Thioredoxin-1 and posttranslational modifications

Thioredoxin System in Insects: Uncovering the Roles of Thioredoxins and Thioredoxin Reductase beyond the Antioxidant Defences

Increased levels of reactive oxygen species (ROS) produced during aerobic metabolism in animals can negatively affect the intracellular redox status, cause oxidative stress and interfere with physiological processes in the cells. The antioxidant defence regulates ROS levels by interplaying diverse enzymes and non-enzymatic metabolites. The thioredoxin system, consisting of the enzyme thioredoxin reductase (TrxR), the redox-active protein thioredoxin (Trx) and NADPH, represent a crucial component of antioxidant defence. It is involved in the signalling and regulation of multiple developmental processes, such as cell proliferation or apoptotic death. Insects have evolved unique variations of TrxR, which resemble mammalian enzymes in overall structure and catalytic mechanisms, but the selenocysteine-cysteine pair in the active site is replaced by a cysteine-cysteine pair typical of bacteria. Moreover, the role of the thioredoxin system in insects is indispensable due to the absence of glutathione reductase, an essential enzyme of the glutathione system. However, the functions of the Trx system in insects are still poorly characterised. In the present review, we provide a critical overview of the current knowledge on the insect Trx system, focusing mainly on TrxR's role in the antioxidant and immune system of model insect species.

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.

Exploring the Thioredoxin System as a Therapeutic Target in Cancer: Mechanisms and Implications

Cells constantly face the challenge of managing oxidants. In aerobic organisms, oxygen (O2) is used for energy production, generating reactive oxygen species (ROS) as byproducts of enzymatic reactions. To protect against oxidative damage, cells possess an intricate system of redox scavengers and antioxidant enzymes, collectively forming the antioxidant defense system. This system maintains the redox equilibrium and enables the generation of localized oxidative signals that regulate essential cellular functions. One key component of this defense is the thioredoxin (Trx) system, which includes Trx, thioredoxin reductase (TrxR), and NADPH. The Trx system reverses oxidation of macromolecules and indirectly neutralizes ROS via peroxiredoxin (Prx). This dual function protects cells from damage accumulation and supports physiological cell signaling. However, the Trx system also shields tumors from oxidative damage, aiding their survival. Due to elevated ROS levels from their metabolism, tumors often rely on the Trx system. In addition, the Trx system regulates critical pathways such as proliferation and neoangiogenesis, which tumors exploit to enhance growth and optimize nutrient and oxygen supply. Consequently, the Trx system is a potential target for cancer therapy. The challenge lies in selectively targeting malignant cells without disrupting the redox equilibrium in healthy cells. The aim of this review article is threefold: first, to elucidate the function of the Trx system; second, to discuss the Trx system as a potential target for cancer therapies; and third, to present the possibilities for inhibiting key components of the Trx system, along with an overview of the latest clinical studies on these inhibitors.

Glutathionyl Hemoglobin and Its Emerging Role as a Clinical Biomarker of Chronic Oxidative Stress

Hemoglobin is one of the proteins that are more susceptible to S-glutathionylation and the levels of its modified form, glutathionyl hemoglobin (HbSSG), increase in several human pathological conditions. The scope of the present review is to provide knowledge about how hemoglobin is subjected to S-glutathionylation and how this modification affects its functionality. The different diseases that showed increased levels of HbSSG and the methods used for its quantification in clinical investigations will be also outlined. Since there is a growing need for precise and reliable methods for markers of oxidative stress in human blood, this review highlights how HbSSG is emerging more and more as a good indicator of severe oxidative stress but also as a key pathogenic factor in several diseases.

Caffeine Inhibits Oxidative Stress- and Low Dose Endotoxemia-Induced Senescence-Role of Thioredoxin-1

The maintenance of Thioredoxin-1 (Trx-1) levels, and thus of cellular redox homeostasis, is vital for endothelial cells (ECs) to prevent senescence induction. One hallmark of EC functionality, their migratory capacity, which depends on intact mitochondria, is reduced in senescence. Caffeine improves the migratory capacity and mitochondrial functionality of ECs. However, the impact of caffeine on EC senescence has never been investigated. Moreover, a high-fat diet, which can induce EC senescence, results in approximately 1 ng/mL lipopolysaccharide (LPS) in the blood. Therefore, we investigated if low dose endotoxemia induces EC senescence and concomitantly reduces Trx-1 levels, and if caffeine prevents or even reverses senescence. We show that caffeine precludes H₂O₂-triggered senescence induction by maintaining endothelial NO synthase (eNOS) levels and preventing the elevation of p21. Notably, 1 ng/mL LPS also increases p21 levels and reduces eNOS and Trx-1 amounts. These effects are completely blocked by co-treatment with caffeine. This prevention of senescence induction is similarly accomplished by the permanent expression of mitochondrial p27, a downstream effector of caffeine. Most importantly, after senescence induction by LPS, a single bolus of caffeine inhibits the increase in p21. This treatment also blocks Trx-1 degradation, suggesting that the reversion of senescence is intimately associated with a normalized redox balance.

The Importance of Thioredoxin-1 in Health and Disease

Thioredoxin-1 (Trx-1) is a multifunctional protein ubiquitously found in the human body. Trx-1 plays an important role in various cellular functions such as maintenance of redox homeostasis, proliferation, and DNA synthesis, but also modulation of transcription factors and control of cell death. Thus, Trx-1 is one of the most important proteins for proper cell and organ function. Therefore, modulation of Trx gene expression or modulation of Trx activity by various mechanisms, including post-translational modifications or protein-protein interactions, could cause a transition from the physiological state of cells and organs to various pathologies such as cancer, and neurodegenerative and cardiovascular diseases. In this review, we not only discuss the current knowledge of Trx in health and disease, but also highlight its potential function as a biomarker.

Natural Molecules Targeting Thioredoxin System and Their Therapeutic Potential

Significance: Thioredoxin (Trx) and thioredoxin reductase are two core members of the Trx system. The system bridges the gap between the universal reducing equivalent NADPH and various biological molecules and plays an essential role in maintaining cellular redox homeostasis and regulating multiple cellular redox signaling pathways. Recent Advance: In recent years, the Trx system has been well documented as an important regulator of many diseases, especially tumorigenesis. Thus, the development of potential therapeutic molecules targeting the system is of great significance for disease treatment. Critical Issues: We herein first discuss the physiological functions of the Trx system and the role that the Trx system plays in various diseases. Then, we focus on the introduction of natural small molecules with potential therapeutic applications, especially the anticancer activity, and review their mechanisms of pharmacological actions via interfering with the Trx system. Finally, we further discuss several natural molecules that harbor therapeutic potential and have entered different clinical trials. Future Directions: Further studies on the functions of the Trx system in multiple diseases will not only improve our understanding of the pathogenesis of many human disorders but also help develop novel therapeutic strategies against these diseases. Antioxid. Redox Signal. 34, 1083-1107.

Glutathione S-Transferases in Cancer

In humans, the glutathione S-transferases (GST) protein family is composed of seven members that present remarkable structural similarity and some degree of overlapping functionalities. GST proteins are crucial antioxidant enzymes that regulate stress-induced signaling pathways. Interestingly, overactive GST proteins are a frequent feature of many human cancers. Recent evidence has revealed that the biology of most GST proteins is complex and multifaceted and that these proteins actively participate in tumorigenic processes such as cell survival, cell proliferation, and drug resistance. Structural and pharmacological studies have identified various GST inhibitors, and these molecules have progressed to clinical trials for the treatment of cancer and other diseases. In this review, we discuss recent findings in GST protein biology and their roles in cancer development, their contribution in chemoresistance, and the development of GST inhibitors for cancer treatment.

The Writers, Readers, and Erasers in Redox Regulation of GAPDH

Glyceraldehyde 3–phosphate dehydrogenase (GAPDH) is a key glycolytic enzyme, which is crucial for the breakdown of glucose to provide cellular energy. Over the past decade, GAPDH has been reported to be one of the most prominent cellular targets of post-translational modifications (PTMs), which divert GAPDH toward different non-glycolytic functions. Hence, it is termed a moonlighting protein. During metabolic and oxidative stress, GAPDH is a target of different oxidative PTMs (oxPTM), e.g., sulfenylation, S-thiolation, nitrosylation, and sulfhydration. These modifications alter the enzyme’s conformation, subcellular localization, and regulatory interactions with downstream partners, which impact its glycolytic and non-glycolytic functions. In this review, we discuss the redox regulation of GAPDH by different redox writers, which introduce the oxPTM code on GAPDH to instruct a redox response; the GAPDH readers, which decipher the oxPTM code through regulatory interactions and coordinate cellular response via the formation of multi-enzyme signaling complexes; and the redox erasers, which are the reducing systems that regenerate the GAPDH catalytic activity. Human pathologies associated with the oxidation-induced dysregulation of GAPDH are also discussed, featuring the importance of the redox regulation of GAPDH in neurodegeneration and metabolic disorders.

Cysteine oxidation and disulfide formation in the ribosomal exit tunnel

Understanding the conformational sampling of translation-arrested ribosome nascent chain complexes is key to understand co-translational folding. Up to now, coupling of cysteine oxidation, disulfide bond formation and structure formation in nascent chains has remained elusive. Here, we investigate the eye-lens protein γB-crystallin in the ribosomal exit tunnel. Using mass spectrometry, theoretical simulations, dynamic nuclear polarization-enhanced solid-state nuclear magnetic resonance and cryo-electron microscopy, we show that thiol groups of cysteine residues undergo S-glutathionylation and S-nitrosylation and form non-native disulfide bonds. Thus, covalent modification chemistry occurs already prior to nascent chain release as the ribosome exit tunnel provides sufficient space even for disulfide bond formation which can guide protein folding.

Positive Regulation of Interleukin-1β Bioactivity by Physiological ROS-Mediated Cysteine S-Glutathionylation

Reactive oxygen species (ROS)-induced cysteine S-glutathionylation is an important posttranslational modification (PTM) that controls a wide range of intracellular protein activities. However, whether physiological ROS can modulate the function of extracellular components via S-glutathionylation is unknown. Using a screening approach, we identified ROS-mediated cysteine S-glutathionylation on several extracellular cytokines. Glutathionylation of the highly conserved Cys-188 in IL-1β positively regulates its bioactivity by preventing its ROS-induced irreversible oxidation, including sulfinic acid and sulfonic acid formation. We show this mechanism protects IL-1β from deactivation by ROS in an in vivo system of irradiation-induced bone marrow (BM) injury. Glutaredoxin 1 (Grx1), an enzyme that catalyzes deglutathionylation, was present and active in the extracellular space in serum and the BM, physiologically regulating IL-1β glutathionylation and bioactivity. Collectively, we identify cysteine S-glutathionylation as a cytokine regulatory mechanism that could be a therapeutic target in the treatment of various infectious and inflammatory diseases.

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.

S-glutathionylation of glyceraldehyde-3-phosphate dehydrogenase induces formation of C150-C154 intrasubunit disulfide bond in the active site of the enzyme

BACKGROUND

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a glycolytic protein involved in numerous non-glycolytic functions. S-glutathionylated GAPDH was revealed in plant and animal tissues. The role of GAPDH S-glutathionylation is not fully understood.

METHODS

Rabbit muscle GAPDH was S-glutathionylated in the presence of H₂O₂ and reduced glutathione (GSH). The modified protein was assayed by MALDI-MS analysis, differential scanning calorimetry, dynamic light scattering, and ultracentrifugation.

RESULTS

Incubation of GAPDH in the presence of H₂O₂ together with GSH resulted in the complete inactivation of the enzyme. In contrast to irreversible oxidation of GAPDH by H₂O₂, this modification could be reversed in the excess of GSH or dithiothreitol. By data of MALDI-MS analysis, the modified protein contained both mixed disulfide between Cys150 and GSH and the intrasubunit disulfide bond between Cys150 and Cys154 (different subunits of tetrameric GAPDH may contain different products). S-glutathionylation results in loosening of the tertiary structure of GAPDH, decreases its affinity to NAD⁺ and thermal stability.

CONCLUSIONS

The mixed disulfide between Cys150 and GSH is an intermediate product of S-glutathionylation: its subsequent reaction with Cys154 results in the intrasubunit disulfide bond in the active site of GAPDH. The mixed disulfide and the C150-C154 disulfide bond protect GAPDH from irreversible oxidation and can be reduced in the excess of thiols. Conformational changes that were observed in S-glutathionylated GAPDH may affect interactions between GAPDH and other proteins (ligands), suggesting the role of S-glutathionylation in the redox signaling.

GENERAL SIGNIFICANCE

The manuscript considers one of the possible mechanisms of redox regulation of cell functions.

Targeting Thioredoxin-1 by dimethyl fumarate induces ripoptosome-mediated cell death

Constitutively active NFκB promotes survival of many cancers, especially T-cell lymphomas and leukemias by upregulating antiapoptotic proteins such as inhibitors of apoptosis (IAPs) and FLICE-like inhibitory proteins (cFLIPs). IAPs and cFLIPs negatively regulate the ripoptosome, which mediates cell death in an apoptotic or necroptotic manner. Here, we demonstrate for the first time, that DMF antagonizes NFκB by suppressing Thioredoxin-1 (Trx1), a major regulator of NFκB transcriptional activity. DMF-mediated inhibition of NFκB causes ripoptosome formation via downregulation of IAPs and cFLIPs. In addition, DMF promotes mitochondrial Smac release and subsequent degradation of IAPs, further enhancing cell death in tumor cells displaying constitutive NFκB activity. Significantly, CTCL patients treated with DMF display substantial ripoptosome formation and caspase-3 cleavage in T-cells. DMF induces cell death predominantly in malignant or activated T-cells. Further, we show that malignant T-cells can die by both apoptosis and necroptosis, in contrast to resting T-cells, which are restricted to apoptosis upon DMF administration. In summary, our data provide new mechanistic insight in the regulation of cell death by targeting NFκB via Trx1 in cancer. Thus, interference with Trx1 activity is a novel approach for treatment of NFκB-dependent tumors.

The Architecture of Thiol Antioxidant Systems among Invertebrate Parasites

The use of oxygen as the final electron acceptor in aerobic organisms results in an improvement in the energy metabolism. However, as a byproduct of the aerobic metabolism, reactive oxygen species are produced, leaving to the potential risk of an oxidative stress. To contend with such harmful compounds, living organisms have evolved antioxidant strategies. In this sense, the thiol-dependent antioxidant defense systems play a central role. In all cases, cysteine constitutes the major building block on which such systems are constructed, being present in redox substrates such as glutathione, thioredoxin, and trypanothione, as well as at the catalytic site of a variety of reductases and peroxidases. In some cases, the related selenocysteine was incorporated at selected proteins. In invertebrate parasites, antioxidant systems have evolved in a diversity of both substrates and enzymes, representing a potential area in the design of anti-parasite strategies. The present review focus on the organization of the thiol-based antioxidant systems in invertebrate parasites. Differences between these taxa and its final mammal host is stressed. An understanding of the antioxidant defense mechanisms in this kind of parasites, as well as their interactions with the specific host is crucial in the design of drugs targeting these organisms.

Adverse Outcomes Associated with Cigarette Smoke Radicals Related to Damage to Protein-disulfide Isomerase

Identification of factors contributing to the development of chronic obstructive pulmonary disease (COPD) is crucial for developing new treatments. An increase in the levels of protein-disulfide isomerase (PDI), a multifaceted endoplasmic reticulum resident chaperone, has been demonstrated in human smokers, presumably as a protective adaptation to cigarette smoke (CS) exposure. We found a similar increase in the levels of PDI in the murine model of COPD. We also found abnormally high levels (4-6 times) of oxidized and sulfenilated forms of PDI in the lungs of murine smokers compared with non-smokers. PDI oxidation progressively increases with age. We begin to delineate the possible role of an increased ratio of oxidized PDI in the age-related onset of COPD by investigating the impact of exposure to CS radicals, such as acrolein (AC), hydroxyquinones (HQ), peroxynitrites (PN), and hydrogen peroxide, on their ability to induce unfolded protein response (UPR) and their effects on the structure and function of PDIs. Exposure to AC, HQ, PN, and CS resulted in cysteine and tyrosine nitrosylation leading to an altered three-dimensional structure of the PDI due to a decrease in helical content and formation of a more random coil structure, resulting in protein unfolding, inhibition of PDI reductase and isomerase activity in vitro and in vivo, and subsequent induction of endoplasmic reticulum stress response. Addition of glutathione prevented the induction of UPR, and AC and HQ induced structural changes in PDI. Exposure to PN and glutathione resulted in conjugation of PDI possibly at active site tyrosine residues. The findings presented here propose a new role of PDI in the pathogenesis of COPD and its age-dependent onset.

Role of thioredoxin nitration in bleomycin-induced pulmonary fibrosis in rats

Oxidant stimulation has been suggested to play an important role in the pathogenesis of idiopathic pulmonary fibrosis (IPF). Our study aimed to investigate the role and mechanisms of thioredoxin (Trx) nitration during the development of IPF. A rat model of IPF was established by intratracheal instillation of bleomycin (BLM). Male Wistar rats were randomly distributed among the control group and BLM-treated group, in which rats were intratracheally instilled with a single dose of BLM (5.0 mg/kg body mass in 1.0 mL phosphate-buffered saline). At 7 or 28 days after instillation the rats were euthanized. Histopathological and biochemical examinations were performed. The activity and protein level of thioredoxin were assessed. The thioredoxin nitration level was determined using immunoprecipitation and immunoblotting techniques. Our results demonstrated that protein tyrosine nitration increased in the BLM-treated group compared with the control group. Trx activity decreased in the BLM group compared with control group, whereas Trx expression and nitration level increased dramatically in the BLM group compared with the control group. Our results indicated that Trx nitration might be involved in the pathogenesis of IPF.

ASK1 mediates the teratogenicity of diabetes in the developing heart by inducing ER stress and inhibiting critical factors essential for cardiac development

Maternal diabetes in mice induces heart defects similar to those observed in human diabetic pregnancies. Diabetes enhances apoptosis and suppresses cell proliferation in the developing heart, yet the underlying mechanism remains elusive. Apoptosis signal-regulating kinase 1 (ASK1) activates the proapoptotic c-Jun NH2-terminal kinase 1/2 (JNK1/2) leading to apoptosis, suggesting a possible role of ASK1 in diabetes-induced heart defects. We aimed to investigate whether ASK1 is activated in the heart and whether deleting the Ask1 gene blocks diabetes-induced adverse events and heart defect formation. The ASK1-JNK1/2 pathway was activated by diabetes. Deleting Ask1 gene significantly reduced the rate of heart defects, including ventricular septal defects (VSDs) and persistent truncus arteriosus (PTA). Additionally, Ask1 deletion diminished diabetes-induced JNK1/2 phosphorylation and its downstream transcription factors and endoplasmic reticulum (ER) stress markers. Consistent with this, caspase activation and apoptosis were blunted. Ask1 deletion blocked the increase in cell cycle inhibitors (p21 and p27) and the decrease in cyclin D1 and D3 and reversed diabetes-repressed cell proliferation. Ask1 deletion also restored the expression of BMP4, NKX2.5, and GATA5, Smad1/5/8 phosphorylation, whose mutations or deletion result in reduced cell proliferation, VSD, and PTA formation. We conclude that ASK1 may mediate the teratogenicity of diabetes through activating the JNK1/2-ER stress pathway and inhibiting cell cycle progression, thereby impeding the cardiogenesis pathways essential for ventricular septation and outflow tract development.

Characterization of a Thioredoxin-1 Gene from Taenia solium and Its Encoding Product

Taenia solium thioredoxin-1 gene (TsTrx-1) has a length of 771 bp with three exons and two introns. The core promoter gene presents two putative stress transcription factor binding sites, one putative TATA box, and a transcription start site (TSS). TsTrx-1 mRNA is expressed higher in larvae than in adult. This gene encodes a protein of 107 amino acids that presents the Trx active site (CGPC), the classical secondary structure of the thioredoxin fold, and the highest degree of identity with the Echinococcus granulosus Trx. A recombinant TsTrx-1 (rTsTrx-1) was produced in Escherichia coli with redox activity. Optimal activity for rTsTrx-1 was at pH 6.5 in the range of 15 to 25°C. The enzyme conserved activity for 3 h and lost it in 24 h at 37°C. rTsTrx-1 lost 50% activity after 1 h and lost activity completely in 24 h at temperatures higher than 55°C. Best storage temperature for rTsTrx-1 was at −70°C. It was inhibited by high concentrations of H₂O₂ and methylglyoxal (MG), but it was inhibited neither by NaCl nor by anti-rTsTrx-1 rabbit antibodies that strongly recognized a ~12 kDa band in extracts from several parasites. These TsTrx-1 properties open the opportunity to study its role in relationship T. solium-hosts.

The structure of the Slrp-Trx1 complex sheds light on the autoinhibition mechanism of the type III secretion system effectors of the NEL family

Salmonella infections are a leading cause of bacterial foodborne illness in the U.S.A. and the European Union Antimicrobial therapy is often administered to treat the infection, but increasingly isolates are being detected that demonstrate resistance to multiple antibiotics. Salmonella enterica contains two virulence-related T3SS (type III secretion systems): one promotes invasion of the intestine and the other one mediates systemic disease. Both of them secrete the SlrP protein acting as E3 ubiquitin ligase in human host cells where it targets Trx1 (thioredoxin-1). SlrP belongs to the NEL family of bacterial E3 ubiquitin ligases that have been observed in two distinct autoinhibitory conformations. We solved the 3D structure of the SlrP-Trx1 complex and determined the Trx1 ubiquitination site. The description of the substrate-binding mode sheds light on the first step of the activation mechanism of SlrP. Comparison with the available structural data of other NEL effectors allowed us to gain new insights into their autoinhibitory mechanism. We propose a molecular mechanism for the regulation of SlrP in which structural constraints sequestrating the NEL domain would be sequentially released. This work thus constitutes a new milestone in the understanding of how these T3SS effectors influence pathogen virulence. It also provides the fundamental basis for future development of new antimicrobials.

Mechanistic insights into the inhibitory effects of palmitoylation on cytosolic thioredoxin reductase and thioredoxin

Overnutrition can lead to oxidative stress, but its underlying mechanism remains unclear. In this study, we report that human liver-derived HepG2 cells utilize cytosolic thioredoxin reductase (TrxR1) and thioredoxin (hTrx1) to defend against the high glucose/palmitate-mediated increase in reactive oxygen species. However, enhanced TrxR1/hTrx1 palmitoylation occurs in parallel with a decrease in their activities under the conditions studied here. An autoacylation process appears to be the major mechanism for generating palmitoylated TrxR1/Trx1 in HepG2 cells. A novel feature of this post-translational modification is the covalent inhibition of TrxR1/hTrx1 by palmitoyl-CoA, an activated form of palmitate. The palmitoyl-CoA/TrxR1 reaction is NADPH-dependent and produces palmitoylated TrxR1 at an active site selenocysteine residue. Conversely, S-palmitoylation occurs at the structural Cys62/Cys69/Cys72 residues but not the active site Cys32/Cys35 residues of hTrx1. Palmitoyl-CoA concentration and the period of incubation with TrxR1/hTrx1 are important factors that influence the inhibitory efficacy of palmitoyl-CoA on TrxR1/hTrx1. Thus, an increase in TrxR1/hTrx1 palmitoylation could be a potential consequence of high glucose/palmitate. The time-dependent inactivation of the NADPH-TrxR1-Trx1 system by palmitoyl-CoA occurs in a biphasic manner - a fast phase followed by a slow phase. Kinetic analysis suggests that the fast phase is consistent with a fast and reversible association between TrxR1/hTrx1 and palmitoyl-CoA. The slow phase is correlated with a slow and irreversible inactivation, in which selenolate/thiolate groups nucleophilically attack the α-carbon of bound palmitoyl-CoA, leading to the formation of thioester/selenoester bonds. hTrx1 can enhance rate of fast phase but limits the rate of slow phase when it is present in a preincubation mixture containing NADPH, TrxR1 and palmitoyl-CoA. Therefore, hTrx1 may provide palmitoylation sites or partially protect the TrxR1 active site selenol/thiol group(s) from palmitoylation. Our data suggest that Se/S-palmitoylation acts as an important modulator of TrxR1/hTrx1 activities, representing a novel potential mechanism that underlies overnutrition-induced events.

Purification and characterization of Taenia crassiceps cysticerci thioredoxin: insight into thioredoxin-glutathione-reductase (TGR) substrate recognition

Thioredoxin (Trx) is an oxidoreductase central to redox homeostasis in cells and is involved in the regulation of protein activity through thiol/disulfide exchanges. Based on these facts, our goal was to purify and characterize cytosolic thioredoxin from Taenia crassiceps cysticerci, as well as to study its behavior as a substrate of thioredoxin-glutathione reductase (TGR). The enzyme was purified >133-fold with a total yield of 9.7%. A molecular mass of 11.7kDa and a pI of 4.84 were measured. Native electrophoresis was used to identify the oxidized and reduced forms of the monomer as well as the presence of a homodimer. In addition to the catalytic site cysteines, cysticerci thioredoxin contains Cys28 and Cys65 residues conserved in previously sequenced cestode thioredoxins. The following kinetic parameters were obtained for the substrate of TGR: a Km of 3.1μM, a kcat of 10s(-1) and a catalytic efficiency of 3.2×10(6)M(-1)s(-1). The negative patch around the α3-helix of Trx is involved in the interaction with TGR and suggests variable specificity and catalytic efficiency of the reductase toward thioredoxins of different origins.

Thioredoxin reductase was nitrated in the aging heart after myocardial ischemia/reperfusion

The age-related loss of anti-oxidant defense reduces recovery from myocardial ischemia/reperfusion injury (MI/R) in aged people. Our previous data showed that inactivation of thioredoxin (Trx) was involved in enhanced aging MI/R injury. Thioredoxin reductase (TrxR), the enzyme known to regulate Trx, is less efficient with age. The aim of the current study was to determine why TrxR activity was reduced and whether reduced TrxR activity contributed to enhanced aging MI/R injury. Both Trx and TrxR activity were decreased in the aging heart, and this difference was further amplified after MI/R. However, MI/R injury did not change TrxR expression between young and aging rats. Increased nitrogen oxide (NOx) but decreased nitric oxide (NO) bioavailability (decreased phosphorylated vasodilator-stimulated phosphoprotein) was observed in aging hearts. Peroxynitrite (ONOO⁻) was increased in aging hearts and was further amplified after MI/R. TrxR nitration in young and aging hearts was detected by immunoprecipitation (anti-nitrotyrosine) followed by immunoblotting (anti-TrxR). Compared with young hearts, TrxR nitration was increased in the aging hearts, and this was further intensified after MI/R. The ONOO⁻ decomposition catalyst (FeTMPyp) reduced TrxR nitration and increased TrxR and Trx activity. More importantly, FeTMPyp attenuated the MI/R injury in aging hearts as evidenced by decreased caspase-3 and malondialdehyde (MDA) concentration and increased cardiac function. Increased ONOO⁻ nitrated TrxR in the aging heart as a post-translational modification, which may be related to the enhanced MI/R injury of aging rats. Interventions that inhibit nitration and restore TrxR activity might be a therapy for attenuating enhanced MI/R injury in aging heart.

Proteomic analyses of the phase transition from acidogenesis to solventogenesis using solventogenic and non-solventogenic Clostridium acetobutylicum strains

The fermentation carried out by the solvent-producing bacterium, Clostridium acetobutylicum, is characterized by two distinct phases: acidogenic and solventogenic phases. Understanding the cellular physiological changes occurring during the phase transition in clostridial fermentation is important for the enhanced production of solvents. To identify protein changes upon entry to stationary phase where solvents are typically produced, we herein analyzed the proteomic profiles of the parental wild type C. acetobutylicum strains, ATCC 824, the non-solventogenic strain, M5 that has lost the solventogenic megaplasmid pSOL1, and the synthetic simplified alcohol forming strain, M5 (pIMP1E1AB) expressing plasmid-based CoA-transferase (CtfAB) and aldehyde/alcohol dehydrogenase (AdhE1). A total of 68 protein spots, corresponding to 56 unique proteins, were unambiguously identified as being differentially present after the phase transitions in the three C. acetobutylicum strains. In addition to changes in proteins known to be involved in solventogenesis (AdhE1 and CtfB), we identified significant alterations in enzymes involved in sugar transport and metabolism, fermentative pathway, heat shock proteins, translation, and amino acid biosynthesis upon entry into the stationary phase. Of these, four increased proteins (AdhE1, CAC0233, CtfB and phosphocarrier protein HPr) and six decreased proteins (butyrate kinase, ferredoxin:pyruvate oxidoreductase, phenylalanyl-tRNA synthetase, adenylosuccinate synthase, pyruvate kinase and valyl-tRNA synthetase) showed similar patterns in the two strains capable of butanol formation. Interestingly, significant changes of several proteins by post-translational modifications were observed in the solventogenic phase. The proteomic data from this study will improve our understanding on how cell physiology is affected through protein levels patterns in clostridia.

The imbalanced redox status in senescent endothelial cells is due to dysregulated Thioredoxin-1 and NADPH oxidase 4

Environmental stressors as well as genetic modifications are known to enhance oxidative stress and aging processes. Mitochondrial and nuclear dysfunctions contribute to the onset of aging. One of the most important redox regulators in primary human endothelial cells is Thioredoxin-1 (Trx-1), a 12 kD protein with additional anti-apoptotic properties. Cellular generators of reactive oxygen species are NADPH oxidases (NOXs), of which NOX4 shows highest expression levels in endothelial cells. Therefore, the aim of the study was to investigate how Trx-1 and NOX4 are regulated during stress-induced premature senescence in endothelial cells. We treated primary human endothelial cells for two weeks with H2O2 to generate stress-induced premature senescence in these cells. In this model senescence-associated β-Galactosidase and nuclear p21 as senescence markers are increased. Moreover, total and mitochondrial reactive oxygen species formation is enhanced. An imbalanced redox homeostasis is detected by elevated NOX4 and decreased Trx-1 levels. This can be rescued by lentiviral expression of Trx-1. Moreover, the lysosomal protease Cathepsin D is over-activated, which results in reduced Trx-1 protein levels. Inhibition of "over-active" Cathepsin D by the specific, cell-permeable inhibitor pepstatin A abolishes the increase in nuclear p21 protein, ROS formation and degradation of Trx-1 protein, thus leading to blockade of stress-induced premature senescence by stabilizing the cellular redox homeostasis. Aortic Trx-1 levels are decreased and Cathepsin D activity is increased in NOX4 transgenic mice exclusively expressing NOX4 in the endothelium when compared to their wildtype littermates. Thus, loss of Trx-1 and upregulation of NOX4 importantly contribute to the imbalance in the redox-status of senescent endothelial cells ex vivo and in vivo.

The thioredoxin system as a therapeutic target in human health and disease

The thioredoxin (Trx) system comprises Trx, truncated Trx (Trx-80), Trx reductase, and NADPH, besides a natural Trx inhibitor, the thioredoxin-interacting protein (TXNIP). This system is essential for maintaining the balance of the cellular redox status, and it is involved in the regulation of redox signaling. It is also pivotal for growth promotion, neuroprotection, inflammatory modulation, antiapoptosis, immune function, and atherosclerosis. As an ubiquitous and multifunctional protein, Trx is expressed in all forms of life, executing its function through its antioxidative, protein-reducing, and signal-transducing activities. In this review, the biological properties of the Trx system are highlighted, and its implications in several human diseases are discussed, including cardiovascular diseases, heart failure, stroke, inflammation, metabolic syndrome, neurodegenerative diseases, arthritis, and cancer. The last chapter addresses the emerging therapeutic approaches targeting the Trx system in human diseases.

Identification of potential protein targets of isothiocyanates by proteomics

Isothiocyanates (ITCs), such as phenethyl isothiocyanate (PEITC) and sulforaphane (SFN), are effective cancer chemopreventive compounds. It is believed that the major mechanism for the cancer preventive activity of ITCs is through the induction of cell cycle arrest and apoptosis. However, the upstream molecular targets of ITCs have been underexplored until recently. To identify proteins that are covalently modified by ITCs, human non-small cell lung cancer A549 cells were treated with (14)C-PEITC and (14)C-SFN, and the cell lysates were extracted for analysis by 2-D gel electrophoresis and mass spectrometry. After superimposing the colloidal Coomassie blue protein staining pattern with the pattern of radioactivity obtained from X-ray films, it was clear that only a small fraction of cellular proteins contained radioactivity, presumably resulting from selective binding with PEITC or SFN via thiocarbamation. More than 30 proteins with a variety of biological functions were identified with high confidence. Here, we report the identities of these potential ITC target proteins and discuss their biological relevance. The discovery of the protein targets may facilitate studies of the mechanisms by which ITCs exert their cancer preventive activity and provide the molecular basis for designing more efficacious ITC compounds.

The role of glutathione S-transferase P in signaling pathways and S-glutathionylation in cancer

Glutathione S-transferase P is abundantly expressed in some mammalian tissues, particularly those associated with malignancies. While the enzyme can catalyze thioether bond formation between some electrophilic chemicals and GSH, novel nondetoxification functions are now ascribed to it. This review summarizes recent material that implicates GSTP in mediating S-glutathionylation of specific clusters of target proteins and in reactions that define a negative regulatory role in some kinase pathways through ligand or protein:protein interactions. It is becoming apparent that GSTP participates in the maintenance of cellular redox homeostasis through a number of convergent and divergent mechanisms. Moreover, drug platforms that have GSTP as a target have produced some interesting preclinical and clinical candidates.

S-glutathionylation: from molecular mechanisms to health outcomes

Redox homeostasis governs a number of critical cellular processes. In turn, imbalances in pathways that control oxidative and reductive conditions have been linked to a number of human disease pathologies, particularly those associated with aging. Reduced glutathione is the most prevalent biological thiol and plays a crucial role in maintaining a reduced intracellular environment. Exposure to reactive oxygen or nitrogen species is causatively linked to the disease pathologies associated with redox imbalance. In particular, reactive oxygen species can differentially oxidize certain cysteine residues in target proteins and the reversible process of S-glutathionylation may mitigate or mediate the damage. This post-translational modification adds a tripeptide and a net negative charge that can lead to distinct structural and functional changes in the target protein. Because it is reversible, S-glutathionylation has the potential to act as a biological switch and to be integral in a number of critical oxidative signaling events. The present review provides a comprehensive account of how the S-glutathionylation cycle influences protein structure/function and cellular regulatory events, and how these may impact on human diseases. By understanding the components of this cycle, there should be opportunities to intervene in stress- and aging-related pathologies, perhaps through prevention and diagnostic and therapeutic platforms.

Regulatory functions of glutathione S-transferase P1-1 unrelated to detoxification

Glutathione S-transferase P1-1 (GSTP) is one member of the family of GSTs and is ubiquitously expressed in human tissues. The literature is replete with reports of high levels of GSTP linked either with cancer incidence or drug resistance, and yet no entirely cogent explanation for these correlations exists. The catalytic detoxification properties of the GST isozyme family have been a primary research focus for the last four decades. However, it has become apparent that they have undergone structural and functional convergence where evolutionary selective pressures have favored the emergence of noncatalytic properties of GSTP that has imbued this isozyme with expanded biological importance. For example, GSTP has now been linked with two cell-signaling functions that are critical to survival. Through protein:protein interactions, GSTP can sequester c-jun N-terminal kinase (JNK) and act as a negative regulator of this stress kinase. Pharmacologically, this activity has been linked with the activity of GSTP inhibitors in stimulating myeloproliferation. In addition, GSTP is linked with the forward S-glutathionylation reaction, a post-translational modification that impacts the function/activity of a number of proteins. Catalytic reversal of S-glutathionylation is well characterized, but the role of GSTP in catalyzing the forward reaction contributes to the "glutathionylation cycle." Moreover, GSTP is itself susceptible to S-glutathionylation, providing an autoregulatory loop for the cycle. Because oxidative stress regulates both S-glutathionylation and JNK-signaling pathways, such links may help to explain the aberrant patterns of GSTP expression in the cancer phenotype. As such, there is an ongoing preclinical and clinical platform of drug discovery and development around GSTP.

Adaptation to oxidative stress, chemoresistance, and cell survival

The discovery of some additional properties and functions of reactive oxygen species (ROS), beyond their toxic effects, provides a novel scenario for the molecular basis and cell regulation of several pathophysiologic processes. ROS are generated by redox-sensitive, prosurvival signaling pathways and function as second messengers in the transduction of several extracellular signals. A complex intracellular redox buffering network has developed to adapt and protect cells against the dangerous effects of oxidative stress. However, pathways involved in ROS-adaptive response may also play a critical role in protecting cells against cytotoxic effects of anticancer agents, thus supporting the hypothesis of a correlation between adaptation/resistance to oxidative stress and resistance to anticancer drugs. This review summarizes the main systems involved in the adaptive responses: an overview on the pathophysiologic relevance of mitochondria on redox-sensitive transcription factors and genes and main antioxidant networks in tumor cells is provided. One of the major aims is to highlight the adaptive mechanisms and their interplay in the intricate connection between oncogenic signaling, oxidative stress, and chemoresistance. Clarification of these mechanisms has tremendous application potential, in terms of developing novel molecular-targeted anticancer therapies and innovative strategies for rational combination of these agents with chemotherapeutic or tumor-specific biologic drugs.

REDOX reaction at ASK1-Cys250 is essential for activation of JNK and induction of apoptosis

ASK1 cysteine oxidation allows JNK activation upon oxidative stress. Trx1 negatively regulates this pathway by reducing the oxidized cysteines of ASK1. However, precisely how oxidized ASK1 is involved in JNK activation and how Trx1 regulates ASK1 oxidoreduction remains elusive. Here, we describe two different thiol reductase activities of Trx1 on ASK1. First, in H(2)O(2)-treated cells, Trx1 reduces the various disulfide bonds generated between cysteines of ASK1 by a rapid and transient action. Second, in untreated cells, Trx1 shows a more stable thiol reductase activity on cysteine 250 (Cys250) of ASK1. After H(2)O(2) treatment, Trx1 dissociates from Cys250, which is not sufficient to activate the ASK1-JNK pathway. Indeed, in untreated cells, a Cys250 to alanine mutant of ASK1 (C250A), which cannot bind Trx1, does not constitutively activate JNK. On the other hand, in H(2)O(2)-treated cells, this mutant (C250A) fails to activate JNK and does not induce apoptosis, although it remains fully phosphorylated on Threonine 838 (Thr838) in its activation loop. Overall, our data show that Cys250 is essential for H(2)O(2)-dependent signaling downstream from ASK1 but at a step subsequent to the phosphorylation of ASK1 Thr838. They also clarify the thiol reductase function of Trx1 on ASK1 activity.

Thioredoxin system inhibitors as mediators of apoptosis for cancer therapy

The thioredoxin (Trx) system is a major antioxidant system integral to maintaining the intracellular redox state. It contains Trx, a redox active protein, which regulates the activity of various enzymes including those that function to counteract oxidative stress within the cell. Trx can also scavenge reactive oxygen species (ROS) and directly inhibits proapoptotic proteins such as apoptosis signal-regulating kinase 1 (ASK1). The oxidized form of Trx is reduced by thioredoxin reductase (TrxR). The cytoplasm and mitochondria contain equivalent Trx systems and inhibition of either system can lead to activation of apoptotic signaling pathways. There are a number of inhibitors with chemotherapy applications that target either Trx or TrxR to induce apoptosis in cancer cells. Suberoylanilide hydroxamic acid (SAHA) is effective against many cancer cells and functions by up-regulating an endogenous inhibitor of Trx. Other compounds target the selenocysteine-containing active site of TrxR. These include gold compounds, platinum compounds, arsenic trioxide, motexafin gadolinium, nitrous compounds, and various flavonoids. Inhibition of TrxR leads to an accumulation of oxidized Trx resulting in cellular conditions that promote apoptosis. In addition, some compounds also convert TrxR to a ROS generating enzyme. The role of Trx system inhibitors in cancer therapy is discussed in this review.

Influence of alpha lipoic acid on antioxidant status in D-galactosamine-induced hepatic injury

D-Galactosamine (GalN)-induced liver injury is associated with reactive oxygen species and oxidative stress. In the present study, we evaluated the effect of alpha lipoic acid (ALA) supplementation on acute GalN-induced oxidative liver injury. Hepatotoxicity induced by single intraperitoneal injection of GalN (500 mg/kg body wt) was evident from increase in lipid peroxidation and serum marker enzymes (asparate transaminase, alanine transaminase, alkaline phosphatase, and lactate dehydrogenase). The decreased activities of enzymic antioxidants (superoxide dismutase, catalase, glutathione peroxidase, and glutathione reductase) as well as glutathione levels were the salient features observed in GalN-induced hepatotoxicity. Pretreatment with ALA (50 mg/kg body weight for 7 days) significantly precluded these changes and prevents the hepatic injury. Hence, this study clearly exemplified that ALA might be a suitable candidate against GalN-induced cellular abnormalities.

Endothelial-specific expression of mitochondrial thioredoxin promotes ischemia-mediated arteriogenesis and angiogenesis

OBJECTIVE

Thioredoxin-2 (Trx2), a major antioxidant protein in mitochondria, enhances nitric oxide bioavailability and inhibits ASK1-dependent apoptosis in endothelial cells (ECs). However, the in vivo role of Trx2 in angiogenesis has not been defined. Here we used EC-specific transgenesis of Trx2 (Trx2-TG) in mice to determine the in vivo function of Trx2 in arteriogenesis and angiogenesis.

METHODS AND RESULTS

In a femoral artery ligation model, Trx2-TG mice had enhanced capacity in limb perfusion recovery and ischemic reserve capacity compared to the nontransgenic littermates. Ischemia-initiated arteriogenesis in the upper limb was augmented in Trx2-TG mice. Trx2-TG mice also showed significantly enhanced capillary formation and maturation in the lower limb. In nontransgenic limb, ischemia specifically induced a downregulation of Trx2 protein, leading to increased oxidative stress, ASK1 activation, and EC apoptosis. In contrast, Trx2-TG maintained a constitutive level of Trx2, reducing the ischemia-induced deleterious responses. We then defined the mechanism by which Trx2 increases angiogenesis using ECs isolated from Trx2-TG mice. Trx2-TG ECs showed increased NO and NO-dependent migration. In addition, these cells were more resistant to oxidative stress-induced activation of ASK1 signaling and apoptosis. Moreover, Trx2-augmented EC survival is NO-independent. To define the relative contributions of Trx2-increased NO and Trx2-reduced ASK1 apoptotic activity to angiogenesis in vivo, we examined Trx2 effects on ischemia-induced angiogenesis in eNOS-deficient mice. The eNOS deletion caused severe impairment in the functional flow recovery in response to ischemia. Trx2 expression in eNOS-KO mice still dramatically inhibited ischemia-induced ASK1 and EC apoptosis, leading to an enhanced functional flow recovery.

CONCLUSIONS

These in vivo and in vitro data support that Trx2 maintains EC function by two parallel pathways-scavenging ROS to increase NO bioavailability and inhibiting ASK1 activity to enhance EC survival, facilitating ischemia-mediated arteriogenesis and angiogenesis.

Protein S-glutathionylation: a regulatory device from bacteria to humans

S-Glutathionylation is the specific post-translational modification of protein cysteine residues by the addition of the tripeptide glutathione, the most abundant and important low-molecular-mass thiol within most cell types. Protein S-glutathionylation is promoted by oxidative or nitrosative stress but also occurs in unstressed cells. It can serve to regulate a variety of cellular processes by modulating protein function and to prevent irreversible oxidation of protein thiols. Recent findings support an essential role for S-glutathionylation in the control of cell-signalling pathways associated with viral infections and with tumour necrosis factor-(-induced apoptosis. Glyceraldehyde-3-phosphate dehydrogenase has recently been implicated in the regulation of endothelin-1 synthesis by a novel, S-glutathionylation-based mechanism involving messenger RNA stability. Moreover, recent studies have identified S-glutathionylation as a redox signalling mechanism in plants.

Force field parameters for S-nitrosocysteine and molecular dynamics simulations of S-nitrosated thioredoxin

Estimation of structural perturbation induced by S-nitrosation is important to understand the mode of cellular signal transduction mediated by nitric oxide. Crystal structures of S-nitrosated proteins have been solved only for a few cases, however, so that molecular dynamics simulation may provide an alternative tool for probing structural perturbation. In this study AMBER-99 force field parameters for S-nitrosocysteine were developed and applied to molecular dynamics simulations of S-nitrosated thioredoxin. Geometry optimization at the level of HF/6-31G * was followed by a restrained electrostatic potential charge-fitting to obtain the atomic charges of S-nitrosocysteine. Force constants for bonds and angles were obtained from generalized AMBER force field. Torsional force constants for CC-SN and CS-NO were determined by fitting the torsional profiles obtained from geometry optimization with those from molecular mechanical energy minimization. Finally molecular dynamics simulations were performed with theses parameters on oxidized and reduced thioredoxin with and without S-nitrosocysteine. In all cases the root-mean-square deviations of alpha-carbons yielded well-behaved trajectories. The CC-SH dihedral angle which fluctuated severely during the simulation became quiet upon S-nitrosation. In conclusion the force field parameters developed in this study for S-nitrosocysteine appear to be suitable for molecular dynamics simulations of S-nitrosated proteins.

Nitric oxide stress induces different responses but mediates comparable protein thiol protection in Bacillus subtilis and Staphylococcus aureus

The nonpathogenic Bacillus subtilis and the pathogen Staphylococcus aureus are gram-positive model organisms that have to cope with the radical nitric oxide (NO) generated by nitrite reductases of denitrifying bacteria and by the inducible NO synthases of immune cells of the host, respectively. The response of both microorganisms to NO was analyzed by using a two-dimensional gel approach. Metabolic labeling of the proteins revealed major changes in the synthesis pattern of cytosolic proteins after the addition of the NO donor MAHMA NONOate. Whereas B. subtilis induced several oxidative stress-responsive regulons controlled by Fur, PerR, OhrR, and Spx, as well as the general stress response controlled by the alternative sigma factor SigB, the more resistant S. aureus showed an increased synthesis rate of proteins involved in anaerobic metabolism. These data were confirmed by nuclear magnetic resonance analyses indicating that NO causes a drastically higher increase in the formation of lactate and butanediol in S. aureus than in B. subtilis. Monitoring the intracellular protein thiol state, we observed no increase in reversible or irreversible protein thiol modifications after NO stress in either organism. Obviously, NO itself does not cause general protein thiol oxidations. In contrast, exposure of cells to NO prior to peroxide stress diminished the irreversible thiol oxidation caused by hydrogen peroxide.

S-glutathionylation: indicator of cell stress and regulator of the unfolded protein response

The specific posttranslational modification of protein cysteine residues by the addition of the tripeptide glutathione is termed S-glutathionylation. This process is promoted by oxidative and nitrosative stress but also occurs in unstressed cells. Altered levels of S-glutathionylation in some proteins have been associated with numerous pathologies, many of which have been linked to redox stress in the endoplasmic reticulum (ER). Proper protein folding is dependent upon controlled redox conditions within the ER, and it seems that ER conditions can in turn affect rates of S-glutathionylation. This article seeks to bring together the ways through which these processes are interrelated and considers the implications of these interrelationships upon therapeutic approaches to disease.

Active caspase-1 is a regulator of unconventional protein secretion

Mammalian cells export most proteins by the endoplasmic reticulum/Golgi-dependent pathway. However, some proteins are secreted via unconventional, poorly understood mechanisms. The latter include the proinflammatory cytokines interleukin(IL)-1beta, IL-18, and IL-33, which require activation by caspase-1 for biological activity. Caspase-1 itself is activated by innate immune complexes, the inflammasomes. Here we show that secretion of the leaderless proteins proIL-1alpha, caspase-1, and fibroblast growth factor (FGF)-2 depends on caspase-1 activity. Although proIL-1alpha and FGF-2 are not substrates of the protease, we demonstrated their physical interaction. Secretome analysis using iTRAQ proteomics revealed caspase-1-mediated secretion of other leaderless proteins with known or unknown extracellular functions. Strikingly, many of these proteins are involved in inflammation, cytoprotection, or tissue repair. These results provide evidence for an important role of caspase-1 in unconventional protein secretion. By this mechanism, stress-induced activation of caspase-1 directly links inflammation to cytoprotection, cell survival, and regenerative processes.

Nonequilibrium thermodynamics of thiol/disulfide redox systems: a perspective on redox systems biology

Understanding the dynamics of redox elements in biologic systems remains a major challenge for redox signaling and oxidative stress research. Central redox elements include evolutionarily conserved subsets of cysteines and methionines of proteins which function as sulfur switches and labile reactive oxygen species (ROS) and reactive nitrogen species (RNS) which function in redox signaling. The sulfur switches depend on redox environments in which rates of oxidation are balanced with rates of reduction through the thioredoxins, glutathione/glutathione disulfide, and cysteine/cystine redox couples. These central couples, which we term redox control nodes, are maintained at stable but nonequilibrium steady states, are largely independently regulated in different subcellular compartments, and are quasi-independent from each other within compartments. Disruption of the redox control nodes can differentially affect sulfur switches, thereby creating a diversity of oxidative stress responses. Systems biology provides approaches to address the complexity of these responses. In the present review, we summarize thiol/disulfide pathway, redox potential, and rate information as a basis for kinetic modeling of sulfur switches. The summary identifies gaps in knowledge especially related to redox communication between compartments, definition of redox pathways, and discrimination between types of sulfur switches. A formulation for kinetic modeling of GSH/GSSG redox control indicates that systems biology could encourage novel therapeutic approaches to protect against oxidative stress by identifying specific redox-sensitive sites which could be targeted for intervention.

Emerging potential of thioredoxin and thioredoxin interacting proteins in various disease conditions

Reactive oxygen species (ROS) are known to be mediators of intracellular signaling pathways. However the excessive production of ROS may be detrimental to the cell as a result of the increased oxidative stress and loss of cell function. Hence, well tuned, balanced and responsive antioxidant systems are vital for proper regulation of the redox status of the cell. The cells are normally able to defend themselves against the oxidative stress induced damage through the use of several antioxidant systems. Even though the free radical scavenging enzymes such as superoxide dismutase (SOD) and catalase can handle huge amounts of reactive oxygen species, should these systems fail some reactive molecules will evade the detoxification process and damage potential targets. In such a scenario, cells recruit certain small molecules and proteins as 'rescue specialists' in case the 'bodyguards' fail to protect potential targets from oxidative damage. The thioredoxin (Trx) system thus plays a vital role in the maintenance of a reduced intracellular redox state which is essential for the proper functioning of each individual cell. Trx alterations have been implicated in many diseases such as cataract formation, ischemic heart diseases, cancers, AIDS, complications of diabetes, hypertension etc. The interactions of Trx with many different proteins and different metabolic and signaling pathways as well as the significant species differences make it an attractive target for therapeutic intervention in many fields of medical science. In this review, we present, the critical roles that thioredoxins play in limiting oxidant stress through either its direct effect as an antioxidant or through its interactions with other key signaling proteins (thioredoxin interacting proteins) and its implications in various disease models.

Molecular dynamics simulations of thioredoxin with S-glutathiolated cysteine-73

Thioredoxin-1 (Trx) becomes inactive when cysteine-73 forms a mixed disulfide with glutathione. This reversible S-glutathiolation may serve as a regulatory mechanism. However, molecular basis for the glutathiolation-induced inhibition has not been established due to the lack of its structure. Molecular dynamics (MD) simulations were performed with Gromacs to obtain structural information on glutathiolated Trx. Glutathiolation did not cause a large change in overall shape of Trx although small local changes in the secondary structures were evident. The glutathione moiety was much more flexible than the peptide and spanned a large conformational space. It remained very close to the active site for a large part of the simulation time. Therefore inhibition of Trx by glutathiolation appears to be due to steric hindrance imposed by the covalently attached glutathione.

The txl1+ gene from Schizosaccharomyces pombe encodes a new thioredoxin-like 1 protein that participates in the antioxidant defence against tert-butyl hydroperoxide

Yeasts are equipped with several putative single-domain thioredoxins located in different subcellular compartments. However, additional proteins containing thioredoxin domains are also encoded by the yeast genomes as described for mammals and other eukaryotic organisms. We report here the characterization of the fission yeast orthologue thioredoxin-like 1 (txl1(+)), which has been previously identified in mammals. Similarly to the human protein, the fission yeast Txl1 is a two-domain protein comprising an N-terminal thioredoxin-like domain and a C-terminal domain of unknown function. Many other yeasts and fungi species contain homologues of txl1(+); however, there is no evidence of txl1(+) orthologues in either Saccharomyces cerevisiae or plants. Txl1 is found in both the nucleus and the cytoplasm of Schizosaccharomyces pombe cells and exhibits a strong reducing activity coupled to thioredoxin reductase. In humans, TXL1 expression is induced by glucose deprivation and overexpression of TXL1 confers resistance against this stress. In contrast, a Sz. pombe Deltatxl1 mutant was not affected in the response against glucose starvation but the Deltatxl1 mutant strain showed a clear hypersensitivity to alkyl hydroperoxide. The mRNA levels of txl1(+) in a h20 strain did not change in response to any oxidative insult (hydrogen peroxide or alkyl hydroperoxide) and the overexpression of an integrated copy of the wild-type txl1(+) gene did not confer a significant increased resistance against alkyl hydroperoxide. Overall, these results indicate that the Txl1 role in the cellular detoxification of alkyl hydroperoxide is exerted through a constitutive transcription of txl1(+).

Nuclear redox-signaling is essential for apoptosis inhibition in endothelial cells--important role for nuclear thioredoxin-1

OBJECTIVE

The redox regulator thioredoxin-1 (Trx) is a potent antioxidative enzyme and exerts important cellular functions. Physiological concentrations of reactive oxygen species (ROS) and of nitric oxide (NO) act as second messengers. Previously, we demonstrated that ROS and NO reduced apoptosis in a Trx-dependent manner. The aim of this study was to determine the underlying mechanisms.

METHODS AND RESULTS

First, we investigated the localization of Trx after H2O2 and NO. Both induced nuclear import of Trx, which required karyopherin-alpha. siRNA against karyopherin-alpha inhibited nuclear import of Trx. Analysis of the Trx amino acid sequence and subsequent immunoprecipitation studies revealed that Trx(K81/82E) is not imported into the nucleus under H2O2 treatment and Trx(K81/82/85E) was retained in the cytosol and induced cell death. Trx(K81/82E) abolished the antiapoptotic capacity of H2O2. Glutathione S-transferase P1 (GST-P1) was identified as one major target regulated by H2O2. siRNA against GST-P1 abolished the antiapoptotic effect of H2O2. Cysteine 69, but not cysteines 32 and 35, which are all required for the complete antiapoptotic function of Trx, is not imported into the nucleus.

CONCLUSION

H2O2-induced nuclear import of Trx depends on karyopherin-alpha and NO. Trx-dependent induction of GST-P1 expression is required for apoptosis inhibition in endothelial cells.

Thiols and the chemoprevention of cancer

Thiols such as glutathione interfere with the complex carcinogenic process. Under stress conditions, they scavenge harmful molecules: Glutathione conjugation of electrophilic carcinogens may prevent tumor initiation, and reduced thiols may defend against oxidative stress. Thus, associated chemopreventive strategies involve enhancement of antioxidant or conjugating capacity by increasing the levels of, particularly, glutathione through precursor application or synthesis stimulation and by inducing the corresponding enzymes. The antioxidant potential of thiols is, however, a part of a more general capacity to regulate redox status even in the absence of unequivocal stress conditions. Redox status controls the activities of various cellular signalling proteins through oxidation or reduction of particular sensor structures that are also mostly thiols. The development of feasible chemotherapeutic strategies on the basis of this complex system of redox-sensitive messenger proteins is a goal in ongoing and future research.

Protein glutathionylation and oxidative stress

Liquid chromatography/electrospray ionization-mass spectrometry (LC/ESI-MS) demonstrated that glutathionyl hemoglobin (Hb) levels are increased in patients with diabetes, hyperlipidemia, uremia and Friedreich's ataxia. Glutathionylation of Hb is enhanced by oxidative stress. High performance liquid chromatography (HPLC) and matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF-MS) have also been developed for the quantification of glutathionyl Hb. Glutathionyl-lens proteins were detected in uremic patients and cataractous aged subjects. Glutathionylation of numerous enzymes is induced by oxidative stress, reduces their catalytic activities and may be involved in protection from the damaging effects of oxidative agents. Thioredoxin, glutaredoxin (thioltransferase) and protein disulfide isomerase are the key enzymes in controlling cellular oxidative stress that catalyze reduction of glutathionyl protein disulfide bonds. Thus, protein glutathionylation is closely associated with oxidative stress.

Stimulation of phosphoinositide 3-kinase/Akt signaling by copper and zinc ions: mechanisms and consequences

The phosphoinositide 3'-kinase (PI3K)/Akt signaling cascade controls cellular processes such as apoptosis and proliferation. Moreover, it is a mediator of insulin effects on target cells and as such is a major regulator of fuel metabolism. The PI3K/Akt cascade was demonstrated to be activated by stressful stimuli, including heat shock and reactive oxygen species (ROS). This minireview focuses on activation of the pathway by exposure of cells to heavy metal ions, Cu2+ and Zn2+. It is hypothesized that stimulation of PI3K/Akt is the molecular mechanism underlying the known insulin-mimetic effects of copper and zinc ions. Following a brief summary of PI3K/Akt signaling and of activation of the cascade by Cu2+ and Zn2+, mechanisms of metal-induced PI3K/Akt activation are discussed with a focus on the role of ROS and of cellular thiols (glutathione, thioredoxin) and protein tyrosine phosphatases in Cu2+ and Zn2+ signaling. Finally, consequences of metal-induced PI3K/Akt activation are discussed, focusing on the modulation of FoxO-family transcription factors by Cu2+ and Zn2+.