Abstract 2042: Modeling estrogen receptor-positive breast cancer bone metastasis to query osteolytic effects of tumor ER signaling

Abstract

Abstract Estrogen receptor α-positive (ER+) breast cancers (BrCAs) have the greatest predilection for forming clinically evident bone metastases (BMETs). To query ERα’s role in osteolytic BrCA BMET progression, tumoral vs bone microenvironment effects of 17β-estradiol (E2) were determined in a human xenograft murine model of ER+ BrCA BMET. Female athymic mice aged 4 weeks (young) or 15 weeks (mature) were inoculated with 1x105 human ER+ MCF-7 BrCA cells via the left cardiac ventricle 2 days post-placement of 60-day release E2 pellets (0.05, 0.10, 0.18, 0.36, & 0.72 mg E2). Osteolytic BMET formation was assessed radiographically and E2effects on bone were determined by DXA and microCT at various time points. The BMET size and proliferation were assessed by immunohistochemical (IHC) analyses at the end of the experiment (42 days). The effect of E2 on tumoral secretion of the osteolytic factor, parathyroid hormone related protein (PTHrP), was determined using a commercially available radioimmunoassay. The incidence and size of osteolytic BMET, which were not evident in the absence of E2 supplementation, were E2 dose-dependent in young mice. In contrast, E2 effects on the bone microenvironment were not dose-dependent, and resulted in identical increases in bone mineral density (BMD) and bone volume (BV/TV). In skeletally immature (young) mice vs skeletally mature mice treated with an identical E2 dose (0.72 mg), which caused significantly different effects on bone turnover, progression of E2-dependent BMET was identical. These results suggest that E2 effects on the tumor, rather than bone, were driving E2-dependency of BMET progression. IHC analysis demonstrated that neither the size of human cytokeratin-positive tumors nor the proportion of Ki67-positive proliferating tumor cells in bone were E2 dose-dependent, suggesting that proliferative effects of E2 could not explain E2-dose dependent differences in osteolytic BMET formation. PTHrP, an osteolytic factor expressed in most clinical BrCA BMET, was E2-inducible ex vivo and secreted in higher levels by tumor cells isolated from BMETs. These results suggest that during ER+ BrCA BMET progression, tumoral effects of E2 not only support ER+ BMET proliferation, but may also have bone-specific effects due to the induction of PTHrP, which may explain the marked osteolytic capacity of the ER+ tumor cells in this model despite a microenvironment where bone volume is markedly increased. Citation Format: Julia N. Cheng, Jennifer B. Frye, Susan A. Whitman, Andrew G. Kunihiro, Madison M. Egan, Julia A. Brickey, Lily A. Alvord, Janet L. Funk. Modeling estrogen receptor-positive breast cancer bone metastasis to query osteolytic effects of tumor ER signaling [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2042.