Selective changes in laminin adhesion and α₆β₄ integrin regulation are associated with the initial steps in keratinocyte maturation

In skin, the distribution of integrins is compartmentalized. Whereas the α₆β₄ integrin complex is polarized to the basal portion of proliferating cells in the basal layer juxtaposed to the basement membrane, α₃β₁ integrin receptors are localized on the cell surface surrounding basal and suprabasal cells, suggesting β₁ integrins mediate both cell-matrix and cell-cell interactions. As initiation of maturation in skin is associated with the detachment of cells from the basement membrane, the early loss of α₆β₄, but not α₃β₁, integrin expression could be a determining factor in the transition from the proliferating to a differentiating keratinocyte. We have studied the regulation of adhesion potential and integrin expression during differentiation of mouse basal keratinocytes cultured in 0.05 mM Ca²⁺ medium and induced to differentiate in 0.12 mM Ca²⁺ medium. Within 12-24 h after elevation of Ca²⁺, a selective loss of the α₆β₄ integrin complex is associated with the induction of the spinous cell marker keratin 1. This early differentiation phenotype coincides with loss of cell attachment mediated by α₆β₄ to laminins 1 and 5 but not to fibronectin or collagen IV. Selective loss of attachment to laminin is also detected in spinous cells isolated from newborn epidermis in vivo. The loss of α₆β₄ protein expression is a consequence of transcriptional and posttranslational events, including reduction in mRNA transcripts, reduced synthesis of the α₆ protein, and enhanced processing of the α₆ and β₄ chains as determined by Western blots and pulse-chase experiments in metabolically labeled keratinocytes. Selective processing of the β₄ intracellular domain is detected before loss of β₄ from the cell surface in basal keratinocytes, and this process is accelerated during differentiation. Whereas early keratinocyte maturation is linked to the selective loss of the α₆β₄ complex, loss of both β₁ and β₄ integrin mRNA and protein occurs as cells proceed to later stages in the differentiation program as induced by 0.5 mM Ca²⁺ or suspension culture. These conditions are characterized by accelerated expression of transglutaminase; reduced keratin 1 protein; loss of adhesion to fibronectin, laminin 1, laminin 5, and collagen IV; and rapid cell death. Contributing to the down-regulation of β₁ integrins during terminal differentiation is a selective sensitivity of α₃β₁ but not α₆β₄ to down-regulation by transforming growth factors β₁ and β₂, factors that are also expressed differentially in normal skin. This study indicates that down-regulation of the α₆β₄ but not β₁ integrins occurs during the initial steps of keratinocyte differentiation and is associated with detachment from the laminin matrix. Such changes could contribute an important signal to initiate the process of terminal keratinocyte differentiation.