Endothelial heme oxygenase-1 induction by hypoxia - Modulation by inducible nitric-oxide synthase and S-nitrosothiols

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 mo dulated by thiols (Foresti, R,, Clark, J, E,, Green, C, J,, and Motterlini Il, (1997) J. Biol, Chem, 272, 18411-18417), Here, we report that up-regula tion of HO-1 in aortic endothelial cells by severe hypoxic conditions (pO(2 ) less than or equal to 2 mm Hg) is preceded by increased inducible NO synt hase and NO synthase activity. This effect is accompanied by oxidation of i ntracellular glutathione and formation of S-nitrosothiols, Incubation of ce lls with a selective inhibitor of inducible NO synthase (S-(2-aminoethyl)-i sothiourea) or a NO scavenger ([2-(4-carboxyphenyl)-4,4,5,5-tetramethylimid azoline-1-oxyl-3-oxide]) significantly attenuated the increase in heme oxyg enase activity caused by reduced oxygen availability. A series of antioxida nt agents did not prevent the elevation in heme oxygenase activity by hypox ia; however, the precursor of glutathione synthesis and thiol donor, N-acet ylcysteine, completely abolished HO-1 induction. We also found that the hyp oxia-mediated increase in endothelial heme oxygenase activity was potentiat ed by the presence of S-nitrosoglutathione. These results indicate that int racellular interaction of thiols with NO is an important determinant in the mechanism leading to HO-1 induction by reduced oxygen levels. We suggest t hat in addition to oxidative stress, HO-1 gene expression can be regulated by redox reactions involving NO and S-nitrosothiols (nitrosative stress), e mphasizing a versatile role for the heme oxygenase pathway in the cellular adaptation to a variety of stressful conditions.