Inhibition of GSK-3 Selectively Reduces Glucose-6-Phosphatase and Phosphoenolpyruvate Carboxykinase Gene Expression

Abstract

A major action of insulin is to regulate the transcription rate of specific genes. The expression of these genes is dramatically altered in type 2 diabetes. For example, the expression of two hepatic genes, glucose-6-phosphatase and PEPCK, is normally inhibited by insulin, but in type 2 diabetes, their expression is insensitive to insulin. An agent that mimics the effect of insulin on the expression of these genes would reduce gluconeogenesis and hepatic glucose output, even in the presence of insulin resistance. The repressive actions of insulin on these genes are dependent on phosphatidylinositol (PI) 3-kinase. However, the molecules that lie between this lipid kinase and the two gene promoters are unknown. Glycogen synthase kinase-3 (GSK-3) is inhibited following activation of PI 3-kinase and protein kinase B. In hepatoma cells, we find that selectively reducing GSK-3 activity strongly reduces the expression of both gluconeogenic genes. The effect is at the level of transcription and is observed with induced or basal gene expression. In addition, GSK-3 inhibition does not result in the subsequent activation of protein kinase B or inhibition of the transcription factor FKHR, which are candidate regulatory molecules for these promoters. Thus, GSK-3 activity is required for basal activity of each promoter. Inhibitors of GSK-3 should therefore reduce hepatic glucose output, as well as increase the synthesis of glycogen from L-glucose. These findings indicate that GSK-3 inhibitors may have greater therapeutic potential for lowering blood glucose levels and treating type 2 diabetes than previously realized. T he cellular actions of insulin include increased glucose transport, glycogen synthesis, and lipogenesis and decreased gluconeogenesis, glycogen, and fat breakdown. The result is reduced hepatic glucose output and increased peripheral glucose utilization. In type 2 diabetes, most of the intracellular actions of insulin are reduced or absent (1-3), yet the identity of the lesions underlying insulin resistance is not clear. To find ways to combat insulin resistance, much research focuses on the insulin-regulated signaling pathways that normally mediate glucose production and/or glucose utilization and that connect the insulin receptor to the proteins that directly mediate each action of insulin. The initial step in insulin signaling involves the binding of insulin to its specific cell surface receptor and activation of the tyrosine kinase activity of the receptor, followed by the phosphorylation of a small subset of proteins, including a group of proteins termed insulin receptor substrates (4). From this point, multiple signaling cascades are activated including the p42/p44 mitogen-activated protein (MAP) kinase cascade, the phosphatidylinositol (PI) 3-kinase-mTOR-p70 S6 kinase cascade, and the PI 3-kinase-PI 3,4,5-Tris phosphate-dependent kinase (PDK)-protein kinase B (PKB) cascade (rev. in 5). The activation of PKB by PDK-1 (5-8) leads to the phosphorylation of multiple cellular proteins including glycogen synt...

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