Integrin α3β1-dependent activation of FAK/Src regulates Rac1-mediated keratinocyte polarization on laminin-5

Upon epidermal wounding, keratinocytes at the wound edge become activated, deposit newly synthesized laminin-5 into the extracellular matrix, and migrate into the wound bed. The interaction between integrin alpha 3 beta 1 and laminin-5 is essential for establishment of a stable, leading lamellipodium and persistent keratinocyte migration. We previously showed that integrin alpha 3 beta 1 activates the Rho family GTPase Rac1 and regulates Rac1-dependent formation of polarized, leading lamellipodia in migrating keratinocytes. In the present study, we explored the role of focal adhesion kinase (FAK) and src signaling in this process. We show that overexpression of the FAK inhibitor FAK-related non-kinase or of the FAK(Y397F) auto-phosphorylation mutant, induced abnormal, non-polarized spreading of keratinocytes on laminin-5. Integrin alpha 3 beta 1 was required for full FAK autophosphorylation at Y397, and subsequent src kinase-dependent phosphorylation of FAK at residues Y861 and Y925, sites responsible for promoting signal transduction downstream of FAK, indicating that alpha 3 beta 1 regulates the coordination of FAK/src signal transcluction. Inhibiting either src kinase activity or FAK signaling interfered with alpha 3 beta 1-mediated Rac1 activation and polarized cell spreading. These findings reveal a novel pathway in migratory keratinocytes wherein alpha 3 beta 1-laminin-5 interactions regulate src kinase signaling through FAK, promoting Rac1 activation and polarized lamellipodium extension.