Role of AP-1 and HIF-1 transcription factors in TGF-beta activation of VEGF expression

Aberrant expression of vascular endothelial growth factor (VEGF) has been demonstrated to be associated with most human solid tumors. Here we report that TGF-beta potently induces VEGF expression in human HT-1080 fibrosarcomas primarily through transcriptional activation with no significant changes in mRNA turnover. The tyrosine kinase inhibitor genistein and AP-1 inhibitor curcumin significantly blocked TGF-beta induction of VEGF expression while SP-1 and MKK1 inhibitors did not. TGF-beta enhanced both AP-1 and HIF-1 DNA binding activities whereas SP-1, AP-2 and NF-1 did not show major changes. Transcriptional reporter assays provided further evidence that TGF-beta augmented both AP-1 and HIF-1 activities. Moreover, TGF-beta-treated HT-1080 cells contained higher levels of HIF-1alpha and c-jun proteins in nuclear extracts. TGF-beta and hypoxia synergistically induced VEGF mRNA expression. Given the fact that most tumors respond to hypoxic stress with increased VEGF expression via HIF-1-dependent transcription, this study identifies for the first time that TGF-beta also increases VEGF mRNA in an AP-l/HIF-1-dependent mechanism and may potentiate the hypoxic response.