EGF-R signaling through Fyn kinase disrupts the function of integrin alpha6 beta 4 at hemidesmosomes: role in epithelial cell migration and carcinoma invasion

W e have examined the mechanism and functional significance of hemidesmosom e disassembly during normal epithelial cell migration and squamous carcinom a invasion. Our findings indicate that a fraction of EGF receptor (EGF-R) c ombines with the hemidesmosomal integrin alpha6 beta4 in both normal and ne oplastic keratinocytes. Activation of the EGF-R causes tyrosine phosphoryla tion of the beta4 cytoplasmic domain and disruption of hemidesmosomes. The Src family kinase inhibitors PP1 and PP2 prevent tyrosine phosphorylation o f beta4 and disassembly of hemidesmosomes without interfering with the acti vation of EGF-R. Coimmunoprecipitation experiments indicate that Fyn and, t o a lesser extent, Yes combine with alpha6 beta4. By contrast, Src and Lck do not associate with alpha6 beta4 to a significant extent. A dominant nega tive form of Fyn, but not Src, prevents tyrosine phosphorylation of beta4 a nd disassembly of hemidesmosomes. These observations suggest that the EGF-R causes disassembly of hemidesmosomes by activating Fyn, which in turn phos phorylates the beta4 cytoplasmic domain. Neoplastic cells expressing domina nt negative Fyn display increased hemidesmosomes and migrate poorly in vitr o in response to EGF. Furthermore, dominant negative Fyn decreases the abil ity of squamous carcinoma cells to invade through Matrigel in vitro and to form lung metastases following intravenous injection in nude mice. These re sults suggest that disruption of hemidesmosomes mediated by Fyn is a prereq uisite for normal keratinocyte migration and squamous carcinoma invasion.