Effect of nonmalignant accessory cells from the metastatic microenvironment on breast cancer cell resistance to antiestrogens.

Abstract

11039 Background: To address the role of the local metastatic microenvironment in the antiestrogen (AE) resistance exhibited by disseminated breast cancer (BrCa), we assembled heterotypic in vitro three-dimensional (3D) tissue cultures comprised of estrogen receptor-positive (ER+) BrCa cells and non-malignant accessory cells from organs frequently targeted by metastatic disease, to model the composition and architecture of metastatic lesions. Methods: MCF7 cells expressing luciferase, cultured alone or with accessory cells, were exposed to dose-ranges of hydroxytamoxifen (4-OHT), fulvestrant or raloxifene. MCF7 cells were also injected s.c. in nude mice either alone or with HS-5 bone marrow stromal cells (BMSCs). Tumor spheroid viability or xenograft growth were measured by bioluminescence. Results: MCF7 response to AEs in 3D conditions (collagen type I and/or Matrigel) was marked by acinar differentiation of tumor spheroids that resembles normal breast epithelium. Co-culture of MCF7 and immortalized accessory cells from the bone (BMSCs; HOBIT and hFOB osteoblasts), brain (SVGp12 astrocytes), liver (THLE3 hepatocytes), and lung (MRC9 fibroblasts) in 3D (but not 2D) conditions, conferred resistance to 4-OHT, raloxifene, and fulvestrant at clinically relevant doses. Heterotypic xenografts (MCF7 + BMSCs) had reduced response to tamoxifen compared with monotypic xenografts (MCF7 alone). BMSCs induced in MCF7 cells downregulation of TGFβ2; upregulation of a transcriptional signature enriched for genes associated with high-grade tumors, including genes involved in interferon response, signaling through EGFR superfamily members, NFkB and other antiapoptotic pathways; and elevated c-Myc protein levels. Exogenous TGFβ2 or neutralizing antibodies against INFγ partially re-sensitized MCF7 cells to 4-OHT in co-cultures. Conclusions: Our results indicate that accessory cells representing the microenvironment of metastatic sites confer AE resistance to BrCa cells in 3D tissue cultures and xenograft models. The mechanisms of stroma-induced AE resistance may reveal new therapeutic targets for refractory BrCa patients with metastatic disease.