Cytotoxicity and genotoxicity of methyleugenol and related congeners - a mechanism of activation for methyleugenol

Methyleugenol is a substituted alkenylbenzene found in a variety of foods, products, and essential oils. In a 2-year bioassay conducted by the Nationa l Toxicology Program, methyleugenol caused neoplastic lesions in the livers of Fischer 344 rats and B6C3F(1) mice. We were interested in the cytotoxic ity and genotoxicity caused by methyleugenol and other alkenylbenzene compo unds: safrole (a known hepatocarcinogen), eugenol, and isoeugenol. The endp oints were evaluated in cultured primary hepatocytes isolated from male Fis cher 344 rats and female B6C3F1 mice. Cytotoxicity was determined by measur ing lactate dehydrogenase (LDH) release, while genotoxicity was determined by using the unscheduled DNA synthesis (UDS) assay. Rat and mouse hepatocyt es showed similar patterns of toxicity for each chemical tested. Methyleuge nol and safrole were relatively non-cytotoxic, but caused UDS at concentrat ions between 10 and 500 muM. In contrast, isoeugenol and eugenol produced c ytotoxicity in hepatocytes with LC50s of similar to 200-300 muM, but did no t cause UDS. Concurrent incubation of 2000 muM cyclohexane oxide (CHO), an epoxide hydrolase competitor, with a non-cytotoxic concentration of methyle ugenol (10 muM) resulted in increased cytotoxicity but had no effect on gen otoxicity. However, incubation of 15 muM pentacholorophenol, a sulfotransfe rase inhibitor, with 10 muM methyleugenol resulted in increased cytotoxicit y but had a significant reduction of genotoxicity. These results suggest th at methyleugenol is similar to safrole in its ability to cause cytotoxicity and genotoxicity in rodents. It appears that the bioactivation of methyleu genol to a DNA reactive electrophile is mediated by a sulfotransferase in r odents, but epoxide formation is not responsible for the observed genotoxic ity. (C) 2000 Elsevier Science B.V. All rights reserved.