Complex interactions between the laminin α4 subunit and integrins regulate endothelial cell behavior in vitro and angiogenesis in vivo

The α4 laminin subunit is a component of the basement membrane of blood vessels where it codistributes with the integrins αvβ3, α3β1, and α6β1. An antibody against the G domain (residues 919-1207; G^919-1207) of the α4 laminin subunit inhibits angiogenesis in a mouse-human chimeric model, indicating the functional importance of this domain. Additional support for the latter derives from the ability of recombinant G^919-1207 to support endothelial cell adhesion. In particular, endothelial cell adhesion to G^919-1207 is half-maximal at 1.4 nM whereas residues 919-1018 and 1016-1207 of the G domain are poor cellular ligands. Function blocking antibodies against integrins αvβ3 and β1 and a combination of antibodies against β3 and α6 integrin subunits inhibit endothelial cell attachment to G^919-1207. Moreover, both αvβ3 and α3β1 integrin bind with high affinity to G^919-1207. Together, our studies demonstrate that the G domain of laminin α4 chain is a specific, high affinity ligand for the αvβ3 and α3β1 integrin heterodimers and that these integrins, together with α6β1, function cooperatively to mediate endothelial cell-α4 laminin interaction and hence blood vessel development. We propose a model based on these data that reconcile apparent discrepancies in the recent literature with regard to the role of the αvβ3 integrin in angiogenesis.