INTEGRIN RECOGNITION OF DIFFERENT CELL-BINDING FRAGMENTS OF LAMININ (P1, E3, E8) AND EVIDENCE THAT ALPHA-6-BETA-1 BUT NOT ALPHA-6-BETA-4 FUNCTIONS AS A MAJOR RECEPTOR FOR FRAGMENT E8

The involvement of integrins in mediating interaction of cells to well-characterized proteolytic fragments (P1, E3, and E8) of laminin was assessed by antibody blocking studies. Cell adhesion to fragment P1 was affected by mAbs against the integrin β1 and β3 subunits and furthermore could be prevented completely by a synthetic peptide containing the Arg-Gly-Asp sequence. Because the β3 antibody-sensitive cell lines expressed the vitronectin receptor (α6β3) at high levels, the involvement of this receptor in cell adhesion to P1 is strongly suggested. Integrin-mediated cell adhesion to E3 is of low affinity and was inhibited by antibodies against the integrin β1 subunit. In contrast, adhesion of some cell types to E3 was not or only partially sensitive to inhibition by anti-integrin subunit antibodies. Cell adhesion to E8 was blocked completed by integrin α6 or β1 antibodies. The α6-specific antibody did not inhibit cell adhesion to E3 or P1. Furthermore, the antibody only blocked adhesion to laminin of those cells that adhered exclusively to the E8 fragment. In addition, expression of α6β1 was closely correlated with the ability of cells to bind to the E8 fragment of laminin. These results indicate that the α6β1 integrin is a specific receptor for the E8 fragment of laminin. Many cell types expressed, instead of or in addition to α6β1 the recently described integrin α6β4. Although the ligand of α6β4 was not identified, it must be different from that of α6β1, because cells that express α6β4, but not α6β1, do not adhere to E8, and cell adhesion to E8 was specifically blocked by β1 specific antibodies. In conclusion, the data indicate that distinct integrin receptors belonging to the β1 or β3 subfamily are involved in adhesion of cells to the various laminin fragments. Adhesion to E3 may also be brought about by other receptor molecules, possibly proteoglycans, not belonging to the integrin family.