POLARIZED EXPRESSION OF INTEGRIN RECEPTORS (ALPHA-6-BETA-4, ALPHA-2-BETA-1, ALPHA-3-BETA-1, AND ALPHA-V-BETA-5) AND THEIR RELATIONSHIP WITH THE CYTOSKELETON AND BASEMENT-MEMBRANE MATRIX IN CULTURED HUMAN KERATINOCYTES

In human keratinocytes cultured in conditions which allow differentiation and stratification and are suitable to reconstitute a fully functional epidermis, alpha-6-beta-4 and two members of the beta-1 integrin family (alpha-2-beta-1 and alpha-3-beta-1) were respectively polarized to the basal and lateral domains of the plasmamembrane both in growing colonies and in the reconstituted epidermis. Conversely, the alpha-v integrin subunit, presumably in association with beta-5, was expressed at the basal surface in growing and migrating but not in stationary keratinocytes. The integrin alpha-6-beta-4: (a) was organized in typical patches which often showed a "leopard skin" pattern where spots corresponded to microfilament-free areas; (b) was not associated with focal contacts containing vinculin and talin but rather corresponded to relatively removed contact areas of the basal membrane as shown by interference reflection microscopy; and (c) was coherent to patches of laminin secreted and deposited underneath the ventral membrane of individual cells. The two beta-1 integrins (alpha-2-beta-1 and alpha-3-beta-1), both endowed with laminin receptor properties, were not associated with focal adhesions under experimental conditions allowing full epidermal maturation but matched the lateral position of vinculin (but not talin), cingulin, and desmoplakin, all makers of intercellular junctions. Often thin strips of laminin were observed in between the lateral aspects of individual basal keratinocytes. The integrin complex alpha-v-beta-5 had a topography similar to that of talin- and vinculin-containing focal adhesions mostly in the peripheral cells of expanding keratinocyte colonies and in coincidence with fibronectin strands. The discrete topography of beta-1 and beta-4 integrins has a functional role in the maintenance of the state of aggregation of cultured keratinocytes since lateral aggregation was impaired by antibodies to beta-1 whereas antibodies to beta-4 prevented cell-matrix adhesion (De Luca, M., R. N. Tamura, S. Kajiji, S. Bondanza, P. Rossino, R. Cancedda, P. C. Marchisio, and V. Quaranta. Proc. Natl. Acad. Sci. USA. 87:6888-6892). Moreover, the surface polarization of integrins followed attachment and depended both on the presence of Ca2+ in the medium and on the integrity of the cytoskeleton. We conclude that our in vitro functional tests and structural data suggest a correlation between the pattern of integrin expression on defined plasmamembrane domains and the mechanism of epidermal assembly.