Regulation of glucose-6-phosphatase gene expression by protein kinase B alpha and the forkhead transcription factor FKHR - Evidence for insulin response unit-dependent and -independent effects of insulin on promoter activity

Glucose-6-phosphatase plays an important role in the regulation of hepatic glucose production, and insulin suppresses glucose-6-phosphatase gene expre ssion. Recent Studies indicate that protein kinase B and Forkhead proteins contribute to insulin-regulated gene expression in the liver. Here, we exam ined the role of protein kinase B and Forkhead proteins in mediating effect s of insulin on glucose-6-phosphatase promoter activity. Transient transfec tion studies with reporter gene constructs demonstrate that insulin suppres ses both basal and dexamethasone/cAMP-induced activity of the glucose-6-pho sphatase promoter in H4IIE hepatoma cells. Both effects are partially mimic ked by coexpression bf protein kinase B alpha. Coexpression of the Forkhead transcription factor FKHR stimulates the glucose-6-phosphatase promoter ac tivity via interaction with an insulin response unit (IRU), and this activa tion is suppressed by protein kinase B, Coexpression of a mutated form of F KHR that cannot be phosphorylated by protein kinase B abolishes the regulat ion of the glucose-6-phosphatase promoter by protein kinase B and disrupts the ability of insulin to regulate the glucose-6-phosphatase promoter via t he IRU, Mutation of the insulin response unit of the glucose-6-phosphatase promoter also prevents the regulation of promoter activity by FKHR and prot ein kinase B but only partially impairs the ability of insulin: to suppress both basal and dexamethasone/cAMP-stimulated promoter function. Taken toge ther, these results indicate that signaling by protein kinase B to Forkhead proteins can account for the ability of insulin to regulate glucose-6-phos phatase promoter activity via the IRU and that other mechanisms that are in dependent of the IRU, protein kinase B, and Forkhead proteins also are impo rtant in mediating effects of in insulin-on glucose-6-phosphatase gene expr ession.