Insulin stimulates hypoxia-inducible factor 1 through a phosphatidylinositol 3-kinase/target of rapamycin-dependent signaling pathway

Hypoxia-inducible factor 1 (HIF-1) is a transcription factor involved in normal mammalian development and in the pathogenesis of several disease states. It consists of two subunits, HIF-1α, which is degraded during normoxia, and HIF-1β, which is constitutively expressed. Activated HIF-1 induces the expression of genes involved in angiogenesis, erythropoiesis, and glucose metabolism. We have previously reported that insulin stimulates vascular endothelial growth factor (VEGF) expression (1). In this study, we show that insulin activates HIF-1, leading to VEGF expression in retinal epithelial cells. Insulin activates HIF-1α protein expression in a dose-dependent manner with a maximum reached within 6 h. The expression of HIF-1α is correlated with the activation of HIF-1 DNA binding activity and the transactivation of a HIF-1-dependent reporter gene. Insulin does not appear to affect HIF-1α mRNA transcription but regulates HIF-1α protein expression through a translation-dependent pathway. The expression of an active form of protein kinase B and treatment of cells with specific inhibitors of phosphatidylinositol 3-kinase (PI3K), MAPK, and target of rapamycin (TOR) show that mainly PI3K and to a lesser extent TOR are required for insulin-induced HIF-1α expression. HIF-1 activity and VEGF expression are also dependent on PI3K- and TOR-dependent signaling. In conclusion, we show here that insulin regulates HIF-1 action through a PI3K/TOR-dependent pathway, resulting in increased VEGF expression.