Peroxiredoxin IV protects cells from oxidative stress by removing H2O2 produced during disulphide formation

Orchestration of secretory protein folding by ER chaperones

The endoplasmic reticulum is a major compartment of protein biogenesis in the cell, dedicated to production of secretory, membrane and organelle proteins. The secretome has distinct structural and post-translational characteristics, since folding in the ER occurs in an environment that is distinct in terms of its ionic composition, dynamics and requirements for quality control. The folding machinery in the ER therefore includes chaperones and folding enzymes that introduce, monitor and react to disulfide bonds, glycans, and fluctuations of luminal calcium. We describe the major chaperone networks in the lumen and discuss how they have distinct modes of operation that enable cells to accomplish highly efficient production of the secretome. This article is part of a Special Issue entitled: Functional and structural diversity of endoplasmic reticulum.

Destroy and exploit: catalyzed removal of hydroperoxides from the endoplasmic reticulum

Peroxidases are enzymes that reduce hydroperoxide substrates. In many cases, hydroperoxide reduction is coupled to the formation of a disulfide bond, which is transferred onto specific acceptor molecules, the so-called reducing substrates. As such, peroxidases control the spatiotemporal distribution of diffusible second messengers such as hydrogen peroxide (H₂O₂) and generate new disulfides. Members of two families of peroxidases, peroxiredoxins (Prxs) and glutathione peroxidases (GPxs), reside in different subcellular compartments or are secreted from cells. This review discusses the properties and physiological roles of PrxIV, GPx7, and GPx8 in the endoplasmic reticulum (ER) of higher eukaryotic cells where H₂O₂ and—possibly—lipid hydroperoxides are regularly produced. Different peroxide sources and reducing substrates for ER peroxidases are critically evaluated. Peroxidase-catalyzed detoxification of hydroperoxides coupled to the productive use of disulfides, for instance, in the ER-associated process of oxidative protein folding, appears to emerge as a common theme. Nonetheless, in vitro and in vivo studies have demonstrated that individual peroxidases serve specific, nonoverlapping roles in ER physiology.

Downregulated expression of peroxiredoxin 4 in granulosa cells from polycystic ovary syndrome

Peroxiredoxin 4 (PRDX4), a member of Peroxiredoxin (PRDX) family, is a typical 2-Cys PRDX. PRDX4 monitors the oxidative burden within cellular compartment and reduces hydrogen peroxide and alkyl hydroperoxide related to oxidative stress and apoptosis. Antioxidant, like PRDX4, may promote follicle development and participate in the pathophysiology of PCOS. In our previous study, we found that PRDX4 was expressed in mice oocyte cumulus oophorus complex, and that PRDX4 could be associated with follicle development. In this study, we explored the expression of PRDX4 in human follicles and possible role of PRDX4 in PCOS pathophysiology. Our data showed that PRDX4 was mainly expressed in granulosa cells in human ovaries. When compared to control group, both PRDX4 mRNA level and protein level decreased in PCOS group. The lowered levels of PRDX4 may relate to oxidative stress in the pathophysiologic progress of PCOS. Furthermore, expression of PRDX4 in the granulosa cells of in vivo or in vitro matured follicles was higher than that in immatured follicles, which suggested that PRDX4 may have a close relationship with follicular development. Altogether, our findings may provide new clues of the pathophysiologic mechanism of PCOS and potential therapeutic strategy using antioxidant, like PRDX4.

Endoplasmic reticulum oxidoreductin-1α (Ero1α) improves folding and secretion of mutant proinsulin and limits mutant proinsulin-induced endoplasmic reticulum stress

Upon chronic up-regulation of proinsulin synthesis, misfolded proinsulin can accumulate in the endoplasmic reticulum (ER) of pancreatic β-cells, promoting ER stress and type 2 diabetes mellitus. In Mutant Ins-gene-induced Diabetes of Youth (MIDY), misfolded mutant proinsulin impairs ER exit of co-expressed wild-type proinsulin, limiting insulin production and leading to eventual β-cell death. In this study we have investigated the hypothesis that increased expression of ER oxidoreductin-1α (Ero1α), despite its established role in the generation of H2O2, might nevertheless be beneficial in limiting proinsulin misfolding and its adverse downstream consequences. Increased Ero1α expression is effective in promoting wild-type proinsulin export from cells co-expressing misfolded mutant proinsulin. In addition, we find that upon increased Ero1α expression, some of the MIDY mutants themselves are directly rescued from ER retention. Secretory rescue of proinsulin-G(B23)V is correlated with improved oxidative folding of mutant proinsulin. Indeed, using three different variants of Ero1α, we find that expression of either wild-type or an Ero1α variant lacking regulatory disulfides can rescue mutant proinsulin-G(B23)V, in parallel with its ability to provide an oxidizing environment in the ER lumen, whereas beneficial effects were less apparent for a redox-inactive form of Ero1. Increased expression of protein disulfide isomerase antagonizes the rescue provided by oxidatively active Ero1. Importantly, ER stress induced by misfolded proinsulin was limited by increased expression of Ero1α, suggesting that enhancing the oxidative folding of proinsulin may be a viable therapeutic strategy in the treatment of type 2 diabetes.

Dynamic regulation of Ero1α and peroxiredoxin 4 localization in the secretory pathway

In the early secretory compartment (ESC), a network of chaperones and enzymes assists oxidative folding of nascent proteins. Ero1 flavoproteins oxidize protein disulfide isomerase (PDI), generating H₂O₂ as a byproduct. Peroxiredoxin 4 (Prx4) can utilize luminal H₂O₂ to oxidize PDI, thus favoring oxidative folding while limiting oxidative stress. Interestingly, neither ER oxidase contains known ER retention signal(s), raising the question of how cells prevent their secretion. Here we show that the two proteins share similar intracellular localization mechanisms. Their secretion is prevented by sequential interactions with PDI and ERp44, two resident proteins of the ESC-bearing KDEL-like motifs. PDI binds preferentially Ero1α, whereas ERp44 equally retains Ero1α and Prx4. The different binding properties of Ero1α and Prx4 increase the robustness of ER redox homeostasis.

The oxidative protein folding machinery in plant cells

Formation of intra-molecular disulfides and concomitant oxidative protein folding is essential for stability and catalytic function of many soluble and membrane-bound proteins in the endomembrane system, the mitochondrial inter-membrane space and the thylakoid lumen. Disulfide generation from free cysteines in nascent polypeptide chains is generally a catalysed process for which distinct pathways exist in all compartments. A high degree of similarities between highly diverse eukaryotic and bacterial systems for generation of protein disulfides indicates functional conservation of key processes throughout evolution. However, while many aspects about molecular function of enzymatic systems promoting disulfide formation have been demonstrated for bacterial and non-plant eukaryotic organisms, it is now clear that the plant machinery for oxidative protein folding displays distinct details, suggesting that the different pathways have been adapted to plant-specific requirements in terms of compartmentation, molecular function and regulation. Here, we aim to evaluate biological diversity by comparing the plant systems for oxidative protein folding to the respective systems from non-plant eukaryotes.

Expression of the endoplasmic reticulum oxidoreductase Ero1α in gastro-intestinal cancer reveals a link between homocysteine and oxidative protein folding

AIM

Ero proteins are central to oxidative protein folding in the endoplasmic reticulum (ER), but their expression varies in a tissue-specific manner. The aim of this work was to establish the expression of Ero1α in the digestive system and to examine the behavior of Ero1α in premalignant Barrett's esophagus, esophageal (OE) and gastric cancers and esophageal cancer cell lines.

RESULTS

Ero1α is expressed in the columnar epithelium of Barrett's tissue, and in OE tumors and gastric tumors. Homocysteine, a precursor in the metabolism of cysteine and methionine, induces the active Ox1 form of Ero1α in the OE cancer cell line OE33.

INNOVATION

These results demonstrate for the first time that Ero1α can sense the level of an amino acid precursor, identifying a potential link between diet, antioxidants, and oxidative protein folding in the ER.

CONCLUSION

The high expression of Ero1α in cancers of the esophagus and stomach demonstrates the importance of ER redox regulation in the gastro-intestinal (GI) tract in health and disease. Proteins and metabolites involved in disulfide bond formation and redox regulation may be suitable targets for both biomarker and drug development in GI cancer.

Principles in redox signaling: from chemistry to functional significance

Reactive oxygen and nitrogen species are currently considered not only harmful byproducts of aerobic respiration but also critical mediators of redox signaling. The molecules and the chemical principles sustaining the network of cellular redox regulated processes are described. Special emphasis is placed on hydrogen peroxide (H(2)O(2)), now considered as acting as a second messenger, and on sulfhydryl groups, which are the direct targets of the oxidant signal. Cysteine residues of some proteins, therefore, act as sensors of redox conditions and are oxidized in a reversible reaction. In particular, the formation of sulfenic acid and disulfide, the initial steps of thiol oxidation, are described in detail. The many cell pathways involved in reactive oxygen species formation are reported. Central to redox signaling processes are the glutathione and thioredoxin systems controlling H(2)O(2) levels and, hence, the thiol/disulfide balance. Lastly, some of the most important redox-regulated processes involving specific enzymes and organelles are described. The redox signaling area of research is rapidly expanding, and future work will examine new pathways and clarify their importance in cellular pathophysiology.

The secretory pathway: exploring yeast diversity

Protein secretion is an essential process for living organisms. In eukaryotes, this encompasses numerous steps mediated by several hundred cellular proteins. The core functions of translocation through the endoplasmic reticulum membrane, primary glycosylation, folding and quality control, and vesicle-mediated secretion are similar from yeasts to higher eukaryotes. However, recent research has revealed significant functional differences between yeasts and mammalian cells, and even among diverse yeast species. This review provides a current overview of the canonical protein secretion pathway in the model yeast Saccharomyces cerevisiae, highlighting differences to mammalian cells as well as currently unresolved questions, and provides a genomic comparison of the S. cerevisiae pathway to seven other yeast species where secretion has been investigated due to their attraction as protein production platforms, or for their relevance as pathogens. The analysis of Candida albicans, Candida glabrata, Kluyveromyces lactis, Pichia pastoris, Hansenula polymorpha, Yarrowia lipolytica, and Schizosaccharomyces pombe reveals that many - but not all - secretion steps are more redundant in S. cerevisiae due to duplicated genes, while some processes are even absent in this model yeast. Recent research obviates that even where homologous genes are present, small differences in protein sequence and/or differences in the regulation of gene expression may lead to quite different protein secretion phenotypes.

Forming disulfides in the endoplasmic reticulum

Protein disulfide bonds are an important co- and post-translational modification for proteins entering the secretory pathway. They are covalent interactions between two cysteine residues which support structural stability and promote the assembly of multi-protein complexes. In the mammalian endoplasmic reticulum (ER), disulfide bond formation is achieved by the combined action of two types of enzyme: one capable of forming disulfides de novo and another able to introduce these disulfides into substrates. The initial process of introducing disulfides into substrate proteins is catalyzed by the protein disulfide isomerase (PDI) oxidoreductases which become reduced and, therefore, have to be re-oxidized to allow for further rounds of disulfide exchange. This review will discuss the various pathways operating in the ER that facilitate oxidation of the PDI oxidoreductases and ultimately catalyze disulfide bond formation in substrate proteins. This article is part of a Special Issue entitled: Functional and structural diversity of endoplasmic reticulum.

Peptide binding by catalytic domains of the protein disulfide isomerase-related protein ERp46

The protein disulfide isomerase (PDI) family member ERp46/endoPDI/thioredoxin domain-containing protein 5 is preferentially expressed in a limited number of tissues, where it may function as a survival factor for nitrosative stress in vivo. It is involved in insulin production as well as in adiponectin signaling and interacts specifically with the redox-regulatory endoplasmic reticulum proteins endoplasmic oxidoreductin 1α (Ero1α) and peroxiredoxin-4. Here, we show that ERp46, although lacking a PDI-like redox-inactive b'-thioredoxin domain with its hydrophobic substrate binding site, is able to bind to a large pool of peptides containing aromatic and basic residues via all three of its catalytic domains (a(0), a and a'), though the a(0) domain may contain the primary binding site. ERp46, which shows relatively higher activity as a disulfide-reductase than as an oxidase/isomerase in vitro compared to PDI and ERp57, possesses chaperone activity in vivo, a property also shared by the C-terminal a' domain. A crystal structure of the a' domain is also presented, offering a view of possible substrate binding sites within catalytic domains of PDI proteins.

The effect of peroxiredoxin 4 on fly physiology is a complex interplay of antioxidant and signaling functions

Peroxiredoxin 4 (Prx4) has been implicated in a wide variety of biological processes, including development, progression of cancer, inflammation, and antioxidant function. The purpose of this study was to provide further insight into its multiple roles at the whole-animal level, using Drosophila. Reduced expression of dPrx4 (up to 90%) resulted in greater sensitivity to oxidative stress, an elevated H₂O₂ flux, and increases in lipid peroxidation, but no effect on longevity. Overexpression at low levels (<2-fold) gave reduced levels of oxidative damage and tended to show an increase in longevity. Flies expressing dPrx4 globally at high levels (>5-fold) had a dramatically reduced life span (by 20-80%) and increased apoptosis. Analysis of these overexpressors revealed an aberrant redistribution of the dPrx4 protein from the endoplasmic reticulum (ER) to cytosol and hemolymph. In addition to the known proapoptotic effects of the cytosolic form of dPrx4, dPrx4 overexpression triggered an NF-κB-mediated proinflammatory response, similar to that observed in cells under ER stress or when microbially challenged. Finally, we provide the first evidence that dPrx4, on secretion into the hemolymph, elicits a JAK/STAT-mediated response. The effects on fly survival and homeostasis appear to represent a combination of differential effects dictated in large part by dPrx4 subcellular and tissue-specific localization.

An involvement of oxidative stress in endoplasmic reticulum stress and its associated diseases

The endoplasmic reticulum (ER) is the major site of calcium storage and protein folding. It has a unique oxidizing-folding environment due to the predominant disulfide bond formation during the process of protein folding. Alterations in the oxidative environment of the ER and also intra-ER Ca²⁺ cause the production of ER stress-induced reactive oxygen species (ROS). Protein disulfide isomerases, endoplasmic reticulum oxidoreductin-1, reduced glutathione and mitochondrial electron transport chain proteins also play crucial roles in ER stress-induced production of ROS. In this article, we discuss ER stress-associated ROS and related diseases, and the current understanding of the signaling transduction involved in ER stress.

Presence of autoantibody against two placental proteins, peroxiredoxin 3 and peroxiredoxin 4, in sera of recurrent pregnancy loss patients

PROBLEM

Recurrent pregnancy loss (RPL) is defined as two or more consecutive abortion before the 20th week of gestation. Autoimmune diseases increase the risk and accounts for at least 20% of RPL. Placenta is a pregnancy unique tissue, and proper formation of placenta is key phenomenal for success of a pregnancy. The aim of this study was to investigate the placental proteins that may act as antibody targets in RPL patients.

METHOD OF STUDY

Total placental proteins were extracted and separated using 2D-PAGE technique. Separated protein spots were transferred on PVDF membrane and blotted with sera from 20 RPL patients and compared with the protein spots that membrane blotted with sera from 20 normal women. Differentially blotted spots were identified by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry mass spectrometry technique. The results of the mass analysis also were confirmed by western blot using mAb and reverse transcriptase polymerase chain reaction technique.

RESULTS

The results indicated that RPL women may produce antibody against Peroxiredoxin 3 (Prx3) and Peroxiredoxin 4 (Prx4).

CONCLUSION

Our results indicate that two placental proteins, Prx3 and Prx4, may act as new placental immune targets. Considering the role of antioxidant defense in the protection of placenta from oxidative stress, production of antibodies against peroxiredoxins 3 and 4 may introduce a new autoimmune hypothesis in RPL, which is needed to be tested in the future works.

Endoplasmic reticulum thiol oxidase deficiency leads to ascorbic acid depletion and noncanonical scurvy in mice

Endoplasmic reticulum (ER) thiol oxidases initiate a disulfide relay to oxidatively fold secreted proteins. We found that combined loss-of-function mutations in genes encoding the ER thiol oxidases ERO1α, ERO1β, and PRDX4 compromised the extracellular matrix in mice and interfered with the intracellular maturation of procollagen. These severe abnormalities were associated with an unexpectedly modest delay in disulfide bond formation in secreted proteins but a profound, 5-fold lower procollagen 4-hydroxyproline content and enhanced cysteinyl sulfenic acid modification of ER proteins. Tissue ascorbic acid content was lower in mutant mice, and ascorbic acid supplementation improved procollagen maturation and lowered sulfenic acid content in vivo. In vitro, the presence of a sulfenic acid donor accelerated the oxidative inactivation of ascorbate by an H(2)O(2)-generating system. Compromised ER disulfide relay thus exposes protein thiols to competing oxidation to sulfenic acid, resulting in depletion of ascorbic acid, impaired procollagen proline 4-hydroxylation, and a noncanonical form of scurvy.

Hyperactivity of the Ero1α oxidase elicits endoplasmic reticulum stress but no broad antioxidant response

Oxidizing equivalents for the process of oxidative protein folding in the endoplasmic reticulum (ER) of mammalian cells are mainly provided by the Ero1α oxidase. The molecular mechanisms that regulate Ero1α activity in order to harness its oxidative power are quite well understood. However, the overall cellular response to oxidative stress generated by Ero1α in the lumen of the mammalian ER is poorly characterized. Here we investigate the effects of overexpressing a hyperactive mutant (C104A/C131A) of Ero1α. We show that Ero1α hyperactivity leads to hyperoxidation of the ER oxidoreductase ERp57 and induces expression of two established unfolded protein response (UPR) targets, BiP (immunoglobulin-binding protein) and HERP (homocysteine-induced ER protein). These effects could be reverted or aggravated by N-acetylcysteine and buthionine sulfoximine, respectively. Because both agents manipulate the cellular glutathione redox buffer, we conclude that the observed effects of Ero1α-C104A/C131A overexpression are likely caused by an oxidative perturbation of the ER glutathione redox buffer. In accordance, we show that Ero1α hyperactivity affects cell viability when cellular glutathione levels are compromised. Using microarray analysis, we demonstrate that the cell reacts to the oxidative challenge caused by Ero1α hyperactivity by turning on the UPR. Moreover, this analysis allowed the identification of two new targets of the mammalian UPR, CRELD1 and c18orf45. Interestingly, a broad antioxidant response was not induced. Our findings suggest that the hyperoxidation generated by Ero1α-C104A/C131A is addressed in the ER lumen and is unlikely to exert oxidative injury throughout the cell.

The H2O2-sensitive HyPer protein targeted to the endoplasmic reticulum as a mirror of the oxidizing thiol-disulfide milieu

Oxidative protein folding in the endoplasmic reticulum (ER) is associated with the formation of native disulfide bonds, which inevitably results in the formation of hydrogen peroxide (H(2)O(2)). Particularly in pancreatic β-cells with their high secretory activity and extremely low antioxidant capacity, the H(2)O(2) molecules generated during oxidative protein folding could represent a significant oxidative burden. Therefore this study was conducted to elucidate the H(2)O(2) generation during disulfide bond formation in insulin-producing RINm5F cells by targeting and specifically expressing the H(2)O(2)-sensitive biosensor HyPer in the ER (ER-HyPer). In addition the influence of overexpression of the H(2)O(2)-metabolizing ER-resident peroxiredoxin IV (PRDXIV) on H(2)O(2) levels was examined. The ER-HyPer fluorescent protein was completely oxidized, whereas HyPer expressed in cytosol, peroxisomes, and mitochondria was prevalently in the reduced state, indicating a high basal H(2)O(2) concentration in the ER lumen. These results could also be confirmed in non-insulin-producing COS-7 cells. Overexpression of PRDXIV in RINm5F cells effectively protected against H(2)O(2)-mediated toxicity; however, it did not affect the fluorescence signal of ER-HyPer. Moreover, the inhibition of de novo protein synthesis and the associated H(2)O(2) generation by cycloheximide had no influence on the ER-HyPer redox state. Taken together, these findings strongly suggest that the H(2)O(2)-sensitive biosensor reflects exclusively the oxidative milieu in the ER and not the H(2)O(2) concentration in the ER lumen.

Suppression of peroxiredoxin 4 in glioblastoma cells increases apoptosis and reduces tumor growth

Glioblastoma multiforme (GBM), the most common and aggressive primary brain malignancy, is incurable despite the best combination of current cancer therapies. For the development of more effective therapies, discovery of novel candidate tumor drivers is urgently needed. Here, we report that peroxiredoxin 4 (PRDX4) is a putative tumor driver. PRDX4 levels were highly increased in a majority of human GBMs as well as in a mouse model of GBM. Reducing PRDX4 expression significantly decreased GBM cell growth and radiation resistance in vitro with increased levels of ROS, DNA damage, and apoptosis. In a syngenic orthotopic transplantation model, Prdx4 knockdown limited GBM infiltration and significantly prolonged mouse survival. These data suggest that PRDX4 can be a novel target for GBM therapies in the future.

Measurement of peroxiredoxin-4 serum levels in rat tissue and its use as a potential marker for hepatic disease

Peroxiredoxin (Prx)-4, a secretable endoplasmic reticulum (ER)-resident isoform of the mammalian Prx family, functions as a thioredoxin-dependent peroxidase. It is acknowledged that Prx-4 plays a role in the detoxification of hydrogen peroxide, and potentially other peroxides, which may be generated during the oxidative folding of proteins and oxidative stress in the ER. The present study was undertaken in order to specifically quantify the tissue levels of Prx-4. To accomplish this, an enzyme-linked immunosorbent assay was developed using a specific polyclonal antibody produced by immunizing a rabbit with native recombinant rat Prx-4 protein. The assay was used to detect Prx-4 in the range of 0.1 and 10 ng/ml, and to investigate tissue distribution in rats. Using this immunoassay, we found that the serum levels of Prx-4 were substantially lower in asymptomatic Long-Evans Cinnamon rats, a rat model of Wilson's disease, compared to normal rats. In addition, the treatment of rat hepatoma cells with N-acetylcysteine led to a significant increase in the release of Prx-4 protein into the medium; thus, it appears likely that the secretion of Prx-4 is associated with the redox state within cells. These results suggest that serum Prx-4 has potential for use as a biomarker for hepatic oxidative stress.

Low-molecular-weight oxidants involved in disulfide bond formation

SIGNIFICANCE

The biogenesis of most secreted and outer membrane proteins involves the formation of structure stabilizing disulfide bonds. Hence knowledge of the mechanisms for their formation is critical for understanding a myriad of cellular processes and associated disease states.

RECENT ADVANCES

Until recently it was thought that members of the Ero1 sulfhydryl oxidase family were responsible for catalyzing the majority of disulfide bond formation in the endoplasmic reticulum. However, multiple eukaryotic organisms are now known to show no or minor phenotypes when these enzymatic pathways are disrupted, suggesting that other pathways can catalyze disulfide bond formation to an extent sufficient to maintain normal physiology.

CRITICAL ISSUES AND FUTURE DIRECTIONS

This lack of a strong phenotype raises multiple questions regarding what pathways are acting and whether they themselves constitute the major route for disulfide bond formation. This review critically examines the potential low molecular oxidants that maybe involved in the catalyzed or noncatalyzed formation of disulfide bonds, with an emphasis on the mammalian endoplasmic reticulum, via an examination of their thermodynamics, kinetics, and availability and gives pointers to help guide future experimental work.

Ero1α regulates Ca(2+) fluxes at the endoplasmic reticulum-mitochondria interface (MAM)

AIMS

The endoplasmic reticulum (ER) is involved in many functions, including protein folding, redox homeostasis, and Ca(2+) storage and signaling. To perform these multiple tasks, the ER is composed of distinct, specialized subregions, amongst which mitochondrial-associated ER membranes (MAM) emerge as key signaling hubs. How these multiple functions are integrated with one another in living cells remains unclear.

RESULTS

Here we show that Ero1α, a key controller of oxidative folding and ER redox homeostasis, is enriched in MAM and regulates Ca(2+) fluxes. Downregulation of Ero1α by RNA interference inhibits mitochondrial Ca(2+) fluxes and modifies the activity of mitochondrial Ca(2+) uniporters. The overexpression of redox active Ero1α increases passive Ca(2+) efflux from the ER, lowering [Ca(2+)](ER) and mitochondrial Ca(2+) fluxes in response to IP3 agonists.

INNOVATION

The unexpected observation that Ca(2+) fluxes are affected by either increasing or decreasing the levels of Ero1α reveals a pivotal role for this oxidase in the early secretory compartment and implies a strict control of its amounts.

CONCLUSIONS

Taken together, our results indicate that the levels, subcellular localization, and activity of Ero1α coordinately regulate Ca(2+) and redox homeostasis and signaling in the early secretory compartment.

Protein disulfide isomerase in redox cell signaling and homeostasis

Thiol proteins may potentially act as redox signaling adaptor proteins, adjusting reactive oxygen species intermediates to specific signals and redox signals to cell homeostasis. In this review, we discuss redox effects of protein disulfide isomerase (PDI), a thioredoxin superfamily oxidoreductase from the endoplasmic reticulum (ER). Abundantly expressed PDI displays ubiquity, interactions with redox and nonredox proteins, versatile effects, and several posttranslational modifications. The PDI family contains >20 members with at least some apparent complementary actions. PDI has oxidoreductase, isomerase, and chaperone effects, the last not directly dependent on its thiols. PDI is a converging hub for pathways of disulfide bond introduction into ER-processed proteins, via hydrogen peroxide-generating mechanisms involving the oxidase Ero1α, as well as hydrogen peroxide-consuming reactions involving peroxiredoxin IV and the novel peroxidases Gpx7/8. PDI is a candidate pathway for coupling ER stress to oxidant generation. Emerging information suggests a convergence between PDI and Nox family NADPH oxidases. PDI silencing prevents Nox responses to angiotensin II and inhibits Akt phosphorylation in vascular cells and parasite phagocytosis in macrophages. PDI overexpression spontaneously enhances Nox activation and expression. In neutrophils, PDI redox-dependently associates with p47phox and supports the respiratory burst. At the cell surface, PDI exerts transnitrosation, thiol reductase, and apparent isomerase activities toward targets including adhesion and matrix proteins and proteases. Such effects mediate redox-dependent adhesion, coagulation/thrombosis, immune functions, and virus internalization. The route of PDI externalization remains elusive. Such multiple redox effects of PDI may contribute to its conspicuous expression and functional role in disease, rendering PDI family members putative redox cell signaling adaptors.

Structure, mechanism, and evolution of Ero1 family enzymes

SIGNIFICANCE

Disulfide bond formation is an essential reaction involved in the folding and maturation of many secreted and membrane proteins. Both prokaryotic and eukaryotic cells utilize various disulfide oxidoreductases and redox-active cofactors to accelerate this oxidative reaction, and higher eukaryotes have diversified and refined these disulfide-introducing cascades over the course of evolution.

RECENT ADVANCES

In the past decade, atomic resolution structures have been solved for an increasing number of disulfide oxidoreductases, thereby revealing the structural and mechanistic basis of cellular disulfide bond formation systems.

CRITICAL ISSUES

In this review, we focus on the evolution, structure, and regulatory mechanisms of endoplasmic reticulum oxidoreductin 1 (Ero1) family enzymes, the primary disulfide bond-generating catalysts in the endoplasmic reticulum (ER). Detailed comparison of Ero1 with other oxidoreductases, such as Prx4, QSOX, Erv1/2, and disulfide bond protein B (DsbB), provides important insight into how this ER-resident flavoenzyme acts in a regulated and specific manner to maintain redox and protein homeostasis in eukaryotic cells.

FUTURE DIRECTIONS

Currently, it is presumed that multiple pathways in addition to that mediated by Ero1 cooperate to achieve oxidative folding of many secretory and membrane proteins in mammalian cells. The important open question is how each oxidative pathway works distinctly or redundantly in response to various cellular conditions.

Vitamin K epoxide reductase contributes to protein disulfide formation and redox homeostasis within the endoplasmic reticulum

Ero1 oxidation of PDI family members drives disulfide bond formation, but parallel pathways support Ero1 function. Relative contributions of known and candidate ER oxidation pathways are ranked by combinatorial RNAi in human hepatoma cells to reveal VKOR as a substantial contributor to ER oxidation, but no role for QSOX1 is observed.

The transfer of oxidizing equivalents from the endoplasmic reticulum (ER) oxidoreductin (Ero1) oxidase to protein disulfide isomerase is an important pathway leading to disulfide formation in nascent proteins within the ER. However, Ero1-deficient mouse cells still support oxidative protein folding, which led to the discovery that peroxiredoxin IV (PRDX4) catalyzes a parallel oxidation pathway. To identify additional pathways, we used RNA interference in human hepatoma cells and evaluated the relative contributions to oxidative protein folding and ER redox homeostasis of Ero1, PRDX4, and the candidate oxidants quiescin-sulfhydryl oxidase 1 (QSOX1) and vitamin K epoxide reductase (VKOR). We show that Ero1 is primarily responsible for maintaining cell growth, protein secretion, and recovery from a reductive challenge. We further show by combined depletion with Ero1 that PRDX4 and, for the first time, VKOR contribute to ER oxidation and that depletion of all three activities results in cell death. Of importance, Ero1, PRDX4, or VKOR was individually capable of supporting cell viability, secretion, and recovery after reductive challenge in the near absence of the other two activities. In contrast, no involvement of QSOX1 in ER oxidative processes could be detected. These findings establish VKOR as a significant contributor to disulfide bond formation within the ER.

Peroxiredoxin 4: a multifunctional biomarker worthy of further exploration

Currently, there is much interest in identifying clinically relevant biomarkers, as they have the potential to be high utility non-invasive tools for early diagnosis and reliable patient monitoring in numerous conditions. Since its discovery almost 15 years ago, research on the ubiquitous antioxidant enzyme peroxiredoxin 4 (Prx4) has culminated in the recognition that Prx4 levels are different in blood drawn from the healthy general population and patients with acute or chronic diseases. In this commentary, the most striking research data from different in vitro approaches, animal models and human observational studies are discussed collectively, highlighting the clinical importance of Prx4 as a multifunctional staging and prognosis biomarker. In this context, the oxidative state of patients may be reflected by intra- and extracellular Prx4 levels, redox state, oligomerization and nitro-oxidative modifications of the enzyme. A consolidated model of the potential role and origin of circulating Prx4 is presented to stimulate further investigations in light of the current biomarker situation.

Structural insights into the peroxidase activity and inactivation of human peroxiredoxin 4

Prx4 (peroxiredoxin 4) is the only peroxiredoxin located in the ER (endoplasmic reticulum) and a proposed scavenger for H2O2. In the present study, we solved crystal structures of human Prx4 in three different redox forms and characterized the reaction features of Prx4 with H2O2. Prx4 exhibits a toroid-shaped decamer constructed of five catalytic dimers. Structural analysis revealed conformational changes around helix α2 and the C-terminal reigon with a YF (Tyr-Phe) motif from the partner subunit, which are required for interchain disulfide formation between Cys87 and Cys208, a critical step of the catalysis. The structural explanation for the restricting role of the YF motif on the active site dynamics is provided in detail. Prx4 has a high reactivity with H2O2, but is susceptible to overoxidation and consequent inactivation by H2O2. Either deletion of the YF motif or dissociation into dimers decreased the susceptibility of Prx4 to overoxidation by increasing the flexibility of Cys87.

Crystal structure of reduced and of oxidized peroxiredoxin IV enzyme reveals a stable oxidized decamer and a non-disulfide-bonded intermediate in the catalytic cycle

Peroxiredoxin IV (PrxIV) is an endoplasmic reticulum-localized enzyme that metabolizes the hydrogen peroxide produced by endoplasmic reticulum oxidase 1 (Ero1). It has been shown to play a role in de novo disulfide formation, oxidizing members of the protein disulfide isomerase family of enzymes, and is a member of the typical 2-Cys peroxiredoxin family. We have determined the crystal structure of both reduced and disulfide-bonded, as well as a resolving cysteine mutant of human PrxIV. We show that PrxIV has a similar structure to other typical 2-Cys peroxiredoxins and undergoes a conformational change from a fully folded to a locally unfolded form following the formation of a disulfide between the peroxidatic and resolving cysteine residues. Unlike other mammalian typical 2-Cys peroxiredoxins, we show that human PrxIV forms a stable decameric structure even in its disulfide-bonded state. In addition, the structure of a resolving cysteine mutant reveals an intermediate in the reaction cycle that adopts the locally unfolded conformation. Interestingly the peroxidatic cysteine in the crystal structure is sulfenylated rather than sulfinylated or sulfonylated. In addition, the peroxidatic cysteine in the resolving cysteine mutant is resistant to hyper-oxidation following incubation with high concentrations of hydrogen peroxide. These results highlight some unique properties of PrxIV and suggest that the equilibrium between the fully folded and locally unfolded forms favors the locally unfolded conformation upon sulfenylation of the peroxidatic cysteine residue.

Protein folding and quality control in the ER

The endoplasmic reticulum (ER) uses an elaborate surveillance system called the ER quality control (ERQC) system. The ERQC facilitates folding and modification of secretory and membrane proteins and eliminates terminally misfolded polypeptides through ER-associated degradation (ERAD) or autophagic degradation. This mechanism of ER protein surveillance is closely linked to redox and calcium homeostasis in the ER, whose balance is presumed to be regulated by a specific cellular compartment. The potential to modulate proteostasis and metabolism with chemical compounds or targeted siRNAs may offer an ideal option for the treatment of disease.

Quantitative proteomics reveals cellular targets of celastrol

Celastrol, a natural substance isolated from plant extracts used in traditional Chinese medicine, has been extensively investigated as a possible drug for treatment of cancer, autoimmune diseases, and protein misfolding disorders. Although studies focusing on celastrol's effects in specific cellular pathways have revealed a considerable number of targets in a diverse array of in vitro models there is an essential need for investigations that can provide a global view of its effects. To assess cellular effects of celastrol and to identify target proteins as biomarkers for monitoring treatment regimes, we performed large-scale quantitative proteomics in cultured human lymphoblastoid cells, a cell type that can be readily prepared from human blood samples. Celastrol substantially modified the proteome composition and 158 of the close to 1800 proteins with robust quantitation showed at least a 1.5 fold change in protein levels. Up-regulated proteins play key roles in cytoprotection with a prominent group involved in quality control and processing of proteins traversing the endoplasmic reticulum. Increased levels of proteins essential for the cellular protection against oxidative stress including heme oxygenase 1, several peroxiredoxins and thioredoxins as well as proteins involved in the control of iron homeostasis were also observed. Specific analysis of the mitochondrial proteome strongly indicated that the mitochondrial association of certain antioxidant defense and apoptosis-regulating proteins increased in cells exposed to celastrol. Analysis of selected mRNA transcripts showed that celastrol activated several different stress response pathways and dose response studies furthermore showed that continuous exposure to sub-micromolar concentrations of celastrol is associated with reduced cellular viability and proliferation. The extensive catalog of regulated proteins presented here identifies numerous cellular effects of celastrol and constitutes a valuable biomarker tool for the development and monitoration of disease treatment strategies.

Protein folding and modification in the mammalian endoplasmic reticulum

Analysis of the human genome reveals that approximately a third of all open reading frames code for proteins that enter the endoplasmic reticulum (ER), demonstrating the importance of this organelle for global protein maturation. The path taken by a polypeptide through the secretory pathway starts with its translocation across or into the ER membrane. It then must fold and be modified correctly in the ER before being transported via the Golgi apparatus to the cell surface or another destination. Being physically segregated from the cytosol means that the ER lumen has a distinct folding environment. It contains much of the machinery for fulfilling the task of protein production, including complex pathways for folding, assembly, modification, quality control, and recycling. Importantly, the compartmentalization means that several modifications that do not occur in the cytosol, such as glycosylation and extensive disulfide bond formation, can occur to secreted proteins to enhance their stability before their exposure to the extracellular milieu. How these various machineries interact during the normal pathway of folding and protein secretion is the subject of this review.

The sarcoplasmic reticulum luminal thiol oxidase ERO1 regulates cardiomyocyte excitation-coupled calcium release and response to hemodynamic load

Two related ER oxidation 1 (ERO1) proteins, ERO1α and ERO1β, dynamically regulate the redox environment in the mammalian endoplasmic reticulum (ER). Redox changes in cysteine residues on intralumenal loops of calcium release and reuptake channels have been implicated in altered calcium release and reuptake. These findings led us to hypothesize that altered ERO1 activity may affect cardiac functions that are dependent on intracellular calcium flux. We established mouse lines with loss of function insertion mutations in Ero1l and Ero1lb encoding ERO1α and ERO1β. The peak amplitude of calcium transients in homozygous Ero1α mutant adult cardiomyocytes was reduced to 42.0 ± 2.2% (n=10, P ≤ 0.01) of values recorded in wild-type cardiomyocytes. Decreased ERO1 activity blunted cardiomyocyte inotropic response to adrenergic stimulation and sensitized mice to adrenergic blockade. Whereas all 12 wild-type mice survived challenge with 4 mg/kg esmolol, 6 of 8 compound Ero1l and Ero1lb mutant mice succumbed to this level of β adrenergic blockade (P ≤ 0.01). In addition, mice lacking ERO1α were partially protected against progressive heart failure in a transaortic constriction model [at 10 wk postprocedure, fractional shortening was 0.31 ± 0.02 in the mutant (n=20) vs. 0.23 ± 0.03 in the wild type (n=18); P ≤ 0.01]. These findings establish a role for ERO1 in calcium homeostasis and suggest that modifying the lumenal redox environment may affect the progression of heart failure.

Multiple ways to make disulfides

Our concept of how disulfides form in proteins entering the secretory pathway has changed dramatically in recent years. The discovery of endoplasmic reticulum (ER) oxidoreductin 1 (ERO1) was followed by the demonstration that this enzyme couples oxygen reduction to de novo formation of disulfides. However, mammals deficient in ERO1 survive and form disulfides, which suggests the presence of alternative pathways. It has recently been shown that peroxiredoxin 4 is involved in peroxide removal and disulfide formation. Other less well-characterized pathways involving quiescin sulfhydryl oxidase, ER-localized protein disulfide isomerase peroxidases and vitamin K epoxide reductase might all contribute to disulfide formation. Here we discuss these various pathways for disulfide formation in the mammalian ER and highlight the central role played by glutathione in regulating this process.

Real-time monitoring of redox changes in the mammalian endoplasmic reticulum

Redox-sensitive GFPs with engineered disulphide bonds have been used previously to monitor redox status in the cytosol and mitochondria of living cells. The usefulness of these redox probes depends on the reduction potential of the disulphide, with low values suiting the cytosol and mitochondrion, and higher values suiting the more oxidising environment of the endoplasmic reticulum (ER). Here, we targeted a modified redox-sensitive GFP (roGFP1-iL), with a relatively high reduction potential, to the ER of mammalian cells. We showed that the disulphide is partially oxidised, allowing roGFP1-iL to monitor changes in ER redox status. When cells were treated with puromycin, the redox balance became more reducing, suggesting that the release of nascent chains from ribosomes alters the ER redox balance. In addition, downregulating Ero1α prevented normal rapid recovery from dithiothreitol (DTT), whereas downregulating peroxiredoxin IV had no such effect. This result illustrates the contribution of the Ero1α oxidative pathway to ER redox balance. This first report of the use of roGFP to study the ER of mammalian cells demonstrates that roGFP1-iL can be used to monitor real-time changes to the redox status in individual living cells.

Glutathione- and non-glutathione-based oxidant control in the endoplasmic reticulum

The redox-active tripeptide glutathione is an endogenous reducing agent that is found in abundance and throughout the cell. In the endoplasmic reticulum (ER), the ratio of glutathione to glutathione disulfide is lower compared with non-secretory organelles. This relatively oxidizing thiol-disulfide milieu is essential for the oxidative folding of nascent proteins in the ER and, at least in part, maintained by the activity of ER-resident endoplasmic oxidoreductin 1 (Ero1) enzymes that oxidize cysteine side chains at the expense of molecular oxygen. Glutathione disulfide and hydrogen peroxide formed as a consequence of Ero1 activity are widely considered as being inoperative and potentially dangerous by-products of oxidative protein folding in the ER. In contrast to this common view, this Commentary highlights the importance of glutathione- and non glutathione-based homeostatic redox control mechanisms in the ER. Stability in the thiol-disulfide system that prominently includes the protein disulfide isomerases is ensured by the contribution of tightly regulated Ero1 activity, ER-resident peroxidases and the glutathione-glutathione-disulfide redox pair that acts as a potent housekeeper of redox balance. Accordingly, the widely held concept that Ero1-mediated over-oxidation in the ER constitutes a common cause of cellular demise is critically re-evaluated.

Different consequences of reactions with hydrogen peroxide and t-butyl hydroperoxide in the hyperoxidative inactivation of rat peroxiredoxin-4

Eukaryotic typical 2-Cys type peroxiredoxin (Prx) is inactivated by hyperoxidation of the peroxidatic cysteine to a sulphinic acid in a catalytic cycle-dependent manner. This inactivation process has been well documented for cytosolic isoforms of Prx. However, such a hyperoxidative inactivation has not fully been investigated in Prx-4, a secretable endoplasmic reticulum-resident isoform, in spite of being a typical 2-Cys type, and details of this process are reported herein. As has been observed in many peroxiredoxins, the peroxidase activity of Prx-4 was almost completely inhibited in the reaction with t-butyl hydroperoxide. On the other hand, when H(2)O(2) was used as the substrate, the peroxidase activity significantly remained after oxidative damage. In spite of these different consequences, mass spectrometric analyses indicated that both reactions resulted in the same oxidative damage, i.e. sulphinic acid formation at the peroxidatic cysteine, suggesting that another cysteine in the active site confers the peroxidase activity. As suggested by the analyses using cysteine-substituted mutants sulphinic acid formation at the peroxidatic cysteine may play a role in the development of the possible alternative mechanism, thereby sustaining the peroxidase activity that prefers H(2)O(2).

Two endoplasmic reticulum PDI peroxidases increase the efficiency of the use of peroxide during disulfide bond formation

Disulfide bond formation in the endoplasmic reticulum by the sulfhydryl oxidase Ero1 family is thought to be accompanied by the concomitant formation of hydrogen peroxide. Since secretory cells can make substantial amounts of proteins that contain disulfide bonds, the production of this reactive oxygen species could have potentially lethal consequences. Here, we show that two human proteins, GPx7 and GPx8, labeled as secreted glutathione peroxidases, are actually endoplasmic reticulum-resident protein disulfide isomerase peroxidases. In vitro, the addition of GPx7 or GPx8 to a folding protein along with protein disulfide isomerase and peroxide enables the efficient oxidative refolding of a reduced denatured protein. Furthermore, both GPx7 and GPx8 interact with Ero1α in vivo, and GPx7 significantly increases oxygen consumption by Ero1α in vitro. Hence, GPx7 and GPx8 may represent a novel route for the productive use of peroxide produced by Ero1α during disulfide bond formation.

Proteostenosis and plasma cell pathophysiology

Plasma cells differentiate from B lymphocytes to sustain antibody production. As professional secretors, they allow dissecting proteostasis in the exocytic compartment, the stresses that protein production entails and their possible roles in signaling. Most plasma cells are short-lived to limit antibody responses. After a few days of intense immunoglobulin production, they undergo apoptosis, offering a unique model of cellular senescence. Recent observations reveal that proteotoxic stresses physiologically contribute to regulate their biogenesis, function and lifespan, explaining partly the sensitivity of multiple myeloma cells to proteasome inhibitors. This essay summarizes these plasma cell lessons, and their general implications for the regulation of proteostasis, cell senescence and cancer therapeutics.

Oxidative protein folding by an endoplasmic reticulum-localized peroxiredoxin

Endoplasmic reticulum (ER) oxidation 1 (ERO1) transfers disulfides to protein disulfide isomerase (PDI) and is essential for oxidative protein folding in simple eukaryotes such as yeast and worms. Surprisingly, ERO1-deficient mammalian cells exhibit only a modest delay in disulfide bond formation. To identify ERO1-independent pathways to disulfide bond formation, we purified PDI oxidants with a trapping mutant of PDI. Peroxiredoxin IV (PRDX4) stood out in this list, as the related cytosolic peroxiredoxins are known to form disulfides in the presence of hydroperoxides. Mouse embryo fibroblasts lacking ERO1 were intolerant of PRDX4 knockdown. Introduction of wild-type mammalian PRDX4 into the ER rescued the temperature-sensitive phenotype of an ero1 yeast mutation. In the presence of an H(2)O(2)-generating system, purified PRDX4 oxidized PDI and reconstituted oxidative folding of RNase A. These observations implicate ER-localized PRDX4 in a previously unanticipated, parallel, ERO1-independent pathway that couples hydroperoxide production to oxidative protein folding in mammalian cells.

Disulfide bonds in ER protein folding and homeostasis

Proteins that are expressed outside the cell must be synthesized, folded, and assembled in a way that ensures they can function in their designate location. Accordingly, these proteins are primarily synthesized in the endoplasmic reticulum (ER), which has developed a chemical environment more similar to that outside the cell. This organelle is equipped with a variety of molecular chaperones and folding enzymes that both assist the folding process, while at the same time exerting tight quality control measures that are largely absent outside the cell. A major post-translational modification of ER-synthesized proteins is disulfide bridge formation, which is catalyzed by the family of protein disulfide isomerases. As this covalent modification provides unique structural advantages to extracellular proteins, multiple pathways to disulfide bond formation have evolved. However, the advantages that disulfide bonds impart to these proteins come at a high cost to the cell. Very recent reports have shed light on how the cell can deal with or even exploit the side reactions of disulfide bond formation to maintain homeostasis of the ER and its folding machinery.

Recycling of peroxiredoxin IV provides a novel pathway for disulphide formation in the endoplasmic reticulum

Disulphide formation in the endoplasmic reticulum (ER) is catalysed by members of the protein disulphide isomerase (PDI) family. These enzymes can be oxidized by the flavoprotein ER oxidoreductin 1 (Ero1), which couples disulphide formation with reduction of oxygen to form hydrogen peroxide (H(2)O(2)). The H(2)O(2) produced can be metabolized by ER-localized peroxiredoxin IV (PrxIV). Continuous catalytic activity of PrxIV depends on reduction of a disulphide within the active site to form a free thiol, which can then react with H(2)O(2). Here, we demonstrate that several members of the PDI family are able to directly reduce this PrxIV disulphide and in the process become oxidized. Furthermore, we show that altering cellular expression of these proteins within the ER influences the efficiency with which PrxIV can be recycled. The oxidation of PDI family members by PrxIV is a highly efficient process and demonstrates how oxidation by H(2)O(2) can be coupled to disulphide formation. Oxidation of PDI by PrxIV may therefore increase efficiency of disulphide formation by Ero1 and also allows disulphide formation via alternative sources of H(2)O(2).