The thioredoxin antioxidant system

Synergistic antibacterial effect of silver and ebselen against multidrug-resistant Gram-negative bacterial infections

Multidrug-resistant (MDR) Gram-negative bacteria account for a majority of fatal infections, and development of new antibiotic principles and drugs is therefore of outstanding importance. Here, we report that five most clinically difficult-to-treat MDR Gram-negative bacteria are highly sensitive to a synergistic combination of silver and ebselen. In contrast, silver has no synergistic toxicity with ebselen on mammalian cells. The silver and ebselen combination causes a rapid depletion of glutathione and inhibition of the thioredoxin system in bacteria. Silver ions were identified as strong inhibitors of Escherichia coli thioredoxin and thioredoxin reductase, which are required for ribonucleotide reductase and DNA synthesis and defense against oxidative stress. The bactericidal efficacy of silver and ebselen was further verified in the treatment of mild and acute MDR E. coli peritonitis in mice. These results demonstrate that thiol-dependent redox systems in bacteria can be targeted in the design of new antibacterial drugs. The silver and ebselen combination offers a proof of concept in targeting essential bacterial systems and might be developed for novel efficient treatments against MDR Gram-negative bacterial infections.

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

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

Chemistry and Enzymology of Disulfide Cross-Linking in Proteins

Cysteine thiols are among the most reactive functional groups in proteins, and their pairing in disulfide linkages is a common post-translational modification in proteins entering the secretory pathway. This modest amino acid alteration, the mere removal of a pair of hydrogen atoms from juxtaposed cysteine residues, contrasts with the substantial changes that characterize most other post-translational reactions. However, the wide variety of proteins that contain disulfides, the profound impact of cross-linking on the behavior of the protein polymer, the numerous and diverse players in intracellular pathways for disulfide formation, and the distinct biological settings in which disulfide bond formation can take place belie the simplicity of the process. Here we lay the groundwork for appreciating the mechanisms and consequences of disulfide bond formation in vivo by reviewing chemical principles underlying cysteine pairing and oxidation. We then show how enzymes tune redox-active cofactors and recruit oxidants to improve the specificity and efficiency of disulfide formation. Finally, we discuss disulfide bond formation in a cellular context and identify important principles that contribute to productive thiol oxidation in complex, crowded, dynamic environments.

AhpC of the mycobacterial antioxidant defense system and its interaction with its reducing partner Thioredoxin-C

Despite the highly oxidative environment of the phagosomal lumen, the need for maintaining redox homeostasis is a critical aspect of mycobacterial biology. The pathogens are equipped with the sophisticated thioredoxin- (Trx) and peroxiredoxin system, including TrxC and the alkyl hydroperoxide reductase subunit C (AhpC), whereby TrxC is one of the reducing partners of AhpC. Here we visualize the redox modulated dodecamer ring formation of AhpC from Mycobacterium bovis (BCG strain; MbAhpC) using electron microscopy and present novel insights into the unique N-terminal epitope (40 residues) of mycobacterial AhpC. Truncations and amino acid substitutions of residues in the unique N-terminus of MbAhpC provide insights into their structural and enzymatic roles, and into the evolutionary divergence of mycobacterial AhpC versus that of other bacteria. These structural details shed light on the epitopes and residues of TrxC which contributes to its interaction with AhpC. Since human cells lack AhpC, the unique N-terminal epitope of mycobacterial AhpC as well as the MbAhpC-TrxC interface represent an ideal drug target.

Thioredoxin reductase from Toxoplasma gondii: an essential virulence effector with antioxidant function

Thioredoxin reductase (TR) can help pathogens resist oxidative-burst injury from host immune cells by maintaining a thioredoxin-reduction state during NADPH consumption. TR is a necessary virulence factor that enables the persistent infection of some parasites. We performed bioinformatics analyses and biochemical assays to characterize the activity, subcellular localization, and genetic ablation of Toxoplasma gondii TR (TgTR), to shed light on its biologic function. We expressed the TgTR protein with an Escherichia coli expression system and analyzed its enzyme activity, reporting a K_m for the recombinant TgTR of 11.47-15.57 μM, using NADPH as a substrate, and 130.48-151.09 μM with dithio-bis-nitrobenzoic acid as a substrate. The TgTR sequence shared homology with that of TR, but lacked a selenocysteine residue in the C-terminal region and was thought to contain 2 flavin adenine dinucleotide (FAD) domains and 1 NADPH domain. In addition, immunoelectron microscopy results showed that TgTR was widely dispersed in the cytoplasm, and we observed that parasite antioxidant capacity, invasion efficiency, and proliferation were decreased in TR-knockout (TR-KO) strains in vitro, although this strain still stimulated the release of reactive oxygen species release in mouse macrophages while being more sensitive to H₂O₂ toxicity in vitro Furthermore, our in vivo results revealed that the survival time of mice infected with the TR-KO strain was significantly prolonged relative to that of mice infected with the wild-type strain. These results suggest that TgTR plays an important role in resistance to oxidative damage and can be considered a virulence factor associated with T. gondii infection.-Xue, J., Jiang, W., Chen, Y., Gong, F., Wang, M., Zeng, P., Xia, C., Wang, Q., Huang, K. Thioredoxin reductase from Toxoplasma gondii: an essential virulence effector with antioxidant function.

GSR is not essential for the maintenance of antioxidant defenses in mouse cochlea: Possible role of the thioredoxin system as a functional backup for GSR

Glutathione reductase (GSR), a key member of the glutathione antioxidant defense system, converts oxidized glutathione (GSSG) to reduced glutathione (GSH) and maintains the intracellular glutathione redox state to protect the cells from oxidative damage. Previous reports have shown that Gsr deficiency results in defects in host defense against bacterial infection, while diquat induces renal injury in Gsr hypomorphic mice. In flies, overexpression of GSR extended lifespan under hyperoxia. In the current study, we investigated the roles of GSR in cochlear antioxidant defense using Gsr homozygous knockout mice that were backcrossed onto the CBA/CaJ mouse strain, a normal-hearing strain that does not carry a specific Cdh23 mutation that causes progressive hair cell degeneration and early onset of hearing loss. Gsr-/- mice displayed a significant decrease in GSR activity and GSH/GSSG ratios in the cytosol of the inner ears. However, Gsr deficiency did not affect ABR (auditory brainstem response) hearing thresholds, wave I amplitudes or wave I latencies in young mice. No histological abnormalities were observed in the cochlea of Gsr-/- mice. Furthermore, there were no differences in the activities of cytosolic glutathione-related enzymes, including glutathione peroxidase and glutamate-cysteine ligase, or the levels of oxidative damage markers in the inner ears between WT and Gsr-/- mice. In contrast, Gsr deficiency resulted in increased activities of cytosolic thioredoxin and thioredoxin reductase in the inner ears. Therefore, under normal physiological conditions, GSR is not essential for the maintenance of antioxidant defenses in mouse cochlea. Given that the thioredoxin system is known to reduce GSSG to GSH in multiple species, our findings suggest that the thioredoxin system can support GSSG reduction in the mouse peripheral auditory system.

What role does the stress response have in congestive heart failure?

This review is concerned with cardiac malfunction as a result of an imbalance in protein proteostasis, the homeostatic balance between protein removal and regeneration in a long remodeling process involving the endoplasmic reticulum (ER) and the unfolded protein response (UPR). The importance of this is of special significance with regard to cardiac function as a high energy requiring muscular organ that has a high oxygen requirement and is highly dependent on mitochondria. The importance of mitochondria is not only concerned with high energy dependence on mitochondrial electron transport, but it also has a role in the signaling between the mitochondria and the ER under stress. Proteins made in the ER are folded as a result of sulfhydryl groups (-SH) and attractive and repulsive reactions in the tertiary structure. We discuss how this matters with respect to an imbalance between muscle breakdown and repair in a stressful environment, especially as a result of oxidative and nitrosative byproducts of mitochondrial activity. The normal repair is a remodeling, but under this circumstance, the cell undergoes or even lysosomal "self eating" autophagy, or even necrosis instead of apoptosis. We shall discuss the relationship of the UPR pathway to chronic congestive heart failure (CHF).

Blue light-induced oxidative stress in live skin

Skin damage from exposure to sunlight induces aging-like changes in appearance and is attributed to the ultraviolet (UV) component of light. Photosensitized production of reactive oxygen species (ROS) by UVA light is widely accepted to contribute to skin damage and carcinogenesis, but visible light is thought not to do so. Using mice expressing redox-sensitive GFP to detect ROS, blue light could produce oxidative stress in live skin. Blue light induced oxidative stress preferentially in mitochondria, but green, red, far red or infrared light did not. Blue light-induced oxidative stress was also detected in cultured human keratinocytes, but the per photon efficacy was only 25% of UVA in human keratinocyte mitochondria, compared to 68% of UVA in mouse skin. Skin autofluorescence was reduced by blue light, suggesting flavins are the photosensitizer. Exposing human skin to the blue light contained in sunlight depressed flavin autofluorescence, demonstrating that the visible component of sunlight has a physiologically significant effect on human skin. The ROS produced by blue light is probably superoxide, but not singlet oxygen. These results suggest that blue light contributes to skin aging similar to UVA.

Thioredoxin A Is Essential for Motility and Contributes to Host Infection of Listeria monocytogenes via Redox Interactions

Microbes employ the thioredoxin system to defend against oxidative stress and ensure correct disulfide bonding to maintain protein function. Listeria monocytogenes has been shown to encode a putative thioredoxin, TrxA, but its biological roles and underlying mechanisms remain unknown. Here, we showed that expression of L. monocytogenes TrxA is significantly induced in bacteria treated with the thiol-specific oxidizing agent, diamide. Deletion of trxA markedly compromised tolerance of the pathogen to diamide, and mainly impaired early stages of infection in human intestinal epithelial Caco-2 cells. In addition, most trxA mutant bacteria were not associated with polymerized actin, and the rare bacteria that were associated with polymerized actin displayed very short tails or clouds during infection. Deletion or constitutive overexpression of TrxA, which was regulated by SigH, severely attenuated the virulence of the pathogen. Transcriptome analysis of L. monocytogenes revealed over 270 genes that were differentially transcribed in the ΔtrxA mutant compared to the wild-type, especially for the virulence-associated genes plcA, mpl, hly, actA, and plcB. Particularly, deletion of TrxA completely reduced LLO expression, and thereby led to a thoroughly impaired hemolytic activity. Expression of these virulence factors are positively regulated by the master regulator PrfA that was found here to use TrxA to maintain its reduced forms for activation. Interestingly, the trxA deletion mutant completely lacked flagella and was non-motile. We further confirmed that this deficiency is attributable to TrxA in maintaining the reduced intracellular monomer status of MogR, the key regulator for flagellar formation, to ensure correct dimerization. In summary, we demonstrated for the first time that L. monocytogenes thioredoxin A as a vital cellular reductase is essential for maintaining a highly reducing environment in the bacterial cytosol, which provides a favorable condition for protein folding and activation, and therefore contributes to bacterial virulence and motility.

Guanylyl cyclase sensitivity to nitric oxide is protected by a thiol oxidation-driven interaction with thioredoxin-1

Nitric oxide (NO) modulates many physiological events through production of cGMP from its receptor, the NO-sensitive guanylyl cyclase (GC1). NO also appears to function in a cGMP-independent manner, via S-nitrosation (SNO), a redox-based modification of cysteine thiols. Previously, we have shown that S-nitrosated GC1 (SNO-GC1) is desensitized to NO stimulation following prolonged NO exposure or under oxidative/nitrosative stress. In animal models of nitrate tolerance and angiotensin II-induced hypertension, decreased vasodilation in response to NO correlates with GC1 thiol oxidation, but the physiological mechanism that resensitizes GC1 to NO and restores basal activity is unknown. Because GC1 interacts with the oxidoreductase protein-disulfide isomerase, we hypothesized that thioredoxin-1 (Trx1), a cytosolic oxidoreductase, could be involved in restoring GC1 basal activity and NO sensitivity because the Trx/thioredoxin reductase (TrxR) system maintains thiol redox homeostasis. Here, by manipulating activity and levels of the Trx1/TrxR system and by using a Trx1-Trap assay, we demonstrate that Trx1 modulates cGMP synthesis through an association between Trx1 and GC1 via a mixed disulfide. A proximity ligation assay confirmed the endogenous Trx1-GC1 complex in cells. Mutational analysis suggested that Cys⁶⁰⁹ in GC1 is involved in the Trx1-GC1 association and modulation of GC1 activity. Functionally, we established that Trx1 protects GC1 from S-nitrosocysteine-induced desensitization. A computational model of Trx1-GC1 interaction illustrates a possible mechanism for Trx1 to maintain basal GC1 activity and prevent/rescue GC1 desensitization to NO. The etiology of some oxidative vascular diseases may very well be explained by the dysfunction of the Trx1-GC1 association.

Measurement and Clinical Significance of Biomarkers of Oxidative Stress in Humans

Oxidative stress is the result of the imbalance between reactive oxygen species (ROS) formation and enzymatic and nonenzymatic antioxidants. Biomarkers of oxidative stress are relevant in the evaluation of the disease status and of the health-enhancing effects of antioxidants. We aim to discuss the major methodological bias of methods used for the evaluation of oxidative stress in humans. There is a lack of consensus concerning the validation, standardization, and reproducibility of methods for the measurement of the following: (1) ROS in leukocytes and platelets by flow cytometry, (2) markers based on ROS-induced modifications of lipids, DNA, and proteins, (3) enzymatic players of redox status, and (4) total antioxidant capacity of human body fluids. It has been suggested that the bias of each method could be overcome by using indexes of oxidative stress that include more than one marker. However, the choice of the markers considered in the global index should be dictated by the aim of the study and its design, as well as by the clinical relevance in the selected subjects. In conclusion, the clinical significance of biomarkers of oxidative stress in humans must come from a critical analysis of the markers that should give an overall index of redox status in particular conditions.

miR-382 Contributes to Renal Tubulointerstitial Fibrosis by Downregulating HSPD1

Redox imbalance plays an important role in the pathogenesis of CKD progression. Previously, we demonstrated that microRNA-382 (miR-382) contributed to TGF-β1-induced loss of epithelial polarity in human kidney epithelial cells, but its role in the development of renal tubulointerstitial fibrosis remains unknown. In this study, we found that with 7 days of unilateral ureteral obstruction (UUO) in mice, the abundance of miR-382 in the obstructed kidney was significantly increased. Meanwhile, the protein expression of heat shock protein 60 (HSPD1), a predicted target of miR-382, was reduced after 7 days of UUO. Expression of 3-nitrotyrosine (3-NT) was upregulated, but expression of thioredoxin (Trx) was downregulated. Anti-miR-382 treatment suppressed the upregulation of miR-382, attenuated renal interstitial fibrosis in the obstructed kidney, and reversed the downregulation of HSPD1/Trx and upregulation of 3-NT after UUO. Furthermore, in vitro study revealed that overexpression of HSPD1 significantly restored Trx expression and reversed TGF-β1-induced loss of E-cadherin, while in vivo study found that direct siRNA-mediated suppression of HSPD1 in the UUO kidney promoted oxidative stress despite miR-382 blockade. Our clinical data showed that upregulation of miR-382/3-NT and downregulation of HSPD1/Trx were also observed in IgA nephropathy patients with renal interstitial fibrosis. These data supported a novel mechanism in which miR-382 targets HSPD1 and contributes to the redox imbalance in the development of renal fibrosis.

DNA damage related crosstalk between the nucleus and mitochondria

The electron transport chain is the primary pathway by which a cell generates energy in the form of ATP. Byproducts of this process produce reactive oxygen species that can cause damage to mitochondrial DNA. If not properly repaired, the accumulation of DNA damage can lead to mitochondrial dysfunction linked to several human disorders including neurodegenerative diseases and cancer. Mitochondria are able to combat oxidative DNA damage via repair mechanisms that are analogous to those found in the nucleus. Of the repair pathways currently reported in the mitochondria, the base excision repair pathway is the most comprehensively described. Proteins that are involved with the maintenance of mtDNA are encoded by nuclear genes and translocate to the mitochondria making signaling between the nucleus and mitochondria imperative. In this review, we discuss the current understanding of mitochondrial DNA repair mechanisms and also highlight the sensors and signaling pathways that mediate crosstalk between the nucleus and mitochondria in the event of mitochondrial stress.

Brevetoxin-2, is a unique inhibitor of the C-terminal redox center of mammalian thioredoxin reductase-1

Karenia brevis, the Florida red tide dinoflagellate produces a suite of neurotoxins known as the brevetoxins. The most abundant of the brevetoxins PbTx-2, was found to inhibit the thioredoxin-thioredoxin reductase system, whereas the PbTx-3 has no effect on this system. On the other hand, PbTx-2 activates the reduction of small disulfides such as 5,5'-dithio-bis-(2-nitrobenzoic acid) by thioredoxin reductase. PbTx-2 has an α, β-unsaturated aldehyde moiety which functions as an efficient electrophile and selenocysteine conjugates are readily formed. PbTx-2 blocks the inhibition of TrxR by the inhibitor curcumin, whereas curcumin blocks PbTx-2 activation of TrxR. It is proposed that the mechanism of inhibition of thioredoxin reduction is via the formation of a Michael adduct between selenocysteine and the α, β-unsaturated aldehyde moiety of PbTx-2. PbTx-2 had no effect on the rates of reactions catalyzed by related enzymes such as glutathione reductase, glutathione peroxidase or glutaredoxin.

All-trans retinoic acid ameliorates hepatic stellate cell activation via suppression of thioredoxin interacting protein expression

Activation of hepatic stellate cells (HSCs) is the effector factor of hepatic fibrosis and hepatocellular carcinoma (HCC) development. Accumulating evidence suggests that retinoic acids (RAs), derivatives of vitamin A, contribute to prevention of liver fibrosis and carcinogenesis, however, regulatory mechanisms of RAs still remain exclusive. To elucidate RA signaling pathway, we previously performed a genome-wide screening of RA-responsive genes by in silico analysis of RA-response elements, and identified 26 RA-responsive genes. We found that thioredoxin interacting protein (TXNIP), which inhibits antioxidant activity of thioredoxin (TRX), was downregulated by all-trans retinoic acid (ATRA). In the present study, we demonstrate that ATRA ameliorates activation of HSCs through TXNIP suppression. HSC activation was attenuated by TXNIP downregulation, whereas potentiated by TXNIP upregulation, indicating that TXNIP plays a crucial role in activation of HSCs. Notably, we showed that TXNIP-mediated HSC activation was suppressed by antioxidant N-acetylcysteine. In addition, ATRA treatment or downregulation of TXNIP clearly declined oxidative stress levels in activated HSCs. These data suggest that ATRA plays a key role in inhibition of HSC activation via suppressing TXNIP expression, which reduces oxidative stress levels.

Tissue- and Condition-Specific Isoforms of Mammalian Cytochrome c Oxidase Subunits: From Function to Human Disease

Cytochrome c oxidase (COX) is the terminal enzyme of the electron transport chain and catalyzes the transfer of electrons from cytochrome c to oxygen. COX consists of 14 subunits, three and eleven encoded, respectively, by the mitochondrial and nuclear DNA. Tissue- and condition-specific isoforms have only been reported for COX but not for the other oxidative phosphorylation complexes, suggesting a fundamental requirement to fine-tune and regulate the essentially irreversible reaction catalyzed by COX. This article briefly discusses the assembly of COX in mammals and then reviews the functions of the six nuclear-encoded COX subunits that are expressed as isoforms in specialized tissues including those of the liver, heart and skeletal muscle, lung, and testes: COX IV-1, COX IV-2, NDUFA4, NDUFA4L2, COX VIaL, COX VIaH, COX VIb-1, COX VIb-2, COX VIIaH, COX VIIaL, COX VIIaR, COX VIIIH/L, and COX VIII-3. We propose a model in which the isoforms mediate the interconnected regulation of COX by (1) adjusting basal enzyme activity to mitochondrial capacity of a given tissue; (2) allosteric regulation to adjust energy production to need; (3) altering proton pumping efficiency under certain conditions, contributing to thermogenesis; (4) providing a platform for tissue-specific signaling; (5) stabilizing the COX dimer; and (6) modulating supercomplex formation.

Protein signature characterizing Helicobacter pylori strains of patients with autoimmune atrophic gastritis, duodenal ulcer and gastric cancer

Background

Helicobacter pylori (H. pylori) represents a key factor in the etiology of autoimmune atrophic gastritis (AAG), duodenal ulcer (DU) and gastric cancer (GC). The aim of this study was to characterize the differential protein expression of H. pylori isolated from gastric biopsies of patients affected by either AAG, DU or GC.

Methods

The H. pylori strains were isolated from endoscopic biopsies from the stomach of patients with gastric disease. Protein profiles of H. pylori were compared by two-dimensional difference in gel electrophoresis (2D-DIGE) coupled with mass spectrometry (MS) for the identification of significantly different spots (Student t-test, p < 0.05).

Results

A total of 47 differentially expressed spots were found between H. pylori isolated from patients with either DU or AAG diseases and those isolated from patients with GC (Anova < 0.05, log fold change >1.5). These spots corresponded to 35 unique proteins. The identity of 7 protein spots was validated after one-dimensional electrophoresis and MS/MS analyses of excised gel portions. In H. pylori isolated from DU-patients a significant increase in proteins with antioxidant activity emerged (AroQ, AspA, FldA, Icd, OorA and ScoB), together with a higher content of proteins counteracting the high acid environment (KatA and NapA). In H. pylori isolated from AAG-patients proteins neutralizing hydrogen concentrations through organic substance metabolic processes decreased (GroL, TrxB and Tuf). In addition, a reduction of bacterial motility (FlhA) was found to be associated with AAG-H. pylori isolates. In GC-H. pylori strains it was found an increase in nucleic acid-binding proteins (e.g. DnaG, Tuf, RpoA, RplU) which may be involved in a higher demand of DNA- and protein-related processes.

Conclusion

Our data suggest the presence of specific protein signatures discriminating among H. pylori isolated from either AAG, DU or GC. Changes in protein expression profiles evaluated by DIGE succeeded in deciphering part of the molecular scenarios associated with the different H. pylori-related gastric diseases.

Electronic supplementary material

The online version of this article (doi:10.1186/s13027-017-0133-x) contains supplementary material, which is available to authorized users.

[Redox-dependent mechanisms of regulation of breast epithelial cell proliferation]

Activation of free radical oxidation in different cell types, including breast epithelial cells, may result in damage to macromolecules, in particular, proteins taking part in regulation of cell proliferation and apoptosis. The glutathione, glutaredoxin and thioredoxin systems play an essential role in maintaining intracellular redox homeostasis. Due to this fact, modulation of cellular redox status under the effect of an SH group inhibitor and an SH group protector may be used as a model for studying the role of redox proteins and glutathione in regulating cell proliferation in different pathological processes. In this study we have evaluated the state of the thioredoxin, glutaredoxin and glutathione systems as well as their role in regulating proliferation of HBL-100 breast epithelial cells under redox status modulation with N-ethylmaleimide (NEM) and 1,4-dithioerythriol (DTE). Modulating the redox status of breast epithelial cells under the effect of NEM and DTE influences the functional activity of glutathione-dependent enzymes, glutaredoxin, thioredoxin, and thioredoxin reductase through changes in the GSH and GSSG concentrations. In HBL-100 cells under redox-status modulation, we have found an increase in the number of cells in the S-phase of the cell cycle and a decrease in the number of cells in the G0/G1 and G2/М phases, as opposed to the values in the intact culture. The proposed model of proliferative activity of cells under redox status modulation may be used for development of new therapeutic approaches for treatment of diseases accompanied by oxidative stress generation.

Physical interaction between human ribonucleotide reductase large subunit and thioredoxin increases colorectal cancer malignancy

Ribonucleotide reductase (RR) is the rate-limiting enzyme in DNA synthesis, catalyzing the reduction of ribonucleotides to deoxyribonucleotides. During each enzymatic turnover, reduction of the active site disulfide in the catalytic large subunit is performed by a pair of shuttle cysteine residues in its C-terminal tail. Thioredoxin (Trx) and glutaredoxin (Grx) are ubiquitous redox proteins, catalyzing thiol-disulfide exchange reactions. Here, immunohistochemical examination of clinical colorectal cancer (CRC) specimens revealed that human thioredoxin1 (hTrx1), but not human glutaredoxin1 (hGrx1), was up-regulated along with human RR large subunit (RRM1) in cancer tissues, and the expression levels of both proteins were correlated with cancer malignancy stage. Ectopically expressed hTrx1 significantly increased RR activity, DNA synthesis, and cell proliferation and migration. Importantly, inhibition of both hTrx1 and RRM1 produced a synergistic anticancer effect in CRC cells and xenograft mice. Furthermore, hTrx1 rather than hGrx1 was the efficient reductase for RRM1 regeneration. We also observed a direct protein-protein interaction between RRM1 and hTrx1 in CRC cells. Interestingly, besides the known two conserved cysteines, a third cysteine (Cys⁷⁷⁹) in the RRM1 C terminus was essential for RRM1 regeneration and binding to hTrx1, whereas both Cys³² and Cys³⁵ in hTrx1 played a counterpart role. Our findings suggest that the up-regulated RRM1 and hTrx1 in CRC directly interact with each other and promote RR activity, resulting in enhanced DNA synthesis and cancer malignancy. We propose that the RRM1-hTrx1 interaction might be a novel potential therapeutic target for cancer treatment.

Quantitative analysis of the cysteine redoxome by iodoacetyl tandem mass tags

The redox conditions that reign inside a cell have a determining effect on a number of biological processes. Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are key redox players and have been linked to a number of pathologies. They have also been shown to play an important regulating role in cell signaling events. On the proteome level, thiol groups of cysteinyl side chains constitute the major targets of ROS and RNS. A number of analytical techniques based on mass spectrometry have been developed to characterize the cysteine redoxome, often facing a number of technical challenges, mostly related to the lability, heterogeneity, and low abundance of the oxidized forms. Furthermore, any posttranslational modification (PTM) quantification method needs to take the parent protein’s expression level into account. While taking all these limitations into consideration, we have developed a quantitative analytical strategy named OxiTMT, based on chemical labeling with iodoacetyl isobaric tandem mass tags (iodoTMT). OxiTMT allowed the generation of quantitative redox data that could be normalized by the protein’s expression profile in up to three different conditions. The method was tested on Escherichia coli with or without an oxidative treatment. Results showed the method to be adequate for the analysis of cysteine PTMs with a good coverage of the cysteine redoxome, especially for the low abundant oxidized species. Some of the challenges that face reporter ion quantification of PTMs by mass spectrometry were also assessed. This study serves as a proof of concept of the established protocol and consequent data treatment step. The use of tandem mass tags opens the ways towards the application of the method to the study of tissues and sera.

Graphical abstract

Electrochemical reverse engineering: A systems-level tool to probe the redox-based molecular communication of biology

The intestine is the site of digestion and forms a critical interface between the host and the outside world. This interface is composed of host epithelium and a complex microbiota which is "connected" through an extensive web of chemical and biological interactions that determine the balance between health and disease for the host. This biology and the associated chemical dialogues occur within a context of a steep oxygen gradient that provides the driving force for a variety of reduction and oxidation (redox) reactions. While some redox couples (e.g., catecholics) can spontaneously exchange electrons, many others are kinetically "insulated" (e.g., biothiols) allowing the biology to set and control their redox states far from equilibrium. It is well known that within cells, such non-equilibrated redox couples are poised to transfer electrons to perform reactions essential to immune defense (e.g., transfer from NADH to O₂ for reactive oxygen species, ROS, generation) and protection from such oxidative stresses (e.g., glutathione-based reduction of ROS). More recently, it has been recognized that some of these redox-active species (e.g., H₂O₂) cross membranes and diffuse into the extracellular environment including lumen to transmit redox information that is received by atomically-specific receptors (e.g., cysteine-based sulfur switches) that regulate biological functions. Thus, redox has emerged as an important modality in the chemical signaling that occurs in the intestine and there have been emerging efforts to develop the experimental tools needed to probe this modality. We suggest that electrochemistry provides a unique tool to experimentally probe redox interactions at a systems level. Importantly, electrochemistry offers the potential to enlist the extensive theories established in signal processing in an effort to "reverse engineer" the molecular communication occurring in this complex biological system. Here, we review our efforts to develop this electrochemical tool for in vitro redox-probing.

Polyamine- and NADPH-dependent generation of ROS during Helicobacter pylori infection: A blessing in disguise

Helicobacter pylori is a Gram-negative bacterium that specifically colonizes the gastric ecological niche. During the infectious process, which results in diseases ranging from chronic gastritis to gastric cancer, the host response is characterized by the activation of the innate immunity of gastric epithelial cells and macrophages. These cells thus produce effector molecules such as reactive oxygen species (ROS) to counteract the infection. The generation of ROS in response to H. pylori involves two canonical pathways: 1) the NADPH-dependent reduction of molecular oxygen to generate O₂^•-, which can dismute to generate ROS; and 2) the back-conversion of the polyamine spermine into spermidine through the enzyme spermine oxidase, leading to H₂O₂ production. Although these products have the potential to affect the survival of bacteria, H. pylori has acquired numerous strategies to counteract their deleterious effects. Nonetheless, ROS-mediated oxidative DNA damage and mutations may participate in the adaptation of H. pylori to its ecological niche. Lastly, ROS have been shown to play a major role in the development of the inflammation and carcinogenesis. It is the purpose of this review to summarize the literature about the production of ROS during H. pylori infection and their role in this infectious gastric disease.

A thioredoxin from antarctic microcrustacean (Euphausia superba): Cloning and functional characterization

Thioredoxins, with a dithiol/disulfide active site (CGPC) are major highly conserved and ubiquitous proteins that are involved in protecting organisms against various oxidative stresses. In the present study, a novel thioredoxin gene was identified in antarctic microcrustacean, Euphausia superba (designated as EsTrx1). The full-length cDNA sequences of EsTrx1 was of 621 bp, containing a 5' untranslated region (UTR) of 45 bp, a 3' UTR of 276 bp and an open reading frame (ORF) of 303 bp encoding a putative protein of 100 amino acids. The predicted molecular weight of EsTrx1 was 11.08 kDa and the theoretical isoelectric point was 4.51. Multiple sequence alignment indicated that the EsTrx1 possessed conserved CGPC redox-active site. EsTrx1 shared 68.6% similarity with the Chinese mitten crab (Eriocheir sinensis) Trx1. The predicted three-dimensional structure of EsTrx1 consisted of a central core of a four-stranded β-sheet and four flanking α-helices. The high similarity of EsTrx1 with Trx1s from other animals together with the phylogenetic analysis indicated that EsTrx1 could be a novel member of Trx1 sub-family. In order to elucidate its biological functions, the recombinant EsTrx1 was constructed and expressed in Escherichia coli BL21 (DE3). Experiments demonstrated that the rEsTrx1 fusion protein possessed the expected redox activity in enzymatic analysis, and be more potent than GSH in antioxidant capacity. These results together indicated that EsTrx1 could be involved in the oxidative stress response of E. superba.

TIGAR knockdown radiosensitizes TrxR1-overexpressing glioma in vitro and in vivo via inhibiting Trx1 nuclear transport

The up-regulation of thioredoxin reductase-1 (TrxR1) is detected in more than half of gliomas, which is significantly associated with increased malignancy grade and recurrence rate. The biological functions of NADPH-dependent TrxR1 are mainly associated with reduced thioredoxin-1 (Trx1) which plays critical roles in cellular redox signaling and tumour radio-resistance. Our previous work has proved that TP53 induced glycolysis and apoptosis regulator (TIGAR) knockdown could notably radiosensitize glioma cells. However, whether TrxR1-overexpressing glioma cells could be re-radiosensitized by TIGAR silence is still far from clear. In the present study, TrxR1 was stably over-expressed in U-87MG and T98G glioma cells. Both in vitro and in vivo data demonstrated that the radiosensitivity of glioma cells was considerably diminished by TrxR1 overexpression. TIGAR abrogation was able to radiosensitize TrxR1-overexpressing gliomas by inhibiting IR-induced Trx1 nuclear transport. Post-radiotherapy, TIGAR low-expression predicted significant longer survival time for animals suffering from TrxR1-overexpessing xenografts, which suggested that TIGAR abrogation might be a promising strategy for radiosensitizing TrxR1-overexpressing glial tumours.

The Arsenic Detoxification System in Corynebacteria: Basis and Application for Bioremediation and Redox Control

Arsenic (As) is widespread in the environment and highly toxic. It has been released by volcanic and anthropogenic activities and causes serious health problems worldwide. To survive arsenic-rich environments, soil and saprophytic microorganisms have developed molecular detoxification mechanisms to survive arsenic-rich environments, mainly by the enzymatic conversion of inorganic arsenate (As^V) to arsenite (As^III) by arsenate reductases, which is then extruded by arsenite permeases. One of these Gram-positive bacteria, Corynebacterium glutamicum, the workhorse of biotechnological research, is also resistant to arsenic. To sanitize contaminated soils and waters, C. glutamicum strains were modified to work as arsenic "biocontainers." Two chromosomally encoded ars operons (ars1 and ars2) are responsible for As resistance. The genes within these operons encode for metalloregulatory proteins (ArsR1/R2), arsenite permeases (Acr3-1/-2), and arsenate reductases (ArsC1/C2/C1'). ArsC1/C2 arsenate reductases are coupled to the low molecular weight thiol mycothiol (MSH) and to the recently discovered mycoredoxin-1 (Mrx-1) present in most Actinobacteria. This MSH/Mrx-1 redox system protects cells against different forms of stress, including reactive oxygen species (ROS), metals, and antibiotics. ROS can modify functional sulfur cysteines by oxidizing the thiol (-SH) to a sulfenic acid (-SOH). These oxidation-sensitive protein cysteine thiols are redox regulated by the MSH/Mrx-1 couple in Corynebacterium and Mycobacterium. In summary, the molecular mechanisms involved in arsenic resistance system in C. glutamicum have paved the way for understanding the cellular response against oxidative stress in Actinobacteria.

Catalase as a sulfide-sulfur oxido-reductase: An ancient (and modern?) regulator of reactive sulfur species (RSS)

Catalase is well-known as an antioxidant dismutating H₂O₂ to O₂ and H₂O. However, catalases evolved when metabolism was largely sulfur-based, long before O₂ and reactive oxygen species (ROS) became abundant, suggesting catalase metabolizes reactive sulfide species (RSS). Here we examine catalase metabolism of H₂S_n, the sulfur analog of H₂O₂, hydrogen sulfide (H₂S) and other sulfur-bearing molecules using H₂S-specific amperometric electrodes and fluorophores to measure polysulfides (H₂S_n; SSP4) and ROS (dichlorofluorescein, DCF). Catalase eliminated H₂S_n, but did not anaerobically generate H₂S, the expected product of dismutation. Instead, catalase concentration- and oxygen-dependently metabolized H₂S and in so doing acted as a sulfide oxidase with a P₅₀ of 20mmHg. H₂O₂ had little effect on catalase-mediated H₂S metabolism but in the presence of the catalase inhibitor, sodium azide (Az), H₂O₂ rapidly and efficiently expedited H₂S metabolism in both normoxia and hypoxia suggesting H₂O₂ is an effective electron acceptor in this reaction. Unexpectedly, catalase concentration-dependently generated H₂S from dithiothreitol (DTT) in both normoxia and hypoxia, concomitantly oxidizing H₂S in the presence of O₂. H₂S production from DTT was inhibited by carbon monoxide and augmented by NADPH suggesting that catalase heme-iron is the catalytic site and that NADPH provides reducing equivalents. Catalase also generated H₂S from garlic oil, diallyltrisulfide, thioredoxin and sulfur dioxide, but not from sulfite, metabisulfite, carbonyl sulfide, cysteine, cystine, glutathione or oxidized glutathione. Oxidase activity was also present in catalase from Aspergillus niger. These results show that catalase can act as either a sulfide oxidase or sulfur reductase and they suggest that these activities likely played a prominent role in sulfur metabolism during evolution and may continue do so in modern cells as well. This also appears to be the first observation of catalase reductase activity independent of peroxide dismutation.

Clinically Evaluated Cancer Drugs Inhibiting Redox Signaling

SIGNIFICANCE

There are a number of redox-active anticancer agents currently in development based on the premise that altered redox homeostasis is necessary for cancer cell's survival. Recent Advances: This review focuses on the relatively few agents that target cellular redox homeostasis to have entered clinical trial as anticancer drugs.

CRITICAL ISSUES

The success rate of redox anticancer drugs has been disappointing compared to other classes of anticancer agents. This is due, in part, to our incomplete understanding of the functions of the redox targets in normal and cancer tissues, leading to off-target toxicities and low therapeutic indexes of the drugs. The field also lags behind in the use biomarkers and other means to select patients who are most likely to respond to redox-targeted therapy.

FUTURE DIRECTIONS

If we wish to derive clinical benefit from agents that attack redox targets, then the future will require a more sophisticated understanding of the role of redox targets in cancer and the increased application of personalized medicine principles for their use. Antioxid. Redox Signal. 26, 262-273.

Transition Metal Homeostasis in Streptococcus pyogenes and Streptococcus pneumoniae

Trace metals such as Fe, Mn, Zn and Cu are essential for various biological functions including proper innate immune function. The host immune system has complicated and coordinated mechanisms in place to either starve and/or overload invading pathogens with various metals to combat the infection. Here, we discuss the roles of Fe, Mn and Zn in terms of nutritional immunity, and also the roles of Cu and Zn in metal overload in relation to the physiology and pathogenesis of two human streptococcal species, Streptococcus pneumoniae and Streptococcus pyogenes. S. pneumoniae is a major human pathogen that is carried asymptomatically in the nasopharynx by up to 70% of the population; however, transition to internal sites can cause a range of diseases such as pneumonia, otitis media, meningitis and bacteraemia. S. pyogenes is a human pathogen responsible for diseases ranging from pharyngitis and impetigo, to severe invasive infections. Both species have overlapping capacity with respect to metal acquisition, export and regulation and how metal homeostasis relates to their virulence and ability to invade and survive within the host. It is becoming more apparent that metals have an important role to play in the control of infection, and with further investigations, it could lead to the potential use of metals in novel antimicrobial therapies.

Multifunctional Mitochondrial Epac1 Controls Myocardial Cell Death

Rationale:

Although the second messenger cyclic AMP (cAMP) is physiologically beneficial in the heart, it largely contributes to cardiac disease progression when dysregulated. Current evidence suggests that cAMP is produced within mitochondria. However, mitochondrial cAMP signaling and its involvement in cardiac pathophysiology are far from being understood.

Objective:

To investigate the role of MitEpac1 (mitochondrial exchange protein directly activated by cAMP 1) in ischemia/reperfusion injury.

Methods and Results:

We show that Epac1 (exchange protein directly activated by cAMP 1) genetic ablation ( Epac1 −/− ) protects against experimental myocardial ischemia/reperfusion injury with reduced infarct size and cardiomyocyte apoptosis. As observed in vivo, Epac1 inhibition prevents hypoxia/reoxygenation–induced adult cardiomyocyte apoptosis. Interestingly, a deleted form of Epac1 in its mitochondrial-targeting sequence protects against hypoxia/reoxygenation–induced cell death. Mechanistically, Epac1 favors Ca 2+ exchange between the endoplasmic reticulum and the mitochondrion, by increasing interaction with a macromolecular complex composed of the VDAC1 (voltage-dependent anion channel 1), the GRP75 (chaperone glucose-regulated protein 75), and the IP3R1 (inositol-1,4,5-triphosphate receptor 1), leading to mitochondrial Ca 2+ overload and opening of the mitochondrial permeability transition pore. In addition, our findings demonstrate that MitEpac1 inhibits isocitrate dehydrogenase 2 via the mitochondrial recruitment of CaMKII (Ca 2+ /calmodulin-dependent protein kinase II), which decreases nicotinamide adenine dinucleotide phosphate hydrogen synthesis, thereby, reducing the antioxidant capabilities of the cardiomyocyte.

Conclusions:

Our results reveal the existence, within mitochondria, of different cAMP–Epac1 microdomains that control myocardial cell death. In addition, our findings suggest Epac1 as a promising target for the treatment of ischemia-induced myocardial damage.

Identification of effector-like proteins in Trichoderma spp. and role of a hydrophobin in the plant-fungus interaction and mycoparasitism

BACKGROUND

Trichoderma spp. can establish beneficial interactions with plants by promoting plant growth and defense systems, as well as, antagonizing fungal phytopathogens in mycoparasitic interactions. Such interactions depend on signal exchange between both participants and can be mediated by effector proteins that alter the host cell structure and function, allowing the establishment of the relationship. The main purpose of this work was to identify, using computational methods, candidates of effector proteins from T. virens, T. atroviride and T. reesei, validate the expression of some of the genes during a beneficial interaction and mycoparasitism and to define the biological function for one of them.

RESULTS

We defined a catalogue of putative effector proteins from T. virens, T. atroviride and T. reesei. We further validated the expression of 16 genes encoding putative effector proteins from T. virens and T. atroviride during the interaction with the plant Arabidopsis thaliana, and with two anastomosis groups of the phytopathogenic fungus Rhizoctonia solani. We found genes which transcript levels are modified in response to the presence of both plant fungi, as well as genes that respond only to either a plant or a fungal host. Further, we show that overexpression of the gene tvhydii1, a Class II hydrophobin family member, enhances the antagonistic activity of T. virens against R. solani AG2. Further, deletion of tvhydii1 results in reduced colonization of plant roots, while its overexpression increases it.

CONCLUSIONS

Our results show that Trichoderma is able to respond in different ways to the presence of a plant or a fungal host, and it can even distinguish between different strains of fungi of a given species. The putative effector proteins identified here may play roles in preventing perception of the fungus by its hosts, favoring host colonization or protecting it from the host's defense response. Finally, the novel effector protein TVHYDII1 plays a role in plant root colonization by T, virens, and participates in its antagonistic activity against R. solani.

Structural variability of E. coli thioredoxin captured in the crystal structures of single-point mutants

Thioredoxin is a ubiquitous small protein that catalyzes redox reactions of protein thiols. Additionally, thioredoxin from E. coli (EcTRX) is a widely-used model for structure-function studies. In a previous paper, we characterized several single-point mutants of the C-terminal helix (CTH) that alter global stability of EcTRX. However, spectroscopic signatures and enzymatic activity for some of these mutants were found essentially unaffected. A comprehensive structural characterization at the atomic level of these near-invariant mutants can provide detailed information about structural variability of EcTRX. We address this point through the determination of the crystal structures of four point-mutants, whose mutations occurs within or near the CTH, namely L94A, E101G, N106A and L107A. These structures are mostly unaffected compared with the wild-type variant. Notably, the E101G mutant presents a large region with two alternative traces for the backbone of the same chain. It represents a significant shift in backbone positions. Enzymatic activity measurements and conformational dynamics studies monitored by NMR and molecular dynamic simulations show that E101G mutation results in a small effect in the structural features of the protein. We hypothesize that these alternative conformations represent samples of the native-state ensemble of EcTRX, specifically the magnitude and location of conformational heterogeneity.

Methanosarcina acetivorans utilizes a single NADPH-dependent thioredoxin system and contains additional thioredoxin homologues with distinct functions

The thioredoxin system plays a central role in the intracellular redox maintenance in the majority of cells. The canonical system consists of an NADPH-dependent thioredoxin reductase (TrxR) and thioredoxin (Trx), a disulfide reductase. Although Trx is encoded in almost all sequenced genomes of methanogens, its incorporation into their unique physiology is not well understood. Methanosarcina acetivorans contains a single TrxR (MaTrxR) and seven Trx (MaTrx1-MaTrx7) homologues. We previously showed that MaTrxR and at least MaTrx7 compose a functional NADPH-dependent thioredoxin system. Here, we report the characterization of all seven recombinant MaTrxs. MaTrx1, MaTrx3, MaTrx4 and MaTrx5 lack appreciable disulfide reductase activity, unlike previously characterized MaTrx2, MaTrx6 and MaTrx7. Enzyme assays demonstrated that, of the MaTrxs, only the reduction of disulfide-containing MaTrx7 is linked to the oxidation of reduced coenzymes. NADPH is shown to be supplied to the MaTrxR-MaTrx7 system through the oxidation of the primary methanogen electron carriers F420H2 and ferredoxin, indicating that it serves as a primary intracellular reducing system in M. acetivorans. Bioinformatic analyses also indicate that the majority of methanogens likely utilize an NADPH-dependent thioredoxin system. The remaining MaTrxs may have specialized functions. MaTrx1 and MaTrx3 exhibited thiol oxidase activity. MaTrx3 and MaTrx6 are targeted to the membrane of M. acetivorans and likely function in the formation and the reduction of disulfides in membrane and/or extracellular proteins, respectively. This work provides insight into the incorporation of Trx into the metabolism of methanogens, and this reveals that methanogens contain Trx homologues with alternative properties and activities.

Dithiol disulphide exchange in redox regulation of chloroplast enzymes in response to evolutionary and structural constraints

Redox regulation of chloroplast enzymes via disulphide reduction is believed to control the rates of CO₂ fixation. The study of the thioredoxin reduction pathways and of various target enzymes lead to the following guidelines.

Mistranslation: from adaptations to applications

BACKGROUND

The conservation of the genetic code indicates that there was a single origin, but like all genetic material, the cell's interpretation of the code is subject to evolutionary pressure. Single nucleotide variations in tRNA sequences can modulate codon assignments by altering codon-anticodon pairing or tRNA charging. Either can increase translation errors and even change the code. The frozen accident hypothesis argued that changes to the code would destabilize the proteome and reduce fitness. In studies of model organisms, mistranslation often acts as an adaptive response. These studies reveal evolutionary conserved mechanisms to maintain proteostasis even during high rates of mistranslation.

SCOPE OF REVIEW

This review discusses the evolutionary basis of altered genetic codes, how mistranslation is identified, and how deviations to the genetic code are exploited. We revisit early discoveries of genetic code deviations and provide examples of adaptive mistranslation events in nature. Lastly, we highlight innovations in synthetic biology to expand the genetic code.

MAJOR CONCLUSIONS

The genetic code is still evolving. Mistranslation increases proteomic diversity that enables cells to survive stress conditions or suppress a deleterious allele. Genetic code variants have been identified by genome and metagenome sequence analyses, suppressor genetics, and biochemical characterization.

GENERAL SIGNIFICANCE

Understanding the mechanisms of translation and genetic code deviations enables the design of new codes to produce novel proteins. Engineering the translation machinery and expanding the genetic code to incorporate non-canonical amino acids are valuable tools in synthetic biology that are impacting biomedical research. This article is part of a Special Issue entitled "Biochemistry of Synthetic Biology - Recent Developments" Guest Editor: Dr. Ilka Heinemann and Dr. Patrick O'Donoghue.

Roles of Catalase and Trehalose in the Protection from Hydrogen Peroxide Toxicity in Saccharomyces cerevisiae

The roles of catalase and trehalose in Saccharomyces cerevisiae subject to hydrogen peroxide (H2O2) treatment were examined by measuring the catalase activity and intracellular trehalose levels in mutants lacking catalase or trehalose synthetase. Intracellular trehalose was elevated but the survival rate after H2O2 treatment remained low in mutants with deletion of the Catalase T gene. On the other hand, deletion of the trehalose synthetase gene increased the catalase activity in mutated yeast to levels higher than those in the wild-type strain, and these mutants exhibited some degree of tolerance to H2O2 treatment. These results suggest that Catalase T is critical in the yeast response to oxidative damage caused by H2O2 treatment, but trehalose also plays a role in protection against H2O2 treatment.

Cytosolic thioredoxin reductase 1 is required for correct disulfide formation in the ER

Folding of proteins entering the secretory pathway in mammalian cells frequently requires the insertion of disulfide bonds. Disulfide insertion can result in covalent linkages found in the native structure as well as those that are not, so-called non-native disulfides. The pathways for disulfide formation are well characterized, but our understanding of how non-native disulfides are reduced so that the correct or native disulfides can form is poor. Here, we use a novel assay to demonstrate that the reduction in non-native disulfides requires NADPH as the ultimate electron donor, and a robust cytosolic thioredoxin system, driven by thioredoxin reductase 1 (TrxR1 or TXNRD1). Inhibition of this reductive pathway prevents the correct folding and secretion of proteins that are known to form non-native disulfides during their folding. Hence, we have shown for the first time that mammalian cells have a pathway for transferring reducing equivalents from the cytosol to the ER, which is required to ensure correct disulfide formation in proteins entering the secretory pathway.

Cancer, Oxidative Stress, and Metastasis

Reactive oxygen species (ROS) are highly reactive molecules that arise from a number of cellular sources, including oxidative metabolism in mitochondria. At low levels they can be advantageous to cells, activating signaling pathways that promote proliferation or survival. At higher levels, ROS can damage or kill cells by oxidizing proteins, lipids, and nucleic acids. It was hypothesized that antioxidants might benefit high-risk patients by reducing the rate of ROS-induced mutations and delaying cancer initiation. However, dietary supplementation with antioxidants has generally proven ineffective or detrimental in clinical trials. High ROS levels limit cancer cell survival during certain windows of cancer initiation and progression. During these periods, dietary supplementation with antioxidants may promote cancer cell survival and cancer progression. This raises the possibility that rather than treating cancer patients with antioxidants, they should be treated with pro-oxidants that exacerbate oxidative stress or block metabolic adaptations that confer oxidative stress resistance.