Abstract 3559: Insulin Receptor Substrates (IRS) Signaling are Essential Regulators of Mitochondrial Function and Cardiomyocyte Survival
It has recently been reported that impaired PI3K signaling leads to cardiac mitochondrial dysfunction and that cardiac insulin receptor deficiency impairs cardiac growth. Signaling from insulin and insulin-like growth factor 1 receptors to PI3K is mediated by insulin receptor substrates (IRS) 1 and 2. Our objective was to determine the contribution of IRS isoforms on cardiac structure, mitochondrial and contractile function. For this purpose we generated mice with cardiomyocyte-specific deletion of IRS1 (CIRS1KO) or IRS2 (CIRS2KO) or IRS1 + IRS2 (CIRS12KO). While the single KOs had normal survival, CIRS12KO mice died from heart failure within the first eleven weeks of life. At the age of 8 weeks, heart weight-to-tibia length ratios, HW/TL were reduced by 11% (p<0.05) in CIRS1KO versus WT. ADP-stimulated mitochondrial oxygen consumption (V-ADP) was decreased by 22% (p<0.05) and ATP synthesis by 16% (p<0.05) in saponin-permeabilized cardiac fibers using pyruvate as substrate but not with palmitoyl carnitine (PC) or glutamate. In contrast, HW/TL was unchanged in CIRS2KO at the age of 8 weeks and was increased at the age of 14 weeks (+7%, p<0.05). Similarly, ADP-stimulated respirations were reduced in CIRS2KO at both ages by −21% and −16% respectively, (p < 0.05) only with pyruvate as substrate. In contrast, CIRS12KO developed dilated cardiomyopathy at the age of 4 weeks (LVDs + 18.9%, LVPWd −12.7%, LVPWs −21.4%, FS −36.7%, EF −26.3%, p<0.05). Histology revealed myofibrillar loss and disarray and increased fibrosis. Using stereology, cardiomyocyte nuclei per area was reduced by 27% and cardiomyocyte cross-sectional area was increased by 33% (both, p<0.05). Using pyruvate, PC or glutamate as substrates, V-ADP and ATP-synthesis rates were reduced by 33– 41%. Transcriptional analysis revealed a coordinate downregulation (by 40 – 60%) of PDK4 and PDH subunits, FAO and OXPHOS genes that were associated with a 35% reduction in expression of PGC-1β. At the protein level, expression of the complex II subunit, succinate-ubiquinol oxidoreductase was reduced by −51% (p<0.01). These data identify a critical role for IRS-mediated signaling in the regulation mitochondrial gene and protein expression, mitochondrial function and cardiomyocyte survival.