Akt/PKB regulates hepatic metabolism by directly inhibiting PGC-1alpha transcription coactivator

Genetic models of PGC-1 and glucose metabolism and homeostasis

Type II diabetes and its complications are a tremendous health burden throughout the world. Our understanding of the changes that lead to glucose imbalance and insulin resistance and ultimately diabetes remain incompletely understood. Many signaling and transcriptional pathways have been identified as being important to maintain normal glucose balance, including that of the peroxisome proliferator activated receptor gamma coactivator (PGC-1) family. This family of transcriptional coactivators strongly regulates mitochondrial and metabolic biology in numerous organs. The use of genetic models of PGC-1s, including both tissue-specific overexpression and knock-out models, has helped to reveal the specific roles that these coactivators play in each tissue. This review will thus focus on the PGC-1s and recently developed genetic rodent models that have highlighted the importance of these molecules in maintaining normal glucose homeostasis.

Role of protein tyrosine phosphatases in the modulation of insulin signaling and their implication in the pathogenesis of obesity-linked insulin resistance

Insulin resistance is a major disorder that links obesity to type 2 diabetes mellitus (T2D). It involves defects in the insulin actions owing to a reduced ability of insulin to trigger key signaling pathways in major metabolic tissues. The pathogenesis of insulin resistance involves several inhibitory molecules that interfere with the tyrosine phosphorylation of the insulin receptor and its downstream effectors. Among those, growing interest has been developed toward the protein tyrosine phosphatases (PTPs), a large family of enzymes that can inactivate crucial signaling effectors in the insulin signaling cascade by dephosphorylating their tyrosine residues. Herein we briefly review the role of several PTPs that have been shown to be implicated in the regulation of insulin action, and then focus on the Src homology 2 (SH2) domain-containing SHP1 and SHP2 enzymes, since recent reports have indicated major roles for these PTPs in the control of insulin action and glucose metabolism. Finally, the therapeutic potential of targeting PTPs for combating insulin resistance and alleviating T2D will be discussed.

Development of a stable cell line with an intact PGC-1α/ERRα axis for screening environmental chemicals

The estrogen-related receptor α (ERRα) and the peroxisome proliferator-activated receptor γ (PPARγ) coactivator 1α (PGC-1α) play critical roles in the control of several physiological functions, including the regulation of genes involved in energy homeostasis. However, little is known about the ability of environmental chemicals to disrupt or modulate this important bioenergetics pathway in humans. The goal of this study was to develop a cell-based assay system with an intact PGC-1α/ERRα axis that could be used as a screening assay for detecting such chemicals. To this end, we successfully generated several stable cell lines expressing PGC-1α and showed that the reporter driven by the native ERRα hormone response unit (AAB-Luc) is active in these cell lines and that the activation is PGC-1α-dependent. Furthermore, we show that this activation can be blocked by the ERRα selective inverse agonist, XCT790. In addition, we find that genistein and bisphenol A further stimulate the reporter activity, while kaempferol has minimal effect. These cell lines will be useful for identifying environmental chemicals that modulate this important pathway.

Insulin receptor signaling in normal and insulin-resistant states

In the wake of the worldwide increase in type-2 diabetes, a major focus of research is understanding the signaling pathways impacting this disease. Insulin signaling regulates glucose, lipid, and energy homeostasis, predominantly via action on liver, skeletal muscle, and adipose tissue. Precise modulation of this pathway is vital for adaption as the individual moves from the fed to the fasted state. The positive and negative modulators acting on different steps of the signaling pathway, as well as the diversity of protein isoform interaction, ensure a proper and coordinated biological response to insulin in different tissues. Whereas genetic mutations are causes of rare and severe insulin resistance, obesity can lead to insulin resistance through a variety of mechanisms. Understanding these pathways is essential for development of new drugs to treat diabetes, metabolic syndrome, and their complications.

Factors affecting insulin-regulated hepatic gene expression

Obesity has become a major concern of public health. A common feature of obesity and related metabolic disorders such as noninsulin-dependent diabetes mellitus is insulin resistance, wherein a given amount of insulin produces less than normal physiological responses. Insulin controls hepatic glucose and fatty acid metabolism, at least in part, via the regulation of gene expression. When the liver is insulin-sensitive, insulin can stimulate the expression of genes for fatty acid synthesis and suppress those for gluconeogenesis. When the liver becomes insulin-resistant, the insulin-mediated suppression of gluconeogenic gene expression is lost, whereas the induction of fatty acid synthetic gene expression remains intact. In the past two decades, the mechanisms of insulin-regulated hepatic gene expression have been studied extensively and many components of insulin signal transduction pathways have been identified. Factors that alter these pathways, and the insulin-regulated hepatic gene expression, have been revealed and the underlying mechanisms have been proposed. This chapter summarizes the recent progresses in our understanding of the effects of dietary factors, drugs, bioactive compounds, hormones, and cytokines on insulin-regulated hepatic gene expression. Given the large amount of information and progresses regarding the roles of insulin, this chapter focuses on findings in the liver and hepatocytes and not those described for other tissues and cells. Typical insulin-regulated hepatic genes, such as insulin-induced glucokinase and sterol regulatory element-binding protein-1c and insulin-suppressed cytosolic phosphoenolpyruvate carboxyl kinase and insulin-like growth factor-binding protein 1, are used as examples to discuss the mechanisms such as insulin regulatory element-mediated transcriptional regulation. We also propose the potential mechanisms by which these factors affect insulin-regulated hepatic gene expression and discuss potential future directions of the area of research.

Differential effects of exposure to maternal obesity or maternal weight loss during the periconceptional period in the sheep on insulin signalling molecules in skeletal muscle of the offspring at 4 months of age

Exposure to maternal obesity before and/or throughout pregnancy may increase the risk of obesity and insulin resistance in the offspring in childhood and adult life, therefore, resulting in its transmission into subsequent generations. We have previously shown that exposure to maternal obesity around the time of conception alone resulted in increased adiposity in female lambs. Changes in the abundance of insulin signalling molecules in skeletal muscle and adipose tissue precede the development of insulin resistance and type 2 diabetes. It is not clear, however, whether exposure to maternal obesity results in insulin resistance in her offspring as a consequence of the impact of increased adiposity on skeletal muscle or as a consequence of the programming of specific changes in the abundance of insulin signalling molecules in this tissue. We have used an embryo transfer model in the sheep to investigate the effects of exposure to either maternal obesity or to weight loss in normal and obese mothers preceding and for one week after conception on the expression and abundance of insulin signalling molecules in muscle in the offspring. We found that exposure to maternal obesity resulted in lower muscle GLUT-4 and Ser 9 phospho-GSK3α and higher muscle GSK3α abundance in lambs when compared to lambs conceived in normally nourished ewes. Exposure to maternal weight loss in normal or obese mothers, however, resulted in lower muscle IRS1, PI3K, p110β, aPKCζ, Thr 642 phospho-AS160 and GLUT-4 abundance in the offspring. In conclusion, maternal obesity or weight loss around conception have each programmed specific changes on subsets of molecules in the insulin signalling, glucose transport and glycogen synthesis pathways in offspring. There is a need for a stronger evidence base to ensure that weight loss regimes in obese women seeking to become pregnant minimize the metabolic costs for the next generation.

Peroxisome proliferator activated receptor α ligands as anticancer drugs targeting mitochondrial metabolism

Tumor cells show metabolic features distinctive from normal tissues, with characteristically enhanced aerobic glycolysis, glutaminolysis and lipid synthesis. Peroxisome proliferator activated receptor α (PPAR α) is activated by nutrients (fatty acids and their derivatives) and influences these metabolic pathways acting antagonistically to oncogenic Akt and c-Myc. Therefore PPAR α can be regarded as a candidate target molecule in supplementary anticancer pharmacotherapy as well as dietary therapeutic approach. This idea is based on hitting the cancer cell metabolic weak points through PPAR α mediated stimulation of mitochondrial fatty acid oxidation and ketogenesis with simultaneous reduction of glucose and glutamine consumption. PPAR α activity is induced by fasting and its molecular consequences overlap with the effects of calorie restriction and ketogenic diet (CRKD). CRKD induces increase of NAD+/NADH ratio and drop in ATP/AMP ratio. The first one is the main stimulus for enhanced protein deacetylase SIRT1 activity; the second one activates AMP-dependent protein kinase (AMPK). Both SIRT1 and AMPK exert their major metabolic activities such as fatty acid oxidation and block of glycolysis and protein, nucleotide and fatty acid synthesis through the effector protein peroxisome proliferator activated receptor gamma 1 α coactivator (PGC-1α). PGC-1α cooperates with PPAR α and their activities might contribute to potential anticancer effects of CRKD, which were reported for various brain tumors. Therefore, PPAR α activation can engage molecular interplay among SIRT1, AMPK, and PGC-1α that provides a new, low toxicity dietary approach supplementing traditional anticancer regimen.

Tff3, as a novel peptide, regulates hepatic glucose metabolism

Type 2 diabetes mellitus (T2DM) is a chronic metabolic disorder strongly associated with hepatic glucose intolerance and insulin resistance. The trefoil peptides are a family of small regulatory proteins and Tff3 is widely expressed in multiple tissues including liver. But the roles of Tff3 in regulation of glucose metabolism and insulin sensitivity in liver remain unclear. Here we show that the hepatic Tff3 expression levels were decreased in ob/ob and high-fat diet-induced obese mice. Overexpression of Tff3 in primary mouse hepatocytes inhibited the expression of gluconeogenic genes, including G6pc, PEPCK and PGC-1α, subsequently decreasing cellular glucose output. GTT and ITT experiments revealed that adenovirus-mediated overexpression of Tff3 in diabetic or obese mice improved glucose tolerance and insulin sensitivity. Collectively, our results indicated that Tff3 peptides are involved in glucose homeostasis and insulin sensitivity, providing a promising peptide on new therapies against the metabolic disorders associated with T2DM.

An overview of the crosstalk between inflammatory processes and metabolic dysregulation during diabetic cardiomyopathy

Metabolic disorders such as obesity, insulin resistance and type 2 diabetes mellitus are all linked to cardiovascular diseases such as cardiac hypertrophy and heart failure. Diabetic cardiomyopathy in particular, is characterized by structural and functional alterations in the heart muscle of people with diabetes that finally lead to heart failure, and which is not directly attributable to coronary artery disease or hypertension. Several mechanisms have been involved in the pathogenesis of diabetic cardiomyopathy, such as alterations in myocardial energy metabolism and calcium signaling. Metabolic disturbances during diabetic cardiomyopathy are characterized by increased lipid oxidation, intramyocardial triglyceride accumulation, and reduced glucose utilization. Overall changes result in enhanced oxidative stress, mitochondrial dysfunction and apoptosis of the cardiomyocytes. On the other hand, the progression of heart failure and cardiac hypertrophy usually entails a local rise in cytokines in cardiac cells and the activation of the proinflammatory transcription factor nuclear factor (NF)-κB. Interestingly, increasing evidences are arising in the recent years that point to a potential link between chronic low-grade inflammation in the heart and metabolic dysregulation. Therefore, in this review we summarize recent new insights into the crosstalk between inflammatory processes and metabolic dysregulation in the failing heart during diabetes, paying special attention to the role of NF-κB and peroxisome proliferator activated receptors (PPARs). In addition, we briefly describe the role of the AMP-activated protein kinase (AMPK), sirtuin 1 (SIRT1) and other pathways regulating cardiac energy metabolism, as well as their relationship with diabetic cardiomyopathy.

Protective effect of HMG CoA reductase inhibitors against running wheel activity induced fatigue, anxiety like behavior, oxidative stress and mitochondrial dysfunction in mice

BACKGROUND

Chronic fatigue stress (CFS) is an important health problem with unknown causes and unsatisfactory prevention strategies, often characterized by long-lasting and debilitating fatigue, myalgia, impairment of neuro-cognitive functions along with other common symptoms. The present study has been designed to explore the protective effect of statins against running wheel activity induced fatigue anxiety.

METHODS

Male albino Laca mice (20-30 g) were subjected to swim stress induced fatigue in a running wheel activity apparatus. Atorvastatin (10, 20 mg/kg, po) and fluvastatin (5, 10 mg/kg, po) were administered daily for 21 days, one hour prior to the animals being subjected to running wheel activity test session of 6 min. Various behavioral tests (running wheel activity, locomotor activity and elevated plus maze test), biochemical parameters (lipid peroxidation, nitrite concentration, glutathione levels and catalase activity) and mitochondrial complex enzyme dysfunctions (complex I, II, III and IV) were subsequently assessed.

RESULTS

Animals exposed to 6 min test session on running wheel for 21 days showed a significant decrease in number of wheel rotations per 6 min indicating fatigue stress like behavior. Treatment with atorvastatin (10 and 20 mg/kg) and fluvastatin (10 mg/kg) for 21 days significantly improved the behavioral alterations [increased number of wheel rotations and locomotor activity, and anxiety like behavior (decreased number of entries and time spent in open arm)], oxidative defence and mitochondrial complex enzyme activities in brain.

CONCLUSION

Present study suggests the protective role of statins against chronic fatigue induced behavioral, biochemical and mitochondrial dysfunctions.

GCN5 acetyltransferase inhibits PGC1α-induced hepatitis B virus biosynthesis

Hepatitis B virus (HBV) biosynthesis is primarily restricted to hepatocytes due to the governing of liver-enriched nuclear receptors (NRs) on viral RNA synthesis. The liver-enriched NR hepatocyte nuclear factor 4α (HNF4α), the key regulator of genes implicated in hepatic glucose metabolism, is also a primary determinant of HBV pregenomic RNA synthesis and HBV replication. Peroxisome proliferator-activated receptor-γ coactivator 1α (PGC1α) coactivates and further enhances the effect of HNF4α on HBV biosynthesis. Here, we showed that the acetyltransferase General Control Non-repressed Protein 5 (GCN5) acetylated PGC1α, leading to alteration of PGC1α from a transcriptionally active state into an inactive state. As a result, the coactivation activity of PGC1α on HBV transcription and replication was suppressed. Apparently, an acetylation site mutant of PGC1α (PGC1αR13) still had coactivation activity as GCN5 could not suppress the coactivation activity of the mutant. Moreover, a catalytically inactive acetyltransferase mutant GCN5m, due to the loss of acetylation activity, failed to inhibit the coactivation function of PGC1α in HBV biosynthesis. Our results demonstrate that GCN5, through its acetyltransferase activity, inhibits PGC1α-induced enhancement of HBV transcription and replication both in vitro and in vivo.

Markers of skeletal muscle mitochondrial function and lipid accumulation are moderately associated with the homeostasis model assessment index of insulin resistance in obese men

Lower skeletal muscle mitochondrial oxidative phosphorylation capacity (OXPHOS) and intramyocellular lipid (IMCL) accumulation have been implicated in the etiology of insulin resistance (IR) in obesity. The purpose of this study was to examine the impact of endurance exercise on biochemical and morphological measures of IMCL and mitochondrial content, and their relationship to IR in obese individuals. We examined mitochondrial content (subunit protein abundance and maximal activity of electron transport chain enzymes), IMCL/mitochondrial morphology in both subsarcolemmal (SS) and intermyofibrillar (IMF) regions by transmission electron microscopy, and intracellular lipid metabolites (diacylglycerol and ceramide) in vastus lateralis biopsies, as well as, the homeostasis model assessment index of IR (HOMA-IR) prior to and following twelve weeks of an endurance exercise regimen in healthy age- and physical activity-matched lean and obese men. Obese men did not show evidence of mitochondrial OXPHOS dysfunction, disproportionate IMCL content in sub-cellular regions, or diacylglycerol/ceramide accretion despite marked IR vs. lean controls. Endurance exercise increased OXPHOS and mitochondrial size and density, but not number of individual mitochondrial fragments, with moderate improvements in HOMA-IR. Exercise reduced SS IMCL content (size, number and density), increased IMF IMCL content, while increasing IMCL/mitochondrial juxtaposition in both regions. HOMA-IR was inversely associated with SS (r = -0.34; P = 0.051) and IMF mitochondrial density (r = -0.29; P = 0.096), IMF IMCL/mitochondrial juxtaposition (r = -0.30; P = 0.086), and COXII (r = -0.32; P = 0.095) and COXIV protein abundance (r = -0.35; P = 0.052); while positively associated with SS IMCL size (r = 0.28; P = 0.119) and SS IMCL density (r = 0.25; P = 0.152). Our findings suggest that once physical activity and cardiorespiratory fitness have been controlled for, skeletal muscle mitochondrial and IMCL profile in obesity may only partially contribute to the development of IR.

PGC1α and mitochondrial metabolism--emerging concepts and relevance in ageing and neurodegenerative disorders

PGC1α is a transcriptional coactivator that is a central inducer of mitochondrial biogenesis in cells. Recent work highlighted that PGC1α can also modulate the composition and functions of individual mitochondria. Therefore, it is emerging that PGC1α is controlling global oxidative metabolism by performing two types of remodelling: (1) cellular remodelling through mitochondrial biogenesis, and (2) organelle remodelling through alteration in the intrinsic properties of mitochondria. The elevated oxidative metabolism associated with increased PGC1α activity could be accompanied by an increase in reactive oxygen species (ROS) that are primarily generated by mitochondria. However, increasing evidence suggests that this is not the case, as PGC1α is also a powerful regulator of ROS removal by increasing the expression of numerous ROS-detoxifying enzymes. Therefore, PGC1α, by controlling both the induction of mitochondrial metabolism and the removal of its ROS by-products, would elevate oxidative metabolism and minimize the impact of ROS on cell physiology. In this Commentary, we discuss how the biogenesis and remodelling of mitochondria that are elicited by PGC1α contribute to an increase in oxidative metabolism and the preservation of ROS homeostasis. Finally, we examine the importance of these findings in ageing and neurodegenerative disorders, conditions that are associated with impaired mitochondrial functions and ROS balance.

Cardioprotection in the aging, diabetic heart: the loss of protective Akt signalling

AIMS

Old age and diabetes are risk factors that often coexist increasing the vulnerability of the heart to the lethal effects of ischaemia-reperfusion injury (IRI). However, to our knowledge, no investigations have examined IRI and cardioprotective signalling in animal models bearing these co-morbidities concomitantly. The ability of the heart to recover following IRI is greatly dependent on its innate cardioprotective potential, in which a central role is played by Akt. We aimed to investigate in an aging diabetic rat model, the susceptibility of the heart to IRI, the achievability of ischaemic preconditioning (IPC) against this lethal event, and the changes in Akt signalling, as the main prosurvival intracellular pathway.

METHODS AND RESULTS

Our data showed that the isolated hearts of aged, diabetic Goto-Kakizaki rats were more susceptible to sub-lethal injury and not amenable to cardioprotection via IPC, compared with younger diabetic rat hearts. Western blot analysis of the heart tissue suggested a chronic up-regulation of Akt phosphorylation, and reduced expression of the mitochondrial regulator PGC-1α and of the anti-oxidant enzyme catalase, potentially due to the Akt up-regulation. Moreover, no further activation of Akt could be achieved following IPC.

CONCLUSION

An increased susceptibility to IRI in the aged, diabetic heart could be a consequence of impaired Akt signalling due to chronic Akt phosphorylation. Additional Akt phosphorylation required for IPC protection may therefore not be possible in the aged, diabetic rat heart and may explain why this cardioprotective manoeuvre cannot be achieved in these hearts.

MicroRNA 33 regulates glucose metabolism

Metabolic diseases are characterized by the failure of regulatory genes or proteins to effectively orchestrate specific pathways involved in the control of many biological processes. In addition to the classical regulators, recent discoveries have shown the remarkable role of small noncoding RNAs (microRNAs [miRNAs]) in the posttranscriptional regulation of gene expression. In this regard, we have recently demonstrated that miR-33a and miR33b, intronic miRNAs located within the sterol regulatory element-binding protein (SREBP) genes, regulate lipid metabolism in concert with their host genes. Here, we show that miR-33b also cooperates with SREBP1 in regulating glucose metabolism by targeting phosphoenolpyruvate carboxykinase (PCK1) and glucose-6-phosphatase (G6PC), key regulatory enzymes of hepatic gluconeogenesis. Overexpression of miR-33b in human hepatic cells inhibits PCK1 and G6PC expression, leading to a significant reduction of glucose production. Importantly, hepatic SREBP1c/miR-33b levels correlate inversely with the expression of PCK1 and G6PC upon glucose infusion in rhesus monkeys. Taken together, these results suggest that miR-33b works in concert with its host gene to ensure a fine-tuned regulation of lipid and glucose homeostasis, highlighting the clinical potential of miR-33a/b as novel therapeutic targets for a range of metabolic diseases.

The protein level of PGC-1α, a key metabolic regulator, is controlled by NADH-NQO1

PGC-1α is a key transcription coactivator regulating energy metabolism in a tissue-specific manner. PGC-1α expression is tightly regulated, it is a highly labile protein, and it interacts with various proteins--the known attributes of intrinsically disordered proteins (IDPs). In this study, we characterize PGC-1α as an IDP and demonstrate that it is susceptible to 20S proteasomal degradation by default. We further demonstrate that PGC-1α degradation is inhibited by NQO1, a 20S gatekeeper protein. NQO1 binds and protects PGC-1α from degradation in an NADH-dependent manner. Using different cellular physiological settings, we also demonstrate that NQO1-mediated PGC-1α protection plays an important role in controlling both basal and physiologically induced PGC-1α protein level and activity. Our findings link NQO1, a cellular redox sensor, to the metabolite-sensing network that tunes PGC-1α expression and activity in regulating energy metabolism.

Dietary iron overload induces visceral adipose tissue insulin resistance

Increased iron stores associated with elevated levels of the iron hormone hepcidin are a frequent feature of the metabolic syndrome. The aim of this study was to assess the effect of dietary iron supplementation on insulin resistance and the role of hepcidin in C57Bl/6 male mice fed a standard or iron-enriched diet for 16 weeks. Iron supplementation increased hepatic iron and serum hepcidin fivefold and led to a 40% increase in fasting glucose due to insulin resistance, as confirmed by the insulin tolerance test, and to threefold higher levels of triglycerides. Iron supplemented mice had lower visceral adipose tissue mass estimated by epididymal fat pad, associated with iron accumulation in adipocytes. Decreased insulin signaling, evaluated by the phospho-Akt/Akt ratio, was detected in the visceral adipose tissue of iron overloaded mice, and gene expression analysis of visceral adipose tissue showed that an iron-enriched diet up-regulated iron-responsive genes and adipokines, favoring insulin resistance, whereas lipoprotein lipase was down-regulated. This resulted in hyperresistinemia and increased visceral adipose tissue expression of suppressor of cytokine signaling-3 (Socs3), a target of resistin and hepcidin implicated in insulin resistance. Acute hepcidin administration down-regulated lipoprotein lipase and up-regulated Socs3 in visceral adipose tissue. In conclusion, we characterized a model of dysmetabolic iron overload syndrome in which an iron-enriched diet induces insulin resistance and hypertriglyceridemia and affects visceral adipose tissue metabolism by a mechanism involving hepcidin up-regulation.

Novel small-molecule PGC-1α transcriptional regulator with beneficial effects on diabetic db/db mice

Peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) has been shown to influence energy metabolism. Hence, we explored a strategy to target PGC-1α expression to treat metabolic syndromes. We developed a high-throughput screening assay that uses the human PGC-1α promoter to drive expression of luciferase. The effects of lead compound stimulation on PGC-1α expression in muscle cells and hepatocytes were investigated in vitro and in vivo. A novel small molecule, ZLN005, led to changes in PGC-1α mRNA levels, glucose uptake, and fatty acid oxidation in L6 myotubes. Activation of AMP-activated protein kinase was involved in the induction of PGC-1α expression. In diabetic db/db mice, chronic administration of ZLN005 increased PGC-1α and downstream gene transcription in skeletal muscle, whereas hepatic PGC-1α and gluconeogenesis genes were reduced. ZLN005 increased fat oxidation and improved the glucose tolerance, pyruvate tolerance, and insulin sensitivity of diabetic db/db mice. Hyperglycemia and dyslipidemia also were ameliorated after treatment with ZLN005. Our results demonstrated that a novel small molecule selectively elevated the expression of PGC-1α in myotubes and skeletal muscle and exerted promising therapeutic effects for treating type 2 diabetes.

Inhibition of carnitine palmitoyltransferase-1 activity alleviates insulin resistance in diet-induced obese mice

Impaired skeletal muscle fatty acid oxidation has been suggested to contribute to insulin resistance and glucose intolerance. However, increasing muscle fatty acid oxidation may cause a reciprocal decrease in glucose oxidation, which might impair insulin sensitivity and glucose tolerance. We therefore investigated what effect inhibition of mitochondrial fatty acid uptake has on whole-body glucose tolerance and insulin sensitivity in obese insulin-resistant mice. C57BL/6 mice were fed a high-fat diet (60% calories from fat) for 12 weeks to develop insulin resistance. Subsequent treatment of mice for 4 weeks with the carnitine palmitoyltransferase-1 inhibitor, oxfenicine (150 mg/kg i.p. daily), resulted in improved whole-body glucose tolerance and insulin sensitivity. Exercise capacity was increased in oxfenicine-treated mice, which was accompanied by an increased respiratory exchange ratio. In the gastrocnemius muscle, oxfenicine increased pyruvate dehydrogenase activity, membrane GLUT4 content, and insulin-stimulated Akt phosphorylation. Intramyocellular levels of lipid intermediates, including ceramide, long-chain acyl CoA, and diacylglycerol, were also decreased. Our results demonstrate that inhibition of mitochondrial fatty acid uptake improves insulin sensitivity in diet-induced obese mice. This is associated with increased carbohydrate utilization and improved insulin signaling in the skeletal muscle, suggestive of an operating Randle Cycle in muscle.

Emerging role of AMP-activated protein kinase in endocrine control of metabolism in the liver

This review summarizes the emerging role of AMP-activated protein kinase (AMPK) in mediating endocrine regulation of metabolic fluxes in the liver. There are a number of hormones which, when acting on the liver, alter AMPK activation. Here we describe those hormones associated with activation and de-activation of AMPK and the potential mechanisms for changes in AMPK activation state. The actions of these hormones, in many cases, are consistent with downstream effects of AMPK signaling thus strengthening the circumstantial case for AMPK-mediated hormone action. In recent years, genetic mouse models have also been used in an attempt to establish the role of AMPK in hormone-stimulated metabolism in the liver. Few experiments have, however, firmly established a causal relationship between hormone action at the liver and AMPK signaling.

Peroxisome proliferator-activated receptor γ coactivator-1α is a central negative regulator of vascular senescence

OBJECTIVE

Cellular senescence influences organismal aging and increases predisposition to age-related diseases, in particular cardiovascular disease, a leading cause of death and disability worldwide. Peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) is a master regulator of mitochondrial biogenesis and function, oxidative stress, and insulin resistance. Senescence is associated with telomere and mitochondrial dysfunction and oxidative stress, implying a potential causal role of PGC-1α in senescence pathogenesis.

APPROACH AND RESULTS

We generated a PGC-1α(+/-)/apolipoprotein E(-/-) mouse model and showed that PGC-1α deficiency promotes a vascular senescence phenotype that is associated with increased oxidative stress, mitochondrial abnormalities, and reduced telomerase activity. PGC-1α disruption results in reduced expression of the longevity-related deacetylase sirtuin 1 (SIRT1) and the antioxidant catalase, and increased expression of the senescence marker p53 in aortas. Further, angiotensin II, a major hormonal inducer of vascular senescence, induces prolonged lysine acetylation of PGC-1α and releases the PGC-1α-FoxO1 complex from the SIRT1 promoter, thus reducing SIRT1 expression. The phosphorylation-defective mutant PGC-1α S570A is not acetylated, is constitutively active for forkhead box O1-dependent SIRT1 transcription, and prevents angiotensin II-induced senescence. Acetylation of PGC-1α by angiotensin II interrupts the PGC-1α-forkhead box O1-SIRT1 feed-forward signaling circuit leading to SIRT1 and catalase downregulation and vascular senescence.

CONCLUSIONS

PGC-1α is a primary negative regulator of vascular senescence. Moreover, the central role of posttranslational modification of PGC-1α in regulating angiotensin II-induced vascular senescence may inform development of novel therapeutic strategies for mitigating age-associated diseases, such as atherosclerosis.

Role of PI3K/Akt signaling in memory CD8 T cell differentiation

The clonal expansion, differentiation into effectors and establishing an immunological memory are crucial components of the adaptive immune response. Following the initial encounter with a pathogen, clonal CD8 T cell expansion yields at least two distinct populations of effector cells, short-lived effector cells (SLECs) and memory precursor effector cells (MPECs). SLECs are the terminally differentiated cells, which play an active role in pathogen clearance and undergo apoptosis once the pathogen is eliminated. In contrast, MPECs persist and give rise to self-renewing memory cells. These memory CD8 T cells maintain a state of heightened alertness and are poised to rapidly respond and swiftly clear the pathogen upon antigen re-encounter. As one of the goals of vaccination is to induce the development of these memory CD8 T cells, understanding the cellular and molecular basis of memory cell differentiation is critical to rational vaccine design. It is clear that memory differentiation is complex and involves multiple interrelated signaling pathways. It is influenced by factors such as the strength and duration of antigen receptor signaling and concurrent exposure to cytokines. Several signaling pathways that influence T cell fate have been recently described, and many culminate in the differential expression of specific transcription factors. Unfortunately, the mechanisms underlying the coordination and confluence of these signaling pathways remain largely unknown. In this review, we will discuss the role of the phosphatidylinositol 3-kinase signaling pathway as a central signaling node, and the function of Akt as a rheostat in orchestrating the differentiation of memory CD8 T cells.

The epidemiology and molecular mechanisms linking obesity, diabetes, and cancer

The worldwide epidemic of obesity is associated with increasing rates of the metabolic syndrome and type 2 diabetes. Epidemiological studies have reported that these conditions are linked to increased rates of cancer incidence and mortality. Obesity, particularly abdominal obesity, is associated with insulin resistance and the development of dyslipidemia, hyperglycemia, and ultimately type 2 diabetes. Although many metabolic abnormalities occur with obesity and type 2 diabetes, insulin resistance and hyperinsulinemia appear to be central to these conditions and may contribute to dyslipidemia and altered levels of circulating estrogens and androgens. In this review, we will discuss the epidemiological and molecular links between obesity, type 2 diabetes, and cancer, and how hyperinsulinemia and dyslipidemia may contribute to cancer development. We will discuss how these metabolic abnormalities may interact with estrogen signaling in breast cancer growth. Finally, we will discuss the effects of type 2 diabetes medications on cancer risk.

The deacetylase Sirt6 activates the acetyltransferase GCN5 and suppresses hepatic gluconeogenesis

Hepatic glucose production (HGP) maintains blood glucose levels during fasting but can also exacerbate diabetic hyperglycemia. HGP is dynamically controlled by a signaling/transcriptional network that regulates the expression/activity of gluconeogenic enzymes. A key mediator of gluconeogenic gene transcription is PGC-1α. PGC-1α's activation of gluconeogenic gene expression is dependent upon its acetylation state, which is controlled by the acetyltransferase GCN5 and the deacetylase Sirt1. Nevertheless, whether other chromatin modifiers-particularly other sirtuins-can modulate PGC-1α acetylation is currently unknown. Herein, we report that Sirt6 strongly controls PGC-1α acetylation. Surprisingly, Sirt6 induces PGC-1α acetylation and suppresses HGP. Sirt6 depletion decreases PGC-1α acetylation and promotes HGP. These acetylation effects are GCN5 dependent: Sirt6 interacts with and modifies GCN5, enhancing GCN5's activity. Lepr(db/db) mice, an obese/diabetic animal model, exhibit reduced Sirt6 levels; ectopic re-expression suppresses gluconeogenic genes and normalizes glycemia. Activation of hepatic Sirt6 may therefore be therapeutically useful for treating insulin-resistant diabetes.

Transcriptional Activity of PGC-1α and NT-PGC-1α Is Differentially Regulated by Twist-1 in Brown Fat Metabolism

Brown fat expresses two PGC-1α isoforms (PGC-1α and NT-PGC-1α) and both play a central role in the regulation of cellular energy metabolism and adaptive thermogenesis by interacting with a wide range of transcription factors including PPARγ, PPARα, ERRα, and NRF1. PGC-1α consists of 797 amino acids, whereas alternative splicing of the PGC-1α gene produces a shorter protein called NT-PGC-1α (aa 1-270). We report in this paper that transcriptional activity of PGC-1α and NT-PGC-1α is differently affected by the transcriptional regulator, Twist-1. Twist-1 suppresses PGC-1α but not NT-PGC-1α. The inhibition of PGC-1α activity by Twist-1 is mediated by direct interaction through the C-terminal region of PGC-1α (aa 353-797). Thus, the absence of the corresponding C-terminal domain in NT-PGC-1α allows NT-PGC-1α to be free from Twist-1-mediated inhibition. Overexpression of Twist-1 in brown adipocytes suppresses transcription of a subset of PGC-1α-target genes involved in mitochondrial fatty acid oxidation and uncoupling (CPT1β, UCP1, and ERRα). In contrast, NT-PGC-1α-mediated induction of these genes is unaffected by Twist-1. These findings show that differences in inhibitory protein-protein interactions of PGC-1α and NT-PGC-1α with Twist-1 lead to differential regulation of their function by Twist-1.

Molecular Mechanism Underlying the Antidiabetic Effects of a Siddha Polyherbal Preparation in the Liver of Type 2 Diabetic Adult Male Rats

A siddha polyherbal preparation consisting of 5 medicinal plants, namely, Asparagus racemosus, Emblica officinalis, Salacia oblonga, Syzygium aromaticum, and Tinospora cordifolia, in equal ratio, was formulated to examine the molecular mechanism by which it exhibits antidiabetic effects in the liver of high-fat and fructose-induced type 2 diabetic rats. The polyherbal preparation treated type 2 diabetic rats showed an increase in insulin receptor, Akt, and glucose transporter2 mRNA levels compared with diabetic rats. Insulin receptor, insulin receptor substrate-2, Akt, phosphorylated Akt substrate of 160kDaThreonine642, α-Actinin-4, β-arrestin-2, and glucose transporter2 proteins were also markedly decreased in diabetic rats, whereas the polyherbal preparation treatment significantly improved the expression of these proteins more than that of metformin-treated diabetic rats. The expression pattern of insulin signaling molecules analyzed in the present study signifies the therapeutic efficacy of the siddha polyherbal preparation.

A physiological increase in insulin suppresses gluconeogenic gene activation in fetal sheep with sustained hypoglycemia

Reduced maternal glucose supply to the fetus and resulting fetal hypoglycemia and hypoinsulinemia activate fetal glucose production as a means to maintain cellular glucose uptake. However, this early activation of fetal glucose production may be accompanied by hepatic insulin resistance. We tested the capacity of a physiological increase in insulin to suppress fetal hepatic gluconeogenic gene activation following sustained hypoglycemia to determine whether hepatic insulin sensitivity is maintained. Control fetuses (CON), hypoglycemic fetuses induced by maternal insulin infusion for 8 wk (HG), and 8 wk HG fetuses that received an isoglycemic insulin infusion for the final 7 days (HG+INS) were studied. Glucose and insulin concentrations were 60% lower in HG compared with CON fetuses. Insulin was 50% higher in HG+INS compared with CON and four-fold higher compared with HG fetuses. Expression of the hepatic gluconeogenic genes, PCK1, G6PC, FBP1, GLUT2, and PGC1A was increased in the HG and reduced in the HG+INS liver. Expression of the insulin-regulated glycolytic and lipogenic genes, PFKL and FAS, was increased in the HG+INS liver. Total FOXO1 protein expression, a gluconeogenic activator, was 60% higher in the HG liver. Despite low glucose, insulin, and IGF1 concentrations, phosphorylation of AKT and ERK was higher in the HG liver. Thus, a physiological increase in fetal insulin is sufficient for suppression of gluconeogenic genes and activation of glycolytic and lipogenic genes in the HG fetal liver. These results demonstrate that fetuses exposed to sustained hypoglycemia have maintained hepatic insulin action in contrast to fetuses exposed to placental insufficiency.

PGC-1 proteins and heart failure

The transcriptional coactivators PGC-1α and PGC-1β are master regulators of oxidative phosphorylation and fatty acid oxidation gene expression. Pressure overload hypertrophy and heart failure are associated with repressed PGC-1α and PGC-1β gene expression. Maintaining expression of PGC-1α and -β preserves contractile function in response to a pathological increase in workload. Here, we discuss the regulation of PGC-1 proteins under conditions of pressure overload hypertrophy and heart failure.

Endurance exercise training increases APPL1 expression and improves insulin signaling in the hepatic tissue of diet-induced obese mice, independently of weight loss

Hepatic insulin resistance is the major contributor to fasting hyperglycemia in type 2 diabetes. The protein kinase Akt plays a central role in the suppression of gluconeogenesis involving forkhead box O1 (Foxo1) and peroxisome proliferator-activated receptor gamma co-activator 1 alpha (PGC-1α), and in the control of glycogen synthesis involving the glycogen synthase kinase beta (GSK3β) in the liver. It has been demonstrated that endosomal adaptor protein APPL1 interacts with Akt and blocks the association of Akt with its endogenous inhibitor, tribbles-related protein 3 (TRB3), improving the action of insulin in the liver. Here, we demonstrated that chronic exercise increased the basal levels and insulin-induced Akt serine phosphorylation in the liver of diet-induced obese mice. Endurance training was able to increase APPL1 expression and the interaction between APPL1 and Akt. Conversely, training reduced both TRB3 expression and TRB3 and Akt association. The positive effects of exercise on insulin action are reinforced by our findings that showed that trained mice presented an increase in Foxo1 phosphorylation and Foxo1/PGC-1α association, which was accompanied by a reduction in gluconeogenic gene expressions (PEPCK and G6Pase). Finally, exercised animals demonstrated increased at basal and insulin-induced GSK3β phosphorylation levels and glycogen content at 24 h after the last session of exercise. Our findings demonstrate that exercise increases insulin action, at least in part, through the enhancement of APPL1 and the reduction of TRB3 expression in the liver of obese mice, independently of weight loss.

Identification of Akt-independent regulation of hepatic lipogenesis by mammalian target of rapamycin (mTOR) complex 2

Mammalian target of rapamycin complex 2 (mTORC2) is a key activator of protein kinases that act downstream of insulin and growth factor signaling. Here we report that mice lacking the essential mTORC2 component rictor in liver (Lrictor(KO)) are unable to respond normally to insulin. In response to insulin, Lrictor(KO) mice failed to inhibit hepatic glucose output. Lrictor(KO) mice also fail to develop hepatic steatosis on a high fat diet and manifest half-normal serum cholesterol levels. This is accompanied by lower levels of expression of SREBP-1c and SREBP-2 and genes of fatty acid and cholesterol biosynthesis. Lrictor(KO) mice had defects in insulin-stimulated Akt Ser-473 and Thr-308 phosphorylation, leading to decreased phosphorylation of Akt substrates FoxO, GSK-3β, PRAS40, AS160, and Tsc2. Lrictor(KO) mice also manifest defects in insulin-activated mTORC1 activity, evidenced by decreased S6 kinase and Lipin1 phosphorylation. Glucose intolerance and insulin resistance of Lrictor(KO) mice could be fully rescued by hepatic expression of activated Akt2 or dominant negative FoxO1. However, in the absence of mTORC2, forced Akt2 activation was unable to drive hepatic lipogenesis. Thus, we have identified an Akt-independent relay from mTORC2 to hepatic lipogenesis that separates the effects of insulin on glucose and lipid metabolism.

Transcript profiling of the ruminant liver indicates a unique program of transcriptional regulation of ketogenic enzymes during food restriction

Ruminants absorb little glucose and rely on hepatic gluconeogenesis and ketogenesis in the fed state to convert short-chain fatty acids produced during digestion into glucose and ketone bodies, respectively. In contrast to the non-ruminant response, fluxes through gluconeogenic and ketogenic pathways decrease during food restriction. Transcriptional regulation responsible for these unique food restriction responses has not been established. To determine the hepatic transcriptional response of ruminants to an acute drop in dietary nutrient supply, 102 yearling heifers were assigned to either ad libitum feeding or 24 h of food withdrawal in a randomized block design. Liver biopsies were obtained for microarray and quantitative real-time PCR analyses of gene expression. Plasma concentrations of non-esterified fatty acids were higher in food restricted heifers, while levels of β-hydroxybutyrate, triacylglycerol, and glucose were decreased. Despite a decline in substrate supply and a lower hepatic production of glucose, expression of the key gluconeogenic enzymes pyruvate carboxylase, phosphoenolpyruvate carboxykinase and fructose-1,6-bisphosphatase was upregulated as in non-ruminants. Downregulation of cholesterolgenic genes and upregulation of fatty acid oxidative genes were consistent with SREBP-2 and PPARα control, respectively. Ketogenesis from short-chain fatty acids was downregulated, contrary to the non-ruminant response to food restriction. Short-chain fatty acids may exert transcriptional control in the ruminant liver similar to that demonstrated in the large intestine of non-ruminants.

Adiponectin increases skeletal muscle mitochondrial biogenesis by suppressing mitogen-activated protein kinase phosphatase-1

Adiponectin enhances mitochondrial biogenesis and oxidative metabolism in skeletal muscle. This study aimed to investigate the underlying mechanisms through which adiponectin induces mitochondrial biogenesis in skeletal muscle. Mitochondrial contents, expression, and activation status of p38 mitogen-activated protein kinase (MAPK) and PPARγ coactivator 1α (PGC-1α) were compared between skeletal muscle samples from adiponectin gene knockout, adiponectin-reconstituted, and control mice. Adenovirus-mediated adiponectin and MAPK phosphatase-1 (MKP1) overexpression were used to verify the relationship of MKP1 and PGC-1α in adiponectin-enhanced mitochondrial biogenesis using cultured C2C12 myotubes and PGC-1α knockout mice. An inhibitory effect of adiponectin on MKP1 gene expression was observed in mouse skeletal muscle and cultured C2C12 myotubes. Overexpression of MKP1 attenuated adiponectin-enhanced mitochondrial biogenesis, with significantly decreased PGC-1α expression and p38 MAPK phosphorylation. Although in vivo adiponectin overexpression reduced MKP1 protein levels, the stimulative effects of adiponectin on mitochondrial biogenesis vanished in skeletal muscle of PGC-1α knockout mice. Therefore, our study indicates that adiponectin enhances p38 MAPK/PGC-1α signaling and mitochondrial biogenesis in skeletal muscle by suppressing MKP1 expression.

A genome-wide siRNA screen to identify modulators of insulin sensitivity and gluconeogenesis

Background

Hepatic insulin resistance impairs insulin’s ability to suppress hepatic glucose production (HGP) and contributes to the development of type 2 diabetes (T2D). Although the interests to discover novel genes that modulate insulin sensitivity and HGP are high, it remains challenging to have a human cell based system to identify novel genes.

Methodology/Principal Findings

To identify genes that modulate hepatic insulin signaling and HGP, we generated a human cell line stably expressing beta-lactamase under the control of the human glucose-6-phosphatase (G6PC) promoter (AH-G6PC cells). Both beta-lactamase activity and endogenous G6PC mRNA were increased in AH-G6PC cells by a combination of dexamethasone and pCPT-cAMP, and reduced by insulin. A 4-gene High-Throughput-Genomics assay was developed to concomitantly measure G6PC and pyruvate-dehydrogenase-kinase-4 (PDK4) mRNA levels. Using this assay, we screened an siRNA library containing pooled siRNA targeting 6650 druggable genes and identified 614 hits that lowered G6PC expression without increasing PDK4 mRNA levels. Pathway analysis indicated that siRNA-mediated knockdown (KD) of genes known to positively or negatively affect insulin signaling increased or decreased G6PC mRNA expression, respectively, thus validating our screening platform. A subset of 270 primary screen hits was selected and 149 hits were confirmed by target gene KD by pooled siRNA and 7 single siRNA for each gene to reduce G6PC expression in 4-gene HTG assay. Subsequently, pooled siRNA KD of 113 genes decreased PEPCK and/or PGC1alpha mRNA expression thereby demonstrating their role in regulating key gluconeogenic genes in addition to G6PC. Last, KD of 61 of the above 113 genes potentiated insulin-stimulated Akt phosphorylation, suggesting that they suppress gluconeogenic gene by enhancing insulin signaling.

Conclusions/Significance

These results support the proposition that the proteins encoded by the genes identified in our cell-based druggable genome siRNA screen hold the potential to serve as novel pharmacological targets for the treatment of T2D.

Orphan nuclear receptor estrogen-related receptor γ (ERRγ) is key regulator of hepatic gluconeogenesis

Glucose homeostasis is tightly controlled by hormonal regulation of hepatic glucose production. Dysregulation of this system is often associated with insulin resistance and diabetes, resulting in hyperglycemia in mammals. Here, we show that the orphan nuclear receptor estrogen-related receptor γ (ERRγ) is a novel downstream mediator of glucagon action in hepatic gluconeogenesis and demonstrate a beneficial impact of the inverse agonist GSK5182. Hepatic ERRγ expression was increased by fasting-dependent activation of the cAMP-response element-binding protein-CRTC2 pathway. Overexpression of ERRγ induced Pck1 and G6PC gene expression and glucose production in primary hepatocytes, whereas abolition of ERRγ gene expression attenuated forskolin-mediated induction of gluconeogenic gene expression. Deletion and mutation analyses of the Pck1 promoter showed that ERRγ directly regulates the Pck1 gene transcription via ERR response elements of the Pck1 promoter as confirmed by ChIP assay and in vivo imaging analysis. We also demonstrate that GSK5182, an inverse agonist of ERRγ, specifically inhibits the transcriptional activity of ERRγ in a PGC-1α dependent manner. Finally, the ERRγ inverse agonist ameliorated hyperglycemia through inhibition of hepatic gluconeogenesis in db/db mice. Control of hepatic glucose production by an ERRγ-specific inverse agonist is a new potential therapeutic approach for the treatment of type 2 diabetes.

Molecular mechanisms for age-associated mitochondrial deficiency in skeletal muscle

The abundance, morphology, and functional properties of mitochondria decay in skeletal muscle during the process of ageing. Although the precise mechanisms remain to be elucidated, these mechanisms include decreased mitochondrial DNA (mtDNA) repair and mitochondrial biogenesis. Mitochondria possess their own protection system to repair mtDNA damage, which leads to defects of mtDNA-encoded gene expression and respiratory chain complex enzymes. However, mtDNA mutations have shown to be accumulated with age in skeletal muscle. When damaged mitochondria are eliminated by autophagy, mitochondrial biogenesis plays an important role in sustaining energy production and physiological homeostasis. The capacity for mitochondrial biogenesis has shown to decrease with age in skeletal muscle, contributing to progressive mitochondrial deficiency. Understanding how these endogenous systems adapt to altered physiological conditions during the process of ageing will provide a valuable insight into the underlying mechanisms that regulate cellular homeostasis. Here we will summarize the current knowledge about the molecular mechanisms responsible for age-associated mitochondrial deficiency in skeletal muscle. In particular, recent findings on the role of mtDNA repair and mitochondrial biogenesis in maintaining mitochondrial functionality in aged skeletal muscle will be highlighted.

The functions and regulation of the PTEN tumour suppressor

The importance of the physiological function of phosphatase and tensin homologue (PTEN) is illustrated by its frequent disruption in cancer. By suppressing the phosphoinositide 3-kinase (PI3K)-AKT-mammalian target of rapamycin (mTOR) pathway through its lipid phosphatase activity, PTEN governs a plethora of cellular processes including survival, proliferation, energy metabolism and cellular architecture. Consequently, mechanisms regulating PTEN expression and function, including transcriptional regulation, post-transcriptional regulation by non-coding RNAs, post-translational modifications and protein-protein interactions, are all altered in cancer. The repertoire of PTEN functions has recently been expanded to include phosphatase-independent activities and crucial functions within the nucleus. Our increasing knowledge of PTEN and pathologies in which its function is altered will undoubtedly inform the rational design of novel therapies.

CITED2 links hormonal signaling to PGC-1α acetylation in the regulation of gluconeogenesis

During fasting, induction of hepatic gluconeogenesis is crucial to ensure proper energy homeostasis. Such induction is dysregulated in type 2 diabetes, resulting in the development of fasting hyperglycemia. Hormonal and nutrient regulation of metabolic adaptation during fasting is mediated predominantly by the transcriptional coactivator peroxisome proliferative activated receptor γ coactivator 1α (PGC-1α) in concert with various other transcriptional regulators. Although CITED2 (CBP- and p300-interacting transactivator with glutamic acid- and aspartic acid-rich COOH-terminal domain 2) interacts with many of these molecules, the role of this protein in the regulation of hepatic gluconeogenesis was previously unknown. Here we show that CITED2 is required for the regulation of hepatic gluconeogenesis through PGC-1α. The abundance of CITED2 was increased in the livers of mice by fasting and in cultured hepatocytes by glucagon-cAMP-protein kinase A (PKA) signaling, and the amount of CITED2 in liver was higher in mice with type 2 diabetes than in non-diabetic mice. CITED2 inhibited the acetylation of PGC-1α by blocking its interaction with the acetyltransferase general control of amino acid synthesis 5-like 2 (GCN5). The consequent downregulation of PGC-1α acetylation resulted in an increase in its transcriptional coactivation activity and an increased expression of gluconeogenic genes. The interaction of CITED2 with GCN5 was disrupted by insulin in a manner that was dependent on phosphoinositide 3-kinase (PI3K)-thymoma viral proto-oncogene (Akt) signaling. Our results show that CITED2 functions as a transducer of glucagon and insulin signaling in the regulation of PGC-1α activity that is associated with the transcriptional control of gluconeogenesis and that this function is mediated through the modulation of GCN5-dependent PGC-1α acetylation. We also found that loss of hepatic CITED2 function suppresses gluconeogenesis in diabetic mice, suggesting it as a therapeutic target for hyperglycemia.

Pten positively regulates brown adipose function, energy expenditure, and longevity

Aging in worms and flies is regulated by the PI3K/Akt/Foxo pathway. Here we extend this paradigm to mammals. Pten(tg) mice carrying additional genomic copies of Pten are protected from cancer and present a significant extension of life span that is independent of their lower cancer incidence. Interestingly, Pten(tg) mice have an increased energy expenditure and protection from metabolic pathologies. The brown adipose tissue (BAT) of Pten(tg) mice is hyperactive and presents high levels of the uncoupling protein Ucp1, which we show is a target of Foxo1. Importantly, a synthetic PI3K inhibitor also increases energy expenditure and hyperactivates the BAT in mice. These effects can be recapitulated in isolated brown adipocytes and, moreover, implants of Pten(tg) fibroblasts programmed with Prdm16 and Cebpβ form subcutaneous brown adipose pads more efficiently than wild-type fibroblasts. These observations uncover a role of Pten in promoting energy expenditure, thus decreasing nutrient storage and its associated damage.

Platelet-rich plasma greatly potentiates insulin-induced adipogenic differentiation of human adipose-derived stem cells through a serine/threonine kinase Akt-dependent mechanism and promotes clinical fat graft maintenance

The potential plasticity and therapeutic utility in tissue regeneration of human adipose-derived stem cells (ASCs) isolated from adult adipose tissue have recently been highlighted. The use of autologous platelet-rich plasma (PRP) represents an alternative strategy in regenerative medicine for the local release of multiple endogenous growth factors. Here we investigated the signaling pathways and effects of PRP and human recombinant insulin on proliferation and adipogenic differentiation of ASCs in vitro. PRP stimulated proliferation (EC(50) = 15.3 ± 1.3% vol/vol), whereas insulin's effect was the opposite (IC(50) = 3.0 ± 0.5 μM). Although PRP alone did not increase adipogenesis, in association with insulin it prevented ASC proliferative arrest, greatly enhanced intracytoplasmic lipid accumulation, strongly increased serine/threonine kinase Akt phosphorylation and mouse monoclonal anti-sterol regulatory element binding protein-1 accumulation, and downregulated Erk-1 activity; adipogenic effects were markedly prevented by the Akt inhibitor wortmannin. PRP with insulin synergistically upregulated fibroblast growth factor receptor (FGFR) and downregulated epidermal growth factor receptor (ErbB) expression; moreover, PRP in association prevented insulin-induced insulin-like growth factor-1 receptor and insulin receptor downregulation. The inhibition of FGFR-1, epidermal growth factor receptor (EGFR), and epidermal growth factor receptor-2 (ErbB2) activity reduced ASC proliferation, but only that of FGFR-1 reduced adipogenesis and Akt phosphorylation, whereas the ErbB2 inhibition effects were the opposite. However, EGFR activity was needed for ErbB2-mediated inhibition of ASC adipogenesis. Clinically, the injection of insulin further ameliorated patients' 1-year PRP-induced fat graft volume maintenance and contour restoring. Our results ascertain that PRP in association with insulin greatly potentiates adipogenesis in human ASCs through a FGFR-1 and ErbB2-regulated Akt mechanism. The ameliorated clinical fat graft maintenance suggests additional useful translational applications of combined PRP-insulin treatment in regenerative medicine.

Systemic elevation of PTEN induces a tumor-suppressive metabolic state

Decremental loss of PTEN results in cancer susceptibility and tumor progression. PTEN elevation might therefore be an attractive option for cancer prevention and therapy. We have generated several transgenic mouse lines with PTEN expression elevated to varying levels by taking advantage of bacterial artificial chromosome (BAC)-mediated transgenesis. The "Super-PTEN" mutants are viable and show reduced body size due to decreased cell number, with no effect on cell size. Unexpectedly, PTEN elevation at the organism level results in healthy metabolism characterized by increased energy expenditure and reduced body fat accumulation. Cells derived from these mice show reduced glucose and glutamine uptake and increased mitochondrial oxidative phosphorylation and are resistant to oncogenic transformation. Mechanistically we find that PTEN elevation orchestrates this metabolic switch by regulating PI3K-dependent and -independent pathways and negatively impacting two of the most pronounced metabolic features of tumor cells: glutaminolysis and the Warburg effect.

PGC-1 coactivators in the cardiovascular system

The beating heart consumes more ATP per weight than any other organ. The machineries required for this are many and complex. Fuel and oxygen must be transported via the vasculature, absorbed by cardiomyocytes, broken down, and regulated to match cellular demands. Much of this occurs in mitochondria, which comprise fully one third of cardiac mass. The PGC-1 proteins are transcriptional coactivators that have emerged as powerful orchestrators of these numerous processes, ensuring their proper coregulation in response to intracellular and extracellular cues. An important role for PGC-1s in cardiac function has been revealed over the past few years, and more recently interest in their role in the vasculature has been burgeoning. We review this literature, focusing on recent developments.

Low-volume interval training improves muscle oxidative capacity in sedentary adults

INTRODUCTION

High-intensity interval training (HIT) increases skeletal muscle oxidative capacity similar to traditional endurance training, despite a low total exercise volume. Much of this work has focused on young active individuals, and it is unclear whether the results are applicable to older less active populations. In addition, many studies have used "all-out" variable-load exercise interventions (e.g., repeated Wingate tests) that may not be practical for all individuals. We therefore examined the effect of a more practical low-volume submaximal constant-load HIT protocol on skeletal muscle oxidative capacity and insulin sensitivity in middle-aged adults, who may be at a higher risk for inactivity-related disorders.

METHODS

Seven sedentary but otherwise healthy individuals (three women) with a mean ± SD age, body mass index, and peak oxygen uptake (VO(2peak)) of 45 ± 5 yr, 27 ± 5 kg·m(-2), and 30 ± 3 mL·kg(-1)·min(-1) performed six training sessions during 2 wk. Each session involved 10 × 1-min cycling at ∼60% of peak power achieved during a ramp VO(2peak) test (eliciting ∼80%-95% of HR reserve) with 1 min of recovery between intervals. Needle biopsy samples (vastus lateralis) were obtained before training and ∼72 h after the final training session.

RESULTS

Muscle oxidative capacity, as reflected by the protein content of citrate synthase and cytochrome c oxidase subunit IV, increased by ∼35% after training. The transcriptional coactivator peroxisome proliferator-activated receptor γ coactivator 1α was increased by ∼56% after training, but the transcriptional corepressor receptor-interacting protein 140 remained unchanged. Glucose transporter protein content increased ∼260%, and insulin sensitivity, on the basis of the insulin sensitivity index homeostasis model assessment, improved by ∼35% after training.

CONCLUSIONS

Constant-load low-volume HIT may be a practical time-efficient strategy to induce metabolic adaptations that reduce the risk for inactivity-related disorders in previously sedentary middle-aged adults.

Integrin-dependent Akt1 activation regulates PGC-1 expression and fatty acid oxidation

BACKGROUND

Poly-N-acetyl glucosamine nanofibers derived from a marine diatom have been used to increase cutaneous wound healing. These nanofibers exert their activity by specifically activating integrins, which makes them a useful tool for dissecting integrin-mediated pathways. We have shown that short-fiber poly-N-acetyl glucosamine nanofiber (sNAG) treatment of endothelial cells results in increased cell motility and metabolic rate in the absence of increased cell proliferation.

RESULTS

Using a Seahorse Bioanalyzer to measure oxygen consumption in real time, we show that sNAG treatment increases oxygen consumption rates, correlated with an integrin-dependent activation of Akt1. Akt1 activation leads to an increase in the expression of the transcriptional coactivator, peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α). This is not due to increased mitochondrial biogenesis, but is associated with an increase in the expression of pyruvate dehydrogenase kinase 4 (PDK4), suggesting regulation of fatty acid oxidation. Blockade of fatty acid oxidation with etomoxir, an O-carnitine palmitoyltransferase-1 inhibitor, blocks the sNAG-dependent increased oxygen consumption. (3)H-palmitate uptake experiments indicate a PDK4-dependent increase in fatty acid oxidation, which is required for nanofiber-induced cell motility.

CONCLUSIONS

Our findings imply a linear pathway whereby an integrin-dependent activation of Akt1 leads to increased PGC-1α and PDK4 expression resulting in increased energy production by fatty acid oxidation.

SIRT3 deficiency and mitochondrial protein hyperacetylation accelerate the development of the metabolic syndrome

Acetylation is increasingly recognized as an important metabolic regulatory posttranslational protein modification, yet the metabolic consequence of mitochondrial protein hyperacetylation is unknown. We find that high-fat diet (HFD) feeding induces hepatic mitochondrial protein hyperacetylation in mice and downregulation of the major mitochondrial protein deacetylase SIRT3. Mice lacking SIRT3 (SIRT3KO) placed on a HFD show accelerated obesity, insulin resistance, hyperlipidemia, and steatohepatitis compared to wild-type (WT) mice. The lipogenic enzyme stearoyl-CoA desaturase 1 is highly induced in SIRT3KO mice, and its deletion rescues both WT and SIRT3KO mice from HFD-induced hepatic steatosis and insulin resistance. We further identify a single nucleotide polymorphism in the human SIRT3 gene that is suggestive of a genetic association with the metabolic syndrome. This polymorphism encodes a point mutation in the SIRT3 protein, which reduces its overall enzymatic efficiency. Our findings show that loss of SIRT3 and dysregulation of mitochondrial protein acetylation contribute to the metabolic syndrome.

Myopathy caused by mammalian target of rapamycin complex 1 (mTORC1) inactivation is not reversed by restoring mitochondrial function

Mammalian target of rapamycin complex 1 (mTORC1) is central to the control of cell, organ, and body size. Skeletal muscle-specific inactivation of mTORC1 in mice results in smaller muscle fibers, fewer mitochondria, increased glycogen stores, and a progressive myopathy that causes premature death. In mTORC1-deficient muscles, peroxisome proliferator-activated receptor gamma coactivator 1-α (PGC-1α), which regulates mitochondrial biogenesis and glucose homeostasis, is strongly down-regulated. Here we tested whether induction of mitochondrial biogenesis pharmacologically or by the overexpression of PGC-1α is sufficient to reverse the phenotype of mice deficient for mTORC1. We show that both approaches normalize mitochondrial function, such as oxidative capacity and expression of mitochondrial genes. However, they do not prevent or delay the progressive myopathy. In addition, we find that mTORC1 has a much stronger effect than PGC-1α on the glycogen content in muscle. This effect is based on the strong activation of PKB/Akt in mTORC1-deficient mice. We also show that activation of PKB/Akt not only affects glycogen synthesis but also diminishes glycogen degradation. Thus, our work provides strong functional evidence that mitochondrial dysfunction in mice with inactivated mTORC1 signaling is caused by the down-regulation of PGC-1α. However, our data also show that the impairment of mitochondria does not lead directly to the lethal myopathy.