The Effect of Pre-pubertal 2,3,7,8-tetrachlorodibenzo-p-dioxin Exposure on Terminal End Bud Differentiation and Genetic Expression in Rat Mammary Glands

Shirou Wu* (1); Julia S. Pereira (1); Ricardo Lopez (1); Fathima Sheriff (1); Kara Snider (1) and Jose Russo (1).

1 Breast Cancer Research Laboratory, Fox Chase Cancer Center, Philadelphia, PA, 19111, USA.

Substantive evidence linking 2,3,7,8-tetracholorodibenzo-p-dioxin (TCDD) exposure to breast cancer risk has predominantly been on the cellular and histological level, due to conflicting and variable-prone epidemiological data. Previous studies have also demonstrated that timing of exposure to TCDD greatly affects risk tumorigenesis. In this study, we investigated the effect of prepubertal TCDD exposure on mammary gland development and genetic expression. Sprague-Dawley rats were orally administered either of two doses (6.67ng/kg of body weight or 20ng/kg bw) of 2,3,7,8-tetracholorodibenzo-p-dioxin and sacrificed at 50 days of age. 4,5 abdominal mammary glands were extracted and prepared with the whole mount technique. Terminal end buds (TEBs) were then counted per tissue sample. We found that rats administered the higher dosage demonstrated a statistically significant greater amount of TEBs. Observations showed that in addition to the TEB count, branching and differentiation were negatively affected in mammary glands of high-dose rats, although no quantitative analysis was conducted. Following morphological examination, gene expression analysis via a microarray found 472 probes over or under-expressed in the high-dose cohort while 8, were over or under-expressed in the low-dose cohort. Among these genes, there are numerous genes that could potentially contribute to mammary carcinogenesis, including ones involving lipid metabolism, known oncogenes, genes involved in the reproductive cycle, genes that could potentially create an environment ill-suited to react to oxidative stress, and genes involved in cellular proliferation and apoptosis. Of these genes, Cyp1b1 was the most upregulated gene in both groups. Environmentally activated Cyp1b1 upregulation has been implicated in mammary carcinogenesis. We observed that the rats treated with the higher dose of TCDD had an increase of TEBs and greatly affected gene expression in the mammary gland. This could imply that TCDD exposure may create environments more suitable for the development of cancer, increasing the risk for mammary carcinogenesis. (Shirou Wu was supported by Huntington Breast Cancer Action Coalition fellowship and this work was supported by NCI and NIEHS Grant UO1 ES012771)

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