Mechanism of Cytochrome Oxidase Dysfunction In Hypoxia and Myocardial Ischemia

Mitochondrial dysfunction is an important mediator of myocardial injury in experimentally induced ischemia‐reperfusion in rodents. Our results show that Cytochrome Oxidase is a direct target of hypoxia/ischemia induced oxidative damage leading to mitochondrial dysfunction. We have shown that, subunits, I, IV and Vb of Cytochrome Oxidase are preferentially phosphorylated and degraded under hypoxic conditions leading to significantly reduced Cytochrome Oxidase activity. We generated stable cell lines with subunits IV and Vb knocked down by siRNA. We report here that loss of these subunits is sufficient to disrupt mitochondrial respirasome function and by using a mitochondria targeted spin trap we demonstrate a marked increase in mitochondrial reactive oxygen species production in these cell lines. Hypoxia caused significantly higher translocation of Protein Kinase A catalytic α (PKA) subunit into mitochondria. We show that mitochondrial targeting of PKA under hypoxia/ischemia is facilitated by induction of cytosolic Casein Kinase II (CK II) activity. Inhibitors of both CKII and PKA reduced loss of Cytochrome Oxidase activity and disruption of mitochondrial ETC supercomplexes under hypoxia. Further, preconditioning of isolated langendorff mouse hearts with these inhibitors significantly protected the hearts from ischemia/reperfusion injury. In summary we report that hypoxia/ischemia induced CKII causes activation and translocation of PKA into mitochondria which in turn phosphorylates Cytochrome Oxidase subunits leading to their degradation, contributing to mitochondrial dysfunction and oxidative stress.