[Show abstract][Hide abstract] ABSTRACT: Mitochondrial dysfunction has been implicated in the development of insulin resistance, however a large variety of association and intervention studies, as well as genetic manipulations in rodents have reported contrasting results. Indeed, even 39 years after the first publication describing a relationship between insulin resistance and diminished mitochondrial function, it is still unclear if a direct relationship exists, and more importantly if changes in mitochondrial capacity are a cause or consequence of insulin resistance. This review will take a journey through the past and summarize the debate about the occurrence of mitochondrial dysfunction and its possible role in causing decreased insulin action in obesity and type 2 diabetes. Evidence will be presented from studies in various human populations, as well as rodents with genetic manipulations of pathways known to affect mitochondrial function and insulin action. Finally, we will discuss if mitochondria are a potential target for the treatment of insulin resistance.
[Show abstract][Hide abstract] ABSTRACT: There is a strong association between aging, diet, and immunity. The effects of macronutrients and energy intake on splanchnic and hepatic lymphocytes were studied in 15 month old mice. The mice were ad-libitum fed 1 of 25 diets varying in the ratios and amounts of protein, carbohydrate, and fat over their lifetime. Lymphocytes in liver, spleen, Peyers patches, mesenteric lymph nodes, and inguinal lymph nodes were evaluated using flow cytometry. Low protein intake reversed aging changes in splenic CD4 and CD8 T cells, CD4:CD8 T cell ratio, memory/effector CD4 T cells and naïve CD4 T cells. A similar influence of total caloric intake in these ad-libitum fed mice was not apparent. Protein intake also influenced hepatic NK cells and B cells, while protein to carbohydrate ratio influenced hepatic NKT cells. Hepatosteatosis was associated with increased energy and fat intake and changes in hepatic Tregs, effector/memory T, and NK cells. Hepatic NK cells were also associated with body fat, glucose tolerance, and leptin levels while hepatic Tregs were associated with hydrogen peroxide production by hepatic mitochondria. Dietary macronutrients, particularly protein, influence splanchnic lymphocytes in old age, with downstream associations with mitochondrial function, liver pathology, and obesity-related phenotype.
The Journals of Gerontology Series A Biological Sciences and Medical Sciences 10/2014; · 4.31 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Excess dietary lipid generally leads to fat deposition and impaired glucose homeostasis, but consumption of fish oil (FO) alleviates many of these detrimental effects. The beneficial effects of FO are thought to be mediated largely via activation of the nuclear receptor peroxisomal-proliferator-activated receptor α (PPARα) by omega-3 polyunsaturated fatty acids and the resulting upregulation of lipid catabolism. However, pharmacological and genetic PPARα manipulations have yielded variable results. We have compared the metabolic effects of FO supplementation and the synthetic PPARα agonist Wy-14,643 (WY) in mice fed a lard-based high-fat diet. In contrast to FO, WY treatment resulted in little protection against diet-induced obesity and glucose intolerance, despite upregulating many lipid metabolic pathways. These differences were likely due to differential effects on hepatic lipid synthesis, with FO decreasing and WY amplifying hepatic lipid accumulation. Our results highlight that the beneficial metabolic effects of FO are likely mediated through multiple independent pathways.
[Show abstract][Hide abstract] ABSTRACT: A critical feature of obesity is enhanced insulin secretion from pancreatic β-cells, enabling the majority of individuals to maintain glycaemic control despite adiposity and insulin resistance. Surprisingly, the factors coordinating this adaptive β-cell response with adiposity have not been delineated. Here we show that fatty acid binding protein 4 (FABP4/aP2) is an adipokine released from adipocytes under obesogenic conditions, such as hypoxia, to augment insulin secretion. The insulinotropic action of FABP4 was identified using an in vitro system that recapitulates adipocyte to β-cell endocrine signalling, with glucose-stimulated insulin secretion (GSIS) as a functional readout, coupled with quantitative proteomics. Exogenous FABP4 potentiated GSIS in vitro and in vivo, and circulating FABP4 levels correlated with GSIS in humans. Insulin inhibited FABP4 release from adipocytes in vitro, in mice and in humans, consistent with feedback regulation. These data suggest that FABP4 and insulin form an endocrine loop coordinating the β-cell response to obesity.
[Show abstract][Hide abstract] ABSTRACT: Lipid deposition in the liver is associated with metabolic disorders including fatty liver disease, type II diabetes, and hepatocellular cancer. The enzymes acetyl-CoA carboxylase 1 (ACC1) and ACC2 are powerful regulators of hepatic fat storage; therefore, their inhibition is expected to prevent the development of fatty liver. In this study we generated liver-specific ACC1 and ACC2 double knockout (LDKO) mice to determine how the loss of ACC activity affects liver fat metabolism and whole-body physiology. Characterization of LDKO mice revealed unexpected phenotypes of increased hepatic triglyceride and decreased fat oxidation. We also observed that chronic ACC inhibition led to hyper-acetylation of proteins in the extra-mitochondrial space. In sum, these data reveal the existence of a compensatory pathway that protects hepatic fat stores when ACC enzymes are inhibited. Furthermore, we identified an important role for ACC enzymes in the regulation of protein acetylation in the extra-mitochondrial space.
[Show abstract][Hide abstract] ABSTRACT: The fundamental questions of what represents a macronutritionally balanced diet and how this maintains health and longevity remain unanswered. Here, the Geometric Framework, a state-space nutritional modeling method, was used to measure interactive effects of dietary energy, protein, fat, and carbohydrate on food intake, cardiometabolic phenotype, and longevity in mice fed one of 25 diets ad libitum. Food intake was regulated primarily by protein and carbohydrate content. Longevity and health were optimized when protein was replaced with carbohydrate to limit compensatory feeding for protein and suppress protein intake. These consequences are associated with hepatic mammalian target of rapamycin (mTOR) activation and mitochondrial function and, in turn, related to circulating branched-chain amino acids and glucose. Calorie restriction achieved by high-protein diets or dietary dilution had no beneficial effects on lifespan. The results suggest that longevity can be extended in ad libitum-fed animals by manipulating the ratio of macronutrients to inhibit mTOR activation.
[Show abstract][Hide abstract] ABSTRACT: Mitochondrial dysfunction has been implicated in the aetiology of many complex diseases, as well as the ageing process. Much of the research on mitochondrial dysfunction has focused on how mitochondrial damage may potentiate pathological phenotypes. The purpose of this review is to draw attention to the less well-studied mechanisms by which the cell adapts to mitochondrial perturbations. This involves communication of stress to the cell and successful induction of quality control responses, which include mitophagy, unfolded protein response, upregulation of antioxidant and DNA repair enzymes, morphological changes, and if all else fails apoptosis. The mitochondrion is an inherently stressful environment and we speculate that dysregulation of stress signaling or an inability to switch on these adaptations during times of mitochondrial stress may underpin mitochondrial dysfunction and hence amount to pathological states over time.
International Journal of Cell Biology 01/2014; 2014:156020.
[Show abstract][Hide abstract] ABSTRACT: Ever since eukaryotes subsumed the bacterial ancestor of mitochondria, the nuclear and mitochondrial genomes have had to closely coordinate their activities, as each encode different subunits of the oxidative phosphorylation (OXPHOS) system. Mitochondrial dysfunction is a hallmark of aging, but its causes are debated. We show that, during aging, there is a specific loss of mitochondrial, but not nuclear, encoded OXPHOS subunits. We trace the cause to an alternate PGC-1α/β-independent pathway of nuclear-mitochondrial communication that is induced by a decline in nuclear NAD(+) and the accumulation of HIF-1α under normoxic conditions, with parallels to Warburg reprogramming. Deleting SIRT1 accelerates this process, whereas raising NAD(+) levels in old mice restores mitochondrial function to that of a young mouse in a SIRT1-dependent manner. Thus, a pseudohypoxic state that disrupts PGC-1α/β-independent nuclear-mitochondrial communication contributes to the decline in mitochondrial function with age, a process that is apparently reversible.
[Show abstract][Hide abstract] ABSTRACT: Fatty acids are essential elements of all cells and have significant roles as energy substrates, components of cellular structure and signalling molecules. The storage of excess energy intake as fat in adipose tissue is an evolutionary advantage aimed at protecting against starvation but in much of today's world humans are faced with an unlimited availability of food and the excessive accumulation of fat is now a major risk for human health especially the development of type 2 diabetes. Since first recognition of the association between fat accumulation, reduced insulin action and increased risk of type 2 diabetes, several mechanisms have been proposed to link excess fatty acid availability to reduced insulin action, some of them competing or contradictory. This review summarises the evidence for these mechanisms in the context of excess dietary fatty acids generating insulin resistance in muscle, the major tissue involved in insulin-stimulated disposal of blood glucose. It also outlines potential problems with models and measurements that may hinder, as well as help improve our understanding of the links between fatty acids and insulin action.
Journal of Endocrinology 12/2013; · 3.59 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Accompanying the high rates of vitamin D deficiency observed in many countries, there is increasing interest in the physiological functions of vitamin D. Vitamin D is recognised to exert extra-skeletal actions in addition to its classic roles in bone and mineral homeostasis. Here we review the evidence for vitamin D's actions in muscle on the basis of observational studies, clinical trials and basic research. Numerous observational studies link vitamin D deficiency with muscle weakness and sarcopaenia. Randomised trials predominantly support an effect of vitamin D supplementation and the prevention of falls in older or institutionalised patients. Studies have also examined the effect of vitamin D in athletic performance, both inferentially by UV radiation and directly by vitamin D supplementation. Effects of vitamin D in muscle metabolic function, specifically insulin sensitivity, are also addressed in this review. At a mechanistic level, animal studies have evaluated the roles of vitamin D and associated minerals, calcium and phosphate, in muscle function. In vitro studies have identified molecular pathways by which vitamin D regulates muscle cell signalling and gene expression. This review evaluates evidence for the various roles of vitamin D in skeletal muscle and discusses controversies that have made this a dynamic field of research. This article is protected by copyright. All rights reserved.
[Show abstract][Hide abstract] ABSTRACT: There is emerging evidence for reduced muscle function in children with Neurofibromatosis type 1 (NF1). We have examined three murine models featuring NF1 deficiency in muscle to study the effect on muscle function as well as any underlying pathophysiology. The Nf1(+/-) mouse exhibited no differences in overall weight, lean tissue mass, fiber size, muscle weakness as measured by grip strength, or muscle atrophy-recovery with limb disuse, although this model lacks many other characteristic features of the human disease. Next, muscle-specific knockout mice (Nf1muscle(-/-)) were generated and they exhibited a failure to thrive leading to neonatal lethality. Intramyocellular lipid accumulations were observed by electron microscopy (EM) and Oil Red O staining. More mature muscle specimens lacking Nf1 expression taken from the limb-specific Nf1Prx1(-/-) conditional knockout line showed a 10-fold increase in muscle triglyceride content. Enzyme assays revealed a significant increase in the activities of oxidative metabolism enzymes in the Nf1Prx1(-/-) mice. Western analyses showed increases in the expression of Fatty Acid Synthase (FAS) and the hormone Leptin, as well as decreased expression of a number of fatty acid transporters in this mouse line. These data support the hypothesis that NF1 is essential for normal muscle function and survival and are the first to suggest a direct link between NF1 and mitochondrial fatty acid metabolism.
Human Molecular Genetics 10/2013; · 6.68 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: α-Actinin-3 deficiency occurs in approximately 16% of the global population due to homozygosity for a common nonsense polymorphism in the ACTN3 gene. Loss of α-actinin-3 is associated with reduced power and enhanced endurance capacity in elite athletes and nonathletes due to "slowing" of the metabolic and physiological properties of fast fibers. Here, we have shown that α-actinin-3 deficiency results in increased calcineurin activity in mouse and human skeletal muscle and enhanced adaptive response to endurance training. α-Actinin-2, which is differentially expressed in α-actinin-3-deficient muscle, has higher binding affinity for calsarcin-2, a key inhibitor of calcineurin activation. We have further demonstrated that α-actinin-2 competes with calcineurin for binding to calsarcin-2, resulting in enhanced calcineurin signaling and reprogramming of the metabolic phenotype of fast muscle fibers. Our data provide a mechanistic explanation for the effects of the ACTN3 genotype on skeletal muscle performance in elite athletes and on adaptation to changing physical demands in the general population. In addition, we have demonstrated that the sarcomeric α-actinins play a role in the regulation of calcineurin signaling.
The Journal of clinical investigation 10/2013; 123(10):4255-4263. · 15.39 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Dietary intake of long-chain fatty acids(LCFA) plays a causative role in insulin resistance and risk of diabetes. Whereas LCFA promote lipid accumulation and insulin resistance, diets rich in medium-chain fatty acids (MCFA) have been associated with increased oxidative metabolism and reduced adiposity, with little deleterious effects on insulin action. The molecular mechanisms underlying these differences between dietary fat subtypes are poorly understood. To investigate this further, we treated C2C12 myotubes with various LCFA(16:0, 18:1n9, 18:2n6) and MCFA (10:0, 12:0), as well as fed mice diets rich in LCFA or MCFA, and investigated fatty acid-induced changes in mitochondrial metabolism and oxidative stress. MCFA-treated cells displayed less lipid accumulation, increased mitochondrial oxidative capacity and less oxidative stress than LCFA-treated cells. These changes were associated with improved insulin action in MCFA-treated myotubes. MCFA-fed mice exhibited increased energy expenditure, reduced adiposity and better glucose tolerance compared to LCFA-mice. Dietary MCFA increased respiration in isolated mitochondria, with a simultaneous reduction in reactive oxygen species generation, and subsequently low oxidative damage. Collectively our findings indicate that in contrast to LCFA, MCFA increase the intrinsic respiratory capacity of mitochondria, without increasing oxidative stress. These effects potentially contribute to the beneficial metabolic actions of dietary MCFA.
The Journal of Lipid Research 09/2013; · 4.73 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In recent years, reversible lysine acylation of proteins has emerged as a major post-translational modification across the cell, and importantly has been shown to regulate many proteins in mitochondria. One key family of deacylase enzymes are the sirtuins, of which SIRT3, SIRT4, and SIRT5 are localised to the mitochondria and regulate acyl modifications in this organelle.
In this review we discuss the emerging role of lysine acylation in the mitochondrion and summarise the evidence that proposes mitochondrial sirtuins are important players in the modulation of mitochondrial energy metabolism in response to external nutrient cues, via their action as lysine deacylases. We also highlight some key areas of mitochondrial sirtuin biology where future research efforts are required.
Lysine deacetylation appears to play some role in regulating mitochondrial metabolism. Recent discoveries of new enzymatic capabilities of mitochondrial sirtuins, including desuccinylation and demalonylation activities, as well as an increasing list of novel protein substrates have identified many new questions regarding the role of mitochondrial sirtuins in the regulation of energy metabolism.
Dynamic changes in the regulation of mitochondrial metabolism may have far-reaching consequences for many diseases, and despite promising initial findings in knockout animals and cell models, the role of the mitochondrial sirtuins requires further exploration in this context.
Biochimica et Biophysica Acta 08/2013; · 4.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Decreasing mitochondrial coupling efficiency has been shown to be an effective therapy for obesity and related metabolic symptoms. Here we identified a novel mitochondrial uncoupler that promoted uncoupled respiration in a cell type-specific manner and investigated its effects on modulation of energy metabolism in vivo and in vitro.
We screened a collection of mitochondrial membrane potential depolarising compounds for a novel chemical uncoupler on isolated skeletal muscle mitochondria using a channel oxygen system. The effect on respiration of metabolic cells (L6 myotubes, 3T3-L1 adipocytes and rat primary hepatocytes) was examined and metabolic pathways sensitive to cellular ATP content were also evaluated. The chronic metabolic effects were investigated in high-fat diet-induced obese mice and standard diet-fed (SD) lean mice.
The novel uncoupler, CZ5, promoted uncoupled respiration in a cell type-specific manner. It stimulated fuel oxidation in L6 myotubes and reduced lipid accumulation in 3T3-L1 adipocytes but did not affect gluconeogenesis or the triacylglycerol content in hepatocytes. The administration of CZ5 to SD mice increased energy expenditure (EE) but did not affect body weight or adiposity. Chronic studies in mice on high-fat diet showed that CZ5 reduced body weight and improved glucose and lipid metabolism via both increased EE and suppressed energy intake. The reduced adiposity was associated with the restoration of expression of key metabolic genes in visceral adipose tissue.
This work demonstrates that a cell type-specific mitochondrial chemical uncoupler may have therapeutic potential for treating high-fat diet-induced metabolic diseases.
[Show abstract][Hide abstract] ABSTRACT: Krüppel-like Factor 3 (KLF3) is a transcriptional regulator that we have shown to be involved in the regulation of adipogenesis in vitro. Here we report that KLF3 null mice are lean and protected from diet-induced obesity and glucose intolerance. On a chow diet, plasma levels of leptin are decreased, and adiponectin is increased. Despite significant reductions in body weight and adiposity, wildtype and knockout animals show equivalent energy intake, expenditure and excretion. To investigate the molecular events underlying these observations, we used microarray analysis to compare gene expression in Klf3(+/+) and Klf3(-/-) tissues. We found that mRNA expression of Fam132a, which encodes a newly identified insulin-sensitizing adipokine, adipolin, is significantly upregulated in the absence of KLF3. We confirmed that KLF3 binds the Fam132a promoter in vitro and in vivo and that this leads to repression of promoter activity. Further, plasma adipolin levels were significantly increased in Klf3(-/-) mice compared to wild-type littermates. Boosting levels of adipolin via targeting of KLF3 offers a novel potential therapeutic strategy for the treatment of insulin resistance.
[Show abstract][Hide abstract] ABSTRACT: AIMS/HYPOTHESIS: While it is well known that diet-induced obesity causes insulin resistance, the precise mechanisms underpinning the initiation of insulin resistance are unclear. To determine factors that may cause insulin resistance, we have performed a detailed time-course study in mice fed a high-fat diet (HFD). METHODS: C57Bl/6 mice were fed chow or an HFD from 3 days to 16 weeks and glucose tolerance and tissue-specific insulin action were determined. Tissue lipid profiles were analysed by mass spectrometry and inflammatory markers were measured in adipose tissue, liver and skeletal muscle. RESULTS: Glucose intolerance developed within 3 days of the HFD and did not deteriorate further in the period to 12 weeks. Whole-body insulin resistance, measured by hyperinsulinaemic-euglycaemic clamp, was detected after 1 week of HFD and was due to hepatic insulin resistance. Adipose tissue was insulin resistant after 1 week, while skeletal muscle displayed insulin resistance at 3 weeks, coinciding with a defect in glucose disposal. Interestingly, no further deterioration in insulin sensitivity was observed in any tissue after this initial defect. Diacylglycerol content was increased in liver and muscle when insulin resistance first developed, while the onset of insulin resistance in adipose tissue was associated with increases in ceramide and sphingomyelin. Adipose tissue inflammation was only detected at 16 weeks of HFD and did not correlate with the induction of insulin resistance. CONCLUSIONS/INTERPRETATION: HFD-induced whole-body insulin resistance is initiated by impaired hepatic insulin action and exacerbated by skeletal muscle insulin resistance and is associated with the accumulation of specific bioactive lipid species.
[Show abstract][Hide abstract] ABSTRACT: We recently showed that bitter melon-derived triterpenoids (BMTs) activate AMPK and increase GLUT4 translocation to the plasma membrane in vitro, and improve glucose disposal in insulin resistant models in vivo. Here we interrogated the mechanism by which these novel compounds activate AMPK, a leading anti-diabetic drug target. BMTs did not activate AMPK directly in an allosteric manner as AMP or the Abbott compound (A-769662) does, nor did they activate AMPK by inhibiting cellular respiration like many commonly used anti-diabetic medications. BMTs increased AMPK activity in both L6 myotubes and LKB1-deficient HeLa cells by 20-35%. Incubation with the CaMKKβ inhibitor, STO-609, completely attenuated this effect suggesting a key role for CaMKKβ in this activation. Incubation of L6 myotubes with the calcium chelator EGTA-AM did not alter this activation suggesting that the BMT-dependent activation was Ca(2+)-independent. We therefore propose that CaMKKβ is a key upstream kinase for BMT-induced activation of AMPK.
PLoS ONE 04/2013; 8(4):e62309. · 3.53 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: AIMS/HYPOTHESIS: Metabolic disorders are commonly investigated using knockout and transgenic mouse models. A variety of mouse strains have been used for this purpose. However, mouse strains can differ in their inherent propensities to develop metabolic disease, which may affect the experimental outcomes of metabolic studies. We have investigated strain-dependent differences in the susceptibility to diet-induced obesity and insulin resistance in five commonly used inbred mouse strains (C57BL/6J, 129X1/SvJ, BALB/c, DBA/2 and FVB/N). METHODS: Mice were fed either a low-fat or a high-fat diet (HFD) for 8 weeks. Whole-body energy expenditure and body composition were then determined. Tissues were used to measure markers of mitochondrial metabolism, inflammation, oxidative stress and lipid accumulation. RESULTS: BL6, 129X1, DBA/2 and FVB/N mice were all susceptible to varying degrees to HFD-induced obesity, glucose intolerance and insulin resistance, but BALB/c mice exhibited some protection from these detrimental effects. This protection could not be explained by differences in mitochondrial metabolism or oxidative stress in liver or muscle, or inflammation in adipose tissue. Interestingly, in contrast with the other strains, BALB/c mice did not accumulate excess lipid (triacylglycerols and diacylglycerols) in the liver; this is potentially related to lower fatty acid uptake rather than differences in lipogenesis or lipid oxidation. CONCLUSIONS/INTERPRETATION: Collectively, our findings indicate that most mouse strains develop metabolic defects on an HFD. However, there are inherent differences between strains, and thus the genetic background needs to be considered carefully in metabolic studies.