MODULATION OF MONOCYTE-ENDOTHELIAL CELL-INTERACTIONS BY PLATELET MICROPARTICLES

Platelets, activated by various agonists, produce microparticles (MP) from the plasma membrane, which are released into the extracellular sp ace. Although the mechanism of MP formation has been clarified, their biological importance remains hi defined. We have recently shown that platelet-derived MP influence platelet and endothelial cell function. In this study, we have further examined the mechanism of cellular acti vation by platelet MP, To address the possibility that they may influe nce monocyte-endothelial interactions, we used an in vitro assay to ex amine their effects on the adhesion of monocytes to human umbilical ve in endothelial cells (HUVEC). Platelet MP increased the adhesion of mo nocytes to HUVEC in a time- and dose-dependent manner. Maximal adhesio n of monocytes to resting HUVEC was observed after 24 h of stimulation with MP. Similar kinetics were observed with U-937 (human promonocyti c leukemia) cells, used as a model for the blood-borne monocyte. Maxim al adhesion of resting monocytes to MP-stimulated HUVEC was observed a fter 5 h of stimulation with MP. The EC(50)s for MP-induced increases in HUVEC, monocyte, and U-937 cell adhesion is 8.74, 43.41, and 10.83 mu g/ml of MP protein, respectively. The induction of monocyte-endothe lial adhesion was mimicked by arachidonic acid isolated from MP, The o bserved increased cellular adhesiveness correlated with MP-induced upr egulation of cell adhesion molecules. MP-stimulated HUVEC increased in tracellular cell adhesion molecule-1 (ICAM-1) but not vascular cell ad hesion molecule-1 (VCAM-1), P-, or E-selectin expression. Monocyte and U-937 lymphocyte function-associated antigen-1 (CD11a/CD18) and macro phage antigen-1 (CD11b/CD18, alpha(m)/beta(2)) were both upregulated u pon MP stimulation, but an increase in p150,95 (CD11c/CD18), very late antigen-1, or ICAM-1 expression was not observed. The functional impo rtance of these changes was demonstrated with blocking antibodies. MP also induced the chemotaxis of U-937 cells in a dose-dependent manner with an EC50 of 4.40 mu g/ml of MP protein. Similarly, arachidonic aci d isolated from MP mimicked the chemotactic response. a role for PKC w as implicated in both adhesion and chemotaxis. GF 109203X, a specific inhibitor of PKC, significantly reduced monocyte-endothelial adhesion, as well as U-937 chemotaxis. The demonstration that platelet MP may m odulate important aspects of endothelial and monocyte function provide s a novel mechanism by which platelets may interact with such cells in human atherosclerosis and inflammation.