Nitric oxide protected against NADPH oxidase-derived superoxide generation in vascular endothelium: Critical role for heme oxygenase-1.

NADPH oxidase-derived superoxide (O) generation is usually considered as an important factor to the pathogenesis of cardiovascular diseases. Nitric oxide (NO), previously viewed as a key regulator of cardiovascular function, is identified to induce expression of heme oxygenase-1 (HO-1), an antioxidant enzyme in blood vessels. In this study, we tested whether NO might modulate NADPH oxidase activity in vascular endothelium via HO-1. In vascular endothelial cells, NO donor (diethylenetriamine/nitric oxide adduct) significantly attenuated NADPH oxidase-derived O generation via a HO-1-dependent mechanism. Mechanistically, the protective effects of NO were (1) linked to increased expression of HO-1, (2) similar to the effects of NADPH oxidase inhibitor, (3) could be abolished by the specific siRNA against HO-1 expression or HO-1 activity inhibitor and (4) similar to the effects of HO-1 end product bilirubin. Moreover, bilirubin seemed to interrupt the assembly and activation of NADPH oxidase. In addition, NO inhibited HO-induced endothelial dysfunction through activation of HO-1. In agreement with these in vitro data, aortic NADPH oxidase activity was increased in NO-deficient mice while the endothelial function was simultaneously impaired. Our results suggested that suppression of NADPH oxidase-dependent oxidative stress may represent a novel mechanism underlying the cardiovascular protection of NO/HO-1 pathway.