Involvement of Phospholipase A2in H2O2-dependent Platelet Activation

Platelet Oxidative Stress and its Relationship with Cardiovascular Diseases in Type 2 Diabetes Mellitus Patients

Enhanced platelet activation and thrombosis are linked to various cardiovascular diseases (CVD). Among other mechanisms, oxidative stress seems to play a pivotal role in platelet hyperactivity. Indeed, upon stimulation by physiological agonists, human platelets generate and release several types of reactive oxygen species (ROS) such as O2 -, H2O2 or OH-, further amplifying the platelet activation response via various signalling pathways, including, formation of isoprostanes, Ca2+ mobilization and NO inactivation. Furthermore, excessive platelet ROS generation, incorporation of free radicals from environment and/or depletion of antioxidants induce pro-oxidant, pro-inflammatory and platelet hyperaggregability effects, leading to the incidence of cardiovascular events. Here, we review the current knowledge regarding the effect of oxidative stress on platelet signaling pathways and its implication in CVD such as type 2 diabetes mellitus. We also summarize the role of natural antioxidants included in vegetables, fruits and medicinal herbs in reducing platelet function via an oxidative stress-mediated mechanism.

Antiplatelet and Anticoagulant Activities of Angelica shikokiana Extract and Its Isolated Compounds

Angelica shikokiana is a Japanese medicinal plant that is used traditionally in several ailments of cardiovascular diseases. However, there is no report regarding its anticoagulant or antiplatelet activities. So this study was designed to screen for such activities (anticoagulant by prothrombin time [PT], activated partial thromboplastin time, and thrombin time assays and antiplatelet activities against adenosine 5′-diphosphate [ADP] and arachidonic acid-induced platelet aggregations) for the methanol extract of the aerial part ( Angelica methanol extract [AME]), its isolated coumarins, flavonoids, and flavonoid metabolites. The AME had potent anticoagulant and antiplatelet activities, and the flavonoid compounds were evidenced to be responsible for such activities. Among coumarins compounds, hyuganin C showed significant prolongation of only PT, while other coumarins were inactive. Similarly, hyuganin C and bergapten were the only active coumarins against ADP-induced platelet aggregation. Compared to the parent compounds, colonic metabolites of the flavonoids had similar anticoagulant and antiplatelet activities, while glucuronides showed sharp decreases in all studied activities. This is the first report showing that the medicinal plant A shikokiana has potent antiplatelet and anticoagulant activities.

Platelet Indices in Patients with Coal Workers' Pneumoconiosis

PURPOSE

Coal workers' pneumoconiosis (CWP) is caused by an accumulation of coal particles in the lung parenchyma. Reactive oxygen species (ROS) play an important role in the pathogenesis of CWP. It is well documented that ROS stimulate platelet activation, adhesion, and aggregation. The aim of the present study was to investigate the relationship between platelet indices and CWP.

METHODS

The demographic features, occupational and medical history, exposure time, and complete blood count of retired coal miners were retrospectively analysed. The control group comprised healthy subjects who had not worked underground. Chest radiographs were evaluated according to the CWP classification of the International Labour Office.

RESULTS

The study population was divided into following groups: 50 controls, 97 without CWP, 142 simple pneumoconiosis (SP), 157 progressive massive fibrosis (PMF). The platelet count, mean platelet volume (MPV), platelet distribution width (PDW), and plateletcrit (PCT) were significantly higher in patients with CWP compared to those in patients without CWP and controls. The platelet count, MPV, and PCT were higher in patients with PMF compared to those in the SP group (p < 0.05). The results of multiple logistic regression analyses indicated that the platelet count, MPV, and PCT were independently associated with the development of CWP.

CONCLUSIONS

Platelet indices may be considered as disease markers for pneumoconiosis in coal miners and as a useful indicator of the progression of pneumoconiosis.

Antiplatelet effects of piplartine, an alkamide isolated from Piper tuberculatum: possible involvement of cyclooxygenase blockade and antioxidant activity

Objectives

Piplartine (piperlongumine; 5,6-dihydro-1-[1-oxo-3-(3,4,5-trimethoxyphenyl]-2(1H) pyridinone) is an alkaloid amide isolated from Piper species (Piperaceae). It has been reported to show multiple pharmacological activities in vitro and in vivo.

Methods

We evaluated the in-vitro antiplatelet effect of piplartine isolated from the roots of P. tuberculatum, on human platelet aggregation induced in platelet-rich plasma by the agonists collagen, adenosine 5′-diphosphate (ADP), arachidonic acid (AA) and thrombin.

Key findings

Piplartine (100μg/ml) caused a 30% inhibition in platelet aggregation when collagen was the agonist. At 200 μg/ml, piplartine significantly inhibited the aggregation induced by arachidonic acid (100%), collagen (59%) or ADP (52%) but not that induced by thrombin. The highest concentration of piplartine (300 μg/ml) inhibited thrombin- (37%), ADP- (71%) and collagen- (98%) induced aggregation. The inhibitory effect of piplartine on ADP-induced platelet aggregation was not modified by pretreatment with pentoxifylline (a phosphodiesterase inhibitor), l-arginine (a substrate for nitric oxide synthase) or ticlopidine (a P2Y12 purinoceptor antagonist). However, aspirin, a well-known inhibitor of cyclooxygenase, greatly increased the inhibitory effect of piplartine on arachidonic-acid-induced platelet aggregation.

Conclusions

The mechanism underlying the piplartine antiplatelet action is not totally clarified. It could be related to the inhibition of cyclooxgenase activity and a decrease in thromboxane A2 formation, similar to that occurring with aspirin. This and other possible mechanisms require further study.

Olive oil isochromans inhibit human platelet reactivity

The effects of certain polyphenolic compounds in red wine, such as resveratrol and quercetin, have been widely investigated to determine the relationship between dietary phenolic compounds and the decreased risk of cardiovascular diseases. However, the effects of polyphenolic compounds contained in other foods, such as olive oil, have received less attention and little information exists regarding the biological activities of the phenol fraction in olive oil. The aim of this study was to evaluate the antiplatelet activity and antioxidant power of two isochromans [1-(3'-methoxy-4'-hydroxy-phenyl)-6,7-dihydroxy-isochroman (encoded L116) and 1-phenyl-6,7-dihydroxy-isochroman (encoded L137)] recently discovered in olive oil and synthesized in our laboratory from hydroxytyrosol. These compounds were effective free radical scavengers and inhibited platelet aggregation and thromboxane release evoked by agonists that induce reactive oxygen species-mediated platelet activation including sodium arachidonate and collagen, but not ADP. Release of tritiated arachidonic acid from platelets was also impaired by L116 and L137. These results indicate that other Mediterranean diet nutraceuticals also exhibit antioxidant activity that could be beneficial in the prevention of vascular diseases.

N-ethylmaleimide-stimulated arachidonic acid release in human platelets

Treatment of human platelets with the alkylating agent N-ethylmaleimide (NEM) induces arachidonic acid release. The effect was time- and dose-dependent. NEM-stimulated arachidonic acid mobilisation could be prevented by pretreating platelets with the cytosolic phospholipase A2 (cPLA2)-specific inhibitor arachidonyltrifluoromethyl ketone. Moreover, the tyrosine kinase inhibitor genistein was able to significantly inhibit arachidonic acid mobilisation. NEM-stimulated release of arachidonic acid appears to be a Ca2+-dependent mechanism, as shown by the observation that arachidonic acid mobilisation was significantly reduced by platelet treatment with EGTA and abolished by preloading platelets with the intracellular chelator 1,2-bis (o-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid tetra (acetoxymethyl) ester (BAPTA/AM). In Fura-2-loaded platelets, NEM was able to significantly increase the intracellular Ca2+ level. The Ca2+ elevation was significantly reduced in the presence of EGTA and suppressed by cell treatment with BAPTA/AM. Arachidonic acid released by NEM produced a significant increase in reactive oxygen species (ROS) intracellular levels, which was partially inhibited by diphenyleneiodonium and almost completely suppressed by 5,8,11,14-eicosatetraynoic acid. In conclusion, the results in this study demonstrate that NEM stimulates arachidonic acid release by cPLA2 activation through intracellular Ca2+ elevation. In addition, tyrosine specific protein kinases seem to be involved in arachidonic acid release. ROS was also shown to be formed during arachidonic acid metabolisation.

The role of (E)-4-hydroxy-2-nonenal in platelet activation by low density lipoprotein and iron

(E)-4-Hydroxy-2-nonenal (HNE) is a highly reactive product of the oxidation of low density lipoprotein (LDL) which increases the platelet aggregation response to various agonists. HNE formation was increased during the enhanced platelet aggregation to thrombin, ADP. A23187 and epinephrine in the presence of LDL. The increase in platelet aggregation and HNE formation by LDL was inhibited by superoxide dismutase and catalase, suggesting superoxide and hydrogen peroxide produced by platelets during aggregation may be at least partly responsible. The responsiveness of platelets to LDL and the accompanying HNE formation was increased further in the presence of ferrous ion. The effect of ferrous ion on both platelet responses and HNE formation was decreased by superoxide dismutase, catalase and the antioxidants dipyridamole and probucol implicating platelet-derived free radicals. Ferrous ion caused an increase in the release of arachidonic acid from platelet membrane phospholipids in the presence of LDL which was probably caused by increased HNE production. The results suggest iron could increase platelet reactivity at sites of vascular injury by increasing HNE formation and promote the development of atherosclerotic lesions.

Hydrogen Peroxide Is Involved in Collagen-Induced Platelet Activation

In this study, we investigated whether (1) collagen-induced platelet aggregation is associated with a burst of H2O2, (2) this oxidant species is involved in the activation of platelets, and (3) the pathways of platelet activation are stimulated by H2O2. Collagen-induced platelet aggregation was associated with production of H2O2, which was abolished by catalase, an enzyme that destroys H2O2. H2O2 production was not observed when ADP or thrombin were used as agonists. Catalase inhibited dose-dependently thromboxane A2 production, release of arachidonic acid from platelet membrane, and Inositol 1,4,5P3 (IP3) formation. In aspirin-treated platelets stimulated with high concentrations of collagen, catalase inhibited platelet aggregation, calcium mobilization, and IP3 production. This study suggests that collagen-induced platelet aggregation is associated with a burst of H2O2 that acts as a second messenger by stimulating the arachidonic acid metabolism and phospholipase C pathway.

Hydrogen Peroxide Is Involved in Collagen-Induced Platelet Activation

In this study, we investigated whether (1) collagen-induced platelet aggregation is associated with a burst of H2O2, (2) this oxidant species is involved in the activation of platelets, and (3) the pathways of platelet activation are stimulated by H2O2. Collagen-induced platelet aggregation was associated with production of H2O2, which was abolished by catalase, an enzyme that destroys H2O2. H2O2 production was not observed when ADP or thrombin were used as agonists. Catalase inhibited dose-dependently thromboxane A2 production, release of arachidonic acid from platelet membrane, and Inositol 1,4,5P3 (IP3) formation. In aspirin-treated platelets stimulated with high concentrations of collagen, catalase inhibited platelet aggregation, calcium mobilization, and IP3 production. This study suggests that collagen-induced platelet aggregation is associated with a burst of H2O2 that acts as a second messenger by stimulating the arachidonic acid metabolism and phospholipase C pathway.

Hyperactivity and increased hydrogen peroxide formation in platelets of NIDDM patients

Free radical activity may contribute to atherosclerotic lesions which in diabetic subjects may frequently lead to vascular complications. It is known that oxidative stress is associated to diabetes. Protein glycation and glucose oxidation could be possible source of free radicals. 28 non insulin dependent diabetic subjects (NIDDM) were examined. 20 healthy subjects matched for age, sex and for the presence of hypertension and hyperlipidemia were also studied. Hydrogen peroxide, measured by intracellular levels of the fluorescent 2,7-dichloro-fluorescein (DCF), was considered as indicative parameter of free radical production. The results showed that in resting platelets the basal level of hydrogen peroxide was significantly higher in diabetic subjects than in controls. Moreover, after stimulation with thrombin, collagen, phorbol myristate acetate (PMA) and platelet activating factor (PAF), platelets of diabetic subjects generated significantly higher amounts of hydrogen peroxide than controls. Moreover, platelet aggregation induced by adenosine 5'-diphosphate (ADP) and plasma beta TG levels were higher in diabetics than in controls. In diabetic patients platelet free radical production and functional activity are increased and therefore could play a role in the elevated thrombotic risk described in diabetes.

Oxygen free radicals and platelet activation

This article reviews our current understanding of the role of oxygen free radicals in platelet activation. Several studies have indicated that platelets, in analogy to other circulating blood cells, are able to produce oxygen free radicals, which are likely to play an important role in the mechanism of platelet activation and aggregation. Platelet activation has been obtained with very low, physiologically relevant concentrations of radicals generated chemically, by leukocytes, and by hemoglobin derived from membrane leakage of erythrocytes. Knowledge of the role of reactive species in platelet physiology is relevant because platelets are brought into close contact with other cells capable of producing free radicals, such as neutrophils, macrophages, and endothelial cells, during the formation of thrombus. The physiopatological importance of these findings is high because it is now emerging that free radicals may have a role in the mechanism of atherosclerosis and its thrombotic complications, where the causative role of platelets is well documented. This background suggests therapeutic interventions with antioxidants as antiplatelet agents to improve the pharmacological effect of classical antiplatelet drug such as aspirin.

Role of hydroxyl radicals in the activation of human platelets

Platelets primed by exposure to subthreshold concentrations of arachidonic acid or collagen are known to be activated by nanomolar levels of hydrogen peroxide. We here demonstrate that this effect is mediated by hydroxyl radicals (OHzero) formed in an extracellular Fenton-like reaction. H2O2-induced platelet aggregation, serotonin release and thromboxane A2 productions were inhibited by OHzero scavengers and by the iron chelator desferrioxamine; hydroxyl radicals were detected directly by ESR measurements of the spin-trapped OHzero adduct. The role of OHzero was confirmed in experiments with exogenously added iron; free or EDTA-bound ferrous iron activated platelets in a process blocked by deoxyribose, mannitol or catalase, whereas ferric iron was without effect unless reductants were included. The activation by OHzero depended on concomitant release of arachidonic acid and was blocked by the phospholipase A2 inhibitors mepacrine and aristolochic acid, and by the Na+/K+ antiporter inhibitor ethylisopropylamiloride. In contrast, neomycin and staurosporin were without effects, indicating that phospholipase C and protein kinase C were not involved in the initial phase of activation. Neither radical formation nor arachidonic acid release was blocked by aspirin. In whole blood aggregation of platelets could be induced by H2O2 generated upon specific stimulation of neutrophils by N-formyl-methionyl-leucyl-phenylalanine; platelet activation and radical formation were blocked by the NADPH oxidase inhibitor diphenyliodonium as well as by catalase and mannitol. These results suggest that reactive oxygen species act as 'second messengers' during the initial phase of the platelet activation process.

Free radical-mediated platelet activation by hemoglobin released from red blood cells

It is known that the rate of thrombus formation depends on interaction between platelets and erythrocytes, but the mechanism of this process has remained obscure. We here show that nanomolar levels of hemoglobin released from damaged red blood cells can induce platelet aggregation. The molecular mechanism is not receptor-based, but involves oxidation of oxyhemoglobin by platelet-derived hydrogen peroxide, with subsequent generation of a small unknown free radical species, detected by ESR spectroscopy. Methemoglobin and carbon monoxide-treated hemoglobin are unable to cause platelet activation or radical formation. The aggregation of platelets induced by hemoglobin is completely blocked by catalase or radical scavengers. These findings indicate a role for a novel extracellular free radical second messenger in the activation of platelets.