Patterns of chloroform-induced regenerative cell proliferation in BDF1 mice correlate with organ specificity and dose-response of tumor formation

It has been reported that chloroform administered to BDF1 mice byinhalation for 2 years at concentrations of 5, 30 or 90 p.p.m. for 6 h/day,5 days/week induced an increase in renal cell tumors in male but not femalemice exposed to the doses of 30 and 90 p.p.m. A small increase in livertumors was statistically significant in the female mice at 90 p.p.m. if theincidences of carcinomas and adenomas were combined. Because chloroform isnot a DNA reactive mutagen, a 13-week time-course and dose-response studywas conducted under conditions of the original bioassay to examine whetherregenerative cell proliferation was an underlying mechanism ofcarcinogenesis. Mice were given bromodeoxyuridine via infusion during thelast 3.5 days prior to necropsy to label cells in S-phase. Chloroforminduced pathology and regenerative cell proliferation, measured as thelabeling index (LI, percentage of cells in S-phase), were assessedmicroscopically and immunohistochemically. Male mice exposed to 30 and 90p.p.m. exhibited a dose-dependent increase in regenerating tubules withinthe renal cortex and up to a 31-fold increase in LI. No renal lesions orincreased LI were observed in females. Increased centrilobular to midzonalhepatocyte degeneration and vacuolation and a 7-fold increase over controlsin the hepatocyte LI were observed in the female mice at 90 p.p.m. at 13weeks. Males exhibited similar pathology, but the increase in LI was notsustained. The observed correlations between cytolethality and regenerativecell proliferation with tumor formation supports extensive evidence thatchloroform induces cancer via a non- genotoxic-cytotoxic mode of action. Aconcentration of 5 p.p.m. is the no-observed-adverse-effect level fornephrotoxicity, cell proliferation and cancer. An appropriate safety factorapplied to this value is a straightforward approach to cancer riskassessment that is consistent with the mode of action of chloroform.