Urokinase receptors promote β1 integrin function through interactions with integrin α3β1

The urokinase receptor (uPAR) is linked to cellular migration through its capacity to promote pericellular proteolysis, regulate integrin function, and mediate cell signaling in response to urokinase (uPA) binding. The mechanisms for these activities remain incompletely defined, although uPAR was recently 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 signaling is alpha3. In uPAR-transfected 293 cells uPAR complexed (> 90%) with alpha3 betai and antibodies to alpha3 blocked uPAR-dependent vitronectin (Vn) adhesion. Soluble uPAR bound to recombinant alpha3 beta1 in uPA-dependent manner (K-d < 20 nM) and binding was blocked by a 17-mer alpha3 beta1 integrin peptide (alpha 325) homologous to the CD11b uPAR-binding site. uPAR colocalized with alpha3 beta1 in MDA-MB-231 cells and uPA (1 nM) enhanced spreading and focal adhesion kinase phosphorylation on fibronectin (Fn) or collagen 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 complexes with alpha3 beta1, promoting direct (Vn) and indirect (Fn, Col) pathways of cell adhesion, the latter a heterotrimeric G protein-dependent mechanism of signaling between alpha3 beta1 and other beta1 integrins.