88PRole of HSP27 in trastuzumab resistance in HER2-overexpressing breast cancer

Background: Overexpression of human epidermal growth factor receptor 2 (HER2) takes place in approximately 25% of breast cancer patients. Trastuzumab (Herceptin®), a FDA-approved HER2-targeting monoclonal antibody, is widely considered as the first line therapy for this patient group. However, despite its clinical benefits, over 70% of the patients frequently suffer from secondary resistance to this drug within one year. Therefore, identifying the resistance mechanism and investigating new therapeutic markers are certainly in need. Methods: To elucidate the in-depth molecular mechanism involved in the resistance, we established two HER2-positive breast cancer cell lines with trastuzumab-resistance. We applied lentiviral shRNA expression system, cell viability assay, real-time PCR, western blot analysis, immunoprecipitation assay, xenograft mouse model for target validation of HSP27 and further evaluation of new applicable small molecule to surmount the resistance. Results: Unique changes in the expression pattern of HSP27 were observed and they showed positive correlation with HER2. We confirmed the interaction between HER2-HSP27 and found that stably silenced HSP27 induced degradation of HER2 and eventually increased susceptibility of trastuzumab-resistant BT474 to Herceptin®. Newly identified compound to overcome the resistance, effectively inhibited the activity of HSP27 by significantly inducing the formation of its abnormal dimers. The candidate compound down-regulated the HER2-related cell signaling pathway without affecting any kinase activities. It also reduced the level of nuclear HER2 and its transcriptional target, cyclin D1, both of which are mentioned to reduce the efficacy toward trastuzumab. The compound in combination with Herceptin®, showed remarkable synergism of anti-cancer effects in trastuzumab-resistant cell lines. Cotreatment of the compound and trastuzumab also significantly induced tumor regression of trastuzumab-resistant cell-implanted xenograft mouse, and also increased their survival rates. Conclusions: Targeting HSP27 by mediating compound-mediated abnormal dimerization would be a novel strategy to overcome trastuzumab-resistance. Legal entity responsible for the study: Ewha Womans University Funding: Bio & Medical Technology Development Program (NRF2014M3A9A9073 908) of the National Research Foundation of Korea (NRF). Disclosure: All authors have declared no conflicts of interest.