Related Experiment Video
Updated: May 6, 2026
Intravital Microscopy of Leukocyte-endothelial and Platelet-leukocyte Interactions in Mesenterial Veins in Mice
Published on: August 13, 2015
Modulation of monocyte-endothelial cell interactions by platelet microparticles
O P Barry1, D Praticò, R C Savani
1Center for Experimental Therapeutics, Department of Pediatrics, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104-6100, USA.
Abstract:
Platelets, activated by various agonists, produce microparticles (MP) from the plasma membrane, which are released into the extracellular space. Although the mechanism of MP formation has been clarified, their biological importance remains ill 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 activation by platelet MP. To address the possibility that they may influence monocyte-endothelial interactions, we used an in vitro assay to examine their effects on the adhesion of monocytes to human umbilical vein endothelial cells (HUVEC). Platelet MP increased the adhesion of monocytes to HUVEC in a time- and dose-dependent manner. Maximal adhesion of monocytes to resting HUVEC was observed after 24 h of stimulation with MP. Similar kinetics were observed with U-937 (human promonocytic leukemia) cells, used as a model for the blood-borne monocyte. Maximal adhesion of resting monocytes to MP-stimulated HUVEC was observed after 5 h of stimulation with MP. The EC50s for MP-induced increases in HUVEC, monocyte, and U-937 cell adhesion is 8.74, 43.41, and 10.83 microg/ml of MP protein, respectively. The induction of monocyte-endothelial adhesion was mimicked by arachidonic acid isolated from MP. The observed increased cellular adhesiveness correlated with MP-induced upregulation of cell adhesion molecules. MP-stimulated HUVEC increased intracellular cell adhesion molecule-1 (ICAM-1) but not vascular cell adhesion molecule-1 (VCAM-1), P-, or E-selectin expression. Monocyte and U-937 lymphocyte function-associated antigen-1 (CD11a/CD18) and macrophage antigen-1 (CD11b/ CD18, alpham/beta2) were both upregulated upon MP stimulation, but an increase in p150,95 (CD11c/CD18), very late antigen-1, or ICAM-1 expression was not observed. The functional importance 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 microg/ml of MP protein. Similarly, arachidonic acid isolated from MP mimicked the chemotactic response. A role for PKC was 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 modulate important aspects of endothelial and monocyte function provides a novel mechanism by which platelets may interact with such cells in human atherosclerosis and inflammation.
Insights
Platelet microparticles (MP) enhance monocyte adhesion to endothelial cells and promote monocyte migration. This interaction, mediated by arachidonic acid and protein kinase C, suggests a role for MP in atherosclerosis and inflammation.
Area of Science:
- Cardiovascular Biology
- Cellular Biology
- Immunology
Background:
- Platelets release microparticles (MP) upon activation, but their biological roles are not fully understood.
- Platelet-derived MP have been shown to influence platelet and endothelial cell functions.
- The specific mechanisms by which MP mediate cellular activation require further investigation.
Purpose of the Study:
- To investigate the mechanism of cellular activation by platelet-derived MP.
- To examine the effects of platelet MP on monocyte-endothelial cell interactions in vitro.
- To determine if platelet MP influence monocyte adhesion and chemotaxis.
Main Methods:
- In vitro adhesion assays using human umbilical vein endothelial cells (HUVEC) and U-937 cells (monocyte model).
- Dose- and time-dependent analysis of MP effects on cellular adhesion and chemotaxis.
- Analysis of cell adhesion molecule expression (ICAM-1, VCAM-1, selectins, integrins) following MP stimulation.
- Investigation of the role of arachidonic acid and protein kinase C (PKC) using isolated arachidonic acid and a PKC inhibitor (GF 109203X).
Main Results:
- Platelet MP significantly increased monocyte and U-937 cell adhesion to HUVEC in a time- and dose-dependent manner.
- MP-induced adhesion was mimicked by arachidonic acid and involved upregulation of ICAM-1 on HUVEC and specific integrins on monocytes.
- Platelet MP also induced dose-dependent chemotaxis of U-937 cells, an effect mimicked by arachidonic acid.
- Protein kinase C (PKC) activation was implicated in both MP-induced adhesion and chemotaxis, as inhibition of PKC significantly reduced these responses.
Conclusions:
- Platelet-derived MP modulate key aspects of endothelial and monocyte function, including adhesion and chemotaxis.
- Arachidonic acid released from MP and protein kinase C signaling play crucial roles in mediating these effects.
- These findings suggest a novel mechanism for platelet interaction with monocytes and endothelial cells, potentially contributing to atherosclerosis and inflammation.
