Abstract 3513: STAT3 in the Matrix of Cardiomyocyte Mitochondria Regulates Opening of the Mitochondrial Permeability Transition Pore
Lack of the signal transducer and activator of transcription 3 (STAT3) abolishes infarct size reduction by ischemic preconditioning and postconditioning. Recently, STAT3 has been identified in mitochondria, where it regulates complex I activity and respiration. Since attenuation of mitochondrial permeability transition pore (MPTP) opening is essential for protection afforded by ischemic pre- and postconditioning, the aim of the present study was to investigate
whether or not STAT3 interacts with mitochondrial proteins regulating MPTP, and
whether or not lack of STAT3 affects calcium-induced MPTP opening.
In subsarcolemmal (SSM) and interfibrillar mitochondria (IFM) isolated from rat left ventricles (LV), immuno-reactivity was detected by Western Blot analysis and confocal laser scan microscopy for STAT3 (and STAT1, but not for STAT5). In SSM and IFM, similar amounts of STAT1 (SSM: 0.4±0.1 arbitrary units (a.u.), IFM: 0.5±0.1 a.u., n=7) and STAT3 (SSM: 1.2±0.1 a.u., IFM: 1.0±0.1 a.u., n=7) were found. Mitochondrial proteins were not contaminated with marker proteins of other cellular compartments. STAT3 was predominantly detected in the matrix of subfractionated rat LV SSM (51.2±8.1% immunoreactivity), whereas only 28.7±8.1% were found in the intermembrane space and 19.1±5.2% in the inner and outer membrane fraction (n=7, p<0.05). STAT3 co-immunoprecipitated with cyclophilin D, a protein regulating MPTP opening. Calcium induced MPTP opening was measured by loss of mitochondrial membrane potential (rhodamine123 fluorescence) in mitochondria from wildtype (WT) mice and from mice with a cardiomyocyte-specific deletion of STAT3 (STAT3-KO). MPTP opened at lower calcium concentrations (82.9±5.9 μM CaCl2, n=12) in STAT3-KO than in WT mitochondria (104.1±9.7 μM CaCl2, n=11, p<0.05). Taken together, STAT1 and STAT3 are localized in the matrix of cardiomyocyte SSM and IFM. Mitochondrial STAT3 binds cyclophilin D and inhibits calcium-induced MPTP opening, thereby potentially contributing to protection against ischemia/reperfusion injury.