Abstract 14279: Insulin Receptor Substrates (IRS) are Essential Regulators of Autophagy and Mitochondrial Function in the Heart
To test the hypothesis that insulin and IGF-1 signaling coordinately suppresses autophagy in the heart, we completely disrupted insulin and IGF-1 signaling by generating mice with combined cardiomyocyte-specific deletion of IRS1 and IRS2 (CIRS12KO). CIRS12KO developed dilated cardiomyopathy by the age of 4 weeks (LVDs +24.8%, FS -36.7%) with severe cardiomyocyte loss (cardiomyocyte nuclei number -26.7%, p<0.05) and increased fibrosis (+2.72fold, p<0.05). The majority of CIRS12KO mice died by eight weeks of age. At one day of age, contractile function was normal and histology revealed absence of fibrosis and preserved cardiac structure. Autophagy as assessed by LC3-II/LC3-I immunoblotting was increased 7.4fold (p<0.05) in CIRS12KO. This was confirmed by increased autophagosome content (+63%, p<0.05) using cadaverine fluorescence and increased numbers of autophagic vesicles in electron micrographs. To determine if excessive autophagy accounted for heart failure in CIRS12KO mice, we decreased autophagy by heterozygote deletion of the autophagy mediator Beclin1 (CIRS12KO x Bec+/-). At two weeks of age, LC3-II/LC3-I ratio was increased in CIRS12KO hearts by 63.3% (p<0.05) but reverted to levels of WT controls following haploinsuffiency for Beclin1. In six-week-old CIRS12KO x Bec+/- mice cardiac function was improved relative to CIRS12KO mice. Left ventricular developed pressure (in vivo) was increased by 34.7%, Max dP/dt and Min dP/dt were increased by 65.5%, 65.0% respectively and the median survival age increased from 7.4 to 16.8 wk. (CIRS12KO x Bec+/- vs. CIRS12KO, p<0.05). In saponin-permeabilized cardiac fibers from two-week-old mice, exposed to succinate/rotenone as substrate, ADP-stimulated mitochondrial oxygen consumption (VADP) and ATP synthesis were equivalently decreased in CIRS12KO hearts irrespective of Beclin1 haploinsuffiency (-30.4 to -38.0% vs. WT respectively, p<0.05 each). Transcriptional analysis revealed equivalent repression of FAO, OXPHOS, and PGC-1α mRNA levels. Together, these data identify IRS signaling as critical regulators of cardiac autophagy and mitochondrial function, which if absent, results in uncontrolled autophagy, mitochondrial dysfunction, myocyte loss and heart failure.
- © 2012 by American Heart Association, Inc.