👤 J Marc Girard

The transcription factor carbohydrate-responsive element-binding protein (ChREBP) has emerged as a central regulator of lipid synthesis in liver because it is required for glucose-induced expression of the glycolytic enzyme liver-pyruvate kinase (L-PK) and acts in synergy with SREBP to induce lipogenic genes such as acetyl-CoA carboxylase (ACC) and fatty acid synthase (FAS). Liver X receptors (LXRs) are also important regulators of the lipogenic pathway, and the recent finding that ChREBP is a direct target of LXRs and that glucose itself can bind and activate LXRs prompted us to study the role of LXRs in the induction of glucose-regulated genes in liver. Using an LXR agonist in wild-type mice, we found that LXR stimulation did not promote ChREBP phosphorylation or nuclear localization in the absence of an increased intrahepatic glucose flux. Furthermore, the induction of ChREBP, L-PK, and ACC by glucose or high-carbohydrate diet was similar in LXRalpha/beta knockout compared with wild-type mice, suggesting that the activation of these genes by glucose occurs by an LXR-independent mechanism. We used fluorescence resonance energy transfer analysis to demonstrate that glucose failed to promote the interaction of LXRalpha/beta with specific cofactors. Finally, siRNA silencing of ChREBP in LXRalpha/beta knockout hepatocytes abrogated glucose-induced expression of L-PK and ACC, further demonstrating the central role of ChREBP in glucose signaling. Taken together, our results demonstrate that glucose is required for ChREBP functional activity and that LXRs are not necessary for the induction of glucose-regulated genes in liver. Show less

Non-alcoholic fatty liver disease is tightly associated with insulin resistance, type 2 diabetes and obesity, but the molecular links between hepatic fat accumulation and insulin resistance are not fully identified. Excessive accumulation of triglycerides (TG) is one the main characteristics of non-alcoholic fatty liver disease and fatty acids utilized for the synthesis of TG in liver are available from the plasma non-esterified fatty acid pool but also from fatty acids newly synthesized through hepatic de novo lipogenesis. Recently, the transcription factor ChREBP (carbohydrate responsive element binding protein) has emerged as a central determinant of lipid synthesis in liver through its transcriptional control of key genes of the lipogenic pathway, including fatty acid synthase and acetyl CoA carboxylase. In this mini-review, we will focus on the importance of ChREBP in the physiopathology of hepatic steatosis and insulin resistance by discussing the physiological and metabolic consequences of ChREBP knockdown in liver of ob/ob mice. Show less

Dysregulations in hepatic lipid synthesis are often associated with obesity and type 2 diabetes, and therefore a perfect understanding of the regulation of this metabolic pathway appears essential to identify potential therapeutic targets. Recently, the transcription factor ChREBP (carbohydrate-responsive element-binding protein) has emerged as a major mediator of glucose action on lipogenic gene expression and as a key determinant of lipid synthesis in vivo. Indeed, liver-specific inhibition of ChREBP improves hepatic steatosis and insulin resistance in obese ob/ob mice. Since ChREBP cellular localization is a determinant of its functional activity, a better knowledge of the mechanisms involved in regulating its nucleo-cytoplasmic shuttling and/or its post-translational activation is crucial in both physiology and physiopathology. Here, we review some of the studies that have begun to elucidate the regulation and function of this key transcription factor in liver. Show less

We evaluated the contribution of three genetic alterations (p53 knockdown, K-RAS(V12), and mutant EGFR) to lung tumorigenesis using human bronchial epithelial cells (HBEC) immortalized with telomerase and Cdk4-mediated p16 bypass. RNA interference p53 knockdown or oncogenic K-RAS(V12) resulted in enhanced anchorage-independent growth and increased saturation density of HBECs. The combination of p53 knockdown and K-RAS(V12) further enhanced the tumorigenic phenotype with increased growth in soft agar and an invasive phenotype in three-dimensional organotypic cultures but failed to cause HBECs to form tumors in nude mice. Growth of HBECs was highly dependent on epidermal growth factor (EGF) and completely inhibited by EGF receptor (EGFR) tyrosine kinase inhibitors, which induced G1 arrest. Introduction of EGFR mutations E746-A750 del and L858R progressed HBECs toward malignancy as measured by soft agar growth, including EGF-independent growth, but failed to induce tumor formation. Mutant EGFRs were associated with higher levels of phospho-Akt, phospho-signal transducers and activators of transcription 3 [but not phospho-extracellular signal-regulated kinase (ERK) 1/2], and increased expression of DUSP6/MKP-3 phosphatase (an inhibitor of phospho-ERK1/2). These results indicate that (a) the HBEC model system is a powerful new approach to assess the contribution of individual and combinations of genetic alterations to lung cancer pathogenesis; (b) a combination of four genetic alterations, including human telomerase reverse transcriptase overexpression, bypass of p16/RB and p53 pathways, and mutant K-RAS(V12) or mutant EGFR, is still not sufficient for HBECs to completely transform to cancer; and (c) EGFR tyrosine kinase inhibitors inhibit the growth of preneoplastic HBEC cells, suggesting their potential for chemoprevention. Show less

Obesity is a metabolic disorder often associated with type 2 diabetes, insulin resistance, and hepatic steatosis. Leptin-deficient (ob/ob) mice are a well-characterized mouse model of obesity in which increased hepatic lipogenesis is thought to be responsible for the phenotype of insulin resistance. We have recently demonstrated that carbohydrate responsive element-binding protein (ChREBP) plays a key role in the control of lipogenesis through the transcriptional regulation of lipogenic genes, including acetyl-CoA carboxylase and fatty acid synthase. The present study reveals that ChREBP gene expression and ChREBP nuclear protein content are significantly increased in liver of ob/ob mice. To explore the involvement of ChREBP in the physiopathology of hepatic steatosis and insulin resistance, we have developed an adenovirus-mediated RNA interference technique in which short hairpin RNAs (shRNAs) were used to inhibit ChREBP expression in vivo. Liver-specific inhibition of ChREBP in ob/ob mice markedly improved hepatic steatosis by specifically decreasing lipogenic rates. Correction of hepatic steatosis also led to decreased levels of plasma triglycerides and nonesterified fatty acids. As a consequence, insulin signaling was improved in liver, skeletal muscles, and white adipose tissue, and overall glucose tolerance and insulin sensitivity were restored in ob/ob mice after a 7-day treatment with the recombinant adenovirus expressing shRNA against ChREBP. Taken together, our results demonstrate that ChREBP is central for the regulation of lipogenesis in vivo and plays a determinant role in the development of the hepatic steatosis and of insulin resistance in ob/ob mice. Show less

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. Show less

Intestinal glucagon-like peptide-1 (GLP-1) is a hormone released into the hepatoportal circulation that stimulates pancreatic insulin secretion. GLP-1 also acts as a neuropeptide to control food intake and cardiovascular functions, but its neural role in glucose homeostasis is unknown. We show that brain GLP-1 controlled whole-body glucose fate during hyperglycemic conditions. In mice undergoing a hyperglycemic hyperinsulinemic clamp, icv administration of the specific GLP-1 receptor antagonist exendin 9-39 (Ex9) increased muscle glucose utilization and glycogen content. This effect did not require muscle insulin action, as it also occurred in muscle insulin receptor KO mice. Conversely, icv infusion of the GLP-1 receptor agonist exendin 4 (Ex4) reduced insulin-stimulated muscle glucose utilization. In hyperglycemia achieved by i.v. infusion of glucose, icv Ex4, but not Ex9, caused a 4-fold increase in insulin secretion and enhanced liver glycogen storage. However, when glucose was infused intragastrically, icv Ex9 infusion lowered insulin secretion and hepatic glycogen levels, whereas no effects of icv Ex4 were observed. In diabetic mice fed a high-fat diet, a 1-month chronic i.p. Ex9 treatment improved glucose tolerance and fasting glycemia. Our data show that during hyperglycemia, brain GLP-1 inhibited muscle glucose utilization and increased insulin secretion to favor hepatic glycogen stores, preparing efficiently for the next fasting state. Show less

Dietary polyunsaturated fatty acids (PUFAs) are potent inhibitors of hepatic glycolysis and lipogenesis. Recently, carbohydrate-responsive element-binding protein (ChREBP) was implicated in the regulation by glucose of glycolytic and lipogenic genes, including those encoding L-pyruvate kinase (L-PK) and fatty acid synthase (FAS). The aim of our study was to assess the role of ChREBP in the control of L-PK and FAS gene expression by PUFAs. We demonstrated in mice, both in vivo and in vitro, that PUFAs [linoleate (C18:2), eicosapentanoic acid (C20:5), and docosahexaenoic acid (C22:6)] suppressed ChREBP activity by increasing ChREBP mRNA decay and by altering ChREBP translocation from the cytosol to the nucleus, independently of an activation of the AMP-activated protein kinase, previously shown to regulate ChREBP activity. In contrast, saturated [stearate (C18)] and monounsaturated fatty acids [oleate (C18:1)] had no effect. Since glucose metabolism via the pentose phosphate pathway is determinant for ChREBP nuclear translocation, the decrease in xylulose 5-phosphate concentrations caused by a PUFA diet favors a PUFA-mediated inhibition of ChREBP translocation. In addition, overexpression of a constitutive nuclear ChREBP isoform in cultured hepatocytes significantly reduced the PUFA inhibition of both L-PK and FAS gene expression. Our results demonstrate that the suppressive effect of PUFAs on these genes is primarily caused by an alteration of ChREBP nuclear translocation. In conclusion, we describe a novel mechanism to explain the inhibitory effect of PUFAs on the genes encoding L-PK and FAS and demonstrate that ChREBP is a pivotal transcription factor responsible for coordinating the PUFA suppression of glycolytic and lipogenic genes. Show less

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. Show less

Hepatic glucokinase (GK) catalyzes the phosphorylation of glucose to glucose 6-phosphate (G6P), a step which is essential for glucose metabolism in liver as well as for the induction of glycolytic and lipogenic genes. The sterol regulatory element-binding protein-1c (SREBP-1c) has emerged as a major mediator of insulin action on hepatic gene expression, but the extent to which its transcriptional effect is caused by an increased glucose metabolism remains unclear. Through the use of hepatic GK knockout mice (hGK-KO) we have shown that the acute stimulation by glucose of l-pyruvate kinase (l-PK), fatty acid synthase (FAS), acetyl-CoA carboxylase (ACC), and Spot 14 genes requires GK expression. To determine whether the effect of SREBP-1c requires GK expression and subsequent glucose metabolism, a transcriptionally active form of SREBP-1c was overexpressed both in vivo and in primary cultures of control and hGK-KO hepatocytes. Our results demonstrate that the synergistic action of SREBP-1c and glucose metabolism via GK is necessary for the maximal induction of l-PK, ACC, FAS, and Spot 14 gene expression. Indeed, in hGK-KO hepatocytes overexpressing SREBP-1c, the effect of glucose on glycolytic and lipogenic genes is lost because of the impaired ability of these hepatocytes to efficiently metabolize glucose, despite a marked increase in low K(m) hexokinase activity. Our studies also reveal that the loss of glucose effect observed in hGK-KO hepatocytes is associated with a decreased in the carbohydrate responsive element-binding protein (ChREBP) gene expression, a transcription factor suggested to mediate glucose signaling in liver. Decreased ChREBP gene expression, achieved using small interfering RNA, results in a loss of glucose effect on endogenous glycolytic (l-PK) and lipogenic (FAS, ACC) gene expression, thereby demonstrating the direct implication of ChREBP in glucose action. Together these results support a model whereby both SREBP-1c and glucose metabolism, acting via ChREBP, are necessary for the dietary induction of glycolytic and lipogenic gene expression in liver. Show less