Human microvascular endothelial cells use β1 and β3 integrin receptor complexes to attach to laminin

Abstract

Microvascular endothelial cells (MEC) use a set of surface receptors to adhere not only to the vascular basement membrane but, during angiogenic stimulation, to the interstitium. We examined how cultured human MEC interact with laminin-rich basement membranes. By using a panel of monoclonal antibodies, we found that MEC cells express a number of integrin-related receptor complexes, including α1β1, α2β1, α3β1, α5β1, α6β1, and αvβ3. Attachment to laminin, a major adhesive protein in basement membranes, was studied in detail. Blocking monoclonal antibodies specific to different integrin receptor complexes showed that the α6β1 complex was important for MEC adhesion to laminin. In addition, blocking antibody also implicated the vitronectin receptor (αvβ3) in laminin adhesion. We used ligand affinity chromatography of detergent-solubilized receptor complexes to further define receptor specificity. On laminin-Sepharose columns, we identified several integrin receptor complexes whose affinity for the ligand was dependent on the type of divalent cation present. Several β1 complexes, including α1β1, α2β1, and α6β1 bound strongly to laminin. In agreement with the antibody blocking experiments, αvβ3 was found to bind well to laminin. However, unlike binding to its other ligands (e.g., vitronectin, fibrinogen, von Willebrand factor), αvβ3 interaction with laminin did not appear to be Arg-GlyAsp (RGD) sensitive. Finally, immunofluorescent staining demonstrated both β1 and β3 complexes in vinculin-positive focal adhesion plaques on the basal surface of MEC adhering to laminin-coated substrates. The results indicate that both these subfamilies of integrin heterodimers are involved in promoting MEC adhesion to laminin and the vascular basement membrane.