Hepatic Glucokinase Is Required for the Synergistic Action of ChREBP and SREBP-1c on Glycolytic and Lipogenic Gene Expression

Abstract

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 Km 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. 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 Km 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. Hepatic glucokinase (GK) 1The abbreviations used are: GK, glucokinase; HK, hexokinase; G6P, glucose 6-phosphate; X5P, xylulose 5-phosphate; SREBP-1c, sterol regulatory element-binding protein-1c; Ad-SREBP-1c, recombinant adenovirus expressing SREBP-1c; Ad-null, adenovirus vector containing no exogenous gene; l-PK, l-pyruvate kinase; FAS, fatty acid synthase; L-CPT I, liver carnitine palmitoyltransferase I; ACC, acetyl-CoA carboxylase; ChREBP, carbohydrate responsive element-binding protein; ChoRE, carbohydrate response element; SRE, sterol regulatory element; RT-PCR, real-time quantitative polymerase chain reaction; HCHO, high carbohydrate diet; siRNA, small interfering RNA; pfu, plaque-forming unit; KO, knockout. 1The abbreviations used are: GK, glucokinase; HK, hexokinase