Evaluation of genotoxicity, pathological lesions, and cell proliferation in livers of rats and mice treated with furan

Abstract

Preliminary results from the National Toxicology Program (NTP) bioassays of furan given by gavage indicate the induction of hepatocellular carcinomas in male F-344 rats and in both sexes of B6C3F1 mice, and cholangiocarcinomas in both sexes of rats. To assess the genotoxicity of furan, chemically induced unscheduled DNA synthesis was evaluated in the in vivo hepatocyte DNA repair assay. Furan did not induce unscheduled DNA synthesis in hepatocytes isolated after single gavage treatment of male F-344 rats (5, 30, and 100 mg/kg) or male B6C3F1 mice (10, 50, 100, and 200 mg/kg). Furan induced cytotoxicity and enhanced cell proliferation were evaluated in livers of rats and mice as events that also might give rise to mutations and/or drive tumor formation. The labeling index (LI, percentage of hepatocyte nuclei in S-phase) was measured histoautoradiographically following a single gavage administration of furan (30 mg/kg, male rats; 50 mg/kg, male mice) followed by an injection of 3H-thymidine 2 hr prior to sacrifice. Hepatocellular necrosis and a sharp increase in LI (23.9 for mice and 17.8 for rats vs. less than 0.5 for controls) was observed 48 hr after treatment with furan, indicative of restorative cell proliferation secondary to cytotoxicity. Hepatocyte proliferation was evaluated also at the highest NTP bioassay dose (15 mg/kg/day for mice and 8 mg/kg/day for rats, 5 days/week) by labeling with 3H-thymidine administered via a 6 day osmotic pump implanted subcutaneously. Necrosis and inflammation were observed along the subcapsular visceral surface of the left or caudate liver lobes, likely due to diffusion of furan directly through the stomach to the liver. After 6 weeks of furan administration, male and female rats, but not mice, exhibited bile duct hyperplasia as well as metaplasia in the areas of fibrosis along the subcapsular visceral surface of the left or caudate liver lobes. The fold increase in hepatocyte LI in treated animals relative to the combined controls measured at weeks 1, 3, and 6 ranged from 39 to 5 for male mice, 18 to 51 for male rats, and 12 to 19 for female rats. Taken together, these data suggest that mechanisms other than direct DNA-reactivity might explain the profile of oncogene mutations observed in the mouse liver tumors, including selective promotion of different subpopulations of preneoplastic cells and/or mutational events secondary to sustained cell proliferation or inflammation. The extensive amount of furan-induced cell proliferation subsequent to cytotoxicity likely had a significant impact on tumor development, and such data should be considered in risk evaluations for this compound.

Comparison of the Hepatotoxicity of Coumarin in the Rat, Mouse, and Syrian Hamster: A Dose and Time Response Study

“…7). As such the present data with coumarin are similar to those obtained in studies with furan (Wilson et al, 1992). Furan has been shown to produce hepatotoxic effects in the rat, a stimulation of hepatocyte labeling index values, and chronic administration results in hepatocellular adenomas/carcinomas and cholangiocarcinomas (Wilson et al, 1992).…”

“…The first class comprises mitogenic agents which produce additive hyperplasia with little or no toxicity, whereas the second class comprises cytotoxic agents where cell proliferation is the result of a regenerative hyperplasia (Loury et al, 1987;Goldsworthy et al, 1991;Grasso and Hinton, 1991;Ames et al, 1993). Examples of such rodent liver cytotoxic agents include carbon tetrachloride, chloroform, and furan (Loury et al, 1987;Grasso et al, 1991;Wilson et al, 1992;Larson et al, 1994).…”

Strain-Specific Enhancement or Inhibition of Coumarin Hepatotoxicity in Mice Following Pretreatment with Two Different Liver Enzyme-Inducing Agents

“…This characteristic lesion is most likely to have resulted from direct diffusion of coumarin across the stomach wall, since the lobe or lobes affected were those in direct apposition to the serosal surface of the stomach. A similar lesion has been observed in rats and mice after oral dosing of another lipophillic substance, furan (Wilson et al, 1992), and has also been attributed to direct transgastric diffusion. Endell and Seidel (1978) showed that, following an oral dose of 350 mg/kg, C3H/He mice were more sensitive to coumarin hepatotoxicity than DBA/2 mice; however, these researchers were unable to demonstrate any strain difference after a lower dose of 250 mg/kg.…”

Hepatobiliary Toxicity of Furan: Identification of Furan Metabolites in Bile of Male F344/N Rats

“…Consistent with the hypothesis that cytotoxicity mediated through binding of cis-2-butene-1,4-dial to critical target proteins is likely to play a key role in furan toxicity and carcinogenicity (Wilson et al, 1992), chemical characterization of furan metabolites in bile provided further evidence to suggest that degraded protein adducts are in vivo metabolites of furan. Based on our findings, however, we cannot conclude that the bile duct epithelium is a primary target of reactive furan metabolites, because biliary toxicity may also occur due to hepatocellular injury.…”