Abstract 3011: FoxO1 and Autophagy Play an Essential Role in Mediating Regression of Hypertrophy During Unloading of Cardiac Myocytes
Forkhead box-O (FoxO) family transcription factors are involved in atrophy and autophagy in skeletal muscle. Autophagy is a bulk degradation process in which cytoplasmic proteins are degraded. We have shown that FoxO1 stimulates autophagy in cardiac myocytes (CMs). We hypothesized that FoxO1 plays an important role in mediating regression of cardiac hypertrophy (CH) after unloading through stimulation of autophagy. Mice were subjected to transverse aortic constriction (TAC) for 1 week, after which the constriction was removed (DeTAC) for 1 week. Regression of hypertrophy was observed after DeTAC, as indicated by LVW/BW (mg/mm) (Sham 3.43, TAC 4.99, DeTAC 3.85, p<0.01) and CM cross-sectional area (μm2) (Sham 198.42, TAC 269.1, DeTAC 184.93, p<0.05). Indicators of autophagosome formation, including LC3-II/ tubulin (Sham 1.0, TAC 1.3, DeTAC 1.9, p<0.05) and GFP-LC3 dots/cell (Sham 0.16, TAC 0.70, DeTAC 1.38, p< 0.01) were significantly increased, whereas p62, a protein degraded by autophagy, was reduced (TAC 0.98, DeTAC 0.72, p<0.01) after DeTAC, suggesting that autophagy is induced. Stimulation of autophagy during DeTAC was accompanied by upregulation of FoxO1 (Sham 1.0, TAC 0.28, DeTAC 1.74, p<0.01). Similar results were obtained in vitro when neonatal rat CMs were subjected to stretch for 36 hours followed by incubation without stretch (destretch). In this model, autophagy was increased after destretch, as indicated by accumulation of LC3-II (3.5 fold) and reduction of p62 (−25%). FoxO1 was significantly upregulated during destretch both at mRNA (2.6 fold, p<0.01) and protein (3.7 fold vs stretch, p<0.05) levels. In order to examine the role of autophagy and FoxO1 in mediating regression of CH during destretch, CMs treated with adenoviruses harboring shRNA-beclin1 or shRNA-FoxO1 were subjected to destretch. CH regression achieved after destretch was significantly attenuated when autophagy was suppressed through beclin1 downregulation or FoxO1 downregulation (control −53%, shRNA-beclin1 −14%, shRNA-FoxO1 −20.5%). These results suggest that autophagy is activated during regression of CH, which is accompanied by upregulation of FoxO1. Autophagy and Foxo1 play an essential role in mediating regression of CH during unloading of CMs.