Distant metastases occur in ∼20% of patients with breast cancer, originate in all breast cancer subtypes, and are the main cause of death in patients. Inhibiting metastatic spread or halting the growth and invasiveness of established metastasis improves survival.

The stroma associated with breast cancer metastasis consists of extracellular matrix (ECM) proteins, including fibrillar collagens, and a cellular component comprised of mesenchymal stem/multipotent stromal cells (MSCs), vessels, and cells of the immune system. MSCs are stromal progenitor cells that participate in tissue maintenance under normal conditions and mediate pathological stromal responses in injury repair and tumorigenesis.

MSCs are recruited to the primary tumor site from the bone marrow in response to tumor derived soluble factors to promote cancer progression through mechanisms that involve cytokines and collagen deposition . Likewise, adipose-derived MSCs have been shown to enhance tumor volume of MDA-MB-231 xenografts and promote cell proliferation and invasion of ovarian cancer. However, the underlying mechanisms of MSC function need further investigation.

Fibrillar collagen is an important component of the primary tumor and the metastasis-associated stroma. In human breast cancer and transgenic mouse models, increased collagen in the tumor stroma correlates with development of metastases. In primary tumors, increased collagen enhances stromal stiffness to promote breast cancer cell invasion and provides a path for cell migration, especially at the tumor edges.

In addition to this mechanical effect, increased collagen initiates intracellular signaling to promote tumorigenesis through specific receptors, including integrins and discoidin domain receptors (DDRs). At present, the role of collagen-initiated signaling at the metastatic site is unknown.

DDR1 and DDR2 are collagen receptor tyrosine kinases: DDR1 is activated by fibrillar (mainly type I and V) and non-fibrillar (type IV) collagen, and DDR2 is exclusively activated by fibrillar collagen. In normal tissues, DDR1 is expressed in epithelial cells, and DDR2 is expressed only in mesenchymal stromal cells, where it regulates ECM synthesis and participates in wound healing. Pathologic DDR2 upregulation has been reported in several human malignancies, including breast cancer, where it is significantly associated with worse survival. In breast cancer cells, DDR2 has been shown to maintain Snail1 activity and an epithelial-to-mesenchymal transition (EMT) phenotype. DDR2 expression in cancer cells and cancer-associated fibroblasts is important for metastasis in the mouse mammary tumor virus-polyoma middle T antigen (MMTV-PyMT) model. However, the underlying mechanisms leading to DDR2 upregulation in cancer, as well as the role of MSC-derived DDR2 in breast cancer progression are unknown.

Authors in the journal Cell Reprots show that DDR2 protein is concordantly upregulated in MSCs and cancer cells in clinical samples of human breast cancer metastasis to several organs. DDR2 expression in MSCs derived from breast cancer metastasis (Met-MSCs) regulates stromal phenotype and function and mediates the effect on breast cancer cell neoplastic functions.

Homozygous slie mutated mice with absent ddr2 developed significantly fewer and smaller syngeneic breast cancer metastases compared to heterozygous slie and wild-type mice. The data reveal that MSC-derived DDR2 initiates a stroma-cancer signaling axis leading to DDR2 upregulation in breast cancer and enhancing growth of metastasis.

Authors provide the foundation to block stromal DDR2 as a potential therapeutic strategy for metastatic breast cancer.