Characterization of the Bone Marrow Microenvironment During the Development of Triple Negative Breast Cancer Metastases

Abstract

Triple negative breast cancer (TNBC) is a subtype that lacks expression of progesterone and estrogen receptors, and does not over-express human epidermal growth factor receptor 2 (HER2). TNBC is not responsive to standard therapies that target these receptors and is associated with poor prognosis, a high rate of recurrence, and aggressive metastasis. The purpose of this study was to characterize the bone marrow microenvironment during the process of primary tumor growth and metastasis in murine TNBC models. We hypothesize that the expression of the inflammatory chemokines KC/CXCL1 and MCP-1/CCL2 in the bone marrow microenvironment promotes the development of breast cancer metastasis. The 2225LM murine tumor has gene expression patterns comparable to human tumors of the basal-like TNBC phenotype and the T11 murine tumor mirrors the claudin-low TNBC phenotype. In our studies, tumors (1-2 mm) are implanted in either the subcutaneous flank or the mammary fat pad of syngeneic, female mice and are excised ~2 weeks later to reliably generate growth of seeded metastases. Previous data from our laboratory found significantly increased levels of KC/CXCL1 and MCP-1/CCL2 in primary tumors of both tumor lines compared to tissue from naïve mice. In the present study, bone marrow is flushed from femurs and tibias of tumor-implanted mice, separated into cellular and soluble components and assayed by ELISA. MCP-1 is significantly elevated in the extracellular component of bone marrow of T11-implanted mice. KC/CXCL1 is not significantly elevated in either the extracellular or intracellular bone marrow of T11-implanted mice. KC/CXCL1 is expressed by inflammatory cells and osteoblasts and is a chemoattractant for inflammatory cells and certain tumor cells. MCP-1/CCL2 is expressed by a variety of inflammatory cells and myeloid progenitor cells and is a chemoattractant for monocytes/macrophages. Our results are consistent with the premise that an increase in MCP-1/CCL2 in the bone marrow promotes the development of claudin-low breast cancer lung metastasis.