REGULATION OF KERATINOCYTE TERMINAL DIFFERENTIATION BY INTEGRIN-EXTRACELLULAR MATRIX INTERACTIONS

Suspension-induced terminal differentiation of human epidermal keratin ocytes can be inhibited by fibronectin through binding to the alpha5be ta1 integrin. We have investigated the effect of fibronectin on expres sion of integrins and proteins of the actin cytoskeleton and have expl ored the nature of the differentiation stimulus by testing different c ombinations of anti-integrin monoclonal antibodies or extracellular ma trix proteins in the suspension assay. Fibronectin prolonged cell surf ace expression of beta1 integrins but did not overcome the inhibition of intracellular transport of integrins that occurs when keratinocytes are placed in suspension. Fibronectin did not prevent the suspension- induced decline in the level of mRNAs encoding the beta1 integrin subu nit, actin, filamin and alpha-actinin; furthermore, the inhibition of terminal differentiation did not depend on the state of assembly of mi crofilaments or microtubules. Terminal differentiation could be partia lly inhibited by an adhesion-blocking monoclonal antibody to the beta1 integrin subunit or by a combination of adhesion blocking antibodies recognising the alpha subunits that associate with beta1 (alpha2, alph a3 and alpha5). Although laminin and type IV collagen do not inhibit t erminal differentiation individually, they were inhibitory when added to cells in combination with a low concentration of fibronectin. We co nclude that the proportion of keratinocyte beta1 integrins occupied by ligand can regulate the initiation of terminal differentiation indepe ndently of the state of assembly of the actin cytoskeleton.