Abstract 16008: FoxO3 Triggers Robust Cardiac Atrophy via BNIP3-dependent Myocyte Autophagy
Introduction: FoxO transcription factors antagonize cardiac hypertrophy induced by load stress and evidence points to FoxO activation in heart disease. However, underlying mechanisms are unknown. In skeletal muscle, FoxO3 triggers autophagy, an evolutionarily ancient mechanism of protein and organelle recycling critical to survival of post-mitotic cells. Based on this, we hypothesized that FoxO antagonizes cardiac growth by activating autophagic flux.
Methods and Results: We engineered transgenic mice harboring a cardiomyocyte-specific constitutively active FoxO3 mutant (caFoxO3flox;αMHC-Mer-Cre-Mer), the expression of which is activated by exposure to tamoxifen. Activation of FoxO3 expression (5-fold over native FoxO3) provoked dramatic and progressive loss of cardiac mass. Within 3 days of transgene activation, heart weight/body weight ratios (HW/BW) declined 30% (±2, p<0.0001, n=9), and 4-day mortality was 100%. Robust increases in cardiac myocyte autophagy were detected: 60-fold increase in LC3-II levels, ±5, p<0.001, n=9; 20-fold (±1.4, p<0.001, n=2) increases in GFP-LC3 puncta. Robust accumulation of autophagosomes was detected by electron microscopy (EM). Despite dramatic cardiac pathology, increases in cardiomyocyte apoptosis were not apparent: TUNEL staining of cardiomyocytes was not increased, caspase activation was not detected, and apoptotic morphology was not seen on EM. Furthermore, both mitochondrial biomass and membrane potential were decreased by 25% and 30% (±2.4, ±3.3 respectively, p<0.001 each, n=3), again consistent with increased autophagic activity. To test the role of BNIP3, a pro-apoptotic protein involved in autophagy in certain contexts, we crossed transgenic animals with BNIP3-null mice. Importantly, the cardiac atrophy phenotype was abolished by BNIP3 inactivation. Finally, sarcomere derangement induced by FoxO3-dependent activation of ubiquitin ligases contributed to development of cardiac dysfunction and rapid lethality.
Conclusions: FoxO3 is a master regulator that governs both the autophagic-lysosomal and proteasomal pathways of protein degradation in heart. FoxO3 antagonizes cardiac growth by activating BNIP3-dependent autophagy.
- © 2010 by American Heart Association, Inc.