NO-mediated activation of heme oxygenase: Endogenous cytoprotection against oxidative stress to endothelium

We investigated the effect of nitric oxide (NO) on the induction of the stress protein heme oxygenase and its protective role in vascular endothelial cells exposed to hydrogen peroxide. Treatment of porcine aortic endothelial cells for 6 h with the NO-releasing compounds (0.1-1 mM) sodium nitroprusside (SNP), S-nitroso-N-acetylpenicillamine (SNAP), and 3-morpholinosydnonimine (SIN-1) resulted in a concentration-dependent increase in heme oxygenase activity. At 1 mM, the activity of heme oxygenase was augmented 8.5-fold with SNP, 5.8-fold with SNAP, and 5.7-fold with SIN-1 over the control value. In contrast, endothelial cells exposed to 100 mu M 8-bromoguanosine 3',5'-cyclic monophosphate, a tissue-permeable analogue that mimics the action of guanosine 3',5'-cyclic monophosphate, did not show any change in heme oxygenase activity. Activation of the inducible NO synthase by the synergistic action of bacterial lipopolysaccharide (250 ng/ml) and interferon-gamma (100 U/ml) also increased endothelial heme oxygenase activity by 3.2-fold (P < 0.05 vs control). Methylene blue (1 mu M), an inhibitor of both NO synthase and guanylate cyclase activities, completely abolished this effect. Cells previously exposed to SNAP and SIN-1 exhibited a significant protection against the cytotoxicity mediated by hydrogen peroxide (250 mu M) (P < 0.05). Conversely, SNP did not show any protective effects, possibly because of catalytic iron released during its chemical decomposition. In fact, the iron chelator deferoxamine (5 mM) completely suppressed the SNP-mediated cytotoxicity and partially attenuated the activity of heme oxygenase to a level equal to that mediated by SIN-1 and SNAP. These results indicate that NO is a determinant in the modulation of the activity of heme oxygenase leading to a major resistance of the endothelium to oxidative stress.