Inactivation of human peroxiredoxin I during catalysis as the result of the oxidation of the catalytic site cysteine to cysteine-sulfinic acid

Cysteine thiol sulfinic acid in plant stress signaling

Cysteine thiols are susceptible to various oxidative posttranslational modifications (PTMs) due to their high chemical reactivity. Thiol-based PTMs play a crucial role in regulating protein functions and are key contributors to cellular redox signaling. Although reversible thiol-based PTMs, such as disulfide bond formation, S-nitrosylation, and S-glutathionylation, have been extensively studied for their roles in redox regulation, thiol sulfinic acid (-SO2H) modification is often perceived as irreversible and of marginal significance in redox signaling. Here, we revisit this narrow perspective and shed light on the redox regulatory roles of -SO2H in plant stress signaling. We provide an overview of protein sulfinylation in plants, delving into the roles of hydrogen peroxide-mediated and plant cysteine oxidase-catalyzed formation of -SO2H, highlighting the involvement of -SO2H in specific regulatory signaling pathways. Additionally, we compile the existing knowledge of the -SO2H reducing enzyme, sulfiredoxin, offering insights into its molecular mechanisms and biological relevance. We further summarize current proteomic techniques for detecting -SO2H and furnish a list of experimentally validated cysteine -SO2H sites across various species, discussing their functional consequences. This review aims to spark new insights and discussions that lead to further investigations into the functional significance of protein -SO2H-based redox signaling in plants.

Oxidative Cysteine Post Translational Modifications Drive the Redox Code Underlying Neurodegeneration and Amyotrophic Lateral Sclerosis

Redox dysregulation, an imbalance between oxidants and antioxidants, is crucial in the pathogenesis of various neurodegenerative diseases. Within this context, the "redoxome" encompasses the network of redox molecules collaborating to maintain cellular redox balance and signaling. Among these, cysteine-sensitive proteins are fundamental for this homeostasis. Due to their reactive thiol groups, cysteine (Cys) residues are particularly susceptible to oxidative post-translational modifications (PTMs) induced by free radicals (reactive oxygen, nitrogen, and sulfur species) which profoundly affect protein functions. Cys-PTMs, forming what is referred to as "cysteinet" in the redox proteome, are essential for redox signaling in both physiological and pathological conditions, including neurodegeneration. Such modifications significantly influence protein misfolding and aggregation, key hallmarks of neurodegenerative diseases such as Alzheimer's, Parkinson's, and notably, amyotrophic lateral sclerosis (ALS). This review aims to explore the complex landscape of cysteine PTMs in the cellular redox environment, elucidating their impact on neurodegeneration at protein level. By investigating specific cysteine-sensitive proteins and the regulatory networks involved, particular emphasis is placed on the link between redox dysregulation and ALS, highlighting this pathology as a prime example of a neurodegenerative disease wherein such redox dysregulation is a distinct hallmark.

The cytosolic hyperoxidation-sensitive and -robust Leishmania peroxiredoxins cPRX1 and cPRX2 are both dispensable for parasite infectivity

Typical two-cysteine peroxiredoxins (2-Cys-PRXs) are H2O2-metabolizing enzymes whose activity relies on two cysteine residues. Protists of the family Trypanosomatidae invariably express one cytosolic 2-Cys-PRX (cPRX1). However, the Leishmaniinae sub-family features an additional isoform (cPRX2), almost identical to cPRX1, except for the lack of an elongated C-terminus with a Tyr-Phe (YF) motif. Previously, cytosolic PRXs were considered vital components of the trypanosomatid antioxidant machinery. Here, we shed new light on the properties, functions and relevance of cPRXs from the human pathogen Leishmania infantum. We show first that LicPRX1 is sensitive to inactivation by hyperoxidation, mirroring other YF-containing PRXs participating in redox signaling. Using genetic fusion constructs with roGFP2, we establish that LicPRX1 and LicPRX2 can act as sensors for H2O2 and oxidize protein thiols with implications for signal transduction. Third, we show that while disrupting the LicPRX-encoding genes increases susceptibility of L. infantum promastigotes to external H2O2in vitro, both enzymes are dispensable for the parasites to endure the macrophage respiratory burst, differentiate into amastigotes and initiate in vivo infections. This study introduces a novel perspective on the functions of trypanosomatid cPRXs, exposing their dual roles as both peroxidases and redox sensors. Furthermore, the discovery that Leishmania can adapt to the absence of both enzymes has significant implications for our understanding of Leishmania infections and their treatment. Importantly, it questions the conventional notion that the oxidative response of macrophages during phagocytosis is a major barrier to infection and the suitability of cPRXs as drug targets for leishmaniasis.

LncFASA promotes cancer ferroptosis via modulating PRDX1 phase separation

Ferroptosis, a unique type of non-apoptotic cell death resulting from iron-dependent lipid peroxidation, has a potential physiological function in tumor suppression, but its underlying mechanisms have not been fully elucidated. Here, we report that the long non-coding RNA (lncRNA) LncFASA increases the susceptibility of triple-negative breast cancer (TNBC) to ferroptosis. As a tumor suppressor, LncFASA drives the formation of droplets containing peroxiredoxin1 (PRDX1), a member of the peroxidase family, resulting in the accumulation of lipid peroxidation via the SLC7A11-GPX4 axis. Mechanistically, LncFASA directly binds to the Ahpc-TSA domain of PRDX1, inhibiting its peroxidase activity by driving liquid-liquid phase separation, which disrupts intracellular ROS homeostasis. Notably, high LncFASA expression indicates favorable overall survival in individuals with breast cancer, and LncFASA impairs the growth of breast xenograft tumors by modulating ferroptosis. Together, our findings illustrate the crucial role of this lncRNA in ferroptosis-mediated cancer development and provide new insights into therapeutic strategies for breast cancer.

The architecture of redox microdomains: Cascading gradients and peroxiredoxins' redox-oligomeric coupling integrate redox signaling and antioxidant protection

In the cytosol of human cells under low oxidative loads, hydrogen peroxide is confined to microdomains around its supply sites, due to its fast consumption by peroxiredoxins. So are the sulfenic and disulfide forms of the 2-Cys peroxiredoxins, according to a previous theoretical analysis [Travasso et al., Redox Biology 15 (2017) 297]. Here, an extended reaction-diffusion model that for the first time considers the differential properties of human peroxiredoxins 1 and 2 and the thioredoxin redox cycle predicts important new aspects of the dynamics of redox microdomains. The peroxiredoxin 1 sulfenates and disulfides are more localized than the corresponding peroxiredoxin 2 forms, due to the former peroxiredoxin's faster resolution step. The thioredoxin disulfides are also localized. As the H2O2 supply rate (vsup) approaches and then surpasses the maximal rate of the thioredoxin/thioredoxin reductase system (V), these concentration gradients become shallower, and then vanish. At low vsup the peroxiredoxin concentration determines the H2O2 concentrations and gradient length scale, but as vsup approaches V, the thioredoxin reductase activity gains influence. A differential mobility of peroxiredoxin disulfide dimers vs. reduced decamers enhances the redox polarity of the cytosol: as vsup approaches V, reduced decamers are preferentially retained far from H2O2 sources, attenuating the local H2O2 buildup. Substantial total protein concentration gradients of both peroxiredoxins emerge under these conditions, and the concentration of reduced peroxiredoxin 1 far from the H2O2 sources even increases with vsup. Altogether, the properties of 2-Cys peroxiredoxins and thioredoxin are such that localized H2O2 supply induces a redox and functional polarization between source-proximal regions (redox microdomains) that facilitate peroxiredoxin-mediated signaling and distal regions that maximize antioxidant protection.

Blood flow patterns switch VEGFR2 activity through differential S-nitrosylation and S-oxidation

Vascular endothelial growth factor receptor-2 (VEGFR2) plays a key role in maintaining vascular endothelial homeostasis. Here, we show that blood flows determine activation and inactivation of VEGFR2 through selective cysteine modifications. VEGFR2 activation is regulated by reversible oxidation at Cys1206 residue. H2O2-mediated VEGFR2 oxidation is induced by oscillatory flow in vascular endothelial cells through the induction of NADPH oxidase-4 expression. In contrast, laminar flow induces the expression of endothelial nitric oxide synthase and results in the S-nitrosylation of VEGFR2 at Cys1206, which counteracts the oxidative inactivation. The shear stress model study reveals that disturbed blood flow operated by partial ligation in the carotid arteries induces endothelial damage and intimal hyperplasia in control mice but not in knock-in mice harboring the oxidation-resistant mutant (C1206S) of VEGFR2. Thus, our findings reveal that flow-dependent redox regulation of the VEGFR2 kinase is critical for the structural and functional integrity of the arterial endothelium.

A role for cytoglobin in regulating intracellular hydrogen peroxide and redox signals in the vasculature

The oxidant hydrogen peroxide serves as a signaling molecule that alters many aspects of cardiovascular functions. Recent studies suggest that cytoglobin - a hemoglobin expressed in the vasculature - may promote electron transfer reactions with proposed functions in hydrogen peroxide decomposition. Here, we determined the extent to which cytoglobin regulates intracellular hydrogen peroxide and established mechanisms. We found that cytoglobin decreased the hyperoxidation of peroxiredoxins and maintained the activity of peroxiredoxin 2 following challenge with exogenous hydrogen peroxide. Cytoglobin promoted a reduced intracellular environment and facilitated the reduction of the thiol-based hydrogen peroxide sensor Hyper7 after bolus addition of hydrogen peroxide. Cytoglobin also limited the inhibitory effect of hydrogen peroxide on glycolysis and reversed the oxidative inactivation of the glycolytic enzyme GAPDH. Our results indicate that cytoglobin in cells exists primarily as oxyferrous cytoglobin (CygbFe 2+ -O 2 ) with its cysteine residues in the reduced form. We found that the specific substitution of one of two cysteine residues on cytoglobin (C83A) inhibited the reductive activity of cytoglobin on Hyper7 and GAPDH. Carotid arteries from cytoglobin knockout mice were more sensitive to glycolytic inhibition by hydrogen peroxide than arteries from wildtype mice. Together, these results support a role for cytoglobin in regulating intracellular redox signals associated with hydrogen peroxide through oxidation of its cysteine residues, independent of hydrogen peroxide reaction at its heme center.

Peroxiredoxins in erythrocytes: far beyond the antioxidant role

The red blood cells (RBCs) are essential to transport oxygen (O2) and nutrients throughout the human body. Changes in the structure or functioning of the erythrocytes can lead to several deficiencies, such as hemolytic anemias, in which an increase in reactive oxidative species generation is involved in the pathophysiological process, playing a significant role in the severity of several clinical manifestations. There are important lines of defense against the damage caused by oxidizing molecules. Among the antioxidant molecules, the enzyme peroxiredoxin (Prx) has the higher decomposition power of hydrogen peroxide, especially in RBCs, standing out because of its abundance. This review aimed to present the recent findings that broke some paradigms regarding the three isoforms of Prxs found in RBC (Prx1, Prx2, and Prx6), showing that in addition to their antioxidant activity, these enzymes may have supplementary roles in transducing peroxide signals, as molecular chaperones, protecting from membrane damage, and maintenance of iron homeostasis, thus contributing to the overall survival of human RBCs, roles that seen to be disrupted in hemolytic anemia conditions.

Overoxidation and Oligomerization of Trypanosoma cruzi Cytosolic and Mitochondrial Peroxiredoxins

Peroxiredoxins (Prxs) have been shown to be important enzymes for trypanosomatids, counteracting oxidative stress and promoting cell infection and intracellular survival. In this work, we investigate the in vitro sensitivity to overoxidation and the overoxidation dynamics of Trypanosoma cruzi Prxs in parasites in culture and in the infection context. We showed that recombinant m-TXNPx, in contrast to what was observed for c-TXNPx, exists as low molecular mass forms in the overoxidized state. We observed that T. cruzi Prxs were overoxidized in epimastigotes treated with oxidants, and a significant proportion of the overoxidized forms were still present at least 24 h after treatment suggesting that these forms are not actively reversed. In in vitro infection experiments, we observed that Prxs are overoxidized in amastigotes residing in infected macrophages, demonstrating that inactivation of at least part of the Prxs by overoxidation occurs in a physiological context. We have shown that m-TXNPx has a redox-state-dependent chaperone activity. This function may be related to the increased thermotolerance observed in m-TXNPx-overexpressing parasites. This study suggests that despite the similarity between protozoan and mammalian Prxs, T. cruzi Prxs have different oligomerization dynamics and sensitivities to overoxidation, which may have implications for their function in the parasite life cycle and infection process.

Functional changes of OsTrxm from reductase to molecular chaperone under heat shock stress

Ubiquitous disulfide reductases, thioredoxins (Trxs), function in the redox balance of all living organisms. Although the roles of the rice (Oryza sativa) Trx m-type isoform (OsTrxm) in chloroplast development have been already published, biochemical and molecular functions of OsTrxm remain to be elucidated for decades. The OsTrxm and its two conserved active cysteine mutant (OsTrxm C95S/C98S, referred to as OsTrxmC/S) proteins in Arabidopsis thaliana were overexpressed to characterize in vivo roles of active cysteines of OsTrxm. Interestingly, the OsTrxm overexpressed variant plants were resistant to heat shock treatment. Especially OsTrxmC/S with higher molecular weight (HMW) complexes showed higher heat tolerance than OsTrxm with lower molecular weight (LMW) structure in Arabidopsis thaliana. To confirm the importance of active cysteines on structural changes under heat stress, OsTrxm and OsTrxmC/S proteins were bacterially expressed and isolated. This study found that two proteins have various structures ranging from LMW to HMW complexes and have potential functions as a disulfide reductase and a molecular chaperone, which has never been reported anywhere. The function of molecular chaperone predominated in the HMW complexes, whereas the disulfide reductase function was observed in LMW forms. These results suggest that the active cysteines of OsTrxm play a critical role in protein structural change as well as heat tolerance in plants.

Peroxiredoxin 5 overexpression decreases oxidative stress and dopaminergic cell death mediated by paraquat

Peroxiredoxins (Prdxs) are thiol-dependent enzymes that scavenge peroxides. Previously, we found that Prdxs were hyperoxidized in a Parkinson's disease model induced by paraquat (PQ), which led to their inactivation, perpetuating reactive oxygen species (ROS) formation. Herein, we evaluated the redox state of the typical 2-Cys-Prx subgroup. We found that PQ induces ROS compartmentalization in different organelles, reflected by the 2-Cys-Prdx hyperoxidation pattern detected by redox eastern blotting. 2-Cys Prdxs are most vulnerable to hyperoxidation, while atypical 2-Cys Peroxiredoxin 5 (Prdx5) is resistant and is expressed in multiple organelles, such as mitochondria, peroxisomes, and cytoplasm. Therefore, we overexpressed human Prdx5 in the dopaminergic SHSY-5Y cell line using the adenoviral vector Ad-hPrdx5. Prdx5 overexpression was confirmed by western blotting and immunofluorescence (IF) and effectively decreased PQ-mediated mitochondrial and cytoplasmic ROS assessed with a mitochondrial superoxide indicator and DHE through IF or flow cytometry. Decreased ROS mediated by Prdx5 in the main subcellular compartments led to overall cell protection against PQ-induced cell death, which was demonstrated by flow cytometry using Annexin V labeling and 7-AAD. Therefore, Prdx5 is an attractive therapeutic target for PD, as its overexpression protects dopaminergic cells from ROS and death, which warrants further experimental animal studies for its subsequent application in clinical trials.

Modelling the Decamerisation Cycle of PRDX1 and the Inhibition-like Effect on Its Peroxidase Activity

Peroxiredoxins play central roles in the detoxification of reactive oxygen species and have been modelled across multiple organisms using a variety of kinetic methods. However, the peroxiredoxin dimer-to-decamer transition has been underappreciated in these studies despite the 100-fold difference in activity between these forms. This is due to the lack of available kinetics and a theoretical framework for modelling this process. Using published isothermal titration calorimetry data, we obtained association and dissociation rate constants of 0.050 µM-4·s-1 and 0.055 s-1, respectively, for the dimer-decamer transition of human PRDX1. We developed an approach that greatly reduces the number of reactions and species needed to model the peroxiredoxin decamer oxidation cycle. Using these data, we simulated horse radish peroxidase competition and NADPH-oxidation linked assays and found that the dimer-decamer transition had an inhibition-like effect on peroxidase activity. Further, we incorporated this dimer-decamer topology and kinetics into a published and validated in vivo model of PRDX2 in the erythrocyte and found that it almost perfectly reconciled experimental and simulated responses of PRDX2 oxidation state to hydrogen peroxide insult. By accounting for the dimer-decamer transition of peroxiredoxins, we were able to resolve several discrepancies between experimental data and available kinetic models.

Biological and Catalytic Properties of Selenoproteins

Selenocysteine is a catalytic residue at the active site of all selenoenzymes in bacteria and mammals, and it is incorporated into the polypeptide backbone by a co-translational process that relies on the recoding of a UGA termination codon into a serine/selenocysteine codon. The best-characterized selenoproteins from mammalian species and bacteria are discussed with emphasis on their biological function and catalytic mechanisms. A total of 25 genes coding for selenoproteins have been identified in the genome of mammals. Unlike the selenoenzymes of anaerobic bacteria, most mammalian selenoenzymes work as antioxidants and as redox regulators of cell metabolism and functions. Selenoprotein P contains several selenocysteine residues and serves as a selenocysteine reservoir for other selenoproteins in mammals. Although extensively studied, glutathione peroxidases are incompletely understood in terms of local and time-dependent distribution, and regulatory functions. Selenoenzymes take advantage of the nucleophilic reactivity of the selenolate form of selenocysteine. It is used with peroxides and their by-products such as disulfides and sulfoxides, but also with iodine in iodinated phenolic substrates. This results in the formation of Se-X bonds (X = O, S, N, or I) from which a selenenylsulfide intermediate is invariably produced. The initial selenolate group is then recycled by thiol addition. In bacterial glycine reductase and D-proline reductase, an unusual catalytic rupture of selenium-carbon bonds is observed. The exchange of selenium for sulfur in selenoproteins, and information obtained from model reactions, suggest that a generic advantage of selenium compared with sulfur relies on faster kinetics and better reversibility of its oxidation reactions.

Antioxidant enzymes that target hydrogen peroxide are conserved across the animal kingdom, from sponges to mammals

Oxygen is the sustenance of aerobic life and yet is highly toxic. In early life, antioxidants functioned solely to defend against toxic effects of reactive oxygen species (ROS). Later, as aerobic metabolisms evolved, ROS became essential for signalling. Thus, antioxidants are multifunctional and must detoxify, but also permit ROS signalling for vital cellular processes. Here we conduct metazoan-wide genomic assessments of three enzymatic antioxidant families that target the predominant ROS signaller, hydrogen peroxide: namely, monofunctional catalases (CAT), peroxiredoxins (PRX), and glutathione peroxidases (GPX). We reveal that the two most evolutionary ancient families, CAT and PRX, exhibit metazoan-wide conservation. In the basal animal lineage, sponges (phylum Porifera), we find all three antioxidant families, but with GPX least abundant. Poriferan CATs are distinct from bilaterian CATs, but the evolutionary divergence is small. Amongst PRXs, subfamily PRX6 is the most conserved, whilst subfamily AhpC-PRX1 is the largest; PRX4 is the only core member conserved from sponges to mammals and may represent the ancestral animal AhpC-PRX1. Conversely, for GPX, the most recent family to arise, only the cysteine-dependent subfamily GPX7 is conserved across metazoans, and common across Porifera. Our analyses illustrate that the fundamental functions of antioxidants have resulted in gene conservation throughout the animal kingdom.

Celastrol suppresses colorectal cancer via covalent targeting peroxiredoxin 1

As a terpenoids natural product isolated from the plant Thunder God Vine, Celastrol is widely studied for its pharmacological activities, including anti-tumor activities. The clinical application of Celastrol is strictly limited due to its severe side effects, whereas previously revealed targets and mechanism of Celastrol seldom reduce its in vivo toxicity via structural optimization. Target identification has a far-reaching influence on the development of innovative drugs, and omics data has been widely used for unbiased target prediction. However, it is difficult to enrich target of specific phenotype from thousands of genes or proteins, especially for natural products with broad promising activities. Here, we developed a text-mining-based web-server tool to enrich targets from omics data of inquired compounds. Then peroxiredoxin 1 (PRDX1) was identified as the ROS-manipulating target protein of Celastrol in colorectal cancer. Our solved high-resolution crystal structure revealed the unique covalent binding mode of Celastrol with PRDX1. New derivative compound 19-048 with improved potency against PRDX1 and selectivity towards PRDX2~PRDX6 were synthesized based on crystal structure analysis. Both Celastrol and 19-048 effectively suppressed the proliferation of colorectal cancer cells. The anti-tumor efficacy of Celastrol and 19-048 was significantly diminished on xenograft nude mice bearing PRDX1 knock-down colorectal cancer cells. Several downstream genes of p53 signaling pathway were dramatically up-regulated with Celastrol or 19-048 treatment. Our findings reveal that the side effects of Celastrol could be reduced via structural modification, and PRDX1 inhibition is promising for the treatment of colorectal cancer.

Inhibition of thioredoxin reductase activity reduces the antioxidant defense capacity of human pluripotent stem cells under conditions of mild but not severe oxidative stress

Pro-oxidative shift in redox balance, usually termed as "oxidative stress", can lead to different cell responses depending on its intensity. Excessive accumulation of reactive oxygen species ("oxidative distress") can cause DNA, lipid and protein damage. Physiological oxidative stimulus ("oxidative eustress"), in turn, can favor cell proliferation and differentiation - the processes of paramount importance primarily for stem cells. Functions of antioxidant enzymes in cells is currently a focus of intense research, however the role of different antioxidant pathways in pluripotent cell responses to oxidative distress/eustress is still under investigation. In this study, we assessed the contribution of the thioredoxin reductase (TrxR)-dependent pathways to maintaining the redox homeostasis in human induced pluripotent stem cells and their differentiated progeny cells under basal conditions and under conditions of oxidative stress of varying intensity. Employing the genetically encoded H₂O₂ biosensor cyto-HyPer and two inhibitors of thioredoxin reductase (auranofin and Tri-1), we show that the reduced activity of TrxR-dependent enzymatic systems leads to the non-cytotoxic disruption of thiol-disulfide metabolism in the cytoplasm of both pluripotent and differentiated cells under basal conditions. Quantifying the cytoplasmic concentrations of peroxide establishing in H₂O₂-stressed cells, we demonstrate that TrxR-dependent pathways contribute to the antioxidant activity in the cell cytoplasm under conditions of mild but not severe oxidative stress in both cell lines tested. The observed effects may testify about a conservative role of the TrxR-controlled enzymatic systems manifested as a response to physiological redox stimuli rather than a protection against the severe oxidative stress.

Molecular characterization and functional analysis of peroxiredoxin 3 (NdPrx3) from Neocaridina denticulata sinensis

Peroxiredoxins (Prxs) widely exist in organisms and can prevent oxidative damage. Here, the characterization and biological function of NdPrx3 from Neocaridina denticulata sinensis were analyzed. The coding sequence of NdPrx3 consists of 684 bp open reading frame (ORF), encoding 227 amino acids with a predicted molecular weight of 24.7 kDa and theoretical pI 6.49. Multiple sequence alignments showed that the conserved domains of NdPrx3, including catalytic triad, dimer interface, decamer interface, peroxidatic, and resolving cysteines, were similar to those of other organisms. The phylogenetic relationship demonstrated that NdPrx3 clustered in the Prx3 class. The highest relative expression of NdPrx3 mRNA was confirmed in gill among the nine tissues from healthy shrimp. The transcript level of NdPrx3 was significantly upregulated from 0 h to 48 h and decreased in 72 h under copper challenge, indicating that NdPrx3 may play an important role in the copper challenge of N. denticulata sinensis. In addition, NdPrx3 was recombinantly expressed in E. coli and purified to one band on SDS-PAGE. The DNA protection of rNdPrx3 was verified. The enzymatic assay of the recombinant NdPrx3 indicated that it had the oxidoreductase function and was stable at a low temperature (10-30 °C).

Modulating antioxidant systems as a therapeutic approach to retinal degeneration

The human retina is facing a big challenge of reactive oxygen species (ROS) from endogenous and exogenous sources. Excessive ROS can cause damage to DNA, lipids, and proteins, triggering abnormal redox signaling, and ultimately lead to cell death. Thus, oxidative stress has been observed in inherited retinal diseases as a common hallmark. To counteract the detrimental effect of ROS, cells are equipped with various antioxidant defenses. In this review, we will focus on the antioxidant systems in the retina and how they can protect retina from oxidative stress. Both small antioxidants and antioxidant enzymes play a role in ROS removal. Particularly, the thioredoxin and glutaredoxin systems, as the major antioxidant systems in mammalian cells, exert functions in redox signaling regulation via modifying cysteines in proteins. In addition, the thioredoxin-like rod-derived cone viability factor (RdCVFL) and thioredoxin interacting protein (TXNIP) can modulate metabolism in photoreceptors and promote their survival. In conclusion, elevating the antioxidant capacity in retina is a promising therapy to curb the progress of inherited retinal degeneration.

The Role of Peroxiredoxins in the Regulation of Sepsis

Oxidative stress, a result of a disturbance in redox homeostasis, is considered to be one of the main aggravating events in the pathogenesis of immune disorders. Peroxiredoxins (Prdxs) are an enzyme family that catalyzes the reduction of peroxides, including hydrogen peroxide, lipid peroxides, and nitrogen peroxides. Although the maintenance of cellular redox homeostasis through Prdxs is essential for surviving in adverse environments, Prdxs also participate in the regulation of cellular signal transduction by modulating the activities of a panel of molecules involved in the signal transduction process. Although Prdxs were discovered as intracellular anti-oxidative enzymes, recent research has revealed that Prdxs also play important roles in the extracellular milieu. Indeed, Prdxs have been shown to have the capacity to activate immune cells through ligation with innate immune receptors such as toll-like receptors (TLRs). In this review, we will summarize the intracellular as well as extracellular roles of Prdxs for and against the pathogenesis of inflammatory disorders including sepsis, hemorrhagic shock, and drug-induced liver injury.

Role of the Redox State of Human Peroxiredoxin-5 on Its TLR4-Activating DAMP Function

Human peroxiredoxin-5 (PRDX5) is a unique redox-sensitive protein that plays a dual role in brain ischemia-reperfusion injury. While intracellular PRDX5 has been reported to act as a neuroprotective antioxidative enzyme by scavenging peroxides, once released extracellularly from necrotic brain cells, the protein aggravates neural cell death by inducing expression of proinflammatory cytokines in macrophages through activation of Toll-like receptor (TLR) 2 (TLR2) and 4 (TLR4). Although recent evidence showed that PRDX5 was able to interact directly with TLR4, little is known regarding the role of the cysteine redox state of PRDX5 on its DAMP function. To gain insights into the role of PRDX5 redox-active cysteine residues in the TLR4-dependent proinflammatory activity of the protein, we used a recombinant human PRDX5 in the disulfide (oxidized) form and a mutant version lacking the peroxidatic cysteine, as well as chemically reduced and hyperoxidized PRDX5 proteins. We first analyzed the oxidation state and oligomerization profile by Western blot, mass spectrometry, and SEC-MALS. Using ELISA, we demonstrate that the disulfide bridge between the enzymatic cysteines is required to allow improved TLR4-dependent IL-8 secretion. Moreover, single-molecule force spectroscopy experiments revealed that TLR4 alone is not sufficient to discriminate the different PRDX5 redox forms. Finally, flow cytometry binding assays show that disulfide PRDX5 has a higher propensity to bind to the surface of living TLR4-expressing cells than the mutant protein. Taken together, these results demonstrate the importance of the redox state of PRDX5 cysteine residues on TLR4-induced inflammation.

The sulfiredoxin-peroxiredoxin redox system regulates the stemness and survival of colon cancer stem cells

Cancer stem cells (CSCs) initiate tumor formation and are known to be resistant to chemotherapy. A metabolic alteration in CSCs plays a critical role in stemness and survival. However, the association between mitochondrial energy metabolism and the redox system remains undefined in colon CSCs. In this study, we assessed the role of the Sulfiredoxin-Peroxiredoxin (Srx-Prx) redox system and mitochondrial oxidative phosphorylation (OXPHOS) in maintaining the stemness and survival of colon CSCs. Notably, Srx contributed to the stability of PrxI, PrxII, and PrxIII proteins in colon CSCs. Increased Srx expression promoted the stemness and survival of CSCs and was important for the maintenance of the mitochondrial OXPHOS system. Furthermore, Nrf2 and FoxM1 led to OXPHOS activation and upregulated expression of Srx-Prx redox system-related genes. Therefore, the Nrf2/FoxM1-induced Srx-Prx redox system is a potential therapeutic target for eliminating CSCs in colon cancer.

Prdx1 Interacts with ASK1 upon Exposure to H2O2 and Independently of a Scaffolding Protein

Hydrogen peroxide (H₂O₂) is a key redox signaling molecule that selectively oxidizes cysteines on proteins. It can accomplish this even in the presence of highly efficient and abundant H₂O₂ scavengers, peroxiredoxins (Prdxs), as it is the Prdxs themselves that transfer oxidative equivalents to specific protein thiols on target proteins via their redox-relay functionality. The first evidence of a mammalian cytosolic Prdx-mediated redox-relay—Prdx1 with the kinase ASK1—was presented a decade ago based on the outcome of a co-immunoprecipitation experiment. A second such redox-relay—Prdx2:STAT3—soon followed, for which further studies provided insights into its specificity, organization, and mechanism. The Prdx1:ASK1 redox-relay, however, has never undergone such a characterization. Here, we combine cellular and in vitro protein–protein interaction methods to investigate the Prdx1:ASK1 interaction more thoroughly. We show that, contrary to the Prdx2:STAT3 redox-relay, Prdx1 interacts with ASK1 at elevated H₂O₂ concentrations, and that this interaction can happen independently of a scaffolding protein. We also provide evidence of a Prdx2:ASK1 interaction, and demonstrate that it requires a facilitator that, however, is not annexin A2. Our results reveal that cytosolic Prdx redox-relays can be organized in different ways and yet again highlight the differentiated roles of Prdx1 and Prdx2.

Structural determinants of multimerization and dissociation in 2-Cys peroxiredoxin chaperone function

Peroxiredoxins (PRDXs) are abundant peroxidases present in all kingdoms of life. Recently, they have been shown to also carry out additional roles as molecular chaperones. To address this emerging supplementary function, this review focuses on structural studies of 2-Cys PRDX systems exhibiting chaperone activity. We provide a detailed understanding of the current knowledge of structural determinants underlying the chaperone function of PRDXs. Specifically, we describe the mechanisms which may modulate their quaternary structure to facilitate interactions with client proteins and how they are coordinated with the functions of other molecular chaperones. Following an overview of PRDX molecular architecture, we outline structural details of the presently best-characterized peroxiredoxins exhibiting chaperone function and highlight common denominators. Finally, we discuss the remarkable structural similarities between 2-Cys PRDXs, small HSPs, and J-domain-independent Hsp40 holdases in terms of their functions and dynamic equilibria between low- and high-molecular-weight oligomers.

The cytosolic tryparedoxin peroxidase from Trypanosoma cruzi induces a pro-inflammatory Th1 immune response in a peroxidatic cysteine-dependent manner

Trypanosoma cruzi cytosolic tryparedoxin peroxidase (c-TXNPx) is a 2-Cys peroxiredoxin (Prx) with an important role in detoxifying host cell oxidative molecules during parasite infection. c-TXNPx is a virulence factor, as its overexpression enhances parasite infectivity and resistance to exogenous oxidation. As Prxs from other organisms possess immunomodulatory properties, we studied the effects of c-TXNPx in the immune response and analysed whether the presence of the peroxidatic cysteine is necessary to mediate these properties. To this end, we used a recombinant c-TXNPx and a mutant version (c-TXNPxC52S) lacking the peroxidatic cysteine. We first analysed the oligomerization profile, oxidation state and peroxidase activity of both proteins by gel filtration, Western blot and enzymatic assay, respectively. To investigate their immunological properties, we analysed the phenotype and functional activity of macrophage and dendritic cells and the T-cell response by flow cytometry after injection into mice. Our results show that c-TXNPx, but not c-TXNPxC52S, induces the recruitment of IL-12/23p40-producing innate antigen-presenting cells and promotes a strong specific Th1 immune response. Finally, we studied the cellular and humoral immune response developed in the context of parasite natural infection and found that only wild-type c-TXNPx induces proliferation and high levels of IFN-γ secretion in PBMC from chronic patients without demonstrable cardiac manifestations. In conclusion, we demonstrate that c-TXNPx possesses pro-inflammatory properties that depend on the presence of peroxidatic cysteine that is essential for peroxidase activity and quaternary structure of the protein and could contribute to rational design of immune-based strategies against Chagas disease.

Pinpointing cysteine oxidation sites by high-resolution proteomics reveals a mechanism of redox-dependent inhibition of human STING

Protein function is regulated by posttranslational modifications (PTMs), among which reversible oxidation of cysteine residues has emerged as a key regulatory mechanism of cellular responses. Given the redox regulation of virus-host interactions, the identification of oxidized cysteine sites in cells is essential to understand the underlying mechanisms involved. Here, we present a proteome-wide identification of reversibly oxidized cysteine sites in oxidant-treated cells using a maleimide-based bioswitch method coupled to mass spectrometry analysis. We identified 2720 unique oxidized cysteine sites within 1473 proteins with distinct abundances, locations, and functions. Oxidized cysteine sites were found in numerous signaling pathways, many relevant to virus-host interactions. We focused on the oxidation of STING, the central adaptor of the innate immune type I interferon pathway, which is stimulated in response to the detection of cytosolic DNA by cGAS. We demonstrated the reversible oxidation of Cys¹⁴⁸ and Cys²⁰⁶ of STING in cells. Molecular analyses led us to establish a model in which Cys¹⁴⁸ oxidation is constitutive, whereas Cys²⁰⁶ oxidation is inducible by oxidative stress or by the natural ligand of STING, 2'3'-cGAMP. Our data suggest that the oxidation of Cys²⁰⁶ prevented hyperactivation of STING by causing a conformational change associated with the formation of inactive polymers containing intermolecular disulfide bonds. This finding should aid the design of therapies targeting STING that are relevant to autoinflammatory disorders, immunotherapies, and vaccines.

Mitochondrial hydrogen peroxide positively regulates neuropeptide secretion during diet-induced activation of the oxidative stress response

Mitochondria play a pivotal role in the generation of signals coupling metabolism with neurotransmitter release, but a role for mitochondrial-produced ROS in regulating neurosecretion has not been described. Here we show that endogenously produced hydrogen peroxide originating from axonal mitochondria (mtH2O2) functions as a signaling cue to selectively regulate the secretion of a FMRFamide-related neuropeptide (FLP-1) from a pair of interneurons (AIY) in C. elegans. We show that pharmacological or genetic manipulations that increase mtH2O2 levels lead to increased FLP-1 secretion that is dependent upon ROS dismutation, mitochondrial calcium influx, and cysteine sulfenylation of the calcium-independent PKC family member PKC-1. mtH2O2-induced FLP-1 secretion activates the oxidative stress response transcription factor SKN-1/Nrf2 in distal tissues and protects animals from ROS-mediated toxicity. mtH2O2 levels in AIY neurons, FLP-1 secretion and SKN-1 activity are rapidly and reversibly regulated by exposing animals to different bacterial food sources. These results reveal a previously unreported role for mtH2O2 in linking diet-induced changes in mitochondrial homeostasis with neuropeptide secretion.

Hyperoxidation of Peroxiredoxins and Effects on Physiology of Drosophila

The catalytic activity of peroxiredoxins (Prx) is determined by the conserved peroxidatic cysteine (Cys_P), which reacts with peroxides to form sulfenic acid (Cys-SOH). Under conditions of oxidative stress, Cys_P is oxidized to catalytically inactive sulfinic (Cys-SO₂) and sulfonic (Cys-SO₃) forms. The Cys-SO₂ form can be reduced in a reaction catalyzed by sulfiredoxin (Srx). To explore the physiological significance of peroxiredoxin overoxidation, we investigated daily variations in the oxidation state of 2-Cys peroxiredoxins in flies of different ages, or under conditions when the pro-oxidative load is high. We found no statistically significant changes in the 2-Cys Prxs monomer:dimer ratio, which indirectly reflects changes in the Prx catalytic activity. However, we found daily variations in Prx-SO_2/3 that were more pronounced in older flies as well as in flies lacking Srx. Unexpectedly, the srx mutant flies did not exhibit a diminished survivorship under normal or oxidative stress conditions. Moreover, the srx mutant was characterized by a higher physiological activity. In conclusion, catalytically inactive forms of Prx-SO_2/3 serve not only as a marker of cellular oxidative burden, but may also play a role in an adaptive response, leading to a positive effect on the physiology of Drosophila melanogaster.

Proteomics characterization of mitochondrial-derived vesicles under oxidative stress

Mitochondria share attributes of vesicular transport with their bacterial ancestors given their ability to form mitochondrial‐derived vesicles (MDVs). MDVs are involved in mitochondrial quality control and their formation is enhanced with stress and may, therefore, play a potential role in mitochondrial‐cellular communication. However, MDV proteomic cargo has remained mostly undefined. In this study, we strategically used an in vitro MDV budding/reconstitution assay on cardiac mitochondria, followed by graded oxidative stress, to identify and characterize the MDV proteome. Our results confirmed previously identified cardiac MDV markers, while also revealing a complete map of the MDV proteome, paving the way to a better understanding of the role of MDVs. The oxidative stress vulnerability of proteins directed the cargo loading of MDVs, which was enhanced by antimycin A (Ant‐A). Among OXPHOS complexes, complexes III and V were found to be Ant‐A‐sensitive. Proteins from metabolic pathways such as the TCA cycle and fatty acid metabolism, along with Fe‐S cluster, antioxidant response proteins, and autophagy were also found to be Ant‐A sensitive. Intriguingly, proteins containing hyper‐reactive cysteine residues, metabolic redox switches, including professional redox enzymes and those that mediate iron metabolism, were found to be components of MDV cargo with Ant‐A sensitivity. Last, we revealed a possible contribution of MDVs to the formation of extracellular vesicles, which may indicate mitochondrial stress. In conclusion, our study provides an MDV proteomics signature that delineates MDV cargo selectivity and hints at the potential for MDVs and their novel protein cargo to serve as vital biomarkers during mitochondrial stress and related pathologies.

Redox Regulation of PTEN by Peroxiredoxins

Phosphatase and tensin homolog deleted on chromosome 10 (PTEN) is known as a tumor suppressor gene that is frequently mutated in numerous human cancers and inherited syndromes. PTEN functions as a negative regulator of PI3K/Akt signaling pathway by dephosphorylating phosphatidylinositol (3, 4, 5)-trisphosphate (PIP3) to phosphatidylinositol (4, 5)-bisphosphate (PIP2), which leads to the inhibition of cell growth, proliferation, cell survival, and protein synthesis. PTEN contains a cysteine residue in the active site that can be oxidized by peroxides, forming an intramolecular disulfide bond between Cys¹²⁴ and Cys⁷¹. Redox regulation of PTEN by reactive oxygen species (ROS) plays a crucial role in cellular signaling. Peroxiredoxins (Prxs) are a superfamily of peroxidase that catalyzes reduction of peroxides and maintains redox homeostasis. Mammalian Prxs have 6 isoforms (I-VI) and can scavenge cellular peroxides. It has been demonstrated that Prx I can preserve and promote the tumor-suppressive function of PTEN by preventing oxidation of PTEN under benign oxidative stress via direct interaction. Also, Prx II-deficient cells increased PTEN oxidation and insulin sensitivity. Furthermore, Prx III has been shown to protect PTEN from oxidation induced by 15s-HpETE and 12s-HpETE, these are potent inflammatory and pro-oxidant mediators. Understanding the tight connection between PTEN and Prxs is important for providing novel therapies. Herein, we summarized recent studies focusing on the relationship of Prxs and the redox regulation of PTEN.

Erythrocytes as a preferential target of oxidative stress in blood

Red blood cells (RBC) are specifically differentiated to transport oxygen and carbon dioxide in the blood and they lack most organelles, including mitochondria. The autoxidation of hemoglobin constitutes a major source of reactive oxygen species (ROS). Nitric oxide, which is produced by endothelial nitric oxide synthase (NOS3) or via the hemoglobin-mediated conversion of nitrite, interacts with ROS and results in the production of reactive nitrogen oxide species. Herein we present an overview of anemic diseases that are closely related to oxidative damage. Because the compensation of proteins by means of gene expression does not proceed in enucleated cells, antioxidative and redox systems play more important roles in maintaining the homeostasis of RBC against oxidative insult compared to ordinary cells. Defects in hemoglobin and enzymes that are involved in energy production and redox reactions largely trigger oxidative damage to RBC. The results of studies using genetically modified mice suggest that antioxidative enzymes, notably superoxide dismutase 1 and peroxiredoxin 2, play essential roles in coping with oxidative damage in erythroid cells, and their absence limits erythropoiesis, the life-span of RBC and consequently results in the development of anemia. The degeneration of the machinery involved in the proteolytic removal of damaged proteins appears to be associated with hemolytic events. The ubiquitin-proteasome system is the dominant machinery, not only for the proteolytic removal of damaged proteins in erythroid cells but also for the development of erythropoiesis. Hence, despite the fact that it is less abundant in RBC compared to ordinary cells, the aberrant ubiquitin-proteasome system may be associated with the development of anemic diseases via the accumulation of damaged proteins, as typified in sickle cell disease, and impaired erythropoiesis.

Redox Pioneer: Professor Sue Goo Rhee

Dr. Sue Goo Rhee is recognized as a Redox Pioneer because he has published five articles in the field of antioxidants and redox signaling that have been cited >1000 times and 69 of his articles in this field have been cited between 100 and 1000 times. Dr. Rhee is known for his discovery of the first three prototypical members of the phospholipase C family, and for the discovery of the ubiquitously expressed peroxiredoxins. Peroxiredoxin catalyzes the thiol-mediated reduction of H₂O₂. These enzymes protect cellular molecules from oxidative damage. Importantly, they also regulate cell signaling by modulating the intracellular levels of H₂O₂ that are induced by signaling agonists. He elucidated the mechanism by which the peroxiredoxins participate in signaling by H₂O₂: Dr. Rhee demonstrated that growth agonists such as epidermal growth factor induce a transient elevation of intracellular H₂O₂ that oxidize the catalytically essential cysteine residue of protein tyrosine phosphatases. The oxidation inactivates the phosphatases, allowing enhanced protein tyrosine phosphorylation to mediate cell signaling. In addition, he established that peroxiredoxins are exquisitely regulated through phosphorylation, glutathionylation, and hyperoxidation of their active site cysteine to cysteine sulfinic acid. Dr. Rhee showed that cysteine oxidation to its sulfinic acid derivative is not irreversible as previously thought. The reduction of hyperoxidized peroxiredoxin is catalyzed by sulfiredoxin. His further investigations implicated cyclic hyperoxidation and reduction of peroxiredoxin in the regulation of certain circadian rhythms.

Molecular profiling and functional delineation of peroxiredoxin 3 (HaPrx3) from the big-belly seahorses (Hippocampus abdominalis) and understanding their immunological responses

Peroxiredoxins (Prxs) are ubiquitously expressed antioxidant proteins that can protect aerobic organisms from oxidative stress. Here, we characterized the HaPrx3 homolog at the molecular level from big-belly seahorse (Hippocampus abdominalis) and analyzed its functional activities. The coding sequence of HaPrx3 consists of 726 bp, which encodes 241 amino acids. The predicted molecular weight and theoretical isoelectric point (pI) of HaPrx3 was 26.20 kDa and 7.04, respectively. Multiple sequence alignments revealed that the arrangements of domains, catalytic triads, dimers, and decamer interfaces of HaPrx3 were conserved among Prx sequences of other organisms. According to the phylogenetic analysis, HaPrx3 is clustered with the teleost Prx3 subclade. The highest transcript level of HaPrx3 was detected in the ovary tissue among fourteen healthy fish tissues. The mRNA levels of HaPrx3 in blood and liver tissues were significantly (P < 0.05) upregulated in response to lipopolysaccharide (LPS), polyinosinic-polycytidylic (poly I:C), Edwardsiella tarda, and Streptococcus iniae, suggesting its involvement in immune responses. Under functional properties, insulin disulfide reduction assay confirmed the oxidoreductase activity of recombinant HaPrx3. A cell viability assay and Hoechst staining indicated cell survival ability and reduction of apoptotic activity, respectively. Moreover, a peroxidase activity assay verified peroxidase activity, while a metal-catalyzed oxidation (MCO) assay indicated the DNA protection ability of HaPrx3. Collectively, it is concluded that HaPrx3 may play a significant role in oxidative stress and immune responses against pathogenic infections in big-belly seahorses.

Peroxiredoxin 1 - Multifunctional antioxidant enzyme, protects from oxidative damages and increases the survival rate of mice exposed to total body irradiation

PURPOSE

Peroxiredoxin 1 (Prx1) is known to be a multifunctional antioxidant enzyme playing an essential role in protecting the organism against oxidative stress. We hypothesized that administration of exogenous recombinant Prx1 may provide additional protection of the mammalian organism during the development of acute oxidative stress induced by ionizing radiation. Hence, the aim of the present work was to study the radioprotective properties of exogenous Prx1.

MATERIALS AND METHODS

Recombinant Prx1 was obtained by genetic engineering. The properties of Prx1 were studied using physicochemical methods. An immunoblotting and ELISA were used for the determination of the level of endogenous and exogenous Prx1 in animal blood. The survival rate of irradiated animals was assessed for 30 days with various modes of administration (intraperitoneal, intramuscular, intravenously) Prx1. Using a hematological analyzer and microscopic analysis, the changes in the level of leukocytes and platelets were assessed in animals that received and did not receive an intravenous injection of Prx1 before irradiation. Genoprotective properties of Prx1 were confirmed by micronucleus test. Real-time PCR was used to investigate the effect of Prx1 on the expression of genes involved in response to oxidative stress.

RESULTS

Recombinant Prx1 was shown to significantly reduce oxidative damage to biological macromolecules. Prx1 is an effective radioprotector which decreases the severity of radiation-induced leuko- and thrombocytopenia, plus protects bone marrow cells from damage. The half-life of Prx1 in the bloodstream is more than 1 h, while within 1 h there is a loss of the antioxidant activity of Prx1 by almost 50%, which limits its use long (2 h) before irradiation. The introduction of Prx1 after irradiation has no significant radiomitigating effect. The most effective way of using Prx1 is intravenous administration shortly (15-30 min) before exposure to ionizing radiation, with a dose reduction factor of 1.3. Under the action of ionizing radiation a dose-dependent appearance of endogenous Prx1 in the bloodstream was also observed. The appearance of Prx1 in the bloodstream alters the expression of stress response genes (especial antioxidant response and DNA repair) in the cells of red bone marrow, promoting the activation of repair processes.

CONCLUSION

The recombinant Prx1 can be considered as an effective radioprotector for minimizing the risks of injury of animal's body by ionizing radiation.

Thiol-based switching mechanisms of stress-sensing chaperones

Thiol-based redox switches evolved as efficient post-translational regulatory mechanisms that enable individual proteins to rapidly respond to sudden environmental changes. While some protein functions need to be switched off to save resources and avoid potentially error-prone processes, protective functions become essential and need to be switched on. In this review, we focus on thiol-based activation mechanisms of stress-sensing chaperones. Upon stress exposure, these chaperones convert into high affinity binding platforms for unfolding proteins and protect cells against the accumulation of potentially toxic protein aggregates. Their chaperone activity is independent of ATP, a feature that becomes especially important under oxidative stress conditions, where cellular ATP levels drop and canonical ATP-dependent chaperones no longer operate. Vice versa, reductive inactivation and substrate release require the restoration of ATP levels, which ensures refolding of client proteins by ATP-dependent foldases. We will give an overview over the different strategies that cells evolved to rapidly increase the pool of ATP-independent chaperones upon oxidative stress and provide mechanistic insights into how stress conditions are used to convert abundant cellular proteins into ATP-independent holding chaperones.

Multiple functions of 2-Cys peroxiredoxins, I and II, and their regulations via post-translational modifications

Peroxiredoxins (Prxs) are an unusual family of thiol-specific peroxidases that possess a binding site for H₂O₂ and rely on a conserved cysteine residue for rapid reaction with H₂O₂. Among 6 mammalian isoforms (Prx I to VI), Prx I and Prx II are mainly found in the cytosol and nucleus. Prx I and Prx II function as antioxidant enzymes and protein chaperone under oxidative distress conditions. Under oxidative eustress conditions, Prx I and Prx II regulate the levels of H₂O₂ at specific area of the cells as well as sense and transduce H₂O₂ signaling to target proteins. Prx I and Prx II are known to be covalently modified on multiple sites: Prx I is hyperoxidized on Cys⁵²; phosphorylated on Ser³², Thr⁹⁰, and Tyr¹⁹⁴; acetylated on Lys⁷, Lys¹⁶, Lys²⁷, Lys³⁵, and Lys¹⁹⁷; glutathionylated on Cys⁵², Cys⁸³, and Cys¹⁷³; and nitrosylated on Cys⁵² and Cys⁸³, whereas Prx II is hyperoxidized on Cys⁵¹; phosphorylated on Thr⁸⁹, Ser¹¹², and Thr¹⁸²; acetylated on Ala² and Lys¹⁹⁶; glutathionylated on Cys⁵¹ and Cys¹⁷²; and nitrosylated on Cys⁵¹ and Cys¹⁷². In this review, we describe how these post-translational modifications affect various functions of Prx I and Prx II.

Redox regulation of tumor suppressor PTEN in cell signaling

Phosphatase and tensin homologs deleted on chromosome 10 (PTEN) is a potent tumor suppressor and often dysregulated in cancers. Cellular PTEN activity is restrained by the oxidation of active-site cysteine by reactive oxygen species (ROS). Recovery of its enzymatic activity predominantly depends on the availability of cellular thioredoxin (Trx) and peroxiredoxins (Prx), both are important players in cell signaling. Trx and Prx undergo redox-dependent conformational changes through the oxidation of cysteine residues at their active sites. Their dynamics are essential for protein functionality and regulation. In this review, we summarized the recent advances regarding the redox regulation of PTEN, with a specific focus on our current state-of-the-art understanding of the redox regulation of PTEN. We also proposed a tight association of the redox regulation of PTEN with Trx dimerization and Prx hyperoxidation, providing guidance for the identification of novel therapeutic targets.

Absence of Cytosolic 2-Cys Prx Subtypes I and II Exacerbates TNF-α-Induced Apoptosis via Different Routes

There are abundant peroxiredoxin (Prx) enzymes, but an increase of cellular H₂O₂ level always happens in apoptotic cells. Here, we show that cellular H₂O₂ switches different apoptosis pathways depending on which type of Prx enzyme is absent. TNF-α-induced H₂O₂ burst preferentially activates the DNA damage-dependent apoptosis pathway in the absence of PrxI. By contrast, the same H₂O₂ burst stimulates the RIPK1-dependent apoptosis pathway in the absence of PrxII by inducing the destruction of cIAP1 in caveolar membrane. Specifically, H₂O₂ induces the oxidation of Cys308 residue in the cIAP1-BIR3 domain, which induces the dimerization-dependent E3 ligase activation. Thus, the reduction in cIAP level by the absence of PrxII triggers cell-autonomous apoptosis in cancer cells and tumors. Such differential functions of PrxI and PrxII are mediated by interaction with H2AX and cIAP1, respectively. Collectively, this study reveals the distinct switch roles of 2-Cys Prx isoforms in apoptosis signaling.

Dual Character of Reactive Oxygen, Nitrogen, and Halogen Species: Endogenous Sources, Interconversions and Neutralization

Oxidative stress resulting from accumulation of reactive oxygen, nitrogen, and halogen species (ROS, RNS, and RHS, respectively) causes the damage of cells and biomolecules. However, over the long evolutionary time, living organisms have developed the mechanisms for adaptation to oxidative stress conditions including the activity of the antioxidant system (AOS), which maintains low intracellular levels of RONS (ROS and RNS) and RHS. Moreover, living organisms have adapted to use low concentrations of these electrophiles for the regulation of cell functions through the reversible post-translational chemical modifications of redox-sensitive amino acid residues in intracellular effectors of signal transduction pathways (protein kinases and protein phosphatases), transcription factors, etc. An important fine-tuning mechanism that ensures involvement of RONS and RHS in the regulation of physiological processes is interconversion between different reactive species. This review focuses on the complex networks of interacting RONS and RHS types and their endogenous sources, such as NOX family of NADPH oxidases, complexes I and III of the mitochondrial electron transport chain, NO synthases, cytochrome P450-containing monooxygenase system, xanthine oxidoreductase, and myeloperoxidases. We highlight that kinetic parameters of reactions involving RONS and RHS determine the effects of these reactive species on cell functions. We also describe the functioning of enzymatic and non-enzymatic AOS components and the mechanisms of RONS and RHS scavenging under physiological conditions. We believe that analysis of interactions between RONS and relationships between different endogenous sources of these compounds will contribute to better understanding of their role in the maintenance of cell redox homeostasis as well as initiation and progression of diseases.

Multiscale Modeling of Thiol Overoxidation in Peroxiredoxins by Hydrogen Peroxide

In this work, we employ a multiscale quantum-classical mechanics (QM/MM) scheme to investigate the chemical reactivity of sulfenic acids toward hydrogen peroxide, both in aqueous solution and in the protein environment of the peroxiredoxin alkyl hydroperoxide reductase E from Mycobacterium tuberculosis (MtAhpE). The reaction of oxidation of cysteine with hydrogen peroxides, catalyzed by peroxiredoxins, is usually accelerated several orders of magnitude in comparison with the analogous reaction in solution. The resulting cysteine sulfenic acid is then reduced in other steps of the catalytic cycle, recovering the original thiol. However, under some conditions, the sulfenic acid can react with another equivalent of oxidant to form a sulfinic acid. This process is called overoxidation and has been associated with redox signaling. Herein, we employed a multiscale scheme based on density function theory calculations coupled to the classical AMBER force field, developed in our group, to establish the molecular basis of thiol overoxidation by hydrogen peroxide. Our results suggest that residues that play key catalytic roles in the oxidation of MtAhpE are not relevant in the overoxidation process. Indeed, the calculations propose that the process is unfavored by this particular enzyme microenvironment.

Enhanced hyperoxidation of peroxiredoxin 2 and peroxiredoxin 3 in the presence of bicarbonate/CO2

Hydrogen peroxide undergoes an equilibrium reaction with bicarbonate/CO₂ to produce peroxymonocarbonate (HCO₄^-). Peroxymonocarbonate is more reactive with thiols than H₂O₂ but it makes up only a small fraction of the H₂O₂ in physiological bicarbonate buffers so the increase in rate of oxidation of low molecular weight thiols is modest. However, for some thiol proteins such as protein tyrosine phosphatases, the rate enhancement is very much greater. We have investigated the effect of bicarbonate/CO₂ on the oxidation of peroxiredoxins (Prdxs) 2 and 3. Using an assay in which reduced Prdx2 inhibits oxidation of horseradish peroxidase by H₂O₂, we saw no difference between phosphate and bicarbonate buffers (pH 7.4). However, hyperoxidation of both Prdxs in bicarbonate was considerably enhanced. Hyperoxidation involves the reaction of the sulfenic acid formed at the active site with a second H₂O₂, and prevents its condensation to a disulfide. Using LC/MS analysis, we determined that the presence of 25 mM bicarbonate/CO₂ increased the ratio of hyperoxidation compared with condensation 6-fold for Prdx2 and 11-fold for Prdx3. These results imply that Prdx hyperoxidation will occur more readily under physiological conditions than appreciated from in vitro experiments, which seldom use bicarbonate buffers. They also raise the possibility that variations in bicarbonate concentration could provide a mechanism for regulating the cellular level of active Prdxs.

Peroxiredoxins and Beyond; Redox Systems Regulating Lung Physiology and Disease

Significance: The lung is a unique organ, as it is constantly exposed to air, and thus it requires a robust antioxidant defense system to prevent the potential damage from exposure to an array of environmental insults, including oxidants. The peroxiredoxin (PRDX) family plays an important role in scavenging peroxides and is critical to the cellular antioxidant defense system. Recent Advances: Exciting discoveries have been made to highlight the key features of PRDXs that regulate the redox tone. PRDXs do not act in isolation as they require the thioredoxin/thioredoxin reductase/NADPH, sulfiredoxin (SRXN1) redox system, and in some cases glutaredoxin/glutathione, for their reduction. Furthermore, the chaperone function of PRDXs, controlled by the oxidation state, demonstrates the versatility in redox regulation and control of cellular biology exerted by this class of proteins. Critical Issues: Despite the long-known observations that redox perturbations accompany a number of pulmonary diseases, surprisingly little is known about the role of PRDXs in the etiology of these diseases. In this perspective, we review the studies that have been conducted thus far to address the roles of PRDXs in lung disease, or experimental models used to study these diseases. Intriguing findings, such as the secretion of PRDXs and the formation of autoantibodies, raise a number of questions about the pathways that regulate secretion, redox status, and immune response to PRDXs. Future Directions: Further understanding of the mechanisms by which individual PRDXs control lung inflammation, injury, repair, chronic remodeling, and cancer, and the importance of PRDX oxidation state, configuration, and client proteins that govern these processes is needed.

Candida glabrata peroxiredoxins, Tsa1 and Tsa2, and sulfiredoxin, Srx1, protect against oxidative damage and are necessary for virulence

Candida glabrata is an opportunistic fungal pathogen that can cause life-threatening infections in immunocompromised patients. To ensure a successful infection, C. glabrata has evolved a variety of strategies to avoid killing within the host. One of these strategies is the resistance to oxidative stress. Here we show that the sulfiredoxin Srx1 and the peroxiredoxins, Tsa1 and Tsa2, are implicated in the oxidative stress response (OSR) and required for virulence. We analyzed null mutations in SRX1, TSA1 and TSA2 and showed that TSA2 and SRX1 are required to respond to oxidative stress. While TSA1 expression is constitutive, SRX1 and TSA2 are induced in the presence of H₂O₂ in a process dependent on H₂O₂ concentration and on both transcription factors Yap1 and Skn7. Msn2 and Msn4 are not necessary for the regulation of SRX1, TSA1 and TSA2. Interestingly, TSA1 and TSA2, which are localized in the cytoplasm, are induced in the presence of neutrophils and required for survival in these phagocytic cells.

A catalytic career: Studies spanning glutamine synthetase, phospholipase C, peroxiredoxin, and the intracellular messenger role of hydrogen peroxide

I learned biochemistry from P. Boon Chock and Earl Stadtman while working on the regulation of Escherichia coli glutamine synthetase as a postdoctoral fellow at the National Institutes of Health. After becoming a tenured scientist at the same institute, my group discovered, purified, and cloned the first three prototypical members of the phospholipase C family and uncovered the mechanisms by which various cell-surface receptors activate these enzymes to generate diacylglycerol and inositol 1,4,5-trisphosphate. We also discovered the family of peroxiredoxin (Prx) enzymes that catalyze the reduction of H₂O₂, and we established that mammalian cells express six Prx isoforms that not only protect against oxidative damage but also mediate cell signaling by modulating intracellular H₂O₂ levels. To validate the signaling role of H₂O₂, we showed that epidermal growth factor induces a transient increase in intracellular H₂O₂ levels, and the essential cysteine residue of protein-tyrosine phosphatases is a target for specific and reversible oxidation by the H₂O₂ produced in such cells. These observations led to a new paradigm in receptor signaling, in which protein tyrosine phosphorylation is achieved not via activation of receptor tyrosine kinases alone but also through concurrent inhibition of protein-tyrosine phosphatases by H₂O₂ Our studies revealed that Prx isozymes are extensively regulated via phosphorylation as well as by hyperoxidation of the active-site cysteine to cysteine sulfinic acid, with the reverse reaction being catalyzed by sulfiredoxin. This reversible hyperoxidation of Prx was further shown to constitute a universal marker for circadian rhythms in all domains of life.

Contribution of Oxidative Stress and Impaired Biogenesis of Pancreatic β-Cells to Type 2 Diabetes

Significance: Type 2 diabetes development involves multiple changes in β-cells, related to the oxidative stress and impaired redox signaling, beginning frequently by sustained overfeeding due to the resulting lipotoxicity and glucotoxicity. Uncovering relationships among the dysregulated metabolism, impaired β-cell "well-being," biogenesis, or cross talk with peripheral insulin resistance is required for elucidation of type 2 diabetes etiology. Recent Advances: It has been recognized that the oxidative stress, lipotoxicity, and glucotoxicity cannot be separated from numerous other cell pathology events, such as the attempted compensation of β-cell for the increased insulin demand and dynamics of β-cell biogenesis and its "reversal" at dedifferentiation, that is, from the concomitantly decreasing islet β-cell mass (also due to transdifferentiation) and low-grade islet or systemic inflammation. Critical Issues: At prediabetes, the compensation responses of β-cells, attempting to delay the pathology progression-when exaggerated-set a new state, in which a self-checking redox signaling related to the expression of Ins gene expression is impaired. The resulting altered redox signaling, diminished insulin secretion responses to various secretagogues including glucose, may lead to excretion of cytokines or chemokines by β-cells or excretion of endosomes. They could substantiate putative stress signals to the periphery. Subsequent changes and lasting glucolipotoxicity promote islet inflammatory responses and further pathology spiral. Future Directions: Should bring an understanding of the β-cell self-checking and related redox signaling, including the putative stress signal to periphery. Strategies to cure or prevent type 2 diabetes could be based on the substitution of the "wrong" signal by the "correct" self-checking signal.

Peroxiredoxin 1 post-transcriptionally regulates snoRNA expression

Peroxiredoxin 1 (Prx1) is a member of the Prx family that detoxifies various peroxide substrates through conserved catalytic cysteine residues with the use of reducing equivalents. In addition to this well-known role of Prx1, we have previously demonstrated that Prx1 also has RNA-binding properties, but its function as an RNA-binding protein (RBP) remains unknown. To characterize the role of Prx1 as an RBP, we pulled down Prx1-RNA complexes and sequenced the target RNAs of Prx1. Through sequencing and further validation studies, we revealed that Prx1 binds to a specific subset of small nucleolar RNAs (snoRNAs) and regulates these molecules at the post-transcriptional level. We also found that active cysteine residues provide a structural and functional link between these two distinct functions of Prx1 (i.e., ROS scavenging and RNA-binding activities). Prx1 functions as a snoRNA-binding protein in its reduced state, and post-transcriptionally regulates the expression of a set of snoRNAs. However, when the active cysteine residues are oxidized, Prx1 loses its activity as a snoRNA-binding protein. This study is the first report describing the novel role of Prx1 as a post-transcriptional regulator of snoRNAs.

A key metabolic integrator, coenzyme A, modulates the activity of peroxiredoxin 5 via covalent modification

Peroxiredoxins (Prdxs) are antioxidant enzymes that catalyse the breakdown of peroxides and regulate redox activity in the cell. Peroxiredoxin 5 (Prdx5) is a unique member of Prdxs, which displays a wider subcellular distribution and substrate specificity and exhibits a different catalytic mechanism when compared to other members of the family. Here, the role of a key metabolic integrator coenzyme A (CoA) in modulating the activity of Prdx5 was investigated. We report for the first time a novel mode of Prdx5 regulation mediated via covalent and reversible attachment of CoA (CoAlation) in cellular response to oxidative and metabolic stress. The site of CoAlation in endogenous Prdx5 was mapped by mass spectrometry to peroxidatic cysteine 48. By employing an in vitro CoAlation assay, we showed that Prdx5 peroxidase activity is inhibited by covalent interaction with CoA in a dithiothreitol-sensitive manner. Collectively, these results reveal that human Prdx5 is a substrate for CoAlation in vitro and in vivo, and provide new insight into metabolic control of redox status in mammalian cells.

The bicarbonate/carbon dioxide pair increases hydrogen peroxide-mediated hyperoxidation of human peroxiredoxin 1

2-Cys peroxiredoxins (Prxs) rapidly reduce H₂O₂, thereby acting as antioxidants and also as sensors and transmitters of H₂O₂ signals in cells. Interestingly, eukaryotic 2-Cys Prxs lose their peroxidase activity at high H₂O₂ levels. Under these conditions, H₂O₂ oxidizes the sulfenic acid derivative of the Prx peroxidatic Cys (C_PSOH) to the sulfinate (C_PSO₂ ^-) and sulfonated (C_PSO₃ ^-) forms, redirecting the C_PSOH intermediate from the catalytic cycle to the hyperoxidation/inactivation pathway. The susceptibility of 2-Cys Prxs to hyperoxidation varies greatly and depends on structural features that affect the lifetime of the C_PSOH intermediate. Among the human Prxs, Prx1 has an intermediate susceptibility to H₂O₂ and was selected here to investigate the effect of a physiological concentration of HCO₃ ^-/CO₂ (25 mm) on its hyperoxidation. Immunoblotting and kinetic and MS/MS experiments revealed that HCO₃ ^-/CO₂ increases Prx1 hyperoxidation and inactivation both in the presence of excess H₂O₂ and during enzymatic (NADPH/thioredoxin reductase/thioredoxin) and chemical (DTT) turnover. We hypothesized that the stimulating effect of HCO₃ ^-/CO₂ was due to HCO₄ ^-, a peroxide present in equilibrated solutions of H₂O₂ and HCO₃ ^-/CO₂ Indeed, additional experiments and calculations uncovered that HCO₄ ^- oxidizes C_PSOH to C_PSO₂ ^- with a second-order rate constant 2 orders of magnitude higher than that of H₂O₂ ((1.5 ± 0.1) × 10⁵ and (2.9 ± 0.2) × 10³ m^-1·s^-1, respectively) and that HCO₄ ^- is 250 times more efficient than H₂O₂ at inactivating 1% Prx1 per turnover. The fact that the biologically ubiquitous HCO₃ ^-/CO₂ pair stimulates Prx1 hyperoxidation and inactivation bears relevance to Prx1 functions beyond its antioxidant activity.