Akt and hypoxia-inducible factor-1 independently enhance tumor growth and angiogenesis

Recent reports have suggested that phosphatidylinositol 3-kinase/Akt signaling can induce angiogenesis and tumor growth by activating the hypoxia-inducible factor-1 (HIF-1). However, the absence of specific biochemical inhibitors of HIF-1 signaling has prevented a direct test of the requirement for HIF-1 activity in Akt-dependent tumorigenesis. To genetically test the relationship between HIF-1 and Akt, activated Akt was expressed in a hepatoma cell line lacking HIF-1. Akt expression was associated with a dramatic increase in tumor size, despite the absence of HIF-1. Tumor size was not further increased in cells with reconstituted HIF-1 activity, indicating that the effects of Akt on tumorigenesis were not limited by the absence of HIF-1. Increased tumor size in Akt-expressing, HIF-deficient cells was associated with vascular endothelial growth factor secretion and tumor vascularization. In addition to vascular endothelial growth factor production, Akt also conferred a cell-autonomous competitive advantage to tumor cells in an in vivo competition experiment. Thus, Akt has potent, HIF-1-independent oncogenic and angiogenic activities.