Abstract 13365: Mst1 Mediates a Cell-protective Mechanism in the Heart Through Phosphorylation of FoxO1 and C/EBP-β
Mammalian sterile 20-like kinase 1 (Mst1), a serine/threonine kinase which is the chief component of the mammalian Hippo pathway, promotes apoptosis and inhibits compensatory cardiac hypertrophy, thereby playing a crucial role in mediating cardiac dysfunction during ischemia/reperfusion and cardiac remodeling after myocardial infarction (MI). Mst1 also stimulates a cell-protective mechanism through Forkhead box O1 (FoxO1) as negative feedback to counteract its strong pro-apoptotic actions. In order to clarify how Mst1 mediates this cell-protective mechanism in the heart, transgenic mice with cardiac-specific overexpression of dominant negative Mst1 (Tg-DN-Mst1) were subjected to 2 hours of prolonged ischemia. Tg-DN-Mst1 hearts exhibited a significantly greater MI/area at risk, as evaluated with tetrazolium chloride staining, than wild-type hearts (60.3±2.2% vs. 50.4±3.3%, p<0.05), suggesting that the cardioprotective effect of Mst1 dominates the pro-apoptotic effect in this experimental condition. FoxO1 upregulated antioxidant genes, including catalase, and inhibited pro-apoptotic genes, including FasL, in cardiomyocytes, and these effects upon gene expression were attenuated when endogenous Mst1 was inhibited. Chromatin immunoprecipitation assays revealed that Mst1 inhibits binding of FoxO1 to the FasL promoter, but promotes binding of FoxO1 to the catalase promoter. Unexpectedly, reporter gene assays indicated that the C/EBP-β binding element in the catalase promoter, but not the FoxO1 binding element, is critical for Mst1-mediated catalase gene upregulation. Physical interaction between FoxO1 and C/EBP-β was enhanced in the presence of Mst1. Mass spectrometry showed that Mst1 phosphorylates C/EBP-β at Thr299 in the leucine zipper domain. Phosphorylation of C/EBP-β at Thr299 by Mst1 facilitated homodimerization of C/EBP-β by promoting leucine zipper formation, thereby facilitating binding of C/EBP-β to DNA. Knockdown of C/EBP-β reversed the protective effects of FoxO1 expression against Mst1-induced apoptosis. In summary, Mst1-mediated phosphorylation of both FoxO1 and C/EBP-β stimulates a cell-protective mechanism by enhancing FoxO1-C/EBP-β interaction and C/EBP-β-mediated transcription in the heart.
- © 2013 by American Heart Association, Inc.