ACTIVATOR-PROTEIN-1 BINDING POTENTIATES THE HYPOXIA-INDUCIBLE FACTOR-1-MEDIATED HYPOXIA-INDUCED TRANSCRIPTIONAL ACTIVATION OF VASCULAR-ENDOTHELIAL GROWTH-FACTOR EXPRESSION IN C6 GLIOMA-CELLS

The endothelial cell-specific mitogen vascular-endothelial growth fact or (VEGF) plays a key role in both physiological and pathological angi ogenesis. The up-regulation of VEGF expression in response to reduced oxygen tension occurs through transcriptional and post-transcriptional mechanisms. To investigate the molecular mechanisms of transcriptiona l activation by hypoxia (1% oxygen), fine mapping of a hypoxia-respons ive region of the human VEGF promoter was carried out using luciferase reporter-gene constructs in C6 glioma cells. Here, we report that the binding site of hypoxia-inducible factor 1 (HIF1) is crucial for the hypoxic induction of VEGF gene expression. However, an enhancer subfra gment containing the HIF1 binding site was not sufficient to confer fu ll hypoxia responsiveness. Addition of upstream sequences restored the full sensitivity to hypoxia induction. This potentiating effect is du e to activator protein 1 binding. The 'potentiating' sequences are una ble to confer hypoxia responsiveness on their own. Our results strongl y suggest that in C6 glioma cells a complex array of trans-acting fact ors facilitates full transcriptional induction of VEGF gene expression by hypoxia.