A MULTICOMPONENT INSULIN-RESPONSE SEQUENCE MEDIATES A STRONG REPRESSION OF MOUSE GLUCOSE-6-PHOSPHATASE GENE-TRANSCRIPTION BY INSULIN

Glucose-6-phosphatase (G6Pase) catalyzes the final step in the glucone ogenic and glycogenolytic pathways. The transcription of the gene enco ding the catalytic subunit of G6Pase is stimulated by glucocorticoids, whereas insulin strongly inhibits both basal G6Pase gene transcriptio n and the stimulatory effect of glucocorticoids. To identify the insul in response sequence (IRS) in the G6Pase promoter through which insuli n mediates its action, we have analyzed the effect of insulin on the b asal expression of mouse G6Pase-chloramphenicol acetyltransferase (CAT ) fusion genes transiently expressed in hepatoma cells. Deletion of th e 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 i nhibits basal G6Pase-CAT expression. The G6Pase promoter region betwee n -198 and -159 contains an IRS, since it can confer an inhibitory eff ect of insulin on the expression of a heterologous fusion gene. This r egion 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 glucos e production characteristic of patients with non-insulin dependent dia betes mellitus.