Antithrombotic effects of peroxynitrite: inhibition and reversal of aggregation in human platelets

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.

The effect of reactive oxygen species on whole blood aggregation and the endothelial cell-platelet interaction in patients with coronary heart disease

Background

The effect of reactive oxygen species (ROS) on platelet function in coronary heart disease (CHD) is complex and poorly defined. Platelet aggregation studies in healthy volunteers have demonstrated contrasting results when platelets are exposed to ROS. We investigated the effect of ROS on whole blood aggregation (WBA) and the endothelial cell-platelet interaction in patients with CHD.

Methods and Results

ROS generated by xanthine and xanthine oxidase caused a concentration-dependent inhibition of WBA in blood from healthy donors and patients with CHD. In patients with CHD, 100 μM xanthine and 100 mU/ml xanthine oxidase inhibited WBA in response to 3 μg/ml collagen by 28.9% (95% CI 15.9%-41.8%, p < 0.001) and in response to 5 μM ADP by 36.0% (95% CI 9.6%-62.4%, p = 0.005). Using nitrotyrosine expression, platelets isolated from patients with CHD were found to be susceptible to peroxynitrite damage. The addition of 1 × 10⁵ cultured endothelial cells inhibited WBA in response to 3 μg/ml collagen by 31.2% (95% CI 12.2%-50.2%, p < 0.05) and in response to 5 μM ADP by 31.6% (95% CI 2.5-60.7%, p < 0.05). Addition of xanthine and xanthine oxidase did not alter this effect, however pre-treatment of endothelial cells with a nitric oxide synthase inhibitor (L-NAME) partly reversed the inhibition.

Conclusion

ROS inhibit WBA in blood from patients with CHD. Whilst endothelial cells also inhibit WBA, the effect is attenuated by L-NAME, suggesting that nitric oxide is likely to remain an important protective mechanism against thrombosis in CHD.

Lipopolysaccharide-stimulated leukocytes contribute to platelet aggregative dysfunction, which is attenuated by catalase in rats

Endotoxemia causes several hematological dysfunctions, including platelet degranulation or disseminated intravascular coagulation, which lead to thrombotic and hemorrhagic events. Here, we tested the hypothesis that bacterial lipopolysaccharide (LPS)-stimulated leukocytes contribute to platelet aggregative dysfunction, and this function is attenuated by antioxidants. Platelet-rich plasma (PRP) was prepared from whole blood of normal and endotoxemic rats. The ability of platelet aggregation was measured by an aggregometer. LPS (50-100 μg/mL) was incubated with PRP, whole blood and PRP with polymorphonuclear leukocytes (PMNs) for 30 minutes, 60 minutes and 90 minutes, and platelet aggregation was detected. LPS-induced platelet aggregative dysfunction was undetectable in intact PRP which was isolated from normal whole blood, whereas it was detected in PRP isolated from endotoxemic rats and LPS-treated whole blood. Moreover, the effect of LPS-induced platelet aggregative dysfunction on intact PRP was observed when the PMNs were added. LPS-induced platelet aggregative dysfunction was significantly attenuated by catalase alone and in combination with N(G)-nitro-L-arginine methyl ester, but not by N(G)-nitro-L-arginine methyl ester alone. These results indicate that LPS-stimulated PMNs modulate platelet aggregation during LPS treatment and the effects are reversed by antioxidants. PMNs serve as an approach to understand LPS-induced platelet aggregative dysfunction during endotoxemia. During this process, the generation of reactive oxygen species, hydrogen peroxide especially, from LPS-stimulated PMNs could be an important potential factor in LPS-induced platelet aggregative dysfunction. Catalase contributes to the prevention of platelet dysfunction during LPS-induced sepsis.

Oxidative risk for atherothrombotic cardiovascular disease

In the vasculature, reactive oxidant species, including reactive oxygen, nitrogen, or halogenating species, and thiyl, tyrosyl, or protein radicals may oxidatively modify lipids and proteins with deleterious consequences for vascular function. These biologically active free radical and nonradical species may be produced by increased activation of oxidant-generating sources and/or decreased cellular antioxidant capacity. Once formed, these species may engage in reactions to yield more potent oxidants that promote transition of the homeostatic vascular phenotype to a pathobiological state that is permissive for atherothrombogenesis. This dysfunctional vasculature is characterized by lipid peroxidation and aberrant lipid deposition, inflammation, immune cell activation, platelet activation, thrombus formation, and disturbed hemodynamic flow. Each of these pathobiological states is associated with an increase in the vascular burden of free radical species-derived oxidation products and, thereby, implicates increased oxidant stress in the pathogenesis of atherothrombotic vascular disease.

Role of reactive nitrogen species in blood platelet functions

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

Redox control of platelet function

There has recently been a dramatic expansion in research in the area of redox biology with systems that utilize thiols to perform redox chemistry being central to redox control. Thiol-based reactions occur in proteins involved in platelet function, including extracellular platelet proteins. The alphaIIbbeta3 fibrinogen receptor contains free thiols that are required for the activation of this receptor to a fibrinogen-binding conformation. This process is under enzymatic control, with protein disulfide isomerase playing a central role in the activation of alphaIIbbeta3. Other integrins, such as the alpha2beta1 collagen receptor on platelets, are also regulated by protein disulfide isomerase and thiol metabolism. Low molecular weight thiols that are found in blood regulate these processes by converting redox sensitive disulfide bonds to thiols and by providing the appropriate redox potential for these reactions. Additional mechanisms of redox control of platelets involve nitric oxide that inhibits platelet responses, and reactive oxygen species that potentiate platelet thrombus formation. Specific nitrosative or oxidative modifications of thiol groups in platelets may modulate platelet function. Since many biologic processes are regulated by redox reactions that involve surface thiols, the extracellular redox state can have an important influence on health and disease status and may be a target for therapeutic intervention.

Endogenous peroxynitrite modulates PGHS-1-dependent thromboxane A2 formation and aggregation in human platelets

Aggregation of activated platelets is considerably mediated by the autocrine action of thromboxane A2 (TxA2) which is formed in a prostaglandin endoperoxide H2 synthase-1 (PGHS-1 or COX-1)-dependent manner. The activity of PGHS-1 can be stimulated by peroxides, an effect termed "peroxide tone", that renders PGHS-1 the key regulatory enzyme in the formation of TxA2. Activated platelets release nitric oxide (*NO) and superoxide (O*2) but their interactions with the prostanoid pathway have been controversially discussed in platelet physiology and pathophysiology. The current study demonstrates that endogenously formed peroxynitrite at nanomolar concentrations, originating from the interaction of *NO and *O2, potently activated PGHS-1, which parallels TxA2 formation and aggregation in human platelets. Inhibition of the endogenous formation of either *NO or O*2 resulted in a concentration-dependent decline of PGHS-1 activity, TxA2 release, and aggregation. The concept of peroxynitrite as modulator of TxA2 formation and aggregation explains the interaction of *NO and O*2 with the PGHS pathway and suggests a mechanism by which antioxidants can regulate PGHS-1-dependent platelet aggregation. This may provide a molecular explanation for the clinically observed hyperreactivity of platelets in high-risk patients and serve as a basis for novel therapeutic interventions.

Potential Benefits of Peroxynitrite

Peroxynitrite (PN) is generated by the reaction of nitric oxide (NO) and superoxide in one of the most rapid reactions in biology. Studies have reported that PN is a cytotoxic molecule that contributes to vascular injury in a number of disease states. However, it has become apparent that PN has beneficial effects including vasodilation, inhibition of platelet aggregation, inhibition of inflammatory cell adhesion, and protection against ischemia/reperfusion injury in the heart. It is our hypothesis that PN may serve to inactivate superoxide and prolong the actions of NO in the circulation. This manuscript reviews the beneficial effects of PN in the cardiovascular system.

Reactive Oxygen Species

Platelets participate not only in thrombus formation but also in the regulation of vessel tone, the development of atherosclerosis, angiogenesis, and in neointima formation after vessel wall injury. It is not surprising, therefore, that the platelet activation cascade (including receptor-mediated tethering to the endothelium, rolling, firm adhesion, aggregation, and thrombus formation) is tightly regulated. In addition to already well-defined platelet regulatory factors, such as nitric oxide (NO), prostacyclin (PGI2), and adenosine, reactive oxygen species (ROS) participate in the regulation of platelet activation. Although exogenously derived ROS are known to affect the regulation of platelet activation, recent data suggest that the platelets themselves generate ROS. Intracellular ROS signaling in activated platelets could be of significant relevance after transient platelet contact with the vessel wall, during the recruitment of additional platelets, and in thrombus formation. This review discusses the potential cellular and enzymatic sources of ROS in platelets, their molecular mechanisms of action in platelet activation, and summarizes in vitro and in vivo evidence for their physiological and potential therapeutic relevance.

Oxidant stress and platelet activation in hypercholesterolemia

Hypercholesterolemia is the dominant risk factor associated with atherothrombotic disorders in the western world. Consequently, much attention has been devoted to defining its role in the pathogenesis of atherosclerosis. It is currently recognized that hypercholesterolemia induces phenotypic changes in the microcirculation that are consistent with oxidative and nitrosative stresses. Superoxide is generated via several cellular systems and, once formed, participates in a number of reactions, yielding various free radicals, such as hydrogen peroxide, peroxynitrite, or oxidized low-density lipoproteins. Once oxidant stress is invoked, characteristic pathophysiologic features ensue, such as platelet activation and lipid peroxidation, which are both involved in the initiation and progression of the atherosclerotic lesions. Thus, therapeutic strategies that act to maintain the normal balance in the oxidant status of the vascular bed may prove effective in reducing the deleterious consequences of hypercholesterolemia.

Interactions of peroxynitrite and other nitrating substances with human platelets: the role of glutathione and peroxynitrite permeability

Platelets labeled with 2',7'-dihydrodichlorofluorescein diacetate (DCF-DA) and stimulated with 50-400nM peroxynitrite (ONOO(-)) produced a rapid increase of the fluorescence signal at 523nm with good linearity and reproducibility. Platelet fluorescence was inhibited by 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), suggesting that HCO(3)(-)/Cl(-) transporter mediated ONOO(-) transport into the platelets. Exposure of platelets to potassium superoxide, hydrogen peroxide, and sodium nitroprusside at concentrations of up to 100 microM did not generate a fluorescence signal. We also studied other nitrating compounds to establish the specificity of the DCF-DA-labeled platelet ONOO(-) assay. A rapid increase of fluorescence was observed when sodium hypochlorite (0.15 to 0.75mM) was added to platelets suspended in a buffered nitrite solution. Exposure of platelets to NO(2), nitroglycerin, and tetranitromethane produced a slow sustained increase of fluorescence. Endogenous glutathione appeared to be an essential factor in the generation of fluorescence by ONOO(-) and other nitrating compounds. We further studied other conditions that increased platelet fluorescence. Stimulation of platelets with thrombin (1U/mL) produced a rapid increase in fluorescence that corresponded to the formation of 20.5nmol ONOO(-) per 10(7) cells, whereas stimulation with collagen and arachidonic acid was without effect. Hypoxia of platelets for 20 and 40min followed by 5min of reoxygenation doubled the fluorescence from these platelets compared with control platelets. Thus, thrombin produced an effect that was likely due to the formation of ONOO(-) in platelets, whereas hypoxia-reoxygenation was likely to cause the formation of an active nitroglutathione-like molecule.

Inhibition of rat platelet aggregation by the diazeniumdiolate nitric oxide donor MAHMA NONOate

1. Inhibition of rat platelet aggregation by the nitric oxide (NO) donor MAHMA NONOate (Z-1-N-methyl-N-[6-(N-methylammoniohexyl)amino]diazen-1-ium-1,2-diolate) was investigated. The aims were to compare its anti-aggregatory effect with vasorelaxation, to determine the effects of the soluble guanylate cyclase inhibitor, ODQ (1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one), and to investigate the possible role of activation of sarco-endoplasmic reticulum calcium-ATPase (SERCA), independent of soluble guanylate cyclase, using thapsigargin. 2 MAHMA NONOate concentration-dependently inhibited sub-maximal aggregation responses to collagen (2-10 micro g ml(-1)) and adenosine diphosphate (ADP; 2 micro M) in platelet rich plasma. It was (i). more effective at inhibiting aggregation induced by collagen than by ADP, and (ii). less potent at inhibiting platelet aggregation than relaxing rat pulmonary artery. 3. ODQ (10 micro M) caused only a small shift (approximately half a log unit) in the concentration-response curve to MAHMA NONOate irrespective of the aggregating agent. 4. The NO-independent activator of soluble guanylate cyclase, YC-1 (3-(5'-hydroxymethyl-2'-furyl)-1-benzyl indazole; 1-100 micro M), did not inhibit aggregation. The cGMP analogue, 8-pCPT-cGMP (8-(4-chlorophenylthio)guanosine 3'5' cyclic monophosphate; 0.1-1 mM), caused minimal inhibition. 5. On collagen-aggregated platelets responses to MAHMA NONOate (ODQ 10 micro M present) were abolished by thapsigargin (200 nM). On ADP-aggregated platelets thapsigargin caused partial inhibition. 6. Results with S-nitrosoglutathione (GSNO) resembled those with MAHMA NONOate. Glyceryl trinitrate and sodium nitroprusside were poor inhibitors of aggregation. 7. Thus inhibition of rat platelet aggregation by MAHMA NONOate (like GSNO) is largely ODQ-resistant and, by implication, independent of soluble guanylate cyclase. A likely mechanism of inhibition is activation of SERCA.

The ingestion of inorganic nitrate increases gastric S-nitrosothiol levels and inhibits platelet function in humans

Platelets play an important role in the development of vascular disease, while vegetarian diets, which are rich in inorganic nitrate, protect against it. This study was performed to assess the effect of potassium nitrate (KNO(3)) ingestion on platelet function in humans. Oral KNO(3) (2 mmol) was given to healthy volunteers and its effect on platelet function assessed by measuring the aggregant effect of collagen. Blood samples were taken for measurement of plasma S-nitrosothiols (RSNO) and platelet cyclic GMP and nitrotyrosine levels. Gastric juice samples were taken for measurement of RSNO. In a separate study, the effect of oral KNO(3) on portal RSNO levels in patients with intrahepatic porto-systemic shunts was assessed. KNO(3) caused a significant increase in gastric RSNO levels, from 0.46 +/- 0.06 to 3.62 +/- 2.82 microM (t(max) 45 min; P < 0.001), and significantly inhibited platelet function (t(max) 60 min; P < 0.001). There was no effect on systemic or portal RSNO, platelet cGMP or platelet nitrotyrosine levels. Oral KNO(3) inhibits platelet aggregation. The time course suggests that gastric RSNO production may be involved in this effect. The protection against vascular events associated with a high intake of vegetables may be due to their high nitrate content.

Nitrolinoleate inhibits platelet activation by attenuating calcium mobilization and inducing phosphorylation of vasodilator-stimulated phosphoprotein through elevation of cAMP

Reactive species formed from nitric oxide (NO) nitrate unsaturated fatty acids such as linoleate (LA) to nitrated derivatives including nitrolinoleate (LNO(2)). The effect of LNO(2) on human platelets was examined to define how nitrated lipids might behave in vivo. LNO(2), but not LA or 3-nitrotyrosine, dose dependently (0.5-10 microm) inhibited thrombin-mediated aggregation of washed human platelets, with concomitant attenuation of P-selectin expression and selective phosphorylation of VASP at the cAMP-dependent protein kinase selective site, serine 157. LNO(2) caused slight mobilization of calcium (Ca(2+)) from intracellular stores but significantly inhibited subsequent thrombin-stimulated Ca(2+) elevations. LNO(2) did not elevate platelet cGMP, and its effects were not blocked with inhibitors of NO signaling (oxyhemoglobin, 1H-[1,2,4]oxadiazole[4,3-a]quinoxalin-1-one. 2-fold elevations in cAMP were found following LNO(2) treatment of platelets, and the adenylyl cyclase inhibitors 2',5'-dideoxyadenosine and SQ22536 partially restored thrombin-stimulated aggregation. Finally, LNO(2) significantly inhibited cAMP hydrolysis to AMP by platelet lysates. These data implicate cAMP in the anti-aggregatory action of LNO(2). The platelet inhibitory actions of LNO(2) indicate that nitration reactions that occur following NO generation in an oxidizing environment can alter the activity of lipids and lend insight into mechanisms by which NO-derived species may modulate the progression of vascular injury.

The role of protein nitration in the inhibition of platelet activation by peroxynitrite

Peroxynitrite at low concentrations (3-10 microM) inhibited agonist-induced platelet aggregation by a mechanism not dependent on the formation of cyclic guanosine monophosphate. Platelets recovered completely from peroxynitrite-induced inhibition within 30 min. Peroxynitrite induced nitration of cytosolic proteins, but this diminished to near basal levels within 60 min of exposure to the oxidant. During this period there was a reduction in tyrosine phosphorylation of specific proteins such as syk, but this was not due to direct nitration of these same proteins. The inhibition of phosphorylation was reversible with platelet proteins recovering the ability to be phosphorylated within 15 min of exposure to peroxynitrite. Conversely, peroxynitrite increased phosphorylation of other proteins, but again these events were not directly linked to nitration. Nitration may affect the phosphorylation of tyrosine residues in a number of proteins, but by an indirect route, possibly by acting on proteins upstream in the signalling cascades. We suggest that low concentrations of peroxynitrite reversibly inhibit platelet aggregation by preventing the phosphorylation of key signalling proteins.

Peroxynitrite attenuates hepatic ischemia-reperfusion injury

In the present study, we examined the effects of peroxynitrite on reperfusion injury using a rat model of hepatic ischemia-reperfusion (HI/R). The left and median lobes of the liver were subjected to 30 min of ischemia, followed by 4 h of reperfusion. Groups A and B rats were sham-operated controls that received vehicle or peroxynitrite; groups C and D rats were subjected to HI/R and received peroxynitrite or vehicle, respectively. A dose of 2 micromol/kg body wt of peroxynitrite, diluted in saline (pH 9.0, 4 degrees C), was administered as a bolus through a portal vein catheter at 0, 60, and 120 min after reperfusion. Results showed that superoxide generation in the ischemic lobes of the liver and plasma alanine aminotransferase (ALT) activity of group C were decreased by 43% and 45%, respectively, compared with group D. Leukocyte accumulations in the ischemic lobes of liver and circulating leukocytes were decreased by 40% and 27%, respectively, in group C vs. D. The ratios of mRNA of P-selectin and intercellular adhesion molecule-1 (ICAM-1) to glyceraldehyde-3-phosphate dehydrogenase (GAPDH) mRNA extracted from the ischemic lobes of the liver of group C were decreased compared with group D. There were no differences between the groups A and B in terms of plasma ALT activity, circulating leukocytes, superoxide generation, and leukocyte infiltration in the ischemic lobes of the liver. Moreover, hemodynamic parameters (i.e., mean arterial blood pressure, cardiac index, stroke index, and systemic vascular resistance) were not significantly different among groups B, C, and D. These results suggest that administration of peroxynitrite via the portal vein only has a local effect. Exogenous peroxynitrite at physiological concentrations attenuates leukocyte-endothelial interaction and reduces leukocyte infiltration. The mechanism of the reduction of leukocyte infiltration into ischemic lobes of the liver appears because of decreased expression of mRNA of P-selectin and ICAM-1. The net effect of administration of peroxynitrite may be to reduce adhesion molecule-mediated, leukocyte-dependent reperfusion injury.

Peroxynitrite-induced apoptosis involves activation of multiple caspases in HL-60 cells

In this study, we show that caspases 2, 3, 6, and 7 were activated during peroxynitrite-induced apoptosis in human leukemia HL-60 cells and that processing of these caspases was accompanied by cleavage of poly(ADP-ribose) polymerase and lamin B. Treatment of cells with DEVD-fluoromethyl ketone (FMK), a selective inhibitor for caspase 3-like proteases, resulted in a marked diminution of apoptotic cells. VAVAD-FMK, an inhibitor of caspase 2, partially inhibited the apoptotic response to peroxynitrite. However, selective inactivation of caspase 6 by VEID-FMK did not affect apoptosis rates. These data suggest that caspase 3-like proteases and caspase 2, but not caspase 6, are required for peroxynitrite-induced apoptosis in this cell type. Moreover, we demonstrate that peroxynitrite treatment stimulated activation of caspases 8 and 9, two initial caspases in the apoptotic signaling pathway, and preincubation of cells with their inhibitor, IETD-FMK, inhibited activation of caspase 3-like proteases and caspase 2 at the concentration that prevents the apoptosis. These observations, together, suggest that caspase 8 and/or caspase 9 mediates activation of caspase 3-like proteases and caspase 2 during the apoptosis induced by peroxynitrite in HL-60 cells.

The nitration of platelet cytosolic proteins during agonist-induced activation of platelets

The nitration of protein tyrosine residues by peroxynitrous acid has been associated with pathological conditions. Here it is shown, using a sensitive competitive enzyme-linked immunosorbent assay and immunoblotting for nitrotyrosine, that spontaneous nitration of specific proteins occurs during a physiological process, the activation of platelets by collagen. One of the main proteins nitrated is vasodilator-stimulated phosphoprotein. Endogenous synthesis of nitric oxide and activity of cyclo-oxygenase were required for the nitration of tyrosine. The nitration was mimicked by addition of peroxynitrite to unstimulated platelets, although the level of nitrotyrosine formation was greater and its distribution among the proteins was less specific.

cGMP inhibits IP3-induced Ca2+ release in intact rat megakaryocytes via cGMP- and cAMP-dependent protein kinases

1. Inhibition of inositol 1,4,5-trisphosphate (IP3) receptor-mediated Ca2+ release by cGMP was examined in intact rat megakaryocytes, by using a combination of single cell fluorescence microscopy to monitor intracellular free calcium ([Ca2+]i) and flash photolysis of caged second messengers. 2. Sodium nitroprusside (SNP), a nitric oxide (NO) donor, and the hydrolysis-resistant cGMP analogue 8-(4-chlorophenylthio)guanosine 3',5'-cyclic monophosphate (pCPT-cGMP) inhibited Ca2+ release induced by photolysis of caged IP3. Neither of them affected the rate of Ca2+ removal from the cytoplasm following photolysis of caged Ca2+. 3. Photolysis of the caged NO donor 3-morpholinosydnonimine (SIN-1) during agonist-induced [Ca2+]i oscillations inhibited Ca2+ release without affecting the rate of Ca2+ uptake and/or extrusion. 4. We conclude that the inhibition of IP3-induced Ca2+ release is the principal mechanism of NO-cGMP-dependent inhibition of [Ca2+]i mobilization. 5. IPG, a specific peptide inhibitor of cGMP-dependent protein kinase (cGMP-PK), blocked the inhibitory effect of pCPT-cGMP, indicating that the inhibition of IP3-induced Ca2+ release by pCPT-cGMP is mediated by cGMP-PK. However, the simultaneous application of both IPG and IP20, a specific peptide inhibitor of cAMP-dependent protein kinase (cAMP-PK), was required to block the inhibitory effect of SNP. These data strongly suggest that NO-cGMP-dependent inhibition of [Ca2+]i mobilization is mediated via the activation of both cGMP-PK and cAMP-PK.

Peroxynitrite induces apoptosis of HL-60 cells by activation of a caspase-3 family protease

Apoptosis is an active process critical for the homeostasis of organisms. Enzymes of the caspase family are responsible for executing this process. We have previously shown that peroxynitrite (ONOO-), a biological product generated from the interaction of nitric oxide and superoxide, induces apoptosis of HL-60 cells. The aim of this study was to elucidate the mechanisms involved in the execution process of peroxynitrite-induced apoptosis. Proteolytic cleavage of poly(ADP-ribose) polymerase, an indication of caspase-3 family protease activation and an early biochemical event accompanying apoptosis, was observed in a time-dependent manner during peroxynitrite-induced apoptosis of HL-60 cells. Activation of caspase-3 during peroxynitrite-induced apoptosis was substantiated by monitoring proteolysis of the caspase-3 proenzyme and by measuring caspase-3 activity with a fluorogenic substrate. Furthermore, pretreatment of HL-60 cells with N-acetyl-Asp-Glu-Val-Asp-aldehyde, a specific inhibitor of caspase-3, but not N-acetyl-Tyr-Val-Ala-Asp-aldehyde, a specific inhibitor of caspase-1, decreased peroxynitrite-induced apoptosis. These results suggest that the activation of a caspase-3 family protease is essential for initiating the execution process of peroxynitrite-induced apoptosis of HL-60 cells.

Reaction of peroxynitrite with HEPES or MOPS results in the formation of nitric oxide donors

We investigated the effects of organic buffers on the NO-like biological activities of ONOO-. In HEPES buffer (50 mM), ONOO- (1 mM) induced a 20-fold increase in endothelial cGMP accumulation and the effect was comparable to that elicited by a maximally active concentration of the NO donor DEA/NO. ONOO- produced a 12-fold increase of cGMP in MOPS buffer (50 mM), but was virtually inactive in phosphate buffer (50 mM). Electrochemical detection of NO showed that the biological effects of ONOO- in HEPES or MOPS were due to accumulation of compounds that released NO in the presence of copper ions. CuCl2-induced formation of NO was completely blocked by the Cu(I) chelator neocuproine but unaffected by the Cu(II) chelator cuprizone, pointing to a Cu(I)-catalyzed decomposition pathway. Formation of NO from ONOO- was not detectable in phosphate buffer, in agreement with the lack of effect of ONOO- on cGMP accumulation in this buffer. These data demonstrate that certain buffer components present in cell culture media may yield artificial results in experiments with authentic ONOO-.

Peroxynitrite reduces myocardial infarct size and preserves coronary endothelium after ischemia and reperfusion in cats

BACKGROUND

Peroxynitrite (ONOO-) is purported to exert cytotoxic effects at high doses. However, physiologically relevant concentrations of ONOO- inhibit polymorphonuclear neutrophil (PMN) adhesion to the endothelium and attenuate PMN-mediated contractile dysfunction in isolated perfused rat hearts. We are unaware of any reports in vivo showing effects of peroxynitrite in myocardial ischemia and reperfusion (MI/R). Thus, the purpose of this study was to examine the in vivo effects of a physiologically relevant concentration of ONOO- (1 micromol/L) in a feline model of MI/R injury.

METHODS AND RESULTS

ONOO- (1 micromol/L) or its vehicle (0.9% NaCl at pH 8.4) was infused intraventricularly, starting 10 minutes before reperfusion in cats subjected to 90 minutes of myocardial ischemia and 4.5 hours of reperfusion. ONOO(-)-treated cats demonstrated marked attenuation of cardiac necrosis after MI/R compared with cats receiving only vehicle (P<.001). Moreover, vasorelaxation of ischemic-reperfused left anterior descending (LAD) coronary artery rings in response to the endothelium-dependent dilators acetylcholine and A23187 was greater in rings isolated from ONOO(-)-treated MI/R cats compared with MI/R cats receiving only vehicle, indicating that postreperfusion coronary vascular endothelial function was preserved by ONOO-. ONOO- also significantly reduced adherence of neutrophils to the ischemic-reperfused LAD coronary endothelium. Immunohistochemical localization of P-selectin was also significantly attenuated in hearts from ONOO(-)-infused MI/R cats.

CONCLUSIONS

These data suggest that physiologically relevant concentrations of ONOO- exert significant cardioprotective and vasculoprotective effects in MI/R in cats, at least partially by attenuating PMN-endothelium interactions.

Peroxynitrite: a two-faced metabolite of nitric oxide

The discovery that nitric oxide (NO) reacts with superoxide (O2.-) forming peroxynitrite (ONOO-) (1) and the proof that this reaction occurs in vivo (2,3) holds enormous implications for the understanding of free radicals in biological systems. Not only in mammalian defense mechanisms against microorganisms, but also in pathophysiology during overexposure of tissues to radicals or other highly reactive species. Peroxynitrite is a highly reactive compound with harmful effects on cells and could therefore be an important microbicidal compound. Furthermore, the reaction of superoxide with NO interferes with NO signalling mechanisms. NO is not only released in response to inflammatory agents by inflammatory cells, but is also an important messenger molecule in paracrine mechanisms and neurotransmission. Whether peroxynitrite formation is a negative side effect of NO and superoxide release, or a functional characteristic is yet to be determined, and will be discussed in this review.