Unraveling the role of estrogen signaling in the breast cancer-bone connection
Authors: Funk Janet , Frye Jennifer , Whitman Susan , Thomas Albiya , Ameneni Geethika , Magee Alyssa , Cheng Julia 
Published: May 2023

Estrogen receptor-alpha positive (ER+) breast cancer is uniquely associated with a high rate of bone metastases (BMET). ER+ breast cancer cells disseminate early to bone where they elude treatment by remaining dormant for decades before poorly understood activation events likely involving ER signaling lead to BMET progression. A lack of relevant pre-clinical ER+ BMET models has impeded our ability to understand these processes and thus identify cures. Having recently demonstrated a specific role for tumoral ER-alpha signaling in promoting tumor-associated osteolysis and BMET progression independent of proliferative effects in nude mouse models of ER+ human breast cancer, we further developed these models to explore effects of ER genomic (nuclear) vs non-genomic membrane-initiated steroid signaling (MISS) on other aspects of ER+ BMET biology, including dormancy. Female nude mice, which require estrogen supplementation for human ER+ tumor growth, were inoculated (intracardiac) with bone-tropic MCF7 cells and treatments effects of 17-ß estradiol (E2, a nuclear and MISS agonist), or estetrol (E4, a fetal estrogen with nuclear agonist/MISS antagonist activity) were compared to vehicle-treated controls. When supplementation began prior to ER+ tumor cell inoculation, neither an E2 dose that can drive MCF7 BMET progression (0.72 mg 60-day pellets) nor an E4 dose that can drive orthotopic MCF7 tumor progression (4 mg/kg/d po) altered bone dissemination of membrane-labeled (Vybrant DiD) MCF-7 cells 72h post-inoculation (p<0.05). In non-supplemented mice, bone-disseminated DiD+ MCF7 cell numbers remained stable for up to one month (p<0.05) without BMET formation, at which time E2 administration triggered a 2.2-fold increase in DiD+ cell number (day 7, p<0.05) followed by robust osteolytic BMET formation (60% incidence by day 28). When mice were instead supplemented beginning 3-days prior to MCF7 cell inoculation, E2-treated mice formed BMET with a high incidence (83% by histology), osteolytic lesions on x-ray (83%), and evidence of anabolic bone effects (18.4% BMD increase [remote from BMET]; 64% increase in osteoblast cell number/mm bone, p<0.05). In contrast, in E4-treated mice, histologic BMET were much smaller with a low incidence (43%), no evidence of osteolytic lesions, and reduced or absent anabolic bone effects (8.1% BMD increase with unchanged osteoblast cell #/mm bone [p<0.01, E2 vs E4]). In conclusion, these studies suggest that ER signaling does not promote bone dissemination but can reactivate dormant ER+ breast cancer cells within bone, initiating a process wherein both nuclear and MISS ER signaling may drive BMET progression. Additional experiments are required to dissect the relevance of tumor vs. bone ER MISS in ER+ BMET and to elucidate the role of ER MISS more broadly as a mediator of estrogenic effects on bone. Metavivor Foundation, NIH (R01CA174926; R03CA181893) This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.