In addition to being key elements in hemostasis and thrombosis, platelets amplify neutrophil function. We aimed to gain further insight into the stimuli, mediators, molecular pathways, and regulation of neutrophil extracellular trap formation mediated by human platelets. Platelets stimulated by lipopolysaccharide, a wall component of gram-negative bacteria, Pam3-cysteine-serine-lysine 4, a mimetic of lipopeptide from gram-positive bacteria, Escherichia coli, Staphylococcus aureus, or physiologic platelet agonists promoting neutrophil extracellular trap formation and myeloperoxidase-associated DNA activity under static and flow conditions. Although P-selectin or glycoprotein IIb/IIIa were not involved, platelet glycoprotein Ib, neutrophil cluster of differentiation 18, and the release of von Willebrand factor and platelet factor 4 seemed to be critical for the formation of neutrophil extracellular traps. The secretion of these molecules depended on thromboxane A(2) production triggered by lipopolysaccharide or Pam3-cysteine-serine-lysine 4 but not on high concentrations of thrombin. Accordingly, aspirin selectively inhibited platelet-mediated neutrophil extracellular trap generation. Signaling through extracellular signal-regulated kinase, phosphatidylinositol 3-kinase, and Src kinases, but not p38 or reduced nicotinamide adenine dinucleotide phosphate oxidase, was involved in platelet-triggered neutrophil extracellular trap release. Platelet-mediated neutrophil extracellular trap formation was inhibited by prostacyclin. Our results support a role for stimulated platelets in promoting neutrophil extracellular trap formation, reveal that an endothelium-derived molecule contributes to limiting neutrophil extracellular trap formation, and highlight platelet inhibition as a potential target for controlling neutrophil extracellular trap cell death.

Summary. Background: Although platelets are anucleated cells, they express several transcription factors that exert nongenomic functions, including the positive and negative regulation of platelet activation. NF-jB is a major transcriptional regulator of genes involved in survival, proliferation and inflammation. Objective: Because platelets play a critical role not only in hemostasis, but also in inflammation and tumor progression, we evaluated the role of NF-jB in platelet physiology. Results: Immunofluorescence, Western blotting and ELISA studies revealed that platelets express IjBa and NF-jB, and that stimulation with thrombin triggers IjBa phosphorylation and degradation and the binding of platelet NF-jB p65 subunit to synthetic olignoucleotides containing the consensus sequence for NF-jB. Two specific unrelated inhibitors of NF-jB activation, BAY 11-7082 and Ro 106-9920, reduced PAC-1 and fibrinogen binding to integrin a IIb b 3 and restricted platelet spreading on immobilized fibrinogen. Both inhibitors impaired aggregation mediated by ADP, epinephrine, collagen or thrombin, but not arachidonic acid. ATP release, TXB 2 formation, P-selectin expression, ERK phosphorylation and cPLA 2 activity stimulated by thrombin were reduced in BAY 11-7082-or Ro 106-9920-treated platelets. Although bleeding time was not affected, ADP-induced platelet aggregation was impaired in mice treated with BAY 11-7082. Conclusions: Our results suggest that NF-jB may be a novel mediator of platelet responses. The blockade of platelet function by NF-jB inhibitors might be relevant in those clinical situations where these drugs are being considered for antitumor and/or anti-inflammatory therapy.

Platelet activation is a critical process during inflammation, thrombosis, and cancer. Here, we show that galectin-1, an endogenous lectin with immunoregulatory properties, plays a key role in human platelet activation and function. Galectin-1 binds to human platelets in a carbohydrate-dependent manner and synergizes with ADP or thrombin to induce platelet aggregation and ATP release. Furthermore, galectin-1 induces F-actin polymerization, up-regulation of P-selectin, and GPIIIa expression; promotes shedding of microvesicles; and triggers conformational changes in GPIIb/IIIa. In addition, exposure to this lectin favors the generation of leukocyte-platelet aggregates. A further mechanistic analysis revealed the involvement of Ca(2+) and cyclic nucleotide-dependent pathways in galectin-1-mediated control of platelet activation. Finally, expression of endogenous galectin-1 in human platelets contributes to ADP-induced aggregation. Our study reveals a novel unrecognized role for galectin-1 in the control of platelet physiology with potential implications in thrombosis, inflammation, and metastasis.