This paper shows that, in confluent human umbilical vein endothelial cell (EC) monolayers, the integrin heterodimers alpha 2 beta 1 and alpha 5 beta 1, but not other members of the beta 1 subfamily, are located at cell-cell contact borders and not at cellular free edges. Also the alpha v chain, but not its most common partner beta 3, that is widely expressed in EC cell-matrix junctions, is found at cell-cell borders. In EC monolayers, the putative ligands of alpha 2 beta 1 and alpha 5 beta 1 receptors, i.e., laminin, collagen type IV, and fibronectin, are also organized in strands corresponding to cell-cell borders. The location of the above integrin receptors is not an artifact of in vitro culture since it has been noted also in explanted islets of the native umbilical vein endothelium. The integrins alpha 2 beta 1 and alpha 5 beta 1 play a role in the maintenance of endothelial monolayer continuity in vitro. Indeed, specific antibodies to alpha 2 beta 1, alpha 5 beta 1, and the synthetic peptide GRGDSP alter its continuity without any initial cell detachment. Moreover, antibodies to alpha 5 beta 1 increase the permeation of macromolecules across confluent EC monolayers. In contrast beta 3 antibodies were ineffective. It is suggested that the relocation of integrins to cell-cell borders is a feature of cells programmed to form polarized monolayers since integrins have a different distribution in nonpolar confluent dermal fibroblasts. The conclusion is that some members of the integrin superfamily collaborate with other intercellular molecules to form lateral junctions and to control both the monolayer integrity and the permeability properties of the vascular endothelial lining. This also suggest that integrins are adhesion molecules provided with a unique biochemical adaptability to different biological functions.

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