MOLECULAR MECHANISMS OF APOPTOSIS IN MYOCARDIAL ISCHEMIA

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The long term goal of this proposal is to elucidate the mechanisms of cardiomyocyte apoptosis following ischemia/reperfusion and its relation to the progression of ventricular remodeling and heart failure. Until recently, myocyte death during myocardial infraction (MI) was considered to be exclusively due to cell necrosis. However, accumulating evidence suggests that apoptotic cell death contributes significantly to myocyte loss during both the acute and chronic phases after ML. In particular, apoptosis may play a major role in reperfusion injury. However, at present, little experimental evidence is available as to how cardiomyocytes undergo apoptosis in response to ischemia/reperfusion. As such, we have established an in vitro model of adult cardiomyocyte apoptosis by hypoxia/reoxygenation and a murine model of MI associated with ventricular remodeling. Our preliminary results suggest that the release of cytochrome c from mitochondria is an early step in cardiomyocyte apoptosis. In addition, members of mitogen activated protein (MAP) kinases, particularly JNK, are rapidly activated in response to ischemia/reperfusion. Evidence from other cell types suggest that persistent activation of the stress-activate kinase JNK leads to apoptosis, possibly by phosphorylating mitochondrial protein Bcl-2. On the other hand, activation of the phosphatidylinositol-3 kinase (PI-3K)/Akt pathway by growth factor stimulation, such as insulin-like growth factor-1 (IGF-1), has been shown to promote cell survival, presumably by phosphorylation of the pro-apoptotic mitochondrial protein Bad. Accordingly, we hypothesize that the loss of outer mitochondrial membrane integrity by ischemia/reperfusion in the critical decision making for irreversible myocyte death by apoptosis, and potentially necrosis as well. We also hypothesize that there are additional anti-apoptotic and pro- apoptotic molecules, other than known Bcl-2 family of proteins, expressed in cardiomyocytes that modulate the mitochondrial response to ischemia/reperfusion. Our Specific Aims of this proposals are: Specific Aim 1: To further characterize our models of cardiomyocyte apoptosis induced by hypoxia/reoxygenation in vitro and myocardia ischemia/reperfusion in vivo. Specific Aim 2: To define the roles of mitochondrial dysfunction, cytochrome c release and Bcl-2 and Bcl-x/L in apoptosis induced by hypoxia/reoxygenation in vitro and ischemia/reperfusion in vivo. Specific Aim 3: To elucidate the roles of stress-activated kinases in the regulation of cardiomyocyte apoptosis during ischemia/reperfusion and identify novel mitochondrial and non- mitochondrial targets of the pro-apoptotic kinase JNK. Specific Aim 4: To elucidate the roles of anti-apoptotic PI-3K/Akt signaling in the regulation of ischemia/reperfusion-induced cardiomyocyte apoptosis and identify novel mitochondrial and non-mitochondrial targets of the anti- apoptotic kinase Akt. We anticipate that elucidation of the mechanisms of cardiomyocyte apoptosis by ischemia/reperfusion and identification of novel myocardial prop-apoptotic and anti-apoptotic molecules will lead to the design of more specific cardiomyocyte protective strategies.