Phosphatidylinositol 3-Kinase and Mek1/2 Are Necessary for Insulin-Like Growth Factor-I-Induced Vascular Endothelial Growth Factor Synthesis in Prostate Epithelial Cells: A Role for Hypoxia-Inducible Factor-1?

Due to the importance of vascular endothelial growth factor (VEGF) in the neovascularization of solid tumors, a clear understanding of how VEGF is regulated in normal and tumor cells is warranted. We investigated insulin-like growth factor (IGF)-I-stimulated signaling pathways that increase the rate of VEGF synthesis in primary cultures of normal prostate epithelial cells (PrEC). IGF-I increased the secretion of VEGF 165 into PrEC growth medium and stimulated transcription of a reporter gene driven by a 1.5-kb region of the VEGF promoter. Inhibition of either phosphatidylinositol 3-kinase (PI3-K) or Mek1/2 signaling pathways completely abrogated the IGF-I-induced increase in VEGF secretion and promoter activity, indicating a dependence on coordinate signaling from both pathways to produce this effect. Levels of the transcription factors hypoxia-inducible factor (HIF)-1 and Fos were elevated in response to IGF-I in a PI3-K-dependent and Mek1/2-dependent manner, respectively. The expression of an activator protein (AP)-1 dominant negative in an immortalized prostate epithelial cell line PZ-HPV-7 suppressed the IGF-I-induced increase in VEGF promoter activity. Mutation of the hypoxia response element (HRE), which mediates hypoxic stimulation of VEGF transcription, did not inhibit the effect of IGF-I on the VEGF promoter, despite the fact that this mutation inhibited PI3-K-stimulated VEGF promoter activity in prostate cancer cells. These data indicate that PI3-K signaling does not increase VEGF transcription through transactivation by HIF-1 at the HRE in normal PrEC. This work also suggests that an additional signal, not stimulated by IGF-I in PrEC, is needed for HIF-1 to stimulate transcription from the VEGF HRE.