br Results br Discussion MSCs derived from the bone

Results
Discussion MSCs derived from the bone marrow and adipose tissue have been shown to promote the growth and metastatic ability of breast cancer and other human malignancies, but the mechanisms are incompletely understood (Barcellos-Hoff et al., 2013, Cuiffo et al., 2014, Del Pozo Martin et al., 2015, Li et al., 2012, Liu et al., 2011). Here, we discover that the collagen receptor DDR2 is upregulated in breast cancer metastasis-associated MSCs and in metastatic breast cancer Tideglusib in clinical samples. Our study shows that the collagen receptor DDR2 mediates MSC-cancer cell crosstalk to enhance breast cancer proliferation, migration and metastasis, at least in part through induction of collagen type I deposition and activation of DDR2 signaling in breast cancer. While previous studies have focused on the role of the primary tumor stroma in cancer progression (Barcellos-Hoff et al., 2013, Cuiffo et al., 2014, Karnoub et al., 2007, Li et al., 2012, Liu et al., 2011), the functions of the metastasis-associated stroma are largely unknown, in part due to the lack of physiologically relevant models. MSCs have been shown to migrate from the bone marrow to the primary and metastatic tumor site in mice, but have been only recently found in human gastric cancer metastasis (Liu et al., 2011, Zhou et al., 2016). We have successfully isolated MSCs from breast cancer metastases, providing evidence that MSCs are present in the breast cancer metastatic niche in humans. Our study directly demonstrates that MSCs migrate with metastatic breast cancer cells to establish the distant metastases. These data advance the understanding of metastasis development and may have implications in the design of anti-metastatic strategies. We find that metastasis-associated MSCs have a normal karyotype, have similar surface marker expression as bone marrow- and adipose-derived MSCs, and exhibit DDR2 pathway activation. DDR2 is a tyrosine kinase receptor expressed in mesenchymal cells, uniquely activated by fibrillar collagens, which are the main components of the ECM (Alves et al., 1995, Fu et al., 2013, Ikeda et al., 2002, Valiathan et al., 2012). DDR2 participates in ECM remodeling during morphogenesis and tissue repair, as well as differentiation and proliferation (Márquez and Olaso, 2014, Olaso et al., 2002). The profound effect of DDR2 in mesenchymal cell and matrix homeostasis is underscored by the finding that mice homozygous for the slie mutation, which removes ddr2, exhibit dwarfism (Kano et al., 2008, Labrador et al., 2001). Recently, DDR2 has been identified as one of 14 genes differentially expressed in bone-marrow-derived MSCs compared to hematopoietic stem cells (Anam and Davis, 2013). However, the function of DDR2 in MSCs has remained unexplored. In this study, we show that DDR2 is a critical regulator of MSC phenotype, collagen I deposition function, and migration capacity. Several studies have demonstrated that DDR2 overexpression and signaling activation in breast cancer cells promotes tumor progression, sparking interest in DDR2 tyrosine kinase as a target for the treatment of breast and other malignancies (Badiola et al., 2011, Kim et al., 2015, Valiathan et al., 2012, Zhang et al., 2013). While no DDR2 mutations have been identified in breast cancer, our lab and other investigators find that DDR2 is overexpressed in over 50% of invasive breast carcinomas compared to none of the normal breast epithelium. In breast cancer samples, DDR2 overexpression in the cancer cells is associated with high collagen in trichrome stains and worse patient survival (Toy et al., 2015, Zhang et al., 2013). Of note, DDR2 is also expressed in the stromal cells of the breast cancer microenvironment (Toy et al., 2015, Zhang et al., 2013). A recent study has demonstrated that in mice, DDR2 expression in cancer cells and in cells of the host tumor microenvironment, including cancer-associated fibroblasts, is critical for breast cancer metastasis in the MMTV-PyMT model (Corsa et al., 2016). However, the mechanisms of DDR2 function in the tumor microenvironment are unclear, and the relationship between DDR2 in stromal and cancer cells has not been considered.