Glucose regulation of the acetyl-CoA carboxylase promoter PI in rat hepatocytes

Normal and Neoplastic Growth Suppression by the Extended Myc Network

Among the first discovered and most prominent cellular oncogenes is MYC, which encodes a bHLH-ZIP transcription factor (Myc) that both activates and suppresses numerous genes involved in proliferation, energy production, metabolism and translation. Myc belongs to a small group of bHLH-ZIP transcriptional regulators (the Myc Network) that includes its obligate heterodimerization partner Max and six "Mxd proteins" (Mxd1-4, Mnt and Mga), each of which heterodimerizes with Max and largely opposes Myc's functions. More recently, a second group of bHLH-ZIP proteins (the Mlx Network) has emerged that bears many parallels with the Myc Network. It is comprised of the Myc-like factors ChREBP and MondoA, which, in association with the Max-like member Mlx, regulate smaller and more functionally restricted repertoires of target genes, some of which are shared with Myc. Opposing ChREBP and MondoA are heterodimers comprised of Mlx and Mxd1, Mxd4 and Mnt, which also structurally and operationally link the two Networks. We discuss here the functions of these "Extended Myc Network" members, with particular emphasis on their roles in suppressing normal and neoplastic growth. These roles are complex due to the temporal- and tissue-restricted expression of Extended Myc Network proteins in normal cells, their regulation of both common and unique target genes and, in some cases, their functional redundancy.

Fructose Metabolism in Cancer

The interest in fructose metabolism is based on the observation that an increased dietary fructose consumption leads to an increased risk of obesity and metabolic syndrome. In particular, obesity is a known risk factor to develop many types of cancer and there is clinical and experimental evidence that an increased fructose intake promotes cancer growth. The precise mechanism, however, in which fructose induces tumor growth is still not fully understood. In this article, we present an overview of the metabolic pathways that utilize fructose and how fructose metabolism can sustain cancer cell proliferation. Although the degradation of fructose shares many of the enzymes and metabolic intermediates with glucose metabolism through glycolysis, glucose and fructose are metabolized differently. We describe the different metabolic fates of fructose carbons and how they are connected to lipogenesis and nucleotide synthesis. In addition, we discuss how the endogenous production of fructose from glucose via the polyol pathway can be beneficial for cancer cells.

Causative and Sanative dynamicity of ChREBP in Hepato-Metabolic disorders

ChREBP is the master regulator of carbohydrate dependent glycolytic and lipogenic flux within metabolic tissues. It plays a vital role in hyper-calorific milieu by activating glycolysis, lipogenesis along with pentose phosphate shunt and glycogen synthesis, fostering immediate reduction in the systemic glycemic levels. Liver being the primary organ to sense disproportionate dietary intake and linked physiological stress, stimulates ChREBP to perform the aforementioned processes. Activated ChREBP also inhibits lipolysis and encourages proper disposal of excessive triglycerides into adipocytes from the liver ablating hepatic intracellular lipid trafficking. Chronic overeating or onset of positive energy balance, hyper-activates ChREBP and signals development, intensification of hepato-metabolic disorders, and allied discrepancies in the whole-body metabolic functioning. ChREBP thus gets negatively connotated as the primary regulator of hepatic disorders, owing to its inherent features as the primary glycemic sensor and the only transcription factor that can transduce glucose-dependent glycolytic and lipogenic signals. Through this review, we - try to recapitulate and emphasize on the sanative events coordinated by ChREBP in several pathophysiological states. In totality, we aim to uncouple the disease-causing aspects of ChREBP from its positive attributes evoked during a metabolic crisis, in hepato-metabolic diseases.

Glucose-6-Phosphate Regulates Hepatic Bile Acid Synthesis in Mice

It is well established that, besides facilitating lipid absorption, bile acids act as signaling molecules that modulate glucose and lipid metabolism. Bile acid metabolism, in turn, is controlled by several nutrient-sensitive transcription factors. Altered intrahepatic glucose signaling in type 2 diabetes associates with perturbed bile acid synthesis. We aimed to characterize the regulatory role of the primary intracellular metabolite of glucose, glucose-6-phosphate (G6P), on bile acid metabolism. Hepatic gene expression patterns and bile acid composition were analyzed in mice that accumulate G6P in the liver, that is, liver-specific glucose-6-phosphatase knockout (L-G6pc^-/- ) mice, and mice treated with a pharmacological inhibitor of the G6P transporter. Hepatic G6P accumulation induces sterol 12α-hydroxylase (Cyp8b1) expression, which is mediated by the major glucose-sensitive transcription factor, carbohydrate response element-binding protein (ChREBP). Activation of the G6P-ChREBP-CYP8B1 axis increases the relative abundance of cholic-acid-derived bile acids and induces physiologically relevant shifts in bile composition. The G6P-ChREBP-dependent change in bile acid hydrophobicity associates with elevated plasma campesterol/cholesterol ratio and reduced fecal neutral sterol loss, compatible with enhanced intestinal cholesterol absorption. Conclusion: We report that G6P, the primary intracellular metabolite of glucose, controls hepatic bile acid synthesis. Our work identifies hepatic G6P-ChREBP-CYP8B1 signaling as a regulatory axis in control of bile acid and cholesterol metabolism.

The ubiquitination ligase SMURF2 reduces aerobic glycolysis and colorectal cancer cell proliferation by promoting ChREBP ubiquitination and degradation

The glucose-responsive transcription factor carbohydrate response element-binding protein (ChREBP) critically promotes aerobic glycolysis and cell proliferation in colorectal cancer cells. It has been reported that ubiquitination may be important in the regulation of ChREBP protein levels and activities. However, the ChREBP-specific E3 ligase and molecular mechanism of ChREBP ubiquitination remains unclear. Using database exploration and expression analysis, we found here that levels of the E3 ligase SMURF2 (Smad-ubiquitination regulatory factor 2) negatively correlate with those of ChREBP in cancer tissues and cell lines. We observed that SMURF2 interacts with ChREBP and promotes ChREBP ubiquitination and degradation via the proteasome pathway. Interestingly, ectopic SMURF2 expression not only decreased ChREBP levels but also reduced aerobic glycolysis, increased oxygen consumption, and decreased cell proliferation in colorectal cancer cells. Moreover, SMURF2 knockdown increased aerobic glycolysis, decreased oxygen consumption, and enhanced cell proliferation in these cells, mostly because of increased ChREBP accumulation. Furthermore, we identified Ser/Thr kinase AKT as an upstream suppressor of SMURF2 that protects ChREBP from ubiquitin-mediated degradation. Taken together, our results indicate that SMURF2 reduces aerobic glycolysis and cell proliferation by promoting ChREBP ubiquitination and degradation via the proteasome pathway in colorectal cancer cells. We conclude that the SMURF2-ChREBP interaction might represent a potential target for managing colorectal cancer.

HCF-1 Regulates De Novo Lipogenesis through a Nutrient-Sensitive Complex with ChREBP

Carbohydrate response element binding protein (ChREBP) is a key transcriptional regulator of de novo lipogenesis (DNL) in response to carbohydrates and in hepatic steatosis. Mechanisms underlying nutrient modulation of ChREBP are under active investigation. Here we identify host cell factor 1 (HCF-1) as a previously unknown ChREBP-interacting protein that is enriched in liver biopsies of nonalcoholic steatohepatitis (NASH) patients. Biochemical and genetic studies show that HCF-1 is O-GlcNAcylated in response to glucose as a prerequisite for its binding to ChREBP and subsequent recruitment of OGT, ChREBP O-GlcNAcylation, and activation. The HCF-1:ChREBP complex resides at lipogenic gene promoters, where HCF-1 regulates H3K4 trimethylation to prime recruitment of the Jumonji C domain-containing histone demethylase PHF2 for epigenetic activation of these promoters. Overall, these findings define HCF-1's interaction with ChREBP as a previously unappreciated mechanism whereby glucose signals are both relayed to ChREBP and transmitted for epigenetic regulation of lipogenic genes.

MondoA/ChREBP: The usual suspects of transcriptional glucose sensing; Implication in pathophysiology

Identification of the Mondo glucose-responsive transcription factors family, including the MondoA and MondoB/ChREBP paralogs, has shed light on the mechanism whereby glucose affects gene transcription. They have clearly emerged, in recent years, as key mediators of glucose sensing by multiple cell types. MondoA and ChREBP have overlapping yet distinct expression profiles, which underlie their downstream targets and separate roles in regulating genes involved in glucose metabolism. MondoA can restrict glucose uptake and influences energy utilization in skeletal muscle, while ChREBP signals energy storage through de novo lipogenesis in liver and white adipose tissue. Because Mondo proteins mediate metabolic adaptations to changing glucose levels, a better understanding of cellular glucose sensing through Mondo proteins will likely uncover new therapeutic opportunities in the context of the imbalanced glucose homeostasis that accompanies metabolic diseases such as type 2 diabetes and cancer. Here, we provide an overview of structural homologies, transcriptional partners as well as the nutrient and hormonal mechanisms underlying Mondo proteins regulation. We next summarize their relative contribution to energy metabolism changes in physiological states and the evolutionary conservation of these pathways. Finally, we discuss their possible targeting in human pathologies.

Is hepatic lipogenesis fundamental for NAFLD/NASH? A focus on the nuclear receptor coactivator PGC-1β

Non-alcoholic fatty liver diseases are the hepatic manifestation of metabolic syndrome. According to the classical pattern of NAFLD progression, de novo fatty acid synthesis has been incriminated in NAFLD progression. However, this hypothesis has been challenged by the re-evaluation of NAFLD development mechanisms together with the description of the role of lipogenic genes in NAFLD and with the recent observation that PGC-1β, a nuclear receptor/transcription factor coactivator involved in the transcriptional regulation of lipogenesis, displays protective effects against NAFLD/NASH progression. In this review, we focus on the implication of lipogenesis and triglycerides synthesis on the development of non-alcoholic fatty liver diseases and discuss the involvement of these pathways in the protective role of PGC-1β toward these hepatic manifestations.

ChREBP Regulates Itself and Metabolic Genes Implicated in Lipid Accumulation in β-Cell Line

Carbohydrate response element binding protein (ChREBP) is an important transcription factor that regulates a variety of glucose-responsive genes in hepatocytes. To date, only two natural isoforms, Chrebpα and Chrebpβ, have been identified. Although ChREBP is known to be expressed in pancreatic β cells, most of the glucose-responsive genes have never been verified as ChREBP targets in this organ. We aimed to explore the impact of ChREBP expression on regulating genes linked to accumulation of lipid droplets, a typical feature of β-cell glucotoxicity. We assessed gene expression in 832/13 cells overexpressing constitutively active ChREBP (caChREBP), truncated ChREBP with nearly identical amino acid sequence to Chrebpβ, or dominant negative ChREBP (dnChREBP). Among multiple ChREBP-controlled genes, ChREBP was sufficient and necessary for regulation of Eno1, Pklr, Mdh1, Me1, Pdha1, Acly, Acaca, Fasn, Elovl6, Gpd1, Cpt1a, Rgs16, Mid1ip1,Txnip, and Chrebpβ. Expression of Chrebpα and Srebp1c were not changed by caChREBP or dnChREBP. We identified functional ChREBP binding sequences that were located on the promoters of Chrebpβ and Rgs16. We also showed that Rgs16 overexpression lead to increased considerable amounts of lipids in 832/13 cells. This phenotype was accompanied by reduction of Cpt1a expression and slight induction of Fasn and Pklr gene in these cells. In summary, we conclude that Chrebpβ modulates its own expression, not that of Chrebpα; it also regulates the expression of several metabolic genes in β-cells without affecting SREBP-1c dependent regulation. We also demonstrate that Rgs16 is one of the ChREBP-controlled genes that potentiate accumulation of lipid droplets in β-cells.

Genome-Wide Analysis of ChREBP Binding Sites on Male Mouse Liver and White Adipose Chromatin

Glucose is an essential nutrient that directly regulates the expression of numerous genes in liver and adipose tissue. The carbohydrate response element-binding protein (ChREBP) links glucose as a signaling molecule to multiple glucose-dependent transcriptional regulatory pathways, particularly genes involved in glycolytic and lipogenic processes. In this study, we used chromatin immunoprecipitation followed by next-generation sequencing to identify specific ChREBP binding targets in liver and white adipose tissue. We found a large number of ChREBP binding sites, which are attributable to 5825 genes in the liver, 2418 genes in white adipose tissue, and 5919 genes in both tissues. The majority of these target genes were involved in known metabolic processes. Pathways in insulin signaling, the adherens junction, and cancers were among the top 5 pathways in both tissues. Motif analysis revealed a consensus sequence CAYGYGnnnnnCRCRTG that was commonly shared by ChREBP binding sites. Putative ChREBP binding sequences were enriched on promoters of genes involved in insulin signaling pathway, insulin resistance, and tumorigenesis.

α-Lipoic acid as a triglyceride-lowering nutraceutical

Considering the current obesity epidemic in the United States (>100 million adults are overweight or obese), the prevalence of hypertriglyceridemia is likely to grow beyond present statistics of ∼30% of the population. Conventional therapies for managing hypertriglyceridemia include lifestyle modifications such as diet and exercise, pharmacological approaches, and nutritional supplements. It is critically important to identify new strategies that would be safe and effective in lowering hypertriglyceridemia. α-Lipoic acid (LA) is a naturally occurring enzyme cofactor found in the human body in small quantities. A growing body of evidence indicates a role of LA in ameliorating metabolic dysfunction and lipid anomalies primarily in animals. Limited human studies suggest LA is most efficacious in situations where blood triglycerides are markedly elevated. LA is commercially available as dietary supplements and is clinically shown to be safe and effective against diabetic polyneuropathies. LA is described as a potent biological antioxidant, a detoxification agent, and a diabetes medicine. Given its strong safety record, LA may be a useful nutraceutical, either alone or in combination with other lipid-lowering strategies, when treating severe hypertriglyceridemia and diabetic dyslipidemia. This review examines the current evidence regarding the use of LA as a means of normalizing blood triglycerides. Also presented are the leading mechanisms of action of LA on triglyceride metabolism.

Multiple E-boxes in the distal promoter of the rat pyruvate carboxylase gene function as a glucose-responsive element

Pyruvate carboxylase (PC) is an anaplerotic enzyme that regulates glucose-induced insulin secretion in pancreatic islets. Dysregulation of its expression is associated with type 2 diabetes. Herein we describe the molecular mechanism underlying the glucose-mediated transcriptional regulation of the PC gene. Incubation of the rat insulin cell line INS-1 832/13 with glucose resulted in a 2-fold increase in PC mRNA expression. Transient transfections of the rat PC promoter-luciferase reporter construct in the above cell line combined with mutational analysis indicated that the rat PC gene promoter contains the glucose-responsive element (GRE), comprising three canonical E-boxes (E1, E3 and E4) and one E-box-like element (E2) clustering between nucleotides –546 and –399, upstream of the transcription start site. Mutation of any of these E-boxes resulted in a marked reduction of glucose-mediated transcriptional induction of the reporter gene. Electrophoretic mobility shift assays revealed that the upstream stimulatory factors 1 and 2 (USF1 and USF2) bind to E1, the Specificity Protein-1 (Sp1) binds to E2, USF2 and the carbohydrate responsive element binding protein (ChREBP) binds to E4, while unknown factors binds to E3. High glucose promotes the recruitment of Sp1 to E2 and, USF2 and ChREBP to E4. Silencing the expression of Sp1, USF2 and ChREBP by their respective siRNAs in INS-1 832/13 cells blunted glucose-induced expression of endogenous PC. We conclude that the glucose-mediated transcriptional activation of the rat PC gene is regulated by at least these three transcription factors.

Soy Protein Suppresses Gene Expression of Acetyl-CoA Carboxylase Alpha from Promoter PI in Rat Liver

Dietary soy protein isolate (SPI) reduces hepatic lipogenesis by suppressing gene expression of lipogenic enzymes, including acetyl-CoA carboxylase (ACC). In order to elucidate the mechanism of this regulation, the effect of dietary SPI on promoter (PI and PII) specific gene expression of ACC alpha was investigated. Rats were fed experimental diets containing SPI or casein as a nitrogen source. SPI feeding decreased the hepatic contents of total ACC mRNA as well as triglyceride (TG) content, but dietary SPI affected the amount of sterol-regulatory element binding protein (SREBP)-1 mRNA and protein very little. The amount of ACC mRNA transcribed from PII promoter containing SRE was not significantly affected by dietary protein, while a significant decrease in PI-generated ACC mRNA content was observed in rats fed the SPI diet. These data suggest that SPI feeding decreased the hepatic contents of ACC alpha mRNA mainly by regulating PI promoter via a nuclear factor(s) other than SREBP-1.

Studies on the antidiabetic activities of Cordyceps militaris extract in diet-streptozotocin-induced diabetic Sprague-Dawley rats

Due to substantial morbidity and high complications, diabetes mellitus is considered as the third “killer” in the world. A search for alternative antidiabetic drugs from herbs or fungi is highly demanded. Our present study aims to investigate the antidiabetic activities of Cordyceps militaris on diet-streptozotocin-induced type 2 diabetes mellitus in rats. Diabetic rats were orally administered with water extract or alcohol extract at 0.05 g/kg and 2 g/kg for 3 weeks, and then, the factors levels related to blood glucose, lipid, free radicals, and even nephropathy were determined. Pathological alterations on liver and kidney were examined. Data showed that, similar to metformin, Cordyceps militaris extracts displayed a significant reduction in blood glucose levels by promoting glucose metabolism and strongly suppressed total cholesterol and triglycerides concentration in serum. Cordyceps militaris extracts exhibit antioxidative effects indicated by normalized superoxide dismutase and glutathione peroxidase levels. The inhibitory effects on blood urea nitrogen, creatinine, uric acid, and protein revealed the protection of Cordyceps militaris extracts against diabetic nephropathy, which was confirmed by pathological morphology reversion. Collectively, Cordyceps militaris extract, a safe pharmaceutical agent, presents excellent antidiabetic and antinephropathic activities and thus has great potential as a new source for diabetes treatment.

Flightless I homolog negatively regulates ChREBP activity in cancer cells

The glucose-responsive transcription factor carbohydrate responsive element binding protein (ChREBP) plays an important role in regulating glucose metabolism in support of anabolic synthesis in both hepatocytes and cancer cells. In order to further investigate the molecular mechanism by which ChREBP regulates transcription, we used a proteomic approach to identify proteins interacting with ChREBP. We found several potential ChREBP-interacting partners, one of which, flightless I homolog (FLII) was verified to interact and co-localize with ChREBP in HCT116 colorectal cancer and HepG2 hepatocellular carcinoma cells. FLII is a member of the gelsolin superfamily of actin-remodeling proteins and can function as a transcriptional co-regulator. The C-terminal 227 amino acid region of ChREBP containing the DNA-binding domain interacted with FLII. Both the N-terminal leucine-rich repeat (LRR) domain and C-terminal gelsolin homolog domain (GLD) of FLII interacted and co-localized with ChREBP. ChREBP and FLII localized in both the cytoplasm and nucleus of cancer cells. Glucose increased expression and nuclear localization of ChREBP, and had minimal effect on the level and distribution of FLII. FLII knockdown using siRNAs increased mRNA and protein levels of ChREBP-activated genes and decreased transcription of ChREBP-repressed genes in cancer cells. Conversely, FLII overexpression negatively regulated ChREBP-mediated transcription in cancer cells. Our findings suggest that FLII is a component of the ChREBP transcriptional complex and negatively regulates ChREBP function in cancer cells.

FOXO1 competes with carbohydrate response element-binding protein (ChREBP) and inhibits thioredoxin-interacting protein (TXNIP) transcription in pancreatic beta cells

Thioredoxin-interacting protein (TXNIP) has emerged as an important factor in pancreatic beta cell biology, and tight regulation of TXNIP levels is necessary for beta cell survival. However, the mechanisms regulating TXNIP expression have only started to be elucidated. The forkhead boxO1 transcription factor (FOXO1) has been reported to up-regulate TXNIP expression in neurons and endothelial cells but to down-regulate TXNIP in liver, and the effects on beta cells have remained unknown. We now have found that FOXO1 binds to the TXNIP promoter in vivo in human islets and INS-1 beta cells and significantly decreases TXNIP expression. TXNIP promoter deletion analyses revealed that an E-box motif conferring carbohydrate response element-binding protein (ChREBP)-mediated, glucose-induced TXNIP expression is necessary and sufficient for this effect, and electromobility shift assays confirmed FOXO1 binding to this site. Moreover, FOXO1 blocked glucose-induced TXNIP expression and reduced glucose-induced ChREBP binding at the TXNIP promoter without affecting ChREBP expression or nuclear localization, suggesting that FOXO1 may compete with ChREBP for binding to the TXNIP promoter. In fact, a FOXO1 DNA-binding mutant (FOXO1-H215R) failed to inhibit TXNIP transcription, and the effects were not restricted to TXNIP as FOXO1 also inhibited transcription of other ChREBP target genes such as liver pyruvate kinase. Together, these results demonstrate that FOXO1 inhibits beta cell TXNIP transcription and suggest that FOXO1 confers this inhibition by interfering with ChREBP DNA binding at target gene promoters. Our findings thereby reveal a novel gene regulatory mechanism and a previously unappreciated cross-talk between FOXO1 and ChREBP, two major metabolic signaling pathways.

Interaction of C/EBP-beta and NF-Y factors constrains activity levels of the nutritionally controlled promoter IA expressing the acetyl-CoA carboxylase-alpha gene in cattle

Background

The enzyme acetyl-CoA carboxylase-alpha (ACC-α) is rate limiting for de novo fatty acid synthesis. Among the four promoters expressing the bovine gene, promoter IA (PIA) is dominantly active in lipogenic tissues. This promoter is in principal repressed but activated under favorable nutritional conditions. Previous analyses already coarsely delineated the repressive elements on the distal promoter but did not resolve the molecular nature of the repressor. Knowledge about the molecular functioning of this repressor is fundamental to understanding the nutrition mediated regulation of PIA activity. We analyzed here the molecular mechanism calibrating PIA activity.

Results

We finely mapped the repressor binding sites in reporter gene assays and demonstrate together with Electrophoretic Mobility Shift Assays that nuclear factor-Y (NF-Y) and CCAAT/enhancer binding protein-β (C/EBPβ) each separately repress PIA activity by binding to their cognate low affinity sites, located on distal elements of the promoter. Simultaneous binding of both factors results in strongest repression. Paradoxically, over expression of NFY factors, but also - and even more so - of C/EBPβ significantly activated the promoter when bound to high affinity sites on the proximal promoter. However, co-transfection experiments revealed that NF-Y may eventually diminish the strong stimulatory effect of C/EBPβ at the proximal PIA in a dose dependent fashion. We validated by chromatin immunoprecipitation, that NF-Y and C/EBP factors may physically interact.

Conclusion

The proximal promoter segment of PIA appears to be principally in an active state, since even minute concentrations of both, NF-Y and C/EBPβ factors can saturate the high affinity activator sites. Higher factor concentrations will saturate the low affinity repressive sites on the distal promoter resulting in reduced and calibrated promoter activity. Based on measurements of the mRNA concentrations of those factors in different tissues we propose that the interplay of both factors may set tissue-specific limits for PIA activity.

Carbon source metabolism and its regulation in cancer cells

Cancer cell proliferation and progression require sufficient supplies of nutrients including carbon sources, nitrogen sources, and molecular oxygen. Particularly, carbon sources and molecular oxygen are critical for the generation of ATP and building blocks, and for the maintenance of intracellular redox status. However, solid tumors frequently outgrow the blood supply, resulting in nutrient insufficiency. Accordingly, cancer cell metabolism shows aberrant biochemical features that are consequences of oncogenic signaling and adaptation. Those adaptive metabolism features, including the Warburg effect and addiction to glutamine, may form the biochemical basis for resistance to chemotherapy and radiation. A better understanding of the regulatory mechanisms that link the signaling pathways to adaptive metabolic reprogramming may identify novel biomarkers for drug development. In this review, we focus on the regulation of carbon source utilization at a cellular level, emphasizing its relevance to proliferative biosynthesis in cancer cells. We summarize the essential needs of proliferating cells and the metabolic features of glucose, lipids, and glutamine, and we review the roles of transcription regulators (i.e., HIF-1, c-Myc, and p53) and two major oncogenic signaling pathways (i.e., PI3K-Akt and MAPK) in regulating the utilization of carbon sources. Finally, the effects of glucose on cell proliferation and perspective from both biochemical and cellular angles are discussed.

Role of transcription factor modifications in the pathogenesis of insulin resistance

Non-alcoholic fatty liver disease (NAFLD) is characterized by fat accumulation in the liver not due to alcohol abuse. NAFLD is accompanied by variety of symptoms related to metabolic syndrome. Although the metabolic link between NAFLD and insulin resistance is not fully understood, it is clear that NAFLD is one of the main cause of insulin resistance. NAFLD is shown to affect the functions of other organs, including pancreas, adipose tissue, muscle and inflammatory systems. Currently efforts are being made to understand molecular mechanism of interrelationship between NAFLD and insulin resistance at the transcriptional level with specific focus on post-translational modification (PTM) of transcription factors. PTM of transcription factors plays a key role in controlling numerous biological events, including cellular energy metabolism, cell-cycle progression, and organ development. Cell type- and tissue-specific reversible modifications include lysine acetylation, methylation, ubiquitination, and SUMOylation. Moreover, phosphorylation and O-GlcNAcylation on serine and threonine residues have been shown to affect protein stability, subcellular distribution, DNA-binding affinity, and transcriptional activity. PTMs of transcription factors involved in insulin-sensitive tissues confer specific adaptive mechanisms in response to internal or external stimuli. Our understanding of the interplay between these modifications and their effects on transcriptional regulation is growing. Here, we summarize the diverse roles of PTMs in insulin-sensitive tissues and their involvement in the pathogenesis of insulin resistance.

High-fat diet causes increased serum insulin and glucose which synergistically lead to renal tubular lipid deposition and extracellular matrix accumulation

Renal tubular lipid accumulation is associated with renal injury in the metabolic syndrome, but its mechanisms are not fully elucidated. The purpose of the present study was to investigate the exact mechanism of renal tubular lipid accumulation in the diet-induced metabolic syndrome. The in vivo experiments showed that a high-fat diet induced hyperglycaemia, hyperinsulinaemia and hypertriacylglycerolaemia, subsequent increases in sterol regulatory element binding protein-1 (SREBP-1) and transforming growth factor-β1 (TGF-β1), lipid droplet deposit in renal tubular cells and interstitial extracellular matrix accumulation in Wistar rats. A human renal proximal tubular epithelial cell line (HKC) was used to determine the direct role of insulin, and the results revealed that insulin induced SREBP-1, fatty acid synthase (FASN), TGF-β1 expressions, lipid droplet and extracellular matrix deposits. Knockdown of SREBP-1 by RNA interference technology significantly inhibited FASN, TGF-β1 up-regulation, lipid and extracellular matrix accumulation caused by insulin. In addition, we found that insulin and high glucose could synergistically increase SREBP-1, FASN, TGF-β1 and fibronectin expressions in HKC cells. These results indicate that high-fat diet-induced increased serum insulin and glucose synergistically cause renal tubular lipid deposit and extracellular matrix accumulation via the SREBP-1 pathway.

Liver X Receptor: an oxysterol sensor and a major player in the control of lipogenesis

De novo fatty acid biosynthesis is also called lipogenesis. It is a metabolic pathway that provides the cells with fatty acids required for major cellular processes such as energy storage, membrane structures and lipid signaling. In this article we will review the role of the Liver X Receptors (LXRs), nuclear receptors that sense oxysterols, in the transcriptional regulation of genes involved in lipogenesis.

PTEN ameliorates high glucose-induced lipid deposits through regulating SREBP-1/FASN/ACC pathway in renal proximal tubular cells

Phosphatase and tensin homology deleted on chromosome ten (PTEN) is a negative regulator of PI3K/Akt pathway, and here we investigated the effect of PTEN on lipogenesis in diabetic rats and high glucose-stimulated human renal proximal tubular cell line (HKC). Decreased PTEN and increased phospho-Akt were found in kidney of diabetic rats, and in vitro research revealed that high glucose attenuated PTEN expression in a time-dependent manner, concomitant with activation of Akt. Again, expression of PTEN significantly inhibited high glucose-caused increased phospho-Akt and lipogenic genes including SREBP-1, fatty acid synthase (FASN) and acetyl-CoA carboxylase (ACC). Furthermore, we confirmed inhibition of TGF-β1 pathway with SB431542 blocked the effect of high glucose on PTEN down-regulation, an increase in phospho-Akt and lipogenesis. These above data suggest that decreased PTEN mediates high glucose-induced lipogenesis in renal proximal tubular cells and TGF-β1 might be involved in PTEN down-regulation.

Replacing dietary glucose with fructose increases ChREBP activity and SREBP-1 protein in rat liver nucleus

Diets high in fructose cause hypertriglyceridemia and insulin resistance in part due to simultaneous induction of gluconeogenic and lipogenic genes in liver. We investigated the mechanism underlying the unique pattern of gene induction by dietary fructose. Male Sprague-Dawley rats (n=6 per group) were meal-fed (4h/d) either 63% (w/w) glucose or 63% fructose diet. After two weeks, animals were killed at the end of the last meal. Nuclear SREBP-1 was 2.2 times higher in fructose-fed rats than glucose-fed rats. Nuclear FoxO1 was elevated 1.7 times in fructose group, but did not reach significance (P=0.08). Unexpectedly, no difference was observed in nuclear ChREBP between two groups. However, ChREBP DNA binding was 3.9x higher in fructose-fed animals without an increase in xylulose-5-phospate, a proposed ChREBP activator. In conclusion, the gene induction by dietary fructose is likely to be mediated in part by simultaneously increased ChREBP activity, SREBP-1 and possibly FoxO1 protein in nucleus.

Lipoic acid improves hypertriglyceridemia by stimulating triacylglycerol clearance and downregulating liver triacylglycerol secretion

Elevated blood triacylglycerol (TG) is a significant contributing factor to the current epidemic of obesity-related health disorders, including type-2 diabetes, nonalcoholic fatty liver disease, and cardiovascular disease. The observation that mice lacking the enzyme sn-glycerol-3-phosphate acyltransferase are protected from insulin resistance suggests the possibility that the regulation of TG synthesis be a target for therapy. Five-week-old Zucker Diabetic Fatty (ZDF) rats were fed a diet containing (R)-alpha-lipoic acid (LA, approximately 200mg/kg body weight per day) for 5 weeks. LA offset the rise in blood and liver TG by inhibiting liver lipogenic gene expression (e.g. sn-glycerol-3-phosphate acyltransferase-1 and diacylglycerol O-acyltransferase-2), lowering hepatic TG secretion, and stimulating clearance of TG-rich lipoproteins. LA-induced TG lowering was not due to the anorectic properties of LA, as pair-fed rats developed hypertriglyceridemia. Livers from LA-treated rats exhibited elevated glycogen content, suggesting dietary carbohydrates were stored as glycogen rather than becoming lipogenic substrate. Although AMP-activated protein kinase (AMPK) reportedly mediates the metabolic effects of LA in rodents, no change in AMPK activity was observed, suggesting LA acted independently of this kinase. The hepatic expression of peroxisome proliferator activated receptor alpha (PPARalpha) target genes involved in fatty acid beta-oxidation was either unchanged or decreased with LA, indicating a different mode of action than for fibrate drugs. Given its strong safety record, LA may have potential clinical applications for the treatment or prevention of hypertriglyceridemia and diabetic dyslipidemia.

Hepatic expression of the SPOT 14 (S14) paralog S14-related (Mid1 interacting protein) is regulated by dietary carbohydrate

The Spot 14 (S14) gene is rapidly up-regulated by signals that induce lipogenesis such as enhanced glucose metabolism and thyroid hormone administration. Previous studies in S14 null mice show that S14 is required for normal lipogenesis in the lactating mammary gland, but not the liver. We speculated that the lack of a hepatic phenotype was due to the expression of a compensatory gene. We recently reported that this gene is likely an S14 paralog that we named S14-Related (S14-R). S14-R is expressed in the liver, but not in the mammary gland. If S14-R compensates for the absence of S14 in the liver, we hypothesized that, like S14, S14-R expression should be glucose responsive. Here, we report that hepatic S14-R mRNA levels increase with high-carbohydrate feeding in mice or within 2 h of treating cultured hepatocytes with elevated glucose. A potential carbohydrate response element (ChoRE) was identified at position -458 of the S14-R promoter. Deletion of or point mutations within the putative S14-R ChoRE led to 50-95% inhibition of the glucose response. Gel-shift analysis revealed that the glucose-activated transcription complex carbohydrate responsive element-binding protein/Max-like protein X (Mlx) binds to the S14-R ChoRE. Finally, S14-R glucose induction is completely blocked when a dominant-negative form of Mlx is overexpressed in primary hepatocytes. In conclusion, our results indicate that the S14-R gene is a glucose-responsive target of carbohydrate responsive element-binding protein/Mlx and suggest that the S14-R protein is a compensatory factor, at least partially responsible for the normal liver lipogenesis observed in the S14 null mouse.

Glucose activates ChREBP by increasing its rate of nuclear entry and relieving repression of its transcriptional activity

Carbohydrate response element-binding protein (ChREBP) is a glucose-responsive transcription factor that activates genes involved in de novo lipogenesis in mammals. The current model for glucose activation of ChREBP proposes that increased glucose metabolism triggers a cytoplasmic to nuclear translocation of ChREBP that is critical for activation. However, we find that ChREBP actively shuttles between the cytoplasm and nucleus in both low and high glucose in the glucose-sensitive beta cell-derived line, 832/13. Glucose stimulates a 3-fold increase in the rate of ChREBP nuclear entry, but trapping ChREBP in the nucleus by mutagenesis or with a nuclear export inhibitor does not lead to constitutive activation. In fact, mutational studies targeting the nuclear export signal of ChREBP also identified a distinct function essential for glucose-dependent transcriptional activation. From this, we conclude that an additional event independent of nuclear translocation is required for activation. The N-terminal segment of ChREBP (amino acids 1-298) has previously been shown to repress activity under basal conditions. This segment has five highly conserved regions, Mondo conserved regions 1-5 (MCR1 to -5). Based on activating mutations in MCR2 and MCR5, we propose that these two regions act coordinately to repress ChREBP in low glucose. In addition, other mutations in MCR2 and mutations in MCR3 were found to prevent glucose activation. Hence, we conclude that both relief of repression and adoption of an activating form are required for ChREBP activation.

Increased de novo lipogenesis and delayed conversion of large VLDL into intermediate density lipoprotein particles contribute to hyperlipidemia in glycogen storage disease type 1a

Glycogen storage disease type 1a (GSD-1a) is a metabolic disorder characterized by fasting-induced hypoglycemia, hepatic steatosis, and hyperlipidemia. The mechanisms underlying the lipid abnormalities are largely unknown. To investigate these mechanisms seven GSD-1a patients and four healthy control subjects received an infusion of [1-(13)C]acetate to quantify cholesterogenesis and lipogenesis. In a subset of patients, [1-(13)C]valine was given to assess lipoprotein metabolism and [2-(13)C]glycerol to determine whole body lipolysis. Cholesterogenesis was 274 +/- 112 mg/d in controls and 641 +/- 201 mg/d in GSD-1a patients (p < 0.01). Plasma triglyceride-palmitate derived from de novo lipogenesis was 7.1 +/- 9.4 and 86.3 +/- 42.5 micromol/h in controls and patients, respectively (p < 0.01). Production of VLDL did not show a consistent difference between the groups, but conversion of VLDL into intermediate density lipoproteins was relatively retarded in all patients (0.6 +/- 0.5 pools/d) compared with controls (4.3 +/- 1.8 pools/d). Fractional catabolic rate of intermediate density lipoproteins was lower in patients (0.8 +/- 0.6 pools/d) compared with controls (3.1 +/- 1.5 pools/d). Whole body lipolysis was similar, i.e., 4.5 +/- 1.9 micromol/kg/min in patients and 3.8 +/- 1.9 micromol/kg/min in controls. Hyperlipidemia in GSD-1a is associated with strongly increased lipid production and a slower relative conversion of VLDL to LDL.

Identification and function of phosphorylation in the glucose-regulated transcription factor ChREBP

In the liver, induction of genes encoding enzymes involved in de novo lipogenesis occurs in response to increased glucose metabolism. ChREBP (carbohydrate-response-element-binding protein) is a basic helix-loop-helix/leucine zipper transcription factor that regulates expression of these genes. To evaluate the potential role of ChREBP phosphorylation in its regulation, we used MS to identify modified residues. In the present paper, we report the detection of multiple phosphorylation sites of ChREBP expressed in hepatocytes, several of which are only observed under high-glucose conditions. Mutation of each of these serine/threonine residues of ChREBP did not alter its ability to respond to glucose. However, mutation of five N-terminal phosphoacceptor sites resulted in a major decrease in activity under high-glucose conditions. These phosphorylated residues are located within a region of ChREBP (amino acids 1-197) that is critical for glucose regulation. Mutation of Ser(56) within this region to an aspartate residue resulted in increased nuclear accumulation and activity under high-glucose conditions. Together, these data suggest that ChREBP activity is regulated by complex multisite phosphorylation patterns involving its N-terminal regulatory region.

Dietary fructose induces a wide range of genes with distinct shift in carbohydrate and lipid metabolism in fed and fasted rat liver

Dietary fructose has been suspected to contribute to development of metabolic syndrome. However, underlying mechanisms of fructose effects are not well characterized. We investigated metabolic outcomes and hepatic expression of key regulatory genes upon fructose feeding under well defined conditions. Rats were fed a 63% (w/w) glucose or fructose diet for 4 h/day for 2 weeks, and were killed after feeding or 24-hour fasting. Liver glycogen was higher in the fructose-fed rats, indicating robust conversion of fructose to glycogen through gluconeogenesis despite simultaneous induction of genes for de novo lipogenesis and increased liver triglycerides. Fructose feeding increased mRNA of previously unidentified genes involved in macronutrient metabolism including fructokinase, aldolase B, phosphofructokinase-1, fructose-1,6-bisphosphatase and carbohydrate response element binding protein (ChREBP). Activity of glucose-6-phosphate dehydrogenase, a key enzyme for ChREBP activation, remained elevated in both fed and fasted fructose groups. In the fasted liver, the fructose group showed lower non-esterified fatty acids, triglycerides and microsomal triglyceride transfer protein mRNA, suggesting low VLDL synthesis even though plasma VLDL triglycerides were higher. In conclusion, fructose feeding induced a broader range of genes than previously identified with simultaneous increase in glycogen and triglycerides in liver. The induction may be in part mediated by ChREBP.

Carbohydrate restriction alters hepatic cholesterol metabolism in guinea pigs fed a hypercholesterolemic diet

The current study was undertaken to evaluate the effect of carbohydrate restriction on hepatic cholesterol metabolism in guinea pigs fed a hypercholesterolemic diet. Hartley male guinea pigs (n = 10 per group) were fed 1 of 3 diets: a diet with a percent energy distribution of 42:23:35 carbohydrate:protein:fat and 0.04% cholesterol (control), a diet with the same macronutrient distribution but with 0.25% cholesterol (HChol), or a carbohydrate-restricted (CR) diet with a percent energy distribution of 11:30:59 carbohydrate:protein:fat and 0.25% cholesterol for 12 wk. There was more accumulation of hepatic cholesterol and triglycerides as well as lower 3-hydroxy-3-methyl glutaryl-CoA reductase messenger RNA abundance in guinea pigs fed the high-cholesterol diets (HChol and CR) (P < 0.01). Guinea pigs fed the CR diet had lower concentrations of hepatic total cholesterol and cholesteryl ester than those fed the HChol diet (P < 0.05). There was no diet effect on hepatic LDL receptor expression. Hepatic acyl CoA cholesteryl acyltransferase (ACAT) activity was lowest in guinea pigs fed the low-cholesterol diet (9.7 +/- 4.8 pmol.min(-1).mg(-1)), intermediate in those fed the CR diet (37.3 +/- 12.4 pmol.min(-1).mg protein(-1)), and highest in guinea pigs fed the HChol diet (55.9 +/- 11.2 pmol.min(-1).mg(-1)). ACAT activity was significantly correlated with hepatic cholesterol (r = 0.715; P < 0.01) and LDL cholesterol (r = 0.59; P < 0.01) for all dietary groups, suggesting a major role of this enzyme in hepatic cholesterol homeostasis and in lipoprotein concentrations. These results indicate that dietary cholesterol increases hepatic lipid accumulation and affects hepatic cholesterol homeostasis. Carbohydrate restriction in the presence of high cholesterol is associated with lower hepatic ACAT activity and an attenuation of hepatic cholesterol accumulation.

Identification of cis-regulatory elements and trans-acting proteins of the rat carbohydrate response element binding protein gene

Carbohydrate response element binding protein (ChREBP) is a transcription factor that activates liver glycolytic and lipogenetic enzyme genes in response to high carbohydrate diet. Here we report the transcriptional regulatory mechanisms for the rat ChREBP gene. Firstly, we determined the transcription initiation site and the nucleotide sequences of the rat ChREBP promoter region encompassing approximately 900bp from the ATG initiation codon. Reporter gene assays demonstrated that the major positive regulatory region exists in the nucleotide sequence between -163 and -32 of the ChREBP gene. This region contains a cluster of putative transcription factor binding elements that consist of two specificity protein 1 (Sp1) binding sites (-66 to -50 and -93 to -78), a sterol regulatory element (-101 to -110), and two nuclear factor-Y (NF-Y) binding sites (-23 to -19 and -131 to -127). Mutations introduced into these sites caused marked reduction of ChREBP promoter activities. Functional synergisms were observed between Sp1/NF-Y and Sp1/sterol regulatory element-binding protein. Additionally, electrophoretic mobility shift assays and chromatin immunoprecipitation assays demonstrated that these factors bound to these elements. Thus, we conclude that functional synergisms between these transcription factors are critical for ChREBP gene transcription.

The promoter for the gene encoding the catalytic subunit of rat glucose-6-phosphatase contains two distinct glucose-responsive regions

Glucose homeostasis requires the proper expression and regulation of the catalytic subunit of glucose-6-phosphatase (G-6-Pase), which hydrolyzes glucose 6-phosphate to glucose in glucose-producing tissues. Glucose induces the expression of G-6-Pase at the transcriptional and posttranscriptional levels by unknown mechanisms. To better understand this metabolic regulation, we mapped the cis-regulatory elements conferring glucose responsiveness to the rat G-6-Pase gene promoter in glucose-responsive cell lines. The full-length (-4078/+64) promoter conferred a moderate glucose response to a reporter construct in HL1C rat hepatoma cells, which was dependent on coexpression of glucokinase. The same construct provided a robust glucose response in 832/13 INS-1 rat insulinoma cells, which are not glucogenic. Glucose also strongly increased endogenous G-6-Pase mRNA levels in 832/13 cells and in rat pancreatic islets, although the induced levels from islets were still markedly lower than in untreated primary hepatocytes. A distal promoter region was glucose responsive in 832/13 cells and contained a carbohydrate response element with two E-boxes separated by five base pairs. Carbohydrate response element-binding protein bound this region in a glucose-dependent manner in situ. A second, proximal promoter region was glucose responsive in both 832/13 and HL1C cells, with a hepatocyte nuclear factor 1 binding site and two cAMP response elements required for glucose responsiveness. Expression of dominant-negative versions of both cAMP response element-binding protein and CAAT/enhancer-binding protein blocked the glucose response of the proximal region in a dose-dependent manner. We conclude that multiple, distinct cis-regulatory promoter elements are involved in the glucose response of the rat G-6-Pase gene.

A critical role for the loop region of the basic helix-loop-helix/leucine zipper protein Mlx in DNA binding and glucose-regulated transcription

The carbohydrate response element (ChoRE) is a cis-acting sequence found in the promoters of genes induced transcriptionally by glucose. The ChoRE is composed of two E box-like motifs that are separated by 5 bp and is recognized by two basic helix–loop–helix/leucine zipper (bHLH/LZ) proteins, ChREBP and Mlx, which heterodimerize to bind DNA. In this study, we demonstrate that two ChREBP/Mlx heterodimers interact to stabilize binding to the tandem E box-like motifs in the ChoRE. Based on a model structure that we generated of ChREBP/Mlx bound to the ChoRE, we hypothesized that intermolecular interactions between residues within the Mlx loop regions of adjacent heterodimers are responsible for stabilizing the complex. We tested this hypothesis by preparing Mlx variants in which the loop region was replaced with that of another family member or mutated at several key residues. These Mlx variants retained their ability to bind to a single perfect E-box motif as a heterodimer with ChREBP, but no longer bound to the ChoRE nor supported glucose responsive activity. In summary, our results support a model in which the loop regions of Mlx play an important functional role in mediating the coordinate binding of ChREBP/Mlx heterodimers to the ChoRE.

Hepatocyte nuclear factor-4alpha contributes to carbohydrate-induced transcriptional activation of hepatic fatty acid synthase

Refeeding a carbohydrate-rich meal after a fast produces a co-ordinated induction of key glycolytic and lipogenic genes in the liver. The transcriptional response is mediated by insulin and increased glucose oxidation, and both signals are necessary for optimal induction of FAS (fatty acid synthase). The glucose-regulated component of FAS promoter activation is mediated in part by ChREBP [ChoRE (carbohydrate response element)-binding protein], which binds to a ChoRE between -7300 and -7000 base-pairs in a carbohydrate-dependent manner. Using in vivo footprinting with nuclei from fasted and refed rats, we identify an imperfect DR-1 (direct repeat-1) element between -7110 and -7090 bp that is protected upon carbohydrate refeeding. Electrophoretic mobility-shift assays establish that this DR-1 element binds HNF-4alpha (hepatocyte nuclear factor 4alpha), and chromatin immunoprecipitation establishes that HNF-4alpha binding to this site is increased approx. 3-fold by glucose refeeding. HNF-4alpha transactivates reporter constructs containing the distal FAS promoter in a DR-1-dependent manner, and this DR-1 is required for full glucose induction of the FAS promoter in primary hepatocytes. In addition, a 3-fold knockdown of hepatocyte HNF-4alpha by small interfering RNA produces a corresponding decrease in FAS gene induction by glucose. Co-immunoprecipitation experiments demonstrate a physical interaction between HNF-4alpha and ChREBP in primary hepatocytes, further supporting an important complementary role for HNF-4alpha in glucose-induced activation of FAS transcription. Taken together, these observations establish for the first time that HNF-4alpha functions in vivo through a DR-1 element in the distal FAS promoter to enhance gene transcription following refeeding of glucose to fasted rats. The findings support the broader view that HNF-4alpha is an integral component of the hepatic nutrient sensing system that co-ordinates transcriptional responses to transitions between nutritional states.

ChREBP*Mlx is the principal mediator of glucose-induced gene expression in the liver

In mammals, glucose-regulated gene expression has been best characterized in the liver, where increased glucose metabolism induces transcription of genes encoding enzymes involved in de novo lipogenesis. ChREBP and Mlx dimerize and function together as a glucose-responsive transcription factor to regulate target genes, such as liver-type pyruvate kinase, acetyl-CoA carboxylase 1, and fatty acid synthase. To identify additional glucose-responsive genes in the liver, we used microarray analysis to compare gene expression patterns in low and high glucose conditions in hepatocytes. Target genes of ChREBP.Mlx were simultaneously identified by gene profiling in the presence or absence of a dominant negative Mlx. Of 224 genes that are induced by glucose, 139 genes (62%) were also inhibited by the dominant negative Mlx. Lipogenic enzyme genes involved in the entire pathway of de novo lipogenesis were found to be glucose-responsive target genes of ChREBP.Mlx. Genes encoding enzymes in other metabolic pathways and numerous regulators of metabolism were also identified. To determine if any of these genes are direct targets of ChREBP.Mlx, we searched for ChoRE-like sequences in the 5'-flanking regions of several genes that responded rapidly to glucose. ChoRE sequences that bound to ChREBP.Mlx and supported a glucose response were identified in two additional genes. Combining all of the known ChoRE sequences, we generated a modified ChoRE consensus sequence, CAYGNGN(5)CNCRTG. In summary, ChREBP.Mlx is the principal transcription factor regulating glucose-responsive genes in the liver and coordinately regulates a family of genes required for glucose utilization and energy storage.

Hepatic gene regulation by glucose and polyunsaturated fatty acids: a role for ChREBP

The liver is a major site for carbohydrate metabolism (glycolysis and glycogen synthesis) and triglyceride synthesis (lipogenesis). In the last decade, increasing evidence has emerged to show that nutrients, in particular, glucose and fatty acids, are able to regulate hepatic gene expression in a transcriptional manner. Indeed, although insulin was long thought to be the major regulator of hepatic gene expression, it is now clear that glucose metabolism rather that glucose itself also contributes substantially to the coordinated regulation of carbohydrate and lipid homeostasis in liver. In fact, the recent discovery of the glucose-signaling transcription factor carbohydrate responsive element binding protein (ChREBP) shed some light on the molecular mechanisms by which glycolytic and lipogenic genes are reciprocally regulated by glucose and fatty acids in liver. Here, we will review some of the recent studies that have begun to elucidate the regulation and function of this key transcription factor in liver. Indeed, a better understanding of the mechanisms by which glucose and fatty acids control hepatic gene expression may provide novel insight into the development of new therapeutic strategies for a better management of diseases involving blood glucose and/or disorders of lipid metabolism.

Glucose-dependent transcriptional regulation by an evolutionarily conserved glucose-sensing module

We report here a novel mechanism for glucose-mediated activation of carbohydrate response element binding protein (ChREBP), a basic helix-loop-helix/leucine zipper (bHLH/ZIP) transcription factor of Mondo family that binds to carbohydrate response element in the promoter of some glucose-regulated genes and activates their expression upon glucose stimulation. Structure-function analysis of ChREBP in a highly glucose-sensitive system using GAL4-ChREBP fusion constructs revealed a glucose-sensing module (GSM) that mediates glucose responsiveness of ChREBP. GSM is conserved among Mondo family members; MondoA, a mammalian paralog of unknown function, and the GSM region of a Drosophila homolog were also found to be glucose responsive. GSM is composed of a low-glucose inhibitory domain (LID) and a glucose-response activation conserved element (GRACE). We have identified a new mechanism accounting for glucose responsiveness of ChREBP that involves specific inhibition of the transactivation activity of GRACE by LID under low glucose concentration and reversal of this inhibition by glucose in an orientation-sensitive manner. The intramolecular inhibition and its release by glucose is a regulatory mechanism that is independent of changes of subcellular localization or DNA binding activity, events that also appear to be involved in glucose responsiveness. This evolutionally conserved mechanism may play an essential role in glucose-responsive gene regulation.

Regulation of rat hepatic L-pyruvate kinase promoter composition and activity by glucose, n-3 polyunsaturated fatty acids, and peroxisome proliferator-activated receptor-alpha agonist

Carbohydrate regulatory element-binding protein (ChREBP), MAX-like factor X (MLX), and hepatic nuclear factor-4alpha (HNF-4alpha) are key transcription factors involved in the glucose-mediated induction of hepatic L-type pyruvate kinase (L-PK) gene transcription. n-3 polyunsaturated fatty acids (PUFA) and WY14643 (peroxisome proliferator-activated receptor alpha (PPARalpha) agonist) interfere with glucose-stimulated L-PK gene transcription in vivo and in rat primary hepatocytes. Feeding rats a diet containing n-3 PUFA or WY14643 suppressed hepatic mRNA(L-PK) but did not suppress hepatic ChREBP or HNF-4alpha nuclear abundance. Hepatic MLX nuclear abundance, however, was suppressed by n-3 PUFA but not WY14643. In rat primary hepatocytes, glucose-stimulated accumulation of mRNA(LPK) and L-PK promoter activity correlated with increased ChREBP nuclear abundance. This treatment also increased L-PK promoter occupancy by RNA polymerase II (RNA pol II), acetylated histone H3 (Ac-H3), and acetylated histone H4 (Ac-H4) but did not significantly impact L-PK promoter occupancy by ChREBP or HNF-4alpha. Inhibition of L-PK promoter activity by n-3 PUFA correlated with suppressed RNA pol II, Ac-H3, and Ac-H4 occupancy on the L-PK promoter. Although n-3 PUFA transiently suppressed ChREBP and MLX nuclear abundance, this treatment did not impact ChREBP-LPK promoter interaction. HNF4alpha-LPK promoter interaction was transiently suppressed by n-3 PUFA. Inhibition of L-PK promoter activity by WY14643 correlated with a transient decline in ChREBP nuclear abundance and decreased Ac-H4 interaction with the L-PK promoter. WY14643, however, had no impact on MLX nuclear abundance or HNF4alpha-LPK promoter interaction. Although overexpressed ChREBP or HNF-4alpha did not relieve n-3 PUFA suppression of L-PK gene expression, overexpressed MLX fully abrogated n-3 PUFA suppression of L-PK promoter activity and mRNA(L-PK) abundance. Overexpressed ChREBP, but not MLX, relieved the WY14643 inhibition of L-PK. In conclusion, n-3 PUFA and WY14643/PPARalpha target different transcription factors to control L-PK gene transcription. MLX, the heterodimer partner for ChREBP, has emerged as a novel target for n-3 PUFA regulation.

Glucose activation of ChREBP in hepatocytes occurs via a two-step mechanism

Carbohydrate response element binding protein (ChREBP) is a transcription factor that mediates glucose-responsive changes in gene expression in hepatocytes. In the current model for glucose regulation, inhibition of ChREBP in low glucose occurs in response to cAMP-dependent protein kinase (PKA)-mediated phosphorylation of residues S196, S626, and/or T666. Activation of ChREBP in conditions of increased glucose results simply from reversal of these inhibitory phosphorylations. To test this model, we analyzed mutant forms of ChREBP that lack one or more of the proposed PKA sites and found that these forms of ChREBP still require glucose for activation. Additionally, cAMP levels in cultured hepatocytes were negligible in low glucose conditions, indicating PKA should not be active. Finally, overall ChREBP phosphorylation did not change in response to altered glucose levels. We conclude that in addition to its repression by PKA, glucose activation of ChREBP involves a second mechanism that is independent of PKA phosphorylation.

Carbohydrate responsive element binding protein (ChREBP) and sterol regulatory element binding protein-1c (SREBP-1c): two key regulators of glucose metabolism and lipid synthesis in liver

In mammals, the regulation of hepatic metabolism plays a key role in whole body energy balance, since the liver is the major site of carbohydrate metabolism (glycolysis and glycogen synthesis) and triglyceride synthesis (lipogenesis). Lipogenesis is regulated through the acute control of key enzyme activities by means of allosteric and covalent modifications. Moreover, the synthesis of most glycolytic and lipogenic enzymes is regulated in response to dietary status, in which glucose, in particular, is a crucial energy nutrient. This latter response occurs in large part through transcriptional regulation of genes encoding glycolytic and lipogenic enzymes. In the past few years, recent advances have been made in understanding the transcriptional regulation of hepatic glycolytic and lipogenic genes by insulin and glucose. Although insulin is a major regulator of hepatic lipogenesis, there is increasing evidence that glucose also contributes to the coordinated regulation of carbohydrate and lipid metabolism in liver. Here, we review the respective roles of the transcription factor sterol regulatory element binding protein-1c (SREBP-1c) in mediating the effect of insulin on hepatic gene expression, and the role of carbohydrate responsive element binding protein (ChREBP) in regulating gene transcription by glucose.

Structure and regulation of acetyl-CoA carboxylase genes of metazoa

Acetyl-CoA carboxylase (ACC) plays a fundamental role in fatty acid metabolism. The reaction product, malonyl-CoA, is both an intermediate in the de novo synthesis of long-chain fatty acids and also a substrate for distinct fatty acyl-CoA elongation enzymes. In metazoans, which have evolved energy storage tissues to fuel locomotion and to survive periods of starvation, energy charge sensing at the level of the individual cell plays a role in fuel selection and metabolic orchestration between tissues. In mammals, and probably other metazoans, ACC forms a component of an energy sensor with malonyl-CoA, acting as a signal to reciprocally control the mitochondrial transport step of long-chain fatty acid oxidation through the inhibition of carnitine palmitoyltransferase I (CPT I). To reflect this pivotal role in cell function, ACC is subject to complex regulation. Higher metazoan evolution is associated with the duplication of an ancestral ACC gene, and with organismal complexity, there is an increasing diversity of transcripts from the ACC paraloges with the potential for the existence of several isozymes. This review focuses on the structure of ACC genes and the putative individual roles of their gene products in fatty acid metabolism, taking an evolutionary viewpoint provided by data in genome databases.

Direct role of ChREBP.Mlx in regulating hepatic glucose-responsive genes

Enzymes required for de novo lipogenesis are induced in mammalian liver after a meal high in carbohydrates. In addition to insulin, increased glucose metabolism initiates an intracellular signaling pathway that transcriptionally regulates genes encoding lipogenic enzymes. A cis-acting sequence, the carbohydrate response element (ChoRE), has been found in the promoter region of several of these genes. ChREBP (carbohydrate response element-binding protein) was recently identified as a candidate transcription factor in the glucose-signaling pathway. We reported that ChREBP requires the heterodimeric partner Max-like factor X (Mlx) to bind to ChoRE sequences. In this study we provide further evidence to support a direct role of Mlx in glucose signaling in the liver. We constructed two different dominant negative forms of Mlx that could dimerize with ChREBP but block its binding to DNA. When introduced into hepatocytes, both dominant negative forms of Mlx inhibited the glucose response of a transfected ChoRE-containing promoter. The glucose response was rescued by adding exogenous wild type Mlx or ChREBP, but not MondoA, a paralog of ChREBP that can also form a heterodimer with Mlx. Furthermore, dominant negative Mlx blocked the induction of glucose-responsive genes from their natural chromosomal context under high glucose conditions. In contrast, genes induced by the insulin and thyroid hormone-signaling pathways were unaffected by dominant negative Mlx. Mlx was present in the glucose-responsive complex of liver nuclear extract from which ChREBP was purified. In conclusion, Mlx is an obligatory partner of ChREBP in regulating lipogenic enzyme genes in liver.

Mechanisms of regulation of gene expression by fatty acids

Fatty acids (FA) regulate the expression of genes involved in lipid and energy metabolism. In particular, two transcription factors, sterol regulatory element binding protein-1c (SREBP-1c) and peroxisome proliferator activated receptor alpha (PPARalpha), have emerged as key mediators of gene regulation by FA. SREBP-1c induces a set of lipogenic enzymes in liver. Polyunsaturated fatty acids (PUFA), but not saturated or monounsaturated FA, suppress the induction of lipogenic genes by inhibiting the expression and processing of SREBP-1c. This unique effect of PUFA suggests that SREBP-1c may regulate the synthesis of unsaturated FA for incorporation into glycerolipids and cholesteryl esters. PPARalpha plays an essential role in metabolic adaptation to fasting by inducing the genes for mitochondrial and peroxisomal FA oxidation as well as those for ketogenesis in mitochondria. FA released from adipose tissue during fasting are considered as ligands of PPARalpha. Dietary PUFA, except for 18:2 n-6, are likely to induce FA oxidation enzymes via PPARalpha as a "feed-forward " mechanism. PPARalpha is also required for regulating the synthesis of highly unsaturated FA, indicating pleiotropic functions of PPARalpha in the regulation of lipid metabolic pathways. It is yet to be determined whether FA regulate other transcription factors such as liver-X receptor, hepatocyte nuclear factor 4, and carbohydrate response element binding protein.