Balancing reactivity against selectivity: the evolution of protein S-nitrosylation as an effector of cell signaling by nitric oxide

Produced by the action of lightning in the atmosphere of the pre-biotic earth, nitric oxide (NO) is a free radical molecule that provided the major nitrogen source for development of life. Remarkably, when atmospheric sources of NO became restrictive, organisms evolved the capacity for NO biosynthesis and NO took on bioregulatory roles. We now recognize NO as an ancestral regulator of diverse and important biological functions, acting throughout the phylogenetic tree. In mammals, NO has been implicated as a pivotal regulator of virtually every major physiological system. The bioactivities of NO, and reactive species derived from NO, arise predominantly from their covalent addition to proteins. Importantly, S-nitrosylation of protein cysteine (Cys) residues has emerged as a preeminent effector of NO bioactivity. How and why NO selectively adds to particular Cys residues in proteins is poorly understood, yet fundamental to how NO communicates its bioactivities. Also, evolutionary pressures that have shaped S-nitrosylation as a biosignaling modality are obscure. Considering recently recognized NO signaling paradigms, we speculate on the origin of NO signaling in biological systems and the molecular adaptations that have endowed NO with the ability to selectively target a subset of protein Cys residues that mediate biosignaling.

Nitric oxide cell signaling: S-nitrosation of Ras superfamily GTPases

The Ras superfamily of small GTPases cycle between inactive GDP-bound and active GTP-bound states to modulate a diverse array of processes involved in cellular growth control. While the basic mechanisms by which GTPase regulatory proteins regulate GTPase substrates have been revealed through numerous studies, detailed studies into the mechanism(s) of free radical-mediated GTPase regulation have only more recently been tackled. This article reviews the mechanism of free radical-mediated GTPase regulation and shows nitric oxide can serve as important regulator of small GTPase proteins (i.e. Ras and RhoA) through protein modifications such as S-nitrosation.

Nitric oxide suppresses NADPH oxidase-dependent superoxide production by S-nitrosylation in human endothelial cells

OBJECTIVE

Endothelial NADPH oxidase is a major source of superoxide in blood vessels and is implicated in the oxidative stress accompanying vascular diseases, including atherosclerosis. Here we investigate the regulation of NADPH oxidase activity by nitric oxide (NO).

METHODS

Human cultured microvascular endothelial cells (HMEC-1) were treated with the NO donors, diethylenetriamine (DETA)-NONOate, S-nitroso-N-acetylpenicillamine (SNAP) or sodium nitroprusside (SNP) for 0.5-24 h. Superoxide production was measured by lucigenin chemiluminescence and dihydroethidium fluorescence, while NADPH oxidase subunit expression was measured via Western blotting. S-nitrosylation was assessed using the 2,3-diaminonapthalene (DAN) assay, and via immunoblotting with an anti-nitrosocysteine antibody.

RESULTS

Specific siRNA reduced Nox2 and Nox4 protein expression and markedly decreased superoxide production in HMEC-1. DETA-NONOate (10-300 micromol/L) suppressed superoxide production in HMEC-1 in a concentration- and time-dependent manner, which was not entirely attributable to stoichiometric reaction with NO, for the effect was observed more than 6 h after removing DETA-NONOate from solution. Similarly, sustained attenuation of superoxide production was achieved with SNP (10-100 micromol/L) and SNAP (10-100 micromol/L). The suppressive effect of NO was not dependent on (1) the sGC/cGMP/PKG pathway, (2) peroxynitrite-formation, (3) reduced protein expression of NADPH oxidase subunits or (4) dissociation of NADPH oxidase subunits. Treatment with NO caused S-nitrosylation of the crucial organizer subunit p47phox, and de-nitrosylation with UV light restored superoxide production.

CONCLUSIONS

NO causes sustained suppression of NADPH oxidase-dependent superoxide production in human endothelial cells by S-nitrosylation of p47phox. These findings highlight a novel approach by which vascular oxidative stress might be suppressed by NO donors.

Decomposition of S-nitrosocysteine via S- to N-transnitrosation

S-nitrosothiols are thought to be important intermediates in nitric oxide signaling pathways. These compounds are unstable, in part, through their ability to donate NO. One model S-nitrosothiol, S-nitrosocysteine, is particularly unstable. Recently, it was proposed that this compound decomposed via intra and intermolecular transfer of the NO group from the sulfur to the nitrogen to form N-nitrosocysteine. This primary nitrosamine is expected to rapidly rearrange to ultimately form a reactive diazonium ion intermediate. To test this hypothesis, we demonstrated that thiirane-2-carboxylic acid is formed during the decomposition of S-nitrosocysteine at neutral pH. Acrylic acid was another product of this reaction. These results indicate that a small but significant amount of S-nitrosocysteine decomposes via S- to N-transnitrosation. The formation of a reactive intermediate in this process indicates the potential for this reaction to contribute to the toxicological properties of nitric oxide.

Adverse effects of the classic antioxidant uric acid in adipocytes: NADPH oxidase-mediated oxidative/nitrosative stress

Uric acid is considered a major antioxidant in human blood that may protect against aging and oxidative stress. Despite its proposed protective properties, elevated levels of uric acid are commonly associated with increased risk for cardiovascular disease and mortality. Furthermore, recent experimental studies suggest that uric acid may have a causal role in hypertension and metabolic syndrome. All these conditions are thought to be mediated by oxidative stress. In this study we demonstrate that differentiation of cultured mouse adipocytes is associated with increased production of reactive oxygen species (ROS) and uptake of uric acid. Soluble uric acid stimulated an increase in NADPH oxidase activity and ROS production in mature adipocytes but not in preadipocytes. The stimulation of NADPH oxidase-dependent ROS by uric acid resulted in activation of MAP kinases p38 and ERK1/2, a decrease in nitric oxide bioavailability, and an increase in protein nitrosylation and lipid oxidation. Collectively, our results suggest that hyperuricemia induces redox-dependent signaling and oxidative stress in adipocytes. Since oxidative stress in the adipose tissue has recently been recognized as a major cause of insulin resistance and cardiovascular disease, hyperuricemia-induced alterations in oxidative homeostasis in the adipose tissue might play an important role in these derangements.

Life history of eNOS: partners and pathways

The complex regulation of eNOS (endothelial nitric oxide synthase) in cardiovascular physiology occurs at multiple stages. eNOS mRNA levels are controlled both at the transcriptional and post-transcriptional phases, and epigenetic mechanisms appear to modulate tissue-specific eNOS expression. The eNOS enzyme reversibly associates with a diverse family of protein partners that regulate eNOS sub-cellular localization, catalytic function, and biological activity. eNOS enzyme activity and sub-cellular localization are intimately controlled by post-translational modifications including phosphorylation, nitrosylation, and acylation. The multiple extra-cellular stimuli affecting eNOS function coordinate their efforts through these key modifications to dynamically control eNOS and NO bioactivity in the vessel wall. This review will focus on the biochemical partners and perturbations of the eNOS protein as this vital enzyme undergoes modulation by diverse signal transduction pathways in the vascular endothelium.

Nitric oxide regulation of protein trafficking in the cardiovascular system

Nitric oxide (NO) is a second messenger with diverse roles in the cardiovascular system, such as inhibiting thrombosis and limiting vascular inflammation. One mechanism by which NO modulates such disparate physiological processes is by regulating protein trafficking within cells. NO inhibits exocytosis of endothelial granules which would otherwise trigger inflammation. NO also blocks platelet secretion of granules that would otherwise activate thrombosis. NO decreases granule trafficking from the Golgi to the plasma membrane by targeting a key component of the exocytic machinery, N-ethylmaleimide sensitive factor (NSF). In contrast to its inhibitory effects on exocytosis, NO accelerates endocytosis. S-nitrosylation of dynamin increases its ability to hydrolyze GTP, assemble in oligomers around a nascent vesicle, and cleave the endocytic vesicle free from the plasma membrane. NO regulation of vesicle trafficking is a molecular mechanism that explains some of the cardiovascular effects of NO, and may be of broad physiological significance.

Nitric oxide controls nuclear export of APE1/Ref-1 through S-nitrosation of cysteines 93 and 310

Apurinic/apyrimidinic endonuclease 1/redox effector factor-1 (APE1/Ref-1, abbreviated as APE1) is a molecule with dual functions in DNA repair and redox regulation of transcription factors. Accumulated work has shown that the biological activities of APE1 are sensitive to oxidative stress; however, whether APE1 functions can be regulated by nitrosative stress remains unknown. In this investigation, we found that S-nitrosoglutathion (GSNO), a nitric oxide donor and also an S-nitrosating agent, effectively stimulated nuclear export of APE1 in a CRM1-independent manner. This nuclear-cytoplasmic translocation was dependent on S-nitrosation modification of APE1, as simultaneous mutation of S-nitrosation target sites Cys93 and Cys310 completely abrogated the cytoplasmic redistribution. The translocation process was reversal and specific, as it could be reversed by reductive reagents, but could not be mimicked by H2O2. In structure, the region aa.64-80 and the beta-strand aa.311-316 in proximity to Cys93 and Cys310 were important for GSNO-induced APE1 relocalization. In addition, a defect of importin-mediated nuclear import pathway was found in the NO-insulted cells, and p50 and HDAC2 were identified as APE1 nuclear export inhibitory proteins. Together, this study may provide a novel molecular mechanism, which links nitrosative stress to APE1-associated physiological and pathological processes.

Buried S-nitrosocysteine revealed in crystal structures of human thioredoxin

We have determined the 1.65 A crystal structure of human thioredoxin-1 after treatment with S-nitrosoglutathione, providing a high-resolution view of this important protein modification and mechanistic insight into protein transnitrosation. Thioredoxin-1 appears to play an intermediary role in cellular S-nitrosylation and is important in numerous biological and pathobiological activities. S-Nitroso modifications of cysteines 62 and 69 are clearly visible in the structure and display planar cis geometries, whereas cysteines 32, 35, and 73 form intra- and intermolecular disulfide bonds. Surprisingly, the Cys 62 nitroso group is completely buried and pointing to the protein interior yet is the most readily formed at neutral pH. The Cys 69 nitroso group is also protected but requires a higher pH for stable formation. The helix intervening between residues 62 and 69 shifts by approximately 0.5 A to accommodate the SNO groups. The crystallographic asymmetric unit contains three independent molecules of thioredoxin, providing three views of the nitrosated protein. The three molecules are in general agreement but display subtle differences, including both cis and trans conformers for Cys 69 SNO in molecule C, and greater disorder in the Cys 62-Cys 69 helix in molecule B. Possible mechanisms for protein transnitrosation with specific geometric requirements and charge stabilization of the nitroxyl disulfide reaction intermediate are discussed.

Signalling by NO-induced protein S-nitrosylation and S-glutathionylation: convergences and divergences

The role of nitric oxide in several signalling routes has been clearly established. In recent years increasing attention has been paid to its ability to produce covalent protein post-translational modifications in conjunction with other reactive oxygen and nitrogen species. Among these, the modification of cysteine residues has been shown to be of particular importance due to the functional relevance of many of them. In this review, we focus on the modification of the cysteine thiol by incorporation of a NO moiety (S-nitrosylation) or of a glutathione moiety (S-glutathionylation). Both modifications are produced by different reactions induced by nitric oxide-related species. We discuss the differences and similarities of both modifications, and their relationships, in regard to the biochemical mechanisms that produce them, including the enzymatic activities that may catalyze some of them and their subcellular compartmentalization. Even when biochemical knowledge is one step ahead of the demonstration of their pathophysiological relevance, we also describe the potential role of both modifications in several processes in which both post-translational modifications are involved.

Nitration of specific tyrosine residues of cytochrome C is associated with caspase-cascade inactivation

Peroxynitrite, a potent oxidative stress inducer, inhibits the mitochondrial electron transfer, induces cell death, and is considered to be involved in the pathology of various diseases. However, the intracellular mechanisms involved in the cell death process are not fully understood. Here we demonstrate that the enhanced nitration of specific tyrosine residues of cytochrome c, which are induced by continuous peroxynitrite exposure, attenuates cytochrome c-induced caspase-9 activation in vitro. Interestingly, cytochrome c nitrated with a single high dose of peroxynitrite preserved its potency, while this did not occur when cytochrome c was treated with continuous peroxynitrite exposure. Although both of these experiments resulted in cytochrome c nitration at the tyrosine residues, it was found that nitration at specific residues was enhanced only when cytochrome c was exposed to continuous peroxynitrite. This is the first report to demonstrate that cytochrome c nitration affects the apoptotic pathway by means of enhancement of nitration at specific tyrosine residues. This result implies that the nitration pattern of cytochrome c may affect the efficacy of the mitochondrial pathway in apoptotic cell death.

Nitric oxide stimulates heme oxygenase-1 gene transcription via the Nrf2/ARE complex to promote vascular smooth muscle cell survival

OBJECTIVE

Previous studies from our laboratory and others found that NO is a potent inducer of heme oxygenase-1 (HO-1) gene transcription in vascular smooth muscle cells (SMC), however, the mechanism responsible for the induction of HO-1 gene expression has not been elucidated. In the present study, we determined the signaling pathway responsible for the induction of HO-1 and its biological significance.

METHODS

Cultured rat aortic SMC were exposed to nitrosative stress by treating cells with various NO donors or with inflammatory cytokines.

RESULTS

Nitrosative stress stimulated an increase in HO-1 mRNA expression and promoter activity in vascular SMC. However, mutation of the antioxidant response element (ARE) in the HO-1 promoter or overexpression of a dominant-negative mutant of NF-E2-related factor-2 (Nrf2) abrogated the activation by NO. Electromobility shift assays using an ARE probe detected a complex that was significantly increased in intensity by NO. In addition, the migration of this complex was retarded by using an antibody directed against Nrf2. NO also increased Nrf2 mRNA expression, total and nuclear Nrf2 levels, and the binding of Nrf2 to the HO-1 promoter. Finally, treatment of SMC with NO stimulated apoptosis that was increased by HO-1 inhibition.

CONCLUSIONS

These results demonstrate that nitrosative stress induces HO-1 gene transcription through the activation of the Nrf2/ARE complex to counteract NO-induced apoptosis of vascular SMC. The capacity of nitrosative stress to activate Nrf2 and stimulate HO-1 gene transcription may represent a critical adaptive response to maintain cell viability at sites of vascular inflammation and atherosclerosis.

N-Nitrosation of Amines by NO2 and NO: A Theoretical Study

Gas-phase nitrosation of amines implies a nonionic pathway different from the nitrosonium nitrosation via acidification of nitrite. Electronic structure calculations discussed in this work suggest a free radical mechanism, in which NO2 abstracts a hydrogen atom from the nitrogen in primary and secondary amines to form an intermediate complex of an aminyl radical and nitrous acid. The aminyl radical intermediate is then quenched by nitric oxide, leading to the formation of nitrosamine. High-level calculations (CBS-QB3) show that alkyl substitutions on amines can activate the H-abstraction reaction. Thus, while H-abstraction from NH3 was found to exhibit a reaction barrier (DeltaH) of 106 kJ/mol, similar calculations indicate that the corresponding barriers decrease to 72 and 45 kJ/mol for methylamine and dimethylamine, respectively. Heterocyclic secondary amines have also been investigated in a similar manner. The five-membered-ring (5-m-r) amine appears to be the most reactive: pyrrolidine (DeltaH=30 kJ/mol), azetidine (DeltaH=44 kJ/mol), piperidine (DeltaH=44 kJ/mol), and aziridine (DeltaH=74 kJ/mol). The reaction barrier for 1H-pyrrole, an aromatic 5-m-r secondary amine, was found to be 59 kJ/mol. The origin of the high activity for the 5-m-r alkylamine stems from a hydrogen-bond-like interaction between the aminyl radical and the nascent nitrous acid molecule. This theoretical study suggests that, in the presence of nitrogen oxides, the gas-phase nitrosation of secondary amines is feasible.

Nitric oxide in chemostat-cultured Escherichia coli is sensed by Fnr and other global regulators: unaltered methionine biosynthesis indicates lack of S nitrosation

We previously elucidated the global transcriptional responses of Escherichia coli to the nitrosating agent S-nitrosoglutathione (GSNO) in both aerobic and anaerobic chemostats, demonstrated the expression of nitric oxide (NO)-protective mechanisms, and obtained evidence of critical thiol nitrosation. The present study was the first to examine the transcriptome of NO-exposed E. coli in a chemostat. Using identical conditions, we compared the GSNO stimulon with the stimulon of NO released from two NO donor compounds {3-[2-hydroxy-1-(1-methyl-ethyl)-2-nitrosohydrazino]-1-propanamine (NOC-5) and 3-(2-hydroxy-1-methyl-2-nitrosohydrazino)-N-methyl-1-propanamine (NOC-7)} simultaneously and demonstrated that there were marked differences in the transcriptional responses to these distinct nitrosative stresses. Exposure to NO did not induce met genes, suggesting that, unlike GSNO, NO does not elicit homocysteine S nitrosation and compensatory increases in methionine biosynthesis. After entry into cells, exogenous methionine provided protection from GSNO-mediated killing but not from NO-mediated killing. Anaerobic exposure to NO led to up-regulation of multiple Fnr-repressed genes and down-regulation of Fnr-activated genes, including nrfA, which encodes cytochrome c nitrite reductase, providing strong evidence that there is NO inactivation of Fnr. Other global regulators apparently affected by NO were IscR, Fur, SoxR, NsrR, and NorR. We tried to identify components of the NorR regulon by performing a microarray comparison of NO-exposed wild-type and norR mutant strains; only norVW, encoding the NO-detoxifying flavorubredoxin and its cognate reductase, were unambiguously identified. Mutation of norV or norR had no effect on E. coli survival in mouse macrophages. Thus, GSNO (a nitrosating agent) and NO have distinct cellular effects; NO more effectively interacts with global regulators that mediate adaptive responses to nitrosative stress but does not affect methionine requirements arising from homocysteine nitrosation.

Proteomic identification of S-nitrosylated proteins in endothelial cells

Nitric oxide (NO) produced in endothelial cells exerts important roles in the vascular system. In recent years, posttranslational modifications induced by NO have been increasingly studied and, among them, cysteine modification by S-nitrosylation (also called S-nitrosation) has been hypothesized to represent a relevant mechanism for cell signaling. Thus, knowledge of the proteins that can be S-nitrosylated in endothelial cells will help to better understand the possible role of this modification. We describe a protocol to identify the S-nitrosylome or S-nitrosoproteome of endothelial cells, based on the specific derivatization of the S-nitrosylation, substituting it by a biotinylation, and the purification of the biotinylated proteins.

Consequences of MnSOD interactions with nitric oxide: nitric oxide dismutation and the generation of peroxynitrite and hydrogen peroxide

The present study demonstrates that manganese superoxide dismutase (MnSOD) (Escherichia coli), binds nitric oxide (*NO) and stimulates its decay under both anaerobic and aerobic conditions. The results indicate that previously observed MnSOD-catalyzed *NO disproportionation (dismutation) into nitrosonium (NO+) and nitroxyl (NO-) species under anaerobic conditions is also operative in the presence of molecular oxygen. Upon sustained aerobic exposure to *NO, MnSOD-derived NO- species initiate the formation of peroxynitrite (ONOO-) leading to enzyme tyrosine nitration, oxidation and (partial) inactivation. The results suggest that both ONOO- decomposition and ONOO(-)-dependent tyrosine residue nitration and oxidation are enhanced by metal centre-mediated catalysis. We show that the generation of ONOO- is accompanied by the formation of substantial amounts of H2O2. MnSOD is a critical mitochondrial antioxidant enzyme, which has been found to undergo tyrosine nitration and inactivation in various pathologies associated with the overproduction of *NO. The results of the present study can account for the molecular specificity of MnSOD nitration in vivo. The interaction of *NO with MnSOD may represent a novel mechanism by which MnSOD protects the cell from deleterious effects associated with overproduction of *NO.

Cardioprotection and mitochondrial S-nitrosation: effects of S-nitroso-2-mercaptopropionyl glycine (SNO-MPG) in cardiac ischemia-reperfusion injury

Mitochondrial dysfunction is a key pathologic event in cardiac ischemia-reperfusion (IR) injury, and protection of mitochondrial function is a potential mechanism underlying ischemic preconditioning (IPC). Acknowledging the role of nitric oxide (NO()) in IPC, it was hypothesized that mitochondrial protein S-nitrosation may be a cardioprotective mechanism. The reagent S-nitroso-2-mercaptopropionyl-glycine (SNO-MPG) was therefore developed to enhance mitochondrial S-nitrosation and elicit cardioprotection. Within cardiomyocytes, mitochondrial proteins were effectively S-nitrosated by SNO-MPG. Consistent with the recent discovery of mitochondrial complex I as an S-nitrosation target, SNO-MPG inhibited complex I activity and cardiomyocyte respiration. The latter effect was insensitive to the NO() scavenger c-PTIO, indicating no role for NO()-mediated complex IV inhibition. A cardioprotective role for reversible complex I inhibition has been proposed, and consistent with this SNO-MPG protected cardiomyocytes from simulated IR injury. Further supporting a cardioprotective role for endogenous mitochondrial S-nitrosothiols, patterns of protein S-nitrosation were similar in mitochondria isolated from Langendorff perfused hearts subjected to IPC, and mitochondria or cells treated with SNO-MPG. The functional recovery of perfused hearts from IR injury was also improved under conditions which stabilized endogenous S-nitrosothiols (i.e. dark), or by pre-ischemic administration of SNO-MPG. Mitochondria isolated from SNO-MPG-treated hearts at the end of ischemia exhibited improved Ca(2+) handling and lower ROS generation. Overall these data suggest that mitochondrial S-nitrosation and complex I inhibition constitute a protective signaling pathway that is amenable to pharmacologic augmentation.

Nitric oxide in blood. The nitrosative-oxidative disequilibrium hypothesis on the pathogenesis of cardiovascular disease

There is growing evidence that altered production and/or spatio-temporal distribution of reactive oxidant species and reactive nitrosative species in blood creates oxidative and/or nitrosative stresses in the failing myocardium and endothelium. This contributes to the abnormal cardiac and vascular phenotypes that characterize cardiovascular disease. These derangements at the system level can now be interpreted at the integrated cellular and molecular levels in terms of effects on signaling elements in the heart and vasculature. The end results of nitric oxide/redox disequilibrium have implications for cardiac and vascular homeostasis and may result in the development of atherosclerosis, myocardial tissue remodelling and hypertrophy. Reactive oxygen species/reactive nitrogen species generation is also attributed to the transit from hypertrophic to apoptotic phenotypes, a possible mechanism of myocardial failure. In this review, we highlight the possible roles of altered production and/or spatio-temporal distribution of reactive oxidant species and reactive nitrosative species in blood on the pathogenesis of the failing cardiovascular system.

Nitrosation ofN-Terminally Blocked Tryptophan and Tryptophan-Containing Peptides by Peroxynitrite

Tryptophan is known to be a major target of oxidative stress and to take part in electron transfer. In proteins, its fluorescence is extinguished after treatment with oxidative agents, like peroxynitrite (ONOO(-)/ONOOH) - the product of the reaction of NO* and superoxide anion (O*(2)(-)) radicals. The main reactions of N-blocked tryptophan derivatives (melatonin or N-acetyl-L-tryptophan) exposed to peroxynitrite at physiological pH are oxidation to formylkynuramine or formylkynurenine, respectively, and nitrosation, which leads to substituted 1-nitrosoindoles. Here we show that peroxynitrite-induced nitrosation is specific to N-blocked L-tryptophan derivatives and is not obtained with free L-tryptophan. Such a nitrosation can be evaluated by using 4,5-diaminofluorescein (DAF-2), which is converted to the fluorescent triazolofluorescein by NO* donors and nitrosating agents. N-acetyl-L-tryptophan was shown to be twice as efficient as melatonin in transferring NO from peroxynitrite to DAF-2. DAF-2 responses were then used to assess the ability of a series of L-tryptophan-containing peptides to give transient N-nitrosoindoles upon treatment with peroxynitrite. Many peptides proved not to be susceptible to nitrosation under these conditions. However, the N-terminally blocked peptide of endothelin-1 (Ac-Asp-Ile-Ile-Trp) reacted in a very similar fashion to melatonin; this shows that tryptophan residue nitrosation could occur when it was exposed to peroxynitrite.

2-(4-Carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide potentiates nitrosation of a heterocyclic amine carcinogen by nitric oxide

Although nitrosation plays an important role in initiation of carcinogenesis, the reactive nitrogen oxygen species (RNOS) mediating this reaction by multiple pathways have not been determined. The heterocyclic amine carcinogen 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) was used as a target to investigate RNOS and pathways for potentiation of nitric oxide (NO)-mediated nitrosation. 2-(4-Carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (CPTIO) oxidizes NO to NO(2)(.) and was used as a tool to investigate NO(2)(.) potentiation of nitrosation. The IQ nitrosation product, 2-nitrosoamino-3-methylimidazo[4,5-f]quinoline ((14)C-N-NO-IQ), was monitored by HPLC. Autoxidation of NO, generated by spermine NONOate (2.4 microM NO/min) for 7.5 min, did not convert 10 microM (14)C-IQ to N-NO-IQ. However, the presence of 15 muM CPTIO resulted in 3 microM N-NO-IQ formation. Potentiation by CPTIO occurred at low and high fluxes of NO, 0.075 to 1.2 microM/min, and over a range of IQ to CPTIO ratios of 0.5 to 10. A significant portion of N-NO-IQ formation was insensitive to azide (10 mM) inhibition, suggesting oxidative nitrosylation. NADH (0.02 mM) did not alter nitrosation by autoxidation, but effectively inhibited potentiation by CPTIO. Ascorbic acid (0.2 mM) and 5,5-dimethyl-1-pyrroline N-oxide (30 mM) inhibited nitrosation with or without CPTIO, while superoxide dismutase was not inhibitory. The RNOS produced by CPTIO had a 27-fold greater affinity for IQ than those produced by autoxidation. Results are consistent with NO(2)(.) or a RNOS like NO(2)(.) potentiating IQ oxidative nitrosylation. Nitrosation occurring at both low and high fluxes of NO can contribute to carcinogenesis.

Nitrosative stress in cancer therapy

Reactive nitrogen species play important roles in cell signalling, but when present at high concentrations they can subject cells to nitrosative stress, which may lead to cell death. Nitric oxide (NOx) is now recognized as playing important roles in cancer aetiology and progression and it can influence the outcome of cancer treatment. It is synthesised by the action of nitric oxide synthases (NOSs) on the amino acid arginine. Although NOx is not highly reactive with biological molecules, it reacts readily with other oxygen radicals to generate highly damaging reactive nitrogen species such as peroxynitrite, nitrogen dioxide and dinitrogen trioxide. These are potent inducers of apoptosis and necrosis. They may also inhibit DNA repair mechanisms, leading to mutation and carcinogenesis. Both inhibition and over-production of NOx have been investigated as strategies for cancer therapy. There is clear evidence that administration of competitive inhibitors of NOS can significantly slow the growth of solid tumors in rodent models, probably by reducing blood flow, and this creates a hypoxic environment that is conducive to the activation of bioreductive anticancer agents. Alternatively, generation of NOx concentrations in the high micromolar range by NOx donor drugs or gene therapy with inducible NOS is directly cytotoxic to cells and has been shown to inhibit tumor growth. At these high concentrations NOx is also an excellent sensitizer to radiation and to some chemotherapeutic agents, particularly cisplatin. Thus, manipulation of NOx levels in tumors offers exciting opportunities to improve the effectiveness of cancer treatment.

Cerebrovascular nitrosative stress mediates neurovascular and endothelial dysfunction induced by angiotensin II

OBJECTIVE

Angiotensin II (AngII) disrupts the regulation of the cerebral circulation through superoxide, a reactive oxygen species (ROS) generated by a nox2-containing NADPH oxidase. We tested the hypothesis that AngII-derived superoxide reacts with nitric oxide (NO) to form peroxynitrite, which, in turn, contributes to the vascular dysfunction.

METHODS AND RESULTS

Cerebral blood flow (CBF) was monitored by laser Doppler flowmetry in the neocortex of anesthetized mice equipped with a cranial window. AngII (0.25+/-0.02 microg/kg/min; intravenous for 30 to 45 minutes) attenuated the cerebral blood flow (CBF) increase produced by topical application of the endothelium-dependent vasodilator acetylcholine (-43+/-1%) and by whisker stimulation (-47+/-1%). AngII also increased the nitration marker 3-nitrotyrosine (3-NT) in cerebral blood vessels, an effect dependent on NO and nox2-derived ROS. Both the cerebrovascular effects of AngII and the nitration were attenuated by pharmacological inhibition or genetic inactivation of NO synthase. The nitration inhibitor uric acid or the peroxynitrite decomposition catalyst FeTPPS abolished AngII-induced cerebrovascular nitration and prevented the cerebrovascular effects of AngII.

CONCLUSIONS

These findings provide evidence that peroxynitrite, formed from NO and nox2-derived superoxide, contributes to the deleterious cerebrovascular effects of AngII. Inhibitors of peroxynitrite action may be valuable tools to counteract the deleterious cerebrovascular effects of AngII-induced hypertension.

On the Mechanism of the Ascorbic Acid-Induced Release of Nitric Oxide fromN-Nitrosated Tryptophan Derivatives: Scavenging of NO by Ascorbyl Radicals

During the past years, there has been increasing interest in endogenous nitric oxide storage compounds. Recently, we briefly reported on the ascorbate-dependent release of nitric oxide ((.)NO) from N-nitrosotryptophan derivatives. In the present study, the underlying mechanism of (.)NO release is studied in more detail, primarily utilizing N-acetyl-N-nitrosotryptophan (NANT) as a model compound. The initial rate of the ascorbate-induced release of nitric oxide has been found to correspond to the rate of NANT decay. In this process, N-acetyltryptophan (NAT) is produced almost quantitatively. The final yield of nitrite amounted to around 90 % with respect to the applied amount of NANT. However, the total release of nitric oxide was only 60 %, as determined by using an FNOCT-4(fluorescent nitric oxide cheletropic trap number 4) assay. Besides nitric oxide, a second volatile product, dinitrogen oxide (N(2)O), has been identified by using (15)N NMR spectrometry, strongly indicating the intermediacy of nitroxyl (HNO). The formation of intermediate ascorbyl radical anions during the NANT-ascorbate reaction has been monitored by using ESR spectrometry. Unexpectedly, it was found that the primary oxidized product of vitamin C, dehydroascorbic acid (DHA), efficiently consumes nitric oxide. Since ESR spectrometry further revealed that ascorbyl radical anions are also generated during the spontaneous decay of DHA, the DHA-nitric oxide reaction is related to recombination of (.)NO with the thus formed ascorbyl radical anions. A conclusively established mechanism of the NANT-ascorbate reaction is presented, with O-nitrosoascorbate as a key intermediate, as additionally supported by CBS-QB3 calculations. The present study suggests that vitamin C and its oxidation products can chemically counterbalance endogenous nitric oxide levels.

Regulation and specificity of S-nitrosylation and denitrosylation

Signal transduction from nitric oxide (NO) is important in physiological processes such as smooth muscle relaxation and neurotransmission. NO signaling occurs through a variety of mechanisms, including S-nitrosylation (SNO) of sulfhydryl groups in protein. A paper in this issue of the journal dissects the regulatory system involved in SNO of caspase-3 and highlights the need for more direct investigations of the complex biological regulation by NO signaling.