Triple-Negative Primary Breast Tumors Induce Supportive Premetastatic Changes in the Extracellular Matrix and Soluble Components of the Lung Microenvironment

The lung is one of the deadliest sites of breast cancer metastasis, particularly in patients with triple-negative (TN) disease. We hypothesized that the presence of a TN primary breast tumor induces changes in the extracellular matrix (ECM) and soluble components of the lung microenvironment that support metastatic behavior. SUM159 (TN) and MCF7 (luminal A) breast cancer cells were injected into mice, and primary breast tumors were established prior to assessing metastatic niche changes. We observed increased CD117(+) hematopoietic progenitor cells in the bone marrow of SUM159 mice versus MCF7 or control mice (p < 0.05). Relative to mice bearing MCF7 tumors and non-tumor controls, mice bearing SUM159 tumors demonstrated enhanced expression of ECM proteins in the lung (fibronectin, tenascin-c and periostin), with similar changes observed in lung fibroblasts treated with extracellular vesicles (EVs) from TN breast cancer cells (p < 0.05). Exposure to lung-conditioned media (LCM) from SUM159 tumor-bearing mice resulted in increased migration/proliferation of both SUM159 and MCF7 cells relative to the control (p < 0.05). In contrast, LCM from MCF-7 tumor-bearing mice had no such effect. LCM from SUM159 tumor-bearing mice contained 16 unique proteins relative to other LCM conditions, including the metastasis-associated proteins CCL7, FGFR4, GM-CSF, MMP3, thrombospondin-1 and VEGF. These findings suggest for the first time that the TN breast cancer molecular subtype may be an important determinant of premetastatic changes to both the ECM and soluble components of the lung, potentially mediated via breast cancer-derived EVs.