Abstract 19297: Cardiac Specific Deletion of Mcl-1 Leads to Impaired Autophagy and Rapid Development of Heart Failure
Mcl-1 is an anti-apoptotic Bcl-2 protein which is expressed at high levels in the heart compared to other anti-apoptotic Bcl-2 proteins. However, very little is known with respect to how Mcl-1 regulates cell survival in cardiomyocytes. To investigate the functional role of Mcl-1 in the heart, we used a loxP/α-MHC-Cre-recombinase system to generate inducible, cardiomyocyte-specific Mcl-1 knockout mice. Mcl-1 deletion led to rapid contractile dysfunction, cardiac hypertrophy, and interstitial fibrosis in the absence of stress. Surprisingly, loss of Mcl-1 did not activate apoptosis. Instead, Mcl-1 deficiency led to necrotic cell death characterized by mitochondrial permeability transition pore (mPTP) opening, mitochondrial swelling, and cellular rupture. To clarify how mPTP opening and necrosis contribute to heart failure in Mcl-1-/- mice, we generated Mcl-1/Cyclophilin D (CypD) double knockouts. CypD is an essential component of the mPTP, and CypD deficiency has been shown to reduce cardiac necrosis. CypD deletion significantly delayed development of heart failure in Mcl-1-/- mice, but did not rescue the phenotype. Although autophagy is an important cellular quality control mechanism, electron microscopy revealed no autophagosome formation in Mcl-1-/- cardiomyocytes. Further analysis revealed that loss of Mcl-1 led to impaired basal autophagic flux. Western blot analysis for autophagy proteins revealed reduced LC3II/I ratios and accumulation of p62 in Mcl-1-/- hearts. Chloroquine injection, which inhibits the fusion between autophagosomes and lysosomes, led to accumulation of LC3II and p62 in WT hearts but not in Mcl-1-/- hearts. To investigate whether autophagy could be induced in Mcl-1-/- hearts, we subjected mice to acute exercise. Autophagy was rapidly enhanced in wild type hearts after 80 minutes of swimming, but no change in autophagy occurred in Mcl-1-/- hearts. Conversely, adenoviral overexpression of Mcl-1 in rat neonatal cardiomyocytes led to increased LC3 processing and autophagy. These data suggest that, in addition to its anti-apoptotic role, Mcl-1 is essential in initiating autophagy in cardiomyocytes. Increased understanding of Mcl-1 mechanisms in the myocardium may provide new therapeutic avenues for heart disease.
- © 2012 by American Heart Association, Inc.