Regulation of the VLA integrin-ligand interactions through the β1 subunit

Abstract

Integrins from the very late activation antigen (VLA) subfamily are involved in cellular attachment to extracellular matrix (ECM) proteins and in intercellular adhesions. It is known that the interaction of integrin proteins with their ligands can be regulated during cellular activation. We have investigated the regulation of different VLA-mediated adhesive interactions through the common β1 chain. We have found that certain anti-β1 antibodies strongly enhance binding of myelomonocytic U-937 cells to fibronectin. This β1-mediated regulatory effect involved both VLA-4 and VLA-5 fibronectin receptors. Moreover, anti-β1 mAb also induced VLA-4-mediated binding to a re-combinant soluble form of its endothelial cell ligand VCAM-1. Non-activated peripheral blood T lymphocytes, unable to mediate VLA-4 interactions with fibronectin or VCAM-1, acquired the ability to bind these ligands in the presence of anti-β1 mAb. The anti-β1-mediated changes in the affinities of β1 integrin for their ligands were comparable to those triggered by different lymphocyte activation agents such as anti-CD3 mAb or phorbol esters. Adhesion of melanoma cells to other ECM proteins such as lamimn or collagen as well as that of α2-transfected K-562 cells to collagen, was also strongly enhanced by anti-β1 mAb. These β1-mediated regulatory effects on different VLA-ligand interactions do not involve changes in cell surface membrane expression of different VLA heterodimers The anti-β1-mediated functional effects required an active metabolism, cytoskeleton integrity and the existence of physiological levels of intracellular calcium as well as a functional Na+/H+ antiporter. β1 antibodies not only increased cell attachment but also promoted spreading and cytoplasmic extension of endothelial cells on plates coated with either fibronectin, collagen or laminin as well as induced the rapid appearance of microspikes in U-937 cells on fibronectin. Moreover, both β1 integrin and the cytoskeletal protein talin colocalized in the anti-β1 induced microspikes. These results emphasize the central role of the common β1 chain in regulating different adhesive functions mediated by VLA integrins as well as cellular morphology.