The Role of Breast Cancer Antiestrogen Resistance 3 (BCAR3) in Breast Tumor Cell Adhesion and Motility

The 5-year survival rate for patients diagnosed with metastatic breast cancer is only 25%. As we move further into the era of personalized medicine, it is critical that we gain a better understanding of the molecular drivers and cell signaling pathways that underlie breast tumor progression and metastasis in order to improve patient survival. The adaptor molecule Breast Cancer Antiestrogen Resistance 3 (BCAR3) functions in cellular processes that contribute to tumor cell adhesion, motility and invasion. Data presented in this thesis demonstrate that BCAR3 localizes to adhesions, promotes Rac1 activity and drives several critical facets of cell motility in invasive breast tumor cells, including membrane protrusion and adhesion disassembly. Interestingly, BCAR3-mediated adhesion disassembly was found to be contingent on its ability to interact with its well-established binding partner, the adaptor molecule p130Cas (Cas). The requirement for BCAR3 in driving this pro-migratory phenotype was underscored by the fact that, when BCAR3 was selectively depleted from invasive breast tumor cells, the cells failed to migrate/invade efficiently or respond properly to growth factor stimuli. Instead, RhoA signaling predominated under these conditions, as evidenced by an increase in RhoA-mediated tension, ROCK-mediated phosphorylation of myosin light chain II, the presence of actin-rich stress fibers, and large, stable ROCK/mDia1-dependent focal adhesions. Together, these data establish that BCAR3 functions as a positive regulator of actin cytoskeletal remodeling and adhesion disassembly in invasive breast tumor cells through its ability to influence the balance between Rac and Rho signaling. Furthermore, while disruption of BCAR3/Cas interaction had no effect on BCAR3 or Cas localization to adhesions, blockade of BCAR3/Cas phenocopied the loss of BCAR3 in that rates of adhesion disassembly were significantly reduced. This suggests that, when in complex with Cas in invasive breast cancer cells, BCAR3 may drive Rac1 activity in adhesions, thus promoting breast tumor cell motility/invasion. Conversely, Rac/Rho reciprocity may be tipped in favor of RhoA signaling in the presence of a BCAR3 molecule that cannot bind to Cas, ultimately leading to deficient adhesion turnover and reduced cell motility.

BCAR3 expression, which is a robust biomarker of a cell signaling axis comprised of Cas and the non-receptor tyrosine kinase c-Src (Src), is upregulated in invasive, triple-negative breast cancer cell lines. Overexpression of BCAR3 in breast cancer cells promotes Src activity, invasive tumor cell behaviors and therapeutic resistance. However, BCAR3 protein expression in human breast tumors had not been reported. To determine the expression pattern of BCAR3 in human breast cancers, 74 human breast tumors and 8 normal breast tissue specimens were analyzed immunohistochemically for BCAR3 expression. BCAR3 protein levels were elevated in high-grade ductal carcinoma in situ (DCIS) and invasive ductal carcinoma (IDC) compared to normal mammary ductal epithelial cells. Moreover, BCAR3 was expressed across multiple subtypes of human breast cancer including triple-negative breast tumors. While BCAR3 protein levels did not correlate with tumor invasiveness or a particular breast cancer subtype, these studies revealed the immense heterogeneity of BCAR3 expression within each tumor subtype. Taking into account these findings, a re-evaluation of the therapeutic and prognostic potential of BCAR3 is put forth that addresses the possible implications of targeting BCAR3, including clinical resistance and the challenges that exist given the heterogeneity of its expression within and between patients. Finally, this work provides a foundation for future studies that aim to determine whether elevated BCAR3 protein levels drive breast tumor progression and/or correlate with clinical outcomes, and whether they can serve as a biomarker to identify tumors that exhibit aggressive subtypes of DCIS and/or high Src activity.