Detection and proteomic identification of S-nitrosylated proteins in endothelial cells

Nitric Oxide Signaling and Sensing in Age-Related Diseases

Nitric oxide (NO) is a key signaling molecule involved in numerous physiological and pathological processes within the human body. This review specifically examines the involvement of NO in age-related diseases, focusing on the cardiovascular, nervous, and immune systems. The discussion delves into the mechanisms of NO signaling in these diseases, emphasizing the post-translational modifications of involved proteins, such as S-nitrosation and nitration. The review also covers the dual nature of NO, highlighting both its protective and harmful effects, determined by concentration, location, and timing. Additionally, potential therapies that modulate NO signaling, including the use of NO donors and nitric oxide synthases (NOSs) inhibitors in the treatment of cardiovascular, neurodegenerative, and oncological diseases, are analyzed. Particular attention is paid to the methods for the determination of NO and its derivatives in the context of illness diagnosis and monitoring. The review underscores the complexity and dual role of NO in maintaining cellular balance and suggests areas for future research in developing new therapeutic strategies.

Roles of Protein S-Nitrosylation in Endothelial Homeostasis and Dysfunction

Significance: Nitric oxide (NO) plays several distinct roles in endothelial homeostasis. Except for activating the guanylyl cyclase enzyme-dependent cyclic guanosine monophosphate signaling pathway, NO can bind reactive cysteine residues in target proteins, a process known as S-nitrosylation (SNO). SNO is proposed to explain the multiple biological functions of NO in the endothelium. Investigating the targets and mechanism of protein SNO in endothelial cells (ECs) can provide new strategies for treating endothelial dysfunction-related diseases. Recent Advances: In response to different environments, proteomics has identified multiple SNO targets in ECs. Functional studies confirm that SNO regulates NO bioavailability, inflammation, permeability, oxidative stress, mitochondrial function, and insulin sensitivity in ECs. It also influences EC proliferation, migration, apoptosis, and transdifferentiation. Critical Issues: Single-cell transcriptomic analysis of ECs isolated from different mouse tissues showed heterogeneous gene signatures. However, litter research focuses on the heterogeneous properties of SNO proteins in ECs derived from different tissues. Although metabolism reprogramming plays a vital role in endothelial functions, little is known about how protein SNO regulates metabolism reprogramming in ECs. Future Directions: Precisely deciphering the effects of protein SNO in ECs isolated from different tissues under different conditions is necessary to further characterize the relationship between protein SNO and endothelial dysfunction-related diseases. In addition, identifying SNO targets that can influence endothelial metabolic reprogramming and the underlying mechanism can offer new views on the crosstalk between metabolism and post-translational protein modification. Antioxid. Redox Signal. 40, 186-205.

bFGF alleviates diabetes-associated endothelial impairment by downregulating inflammation via S-nitrosylation pathway

Protein S-nitrosylation is a reversible protein modification implicated in both physiological and pathophysiological regulation of protein function. However, the relationship between dysregulated S-nitrosylation homeostasis and diabetic vascular complications remains incompletely understood. Here, we demonstrate that basic fibroblast growth factor (bFGF) is a key regulatory link between S-nitrosylation homeostasis and inflammation, and alleviated endothelial dysfunction and angiogenic defects in diabetes. Subjecting human umbilical vein endothelial cells (HUVECs) to hyperglycemia and hyperlipidemia significantly decreased endogenous S-nitrosylated proteins, including S-nitrosylation of inhibitor kappa B kinase β (IKKβ^C179) and transcription factor p65 (p65^C38), which was alleviated by bFGF co-treatment. Pretreatment with carboxy-PTIO (c-PTIO), a nitric oxide scavenger, abolished bFGF-mediated S-nitrosylation increase and endothelial protection. Meanwhile, nitrosylation-resistant IKKβ^C179S and p65^C38S mutants exacerbated endothelial dysfunction in db/db mice, and in cultured HUVECs subjected to hyperglycemia and hyperlipidemia. Mechanistically, bFGF-mediated increase of S-nitrosylated IKKβ and p65 was attributed to synergistic effects of increased endothelial nitric oxide synthase (eNOS) and thioredoxin (Trx) activity. Taken together, the endothelial protective effect of bFGF under hyperglycemia and hyperlipidemia can be partially attributed to its role in suppressing inflammation via the S-nitrosylation pathway.

Polymeric Nitric Oxide Delivery Nanoplatforms for Treating Cancer, Cardiovascular Diseases, and Infection

The shortened Abstract is as follows: Therapeutic gas nitric oxide (NO) has demonstrated the unique advances in biomedical applications due to its prominent role in regulating physiological/pathophysiological activities in terms of vasodilation, angiogenesis, chemosensitizing effect, and bactericidal effect. However, it is challenging to deliver NO, due to its short half-life (<5 s) and short diffusion distances (20-160 µm). To address these, various polymeric NO delivery nanoplatforms (PNODNPs) have been developed for cancer therapy, antimicrobial and cardiovascular therapeutics, because of the important advantages of polymeric delivery nanoplatforms in terms of controlled release of therapeutics and the extremely versatile nature. This reviews highlights the recent significant advances made in PNODNPs for NO storing and targeting delivery. The ideal and unique criteria that are required for PNODNPs for treating cancer, cardiovascular diseases and infection, respectively, are summarized. Hopefully, effective storage and targeted delivery of NO in a controlled manner using PNODNPs could pave the way for NO-sensitized synergistic therapy in clinical practice for treating the leading death-causing diseases.

Role of GSH and Iron-Sulfur Glutaredoxins in Iron Metabolism-Review

Glutathione (GSH) was initially identified and characterized for its redox properties and later for its contributions to detoxification reactions. Over the past decade, however, the essential contributions of glutathione to cellular iron metabolism have come more and more into focus. GSH is indispensable in mitochondrial iron-sulfur (FeS) cluster biosynthesis, primarily by co-ligating FeS clusters as a cofactor of the CGFS-type (class II) glutaredoxins (Grxs). GSH is required for the export of the yet to be defined FeS precursor from the mitochondria to the cytosol. In the cytosol, it is an essential cofactor, again of the multi-domain CGFS-type Grxs, master players in cellular iron and FeS trafficking. In this review, we summarize the recent advances and progress in this field. The most urgent open questions are discussed, such as the role of GSH in the export of FeS precursors from mitochondria, the physiological roles of the CGFS-type Grx interactions with BolA-like proteins and the cluster transfer between Grxs and recipient proteins.

The Peroxidatic Thiol of Peroxiredoxin 1 is Nitrosated by Nitrosoglutathione but Coordinates to the Dinitrosyl Iron Complex of Glutathione

Protein S-nitrosation is an important consequence of NO^●·metabolism with implications in physiology and pathology. The mechanisms responsible for S-nitrosation in vivo remain debatable and kinetic data on protein S-nitrosation by different agents are limited. 2-Cys peroxiredoxins, in particular Prx1 and Prx2, were detected as being S-nitrosated in multiple mammalian cells under a variety of conditions. Here, we investigated the kinetics of Prx1 S-nitrosation by nitrosoglutathione (GSNO), a recognized biological nitrosating agent, and by the dinitrosyl-iron complex of glutathione (DNIC-GS; [Fe(NO)₂(GS)₂]⁻), a hypothetical nitrosating agent. Kinetics studies following the intrinsic fluorescence of Prx1 and its mutants (C83SC173S and C52S) were complemented by product analysis; all experiments were performed at pH 7.4 and 25 ℃. The results show GSNO-mediated nitrosation of Prx1 peroxidatic residue (k+NOCys52 = 15.4 ± 0.4 M⁻¹. s⁻¹) and of Prx1 Cys⁸³ residue (k+NOCys83 = 1.7 ± 0.4 M⁻¹. s⁻¹). The reaction of nitrosated Prx1 with GSH was also monitored and provided a second-order rate constant for Prx1Cys⁵²NO denitrosation of k−NOCys52 = 14.4 ± 0.3 M⁻¹. s⁻¹. In contrast, the reaction of DNIC-GS with Prx1 did not nitrosate the enzyme but formed DNIC-Prx1 complexes. The peroxidatic Prx1 Cys was identified as the residue that more rapidly replaces the GS ligand from DNIC-GS (kDNICCys52 = 7.0 ± 0.4 M⁻¹. s⁻¹) to produce DNIC-Prx1 ([Fe(NO)₂(GS)(Cys⁵²-Prx1)]⁻). Altogether, the data showed that in addition to S-nitrosation, the Prx1 peroxidatic residue can replace the GS ligand from DNIC-GS, forming stable DNIC-Prx1, and both modifications disrupt important redox switches.

Exogenous application of nitric oxide donors regulates short-term flooding stress in soybean

Short-term water submergence to soybean (Glycine max L.) create hypoxic conditions hindering plant growth and productivity. Nitric oxide (NO) is considered a stress-signalling and stress-evading molecule, however, little is known about its role during flooding stress. We elucidated the role of sodium nitroprusside (SNP) and S-nitroso L-cysteine (CySNO) as NO donor in modulation of flooding stress-related bio-chemicals and genetic determinants of associated nitrosative stress to Daewon and Pungsannamul soybean cultivars after 3 h and 6 h of flooding stress. The results showed that exogenous SNP and CysNO induced glutathione activity and reduced the resulting superoxide anion contents during short-term flooding in Pungsannamul soybean. The exo- SNP and CysNO triggered the endogenous S-nitrosothiols, and resulted in elevated abscisic acid (ABA) contents in both soybean cultivars overtime. To know the role of ABA and NO related genes in short-term flooding stress, the mRNA expression of S-nitrosoglutathione reductase (GSNOR1), NO overproducer1 (NOX1) and nitrate reductase (NR), Timing of CAB expression1 (TOC1), and ABA-receptor (ABAR) were assessed. The transcripts accumulation of GSNOR1, NOX1, and NR being responsible for NO homeostasis, were significantly high in response to early or later phases of flooding stress. ABAR and TOC1 showed a decrease in transcript accumulation in both soybean plants treated with exogenous SNP and CySNO. The exo- SNP and CySNO could impinge a variety of biochemical and transcriptional programs that can mitigate the negative effects of short-term flooding stress in soybean.

S-nitrosylation of cytoskeletal proteins

Nitric oxide has pronounced effects on cellular functions normally associated with the cytoskeleton, including cell motility, shape, contraction, and mitosis. Protein S-nitrosylation, the covalent addition of a NO group to a cysteine sulfur, is a signaling pathway for nitric oxide that acts in parallel to cyclic guanosine monophosphate (cGMP), but is poorly studied compared to the latter. There is growing evidence that S-nitrosylation of cytoskeletal proteins selectively alters their function. We review that evidence, and find that S-nitrosylation of cytoskeletal targets has complementary but distinct effects to cyclic-GMP in motile and contractile cells-promoting cell migration, and biasing muscle contraction toward relaxation. However, the effects of S-nitrosylation on a host of cytoskeletal proteins and functions remains to be explored.

S-nitrosylation and its role in breast cancer angiogenesis and metastasis

S-nitrosylation, the modification by nitric oxide of free sulfhydryl groups in cysteines, has become an important regulatory mechanism in carcinogenesis and metastasis. S-nitrosylation of targets in tumor cells contributes to metastasis regulating epithelial to mesenchymal transition, migration and invasion. In the tumor environment, the role of S-nitrosylation in endothelium has not been addressed; however, the evidence points out that S-nitrosylation of endothelial proteins may regulate angiogenesis, adhesion of tumor cells to the endothelium, intra and extravasation of tumor cells and contribute to metastasis.

Thioredoxin and glutaredoxin regulate metabolism through different multiplex thiol switches

The aim of the present study was to define the role of Trx and Grx on metabolic thiol redox regulation and identify their protein and metabolite targets. The hepatocarcinoma-derived HepG2 cell line under both normal and oxidative/nitrosative conditions by overexpression of NO synthase (NOS3) was used as experimental model. Grx1 or Trx1 silencing caused conspicuous changes in the redox proteome reflected by significant changes in the reduced/oxidized ratios of specific Cys's including several glycolytic enzymes. Cys⁹¹ of peroxiredoxin-6 (PRDX6) and Cys¹⁵³ of phosphoglycerate mutase-1 (PGAM1), that are known to be involved in progression of tumor growth, are reported here for the first time as specific targets of Grx1. A group of proteins increased their Cys_RED/Cys_OX ratio upon Trx1 and/or Grx1 silencing, including caspase-3 Cys¹⁶³, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) Cys²⁴⁷ and triose-phosphate isomerase (TPI) Cys²⁵⁵ likely by enhancement of NOS3 auto-oxidation. The activities of several glycolytic enzymes were also significantly affected. Glycolysis metabolic flux increased upon Trx1 silencing, whereas silencing of Grx1 had the opposite effect. Diversion of metabolic fluxes toward synthesis of fatty acids and phospholipids was observed in siRNA-Grx1 treated cells, while siRNA-Trx1 treated cells showed elevated levels of various sphingomyelins and ceramides and signs of increased protein degradation. Glutathione synthesis was stimulated by both treatments. These data indicate that Trx and Grx have both, common and specific protein Cys redox targets and that down regulation of either redoxin has markedly different metabolic outcomes. They reflect the delicate sensitivity of redox equilibrium to changes in any of the elements involved and the difficulty of forecasting metabolic responses to redox environmental changes.

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Trx1 and Grx1 Cys redox targets are abundant among Glycolytic enzymes.

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PRDX6-Cys⁹¹ and PGAM-Cys¹⁵³ are specific targets of Grx1.

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Down regulation of thioredoxin and glutaredoxin have different metabolic outcomes.

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Glutathione synthesis and membrane lipid composition are sensitive to Trx1 and Grx1 down regulation.

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Redoxins down regulation also induce target Cys reductive changes under NOS3 overexpression.

S-nitrosocysteine-responsive genes modulate diverse regulatory pathways in Oryza sativa: a transcriptome profiling study

Rice (Oryza sativa L.) is a major food crop and also a well-established genetic model. Nitric oxide (NO) and its derivatives are important signalling molecules that actively participate in various signalling pathways in response to different stresses. In this study, we performed RNA-seq mediated transcriptomic analysis of rice after treatment with the nitric oxide donor, S-nitroso-L-cysteine (CySNO), generating an average of 37.5 and 41.5 million reads from control and treated leaf samples respectively. More than 95% of the reads were successfully mapped to the O. sativa reference genome yielding a total of 33539 differentially expressed genes (DEGs, P < 0.05). Further analyses identified 825 genes with at least 2-fold change in the expression following treatment with CySNO (P < 0.01). The DEGs identified were involved in diverse molecular functions such as catalytic activity, binding, transport, and receptor activity and were mostly located in the membrane, organelles such as nucleus, Golgi apparatus and mitochondria. DEGs also contained several genes that regulate responses to abiotic stresses such as drought, heat, cold and salt stress and biotic stresses. We also found significantly similar expression patterns of CySNO-responsive DEGs of rice with the CySNO-responsive DEGs of Arabidopsis in a previous study. Expression patterns of genes involved in key biological functions were verified using quantitative real time (qRT)-PCR. The findings of this study suggest that NO regulates the transcriptional control of genes involved in a wide variety of physiological functions in rice, and that NO-mediated transcriptional networks are highly conserved across the plant kingdom. This study provides useful information regarding the transcriptional response of plants to nitrosative stress.

Nitric oxide signalling in cardiovascular health and disease

Nitric oxide (NO) signalling has pleiotropic roles in biology and a crucial function in cardiovascular homeostasis. Tremendous knowledge has been accumulated on the mechanisms of the nitric oxide synthase (NOS)-NO pathway, but how this highly reactive, free radical gas signals to specific targets for precise regulation of cardiovascular function remains the focus of much intense research. In this Review, we summarize the updated paradigms on NOS regulation, NO interaction with reactive oxidant species in specific subcellular compartments, and downstream effects of NO in target cardiovascular tissues, while emphasizing the latest developments of molecular tools and biomarkers to modulate and monitor NO production and bioavailability.

Transcriptome profile of NO-induced Arabidopsis transcription factor genes suggests their putative regulatory role in multiple biological processes

TFs are important proteins regulating plant responses during environmental stresses. These insults typically induce changes in cellular redox tone driven in part by promoting the production of reactive nitrogen species (RNS). The main source of these RNS is nitric oxide (NO), which serves as a signalling molecule, eliciting defence and resistance responses. To understand how these signalling molecules regulate key biological processes, we performed a large scale S-nitrosocysteine (CySNO)-mediated RNA-seq analysis. The DEGs were analysed to identify potential regulatory TFs. We found a total of 673 (up- and down-regulated) TFs representing a broad range of TF families. GO-enrichment and MapMan analysis suggests that more than 98% of TFs were mapped to the Arabidopsis thaliana genome and classified into pathways like hormone signalling, protein degradation, development, biotic and abiotic stress, etc. A functional analysis of three randomly selected TFs, DDF1, RAP2.6, and AtMYB48 identified a regulatory role in plant growth and immunity. Loss-of-function mutations within DDF1 and RAP2.6 showed compromised basal defence and effector triggered immunity, suggesting their positive role in two major plant defence systems. Together, these results imply an important data representing NO-responsive TFs that will help in exploring the core mechanisms involved in biological processes in plants.

Identification of S-Nitrosylated and Reversibly Oxidized Proteins by Fluorescence Switch and Complementary Techniques

S-nitrosylation and other reversible oxidative posttranslational modifications of proteins are part of the nonclassical mechanisms of nitric oxide signaling. The biotin switch technique for specifically labeling S-nitrosylated proteins opened the way to proteomic identification of these modifications. Since then, several variations and adaptations of the original method have been applied.We describe here the protocols of several techniques that can be used for the proteomic identification of these modifications, as well as for the detailed characterization of the modification of individual proteins. The fluorescence switch technique allows the proteomic identification of S-nitrosylated proteins based on their fluorescent labeling coupled to electrophoretic separation, as well as the comparison of the overall modification in different samples. The redox fluorescence switch is an adaptation to detect all the proteins reversibly oxidized in cysteine residues. We also describe the protocols of complementary techniques that allow comparing the extent of modification of individual proteins in several conditions by biotin switch, and the identification of modified residues by differential labeling adapted for mass spectrometry identification.

Comprehensive analysis of plant rapid alkalization factor (RALF) genes

Receptor mediated signal carriers play a critical role in the regulation of plant defense and development. Rapid alkalization factor (RALF) proteins potentially comprise important signaling components which may have a key role in plant biology. The RALF gene family contains large number of genes in several plant species, however, only a few RALF genes have been characterized to date. In this study, an extensive database search identified 39, 43, 34 and 18 RALF genes in Arabidopsis, rice, maize and soybean, respectively. These RALF genes were found to be highly conserved across the 4 plant species. A comprehensive analysis including the chromosomal location, gene structure, subcellular location, conserved motifs, protein structure, protein-ligand interaction and promoter analysis was performed. RALF genes from four plant species were divided into 7 groups based on phylogenetic analysis. In silico expression analysis of these genes, using microarray and EST data, revealed that these genes exhibit a variety of expression patterns. Furthermore, RALF genes showed distinct expression patterns of transcript accumulation in vivo following nitrosative and oxidative stresses in Arabidopsis. Predicted interaction between RALF and heme ligand also showed that RALF proteins may contribute towards transporting or scavenging oxygen moieties. This suggests a possible role for RALF genes during changes in cellular redox status. Collectively, our data provides a valuable resource to prime future research in the role of RALF genes in plant growth and development.

Nitric Oxide Mediated Transcriptome Profiling Reveals Activation of Multiple Regulatory Pathways in Arabidopsis thaliana

Imbalance between the accumulation and removal of nitric oxide and its derivatives is a challenge faced by all plants at the cellular level, and is especially important under stress conditions. Exposure of plants to various biotic and abiotic stresses causes rapid changes in cellular redox tone potentiated by the rise in reactive nitrogen species that serve as signaling molecules in mediating defensive responses. To understand mechanisms mediated by these signaling molecules, we performed a large-scale analysis of the Arabidopsis transcriptome induced by nitrosative stress. We generated an average of 84 and 91 million reads from three replicates each of control and 1 mM S-nitrosocysteine (CysNO)-infiltrated Arabidopsis leaf samples, respectively. After alignment, more than 95% of all reads successfully mapped to the reference and 32,535 genes and 55,682 transcripts were obtained. CysNO infiltration caused differential expression of 6436 genes (3448 up-regulated and 2988 down-regulated) and 6214 transcripts (3335 up-regulated and 2879 down-regulated) 6 h post-infiltration. These differentially expressed genes were found to be involved in key physiological processes, including plant defense against various biotic and abiotic stresses, hormone signaling, and other developmental processes. After quantile normalization of the FPKM values followed by student's T-test (P < 0.05) we identified 1165 DEGs (463 up-regulated and 702 down-regulated) with at least 2-folds change in expression after CysNO treatment. Expression patterns of selected genes involved in various biological pathways were verified using quantitative real-time PCR. This study provides comprehensive information about plant responses to nitrosative stress at transcript level and would prove helpful in understanding and incorporating mechanisms associated with nitrosative stress responses in plants.

Hypercysteinemia promotes atherosclerosis by reducing protein S-nitrosylation

Protein S-nitrosylation plays important role in the regulation of cardiovascular functions in nitric oxide (NO) Pathway. Hypercysteinemia (HHcy) is an independently risk factor for atherosclerosis. We hypothesized that HHcy promotes atherosclerosis by reducing level of vascular protein S-nitrosylation. The aim of present study is to investigate effect of HHcy on vascular protein S-nitrosylation. A total of 45 male apoE-/- mice were randomly divided into three groups. The control group was fed a Western-type diet. The HHcy group was fed a diet containing 4.4% L-methionine, and the HHcy+NONOate group was fed a diet containing 4.4% L-methionine and administrated NONOate (ip). Human umbilical vein endothelial cells were performed for in vitro experiment. Plasma lipids were measured every 4 weeks. After 12 weeks, aortic atherosclerotic lesion areas were detected as well as cellular components. The levels of plasma homocysteine (Hcy) and NO were measured. S-nitrosylation was detected using immunofluorescence, and further confirmed by biotin switch method. We found that compared with the control group, Hcy levels, and atherosclerotic plaque, and content of vascular smooth muscle cells and macrophages in lesions significantly increased, and levels of NO significantly decreased in the HHcy group. However, NONOate reverses this effect. In addition, Hcy significantly reduced protein S-nitrosylation in human umbilical vein endothelial cells. This reduction of protein S-nitrosylation was accompanied by reduced levels of NO. Our results suggested that Hcy promoted atherosclerosis by inhibiting vascular protein S-nitrosylation.

Peroxiredoxin post-translational modifications by redox messengers

Peroxiredoxins (Prxs) are a family of thiol peroxidases that participate in hydroperoxide detoxification and regulates H₂O₂ signaling. In mammals, the four typical 2-Cys Prxs (Prxs 1, 2, 3 and 4) are known to regulate H₂O₂-mediated intracellular signaling. The 2 catalytic cysteines of 2-Cys Prxs, the so-called peroxidatic and resolving cysteines, are regulatory switches that are prone to react with redox signaling molecules. We investigated the respective modifications induced by H₂O₂, NO and H₂S in the murine macrophage cell line RAW264.7 by mass spectrometry and immunoblotting after separating 2-Cys Prxs by one-dimensional or two-dimensional PAGE. We found that H₂S, unlike NO, does not prevent H₂O₂-mediated sulfinylation of 2-Cys Prxs and that Prx2 is more sensitive to NO-mediated protection against sulfinylation by peroxides. We also observed that cells exposed to exogenous NO, released by Cys-SNO or DETA-NO, or producing NO upon stimulation by IFN-γ and LPS, present an acidic form of Prx1 whose modification is consistent with S-homocysteinylation of its peroxidatic cysteine.

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NO and H₂O₂ differently modify 2-Cys peroxiredoxins.

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Prx1 is more prone to modifications by NO than the other 2-Cys Prxs.

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NO promotes S-homocysteinylation of Prx1 peroxidatic cysteine.

Global analysis of S-nitrosylation sites in the wild type (APP) transgenic mouse brain-clues for synaptic pathology

Alzheimer's disease (AD) is characterized by an early synaptic loss, which strongly correlates with the severity of dementia. The pathogenesis and causes of characteristic AD symptoms are not fully understood. Defects in various cellular cascades were suggested, including the imbalance in production of reactive oxygen and nitrogen species. Alterations in S-nitrosylation of several proteins were previously demonstrated in various AD animal models and patients. In this work, using combined biotin-switch affinity/nano-LC-MS/MS and bioinformatic approaches we profiled endogenous S-nitrosylation of brain synaptosomal proteins from wild type and transgenic mice overexpressing mutated human Amyloid Precursor Protein (hAPP). Our data suggest involvement of S-nitrosylation in the regulation of 138 synaptic proteins, including MAGUK, CamkII, or synaptotagmins. Thirty-eight proteins were differentially S-nitrosylated in hAPP mice only. Ninety-five S-nitrosylated peptides were identified for the first time (40% of total, including 33 peptides exclusively in hAPP synaptosomes). We verified differential S-nitrosylation of 10 (26% of all identified) synaptosomal proteins from hAPP mice, by Western blotting with specific antibodies. Functional enrichment analysis linked S-nitrosylated proteins to various cellular pathways, including: glycolysis, gluconeogenesis, calcium homeostasis, ion, and vesicle transport, suggesting a basic role of this post-translational modification in the regulation of synapses. The linkage of SNO-proteins to axonal guidance and other processes related to APP metabolism exclusively in the hAPP brain, implicates S-nitrosylation in the pathogenesis of Alzheimer's disease.

Targeted disruption of inducible nitric oxide synthase protects against aging, S-nitrosation, and insulin resistance in muscle of male mice

Accumulating evidence has demonstrated that S-nitrosation of proteins plays a critical role in several human diseases. Here, we explored the role of inducible nitric oxide synthase (iNOS) in the S-nitrosation of proteins involved in the early steps of the insulin-signaling pathway and insulin resistance in the skeletal muscle of aged mice. Aging increased iNOS expression and S-nitrosation of major proteins involved in insulin signaling, thereby reducing insulin sensitivity in skeletal muscle. Conversely, aged iNOS-null mice were protected from S-nitrosation-induced insulin resistance. Moreover, pharmacological treatment with an iNOS inhibitor and acute exercise reduced iNOS-induced S-nitrosation and increased insulin sensitivity in the muscle of aged animals. These findings indicate that the insulin resistance observed in aged mice is mainly mediated through the S-nitrosation of the insulin-signaling pathway.

Nitrosothiols in the immune system: signaling and protection

SIGNIFICANCE

In the immune system, nitric oxide (NO) has been mainly associated with antibacterial defenses exerted through oxidative, nitrosative, and nitrative stress and signal transduction through cyclic GMP-dependent mechanisms. However, S-nitrosylation is emerging as a post-translational modification (PTM) involved in NO-mediated cell signaling.

RECENT ADVANCES

Precise roles for S-nitrosylation in signaling pathways have been described both for innate and adaptive immunity. Denitrosylation may protect macrophages from their own S-nitrosylation, while maintaining nitrosative stress compartmentalized in the phagosomes. Nitrosothiols have also been shown to be beneficial in experimental models of autoimmune diseases, mainly through their role in modulating T-cell differentiation and function.

CRITICAL ISSUES

Relationship between S-nitrosylation, other thiol redox PTMs, and other NO-signaling pathways has not been always taken into account, particularly in the context of immune responses. Methods for assaying S-nitrosylation in individual proteins and proteomic approaches to study the S-nitrosoproteome are constantly being improved, which helps to move this field forward.

FUTURE DIRECTIONS

Integrated studies of signaling pathways in the immune system should consider whether S-nitrosylation/denitrosylation processes are among the PTMs influencing the activity of key signaling and adaptor proteins. Studies in pathophysiological scenarios will also be of interest to put these mechanisms into broader contexts. Interventions modulating nitrosothiol levels in autoimmune disease could be investigated with a view to developing new therapies.

Nitric oxide activates intradomain disulfide bond formation in the kinase loop of Akt1/PKBα after burn injury

Severe burn injury is an acute inflammatory state with massive alterations in gene expression and levels of growth factors, cytokines and free radicals. During the catabolic processes, changes in insulin sensitivity and skeletal muscle wasting (unintended loss of 5–15% of lean body mass) are observed clinically. Here, we reveal a novel molecular mechanism of Akt1/protein kinase Bα (Akt1/PKBα) regulated via cross-talking between dephosphorylation of Thr³⁰⁸ and S-nitrosylation of Cys²⁹⁶ post severe burn injury, which were characterized using nano-LC interfaced with tandem quadrupole time-of-fight mass spectrometry (Q-TOF)^micro tandem mass spectrometry in both in vitro and in vivo studies. For the in vitro studies, Akt1/PKBα was S-nitrosylated with S-nitrosoglutathione and derivatized by three methods. The derivatives were isolated by SDS-PAGE, trypsinized and analyzed by the tandem MS. For the in vivo studies, Akt1/PKBα in muscle lysates from burned rats was immuno-precipitated, derivatized with HPDP-Biotin and analyzed as above. The studies demonstrated that the NO free radical reacts with the free thiol of Cys²⁹⁶ to produce a Cys²⁹⁶-SNO intermediate which accelerates interaction with Cys³¹⁰ to form Cys²⁹⁶-Cys³¹⁰ in the kinase loop. MS/MS sequence analysis indicated that the dipeptide, linked via Cys²⁹⁶-Cys³¹⁰, underwent dephosphorylation at Thr³⁰⁸. These effects were not observed in lysates from sham animals. As a result of this dual effect of burn injury, the loose conformation that is slightly stabilized by the Lys²⁹⁷-Thr³⁰⁸ salt bridge may be replaced by a more rigid structure which may block substrate access. Together with the findings of our previous report concerning mild IRS-1 integrity changes post burn, it is reasonable to conclude that the impaired Akt1/PKBα has a major impact on FOXO3 subcellular distribution and activities.

Peroxiredoxins and sulfiredoxin at the crossroads of the NO and H2O2 signaling pathways

Peroxiredoxins (Prxs) are a family of peroxidases that maintain thiol homeostasis by catalyzing the reduction of organic hydroperoxides, H2O2, and peroxynitrite. Eukaryotic 2-Cys-Prxs, also referred to as typical Prxs, can be inactivated by oxidation of the catalytic cysteine to sulfinic acid, which may regulate the intracellular messenger function of H2O2. A small redox protein, sulfiredoxin (Srx), has been shown to reduce sulfinylated 2-Cys-Prxs and thus to regenerate active 2-Cys-Prxs. We previously reported that cytokine-induced nitric oxide (NO) intervenes in this pathway by decreasing the level of 2-Cys overoxidation and by upregulating Srx through the activation of the transcription factor nuclear factor erythroid 2-related factor (Nrf2). Here, we describe the methods used to monitor the interplay between NO and H2O2 in the regulation of the Prx/Srx system in immunostimulated macrophages, which produce both reactive oxygen species and NO.

Cell signalling by reactive lipid species: new concepts and molecular mechanisms

The process of lipid peroxidation is widespread in biology and is mediated through both enzymatic and non-enzymatic pathways. A significant proportion of the oxidized lipid products are electrophilic in nature, the RLS (reactive lipid species), and react with cellular nucleophiles such as the amino acids cysteine, lysine and histidine. Cell signalling by electrophiles appears to be limited to the modification of cysteine residues in proteins, whereas non-specific toxic effects involve modification of other nucleophiles. RLS have been found to participate in several physiological pathways including resolution of inflammation, cell death and induction of cellular antioxidants through the modification of specific signalling proteins. The covalent modification of proteins endows some unique features to this signalling mechanism which we have termed the ‘covalent advantage’. For example, covalent modification of signalling proteins allows for the accumulation of a signal over time. The activation of cell signalling pathways by electrophiles is hierarchical and depends on a complex interaction of factors such as the intrinsic chemical reactivity of the electrophile, the intracellular domain to which it is exposed and steric factors. This introduces the concept of electrophilic signalling domains in which the production of the lipid electrophile is in close proximity to the thiol-containing signalling protein. In addition, we propose that the role of glutathione and associated enzymes is to insulate the signalling domain from uncontrolled electrophilic stress. The persistence of the signal is in turn regulated by the proteasomal pathway which may itself be subject to redox regulation by RLS. Cell death mediated by RLS is associated with bioenergetic dysfunction, and the damaged proteins are probably removed by the lysosome-autophagy pathway.

SILAM for quantitative proteomics of liver Akt1/PKBα after burn injury

Akt1/protein kinase Bα (Akt1/PKBα) is a downstream mediator of the insulin signaling system. In this study we explored mechanism(s) for its role in burn injury. Akt1/PKBα in liver extracts from mice with burn injury fed with (²H₇)-L-Leu was immunoprecipitated and isolated with SDS-PAGE. Two tryptic peptides, one in the kinase loop and a control peptide just outside of the loop were sequenced via nano-LC interfaced with quadruple time-of-flight tandem mass spectrometry (Q-TOF tandem MS). Their relative isotopologue abundances were determined by stable isotope labeling by amino acids in mammalians (SILAM). Relative quantifications based on paired heavy/light peptides were obtained in 3 steps. The first step included homogenization of mixtures of equal amounts of tissue from burned and sham-treated animals (i.e., isotope dilution) and acquisition of uncorrected data based on parent monoisotopic MS ion ratios. The second step included determination of isotopic enrichment of the kinase from burned mice on Day 7 and the third step enrichment correction of partially labeled heavy and light monoisotopic MS ion ratios for relative quantification of bioactivity (loop peptide) and expression level (control peptide). Protein synthesis and enrichment after injury were found to be dependent on tissue and turnover of individual proteins. Three heavy and light monoisotopic ion ratios for albumin peptides from burned mice indicated ~55% enrichment and ~16.7-fold downregulation. In contract, serum amyloid P had ~66% enrichment and was significantly upregulated. Akt1/PKBα had ~56% enrichment and kinase level in response to the burn injury was upregulated compared with the control peptide. However, kinase bioactivity, represented by the Cys²⁹⁶ peptide, was significantly reduced. Overall, we demonstrated that i) quantitative proteomics can be performed without completely labeled mice; ii) measurement of enrichment of acyl-tRNAs is unnecessary and iii) Cys²⁹⁶ plays an important role in kinase activity after burn injury.

Distinction of thioredoxin transnitrosylation and denitrosylation target proteins by the ICAT quantitative approach

S-Nitrosylation is a reversible PTM for regulating protein function. Thioredoxin-1 (Trx1) catalyzes either transnitrosylation or denitrosylation of specific proteins, depending on the redox status of the cysteines within its conserved oxidoreductase CXXC motif. With a disulfide bond formed between the two catalytic cysteines, Trx1 is not only inactive as a denitrosylase, but it may also be nitrosylated at Cys73 and serve as a transnitrosylating agent. Identification of Trx1-mediated transnitrosylation or denitrosylation targets will contribute to a better understanding of Trx1's function. Previous experimental approaches based on the attenuation of CXXC oxidoreductase activity cannot readily distinguish Trx1 transnitrosylation targets from denitrosylation targets. In this study, we used the ICAT method in conjunction with the biotin switch technique to differentiate Trx1 transnitrosylation targets from denitrosylation target proteins from neuroblastoma cells. We demonstrate that the ICAT approach is effective for quantitative identification of putative Trx1 transnitrosylation and denitrosylation target peptides. From these analyses, we confirmed reports that peroxiredoxin 1 is a Trx1 transnitrosylation, but not a denitrosylation target, and we found several other proteins, including cyclophilin A to be modulated in this manner. Unexpectedly, we found that many nitrosylation sites are reversibly regulated by Trx1, suggesting a more prominent role for Trx1 in regulating S-nitrosylation.

S-nitrosylation of proteins: a new insight into endothelial cell function regulated by eNOS-derived NO

Nitric oxide (NO) is a messenger molecule that is highly diffusible and short-lived. Despite these two characteristics, seemingly unsuitable for intracellular reactions, NO modulates a variety of cellular processes via the mechanism of S-nitrosylation. An important factor that determines the specificity of S-nitrosylation as a signaling mechanism is the compartmentalization of nitric oxide synthase (NOS) with its target proteins. Endothelial NOS (eNOS) is unique among the NOS family members by being localized mainly near specific intracellular membrane domains including the cytoplasmic face of the Golgi apparatus and plasma membrane caveolae. Nitric oxide produced by eNOS localized on the Golgi apparatus can react with thiol groups on nearby Golgi proteins via a redox mechanism resulting in S-nitrosylation of these proteins. This modification influences their function as regulators of cellular processes such as protein trafficking (e.g., exocytosis and endocytosis), redox state, and cell cycle. Thus, eNOS-derived NO regulates a wide range of endothelial cell functions, such as inflammation, apoptosis, permeability, migration, and cell growth.

The disulfide proteome and other reactive cysteine proteomes: analysis and functional significance

Ten years ago, proteomics techniques designed for large-scale investigations of redox-sensitive proteins started to emerge. The proteomes, defined as sets of proteins containing reactive cysteines that undergo oxidative post-translational modifications, have had a particular impact on research concerning the redox regulation of cellular processes. These proteomes, which are hereafter termed "disulfide proteomes," have been studied in nearly all kingdoms of life, including animals, plants, fungi, and bacteria. Disulfide proteomics has been applied to the identification of proteins modified by reactive oxygen and nitrogen species under stress conditions. Other studies involving disulfide proteomics have addressed the functions of thioredoxins and glutaredoxins. Hence, there is a steadily growing number of proteins containing reactive cysteines, which are probable targets for redox regulation. The disulfide proteomes have provided evidence that entire pathways, such as glycolysis, the tricarboxylic acid cycle, and the Calvin-Benson cycle, are controlled by mechanisms involving changes in the cysteine redox state of each enzyme implicated. Synthesis and degradation of proteins are processes highly represented in disulfide proteomes and additional biochemical data have established some mechanisms for their redox regulation. Thus, combined with biochemistry and genetics, disulfide proteomics has a significant potential to contribute to new discoveries on redox regulation and signaling.

Identification of S-nitrosylation of proteins of Helicobacter pylori in response to nitric oxide stress

Innate and adaptive immune responses are activated in humans when Helicobacter pylori invades the gastric mucosa. Nitric oxide (NO) and reactive nitrogen species are important immune effectors, which can exert their functions through oxidation and S-nitrosylation of proteins. S-nitrosoglutathione and sodium nitroprus-side were used as NO donors and H. pylori cells were incubated with these compounds to analyze the inhibitory effect of NO. The suppressing effect of NO on H. pylori has been shown in vitro. Furthermore, the proteins modified by S-nitrosylation in H. pylori were identified through the biotin switch method in association with matrix-assisted laser desorption ionization/time-of-flight tandem mass spectrometry (MALDI-TOF-MS/MS). Five S-nitrosylated proteins identified were a chaperone and heat-shock protein (GroEL), alkyl hydroperoxide reductase (TsaA), urease alpha subunit (UreA), HP0721, and HP0129. Importantly, S-nitrosylation of TsaA and UreA were confirmed using purified recombinant proteins. Considering the importance of these enzymes in antioxidant defenses, adherence, and colonization, NO may exert its antibacterial actions by targeting enzymes through S-nitrosylation. Identification of protein S-nitrosylation may contribute to an understanding of the antibacterial actions of NO. Our findings provide an insight into potential targets for the development of novel therapeutic agents against H. pylori infection.

S-alkylating labeling strategy for site-specific identification of the s-nitrosoproteome

S-nitrosylation, a post-translational modification of cysteine residues induced by nitric oxide, mediates many physiological functions. Due to the labile nature of S-nitrosylation, detection by mass spectrometry (MS) is challenging. Here, we developed an S-alkylating labeling strategy using the irreversible biotinylation on S-nitrosocysteines for site-specific identification of the S-nitrosoproteome by LC-MS/MS. Using COS-7 cells without endogenous nitric oxide synthase, we demonstrated that the S-alkylating labeling strategy substantially improved the blocking efficiency of free cysteines, minimized the false-positive identification caused by disulfide interchange, and increased the digestion efficiency for improved peptide identification using MS analyses. Using this strategy, we identified total 586 unique S-nitrosylation sites corresponding to 384 proteins in S-nitroso-N-acetylpenicillamine (SNAP)/l-cysteine-treated mouse MS-1 endothelial cells, including 234 previously unreported S-nitrosylated proteins. When the topologies of 84 identified transmembrane proteins were further analyzed, their S-nitrosylation sites were found to mostly face the cytoplasmic side, implying that S-nitrosylation occurs in the cytoplasm. In addition to the previously known acid/basic motifs, the ten deduced consensus motifs suggested that combination of local hydrophobicity and acid/base motifs in the tertiary structure contribute to the specificity of S-nitrosylation. Moreover, the S-nitrosylated cysteines showed preference on beta-strand, having lower relative surface accessibility at the S-nitrosocysteines.

Redox regulatory mechanism of transnitrosylation by thioredoxin

Transnitrosylation and denitrosylation are emerging as key post-translational modification events in regulating both normal physiology and a wide spectrum of human diseases. Thioredoxin 1 (Trx1) is a conserved antioxidant that functions as a classic disulfide reductase. It also catalyzes the transnitrosylation or denitrosylation of caspase 3 (Casp3), underscoring its central role in determining Casp3 nitrosylation specificity. However, the mechanisms that regulate Trx1 transnitrosylation and denitrosylation of specific targets are unresolved. Here we used an optimized mass spectrometric method to demonstrate that Trx1 is itself nitrosylated by S-nitrosoglutathione at Cys(73) only after the formation of a Cys(32)-Cys(35) disulfide bond upon which the disulfide reductase and denitrosylase activities of Trx1 are attenuated. Following nitrosylation, Trx1 subsequently transnitrosylates Casp3. Overexpression of Trx1(C32S/C35S) (a mutant Trx1 with both Cys(32) and Cys(35) replaced by serine to mimic the disulfide reductase-inactive Trx1) in HeLa cells promoted the nitrosylation of specific target proteins. Using a global proteomics approach, we identified 47 novel Trx1 transnitrosylation target protein candidates. From further bioinformatics analysis of this set of nitrosylated peptides, we identified consensus motifs that are likely to be the determinants of Trx1-mediated transnitrosylation specificity. Among these proteins, we confirmed that Trx1 directly transnitrosylates peroxiredoxin 1 at Cys(173) and Cys(83) and protects it from H(2)O(2)-induced overoxidation. Functionally, we found that Cys(73)-mediated Trx1 transnitrosylation of target proteins is important for protecting HeLa cells from apoptosis. These data demonstrate that the ability of Trx1 to transnitrosylate target proteins is regulated by a crucial stepwise oxidative and nitrosative modification of specific cysteines, suggesting that Trx1, as a master regulator of redox signaling, can modulate target proteins via alternating modalities of reduction and nitrosylation.

Identification of S-nitrosylated targets of thioredoxin using a quantitative proteomic approach

Reversible protein cysteine nitrosylation (S-nitrosylation) is a common mechanism utilized in signal transduction and other diverse cellular processes. Protein denitrosylation is largely mediated by cysteine denitrosylases, but the functional scope and significance of these enzymes are incompletely defined, in part due to limited information on their cognate substrates. Here, using Jurkat cells, we employed stable isotope labeling by amino acids in cell culture (SILAC), coupled to the biotin switch technique and mass spectrometry, to identify 46 new substrates of one denitrosylase, thioredoxin 1. These substrates are involved in a wide range of cellular functions including cytoskeletal organization, cellular metabolism, signal transduction, and redox homeostasis. We also identified multiple S-nitrosylated proteins that are not substrates of thioredoxin 1. A verification of our principal findings was made in a second cell type (RAW264.7 cells). Our results point to thioredoxin 1 as a major protein denitrosylase in mammalian cells and demonstrate the utility of quantitative proteomics for large-scale identification of denitrosylase substrates.

S-nitrosoproteome in endothelial cells revealed by a modified biotin switch approach coupled with Western blot-based two-dimensional gel electrophoresis

NO-mediated S-nitrosation of cysteine residues has been recognized as a fundamental post-translational modification. S-Nitrosation of endothelial cell (EC) proteins can alter function and affect vascular homeostasis. Trace amounts of S-nitrosoproteins in endothelial cells (ECs) in vivo coupled with lability of the S-nitroso bond have hindered a comprehensive characterization. We demonstrate a convenient and reliable method, requiring minimal sample, for the screening and identification of S-nitrosoproteins. ECs treated with the NO donor S-nitroso-N-acetylpenicillamine (SNAP) were subjected to the biotin switch method of labeling, then detected by analytical Western blot-based two-dimensional gel electrophoresis (2-DE). More than 89 SNAP-increased S-nitrosoproteins were detected and 28 of these were successfully excised from preparative 2-DE gel and identified by LC-MS/MS. Moreover, the nitrosocysteine residue for each protein (HSPA9/368, beta-actin/16, TMP3/170, vimentin/328) was also determined, and the relative ratio of S-nitrosation/non-S-nitrosation for Cys328 of vimentin was estimated using MASIC software. By the combination of the biotin switch method with 2-DE and Western blot analysis, S-nitrosoproteins can be screened and characterized by MS, providing a basis for further study of the physiological significance of each S-nitrosoproteins.

Aspirin attenuates insulin resistance in muscle of diet-induced obese rats by inhibiting inducible nitric oxide synthase production and S-nitrosylation of IRbeta/IRS-1 and Akt

AIM/HYPOTHESIS

High-dose aspirin treatment improves fasting and postprandial hyperglycaemia in patients with type 2 diabetes, as well as in animal models of insulin resistance associated with obesity and sepsis. In this study, we investigated the effects of aspirin treatment on inducible nitric oxide synthase (iNOS)-mediated insulin resistance and on S-nitrosylation of insulin receptor (IR)-beta, IRS-1 and protein kinase B (Akt) in the muscle of diet-induced obese rats and also in iNos (also known as Nos2)-/- mice on high fat diet.

METHODS

Aspirin (120 mg kg-1 day-1 for 2 days) or iNOS inhibitor (L-NIL; 80 mg/kg body weight) were administered to diet-induced obese rats or mice and iNOS production and insulin signalling were investigated. S-nitrosylation of IRbeta/IRS-1 and Akt was investigated using the biotin switch method.

RESULTS

iNOS protein levels increased in the muscle of diet-induced obese rats, associated with an increase in S-nitrosylation of IRbeta, IRS-1 and Akt. These alterations were reversed by aspirin treatment, in parallel with an improvement in insulin signalling and sensitivity, as measured by insulin tolerance test and glucose clamp. Conversely, while aspirin reversed the increased phosphorylation of IkappaB kinase beta and c-Jun amino-terminal kinase, as well as IRS-1 serine phosphorylation in diet-induced obese rats and iNos -/- mice on high-fat diet, these alterations were not associated with the improvement of insulin action induced by this drug.

CONCLUSIONS/INTERPRETATION

Our data demonstrate that aspirin treatment not only reduces iNOS protein levels, but also S-nitrosylation of IRbeta, IRS-1 and Akt. These changes are associated with improved insulin resistance and signalling, suggesting a novel mechanism of insulin sensitisation evoked by aspirin treatment.

Structural analysis of cysteine S-nitrosylation: a modified acid-based motif and the emerging role of trans-nitrosylation

S-Nitrosylation, the selective and reversible addition of nitric oxide (NO) moiety to cysteine (Cys) sulfur in proteins, regulates numerous cellular processes. In recent years, proteomic approaches that are capable of identifying nitrosylated Cys residues have been developed. However, the features underlying the specificity of Cys modification with NO remain poorly defined. Previous studies suggested that S-nitrosylated Cys may be flanked by an acid-base motif or hydrophobic areas and show high reactivity, low pK(a), and high sulfur atom exposure. In the current study, we prepared an extensive, manually curated data set of proteins with S-nitrosothiols, accounting for a variety of biochemical functions, organisms of origin, and physiological responses to NO. Analysis of this generic NO-Cys data set revealed that proximal acid-base motif, Cys pK(a), sulfur atom exposure, and Cys conservation or hydrophobicity in the vicinity of the modified Cys do not define the specificity of S-nitrosylation. Instead, this analysis revealed a revised acid-base motif, which is located more distantly to the Cys and has its charged groups exposed. We hypothesize that, rather than being strictly used for direct activation of Cys, the modified acid-base motif is engaged in protein-protein interactions thereby contributing to trans-nitrosylation as an important and widespread mechanism for reversible modification of Cys with NO moiety. For proteins lacking the revised motif, we discuss alternative mechanisms including a potential role of nitrosoglutathione as a trans-acting agent.

Unraveling the S-nitrosoproteome: tools and strategies

One of the major tasks to be accomplished in the postgenomic era is the characterization of PTMs in proteins. The S-nitrosation of protein thiols is a redox-based PTM that modulating enzymatic activity, subcellular localization, complex formation, and degradation of proteins, largely contributes to the complexity of cellular proteomes. Although the detection of S-nitrosated proteins is problematical due to the lability of S-nitrosothiols, with the improvement of molecular tools an increasing range of proteins has been shown to undergo S-nitrosation. We here review recent proteomic approaches for the systematic assessment of potential targets for protein S-nitrosation. The development of new analytical methods and strategies over the past several years now allows us to investigate the nitrosoproteome on a global scale.

Shear flow increases S-nitrosylation of proteins in endothelial cells

AIMS

Endothelial cells (ECs) constantly exposed to shear flow increase nitric oxide production via the activation of endothelial nitric oxide synthase. Nitric oxide-mediated S-nitrosylation has recently been identified as an important post-translational modification that may alter signalling and/or protein function. S-nitrosylation of endothelial proteins after shear flow treatment has not been fully explored. In this study, the CyDye switch method was utilized to examine S-nitrosylated proteins in ECs after exposure to shear flow.

METHODS AND RESULTS

Human umbilical vein ECs were subjected to shear flow for 30 min, and S-nitrosylated proteins were detected by the CyDye switch method. In principle, free thiols in proteins become blocked by alkylation, the S-nitrosylated bond is reduced by ascorbate, and then CyDye labels proteins. Proteins that separately labelled with Cy3 or Cy5 were mixed and subjected to two-dimensional gel electrophoresis for further analysis. More than 100 S-nitrosoproteins were detected in static and shear-treated ECs. Among these, 12 major proteins of heterogeneous function showed a significant increase in S-nitrosylation following shear flow. The S-nitrosylated residues in tropomyosin and vimentin, which were localized in the hydrophobic motif of each protein, were identified as Cys170 and Cys328, respectively.

CONCLUSION

Post-translational S-nitrosylation of proteins in ECs can be detected by a reliable CyDye switch method. This flow-induced S-nitrosylation of endothelial proteins may be essential for the adaptation and remodelling of ECs under flow conditions.

Redox control of platelet function

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

S-Nitrosylation of the epidermal growth factor receptor: a regulatory mechanism of receptor tyrosine kinase activity

Nitric oxide (NO) donors inhibit the epidermal growth factor (EGF)-dependent auto(trans)phosphorylation of the EGF receptor (EGFR) in several cell types in which NO exerts antiproliferative effects. We demonstrate in this report that NO inhibits, whereas NO synthase inhibition potentiates, the EGFR tyrosine kinase activity in NO-producing cells, indicating that physiological concentrations of NO were able to regulate the receptor activity. Depletion of intracellular glutathione enhanced the inhibitory effect of the NO donor 1,1-diethyl-2-hydroxy-2-nitrosohydrazine (DEA/NO) on EGFR tyrosine kinase activity, supporting the notion that such inhibition was a consequence of an S-nitrosylation reaction. Addition of DEA/NO to cell lysates resulted in the S-nitrosylation of a large number of proteins including the EGFR, as confirmed by the chemical detection of nitrosothiol groups in the immunoprecipitated receptor. We prepared a set of seven EGFR(C --> S) substitution mutants and demonstrated in transfected cells that the tyrosine kinase activity of the EGFR(C166S) mutant was completely resistant to NO, whereas the EGFR(C305S) mutant was partially resistant. In the presence of EGF, DEA/NO significantly inhibited Akt phosphorylation in cells transfected with wild-type EGFR, but not in those transfected with C166S or C305S mutants. We conclude that the EGFR can be posttranslationally regulated by reversible S-nitrosylation of C166 and C305 in living cells.

Nitrosothiol reactivity profiling identifies S-nitrosylated proteins with unexpected stability

Nitric oxide (NO) regulates protein function by S-nitrosylation of cysteine to form nitrosothiols. Nitrosothiols are highly susceptible to nonenzymatic degradation by cytosolic reducing agents. Here we show that although most protein nitrosothiols are rapidly degraded by cytosolic reductants, a small subset form unusually stable S-nitrosylated proteins. Our findings suggest that stable S-nitrosylation reflects a protein conformation change that shields the nitrosothiol. To identify stable protein nitrosothiols, we developed a proteomic method for profiling S-nitrosylation. We examined the stability of over 100 S-nitrosylated proteins, and identified 10 stable nitrosothiols. These proteins remained S-nitrosylated in cells after NO synthesis was inhibited, unlike most S-nitrosylated proteins. Taken together, our data identify a class of NO targets that form stable nitrosothiols in the cell and are likely to mediate the persistent cellular effects of NO.

Proteomic analysis of protein S-nitrosylation

Nitric oxide (NO) produces covalent PTMs of specific cysteine residues, a process known as S-nitrosylation. This route is dynamically regulated and is one of the major NO signalling pathways known to have strong and dynamic interactions with redox signalling. In agreement with this scenario, binding of NO to key cysteine groups can be linked to a broad range of physiological and pathological cellular events, such as smooth muscle relaxation, neurotransmission and neurodegeneration. The characterization of S-nitrosylated residues and the functional relevance of this protein modification are both essential information needed to understand the action of NO in living organisms. In this review, we focus on recent advances in this field and on state-of-the-art proteomic approaches which are aimed at characterizing the S-nitrosylome in different biological backgrounds.

Selective fluorescent labeling of S-nitrosothiols (S-FLOS): a novel method for studying S-nitrosation

Protein S-nitrosation is a reversible post-translation modification critical for redox-sensitive cell signaling that is typically studied using the Biotin Switch method. This method and subsequent modifications usually require avidin binding or Western blot analysis to detect biotin labeled proteins. We describe here a modification of the Biotin Switch assay that eliminates the need for Western blot or avidin enrichment protocols and allows direct comparison of the S-nitrosation state proteins from two different samples in the same gel lane or on the same 2D gel. This S-FLOS method offers detection, identification and quantification of S-nitrosated proteins, with the potential for site-specific identification of nitrosation events.

Alteration of S-nitrosothiol homeostasis and targets for protein S-nitrosation in human hepatocytes

The liver is one organ clearly influenced by nitric oxide (NO), and acute and chronic exposure to this substance has been associated with distinct patterns of liver disease. Disruption or deregulation of S-nitrosothiol (SNO) signalling leads to impairment of cellular function and disease, and this study was aimed to identify potential targets for protein S-nitrosation during alteration of SNO homeostasis in human hepatocytes. Cells were treated with S-nitroso-L-cysteine (CSNO), an effective physiological nitrosothiol for delivering NO bioactivity to cells. Treatment with CSNO augmented the levels of S-nitrosoproteins detected both by chemiluminescence and the biotin switch method. CSNO treatment also increased S-nitrosoglutathione reductase (GSNOR) activity that returned SNO content to basal levels. This increased enzymatic activity was related to augmented levels of ADH-5 mRNA, the gene encoding for GSNOR in humans. In addition, the treatment with the SNO also increased cell death. Twenty S-nitrosoproteins were identified in CSNO-treated hepatocytes, including mitochondrial aldehyde dehydrogenase, protein disulphide isomerase, Hsp60, GRP75 and Raf kinase inhibitor protein. The identification in the S-nitrosatable proteome of proteins involved in metabolism, maintenance of cellular homeostasis and signalling points to the relevance of protein S-nitrosation to the physiology and pathophysiology of human hepatocytes.