Abstract 1776: Mammalian Sterile 20 Like Kinase 1 (Mst1) is an Endogenous Negative Regulator of Autophagy in the Heart
Autophagy is a major mechanism of protein degradation, which participates in protein quality control in the heart. Signaling mechanisms controlling autophagy in the heart are poorly understood. Autophagy is markedly suppressed in mice with cardiac specific overexpression of mammalian STE 20 like-kinase1 (Tg-Mst1), a model of dilated cardiomyopathy. In Tg-Mst1, suppression of autophagy was accompanied by accumulation of polyubiquitinated proteins. The ubiquitin- and LC3-binding protein p62, known to be degraded by autophagy, was accumulated in Tg-Mst1 mice (3.4±0.1 fold, vs. non-transgenic (NTg) mice, p<0.01). The activity of Mst1 was suppressed by glucose deprivation (GD) in cardiac myocytes (0.28±0.01 fold, p<0.01), whereas restoration of Mst1 activity attenuated GD-induced autophagy, as determined by LC3-II/ LC3-I (Control 1.00±0.03; GD 1.48±0.02; GD with Mst1 1.12±0.04* *p<0.01, vs GD). Autophagosome (AP) formation by starvation, as evaluated by GFP-LC3 dots, was suppressed in Tg-Mst1 compared to NTg mice (43±3, 269±7, p< 0.01). Inhibition of GD-induced accumulation of AP was observed even in the presence of chloroquine, an inhibitor of lysosomal degradation, suggesting that Mst1 inhibits AP formation. Phosphorylation of AMPK at Thr 172 (0.56±0.01 fold, p<0.01) and eukaryotic elongation factor-2 (eEF2) at Thr 56 (0.34±0.1 fold, p<0.01), positive mediators of autophagy, was suppressed, whereas phosphorylation of mTOR at Ser 2448 (1.28±0.02 fold, p<0.01), a negative regulator of autophagy, was increased in Tg-Mst1 compared with NTg mice. Increases in GFP-LC3 dots under GD were decreased even in the presence of rapamycin (Rap), an inhibitor of mTOR, when Mst1 is overexpressed in cultured cardiac myocytes (Rap 185±11, Rap+Mst1 49±6, p<0.01), suggesting that Mst1 also acts downstream of mTOR. These results suggest that Mst1 is a negative regulator of autophagy in cardiac myocytes. Activation of Mst1, which occurs during cardiac stress, inhibits, whereas inhibition of Mst1 during starvation stimulates, autophagy in cardiac myocytes. Mst1 inhibits autophagy by inhibiting signaling mechanisms both upstream and downstream of mTOR. In conclusion, Mst1 critically regulates protein turnover in the heart through suppression of autophagy.