CALCIUM-INDUCED CHANGES IN DISTRIBUTION AND SOLUBILITY OF CADHERINS, INTEGRINS AND THEIR ASSOCIATED CYTOPLASMIC PROTEINS IN HUMAN KERATINOCYTES

Studies with cultured human epidermal keratinocytes have shown that st ratification, the movement of differentiating cells out of the basal l ayer, involves changes in cell-extracellular matrix and cell-cell adhe siveness mediated by receptors of the integrin and cadherin families, respectively. Keratinocytes normally lose their integrins when they in itiate terminal differentiation. However, when stratification is inhib ited by a low concentration of calcium ions in the medium (0.1 mM) or by addition of antibodies to P- and E-cadherin in standard medium (1.8 mM calcium ions), differentiating, involucrin-positive, cells continu e to express functional integrins. In order to investigate the mechani sm by which cadherins may regulate integrin expression, we have examin ed the distribution and detergent solubility of the receptors and asso ciated cytoplasmic proteins in keratinocytes grown as a monolayer in l ow calcium medium or transferred to standard medium to induce stratifi cation. Within 1 hour of raising the concentration of calcium ions, in tegrins, cadherins, alpha-catenin, beta-catenin, plakoglobin, vinculin and alpha-actinin appeared to accumulate at cell-cell borders, wherea s the focal contact proteins, paxillin and talin, did not. The change in distribution was correlated with decreased solubility in 0.5% Trito n X-100 of some of the proteins examined, but the integrins, alpha-act inin, paxillin and talin remained completely soluble. Addition of cyto chalasin D inhibited both the redistribution of proteins and subsequen t stratification of involucrin-positive cells. Cycloheximide treatment allowed protein redistribution and stratification, but involucrin-pos itive cells continued to express integrins. These results suggest that stratification requires the interactions of cadherins and integrins w ith the actin cytoskeleton and that the selective loss of integrins fr om differentiating cells requires de novo protein synthesis.