It has been reported that chloroform administered to male B6C3F(1) mice at doses of 1.38 and 277 mg/kg/day in corn oil by gavage 5 days/week for 2 years resulted in incidences of hepatocellular carcinomas of 36 and 98% relative to an incidence in controls of 6%. Cytotoxicity and regenerative cell proliferation have been implicated in the tumorigenic process for this nongenotoxic compound. Although chloroform is known to be nephrotoxic in the male mouse, no treatment-related increase was observed in the frequency of kidney tumors. To better understand the relationship of these endpoints, this study evaluated chloroform-induced cytotoxicity and cell proliferation in the liver and kidney under conditions of the cancer study. B6C3F(1) mice were administered oral doses of 0, 34, 90, 138, or 277 mg/kg/day of chloroform dissolved in corn oil for 4 days or 5 days/week for 3 weeks. Bromo-2'-deoxyuridine (BrdU) was administered via osmotic pumps implanted 3,5 days prior to necropsy to label cells in S-phase. Cell proliferation was evaluated in tissue sections immunohistochemically as the percentage of cells in S-phase (nuclear labeling index; LI). Mice given 34 and 90 mg/kg/day by gavage had mild degenerative changes in centrilobular hepatocytes after 4 days of treatment, which was absent at 3 weeks. Centrilobular necrosis was observed in mice given 138 or 277 mg/kg chloroform for 4 days, with increased severity of necrosis at 3 weeks. The LI in the livers of mice was increased in a dose-dependent manner at all chloroform doses following 4 days of treatment. The hepatic LI was similarly elevated at 3 weeks in the 277-mg/kg dose group, but had declined in the lower dose groups, and was no longer elevated above controls at 34 or 90 mg/kg/day. Kidneys from mice in all dose groups exhibited a dose-related acute tubular necrosis after 4 days of dosing. After 3 weeks of dosing, regenerating tubules were observed in the lower dose groups, while mice treated with 277 mg/kg had severe nephropathy characterized by degeneration, necrosis, and regeneration affecting all of the proximal tubules. The LI in the proximal tubules was increased at all doses after 4 days of treatment. At the end of 3 weeks of dosing, the LI in the renal cortex had decreased at all dose levels and was no longer significantly different from controls in the 34- and 90-mg/kg/day groups. The sustained increase in LI seen in the livers of mice administered not only the maximum tolerated dose (MTD), but also one-half the MTD is consistent with the hypothesis that events secondary to cytolethality and regenerative cell proliferation play a key role in chloroform-induced mouse liver cancer. The observation of toxicity and acute regenerative cell proliferation in the kidney without subsequent tumor formation may be because the severity of the nephrotoxicity in some way actually interfered with tumor formation. Alternatively, the mouse liver may simply be a more sensitive target organ than the kidney to tumor formation induced through mechanisms secondary to regenerative cell proliferation. (C) 1994 Society of Toxicology.
