The Function Of Tumor Suppressor Gene ATBF1in Breast Epithelium Differentiation And Mammary Gland Development

Breast cancer is the most popular cancer in women around the world. Recently, the breast cancer survival has been greatly reduced based on the discovery of rumor-associated genes. However, a large number of these genes remain unclear. ATBF1is a candidate tumor suppressor gene that interacts with estrogen receptor (ER) to inhibit the function of estrogen-ER signaling in gene regulation and cell proliferation control in breast cancer cells. In clinical samples, down-regulation of ATBF1is associated with adverse features of breast cancer including ER positivity. Moreover, PR is an important target gene of estrogen-ER signaling pathway and PR is always positive in ER positive breast cancer cells. In this dissertation, we clarify the role of ATBF1to breast epithelium proliferation, differentiation, mammary gland development and breast cancer tumorigenesis based on the relationship between ATBF1and estrogen-ER or progesterone-PR signaling pathway, respectively.First, We examined whether Atbfl and its interaction with ER modulate the development of pubertal mammary gland, where estrogen is the predominant steroid hormone. In anin vitro model of cell differentiation, i.e., MCF10A cells cultured in Matrigel, ATBF1expression was significantly increased, and knockdown of ATBF1inhibited acinus formation.During mouse mammary gland development, Atbf1was expressed at varying levels at different stages, with higher levels during puberty, lower during pregnancy, and the highest during lactation. Knockout of Atbfl at the onset of puberty enhanced ductal elongation and bifurcation and promoted cell proliferation in both ducts and terminal end buds of pubertal mammary glands. Enhanced cell proliferation primarily occurred in ER-positive cells and was accompanied by increased expression of ER target genes. Furthermore, inactivation of Atbf1reduced the expression of basal cell markers (CK5, CK14and CD44) but not luminal cell markers. These findings indicate that Atbfl plays a role in the development of pubertal mammary gland likely by modulating the function of estrogen-ER signaling in luminal cells and by modulating gene expression in basal cells. Second, progesterone (Pg) is an essential steroid hormone for mammary epithelial stem/progenitor cells, mammary gland development and breast carcinogenesis. Whereas it is believed that the role of Pg-PR is mediated by key transcription factors, such factors largely remained to be discovered. In this study, we found that ATBF1was a transcriptional target of the Pg/PR signaling in mammary epithelial cells. Pg treatment dramatically increased ATBF1expression at both mRNA and protein levels in vitro and in vivo. Induction of ATBF1was PR-dependent, as it only occurred in PR-positive but not in PR-negative cells, and pretreatment with progesterone antagonist RU-486or RNAi-mediated knockdown of PR abolished the upregulation of ATBF1by Pg. In addition, promoter-reporter assay and ChIP assay showed that Pg-activated PR directly bound to the promoter of ATBF1to induce its transcription. Functionally, reduction of ATBF1expression prevented the function of Pg in promoting progenitor cell transition, as indicated by colony formation in a Matrigel culture assay and expression of stem cell markers CD49f and CD44. These findings suggest that ATBF1plays a crucial role in the Pg/PR signaling pathway in mammary epithelial cells.