Distinct ligand binding sites in integrin α3β1 regulate matrix adhesion and cell-cell contact

Abstract

The integrin α3β1 mediates cellular adhesion to the matrix ligand laminin-5. A second integrin ligand, the urokinase receptor (uPAR), associates with α3β1 via a surface loop within the α3 β-propeller (residues 242-246) but outside the laminin binding region, suggesting that uPAR-integrin interactions could signal differently from matrix engagement. To explore this, α3-/- epithelial cells were reconstituted with wild-type (wt) α3 or α3 with Ala mutations within the uPAR-interacting loop (H245A or R244A). Wt or mutant-bearing cells showed comparable expression and adhesion to laminin-5. Cells expressing wt α3 and uPAR dissociated in culture, with increased Src activity, up-regulation of SLUG, and down-regulation of E-cadherin and γ-catenin. Src kinase inhibition or expression of Src 1-251 restored the epithelial phenotype. The H245A and R244A mutants were unaffected by coexpression of uPAR. We conclude that α3β1 regulates both cell-cell contact and matrix adhesion, but through distinct protein interaction sites within its β-propeller. These studies reveal an integrin- and Src-dependent pathway for SLUG expression and mesenchymal transition.