Abstract 3992: Insulin Receptor Substrates (IRS) are Critical Regulators of Autophagy and Cardiomyocyte Survival
A transient perinatal increase in cardiomyocyte autophagy plays an important role in mammalian survival, but the mechanisms that mediate the suppression of autophagy once feeding is established are not known. To test the hypothesis that insulin signaling mediates these effects, we completely disrupted insulin and IGF-1 signaling in the heart by generating mice with cardiomyocyte-restricted KO of IRS1 and IRS2 (CIRS12KO). CIRS12KO died from heart failure within the first eleven weeks of life, and at the time of death exhibited severe myocyte loss and replacement fibrosis. At 24h of age, cardiac contractile function and expression of genes involved in oxidative phosphorylation (OXPHOS) (Ndufa9, COX4i1) and their transcriptional regulators PGC-1α and PGC-1β were normal. However, phosphorylation of insulin-activated targets such as Akt (Ser473) and S6 (Ser235/236) were reduced by −44.7%, −85.1% respectively (p<0.05). Autophagy as quantified by cadaverine fluorescence was increased by 63% (p<0.05). Electron microscopy revealed increased numbers of autophagic vesicles and reduced mitochondrial content. Although light microscopy revealed normal gross histology and the absence of fibrosis in hearts of 24-hour-old mice, the ratio of TUNEL positive to total nuclei (TUN/nuc) was increased 3.53 fold compared to wildtype (p<0.05). By 4 weeks of age, contractile function was severely impaired (EF -26.3%, FS -36.7%, p<0.05) and myofibrillar loss and increased fibrosis (2.72fold, p<0.05) were clearly evident. Autophagy rates remained increased by 35% (p=0.053) as was the TUN/nuc ratio (2.34fold, p<0.05). By this stage, OXPHOS (Ndufa9, Ndufv1, Uqcrc1, Cox4i1, Cox5b) as well as PGC-1α and PGC-1β gene expression was diminished by 54–72% (p<0.05% each). Thus insulin signaling is a critical regulator of myocardial autophagic flux and loss of IRS signals prevents the physiological suppression of autophagy that occurs when feeding is established and insulin levels rise, which if absent results in uncontrolled autophagy, apoptosis, myocyte loss and heart failure.