Protein S-nitrosylation: purview and parameters

Protein thiol oxidation in health and disease: techniques for measuring disulfides and related modifications in complex protein mixtures

Oxidant species are known to contribute to disease and dysfunction in biological systems. However, evidence has been progressively accumulating that demonstrates a more fundamental role for many oxidant species in the regulation of everyday function of healthy cells. Redox dependent signaling events involving the post-translational, oxidative modification of proteins has now been accepted as an important regulatory process, although the full extent of such mechanisms is yet to be determined. Some protein cysteinyl thiols are known to be susceptible to a number of redox-dependent modifications, including an interchange between the reduced thiol and several different oxidized disulfide states. Here, the role of oxidants as regulatory entities is reviewed, as are the many different ways protein disulfide formation can be analysed in complex protein mixtures. This includes an overview of many of the Proteomic strategies that can be used to identify proteins that form disulfides when pro-oxidizing conditions arise in cells, as well as related methods for studying intermediates that may precede disulfide formation.

Sphingolipid signaling and redox regulation

Sphingolipids including ceramide and its derivatives such as ceramide-1-phosphate, glycosyl-ceramide, and sphinogosine (-1-phosphate) are now recognized as novel intracellular signal mediators for regulation of inflammation, apoptosis, proliferation, and differentiation. One of the important and regulated steps in these events is the generation of these sphingolipids via hydrolysis of sphingomyelin through the action of sphingomyelinases (SMase). Several lines of evidence suggest that reactive oxygen species (ROS; O2-, H2O2, and OH-,) and reactive nitrogen species (RNS; NO, and ONOO-) and cellular redox potential, which is mainly regulated by cellular glutathione (GSH), are tightly linked to the regulation of SMase activation. On the other hand, sphingolipids are also known to play an important role in maintaining cellular redox homeostasis through regulation of NADPH oxidase, mitochondrial integrity, and antioxidant enzymes. Therefore, this paper reviews the relationship between cellular redox and sphingolipid metabolism and its biological significance.

Nitric oxide/redox-based signalling as a therapeutic target for penile disorders

Oxidative and/or nitrosative stress is implicated in the pathogeneses of assorted penile disorders of clinical significance, notably erectile dysfunction, priapism and penile fibrosis. It is becoming increasingly recognised that the generation and activity of reactive oxygen and nitrogen species in the penis influence vascular homeostasis of this organ, with adverse effects exerted at cellular and molecular levels. Furthermore, these elements may interact with molecular signalling pathways operating in the penis, modulating their functional roles. This interaction in particular suggests that by accessing molecular targets associated with oxidative/nitrosative stress in the penis, new pharmacotherapeutic approaches may be developed to promote normal erectile ability and preserve erectile tissue health. This notion pertains to, but also extends beyond, interventions which predictably target components of the nitric oxide-based signal transduction pathway for the on-demand treatment of erectile dysfunction. The next line of pharmaceuticals for disorders of the penis, in general, may well spawn from an integrative understanding of the complex regulatory interactions influenced by, as well as influencing nitric oxide signalling in this organ.

S-Nitrosogluthathione reductase activity of amphioxus ADH3: insights into the nitric oxide metabolism

Nitric oxide (NO) is a signalling molecule involved in many physiological functions. An important via of NO action is through the S-nitrosylation of proteins, a post-translational modification that regulates the activity of enzymes, protein-protein interactions and signal transduction pathways. Alcohol dehydrogenase class III (ADH3) recognises S-nitrosoglutathione (GSNO), the main reservoir of non-protein S-nitrosothiol, and functions as an effective GSNO reductase (GSNOR) and as a safeguard against nitrosative stress. To investigate the evolutionary conservation of this metabolic role, we have produced recombinant Branchiostoma floridae ADH3. Pure preparations of ADH3 showed 2-fold higher activity as GSNOR than as formaldehyde dehydrogenase, the previously assumed main role for ADH3. To correlate ADH3 expression in the gut with areas of NO production, we analysed the tissue distribution of the nitric oxide synthase (NOS) enzyme in amphioxus larvae. Immunostaining of the NOS enzyme revealed expression in the gut and in the dorsal region of the club-shaped gland. Co-localization in the gut supports the ADH3 and NOS joint contribution to the NO/SNO homeostasis.

Atorvastatin-induced cardioprotection is mediated by increasing inducible nitric oxide synthase and consequent S-nitrosylation of cyclooxygenase-2

We determined the effects of cyclooxygenase-1 (COX-1; SC-560), COX-2 (SC-58125), and inducible nitric oxide synthase (iNOS; 1400W) inhibitors on atorvastatin (ATV)-induced myocardial protection and whether iNOS mediates the ATV-induced increases in COX-2. Sprague-Dawley rats received 10 mg ATV·kg−1·day−1 added to drinking water or water alone for 3 days and received intravenous SC-58125, SC-560, 1400W, or vehicle alone. Anesthesia was induced with ketamine and xylazine and maintained with isoflurane. Fifteen minutes after intravenous injection rats underwent 30-min myocardial ischemia followed by 4-h reperfusion [infarct size (IS) protocol], or the hearts were explanted for biochemical analysis and immunoblotting. Left ventricular weight and area at risk (AR) were comparable among groups. ATV reduced IS to 12.7% (SD 3.1) of AR, a reduction of 64% vs. 35.1% (SD 7.6) in the sham-treated group ( P < 0.001). SC-58125 and 1400W attenuated the protective effect without affecting IS in the non-ATV-treated rats. ATV increased calcium-independent NOS (iNOS) [11.9 (SD 0.8) vs. 3.9 (SD 0.1) × 1,000 counts/min; P < 0.001] and COX-2 [46.7 (SD 1.1) vs. 6.5 (SD 1.4) pg/ml of 6-keto-PGF1α; P < 0.001] activity. Both SC-58125 and 1400W attenuated this increase. SC-58125 did not affect iNOS activity, whereas 1400W blocked iNOS activity. COX-2 was S-nitrosylated in ATV-treated but not sham-treated rats or rats pretreated with 1400W. COX-2 immunoprecipitated with iNOS but not with endothelial nitric oxide synthase. We conclude that ATV reduced IS by increasing the activity of iNOS and COX-2, iNOS is upstream to COX-2, and iNOS activates COX-2 by S-nitrosylation. These results are consistent with the hypothesis that preconditioning effects are mediated via PG.

Identification of S-nitrosylation motifs by site-specific mapping of the S-nitrosocysteine proteome in human vascular smooth muscle cells

S-nitrosylation, the selective modification of cysteine residues in proteins to form S-nitrosocysteine, is a major emerging mechanism by which nitric oxide acts as a signaling molecule. Even though nitric oxide is intimately involved in the regulation of vascular smooth muscle cell functions, the potential protein targets for nitric oxide modification as well as structural features that underlie the specificity of protein S-nitrosocysteine formation in these cells remain unknown. Therefore, we used a proteomic approach using selective peptide capturing and site-specific adduct mapping to identify the targets of S-nitrosylation in human aortic smooth muscle cells upon exposure to S-nitrosocysteine and propylamine propylamine NONOate. This strategy identified 20 unique S-nitrosocysteine-containing peptides belonging to 18 proteins including cytoskeletal proteins, chaperones, proteins of the translational machinery, vesicular transport, and signaling. Sequence analysis of the S-nitrosocysteine-containing peptides revealed the presence of acid/base motifs, as well as hydrophobic motifs surrounding the identified cysteine residues. High-resolution immunogold electron microscopy supported the cellular localization of several of these proteins. Interestingly, seven of the 18 proteins identified are localized within the ER/Golgi complex, suggesting a role for S-nitrosylation in membrane trafficking and ER stress response in vascular smooth muscle.

MyD88 and NOS2 are essential for toll-like receptor 4-mediated survival effect in cardiomyocytes

Innate immune system such as Toll-like receptor 4 (TLR4) represents the first line of defense against infection. In addition to its pivotal role in host immunity, recent studies have suggested that TLR4 may play a broader role in mediating tissue inflammation and cell survival in response to noninfectious injury. We and other investigators have reported that cardiac TLR4 signaling is dynamically modulated in ischemic myocardium and that activation of TLR4 confers a survival benefit in the heart and in isolated cardiomyocytes. However, the signaling pathways leading to these effects are not completely understood. Here, we investigate the role of MyD88, an adaptor protein of TLR4 signaling, and inducible nitric oxide synthase (NOS2) in mediating TLR4-induced cardiomyocyte survival in an in vitro model of apoptosis. Serum deprivation induced a significant increase in the number of apoptotic cardiomyocytes as demonstrated by transferase-mediated dUTP nick-end labeling (TUNEL) assay, nuclear morphology, DNA laddering, and DNA-histone ELISA. Lipopolysaccharide (LPS), a TLR4 agonist, activated TLR4 signaling and led to significant reduction in apoptotic cardiomyocytes and improved cellular function of surviving cardiomyocytes with enhanced Ca(2+) transients and cell shortening. We found that both TLR4 and MyD88 are required for the LPS-induced beneficial effects as demonstrated by improved survival and function in wild-type but not in TLR4(-/-) or MyD88(-/-) cardiomyocytes. Moreover, genetic deletion or pharmacological inhibition of NOS2 abolished survival and functional rescue of cardiomyocytes treated with LPS. Taken together, these data suggest that TLR4 protects cardiomyocytes from stress-induced injury through MyD88- and NOS2-dependent mechanisms.

Sodium nitroprusside promotes IRP2 degradation via an increase in intracellular iron and in the absence of S nitrosylation at C178

In iron-replete cells the posttranscriptional regulator IRP2 undergoes ubiquitination and proteasomal degradation. A similar response occurs in cells exposed to sodium nitroprusside (SNP), an NO-releasing drug. It has been proposed that nitroprusside ([Fe(CN)5NO]2-) fails to donate iron into cells and that it promotes IRP2 degradation via S nitrosylation at C178. This residue is located within a stretch of 73 amino acids, earlier proposed to define an iron-dependent degradation domain. Surprisingly, we show that IRP2 bearing a C178S mutation or a Delta73 deletion is sensitive to degradation not only by ferric ammonium citrate (FAC) but also by SNP. Moreover, FAC and SNP attenuate the RNA-binding activities of IRP2 and its homologue IRP1 with similar kinetics. Actinomycin D, cycloheximide, succinylacetone, and dimethyl-oxalylglycine antagonize IRP2 degradation in response to both FAC and SNP, suggesting a common mechanistic basis. IRP2 is not only sensitive to fresh, but also to photodegraded SNP and remains unaffected by S-nitrosoglutathione (GSNO), an established nitrosation agent. Importantly, both fresh and photodegraded SNP, but not GSNO, promote a >4-fold increase in the calcein-accessible labile iron pool. Collectively, these results suggest that IRP2 degradation by SNP does not require S nitrosylation but rather represents a response to iron loading.

Arginase modulates myocardial contractility by a nitric oxide synthase 1-dependent mechanism

Cardiac myocytes contain two constitutive NO synthase (NOS) isoforms with distinct spatial locations, which allows for isoform-specific regulation. One regulatory mechanism for NOS is substrate (l-arginine) bioavailability. We tested the hypothesis that arginase (Arg), which metabolizes l-arginine, constrains NOS activity in the cardiac myocyte in an isoform-specific manner. Arg activity was detected in both rat heart homogenates and isolated myocytes. Although both Arg I and II mRNA and protein were present in whole heart, Arg II alone was found in isolated myocytes. Arg inhibition with S-(2-boronoethyl)-l-cysteine (BEC) augmented Ca(2+)-dependent NOS activity and NO production in myocytes, which did not depend on extracellular l-arginine. Arg II coimmunoprecipited with NOS1 but not NOS3. Isolation of myocyte mitochondrial fractions in combination with immuno-electron microscopy demonstrates that Arg II is confined primarily to the mitochondria. Because NOS1 positively modulates myocardial contractility, we determined whether Arg inhibition would increase basal myocardial contractility. Consistent with our hypothesis, Arg inhibition increased basal contractility in isolated myocytes by a NOS-dependent mechanism. Both the Arg inhibitors N-hydroxy-nor-l-arginine and BEC dose-dependently increased basal contractility in rat myocytes, which was inhibited by both nonspecific and NOS1-specific NOS inhibitors N(G)-nitro-l-arginine methyl ester and S-methyl-l-thiocitrulline, respectively. Also, BEC increased contractility in isolated myocytes from WT and NOS3 but not NOS1 knockout mice. We conclude that mitochondrial Arg II negatively regulates NOS1 activity, most likely by limiting substrate availability in its microdomain. These findings have implications for therapy in pathophysiologic states such as aging and heart failure in which myocardial NO signaling is disrupted.

S-nitrosylation and permeation through connexin 43 hemichannels in astrocytes: induction by oxidant stress and reversal by reducing agents

Marked increase in cell permeability ascribed to open connexin (Cx)43 hemichannels is induced by metabolic inhibition (MI) of cortical astrocytes in culture, but the molecular mechanisms are not established. Dephosphorylation and/or oxidation of Cx43 hemichannels was proposed as a potential mechanism to increase their open probability. We now demonstrate that MI increases the number of hemichannels on the cell surface assayed by biotinylation and Western blot, and that this change is followed by increased dephosphorylation and S-nitrosylation. The increase in rate of dye uptake caused by MI is comparable to the increase in surface expression; thus, open probability and permeation per hemichannel may be unchanged. Reducing agents did not affect dephosphorylation of Cx43 hemichannels but reduced dye uptake and S-nitrosylation. Uptake was also reduced by elevated intracellular but not extracellular levels of reduced glutathione. Moreover, nitric oxide donors induced dye uptake and nitrosylation of surface Cx43 but did not affect its abundance or phosphorylation. Thus, permeability per channel is increased, presumably because of increase in open probability. We propose that increased dye uptake induced by MI is mediated by an increased number of Cx43 hemichannels in the surface and is associated with multiple molecular changes, among which nitrosylation of intracellular Cx43 cysteine residues may be a critical factor.

Nitric oxide and posttranslational modification of the vascular proteome: S-nitrosation of reactive thiols

Nitric oxide (NO*) is known to exert its effects via guanylyl cyclase and cyclic GMP-dependent pathways and by cyclic GMP-independent pathways, including the posttranslational modification of proteins. Much ongoing research is focused on defining the mechanisms of NO*-mediated protein modification, the identity and function of the modified proteins, and the significance of these changes in health and disease. S-nitrosation or thionitrite formation has only been found on a limited number of residues in a subset of proteins in in vitro and in vivo studies. Protein S-nitrosation also appears to be reversible. There are several theories about the in vivo S-nitrosating agent, and most suggest a role for oxidation products of NO* in this process. Flux in cellular S-nitrosoprotein pools appears to be regulated by NO* availability and is redox-sensitive. An analysis of S-nitrosation in candidate proteins has clarified the mechanism by which NO* regulates enzymatic and cellular functions. These findings suggest the utility of using proteomic methods to identify unique targets for protein S-nitrosation to understand further the molecular mechanisms of the effects of NO*.

GAPDH as a sensor of NO stress

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a classic glycolytic enzyme, and accumulating evidence has suggested that GAPDH is a multi-functional protein. In particular, its role as a mediator for cell death has been highlighted. For the last decade, many groups reported that a pool of GAPDH translocates to the nucleus under a variety of stressors, most of which are associated with oxidative stress. At the molecular level, sequential steps lead to nuclear translocation of GAPDH during cell death as follows: first, a catalytic cysteine in GAPDH (C150 in rat GAPDH) is S-nitrosylated by nitric oxide (NO) that is generated from inducible nitric oxide synthase (iNOS) and/or neuronal NOS (nNOS); second, the modified GAPDH becomes capable of binding with Siah1, an E3 ubiquitin ligase, and stabilizes it; third, the GAPDH-Siah protein complex translocates to the nucleus, dependent on Siah1's nuclear localization signal, and degrades Siah1's substrates in the nucleus, which results in cytotoxicity. A recent report suggests that GAPDH may be genetically associated with late-onset of Alzheimer's disease. (-)-deprenyl, which has originally been used as a monoamine oxidase inhibitor for Parkinson's disease, binds to GAPDH and displays neuroprotective actions, but its molecular mechanism is still unclear. The NO/GAPDH/Siah1 death cascade will contribute to the molecular understanding of a role of GAPDH in neurodegenerative disorders and help to establish novel therapeutic strategies.

S-nitrosothiol signaling in respiratory biology

Genetic and biochemical data demonstrate a pivotal role for S-nitrosothiols (SNOs) in mediating the actions of nitric oxide synthases (NOSs). SNOs serve to convey NO bioactivity and to regulate protein function. This understanding is of immediate interest to the pulmonary clinical and research communities. This article reviews the following: (1) biochemical and cellular evidence that SNOs in amino acids, peptides, and proteins elicit NOS-dependent signaling in the respiratory system and (2) studies that link SNO signaling to pulmonary medicine. SNO-mediated signaling is involved in the regulation of minute ventilation, ventilation-perfusion matching, pulmonary arterial pressure, basal airway tone, and respiratory and peripheral muscle function. Derangements in SNO signaling are implicated in many disorders relevant to pulmonary and critical care medicine, including apnea, hypoxemia, pulmonary hypertension, asthma, cystic fibrosis, pneumonia, and septic shock.

Albumin-bound bilirubin interacts with nitric oxide by a redox mechanism

Bilirubin, the final product of heme catabolism, plays a crucial role in the cellular defense against oxidative and nitrosative stress. This study investigated the interaction of albumin-bound bilirubin, the circulating form of the bile pigment, with nitric oxide (NO), a gaseous modulator involved in many physiological functions but able to induce cytotoxicity and cell death if produced in excess. A short-lived endogenous S-nitrosothiol such as S-nitroso-cysteine was used as NO donor. In PBS without chelators, bilirubin was bound to human serum albumin with an apparent affinity of 1.6 +/- 0.2 microM (n = 4). Furthermore, albumin (2-20 microM) dose-dependently increased the half-life of BR (10 microM) exposed to S-nitroso-cysteine (100 microM) of 2.4 +/- 0.4 times (n = 4). Albumin-bound bilirubin was almost completely oxidized by S-nitroso-cysteine-derived NO, and biliverdin was the major product formed; this reaction seemed to be rather specific for albumin-bound bilirubin because when free bilirubin was reacted with S-nitroso-cysteine the formation of biliverdin was significantly lower. Uric acid and reduced glutathione, two well-known plasma antioxidants, at physiological concentrations protected albumin-bound bilirubin from NO-mediated oxidation. Taken together, these data suggest that albumin-bound bilirubin maintains its ability to interact with NO also in the bloodstream counteracting extracellular nitrosative reactions.

Nitric oxide donor-induced persistent inhibition of cell adhesion protein expression and NFkappaB activation in endothelial cells

Nitric oxide (NO), applied by inhalation or released from NO donors, has been used to reduce the expression of cell adhesion molecules (CAM) and ameliorate other consequences of ischemia/reperfusion (I/R) injury. In this study, we have assessed the time frames of pretreatment and of the duration of the preconditioned state using human umbilical vein endothelial cells (HUVECs) and the NO donor, SNAP, in combination with cysteine. The induction of vascular cell adhesion molecule (VCAM), intercellular adhesion molecule (ICAM) and E-selectin by the cytokines TNFalpha and IL-1beta, and by bacterial lipopolysaccharide (LPS) was reduced by SNAP/Cys preincubation (30 min, 1mM) to less than 10% of controls. This refractory state in respect to cytokine-induced CAM expression persisted for 6h after washout of the NO donor in the combination TNFalpha/VCAM, and a partial block was still observed after 8h. The effect was not mediated by the cGMP pathway, as was demonstrated by using the inhibitor of guanylyl cyclase, ODQ, and the cGMP analogue, 8-Br-cGMP. The TNFalpha-induced expression of CAM was exclusively dependent on the transcription factor NFkappaB since the inhibitor of NFkappaB activation, BAY 11-7082, completely blocked the induction. The TNFalpha-induced phosphorylation and degradation of the inhibitor of kappaB (IkappaBalpha) was suppressed for up to 8h after SNAP/Cys pretreatment. The inhibitory S-nitrosation of IkappaB kinase (IKKbeta), as assessed by the biotin-switch-procedure and immunoprecipitation, was only detectable immediately after SNAP/Cys incubation but not at later time points. In summary, a short preincubation of HUVEC with SNAP/Cys results in a persistent suppression of NFkappaB-dependent expression of CAM. The stabilization of IkappaBalpha over the same time span may be causally related to this effect.

Requirement for multiple activation signals by anti-inflammatory feedback in macrophages

Pathogen killing is one of the primary roles of macrophages, utilizing potent effectors such as nitric oxide (NO) and involving other cellular machinery including iron regulatory apparatus. Macrophages become strongly activated upon receipt of appropriate signaling with cytokines and pathogen-derived endotoxins. However, they must resist activation in the absence of decisive signaling due to the energetic demands of activation coupled with the toxic nature of effector molecules to surrounding tissues. We have developed a mathematical model of the modular biochemical network of macrophages involved with activation, pathogen killing and iron regulation. This model requires synergistic interaction of multiple activation signals to overcome the quiescent state. To achieve a trade-off between macrophage quiescence and activation, strong activation signals are modulated via negative regulation by NO. In this way a single activation signal is insufficient for complete activation. In addition, our results suggest that iron regulation is usually controlled by activation signals. However, under conditions of partial macrophage activation, exogenous iron levels play a key role in regulating NO production. This model will be useful for evaluating macrophage control of intracellular pathogens in addition to the biochemical mechanisms examined here.

Nitric oxide regulates endocytosis by S-nitrosylation of dynamin

The GTPase dynamin regulates endocytic vesicle budding from the plasma membrane, but the molecular mechanisms involved remain incompletely understood. We report that dynamin, which interacts with NO synthase, is S-nitrosylated at a single cysteine residue (C607) after stimulation of the beta(2) adrenergic receptor. S-nitrosylation increases dynamin self-assembly and GTPase activity and facilitates its redistribution to the membrane. A mutant protein bearing a C607A substitution does not self-assemble properly or increase its enzymatic activity in response to NO. In NO-generating cells, expression of dynamin C607A, like the GTPase-deficient dominant-negative K44A dynamin, inhibits both beta(2) adrenergic receptor internalization and bacterial invasion. Furthermore, exogenous or endogenously produced NO enhances internalization of both beta(2) adrenergic and epidermal growth factor receptors. Thus, NO regulates endocytic vesicle budding by S-nitrosylation of dynamin. Collectively, our data suggest a general NO-dependent mechanism by which the trafficking of receptors may be regulated and raise the idea that pathogenic microbes and viruses may induce S-nitrosylation of dynamin to facilitate cellular entry.

Endothelial nitric oxide synthase is segregated from caveolin-1 and localizes to the leading edge of migrating cells

The enzyme endothelial Nitric Oxide Synthase (eNOS) is involved in key physiological and pathological processes, including cell motility and apoptosis. It is widely believed that at the cell surface eNOS is localized in caveolae, where caveolin-1 negatively regulates its activity, however, there are still uncertainties on its intracellular distribution. Here, we applied high resolution confocal microscopy to investigate the surface distribution of eNOS in transfected HeLa cells and in human umbilical vein endothelial cells (HUVEC) endogenously expressing the enzyme. In confluent and non-confluent HUVEC and HeLa cells, we failed to detect substantial colocalization between eNOS and caveolin-1 at the cell surface. Instead, in non-confluent cells, eNOS was concentrated in ruffles and at the leading edge of migrating cells, colocalizing with actin filaments and with the raft marker ganglioside G(M1), and well segregated from caveolin-1, which was restricted to the posterior region of the cells. Treatments that disrupted microfilaments caused loss of eNOS from the cell surface and decreased Ca(2+)-stimulated activity, suggesting a role of the cytoskeleton in the localization and function of the enzyme. Our results provide a morphological correlate for the role of eNOS in cell migration and raise questions on the site of interaction between eNOS and caveolin-1.

SNOSID, a proteomic method for identification of cysteine S-nitrosylation sites in complex protein mixtures

Reversible addition of NO to Cys-sulfur in proteins, a modification termed S-nitrosylation, has emerged as a ubiquitous signaling mechanism for regulating diverse cellular processes. A key first-step toward elucidating the mechanism by which S-nitrosylation modulates a protein's function is specification of the targeted Cys (SNO-Cys) residue. To date, S-nitrosylation site specification has been laboriously tackled on a protein-by-protein basis. Here we describe a high-throughput proteomic approach that enables simultaneous identification of SNO-Cys sites and their cognate proteins in complex biological mixtures. The approach, termed SNOSID (SNO Site Identification), is a modification of the biotin-swap technique [Jaffrey, S. R., Erdjument-Bromage, H., Ferris, C. D., Tempst, P. & Snyder, S. H. (2001) Nat. Cell. Biol. 3, 193-197], comprising biotinylation of protein SNO-Cys residues, trypsinolysis, affinity purification of biotinylated-peptides, and amino acid sequencing by liquid chromatography tandem MS. With this approach, 68 SNO-Cys sites were specified on 56 distinct proteins in S-nitrosoglutathione-treated (2-10 microM) rat cerebellum lysates. In addition to enumerating these S-nitrosylation sites, the method revealed endogenous SNO-Cys modification sites on cerebellum proteins, including alpha-tubulin, beta-tubulin, GAPDH, and dihydropyrimidinase-related protein-2. Whereas these endogenous SNO proteins were previously recognized, we extend prior knowledge by specifying the SNO-Cys modification sites. Considering all 68 SNO-Cys sites identified, a machine learning approach failed to reveal a linear Cys-flanking motif that predicts stable transnitrosation by S-nitrosoglutathione under test conditions, suggesting that undefined 3D structural features determine S-nitrosylation specificity. SNOSID provides the first effective tool for unbiased elucidation of the SNO proteome, identifying Cys residues that undergo reversible S-nitrosylation.

Inducible nitric oxide synthase binds, S-nitrosylates, and activates cyclooxygenase-2

Cyclooxygenase-2 (COX-2) and inducible nitric oxide synthase (iNOS) are two major inflammatory mediators. Here we show that iNOS specifically binds to COX-2 and S-nitrosylates it, enhancing COX-2 catalytic activity. Selectively disrupting iNOS-COX-2 binding prevented NO-mediated activation of COX-2. This synergistic molecular interaction between two inflammatory systems may inform the development of anti-inflammatory drugs.

Mechanisms for nitric oxide synthesis in plants

The discovery that nitric oxide (NO) acts as a signal fundamentally shifted our understanding of free radicals from toxic by-products of oxidative metabolism to key regulators of cellular functions. This discovery has led to intense investigation into the synthesis of NO in both animals and plants. Nitric oxide synthases (NOS) are the primary sources of NO in animals and are complex, highly regulated enzymes that oxidize arginine to NO and citrulline. Plant NO synthesis, however, appears more complex and includes both nitrite and arginine-dependent mechanisms. The components of the arginine pathway have been elusive as no known orthologues of animal NOS exist in plants. An Arabidopsis gene (AtNOS1) has been identified that is needed for NO synthesis in vivo and has biochemical properties similar to animal cNOS, yet it has no sequence similarity to any known animal NOS. An Atnos1 insertion mutant has been useful for genetic studies of NO regulation and for uncovering new roles for NO signalling. The elucidation of plant NO synthesis promises to yield novel mechanisms that may be applicable to animal systems.

Nitric oxide and the vascular endothelium

The vascular endothelium synthesises the vasodilator and anti-aggregatory mediator nitric oxide (NO) from L-arginine. This action is catalysed by the action of NO synthases, of which two forms are present in the endothelium. Endothelial (e)NOS is highly regulated, constitutively active and generates NO in response to shear stress and other physiological stimuli. Inducible (i)NOS is expressed in response to immunological stimuli, is transcriptionally regulated and, once activated, generates large amounts of NO that contribute to pathological conditions. The physiological actions of NO include the regulation of vascular tone and blood pressure, prevention of platelet aggregation and inhibition of vascular smooth muscle proliferation. Many of these actions are a result of the activation by NO of the soluble guanylate cyclase and consequent generation of cyclic guanosine monophosphate (cGMP). An additional target of NO is the cytochrome c oxidase, the terminal enzyme in the electron transport chain, which is inhibited by NO in a manner that is reversible and competitive with oxygen. The consequent reduction of cytochrome c oxidase leads to the release of superoxide anion. This may be an NO-regulated cell signalling system which, under certain circumstances, may lead to the formation of the powerful oxidant species, peroxynitrite, that is associated with a variety of vascular diseases.

Nitric oxide protects against mitochondrial permeabilization induced by glutathione depletion: Role of S-nitrosylation?

Nitric oxide (NO) is known to mediate a multitude of biological effects including inhibition of respiration at cytochrome c oxidase (COX), formation of peroxynitrite (ONOO-) by reaction with mitochondrial superoxide (O2*-), and S-nitrosylation of proteins. In this study, we investigated pathways of NO metabolism in lymphoblastic leukemic CEM cells in response to glutathione (GSH) depletion. We found that NO blocked mitochondrial protein thiol oxidation, membrane permeabilization, and cell death. The effects of NO were: (1) independent of respiratory chain inhibition since protection was also observed in CEM cells lacking mitochondrial DNA (rho0) which do not possess a functional respiratory chain and (2) independent of ONOO- formation since nitrotyrosine (a marker for ONOO- formation) was not detected in extracts from cells treated with NO after GSH depletion. However, NO increased the level of mitochondrial protein S-nitrosylation (SNO) determined by the Biotin Switch assay and by the release of NO from mitochondrial fractions treated with mercuric chloride (which cleaves SNO bonds to release NO). In conclusion, these results indicate that NO blocks cell death after GSH depletion by preserving the redox status of mitochondrial protein thiols probably by a mechanism that involves S-nitrosylation of mitochondrial protein thiols.

Function of cGMP-Dependent Protein Kinases as Revealed by Gene Deletion

Over the past few years, a wealth of biochemical and functional data have been gathered on mammalian cGMP-dependent protein kinases (cGKs). In mammals, three different kinases are encoded by two genes. Mutant and chimeric cGK proteins generated by molecular biology techniques yielded important biochemical knowledge, such as the function of the NH2-terminal domains of cGKI and cGKII, the identity of the cGMP-binding sites of cGKI, and the substrate specificity of the enzymes. Genetic approaches have proven especially useful for the analysis of the biological functions of cGKs. Recently, some of the in vivo targets and mechanisms leading to changes in neuronal adaptation, smooth muscle relaxation and growth, intestinal water secretion, bone growth, renin secretion, and other important functions have been identified. These data show that cGKs are signaling molecules involved in many biological functions.

Modulation of nitric oxide bioactivity by plant haemoglobins

Nitric oxide (NO) is a highly reactive signalling molecule that has numerous targets in plants. Both enzymatic and non-enzymatic synthesis of NO has been detected in several plant species, and NO functions have been characterized during diverse physiological processes such as plant growth, development, and resistance to biotic and abiotic stresses. This wide variety of effects reflects the basic signalling mechanisms that are utilized by virtually all mammalian and plant cells and suggests the necessity of detoxification mechanisms to control the level and functions of NO. During the last two years an increasing number of reports have implicated non-symbiotic haemoglobins as the key enzymatic system for NO scavenging in plants, indicating that the primordial function of haemoglobins may well be to protect against nitrosative stress and to modulate NO signalling functions. The biological relevance of plant haemoglobins during specific conditions of plant growth and stress, and the existence of further enzymatic and non-enzymatic NO scavenging systems, suggest the existence of precise NO modulation mechanisms in plants, as observed for different NO sources.

Nitric oxide synthase localization in the rat neutrophils: immunocytochemical, molecular, and biochemical studies

Nitric oxide (NO) modulates diverse functions of polymorphonuclear neutrophils (PMNs), but localization of NO synthase (NOS) and identification of its interacting proteins remain the least defined. The present study discerns subcellular distribution of NOS and caveolin-1, a prominent NOS-interacting protein in rat PMNs. Localization of NOS was explored by confocal and immunogold electron microscopy, and its activity was assessed by L-[3H] arginine and 4,5-diaminofluorescein diacetate (DAF-2DA). Reverse transcriptase-polymerase chain reaction using NOS primers and Western blotting demonstrated the presence of neuronal NOS (nNOS) and inducible NOS (iNOS) in PMNs. Immunocytochemical studies exhibited distribution of nNOS and iNOS in cytoplasm and nucleus, and L-[3H] citrulline formation and DAF fluorescence confirmed NOS activity in both fractions. NOS activity correlated positively with calmodulin concentration in both of the fractions. nNOS and iNOS colocalized with caveolin-1, as evidenced by immunocytochemical and immunoprecipitation studies. The results thus provide first evidence of nNOS and iNOS in the nuclear compartment and suggest NOS interaction with caveolin-1 in rat PMNs.

Differential inhibition of Arabidopsis methionine adenosyltransferases by protein S-nitrosylation

In animals, protein S-nitrosylation, the covalent attachment of NO to the thiol group of cysteine residues, is an intensively investigated posttranslational modification, which regulates many different processes. A growing body of evidence suggests that this type of redox-based regulation mechanism plays a pivotal role in plants, too. Here we report the molecular mechanism for S-nitrosylation of methionine adenosyltransferase (MAT) of Arabidopsis thaliana, thereby presenting the first detailed characterization of S-nitrosylation in plants. We cloned three MAT isoforms of Arabidopsis and tested the effect of NO on the activity of the purified, recombinant proteins. Our data showed that incubation with GSNO resulted in blunt, reversible inhibition of MAT1, whereas MAT2 and MAT3 were not significantly affected. Cys-114 of MAT1 was identified as the most promising target of NO-induced inhibition of MAT1, because this residue is absent in MAT2 and MAT3. Structural analysis of MAT1 revealed that Cys-114 is located nearby the putative substrate binding site of this enzyme. Furthermore, Cys-114 is flanked by S-nitrosylation-promoting amino acids. The inhibitory effect of GSNO was drastically reduced when Cys-114 of MAT1 was replaced by arginine, and mass spectrometric analyses of Cys-114-containing peptides obtained after chymotryptic digestion demonstrated that Cys-114 of MAT1 is indeed S-nitrosylated. Because MAT catalyzes the synthesis of the ethylene precursor S-adenosylmethionine and NO is known to influence ethylene production in plants, this enzyme probably mediates the cross-talk between ethylene and NO signaling.

Ascorbic acid reduction of microtubule protein disulfides and its relevance to protein S-nitrosylation assays

The biotin switch assay was developed to aid in the identification of S-nitrosylated proteins in different cell types. However, our work with microtubule proteins including tubulin and its associated proteins tau and microtubule-associated protein-2 shows that ascorbic acid is not a selective reductant of protein S-nitrosothiols as described in the biotin switch assay. Herein we show that ascorbic acid reduces protein disulfides in tubulin, tau, and microtubule-associated protein-2 that are formed by peroxynitrite anion. Reduction of microtubule-associated protein disulfides by ascorbic acid following peroxynitrite treatment restores microtubule polymerization kinetics to control levels. We also show that ascorbic acid reduces the disulfide dithiobis(2-nitrobenzoic acid), a reagent commonly used to detect protein thiols. Not only do we describe a new reactivity of ascorbic acid with microtubule proteins but we expose an important limitation when using the biotin switch assay to detect protein S-nitrosylation.

Nitric oxide regulates the proliferation of chick embryo retina cells by a cyclic GMP-independent mechanism

Nitric oxide (NO) is an intercellular messenger involved in many physiological and pathological processes of vertebrate and invertebrate animal tissues. In the embryonic chick retina, nitric oxide synthase (NOS) activity and a system for l-arginine transport between neurons and glial cells were described, supporting the idea that nitric oxide is a critical molecule during retinal development. In the present work we show that nitric oxide is a modulator of cell proliferation in chick embryo retina. Mixed cultures of retinal neurons and glial cells were submitted to [(3)H]-thymidine incorporation after drug treatment. Incubation for 24h with the NO donors S-nitroso-N-acetyl-penicillamine (SNAP) or Spermine nitric oxide (SpNO) complex promoted a decrease of approximately 70% of [(3)H]-thymidine incorporation in a dose-dependent manner. SNAP did not increase Lactate dehydrogenase release and its effect was not mimicked by 8-bromo cyclic GMP, or blocked by the guanylate cyclase inhibitor 1H-[1,2,4]oxadiazole[4,3-a]quinoxalin-1-one (ODQ), indicating that the effect was not due to cell death or mediated by increases of cyclic GMP levels. The inhibition was completely prevented by dithiotreitol (DTT), strongly indicating the participation of an S-nitrosylation mechanism. SNAP blocked the increase of [(3)H]-thymidine incorporation induced by ATP. Using purified cultures of glial cells we showed that the NO donor SNAP produced an inhibition of 50% in cell proliferation and did stimulate ERK1/2 phosphorylation, indicating that the inhibition of this pathway was not involved in its cytostatic effect. [(3)H]-Thymidine autoradiography of mixed cultures showed labeling of oval nuclei of glial flat cells. The injection of eggs with SNAP also did promote an intense inhibition of [(3)H]-thymidine incorporation in retinas from 9-day-old embryos. These data suggest that nitric oxide affects the proliferation of chick embryo retina glial cells in culture or "in vivo" through cyclic GMP and ERK-independent pathways.

A mechanism underlying AMPA receptor trafficking during cerebellar long-term potentiation

Long-term potentiation (LTP) is mediated by the activity-driven delivery of GluR1 glutamate receptors via Ca2+/calmodulin-dependent protein kinase II activity in various brain regions. Recently, postsynaptic LTP was shown to be induced at parallel fiber-Purkinje cell synapses by stimulating the parallel fibers at 1 Hz or applying a NO donor. Here, we demonstrate that NO-evoked postsynaptic LTP in mice cerebellum was blocked by botulinum toxin and enhanced by prior treatment with phorbol ester, which is known to induce GluR2 endocytosis. Interestingly, such LTP was not affected by a Ca2+/calmodulin-dependent protein kinase II inhibitor or a peptide binding to a protein interacting with C kinase 1, but was blocked by a peptide binding to N-ethylmaleimide-sensitive factor, which specifically binds to GluR2. Therefore, although the synaptic incorporation of GluR2 has been reported to be a constitutive pathway, NO-induced postsynaptic LTP in Purkinje cells is likely mediated by a pathway involving N-ethylmaleimide-sensitive factor-dependent GluR2 trafficking.

Subcellular targeting and differential S-nitrosylation of endothelial nitric-oxide synthase

Endothelial nitric-oxide synthase (eNOS) undergoes a complex pattern of post-translational modifications that regulate its activity. We have recently reported that eNOS is constitutively S-nitrosylated in endothelial cells and that agonists promote eNOS denitrosylation concomitant with enzyme activation (Erwin, P. A., Lin, A. J., Golan, D. E., and Michel, T. (2005), J. Biol. Chem. 280, 19888-19894). In the present studies, we use mass spectrometry to confirm that the zinc-tetrathiolate cysteines of eNOS are S-nitrosylated. eNOS targeting to the plasma membrane is necessary for enzyme S-nitrosylation, and we report that translocation between cellular compartments is necessary for dynamic eNOS S-nitrosylation. We transfected cells with cDNA encoding wild-type eNOS, which is membrane-targeted, or with acylation-deficient mutant eNOS (Myr-), which is expressed solely in the cytosol. While wild-type eNOS is robustly S-nitrosylated, we found that S-nitrosylation of the Myr- eNOS mutant is nearly abolished. When we transfected cells with a fusion protein in which Myr- eNOS is ligated to the CD8-transmembrane domain (CD8-Myr-), we found that CD8-Myr- eNOS, which does not undergo dynamic subcellular translocation, is hypernitrosylated relative to wild-type eNOS. Furthermore, we found that when endothelial cells transfected with wild-type or CD8-Myr- eNOS are stimulated with eNOS agonist, only wild-type eNOS is denitrosylated; CD8-Myr- eNOS S-nitrosylation is unchanged. These findings indicate that subcellular targeting is a critical determinant of eNOS S-nitrosylation. Finally, we show that eNOS S-nitrosylation can be detected in intact arterial preparations from mouse and that eNOS S-nitrosylation is a dynamic agonist-modulated process in intact blood vessels. These studies suggest that receptor-regulated eNOS S-nitrosylation may represent an important determinant of NO-dependent signaling in the vascular wall.

NO-donating aspirin induces phase II enzymes in vitro and in vivo

Modulation of drug metabolizing enzymes, leading to facilitated elimination of carcinogens represents a successful strategy for cancer chemoprevention. Nitric oxide-donating aspirin (NO-ASA) is a promising agent for the prevention of colon and other cancers. We studied the effect of NO-ASA on drug metabolizing enzymes in HT-29 human colon adenocarcinoma and Hepa 1c1c7 mouse liver adenocarcinoma cells and in Min mice treated with NO-ASA for 3 weeks. In these cell lines, NO-ASA induced the activity and expression of NAD(P)H:quinone oxireductase (NQO) and glutathione S-transferase (GST). Compared with untreated Min mice, NO-ASA increased in the liver the activity (nmol/min/mg; mean+/-SEM for all) of NQO (85+/-6 versus 128+/-11, P<0.05) and GST (2560+/-233 versus 4254+/-608, P<0.005) and also in the intestine but not in the kidney; the expression of NQO1 and GST P1-1 was also increased. NO-ASA had only a marginal effect on P450 1A1 and P450 2E1, two phase I enzymes. The release of NO from NO-ASA, determined with a selective microelectrode was paralleled by the induction of NQO1 and abrogated by NO scavengers; an exogenous NO donor also induced the expression of NQO1. NO-ASA induced concentration-dependently the translocation of Nrf2 into the nucleus as documented by immunofluorescence and immunoblotting; this paralleled the induction of NQO1 and GST P1-1. Thus NO-ASA induces phase II enzymes, at least in part, through the action of NO that it releases and by modulating the Keap1-Nrf2 pathway; this effect may be part of its mechanism of action against colon and other cancers.

Regulation of Weibel–Palade Body Exocytosis

Weibel-Palade bodies (WPBs) are endothelial granules that store von Willebrand factor (VWF), P-selectin, and other vascular modulators. Endothelial cells secrete WPBs in response to vascular injury, releasing VWF, which triggers platelet rolling, and externalizing P-selectin, which activates leukocyte trafficking. Endothelial exocytosis is one of the earliest responses to vascular damage and plays a pivotal role in thrombosis and inflammation. This review examines the regulation of WPB exocytosis-the exocytic machinery, activators, and inhibitors of exocytosis-and speculates about the development of novel anti-exocytic drugs.

The N-end rule pathway as a nitric oxide sensor controlling the levels of multiple regulators

The conjugation of arginine to proteins is a part of the N-end rule pathway of protein degradation. Three amino (N)-terminal residues--aspartate, glutamate and cysteine--are arginylated by ATE1-encoded arginyl-transferases. Here we report that oxidation of N-terminal cysteine is essential for its arginylation. The in vivo oxidation of N-terminal cysteine, before its arginylation, is shown to require nitric oxide. We reconstituted this process in vitro as well. The levels of regulatory proteins bearing N-terminal cysteine, such as RGS4, RGS5 and RGS16, are greatly increased in mouse ATE1-/- embryos, which lack arginylation. Stabilization of these proteins, the first physiological substrates of mammalian N-end rule pathway, may underlie cardiovascular defects in ATE1-/- embryos. Our findings identify the N-end rule pathway as a new nitric oxide sensor that functions through its ability to destroy specific regulatory proteins bearing N-terminal cysteine, at rates controlled by nitric oxide and apparently by oxygen as well.

Multiple potassium channels mediate nitric oxide-induced inhibition of rat vascular smooth muscle cell proliferation

Several nitric oxide (NO) effects in the cardiovascular system are mediated by soluble guanylate cyclase (sGC) activation but potassium channels (KC) are also emerging as important effectors of NO actions. We investigated the relationship among vascular smooth muscle cell proliferation, NO, cyclic GMP, and KC using the A7r5 smooth muscle cell line derived from rat aorta. NO donors (two nitrosothiols, S-nitroso-acetyl-d,l-penicillamine, SNAP, and S-nitroso-glutathione, GSNO, and an organic nitrate, glyceryl trinitrate, GTN; 1-1000 microM) dose-dependently inhibited cell proliferation. ODQ (a selective inhibitor of sGC; 0.1 and 1 microM) and KT5823 (a selective inhibitor of cGMP-dependent protein kinase, 1 microM) prevented NO effects, confirming that sGC is a key target. In this report, we show that tetraethylammonium (TEA, a non-selective blocker of KC, 300 microM), and 4-aminopyridine (a selective blocker of voltage-dependent KC, 100 microM) prevented SNAP inhibitory effects on cell proliferation, whereas glibenclamide (a selective blocker of ATP-dependent KC, 1 microM) was ineffective. Iberiotoxin (a selective blocker of high conductance calcium-activated KC, 100 nM), as well charybdotoxin (a blocker of high and intermediate conductance calcium-activated KC, 100 nM) and apamine (a selective blocker of small conductance calcium-activated KC, 100 nM), blocked the antiproliferative effect induced by SNAP. NS1619 (an opener of high conductance calcium-activated KC, 1-100 microM), inhibited cell proliferation. In addition, sub-effective concentrations of ODQ (100 nM) and TEA (10 microM) synergized in blocking SNAP antiproliferative effects. Thus, voltage-dependent and calcium-activated but not ATP-dependent KC appear to have a prominent role, besides sGC activation, in NO-induced inhibition of vascular smooth muscle cell proliferation.

Avicinylation (thioesterification): a protein modification that can regulate the response to oxidative and nitrosative stress

Avicins are a recently discovered family of plant-derived terpenoid molecules that possess proapoptotic, antiinflammatory, and cytoprotective properties in mammalian cells. Previous work demonstrating that avicins can exert their effects by suppressing or activating the redox-sensitive transcription factors NF-kappaB and nuclear factor-erythroid 2 p45-related factor (Nrf2), respectively, has raised the idea that they may react with critical cysteine residues. To understand the molecular mechanism through which avicins regulate protein function, we examined their effects on the paradigmatic redox-responsive transcriptional activator, OxyR of Escherichia coli, which protects bacterial cells against oxidative and nitrosative stresses. In vitro transcription assays demonstrated that avicins activate OxyR and its target genes katG and oxyS in a DTT-reversible manner. In addition, katG-dependent hydroperoxidase I activity was enhanced in avicin-treated bacteria. Mass spectrometric analysis of activated OxyR revealed thioesterification of the critical regulatory cysteine, Cys-199, to an avicin fragment comprising the outer monoterpene side chain. Our results indicate that avicinylation can induce adaptive responses that protect cells against oxidative or nitrosative stress. More generally, transesterification may represent a previously undescribed thiol-directed posttranslational modification, which extends the code for redox regulation of protein function.

Inhibition of mitochondrial aldehyde dehydrogenase by nitric oxide-mediated S-nitrosylation

Mitochondrial aldehyde dehydrogenase (ALDH2) is responsible for the metabolism of acetaldehyde and other toxic lipid aldehydes. Despite many reports about the inhibition of ALDH2 by toxic chemicals, it is unknown whether nitric oxide (NO) can alter the ALDH2 activity in intact cells or in vivo animals. The aim of this study was to investigate the effects of NO on ALDH2 activity in H4IIE-C3 rat hepatoma cells. NO donors such as S-nitrosoglutathione (GSNO), S-nitroso-N-acetylpenicillamine, and 3-morpholinosydnonimine significantly increased the nitrite concentration while they inhibited the ALDH2 activity. Addition of GSH-ethylester (GSH-EE) completely blocked the GSNO-mediated ALDH2 inhibition and increased nitrite concentration. To directly demonstrate the NO-mediated S-nitrosylation and inactivation, ALDH2 was immunopurified from control or GSNO-treated cells and subjected to immunoblot analysis. The anti-nitrosocysteine antibody recognized the immunopurified ALDH2 only from the GSNO-treated samples. All these results indicate that S-nitrosylation of ALDH2 in intact cells leads to reversible inhibition of ALDH2 activity.

Principles for characterizing the potential human health effects from exposure to nanomaterials: elements of a screening strategy

The rapid proliferation of many different engineered nanomaterials (defined as materials designed and produced to have structural features with at least one dimension of 100 nanometers or less) presents a dilemma to regulators regarding hazard identification. The International Life Sciences Institute Research Foundation/Risk Science Institute convened an expert working group to develop a screening strategy for the hazard identification of engineered nanomaterials. The working group report presents the elements of a screening strategy rather than a detailed testing protocol. Based on an evaluation of the limited data currently available, the report presents a broad data gathering strategy applicable to this early stage in the development of a risk assessment process for nanomaterials. Oral, dermal, inhalation, and injection routes of exposure are included recognizing that, depending on use patterns, exposure to nanomaterials may occur by any of these routes. The three key elements of the toxicity screening strategy are: Physicochemical Characteristics, In Vitro Assays (cellular and non-cellular), and In Vivo Assays. There is a strong likelihood that biological activity of nanoparticles will depend on physicochemical parameters not routinely considered in toxicity screening studies. Physicochemical properties that may be important in understanding the toxic effects of test materials include particle size and size distribution, agglomeration state, shape, crystal structure, chemical composition, surface area, surface chemistry, surface charge, and porosity. In vitro techniques allow specific biological and mechanistic pathways to be isolated and tested under controlled conditions, in ways that are not feasible in in vivo tests. Tests are suggested for portal-of-entry toxicity for lungs, skin, and the mucosal membranes, and target organ toxicity for endothelium, blood, spleen, liver, nervous system, heart, and kidney. Non-cellular assessment of nanoparticle durability, protein interactions, complement activation, and pro-oxidant activity is also considered. Tier 1 in vivo assays are proposed for pulmonary, oral, skin and injection exposures, and Tier 2 evaluations for pulmonary exposures are also proposed. Tier 1 evaluations include markers of inflammation, oxidant stress, and cell proliferation in portal-of-entry and selected remote organs and tissues. Tier 2 evaluations for pulmonary exposures could include deposition, translocation, and toxicokinetics and biopersistence studies; effects of multiple exposures; potential effects on the reproductive system, placenta, and fetus; alternative animal models; and mechanistic studies.

Brain cholinergic vulnerability: Relevance to behavior and disease

The major populations of cholinergic neurons in the brain include two "projection" systems, located in the pontine reticular formation and in the basal forebrain. These two complexes comprise, in part, the anatomical substrates for the "ascending reticular activating system" (ARAS). The pontine cholinergic system relays its rostral influences mainly through thalamic intralaminar nuclei, but it also connects to the basal forebrain and provides a minor innervation of cortex. The basal forebrain cholinergic complex (BFCC) projects directly to cortex and hippocampus, and has a minor connection with the thalamus. Recent data reveal that a parallel system of basal forebrain GABAergic projection neurons innervates cortex/hippocampus in a way that seems to complement the BFCC. Generally, the picture developed from more than 50 years of research is consistent with a "global" influence of these two ascending cholinergic projections on cortical and hippocampal regions. Seemingly, the BFCC acts in tandem or in parallel with the pontine cholinergic projection to activate the electro-encephalogram, increase cerebral blood flow, regulate sleep-wake cycling, and modulate cognitive function. There are quite a number and variety of human brain conditions, notably including Alzheimer's disease, in which degeneration of basal forebrain cholinergic neurons has been documented. Whether the corticopetal GABA system is affected by disease has not been established. Studies of degeneration of the pontine projection are limited, but the available data suggest that it is relatively preserved in Alzheimer's disease. Hypotheses of BFCC degeneration include growth factor deprivation, intracellular calcium dysfunction, amyloid excess, inflammation, and mitochondrial abnormalities/oxidative stress. But, despite considerable research conducted over several decades, the exact mechanisms underlying brain cholinergic vulnerability in human disease remain unclear.

Nitric oxide and multiple sclerosis

Nitric oxide (NO) is a free radical signaling molecule with remarkably complex biochemistry. Its involvement in multiple sclerosis (MS) had been postulated soon after the discovery of the critical role NO plays in inflammation. However, the extent of NO's contribution to MS is not yet understood, party due to the often opposing roles that NO can play in cellular processes. This review briefly covers new developments in the area of NO that may be relevant to MS. It also describes recent progress in understanding the role of NO in MS, new potential targets of the action of NO in the cell, and prospects for NO-based therapies.

Pulmonary and systemic nitric oxide metabolites in a baboon model of neonatal chronic lung disease

We report on developmental changes of pulmonary and systemic nitric oxide (NO) metabolites in a baboon model of chronic lung disease with or without exposure to inhaled NO. The plasma levels of nitrite and nitrate, staining for S-nitrosothiols and 3-nitrotyrosine in the large airways, increased between 125 d and 140 d of gestation (term 185 d) in animals developing in utero. The developmental increase in NO-mediated protein modifications was not interrupted by delivery at 125 d of gestation and mechanical ventilation for 14 d, whereas plasma nitrite and nitrate levels increased in this model. Exposure to inhaled NO resulted in a further increase in plasma nitrite and nitrate and an increase in plasma S-nitrosothiol without altering lung NO synthase expression. These data demonstrate a developmental progression in levels of pulmonary NO metabolites that parallel known maturational increases in total NO synthase activity in the lung. Despite known suppression of total pulmonary NO synthase activity in the chronic lung disease model, pulmonary and systemic NO metabolite levels are higher than in the developmental control animals. Thus, a deficiency in NO production and biological function in the premature baboon was not apparent by the detection and quantification of these surrogate markers of NO production.

Nitric oxide bioavailability in malaria

Rational development of adjunct or anti-disease therapy for severe Plasmodium falciparum malaria requires cellular and molecular definition of malarial pathogenesis. Nitric oxide (NO) is a potential target for such therapy but its role during malaria is controversial. It has been proposed that NO is produced at high levels to kill Plasmodium parasites, although the unfortunate consequence of elevated NO levels might be impaired neuronal signaling, oxidant damage and red blood cell damage that leads to anemia. In this case, inhibitors of NO production or NO scavengers might be an effective adjunct therapy. However, increasing amounts of evidence support the alternate hypothesis that NO production is limited during malaria. Furthermore, the well-documented NO scavenging by cell-free plasma hemoglobin and superoxide, the levels of which are elevated during malaria, has not been considered. Low NO bioavailability in the vasculature during malaria might contribute to pathologic activation of the immune system, the endothelium and the coagulation system: factors required for malarial pathogenesis. Therefore, restoring NO bioavailability might represent an effective anti-disease therapy.

Differential contribution of nitric oxide and cGMP to the stimulatory effects of growth hormone-releasing hormone and low-concentration somatostatin on growth hormone release from somatotrophs

There is increasing evidence that nitric oxide (NO) produced by NO synthase (NOS), and their signalling partners, guanylyl cyclase and cGMP, play a relevant role in growth hormone (GH) secretion from somatotrophs. We previously demonstrated that both GH-releasing hormone (GHRH; 10(-8) M) and low concentrations of somatostatin (10(-15) M) stimulate pig GH release in vitro, whereas a high somatostatin concentration (10(-7) M) inhibits GHRH-induced GH secretion. To ascertain the possible contribution of the NOS-NO and guanylyl cyclase-cGMP routes to these responses, cultures of pituitary cells from prepubertal female pigs were treated (30 min) with GHRH (10(-8) M) or somatostatin (10(-7) or 10(-15) M) in the absence or presence of activators or blockers of key steps of these signalling cascades, and GH release was measured. Two distinct activators of NO route, SNAP (5x10(-4) M) or L-AME (10(-3) M), similarly stimulated GH release when applied alone (with this effect being blocked by 10(-7) M somatostatin), but did not alter the stimulatory effect of GHRH or 10(-15) M somatostatin. Conversely, two NO pathway inhibitors, NAME (10(-5) M) or haemoglobin (20 microg/ml) similarly blocked GHRH- or 10(-15) M somatostatin-stimulated GH release. 8-Br-cGMP (10(-8) to 10(-4) M) strongly stimulated GH release, suggesting that cGMP may function as a subsequent step in the NO pathway in this system. Interestingly, 10(-7) M somatostatin did not inhibit the stimulatory effect of 8-Br-cGMP. Moreover, although 8-Br-cGMP did not modify the effect of GHRH, it enhanced GH release stimulated by 10(-15) M somatostatin. Accordingly, a specific guanylyl cyclase inhibitor, LY-83, 583 (10(-5) M) did not alter 10(-15) M somatostatin-induced GH release, whereas it blocked GHRH-induced GH secretion. These results demonstrate for the first time that the NOS/NO signalling pathway contributes critically to the stimulatory effects of both GHRH and low-concentration somatostatin on GH release, and that, conversely, the subsequent guanylyl cyclase/cGMP step only mediates GHRH- and not low-concentration somatostatin-induced GH secretion from somatotrophs.

Endothelial thrombomodulin induces Ca2+ signals and nitric oxide synthesis through epidermal growth factor receptor kinase and calmodulin kinase II

Endothelial membrane-bound thrombomodulin is a high affinity receptor for thrombin to inhibit coagulation. We previously demonstrated that the thrombin-thrombomodulin complex restrains cell proliferation mediated through protease-activated receptor (PAR)-1. We have now tested the hypothesis that thrombomodulin transduces a signal to activate the endothelial nitric-oxide synthase (NOS3) and to modulate G protein-coupled receptor signaling. Cultured human umbilical vein endothelial cells were stimulated with thrombin or a mutant of thrombin that binds to thrombomodulin and has no catalytic activity on PAR-1. Thrombin and its mutant dose dependently activated NO release at cell surface. Pretreatment with anti-thrombomodulin antibody suppressed NO response to the mutant and to low thrombin concentration and reduced by half response to high concentration. Thrombin receptor-activating peptide that only activates PAR-1 and high thrombin concentration induced marked biphasic Ca2+ signals with rapid phosphorylation of PLC(beta3) and NOS3 at both serine 1177 and threonine 495. The mutant thrombin evoked a Ca2+ spark and progressive phosphorylation of Src family kinases at tyrosine 416 and NOS3 only at threonine 495. It activated rapid phosphatidylinositol-3 kinase-dependent NO synthesis and phosphorylation of epidermal growth factor receptor and calmodulin kinase II. Complete epidermal growth factor receptor inhibition only partly reduced the activation of phospholipase Cgamma1 and NOS3. Prestimulation of thrombomodulin did not affect NO release but reduced Ca2+ responses to thrombin and histamine, suggesting cross-talks between thrombomodulin and G protein-coupled receptors. This is the first demonstration of an outside-in signal mediated by the cell surface thrombomodulin receptor to activate NOS3 through tyrosine kinase-dependent pathway. This signaling may contribute to thrombomodulin function in thrombosis, inflammation, and atherosclerosis.

S-nitrosoglutathione inhibits alpha1-adrenergic receptor-mediated vasoconstriction and ligand binding in pulmonary artery

Endogenous nitric oxide donor compounds (S-nitrosothiols) contribute to low vascular tone by both cGMP-dependent and -independent pathways. We have reported that S-nitrosoglutathione (GSNO) inhibits 5-hydroxytryptamine (5-HT)-mediated pulmonary vasoconstriction via a cGMP-independent mechanism likely involving S-nitrosylation of its G protein-coupled receptor (GPCR) system. Because catecholamines, like 5-HT, constrict lung vessels via a GPCR coupled to G(q), we hypothesized that S-nitrosothiols modify the alpha1-adrenergic GPCR system to inhibit pulmonary vasoconstriction by receptor agonists, e.g., phenylephrine (PE). Rat pulmonary artery rings were pretreated for 30 min with and without an S-nitrosothiol, either GSNO or S-nitrosocysteine (CSNO), and constricted with sequential concentrations of PE (10(-8)-10(-6) M). Effective cGMP-dependence was tested in rings pretreated with soluble guanylate cyclase inhibitors {either 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) or LY-83583} or G kinase inhibitor (KT-5823), and a thiol reductant [dithiothreitol (DTT)] was used to test reversibility of S-nitrosylation. Both S-nitrosothiols attenuated the PE dose response. The GSNO effect was not prevented by LY-83583, ODQ, or KT-5823, indicating cGMP independence. GSNO inhibition was reversed by DTT, consistent with S-nitrosylation or other GSNO-mediated cysteine modifications. In CSNO-treated lung protein, the alpha1-adrenergic receptor was shown to undergo S-nitrosylation in vitro using a biotin switch assay. Studies of alpha1-adrenergic receptor subtype expression and receptor density by saturation binding with 125I-HEAT showed that GSNO decreased alpha1-adrenergic receptor density but did not alter affinity for antagonist or agonist. These data demonstrate a novel cGMP-independent mechanism of reversible alpha1-adrenergic receptor inhibition by S-nitrosothiols.

NO news is good news for plants

The organization of redox signaling and the use of nitric oxide (NO) to transmit information, modulate biological processes or create cellular damage are highly complex. Recent reports provide an exceptional picture of NO production, of the regulation of NO bioactivity through detoxification reactions and of biochemical events by which NO transduces signals into cellular responses, in particular during disease resistance. Furthermore, other exciting reports on NO function in germination, growth and reproduction support the view that NO is a 'do it all' molecule that plays a crucial role during the entire lifespan of the plant.

Thioredoxin catalyzes the S-nitrosation of the caspase-3 active site cysteine

Nitric oxide (NO) signaling through the formation of cGMP is well established; however, there seems to be an increasing role for cGMP-independent NO signaling. Although key molecular details remain unanswered, S-nitrosation represents an example of cGMP-independent NO signaling. This modification has garnered recent attention as it has been shown to modulate the function of several important biochemical pathways. Although an analogy to O-phosphorylation can be drawn, little is known about protein nitrosothiol regulation in vivo. In solution, NO readily reacts with oxygen to yield a nitrosating agent, but this process alone provides no specificity for nitrosation. This lack of specificity is exemplified by the in vitro poly-S-nitrosation of caspase-3 (Casp-3, ref. 6) and the ryanodine receptor. Previous in vivo work with Casp-3 suggests that a protein-assisted process may be responsible for selective S-nitrosation of the catalytic cysteine (Cys163). We demonstrated that a single cysteine in thioredoxin (Trx) is capable of a targeted, reversible transnitrosation reaction with Cys163 of Casp-3. A greater understanding of how S-nitrosation is mediated has broad implications for cGMP-independent signaling. The example described here also suggests a new role for Trx in the regulation of apoptosis.