The most life-threatening aspects of cancer are invasion and metastasis. In recent decades, evidence has been accumulating showing that the urokinase-type plasminogen activator (uPA) and its glycolipid-anchored cell surface receptor (uPAR) are engaged in various signal transduction events related to growth, invasion, and metastasis of malignant tumors. The evidence comes from the observation that high levels of uPA and uPAR in human tumors is associated with a poor patient prognosis, the basic idea being that the serine proteinase uPA released in tumors catalyses the activation of plasmin, which in turn catalyses degradation of extracellular matrix (ECM), and thus facilitates cancer cell invasion into surrounding tissue. Surprisingly, a high tumor level of the uPA inhibitor plasminogen activator inhibitor-1 (PAI-1)is also associated with a poor prognosis, the PAI-1 level in fact being one of the most informative biochemical prognostic markers. In addition to inhibiting uPA, PAI-1 interacts with vitronectin (VN), which is a relatively unique adhesive protein not only because PAI-1 binds to it with high affinity and specificity but also because cells can attach to it through integrins, uPAR, or both. The binding of PAI-1 and uPAR to VN clearly suggests a functional interaction between VN and the plasminogen activation system. A long-term goal of our laboratory is to define these functional interactions between VN, uPAR, and integrins (i.e., αVß3 and αVß5), and to understand to what extend PAI-1 contributes to the regulation of these interactions. Since cell migration is a critical component of invasion and metastasis, and it is clear that regulated changes in the affinity state of adhesion receptors (e.g., integrins and uPAR) are essential for optimal cell motility, our group is especially interested in the role of PAI-1 in the regulation of cell adhesion. We have expressed a variety of recombinant VN variants and PAI-1 mutants and established several cell lines from mouse strains which are genetically deficient in VN, PAI-1 or uPAR. Analysis of theses and other, tumor-derived cell lines in in vitro migration and invasion assays revealed a highly uPAR-dependent mechanism by which PAI-1 induces cell detachment from the ECM. We found that PAI-1 is able to induce deactivation of uPAR and matrix-engaged integrins, and to promote their endocytosis and recycling (Czekay et al., 2004). This novel deadhesive activity of PAI-1 is directed against a variety of integrins and suggests a role for PAI-1 in promoting rapid cycles of cell attachment and cell detachment, thus increasing cell motility. Interestingly, this process is independent of the role of PAI-1 as an enzymatic inhibitor of uPA. We are in the process of establishing adenoviral/retroviral constructs to regulate expression levels of PAI-1, integrins, and uPAR to better understand the precise conditions needed for the deadhesive function of PAI-1. In this regard, we are developing an in vivo mouse tumor model in order to investigate the contribution of the functional interaction between uPAR and integrins, and between those adhesion receptors and the ECM (e.g., VN). As a new interest in the laboratory we will focus on the identification of signaling pathways that are activated by either uPAR/integrin or uPAR/integrin/VN interaction and on how they may contribute to increased cell migration and invasion.