Metacaspase activity of Arabidopsis thaliana is regulated by S-nitrosylation of a critical cysteine residue

Nitric oxide (NO) regulates a number of signaling functions in both animals and plants under several physiological and pathophysiological conditions. S-Nitrosylation linking a nitrosothiol on cysteine residues mediates NO signaling functions of a broad spectrum of mammalian proteins, including caspases, the main effectors of apoptosis. Metacaspases are suggested to be the ancestors of metazoan caspases, and plant metacaspases have previously been shown to be genuine cysteine proteases that autoprocess in a manner similar to that of caspases. We show that S-nitrosylation plays a central role in the regulation of the proteolytic activity of Arabidopsis thaliana metacaspase 9 (AtMC9) and hypothesize that this S-nitrosylation affects the cellular processes in which metacaspases are involved. We found that AtMC9 zymogens are S-nitrosylated at their active site cysteines in vivo and that this posttranslational modification suppresses both AtMC9 autoprocessing and proteolytic activity. However, the mature processed form is not prone to NO inhibition due to the presence of a second S-nitrosylation-insensitive cysteine that can replace the S-nitrosylated cysteine residue within the catalytic center of the processed AtMC9. This cysteine is absent in caspases and paracaspases but is conserved in all reported metacaspases.     

Proteomic analysis of S-nitrosylated proteins in Arabidopsis thaliana undergoing hypersensitive response

Nitric oxide (NO) has a fundamental role in the plant hypersensitive disease resistance response (HR), and S-nitrosylation is emerging as an important mechanism for the transduction of its bioactivity. A key step toward elucidating the mechanisms by which NO functions during the HR is the identification of the proteins that are subjected to this PTM. By using a proteomic approach involving 2-DE and MS we characterized, for the first time, changes in S-nitrosylated proteins in Arabidopsis thaliana undergoing HR. The 16 S-nitrosylated proteins identified are mostly enzymes serving intermediary metabolism, signaling and antioxidant defense. The study of the effects of S-nitrosylation on the activity of the identified proteins and its role during the execution of the disease resistance response will help to understand S-nitrosylation function and significance in plants.     

Inhibition of protein-tyrosine phosphatases by mild oxidative stresses is dependent on S-nitrosylation

Previous studies have shown that a Ca(2+)-dependent nitric-oxide synthase (NOS) is activated as part of a cellular response to low doses of ionizing radiation. Genetic and pharmacological inhibitor studies linked this NO signaling to the radiation-induced activation of ERK1/2. Herein, a mechanism for the radiation-induced activation of Tyr phosphorylation-dependent pathways (e.g. ERK1/2) involving the inhibition of protein-Tyr phosphatases (PTPs) by S-nitrosylation is tested. The basis for this mechanism resides in the redox-sensitive active site Cys in PTPs. These studies also examined oxidative stress induced by low concentrations of H(2)O(2). S-Nitrosylation of total cellular PTP and immunopurified SHP-1 and SHP-2 was detected as protection of PTP enzymatic activity from alkylation by N-ethylmaleimide and reversal by ascorbate. Both radiation and H(2)O(2) protected PTP activity from alkylation by a mechanism reversible by ascorbate and inhibited by NOS inhibitors or expression of a dominant negative mutant of NOS-1. Radiation and H(2)O(2) stimulated a transient increase in cytoplasmic free [Ca(2+)]. Radiation, H(2)O(2), and the Ca(2+) ionophore, ionomycin, also stimulated NOS activity, and this was associated with an enhanced S-nitrosylation of the active site Cys(453) determined by isolation of S-nitrosylated wild type but not active site Cys(453) --> Ser SHP-1 mutant by the "biotin-switch" method. Thus, one consequence of oxidative stimulation of NO generation is S-nitrosylation and inhibition of PTPs critical in cellular signal transduction pathways. These results support the conclusion that a mild oxidative signal is converted to a nitrosative one due to the better redox signaling properties of NO.     

Nitric oxide-mediated inhibition of taurocholate uptake involves S-nitrosylation of NTCP

The sodium-taurocholate (TC) cotransporting polypeptide (NTCP) facilitates bile formation by mediating sinusoidal Na(+)-TC cotransport. During sepsis-induced cholestasis, there is a decrease in NTCP-dependent uptake of bile acids and an increase in nitric oxide (NO) levels in hepatocytes. In rat hepatocytes NO inhibits Na(+)-dependent uptake of taurocholate. The aim of this study was to extend these findings to human NTCP and to further investigate the mechanism by which NO inhibits TC uptake. Using a human hepatoma cell line stably expressing NTCP (HuH-NTCP), we performed experiments with the NO donors sodium nitroprusside and S-nitrosocysteine and demonstrated that NO inhibits TC uptake in these cells. Kinetic analyses revealed that NO significantly decreased the V(max) but not the K(m) of TC uptake by NTCP, indicating noncompetitive inhibition. NO decreased the amount of NTCP in the plasma membrane, providing a molecular mechanism for the noncompetitive inhibition of TC uptake. One way that NO can modify protein function is through a posttranslational modification known as S-nitrosylation: the binding of NO to cysteine thiols. Using a biotin switch technique we observed that NTCP is S-nitrosylated under conditions in which NO inhibits TC uptake. Moreover, dithiothreitol reversed NO-mediated inhibition of TC uptake and S-nitrosylation of NTCP, indicating that NO inhibits TC uptake via modification of cysteine thiols. In addition, NO treatment led to a decrease in Ntcp phosphorylation. Taken together these results indicate that the inhibition of TC uptake by NO involves S-nitrosylation of NTCP.     

Protein S-nitrosylation: What's going on in plants?

Nitric oxide (NO) is now recognized as a key regulator of plant physiological processes. Understanding the mechanisms by which NO exerts its biological functions has been the subject of extensive research. Several components of the signaling pathways relaying NO effects in plants, including second messengers, protein kinases, phytohormones, and target genes, have been characterized. In addition, there is now compelling experimental evidence that NO partly operates through posttranslational modification of proteins, notably via S-nitrosylation and tyrosine nitration. Recently, proteome-wide scale analyses led to the identification of numerous protein candidates for S-nitrosylation in plants. Subsequent biochemical and in silico structural studies revealed certain mechanisms through which S-nitrosylation impacts their functions. Furthermore, first insights into the physiological relevance of S-nitrosylation, particularly in controlling plant immune responses, have been recently reported. Collectively, these discoveries greatly extend our knowledge of NO functions and of the molecular processes inherent to signal transduction in plants.     

Nitric oxide-induced S-glutathionylation and inactivation of glyceraldehyde-3-phosphate dehydrogenase

S-Nitrosylation of protein thiol groups by nitric oxide (NO) is a widely recognized protein modification. In this study we show that nitrosonium tetrafluoroborate (BF4NO), a NO+ donor, modified the thiol groups of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) by S-nitrosylation and caused enzyme inhibition. The resultant protein-S-nitrosothiol was found to be unstable and to decompose spontaneously, thereby restoring enzyme activity. In contrast, the NO-releasing compound S-nitrosoglutathione (GSNO) promoted S-glutathionylation of a thiol group of GAPDH both in vitro and under cellular conditions. The GSH-mixed protein disulfide formed led to a permanent enzyme inhibition, but upon dithiothreitol addition a functional active GAPDH was recovered. This S-glutathionylation is specific for GSNO because GSH itself was unable to produce protein-mixed disulfides. During cellular nitrosative stress, the production of intracellular GSNO might channel signaling responses to form protein-mixed disulfide that can regulate intracellular function.     

Nitric oxide,NOC-12, and S-nitrosoglutathione modulate the skeletal muscle calcium release channel/ryanodine receptor by different mechanisms. An allosteric function for O2 in S-nitrosylation of the channel

The skeletal muscle Ca(2+) release channel/ryanodine receptor (RyR1) contains approximately 50 thiols per subunit. These thiols have been grouped according to their reactivity/responsiveness toward NO, O(2), and glutathione, but the molecular mechanism enabling redox active molecules to modulate channel activity is poorly understood. In the case of NO, very low concentrations (submicromolar) activate RyR1 by S-nitrosylation of a single cysteine residue (Cys-3635), which resides within a calmodulin binding domain. S-Nitrosylation of Cys-3635 only takes place at physiological tissue O(2) tension (pO(2); i.e. approximately 10 mm Hg) but not at pO(2) approximately 150 mm Hg. Two explanations have been offered for the loss of RyR1 responsiveness to NO at ambient pO(2), i.e. Cys-3635 is oxidized by O(2) versus O(2) subserves an allosteric function (Eu, J. P., Sun, J. H., Xu, L., Stamler, J. S., and Meissner, G. (2000) Cell 102, 499-509). Here we report that the NO donors NOC-12 and S-nitrosoglutathione both activate RyR1 by release of NO but do so independently of pO(2). Moreover, NOC-12 activates the channel by S-nitrosylation of Cys-3635 and thereby reverses channel inhibition by calmodulin. In contrast, S-nitrosoglutathione activates RyR1 by oxidation and S-nitrosylation of thiols other than Cys-3635 (and calmodulin is not involved). Our results suggest that the effect of pO(2) on RyR1 S-nitrosylation is exerted through an allosteric mechanism.     

蛋白亚硝基化研究进展及其在植物抗病中的作用

蛋白亚硝基化（S-nitrosylation）是一种在一氧化氮作用下与蛋白半胱氨酸巯基共价结合,使巯基-SH转化为-SNO的反应。作为一种氧化还原依赖的翻译后调控形式,蛋白亚硝基化对多种蛋白的功能具有调节作用,越来越多的证据表明蛋白亚硝基化在植物抗病中发挥重要的作用。简要介绍了蛋白巯基亚硝基化的特点、检测方法、功能研究以及在植物抗病调节方面的最新进展。     

S-nitrosothiols react preferentially with zinc thiolate clusters of metallothionein III through transnitrosation

Metallothionein (MT) is a two-domain protein with zinc thiolate clusters that bind and release zinc depending on the redox states of the sulfur ligands. Since S-nitrosylation of cysteine is considered a prototypic cellular redox signaling mechanism, we here investigate the reactions of S-nitrosothiols with different isoforms of MT. MT-III is significantly more reactive than MT-I/II toward S-nitrosothiols, whereas the reactivity of all three isoforms toward reactive oxygen species is comparable. A cellular system, in which all three MTs are similarly effective in protecting rat embryonic cortical neurons in primary culture against hydrogen peroxide but where MT-III has a much more pronounced effect of protecting against S-nitrosothiols, confirms this finding. MT-III is the only isoform with consensus acid-base sequence motifs for S-nitrosylation in both domains. Studies with synthetic and zinc-reconstituted domain peptides demonstrate that S-nitrosothiols indeed release zinc from both the alpha- and the beta-domain of MT-III. S-Nitrosylation occurs via transnitrosation, a mechanism that differs fundamentally from that of previous studies of reactions of MT with NO*. Our data demonstrate that zinc thiolate bonds are targets of S-nitrosothiol signaling and further indicate that MT-III is biologically specific in converting NO signals to zinc signals. This could bear importantly on the physiological action of MT-III, whose biological activity as a neuronal growth inhibitory factor is unique, and for brain diseases that have been related to oxidative or nitrosative stress.     

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.     

Say NO to neurodegeneration: role of S-nitrosylation in neurodegenerative disorders

Nitric oxide (NO) is an important signaling molecule that controls a wide range of biological processes. One of the signaling mechanisms of NO is through the S-nitrosylation of cysteine residues on proteins. S-nitrosylation is now regarded as an important redox signaling mechanism in the regulation of different cellular and physiological functions. However, deregulation of S-nitrosylation has also been linked to various human diseases such as neurodegenerative disorders. Nitrosative stress has long been considered as a major mediator in the development of neurodegeneration, but the molecular mechanism of how NO can contribute to neurodegeneration is not completely clear. Early studies suggested that nitration of proteins, which can induce protein aggregation might contribute to the neurodegenerative process. However, several recent studies suggest that S-nitrosylation of proteins that are important for neuronal survival contributes substantially in the development of various neurodegenerative disorders. Thus, in-depth understanding of the mechanism of neurodegeneration in relation to S-nitrosylation will be critical for the development of therapeutic treatment against these neurodegenerative diseases.     

Effects of endogenous ligands on the biological role of human serum albumin in S-nitrosylation

Many proteins have been identified as targets for S-nitrosylation, including structural and signaling proteins, and ion channels. S-nitrosylation plays an important role in regulating their activity and function. We used human serum albumin (HSA), a major endogenous NO traffic protein, and studied the effect of mediators on S-nitrosylation processes which control NO bioactivity. By using NOC-7, S-nitrosoglutathione, and activated RAW264.7 cells as NO-donors we found that high-affinity binding of endogenous ligands (Cu²⁺, bilirubin and fatty acid) can affect these processes. It is likely that the same effects take place in many clinical situations characterized by increased fatty acid concentrations in plasma such as type II diabetes and the metabolic syndrome. Thus, endogenous ligands, changing their plasma concentrations, could be a novel type of mediator of S-nitrosylation not only in the case of HSA but also for other target proteins.     

Receptor-regulated dynamic S-nitrosylation of endothelial nitric-oxide synthase in vascular endothelial cells

The endothelial isoform of nitric-oxide synthase (eNOS) is regulated by a complex pattern of post-translational modifications. In these studies, we show that eNOS is dynamically regulated by S-nitrosylation, the covalent adduction of nitric oxide (NO)-derived nitrosyl groups to the cysteine thiols of proteins. We report that eNOS is tonically S-nitrosylated in resting bovine aortic endothelial cells and that the enzyme undergoes rapid transient denitrosylation after addition of the eNOS agonist, vascular endothelial growth factor. eNOS is thereafter progressively renitrosylated to basal levels. The receptor-mediated decrease in eNOS S-nitrosylation is inversely related to enzyme phosphorylation at Ser(1179), a site associated with eNOS activation. We also document that targeting of eNOS to the cell membrane is required for eNOS S-nitrosylation. Acylation-deficient mutant eNOS, which is targeted to the cytosol, does not undergo S-nitrosylation. Using purified eNOS, we show that eNOS S-nitrosylation by exogenous NO donors inhibits enzyme activity and that eNOS inhibition is reversed by denitrosylation. We determine that the cysteines of the zinc-tetrathiolate that comprise the eNOS dimer interface are the targets of S-nitrosylation. Mutation of the zinc-tetrathiolate cysteines eliminates eNOS S-nitrosylation but does not eliminate NO synthase activity, arguing strongly that disruption of the zinc-tetrathiolate does not necessarily lead to eNOS monomerization in vivo. Taken together, these studies suggest that eNOS S-nitrosylation may represent an important mechanism for regulation of NO signaling pathways in the vascular wall.     

New insights into protein S-nitrosylation. Mitochondria as a model system

The biological effects of nitric oxide (NO) are in significant part mediated through S-nitrosylation of cysteine thiol. Work on model thiol substrates has raised the idea that molecular oxygen (O(2)) is required for S-nitrosylation by NO; however, the relevance of this mechanism at the low physiological pO(2) of tissues is unclear. Here we have used a proteomic approach to study S-nitrosylation reactions in situ. We identify endogenously S-nitrosylated proteins in subcellular organelles, including dihydrolipoamide dehydrogenase and catalase, and show that these, as well as hydroxymethylglutaryl-CoA synthase and sarcosine dehydrogenase (SarDH), are S-nitrosylated by NO under strictly anaerobic conditions. S-Nitrosylation of SarDH by NO is best rationalized by a novel mechanism involving the covalently bound flavin of the enzyme. We also identify a set of mitochondrial proteins that can be S-nitrosylated through multiple reaction channels, including anaerobic/oxidative, NO/O(2), and GSNO-mediated transnitrosation. Finally, we demonstrate that steady state levels of S-nitrosylation are higher in mitochondrial extracts than the intact organelles, suggesting the importance of denitrosylation reactions. Collectively, our results provide new insight into the determinants of S-nitrosothiol levels in subcellular compartments.     

Neuronal protection and destruction by NO

Nitric oxide (NO)-related species include different redox states of the NO group, which have recently been reported to exist endogenously in biological tissues including the brain. The importance of these different NO-related species is that their distinct chemical reactivities can influence the life and death of neurons in response to various insults. In the case of NO+ equivalents (having one less electron than NO.), the mechanism of reaction often involves S-nitrosylation or transfer of the NO group to the sulfhydryl of a cysteine residue (or more properly to a thiolate anion) to form an RS-NO; further oxidation of critical thiols can possibly then form disulfide bonds from neighboring cysteine residues. We have mounted both physiological and chemical evidence that N-methyl-D-aspartate receptor (NMDAR) activity and caspase enzyme activity can be decreased by S-nitrosylation, as can other signaling molecules involved in neuronal apoptotic pathways, to afford neuroprotection. Over the past 5 years, beginning with our report on the NMDAR, evidence has accumulated that S-nitrosylation can regulate the biological activity of a great variety of proteins, in some ways akin to phosphorylation. Thus, this chemical reaction is gaining acceptance as a newly-recognized molecular switch to control protein function via reactive thiol groups, such as those encountered on the NMDAR and in the active site of caspases. One method of producing S-nitrosylation of the NMDAR and caspases is the administration of nitroglycerin, and nitroglycerin can be neuroprotective in acute focal ischemia/reperfusion models via mechanisms other than increasing cerebral blood flow. In contrast, NO* itself does not appear to react with thiol under physiological conditions. In fact, the favored reaction of NO* is with O2*- (superoxide anion) to form ONOO- (peroxynitrite), which can lead to neurotoxicity. A third NO-related species with one added electron compared to NO* is nitroxyl anion (NO-). NO- -unlike NO* but reminiscent of NO+ transfer - reacts with critical thiol groups of the NMDA receptor to curtail excessive Ca2+ influx and thus provide neuroprotection from excitotoxic insults.     

S-Nitrosylation of Akt by organic nitrate delays revascularization and the recovery of cardiac function in mice following myocardial infarction

The effects of long-term nitrate therapy are compromised due to protein S-Nitrosylation, which is mediated by nitric oxide (NO). This study is to determine the role of Akt S-Nitrosylation in the recovery of heart functions after ischaemia. In recombinant Akt protein and in HEK293 cells, NO donor decreased Akt activity and induced Akt S-Nitrosylation, but was abolished if Akt protein was mutated by replacing cysteine 296/344 with alanine (Akt-C296/344A). In endothelial cells, NO induced Akt S-Nitrosylation, reduced Akt activity and damaged multiple cellular functions including proliferation, migration and tube formation. These alterations were ablated if cells expressed Akt-C296/344A mutant. In Apoe^−/− mice, nitroglycerine infusion increased both Akt S-Nitrosylation and infarct size, reduced Akt activity and capillary density, and delayed the recovery of cardiac function in ischaemic hearts, compared with mice infused with vehicle. Importantly, these in vivo effects of nitroglycerine in Apoe^−/− mice were remarkably prevented by adenovirus-mediated enforced expression of Akt-C296/344A mutant. In conclusion, long-term usage of organic nitrate may inactivate Akt to delay ischaemia-induced revascularization and the recovery of cardiac function through NO-mediated S-Nitrosylation.     

Reprint of "oxidative alterations of cyclooxygenase during atherogenesis" [Prostag. Oth. Lipid. M. 80 (2006) 1-14

Nitric oxide (*NO) and eicosanoids are critical mediators of physiological and pathophysiological processes. They include inflammation and atherosclerosis. *NO production and eicosanoid synthesis become disrupted during atherosclerosis and thus, it is important to understand the mechanisms that may contribute to this outcome. We, and others, have shown that nitrogen oxide (NOx) species modulate cyclooxygenase (COX; also known as prostaglandin H2 synthase) activity and alter eicosanoid production. We have determined that peroxynitrite (ONOO-) has multiple effects on COX activity. ONOO- can provide the peroxide tone necessary for COX activation, such that simultaneous exposure of COX to its arachidonic acid substrate and ONOO- results in increased eicosanoid production. Alternatively, in the absence of arachidonic acid, ONOO- can modify COX through nitration of an essential tyrosine residue (Tyr385) such that it is incapable of catalysis. In this regard, we have shown that COX nitration occurs in human atherosclerotic tissue and in aortic lesions from ApoE-/- mice kept on a high fat diet. Additionally, we have demonstrated that Tyr nitration in ApoE-/- mice is dependent on the inducible form of NO synthase (iNOS). Under conditions where ONOO- persists and arachidonic acid is not immediately available, the cell may try to correct the situation by responding to ONOO- and releasing arachidonic acid via a signaling pathway to favor COX activation. Other post-translational modifications of COX by NOx species include S-nitrosation of cysteine (Cys) residues (which may have an activating effect) and Cys oxidation. The central focus of this review will include a discussion of how NOx species alter COX activity at the molecular level and how these modifications may contribute to altered eicosanoid output during atherosclerosis and lesion development.     

Nitric oxide-induced S-nitrosylation of glyceraldehyde-3-phosphate dehydrogenase inhibits enzymatic activity and increases endogenous ADP-ribosylation.

Using conditions that produced chronic inflammation in rat liver, we were able to find a correlation between induction of nitric oxide production and inhibition of glyceraldehyde-3-phosphate dehydrogenase (GAPDH; EC 1.2.1.12). This enzyme is a tetramer composed of identical M(r) 37,000 subunits. The tetramer contains 16 thiol groups, four of which are essential for enzymatic activity. Our information indicates that four thiol groups are S-nitrosylated by exposure to authentic nitric oxide (NO) gas. Furthermore, NO decreased GAPDH activity while increasing its auto-ADP-ribosylation. Reduced nicotinamide adenine dinucleotide and dithiothreitol are required for the S-nitrosylation of GAPDH caused by the NO-generating compound sodium nitroprusside. Our results suggests that a new and important action of nitric oxide on cells is the S-nitrosylation and inactivation of GAPDH. S-Nitrosylation of GAPDH may be a key covalent modification of multiple regulatory consequences in chronic liver inflammation.