Contrasting effects of NO and peroxynitrites on HSP70 expression and apoptosis in human monocytes

The free radicals nitric oxide (. NO) and superoxide (O-2(-).) react to form peroxynitrite (ONOO-), a highly toxic oxidant species. In this study we investigated the respective effects of NO and ONOO- in monocytes from healthy human donors. Purified monocytes were incubated for 6 or 16 h with a pure NO donor (S-nitroso-N-acetyl-DL-penicillamine, 0-2 mM), an . NO/ONOO- donor (3-morpholinosydnonimine chlorhydrate, 0-2 mM) with and without superoxide dismutase (200 IU/ml), or pure ONOO-. We provide evidence that 3-morpholinosydnonimine chlorhydrate alone represents a strong stress to human monocytes leading to a dose-dependent increase in heat shock protein-70 (HSP70) expression, mitochondrial membrane depolarization, and cell death by apoptosis and necrosis. These phenomena were abolished by superoxide dismutase, suggesting that ONOO-, but not . NO, was responsible for the observed effects. This observation was further strengthened by the absence of a stress response in cells exposed to S-nitroso-N-acetyl-DL-penicillamine. Conversely, exposure of cells to ONOO- alone also induced mitochondrial membrane depolarization and cell death by apoptosis and necrosis. Thus ONOO- formation may well explain the toxic effect generally attributed to . NO.