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 t he 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 cel l surface surrounding basal and suprabasal cells, suggesting beta(1) i ntegrins mediate both cell-matrix and cell-cell interactions. As initi ation of maturation in skin is associated with the detachment of cells from the basement membrane, the early loss of alpha(6) beta(4) but no t alpha(3) beta(1) integrin expression could be a determining factor i n the transition from the proliferating to a differentiating keratinoc yte. We have studied the regulation of adhesion potential and integrin expression during differentiation of mouse basal keratinocytes cultur ed 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 t he alpha(6) beta(4) integrin complex is associated with the induction of the spinous cell marker keratin 1. This early differentiation pheno type coincides with loss of cell attachment mediated by alpha(6) beta( 4) to laminins 1 and 5 but not to fibronectin or collagen IV. Selectiv e loss of attachment to laminin is also detected in spinous cells isol ated from newborn epidermis in vivo. The loss of alpha(6) beta(4) prot ein expression is a consequence of transcriptional and posttranslation al events, including reduction in mRNA transcripts, reduced synthesis of the alpha(6) protein, and enhanced processing of the alpha(6) and b eta(4), chains as determined by Western blots and pulse-chase experime nts in metabolically labeled keratinocytes. Selective processing of th e beta(4) intracellular domain is detected before loss of beta(4) from the cell surface in basal keratinocytes, and this process is accelera ted during differentiation. Whereas early keratinocyte maturation is l inked to the selective loss of the alpha(6) beta(4) complex, loss of b oth beta(1) and beta(4) integrin mRNA and protein occurs as cells proc eed to later stages in the differentiation program as induced by 0.5 m M Ca2+ or suspension culture. These conditions are characterized by ac celerated expression of transglutaminase; reduced keratin 1 protein; l oss 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 o f alpha(3) beta(1) but not alpha(6) beta(4) to down-regulation by tran sforming growth factors beta(1) and beta(2), factors that are also exp ressed differentially in normal skin. This study indicates that down-r egulation of the alpha(6) beta(4) but not beta(1) integrins occurs dur ing the initial steps of keratinocyte differentiation and is associate d with detachment from the laminin matrix. Such changes could contribu te an important signal to initiate the process of terminal keratinocyt e differentiation.