Abstract 13422: Mitochondrial Reductive Stress Induced by Over-Expression of SOD2 Enhances Cardiac Function
Previously, it was reported that over-expression of a mutant human alphaB-Crystallin induces reductive stress and a cardiomyopathic phenotype in mice. However, the stress imposed by altered protein folding induced by the mutant alphaB-Crystallin may not necessarily apply to all forms of reductive stress. Accordingly, we hypothesized that SOD2 over-expression in cardiac mitochondria would produce reductive stress, but not necessarily a pathological cardiac phenotype. To test this we studied transgenic mice with cardiac myocyte (cardiac myosin heavy chain promoter) over-expression of human SOD2 (SOD2-tg). We found no detectable mitochondria-mediated •O2- generation (EPR analysis), lower sensitivity to antimycin A (less •O2- generation), and lower redox activity (oxidation of cyclic hydroxylamine to stable nitroxide [in nmol nitroxide produced/min/mg protein]; WT: 2.0±0.3 vs SOD2-tg: 0.8±0.1). In SOD2-tg mitochondria (compared to WT), GSSG concentration was decreased by 37.2±4.5%, and GSH/GSSG ratio was increased by 66.7±6.8%. These results support a more reducing milieu in the mitochondria of the SOD2-tg heart. Cardiac mitochondria isolated from SOD2-tg exhibited a marked decline in state 3 O2 consumption rate (OCR, by 30.1±3.6%), state 4 OCR (by 18±7%), and FCCP-induced uncoupling OCR (by 41.2±6.2%). However, no significant differences in the respiratory control index between WT and SOD2-tg (6.3±0.3 vs. 5.7±0.5, p>0.05),suggesting a down-regulation of the bioenergetic threshold for the SOD2-tg mitochondria. EPR analysis of the isolated mitochondria at 77 oK further showed significant decrease of semiquinone radical in the SOD2-tg vs WT at the Qi site (by 69.4±9.9%). We studied cardiac function in WT and SOD2-tg mice (m-mode echocardiography). SOD2-tg had a significantly greater ejection fraction (90±1%) compared to wild types (74±2) to and decreased basal rate pressure product (RPP in bpm*mmHg,34615 to 24003), suggesting supernormal, not impaired cardiac function. In conclusion, although SOD2 overexpression-dependent reductive stress triggers a down-regulation of mitochondrial bioenergetic threshold in murine heart and induces a more reductive milieu, the result of these changes produce super-normal ventricular function.
- © 2013 by American Heart Association, Inc.