Abstract 148: Mst1 Mediates Cell-protective Mechanisms In The Heart Through Phosphorylation Of FoxO1 And C/EBP-beta

Abstract

Forkhead box O1 (FoxO1) regulates both cell survival and death in the heart. The function of FoxO1 is modulated by posttranslational modifications, including phosphorylation. Mst1 (mammalian sterile 20-like kinase 1), a stress-activated pro-apoptotic kinase, phosphorylates FoxO1 at Ser209/216/231/232. We investigated the molecular mechanism by which FoxO1 regulates both cell survival and death in the heart. Overexpression of FoxO1 significantly suppressed, whereas downregulation of endogenous FoxO1 exacerbated, Mst1 activity and Mst1-induced apoptosis. In the presence of Mst1, FoxO1 downregulated pro-apoptotic genes, including FasL, but upregulated antioxidant genes, including catalase. Chromatin immunoprecipitation assays revealed that Mst1 attenuated FoxO1 binding to the FasL promoter, but it enhanced binding to the catalase promoter. Reporter gene assays showed that C/EBP-β binding elements, but not the FoxO binding ones, in the catalase promoter, are critical for Mst1-mediated upregulation of the catalase gene. FoxO1 interacted with C/EBP-β and their interaction was enhanced in the presence of Mst1. Mass spectrometry analyses revealed that Mst1 phosphorylates C/EBP-β at Thr 299 in its leucine zipper domain. Downregulation of endogenous C/EBP-β reversed the cell-protective effects of FoxO1 expression against Mst1-induced apoptosis. We evaluated the role of Mst1-mediated phosphorylation of FoxO1 and C/EBP-β in mediating survival and death of cardiomyocytes in response to ischemia/reperfusion (I/R). The size of myocardial infarction (MI) in left ventricle (LV) after I/R was significantly greater in cardiac-specific FoxO1 knockout ( c-FoxO1 -/- ) mice than in wild-type mice (MI/area at risk: 59±2, 41±1%, p <0.05, n=6). Injection of a C/EBP-β phospho-mimetic mutant adenovirus into the LV of c-FoxO1 -/- significantly reduced the I/R-induced LV infarct size observed in c-FoxO1 -/- (50±2%, p <0.05, n=6). In summary, Mst1-mediated phosphorylation of FoxO1 inhibits the DNA binding of FoxO1 to the cell death promoting genes whereas it enhances FoxO1-C/EBP-β interaction, Mst1-mediated phosphorylation of C/EBP-β, and C/EBP-β-mediated transcription, which activates cell-protective mechanisms in the heart.