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

Hepatitis C virus NS5A and core proteins induce oxidative stress-mediated calcium signalling alterations in hepatocytes

BACKGROUND/AIMS

The hepatitis C virus (HCV) structural core and non-structural NS5A proteins induce in liver cells a series of intracellular events, including elevation of reactive oxygen and nitrogen species (ROS/RNS). Since oxidative stress is associated to altered intracellular Ca(2+) homeostasis, we aimed to investigate the effect of these proteins on Ca(2+) mobilization in human hepatocyte-derived transfected cells, and the protective effect of quercetin treatment.

METHODS

Ca(2+) mobilization and actin reorganization were determined by spectrofluorimetry. Production of ROS/RNS was determined by flow cytometry.

RESULTS

Cells transfected with NS5A and core proteins showed enhanced ROS/RNS production and resting cytosolic Ca(2+) concentration, and reduced Ca(2+) concentration into the stores. Phenylephrine-evoked Ca(2+) release, Ca(2+) entry and extrusion by the plasma membrane Ca(2+)-ATPase were significantly reduced in transfected cells. Similar effects were observed in cytokine-activated cells. Phenylephrine-evoked actin reorganization was reduced in the presence of core and NS5A proteins. These effects were significantly prevented by quercetin. Altered Ca(2+) mobilization and increased calpain activation were observed in replicon-containing cells.

CONCLUSIONS

NS5A and core proteins induce oxidative stress-mediated Ca(2+) homeostasis alterations in human hepatocyte-derived cells, which might underlie the effects of both proteins in the pathogenesis of liver disorders associated to HCV infection.

Melatonin, as an adjuvant-like agent, enhances platelet responsiveness

Melatonin exerts immunomodulatory actions that enhance the magnitude and quality of immune responses specific for certain antigens; this has raised the possibility of using melatonin to design novel vaccine adjuvant systems. The present study investigated the effect of subcutaneous slow-release melatonin implants and subcutaneous melatonin injections on the responsiveness of circulating platelets in sheep after vaccination against Dichelobacter nodosus (A1 and C serotypes), the bacterium that causes ovine footrot, a major cause of lameness in sheep. The experiments were carried out in sheep from a farm located in an area of Mediterranean-type ecosystem. Plasma melatonin levels were determined by radioimmunoassay, sheep platelet aggregation was monitored using an aggregometer and Ca2+ mobilization was determined by spectrofluorimetry using fura-2. Administration of melatonin either by implants or subcutaneous injections increased plasma melatonin concentrations, an effect that was found to be greater and more sustained when melatonin was administered via implants. Vaccination per se, as well as melatonin, increased the percentage and rate of platelet aggregation and reduced the lag-time in response to the physiological agonist thrombin, an effect that was found to be significantly greater when melatonin was administered to vaccinated animals. Melatonin enhanced thrombin-evoked Ca2+ release and entry and further increased Ca2+ mobilization observed in platelets from vaccinated sheep. These observations suggest that the use of melatonin, as a novel adjuvant, induces beneficial effects on platelet function and haemostasis, and opens new perspectives for therapeutic manipulation of immune responses to vaccination.

Effect of homocysteine on calcium mobilization and platelet function in type 2 diabetes mellitus

Type 2 diabetes mellitus induces a characteristic platelet hyperactivity that might be due to several factors including oxidative stress and abnormal intracellular Ca2+ homeostasis. Hyperhomocysteinaemia is considered a risk factor in the development of thrombosis although its effect on platelet function and the mechanisms involved are still poorly understood. Here we show that homocysteine (Hcy) induce a concentration-dependent increase in endogenous production of reactive oxygen species (ROS), which was significantly greater in platelets from diabetic patients than in controls. Platelet treatment with Hcy resulted in Ca2+ release from the dense tubular system and the acidic stores. Ca2+ mobilisation-induced by Hcy consisted in two components, an initial slow increase in intracellular free Ca2+ concentration ([Ca2+]i) and a rapid and marked increase in [Ca2+]i, the second leading to the activation of platelet aggregation. As well as ROS generation, Ca2+ mobilization and platelet aggregation were significantly greater in platelets from diabetic donors than in controls, which indicate that platelets from diabetic donors are more sensitive to Hcy. These findings, together with the hyperhomocysteinaemia reported in diabetic patients, strongly suggest that Hcy might be considered a risk factor in the development of cardiovascular complications associated to type 2 diabetes mellitus.

Effect of homocysteine on calcium mobilization and platelet function in type 2 diabetes mellitus

Type 2 diabetes mellitus induces a characteristic platelet hyperactivity that might be due to several factors including oxidativ stress and abnormal intracellular Ca²⁺ homeostasis. Hyperhomocysteinaemia is considered a risk factor in the development of thrombosis although its effect on platelet function and the mechanisms involved are still poorly understood. Here we show tha homocysteine induce a concentration-dependent increase in endogenous production of reactive oxygen species (ROS), which was significantly greater in platelets from diabetic patients than in controls. Platelet treatment with homocysteine resulted in Ca²⁺ release from the dense tubular system and the acidic stores. Ca²⁺ mobilization-induced by homocysteine consisted in two components, an initial slow increase in intracellular free Ca ⁺ concentration ([Ca ⁺]i) and a rapid and marked increase in [Ca²⁺]i, th second leading to the activation of platelet aggregation. As well as ROS generation, Ca²⁺ mobilization and platelet aggregation were significantly greater in platelets from diabetic donors than in controls, which indicate that platelets from diabetic donors are more sensitive to homocysteine. These findings, together with the hyperhomocysteinaemia reported in diabetic patients, strongly suggest that homocysteine might be considered a risk factor in the development of cardiovascular complications associated to type 2 diabetes mellitus.

Hydrogen peroxide-induced stroke: elucidation of the mechanism in vivo

OBJECT

Hydrogen peroxide (H2O2) is used as a hemostatic agent in many neurosurgery centers. The authors used a 3% H2O2 solution for final hemostasis after removal of a left insular tumor. Immediately afterward, air bubbles were observed within the lumen of the polar temporal artery. Postoperative MR imaging revealed punctate areas of infarction in the lenticulostriate artery territory. The authors designed an experimental study to elucidate the mechanism of remote O2 emboli and reactive O2 species-related vasoactive responses and thrombus formation.

METHODS

In this study, H2O2 irrigation was used in mice with either an intact pial layer or after the pia mater was removed through a corticotomy. Normal saline irrigation was used in the corresponding control groups. Vessels were examined for intravascular O2 emboli under the microscope. Tissue sections were then obtained and stained with H & E and the 3-nitrotyrosine (3-NT) antibody to evaluate intravascular thrombus formation and peroxynitrite reaction, respectively.

RESULTS

Multiple bubbles were observed within the lumen of the vessels after exposure to H2O2 regardless of whether the pial layer was destroyed or intact. Immunofluorescent staining for 3-NT showed an abundant positive reaction in the vessel walls of all animals exposed to H2O2 as well as vascular occlusion with acute thrombus formation. Samples taken from the animals that received saline showed no positive staining for 3-NT and no vascular occlusion.

CONCLUSIONS

Exposure to H2O2 may cause serious ischemic complications. The formation of peroxynitrite may cause vasoactive responses to H2O2 and platelet aggregation/thrombus formation, and the free diffusion of H2O2 through the vessel walls and its conversion to water and O2 leads to O2 bubbles within the closed vessel lumen. If used intradurally, H2O2 may have deleterious ischemic effects, and it can only be used carefully in open extradural spaces.

Olive tree wood phenolic compounds with human platelet antiaggregant properties

Oleuropein and (+)-cycloolivil are natural polyphenolic compounds with a significant radical scavenging activity present in olive tree. We have investigated the antiaggregant effects of oleuropein and (+)-cycloolivil isolated from an ethyl acetate extract of olive tree wood. Oleuropein and (+)-cycloolivil reduced the ability of thrombin to stimulate platelet aggregation. Both compounds reduced thrombin-evoked Ca(2+) release and entry to a similar extent to hydroxytyrosol. This effect was greater in platelets from patients with type 2 diabetes mellitus than in controls. Thrombin-, thapsigargin- and 2,5-di-(tert-butyl)-1,4-hydroquinone (TBHQ)-evoked protein tyrosine phosphorylation, which is involved in Ca(2+) signalling and platelet aggregation, is inhibited by oleuropein and (+)-cycloolivil. oleuropein and (+)-cycloolivil are natural oxygen radical scavengers that reduce thrombin-induced protein tyrosine phosphorylation, Ca(2+) signalling and platelet aggregation. These observations suggest that oleuropein and (+)-cycloolivil may prevent thrombotic complications associated to platelet hyperaggregability and be the base for the development of antiaggregant therapeutic strategies.

Pigment epithelium-derived factor (PEDF) inhibits collagen-induced platelet activation by reducing intraplatelet nitrotyrosine levels

We have recently found that pigment epithelium-derived factor (PEDF), a glycoprotein with a potent neuronal differentiating activity, not only inhibits platelet aggregation and P-selectin expression, but also suppresses occlusive thrombus formation in rats through its anti-oxidative properties. These findings suggest that PEDF may play a protective role against atherothrombosis. However, the underlying molecular mechanism by which PEDF inhibited platelet aggregation and activation in vitro is fully understood. Since nitric oxide (NO) suppresses platelet aggregation and activation, it is conceivable that PEDF could inhibit platelet activation by suppressing the inactivation of NO and subsequent formation of peroxynitrite via its anti-oxidative properties. In this study, we examined whether PEDF reduced intraplatelet nitrotyrosine levels, a marker of protein nitration by peroxynitrite, and subsequently suppressed platelet-derived growth factor-AB (PDGF-AB) production by collage-exposed platelets. PEDF was found to significantly reduce the collagen-elicited intraplatelet nitrotyrosine formation and PDGF-AB secretion by platelets. The present study demonstrated for the first time that PEDF could inhibit the collagen-induced platelet activation by suppressing nitrotyrosine formation. PEDF may inhibit the platelet activation by suppressing the inactivation of NO and subsequent formation of peroxynitrite via its anti-oxidative properties.

Thrombin induces activation and translocation of Bid, Bax and Bak to the mitochondria in human platelets

BACKGROUND

Thrombin is a physiological platelet agonist that activates apoptotic events, including cytochrome c release and phosphatidylserine exposure; however, the mechanisms underlying these events remain unclear.

OBJECTIVES

The present study is aimed to investigate whether thrombin induces activation and mitochondrial translocation of Bid, Bax and Bak.

METHODS

Changes in the mitochondrial membrane potential were registered using the dye JC-1; Bid, Bax and Bak translocation to the mitochondria was detected by immunoprecipitation and Western blotting in samples from mitochondrial and cytosolic fractions.

RESULTS

Treatment of platelets with thrombin or ADP induces activation and mitochondrial association of active Bid, Bax and Bak. Translocation of Bid and Bax to the mitochondria was reduced by cytochalasin D, latrunculin A or jasplakinolide. Platelet exposure to exogenous H(2)O(2) (10 microm) results in activation of Bid and Bax, which was found to be similar to the effect of thrombin. Thrombin evokes mitochondrial membrane depolarization, which is attenuated by catalase.

CONCLUSION

Our results indicate that thrombin induces activation and mitochondrial translocation of Bid, Bax and Bak, which is likely to be one of the apoptotic events in human platelets.

Platelet signalling abnormalities in patients with type 2 diabetes mellitus: a review

The hyperactivation of platelets is involved in the cardiovascular complications associated with type 2 diabetes mellitus. Altered platelet behavior contributes to the angiopathies associated with diabetes. A number of mechanisms involved in platelet activation are altered in diabetes. Platelets from type 2 diabetic patients show an enhanced endogenous reactive oxygen species production and a reduced antioxidant capability, which increase the activity of several tyrosine kinases, such as the Bruton's tyrosine kinase, MAP kinases or proteins of the SRC family. Oxidative stress is also involved in the abnormal intracellular calcium homeostasis observed in platelets from type 2 diabetics, including an enhanced resting cytosolic calcium concentration and calcium release and entry in response to agonists. Moreover, diabetes alters the bioavailability of nitric oxide in platelets. Basal nitric oxide synthase activity is reduced in homogenates of platelets obtained from patients with type 2 diabetes mellitus. The study of these abnormalities might be helpful in the development of new pharmacological strategies to reduce platelet activation in type 2 diabetes mellitus.

Cinnamtannin B-1 as an antioxidant and platelet aggregation inhibitor

Cinnamtannin B-1 is a naturally occurring trimeric A-type proanthocyanidin, present in a limited number of plants, which exhibits a large number of cellular actions mostly derived from its antioxidant properties. Cinnamtannin B-1 modulates several biological processes such as changes in cytosolic free Ca(2+) concentration, endogenous reactive oxygen species generation, protein tyrosine phosphorylation and platelet aggregation. Proanthocyanidins, such as cinnamtannin B-1, have been reported to exert antitumoral activity mediated by a selective proapoptotic action in a number of tumoral cell lines associated with antiapoptotic activity in normal cells. The opposite effects of proanthocyanidins in normal and tumoral cells suggest that these compounds might be the base for therapeutic strategies directed selectively against tumoral cells. In addition, cinnamtannin B-1 shows antithrombotic actions through inhibition, in platelets, of endogenous ROS generation, Ca(2+) mobilization and, subsequently, aggregation. This has been reported to be especially relevant in platelets from diabetic patients, where cinnamtannin B-1 reverses both platelet hypersensitivity and hyperactivity. Considering the large number of cellular effects of cinnamtannin B-1 the development of therapeutic strategies for thrombotic disorders or certain types of cancer deserves further studies. This review summarizes the current knowledge on the actions and relevance of the signalling pathways modulated by cinnamtannin B-1.

STIM1 regulates acidic Ca2+ store refilling by interaction with SERCA3 in human platelets

Ca(2+) mobilization regulates a wide variety of cellular functions. Platelets possess agonist-releasable Ca(2+) stores in acidic organelles where sarcoendoplasmic reticulum Ca(2+)-ATPase-3 (SERCA) pump is involved in store refilling. Stromal interaction molecule 1 (STIM1), which has been presented as a central regulator of platelet function, is a Ca(2+) sensor of the intracellular Ca(2+) stores. Here we present that STIM1 is required for acidic store refilling. Electrotransjection of cells with anti-STIM1 (Y(231)-K(243)) antibody, directed towards a cytoplasmic sequence of STIM1, significantly reduced acidic store refilling, which was tested by remobilizing Ca(2+) from the acidic stores using 2,5-di-(t-butyl)-1,4-hydroquinone (TBHQ) after a brief refilling period that followed thrombin stimulation. Platelet treatment with thrombin or thapsigargin in combination with ionomycin, to induce extensive Ca(2+) store depletion, resulted in a transient increase in the interaction between STIM1 and SERCA3, reaching a maximum 30 s after stimulation. The coupling between STIM1 and SERCA3 was abolished by electrotransjection with anti-STIM1 antibody. The interaction between STIM1 and SERCA3 induced by thrombin or by treatment with thapsigargin plus ionomycin is reduced in platelets from type 2 diabetic patients, as well as Ca(2+) reuptake into the acidic Ca(2+) stores. These findings provide evidence for a role of STIM1 in acidic store refilling in platelets probably acting as a Ca(2+) sensor and regulating the activity of SERCA3. This action is impaired in platelets from type 2 diabetics, which might lead to the enhanced cytosolic Ca(2+) concentration observed and, therefore, in platelet hyperactivity.

The mechanism of anti-platelet activity of davallialactone: involvement of intracellular calcium ions, extracellular signal-regulated kinase 2 and p38 mitogen-activated protein kinase

This study was designed to investigate the effect of davallialactone, which was isolated from the mushroom Inonotus xeranticus, on platelet aggregation induced by collagen, thrombin and ADP. We found that davallialactone dose-dependently inhibited platelet aggregation that was stimulated either by collagen (2.5 microg/ml), a potent ligand of integrin alpha2beta1 and glycoprotein VI, or by thrombin (0.1U/ml), a potent agonist of the protease-activated receptors (PARs) PAR1 and PAR3. In addition, davallialactone inhibited platelet aggregation induced by ADP, an agonist of P2Y receptor. To understand the mechanism of anti-platelet activity, we determined whether davallialactone affected the downstream signaling in collagen-activated platelets. Using the fura-2/AM fluorometric assay, we found that davallialactone dose-dependently inhibited intracellular calcium concentration levels ([Ca2+]i). Moreover, davallialactone inhibited the phosphorylation of extracellular signal-regulated protein kinase (ERK)-2 and p38 mitogen-activated protein kinase (MAPK), in a dose-dependent manner. The tyrosine phosphorylation of 60 and 85kDa proteins, which were activated by collagen, were differentially inhibited by davallialactone. Taken together, these data suggest that davallialactone may have potential anti-platelet aggregation activity via suppression of intracellular downstream signaling pathways.

Natriuretic peptides in vascular physiology and pathology

Four major natriuretic peptides have been isolated: atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), C-type natriuretic peptide (CNP), and Dendroaspis-type natriuretic peptide (DNP). Natriuretic peptides play an important role in the regulation of cardiovascular homeostasis maintaining blood pressure and extracellular fluid volume. The classical endocrine effects of natriuretic peptides to modulate fluid and electrolyte balance and vascular smooth muscle tone are complemented by autocrine and paracrine actions that include regulation of coronary blood flow and, therefore, myocardial perfusion; modulation of proliferative responses during myocardial and vascular remodeling; and cytoprotective anti-ischemic effects. The actions of natriuretic peptides are mediated by the specific binding of these peptides to three cell surface receptors: type A natriuretic peptide receptor (NPR-A), type B natriuretic peptide receptor (NPR-B), and type C natriuretic peptide receptor (NPR-C). NPR-A and NPR-B are guanylyl cyclase receptors that increase intracellular cGMP concentration and activate cGMP-dependent protein kinases. NPR-C has been presented as a clearance receptor and its activation also results in inhibition of adenylyl cyclase activity. The wide range of effects of natriuretic peptides might be the base for the development of new therapeutic strategies of great benefit in patients with cardiovascular problems including coronary artery disease or heart failure. This review summarizes current literature concerning natriuretic peptides, their receptors and their effects on fluid/electrolyte balance, and vascular and cardiac physiology and pathology, including primary hypertension and myocardial infarction. In addition, we will attempt to provide an update on important issues regarding natriuretic peptides in congestive heart failure.

Peroxynitrite: biochemistry, pathophysiology and development of therapeutics

Peroxynitrite--the product of the diffusion-controlled reaction of nitric oxide with superoxide radical--is a short-lived oxidant species that is a potent inducer of cell death. Conditions in which the reaction products of peroxynitrite have been detected and in which pharmacological inhibition of its formation or its decomposition have been shown to be of benefit include vascular diseases, ischaemia-reperfusion injury, circulatory shock, inflammation, pain and neurodegeneration. In this Review, we first discuss the biochemistry and pathophysiology of peroxynitrite and then focus on pharmacological strategies to attenuate the toxic effects of peroxynitrite. These include its catalytic reduction to nitrite and its isomerization to nitrate by metalloporphyrins, which have led to potential candidates for drug development for cardiovascular, inflammatory and neurodegenerative diseases.

Involvement of SNARE proteins in thrombin-induced platelet aggregation: Evidence for the relevance of Ca2+ entry

Thrombin induces platelet activation through a variety of intracellular mechanisms, including Ca(2+) mobilization. The protein of the exocytotic machinery SNAP-25, but not VAMPs, is required for store-operated Ca(2+) entry, the main mechanism for Ca(2+) influx in platelets. Hence, we have investigated the role of the SNAP-25 and VAMPs in thrombin-induced platelet aggregation. Platelet stimulation with thrombin or selective activation of thrombin receptors PAR-1, PAR-4 or GPIb-IX-V results in platelet aggregation that, except for GPIb-IX-V receptor, requires Ca(2+) entry for full activation. Depletion of the intracellular Ca(2+) stores using pharmacological tools was unable to induce aggregation except when cytosolic Ca(2+) concentration reached a critical level (around 1.5 microM). Electrotransjection of cells with anti-SNAP-25 antibody reduced thrombin-evoked platelet aggregation, while electrotransjection of anti-VAMP-1, -2 and -3 antibody had no effect. These findings support a role for SNAP-25 but not VAMP-1, -2 and -3 in platelet aggregation, which is likely mediated by the regulation of Ca(2+) mobilization in human platelets.

Characterization of the Intracellular Mechanisms Involved in the Antiaggregant Properties of Cinnamtannin B-1 from Bay Wood in Human Platelets

Cinnamtannin B-1, a natural A-type proanthocyanidin recently identified as a radical scavenger component of Laurus nobilis L., exerts antiaggregant and antiapoptotic effects in human platelets. Here, we have investigated the intracellular mechanisms involved in the antiaggregant effects of cinnamtannin B-1. Cinnamtannin B-1 showed a greater free radical scavenging activity than vitamin C, vitamin E, or Trolox, among other antioxidants and reduced thrombin-evoked tubulin reorganization and platelet aggregation. Thrombin-evoked activation of Btk and pp60(src) was also inhibited by cinnamtannin B-1. In conclusion, we show that cinnamtannin B-1 is a powerful oxygen radical scavenger that reduces thrombin-evoked microtubular remodeling and activation of the tyrosine kinases Btk and pp60(src), which leads to inhibition of platelet aggregation. These observations suggest that cinnamtannin B-1 may prevent thrombotic complications associated to platelet hyperaggregability and hyperactivity, although further studies are necessary to establish appropriate therapeutic strategies.

Lipid Metabolism in Rats is Modified by Nitric Oxide Availability Through a Ca++-Dependent Mechanism

We studied lipid metabolism and the antioxidant defense system in plasma and liver of rats fed diets supplemented with L(omega)-nitro-L-arginine methyl ester (L-NAME), isosorbide dinitrate (DIS), L-arginine (Arg), or the associations of these drugs. Liver hydroperoxide and thiobarbituric-acid-reactive substance (TBARS) levels were decreased by Arg and increased by L-NAME or DIS treatments. Oxidized glutathione and conjugated dienes were increased by DIS. Nitrate + nitrite levels and serum calcium ([Ca(++)]) were incremented by Arg or DIS and reduced by L-NAME. Superoxide dismutase and catalase activities decreased under Arg treatment, while L-NAME or DIS caused stimulation. Liver high-density lipoprotein (HDL) cholesterol was increased by DIS or NAME (alone or associated with Arg). Free fatty acids and neutral and polar lipids were increased by Arg, L: -NAME, and DIS. However, predominating phospholipid synthesis increased the neutral/polar ratio. Decreased levels of nitric oxide (NO) (low [Ca(++)]) was directly associated with increased fatty acid synthetase, decreased phospholipase A(2), carnitine-palmitoyl transferase, and fatty acid desaturase activities. Raised NO (high [Ca(++)]) inversely correlated with increased phospholipase-A(2) and acyl-coenzyme A (CoA) synthetase and decreased fatty acid synthetase and beta-oxidation rate. Arg or DIS produced changes that were partially reverted by association with L-NAME. Based on these observations, prolonged therapeutical approaches using drugs that modify NO availability should be carefully considered.

Antioxidant and antiplatelet effects of rosuvastatin in a hamster model of prediabetes

The objectives of this study were to determine the relationships among Type II diabetes (T2DM)-dependent elevations in platelet-derived reactive oxygen species (ROS), platelet-surface protein disulfide isomerase (psPDI) NO-releasing activity, and platelet aggregation and to evaluate the efficacy of rosuvastatin in normalizing these parameters in primary cells derived from a hamster model of prediabetic insulin resistance induced by fructose feeding. Platelets from rosuvastatin-treated non-fructose-fed (NFF) and fructose-fed (FF) hamsters were analyzed for aggregability and psPDI-denitrosation activity. Platelets from NFF animals treated with xanthine/xanthine oxidase (X/XO) were assessed for the same parameters and primary aortic endothelial cells (AEC) cultivated with a range of [rosuvastatin] +/- mevalonate were analyzed for ROS production. Platelets from FF hamsters displayed statistically significant enhanced ROS production, diminished psPDI-mediated NO-releasing activity, and hyperaggregability. Suggestively, platelets from NFF animals treated with X/XO displayed characteristics similar to platelets from FF animals. Rosuvastatin elicited a normalizing effect on all parameters measured in platelets from FF animals. Further, ROS production in primary AEC from FF animals could be blunted to that of NFF animals by concentrations of rosuvastatin in the range of those achieved in the bloodstream. Diminished psPDI-dependent NO-releasing activity and increased initial aggregation rates of FF platelets may result from elevated vascular ROS production under conditions of insulin resistance. Normalization of ROS production and platelet aggregation by rosuvastatin indicates its potential use as a vasculoprotective agent.

Intracellular Ca2+ homeostasis and aggregation in platelets are impaired by ethanol through the generation of H2O2 and oxidation of sulphydryl groups

The mechanisms involved in the effect of ethanol on Ca2+ entry and aggregability have been investigated in human platelets in order to shed new light on the pathogenesis of alcohol consumption. Ethanol (50 mM) induced H2O2 production in platelets by Ca2+-dependent and independent mechanisms. Ca2+ entry induced by ethanol was impaired by catalase. Ethanol reduced SOCE mediated by depletion of the 2,5-di-(tert-butyl)-1,4-hydroquinone (TBHQ)-sensitive acidic stores but enhances SOCE regulated by the dense tubular system. This effect was abolished by treatment with catalase or the sulphydryl group reducing agent dithiotreitol (DTT). Similarly, the anti-aggregant effect of ethanol was prevented by platelet treatment with catalase or DTT. In conclusion we provide considerable evidence that ethanol alters Ca2+ entry and reduces thrombin-induced aggregation as a result of the generation of H2O2 and the oxidation of sulphydryl groups in human platelets.

Membrane transport of hydrogen peroxide

Hydrogen peroxide (H2O2) belongs to the reactive oxygen species (ROS), known as oxidants that can react with various cellular targets thereby causing cell damage or even cell death. On the other hand, recent work has demonstrated that H2O2 also functions as a signalling molecule controlling different essential processes in plants and mammals. Because of these opposing functions the cellular level of H2O2 is likely to be subjected to tight regulation via processes involved in production, distribution and removal. Substantial progress has been made exploring the formation and scavenging of H2O2, whereas little is known about how this signal molecule is transported from its site of origin to the place of action or detoxification. From work in yeast and bacteria it is clear that the diffusion of H2O2 across membranes is limited. We have now obtained direct evidence that selected aquaporin homologues from plants and mammals have the capacity to channel H2O2 across membranes. The main focus of this review is (i) to summarize the most recent evidence for a signalling role of H2O2 in various pathways in plants and mammals and (ii) to discuss the relevance of specific transport of H2O2.

Alterations in intracellular calcium homeostasis and platelet aggregation induced by ethanol

The in vitro effects of ethanol on intracellular Ca(2+) homeostasis and tyrosine phosphorylation have been investigated in human platelets in order to clarify the cellular mechanisms underlying its described anti-aggregant effects. Ethanol (1-50 mM) reduced, in a dose-dependent manner, the rate and amplitude of aggregation and attenuated the phosphotyrosine content both induced by 0.1U/ml of the physiological ligand, thrombin. Thrombin-induced Ca(2+) entry to the cytosol was significantly reduced, and capacitative Ca(2+) entry (CCE) significantly altered, by 50 mM ethanol, so that ethanol reduces CCE mediated by depletion of the 2,5-di-(tert-butyl)-1,4-hydroquinone (TBHQ)-sensitive store but enhances CCE induced by the TBHQ-insensitive pool. In conclusion, we provide considerable evidence that ethanol reduces thrombin-induced aggregation, which is likely a result of a significant inhibition of Ca(2+) entry, as well as a reduction in the activity of protein tyrosine kinases.

Early caspase-3 activation independent of apoptosis is required for cellular function

A number of pro-apoptotic stimuli induce the activation of caspase-9, an initiator protease that activates executioner caspases, such as caspase-3, leading to the development of programmed cell death. Here we demonstrate that cell (platelets and pancreatic acinar cells) stimulation with agonists induces a bimodal activation of caspase-3. The early caspase-3 activation occurs within 1 min of stimulation and is independent on caspase-9 or mitochondrial cytochrome c release suggesting that is a non-apoptotic event. The ability of agonists to induce early activation of caspase-3 is similar to that observed for other physiological processes. Activation of caspase-3 by physiological concentrations of cellular agonists, including thrombin or CCK-8, is independent of rises in cytosolic calcium concentration but requires PKC activation, and is necessary for agonist-induced activation of the tyrosine kinases Btk and pp60src and for several cellular functions, including store-operated calcium entry, platelet aggregation, or pancreatic secretion. Thus, early activation of caspase-3 seems to be a non-apoptotic event required for cellular function.