Human Microvascular Endothelial Cells Use β 1 and β 3 Integrin Receptor Complexes to Attach to Laminin

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 α 1β 1, α 2β 1,α 3β 1,α 5β 1,α 6β 1, and α vβ 3. 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 α 6β 1 complex was important for MEC adhesion to laminin. In addition, blocking antibody also implicated the vitronectin receptor (α vβ 3) 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 β 1 complexes, including α 1β 1,α 2β 1, and α 6β 1 bound strongly to laminin. In agreement with the antibody blocking experiments, α vβ 3 was found to bind well to laminin. However, unlike binding to its other ligands (e.g., vitronectin, fibrinogen, von Willebrand factor), α vβ 3 interaction with laminin did not appear to be Arg-Gly-Asp (RGD) sensitive. Finally, immunofluorescent staining demonstrated both β 1 and β 3 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.