The NF?B pathway promotes endocrine therapy tolerance through activation of integrated stress response in ER positive breast cancer

While estrogen receptor positive breast tumors generally respond well to endocrine therapy (ET), up to 40% of patients will experience relapse, either while on endocrine therapy or after ET is completed. We have previously shown that the selective pressure of tamoxifen activates the pro-survival NFkB pathway in patient tumors, breast cancer cell lines, and breast cancer xenograft tumors, and that this activation allows for survival of a population of ET-tolerant cells, which can contribute to relapse after ET withdrawal. Here, we utilized single cell RNA-sequencing to characterize survival pathways in NFkB+, ET-tolerant cells and identified activation of the integrated stress response as a critical survival mechanism. Moreover, we found key players in this pathway are regulated by ET in patient derived organoids. In addition, we developed an ET-tolerant gene signature that can be found in metastatic cell populations and that predicts poor outcome to ET. Our findings suggest that co-targeting of ER and key players in the integrated stress response may be a viable therapeutic strategy to eliminate cells that survive the selective pressure of ET. Overall design: scRNA-seq (OneCellBio)