Abstract B20: Role of ErbB3 in tumorigenesis and drug resistance in ErbB2-driven breast cancer

Introduction: Overexpression of erbB3 is frequently observed in erbB2 altered breast cancers. Numerous studies indicate that the expression of erbB3 plays an important role in erbB2-driven breast cancer formation and progression. In this study, we take advantage of the mammary tumors derived from MMTV-neu transgenic mice and the tumor xenografts established from human erbB2-overexpressing (erbB2+) breast cancer cells to further elucidate the role of erbB3 in tumorigenesis and resistance to chemotherapy and erbB2-targeted therapy. Methods: Immunohistochemical analyses were performed to study the expression and phosphorylation of rat c-neu/ErbB2 and mouse erbB3 in mammary tumors. Co-immunoprecipitation (IP) analysis was employed to examine the heterodimerization between erbB2 and erbB3. Cell growth (MTS) assays were used to determine cell viability. Lentiviral vector containing shRNA was used to specifically knockdown erbB3. Flow cytometric assays were performed to define cell cycle distribution. Western blot analyses were performed to assess the expression and activation of proteins. The tumor xenografts were established by inoculation of BT474-HR20 cells (trastuzumab-resistant subline derived from BT474 cells) into nude mice. The tumor-bearing mice were treated with paclitaxel/trastuzumab and/or an anti-erbB3 monoclonal antibody (Ab, MM-121/SAR256212) to determine whether an erbB3-targeted therapy may enhance paclitaxel/trastuzumab9s antitumor activity. Results: The expression status of erbB2 and/or erbB3 had no effect on tumor latency, however, the mammary tumors with co-expression of both erbB2 and erbB3 receptors grew significantly faster than those with either erbB2 or erbB3 expression alone. The tumors with co-expression of both receptors showed higher levels of phosphorylated-erbB2 (P-erbB2) and P-erbB3. The interaction between the rat c- neu transgene encoded erbB2 and mouse erbB3 was confirmed via IP assays in the mammary tumors. Elevated expression of erbB3 enhanced cell proliferation, whereas knockdown of erbB3 inhibited proliferation of erbB2+ breast cancer cells. The combinations of trastuzumab with an erbB3 blocking Ab significantly reduced cell viability in several erbB2+ breast cancer cell lines. Furthermore, MM-121 in combination with paclitaxel or trastuzumab (as compared to either alone) dramatically inhibited tumor growth in a mouse xenograft model. Conclusion: Activation of the erbB3 signaling plays a pivotal role in the development of erbB2-driven breast cancer. Strategies against erbB3 abrogates erbB2-mediated paclitaxel / trastuzumab resistance in breast cancer. Our data provide a strong basis to explore the therapeutic potential of erbB3-targeted therapy in combination with paclitaxel/ trastuzumab in breast cancer patients whose tumors overexpress erbB2. Citation Format: Hui Lyu, Jingcao Huang, Susan M. Edgerton, Ann D. Thor, Bolin Liu. Role of ErbB3 in tumorigenesis and drug resistance in ErbB2-driven breast cancer. [abstract]. In: Proceedings of the AACR Precision Medicine Series: Drug Sensitivity and Resistance: Improving Cancer Therapy; Jun 18-21, 2014; Orlando, FL. Philadelphia (PA): AACR; Clin Cancer Res 2015;21(4 Suppl): Abstract nr B20.