HEPATOCYTE NUCLEAR FACTOR-I ACTS AS AN ACCESSORY FACTOR TO ENHANCE THE INHIBITORY-ACTION OF INSULIN ON MOUSE GLUCOSE-6-PHOSPHATASE GENE-TRANSCRIPTION

Glucose-6-phosphatase catalyzes the terminal step in the gluconeogenic and glycogenolytic pathways. Transcription of the gene encoding the g lucose-6-phosphatase catalytic subunit (G6Pase) is stimulated by cAMP and glucocorticoids whereas insulin strongly inhibits both this induct ion and basal G6Pase gene transcription. Previously, we have demonstra ted that the maximum repression of basal G6Pase gene transcription by insulin requires two distinct promoter regions, designated A (from -27 1 to -199) and B (from -198 to -159), Region B contains an insulin res ponse sequence because it can confer an inhibitory effect of insulin o n the expression of a heterologous fusion gene. By contrast, region A fails to mediate an insulin response in a heterologous context, and th e mutation of region B within an otherwise intact promoter almost comp letely abolishes the effect of insulin on basal G6Pase gene transcript ion. Therefore, region A is acting as an accessory element to enhance the effect of insulin, mediated through region B, on G6Pase gene trans cription. Such an arrangement is a common feature of cAMP and glucocor ticoid-regulated genes hut has not been previously described for insul in. A combination of fusion gene and protein-binding analyses revealed that the accessory factor binding region A is hepatocyte nuclear fact or-1. Thus, despite the usually antagonistic effects of cAMP/glucocort icoids and insulin, all three agents are able to use the same factor t o enhance their action on gene transcription. The potential role of G6 Pase overexpression in the pathophysiology of MODY3 and 5, rare forms of diabetes caused by hepatocyte nuclear factor-1 mutations, is discus sed.