DDR2 positive endothelial cells promote angiogenesis and metastasis in breast tumors

The collagen receptor DDR2, a receptor tyrosine kinase, contributes to breast cancer progression through actions in both invasive tumor cells and tumor stromal cells. In the tumor stroma, DDR2 is predominantly expressed by cancer associated fibroblasts (CAFs) and therein contributes to the formation of a tumor permissive ECM which facilitates metastasis. Through an analysis of human breast tumor scRNAseq data and multiplex immunohistochemistry, we noted that DDR2 is also expressed by a subset of tumor CD31 + endothelial and LYVE1 + /CD31 + lymphatic cells. To determine whether this endothelial cell DDR2 + population contributes to breast cancer progression, and if so how, we genetically depleted Ddr2 fl/fl from endothelial cells using inducible Cdh5-Cre ERT2 in the context of MMTV-PyMT mouse breast tumor model. In endothelial Ddr2 -/- breast tumors there was no significant difference in primary tumor burden or total number of CD31 + endothelial cells within the tumor; however, there was a significant reduction in lung metastases. Closer analyses revealed a decrease in CD31 + vessel length and area in primary tumors from these mice and an associated increase in hypoxia. When VEGF-containing Matrigel plugs were implanted subcutaneously in ubiquitous DDR2 null mice, CD31 + vessel growth into the plug was reduced. Vessel outgrowth from aortic rings from global DDR2 null mice was decreased. These data suggested some migratory defect in Ddr2 -/- endothelial cells. To test this possibility directly, we developed, characterized, and utilized a microfluidic device wherein ECs are subjected to mechanical fluid flow within a 3D fibrin matrix. Genetic depletion of Ddr2 and antibody inhibition of DDR2 both resulted in reduced EC migration. In summary, we find that DDR2 is present in a subset of ECs, possibly tip cells, and impacts tumoral and non-tumoral angiogenesis in vivo and in vitro.