Endothelial Heme Oxygenase-1 Induction by Hypoxia

The stress protein heme oxygenase-1 (HO-1) is induced in endothelial cells exposed to nitric oxide (NO)-releasing agents, and this process is finely modulated by thiols (Foresti, R., Clark, J. E., Green, C. J., and Motterlini R. (1997) J. Biol. Chem. 272, 18411–18417). Here, we report that up-regulation of HO-1 in aortic endothelial cells by severe hypoxic conditions (pO2 ≤ 2 mm Hg) is preceded by increased inducible NO synthase and NO synthase activity. This effect is accompanied by oxidation of intracellular glutathione and formation of S-nitrosothiols. Incubation of cells with a selective inhibitor of inducible NO synthase (S-(2-aminoethyl)-isothiourea) or a NO scavenger ([2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide]) significantly attenuated the increase in heme oxygenase activity caused by reduced oxygen availability. A series of antioxidant agents did not prevent the elevation in heme oxygenase activity by hypoxia; however, the precursor of glutathione synthesis and thiol donor,N-acetylcysteine, completely abolished HO-1 induction. We also found that the hypoxia-mediated increase in endothelial heme oxygenase activity was potentiated by the presence ofS-nitrosoglutathione. These results indicate that intracellular interaction of thiols with NO is an important determinant in the mechanism leading to HO-1 induction by reduced oxygen levels. We suggest that in addition to oxidative stress, HO-1 gene expression can be regulated by redox reactions involving NO andS-nitrosothiols (nitrosative stress), emphasizing a versatile role for the heme oxygenase pathway in the cellular adaptation to a variety of stressful conditions.