Our work has been designed for testing the hypothesis that estrogenically active environmental chemicals influence the lifetime risk of developing cancer when exposures occur during critical early periods of breast development. For accomplishing this objective Sprague-Dawley rats were used for testing the effects of prenatal and prepubertal exposures to a low (LD) and a high dose (HD) of the hormonally-active compounds bisphenol A (BPA), butyl benzyl phthalate (BBP), and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on the genomic and proteomic signatures of mammary glands of 21, 35, 50 or 100 day-old rats. Changes in endocrinological parameters and mammary gland morphology, cell proliferation, gene and protein expression at these ages, which represent critical stages of development and differentiation, are correlated with the response of the mammary gland to chemical carcinogens. In addition, proteins identified in the serum collected from rats at these various stages serve as biomarkers of exposure and risk; correlation of their presence with those found in the serum of a cohort of girls traversing similar windows of development constitutes the basis for prospectively investigating the relationship between hormonally active exposures and pubertal milestones. In rats prenatally or prepubertally-exposed to LD or HD BPA or BBP none of the following parameters were affected: body weight, age at vaginal opening, uterine weight, and estrous cyclicity. Prepubertal HD TCDD exposure, on the other hand, significantly decreased body weight at 21, 35, and 50, but not at 100 days of age, and both, LD and HD TCDD decreased uterine weight at day 35, delayed vaginal opening, and induced irregular estrous cycles. At genomic level, on the other hand, prepubertal exposure to HD BPA resulted in a greater number of dysregulated genes and a greater number of chemically-induced mammary carcinomas than prenatal exposure, an indication that prepuberty is a period of greater sensitivity to the effect of BPA for the mammary gland to undergo specific genomic changes that determine its susceptibility to carcinogenesis. Although these effects were not elicited by the other compounds, the genomic changes induced in the rat mammary gland by BPA, BBP, and TCDD exposure identified a specific set of genes relevant to humans and led us to conclude that each compound at each specific window of exposure selectively affects defined biological processes, cellular components, and molecular functions. In addition, in vitro treatment of human breast epithelial cells with BPA and BBP induces cell transformation and changes in genes dysregulated in animals treated with these compounds in vivo and that exhibit single nucleotide polymorphism (SNP) in the buccal mucosa of the cohort of girls included in these studies. These results have provided new sets of genomic markers and new paradigms for the understanding of the role of environmental exposures in the human population cancer risk.
(This work was supported by NCI and NIEHS Grant UO1 ES012771).