A multicomponent insulin response sequence mediates a strong repression of mouse glucose-6-phosphatase gene transcription by insulin

Abstract

Glucose-6-phosphatase (G6Pase) catalyzes the final step in the gluconeogenic and glycogenolytic pathways. The transcription of the gene encoding the catalytic subunit of G6Pase is stimulated by glucocorticoids, whereas insulin strongly inhibits both basal G6Pase gene transcription and the stimulatory effect of glucocorticoids. To identify the insulin response sequence (IRS) in the G6Pase promoter through which insulin mediates its action, we have analyzed the effect of insulin on the basal expression of mouse G6Pase-chloramphenicol acetyltransferase (CAT) fusion genes transiently expressed in hepatoma cells. Deletion of the G6Pase promoter sequence between -271 and -199 partially reduces the inhibitory effect of insulin, whereas deletion of additional sequence between -198 and -159 completely abolishes the insulin response. The presence of this multicomponent IRS may explain why insulin potently inhibits basal G6Pase-CAT expression. The G6Pase promoter region between -198 and -159 contains an IRS, since it can confer an inhibitory effect of insulin on the expression of a heterologous fusion gene. This region contains three copies of the T(G/A)TTTTG sequence, which is the core motif of the phosphoenolpyruvate carboxykinase (PEPCK) gene IRS. This suggests that a coordinate increase in both G6Pase and PEPCK gene transcription is likely to contribute to the increased hepatic glucose production characteristic of patients with non-insulin-dependent diabetes mellitus.