LAUSR

Background: HER2 (Human epithelial growth factor receptor 2)-targeting therapies have been approved for patients with HER2-positive breast and gastric cancer and use have been attempted in various solid tumor types, including biliary tract cancer. However, resistance mechanism remains as a major challenge of HER2-targeting therapies. YAP (Yes-associated protein) is a major downstream effector of Hippo pathway, and it plays an essential role in cancer cell proliferation, survival and differentiation. Moreover, YAP is emerging as a key player of resistance mechanism of cytotoxic and targeted drugs. Yap is also an important immunosuppressive molecule as it works as a negative regulator of T cell tumor infiltration. In this study, we intend to elucidate the role of YAP in mechanism of trastuzumab resistance and T cell immune response in HER2-positive cancer cells. Methods: We established four trastuzumab-resistant (HR) cell lines (N87HR, SNU216HR, SNU2670HR and SNU2773HR) from HER2-postive gastric and biliary tract cancer cell lines. To inhibit the function of YAP, siRNA and Verteporfin (YAP-TEAD inhibitor) were used. MTT assay, cell cycle analysis, western blot and migration assay were performed to analyze the antitumor effects through YAP downregulation. In order to evaluate immune-modulation by YAP, human PBMC co-culture was used and immune markers were analyzed by RT-PCR and flow cytometry. Mouse xenograft models were established using SNU2773 and SNU2773HR cells. Results: We confirmed that the expressions of pYAP and YAP were elevated in HR cells (SNU216HR, SNU2670HR and SNU2773HR) compared to parental cells. Reducing the expressed YAP in HR cells inhibited tumor cell growth and migration and induced apoptosis. Immune suppression markers such as PD-L1, CD155, and galectin-9 were effectively decreased, while CD80, a stimulation marker, was increased by verteporfin treatment. Also, when YAP was decreased, CCL5 and CXCL10, well known CD8+ T cell recruitment cytokines, were increased. In HR cells treated with siYAP and verteporfin, there was a trend of increasing CD4+ and CD8+ T cells when co-culture with PBMC. In vivo experiment data showed greater tumor growth inhibition effects with SNU2773HR than SNU2773 xenograft models when treated with verteporfin. Conclusion: The expression of YAP is elevated in trastuzumab-resistant (HR) cells and inhibition of YAP shows anti-tumor effects and activation of T cell responses. Collectively, our data suggests that the inhibition of YAP is one of many promising strategies to overcome trastuzumab resistance and T cell regulatory mechanisms in HER2-positive cancers. Citation Format: Ah Rong Nam, Jeesun Yoon, Kyoung-Seok Oh, Jae-Min Kim, Ju-Hee Bang, Yoojin Jeong, Sea Young Choo, Hyo Jung Kim, Su In Lee, Tae-Yong Kim, Do-Youn Oh. Targeting of YAP overcomes trastuzumab-resistance and promotes immune responses in HER2-positive cancers. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 3872.