Induction of heme oxygenase-1 by hypoxia and free radicals in human dermalfibroblasts

Heme oxygenase-1 (HO-1) catalyzes the rate-limiting step in heme catabolism and presumably is involved in cellular iron homeostasis. It is induced by a variety of cellular stresses, including oxygen deprivation and free radic al-mediated stress. We examined induction of HO-1 mRNA in shin fibroblasts and investigated the mechanism by which it occurs. Hypoxia did not appear t o act via induction of oxygen free radicals: induction of HO-1 was not sens itive to the free radical scavenger GSH or other antioxidants. Moreover, hy poxia did not increase steady-state levels of free radicals generated by fi broblasts. In contrast, HO-1 induction by the oxidants, H2O2 and carbonyl c yanide m-chlorophenylhydrazone (CCCP) was significantly attenuated in the p resence of free radical scavengers. This correlated with increased levels o f free radical production in fibroblasts treated with these oxidants. Iron depletion by desferrioxamine mesylate, a specific iron complexon, completel y inhibited hypoxic stimulation of HO-1 but did not attenuate the effect of H2O2 and CCCP on HO-1 mRNA. Addition of Fe2+, Fe3+, or holo-transferrin to fibroblasts increased levels of HO-1 mRNA. Treatment of cells with hypoxia , but not H2O2 or an exogenous source of iron, significantly increased the half-life of HO-1 mRNA. The data suggest hypoxia regulates HO-1 gene expres sion by aspecific posttranscriptional mechanism: stabilization of mRNA. Hyp oxia has previously been shown to increase fibroblast collagen synthesis an d is thought to play a role in pathogenesis of systemic sclerosis (SSc). Sk in fibroblasts isolated from patients with SSc demonstrated significantly s tronger induction of HO-1 by hypoxia than did fibroblasts from normal contr ols. We hypothesize that exposure of SSc fibroblasts to hypoxic conditions leads to in vivo selective proliferation of cells that adapt to hypoxia.