Oxidation of mitochondrial peroxiredoxin 3 during the initiation of receptor-mediated apoptosis

Exploring the role of PRDX4 in the development of uterine corpus endometrial carcinoma

Peroxicedoxin 4 (PRDX4), a member of the peroxicedoxins (PRDXs), has been reported in many cancer-related studies, but its role in uterine corpus endometrial carcinoma (UCEC) is not fully understood. In the present study, we found that PRDX4 was highly expressed in UCEC tissues and cell lines through the combination of bioinformatics analysis and experiments, and elevated PRDX4 levels were associated with poor prognosis. Knockdown of PRDX4 significantly blocked the proliferation and migration of the UCEC cell line Ishikawa and reduced degree of cell confluence. These findings highlight the oncogenic role of PRDX4 in UCEC. In addition, genes that interact with PRDX4 in UCEC were MT-ATP8, PBK, and PDIA6, and we speculated that these genes interacted with each other to promote disease progression in UCEC. Thus, PRDX4 is a potential diagnostic biomarker for UCEC, and targeting PRDX4 may be a potential therapeutic strategy for patients with UCEC.

Changes to insulin sensitivity in glucose clearance systems and redox following dietary supplementation with a novel cysteine-rich protein: A pilot randomized controlled trial in humans with type-2 diabetes

We recently developed a novel keratin-derived protein (KDP) rich in cysteine, glycine, and arginine, with the potential to alter tissue redox status and insulin sensitivity. The KDP was tested in 35 human adults with type-2 diabetes mellitus (T2DM) in a 14-wk randomised controlled pilot trial comprising three 2×20 g supplemental protein/day arms: KDP-whey (KDPWHE), whey (WHEY), non-protein isocaloric control (CON), with standardised exercise. Outcomes were measured morning fasted and following insulin-stimulation (80 mU/m2/min hyperinsulinaemic-isoglycaemic clamp). With KDPWHE supplementation there was good and very-good evidence for moderate-sized increases in insulin-stimulated glucose clearance rate (GCR; 26%; 90% confidence limits, CL 2%, 49%) and skeletal-muscle microvascular blood flow (46%; 16%, 83%), respectively, and good evidence for increased insulin-stimulated sarcoplasmic GLUT4 translocation (18%; 0%, 39%) vs CON. In contrast, WHEY did not effect GCR (-2%; -25%, 21%) and attenuated HbA1c lowering (14%; 5%, 24%) vs CON. KDPWHE effects on basal glutathione in erythrocytes and skeletal muscle were unclear, but in muscle there was very-good evidence for large increases in oxidised peroxiredoxin isoform 2 (oxiPRX2) (19%; 2.2%, 35%) and good evidence for lower GPx1 concentrations (-40%; -4.3%, -63%) vs CON; insulin stimulation, however, attenuated the basal oxiPRX2 response (4%; -16%, 24%), and increased GPx1 (39%; -5%, 101%) and SOD1 (26%; -3%, 60%) protein expression. Effects of KDPWHE on oxiPRX3 and NRF2 content, phosphorylation of capillary eNOS and insulin-signalling proteins upstream of GLUT4 translocation AktSer437 and AS160Thr642 were inconclusive, but there was good evidence for increased IRSSer312 (41%; 3%, 95%), insulin-stimulated NFκB-DNA binding (46%; 3.4%, 105%), and basal PAK-1Thr423/2Thr402 phosphorylation (143%; 66%, 257%) vs WHEY. Our findings provide good evidence to suggest that dietary supplementation with a novel edible keratin protein in humans with T2DM may increase glucose clearance and modify skeletal-muscle tissue redox and insulin sensitivity within systems involving peroxiredoxins, antioxidant expression, and glucose uptake.

Regulation of peroxiredoxin-3 gene expression under basal and hyperglycemic conditions: Key roles for transcription factors Sp1, CREB and NF-κB

Peroxiredoxin-3 (Prx-3), a thioredoxin-dependent peroxidase located exclusively in the mitochondrial matrix, catalyses peroxides/peroxinitrites. Altered levels of Prx-3 is associated with diabetic cardiomyopathy (DCM). However, molecular mechanisms of Prx-3 gene regulation remain partially understood. We undertook a systemic analysis of the Prx-3 gene to identify the key motifs and transcriptional regulatory molecules. Transfection of promoter-reporter constructs in the cultured cells identified -191/+20 bp domain as the core promoter region. Stringent in silico analysis of this core promoter revealed putative binding sites for specificity protein 1 (Sp1), cAMP response element-binding protein (CREB) and nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB). Interestingly, while co-transfection of the -191/+20 bp construct with Sp1/CREB plasmid diminished Prx3 promoter-reporter activity, mRNA and protein levels, co-transfection with NF-κB expression plasmid augmented the same. Consistently, inhibition of Sp1/CREB/NF-κB expression reversed the promoter-reporter activity, mRNA and protein levels of Prx-3, thereby confirming their regulatory effects. ChIP assays provided evidence for interactions of Sp1/CREB/NF-κB with the Prx-3 promoter. H9c2 cells treated with high glucose as well as streptozotocin (STZ)-treated diabetic rats showed time-dependent reduction in promoter activity, endogenous transcript and protein levels of Prx-3. Augmentation of Sp1/CREB protein levels and their strong binding with Prx-3 promoter are responsible for diminished Prx-3 levels under hyperglycemia. The activation/increase in the NF-κB expression under hyperglycemia was not sufficient to restore the reduction of endogenous Prx-3 levels owing to its weak binding affinity. Taken together, this study elucidates the previously unknown roles of Sp1/CREB/NF-κB in regulating Prx-3 gene expression under hyperglycemic condition.

CD36 Signal Transduction in Metabolic Diseases: Novel Insights and Therapeutic Targeting

The cluster of differentiation 36 (CD36) is a scavenger receptor present on various types of cells and has multiple biological functions that may be important in inflammation and in the pathogenesis of metabolic diseases, including diabetes. Here, we consider recent insights into how the CD36 response becomes deregulated under metabolic conditions, as well as the therapeutic benefits of CD36 inhibition, which may provide clues for developing strategies aimed at the treatment or prevention of diabetes associated with metabolic diseases. To facilitate this process further, it is important to pinpoint regulatory mechanisms that are relevant under physiological and pathological conditions. In particular, understanding the mechanisms involved in dictating specific CD36 downstream cellular outcomes will aid in the discovery of potent compounds that target specific CD36 downstream signaling cascades.

Hyperoxidation of Peroxiredoxins and Effects on Physiology of Drosophila

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

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

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

High glucose-induced PRDX3 acetylation contributes to glucotoxicity in pancreatic β-cells: Prevention by Teneligliptin

Chronic hyperglycemia has deleterious effects on pancreatic β-cell function and survival in type 2 diabetes (T2D) due to the low expression level of endogenous antioxidants in the β-cells. Peroxiredoxin-3 (PRDX3) is a mitochondria specific H202 scavenger and protects the cell from mitochondrial damage. However, nothing is known about how glucotoxicity influences PRDX3 function in the pancreatic beta cells. Exposure of rat insulinoma INS-1 cells and human beta cells (1.1B4) to high glucose conditions (30mM) stimulated acetylation of PRDX3 which facilitates its hyper-oxidation causing mitochondrial dysfunction by SIRT1 degradation. SIRT1 deficiency induces beta cell apoptosis via NOX-JNK-p66Shc signalosome activation. Herein we investigated the direct effect of Teneligliptin, a newer DPP-4 inhibitor on beta-cell function and survival in response to high glucose conditions. Teneligliptin treatment enhances SIRT1 protein levels and activity by USP22, an ubiquitin specific peptidase. Activated SIRT1 prevents high glucose-induced PRDX3 acetylation by SIRT3 resulted in inhibition of PRDX3 hyper-oxidation thereby strengthening the mitochondrial antioxidant defense. Notably, we identify PRDX3 as a novel SIRT3 target and show their physical interaction. Intriguingly, inhibition of SIRT1 activity by EX-527 or SIRT1 siRNA knockdown exacerbated the SIRT3 mediated PRDX3 deacetylation which leads to peroxiredoxin-3 hyper-oxidation and beta-cell apoptosis by the activation of NOX-JNK-p66Shc signalosome. Collectively, our results unveil a novel and first direct effect of high glucose on PRDX3 acetylation on beta-cell dysfunction by impaired antioxidant defense and SIRT1 mediated SIRT3-PRDX3 activation by Teneligliptin suppresses high glucose-mediated mitochondrial dysfunction.

Mitochondria-targeted magnolol inhibits OXPHOS, proliferation, and tumor growth via modulation of energetics and autophagy in melanoma cells

Introduction:

Melanoma is an aggressive form of skin cancer for which there are no effective drugs for prolonged treatment. The existing kinase inhibitor antiglycolytic drugs (B-Raf serine/threonine kinase or BRAF inhibitors) are effective for a short time followed by a rapid onset of drug resistance.

Presentation of case:

Here, we show that a mitochondria-targeted analog of magnolol, Mito-magnolol (Mito-MGN), inhibits oxidative phosphorylation (OXPHOS) and proliferation of melanoma cells more potently than untargeted magnolol. Mito-MGN also inhibited tumor growth in murine melanoma xenografts. Mito-MGN decreased mitochondrial membrane potential and modulated energetic and mitophagy signaling proteins.

Discussion:

Results indicate that Mito-MGN is significantly more potent than the FDA-approved OXPHOS inhibitor in inhibiting proliferation of melanoma cells.

Conclusion:

These findings have implications in the treatment of melanomas with enhanced OXPHOS status due to metabolic reprogramming or drug resistance.

Kinetic modeling of H2O2 dynamics in the mitochondria of HeLa cells

Hydrogen peroxide (H2O2) promotes a range of phenotypes depending on its intracellular concentration and dosing kinetics, including cell death. While this qualitative relationship has been well established, the quantitative and mechanistic aspects of H2O2 signaling are still being elucidated. Mitochondria, a putative source of intracellular H2O2, have recently been demonstrated to be particularly vulnerable to localized H2O2 perturbations, eliciting a dramatic cell death response in comparison to similar cytosolic perturbations. We sought to improve our dynamic and mechanistic understanding of the mitochondrial H2O2 reaction network in HeLa cells by creating a kinetic model of this system and using it to explore basal and perturbed conditions. The model uses the most current quantitative proteomic and kinetic data available to predict reaction rates and steady-state concentrations of H2O2 and its reaction partners within individual mitochondria. Time scales ranging from milliseconds to one hour were simulated. We predict that basal, steady-state mitochondrial H2O2 will be in the low nM range (2-4 nM) and will be inversely dependent on the total pool of peroxiredoxin-3 (Prx3). Neglecting efflux of H2O2 to the cytosol, the mitochondrial reaction network is expected to control perturbations well up to H2O2 generation rates ~50 μM/s (0.25 nmol/mg-protein/s), above which point the Prx3 system would be expected to collapse. Comparison of these results with redox Western blots of Prx3 and Prx2 oxidation states demonstrated reasonable trend agreement at short times (≤ 15 min) for a range of experimentally perturbed H2O2 generation rates. At longer times, substantial efflux of H2O2 from the mitochondria to the cytosol was evidenced by peroxiredoxin-2 (Prx2) oxidation, and Prx3 collapse was not observed. A refined model using Monte Carlo parameter sampling was used to explore rates of H2O2 efflux that could reconcile model predictions of Prx3 oxidation states with the experimental observations.

Mitochondria-targeted paraquat and metformin mediate ROS production to induce multiple pathways of retrograde signaling: A dose-dependent phenomenon

The mitochondrial electron transport chain is a major source of reactive oxygen species (ROS) and is also a target of ROS, with an implied role in the stabilization of hypoxia-inducible factor (HIF) and induction of the AMPK pathway. Here we used varying doses of two agents, Mito-Paraquat and Mito-Metformin, that have been conjugated to cationic triphenylphosphonium (TPP⁺) moiety to selectively target them to the mitochondrial matrix compartment, thereby resulting in the site-specific generation of ROS within mitochondria. These agents primarily induce superoxide (O₂^•-) production by acting on complex I. In Raw264.7 macrophages, C2C12 skeletal myocytes, and HCT116 adenocarcinoma cells, we show that mitochondria-targeted oxidants can induce ROS (O₂^•- and H₂O₂). In all three cell lines tested, the mitochondria-targeted agents disrupted membrane potential and activated calcineurin and the Cn-dependent retrograde signaling pathway. Hypoxic culture conditions also induced Cn activation and HIF1α activation in a temporally regulated manner, with the former appearing at shorter exposure times. Together, our results indicate that mitochondrial oxidant-induced retrograde signaling is driven by disruption of membrane potential and activation of Ca²⁺/Cn pathway and is independent of ROS-induced HIF1α or AMPK pathways.

The Role of CD36 in Type 2 Diabetes Mellitus: β-Cell Dysfunction and Beyond

Impaired β-cell function is the key pathophysiology of type 2 diabetes mellitus, and chronic exposure of nutrient excess could lead to this tragedy. For preserving β-cell function, it is essential to understand the cause and mechanisms about the progression of β-cells failure. Glucotoxicity, lipotoxicity, and glucolipotoxicity have been suggested to be a major cause of β-cell dysfunction for decades, but not yet fully understood. Fatty acid translocase cluster determinant 36 (CD36), which is part of the free fatty acid (FFA) transporter system, has been identified in several tissues such as muscle, liver, and insulin-producing cells. Several studies have reported that induction of CD36 increases uptake of FFA in several cells, suggesting the functional interplay between glucose and FFA in terms of insulin secretion and oxidative metabolism. However, we do not currently know the regulating mechanism and physiological role of CD36 on glucolipotoxicity in pancreatic β-cells. Also, the downstream and upstream targets of CD36 related signaling have not been defined. In the present review, we will focus on the expression and function of CD36 related signaling in the pancreatic β-cells in response to hyperglycemia and hyperlipidemia (ceramide) along with the clinical studies on the association between CD36 and metabolic disorders.

Peroxiredoxin 6 Inhibits Osteogenic Differentiation and Bone Formation Through Human Dental Pulp Stem Cells and Induces Delayed Bone Development

Aims: Peroxiredoxins (PRDXs) are thiol-specific antioxidant enzymes that regulate redox balance that are critical for maintaining the cellular potential for self-renewal and stemness. Stem cell-based regenerative medicine is a promising approach in tissue reconstruction. However, to obtain functional cells for use in clinical applications, stem cell technology still requires improvements. Results: In this study, we found that PRDX6 levels were decreased during osteogenic differentiation in human dental pulp stem cells (hDPSCs). hDPSCs stably expressing Myc-PRDX6 (hDPSC/myc-PRDX6) inhibited cell growth in hDPSCs during osteogenic differentiation and impaired osteogenic phenotypes such as alkaline phosphatase (ALP) activity, mineralized nodule formation, and osteogenic marker genes [ALP and osteocalcin (OCN)]. hDPSC cell lines stably expressing mutant glutathione peroxidase (PRDX6(C47S)) and independent phospholipase A2 (PRDX6(S32A)) were also generated. Each mutant form of PRDX6 abolished the impaired osteogenic phenotypes, the transforming growth factor-β-mediated Smad2 and p38 pathways, and RUNX2 expression. Furthermore, in vivo experiments revealed that hDPSC/myc-PRDX6 suppressed hDPSC-based bone regeneration in calvarial defect mice, and newborn PRDX6 transgenic mice exhibited delayed bone development and reduced RUNX2 expression. Innovation and Conclusion: These findings illuminate the effects of PRDX6 during osteogenic differentiation of hDPSCs, and also suggest that regulating PRDX6 may improve the clinical utility of stem cell-based regenerative medicine for the treatment of bone diseases. Antioxid. Redox Signal. 30, 1969-1982.

CD36 dependent redoxosomes promotes ceramide-mediated pancreatic β-cell failure via p66Shc activation

Altered metabolism is implicated in the pathogenesis of beta-cell failure in type 2 diabetes (T2D). Plasma and tissue levels of ceramide species play positive roles in inflammatory and oxidative stress responses in T2D. However, oxidative targets and mechanisms underlying ceramide signaling are unclear. We investigated the role of CD36-dependent redoxosome (redox-active endosome), a membrane-based signaling agent, in ceramide-induced beta-cell dysfunction and failure. Exposure of beta cells to C2-ceramide (N-acetyl-sphingosine) induced a CD36-dependent non-receptor tyrosine kinase Src-mediated redoxosome (Vav2-Rac1-NOX) formation. Activated Rac1-GTP-NADPH oxidase complex induced c-Jun-N-terminal kinase (JNK) activation and nuclear factor (NF)-kB transcription, which was associated with thioredoxin-interacting protein (TXNIP) upregulation and thioredoxin activity suppression. Upregulated JNK expression induced p66Shc serine36 phosphorylation and peroxiredoxin-3 hyperoxidation, causing beta-cell apoptosis via mitochondrial dysfunction. CD36 inhibition by sulfo-N-succinimidyl oleate (SSO) or CD36 siRNA blocked C2-ceramide-induced redoxosome activation, thereby decreasing JNK-dependent p66Shc serine36 phosphorylation. CD36 inhibition downregulated TXNIP expression and promoted thioredoxin activity via enhanced thioredoxin reductase activity, which prevented peroxiredoxin-3 oxidation. CD36 inhibition potentiated glucose-stimulated insulin secretion and prevented beta-cell apoptosis. Our results reveal a new role of CD36 during early molecular events that lead to Src-mediated redoxosome activation, which contributes to ceramide-induced pancreatic beta-cell dysfunction and failure.

Rapid peroxynitrite reduction by human peroxiredoxin 3: Implications for the fate of oxidants in mitochondria

Mitochondria are main sites of peroxynitrite formation. While at low concentrations mitochondrial peroxynitrite has been associated with redox signaling actions, increased levels can disrupt mitochondrial homeostasis and lead to pathology. Peroxiredoxin 3 is exclusively located in mitochondria, where it has been previously shown to play a major role in hydrogen peroxide reduction. In turn, reduction of peroxynitrite by peroxiredoxin 3 has been inferred from its protective actions against tyrosine nitration and neurotoxicity in animal models, but was not experimentally addressed so far. Herein, we demonstrate the human peroxiredoxin 3 reduces peroxynitrite with a rate constant of 1 × 10⁷ M^-1 s^-1 at pH 7.8 and 25 °C. Reaction with hydroperoxides caused biphasic changes in the intrinsic fluorescence of peroxiredoxin 3: the first phase corresponded to the peroxidatic cysteine oxidation to sulfenic acid. Peroxynitrite in excess led to peroxiredoxin 3 hyperoxidation and tyrosine nitration, oxidative post-translational modifications that had been previously identified in vivo. A significant fraction of the oxidant is expected to react with CO₂ and generate secondary radicals, which participate in further oxidation and nitration reactions, particularly under metabolic conditions of active oxidative decarboxylations or increased hydroperoxide formation. Our results indicate that both peroxiredoxin 3 and 5 should be regarded as main targets for peroxynitrite in mitochondria.

Peroxiredoxin interaction with the cytoskeletal-regulatory protein CRMP2: Investigation of a putative redox relay

Hydrogen peroxide (H₂O₂) acts as a signaling molecule in cells by oxidising cysteine residues in regulatory proteins such as phosphatases, kinases and transcription factors. It is unclear exactly how many of these proteins are specifically targeted by H₂O₂ because they appear too unreactive to be directly oxidised. One proposal is that peroxiredoxins (Prxs) initially react with H₂O₂ and then oxidise adjacent proteins via a thiol relay mechanism. The aim of this study was to identify constitutive interaction partners of Prx2 in Jurkat T-lymphoma cells, in which thiol protein oxidation occurs at low micromolar concentrations of H₂O₂. Immunoprecipitation and proximity ligation assays identified a physical interaction between collapsin response mediator protein 2 (CRMP2) and cytoplasmic Prx2. CRMP2 regulates microtubule structure during lymphocyte migration and neuronal development. Exposure of Jurkat cells to low micromolar levels of H₂O₂ caused rapid and reversible oxidation of CRMP2, in parallel with Prx2 oxidation, despite purified recombinant CRMP2 protein reacting slowly with H₂O₂ (k~1 M^-1s^-1). Lowering Prx expression should inhibit oxidation of proteins oxidised by a relay mechanism, however knockout of Prx2 had no effect on CRMP2 oxidation. CRMP2 also interacted with Prx1, suggesting redundancy in single knockout cells. Prx 1 and 2 double knockout Jurkat cells were not viable. An interaction between Prx2 and CRMP2 was also detected in other human and rodent cells, including primary neurons. However, low concentrations of H₂O₂ did not cause CRMP2 oxidation in these cells. This indicates a cell-type specific mechanism for promoting CRMP2 oxidation in Jurkat cells, with insufficient evidence to attribute oxidation to a Prx-dependent redox relay.

Integrative proteomics and immunochemistry analysis of the factors in the necrosis and repair in acetaminophen-induced acute liver injury in mice

Acetaminophen (APAP) overdose-induced acute liver injury (AILI) is a significant clinical problem worldwide, the hepatotoxicity mechanisms are well elucidated, but the factors involved in the necrosis and repair still remain to be investigated. APAP was injected intraperitoneally in male Institute of Cancer Research (ICR) mice. Quantitative proteome analysis of liver tissues was performed by 2-nitrobenzenesulfenyl tagging, two-dimensional-nano high-performance liquid chromatography separation, and matrix-assisted laser desorption/ionization-time of flight mass spectrometry analysis. Diffrenetial proteins were verified by the immunochemistry method. 36 and 44 differentially expressed proteins were identified, respectively, at 24 hr after APAP (200 or 300 mg·kg ^-1 ) administration. The decrease in the mitochondrial protective proteins Prdx6, Prdx3, and Aldh2 accounted for the accumulation of excessive reactive oxygen species (ROS) and aldehydes, impairing mitochondria structure and function. The Gzmf combined with Bax and Apaf-1 jointly contributed to the necrosis. The blockage of Stat3 activation led to the overexpression of unphosphorylated Stat3 and the overproduction of Bax. The overexpression of unphosphorylated Stat3 represented necrosis; the alternation from Stat3 to p-Stat3 in necrotic regions represented hepatocytes from death to renewal. The high expressions of P4hα1, Ncam, α-SMA, and Cygb were involved in the liver repair, they were not only the markers of activated HSC but also represented an intermediate stage of hepatocytes from damage or necrosis to renewal. Our data provided a comprehensive report on the profile and dynamic changes of the liver proteins in AILI; the involvement of Gzmf and the role of Stat3 in necrosis were revealed; and the role of hepatocyte in liver self-repair was well clarified.

Cancer-Associated Function of 2-Cys Peroxiredoxin Subtypes as a Survival Gatekeeper

Cancer cells are abnormal cells that do not comply with tissue homeostasis but undergo uncontrolled proliferation. Such abnormality is driven mostly by somatic mutations on oncogenes and tumor suppressors. Cancerous mutations show intra-tumoral heterogeneity across cancer types and eventually converge into the self-activation of proliferative signaling. While transient production of intracellular reactive oxygen species (ROS) is essential for cell signaling, its persistent production is cytotoxic. Thus, cancer cells require increased levels of intracellular ROS for continuous proliferation, but overexpress cellular peroxidase enzymes, such as 2-Cys peroxiredoxins, to maintain ROS homeostasis. However, suppression of 2-Cys peroxiredoxins has also been reported in some metastatic cancers. Hence, the cancer-associated functions of 2-Cys peroxiredoxins must be illuminated in the cellular context. In this review, we describe the distinctive signaling roles of 2-Cys peroxiredoxins beyond their intrinsic ROS-scavenging role in relation to cancer cell death and survival.

Mitochondrial H2O2 Generation Using a Tunable Chemogenetic Tool To Perturb Redox Homeostasis in Human Cells and Induce Cell Death

Among reactive oxygen species (ROS), H₂O₂ alone acts as a signaling molecule that promotes diverse phenotypes depending on the intracellular concentration. Mitochondria have been suggested as both sources and sinks of cellular H₂O₂, and mitochondrial dysfunction has been implicated in diseases such as cancer. A genetically encoded H₂O₂ generator, d-amino acid oxidase (DAAO), was targeted to the mitochondria of human cells, and its utility in investigating cellular response to a range of H₂O₂ doses over time was assessed. Organelle-specific peroxiredoxin dimerization and protein S-glutathionylation were measured as indicators of increased H₂O₂ flux due to the activity of DAAO. Cell death was observed in a concentration- and time-dependent manner, and protein oxidation shifted in localization as the dose increased. This work presents the first systematic study of H₂O₂-specific perturbation of mitochondria in human cells, and it reveals a marked sensitivity of this organelle to increases in H₂O₂ in comparison with prior studies that targeted the cytosol.

Heat-shock protein B1 upholds the cytoplasm reduced state to inhibit activation of the Hippo pathway in H9c2 cells

Heat-shock protein B1 (HSPB1) is a multifunctional protein that protects against oxidative stress; however, its function in antioxidant pathways remains largely unknown. Here, we sought to determine the roles of HSPB1 in H9c2 cells subjected to oxidative stress. Using nonreducing sodium dodecyl sulfate polyacrylamide gel electrophoresis, we found that increased HSPB1 expression promoted the reduced states of glutathione reductase (GR), peroxiredoxin 1 (Prx1), and thioredoxin 1, whereas knockdown of HSPB1 attenuated these responses following oxidative stress. Increased HSPB1 expression promoted the activation of GR and thioredoxin reductase. Conversely, knockdown of HSPB1 attenuated these responses following oxidative stress. Importantly, overexpression of HSPB1 promoted the complex formation between HSPB1 and oxidized Prx1, leading to dephosphorylation of STE-mammalian STE20-like kinase 1 (MST1) in H9c2 cells exposed to H₂ O ₂ , whereas downregulation of HSPB1 induced the opposite results. Mechanistically, HSPB1 regulated the Hippo pathway by enhancing the dephosphorylation of MST1, resulting in reduced phosphorylation of LATS1 and Yes-associated protein (YAP). Moreover, HSPB1 regulated YAP-dependent gene expression. Thus, HSPB1 promoted the reduced state of endogenous antioxidant pathways following oxidative stress in H9c2 cells and improved the redox state of the cytoplasm via modulation of the Hippo signaling pathway.

Detection of mitochondria-generated reactive oxygen species in cells using multiple probes and methods: Potentials, pitfalls, and the future

Reactive oxygen and nitrogen species (ROS/RNS) such as superoxide (O₂^̇̄), hydrogen peroxide, lipid hydroperoxides, peroxynitrite, and hypochlorous and hypobromous acids play a key role in many pathophysiological processes. Recent studies have focused on mitochondrial ROS as redox signaling species responsible for promoting cell division, modulating and regulating kinases and phosphatases, and activating transcription factors. Many ROS also stimulate cell death and senescence. The extent to which these processes occur is attributed to ROS levels (low or high) in cells. However, the exact nature of ROS remains unknown. Investigators have used redox-active probes that, upon oxidation by ROS, yield products exhibiting fluorescence, chemiluminescence, or bioluminescence. Mitochondria-targeted probes can be used to detect ROS generated in mitochondria. However, because most of these redox-active probes (untargeted and mitochondria-targeted) are oxidized by several ROS species, attributing redox probe oxidation to specific ROS species is difficult. It is conceivable that redox-active probes are oxidized in common one-electron oxidation pathways, resulting in a radical intermediate that either reacts with another oxidant (including oxygen to produce O₂^̇̄) and forms a stable fluorescent product or reacts with O₂^̇̄ to form a fluorescent marker product. Here, we propose the use of multiple probes and complementary techniques (HPLC, LC-MS, redox blotting, and EPR) and the measurement of intracellular probe uptake and specific marker products to identify specific ROS generated in cells. The low-temperature EPR technique developed to investigate cellular/mitochondrial oxidants can easily be extended to animal and human tissues.

Peroxiredoxin Involvement in the Initiation and Progression of Human Cancer

SIGNIFICANCE

It has been proposed that cancer cells are heavily dependent on their antioxidant defenses for survival and growth. Peroxiredoxins are a family of abundant thiol-dependent peroxidases that break down hydrogen peroxide, and they have a central role in the maintenance and response of cells to alterations in redox homeostasis. As such, they are potential targets for disrupting tumor growth. Recent Advances: Genetic disruption of peroxiredoxin expression in mice leads to an increased incidence of neoplastic disease, consistent with a role for peroxiredoxins in protecting genomic integrity. In contrast, many human tumors display increased levels of peroxiredoxin expression, suggesting that strengthened antioxidant defenses provide a survival advantage for tumor progression. Peroxiredoxin inhibitors are being developed and explored as therapeutic agents in different cancer models.

CRITICAL ISSUES

It is important to complement peroxiredoxin knockout and expression studies with an improved understanding of the biological function of the peroxiredoxins. Although current results can be interpreted within the context that peroxiredoxins scavenge hydroperoxides, some peroxiredoxin family members appear to have more complex roles in regulating the response of cells to oxidative stress through protein interactions with constituents of other signaling pathways.

FUTURE DIRECTIONS

Further mechanistic information is required for understanding the role of oxidative stress in cancer, the function of peroxiredoxins in normal versus cancer cells, and for the design and testing of specific peroxiredoxin inhibitors that display selectivity to malignant cells. Antioxid. Redox Signal. 28, 591-608.

Liver proteome response of pre-harvest Atlantic salmon following exposure to elevated temperature

Background

Atlantic salmon production in Tasmania (Southern Australia) occurs near the upper limits of the species thermal tolerance. Summer water temperatures can average over 19 °C over several weeks and have negative effects on performance and health. Liver tissue exerts important metabolic functions in thermal adaptation. With the aim of identifying mechanisms underlying liver plasticity in response to chronic elevated temperature in Atlantic salmon, label-free shotgun proteomics was used to explore quantitative protein changes after 43 days of exposure to elevated temperature.

Results

A total of 276 proteins were differentially (adjusted p-value < 0.05) expressed between the control (15 °C) and elevated (21 °C) temperature treatments. As identified by Ingenuity Pathway Analysis (IPA), transcription and translation mechanisms, protein degradation via the proteasome, and cytoskeletal components were down-regulated at elevated temperature. In contrast, an up-regulated response was identified for NRF2-mediated oxidative stress, endoplasmic reticulum stress, and amino acid degradation. The proteome response was paralleled by reduced fish condition factor and hepato-somatic index at elevated temperature.

Conclusions

The present study provides new evidence of the interplay among different cellular machineries in a scenario of heat-induced energy deficit and oxidative stress, and refines present understanding of how Atlantic salmon cope with chronic exposure to temperature near the upper limits of thermal tolerance.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4517-0) contains supplementary material, which is available to authorized users.

Control of Mitochondrial Remodeling by the ATPase Inhibitory Factor 1 Unveils a Pro-survival Relay via OPA1

The ubiquitously expressed ATPase inhibitory factor 1 (IF1) is a mitochondrial protein that blocks the reversal of the F1Fo-ATPsynthase, preventing dissipation of cellular ATP and ischemic damage. IF1 suppresses programmed cell death, enhancing tumor invasion and chemoresistance, and is expressed in various types of human cancers. In this study, we examined its effect on mitochondrial redox balance and apoptotic cristae remodeling, finding that, by maintaining ATP levels, IF1 reduces glutathione (GSH) consumption and inactivation of peroxiredoxin 3 (Prx3) during apoptosis. This correlates with inhibition of metallopeptidase OMA1-mediated processing of the pro-fusion dynamin-related protein optic atrophy 1 (OPA1). Stabilization of OPA1 impedes cristae remodeling and completion of apoptosis. Taken together, these data suggest that IF1 acts on both mitochondrial bioenergetics and structure, is involved in mitochondrial signaling in tumor cells, and may underlie their proliferative capacity.

NOX2 amplifies acetaldehyde-mediated cardiomyocyte mitochondrial dysfunction in alcoholic cardiomyopathy

Alcoholic cardiomyopathy (ACM) resulting from excess alcohol consumption is an important cause of heart failure (HF). Although it is assumed that the cardiotoxicity of the ethanol (EtOH)-metabolite acetaldehyde (ACA) is central for its development and progression, the exact mechanisms remain obscure. Murine cardiomyocytes (CMs) exposed to ACA or EtOH showed increased superoxide (O₂^•−) levels and decreased mitochondrial polarization, both being normalized by NADPH oxidase (NOX) inhibition. C57BL/6 mice and mice deficient for the ACA-degrading enzyme mitochondrial aldehyde dehydrogenase (ALDH-2^−/−) were fed a 2% EtOH diet for 5 weeks creating an ACA-overload. 2% EtOH-fed ALDH-2^−/− mice exhibited a decreased cardiac function, increased heart-to-body and lung-to-body weight ratios, increased cardiac levels of the lipid peroxidation product malondialdehyde (MDA) as well as increased NOX activity and NOX2/glycoprotein 91^phox (NOX2/gp91^phox) subunit expression compared to 2% EtOH-fed C57BL/6 mice. Echocardiography revealed that ALDH-2^−/−/gp91^phox−/− mice were protected from ACA-overload-induced HF after 5 weeks of 2% EtOH-diet, demonstrating that NOX2-derived O₂^•− contributes to the development of ACM. Translated to human pathophysiology, we found increased gp91^phox expression in endomyocardial biopsies of ACM patients. In conclusion, ACM is promoted by ACA-driven mitochondrial dysfunction and can be improved by ablation of NOX2/gp91^phox. NOX2/gp91^phox therefore might be a potential pharmacological target to treat ACM.

Enhanced targeting of mitochondrial peroxide defense by the combined use of thiosemicarbazones and inhibitors of thioredoxin reductase

Peroxiredoxin-3 (Prx3) accounts for about 90% of mitochondrial peroxidase activity, and its marked upregulation in many cancers is important for cell survival. Prx3 oxidation can critically alter peroxide signaling and defense and can be a seminal event in promoting cell death. Here it is shown that this mechanism can be exploited pharmacologically by combinations of clinically available drugs that compromise Prx3 function in different ways. Clinically relevant levels of the thiosemicarbazone iron chelators triapine (Tp) and 2,2'-Dipyridyl-N,N-dimethylsemicarbazone (Dp44mT) promote selective oxidation of mitochondrial Prx3, but not cytosolic Prx1, in multiple human lung and ovarian cancer lines. Decreased cell survival closely correlates with Prx3 oxidation. However, Prx3 oxidation is not merely an indicator of cell death as cytotoxic concentrations of cisplatin do not cause Prx3 oxidation. The siRNA-mediated suppression of either Prx3 or thioredoxin-2, which supports Prx3, enhances Tp's cytotoxicity. Tp-mediated Prx3 oxidation is driven by enhanced peroxide generation, but not by nitric oxide. Many tumors overexpress thioredoxin reductase (TrxR) which supports Prx activity. Direct inhibitors of TrxR (e.g. auranofin, cisplatin) markedly enhanced Tp's cytotoxicity, and auranofin enhanced Prx3 oxidation by low dose Tp. Together, these results support an important role for Prx3 oxidation in the cytotoxicity of Tp, and demonstrate that TrxR inhibitors can significantly enhance Tp's cytotoxicity. Thiosemicarbazone-based regimens could prove effective for targeting Prx3 in a variety of cancers.

Overview on Peroxiredoxin

Peroxiredoxins (Prxs) are a very large and highly conserved family of peroxidases that reduce peroxides, with a conserved cysteine residue, designated the "peroxidatic" Cys (CP) serving as the site of oxidation by peroxides (Hall et al., 2011; Rhee et al., 2012). Peroxides oxidize the CP-SH to cysteine sulfenic acid (CP-SOH), which then reacts with another cysteine residue, named the "resolving" Cys (CR) to form a disulfide that is subsequently reduced by an appropriate electron donor to complete a catalytic cycle. This overview summarizes the status of studies on Prxs and relates the following 10 minireviews.

Utilizing Natural and Engineered Peroxiredoxins As Intracellular Peroxide Reporters

It is increasingly apparent that nature evolved peroxiredoxins not only as H2O2 scavengers but also as highly sensitive H2O2 sensors and signal transducers. Here we ask whether the H2O2 sensing role of Prx can be exploited to develop probes that allow to monitor intracellular H2O2 levels with unprecedented sensitivity. Indeed, simple gel shift assays visualizing the oxidation of endogenous 2-Cys peroxiredoxins have already been used to detect subtle changes in intracellular H2O2 concentration. The challenge however is to create a genetically encoded probe that offers real-time measurements of H2O2 levels in intact cells via the Prx oxidation state. We discuss potential design strategies for Prx-based probes based on either the redox-sensitive fluorophore roGFP or the conformation-sensitive fluorophore cpYFP. Furthermore, we outline the structural and chemical complexities which need to be addressed when using Prx as a sensing moiety for H2O2 probes. We suggest experimental strategies to investigate the influence of these complexities on probe behavior. In doing so, we hope to stimulate the development of Prx-based probes which may spearhead the further study of cellular H2O2 homeostasis and Prx signaling.

Peroxiredoxins and the Regulation of Cell Death

Cell death pathways such as apoptosis can be activated in response to oxidative stress, enabling the disposal of damaged cells. In contrast, controlled intracellular redox events are proposed to be a significant event during apoptosis signaling, regardless of the initiating stimulus. In this scenario oxidants act as second messengers, mediating the post-translational modification of specific regulatory proteins. The exact mechanism of this signaling is unclear, but increased understanding offers the potential to promote or inhibit apoptosis through modulating the redox environment of cells. Peroxiredoxins are thiol peroxidases that remove hydroperoxides, and are also emerging as important players in cellular redox signaling. This review discusses the potential role of peroxiredoxins in the regulation of apoptosis, and also their ability to act as biomarkers of redox changes during the initiation and progression of cell death.

Selective superoxide generation within mitochondria by the targeted redox cycler MitoParaquat

Superoxide is the proximal reactive oxygen species (ROS) produced by the mitochondrial respiratory chain and plays a major role in pathological oxidative stress and redox signaling. While there are tools to detect or decrease mitochondrial superoxide, none can rapidly and specifically increase superoxide production within the mitochondrial matrix. This lack impedes progress, making it challenging to assess accurately the roles of mitochondrial superoxide in cells and in vivo. To address this unmet need, we synthesized and characterized a mitochondria-targeted redox cycler, MitoParaquat (MitoPQ) that comprises a triphenylphosphonium lipophilic cation conjugated to the redox cycler paraquat. MitoPQ accumulates selectively in the mitochondrial matrix driven by the membrane potential. Within the matrix, MitoPQ produces superoxide by redox cycling at the flavin site of complex I, selectively increasing superoxide production within mitochondria. MitoPQ increased mitochondrial superoxide in isolated mitochondria and cells in culture ~a thousand-fold more effectively than untargeted paraquat. MitoPQ was also more toxic than paraquat in the isolated perfused heart and in Drosophila in vivo. MitoPQ enables the selective generation of superoxide within mitochondria and is a useful tool to investigate the many roles of mitochondrial superoxide in pathology and redox signaling in cells and in vivo.

Mitochondrial peroxiredoxin 3 (Prx3) from rock bream (Oplegnathus fasciatus): immune responses and role of recombinant Prx3 in protecting cells from hydrogen peroxide induced oxidative stress

Pathogenic infections and environmental factors cause a variety of stresses in fish including oxidative stress by rapid elevation of reactive oxygen species (ROS) and reactive nitrogen species (RNS). Transcriptional activation and expression of antioxidant enzymes are essential for reducing the oxidative stress. In this study, we present the molecular characterization, immune responses and ROS scavenging activity of mitochondrial peroxiredoxin 3 from Oplegnathus fasciatus (RbPrx3). Coding sequence (CDS) of RbPrx3 contains 248 amino acids polypeptide which consists of highly conserved peroxiredoxin super family domain and two cysteine residues. Pairwise sequence comparison revealed that RbPrx3 has the greatest identity (94.8%) to Sparus aurata Prx3. Transcriptional analysis of RbPrx3 indicated the ubiquitously expressed mRNA in wide array of organs showing the highest expression in the liver of rock bream. Upon immune challenge of Edwardsiella tarda, Streptococcus iniae, rock bream iridovirus (RBIV) and lipopolysaccharide (LPS), RbPrx3 mRNA level was up-regulated in immunocompetent liver tissues compared to unchallenged fish. Purified recombinant RbPrx3 treated THP-1 cells showed higher survival rate against H(2)O(2) induced oxidative stress and significantly reduced the level of intracellular ROS. Overall results from our study suggest that RbPrx3 may be involved in broader functions such as regulating oxidative stresses by scavenging ROS and activating immune responses in rock bream.

Comparative proteomics analysis of host cells infected with Brucella abortus A19

We carried out a proteomic analysis of THP-1-derived macrophages with and without Brucella abortus A19 (B. abortus A19) infection in order to study the cellular responses to B. abortus A19. The proteins were analyzed at different time points after infection with 2DE followed by MALDI-TOF/TOF identification. Comparative analysis of multiple 2DE gels revealed that the majority of changes in protein abundance appeared between 48 and 96 h after infection. MS identified 44 altered proteins, including 20 proteins increased in abundance and 24 proteins decreased in abundance, which were found to be involved in cytoskeleton, signal transduction, energy metabolism, host macromolecular biosynthesis, and stress response. Moreover, 22 genes corresponding to the altered proteins were quantified by real-time RT-PCR to examine the transcriptional profiles between infected and uninfected THP-1-derived macrophages. Finally, we mapped the altered pathways and networks using ingenuity pathway analysis, which suggested that the altered protein species were heavily favored germ cell-Sertoli cell junction signaling as the primary pathway. Furthermore, mechanisms of viral exit from host cell and macrophage stimulating protein-recepteur d'origine nantais signaling appeared to be major pathways modulated in infected cells. This study effectively provides useful dynamic protein-related information concerning B. abortus infection in macrophages.

2-cys peroxiredoxins: emerging hubs determining redox dependency of Mammalian signaling networks

Mammalian cells have a well-defined set of antioxidant enzymes, which includes superoxide dismutases, catalase, glutathione peroxidases, and peroxiredoxins. Peroxiredoxins are the most recently identified family of antioxidant enzymes that catalyze the reduction reaction of peroxides, such as H2O2. In particular, typical 2-Cys peroxiredoxins are the featured peroxidase enzymes that receive the electrons from NADPH by coupling with thioredoxin and thioredoxin reductase. These enzymes distribute throughout the cellular compartments and, therefore, are thought to be broad-range antioxidant defenders. However, recent evidence demonstrates that typical 2-Cys peroxiredoxins play key signal regulatory roles in the various signaling networks by interacting with or residing near a specific redox-sensitive molecule. These discoveries help reveal the redox signaling landscape in mammalian cells and may further provide a new paradigm of therapeutic approaches based on redox signaling.

Peroxiredoxins as biomarkers of oxidative stress

BACKGROUND

Peroxiredoxins (Prxs) are a class of abundant thiol peroxidases that degrade hydroperoxides to water. Prxs are sensitive to oxidation, and it is hypothesized that they also act as redox sensors. The accumulation of oxidized Prxs may indicate disruption of cellular redox homeostasis.

SCOPE OF REVIEW

This review discusses the biochemical properties of the Prxs that make them suitable as endogenous biomarkers of oxidative stress, and describes the methodology available for measuring Prx oxidation in biological systems.

MAJOR CONCLUSIONS

Two Prx oxidation products accumulate in cells under increased oxidative stress: an intermolecular disulfide and a hyperoxidized form. Methodologies are available for measuring both of these redox states, and oxidation has been reported in cells and tissues under oxidative stress from external or internal sources.

GENERAL SIGNIFICANCE

Monitoring the oxidation state of Prxs provides insight into disturbances of cellular redox homeostasis, and complements the use of exogenous probes of oxidative stress. This article is part of a Special Issue entitled Current methods to study reactive oxygen species - pros and cons and biophysics of membrane proteins. Guest Editor: Christine Winterbourn.

Peroxiredoxin-3 is overexpressed in prostate cancer and promotes cancer cell survival by protecting cells from oxidative stress

Objective:

We have previously identified peroxiredoxin-3 (PRDX-3) as a cell-surface protein that is androgen regulated in the LNCaP prostate cancer (PCa) cell line. PRDX-3 is a member of the peroxiredoxin family that are responsible for neutralising reactive oxygen species.

Experimental design:

PRDX-3 expression was examined in tissue from 32 patients using immunohistochemistry. Subcellular distribution was determined using confocal microscopy. PRDX-3 expression was determined in antiandrogen-resistant cell lines by western blotting and quantitative RT–PCR. The pathways of PRDX-3 overexpression and knockdown on apoptosis and response to oxidative stress were investigated using protein arrays.

Results:

PRDX-3 is upregulated in a number of endocrine-regulated tumours; in particular in PCa and prostatic intraepithelial neoplasia. Although the majority of PRDX-3 is localised to the mitochondria, we have confirmed that PRDX-3 at the cell membrane is androgen regulated. In antiandrogen-resistant LNCaP cell lines, PRDX-3 is upregulated at the protein but not RNA level. Resistant cells also possess an upregulation of the tricarboxylic acid (TCA) pathway and resistance to H₂O₂-induced apoptosis through a failure to activate pro-apoptotic pathways. Knockdown of PRDX-3 restored H₂O₂ sensitivity.

Conclusion:

Our results suggest that PRDX-3 has an essential role in regulating oxidation-induced apoptosis in antiandrogen-resistant cells. PRDX-3 may have potential as a therapeutic target in castrate-independent PCa.

Anti-apoptotic role of peroxiredoxin III in cervical cancer cells

As a member of peroxiredoxin (Prx) family, PrxIII is predominantly located in mitochondria and plays an important role as a scavenger of reactive oxygen species (ROS). Since previous reports demonstrated over-expression of PrxIII in cervical cancer, we conducted the present study to investigate the significance of PrxIII in cervical cancer development and/or progression.

Cervical cancer cells were cultured from tissues derived from cervical cancer patients. After successful knockdown of PrxIII expression by small interfering RNA, we evaluated ROS level, viable cell number, and apoptosis of cervical cancer cells along with the culture time.

The production of ROS was increased in cervical cancer cells as compared with normal cervical epithelia. Knockdown of PrxIII expression induced up-regulation of other Prx members including PrxI, PrxII, and PrxV. ROS level was higher in down-regulated cervical cancer cells than in controls and the difference was increasing with culture time. We also observed increased apoptosis and decreased viable cell number in down-regulated cervical cancer cells.

Our results suggest that PrxIII is an indispensable ROS scavenger, which protects tumor cells against oxidative damage and subsequent apoptosis.

Mitochondrial thiols in the regulation of cell death pathways

SIGNIFICANCE

Regulation of mitochondrial H(2)O(2) homeostasis and its involvement in the regulation of redox-sensitive signaling and transcriptional pathways is the consequence of the concerted activities of the mitochondrial energy- and redox systems.

RECENT ADVANCES

The energy component of this mitochondrial energy-redox axis entails the formation of reducing equivalents and their flow through the respiratory chain with the consequent electron leak to generate [Formula: see text] and H(2)O(2). The mitochondrial redox component entails the thiol-based antioxidant system, largely accounted for by glutathione- and thioredoxin-based systems that support the activities of glutathione peroxidases, peroxiredoxins, and methionine sulfoxide reductase. The ultimate reductant for these systems is NADPH: mitochondrial sources of NADPH are the nicotinamide nucleotide transhydrogenase, isocitrate dehydrogenase-2, and malic enzyme. NADPH also supports the glutaredoxin activity that regulates the extent of S-glutathionylation of mitochondrial proteins in response to altered redox status.

CRITICAL ISSUES

The integrated network of these mitochondrial thiols constitute a regulatory device involved in the maintenance of steady-state levels of H(2)O(2), mitochondrial and cellular redox and metabolic homeostasis, as well as the modulation of cytosolic redox-sensitive signaling; disturbances of this regulatory device affects transcription, growth, and ultimately influences cell survival/death.

FUTURE DIRECTIONS

The modulation of key mitochondrial thiol proteins, which participate in redox signaling, maintenance of the bioenergetic machinery, oxidative stress responses, and cell death programming, provides a pivotal direction in developing new therapies towards the prevention and treatment of several diseases.

Methylglyoxal has deleterious effects on thioredoxin in human aortic endothelial cells

Methylglyoxal (MG), a precursor of advanced glycation end products (AGEs), is elevated in diabetic patient's plasma. Some studies have demonstrated that MG induces oxidative stress and apoptosis. Thioredoxin (Trx) is a cytoprotective protein with anti-oxidative and anti-apoptosis functions. In this study, we examined the effects of MG on Trx in human aortic endothelial cells (HAECs). MG increased oxidized-hydroethidine fluorescence intensity, suggesting intracellular accumulation of reactive oxygen species. Flow cytometric analyses with annexin-V/propidium iodide double staining revealed that cells incubated with MG displayed features characteristic of apoptosis. The condensation of chromatin, the release of cytochrome c into cytosol, and the collapse of mitochondrial membrane potential by MG were observed. The exposure to MG decreased Trx protein levels through transcription regulation. MG induced the oxidative damage of peroxiredoxin, a Trx-dependent peroxidase. These results suggest that MG has deleterious effects on Trx in HAECs, which may be contribute to oxidative stress and apoptosis.

Inactivation of pyruvate dehydrogenase kinase 2 by mitochondrial reactive oxygen species

Reactive oxygen species are byproducts of mitochondrial respiration and thus potential regulators of mitochondrial function. Pyruvate dehydrogenase kinase 2 (PDHK2) inhibits the pyruvate dehydrogenase complex, thereby regulating entry of carbohydrates into the tricarboxylic acid (TCA) cycle. Here we show that PDHK2 activity is inhibited by low levels of hydrogen peroxide (H(2)O(2)) generated by the respiratory chain. This occurs via reversible oxidation of cysteine residues 45 and 392 on PDHK2 and results in increased pyruvate dehydrogenase complex activity. H(2)O(2) derives from superoxide (O(2)(.)), and we show that conditions that inhibit PDHK2 also inactivate the TCA cycle enzyme, aconitase. These findings suggest that under conditions of high mitochondrial O(2)(.) production, such as may occur under nutrient excess and low ATP demand, the increase in O(2)() and H(2)O(2) may provide feedback signals to modulate mitochondrial metabolism.

The effects of chromium(VI) on the thioredoxin system: implications for redox regulation

Hexavalent chromium [Cr(VI)] compounds are highly redox active and have long been recognized as potent cytotoxins and carcinogens. The intracellular reduction of Cr(VI) generates reactive Cr intermediates, which are themselves strong oxidants, as well as superoxide, hydrogen peroxide, and hydroxyl radical. These probably contribute to the oxidative damage and effects on redox-sensitive transcription factors that have been reported. However, the identification of events that initiate these signaling changes has been elusive. More recent studies show that Cr(VI) causes irreversible inhibition of thioredoxin reductase (TrxR) and oxidation of thioredoxin (Trx) and peroxiredoxin (Prx). Mitochondrial Trx2/Prx3 are more sensitive to Cr(VI) treatment than cytosolic Trx1/Prx1, although both compartments show thiol oxidation with higher doses or longer treatments. Thiol redox proteomics demonstrate that Trx2, Prx3, and Trx1 are among the most sensitive proteins in cells to Cr(VI) treatment. Their oxidation could therefore represent initiating events that have widespread implications for protein thiol redox control and for multiple aspects of redox signaling. This review summarizes the effects of Cr(VI) on the TrxR/Trx system and how these events could influence a number of downstream redox signaling systems that are influenced by Cr(VI) exposure. Some of the signaling events discussed include the activation of apoptosis signal regulating kinase and MAP kinases (p38 and JNK) and the modulation of a number of redox-sensitive transcription factors including AP-1, NF-κB, p53, and Nrf2.

Lymphocytes from rheumatoid arthritis patients have elevated levels of intracellular peroxiredoxin 2, and a greater frequency of cells with exofacial peroxiredoxin 2, compared with healthy human lymphocytes

Peroxiredoxin 2 has immune regulatory functions, but its expression in human peripheral blood lymphocytes and levels in extracellular fluid in healthy subjects and rheumatoid arthritis patients are poorly described. In the present study, the median intracellular peroxiredoxin 2 protein content of lymphocytes from rheumatoid arthritis patients was more than two-fold higher compared with healthy subjects’ lymphocytes. Intracellular peroxiredoxin 3 levels were similar in healthy and rheumatoid arthritis lymphocytes. Flow cytometry detected peroxiredoxin 2 on the surface of ca. 8% of T cells and ca. 56% of B cells (median % values) of all subjects analyzed. Exofacial thioredoxin-1 was also observed. In the total lymphocyte population from rheumatoid arthritis patients, few cells (median, 6%) displayed surface peroxiredoxin 2. In contrast, a significantly increased proportion of interleukin-17^+ve lymphocytes were exofacially peroxiredoxin 2^+ve (median, 39%). Prdx2 was also detected in human extracellular fluids. We suggest that crucial inflammatory cell subsets, i.e. interleukin-17^+ve T cells, exhibit increased exofacial redox-regulating enzymes and that peroxiredoxin 2 may be involved in the persistence of pro-inflammatory cells in chronic inflammation.

Mitochondrial thiols in antioxidant protection and redox signaling: distinct roles for glutathionylation and other thiol modifications

SIGNIFICANCE

The mitochondrial matrix contains much of the machinery at the heart of metabolism. This compartment is also exposed to a high and continual flux of superoxide, hydrogen peroxide, and related reactive species. To protect mitochondria from these sources of oxidative damage, there is an integrated set of thiol systems within the matrix comprising the thioredoxin/peroxiredoxin/methionine sulfoxide reductase pathways and the glutathione/glutathione peroxidase/glutathione-S-transferase/glutaredoxin pathways that in conjunction with protein thiols prevent much of this oxidative damage. In addition, the changes in the redox state of many components of these mitochondrial thiol systems may transduce and relay redox signals within and through the mitochondrial matrix to modulate the activity of biochemical processes.

RECENT ADVANCES

Here, mitochondrial thiol systems are reviewed, and areas of uncertainty are pointed out, focusing on recent developments in our understanding of their roles.

CRITICAL ISSUES

The areas of particular focus are on the multiple, overlapping roles of mitochondrial thiols and on understanding how these thiols contribute to both antioxidant defenses and redox signaling.

FUTURE DIRECTIONS

Recent technical progress in the identification and quantification of thiol modifications by redox proteomics means that many of the questions raised about the multiple roles of mitochondrial thiols can now be addressed.

Biological targets of isothiocyanates

BACKGROUND

Isothiocyanates are phytochemicals with a broad array of effects in biological systems. Bioactivity includes the stimulation of cellular antioxidant systems, induction of apoptosis and interference with cytokine production and activity. Epidemiological evidence and experimental studies indicate that naturally occurring isothiocyanates and synthetic derivatives have anti-cancer and anti-inflammatory properties.

SCOPE OF REVIEW

This review focuses on the molecular targets of isothiocyanates, and how target modification translates into a biological response.

MAJOR CONCLUSIONS

Isothiocyanates may mediate their effects via direct protein modification or indirectly by disruption of redox homeostasis and increased thiol oxidation. Some target proteins have been identified, but in-depth searches with new techniques are needed to reveal novel targets. Site-directed mutagenesis and isothiocyanate structure-activity relationships will assist in determining the biological significance of specific modifications.

GENERAL SIGNIFICANCE

Target identification is important for rational drug design and exploiting the therapeutic potential of isothiocyanates. It also provides insight into the diverse pathways that these compounds regulate.

The intracellular redox stress caused by hexavalent chromium is selective for proteins that have key roles in cell survival and thiol redox control

Hexavalent chromium [Cr(VI)] compounds (e.g. chromates) are strong oxidants that readily enter cells where they are reduced to reactive Cr intermediates that can directly oxidize some cell components and can promote the generation of reactive oxygen and nitrogen species. Inhalation is a major route of exposure which directly exposes the bronchial epithelium. Previous studies with non-cancerous human bronchial epithelial cells (BEAS-2B) demonstrated that Cr(VI) treatment results in the irreversible inhibition of thioredoxin reductase (TrxR) and the oxidation of thioredoxins (Trx) and peroxiredoxins (Prx). The mitochondrial Trx/Prx system is somewhat more sensitive to Cr(VI) than the cytosolic Trx/Prx system, and other redox-sensitive mitochondrial functions are subsequently affected including electron transport complexes I and II. Studies reported here show that Cr(VI) does not cause indiscriminant thiol oxidation, and that the Trx/Prx system is among the most sensitive of cellular protein thiols. Trx/Prx oxidation is not unique to BEAS-2B cells, as it was also observed in primary human bronchial epithelial cells. Increasing the intracellular levels of ascorbate, an endogenous Cr(VI) reductant, did not alter the effects on TrxR, Trx, or Prx. The peroxynitrite scavenger MnTBAP did not protect TrxR, Trx, Prx, or the electron transport chain from the effects of Cr(VI), implying that peroxynitrite is not required for these effects. Nitration of tyrosine residues of TrxR was not observed following Cr(VI) treatment, further ruling out peroxynitrite as a significant contributor to the irreversible inhibition of TrxR. Cr(VI) treatments that disrupt the TrxR/Trx/Prx system did not cause detectable mitochondrial DNA damage. Overall, the redox stress that results from Cr(VI) exposure shows selectivity for key proteins which are known to be important for redox signaling, antioxidant defense, and cell survival.

The iron-chelating drug triapine causes pronounced mitochondrial thiol redox stress

Triapine (Tp) is an iron chelator with activity against several types of cancer. Iron-Tp [Fe(III)(Tp)(2)] can be redox-cycled to generate reactive oxygen species that may contribute to its cytotoxicity. However, evidence for this mechanism in cells is limited. The cytosolic and mitochondrial thioredoxins (Trx1 and Trx2, respectively) are essential for cell survival. They are normally maintained in the reduced state, and support the function of many intracellular proteins including the peroxiredoxins (Prxs). Their redox status can indicate oxidant stress in their respective subcellular compartments. Tp treatment of human lung A549 cells caused almost complete oxidation of Trx2 and its dependent peroxiredoxin (Prx3), but there was no effect on Trx1 redox status. Significant inhibition of total TrxR activity did not occur until Tp levels were 4-fold above those needed to cause Trx2 oxidation. While Tp caused a 36-45% decline in reduced glutathione (GSH) levels, GSH accounted for >99% of the total glutathione in the absence and presence of Tp. In vitro studies demonstrated that cysteine reduces Fe(III)(Tp)(2) to Fe(II)(Tp)(2), and cysteine was faster and more efficient than reduced glutathione (GSH) in this regard. Fe(III)(Tp)(2) also mediated the oxidation of purified Trx2 in vitro. Thus, Fe(III)(Tp)(2) itself, and/or various reactive species that may result from its redox cycling, could account for Trx2 and Prx3 oxidation in Tp-treated cells. The striking difference between the effects on Trx2 and Trx1 implies a pronounced thiol redox stress that is largely directed at the mitochondria. These previously unrecognized effects of Tp could contribute to its overall cytotoxicity.

Both thioredoxin 2 and glutaredoxin 2 contribute to the reduction of the mitochondrial 2-Cys peroxiredoxin Prx3

The proteins from the thioredoxin family are crucial actors in redox signaling and the cellular response to oxidative stress. The major intracellular source for oxygen radicals are the components of the respiratory chain in mitochondria. Here, we show that the mitochondrial 2-Cys peroxiredoxin (Prx3) is not only substrate for thioredoxin 2 (Trx2), but can also be reduced by glutaredoxin 2 (Grx2) via the dithiol reaction mechanism. Grx2 reduces Prx3 exhibiting catalytic constants (K(m), 23.8 μmol·liter(-1); V(max), 1.2 μmol·(mg·min)(-1)) similar to Trx2 (K(m), 11.2 μmol·liter(-1); V(max), 1.1 μmol·(mg·min)(-1)). The reduction of the catalytic disulfide of the atypical 2-Cys Prx5 is limited to the Trx system. Silencing the expression of either Trx2 or Grx2 in HeLa cells using specific siRNAs did not change the monomer:dimer ratio of Prx3 detected by a specific 2-Cys Prx redox blot. Only combined silencing of the expression of both proteins led to an accumulation of oxidized protein. We further demonstrate that the distribution of Prx3 in different mouse tissues is either linked to the distribution of Trx2 or Grx2. These results introduce Grx2 as a novel electron donor for Prx3, providing further insights into pivotal cellular redox signaling mechanisms.