Cloning of the rat pyruvate dehydrogenase kinase 4 gene promoter: activation of pyruvate dehydrogenase kinase 4 by the peroxisome proliferator-activated receptor gamma coactivator

CcpA-Knockout Staphylococcus aureus Induces Abnormal Metabolic Phenotype via the Activation of Hepatic STAT5/PDK4 Signaling in Diabetic Mice

Catabolite control protein A (CcpA), an important global regulatory protein, is extensively found in S. aureus. Many studies have reported that CcpA plays a pivotal role in regulating the tricarboxylic acid cycle and pathogenicity. Moreover, the CcpA-knockout Staphylococcus aureus (S. aureus) in diabetic mice, compared with the wild-type, showed a reduced colonization rate in the tissues and organs and decreased inflammatory factor expression. However, the effect of CcpA-knockout S. aureus on the host's energy metabolism in a high-glucose environment and its mechanism of action remain unclear. S. aureus, a common and major human pathogen, is increasingly found in patients with obesity and diabetes, as recent clinical data reveal. To address this issue, we generated CcpA-knockout S. aureus strains with different genetic backgrounds to conduct in-depth investigations. In vitro experiments with high-glucose-treated cells and an in vivo model study with type 1 diabetic mice were used to evaluate the unknown effect of CcpA-knockout strains on both the glucose and lipid metabolism phenotypes of the host. We found that the strains caused an abnormal metabolic phenotype in type 1 diabetic mice, particularly in reducing random and fasting blood glucose and increasing triglyceride and fatty acid contents in the serum. In a high-glucose environment, CcpA-knockout S. aureus may activate the hepatic STAT5/PDK4 pathway and affect pyruvate utilization. An abnormal metabolic phenotype was thus observed in diabetic mice. Our findings provide a better understanding of the molecular mechanism of glucose and lipid metabolism disorders in diabetic patients infected with S. aureus.

Interference with the HNF4-dependent gene regulatory network diminishes ER stress in hepatocytes

In all eukaryotic cell types, the unfolded protein response (UPR) upregulates factors that promote protein folding and misfolded protein clearance to help alleviate endoplasmic reticulum (ER) stress. Yet ER stress in the liver is uniquely accompanied by the suppression of metabolic genes, the coordination and purpose of which is largely unknown. Here, we used unsupervised machine learning to identify a cluster of correlated genes that were profoundly suppressed by persistent ER stress in the liver. These genes, which encode diverse functions including metabolism, coagulation, drug detoxification, and bile synthesis, are likely targets of the master regulator of hepatocyte differentiation HNF4α. The response of these genes to ER stress was phenocopied by liver-specific deletion of HNF4 α. Strikingly, while deletion of HNF4α exacerbated liver injury in response to an ER stress challenge, it also diminished UPR activation and partially preserved ER ultrastructure, suggesting attenuated ER stress. Conversely, pharmacological maintenance of hepatocyte identity in vitro enhanced sensitivity to stress. Several pathways potentially link HNF4α to ER stress sensitivity, including control of expression of the tunicamycin transporter MFSD2A; modulation of IRE1/XBP1 signaling; and regulation of Pyruvate Dehydrogenase. Together, these findings suggest that HNF4α activity is linked to hepatic ER homeostasis through multiple mechanisms.

Combined metabolomic and transcriptomic profiling approaches reveal the cardiac response to high-fat diet

The response of vital organs to different types of nutrition or diet is a fundamental question in physiology. We examined the cardiac response to 4 weeks of high-fat diet in mice, measuring cardiac metabolites and mRNA. Metabolomics showed dramatic differences after a high-fat diet, including increases in several acyl-carnitine species. The RNA-seq data showed changes consistent with adaptations to use more fatty acid as substrate and an increase in the antioxidant protein catalase. Changes in mRNA were correlated with changes in protein level for several highly responsive genes. We also found significant sex differences in both metabolomics and RNA-seq datasets, both at baseline and after high fat diet. This work reveals the response of a vital organ to dietary intervention at both metabolomic and transcriptomic levels, which is a fundamental question in physiology. This work also reveals significant sex differences in cardiac metabolites and gene expression.

Pyruvate dehydrogenase kinases (PDKs): an overview toward clinical applications

Pyruvate dehydrogenase kinase (PDK) can regulate the catalytic activity of pyruvate decarboxylation oxidation via the mitochondrial pyruvate dehydrogenase complex, and it further links glycolysis with the tricarboxylic acid cycle and ATP generation. This review seeks to elucidate the regulation of PDK activity in different species, mainly mammals, and the role of PDK inhibitors in preventing increased blood glucose, reducing injury caused by myocardial ischemia, and inducing apoptosis of tumor cells. Regulations of PDKs expression or activity represent a very promising approach for treatment of metabolic diseases including diabetes, heart failure, and cancer. The future research and development could be more focused on the biochemical understanding of the diseases, which would help understand the cellular energy metabolism and its regulation by pharmacological effectors of PDKs.

Reprogramming of glucose metabolism of cumulus cells and oocytes and its therapeutic significance

The aim of this review is to summarize our current understanding of the molecular mechanism for the glucose metabolism, especially pyruvate dehydrogenase (PDH), during oocyte maturation, as well as future perspectives of therapeutic strategies for aging focusing on metabolic regulation between aerobic glycolysis and the tricarboxylic acid (TCA) cycle/oxidative phosphorylation (OXPHOS). Each keyword alone or in combination was used to search from PubMed. Glucose metabolism is a dynamic process involving "On" and "Off" switches by the pyruvate dehydrogenase kinase (PDK)-PDH axis, which is crucial for energy metabolism and mitochondrial efficiency in cumulus cell differentiation and oocyte maturation. Activation of PDK suppresses the conversion of pyruvate to acetyl-coenzyme A (acetyl-CoA) through the inactivation of PDH, which allows the cumulus cells to supply sufficient amounts of pyruvate, lactate, and nicotinamide adenine dinucleotide phosphate (NADPH) to the oocytes. On the other hand, inactivation of PDK in oocytes can produce adenosine triphosphate (ATP) through a metabolic shift from aerobic glycolysis to the TCA cycle/OXPHOS. The metabolic balance between aerobic glycolysis and TCA cycle/OXPHOS presents us with a number of enzymes, ligands, receptors, and antioxidants that are potential therapeutic targets, some of which have already been successfully pursued to improve fertility outcomes. However, there are also many reports that question their efficacy. In conclusion, understanding the molecular mechanisms involved in the PDK-PDH axis is a crucial step to advance in novel therapeutic strategies to improve oocyte quality.

The less weight loss due to modest food restriction drove more fat accumulation in striped hamsters refed with high-fat diet

Food restriction (FR) has been commonly used to decrease body fat, reducing the risk of overweight in humans and animals. However, the lost weight has been shown to be followed by overweight when food restriction ends. It remains uncertain whether the weight loss drives the overweight, or not. In the present study, striped hamsters were restricted by 15%, 30% and 40% of ad libitum food intake for 2 weeks, followed by high-fat refeeding for 6 weeks (FR15%-Re, FR30%-Re and FR40%-Re). The hamsters in FR15%, FR30% and FR40% groups decreased by 21.1%, 37.8% and 50.0% in fat mass (P < 0.01), and 16.8%, 42.8% and 53.4% in leptin levels (P < 0.01) compared with the hamsters fed ad libitum. The FR15%-Re, FR30%-Re and FR40%-Re groups showed 77.0%, 37.2% and 23.7% more body fat than ad libitum group (P < 0.01). The FR15%-Re group showed considerable decreases in gene expression of arcuate nucleus co-expressing proopiomelanocortin (POMC), cocaine - and amphetamineregulated transcript (CART) and the long isoform of leptin receptor (LepRb) in the hypothalamus and of several genes associated with fatty acid transport to mitochondria and β-oxidation in brown adipose tissue and liver. It suggests that less weight loss is likely to drive more fat accumulation when food restriction ends, in which the impaired function of LepRb, POMC and CART in the brain and fatty acid oxidation in brown adipose tissue and liver may be involved.

Effects of the Activation of Three Major Hepatic Akt Substrates on Glucose Metabolism in Male Mice

Insulin suppresses glucose output from the liver via Akt activation; however, which substrate of Akt plays the major role in transducing this effect is unclear. We tested the postnatal expression of Akt-unresponsive, constitutively active mutants of three major Akt substrates widely considered to regulate glucose metabolism [i.e., FoxO1, PGC1α, and glycogen synthase kinase-3β (GSK3β)] using adenoviral gene delivery to the mouse liver. We performed physiological hyperinsulinemic-euglycemic clamp studies using these mice under awake and nonrestrained conditions with blood sampling via an arterial catheter. Hepatic expression of constitutively active FoxO1 induced significant hepatic and systemic insulin resistance. However, neither the expression of constitutively active PGC1α nor that of GSK3β significantly changed insulin sensitivity. Simultaneous expression of all three mutants together induced no further insulin resistance compared with that of the FoxO1 mutant. The glycogen content in the liver was significantly reduced by constitutively active GSK3β expression. In cultured hepatocytes, constitutively active PGC1α induced markedly stronger transcriptional enhancement of gluconeogenic key enzymes than did constitutively active FoxO1. From these results, we conclude that FoxO1 has the most prominent role in transducing insulin's effect downstream from Akt to suppress hepatic glucose output, involving mechanisms independent of the transcriptional regulation of key gluconeogenic enzymes.

FoxO1 regulates myocardial glucose oxidation rates via transcriptional control of pyruvate dehydrogenase kinase 4 expression

Pyruvate dehydrogenase (PDH) is the rate-limiting enzyme for glucose oxidation and a critical regulator of metabolic flexibility during the fasting to feeding transition. PDH is regulated via both PDH kinases (PDHK) and PDH phosphatases, which phosphorylate/inactivate and dephosphorylate/activate PDH, respectively. Our goal was to determine whether the transcription factor forkhead box O1 (FoxO1) regulates PDH activity and glucose oxidation in the heart via increasing the expression of Pdk4, the gene encoding PDHK4. To address this question, we differentiated H9c2 myoblasts into cardiac myocytes and modulated FoxO1 activity, after which Pdk4/PDHK4 expression and PDH phosphorylation/activity were assessed. We assessed binding of FoxO1 to the Pdk4 promoter in cardiac myocytes in conjunction with measuring the role of FoxO1 on glucose oxidation in the isolated working heart. Both pharmacological (1 µM AS1842856) and genetic (siRNA mediated) inhibition of FoxO1 decreased Pdk4/PDHK4 expression and subsequent PDH phosphorylation in H9c2 cardiac myocytes, whereas 10 µM dexamethasone-induced Pdk4/PDHK4 expression was abolished via pretreatment with 1 µM AS1842856. Furthermore, transfection of H9c2 cardiac myocytes with a vector expressing FoxO1 increased luciferase activity driven by a Pdk4 promoter construct containing the FoxO1 DNA-binding element region, but not in a Pdk4 promoter construct lacking this region. Finally, AS1842856 treatment in fasted mice enhanced glucose oxidation rates during aerobic isolated working heart perfusions. Taken together, FoxO1 directly regulates Pdk4 transcription in the heart, thereby controlling PDH activity and subsequent glucose oxidation rates.NEW & NOTEWORTHY Although studies have shown an association between FoxO1 activity and pyruvate dehydrogenase kinase 4 expression, our study demonstrated that pyruvate dehydrogenase kinase 4 is a direct transcriptional target of FoxO1 (but not FoxO3/FoxO4) in the heart. Furthermore, we report here, for the first time, that FoxO1 inhibition increases glucose oxidation in the isolated working mouse heart.

Early induction of pyruvate dehydrogenase kinase 4 by retinoic acids in adipocytes

SCOPE

Vitamin A and its metabolites, such as retinoic acids (RA), are related to metabolic diseases, in particular insulin resistance and obesity. Here, we studied the roles of 9-cis RA and all-trans RA on the regulation of pyruvate dehydrogenase kinase 4 (PDK4), an enzyme involved in fatty acid reesterification, which is a crucial metabolic pathway in adipose tissue (AT) lipid homeostasis.

METHODS AND RESULTS

9-cis RA and all-trans RA treatment of human and murine AT explants, as well as adipocytes (3T3-F442A cell line) induces PDK4 expression both at the mRNA and the protein level, via a transcriptional mechanism. Using site-directed mutagenesis and chomatin immuno-precipitation, we showed that this activation involves two new RA responsive elements in the Pdk4 promoter, RAREa (DR1: -125/-112) and RAREb (DR1: -86/-73), specific to AT. Furthermore, even though endogeneous Pdk4 gene was upregulated by RA in Fao cells, a rat hepatoma cell line, the induction did not occur through the newly found RAREs.

CONCLUSION

In this study, we showed that adipocyte PDK4 gene is a new target of the vitamin A derived RA and might participate to the reduced fatty acid efflux from the adipocyte, a step that plays an important role in the developement of metabolic diseases.

High protein/fish oil diet prevents hepatic steatosis in NONcNZO10 mice; association with diet/genetics-regulated micro-RNAs

OBJECTIVE

NONcNZO10 (NZ10) mice are predisposed to obesity and develop type 2 diabetes (T2D) and hepatic steatosis even when maintained on a control diet (CD) of 6% fat. Studies were designed to determine whether this extreme susceptibility phenotype could be alleviated by diet and if so the molecular targets of diet.

METHODS

NZ10 and SWR/J (SWR) control mice were fed a CD or a test diet of high protein and fish oil (HPO) for 19 weeks and then analyzed for steatosis, blood chemistry, hepatic gene and micro-RNA expression.

RESULTS

HPO diet prevented steatosis, significantly increased serum adiponectin and reduced serum cholesterol and triglycerides only in NZ10 mice. The HPO diet repressed hepatic expression of fatty acid metabolic regulators including PPAR-γ, sterol regulatory element-binding protein-c1, peroxisome proliferator-activated receptor gamma co-activator-1, fatty acid synthase, fatty acid binding protein-4, and apolipoprotein A4 genes only in NZ10 mice. Also repressed by a HPO diet were adiponectinR2 receptor, leptin-R, PPAR-α, pyruvate dehydrogenase kinase isoforms 2 and 4, AKT2 and GSK3β. Micro-RNA (miR) arrays identified miRs that were diet and/or genetics regulated. QRTPCR confirmed increased expression of miR-205 and suppression of a series of miRs including miRs-411, 155, 335 and 21 in the NZ10-HPO group, each of which are implicated in the progression of diabetes and/or steatosis. Evidence is presented that miR-205 co-regulates with PPARγ and may regulate fibrosis and EMT during the progression of steatosis in the livers of NZ10-CD mice. The dietary responses of miR-205 are tissue-specific with opposite effects in adipose and liver.

CONCLUSION

The results confirm that a HPO diet overrides the genetic susceptibility of NZ10 mice and this correlates with the suppression of key genes and perhaps micro-RNAs involved in hyperglycemia, dyslipidemia and inflammation including master PPAR regulators, adiponectin and leptin receptors.

Forkhead box transcription factor 1: role in the pathogenesis of diabetic cardiomyopathy

Diabetic cardiomyopathy (DCM) is a disorder of the heart muscle in people with diabetes that can occur independent of hypertension or vascular disease. The underlying mechanism of DCM is incompletely understood. Some transcription factors have been suggested to regulate the gene program intricate in the pathogenesis of diabetes prompted cardiac injury. Forkhead box transcription factor 1 is a pleiotropic transcription factor that plays a pivotal role in a variety of physiological processes. Altered FOXO1 expression and function have been associated with cardiovascular diseases, and the important role of FOXO1 in DCM has begun to attract attention. In this review, we focus on the FOXO1 pathway and its role in various processes that have been related to DCM, such as metabolism, oxidative stress, endothelial dysfunction, inflammation and apoptosis.

Soybean Oil Is More Obesogenic and Diabetogenic than Coconut Oil and Fructose in Mouse: Potential Role for the Liver

The obesity epidemic in the U.S. has led to extensive research into potential contributing dietary factors, especially fat and fructose. Recently, increased consumption of soybean oil, which is rich in polyunsaturated fatty acids (PUFAs), has been proposed to play a causal role in the epidemic. Here, we designed a series of four isocaloric diets (HFD, SO-HFD, F-HFD, F-SO-HFD) to investigate the effects of saturated versus unsaturated fat, as well as fructose, on obesity and diabetes. C57/BL6 male mice fed a diet moderately high in fat from coconut oil and soybean oil (SO-HFD, 40% kcal total fat) showed statistically significant increases in weight gain, adiposity, diabetes, glucose intolerance and insulin resistance compared to mice on a diet consisting primarily of coconut oil (HFD). They also had fatty livers with hepatocyte ballooning and very large lipid droplets as well as shorter colonic crypt length. While the high fructose diet (F-HFD) did not cause as much obesity or diabetes as SO-HFD, it did cause rectal prolapse and a very fatty liver, but no balloon injury. The coconut oil diet (with or without fructose) increased spleen weight while fructose in the presence of soybean oil increased kidney weight. Metabolomics analysis of the liver showed an increased accumulation of PUFAs and their metabolites as well as γ-tocopherol, but a decrease in cholesterol in SO-HFD. Liver transcriptomics analysis revealed a global dysregulation of cytochrome P450 (Cyp) genes in SO-HFD versus HFD livers, most notably in the Cyp3a and Cyp2c families. Other genes involved in obesity (e.g., Cidec, Cd36), diabetes (Igfbp1), inflammation (Cd63), mitochondrial function (Pdk4) and cancer (H19) were also upregulated by the soybean oil diet. Taken together, our results indicate that in mice a diet high in soybean oil is more detrimental to metabolic health than a diet high in fructose or coconut oil.

Role of sirtuin 1 in the regulation of hepatic gene expression by thyroid hormone

Sirtuin 1 (SIRT1) is a nuclear deacetylase that modulates lipid metabolism and enhances mitochondrial activity. SIRT1 targets multiple transcription factors and coactivators. Thyroid hormone (T(3)) stimulates the expression of hepatic genes involved in mitochondrial fatty acid oxidation and gluconeogenesis. We reported that T(3) induces genes for carnitine palmitoyltransferase (cpt1a), pyruvate dehydrogenase kinase 4 (pdk4), and phosphoenolpyruvate carboxykinase (pepck). SIRT1 increases the expression of these genes via the activation of several factors, including peroxisome proliferator-activated receptor α, estrogen-related receptor α, and peroxisome proliferator-activated receptor γ coactivator (PGC-1α). Previously, we reported that PGC-1α participates in the T(3) induction of cpt1a and pdk4 in the liver. Given the overlapping targets of T(3) and SIRT1, we investigated whether SIRT1 participated in the T(3) regulation of these genes. Resveratrol is a small phenolic compound whose actions include the activation of SIRT1. Addition of resveratrol increased the T(3) induction of the pdk4 and cpt1a genes in hepatocytes. Furthermore, expression of SIRT1 in hepatocytes mimicked resveratrol in the regulation of gene expression by T(3). The deacetylase activity of SIRT1 was required and PGC-1α was deacetylated following addition of T(3). We found that SIRT1 interacted directly with T(3) receptor (TRβ). Knockdown of SIRT1 decreased the T(3) induction of cpt1a and pdk4 and reduced the T(3) inhibition of sterol response element binding protein (srebp-1c) both in isolated hepatocytes and in rat liver. Our results indicate that SIRT1 contributes to the T(3) regulation of hepatic genes.

Genome-wide analysis of FoxO1 binding in hepatic chromatin: potential involvement of FoxO1 in linking retinoid signaling to hepatic gluconeogenesis

The forkhead transcription factor FoxO1 is a critical regulator of hepatic glucose and lipid metabolism, and dysregulation of FoxO1 function has been implicated in diabetes and insulin resistance. We globally identified FoxO1 occupancy in mouse hepatic chromatin on a genome-wide level by chromatin immunoprecipitation coupled with high-throughput DNA sequencing (ChIP-seq). To establish the specific functional significance of FoxO1 against other FoxO proteins, ChIP-seq was performed with chromatin from liver-specific FoxO1 knockout and wild-type mice. Here we identified 401 genome-wide FoxO1-binding locations. Motif search reveals a sequence element, 5′ GTAAACA 3′, consistent with a previously known FoxO1-binding site. Gene set enrichment analysis shows that the data from FoxO1 ChIP-seq are highly correlated with the global expression profiling of genes regulated by FoxO1, demonstrating the functional relevance of our FoxO1 ChIP-seq study. Interestingly, gene ontology analysis reveals the functional significance of FoxO1 in retinoid metabolic processes. We show here that FoxO1 directly binds to the genomic sites for the genes in retinoid metabolism. Notably, deletion of FoxO1 caused a significantly reduced induction of Pck1 and Pdk4 in response to retinoids. As Pck1 and Pdk4 are downstream targets of retinoid signaling, these results suggest that FoxO1 plays a potential role in linking retinoid metabolism to hepatic gluconeogenesis.

Regulation of pyruvate dehydrogenase kinase 4 (PDK4) by CCAAT/enhancer-binding protein beta (C/EBPbeta)

The conversion of pyruvate to acetyl-CoA in mitochondria is catalyzed by the pyruvate dehydrogenase complex (PDC). Activity of PDC is inhibited by phosphorylation via the pyruvate dehydrogenase kinases (PDKs). Here, we examined the regulation of Pdk4 gene expression by the CCAAT/enhancer-binding protein β (C/EBPβ). C/EBPβ modulates the expression of multiple hepatic genes including those involved in metabolism, development, and inflammation. We found that C/EBPβ induced Pdk4 gene expression and decreased PDC activity. This transcriptional induction was mediated through two C/EBPβ binding sites in the Pdk4 promoter. C/EBPβ participates in the hormonal regulation of gluconeogenic genes. Previously, we reported that Pdk4 was induced by thyroid hormone (T(3)). Therefore, we investigated the role of C/EBPβ in the T(3) regulation of Pdk4. T(3) increased C/EBPβ abundance in primary rat hepatocytes. Knockdown of C/EBPβ with siRNA diminished the T(3) induction of the Pdk4 and carnitine palmitoyltransferase (Cpt1a) genes. CPT1a is an initiating step in the mitochondrial oxidation of long chain fatty acids. Our results indicate that C/EBPβ stimulates Pdk4 expression and participates in the T(3) induction of the Cpt1a and Pdk4 genes.

Role of PPARα in Hepatic Carbohydrate Metabolism

Tight control of storage and synthesis of glucose during nutritional transitions is essential to maintain blood glucose levels, a process in which the liver has a central role. PPARα is the master regulator of lipid metabolism during fasting, but evidence is emerging for a role of PPARα in balancing glucose homeostasis as well. By using PPARα ligands and PPARα(-/-) mice, several crucial genes were shown to be regulated by PPARα in a direct or indirect way. We here review recent evidence that PPARα contributes to the adaptation of hepatic carbohydrate metabolism during the fed-to-fasted or fasted-to-fed transition in rodents.

Peroxisome proliferator activated receptor alpha (PPARalpha) and PPAR gamma coactivator (PGC-1alpha) induce carnitine palmitoyltransferase IA (CPT-1A) via independent gene elements

Long chain fatty acids and pharmacologic ligands for the peroxisome proliferator activated receptor alpha (PPARalpha) activate expression of genes involved in fatty acid and glucose oxidation including carnitine palmitoyltransferase-1A (CPT-1A) and pyruvate dehydrogenase kinase 4 (PDK4). CPT-1A catalyzes the transfer of long chain fatty acids from acyl-CoA to carnitine for translocation across the mitochondrial membranes and is an initiating step in the mitochondrial oxidation of long chain fatty acids. PDK4 phosphorylates and inhibits the pyruvate dehydrogenase complex (PDC) which catalyzes the conversion of pyruvate to acetyl-CoA in the glucose oxidation pathway. The activity of CPT-1A is modulated both by transcriptional changes as well as by malonyl-CoA inhibition. In the liver, CPT-1A and PDK4 gene expression are induced by starvation, high fat diets and PPARalpha ligands. Here, we characterized a binding site for PPARalpha in the second intron of the rat CPT-1A gene. Our studies indicated that WY14643 and long chain fatty acids induce CPT-1A gene expression through this element. In addition, we found that mutation of the PPARalpha binding site reduced the expression of CPT-1A-luciferase vectors in the liver of fasted rats. We had demonstrated previously that CPT-1A was stimulated by the peroxisome proliferator activated receptor gamma coactivator (PGC-1) via sequences in the first intron of the rat CPT-1A gene. Surprisingly, PGC-1alpha did not enhance CPT-1A transcription through the PPARalpha binding site in the second intron. Following knockdown of PGC-1alpha with short hairpin RNA, the CPT-1A and PDK4 genes remained responsive to WY14643. Overall, our studies indicated that PPARalpha and PGC-1alpha stimulate transcription of the CPT-1A gene through different regions of the CPT-1A gene.

Regulation of pyruvate dehydrogenase kinase isoform 4 (PDK4) gene expression by glucocorticoids and insulin

The pyruvate dehydrogenase complex (PDC) catalyzes the conversion of pyruvate to acetyl-CoA in mitochondria and is a key regulatory enzyme in the oxidation of glucose to acetyl-CoA. Phosphorylation of PDC by the pyruvate dehydrogenase kinases (PDK) inhibits its activity. The expression of the pyruvate dehydrogenase kinase 4 (PDK4) gene is increased in fasting and other conditions associated with the switch from the utilization of glucose to fatty acids as an energy source. Transcription of the PDK4 gene is elevated by glucocorticoids and inhibited by insulin. In this study, we have investigated the factors involved in the regulation of the PDK4 gene by these hormones. Glucocorticoids stimulate PDK4 through two glucocorticoid receptor (GR) binding sites located more than 6000 base pairs upstream of the transcriptional start site. Insulin inhibits the glucocorticoid induction in part by causing dissociation of the GR from the promoter. Previously, we found that the estrogen related receptor alpha (ERRalpha) stimulates the expression of PDK4. Here, we determined that one of the ERRalpha binding sites contributes to the insulin inhibition of PDK4. A binding site for the forkhead transcription factor (FoxO1) is adjacent to the ERRalpha binding sites. FoxO1 participates in the glucocorticoid induction of PDK4 and the regulation of this gene by insulin. Our data demonstrate that glucocorticoids and insulin each modulate PDK4 gene expression through complex hormone response units that contain multiple factors.

Regulation of pyruvate dehydrogenase kinase 4 (PDK4) by thyroid hormone: role of the peroxisome proliferator-activated receptor gamma coactivator (PGC-1 alpha)

PDK4 (pyruvate dehydrogenase kinase 4) regulates pyruvate oxidation through the phosphorylation and inhibition of the pyruvate dehydrogenase complex (PDC). PDC catalyzes the conversion of pyruvate to acetyl-CoA and is an important control point in glucose and pyruvate metabolism. PDK4 gene expression is stimulated by thyroid hormone (T(3)), glucocorticoids, and long chain fatty acids. The effects of T(3) on gene expression in the liver are mediated via the thyroid hormone receptor. Here, we have identified two binding sites for thyroid hormone receptor beta in the promoter of the rat PDK4 (rPDK4) gene. In addition, we have investigated the role of transcriptional coactivators and found that the PGC-1 alpha (peroxisome proliferator-activated receptor gamma coactivator) enhances the T(3) induction of rPDK4. Following T(3) administration, there is an increase in the association of PGC-1 alpha with the rPDK4 promoter. Interestingly, this increased association is with the proximal rPDK4 promoter rather than the distal region of the gene that contains the T(3) response elements. Administration of T(3) to hypothyroid rats elevated the abundance of PGC-1 alpha mRNA and protein in the liver. In addition, we observed greater association of PGC-1 alpha not only with the rPDK4 gene but also with phosphoenolpyruvate carboxykinase and CPT-1a (carnitine palmitoyltransferase 1a) genes. Knockdown of PGC-1 alpha in rat hepatocytes reduced the T(3) induction of PDK4, PEPCK, and CPT-1a genes. Our results indicate that T(3) regulates PGC-1 alpha abundance and association with hepatic genes, and in turn PGC-1 alpha is an important participant in the T(3) induction of selected genes.

DAX-1 acts as a novel corepressor of orphan nuclear receptor HNF4alpha and negatively regulates gluconeogenic enzyme gene expression

DAX-1 (dosage-sensitive sex reversal adrenal hypoplasia congenital critical region on X chromosome, gene 1) is an atypical member of the nuclear receptor family and acts as a corepressor of a number of nuclear receptors. HNF4alpha (hepatocyte nuclear factor 4alpha) is a liver-enriched transcription factor that controls the expression of a variety of genes involved in cholesterol, fatty acid, and glucose metabolism. Here we show that DAX-1 inhibits transcriptional activity of HNF4alpha and modulates hepatic gluconeogenic gene expression. Hepatic DAX-1 expression is increased by insulin and SIK1 (salt-inducible kinase 1), whereas it is decreased in high fat diet-fed and diabetic mice. Coimmunoprecipitation assay from mouse liver samples depicts that endogenous DAX-1 interacts with HNF4alpha in vivo. In vivo chromatin immunoprecipitation assay affirms that the recruitment of DAX-1 on the phosphoenolpyruvate carboxykinase (PEPCK) gene promoter is inversely correlated with the recruitment of PGC-1alpha and HNF4alpha under fasting and refeeding, showing that DAX-1 could compete with the coactivator PGC-1alpha for binding to HNF4alpha. Adenovirus-mediated expression of DAX-1 decreased both HNF4alpha- and forskolin-mediated gluconeogenic gene expressions. In addition, knockdown of DAX-1 partially reverses the insulin-mediated inhibition of gluconeogenic gene expression in primary hepatocytes. Finally, DAX-1 inhibits PEPCK and glucose-6-phosphatase gene expression and significantly lowers fasting blood glucose level in high fat diet-fed mice, suggesting that DAX-1 can modulate hepatic gluconeogenesis in vivo. Overall, this study demonstrates that DAX-1 acts as a corepressor of HNF4alpha to negatively regulate hepatic gluconeogenic gene expression in liver.

Comparative study of sesame lignans (sesamin, episesamin and sesamolin) affecting gene expression profile and fatty acid oxidation in rat liver

The impact of sesamin, episesamin and sesamolin (sesame lignans) on hepatic gene expression profiles was compared with a DNA microarray. Male Sprague-Dawley rats were fed experimental diets containing 0.2% sesamin, episesamin or sesamolin, and a control diet free of lignans for 15 d. Compared to a lignan-free diet, a diet containing sesamin, episesamin and sesamolin caused 1.5- and 2-fold changes in the expression of 128 and 40, 526 and 152, and 516 and 140 genes, respectively. The lignans modified not only the mRNA levels of many enzymes involved in hepatic fatty acid oxidation, but also those of proteins involved in the transportation of fatty acids into hepatocytes and their organelles, and regulating hepatic concentrations of carnitine, CoA and malonyl-CoA. It is apparent that sesame lignans stimulate hepatic fatty acid oxidation by affecting the gene expression of various proteins regulating hepatic fatty acid metabolism. We also observed that lignans modified the gene expression of various proteins involved in hepatic lipogenesis, cholesterogenesis and glucose metabolism. The changes were generally greater with episesamin and sesamolin than with sesamin. In terms of the amounts accumulated in serum and the liver, the lignans ranked in the order sesamolin, episesamin and sesamin. The differences in bio-availability among these lignans appear to be important to their divergent physiological activities.

Rapid exercise-induced changes in PGC-1α mRNA and protein in human skeletal muscle

The mRNA of the nuclear coactivator peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) increases during prolonged exercise and is influenced by carbohydrate availability. It is unknown if the increases in mRNA reflect the PGC-1α protein or if glycogen stores are an important regulator. Seven male subjects [23 ± 1.3 yr old, maximum oxygen uptake (V̇o2 max) 48.4 ± 0.8 ml·kg−1·min−1] exercised to exhaustion (∼2 h) at 65% V̇o2 max followed by ingestion of either a high-carbohydrate (HC) or low-carbohydrate (LC) diet (7 or 2.9 g·kg−1·day−1, respectively) for 52 h of recovery. Glycogen remained depressed in LC ( P < 0.05) while returning to resting levels by 24 h in HC. PGC-1α mRNA increased both at exhaustion (3-fold) and 2 h later (6.2-fold) ( P < 0.05) but returned to rest levels by 24 h. PGC-1α protein increased ( P < 0.05) 23% at exhaustion and remained elevated for at least 24 h ( P < 0.05). While there was no direct treatment effect (HC vs. LC) for PGC-1α mRNA or protein, there was a linear relationship between the changes in glycogen and those in PGC-1α protein during exercise and recovery ( r = −0.68, P < 0.05). In contrast, PGC-1β did not increase with exercise but rather decreased ( P < 0.05) below rest level at 24 and 52 h, and the decrease was greater ( P < 0.05) in LC. PGC-1α protein content increased in prolonged exercise and remained upregulated for 24 h, but this could not have been predicted by the changes in mRNA. The β-isoform declined rather than increasing, and this was greater when glycogen was not resynthesized to rest levels.

Sequential changes in the expression of genes involved in lipid metabolism in adipose tissue and liver in response to fasting

The aim of this study was to provide a sequential analysis of the expression patterns of key genes involved in lipid metabolism in white adipose tissue (WAT) and liver and their relationship with blood parameters in response to fasting. Adult male rats were studied under different feeding conditions: feeding state, after 4, 8, or 24 h fasting, and after 3 h refeeding after 8 h fasting. Blood parameters and the expression of genes involved in lipogenesis and lipolysis in WAT and liver were analyzed. mRNA levels of genes involved in lipogenesis in liver (SREBP1c, FAS, and GPAT) had already decreased after 4 h fasting, as well as those of PPARgamma in WAT, whereas the decrease in SREBP1c, FAS, GPAT, and GLUT4 mRNA levels in WAT was observed after 8 h. Concerning lipolytic and fatty-acid-oxidation-related genes, liver PPARalpha, FGF21, CPT1, and PDK4 mRNA levels increased after 8 h fasting and those of ACOX1 after 24 h, and in WAT, ATGL, and CPT1 mRNA levels were greater after 24 h. Three hours refeeding increased the expression levels of PPARgamma in WAT, SREBP1c in both liver and WAT, and GPAT in liver, and decreased the expression levels of PPARalpha, CPT1, and PDK4 in liver. These results give new insight into the different adaptive time course response to fasting in the expression of genes involved in lipid metabolism, thus pointing out the very rapid response of lipogenic genes, particularly in liver, and the later response of lipolytic genes, particularly in WAT.

CCAAT/enhancer binding protein-beta is a transcriptional regulator of peroxisome-proliferator-activated receptor-gamma coactivator-1alpha in the regenerating liver

The transcriptional coactivator peroxisome-proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha) is induced in the liver in response to fasting and coordinates the activation of targets necessary for increasing energy production for gluconeogenesis and ketogenesis. After partial hepatectomy, the liver must restore its mass while maintaining metabolic homeostasis to ensure survival. Here we report that PGC-1alpha is rapidly and dramatically induced after hepatectomy, with an amplitude of induction that exceeds the fasting response. Maximal activation of PGC-1alpha after hepatectomy is dependent on the basic leucine zipper transcription factor, CCAAT/enhancer binding protein-beta (C/EBPbeta), a critical factor in hepatocyte proliferation. We demonstrate in vivo C/EBPbeta binding to C/EBP and cAMP response element sites in the PGC-1alpha promoter and show that the C/EBP site is essential for PGC-1alpha activation. Expression of the PGC-1alpha target, carnitine palmitoyl transferase 1a, the rate-limiting enzyme in fatty acid beta-oxidation, and of long-chain acyl-coenzyme A dehydrogenase, an enzyme involved in beta-oxidation of long chain fatty acids, was significantly reduced in C/EBPbeta(-/-) livers after hepatectomy. These findings identify C/EBPbeta as a direct activator of PGC-1alpha in the regenerating liver. The demonstration of a functional link between C/EBPbeta and PGC-1alpha activation provides a likely mechanism for how upstream signaling pathways in the regenerating liver can enable the adaptation to the changed metabolic status.

Genome-wide co-expression based prediction of differential expressions

MOTIVATION

Microarrays have been widely used for medical studies to detect novel disease-related genes. They enable us to study differential gene expressions at a genomic level. They also provide us with informative genome-wide co-expressions. Although many statistical methods have been proposed for identifying differentially expressed genes, genome-wide co-expressions have not been well considered for this issue. Incorporating genome-wide co-expression information in the differential expression analysis may improve the detection of disease-related genes.

RESULTS

In this study, we proposed a statistical method for predicting differential expressions through the local regression between differential expression and co-expression measures. The smoother span parameter was determined by optimizing the rank correlation between the observed and predicted differential expression measures. A mixture normal quantile-based method was used to transform data. We used the gene-specific permutation procedure to evaluate the significance of a prediction. Two published microarray data sets were analyzed for applications. For the data set collected for a prostate cancer study, the proposed method identified many genes with weak differential expressions. Several of these genes have been shown in literature to be associated with the disease. For the data set collected for a type 2 diabetes study, no significant genes could be identified by the traditional methods. However, the proposed method identified many genes with significantly low false discovery rates.

AVAILABILITY

The R codes are freely available at http://home.gwu.edu/~ylai/research/CoDiff, where the gene lists ranked by our method are also provided as the Supplementary Material.

PGC-1β is downregulated by training in human skeletal muscle: no effect of training twice every second day vs. once daily on expression of the PGC-1 family

We hypothesized that the peroxisome proliferator-activated receptor-γ coactivator-1 (PGC-1) family of transcriptional coactivators (PGC-1α, PGC-1β, and PRC) is differentially regulated by training once daily vs. training twice daily every second day and that this difference might be observed in the acute response to endurance exercise. Furthermore, we hypothesized that expression levels of the PGC-1 family differ with muscular fiber-type composition. Thus, before and after 10 wk of knee extensor endurance training, training one leg once daily and the other leg twice daily every second day, keeping the total amount of training for the legs equal, skeletal muscle mRNA expression levels of PGC-1α, PGC-1β, and PRC were determined in young healthy men ( n = 7) in response to 3 h of acute exercise. No significant difference was found between the two legs, suggesting that regulation of the PGC-1 family is independent of training protocol. Training decreased PGC-1β in both legs, whereas PGC-1α was increased, but not significantly, in the leg training once daily. PRC did not change with training. Both PGC-1α and PRC were increased by acute exercise both before and after endurance training, whereas PGC-1β did not change. The mRNA levels of the PGC-1 family were examined in different types of human skeletal muscle (triceps, soleus, and vastus lateralis; n = 7). Only the expression level of PGC-1β differed and correlated inversely with percentage of type I fibers. In conclusion, there was no difference between training protocols on the acute exercise and training response of the PGC-1 family. However, training caused a decrease in PGC-1β mRNA levels.

The STAT5A-mediated induction of pyruvate dehydrogenase kinase 4 expression by prolactin or growth hormone in adipocytes

The purpose of this study was to determine whether pyruvate dehydrogenase kinase (PDK)4 was expressed in adipocytes and whether PDK4 expression was hormonally regulated in fat cells. Both Northern blot and Western blot analyses were conducted on samples isolated from 3T3-L1 adipocytes after various treatments with prolactin (PRL), growth hormone (GH), and/or insulin. Transfection of PDK4 promoter reporter constructs was performed. In addition, glucose uptake measurements were conducted. Our studies demonstrate that PRL and porcine GH can induce the expression of PDK4 in 3T3-L1 adipocytes. Our studies also show that insulin pretreatment can attenuate the ability of these hormones to induce PDK4 mRNA expression. In addition, we identified a hormone-responsive region in the murine PDK4 promoter and characterized a STAT5 binding site in this region that mediates the PRL (sheep) and GH (porcine) induction in PDK4 expression in 3T3-L1 adipocytes. PDK4 is a STAT5A target gene. PRL is a potent inducer of PDK4 protein levels, results in an inhibition of insulin-stimulated glucose transport in fat cells, and likely contributes to PRL-induced insulin resistance.

Characterization of the transactivation domain in the peroxisome-proliferator-activated receptor gamma co-activator (PGC-1)

The PGC-1s (peroxisome-proliferator-activated receptor gamma co-activators) are a family of transcriptional regulators that induce the expression of various metabolic genes. PGC-1 proteins stimulate genes involved in mitochondrial biogenesis, fatty acid oxidation and hepatic gluconeogenesis. Previous studies have demonstrated that the PGC-1alpha and beta isoforms interact with nuclear receptors through the conserved LXXLL (leucine-X-X-leucine-leucine) motifs. In the present study, we have investigated the mechanisms by which these PGC-1 isoforms stimulate gene expression. We have determined that the N-terminus of PGC-1 is responsible for transcriptional activation. Two conserved peptide motifs were identified in the N-terminus of PGC-1alpha and beta isoforms. These domains were named AD1 and AD2 (activation domain 1 and 2). Deletion of both of these motifs decreased the induction of various PGC-1-regulated genes including the PEPCK (phosphoenolpyruvate carboxykinase) and CPT-I (carnitine palmitoyltransferase-I) genes. It was determined that amino acids containing a negative charge in AD1 and the leucine residues in AD2 were important for the transcriptional induction of the PEPCK and CPT-I genes. Disruption of the AD motifs did not diminish the ability of the PGC-1alpha protein to associate with the PEPCK or CPT-I genes. In addition, deletion of the AD domains did not eliminate the ability of PGC-1alpha to interact with the thyroid hormone receptor. The data indicate that the AD1 and AD2 motifs mediate the induction of many PGC-1- responsive genes, but they do not contribute to the recruitment of PGC-1 to target genes.

Effects of cannabinoid receptors on skeletal muscle oxidative pathways

The endocannabinoids, a recently discovered endogenous, lipid derived, signaling system regulating energy metabolism, have effects on central and peripheral energy metabolism predominantly via the cannabinoid receptor type 1 (CB1). CB1 is expressed centrally in the hypothalamus and nucleus accumbens and peripherally in adipocytes and skeletal muscle. This study determined the effect of endocannabinoids on the expression of genes regulating energy metabolism in human skeletal muscle. Primary cultures of myotubes (lean and obese; n=3/group) were treated with the cannabinoid receptor agonist, anandamide (AEA) (0.2 and 5microM) and the CB1 specific antagonist AM251 (0.2 and 5microM) separately and in combination for 24h. The expression of mRNA for AMP-activated protein kinase (AMPK) alpha 1 (alpha1) and alpha 2 (alpha2), pyruvate dehydrogenase kinase 4 (PDK4) and peroxisome proliferator-activated receptor-gamma co-activator-1alpha (PGC-1alpha) were determined using 'Real Time' RT-PCR. AMPKalpha1 mRNA increased in lean and obese myotubes in response to AM251 (P<0.05). AEA inhibited the effect of AM251 on AMPKalpha1 mRNA levels in myotubes from lean and obese subjects (P<0.05); the dose-response curve was shifted to the left in the obese. In response to AM251, irrespective of the presence of AEA, PDK4 expression was decreased in lean and obese myotubes (P<0.05). Taken together these data suggest that endocannabinoids regulate pathways affecting skeletal muscle oxidation, effects particularly evident in myotubes from obese individuals.

Regulation of carnitine palmitoyltransferase I (CPT-Ialpha) gene expression by the peroxisome proliferator activated receptor gamma coactivator (PGC-1) isoforms

The peroxisome proliferator activated receptor gamma coactivators (PGC-1) have important roles in mitochondrial biogenesis and metabolic control in a variety of tissues. There are multiple isoforms of PGC-1 including PGC-1alpha and PGC-1beta. Both the PGC-1alpha and beta isoforms promote mitochondrial biogenesis and fatty acid oxidation, but only PGC-1alpha stimulates gluconeogenesis in the liver. Carnitine palmitoyltransferase I (CPT-I) is a key enzyme regulating mitochondrial fatty acid oxidation. In these studies, we determined that PGC-1beta stimulated expression of the "liver" isoform of CPT-I (CPT-Ialpha) but that PGC-1beta did not induce pyruvate dehydrogenase kinase 4 (PDK4) which is a regulator of pyruvate metabolism. The CPT-Ialpha gene is induced by thyroid hormone. We found that T3 increased the expression of PGC-1beta and that PGC-1beta enhanced the T3 induction of CPT-Ialpha. The thyroid hormone receptor interacts with PGC-1beta in a ligand dependent manner. Unlike PGC-1alpha, the interaction of PGC-1beta and the T3 receptor does not occur exclusively through the leucine-X-X-leucine-leucine motif in PGC-1beta. We have found that PGC-1beta is associated with the CPT-Ialpha gene in vivo. Overall, our results demonstrate that PGC-1beta is a coactivator in the T3 induction of CPT-Ialpha and that PGC-1beta has similarities and differences with the PGC-1alpha isoform.

Glucocorticoids produce whole body insulin resistance with changes in cardiac metabolism

Insulin resistance is viewed as an insufficiency in insulin action, with glucocorticoids being recognized to play a key role in its pathogenesis. With insulin resistance, metabolism in multiple organ systems such as skeletal muscle, liver, and adipose tissue is altered. These metabolic alterations are widely believed to be important factors in the morbidity and mortality of cardiovascular disease. More importantly, clinical and experimental studies have established that metabolic abnormalities in the heart per se also play a crucial role in the development of heart failure. Following glucocorticoids, glucose utilization is compromised in the heart. This attenuated glucose metabolism is associated with altered fatty acid supply, composition, and utilization. In the heart, elevated fatty acid use has been implicated in a number of metabolic, morphological, and mechanical changes and, more recently, in "lipotoxicity". In the present article, we review the action of glucocorticoids, their role in insulin resistance, and their influence in modulating peripheral and cardiac metabolism and heart disease.

[Transcriptional regulation of metabolic switching PDK4 gene under various physiological conditions]

Pyruvate dehydrogenase kinase 4 (PDK4) phosphorylates and inactivates the pyruvate dehydrogenase complex to respond to physiologic conditions. This response switches the energy source from glucose to fatty acids to maintain blood glucose levels. Transcription of the PDK4 gene is activated by fasting or by the administration of a peroxisome proliferator-activated receptor alpha (PPARalpha) ligand in a tissue-specific manner. However, the two mechanisms to induce PDK4 mRNA as well as the relationship between the two have not been studied in detail. In this study, we show that the two mechanisms are independent, at least in the mouse skeletal muscle, and that estrogen-related receptor alpha (ERRalpha) is directly involved in the PPARalpha-independent transcriptional activation of the PDK4 gene with peroxisome proliferator-activated receptor gamma co-activator 1alpha (PGC-1alpha) as a specific partner. The latter conclusion is based on the following evidence: 1) Deletion and point mutation analyses of the cloned mouse PDK4 gene promoter sequence identified an exact possible ERRalpha-binding motif as the PGC-1alpha responsive element. 2) The overexpression of ERRalpha by cotransfection enhanced, and the knocking down of it by specific shRNAs diminished, the PGC-1alpha-dependent activation. 3) Specific binding of ERRalpha to the identified PGC-1alpha-responsive sequence of the mouse PDK4 promoter was confirmed in the electrophoresis mobility shift assay using anti-ERRalpha antibodies. These results suggest that PGC-1alpha plays an essential role not only in regulating the amounts of energy creating enzymes, but also at the step of metabolic switching with unevenly distributed tissue transcription factors such as ERRalpha in the skeletal muscle, thus harmonizing tissue-specific functions and energy metabolism.

Advances in molecular biology of hibernation in mammals

Mammalian hibernation is characterized by profound reductions in metabolism, oxygen consumption and heart rate. As a result, the animal enters a state of suspended animation where core body temperatures can plummet as low as -2.9 degrees C. Not only can hibernating mammals survive these physiological extremes, but they also return to a normothermic state of activity without reperfusion injury or other ill effects. This review examines recent findings on the genes, proteins and small molecules that control the induction and maintenance of hibernation in mammals. The molecular events involved with remodeling metabolism, inducing hypothermia and maintaining organ function are discussed and considered with respect to analogous processes in non-hibernating mammals such as mice and humans. The advent of sequenced genomes from three distantly related hibernators, a bat, hedgehog and ground squirrel, provides additional opportunities for molecular biologists to explore the mechanistic aspects of this biological adaptation in greater detail.

Estrogen-related receptors stimulate pyruvate dehydrogenase kinase isoform 4 gene expression

The pyruvate dehydrogenase complex (PDC) catalyzes the conversion of pyruvate to acetyl-CoA in mitochondria and is a key regulatory enzyme in the oxidation of glucose to acetyl-CoA. Phosphorylation of PDC by the pyruvate dehydrogenase kinases (PDK2 and PDK4) inhibits PDC activity. Expression of the PDK genes is elevated in diabetes, leading to the decreased oxidation of pyruvate to acetyl-CoA. In these studies we have investigated the transcriptional regulation of the PDK4 gene by the estrogen-related receptors (ERRalpha and ERRgamma). The ERRs are orphan nuclear receptors whose physiological roles include the induction of fatty acid oxidation in heart and muscle. Previously, we found that the peroxisome proliferator-activated receptor gamma coactivator (PGC-1alpha) stimulates the expression of PDK4. Here we report that ERRalpha and ERRgamma stimulate the PDK4 gene in hepatoma cells, suggesting a novel role for ERRs in controlling pyruvate metabolism. In addition, both ERR isoforms recruit PGC-1alpha to the PDK4 promoter. Insulin, which decreases the expression of the PDK4 gene, inhibits the induction of PDK4 by ERRalpha and ERRgamma. The forkhead transcription factor (FoxO1) binds the PDK4 gene and contributes to the induction of PDK4 by ERRs and PGC-1alpha. Insulin suppresses PDK4 expression in part through the dissociation of FoxO1 and PGC-1alpha from the PDK4 promoter. Our data demonstrate a key role for the ERRs in the induction of hepatic PDK4 gene expression.

Mechanisms underlying regulation of the expression and activities of the mammalian pyruvate dehydrogenase kinases

The mechanisms that control mammalian pyruvate dehydrogenase complex (PDC) activity include its phosphorylation (inactivation) by a family of pyruvate dehydrogenase kinases (PDKs 1 - 4). Here we review new developments in the regulation of the activities and expression of the PDKs, in particular PDK2 and PDK4, in relation to glucose and lipid homeostasis. This review describes recent advances relating to the acute and long-term modes of regulation of the PDKs, with particular emphasis on the regulatory roles of nuclear receptors including peroxisome proliferator-activated receptor (PPAR) alpha and Liver X receptor (LXR), PPAR gamma coactivator alpha (PGC-1alpha) and insulin, and the impact of changes in PDK activity and expression in glucose and lipid homeostasis. Since PDK4 may assist in lipid clearance when there is an imbalance between lipid delivery and oxidation, it may represent an attractive target for interventions aimed at rectifying abnormal lipid as well as glucose homeostasis in disease states.

Identification of ERRalpha as a specific partner of PGC-1alpha for the activation of PDK4 gene expression in muscle

Pyruvate dehydrogenase kinase 4 (PDK4) is a key regulatory enzyme involved in switching the energy source from glucose to fatty acids in response to physiological conditions. Transcription of the PDK4 gene is activated by fasting or by the administration of a PPARalpha ligand in a tissue-specific manner. Here, we show that the two mechanisms are independent, and that ERRalpha is directly involved in PPARalpha-independent transcriptional activation of the PDK4 gene with PGC-1alpha as a specific partner. This conclusion is based on the following evidence. First, detailed mutation analyses of the cloned PDK4 gene promoter sequence identified a possible ERRalpha-binding motif as the PGC-1alpha responsive element. Second, overexpression of ERRalpha by cotransfection enhanced, and the knockout of it by shRNAs diminished, PGC-1alpha-dependent activation. Third, specific binding of ERRalpha to the identified PGC-1alpha responsive sequence was confirmed by the electrophoresis mobility shift assay. Finally, cell-type-specific responsiveness to PGC-1alpha was observed and this could be explained by differences in the expression levels of ERRalpha, however, ectopic expression of ERRalpha in poorly responsive cells did not restore PGC-1alpha responsiveness, indicating that ERRalpha is necessary, but not sufficient for the response.

Regulation of the pyruvate dehydrogenase complex

The PDC (pyruvate dehydrogenase complex) plays a central role in the maintenance of glucose homoeostasis in mammals. The carbon flux through the PDC is meticulously controlled by elaborate mechanisms involving post-translational (short-term) phosphorylation/dephosphorylation and transcriptional (long-term) controls. The former regulatory mechanism involving multiple phosphorylation sites and tissue-specific distribution of the dedicated kinases and phosphatases is not only dependent on the interactions among the catalytic and regulatory components of the complex but also sensitive to the intramitochondrial redox state and metabolite levels as indicators of the energy status. Furthermore, differential transcriptional controls of the regulatory components of PDC further add to the complexity needed for long-term tuning of PDC activity for the maintenance of glucose homoeostasis during normal and disease states.