The role of thioredoxin in the regulation of cellular processes by S-nitrosylation. Biochim Biophys Acta Gen Subj. 2012;1820:689-700. [PMID: 21878369 DOI: 10.1016/j.bbagen.2011.08.012]

The hunt for transnitrosylase

The biochemical interplay between antioxidants and pro-oxidants maintains the redox homeostatic balance of the cell, which, when perturbed to moderate or high extents, has been implicated in the onset and/or progression of chronic diseases such as diabetes mellitus, cancer, and neurodegenerative diseases. Thioredoxin, glutaredoxin, and lipoic acid-like thiol oxidoreductase systems constitute a unique ensemble of robust cellular antioxidant defenses, owing to their indispensable roles as S-denitrosylases, S-deglutathionylases, and disulfide reductants in maintaining a reduced free thiol state with biological relevance. Thus, in cells subjected to nitrosative stress, cellular antioxidants will S-denitrosylate their cognate S-nitrosoprotein substrates, rather than participate in trans-S-nitrosylation via protein-protein interactions. Researchers have been at the forefront of vaguely establishing the concept of 'transnitrosylation' and its influence on pathophysiology with experimental evidence from in vitro studies that lack proper biochemical logic. The suggestive and reiterative use of antioxidants as transnitrosylases in the scientific literature leaves us on a cliffhanger with several open-ended questions that prompted us to 'hunt' for scientific logic behind the trans-S-nitrosylation chemistry. Given the gravity of the situation and to look at the bigger picture of 'trans-S-nitrosylation', we aim to present a novel attempt at justifying the hesitance in accepting antioxidants as capable of transnitrosylating their cognate protein partners and reflecting on the need to resolve the controversy that would be crucial from the perspective of understanding therapeutic outcomes involving such cellular antioxidants in disease pathogenesis. Further characterization is required to identify the regulatory mechanisms or conditions where an antioxidant like Trx, Grx, or DJ-1 can act as a cellular transnitrosylase.

The Role of Thioredoxin System in Shank3 Mouse Model of Autism

Autism spectrum disorder (ASD) is a complex neurodevelopmental disorder characterized by difficulties in social interaction and communication, repetitive behaviors, and restricted interests. Unfortunately, the underlying molecular mechanism behind ASD remains unknown. It has been reported that oxidative and nitrosative stress are strongly linked to ASD. We have recently found that nitric oxide (NO•) and its products play an important role in this disorder. One of the key proteins associated with NO• is thioredoxin (Trx). We hypothesize that the Trx system is altered in the Shank3 KO mouse model of autism, which may lead to a decreased activity of the nuclear factor erythroid 2-related factor 2 (Nrf2), resulting in oxidative stress, and thus, contributing to ASD-related phenotypes. To test this hypothesis, we conducted in vivo behavioral studies and used primary cortical neurons derived from the Shank3 KO mice and human SH-SY5Y cells with SHANK3 mutation. We showed significant changes in the levels and activity of Trx redox proteins in the Shank3 KO mice. A Trx1 inhibitor PX-12 decreased Trx1 and Nrf2 expression in wild-type mice, causing abnormal alterations in the levels of synaptic proteins and neurotransmission markers, and an elevation of nitrosative stress. Trx inhibition resulted in an ASD-like behavioral phenotype, similar to that of Shank3 KO mice. Taken together, our findings confirm the strong link between the Trx system and ASD pathology, including the increased oxidative/nitrosative stress, and synaptic and behavioral deficits. The results of this study may pave the way for identifying novel drug targets for ASD.

Leveraging the redundancy of S-denitrosylases in response to S-nitrosylation of caspases: Experimental strategies and beyond

Redox-based protein posttranslational modifications, such as S-nitrosylation of critical, active site cysteine thiols have garnered significant clinical attention and research interest, reasoning for one of the crucial biological implications of reactive messenger molecule, nitric oxide in the cellular repertoire. The stringency of the S-(de)nitrosylation-based redox switch governs the activity and contribution of several susceptible enzymes in signal transduction processes and diverse pathophysiological settings, thus establishing it as a transient yet reasonable, and regulated mechanism of NO adduction and release. Notably, endogenous proteases like cytosolic and mitochondrial caspases with a molecular weight ranging from 33 to 55 kDa are susceptible to performing this biochemistry in the presence of major oxidoreductases, which further unveils the enormous redox-mediated regulational control of caspases in the etiology of diseases. In addition to advancing the progress of the current state of understanding of 'redox biochemistry' in the field of medicine and enriching the existing dynamic S-nitrosoproteome, this review stands as a testament to an unprecedented shift in the underpinnings for redundancy and redox relay between the major redoxin/antioxidant systems, fine-tuning of which can command the apoptotic control of caspases at the face of nitro-oxidative stress. These intricate functional overlaps and cellular backups, supported rationally by kinetically favorable reaction mechanisms suggest the physiological relevance of identifying and involving such cognate substrates for cellular S-denitrosylases that can shed light on the bigger picture of extensively proposing targeted therapies and redox-based drug designing to potentially alleviate the side effects of NOx/ROS in disease pathogenesis.

Protein S-nitrosylation is involved in valproic acid-promoted neuronal differentiation of adipose tissue-derived stem cells

Neuronal differentiation of adipose tissue-derived stem cells (ASCs) is greatly promoted by valproic acid (VPA) with cAMP elevating agents thorough NO signaling pathways, but its mechanism is not fully understood. In the present study, we investigate the involvement of protein S-nitrosylation in the VPA-promoted neuronal differentiation of ASCs. The whole amount of S-nitrosylated protein was increased by the treatment with VPA alone for three days in ASCs. An inhibitor of thioredoxin reductase (TrxR), auranofin, further increased the amount of S-nitrosylated protein and enhances the VPA-promoted neuronal differentiation in ASCs. On the contrary, another inhibitor of TrxR, dinitrochlorobenzene, inhibited the VPA-promoted neuronal differentiation in ASCs even with cAMP elevating agents, which was accompanied by unexpectedly decreased S-nitrosylated protein. It was considered from these results that increased protein S-nitrosylation is involved in VPA-promoted neuronal differentiation of ASCs. By the proteomic analysis of S-nitrosylated protein in VPA-treated ASCs, no identified proteins could be specifically related to VPA-promoted neuronal differentiation. The identified proteins, however, included those involved in the metabolism of substances regulating neuronal differentiation, such as aspartate and glutamate.

Experimental pharmacological approaches to reverse impaired awareness of hypoglycemia-a review

The colossal global burden of diabetes management is compounded by the serious complication of hypoglycemia. Protective physiologic hormonal and neurogenic counterregulatory responses to hypoglycemia are essential to preserve glucose homeostasis and avert serious morbidity. With recurrent exposure to hypoglycemic episodes over time, these counterregulatory responses to hypoglycemia can diminish, resulting in an impaired awareness of hypoglycemia (IAH). IAH is characterized by sudden neuroglycopenia rather than preceding cautionary autonomic symptoms. IAH increases the risk of subsequent sudden and severe hypoglycemic episodes in patients with diabetes. The postulated causative mechanisms behind IAH are complex and varied. It is therefore challenging to identify a single effective therapeutic strategy. In this review, we closely examine the efficacy and feasibility of a myriad of pharmaceutical interventions in preventing and treating IAH as described in clinical and preclinical studies. Pharmaceutical agents outlined include N-acetyl cysteine, GABA A receptor blockers, opioid receptor antagonists, AMP activated protein kinase agonists, potassium channel openers, dehydroepiandrosterone, metoclopramide, antiadrenergic agents, antidiabetic agents and glucagon.

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.

The Role of Post-Translational Modifications in Regulation of NLRP3 Inflammasome Activation

Pathogen-associated molecular patterns (PAMPs) and danger-associated molecular patterns (DAMPs) induce NLRP3 inflammasome activation, and subsequent formation of active caspase-1 as well as the maturation of interleukin-1β (IL-1β) and gasdermin D (GSDMD), mediating the occurrence of pyroptosis and inflammation. Aberrant NLRP3 inflammasome activation causes a variety of diseases. Therefore, the NLRP3 inflammasome pathway is a target for prevention and treatment of relative diseases. Recent studies have suggested that NLRP3 inflammasome activity is closely associated with its post-translational modifications (PTMs). This review focuses on PTMs of the components of the NLRP3 inflammasome and the resultant effects on regulation of its activity to provide references for the exploration of the mechanisms by which the NLRP3 inflammasome is activated and controlled.

Digital microfluidic platform assembled into a home-made studio for sample preparation and colorimetric sensing of S-nitrosocysteine

Digital microfluidics (DMF) is a versatile lab-on-a-chip platform that allows integration with several types of sensors and detection techniques, including colorimetric sensors. Here, we propose, for the first time, the integration of DMF chips into a mini studio containing a 3D-printed holder with previously fixed UV-LEDs to promote sample degradation on the chip surface before a complete analytical procedure involving reagent mixture, colorimetric reaction, and detection through a webcam integrated on the equipment. As a proof-of-concept, the feasibility of the integrated system was successfully through the indirect analysis of S-nitrosocysteine (CySNO) in biological samples. For this purpose, UV-LEDs were explored to perform the photolytic cleavage of CySNO, thus generating nitrite and subproducts directly on DMF chip. Nitrite was then colorimetrically detected based on a modified Griess reaction, in which reagents were prepared through a programable movement of droplets on DMF devices. The assembling and the experimental parameters were optimized, and the proposed integration exhibited a satisfactory correlation with the results acquired using a desktop scanner. Under the optimal experimental conditions, the obtained CySNO degradation to nitrite was 96%. Considering the analytical parameters, the proposed approach revealed linear behavior in the CySNO concentration range between 12.5 and 400 μmol L^-1 and a limit of detection equal to 2.8 μmol L^-1. Synthetic serum and human plasma samples were successfully analyzed, and the achieved results did not statistically differ from the data recorded by spectrophotometry at the confidence level of 95%, thus indicating the huge potential of the integration between DMF and mini studio to promote complete analysis of lowmolecular weight compounds.

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.

Thioredoxin 1 promotes autophagy through transnitrosylation of Atg7 during myocardial ischemia

Modification of cysteine residues by oxidative and nitrosative stress affects structure and function of proteins, thereby contributing to the pathogenesis of cardiovascular disease. Although the major function of thioredoxin 1 (Trx1) is to reduce disulfide bonds, it can also act as either a denitrosylase or transnitrosylase in a context-dependent manner. Here we show that Trx1 transnitrosylates Atg7, an E1-like enzyme, thereby stimulating autophagy. During ischemia, Trx1 was oxidized at Cys32-Cys35 of the oxidoreductase catalytic center and S-nitrosylated at Cys73. Unexpectedly, Atg7 Cys545-Cys548 reduced the disulfide bond in Trx1 at Cys32-Cys35 through thiol-disulfide exchange and this then allowed NO to be released from Cys73 in Trx1 and transferred to Atg7 at Cys402. Experiments conducted with Atg7 C402S-knockin mice showed that S-nitrosylation of Atg7 at Cys402 promotes autophagy by stimulating E1-like activity, thereby protecting the heart against ischemia. These results suggest that the thiol-disulfide exchange and the NO transfer are functionally coupled, allowing oxidized Trx1 to mediate a salutary effect during myocardial ischemia through transnitrosylation of Atg7 and stimulation of autophagy.

Graphene Oxide Enhances Biogenesis and Release of Exosomes in Human Ovarian Cancer Cells

BACKGROUND

Exosomes, which are nanovesicles secreted by almost all the cells, mediate intercellular communication and are involved in various physiological and pathological processes. We aimed to investigate the effects of graphene oxide (GO) on the biogenesis and release of exosomes in human ovarian cancer (SKOV3) cells.

METHODS

Exosomes were isolated using ultracentrifugation and ExoQuick and characterized by various analytical techniques. The expression levels of exosome markers were analyzed via quantitative reverse transcription-polymerase chain reaction and enzyme-linked immunosorbent assay.

RESULTS

Graphene oxide (10-50 μg/mL), cisplatin (2-10 μg/mL), and C6-ceramide (5-25 μM) inhibited the cell viability, proliferation, and cytotoxicity in a dose-dependent manner. We observed that graphene oxide (GO), cisplatin (CIS), and C6-Ceramide (C6-Cer) stimulated acetylcholine esterase and neutral sphingomyelinase activity, total exosome protein concentration, and exosome counts associated with increased level of apoptosis, oxidative stress and endoplasmic reticulum stress. In contrast, GW4869 treatment inhibits biogenesis and release of exosomes. We observed that the human ovarian cancer cells secreted exosomes with typical cup-shaped morphology and surface protein biomarkers. The expression levels of TSG101, CD9, CD63, and CD81 were significantly higher in GO-treated cells than in control cells. Further, cytokine and chemokine levels were significantly higher in exosomes isolated from GO-treated SKOV3 cells than in those isolated from control cells. SKOV3 cells pre-treated with N-acetylcysteine or GW4869 displayed a significant reduction in GO-induced exosome biogenesis and release. Furthermore, endocytic inhibitors decrease exosome biogenesis and release by impairing endocytic pathways.

CONCLUSION

This study identifies GO as a potential tool for targeting the exosome pathway and stimulating exosome biogenesis and release. We believe that the knowledge acquired in this study can be potentially extended to other exosome-dominated pathologies and model systems. Furthermore, these nanoparticles can provide a promising means to enhance exosome production in SKOV3 cells.

S-Denitrosylation: A Crosstalk between Glutathione and Redoxin Systems

S-nitrosylation of proteins occurs as a consequence of the derivatization of cysteine thiols with nitric oxide (NO) and is often associated with diseases and protein malfunction. Aberrant S-nitrosylation, in addition to other genetic and epigenetic factors, has gained rapid importance as a prime cause of various metabolic, respiratory, and cardiac disorders, with a major emphasis on cancer and neurodegeneration. The S-nitrosoproteome, a term used to collectively refer to the diverse and dynamic repertoire of S-nitrosylated proteins, is relatively less explored in the field of redox biochemistry, in contrast to other covalently modified versions of the same set of proteins. Advancing research is gradually unveiling the enormous clinical importance of S-nitrosylation in the etiology of diseases and is opening up new avenues of prompt diagnosis that harness this phenomenon. Ever since the discovery of the two robust and highly conserved S-nitrosoglutathione reductase and thioredoxin systems as candidate denitrosylases, years of rampant speculation centered around the identification of specific substrates and other candidate denitrosylases, subcellular localization of both substrates and denitrosylases, the position of susceptible thiols, mechanisms of S-denitrosylation under basal and stimulus-dependent conditions, impact on protein conformation and function, and extrapolating these findings towards the understanding of diseases, aging and the development of novel therapeutic strategies. However, newer insights in the ever-expanding field of redox biology reveal distinct gaps in exploring the crucial crosstalk between the redoxins/major denitrosylase systems. Clarifying the importance of the functional overlap of the glutaredoxin, glutathione, and thioredoxin systems and examining their complementary functions as denitrosylases and antioxidant enzymatic defense systems are essential prerequisites for devising a rationale that could aid in predicting the extent of cell survival under high oxidative/nitrosative stress while taking into account the existence of the alternative and compensatory regulatory mechanisms. This review thus attempts to highlight major gaps in our understanding of the robust cellular redox regulation system, which is upheld by the concerted efforts of various denitrosylases and antioxidants.

NO news: S-(de)nitrosylation of cathepsins and their relationship with cancer

Tumor formation and progression have been much of a study over the last two centuries. Recent studies have seen different developments for the early diagnosis and treatment of the disease; some of which even promise survival of the patient. Cysteine proteases, mainly cathepsins have been unequivocally identified as putative worthy players of redox imbalance that contribute to the premonition and further progression of cancer by interfering in the normal extracellular and intracellular proteolysis and initiating a proteolytic cascade. The present review article focuses on the study of cancer so far, while establishing facts on how future studies focused on the cellular interrelation between nitric oxide (NO) and cancer, can direct their focus on cathepsins. For a tumor cell to thrive and synergize a cancerous environment, different mutations in the proteolytic and signaling pathways and the proto-oncogenes, oncogenes, and the tumor suppressor genes are made possible through cellular biochemistry and some cancer-stimulating environmental factors. The accumulated findings show that S-nitrosylation of cathepsins under the influence of NO-donors can prevent the invasion of cancer and cause cancer cell death by blocking the activity of cathepsins as well as the major denitrosylase systems using a multi-way approach. Faced with a conundrum of how to fill the gap between the dodging of established cancer hallmarks with cathepsin activity and gaining appropriate research/clinical accreditation using our hypothesis, the scope of this review also explores the interplay and crosstalk between S-nitrosylation and S-(de)nitrosylation of this protease and highlights the utility of charging thioredoxin (Trx) reductase inhibitors, low-molecular-weight dithiols, and Trx mimetics using efficient drug delivery system to prevent the denitrosylation or regaining of cathepsin activity in vivo. In foresight, this raises the prospect that drugs or novel compounds that target cathepsins taking all these factors into consideration could be deployed as alternative or even better treatments for cancer, though further research is needed to ascertain the safety, efficiency and effectiveness of this approach.

Ribonucleotide reductase: Implications of thiol S-nitrosylation and tyrosine nitration for different subunits

Ribonucleotide reductase (RNR) is a multi-subunit enzyme responsible for catalyzing the rate-limiting step in the production of deoxyribonucleotides essential for DNA synthesis and repair. The active RNR complex is composed of multimeric R1 and R2 subunits. The RNR catalysis involves the formation of tyrosyl radicals in R2 subunits and thiyl radicals in R1 subunits. Despite the quaternary structure and cofactor diversity, all the three classes of RNR have a conserved cysteine residue at the active site which is converted into a thiyl radical that initiates the substrate turnover, suggesting that the catalytic mechanism is somewhat similar for all three classes of the RNR enzyme. Increased RNR activity has been associated with malignant transformation, cancer cell growth, and tumorigenesis. Efforts concerning the understanding of RNR inhibition in designing potent RNR inhibitors/drugs as well as developing novel approaches for antibacterial, antiviral treatments, and cancer therapeutics with improved radiosensitization have been made in clinical research. This review highlights the precise and potent roles of NO in RNR inhibition by targeting both the subunits. Under nitrosative stress, the thiols of the R1 subunits have been found to be modified by S-nitrosylation and the tyrosyl radicals of the R2 subunits have been modified by nitration. In view of the recent advances and progresses in the field of nitrosative modifications and its fundamental role in signaling with implications in health and diseases, the present article focuses on the regulations of RNR activity by S-nitrosylation of thiols (R1 subunits) and nitration of tyrosyl residues (R2 subunits) which will further help in designing new drugs and therapies.

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.

S-Nitrosylation of RhoGAP Myosin9A Is Altered in Advanced Diabetic Kidney Disease

The molecular pathogenesis of diabetic kidney disease progression is complex and remains unresolved. Rho-GAP MYO9A was recently identified as a novel podocyte protein and a candidate gene for monogenic FSGS. Myo9A involvement in diabetic kidney disease has been suggested. Here, we examined the effect of diabetic milieu on Myo9A expression in vivo and in vitro. We determined that Myo9A undergoes S-nitrosylation, a post-translational modification dependent on nitric oxide (NO) availability. Diabetic mice with nodular glomerulosclerosis and severe proteinuria associated with doxycycline-induced, podocyte-specific VEGF ₁₆₄ gain-of-function showed markedly decreased glomerular Myo9A expression and S-nitrosylation, as compared to uninduced diabetic mice. Immortalized mouse podocytes exposed to high glucose revealed decreased Myo9A expression, assessed by qPCR, immunoblot and immunocytochemistry, and reduced Myo9A S-nitrosylation (SNO-Myo9A), assessed by proximity link assay and biotin switch test, functionally resulting in abnormal podocyte migration. These defects were abrogated by exposure to a NO donor and were not due to hyperosmolarity. Our data demonstrate that high-glucose induced decrease of both Myo9A expression and SNO-Myo9A is regulated by NO availability. We detected S-nitrosylation of Myo9A interacting proteins RhoA and actin, which was also altered by high glucose and NO dependent. RhoA activity inversely related to SNO-RhoA. Collectively, data suggest that dysregulation of SNO-Myo9A, SNO-RhoA and SNO-actin may contribute to the pathogenesis of advanced diabetic kidney disease and may be amenable to therapeutic targeting.

Palladium Nanoparticle-Induced Oxidative Stress, Endoplasmic Reticulum Stress, Apoptosis, and Immunomodulation Enhance the Biogenesis and Release of Exosome in Human Leukemia Monocytic Cells (THP-1)

Background

Exosomes are endosome-derived nano-sized vesicles that have emerged as important mediators of intercellular communication and play significant roles in various diseases. However, their applications are rigorously restricted by the limited secretion competence of cells. Therefore, strategies to enhance the production and functions of exosomes are warranted. Studies have shown that nanomaterials can significantly enhance the effects of cells and exosomes in intercellular communication; however, how palladium nanoparticles (PdNPs) enhance exosome release in human leukemia monocytic cells (THP-1) remains unclear. Therefore, this study aimed to address the effect of PdNPs on exosome biogenesis and release in THP-1 cells.

Methods

Exosomes were isolated by ultracentrifugation and ExoQuick^TM and characterized by dynamic light scattering, nanoparticle tracking analysis system, scanning electron microscopy, transmission electron microscopy, EXOCET^TM assay, and fluorescence polarization. The expression levels of exosome markers were analyzed via quantitative reverse transcription-polymerase chain reaction and enzyme-linked immunosorbent assay.

Results

PdNP treatment enhanced the biogenesis and release of exosomes by inducing oxidative stress, endoplasmic reticulum stress, apoptosis, and immunomodulation. The exosomes were spherical in shape and had an average diameter of 50–80 nm. Exosome production was confirmed via total protein concentration, exosome counts, acetylcholinesterase activity, and neutral sphingomyelinase activity. The expression levels of TSG101, CD9, CD63, and CD81 were significantly higher in PdNP-treated cells than in control cells. Further, cytokine and chemokine levels were significantly higher in exosomes isolated from PdNP-treated THP-1 cells than in those isolated from control cells. THP-1 cells pre-treated with N-acetylcysteine or GW4869 showed significant decreases in PdNP-induced exosome biogenesis and release.

Conclusion

To our knowledge, this is the first study showing that PdNPs stimulate exosome biogenesis and release and simultaneously increase the levels of cytokines and chemokines by modulating various physiological processes. Our findings suggest a reasonable approach to improve the production of exosomes for various therapeutic applications.

Neurodegeneration: Impact of S-nitrosylated Parkin, DJ-1 and PINK1 on the pathogenesis of Parkinson's disease

Parkinson's disease (PD) is one of the fastest-growing neurodegenerative disorders of increasing global prevalence. It represents the second most common movement disorder after tremor and the second most common neurodegenerative disorder after Alzheimer's disease. The incidence rate of idiopathic PD increases steadily with age, however, some variants of autosomal recessive inheritance are present with an early age-at-onset (ARPD). Approximately 50 percent of ARPD cases have been linked to bi-allelic mutations in genes encoding Parkin, DJ-1, and PINK1. Each protein has been implicated in maintaining proper mitochondrial function, which is particularly important for neuronal health. Aberrant post-translational modifications of these proteins may disrupt their cellular functions and thus contributing to the development of idiopathic PD. Some post-translational modifictions can be attributed to the dysregulation of potentially harmful reactive oxygen and nitrogen species inside the cell, which promote oxidative and nitrosative stress, respectively. Unlike oxidative modifications, the covalent modification by Nitric Oxide under nitrosative stress, leading to S-nitrosylation of Parkin, DJ-1; and PINK1, is less studied. Here, we review the available literature on S-nitrosylation of these three proteins, their implications in the pathogenesis of PD, and provide an overview of currently known, denitrosylating systems in eukaryotic cells.

Exploiting S-nitrosylation for cancer therapy: facts and perspectives

S-nitrosylation, the post-translational modification of cysteines by nitric oxide, has been implicated in several cellular processes and tissue homeostasis. As a result, alterations in the mechanisms controlling the levels of S-nitrosylated proteins have been found in pathological states. In the last few years, a role in cancer has been proposed, supported by the evidence that various oncoproteins undergo gain- or loss-of-function modifications upon S-nitrosylation. Here, we aim at providing insight into the current knowledge about the role of S-nitrosylation in different aspects of cancer biology and report the main anticancer strategies based on: (i) reducing S-nitrosylation-mediated oncogenic effects, (ii) boosting S-nitrosylation to stimulate cell death, (iii) exploiting S-nitrosylation through synthetic lethality.

Investigating the Thioredoxin and Glutathione Systems' Response in Lymphoma Cells after Treatment with [Au(d2pype)2]CL

Lymphoma is a blood cancer comprising various subtypes. Although effective therapies are available, some patients fail to respond to treatment and can suffer from side effects. Antioxidant systems, especially the thioredoxin (Trx) and glutathione (GSH) systems, are known to enhance cancer cell survival, with thioredoxin reductase (TrxR) recently reported as a potential anticancer target. Since the GSH system can compensate for some Trx system functions, we investigated its response in three lymphoma cell lines after inhibiting TrxR activity with [Au(d2pype)2]Cl, a known TrxR inhibitor. [Au(d2pype)2]Cl increased intracellular reactive oxygen species (ROS) levels and induced caspase-3 activity leading to cell apoptosis through inhibiting both TrxR and glutathione peroxidase (Gpx) activity. Expression of the tumour suppresser gene TXNIP increased, while GPX1 and GPX4 expression, which are related to poor prognosis of lymphoma patients, decreased. Unlike SUDHL2 and SUDHL4 cells, which exhibited a decreased GSH/GSSG ratio after treatment, in KMH2 cells the ratio remained unchanged, while glutathione reductase and glutaredoxin expression increased. Since KMH2 cells were less sensitive to treatment with [Au(d2pype)2]Cl, the GSH system may play a role in protecting cells from apoptosis after TrxR inhibition. Overall, our study demonstrates that inhibition of TrxR represents a valid therapeutic approach for lymphoma.

Modulatory mechanisms of NLRP3: Potential roles in inflammasome activation

The NLRP3 inflammasome regulates innate immune and inflammatory responses by promoting pro-inflammatory cytokines such as IL-18 and IL-1β. NLRP3 is one of the main factors restricting the activation of the inflammasome, which is closely related to the abundance and localization of NLRP3. A substantial number of studies have focused on specifically targeting NLRP3 to develop inhibitors against NLRP3 inflammasome. Here, we succinctly review the regulation of NLRP3 expression at DNA/chromosome, transcriptional, post-transcriptional, and translation levels. These are critical for the fine regulation of the NLRP3 inflammasome.

Structural and functional insights into nitrosoglutathione reductase from Chlamydomonas reinhardtii

Protein S-nitrosylation plays a fundamental role in cell signaling and nitrosoglutathione (GSNO) is considered as the main nitrosylating signaling molecule. Enzymatic systems controlling GSNO homeostasis are thus crucial to indirectly control the formation of protein S-nitrosothiols. GSNO reductase (GSNOR) is the key enzyme controlling GSNO levels by catalyzing its degradation in the presence of NADH. Here, we found that protein extracts from the microalga Chlamydomonas reinhardtii catabolize GSNO via two enzymatic systems having specific reliance on NADPH or NADH and different biochemical features. Scoring the Chlamydomonas genome for orthologs of known plant GSNORs, we found two genes encoding for putative and almost identical GSNOR isoenzymes. One of the two, here named CrGSNOR1, was heterologously expressed and purified. Its kinetic properties were determined and the three-dimensional structures of the apo-, NAD⁺- and NAD⁺/GSNO-forms were solved. These analyses revealed that CrGSNOR1 has a strict specificity towards GSNO and NADH, and a conserved folding with respect to other plant GSNORs. The catalytic zinc ion, however, showed an unexpected variability of the coordination environment. Furthermore, we evaluated the catalytic response of CrGSNOR1 to thermal denaturation, thiol-modifying agents and oxidative modifications as well as the reactivity and position of accessible cysteines. Despite being a cysteine-rich protein, CrGSNOR1 contains only two solvent-exposed/reactive cysteines. Oxidizing and nitrosylating treatments have null or limited effects on CrGSNOR1 activity and folding, highlighting a certain resistance of the algal enzyme to redox modifications. The molecular mechanisms and structural features underlying the response to thiol-based modifications are discussed.

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Chlamydomonas protein extracts catalyze NAD(P)H-dependent GSNO degradation.

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Chlamydomonas GSNOR1 is a zinc-containing protein strictly relying on GSNO and NADH.

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The 3D-structure of CrGSNOR1 revealed a conserved folding with other plant GSNORs.

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CrGSNOR1 contains only two solvent-exposed/reactive cysteines.

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Oxidizing and nitrosylating treatments have limited effects on CrGSNOR1 activity.

All Roads Lead to Auxin: Post-translational Regulation of Auxin Transport by Multiple Hormonal Pathways

Auxin is a key hormonal regulator, that governs plant growth and development in concert with other hormonal pathways. The unique feature of auxin is its polar, cell-to-cell transport that leads to the formation of local auxin maxima and gradients, which coordinate initiation and patterning of plant organs. The molecular machinery mediating polar auxin transport is one of the important points of interaction with other hormones. Multiple hormonal pathways converge at the regulation of auxin transport and form a regulatory network that integrates various developmental and environmental inputs to steer plant development. In this review, we discuss recent advances in understanding the mechanisms that underlie regulation of polar auxin transport by multiple hormonal pathways. Specifically, we focus on the post-translational mechanisms that contribute to fine-tuning of the abundance and polarity of auxin transporters at the plasma membrane and thereby enable rapid modification of the auxin flow to coordinate plant growth and development.

When S-Nitrosylation Gets to Mitochondria: From Signaling to Age-Related Diseases

Significance: Cysteines have an essential role in redox signaling, transforming an oxidant signal into a biological response. Among reversible cysteine post-translational modifications, S-nitrosylation acts as a redox-switch in several pathophysiological states, such as ischemia/reperfusion, synaptic transmission, cancer, and muscular dysfunctions. Recent Advances: Growing pieces of in vitro and in vivo evidence argue for S-nitrosylation being deeply involved in development and aging, and playing a role in the onset of different pathological states. New findings suggest it being an enzymatically regulated cellular process, with deep impact on mitochondrial structure and function, and in cellular metabolism. In light of this, the recent discovery of the denitrosylase S-nitrosoCoA (coenzyme A) reductase takes on even greater importance and opens new perspectives on S-nitrosylation as a general mechanism of cellular homeostasis. Critical Issues: Based on these recent findings, we aim at summarizing and elaborating on the established and emerging crucial roles of S-nitrosylation in mitochondrial metabolism and mitophagy, and provide an overview of the pathophysiological effects induced by its deregulation. Future Directions: The identification of new S-nitrosylation targets, and the comprehension of the mechanisms through which S-nitrosylation modulates specific classes of proteins, that is, those impinging on diverse mitochondrial functions, may help to better understand the pathophysiology of aging, and propose lines of intervention to slow down or extend the onset of aging-related diseases.

Upregulation of antioxidant thioredoxin by antidepressants fluoxetine and venlafaxine

RATIONALE

Selective serotonin reuptake inhibitors (SSRIs) and serotonin-norepinephrine reuptake inhibitors (SNRIs) are the most commonly used drugs for the treatment of depression. Studies have shown that chronic treatment with SSRIs and SNRIs produces a protective effect against oxidative stress. Thioredoxin (Trx) is an antioxidant protein that reverses protein cysteine oxidation and facilitates scavenging reactive oxygen species.

OBJECTIVES

The current study is to determine whether the SSRI fluoxetine and the SNRI venlafaxine regulate Trx and protect neuronal cells against protein cysteine oxidation.

METHODS

HT22 mouse hippocampal cells were incubated with fluoxetine or venlafaxine for 5 days. Protein levels of Trx, Trx reductase (TrxR), and Trx-interacting protein (Txnip) were measured by immunoblotting analysis. Trx and TrxR activities were analyzed by spectrophotometric method. Protein cysteine sulfenylation was measured by dimedone-conjugation assay, while nitrosylation was measured by biotin-switch assay.

RESULTS

We found that treatment with fluoxetine or venlafaxine for 5 days increased Trx and TrxR protein levels but produced no effect on Txnip protein levels. These treatments also increased Trx and TrxR activities. Although treatment with fluoxetine or venlafaxine alone had no effect on sulfenylated and nitrosylated protein levels, both drugs inhibited H₂O₂-increased sulfenylated protein levels and nitric oxide donor nitrosoglutathione-increased nitrosylated protein levels. Stress increases risk of depression. We also found that treatment with fluoxetine or venlafaxine for 5 days inhibited stress hormone corticosterone-increased total sulfenylated and nitrosylated protein levels.

CONCLUSIONS

Our findings suggest that chronic treatment with antidepressants may upregulate Trx, subsequently inhibiting protein sulfenylation and nitrosylation, which may contribute to the protective effect of antidepressants against oxidative stress. Our findings also indicate that thioredoxin is a potential therapeutic target for the treatment of depression.

Substrate specificity of thioredoxins and glutaredoxins - towards a functional classification

The spatio-temporal reduction and oxidation of protein thiols is an essential mechanism in signal transduction in all kingdoms of life. Thioredoxin (Trx) family proteins efficiently catalyze thiol-disulfide exchange reactions and the proteins are widely recognized for their importance in the operation of thiol switches. Trx family proteins have a broad and at the same time very distinct substrate specificity – a prerequisite for redox switching. Despite of multiple efforts, the true nature for this specificity is still under debate. Here, we comprehensively compare the classification/clustering of various redoxins from all domains of life based on their similarity in amino acid sequence, tertiary structure, and their electrostatic properties. We correlate these similarities to the existence of common interaction partners, identified in various previous studies and suggested by proteomic screenings. These analyses confirm that primary and tertiary structure similarity, and thereby all common classification systems, do not correlate to the target specificity of the proteins as thiol-disulfide oxidoreductases. Instead, a number of examples clearly demonstrate the importance of electrostatic similarity for their target specificity, independent of their belonging to the Trx or glutaredoxin subfamilies.

Thiol-Redox Regulation in Lung Development and Vascular Remodeling

Significance: Redox homeostasis is finely tuned and governed by distinct intracellular mechanisms. The dysregulation of this either by external or internal events is a fundamental pathophysiologic base for many pulmonary diseases. Recent Advances: Based on recent discoveries, it is increasingly clear that cellular redox state and oxidation of signaling molecules are critical modulators of lung disease and represent a final common pathway that leads to poor respiratory outcomes. Critical Issues: Based on the wide variety of stimuli that alter specific redox signaling pathways, improved understanding of the disease and patient-specific alterations are needed for the development of therapeutic targets. Further Directions: For the full comprehension of redox signaling in pulmonary disease, it is essential to recognize the role of reactive oxygen intermediates in modulating biological responses. This review summarizes current knowledge of redox signaling in pulmonary development and pulmonary vascular disease.

[Selenium compounds in redox regulation of inflammation and apoptosis]

Monocytes and macrophages play a key role in the development of inflammation: under the action of lipopolysaccharides (LPS), absorbed from the intestine, monocytes and macrophages form reactive oxygen species (ROS) and cytokines, this leads to the development of oxidative stress, inflammation and/or apoptosis in all types of tissues. In the cells LPS induce an "internal" TLR4-mediated MAP-kinase inflammatory signaling pathway and cytokines through the superfamily of tumor necrosis factor receptor (TNFR) and the "death domain" (DD) initiate an "external" caspase apoptosis cascade or necrosis activation that causes necroptosis. Many of the proteins involved in intracellular signaling cascades (MYD88, ASK1, IKKa/b, NF-kB, AP-1) are redox-sensitive and their activity is regulated by antioxidants thioredoxin, glutaredoxin, nitroredoxin, and glutathione. Oxidation of these signaling proteins induced by ROS enhances the development of inflammation and apoptosis, and their reduction with antioxidants, on the contrary, stabilizes the signaling cascades speed, preventing the vicious circle of oxidative stress, inflammation and apoptosis that follows it. Antioxidant (AO) enzymes thioredoxin reductase (TRXR), glutaredoxin reductase (GLRXR), glutathione reductase (GR) are required for reduction of non-enzymatic antioxidants (thioredoxin, glutaredoxin, nitroredoxin, glutathione), and AO enzymes (SOD, catalase, GPX) are required for ROS deactivation. The key AO enzymes (TRXR and GPX) are selenium-dependent; therefore selenium deficiency leads to a decrease in the body's antioxidant defense, the development of oxidative stress, inflammation, and/or apoptosis in various cell types. Nrf2-Keap1 signaling pathway activated by selenium deficiency and/or oxidative stress is necessary to restore redox homeostasis in the cell. In addition, expression of some genes is changed with selenium deficiency. Consequently, growth and proliferation of cells, their movement, development, death, and survival, as well as the interaction between cells, the redox regulation of intracellular signaling cascades of inflammation and apoptosis, depend on the selenium status of the body. Prophylactic administration of selenium-containing preparations (natural and synthetic (organic and inorganic)) is able to normalize the activity of AO enzymes and the general status of the body. Organic selenium compounds have a high bioavailability and, depending on their concentration, can act both as selenium donors to prevent selenium deficiency and as antitumor drugs due to their toxicity and participation in the regulation of signaling pathways of apoptosis. Known selenorganic compounds diphenyldiselenide and ethaselen share similarity with the Russian organo selenium compound, diacetophenonylselenide (DAPS-25), which serves as a source of bioavailable selenium, exhibits a wide range of biological activity, including antioxidant activity, that governs cell redox balance, inflammation and apoptosis regulation.

Extracellular vesicle-mediated macrophage activation: An insight into the mechanism of thioredoxin-mediated immune activation

Extracellular vesicles (EVs) generated from redox active anticancer drugs are released into the extracellular environment. These EVs contain oxidized molecules and trigger inflammatory responses by macrophages. Using a mouse model of doxorubicin (DOX)-induced tissue injury, we previously found that the major sources of circulating EVs are from heart and liver, organs that are differentially affected by DOX. Here, we investigated the effects of EVs from cardiomyocytes and those from hepatocytes on macrophage activation. EVs from H9c2 rat cardiomyocytes (H9c2 EVs) and EVs from FL83b mouse hepatocytes (FL83 b EVs) have different levels of protein-bound 4-hydroxynonenal and thus different immunostimulatory effects on mouse RAW264.7 macrophages. H9c2 EVs but not FL83 b EVs induced both pro-inflammatory and anti-inflammatory macrophage activation, mediated by NFκB and Nrf-2 pathways, respectively. DOX enhanced the effects of H9c2 EVs but not FL83 b EVs. While EVs from DOX-treated H9c2 cells (H9c2 DOXEVs) suppressed mitochondrial respiration and increased glycolysis of macrophages, EVs from DOX-treated FL83b cells (FL83b DOXEVs) enhanced mitochondrial reserve capacity. Mechanistically, the different immunostimulatory functions of H9c2 EVs and FL83 b EVs are regulated, in part, by the redox status of the cytoplasmic thioredoxin 1 (Trx1) of macrophages. H9c2 DOXEVs lowered the level of reduced Trx1 in cytoplasm while FL83b DOXEVs did the opposite. Trx1 overexpression alleviated the effect of H9c2 DOXEVs on NFκB and Nrf-2 activation and prevented the upregulation of their target genes. Our findings identify EVs as a novel Trx1-mediated redox mediator of immune response, which greatly enhances our understanding of innate immune responses during cancer therapy.

Urban Particulate Matter-Induced Decomposition of S-Nitrosoglutathione Relevant to Aberrant Nitric Oxide Biological Signaling

Exposure to airborne particulate matter (PM) is associated with hazardous effects on human health. Soluble constituents of PM may be released in biological fluids and disturb the precisely tuned nitric oxide signaling processes. The influence of aqueous extracts from two types of airborne urban PM (SRM 1648a, a commercially available sample, and KR PM2.5, a sample collected "in-house" in Krakow, Poland) on the stability of S-nitrosoglutathione (GSNO) was investigated. The particle interfaces had no direct effect on the studied reaction, but extracts obtained from both samples facilitated NO release from GSNO. The effectiveness of NO release was significantly affected by glutathione (GSH) and ascorbic acid (AscA). Examination of the combined influence of Cu²⁺ , Fe³⁺ , and reductants on GSNO stability revealed copper to be the main GSNO decomposing species. Computational models of nitrosothiols interacting with metal oxide substrates and solvated metal ions support these claims. The study stresses the importance of the interplay between metal ions and biological reductants in S-nitrosothiols decomposition.

Exogenous recombinant human thioredoxin-1 prevents acetaminophen-induced liver injury by scavenging oxidative stressors, restoring the thioredoxin-1 system and inhibiting receptor interacting protein-3 overexpression

Thioredoxin-1 (Trx-1) is a potent therapeutic agent against a variety of diseases because of its actions as an antioxidant and regulator of apoptosis. N-acetyl-p-aminophenol (APAP), commonly known as acetaminophen, generates excessive oxidative stress and triggers hepatocyte cell death, exemplified by regulated necrosis. In the present study, we investigated whether APAP-induced liver injury in a mouse model is associated with "necroptosis," and if pretreatment with recombinant Trx-1 prevents the hepatic injury caused by APAP overdose. We also explored the mechanism underlying the preventive action of Trx-1 against APAP-induced hepatic injury. In a prevention study, C3H/he mice received different doses (0, 10, 50 or 100 mg kg^-1 body weight) of recombinant human Trx-1 intraperitoneally, followed by a single oral dose of 300 mg kg^-1 of APAP. In this experimental paradigm, liver injury and lethality were markedly decreased in rhTrx-1-pretreated mice. In survival experiments, mice received rhTrx-1 followed by oral administration of a lethal dose of APAP. APAP overdose caused a series of liver toxicity-associated events, beginning with overexpression of c-fos, excessive production of reactive oxygen species and reactive nitrogen species (RNS) and leading to decreased endogenous Trx-1 expression and activation of JNK signaling pathways. Pretreatment with rhTrx-1 inhibited all of these toxicological manifestations of APAP. In addition, rhTrx-1 significantly reduced the expression of RIP-3, a critical necrosome component. Taken together, our findings indicate that rhTrx-1 prevents APAP-induced liver injury through multiple action mechanisms, including scavenging reactive oxygen species and reactive nitrogen species, restoring endogenous Trx-1 levels and inhibiting RIP-3 overexpression.

Oxidative Signaling Response to Cadmium Exposure

Changes in the intracellular thiol-disulfide balance are considered major determinants in the redox status/signaling of the cell. Cellular signaling is very sensitive to both exogenous and intracellular redox status and respond to many exogenous pro-oxidative or oxidative stresses. Redox status has dual effects on upstream signaling systems and downstream transcription factors. Redox signaling pathways use reactive oxygen species (ROS) to transfer signals from different sources to the nucleus to regulate such functions as growth, differentiation, proliferation, and apoptosis. Mitogen-activated protein kinases are activated by numerous cellular stresses and ligand-receptor bindings. An imbalance in the oxidant/antioxidant system, either resulting from excessive ROS/reactive nitrogen species production and/or antioxidant system impairment, leads to oxidative stress. Glutathione (GSH) is known to play a critical role in the cellular defense against unregulated oxidative stress in mammalian cells and involvement of large molecular antioxidants include classical antioxidant enzymes, such as superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), and glutathione reductase (GR). Cadmium (Cd), a potent toxic heavy metal, is a widespread environmental contaminant. It is known to cause renal dysfunction, hepatic toxicity, genotoxicity, and apoptotic effects depending on the dose, route, and duration of exposure. This review examines the signaling pathways and mechanisms of activation of transcription factors by Cd-induced oxidative stress thus representing an important basis for understanding the mechanisms of Cd effect on the cells.

Thioredoxin-1 Overexpression in the Ventromedial Nucleus of the Hypothalamus Preserves the Counterregulatory Response to Hypoglycemia During Type 1 Diabetes in Male Rats

We previously showed that the glutathione precursor, N-acetylcysteine (NAC), prevented hypoglycemia-associated autonomic failure (HAAF) and impaired activation of ventromedial hypothalamus (VMH) glucose-inhibited (GI) neurons by low glucose after recurrent hypoglycemia (RH) in nondiabetic rats. However, NAC does not normalize glucose sensing by VMH GI neurons when RH occurs during diabetes. We hypothesized that recruiting the thioredoxin (Trx) antioxidant defense system would prevent HAAF and normalize glucose sensing after RH in diabetes. To test this hypothesis, we overexpressed Trx-1 (cytosolic form of Trx) in the VMH of rats with streptozotocin (STZ)-induced type 1 diabetes. The counterregulatory response (CRR) to hypoglycemia in vivo and the activation of VMH GI neurons in low glucose using membrane potential sensitive dye in vitro was measured before and after RH. VMH Trx-1 overexpression normalized both the CRR and glucose sensing by VMH GI neurons in STZ rats. VMH Trx-1 overexpression also lowered the insulin requirement to prevent severe hyperglycemia in STZ rats. However, like NAC, VMH Trx-1 overexpression did not prevent HAAF or normalize activation of VMH GI neurons by low glucose in STZ rats after RH. We conclude that preventing HAAF in type 1 diabetes may require the recruitment of both antioxidant systems.

Regulation of protein function by S-nitrosation and S-glutathionylation: processes and targets in cardiovascular pathophysiology

Decades of chemical, biochemical and pathophysiological research have established the relevance of post-translational protein modifications induced by processes related to oxidative stress, with critical reflections on cellular signal transduction pathways. A great deal of the so-called ‘redox regulation’ of cell function is in fact mediated through reactions promoted by reactive oxygen and nitrogen species on more or less specific aminoacid residues in proteins, at various levels within the cell machinery. Modifications involving cysteine residues have received most attention, due to the critical roles they play in determining the structure/function correlates in proteins. The peculiar reactivity of these residues results in two major classes of modifications, with incorporation of NO moieties (S-nitrosation, leading to formation of proteinS-nitrosothiols) or binding of low molecular weight thiols (S-thionylation, i.e. in particularS-glutathionylation,S-cysteinylglycinylation andS-cysteinylation). A wide array of proteins have been thus analyzed in detail as far as their susceptibility to either modification or both, and the resulting functional changes have been described in a number of experimental settings. The present review aims to provide an update of available knowledge in the field, with a special focus on the respective (sometimes competing and antagonistic) roles played by proteinS-nitrosations andS-thionylations in biochemical and cellular processes specifically pertaining to pathogenesis of cardiovascular diseases.

Basal S-Nitrosylation Is the Guardian of Tissue Homeostasis

Recent studies have uncovered that nitric oxide (NO) signaling is largely conducted by S-nitrosylation, involving >3000 proteins. The nitrosyl group could then travel further by transnitrosylation or be secreted, enabling regulation of the whole tissue. A subset of proteins are constitutively S-nitrosylated, playing roles in the regulation of tissue homeostasis.

Functional research and molecular mechanism of Kainic acid-induced denitrosylation of thioredoxin-1 in rat hippocampus

Thioredoxin-1 (Trx1) has long been recognized as a redox regulator, and is implicated in the inhibition of cell apoptosis. Trx1 is essential for the maintenance of the S-nitrosylation of molecules in cells. The S-nitrosylation of Trx1 is essential for the physiological function such as preservation of the redox regulatory activity. The mechanisms underlying Trx1 denitrosylation induced by kainate acid (KA) injection still remain uncharacterized. Our results showed that the S-nitrosylation levels of Trx1 were decreased subsequent to KA injection and that the glutamate receptor 6 (GluR6) antagonist NS102 could inhibit the denitrosylation of Trx1. Moreover, the denitrosylation of Trx1 following KA treatment could be suppressed by the Fas ligand (FasL) antisense oligodeoxynucleotides (AS-ODNs), the Trx reductase (TrxR) inhibitor dinitrochlorobenzene (DNCB), or the Nitric oxide (NO) donors sodium nitroprusside (SNP) and S-nitrosoglutathione (GSNO). Subsequently, these mechanisms were morphologically validated by cresyl violet staining, in situ TUNEL staining to detect the survival of CA1 and CA3/DG pyramidal neurons. NS102, FasL AS-ODNs, GSNO and SNP could provide neuroprotection of the pyramidal neurons of CA1 and CA3/dentate gyrus (DG) regions by attenuating Trx1 denitrosylation. Our results also showed that the denitrosylation of Trx1 induced by KA injection can active the caspase-3 which results in apoptosis.

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.

Modulation of signaling mechanisms in the heart by thioredoxin 1

Myocardial ischemia/reperfusion and heart failure are the major cardiac conditions in which an imbalance between oxidative stress and anti-oxidant mechanisms is observed. The myocardium has endogenous reducing mechanisms, including the thioredoxin (Trx) and glutathione systems, that act to scavenge reactive oxygen species (ROS) and reduce oxidized proteins. The Trx system consists of Trx, Trx reductase (TrxR), and an electron donor, NADPH, where Trx is maintained in a reduced state in the presence of TrxR and NADPH. Trx1, a major isoform of Trx, is abundantly expressed in the heart and exerts its oxidoreductase activity through conserved Cys32 and Cys35, reducing oxidized proteins through thiol disulfide exchange reactions. In this review, we will focus on molecular targets of Trx1 in the heart, including transcription factors, microRNAs, histone deactylases, and protein kinases. We will then discuss how Trx1 regulates the functions of its targets, thereby affecting the extent of myocardial injury caused by myocardial ischemia/reperfusion and the progression of heart failure.

The role of S-nitrosoglutathione reductase (GSNOR) in human disease and therapy

S-nitrosoglutathione reductase (GSNOR), or ADH5, is an enzyme in the alcohol dehydrogenase (ADH) family. It is unique when compared to other ADH enzymes in that primary short-chain alcohols are not its principle substrate. GSNOR metabolizes S-nitrosoglutathione (GSNO), S-hydroxymethylglutathione (the spontaneous adduct of formaldehyde and glutathione), and some alcohols. GSNOR modulates reactive nitric oxide (•NO) availability in the cell by catalyzing the breakdown of GSNO, and indirectly regulates S-nitrosothiols (RSNOs) through GSNO-mediated protein S-nitrosation. The dysregulation of GSNOR can significantly alter cellular homeostasis, leading to disease. GSNOR plays an important regulatory role in smooth muscle relaxation, immune function, inflammation, neuronal development and cancer progression, among many other processes. In recent years, the therapeutic inhibition of GSNOR has been investigated to treat asthma, cystic fibrosis and interstitial lung disease (ILD). The direct action of •NO on cellular pathways, as well as the important regulatory role of protein S-nitrosation, is closely tied to GSNOR regulation and defines this enzyme as an important therapeutic target.

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.

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.

To eat, or NOt to eat: S-nitrosylation signaling in autophagy

Autophagy is the main catabolic cellular process through which cells adapt their needs (e.g., growth and proliferation) to environmental availability of nutrients (e.g., amino acid and glucose) and growth factors. The rapid activation of the autophagy response essentially depends on protein post-translational modifications (PTMs), which act as molecular switches triggering signaling cascades. Deregulation of autophagy contributes to pathological conditions, such as cancer and neurodegeneration. Therefore, understanding how PTMs affect the occurrence of autophagy is of the highest importance for clinical applications. Besides phosphorylation and ubiquitylation, which represent the best known examples of PTMs, redox-based modifications are also emerging as contributing to the regulation of intracellular signaling. Of note, S-nitrosylation of cysteine residues is a redox PTM and is the principal mechanism of nitric oxide-based signaling. Results emerging in recent years suggest that NO has a role in modulating autophagy. However, the function of S-nitrosylation in autophagy regulation remains still unveiled. By this review, we describe the upstream events regulating autophagy activation focusing on recently published evidence implying a S-nitrosylation-dependent regulation.

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.