Xenochemical Exposure and DNA Methylation

Masiko Negishi
Laboratory of Reproductive and Developmental Toxicology; NIEHS

We will introduce two systems in which DNA methylation may affect susceptibility to xenochemical toxicity and carcinogenicity: liver microsomal cytochromes P450 (CYPs) and diethylstilbestrol (DES)-induced uterine tumor. CYPs play a major role in metabolic detoxification/activation of xenochemicals and often exhibit sexually dimorphic expression. Xenochemical metabolism by these sex-specific CYPs may lead to sex-dependent response to protoxins, procarcinogens and prodrugs. In mice, the female-specific CYP2A4 (testosterone 15a-hydroxylase) and the male-specific CYP2D9 (testosterone 16 a-hydroxylase) are the best characterized sex-specific CYP genes. The regulatory element of their promoter sequences contains a potential DNA methylation site. These CpG sites are developmentally demethylated in accordance with the sex-specific expressions of the corresponding CYP genes. Sexually dimorphic demethylation such as these may alter physiological and pathological responses to xenochemical exposure.

Mice treated by DES prenatally or neonatally develop uterine tumors in adult life. The estrogen-responsive lactoferrin gene in mouse uterus has been used as a maker of the tumor development in the DES-treated mice. We examined the DNA methylation pattern of the lactoferrin promoter in the mouse uterus. The CpG/-464 site was found to be abnormally demethylated as a result of neonatal exposure to DES in the mature uteri. Although this abnormal demethylation appears to be imprinted, the actual process of the demethylation is under hormonal control since CpG/-464 remained methylated in the neonatally DES-treated and ovariectomized mice. This mode of the demethylation is reminiscent of the uterine tumor formation in neonatally DES-treated adult mice. DNA methylation by xenochemical exposure such as DES may have serious consequences in xenochemical-induced tumor development.