LAUSR

Estrogen receptor α-positive (ER+) breast cancers (BrCAs) have the greatest predilection for forming bone metastases (BMETs). To begin to query ERα9s 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. For these studies, naive or 17β-estradiol (E2)-treated 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). For studies examining E2 effects on bone, MCF-7 cells were not inoculated. Osteolytic lesion formation was assessed by hind limb radiographs, and E2 effects on bone were assessed by DXA (Faxitron), μCT (Scanco), and biochemical markers of bone formation or resorption (P1NP and TRAcP, respectively; Immunodiagnostic Systems). BMET incidence and lesion size in young mice were E2 dose-dependent, achieving 100% incidence at the highest dose tested, with smaller lesions and lower incidence occurring in response to lower doses, and no lesions forming in the absence of E2 even after 8 months. 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), primarily via an increase in bone formation. To further query the importance of tumoral vs bone effects of E2 in driving ER+ BrCA progression, the tumoral effects of an identical E2 dose (0.72 mg) were compared in the skeletally immature young mice, which accrued increased bone (BV/TV) due to increases in bone formation in response to E2, vs skeletally mature mice, where E2-induced increases in bone were less marked and primarily driven by decreases in bone resorption. In these two very different bone milieus, progression of E2-dependent BMET was identical. While the relative roles of E2-induced tumor cell proliferation vs osteolytic factor secretion cannot be distinguished here, the development of osteolytic lesions similar in size to those occurring in ER- models driven by tumoral secretion of osteolytic factors, despite a marked increase in bone volume, suggests that tumoral effects of E2 may drive osteolytic ER+ BrCA BMET progression in a bone-specific fashion. Citation Format: Julia N. Cheng, Jennifer B. Frye, Susan A. Whitman, Andrew G. Kunihiro, Madison M. Egan, Julia A. Brickey, Janet L. Funk. Tumor-specific effects of estrogen drive a murine model of human estrogen receptor-positive breast cancer bone metastasis independent of bone microenvironment changes [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2116.