Cooperation of yeast peroxiredoxins Tsa1p and Tsa2p in the cellular defense against oxidative and nitrosative stress

EOS1, whose deletion confers sensitivity to oxidative stress, is involved in N-glycosylation in Saccharomyces cerevisiae

The deletion strain of Saccharomyces cerevisiae YNL080c (designed as EOS1) was identified as a strain sensitive to high-sucrose stress in our previous report [A. Ando, F. Tanaka, Y. Murata, H. Takagi, J. Shima, Identification and classification of genes required for tolerance to high sucrose stress revealed by genome-wide screening of Saccharomyces cerevisiae, FEMS Yeast Res. 6 (2006) 249-267]. Delta eos1 showed higher sensitivity to oxidative stress than to high-sucrose stress. Immunofluorescence microscopic and cellular fractionation analyses suggested that Eos1 localizes in the endoplasmic reticulum membrane. We found that the deletion of EOS1 enhances tunicamycin tolerance and that in Delta eos1 the transcription level of KAR2, which is the ER stress-inducible gene, was much lower than that in the wild-type strain (BY4741) when exposed to tunicamycin. The inhibition of the N-glycosylation of carboxypeptidase Y and invertase activity caused by the addition of tunicamycin was depressed in Delta eos1, suggesting that EOS1 may be involved in N-glycosylation of the cellular proteins.

Specific aquaporins facilitate the diffusion of hydrogen peroxide across membranes

The metabolism of aerobic organisms continuously produces reactive oxygen species. Although potentially toxic, these compounds also function in signaling. One important feature of signaling compounds is their ability to move between different compartments, e.g. to cross membranes. Here we present evidence that aquaporins can channel hydrogen peroxide (H2O2). Twenty-four aquaporins from plants and mammals were screened in five yeast strains differing in sensitivity toward oxidative stress. Expression of human AQP8 and plant Arabidopsis TIP1;1 and TIP1;2 in yeast decreased growth and survival in the presence of H2O2. Further evidence for aquaporin-mediated H2O2 diffusion was obtained by a fluorescence assay with intact yeast cells using an intracellular reactive oxygen species-sensitive fluorescent dye. Application of silver ions (Ag+), which block aquaporin-mediated water diffusion in a fast kinetics swelling assay, also reversed both the aquaporin-dependent growth repression and the H2O2-induced fluorescence. Our results present the first molecular genetic evidence for the diffusion of H2O2 through specific members of the aquaporin family.

Arrhythmia and neuronal/endothelial myocyte uncoupling in hyperhomocysteinemia

Elevated levels of homocysteine (Hcy) known as hyperhomocysteinemia (HHcy) are associated with arrhythmogenesis and sudden cardiac death (SCD). Hcy decreases constitutive neuronal and endothelial nitric oxide (NO), and cardiac diastolic relaxation. Hcy increases the iNOS/NO, peroxynitrite, mitochondrial NADPH oxidase, and suppresses superoxide dismutase (SOD) and redoxins. Hcy activates matrix metalloproteinase (MMP), disrupts connexin-43 and increases collagen/elastin ratio. The disruption of connexin-43 and accumulation of collagen (fibrosis) disrupt the normal pattern of cardiac conduction and attenuate NO transport from endothelium to myocyte (E-M) causing E-M uncoupling, leading to a pro-arrhythmic environment. The goal of this review is to elaborate the mechanism of Hcy-mediated iNOS/NO in E-M uncoupling and SCD. It is known that Hcy creates arrhythmogenic substrates (i.e. increase in collagen/elastin ratio and disruption in connexin-43) and exacerbates heart failure during chronic volume overload. Also, Hcy behaves as an agonist to N-methyl-D-aspartate (NMDA, an excitatory neurotransmitter) receptor-1, and blockade of NMDA-R1 reduces the increase in heart rate-evoked by NMDA-analog and reduces SCD. This review suggest that Hcy increases iNOS/NO, superoxide, metalloproteinase activity, and disrupts connexin-43, exacerbates endothelial-myocyte uncoupling and cardiac failure secondary to inducing NMDA-R1.

The diverse antioxidant systems of Helicobacter pylori

The gastric pathogen Helicobacter pylori induces a strong inflammatory host response, yet the bacterium maintains long-term persistence in the host. H. pylori combats oxidative stress via a battery of diverse activities, some of which are unique or newly described. In addition to using the well-studied bacterial oxidative stress resistance enzymes superoxide dismutase and catalase, H. pylori depends on a family of peroxiredoxins (alkylhydroperoxide reductase, bacterioferritin co-migratory protein and a thiol-peroxidase) that function to detoxify organic peroxides. Newly described antioxidant proteins include a soluble NADPH quinone reductase (MdaB) and an iron sequestering protein (NapA) that has dual roles - host inflammation stimulation and minimizing reactive oxygen species production within H. pylori. An H. pylori arginase attenuates host inflammation, a thioredoxin required as a reductant for many oxidative stress enzymes is also a chaperon, and some novel properties of KatA and AhpC were discovered. To repair oxidative DNA damage, H. pylori uses an endonuclease (Nth), DNA recombination pathways and a newly described type of bacterial MutS2 that specifically recognizes 8-oxoguanine. A methionine sulphoxide reductase (Msr) plays a role in reducing the overall oxidized protein content of the cell, although it specifically targets oxidized Met residues. H. pylori possess few stress regulator proteins, but the key roles of a ferric uptake regulator (Fur) and a post-transcriptional regulator CsrA in antioxidant protein expression are described. The roles of all of these antioxidant systems have been addressed by a targeted mutant analysis approach and almost all are shown to be important in host colonization. The described antioxidant systems in H. pylori are expected to be relevant to many bacterial-associated diseases, as genes for most of the enzymes carrying out the newly described roles are present in a number of pathogenic bacteria.

A mosquito 2-Cys peroxiredoxin protects against nitrosative and oxidative stresses associated with malaria parasite infection

Malaria parasite infection in anopheline mosquitoes induces nitrosative and oxidative stresses that limit parasite development, but also damage mosquito tissues in proximity to the response. Based on these observations, we proposed that cellular defenses in the mosquito may be induced to minimize self-damage. Specifically, we hypothesized that peroxiredoxins (Prxs), enzymes known to detoxify reactive oxygen species (ROS) and reactive nitrogen oxide species (RNOS), protect mosquito cells. We identified an Anopheles stephensi 2-Cys Prx ortholog of Drosophila melanogaster Prx-4783, which protects fly cells against oxidative stresses. To assess function, AsPrx-4783 was overexpressed in D. melanogaster S2 and in A. stephensi (MSQ43) cells and silenced in MSQ43 cells with RNA interference before treatment with various ROS and RNOS. Our data revealed that AsPrx-4783 and DmPrx-4783 differ in host cell protection and that AsPrx-4783 protects A. stephensi cells against stresses that are relevant to malaria parasite infection in vivo, namely nitric oxide (NO), hydrogen peroxide, nitroxyl, and peroxynitrite. Further, AsPrx-4783 expression is induced in the mosquito midgut by parasite infection at times associated with peak nitrosative and oxidative stresses. Hence, whereas the NO-mediated defense response is toxic to both host and parasite, AsPrx-4783 may shift the balance in favor of the mosquito.

Plasmodium falciparum 2-Cys peroxiredoxin reacts with plasmoredoxin and peroxynitrite

Thioredoxin peroxidase 1 (TPx1) of the malarial parasite Plasmodium falciparum is a 2-Cys peroxiredoxin involved in the detoxification of reactive oxygen species and - as shown here - of reactive nitrogen species. As novel electron acceptor of reduced TPx1, we characterised peroxynitrite; the rate constant for ONOO- reduction by the enzyme (1 x 10(6) M(-1) s(-1) at pH 7.4 and 37 degrees C) was determined by stopped-flow measurements. As reducing substrate of TPx1, we identified - aside from thioredoxin - plasmoredoxin; this 22-kDa protein occurs only in malarial parasites. When studying the potential roles of Cys74 and Cys170 of Tpx1 in catalysis, as well as in oligomerisation behaviour, we found that replacement of Cys74 by Ala influenced neither the dimerisation nor enzymatic activity of TPx1. In the C170A mutant, however, the kcat/Km for reduced Trx as a substrate was shown to be approximately 50-fold lower and, in contrast to the wild-type enzyme, covalently linked dimers were not formed. For the catalytic cycle of TPx1, we conclude that oxidation of the peroxidatic Cys50 by the oxidising substrate is followed by the formation of an intermolecular disulfide bond between Cys50 and Cys170' of the second subunit, which is then attacked by an external electron donor such as thioredoxin or plasmoredoxin.

Effect of nitrosative stress on Schizosaccharomyces pombe: inactivation of glutathione reductase by peroxynitrite

Oxidative stress has been shown to alter cellular redox status in various cell types. Changes in expressions of several antioxidative and antistress-responsive genes along with activation or inactivation of various proteins were also reported during oxidative insult as well as during nitrosative stress. In the present study, we show the effect of nitrosative stress on cellular redox status of fission yeast Schizosaccharomyces pombe. This is the first report of S-nitrosoglutathione (GSNO) reductase activity in S. pombe and its inactivation by GSNO. We also show the inactivation of glutathione reductase (GR) and glutathione peroxidase in the presence of various reactive nitrogen species in vivo. In addition, we first observe the inactivation of GR by peroxynitrite in vivo using S. pombe cells and also similar observations under in vitro conditions. An immunoreactive band against monoclonal anti-3-nitrotyrosine antibody confirms the modification of GR under in vitro conditions. We also show the effect of nitrosative stress on Deltapap1 cells of S. pombe, which are more sensitive to nitrosative stress, indicating the involvement of Pap1 in the protection against nitrosative stress. Finally, exposure of S. pombe cells to reactive nitrogen species reveals an important role of cellular thiol pool in protection against nitrosative stress.

The role of peroxiredoxin II in radiation-resistant MCF-7 breast cancer cells

Although several signaling pathways have been suggested to be involved in the cellular response to ionizing radiation, the molecular basis of tumor resistance to radiation remains elusive. We have developed a unique model system based upon the MCF-7 human breast cancer cell line that became resistant to radiation treatment (MCF+FIR30) after exposure to chronic ionizing radiation. By proteomics analysis, we found that peroxiredoxin II (PrxII), a member of a family of peroxidases, is up-regulated in the radiation-derived MCF+FIR3 cells but not in the MCF+FIS4 cells that are relatively sensitive to radiation. Both MCF+FIR3 and MCF+FIS4 cell lines are from MCF+FIR30 populations. Furthermore, the resistance to ionizing radiation can be partially reversed by silencing the expression of PrxII by PrxII/small interfering RNA treatment of MCF+FIR3 resistant cells, suggesting that PrxII is not the sole factor responsible for the resistant phenotype. The relevance of this mechanism was further confirmed by the increased radioresistance in PrxII-overexpressing MCF+FIS4 cells when compared with vector control cells. The up-regulation of the PrxII protein in radioresistant cancer cells suggested that human peroxiredoxin plays an important role in eliminating the generation of reactive oxygen species by ionizing radiation. The present finding, together with the observation that PrxII is also up-regulated in response to ionizing radiation in other cell systems, strengthens the hypothesis that the PrxII antioxidant protein is involved in the cellular response to ionizing radiation and functions to reduce the intracellular reactive oxygen species levels, resulting in increased resistance of breast cancer cells to ionizing radiation.

Cigarette smoke extract induces changes in growth and gene expression of Saccharomyces cerevisiae

The response of Saccharomyces cerevisiae cells to an aqueous extract of cigarette smoke was studied. Exposure to cigarette smoke extract inhibits yeast growth and results in global changes in gene expression spanning many functional classes of genes. Genes involved in response to oxidative stress are upregulated after a brief exposure to cigarette smoke extract. The effects of cigarette smoke extract on yeast growth can be reversed by treatment with anti-oxidants. Mutants lacking superoxide dismutase gene were hypersensitive to cigarette smoke exposure. YAP1 is a central transcriptional regulator of oxidative stress in yeast. YAP1 dependent expression of beta-galactosidase was enhanced following exposure to cigarette smoke. The overall agreement between our observations and the recently reported effects of cigarette smoke on gene expression in rodent and human cells suggests that yeast can be used as a model system in toxicogenomics studies for monitoring toxic agents and studying the cellular and molecular consequences of exposure to potentially toxic agents.

The moonlighting protein Tsa1p is implicated in oxidative stress response and in cell wall biogenesis inCandida albicans

Candida albicans is one of the most common fungal pathogens in humans. The cell wall is the first contact site between host and pathogen and thus is critical for colonization and infection of the host. We have identified Tsa1p, a protein that is differentially localized to the cell wall of C. albicans in hyphal cells but remains in the cytosol and nucleus in yeast-form cells. This is different from Saccharomyces cerevisiae, where the homologous protein solely has been found in the cytoplasm. We report here that TSA1 confers resistance towards oxidative stress as well as is involved in the correct composition of hyphal cell walls. However, no significant change of the cell wall composition was observed in a TSA1 deletion strain in yeast-form cells, which is in good agreement with the observation that Tsa1p is absent from the yeast-form cell wall. This indicates that Tsa1p of C. albicans might represent a moonlighting protein with specific functions correlating to its respective localization. Furthermore, the translocation of Tsa1p to the hyphal cell wall of C. albicans depends on Efg1p, suggesting a contribution of the cAMP/PKA pathway to the localization of this protein. In a strain deleted for TUP1 that filaments constitutively Tsa1p can be found in the cell wall under all conditions tested, confirming the result that Tsa1p localization to the cell wall is correlated to the morphology of C. albicans.

Peroxiredoxins: a historical overview and speculative preview of novel mechanisms and emerging concepts in cell signaling

The observation that purified yeast glutamine synthetase is rapidly inactivated in a thiol-containing buffer yet retains activity in crude extracts containing the same thiol led to our discovery of an enzyme that protects against oxidation in a thiol-containing system. This novel antioxidant enzyme was shown to reduce hydroperoxides and, more recently, peroxynitrite with the use of electrons provided by a physiological thiol like thioredoxin. It defined a family of proteins, present in organisms from all kingdoms, that was named peroxiredoxin (Prx). All Prx enzymes contain a conserved Cys residue that undergoes a cycle of peroxide-dependent oxidation and thiol-dependent reduction during catalysis. Mammalian cells express six isoforms of Prx (Prx I to VI), which are classified into three subgroups (2-Cys, atypical 2-Cys, and 1-Cys) based on the number and position of Cys residues that participate in catalysis. The relative abundance of Prx enzymes in mammalian cells appears to protect cellular components by removing the low levels of peroxides produced as a result of normal cellular metabolism. During catalysis, the active site cysteine is occasionally overoxidized to cysteine sulfinic acid. Contrary to the general belief that oxidation to the sulfinic state is an irreversible process in cells, studies on the fate of the overoxidized Prx species revealed a mechanism by which the catalytically active thiol form is recovered. This sulfinic reduction is a slow, ATP-dependent process that is specific to 2-Cys Prx isoforms. This reversible overoxidation may represent an adaptation unique to eukaryotic cells that accommodates the intracellular messenger function of H(2)O(2), but experimental validation of such speculation is yet to come.

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

Although activation of the AP-1-like transcription factor Pap1 in Schizosaccharomyces pombe is important for oxidative stress-induced gene expression, this activation is delayed at higher concentrations of peroxide. Here, we reveal that the 2-Cys peroxiredoxin (2-Cys Prx) Tpx1 is required for the peroxide-induced activation of Pap1. Tpx1, like other eukaryotic 2-Cys Prxs, is highly sensitive to oxidation, which inactivates its thioredoxin peroxidase activity. Our data suggest that the reduced thioredoxin peroxidase-active form of Tpx1 is required for the peroxide-induced oxidation and nuclear accumulation of Pap1. Indeed, in contrast to the previously described role for Tpx1 in the activation of the Sty1 stress-activated protein kinase by peroxide, we find that both catalytic cysteines of Tpx1 are required for Pap1 activation. Moreover, overexpression of the conserved sulfiredoxin Srx1, which interacts with and reduces Tpx1, allows rapid activation of Pap1 at higher concentrations of H(2)O(2). Conversely, loss of Srx1 prevents the reduction of oxidized Tpx1 and prolongs the inhibition of Pap1 activation. Collectively, these data suggest that redox regulation of the thioredoxin peroxidase activity of Tpx1 acts as a molecular switch controlling the transcriptional response to H(2)O(2). Furthermore, they reveal that a single eukaryotic 2-Cys Prx regulates peroxide signaling by multiple independent mechanisms.

Human peroxiredoxin 5 is a peroxynitrite reductase

Peroxiredoxins are an ubiquitous family of peroxidases widely distributed among prokaryotes and eukaryotes. Peroxiredoxin 5, which is the last discovered mammalian member, was previously shown to reduce peroxides with the use of reducing equivalents derived from thioredoxin. We report here that human peroxiredoxin 5 is also a peroxynitrite reductase. Analysis of peroxiredoxin 5 mutants, in which each of the cysteine residues was mutated, suggests that the nucleophilic attack on the O-O bond of peroxynitrite is performed by the N-terminal peroxidatic Cys(47). Moreover, with the use of pulse radiolysis, we show that human peroxiredoxin 5 reduces peroxynitrite with an unequalled high rate constant of (7+/-3)x10(7) M(-1)s(-1).

Cytosolic Thioredoxin Peroxidase I and II Are Important Defenses of Yeast against Organic Hydroperoxide Insult

The cytosolic thioredoxin peroxidase II (cTPxII/Tsa2p) from Saccharomyces cerevisiae shares 86% identity with the relatively well characterized cytosolic thioredoxin peroxidase I (cTPxI/Tsa1p). In contrast to cTPxI protein, cTPxII is not abundant and is highly inducible by peroxides. Here, we describe a unique phenotype for DeltacTPxII strain; these cells were highly sensitive to tert-butylhydroperoxide (TBHP) but presented resistance to H(2)O(2) in fermentative and respiratory conditions. In contrast, DeltacTPxI strain was very sensitive to both TBHP and H(2)O(2), whatever the carbon source present in the media. These differences in the response of mutant cells to the different kinds of peroxide insult could not be attributed to enzymatic properties of cTPxI and cTPxII since the recombinant proteins showed similar in vitro efficiencies (K(cat) /K(m)) in the removals of both kinds of peroxide. This specific sensitivity of DeltacTPxII cells to TBHP could not be related to the expression pattern of TSA2 (cytosolic thioredoxin peroxidase II gene) either, since this gene is highly inducible by both H(2)O(2) and TBHP when cells were grown in different conditions. Finally, peroxide-removing assays were performed and showed that catalase activity increased significantly only in DeltacTPxII cells, which appear to be related with the resistance of this strain to H(2)O(2). Taken together, present data indicate that cTPxII and cTPxI are key components of the yeast defense system against organic peroxide insult. In regard to the stress induced by H(2)O(2), catalases (peroxisomal and/or cytosolic) and cTPxII seemed to cooperate with cTPxI in the defense of yeast against this oxidant.

Two linked genes of Leishmania infantum encode tryparedoxins localised to cytosol and mitochondrion

Tryparedoxins are components of the hydroperoxide detoxification cascades of Kinetoplastida, where they mediate electron transfer between trypanothione and a peroxiredoxin, which reduces hydroperoxides and possibly peroxynitrite. Tryparedoxins may also be involved in DNA synthesis, by their capacity to reduce ribonucleotide reductase. Here we report on the isolation of two tryparedoxin genes from Leishmania infantum, Li7XN1 and LiTXN2, which share the same genetic locus. These genes are both single copy and code for two active tryparedoxin enzymes, LiTXN1 and LiTXN2, with different biochemical and biological features. LiTXN1 is located to the cytosol and is upregulated in the infectious forms of the parasite, strongly suggesting that it might play an important role during infection. LiTXN2 is the first mitochondrial tryparedoxin described in Kinetoplastida. Biochemical assays performed on the purified recombinant proteins have shown that LiTXN1 preferentially reduces the cytosolic L. infantum peroxiredoxins, LicTXNPx1 and LicTXNPx2, whereas LiTXN2 has a higher specific activity for a mitochondrial peroxiredoxin, LimTXNPx. Kinetically, the two tryparedoxins follow a ping-pong mechanism and show no saturation. We suggest that LiTXN1 and LiTXN2 are part of two distinct antioxidant machineries, one cytosolic, the other mitochondrial, that complement each other to ensure effective defence from several sources of oxidants throughout the development of L. infantum.

Attachment of the ubiquitin-related protein Urm1p to the antioxidant protein Ahp1p

Urm1p is a ubiquitin-related protein that serves as a posttranslational modification of other proteins. Urm1p conjugation has been implicated in the budding process and in nutrient sensing. Here, we have identified the first in vivo target for the urmylation pathway as the antioxidant protein Ahp1p. The attachment of Urm1p to Ahp1p requires the E1 for the urmylation pathway, Uba4p. Loss of the urmylation pathway components results in sensitivity to a thiol-specific oxidant, as does loss of Ahp1p, implying that urmylation has a role in an oxidative-stress response. Moreover, treatment of cells with thiol-specific oxidants affects the abundance of Ahp1p-Urm1p conjugates. These results suggest that the conjugation of Urm1p to Ahp1p could regulate the function of Ahp1p in antioxidant stress response in Saccharomyces cerevisiae.

Mutator genes for suppression of gross chromosomal rearrangements identified by a genome-wide screening in Saccharomyces cerevisiae

Different types of gross chromosomal rearrangements (GCRs), including translocations, interstitial deletions, terminal deletions with de novo telomere additions, and chromosome fusions, are observed in many cancers. Multiple pathways, such as S-phase checkpoints, DNA replication, recombination, chromatin remodeling, and telomere maintenance that suppress GCRs have been identified. To experimentally expand our knowledge of other pathway(s) that suppress GCRs, we developed a generally applicable genome-wide screening method. In this screen, we identified 10 genes (ALO1, CDC50, CSM2, ELG1, ESC1, MMS4, RAD5, RAD18, TSA1, and UFO1) that encode proteins functioning in the suppression of GCRs. Moreover, the breakpoint junctions of GCRs from these GCR mutator mutants were determined with modified breakpoint-mapping methods. We also identified nine genes (AKR1, BFR1, HTZ1, IES6, NPL6, RPL13B, RPL27A, RPL35A, and SHU2) whose mutations generated growth defects with the pif1Delta mutation. In addition, we found that some of these mutations changed the telomere size.

Glutathione and thioredoxin peroxidases mediate susceptibility of yeast mitochondria to Ca(2+)-induced damage

The effect of thioredoxin peroxidases on the protection of Ca(2+)-induced inner mitochondrial membrane permeabilization was studied in the yeast Saccharomyces cerevisiae using null mutants for these genes. Since deletion of a gene can promote several other effects besides the absence of the respective protein, characterizations of the redox state of the mutant strains were performed. Whole cellular extracts from all the mutants presented lower capacity to decompose H(2)O(2) and lower GSH/GSSG ratios, as expected for strains deficient for peroxide-removing enzymes. Interestingly, when glutathione contents in mitochondrial pools were analyzed, all mutants presented lower GSH/GSSG ratios than wild-type cells, with the exception of DeltacTPxI strain (cells in which cytosolic thioredoxin peroxidase I gene was disrupted) that presented higher GSH/GSSG ratio. Low GSH/GSSG ratios in mitochondria increased the susceptibility of yeast to damage induced by Ca(2+) as determined by membrane potential and oxygen consumption experiments. However, H(2)O(2) removal activity appears also to be important for mitochondria protection against permeabilization because exogenously added catalase strongly inhibited loss of mitochondrial potential. Moreover, exogenously added recombinant peroxiredoxins prevented inner mitochondrial membrane permeabilization. GSH/GSSG ratios decreased after Ca(2+) addition, suggesting that reactive oxygen species (ROS) probably mediate this process. Taken together our results indicate that both mitochondrial glutathione pools and peroxide-removing enzymes are key components for the protection of yeast mitochondria against Ca(2+)-induced damage.

Regeneration of peroxiredoxins by p53-regulated sestrins, homologs of bacterial AhpD

Acting as a signal, hydrogen peroxide circumvents antioxidant defense by overoxidizing peroxiredoxins (Prxs), the enzymes that metabolize peroxides. We show that sestrins, a family of proteins whose expression is modulated by p53, are required for regeneration of Prxs containing Cys-SO(2)H, thus reestablishing the antioxidant firewall. Sestrins contain a predicted redox-active domain homologous to AhpD, the enzyme catalyzing the reduction of a bacterial Prx, AhpC. Purified Hi95 (sestrin 2) protein supports adenosine triphosphate-dependent reduction of overoxidized PrxI in vitro, indicating that unlike AhpD, which is a disulfide reductase, sestrins are cysteine sulfinyl reductases. As modulators of peroxide signaling and antioxidant defense, sestrins constitute potential therapeutic targets.

Peroxiredoxin-null Yeast Cells Are Hypersensitive to Oxidative Stress and Are Genomically Unstable

Peroxiredoxins are a family of abundant peroxidases found in all organisms. Although these antioxidant enzymes are thought to be critically involved in cellular defense and redox signaling, their exact physiological roles are largely unknown. In this study, we took a genetic approach to address the functions of peroxiredoxins in budding yeast. We generated and characterized a yeast mutant lacking all five peroxiredoxins. The quintuple peroxiredoxin-null mutant was still viable, though the growth rate was lower under normal aerobic conditions. Although peroxiredoxins are not essential for cell viability, peroxiredoxin-null yeast cells were more susceptible to oxidative and nitrosative stress. In the complete absence of peroxiredoxins, the expression of other antioxidant proteins including glutathione peroxidase and glutathione reductase was induced. In addition, the quintuple mutant was hypersensitive to glutathione depletion. Thus, the glutathione system might cooperate with other antioxidant enzymes to compensate for peroxiredoxin deficiency. Interestingly, the peroxiredoxinnull yeast cells displayed an increased rate of spontaneous mutations that conferred resistance to canavanine. This mutator phenotype was rescued by yeast peroxiredoxin Tsa1p, but not by its active-site mutant defective for peroxidase activity. Our findings suggest that the antioxidant function of peroxiredoxins is important for maintaining genome stability in eukaryotic cells.

Two Enzymes in One

Although a great deal is known biochemically about peroxiredoxins (Prxs), little is known about their real physiological function. We show here that two cytosolic yeast Prxs, cPrxI and II, which display diversity in structure and apparent molecular weights (MW), can act alternatively as peroxidases and molecular chaperones. The peroxidase function predominates in the lower MW forms, whereas the chaperone function predominates in the higher MW complexes. Oxidative stress and heat shock exposure of yeasts causes the protein structures of cPrxI and II to shift from low MW species to high MW complexes. This triggers a peroxidase-to-chaperone functional switch. These in vivo changes are primarily guided by the active peroxidase site residue, Cys(47), which serves as an efficient "H(2)O(2)-sensor" in the cells. The chaperone function of these proteins enhances yeast resistance to heat shock.

Complementation of the yeast deletion mutant DeltaNCE103 by members of the beta class of carbonic anhydrases is dependent on carbonic anhydrase activity rather than on antioxidant activity

In recent years, members of the beta class of CAs (carbonic anhydrases) have been shown to complement Delta NCE103, a yeast strain unable to grow under aerobic conditions. The activity required for complementation of Delta NCE103 by tobacco chloroplast CA was studied by site-directed mutagenesis. E196A (Glu196-->Ala), a mutated tobacco CA with low levels of CA activity, complemented Delta NCE103. To determine whether restoration of Delta NCE103 was due to residual levels of CA activity or whether it was related to previously proposed antioxidant activity of CAs [Götz, Gnann and Zimmermann (1999) Yeast 15, 855-864], additional complementation analysis was performed using human CAII, an alpha CA structurally unrelated to the beta class of CAs to which the tobacco protein belongs. Human CAII complemented Delta NCE103, strongly arguing that CA activity is responsible for the complementation of Delta NCE103. Consistent with this conclusion, recombinant NCE103 synthesized in Escherichia coli shows CA activity, and Delta NCE103 expressing the tobacco chloroplast CA exhibits the same sensitivity to H2O2 as the wild-type strain.

Poplar peroxiredoxin Q. A thioredoxin-linked chloroplast antioxidant functional in pathogen defense

Peroxiredoxins are ubiquitous thioredoxin- or glutaredoxin-dependent peroxidases, the function of which is to destroy peroxides. Peroxiredoxin Q, one of the four plant subtypes, is a homolog of the bacterial bacterioferritin comigratory proteins. We show here that the poplar (Populus tremula x Populus tremuloides) protein acts as a monomer with an intramolecular disulfide bridge between two conserved cysteines. A wide range of electron donors and substrates was tested. Unlike type II peroxiredoxin, peroxiredoxin Q cannot use the glutaredoxin or cyclophilin isoforms tested, but various cytosolic, chloroplastic, and mitochondrial thioredoxins are efficient electron donors with no marked specificities. The redox midpoint potential of the peroxiredoxin Q catalytic disulfide is -325 mV at pH 7.0, explaining why the wild-type protein is reduced by thioredoxin but not by glutaredoxin. Additional evidence that thioredoxin serves as a donor comes from the formation of heterodimers between peroxiredoxin Q and monocysteinic mutants of spinach (Spinacia oleracea) thioredoxin m. Peroxiredoxin Q can reduce various alkyl hydroperoxides, but with a better efficiency for cumene hydroperoxide than hydrogen peroxide and tertiary butyl hydroperoxide. The use of immunolocalization and of a green fluorescence protein fusion construct indicates that the transit sequence efficiently targets peroxiredoxin Q to the chloroplasts and especially to those of the guard cells. The expression of this protein and of type II peroxiredoxin is modified in response to an infection by two races of Melampsora larici-populina, the causative agent of the poplar rust. In the case of an hypersensitive response, the peroxiredoxin expression increased, whereas it decreased during a compatible interaction.

Identification of multiple transcripts for antioxidant protein 2 (Aop2): differential regulation by oxidative stress and growth factors

Antioxidant protein 2 is a unique member of the thiol-specific antioxidant family of proteins known to reduce reactive oxygen species in the presence of thiol-containing electron donors. It is also a candidate atherosclerosis susceptibility gene in mice. In the present study, we sought to characterize the transcripts of this gene, and determine which, if any, are regulated by conditions associated with oxidative stress. We have identified multiple Aop2 transcripts by Northern blot, each exhibiting a unique tissue distribution. These include the previously reported 1.47 kb major transcript, two alternative Aop2 transcripts found exclusively in liver, and a testis-specific transcript believed to be the highly related intronless gene Aop2-rs1. Treatment of a murine hepatocyte cell line with glucose oxidase led to the specific and transient induction of the 1.47 kb transcript, while the 3.1 kb transcript was regulated by serum deprivation and re-stimulation with either keratinocyte growth factor or serum in a time-dependent manner. Since these ROS-inducing stimuli involve different mechanisms of action and cellular responses, our data suggest that alternative Aop2 transcripts may play distinct roles in different oxidative stress responses, and possibly in atherosclerosis.

Active site mutagenesis and phospholipid hydroperoxide reductase activity of poplar type II peroxiredoxin

The nature of the active site and the substrate specificity of poplar type II peroxiredoxin, an enzyme which preferentially uses glutaredoxin as an electron donor, were investigated in this study. The type II peroxiredoxin is able to use phospholipid hydroperoxide nearly as efficiently as hydrogen peroxide. Two of the hyper-conserved amino acid residues in peroxiredoxins have been altered, by site-directed mutagenesis, generating the mutants T48V and R129Q. The two mutant proteins are inactive with hydrogen peroxide or tertiary butyl hydroperoxide as substrates. On the other hand, the mutant enzymes catalyse the degradation of cumene hydroperoxide with low efficiency. This suggests that the thiol-dependent regeneration process of the catalytic cysteine is not affected by the mutations and that all substrates are not accommodated identically in the active site.

ATP-dependent reduction of cysteine-sulphinic acid by S. cerevisiae sulphiredoxin

Proteins contain thiol-bearing cysteine residues that are sensitive to oxidation, and this may interfere with biological function either as 'damage' or in the context of oxidant-dependent signal transduction. Cysteine thiols oxidized to sulphenic acid are generally unstable, either forming a disulphide with a nearby thiol or being further oxidized to a stable sulphinic acid. Cysteine-sulphenic acids and disulphides are known to be reduced by glutathione or thioredoxin in biological systems, but cysteine-sulphinic acid derivatives have been viewed as irreversible protein modifications. Here we identify a yeast protein of relative molecular mass M(r) = 13,000, which we have named sulphiredoxin (identified by the US spelling 'sulfiredoxin', in the Saccharomyces Genome Database), that is conserved in higher eukaryotes and reduces cysteine-sulphinic acid in the yeast peroxiredoxin Tsa1. Peroxiredoxins are ubiquitous thiol-containing antioxidants that reduce hydroperoxides and control hydroperoxide-mediated signalling in mammals. The reduction reaction catalysed by sulphiredoxin requires ATP hydrolysis and magnesium, involving a conserved active-site cysteine residue which forms a transient disulphide linkage with Tsa1. We propose that reduction of cysteine-sulphinic acids by sulphiredoxin involves activation by phosphorylation followed by a thiol-mediated reduction step. Sulphiredoxin is important for the antioxidant function of peroxiredoxins, and is likely to be involved in the repair of proteins containing cysteine-sulphinic acid modifications, and in signalling pathways involving protein oxidation.

Disruption of the Plasmodium falciparum 2-Cys peroxiredoxin gene renders parasites hypersensitive to reactive oxygen and nitrogen species

In parasitism, Plasmodium falciparum is exposed to toxic reactive oxygen species and reactive nitrogen species (RNS). Peroxiredoxins (Prx) are ubiquitously distributed antioxidant enzymes. In bacteria and yeast, Prx have also been implicated in detoxifying RNS. Here, we used a gene targeting strategy to investigate the physiological role of 2-Cys Prx of P. falciparum, PfTPx-1, in living parasite cells. The PfTPx-1-null parasite line was more sensitive to paraquat (a superoxide donor) and sodium nitroprusside (a nitric oxide donor), than wildtype. These findings suggest that PfTPx-1 protects the parasite cells from oxidative and nitrosative stresses.

Thioredoxin peroxidases can foster cytoprotection or cell death in response to different stressors: over- and under-expression of thioredoxin peroxidase in Drosophila cells

Recently, we identified a set of five genes constituting the peroxiredoxin gene family in Drosophila melanogaster [Radyuk, Klichko, Spinola, Sohal and Orr (2001) Free Radical Biol. Med. 31, 1090-1100]. This set includes two abundant thioredoxin peroxidase (TPx) species, namely Drosophila peroxiredoxin DPx-4783, a cytosolic TPx and DPx-5037, a mitochondrial TPx. Overexpression of either one of them in Drosophila S2 cells conferred increased resistance to toxicity induced by hydrogen peroxide, paraquat or cadmium. To understand further the functional roles of these enzymes in vivo, we report in the present study the effects of decreased expression, using RNA interference, on the response of S2 cells to different stressors. When either of the TPxs was blocked, cells became relatively more susceptible to oxidative stress caused by exposure to hydrogen peroxide or paraquat, but were unaffected when challenged with copper and heat stress. In contrast, TPx overexpressing cells were more susceptible to copper and heat stress when compared with control cells and exhibited DNA fragmentation. Furthermore, when cells were supplemented with N -acetyl-L-cysteine together with copper, there was a clear negative effect on cell survival, which was exacerbated by TPx overexpression. Manipulations in the levels of TPxs demonstrated that, under different stress conditions, these enzymes might have both beneficial and detrimental effects on Drosophila cell viability.

The trypanothione-thiol system in Trypanosoma cruzi as a key antioxidant mechanism against peroxynitrite-mediated cytotoxicity

Peroxynitrite, the reaction product between superoxide (O(*2)) and nitric oxide (*NO), is a powerful oxidizing species that contributes to macrophage competence against pathogens. In this context, peroxynitrite appears to play an important role in controlling infection by Trypanosoma cruzi, the unicellular parasite responsible for Chagas disease. T. cruzi contains various enzyme systems for the decomposition of hydroperoxides, all of which involve the participation of the low-molecular-weight dithiol trypanothione (N(1),N(8)-bis(glutathionyl)spermidine) as a critical redox partner. A large fraction of the trypanothione-dependent antioxidant capacity of T. cruzi is linked to the tryparedoxin-tryparedoxin peroxidase system which has critical protein thiol groups. In this report we demonstrate that dihydrotrypanothione is readily consumed during peroxynitrite challenge to cells to yield the corresponding trypanothione disulfide. On the other hand, glutathione, which is present in T. cruzi at lower concentrations than trypanothione, is consumed to a much lesser extent and mainly evolves to glutathione-protein mixed disulfides. The inhibition of glutathione biosynthesis by buthionine sulfoximine, which decreases glutathione concentration to 10% of control after 20 h, neither affects the concentration of dihydrotrypanothione nor sensitizes T. cruzi to peroxynitrite-mediated cytotoxicity. On the other hand, pretreatment of T. cruzi with diamide, which leads to a significant depletion (>70%) of dihydrotrypanothione, largely increases the extent of cellular nitration and inhibition of cell growth caused by peroxynitrite. Altogether, our findings support a key protective role for dihydrotrypanothione and the trypanothione-dependent antioxidant system in T. cruzi against peroxynitrite, which may facilitate the survival of trypanosomes within the oxidative environment of activated macrophages.

Nitrosative stress on yeast: inhibition of glyoxalase-I and glyceraldehyde-3-phosphate dehydrogenase in the presence of GSNO

Under nitrosative stressed condition intracellular GSNO accumulation is common to all cell types. Conserved NADH-dependent GSNO reductase was reported previously as an important cellular protective measure against this. In spite of the constitutive nature of the enzyme, we observed in vivo inactivation of two important enzymes-glyoxalase-I and glyceraldehyde-3-phosphate dehydrogenase under 5 mM GSNO stress in two budding yeasts, though with difference in their sensitivity. Former was more susceptible to inactivation in in vitro condition, too. In this study, we explored the competitive nature of yeast glyoxalase-I inhibition by GSNO. GSNO actually competes with GSH substrate-binding site of the enzyme.

Role of peroxidoxins in Leishmania chagasi survival. Evidence of an enzymatic defense against nitrosative stress

The mechanisms by which Leishmania parasites survive exposure to highly reactive oxygen (ROS) and nitrogen (RNS) species within phagosomes of macrophages are not well known. Recently it has been shown that RNS alone is sufficient and necessary to control Leishmania donovani infection in mice (Murray, H. W., and Nathan, C. F. (1999) J. Exp. Med. 189, 741-746). No enzymatic defense against RNS has been discovered in Leishmania to date. We have previously isolated two peroxidoxins (LcPxn1 and LcPxn2) from Leishmania chagasi and showed that recombinant LcPxn1 protein was capable of detoxifying hydrogen peroxide, hydroperoxide, and hydroxyl radicals (Barr, S. D., and Gedamu, L. (2001) J. Biol. Chem. 276, 34279-34287). In further characterizing the physiological role of peroxidoxins in Leishmania survival, we show here that recombinant LcPxn1 protein can detoxify RNS in addition to ROS, whereas recombinant LcPxn2 protein can only detoxify hydrogen peroxide. LcPxn1 and LcPxn2 are localized to the cytoplasm, and overexpression of LcPxn1 in L. chagasi parasites enhanced survival when exposed to exogenous ROS and RNS and enhanced survival within U937 macrophage cells. Site-directed mutagenesis studies revealed that the conserved Cys-52 residue is essential for detoxifying hydrogen peroxide, t-butyl hydroperoxide, and hydroxyl radicals, whereas the conserved Cys-173 residue is essential for detoxifying t-butyl hydroperoxide and peroxynitrite. This is the first report of an enzymatic defense against RNS in Leishmania.

Transcriptional regulation of yeast peroxiredoxin gene TSA2 through Hap1p, Rox1p, and Hap2/3/5p

In Saccharomyces cerevisiae, the transcription of peroxiredoxin gene TSA2 is responsive to various reactive oxygen and nitrogen species. Redox-regulated transcriptional activators Yap1p, Skn7p, Msn2p/Msn4p have been shown to play a role in regulating TSA2 expression. In this study we show that the transcription of TSA2 is under complex control involving additional transcription factors Hap1p, Rox1p, and Hap2/3/5p. Deletion of HAP1 led to a 50% reduction of TSA2 transcriptional activity. As an intracellular oxygen sensor, heme stimulated TSA2 transcription by activating Hap1p. The induction of TSA2 by H(2)O(2) is also mediated in part through Hap1p. Countering the effects of Hap1p was a transcriptional repressor Rox1p. Deletion of ROX1 or mutation of Rox1p-binding site significantly activated TSA2 transcription. In addition, TSA2 activity was diminished in hap2Delta, hap3Delta, hap4Delta, and hap5Delta strains, but was stimulated upon overexpression of Hap4p. Hap2/3/5p may cooperate with Msn2/4p to activate TSA2 after diauxic shift. Finally, we demonstrated a role for kinases Ras1/2p and Hog1p in Msn2/4p-dependent activation of TSA2. In particular, Hog1p mediated the response of TSA2 to osmotic and oxidative stress. Taken together, our findings suggest that the expression of TSA2 is regulated by a group of transcription factors responsive differentially to stress conditions.

Functional complementation in yeast reveals a protective role of chloroplast 2-Cys peroxiredoxin against reactive nitrogen species

The importance of nitric oxide (NO) as a signaling molecule to various plant physiological and pathophysiological processes is becoming increasingly evident. However, little is known about how plants protect themselves from nitrosative and oxidative damage mediated by NO and NO-derived reactive nitrogen species (RNS). Peroxynitrite, the product of the reaction between NO and superoxide anion, is considered to play a central role in RNS-induced cytotoxicity, as a result of its potent ability to oxidize diverse biomolecules. Employing heterologous expression in bacteria and yeast, we investigated peroxynitrite-scavenging activity in plants of 2-Cys peroxiredoxin (2CPRX), originally identified as a hydroperoxide-reducing peroxidase that is ubiquitously distributed among organisms. The putative mature form of a chloroplast-localized 2CPRX from Arabidopsis thaliana was overproduced in Escherichia coli as an amino-terminally hexahistidine-tagged fusion protein. The purified recombinant 2CPRX, which was catalytically active as peroxidase, efficiently prevented the peroxynitrite-induced oxidation of a sensitive compound. We also examined in vivo the ability of the Arabidopsis 2CPRX to complement the 2CPRX deficiency of a Saccharomyces cerevisiae mutant. Functional expression in the mutant strain of the Arabidopsis 2CPRX not only increased cellular tolerance to hydrogen peroxide, but also complemented the hypersensitive growth defect induced by nitrite-mediated cytotoxicity. The complemented cells significantly enhanced the capacity to reduce RNS-mediated oxidative damages. The results presented here demonstrate a new role of plant 2CPRX as a critical determinant of the resistance to RNS, and support the existence of a plant enzymatic basis for RNS metabolism.