Peroxiredoxins as Preferential Targets in H2O2-Induced Signaling

Reactive oxygen species and cell signaling. Review

Reactive oxygen species (ROS) is a term encompassing a group of highly reactive oxygen-derived molecules. In physiological systems, ROS production exists in concert with antioxidant defenses, which safeguard cells against higher, toxic levels of ROS. Oxidative stress, coined as "oxidative distress", is "a serious imbalance between the generation of ROS and antioxidant defenses in favor of ROS, causing excessive oxidative damage to biomolecules". At physiological levels, ROS are essential for many cellular processes, which is known as "oxidative eustress". Oxidants like hydrogen peroxide (H2O2) activate signaling pathways like mitogen-activated protein kinases (MAPK)s and phosphoinositide 3-kinase (PI3K)/protein kinase B (Akt). ROS activate transcription factors like nuclear factor erythroid 2-related factor 2 (Nrf2), hypoxia-inducible factor 1α (HIF-1α), activator protein 1 (AP-1), and nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB). Redox signaling through H2O2 mainly occurs through reversible oxidation of protein cysteine thiolate residues (RS-) to form sulfenic acids (RSOH). An unresolved question is that the reaction rate of H2O2 with protein thiols is very low. In cells, the reaction of H2O2 with protein thiols is likely to be outcompeted by faster reactions of H2O2 with peroxiredoxins and glutathione peroxidases. A novel mechanism being explored is that H2O2 could react with peroxiredoxins that act as reactive redox sensor proteins, leading to peroxiredoxin-mediated relays. Very few redox signaling pathways have been well characterized. Improved understanding of precise mechanisms by which ROS regulate signaling pathways and the role of cellular sensors, is essential for deciphering their roles in physiological and pathological conditions.

Studying the Human Microbiota: Advances in Understanding the Fundamentals, Origin, and Evolution of Biological Timekeeping

The recently observed circadian oscillations of the intestinal microbiota underscore the profound nature of the human-microbiome relationship and its importance for health. Together with the discovery of circadian clocks in non-photosynthetic gut bacteria and circadian rhythms in anucleated cells, these findings have indicated the possibility that virtually all microorganisms may possess functional biological clocks. However, they have also raised many essential questions concerning the fundamentals of biological timekeeping, its evolution, and its origin. This narrative review provides a comprehensive overview of the recent literature in molecular chronobiology, aiming to bring together the latest evidence on the structure and mechanisms driving microbial biological clocks while pointing to potential applications of this knowledge in medicine. Moreover, it discusses the latest hypotheses regarding the evolution of timing mechanisms and describes the functions of peroxiredoxins in cells and their contribution to the cellular clockwork. The diversity of biological clocks among various human-associated microorganisms and the role of transcriptional and post-translational timekeeping mechanisms are also addressed. Finally, recent evidence on metabolic oscillators and host-microbiome communication is presented.

Oxidative Stress in Healthy and Pathological Red Blood Cells

Red cell diseases encompass a group of inherited or acquired erythrocyte disorders that affect the structure, function, or production of red blood cells (RBCs). These disorders can lead to various clinical manifestations, including anemia, hemolysis, inflammation, and impaired oxygen-carrying capacity. Oxidative stress, characterized by an imbalance between the production of reactive oxygen species (ROS) and the antioxidant defense mechanisms, plays a significant role in the pathophysiology of red cell diseases. In this review, we discuss the most relevant oxidant species involved in RBC damage, the enzymatic and low molecular weight antioxidant systems that protect RBCs against oxidative injury, and finally, the role of oxidative stress in different red cell diseases, including sickle cell disease, glucose 6-phosphate dehydrogenase deficiency, and pyruvate kinase deficiency, highlighting the underlying mechanisms leading to pathological RBC phenotypes.

Biophysical tools to study the oligomerization dynamics of Prx1-class peroxiredoxins

Peroxiredoxins (Prx) are ubiquitous, highly conserved peroxidases whose activity depends on catalytic cysteine residues. The Prx1-class of the peroxiredoxin family, also called typical 2-Cys Prx, organize as head-to-tail homodimers containing two active sites. The peroxidatic cysteine CP of one monomer reacts with the peroxide substrate to form sulfenic acid that reacts with the resolving cysteine (CR) of the adjacent subunit to form an intermolecular disulfide, that is reduced back by the thioredoxin/thioredoxin reductase/NADPH system. Although the minimal catalytic unit is the dimer, these Prx oligomerize into (do)decamers. In addition, these ring-shaped decamers can pile-up into high molecular weight structures. Prx not only display peroxidase activity reducing H2O2, peroxynitrous acid and lipid hydroperoxides (antioxidant enzymes), but also exhibit holdase activity protecting other proteins from unfolding (molecular chaperones). Highly relevant is their participation in redox cellular signaling that is currently under active investigation. The different activities attributed to Prx are strongly ligated to their quaternary structure. In this review, we will describe different biophysical approaches used to characterize the oligomerization dynamics of Prx that include the classical size-exclusion chromatography, analytical ultracentrifugation, calorimetry, and also fluorescence anisotropy and lifetime measurements, as well as mass photometry.

Temporal Coordination of the Transcription Factor Response to H2O2 stress

The p53 and FOXO transcription factors (TFs) share many similarities despite their distinct evolutionary origins. Both TFs are activated by a variety of cellular stresses and upregulate genes in similar pathways including cell-cycle arrest and apoptosis. Oxidative stress from excess H2O2 activates both FOXO1 and p53, yet whether they are activated at the same time is unclear. Here we found that cells respond to high H2O2 levels in two temporal phases. In the first phase FOXO1 rapidly shuttles to the nucleus while p53 levels remain low. In the second phase FOXO1 exits the nucleus and p53 levels rise. We found that other oxidative stress induced TFs are activated in the first phase with FOXO1 (NF-κB, NFAT1), or the second phase with p53 (NRF2, JUN) but not both following H2O2 stress. The two TF phases result in large differences in gene expression patterns. Finally, we provide evidence that 2-Cys peroxiredoxins control the timing of the TF phases in response to H2O2.

Distinct mechanisms underlie H2O2 sensing in C. elegans head and tail

Environmental oxidative stress threatens cellular integrity and should therefore be avoided by living organisms. Yet, relatively little is known about environmental oxidative stress perception. Here, using microfluidics, we showed that like I2 pharyngeal neurons, the tail phasmid PHA neurons function as oxidative stress sensing neurons in C. elegans, but display different responses to H₂O₂ and light. We uncovered that different but related receptors, GUR-3 and LITE-1, mediate H₂O₂ signaling in I2 and PHA neurons. Still, the peroxiredoxin PRDX-2 is essential for both, and might promote H₂O₂-mediated receptor activation. Our work demonstrates that C. elegans can sense a broad range of oxidative stressors using partially distinct H₂O₂ signaling pathways in head and tail sensillae, and paves the way for further understanding of how the integration of these inputs translates into the appropriate behavior.

Fluorescence Lifetime Phasor Analysis of the Decamer-Dimer Equilibrium of Human Peroxiredoxin 1

Protein self-assembly is a common feature in biology and is often required for a myriad of fundamental processes, such as enzyme activity, signal transduction, and transport of solutes across membranes, among others. There are several techniques to find and assess homo-oligomer formation in proteins. Naturally, all these methods have their limitations, meaning that at least two or more different approaches are needed to characterize a case study. Herein, we present a new method to study protein associations using intrinsic fluorescence lifetime with phasors. In this case, the method is applied to determine the equilibrium dissociation constant (KD) of human peroxiredoxin 1 (hPrx1), an efficient cysteine-dependent peroxidase, that has a quaternary structure comprised of five head-to-tail homodimers non-covalently arranged in a decamer. The hPrx1 oligomeric state not only affects its activity but also its association with other proteins. The excited state lifetime of hPrx1 has distinct values at high and low concentrations, suggesting the presence of two different species. Phasor analysis of hPrx1 emission lifetime allowed for the identification and quantification of hPrx1 decamers, dimers, and their mixture at diverse protein concentrations. Using phasor algebra, we calculated the fraction of hPrx1 decamers at different concentrations and obtained KD (1.1 × 10-24 M4) and C0.5 (1.36 μM) values for the decamer-dimer equilibrium. The results were validated and compared with size exclusion chromatography. In addition, spectral phasors provided similar results despite the small differences in emission spectra as a function of hPrx1 concentration. The phasor approach was shown to be a highly sensitive and quantitative method to assess protein oligomerization and an attractive addition to the biophysicist's toolkit.

Bioinformatic Analyses of Peroxiredoxins and RF-Prx: A Random Forest-Based Predictor and Classifier for Prxs

Peroxiredoxins (Prxs) are a protein superfamily, present in all organisms, that play a critical role in protecting cellular macromolecules from oxidative damage but also regulate intracellular and intercellular signaling processes involving redox-regulated proteins and pathways. Bioinformatic approaches using computational tools that focus on active site-proximal sequence fragments (known as active site signatures) and iterative clustering and searching methods (referred to as TuLIP and MISST) have recently enabled the recognition of over 38,000 peroxiredoxins, as well as their classification into six functionally relevant groups. With these data providing so many examples of Prxs in each class, machine learning approaches offer an opportunity to extract additional information about features characteristic of these protein groups.In this study, we developed a novel computational method named "RF-Prx" based on a random forest (RF) approach integrated with K-space amino acid pairs (KSAAP) to identify peroxiredoxins and classify them into one of six subgroups. Our process performed in a superior manner compared to other machine learning classifiers. Thus the RF approach integrated with K-space amino acid pairs enabled the detection of class-specific conserved sequences outside the known functional centers and with potential importance. For example, drugs designed to target Prx proteins would likely suffer from cross-reactivity among distinct Prxs if targeted to conserved active sites, but this may be avoidable if remote, class-specific regions could be targeted instead.

Screening compound libraries for H2O2-mediated cancer therapeutics using a peroxiredoxin-based sensor

Compounds that modulate H₂O₂ reaction networks have applications as targeted cancer therapeutics, as a subset of cancers exhibit sensitivity to this redox signal. Previous studies to identify therapeutics that induce oxidants have relied upon probes that respond to many different oxidants in cells, and thus do not report on only H₂O₂, a redox signal that selectively oxidizes proteins. Here we use a genetically encoded fluorescent probe for human peroxiredoxin-2 (Prx2) oxidation in screens for small-molecule compounds that modulate H₂O₂ pathways. We further characterize cellular responses to several compounds selected from the screen. Our results reveal that some, but not all, of the compounds enact H₂O₂-mediated toxicity in cells. Among them, SMER3, an antifungal, has not been reported as an oxidant-inducing drug. Several drugs, including cisplatin, that previously have been shown to induce reactive oxygen species (ROS) do not appear to oxidize Prx2, suggesting H₂O₂ is not among the ROS induced by those drugs.

Non-thermal plasma-induced DMPO-OH yields hydrogen peroxide

Recent developments in electronics have enabled the medical applications of non-thermal plasma (NTP), which elicits reactive oxygen species (ROS) and reactive nitrogen species (RNS), such as hydroxyl radical (●OH), hydrogen peroxide (H2O2), singlet oxygen (1O2), superoxide (O2●-), ozone, and nitric oxide at near-physiological temperatures. In preclinical studies or human clinical trials, NTP promotes blood coagulation, eradication of bacterial, viral and biofilm-related infections, wound healing, and cancer cell death. To elucidate the solution-phase biological effects of NTP in the presence of biocompatible reducing agents, we employed electron paramagnetic resonance (EPR) spectroscopy to quantify ●OH using a spin-trapping probe, 5,5-dimethyl-1-pyrroline-N-oxide (DMPO); 1O2 using a fluorescent probe; and O2●- and H2O2 using luminescent probes in the presence of thiols or tempol. NTP-induced ●OH was significantly scavenged by dithiothreitol (DTT), reduced glutathione (GSH), and oxidized glutathione (GSSG) in 2 or 5 mM DMPO. NTP-induced O2●- was significantly scavenged by 10 μM DTT and GSH, while 1O2 was not efficiently scavenged by these compounds. GSSG degraded H2O2 more effectively than GSH and DTT, suggesting that the disulfide bonds reacted with H2O2. In the presence of 1-50 mM DMPO, NTP-induced H2O2 quantities were unchanged. The inhibitory effect of tempol concentration (50 and 100 μM) on H2O2 production was observed in 1 and 10 mM DMPO, whereas it became ineffective in 50 mM DMPO. Furthermore, DMPO-OH did not interact with tempol. These results suggest that DMPO and tempol react competitively with O2●-. Further studies are warranted to elucidate the interaction between NTP-induced ROS and biomolecules.

A guide to genetically encoded tools for the study of H2 O2

Cell metabolism heavily relies on the redox reactions that inevitably generate reactive oxygen species (ROS). It is now well established that ROS fluctuations near basal levels coordinate numerous physiological processes in living organisms, thus exhibiting regulatory functions. Hydrogen peroxide, the most long-lived ROS, is a key contributor to ROS-dependent signal transduction in the cell. H₂ O₂ is known to impact various targets in the cell; therefore, the question of how H₂ O₂ modulates physiological processes in a highly specific manner is central in redox biology. To resolve this question, novel genetic tools have recently been created for detecting H₂ O₂ and emulating its generation in living organisms with unmatched spatiotemporal resolution. Here, we review H₂ O₂ -sensitive genetically encoded fluorescent sensors and opto- and chemogenetic tools for controlled H₂ O₂ generation.

Peroxiredoxins-The Underrated Actors during Virus-Induced Oxidative Stress

Enhanced production of reactive oxygen species (ROS) triggered by various stimuli, including viral infections, has attributed much attention in the past years. It has been shown that different viruses that cause acute or chronic diseases induce oxidative stress in infected cells and dysregulate antioxidant its antioxidant capacity. However, most studies focused on catalase and superoxide dismutases, whereas a family of peroxiredoxins (Prdx), the most effective peroxide scavengers, were given little or no attention. In the current review, we demonstrate that peroxiredoxins scavenge hydrogen and organic peroxides at their physiological concentrations at various cell compartments, unlike many other antioxidant enzymes, and discuss their recycling. We also provide data on the regulation of their expression by various transcription factors, as they can be compared with the imprint of viruses on transcriptional machinery. Next, we discuss the involvement of peroxiredoxins in transferring signals from ROS on specific proteins by promoting the oxidation of target cysteine groups, as well as briefly demonstrate evidence of nonenzymatic, chaperone, functions of Prdx. Finally, we give an account of the current state of research of peroxiredoxins for various viruses. These data clearly show that Prdx have not been given proper attention despite all the achievements in general redox biology.

Genetically increasing flux through β-oxidation in skeletal muscle increases mitochondrial reductive stress and glucose intolerance

Elevated mitochondrial hydrogen peroxide (H₂O₂) emission and an oxidative shift in cytosolic redox environment have been linked to high-fat-diet-induced insulin resistance in skeletal muscle. To test specifically whether increased flux through mitochondrial fatty acid oxidation, in the absence of elevated energy demand, directly alters mitochondrial function and redox state in muscle, two genetic models characterized by increased muscle β-oxidation flux were studied. In mice overexpressing peroxisome proliferator-activated receptor-α in muscle (MCK-PPARα), lipid-supported mitochondrial respiration, membrane potential (ΔΨ_m), and H₂O₂ production rate (JH₂O₂) were increased, which coincided with a more oxidized cytosolic redox environment, reduced muscle glucose uptake, and whole body glucose intolerance despite an increased rate of energy expenditure. Similar results were observed in lipin-1-deficient, fatty-liver dystrophic mice, another model characterized by increased β-oxidation flux and glucose intolerance. Crossing MCAT (mitochondria-targeted catalase) with MCK-PPARα mice normalized JH₂O₂ production, redox environment, and glucose tolerance, but surprisingly, both basal and absolute insulin-stimulated rates of glucose uptake in muscle remained depressed. Also surprising, when placed on a high-fat diet, MCK-PPARα mice were characterized by much lower whole body, fat, and lean mass as well as improved glucose tolerance relative to wild-type mice, providing additional evidence that overexpression of PPARα in muscle imposes more extensive metabolic stress than experienced by wild-type mice on a high-fat diet. Overall, the findings suggest that driving an increase in skeletal muscle fatty acid oxidation in the absence of metabolic demand imposes mitochondrial reductive stress and elicits multiple counterbalance metabolic responses in an attempt to restore bioenergetic homeostasis.NEW & NOTEWORTHY Prior work has suggested that mitochondrial dysfunction is an underlying cause of insulin resistance in muscle because it limits fatty acid oxidation and therefore leads to the accumulation of cytotoxic lipid intermediates. The implication has been that therapeutic strategies to accelerate β-oxidation will be protective. The current study provides evidence that genetically increasing flux through β-oxidation in muscle imposes reductive stress that is not beneficial but rather detrimental to metabolic regulation.

The Reducible Disulfide Proteome of Synaptosomes Supports a Role for Reversible Oxidations of Protein Thiols in the Maintenance of Neuronal Redox Homeostasis

The mechanisms by which neurons maintain redox homeostasis, disruption of which is linked to disease, are not well known. Hydrogen peroxide, a major cellular oxidant and neuromodulator, can promote reversible oxidations of protein thiols but the scope, targets, and significance of such oxidations occurring in neurons, especially in vivo, are uncertain. Using redox phenylarsine oxide (PAO)-affinity chromatography, which exploits the high-affinity of trivalent arsenicals for protein dithiols, this study investigated the occurrence of reducible and, therefore, potentially regulatory, protein disulfide bonds in Triton X-100-soluble protein fractions from isolated nerve-endings (synaptosomes) prepared from rat brains. Postmortem oxidations of protein thiols were limited by rapidly freezing the brains following euthanasia and, later, homogenizing them in the presence of the N-ethylmaleimide to trap reduced thiols. The reducible disulfide proteome comprised 5.4% of the total synaptosomal protein applied to the immobilized PAO columns and was overrepresented by pathways underlying ATP synaptic supply and demand including synaptic vesicle trafficking. The alpha subunits of plasma membrane Na⁺, K⁺-ATPase and the mitochondrial ATP synthase were particularly abundant proteins of the disulfide proteome and were enriched in this fraction by 3.5- and 6.7-fold, respectively. An adaptation of the commonly used "biotin-switch" method provided additional support for selective oxidation of thiols on the alpha subunit of the ATP synthase. We propose that reversible oxidations of protein thiols may underlie a coordinated metabolic response to hydrogen peroxide, serving to both control redox signaling and protect neurons from oxidant stress.

The Metabolite Urolithin-A Ameliorates Oxidative Stress in Neuro-2a Cells, Becoming a Potential Neuroprotective Agent

Urolithin A is a metabolite generated from ellagic acid and ellagitannins by the intestinal microbiota after consumption of fruits such as pomegranates or strawberries. The objective of this study was to determine the cytoprotective capacity of this polyphenol in Neuro-2a cells subjected to oxidative stress, as well as its direct radical scavenging activity and properties as an inhibitor of oxidases. Cells treated with this compound and H₂O₂ showed a greater response to oxidative stress than cells only treated with H₂O₂, as mitochondrial activity (MTT assay), redox state (ROS formation, lipid peroxidation), and the activity of antioxidant enzymes (CAT: catalase, SOD: superoxide dismutase, GR: glutathione reductase, GPx: glutathione peroxidase) were significantly ameliorated; additionally, urolithin A enhanced the expression of cytoprotective peroxiredoxins 1 and 3. Urolithin A also acted as a direct radical scavenger, showing values of 13.2 μM Trolox Equivalents for Oxygen Radical Absorbance Capacity (ORAC) and 5.01 µM and 152.66 µM IC₅₀ values for superoxide and 2,2-diphenyss1-picrylhydrazyl (DPPH) radicals, respectively. Finally, inhibition of oxidizing enzymes, such as monoamine oxidase A and tyrosinase, was also detected in a dose-dependent manner. The cytoprotective effects of urolithin A could be attributed to the improvement of the cellular antioxidant battery, but also to its role as a direct radical scavenger and enzyme inhibitor of oxidases.

A role for peroxiredoxins in H2O2- and MEKK-dependent activation of the p38 signaling pathway

The p38 mitogen-activated protein kinase (MAPK) signaling pathway plays an important role in the cellular response to various stresses and its deregulation accompanies pathological conditions such as cancer and chronic inflammation. Hydrogen peroxide (H2O2) is a well-established activator of the p38 MAPK signaling pathway. However, the mechanisms of H2O2-induced p38 activation are not yet fully understood. In Drosophila cells, we find that H2O2-induced activation of p38 depends on the MAPK kinase kinase (MAP3K) Mekk1. In line with the emerging role of peroxiredoxins as H2O2 sensors and signal transmitters we observe an H2O2-dependent interaction between Mekk1 and the cytosolic peroxiredoxin of Drosophila, Jafrac1. In human cells, MEKK4 (the homologue of Mekk1) and peroxiredoxin-2 (Prx2) interact in a similar manner, suggesting an evolutionarily conserved mechanism. In both organisms, H2O2 induces transient disulfide-linked conjugates between the MAP3K and a typical 2-Cys peroxiredoxin. We propose that these conjugates represent the relaying of oxidative equivalents from H2O2 to the MAP3K and that the oxidation of Mekk1/MEKK4 leads to the downstream activation of p38 MAPK. Indeed, the depletion of cytosolic 2-Cys peroxiredoxins in human cells diminished H2O2-induced activation of p38 MAPK.

Molecular characterization of novel mitochondrial peroxiredoxins from the Antarctic emerald rockcod and their gene expression in response to environmental warming

In the present study we describe the molecular characterization of the two paralogous mitochondrial peroxiredoxins from Trematomus bernacchii, a teleost that plays a pivotal role in the Antarctic food chain. The two putative amino acid sequences were compared with orthologs from other fish, highlighting a high percentage of identity and similarity with the respective variant, in particular for the residues that are essential for the characteristic peroxidase activity of these enzymes. The temporal expression of Prdx3 and Prdx5 mRNAs in response to short-term thermal stress showed a general upregulation of prdx3, suggesting that this isoform is the most affected by temperature increase. These data, together with the peculiar differences between the molecular structures of the two mitochondrial Prdxs in T. bernacchii as well as in the tropical species Stegastes partitus, suggest an adaptation that allowed these poikilothermic aquatic vertebrates to colonize very different environments, characterized by different temperature ranges.

Nutrient Metabolism, Subcellular Redox State, and Oxidative Stress in Pancreatic Islets and β-Cells

Insulin-secreting pancreatic β-cells play a critical role in blood glucose homeostasis and the development of type 2 diabetes (T2D) in the context of insulin resistance. Based on data obtained at the whole cell level using poorly specific chemical probes, reactive oxygen species (ROS) such as superoxide and hydrogen peroxide have been proposed to contribute to the stimulation of insulin secretion by nutrients (positive role) and to the alterations of cell survival and secretory function in T2D (negative role). This raised the controversial hypothesis that any attempt to decrease β-cell oxidative stress and apoptosis in T2D would further impair insulin secretion. Over the last decade, the development of genetically-encoded redox probes that can be targeted to cellular compartments of interest and are specific of redox couples allowed the evaluation of short- and long-term effects of nutrients on β-cell redox changes at the subcellular level. The data indicated that the nutrient regulation of β-cell redox signaling and ROS toxicity is far more complex than previously thought and that the subcellular compartmentation of these processes cannot be neglected when evaluating the mechanisms of ROS production or the efficacy of antioxidant enzymes and antioxidant drugs under glucolipotoxic conditions and in T2D. In this review, we present what is currently known about the compartmentation of redox homeostatic systems and tools to investigate it. We then review data about the effects of nutrients on β-cell subcellular redox state under normal conditions and in the context of T2D and discuss challenges and opportunities in the field.

Catalysis of Peroxide Reduction by Fast Reacting Protein Thiols

Life on Earth evolved in the presence of hydrogen peroxide, and other peroxides also emerged before and with the rise of aerobic metabolism. They were considered only as toxic byproducts for many years. Nowadays, peroxides are also regarded as metabolic products that play essential physiological cellular roles. Organisms have developed efficient mechanisms to metabolize peroxides, mostly based on two kinds of redox chemistry, catalases/peroxidases that depend on the heme prosthetic group to afford peroxide reduction and thiol-based peroxidases that support their redox activities on specialized fast reacting cysteine/selenocysteine (Cys/Sec) residues. Among the last group, glutathione peroxidases (GPxs) and peroxiredoxins (Prxs) are the most widespread and abundant families, and they are the leitmotif of this review. After presenting the properties and roles of different peroxides in biology, we discuss the chemical mechanisms of peroxide reduction by low molecular weight thiols, Prxs, GPxs, and other thiol-based peroxidases. Special attention is paid to the catalytic properties of Prxs and also to the importance and comparative outlook of the properties of Sec and its role in GPxs. To finish, we describe and discuss the current views on the activities of thiol-based peroxidases in peroxide-mediated redox signaling processes.

Unraveling the effects of peroxiredoxin 2 nitration; role of C-terminal tyrosine 193

Peroxiredoxins (Prx) are enzymes that efficiently reduce hydroperoxides through active participation of cysteine residues (C_P, C_R). The first step in catalysis, the reduction of peroxide substrate, is fast, 10⁷ - 10⁸ M^-1s^-1 for human Prx2. In addition, the high intracellular concentration of Prx positions them not only as good antioxidants but also as central players in redox signaling pathways. These biological functions can be affected by post-translational modifications that could alter the peroxidase activity and/or interaction with other proteins. In particular, inactivation by hyperoxidation of C_P, which occurs when a second molecule of peroxide reacts with the C_P in the sulfenic acid form, modulates their participation in redox signaling pathways. The higher sensitivity to hyperoxidation of some Prx has been related to the presence of structural motifs that disfavor disulfide formation at the active site, making the C_P sulfenic acid more available for hyperoxidation or interaction with a redox protein target. We previously reported that treatment of human Prx2 with peroxynitrite results in tyrosine nitration, a post-translational modification on non-catalytic residues, yielding a more active peroxidase with higher resistance to hyperoxidation. In this work, studies on various mutants of hPrx2 confirm that the presence of the tyrosyl side-chain of Y193, belonging to the C-terminal YF motif of eukaryotic Prx, is necessary to observe the increase in Prx2 resistance to hyperoxidation. Moreover, our results underline the critical role of this structural motif on the rate of disulfide formation that determines the differential participation of Prx in redox signaling pathways.

The role of thiols in antioxidant systems

The sulfur biochemistry of the thiol group endows cysteines with a number of highly specialized and unique features that enable them to serve a variety of different functions in the cell. Typically highly conserved in proteins, cysteines are predominantly found in functionally or structurally crucial regions, where they act as stabilizing, catalytic, metal-binding and/or redox-regulatory entities. As highly abundant low molecular weight thiols, cysteine thiols and their oxidized disulfide counterparts are carefully balanced to maintain redox homeostasis in various cellular compartments, protect organisms from oxidative and xenobiotic stressors and partake actively in redox-regulatory and signaling processes. In this review, we will discuss the role of protein thiols as scavengers of hydrogen peroxide in antioxidant enzymes, use thiol peroxidases to exemplify how protein thiols contribute to redox signaling, provide an overview over the diverse set of low molecular weight thiol-based redox systems found in biology, and illustrate how thiol-based redox systems have evolved not only to protect against but to take full advantage of a world full of molecular oxygen.

Kinetics of formation and reactivity of the persulfide in the one-cysteine peroxiredoxin from Mycobacterium tuberculosis

Hydrogen sulfide (H₂S) participates in prokaryotic metabolism and is associated with several physiological functions in mammals. H₂S reacts with oxidized thiol derivatives (i.e. disulfides and sulfenic acids) and thereby forms persulfides, which are plausible transducers of the H₂S-mediated signaling effects. The one-cysteine peroxiredoxin alkyl hydroperoxide reductase E from Mycobacterium tuberculosis (MtAhpE-SH) reacts fast with hydroperoxides, forming a stable sulfenic acid (MtAhpE-SOH), which we chose here as a model to study the interactions between H₂S and peroxiredoxins (Prx). MtAhpE-SOH reacted with H₂S, forming a persulfide (MtAhpE-SSH) detectable by mass spectrometry. The rate constant for this reaction was (1.4 ± 0.2) × 10³ m^-1 s^-1 (pH 7.4, 25 °C), six times higher than that reported for the reaction with the main low-molecular-weight thiol in M. tuberculosis, mycothiol. H₂S was able to complete the catalytic cycle of MtAhpE and, according to kinetic considerations, it could represent an alternative substrate in M. tuberculosis. MtAhpE-SSH reacted 43 times faster than did MtAhpE-SH with the unspecific electrophile 4,4'-dithiodipyridine, a disulfide that exhibits no preferential reactivity with peroxidatic cysteines, but MtAhpE-SSH was less reactive toward specific Prx substrates such as hydrogen peroxide and peroxynitrite. According to molecular dynamics simulations, this loss of specific reactivity could be explained by alterations in the MtAhpE active site. MtAhpE-SSH could transfer its sulfane sulfur to a low-molecular-weight thiol, a process likely facilitated by the low pK_a of the leaving thiol MtAhpE-SH, highlighting the possibility that Prx participates in transpersulfidation. The findings of our study contribute to the understanding of persulfide formation and reactivity.

Differential peroxiredoxin hyperoxidation regulates MAP kinase signaling in human articular chondrocytes

The peroxiredoxin (Prx) family of Cys-dependent peroxidases control intracellular levels of H₂O₂ and can regulate signal transduction. Inhibition of the Prxs, through hyperoxidation amongst other mechanisms, leads to oxidative stress conditions that can alter homeostatic signaling. To determine the effects oxidation of Prx1-Prx3 has on MAP kinase and IGF-1 signaling events in human chondrocytes, this study used 2-methyl-1,4-naphthoquinone (menadione) and 2,3-dimethyl-1,4-naphthoquinone (DMNQ) as H₂O₂-generating tools due to their differential mechanisms of action. Menadione and DMNQ generated similar levels of intracellular H₂O₂ as determined using the biosensor Orp1-roGFP and by measuring Prx redox status. However, menadione generated higher levels of mitochondrial H₂O₂ associated with Prx3 hyperoxidation and phosphorylation of Prx1 while DMNQ treatment was associated with hyperoxidation of cytosolic Prx1 and Prx2 but not mitochondrial Prx3. Both menadione and DMNQ induced sustained phosphorylation of p38 but only DMNQ activated JNK. Menadione but not DMNQ inhibited IGF-1-induced Akt phosphorylation. Chondrocytes transduced with an adenoviral vector to overexpress Prx3 displayed decreased PrxSO_2/3 formation in response to menadione which was associated with restoration of IGF-1-mediated Akt signaling and inhibition of p38 phosphorylation. Prx1 and Prx2 overexpression had no effects on Prx redox status but Prx1 overexpression enhanced basal Akt phosphorylation. These results suggest that hyperoxidation of specific Prx isoforms is associated with distinct cell signaling events and identify Prx3 redox status as an important regulator of anabolic and catabolic signal transduction. Targeted strategies to prevent mitochondrial Prx3 hyperoxidation could be useful in maintaining cellular redox balance and homeostatic signaling.

Protein Promiscuity in H2O2 Signaling

SIGNIFICANCE

Decrypting the cellular response to oxidative stress relies on a comprehensive understanding of the redox signaling pathways stimulated under oxidizing conditions. Redox signaling events can be divided into upstream sensing of oxidants, midstream redox signaling of protein function, and downstream transcriptional redox regulation. Recent Advances: A more and more accepted theory of hydrogen peroxide (H₂O₂) signaling is that of a thiol peroxidase redox relay, whereby protein thiols with low reactivity toward H₂O₂ are instead oxidized through an oxidative relay with thiol peroxidases.

CRITICAL ISSUES

These ultrareactive thiol peroxidases are the upstream redox sensors, which form the first cellular port of call for H₂O₂. Not all redox-regulated interactions between thiol peroxidases and cellular proteins involve a transfer of oxidative equivalents, and the nature of redox signaling is further complicated through promiscuous functions of redox-regulated "moonlighting" proteins, of which the precise cellular role under oxidative stress can frequently be obscured by "polygamous" interactions. An ultimate goal of redox signaling is to initiate a rapid response, and in contrast to prokaryotic oxidant-responsive transcription factors, mammalian systems have developed redox signaling pathways, which intersect both with kinase-dependent activation of transcription factors, as well as direct oxidative regulation of transcription factors through peroxiredoxin (Prx) redox relays.

FUTURE DIRECTIONS

We highlight that both transcriptional regulation and cell fate can be modulated either through oxidative regulation of kinase pathways, or through distinct redox-dependent associations involving either Prxs or redox-responsive moonlighting proteins with functional promiscuity. These protein associations form systems of crossregulatory networks with multiple nodes of potential oxidative regulation for H₂O₂-mediated signaling.

LAURDAN fluorescence and phasor plots reveal the effects of a H2O2 bolus in NIH-3T3 fibroblast membranes dynamics and hydration

Fluorescence spectroscopy, coupled with microscopy, opens new frontiers for the study of dynamic processes with high spatio-temporal resolution. The application of phasor plots to FLIM and hyperspectral imaging demonstrate unprecedented capabilities to study complex photophysics at the subcellular level. Using these approaches we studied the effects of an H2O2 bolus on NIH-3T3 membranes dynamics monitored by LAURDAN fluorescence. Exposure of NIH-3T3 cells to a bolus of H2O2 modifies the cell membranes and, in particular, the plasma membrane in a complex manner. The LAURDAN results reveal that the peroxide treatment decreases membrane fluidity but surprisingly increases dipolar relaxation around the excited probe. Using the Multidimensional-phasor approach we elucidated the complex photophysics of LAURDAN incorporated into cell membrane after H2O2 exposure. The results indicate the occurrence of LAURDAN fast-diffusion from gel↔ld phases in membranes exposed to a H2O2 bolus. An ad hoc hypothesis is presented to interpret the results in the context of H2O2 oxidative distress/eustress.

Differential parameters between cytosolic 2-Cys peroxiredoxins, PRDX1 and PRDX2

Peroxiredoxins are thiol-dependent peroxidases that function in peroxide detoxification and H₂ O₂ induced signaling. Among the six isoforms expressed in humans, PRDX1 and PRDX2 share 97% sequence similarity, 77% sequence identity including the active site, subcellular localization (cytosolic) but they hold different biological functions albeit associated with their peroxidase activity. Using recombinant human PRDX1 and PRDX2, the kinetics of oxidation and hyperoxidation with H₂ O₂ and peroxynitrite were followed by intrinsic fluorescence. At pH 7.4, the peroxidatic cysteine of both isoforms reacts nearly tenfold faster with H₂ O₂ than with peroxynitrite, and both reactions are orders of magnitude faster than with most protein thiols. For both isoforms, the sulfenic acids formed are in turn oxidized by H₂ O₂ with rate constants of ca 2 × 10³ M^-1 s^-1 and by peroxynitrous acid significantly faster. As previously observed, a crucial difference between PRDX1 and PRDX2 is on the resolution step of the catalytic cycle, the rate of disulfide formation (11 s^-1 for PRDX1, 0.2 s^-1 for PRDX2, independent of the oxidant) which correlates with their different sensitivity to hyperoxidation. This kinetic pause opens different pathways on redox signaling for these isoforms. The longer lifetime of PRDX2 sulfenic acid allows it to react with other protein thiols to translate the signal via an intermediate mixed disulfide (involving its peroxidatic cysteine), whereas PRDX1 continues the cycle forming disulfide involving its resolving cysteine to function as a redox relay. In addition, the presence of C83 on PRDX1 imparts a difference on peroxidase activity upon peroxynitrite exposure that needs further study.

The Multifaceted Impact of Peroxiredoxins on Aging and Disease

SIGNIFICANCE

Peroxiredoxins (Prxs), a family of thiol-associated peroxidases, are purported to play a major role in sensing and managing hydrogen peroxide concentrations and transducing peroxide-derived signals. Recent Advances: Prxs can act as detoxifying factors and impart effects to cells that can be either sparing or suicidal. Advances have been made to address the qualitative changes in Prx function in response to quantitative changes in the signal level and to understand how Prx activity could be affected by their own substrates. Here we rationalize the basis for both positive and negative effects on signaling pathways and cell physiology, summarizing data from model organisms, including invertebrates.

CRITICAL ISSUES

Resolving the relationship between the promiscuous behavior of reactive oxygen species and the specificity of Prxs toward different targets in redox-sensitive signaling pathways is a key area of research. Attempts to understand Prx function and underlying mechanisms were conducted in vitro or in vivo under nonphysiological conditions, leaving the physiological relevance yet to be defined. Other issues: Why despite the high degree of homology and similarities in subcellular and tissue distribution between Prxs do they display differential effects on signaling? How is the specificity of post-translational protein modifications determined? Other than chaperone-like activity, how do hyperoxidized Prxs function?

FUTURE DIRECTIONS

Genetic models with mutated catalytic and resolving cysteines should be further exploited to dissect the functional significance of individual Prxs in their different states together with their alternative reducing partners. Such an analysis may then be extended to help identify Prx-specific targets.

Cellular Timekeeping: It's Redox o'Clock

Mounting evidence in recent years supports the extensive interaction between the circadian and redox systems. The existence of such a relationship is not surprising because most organisms, be they diurnal or nocturnal, display daily oscillations in energy intake, locomotor activity, and exposure to exogenous and internally generated oxidants. The transcriptional clock controls the levels of many antioxidant proteins and redox-active cofactors, and, conversely, the cellular redox poise has been shown to feed back to the transcriptional oscillator via redox-sensitive transcription factors and enzymes. However, the circadian cycles in the S-sulfinylation of the peroxiredoxin (PRDX) proteins constituted the first example of an autonomous circadian redox oscillation, which occurred independently of the transcriptional clock. Importantly, the high phylogenetic conservation of these rhythms suggests that they might predate the evolution of the transcriptional oscillator, and therefore could be a part of a primordial circadian redox/metabolic oscillator. This discovery forced the reappraisal of the dogmatic transcription-centered view of the clockwork and opened a new avenue of research. Indeed, the investigation into the links between the circadian and redox systems is still in its infancy, and many important questions remain to be addressed.

Differential Kinetics of Two-Cysteine Peroxiredoxin Disulfide Formation Reveal a Novel Model for Peroxide Sensing

Two-cysteine peroxiredoxins (Prx) have a three-step catalytic cycle consisting of (1) reduction of peroxide and formation of sulfenic acid on the enzyme, (2) condensation of the sulfenic acid with a thiol to form disulfide, also known as resolution, and (3) reduction of the disulfide by a reductant protein. By following changes in protein fluorescence, we have studied the pH dependence of reaction 2 in human peroxiredoxins 1, 2, and 5 and in Salmonella typhimurium AhpC and obtained rate constants for the reaction and p K_a values of the thiol and sulfenic acid involved for each system. The observed reaction 2 rate constant spans 2 orders of magnitude, but in all cases, reaction 2 appears to be slow compared to the same reaction in small-molecule systems, making clear the rates are limited by conformational features of the proteins. For each Prx, reaction 2 will become rate-limiting at some critical steady-state concentration of H₂O₂ producing the accumulation of Prx as sulfenic acid. When this happens, an alternative and faster-resolving Prx (or other peroxidase) may take over the antioxidant role. The accumulation of sulfenic acid Prx at distinct concentrations of H₂O₂ is embedded in the kinetic limitations of the catalytic cycle and may constitute the basis of a H₂O₂-mediated redox signal transduction pathway requiring neither inactivation nor posttranslational modification. The differences in the rate constants of resolution among Prx coexisting in the same compartment may partially explain their complementation in antioxidant function and stepwise sensing of H₂O₂ concentration.

The Conundrum of Hydrogen Peroxide Signaling and the Emerging Role of Peroxiredoxins as Redox Relay Hubs

SIGNIFICANCE

Hydrogen peroxide (H₂O₂) is known to act as a messenger in signal transduction. How H₂O₂ leads to selective and efficient oxidation of specific thiols on specific signaling proteins remains one of the most important open questions in redox biology. Recent Advances: Increasing evidence implicates thiol peroxidases as mediators of protein thiol oxidation. Recently, this evidence has been extended to include the peroxiredoxins (Prxs). Prxs are exceptionally sensitive to H₂O₂, abundantly expressed and capture most of the H₂O₂ that is generated inside cells.

CRITICAL ISSUES

The overall prevalence and importance of Prx-based redox signaling relays are still unknown. The same is true for alternative mechanisms of redox signaling.

FUTURE DIRECTIONS

It will be important to clarify the relative contributions of Prx-mediated and direct thiol oxidation to H₂O₂ signaling. Many questions relating to Prx-based redox relays remain to be answered, including their mechanism, structural organization, and the potential role of adaptor proteins. Antioxid. Redox Signal. 28, 558-573.

Early cysteine-dependent inactivation of 26S proteasomes does not involve particle disassembly

Under oxidative stress 26S proteasomes suffer reversible disassembly into its 20S and 19S subunits, a process mediated by HSP70. This inhibits the degradation of polyubiquitinated proteins by the 26S proteasome and allows the degradation of oxidized proteins by a free 20S proteasome. Low fluxes of antimycin A-stimulated ROS production caused dimerization of mitochondrial peroxiredoxin 3 and cytosolic peroxiredoxin 2, but not peroxiredoxin overoxidation and overall oxidation of cellular protein thiols. This moderate redox imbalance was sufficient to inhibit the ATP stimulation of 26S proteasome activity. This process was dependent on reversible cysteine oxidation. Moreover, our results show that this early inhibition of ATP stimulation occurs previous to particle disassembly, indicating an intermediate step during the redox regulation of the 26S proteasome with special relevance under redox signaling rather than oxidative stress conditions.

A role for 2-Cys peroxiredoxins in facilitating cytosolic protein thiol oxidation

Hydrogen peroxide (H₂O₂) acts as a signaling messenger by triggering the reversible oxidation of redox-regulated proteins. It remains unclear how proteins can be oxidized by signaling levels of H₂O₂ in the presence of peroxiredoxins, which are highly efficient peroxide scavengers. Here we show that the rapid formation of disulfide bonds in cytosolic proteins is enabled, rather than competed, by cytosolic 2-Cys peroxiredoxins. Under the conditions tested, the combined deletion or depletion of cytosolic peroxiredoxins broadly frustrated H₂O₂-dependent protein thiol oxidation, which is the exact opposite of what would be predicted based on the assumption that H₂O₂ oxidizes proteins directly. We find that peroxiredoxins enable rapid and sensitive protein thiol oxidation by relaying H₂O₂-derived oxidizing equivalents to other proteins. Although these findings do not rule out the existence of Prx-independent H₂O₂ signaling mechanisms, they suggest a broader role for peroxiredoxins as sensors and transmitters of H₂O₂ signals than hitherto recognized.

Regulatory Role of Redox Balance in Determination of Neural Precursor Cell Fate

In 1990s, reports of discovery of a small group of cells capable of proliferation and contribution to formation of new neurons in the central nervous system (CNS) reversed a century-old concept on lack of neurogenesis in the adult mammalian brain. These cells are found in all stages of human life and contribute to normal cellular turnover of the CNS. Therefore, the identity of regulating factors that affect their proliferation and differentiation is a highly noteworthy issue for basic scientists and their clinician counterparts for therapeutic purposes. The cues for such control are embedded in developmental and environmental signaling through a highly regulated tempo-spatial expression of specific transcription factors. Novel findings indicate the importance of reactive oxygen species (ROS) in the regulation of this signaling system. The elusive nature of ROS signaling in many vital processes from cell proliferation to cell death creates a complex literature in this field. Here, we discuss the emerging thoughts on the importance of redox regulation of proliferation and maintenance in mammalian neural stem and progenitor cells under physiological and pathological conditions. The current knowledge on ROS-mediated changes in redox-sensitive proteins that govern the molecular mechanisms in proliferation and differentiation of these cells is reviewed.

Thioredoxin reductase 1 and NADPH directly protect protein tyrosine phosphatase 1B from inactivation during H2O2 exposure

Regulation of growth factor signaling involves reversible inactivation of protein tyrosine phosphatases (PTPs) through the oxidation and reduction of their active site cysteine. However, there is limited mechanistic understanding of these redox events and their co-ordination in the presence of cellular antioxidant networks. Here we investigated interactions between PTP1B and the peroxiredoxin 2 (Prx2)/thioredoxin 1 (Trx1)/thioredoxin reductase 1 (TrxR1) network. We found that Prx2 becomes oxidized in PDGF-treated fibroblasts, but only when TrxR1 has first been inhibited. Using purified proteins, we also found that PTP1B is relatively insensitive to inactivation by H₂O₂ but found no evidence for a relay mechanism in which Prx2 or Trx1 facilitates PTP1B oxidation. Instead, these proteins prevented PTP1B inactivation by H₂O₂ Intriguingly, we discovered that TrxR1/NADPH directly protects PTP1B from inactivation when present during the H₂O₂ exposure. This protection was dependent on the concentration of TrxR1 and independent of Trx1 and Prx2. The protection was blocked by auranofin and required an intact selenocysteine residue in TrxR1. This activity likely involves reduction of the sulfenic acid intermediate form of PTP1B by TrxR1 and is therefore distinct from the previously described reactivation of end-point oxidized PTP1B, which requires both Trx1 and TrxR1. The ability of TrxR1 to directly reduce an oxidized phosphatase is a novel activity that can help explain previously observed increases in PTP1B oxidation and PDGF receptor phosphorylation in TrxR1 knockout cells. The activity of TrxR1 is therefore of potential relevance for understanding the mechanisms of redox regulation of growth factor signaling pathways.

Redox modulation of thimet oligopeptidase activity by hydrogen peroxide

Thimet oligopeptidase (EC 3.4.24.15, TOP) is a cytosolic mammalian zinc protease that can process a diversity of bioactive peptides. TOP has been pointed out as one of the main postproteasomal enzymes that process peptide antigens in the MHC class I presentation route. In the present study, we describe a fine regulation of TOP activity by hydrogen peroxide (H₂O₂). Cells from a human embryonic kidney cell line (HEK293) underwent an ischemia/reoxygenation-like condition known to increase H₂O₂ production. Immediately after reoxygenation, HEK293 cells exhibited a 32% increase in TOP activity, but no TOP activity was observed 2 h after reoxygenation. In another model, recombinant rat TOP (rTOP) was challenged by H₂O₂ produced by rat liver mitoplasts (RLMt) alone, and in combination with antimycin A, succinate, and antimycin A plus succinate. In these conditions, rTOP activity increased 17, 30, 32 and 38%, respectively. Determination of H₂O₂ concentration generated in reoxygenated cells and mitoplasts suggested a possible modulation of rTOP activity dependent on the concentration of H₂O₂. The measure of pure rTOP activity as a function of H₂O₂ concentration corroborated this hypothesis. The data fitted to an asymmetrical bell-shaped curve in which the optimal activating H₂O₂ concentration was 1.2 nM, and the maximal inhibition (75% about the control) was 1 μm. Contrary to the oxidation produced by aging associated with enzyme oligomerization and inhibition, H₂O₂ oxidation produced sulfenic acid and maintained rTOP in the monomeric form. Consistent with the involvement of rTOP in a signaling redox cascade, the H₂O₂-oxidized rTOP reacted with dimeric thioredoxin-1 (TRx-1) and remained covalently bound to one subunit of TRx-1.

N-acetylcysteine improves the quality of red blood cells stored for transfusion

Storage inflicts a series of changes on red blood cells (RBC) that compromise the cell survival and functionality; largely these alterations (storage lesions) are due to oxidative modifications. The possibility of improving the quality of packed RBC stored for transfusion including N-acetylcysteine (NAC) in the preservation solution was explored. Relatively high concentrations of NAC (20-25 mM) were necessary to prevent the progressive leakage of hemoglobin, while lower concentrations (≥2.5 mM) were enough to prevent the loss of reduced glutathione during the first 21 days of storage. Peroxiredoxin-2 was also affected during storage, with a progressive accumulation of disulfide-linked dimers and hetero-protein complexes in the cytosol and also in the membrane of stored RBC. Although the presence of NAC in the storage solution was unable to avoid the formation of thiol-mediated protein complexes, it partially restored the capacity of the cell to metabolize H₂O₂, indicating the potential use of NAC as an additive in the preservation solution to improve RBC performance after transfusion.

The role of sulfenic acids in cellular redox signaling: Reconciling chemical kinetics and molecular detection strategies

The reversible oxidation of protein cysteine residues is well recognized as an important regulatory mechanism in redox-dependent cell signaling. Cysteine oxidation is diverse in nature and involves various post-translational modifications (sulfenic acids, disulfides, etc.) and the specific functional or structural impact of these specific oxidative events is still poorly understood. The proximal product of protein cysteine oxidation by biological reactive oxygen species (ROS) is sulfenic acid (Cys-SOH), and experimental evidence is accruing for the formation of Cys-SOH as intermediate in protein cysteine oxidation in various biological settings. However, the plausibility of protein Cys-SH oxidation by ROS has often been put in question because of slow reaction kinetics compared to more favorable reactions with abundant thiol-based reductants such as peroxiredoxins (Prx) or glutathione (GSH). This commentary aims to address this controversy by highlighting the unique physical properties in cells that may restrict ROS diffusion and allow otherwise less favorable cysteine oxidation of proteins. Some limitations of analytical tools to assess Cys-SOH are also discussed. We conclude that formation of Cys-SOH in biological systems cannot always be predicted based on kinetic analyses in homogenous solution, and may be facilitated by unique structural and physical properties of Cys-containing proteins within e.g. signaling complexes.

Crosstalk between calcium and reactive oxygen species signaling in cancer

The interplay between Ca²⁺ and reactive oxygen species (ROS) signaling pathways is well established, with reciprocal regulation occurring at a number of subcellular locations. Many Ca²⁺ channels at the cell surface and intracellular organelles, including the endoplasmic reticulum and mitochondria are regulated by redox modifications. In turn, Ca²⁺ signaling can influence the cellular generation of ROS, from sources such as NADPH oxidases and mitochondria. This relationship has been explored in great depth during the process of apoptosis, where surges of Ca²⁺ and ROS are important mediators of cell death. More recently, coordinated and localized Ca²⁺ and ROS transients appear to play a major role in a vast variety of pro-survival signaling pathways that may be crucial for both physiological and pathophysiological functions. While much work is required to firmly establish this Ca²⁺-ROS relationship in cancer, existing evidence from other disease models suggests this crosstalk is likely of significant importance in tumorigenesis. In this review, we describe the regulation of Ca²⁺ channels and transporters by oxidants and discuss the potential consequences of the ROS-Ca²⁺ interplay in tumor cells.

Happily (n)ever after: Aging in the context of oxidative stress, proteostasis loss and cellular senescence

Aging is a complex phenomenon and its impact is becoming more relevant due to the rising life expectancy and because aging itself is the basis for the development of age-related diseases such as cancer, neurodegenerative diseases and type 2 diabetes. Recent years of scientific research have brought up different theories that attempt to explain the aging process. So far, there is no single theory that fully explains all facets of aging. The damage accumulation theory is one of the most accepted theories due to the large body of evidence found over the years. Damage accumulation is thought to be driven, among others, by oxidative stress. This condition results in an excess attack of oxidants on biomolecules, which lead to damage accumulation over time and contribute to the functional involution of cells, tissues and organisms. If oxidative stress persists, cellular senescence is a likely outcome and an important hallmark of aging. Therefore, it becomes crucial to understand how senescent cells function and how they contribute to the aging process. This review will cover cellular senescence features related to the protein pool such as morphological and molecular hallmarks, how oxidative stress promotes protein modifications, how senescent cells cope with them by proteostasis mechanisms, including antioxidant enzymes and proteolytic systems. We will also highlight the nutritional status of senescent cells and aged organisms (including human clinical studies) by exploring trace elements and micronutrients and on their importance to develop strategies that might increase both, life and health span and postpone aging onset.

The NADPH Oxidases DUOX1 and NOX2 Play Distinct Roles in Redox Regulation of Epidermal Growth Factor Receptor Signaling

The epidermal growth factor receptor (EGFR) plays a critical role in regulating airway epithelial homeostasis and responses to injury. Activation of EGFR is regulated by redox-dependent processes involving reversible cysteine oxidation by reactive oxygen species (ROS) and involves both ligand-dependent and -independent mechanisms, but the precise source(s) of ROS and the molecular mechanisms that control tyrosine kinase activity are incompletely understood. Here, we demonstrate that stimulation of EGFR activation by ATP in airway epithelial cells is closely associated with dynamic reversible oxidation of cysteine residues via sequential sulfenylation and S-glutathionylation within EGFR and the non-receptor-tyrosine kinase Src. Moreover, the intrinsic kinase activity of recombinant Src or EGFR was in both cases enhanced by H₂O₂ but not by GSSG, indicating that the intermediate sulfenylation is the activating modification. H₂O₂-induced increase in EGFR tyrosine kinase activity was not observed with the C797S variant, confirming Cys-797 as the redox-sensitive cysteine residue that regulates kinase activity. Redox-dependent regulation of EGFR activation in airway epithelial cells was found to strongly depend on activation of either the NADPH oxidase DUOX1 or the homolog NOX2, depending on the activation mechanism. Whereas DUOX1 and Src play a primary role in EGFR transactivation by wound-derived signals such as ATP, direct ligand-dependent EGFR activation primarily involves NOX2 with a secondary role for DUOX1 and Src. Collectively, our findings establish that redox-dependent EGFR kinase activation involves a dynamic and reversible cysteine oxidation mechanism and that this activation mechanism variably involves DUOX1 and NOX2.

Quantitative measures for redox signaling

Redox signaling is now recognized as an important regulatory mechanism for a number of cellular processes including the antioxidant response, phosphokinase signal transduction and redox metabolism. While there has been considerable progress in identifying the cellular machinery involved in redox signaling, quantitative measures of redox signals have been lacking, limiting efforts aimed at understanding and comparing redox signaling under normoxic and pathogenic conditions. Here we have outlined some of the accepted principles for redox signaling, including the description of hydrogen peroxide as a signaling molecule and the role of kinetics in conferring specificity to these signaling events. Based on these principles, we then develop a working definition for redox signaling and review a number of quantitative methods that have been employed to describe signaling in other systems. Using computational modeling and published data, we show how time- and concentration- dependent analyses, in particular, could be used to quantitatively describe redox signaling and therefore provide important insights into the functional organization of redox networks. Finally, we consider some of the key challenges with implementing these methods.

Disulfide Stress Targets Modulators of Excitotoxicity in Otherwise Healthy Brains

Oxidative stress is a long-hypothesized cause of diverse neurological and psychiatric disorders but the pathways by which physiological redox perturbations may detour healthy brain development and aging are unknown. We reported recently (Foley et al., Neurochem Res 39:2030-2039, 2014) that two-electron oxidations, to disulfides, of protein vicinal thiols can vary markedly in association with more modest oxidations of the glutathione redox couple in brains from healthy adolescent rats whereas levels of protein S-glutathionylation were low and unchanged. Here, we demonstrate that the selective oxidations of protein vicinal thiols, occurring only in the more oxidized brains under study, were linked specifically to a peroxide stress as evidenced by increased oxidations, to disulfides, of the presumed catalytic vicinal thiols of peroxiredoxins 1 and 2. Moreover, we identify the catalytic subunit(s) of Na⁺, K⁺-ATPase, tubulins, glyceraldehyde-3-phosphate dehydrogenase, and protein phosphatase 1, all of which can modulate glutamate neurotransmission and the vulnerability of neurons to excitotoxicity, as non-peroxidase proteins exhibiting prominent oxidations of vicinal thiols. The two-electron pathway, demonstrated here, linking physiological redox perturbations in otherwise healthy brains to protein determinants of excitotoxicity, suggests an alternative to free radical pathways by which oxidative stress may impact brain development and aging.

Typical 2-Cys peroxiredoxins in human parasites: Several physiological roles for a potential chemotherapy target

Peroxiredoxins (Prxs) are ubiquitary proteins able to play multiple physiological roles, that include thiol-dependent peroxidase, chaperone holdase, sensor of H2O2, regulator of H2O2-dependent signal cascades, and modulator of the immune response. Prxs have been found in a great number of human pathogens, both eukaryotes and prokaryotes. Gene knock-out studies demonstrated that Prxs are essential for the survival and virulence of at least some of the pathogens tested, making these proteins potential drug targets. However, the multiplicity of roles played by Prxs constitutes an unexpected obstacle to drug development. Indeed, selective inhibitors of some of the functions of Prxs are known (namely of the peroxidase and holdase functions) and are here reported. However, it is often unclear which function is the most relevant in each pathogen, hence which one is most desirable to inhibit. Indeed there are evidences that the main physiological role of Prxs may not be the same in different parasites. We here review which functions of Prxs have been demonstrated to be relevant in different human parasites, finding that the peroxidase and chaperone activities figure prominently, whereas other known functions of Prxs have rarely, if ever, been observed in parasites, or have largely escaped detection thus far.

Mitochondrial reactive oxygen species: Do they extend or shorten animal lifespan?

Testing the predictions of the Mitochondrial Free Radical Theory of Ageing (MFRTA) has provided a deep understanding of the role of reactive oxygen species (ROS) and mitochondria in the aging process. However those data, which support MFRTA are in the majority correlative (e.g. increasing oxidative damage with age). In contrast the majority of direct experimental data contradict MFRTA (e.g. changes in ROS levels do not alter longevity as expected). Unfortunately, in the past, ROS measurements have mainly been performed using isolated mitochondria, a method which is prone to experimental artifacts and does not reflect the complexity of the in vivo process. New technology to study different ROS (e.g. superoxide or hydrogen peroxide) in vivo is now available; these new methods combined with state-of-the-art genetic engineering technology will allow a deeper interrogation of, where, when and how free radicals affect aging and pathological processes. In fact data that combine these new approaches, indicate that boosting mitochondrial ROS in lower animals is a way to extend both healthy and maximum lifespan. In this review, I discuss the latest literature focused on the role of mitochondrial ROS in aging, and how these new discoveries are helping to better understand the role of mitochondria in health and disease. This article is part of a Special Issue entitled 'EBEC 2016: 19th European Bioenergetics Conference, Riva del Garda, Italy, July 2-6, 2016', edited by Prof. Paolo Bernardi.

Oxidative Stress Promotes Peroxiredoxin Hyperoxidation and Attenuates Pro-survival Signaling in Aging Chondrocytes

Oxidative stress-mediated post-translational modifications of redox-sensitive proteins are postulated as a key mechanism underlying age-related cellular dysfunction and disease progression. Peroxiredoxins (PRX) are critical intracellular antioxidants that also regulate redox signaling events. Age-related osteoarthritis is a common form of arthritis that has been associated with mitochondrial dysfunction and oxidative stress. The objective of this study was to determine the effect of aging and oxidative stress on chondrocyte intracellular signaling, with a specific focus on oxidation of cytosolic PRX2 and mitochondrial PRX3. Menadione was used as a model to induce cellular oxidative stress. Compared with chondrocytes isolated from young adult humans, chondrocytes from older adults exhibited higher levels of PRX1-3 hyperoxidation basally and under conditions of oxidative stress. Peroxiredoxin hyperoxidation was associated with inhibition of pro-survival Akt signaling and stimulation of pro-death p38 signaling. These changes were prevented in cultured human chondrocytes by adenoviral expression of catalase targeted to the mitochondria (MCAT) and in cartilage explants from MCAT transgenic mice. Peroxiredoxin hyperoxidation was observedin situin human cartilage sections from older adults and in osteoarthritic cartilage. MCAT transgenic mice exhibited less age-related osteoarthritis. These findings demonstrate that age-related oxidative stress can disrupt normal physiological signaling and contribute to osteoarthritis and suggest peroxiredoxin hyperoxidation as a potential mechanism.

Structural changes upon peroxynitrite-mediated nitration of peroxiredoxin 2; nitrated Prx2 resembles its disulfide-oxidized form

Peroxiredoxins are cys-based peroxidases that function in peroxide detoxification and H2O2-induced signaling. Human Prx2 is a typical 2-Cys Prx arranged as pentamers of head-to-tail homodimers. During the catalytic mechanism, the active-site cysteine (CP) cycles between reduced, sulfenic and disulfide state involving conformational as well as oligomeric changes. Several post-translational modifications were shown to affect Prx activity, in particular CP overoxidation which leads to inactivation. We have recently reported that nitration of Prx2, a post-translational modification on non-catalytic tyrosines, unexpectedly increases its peroxidase activity and resistance to overoxidation. To elucidate the cross-talk between this post-translational modification and the enzyme catalysis, we investigated the structural changes of Prx2 after nitration. Analytical ultracentrifugation, UV absorption, circular dichroism, steady-state and time-resolved fluorescence were used to connect catalytically relevant redox changes with tyrosine nitration. Our results show that the reduced nitrated Prx2 structurally resembles the disulfide-oxidized native form of the enzyme favoring a locally unfolded conformation that facilitates disulfide formation. These results provide structural basis for the kinetic analysis previously reported, the observed increase in activity and the resistance to overoxidation of the peroxynitrite-treated enzyme.

Protein Sulfenylation: A Novel Readout of Environmental Oxidant Stress

Oxidative stress is a commonly cited mechanism of toxicity of environmental agents. Ubiquitous environmental chemicals such as the diesel exhaust component 1,2-naphthoquinone (1,2-NQ) induce oxidative stress by redox cycling, which generates hydrogen peroxide (H2O2). Cysteinyl thiolate residues on regulatory proteins are subjected to oxidative modification by H2O2 in physiological contexts and are also toxicological targets of oxidant stress induced by environmental contaminants. We investigated whether exposure to environmentally relevant concentrations of 1,2-NQ can induce H2O2-dependent oxidation of cysteinyl thiols in regulatory proteins as a readout of oxidant stress in human airway epithelial cells. BEAS-2B cells were exposed to 0-1000 μM 1,2-NQ for 0-30 min, and levels of H2O2 were measured by ratiometric spectrofluorometry of HyPer. H2O2-dependent protein sulfenylation was measured using immunohistochemistry, immunoblotting, and isotopic mass spectrometry. Catalase overexpression was used to investigate the relationship between H2O2 generation and protein sulfenylation in cells exposed to 1,2-NQ. Multiple experimental approaches showed that exposure to 1,2-NQ at concentrations as low as 3 μM induces H2O2-dependent protein sulfenylation in BEAS-2B cells. Moreover, the time of onset and duration of 1,2-NQ-induced sulfenylation of the regulatory proteins GAPDH and PTP1B showed significant differences. Oxidative modification of regulatory cysteinyl thiols in human lung cells exposed to relevant concentrations of an ambient air contaminant represents a novel marker of oxidative environmental stress.