Summary
Interactions between the uPA receptor (uPAR) and various inte-grins, including α5β1, are known to modulate integrin-dependent cell adhesion, and we have shown that the
integrin-associated tetraspanin protein CD82 down-regulates uPAR-dependent plasminogen
activation by affecting α5β1 cellular localisation. Here we have investigated whether overexpression of α5β1 directly affects uPAR-dependent pericellular proteolysis. CHO cells overexpressing
α5β1 were found to activate plasminogen at a rate up to 18-fold faster than B2CHO cells
which are α5-deficient. This effect was dependent on the activation state of α5β1, as it was maximal in the presence of Mn2+. To determine the role of uPAR-α5β1 interactions in this effect, we determined the adhesion of these cells to immobilised
soluble uPAR (suPAR). Neither cell-type was found to adhere to suPAR, but both cell
types were found to adhere to an anti-uPAR monoclonal antibody in a uPAR- and integrin-dependent
manner. This adhesion was 10-fold greater in the absence of α5β1, possibly implicating the involvement of non-α5-integrins. Soluble forms of the various components were used to investigate the molecular
basis of these effects, but no direct interactions could be demonstrated between α5β1 and either uPAR, uPA or uPA-uPAR complex. This suggests that assembly of these components
on the plasma membrane is required to influence uPAR function, increasing uPAR-dependent
pericellular proteolysis and decreasing uPAR-dependent cell adhesion. These interactions
may be modified by other integrins, suggesting a complex interplay between uPAR and
integrins on the cell surface with the potential to regulate invasive cell migration.
Keywords
uPA - uPAR - plasminogen activation - integrins - cell adhesion