Abstract P5-07-03: The nuclear coactivator amplified in breast cancer 1 maintains tumor initiating cells during development of ductal carcinoma in situ (2013)

Abstract Introduction: Ductal Carcinoma In Situ (DCIS) has become the most diagnosed breast tumor type in North America. Amplified In Breast Cancer 1 (AIB1) is amplified in 5 to 10% of primary breast tumors. We have recently demonstrated that AIB1 causes development of DCIS lesions in the mammary gland of 30% of the AIB1 transgenic mice. Our laboratory has also shown that deletion of one allele of AIB1 prevents HER2-induced mammary tumor development in mice, suggesting a pivotal role of AIB1 during earlier stage of breast cancer. We hypothesize that AIB1-mediated pathways promote the transformation of the mammary epithelial cells and the alterations of the functional integrity of the mammary gland. By contrast, decrease in AIB1 functionality may result in prevented DCIS initiation and development. Results: We show by immunohistochemistry that AIB1 is expressed at low levels in normal breast but is highly expressed in DCIS lesions. We investigated the impact of decreasing AIB1 expression on the early transformed MCFDCIS cell line. MCFDCIS cells generate multiacinar disorganized structures with filling of the lumen, loss of polarization and escape from the proliferative suppression. Reduction of AIB1 in human MCFDCIS cells restored a more normal mammary acinar structure with basement membrane in 3D growth in Matrigel. Then, we studied the effect of regulating AIB1 on the histopathology development of the DCIS lesions both prior to DCIS development or in existing MCFDCIS lesions in a xenograft mouse model by subcutaneously injecting MCFDCIS cells deficient or not in AIB1 into nude mice. The MCFDCIS xenografted tumors obtained reproduced the human breast cancer progression and displayed multiple comedo DCIS-like lesions. Decreased levels of AIB1 in MCFDCIS cells, inhibited tumor growth and led to smaller, necrotic lesions. To investigate how AIB1 affects initiation and progression of DCIS, we compared global gene expression changes in MCFDCIS cells +/-AIB1 shRNA in vitro using cDNA array analysis. Among the most significant expression changes observed were genes that are differentially expressed in Breast Cancer Initiating Cells (BCIC) including CD24 and members of the NOTCH signaling pathway (DLL1 and DLL3). We demonstrated by flow cytometric and immunohistochemical analysis that AIB1 reduction in MCFDCIS cells was correlated with significant reduction in the CD24-/CD44+ BCIC population, and a decrease in myoepithelial progenitor cells in the DCIS lesions in vitro and in vivo. Loss of AIB1 in MCFDCIS cells was also accompanied by a loss of expression of NOTCH 2, 3 and 4, JAG2, HES1, GATA3, HER2 and HER3 in vivo. Conclusions: These data indicate that AIB1 plays a central role in the initiation and maintenance of DCIS and that reduction of AIB1 causes loss of BCIC, loss of components of the NOTCH, HER2 and HER3 signaling pathways and fewer DCIS myoepithelial progenitor cells in vivo. We propose that increased expression of AIB1, through maintenance of BCIC, facilitates formation of DCIS, a necessary step prior to development of invasive disease. Thus, targeting AIB1 may represent a new therapeutic paradigm especially for women with AIB1-positive primary tumor. Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P5-07-03.