Tyrosine kinase independent actions of DDR2 in tumor cells and CAFs influence tumor invasion, migration, and metastasis
Both tumor cell-intrinsic signals and tumor cell-extrinsic signals from cells within the tumor microenvironment influence tumor cell dissemination and metastasis. The fibrillar collagen receptor tyrosine kinase discoidin domain receptor 2, DDR2, is essential for breast cancer metastasis in mouse models, and high expression of DDR2 in tumor and tumor stromal cells is strongly associated with poorer clinical outcomes. DDR2 tyrosine kinase activity was hypothesized to be required for DDR2's metastatic activity, however, inhibition of DDR2 tyrosine kinase activity, along with other RTKs, has failed to provide clinically relevant responses in metastatic patients. Here, we show that tyrosine kinase-activity independent action of DDR2 in tumor cells can support Matrigel invasion and in vivo metastasis. Paracrine actions of DDR2 in tumor cells and CAFs also support tumor invasion, migration, and lung colonization in vivo. These data suggest that tyrosine kinase independent function of DDR2 could explain failures of TKI treatment in metastatic breast cancer patients and highlight the need for alternate therapeutic strategies that inhibit both tyrosine kinase-dependent and independent actions of RTKs in the treatment of breast cancer.