Abstract
Binding of typeโ1 plasminogen activator inhibitor (PAIโ1) to cell surface urokinase (uPA) promotes inactivation and internalization of adhesion receptors (e.g., urokinase receptor (uPAR), integrins) and leads to cell detachment from a variety of extracellular matrices. In this report, we begin to examine the mechanism of this process. We show that neither specific antibodies to uPA, nor active site inhibitors of uPA, can detach the cells. Thus, cell detachment is not simply the result of the binding of macromolecules to uPA and/or of the inactivation of uPA. We further demonstrate that another uPA inhibitor, protease nexinโ1 (PNโ1), also stimulates cell detachment in a uPA/uPARโdependent manner. The binding of both inhibitors to uPA leads to the specific inactivation of the matrixโengaged integrins and the subsequent detachment of these integrins from the underlying extracellular matrix (ECM). This inhibitorโmediated inactivation of integrins requires direct interaction between uPAR and those integrins since cells attached to the ECM through integrins incapable of binding uPAR do not respond to the presence of either PAIโ1 of PNโ1. Although both inhibitors initiate the clearance of uPAR, only PAIโ1 triggers the internalization of integrins. However, cell detachment by PAIโ1 or PNโ1 does not depend on the endocytosis of these integrins since cell detachment was also observed when clearance of these integrins was blocked. Thus, PAIโ1 and PNโ1 induce cell detachment through two slightly different mechanisms that affect integrin metabolism. These differences may be important for distinct cellular processes that require controlled changes in the subcellular localization of these receptors. J. Cell. Physiol. 220: 655โ663, 2009. ยฉ 2009 WileyโLiss, Inc.