Abstract 4392: Yes1 is the key molecule for the resistance to trastuzumab in breast cancer, and dasatinib overcomes the resistance

Proceedings: AACR 107th Annual Meeting 2016; April 16-20, 2016; New Orleans, LA Background: Overexpression of human epidermal growth factor receptor 2 (HER2) was observed in approximately 15-23% of breast cancers and they are classified as HER2-positive breast cancer. Trastuzumab is a therapeutic drug for the first choice for HER2-positive breast cancer, showing good response. However, acquired resistance to trastuzumab is one of the critical clinical issues in breast cancer treatment, especially in the patients with recurrent breast cancer. Therefore, it is necessary to develop the effective therapy to overcome the resistance. In this study, we established a trastuzumab-resistant breast cancer cell line from a trastuzumab-sensitive cell line with HER2 amplification (BT-474). We analyzed the mechanisms of resistance to trastuzumab and demonstrated the anti-tumor effect of dasatinib. Methods: Trastuzumab-resistant breast cancer cell line (BT-474-R) was established by treating BT-474 cells for long-term exposure with increasing doses of trastuzumab (from 0.1 μg/mL up to 40 μg/mL). Expression and activation of HER2 and its related molecules were investigated using western blotting and real-time PCR. Cell viability was evaluated using MTS assay. Cell cycle was analyzed using flow cytometry. Results: Proto-oncogene tyrosine-protein kinase Yes1, which is one of the Src family members, was amplified, overexpressed and activated in BT-474-R. HER2 and Akt were also activated. Silencing of Yes1 by siRNA induced BT-474-R to recover sensitivity to trastuzumab. Pharmaceutical inhibition of Yes1 by Src inhibitor dasatinib was also effective to recovery sensitivity to trastuzumab in BT-474-R. Combination treatment of dasatinib and trastuzumab induced down-regulation of signaling molecules such as HER2 and Akt. Moreover, these combination treatments induced G1-phase cell-cycle arrest and apoptosis. Conclusion: Yes1 plays an important role in acquired resistance to trastuzumab in HER2-positive breast cancer. Our data also suggest that pharmacological inhibition of Yes1 may become the new strategy to overcome resistance to trastuzumab. Citation Format: Hiromasa Yamamoto, Tatsuaki Takeda, Hirotaka Kanzaki, Ken Suzawa, Kei Namba, Hiroki Sato, Hidejiro Torigoe, Mototsugu Watanabe, Yuho Maki, Junichi Soh, Hiroaki Asano, Kazunori Tsukuda, Shinichiro Miyoshi, Yoshihisa Kitamura, Toshiaki Sendo, Shinichi Toyooka. Yes1 is the key molecule for the resistance to trastuzumab in breast cancer, and dasatinib overcomes the resistance. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4392.