Abstract Fri165: Novel mechanism of hypoxia inducible factors 1 alpha (HIF-1α) regulation

Hypoxia is a key factor in ischemia contributing to heart failure/stroke. Dysfunction of β-adrenergic receptors (βARs) that are key regulators of cardiac function underlies deleterious outcomes. We showed that acute hypoxia activates Phosphoinositide 3-kinase γ (PI3Kγ) in the endosomes impairing βARs resensitization leading to cardiac dysfunction and HIF-1α stabilization. In contrast to wild-type (WT) mice, subjecting PI3Kγ knock out (KO) mice to acute hypoxia resulted in preserved cardiac function, reduced βAR phosphorylation, and increased βAR-associated PP2A activity. Notably, HIF-1α did not accumulate in PI3Kγ KO mice despite hypoxia, showing a novel upstream role for PI3Kγ in HIF-1α regulation. To investigate the role of PI3Kγ in HIF-1α regulation, we generated transgenic (Tg) mice with cardiomyocyte specific expression of wild type PI3Kγ under α-MHC promoter (PI3Kγ-WT). Immunoblotting of cardiac lysates revealed significant accumulation of HIF-1α in the PI3Kγ WT Tg even at baseline, while it was completely absent in the PI3Kγ KO and Non-Tg (NTg) controls. Similarly, overexpression of PI3Kγ-WT in HEK 293 cells leads to stabilization of HIF-1α even in normoxia while there was minimal HIF-1α stabilization in the parental HEK 293 cells. Furthermore, treatment of HEK 293 cells expressing PI3Kγ-WT with PI3K inhibitor wortmannin or overexpression of inactive PI3Kγ (deletion in the ATP binding site, PI3K-inact) leads to loss in HIF-1α stabilization in normoxia showing that stabilization HIF-1α is kinase-dependent mechanism. These findings establish PI3Kγ as a novel regulator of HIF-1α that could bypass hypoxia-mediated regulation of HIF-1α to mediate unique transcriptional response underlying the anti-apoptotic role of PI3Kγ. Moreover, our cellular and cardiac studies show that PI3Kγ regulates HIF-1α stabilization through Von Hippel-Lindau (VHL) that is a key ubiquitin ligase mediating HIF-1α degradation and mechanisms of VHL regulation by PI3Kγ will be discussed in our presentation. Understanding the fundamental mechanism of HIF-1α stabilization by PI3Kγ will help to develop potential therapeutic strategies to mediate HIF-1α stabilization to accelerate acute adaptive signaling in response to myocardial infarction and stroke.