Evidence for a NO synthase in porcine platelets which is stimulated during activation/aggregation

Inflammatory Mediators of Platelet Activation: Focus on Atherosclerosis and COVID-19

Background: Atherosclerotic cardiovascular diseases are characterized by a dysregulated inflammatory and thrombotic state, leading to devastating complications with increased morbidity and mortality rates. Summary: In this review article, we present the available evidence regarding the impact of inflammation on platelet activation in atherosclerosis. Key messages: In the context of a dysfunctional vascular endothelium, structural alterations by means of endothelial glycocalyx thinning or functional modifications through impaired NO bioavailability and increased levels of von Willebrand factor result in platelet activation. Moreover, neutrophil-derived mediators, as well as neutrophil extracellular traps formation, have been implicated in the process of platelet activation and platelet-leukocyte aggregation. The role of pro-inflammatory cytokines is also critical since their receptors are also situated in platelets while TNF-α has also been found to induce inflammatory, metabolic, and bone marrow changes. Additionally, important progress has been made towards novel concepts of the interaction between inflammation and platelet activation, such as the toll-like receptors, myeloperoxidase, and platelet factor-4. The accumulating evidence is especially important in the era of the coronavirus disease-19 pandemic, characterized by an excessive inflammatory burden leading to thrombotic complications, partially mediated by platelet activation. Lastly, recent advances in anti-inflammatory therapies point towards an anti-thrombotic effect secondary to diminished platelet activation.

L-lysine protects C2C12 myotubes and 3T3-L1 adipocytes against high glucose damages and stresses

Hyperglycemia is a hallmark of diabetes, which is associated with protein glycation and misfolding, impaired cell metabolism and altered signaling pathways result in endoplasmic reticulum stress (ERS). We previously showed that L-lysine (Lys) inhibits the nonenzymatic glycation of proteins, and protects diabetic rats and type 2 diabetic patients against diabetic complications. Here, we studied some molecular aspects of the Lys protective role in high glucose (HG)-induced toxicity in C2C12 myotubes and 3T3-L1 adipocytes. C2C12 and 3T3-L1 cell lines were differentiated into myotubes and adipocytes, respectively. Then, they were incubated with normal or high glucose (HG) concentrations in the absence/presence of Lys (1 mM). To investigate the role of HG and/or Lys on cell apoptosis, oxidative status, unfolded protein response (UPR) and autophagy, we used the MTT assay and flow cytometry, spectrophotometry and fluorometry, RT-PCR and Western blotting, respectively. In both cell lines, HG significantly reduced cell viability and induced apoptosis, accompanying with the significant increase in reactive oxygen species (ROS) and nitric oxide (NO). Furthermore, the spliced form of X-box binding protein 1 (XBP1), at both mRNA and protein levels, the phosphorylated eukaryotic translation initiation factor 2α (p-eIf2α), and the Light chain 3 (LC3)II/LC3I ratio was also significantly increased. Lys alone had no significant effects on most of these parameters; but, treatment with HG plus Lys returned them all to, or close to, the normal values. The results indicated the protective role of Lys against glucotoxicity induced by HG in C2C12 myotubes and 3T3-L1 adipocytes.

Hydrogen Sulfide and Platelets: A Possible Role in Thrombosis

Platelets are circulating blood elements with key roles in haemostasis and thrombosis. Platelets are activated by a range of stimuli including exposed subendothelial components. Haemostasis also depends upon the effects of inhibitory substances, including the gasotransmitter nitric oxide whose effects on platelets are well documented. Evidence is also emerging to suggest that H2S is generated enzymatically by platelets and can impact their function. Exposure of platelets to H2S from slow-release compounds inhibits aggregation and exerted anti-thrombotic effects in vivo. The mechanisms by which H2S impacts platelet function and the importance of interactions between H2S and other gasotransmitters remain unclear. H2S is therefore emerging as a potentially important regulator of platelet activation and thrombosis. Further study is required to evaluate its importance as a regulator of platelet physiology and associated pathological conditions such as myocardial infarction and stroke.

Abnormal platelet aggregation in idiopathic pulmonary arterial hypertension: role of nitric oxide

Idiopathic pulmonary arterial hypertension (IPAH) is a rare and progressive disease. Several processes are believed to lead to the fatal progressive pulmonary arterial narrowing seen in IPAH including vasoconstriction, cellular proliferation inflammation, vascular remodeling, abnormalities in the lung matrix, and in situ thrombosis. Nitric oxide (NO) produced by NO synthases (NOS) is a potent vasodilator and plays important roles in many other processes including platelet function. Reduced NO levels in patients with IPAH are known to contribute to the development of pulmonary hypertension and its complications. Platelet defects have been implied in IPAH, but original research supporting this hypothesis has been limited. Normal platelets are known to have NOS activity, but little is known about NOS expression and NO production by platelets in patients with IPAH. Here we characterized the phenotype of the platelets in IPAH and show a defect in their ability to be activated in vitro by thrombin receptor activating protein but not adenosine diphosphate. We also show that endothelial NOS (eNOS) levels in these platelets are reduced and demonstrate that NO is an important regulator of platelet function. Thus reduced levels of eNOS in platelets could impact their ability to regulate their own function appropriately.

Role of reactive nitrogen species in blood platelet functions

Blood platelets, in analogy to other circulating blood cells, can generate reactive oxygen/nitrogen species (ROS/RNS) that may behave as second messengers and may regulate platelet functions. Accumulating evidence suggest a role of ROS/RNS in platelet activation. On the other hand, an increased production of ROS/RNS causes oxidative stress, and thus, may contribute to the development of different diseases, including vascular complications, inflammatory and psychiatric illnesses. Oxidative stress in platelets leads to chemical changes in a wide range of their components, and platelet proteins may be initial targets of ROS/RNS action. It has been demonstrated that reaction of proteins with ROS/RNS results in the oxidation and nitration of some amino acid residues, formation of aggregates or fragmentation of proteins. In oxidized proteins new carbonyl groups and protein hydroperoxides are also formed. In platelets, low molecular weight thiols such as glutathione (GSH), cysteine and cysteinylglycine and protein thiols may be also target for ROS/RNS action. This review describes the chemical structure and biological activities of reactive nitrogen species, mainly nitric oxide ((*)NO) and peroxynitrite (ONOO(-)) and their effects on blood platelet functions, and the mechanisms involved in their action on platelets.

Augmentation of U46619 induced human platelet aggregation by aspirin

Aspirin is known to suppress platelet function markedly. However, aspirin at concentrations higher than 1 mM was observed to augment 1.3 microM U46619 (a stable thromboxane receptor (TP receptor) agonist) induced human platelet aggregation in this study. Moreover, at a concentration as low as 250 microM aspirin increased the aggregation induced by U46619 in 13% of normal and healthy individuals. The degree of platelet aggregation and the amount of ATP release were enhanced in U46619 stimulated platelet rich plasma by the addition of aspirin (>250 microM). U46619 was previously reported to inhibit forskolin-stimulated adenyl cyclase and to reduce the cAMP formation. Both of the augmentation effects of aspirin on U46619-induced aggregation and ATP release were blocked by MeSAMP, a P2Y(12) receptor antagonist. U46619 induced aggregation was suppressed by the addition of ADP scavenger (CP/CPK) with no significant change on ATP measured and the effect of CP/CPK could not be reversed by aspirin. In addition, aspirin augmented the inhibitory effect of U46619 on the cAMP production. Our present results suggested that the potentiation effect of aspirin on U46619 induced aggregation was related with the secreted ADP and the subsequent P2Y(12)/Gi related signaling.

Unresolved roles of platelet nitric oxide synthase

Endothelial-derived nitric oxide (NO) is a key regulator of platelet function, inhibiting both adhesion to the extracellular matrix and aggregation at sites of vascular injury. Platelets also have the capacity to synthesize and release bioactive NO, which is thought to make a significant contribution to the vascular pool of NO. The regulation of platelet NO production is poorly understood and studies examining the physiological role of platelet-derived NO have produced contradictory and controversial findings. In the present article, we discuss the current understanding of the biochemical and molecular regulation of platelet NO synthesis and outline the potential physiological and clinical significance of this molecule.

Nitric oxide and wound repair: role of cytokines?

Wound healing involves platelets, inflammatory cells, fibroblasts, and epithelial cells. All of these cell types are capable of producing nitric oxide (NO), either constitutively or in response to inflammatory cytokines, through the activity of nitric oxide synthases (NOSs): eNOS (NOS3; endothelial NOS) and iNOS (NOS2; inducible NOS), respectively. Indeed, pharmacological inhibition or gene deletion of these enzymes impairs wound healing. The wound healing mechanisms that are triggered by NO appear to be diverse, involving inflammation, angiogenesis, and cell proliferation. All of these processes are controlled by defined cytokine cascades; in many cases, NO appears to modulate these cytokines. In this review, we summarize the history and present state of research on the role of NO in wound healing within the framework of modulation of cytokines.

Modulation of human platelet aggregation by the phosphodiesterase type 5 inhibitor sildenafil

The aim of this study was to investigate if the phospodiesterase type 5 inhibitor sildenafil inhibits collagen- or ADP-induced human platelet aggregation and bleeding time. To investigate this, two studies were designed. In the first, a single oral dose of sildenafil, 100 mg, was administered to healthy men. Bleeding time was determined and agonist (ADP and collagen)-induced platelet aggregation (ex vivo in platelet rich plasma) was measured 0, 1, and 4 h after application. In the second, a single oral dose of sildenafil, 50 mg, was administered and, in addition to the parameters in the first study, we also determined the platelet aggregation after 24 h and measured the effect of a nitric oxide donor (S-nitroso-N-acetylpenicillamine [SNAP]) in combination to mimic a physiologic nitric oxide release from the endothelium. The bleeding time of 1 h after sildenafil medication (100 mg) was significantly prolonged but recovered toward control values after 4 h, whereas application of sildenafil at a lower dose (50 mg) did not alter the bleeding time. Sildenafil (100 and 50 mg) did not inhibit the ADP-induced aggregation, whereas the collagen-induced aggregation (100 mg) was markedly reduced after 1 h and significantly inhibited 4 h after application. This inhibitory effect was overcome by higher concentrations of collagen. SNAP (0.5 microM) induced an inhibition of platelet aggregation that was potentiated after taking sildenafil (50 mg, 1 and 4 h afterward) and abrogated after 24 h. These data indicates that sildenafil may inhibit collagen-induced platelet aggregation ex vivo. After co-administration of nitric oxide, collagen- and ADP-induced platelet aggregation was significantly inhibited, which may reflect physiologic conditions of an in vivo system.

Expression of an endothelial-type nitric oxide synthase isoform in human neutrophils: modification by tumor necrosis factor-alpha and during acute myocardial infarction

OBJECTIVES

The purpose of this study was to determine whether human neutrophils express an endothelial-type nitric oxide synthase (eNOS), and to study the effect of tumor necrosis factor-alpha (TNF-alpha) on its expression.

BACKGROUND

Several studies have demonstrated the presence of a constitutively expressed nitric oxide svnthase (NOS) in neutrophils. Cardiovascular disease is characterized by increased levels of plasma TNF-alpha, a cytokine that has demonstrated eNOS messenger ribonucleic acid (mRNA) destabilization in cultured endothelial cells.

METHODS

Neutrophils were obtained from healthy volunteers and from patients with acute myocardial infarction (AMI).

RESULTS

Human neutrophils express eNOS mRNA and eNOS protein. Stimulation of neutrophils with TNF-alpha decreased eNOS protein expression by reducing eNOS mRNA stabilization. In the present study, we also show that the cytosol of human neutrophils contains proteins that bind to a specific region within the 3'-untranslated region (3'-UTR) of eNOS mRNA. Tumor necrosis factor-alpha increased the binding of the cytosolic proteins to the 3'-UTR of eNOS mRNA. Simvastatin reduced the TNF-alpha-related binding activity of neutrophil cytosolic proteins to eNOS mRNA, which was associated with its protective effect on eNOS protein expression. The in vivo reproduction of the in vitro findings was performed in neutrophils obtained from patients with AMI and showed a diminished expression of eNOS protein, which was associated with increased binding of the cytosolic proteins.

CONCLUSIONS

These observations demonstrate that human neutrophils express eNOS, which is downregulated by TNF-alpha and during AMI. This effect is associated with increased binding of neutrophil cytosolic proteins to the 3'-UTR of eNOS mRNA.

A 4-trifluoromethyl derivative of salicylate, triflusal, stimulates nitric oxide production by human neutrophils: role in platelet function

BACKGROUND

The thrombotic process is a multicellular phenomenon in which not only platelets but also neutrophils are involved. Recent in vitro studies performed in our laboratory have demonstrated that triflusal, a 4-trifluoromethyl derivative of salicylate, reduced platelet aggregation not only by inhibiting thromboxane A2 production but also by stimulating nitric oxide (NO) generation by neutrophils. The aim of the present study was to evaluate whether oral treatment of healthy volunteers with triflusal could modify the ability of their neutrophils to produce NO and to test the role of the NO released by neutrophils in the modulation of ADP-induced platelet aggregation and alpha-granule secretion.

METHODS

The study was performed in 12 healthy volunteers who were orally treated with triflusal (600 mg day-1) for 5 days. Flow cytometric detection of platelet surface expression of P-selectin was used as a measure of the ability of platelets to release the contents of their alpha-granules.

RESULTS

After treatment with triflusal, there was an increase in NO production by neutrophils and an increase in endothelial nitric oxide synthase (eNOS) protein expression in neutrophils. A potentiation of the inhibition of platelet aggregation by neutrophils was reversed by incubating neutrophils with both an L-arginine antagonist, NG-nitro-L-arginine methyl ester (L-NAME) and an NO scavenger, 2-(4-carboxyphenyl)-4,4,5,5 tetramethylimidazoline 1-oxyl 3-oxide (C-PTIO). A slight decrease in P-selectin surface expression on platelets was found which was not modified by the presence of neutrophils and therefore by the neutrophil-derived NO. Exogenous NO released by sodium nitroprusside dose-dependently inhibited both ADP-stimulated alpha-granule secretion and platelet aggregation. Therefore, platelet aggregation showed a greater sensitivity to be inhibited by exogenous NO than P-selectin expression.

CONCLUSION

Oral treatment of healthy volunteers with triflusal stimulated NO production and eNOS protein expression in their neutrophils. After triflusal treatment, the neutrophils demonstrated a higher ability to prevent ADP-induced platelet aggregation. However, the neutrophils and the endogenous NO generated by them failed to modify P-selectin expression in ADP-activated platelets.

Minor involvement of nitric oxide during chronic endotoxemia in anesthetized pigs

To study the modifications of hepatic blood flow and hepatic function over time during endotoxemia, 10 pigs received a continuous intravenous infusion of endotoxin (Endo, 160 ng. kg(-1). h(-1)) over 18 h and 7 control (Ctrl) animals received a saline infusion. The involvement of nitric oxide (NO) in this endotoxic model was assessed by measuring plasma concentrations of NO(-)(2), NO(-)(3), and cGMP, by testing vascular reactivity to ACh, and by evaluating inducible NO synthase (NOS 2) expression in hepatic biopsies. Endotoxin induced hypotensive and normokinetic shock in association with few modifications of hepatic blood flow, and hepatic injury was observed in both groups. Endotoxin did not increase plasma concentrations of NO(-)(2), NO(-)(3), and cGMP. The ACh-dependent decrease of mean arterial pressure was reduced in Endo pigs, whereas a minor difference was observed between Ctrl and Endo pigs for ACh-dependent modification of hepatic perfusion. Hepatic NOS 2 mRNA was not detected in Ctrl pigs. In Endo pigs, NOS 2 protein expression was detected only in tissues surrounding the portal vein and the inferior vena cava, whereas NOS 2 mRNA was expressed in all hepatic biopsies. Thus, although endotoxemia induces NOS 2 expression in the liver, our findings show that NO involvement is lower in pigs than in rodents during endotoxemia.