Thiol-based antioxidants elicit mitochondrial oxidation via respiratory complex III

Cysteine induces mitochondrial reductive stress in glioblastoma through hydrogen peroxide production

Glucose and amino acid metabolism are critical for glioblastoma (GBM) growth, but little is known about the specific metabolic alterations in GBM that are targetable with FDA-approved compounds. To investigate tumor metabolism signatures unique to GBM, we interrogated The Cancer Genome Atlas for alterations in glucose and amino acid signatures in GBM relative to other human cancers and found that GBM exhibits the highest levels of cysteine and methionine pathway gene expression of 32 human cancers. Treatment of patient-derived GBM cells with the FDA-approved single cysteine compound N-acetylcysteine (NAC) reduced GBM cell growth and mitochondrial oxygen consumption, which was worsened by glucose starvation. Normal brain cells and other cancer cells showed no response to NAC. Mechanistic experiments revealed that cysteine compounds induce rapid mitochondrial H2O2 production and reductive stress in GBM cells, an effect blocked by oxidized glutathione, thioredoxin, and redox enzyme overexpression. From analysis of the clinical proteomic tumor analysis consortium (CPTAC) database, we found that GBM cells exhibit lower expression of mitochondrial redox enzymes than four other cancers whose proteomic data are available in CPTAC. Knockdown of mitochondrial thioredoxin-2 in lung cancer cells induced NAC susceptibility, indicating the importance of mitochondrial redox enzyme expression in mitigating reductive stress. Intraperitoneal treatment of mice bearing orthotopic GBM xenografts with a two-cysteine peptide induced H2O2 in brain tumors in vivo. These findings indicate that GBM is uniquely susceptible to NAC-driven reductive stress and could synergize with glucose-lowering treatments for GBM.

CHCHD2 mediates glioblastoma cell proliferation, mitochondrial metabolism, hypoxia‑induced invasion and therapeutic resistance

Glioblastoma (GBM) is the most common and malignant primary brain tumor affecting adults and remains incurable. The mitochondrial coiled‑coil‑helix‑coiled‑coil‑helix domain‑containing protein 2 (CHCHD2) has been demonstrated to mediate mitochondrial respiration, nuclear gene expression and cell migration; however, evidence of this in GBM is lacking. In the present study, it was hypothesized that CHCHD2 may play a functional role in U87 GBM cells expressing the constitutively active epidermal growth factor receptor variant III (EGFRvIII). The amplification of the CHCHD2 gene was found to be associated with a decreased patient overall and progression‑free survival. The CHCHD2 mRNA levels were increased in high‑vs. low‑grade glioma, IDH‑wt GBMs, and in tumor vs. non‑tumor tissue. Additionally, CHCHD2 protein expression was greatest in invasive, EGFRvIII‑expressing patient‑derived samples. The CRISPR‑Cas9‑mediated knockout of CHCHD2 in EGFRvIII‑expressing U87 cells resulted in an altered mitochondrial respiration and glutathione status, in decreased cell growth and invasion under both normoxic and hypoxic conditions, and in an enhanced sensitivity to cytotoxic agents. CHCHD2 was distributed in both the mitochondria and nuclei of U87 and U87vIII cells, and the U87vIII cells exhibited a greater nuclear expression of CHCHD2 compared to isogenic U87 cells. Incubation under hypoxic conditions, serum starvation and the reductive unfolding of CHCHD2 induced the nuclear accumulation of CHCHD2 in both cell lines. Collectively, the findings of the present study indicate that CHCHD2 mediates a variety of GBM characteristics, and highlights mitonuclear retrograde signaling as a pathway of interest in GBM cell biology.

Expected and Unexpected Effects of Pharmacological Antioxidants

In this review, we have collected the existing data on the bioactivity of antioxidants (N-acetylcysteine, polyphenols, vitamin C) which are traditionally used in experimental biology and, in some cases, in the clinic. Presented data show that, despite the capacity of these substances to scavenge peroxides and free radicals in cell-free systems, their ability to exhibit these properties in vivo, upon pharmacological supplementation, has not been confirmed so far. Their cytoprotective activity is explained mainly by the ability not to suppress, but to activate multiple redox pathways, which causes biphasic hormetic responses and highly pleiotropic effects in cells. N-acetylcysteine, polyphenols, and vitamin C affect redox homeostasis by generating low-molecular-weight redox-active compounds (H₂O₂ or H₂S), known for their ability to stimulate cellular endogenous antioxidant defense and promote cytoprotection at low concentrations but exert deleterious effects at high concentrations. Moreover, the activity of antioxidants strongly depends on the biological context and mode of their application. We show here that considering the biphasic and context-dependent response of cells on the pleiotropic action of antioxidants can help explain many of the conflicting results obtained in basic and applied research and build a more logical strategy for their use.

Antiproliferative effects of mitochondria-targeted N-acetylcysteine and analogs in cancer cells

N-acetylcysteine (NAC) has been used as an antioxidant drug in tumor cells and preclinical mice tumor xenografts, and it improves adaptive immunotherapy in melanoma. NAC is not readily bioavailable and is used in high concentrations. The effects of NAC have been attributed to its antioxidant and redox signaling role in mitochondria. New thiol-containing molecules targeted to mitochondria are needed. Here, mitochondria-targeted NAC with a 10-carbon alkyl side chain attached to a triphenylphosphonium group (Mito₁₀-NAC) that is functionally similar to NAC was synthesized and studied. Mito₁₀-NAC has a free sulfhydryl group and is more hydrophobic than NAC. Mito₁₀-NAC is nearly 2000-fold more effective than NAC in inhibiting several cancer cells, including pancreatic cancer cells. Methylation of NAC and Mito₁₀-NAC also inhibited cancer cell proliferation. Mito₁₀-NAC inhibits mitochondrial complex I-induced respiration and, in combination with monocarboxylate transporter 1 inhibitor, synergistically decreased pancreatic cancer cell proliferation. Results suggest that the antiproliferative effects of NAC and Mito₁₀-NAC are unlikely to be related to their antioxidant mechanism (i.e., scavenging of reactive oxygen species) or to the sulfhydryl group-dependent redox modulatory effects.

Phase II trial of cytarabine and mitoxantrone with devimistat in acute myeloid leukemia

Devimistat is a TCA cycle inhibitor. A previously completed phase I study of devimistat in combination with cytarabine and mitoxantrone in patients with relapsed or refractory AML showed promising response rates. Here we report the results of a single arm phase II study (NCT02484391). The primary outcome of feasibility of maintenance devimistat following induction and consolidation with devimistat in combination with high dose cytarabine and mitoxantrone was not met, as maintenance devimistat was only administered in 2 of 21 responders. The secondary outcomes of response (CR + CRi) and median survival were 44% (21/48) and 5.9 months respectively. There were no unexpected toxicities observed. An unplanned, post-hoc analysis of the phase I and II datasets suggests a trend of a dose response in older but not younger patients. RNA sequencing data from patient samples reveals an age-related decline in mitochondrial gene sets. Devimistat impairs ATP synthesis and we find a correlation between mitochondrial membrane potential and sensitivity to chemotherapy. Devimistat also induces mitochondrial reactive oxygen species and turnover consistent with mitophagy. We find that pharmacological or genetic inhibition of mitochondrial fission or autophagy sensitizes cells to devimistat. These findings suggest that an age related decline in mitochondrial quality and autophagy may be associated with response to devimistat however this needs to be confirmed in larger cohorts with proper trial design.

Combining cytarabine and mitoxantrone with the tricarboxylic acid cycle inhibitor devimistat has been reported in a phase I clinical trial with relapsed or refractory acute myeloid leukaemia (AML). Here, the authors report the outcomes of a phase II study, analyse samples from both phases and perform preclinical analyses that show mitochondrial fission or autophagy inhibition sensitizes AML cells to devimistat.

Induction of Premature Cell Senescence Stimulated by High Doses of Antioxidants Is Mediated by Endoplasmic Reticulum Stress

In our previous study, we found that high doses of several substances with antioxidant capacities (Tempol, resveratrol, diphenyleneiodonium) can cause genotoxic stress and induce premature senescence in the human mesenchymal stem cells (MSCs). Here, using whole-transcriptome analysis, we revealed the signs of endoplasmic reticulum stress and unfolded protein response (UPR) in MSCs stressed with Tempol and resveratrol. In addition, we found the upregulation of genes, coding the UPR downstream target APC/C, and E3 ubiquitin ligase that regulate the stability of cell cycle proteins. We performed the molecular analysis, which further confirmed the untimely degradation of APC/C targets (cyclin A, geminin, and Emi1) in MSCs treated with antioxidants. Human fibroblasts responded to antioxidant applications similarly. We conclude that endoplasmic reticulum stress and impaired DNA synthesis regulation can be considered as potential triggers of cell damage and premature senescence stimulated by high-dose antioxidant treatments.

Reduced mitochondrial complex II activity enhances cell death via intracellular reactive oxygen species in STHdhQ111 striatal neurons with mutant huntingtin

Huntington's disease (HD) is an inherited neurodegenerative disorder caused by CAG repeat expansion in the huntingtin (HTT) gene. Here, we examined the effects of antioxidants on 3-nitropropionic acid (3-NP; a mitochondrial complex II inhibitor)-induced mitochondrial dysfunction and cell death in STHdhQ111 striatal cells carrying homozygous mutant HTT with extended CAG repeats compared with those in STHdhQ7 striatal cells. 3-NP reduced cell viability and increased cell death both in STHdhQ111 and STHdhQ7, and the cytotoxicity was markedly attenuated by antioxidants (N-acetyl-l-cysteine and edaravone). Furthermore, 3-NP increased intracellular reactive oxygen species (ROS) production in both cell lines, and this increase was inhibited by antioxidants. Mitochondrial ROS was also increased by 3-NP in STHdhQ111 but not in STHdhQ7, and this increase was significantly inhibited by edaravone. Mitochondrial membrane potential (MMP) was lower in STHdhQ111 than that in STHdhQ7, and antioxidants prevented 3-NP-induced MMP decrease in STHdhQ111.3-NP enhanced oligomerization of dynamin-related protein 1 (Drp1), a protein that promotes mitochondrial fission in both cells, and both antioxidants prevented the increase in oligomerization. These results suggest that reduced mitochondrial complex II activity enhances cell death via intracellular ROS production and Drp1 oligomerization in striatal cells with mutant HTT and antioxidants may reduce striatal cell death.

Taurine Reverses Oxidative Damages and Restores the Muscle Function in Overuse of Exercised Muscle

Exercise-induced oxidative stress is linked with the expression level of endogenous antioxidants, but these antioxidants cannot overcome all oxidative stress-related damages in the cells, particularly when cells are under physiological stress. Sometimes, compounds are needed for cellular function, which are produced/activated within the cells, and these compounds can be synthesized by performing exercise, especially high-performance exercise. Taurine is a sulfur-containing amino acid used for various physiological functions. However, its synthesis and accumulation under the oxidative environment may be compromised. Recently, we have shown that taurine level is increased during exercise performance with a decrease in oxidative damage in overused muscles. Other studies have also shown that short-term supplementation with taurine increased physiological performance during severe work intensities, suggesting the role of taurine in improving muscle performance during exercise. However, its precursor cysteine is used in the synthesis of other compounds like GSH and Coenzyme A, which are important for regulating the redox system and energy homeostasis. It is, therefore, important to understand whether taurine synthesis within the cells can blunt the activity of other compounds that are beneficial in preventing oxidative damage during intense exercise. Furthermore, it is important to understand whether taurine supplementation can prevent the conditions observed in the physiological stress of muscles. This review discusses how taurine synthesis could alter exercise-induced ROS generation and the relationship between the physiological stress of muscle and subsequent improvements in exercise performance.

Neuron Activity Dependent Redox Compartmentation Revealed with a Second Generation Red-Shifted Ratiometric Sensor

Oxidative stress is a hallmark of several aging and trauma related neurological disorders, but the precise details of how altered neuronal activity elicits subcellular redox changes have remained difficult to resolve. Current redox sensitive dyes and fluorescent proteins can quantify spatially distinct changes in reactive oxygen species levels, but multicolor probes are needed to accurately analyze compartment-specific redox dynamics in single cells that can be masked by population averaging. We previously engineered genetically encoded red-shifted redox-sensitive fluorescent protein sensors using a Förster resonance energy transfer relay strategy. Here, we developed a second-generation excitation ratiometric sensor called rogRFP2 with improved red emission for quantitative live-cell imaging. Using this sensor to measure activity-dependent redox changes in individual cultured neurons, we observed an anticorrelation in which mitochondrial oxidation was accompanied by a concurrent reduction in the cytosol. This behavior was dependent on the activity of Complex I of the mitochondrial electron transport chain and could be modulated by the presence of cocultured astrocytes. We also demonstrated that the red fluorescent rogRFP2 facilitates ratiometric one- and two-photon redox imaging in rat brain slices and Drosophila retinas. Overall, the proof-of-concept studies reported here demonstrate that this new rogRFP2 redox sensor can be a powerful tool for understanding redox biology both in vitro and in vivo across model organisms.

High doses of synthetic antioxidants induce premature senescence in cultivated mesenchymal stem cells

Stress-induced premature senescence program is known to be activated in cells by various genotoxic stressors, and oxidative stress is considered to be the main of those. To this end, many studies discover antioxidants as protective anti-aging agents. In the current study, we examined the effects of different antioxidants (Tempol, resveratrol, NAC, DPI) on the mesenchymal stem cells maintained in normal physiological conditions. We used high, but non-cytotoxic antioxidant doses which are widely used in laboratory practice to protect cells from oxidative damage. We show that these substances induce reversible block of cell proliferation and do not cause any genotoxic effects when applied to the quiescent cells. However, the same doses of the same substances, when applied to the proliferating cells, can induce irreversible cell cycle arrest, DNA strand breaks accumulation and DNA damage response activation. As a consequence, antioxidant-induced DNA damage results in the stress-induced premature senescence program activation. We conclude that high doses of antioxidants, when applied to the proliferating cells that maintain physiological levels of reactive oxygen species, can cause DNA damage and induce premature senescence which suggests to re-estimate believed unconditional anti-aging antioxidant properties.

Sodium Thiosulfate Preconditioning Ameliorates Ischemia/Reperfusion Injury in Rat Hearts Via Reduction of Oxidative Stress and Apoptosis

PURPOSE

Sodium thiosulfate (STS) has of late been proven efficacious in models of urolithiasis and vascular calcification. However, its cardiovascular effects on ischemia reperfusion injury (IR) have not been revealed. Being an antioxidant and calcium chelator, it is assumed to play a vital role in IR as ROS production and calcium overload are major perpetrators of IR injury.

METHODS

The cardioprotective effect of STS was evaluated in vitro using H9C2 cardiomyocytes and in vivo using both isolated rat heart and intact left anterior descending artery (LAD) occlusion models of ischemia reperfusion injury. Finally, in silico tools were utilized to establish its possible mode of action. Myocardial injury markers and expression of apoptotic proteins were studied along with myocardial histopathology.

RESULTS

STS of 1 mM recovered H9C2 cells from glucose oxidase/catalase-induced apoptosis. The isolated rat heart treated with STS prior to IR injury improved its hemodynamics and reduced the infarct size to 9%. This was supported by the absence of derangement of cardiac fibers from H&E stained section of LAD-occluded rats. Plasma troponin levels decreased by 15% compared to IR and the myocardium showed diminished apoptotic proteins. An in silico docking analysis revealed higher binding affinity of STS for caspase-3 with a binding energy of - 60.523 kcal/mol for the complex.

CONCLUSION

The effectiveness of STS as a cardioprotective agent is attributed to the reduction of apoptosis by binding to the active site of caspase-3 in silico, which was substantiated by the reduced expression of caspase-3 and poly ADP ribose polymerase levels.

Extending roGFP Emission via Förster-Type Resonance Energy Transfer Relay Enables Simultaneous Dual Compartment Ratiometric Redox Imaging in Live Cells

Reactive oxygen species (ROS) mediate both intercellular and intraorganellar signaling, and ROS propagate oxidative stress between cellular compartments such as mitochondria and the cytosol. Each cellular compartment contains its own sources of ROS as well as antioxidant mechanisms, which contribute to dynamic fluctuations in ROS levels that occur during signaling, metabolism, and stress. However, the coupling of redox dynamics between cellular compartments has not been well studied because of the lack of available sensors to simultaneously measure more than one subcellular compartment in the same cell. Currently, the redox-sensitive green fluorescent protein, roGFP, has been used extensively to study compartment-specific redox dynamics because it provides a quantitative ratiometric readout and it is amenable to subcellular targeting as a genetically encoded sensor. Here, we report a new family of genetically encoded fluorescent protein sensors that extend the fluorescence emission of roGFP via Förster-type resonance energy transfer to an acceptor red fluorescent protein for dual-color live-cell microscopy. We characterize the redox and optical properties of the sensor proteins, and we demonstrate that they can be used to simultaneously measure cytosolic and mitochondrial ROS in living cells. Furthermore, we use these sensors to reveal cell-to-cell heterogeneity in redox coupling between the cytosol and mitochondria when neuroblastoma cells are exposed to reductive and metabolic stresses.

Reactive oxygen species (ROS) are a key determinant of cancer's metabolic phenotype

Cancer cells exhibit a wide range of metabolic phenotypes, ranging from strict aerobic glycolysis to increased mitochondrial respiration. The cause and utility of this metabolic variation is poorly understood. Given that cancer cells experience heavy selection within their microenvironment, survival requires metabolic adaptation to both extracellular and intracellular conditions. Herein, we suggest that reactive oxygen species (ROS) are a key determinant of cancer's metabolic phenotype. Intracellular ROS levels can be modified by an assortment of critical parameters including oxygenation, glucose availability and growth factors. ROS act as integrators of environmental information as well as downstream effectors of signaling pathways. Maintaining ROS within a narrow range allows malignant cells to enhance growth and invasion while limiting their apoptotic susceptibility. Cancer cells actively modify their metabolism to optimize intracellular ROS levels and thereby improve survival. Furthermore, we highlight distinct metabolic phenotypes in response to oxidative stress and their tumorigenic drivers.

N‑acetylcysteine induces apoptosis via the mitochondria‑dependent pathway but not via endoplasmic reticulum stress in H9c2 cells

N-acetylcysteine (NAC), a precursor of glutathione, is a widely used thiol-containing antioxidant and modulator of the intracellular redox state. Our previous study demonstrated that excess reduced glutathione (GSH) from NAC treatment paradoxically led to a reduction in glutathione redox potential, increased mitochondrial oxidation and caused cytotoxicity at lower reactive oxygen species levels in H9c2 cells. However, no detailed data are available on the molecular mechanisms of NAC-induced cytotoxicity on H9c2 cells. In the present study, it was demonstrated that NAC-induced cytotoxicity towards H9c2 cells was associated with apoptosis. The activation of caspase-9 and −3, and cleavage of procaspase-9 and −3, but not of caspase-8, were involved in NAC-induced apoptosis. The dissipation of mitochondrial transmembrane potential, release of cytochrome c, translocation of B cell lymphoma-2 (Bcl-2)-associated X protein (Bax) to the mitochondria, and the increased ratio of Bax/Bcl-2 mRNA indicated that NAC treatment-induced apoptosis occurred mainly through the mitochondria-dependent pathway. Redox western blot analysis demonstrated that NAC did not disrupt the highly oxidized environment of the endoplasmic reticulum, which was indicated by maintenance of the oxidized form of protein disulfide isomerase, an essential chaperone in the formation of disulfide bond formation in the endoplasmic reticulum. In addition, no significant changes in the expression of binding immunoglobulin protein or C/EBP homologous protein were apparent in the process of NAC-induced apoptosis. Taken together, the present study demonstrated for the first time, to the best of our knowledge, that NAC induced apoptosis via the mitochondria-dependent pathway but not via endoplasmic reticulum stress in H9c2 cells, and the exogenous GSH from NAC did not alter the oxidized milieu of the endoplasmic reticulum.

Methyl-4-phenylpyridinium (MPP+) differentially affects monoamine release and re-uptake in murine embryonic stem cell-derived dopaminergic and serotonergic neurons

1-Methyl-4-phenyl-1,2,5,6-tetrahydropyridine (MPTP) is known to selectively damage dopaminergic (DA) cells in the substantia nigra and to produce symptoms which are alike to those observed in Parkinson's disease (PD). Based on the similarity between MPTP-induced neurotoxicity and PD-related neuropathology, application of MPTP or its metabolite methyl-4-phenylpyridinium (MPP+) was successfully established in experimental rodent models to study PD-related neurodegenerative events. MPP+ is taken up by the dopamine transporter (DAT) into DA neurons where it exerts its neurotoxic action on mitochondria by affecting complex I of the respiratory chain. MPP+ is also a high affinity substrate for the serotonin transporter (SERT), however little is known about possible toxic effects of MPP+ on serotonergic (5-HT) neurons. In order to compare cell type-specific effects of MPP+ treatment, we have differentiated mouse embryonic stem (ES) cells into DA and 5-HT neurons and studied the impact of MPP+ treatment on both types of monoaminergic neurons in vitro. MPP+ treatment impacts on mitochondrial membrane potential in DA as well as 5-HT ES cell-derived neurons. Although mitochondria metabolisms are similarly affected, synaptic vesicle cycling is only impaired in DA ES cell-derived neurons. Most importantly we show that MPP+ induces DAT externalization in DA neurons, but internalization of SERT in 5-HT neurons. This diverse MPP+-induced transporter trafficking is reflected by elevated substrate uptake in DA neurons, and diminished substrate uptake in 5-HT neurons. In summary, our experimental data point toward differential effects of MPP+ intoxication on neurotransmitter release and re-uptake in different types of monoaminergic neurons.

Comparison of neurons derived from mouse P19, rat PC12 and human SH-SY5Y cells in the assessment of chemical- and toxin-induced neurotoxicity

Background

Exposure to chemicals might be toxic to the developing brain. There is a need for simple and robust in vitro cellular models for evaluation of chemical-induced neurotoxicity as a complement to traditional studies on animals. In this study, neuronally differentiated mouse embryonal carcinoma P19 cells (P19 neurons) were compared with human neuroblastoma SH-SY5Y cells and rat adrenal pheochromocytoma PC12 cells for their ability to detect toxicity of methylmercury (MeHg), okadaic acid and acrylamide.

Methods

Retinoic acid-treated P19 and SH-SY5Y cells and nerve growth factor-stimulated PC12 cells, allowed to differentiate for 6 days, were exposed to MeHg, okadaic acid and acrylamide for 48 h. Cell survival and neurite outgrowth were assessed with the calcein-AM assay and fluorescence detection of antibodies against the cytoskeletal neuron-specific protein βIII-tubulin, respectively. The effects of glutathione (GSH) and the potent inhibitor of GSH synthesis buthionine sulfoximine (BSO) on the MeHg induced-toxicity were assessed using the PrestoBlue™ cell viability assay and the TMRE mitochondrial membrane potential assay.

Results

Differentiated P19 cells developed the most extensive neuronal network among the three cell models and were the most sensitive neuronal model to detect neurotoxic effects of the test compounds. MeHg produced a concentration-dependent toxicity in differentiated P19 cells and SH-SY5Y cells, with statistically significant effects at concentrations from 0.1 μM in the P19 neurons and 1 μM in the SH-SY5Y cells. MeHg induced a decrease in the cellular metabolic activity and mitochondrial membrane potential (ΔΨm) in the differentiated P19 cells and SH-SY5Y cells, that were attenuated by GSH. Okadaic acid and acrylamide also showed statistically significant toxicity in the P19 neurons, but not in the SH-SY5Y cells or the P12 cells.

Conclusions

P19 neurons are more sensitive to detect cytotoxicity of MeHg, okadaic acid and acrylamide than retinoic acid-differentiated SH-SY5Y cells and nerve growth factor-treated PC12 cells. P19 neurons are at least as sensitive as differentiated SH-SY5Y cells to detect the loss of mitochondrial membrane potential produced by MeHg and the protective effects of extracellular GSH on MeHg toxicity. P19 neurons may be a useful model to study neurotoxic effects of chemicals.

Protein S-glutathionylation alters superoxide/hydrogen peroxide emission from pyruvate dehydrogenase complex

Pyruvate dehydrogenase (Pdh) is a vital source of reactive oxygen species (ROS) in several different tissues. Pdh has also been suggested to serve as a mitochondrial redox sensor. Here, we report that O₂^•-/ H₂O₂ emission from pyruvate dehydrogenase (Pdh) is altered by S-glutathionylation. Glutathione disulfide (GSSG) amplified O₂^•-/ H₂O₂ production by purified Pdh during reverse electron transfer (RET) from NADH. Thiol oxidoreductase glutaredoxin-2 (Grx2) reversed these effects confirming that Pdh is a target for S-glutathionylation. S-glutathionylation had the opposite effect during forward electron transfer (FET) from pyruvate to NAD⁺ lowering O₂^•-/ H₂O₂ production. Immunoblotting for protein glutathione mixed disulfides (PSSG) following diamide treatment confirmed that purified Pdh can be S-glutathionylated. Similar observations were made with mouse liver mitochondria. S-glutathionylation catalysts diamide and disulfiram significantly reduced pyruvate or 2-oxoglutarate driven O₂^•-/ H₂O₂ production in liver mitochondria, results that were confirmed using various Pdh, 2-oxoglutarate dehydrogenase (Ogdh), and respiratory chain inhibitors. Immunoprecipitation of Pdh and Ogdh confirmed that either protein can be S-glutathionylated by diamide and disulfiram. Collectively, our results demonstrate that the S -glutathionylation of Pdh alters the amount of ROS formed by the enzyme complex. We also confirmed that Ogdh is controlled in a similar manner. Taken together, our results indicate that the redox sensing and ROS forming properties of Pdh and Ogdh are linked to S-glutathionylation.

GPx1 knockdown suppresses chondrogenic differentiation of ATDC5 cells through induction of reductive stress

Glutathione peroxidase 1 (GPx1) is a selenium (Se)-containing protein and is induced in cartilage formation. GPx1 eliminates reactive oxygen species (ROS), which are required for chondrogenic induction. The physiological properties of GPx1 in cartilage and the redox mechanisms involved are not known. The effects of GPx1 on chondrogenic differentiation of ATDC5 cells were examined through short hairpin RNA-mediated gene silencing. The results demonstrated that GPx1 knockdown impaired gene expression of sex determining region Y-box 9, collagen II (Col II), and aggrecan. GPx1 knockdown suppressed the accumulation of cartilage glycosaminoglycans (GAGs) and the proliferation of chondrocyte. GPx1 knockdown also induced cell apoptosis. However, cell sensitivity toward exogenous oxidative stress was not increased after GPx1 knockdown. Unexpectedly, GPx1 knockdown not only induced oxidative stress characterized by the increased production of ROS but also caused reductive stress indicated by an elevation of glutathione (GSH)/oxidized GSH (GSSG) ratio. Furthermore, GPx1 knockdown-mediated reductive and oxidative stress could be antagonized by a thiol-oxidizing agent diamide and a thiol-containing compound N-acetylcysteine (NAC), respectively. Moreover, NAC attenuated GPx1 knockdown-induced cell apoptosis, while diamide prevented GPx1 knockdown-suppressed chondrocyte proliferation. Finally, diamide but not NAC could rescue GPx1 knockdown-mediated impaired chondrogenic differentiation. In summary, GPx1 is essential for chondrogenic induction in ATDC5 cells mainly through modulation of intracellular GSH/GSSG ratio, rather than an antioxidant enzyme to detoxify ROS. In addition, GPx1 knockdown-induced impaired chondrogenesis may participate in the pathogenesis of the endemic osteoarthropathy due to Se deficiency. These observations offer novel insights for the development of therapeutic target during cartilage degeneration.

Distinct responses of compartmentalized glutathione redox potentials to pharmacologic quinones targeting NQO1

Deoxynyboquinone (DNQ), a potent novel quinone-based antineoplastic agent, selectively kills solid cancers with overexpressed cytosolic NAD(P)H:quinone oxidoreductase-1 (NQO1) via excessive ROS production. A genetically encoded redox-sensitive probe was used to monitor intraorganellar glutathione redox potentials (EGSH) as a direct indicator of cellular oxidative stress following chemotherapeutic administration. Beta-lapachone (β-lap) and DNQ-induced spatiotemporal redox responses were monitored in human lung A549 and pancreatic MIA-PaCa-2 adenocarcinoma cells incubated with or without dicumarol and ES936, potent NQO1 inhibitors. Immediate oxidation of EGSH in both the cytosol and mitochondrial matrix was observed in response to DNQ and β-lap. The DNQ-induced cytosolic oxidation was fully prevented with NQO1 inhibition, whereas mitochondrial oxidation in A549 was NQO1-independent in contrast to MIA-PaCa-2 cells. However, at pharmacologic concentrations of β-lap both quinone-based substrates directly oxidized the redox probe, a possible sign of off-target reactivity with cellular thiols. Together, these data provide new evidence that DNQ's direct and discerning NQO1 substrate specificity underlies its pharmacologic potency, while β-lap elicits off-target responses at its effective doses.

An unexplored role for Peroxiredoxin in exercise-induced redox signalling?

Peroxiredoxin (PRDX) is a ubiquitous oxidoreductase protein with a conserved ionised thiol that permits catalysis of hydrogen peroxide (H2O2) up to a million times faster than any thiol-containing signalling protein. The increased production of H2O2 within active tissues during exercise is thought to oxidise conserved cysteine thiols, which may in turn facilitate a wide variety of physiological adaptations. The precise mechanisms linking H2O2 with the oxidation of signalling thiol proteins (phosphates, kinases and transcription factors) are unclear due to these proteins' low reactivity with H2O2 relative to abundant thiol peroxidases such as PRDX. Recent work has shown that following exposure to H2O2 in vitro, the sulfenic acid of the PRDX cysteine can form mixed disulphides with transcription factors associated with cell survival. This implicates PRDX as an 'active' redox relay in transmitting the oxidising equivalent of H2O2 to downstream proteins. Furthermore, under oxidative stress, PRDX can form stable oxidised dimers that can be secreted into the extracellular space, potentially acting as an extracellular 'stress' signal. There is extensive literature assessing non-specific markers of oxidative stress in response to exercise, however the PRDX catalytic cycle may offer a more robust approach for measuring changes in redox balance following exercise. This review discusses studies assessing PRDX-mediated cellular signalling and integrates the recent advances in redox biology with investigations that have examined the role of PRDX during exercise in humans and animals. Future studies should explore the role of PRDX as a key regulator of peroxide mediated-signal transduction during exercise in humans.

Live-cell imaging approaches for the investigation of xenobiotic-induced oxidant stress

BACKGROUND

Oxidant stress is arguably a universal feature in toxicology. Research studies on the role of oxidant stress induced by xenobiotic exposures have typically relied on the identification of damaged biomolecules using a variety of conventional biochemical and molecular techniques. However, there is increasing evidence that low-level exposure to a variety of toxicants dysregulates cellular physiology by interfering with redox-dependent processes.

SCOPE OF REVIEW

The study of events involved in redox toxicology requires methodology capable of detecting transient modifications at relatively low signal strength. This article reviews the advantages of live-cell imaging for redox toxicology studies.

MAJOR CONCLUSIONS

Toxicological studies with xenobiotics of supra-physiological reactivity require careful consideration when using fluorogenic sensors in order to avoid potential artifacts and false negatives. Fortunately, experiments conducted for the purpose of validating the use of these sensors in toxicological applications often yield unexpected insights into the mechanisms through which xenobiotic exposure induces oxidant stress.

GENERAL SIGNIFICANCE

Live-cell imaging using a new generation of small molecule and genetically encoded fluorophores with excellent sensitivity and specificity affords unprecedented spatiotemporal resolution that is optimal for redox toxicology studies. This article is part of a Special Issue entitled Air Pollution, edited by Wenjun Ding, Andrew J. Ghio and Weidong Wu.