[show abstract][hide abstract] ABSTRACT: Obesity-related diseases such as diabetes and dyslipidemia result from metabolic alterations including the defective conversion, storage and utilization of nutrients, but the central mechanisms that regulate this process of nutrient partitioning remain elusive. As positive regulators of feeding behaviour, agouti-related protein (AgRP) producing neurons are indispensible for the hypothalamic integration of energy balance. Here, we demonstrate a role for AgRP-neurons in the control of nutrient partitioning. We report that ablation of AgRP-neurons leads to a change in autonomic output onto liver, muscle and pancreas affecting the relative balance between lipids and carbohydrates metabolism. As a consequence, mice lacking AgRP-neurons become obese and hyperinsulinemic on regular chow but display reduced body weight gain and paradoxical improvement in glucose tolerance on high-fat diet. These results provide a direct demonstration of a role for AgRP-neurons in the coordination of efferent organ activity and nutrient partitioning, providing a mechanistic link between obesity and obesity-related disorders.
[show abstract][hide abstract] ABSTRACT: Insulin resistance is a major characteristic of obesity and type 2 diabetes (T2DM). During the last decade, endoplasmic reticulum (ER) stress has emerged as a new player in this field and a considerable number of recent studies have pointed out its role in the onset of insulin resistance (IR). ER stress appears to act directly as a negative modulator of the insulin signaling pathway but also indirectly by promoting lipid accumulation. This review aims to summarize and decipher the abundant new literature concerning the emerging and multifaceted involvement of ER stress in the development of metabolic dysfunctions in insulin target tissues.
Trends in Endocrinology and Metabolism 07/2012; 23(8):381-90. · 8.90 Impact Factor
[show abstract][hide abstract] ABSTRACT: In liver, the glucose-responsive transcription factor ChREBP plays a critical role in converting excess carbohydrates into triglycerides through de novo lipogenesis. Although the importance of ChREBP in glucose sensing and hepatic energy utilization is strongly supported, the mechanism driving its activation in response to glucose in the liver is not fully understood. Indeed, the current model of ChREBP activation, which depends on Serine 196 and Threonine 666 dephosphorylation, phosphatase 2A (PP2A) activity, and xylulose 5-phosphate (X5P) as a signaling metabolite, has been challenged.
We inhibited PP2A activity in HepG2 cells through the overexpression of SV40 small t antigen and addressed the importance of ChREBP dephosphorylation on Ser-196 using a phospho-specific antibody. To identify the exact nature of the metabolite signal required for ChREBP activity in liver, we focused on the importance of G6P synthesis in liver cells, through the modulation of glucose 6-phosphate dehydrogenase (G6PDH) activity, the rate-limiting enzyme of the pentose phosphate pathway in hepatocytes, and in HepG2 cells using both adenoviral and siRNA approaches.
In contrast to the current proposed model, our study reports that PP2A activity is dispensable for ChREBP activation in response to glucose and that dephosphorylation on Ser-196 is not sufficient to promote ChREBP nuclear translocation in the absence of a rise in glucose metabolism. By deciphering the respective roles of G6P and X5P as signaling metabolites, our study reveals that G6P produced by GK, but not X5P, is essential for both ChREBP nuclear translocation and transcriptional activity in response to glucose in liver cells.
Altogether, our study, by reporting that G6P is the glucose-signaling metabolite, challenges the PP2A/X5P-dependent model currently described for ChREBP activation in response to glucose in liver.
Journal of Hepatology 08/2011; 56(1):199-209. · 9.86 Impact Factor
[show abstract][hide abstract] ABSTRACT: Laforin is a dual specificity protein phosphatase involved in Lafora disease (LD), a fatal form of progressive myoclonus epilepsy characterized by neurodegeneration and the presence of intracellular polyglucosan inclusions (Lafora bodies) in different tissues. In this work, we describe that mice lacking laforin (epm2a-/-) have enhanced insulin response leading to altered whole-body energy balance. This enhanced insulin response overactivates the Akt pathway which increases glucose uptake in the heart, resulting in increased glycogen levels and the formation of polyglucosan inclusions. In addition, enhanced insulin response resulted in increased liver lipid biosynthesis, resulting in hepatic steatosis. On the contrary, overexpression in rat hepatoma FTO2B cells of native laforin but not of a form lacking phosphatase activity (C266S) resulted in attenuation of insulin signaling. These results define laforin as a new regulator of insulin sensitivity, which provides novel insights into LD pathogenesis and identifies this phosphatase as a potential novel component of the insulin signaling cascade.
Human Molecular Genetics 07/2011; 20(13):2571-84. · 7.69 Impact Factor
[show abstract][hide abstract] ABSTRACT: "Double-stranded RNA-dependent protein kinase links pathogen sensing with stress and metabolic homeostasis" by Nakamura T, Furuhashi M, Li P, Cao H, Tuncman G, Sonenberg N, Gorgun CZ, Hotamisligil GS. Cell 140, 338-348, February 5, 2010. Copyright 2010. Abstract reprinted with permission from Elsevier. As chronic inflammation is a hallmark of obesity, pathways that integrate nutrient- and pathogen sensing pathways are of great interest in understanding the mechanisms of insulin resistance, type 2 diabetes, and other chronic metabolic pathologies. Here, we provide evidence that double-stranded RNA-dependent protein kinase (PKR) can respond to nutrient signals as well as endoplasmic reticulum (ER) stress and coordinate the activity of other critical inflammatory kinases such as the c-Jun N-terminal kinase (JNK) to regulate insulin action and metabolism. PKR also directly targets and modifies insulin receptor substrate and hence integrates nutrients and insulin action with a defined pathogen response system. Dietary and genetic obesity features marked activation of PKR in adipose and liver tissues and absence of PKR alleviates metabolic deterioration due to nutrient or energy excess in mice. These findings demonstrate PKR as a critical component of an inflammatory complex that responds to nutrients and organelle dysfunction.
Journal of Hepatology 02/2011; 54(6):1307-9. · 9.86 Impact Factor
[show abstract][hide abstract] ABSTRACT: Patients with non-alcoholic fatty liver disease are characterised by a decreased n-3/n-6 polyunsaturated fatty acid (PUFA) ratio in hepatic phospholipids. The metabolic consequences of n-3 PUFA depletion in the liver are poorly understood. We have reproduced a drastic drop in n-3 PUFA among hepatic phospholipids by feeding C57Bl/6J mice for 3 months with an n-3 PUFA depleted diet (DEF) versus a control diet (CT), which only differed in the PUFA content. DEF mice exhibited hepatic insulin resistance (assessed by euglycemic-hyperinsulinemic clamp) and steatosis that was associated with a decrease in fatty acid oxidation and occurred despite a higher capacity for triglyceride secretion. Microarray and qPCR analysis of the liver tissue revealed higher expression of all the enzymes involved in lipogenesis in DEF mice compared to CT mice, as well as increased expression and activation of sterol regulatory element binding protein-1c (SREBP-1c). Our data suggest that the activation of the liver X receptor pathway is involved in the overexpression of SREBP-1c, and this phenomenon cannot be attributed to insulin or to endoplasmic reticulum stress responses. In conclusion, n-3 PUFA depletion in liver phospholipids leads to activation of SREBP-1c and lipogenesis, which contributes to hepatic steatosis.
PLoS ONE 01/2011; 6(8):e23365. · 3.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: AMPK is a kinase involved in cell energy homeostasis. In this issue of Cell Metabolism, Chopra and colleagues (2011) reveal a new function of AMPK in stimulating fat absorption. AMPK enhances the expression of the bile acid transporter, BSEP, by activating the coactivator SRC-2, thus promoting bile acid secretion.
[show abstract][hide abstract] ABSTRACT: The adiponutrin/PNPLA3 (patatin-like phospholipase domain-containing protein 3) variant I148M has recently emerged as an important marker of human fatty liver disease. In order to understand the role of the adiponutrin/PNPLA3 protein, we investigated the regulation of its expression in both human and mouse hepatocytes.
Adiponutrin/PNPLA3 and lipogenic enzyme expression was determined by real-time PCR analysis in a wide panel of analysis in vivo in the mouse liver and in vitro in murine hepatocytes and human hepatocyte cell lines infected with ChREBP or SREBP1c-expressing adenoviruses.
We show that in the mouse liver, adiponutrin/PNPLA3 gene expression is under the direct transcriptional control of ChREBP (carbohydrate-response element-binding protein) and SREBP1c (sterol regulatory element binding protein1c) in response to glucose and insulin, respectively. In silico analysis revealed the presence of a ChoRE (carbohydrate response element) and of a SRE (sterol response element) binding site on the mouse adiponutrin/PNPLA3 gene promoter. Point mutation analysis in reporter gene assays identified the functional response of these two binding sites in the mouse adiponutrin/PNPLA3 promoter. In contrast, in human immortalized hepatocytes and in HepG2 hepatoma cells, only SREBP1c was able to induce adiponutrin/PNPLA3 expression, whereas ChREBP was unable to modulate its expression.
All together, our results suggest that adiponutrin/PNPLA3 is regulated by two key factors of the glycolytic and lipogenic pathways, raising the question of its implication in the metabolism of carbohydrates and lipids.
Journal of Hepatology 11/2010; 55(1):145-53. · 9.86 Impact Factor
[show abstract][hide abstract] ABSTRACT: To examine the role of endoplasmic reticulum stress in the regulation of hepatic lipid metabolism and its contribution to the development of hepatic steatosis.
Endoplasmic reticulum stress activation has been reported in most models of hepatic steatosis in rodents and humans and its contribution to hepatic fat deposition has been recently documented. The main metabolic pathway affected by endoplasmic reticulum stress is lipogenesis. Endoplasmic reticulum stress activates the proteolytic cleavage of the lipogenic transcription factor sterol regulatory element binding protein-1c leading to the induction of lipogenic enzyme expression. A role for X box-binding protein 1, an endoplasmic reticulum stress-activated transcription factor, has also recently emerged. Endoplasmic reticulum stress, by inhibiting apoB100 secretion, has associated with impaired VLDL secretion. In rodents, treatments with molecular or chemical chaperones that reduce endoplasmic reticulum stress markers have fully demonstrated their efficiency in the treatment of hepatic steatosis.
Manipulating endoplasmic reticulum stress pathway yields encouraging results for the treatment of hepatic steatosis in rodents. However, activation of unfolded protein response is a physiological mechanism, which is particularly important for secretory cells such as hepatocytes and the long-term consequences of such treatments should be cautiously evaluated.
Current opinion in lipidology 06/2010; 21(3):239-46. · 6.13 Impact Factor
[show abstract][hide abstract] ABSTRACT: Nutritional hepatic disorders are spreading worldwide associated to obesity and type 2 diabetes. The underlying mechanisms leading to the development of hepatic steatosis and its complications are not fully understood. The endoplasmic reticulum (ER) stress response has recently been proposed to play a crucial role in the setting of these pathologies. This review will evaluate the late discoveries highlighting ER stress as a major actor in the development of nutritional liver diseases.
Activation of ER stress has been reported in the fatty liver of obese rodents and obese individuals. The mechanisms by which ER stress leads to the development of hepatic steatosis have been recently documented. ER stress has been shown to directly activate the lipogenic transcription factor SREBP-1c (sterol regulatory element binding protein-1c) conducting to an induction of the lipogenic pathway. ER stress activation is also associated with impaired VLDL (very low density lipoprotein) secretion. ER stress could also have a role in hepatic steatosis progression by triggering inflammation and fibrosis. In rodents, therapies aiming to reduce ER stress have fully demonstrated their efficiency in the treatment of hepatic steatosis.
ER stress has been recently involved in the development of hepatic steatosis. Thus, ER stress could represent in the future an eligible therapeutic target for the treatment of nonalcoholic fatty liver disease. However, as ER stress is a fundamental mechanism involved in cell survival, any modification of this pathway must be carefully assessed.
Current opinion in clinical nutrition and metabolic care. 10/2009; 12(6):575-82.
[show abstract][hide abstract] ABSTRACT: Chronic hepatitis C virus (HCV) infection is associated with altered lipid metabolism and hepatocellular steatosis. Virus-induced steatosis is a cytopathic effect of HCV replication. The goal of this study was to examine the mechanisms underlying HCV-induced lipid metabolic defects in a transgenic mouse model expressing the full HCV protein repertoire at levels corresponding to natural human infection. In this model, expression of the HCV full-length open reading frame was associated with hepatocellular steatosis and reduced plasma triglyceride levels. Triglyceride secretion was impaired, whereas lipogenesis was activated. Increased lipogenic enzyme transcription was observed, resulting from maturational activation and nuclear translocation of sterol regulatory element-binding protein 1c (SREBP1c). However, endoplasmic reticulum (ER) stress markers were expressed at similar levels in both HCV transgenic mice and their wild type counterparts, suggesting that SREBP1c proteolytic cleavage in the presence of HCV proteins was independent of ER stress. In conclusion, transgenic mice expressing the HCV full-length polyprotein at low levels have decreased plasma triglyceride levels and develop hepatocellular steatosis in the same way as HCV-infected patients. In these mice, SREBP1c activation by one or several HCV proteins induces de novo triglyceride synthesis via the lipogenic pathway, in a manner independent of ER stress, whereas triglyceride secretion is simultaneously reduced.
Journal of Biological Chemistry 10/2009; 284(48):33466-74. · 4.65 Impact Factor
[show abstract][hide abstract] ABSTRACT: Hepatic steatosis is present in insulin-resistant obese rodents and is concomitant with active lipogenesis. Hepatic lipogenesis depends on the insulin-induced activation of the transcription factor SREBP-1c. Despite prevailing insulin resistance, SREBP-1c is activated in the livers of genetically and diet-induced obese rodents. Recent studies have reported the presence of an ER stress response in the livers of obese ob/ob mice. To assess whether ER stress promotes SREBP-1c activation and thus contributes to lipogenesis, we overexpressed the chaperone glucose-regulated protein 78 (GRP78) in the livers of ob/ob mice using an adenoviral vector. GRP78 overexpression reduced ER stress markers and inhibited SREBP-1c cleavage and the expression of SREBP-1c and SREBP-2 target genes. Furthermore, hepatic triglyceride and cholesterol contents were reduced, and insulin sensitivity improved, in GRP78-injected mice. These metabolic improvements were likely mediated by restoration of IRS-2 expression and tyrosine phosphorylation. Interestingly, GRP78 overexpression also inhibited insulin-induced SREBP-1c cleavage in cultured primary hepatocytes. These findings demonstrate that GRP78 inhibits both insulin-dependent and ER stress-dependent SREBP-1c proteolytic cleavage and explain the role of ER stress in hepatic steatosis in obese rodents.
The Journal of clinical investigation 05/2009; 119(5):1201-15. · 15.39 Impact Factor
[show abstract][hide abstract] ABSTRACT: Sterol-regulatory element binding proteins (SREBPs) control the expression of genes involved in fatty acid and cholesterol biosynthesis. Using microarrays, we observed that mature SREBP-1 also induced the expression of genes unrelated to lipid metabolism, such as heme oxygenase 1 (HMOX1), plasma glutathione peroxidase, the phosphatidylinositol-3 kinase regulatory subunit p55 gamma, synaptic vesicle glycoprotein 2A, and COTE1. The expression of these genes was repressed upon addition of sterols, which block endogenous SREBP cleavage, and was induced by the statin drug mevinolin. Stimulation of fibroblasts with platelet-derived growth factor, which activates SREBP-1, had a similar effect. Fasted mice that were refed with a high-carbohydrate diet presented an increased expression of HMOX1 and p55 gamma in the liver. Overall, the transcriptional signature of SREBP-1 in fibroblasts stimulated by growth factors was very similar to that described in liver cells. We analyzed the HMOX1 promoter and found one SREBP binding site of the E-box type, which was required for regulation by SREBP-1a and SREBP-1c but was insensitive to SREBP-2. In conclusion, our data suggest that SREBP-1 regulates the expression of stress response and signaling genes, which could contribute to the metabolic response to insulin and growth factors in various tissues.
The Journal of Lipid Research 08/2007; 48(7):1628-36. · 4.39 Impact Factor
[show abstract][hide abstract] ABSTRACT: The endoplasmic reticulum (ER) is the first compartment in the secretory pathway. In the ER, proteins fold into their native configuration and are modified by post-translational modifications. Perturbations that alter ER homeostasis therefore disrupt folding and lead to the accumulation of unfolded proteins. These perturbations include modifications of Ca2+ homeostasis, increased demand for protein folding due to elevated synthesis of proteins in specialized cells or expression of a mutant misfolded protein. To limit accumulation of unfolded proteins, the cells have developed a specialized pathway : the unfolded protein response (UPR). UPR involves the activation of three transmembrane proteins of the ER : the PKR-like ER protein kinase (PERK), the activating transcription factor 6 (ATF6) and the inositol requiring enzyme 1 (IRE-1). The activation of all three components of the UPR depends on the dissociation of the luminal chaperone BiP/GRP78 from the luminal part of these proteins. Once activated, these pathways down-regulate protein synthesis through the phosphorylation of eiF2 (eucaryotic translation initiation factor 2) and up-regulate the transcription of genes which encode ER chaperones, protein folding enzymes and components of the ER-associated degradation system (ERAD). Growing evidences indicate that UPR signaling plays critical roles in nutrient sensing, differentiation of secretory cells such as pancreatic b cell and antibody producing plasma cells, glucose homeostasis and in the development of pathologies linked to accumulation of aggregated proteins.
Medecine sciences: M/S 04/2007; 23(3):291-6. · 0.56 Impact Factor
[show abstract][hide abstract] ABSTRACT: Metformin is an anti-diabetic drug commonly used to treat cycle disorders and anovulation in women with polycystic ovary syndrome. However, the effects and molecular mechanism of metformin in the ovary are not entirely understood. We investigated the effects of this drug on steroidogenesis and proliferation in rat granulosa cells. Metformin (10 mM) treatment for 48 h reduced progesterone and estradiol (E2) production in both basal conditions and under FSH stimulation. It also decreased the levels of the HSD3B, CYP11A1, STAR, and CYP19A1 proteins in response to FSH (10(-8) M) and of HSD3B in the basal state only. Metformin treatment (10 mM, 24 h) also reduced cell proliferation and the levels of CCND2 and CCNE proteins without affecting cell viability, both in the basal state and in response to FSH. Furthermore, metformin treatment for 1 h simultaneously increased the Thr172 phosphorylation of PRKAA (adenosine 5' monophosphate-activated protein kinase alpha) and the Ser79 phosphorylation of ACACA (acetyl-Coenzyme A carboxylase alpha). The adenovirus-mediated production of dominant-negative PRKAA totally abolished the effects of metformin on progesterone secretion, HSD3B and STAR protein production, and MAPK3/1 phosphorylation. Conversely, total inhibition of PRKAA Thr172 phosphorylation with the dominant-negative PRKAA adenovirus did not restore the decrease in E2 production and cell proliferation induced by metformin. Our results therefore strongly suggest that metformin reduces progesterone production via a PRKAA-dependent mechanism, whereas PRKAA activation is not essential for the decrease in E2 production and cell growth induced by metformin in rat granulosa cells.
Biology of Reproduction 10/2006; 75(3):342-51. · 4.03 Impact Factor
[show abstract][hide abstract] ABSTRACT: Obesity is a metabolic disorder often associated with type 2 diabetes, insulin resistance, and hepatic steatosis. Leptin-deficient (ob/ob) mice are a well-characterized mouse model of obesity in which increased hepatic lipogenesis is thought to be responsible for the phenotype of insulin resistance. We have recently demonstrated that carbohydrate responsive element-binding protein (ChREBP) plays a key role in the control of lipogenesis through the transcriptional regulation of lipogenic genes, including acetyl-CoA carboxylase and fatty acid synthase. The present study reveals that ChREBP gene expression and ChREBP nuclear protein content are significantly increased in liver of ob/ob mice. To explore the involvement of ChREBP in the physiopathology of hepatic steatosis and insulin resistance, we have developed an adenovirus-mediated RNA interference technique in which short hairpin RNAs (shRNAs) were used to inhibit ChREBP expression in vivo. Liver-specific inhibition of ChREBP in ob/ob mice markedly improved hepatic steatosis by specifically decreasing lipogenic rates. Correction of hepatic steatosis also led to decreased levels of plasma triglycerides and nonesterified fatty acids. As a consequence, insulin signaling was improved in liver, skeletal muscles, and white adipose tissue, and overall glucose tolerance and insulin sensitivity were restored in ob/ob mice after a 7-day treatment with the recombinant adenovirus expressing shRNA against ChREBP. Taken together, our results demonstrate that ChREBP is central for the regulation of lipogenesis in vivo and plays a determinant role in the development of the hepatic steatosis and of insulin resistance in ob/ob mice.
[show abstract][hide abstract] ABSTRACT: AMP-activated protein kinase (AMPK) is a fuel sensor in glucose, lipid, and cholesterol metabolism. Using RT-PCR and Western blot, AMPK subunits mRNAs (alpha1/2, beta1/2, and gamma1/2) and proteins (alpha1/2 and beta1/2) can be found in the hen preovulatory follicles and precisely in both granulosa and theca cells. These preovulatory follicles are organized in a hierarchy according to their size (F5/6 to F1). The smallest number (F1) corresponds to the largest size and the latest mature stage. Phosphorylation of AMPKalpha on Thr172 and of acetyl-CoA carboxylase on Ser79 are higher in F4 and F3 than in F1 granulosa cells. However, they are not affected in F4-F1 theca cells. Treatment with 1 mM 5-amino-imidazole-4-carboxyamide-1-beta-D-ribofuranoside (AICAR), an activator of AMPK, dose dependently increased phosphorylation of AMPKalpha on Thr172 in primary F3/4 and F1 granulosa cells. In the absence of FSH, AICAR treatment increased progesterone, P450 side chain cleavage and steroidogenic acute regulatory (StAR) production in both F3/4 and F1 granulosa cells. However, in the presence of FSH, AICAR treatment for 36 h increased progesterone secretion, StAR protein levels and reduced extracellular signal-regulated kinase (ERK)1/2 phosphorylation in F3/4 granulosa cells. Opposite data were observed in F1 granulosa cells. Adenovirus-mediated expression of dominant-negative AMPK totally restored the effects of AICAR on FSH-induced progesterone secretion, StAR protein production, and ERK1/2 phosphorylation in F3/4 and F1 granulosa cells. Using a specific inhibitor of ERK1/2 (U0126), we also showed that this kinase is a negative regulator of the FSH-induced progesterone secretion in F3/4 and F1 granulosa cells, suggesting that AICAR-mediated AMPK activation modifies FSH-induced progesterone secretion differently through the ERK1/2 signaling pathway in hen F3/4 and F1 granulosa cells.
Journal of Endocrinology 08/2006; 190(1):85-97. · 4.06 Impact Factor
[show abstract][hide abstract] ABSTRACT: Design, synthesis and structure-activity relationships of benzimidazole derivatives as activators of the AMP-activated protein kinase (AMPK) are presented in this paper. AMPK is the central component of a protein kinase cascade that plays a key role in the regulation of energy balance. Once activated, AMPK initiates a series of responses that are aimed at restoring the energy balance of the cell and recent studies have indicated that AMPK plays an important role in regulation of the whole-body energy metabolism. The following study based on the lead compound S27847 involved modification of three regions of this compound. Preliminary structure-activity relationships are being described.
[show abstract][hide abstract] ABSTRACT: AMP-activated protein kinase (AMPK) is involved in cellular energy homeostasis. Its functions have been extensively studied in muscles and liver. AMPK stimulates pathways which increase energy production (glucose transport, fatty acid oxidation) and switches off pathways which consume energy (lipogenesis, protein synthesis, gluconeogenesis). This has led to the concept that AMPK has an interesting pharmaceutical potential in situations of insulin resistance and it is indeed the target of existing drugs and hormones which improve insulin sensitivity. Adipose tissue is a key player in energy metabolism through the release of substrates and hormones involved in metabolism and insulin sensitivity. Activation of AMPK in adipose tissue can be achieved through situations such as fasting and exercise. Leptin and adiponectin as well as hypoglycaemic drugs are activators of adipose tissue AMPK. This activation probably involves changes in the AMP/ATP ratio and the upstream kinase LKB1. When activated, AMPK limits fatty acid efflux from adipocytes and favours local fatty acid oxidation. Since fatty acids have a key role in insulin resistance, especially in muscles, activating AMPK in adipose tissue might be found to be beneficial in insulin-resistant states, particularly as AMPK activation also reduces cytokine secretion in adipocytes.
The Journal of Physiology 08/2006; 574(Pt 1):55-62. · 4.38 Impact Factor