Nitric oxide enhances the manganese superoxide dismutase-dependent suppression of proliferation in HT-1080 fibrosarcoma cells

The overexpression of manganese superoxide dismutase (MnSOD), an enzyme tha t catalyzes the removal of superoxide (O-2(radical anion)) from the mitocho ndria, has been shown to be closely associated with tumor regression in viv o and loss of the malignant phenotype in vitro. To investigate the mechanis m by which MnSOD overexpression mediates this reversal, we have established 29 independent, clonal MnSOD-overexpressing HT-1080 fibrosarcoma cells, Mn SOD activity is inversely correlated with cell proliferation in our cell li nes. Incubating cells in 3% oxygen can prevent the inhibition of cellular p roliferation mediated by MnSOD, suggesting that oxygen is a prerequisite co mponent of the MnSOD-dependent proliferative inhibition, Confocal laser mic roscopy was used in combination with the oxidant-sensitive fluorescent dyes dihydrorhodamine-123, dihydroethidium, and 2',7'-dichlorodihydrofluorescei n diacetate to determine the oxidizing capacity of the MnSOD-overexpressing cells, When compared with parental or control cell lines, there was a sign ificant decrease in the rate of oxidation of the fluorophores in the MnSOD- overexpressing cell lines, Thus, an increase in the oxidizing capacity of t he cells does not appear to mediate the inhibition of proliferation associa ted with MnSOD overexpression. Superoxide dismutase has also been shown to enhance the cytotoxic activity of NO . toward tumor cells, In this study, w e have shown that MnSOD overexpression enhances the cytostatic action of th e NO . donors, sodium nitroprusside, 3-morpholinosydnonomine, and (Z)-1-[2- aminethyl)-N-(2-ammonioethyl)amino]diaten-1-ium-1,2-diolate in a dose-depen dent manner, In addition, the NO . toxicity is blocked by oxyhemoglobin, a NO . scavenger. Our findings suggest that NO . may play a role in the rever sal of tumorigenicity associated with MnSOD overexpression.