Molecular regulators of cardiac myocyte apoptosis during ischemia
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Recent studies have demonstrated that cardiac myocytes undergo apoptosis in response to ischemic insults. Little is known, however, about the molecular regulation of myocyte apoptosis. To elucidate these regulatory pathways, a mouse model of myocardial infarction (MI) was generated. Coronary occlusion resulted in myocyte apoptosis in hypoperfused, hypoxic regions of the myocardium. Despite this correlation, p53, an inducer of myocyte apoptosis which has been shown to be upregulated by hypoxia, was dispensable for apoptosis during MI. This result is consistent with the existence of multiple, redundant death-inducing pathways, at least one of which is p53-independent. The likely point of convergence of these pathways is activation of the caspase family of cysteine proteases. In fact, activation of caspase-3 was observed in cardiac myocytes following MI. Furthermore, caspase activity is necessary for the progression of myocyte apoptosis, since administration of caspase inhibitors blocked apoptosis during MI.;To more closely examine the molecular mechanism of cardiac myocyte apoptosis, an in vitro model of ischemia was established in which primary cultures of neonatal rat myocytes were subjected to serum and glucose deprivation, in the presence of 2-deoxy-D-glucose, a non-metabolizable glucose analogue. Biochemical and morphologic indicators of apoptosis, such as nuclear fragmentation, internucleosomal DNA degradation and processing of caspase substrates, were evident 12--18 hours following initiation of treatment. All of these events were blocked upon inhibition of caspase activity. The mechanism by which caspases were activated in ischemic myocytes was then explored. Cytochrome c, normally located at the inner mitochondrial membrane, was released into the cytosol in apoptotic myocytes, independently of caspase activity. This release is postulated to lead to activation of the caspases. In fact, both caspase-9 and caspase-3 were proteolytically processed in ischemic cells. These results for the first time demonstrate the involvement of the mitochondrial. death pathway in mediating myocyte apoptosis.;These experiments have delineated some of the primary regulators of cardiac myocyte apoptosis during ischemia. In particular, caspases play a central role in myocyte apoptosis. This suggests that caspase inhibition, in combination with standard reperfusion therapy, may provide a novel approach to the treatment of ischemic heart disease.
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