OR16-2 Steroid Receptor Co-Activator Complexes Regulate Metabolic PFKFB Enzymes to Drive Therapy Resistant ER+ Breast Cancer

Recurrence of metastatic breast cancer stemming from acquired endocrine and chemotherapy resistance remains a health burden for women with luminal (ER+) breast cancer. ER+ tumor cells can remain viable but quiescent for years to decades. Contributing factors to metastatic spread include the survival and expansion of breast cancer stem cells (CSCs). Breast CSCs are poorly proliferative and frequently exist as a minority population in recurrent tumors. Our objective is to define novel signaling pathways that govern therapy resistance in ER+ breast cancer. We previously showed that cytoplasmic complexes composed of steroid receptor (SR) co-activators, PELP1 and SRC-3, modulate breast CSC expansion through upregulation of the HIF-activated metabolic target genes PFKFB3 and PFKFB4. PELP1 interacts with PFKFB proteins, and inhibition of PFKFB3 and PFKFB4 kinase activity blocks PELP1-induced tumorspheres and protein-protein interactions with SRC-3. PFKFB inhibitors in combination with ER targeted therapies blocked proliferation and tumorsphere formation in multiple models of advanced breast cancer, including paclitaxel (TaxR) and tamoxifen (TamR) resistant models and ER+ patient-derived organoids (PDxO). Chemotherapy and endocrine therapy resistant models contained increased levels of cytoplasmic PELP1, PELP1/SRC-3 complexes, and phenocopied PELP1/SRC-3/PFKFB biology and CSC behavior. To better understand CSC-mediated pathways in the context of therapy resistance, we performed RNA-seq on 3D-cultured TaxR tumorspheres to enrich for CSCs. Pathway analysis revealed that TaxR tumorspheres upregulate stem (Sox2-Oct4-Nanog), HIF, and progesterone mediated pathways. Progesterone receptor (PR) is a known contributor to CSC populations, and we observed increased PR mRNA and protein expression in TaxR cells grown as tumorspheres. Additionally, in vivo emergence of circulating tumor cell (CTC) populations was observed in ER+ TaxR mammary intraductal (MIND) xenografts. Together, our data suggest that PELP1, SRC-3, and PFKFBs cooperate to drive ER+ tumor cells that include CSCs and CTCs. Our findings are applicable to therapy resistant models, indicating that SR co-activator complexes are a key mediator of resistance in ER+ breast cancer and may cooperate with PR to promote therapy resistant CSCs. Identifying non-ER pharmacological targets offers a useful approach to blocking metastatic escape from standard of care ER/estrogen (E2)-targeted strategies to overcome endocrine and chemotherapy resistance in ER+ breast cancer. Presentation: Sunday, June 12, 2022 11:15 a.m. - 11:30 a.m.