Abstract Body: Introduction: Mammalian sterile 20-like kinase 1 (Mst1), a major component of the Hippo pathway, promotes cell death and differentiation while inhibiting cell growth, thereby contributing to the development of heart failure. We recently discovered that Mst1 phosphorylates protein kinase RNA-like endoplasmic reticulum kinase (PERK), a kinase responsible for the integrated stress response (ISR).

Methods and results: Mst1 induces phosphorylation of the cytoplasmic domain of PERK, leading to its activation. To evaluate the role of phosphorylation of PERK in the heart, we generated knock-in mice expressing a PERK mutant that is not phosphorylated by Mst1 (PERK-KI). Although PERK was activated by tunicamycin, an ER stress inducer, it was not activated by transverse aortic constriction (TAC) in PERK-KI mice in vivo, suggesting that Mst1 mediates activation of PERK during TAC. To investigate the effect of Mst1-induced PERK phosphorylation on cardiac function, cardiac-specific Mst1 transgenic mice (Tg-Mst1) were crossed with heterozygous (h) PERK-KI mice (Tg-Mst1-hPERK-KI). The level of phosphorylation of eIF2α was decreased in Tg-Mst1-hPERK-KI mice compared to in Tg-Mst1 mice (0.45-fold, p<0.05), confirming that Mst1-induced activation of PERK is inhibited in hPERK-KI mice. Decreases in left ventricular ejection fraction (LVEF) in Tg-Mst1 was alleviated in Tg-Mst1-hPERK-KI mice (46% vs Tg-Mst1 34%, p<0.05), which was accompanied by reduced fibrosis (3.5% vs Tg-Mst1-hPERK-KI 2.2%, p<0.05) in the heart. Since Mst1 is activated by TAC (2.4-fold), we investigated the effect of TAC in hPERK-KI mice. Cardiac dysfunction induced by 8 weeks of TAC was also alleviated in hPERK-KI mice (LVEF: 67% vs WT 46%, p<0.05). It has been shown that the protein level of angiotensin II type 1 receptor (AT1R), a critical mediator of pathological cardiac hypertrophy, is increased by the ISR. We found that AT1R expression was increased in Tg-Mst1 mice but attenuated in Tg-Mst1-hPERK-KI mice (2.18-fold, p<0.05).

Conclusion: Mst1 mediates pathological hypertrophy through activation of PERK. Mst1-induced activation of PERK was accompanied by upregulation of AT1R, a protein known to be regulated by the ISR.