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/

Insulin regulates the expression of multiple hepatic genes through a c onserved insulin response sequence (IRS) (CAAAAC/TAA) by an as yet und etermined mechanism, Protein kinase B/Akt (PKB/Akt), a member of the P KA/PKC serine/threonine kinase family, functions downstream from phosp hatidylinositol 3'-kinase (PI3K) in mediating effects of insulin on gl ucose transport and glycogen synthesis, We asked whether PKB/Akt media tes sequence-specific effects of insulin on hepatic gene expression us ing the model of the insulin-like growth factor binding protein-1 (IGF BP-1) promoter. Insulin lowers IGFBP-1 mRNA levels, inhibits IGFBP-1 p romoter activity, and activates PKB/Akt in HepG2 hepatoma cells throug h a PI3K-dependent, rapamycin insensitive mechanism, Constitutively ac tive PI3K and PKB/ Akt are each sufficient to mediate effects of insul in 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 pro- meter activity, The IGFBP-1 promoter contains two IRSs each of which is sufficient to mediate sequence-specific eff ects of insulin, PI3K, and PKB/Akt on promoter activity, Highly relate d IRSs from the phosphoenolpyruvate carboxykinase and apolipoprotein C III genes also are effective in this setting, These results indicate t hat PKB/Akt functions downstream from PI3R in mediating sequence-speci fic effects of insulin on the expression of IGFBP-1 and perhaps multip le hepatic genes through a conserved IRS.