Insulin-like growth factor binding protein-1 (IGFBP-1) gene expression is stimulated by glucocorticoids and suppressed by insulin in the liver. Insulin response sequences (IRSs) mediate effects of insulin on basal promoter function, while glucocorticoids stimulate promoter activity through a contiguous glucocorticoid response element (GRE). Here, we examined the role of IRS-dependent and -independent mechanisms in mediating insulin and glucocorticoids effects on IGFBP-1 promoter activity.

Dexamethasone (Dex) stimulates IGFBP-1 promoter activity in HepG2 cells and mutation of IRSs reduces this effect, indicating that IRS-associated factors enhance glucocorticoid effects on promoter function. Conversely, insulin inhibits basal promoter activity by 40% and Dex-stimulated promoter activity by 65%, indicating that glucocorticoids enhance the ability of insulin to suppress promoter activity. Mutation of IRSs completely disrupts the insulin effect on basal promoter activity, and reduces but does not abolish inhibition of Dex-stimulated promoter activity, indicating that the insulin suppresses glucocorticoid-stimulated promoter activity through both IRS-dependent and -independent mechanisms. IRS-independent effects of insulin are context-dependent, since insulin does not suppress glucocorticoid-stimulated activity of a promoter containing multiple GREs. Cotransfection studies indicate that suppression of PGC-1, an insulin-regulated coactivator of GR, is not required for this effect of insulin. Studies with pharmacological inhibitors indicate that both phosphatidylinositol-3' kinase and mitogen activated kinase kinase pathways contribute to IRS-independent effects. These studies indicate that glucocorticoids and IRS-associated factors function together to mediate effects of insulin and glucocorticoids on promoter activity, and that glucocorticoid treatment creates a complex environment where insulin regulates IGFBP-1 expression through both IRS-dependent and IRS-independent mechanisms.