Functional down-regulation of α5β1 integrin in keratinocytes is reversible but commitment to terminal differentiation is not

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Abstract
Extracellular matrix receptors of the integrin family have a dual role in the epidermis, regulating both adhesion and differentiation. Loss of contact with the extra-cellular matrix causes keratinocytes to become commit-ted to terminal differentiation, and results in a decrease in the ability of the α5β1 integrin to bind fibronectin. We have investigated whether the decrease in ligand-binding ability is reversible and, if so, whether commitment to terminal differentiation can also be reversed. Keratinocytes that had been placed in suspension for 5 hours to induce commitment were compared with the starting population (0 hour cells) in the presence or absence of 8A2, an activating anti-β 1 antibody. 8A2 IgG or FAb fragments increased the amount of α5β1 in cell extracts that bound to fibronectin-Sepharose and in the presence of 8A2 the amount of bound α5β1 in 0 hour and 5 hour extracts was equal. 8A2 also restored α5β1function in adhesion assays of intact 5 hour cells. Ca2+, Mg2+ and Mn2+ alone, at concentrations of up to 1 mM, did not increase the adhesiveness of 5 hour cells relative to 0 hour cells; however, the effect of 8A2 on keratinocytes was dependent on Ca2+. Although 8A2 restored 5 1 ligand-binding ability it did not prevent committed cells from withdrawing from the cell cycle and expressing involucrin, a differentiation marker. The results suggest that loss of matrix contact triggers two distinct events in keratinocytes: a reversible change in α5β1 conformation and generation of an irreversible signal through the receptor that culminates in terminal differentiation.

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