Abstract 1524: Gene 33 Promotes Hypoxia-Induced Cardiomyocyte Apoptosis by Suppressing Growth Factor-Mediated AKT and ERK Survival Signaling
Ischemia in the heart triggers cardiomyocyte apoptosis and necrosis. Loss of cardiomyocytes by apoptosis contributes significantly to ischemic cardiomyopathy. Despite much research effort, the mechanisms controlling cardiomyocyte apoptosis in response to ischemia are not well understood. Most published studies have focused on conventional mediators of apoptosis such as caspases or members of the BCL2 family of proteins. Transcriptionally induced mediators of cardiomyocyte apoptosis are much less appreciated. Here we report that Gene 33 (also named MIG6, RALT or ERRFI1) mediates cardiomyocyte apoptosis in response to hypoxia and that Gene 33 is strongly induced by ischemic insults in the mouse heart. Gene 33 is an adaptor protein known to bind to ErbB family receptor tyrosine kinases and to suppress their tyrosine phosphorylation and the downstream signaling events. We find that ectopic adenoviral vector-mediated expression of Gene 33 in neonatal rat cardiomyocytes is potently apoptogenic. Co-expression of constitutively active AKT (myr-AKT) and/or MEK-1 (MEK-DD) with Gene 33 partially rescues cardiomyocytes from Gene 33-induced cell death. Ectopic expression of Gene 33 in neonatal rat cardiomyocyte reduces basal ERK and AKT activities and blunts ErbB and IGF-1 signaling to ERK and AKT. This is mediate, at least in part, by preventing ligand-induced receptor tyrosine phosphorylation. We find that in neonatal rat cardiomyocytes endogenous Gene 33 is strongly induced by hypoxia. This is accompanied by elevated phosphorylation of EGFR, ERK, AKT and FKHR, increased activities of caspase 3 and 9, and higher number of TUNEL positive cells. RNAi knockdown of endogenous Gene 33 further enhances phosphorylation of ERK, AKT and FKHR, reduces activation of caspase 3 and 9, and reduces number of TUNEL positive cells. Furthermore, myocardial infarction and ischemia/reperfusion induce Gene 33 expression in the infarct regions of the left ventricular tissue. These findings identify a novel pro-apoptotic factor that promotes cardiomyocyte apoptosis by inhibiting growth factor-mediated survival signals. We propose a model in which cardiomyocyte Gene 33 is induced by ischemia and promotes apoptosis thereby contributes to ischemic injury in the heart.