Carboxypeptidase cathepsin X mediates β2-integrin-dependent adhesion of differentiated U-937 cells

Cathepsin X is a lysosomal carboxypeptidase with a potential role in processes of inflammation and immune response. The integrin-binding motifs RGD and ECD, present in the pro- and in mature forms of cathepsin X, respectively, suggest that this enzyme might have a function in cell signaling and adhesion. In this study, we report that cysteine protease inhibitors E-64 and CA-074 and 2F12 monoclonal antibody, all of which inhibit cathepsin X activity, significantly reduced adhesion of differentiated U-937 cells to polystyrene- and fibrinogen-coated surfaces via Mac-1 integrin receptor, whereas their binding to vitronectin, fibronectin or Matrigel was not affected. On the other hand, cathepsin X, added to differentiating U-937 cells, stimulated their adhesion. Using confocal microscopy, we demonstrated that the pro-form of cathepsin X was co-localized with β2 and β3 integrin subunits and its mature form solely with the β2 integrin subunit with the most intense signal in cell–cell junctions in differentiated U-937 cells and in co-cultures with endothelial cells. Our results indicate that active cathepsin X mediates the function of β2 integrin receptors during cell adhesion and that it could also be involved in other processes associated with β2 integrin receptors such as phagocytosis and T cell activation.