PROJECT SUMMARY It is now appreciated that the tumor microenvironment contributes to tumor cell spread and response to therapy. The tumor environment (or stroma) consists of non-tumor cells, structural proteins, and embedded growth factor and cytokines, and in breast tumors these components differ from their normal tissue counterparts in composition, architecture, and function. A major challenge then to the prevention and treatment of breast cancer metastasis is to understand how the tumor environment influences tumor cell invasive/migratory capacity. We have identified a novel collagen I stimulated DDR2 (a RTK) signaling pathway that is active in both tumor cells and tumor stromal cells and a critical regulator of breast cancer metastasis. We have genetically validated DDR2 as a novel and viable therapeutic target for prevention and treatment of metastatic disease. Because of its actions, inhibition of DDR2 activity in tumors should affect both tumor cells, tumor stroma and the communication between tumors and their environment. In tumor cells, we have shown that DDR2 controls collective migration/invasion through the collagen- rich tumor ECM to access blood vessels. In new work we have found that the action of DDR2 in the tumor stroma controls ECM production, collagen fiber organization, and angiogenesis, yet we do not know in which cells within the tumor stroma DDR2 is important for these effects and how. Furthermore, we have found that DDR2 exhibits distinct kinase-dependent and kinase-independent cellular effects that could pose challenges to therapies directed only at inhibiting kinase activity. Since the action of DDR2 within cells of the primary tumor stroma is important for metastasis, this proposal is focused on determining its function(s) in the tumor stroma. There are three specific aims. In the first aim we will determine in which tumor stroma cells the action of DDR2 contributes to changes to tumor fibrosis and collagen organization, and how. Second, we will determine whether the endothelial cell intrinsic action of DDR2 contribute to the breast tumor angiogenesis and how. Finally, in the third aim we will determine the in vivo consequences for metastasis and mechanism(s) of action of kinase-independent and kinase-dependent DDR2 in breast tumor cells and breast tumor CAFs.