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Retinoid metabolism and nuclear receptor responses: New insights into coordinated regulation of the PPAR–RXR complex
Abstract
Retinoids, naturally-occurring vitamin A derivatives, regulate metabolism by activating specific nuclear receptors, including the retinoic acid receptor (RAR) and the retinoid X receptor (RXR). RXR, an obligate heterodimeric partner for other nuclear receptors, including peroxisome proliferator-activated receptors (PPARs), helps coordinate energy balance. Recently, many groups have identified new connections between retinoid metabolism and PPAR responses. We found that retinaldehyde (Rald), a molecule that can yield RA through the action of retinaldehyde dehydrogenases (Raldh), is present in fat in vivo and can inhibit PPAR gamma-induced adipogenesis. In vitro, Rald inhibits RXR and PPAR gamma activation. Raldh1-deficient mice have increased Rald levels in fat, higher metabolic rates and body temperatures, and are protected against diet-induced obesity and insulin resistance. Interestingly, one specific asymmetric beta-carotene cleavage product, apo-14'-carotenal, can also inhibit PPAR gamma and PPAR alpha responses. These data highlight how pathways of beta-carotene metabolism and specific retinoid metabolites may have direct distinct metabolic effects.
... Vitamin A (retinol) is a micronutrient with a wide range of biological functions such as vision, tissue differentiation and immunity [1,2]. Its physiological functions are mainly mediated through its active metabolite, retinoic acid [1,3]. ...
... Vitamin A (retinol) is a micronutrient with a wide range of biological functions such as vision, tissue differentiation and immunity [1,2]. Its physiological functions are mainly mediated through its active metabolite, retinoic acid [1,3]. Recent evidence has shown that vitamin A is also implicated in the pathogenesis of obesity, diabetes, cancer and cardiovascular disease [4][5][6][7]. ...
... Liver plays a pivotal role in the homeostasis of vitamin A via regulating the transport, storage, production and metabolism of retinoids [8,9], where dietary retinol is first oxidized to retinaldehyde by both cytosolic alcohol dehydrogenases (ADHs) and microsomal retinol dehydrogenases (RDHs) [10,11] and subsequently oxidized to retinoic acid by retinaldehyde dehydrogenases (RALDHs) [12]. The conversion of retinaldehyde into retinoic acid is irreversible and considered to be the rate-limiting step in retinoic acid biosynthesis [1,3,13,14]. ...
Obesity and insulin resistance are associated with overexpression of retinaldehyde dehydrogenase 1 (RALDH1). We aimed to investigate the roles of hepatic RALDH1 induction in glucose metabolism impairment using mice fed with high‐fat diet (HFD). Mice were fed with HFD for 8 weeks and treated with RALDH inhibitor citral for another 4 weeks. Oral glucose tolerance test (OGTT), pyruvate tolerance test (PTT) and insulin tolerance test (ITT) were operated. Expressions of Phosphoenolpyruvate carboxykinase 1 (PCK1), glucokinase (GCK) and RALDH1 were measured. Therapeutic effects of citral were also conducted in diabetic rats. Effects of retinaldehyde on PCK1 and GCK expressions were examined in rat primary hepatocytes and HepG2 cells. The results showed that HFD mice were characterized by hyperlipidaemia and insulin resistance, accompanied by significantly increased RALDH1 activity and expression. Citral (10 and 50 mg/kg) ameliorated HFD‐induced hyperlipidaemia and insulin resistance, as evidenced by the improved fasting glucose, insulin levels and lipid profiles. OGTT and PTT demonstrated that citral reversed HFD‐induced glucose disposal impairment and glucose production enhancement. Citral also reversed the increased PCK1 expression and decreased GCK expression by HFD. Citral therapeutic effects were reconfirmed in diabetic rats. In vitro data indicated that retinaldehyde had the strongest PCK1 induction in primary hepatocytes of diabetic rats compared with HFD rats and control rats, in line with the increased RALDH1 expression. Citral reversed the retinaldehyde‐induced PCK1 expression in primary rat hepatocytes and HepG2 cells. In conclusion, RALDH1 induction impaired glucose metabolism partly via modulating PCK1 and GCK expression. Citral improved glucose metabolism through inhibiting RALDH activity.
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... Among RA isomers, ATRA can bind to and activate RARs both in vivo and in vitro. 9-cis-RA can activate RXRs and RARs in vitro, but it does not work in the body [23]. 13-cis-RA makes up about 25% of circulating retinoic acid levels [24]. ...
... 9-cis RA has been detected at extremely low levels. Therefore, most of the in vivo activation of all RAR subtypes (α, β and γ) is considered to be ATRA-mediated [23]. RARs are members of the steroid/vitamin D/thyroid hormone receptor family, the pleiotropic effect of ATRA is mediated through its combination with RARs [18], and regulates many biological functions, such as embryonic development, organogenesis, homeostasis, vision, immune function, and reproduction [25]. ...
Atherosclerosis is a major risk factor for myocardial infarction and ischemic stroke, which are the leading cause of death worldwide. All-trans retinoic acid (ATRA) is a natural derivative of essential vitamin A. Numerous studies have shown that ATRA plays an important role in cell proliferation, cell apoptosis, cell differentiation, and embryonic development. All-trans retinoic acid (ATRA) is a ligand of retinoic acid receptors that regulates various biological processes by activating retinoic acid signals. In this paper, the metabolic processes of ATRA were reviewed, with emphasis on the effects of ATRA on inflammatory cells involved in the process of atherosclerosis.
... ALDH1A1 is the key enzyme involved in the synthesis of γ-aminobutyric acid (GABA) in dopamine neurons 25 . ALDH1A1 knockout mice resist the high fat-induced insulin resistance and obesity and display increased metabolic rates; reduced fasting glucose levels by the inhibition of gluconeogenesis and reduced hepatic triacylglycerol synthesis [26][27][28][29] . High expression and activity of ALDH1A1 is a characteristic feature of stem cells of normal tissues and cancer stem cells and this property has been exploited for the isolation of stem cells. ...
... Our findings have far-reaching implications on the physiological roles of PRMT3 on diverse cellular processes regulated by ALDH1A1, such as development, differentiation, adipogenesis, gluconeogenesis, initiation of meiosis, and synthesis of γaminobutyric acid (GABA) in dopamine neurons [25][26][27][28][29]57 . For instance, PRMT3-mediated inhibition of ALDH1A1 activity might lead to the accumulation of retinaldehyde, which in turn might inhibit adipogenesis 26 . ...
Protein arginine methyltransferase 3 (PRMT3) regulates protein functions by introducing asymmetric dimethylation marks at the arginine residues in proteins. However, very little is known about the interaction partners of PRMT3 and their functional outcomes. Using yeast-two hybrid screening, we identified Retinal dehydrogenase 1 (ALDH1A1) as a potential interaction partner of PRMT3 and confirmed this interaction using different methods. ALDH1A1 regulates variety of cellular processes by catalyzing the conversion of retinaldehyde to retinoic acid. By molecular docking and site-directed mutagenesis, we identified the specific residues in the catalytic domain of PRMT3 that facilitate interaction with the C-terminal region of ALDH1A1. PRMT3 inhibits the enzymatic activity of ALDH1A1 and negatively regulates the expression of retinoic acid responsive genes in a methyltransferase activity independent manner. Our findings show that in addition to regulating protein functions by introducing methylation modifications, PRMT3 could also regulate global gene expression through protein-protein interactions.
... The two RA isomers are detected in vivo: the most abundant all-trans-RA and 13 cis RA (13 cRA), which is found in lower concentrations in mice as well as in humans [9] although some studies indicate that RA cannot be synthesized in animals and must be taken from their diet [10]. The conversion of retinal into RA is irrversible and considered to be the rate-limiting step in RA biosynthesis [11]. The physiological functions of retinol are mainly mediated by its active acid metabolite, RA, which regulates more than 650 gene expressions by activating nuclear receptors RAR and RXR. ...
... All-trans RA bind to RARs, whereas 9-cis RA binds to both RARs and RXRs. In contrast, 13 cRA does not exhibit specific binding to RXRs, and has a 100-fold lower affinity to RARs than all-trans RA or 9-cis RA [11]. ...
Vitamin A and its metabolites are key regulators of the development of adipose tissue and associated metabolic complications. The aim of this study was to determine the effect of high fat diet and 13-cis retinoic acid (13 cRA) application on metabolic parameters, adipogenic and inflammatory indicators in female Lewis rats. Female rats of Lewis strain were fed standard laboratory diet (STD) and high fat diet (HFD, 45% of saturated fatty acids) during 30 days. The groups were divided into additional 3 groups (6 rats each): two experimental groups that received 13 cRA orally on a daily basis during 30 days (7.5 mg/kg and 15 mg/kg, respectively) and the control group that was given sunflower oil. Animals were sacrificed after 60 days. Feeding of Lewis rats with chronic HFD diet with 13 cRA supplementation increased weight gain, adiposity index, dyslipidaemia, hyperleptinaemia, insulin resistance, VLDL concentrations, oxidative stress and atherogenic indices. Administration of 13 cRA in Lewis rats fed STD did not change the weight of the animals, but it slightly increased the atherogenic parameters. 13 cRA and HFD affect metabolic parameters, glucose and lipid metabolism in Lewis rats and its administration has a completely different effect on metabolism in rats fed STD, highlighting the complex role of vitamin A supplementation in obesity. Other factors, such as genetics, age, sex, adipose tissue distribution, also must be taken into consideration.
... 9-cis-retinoic acid (9cRA) and all-trans retinoic acid (atRA) are all bioactive retinoids that belong to vitamin A metabolites (Heine et al., 2018). 9cRA is generated from atRA by spontaneous isomerization in vivo (Yu et al., 2017;Heine et al., 2018) and they can regulate metabolism by activating two specific nuclear receptors--retinoic acid receptor (RAR) and retinoic X receptor (RXR) (Egea et al., 2000;Ziouzenkova and Plutzky, 2008;Abed et al., 2017). Interestingly, RARs can bind both atRA and 9cRA, whereas RXRs exclusively binds 9cRA (Heyman et al., 1992;Egea et al., 2000;Abed et al., 2017). ...
... Moreover, retinoic acid signaling is also existed in the starfish Patiria pectinifera, and serves as a regulator in metamorphosis process (Yamakawa et al., 2018). As an obligate heterodimeric partner for other nuclear receptors, including PPARs (α, δ/β, and γ), which are ligandactivated transcription factors that regulate gene expression (Ziouzenkova et al., 2002), RXR helps coordinate energy balance (Ziouzenkova and Plutzky, 2008). PPARα heterodimerizes with RXRβ, and they cooperate to activate the acyl-CoA oxidase gene promoter (Keller et al., 1993). ...
Melatonin is a highly conserved hormone in evolutionary history. It occurs in numerous organisms and plays a role in the endocrine and immune systems. Locomotor behavior is a basic behavior in animals and is an important indicator of circadian rhythms, which are coordinated by the nervous and endocrine systems. To date, the effect of melatonin on locomotor behavior has been studied in vertebrates, including syrian hamsters, sparrows, rats, zebrafish, goldfish, and flatworms. However, there have been few studies of the effects of melatonin on locomotor behavior in marine invertebrates. The goals of present study were to show the existence of melatonin in the sea cucumber Apostichopus japonicus and to evaluate its effect on locomotor activity. In addition, muscle tissues from control and melatonin-treated sea cucumbers were tested using ultra performance liquid chromatography and quadrupole time-of-flight mass spectrometry (UPLC-Q-TOF-MS) to determine the changes of metabolic activity in muscle. Melatonin was present in the coelomic fluid of A. japonicus at a concentration of ∼135.0 ng/L. The total distance traveled and number steps taken over 9 h after melatonin administration decreased with increasing concentration of the melatonin dose. Mean and maximum velocity of movement and stride length and stride frequency also decreased, but their differences were not statistically significant. Overall, these results suggest that melatonin administration had a sedative effect on A. japonicus. The levels of 22 different metabolites were altered in the muscle tissues of melatonin-treated sea cucumbers. Serotonin, 9-cis retinoic acid, all-trans retinoic acid, flavin mononucleotide in muscles were downregulated after melatonin administration. Moreover, a high free fatty acid (FFA) concentration and a decrease in the adenosine 5′-triphosphate (ATP) concentration in the muscle tissues of the melatonin-treated group were detected as well. These results suggest that the sedative effect of melatonin involves some other metabolic pathways, and the reduced locomotor modulator—serotonin, inhibited fatty acid oxidation and disturbed oxidative phosphorylation are potential physiological mechanisms that result in the inhibitory effect of melatonin on locomotion in sea cucumbers.
... The activation of all RAR isoforms in vivo is specifically mediated by ATRA, whereas RXRs are activated by their own ligand, 9-cis RA. This molecule has demonstrated high-affinity RXRs binding in vitro, but was not detected in cells unless ATRA was present first or added (10)(11)(12). ...
Background/aim:
Myelodysplastic syndromes (MDS) are clinically heterogeneous hematological malignancies with an increased risk of transformation to acute myeloid leukemia, emphasizing the importance of identifying new diagnostic and prognostic markers. This study sought to investigate the predictive ability of all-trans retinoic acid (ATRA)-dependent nuclear transcription factors RARα and PPARβ/δ gene expression in MDS patients.
Materials and methods:
Peripheral blood specimens were collected from 49 MDS patients and 15 healthy volunteers. The specimens were further separated in Ficoll density gradient to obtain the mononuclear cells fractions. Gene expression analysis was carried out using quantitative real-time polymerase chain reaction (qRT-PCR) technique.
Results:
In the mononuclear cell fractions of MDS patients, RARα expression was increased (p<0.05) and PPARβ/δ expression was decreased (p<0.01) compared to healthy volunteers. When RARα and PPARβ/δ expression was compared in groups of MDS patients with different risks of disease progression, no statistically significant difference was found for RARα expression, while PPARβ/δ expression was significantly lower in the high-risk group of patients compared to the low-risk group (p<0.05). The expression of RARα was significantly associated with overall survival (p<0.05). ROC analysis showed that the expression of PPARβ/δ, rather than RARα expression, could have potential diagnostic value for MDS patients (AUC=0.75, p=0.003 and AUC=0.65, p=0.081, respectively).
Conclusion:
RARα and PPARβ/δ genes are putative biomarkers that may be associated with the diagnosis and prognosis of MDS.
... Reprogramming of the cellular metabolism is considered one of the core hallmarks of cancer, but little is still known about the mechanisms that drive this shift in the metabolism [1]. The CpGs were enriched for enhancers and binding regions of TFs involved in metabolism such as the HNF family of TFs [26][27][28] and RXRA [29]. Moreover, we found the emQTL to be more than expected by chance in GeneHancer and IM-PET loops. ...
Aberrant DNA methylation contributes to gene expression deregulation in cancer. However, these alterations precise regulatory role and clinical implications are still not fully understood. In this study, we performed expression-methylation Quantitative Trait Loci (emQTL) analysis to identify deregulated cancer-driving transcriptional net-works linked to CpG demethylation pan-cancer. By analyzing 33 cancer types from The Cancer Genome Atlas, we identified and confirmed significant correlations between CpG methylation and gene expression (emQTL) in cis and trans, both across and within cancer types. Bipartite network analysis of the emQTL revealed groups of CpGs and genes related to important biological processes involved in carcinogenesis; specifically, we identified three types of emQTL networks associated with alterations linked to the regulation of proliferation, metabolism, and hormone-signaling. These bipartite communities were characterized by loss of enhancer methylation in transcription factor binding regions (TFBRs) located in enhancers. The underlying CpGs were topologically linked to upregulated genes through chromatin loops. Loss of enhancer methylation and target genes were exemplified in pancreatic cancer. Penalized Cox regression analysis showed a significant prognostic impact of the pan-cancer emQTL. Taken together, our integrative pan-cancer analysis reveals a common architecture of aberrant DNA demethylation that illustrates a convergence of pathological regulatory mechanisms across cancer types.
... Retinoic acid (RA), the active metabolite of vitamin A, has been reported to modulate essential physiological processes, including vision, reproduction, proliferation, differentiation, and apoptosis [20,21]. In vitro evidence demonstrated that vitamin A is helpful in depolymerizing α-Syn aggregates [22]. ...
Parkinson's disease (PD), the second-most prevalent neurodegenerative disorder, is characterized by the aberrant deposition of α-synuclein (α-Syn) aggregation in neurons. Recent reports have shown that retinoic acid (RA) ameliorates motor deficits. However, the underlying molecular mechanisms remain unclear. In this article, we investigated the effects of RA on cellular and animal models of PD. We found that RA is beneficial for neuronal survival in PD-associated models. In α-Syn preformed fibrils-treated mice, RA administration relieved the formation of intracellular inclusions, dopaminergic neuronal loss, and behavioral deficits. α-Syn preformed fibrils-treated SH-SY5Y cells manifested decreased cell viability, apoptosis, α-Syn aggregation, and autophagy defects. All these negative phenomena were alleviated by RA. More importantly, RA could inhibit the neurotoxicity via inhibiting α-Syn preformed fibrils-induced STAT1-PARP1 signaling, which could also be antagonized by IFN-γ. In conclusion, RA could hinder α-Syn preformed fibrils-induced toxicity by inhibiting STAT1-PARP1 signaling. Thus, we present new insight into RA in PD management.
... Upon ligand activation, RAR heterodimerizes with RXR which integrates a unique transcriptional network dependent on retinoid metabolism and controls the function of other nuclear receptors. RXR can also form heterodimers with many key transcriptional sensors including peroxisome proliferator-activated receptors, the vitamin D receptor,the liver X receptor, farnesoid X receptor, that help maintain homeostasis in many pathways.68 Furthermore, former studies clearly indicated that V A deprivation is directly involved in motor neurons pathology.69 ...
Introduction
A recent approach to cure neurodegenerative diseases is to reprogram fibroblasts into functioning neurons using multiple exogenous transcription factors (TFs) and micro‐RNAs. Administering agents that can endogenously induce these TFs may bypass the limitations of this approach. Astrocytes may represent a part of the extrahepatic‐stellate system involved in vitamin‐A (VA) homeostasis. Activated‐stellate cells lose their VA‐storage capacity, and this was previously applied for hepatic‐stellate cells (HSCs) targeting to treat liver fibrosis. Accordingly, it is hypothesized that Parkinson's disease (PD) may be coupled with retinoid depletion that may extract VA from VA‐rich‐HSCs triggering liver fibrosis. Thus, VA administration may selectively target VA‐deficient reactive astrocytes and HSCs. Besides, VA has the regenerative capability and may induce endogenous‐TFs generation.
Methods
Fluorescently labeled VA‐coupled liposomes (FLV) were traced using confocal laser microscope in rats with induced PD for detecting brain accumulation and uptake into fluorescently labeled astrocytes. Liver fibrosis associated with PD was assessed biochemically and histopathologically, while VA deficiency was confirmed by assessing retinol‐binding protein gene expression in the brain and liver. Multiple VA doses were tested for reversing PD‐associated liver fibrosis, generating TFs (involved in reprograming astrocytes/fibroblasts into different neuronal types) and capability of dopaminergic‐neurons regeneration.
Results
Fluorescently labeled VA‐coupled liposomes revealed selective brain accumulation and uptake into astrocytes. PD was associated with significant liver fibrosis and VA deficiency in the brain and liver. Furthermore, VA‐medium dose (VAMD) was the optimum one for reversing PD‐associated liver fibrosis, generating multiple astrocytes/fibroblasts reprogramming TFs, regenerating dopaminergic neurons, and improving PD.
Conclusion
VA‐medium dose pursued brain targeting in PD with the potential capability of regenerating neurons and restoring dopaminergic transmission. This may place this therapy as an essential treatment in PD management protocol.
... The retinol metabolic pathway was also downregulated in the HC cows, with lower expression of RBP4 and TTR (encoding retinol binding protein and transthyretin, respectively), both of which act as retinol transporters in the blood. Interaction of retinol with the PPAR signalling pathway influences transcription of many other downstream genes [58]. APOA2 and APOC3 together with APOH are also part of the cholesterol metabolic pathway. ...
The functionality of circulating leukocytes in dairy cows is suppressed after calving, with negative energy balance as a risk factor. Leukocyte transcriptomic profiles were compared separately in 44 multiparous (MP) and 18 primiparous (PP) Holstein–Friesian cows receiving diets differing in concentrate proportion to test whether immune dysfunction could be mitigated by appropriate nutrition. After calving, cows were offered either (1) low concentrate (LC); (2) medium concentrate (MC) or (3) high concentrate (HC) diets with proportions of concentrate to grass silage of 30%:70%, 50%:50% and 70%:30%, respectively. Cow phenotype data collected included circulating metabolites, milk yield and health and fertility records. RNA sequencing of circulating leukocytes at 14 days in milk was performed. The HC diet improved energy balance in both age groups. There were more differentially expressed genes in PP than MP cows (460 vs. 173, HC vs. LC comparison) with few overlaps. The MP cows on the LC diet showed upregulation of the complement and coagulation cascade and innate immune defence mechanisms against pathogens and had a trend of more cases of mastitis and poorer fertility. In contrast, the PP cows on the HC diet showed greater immune responses based on both gene expression and phenotypic data and longer interval of calving to conception. The leukocytes of MP and PP cows therefore responded differentially to the diets between age, nutrient supply and immunity affecting their health and subsequent fertility.
... In target cells, the active form of vitamin A mediates a pleiotropic effect via nuclear receptors or indirect nongenomic interactions with target genes. The signaling pathways of vitamin A have been reported to be highly complex because they can interact with various nuclear receptors (Ziouzenkova & Plutzky, 2008). ...
Vitamin A is an essential micronutrient whose deficiency is still a major health concern in many regions of the world. It plays an essential role in human growth and development, immunity, and vision, but may also help prevent several other chronic diseases. The total amount of vitamin A in the human diet often falls below the recommended dietary allowance of approximately 900-1000 μg/day for a healthy adult. Moreover, a significant proportion of vitamin A may be degraded during food processing, storage, and distribution, thereby reducing its bioactivity. Finally, the vitamin A in some foods has a relatively low bioavailabil-ity, which further reduces its efficacy. The World Health Organization has recommended fortification of foods and beverages as a safe and cost-effective means of addressing vitamin A deficiency. However, there are several factors that must be overcome before effective fortified foods can be developed, including the low solubility, chemical stability, and bioavailability of this oil-soluble vitamin. Consequently , strategies are required to evenly disperse the vitamin throughout food matrices, to inhibit its chemical degradation, to avoid any adverse interactions with any other food components, to ensure the food is palatable, and to increase its bioavailability. In this review article, we discuss the chemical, physical, and nutritional attributes of vitamin A, its main dietary sources, the factors contributing to its current deficiency, and various strategies to address these deficiencies, including diet diversification, biofortification, and food fortification.
... In target cells, the active form of vitamin A mediates a pleiotropic effect via nuclear receptors or indirect nongenomic interactions with target genes. The signaling pathways of vitamin A have been reported to be highly complex because they can interact with various nuclear receptors (Ziouzenkova & Plutzky, 2008). ...
Vitamin A is an essential micronutrient whose deficiency is still a major health concern in many regions of the world. It plays an essential role in human growth and development, immunity, and vision, but may also help prevent several other chronic diseases. The total amount of vitamin A in the human diet often falls below the recommended dietary allowance of approximately 900–1000 𝜇 g/day for a healthy adult. Moreover, a significant proportion of vitamin A may be degraded during food processing, storage, and distribution, thereby reducing its bioactivity. Finally, the vitamin A in some foods has a relatively low bioavailability, which further reduces its efficacy. The World Health Organization has recommended fortification of foods and beverages as a safe and cost-effective means of addressing vitamin A deficiency. However, there are several factors that must be overcome before effective fortified foods can be developed, including the low solubility, chemical stability, and bioavailability of this oil-soluble vitamin. Consequently, strategies are required to evenly disperse the vitamin throughout food matrices, to inhibit its chemical degradation, to avoid any adverse interactions with any other food components, to ensure the food is palatable, and to increase its bioavailability. In this review article, we discuss the chemical, physical, and nutritional attributes of vitamin A, its main dietary sources, the factors contributing to its current deficiency, and various strategies to address these deficiencies, including diet diversification, biofortification, and food fortification.
... During studies on β-carotene and adipocyte differentiation, Ziouzenkova et al. [31,32] presented evidence that β-apo-14 -carotenal acted as a transcriptional repressor of RARα, RXRα, PPARα, PPARγ, LXRα, and LXRβ. They used standard NR-LBD-Gal4 reporter assays in transfected cells. ...
Naturally occurring retinoids (retinol, retinal, retinoic acid, retinyl esters) are a subclass of β-apocarotenoids, defined by the length of the polyene side chain. Provitamin A carotenoids are metabolically converted to retinal (β-apo-15-carotenal) by the enzyme β-carotene-15,15′-dioxygenase (BCO1) that catalyzes the oxidative cleavage of the central C=C double bond. A second enzyme β-carotene-9′-10′-dioxygenase cleaves the 9′,10′ bond to yield β-apo-10′-carotenal and β-ionone. Chemical oxidation of the other double bonds leads to the generation of other β-apocarotenals. Like retinal, some of these β-apocarotenals are metabolically oxidized to the corresponding β-apocarotenoic acids or reduced to the β-apocarotenols, which in turn are esterified to β-apocarotenyl esters. Other metabolic fates such as 5,6-epoxidation also occur as for retinoids. Whether the same enzymes are involved remains to be understood. β-Apocarotenoids occur naturally in plant-derived foods and, therefore, are present in the diet of animals and humans. However, the levels of apocarotenoids are relatively low, compared with those of the parent carotenoids. Moreover, human studies show that there is little intestinal absorption of intact β-apocarotenoids. It is possible that they are generated in vivo under conditions of oxidative stress. The β-apocarotenoids are structural analogs of the naturally occurring retinoids. As such, they may modulate retinoid metabolism and signaling. In deed, those closest in size to the C-20 retinoids—namely, β-apo-14′-carotenoids (C-22) and β-apo-13-carotenone (C-18) bind with high affinity to purified retinoid receptors and function as retinoic acid antagonists in transactivation assays and in retinoic acid induction of target genes. The possible pathophysiologic relevance in human health remains to be determined.
... The stimulation of the plethora of gene cascades implicated in a wide range of body functions takes place following the selective interaction of PPAR with DNA sequences [24]. The interaction of PPAR as well as RXR heterodimers with the co-repressor complexes and the repression of transcription of genes occur during the non-appearance of ligands [25]. In contrast, the presence as well as interaction of natural (FA, and related compounds), and synthetic ([4-[3-(4-Acetyl-3-hydroxy-2-propylphenoxy)propoxy]phenoxy]acetic acid (L-165041), 2- [2-methyl-4-[[4-methyl-2-[4-(trifluoromethyl)phenyl]-1,3-thiazol-5-yl]methylsulfonyl]phenoxy]acetic acid (GW-501516), glitazones/thiazolidinediones (TZDs), and fibrates) and ligands provokes a configurational transition inside the PPAR, prompting co-repressor amino acid chain to detach and the co-activator amino acid chain to engage in order to elevate target gene transcription [16,23,26]. ...
One of the utmost frequently emerging neurodegenerative diseases, Parkinson's disease (PD), comprehend the forfeit of dopamine (DA)-generating nerve cells in the substantia nigra pars compacta (SN-PC). The etiology and pathogenesis underlying the emergence of PD is still obscure. However, expanding corroboration encourages the involvement of genetic and environmental factors in the etiology of PD. The destruction of numerous cellular components, namely oxidative stress, ubiquitin-proteasome system (UPS) dysfunction, autophagy-lysosome system dysfunction, neuroinflammation and programmed cell death, and mitochondrial dysfunction partake in the pathogenesis of PD. Present-day pharmacotherapy can alleviate the manifestations, but no therapy has been demonstrated to cease disease progression. Peroxisome proliferator-activated receptors (PPARs) are ligand-directed transcription factors pertaining to the class of nuclear hormone receptors (NHR), and are implicated in the modulation of mitochondrial operation, inflammation, wound healing, redox equilibrium, and metabolism of blood sugar and lipids. Numerous PPAR agonists have been recognized to safeguard nerve cells from oxidative destruction, inflammation, and programmed cell death in PD and other neurodegenerative diseases. Additionally, various investigations suggest that regular administration of PPAR-activating non-steroidal anti-inflammatory drugs (NSAIDs) (ibuprofen, indomethacin), and leukotriene receptor antagonists (montelukast) were related to the de-escalated evolution of neurodegenerative diseases. The present review elucidates the emerging evidence enlightening the neuroprotective outcomes of PPAR agonists in in vivo and in vitro models experiencing PD. Existing articles up to the present were procured through PubMed, MEDLINE, etc., utilizing specific keywords spotlighted in this review. Furthermore, the authors aim to provide insight into the neuroprotective actions of PPAR agonists by outlining the pharmacological mechanism. As a conclusion, PPAR agonists exhibit neuroprotection through modulating the expression of a group of genes implicated in cellular survival pathways, and may be a propitious target in the therapy of incapacitating neurodegenerative diseases like PD.
... RXR, an obligate heterodimeric partner of many nuclear receptors (NR), occupies a central place in NR signaling and plays a critical role in maintaining energy homeostasis (62). In human liver, RXR could promote fatty acid oxidation by forming a heterodimer with peroxisome proliferator-activated receptor alpha (PPARα) which could be activated by adiponectin to magnify this signaling (63). ...
Screening functional food ingredients (FFI) from medicinal and edible plants (MEP) has still remained a great challenge due to the complexity of MEP and its obscure function mechanisms. Herein, an integrated strategy based on sequential metabolites identification approach, network pharmacology, molecular docking, and surface plasmon resonance (SPR) analysis was proposed for quickly identifying the active constituents in MEP. First, the sequential biotransformation process of MEP, including intestinal absorption and metabolism, and hepatic metabolism, was investigated by oral gavage, and intestinal perfusion with venous sampling method. Then the blood samples were analyzed by UPLC-Q Exactive Orbitrap HRMS. Second, the network pharmacology approach was used to explore the potential targets and possible mechanisms of the in vivo metabolites of MEP. Third, molecular docking and SPR approaches were used to verify the specific interactions between protein targets and representative ingredients. The proposed integrated strategy was successfully used to explore the heptoprotective components and the underlying molecular mechanism of Paeoniae Radix Alba (PRA). A total of 44 compounds were identified in blood samples, including 17 porotypes and 27 metabolites. The associated metabolic pathways were oxidation, methylation, sulfation, and glucuronidation. After further screening, 31 bioactive candidates and 377 related targets were obtained. In addition, the bioactive components contained in PRA may have therapeutic potentials for non-alcoholic fatty liver disease (NAFLD). The above results demonstrated the proposed strategy may provide a feasible tool for screening FFI and elaborating the complex function mechanisms of MEP.
... While direct targeting of developmental pathways (such as Shh, Wnt, Notch pathways) may be curtailed by the complex and pleiotropic homeostatic roles they exert in healthy tissues [47,97,98], some key nuclear receptors (e.g. PPARγ, RAR/RXR and VDR) [35,42,[99][100][101] and lineage-promoting transcription factors [75] amenable to targeting by small molecules [28] represent appealing candidates for differentiation-inducing therapies. ...
Cancer has been traditionally viewed as a disease characterised by excessive and uncontrolled proliferation, leading to the development of cytotoxic therapies against highly proliferating malignant cells. However, tumours frequently relapse due to the presence of slow-cycling cancer stem cells eluding chemo and radiotherapy. Since these malignant stem cells are largely undifferentiated, inducing their lineage commitment has been proposed as a potential intervention strategy to deplete tumours from their most resistant components. Pro-differentiation approaches have thus far yielded clinical success in the reversion of acute promyelocytic leukaemia (APL), and new developments are fast widening their therapeutic applicability to solid carcinomas. Recent advances in cancer differentiation discussed here highlight the potential and outstanding challenges of differentiation-based approaches.
... The PPARs are nuclear receptor proteins that work as transcription factors regulating the expression of genes involved in adipogenesis and adipocyte differentiation (Kersten et al., 2000;Dunning et al., 2014). RXR is a nuclear receptor that is usually combined with a PPAR protein as a heterodimer that bonds with obesogens and regulates gene expression (Ziouzenkova and Plutzky, 2008). The heterodimer of PPAR-RXR has been documented to be a key regulator in the TBT-induced obesogenic effect (Grun and Blumberg, 2006;Treviño et al., 2016). ...
The widely used antifoulant tributyltin chloride (TBT) is highly toxic to aquatic organisms. In the present study, four-week-old Japanese medaka (Oryzias latipes) juveniles were orally exposed to TBT at 1 and 10 ng/g bw/d for 1, 2, and 4 weeks, respectively. Half of the tested medaka juveniles showed bone morphology alterations in both 1 and 10 ng/g bw/d TBT 4-week exposure groups. Nile Red (NR) staining showed that the juveniles exposed to 1 ng/g bw/d TBT for 2 and 4 weeks had significantly enlarged adipocyte areas. The mRNA-Seq analysis indicated that 1 ng/g bw/d TBT exposure for 2 weeks affected bone morphology through developmental processes. The GO and KEGG analyses suggested that the adipogenic effect of TBT observed in this study may be induced by metabolic processes, oxidative phosphorylation, and fatty acid degradation and metabolism pathways. Therefore, both morphological observation and mRNA-Seq analysis showed obesogenic effects and developmental toxicity of TBT to juvenile Japanese medaka.
... We found that a 1% increase in FE was associated with a 26% decrease in Aldh1a1 expression levels. In fact, Aldh1a1 is a key enzyme in the synthesis of 9-cis retinoic acid, the potent agonist of both the retinoic acid receptor (RAR) and retinoid X receptor (RXR) (Ziouzenkova & Plutzky, 2008). Deficiency of Aldh1a1 inhibited 9-cis retinoic acid synthesis, leading to significantly attenuated hepatic glucose production as well as hepatic triacylglycerol synthesis (Kiefer et al., 2012). ...
Background:
Feed efficiency (FE) is an important trait for livestock and humans. While the livestock industry focuses on increasing FE, in the current obesogenic society it is more of interest to decrease FE. Hence, understanding mechanisms involved in the regulation of FE and particularly how it can be decreased would help tremendously in counteracting the obesity pandemic. However, it is difficult to accurately measure or calculate FE in humans. In this study, we aimed to address this challenge by developing a hierarchical dynamic model based on humanized mouse data.
Methods:
We analyzed existing experimental data derived from 105 APOE*3-Leiden.CETP (E3L.CETP) mice fed a high-fat high-cholesterol (HFHC) diet for 1 (N = 20), 2 (N = 19), 3 (N = 20), and 6 (N = 46) month. We developed an ordinary differential equation (ODE) based model to estimate the FE based on the longitudinal data of body weight and food intake. Since the liver plays an important role in maintaining metabolic homeostasis, we evaluated associations between FE and hepatic gene expression levels. Depending on the feeding duration, we observed different relationships between FE and hepatic gene expression levels.
Results:
After 1-month feeding of HFHC diet, we observed that FE was associated with vitamin A metabolism, arachidonic acid metabolism, and the PPAR signaling pathway. After 3- and 6-month feeding of HFHC diet, we observed that FE was associated most strongly with expression levels of Spink1 and H19, genes involved in cell proliferation and glucose metabolism, respectively.
Conclusions:
In conclusion, our analysis suggests that various biological processes such as vitamin A metabolism, hepatic response to inflammation, and cell proliferation associate with FE at different stages of diet-induced obesity.
... The PPARs also heterodimerize with other nuclear receptors such as retinoid X receptor (RXR) (Dawson and Xia, 2012;Qi et al., 2000). Retinoids such as retinal, retinaldehyde, and apo14 modulate RXR and PPAR both in vitro and in vivo (Ziouzenkova et al., 2007a(Ziouzenkova et al., , 2007bZiouzenkova and Plutzky, 2008). We observed significant upregulation of genes involved in retinol metabolism, including rdh12l, rdh20, retsat, dhrs4, dhrs9, bco1, dgat1a suggesting that PPAR-RXR crosstalk is affected. ...
Human consumption of cannabinoid-containing products during early life or pregnancy is rising. However, information about the molecular mechanisms involved in early life stage Δ9-tetrahydrocannabinol (THC) and cannabidiol (CBD) toxicities is critically lacking. Here, larval zebrafish (Danio rerio) were used to measure THC and CBD-mediated changes on transcriptome and the roles of cannabinoid receptors (Cnr) 1 and 2 and peroxisome proliferator activator receptor γ (PPARγ) in developmental toxicities. Transcriptomic profiling of 96-hour post fertilization (hpf) cnr+/+ embryos exposed (6-96 hpf) to 4 μM THC or 0.5 μM CBD showed differential expression of 904 and 1095 genes for THC and CBD, respectively, with 360 in common. KEGG pathways enriched in the THC and CBD datasets included those related to drug, retinol, and steroid metabolism and PPAR signaling. THC exposure caused increased mortality and deformities (pericardial and yolk sac edemas, reduction in length) in cnr1-/- and cnr2-/- fish compared to cnr+/+ suggesting Cnr receptors are involved in protective pathways. Conversely, the cnr1-/- larvae were more resistant to CBD-induced malformations, mortality, and behavioral alteration implicating Cnr1 in CBD-mediated toxicity. Behavior (decreased distance travelled) was the most sensitive endpoint to THC and CBD exposure. Co-exposure to the PPARγ inhibitor GW9662 and CBD in cnr+/+ and cnr2-/- strains caused more adverse outcomes compared to CBD alone, but not in the cnr1-/- fish, suggesting that PPARγ plays a role in CBD metabolism downstream of Cnr1. Collectively, PPARγ, Cnr1, and Cnr2 play important roles in the developmental toxicity of cannabinoids with Cnr1 being the most critical.
... The American Journal of Pathologyajp.amjpathol.org 9 heterodimers with retinoid X receptors and is a regulator of adipocyte differentiation. 37 Ectopic expression of PPARg in nonadipogenic embryonic fibroblasts stimulates the adipocyte gene transcription program and drives adipogenesis, 38 whereas no other single factor can induce adipogenesis in the absence of PPARg. 39 Several transcription factors have been shown to be able to induce expression of PPARg, although STAT6 was not among them in one review focusing on adipose tissue. ...
Solitary fibrous tumors (SFTs) harbor activating NAB2-STAT6 gene fusions. Different variants of the NAB2-STAT6 gene fusion have been associated with distinct clinico-pathological features. Lipomatous SFTs are a morphological variant of SFTs characterized by a fat-forming tumor component. The aim of the current study was to evaluate NAB2-STAT6 fusion variants and further molecular genetic features in a cohort of lipomatous SFTs. A hybrid-capture based next-generation sequencing panel was employed to detect NAB2-STAT6 gene fusions at the RNA level. Additionally, the RNA expression levels of 507 genes were evaluated using this panel, and were compared to a control cohort of non-lipomatous SFTs. Notably, 5/11 (45%) of lipomatous SFTs in the current series harbored the uncommon NAB2 exon 4 - STAT6 exon 4 gene fusion variant, which is observed in only 0.9 – 1.4% of non-lipomatous SFTs. Furthermore, lipomatous SFTs displayed significant differences in gene expression compared to their non-lipomatous counterparts, including upregulation of the gene Peroxisome Proliferator Activated Receptor Gamma (PPARG). PPARγ is a nuclear receptor regulating adipocyte differentiation, providing a possible explanation for the fat-forming component in lipomatous SFTs. In summary, the current study provides a possible molecular genetic basis for the distinct morphological features of lipomatous SFTs.
... Interestingly, in Young-HLZ, the observations that (1) the higher circulating levels of retinol, the increase of which has been previously shown to be linked to PPARalpha induction [93] and (2) the higher DHA/EPA ratio, previously shown to be a marker of increased peroxisomal beta-oxidation induced by PPAR-alpha [94], together support the hypothesis of enhanced PPAR-alpha activity. ...
To evaluate whether a peculiar plasma profile of fatty acids and endocannabinoidome (eCBome)-related mediators may be associated to longevity, we assessed them in octogenarians (Old; n=42) living in the east-central mountain area of Sardinia, a High-Longevity Zone (HLZ), compared to sexagenarian (Young; n=21) subjects from the same area, and to Olds (n=22) from the Northern Sardinia indicated as Lower-Longevity Zone (LLZ). We found significant increases in conjugated linoleic acid (CLA) and heptadecanoic acid (17:0) levels in Old-HLZ with respect to younger subjects and Old-LLZ subjects. Young-HLZ subjects exhibited higher circulating levels of pentadecanoic acid (15:0) and retinol. Palmitoleic acid (POA) was elevated in both Young and Old subjects from the HLZ. eCBome profile showed a significantly increased plasma level of the two endocannabinoids, N-arachidonoyl-ethanolamine (AEA) and 2-arachidonoyl-glycerol (2-AG) in Old-HLZ subjects compared to Young-HLZ and Old-LLZ respectively. In addition, we found increased N-oleoyl-ethanolamine (OEA), 2-linoleoyl-glycerol (2-LG) and 2-oleoyl-glycerol (2-OG) levels in Old-HLZ group with respect to Young-HLZ (as for OEA an d 2-LG) and both the Old-LLZ and Young-HLZ for 2-OG. The endogenous metabolite of docosahexaenoic acid (DHA), N-docosahexaenoyl-ethanolamine (DHEA) was significantly increased in Old-HLZ subjects. In conclusion, our results suggest that in the HLZ area, Young and Old subjects exhibited a favourable, albeit distinctive, fatty acids and eCBome profile that may be indicative of a metabolic pattern potentially protective from adverse chronic conditions. These factors could point to a suitable physiological metabolic pattern that may counteract the adverse stimuli leading to age-related disorders such as neurodegenerative and metabolic diseases.
... For over a decade, adipose tissue has been considered a main storage site for vitamin A-derived metabolites (48,51), which function at the transcriptional level to control the expression of genes involved in fuel metabolism (refs. 52, 53 and reviewed in ref. 54) and mitochondrial biogenesis (49). A handful of alcohol and retinol dehydrogenase enzymes are capable of oxidizing retinol to retinaldehyde to facilitate the synthesis of these regulatory molecules. ...
The relationship between adiposity and metabolic health is well established. However, very little is known about the fat depot, known as paracardial fat (pCF), located superior to and surrounding the heart. Here, we show that pCF remodels with aging and a high-fat diet and that the size and function of this depot are controlled by alcohol dehydrogenase 1 (ADH1), an enzyme that oxidizes retinol into retinaldehyde. Elderly individuals and individuals with obesity have low ADH1 expression in pCF, and in mice, genetic ablation of Adh1 is sufficient to drive pCF accumulation, dysfunction, and global impairments in metabolic flexibility. Metabolomics analysis revealed that pCF controlled the levels of circulating metabolites affecting fatty acid biosynthesis. Also, surgical removal of the pCF depot was sufficient to rescue the impairments in cardiometabolic flexibility and fitness observed in Adh1-deficient mice. Furthermore, treatment with retinaldehyde prevented pCF remodeling in these animals. Mechanistically, we found that the ADH1/retinaldehyde pathway works by driving PGC-1α nuclear translocation and promoting mitochondrial fusion and biogenesis in the pCF depot. Together, these data demonstrate that pCF is a critical regulator of cardiometabolic fitness and that retinaldehyde and its generating enzyme ADH1 act as critical regulators of adipocyte remodeling in the pCF depot.
... RAR and PPAR are heterodimeric partners of the retinoid X receptor (RXR) family, whose activity is regulated by the presence of retinoic acid (RA) or/and all-trans-retinoic acid (ATRA). 44 RA maintains an immune tolerant state in dendritic cells by preventing induced expansion of Th17 cells. 45 ...
Background
SARS‐CoV‐2 triggers a dysregulated innate immune system activation. As the mevalonate pathway (MVP) prevents the activation of inflammasomes and cytokine release and regulates endosomal transport, compromised signaling could be associated with the pathobiology of COVID‐19.
Prior transcriptomic studies of host cells in response to SARS‐CoV‐2 infection have not reported to date the effects of SARS‐CoV‐2 on the MVP. In this study, we accessed public datasets to report in‐silico investigations into gene expression. In addition, we proposed candidate genes which are thought to have a direct association with the pathogenesis of COVID‐19, and which may be dependent on signals derived from the MVP.
Results
Our results revealed dysregulation of genes involved in the MVP. These results were not found when investigating the gene expression data from host cells infected with H3N2 influenza virus, H1N1 influenza virus (IAV) or respiratory syncytial virus (RSV). Our manually curated gene set showed significant gene expression variability in A549 cells infected with SARS‐CoV‐2, as per Blanco‐Melo, et al. dataset (GSE147507).
Conclusions
In light of the present findings, SARS‐CoV‐2 could hijack the MVP, leading to hyperinflammatory responses. Prompt reconstitution of this pathway with available agents should be considered in future studies.
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... Intracellular retinoic acid availability and nuclear transport is facilitated by cellular retinoic acid-binding proteins and fatty acid binding protein 4 (28)(29)(30). Retinoic acid signals predominantly through the nuclear receptors retinoic acid receptors (RAR), retinoid X receptors (RXR) and peroxisome proliferator-activated receptors (PPAR) (24,31) and is thus a potent regulator or gene transcription (Figure 1). While 9-cis retinoic acid has been found to be a potent ligand for RXR, its physiological relevance is under debate (32,33). ...
In recent years, brown adipose tissue (BAT) has gained significance as a metabolic organ dissipating energy through heat production. Promotion of a thermogenic program in fat holds great promise as potential therapeutic tool to counteract weight gain and related sequelae. Current research efforts are aimed at identifying novel pathways regulating brown fat function and the transformation of white adipocytes into BAT-like cells, a process called "browning." Besides numerous genetic factors some circulating molecules can act as mediators of adipose tissue thermogenesis. Vitamin A metabolites, the retinoids, are potent regulators of gene transcription through nuclear receptor signaling and are thus involved in a plethora of metabolic processes. Accumulating evidence links retinoid action to brown fat function and browning of WAT mainly via orchestrating a transcriptional BAT program in adipocytes including expression of key thermogenic genes such as uncoupling protein 1. Here we summarize the current understanding how retinoids play a role in adipose tissue thermogenesis through transcriptional control of thermogenic gene cassettes and potential non-genomic mechanisms.
... Retinoid metabolism (vitamin A and its derivatives) is known to be integral in regulating energy balance through actions in adipose tissue and the liver [10e12]. Retinoids are diverse signalling molecules that can modulate the activity of the nuclear receptors retinoic acid receptor (RAR) and retinoid X receptor (RXR) [13,14]. Physiologic retinoid concentrations depend on vitamin A intake, tissue storage and subsequent modification through a complex enzymatic network. ...
Objective:
Transformation of white into brown fat ("browning") reduces obesity in many preclinical models and holds great promise as therapeutic concept in metabolic disease. Vitamin A metabolites (retinoids) have been linked to thermogenic programming of adipose tissue, however the physiologic importance of systemic retinoid transport for adipose tissue browning and adaptive thermogenesis is unknown.
Methods:
We performed cold exposure studies in mice and humans and used a genetic model of defective vitamin A transport, the retinol binding protein deficient (Rbp-/-) mouse, to study the effects of cooling on systemic vitamin A and the relevance of intact retinoid transport on cold-induced adipose tissue browning.
Results:
We show that cold stimulation in mice and humans leads to an increase in circulating retinol and its plasma transporter Rbp. In Rbp-/- mice thermogenic programming of adipocytes and oxidative mitochondrial function are dramatically impaired in subcutaneous white fat, which renders Rbp-/- mice more cold sensitive. In contrast, retinol stimulation in primary human adipocytes promotes thermogenic gene expression and mitochondrial respiration. In humans, cold-mediated retinol increase is associated with a shift in oxidative substrate metabolism suggestive of higher lipid utilization.
Conclusions:
Systemic vitamin A levels are regulated by cold exposure in mice and humans and intact retinoid transport is essential for cold-induced adipose tissue browning and adaptive thermogenesis.
... Retinoic acid (RA) is a metabolite of β-carotene and is transformed by the enzymes in the intestinal mucosa (Mudroňová et al., 2018). RA activates its signaling pathway predominantly by binding to the heterodimeric nuclear receptors, RARs and RXRs (Pantazi et al., 2015;Ziouzenkova & Plutzky, 2008). RA has several functions that contribute to a potent mucosal IgA response, including facilitating the proliferation and differentiation of immunocompetent cells (Pantazi et al., 2015;Rühl, 2007), amplifying the induction of IgA class switching and imprinting of the gut-homing receptors on T and B cells (Mora & von Andrian, 2009). ...
β‐carotene is a robust modulator of mucosal barriers, and it can amplify the immunoglobulin A (IgA) response via the retinoic acid (RA)–mediated pathway. We investigated the influence of β‐carotene on intestinal barriers in layer‐type cockerels. In this study, β‐carotene has a positive influence on growth performance and intestinal morphology. β‐carotene remarkably enhanced serum secretory immunoglobulin A (sIgA) levels, jejunal mucosal sIgA, and IgA concentrations. β‐Carotene significantly enhanced mRNA expression levels of IgA, CC chemokine receptor‐9 (CCR9), polymeric immunoglobulin receptor (pIgR), and retinoic acid receptor α (RARα) in the ileal tissues and pIgR in the jejunal tissues. β‐Carotene improves mRNA expression of intestinal barrier‐related proteins including: mucin‐2 (MUC‐2), zonula occludens‐2 (ZO‐2), occludins (OCLN), and zonula occludens‐1 (ZO‐1) in the ileal tissues. Moreover, β‐carotene decreased the levels of Escherichia coli and elevates the levels of Lactobacillus. The results indicate that β‐carotene can promote growth performance and contribute to the gradual development of intestinal barriers in Hyline Brown chicks. This study enriches our knowledge about the effects of β‐carotene on intestinal barrier and highlights a theoretical basis of β‐carotene application in the poultry industry.
... As a member of most nuclear receptors, PPARG has a ligand-dependent activation domain, DNA binding domain, hinge domain, and ligand binding domain. PPARG can bind with retinoic acid receptor a (RXRa) to form heterodimers and then bind to the target gene PPRE (peroxisome proliferator response elements) to activate the expression of target genes (32). Therefore, retinoic acid can affect fat production through the interaction between RXRa and PPARG. ...
As a member of the Iroquois homeobox gene family, the IRX3 gene plays an important role in regulating the growth, development and fat deposition of chordates. In the present study, we found that the bovine IRX3 gene was highly expressed in lung, kidney, heart, subcutaneous fat and longissimus dorsi muscle using real-time PCR. We cloned the full-length sequence of the bovine IRX3 gene promoter and constructed eight series of 5′ deletion promoter plasmid luciferase reporter assays, then transfected them to 3T3-L1 and C2C12 cell lines to detect its core promoter regions. The results showed that the core promoter of bovine IRX3 was located within a -292/-42 bp region relative to the transcriptional start site (TSS). Furthermore, sequence analysis identified eight CpG islands in the promoter region. An chromatin immunoprecipitation (ChIP) assay in combination with site-directed mutation and siRNA interference demonstrated that SREBF2 and PPARG binding occurs in region -292/-42 and is essential in bovine IRX3 transcription. These results lay an important theoretical foundation for exploring the molecular regulation mechanism of the IRX3 gene in bovine fat deposition.
... One of the most often claimed is the likely difference between rodents and humans in terms of functional relevance. Despite of that, current data support its human role showing that there is room for exploration and even for a potential development of new approaches and rational therapies [256]. Data obtained in rodents indicates that chronic AMPK activation protects against high-fat diet-induced obesity through both UCP1 dependent and independent mechanisms. ...
The hypothalamus is a brain region in charge of many vital functions. Among them, BAT thermogenesis represents an essential physiological function to maintain body temperature. In the metabolic context, it has now been established that energy expenditure attributed to BAT function can contribute to the energy balance in a substantial extent. Thus, therapeutic interest in this regard has increased in the last years and some studies have shown that BAT function in humans can make a real contribution to improve diabetes and obesity-associated diseases. Nevertheless, how the hypothalamus controls BAT activity is still not fully understood. Despite the fact that much has been known about the mechanisms that regulate BAT activity in recent years, and that the central regulation of thermogenesis offers a very promising target, many questions remain still unsolved. Among them, the possible human application of knowledge obtained from rodent studies, and drug administration strategies able to specifically target the hypothalamus. Here, we review the current knowledge of homeostatic regulation of BAT, including the molecular insights of brown adipocytes, its central control, and its implication in the development of obesity.
... Therefore, antioxidants may have a protective role against DEHP induced insulin resistance. In addition, certain antioxidants such as beta-carotene have been shown to inhibit proliferator-activated receptors (PPARs), which may protect against adiposity and insulin resistance (Ziouzenkova and Plutzky, 2008). In experimental studies, rats treated with DEHP showed impaired insulin signal transduction and elevated blood glucose levels compared with controls. ...
... In the absence of ALDH1A1, both adipogenesis in vitro and dietinduced fat formation in vivo are markedly impaired. ALDH1A1 deficiency increases retinaldehyde levels that inhibits RXR and PPARG activation, inhibiting PPARG-induced adipogenesis [52]. In diabetic DSCs, adipogenic differentiation is also prevented by the downregulation of the PPARG gene. ...
Diabetic foot ulcers (DFUs) are lesions that involve loss of epithelium and dermis, sometimes involving deep structures, compartments, and bones. The aim of this work is to investigate the innate regenerative properties of dermal tissue around ulcers by the identification and analysis of resident dermal stem cells (DSCs). Dermal samples were taken at the edge of DFUs, and genes related to the wound healing process were analyzed by the real-time PCR array. The DSCs were isolated and analyzed by immunofluorescence, flow cytometry, and real-time PCR array to define their stemness properties. The gene expression profile of dermal tissue showed a dysregulation in growth factors, metalloproteinases, collagens, and integrins involved in the wound healing process. In the basal condition, diabetic DSCs adhered on the culture plate with spindle-shaped fibroblast-like morphology. They were positive to the mesenchymal stem cells markers CD44, CD73, CD90, and CD105, but negative for the hematopoietic markers CD14, CD34, CD45, and HLA-DR. In diabetic DSCs, the transcription of genes related to self-renewal and cell division were equivalent to that in normal DSCs. However, the expression of CCNA2, CCND2, CDK1, ALDH1A1, and ABCG2 was downregulated compared with that of normal DSCs. These genes are also related to cell cycle progression and stem cell maintenance. Further investigation will improve the understanding of the molecular mechanisms by which these genes together govern cell proliferation, revealing new strategies useful for future treatment of DFUs.
... where RAL has a key role. Indeed, Rxr/Ppar heterodimers bind to peroxisome proliferator response elements in the promoters of different genes involved in lipid metabolism and lipogenesis, acting as transcriptional regulators (Chawla et al., 2001;Ziouzenkova and Plutzky, 2008). The present study shows that the highest expression levels of nuclear receptors (rara, rxra and pparg) occur right before the onset of meiosis in males, while in females, the increase during meiosis resumption is only true for pparg. ...
Retinoic Acid (RA) is a vitamin A derivative present in many biological processes including embryogenesis, organ development and cell differentiation. The RA signaling pathway is essential for the onset of meiosis in tetrapods, although its role in fish reproduction needs further evidence. This study reports the expression profiles of several genes involved in this pathway during sex differentiation and the first reproductive season in European sea bass (Dicentrarchus labrax) gonads. The assessed genes are representative of several steps of the pathway including retinol transport, RA synthesis, nuclear receptors, RA transport and degradation. The study includes a synteny analysis of stra8, a tetrapod meiosis gatekeeper, in several taxa. The results show that, these genes were overexpressed during early gonad development and their expression decreased during meiosis progression in males and during vitellogenesis in females. Specifically, a decrease of cyp26a1, involved in RA degradation, together with an increase of aldh1a2 and aldh1a3, in charge of RA-synthesis, might ensure the availability of high RA levels at the time of meiosis in males and females. Moreover, the absence of stra8 in the European sea bass genome, as well as the conserved genomic neighbourhood found in other taxa, suggest a stra8 independent signaling for RA during meiosis. Taken together, our results might help to better understand the role of RA signaling in teleost gonad development.
... Retinoid metabolites act on lipid signalling pathways by activating RXRs which in turn form heterodimers with PPAR-lipid complexes (Fig 4). The PPAR complex controls transcription of target genes involved in lipid signalling and metabolism [71][72][73]. Several genes acting downstream of the nuclear receptor PPARα/RXRα complex were also found to be differently expressed in F 1 high ARA livers. ...
Disproportionate high intake of n-6 polyunsaturated fatty acids (PUFAs) in the diet is considered as a major human health concern. The present study examines changes in the hepatic gene expression pattern of adult male zebrafish progeny associated with high levels of the n-6 PUFA arachidonic acid (ARA) in the parental diet. The parental generation (F0) was fed a diet which was either low (control) or high in ARA (high ARA). Progenies of both groups (F1) were given the control diet. No differences in body weight were found between the diet groups within adult stages of either F0 or F1 generation. Few differentially expressed genes were observed between the two dietary groups in the F0 in contrast to the F1 generation. Several links were found between the previous metabolic analysis of the parental fish and the gene expression analysis in their adult progeny. Main gene expression differences in the progeny were observed related to lipid and retinoid metabolism by PPARα/RXRα playing a central role in mediating changes to lipid and long-chain fatty acid metabolism. The enrichment of genes involved in β-oxidation observed in the progeny, corresponded to the increase in peroxisomal β-oxidative degradation of long-chain fatty acids in the parental fish metabolomics data. Similar links between the F0 and F1 generation were identified for the methionine cycle and transsulfuration pathway in the high ARA group. In addition, estrogen signalling was found to be affected by parental high dietary ARA levels, where gene expression was opposite directed in F1 compared to F0. This study shows that the dietary n-3/n-6 PUFA ratio can alter gene expression patterns in the adult progeny. Whether the effect is mediated by permanent epigenetic mechanisms regulating gene expression in developing gametes needs to be further investigated.
... Many aspects of RXR, including genetic variation, specific retinoid molecules modulated its activity, the enzymes involved in generating retinoid modulators, the binding proteins that handle retinoid transport and delivery may influence RXR responses and functional PPAR effects [48][49][50]. ...
Peroxisome proliferator-activated receptors (PPAR) belong to nuclear super family of ligand-
activated transcriptional factors. Name of this group of nuclear receptors was derived from their
ability to stimulate proliferation of peroxisomes, which was described in rats in 1990. In
humans, there are\we distinguish three isoforms of nuclear receptor PPAR: PPARα, PPARβ/δ
a PPARγ. This isoforms differ by their distribution and specific function [2]. PPAR is nutrient
sensor, which regulates a number of homeostatic functions, such as glucose and lipid metabolism.
PPAR regulates uptake, utilization, oxidation and storage of fatty acids. Also, PPAR regulates
apoptosis, growth and migration of cells, modulates oxidative stress and inflammatory disease in
cardiovascular system and is particularly involved in vascular tonus regulation. Disruptions
this pathways can result in development of several diseases and pathological states, such as
obesity, obesity-induced inflammation, atherosclerosis, diabetes mellitus, metabolic syndrome
and hypertension. PPAR activation leads to trans activation and trans-repression, which can result in induction
and repression of gene expression. Ligand-activated PPAR creates heterodimer with retinoic
receptor, which is subsequently bound on sequence- specific target element in promoter region
of target genes.
... Many aspects of RXR, including genetic variation, specific retinoid molecules modulated its activity, the enzymes involved in generating retinoid modulators, the binding proteins that handle retinoid transport and delivery may influence RXR responses and functional PPAR effects [48][49][50]. ...
Type 2 diabetes (T2D) is often accompanied with an induction of retinaldehyde dehydrogenase 1 (RALDH1 or ALDH1A1) expression and a consequent decrease in hepatic retinaldehyde (Rald) levels. However, the role of hepatic Rald deficiency in T2D progression remains unclear. In this study, we demonstrated that reversing T2D-mediated hepatic Rald deficiency by Rald or citral treatments, or liver-specific Raldh1 silencing substantially lowered fasting glycemia levels, inhibited hepatic glucogenesis, and downregulated phosphoenolpyruvate carboxykinase 1 (PCK1) and glucose-6-phosphatase (G6PC) expression in diabetic db/db mice. Fasting glycemia and Pck1/G6pc mRNA expression levels were strongly negatively correlated with hepatic Rald levels, indicating the involvement of hepatic Rald depletion in T2D deterioration. A similar result that liver-specific Raldh1 silencing improved glucose metabolism was also observed in high-fat diet-fed mice. In primary human hepatocytes and oleic acid-treated HepG2 cells, Rald or Rald + RALDH1 silencing resulted in decreased glucose production and downregulated PCK1/G6PC mRNA and protein expression. Mechanistically, Rald downregulated direct repeat 1-mediated PCK1 and G6PC expression by antagonizing retinoid X receptor α, as confirmed by luciferase reporter assays and molecular docking. These results highlight the link between hepatic Rald deficiency, glucose dyshomeostasis, and the progression of T2D, whilst also suggesting RALDH1 as a potential therapeutic target for T2D.
Cancer cachexia is a metabolic disease affecting multiple organs and characterized by loss of adipose and muscle tissues. Metabolic dysregulated of adipose tissue has a crucial role in cancer cachexia. β-Carotene (BC) is stored in adipose tissues and increases muscle mass and differentiation. However, its regulatory effects on adipose tissues in cancer cachexia have not been investigated yet. In this study, we found that BC supplementations could inhibit several cancer cachexia-related changes, including decreased carcass-tumor (carcass weight after tumor removal), adipose weights, and muscle weights in CT26-induced cancer cachexia mice. Moreover, BC supplementations suppressed cancer cachexia-induced lipolysis, fat browning, hepatic gluconeogenesis, and systemic inflammation. Altered diversity and composition of gut microbiota in cancer cachexia were restored by the BC supplementations. BC treatments could reverse the down-regulated adipogenesis and dysregulated mitochondrial respiration and glycolysis in adipocytes and colon cancer organoid co-culture systems. Taken together, these results suggest that BC can be a potential therapeutic strategy for cancer cachexia.
High feed utilization efficiency can reduce agricultural costs and pollution. However, the feed conversion ratio (FCR) of local sheep breeds is low in China, so improving FCR of sheep is an important economic and environmental issue. However, the molecular mechanism affecting feed utilization remains unclear. In this study, transcriptome sequencing was used to compare the differences between small intestinal tissues from sheep with high and low feed conversion. The results identified 145 differentially expressed genes (DEGs) between the high and low FCR groups, of which 80 were upregulated and 65 were downregulated. Gene Ontology annotation (GO) and KEGG pathway analysis showed that DEGs were mainly enriched in metabolic processes and lipid metabolism pathways. Glucagon-like peptide 2 receptor (GLP2R) showed a significant negative correlation with feed intake (FI), whereas Lysophosphatidate acyltransferase (AGPAT1) was positively correlated with FCR. Adiponectin receptor2 (ADIPOR2) was negatively correlated with FCR. They may be considered as candidate genes that play a crucial role in feed utilization. Our data provided many valuable candidate genes that will help to study the molecular mechanisms of feed utilization in sheep.
Kidney disease (KD) is characterized by the presence of elevated oxidative stress, and this is postulated as contributing to the high cardiovascular morbidity and mortality in these individuals. Chronic KD (CKD) is related to high grade inflammatory condition and pro-oxidative state that aggravates the progression of the disease by damaging primary podocytes. Liposoluble vitamins (vitamin A and E) are potent dietary antioxidants that have also anti-inflammatory and antiapoptotic functions. Vitamin deficits in CKD patients are a common issue, and multiple causes are related to them: Anorexia, dietary restrictions, food cooking methods, dialysis losses, gastrointestinal malabsorption, etc. The potential benefit of retinoic acid (RA) and α-tocopherol have been described in animal models and in some human clinical trials. This review provides an overview of RA and α tocopherol in KD.
Background and Objectives: Retinoids are naturally occurring vitamin A derivatives that regulate cellular processes and metabolism. In particular, retinoids play a key role in cellular proliferation by binding to retinoic acid receptors (RAR)-alpha, beta, and gamma. Considering the functional role of nasal mucosa where active cell regeneration occurs, RAR may play a role in tissue remodeling of the human nasal mucosa.Methods: In this study, we investigated the expression and distribution pattern of RAR using reverse transcription-polymerase chain reaction (RT-PCR), immunohistochemistry (IHC) and Western blot in normal ethmoid mucosa (NE), chronic rhinosinusitis (IE) and polyp (P).Results: IE and P samples showed higher expression levels of RAR in RT-PCR and Western blot than NE samples. RAR reactivity was also observed in the NE group, which indicates that cell regeneration also occurs in normal condition. Through IHC, we found the localization of RAR. RAR-α was distributed in the epithelial cells, submucosal glands, and endothelial cells. RAR-β was located in the basal epithelium, while RAR-γ was present in goblet cells and submucosal glands. The staining intensity of RAR-α, β and γ was higher than that in the NE group. Especially in the P group, RARs were abundantly distributed in the stalks of polyps.Conclusion: The stalk region contains a lot of collagen and fibroblasts to support polyp formation, and the greater amount of RAR in the stalk suggested that RARs may be associated with angiogenesis and cell proliferation. Accordingly, elevated RAR levels in chronic rhinosinusitis could indicate that RARs play a critical role in cell regeneration, angiogenesis and immunomodulation under inflammatory conditions in the human nasal mucosa.
A cosmeceutical should be represented a topical preparation sold as a cosmetic but with an observable pharmaceutical action. Retinoids define a class of substances comprising vitamin A (retinol) and its naturally and synthetic derivatives. Retinoic acid is oxidized to a less active metabolite, 4‐oxoretinoic acid, or converted to retinoyl glucuronide, whereas retinol is converted to retinyl glucuronide. Vitamin A is present in the epidermis as free and esterified retinol at an average concentration of 1–2 nmol/g. Natural retinoids may be efficient when used in topical application, but they are irritant and unstable following exposure to the sunlight. The genomic effects of topical retinoids are the consequence of the modulation of gene expression following their binding to nuclear retinoic acid receptor or retinoid X receptors. Retinol and retinyl esters have been incorporated into many skin products. Cosmeceutical retinoids may be useful in the treatment of skin aging, acne, and pigmentation disorders.
Ethnopharmacological relevance
Pomegranate, Punica granatum L., has been used in traditional medicine in China and several regions of the world including Ayurveda, Islamic, and Persian for the treatment of atherosclerosis, diabetes, hypertension, hyperlipidemia, and several types of cancer, as well as for peptic ulcer and oral diseases for hundreds of years. Presently, pomegranate is treated as both a “medicine food homology” herbal medicine and a healthy food supplemental product.
Aim of the study
The aim of this work is to develop an overview of pomegranate in the context of the status of its traditional medicine theories, the spread along the Silk Road, ethnopharmacological uses, chemical compositions, pharmacological activities, toxicology, and the involved pathways.
Materials and Methods
Information on P. granatum L. was acquired from published materials, including monographs on medicinal plants, ancient and modern recorded classical texts; and pharmacopoeias and electronic databases (PubMed, Science Direct, Web of Science, Google Scholar, CNKI, and Wanfang Data).
Results
Pomegranate has been used in many traditional medical systems throughout history. It is widely cultivated in Central Asia and spread throughout China along the Silk Road. Many phytochemicals, such as tannins, organic acids, flavonoids, alkaloids, and volatile oils have been identified from different parts of pomegranate, these compounds have a wide range of activities, including antioxidant, antimicrobial, and anti-oncogenic properties, as well as conferring resistance to cerebrovascular disease. Furthermore, A summary of the four promising pharmacological pathways is provided.
Conclusions
The traditional uses, chemical compositions, pharmacological activities, and signaling pathways of pomegranate are summarized comprehensively in the review. It can be treated as a guidance for the future clinical and basic research. The information provided in this review will be very useful for further studies to develop novel therapeutic directions for application of pomegranate.
Overexpression of aldehyde dehydrogenase 1A1 (ALDH1A1) is associated with the occurrence and development of obesity and insulin resistance. Herein, a series of coumarin-based ALDH1A1 inhibitors were designed, synthesized and evaluated. Among them, compounds 10, 14 and 26 exhibited potent inhibitory activity against ALDH1A1 and high selectivity over ALDH1A2, ALDH1A3, ALDH2 and ALDH3A1. Optimized compound 10 showed markedly improved pharmacokinetic characters and ADME profiles comparing to the lead compound 1. In vitro study demonstrated that 10 alleviated palmitic acid-induced impairment of glucose consumption in HepG2 cells.
Background
Stevens-Johnson syndrome (SJS), a blistering disorder of the skin and mucous membrane, leads to ocular morbidity in >60% of cases. Retinoids are vital micronutrients for vision, regulating corneal and conjunctival cell proliferation, differentiation and immune function. This prospective case–control study probed for alterations in retinoid metabolism by evaluating retinoic acid receptor signalling in the conjunctival cells of patients with SJS.
Methods
Imprints were collected from the bulbar conjunctiva of patients with chronic SJS. The gene expression of retinoic acid receptors, namely, RXRA, RARA, RARG, RORA; the fibrosis marker TGFβ and its receptor TGFβRII ; the transcription factors PPAR-γ, STRA6 and Stat3 ; the enzymes aldehyde dehydrogenase ( ALDH1a1 ), alpha-1 antitrypsin ( A1AT ); and the Cyp genes Cyp26a1 and Cyp26b1 were assessed by quantitative PCR in patients with SJS pre-mucous (n = 34) and post-mucous membrane graft (MMG) intervention (n=19) in comparison with age-matched/sex-matched healthy controls (n=20). Western blot analysis of ALDH1a1, RARA and RARG were done in the conjunctival imprint cells.
Results
The transcript levels of ALDH1a1, RXRA, RORA, STRA6, Cyp26a1 and Cyp26b1 were decreased around 4, 26, 17, 129, 9 and 8 folds, respectively, and RARA, RARG, PPAR-γ, TGFβ, TGFβRII were increased by 12, 15, 51, 16 and 87 folds, respectively, in SJS conjunctiva at the pre-MMG stage. The changes in RORA, Cyp26a1, Cyp26b1, RARA and Stat3 were statistically significant (p<0.05). Changes in protein expression of ALDH1a1, RARA and RARG supported the gene expression changes.
Conclusions
The study provides the first experimental insight into the role of retinoid metabolism in the ocular sequelae of chronic SJS.
Carotenoids can be enzymatically converted to apocarotenoids by carotenoid cleavage dioxygenases. Insect genomes encode only one member of this ancestral enzyme family. We cloned and characterized the ninaB genes from the silk worm (Bombyx mori) and the flour beetle (Tribolium castaneum). We expressed BmNinaB and TcNinaB in E. coli and analyzed their biochemical properties. Both enzymes catalyzed a conversion of carotenoids into cis-retinoids. The enzymes catalyzed a combined trans to cis isomerization at the C11, C12 double bond and oxidative cleavage reaction at the C15, C15′ bond of the carotenoid carbon backbone. Analyses of the spatial and temporal expression patterns revealed that ninaB genes were differentially expressed during the beetle and moth life cycles with high expression in reproductive organs. In Bombyx mori, ninaB was almost exclusively expressed in female reproductive organs of the pupa and adult. In Tribolium castaneum, low expression was found in reproductive organs of females but high expressions in male reproductive organs of the pupa and imagoes. We performed RNAi experiments to characterize the role of NinaB in insect reproduction. We observed that RNAi treatment significantly decreased the expression levels of BmninaB and TcninaB and reduced the egg laying capacity of both insects. Together, our study revealed that NinaB's unique enzymatic properties are well conserved among insects and implicate NinaB function in insect reproduction.
Cell death is a perpetual feature of tissue microenvironments; each day under homeostatic conditions, billions of cells die and must be swiftly cleared by phagocytes. However, cell death is not limited to this natural turnover—apoptotic cell death can be induced by infection, inflammation, or severe tissue injury. Phagocytosis of apoptotic cells is thus coupled to specific functions, from the induction of growth factors that can stimulate the replacement of dead cells to the promotion of tissue repair or tissue remodeling in the affected site. In this review, we outline the mechanisms by which phagocytes sense apoptotic cell death and discuss how phagocytosis is integrated with environmental cues to drive appropriate responses.
The occurrence of 13-cis-retinoic acid as an endogenous component in human serum has been confirmed by cochromatography with standards in both normal-phase and reverse-phase high-performance liquid chromatographic (HPLC) system, by the lambda max of its UV spectrum recorded simultaneously with the HPLC run, and by chromatography of its methyl derivative. The method using solid-phase extraction followed by a gradient reverse-phase HPLC procedure with an internal standard and sensitive UV detector, provides an efficient and sensitive technique for the separation and quantification of serum 13-cis- and all-trans-retinoic acid. Serum levels of 13-cis- and all-trans-retinoic acid in 26 fasting volunteers ranged from 1.0 to 2.2 ng/ml (mean +/- SEM = 1.4 +/- 0.3 ng/ml) and from 1.1 to 1.9 ng/ml (mean +/- SEM = 1.4 +/- 0.2 ng/ml), respectively. The levels determined by a liquid-liquid double-phase extraction method were 90% higher in both 13-cis- and all- trans-retinoic acid than those from a solid-phase extraction. Human small intestine can isomerize all-trans-retinoic acid. 13-cis-Retinoic acid is the predominant cis isomer after incubation of intestinal mucosa homogenates with all-trans-retinoic acid. Moreover, the concentration of retinoic acid in serum is related to diet in that the level of total retinoic acid was 36% higher (n = 10) 2 h after a nonstandard breakfast than in fasting subjects.
In obesity and type 2 diabetes, expression of the GLUT4 glucose transporter is decreased selectively in adipocytes. Adipose-specific Glut4 (also known as Slc2a4) knockout (adipose-Glut4
-/-) mice show insulin resistance secondarily in muscle and liver. Here we show, using DNA arrays, that expression of retinol binding protein-4 (RBP4) is elevated in adipose tissue of adipose-Glut4
-/- mice. We show that serum RBP4 levels are elevated in insulin-resistant mice and humans with obesity and type 2 diabetes. RBP4 levels are normalized by rosiglitazone, an insulin-sensitizing drug. Transgenic overexpression of human RBP4 or injection of recombinant RBP4 in normal mice causes insulin resistance. Conversely, genetic deletion of Rbp4 enhances insulin sensitivity. Fenretinide, a synthetic retinoid that increases urinary excretion of RBP4, normalizes serum RBP4 levels and improves insulin resistance and glucose intolerance in mice with obesity induced by a high-fat diet. Increasing serum RBP4 induces hepatic expression of the gluconeogenic enzyme phosphoenolpyruvate carboxykinase (PEPCK) and impairs insulin signalling in muscle. Thus, RBP4 is an adipocyte-derived 'signal' that may contribute to the pathogenesis of type 2 diabetes. Lowering RBP4 could be a new strategy for treating type 2 diabetes.
The acute and chronic effects of vitamin A toxicity are well documented in the literature. Emerging evidence suggests that subtoxicity without clinical signs of toxicity may be a growing concern, because intake from preformed sources of vitamin A often exceeds the recommended dietary allowances (RDA) for adults, especially in developed countries. Osteoporosis and hip fracture are associated with preformed vitamin A intakes that are only twice the current RDA. Assessing vitamin A status in persons with subtoxicity or toxicity is complicated because serum retinol concentrations are nonsensitive indicators in this range of liver vitamin A reserves. The metabolism in well-nourished persons of preformed vitamin A, provided by either liver or supplements, has been studied by several research groups. To control vitamin A deficiency, large therapeutic doses are administered in developing countries to women and children, who often are undernourished. Nevertheless, little attention has been given to the short-term kinetics (ie, after absorption but before storage) of a large dose of vitamin A or to the short- and long-term effects of such a dose given to lactating women on serum and breast-milk concentrations of retinol and its metabolites. Moreover, appropriate dosing regimens have not been systematically evaluated to ascertain the quantitative improvement in vitamin A status of the women and children who receive these supplements. The known acute and chronic effects of vitamin A toxicity have been reported previously. However, further research is needed to ascertain the areas of the world in which subclinical toxicity exists and to evaluate its effects on overall health and well-being.
The occurrence of 13-cis-retinoic acid as an endogenous component in human serum has been confirmed by cochromatography with standards in both normal-phase and reverse-phase high-performance liquid chromatographic (HPLC) system, by the lambda max of its UV spectrum recorded simultaneously with the HPLC run, and by chromatography of its methyl derivative. The method using solid-phase extraction followed by a gradient reverse-phase HPLC procedure with an internal standard and sensitive UV detector, provides an efficient and sensitive technique for the separation and quantification of serum 13-cis- and all-trans-retinoic acid. Serum levels of 13-cis- and all-trans-retinoic acid in 26 fasting volunteers ranged from 1.0 to 2.2 ng/ml (mean +/- SEM = 1.4 +/- 0.3 ng/ml) and from 1.1 to 1.9 ng/ml (mean +/- SEM = 1.4 +/- 0.2 ng/ml), respectively. The levels determined by a liquid-liquid double-phase extraction method were 90% higher in both 13-cis- and all- trans-retinoic acid than those from a solid-phase extraction. Human small intestine can isomerize all-trans-retinoic acid. 13-cis-Retinoic acid is the predominant cis isomer after incubation of intestinal mucosa homogenates with all-trans-retinoic acid. Moreover, the concentration of retinoic acid in serum is related to diet in that the level of total retinoic acid was 36% higher (n = 10) 2 h after a nonstandard breakfast than in fasting subjects.
To gain insight into the function of peroxisome proliferator-activated receptor (PPAR) isoforms in mammals, we have cloned and characterized two PPAR alpha-related cDNAs (designated PPAR gamma and -delta, respectively) from mouse. The three PPAR isoforms display widely divergent patterns of expression during embryogenesis and in the adult. Surprisingly, PPAR gamma and -delta are not activated by pirinixic acid (Wy 14,643), a potent peroxisome proliferator and activator of PPAR alpha. However, PPAR gamma and -delta are activated by the structurally distinct peroxisome proliferator LY-171883 and linoleic acid, respectively, indicating that each of the isoforms can act as a regulated activator of transcription. These data suggest that tissue-specific responsiveness to peroxisome proliferators, including certain fatty acids, is in part a consequence of differential expression of multiple, pharmacologically distinct PPAR isoforms.
Peroxisome proliferator-activated receptors (PPARs) are nuclear hormone receptors controlling the expression of genes involved in lipid homeostasis. PPARs activate gene transcription in response to a variety of compounds including hypolipidemic drugs as well as natural fatty acids. From the plethora of PPAR activators, Scatchard analysis of receptor-ligand interactions has thus far identified only four ligands. These are the chemotactic agent leukotriene B4 and the hypolipidemic drug Wy 14,643 for the alpha-subtype and a prostaglandin J2 metabolite and synthetic antidiabetic thiazolidinediones for the gamma-subtype. Based on the hypothesis that ligand binding to PPAR would induce interactions of the receptor with transcriptional coactivators, we have developed a novel ligand sensor assay, termed coactivator-dependent receptor ligand assay (CARLA). With CARLA we have screened several natural and synthetic candidate ligands and have identified naturally occurring fatty acids and metabolites as well as hypolipidemic drugs as bona fide ligands of the three PPAR subtypes from Xenopus laevis. Our results suggest that PPARs, by their ability to interact with a number of structurally diverse compounds, have acquired unique ligand-binding properties among the superfamily of nuclear receptors that are compatible with their biological activity.
The retinoid X receptor (RXR), a member of the superfamily of hormone nuclear receptors, is a ligand-inducible transcription factor that is activated by the vitamin A derivative 9-cis-retinoic acid. We previously showed that RXR self-associates into tetramers with a high affinity and that ligand binding induces rapid dissociation of receptor tetramers to smaller species. Here, the RXR region that is responsible for mediating tetramer formation is identified. It is shown that this interface, which we term the "tetramerization domain," critically contains two consecutive phenylalanine residues located at the C-terminal region of the receptor. Mutation of these residues is sufficient to disrupt RXR tetramers without affecting the overall fold of the protein or interfering with ligand binding, dimer formation, or DNA binding by the receptor. Nevertheless, the tetramer-impaired mutant was found to be transcriptionally defective. The newly characterized tetramerization domain and the previously identified main dimerization interface of RXR act autonomously to affect separate intersubunit interactions that, overall, lead to formation of tetramers. Protein-protein interactions mediated by the tetramerization domain, but not those that involve the dimerization interface, are disrupted following ligand binding by RXR. Overall, these data attest to the specificity of the interaction and implicate the tetramerization interface in playing a direct role in regulating transcriptional activation by RXR.
To determine the functions of retinoic acid receptors RAR and RXR, we have systematically knocked-out their genes by homologous recombination in the embryonic stem cells and generated null-mutant mice. This approach has allowed us to perform a genetic dissection of the retinoic acid signalling pathway.
Understanding of the stereospecificity of enzymatic reactions that regenerate the universal chromophore required to sustain vision in vertebrates, 11-cis-retinal, is needed for an accurate molecular model of retinoid transformations. In rod outer segments (ROS), the redox reaction involves all-trans-retinal and pro-S-NADPH that results in the production of pro-R-all-trans-retinol. A recently identified all-trans-retinol dehydrogenase (photoreceptor retinol dehydrogenase) displays identical stereospecificity to that of the ROS enzyme(s). This result is unusual, because photoreceptor retinol dehydrogenase is a member of a short chain alcohol dehydrogenase family, which is often pro-S-specific toward their hydrophobic alcohol substrates. The second redox reaction occurring in retinal pigment epithelium, oxidation of 11-cis-retinol, which is largely catalyzed by abundantly expressed 11-cis-retinol dehydrogenase, is pro-S-specific to both 11-cis-retinol and NADH. However, there is notable presence of pro-R-specific activities. Therefore, multiple retinol dehydrogenases are involved in regeneration of 11-cis-retinal. Finally, the cellular retinaldehyde-binding protein-induced isomerization of all-trans-retinol to 11-cis-retinol proceeds with inversion of configuration at the C(15) carbon of retinol. Together, these results provide important additions to our understanding of retinoid transformations in the eye and a prelude for in vivo studies that ultimately may result in efficient pharmacological intervention to restore and prevent deterioration of vision in several inherited eye diseases.
Bile salt export pump (BSEP) is a major bile acid transporter in the liver. Mutations in BSEP result in progressive intrahepatic cholestasis, a severe liver disease that impairs bile flow and causes irreversible liver damage. BSEP is a target for inhibition and down-regulation by drugs and abnormal bile salt metabolites, and such inhibition and down-regulation may result in bile acid retention and intrahepatic cholestasis. In this study, we quantitatively analyzed the regulation of BSEP expression by FXR ligands in primary human hepatocytes and HepG2 cells. We demonstrate that BSEP expression is dramatically regulated by ligands of the nuclear receptor farnesoid X receptor (FXR). Both the endogenous FXR agonist chenodeoxycholate (CDCA) and synthetic FXR ligand GW4064 effectively increased BSEP mRNA in both cell types. This up-regulation was readily detectable at as early as 3 h, and the ligand potency for BSEP regulation correlates with the intrinsic activity on FXR. These results suggest BSEP as a direct target of FXR and support the recent report that the BSEP promoter is transactivated by FXR. In contrast to CDCA and GW4064, lithocholate (LCA), a hydrophobic bile acid and a potent inducer of cholestasis, strongly decreased BSEP expression. Previous studies did not identify LCA as an FXR antagonist ligand in cells, but we show here that LCA is an FXR antagonist with partial agonist activity in cells. In an in vitro co-activator association assay, LCA decreased CDCA- and GW4064-induced FXR activation with an IC(50) of 1 microm. In HepG2 cells, LCA also effectively antagonized GW4064-enhanced FXR transactivation. These data suggest that the toxic and cholestatic effect of LCA in animals may result from its down-regulation of BSEP through FXR. Taken together, these observations indicate that FXR plays an important role in BSEP gene expression and that FXR ligands may be potential therapeutic drugs for intrahepatic cholestasis.
Peroxisome proliferator-activated receptor gamma (PPAR-gamma) plays a key role in adipocyte differentiation and insulin sensitivity. Its synthetic ligands, the thiazolidinediones (TZD), are used as insulin sensitizers in the treatment of type 2 diabetes. These compounds induce both adipocyte differentiation in cell culture models and promote weight gain in rodents and humans. Here, we report on the identification of a new synthetic PPARgamma antagonist, the phosphonophosphate SR-202, which inhibits both TZD-stimulated recruitment of the coactivator steroid receptor coactivator-1 and TZD-induced transcriptional activity of the receptor. In cell culture, SR-202 efficiently antagonizes hormone- and TZD-induced adipocyte differentiation. In vivo, decreasing PPARgamma activity, either by treatment with SR-202 or by invalidation of one allele of the PPARgamma gene, leads to a reduction of both high fat diet-induced adipocyte hypertrophy and insulin resistance. These effects are accompanied by a smaller size of the adipocytes and a reduction of TNFalpha and leptin secretion. Treatment with SR-202 also dramatically improves insulin sensitivity in the diabetic ob/ob mice. Thus, although we cannot exclude that its actions involve additional signaling mechanisms, SR-202 represents a new selective PPARgamma antagonist that is effective both in vitro and in vivo. Because it yields both antiobesity and antidiabetic effects, SR-202 may be a lead for new compounds to be used in the treatment of obesity and type 2 diabetes.
Increased levels of triglyceride-rich lipoproteins provoke lipid accumulation in the artery wall, triggering early inflammatory responses central to atherosclerosis like endothelial adhesion molecule expression. The endogenous mechanisms limiting such reactions remain poorly defined. Lipoprotein lipase (LPL) plays a central role in lipid metabolism by hydrolyzing triglyceride rich lipoproteins and releasing fatty acids. We found that LPL treatment reversed tumor necrosis factor alpha and very low-density lipoprotein (VLDL)-stimulated endothelial vascular cell adhesion molecule 1 (VCAM1) induction and VCAM1 promoter responses, thus recapitulating effects reported with synthetic peroxisome proliferator-activated receptor (PPAR) agonists. In fact, these LPL effects on VCAM1 were absent in endothelial cells isolated from PPAR alpha-deficient mice. This finding suggests a novel antiinflammatory role for LPL. Further studies reveal specificity for PPAR activation through lipolysis in regards to lipoprotein substrate (VLDL > LDL > HDL), PPAR isoform (PPAR alpha > PPAR delta > PPAR gamma), and among fatty acid-releasing lipases. These PPAR responses required intact LPL catalytic activity. In vivo, transgenic mice overexpressing LPL had increased peroxisome proliferation, but not in the genetic absence of PPAR alpha. Although human plasma possesses minimal PPAR alpha activation despite containing abundant free fatty acids, marked PPAR alpha activation is seen with human plasma after LPL is added in vitro or systemically released in vivo. These data suggest a previously uncharacterized pathway in which the key lipolytic enzyme LPL can act on circulating lipoproteins to generate PPAR alpha ligands, providing a potentially important link between lipoprotein metabolism and distal PPAR alpha transcriptional effects.
Beta-carotene 15,15'-monooxygenase (BCM) catalyzes the first step of vitamin A biosynthesis from provitamin A carotenoids. We wished to determine the factors underlying the transcriptional regulation of this gene. After cloning of the 40 kilobase pair (kbp) mouse Bcm gene and determination of its genomic organization, analysis of the 2 kb 5'-flanking region showed several putative transcription factor binding sites including TATA box, a peroxisome proliferator response element (PPRE), AP2, and bHLH. The 2 kb fragment drove specific luciferase gene expression in vitro only in cell lines that express BCM (TC7, PF11, and monkey retinal pigment epithelium). Nucleotides -41 to +163, and -60 to +163 drove basal and specific Bcm transcriptional activity, respectively. Site-directed mutagenesis and gel shift experiments demonstrate that PPRE was essential for Bcm promoter specificity and that the peroxisome proliferator activated receptor (PPAR) gamma (PPARgamma) specifically binds to this element. Furthermore, cotransfection experiments and pharmacological treatments in vitro, using the specific PPARgamma agonists LY17883 and ciglitazone, demonstrate that the PPRE element confers peroxisome proliferator responsiveness via the PPARgamma and retinoid X receptor-alpha heterodimer. Treatment of mice with the PPARalpha/gamma agonist WY14643 increases BCM protein expression in liver. Thus PPAR is a key transcription factor for the transcriptional regulation of the Bcm gene, suggesting a broader function for PPARs in the regulation of carotenoid metabolism metabolism that is consistent with their established role in neutral lipid metabolism and transport.
Two isomers of retinoic acid (RA) may be necessary as ligands for retinoid signaling: all-trans-RA for RA receptors (RARs) and 9-cis-RA for retinoid X receptors (RXRs). This was explored by using retinaldehyde dehydrogenase (Raldh)2-/- mouse embryos lacking mesodermal RA synthesis that display early growth arrest unless rescued by all-trans-RA administration. Because isomerization of all-trans-RA to 9-cis-RA can occur, it is unclear whether both ligands are needed for rescue. We show here that an RAR-specific ligand can rescue Raldh2-/- embryos as efficiently as all-trans-RA, whereas an RXR-specific ligand has no effect. Further, whereas all-trans-RA was detected in embryos, 9-cis-RA was undetectable unless a supraphysiological dose of all-trans-RA was administered, revealing that 9-cis-RA is of pharmacological but not physiological significance. Because 9-cis-RA is undetectable and unnecessary for Raldh2-/- rescue, and others have shown that 4-oxo-RA is unnecessary for mouse development, all-trans-RA emerges as the only ligand clearly necessary for retinoid receptor signaling.
Vitamin A (retinol) is a nutrient that is essential for developmental regulation but toxic in large amounts. Previous genetic studies have revealed that alcohol dehydrogenase Adh1 is required for efficient clearance of excess retinol to prevent toxicity, thus demonstrating that the mechanism involves oxidation of excess retinol to retinoic acid (RA). Whereas Adh1 plays a dominant role in the first step of the clearance pathway (oxidation of retinol to retinaldehyde), it is unknown what controls the second step (oxidation of retinaldehyde to RA). We now present genetic evidence that aldehyde dehydrogenase Aldh1a1, also known as retinaldehyde dehydrogenase Raldh1, plays a dominant role in the second step of retinol clearance in adult mice. Serum RA levels following a 50 mg/kg dose of retinol were reduced 72% in Raldh1-/- mice and 82% in Adh1-/- mice. This represented reductions in RA synthesis of 77-78% for each mutant after corrections for altered RA degradation in each. After retinol dosing, serum retinaldehyde was increased 2.5-fold in Raldh1-/- mice (indicating defective retinaldehyde clearance) and decreased 3-fold in Adh1-/- mice (indicating defective retinaldehyde synthesis). Serum retinol clearance following retinol administration was decreased 7% in Raldh1-/- mice and 69% in Adh1-/- mice. LD50 studies indicated a small increase in retinol toxicity in Raldh1-/- mice and a large increase in Adh1-/- mice. These observations demonstrate that Raldh1 functions downstream of Adh1 in the oxidative metabolism of excess retinol and that toxicity correlates primarily with accumulating retinol rather than retinaldehyde.
Retinoic acid (RA) modulates transcription of numerous target genes, thereby regulating a myriad of biological processes.
It is well established that RA functions by activating retinoic acid receptors (RARs), which, in turn, control cell differentiation,
proliferation, and apoptosis. However, perplexing reports of diverse and sometime opposing actions of RA have been published.
Hence, while RA induces apoptosis and inhibits cell growth in some settings, it potentiates proliferation and acts as an anti-apoptotic
agent in others. These observations raise the possibility that signaling pathways other than RAR may be involved in mediating
RA activities. Here we show that RA is a high affinity ligand for another nuclear receptor, namely the orphan receptor peroxisome
proliferator-activated receptor (PPAR) β/δ. We demonstrate that while RA does not activate PPARα and PPARγ, it binds to PPARβ/δ
with nanomolar affinity, modulates the conformation of the receptor, promotes interaction with the coactivator SRC-1, and
efficiently activates PPARβ/δ-mediated transcription. Transcriptional signaling by RA is thus exerted by a dual pathway, providing
a rationale for understanding divergent cellular responses to this hormone.
As a promiscuous dimerization partner the retinoid X receptor (RXR) can contribute to signaling by multiple nuclear receptors.
However, the impact of RXR cosignaling and the possible existence of an RXR homodimer signaling pathway are largely unexplored.
We report here on the separation of RXR homo- and heterodimerization as an essential step towards the elucidation of the roles
of RXR homo- and heterodimers in retinoid-rexinoid signaling. RXR homodimerization was specifically disrupted by single mutations
in the RXR dimerization interface. In contrast, even multiple mutations did not fully impair RXR heterodimerization with retinoic
acid receptor (RAR). Importantly, the mutation of mouse RXRα (mRXRα) Tyr402 substantially weakened RAR heterodimerization
while concomitantly increasing homodimerization. Not only did this lead to cooperatively enhanced RXR homodimer binding to
DR1 or DR5 elements, but unexpectedly, the mutant acquired significant binding efficiency for noncognate DR3 or DR4 elements
as well. The increased stability of RXR homodimers on DR1 correlated with increased transcriptional activity of mRXRαY402A on DR1-based reporter genes. Weak, if any, heterodimerization was observed with thyroid, vitamin D3, or peroxisome proliferator-activating receptors. A model accounting for the structural impact of the Tyr402 mutation on
dimerization is discussed. These results provide the basis for a genetic replacement of wild-type RXRs by mutants like mRXRαY402A to elucidate the physiological impact of RXR homo- and heterodimerization.
Obesity and type II diabetes are closely related metabolic diseases with an increasing incidence worldwide. No clear-cut pharmacological treatment for these complex metabolic disturbances is available despite current efforts. New directions and perspectives for the pharmacological or nutritional treatment of these diseases should be defined. In recent years, a growing body of evidence shows that retinoids and retinoic acid receptors are involved in the control of biological aspects (e.g. adiposity and energy expenditure mechanisms), which offers great potential for research on the treatment of obesity and type II diabetes. All-trans retinoic acid is known to inhibit adipocyte differentiation, whereas, molecules activating the retinoid X-receptor (rexinoids) promote the differentiation of adipocytes. Treatment with rexinoids ameliorates glycemic control in rodent models of type II diabetes and obesity, although other findings indicate similar positive effects by inhibiting the receptor. Moreover, natural products of dietary origin, such as phytanic acid can activate RXR and thus, trigger adipose cell differentiation. Finally, the activation of retinoic acid receptors or retinoid X receptors has been reported to induce the gene expression of uncoupling proteins, which are mitochondrial proteins involved in the regulation of energy expenditure and fatty acid metabolism. Further research is required to exploit the capacities of the retinoid-dependent pathways of regulation of adiposity, insulin sensitivity and energy expenditure for drug development in metabolic disturbances.
Although triglyceride-rich particles, such as very low-density lipoprotein (VLDL), contribute significantly to human atherogenesis, the molecular basis for lipoprotein-driven pathogenicity is poorly understood. We demonstrate that in macrophages, VLDL functions as a transcriptional regulator via the activation of the nuclear receptor peroxisome proliferator-activated receptor delta. The signaling components of native VLDL are its triglycerides, whose activity is enhanced by lipoprotein lipase. Generation of peroxisome proliferator-activated receptor delta null macrophages verifies the absolute requirement of this transcription factor in mediating the VLDL response. Thus, our data reveal a pathway through which dietary triglycerides and VLDL can directly regulate gene expression in atherosclerotic lesions.
Peroxisome proliferator-activated receptors (PPARs) are nuclear hormone receptors that regulate gene transcription in response to peroxisome proliferators and fatty acids. PPARs also play an important role in the regulation of adipocyte differentiation. It is unclear, however, what naturally occurring compounds activate each of the PPAR subtypes. To address this issue, a screening assay was established using heterologous fusions of the bacterial tetracycline repressor to several members of the peroxisome proliferator-activated receptor (PPAR) family. This assay was employed to compare the activation of PPAR family members by known PPAR activators including peroxisome proliferators and fatty acids. Interestingly, the activation of PPARs by fatty acids was partially inhibited by the cyclooxygenase inhibitor indomethacin, which prevents prostaglandin synthesis. Indeed, prostaglandins PGA1 and 2, PGD1 and 2, and PGJ2-activated PPARs, while a number of other prostaglandins had no effect. We also screened a variety of hydroxyeicosatetraenoic acids (HETEs) for the ability to activate PPARs. 8(S)-HETE, but not other (S)-HETEs, was a strong activator of PPAR alpha. Remarkably, PPAR activation by 8(S)-HETE was stereoselective. In addition, 8(S)-HETE was able to induce differentiation of 3T3-L1 preadipocytes. These results indicate that PPARs are differentially activated by naturally occurring eicosanoids and related molecules.
The process of adipogenesis is known to involve the interplay of several transcription factors. Activation of one of these factors, the nuclear hormone receptor PPARγ, is known to promote fat cell differentiation in vitro. Whether PPARγ is required for this process in vivo has remained an open question because a viable loss-of-function model for PPARγ has been lacking. We demonstrate here that mice chimeric for wild-type and PPARγ null cells show little or no contribution of null cells to adipose tissue, whereas most other organs examined do not require PPARγ for proper development. In vitro, the differentiation of ES cells into fat is shown to be dependent on PPARγ gene dosage. These data provide direct evidence that PPARγ is essential for the formation of fat.
On page 845 in the first paragraph of the “All Randomized Trials” subsection, the sentence that read “Heterogeneity was not significant (I²=18.6%, P=.10)” should have read “Heterogeneity was significant (I²=18.9%, P=.10).” In the following sentence that begins “Adjusted-rank correlation test (P=.08), but not the regression asymmetry test (P=.26), suggested the bias among trials,” the respective P values should have read “(P=.09)” and “(P=.24).” In the second paragraph of the same subsection, the portion of the sentence that begins on page 845: “Univariate meta-regression analyses revealed significant influences of dose of beta carotene (RR, 1.004; 95% CI, 1.001-1.007; P=.012),” the P value should have been equal to “.014.” In the latter part of the same sentence that falls on page 847, the P value for the dose of selenium that read “P=.002” should have read “P=.001.” In the following part of the sentence, the upper confidence limit that read “1.29” should have read “1.30.” In the third paragraph of the same subsection, on page 847, the P value for the “multivariate meta-regression” for dose of selenium that read “P=.005” should have read “P=.004,” the lower confidence limit for low-bias risk trials that read “1.05” should have read “1.04,” and the P value for the low-bias risk trials in the same sentence that read “P=.005” should have read “P=.006.”
In Table 5 on page 853, the RR (95% CI) in the “Beta carotene given singly” row that read “1.06 (1.01-1.11)” should have read “1.05 (1.00-1.11)” and the I² value that read “5.4” should have read “11.8.” In the “Beta carotene given in combination with other antioxidant supplements” row, the I² value that read “55.6” should have read “55.5.” In the “Beta carotene given singly or in combination with other antioxidant supplements” row, the CI range that read “(0.96-1.08)” should have read “(0.95-1.07)” and the I2 value that read “52.2” should have read “52.5.” In the “Beta carotene given singly or in combination with other antioxidant supplements after exclusion of high-bias risk and selenium trials” row, the I² value that read 36.8” should have read “34.4” In the “Vitamin E given singly” row, the number of study participants that read “47 007” should have read “41 341.” In the “Vitamin E given in combination with other antioxidant supplements” row, the RR that read “1.01” should have read “1.00” and the I² value that read “17.2” should have read “16.9.” In the “Vitamin E given singly or in combination with other antioxidant supplements” row, the I²value that read “2.8” should have read “2.4.” In the “Vitamin E given singly or in combination with other antioxidant supplements after exclusion of high-bias risk and selenium trials” row, the list of references should have included reference 87 and excluded 95.
Acute promyelocytic leukemia is a form of acute myelogenous leukemia, characterized by the t(15;17) chromososmal translocation and the presence of the abnormal PML-RARalpha fusion protein. All-trans-retinoic acid is a potent agent for the treatment of this fatal subtype of AML, and is particularly effective when combined with cytotoxic chemotherapy. The important biological activities of all-trans-retinoic acid in vitro and in vivo have provoked extensive studies over the years, aimed to define the mechanisms by which it induces its antileukemic effects. It is now well established that all-trans-retinoic acid when administered at pharmacological doses can reverse the dominant-negative effects that the PML-RARalpha oncoprotein exhibits on the functions of the wild type PML and RARalpha proteins. All-trans-retinoic acid induces gene transcription via retinoic acid responsive elements (RARE) that are present in the promoters of retinoid-responsive genes that ultimately result in the production of protein products that regulate leukemic cell differentiation and induce cell-cycle arrest. There is now accumulating evidence that additional signalling pathways are activated during all-trans-retinoic acid-treatment of cells, involving Stat-proteins, tyrosine kinases and mitogen-activated protein (Map) kinases. This review summarizes the current knowledge on the signalling cascades activated by all-trans-retinoic acid in APL cells. The clinical implications and potential translational applications from the accumulating knowledge in the field are also discussed.
A number of products including apocarotenal, epoxycarotenal, apocarotenone, and epoxycarotenone generated by lipoxygenase (LOX) catalyzed co-oxidation of beta-carotene have been tentatively identified through the use of GC/MS and HPLC combined with photodiode array detection. Because of the large number of high molecular weight products detected and their probable chemical structures, a co-oxidation mechanism is proposed that involves random attack along the alkene chain of the carotenoid by a LOX-generated linoleoylperoxyl radical. It is suggested that a direct release from the enzyme of the radical, which initiates the co-oxidation of beta-carotene, is greater for pea LOX-3 than for pea LOX-2 or soybean LOX-1. It is proposed that further products may be formed by free radical propagated reactions and that the formation of 1,10- and 1,14-dicarbonyl compounds may arise by secondary oxidation of the primary products.
All-trans retinoic acid (RA) has previously been shown to modulate the transcriptional properties of the retinoic acid receptor (RAR) and retinoid X receptor (RXR). The inability of all-trans RA to bind to RXR suggests that it may be metabolized to a more active high affinity ligand. We report here an experimental approach that has identified 9-cis RA as an RXR ligand. It is up to 40-fold more potent than all-trans RA in transfection assays and binds with high affinity. The production of 9-cis RA in cultured cells and the identification of this molecule in liver and kidney demonstrates the existence of this molecule in living organisms. The discovery of this novel hormone points to the key role retinoid metabolism may have in generating new signaling pathways.
The anticarcinogenic action of carotenoids such as beta-carotene has been frequently ascribed to their antioxidant properties. However, very little is actually known about the nature of the antioxidant reaction or the products that are formed. beta-Carotene was exposed to either spontaneous autoxidation conditions or to radical-initiated autoxidation conditions. The products were separated by reverse-phase HPLC, and individual peaks were characterized with an on-line diode array detector. Carbonyl products were isolated and characterized by several procedures, including borohydride reduction to the corresponding alcohols, derivatization with O-ethyl-hydroxylamine to the corresponding O-ethyl-oximes of the carbonyls, and analysis by GC-MS. Under the conditions of the experiments, the formation of a homologous series of carbonyl products was demonstrated, including beta-apo-13-carotenone, retinal, beta-apo-14'-carotenal, beta-apo-12'-carotenal, and beta-apo-10'-carotenal. Several very hydrophobic compounds were formed, which have not been previously identified. In addition, the products of NaOCl-treatment of beta-carotene were analyzed, and shown to be significantly different from the autoxidation products. This type of product analysis should be useful in determining the nature of the oxidants reacting with beta-carotene in vivo.
Whether the conversion of beta-carotene into retinoids involves an enzymatic excentric cleavage mechanism was examined in vitro with homogenates prepared from human, monkey, ferret, and rat tissue. Using high-performance liquid chromatography, significant amounts of beta-apo-12'-, -10'-, and -8'-carotenals, retinal, and retinoic acid were found after incubation of intestinal homogenates of the four different species with beta-carotene in the presence of NAD+ and dithiothreitol. No beta-apo-carotenals or retinoids were detected in control incubations done without tissue homogenates. The production of beta-apo-carotenals was linear for 30 min and up to tissue protein concentrations of 1.5 mg/ml. The rate of formation of beta-apo-carotenals from 2 microM beta-carotene was about 7- to 14-fold higher than the rate of retinoid formation in intestinal homogenates, and the rate of beta-apo-carotenal production was fivefold greater in primate intestine vs rat or ferret intestine (P less than 0.05). The amounts of beta-apo-carotenals and retinoids formed were markedly reduced when NAD+ was replaced by NADH, or when dithiothreitol and cofactors were deleted from the incubation mixture. Both beta-apo-carotenal and retinoid production from beta-carotene were inhibited completely by adding disulfiram, an inhibitor of sulfhydryl-containing enzymes. Incubation of beta-carotene with liver, kidney, lung, and fat homogenates from each species also resulted in the appearance of beta-apo-carotenals and retinoids. The identification of three unknown compounds which might be excentric cleavage products is ongoing. These data support the existence of an excentric cleavage mechanism for beta-carotene conversion.
Retinol at levels above 0.5 n moles/mg protein and retinoic acid at levels above 0.25 n moles/mg protein are shown to uncouple mitochondrial oxidative phosphorylation. Uncoupling is measured with the substrates ascorbate plus N,N,N',N'-tetra-methyl-p-phenylenediamine, succinate and beta-hydroxybutyrate. These experiments indicate that the uncoupling effects of retinol and retinoic acid are not site specific but likely are the result of the vitamin being a general membrane bilayer disrupting agent.
Competition of all-trans-retinol and all-trans-retinaldehyde with 3H-labeled all-trans-retinoic acid (RA) for binding to retinoic acid receptors (RARs) was examined in human neuroblastoma cell nuclear extracts. All-trans-retinol was 35-fold less potent than all-trans-RA, whereas all-trans-retinaldehyde was 500-fold less active in binding to the nuclear receptors. To confirm that all-trans-retinol binds to RARs, experiments were carried out with RARs alpha, beta, and gamma expressed as bacterial fusion proteins. All-trans-retinol was only 4- to 7-fold less potent than all-trans-RA in binding to all three RAR subtypes. The all-trans-retinol binding observed was not the result of metabolism of retinol to RA or some other active compound during the binding experiment. Retinyl acetate was virtually inactive in competition binding experiments, while very slight activity was observed with 13-cis-RA and all-trans-retinaldehyde. Significant competition occurred with 4-hydroxy-RA and 4-keto-RA, which were 15- to 40-fold less potent than all-trans-RA. The 9-cis isomer of RA was equipotent with all-trans-retinol in these studies. These results suggest that all-trans-retinol cannot be excluded as a physiologically significant ligand for RAR-mediated gene expression.
Metabolic activation of retinol into the hormone retinoic acid and metabolism of retinoic acid entail essential aspects of retinoid biology that seem interdependent with functions of retinoid binding-proteins. Cellular retinol binding protein and cellular retinoic acid binding protein enjoy widespread expression and, where expressed, their liganded forms represent the major physiological forms of retinol and retinoic acid, respectively. These retinoid binding proteins may protect cells from the amphipathic properties of retinoids and protect the structurally sensitive retinoids from the cellular milieu. Starting from the perspective that the enzymes most likely to metabolize retinoids in vivo might recognize the major forms of retinoids that occur in vivo, several laboratories have produced results that support a model of retinoid metabolism with prominent roles for the cellular retinoid binding proteins. In this model, liganded cellular retinoid binding proteins serve as substrates for the metabolism of some retinoids (retinol, retinoic acid), restricting access to those enzymes that recognize both the binding protein and the retinoid. Other retinoids (3,4-didehydroretinol, 4-oxo-retinoic acid) liganded to binding-proteins have their metabolism arrested. In its unliganded form, at least one retinoid binding protein (cellular retinol binding protein) serves as a retinoid concentration-sensitive modulator of enzymes that catalyze retinol metabolism. This review will describe the model and the intrinsic relationships among retinoid-specific enzymes and retinoid binding proteins.
Retinoic acid receptors (RAR), thyroid hormone receptors (TR), peroxisome proliferator activated receptors (PPARs) and the orphan receptor, LXR, bind preferentially to DNA as heterodimers with a common partner, retinoid X receptor (RXR), to regulate transcription. We investigated whether RXR-selective agonists replicate the activity of ligands for several of these receptors? We demonstrate here that RXR-selective ligands (referred to as rexinoids) function as RXR heterodimer-selective agonists, activating RXR: PPARgamma and RXR:LXR dimers but not RXR:RAR or RXR:TR heterodimers. Because PPARgamma is a target for antidiabetic agents, we investigated whether RXR ligands could alter insulin and glucose signalling. In mouse models of noninsulin-dependent diabetes mellitus (NIDDM) and obesity, RXR agonists function as insulin sensitizers and can decrease hyperglycaemia, hypertriglyceridaemia and hyperinsulinaemia. This antidiabetic activity can be further enhanced by combination treatment with PPARgamma agonists, such as thiazolidinediones. These data suggest that the RXR:PPARgamma heterodimer is a single-function complex serving as a molecular target for treatment of insulin resistance. Activation of the RXR:PPARgamma dimer with rexinoids may provide a new and effective treatment for NIDDM.
Epidemiological data suggest that the intake of antioxidants such as alpha-tocopherol (vitamin E) and beta-carotene has an inverse correlation with the incidence of coronary heart disease. The results from clinical trials of antioxidant supplementation in people with known coronary heart disease are inconclusive.
We studied the frequency of major coronary events in 1862 men enrolled in the alpha-tocopherol beta-carotene Cancer Prevention Study (smokers aged between 50 and 69 years) who had a previous myocardial infarction. In this randomised, double-blind. placebo-controlled study, men had received dietary supplements of alpha-tocopherol (50 mg/day), beta-carotene (20 mg/day), both, or placebo. The median follow-up was 5.3 years. The endpoint of this substudy was the first major coronary event after randomisation. Analyses were by intention to treat.
424 major coronary events (non-fatal myocardial infarction and fatal coronary heart disease) occurred during follow-up. There were no significant differences in the number of major coronary events between any supplementation group and the placebo group (alpha-tocopherol 94/466; beta-carotene 113/461; alpha-tocopherol and beta-carotene 123/497; placebo 94/438 [log-rank test, p = 0.25]). There were significantly more deaths from fatal coronary heart disease in the beta-carotene (74/461, multivariate-adjusted relative risk 1.75 [95% CI 1.16-2.64], p = 0.007) and combined alpha-tocopherol and beta-carotene groups (67/497, relative risk 1.58 [1.05-2.40], p = 0.03) than in the placebo group (39/438), but there was no significant increase in the alpha-tocopherol supplementation group (54/466, relative risk 1.33 [0.86-2.05], p = 0.20).
The proportion of major coronary events in men with a previous myocardial infarction who smoke was not decreased with either alpha-tocopherol or beta-carotene supplements. In fact, the risk of fatal coronary heart disease increased in the groups that received either beta-carotene or the combination of alpha-tocopherol and beta-carotene; there was a non-significant trend of increased deaths in the alpha-tocopherol group. We do not recommend the use of alpha-tocopherol or beta-carotene supplements in this group of patients.
Bile acids are essential for the solubilization and transport of dietary lipids and are the major products of cholesterol catabolism. Results presented here show that bile acids are physiological ligands for the farnesoid X receptor (FXR), an orphan nuclear receptor. When bound to bile acids, FXR repressed transcription of the gene encoding cholesterol 7alpha-hydroxylase, which is the rate-limiting enzyme in bile acid synthesis, and activated the gene encoding intestinal bile acid-binding protein, which is a candidate bile acid transporter. These results demonstrate a mechanism by which bile acids transcriptionally regulate their biosynthesis and enterohepatic transport.
This article presents a model that integrates the functions of retinoid-binding proteins with retinoid metabolism. One of these proteins, the widely expressed (throughout retinoid target tissues and in all vertebrates) and highly conserved cellular retinol-binding protein (CRBP), sequesters retinol in an internal binding pocket that segregates it from the intracellular milieu. The CRBP-retinol complex appears to be the quantitatively major form of retinol in vivo, and may protect the promiscuous substrate from nonenzymatic degradation and/or non-specific enzymes. For example, at least seven types of dehydrogenases catalyze retinal synthesis from unbound retinol in vitro (NAD+ vs. NADP+ dependent, cytosolic vs. microsomal, short-chain dehydrogenases/reductases vs. medium-chain alcohol dehydrogenases). But only a fraction of these (some of the short-chain de-hydrogenases/reductases) have the fascinating additional ability of catalyzing retinal synthesis from CRBP-bound retinol as well. Similarly, CRBP and/or other retinoid-binding proteins function in the synthesis of retinal esters, the reduction of retinal generated from intestinal beta-carotene metabolism, and retinoic acid metabolism. The discussion details the evidence supporting an integrated model of retinoid-binding protein/metabolism. Also addressed are retinoid-androgen interactions and evidence incompatible with ethanol causing fetal alcohol syndrome by competing directly with retinol dehydrogenation to impair retinoic acid biosynthesis.
Biochemical studies indicate that alcohol dehydrogenase (ADH) metabolizes retinol to retinal, and that aldehyde dehydrogenase (ALDH) metabolizes retinal to retinoic acid, a molecule essential for growth and development. Summarized herein are several genetic studies supporting in vivo functions for ADH and ALDH in retinoic acid synthesis. Gene targeting was used to create knockout mice for either Adh1 or Adh4. Both knockout mice were viable and fertile without obvious defects. However, when wild-type and Adh4 knockout mice were subjected to vitamin A deficiency during gestation, the survival rate at birth was 3.3-fold lower for Adh4 knockout mice. When adult mice were examined for production of retinoic acid following retinol administration, Adh1 knockout mice exhibited 10-fold lower retinoic acid levels in liver compared with wild-type, whereas Adh4 knockout mice differed from wild-type by less than 2-fold. Thus, Adh1 plays a major role in the metabolism of a large dose of retinol to retinoic acid in adults, whereas Adh4 plays a role in maintaining sufficient retinol metabolism for development during retinol deficiency. ALDHs were examined by overexpression studies in frog embryos. Injection of mRNAs for either mouse Raldh1 or Raldh2 stimulated retinoic acid synthesis in frog embryos at the blastula stage when retinoic acid is normally undetectable. Overexpression of human ALDH2, human ALDH3, and mouse Aldh-pb did not stimulate retinoic acid production. In addition, Raldh2 knockout mice exhibit embryonic lethality with defects in retinoid-dependent tissues. Overall, these studies provide genetic evidence that Adh1, Adh4, Raldh1, and Raldh2 encode retinoid dehydrogenases involved in retinoic acid synthesis in vivo.
The ability of vitamin A (retinol) to control growth and development depends upon tissue-specific metabolism of retinol to retinoic acid (RA). RA then functions as a ligand for retinoid receptor signaling. Mouse genetic studies support a role for cytosolic alcohol dehydrogenases (ADH) in the first step (oxidation of retinol to retinaldehyde) and a role for cytosolic retinaldehyde dehydrogenases (RALDH) in the second step (oxidation of retinaldehyde to RA). Mice lacking ADH3 have reduced survival and a growth defect that can be rescued by dietary retinol supplementation, whereas the effect of a loss of ADH1 or ADH4 is noticed only in mice subjected to vitamin A excess or deficiency, respectively. Also, genetic deficiency of both ADH1 and ADH4 does not have additive effects, verifying separate roles for these enzymes in retinoid metabolism. As for the second step of RA synthesis, a null mutation of RALDH2 is embryonic lethal, eliminating most mesodermal RA synthesis, whereas loss of RALDH1 eliminates RA synthesis only in the embryonic dorsal retina with no obvious effect on development. Analysis of RA-rescued RALDH2 mutants has also revealed that RALDH3 and at least one additional enzyme produce RA tissue-specifically in embryos. Collectively, these genetic findings indicate that metabolism of retinol to retinaldehyde is not tissue-restricted as it is catalyzed by ubiquitously-expressed ADH3 (a low activity form) as well as by tissue-specifically expressed ADH1 and ADH4 (high activity forms). In contrast, further metabolism of retinaldehyde to RA is tissue-restricted as all enzymes identified are tissue-specific. An important concept to emerge is that selective expression of enzymes catalyzing the second step is what limits the tissues that can completely metabolize retinol to RA to initiate retinoid signaling.
Beyond their classical nutritional roles, nutrients modify gene expression and function in
target cells and, by so doing, affect many fundamental biological processes. An emerging example,
which is the focus of this review, is the involvement of vitamin A in the regulation of the level
and functioning of body fat reserves. Retinoic acid, the carboxylic acid form of vitamin A, is a
transcriptional activator of the genes encoding uncoupling proteins, and results in animals indicate
that whole body thermogenic capacity is related to the vitamin A status. Retinoic acid also
influences adipocyte differentiation and survival, with high doses inhibiting and low doses promoting
adipogenesis of preadipose cells in culture. Moreover, vitamin A status can influence the
development and function of adipose tissues in whole animals, with a low vitamin A status favouring
increased fat deposition.
In early 1900s, based on indirect evidence, Steenbock and Morton independently predicted that beta-carotene could be the biological precursor of vitamin A, although this notion was contested by others. In the 1930s, Thomas Moore showed the in vivo formation of vitamin A from beta-carotene. But it was not until Jim Olson and DeWitt Goodman independently showed in 1965 the formation of retinal, the aldehyde form of vitamin A from beta-carotene in cell-free extracts of liver and intestine, that this vital pathway of beta-carotene was recognized. Despite compelling evidence in several experimental systems for the central cleavage of beta-carotene to retinal by many investigators, there were some careful independent studies by Glover et al., Ganguly et al., Hansen and Meret and Krinsky et al. showing the eccentric cleavage of beta-carotene resulting in the formation of apocarotenoids both in vivo and in vitro. In an attempt to resolve this controversial issue, we revisited this problem in 1989 and showed beyond doubt the formation of retinal as the sole enzymatic product of a cytosolic enzyme from rabbit and rat intestinal mucosa by mass spectrometry and tracer analysis of the crystallized product. This was confirmed in 1996 by Nagao using the pig intestinal extract. Yeum et al. confirmed in 2000 that retinal is the sole product of beta-carotene cleavage in the presence of alpha-tocopherol, and that the observed formation of apocarotenoids occurs only in the absence of an antioxidant like alpha-tocopherol. In the same year, Barua and Olson also concluded from their in vivo studies in rats that central cleavage is by far the major pathway for the formation of vitamin A from beta-carotene. Beta, beta-carotene 15,15'-dioxygenase (EC 1.13.11.21) is the key enzyme that cleaves beta-carotene into two molecules of retinal. It is a cytosolic enzyme primarily localized in the duodenal mucosa although it has been found in liver. It is a 66 kDa sulfhydryl protein, requires molecular oxygen and is activated by ferrous ions. It is highly specific for 15:15' ethylenic bond of carotenoids although it has fairly broad specificity towards a number of carotenoids with at least one intact beta-ionone ring. The dioxygenase was recently cloned from Drosophila melanogaster and from the chicken intestine. The recombinant protein was found to form retinal as the sole cleavage product of beta-carotene. No apo-carotenoids were formed. Therefore, it is unequivocally proven that the major, if not the sole, pathway of beta-carotene cleavage to vitamin A is by oxidative cleavage of the central ethylenic bond of beta-carotene to yield two molecules of retinal. Most recently, human dioxygenase has also been cloned. Thus, the wisdom, vision and epoch-making mission of Jim Olson in the science of beta-carotene metabolism have been accomplished. I have no doubt that the impact of his original discovery of the dioxygenase and its importance in vitamin A nutriture should be forthcoming in the near future.
To examine connections between triglyceride metabolism and inflammation, especially as they relate to transcriptional regulation through peroxisomal proliferator activated receptors activation.
Peroxisomal proliferator activated receptors, members of the steroid hormone nuclear receptor family, have been of particular interest as a mechanism through which different dietary components might control gene expression. Extensive prior work has defined the central role peroxisomal proliferator activated receptors play in many key metabolic responses, including glucose control and lipid metabolism. Emerging evidence suggests peroxisomal proliferator activated receptor activation may limit inflammation and atherosclerosis. The demonstration that certain fatty acids can activate peroxisomal proliferator activated receptors belies the potential link between nutritional components and peroxisomal proliferator activated receptor responses. Interest in this connection had been heightened by recent evidence that lipolysis in certain situations can both generate peroxisomal proliferator activated receptor ligands and limit some known inflammatory responses.
Lipolytic peroxisomal proliferator activated receptor activation suggests new ways in which to reconsider triglycerides and the distal consequences of their metabolism, including the possible effects on inflammation and atherosclerosis.
RXR, a member of the superfamily of nuclear hormone receptors, regulates gene transcription in response to 9-cis-retinoic acid. We previously showed that, among nuclear receptors, RXR is unique in that it self-associates into homotetramers, and that these tetramers dissociate rapidly upon ligation. Here, we report that binding of RXR tetramers to DNA containing two RXR response elements results in a dramatic DNA-looping. RXR can thus juxtapose distant DNA sequences, enabling transcriptional regulation by far-upstream factors. We show that RXR functions as a DNA architectural factor and that, while this activity is regulated by 9-cis-retinoic acid, it is distinct from and independent of the receptor's intrinsic transcriptional activity. The data establish RXR as the first identified architectural factor whose activity is regulated by a small ligand, and demonstrate a novel mechanism of transcriptional regulation by retinoids.
We report a sensitive LC (liquid chromatography)/MS/MS assay using selected reaction monitoring to quantify RA (retinoic acid), which is applicable to biological samples of limited size (10-20 mg of tissue wet weight), requires no sample derivatization, provides mass identification and resolves atRA (all-trans-RA) from its geometric isomers. The assay quantifies over a linear range of 20 fmol to 10 pmol, and has a 10 fmol limit of detection at a signal/noise ratio of 3. Coefficients of variation are: instrumental, 0.5-2.9%; intra-assay, 5.4+/-0.4%; inter-assay 8.9+/-1.0%. An internal standard (all-trans-4,4-dimethyl-RA) improves accuracy by confirming extraction efficiency and revealing handling-induced isomerization. Tissues of 2-4-month-old C57BL/6 male mice had atRA concentrations of 7-9.6 pmol/g and serum atRA of 1.9+/-0.6 pmol/ml (+/-S.E.M.). Tissue 13-cis-RA ranged from 2.9 to 4.2 pmol/g, and serum 13-cis-RA was 1.2+/-0.3 pmol/ml. CRBP (cellular retinol-binding protein)-null mouse liver had atRA approximately 30% lower than wild-type (P<0.05), but kidney, testis, brain and serum atRA were similar to wild-type. atRA in brain areas of 12-month-old female C57BL/6 mice were (+/-S.E.M.): whole brain, 5.4+/-0.4 pmol/g; cerebellum, 10.7+/-0.3 pmol/g; cortex, 2.6+/-0.4 pmol/g; hippocampus, 8.4+/-1.2 pmol/g; striatum, 15.3+/-4.7 pmol/g. These data provide the first analytically robust quantification of atRA in animal brain and in CRBP-null mice. Direct measurements of endogenous RA should have a substantial impact on investigating target tissues of RA, mechanisms of RA action, and the relationship between RA and chronic disease.
The principal abnormalities of the metabolic syndrome are abdominal obesity, atherogenic dyslipidemia, hypertension, insulin resistance, inflammation, and prothrombotic states. This review focuses on the retinoid X receptor (RXR) and its partners in the metabolic syndrome. RXR and its partners are nuclear receptors that function as ligand-dependent transcription factors. The ligands are lipids, and the system functions as a cellular lipid sensor. Agonists and inhibitors of these receptors are promising treatments for this widespread syndrome.
The mouse liver 16,000 g fraction, which contains peroxisomes, reduces all-trans-retinal, but has limited ability to dehydrogenate retinol enzymatically. Feeding mice for 2 weeks with a diet containing clofibrate (0.5%, w/w), a PPAR alpha ligand and peroxisome proliferator, increased the 16,000 g fraction approximately 2-fold in protein, approximately 2-fold in specific activity of retinal reduction, and approximately 4-fold in retinal reductase units compared to controls, and caused a 50% decrease in liver retinol. An increase in both reductase specific activity and units indicates that clofibrate/PPAR alpha induced expression of retinal-reducing enzymes(s), in addition to increasing reductase(s) content. We expressed a cDNA from the NCBI data bank that encodes a peroxisome short-chain dehydrogenase/reductase. The enzyme, mouse retinal reductase (RRD, also known as human 2,4-dienoyl-CoA reductase), reduces all-trans-retinal [V(m) = 40 nmol min(-1) (mg of protein)(-1); K(0.5) = 2.3 microM] and has 4- and 60-fold less activity with 13-cis-retinal and 9-cis-retinal, respectively. Recombinant RRD functions with both unbound and CRBP(I) (cellular retinol-binding protein)-bound retinal, but apo-CRBP(I) inhibits the reductase. RRD mRNA expression was initiated on embryo day 7. Most adult tissues assayed expressed the mRNA. Liver, kidney, and heart had the most intense expression, with much less intense expression in brain, spleen, and lung. Clofibrate feeding increased the amount of RRD protein in the 16,000 g fraction of liver, consistent with the clofibrate-induced increase in reductase activity. These data relate retinoid metabolism, PPAR alpha, peroxisomes, and RRD, and are consistent with a further function of CRBP(I) in retinoid metabolism.
Retinoylation (acylation of proteins by retinoic acid) is considered as one mechanism of retinoic acid (RA) action occurring in cells in vitro and in vivo. Previously, our studies showed that in rat tissues the formation of retinoyl-CoA from RA, the first step of retinoylation, required ATP, CoA and MgCl(2). In the current study, we examined whether the transfer of retinoyl-CoA into proteins, the second step of retinoylation, occurs in rat tissues. [(3)H]-Labeled-retinoyl-CoA bound covalently to proteins in rat liver, kidney, testis, and brain. The levels of incorporation of retinoyl-CoA into proteins were higher in vitamin A-deficient rats than in normal ones. The formation of retinoylated proteins depended on the incubation time, and the concentrations of retinoyl-CoA and homogenate. The reaction was suppressed by fatty acyl-CoAs and palmitic acid, but not by arachidonic acid. The Vmax and Km values for retinoyl-CoA in the formation of retinoylated proteins using a crude liver extract were estimated to be 2,597.3 pmol/min/mg protein and 9.5 x 10(-5) M, respectively. Retinoylated proteins formed from retinoyl-CoA, including a 17 kDa protein exhibiting high radioactivity, disappeared in the presence of 2-mercaptoethanol, indicating that RA was linked to the proteins through a thioester bond. These results demonstrate that retinoylation in rat tissues occurs via retinoyl-CoA formed from RA. This process may play a significant physiological role in cells.