Protein kinase Cδ-mediated phosphorylation of α6β4 is associated with reduced integrin localization to the hemidesmosome and decreased keratinocyte attachment

In mammalian epidermis, expression of the α6β4 integrin is restricted to the hemidesmosome complexes, which connect the proliferative basal cell layer with the underlying basement membrane. Keratinocyte differentiation is associated with down-regulation of α6β4 expression and detachment of keratinocytes from the basement membrane. Neoplastic keratinocytes delay maturation, proliferate suprabasally, and retain the expression of the α6β4 integrin in suprabasal cells disassociated from the hemidesmosomes. We now show that the α6β4 integrin is a substrate for serine phosphorylation by protein kinase C in keratinocytes. Furthermore, protein kinase C-mediated phosphorylation of α6β4 is associated with redistribution of this integrin from the hemidesmosome to the cytosol. Specifically, in vitro kinase assays identified the protein kinase Cδ as the primary isoform phosphorylating α6 and β4 integrin subunits. Using recombinant protein kinase C adenoviruses, overexpression of protein kinase Cδ but not protein kinase Cα in primary keratinocytes increased β4 serine phosphorylation, decreased α6β4 localization to the hemidesmosome complexes, and reduced keratinocyte attachment. Taken together, these results establish a link between protein kinase Cδ-mediated serine phosphorylation of α6β4 integrin and its effects on α6β4 subcellular localization and keratinocyte attachment to the laminin underlying matrix.