Plasmin abrogates alpha v beta 5-mediated adhesion of a human keratinocyte cell line (HaCaT) to vitronectin

At cellular surfaces, urokinase-type plasminogen activator (uPA) is bound to a specific receptor (uPA-R). When bound to this receptor, uPA activates plasminogen, which is derived from plasma or the interstitial fluids. Thus, plasmin is provided for proteolysis of pericellular proteinaceous substrates. Here we demonstrate by immunocytology and laser scan microscopy that in the human keratinocyte cell line HaCaT uPA-R and uPA are localized together with the integrin alpha v beta 5 in focal contacts. Via the integrin alpha v beta 5, HaCaT cells adhere to vitronectin in a RGD-dependent manner. Plasmin interfered with the alpha v beta 5-mediated keratinocyte adhesion to vitronectin, most likely via cleavage of vitronectin and destruction of its cell binding function. Our findings demonstrate that plasmin, when generated by the uPA-dependent cell surface-associated pathway of plasminogen activation, can abrogate the cell-binding function of vitronectin and can thus disturb the adhesive interaction with this matrix molecule. In focal contacts molecules are assembled that are crucial for adhesion to vitronectin (i.e., the integrin alpha v beta 5), as well as for the generation of plasmin (i.e., uPA-R and uPA), which can negatively influence the binding interaction. We suggest that the plasmin-mediated abrogation of the interaction between the integrin alpha v beta 5 and vitronectin is a pathway of negative regulation; the codistribution of uPA-R/uPA and alpha v beta 5 in focal contacts may restrict this process to areas of cell/matrix contact.