MAPK and Akt act cooperatively but independently on hypoxia inducible factor-1 alpha in rasV12 upregulation of VEGF

Oncogenic ras upregulates the expression of VEGF through the activation of the transcriptional enhancer hypoxia inducible factor-1 alpha (HIF-1 alpha) by a still poorly understood mechanism. Here, we demonstrate that both the Raf/MEK/MAPK and the PI3 kinase/Akt signaling pathways potently and additi vely stimulate the expression from a hypoxia response element (HRE) within the 5'flanking region of the VEGF promoter. Interestingly, while MAPK appea rs to specifically upregulate the transactivation activity of HIF-1 alpha t hrough direct phosphorylation of its regulatory/inhibitory domain, GSK-3, a downstream target of Akt, directly phosphorylates the HIF-1 alpha oxygen-d ependent degradation domain. These results suggest a novel mechanism whereb y two divergent signaling pathways emerging from Ras may cooperatively but independently regulate the activity of a HIF-1 alpha, thereby promoting the expression of a potent angiogenic mediator.