Endothelial cells promote triple-negative breast cancer cell metastasis

Endothelial cells (ECs) in the tumor microenvironment have been reported to play a more active role in solid tumor growth and metastatic dissemination than simply providing the physical structure to form conduits for blood flow; however, the involvement of ECs in the process of triple-negative breast cancer (TNBC) metastasis has not been addressed. Here, we demonstrate that ECs-when mixed with TNBC cells-could increase TNBC cell metastatic potency. After treatment with TGF-β to induce endothelial-mesenchymal transition (EMT), TNBC cells could produce plasminogen activator inhibitor-1 (PAI-1) and stimulate the expression and secretion of the chemokine, CCL5, from ECs, which then acts in a paracrine fashion on TNBC cells to enhance their migration, invasion, and metastasis. CCL5, in turn, accelerates TNBC cell secretion of PAI-1 and promotes TNBC cell metastasis, thus forming a positive feedback loop. Moreover, this enhanced metastatic ability is reversible and dependent on CCL5 signaling