Normal cardiac function relies on highly coordinated intracellular events, such as calcium cycling and contraction, with adequate mitochondrial energy metabolism. However, mitochondrial respiration unavoidably produces reactive oxygen species (ROS) as electrons leak from the electron transport chain (ETC). Complex I of the ETC is believed to be the major site for ROS generation in the mitochondria. However, suppression of Complex I activity by chemical inhibitors leads to oxidative cell damage. In this study, we used a genetic model of Complex I deficiency, in which a key component of Complex I, Ndufs4, was deleted in the heart, to determine the causal role of Complex I in ischemia-reperfusion-induced oxidative stress in adult cardiac myocytes. Germline deletion of Ndufs4 in the heart (Ndufs4H-/-) leads to a ~75% decline of Complex I activity in cardiac mitochondria without obvious disease phenotype in the mice. As predicted, the mitochondrial respiration-coupled superoxide production events, superoxide flashes, were significantly decreased at baseline in the Ndufs4H-/- myocytes. Respiration substrate (pyruvate, 20 mM) failed to stimulate mitochondrial superoxide flash production in Ndufs4H-/- myocytes. This is accompanied by the slightly decreased steady state intracellular and mitochondrial ROS levels determined by the targeted H2O2 indicator, Hyper. The intracellular redox homeostasis is also tilted toward more reduced state, since the NADH/NAD ratio increased 67%. Surprisingly, ischemia reperfusion mimetic treatment of the myocytes caused dramatic increase in mitochondrial ROS production in Ndufs4H-/- groups, which contributed to the elevated overall cellular oxidative status. Overexpression of catalase in the mitochondria prevented these effects. Mechanistically, increased reducing equivalent (NADH) contributed to the dramatic ROS production during ischemia and reperfusion in Ndufs4H-/- myocytes. In summary, mitochondrial Complex I plays a critical role in controlling mitochondrial and cytosolic ROS homeostasis under normal conditions, and compromised Complex I function leads to accumulation of electron donors that paradoxically promote ROS production during ischemia reperfusion.
Molecular Mechanism Underlying Hypoxic Preconditioning-Promoted Mitochondrial Translocation of DJ-1 in Hypoxia/Reoxygenation H9c2 Cells