Abstract 5886: Translational reprogramming of breast cancer and its role in tamoxifen resistance

Abstract

Abstract Breast cancer is the most commonly diagnosed malignancy among women in the U.S. and the second leading cause of cancer-related deaths in women. The estrogen receptor-positive (ER+) breast cancer subtype comprises the majority (70-80%) of breast cancer cases and the majority of late recurrences. Anti-estrogen therapy with tamoxifen remains a cornerstone of adjuvant therapy for ER+ breast cancers in the premenopausal setting, but resistance is inevitable for a third of patients, leading to metastatic disease, which is responsible for the majority of breast cancer deaths. Our work demonstrates that tamoxifen resistance in ER+ breast cancer involves selective mRNA translation mediated by hyperactivation of mTOR and MNK1, two kinases that play major roles in mRNA translation. The hyperactivation of mTOR and MNK1 converges on eukaryotic translation initiation factor 4E (eIF4E) to increase its availability and mRNA selectivity. Using tamoxifen-sensitive and -resistant ER+ cells, we conducted genome-wide transcriptomic and translatomic analyses +/- silencing of eIF4E. We identified key genes responsible for tamoxifen resistance, whose mRNAs are selectively translationally upregulated with increased levels of eIF4E. RUNX2 is a key mRNA that is selectively translationally upregulated, which we show provides resistance by opposing estrogenic signaling and possibly phenotypically reprogramming the cell. Silencing of RUNX2 strongly resensitizes the cells to tamoxifen. We also show that RUNX2 impairs cellular proliferation and cell survival of previously drug-resistant cells in the presence of tamoxifen. Silencing of key translationally upregulated mRNAs, such as RUNX2, can partially restore endocrine therapy sensitivity. Using a tamoxifen-resistant ER+ PDX model, and patient-derived sensitive and resistant tumor tissues (no recurrence or recurrence within 5 years), we validated the hyperactivation of mTOR and MNK1, the overexpression of eIF4E, and selective mRNA translation in tamoxifen resistance. We hypothesize that selection for increased expression of RUNX2 and other selectively translated mRNAs promotes a more “basal-like,” estrogen-independent phenotype. We aim to determine how tamoxifen resistance translationally reprograms breast cancer cells to a different breast cancer phenotype (e.g., cancer stem cell-like, basal-like, luminal with disrupted ER signaling). Citation Format: Sofia Bakogianni, Phillip Geter, Amanda Ernlund, Rezina Arju, Abhilash Gadi, Robert J. Schneider. Translational reprogramming of breast cancer and its role in tamoxifen resistance [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 5886.