Integrin activation by regulated dimerization and oligomerization of platelet endothelial cell adhesion molecule (PECAM)-1 from within the cell

Platelet endothelial cell adhesion molecule (PECAM)-1 is a 130-kD transmemb rane glycoprotein having six Ig homology domains within its extracellular d omain and an immunoreceptor tyrosine-based inhibitory motif within its cyto plasmic domain. Previous studies have shown that addition of bivalent anti- PECAM-1 mAbs to the surface of T cells, natural killer cells, neutrophils, or platelets result in increased cell adhesion to immobilized integrin liga nds. However, the mechanism by which this occurs is not clear, and it is po ssible that anti-PECAM-1 mAbs elicit this effect by simply sequestering PEC AM-1, via antibody-induced patching and capping, away from stimulatory rece ptors that it normally regulates. To determine whether dimerization or olig omerization of PECAM-1 directly initiates signal transduction pathways that affect integrin function in an antibody-independent manner, stable human e mbryonic kidney-293 cell lines were produced that expressed chimeric PECAM- 1 cDNAs containing one or two FK506-binding protein (FKBP) domains at their COOH terminus. Controlled dimerization initiated byaddition of the bivalen t, membrane-permeable FKBP dimerizer, AP1510, nearly doubled homophilic bin ding capacity, whereas AP1510-induced oligomers favored cis PECAM-1/PECAM-1 associations within the plane of the plasma membrane at the expense of tra ns homophilic adhesion. Importantly, AP1510-induced oligomerization resulte d in a marked increase in both adherence and spreading of PECAM/FKBP-2-tran sfected cells on immobilized fibronectin, a reaction that was mediated by t he integrin alpha (5)beta (1). These data demonstrate that signals required for integrin activation can be elicited by clustering of PECAM-1 from insi de the cell, and suggest that a dynamic equilibrium between PECAM-1 monomer s, dimers, and oligomers may control cellular activation signals that influ ence the adhesive properties of vascular cells that express this novel memb er of the immunoreceptor tyrosine-based inhibitory motif family of regulato ry receptors.