Transcriptional coregulators in the control of energy homeostasis

PGC-1α expression is increased in leukocytes in experimental acute pancreatitis

Severe acute pancreatitis (AP) induces a systemic inflammatory disease that is responsible for high mortality rates, particularly when it is complicated by infection. Therefore, differentiating sepsis from the systemic inflammation caused by AP is a serious clinical challenge. Considering the high metabolic rates of leukocytes in response to stress induced by infection, we hypothesized that the transcription coactivator peroxisome proliferator-activated receptor gamma coactivator 1 (PGC-1α), a master regulator of mitochondrial biogenesis and function, would be distinctly expressed during inflammation or infection and, therefore, could constitute a useful marker to differentiate between these two conditions. Rats were subjected to injection of taurocholate into the main pancreatic duct, which caused a severe AP with high amylase levels and white blood cell counts. In these animals, a marked increase in PGC-1α mRNA levels in circulating leukocytes was observed 48 h after the surgical procedure, a time when bacteremia is present. Antibiotic treatment abolished PGC-1α up-regulation. Moreover, PGC-1α expression was higher in peritoneal macrophages from animals subjected to a bacterial insult (cecal ligation and puncture) than in animals with AP. In isolated macrophages, we also observed that PGC-1α expression is more prominent in the presence of a phagocytic stimulus (zymosan) when compared to lipopolysaccharide-induced aseptic inflammation. Moreover, abolishing PGC-1α expression with antisense oligos impaired zymosan phagocytosis. Together, these findings suggest that PGC-1α is differentially expressed during aseptic inflammation and infection and that it is necessary for adequate phagocytosis. These results could be useful in developing new tests for differentiating infection from inflammation for clinical purposes in patients with AP.

Ellagic acid suppresses lipid accumulation by suppressing early adipogenic events and cell cycle arrest

Ellagic acid (EA) is a natural polyphenol found in various fruits and vegetables. In this study, we examined the inhibitory effect of EA on fat accumulation in 3T3-L1 cells during adipogenesis. Our data showed that EA reduced fat accumulation by down-regulating adipogenic markers such as peroxisome proliferator activated receptor γ (PPARγ) and the CCAAT/enhancer binding protein α (C/EBPα) at the mRNA and protein levels in a dose-dependent manner. We found that the decrease in adipogenic markers resulted from reduced expression of some early adipogenic transcription factors such as KLF4, KLF5, Krox20, and C/EBPβ within 24 h. Also, these inhibitions were correlated with down-regulation of TG synthetic enzymes, causing inhibition of triglyceride (TG) levels in 3T3-L1 cells investigated by ORO staining and in zebrafish investigated by TG assay. Additionally, the cell cycle analysis showed that EA inhibited cell cycle progression by arresting cells at the G0/G1 phase.

Hepatic TRAP80 selectively regulates lipogenic activity of liver X receptor

Inflammation in response to excess low-density lipoproteins in the blood is an important driver of atherosclerosis development. Due to its ability to enhance ATP-binding cassette A1-dependent (ABCA1-dependent) reverse cholesterol transport (RCT), liver X receptor (LXR) is an attractive target for the treatment of atherosclerosis. However, LXR also upregulates the expression of sterol regulatory element-binding protein 1c (SREBP-1c), leading to increased hepatic triglyceride synthesis, an independent risk factor for atherosclerosis. Here, we developed a strategy to separate the favorable and unfavorable effects of LXR by exploiting the specificity of the coactivator thyroid hormone receptor-associated protein 80 (TRAP80). Using human hepatic cell lines, we determined that TRAP80 selectively promotes the transcription of SREBP-1c but not ABCA1. Adenovirus-mediated expression of shTRAP80 inhibited LXR-dependent SREBP-1c expression and RNA polymerase II recruitment to the LXR responsive element (LXRE) of SREBP-1c, but not to the LXRE of ABCA1. In murine models, liver-specific knockdown of TRAP80 ameliorated liver steatosis and hypertriglyceridemia induced by LXR activation and maintained RCT stimulation by the LXR ligand. Together, these data indicate that TRAP80 is a selective regulator of hepatic lipogenesis and is required for LXR-dependent SREBP-1c activation. Moreover, targeting the interaction between TRAP80 and LXR should facilitate the development of potential LXR agonists that effectively prevent atherosclerosis.

Targeting steroid receptor coactivator 1 with antisense oligonucleotides increases insulin-stimulated skeletal muscle glucose uptake in chow-fed and high-fat-fed male rats

The steroid receptor coactivator 1 (SRC1) regulates key metabolic pathways, including glucose homeostasis. SRC1(-/-) mice have decreased hepatic expression of gluconeogenic enzymes and a reduction in the rate of endogenous glucose production (EGP). We sought to determine whether decreasing hepatic and adipose SRC1 expression in normal adult rats would alter glucose homeostasis and insulin action. Regular chow-fed and high-fat-fed male Sprage-Dawley rats were treated with an antisense oligonucleotide (ASO) against SRC1 or a control ASO for 4 wk, followed by metabolic assessments. SRC1 ASO did not alter basal EGP or expression of gluconeogenic enzymes. Instead, SRC1 ASO increased insulin-stimulated whole body glucose disposal by ~30%, which was attributable largely to an increase in insulin-stimulated muscle glucose uptake. This was associated with an approximately sevenfold increase in adipose expression of lipocalin-type prostaglandin D2 synthase, a previously reported regulator of insulin sensitivity, and an approximately 70% increase in plasma PGD2 concentration. Muscle insulin signaling, AMPK activation, and tissue perfusion were unchanged. Although GLUT4 content was unchanged, SRC1 ASO increased the cleavage of tether-containing UBX domain for GLUT4, a regulator of GLUT4 translocation. These studies point to a novel role of adipose SRC1 as a regulator of insulin-stimulated muscle glucose uptake.

Natural product agonists of peroxisome proliferator-activated receptor gamma (PPARγ): a review

Agonists of the nuclear receptor PPARγ are therapeutically used to combat hyperglycaemia associated with the metabolic syndrome and type 2 diabetes. In spite of being effective in normalization of blood glucose levels, the currently used PPARγ agonists from the thiazolidinedione type have serious side effects, making the discovery of novel ligands highly relevant. Natural products have proven historically to be a promising pool of structures for drug discovery, and a significant research effort has recently been undertaken to explore the PPARγ-activating potential of a wide range of natural products originating from traditionally used medicinal plants or dietary sources. The majority of identified compounds are selective PPARγ modulators (SPPARMs), transactivating the expression of PPARγ-dependent reporter genes as partial agonists. Those natural PPARγ ligands have different binding modes to the receptor in comparison to the full thiazolidinedione agonists, and on some occasions activate in addition PPARα (e.g. genistein, biochanin A, sargaquinoic acid, sargahydroquinoic acid, resveratrol, amorphastilbol) or the PPARγ-dimer partner retinoid X receptor (RXR; e.g. the neolignans magnolol and honokiol). A number of in vivo studies suggest that some of the natural product activators of PPARγ (e.g. honokiol, amorfrutin 1, amorfrutin B, amorphastilbol) improve metabolic parameters in diabetic animal models, partly with reduced side effects in comparison to full thiazolidinedione agonists. The bioactivity pattern as well as the dietary use of several of the identified active compounds and plant extracts warrants future research regarding their therapeutic potential and the possibility to modulate PPARγ activation by dietary interventions or food supplements.

In utero exposure to prepregnancy maternal obesity and postweaning high-fat diet impair regulators of mitochondrial dynamics in rat placenta and offspring

The proportion of pregnant women who are obese at conception continues to rise. Compelling evidence suggests the intrauterine environment is an important determinant of offspring health. Maternal obesity and unhealthy diets are shown to promote metabolic programming in the offspring. Mitochondria are maternally inherited, and we have previously shown impaired mitochondrial function in rat offspring exposed to maternal obesity in utero. Mitochondrial health is maintained by mitochondrial dynamics, or the processes of fusion and fission, which serve to repair damaged mitochondria, remove irreparable mitochondria, and maintain mitochondrial morphology. An imbalance between fusion and fission has been associated with obesity, insulin resistance, and reproduction complications. In the present study, we examined the influence of maternal obesity and postweaning high-fat diet (HFD) on key regulators of mitochondrial fusion and fission in rat offspring at important developmental milestones which included postnatal day (PND)35 (2 wk HFD) and PND130 (∼16 wk HFD). Our results indicate HFD-fed offspring had reduced mRNA expression of presenilin-associated rhomboid-like (PARL), optic atrophy (OPA)1, mitofusin (Mfn)1, Mfn2, fission (Fis)1, and nuclear respiratory factor (Nrf)1 at PND35, while OPA1 and Mfn2 remained decreased at PND130. Putative transcriptional regulators of mitochondrial dynamics were reduced in rat placenta and offspring liver and skeletal muscle [peroxisome proliferator-activated receptor gamma coactivator (PGC1)α, PGC1β, and estrogen-related receptor (ERR)α], consistent with indirect calorimetry findings revealing reduced energy expenditure and impaired fat utilization. Overall, maternal obesity detrimentally alters mitochondrial targets that may contribute to impaired mitochondrial health and increased obesity susceptibility in later life.

Protection against high-fat diet-induced obesity in Helz2-deficient male mice due to enhanced expression of hepatic leptin receptor

Obesity arises from impaired energy balance, which is centrally coordinated by leptin through activation of the long form of leptin receptor (Leprb). Obesity causes central leptin resistance. However, whether enhanced peripheral leptin sensitivity could overcome central leptin resistance remains obscure. A peripheral metabolic organ targeted by leptin is the liver, with low Leprb expression. We here show that mice fed a high-fat diet (HFD) and obese patients with hepatosteatosis exhibit increased expression of hepatic helicase with zinc finger 2, a transcriptional coactivator (Helz2), which functions as a transcriptional coregulator of several nuclear receptors, including peroxisome proliferator-activated receptor γ in vitro. To explore the physiological importance of Helz2, we generated Helz2-deficient mice and analyzed their metabolic phenotypes. Helz2-deficient mice showing hyperleptinemia associated with central leptin resistance were protected against HFD-induced obesity and had significantly up-regulated hepatic Leprb expression. Helz2 deficiency and adenovirus-mediated liver-specific exogenous Leprb overexpression in wild-type mice significantly stimulated hepatic AMP-activated protein kinase on HFD, whereas Helz2-deficient db/db mice lacking functional Leprb did not. Fatty acid-β oxidation was increased in Helz2-deficeint hepatocytes, and Helz2-deficient mice revealed increased oxygen consumption and decreased respiratory quotient in calorimetry analyses. The enhanced hepatic AMP-activated protein kinase energy-sensing pathway in Helz2-deficient mice ameliorated hyperlipidemia, hepatosteatosis, and insulin resistance by reducing lipogenic gene expression and stimulating lipid-burning gene expression in the liver. These findings together demonstrate that Helz2 deficiency ameliorates HFD-induced metabolic abnormalities by stimulating endogenous hepatic Leprb expression, despite central leptin resistance. Hepatic HELZ2 might be a novel target molecule for the treatment of obesity with hepatosteatosis.

Hepatic SRC-1 activity orchestrates transcriptional circuitries of amino acid pathways with potential relevance for human metabolic pathogenesis

Disturbances in amino acid metabolism are increasingly recognized as being associated with, and serving as prognostic markers for chronic human diseases, such as cancer or type 2 diabetes. In the current study, a quantitative metabolomics profiling strategy revealed global impairment in amino acid metabolism in mice deleted for the transcriptional coactivator steroid receptor coactivator (SRC)-1. Aberrations were hepatic in origin, because selective reexpression of SRC-1 in the liver of SRC-1 null mice largely restored amino acids concentrations to normal levels. Cistromic analysis of SRC-1 binding sites in hepatic tissues confirmed a prominent influence of this coregulator on transcriptional programs regulating amino acid metabolism. More specifically, SRC-1 markedly impacted tyrosine levels and was found to regulate the transcriptional activity of the tyrosine aminotransferase (TAT) gene, which encodes the rate-limiting enzyme of tyrosine catabolism. Consequently, SRC-1 null mice displayed low TAT expression and presented with hypertyrosinemia and corneal alterations, 2 clinical features observed in the human syndrome of TAT deficiency. A heterozygous missense variant of SRC-1 (p.P1272S) that is known to alter its coactivation potential, was found in patients harboring idiopathic tyrosinemia-like disorders and may therefore represent one risk factor for their clinical symptoms. Hence, we reinforce the concept that SRC-1 is a central factor in the fine orchestration of multiple pathways of intermediary metabolism, suggesting it as a potential therapeutic target that may be exploitable in human metabolic diseases and cancer.

Calcium signalling in sensory neurones and peripheral glia in the context of diabetic neuropathies

Peripheral sensory nervous system is comprised of neurones with their axons and neuroglia that includes satellite glial cells in sensory ganglia, myelinating, non-myelinating and perisynaptic Schwann cells. Pathogenesis of peripheral diabetic polyneuropathies is associated with aberrant function of both neurones and glia. Deregulated Ca(2+) homoeostasis and aberrant Ca(2+) signalling in neuronal and glial elements contributes to many forms of neuropathology and is fundamental to neurodegenerative diseases. In diabetes both neurones and glia experience metabolic stress and mitochondrial dysfunction which lead to deregulation of Ca(2+) homeostasis and Ca(2+) signalling, which in their turn lead to pathological cellular reactions contributing to development of diabetic neuropathies. Molecular cascades responsible for Ca(2+) homeostasis and signalling, therefore, can be regarded as potential therapeutic targets.

Transcriptional coregulators: fine-tuning metabolism

Metabolic homeostasis requires that cellular energy levels are adapted to environmental cues. This adaptation is largely regulated at the transcriptional level, through the interaction between transcription factors, coregulators, and the basal transcriptional machinery. Coregulators, which function as both metabolic sensors and transcriptional effectors, are ideally positioned to synchronize metabolic pathways to environmental stimuli. The balance between inhibitory actions of corepressors and stimulatory effects of coactivators enables the fine-tuning of metabolic processes. This tight regulation opens therapeutic opportunities to manage metabolic dysfunction by directing the activity of cofactors toward specific transcription factors, pathways, or cells/tissues, thereby restoring whole-body metabolic homeostasis.

The nucleolus—guardian of cellular homeostasis and genome integrity

All organisms sense and respond to conditions that stress their homeostasis by downregulating the synthesis of rRNA and ribosome biogenesis, thus designating the nucleolus as the central hub in coordinating the cellular stress response. One of the most intriguing roles of the nucleolus, long regarded as a mere ribosome-producing factory, is its participation in monitoring cellular stress signals and transmitting them to the RNA polymerase I (Pol I) transcription machinery. As rRNA synthesis is a most energy-consuming process, switching off transcription of rRNA genes is an effective way of saving the energy required to maintain cellular homeostasis during acute stress. The Pol I transcription machinery is the key convergence point that collects and integrates a vast array of information from cellular signaling cascades to regulate ribosome production which, in turn, guides cell growth and proliferation. This review focuses on the mechanisms that link cell physiology to rDNA silencing, a prerequisite for nucleolar integrity and cell survival.

Polyphenol-rich blackcurrant extract prevents inflammation in diet-induced obese mice

Obesity is closely associated with chronic, low-grade inflammation. We investigated if polyphenol-rich blackcurrant extract (BCE) can prevent inflammation in vivo. Male C57BL/6J mice were fed a modified AIN-93M control diet containing high fat/high cholesterol (16% fat, 0.25% cholesterol by weight) or the control diet supplemented with 0.1% BCE (wt/wt) for 12 weeks. In BCE-fed mice, the percentage of body weight and adipocyte size of the epididymal fat were significantly lower than those of control mice. There were fewer crown-like structures (CLS) with concomitant decreases in F4/80, cluster of differentiation 68 and inhibitor of nuclear factor κB kinase ε (IKKε) mRNA in the epididymal adipose of BCE-fed mice. F4/80 and IKKε mRNA levels were positively correlated with CLS number. In the skeletal muscle of mice fed with BCE, mRNA expression of genes involved in energy expenditure and mitochondrial biogenesis, including PPARα, PPARδ, UCP-2, UCP-3 and mitochondrial transcription factor A, were significantly increased. When splenocytes from BCE-fed mice were stimulated by lipopolysaccharides, tumor necrosis factor α and interleukin-1β mRNA were significantly lower than control splenocytes. Together, the results suggest that BCE supplementation decreases obesity-induced inflammation in adipose tissue and splenocytes, at least in part, by modulating energy metabolism in skeletal muscle.

Differential roles of PPARγ vs TR4 in prostate cancer and metabolic diseases

Peroxisome proliferator-activated receptor γ (PPARγ, NR1C3) and testicular receptor 4 nuclear receptor (TR4, NR2C2) are two members of the nuclear receptor (NR) superfamily that can be activated by several similar ligands/activators including polyunsaturated fatty acid metabolites, such as 13-hydroxyoctadecadienoic acid and 15-hydroxyeicosatetraenoic acid, as well as some anti-diabetic drugs such as thiazolidinediones (TZDs). However, the consequences of the transactivation of these ligands/activators via these two NRs are different, with at least three distinct phenotypes. First, activation of PPARγ increases insulin sensitivity yet activation of TR4 decreases insulin sensitivity. Second, PPARγ attenuates atherosclerosis but TR4 might increase the risk of atherosclerosis. Third, PPARγ suppresses prostate cancer (PCa) development and TR4 suppresses prostate carcinogenesis yet promotes PCa metastasis. Importantly, the deregulation of either PPARγ or TR4 in PCa alone might then alter the other receptor's influences on PCa progression. Knocking out PPARγ altered the ability of TR4 to promote prostate carcinogenesis and knocking down TR4 also resulted in TZD treatment promoting PCa development, indicating that both PPARγ and TR4 might coordinate with each other to regulate PCa initiation, and the loss of either one of them might switch the other one from a tumor suppressor to a tumor promoter. These results indicate that further and detailed studies of both receptors at the same time in the same cells/organs may help us to better dissect their distinct physiological roles and develop better drug(s) with fewer side effects to battle PPARγ- and TR4-related diseases including tumor and cardiovascular diseases as well as metabolic disorders.

Epidemiological transition of colorectal cancer in developing countries: Environmental factors, molecular pathways, and opportunities for prevention

Colorectal cancer (CRC) is one of the leading causes of cancer and cancer-related mortality worldwide. The disease has been traditionally a major health problem in industrial countries, however the CRC rates are increasing in the developing countries that are undergoing economic growth. Several environmental risk factors, mainly changes in diet and life style, have been suggested to underlie the rise of CRC in these populations. Diet and lifestyle impinge on nuclear receptors, on the intestinal microbiota and on crucial molecular pathways that are implicated in intestinal carcinogenesis. In this respect, the epidemiological transition in several regions of the world offers a unique opportunity to better understand CRC carcinogenesis by studying the disease phenotypes and their environmental and molecular associations in different populations. The data from these studies may have important implications for the global prevention and treatment of CRC.

Allele compensation in tip60+/- mice rescues white adipose tissue function in vivo

Adipose tissue is a key regulator of energy homestasis. The amount of adipose tissue is largely determined by adipocyte differentiation (adipogenesis), a process that is regulated by the concerted actions of multiple transcription factors and cofactors. Based on in vitro studies in murine 3T3-L1 preadipocytes and human primary preadipocytes, the transcriptional cofactor and acetyltransferase Tip60 was recently identified as an essential adipogenic factor. We therefore investigated the role of Tip60 on adipocyte differentiation and function, and possible consequences on energy homeostasis, in vivo. Because homozygous inactivation results in early embryonic lethality, Tip60+/− mice were used. Heterozygous inactivation of Tip60 had no effect on body weight, despite slightly higher food intake by Tip60+/− mice. No major effects of heterozygous inactivation of Tip60 were observed on adipose tissue and liver, and Tip60+/− displayed normal glucose tolerance, both on a low fat and a high fat diet. While Tip60 mRNA was reduced to 50% in adipose tissue, the protein levels were unaltered, suggesting compensation by the intact allele. These findings indicate that the in vivo role of Tip60 in adipocyte differentiation and function cannot be properly addressed in Tip60+/− mice, but requires the generation of adipose tissue-specific knock out animals or specific knock-in mice.

Role of mitochondria in nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) affects about 30% of the general population in the United States and includes a spectrum of disease that includes simple steatosis, non-alcoholic steatohepatitis (NASH), fibrosis and cirrhosis. Significant insight has been gained into our understanding of the pathogenesis of NALFD; however the key metabolic aberrations underlying lipid accumulation in hepatocytes and the progression of NAFLD remain to be elucidated. Accumulating and emerging evidence indicate that hepatic mitochondria play a critical role in the development and pathogenesis of steatosis and NAFLD. Here, we review studies that document a link between the pathogenesis of NAFLD and hepatic mitochondrial dysfunction with particular focus on new insights into the role of impaired fatty acid oxidation, the transcription factor peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α), and sirtuins in development and progression of NAFLD.

TIPping the balance in adipogenesis: USP7-mediated stabilization of Tip60

Adipogenesis is regulated by a complex interplay between transcription factors, in concert with-among others-transcriptional cofactors, signaling cascades and miRNAs. Several studies have implicated the transcriptional cofactor and acetyltransferase Tip60 in PPARγ signaling and adipocyte differentiation. Since Tip60 protein levels, but not mRNA levels, are upregulated during adipogenesis, and since Tip60 can be degraded by the proteasome, we hypothesized that Tip60 protein may be stabilized through deubiquitination during adipogenesis. Indeed, Tip60 is protected from proteasomal degeradation by the deubiquitinase USP7, which is particularly important for mitotic clonal expansion (MCE), an early step in adipogenesis. Besides this novel role in early differentiation, earlier studies indicated that Tip60 is also important during the later stages of differentiation, indicating a dual role for this protein in adipogenesis. Our recent study sheds new light on the role of Tip60 in cellular differentiation and provide new insights into the importance of a regulatory process that has not been studied intensively in adipogenesis: protein (de)ubiquitination.

Prenatal Hypoxia Reduces Mitochondrial Protein Levels and Cytochrome c Oxidase Activity in Offspring Guinea Pig Hearts

Prenatal hypoxia (HPX) reduces mitochondrial cytochrome c oxidase (CCO and COX) activity in fetal guinea pig (GP) hearts. The aim of this study was to quantify the lasting effects of chronic prenatal HPX on cardiac mitochondrial enzyme activity and protein expression in offspring hearts. Pregnant GPs were exposed to either normoxia (NMX) or HPX (10.5%O₂) during the last 14 days of pregnancy. Both NMX and HPX fetuses, delivered vaginally, were housed under NMX conditions until 90 days of age. Total RNA and mitochondrial fractions were isolated from hearts of anesthetized NMX and HPX offspring and showed decreased levels of CCO but not medium-chain acyl dehydrogenase activity, protein levels of nuclear- and mitochondrial-encoded COX4 and COX1, respectively, and messenger RNA expression of peroxisome proliferator-activated receptor gamma coactivator 1-alpha, COX5b, and 4.1 compared to NMX controls. Prenatal HPX may alter mitochondrial function in the offspring by disrupting protein expression associated with the respiratory chain.

The novel dipeptidyl peptidase-4 inhibitor teneligliptin prevents high-fat diet-induced obesity accompanied with increased energy expenditure in mice

Dipeptidyl peptidase-4 (DPP-4)-deficient mice exhibit prevention of obesity with increased energy expenditure, whereas currently available DPP-4 inhibitors do not induce similar changes. We investigated the impact of the novel DPP-4 inhibitor teneligliptin on body weight, energy expenditure, and obesity-related manifestations in diet-induced obese mice. Six-weeks-old C57BL/6N mice were fed a high-fat diet (60%kcal fat) ad libitum and administered teneligliptin (30 or 60mg/kg) via drinking water for 10 weeks. Mice fed a high-fat diet showed accelerated body weight gain. In contrast, compared with the vehicle group, the administration of teneligliptin reduced body weight to 88% and 71% at dose of 30mg/kg/day and 60mg/kg/day, respectively. Although there was no change in locomotor activity, indirect calorimetry studies showed that teneligliptin (60mg/kg) increased oxygen consumption by 22%. Adipocyte hypertrophy and hepatic steatosis induced by a high-fat diet were suppressed by teneligliptin. The mean adipocyte size in the 60-mg/kg treatment group was 44% and hepatic triglyceride levels were 34% of the levels in the vehicle group. Furthermore, treatment with teneligliptin (60mg/kg) reduced plasma levels of insulin to 40% and increased the glucose infusion rate to 39%, as measured in the euglycemic clamp study, indicating its beneficial effect on insulin resistance. We showed for the first time that the DPP-4 inhibitor prevents obesity and obesity-related manifestations with increased energy expenditure. Our findings suggest the potential utility of teneligliptin for the treatment of a broad spectrum of metabolic disorders related to obesity beyond glycemic control.

Electrical stimulation modulates Wnt signaling and regulates genes for the motor endplate and calcium binding in muscle of rats with spinal cord transection

BACKGROUND

Spinal cord injury (SCI) results in muscle atrophy and a shift of slow oxidative to fast glycolytic fibers. Electrical stimulation (ES) at least partially restores muscle mass and fiber type distribution. The objective of this study was to was to characterize the early molecular adaptations that occur in rat soleus muscle after initiating isometric resistance exercise by ES for one hour per day for 1, 3 or 7 days when ES was begun 16 weeks after SCI. Additionally, changes in mRNA levels after ES were compared with those induced in soleus at the same time points after gastrocnemius tenotomy (GA).

RESULTS

ES increased expression of Hey1 and Pitx2 suggesting increased Notch and Wnt signaling, respectively, but did not normalize RCAN1.4, a measure of calcineurin/NFAT signaling, or PGC-1ß mRNA levels. ES increased PGC-1α expression but not that of slow myofibrillar genes. Microarray analysis showed that after ES, genes coding for calcium binding proteins and nicotinic acetylcholine receptors were increased, and the expression of genes involved in blood vessel formation and morphogenesis was altered. Of the 165 genes altered by ES only 16 were also differentially expressed after GA, of which 12 were altered in the same direction by ES and GA. In contrast to ES, GA induced expression of genes related to oxidative phosphorylation.

CONCLUSIONS

Notch and Wnt signaling may be involved in ES-induced increases in the mass of paralyzed muscle. Molecular adaptations of paralyzed soleus to resistance exercise are delayed or defective compared to normally innervated muscle.

SIRT1 is a direct coactivator of thyroid hormone receptor β1 with gene-specific actions

Sirtuin 1 (SIRT1) NAD⁺-dependent deacetylase regulates energy metabolism by modulating expression of genes involved in gluconeogenesis and other liver fasting responses. While many effects of SIRT1 on gene expression are mediated by deacetylation and activation of peroxisome proliferator activated receptor coactivator α (PGC-1α), SIRT1 also binds directly to DNA bound transcription factors, including nuclear receptors (NRs), to modulate their activity. Since thyroid hormone receptor β1 (TRβ1) regulates several SIRT1 target genes in liver and interacts with PGC-1α, we hypothesized that SIRT1 may influence TRβ1. Here, we confirm that SIRT1 cooperates with PGC-1α to enhance response to triiodothyronine, T₃. We also find, however, that SIRT1 stimulates TRβ1 activity in a manner that is independent of PGC-1α but requires SIRT1 deacetylase activity. SIRT1 interacts with TRβ1 in vitro, promotes TRβ1 deacetylation in the presence of T₃ and enhances ubiquitin-dependent TRβ1 turnover; a common response of NRs to activating ligands. More surprisingly, SIRT1 knockdown only strongly inhibits T₃ response of a subset of TRβ1 target genes, including glucose 6 phosphatase (G-6-Pc), and this is associated with blockade of TRβ1 binding to the G-6-Pc promoter. Drugs that target the SIRT1 pathway, resveratrol and nicotinamide, modulate T₃ response at dual TRβ1/SIRT1 target genes. We propose that SIRT1 is a gene-specific TRβ1 co-regulator and TRβ1/SIRT1 interactions could play important roles in regulation of liver metabolic response. Our results open possibilities for modulation of subsets of TR target genes with drugs that influence the SIRT1 pathway.

Apoptosis induced by PGC-1β in breast cancer cells is mediated by the mTOR pathway

The peroxisome proliferator-activated receptor-γ (PPAR-γ) coactivator-1β (PGC-1β) is a well-established regulator of mitochondrial biogenesis. However, the underlying mechanism of PGC-1β action remains elusive. This study reveals that knockdown of endogenous PGC-1β by short-hairpin RNA (shRNA) leads to a decrease in the expression of mammalian target of rapamycin (mTOR) pathway-related genes in MDA-MB-231 cells. After knockdown of PGC-1β, phosphorylation of AMP-activated protein kinase (AMPK), phosphorylation of Rictor on Thr1135, Raptor and S6 protein was inhibited. However, Akt phosphorylation on Ser473 was upregulated and cell apoptosis occurred. In particular, we demonstrate that the levels of PGC-1β and mTOR correlated with overall mitochondrial activity. These results provide new evidence that cell apoptosis is orchestrated by the balance between several signaling pathways, and that PGC-1β takes part in these events in breast cancer cells mediated by the mTOR signaling pathway.

Adaptability to hypobaric hypoxia is facilitated through mitochondrial bioenergetics: an in vivo study

BACKGROUND AND PURPOSE

High-altitude pulmonary oedema (HAPE) experienced under high-altitude conditions is attributed to mitochondrial redox distress. Hence, hypobaric hypoxia (HH)-induced alteration in expression of mitochondrial biogenesis and dynamics genes was determined in rat lung. Further, such alteration was correlated with expression of mitochondrial DNA (mtDNA)-encoded oxidative phosphorylation (mtOXPHOS) genes. The prophylactic effect of dexamethasone (DEX) in counteracting the HH-induced mitochondrial distress was used as control to understand adaptation to high-altitude exposure.

EXPERIMENTAL APPROACH

Rats pretreated with DEX were exposed to normobaric normoxia (NN) or HH. HH-induced injury was assessed as an increase in lung water content, tissue damage and oxidant generation. Mitochondrial number, mtDNA content and mtOXPHOS activities were measured to determine mitochondrial function. The expression of mitochondrial biogenesis, dynamics and mtOXPHOS genes was studied.

KEY RESULTS

HH-induced lung injury was associated with decreased mitochondrial number, mtDNA content and mtOXPHOS activities. HH exposure decreased the nuclear gene oestrogen-related receptor-α (ERRα), which interacts with PPAR-γ coactivator-1α (PGC-1α) in controlling mitochondrial metabolism. Consequently, mtOXPHOS transcripts are repressed under HH. Further, HH modulated mitochondrial dynamics by decreasing mitofusin 2 (Mfn2) and augmenting fission 1 (Fis1) and dynamin-related protein 1 (Drp1) expression. Nevertheless, DEX treatment under NN (i.e. adaptation to HH) did not affect mitochondrial biogenesis and dynamics, but increased mtOXPHOS transcripts. Further, mtOXPHOS activities increased together with reduced oxidant generation. Also, DEX pretreatment normalized ERRα along with mitochondrial dynamics genes and increased mtOXPHOS transcripts to elicit the mitochondrial function under HH.

CONCLUSIONS AND IMPLICATIONS

HH stress (HAPE)-mediated mitochondrial dysfunction is due to repressed ERRα and mtOXPHOS transcripts. Thus, ERRα-mediated protection of mitochondrial bioenergetics might be the likely candidate required for lung adaptation to HH.

Bioenergetic flux, mitochondrial mass and mitochondrial morphology dynamics in AD and MCI cybrid cell lines

Bioenergetic dysfunction occurs in Alzheimer's disease (AD) and mild cognitive impairment (MCI), a clinical syndrome that frequently precedes symptomatic AD. In this study, we modeled AD and MCI bioenergetic dysfunction by transferring mitochondria from MCI, AD and control subject platelets to mtDNA-depleted SH-SY5Y cells. Bioenergetic fluxes and bioenergetics-related infrastructures were characterized in the resulting cytoplasmic hybrid (cybrid) cell lines. Relative to control cybrids, AD and MCI cybrids showed changes in oxygen consumption, respiratory coupling and glucose utilization. AD and MCI cybrids had higher ADP/ATP and lower NAD+/NADH ratios. AD and MCI cybrids exhibited differences in proteins that monitor, respond to or regulate cell bioenergetic fluxes including HIF1α, PGC1α, SIRT1, AMPK, p38 MAPK and mTOR. Several endpoints suggested mitochondrial mass increased in the AD cybrid group and probably to a lesser extent in the MCI cybrid group, and that the mitochondrial fission-fusion balance shifted towards increased fission in the AD and MCI cybrids. As many of the changes we observed in AD and MCI cybrid models are also seen in AD subject brains, we conclude reduced bioenergetic function is present during very early AD, is not brain-limited and induces protean retrograde responses that likely have both adaptive and mal-adaptive consequences.

Novel pathogenic pathways in diabetic neuropathy

Diabetic peripheral neuropathy (DPN) is a common complication affecting more than one third of diabetes mellitus (DM) patients. Although all cellular components participating in peripheral nerve function are exposed to and affected by the metabolic consequences of DM, nodal regions, areas of intense interactions between Schwann cells and axons, may be particularly sensitive to DM-induced alterations. Nodes are enriched in insulin receptors, glucose transporters, Na(+) and K(+) channels, and mitochondria, all implicated in the development and progression of DPN. Latest results particularly reinforce the idea that changes in ion-channel function and energy metabolism, both of which depend on axon-glia crosstalk, are among the important contributors to DPN. These insights provide a basis for new therapeutic approaches aimed at delaying or reversing DPN.

Pharmacological approaches to restore mitochondrial function

Mitochondrial dysfunction is not only a hallmark of rare inherited mitochondrial disorders but also implicated in age-related diseases, including those that affect the metabolic and nervous system, such as type 2 diabetes and Parkinson's disease. Numerous pathways maintain and/or restore proper mitochondrial function, including mitochondrial biogenesis, mitochondrial dynamics, mitophagy and the mitochondrial unfolded protein response. New and powerful phenotypic assays in cell-based models as well as multicellular organisms have been developed to explore these different aspects of mitochondrial function. Modulating mitochondrial function has therefore emerged as an attractive therapeutic strategy for several diseases, which has spurred active drug discovery efforts in this area.

Epigenetic methylations and their connections with metabolism

Metabolic pathways play fundamental roles in several processes that regulate cell physiology and adaptation to environmental changes. Altered metabolic pathways predispose to several different pathologies ranging from diabetes to cancer. Specific transcriptional programs tightly regulate the enzymes involved in cell metabolism and dictate cell fate regulating the differentiation into specialized cell types that contribute to metabolic adaptation in higher organisms. For these reasons, it is of extreme importance to identify signaling pathways and transcription factors that positively and negatively regulate metabolism. Genomic organization allows a plethora of different strategies to regulate transcription. Importantly, large evidence suggests that the quality of diet and the caloric regimen can influence the epigenetic state of our genome and that certain metabolic pathways are also epigenetically controlled reveling a tight crosstalk between metabolism and epigenomes. Here we focus our attention on methylation-based epigenetic reactions, on how different metabolic pathways control these activities, and how these can influence metabolism. Altogether, the recent discoveries linking these apparent distant areas reveal that an exciting field of research is emerging.

Bridging epigenomics and complex disease: the basics

The DNA sequence largely defines gene expression and phenotype. However, it is becoming increasingly clear that an additional chromatin-based regulatory network imparts both stability and plasticity to genome output, modifying phenotype independently of the genetic blueprint. Indeed, alterations in this "epigenetic" control layer underlie, at least in part, the reason for monozygotic twins being discordant for disease. Functionally, this regulatory layer comprises post-translational modifications of DNA and histones, as well as small and large noncoding RNAs. Together these regulate gene expression by changing chromatin organization and DNA accessibility. Successive technological advances over the past decade have enabled researchers to map the chromatin state with increasing accuracy and comprehensiveness, catapulting genetic research into a genome-wide era. Here, aiming particularly at the genomics/epigenomics newcomer, we review the epigenetic basis that has helped drive the technological shift and how this progress is shaping our understanding of complex disease.

Sargassum polycystum reduces hyperglycaemia, dyslipidaemia and oxidative stress via increasing insulin sensitivity in a rat model of type 2 diabetes

BACKGROUND

Sargassum polycystum, a brown seaweed, contains various nutrients and bioactive compounds that have antioxidant and healing properties. The research hypothesises that antioxidants and pigments in dietary S. polycystum extracts can improve insulin sensitivity, blood sugar levels and blood lipid levels in a rat model of type 2 diabetes. The diabetes was induced by a high-sugar, high-fat diet for 16 weeks to enhance insulin resistance, followed by a low-dose intraperitoneal injection of streptozotocin (35 mg kg(-1) body weight). The doses of S. polycystum tested on diabetic rats were 150 and 300 mg kg(-1) body weight for the ethanolic extract or 150 and 300 mg kg(-1) for the water extract. Normal rats, untreated diabetic and metformin-treated diabetic rats (n = 6) were used as control.

RESULTS

Both doses of the alcohol extract of S. polycystum and the 300 mg kg(-1) water extract, significantly reduced blood glucose and glycosylated haemoglobin (HbA1C ) levels. Serum total cholesterol, triglyceride levels and plasma atherogenic index were significantly decreased after 22 days treatment in all seaweed groups. Unlike metformin, S. polycystum did not significantly change plasma insulin in the rats, but increased the response to insulin.

CONCLUSION

The consumption of either ethanolic or water extracts of S. polycystum dose dependently reduced dyslipidaemia in type 2 diabetic rats. S. polycystum is a potential insulin sensitiser, for a comestible complementary therapy in the management of type 2 diabetes which can help reduce atherogenic risk.

Ablation of TRIP-Br2, a regulator of fat lipolysis, thermogenesis and oxidative metabolism, prevents diet-induced obesity and insulin resistance

Obesity develops as a result of altered energy homeostasis favoring fat storage. Here we describe a new transcription co-regulator for adiposity and energy metabolism, SERTA domain containing 2 (TRIP-Br2, also called SERTAD2). TRIP-Br2-null mice are resistant to obesity and obesity-related insulin resistance. Adipocytes of these knockout mice showed greater stimulated lipolysis secondary to enhanced expression of hormone sensitive lipase (HSL) and β3-adrenergic (Adrb3) receptors. The knockout mice also have higher energy expenditure because of increased adipocyte thermogenesis and oxidative metabolism caused by upregulating key enzymes in their respective processes. Our data show that a cell-cycle transcriptional co-regulator, TRIP-Br2, modulates fat storage through simultaneous regulation of lipolysis, thermogenesis and oxidative metabolism. These data, together with the observation that TRIP-Br2 expression is selectively elevated in visceral fat in obese humans, suggests that this transcriptional co-regulator is a new therapeutic target for counteracting the development of obesity, insulin resistance and hyperlipidemia.

TSC22D4 is a molecular output of hepatic wasting metabolism

In mammals, proper storage and distribution of lipids in and between tissues is essential for the maintenance of energy homeostasis. Here, we show that tumour growth triggers hepatic metabolic dysfunction as part of the cancer cachectic phenotype, particularly by reduced hepatic very-low-density-lipoprotein (VLDL) secretion and hypobetalipoproteinemia. As a molecular cachexia output pathway, hepatic levels of the transcription factor transforming growth factor beta 1-stimulated clone (TSC) 22 D4 were increased in cancer cachexia. Mimicking high cachectic levels of TSC22D4 in healthy livers led to the inhibition of hepatic VLDL release and lipogenic genes, and diminished systemic VLDL levels under both normal and high fat dietary conditions. Liver-specific ablation of TSC22D4 triggered hypertriglyceridemia through the induction of hepatic VLDL secretion. Furthermore, hepatic TSC22D4 expression levels were correlated with the degree of body weight loss and VLDL hypo-secretion in cancer cachexia, and TSC22D4 deficiency rescued tumour cell-induced metabolic dysfunction in hepatocytes. Therefore, hepatic TSC22D4 activity may represent a molecular rationale for peripheral energy deprivation in subjects with metabolic wasting diseases, including cancer cachexia.

Mechanisms of divergent effects of activated peroxisome proliferator-activated receptor-γ on mitochondrial citrate carrier expression in 3T3-L1 fibroblasts and mature adipocytes

The citrate carrier (CIC), a nuclear-encoded protein located in the mitochondrial inner membrane, plays an important metabolic role in the transport of acetyl-CoA from the mitochondrion to the cytosol in the form of citrate for fatty acid and cholesterol synthesis. Citrate has been reported to be essential for fibroblast differentiation into fat cells. Because peroxisome proliferator-activated receptor-gamma (PPARγ) is known to be one of the master regulators of adipogenesis, we aimed to study the regulation of CIC by the PPARγ ligand rosiglitazone (BRL) in 3T3-L1 fibroblasts and in adipocytes. We demonstrated that BRL up-regulated CIC mRNA and protein levels in fibroblasts, while it did not elicit any effects in mature adipocytes. The enhancement of CIC levels upon BRL treatment was reversed using the PPARγ antagonist GW9662, addressing how this effect was mediated by PPARγ. Functional experiments using a reporter gene containing rat CIC promoter showed that BRL enhanced CIC promoter activity. Mutagenesis studies, electrophoretic-mobility-shift assay and chromatin-immunoprecipitation analysis revealed that upon BRL treatment, PPARγ and Sp1 are recruited on the Sp1-containing region within the CIC promoter, leading to an increase in CIC expression. In addition, mithramycin, a specific inhibitor for Sp1-DNA binding activity, abolished the PPARγ-mediated up-regulation of CIC in fibroblasts. The stimulatory effects of BRL disappeared in mature adipocytes in which PPARγ/Sp1 complex recruited SMRT corepressor to the Sp1 site of the CIC promoter. Taken together, our results contribute to clarify the molecular mechanisms by which PPARγ regulates CIC expression during the differentiation stages of fibroblasts into mature adipocytes.

Bezafibrate administration improves behavioral deficits and tau pathology in P301S mice

Peroxisome proliferator-activated receptors (PPARs) are ligand-mediated transcription factors, which control both lipid and energy metabolism and inflammation pathways. PPARγ agonists are effective in the treatment of metabolic diseases and, more recently, neurodegenerative diseases, in which they show promising neuroprotective effects. We studied the effects of the pan-PPAR agonist bezafibrate on tau pathology, inflammation, lipid metabolism and behavior in transgenic mice with the P301S human tau mutation, which causes familial frontotemporal lobar degeneration. Bezafibrate treatment significantly decreased tau hyperphosphorylation using AT8 staining and the number of MC1-positive neurons. Bezafibrate treatment also diminished microglial activation and expression of both inducible nitric oxide synthase and cyclooxygenase 2. Additionally, the drug differentially affected the brain and brown fat lipidome of control and P301S mice, preventing lipid vacuoles in brown fat. These effects were associated with behavioral improvement, as evidenced by reduced hyperactivity and disinhibition in the P301S mice. Bezafibrate therefore exerts neuroprotective effects in a mouse model of tauopathy, as shown by decreased tau pathology and behavioral improvement. Since bezafibrate was given to the mice before tau pathology had developed, our data suggest that bezafibrate exerts a preventive effect on both tau pathology and its behavioral consequences. Bezafibrate is therefore a promising agent for the treatment of neurodegenerative diseases associated with tau pathology.

SUMO-specific protease 1 regulates mitochondrial biogenesis through PGC-1α

Peroxisome proliferator-activated receptor γ (PPARγ) coactivator 1α (PGC-1α) is a master regulator of mitochondrial biogenesis in response to changes in the cellular environment, physiological or pathological status of mammals. PGC-1α is known to be modified by SUMO (Small Ubiquitin-like Modifier). However, it is not known whether SUMOylation could affect the function of PGC-1α in mitochondrial biogenesis and that how PGC-1α SUMOylation is regulated. In this study, we have identified the role of Sentrin/SUMO-specific protease 1 (SENP1) as a specific SUMO protease to regulate SUMOylation status of PGC-1α. More importantly, we have also found that SENP1 promotes PGC-1α transcription activity, which is essential for the expression of mitochondrial genes and subsequently mitochondrial biogenesis. Thus, we reveal that the SUMOylation of PGC-1α controlled by SENP1 plays an important role in mitochondrial biogenesis and function.

Mitochondrial function in vascular endothelial cell in diabetes

Micro- and macrovascular complications are commonly seen in diabetic patients and endothelial dysfunction contributes to the development and progression of the complications. Abnormal functions in endothelial cells lead to the increase in vascular tension and atherosclerosis, followed by systemic hypertension as well as increased incidence of ischemia and stroke in diabetic patients. Mitochondria are organelles serving as a source of energy production and as regulators of cell survival (e.g., apoptosis and cell development) and ion homeostasis (e.g., H(+), Ca(2+)). Endothelial mitochondria are mainly responsible for generation of reactive oxygen species (ROS) and maintaining the Ca(2+) concentration in the cytosol. There is increasing evidence that mitochondrial morphological and functional changes are implicated in vascular endothelial dysfunction. Enhanced mitochondrial fission and/or attenuated fusion lead to mitochondrial fragmentation and disrupt the endothelial physiological function. Abnormal mitochondrial biogenesis and disturbance of mitochondrial autophagy increase the accumulation of damaged mitochondria, such as irreversibly depolarized or leaky mitochondria, and facilitate cell death. Augmented mitochondrial ROS production and Ca(2+) overload in mitochondria not only cause the maladaptive effect on the endothelial function, but also are potentially detrimental to cell survival. In this article, we review the physiological and pathophysiological role of mitochondria in endothelial function with special focus on diabetes.

Mechanisms of thyroid hormone action

Our understanding of thyroid hormone action has been substantially altered by recent clinical observations of thyroid signaling defects in syndromes of hormone resistance and in a broad range of conditions, including profound mental retardation, obesity, metabolic disorders, and a number of cancers. The mechanism of thyroid hormone action has been informed by these clinical observations as well as by animal models and has influenced the way we view the role of local ligand availability; tissue and cell-specific thyroid hormone transporters, corepressors, and coactivators; thyroid hormone receptor (TR) isoform-specific action; and cross-talk in metabolic regulation and neural development. In some cases, our new understanding has already been translated into therapeutic strategies, especially for treating hyperlipidemia and obesity, and other drugs are in development to treat cardiac disease and cancer and to improve cognitive function.

Using gene-network landscape to dissect genotype effects of TCF7L2 genetic variant on diabetes and cardiovascular risk

The single nucleotide polymorphism (SNP) within the TCF7L2 gene, rs7903146, is, to date, the most significant genetic marker associated with Type 2 diabetes mellitus (T2DM) risk. Nonetheless, its functional role in disease pathology is poorly understood. The aim of the present study was to investigate, in vascular smooth muscle cells from 92 patients undergoing aortocoronary bypass surgery, the contribution of this SNP in T2DM using expression levels and expression correlation comparison approaches, which were visually represented as gene interaction networks. Initially, the expression levels of 41 genes (seven TCF7L2 splice forms and 40 other T2DM relevant genes) were compared between rs7903146 wild-type (CC) and T2DM-risk (CT + TT) genotype groups. Next, we compared the expression correlation patterns of these 41 genes between groups to observe if the relationships between genes were different. Five TCF7L2 splice forms and nine genes showed significant expression differences between groups. RXRα gene was pinpointed as showing the most different expression correlation pattern with other genes. Therefore, T2DM risk alleles appear to be influencing TCF7L2 splice form's expression in vascular smooth muscle cells, and RXRα gene is pointed out as a treatment target candidate for risk reduction in individuals with high risk of developing T2DM, especially individuals harboring TCF7L2 risk genotypes.

LRH-1-dependent glucose sensing determines intermediary metabolism in liver

Liver receptor homolog 1 (LRH-1), an established regulator of cholesterol and bile acid homeostasis, has recently emerged as a potential drug target for liver disease. Although LRH-1 activation may protect the liver against diet-induced steatosis and insulin resistance, little is known about how LRH-1 controls hepatic glucose and fatty acid metabolism under physiological conditions. We therefore assessed the role of LRH-1 in hepatic intermediary metabolism. In mice with conditional deletion of Lrh1 in liver, analysis of hepatic glucose fluxes revealed reduced glucokinase (GCK) and glycogen synthase fluxes as compared with those of wild-type littermates. These changes were attributed to direct transcriptional regulation of Gck by LRH-1. Impaired glucokinase-mediated glucose phosphorylation in LRH-1-deficient livers was also associated with reduced glycogen synthesis, glycolysis, and de novo lipogenesis in response to acute and prolonged glucose exposure. Accordingly, hepatic carbohydrate response element-binding protein activity was reduced in these animals. Cumulatively, these data identify LRH-1 as a key regulatory component of the hepatic glucose-sensing system required for proper integration of postprandial glucose and lipid metabolism.

Effects of high-fat diet and physical activity on pyruvate dehydrogenase kinase-4 in mouse skeletal muscle

Background

The expression of PDK4 is elevated by diabetes, fasting and other conditions associated with the switch from the utilization of glucose to fatty acids as an energy source. It is previously shown that peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α), a master regulator of energy metabolism, coactivates in cell lines pyruvate dehydrogenase kinase-4 (PDK4) gene expression via the estrogen-related receptor α (ERRα). We investigated the effects of long-term high-fat diet and physical activity on the expression of PDK4, PGC-1α and ERRα and the amount and function of mitochondria in skeletal muscle.

Methods

Insulin resistance was induced by a high-fat (HF) diet for 19 weeks in C57BL/6 J mice, which were either sedentary or with access to running wheels. The skeletal muscle expression levels of PDK4, PGC-1α and ERRα were measured and the quality and quantity of mitochondrial function was assessed.

Results

The HF mice were more insulin-resistant than the low-fat (LF) -fed mice. Upregulation of PDK4 and ERRα mRNA and protein levels were seen after the HF diet, and when combined with running even more profound effects on the mRNA expression levels were observed. Chronic HF feeding and voluntary running did not have significant effects on PGC-1α mRNA or protein levels. No remarkable difference was found in the amount or function of mitochondria.

Conclusions

Our results support the view that insulin resistance is not mediated by the decreased qualitative or quantitative properties of mitochondria. Instead, the role of PDK4 should be contemplated as a possible contributor to high-fat diet-induced insulin resistance.

S6 kinase 2 deficiency enhances ketone body production and increases peroxisome proliferator-activated receptor alpha activity in the liver

Nutrient homeostasis is tightly regulated by the balance between energy production and utilization. During fasting, production of ketone bodies as an alternative energy source is critical to maintain nutrient homeostasis. An important component in the nutrient-sensitive signaling pathway is S6 kinase 2 (S6K2), a downstream effector of mammalian target of rapamycin. Here, we show that mice lacking S6K2 exhibit elevated levels of ketone bodies and enhanced peroxisome proliferator-activated receptor alpha (PPARα) activity upon nutrient availability. Consistent with this, knockdown of S6K2 increases the transcriptional activity of PPARα. S6K2 suppresses PPARα by associating with its corepressor, nuclear receptor corepressor 1 (NCoR1), and by inducing the recruitment of NCoR1 to the nucleus. Moreover, ob/ob mice, a genetic model of obesity, have markedly elevated S6K2 activity, and S6K2 was strongly associated with NCoR1 in the nucleus of liver cells.

CONCLUSION

Our findings suggest that S6K2 regulates hepatic energy homeostasis by repressing PPARα activity and point to its potential relevance for therapeutic strategies designed to modulate S6K2 activity as a treatment for deregulated ketone body production.

Transcriptional analysis of brown adipose tissue in leptin-deficient mice lacking inducible nitric oxide synthase: evidence of the role of Med1 in energy balance

Leptin and nitric oxide (NO) are implicated in the control of energy homeostasis. The aim of the present study was to examine the impact of the absence of the inducible NO synthase (iNOS) gene on the regulation of energy balance in ob/ob mice analyzing the changes in gene expression levels in brown adipose tissue (BAT). Double knockout (DBKO) mice simultaneously lacking the ob and iNOS genes were generated and the expression of genes involved in energy balance including fatty acid and glucose metabolism as well as mitochondrial genes were analyzed by microarrays. DBKO mice exhibited an improvement in energy balance with a decrease in body weight (P < 0.001), total fat pads (P < 0.05), and food intake (P < 0.05), as well as an enhancement in BAT function compared with ob/ob mice. To better understand the molecular events associated with this improvement, BAT gene expression was analyzed. Of particular interest, gene expression levels of the key subunit of the Mediator complex Med1 was upregulated (P < 0.05) in DBKO mice. Real-time PCR and immunohistochemistry further confirmed this data. Med1 is implicated in adipogenesis, lipid metabolic and biosynthetic processes, glucose metabolism, and mitochondrial metabolic pathways. Med1 plays an important role in the transcriptional control of genes implicated in energy homeostasis, suggesting that the improvement in energy balance and BAT function of the DBKO mice is mediated, at least in part, through the transcription coactivator Med1.

Impaired adenosine monophosphate-activated protein kinase signalling in dorsal root ganglia neurons is linked to mitochondrial dysfunction and peripheral neuropathy in diabetes

Mitochondrial dysfunction occurs in sensory neurons and may contribute to distal axonopathy in animal models of diabetic neuropathy. The adenosine monophosphate-activated protein kinase and peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) signalling axis senses the metabolic demands of cells and regulates mitochondrial function. Studies in muscle, liver and cardiac tissues have shown that the activity of adenosine monophosphate-activated protein kinase and PGC-1α is decreased under hyperglycaemia. In this study, we tested the hypothesis that deficits in adenosine monophosphate-activated protein kinase/PGC-1α signalling in sensory neurons underlie impaired axonal plasticity, suboptimal mitochondrial function and development of neuropathy in rodent models of type 1 and type 2 diabetes. Phosphorylation and expression of adenosine monophosphate-activated protein kinase/PGC-1α and mitochondrial respiratory chain complex proteins were downregulated in dorsal root ganglia of both streptozotocin-diabetic rats and db/db mice. Adenoviral-mediated manipulation of endogenous adenosine monophosphate-activated protein kinase activity using mutant proteins modulated neurotrophin-directed neurite outgrowth in cultures of sensory neurons derived from adult rats. Addition of resveratrol to cultures of sensory neurons derived from rats after 3-5 months of streptozotocin-induced diabetes, significantly elevated adenosine monophosphate-activated protein kinase levels, enhanced neurite outgrowth and normalized mitochondrial inner membrane polarization in axons. The bioenergetics profile (maximal oxygen consumption rate, coupling efficiency, respiratory control ratio and spare respiratory capacity) was aberrant in cultured sensory neurons from streptozotocin-diabetic rats and was corrected by resveratrol treatment. Finally, resveratrol treatment for the last 2 months of a 5-month period of diabetes reversed thermal hypoalgesia and attenuated foot skin intraepidermal nerve fibre loss and reduced myelinated fibre mean axonal calibre in streptozotocin-diabetic rats. These data suggest that the development of distal axonopathy in diabetic neuropathy is linked to nutrient excess and mitochondrial dysfunction via defective signalling of the adenosine monophosphate-activated protein kinase/PGC-1α pathway.

Kruppel-like factor 15 regulates skeletal muscle lipid flux and exercise adaptation

The ability of skeletal muscle to enhance lipid utilization during exercise is a form of metabolic plasticity essential for survival. Conversely, metabolic inflexibility in muscle can cause organ dysfunction and disease. Although the transcription factor Kruppel-like factor 15 (KLF15) is an important regulator of glucose and amino acid metabolism, its endogenous role in lipid homeostasis and muscle physiology is unknown. Here we demonstrate that KLF15 is essential for skeletal muscle lipid utilization and physiologic performance. KLF15 directly regulates a broad transcriptional program spanning all major segments of the lipid-flux pathway in muscle. Consequently, Klf15-deficient mice have abnormal lipid and energy flux, excessive reliance on carbohydrate fuels, exaggerated muscle fatigue, and impaired endurance exercise capacity. Elucidation of this heretofore unrecognized role for KLF15 now implicates this factor as a central component of the transcriptional circuitry that coordinates physiologic flux of all three basic cellular nutrients: glucose, amino acids, and lipids.