FUNCTIONAL DOWN-REGULATION OF ALPHA(5)BETA(1) INTEGRIN IN KERATINOCYTES IS REVERSIBLE BUT COMMITMENT TO TERMINAL DIFFERENTIATION IS NOT

Extracellular matrix receptors of the integrin family have a dual role in the epidermis, regulating both adhesion and differentiation. Loss of contact with the extracellular matrix causes keratinocytes to becom e committed to terminal differentiation, and results in a decrease in the ability of the alpha(5) beta(1) integrin to bind fibronectin. We h ave investigated whether the decrease in ligand-binding ability is rev ersible and, if so, whether commitment to terminal differentiation can also be reversed. Keratinocytes that had been placed in suspension fo r 5 hours to induce commitment were compared with the starting populat ion (0 hour cells) in the presence or absence of 8A2, an activating an ti-beta(1) antibociy. 8A2 IgG or FAb fragments increased the amount of alpha(5) beta(1) in cell extracts that bound to fibronectin-Sepharose and in the presence of 8A2 the amount of bound alpha(5) beta(1) in 0 hour and 5 hour extracts was equal. 8A2 also restored alpha(5) beta(1) function in adhesion assays of intact 5 hour cells. Ca2+, Mg2+ and Mn 2+ alone, at concentrations of up to 1 mM, did not increase the adhesi veness of 5 hour cells relative to 0 hour cells; however, the effect o f 8A2 on keratinocytes was dependent on Ca2+. Although 8A2 restored al pha(5) beta(1) ligand-binding ability it did not prevent committed cel ls from withdrawing from the cell cycle and expressing involucrin, a d ifferentiation marker. The results suggest that loss of matrix contact triggers two distinct events in keratinocytes: a reversible change in alpha(5) beta(1) conformation and generation of an irreversible signa l through the receptor that culminates in terminal differentiation.