Dentin Renaud, Pégorier Jean-Paul, Benhamed Fadila, Foufelle Fabienne, Ferré Pascal, Fauveau Véronique, Magnuson Mark A, Girard Jean, Postic Catherine
Département d'Endocrinologie, Institut Cochin, INSERM U567, CNRS UMR8104, Université René Descartes, 24 rue du Faubourg Saint Jacques, 75014 Paris, France.
J Biol Chem. 2004 May 7;279(19):20314-26. doi: 10.1074/jbc.M312475200. Epub 2004 Feb 25.
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.
肝葡萄糖激酶（GK）催化葡萄糖磷酸化生成6-磷酸葡萄糖（G6P），这一步骤对于肝脏中的葡萄糖代谢以及糖酵解和脂肪生成基因的诱导至关重要。固醇调节元件结合蛋白-1c（SREBP-1c）已成为胰岛素对肝脏基因表达作用的主要介导因子，但其转录效应在多大程度上是由葡萄糖代谢增加引起的仍不清楚。通过使用肝GK基因敲除小鼠（hGK-KO），我们已经表明，葡萄糖对l-丙酮酸激酶（l-PK）、脂肪酸合酶（FAS）、乙酰辅酶A羧化酶（ACC）和Spot 14基因的急性刺激需要GK表达。为了确定SREBP-1c的作用是否需要GK表达及随后的葡萄糖代谢，在体内以及对照和hGK-KO肝细胞的原代培养物中过表达了具有转录活性的SREBP-1c形式。我们的结果表明，SREBP-1c与通过GK进行的葡萄糖代谢之间的协同作用对于最大程度诱导l-PK、ACC、FAS和Spot 14基因表达是必需的。实际上，在过表达SREBP-1c的hGK-KO肝细胞中，尽管低K（m）己糖激酶活性显著增加，但由于这些肝细胞有效代谢葡萄糖的能力受损，葡萄糖对糖酵解和脂肪生成基因的作用丧失。我们的研究还揭示，在hGK-KO肝细胞中观察到的葡萄糖效应丧失与碳水化合物反应元件结合蛋白（ChREBP）基因表达降低有关，ChREBP是一种被认为介导肝脏中葡萄糖信号传导的转录因子。使用小干扰RNA降低ChREBP基因表达会导致葡萄糖对内源糖酵解（l-PK）和脂肪生成（FAS、ACC）基因表达的作用丧失，从而证明ChREBP直接参与葡萄糖作用。这些结果共同支持了一种模型，即SREBP-1c和通过ChREBP起作用的葡萄糖代谢对于饮食诱导肝脏中糖酵解和脂肪生成基因表达都是必需的。
