Potential role for focal adhesion kinase in migrating and proliferating keratinocytes near epidermal wounds and in culture.

In normal, differentiating skin, hemidesmosomes make the stable attachment of basal epidermal keratinocytes to the dermis by linking the cytoplasmic keratin intermediate filaments to components of the basal lamina. In contrast, laterally migrating and proliferating basal keratinocytes in culture and presumably in repairing wounds use focal adhesions to form dynamic attachments to the dermis by linking actin microfilaments to the extracellular matrix. Focal adhesion kinase (FAK), a non-receptor protein tyrosine kinase concentrated along with phosphotyrosine-containing proteins in the focal adhesions of some cultured cells, is activated in vitro when cells attach, form focal adhesions, and spread. This report finds that FAK is activated, as determined from its increased phosphotyrosine content and from its increased labeling with [gamma-32P]ATP, in immunoprecipitates from human cultured keratinocytes attached and spreading on fibronectin compared to those attached but not spreading on polylysine. Furthermore, immunofluorescence shows that both FAK and phosphotyrosine are concentrated in the focal adhesions of cultured keratinocytes attached and spreading on extracellular matrix components known to facilitate cellular migration (fibronectin, collagens I or IV, and epiligrin). Finally, immunohistochemistry localizes FAK to the epidermal-dermal junction in repairing partial thickness burn wounds. FAK is found at the epidermal-dermal junction at sites and times which coincide with actively migrating or rapidly proliferating basal keratinocytes, suggesting that this distribution represents FAK concentrated and activated in adhesions analogous to the focal adhesions seen in cultured cells. Hence, FAK appears to have an important in vivo role in the reepithelialization of human wounds.