Sqstm1/p62 Regulates Hypoxia Inducible Factor 1 Alpha Stabilization And Protects The Heart From Hypoxic Injury

Ischemic heart disease (IHD) is characterized by cardiac tissue hypoxia, dysregulatedmetabolism, and cell death. Hypoxia inducible factor 1a (HIF1α) is a major component of thehypoxia pathway that regulates metabolic and angiogenic genes. Under normoxia, HIF1α getshydroxylated by the prolyl hydroxylases (PHDs), followed by its ubiquitination, and degradationby the Ubiquitin Proteasome System (UPS). Given the short half-life of HIF1α (>5mins), PHDinhibitors are employed to stabilize HIF1α and improve cardiac function in animal models ofischemia/infarction. Because PHD inhibitors exert off-target effects, alternative strategies tostabilize HIF1α are needed. In cancer cells, p62 stabilizes HIF1α by binding PHD3. Wehypothesized that p62 would stabilize HIF1α and provides cardioprotection from hypoxia. Wegenerated mice with tamoxifen-inducible cardiomyocyte-specific p62 deletion (cip62KO mice)and exposed them to 7% O2 for 6h. Compared to wild-type (WT) littermates, cip62KO micedisplayed increased oxidative stress and enhanced cell death. Hypoxia caused contractiledysfunction in cip62KO vs. WT mice. To gain mechanistic insights as to how lack of p62exacerbated hypoxic injury, cultured H9c2 cardiac myoblasts were exposed to 21% (normoxia)or 1% (hypoxia) O2 for 24h. Hypoxia increased HIF1α and p62 protein expression in H9c2 cells(p<0.05). To determine whether hypoxia-induced p62 accumulation is required for HIF1αstabilization, H9c2 cells were transfected with p62 or scrambled (ctrl) siRNA for 48h, andexposed to hypoxia. Compared to ctrl siRNA cells, hypoxia-induced HIF1α protein accumulationwas reduced (p<0.05) after p62 knockdown. Additionally, hypoxia increased expression ofHIF1α downstream targets: Egln1 , Vegfa , Bnip3 and Hmox1 mRNA, in ctrl siRNA cells, but theresponse was blunted (p<0.05) after p62 knockdown. These data indicate that p62 contributes tohypoxia-induced HIF1α stabilization and transcriptional activation. Defining how p62contributes to HIF1α stabilization and hypoxia tolerance is relevant clinically and could identifyp62 as a therapeutic target for treating IHD.