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.