SAT-311 Investigating the Effects of Non-Estrogenic Reproductive Hormones on Human Estrogen Receptor-Positive Breast Cancer Bone Metastases in a Murine Model

Abstract Disclosure: A.L. Babcock: None. J.B. Frye: None. S.A. Whitman: None. A.M. Rossi: None. J.N. Farr: None. S. Ehsani: None. J.L. Funk: None. The role of estrogen in driving the progression of estrogen receptor-positive (ER+) breast cancer (BC) is well-established, yet pre-clinical studies investigating the involvement of tumoral progesterone (PR) and androgen (AR) receptors show conflicting results. This inconsistency is likely due to the promiscuity of steroid receptor ligands and the varying levels of steroid hormones and their receptors within the tumor microenvironment. Although bone is the primary site of metastasis in ER+ metastatic breast cancer, the specific contributions of tumoral PR and AR receptors (found in 59% and 87% of clinical ER+/HER2- bone metastases [BMET], respectively) to progression at this site remain poorly understood. To examine this question, studies were conducted to assess the effects of ovariectomy (OVX), a model of surgical menopause that significantly lowers endogenous estrogen, as well as progesterone and androgen levels, in a pre-clinical model of human ER+ bone metastases that is dependent on supplementation with 17ß-estradiol (E2). In vivo E2-driven osteolytic BMET progression (radiographic and histologic) was compared in OVX (at 9 weeks of age) vs ovary-intact mice using skeletally mature female nude mice supplemented with a low dose of E2 (0.36 mg 60-day pellets) before intracardiac inoculation (at 10 weeks of age) with BMET-derived human BC ER+ MCF7 cells (43.4M cells). Effects of OVX on E2-supplemented orthotopic tumor progression was also examined. Human ER+ MCF7 cells and MCF7-BMET derived cell lines expressed PR and AR in vitro as determined by Western analysis, albeit with lower levels of AR and the PR B isoform in most BMET-derived lines, including 43.4M cells used here. In vivo, the incidence of osteolytic ER+ 43.4M BMET formation was the same in E2-supplemented OVX vs intact mice. However, osteolytic lesion size was reduced by 40.6% in OVX mice. Bone mineral density (BMD) of tumor-free proximal femurs, while slightly lower in E2-supplemented OVX (vs intact mice), was not statistically different. PR and AR proteins were detected in BMET in OVX and intact mice by immunohistochemical staining. However, staining intensity was reduced in OVX’d mice. The anti-tumoral effects of OVX extended beyond bone; E2-driven orthotopic 43.4M tumor growth was also reduced (40% reduction in tumor weight, p=0.03). These results are consistent with the hypothesis that tumoral AR and PR signaling may enhance bone metastasis progression in this human ER+ BMET model. Furthermore, while ovariectomy to reduce estrogen levels is a common method for modeling natural menopause in pre-clinical studies, effects on other steroid receptor signaling pathways may have lower relevance for modeling ovary-intact menopause. Experiments are ongoing to examine effects of a follicle-deplete, ovary-intact model of menopause on ER+ BMET progression. Presentation: Saturday, July 12, 2025