Abstract 218: Mst1 Promotes Cardiac Myocyte Apoptosis through PERK-Mediated Stimulation of the Unfolded Protein Response
Mammalian sterile 20-like kinase 1 (Mst1) is a serine threonine kinase, which plays an important role in mediating apoptosis in response to ischemia/reperfusion and cardiac remodeling in the heart. In transgenic mice with cardiac specific overexpression of Mst1 (Tg-Mst1), there was no hypertrophy despite the fact that the mice developed dilated cardiomyopathy with elevated wall stress, suggesting that Mst1 also inhibits compensatory hypertrophy. We have shown previously that Mst1 negatively regulates cardiac hypertrophy under pressure overload in part through activation of PERK, an endoplasmic reticulum stress sensor kinase, and subsequent induction of the unfolded protein response (UPR). Here, we examined whether Mst1-induced stimulation of the UPR also contributes to induction of apoptosis. Adenovirus-mediated overexpression of Mst1 in cardiac myocytes upregulated CHOP, a pro-apoptotic transcription factor activated under the UPR, 3.79 fold (p<0.05). Expression of dominant negative PERK (DN-PERK) abolished Mst1-induced increases in expression of CHOP in cardiac myocytes. Furthermore, DN-PERK significantly inhibited Mst1-induced increases in apoptosis in cardiac myocytes as determined by cytoplasmic accumulation of mono- and oligonucleosomes (Mst1 2.23, Mst1+DN-PERK 1.53, p<0.05). shRNA-mediated knock-down of CHOP significantly inhibited Mst1-induced increases in apoptosis in cardiac myocytes (Mst1 2.52, Mst1+shRNA CHOP 1.90, p<0.05). In order to test the role of CHOP in mediating apoptosis in vivo, we crossed Tg-Mst1 and CHOP −/− mice. Although Tg-Mst1 exhibited a significant increase in TUNEL positive cardiac myocytes compared with non-transgenic control (0.3% vs 0.04%p<0.05), Tg-Mst1 generated under CHOP null background (Tg-Mst1- CHOP−/−) exhibited significantly reduced levels of TUNEL positive myocytes (0.08% p<0.05 vs Tg-Mst1). Left ventricular ejection fraction was significantly greater in Tg-Mst1-CHOP−/− than in Tg-Mst1 mice (53.7%vs 43.5%, p<0.05). These results suggest that the PERK-CHOP pathway plays a significant role in mediating the proapoptotic function of Mst1. Thus, the UPR mediates key functions of Mst1, namely inhibition of compensatory cardiac hypertrophy and stimulation of apoptosis.