EXPRESSION AND FUNCTION OF THE CYTOPLASMIC VARIANTS OF THE INTEGRIN-ALPHA-6 SUBUNIT IN TRANSFECTED K562 CELLS - ACTIVATION-DEPENDENT ADHESION AND INTERACTION WITH ISOFORMS OF LAMININ

Two variants of the cytoplasmic domain of the integrin alpha6 subunit have been identified (alpha6A and alpha6B). To determine the role of each variant in mediating cell adhesion to laminin, we have independently expressed the alpha6A and alpha6B subunits in K562 cells. Both variants associated with endogenous beta1 and were present at comparable levels on the surface of transfected K562 cells. After activation with phorbol ester (phorbol 12-myristate 13-acetate; PMA) or the stimulatory anti-beta1 antibody TS2/16, alpha6Abeta1 as well as alpha6Bbeta1 mediated cell adhesion to laminin and more specifically to its fragment E8. Furthermore, both integrin variants interacted with the laminin isoforms kalinin and merosin. Cell adhesion to laminin isoforms was inhibited by the alpha6-specific monoclonal antibody GoH3. PMA was less efficient in stimulating adhesion than TS2/16 and stimulated adhesion of alpha6B transfectants better than of alpha6A transfectants. In contrast, TS2/16 stimulated the adhesion of the alpha6A and alpha6B transfectants to laminin to a similar extent. These findings indicate that the cells may regulate the activation of the two alpha6 variants independently. Activation by PMA was associated with the phosphorylation of both alpha6A and alpha6B subunits, but there was no relationship between the degree of phosphorylation and the ability of the transfectants to adhere to laminin since alpha6A became phosphorylated much more strongly by PMA than alpha6B. Thus, both alpha6Abeta1 and alpha6Bbeta1 on K562 cells are activation-dependent receptors for different isoforms of laminin.