Iron regulates hyperoxia-dependent human heme oxygenase 1 gene expression in pulmonary endothelial cells

The endothelium of the lung is sensitive to the toxic effects of oxygen, an d early evidence of toxicity is characterized by protein leak and extravasa tion of red blood cells. The overproduction of oxygen free radicals plays a critical role in the pathophysiology of a hyperoxic lung injury, Recently, heme oxygenase 1 (HO-1), the rate-limiting enzyme in the metabolism of hem e, has been found to have a protective role in oxidant injury. Our laborato ry and others have identified HO-1 as a hyperoxia-inducible protein. In thi s study, we characterized HO-1 expression and evaluated its regulation in h uman pulmonary endothelial cells. Hyperoxia results in a relatively small i ncrease in HO-1 expression; however, this induction is potentiated by heme and dramatically potentiated in the presence of free iron. This is probably more reflective of the in vivo situation in which there is extravasation o f heme and iron products. We also found that HO-1 expression depended on ch elatable iron. The iron chelator desferrioxamine not only inhibited the iro n-dependent potentiation of HO-1 in response to hyperoxia but also inhibite d both hyperoxia and basal expression. On the basis of inhibitor studies an d nuclear run-on assays, we demonstrated that this induction is transcripti onally dependent. We also evaluated 4.5 kb of the human HO-1 promoter regio n and demonstrated that this region has promoter activity to the stimulus h eme; however, there was no evidence of promoter activity to either iron or hyperoxia. This diversity of promoter activity to heme, heavy metals, and h yperoxia is unique to the human HO-1 gene.