Platelet alpha-granule secretion and its modification by SC-57101A, a GPIIb/IIIa antagonist

Platelets and their chemokines in atherosclerosis-clinical applications

The concept of platelets as important players in the process of atherogenesis has become increasingly accepted due to accumulating experimental and clinical evidence. Despite the progress in understanding the molecular details of atherosclerosis, particularly by using animal models, the inflammatory and thrombotic roles of activated platelet s especially in the human system remain difficult to dissect, as often only the complications of atherosclerosis, i.e., stroke and myocardial infarction are definable but not the plaque burden. Platelet indices including platelet count and mean platelet volume (MPV) and soluble mediators released by activated platelets are associated with atherosclerosis. The chemokine CXCL4 has multiple atherogenic activities, e.g., altering the differentiation of T cells and macrophages by inhibiting neutrophil and monocyte apoptosis and by increasing the uptake of oxLDL and synergizing with CCL5. CCL5 is released and deposited on endothelium by activated platelets thereby triggering atherogenic monocyte recruitment, which can be attenuated by blocking the corresponding chemokine receptor CCR5. Atheroprotective and plaque stabilizing properties are attributed to CXCL12, which plays an important role in regenerative processes by attracting progenitor cells. Its release from luminal attached platelets accelerates endothelial healing after injury. Platelet surface molecules GPIIb/IIIa, GP1bα, P-selectin, JAM-A and the CD40/CD40L dyade are crucially involved in the interaction with endothelial cells, leukocytes and matrix molecules affecting atherogenesis. Beyond the effects on the arterial inflammatory infiltrate, platelets affect cholesterol metabolism by binding, modifying and endocytosing LDL particles via their scavenger receptors and contribute to the formation of lipid laden macrophages. Current medical therapies for the prevention of atherosclerotic therapies enable the elucidation of mechanisms linking platelets to inflammation and atherosclerosis.

Human neutrophil alpha-defensins induce formation of fibrinogen and thrombospondin-1 amyloid-like structures and activate platelets via glycoprotein IIb/IIIa

BACKGROUND

Human neutrophil α-defensins (HNPs) are important constituents of the innate immune system. Beyond their antimicrobial properties, HNPs also have pro-inflammatory features. While HNPs in plasma from healthy individuals are barely detectable, their level is strongly elevated in septic plasma and plasma from patients with acute coronary syndromes.

OBJECTIVES

As thrombosis and inflammation are intertwined processes and activation of human polymorphonuclear leukocytes (PMNL) and subsequent degranulation is associated with full activation of surrounding platelets, we studied the effect of HNPs on platelet function.

METHODS

The effect of HNPs on platelet activation parameters and apoptosis was investigated via aggregometry, flow cytometry, confocal microscopy and the ELISA technique.

RESULTS

It was found that HNPs activate platelets in pathophysiologically relevant doses, inducing fibrinogen and thrombospondin-1 binding, aggregation, granule secretion, sCD40L shedding, and procoagulant activity. HNPs bound directly to the platelet membrane, induced membrane pore formation, microparticle formation, mitochondrial membrane depolarization and caspase-3-activity. Confocal microscopy revealed the HNP-induced formation of polymeric fibrinogen and thrombospondin-1 amyloid-like structures, which bound microorganisms. Platelets adhered to these structures and formed aggregates. Blocking of glycoprotein IIb/IIIa (GPIIb/IIIa) markedly inhibited HNP-induced platelet activation. In addition, heparin, heparinoid, serpins and α(2)-macroglobulin, which all bind to HNPs, blocked HNP-1-induced platelet activation in contrast to direct thrombin inhibitors such as hirudin.

CONCLUSIONS

HNPs activate platelets and induce platelet apoptosis by formation of amyloid-like proteins. As these structures entrapped bacteria and fungi, they might reflect an additional function of HNPs in host defense. The described mechanism links again thrombosis and infection.

Saxatilin, a Snake Venom Disintegrin, Regulates Platelet Activation Associated with Human Vascular Endothelial Cell Migration and Invasion

BACKGROUND

Platelet activation results in platelet aggregation and the secretion of granules, which contain a variety of constituents including nonprotein molecules, adhesive proteins and hydrolases. The platelet-derived supernatant (PDS), which contains these granules, is known to trigger the activation of endothelium and chemotaxis of monocytes.

METHODS AND RESULTS

PDS derived from collagen-activated platelets stimulated human umbilical vein endothelial cell (HUVEC) migration and invasion, as measured through the use of a Boyden chamber. This collagen-induced PDS also triggered integrin alpha(v)beta(3) upregulation in HUVECs. The inclusion of a neutralizing antibody to platelet-derived growth factor (PDGF)-B abolished HUVEC migration/invasion and integrin alpha(v)beta(3) upregulation, showing that PDGF-AB mediates the proangiogenic effects of collagen-activated PDS. Saxatilin, a snake venom disintegrin known to interrupt platelet aggregation by antagonizing integrin alpha(IIb)beta(3), inhibited the collagen-induced platelet activation and abolished the angiogenic properties of PDS. Saxatilin also inhibited the collagen-induced phosphorylation of Syk, a key mediator of inside-out signaling in platelet activation.

CONCLUSION

Saxatilin inhibits platelet activation, platelet PDGF-AB release as well as subsequent endothelial cell migration and invasion.