Platelet endothelial cell adhesion molecule‐1 regulates collagen‐stimulated platelet function by modulating the association of phosphatidylinositol 3‐kinase with Grb‐2‐associated binding protein‐1 and linker for activation of T cells

Background: Platelet activation by collagen depends on signals transduced by the glycoprotein (GP)VI–Fc receptor (FcR)γ‐chain collagen receptor complex, which involves recruitment of phosphatidylinositol 3‐kinase (PI3K) to phosphorylated tyrosines in the linker for activation of T cells (LAT). An interaction between the p85 regulatory subunit of PI3K and the scaffolding molecule Grb‐2‐associated binding protein‐1 (Gab1), which is regulated by binding of the Src homology 2 domain‐containing protein tyrosine phosphatase‐2 (SHP‐2) to Gab1, has been shown in other cell types to sustain PI3K activity to elicit cellular responses. Platelet endothelial cell adhesion molecule‐1 (PECAM‐1) functions as a negative regulator of platelet reactivity and thrombosis, at least in part by inhibiting GPVI–FcRγ‐chain signaling via recruitment of SHP‐2 to phosphorylated immunoreceptor tyrosine‐based inhibitory motifs in PECAM‐1. Objective: To investigate the possibility that PECAM‐1 regulates the formation of the Gab1–p85 signaling complexes, and the potential effect of such interactions on GPVI‐mediated platelet activation in platelets. Methods: The ability of PECAM‐1 signaling to modulate the LAT signalosome was investigated with immunoblotting assays on human platelets and knockout mouse platelets. Results: PECAM‐1‐associated SHP‐2 in collagen‐stimulated platelets binds to p85, which results in diminished levels of association with both Gab1 and LAT and reduced collagen‐stimulated PI3K signaling. We therefore propose that PECAM‐1‐mediated inhibition of GPVI‐dependent platelet responses result, at least in part, from recruitment of SHP‐2–p85 complexes to tyrosine‐phosphorylated PECAM‐1, which diminishes the association of PI3K with activatory signaling molecules, such as Gab1 and LAT.