Role of heme oxygenase in preserving vascular bioactive NO

Beyond its vasodilator role, vascular nitric oxide (NO), which is synthesized by endothelial NO synthase (eNOS) via its activation, has been shown to play a number of other beneficial roles in the vascular system; it inhibits proliferation of vascular smooth muscle cells, prevents platelet aggregation, and regulates endothelial apoptosis. Such beneficial roles have been shown to be implicated in the regulation of endothelial functions. A loss of NO bioavailability that may result either from decreased eNOS expression and activity or from increased NO degradation is associated with endothelial dysfunction, a key factor in the development of vascular diseases. Heme oxygenase-1 (HO-1), an inducible enzyme, catalyzes the oxidative degradation of heme to free iron, carbon monoxide, and biliverdin, the latter being subsequently converted into bilirubin. In the vascular system, HO-1 and heme degradation products perform important physiological functions, which are ultimately linked to the protection of vascular cells. Studies have shown that HO-1 and heme degradation products exert vasodilatory, antioxidant, anti-inflammatory, antiproliferative and anti-apoptotic effects on vascular cells. Interestingly, these effects of HO-1 and its by-products are similar, at least in part, to those of eNOS-derived NO; this similarity may prompt investigators to study a possible relationship between eNOS-derived NO and HO-1 pathways. Many studies have been reported, and accumulating evidence suggests that HO-1 and heme degradation products can improve vascular function, at least in part, by compensating for the loss of NO bioavailability. This paper will provide the possible pathway explaining how HO-1 and heme degradation products can preserve vascular NO.