Abstract 357: Up-Regulation of Cytochrome Oxidase III after Myocardial Infarction Impairs Mitochondrial Respiration and Cardiac Cell Survival
Cytochrome C oxidase (COX) is the terminal oxidase of the mitochondrial electron transport chain and is composed of 13 subunits, among which the subunits COX I and COX II are the main catalytic components, but the function of COX III is not clear. Because myocardial ischemia affects mitochondrial oxidative metabolism, we hypothesized that COX activity and expression would be affected during post-ischemic cardiomyopathy. This hypothesis was tested in a monkey model after chronic (2 months) myocardial infarction (MI; n=6) and followed by heart failure (HF; n=9) induced by 3 weeks of ventricular pacing. In that model, COX I protein expression was significantly decreased by 3-fold after MI and by 4-fold after HF (P<0.05 versus sham), whereas COX II expression remained unchanged. Surprisingly, COX III protein expression significantly increased by 5-fold after MI, and further by 10-fold after HF when compared to sham (both, P<0.05 versus sham). The physiological role of COX III and its potential impact on COX composition and activity were examined further in vitro. Overexpression of COX III in mitochondria of HL-1 cells resulted in a dose-dependent, up to 80% decrease in protein expression of COX I, 60% decrease in global COX activity, 60% decrease in cell viability, and up to 3-fold increase in apoptosis as measured by TUNEL and caspase-3 activity (all values, P<0.05 versus LacZ control). Addition of H2O2 decreased cell viability by 47±3% upon overexpression of COX III, but only by 12±5% in control conditions (P<0.05 between groups). COX III expression also significantly increased by 1.5-fold in HL-1 cells exposed to the pro-apoptotic agents staurosporine and chelerythrine (P<0.05 versus vehicle). We conclude that the ischemic stress induced by MI in vivo leads to a specific up-regulation of COX III, which can be reproduced in vitro by inducing cell stress. Increased protein expression of COX III is sufficient to down-regulate the expression of the catalytic subunit COX I, to impair overall COX oxidative activity, and to increase cell death by apoptosis. Therefore, up-regulation of COX III expression may contribute to the increased susceptibility to apoptosis following MI and subsequent HF.