Abstract 221: MitoROMK Expression Mitigates the Cellular Response to Oxidative Stress

Abstract

We have recently reported that mitoROMK overexpression protects against, while knockdown of native ROMK exacerbates, oxidative stress-induced cell death and have proposed mitoROMK as the molecular correlate of the cardioprotective mitoKATP channel (Foster, Ho, et al Circ Res 2012). In order to better understand the mechanisms of protection, here we examined how alterations of ROMK expression affect glutathione redox responses during acute oxidative stress, utilizing cytoplasmic or mitochondrially targeted, genetically-encoded GSSG sensors. The tandem protein construct Grx1-roGFP2, a fusion of human glutaredoxin and redox sensitive GFP (Gutscher et al, Nat Methods 2008) was incorporated into adenoviral gene transfer vectors for expression in cardiac-derived H9C2 cells. Cells were subjected to treatment with the Complex III inhibitor Antimycin A (50µM) to induce endogenous mitochondrial reactive oxygen species (ROS) production, and changes in the glutathione redox potential were determined ratiometrically. MitoROMK overexpressing cells showed a significantly lower rate of mitochondrial matrix GSH oxidation secondary to H2O2 scavenging compared to both control and ROMK knockdown cell lines, indicating enhanced ability to maintain ROS balance. Increases in cytoplasmic GSSG were also blunted by mitoROMK overexpression. To determine if ROMK expression affected ROS-induced mitochondrial calcium overload, we also employed the mitochondrially-targeted ratiometric Ca2+ probe GEM-GECO (Zhao et al Science 2011). The same Antimycin A challenge led to large mitochondrial Ca2+ increase in control and knockdown cell lines, but this response was significantly suppressed in mitoROMK overexpressing cells. This could not be explained by differences in baseline mitochondrial membrane potential (ΔΨm; determined using the potentiometric probe tetramethylrhodamine methyl ester), because both control cells and ROMK overexpressors showed similar ΔΨm, while ROMK knockdown cells were more depolarized. The results suggest that enhanced cell survival in mitoROMK overexpressing cells is mediated by improved mitochondrial ROS balance and calcium handling, providing a key molecular link between mitochondrial K+ channels and cardioprotection.