Protein Kinase B/Akt Mediates Effects of Insulin on Hepatic Insulin-like Growth Factor-binding Protein-1 Gene Expression through a Conserved Insulin Response Sequence

Abstract

Insulin regulates the expression of multiple hepatic genes through a conserved insulin response sequence (IRS) (CAAAAC/TAA) by an as yet undetermined mechanism. Protein kinase B/Akt (PKB/Akt), a member of the PKA/PKC serine/threonine kinase family, functions downstream from phosphatidylinositol 3'-kinase (PI3K) in mediating effects of insulin on glucose transport and glycogen synthesis. We asked whether PKB/Akt mediates sequence-specific effects of insulin on hepatic gene expression using the model of the insulin-like growth factor binding protein-1 (IGFBP-1) promoter. Insulin lowers IGFBP-1 mRNA levels, inhibits IGFBP-1 promoter activity, and activates PKB/Akt in HepG2 hepatoma cells through a PI3K-dependent, rapamycin-insensitive mechanism. Constitutively active PI3K and PKB/Akt are each sufficient to mediate effects of insulin on the IGFBP-1 promoter in a nonadditive fashion. Dominant negative K179 PKB/Akt disrupts the ability of insulin and PI3K to activate PKB/Akt and to inhibit promoter activity. The IGFBP-1 promoter contains two IRSs each of which is sufficient to mediate sequence-specific effects of insulin, PI3K, and PKB/Akt on promoter activity. Highly related IRSs from the phosphoenolpyruvate carboxykinase and apolipoprotein CIII genes also are effective in this setting. These results indicate that PKB/Akt functions downstream from PI3K in mediating sequence-specific effects of insulin on the expression of IGFBP-1 and perhaps multiple hepatic genes through a conserved IRS.