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Heme oxygenase-1 (HO-1) is one of several enzymes induced by hepatotoxicants, and is thought to have an important protective role against cellular stress during liver inflammation and injury. The objective of the present study was to evaluate the role of HO-1 in estradiol-induced liver injury. A single dose of ethinyl estradiol (500 mg/kg, po) resulted in mild liver injury. Repeated administration of ethinyl estradiol (500 mg/kg/day for 4 days, po) resulted in no detectable liver injury or dysfunction. Using RT-PCR analysis, we demonstrate that HO-1 gene expression in whole liver tissue is elevated (>20-fold) after the single dose of ethinyl estradiol. The number and intensity of HO-1 immunoreactive macrophages were increased after the single dose of ethinyl estradiol. HO-1 expression was undetectable in hepatic parenchymal cells from rats receiving Methocel control or a single dose of ethinyl estradiol, however cytosolic HO-1 immunoreactivity in these cells after repeated dosing of ethinyl estradiol was pronounced. The increases in HO-1 mRNA and HO-1 immunoreactivity following administration of a single dose of ethinyl estradiol suggested that this enzyme might be responsible for the observed protection of the liver during repeated dosing. To investigate the effect of HO-1 expression on ethinyl estradiol-induced hepatotoxicity, rats were pretreated with hemin (50 μmol/kg, ip, a substrate and inducer of HO-1), with tin protoporphyrin IX (60 μmol/kg, ip, an HO-1 inhibitor), or with gadolinium chloride (10 mg/kg, iv, an inhibitor/toxin of Kupffer cells) 24 h before ethinyl estradiol treatment. Pretreatment with modulators of HO-1 expression and activity had generally minimal effects on ethinyl estradiol-induced liver injury. These data suggest that HO-1 plays a limited role in antioxidant defense against ethinyl estradiol-induced oxidative stress and hepatotoxicity, and suggests that other coordinately induced enzymes are responsible for protection observed with repeated administration of high doses of this compound.