Oxidation of a eukaryotic 2-Cys peroxiredoxin is a molecular switch controlling the transcriptional response to increasing levels of hydrogen peroxide

Quantifying redox transcription factor dynamics as a tool to investigate redox signalling

A critical feature of the cellular antioxidant response is the induction of gene expression by redox-sensitive transcription factors. In many cells, activating these transcription factors is a dynamic process involving multiple redox steps, but it is unclear how these dynamics should be measured. Here, we show how the dynamic profile of the Schizosaccharomyces pombe Pap1 transcription factor is quantifiable by three parameters: signal amplitude, signal time and signal duration. In response to increasing hydrogen peroxide concentrations, the Pap1 amplitude decreased while the signal time and duration showed saturable increases. In co-response plots, these parameters showed a complex, non-linear relationship to the mRNA levels of four Pap1-regulated genes. We also demonstrate that hydrogen peroxide and tert-butyl hydroperoxide trigger quantifiably distinct Pap1 activation profiles and transcriptional responses. Based on these findings, we propose that different oxidants and oxidant concentrations modulate the Pap1 dynamic profile, leading to specific transcriptional responses. We further show how the effect of combination and pre-exposure stresses on Pap1 activation dynamics can be quantified using this approach. This method is therefore a valuable addition to the redox signalling toolbox that may illuminate the role of dynamics in determining appropriate responses to oxidative stress.

Temporal coordination of the transcription factor response to H2O2 stress

Oxidative stress from excess H2O2 activates transcription factors that restore redox balance and repair oxidative damage. Although many transcription factors are activated by H2O2, it is unclear whether they are activated at the same H2O2 concentration, or time. Dose-dependent activation is likely as oxidative stress is not a singular state and exhibits dose-dependent outcomes including cell-cycle arrest and cell death. Here, we show that transcription factor activation is both dose-dependent and coordinated over time. Low levels of H2O2 activate p53, NRF2 and JUN. Yet under high H2O2, these transcription factors are repressed, and FOXO1, NF-κB, and NFAT1 are activated. Time-lapse imaging revealed that the order in which these two groups of transcription factors are activated depends on whether H2O2 is administered acutely by bolus addition, or continuously through the glucose oxidase enzyme. Finally, we provide evidence that 2-Cys peroxiredoxins control which group of transcription factors are activated.

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.

A peroxiredoxin-P38 MAPK scaffold increases MAPK activity by MAP3K-independent mechanisms

Peroxiredoxins (Prdxs) utilize reversibly oxidized cysteine residues to reduce peroxides and promote H2O2 signal transduction, including H2O2-induced activation of P38 MAPK. Prdxs form H2O2-induced disulfide complexes with many proteins, including multiple kinases involved in P38 MAPK signaling. Here, we show that a genetically encoded fusion between a Prdx and P38 MAPK is sufficient to hyperactivate the kinase in yeast and human cells by a mechanism that does not require the H2O2-sensing cysteine of the Prdx. We demonstrate that a P38-Prdx fusion protein compensates for loss of the yeast scaffold protein Mcs4 and MAP3K activity, driving yeast into mitosis. Based on our findings, we propose that the H2O2-induced formation of Prdx-MAPK disulfide complexes provides an alternative scaffold and signaling platform for MAPKK-MAPK signaling. The demonstration that formation of a complex with a Prdx is sufficient to modify the activity of a kinase has broad implications for peroxide-based signal transduction in eukaryotes.

Fission yeast Cdc14-like phosphatase Flp1/Clp1 modulates the transcriptional response to oxidative stress

Reactive oxygen species (ROS) are an important source of cellular damage. When ROS intracellular levels increase, oxidative stress takes place affecting DNA stability and metabolic functions. To prevent these effects, stress-activated protein kinases (SAPKs) delay cell cycle progression and induce a transcriptional response that activates antioxidant mechanisms ensuring cell adaptation and survival. Fission yeast Cdc14-like phosphatase Flp1 (also known as Clp1) has a well-established role in cell cycle regulation. Moreover, Flp1 contributes to checkpoint activation during replication stress. Here, we show that Flp1 has a role in fine-tuning the cellular oxidative stress response. Phosphorylation-dependent nucleolar release of Flp1 in response to oxidative stress conditions plays a role in the cellular transcriptional response. Thus, Flp1 ablation increases the transcriptional response to oxidative stress, in both intensity and duration, upregulating both Atf1/Pcr1- and Pap1-dependent stress induced genes. Remarkably, we found that Flp1 interacts with the Atf1/Pcr1 complex with Pcr1 acting as a direct substrate. Our results provide evidence that Flp1 modulates the oxidative stress response by limiting the Atf1/Pcr1-mediated transcription.

The role of peroxiredoxins PrxA and PrxB in the antioxidant response, carbon utilization and development in Aspergillus nidulans

In addition to their role in the breakdown of H2O2, some peroxiredoxins (Prxs) have chaperone and H2O2 sensing functions. Acting as an H2O2 sensor, Prx Gpx3 transfers the oxidant signal to the transcription factor Yap1, involved in the antioxidant response in Saccharomyces cerevisiae. We have shown that Aspergillus nidulans Yap1 ortholog NapA is necessary for the antioxidant response, the utilization of arabinose, fructose and ethanol, and for proper development. To address the Prx roles in these processes, we generated and characterized mutants lacking peroxiredoxins PrxA, PrxB, PrxC, or TpxC. Our results show that the elimination of peroxiredoxins PrxC or TpxC does not produce any distinguishable phenotype. In contrast, the elimination of atypical 2-cysteine peroxiredoxins PrxA and PrxB produce different mutant phenotypes. ΔprxA, ΔnapA and ΔprxA ΔnapA mutants are equally sensitive to H2O2 and menadione, while PrxB is dispensable for this. However, the sensitivity of ΔprxA and ΔprxA ΔnapA mutants is increased by the lack of PrxB. Moreover, PrxB is required for arabinose and ethanol utilization and fruiting body cell wall pigmentation. PrxA expression is partially independent of NapA, and the replacement of peroxidatic cysteine 61 by serine (C61S) is enough to cause oxidative stress sensitivity and prevent NapA nuclear accumulation in response to H2O2, indicating its critical role in H2O2 sensing. Our results show that despite their high similarity, PrxA and PrxB play differential roles in Aspergillus nidulans antioxidant response, carbon utilization and development.

Temporal Coordination of the Transcription Factor Response to H2O2 stress

Oxidative stress from excess H2O2 activates transcription factors (TFs) that restore redox balance and repair oxidative damage. Though many TFs are activated by H2O2, it is unknown whether they are activated at the same H2O2 concentration or time after H2O2 stress. We found TF activation is tightly coordinated over time and dose dependent. We first focused on p53 and FOXO1 and found that in response to low H2O2, p53 is activated rapidly while FOXO1 remains inactive. In contrast, cells respond to high H2O2 in two temporal phases. In the first phase FOXO1 rapidly shuttles to the nucleus while p53 remains inactive. In the second phase FOXO1 shuts off and p53 levels rise. Other TFs are activated in the first phase with FOXO1 (NF-κB, NFAT1), or the second phase with p53 (NRF2, JUN), but not both. The two phases result in large differences in gene expression. Finally, we provide evidence that 2-Cys peroxiredoxins control which TF are activated and the timing of TF activation.

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.

Schizosaccharomyces pombe MAP kinase Sty1 promotes survival of Δppr10 cells with defective mitochondrial protein synthesis

Deletion of the Schizosaccharomyces pombe pentatricopeptide repeat gene ppr10 severely impairs mitochondrial translation, resulting in defective oxidative phosphorylation (OXPHOS). ppr10 deletion also induces iron starvation response, resulting in increased reactive oxygen species (ROS) production and reduced viability under fermentative conditions. S. pombe has two principal stress-response pathways, which are mediated by the mitogen-activated protein kinase Sty1 and the basic leucine zipper transcription factor Pap1, respectively. In this study, we examined the roles of Sty1 and Pap1 in the cellular response to the mitochondrial translation defect caused by ppr10 deletion. We found that ppr10 deletion resulted in two waves of stress protein activation. The early response occurred in exponential phase and resulted in the expression of a subset of stress proteins including Gst2 and Obr1. The upregulation of some of these stress proteins in Δppr10 cells in early response is dependent on the basal nuclear levels of Sty1 or Pap1. The late response occurred in early stationary phase and coincided with the stable localization of Sty1 and Pap1 in the nucleus, presumably resulting in persistent activation of a large set of stress proteins. Deletion of sty1 in Δppr10 cells caused severe defects in cell division and growth, and further impaired cell viability. Deletion of the mitochondrial superoxide dismutase gene sod2 whose expression is controlled by Sty1 severely inhibited the growth of Δppr10 cells. Overexpression of sod2 improves the viability of Δppr10 cells. Our results support an important role for Sty1 in counteracting stress induced by ppr10 deletion under fermentative growth conditions.

High cysteine concentrations in cell culture media lead to oxidative stress and reduced bioprocess performance of recombinant CHO cells

Cysteine is considered an essential amino acid in the cultivation of Chinese hamster ovary (CHO) cells. An optimized cysteine supply during fed-batch cultivation supports the protein production capacity of recombinant CHO cell lines. However, we observed that CHO production cell lines seeded at low cell densities in chemically defined media enriched with, cysteine greater than 2.5 mM resulted in markedly reduced cell growth during passaging, hampering seed train performance and scale-up. To investigate the underlying mechanism, seeding cell densities and initial cysteine concentrations ranging from low to high cysteine concentrations were varied followed by an analysis of cell culture performance. Additionally, cell cycle analysis, intracellular quantification of reactive oxygen species (ROS) as well as transcriptomic analyses by next-generation sequencing were carried out. Our results demonstrate that CHO cells seeded at low cell densities at high initial cysteine concentrations encountered increased oxidative stress leading to a p21-mediated cell cycle arrest in the G1/S phase. The resulting oxidative stress caused redox imbalance in the endoplasmic reticulum and activation of the unfolded protein response as well as the major antioxidant nuclear factor-like 2 response pathways. We were able to identify potential signature genes related to oxidative stress and the inhibition of the pentose phosphate pathway. Finally, we present that seeding cells at a higher concentration counteract oxidative stress in cysteine-enriched cell culture media. This article is protected by copyright. All rights reserved.

Experimental Approaches for Investigating Disulfide-Based Redox Relays in Cells

Reversible oxidation of cysteine residues within proteins occurs naturally during normal cellular homeostasis and can increase during oxidative stress. Cysteine oxidation often leads to the formation of disulfide bonds, which can impact protein folding, stability, and function. Work in both prokaryotic and eukaryotic models over the past five decades has revealed several multiprotein systems that use thiol-dependent oxidoreductases to mediate disulfide bond reduction, formation, and/or rearrangement. Here, I provide an overview of how these systems operate to carry out disulfide exchange reactions in different cellular compartments, with a focus on their roles in maintaining redox homeostasis, transducing redox signals, and facilitating protein folding. Additionally, I review thiol-independent and thiol-dependent approaches for interrogating what proteins partner together in such disulfide-based redox relays. While the thiol-independent approaches rely either on predictive measures or standard procedures for monitoring protein-protein interactions, the thiol-dependent approaches include direct disulfide trapping methods as well as thiol-dependent chemical cross-linking. These strategies may prove useful in the systematic characterization of known and newly discovered disulfide relay mechanisms and redox switches involved in oxidant defense, protein folding, and cell signaling.

Oxidative stress response pathways in fungi

Fungal response to any stress is intricate, specific, and multilayered, though it employs only a few evolutionarily conserved regulators. This comes with the assumption that one regulator operates more than one stress-specific response. Although the assumption holds true, the current understanding of molecular mechanisms that drive response specificity and adequacy remains rudimentary. Deciphering the response of fungi to oxidative stress may help fill those knowledge gaps since it is one of the most encountered stress types in any kind of fungal niche. Data have been accumulating on the roles of the HOG pathway and Yap1- and Skn7-related pathways in mounting distinct and robust responses in fungi upon exposure to oxidative stress. Herein, we review recent and most relevant studies reporting the contribution of each of these pathways in response to oxidative stress in pathogenic and opportunistic fungi after giving a paralleled overview in two divergent models, the budding and fission yeasts. With the concept of stress-specific response and the importance of reactive oxygen species in fungal development, we first present a preface on the expanding domain of redox biology and oxidative stress.

Basis for using thioredoxin as an electron donor by Schizosaccharomyces pombe Gpx1 and Tpx1

Glutathione (GSH) peroxidases (GPxs or GSHPx) and thioredoxin (Trx) peroxidases (TPxs) are two classes of peroxidases that catalyze the reduction of peroxides. GPxs and TPxs generally use GSH or Trx, respectively, to recycle the oxidized cysteine (Cys) residue in the protein. However, it is unclear why unlike human GPxs, the Schizosaccharomyces pombe Gpx1 (spGpx1) prefers Trx over GSH for recycling of the active-site peroxidatic Cys residue. Here, we compared spGpx1 and S. pombe Tpx1 (spTpx1) protein sequences with those of their respective homologs in Saccharomyces cerevisiae and humans. Our analysis revealed that like spTpx1, spGpx1 contains a pair of conserved Cys residues (Cys36 and Cys82). These two conserved Cys residues are named peroxidatic and resolving Cys residues, respectively, and are found only in GPxs and TPxs that prefer Trx as an electron donor. Our analysis suggested that Cys36 and Cys82 in spGpx1 are most likely to form a disulfide bond upon oxidation of Cys36. Molecular modelling predicted that a conformational change might be required for the formation of this disulfide bond. Evolutionary analysis suggested that fungal GPxs and TPxs are related by divergent evolution from a common ancestor. Our analyses support a prediction that while spGpx1 and spTpx1 are phylogenetically and functionally different, they evolved from a common ancestor and use a similar mechanism for recycling of the active-site peroxidatic Cys residue.

spGpx1 contains two conserved Cys residues (Cys36 and Cys82), which may form a disulfide bond upon oxidation and the reduction of this disulfide bond is most likely to be mediated by Trx in vivo.

Fungal GPxs and TPxs evolved from a common ancestor.

Monitoring cytosolic H2O2 fluctuations arising from altered plasma membrane gradients or from mitochondrial activity

Genetically encoded probes monitoring H₂O₂ fluctuations in living organisms are key to decipher redox signaling events. Here we use a new probe, roGFP2-Tpx1.C169S, to monitor pre-toxic fluctuations of peroxides in fission yeast, where the concentrations linked to signaling or to toxicity have been established. This probe is able to detect nanomolar fluctuations of intracellular H₂O₂ caused by extracellular peroxides; expression of human aquaporin 8 channels H₂O₂ entry into fission yeast decreasing membrane gradients. The probe also detects H₂O₂ bursts from mitochondria after addition of electron transport chain inhibitors, the extent of probe oxidation being proportional to the mitochondrial activity. The oxidation of this probe is an indicator of steady-state levels of H₂O₂ in different genetic backgrounds. Metabolic reprogramming during growth in low-glucose media causes probe reduction due to the activation of antioxidant cascades. We demonstrate how peroxiredoxin-based probes can be used to monitor physiological H₂O₂ fluctuations.

Reliable methods of measuring intracellular H₂O₂ fluctuations are necessary to advance redox biology. Here the authors design a H₂O₂ sensor based on the fission yeast peroxiredoxin Tpx1 to sense nanomolar fluctuations of intracellular H₂O₂ in response to genetic and environmental perturbations.

Hyperoxidation of mitochondrial peroxiredoxin limits H2 O2 -induced cell death in yeast

Hydrogen peroxide (H₂O₂) plays important roles in cellular signaling, yet nonetheless is toxic at higher concentrations. Surprisingly, the mechanism(s) of cellular H₂O₂ toxicity remain poorly understood. Here, we reveal an important role for mitochondrial 1‐Cys peroxiredoxin from budding yeast, Prx1, in regulating H₂O₂‐induced cell death. We show that Prx1 efficiently transfers oxidative equivalents from H₂O₂ to the mitochondrial glutathione pool. Deletion of PRX1 abrogates glutathione oxidation and leads to a cytosolic adaptive response involving upregulation of the catalase, Ctt1. Both of these effects contribute to improved cell viability following an acute H₂O₂ challenge. By replacing PRX1 with natural and engineered peroxiredoxin variants, we could predictably induce widely differing matrix glutathione responses to H₂O₂. Therefore, we demonstrated a key role for matrix glutathione oxidation in driving H₂O₂‐induced cell death. Finally, we reveal that hyperoxidation of Prx1 serves as a switch‐off mechanism to limit oxidation of matrix glutathione at high H₂O₂ concentrations. This enables yeast cells to strike a fine balance between H₂O₂ removal and limitation of matrix glutathione oxidation.

Yeast AP-1 like transcription factors (Yap) and stress response: a current overview

Yeast adaptation to stress has been extensively studied. It involves large reprogramming of genome expression operated by many, more or less specific, transcription factors. Here, we review our current knowledge on the function of the eight Yap transcription factors (Yap1 to Yap8) in Saccharomyces cerevisiae, which were shown to be involved in various stress responses. More precisely, Yap1 is activated under oxidative stress, Yap2/Cad1 under cadmium, Yap4/Cin5 and Yap6 under osmotic shock, Yap5 under iron overload and Yap8/Arr1 by arsenic compounds. Yap3 and Yap7 seem to be involved in hydroquinone and nitrosative stresses, respectively. The data presented in this article illustrate how much knowledge on the function of these Yap transcription factors is advanced. The evolution of the Yap family and its roles in various pathogenic and non-pathogenic fungal species is discussed in the last section.

Protection from Disulfide Stress by Inhibition of Pap1 Nuclear Export in Schizosaccharomyces pombe

Appropriate subcellular localization of regulatory factors is critical for cellular function. Pap1, a nucleocytoplasmic shuttling transcription factor of Schizosaccharomyces pombe, is redox regulated for localization and antistress function. In this study, we find that overproduction of a peptide conjugate containing the nuclear export signal of Oxs1, a conserved eukaryotic protein that, along with Pap1, regulates certain diamide responsive genes, can retain Pap1 in the nucleus before stress by competing for nuclear export. The nuclear retention of Pap1 upregulates several drug resistance genes to prime the cells for higher tolerance to disulfide stress. Overproduction of Oxs1 also upregulates these same genes, not by competing for export but by binding directly to the drug resistance gene promoters for Pap1-mediated activation. Of medical relevance is that this may suggest a gene therapy approach of using nuclear export signal conjugates to suppress the nuclear export of biomolecules.

Mechanisms and Applications of Redox-Sensitive Green Fluorescent Protein-Based Hydrogen Peroxide Probes

SIGNIFICANCE

Genetically encoded hydrogen peroxide (H₂O₂) sensors, based on fusions between thiol peroxidases and redox-sensitive green fluorescent protein 2 (roGFP2), have dramatically broadened the available "toolbox" for monitoring cellular H₂O₂ changes. Recent Advances: Recently developed peroxiredoxin-based probes such as roGFP2-Tsa2ΔC_R offer considerably improved H₂O₂ sensitivity compared with previously available genetically encoded sensors and now permit dynamic, real-time, monitoring of changes in endogenous H₂O₂ levels.

CRITICAL ISSUES

The correct understanding and interpretation of probe read-outs is crucial for their meaningful use. We discuss probe mechanisms, potential pitfalls, and best practices for application and interpretation of probe responses and highlight where gaps in our knowledge remain.

FUTURE DIRECTIONS

The full potential of the newly available sensors remains far from being fully realized and exploited. We discuss how the ability to monitor basal H₂O₂ levels in real time now allows us to re-visit long-held ideas in redox biology such as the response to ischemia-reperfusion and hypoxia-induced reactive oxygen species production. Further, recently proposed circadian cycles of peroxiredoxin hyperoxidation might now be rigorously tested. Beyond their application as H₂O₂ probes, roGFP2-based H₂O₂ sensors hold exciting potential for studying thiol peroxidase mechanisms, inactivation properties, and the impact of post-translational modifications, in vivo. Antioxid. Redox Signal. 29, 552-568.

Tyrosine substitution of a conserved active-site histidine residue activates Plasmodium falciparum peroxiredoxin 6

Peroxiredoxins efficiently remove hydroperoxides and peroxynitrite in pro- and eukaryotes. However, isoforms of one subfamily of peroxiredoxins, the so-called Prx6-type enzymes, usually have very low activities in standard peroxidase assays in vitro. In contrast to other peroxiredoxins, Prx6 homologues share a conserved histidyl residue at the bottom of the active site. Here we addressed the role of this histidyl residue for redox catalysis using the Plasmodium falciparum homologue PfPrx6 as a model enzyme. Steady-state kinetics with tert-butyl hydroperoxide (tBuOOH) revealed that the histidyl residue is nonessential for Prx6 catalysis and that a replacement with tyrosine can even increase the enzyme activity four- to six-fold in vitro. Stopped-flow kinetics with reduced PfPrx6^WT , PfPrx6^C128A , and PfPrx6^H39Y revealed a preference for H₂ O₂ as an oxidant with second order rate constants for H₂ O₂ and tBuOOH around 2.5 × 10⁷ M^-1 s^-1 and 3 × 10⁶ M^-1 s^-1 , respectively. Differences between the oxidation kinetics of PfPrx6^WT , PfPrx6^C128A , and PfPrx6^H39Y were observed during a slower second-reaction phase. Our kinetic data support the interpretation that the reductive half-reaction is the rate-limiting step for PfPrx6 catalysis in steady-state measurements. Whether the increased activity of PfPrx6^H39Y is caused by a facilitated enzyme reduction because of a destabilization of the fully folded enzyme conformation remains to be analyzed. In summary, the conserved histidyl residue of Prx6-type enzymes is non-essential for catalysis, PfPrx6 is rapidly oxidized by hydroperoxides, and the gain-of-function mutant PfPrx6^H39Y might provide a valuable tool to address the influence of conformational changes on the reactivity of Prx6 homologues.

Beyond ROS clearance: Peroxiredoxins in stress signaling and aging

Antioxidants were long predicted to have lifespan-promoting effects, but in general this prediction has not been well supported. While some antioxidants do seem to have a clear effect on longevity, this may not be primarily as a result of their role in the removal of reactive oxygen species, but rather mediated by other mechanisms such as the modulation of intracellular signaling. In this review we discuss peroxiredoxins, a class of proteinaceous antioxidants with redox signaling and chaperone functions, and their involvement in regulating longevity and stress resistance. Peroxiredoxins have a clear role in the regulation of lifespan and survival of many model organisms, including the mouse, Caenorhabditis elegans and Drosophila melanogaster. Recent research on peroxiredoxins - in these models and beyond - has revealed surprising new insights regarding the interplay between peroxiredoxins and longevity signaling, which will be discussed here in detail. As redox signaling is emerging as a potentially important player in the regulation of longevity and aging, increased knowledge of these fascinating antioxidants and their mode(s) of action is paramount.

Using in vivo oxidation status of one- and two-component redox relays to determine H2O2 levels linked to signaling and toxicity

Background

Hydrogen peroxide (H₂O₂) is generated as a by-product of metabolic reactions during oxygen use by aerobic organisms, and can be toxic or participate in signaling processes. Cells, therefore, need to be able to sense and respond to H₂O₂ in an appropriate manner. This is often accomplished through thiol switches: Cysteine residues in proteins that can act as sensors, and which are both scarce and finely tuned. Bacteria and eukaryotes use different types of such sensors—either a one-component (OxyR) or two-component (Pap1-Tpx1) redox relay, respectively. However, the biological significance of these two different signaling modes is not fully understood, and the concentrations and peroxides driving those types of redox cascades have not been determined, nor the intracellular H₂O₂ levels linked to toxicity. Here we elucidate the characteristics, rates, and dynamic ranges of both systems.

Results

By comparing the activation of both systems in fission yeast, and applying mathematical equations to the experimental data, we estimate the toxic threshold of intracellular H₂O₂ able to halt aerobic growth, and the temporal gradients of extracellular to intracellular peroxides. By calculating both the oxidation rates of OxyR and Tpx1 by peroxides, and their reduction rates by the cellular redoxin systems, we propose that, while Tpx1 is a sensor and an efficient H₂O₂ scavenger because it displays fast oxidation and reduction rates, OxyR is strictly a H₂O₂ sensor, since its reduction kinetics are significantly slower than its oxidation by peroxides, and therefore, it remains oxidized long enough to execute its transcriptional role. We also show that these two paradigmatic H₂O₂-sensing models are biologically similar at pre-toxic peroxide levels, but display strikingly different activation behaviors at toxic doses.

Conclusions

Both Tpx1 and OxyR contain thiol switches, with very high reactivity towards peroxides. Nevertheless, the fast reduction of Tpx1 defines it as a scavenger, and this efficient recycling dramatically changes the Tpx1-Pap1 response to H₂O₂ and connects H₂O₂ sensing to the redox state of the cell. In contrast, OxyR is a true H₂O₂ sensor but not a scavenger, being partially insulated from the cellular electron donor capacity.

Electronic supplementary material

The online version of this article (10.1186/s12915-018-0523-6) contains supplementary material, which is available to authorized users.

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.

Hyperoxidation of Peroxiredoxins: Gain or Loss of Function?

SIGNIFICANCE

In 2003, structural studies revealed that eukaryotic 2-Cys peroxiredoxins (Prx) have evolved to be sensitive to inactivation of their thioredoxin peroxidase activity by hyperoxidation (sulfinylation) of their peroxide-reacting catalytic cysteine. This was accompanied by the unexpected discovery, that the sulfinylation of this cysteine was reversible in vivo and the identification of a new enzyme, sulfiredoxin, that had apparently co-evolved specifically to reduce hyperoxidized 2-Cys Prx, restoring their peroxidase activity. Together, these findings have provided the impetus for multiple studies investigating the purpose of this reversible, Prx hyperoxidation. Recent Advances: It has been suggested that inhibition of the thioredoxin peroxidase activity by hyperoxidation can both promote and inhibit peroxide signal transduction, depending on the context. Prx hyperoxidation has also been proposed to protect cells against reactive oxygen species (ROS)-induced damage, by preserving reduced thioredoxin and/or by increasing non-peroxidase chaperone or signaling activities of Prx.

CRITICAL ISSUES

Here, we will review the evidence in support of each of these proposed functions, in view of the in vivo contexts in which Prx hyperoxidation occurs, and the role of sulfiredoxin. Thus, we will attempt to explain the basis for seemingly contradictory roles for Prx hyperoxidation in redox signaling.

FUTURE DIRECTIONS

We provide a rationale, based on modeling and experimental studies, for why Prx hyperoxidation should be considered a suitable, early biomarker for damaging levels of ROS. We discuss the implications that this has for the role of Prx in aging and the detection of hyperoxidized Prx as a conserved feature of circadian rhythms. Antioxid. Redox Signal. 28, 574-590.

Role/s of 'Antioxidant' Enzymes in Ageing

Reactive oxygen species (ROS), generated externally and during aerobic metabolism, are a potent cause of cell damage. Oxidative damage is a feature of many diseases and ageing, including age-associated diseases, such as diabetes, cancer, cardiovascular and neurodegenerative diseases. Indeed, this association helped lead to the widely expounded 'Free Radical Theory of Aging', proposing that the accumulation of ROS-induced damage is the underlying cause of ageing. In the last decade, it has become apparent that ROS play more complex roles in ageing than simply causing damage. This includes the induction of signalling pathways that protect against/repair cell damage. Cells encode a variety of enzymes that metabolise ROS, some of which reduce them to less reactive species. In this chapter, we review the evidence that manipulating the levels of these enzymes has any effect/s on ageing. We will also highlight a few examples illustrating why it is an over-simplification to describe the activities of some of these enzymes as 'antioxidants'. We discuss how these studies have helped refine our view of how ROS and ROS-metabolising enzymes contribute to the ageing process.

Stress Adaptation

Fungal species display an extraordinarily diverse range of lifestyles. Nevertheless, the survival of each species depends on its ability to sense and respond to changes in its natural environment. Environmental changes such as fluctuations in temperature, water balance or pH, or exposure to chemical insults such as reactive oxygen and nitrogen species exert stresses that perturb cellular homeostasis and cause molecular damage to the fungal cell. Consequently, fungi have evolved mechanisms to repair this damage, detoxify chemical insults, and restore cellular homeostasis. Most stresses are fundamental in nature, and consequently, there has been significant evolutionary conservation in the nature of the resultant responses across the fungal kingdom and beyond. For example, heat shock generally induces the synthesis of chaperones that promote protein refolding, antioxidants are generally synthesized in response to an oxidative stress, and osmolyte levels are generally increased following a hyperosmotic shock. In this article we summarize the current understanding of these and other stress responses as well as the signaling pathways that regulate them in the fungi. Model yeasts such as Saccharomyces cerevisiae are compared with filamentous fungi, as well as with pathogens of plants and humans. We also discuss current challenges associated with defining the dynamics of stress responses and with the elaboration of fungal stress adaptation under conditions that reflect natural environments in which fungal cells may be exposed to different types of stresses, either sequentially or simultaneously.

Lack of a peroxiredoxin suppresses the lethality of cells devoid of electron donors by channelling electrons to oxidized ribonucleotide reductase

The thioredoxin and glutaredoxin pathways are responsible of recycling several enzymes which undergo intramolecular disulfide bond formation as part of their catalytic cycles such as the peroxide scavengers peroxiredoxins or the enzyme ribonucleotide reductase (RNR). RNR, the rate-limiting enzyme of deoxyribonucleotide synthesis, is an essential enzyme relying on these electron flow cascades for recycling. RNR is tightly regulated in a cell cycle-dependent manner at different levels, but little is known about the participation of electron donors in such regulation. Here, we show that cytosolic thioredoxins Trx1 and Trx3 are the primary electron donors for RNR in fission yeast. Unexpectedly, trx1 transcript and Trx1 protein levels are up-regulated in a G1-to-S phase-dependent manner, indicating that the supply of electron donors is also cell cycle-regulated. Indeed, genetic depletion of thioredoxins triggers a DNA replication checkpoint ruled by Rad3 and Cds1, with the final goal of up-regulating transcription of S phase genes and constitutive RNR synthesis. Regarding the thioredoxin and glutaredoxin cascades, one combination of gene deletions is synthetic lethal in fission yeast: cells lacking both thioredoxin reductase and cytosolic dithiol glutaredoxin. We have isolated a suppressor of this lethal phenotype: a mutation at the Tpx1-coding gene, leading to a frame shift and a loss-of-function of Tpx1, the main client of electron donors. We propose that in a mutant strain compromised in reducing equivalents, the absence of an abundant and competitive substrate such as the peroxiredoxin Tpx1 has been selected as a lethality suppressor to favor RNR function at the expense of the non-essential peroxide scavenging function, to allow DNA synthesis and cell growth.

The essential enzyme ribonucleotide reductase (RNR), the rate-limiting enzyme of deoxyribonucleotide synthesis, relies on the thioredoxin and glutaredoxin electron flow cascades for recycling. RNR is tightly regulated in a cell cycle-dependent manner at different levels. Here, we show that cytosolic thioredoxin Trx1 is the primary electron donor for RNR in fission yeast, and that trx1 transcript and protein levels are up-regulated at G1-to-S phase transition. Genetic depletion of thioredoxins triggers the DNA replication checkpoint up-regulating RNR synthesis. Furthermore, deletion of the genes coding for thioredoxin reductase and dithiol glutaredoxin is synthetic lethal, and we show that a loss-of-function mutation at the peroxiredoxin Tpx1-coding gene acts as a genetic suppressor. We propose that in a mutant strain compromised in reducing equivalents, the absence of an abundant and competitive substrate of redoxins, the peroxiredoxin Tpx1, has been selected as a lethality suppressor to favor channeling of electrons to the essential RNR.

Proteomic Characterization of Reversible Thiol Oxidations in Proteomes and Proteins

SIGNIFICANCE

Reactive oxygen species are produced during normal metabolism in cells, and their excesses have been implicated in protein damage and toxicity, as well as in the activation of signaling events. In particular, hydrogen peroxide participates in the regulation of different physiological processes as well as in the induction of antioxidant cascades, and often the redox molecular events triggering these pathways are based on reversible cysteine (Cys) oxidation. Recent Advances: Increases in peroxides can cause the accumulation of reversible Cys oxidations in proteomes, which may be either protecting thiols from irreversible oxidations or may just be reporters of future toxicity. It is also becoming clear, however, that only a few proteins, such as the bacterial OxyR or peroxidases, can suffer direct oxidation of their Cys residues by hydrogen peroxide and, therefore, may be the only true sensors initiating signaling events.

CRITICAL ISSUES

We will in this study describe some of the methodologies used to characterize at the proteome level reversible thiol oxidations, specifically those combining gel-free approaches with mass spectrometry. In the second part of this review, we will summarize some of the electrophoretic and proteomic techniques used to monitor Cys oxidation at the protein level, needed to confirm that a protein contains redox Cys involved in signaling relays, using as examples some of the best characterized redox sensors such as bacterial OxyR or yeast Tpx1/Pap1.

FUTURE DIRECTIONS

While Cys oxidations are often detected in proteomes and in specific proteins, major efforts have to be made to establish that they are physiologically relevant. Antioxid. Redox Signal. 26, 329-344.

Multiple Functions and Regulation of Mammalian Peroxiredoxins

Peroxiredoxins (Prxs) constitute a major family of peroxidases, with mammalian cells expressing six Prx isoforms (PrxI to PrxVI). Cells produce hydrogen peroxide (H₂O₂) at various intracellular locations where it can serve as a signaling molecule. Given that Prxs are abundant and possess a structure that renders the cysteine (Cys) residue at the active site highly sensitive to oxidation by H₂O₂, the signaling function of this oxidant requires extensive and highly localized regulation. Recent findings on the reversible regulation of PrxI through phosphorylation at the centrosome and on the hyperoxidation of the Cys at the active site of PrxIII in mitochondria are described in this review as examples of such local regulation of H₂O₂ signaling. Moreover, their high affinity for and sensitivity to oxidation by H₂O₂ confer on Prxs the ability to serve as sensors and transducers of H₂O₂ signaling through transfer of their oxidation state to bound effector proteins.

Oxidative stress response pathways: Fission yeast as archetype

Schizosaccharomyces pombe is a popular model eukaryotic organism to study diverse aspects of mammalian biology, including responses to cellular stress triggered by redox imbalances within its compartments. The review considers the current knowledge on the signaling pathways that govern the transcriptional response of fission yeast cells to elevated levels of hydrogen peroxide. Particular attention is paid to the mechanisms that yeast cells employ to promote cell survival in conditions of intermediate and acute oxidative stress. The role of the Sty1/Spc1/Phh1 mitogen-activated protein kinase in regulating gene expression at multiple levels is discussed in detail.

Increasing extracellular H2O2 produces a bi-phasic response in intracellular H2O2, with peroxiredoxin hyperoxidation only triggered once the cellular H2O2-buffering capacity is overwhelmed

Reactive oxygen species, such as H2O2, can damage cells but also promote fundamental processes, including growth, differentiation and migration. The mechanisms allowing cells to differentially respond to toxic or signaling H2O2 levels are poorly defined. Here we reveal that increasing external H2O2 produces a bi-phasic response in intracellular H2O2. Peroxiredoxins (Prx) are abundant peroxidases which protect against genome instability, ageing and cancer. We have developed a dynamic model simulating in vivo changes in Prx oxidation. Remarkably, we show that the thioredoxin peroxidase activity of Prx does not provide any significant protection against external rises in H2O2. Instead, our model and experimental data are consistent with low levels of extracellular H2O2 being efficiently buffered by other thioredoxin-dependent activities, including H2O2-reactive cysteines in the thiol-proteome. We show that when extracellular H2O2 levels overwhelm this buffering capacity, the consequent rise in intracellular H2O2 triggers hyperoxidation of Prx to thioredoxin-resistant, peroxidase-inactive form/s. Accordingly, Prx hyperoxidation signals that H2O2 defenses are breached, diverting thioredoxin to repair damage.

Diverse fission yeast genes required for responding to oxidative and metal stress: Comparative analysis of glutathione-related and other defense gene deletions

Living organisms have evolved multiple sophisticated mechanisms to deal with reactive oxygen species. We constructed a collection of twelve single-gene deletion strains of the fission yeast Schizosaccharomyces pombe designed for the study of oxidative and heavy metal stress responses. This collection contains deletions of biosynthetic enzymes of glutathione (Δgcs1 and Δgsa1), phytochelatin (Δpcs2), ubiquinone (Δabc1) and ergothioneine (Δegt1), as well as catalase (Δctt1), thioredoxins (Δtrx1 and Δtrx2), Cu/Zn- and Mn- superoxide dismutases (SODs; Δsod1 and Δsod2), sulfiredoxin (Δsrx1) and sulfide-quinone oxidoreductase (Δhmt2). First, we employed metabolomic analysis to examine the mutants of the glutathione biosynthetic pathway. We found that ophthalmic acid was produced by the same enzymes as glutathione in S. pombe. The identical genetic background of the strains allowed us to assess the severity of the individual gene knockouts by treating the deletion strains with oxidative agents. Among other results, we found that glutathione deletion strains were not particularly sensitive to peroxide or superoxide, but highly sensitive to cadmium stress. Our results show the astonishing diversity in cellular adaptation mechanisms to various types of oxidative and metal stress and provide a useful tool for further research into stress responses.

Peroxiredoxins in Regulation of MAPK Signalling Pathways; Sensors and Barriers to Signal Transduction

Peroxiredoxins are highly conserved and abundant peroxidases. Although the thioredoxin peroxidase activity of peroxiredoxin (Prx) is important to maintain low levels of endogenous hydrogen peroxide, Prx have also been shown to promote hydrogen peroxide-mediated signalling. Mitogen activated protein kinase (MAPK) signalling pathways mediate cellular responses to a variety of stimuli, including reactive oxygen species (ROS). Here we review the evidence that Prx can act as both sensors and barriers to the activation of MAPK and discuss the underlying mechanisms involved, focusing in particular on the relationship with thioredoxin.

Physiological relevance of plant 2-Cys peroxiredoxin overoxidation level and oligomerization status

Peroxiredoxins are ubiquitous thioredoxin-dependent peroxidases presumed to display, upon environmental constraints, a chaperone function resulting from a redox-dependent conformational switch. In this work, using biochemical and genetic approaches, we aimed to unravel the factors regulating the redox status and the conformation of the plastidial 2-Cys peroxiredoxin (2-Cys PRX) in plants. In Arabidopsis, we show that in optimal growth conditions, the overoxidation level mainly depends on the availability of thioredoxin-related electron donors, but not on sulfiredoxin, the enzyme reducing the 2-Cys PRX overoxidized form. We also observed that upon various physiological temperature, osmotic and light stress conditions, the overoxidation level and oligomerization status of 2-Cys PRX can moderately vary depending on the constraint type. Further, no major change was noticed regarding protein conformation in water-stressed Arabidopsis, barley and potato plants, whereas species-dependent up- and down-variations in overoxidation were observed. In contrast, both 2-Cys PRX overoxidation and oligomerization were strongly induced during a severe oxidative stress generated by methyl viologen. From these data, revealing that the oligomerization status of plant 2-Cys PRX does not exhibit important variation and is not tightly linked to the protein redox status upon physiologically relevant environmental constraints, the possible in planta functions of 2-Cys PRX are discussed.

Microbial 2-Cys Peroxiredoxins: Insights into Their Complex Physiological Roles

The peroxiredoxins (Prxs) constitute a very large and highly conserved family of thiol-based peroxidases that has been discovered only very recently. We consider here these enzymes through the angle of their discovery, and of some features of their molecular and physiological functions, focusing on complex phenotypes of the gene mutations of the 2-Cys Prxs subtype in yeast. As scavengers of the low levels of H2O2 and as H2O2 receptors and transducers, 2-Cys Prxs have been highly instrumental to understand the biological impact of H2O2, and in particular its signaling function. 2-Cys Prxs can also become potent chaperone holdases, and unveiling the in vivo relevance of this function, which is still not established, should further increase our knowledge of the biological impact and toxicity of H2O2. The diverse molecular functions of 2-Cys Prx explain the often-hard task of relating them to peroxiredoxin genes phenotypes, which underscores the pleiotropic physiological role of these enzymes and complex biologic impact of H2O2.

Sulfiredoxin Promotes Colorectal Cancer Cell Invasion and Metastasis through a Novel Mechanism of Enhancing EGFR Signaling

Sulfiredoxin (SRXN1/Srx) is a multifunction enzyme with a primary antioxidant role of reducing the overoxidized inactive form of peroxiredoxins (Prxs). The function and mechanisms of Srx in cancer development are not well understood. Here, Srx is preferentially expressed in human colorectal cancer cells but not in normal colon epithelial cells. Loss-of-function studies demonstrate that knockdown of Srx in poorly differentiated colorectal cancer cells not only leads to the inhibition of colony formation and cell invasion in vitro, but also reduces tumor xenograft growth and represses metastasis to distal organs in a mouse orthotopic implantation model. Notably, exactly opposite effects were observed in gain-of-function experiments when Srx was ectopically expressed in well-differentiated colorectal cancer cells. Mechanistically, expression of Srx enhances the activation of MAPK signaling through increasing the C-terminal tyrosine phosphorylation levels of EGFR. This function of Srx is mediated through its inhibition of EGFR acetylation at K1037, a novel posttranslational modification of EGFR in human colorectal cancer cells identified by liquid chromatography-electrospray ionization-tandem mass spectrometry (LC/ESI/MS-MS) proteomic analysis. Furthermore, abolishment of K1037 acetylation in human colorectal cancer cells by site-specific mutagenesis leads to sustained activation of EGFR-MAPK signaling. Combined, these data reveal that Srx promotes colorectal cancer cell invasion and metastasis through a novel mechanism of enhancing EGFR signaling.

IMPLICATIONS

Sulfiredoxin is a critical oncogenic protein that can be used as a molecular target to develop therapeutics for patients with metastatic colorectal cancer.

Peroxynitrite and hydrogen peroxide elicit similar cellular stress responses mediated by the Ccp1 sensor protein

Peroxynitrite [ONOO(H)] is an oxidant associated with deleterious effects in cells. Because it is an inorganic peroxide that reacts rapidly with peroxidases, we speculated that cells may respond to ONOO(H) and H2O2 challenge in a similar manner. We exposed yeast cells to SIN-1, a well-characterized ONOO(H) generator, and observed stimulation of catalase and peroxiredoxin (Prx) activities. Previously, we reported that H2O2 challenge increases these activities in wild-type cells and in cells producing the hyperactive mutant H2O2 sensor Ccp1(W191F) but not in Ccp1-knockout cells (ccp1Δ). We find here that the response of ccp1Δ and ccp1(W191F) cells to SIN-1 mirrors that to H2O2, identifying Ccp1 as a sensor of both peroxides. SIN-1 simultaneously releases (•)NO and O2(•-), which react to form ONOO(H), but exposure of the three strains separately to an (•)NO donor (spermine-NONOate) or an O2(•-) generator (paraquat) mainly depresses catalase or Prx activity, whereas co-challenge with the NONOate and paraquat stimulates these activities. Because Ccp1 appears to sense ONOO(H) in cells, we examined its reaction with ONOO(H) in vitro and found that peroxynitrous acid (ONOOH) rapidly (k2>10(6)M(-1)s(-1)) oxidizes purified Ccp1 to an intermediate with spectral and ferrocytochrome-oxidizing properties indistinguishable from those of its well-characterized compound I formed with H2O2. Importantly, the nitrite released from ONOOH is not oxidized to (•)NO2 by Ccp1(׳)s compound I, unlike peroxidases involved in immune defense. Overall, our results reveal that yeast cells mount a common antioxidant response to ONOO(H) and H2O2, with Ccp1 playing a pivotal role as an inorganic peroxide sensor.

Intermolecular disulfide-dependent redox signalling

Until recently, ROS (reactive oxygen species) were often seen as merely damaging agents. However, small, but significant, amounts of hydrogen peroxide (H2O2) are also being produced upon, for instance, NADPH-oxidase activation in response to growth factor signalling and as a by-product of mitochondrial respiration. H2O2 perturbs the local cellular redox state and this results in specific and reversible cysteine oxidation in target proteins, thereby translating the redox state into a signal that ultimately leads to an appropriate cellular response. This phenomenon of signalling through cysteine oxidation is known as redox signalling and has recently been shown to be involved in a wide range of physiological processes. Cysteine residue oxidation can lead to a range of post-translational modifications, one of which is the formation of intermolecular disulfides. In the present mini-review we will give a number of examples of proteins regulated by intermolecular disulfides and discuss a recently developed method to screen for these interactions. The consequences of the regulation of the FOXO4 (forkhead box O4) transcription factor by formation of intermolecular disulfides with both TNPO1 (transportin 1) and p300/CBP [CREB (cAMP-response-element-binding protein)-binding protein] are discussed in more detail.

Role of sulfiredoxin in systemic diseases influenced by oxidative stress

Sulfiredoxin is a recently discovered member of the oxidoreductases family which plays a crucial role in thiol homoeostasis when under oxidative stress. A myriad of systemic disorders have oxidative stress and reactive oxygen species as the key components in their etiopathogenesis. Recent studies have evaluated the role of this enzyme in oxidative stress mediated diseases such as atherosclerosis, chronic obstructive pulmonary disease and a wide array of carcinomas. Its action is responsible for the normal functioning of cells under oxidative stress and the promotion of cell survival in cancerous cells. This review will highlight the cumulative effects of sulfiredoxin in various systemic disorders with a strong emphasis on its target activity and the factors influencing its expression in such conditions.

The fission yeast Schizosaccharomyces pombe as a model to understand how peroxiredoxins influence cell responses to hydrogen peroxide

As a more selectively reactive oxygen species, H2O2 (hydrogen peroxide) has been co-opted as a signalling molecule, but high levels can still lead to lethal amounts of cell damage. 2-Cys Prxs (peroxiredoxins) are ubiquitous thioredoxin peroxidases which utilize reversibly oxidized catalytic cysteine residues to reduce peroxides. As such, Prxs potentially make an important contribution to the repertoire of cell defences against oxidative damage. Although the abundance of eukaryotic 2-Cys Prxs suggests an important role in maintaining cell redox, the surprising sensitivity of their thioredoxin peroxidase activity to inactivation by H2O2 has raised questions as to their role as an oxidative stress defence. Indeed, work in model yeast has led the way in revealing that Prxs do much more than simply remove peroxides and have even uncovered circumstances where their thioredoxin peroxidase activity is detrimental. In the present paper, we focus on what we have learned from studies in the fission yeast Schizosaccharomyces pombe about the different roles of 2-Cys Prxs in responses to H2O2 and discuss the general implications of these findings for other systems.

A genetic approach to study H2O2 scavenging in fission yeast--distinct roles of peroxiredoxin and catalase

The main peroxiredoxin in Schizosaccharomyces pombe, Tpx1, is important to sustain aerobic growth, and cells lacking this protein are only able to grow on solid plates under anaerobic conditions. We have found that deletion of the gene coding for thioredoxin reductase, trr1, is a suppressor of the sensitivity to aerobic growth of Δtpx1 cells, so that cells lacking both proteins are able to grow on solid plates in the presence of oxygen. We have investigated this suppression effect, and determined that it depends on the presence of catalase, which is constitutively expressed in Δtrr1 cells in a transcription factor Pap1-dependent manner. A complete characterization of the repertoire of hydrogen peroxide scavenging activities in fission yeast suggests that Tpx1 is the only enzyme with sufficient sensitivity for peroxides and cellular abundance as to control the low levels produced during aerobic growth, catalase being the next barrier of detoxification when the steady-state levels of peroxides are increased in Δtpx1 cells. Gpx1, the only glutathione peroxidase encoded by the S. pombe genome, only has a minor secondary role when extracellular peroxides are added. Our study proposes non-overlapping roles for the different hydrogen peroxide scavenging activities of this eukaryotic organism.

The oxidative stress responsive transcription factor Pap1 confers DNA damage resistance on checkpoint-deficient fission yeast cells

Eukaryotic cells invoke mechanisms to promote survival when confronted with cellular stress or damage to the genome. The protein kinase Chk1 is an integral and conserved component of the DNA damage response pathway. Mutation or inhibition of Chk1 results in mitotic death when cells are exposed to DNA damage. Oxidative stress activates a pathway that results in nuclear accumulation of the bZIP transcription factor Pap1. We report the novel finding that fission yeast Pap1 confers resistance to drug- and non-drug-induced DNA damage even when the DNA damage checkpoint is compromised. Multi-copy expression of Pap1 restores growth to chk1-deficient cells exposed to camptothecin or hydroxyurea. Unexpectedly, increased Pap1 expression also promotes survival of chk1-deficient cells with mutations in genes encoding DNA ligase (cdc17) or DNA polymerase δ (cdc6), but not DNA replication initiation mutants. The ability of Pap1 to confer resistance to DNA damage was not specific to chk1 mutants, as it also improved survival of rad1- and rad9-deficient cells in the presence of CPT. To confer resistance to DNA damage Pap1 must localize to the nucleus and be transcriptionally active.

Thiol-based H2O2 signalling in microbial systems

Cysteine residues, and in particular their thiolate groups, react not only with reactive oxygen species but also with electrophiles and with reactive nitrogen species. Thus, cysteine oxidation has often been linked to the toxic effects of some of these reactive molecules. However, thiol-based switches are common in protein sensors of antioxidant cascades, in both prokaryotic and eukaryotic organisms. We will describe here three redox sensors, the transcription factors OxyR, Yap1 and Pap1, which respond by disulfide bond formation to hydrogen peroxide stress, focusing specially on the differences among the three peroxide-sensing mechanisms.

Ybp1 and Gpx3 signaling in Candida albicans govern hydrogen peroxide-induced oxidation of the Cap1 transcription factor and macrophage escape

AIMS

As Candida albicans is the major fungal pathogen of humans, there is an urgent need to understand how this pathogen evades toxic reactive oxygen species (ROS) generated by the host immune system. A key regulator of antioxidant gene expression, and thus ROS resistance, in C. albicans is the AP-1-like transcription factor Cap1. Despite this, little is known regarding the intracellular signaling mechanisms that underlie the oxidation and activation of Cap1. Therefore, the aims of this study were; (i) to identify the regulatory proteins that govern Cap1 oxidation, and (ii) to investigate the importance of Cap1 oxidation in C. albicans pathogenesis.

RESULTS

In response to hydrogen peroxide (H2O2), but not glutathione-depleting/modifying oxidants, Cap1 oxidation, nuclear accumulation, phosphorylation, and Cap1-dependent gene expression, is mediated by a glutathione peroxidase-like enzyme, which we name Gpx3, and an orthologue of the Saccharomyces cerevisiae Yap1 binding protein, Ybp1. In addition, Ybp1 also functions to stabilise Cap1 and this novel function is conserved in S. cerevisiae. C. albicans cells lacking Cap1, Ybp1, or Gpx3, are unable to filament and thus, escape from murine macrophages after phagocytosis, and also display defective virulence in the Galleria mellonella infection model.

INNOVATION

Ybp1 is required to promote the stability of fungal AP-1-like transcription factors, and Ybp1 and Gpx3 mediated Cap1-dependent oxidative stress responses are essential for the effective killing of macrophages by C. albicans.

CONCLUSION

Activation of Cap1, specifically by H2O2, is a prerequisite for the subsequent filamentation and escape of this fungal pathogen from the macrophage.

A peroxiredoxin promotes H2O2 signaling and oxidative stress resistance by oxidizing a thioredoxin family protein

H₂O₂ can cause oxidative damage associated with age-related diseases such as diabetes and cancer but is also used to initiate diverse responses, including increased antioxidant gene expression. Despite significant interest, H₂O₂-signaling mechanisms remain poorly understood. Here, we present a mechanism for the propagation of an H₂O₂ signal that is vital for the adaptation of the model yeast, Schizosaccharomyces pombe, to oxidative stress. Peroxiredoxins are abundant peroxidases with conserved antiaging and anticancer activities. Remarkably, we find that the only essential function for the thioredoxin peroxidase activity of the Prx Tpx1(hPrx1/2) in resistance to H₂O₂ is to inhibit a conserved thioredoxin family protein Txl1(hTxnl1/TRP32). Thioredoxins regulate many enzymes and signaling proteins. Thus, our discovery that a Prx amplifies an H₂O₂ signal by driving the oxidation of a thioredoxin-like protein has important implications, both for Prx function in oxidative stress resistance and for responses to H₂O₂.

•

The thioredoxin-like protein Txl1 is oxidized in response to H₂O₂

•

The thioredoxin peroxidase activity of the Prx Tpx1 is required for oxidation of Txl1

•

The AP-1-like transcription factor Pap1 is an in vivo substrate for Txl1

•

Tpx1’s thioredoxin peroxidase activity provides H₂O₂ resistance by regulating Txl1