OR05-04 Steroid Receptor Co-Activators Complexes Cooperate with Progesterone Receptors (PR) to Reprogram Metabolic Pathways that Drive Therapy Resistant Populations in ER+ Breast Cancer

Late recurrence of metastatic disease stemming from acquired therapy resistance remains a significant health burden for women with ER+ breast cancer. Disseminated ER+ tumor cell populations can remain quiescent for years to decades, and contributing factors include breast cancer stem cells (CSCs), which are non-proliferative and frequently exist as a minority population in recurrent therapy-resistant tumors. Progesterone receptors (PR) are known drivers of normal stem and breast CSCs. Our objective was to define novel signaling pathways governing cell fate transitions involved in driving therapy resistance in ER+ breast cancer. We reported that cytoplasmic complexes composed of steroid receptor (SR) co-activators, PELP1 and SRC-3, drive breast CSC outgrowth. SRC-3 knockdown abrogated PELP1-induced CSC expansion and target genes required for cell survival, suggesting an essential role for PELP1/SRC-3 complexes. PELP1 also forms a signaling and transcriptional complex with ER and PR-B. Phospho-PR species are key mediators of stemness in ER+ breast cancer models. Accordingly, PR knockdown and antiprogestins disrupted PELP1/SRC-3 complexes and blocked PELP1-induced breast CSC outgrowth. Mammary stem cell (MaSC) populations were increased in vivo in MMTV-tTA;TRE-cyto-PELP1 transgenic mice as well as in MMTV-tTA;TRE-hPR-B mice. To better understand PELP1-mediated pathways, we performed RNA-seq on MCF-7 PELP1+ models grown in tumorsphere conditions to enrich for CSC populations (ALDH+, CD44+/CD24-). Cytoplasmic PELP1-expressing cells had a different global gene profile relative to WT PELP1 (i.e. nuclear). Gene sets associated with stem cell biology, hypoxic stress, and cancer metabolism were differentially regulated, including members of the glycolytic bi-functional kinase/phosphatase PFKFB family. Seahorse metabolic phenotyping demonstrated cytoplasmic PELP1 influences metabolism by increasing both glycolysis and mitochondrial respiration. Cytoplasmic PELP1 interacted strongly with PFKFB3 and PFKFB4, and inhibition of PFKFB3 or PFKFB4 kinase activity blocked PELP1-induced tumorspheres and protein-protein interactions with SRC-3. Additionally, antiprogestin and PFKFB inhibitors were synergistic when combined with ER+ targeted therapies. These aspects of PELP1/SRC-3 biology were phenocopied in therapy resistant models (tamoxifen resistant [TamR], paclitaxel resistant [TaxR]). Together, our data suggest that PELP1, SRC-3, PR, and PFKFBs form complexes that reprogram cellular metabolism to drive breast CSC expansion. Identifying the mechanisms that regulate recurrent ER+ tumor cell populations will enable specific targeting within heterogeneous breast tumors and may lead to the development of non-ER targets that can be used in combination with endocrine treatments to overcome therapy resistance.