Metabolism of chloroform by cytochrome P450 2E1 is required for induction of toxicity in the liver, kidney, and nose of male mice

Chloroform is a nongenotoxic-cytotoxic liver and kidney carcinogen and nasa l toxicant in some strains and sexes of rodents. Substantial evidence indic ates that tumor induction is secondary to events associated with cytolethal ity and regenerative cell proliferation. Therefore, pathways leading to tox icity, such as metabolic activation, become critical information in mechani sm-based risk assessments. The purpose of this study was to determine the d egree to which chloroform-induced cytotoxicity is dependent on the cytochro mes P450 in general and P450 2E1 in particular. Male B6C3F(1), Sv/129 wild- type (Cyp2e1+/+), and Sv/129 CYP2E1 knockout (Cyp2e1-/- or Cyp2e1-null) mic e were exposed 6 h/day for 4 consecutive days to 90 ppm chloroform by inhal ation. Parallel control and treated groups, excluding Cyp2e1-null mice, als o received an i.p. injection (150 mg/kg) of the irreversible cytochrome P45 0 inhibitor 1-aminobenzotriazole (ABT) twice on the day before exposures be gan and 1 h before every exposure. Cells in S-phase were labeled by infusio n of BrdU via an implanted osmotic pump for 3.5 days prior to necropsy, and the labeling index was quantified immunohistochemically. B6C3F(1) and Sv/1 29 wild-type mice exposed to chloroform alone had extensive hepatic and ren al necrosis with significant regenerative cell proliferation. These animals had minimal toxicity in the nasal turbinates with focal periosteal cell pr oliferation. Administration of ABT completely protected against the hepatic , renal, and nasal toxic effects of chloroform. Induced pathological change s and regenerative cell proliferation were absent in these target sites in Cyp2e1-/- mice exposed to 90 ppm chloroform. These findings indicate that m etabolism is obligatory for the development of chloroform-induced hepatic, renal, and nasal toxicity and that cytochrome P450 2E1 appears to be the on ly enzyme responsible for this cytotoxic-related metabolic conversion under these exposure conditions. (C) 1999 Academic Press.