Hepatic expression of heme oxygenase-1 and antioxidant response element-mediated genes following administration of ethinyl estradiol to rats.

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 micromol/kg, ip, a substrate and inducer of HO-1), with tin protoporphyrin IX (60 micromol/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.

Peroxides and macrophages in the toxicity of fine particulate matter in rats.

Epidemiologists have observed a positive association between human morbidity and mortality and the atmospheric concentrations of fine particulate matter (PM), but the mechanisms underlying the toxic effects of PM have not been elucidated. Various components of ambient PM have been implicated in toxicity (including ultrafine particles, transition metals, organics and oxidants). Our research focused on hydrogen peroxide (H2O2). We speculated that fine PM transports H2O2 into the lower lung, leading to tissue injury and to accumulation and activation of macrophages in these regions. The macrophages release cytotoxic mediators and proinflammatory cytokines that contribute to the pathogenesis of tissue injury. To test this hypothesis, we conducted studies to determine (1) whether tissue injury induced by aerosols is mediated by cytotoxic H2O2 carried into the lower lung by fine particles and (2) whether exposure of rats to fine PM leads to accumulation of activated macrophages in the lung. For our studies, systems were designed to generate model atmospheric fine PM and atmospheric peroxides consisting of an ammonium sulfate [(NH4)2SO4] aerosol (mass median diameter, 0.46 +/- 0.14 microm) and H2O2. We also constructed a 6-port nose-only exposure chamber. Female Sprague Dawley rats were exposed for 2 hours to aerosols consisting of (NH4)2SO4 (430 microg/m3), (NH4)2SO4 + 10, 20 or 100 ppb H2O2, vapor-phase H2O2 (10, 20 or 100 ppb), or particle-free air. Studies using oxygen-18 (18O)-labeled H2O2 were conducted to validate the transport of H2O2 into the lower lung with (NH4)2SO4. Rats were killed immediately (0 hours) or 24 hours after exposure. Compared with control animals, inhalation of (NH4)2SO4 and H2O2, alone or in combination, had no major effect on cell number or viability, protein content, or lactate dehydrogenase (LDH) levels in bronchoalveolar lavage (BAL) fluid collected either immediately or 24 hours after exposure. However, electron microscopy revealed that a larger number of neutrophils in pulmonary capillaries adhered to the vascular endothelium, especially in lungs of rats exposed to (NH4)2SO4 + H2O2. Inhalation of (NH4)2SO4 + H2O2 was also found to be associated with altered macrophage functional activity. Thus, exposing rats to (NH4)2SO4 + 20 ppb H2O2 or 20 ppb H2O2 alone caused a level of tumor necrosis factor alpha (TNF-alpha) production by lung macrophages that was higher than in controls. This higher level was observed immediately after exposure and persisted for at least 24 hours. Greater TNF-alpha production was also detected 24 hours after exposure to (NH4)2SO4 + 10 ppb H2O2. Immediately after rats inhaled (NH4)2SO4 + 10 ppb H2O2 or 20 ppb H2O2 alone, we also observed a transiently higher production of superoxide anion (O2-) by alveolar macrophages. Macrophages isolated 24 hours after exposure to 20 ppb H2O2 also produced larger quantities of superoxide anion. In contrast, immediately after exposure, macrophages from rats exposed to (NH4)2SO4 + 10 ppb H2O2 or to 20 ppb H2O2 alone generated less nitric oxide (NO). Reduced nitric oxide production was also observed 24 hours after exposure to (NH4)2SO4 + 10 ppb H2O2 or to 10 or 20 ppb H2O2 alone. Reduced nitric oxide production may have been due to superoxide anion-driven formation of peroxynitrite (ONOO-) anions. In this regard, nitrotyrosine, an in vivo marker of peroxynitrite, was detected in lung tissue immediately after rats were exposed to (NH4)2SO4 + H2O2 or to H2O2 alone (10 or 20 ppb). We also found that alveolar macrophages from rats exposed to (NH4)2SO4 + H2O2 showed a greater expression of the antioxidant enzyme heme oxygenase-1 (HO-1) when stimulated with lipopolysaccharide (LPS) and interferon-gamma (IFN-gamma). Similar results were observed after exposure of rats to an organic peroxide aerosol (cumene hydroperoxide). Taken together, the results of our studies demonstrate that biological effects of inhaled H2O2 are augmented by fine PM. Moreover, tissue injury induced by (NH4)2SO4 + H2O2 may be related to altered production of cytotoxic mediators by alveolar macrophages. Determining the relevance of these toxicologic results to human health will be important in future studies for evaluating the risk of exposure.