Furan is a potent hepatotoxin and hepatocarcinogen in rodents, causing hepatocellular adenomas and carcinomas in rats and mice, and high incidences of cholangiocarcinomas in rats at doses ≥ 2 mg/kg bw. A genotoxic mode of action cannot be excluded for furan-induced tumor formation. Furan is metabolized by cytochrome P450 (CYP) enzymes, predominantly CYP2E1, to its major metabolite cis-2-butene-1,4-dial (BDA, maleic dialdehyde), a highly reactive electrophile identified as the key mediator of furan toxicity and carcinogenicity. Furan-mediated effects on glutathione (GSH) levels and cell viability can be suppressed by the CYP inhibitor 1-phenylimidazole and increased by pretreatment of rats with acetone (a CYP2E1-inducing agent), indicating that furan cytotoxicity depends on its metabolic activation. BDA has been shown to react with cellular nucleophiles such as GSH and amino acids and to cause cross-links between thiols and amino groups, giving rise to lactam and pyrrole derivatives. Furan reduced the percentage of DNA in the comet tail in turkey liver fetal hepatocytes. Furan was also shown to induce chromosomal aberrations and sister chromatid exchanges (SCEs) in Chinese hamster ovary (CHO) cells. A statistically significant increase of micronucleated cells was recently reported in the spleen of furan-treated mice. A reduction of percentage of DNA in comet tail in liver cells was observed following treatment of turkey fetuses in ovo. Exposure to furan at doses associated with increased tumor incidences initially causes hepatocellular necrosis, accompanied by inflammation and sustained regenerative proliferation of hepatocytes, which may present key events in furan-induced hepatocellular carcinogenicity. Subcapsular and centrilobular necrosis accompanied by markedly increased liver enzymes is the primary response to furan treatment. The involvement of inflammatory processes in furan toxicity is also reflected by increased expression of cytokines and other inflammation-associated genes, such as IFN-γ, IL-1β, IL-6, IL-10, and components of the complement system, which may, however, also derive from lesions involving the biliary tract. Indeed, increased production of reactive oxygen species in response to furan is suggested by immunohistochemical detection of 8-oxo-dG within nuclei of hepatocytes of centrilobular areas following high-dose exposure and changes in the expression of genes responsive to oxidative stress in rats and/or mice. (A15446)