Hydrogen peroxide and peroxynitrite enhance Ca2+ mobilization and aggregation in platelets from type 2 diabetic patients

Vitamin D Supplementation Modulates Platelet-Mediated Inflammation in Subjects With Type 2 Diabetes: A Randomized, Double-Blind, Placebo-Controlled Trial

Background

Recently, our group identified increased platelet-mediated inflammation in type 2 diabetes (T2DM) patients, and it is a well-established risk factor for diabetes complications, particularly for the development of cardiovascular diseases (CVD). Furthermore, vitamin D is reported to play an important role in the modulation of platelet hyperactivity and immune function, although the effect of vitamin D on platelet-mediated inflammation is not well studied. Hence, we aimed to investigate the effect of vitamin D supplementation on platelet-mediated inflammation in T2DM patients.

Methods

After screening a total of 201 subjects, our randomized, double-blind, placebo-controlled trial included 59 vitamin-D-deficient T2DM subjects, and the participants were randomly assigned to placebo (n = 29) or vitamin D3 (n = 30) for 6 months. Serum vitamin D metabolite levels, immunome profiling, platelet activation, and platelet-immune cell aggregate formation were measured at baseline and at the end of the study. Similarly, the serum levels of inflammatory cytokines/chemokines were assessed by a multiplex assay.

Results

Six months of vitamin D supplementation increases the serum vitamin D3 and total 25(OH)D levels from the baseline (p < 0.05). Vitamin D supplementation does not improve glycemic control, and no significant difference was observed in immune cells. However, platelet activation and platelet immune cell aggregates were altered after the vitamin D intervention (p < 0.05). Moreover, vitamin D reduces the serum levels of IL-18, TNF-α, IFN-γ, CXCL-10, CXCL-12, CCL-2, CCL-5, CCL-11, and PF-4 levels compared to the baseline levels (p < 0.05). Our ex vivo experiment confirms that a sufficient circulating level of vitamin D reduces platelet activation and platelet intracellular reactive oxygen species.

Conclusion

Our study results provide evidence that vitamin D supportive therapy may help to reduce or prevent the disease progression and cardiovascular risk in T2DM patients by suppressing oxidative stress and platelet-mediated inflammation.

Clinical Trial Registration

Clinical Trial Registry of India: CTRI/2019/01/016921.

An Insight into Phytochemical, Pharmacological, and Nutritional Properties of Arbutus unedo L. from Morocco

Arbutus unedo L. (Ericaceae) is an evergreen shrub widely distributed in the Mediterranean region, particularly through the Moroccan forests. It is an important medicinal plant of great scientific interest due to its nutritional, pharmacological, and chemical properties. The objective of this review is to provide insights into traditional medicinal uses and phytochemical and pharmacological properties of A. unedo from Morocco. In Morocco, the plant has been used as a traditional medicine to treat several pathological conditions. Many phytochemical compounds have been reported in the plant, of which vitamins, carotenoids, flavonoids, polyphenols, tannins, and their derivatives are the most prevalent. Leaves and fruits of A. unedo contain the most significant number of phytochemicals among the species. Furthermore, researchers have demonstrated that A. unedo exhibited antioxidant, anticancer, antibacterial, antidiabetic, antiaggregant, and antihypertensive activities due to the presence of many biochemical compounds with health-promoting properties. According to different toxicity tests, the use of A. unedo is devoid of any significant side effects and/or toxicity. Despite its nutraceutical and health-promoting properties, Moroccan A. unedo remains underexploited mainly, and most of its traditional uses have not yet undergone scientific evidence-based research; therefore, improved knowledge about the potential value of the plant would allow understanding of its biological activity based on its phytochemical compounds that may contribute to the species preservation and valorization.

Novel perspectives on redox signaling in red blood cells and platelets in cardiovascular disease

The fundamental physiology of circulating red blood cells (RBCs) and platelets involving regulation of oxygen transport and hemostasis, respectively, are well-described in the literature. Their abundance in the circulation and their interaction with the vascular wall and each other have attracted the attention of other putative physiological and pathophysiological effects of these cells. RBCs and platelets are both important regulators of redox balance harboring powerful pro-oxidant and anti-oxidant (enzymatic and non-enzymatic) capacities. They are also involved in the regulation of vascular tone mainly via export of nitric oxide bioactivity and adenosine triphosphate. Of further importance are emerging observations that these cells undergo functional alterations when exposed to risk factors for cardiovascular disease and during developed cardiometabolic diseases. Under these conditions, the RBCs and platelets contribute to increased oxidative stress by their formation of reactive species including superoxide anion radical, hydrogen peroxide and peroxynitrite. These alterations trigger key changes in the vascular wall characterized by enhanced oxidative stress, reduced nitric oxide bioavailability and endothelial dysfunction. Additional pathophysiological effects are triggered in the heart resulting in increased susceptibility to ischemia-reperfusion injury with impairment in cardiac function. Pharmacological interventions aiming at restoring circulating cell function has been shown to exert marked beneficial effects on cardiovascular function. In this review, we summarize the current knowledge of RBC and platelet biology with special focus on redox biology, their roles in the development of cardiovascular disease and potential therapeutic strategies targeting RBC and platelet dysfunction. Finally, the complex and scarcely understood interaction between RBCs and platelets is discussed.

ROS in Platelet Biology: Functional Aspects and Methodological Insights

Reactive oxygen species (ROS) and mitochondria play a pivotal role in regulating platelet functions. Platelet activation determines a drastic change in redox balance and in platelet metabolism. Indeed, several signaling pathways have been demonstrated to induce ROS production by NAPDH oxidase (NOX) and mitochondria, upon platelet activation. Platelet-derived ROS, in turn, boost further ROS production and consequent platelet activation, adhesion and recruitment in an auto-amplifying loop. This vicious circle results in a platelet procoagulant phenotype and apoptosis, both accounting for the high thrombotic risk in oxidative stress-related diseases. This review sought to elucidate molecular mechanisms underlying ROS production upon platelet activation and the effects of an altered redox balance on platelet function, focusing on the main advances that have been made in platelet redox biology. Furthermore, given the increasing interest in this field, we also describe the up-to-date methods for detecting platelets, ROS and the platelet bioenergetic profile, which have been proposed as potential disease biomarkers.

A novel combinatorial technique for simultaneous quantification of oxygen radicals and aggregation reveals unexpected redox patterns in the activation of platelets by different physiopathological stimuli

The regulation of platelets by oxidants is critical for vascular health and may explain thrombotic complications in diseases such as diabetes and dementia, but remains poorly understood. Here, we describe a novel technique combining electron paramagnetic resonance spectroscopy and turbidimetry, which has been utilized to monitor simultaneously platelet activation and oxygen radical generation. This technique has been used to investigate the redox-dependence of human and mouse platelets. Using selective peptide inhibitors of NADPH oxidases (NOXs) on human platelets and genetically modified mouse platelets (NOX1^−/− or NOX2^−/−), we discovered that: 1) intracellular but not extracellular superoxide anion generated by NOX is critical for platelet activation by collagen; 2) superoxide dismutation to hydrogen peroxide is required for thrombin-dependent activation; 3) NOX1 is the main source of oxygen radicals in response to collagen, while NOX2 is critical for activation by thrombin; 4) two platelet modulators, namely oxidized low density lipoproteins (oxLDL) and amyloid peptide β (Aβ), require activation of both NOX1 and NOX2 to pre-activate platelets. This study provides new insights into the redox dependence of platelet activation. It suggests the possibility of selectively inhibiting platelet agonists by targeting either NOX1 (for collagen) or NOX2 (for thrombin). Selective inhibition of either NOX1 or NOX2 impairs the potentiatory effect of tested platelet modulators (oxLDL and Aβ), but does not completely abolish platelet hemostatic function. This information offers new opportunities for the development of disease-specific antiplatelet drugs with limited bleeding side effects by selectively targeting one NOX isoenzyme.

The Main Determinants of Diabetes Mellitus Vascular Complications: Endothelial Dysfunction and Platelet Hyperaggregation

Diabetes mellitus is a common disease that affects 3–5% of the general population in Italy. In some countries of northern Europe or in North America, it can even affect 6–8% of the population. Of great concern is that the number of cases of diabetes is constantly increasing, probably due to the increase in obesity and the sedentary nature of the population. According to the World Health Organization, in the year 2030 there will be 360 million people with diabetes, compared to 170 million in 2000. This has important repercussions on the lives of patients and their families, and on health systems that offer assistance to patients. In this review, we try to describe in an organized way the pathophysiological continuity between diabetes mellitus, endothelial dysfunction, and platelet hyperaggregation, highlighting the main molecular mechanisms involved and the interconnections.

Conflicting actions of 4-vinylcatechol in rat lymphocytes under oxidative stress induced by hydrogen peroxide

4-Vinylcatechol (4VC) has been identified as an aroma compound in roasted foods, especially coffee. It is also a component in traditional herbal medicines. This compound may be subconsciously ingested through foods and herbs. Recent experimental evidence has shown that 4VC possesses an antioxidative action. However, the antioxidative action of 4VC at cellular levels is not well characterized. The effects of 4VC (0.1-100 µM) were examined on rat thymic lymphocytes without and with oxidative stress induced by 300 µM hydrogen peroxide (H₂O₂). Cell treatment with 100 µM 4VC alone for 4 h significantly increased the population of dead cells. Thus, 4VC at 100 µM or above elicits cytotoxicity. However, 4VC at sublethal concentrations (1-10 µM) significantly attenuated the H₂O₂-induced increase in cell lethality in a concentration-dependent manner. While application of 10 µM 4VC slowed the process of cell death induced by H₂O₂, 4VC did not antagonize the H₂O₂-induced reduction of cellular nonprotein thiols. Although 4VC at 10 µM did not affect intracellular Ca²⁺ and Zn²⁺ levels, the agent potentiated the H₂O₂-induced increases in these levels. These actions of 10 µM 4VC are adverse to the cells under the oxidative stress. However, 10 µM 4VC partly attenuated the cell death induced by 100 nM A23187, a calcium ionophore. There are conflicting actions of 4VC at 1-100 µM on the cells under oxidative stress although the agent is used for an antioxidant. Thus, caution is required when using 4VC as a therapeutic agent.

Stanniocalcin 2 Regulates Non-capacitative Ca2+ Entry and Aggregation in Mouse Platelets

Stanniocalcin 2 (STC2) is a fish protein that controls body Ca²⁺ and phosphate metabolism. STC2 has also been described in mammals, and as platelet function highly depends on both extracellular and intracellular Ca²⁺, we have explored its expression and function in these cells. STC2^−/− mice exhibit shorter tail bleeding time than WT mice. Platelets from STC2-deficient mice showed enhanced aggregation, as well as enhanced Ca²⁺ mobilization in response to the physiological agonist thrombin (Thr) and the diacylglycerol analog, OAG, a selective activator of the non-capacitative Ca²⁺ entry channels. Interestingly, platelets from STC2^−/− mice exhibit attenuated interaction between STIM1 and Orai1 in response to Thr, thus suggesting that STC2 is required for Thr-evoked STIM1-Orai1 interaction and the subsequent store-operated Ca²⁺ entry (SOCE). We have further assessed possible changes in the expression of the most relevant channels involved in non-capacitative Ca²⁺ entry in platelets. Then, protein expression of Orai3, TRPC3 and TRPC6 were evaluated by Western blotting, and the results revealed that while the expression of Orai3 was enhanced in the STC2-deficient mice, others like TRPC3 and TRPC6 remains almost unaltered. Summarizing, our results provide for the first time evidence for a role of STC2 in platelet physiology through the regulation of agonist-induced Ca²⁺ entry, which might be mediated by the regulation of Orai3 channel expression.

Revisiting the safety of aspartame

Aspartame is a synthetic dipeptide artificial sweetener, frequently used in foods, medications, and beverages, notably carbonated and powdered soft drinks. Since 1981, when aspartame was first approved by the US Food and Drug Administration, researchers have debated both its recommended safe dosage (40 mg/kg/d) and its general safety to organ systems. This review examines papers published between 2000 and 2016 on both the safe dosage and higher-than-recommended dosages and presents a concise synthesis of current trends. Data on the safe aspartame dosage are controversial, and the literature suggests there are potential side effects associated with aspartame consumption. Since aspartame consumption is on the rise, the safety of this sweetener should be revisited. Most of the literature available on the safety of aspartame is included in this review. Safety studies are based primarily on animal models, as data from human studies are limited. The existing animal studies and the limited human studies suggest that aspartame and its metabolites, whether consumed in quantities significantly higher than the recommended safe dosage or within recommended safe levels, may disrupt the oxidant/antioxidant balance, induce oxidative stress, and damage cell membrane integrity, potentially affecting a variety of cells and tissues and causing a deregulation of cellular function, ultimately leading to systemic inflammation.

Hydrogen peroxide attenuates refilling of intracellular calcium store in mouse pancreatic acinar cells

Intracellular calcium (Ca²⁺) oscillation is an initial event in digestive enzyme secretion of pancreatic acinar cells. Reactive oxygen species are known to be associated with a variety of oxidative stress-induced cellular disorders including pancreatitis. In this study, we investigated the effect of hydrogen peroxide (H₂O₂) on intracellular Ca²⁺ accumulation in mouse pancreatic acinar cells. Perfusion of H₂O₂ at 300 µM resulted in additional elevation of intracellular Ca²⁺ levels and termination of oscillatory Ca²⁺ signals induced by carbamylcholine (CCh) in the presence of normal extracellular Ca²⁺. Antioxidants, catalase or DTT, completely prevented H₂O₂-induced additional Ca²⁺ increase and termination of Ca²⁺ oscillation. In Ca²⁺-free medium, H₂O₂ still enhanced CCh-induced intracellular Ca²⁺ levels and thapsigargin (TG) mimicked H₂O₂-induced cytosolic Ca²⁺ increase. Furthermore, H₂O₂-induced elevation of intracellular Ca²⁺ levels was abolished under sarco/endoplasmic reticulum Ca²⁺ ATPase-inactivated condition by TG pretreatment with CCh. H₂O₂ at 300 µM failed to affect store-operated Ca²⁺ entry or Ca²⁺ extrusion through plasma membrane. Additionally, ruthenium red, a mitochondrial Ca²⁺ uniporter blocker, failed to attenuate H₂O₂-induced intracellular Ca²⁺ elevation. These results provide evidence that excessive generation of H₂O₂ in pathological conditions could accumulate intracellular Ca²⁺ by attenuating refilling of internal Ca²⁺ stores rather than by inhibiting Ca²⁺ extrusion to extracellular fluid or enhancing Ca²⁺ mobilization from extracellular medium in mouse pancreatic acinar cells.

Unaltered Angiogenesis-Regulating Activities of Platelets in Mild Type 2 Diabetes Mellitus despite a Marked Platelet Hyperreactivity

Type 2 diabetes mellitus (T2DM) is associated with platelet dysfunction and impaired angiogenesis. Aim of the study is to investigate if platelet dysfunction might hamper platelet angiogenic activities in T2DM patients. Sixteen T2DM patients and gender/age-matched non-diabetic controls were studied. Flow cytometry and endothelial colony forming cell (ECFC) tube formation on matrigel were used to assess platelet reactivity and angiogenic activity, respectively. Thrombin receptor PAR1-activating peptide (PAR1-AP) induced higher platelet P-selectin expression, and evoked more rapid and intense platelet annexin V binding in T2DM patients, seen as a more rapid increase of annexin V⁺ platelets (24.3±6.4% vs 12.6±3.8% in control at 2 min) and a higher elevation (30.9±5.1% vs 24.3±3.0% at 8 min). However, PAR1-AP and PAR4-AP induced similar releases of angiogenic regulators from platelets, and both stimuli evoked platelet release of platelet angiogenic regulators to similar extents in T2DM and control subjects. Thus, PAR1-stimulated platelet releasate (PAR1-PR) and PAR4-PR similarly enhanced capillary-like network/tube formation of ECFCs, and the enhancements did not differ between T2DM and control subjects. Direct supplementation of platelets to ECFCs at the ratio of 1:200 enhanced ECFC tube formation even more markedly, leading to approximately 100% increases of the total branch points of ECFC tube formation, for which the enhancements were also similar between patients and controls. In conclusion, platelets from T2DM subjects are hyperreactive. Platelet activation induced by high doses of PAR1-AP, however, results in similar releases of angiogenic regulators in mild T2DM and control subjects. Platelets from T2DM and control subjects also demonstrate similar enhancements on ECFC angiogenic activities.

Store-operated calcium entry and diabetic complications

Store-operated Ca(2+) entry (SOCE) is mediated by the store-operated Ca(2+) channel (SOC) that opens upon depletion of internal Ca(2+) stores following activation of G protein-coupled receptors or receptor tyrosine kinases. Over the past two decades, the physiological and pathological relevance of SOCE has been extensively studied. Recently, accumulating evidence suggests associations of altered SOCE with diabetic complications. This review focuses on the implication of SOCE as it pertains to various complications resulting from diabetes. We summarize recent findings by us and others on the involvement of abnormal SOCE in the development of diabetic complications, such as diabetic nephropathy and diabetic vasculopathy. The underlying mechanisms that mediate the diabetes-associated alterations of SOCE are also discussed. The SOCE pathway may be considered as a potential therapeutic target for diabetes-associated diseases.

Second Messenger-Operated Calcium Entry Through TRPC6

Canonical transient receptor potential 6 (TRPC6) proteins assemble into heteromultimeric structures forming non-selective cation channels. In addition, many TRPC6-interacting proteins have been identified like some enzymes, channels, pumps, cytoskeleton-associated proteins, immunophilins, or cholesterol-binding proteins, indicating that TRPC6 are engaged into macromolecular complexes. Depending on the cell type and the experimental conditions used, TRPC6 activity has been reported to be controlled by diverse modalities. For instance, the second messenger diacylglycerol, store-depletion, the plant extract hyperforin or H2O2 have all been shown to trigger the opening of TRPC6 channels. A well-characterized consequence of TRPC6 activation is the elevation of the cytosolic concentration of Ca(2+). This latter response can reflect the entry of Ca(2+) through open TRPC6 channels but it can also be due to the Na(+)/Ca(2+) exchanger (operating in its reverse mode) or voltage-gated Ca(2+) channels (recruited in response to a TRPC6-mediated depolarization). Although TRPC6 controls a diverse array of biological functions in many tissues and cell types, its pathophysiological functions are far from being fully understood. This chapter covers some key features of TRPC6, with a special emphasis on their biological significance in kidney and blood cells.

Integrative proteome analysis of Brachypodium distachyon roots and leaves reveals a synergetic responsive network under H2O2 stress

The plant oxidative stress response is vital for defense against various abiotic and biotic stresses. In this study, ultrastructural changes and the proteomic response to H2O2 stress in roots and leaves of the model plant Brachypodium distachyon were studied. Transmission electron microscopy (TEM) showed that the ultrastructural damage in roots was more serious than in leaves. Particularly, the ultrastructures of organelles and the nucleus in root tip cells were damaged, leading to the inhibition of normal biological activities of roots, which then spread throughout the plant. Based on two-dimensional electrophoresis (2-DE) and MALDI-TOF/TOF-MS, 84 and 53 differentially accumulated protein (DAP) spots representing 75 and 45 unique proteins responsive to H2O2 stress in roots and leaves, respectively, were identified. These protein species were mainly involved in signal transduction, energy metabolism, redox homeostasis/stress defense, protein folding/degradation, and cell wall/cell structure. Interestingly, two 14-3-3 proteins (GF14-B and GF14-D) were identified as DAPs in both roots and leaves. Protein-protein interaction (PPI) analysis revealed a synergetic H2O2-responsive network.

Evaluation of the antiaggregant activity of ascorbyl phenolic esters with antioxidant properties

Beneficial effects of the antioxidant L-ascorbic acid (Asc) in human health are well known. Its particular role in hemostasis deserves further consideration, since it has been described a dose-dependent effect of Asc in platelet activity. Contrary, it has been demonstrated that phenolic compounds have inhibitory effects on platelet aggregation stimulated by the physiological agonist thrombin (Thr). Here, we have evaluated the actions of three synthetic phenolic esters of Asc: L-ascorbyl 6-protocatechuate (Prot Asc), L-ascorbyl 6-gallate (Gal Asc), and L-ascorbyl 6-caffeate (Caf Asc). All these Asc derivatives exhibited greater radical scavenging activity than Asc, and in experiments using human platelets from healthy subjects, they do not evoke changes in platelet viability upon their administration. Nevertheless, these compounds altered platelet calcium homeostasis in response to Thr, although Prot Asc induced a smaller effect than Gal Asc, Caf Asc, and Asc. As a consequence, platelet aggregation was also impaired by these compounds, reporting Prot Asc and Caf Asc a weaker antiaggregant action than Gal Asc and Asc. Treatments with Gal Asc and Caf Asc altered in larger extent the phosphorylation pattern of pp60(Src) and mammalian target of rapamycin (mTOR) evoked by stimulating human platelets with Thr. Summarizing, Prot Asc is the ascorbyl phenolic ester with the strongest antioxidant properties and weakest antiaggregant actions, and its use as antioxidant may be safer than the rest of derivatives in order to prevent thrombotic alteration in patients that need treatment with antioxidant therapies.

P66Shc mediates increased platelet activation and aggregation in hypercholesterolemia

BACKGROUND AND HYPOTHESIS

Hypercholesterolemia leads to a prothrombotic phenotype. Platelet hyperactivity associated with hypercholesterolemia has been attributed, in part, to oxidative stress. P66Shc is a well-known determinant of cellular and organismal oxidative stress. However, its role in platelet biology is not known. We hypothesized that p66Shc mediates platelet hyperactivation and hyperaggregation in hypercholesterolemia.

METHODS AND RESULTS

P66Shc was expressed in both human and mouse platelets, as determined by qRT-PCR and immunoblotting. Mouse platelet p66Shc expression was upregulated by hypercholesterolemia induced by high-fat diet feeding. Compared to wild-type mice, high-fat diet-induced p66Shc expression in platelets was suppressed in transgenic mice expressing a short hairpin RNA targeting p66Shc (p66ShcRNAi). High-fat diet feeding of wild-type mice amplified surface P-selectin expression on platelets stimulated by the thrombin receptor agonist protease-activated receptor-4 (PAR4), and increased aggregation of platelets induced by thrombin. These exaggerated platelet responses induced by high-fat diet feeding were significantly blunted in p66ShcRNAi mice. Finally, thrombin-stimulated platelet reactive oxygen species were suppressed in p66ShcRNAi mice.

CONCLUSIONS

Hypercholesterolemia stimulates p66Shc expression in platelets, promoting platelet oxidative stress, hyperreactivity and hyperaggregation via p66Shc.

Antioxidative effects of extracts from Trifolium species on blood platelets exposed to oxidative stress

Clovers (Trifolium) may possess a significant therapeutic potential, but the effects of compounds from these plants on blood platelets and haemostasis have been poorly recognized. The present study was designed to evaluate the antioxidative action of extracts from three species of clovers: Trifolium pratense, Trifolium pallidum and Trifolium scabrum in the protection of human blood platelets in vitro. Platelet suspensions were pre-incubated with crude extract and phenolic fraction of T. pratense or phenolic fractions of T. scabrum and T. pallidum, at the final concentrations of 0.5-50 μg/ml. Then, for the induction of oxidative stress, 100 μM peroxynitrite was added. The antioxidative activity of plant extracts was assessed by measurements of the level of 3-nitrotyrosine, thiol groups and lipid peroxidation products (hydroperoxides and thiobarbituric acid-reactive substances). Despite the significant differences in the composition of the investigated extracts, we observed antioxidative effects of all used mixtures. The presence of Trifolium extracts considerably reduced the peroxynitrite-mediated modifications of proteins and diminished peroxidation of lipids in platelets. Our results indicate on a strong antioxidative activity of the tested extracts-statistically significant effects were found even for the lowest concentrations (0.5 μg/ml) of all extracts. This action may be useful in the protection of blood components, very susceptible to oxidative modifications. The obtained results suggest that the examined clovers are a promising source of compounds, valuable for the protection against oxidative stress-induced damage to blood platelets.

Platelet function profiles in patients with diabetes mellitus

Patients with diabetes mellitus (DM) are at high risk for several cardiovascular disorders such as coronary heart disease, stroke, peripheral arterial disease, and congestive heart failure. DM has reached epidemic proportions and its strong association with coronary artery disease is responsible for increased cardiovascular morbidity and mortality. DM patients are characterized by platelet hyperreactivity, which contribute to the enhanced atherothrombotic risk of these subjects. Several mechanisms are involved in the hyperreactive platelet phenotype characterizing DM patients. Furthermore, a large proportion of DM patients show inadequate response to standard antiplatelet treatments and high rate of adverse recurrent cardiovascular events despite compliance with standard antiplatelet treatment regimens. Therefore, new antiplatelet treatment regimens are warranted in DM patients to reduce their atherothrombotic risk. The present manuscript provides an overview on the current status of knowledge on platelet function profiles in patients with DM and therapeutic considerations.

Exacerbation of glycoprotein VI-dependent platelet responses in a rhesus monkey model of Type 1 diabetes

Thrombosis is a life-threatening complication of diabetes. Platelet reactivity is crucial to thrombus formation, particularly in arterial vessels and in thrombotic complications causing myocardial infarction or ischaemic stroke, but diabetic patients often respond poorly to current antiplatelet medication. In this study, we used a nonhuman primate model of Type 1 diabetes to measure early downstream signalling events following engagement of the major platelet collagen receptor, glycoprotein (GP)VI. Diabetic monkeys were given enough insulin to maintain their blood glucose levels either at ~8 mM (well-controlled diabetes) or ~15 mM (poorly controlled diabetes). Flow cytometric analysis was used to measure platelet reactive oxygen species (ROS) generation, calcium mobilisation, receptor surface expression, and immature platelet fraction. We observed exacerbated intracellular ROS and calcium flux associated with engagement of GPVI in monkeys with poorly controlled diabetes. GPVI surface levels did not differ between healthy monkeys or the two diabetic groups. Treatment of platelets with the specific Syk inhibitor BAY61-3606 inhibited GPVI-dependent ROS and, importantly, reduced ROS generation in the poorly controlled diabetes group to that observed in healthy monkeys. These data indicate that glycaemic control is important in reducing GPVI-dependent platelet hyperreactivity and point to a potential antithrombotic therapeutic benefit of Syk inhibition in hyperglycaemic diabetes.

Thrombin-stimulated discharge of calcium stores in human platelets: analysis of experimental data

The purpose of this research was to analyze experimental data concerning thrombin-stimulated discharge of calcium stores in human platelets contained in calcium-free medium in view of better understanding the mechanisms involved in calcium fluxes. The model curves are reasonably close to experimental data; the parameters of the models are related to the properties of the entities responsible for control or maintenance of cytosolic calcium concentration. It has been shown that: (a) time-course of calcium concentration in cytosol of human platelets can be acceptably modeled on the basis of reasonable assumptions concerning agonist stimulated calcium redistribution in cellular compartments; (b) those assumptions are of fundamental importance for the model (c) some parameters of the model (taken arbitrarily) cannot be estimated independently of others from fitting the model to experimental data available; (d) special experiments are necessary to determine the unknown parameters; (e) agonist-stimulated change of the permeability of endomembrane of calcium stores can be regarded as a pulse of the permeability; it can be modeled as a sequence of transitions of the system from inactive to active and to inactive state again.

ITAM receptor-mediated generation of reactive oxygen species in human platelets occurs via Syk-dependent and Syk-independent pathways

BACKGROUND

Ligation of the platelet-specific collagen receptor, GPVI/FcRγ, causes rapid, transient disulfide-dependent homodimerization, and the production of intracellular reactive oxygen species (ROS) generated by the NADPH oxidase, linked to GPVI via TRAF4.

OBJECTIVES

The aim of this study was to evaluate the role of early signaling events in ROS generation following engagement of either GPVI/FcRγ or a second immunoreceptor tyrosine-based activation motif (ITAM)-containing receptor on platelets, FcγRIIa.

METHODS AND RESULTS

Using an H(2) DCF-DA-based flow cytometric assay to measure intracellular ROS, we show that treatment of platelets with either the GPVI agonists, collagen-related peptide (CRP) or convulxin (Cvx), or the FcγRIIa agonist 14A2, increased intraplatelet ROS; other platelet agonists such as ADP and TRAP did not. Basal ROS in platelet-rich plasma from 14 healthy donors displayed little inter-individual variability. CRP, Cvx or 14A2 induced an initial burst of ROS within 2 min followed by additional ROS reaching a plateau after 15-20 min. The Syk inhibitor BAY61-3606, which blocks ITAM-dependent signaling, had no effect on the initial ROS burst, but completely inhibited the second phase.

CONCLUSIONS

Together, these results show for the first time that ROS generation downstream of GPVI or FcγRIIa consists of two distinct phases: an initial Syk-independent burst followed by additional Syk-dependent generation.

Platelet dysfunction in vascular pathologies and how can it be treated

Cardiovascular diseases are one of the leading causes of morbidity and mortality in industrialized countries, and although many processes play a role in the development of vascular disease, thrombosis is the primary event that precipitates stroke and acute coronary syndromes. The blood platelets are of significant importance in medicine. These cells are involved in many physiological processes, particularly haemostasis through their ability to aggregate and form clots in response to activation. In addition, these dynamic cells display activities that extend beyond thrombosis, including an important role in initiating and sustaining vascular inflammation. The expansion of knowledge from basic and clinical research has highlighted the critical position of platelets in several inflammatory diseases such as arthritis and atherosclerosis. Platelets are emerging as important mediators of inflammation and provide important signals to mediate phenotype of other blood and vascular cells. The important role of platelets in arterial thrombosis and the onset of acute myocardial infarction after atherosclerotic plaque rupture make inhibition of platelet aggregation a critical step in preventing thrombotic events associated with stroke, heart attack, and peripheral arterial thrombosis. However, the use of platelet inhibitors for thrombosis prevention must seek a delicate balance between inhibiting platelet activation and an associated increased bleeding risk. The aim of this review is to up-date the knowledge on platelets physiology and dysfunction in pathologies, such as diabetes mellitus, hypercholesterolemia, and hypertension, emphasizing the link between platelets and the inflammation-related atherosclerosis. The review evaluates the opportunities offered by the novel platelet inhibitors to efficiently alleviate the thrombotic events.

Reactive oxygen and nitrogen species modulate the ex-vivo effects of LPS on platelet adhesion to fibrinogen

AIMS

Excessive production of nitric oxide (NO) and reactive oxygen species (ROS) in sepsis modulates different cell functions. Since the sepsis severity is associated with the degree of platelet activation, we decided to investigate the role of systemic generation of NO and ROS in modulating the platelet adhesion of lipopolysaccharide (LPS)-treated rats.

MAIN METHODS

Platelet adhesion was evaluated using fibrinogen-coated 96-well microtiter plates. Cyclic GMP levels were measured using enzyme immunoassay kit.

KEY FINDINGS

Treatment of rats with LPS significantly increased spontaneous platelet adhesion, but reduced the thrombin-activated platelet adhesion when compared with control rats. Chronic treatment of rats with the NO synthase inhibitor L-NAME (20 mg/rat/day, 7 days) prior to LPS injection normalized the increased adhesion in non-activated platelets, but failed to affect the adhesion in thrombin-activated platelets. The cGMP levels were modified neither in non-activated nor in thrombin-activated platelets of LPS-treated rats when compared with control rats. The incubation of non-activated platelets with the O2- scavenger PEG-SOD reversed the stimulatory effect of LPS on spontaneous adhesion, but had no effect in stimulated-platelet adhesion of non-treated or LPS-treated groups. Moreover, pretreatment of rats with the antioxidant N-acetylcysteine (NAC; 150 mg/kg) prevented the increase of non-activated platelet adhesion, and significantly reduced the inhibitory effect of LPS on thrombin-stimulated adhesion.

SIGNIFICANCE

Our findings suggest that in LPS-treated rats, NO plays an important modulatory role only in non-stimulated platelet adhesion through cGMP-independent mechanisms, while ROS, directly or by affecting the redox state of the animals, modulates both non-activated and thrombin-activated platelet adhesion.

Tamoxifen promotes superoxide production in platelets by activation of PI3-kinase and NADPH oxidase pathways

BACKGROUND

Tamoxifen is a selective estrogen receptor antagonist that is widely used for treatment and prevention of breast cancer. However, tamoxifen use can lead to an increased incidence of thrombotic events. The reason for this adverse event remains unknown. Previous studies showed that tamoxifen and its active metabolite Z-4-hydroxytamoxifen rapidly increased intracellular free calcium ([Ca(2+)](i)) in human platelets by a non-genomic mechanism that involved the activation of phospholipase C. Platelets play a pivotal role in thrombosis and Ca(2+) elevation is a central event in platelet activation. Therefore the mechanism by which tamoxifen activated Ca(2+) entry into platelets was investigated.

METHODS

[Ca(2+)](i) was measured using the fluorescent indicator fura-2 and reactive oxygen species were measured using lucigenin in isolated human platelets.

RESULTS

Tamoxifen analogs E-4-hydroxytamoxifen, with weak activity at the nuclear estrogen receptor and Z-4-hydroxytamoxifen, with strong activity at nuclear estrogen receptor, were equally active at increasing [Ca(2+)](i) and synergizing with ADP and thrombin to increase [Ca(2+)](i) in platelets. This result suggests that the effects of tamoxifen and E- and Z-4-hydroxytamoxifen to increase [Ca(2+)](i) are not mediated by the classical genomic estrogen receptor. The effects of tamoxifen to increase [Ca(2+)](i) were strongly inhibited by apocynin and apocynin dimer. This suggests that tamoxifen activates NADPH oxidase which leads to superoxide generation and in turn caused an increase in [Ca(2+)](i). Free radical scavengers TEMPO and TEMPOL also inhibited tamoxifen-induced [Ca(2+)](i) elevation. Inhibition of phosphoinositide-3-kinase (PI3-kinase), an upstream effector of NADPH oxidase with wortmannin and LY-294,002 also caused substantial inhibition of tamoxifen-induced elevation of [Ca(2+)](i).

CONCLUSION

Tamoxifen increases [Ca(2+)](i) in human platelets by a non-genomic mechanism. Tamoxifen activates phospholipase Cγ as well as PI3-kinase and NADPH oxidase pathway to generate superoxide which causes the release of Ca(2+) from the endoplasmic reticulum, and promotes Ca(2+) influx into the platelets.

Attenuated store-operated divalent cation entry and association between STIM1, Orai1, hTRPC1 and hTRPC6 in platelets from type 2 diabetic patients

Agonist-evoked Ca(2+) entry has been reported to be enhanced in platelets from type 2 diabetic patients, which results in altered platelet responsiveness and cardiovascular complications. The present study is aimed to investigate whether store-operated divalent cation entry, a major Ca(2+) entry pathway, is altered in platelets from diabetic patients. Store-operated divalent cation entry was estimated by determination of Mn(2+) entry. Association between STIM1, Orai1, hTRPC1 and hTRPC6 was detected by co-immunoprecipitation and Western blotting. In the presence of specific purinergic and serotoninergic receptor antagonists Mn(2+) entry, induced by thapsigargin (TG), was reduced in platelets from diabetic donors as compared to healthy controls. Treatment with TG or the agonist thrombin enhanced co-immunoprecipitation of STIM1 with Orai1, hTRPC1 and hTRPC6 in platelets from healthy donors, a response that was significantly reduced in platelets from diabetic patients. Our results indicate that store-operated divalent cation entry is reduced in platelets from type 2 diabetic subjects, which is likely mediated by impairment of the association of STIM1 with the channel subunits Orai1, hTRPC1 and hTRPC6 and might be involved in the pathogenesis of the altered platelet responsiveness observed in diabetic patients.

Ca2+ leakage rate from agonist-sensitive intracellular pools is altered in platelets from patients with type 2 diabetes

Platelets from patients with type 2 diabetes show abnormalities in intracellular Ca(2+) homeostasis that are involved in platelet hyperaggregability and the development of thrombotic complications. Different Ca(2+) transport mechanisms have been reported to be altered in platelets from patients with type 2 diabetes, including the sarcoendoplasmic and plasma membrane Ca(2+)-ATPases, plasma membrane Ca(2+) channels, or the Na(+)/Ca(2+) exchanger. Here, we have investigated whether passive Ca(2+) leak from the stores is altered in platelets from patients with type 2 diabetes. Resting cytosolic Ca(2+) concentration ([Ca(2+)](i)) was found to be greater in platelets from patients with type 2 diabetes than in healthy controls. In a Ca(2+)-free medium, platelet stimulation with thrombin or ADP evokes a rapid and transient increase in [Ca(2+)](i) that was found to be greater in patients with diabetes than in healthy controls. Sequential or combined inhibition of Ca(2+) extrusion and Ca(2+) sequestration into the stores reduced the difference between the responses to agonists in patients with diabetes and healthy controls, although agonist-induced Ca(2+) efflux from the stores was still significantly greater in patients with diabetes. Ca(2+) leak from the dense tubular system or the acidic stores, induced by a low concentration of thapsigargin or 2,5-di-(t-butyl)-1,4-hydroquinone (TBHQ), respectively, was clearly greater in patients with diabetes than in controls, and was not significantly modified by treatment with 2-APB. These findings indicate that passive Ca(2+) leakage rate from the intracellular stores in platelets is significantly enhanced in patients with type 2 diabetes mellitus and this might explain the increased resting [Ca(2+)](i).

Adiponectin Inhibits Hyperlipidemia-Induced Platelet Aggregation via Attenuating Oxidative/Nitrative Stress

Adiponectin acts as an endogenous antithrombotic factor. However, the mechanisms underlying the inhibition of platelet aggregation by adiponectin still remain elusive. The present study was designed to test whether adiponectin inhibits platelet aggregation by attenuation of oxidative/nitrative stress. Adult rats were fed a regular or high-fat diet for 14 weeks. The platelet was immediately separated and stimulated with recombinant full-length adiponectin (rAPN) or not. The platelet aggregation, nitric oxide (NO) and superoxide production, endothelial nitric oxide synthase (eNOS)/inducible NOS (iNOS) expression, and antioxidant capacity were determined. Treatment with rAPN inhibited hyperlipidemia-induced platelet aggregation (P<0.05). Interestingly, total NO, a crucial molecule depressing platelet aggregation and thrombus formation，was significantly reduced, rather than increased in rAPN-treated platelets. Treatment with rAPN markedly decreased superoxide production (-62 %, P<0.05) and enhanced antioxidant capacity (+38 %, P<0.05) in hyperlipidemic platelets. Hyperlipidemia-induced reduced eNOS phosphorylation and increased iNOS expression were significantly reversed following rAPN treatment (P<0.05, P<0.01, respectively). Taken together, these data suggest that adiponectin is an adipokine that suppresses platelet aggregation by enhancing eNOS activation and attenuating oxidative/nitrative stress including blocking iNOS expression and superoxide production.

Platelet function in patients with diabetes mellitus: from a theoretical to a practical perspective

Patients with diabetes mellitus have an increased prevalence of vascular disease. Pathologic thrombosis associated with atherosclerotic plaque rupture is a major cause of morbidity and mortality. Platelets are intimately involved in the initiation and propagation of thrombosis. Evidence suggests that platelets from patients with type 2 diabetes have increased reactivity and baseline activation compared to healthy controls. We review the pathophysiology of platelet hyperreactivity in DM patients and its implications in clinical practice, with particular focus on acute coronary syndromes, percutaneous coronary intervention, and novel antiplatelet agents.

Role of oxidant scavengers in the prevention of Ca²+ homeostasis disorders

A number of disorders, such as Alzheimer disease and diabetes mellitus, have in common the alteration of the redox balance, resulting in an increase in reactive oxygen species (ROS) generation that might lead to the development of apoptosis and cell death. It has long been known that ROS can significantly alter Ca²+ mobilization, an intracellular signal that is involved in the regulation of a wide variety of cellular functions. Cells have a limited capability to counteract the effects of oxidative stress, but evidence has been provided supporting the beneficial effects of exogenous ROS scavengers. Here, we review the effects of oxidative stress on intracellular Ca²+ homeostasis and the role of antioxidants in the prevention and treatment of disorders associated to abnormal Ca²+ mobilization induced by ROS.

Platelet abnormalities in diabetes mellitus

Patients with diabetes mellitus (DM) have accelerated atherosclerosis, which is the main underlying factor contributing to the high risk of atherothrombotic events in these patients. Atherothrombotic complications are the leading cause of morbidity and mortality in patients with DM. Among factors contributing to the prothrombotic condition which characterise patients with DM, platelet hyperreactivity plays a pivotal role. Platelets of DM patients are characterised by dysregulation of several signalling pathways leading to intensified adhesion, activation and aggregation. Multiple mechanisms are involved in platelet dysfunction of patients with DM, which can be categorised as follows: a) hyperglycaemia, b) insulin deficiency and resistance, c) associated metabolic conditions, and d) other cellular abnormalities.The present manuscript aims to provide an overview on the current status of knowledge on platelet abnormalities that characterise patients with DM.

Mechanisms underlying the inhibitory effects of lipopolysaccharide on human platelet adhesion

Alterations in platelet aggregation in septic conditions are well established. However, little is known about the effects of lipopolysaccharide (LPS) on platelet adhesion. We have therefore investigated the effects of LPS in human platelet adhesion, using an in vitro model of platelet adhesion to fibrinogen-coated wells. Microtiter plates were coated with human fibrinogen, after which washed platelets (6 x 10(8) platelets/ml) were allowed to adhere. Adherent platelets were quantified through measurement of acid phosphatase activity. Calcium mobilization in Fura2-AM-loaded platelets was monitored with a spectrofluorimeter. Platelet flow cytometry in thrombin-stimulated platelets was performed using monoclonal mouse anti-platelet GPIIb/IIIa antibody (PAC-1). Prior incubation of washed platelets with LPS (0.01-300 microg/ml) for 5 to 60 min concentration- and time-dependently inhibited non-activated platelet adhesion. In thrombin-activated (50 mU/ml) platelets, LPS inhibited the adhesion to a significantly lesser extent than non-activated platelets. Cyclohexamide, superoxide dismutase polyethylene glycol (PEG-SOD) or catalase polyethylene glycol did not affect the LPS responses. No alterations in cyclic GMP levels were seen after platelet incubation with LPS, except with the highest concentration employed (300 microg/ml) where an increase of 36% (P < 0.05) was observed. Thrombin increased by 7.5-fold the internal Ca(2+) platelet levels, an effect markedly inhibited by LPS. Thrombin induced concentration-dependent platelet GPIIb/IIIa activation, but LPS failed to affect the activation state of this membrane glycoprotein. In conclusion, LPS inhibits human platelet adhesion to fibrinogen by mechanisms involving blockade of external Ca(2+), independently of cGMP generation and activation of GPIIb/IIIa complex.

Synthesis and evaluation of the platelet antiaggregant properties of phenolic antioxidants structurally related to rosmarinic acid

Polyphenols, such as rosmarinic acid, are widely distributed natural products with relevant antioxidant activity. Oxidative stress plays an important role in the pathogenesis of a number of disorders. Here, we report on the synthesis and biological effects of the polyphenolic esters hydroxytyrosyl gallate (1), hydroxytyrosyl protocatechuate (2) and hydroxytyrosyl caffeate (3), structurally related to rosmarinic acid. The three compounds showed a greater free radical scavenging activity than their precursors and also than rosmarinic acid. Esters 1 and 3 significantly reduced thrombin-evoked platelet aggregation, which is likely mediated to the attenuation of thrombin-stimulated Ca(2+) release and entry. The three compounds reduced the ability of platelets to accumulate Ca(2+) in the intracellular stores, probably by enhancing the Ca(2+) leakage rate and reduced store-operated Ca(2+) entry in these cells. These observations suggest that the structurally-simplified analogs to rosmarinic acid, compounds 1 and 3, might be the base of therapeutic strategies to prevent thrombotic complications associated to platelet hyperaggregability due to oxidative stress.

Metabolism of hydrogen peroxide between diapause and non-diapause eggs of the silkworm, Bombyx mori during chilling at 5 degrees C

When diapause and non-diapause eggs of the same bivoltine strain of Bombyx mori were chilled at 5 degrees C for more than 30 days, the hatchability of diapause eggs increased while that of non-diapause eggs decreased, respectively. To investigate the relationship between effects of chilling on the hatchability and the metabolism of hydrogen peroxide (H2O2), content of H2O2 and activities of superoxide dismutase (SOD), xanthine oxidase (XO), and catalase (CAT) between diapause and non-diapause eggs were determined during the chilling at 5 degrees C. The significant enhancement of H2O2 occurred prior to the quick increase of the hatchability in diapause eggs and coincided with the quick decline of the hatchability in non-diapause eggs, respectively. Diapause eggs contained significantly higher H2O2 and XO activity and lower CAT activity compared to non-diapause eggs. Our results showed that there were significant differences in the metabolism of H2O2 between diapause and non-diapause eggs during chilling and that significant enhancement of H2O2 may be involved in the diapause termination of diapause eggs and the cell damage of non-diapause eggs.

A common carcinogen benzo[a]pyrene causes neuronal death in mouse via microglial activation

Background

Benzo[a]pyrene (B[a]P) belongs to a class of polycyclic aromatic hydrocarbons that serve as micropollutants in the environment. B[a]P has been reported as a probable carcinogen in humans. Exposure to B[a]P can take place by ingestion of contaminated (especially grilled, roasted or smoked) food or water, or inhalation of polluted air. There are reports available that also suggests neurotoxicity as a result of B[a]P exposure, but the exact mechanism of action is unknown.

Methodology/Principal Findings

Using neuroblastoma cell line and primary cortical neuron culture, we demonstrated that B[a]P has no direct neurotoxic effect. We utilized both in vivo and in vitro systems to demonstrate that B[a]P causes microglial activation. Using microglial cell line and primary microglial culture, we showed for the first time that B[a]P administration results in elevation of reactive oxygen species within the microglia thereby causing depression of antioxidant protein levels; enhanced expression of inducible nitric oxide synthase, that results in increased production of NO from the cells. Synthesis and secretion of proinflammatory cytokines were also elevated within the microglia, possibly via the p38MAP kinase pathway. All these factors contributed to bystander death of neurons, in vitro. When administered to animals, B[a]P was found to cause microglial activation and astrogliosis in the brain with subsequent increase in proinflammatory cytokine levels.

Conclusions/Significance

Contrary to earlier published reports we found that B[a]P has no direct neurotoxic activity. However, it kills neurons in a bystander mechanism by activating the immune cells of the brain viz the microglia. For the first time, we have provided conclusive evidence regarding the mechanism by which the micropollutant B[a]P may actually cause damage to the central nervous system. In today's perspective, where rising pollution levels globally are a matter of grave concern, our study throws light on other health hazards that such pollutants may exert.

Antioxidants change platelet responses to various stimulating events

The role of platelets in hemostasis may be influenced by alteration of the platelet redox state-the presence of antioxidants and the formation of reactive oxygen and nitrogen species. We investigated the effects of two antioxidants, resveratrol and trolox, on platelet activation. Trolox and resveratrol inhibited aggregation of washed platelets and platelet-rich plasma activated by ADP, collagen, and thrombin receptor-activating peptide. Resveratrol was a more effective agent in reducing platelet static and dynamic adhesion in comparison with trolox. The antioxidant capacity of resveratrol was, however, the same as that of trolox. After incubation of platelets with antioxidants, the resveratrol intraplatelet concentration was about five times lower than the intracellular concentration of trolox. Although both antioxidants comparably lowered hydroxyl radical and malondialdehyde production in platelets stimulated with collagen, TxB(2) levels were decreased by resveratrol much more effectively than by trolox. Cyclooxygenase 1 was inhibited by resveratrol and not by trolox. Our data indicate that antioxidants, apart from nonspecific redox or radical-quenching mechanisms, inhibit platelet activation also by specific interaction with target proteins. The results also show the importance of studying platelet activation under conditions of real blood flow in contact with reactive surfaces, e.g., using dynamic adhesion experiments.

Endogenously generated reactive oxygen species reduce PMCA activity in platelets from patients with non-insulin-dependent diabetes mellitus

Intracellular Ca2+ homeostasis in platelets of patients with non-insulin-dependent diabetes mellitus (NIDDM) has been reported to be altered, leading to an increased adhesiveness and spontaneous aggregation. Among the disturbed Ca2+ mechanism in platelets from NIDDM subjects, a reduced Ca2+ extrusion by the plasma membrane Ca2+-ATPase (PMCA) is especially relevant, maintaining an elevated cytosolic free Ca2+ concentration that results in platelet hypersensitivity. Here we show that treatment of platelets from NIDDM patients with 300 U/mL catalase or 5 mM D-mannitol, which prevent H2O2- and hydroxyl radicals-mediated oxidative stress, respectively, increases Ca2+ extrusion after treatment with thapsigargin (TG) plus ionomycin (Iono). In contrast, 1 mM trolox, a scavenger of ONOO-, did not alter TG + Iono-induced response. Catalase and D-mannitol reversed the enhanced tyrosine phosphorylation of PMCA induced by TG + Iono in NIDDM patients. These findings open up new horizon for the development of therapeutic strategies to palliate cardiovascular disorders in NIDDM.

N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine induces apoptosis through the activation of caspases-3 and -8 in human platelets. A role for endoplasmic reticulum stress

BACKGROUND

Apoptosis or programmed cell death involves a number of biochemical events, including the activation of caspases, which lead to specific cell morphology changes and ultimately cell death. Traditionally, two apoptotic pathways have been described: the cell-surface death receptor-dependent extrinsic pathway and the mitochondria-dependent intrinsic pathway. Alternatively, apoptosis has been reported to be induced by endoplasmic reticulum (ER) stress, which is mainly induced by a reduction in intraluminal free Ca(2+) concentration ([Ca(2+)](ER)).

OBJECTIVES

The present study aimed to investigate the development of apoptotic events after ER stress induced by N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN), an ER Ca(2+) chelator, in human platelets.

METHODS

Changes in cytosolic free Ca(2+) concentration, caspase activity and phosphatidylserine externalization were determined by fluorimetric techniques.

RESULTS

Our results indicate that TPEN reduces the amount of free Ca(2+) releasable by the Ca(2+)-mobilizing agonist thrombin. TPEN induced activation of caspase-3, -8 and -9 and subsequent phosphatidylserine externalization. The ability of TPEN to induce phosphatidylserine externalization was smaller than that of thrombin. In addition, TPEN was able to induce phosphorylation of the eukaryotic initiation factor 2 alpha (eIF2 alpha). TPEN-mediated caspase-3 activation requires functional caspase-8, but is independent of H(2)O(2) generation. Activation of caspase-3 and -8 by TPEN was prevented by salubrinal, an agent that prevents ER stress-induced apoptosis.

CONCLUSION

These findings provide experimental evidence for the existence of ER stress-mediated apoptosis in human platelets, a process that might limit platelet life span upon prolonged stimulation with agonists.

Dynamic interaction of hTRPC6 with the Orai1-STIM1 complex or hTRPC3 mediates its role in capacitative or non-capacitative Ca(2+) entry pathways

TRPC (canonical transient receptor potential) channel subunits have been shown to assemble into homo- or hetero-meric channel complexes, including different Ca2+-handling proteins, required for the activation of CCE (capacitative Ca2+ entry) or NCCE (non-CCE) pathways. In the present study we found evidence for the dynamic interaction between endogenously expressed hTRPC6 (human TRPC6) with either both Orai1 and STIM1 (stromal interaction molecule 1) or hTRPC3 to participate in CCE or NCCE. Electrotransjection of cells with an anti-hTRPC6 antibody, directed towards the C-terminal region, reduces CCE induced by TPEN [N,N,N',N'-tetrakis-(2-pyridylmethyl)-ethylenediamine], which reduces the intraluminal free Ca2+ concentration. Cell stimulation with thrombin or extensive Ca2+-store depletion by TG (thapsigargin)+ionomycin enhanced the interaction between hTRPC6 and the CCE proteins Orai1 and STIM1. In contrast, stimulation with the diacylglycerol analogue OAG (1-oleoyl-2-acetyl-sn-glycerol) displaces hTRPC6 from Orai1 and STIM1 and enhances the association between hTRPC6 and hTRPC3. The interaction between hTRPC6 and hTRPC3 was abolished by dimethyl-BAPTA [1,2-bis-(o-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid] loading, which indicates that this phenomenon is Ca2+-dependent. These findings support the hypothesis that hTRPC6 participates both in CCE and NCCE through its interaction with the Orai1-STIM1 complex or hTRPC3 respectively.