It has long been appreciated that the processes of tissue morphogenesis and tumor initiation/progression share many common features. This is particularly the case for signaling networks involved in cell proliferation, survival, and motility. The focus of the current study is on a signaling complex comprised of the adaptor molecules p130Cas (Cas) and Breast Cancer Antiestrogen Resistance 3 (BCAR3), which together are potent activators of the non-receptor tyrosine kinase c-Src. BCAR3 and Cas are present in a stable molecular complex in breast tumor cells where they have been shown to play a role in the regulation of cell adhesion and motility, both of which are obligate elements of tumor cell proliferation and invasion. In this study, we sought to determine whether the biological activities attributed to BCAR3/Cas in breast cancer cells were dependent upon their interaction, and whether these molecules also might function during breast morphogenesis.

We show that the entire pool of BCAR3 is in complex with Cas in invasive breast tumor cells and that these proteins co-localize in dynamic cellular adhesions. While accumulation of BCAR3 in adhesions does not require Cas binding, a direct interaction between BCAR3 and Cas is necessary for efficient dissociation of BCAR3 from adhesions. The dissociation rates of Cas and two other adhesion proteins, α-actinin and talin, are also significantly slower in the presence of a Cas-binding mutant of BCAR3, suggesting that turnover of the entire adhesion complex is delayed under these conditions. As is the case for adhesion turnover, BCAR3-Cas interactions were found to be important for BCAR3-mediated breast tumor cell invasion in Matrigel. Previous work showed that BCAR3 is a potent activator of Rac1, which in turn is an important regulator of adhesion dynamics and invasion. We show that, as is the case for adhesion turnover, Rac1 activation by BCAR3 also requires its interaction with Cas. Together, these data show that the ability of BCAR3 to promote adhesion disassembly, tumor cell invasion, and Rac1 activity, are all dependent on its ability to bind to Cas, thus implicating the BCAR3/Cas complex as an important functional unit in invasive breast tumor cells. To determine whether this was also the case in breast tumors, we set out to measure BCAR3 protein expression in tumor samples. We reasoned that BCAR3 expression could potentially serve as a surrogate marker for the activity of the BCAR3/Cas complex since all of the BCAR3 in breast tumor cells is associated with Cas. Preliminary data indicate that BCAR3 protein levels are extremely low in normal mammary epithelial cells but that its expression is elevated in multiple breast tumor subtypes. BCAR3 is also expressed during tumor progression in the MMTV-polyoma virus middle T murine breast tumor model. Studies are now being performed in the mouse to determine whether BCAR3 expression in breast tumors promotes tumor progression and metastasis. If this is the case, then targeting the BCAR3/Cas complex and its downstream effectors could be a viable approach for treating invasive breast cancers.

Like many signaling pathways that are involved in cancer development, we hypothesize that the BCAR3/Cas signaling complex also plays a role in normal morphogenesis. Analysis of BCAR3 protein in developing mammary glands showed that BCAR3 expression is highest during puberty and is downregulated in fully developed glands. We are currently modeling mammary gland morphogenesis using organoid cultures to investigate the potential role of BCAR3/Cas signaling in this process. Determining how signaling through BCAR3/Cas supports mammary gland development will contribute to our understanding of how elevated signaling through this complex may promote breast cancer progression.

Citation Format: Allison M. Cross, Ashley L. Wilson, Michael S. Guerrero, Keena S. Thomas, Alexia I. Bachir, Ryan A. Llewellyn, Kristopher E. Kubow, A Rick Horwitz, Amy H. Bouton. A role for the BCAR3/Cas signaling complex in breast tumor progression, metastasis and normal breast morphogenesis. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Metastasis; 2015 Nov 30-Dec 3; Austin, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(7 Suppl):Abstract nr B17.