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 alpha 6 subunit have been identified (alpha 6A and alpha 6B). To determine the role of each variant in mediating cell adhesion to laminin, we have independently expressed the alpha 6A and alpha 6B subunits in K562 cells. Both variants associated with endogenous beta 1 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-beta 1 antibody TS2/16, alpha 6A beta 1 as well as alpha 6B beta 1 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 alpha 6-specific monoclonal antibody GoH3. PMA was less efficient in stimulating adhesion than TS2/16 and stimulated adhesion of alpha 6B transfectants better than of alpha 6A transfectants. In contrast, TS2/16 stimulated the adhesion of the alpha 6A and alpha 6B transfectants to laminin to a similar extent. These findings indicate that the cells may regulate the activation of the two alpha 6 variants independently. Activation by PMA was associated with the phosphorylation of both alpha 6A and alpha 6B subunits, but there was no relationship between the degree of phosphorylation and the ability of the transfectants to adhere to laminin since alpha 6A became phosphorylated much more strongly by PMA than alpha 6B. Thus, both alpha 6A beta 1 and alpha 6B beta 1 on K562 cells are activation-dependent receptors for different isoforms of laminin.