Urokinase receptors promote beta 1 integrin function through interactions with integrin alpha 3 beta 1

The urokinase receptor (uPAR) is linked to cellular migration through its c apacity to promote pericellular proteolysis, regulate integrin function, an d mediate cell signaling in response to urokinase (uPA) binding. The mechan isms for these activities remain incompletely defined, although uPAR was re cently identified as a cis-acting ligand for the beta2 integrin CD11b/CD18 (Mac-1). Here we show that a major beta1 integrin partner for uPAR/uPA sign aling is alpha3. In uPAR-transfected 293 cells uPAR complexed (> 90%) with alpha3 betai and antibodies to alpha3 blocked uPAR-dependent vitronectin (V n) adhesion. Soluble uPAR bound to recombinant alpha3 beta1 in uPA-dependen t manner (K-d < 20 nM) and binding was blocked by a 17-mer alpha3 beta1 int egrin peptide (alpha 325) homologous to the CD11b uPAR-binding site. uPAR c olocalized with alpha3 beta1 in MDA-MB-231 cells and uPA (1 nM) enhanced sp reading and focal adhesion kinase phosphorylation on fibronectin (Fn) or co llagen type I (Col) in a pertussis toxin- and alpha 325-sensitive manner. A critical role of alpha3 beta1 in uPA signaling was verified by studies of epithelial cells from alpha3-deficient mice. Thus, uPAR preferentially comp lexes with alpha3 beta1, promoting direct (Vn) and indirect (Fn, Col) pathw ays of cell adhesion, the latter a heterotrimeric G protein-dependent mecha nism of signaling between alpha3 beta1 and other beta1 integrins.