MECHANISMS OF CARDIOMYOCYTE CELL DEATH BY APOPTOSIS

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SUMMARY OF WORK Cardiac cell loss occurs in response to acute ischemic injury, during heart failure, and during the mormal aging of the heart. Cell loss is due predominantly to the death of cardiac myocytes and is mediated in large part by apoptosis. Because cardiomyocyte (CM) loss is irreversible and permanently damaging to the heart, its prevention is likely to have important beneficial consequences to health. We have used hypoxia as an in vitro surrogate for ischemia induce cardiomyocyte cell death by apoptosis in neonatal cardiomyocytes. We have shown that in these cells there is increased expression and transactivating ability by the tumor suppressor genes, p53 and p73, that accompanies the onset of apoptotic cell death and that forced expression of either p53 or p21/WAF1, an important downstream target of p53/p73, in normoxi CMs causes apoptosis. We have also shown that apoptosis in these cells does not result in PARP cleavage, a well-characterized substrate for the executioner caspase 3. Instead, expression of initiator caspase 2 (which in other cell types engages the "death machinery" independently of caspase 3)is increased in response to hypoxia while the expression of its endogenous and muscle-specific inhibitor, known as ARC, is decreased. Because excessive adrenergic drive is thought to be a contributing factor to cell loss associated with heart failure, we have examined the effects of adrenergic stimulation on cardiomyocyte cell death and survival. We have found that beta2-adrenergic stimulation protects neonatal cardiomyocytes from hypoxia-induced apoptosis, while beta1-adrenergic stimulations potentiates it. The protective effects of beta2 stimulation are mediated through a pertussus toxin (PTX)-sensitive pathway. Our current studies are directed at demonstrating a critical role for a caspase-2 selective pathway and at understanding how adrenergic signaling pathways interact with signals initiated by p53/p73/p21 to effect p53/p21 engage the "death machinery" in cardiomyocytes.