Selective changes in laminin adhesion and alpha 6 beta 4 integrin regulation are associated with the initial steps in keratinocyte maturation.

In skin, the distribution of integrins is compartmentalized. Whereas the alpha 6 beta 4 integrin complex is polarized to the basal portion of proliferating cells in the basal layer juxtaposed to the basement membrane, alpha 3 beta 1 integrin receptors are localized on the cell surface surrounding basal and suprabasal cells, suggesting beta 1 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 alpha 6 beta 4, but not alpha 3 beta 1, 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 culture 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 alpha 6 beta 4 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 alpha 6 beta 4 to laminins 1 and 5 but not a 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 alpha 6 and beta 4 protein expression is a consequence of transcriptional and posttranscriptional events, including reduction in mRNA transcripts, reduced synthesis of the alpha 6 protein, and enhanced processing of the alpha 6 and beta 4 chains as determined by Western blots and pulse-chase experiments in metabolically labeled keratinocytes. Selective processing of the beta 4 intracellular domain is detected before loss of beta 4 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 alpha 6 beta 4 complex, loss of both beta 1 and beta 4 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 beta 1 integrins during terminal differentiation is a selective sensitivity of alpha 3 beta 1 but not alpha 6 beta 4 to down-regulation by transforming growth factors beta 1 and beta 2, factors that are also expressed differentially in normal skin. This study indicates that down-regulation of the alpha 6 beta 4 but not beta 1 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.