MDM2 Regulates Hypoxic Hypoxia-inducible Factor 1α Stability in an E3 Ligase, Proteasome, and PTEN-Phosphatidylinositol 3-Kinase-AKT-dependent Manner

Hypoxia-inducible factor 1 (HIF1) is a heterodimeric transcription factor containing an inducibly expressed HIF1 alpha subunit and a constitutively expressed HIF1 beta subunit. Under hypoxic conditions, the HIF1 alpha subunit accumulates because of a decrease in the rate of proteolytic degradation, and the resulting HIF1 alpha-HIF1 beta heterodimers undergo post-translational modifications that promote transactivation. Previous reports suggest that amplified signaling through PI3K enhances HIF1-dependent gene expression; however, its role is controversial, and the mechanism is unclear. Using genetically engineered PTEN-deficient cell lines, we demonstrate that PTEN specifically inhibited the accumulation of HIF1 alpha in response to hypoxia. Furthermore, we report that in glioblastoma cell lines, inhibition of PI3K pathway, using pan as well as isoform-specific PI3K inhibitors SF1126, PF4691502, BEZ-235, GDC0941, and TGX221 blocked the induction of HIF1 alpha protein and its targets vascular endothelial growth factor, HK1, and GLUT1 mRNA in response to hypoxia. Herein, we describe the first evidence that HIF1 alpha can be degraded under hypoxic conditions via the 26 S proteasome and that MDM2 is the E3 ligase that induces the hypoxic degradation of HIF1 alpha. Moreover, the action of MDM2 on HIF1 alpha under hypoxia occurs in the cytoplasm and is controlled by the PTEN-PI3K-AKT signaling axis. These data strongly suggest a new role for PTEN in the regulation of HIF1 alpha and importantly that PI3K-AKT activation is required for the hypoxic stabilization of HIF1 alpha and that hypoxia alone is not sufficient to render HIF1 alpha resistant to proteasomal cleavage and degradation. Moreover, these findings suggest new therapeutic considerations for PI3K and/or AKT inhibitors for cancer therapeutics.