Abstract 3782: Steroid receptor co-activator complexes cooperate to reprogram metabolic pathways and drive therapy resistance in luminal breast cancer
Authors: Ostrander Julie Hanson , Truong Thu H. , Benner Elizabeth , Dwyer Amy R. , Lange Carol A. 
Published: September 2020

Abstract
Late recurrence of metastatic disease stemming from acquired therapy resistance remains a significant health burden for women with estrogen receptor (ER) positive 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 both normal stem and breast CSC outgrowth. 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 cytoplasmic PELP1-induced CSC expansion and target genes required for cell survival, suggesting an essential role for PELP1/SRC-3 complexes in breast CSC outgrowth. PELP1 also forms a signaling and transcriptional complex with ERα 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. Moreover, mammary stem cell (MaSC) populations were increased in vivo in MMTV-tTA;TRE-cyto-PELP1 transgenic mouse models 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 breast CSC populations (ALDH+, CD44+/CD24-). Cytoplasmic PELP1-expressing cells had a different global gene profile relative to cells expressing 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 that cytoplasmic PELP1 influences cellular 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 tumorsphere formation 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 signaling complexes that reprogram cellular metabolism to drive expansion of breast CSCs. Identifying the molecular mechanisms that regulate recurrent ER+ tumor cell populations will enable specific targeting within heterogeneous breast tumors. Our work may lead to the development of non-ER targets that can be used in combination with current endocrine treatments and PR-targeting strategies to overcome therapy resistance in ER+ breast cancer.
Citation Format: Julie Hanson Ostrander, Thu H. Truong, Elizabeth Benner, Amy R. Dwyer, Carol A. Lange. Steroid receptor co-activator complexes cooperate to reprogram metabolic pathways and drive therapy resistance in luminal breast cancer [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 3782.