Urokinase plasminogen activator (uPA) has been implicated in fibrinolysis and diverse processes that involve cell migration such as atherosclerosis, angiogenesis, wound repair, and tumor metastases. Studies in mice with targeted deletion of the genes for uPA and uPAR indicate that uPA induces transmembrane signaling through pathways discrete from those involved in binding to its glycolipid-anchored receptor (uPAR), but to date the details of this mechanism have not been elucidated. uPA is composed of a receptor-binding growth factor domain, a kringle domain, the function of which is unknown, and a protease domain. Recent studies from our laboratories demonstrate that the kringle of uPA binds to vascular smooth muscle cells (VSMC) and delivers a signal that potentiates uPA-mediated VSMC contraction and cell migration. Further, recognition of the kringle mediates clearance of uPA from cell surfaces, suggesting the kringle domain represents an important control point in VSMC function. We now propose to extend these studies and to examine these newly described properties of the uPA kringle in greater detail through five inter-related specific aims. 1) We will characterize the interaction of the uPA kringle with the low-density lipoprotein receptor/alpha2 macroglobulin receptor which mediates uPA degradation. 2) We will isolate and identify the uPA-kringle binding protein on VSMC. 3) We will examine the kringle-mediated signal transduction pathway in VSMC. 4) We will study the interaction between kringle binding protein-dependent and uPAR-dependent signal transduction events in VSMC using uPA variants capable of activation either or both pathways. 5) We will examine the role of the uPA kringle and kringle binding protein in vascular wall remodeling in a rat model of intravascular trauma. These studies will provide insight into a newly described uPA-mediated signal transduction pathway involved in smooth muscle cell migration and vascular repair. Identification of the kringle binding protein may provide an opportunity to modulate the contribution of vascular smooth muscle cell proliferation and migration to atherosclerosis and restenosis.