Breast Cancer Antiestrogen Resistance-3 (BCAR3) in Mammary Gland Development and Breast Cancer

Despite increased early detection and improved treatment options, breast cancer remains the second leading cause of cancer deaths among women. The majority of breast cancer mortalities are the consequence of therapeutic resistant metastatic disease. A better understanding of the genetic alterations and signaling pathways involved in breast cancer progression and therapeutic resistance is required to identify new and better therapeutic targets to combat this disease. Breast Cancer Antiestrogen-3 (BCAR3) has been identified as an adaptor molecule that is upregulated in aggressive breast cancer cell lines, where it contributes to increased proliferation, migration, and invasion. The work presented in this thesis focuses on understanding BCAR3 signaling in breast cancer progression as well as mammary gland morphogenesis. The data presented demonstrate that BCAR3 controls adhesion turnover, migration, and invasion through interactions with the adaptor molecule p130Cas (Cas). In addition, BCAR3 was found to be upregulated and differentially expressed during tumor progression in the MMTV-polyoma middle T (PyMT) mouse model of spontaneous breast cancer. Preliminary xenograft studies in mice reveal that BCAR3 expression accelerates tumor formation and controls total tumor burden in MDA-MB-231 breast tumors. Future studies are needed to determine if BCAR3 can regulate the growth of established tumors and promote metastasis, and if interactions with Cas are required for its functions in vivo.
Notably, many of the signaling pathways that regulate tumor progression are also involved in normal development. Thus, by gaining a better understanding of how proteins regulate normal development, we can improve our understanding of how they can be used and/or disrupted to promote cancer progression. BCAR3 expression was found to be upregulated in mammary glands of pubertal and pregnant mice. Despite the established
proliferative, migratory, and invasive functions of BCAR3 in breast cancer cells, BCAR3 does not appear to promote these functions in mammary epithelial cells during mammary morphogenesis. Preliminary studies analyzing mammary glands of BCAR3 knockout mice revealed enhanced ductal outgrowth and reduced numbers of terminal end buds during puberty, a phenotype consistent with accelerated mammary gland development. These
data suggest that BCAR3 may normally function to suppress mammary gland
development. Further studies are needed to determine if the putative suppressive role of BCAR3 during mammary gland development stems from a function of BCAR3 in the epithelial or stromal cells of the gland. Understanding how BCAR3 acts to suppress normal mammary gland development may provide insight into as-yet-unknown functions of BCAR3 in epithelial and non-epithelial cells.
Based on the work presented in this thesis, and the established functions of
BCAR3, we propose that BCAR3 may be a useful biomarker and/or therapeutic target for breast cancer. Future studies are needed to determine if there is a correlation between BCAR3 protein expression and tumor subtype, tumor grade, metastasis, therapeutic response, and relapse-free and overall survival. These data will in turn determine whether BCAR3 could serve as biomarker for metastasis and/or sensitivity to Src and TGF-beta inhibitors. Additionally, if the BCAR3/Cas complex proves to be a driver of breast tumor growth and progression, it will be important to explore novel approaches for targeting the
complex, including BCAR3-specific siRNAs or small molecule and/or peptide inhibitors to block BCAR3/Cas interactions.