Discovery of New Small Molecules Targeting the Vitronectin-Binding Site of the Urokinase Receptor That Block Cancer Cell Invasion

Abstract

Besides focusing urokinase (uPA) proteolytic activity on the cell membrane, the uPA receptor (uPAR) is able to bind vitronectin, via a direct binding site. Furthermore, uPAR interacts with other cell surface receptors, such as integrins, receptor tyrosine kinases, and chemotaxis receptors, triggering cell-signaling pathways that promote tumor progression. The ability of uPAR to coordinate binding and degradation of extracellular matrix (ECM) and cell signaling makes it an attractive therapeutic target in cancer. We used structure-based virtual screening (SB-VS) to search for small molecules targeting the uPAR-binding site for vitronectin. Forty-one compounds were identified and tested on uPAR-negative HEK-293 epithelial cells transfected with uPAR (uPAR-293 cells), using the parental cell line transfected with the empty vector (V-293 cells) as a control. Compounds 6 and 37 selectively inhibited uPAR-293 cell adhesion to vitronectin and the resulting changes in cell morphology and signal transduction, without exerting any effect on V-293 cells. Compounds 6 and 37 inhibited uPAR-293 cell binding to vitronectin with IC50 values of 3.6 and 1.2 μmol/L, respectively. Compounds 6 and 37 targeted S88 and R91, key residues for uPAR binding to vitronectin but also for uPAR interaction with the fMLF family of chemotaxis receptors (fMLF-Rs). As a consequence, compounds 6 and 37 impaired uPAR-293 cell migration toward fetal calf serum (FCS), uPA, and fMLF, likely by inhibiting the interaction between uPAR and FPR1, the high affinity fMLF-R. Both compounds blocked in vitro ECM invasion of several cancer cell types, thus representing new promising leads for pharmaceuticals in cancer.