α6β4 integrin regulates keratinocyte chemotaxis through differential GTPase activation and antagonism of α3β1 integrin

Growth factor-induced cell migration and proliferation are essential for epithelial wound repair. Cell migration during wound repair also depends upon expression of laminin-5, a ligand for alpha6beta4 integrin. We investigated the role of alpha6beta4 integrin in laminin-5-dependent keratinocyte migration by re-expressing normal or attachment-defective alpha6beta4 integrin in beta4 integrin null keratinocytes. We found that expression of beta4 integrin in either a ligand bound or ligand unbound state was necessary and sufficient for EGF-induced cell migration. In a ligand bound state, beta4 integrin supported EGF-induced cell migration though sustained activation of Racl. In the absence of alpha6beta4 integrin ligation, Rac1 activation became tempered and EGF chemotaxis proceeded through an alternate mechanism that depended upon alpha4beta1 integrin and was characterized by cell scattering. alpha3beta1 integrin also relocalated from cell-cell contacts to sites of basal clustering where it displayed increased conformational activation. The aberrant distribution and activation of alpha3beta1 integrin in attachment-defective beta4 cells could be reversed by the activation of Rac1. Conversely, in WT beta4 cells the normal cell-cell localization of alpha3beta1 integrin became aberrant after the inhibition of Racl. These studies indicate that the extracellular domain of beta4 integrin, through its ability to bind ligand, functions to integrate the divergent effects of growth factors on the cytoskeleton and adhesion receptors so that coordinated keratinocyte migration can be achieved.