Hypoxia-activated apoptosis of cardiac myocytes requires reoxygenation or a pH shift and is independent of p53

Ischemia and reperfusion activate cardiac myocyte apoptosis, which may be a n important feature in the progression of ischemic heart disease. The relat ive contributions of ischemia and reperfusion to apoptotic signal transduct ion have not been established. We report here that severe chronic hypoxia a lone does not cause apoptosis of cardiac myocytes in culture. When rapidly contracting cardiac myocytes were exposed to chronic hypoxia, apoptosis occ urred only when there was a decrease in extracellular pH ([pH](o)). Apoptos is did not occur when [pH], was neutralized. Addition of acidic medium from hypoxic cultures or exogenous lactic acid stimulated apoptosis in aerobic myocytes. Hypoxia-acidosis-mediated cell death was independent of p53: equi valent apoptosis occurred in cardiac myocytes isolated from wild-type and p 53 knockout mice, and hypoxia caused no detectable change in p53 abundance or p53-dependent transcription. Reoxygenation of hypoxic cardiac myocytes i nduced apoptosis in 25-30% of the cells and was also independent of p53 by the same criteria. Finally, equivalent levels of apoptosis, as demonstrated by DNA fragmentation, were induced by ischemia-reperfusion, but not by isc hemia alone, of Langendorff-perfused hearts from wild-type and p53 knockout mice. We conclude that acidosis, reoxygenation, and reperfusion, but not h ypoxia (or ischemia) alone, are strong stimuli for programmed cell death th at is substantially independent of p53.