Nitric oxide and the thioredoxin system: a complex interplay in redox regulation

S-Glutathionylation and S-Nitrosylation as Modulators of Redox-Dependent Processes in Cancer Cell

Development of oxidative/nitrosative stress associated with the activation of oncogenic pathways results from the increase in the generation of reactive oxygen and nitrogen species (ROS/RNS) in tumor cells, where they can have a dual effect. At high concentrations, ROS/RNS cause cell death and limit tumor growth at certain phases of its development, while their low amounts promote oxidative/nitrosative modifications of key redox-dependent residues in regulatory proteins. The reversibility of such modifications as S-glutathionylation and S-nitrosylation that proceed through the electrophilic attack of ROS/RNS on nucleophilic Cys residues ensures the redox-dependent switch in the activity of signaling proteins, as well as the ability of these compounds to control cell proliferation and programmed cell death. The content of S-glutathionylated and S-nitrosylated proteins is controlled by the balance between S-glutathionylation/deglutathionylation and S-nitrosylation/denitrosylation, respectively, and depends on the cellular redox status. The extent of S-glutathionylation and S-nitrosylation of protein targets and their ratio largely determine the status and direction of signaling pathways in cancer cells. The review discusses the features of S-glutathionylation and S-nitrosylation reactions and systems that control them in cancer cells, as well as their relationship with redox-dependent processes and tumor growth.

Thioredoxin Signaling Pathways in Cancer

Significance: Thioredoxin (Trx) is a powerful antioxidant that reduces protein disulfides to maintain redox stability in cells and is involved in regulating multiple redox-dependent signaling pathways. Recent Advance: The current accumulation of findings suggests that Trx participates in signaling pathways that interact with various proteins to manipulate their dynamic regulation of structure and function. These network pathways are critical for cancer pathogenesis and therapy. Promising clinical advances have been presented by most anticancer agents targeting such signaling pathways. Critical Issues: We herein link the signaling pathways regulated by the Trx system to potential cancer therapeutic opportunities, focusing on the coordination and strengths of the Trx signaling pathways in apoptosis, ferroptosis, immunomodulation, and drug resistance. We also provide a mechanistic network for the exploitation of therapeutic small molecules targeting the Trx signaling pathways. Future Directions: As research data accumulate, future complex networks of Trx-related signaling pathways will gain in detail. In-depth exploration and establishment of these signaling pathways, including Trx upstream and downstream regulatory proteins, will be critical to advancing novel cancer therapeutics. Antioxid. Redox Signal. 38, 403-424.

Evaluation of the Effect of the Fibroblast Growth Factor Type 2 (FGF-2) Administration on Placental Gene Expression in a Murine Model of Preeclampsia Induced by L-NAME

The abnormal implantation of the trophoblast during the first trimester of pregnancy precedes the appearance of the clinical manifestations of preeclampsia (PE), which is a hypertensive disorder of pregnancy. In a previous study, which was carried out in a murine model of PE that was induced by NG-nitro-L-arginine methyl ester (L-NAME), we observed that the intravenous administration of fibroblast growth factor 2 (FGF2) had a hypotensive effect, improved the placental weight gain and attenuated the fetal growth restriction, and the morphological findings that were induced by L-NAME in the evaluated tissues were less severe. In this study, we aimed to determine the effect of FGF2 administration on the placental gene expression of the vascular endothelial growth factor (VEGFA), VEGF receptor 2 (VEGFR2), placental growth factor, endoglin (ENG), superoxide dismutase 1 (SOD1), catalase (CAT), thioredoxin (TXN), tumor protein P53 (P53), BCL2 apoptosis regulator, Fas cell surface death receptor (FAS), and caspase 3, in a Sprague Dawley rat PE model, which was induced by L-NAME. The gene expression was determined by a real-time polymerase chain reaction using SYBR green. Taking the vehicle or the L-NAME group as a reference, there was an under expression of placental VEGFA, VEGFR2, ENG, P53, FAS, SOD1, CAT, and TXN genes in the group of L-NAME + FGF2 (p < 0.05). The administration of FGF2 in the murine PE-like model that was induced by L-NAME reduced the effects that were generated by proteinuria and the increased BP, as well as the response of the expression of genes that participate in angiogenesis, apoptosis, and OS. These results have generated valuable information regarding the identification of molecular targets for PE and provide new insights for understanding PE pathogenesis.

New insights on selenoproteins and neuronal function

Fifty years have passed since the discovery of the first selenoprotein by Rotruck and colleagues. In that time, the essential nature of selenium has come to light including the dependence of the brain on selenium to function properly. Animal models have shown that a lack of certain selenoproteins in the brain is detrimental for neuronal health, sometimes leading to neurodegeneration. There is also potential for selenoprotein-mediated redox balance to impact neuronal activity, including neurotransmission. Important insights on these topics have been gained over the past several years. This review briefly summarizes the known roles of specific selenoproteins in the brain while highlighting recent advancements regarding selenoproteins in neuronal function. Hypothetical models of selenoprotein function and emerging topics in the field are also provided.

Deciphering the Path of S-nitrosation of Human Thioredoxin: Evidence of an Internal NO Transfer and Implication for the Cellular Responses to NO

Nitric oxide (NO) is a free radical with a signaling capacity. Its cellular functions are achieved mainly through S-nitrosation where thioredoxin (hTrx) is pivotal in the S-transnitrosation to specific cellular targets. In this study, we use NMR spectroscopy and mass spectrometry to follow the mechanism of S-(trans)nitrosation of hTrx. We describe a site-specific path for S-nitrosation by measuring the reactivity of each of the 5 cysteines of hTrx using cysteine mutants. We showed the interdependence of the three cysteines in the nitrosative site. C73 is the most reactive and is responsible for all S-transnitrosation to other cellular targets. We observed NO internal transfers leading to C62 S-nitrosation, which serves as a storage site for NO. C69-SNO only forms under nitrosative stress, leading to hTrx nuclear translocation.

Apoptosis signal-regulating kinase 1 (ASK1) inhibition reduces endothelial cytokine production without improving permeability after toll-like receptor 4 (TLR4) challenge

Sepsis represents a life-threatening event often mediated by the host's response to pathogens such as gram-negative organisms, which release the proinflammatory lipopolysaccharide (LPS). Within the endothelium, the mitogen-activated protein kinase (MAPK) pathway is an important driver of endothelial injury during sepsis, of which oxidant-sensitive apoptosis signal-regulating kinase 1 (ASK1) is postulated to be a critical upstream regulator. We hypothesized that ASK1 would play a key role in endothelial inflammation during bacterial challenge. Utilizing RNA sequencing data from patients and cultured human microvascular endothelial cells (HMVECs), ASK1 expression was increased in sepsis and after LPS challenge. Two ASK1 inhibitors, GS444217 and MSC2023964A, reduced cytokine production in HMVECs following LPS stimulation, but had no effect on permeability as measured by transendothelial electrical resistance and intercellular space. MAPKs are known to interact with endothelial nitric oxide synthase (eNOS) and ASK1 expression levels correlated with eNOS expression in patients with septic shock. In addition, eNOS physically interacted with ASK1, though this interaction was not altered by ASK1 inhibition, nor did inhibition alter MAPK p38 activity. Instead, among MAPKs, ASK1 inhibition only impaired LPS-induced JNK phosphorylation. The reduction in JNK activation caused by ASK1 inhibition impaired JNK-mediated cytokine production without affecting permeability. Thus, LPS triggers JNK-dependent cytokine production that requires ASK1 activation, but both its effects on permeability and activation of p38 are ASK1-independent. These data demonstrate how distinct MAPK signaling pathways regulate endothelial inflammatory outputs during acute infectious challenge.

Molecular characterization and immune regulatory, antioxidant, and antiapoptotic activities of thioredoxin domain-containing protein 17 (TXNDC17) in yellowtail clownfish (Amphiprion clarkii)

Thioredoxin domain-containing protein 17 (TXNDC17) is an important, highly conserved oxidoreductase protein, ubiquitously expressed in all living organisms. It is a small (~14 kDa) protein mostly co-expressed with thioredoxin 1 (TRx1). In the present study, we obtained the TXNDC17 gene sequence from a previously constructed yellowtail clownfish (Amphiprion clarkii) (AcTXNDC17) database and studied its phylogeny as well as the protein's molecular characteristics, antioxidant, and antiapoptotic effects. The full length of the AcTXNDC17 cDNA sequence was 862 bp with a 372 bp region encoding a 123 amino acid (aa) protein. The predicted molecular mass and isoelectric point of AcTXNDC17 were 14.2 kDa and 5.75, respectively. AcTXNDC17 contained a TRX-related protein 14 domain and a highly conserved N-terminal Cys⁴³-Pro⁴⁴-Asp⁴⁵-Cys⁴⁶ motif. qPCR analysis revealed that AcTXNDC17 transcripts were ubiquitously and differently expressed in all the examined tissues. AcTXNDC17 expression in the spleen tissue was significantly upregulated in a time-dependent manner upon stimulation with lipopolysaccharide (LPS), polyinosinic-polycytidylic (poly I:C), and Vibrio harveyi. Besides, LPS-induced intrinsic apoptotic pathway (TNF-α, caspase-8, Bid, cytochrome C, caspase-9, and caspase-3) gene expression was significantly lower in AcTXNDC17-overexpressing RAW264.7 cells, as were NF-κB activation and nitric oxide (NO) production. Furthermore, the viability of H₂O₂-stimulated macrophages was significantly improved under AcTXNDC17 overexpression. Collectively, our findings indicate that AcTXNDC17 is involved in the innate immune response of the yellowtail clownfish.

Tracing the path of inhaled nitric oxide: Biological consequences of protein nitrosylation

Nitric oxide (NO) is a comprehensive regulator of vascular and airway tone. Endogenous NO produced by nitric oxide synthases regulates multiple signaling cascades, including activation of soluble guanylate cyclase to generate cGMP, relaxing smooth muscle cells. Inhaled NO is an established therapy for pulmonary hypertension in neonates, and has been recently proposed for the treatment of hypoxic respiratory failure and acute respiratory distress syndrome due to COVID-19. In this review, we summarize the effects of endogenous and exogenous NO on protein S-nitrosylation, which is the selective and reversible covalent attachment of a nitrogen monoxide group to the thiol side chain of cysteine. This posttranslational modification targets specific cysteines based on the acid/base sequence of surrounding residues, with significant impacts on protein interactions and function. S-nitrosothiol (SNO) formation is tightly compartmentalized and enzymatically controlled, but also propagated by nonenzymatic transnitrosylation of downstream protein targets. Redox-based nitrosylation and denitrosylation pathways dynamically regulate the equilibrium of SNO-proteins. We review the physiological roles of SNO proteins, including nitrosohemoglobin and autoregulation of blood flow through hypoxic vasodilation, and pathological effects of nitrosylation including inhibition of critical vasodilator enzymes; and discuss the intersection of NO source and dose with redox environment, in determining the effects of protein nitrosylation.

Glutathione in Protein Redox Modulation through S-Glutathionylation and S-Nitrosylation

S-glutathionylation and S-nitrosylation are reversible post-translational modifications on the cysteine thiol groups of proteins, which occur in cells under physiological conditions and oxidative/nitrosative stress both spontaneously and enzymatically. They are important for the regulation of the functional activity of proteins and intracellular processes. Connecting link and “switch” functions between S-glutathionylation and S-nitrosylation may be performed by GSNO, the generation of which depends on the GSH content, the GSH/GSSG ratio, and the cellular redox state. An important role in the regulation of these processes is played by Trx family enzymes (Trx, Grx, PDI), the activity of which is determined by the cellular redox status and depends on the GSH/GSSG ratio. In this review, we analyze data concerning the role of GSH/GSSG in the modulation of S-glutathionylation and S-nitrosylation and their relationship for the maintenance of cell viability.

S-nitrosocysteine and glutathione depletion synergize to induce cell death in human tumor cells: Insights into the redox and cytotoxic mechanisms

Nitric oxide (NO)-dependent signaling and cytotoxic effects are mediated in part via protein S-nitrosylation. The magnitude and duration of S-nitrosylation are governed by the two main thiol reducing systems, the glutathione (GSH) and thioredoxin (Trx) antioxidant systems. In recent years, approaches have been developed to harness the cytotoxic potential of NO/nitrosylation to inhibit tumor cell growth. However, progress in this area has been hindered by insufficient understanding of the balance and interplay between cellular nitrosylation, other oxidative processes and the GSH/Trx systems. In addition, the mechanistic relationship between thiol redox imbalance and cancer cell death is not fully understood. Herein, we explored the redox and cellular effects induced by the S-nitrosylating agent, S-nitrosocysteine (CysNO), in GSH-sufficient and -deficient human tumor cells. We used l-buthionine-sulfoximine (BSO) to induce GSH deficiency, and employed redox, biochemical and cellular assays to interrogate molecular mechanisms. We found that, under GSH-sufficient conditions, a CysNO challenge (100-500 μM) results in a marked yet reversible increase in protein S-nitrosylation in the absence of appreciable S-oxidation. In contrast, under GSH-deficient conditions, CysNO induces elevated and sustained levels of both S-nitrosylation and S-oxidation. Experiments in various cancer cell lines showed that administration of CysNO or BSO alone commonly induce minimal cytotoxicity whereas BSO/CysNO combination therapy leads to extensive cell death. Studies in HeLa cancer cells revealed that treatment with BSO/CysNO results in dual inhibition of the GSH and Trx systems, thereby amplifying redox stress and causing cellular dysfunction. In particular, BSO/CysNO induced rapid oxidation and collapse of the actin cytoskeletal network, followed by loss of mitochondrial function, leading to profound and irreversible decrease in ATP levels. Further observations indicated that BSO/CysNO-induced cell death occurs via a caspase-independent mechanism that involves multiple stress-induced pathways. The present findings provide new insights into the relationship between cellular nitrosylation/oxidation, thiol antioxidant defenses and cell death. These results may aid future efforts to develop NO/redox-based anticancer approaches.

Slight Deuterium Enrichment in Water Acts as an Antioxidant: Is Deuterium a Cell Growth Regulator?

Small admixtures in water, e.g. of metal ions, often act as cell growth regulators. Here we report that enrichment of deuterium content in water, normally found at 8 mm concentration, two-three folds increases cell proliferation and lowers the oxidative stress level as well. Acting as an anti-oxidant, deuterium-enriched water prevents the toxic effect of such oxidative agents as hydrogen peroxide and auranofin. This action is opposite to that of deuterium depletion that is known to suppress cell growth and induce oxidative stress in mitochondria. We thus hypothesize that deuterium may be a natural cell growth regulator that controls mitochondrial oxidation-reduction balance. Because growth acceleration is reduced approximately by half by addition to water a minute amount (0.15%) of ¹⁸O isotope, at least part of the deuterium effect on cell growth can be explained by the isotopic resonance phenomenon. A slight (≈2-fold) enrichment of deuterium in water accelerates human cell growth. Quantitative MS based proteomics determined changes in protein abundances and redox states and found that deuterium-enriched water acts mainly through decreasing ROS production in mitochondria. This action is opposite to that of deuterium depletion that suppresses cell growth by inducing oxidative stress. Thus deuterium may be a natural cell growth regulator that controls mitochondrial oxidation-reduction balance. The role of isotopic resonance in this effect was validated by further experiments on bacteria.

ROS-associated immune response and metabolism: a mechanistic approach with implication of various diseases

The immune system plays a pivotal role in maintaining the defense mechanism against external agents and also internal danger signals. Metabolic programming of immune cells is required for functioning of different subsets of immune cells under different physiological conditions. The field of immunometabolism has gained ground because of its immense importance in coordination and balance of immune responses. Metabolism is very much related with production of energy and certain by-products. Reactive oxygen species (ROS) are generated as one of the by-products of various metabolic pathways. The amount, localization of ROS and redox status determine transcription of genes, and also influences the metabolism of immune cells. This review discusses ROS, metabolism of immune cells at different cellular conditions and sheds some light on how ROS might regulate immunometabolism.

Oxidants, Antioxidants and Thiol Redox Switches in the Control of Regulated Cell Death Pathways

It is well appreciated that biological reactive oxygen and nitrogen species such as hydrogen peroxide, superoxide and nitric oxide, as well as endogenous antioxidant systems, are important modulators of cell survival and death in diverse organisms and cell types. In addition, oxidative stress, nitrosative stress and dysregulated cell death are implicated in a wide variety of pathological conditions, including cancer, cardiovascular and neurological diseases. Therefore, much effort is devoted to elucidate the molecular mechanisms linking oxidant/antioxidant systems and cell death pathways. This review is focused on thiol redox modifications as a major mechanism by which oxidants and antioxidants influence specific regulated cell death pathways in mammalian cells. Growing evidence indicates that redox modifications of cysteine residues in proteins are involved in the regulation of multiple cell death modalities, including apoptosis, necroptosis and pyroptosis. In addition, recent research suggests that thiol redox switches play a role in the crosstalk between apoptotic and necrotic forms of regulated cell death. Thus, thiol-based redox circuits provide an additional layer of control that determines when and how cells die.

Redox Homeostasis in Photosynthetic Organisms: Novel and Established Thiol-Based Molecular Mechanisms

Significance: Redox homeostasis consists of an intricate network of reactions in which reactive molecular species, redox modifications, and redox proteins act in concert to allow both physiological responses and adaptation to stress conditions. Recent Advances: This review highlights established and novel thiol-based regulatory pathways underlying the functional facets and significance of redox biology in photosynthetic organisms. In the last decades, the field of redox regulation has largely expanded and this work is aimed at giving the right credit to the importance of thiol-based regulatory and signaling mechanisms in plants. Critical Issues: This cannot be all-encompassing, but is intended to provide a comprehensive overview on the structural/molecular mechanisms governing the most relevant thiol switching modifications with emphasis on the large genetic and functional diversity of redox controllers (i.e., redoxins). We also summarize the different proteomic-based approaches aimed at investigating the dynamics of redox modifications and the recent evidence that extends the possibility to monitor the cellular redox state in vivo. The physiological relevance of redox transitions is discussed based on reverse genetic studies confirming the importance of redox homeostasis in plant growth, development, and stress responses. Future Directions: In conclusion, we can firmly assume that redox biology has acquired an established significance that virtually infiltrates all aspects of plant physiology.

Protein S-Nitrosylation: Determinants of Specificity and Enzymatic Regulation of S-Nitrosothiol-Based Signaling

SIGNIFICANCE

Protein S-nitrosylation, the oxidative modification of cysteine by nitric oxide (NO) to form protein S-nitrosothiols (SNOs), mediates redox-based signaling that conveys, in large part, the ubiquitous influence of NO on cellular function. S-nitrosylation regulates protein activity, stability, localization, and protein-protein interactions across myriad physiological processes, and aberrant S-nitrosylation is associated with diverse pathophysiologies. Recent Advances: It is recently recognized that S-nitrosylation endows S-nitroso-protein (SNO-proteins) with S-nitrosylase activity, that is, the potential to trans-S-nitrosylate additional proteins, thereby propagating SNO-based signals, analogous to kinase-mediated signaling cascades. In addition, it is increasingly appreciated that cellular S-nitrosylation is governed by dynamically coupled equilibria between SNO-proteins and low-molecular-weight SNOs, which are controlled by a growing set of enzymatic denitrosylases comprising two main classes (high and low molecular weight). S-nitrosylases and denitrosylases, which together control steady-state SNO levels, may be identified with distinct physiology and pathophysiology ranging from cardiovascular and respiratory disorders to neurodegeneration and cancer.

CRITICAL ISSUES

The target specificity of protein S-nitrosylation and the stability and reactivity of protein SNOs are determined substantially by enzymatic machinery comprising highly conserved transnitrosylases and denitrosylases. Understanding the differential functionality of SNO-regulatory enzymes is essential, and is amenable to genetic and pharmacological analyses, read out as perturbation of specific equilibria within the SNO circuitry.

FUTURE DIRECTIONS

The emerging picture of NO biology entails equilibria among potentially thousands of different SNOs, governed by denitrosylases and nitrosylases. Thus, to elucidate the operation and consequences of S-nitrosylation in cellular contexts, studies should consider the roles of SNO-proteins as both targets and transducers of S-nitrosylation, functioning according to enzymatically governed equilibria.

Virtual screening-guided discovery of thioredoxin reductase inhibitors

The selenoprotein thioredoxin reductase (TXNRD) is a promising therapeutic target for cancer. To discover novel TXNRD inhibitors, a library of α, β-unsaturated carbonyl compounds were applied in structure-based virtual screening for the selection of hit compounds. Fifteen top-ranked compounds were further validated experimentally, exhibiting potent inhibition of TXNRD and remarkable cytotoxicity to cancer cells. The further binding mode analysis indicated that multiple noncovalent interactions between the inhibitors and the active pocket of TXNRD facilitated the formation of covalent bonds between the Sec498 on TXNRD and the α, β-unsaturated carbonyl groups on inhibitors. Results from both simulations and experiments demonstrated that Sec498 is the prime interaction site for the inhibition of TXNRD. Taking compound 7 as an example, the inhibition of TXNRD by compounds promoted oxidative stress-mediated apoptosis of cancer cells. Given these findings, novel TXNRD inhibitors may be discovered and introduced to the growing fields of small molecule drugs against TXNRD.

Molecular recognition of S-nitrosothiol substrate by its cognate protein denitrosylase

Protein S-nitrosylation mediates a large part of nitric oxide's influence on cellular function by providing a fundamental mechanism to control protein function across different species and cell types. At steady state, cellular S-nitrosylation reflects dynamic equilibria between S-nitrosothiols (SNOs) in proteins and small molecules (low-molecular-weight SNOs) whose levels are regulated by dedicated S-nitrosylases and denitrosylases. S-Nitroso-CoA (SNO-CoA) and its cognate denitrosylases, SNO-CoA reductases (SCoRs), are newly identified determinants of protein S-nitrosylation in both yeast and mammals. Because SNO-CoA is a minority species among potentially thousands of cellular SNOs, SCoRs must preferentially recognize this SNO substrate. However, little is known about the molecular mechanism by which cellular SNOs are recognized by their cognate enzymes. Using mammalian cells, molecular modeling, substrate-capture assays, and mutagenic analyses, we identified a single conserved surface Lys (Lys-127) residue as well as active-site interactions of the SNO group that mediate recognition of SNO-CoA by SCoR. Comparing SCoR^K127A versus SCoR^WT HEK293 cells, we identified a SNO-CoA-dependent nitrosoproteome, including numerous metabolic protein substrates. Finally, we discovered that the SNO-CoA/SCoR system has a role in mitochondrial metabolism. Collectively, our findings provide molecular insights into the basis of specificity in SNO-CoA-mediated metabolic signaling and suggest a role for SCoR-regulated S-nitrosylation in multiple metabolic processes.

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.

Roles of mammalian glutathione peroxidase and thioredoxin reductase enzymes in the cellular response to nitrosative stress

Mammalian cells employ elaborate antioxidant systems to effectively handle reactive oxygen and nitrogen species (ROS and RNS). At the heart of these systems operate two selenoprotein families consisting of glutathione peroxidase (GPx) and thioredoxin reductase (TrxR) enzymes. Although mostly studied in the context of oxidative stress, considerable evidence has amassed to indicate that these selenoenzymes also play important roles in nitrosative stress responses. GPx and TrxR, together with their redox partners, metabolize nitrosothiols and peroxynitrite, two major RNS. As such, these enzymes play active roles in the cellular defense against nitrosative stress. However, under certain conditions, these enzymes are inactivated by nitrosothiols or peroxynitrite, which may exacerbate oxidative and nitrosative stress in cells. The selenol groups in the active sites of GPx and TrxR enzymes are critically involved in these beneficial and detrimental processes. Further elucidation of the biochemical interactions between distinct RNS and GPx/TrxR will lead to a better understanding of the roles of these selenoenzymes in cellular homeostasis and disease.

Thioredoxin-related protein of 14 kDa as a modulator of redox signalling pathways

Thioredoxin-related protein of 14 kDa (TRP14; also named TXNDC17 for thioredoxin domain-containing protein 17) is a highly conserved and ubiquitously expressed oxidoreductase. It is expressed in parallel with thioredoxin 1 (Trx1, TXN; TXN1), an efficient substrate for the mammalian cytosolic selenoprotein thioredoxin reductase 1 (TrxR1; TXNRD1). However, TRP14, in sharp contrast to Trx1, cannot support the activities of ribonucleotide reductase, peroxiredoxins or methionine sulfoxide reductases, thus is unable to directly support cell proliferation or antioxidant defence through these pathways. However, TRP14 has been shown to efficiently reduce l-cystine, which thereby indirectly supports glutathione synthesis. TRP14 can also suppress NF-κB signalling, is functionally linked to STAT3 signalling, and can directly reactivate oxidized protein-tyrosine phosphatase PTP1B. Furthermore, TRP14 can efficiently reduce persulfidated or nitrosylated cysteine residues in many proteins, thereby having the capacity to modulate signalling through hydrogen sulfide or NO. Additional bioinformatics analyses and observations suggest further roles for TRP14; therefore, further studies of its functions are warranted. Collectively, the results available suggest that TRP14 is a member of the thioredoxin system dedicated to the control of cellular redox signalling pathways. LINKED ARTICLES: This article is part of a themed section on Chemical Biology of Reactive Sulfur Species. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.4/issuetoc.

Nitric oxide metabolites in hypoxia, freezing, and hibernation of the wood frog, Rana sylvatica

Nitric oxide (NO) is a gaseous free radical that in diverse organisms performs many signaling and protective functions, such as vasoregulation, inhibition of apoptosis, antioxidation, and metabolic suppression. Increased availability of NO may be especially important during life-history periods when organisms contend with multiple stresses. We investigated dynamics of the NO metabolites, nitrite (NO₂^-) and nitrate (NO₃^-), in the blood plasma, heart, liver, and skeletal muscle of the wood frog (Rana sylvatica), an amphibian that endures chronic cold, freezing, hypoxia, dehydration, and extended aphagia during hibernation. We found elevated concentrations of NO₂^- and/or NO₃^- in the plasma (up to 4.1-fold), heart (3.1-fold), and liver (up to 4.1-fold) of frogs subjected to experimental hypoxia (24 h, 4 °C), and in the liver (up to 3.8-fold) of experimentally frozen frogs (48 h, - 2.5 °C), suggesting that increased NO availability aids in survival of these stresses. During a 38-week period of simulated hibernation, NO₂^- and/or NO₃^- increased in the plasma (up to 10.4-fold), heart (up to 3.3-fold), and liver (5.0-fold) during an initial 5-week winter-acclimatization regimen and generally remained elevated thereafter. In hibernation, plasma NO₂^- was higher in frogs indigenous to Interior Alaska than in conspecifics from a temperate locale (southern Ohio), suggesting that NO availability is matched to the severity of environmental conditions prevailing in winter. The comparatively high NO availability in R. sylvatica, a stress-tolerant species, together with published values for other species, suggest that the NO protection system is of general importance in the stress adaptation of vertebrates.

Computational Structural Biology of S-nitrosylation of Cancer Targets

Nitric oxide (NO) plays an essential role in redox signaling in normal and pathological cellular conditions. In particular, it is well known to react in vivo with cysteines by the so-called S-nitrosylation reaction. S-nitrosylation is a selective and reversible post-translational modification that exerts a myriad of different effects, such as the modulation of protein conformation, activity, stability, and biological interaction networks. We have appreciated, over the last years, the role of S-nitrosylation in normal and disease conditions. In this context, structural and computational studies can help to dissect the complex and multifaceted role of this redox post-translational modification. In this review article, we summarized the current state-of-the-art on the mechanism of S-nitrosylation, along with the structural and computational studies that have helped to unveil its effects and biological roles. We also discussed the need to move new steps forward especially in the direction of employing computational structural biology to address the molecular and atomistic details of S-nitrosylation. Indeed, this redox modification has been so far an underappreciated redox post-translational modification by the computational biochemistry community. In our review, we primarily focus on S-nitrosylated proteins that are attractive cancer targets due to the emerging relevance of this redox modification in a cancer setting.

Role of Thioredoxin in Age-Related Hypertension

PURPOSE OF REVIEW

Although the roles of oxidant stress and redox perturbations in hypertension have been the subject of several reviews, role of thioredoxin (Trx), a major cellular redox protein in age-related hypertension remains inadequately reviewed. The purpose of this review is to bring readers up-to-date with current understanding of the role of thioredoxin in age-related hypertension.

RECENT FINDINGS

Age-related hypertension is a major underlying cause of several cardiovascular disorders, and therefore, intensive management of blood pressure is indicated in most patients with cardiovascular complications. Recent studies have shown that age-related hypertension was reversed and remained lowered for a prolonged period in mice with higher levels of human Trx (Trx-Tg). Additionally, injection of human recombinant Trx (rhTrx) decreased hypertension in aged wild-type mice that lasted for several days. Both Trx-Tg and aged wild-type mice injected with rhTrx were normotensive, showed increased NO production, decreased arterial stiffness, and increased vascular relaxation. These studies suggest that rhTrx could potentially be a therapeutic molecule to reverse age-related hypertension in humans. The reversal of age-related hypertension by restoring proteins that have undergone age-related modification is conceptually novel in the treatment of hypertension. Trx reverses age-related hypertension via maintaining vascular redox homeostasis, regenerating critical vasoregulatory proteins oxidized due to advancing age, and restoring native function of proteins that have undergone age-related modifications with loss-of function. Recent studies demonstrate that Trx is a promising molecule that may ameliorate or reverse age-related hypertension in older adults.

Thioredoxin-Dependent Decomposition of Protein S-Nitrosothiols

The addition of nitric oxide to cysteine moieties of proteins results in the formation of S-nitrosothiols (SNO) that have emerged as important posttranslational signaling cues in a wide variety of eukaryotic processes. While formation of protein-SNO is largely nonenzymatic, the conserved family of Thioredoxin (TRX) enzymes are capable of selectively reducing protein-SNO. Consequently, TRX enzymes are thought to provide reversibility and specificity to protein-SNO signaling networks. Here, we describe an in vitro methodology based on enzymatic oxidoreductase and biotin-switch techniques, allowing for the detection of protein-SNO targets of TRX enzymes. We show that this methodology identifies both global and specific protein-SNO targets of TRX in plant cell extracts.

Molecular mechanisms accompanying nitric oxide signalling through tyrosine nitration and S-nitrosylation of proteins in plants

Nitric oxide (NO) signalling in plants is responsible for modulation of a variety of plant developmental processes. Depending on the tissue system, the signalling of NO-modulated biochemical responses majorly involves the processes of tyrosine nitration or S-nitrosylation of specific proteins/enzymes. It has further been observed that there is a significant impact of various biotic/abiotic stress conditions on the extent of tyrosine nitration and S-nitrosylation of various metabolic enzymes, which may act as a positive or negative modulator of the specific routes associated with adaptive mechanisms employed by plants under the said stress conditions. In addition to recent findings on the modulation of enzymes of primary metabolism by NO through these two biochemical mechanisms, a major mechanism for regulating the levels of reactive oxygen species (ROS) under stress conditions has also been found to be through tyrosine nitration or S-nitrosylation of ROS-scavenging enzymes. Recent investigations have further highlighted the differential manner in which the ROS-scavenging enzymes may be S-nitrosylated and tyrosine nitrated, with reference to their tissue distribution. Keeping in mind the very recent findings on these aspects, the present review has been prepared to provide an analytical view on the significance of protein tyrosine nitration and S-nitrosylation in plant development.

S-nitrosylation/denitrosylation as a regulatory mechanism of salt stress sensing in sunflower seedlings

Nitric oxide (NO) and various reactive nitrogen species produced in cells in normal growth conditions, and their enhanced production under stress conditions are responsible for a variety of biochemical aberrations. The present findings demonstrate that sunflower seedling roots exhibit high sensitivity to salt stress in terms of nitrite accumulation. A significant reduction in S-nitrosoglutathione reductase (GSNOR) activity is evident in response to salt stress. Restoration of GSNOR activity with dithioerythritol shows that the enzyme is reversibly inhibited under conditions of 120 mM NaCl. Salt stress-mediated S-nitrosylation of cytosolic proteins was analyzed in roots and cotyledons using biotin-switch assay. LC-MS/MS analysis revealed opposite patterns of S-nitrosylation in seedling cotyledons and roots. Salt stress enhances S-nitrosylation of proteins in cotyledons, whereas roots exhibit denitrosylation of proteins. Highest number of proteins having undergone S-nitrosylation belonged to the category of carbohydrate metabolism followed by other metabolic proteins. Of the total 61 proteins observed to be regulated by S-nitrosylation, 17 are unique to cotyledons, 4 are unique to roots whereas 40 are common to both. Eighteen S-nitrosylated proteins are being reported for the first time in plant systems, including pectinesterase, phospholipase d-alpha and calmodulin. Further physiological analysis of glyceraldehyde-3-phosphate dehydrogenase and monodehydroascorbate reductase showed that salt stress leads to a reversible inhibition of both these enzymes in cotyledons. However, seedling roots exhibit enhanced enzyme activity under salinity stress. These observations implicate the role of S-nitrosylation and denitrosylation in NO signaling thereby regulating various enzyme activities under salinity stress in sunflower seedlings.

Innate immune evasion strategies against Cryptococcal meningitis caused by Cryptococcus neoformans

As an infectious fungus that affects the respiratory tract, Cryptococcus neoformans (C. neoformans) commonly causes asymptomatic pulmonary infection. C. neoformans may target the brain instead of the lungs and cross the blood-brain barrier (BBB) in the early phase of infection; however, this is dependent on successful evasion of the host innate immune system. During the initial stage of fungal infection, a complex network of innate immune factors are activated. C. neoformans utilizes a number of strategies to overcome the anti-fungal mechanisms of the host innate immune system and cross the BBB. In the present review, the defensive mechanisms of C. neoformans against the innate immune system and its ability to cross the BBB were discussed, with an emphasis on recent insights into the activities of anti-phagocytotic and anti-oxidative factors in C. neoformans.

Synthesis of naphthazarin derivatives and identification of novel thioredoxin reductase inhibitor as potential anticancer agent

Mammalian thioredoxin reductase (TrxR) enzymes play a crucial role in regulating multiple redox-based signaling pathways and have attracted increasing attention as promising anticancer drug targets. We report here the synthesis of a panel of naphthazarin derivatives and discovery of 2-methyl-5,8-dihydroxy-1,4-naphthoquinone (3, 2-methylnaphthazarin) as a potent cytotoxic agent with a submicromolar half maximal inhibitory concentration to the human promyelocytic leukemia HL-60 cells. Mechanism studies reveal that the compound selectively inhibits TrxR to induce oxidative stress-mediated apoptosis of HL-60 cells. Knockdown of TrxR sensitizes the cells to 3 insults, while overexpression of the functional enzyme confers resistance to the compound treatment, underpinning the physiological significance of targeting TrxR by 3. Clarification of the interaction of compound 3 with TrxR unveils a mechanism underlying the cellular action of the compound, and sheds light in considering development of the compound as a potential cancer chemotherapeutic agent.

Hydrogen peroxide production is affected by oxygen levels in mammalian cell culture

Although oxygen levels in the extracellular space of most mammalian tissues are just a few percent, under standard cell culture conditions they are not regulated and are often substantially higher. Some cellular sources of reactive oxygen species, like NADPH oxidase 4, are sensitive to oxygen levels in the range between 'normal' physiological (typically 1-5%) and standard cell culture (up to 18%). Hydrogen peroxide in particular participates in signal transduction pathways via protein redox modifications, so the potential increase in its production under standard cell culture conditions is important to understand. We measured the rates of cellular hydrogen peroxide production in some common cell lines, including C2C12, PC-3, HeLa, SH-SY5Y, MCF-7, and mouse embryonic fibroblasts (MEFs) maintained at 18% or 5% oxygen. In all instances the rate of hydrogen peroxide production by these cells was significantly greater at 18% oxygen than at 5%. The increase in hydrogen peroxide production at higher oxygen levels was either abolished or substantially reduced by treatment with GKT 137831, a selective inhibitor of NADPH oxidase subunits 1 and 4. These data indicate that oxygen levels experienced by cells in culture influence hydrogen peroxide production via NADPH oxidase 1/4, highlighting the importance of regulating oxygen levels in culture near physiological values. However, we measured pericellular oxygen levels adjacent to cell monolayers under a variety of conditions and with different cell lines and found that, particularly when growing at 5% incubator oxygen levels, pericellular oxygen was often lower and variable. Together, these observations indicate the importance, and difficulty, of regulating oxygen levels experienced by cells in culture.

The Deep Thioredoxome in Chlamydomonas reinhardtii: New Insights into Redox Regulation

Thiol-based redox post-translational modifications have emerged as important mechanisms of signaling and regulation in all organisms, and thioredoxin plays a key role by controlling the thiol-disulfide status of target proteins. Recent redox proteomic studies revealed hundreds of proteins regulated by glutathionylation and nitrosylation in the unicellular green alga Chlamydomonas reinhardtii, while much less is known about the thioredoxin interactome in this organism. By combining qualitative and quantitative proteomic analyses, we have comprehensively investigated the Chlamydomonas thioredoxome and 1188 targets have been identified. They participate in a wide range of metabolic pathways and cellular processes. This study broadens not only the redox regulation to new enzymes involved in well-known thioredoxin-regulated metabolic pathways but also sheds light on cellular processes for which data supporting redox regulation are scarce (aromatic amino acid biosynthesis, nuclear transport, etc). Moreover, we characterized 1052 thioredoxin-dependent regulatory sites and showed that these data constitute a valuable resource for future functional studies in Chlamydomonas. By comparing this thioredoxome with proteomic data for glutathionylation and nitrosylation at the protein and cysteine levels, this work confirms the existence of a complex redox regulation network in Chlamydomonas and provides evidence of a tremendous selectivity of redox post-translational modifications for specific cysteine residues.

Thioredoxin promotes survival signaling events under nitrosative/oxidative stress associated with cancer development

Accumulating mutations may drive cells into the acquisition of abnormal phenotypes that are characteristic of cancer cells. Cancer cells feature profound alterations in proliferation programs that result in a new population of cells that overrides normal tissue construction and maintenance programs. To achieve this goal, cancer cells are endowed with up regulated survival signaling pathways. They also must counteract the cytotoxic effects of high levels of nitric oxide (NO) and of reactive oxygen species (ROS), which are by products of cancer cell growth. Accumulating experimental evidence associates cancer cell survival with their capacity to up-regulate antioxidant systems. Elevated expression of the antioxidant protein thioredoxin-1 (Trx1) has been correlated with cancer development. Trx1 has been characterized as a multifunctional protein, playing different roles in different cell compartments. Trx1 migrates to the nucleus in cells exposed to nitrosative/oxidative stress conditions. Trx1 nuclear migration has been related to the activation of transcription factors associated with cell survival and cell proliferation. There is a direct association between the p21Ras-ERK1/2 MAP Kinases survival signaling pathway and Trx1 nuclear migration under nitrosative stress. The expression of the cytoplasmic protein, the thioredoxin-interacting protein (Txnip), determines the change in Trx1 cellular compartmentalization. The anti-apoptotic actions of Trx1 and its denitrosylase activity occur in the cytoplasm and serve as important regulators of cell survival. Within this context, this review focuses on the participation of Trx1 in cells under nitrosative/oxidative stress in survival signaling pathways associated with cancer development.

Detection of ROS Induced Proteomic Signatures by Mass Spectrometry

Reversible and irreversible post-translational modifications (PTMs) induced by endogenously generated reactive oxygen species (ROS) in regulatory enzymes and proteins plays an essential role in cellular signaling. Almost all cellular processes including metabolism, transcription, translation and degradation have been identified as containing redox regulated proteins. Specific redox modifications of key amino acids generated by ROS offers a dynamic and versatile means to rapidly alter the activity or functional structure of proteins in response to biochemical, environmental, genetic and pathological perturbations. How the proteome responds to these stimuli is of critical importance in oxidant physiology, as it can regulate the cell stress response by reversible and irreversible PTMs, affecting protein activity and protein-protein interactions. Due to the highly labile nature of many ROS species, applying redox proteomics can provide a signature footprint of the ROS species generated. Ideally redox proteomic approaches would allow; (1) the identification of the specific PTM, (2) identification of the amino acid residue that is modified and (3) the percentage of the protein containing the PTM. New developments in MS offer the opportunity of a more sensitive targeted proteomic approach and retrospective data analysis. Subsequent bioinformatics analysis can provide an insight into the biochemical and physiological pathways or cell signaling cascades that are affected by ROS generation. This mini-review will detail current redox proteomic approaches to identify and quantify ROS induced PTMs and the subsequent effects on cellular signaling.

Targeting the Thioredoxin System for Cancer Therapy

Thioredoxin (Trx) and thioredoxin reductase (TrxR) are essential components of the Trx system which plays pivotal roles in regulating multiple cellular redox signaling pathways. In recent years TrxR/Trx have been increasingly recognized as an important modulator of tumor development, and hence targeting TrxR/Trx is a promising strategy for cancer treatment. In this review we first discuss the structural details of TrxR, the functions of the Trx system, and the rational of targeting TrxR/Trx for cancer treatment. We also highlight small-molecule TrxR/Trx inhibitors that have potential anticancer activity and review their mechanisms of action. Finally, we examine the challenges of developing TrxR/Trx inhibitors as anticancer agents and perspectives for selectively targeting TrxR/Trx.

The Plasma Proteome Is Associated with Anthropometric Status of Undernourished Nepalese School-Aged Children

Background: Malnutrition affects body growth, size, and composition of children. Yet, few functional biomarkers are known to be associated with childhood morphology.

Objective: This cross-sectional study examined associations of anthropometric indicators of height, musculature, and fat mass with plasma proteins by using proteomics in a population cohort of school-aged Nepalese children.

Methods: Height, weight, midupper arm circumference (MUAC), triceps and subscapular skinfolds, upper arm muscle area (AMA), and arm fat area (AFA) were assessed in 500 children 6–8 y of age. Height-for-age z scores (HAZs), weight-for-age z scores (WAZs), and body mass index–for-age z scores (BAZs) were derived from the WHO growth reference. Relative protein abundance was quantified by using tandem mass spectrometry. Protein-anthropometry associations were evaluated by linear mixed-effects models and identified as having a false discovery rate (q) <5%.

Results: Among 982 proteins, 1, 10, 14, and 17 proteins were associated with BAZ, HAZ, MUAC, and AMA, respectively (q < 0.05). Insulin-like growth factor (IGF)-I, 2 IGF-binding proteins, and carnosinase-1 were associated with both HAZ and AMA. Proteins involved in nutrient transport, activation of innate immunity, and bone mineralization were associated with HAZ. Several extracellular matrix proteins were positively associated with AMA alone. The proteomes of MUAC and AMA substantially overlapped, whereas no proteins were associated with AFA or triceps and subscapular skinfolds. Myosin light-chain kinase, possibly reflecting leakage from muscle, was inversely associated with BAZ. The proteome of WAZ was the largest (n = 33) and most comprehensive, including proteins involved in neural development and oxidative stress response, among others.

Conclusions: Plasma proteomics confirmed known biomarkers of childhood growth and revealed novel proteins associated with lean mass in chronically undernourished children. Identified proteins may serve as candidates for assessing growth and nutritional status of children in similar undernourished settings. The antenatal micronutrient supplementation trial yielding the study cohort of children was registered at clinicaltrials.gov as NCT00115271.

Nitrosothiol-Trapping-Based Proteomic Analysis of S-Nitrosylation in Human Lung Carcinoma Cells

Nitrosylation of cysteines residues (S-nitrosylation) mediates many of the cellular effects of nitric oxide in normal and diseased cells. Recent research indicates that S-nitrosylation of certain proteins could play a role in tumor progression and responsiveness to therapy. However, the protein targets of S-nitrosylation in cancer cells remain largely unidentified. In this study, we used our recently developed nitrosothiol trapping approach to explore the nitrosoproteome of human A549 lung carcinoma cells treated with S-nitrosocysteine or pro-inflammatory cytokines. Using this approach, we identified about 300 putative nitrosylation targets in S-nitrosocysteine-treated A549 cells and approximately 400 targets in cytokine-stimulated cells. Among the more than 500 proteins identified in the two screens, the majority represent novel targets of S-nitrosylation, as revealed by comparison with publicly available nitrosoproteomic data. By coupling the trapping procedure with differential thiol labeling, we identified nearly 300 potential nitrosylation sites in about 150 proteins. The proteomic results were validated for several proteins by an independent approach. Bioinformatic analysis highlighted important cellular pathways that are targeted by S-nitrosylation, notably, cell cycle and inflammatory signaling. Taken together, our results identify new molecular targets of nitric oxide in lung cancer cells and suggest that S-nitrosylation may regulate signaling pathways that are critically involved in lung cancer progression.

Regulation of Anticancer Styrylpyrone Biosynthesis in the Medicinal Mushroom Inonotus obliquus Requires Thioredoxin Mediated Transnitrosylation of S-nitrosoglutathione Reductase

The medicinal macrofungus Inonotus obliquus widely utilized as folk medicine in Russia and Baltic countries is a source of phenylpropanoid-derived styrylpyrone polyphenols that can inhibit tumor proliferation. Insights into the regulatory machinery that controls I. obliquus styrylpyrone polyphenol biosynthesis will enable strategies to increase the production of these molecules. Here we show that Thioredoxin (Trx) mediated transnitrosylation of S-nitrosoglutathione reductase (GSNOR) underpins the regulation of styrylpyrone production, driven by nitric oxide (NO) synthesis triggered by P. morii coculture. NO accumulation results in the S-nitrosylation of PAL and 4CL required for the synthesis of precursor phenylpropanoids and styrylpyrone synthase (SPS), integral to the production of styrylpyrone, inhibiting their activities. These enzymes are targeted for denitrosylation by Trx proteins, which restore their activity. Further, this Trx S-nitrosothiol (SNO) reductase activity was potentiated following S-nitrosylation of Trx proteins at a non-catalytic cysteine (Cys) residue. Intriguingly, this process was counterbalanced by Trx denitrosylation, mediated by Trx-dependent transnitrosylation of GSNOR. Thus, unprecedented interplay between Trx and GSNOR oxidoreductases regulates the biosynthesis of styrylpyrone polyphenols in I. obliquus.

How are nitrosothiols formed de novo in vivo?

The biological mechanisms of de novo formation of cellular nitrosothiols (as opposed to transnitrosation) are reviewed. The approach is to introduce chemical foundations for each mechanism, followed by evidence in biological systems. The general categories include mechanisms involving nitrous acid, NO autoxidation and oxidant stress, redox active and inactive metal ions, and sulfide/persulfide. Important conclusions/speculations are that de novo cellular thiol nitrosation (1) is an oxidative process, and so should be considered within the family of other thiol oxidative modifications, (2) may not involve a single dominant process but depends on the specific conditions, (3) does not involve O₂ under at least some conditions, and (4) may serve to provide a "substrate pool" of protein cysteine nitrosothiol which could, through subsequent enzymatic transnitrosation/denitrosation, be "rearranged" to accomplish the specificity and regulatory control required for effective post-translational signaling.

Application of a Thioredoxin-Trapping Mutant for Analysis of the Cellular Nitrosoproteome

Nitric oxide influences a wide range of cellular functions through S-nitrosylation, a redox-dependent posttranslational protein modification that involves attachment of a nitroso moiety to a reactive thiol group. Over the past two decades, S-nitrosylation has emerged as a ubiquitous mechanism for controlling the activity, subcellular localization, and molecular interactions of proteins, thereby influencing many cellular processes. In addition, recent studies have indicated that aberrant S-nitrosylation may lead to cellular dysfunction and damage. Despite significant advances in the field, progress has been hindered by challenges related to the analysis of S-nitrosylation by large-scale proteomic approaches. This chapter describes the application of a thioredoxin-trapping mutant for proteomic analysis of S-nitrosylation. Thioredoxin is a ubiquitous oxidoreductase directly involved in denitrosylation reactions. The presented method relies upon mechanism-based trapping, whereby a recombinant thioredoxin trap mutant captures nitrosylated proteins, which are subsequently isolated and identified by mass spectrometry. This nitrosothiol-trapping procedure can expand upon and complement currently available methods for the analysis of the nitrosoproteome.

Protective Effects of Selol Against Sodium Nitroprusside-Induced Cell Death and Oxidative Stress in PC12 Cells

Selol is an organic selenitetriglyceride formulation containing selenium at +4 oxidation level that can be effectively incorporated into catalytic sites of of Se-dependent antioxidants. In the present study, the potential antioxidative and cytoprotective effects of Selol against sodium nitroprusside (SNP)-evoked oxidative/nitrosative stress were investigated in PC12 cells and the underlying mechanisms analyzed. Spectrophoto- and spectrofluorimetic methods as well as fluorescence microscopy were used in this study; mRNA expression was quantified by real-time PCR. Selol dose-dependently improved the survival and decreased the percentage of apoptosis in PC12 cells exposed to SNP. To determine the mechanism of this protective action, the effect of Selol on free radical generation and on antioxidative potential was evaluated. Selol offered significant protection against the elevation of reactive oxidative species (ROS) evoked by SNP. Moreover, this compound restored glutathione homeostasis by ameliorating the SNP-evoked disturbance of GSH/GSSG ratio. The protective effect exerted by Selol was associated with the prevention of SNP-mediated down-regulation of antioxidative enzymes: glutathione peroxidase (Se-GPx), glutathione reductase (GR), and thioredoxin reductase (TrxR). Finally, GPx inhibition significantly abolished the cytoprotective effect of Selol. In conclusion, these results suggest that Selol effectively protected PC12 cells against SNP-induced oxidative damage and death by adjusting free radical levels and antioxidant system, and suppressing apoptosis. Selol could be successfully used in the treatments of diseases that involve oxidative stress and resulting apoptosis.

Inhibitory nitrosylation of mammalian thioredoxin reductase 1: Molecular characterization and evidence for its functional role in cellular nitroso-redox imbalance

Mammalian thioredoxin 1 (Trx1) and the selenoprotein Trx reductase 1 (TrxR1) are key cellular enzymes that function coordinately in thiol-based redox regulation and signaling. Recent studies have revealed that the Trx1/TrxR1 system has an S-nitrosothiol reductase (denitrosylase) activity through which it can regulate nitric oxide-related cellular processes. In this study we revealed that TrxR1 is itself susceptible to nitrosylation, characterized the underlying mechanism, and explored its functional significance. We found that nitrosothiol or nitric oxide donating agents rapidly and effectively inhibited the activity of recombinant or endogenous TrxR1. In particular, the NADPH-reduced TrxR1 was partially and reversibly inhibited upon exposure to low concentrations (<10μM) of S-nitrosocysteine (CysNO) and markedly and continuously inhibited at higher doses. Concurrently, TrxR1 very efficiently reduced low, but not high, levels of CysNO. Biochemical and mass spectrometric analyses indicated that its active site selenocysteine residue renders TrxR1 highly susceptible to nitrosylation-mediated inhibition, and revealed both thiol and selenol modifications at the two redox active centers of the enzyme. Studies in HeLa cancer cells demonstrated that endogenous TrxR1 is sensitive to nitrosylation-dependent inactivation and pointed to an important role for glutathione in reversing or preventing this process. Notably, depletion of cellular glutathione with l-buthionine-sulfoximine synergized with nitrosating agents in promoting sustained nitrosylation and inactivation of TrxR1, events that were accompanied by significant oxidation of Trx1 and extensive cell death. Collectively, these findings expand our knowledge of the role and regulation of the mammalian Trx system in relation to cellular nitroso-redox imbalance. The observations raise the possibility of exploiting the nitrosylation susceptibility of TrxR1 for killing tumor cells.

Catabolism and safety of supplemental L-arginine in animals

L-arginine (Arg) is utilized via multiple pathways to synthesize protein and low-molecular-weight bioactive substances (e.g., nitric oxide, creatine, and polyamines) with enormous physiological importance. Furthermore, Arg regulates cell signaling pathways and gene expression to improve cardiovascular function, augment insulin sensitivity, enhance lean tissue mass, and reduce obesity in humans. Despite its versatile roles, the use of Arg as a dietary supplement is limited due to the lack of data to address concerns over its safety in humans. Data from animal studies are reviewed to assess arginine catabolism and the safety of long-term Arg supplementation. The arginase pathway was responsible for catabolism of 76-85 and 81-96 % Arg in extraintestinal tissues of pigs and rats, respectively. Dietary supplementation with Arg-HCl or the Arg base [315- and 630-mg Arg/(kg BW d) for 91 d] had no adverse effects on male or female pigs. Similarly, no safety issues were observed for male or female rats receiving supplementation with 1.8- and 3.6-g Arg/(kg BW d) for at least 91 d. Intravenous administration of Arg-HCl to gestating sheep at 81 and 180 mg Arg/(kg BW d) is safe for at least 82 and 40 d, respectively. Animals fed conventional diets can well tolerate large amounts of supplemental Arg [up to 630-mg Arg/(kg BW d) in pigs or 3.6-g Arg/(kg BW d) in rats] for 91 d, which are equivalent to 573-mg Arg/(kg BW d) for humans. Collectively, these results can help guide studies to determine the safety of long-term oral administration of Arg in humans.

Dual targeting of the thioredoxin and glutathione systems in cancer and HIV

Although the use of antioxidants for the treatment of cancer and HIV/AIDS has been proposed for decades, new insights gained from redox research have suggested a very different scenario. These new data show that the major cellular antioxidant systems, the thioredoxin (Trx) and glutathione (GSH) systems, actually promote cancer growth and HIV infection, while suppressing an effective immune response. Mechanistically, these systems control both the redox- and NO-based pathways (nitroso-redox homeostasis), which subserve innate and cellular immune defenses. Dual inhibition of the Trx and GSH systems synergistically kills neoplastic cells in vitro and in mice and decreases resistance to anticancer therapy. Similarly, the population of HIV reservoir cells that constitutes the major barrier to a cure for AIDS is exquisitely redox sensitive and could be selectively targeted by Trx and GSH inhibitors. Trx and GSH inhibition may lead to a reprogramming of the immune response, tilting the balance between the immune system and cancer or HIV in favor of the former, allowing elimination of diseased cells. Thus, therapies based on silencing of the Trx and GSH pathways represent a promising approach for the cure of both cancer and AIDS and warrant further investigation.

When Bad Guys Become Good Ones: The Key Role of Reactive Oxygen Species and Nitric Oxide in the Plant Responses to Abiotic Stress

The natural environment of plants is composed of a complex set of abiotic stresses and their ability to respond to these stresses is highly flexible and finely balanced through the interaction between signaling molecules. In this review, we highlight the integrated action between reactive oxygen species (ROS) and reactive nitrogen species (RNS), particularly nitric oxide (NO), involved in the acclimation to different abiotic stresses. Under stressful conditions, the biosynthesis transport and the metabolism of ROS and NO influence plant response mechanisms. The enzymes involved in ROS and NO synthesis and scavenging can be found in different cells compartments and their temporal and spatial locations are determinant for signaling mechanisms. Both ROS and NO are involved in long distances signaling (ROS wave and GSNO transport), promoting an acquired systemic acclimation to abiotic stresses. The mechanisms of abiotic stresses response triggered by ROS and NO involve some general steps, as the enhancement of antioxidant systems, but also stress-specific mechanisms, according to the stress type (drought, hypoxia, heavy metals, etc.), and demand the interaction with other signaling molecules, such as MAPK, plant hormones, and calcium. The transduction of ROS and NO bioactivity involves post-translational modifications of proteins, particularly S-glutathionylation for ROS, and S-nitrosylation for NO. These changes may alter the activity, stability, and interaction with other molecules or subcellular location of proteins, changing the entire cell dynamics and contributing to the maintenance of homeostasis. However, despite the recent advances about the roles of ROS and NO in signaling cascades, many challenges remain, and future studies focusing on the signaling of these molecules in planta are still necessary.

Protein S-nitrosylation in photosynthetic organisms: A comprehensive overview with future perspectives

BACKGROUND

The free radical nitric oxide (NO) and derivative reactive nitrogen species (RNS) play essential roles in cellular redox regulation mainly through protein S-nitrosylation, a redox post-translational modification in which specific cysteines are converted to nitrosothiols.

SCOPE OF VIEW

This review aims to discuss the current state of knowledge, as well as future perspectives, regarding protein S-nitrosylation in photosynthetic organisms.

MAJOR CONCLUSIONS

NO, synthesized by plants from different sources (nitrite, arginine), provides directly or indirectly the nitroso moiety of nitrosothiols. Biosynthesis, reactivity and scavenging systems of NO/RNS, determine the NO-based signaling including the rate of protein nitrosylation. Denitrosylation reactions compete with nitrosylation in setting the levels of nitrosylated proteins in vivo.

GENERAL SIGNIFICANCE

Based on a combination of proteomic, biochemical and genetic approaches, protein nitrosylation is emerging as a pervasive player in cell signaling networks. Specificity of protein nitrosylation and integration among different post-translational modifications are among the major challenges for future experimental studies in the redox biology field. This article is part of a Special Issue entitled: Plant Proteomics--a bridge between fundamental processes and crop production, edited by Dr. Hans-Peter Mock.