Phosphatidylinositol 3-kinase/Akt signaling is neither required for hypoxic stabilization of HIF-1 alpha nor sufficient for HIF-1-dependent target gene transcription

Abstract

The serine/threonine kinase Akt/PKB and the oxygen-responsive transcription factor HIF-1 share the ability to induce such processes as angiogenesis, glucose uptake, and glycolysis. Akt activity and HIF-1 are both essential for development and implicated in tumor growth. Upon activation by products of phosphatidylinositol 3-kinase (PI3K), Akt phosphorylates downstream targets that stimulate growth and inhibit apoptosis. Previous reports suggest that Akt may achieve its effects on angiogenesis and glucose metabolism by stimulating HIF-1 activity. We report here that, whereas serum stimulation can induce a slight accumulation of HIF-1 alpha protein in a PI3K/Akt pathway-dependent fashion, hypoxia induces much higher levels of HIF-1 alpha protein and HIF-1 DNA binding activity independently of PI3K and mTOR activity. In addition, we find the effects of constitutively active Akt on HIF-1 activity are cell-type specific. High levels of Akt signaling can modestly increase HIF-1 alpha protein, but this increase does not affect HIF-1 target gene expression. Therefore, the PI3K/Akt pathway is not necessary for hypoxic induction of HIF-1 subunits or activity, and constitutively active Akt is not itself sufficient to induce HIF-1 activity.