The liver provides for long-term energy needs of the body by converting excess carbohydrate into fat for storage. Insulin is one factor that promotes hepatic lipogenesis, but there is increasing evidence that glucose also contributes to the coordinated regulation of carbohydrate and fat metabolism in liver by mechanisms that are independent of insulin. In this study, we show that the transcription factor, carbohydrate response element-binding protein (ChREBP), is required both for basal and carbohydrate-induced expression of several liver enzymes essential for coordinated control of glucose metabolism, fatty acid, and the synthesis of fatty acids and triglycerides in vivo.

Carbohydrate response element (ChRE)-binding protein (ChREBP) is a recently discovered transcription factor that is activated in response to high glucose concentrations in liver independently of insulin. ChREBP was first identified by its ability to bind the ChRE of the liver pyruvate kinase (LPK) gene. We recently reported that the increase in expression of multiple liver lipogenic enzyme mRNAs elicited by feeding a high-carbohydrate diet as well as that of LPK mRNA is markedly reduced in mice lacking ChREBP gene expression (ChREBP ؊/؊ ) in comparison to WT mice. The present study provides evidence for a direct and dominant role of ChREBP in the glucose regulation of two key liver lipogenic enzymes, acetyl-CoA carboxylase (ACC) and fatty acid synthase (FAS). ACC, FAS, and LPK mRNA levels were higher in WT hepatocytes cultured with high (25 mM) rather than low (5.5 mM) glucose medium, but there was no effect of glucose concentration on these mRNA levels in ChREBP ؊/؊ hepatocytes. Similarly, reporter constructs containing ACC, FAS, or LPK gene ChREs were responsive to glucose when transfected into WT but not ChREBP ؊/؊ hepatocytes, and glucose transactivation of the constructs in ChREBP ؊/؊ hepatocytes was restored by cotransfection with a ChREBP expression plasmid. ChREBP binding to ACC, FAS, and LPK ChRE sequences in vitro was demonstrated by electrophoretic mobility super shift assays. In vivo binding of ChREBP to ACC, FAS, and LPK gene promoters in intact liver nuclei from rats fed a high-carbohydrate diet was demonstrated by using a formaldehyde crosslinking and chromatin immunoprecipitation procedure.

The transcription factor carbohydrate response element-binding protein (ChREBP) mediates insulin-independent, glucose-stimulated gene expression of multiple liver enzymes responsible for converting excess carbohydrate to fatty acids for long-term storage. To investigate ChREBP's role in the development of obesity and obesity-associated metabolic dysregulation, ChREBP-deficient mice were intercrossed with ob/ob mice. As a result of deficient leptin expression, ob/ob mice overeat, become obese and resistant to insulin, and display marked elevations in hepatic lipogenesis, gluconeogenesis, and plasma glucose and triglycerides. mRNA expression of all hepatic lipogenic enzymes was significantly lower in ob/ob-ChREBP-/- than in ob/ob mice, resulting in decreased hepatic fatty acid synthesis and normalization of plasma free fatty acid and triglyceride levels. Overall weight gain in addition to adiposity was reduced in the doubly deficient mice. The former was largely attributable to decreased food intake and may result from decreased hypothalamic expression of the appetite-stimulating neuropeptide agouti-related protein. mRNA expression and activity of gluconeogenic enzymes also was lower in the doubly deficient mice, contributing to significantly lower blood glucose levels. The results of this study suggest that inactivation of ChREBP expression not only reduces fat synthesis and obesity in ob/ob mice but also results in improved glucose tolerance and appetite control.

Edited by Robert Barouki Keywords:Fibroblast growth factor 21 Carbohydrate response element binding protein Rat hepatocyte Liver type pyruvate kinase Fatty acid synthase a b s t r a c t Fibroblast growth factor 21 (FGF21) has beneficial effects of improving the plasma glucose and lipid profiles in diabetic rodents. Here, we investigated carbohydrate response element binding protein (ChREBP) involvement in the regulation of FGF21 mRNA expression in liver. Glucose stimulation and adenoviral overexpression of dominant active ChREBP increased FGF21 mRNA. Consistently, adenoviral expression of dominant negative Mlx inhibited glucose induction of FGF21 mRNA. Furthermore, deletion studies of mouse FGF21 gene promoter (À2000 to +65 bp) revealed a glucose responsive region between À74 and À52 bp. These findings suggest that FGF21 expression is regulated by ChREBP.

One subtype of maturity-onset diabetes of the young (MODY)-3 results from mutations in the gene encoding hepatocyte nuclear factor (HNF)-1␣. We generated transgenic mice expressing a naturally occurring dominant-negative form of human HNF-1␣ (P291fsinsC) in pancreatic ␤-cells. A progressive hyperglycemia with age was seen in these transgenic mice, and the mice developed diabetes with impaired glucose-stimulated insulin secretion. The pancreatic islets exhibited abnormal architecture with reduced expression of glucose transporter (GLUT2) and E-cadherin. Blockade of Ecadherin-mediated cell adhesion in pancreatic islets abolished the glucose-stimulated increases in intracellular Ca 2؉ levels and insulin secretion, suggesting that loss of E-cadherin in ␤-cells is associated with impaired insulin secretion. There was also a reduction in ␤-cell number (50%), proliferation rate (15%), and pancreatic insulin content (45%) in 2-day-old transgenic mice and a further reduction in 4-week-old animals. Our findings suggest various roles for HNF-1␣ in normal glucose metabolism, including the regulation of glucose transport, ␤-cell growth, and ␤-cell-to-␤-cell communication.

Abstract. Excess carbohydrate intake leads to fat accumulation and insulin resistance. Glucose and insulin coordinately regulate de novo lipogenesis from glucose in the liver, and insulin activates several transcription factors including SREBP1c and LXR, while those activated by glucose remain unknown. Recently, a carbohydrate response element binding protein (ChREBP), which binds to the carbohydrate response element (ChoRE) in the promoter of rat liver type pyruvate kinase (LPK), has been identified. The target genes of ChREBP are involved in glycolysis, lipogenesis, and gluconeogenesis. Although the regulation of ChREBP remains unknown in detail, the transactivity of ChREBP is partly regulated by a phosphorylation/dephosphorylation mechanism. During fasting, protein kinase A and AMP-activated protein kinase phosphorylate ChREBP and inactivate its transactivity. During feeding, xylulose-5-phosphate in the hexose monophosphate pathway activates protein phosphatase 2A, which dephosphorylates ChREBP and activates its transactivity. ChREBP controls 50% of hepatic lipogenesis by regulating glycolytic and lipogenic gene expression. In ChREBP -/-mice, liver triglyceride content is decreased and liver glycogen content is increased compared to wild-type mice. These results indicate that ChREBP can regulate metabolic gene expression to convert excess carbohydrate into triglyceride rather than glycogen. Furthermore, complete inhibition of ChREBP in ob/ob mice reduces the effects of the metabolic syndrome such as obesity, fatty liver, and glucose intolerance. Thus, further clarification of the physiological role of ChREBP may be useful in developing treatments for the metabolic syndrome.

This is the first report that HIF-1alpha is associated with the occurrence of type 2 diabetes and suggests that the P582S HIF-1alpha mutation should be assessed in larger studies as a risk factor for type 2 diabetes.

Carbohydrate-response element-binding protein (ChREBP) has been identified as a transcription factor that binds to carbohydrate response element in the promoter of pyruvate kinase, liver and red blood cells. ChREBP is activated by metabolites derived from glucose and suppressed by adenosine monophosphate (AMP), ketone bodies and cyclic cAMP. ChREBP regulates gene transcription related to glucose and lipid metabolism. Findings from knockout mice and human subjects suggest that ChREBP helps to induce hepatic steatosis, dyslipidemia, and glucose intolerance. Moreover, in tumor cells, ChREBP promotes aerobic glycolysis through p53 inhibition, resulting in tumor cell proliferation. Anti-diabetic and anti-lipidemic drugs such as atorvastatin, metformin, bile acid sequestrants, docosahexaenoic acid and eicosapentaenoic acid may affect ChREBP transactivity. Secretory proteins such as fibroblast growth factor 21 and ANGPTL8 (Betatrophin) may be promising candidates for biologic markers reflecting ChREBP transactivity. Thus, ChREBP is associated with metabolic diseases and cancers, and may be a link between them.