Urokinase receptor-dependent upregulation of smooth muscle cell adhesion to vitronectin by urokinase

The plasminogen activator system has been implicated in the modulation of t he response to vascular injury. Although urokinase-type plasminogen activat or (uPA) and its receptor (uPAR) may enhance matrix degradation as well as migration and invasion by smooth muscle cells (SMCs), their roles in cell a dhesion are uncertain. Therefore, we examined the ability of uPA and uPAR t o modulate adhesion of cultured human vascular SMCs to various matrices. We demonstrated a dose-dependent stimulation of adhesion by single-chain uPk (scuPA) to vitronectin (maximum 1.55-fold [+/-0.04-fold] increase, 10 nmol/ L, P<0.002) but not to laminin, collagen I, or collagen IV. Baseline adhesi on to vitronectin was completely inhibited by both EDTA and RGD peptide but was restored to >40% of control in the presence of scuPA (P=0.001 and 0.04 6, respectively). Adhesion to vitronectin was also significantly enhanced b y the amino-terminal fragment of uPA (P=0.007) and two-chain, high-molecula r-weight uPA (P<0.01) but not by the low-molecular-weight fragment of uPA, which lacks the receptor-binding domain. Aprotinin, a plasmin inhibitor, ha d no effect on baseline or scuPA-stimulated adhesion, suggesting a plasmin- independent process. Preincubation of scuPA with soluble uPAR inhibited scu PA stimulation of adhesion by 88+/-14% (P=0.01), as did pretreatment of SMC s with phosphatidylinositol-specific phospholipase C, which removes glycoph osphatidylinositol-anchored proteins, including uPAR. Antibodies to both al pha(v)beta(5) and alpha(v)beta(5) integrin inhibited baseline adhesion but not scuPA stimulation. Finally, coating plates with scuPA alone enabled cel l adhesion, which could be inhibited by both soluble uPAR and anti-uPAR ant ibodies. These data suggest that uPA stimulates adhesion of SMCs specifical ly to vitronectin and that it is mediated by an interaction with uPAR. Upre gulation of both proteins after vascular injury may facilitate migration th rough stimulation of both matrix degradation and cell adhesion.