Selective pressure of endocrine therapy activates the integrated stress response through NFκB signaling in a subpopulation of ER positive breast cancer cells

Background: While estrogen receptor (ER) 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 previously demonstrated that the selective pressure of tamoxifen activates the NF kappa B pathway in ER + patient tumors, breast cancer cell lines, and breast cancer xenograft tumors, and that this activation allows for survival of a subpopulation of NF kappa B+ cells that contribute to cell regrowth and tumor relapse after ET withdrawal. However, the mechanisms contributing to the expansion of an NF kappa B+ cell population on ET are unknown. Methods: Here, we utilized single-cell RNA sequencing and bioinformatics approaches to characterize the NF kappa B+ cell population and its clinical relevance. Follow-up studies were conducted to validate our findings and assess the function of the integrated stress response pathway in breast cancer cell lines and patient-derived models. Results: We found that the NF kappa B+ population that arises in response to ET is a preexisting population is enriched under the selective pressure of ET Based on the preexisting NF kappa B+ cell population, we developed a gene signature and found that it is predictive of tumor relapse when expressed in primary ER+ tumors and is retained in metastatic cell populations. Moreover, we identified that the integrated stress response (ISR), as indicated by increased phosphorylation of eIF2 alpha, occurs in response to ET and contributes to clonogenic growth under the selective pressure of ET. Conclusions: Taken together, our findings suggest that a cell population with active NF kappa B and ISR signaling can survive and expand under the selective pressure of ET and that targeting this population may be a viable therapeutic strategy to improve patient outcome by eliminating cells that survive ET. Understanding the mechanisms by which breast cancer cells survive the selective pressure of ET may improve relapse rates and overall outcome for patients with ER+ breast tumors.

Automated summary

The NF kappa B+ cell population expands during endocrine therapy (ET), and this population has clinical relevance. A gene signature based on the preexisting NF kappa B+ cell population is predictive of tumor relapse. The integrated stress response (ISR) occurs during ET and contributes to clonogenic growth under the selective pressure of ET.