Human exposure to environmental chemicals is considered a contributing factor to increased human susceptibility to a number of diseases including diabetes and cancer. The mechanism(s) of action of these chemicals are not well understood, however, a great number of them are metabolized to epoxide intermediates via CYP2E1. We therefore hypothesized that oxidation is a pre-requisite for the induction of toxicity, mutagenicity,and carcinogenicity of such chemicals. In the curent investigations, we selected urethane and 1-bromopropane as model chemicals to assess the role of CYP2E1-mediated oxidation in the carcinogencity and reproductive toxicity of the 2 chemicals, respectively. ? Urethane is an established animal carcinogen and has been classified as reasonably anticipated to be a human carcinogen. Until recently, urethane metabolism via esterases was considered the main metabolic pathway of this chemical. However, recent studies in this laboratory showed that CYP2E1, and not esterase, is the primary enzyme responsible for urethane oxidation. Subsequent studies demonstrated significant inhibition of urethane induced genotoxicity and cell proliferation in Cyp2e1-/- compared to Cyp2e1+/+ mice. Using Cyp2e1-/- mice, current studies were undertaken to assess the relationships between urethane metabolism and carcinogenicity. Urethane was administered via gavage at 1, 10, or 100 mg/kg/day, 5 days/week, for 6 weeks. Animals were kept without chemical administration for 7 months after which they were euthanized, and urethane carcinogenicity was assessed. Microscopic examination showed a significant reduction in the incidences of liver hemangiomas and hemangiosarcomas in Cyp2e1-/- compared to Cyp2e+/+ mice. Lung nodules increased in a dose-dependent manner and were less prevalent in Cyp2e1-/- compared to Cyp2e+/+ mice. Microscopic alterations included bronchoalveolar adenomas, and in one Cyp2e1+/+ mouse treated with 100 mg/kg urethane, a bronchoalveolar carcinoma was diagnosed. Significant reduction in the incidence of adenomas and the number of adenomas/lung were observed in Cyp2e1-/- compared to Cyp2e1+/+ mice. In the Harderian gland, the incidences of hyperplasia and adenomas were significantly lower in Cyp2e1-/- compared to Cyp2e+/+ mice at the 10 mg/kg dose, with no significant differences observed at the high or low doses. In conclusion, this work demonstrated a significant reduction of urethane-induced carcinogenicity in Cyp2e1-/- compared to Cyp2e1+/+ mice and proved that CYP2E1-mediated oxidation plays an essential role in urethane-induced carcinogenicity.? 1-Bromopropane (1-BrP) induces dose- and time-dependent reproductive organ toxicity and reduced sperm motility in rodents. The contribution of CYP2E1 to 1-BrP metabolism and to induction of male reproductive toxicity was investigated using Cyp2e+/+ and Cyp2e1-/- mice. In gas uptake inhalation studies, the elimination half-life of 1,2,3-13C-1-BrP was longer in Cyp2e1-/- mice relative to Cyp2e+/+ (3.2 vs. 1.3 hr). Urinary metabolites were identified by 13C-NMR. The mercapturic acid of 1-bromo-2-hydroxypropane (2OHBrP) was the major urinary metabolite in Cyp2e+/+ mice and products? of conjugation of 1-BrP with GSH were insignificant. The ratio of GSH conjugation to 2-hydroxylation increased 5 fold in Cyp2e1-/- mice relative to Cyp2e+/+. After 1-BrP exposure, hepatic glutathione was decreased by 76% in Cyp2e+/+ mice vs. 47% in Cyp2e1-/- mice. Despite a 170% increase in 1-BrP exposure in Cyp2e1-/- vs. Cyp2e+/+ mice, sperm motility in exposed Cyp2e1-/- mice did not change relative to unexposed matched controls. This suggests that metabolites produced through CYP2E1-mediated oxidation may be responsible for 1-BrP induced sperm toxicity. Both 1-BrP and 2OHBrP inhibited the motility of sperm obtained from Cyp2e+/+ mice in vitro. However, only 2OHBrP reduced the motility of sperm obtained from Cyp2e1-/- mice in vitro, suggesting that conversion of parent compound to 2OHBrP within the spermatozoa may contribute, at least in part to reduced motility. Overall, these data suggest that metabolism of 1-BrP is mediated in part by CYP2E1 and activation of 1BrP via this enzyme may contribute to the male reproductive toxicity of this chemical.
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