There is widespread concern that exposure to environmental chemicals may be contributing to human reproductive and developmental disorders. Male germ cell mutagenicity is an area of growing concern and dominant lethality testing indicates that paternal exposure to environmental germ cell mutagens could lead to adverse outcomes in the survival and health of offspring. Many epoxides (ethylene oxide, propylene oxide, glycidol, styrene oxide) and epoxide-forming chemicals (acrylamide, acrylonitrile, 1,3-butadiene, urethane) are reported to cause dominant lethal mutations. The mechanism of action of these chemicals is not well understood, however, most of these environmental chemicals are metabolized to epoxide intermediates via CYP2E1. We therefore hypothesized that epoxidation of these chemicals may be responsible for the induction of germ cell mutagenicity. Using acrylamide as a model chemical, studies were undertaken to investigate the metabolic and molecular basis for the induction of germ cell mutagenicity by epoxide-forming chemicals. Acrylamide is an animal carcinogen and probable human carcinogen present in appreciable amounts in heated carbohydrate-rich foodstuffs. It is also a germ cell mutagen, inducing dominant lethal mutations and heritable chromosomal translocations in postmeiotic sperm of treated mice. Acrylamide?s affinity for male germ cells has sometimes been overlooked in assessing its toxicity and defining human health risks. Previous investigations of acrylamide?s germ cell activity in mice showed stronger effects after repeated administration of low doses compared to a single high dose, suggesting the possible involvement of a stable metabolite. A key oxidative metabolite of acrylamide is the epoxide, glycidamide, generated by cytochrome P4502E1 (CYP2E1). To explore the role of CYP2E1 metabolism in the germ cell mutagenicity of acrylamide, CYP2E1-null and wild-type male mice were treated by intraperitoneal injection with 0, 12.5, 25, or 50 mg acrylamide/5 ml saline/kg/day for 5 consecutive days. At defined times after exposure, males were mated to untreated B6C3F1 females. Females were sacrificed in late gestation and uterine contents were examined. Dose-related increases in resorption moles (chromosomally aberrant embryos) and decreases in the numbers of pregnant females and the proportion of living fetuses were seen in females mated to acrylamide-treated wild-type mice. No changes in any fertility parameters were seen in females mated to acrylamide-treated CYP2E1-null mice. Our results constitute the first unequivocal demonstration that acrylamide-induced germ cell mutations in male mice require CYP2E1-mediated epoxidation of acrylamide. Public Health or Environmental Health Significance: Cytochrome P450 2E1 (CYP2E1) is responsible for the bioactivation of a variety of environmentally important xenobiotics including 1,3-butadiene, acetaldehyde, acetaminophen, aniline, benzene, carbon tetrachloride, trichloroethylene, dichloroethylene, ethylene glycol, vinyl chloride, and nitrosamines. Most of these chemicals undergo oxidative metabolism via the cytochrome P450 enzymes to form epoxide intermediates that are thought to play a role in human carcinogenesis. Epoxides are three membered cyclic ethers and are among the most potent known mutagens, reproductive toxins, and carcinogens. Environmental chemicals that are metabolized to epoxides share a similar pattern of activity. Further, genetic polymorphisms in CYP2E1 were considered important risk factors in the development of xenobiotic-induced diseases including reproductive toxicity, mutagenicity, and carcinogenicity in humans. Once the mechanism of action of epoxides is established, the present approach may be used to predict the effects of chemicals that are metabolized to epoxides. Further, once the role of epoxides in environmentally caused human diseases is established, a more accurate assessment of human risks to epoxide-forming chemicals may be accomplished and the connection between metabolism of chemicals, carcinogenicity, reproductive toxicity, and CYP polymorphisms in humans may be established. In addition, many of the model chemicals that we use in our research are of interest to the NTP and therefore our work will have the additional advantage of complementing the NTP mission. Finally, current work suggest that CYP-null mice constitute appropriate models for the investigation of the role of oxidative metabolism in the toxicity, mutagenicity, and carcinogenicity of a large number of structurally diverse environmental chemicals.

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National Institute of Environmental Health Sciences (NIEHS)
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Ghanayem, Burhan I; Bai, Re; Burka, Leo T (2009) Effect of dose volume on the toxicokinetics of acrylamide and its metabolites and 2-deoxy-D-glucose. Drug Metab Dispos 37:259-63
Ghanayem, Burhan I (2007) Inhibition of urethane-induced carcinogenicity in cyp2e1-/- in comparison to cyp2e1+/+ mice. Toxicol Sci 95:331-9
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Kim, Dojung; Ghanayem, Burhan I (2006) Comparative metabolism and disposition of trichloroethylene in Cyp2e1-/-and wild-type mice. Drug Metab Dispos 34:2020-7
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Hoffler, Undi; Dixon, Darlene; Peddada, Shyamal et al. (2005) Inhibition of urethane-induced genotoxicity and cell proliferation in CYP2E1-null mice. Mutat Res 572:58-72
El Hadri, L; Chanas, B; Ghanayem, B I (2005) Comparative metabolism of methacrylonitrile and acrylonitrile to cyanide using cytochrome P4502E1 and microsomal epoxide hydrolase-null mice. Toxicol Appl Pharmacol 205:116-25

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