Abstract 5236: Development of chronic disease model of metastatic estrogen receptor positive (ER+) breast cancer (BC)
Authors: Ganapathy Vidya , Banach-Petrosky Whitney , Xie Wen , Nienhuis Hilde , Reiss Michael 
Published: June 2012

Abstract
Expression of estrogen and progesterone receptors (ER, PR) in metastatic BC (MBC) strongly predicts for estrogen-dependence and, hence, for response to estrogen withdrawal (EWD) and to pharmacological anti-estrogens (AE). However, the choice of agents and their sequence is empiric and lacks a solid scientific foundation, largely because we lack in vivo models that accurately replicate chronic human ER+ MBC. In order to generate bone(marrow) metastasis models, highly bone tropic MCF7 or T47D cells (HER2-) or BT474 (HER2+) ER+, PR+ BC cells were selected in vivo by repeated rounds of intratibia injection, harvest and reinjection in nude mice. Histologically, MCF-7- and T47D-derived tumors fill the marrow sinusoids, with minimal osteolysis. In contrast, ER+, HER2+ BT474 cells give rise to expansile osteolytic metastases associated with osteoclast activation, reminiscent of the phenotype seen in basal-like MBC models. In order to test the estrogen-dependence of bone metastases in vivo, bone tropic MCF7 cells were injected into the tibiae of oophorectomized mice. One group received estradiol (E2) supplementation, while the control group did not. Serial in vivo imaging confirmed the appearance of mixed osteolytic and -blastic bone lesions in the E2-treated group but not in the control group. In addition, we were able to detect regional metastases to the adjacent femur and fibula. At 12 weeks, E2 pellets were removed from the treatment group. E2 deprivation induced rapid resolution of lytic lesions and pathological fractures associated with dramatic callus formation. The experiment was terminated 18 weeks later. Histologically, the residual tibial tumors were epithelioid, and still expressed cytokeratin, E-cadherin, ER and PR. Primary cell lines were established from 5 of the 5 animals. Of these, 2 were still E2-dependent in vitro and ER+, PR+. Of the 3 remaining E2-independent lines, one was ER-, PR-, and one was ER+, PR-. To determine whether or not the MCF7 cells inoculated into the control mice had retained viability and tumorigenic potential even in the absence of exogenous estrogen, E2 pellets were introduced at 12 weeks after intra-tibia injection. Ten weeks later, histological analysis failed to detect any tumor growth. However, we were able to isolate a new tertiary MCF7 subclone from 1 of 5 of these mice. This cell line is E2-dependent in vitro and ER+, PR+. In conclusion, bone-tropic MCF7-derived BC clones retain E2-dependence in a bone marrow microenvironment in vivo. Skeletal metastases respond rapidly to EWD, followed by the emergence of E2-independent subclones. In addition, in the absence of E2, bone tropic MCF7 cells are able to remain dormant but viable in a bone marrow microenvironment in vivo for prolonged periods of time. Thus, this in vivo mouse model faithfully reproduces chronic ER+, PR+ MBC and will allow us, for the first time, to develop rational treatment strategies for this disease.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 5236. doi:10.1158/1538-7445.AM2011-5236