HEPATIC NUCLEAR FACTOR-3-REGULATED AND HORMONE-REGULATED EXPRESSION OF THE PHOSPHOENOLPYRUVATE CARBOXYKINASE AND INSULIN-LIKE GROWTH FACTOR-BINDING PROTEIN-1 GENES

The rate of transcription of the hepatic phosphoenolpyruvate carboxyki nase (PEPCK) and insulin-like growth factor-binding protein 1 (IGFBP-1 ) genes is stimulated by glucocorticoids and inhibited by insulin. In both cases, the effect of insulin is dominant, since it suppresses bot h basal and glucocorticoid-stimulated PEPCK or IGFBP-1 gene transcript ion. Analyses of both promoters by transfection of PEPCK or IGFBP-1-ch loramphenicol acetyltransferase fusion genes into rat hepatoma cells h as led to the identification of insulin response sequences (IRSs) in b oth genes. The core IRS, T(G/A)TTTTG, is the same in both genes, but t he PEPCK promoter has a single copy of this element whereas the IGFBP- 1 promoter has two copies arranged as an inverted palindrome. The IGFB P-1 IRS and PEPCK IRS both bind the alpha and beta forms of hepatic nu clear factor 3 (HNF-3), although the latter does so with a sixfold-low er relative affinity. Both the PEPCK and the IGFBP-1 IRSs also functio n as accessory factor binding sites required for the full induction of gene transcription by glucocorticoids. A combination of transient tra nsfection and DNA binding studies suggests that HNF-3 is the accessory factor that supports glucocorticoid-induced gene transcription. In bo th genes, the HNF-3 binding site overlaps the IRS core motif(s). A mod el in which insulin is postulated to mediate its negative effect on gl ucocorticoid-induced PEPCK and IGFBP-1 gene transcription indirectly b y inhibiting HNF-3 action is proposed.