Urokinase-receptor/integrin complexes are functionally involved in adhesion and progression of human breast cancer in vivo

Interactions between specific cell-surface molecules, which include the uro kinase receptor (uPAR) and integrins, are crucial to processes of tumor inv asion and metastasis. Here we demonstrate that uPAR and beta1-integrins may cluster at distinct sites at the cell surface of metastatic MDA-MB-231 bre ast cancer cells and form functional complexes. Attachment assays performed in the presence of a synthetic peptide (p25), which interferes with the fo rmation of uPAR-integrin complexes, reveal that uPAR is able to regulate th e adhesive function of integrins in breast cancer cells. On dissociation of the uPAR-integrin complexes by p25, tumor cell attachment to the extracell ular matrix was either decreased (vitronectin) or increased (fibronectin). Moreover, the tumor cells display remarkable morphological changes when cul tured on fibronectin in the continuous presence of p25, leading to increase d cell spreading and attachment. In marked contrast to control conditions, increased cellular adhesion to fibronectin after p25 treatment was entirely beta1-integrin-mediated. The role of uPAR-integrin complexes in tumor prog ression was studied in an in vivo bone xenograft model. Stably transfected MDA-MB-231 cells that overexpress p25 showed a significant reduction in tum or progression in bone (P less than or equal to 0.0001 versus mock-control) . In line with these observations, continuous administration of p25 (25 mug /mouse/day, osmotic minipumps) for 28 days resulted in significantly reduce d tumor progression of MDA-MB-231 cells in bone (P less than or equal to 0. 005) when compared to scrambled control peptide. In conclusion, our data de monstrate that uPAR can act as an adhesion receptor in breast cancer and is capable of regulating integrin function. Our findings strongly suggest tha t adhesive and proteolytic events are tightly associated in metastatic brea st cancer cells and that functional integrin-uPAR complexes are involved in tumor progression in vivo.