Distinct structural requirements for binding of the integrins alphavbeta6, alphavbeta3, alphavbeta5, alpha5beta1 and alpha9beta1 to osteopontin.

The extracellular matrix protein, osteopontin, is a ligand for several members of the integrin family, including alpha5beta1, alphavbeta3, alphavbeta5 and alpha9beta1. Osteopontin is a substrate for a number of extracellular proteases, including thrombin and the metalloproteases MMP-3 and MMP-7, which cleave osteopontin at sites close to or within the mapped integrin binding sites. Using affinity chromatography and cell adhesion assays, we now identify the integrin alphavbeta6 as an additional osteopontin receptor. Utilizing a series of recombinant forms of osteopontin, we compared the structural requirements for alphavbeta6 binding with those for the 4 other osteopontin-binding integrins. Like alpha5beta1, alphavbeta3 and alphavbeta5 (but not alpha9beta1), alphavbeta6 binds to the RGD site in osteopontin, since RGD peptide or mutation of this site to RAA completely inhibits alphavbeta6-mediated cell adhesion. For both alpha9beta1 and alpha5beta1, the N-terminal fragment generated by thrombin cleavage is a much better ligand than full length osteopontin, whereas thrombin-cleavage does not appear to be required for optimal adhesion to alphavbeta3, alphavbeta5 or alphavbeta6. A recombinant fragment predicted to be generated by MMP cleavage no longer supported alpha5beta1 or alpha9beta1-mediated adhesion, but adhesion mediated by alphavbeta5 or alphavbeta6 was unaffected. Finally, adhesion of alphavbeta5 or alphavbeta6 was inhibited by mutation of two aspartic acid residues upstream of the RGD site, whereas adhesion mediated by alphavbeta3, alpha5beta1 or alpha9beta1 was unaffected by these mutations. These results suggest that the hierarchy of integrin interactions with osteopontin can undergo complex regulation at least in part through the action of extracellular proteases.