Tumor-associated soluble uPAR-directed endothelial cell motility and tumor angiogenesis.

The expression of urokinase-type plasminogen activator (uPA) receptor (uPAR) correlates with the malignant phenotype of various cancers. The soluble form of uPAR (s-uPAR) is present in the circulation of cancer patients, but the role of s-uPAR in endothelial cell migration is poorly understood. Therefore, we examined the role of tumor-associated s-uPAR on endothelial cell motility and angiogenesis. Here, we present evidence that tumor-associated s-uPAR augments the migration of human umbilical vein endothelial cells (HUVECs). When grown on tumor-conditioned medium, the membrane fraction of HUVECs had increased localization of s-uPAR onto its cell membrane. Colocalization studies for GM1 ganglioside receptor and uPAR further demonstrated s-uPAR recruitment onto lipid rafts of HUVECs. Immunoblot analysis for uPAR in lipid raft fractions confirmed s-uPAR recruiting onto HUVECs' membrane. Further, s-uPAR induced Rac1-mediated cell migration while either function-blocking uPAR antibodies or dominant-negative mutant Rac1 expression in HUVECs-mitigated s-uPAR-enhanced cell migration. In addition, orthotopic implantation of uPAR-overexpressing cells resulted in a significant increase in circulating s-uPAR in blood serum and invasive nature of tumor and tumor vasculature in mice. Collectively, this data provide insight into tumor-associated s-uPAR-directed migration of endothelial cells and its subsequent influence on tumor angiogenesis.