CREB regulates hepatic gluconeogenesis through the coactivator PGC-1

PGC-1α, un co-activateur transcriptionnel impliqué dans le métabolisme

Transcriptional coactivators can be important targets for physiologic regulation. PPARgamma coactivator-1alpha (PGC-1alpha), in cooperation with several transcription factors, has emerged as a key regulator of several aspects of mammalian energy metabolism including mitochondrial biogenesis, adaptive thermogenesis in brown adipose tissue, glucose uptake, fiber type-switching in skeletal muscle, gluconeogenesis in liver and insulin secretion from pancreas. Recent studies have shown a reduced expression of PGC-1alpha in skeletal muscle of diabetic and prediabetic humans. Moreover, expression of PGC-1alpha in white fat cells activates a broad program of adaptive thermogenesis characteristic of brown fat cells. PGC-1alpha could be a target for antiobesity or diabetes drugs. The aim of this article was to summarize the molecular mechanisms and biological programs controlled by the transcriptional coactivator PGC-1alpha.

SIRT3, a mitochondrial sirtuin deacetylase, regulates mitochondrial function and thermogenesis in brown adipocytes

SIRT3 is one of the seven mammalian sirtuin homologs of the yeast Sir2 gene, which mediates the effect of caloric restriction on life span extension in yeast and Caenorhabditis elegans. Because adipose tissue is essential in energy homeostasis and also plays a role in life span determination, we decided to investigate the function of sirtuin members in fat. We report here that murine SIRT3 is expressed in brown adipose tissue and is localized on the mitochondria inner membrane. Caloric restriction activates SIRT3 expression in both white and brown adipose. Additionally, cold exposure up-regulates SIRT3 expression in brown fat, whereas elevated climate temperature reduces the expression. Enforced expression of SIRT3 in the HIB1B brown adipocytes enhances the expression of the uncoupling protein PGC-1alpha, UCP1, and a series of mitochondria-related genes. Both ADP-ribosyltransferase and deacetylase activities of SIRT3 are required for this action. Furthermore, the SIRT3 deacetylase mutant exhibits a dominant negative effect by inhibiting UCP1 expression. This inhibitive effect can be abolished by the coexpression of PGC-1alpha, indicating a major role of PGC-1alpha in the SIRT3 action. In addition, SIRT3 stimulates CREB phosphorylation, which reportedly activates PGC-1alpha promoter directly. Functionally, sustained expression of SIRT3 decreases membrane potential and reactive oxygen species production while increasing cellular respiration. Finally, SIRT3, along with genes related to mitochondrial function, is down-regulated in the brown adipose tissue of several genetically obese mice. In summary, our results demonstrate that SIRT3 activates mitochondria functions and plays an important role in adaptive thermogenesis in brown adipose.

Thioredoxin-interacting protein deficiency disrupts the fasting-feeding metabolic transition

Through a positional cloning approach, the thioredoxin-interacting protein gene (Txnip) was recently identified as causal for a form of combined hyperlipidemia in mice (Bodnar, J. S., A. Chatterjee, L. W. Castellani, D. A. Ross, J. Ohmen, J. Cavalcoli, C. Wu, K. M. Dains, J. Catanese, M. Chu, S. S. Sheth, K. Charugundla, P. Demant, D. B. West, P. de Jong, and A. J. Lusis. 2002. Positional cloning of the combined hyperlipidemia gene Hyplip1. Nat. Genet. 30: 110-116). We now show that Txnip-deficient mice in the fed state exhibit a metabolic profile similar to fasted mice, including increased levels of plasma ketone bodies and free fatty acids, decreased glucose, and increased hepatic expression of peroxisome proliferator-activated receptor-gamma coactivator-1alpha, phosphoenolpyruvate carboxykinase, glucose-6-phosphatase, and acyl-CoA oxidase. Dramatic differences in the expression of key metabolic enzymes were also observed in other tissues, and the fat-to-muscle ratio of Txnip-deficient mice was increased by approximately 40%. We demonstrate an effect of Txnip on the redox status, as the Txnip-deficient mice in the fed state had a significant increase in the ratio of NADH to NAD(+). Surprisingly, we observed that Txnip-deficient mice and wild-type mice had similar levels of thioredoxin activity, suggesting that the effects of Txnip deficiency may be mediated in part by other interactions. These results indicate a role for Txnip in the metabolic response to feeding and the maintenance of the redox status.

Up-regulation of PPARγ coactivator-1α as a strategy for preventing and reversing insulin resistance and obesity

Excessive accumulation of triglycerides and certain fatty acid derivatives in skeletal muscle and other tissues appears to mediate many of the adverse effects of insulin resistance syndrome. Although fatty diets and obesity can promote such accumulation, deficient capacity for fatty acid oxidation can also contribute in this regard. Indeed, in subjects who are insulin resistant, diabetic, and/or obese, fatty acid oxidation by skeletal muscle tends to be inefficient, reflecting decreased expression of mitochondria and mitochondrial enzymes in muscle. This phenomenon is not corrected by weight loss, is not simply reflective of subnormal physical activity, and is also seen in lean first-degree relatives of diabetics; thus, it appears to be primarily attributable to genetic factors. Recent studies indicate that decreased expression of PPARgamma coactivator-1alpha (PGC-1alpha), a "master switch" which induces mitochondrial biogenesis by supporting the transcriptional activity of the nuclear respiratory factors, may largely account for the diminished oxidative capacity of subjects prone to insulin resistance. Thus, feasible measures which up-regulate PGC-1alpha may be useful for preventing and treating insulin resistance and obesity. These may include exercise training, metformin and other agents which stimulate AMP-activated kinase, high-dose biotin, and PPARdelta agonists. Drugs which are specific agonists for PPARdelta show remarkable efficacy in rodent models of insulin resistance, diabetes, and obesity, and are currently being evaluated clinically. Phytanic acid, a branched-chain fatty acid found in omnivore diets, can also activate PPARdelta, and thus should be examined with respect to its impact on mitochondrial biogenesis and insulin sensitivity.

Correction of insulin resistance and the metabolic syndrome

Insulin resistance is a common phenomenon of the metabolic syndrome, which is clinically characterized by a clustering of various cardiovascular risk factors in a single individual and a higher prevalence of respective complications, such as coronary heart disease and stroke. At the cellular level, insulin resistance is defined as a reduced insulin action, which can affect not only glucose uptake, but also gene regulation. Elucidation of novel signaling networks within the cell which are mediating and affecting insulin action will reveal many new genes and drug targets that are potentially of clinical relevance in the future. In this chapter, we propose that the metabolic syndrome might be a clinical consequence of altered gene regulation. This is illuminated in the context of transcription factors, e.g., sterol regulatory element binding proteins (SREBPs), coupling signals from nutrients, metabolites, and hormones at the gene regulatory level with pathobiochemical features of increased lipid accumulation in lean nonadipose tissues. The phenomenon of ectopic lipid accumulation (lipotoxicity) appears to be a novel link between insulin resistance, obesity, and possibly other features of the metabolic syndrome. Therefore, the investigation of specific gene regulatory networks and their alterations might be a clue to understanding the development and clustering of different cardiovascular risk factors in different individuals. As cellular sensors transcription factors--as common denominators of gene regulatory networks--might thereby also determine the susceptibility of individuals to cardiovascular risk factors and their complications.

Lipotoxicity, an imbalance between lipogenesis de novo and fatty acid oxidation

Obesity and type 2 diabetes mellitus are the major noncommunicable public health problem of the 21st century. The best strategy to tackle this problem is to develop strategies to prevent/treat obesity. However, it is becoming clear that despite successful research identifying weight regulatory pathways, the development of the obesity epidemic is outpacing scientific progress. The lack of success controlling the obesity epidemic in an aging population will result in another subsequent uncontrolled epidemic of complications. Our research focuses on the mechanisms causing lipotoxicity aiming to identify suitable strategies to prevent or at least retard the development of the metabolic syndrome. Previous work using transgenic and knockout mouse models has shown an interplay between white adipose tissue and skeletal muscle linking fatty acid (FA) synthesis with reciprocal effects on FA oxidation. Work from our lab and others suggests that defective adipose tissue is a key link between obesity, insulin resistance and type 2 diabetes mellitus by promoting the development of lipotoxicity in peripheral tissues. We propose a series of models to describe the process by which the adipose tissue could react to an energy-rich environment and responds depending on genetic and physiological factors, impacting on the functions of other peripheral tissues. We suggest that by examining hypotheses that encompass multiple organs and the partitioning of energy between these organs, a suitable strategy can be devised for the treatment of chronic obesity.

Defects in adaptive energy metabolism with CNS-linked hyperactivity in PGC-1alpha null mice

PGC-1alpha is a coactivator of nuclear receptors and other transcription factors that regulates several metabolic processes, including mitochondrial biogenesis and respiration, hepatic gluconeogenesis, and muscle fiber-type switching. We show here that, while hepatocytes lacking PGC-1alpha are defective in the program of hormone-stimulated gluconeogenesis, the mice have constitutively activated gluconeogenic gene expression that is completely insensitive to normal feeding controls. C/EBPbeta is elevated in the livers of these mice and activates the gluconeogenic genes in a PGC-1alpha-independent manner. Despite having reduced mitochondrial function, PGC-1alpha null mice are paradoxically lean and resistant to diet-induced obesity. This is largely due to a profound hyperactivity displayed by the null animals and is associated with lesions in the striatal region of the brain that controls movement. These data illustrate a central role for PGC-1alpha in the control of energy metabolism but also reveal novel systemic compensatory mechanisms and pathogenic effects of impaired energy homeostasis.

Identification of small-molecule antagonists that inhibit an activator: coactivator interaction

Phosphorylation of the cAMP response element binding protein (CREB) at Ser-133 in response to hormonal stimuli triggers cellular gene expression via the recruitment of the histone acetylase coactivator paralogs CREB binding protein (CBP) and p300 to the promoter. The NMR structure of the CREB:CBP complex, using relevant interaction domains called KID and KIX, respectively, reveals a shallow hydrophobic groove on the surface of KIX that accommodates an amphipathic helix in phospho (Ser-133) KID. Using an NMR-based screening approach on a preselected small-molecule library, we identified several compounds that bind to different surfaces on KIX. One of these, KG-501 (2-naphthol-AS-E-phosphate), targeted a surface distal to the CREB binding groove that includes Arg-600, a residue that is required for the CREB:CBP interaction. When added to live cells, KG-501 disrupted the CREB: CBP complex and attenuated target gene induction in response to cAMP agonist. These results demonstrate the ability of small molecules to interfere with second-messenger signaling cascades by inhibiting specific protein-protein interactions in the nucleus.

ETO/MTG8 is an inhibitor of C/EBPbeta activity and a regulator of early adipogenesis

The putative transcriptional corepressor ETO/MTG8 has been extensively studied due to its involvement in a chromosomal translocation causing the t(8;21) form of acute myeloid leukemia. Despite this, the role of ETO in normal physiology has remained obscure. Here we show that ETO is highly expressed in preadipocytes and acts as an inhibitor of C/EBPbeta during early adipogenesis, contributing to its characteristically delayed activation. ETO prevents both the transcriptional activation of the C/EBPalpha promoter by C/EBPbeta and its concurrent accumulation in centromeric sites during early adipogenesis. ETO expression rapidly reduces after the initiation of adipogenesis, and this is essential to the normal induction of adipogenic gene expression. These findings define, for the first time, a molecular role for ETO in normal physiology as an inhibitor of C/EBPbeta and a novel regulator of early adipogenesis.

Peroxisomal proliferator-activated receptor-gamma coactivator-1 alpha (PGC-1 alpha) enhances the thyroid hormone induction of carnitine palmitoyltransferase I (CPT-I alpha)

Carnitine palmitoyltransferase I (CPT-I) catalyzes the rate-controlling step in the pathway of mitochondrial fatty acid oxidation. Thyroid hormone will stimulate the expression of the liver isoform of CPT-I (CPT-I alpha). This induction of CPT-I alpha gene expression requires the thyroid hormone response element in the promoter and sequences within the first intron. The peroxisomal proliferator-activated receptor-gamma coactivator-1 alpha (PGC-1 alpha) is a coactivator that promotes mitochondrial biogenesis, mitochondrial fatty acid oxidation, and hepatic gluconeogenesis. In addition, PGC-1 alpha will stimulate the expression of CPT-I alpha in primary rat hepatocytes. Here we report that thyroid hormone will increase PGC-1 alpha mRNA and protein levels in rat hepatocytes. In addition, overexpression of PGC-1 alpha will enhance the thyroid hormone induction of CPT-I alpha indicating that PGC-1 alpha is a coactivator for thyroid hormone. By using chromatin immunoprecipitation assays, we show that PGC-1 alpha is associated with both the thyroid hormone response element in the CPT-I alpha gene promoter and the first intron of the CPT-I alpha gene. Our data demonstrate that PGC-1 alpha participates in the stimulation of CPT-I alpha gene expression by thyroid hormone and suggest that PGC-1 alpha is a coactivator for thyroid hormone.

Cold-induced PGC-1alpha expression modulates muscle glucose uptake through an insulin receptor/Akt-independent, AMPK-dependent pathway

Peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha) participates in control of expression of genes involved in adaptive thermogenesis, muscle fiber type differentiation, and fuel homeostasis. The objective of the present study was to evaluate the participation of cold-induced PGC-1alpha expression in muscle fiber type-specific activity of proteins that belong to the insulin-signaling pathway. Rats were exposed to 4 degrees C for 4 days and acutely treated with insulin in the presence or absence of an antisense oligonucleotide to PGC-1alpha. Cold exposure promoted a significant increase of PGC-1alpha and uncoupling protein-3 protein expression in type I and type II fibers of gastrocnemius muscle. In addition, cold exposure led to higher glucose uptake during a hyperinsulinemic clamp, which was accompanied by higher expression and membrane localization of GLUT4 in both muscle fiber types. Cold exposure promoted significantly lower insulin-induced tyrosine phosphorylation of the insulin receptor (IR) and Ser473 phosphorylation of acute transforming retrovirus thymoma (Akt) and an insulin-independent increase of Thr172 phosphorylation of adenosine 5'-monophosphate-activated protein kinase (AMPK). Inhibition of PGC-1alpha expression in cold-exposed rats by antisense oligonucleotide treatment diminished glucose clearance rates during a hyperinsulinemic clamp and reduced expression and membrane localization of GLUT4. Reduction of PGC-1alpha expression resulted in no modification of insulin-induced tyrosine phosphorylation of the IR and Ser473 phosphorylation of Akt. Finally, reduction of PGC-1alpha resulted in lower Thr172 phosphorylation of AMPK. Thus cold-induced hyperexpression of PGC-1alpha participates in control of skeletal muscle glucose uptake through a mechanism that controls GLUT4 expression and subcellular localization independent of the IR and Akt activities but dependent on AMPK.

Nuclear receptor signaling and cardiac energetics

The heart has a tremendous capacity for ATP generation, allowing it to function as an efficient pump throughout the life of the organism. The adult myocardium uses either fatty acid or glucose oxidation as its main energy source. Under normal conditions, the adult heart derives most of its energy through oxidation of fatty acids in mitochondria. However, the myocardium has a remarkable ability to switch between carbohydrate and fat fuel sources so that ATP production is maintained at a constant rate in diverse physiological and dietary conditions. This fuel selection flexibility is important for normal cardiac function. Although cardiac energy conversion capacity and metabolic flux is modulated at many levels, an important mechanism of regulation occurs at the level of gene expression. The expression of genes involved in multiple energy transduction pathways is dynamically regulated in response to developmental, physiological, and pathophysiological cues. This review is focused on gene transcription pathways involved in short- and long-term regulation of myocardial energy metabolism. Much of our knowledge about cardiac metabolic regulation comes from studies focused on mitochondrial fatty acid oxidation. The genes involved in this key energy metabolic pathway are transcriptionally regulated by members of the nuclear receptor superfamily, specifically the fatty acid-activated peroxisome proliferator-activated receptors (PPARs) and the nuclear receptor coactivator, PPARgamma coactivator-1alpha (PGC-1alpha). The dynamic regulation of the cardiac PPAR/PGC-1 complex in accordance with physiological and pathophysiological states will be described.

PML-RARA-RXR oligomers mediate retinoid and rexinoid/cAMP cross-talk in acute promyelocytic leukemia cell differentiation

PML-RARA was proposed to initiate acute promyelocytic leukemia (APL) through PML-RARA homodimer-triggered repression. Here, we examined the nature of the PML-RARA protein complex and of its DNA targets in APL cells. Using a selection/amplification approach, we demonstrate that PML-RARA targets consist of two AGGTCA elements in an astonishing variety of orientations and spacings, pointing to highly relaxed structural constrains for DNA binding and identifying a major gain of function of this oncogene. PML-RARA-specific response elements were identified, which all conveyed a major transcriptional response to RA only in APL cells. In these cells, we demonstrate that PML-RARA oligomers are complexed to RXR. Directly probing PML-RARA function in APL cells, we found that the differentiation enhancer cyclic AMP (cAMP) boosted transcriptional activation by RA. cAMP also reversed the normal silencing (subordination) of the transactivating function of RXR when bound to RARA or PML-RARA, demonstrating that the alternate rexinoid/cAMP-triggered APL differentiation pathway also activates PML-RARA targets. Finally, cAMP restored both RA-triggered differentiation and PML-RARA transcriptional activation in mutant RA-resistant APL cells. Collectively, our findings directly demonstrate that APL cell differentiation parallels transcriptional activation through PML-RARA-RXR oligomers and that those are functionally targeted by cAMP, identifying this agent as another oncogene-targeted therapy.

The developmental regulation of peroxisome proliferator-activated receptor-gamma coactivator-1alpha expression in the liver is partially dissociated from the control of gluconeogenesis and lipid catabolism

The developmental regulation of peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha) gene expression was studied in mice and compared with that of marker genes of liver energy metabolism. The PGC-1alpha gene was highly expressed in fetal liver compared with that in adults and remained high in neonatal liver. The regulation of PGC-1alpha gene expression during the fetal and early neonatal periods was dissociated from that of gluconeogenic genes, i.e. the phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase) genes. Only under the effects of starvation was PGC-1alpha gene expression induced in parallel to phosphoenolpyruvate carboxykinase and G6Pase mRNAs during the perinatal period. Furthermore, the PGC-1alpha gene was not regulated as part of the developmental program of gene expression associated with the maturation of hepatic gluconeogenesis, as revealed by the impaired PEPCK and G6Pase gene expression but unaltered PGC-1alpha mRNA levels in CCAAT/enhancer-binding protein-alpha-null fetus and neonates. Regulation of the PGC-1alpha gene and that of mitochondrial 3-hydroxy-3-methyl-glutaryl-coenzyme A synthase, acyl-coenzyme A oxidase, and long-chain acyl-coenzyme dehydrogenase, marker genes of lipid catabolism, were dissociated in fetuses and neonates. The expression of lipid catabolism genes was down-regulated in fasted neonates, whereas PGC-1alpha was oppositely regulated. The independent regulation of PGC-1alpha and lipid catabolism genes was also found in peroxisome proliferator-activated receptor-alpha-null neonates, in which PGC-1alpha mRNA levels were unaffected whereas gene expression for 3-hydroxy-3-methyl-glutaryl-coenzyme A synthase and acyl-coenzyme A oxidase was impaired. Thus, regulation of the PGC-1alpha gene is partially dissociated from the patterns of regulation of hepatic genes encoding enzymes involved in gluconeogenesis and lipid catabolism during fetal ontogeny and in response to the initiation of lactation.

Deacetylase inhibitors increase muscle cell size by promoting myoblast recruitment and fusion through induction of follistatin

Fusion of undifferentiated myoblasts into multinucleated myotubes is a prerequisite for developmental myogenesis and postnatal muscle growth. We report that deacetylase inhibitors favor the recruitment and fusion of myoblasts into preformed myotubes. Muscle-restricted expression of follistatin is induced by deacetylase inhibitors and mediates myoblast recruitment and fusion into myotubes through a pathway distinct from those utilized by either IGF-1 or IL-4. Blockade of follistatin expression by RNAi-mediated knockdown, functional inactivation with either neutralizing antibodies or the antagonist protein myostatin, render myoblasts refractory to HDAC inhibitors. Muscles from animals treated with the HDAC inhibitor trichostatin A display increased production of follistatin and enhanced expression of markers of regeneration following muscle injury. These data identify follistatin as a central mediator of the fusigenic effects exerted by deacetylase inhibitors on skeletal muscles and establish a rationale for their use to manipulate skeletal myogenesis and promote muscle regeneration.

Regulation of PPARgamma coactivator 1alpha (PGC-1alpha) signaling by an estrogen-related receptor alpha (ERRalpha) ligand

Peroxisome proliferator-activated receptor gamma (PPARgamma) coactivator 1alpha (PGC-1alpha) is a transcriptional coactivator that is a key component in the regulation of energy production and utilization in metabolic tissues. Recent work has identified PGC-1alpha as a strong coactivator of the orphan nuclear receptor estrogen-related receptor alpha (ERRalpha), implicating ERRalpha as a potential mediator of PGC-1alpha action. To understand the role of ERRalpha in PGC-1alpha signaling, a parallel approach of high-throughput screening and gene-expression analysis was used to identify ERRalpha small-molecule regulators and target genes. We report here the identification of a potent and selective ERRalpha inverse agonist that interferes effectively with PGC-1alpha/ERRalpha-dependent signaling. This inverse agonist inhibits the constitutive activity of ERRalpha in both biochemical and cell-based assays. Also, we demonstrate that monoamine oxidase B is an ERRalpha target gene whose expression is regulated by PGC-1alpha and ERRalpha and inhibited by the ERRalpha inverse agonist. The discovery of potent and selective ERRalpha modulators and their effect on PGC-1alpha signaling provides mechanistic insight into gene regulation by PGC-1alpha. These findings validate ERRalpha as a promising therapeutic target in the treatment of metabolic disorders, including diabetes and obesity.

Insulin regulation of hepatic gluconeogenesis through phosphorylation of CREB-binding protein

Hepatic gluconeogenesis is essential for maintenance of normal blood glucose concentrations and is regulated by opposing stimulatory (cyclic adenosine monophosphate, cAMP) and inhibitory (insulin) signaling pathways. The cAMP signaling pathway leads to phosphorylation of cAMP response element-binding (CREB) protein, resulting in recruitment of the coactivators CREB-binding protein (CBP) and p300 and subsequent activation of gluconeogenesis. Insulin signaling leads to phosphorylation of CBP at serine 436, a residue near its CREB-interacting domain, but it is unknown whether this event modulates cAMP signaling. Here, we show in vitro and in 'knock-in' mice that a mutant CBP (S436A) is aberrantly recruited to CREB protein, resulting in inappropriate activation of gluconeogenesis in the fed state and glucose intolerance resulting from increased hepatic glucose production. We propose that insulin signaling may directly regulate many cAMP signaling pathways at the transcriptional level by controlling CBP recruitment.

p38 mitogen-activated protein kinase is the central regulator of cyclic AMP-dependent transcription of the brown fat uncoupling protein 1 gene

It is well established that catecholamine-stimulated thermogenesis in brown fat requires beta-adrenergic elevations in cyclic AMP (cAMP) to increase expression of the uncoupling protein 1 (UCP1) gene. However, little is known about the downstream components of the signaling cascade or the relevant transcription factor targets thereof. Here we demonstrate that cAMP- and protein kinase A-dependent activation of p38 mitogen-activated protein kinase (MAPK) in brown adipocytes is an indispensable step in the transcription of the UCP1 gene in mice. By phosphorylating activating transcription factor 2 (ATF-2) and peroxisome proliferator-activated receptor gamma (PPARgamma) coativator 1alpha (PGC-1alpha), members of two distinct nuclear factor families, p38 MAPK controls the expression of the UCP1 gene through their respective interactions with a cAMP response element and a PPAR response element that both reside within a critical enhancer motif of the UCP1 gene. Activation of ATF-2 by p38 MAPK additionally serves as the cAMP sensor that increases expression of the PGC-1alpha gene itself in brown adipose tissue. In conclusion, our findings illustrate that by orchestrating the activity of multiple transcription factors, p38 MAPK is a central mediator of the cAMP signaling mechanism of brown fat that promotes thermogenesis.

Hepatocytes: critical for glucose homeostasis

Maintaining blood glucose levels within a narrow range is a critical physiological function requiring multiple metabolic pathways and involving several cell types, including a prominent role for hepatocytes. Under hormonal control, hepatocytes can respond to either feeding or fasting conditions by storing or producing glucose as necessary. In the fasting state, the effects of glucagon avoid hypoglycemia by stimulating glucogenesis and glycogenolysis and initiating hepatic glucose release. Postprandially, insulin prevents hyperglycemia, in part, by suppressing hepatic gluconeogenesis and glycogenolysis and facilitating hepatic glycogen synthesis. Both transcriptional regulation of rate limiting enzymes and modulation of enzyme activity through phosphorylation and allosteric regulation are involved. Type 2 diabetes mellitus is the most common serious metabolic condition in the world, and results from a subnormal response of tissues to insulin (insulin resistance) and a failure of the insulin-secreting beta cells to compensate. In type 2 diabetes, glucose is overproduced by the hepatocyte and is ineffectively metabolized by other organs. Impairments in the insulin signal transduction pathway appear to be critical lesions contributing to insulin resistance and type 2 diabetes.

Selective stimulation of G-6-Pase catalytic subunit but not G-6-P transporter gene expression by glucagon in vivo and cAMP in situ

We recently compared the regulation of glucose-6-phosphatase (G-6-Pase) catalytic subunit and glucose 6-phosphate (G-6-P) transporter gene expression by insulin in conscious dogs in vivo (Hornbuckle LA, Edgerton DS, Ayala JE, Svitek CA, Neal DW, Cardin S, Cherrington AD, and O'Brien RM. Am J Physiol Endocrinol Metab 281: E713-E725, 2001). In pancreatic-clamped, euglycemic conscious dogs, a 5-h period of hypoinsulinemia led to a marked increase in hepatic G-6-Pase catalytic subunit mRNA; however, G-6-P transporter mRNA was unchanged. Here, we demonstrate, again using pancreatic-clamped, conscious dogs, that glucagon is a candidate for the factor responsible for this selective induction. Thus glucagon stimulated G-6-Pase catalytic subunit but not G-6-P transporter gene expression in vivo. Furthermore, cAMP stimulated endogenous G-6-Pase catalytic subunit gene expression in HepG2 cells but had no effect on G-6-P transporter gene expression. The cAMP response element (CRE) that mediates this induction was identified through transient transfection of HepG2 cells with G-6-Pase catalytic subunit-chloramphenicol acetyltransferase fusion genes. Gel retardation assays demonstrate that this CRE binds several transcription factors including CRE-binding protein and CCAAT enhancer-binding protein.

Complex haplotypes of the PGC-1alpha gene are associated with carbohydrate metabolism and type 2 diabetes

Peroxisome proliferator-activated receptor coactivator-1alpha (PGC-1alpha) is a transcriptional coactivator implicated in transcriptional programs of hepatic gluconeogenesis, oxidative phosphorylation, and insulin release by beta-cells. To study associations of the PGC-1alpha gene locus with carbohydrate metabolism and type 2 diabetes in humans, we identified several polymorphisms in the promoter region that were located in a haplotype block distinct from a second haplotype block containing part of intron 2 and extending beyond exon 13. Each block contained five common haplotypes. Oral glucose tolerance testing revealed associations of promoter haplotype combinations with 30- and 60-min postload plasma glucose levels, whereas haplotypes in both blocks were associated with indexes of beta-cell function. The associations of promoter haplotypes are supported by functional studies showing that some polymorphisms are located in transcription factor binding sites and affect transactivation in an allele-specific manner. By comparing patients with type 2 diabetes and control subjects, we observed borderline significant differences of four-loci haplotype distributions in the downstream haplotype block. Moreover, the haplotype that was associated with the strongest insulin response to glucose conferred the lowest risk of type 2 diabetes (P < 0.01). Thus, the PGC-1alpha gene locus influences carbohydrate metabolism and contributes to type 2 diabetes in the population studied.

A polymorphic autoregulatory hormone response element in the human estrogen-related receptor alpha (ERRalpha) promoter dictates peroxisome proliferator-activated receptor gamma coactivator-1alpha control of ERRalpha expression

The orphan nuclear estrogen-related receptor alpha (ERRalpha) and transcriptional cofactor peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1alpha) are involved in the regulation of energy metabolism. Recently, extensive cross-talk between PGC-1alpha and ERRalpha has been demonstrated. The presence of PGC-1alpha is associated with an elevated expression of ERRalpha, and the two proteins can influence the transcriptional activities of one another. Using a candidate gene approach to detect regulatory variants within genes encoding nuclear receptors, we have identified a 23-bp sequence (ESRRA23) containing two nuclear receptor recognition half-site motifs that is present in 1-4 copies within the promoter of the human ESRRA gene encoding ERRalpha. The ESRRA23 sequence contains a functional ERR response element that is specifically bound by ERRalpha, and chromatin immunoprecipitation shows that endogenous ERRalpha occupies its own promoter in vivo. Strikingly, introduction of PGC-1alpha in HeLa cells by transient transfection induces the activity of the ESRRA promoter in a manner that is dependent on the presence of the ESRRA23 element and on its dosage. Coexpression of ERRalpha and PGC-1alpha results in a synergistic activation of the ESRRA promoter. In experiments using ERRalpha null fibroblasts, the ability of PGC-1alpha to stimulate the ESRRA promoter is considerably reduced but can be restored by addition of ERRalpha. Taken together, these results demonstrate that an interdependent ERRalpha/PGC-1alpha-based transcriptional pathway targets the ESRRA23 element to dictate the level of ERRalpha expression. This study further suggests that this regulatory polymorphism may provide differential responses to ERRalpha/PGC-1alpha-mediated metabolic cues in the human population.

Insulin/Foxo1 pathway regulates expression levels of adiponectin receptors and adiponectin sensitivity

Adiponectin/Acrp30 is a hormone secreted by adipocytes, which acts as an antidiabetic and antiatherogenic adipokine. We reported previously that AdipoR1 and -R2 serve as receptors for adiponectin and mediate increased fatty acid oxidation and glucose uptake by adiponectin. In the present study, we examined the expression levels and roles of AdipoR1/R2 in several physiological and pathophysiological states such as fasting/refeeding, obesity, and insulin resistance. Here we show that the expression of AdipoR1/R2 in insulin target organs, such as skeletal muscle and liver, is significantly increased in fasted mice and decreased in refed mice. Insulin deficiency induced by streptozotocin increased and insulin replenishment reduced the expression of AdipoR1/R2 in vivo. Thus, the expression of AdipoR1/R2 appears to be inversely correlated with plasma insulin levels in vivo. Interestingly, the incubation of hepatocytes or myocytes with insulin reduced the expression of AdipoR1/R2 via the phosphoinositide 3-kinase/Foxo1-dependent pathway in vitro. Moreover, the expressions of AdipoR1/R2 in ob/ob mice were significantly decreased in skeletal muscle and adipose tissue, which was correlated with decreased adiponectin binding to membrane fractions of skeletal muscle and decreased AMP kinase activation by adiponectin. This adiponectin resistance in turn may play a role in worsening insulin resistance in ob/ob mice. In conclusion, the expression of AdipoR1/R2 appears to be inversely regulated by insulin in physiological and pathophysiological states such as fasting/refeeding, insulin deficiency, and hyper-insulinemia models via the insulin/phosphoinositide 3-kinase/Foxo1 pathway and is correlated with adiponectin sensitivity.

PGC-1 promotes insulin resistance in liver through PPAR-alpha-dependent induction of TRB-3

Insulin resistance is a major hallmark in the development of type 2 diabetes, which is characterized by an impaired ability of insulin to inhibit glucose output from the liver and to promote glucose uptake in muscle. The nuclear hormone receptor coactivator PGC-1 (peroxisome proliferator-activated (PPAR)-gamma coactivator-1) has been implicated in the onset of type 2 diabetes. Hepatic PGC-1 expression is elevated in mouse models of this disease, where it promotes constitutive activation of gluconeogenesis and fatty acid oxidation through its association with the nuclear hormone receptors HNF-4 and PPAR-alpha, respectively. Here we show that PGC-1-deficient mice, generated by adenoviral delivery of PGC-1 RNA interference (RNAi) to the liver, experience fasting hypoglycemia. Hepatic insulin sensitivity was enhanced in PGC-1-deficient mice, reflecting in part the reduced expression of the mammalian tribbles homolog TRB-3, a fasting-inducible inhibitor of the serine-threonine kinase Akt/PKB (ref. 6). We show here that, in the liver, TRB-3 is a target for PPAR-alpha. Knockdown of hepatic TRB-3 expression improved glucose tolerance, whereas hepatic overexpression of TRB-3 reversed the insulin-sensitive phenotype of PGC-1-deficient mice. These results indicate a link between nuclear hormone receptor and insulin signaling pathways, and suggest a potential role for TRB-3 inhibitors in the treatment of type 2 diabetes.

Bile acids regulate gluconeogenic gene expression via small heterodimer partner-mediated repression of hepatocyte nuclear factor 4 and Foxo1

Bile acid homeostasis is tightly controlled by the feedback mechanism in which an atypical orphan nuclear receptor (NR) small heterodimer partner (SHP) inactivates several NRs such as liver receptor homologue-1 and hepatocyte nuclear factor 4. Although NRs have been implicated in the transcriptional regulation of gluconeogenic genes, the effect of bile acids on gluconeogenic gene expression remained unknown. Here, we report that bile acids inhibit the expression of gluconeogenic genes, including glucose-6-phosphatase (G6Pase), phosphoenolpyruvate carboxykinase, and fructose 1,6-bis phosphatase in an SHP-dependent fashion. Cholic acid diet decreased the mRNA levels of these gluconeogenic enzymes, whereas those of SHP were increased. Reporter assays demonstrated that the promoter activity of phosphoenolpyruvate carboxykinase and fructose 1,6-bis phosphatase via hepatocyte nuclear factor 4, or that of G6Pase via the forkhead transcription factor Foxo1, was down-regulated by treatment with chenodeoxicholic acid and with transfected SHP. Remarkably, Foxo1 interacted with SHP in vivo and in vitro, which led to the repression of Foxo1-mediated G6Pase transcription by competition with a coactivator cAMP response element-binding protein-binding protein. These findings reveal a novel mechanism by which bile acids regulate gluconeogenic gene expression via an SHP-dependent regulatory pathway.

Retinoblastoma protein functions as a molecular switch determining white versus brown adipocyte differentiation

Adipocyte precursor cells give raise to two major cell populations with different physiological roles: white and brown adipocytes. Here we demonstrate that the retinoblastoma protein (pRB) regulates white vs. brown adipocyte differentiation. Functional inactivation of pRB in wild-type mouse embryo fibroblasts (MEFs) and white preadipocytes by expression of simian virus 40 large T antigen results in the expression of the brown fat-specific uncoupling protein 1 (UCP-1) in the adipose state. Retinoblastoma gene-deficient (Rb-/-) MEFs and stem cells, but not the corresponding wild-type cells, differentiate into adipocytes with a gene expression pattern and mitochondria content resembling brown adipose tissue. pRB-deficient MEFs exhibit an increased expression of the Forkhead transcription factor Foxc2 and its target gene cAMP-dependent protein kinase regulatory subunit RIalpha, resulting in increased cAMP sensitivity. Suppression of cAMP-dependent protein kinase activity in Rb(-/-)MEFs blocked the brown adipocyte-like gene expression pattern without affecting differentiation per se. Immunohistochemical studies revealed that pRB is present in the nuclei of white but not brown adipocyte precursor cells at a developmental stage where both cell types begin to accumulate lipid and brown adipocytes express UCP-1. Furthermore, pRB rapidly undergoes phosphorylation upon cold-induced neodifferentiation and up-regulation of UCP-1 expression in brown adipose tissue. Finally, down-regulation of pRB expression accompanies transdifferentiation of white into brown adipocytes in response to beta3-adrenergic receptor agonist treatment. We propose that pRB acts as a molecular switch determining white vs. brown adipogenesis, suggesting a previously uncharacterized function of this key cell cycle regulator in adipocyte lineage commitment and differentiation.

Regulation of transcriptional coactivator PGC-1alpha

Transcriptional coregulators modulate the activity of transcription factors and are required for the proper regulation of gene expression. One transcriptional coactivator, peroxisome proliferator-activated receptor-gamma coactivator 1alpha (PGC-1alpha), plays an important role in the control of energy metabolism and has been associated with type 2 diabetes. A recent paper by Fan et al. provides new information about the posttranslational regulation of PGC-1alpha activity. This Perspective discusses the implications of these findings with respect to diabetes and aging.

Expression of the thermogenic nuclear hormone receptor coactivator PGC-1alpha is reduced in the adipose tissue of morbidly obese subjects

Peroxisome proliferator-activated receptor gamma coactivator 1alpha (PGC1alpha) is an accessory protein which can potentiate the transcriptional activation function of many nuclear hormone receptors. Its tissue distribution and physiological studies suggest that its principal in vivo roles are to promote cold-induced thermogenesis, mitochondrial biogenesis, hepatic gluconeogenesis, and fatty acid beta-oxidation. It is expressed in the white adipose tissue of both humans and rodents, and in rodents it has been suggested to mediate in part the leptin-induced conversion of white adipocytes from fat storing to fat oxidising cells. In this study, quantitative real-time PCR has been used in human tissue to demonstrate that (1) PGC1alpha mRNA levels in subcutaneous fat are three-fold lower in morbidly obese than in slim subjects; (2) there are no differences in PGC1alpha mRNA between omental and subcutaneous mature adipocytes; (3) there is a robust induction of PGC1alpha expression during subcutaneous human preadipocyte differentiation ex vivo. Whether low PGC1alpha expression is a prelude to the development of obesity, or a consequence of that obesity, attempts to upregulate endogenous white adipose tissue expression may prove a valuable new avenue to explore in obesity therapy.

Evidence of a novel quantitative-trait locus for obesity on chromosome 4p in Mexican Americans

Although several genomewide scans have identified quantitative-trait loci influencing several obesity-related traits in humans, genes influencing normal variation in obesity phenotypes have not yet been identified. We therefore performed a genome scan of body mass index (BMI) on Mexican Americans, a population prone to obesity and diabetes, using a variance-components linkage analysis to identify loci that influence BMI. We used phenotypic data from 430 individuals (26% diabetics, 59% females, mean age +/- SD = 43 +/- 17 years, mean BMI +/- SD = 30.0 +/- 6.7, mean leptin (ng/ml) +/- SD = 22.1 +/- 17.1) distributed across 27 low-income Mexican American pedigrees who participated in the San Antonio Family Diabetes Study (SAFDS) for whom a 10-15-cM map is available. In this genomewide search, after accounting for the covariate effects of age, sex, diabetes, and leptin, we identified a genetic region exhibiting the most highly significant evidence for linkage (LOD 4.5) with BMI on chromosome 4p (4p15.1) at 42 cM, near marker D4S2912. This linkage result has been confirmed in an independent linkage study of severe obesity in Utah pedigrees. Two strong positional candidates, the human peroxisome proliferator-activated receptor gamma coactivator 1 (PPARGC1) and cholecystokinin A receptor (CCKAR) with major roles in the development of obesity, are located in this region. In conclusion, we identified a major genetic locus influencing BMI on chromosome 4p in Mexican Americans.

PE-1/METS, an antiproliferative Ets repressor factor, is induced by CREB-1/CREM-1 during macrophage differentiation

The molecular mechanisms involved in regulating the balance between cellular proliferation and differentiation remain poorly understood. Members of the Ets-domain family of transcription factors are candidates for proteins that might differentially regulate cell cycle control and cell type-specific genes during the differentiation of myeloid progenitor cells. The Ets repressor PE-1/METS has been suggested to contribute to growth arrest during terminal macrophage differentiation by repressing Ets target genes involved in Ras-dependent proliferation. An important feature of this regulatory model is that PE-1/METS is itself induced by the program of macrophage differentiation elicited by M-CSF. Here, we present evidence that the PE-1/METS gene is a transcriptional target of the cyclic AMP response element-binding protein-1 (CREB-1). CREB-1 expression is dramatically up-regulated during macrophage differentiation and phosphorylation of CREB-1 and the related factor CREM-1 are stimulated by M-CSF in a SAPK2/p38-dependent manner. Chromatin immunoprecipitation experiments demonstrate that CREB-1/CREM-1 are recruited to the PE-1/METS promoter as well as to the promoters of other genes that are up-regulated during terminal macrophage differentiation. Overexpression of CREB-1 stimulates the activities of the PE-1/METS, and macrosialin promoters, while expression of a dominant negative form of CREB-1 during macrophage differentiation inhibits expression of the PE-1/METS and macrosialin genes. Inhibition of CREB function also results in reduced expression of CD54 and impaired cell adhesion. Taken together, these findings reveal new roles of CREB-1/CREM-1 as regulators of macrophage differentiation.

Suppression of mitochondrial respiration through recruitment of p160 myb binding protein to PGC-1alpha: modulation by p38 MAPK

The transcriptional coactivator PPAR gamma coactivator 1 alpha (PGC-1alpha) is a key regulator of metabolic processes such as mitochondrial biogenesis and respiration in muscle and gluconeogenesis in liver. Reduced levels of PGC-1alpha in humans have been associated with type II diabetes. PGC-1alpha contains a negative regulatory domain that attenuates its transcriptional activity. This negative regulation is removed by phosphorylation of PGC-1alpha by p38 MAPK, an important kinase downstream of cytokine signaling in muscle and beta-adrenergic signaling in brown fat. We describe here the identification of p160 myb binding protein (p160MBP) as a repressor of PGC-1alpha. The binding and repression of PGC-1alpha by p160MBP is disrupted by p38 MAPK phosphorylation of PGC-1alpha. Adenoviral expression of p160MBP in myoblasts strongly reduces PGC-1alpha's ability to stimulate mitochondrial respiration and the expression of the genes of the electron transport system. This repression does not require removal of PGC-1alpha from chromatin, suggesting that p160MBP is or recruits a direct transcriptional suppressor. Overall, these data indicate that p160MBP is a powerful negative regulator of PGC-1alpha function and provide a molecular mechanism for the activation of PGC-1alpha by p38 MAPK. The discovery of p160MBP as a PGC-1alpha regulator has important implications for the understanding of energy balance and diabetes.

Role of STAT-3 in regulation of hepatic gluconeogenic genes and carbohydrate metabolism in vivo

The transcription factor, signal transducer and activator of transcription-3 (STAT-3) contributes to various physiological processes. Here we show that mice with liver-specific deficiency in STAT-3, achieved using the Cre-loxP system, show insulin resistance associated with increased hepatic expression of gluconeogenic genes. Restoration of hepatic STAT-3 expression in these mice, using adenovirus-mediated gene transfer, corrected the metabolic abnormalities and the alterations in hepatic expression of gluconeogenic genes. Overexpression of STAT-3 in cultured hepatocytes inhibited gluconeogenic gene expression independently of peroxisome proliferator-activated receptor-gamma coactivator-1 alpha (PGC-1 alpha), an upstream regulator of gluconeogenic genes. Liver-specific expression of a constitutively active form of STAT-3, achieved by infection with an adenovirus vector, markedly reduced blood glucose, plasma insulin concentrations and hepatic gluconeogenic gene expression in diabetic mice. Hepatic STAT-3 signaling is thus essential for normal glucose homeostasis and may provide new therapeutic targets for diabetes mellitus.

11beta-Hydroxysteroid dehydrogenase Type 1 in obesity and Type 2 diabetes

Obesity and Type 2 diabetes mellitus are associated with abnormal regulation of glucocorticoid metabolism that are highlighted by clinical similarities between the sequelae of insulin resistance and Cushing's syndrome, as well as glucocorticoids' functional antagonism to insulin. 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1) activates functionally inert glucocorticoid precursors (cortisone) to active glucocorticoids (cortisol) within insulin target tissues, such as adipose tissue, thereby regulating local glucocorticoid action. Recent data, mainly from rodents, provide considerable evidence for a causal role of 11beta-HSD1 for the development of visceral obesity and Type 2 diabetes though data in humans are not unequivocal. This review summarizes current evidence on a possible role of 11beta-HSD1 for development of the metabolic syndrome, raising the possibility of novel therapeutic options for the treatment of Type 2 diabetes by inhibition or down-regulation of 11beta-HSD1 activity.

CREB controls hepatic lipid metabolism through nuclear hormone receptor PPAR-gamma

Fasting triggers a series of hormonal cues that promote energy balance by inducing glucose output and lipid breakdown in the liver. In response to pancreatic glucagon and adrenal cortisol, the cAMP-responsive transcription factor CREB activates gluconeogenic and fatty acid oxidation programmes by stimulating expression of the nuclear hormone receptor coactivator PGC-1 (refs 2-5). In parallel, fasting also suppresses lipid storage and synthesis (lipogenic) pathways, but the underlying mechanism is unknown. Here we show that mice deficient in CREB activity have a fatty liver phenotype and display elevated expression of the nuclear hormone receptor PPAR-gamma, a key regulator of lipogenic genes. CREB inhibits hepatic PPAR-gamma expression in the fasted state by stimulating the expression of the Hairy Enhancer of Split (HES-1) gene, a transcriptional repressor that is shown here to be a mediator of fasting lipid metabolism in vivo. The coordinate induction of PGC-1 and repression of PPAR-gamma by CREB during fasting provides a molecular rationale for the antagonism between insulin and counter-regulatory hormones, and indicates a potential role for CREB antagonists as therapeutic agents in enhancing insulin sensitivity in the liver.

A tissue-restricted cAMP transcriptional response: SOX10 modulates alpha-melanocyte-stimulating hormone-triggered expression of microphthalmia-associated transcription factor in melanocytes

alpha-Melanocyte-stimulating hormone (MSH) utilizes cAMP to trigger pigmentation of melanocytes via activation of melanocyte-restricted microphthalmia-associated transcription factor (M-MITF) expression. M-MITF is a melanocyte-restricted helix-loop-helix transcription factor capable of transactivating promoters for multiple genes whose products modulate pigmentation. Although M-MITF promoter activation by MSH is known to occur through a conserved cAMP-response element (CRE), it remains unclear how this CRE exhibits such exquisitely tissue-restricted responsiveness. Here we show that cAMP-mediated CRE-binding protein activation of the M-MITF promoter requires a second DNA element located approximately 100 bp upstream, a site that is bound and activated by SOX10. Mutations in the SOX10 transcription factor, like MITF, results in a disorder known as Waardenburg Syndrome. The cAMP response of the M-MITF promoter was analyzed in melanoma and neuroblastoma cells (which are neural crest-derived but lack both M-MITF and SOX10 expression). M-MITF promoter responsiveness to cAMP was found to depend upon SOX10, and reciprocally, SOX10 transactivation was dependent upon the CRE. Ectopic SOX10 expression, in cooperation with cAMP signaling, activated the M-MITF promoter function and the expression of measurable endogenous M-MITF transcripts in neuroblastoma cells. SOX10dom, a mutant allele, failed to cooperate with cAMP in neuroblastoma cells and attenuated the cAMP responsiveness of the M-MITF promoter in melanoma cells. These observations demonstrate a means whereby the ubiquitous cAMP signaling machinery is harnessed to produce a highly tissue-restricted transcriptional response by cooperating with architectural factors, in this case SOX10.

Compensation by the muscle limits the metabolic consequences of lipodystrophy in PPAR gamma hypomorphic mice

Peroxisome proliferator-activated receptor gamma (PPAR gamma) is a nuclear receptor, which controls adipocyte differentiation. We targeted with homologous recombination the PPAR gamma 2-specific exon B, resulting in a white adipose tissue knockdown of PPAR gamma. Although homozygous (PPAR gamma hyp/hyp) mice are born with similar weight as the WT mice, the PPAR gamma hyp/hyp animals become growth retarded and develop severe lipodystrophy and hyperlipidemia. Almost half of these PPAR gamma hyp/hyp mice die before adulthood, whereas the surviving PPAR gamma hyp/hyp animals overcome the growth retardation, yet remain lipodystrophic. In contrast to most lipodystrophic models, the adult PPAR gamma hyp/hyp mice only have mild glucose intolerance and do not have a fatty liver. These metabolic consequences of the lipodystrophy are relatively benign because of the induction of a compensatory gene expression program in the muscle that enables efficient oxidation of excess lipids. The PPAR gamma hyp/hyp mice unequivocally demonstrate that PPAR gamma is the master regulator of adipogenesis in vivo and establish that lipid and glucose homeostasis can be relatively well maintained in the absence of white adipose tissue.

Repression of p65 transcriptional activation by the glucocorticoid receptor in the absence of receptor-coactivator interactions

Glucocorticoids are among the most potent antiinflammatory agents, acting through the glucocorticoid receptor (GR) to suppress gene expression of a variety of cytokines. This appears to be via transcriptional interference (or transrepression) of key regulatory factors such as nuclear factor-kappaB and activator protein 1. Ligand-bound GR can also activate gene transcription (transactivation) via direct binding to glucocorticoid response elements. Transactivation by GR is potentiated by accessory coactivators such as steroid receptor coactivator 1 and peroxisome proliferator-activated receptor gamma coactivator 1, whereas the role of these proteins in transrepression is unclear.Here, we show that GR can recruit several coactivator receptor interacting domains in a ligand-dependent manner. All interactions require the charge clamp defined by K579/E755, while a subset also requires a second charge clamp defined by R585/D590, within the GR ligand-binding domain. A point mutation, E755A, abolished all GR-receptor interacting domain interactions and led to a decrease in GR-mediated transactivation, but did not significantly affect GR-mediated transrepression of Gal4-p65 activity. Overexpression of a GR-interacting coactivator peptide blocked transactivation but did not affect transrepression of p65 or TNFalpha-induced IL-6 promoter activity. Finally, the GR antagonist RU486 did not recruit coactivators to GR but maintained the ability to transrepress p65 activity. Our data suggest that different coactivators utilize distinct contact points to interact with GR. Although GR interactions with specific coactivators are critical for transactivation, they appear to be dispensable for at least certain aspects of GR-mediated transrepression of nuclear factor-kappaB. This is consistent with the notion that all GR- mediated repression is not intrinsically linked to activation and can be separated mechanistically.

Coordinated control of cholesterol catabolism to bile acids and of gluconeogenesis via a novel mechanism of transcription regulation linked to the fasted-to-fed cycle

Bile acid metabolism plays an essential role in cholesterol homeostasis and is critical for the initiation of atherosclerotic disease. However, despite the recent advances, the molecular mechanisms whereby bile acids regulate gene transcription and cholesterol homeostasis in mammals still need further investigations. Here, we show that bile acids suppress transcription of the gene (CYP7A1) encoding cholesterol 7alpha-hydroxylase, the rate-limiting enzyme in bile acid biosynthesis, also through an unusual mechanism not involving the bile acid nuclear receptor, farnesoid X receptor. By performing cell-based reporter assays, protein/protein interaction, and chromatin immunoprecipitation assays, we demonstrate that bile acids impair the recruitment of peroxisome proliferator-activated receptor-gamma coactivator-1alpha and cAMP response element-binding protein-binding protein by hepatocyte nuclear factor-4alpha, a master regulator of CYP7A1. We also show for the first time that bile acids inhibit transcription of the gene (PEPCK) encoding phosphoenolpyruvate carboxykinase, the rate-limiting enzyme in gluconeogenesis, through the same farnesoid X receptor-independent mechanism. Chromatin immunoprecipitation assay revealed that bile acid-induced dissociation of coactivators from hepatocyte nuclear factor-4alpha decreased the recruitment of RNA polymerase II to the core promoter and downstream in the 3'-untranslated regions of these two genes, reflecting the reduction of gene transcription. Finally, we found that Cyp7a1 expression was stimulated in fasted mice in parallel to Pepck, whereas the same genes were repressed by bile acids. Collectively, these results reveal a novel regulatory mechanism that controls gene transcription in response to extracellular stimuli and argue that the transcription regulation by bile acids of genes central to cholesterol and glucose metabolism should be viewed dynamically in the context of the fasted-to-fed cycle.

Novel concepts in insulin regulation of hepatic gluconeogenesis

The regulation of hepatic gluconeogenesis is an important process in the adjustment of the blood glucose level, and pathological changes in the glucose production of the liver are a central characteristic in type 2 diabetes. The pharmacological intervention in signaling events that regulate the expression of the key gluconeogenic enzymes phosphoenolpyruvate carboxykinase (PEPCK) and the catalytic subunit glucose-6-phosphatase (G-6-Pase) is regarded as a potential strategy for the treatment of metabolic aberrations associated with this disease. However, such intervention requires a detailed understanding of the molecular mechanisms involved in the regulation of this process. Glucagon and glucocorticoids are known to increase hepatic gluconeogenesis by inducing the expression of PEPCK and G-6-Pase. The coactivator protein PGC-1 has been identified as an important mediator of this regulation. In contrast, insulin is known to suppress both PEPCK and G-6-Pase gene expression by the activation of PI 3-kinase. However, PI 3-kinase-independent pathways can also lead to the inhibition of gluconeogenic enzymes. This review focuses on signaling mechanisms and nuclear events that transduce the regulation of gluconeogenic enzymes.

Candidate gene association study in type 2 diabetes indicates a role for genes involved in beta-cell function as well as insulin action

Type 2 diabetes is an increasingly common, serious metabolic disorder with a substantial inherited component. It is characterised by defects in both insulin secretion and action. Progress in identification of specific genetic variants predisposing to the disease has been limited. To complement ongoing positional cloning efforts, we have undertaken a large-scale candidate gene association study. We examined 152 SNPs in 71 candidate genes for association with diabetes status and related phenotypes in 2,134 Caucasians in a case-control study and an independent quantitative trait (QT) cohort in the United Kingdom. Polymorphisms in five of 15 genes (33%) encoding molecules known to primarily influence pancreatic beta-cell function-ABCC8 (sulphonylurea receptor), KCNJ11 (KIR6.2), SLC2A2 (GLUT2), HNF4A (HNF4alpha), and INS (insulin)-significantly altered disease risk, and in three genes, the risk allele, haplotype, or both had a biologically consistent effect on a relevant physiological trait in the QT study. We examined 35 genes predicted to have their major influence on insulin action, and three (9%)-INSR, PIK3R1, and SOS1-showed significant associations with diabetes. These results confirm the genetic complexity of Type 2 diabetes and provide evidence that common variants in genes influencing pancreatic beta-cell function may make a significant contribution to the inherited component of this disease. This study additionally demonstrates that the systematic examination of panels of biological candidate genes in large, well-characterised populations can be an effective complement to positional cloning approaches. The absence of large single-gene effects and the detection of multiple small effects accentuate the need for the study of larger populations in order to reliably identify the size of effect we now expect for complex diseases.

PGC-1beta in the regulation of hepatic glucose and energy metabolism

Peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1alpha) is a transcriptional coactivator that regulates multiple aspects of cellular energy metabolism, including mitochondrial biogenesis, hepatic gluconeogenesis, and beta-oxidation of fatty acids. PGC-1alpha mRNA levels are increased in both type-1 and type-2 diabetes and may contribute to elevated hepatic glucose production in diabetic states. We have recently described PGC-1beta, a novel transcriptional coactivator that is a homolog of PGC-1alpha. Although PGC-1beta shares significant sequence similarity and tissue distribution with PGC-1alpha, the biological activities of PGC-1beta in the regulation of cellular metabolism is unknown. In this study, we used an adenoviral-mediated expression system to study the function of PGC-1beta both in cultured hepatocytes and in the liver of rats. PGC-1beta, like PGC-1alpha, potently induces the expression of an array of mitochondrial genes involved in oxidative metabolism. However, in contrast to PGC-1alpha, PGC-1beta poorly activates the expression of gluconeogenic genes in hepatocytes or liver in vivo, illustrating that these two coactivators play distinct roles in hepatic glucose metabolism. The reduced ability of PGC-1beta to induce gluconeogenic genes is due, at least in part, to its inability to physically associate with and coactivate hepatic nuclear receptor 4alpha (HNF4alpha) and forkhead transcription factor O1 (FOXO1), two critical transcription factors that mediate the activation of gluconeogenic gene expression by PGC-1alpha. These data illustrate that PGC-1beta and PGC-1alpha have distinct arrays of activities in hepatic energy metabolism.

Localization and hormonal control of serine dehydratase during metabolic acidosis differ markedly from those of phosphoenolpyruvate carboxykinase in rat kidney

Serine dehydratase (SDH) is abundant in the rat liver but scarce in the kidney. When administrated with dexamethasone, the renal SDH activity was augmented 20-fold, whereas the hepatic SDH activity was affected little. In situ hybridization and immunohistochemistry revealed that SDH was localized to the proximal straight tubule of the nephron. To address the role of this hormone, rats were made acidotic by gavage of NH(4)Cl. Twenty-two hours later, the SDH activity was increased three-fold along with a six-fold increment in the phosphoenolpyruvate carboxykinase (PEPCK) activity, a rate-limiting enzyme of gluconeogenesis. PEPCK, which is localized to the proximal tubules under the normal condition, spreads throughout the entire cortex to the outer medullary rays by acidosis, whereas SDH does not change regardless of treatment with dexamethasone or NH(4)Cl. When NH(4)Cl was given to adrenalectomized rats, in contrast to the SDH activity no longer increasing, the PEPCK activity responded to acidosis to the same extent as in the intact rats. A simultaneous administration of dexamethasone and NH(4)Cl into the adrenalectomized rats fully restored the SDH activity, demonstrating that the rise in the SDH activity during acidosis is primarily controlled by glucocorticoids. The present findings clearly indicate that the localization of SDH and its hormonal regulation during acidosis are strikingly different from those of PEPCK.

The AmCREB gene is an ortholog of the mammalian CREB/CREM family of transcription factors and encodes several splice variants in the honeybee brain

The transcription factor CREB (cAMP response element binding protein) is required for the switch from short-term to long-term synaptic plasticity and from short-term to long-term memory. Its activity is regulated by the cAMP-dependent signalling cascade, which has been shown to play a crucial role in the honeybee's long-term memory formation. To elucidate the role of the CREB in honeybee memory formation we analysed a CREB-homologous gene, AmCREB, which is expressed as several transcripts in the honeybee brain. Eight transcripts have been identified (AmCREB 1-8) that are generated by alternate splicing. One antibody generated against a subset of these variants reveals a cytosolic localization in the mushroom body alpha-lobes, the glomeruli of the antennal lobes, the protocerebral lobes, the central complex and in the optical lobes.

Dexamethasone induction of hypertension and diabetes is PPAR-alpha dependent in LDL receptor-null mice

Hypertension and diabetes are common side effects of glucocorticoid treatment. To determine whether peroxisome proliferator-activated receptor-alpha (PPAR-alpha) mediates these sequelae, mice deficient in low-density lipoprotein receptor (Ldlr-/-), with (Ppara+/+) or without (Ppara-/-) PPAR-alpha, were treated chronically with dexamethasone. Ppara+/+, but not Ppara-/-, mice developed hyperglycemia, hyperinsulinemia and hypertension. Similar effects on glucose metabolism were seen in a different model using C57BL/6 mice. Hepatic gluconeogenic gene expression was increased and insulin-mediated suppression of endogenous glucose production was less effective in dexamethasone-treated Ppara+/+ mice. Adenoviral reconstitution of PPAR-alpha in the livers of nondiabetic, normotensive, dexamethasone-treated Ppara-/- mice induced hyperglycemia, hyperinsulinemia and increased gluconeogenic gene expression. It also increased blood pressure, renin activity, sympathetic nervous activity and renal sodium retention. Human hepatocytes treated with dexamethasone and the PPAR-alpha agonist Wy14,643 induced PPARA and gluconeogenic gene expression. These results identify hepatic activation of PPAR-alpha as a mechanism underlying glucocorticoid-induced insulin resistance.

Dietary fat interacts with QTLs controlling induction of Pgc-1 alpha and Ucp1 during conversion of white to brown fat

To identify novel regulatory factors controlling induction of the brown adipocyte-specific mitochondrial uncoupling protein (Ucp1) mRNA in the retroperitoneal white fat depot, we previously mapped quantitative trait loci (QTLs) that control this trait to chromosomes 2, 3, 8, and 19. Since the peroxisome proliferator activator receptor-gamma coactivator-1alpha (PGC-1alpha) regulates Ucp1 and other genes of energy metabolism, we have evaluated whether the QTLs controlling Ucp1 mRNA levels also modulate Pgc-1alpha mRNA levels by analysis of backcross progeny from the A/J and C57BL/6J strains of mice. The results indicate that a locus on chromosome 3 orchestrates expression of Pgc-1alpha and Ucp1 in retroperitoneal fat of mice fed a low-fat diet; however, the effect of this locus on Pgc-1alpha is lost, and a significant correlation between Ucp1 and Pgc-1alpha is severely reduced in mice fed a high-fat diet. An additional QTL located on chromosome 5 has also been identified for the selective regulation of Ucp1 mRNA levels. Similar to the effects of a high-fat diet on the chromosome 3 QTL, linkage of the chromosome 5 QTL is also lost in mice on a high-fat diet. Thus dietary fat has a profound influence on PGC-1alpha-regulated pathways controlling energy metabolism in white fat. The allelic variation observed in the regulation of Ucp1 and Pgc-1alpha expression in brown adipocytes of white fat but not interscapular brown fat suggests that fundamentally different regulatory mechanisms exist to control the thermogenic capacities of these tissues.