Glutathione and Glutaredoxin-Key Players in Cellular Redox Homeostasis and Signaling

This Special Issue of Antioxidants on Glutathione (GSH) and Glutaredoxin (Grx) was designed to collect review articles and original research studies focused on advancing the current understanding of the roles of the GSH/Grx system in cellular homeostasis and disease processes. The tripeptide glutathione (GSH) is the most abundant non-enzymatic antioxidant/nucleophilic molecule in cells. In addition to various metabolic reactions involving GSH and its oxidized counterpart GSSG, oxidative post-translational modification (PTM) of proteins has been a focal point of keen interest in the redox field over the last few decades. In particular, the S-glutathionylation of proteins (protein-SSG formation), i.e., mixed disulfides between GSH and protein thiols, has been studied extensively. This reversible PTM can act as a regulatory switch to interconvert inactive and active forms of proteins, thereby mediating cell signaling and redox homeostasis. The unique architecture of the GSH molecule enhances its relative abundance in cells and contributes to the glutathionyl specificity of the primary catalytic activity of the glutaredoxin enzymes, which play central roles in redox homeostasis and signaling, and in iron metabolism in eukaryotes and prokaryotes under physiological and pathophysiological conditions. The class-1 glutaredoxins are characterized as cytosolic GSH-dependent oxidoreductases that catalyze reversible protein S-glutathionylation specifically, thereby contributing to the regulation of redox signal transduction and/or the protection of protein thiols from irreversible oxidation. This Special Issue includes nine other articles: three original studies and six review papers. Together, these ten articles support the central theme that GSH/Grx is a unique system for regulating thiol-redox hemostasis and redox-signal transduction, and the dysregulation of the GSH/Grx system is implicated in the onset and progression of various diseases involving oxidative stress. Within this context, it is important to appreciate the complementary functions of the GSH/Grx and thioredoxin systems not only in thiol-disulfide regulation but also in reversible S-nitrosylation. Several potential clinical applications have emerged from a thorough understanding of the GSH/Grx redox regulatory system at the molecular level, and in various cell types in vitro and in vivo, including, among others, the concept that elevating Grx content/activity could serve as an anti-fibrotic intervention; and discovering small molecules that mimic the inhibitory effects of S-glutathionylation on dimer association could identify novel anti-viral agents that impact the key protease activities of the HIV and SARS-CoV-2 viruses. Thus, this Special Issue on Glutathione and Glutaredoxin has focused attention and advanced understanding of an important aspect of redox biology, as well as spawning questions worthy of future study.

Critical Roles of Glutaredoxin in Brain Cells-Implications for Parkinson's Disease

SIGNIFICANCE

Glutaredoxin (Grx)1, an evolutionarily conserved and ubiquitous enzyme, regulates redox signal transduction and protein redox homeostasis by catalyzing reversible S-glutathionylation. Grx1 plays different roles in different cell types. In Parkinson's disease (PD), Grx1 regulates apoptosis signaling in dopaminergic neurons, so that loss of Grx1 leads to increased cell death; in microglial cells, Grx1 regulates proinflammatory signaling, so that upregulation of Grx1 promotes cytokine production. Here we examine the regulatory roles of Grx1 in PD with a view toward therapeutic innovation. Recent Advances: In postmortem midbrain PD samples, Grx1 was decreased relative to controls, specifically within dopaminergic neurons. In Caenorhabditis elegans models of PD, loss of the Grx1 homologue led to exacerbation of the neurodegenerative phenotype. This effect was partially relieved by overexpression of neuroprotective DJ-1, consistent with regulation of DJ-1 content by Grx1. Increased GLRX copy number in PD patients was associated with earlier PD onset; and Grx1 levels correlated with levels of proinflammatory tumor necrosis factor-α in mouse and human brain samples. In vitro studies showed Grx1 to be upregulated on proinflammatory activation of microglia. Direct overexpression of Grx1 increased microglial activation; silencing Grx1 diminished activation. Grx1 upregulation in microglia corresponded to increased neuronal cell death in coculture. Overall, these studies identify competing roles of Grx1 in PD etiology.

CRITICAL ISSUES

The dilemma regarding Grx1 as a PD therapeutic target is whether to stimulate its upregulation for neuroprotection or inhibit its proinflammatory activity.

FUTURE DIRECTIONS

Further investigation is needed to understand the preponderant role of Grx1 regarding dopaminergic neuronal survival.

Motor and non-motor features of Parkinson's disease in LRRK2 G2019S carriers versus matched controls

INTRODUCTION

LRRK2 G2019S mutation carriers with Parkinson's disease (PD) have been generally indistinguishable from those with idiopathic PD, with the exception of variable differences in some motor and non-motor domains, including cognition, gait, and balance. LRRK2 G2019S is amongst the most common genetic etiologies for PD, particularly in Ashkenazi Jewish (AJ) populations.

METHODS

This cross-sectional data collection study sought to clarify the phenotype of LRRK2 G2019S mutation carriers with PD. Primary endpoints were the Movement Disorder Society Unified Parkinson Disease Rating Scale (MDS-UPDRS) and Montreal Cognitive Assessment (MoCA). Other motor and non-motor data were also assessed. The Mann-Whitney U Test was utilized to compare LRRK2 G2019S carriers with PD (LRRK2+) with non-carrier PD controls who were matched for age, gender, education, and PD duration. Survival analyses and log rank tests were utilized to compare interval from onset of PD to development of motor and non-motor complications.

RESULTS

We screened 251 subjects and 231 completed the study, of whom 9 were LRRK2+, including 7 AJ subjects. 22.73% of AJ subjects with a family history of PD (FH) and 12.96% of AJ subjects without a FH were LRRK2+. There were no significant differences between the 9 LRRK2+ subjects and 19 matched PD controls in MDS-UPDRS, MoCA, or other motor and non-motor endpoints.

CONCLUSION

Prevalence of the LRRK2 G2019S mutation in AJ and non-AJ subjects in our study population in Cleveland, Ohio was comparable to other clinical studies. There were no significant motor or non-motor differences between LRRK2+ PD and matched PD controls.

Novel chloroacetamido compound CWR-J02 is an anti-inflammatory glutaredoxin-1 inhibitor

Glutaredoxin (Grx1) is a ubiquitously expressed thiol-disulfide oxidoreductase that specifically catalyzes reduction of S-glutathionylated substrates. Grx1 is known to be a key regulator of pro-inflammatory signaling, and Grx1 silencing inhibits inflammation in inflammatory disease models. Therefore, we anticipate that inhibition of Grx1 could be an anti-inflammatory therapeutic strategy. We used a rapid screening approach to test 504 novel electrophilic compounds for inhibition of Grx1, which has a highly reactive active-site cysteine residue (pKa 3.5). From this chemical library a chloroacetamido compound, CWR-J02, was identified as a potential lead compound to be characterized. CWR-J02 inhibited isolated Grx1 with an IC₅₀ value of 32 μM in the presence of 1 mM glutathione. Mass spectrometric analysis documented preferential adduction of CWR-J02 to the active site Cys-22 of Grx1, and molecular dynamics simulation identified a potential non-covalent binding site. Treatment of the BV2 microglial cell line with CWR-J02 led to inhibition of intracellular Grx1 activity with an IC₅₀ value (37 μM). CWR-J02 treatment decreased lipopolysaccharide-induced inflammatory gene transcription in the microglial cells in a parallel concentration-dependent manner, documenting the anti-inflammatory potential of CWR-J02. Exploiting the alkyne moiety of CWR-J02, we used click chemistry to link biotin azide to CWR-J02-adducted proteins, isolating them with streptavidin beads. Tandem mass spectrometric analysis identified many CWR-J02-reactive proteins, including Grx1 and several mediators of inflammatory activation. Taken together, these data identify CWR-J02 as an intracellularly effective Grx1 inhibitor that may elicit its anti-inflammatory action in a synergistic manner by also disabling other pro-inflammatory mediators. The CWR-J02 molecule provides a starting point for developing more selective Grx1 inhibitors and anti-inflammatory agents for therapeutic development.

Upregulation of Glutaredoxin-1 Activates Microglia and Promotes Neurodegeneration: Implications for Parkinson's Disease

AIMS

Neuroinflammation and redox dysfunction are recognized factors in Parkinson's disease (PD) pathogenesis, and diabetes is implicated as a potentially predisposing condition. Remarkably, upregulation of glutaredoxin-1 (Grx1) is implicated in regulation of inflammatory responses in various disease contexts, including diabetes. In this study, we investigated the potential impact of Grx1 upregulation in the central nervous system on dopaminergic (DA) viability.

RESULTS

Increased GLRX copy number in PD patients was associated with earlier PD onset, and Grx1 levels correlated with levels of proinflammatory tumor necrosis factor-alpha (TNF-α) in mouse and human brain samples, prompting mechanistic in vitro studies. Grx1 content/activity in microglia was upregulated by lipopolysaccharide (LPS), or TNF-α, treatment. Adenoviral overexpression of Grx1, matching the extent of induction by LPS, increased microglial activation; Grx1 silencing diminished activation. Selective inhibitors/probes of nuclear factor κB (NF-κB) activation revealed glrx1 induction to be mediated by the Nurr1/NF-κB axis. Upregulation of Grx1 in microglia corresponded to increased death of neuronal cells in coculture. With a mouse diabetes model of diet-induced insulin resistance, we found upregulation of Grx1 in brain was associated with DA loss (decreased tyrosine hydroxylase [TH]; diminished TH-positive striatal axonal terminals); these effects were not seen with Grx1-knockout mice.

INNOVATION

Our results indicate that Grx1 upregulation promotes neuroinflammation and consequent neuronal cell death in vitro, and synergizes with proinflammatory insults to promote DA loss in vivo. Our findings also suggest a genetic link between elevated Grx1 and PD development.

CONCLUSION

In vitro and in vivo data suggest Grx1 upregulation promotes neurotoxic neuroinflammation, potentially contributing to PD. Antioxid. Redox Signal. 25, 967-982.

Regulation of DJ-1 by Glutaredoxin 1 in Vivo: Implications for Parkinson's Disease

Parkinson's disease (PD) is the second most common neurodegenerative disease worldwide, caused by the degeneration of the dopaminergic neurons in the substantia nigra. Mutations in PARK7 (DJ-1) result in early onset autosomal recessive PD, and oxidative modification of DJ-1 has been reported to regulate the protective activity of DJ-1 in vitro. Glutathionylation is a prevalent redox modification of proteins resulting from the disulfide adduction of the glutathione moiety to a reactive cysteine-SH, and glutathionylation of specific proteins has been implicated in regulation of cell viability. Glutaredoxin 1 (Grx1) is the principal deglutathionylating enzyme within cells, and it has been reported to mediate protection of dopaminergic neurons in Caenorhabditis elegans; however many of the functional downstream targets of Grx1 in vivo remain unknown. Previously, DJ-1 protein content was shown to decrease concomitantly with diminution of Grx1 protein content in cell culture of model neurons (SH-SY5Y and Neuro-2A lines). In the current study we aimed to investigate the regulation of DJ-1 by Grx1 in vivo and characterize its glutathionylation in vitro. Here, with Grx(-/-) mice we provide show that Grx1 regulates protein levels of DJ-1 in vivo. Furthermore, with model neuronal cells (SH-SY5Y) we observed decreased DJ-1 protein content in response to treatment with known glutathionylating agents, and with isolated DJ-1 we identified two distinct sites of glutathionylation. Finally, we found that overexpression of DJ-1 in the dopaminergic neurons partly compensates for the loss of the Grx1 homologue in a C. elegans in vivo model of PD. Therefore, our results reveal a novel redox modification of DJ-1 and suggest a novel regulatory mechanism for DJ-1 content in vivo.

Serum levels of lipid metabolites in age-related macular degeneration

Age-related macular degeneration (AMD) is a neurodegenerative disease that causes adult-onset blindness. There are 2 forms of this progressive disease: wet and dry. Currently there is no cure for AMD, but several treatment options have started to emerge making early detection critical for therapeutic success. Analysis of the eyes of Abca4(-/-)Rdh8(-/-) mice that display light-induced retinal degeneration indicates that 11-cis-retinal and docosahexaenoic acid (DHA) levels were significantly decreased as compared with the eyes of control dark-adapted C57BL/6J mice. In addition, exposure to intense light correlated with higher levels of prostaglandin G2 in the eyes of Abca4(-/-)Rdh8(-/-) mice. Intense light exposure also lowered DHA levels in the eyes of wild-type C57BL/6J mice without discernible retinal degeneration. Analysis of human serum from patients with AMD recapitulated these dysregulated DHA levels and revealed dysregulation of arachidonic acid (AA) levels as well (∼32% increase in patients with AMD compared with average levels in healthy individuals). From these observations, we then built a statistical model that included levels of DHA and AA from human serum. This model had a 74% probability of correctly identifying patients with AMD from controls. Addition of a genetic analysis for one of the most prevalent amino acid substitutions in the age-related maculopathy susceptibility 2 gene linked to AMD, Ala(69)→Ser, did not improve the statistical model. Thus, we have characterized a reliable method with the potential to detect AMD without a genetic component, paving the way for a larger-scale clinical evaluation. Our studies on mouse models along with the analysis of human serum suggest that our small molecule-based model may serve as an effective tool to estimate the risk of developing AMD.

The roles of redox enzymes in Parkinson's disease: Focus on glutaredoxin

Parkinson's disease (PD) results from the loss of dopaminergic neurons in the substantia nigra portion of the midbrain, and represents the second most common neurodegenerative disease in the world. Although the etiology of PD is currently unclear, oxidative stress and redox dysfunction are generally understood to play key roles in PD pathogenesis and progression. Aging and environmental factors predispose cells to adverse effects of redox changes. In addition to these factors, genetic mutations linked to PD have been observed to disrupt the redox balance. Mutations in leucine-rich repeat kinase 2 (LRRK2) are associated with autosomal dominant PD, and several of these mutations have also been shown to increase the levels of reactive oxygen species in cells. Anti-oxidant proteins are necessary to restore the redox balance and maintain cell viability. Over the past decade studies have started to demonstrate the critical importance for redox proteins mediating neuronal protection in models of PD. This commentary briefly describes some of the factors hypothesized to contribute to PD, specifically regarding the redox changes that occur in PD. Dysregulation of redox proteins in PD is highlighted by some of the work detailing the roles of peroxiredoxin-3 and thioredoxin-1 in models of PD. In an attempt to generate novel therapies for PD, several potent inhibitors of LRRK2 have been developed. The use of these compounds, both as tools to understand the biology of LRRK2 and as potential therapeutic strategies is also discussed. This mini-review then provides a historical prospective on the discovery and characterization of glutaredoxin (Grx1), and briefly describes current understanding of the role of Grx1 in PD. The review concludes by highlighting our recent publication describing the novel role for Grx1 in mediating dopaminergic neuronal protection both in vitro and in vivo.

Glutaredoxin deficiency exacerbates neurodegeneration in C. elegans models of Parkinson's disease

Parkinson's disease (PD) is characterized by selective degeneration of dopaminergic neurons. Although the etiology of PD remains incompletely understood, oxidative stress has been implicated as an important contributor in the development of PD. Oxidative stress can lead to oxidation and functional perturbation of proteins critical to neuronal survival. Glutaredoxin 1 (Grx1) is an evolutionally conserved antioxidant enzyme that repairs protein oxidation by reversing the oxidative modification of cysteine known as S-glutathionylation. We aimed to explore the regulatory role of Grx1 in PD. We first examined the levels of Grx1 in postmortem midbrain samples from PD patients, and observed that Grx1 content is decreased in PD, specifically within the dopaminergic neurons. We subsequently investigated the potential role of Grx1 deficiency in PD pathogenesis by examining the consequences of loss of the Caenorhabditis elegans Grx1 homolog in well-established worm models of familial PD caused by overexpression of pathogenic human LRRK2 mutants G2019S or R1441C. We found that loss of the Grx1 homolog led to significant exacerbation of the neurodegenerative phenotype in C. elegans overexpressing the human LRRK2 mutants. Re-expression in the dopaminergic neurons of the active, but not a catalytically inactive form of the Grx1 homolog rescued the exacerbated phenotype. Loss of the Grx1 homolog also exacerbated the neurodegenerative phenotype in other C. elegans models, including overexpression of human α-synuclein and overexpression of tyrosine hydroxylase (a model of sporadic PD). Therefore, our results reveal a novel neuroprotective role of glutaredoxin against dopaminergic neurodegeneration in models of familial and sporadic PD.

N-acetyl-L-cysteine sensitizes pancreatic cancers to gemcitabine by targeting the NFκB pathway

First-line therapy for pancreatic cancer is gemcitabine. Although tumors may initially respond to the gemcitabine treatment, soon tumor resistance develops leading to treatment failure. Previously, we demonstrated in human MIA PaCa-2 pancreatic cancer cells that N-acetyl-l-cysteine (NAC), a glutathione (GSH) precursor, prevents NFκB activation via S-glutathionylation of p65-NFκB, thereby blunting expression of survival genes. In this study, we documented the molecular sites of S-glutathionylation of p65, and we investigated whether NAC can suppress NFκB signaling and augment a therapeutic response to gemcitabine in vivo. Mass spectrometric analysis of S-glutathionylated p65-NFκB protein in vitro showed post-translational modifications of cysteines 38, 105, 120, 160 and 216 following oxidative and nitrosative stress. Circular dichroism revealed that S-glutathionylation of p65-NFκB did not change secondary structure of the protein, but increased tryptophan fluorescence revealed altered tertiary structure. Gemcitabine and NAC individually were not effective in decreasing MIA PaCa-2 tumor growth in vivo. However, combination treatment with NAC and gemcitabine decreased tumor growth by approximately 50%. NAC treatment also markedly enhanced tumor apoptosis in gemcitabine-treated mice. Compared to untreated tumors, gemcitabine treatment alone increased p65-NFκB nuclear translocation (3.7-fold) and DNA binding (2.5-fold), and these effects were blunted by NAC. In addition, NAC plus gemcitabine treatment decreased anti-apoptotic XIAP protein expression compared to gemcitabine alone. None of the treatments, however, affected extent of tumor hypoxia, as assessed by EF5 staining. Together, these results indicate that adjunct therapy with NAC prevents NFκB activation and improves gemcitabine chemotherapeutic efficacy.

Aging-dependent changes in rat heart mitochondrial glutaredoxins--Implications for redox regulation

Clinical and animal studies have documented that hearts of the elderly are more susceptible to ischemia/reperfusion damage compared to young adults. Recently we found that aging-dependent increase in susceptibility of cardiomyocytes to apoptosis was attributable to decrease in cytosolic glutaredoxin 1 (Grx1) and concomitant decrease in NF-κB-mediated expression of anti-apoptotic proteins. Besides primary localization in the cytosol, Grx1 also exists in the mitochondrial intermembrane space (IMS). In contrast, Grx2 is confined to the mitochondrial matrix. Here we report that Grx1 is decreased by 50–60% in the IMS, but Grx2 is increased by 1.4–2.6 fold in the matrix of heart mitochondria from elderly rats. Determination of in situ activities of the Grx isozymes from both subsarcolemmal (SSM) and interfibrillar (IFM) mitochondria revealed that Grx1 was fully active in the IMS. However, Grx2 was mostly in an inactive form in the matrix, consistent with reversible sequestration of the active-site cysteines of two Grx2 molecules in complex with an iron–sulfur cluster. Our quantitative evaluations of the active/inactive ratio for Grx2 suggest that levels of dimeric Grx2 complex with iron–sulfur clusters are increased in SSM and IFM in the hearts of elderly rats. We found that the inactive Grx2 can be fully reactivated by sodium dithionite or exogenous superoxide production mediated by xanthine oxidase. However, treatment with rotenone, which generates intramitochondrial superoxide through inhibition of mitochondrial respiratory chain Complex I, did not lead to Grx2 activation. These findings suggest that insufficient ROS accumulates in the vicinity of dimeric Grx2 to activate it in situ.

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Glutaredoxins play key roles in cellular redox regulation, which is sensitive to aging-dependent dysregulation.

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Grx1 is diminished in the intermembrane space of mitochondria from aged heart; matrix Grx2 is increased but mostly in an inactive form.

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The inactive Grx2 is selectively activated by superoxide.

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Mitochondrial glutaredoxin changes may contribute to dysregulation of redox homeostasis during aging.

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Changes in in situ activities of heart mitochondrial Grx1 and Grx2 with aging provide mechanistic insights for future studies.

Kinase inhibitors arrest neurodegeneration in cell and C. elegans models of LRRK2 toxicity

Mutations in leucine-rich repeat kinase 2 (LRRK2) are the most frequent known cause of late-onset Parkinson's disease (PD). To explore the therapeutic potential of small molecules targeting the LRRK2 kinase domain, we characterized two LRRK2 kinase inhibitors, TTT-3002 and LRRK2-IN1, for their effects against LRRK2 activity in vitro and in Caenorhabditis elegans models of LRRK2-linked neurodegeneration. TTT-3002 and LRRK2-IN1 potently inhibited in vitro kinase activity of LRRK2 wild-type and mutant proteins, attenuated phosphorylation of cellular LRRK2 and rescued neurotoxicity of mutant LRRK2 in transfected cells. To establish whether LRRK2 kinase inhibitors can mitigate pathogenesis caused by different mutations including G2019S and R1441C located within and outside of the LRRK2 kinase domain, respectively, we evaluated effects of TTT-3002 and LRRK2-IN1 against R1441C- and G2019S-induced neurodegeneration in C. elegans models. TTT-3002 and LRRK2-IN1 rescued the behavioral deficit characteristic of dopaminergic impairment in transgenic C. elegans expressing human R1441C- and G2019S-LRRK2. The inhibitors displayed nanomolar to low micromolar rescue potency when administered either pre-symptomatically or post-symptomatically, indicating both prevention and reversal of the dopaminergic deficit. The same treatments also led to long-lasting prevention and rescue of neurodegeneration. In contrast, TTT-3002 and LRRK2-IN1 were ineffective against the neurodegenerative phenotype in transgenic worms carrying the inhibitor-resistant A2016T mutation of LRRK2, suggesting that they elicit neuroprotective effects in vivo by targeting LRRK2 specifically. Our findings indicate that the LRRK2 kinase activity is critical for neurodegeneration caused by R1441C and G2019S mutations, suggesting that kinase inhibition of LRRK2 may represent a promising therapeutic strategy for PD.

Protein-thiol oxidation and cell death: regulatory role of glutaredoxins

SIGNIFICANCE

Glutaredoxin (Grx) is the primary enzyme responsible for catalysis of deglutathionylation of protein-mixed disulfides with glutathione (GSH) (protein-SSG). This reversible post-translational modification alters the activity and function of many proteins important in regulation of critical cellular processes. Aberrant regulation of protein glutathionylation/deglutathionylation reactions due to changes in Grx activity can disrupt both apoptotic and survival signaling pathways.

RECENT ADVANCES

Grx is known to regulate the activity of many proteins through reversible glutathionylation, such as Ras, Fas, ASK1, NFκB, and procaspase-3, all of which play important roles in control of apoptosis. Reactive oxygen species and/or reactive nitrogen species mediate oxidative modifications of critical Cys residues on these apoptotic mediators, facilitating protein-SSG formation and thereby altering protein function and apoptotic signaling.

CRITICAL ISSUES

Much of what is known about the regulation of apoptotic mediators by Grx and reversible glutathionylation has been gleaned from in vitro studies of discrete apoptotic pathways. To relate these results to events in vivo it is important to examine changes in protein-SSG status in situ under natural cellular conditions, maintaining relevant GSH:GSSG ratios and using appropriate inducers of apoptosis.

FUTURE DIRECTIONS

Apoptosis is a highly complex, tightly regulated process involving many different checks and balances. The influence of Grx activity on the interconnectivity among these various pathways remains unknown. Knowledge of the effects of Grx is essential for developing novel therapeutic approaches for treating diseases involving dysregulated apoptosis, such as cancer, heart disease, diabetes, and neurodegenerative diseases, where alterations in redox homeostasis are hallmarks for pathogenesis.

Abstract 1316: N-acetyl-L-cysteine sensitizes pancreatic cancer therapy by targeting NFκB pathway: Translation into animal model

Pancreatic cancer has the worst prognosis of all major cancers with 97% treatment failure. First-line systemic therapy for pancreatic cancer is the antimetabolite gemcitabine. Although, tumors may initially respond to the gemcitabine treatment, soon tumor resistance and excessive toxicity develops leading to treatment failure. Efforts to optimize and improve the efficacy of gemcitabine therapy through combination with other agents have little success. Previously, we demonstrated that N-acetyl-L-cysteine (NAC), a glutathione (GSH) precursor prevents NFκB activation and promotes pancreatic cancer cell death. The mechanism of inactivation of NFκB by NAC supplementation is a post-translational sulfhydryl modification called S-glutathionylation on p65-NFκB that inhibits its DNA-binding capacity and thus prevents activation of the survival genes. Mass spectrometric analysis indicated that four of the 9 cysteines of p65 are potentially S-glutathionylated. Our hypothesis is that chemoresistance against gemcitabine arises from a combination of hypoxia- and gemcitabine-induced NFκB activation. The objective of this study is to test the hypothesis and investigate if thiol donors like NAC can suppress NFκB signaling and augment a therapeutic response to gemcitabine in vivo. Here we determined the anti-tumor effect of gemcitabine and NAC on pancreatic tumors in mice. MIA PaCa-2 pancreatic cancer cells were injected into nude mice to grow subcutaneous tumors. Treatment groups were untreated, NAC, gemcitabine and gemcitabine+NAC. Results indicated that neither gemcitabine nor NAC by itself was effective in decreasing tumor growth. However, NAC and gemcitabine in combination inhibited tumor growth by 33%. Gemcitabine activates NFκB, although the mechanism for this phenomenon is unclear. NFκB activity was indeed increased in gemcitabine-treated tumors (3-fold) and combination with NAC treatment blunted this effect (p<0.005). The results confirm gemcitabine-induced NFκB activation and NAC bioavailability in tumors. To assess whether combined treatment of gemcitabine and NAC induces apoptosis, we performed TUNEL staining in tissues. Gemcitabine plus NAC increased apoptosis by 35-fold compared to gemcitabine alone. Taken together, our data suggest that NAC first inhibits NFκB activation arising from hypoxic regions of tumors and secondarily block gemcitabine-activated NFκB in hypoxic and normoxic regions of tumors, and therefore sensitizes cells to gemcitabine-induced killing. This could be used as an effective adjunct to gemcitabine chemotherapy of pancreatic cancer.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1316. doi:1538-7445.AM2012-1316

Posttranslational modification of cysteine in redox signaling and oxidative stress: Focus on s-glutathionylation

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) have become recognized as second messengers for initiating and/or regulating vital cellular signaling pathways, and they are known also as deleterious mediators of cellular stress and cell death. ROS and RNS, and their cross products like peroxynitrite, react primarily with cysteine residues whose oxidative modification leads to functional alterations in the proteins. In this Forum, the collection of six review articles presents a perspective on the broad biological impact of cysteine modifications in health and disease from the molecular to the cellular and organismal levels, focusing in particular on reversible protein-S-glutathionylation and its central role in transducing redox signals as well as protecting proteins from irreversible cysteine oxidation. The Forum review articles consider the role of S-glutationylation in regulation of the peroxiredoxin enzymes, the special redox environment of the mitochondria, redox regulation pertinent to the function of the cardiovascular system, mechanisms of redox-activated apoptosis in the pulmonary system, and the role of glutathionylation in the initiation, propagation, and treatment of neurodegenerative diseases. Several common themes emerge from these reviews; notably, the probability of crosstalk between signaling/regulation mechanisms involving protein-S-nitrosylation and protein-S-glutathionylation, and the need for quantitative analysis of the relationship between specific cysteine modifications and corresponding functional changes in various cellular contexts.

Mechanisms of altered redox regulation in neurodegenerative diseases--focus on S--glutathionylation

SIGNIFICANCE

Neurodegenerative diseases are characterized by progressive loss of neurons. A common feature is oxidative stress, which arises when reactive oxygen species (ROS) and/or reactive nitrogen species (RNS) exceed amounts required for normal redox signaling. An imbalance in ROS/RNS alters functionality of cysteines and perturbs thiol-disulfide homeostasis. Many cysteine modifications may occur, but reversible protein mixed disulfides with glutathione (GSH) likely represents the common steady-state derivative due to cellular abundance of GSH and ready conversion of cysteine-sulfenic acid and S-nitrosocysteine precursors to S-glutathionylcysteine disulfides. Thus, S-glutathionylation acts in redox signal transduction and serves as a protective mechanism against irreversible cysteine oxidation. Reversal of protein-S-glutathionylation is catalyzed specifically by glutaredoxin which thereby plays a critical role in cellular regulation. This review highlights the role of oxidative modification of proteins, notably S-glutathionylation, and alterations in thiol homeostatic enzyme activities in neurodegenerative diseases, providing insights for therapeutic intervention.

RECENT ADVANCES

Recent studies show that dysregulation of redox signaling and sulfhydryl homeostasis likely contributes to onset/progression of neurodegeneration. Oxidative stress alters the thiol-disulfide status of key proteins that regulate the balance between cell survival and cell death.

CRITICAL ISSUES

Much of the current information about redox modification of key enzymes and signaling intermediates has been gleaned from studies focused on oxidative stress situations other than the neurodegenerative diseases.

FUTURE DIRECTIONS

The findings in other contexts are expected to apply to understanding neurodegenerative mechanisms. Identification of selectively glutathionylated proteins in a quantitative fashion will provide new insights about neuropathological consequences of this oxidative protein modification.

HSV infection induces production of ROS, which potentiate signaling from pattern recognition receptors: role for S-glutathionylation of TRAF3 and 6

The innate immune response constitutes the first line of defense against infections. Pattern recognition receptors recognize pathogen structures and trigger intracellular signaling pathways leading to cytokine and chemokine expression. Reactive oxygen species (ROS) are emerging as an important regulator of some of these pathways. ROS directly interact with signaling components or induce other post-translational modifications such as S-glutathionylation, thereby altering target function. Applying live microscopy, we have demonstrated that herpes simplex virus (HSV) infection induces early production of ROS that are required for the activation of NF-κB and IRF-3 pathways and the production of type I IFNs and ISGs. All the known receptors involved in the recognition of HSV were shown to be dependent on the cellular redox levels for successful signaling. In addition, we provide biochemical evidence suggesting S-glutathionylation of TRAF family proteins to be important. In particular, by performing mutational studies we show that S-glutathionylation of a conserved cysteine residue of TRAF3 and TRAF6 is important for ROS-dependent activation of innate immune pathways. In conclusion, these findings demonstrate that ROS are essential for effective activation of signaling pathways leading to a successful innate immune response against HSV infection.

Herpes simplex virus (HSV) type 1 and 2 are important human pathogens, which can give rise to severe diseases during both primary and recurrent infections. In addition to activating “classical” innate and adaptive immune responses, many infections stimulate other cellular activities such as and production of reactive oxygen species (ROS). However, there is little knowledge on the cross-talk between ROS and the innate antiviral response. In this article we show that HSV infection leads to production of ROS, and that ROS play a critical role in activation of innate immune responses to these viruses. At the mechanistic level, we show that ROS stimulate glutathionylation (a protein modification) of the signaling molecules TRAF3 and 6, which promotes redox-sensitive signaling. Our data support the idea that the innate immune system not only detects specific HSV molecules but also senses the cellular oxidative stress level, and integrates this into the innate immune response to infections.

Levodopa activates apoptosis signaling kinase 1 (ASK1) and promotes apoptosis in a neuronal model: implications for the treatment of Parkinson's disease

Oxidative stress is implicated in the etiology of Parkinson's disease (PD), the second most common neurodegenerative disease. PD is treated with chronic administration of l-3,4-dihydroxyphenylalanine (levodopa, L-DOPA), and typically, increasing doses are used during progression of the disease. Paradoxically, L-DOPA is a pro-oxidant and induces cell death in cellular models of PD through disruption of sulfhydryl homeostasis involving loss of the thiol-disulfide oxidoreductase functions of the glutaredoxin (Grx1) and thioredoxin (Trx1) enzyme systems [Sabens, E. A., Distler, A. M., and Mieyal, J. J. (2010) Biochemistry 49 (12), 2715-2724]. Considering this loss of both Grx1 and Trx1 activities upon L-DOPA treatment, we sought to elucidate the mechanism(s) of L-DOPA-induced apoptosis. In other contexts, both the NFκB (nuclear factor κB) pathway and the ASK1 (apoptosis signaling kinase 1) pathway have been shown to be regulated by both Grx1 and Trx1, and both pathways have been implicated in cell death signaling in model systems of PD. Moreover, mixed lineage kinase (MLK) has been considered as a potential therapeutic target for PD. Using SHSY5Y cells as model dopaminergic neurons, we found that NFκB activity was not altered by L-DOPA treatment, and the selective MLK inhibitor (CEP-1347) did not protect the cells from L-DOPA. In contrast, ASK1 was activated with L-DOPA treatment as indicated by phosphorylation of its downstream mitogen-activated protein kinases (MAPK), p38 and JNK. Chemical inhibition of either p38 or JNK provided protection from L-DOPA-induced apoptosis. Moreover, direct knockdown of ASK1 protected from L-DOPA-induced neuronal cell death. These results identify ASK1 as the main pro-apoptotic pathway activated in response to L-DOPA treatment, implicating it as a potential target for adjunct therapy in PD.

Glutaredoxin regulates apoptosis in cardiomyocytes via NFkappaB targets Bcl-2 and Bcl-xL: implications for cardiac aging

Cardiomyocyte apoptosis is a well-established contributor to irreversible injury following myocardial infarction (MI). Increased cardiomyocyte apoptosis is associated also with aging in animal models, exacerbated by MI; however, mechanisms for this increased sensitivity to oxidative stress are unknown. Protein mixed-disulfide formation with glutathione (protein glutathionylation) is known to change the function of intermediates that regulate apoptosis. Since glutaredoxin (Grx) specifically catalyzes protein deglutathionylation, we examined its status with aging and its influence on regulation of apoptosis. Grx1 content and activity are decreased by approximately 40% in elderly (24-mo) Fischer 344 rat hearts compared to adult (6-mo) controls. A similar extent of Grx1 knockdown in H9c2 cardiomyocytes led to increased apoptosis, decreased NFkappaB-dependent transcriptional activity, and decreased production (mRNA and protein) of anti-apoptotic NFkappaB target genes, Bcl-2 and Bcl-xL. Knockdown of Bcl-2 and/or Bcl-xL in wild-type H9c2 cells to the same extent ( approximately 50%) as observed in Grx1-knockdown cells increased baseline apoptosis; and knockdown of Bcl-xL, but not Bcl-2, also increased oxidant-induced apoptosis analogous to Grx1-knockdown cells. Natural Grx1-deficient cardiomyocytes isolated from elderly rats also displayed diminished NFkappaB activity and Bcl-xL content. Taken together, these data indicate diminution of Grx1 in elderly animals contributes to increased apoptotic susceptibility via regulation of NFkappaB function.

Levodopa deactivates enzymes that regulate thiol-disulfide homeostasis and promotes neuronal cell death: implications for therapy of Parkinson's disease

Parkinson's disease (PD), characterized by dopaminergic neuronal loss, is attributed to oxidative stress, diminished glutathione (GSH) levels, mitochondrial dysfunction, and protein aggregation. Treatment of PD involves chronic administration of Levodopa (l-DOPA) which is a pro-oxidant and may disrupt sulfhydryl homeostasis. The goal of these studies is to elucidate the effects of l-DOPA on thiol homeostasis in a model akin to PD, i.e., immortalized dopaminergic neurons (SHSY5Y cells) with diminished GSH content. These neurons exhibit hypersensitivity to l-DOPA-induced cell death, which is attributable to concomitant inhibition of the intracellular thiol disulfide oxidoreductase enzymes. Glutaredoxin (Grx) was deactivated in a dose-dependent fashion, but its content was unaffected. Glutathione disulfide (GSSG) reductase (GR) activity was not altered. Selective knockdown of Grx resulted in an increased level of apoptosis, documenting the role of the Grx system in neuronal survival. l-DOPA treatments also led to decreased activities of thioredoxin (Trx) and thioredoxin reductase (TR), concomitant with diminution of their cellular contents. Selective chemical inhibition of TR activity led to an increased level of apoptosis, documenting the Trx system's contribution to neuronal viability. To investigate the mechanism of inhibition at the molecular level, we treated the each isolated enzyme with oxidized l-DOPA. GR, Trx, and TR activities were little affected. However, Grx was inactivated in a time- and concentration-dependent fashion indicative of irreversible adduction of dopaquinone to its nucleophilic active-site Cys-22, consistent with the intracellular loss of Grx activity but not Grx protein content after l-DOPA treatment. Overall l-DOPA is shown to impair the collaborative contributions of the Grx and Trx systems to neuron survival.

Covalent cross-linking of glutathione and carnosine to proteins by 4-oxo-2-nonenal

The lipid oxidation product 4-oxo-2-nonenal (ONE) derived from peroxidation of polyunsaturated fatty acids is a highly reactive protein cross-linking reagent. The major family of cross-links reflects conjugate addition of side chain nucleophiles such as sulfhydryl or imidazole groups to the C triple bond C of ONE to give either a 2- or 3-substituted 4-ketoaldehyde, which then undergoes Paal-Knorr condensation with the primary amine of protein lysine side chains. If ONE is intercepted in biological fluids by antielectrophiles such as glutathione (GSH) or beta-alanylhistidine (carnosine), this would lead to circulating 4-ketoaldehydes that could then bind covalently to the protein Lys residues. This phenomenon was investigated by SDS-PAGE and mass spectrometry (matrix-assisted laser desorption/ionization time-of-flight and LC-ESI-MS/MS with both tryptic and chymotryptic digestion). Under the reaction conditions of 0.25-2 mM ONE, 1 mM GSH or carnosine, 0.25 mM bovine beta-lactoglobulin (beta-LG), and 100 mM phosphate buffer (pH 7.4, 10% ethanol) for 24 h at 37 degrees C, virtually every Lys of beta-LG was found to be fractionally cross-linked to GSH. Cross-linking of Lys to carnosine was less efficient. Using cytochrome c and RNase A, we showed that ONE becomes more protein-reactive in the presence of GSH, whereas protein modification by 4-hydroxy-2-nonenal is inhibited by GSH. Stable antielectrophile-ONE-protein cross-links may serve as biomarkers of oxidative stress and may represent a novel mechanism of irreversible protein glutathionylation.

Dynamics of glutathione and ophthalmate traced with 2H-enriched body water in rats and humans

We developed a LC-MS-MS assay of the (2)H labeling of free glutathione (GSH) and bound glutathione [GSSR; which includes all DTT-reducible forms, primarily glutathione disulfide (GSSG) and mixed disulfides with proteins] and ophthalmate (an index of GSH depletion) labeled from (2)H-enriched body water. In rats whose body water was 2.5% (2)H enriched for up to 31 days, GSH labeling follows a complex pattern because of different rates of labeling of its constitutive amino acids. In rats infused with [(13)C(2),(15)N-glycine]glutathione, the rate of appearance of plasma GSH was 2.1 micromol.min(-1).kg(-1), and the half-life of plasma GSH/GSSR was 6-8 min. In healthy humans whose body fluids were 0.5% (2)H enriched, the (2)H labeling of GSH/GSSR and ophthalmate can be precisely measured after 4 h, with GSH being more rapidly labeled than GSSR. Since plasma GSH/GSSR derives mostly from liver, this technique opens the way to 2) probe noninvasively the labeling pattern and redox status of the liver GSH system in humans and 2) assess the usefulness of ophthalmate as an index of GSH depletion.

Deglutathionylation of 2-Cys peroxiredoxin is specifically catalyzed by sulfiredoxin

Reversible protein glutathionylation plays a key role in cellular regulation and cell signaling and protects protein thiols from hyperoxidation. Sulfiredoxin (Srx), an enzyme that catalyzes the reduction of Cys-sulfinic acid derivatives of 2-Cys peroxiredoxins (2-Cys Prxs), has been shown to catalyze the deglutathionylation of actin. We show that deglutathionylation of 2-Cys Prx, a family of peroxidases, is specifically catalyzed by Srx. Using the ubiquitously expressed member of 2-Cys Prx, Prx I, we revealed the following. (i) Among its four Cys residues, Cys(52), Cys(83), and Cys(173) can be glutathionylated in vitro. Deglutathionylation with Cys mutants showed that Cys(83) and Cys(173) were preferentially catalyzed by Srx, with glutathionylated Srx as the reaction intermediate, whereas glutaredoxin I was more favorable for deglutathionylating Cys(52). (ii) Studies using site-directed mutagenesis coupled with binding and deglutathionylation activities revealed that Pro(174) and Pro(179) of Prx I and Tyr(92) of Srx are essential for both activities. Furthermore, relative to glutaredoxin I, Srx exhibited negligible deglutathionylation activity for glutathionylated cysteine and glutathionylated BSA. These results indicate that Srx is specific for deglutathionylating Prx I due to its favorable affinity for Prx I. To assess the biological relevance of these observations, we showed that Prx I is glutathionylated in A549 and HeLa cells under modest levels of H(2)O(2). In addition, the level of glutathionylated Prx I was substantially elevated in small interfering RNA-mediated Srx-knocked down cells, whereas the reverse was observed in Srx-overexpressing cells. However, glutathionylation of Prx V, not known to bind to Srx, was not affected by the change in Srx expression levels.

Mechanistic and kinetic details of catalysis of thiol-disulfide exchange by glutaredoxins and potential mechanisms of regulation

Glutaredoxins are small, heat-stable proteins that exhibit a characteristic thioredoxin fold and a CXXC/S active-site motif. A variety of glutathione (GSH)-dependent catalytic activities have been attributed to the glutaredoxins, including reduction of ribonucleotide reductase, arsenate, and dehydroascorbate; assembly of iron sulfur cluster complexes; and protein glutathionylation and deglutathionylation. Catalysis of reversible protein glutathionylation by glutaredoxins has been implicated in regulation of redox signal transduction and sulfhydryl homeostasis in numerous contexts in health and disease. This forum review is presented in two parts. Part I is focused primarily on the mechanism of the deglutathionylation reaction catalyzed by prototypical dithiol glutaredoxins, especially human Grx1 and Grx2. Grx-catalyzed protein deglutathionylation proceeds by a nucleophilic, double-displacement mechanism in which rate enhancement is attributed to special reactivity of the low pK(a) cysteine at its active site, and to increased nucleophilicity of the second substrate, GSH. Glutaredoxins (and Grx domains) have been identified in most organisms, and many exhibit deglutathionylation or other activities or both. Further characterization according to glutathionyl selectivity, physiological substrates, and intracellular roles may lead to subclassification of this family of enzymes. Part II presents potential mechanisms for in vivo regulation of Grx activity, providing avenues for future studies.

Glutaredoxin regulates autocrine and paracrine proinflammatory responses in retinal glial (muller) cells

Protein S-glutathionylation is a reversible redox-dependent post-translational modification. Many cellular functions and signal transduction pathways involve proteins whose cysteine-dependent activities are modulated by glutathionylation. Glutaredoxin (Grx1) plays a key role in such regulation because it is a specific and efficient catalyst of deglutathionylation. We recently reported an increase in Grx1 in retinae of diabetic rats and in rat retinal Müller glial cells (rMC-1) cultured in high glucose. This up-regulation of Grx1 was concomitant with NFkappaB activation and induction of intercellular adhesion molecule-1 (ICAM-1). This proinflammatory response was replicated by adenoviral-directed up-regulation of Grx1 in cells in normal glucose. The site of regulation of NFkappaB was localized to the cytoplasm, where IkappaB kinase (IKK) is a master regulator of NFkappaB activation. In the current study, inhibition of IKK activity abrogated the increase in ICAM-1 induced by high glucose or by adenoviral-directed up-regulation of Grx1. Conditioned medium from the Müller cells overexpressing Grx1 was added to fresh cultures of Müller or endothelial cells and elicited increases in the Grx1 and ICAM-1 proteins in these cells. These effects correlate with a novel finding that secretion of interleukin-6 was elevated in the cultures of Grx overexpressing cells. Also, pure interleukin-6 increased Grx1 and ICAM-1 in the rMC-1 cells. Thus, Grx1 appears to play an important role in both autocrine and paracrine proinflammatory responses. Furthermore, IKKbeta isolated from Müller cells in normal glucose medium was found to be glutathionylated on Cys-179. Hence Grx-mediated activation of IKK via deglutathionylation may play a central role in diabetic complications in vivo where Grx1 is increased.

Molecular mechanisms and clinical implications of reversible protein S-glutathionylation

Sulfhydryl chemistry plays a vital role in normal biology and in defense of cells against oxidants, free radicals, and electrophiles. Modification of critical cysteine residues is an important mechanism of signal transduction, and perturbation of thiol-disulfide homeostasis is an important consequence of many diseases. A prevalent form of cysteine modification is reversible formation of protein mixed disulfides (protein-SSG) with glutathione (GSH). The abundance of GSH in cells and the ready conversion of sulfenic acids and S-nitroso derivatives to S-glutathione mixed disulfides suggests that reversible S-glutathionylation may be a common feature of redox signal transduction and regulation of the activities of redox sensitive thiol-proteins. The glutaredoxin enzyme has served as a focal point and important tool for evolution of this regulatory mechanism, because it is a specific and efficient catalyst of protein-SSG deglutathionylation. However, mechanisms of control of intracellular Grx activity in response to various stimuli are not well understood, and delineation of specific mechanisms and enzyme(s) involved in formation of protein-SSG intermediates requires further attention. A large number of proteins have been identified as potentially regulated by reversible S-glutathionylation, but only a few studies have documented glutathionylation-dependent changes in activity of specific proteins in a physiological context. Oxidative stress is a hallmark of many diseases which may interrupt or divert normal redox signaling and perturb protein-thiol homeostasis. Examples involving changes in S-glutathionylation of specific proteins are discussed in the context of diabetes, cardiovascular and lung diseases, cancer, and neurodegenerative diseases.

Kinetic and mechanistic characterization and versatile catalytic properties of mammalian glutaredoxin 2: implications for intracellular roles

Glutaredoxin (Grx)-catalyzed deglutathionylation of protein-glutathione mixed disulfides (protein-SSG) serves important roles in redox homeostasis and signal transduction, regulating diverse physiological and pathophysiological events. Mammalian cells have two Grx isoforms: Grx1, localized to the cytosol and mitochondrial intermembrane space, and Grx2, localized primarily to the mitochondrial matrix [Pai, H. V., et al. (2007) Antioxid. Redox Signaling 9, 2027-2033]. The catalytic behavior of Grx1 has been characterized extensively, whereas Grx2 catalysis is less well understood. We observed that human Grx1 and Grx2 exhibit key catalytic similarities, including selectivity for protein-SSG substrates and a nucleophilic, double-displacement, monothiol mechanism exhibiting a strong commitment to catalysis. A key distinction between Grx1- and Grx2-mediated deglutathionylation is decreased catalytic efficiency ( k cat/ K M) of Grx2 for protein deglutathionylation (due primarily to a decreased k cat), reflecting a higher p K a of its catalytic cysteine, as well as a decreased enhancement of nucleophilicity of the second substrate, GSH. As documented previously for hGrx1 [Starke, D. W., et al. (2003) J. Biol. Chem. 278, 14607-14613], hGrx2 catalyzes glutathione-thiyl radical (GS (*)) scavenging, and it also mediates GS transfer (protein S-glutathionylation) reactions, where GS (*) serves as a superior glutathionyl donor substrate for formation of GAPDH-SSG, compared to GSNO and GSSG. In contrast to its lower k cat for deglutathionylation reactions, Grx2 promotes GS-transfer to the model protein substrate GAPDH at rates equivalent to those of Grx1. Estimation of Grx1 and Grx2 concentrations within mitochondria predicts comparable deglutathionylation activities within the mitochondrial subcompartments, suggesting localized regulatory functions for both isozymes.

Regulation by reversible S-glutathionylation: molecular targets implicated in inflammatory diseases

S-glutathionylation is a reversible post-translational modification that continues to gain eminence as a redox regulatory mechanism of protein activity and associated cellular functions. Many diverse cellular proteins such as transcription factors, adhesion molecules, enzymes, and cytokines are reported to undergo glutathionylation, although the functional impact has been less well characterized. De-glutathionylation is catalyzed specifically and efficiently by glutaredoxin (GRx, aka thioltransferase), and facile reversibility is critical in determining the physiological relevance of glutathionylation as a means of protein regulation. Thus, studies with cohesive themes addressing both the glutathionylation of proteins and the corresponding impact of GRx are especially useful in advancing understanding. Reactive oxygen species (ROS) and redox regulation are well accepted as playing a role in inflammatory processes, such as leukostasis and the destruction of foreign particles by macrophages. We discuss in this review the current implications of GRx and/or glutathionylation in the inflammatory response and in diseases associated with chronic inflammation, namely diabetes, atherosclerosis, inflammatory lung disease, cancer, and Alzheimer's disease, and in viral infections.

What is the functional significance of the unique location of glutaredoxin 1 (GRx1) in the intermembrane space of mitochondria?

Glutaredoxins (GRx) catalyze reversible protein glutathionylation. They are implicated in sulfhydryl homeostasis and regulation of redox signal transduction, controlling various cellular processes like DNA synthesis, defense against oxidative stress, apoptosis signaling, and DNA-binding of transcription factors. Two isoforms of GRx are well characterized in mammals: GRx1, the "cytosolic" form, and GRx2, the "mitochondrial" form. Here we report documentation of GRx1 in mitochondria, localized exclusively in the intermembrane space and segregated from GRx2, localized exclusively in the mitochondrial matrix. We hypothesize that GRx1 and GRx2 in their unique locations regulate different functions of the mitochondria via reversible S-glutathionylation.

Targeted disruption of the glutaredoxin 1 gene does not sensitize adult mice to tissue injury induced by ischemia/reperfusion and hyperoxia

To understand the physiological function of glutaredoxin, a thiotransferase catalyzing the reduction of mixed disulfides of protein and glutathione, we generated a line of knockout mice deficient in the cytosolic glutaredoxin 1 (Grx1). To our surprise, mice deficient in Grx1 were not more susceptible to acute oxidative insults in models of heart and lung injury induced by ischemia/reperfusion and hyperoxia, respectively, suggesting that either changes in S-glutathionylation status of cytosolic proteins are not the major cause of such tissue injury or developmental adaptation in the Glrx1-knockout animals alters the response to oxidative insult. In contrast, mouse embryonic fibroblasts (MEFs) isolated from Grx1-deficient mice displayed an increased vulnerability to diquat and paraquat, but they were not more susceptible to cell death induced by hydrogen peroxide (H(2)O(2)) and diamide. A deficiency in Grx1 also sensitized MEFs to protein S-glutathionylation in response to H(2)O(2) treatment and retarded deglutathionylation of the S-glutathionylated proteins, especially for a single prominent protein band. Additional experiments showed that MEFs lacking Grx1 were more tolerant to apoptosis induced by tumor necrosis factor alphaplus actinomycin D. These findings suggest that various oxidants may damage the cells via distinct mechanisms in which the action of Grx1 may or may not be protective and Grx1 may exert its function on specific target proteins.

Mechanisms of reversible protein glutathionylation in redox signaling and oxidative stress

Reversible protein S-glutathionylation (protein-SSG) is an important post-translational modification, providing protection of protein cysteines from irreversible oxidation and serving to transduce redox signals. Analogous to phosphatases, glutaredoxin (GRx) enzymes catalyze deglutathionylation of proteins, regulating diverse intracellular signaling pathways. Recently, other enzymes have been reported to exhibit deglutathionylating activity, but their contribution to intracellular protein deglutathionylation is uncertain. Currently, no enzyme has been shown to serve as a catalyst of S-glutathionylation in situ, although potential prototypes are reported, including human GRx1 and the pi isoform of glutathione-S-transferase (GSTpi). Further insight into cellular mechanisms of protein glutathionylation and deglutathionylation will enrich our understanding of redox signal transduction and potentially identify new therapeutic targets for diseases in which oxidative stress perturbs normal redox signaling. Accordingly, this review focuses primarily on mechanisms of catalysis in mammalian systems.

Glutathione supplementation potentiates hypoxic apoptosis by S-glutathionylation of p65-NFkappaB

In murine embryonic fibroblasts, N-acetyl-L-cysteine (NAC), a GSH generating agent, enhances hypoxic apoptosis by blocking the NFkappaB survival pathway (Qanungo, S., Wang, M., and Nieminen, A. L. (2004) J. Biol. Chem. 279, 50455-50464). Here, we examined sulfhydryl modifications of the p65 subunit of NFkappaB that are responsible for NFkappaB inactivation. In MIA PaCa-2 pancreatic cancer cells, hypoxia increased p65-NFkappaB DNA binding and NFkappaB transactivation by 2.6- and 2.8-fold, respectively. NAC blocked these events without having an effect on p65-NFkappaB protein levels and p65-NFkappaB nuclear translocation during hypoxia. Pharmacological inhibition of the NFkappaB pathway also induced hypoxic apoptosis, indicating that the NFkappaB signaling pathway is a major protective mechanism against hypoxic apoptosis. In cell lysates after hypoxia and treatment with N-ethylmaleimide (thiol alkylating agent), dithiothreitol (disulfide reducing agent) was not able to increase binding of p65-NFkappaB to DNA, suggesting that most sulfhydryls in p65-NFkappaB protein were in reduced and activated forms after hypoxia, thereby being blocked by N-ethylmaleimide. In contrast, with hypoxic cells that were also treated with NAC, dithiothreitol increased p65-NFkappaB DNA binding. Glutaredoxin (GRx), which specifically catalyzes reduction of protein-SSG mixed disulfides, reversed inhibition of p65-NFkappaB DNA binding in extracts from cells treated with hypoxia plus NAC and restored NFkappaB activity. This finding indicated that p65-NFkappaB-SSG was formed in situ under hypoxia plus NAC conditions. In cells, knock-down of endogenous GRx1, which also promotes protein glutathionylation under hypoxic radical generating conditions, prevented NAC-induced NFkappaB inactivation and hypoxic apoptosis. The results indicate that GRx-dependent S-glutathionylation of p65-NFkappaB is most likely responsible for NAC-mediated NFkappaB inactivation and enhanced hypoxic apoptosis.

Glutaredoxin regulates nuclear factor kappa-B and intercellular adhesion molecule in Müller cells: model of diabetic retinopathy

Reversible S-glutathionylation of proteins is a focal point of redox signaling and cellular defense against oxidative stress. This post-translational modification alters protein function, and its reversal (deglutathionylation) is catalyzed specifically and efficiently by glutaredoxin (GRx, thioltransferase), a thioldisulfide oxidoreductase. We hypothesized that changes in glutaredoxin might be important in the development of diabetic retinopathy, a condition characterized by oxidative stress. Indeed, GRx protein and activity were increased in retinal homogenates from streptozotocin-diabetic rats. Also, incubation of rat retinal Müller cells (rMC-1) in normal glucose (5 mm) or diabetic-like glucose (25 mm) medium led to selective upregulation of GRx in contrast to thioredoxin, the other thioldisulfide oxidoreductase system. Under analogous conditions, NF-kappaB (p50-p65) translocated to the nucleus, and expression of ICAM-1 (intercellular adhesion molecule-1), a transcriptional product of NF-kappaB, increased. Proinflammatory ICAM-1 is increased in diabetic retinae, and it is implicated in pathogenesis of retinopathy. To evaluate the role of GRx in mediating these changes, intracellular GRx content and activity in rMC-1 cells were increased independently under normal glucose via infection with an adenoviral GRx1 construct (Ad-GRx). rMC-1 cells exhibited adenovirus concentration-dependent increases in GRx and corresponding increases in NF-kappaB nuclear translocation, NF-kappaB luciferase reporter activity, and ICAM-1 expression. Blocking the increase in GRx1 via small interfering RNA in rMC-1 cells in high glucose prevented the increased ICAM-1 expression. These data suggest that redox regulation by glutaredoxin in retinal glial cells is perturbed by hyperglycemia, leading to NF-kappaB activation and a pro-inflammatory response. Thus, GRx may represent a novel therapeutic target to inhibit diabetic retinopathy.

Molecular mechanism of glutathione-mediated protection from oxidized low-density lipoprotein-induced cell injury in human macrophages: role of glutathione reductase and glutaredoxin

Macrophage death is a hallmark of advanced atherosclerotic plaque, and oxidized low-density lipoprotein (OxLDL) found in these lesions is believed to contribute to macrophage injury. However, the underlying mechanisms of this phenomenon are only poorly understood. Here we show that in human monocyte-derived macrophages, OxLDL depleted intracellular glutathione (GSH) and inhibited glutathione reductase, resulting in a marked diminution of the glutathione/glutathione disulfide ratio. In the absence of OxLDL, an 80% depletion of intracellular GSH levels did not affect cell viability, but glutathione depletion dramatically increased OxLDL-induced cell death. Conversely, supplementation of intracellular GSH stores with glutathione diethyl ester substantially diminished OxLDL toxicity. OxLDL also promoted protein-S-glutathionylation, which was increased in macrophages pretreated with the glutathione reductase inhibitor BCNU. Knockdown experiments with siRNA directed against glutathione reductase and glutaredoxin showed that both enzymes are essential for the protection of macrophages against OxLDL. Finally, the peroxyl-radical scavenger Trolox did not prevent GSH depletion but completely blocked OxLDL-induced protein-S-glutathionylation and cell death. These data suggest that OxLDL promotes ROS formation and protein-S-glutathionylation by a mechanism independent from its effect on GSH depletion. Neither mechanism was sufficient to induce macrophage injury, but when stimulated concurrently, these pathways promoted the accumulation of protein-glutathione mixed disulfides and cell death.

Selective inactivation of glutaredoxin by sporidesmin and other epidithiopiperazinediones

Glutaredoxin (thioltransferase) is a thiol-disulfide oxidoreductase that displays efficient and specific catalysis of protein-SSG deglutathionylation and is thereby implicated in homeostatic regulation of the thiol-disulfide status of cellular proteins. Sporidesmin is an epidithiopiperazine-2,5-dione (ETP) fungal toxin that disrupts cellular functions likely via oxidative alteration of cysteine residues on key proteins. In the current study sporidesmin inactivated human glutaredoxin in a time- and concentration-dependent manner. Under comparable conditions other thiol-disulfide oxidoreductase enzymes, glutathione reductase, thioredoxin, and thioredoxin reductase, were unaffected by sporidesmin. Inactivation of glutaredoxin required the reduced (dithiol) form of the enzyme, the oxidized (intramolecular disulfide) form of sporidesmin, and molecular oxygen. The inactivated glutaredoxin could be reactivated by dithiothreitol only in the presence of urea, followed by removal of the denaturant, indicating that inactivation of the enzyme involves a conformationally inaccessible disulfide bond(s). Various cysteine-to-serine mutants of glutaredoxin were resistant to inactivation by sporidesmin, suggesting that the inactivation reaction specifically involves at least two of the five cysteine residues in human glutaredoxin. The relative ability of various epidithiopiperazine-2,5-diones to inactivate glutaredoxin indicated that at least one phenyl substituent was required in addition to the epidithiodioxopiperazine moiety for inhibitory activity. Mass spectrometry of the modified protein is consistent with formation of intermolecular disulfides, containing one adducted toxin per glutaredoxin but with elimination of two sulfur atoms from the detected product. We suggest that the initial reaction is between the toxin sulfurs and cysteine 22 in the glutaredoxin active site. This study implicates selective modification of sulfhydryls of target proteins in some of the cytotoxic effects of the ETP fungal toxins and their synthetic analogues.

Computational and mutational analysis of human glutaredoxin (thioltransferase): probing the molecular basis of the low pKa of cysteine 22 and its role in catalysis

Human glutaredoxin (GRx), also known as thioltransferase, is a 12 kDa thiol-disulfide oxidoreductase that is highly selective for reduction of glutathione-containing mixed disulfides. The apparent pK(a) for the active site Cys22 residue is approximately 3.5. Previously we observed that the catalytic enhancement by glutaredoxin could be ascribed fully to the difference between the pK(a) of its Cys22 thiol moiety and the pK(a) of the product thiol, each acting as a leaving group in the enzymatic and nonenzymatic reactions, respectively [Srinivasan et al. (1997), Biochemistry 36, 3199-3206]. Continuum electrostatic calculations suggest that the low pK(a) of Cys22 results primarily from stabilization of the thiolate anion by a specific ion-pairing with the positively charged Lys19 residue, although hydrogen bonding interactions with Thr21 also appear to contribute. Variants of Lys19 were considered to further assess the predicted role of Lys19 on the pK(a) of Cys22. The variants K19Q and K19L were generated by molecular modeling, and the pK(a) value for Cys22 was calculated for each variant. For K19Q, the predicted Cys22 pK(a) is 7.3, while the predicted value is 8.3 for K19L. The effects of the mutations on the interaction energy between the adducted glutathionyl moiety and GRx were roughly estimated from the van der Waals and electrostatic energies between the glutathionyl moiety and proximal protein residues in a mixed disulfide adduct of GRx and glutathione, i.e., the GRx-SSG intermediate. The values for the K19 mutants differed by only a small amount compared to those for the wild type enzyme intermediate. Together, the computational analysis predicted that the mutant enzymes would have markedly reduced catalytic rates while retaining the glutathionyl specificity displayed by the wild type enzyme. Accordingly, we constructed and characterized the K19L and K19Q mutants of two forms of the GRx enzyme. Each of the mutants retained glutathionyl specificity as predicted and displayed diminution in activity, but the decreases in activity were not to the extent predicted by the theoretical calculations. Changes in the respective Cys22-thiol pK(a) values of the mutant enzymes, as shown by pH profiles for iodoacetamide inactivation of the respective enzymes, clearly revealed that the K19-C22 ion pair cannot fully account for the low pK(a) of the Cys22 thiol. Additional contributions to stabilization of the Cys22 thiolate are likely donated by Thr21 and the N-terminal partial positive charge of the neighboring alpha-helix.

Role of vicinal protein thiols in radiation and cytotoxic responses

Glutathione (GSH) and more recently protein thiols (P-SH) have been found to play a major role in cellular radiation response. However, the effects of protein vicinal thiols, which are important for the functions of several major enzymes, on cellular responses to radiation have not been clearly delineated. Here we investigated the effects of depleting GSH and protein vicinal thiols (HS-P-SH) and P-SH on cell toxicity and radiation response. We used hydroxyethyldisulfide (HEDS, beta-mercaptoethanol-disulfide) alone and in combination with phenylarsine oxide (PAO) to alter P-SH, HS-P-SH and GSH. HEDS, a direct substrate for thioredoxin reductase and an indirect substrate for glutaredoxin (thioltransferase), did not alter protein vicinal thiols in cells. However, PAO, which specifically forms a covalent adduct with vicinal thiols, blocked bioreduction of HEDS; there was a concomitant and yet unexplained decrease in K1 cell GSH in the presence of HEDS and PAO. G6PD+ (K1) and G6PD- (E89) cells treated with L-buthionine sulfoximine (L-BSO) for 72 h to deplete GSH followed by PAO showed an increased cytotoxic response. However, the surviving E89 cells showed a 10,000-fold greater radiation lethality than the K1 cells. The effects of rapid depletion of GSH by a combination of L-BSO and dimethyfumarate (DMF), a glutathione-S-transferase substrate, were also investigated. Under these conditions, PAO radiosensitized the E89 cells more than 1000-fold over the K1 cells. The potential mechanisms for the altered response may be related to the inhibition of thioredoxin reductase and glutaredoxin. Both are key enzymes involved in DNA synthesis, protein homeostasis and cell survival. With GSH removed, vicinal thiols appear to play a critical role in determining cell survival and radiosensitivity. Decreasing P-SH and removing GSH and vicinal thiols is extremely toxic to K1 and E89 cells. We conclude that radiation sensitivity and cell survival are dependent on vicinal thiol and GSH. In the former and latter cases, the protein thiols are also important.

A novel thiol oxidation-based mechanism for adriamycin-induced cell injury in human macrophages

Adriamycin is a widely used antitumor antibiotic, but its use has been limited by its cytotoxicity in both cardiomyocytes and non-cardiac tissues. While adriamycin's ability to redox cycle via one-electron transfer reactions and generate ROS is thought to promote cardiotoxicity, the mechanisms involved in non-cardiac tissue injury are not clear. Here we show that prolonged exposure (48 h) of human monocyte-derived macrophages to adriamycin at concentrations as low as 1 microM promotes caspase-independent cell death. Treatment of cells with scavengers of superoxide and peroxyl radicals blocked adriamycin-induced oxidation of dichlorodihydrofluorescein (DCFH) but did not prevent macrophage injury. Macrophages treated with either adriamycin or the thiol oxidant diamide showed elevated levels of glutathione disulfide and increased protein-S-glutathionylation prior to cell injury, indicating that thiol oxidation is involved in adriamycin-induced macrophage death. Furthermore, inhibition of glutathione reductase (GR) with 1,3-bis[2-chloroethyl]-1-nitrosourea or transfection of macrophages with small inhibitory RNA (siRNA) directed against GR or glutaredoxin (Grx) potentiated adriamycin-induced macrophage injury. Thus, both GR and Grx appear to play a crucial role in protecting macrophages from adriamycin-induced cell injury. These findings suggest a new mechanism for adriamycin-induced tissue injury whereby thiol oxidation, rather than one-electron redox cycling and ROS generation, mediates adriamycin-induced cell damage.

Glutaredoxin: role in reversible protein s-glutathionylation and regulation of redox signal transduction and protein translocation

Reversible posttranslational modifications on specific amino acid residues can efficiently regulate protein functions. O-Phosphorylation is the prototype and analogue to the rapidly emerging mechanism of regulation known as S-glutathionylation. The latter is being recognized as a potentially widespread form of modulation of the activities of redox-sensitive thiol proteins, especially those involved in signal transduction pathways and translocation. The abundance of reduced glutathione in cells and the ready conversion of sulfenic acids and S-nitroso derivatives to S-glutathione mixed disulfides support the notion that reversible S-glutathionylation is likely to be the preponderant mode of redox signal transduction. The glutaredoxin enzyme has served as a focal point and important tool for evolution of this regulatory mechanism because of its characterization as a specific and efficient catalyst of protein-SSG de-glutathionylation (akin to phosphatases). Identification of specific mechanisms and enzyme(s) that catalyze formation of protein-SSG intermediates, however, is largely unknown and represents a prime objective for furthering understanding of this evolving mechanism of cellular regulation. Several proteomic approaches, including the use of cysteine-reactive fluorescent and radiolabel probes, have been developed to detect arrays of proteins whose cysteine residues are modified in response to oxidants, thus identifying them as potential interconvertible proteins to be regulated by redox signaling (glutathionylation). Specific criteria were used to evaluate current data on cellular regulation via S-glutathionylation. Among many proteins under consideration, actin, protein tyrosine phosphatase-1B, and Ras stand out as the best current examples for establishing this regulatory mechanism.

Stable and controllable RNA interference: Investigating the physiological function of glutathionylated actin

RNA interference is an effective method to silence specific gene expression. Its application to mammalian cells, however, has been hampered by various shortcomings. Recently, it was reported that introduction of 22-bp double-stranded RNAs (dsRNAs) would specifically suppress expression of endogenous and heterogeneous genes in various mammalian cell lines. However, using this method, we failed to knock out proteins of interest effectively. Here we report the development of a stable and controllable method for generating dsRNA intracellularly. Tetracycline-responsive transactivator-containing cells were transfected with a vector capable of tetracycline-induced bidirectionally overexpressing sense and antisense RNA to form dsRNA in vivo. With this method, glutaredoxin, monitored by Western blot, was knocked out by overexpressing 290-base sense and antisense RNA in NIH 3T3 cells controlled by tetracycline or doxycycline. By using these glutaredoxin knocked-out cells, we have demonstrated that actin deglutathionylation plays a key role in growth factor-mediated actin polymerization, translocalization, and reorganization near the cell periphery.

Glutathione-thiyl radical scavenging and transferase properties of human glutaredoxin (thioltransferase). Potential role in redox signal transduction

Glutaredoxin (GRx, thioltransferase) is implicated in cellular redox regulation, and it is known for specific and efficient catalysis of reduction of protein-S-S-glutathione-mixed disulfides (protein-SSG) because of its remarkably low thiol pK(a) ( approximately 3.5) and its ability to stabilize a catalytic S-glutathionyl intermediate (GRx-SSG). These unique properties suggested that GRx might also react with glutathione-thiyl radicals (GS(.)) and stabilize a disulfide anion radical intermediate (GRx-SSG), thereby facilitating the conversion of GS(.) to GSSG or transfer of GS(.) to form protein-SSG. We found that GRx catalyzes GSSG formation in the presence of GS-thiyl radical generating systems (Fe(2+)/ADP/H(2)O(2) + GSH or horseradish peroxidase/H(2)O(2) + GSH). Catalysis is dependent on O(2) and results in concomitant superoxide formation, and it is distinguished from glutathione peroxidase-like activity. With the horseradish peroxidase system and [(35)S]GSH, GRx enhanced the rate of GS-radiolabel incorporation into GAPDH. GRx also enhanced the rate of S-glutathionylation of glyceraldehyde-3-phosphate dehydrogenase with GSSG or S-nitrosoglutathione, but these glutathionyl donors were much less efficient. Both actin and protein-tyrosine phosphatase-1B were superior substrates for GRx-facilitated S-glutathionylation with GS-radical. These studies characterize GRx as a versatile catalyst, facilitating GS-radical scavenging and S-glutathionylation of redox signal mediators, consistent with a critical role in cellular regulation.

Reversible inactivation of alpha-ketoglutarate dehydrogenase in response to alterations in the mitochondrial glutathione status

In a previous study, we found that treatment of rat heart mitochondria with H(2)O(2) resulted in a decline and subsequent recovery in the rate of state 3 NADH-linked respiration. These effects were shown to be mediated by reversible alterations in NAD(P)H utilization and in the activities of specific Krebs cycle enzymes alpha-ketoglutarate dehydrogenase (KGDH) and succinate dehydrogenase. The purpose of the current study was to examine potential mechanism(s) by which H(2)O(2) reversibly alters KGDH activity. We report here that inactivation is not simply due to direct interaction of H(2)O(2) with KGDH. In addition, incubation of mitochondria with deferroxamine, an iron chelator, or 1,3-dimethyl-2-thiourea, an oxygen radical scavenger, prior to addition of H(2)O(2) did not alter the rate or extent of inactivation. Thus, inactivation does not appear to involve a more potent oxygen radical formed upon metal-catalyzed oxidation. Inactive KGDH from H(2)O(2)-treated mitochondria was reactivated with dithiothreitol, implicating oxidation of a protein sulfhydryl(s). However, the thioredoxin system had no effect, indicating that enzyme inactivation is not due to the formation of intra- or intermolecular disulfide(s) or a sulfenic acid. Upon incubation of mitochondria with H(2)O(2), reduced GSH levels fell rapidly prior to enzyme inactivation but recovered at the same time as enzyme activity. Importantly, treatment of inactive KGDH with glutaredoxin facilitated the GSH-dependent recovery of KGDH activity. Glutaredoxin is characterized as a specific and efficient catalyst of protein deglutathionylation. Thus, the results of the current study indicate that KGDH activity appears to be modulated through enzymatic glutathionylation and deglutathionylation. These studies demonstrate a novel mechanism by which KGDH activity and mitochondrial function can be modulated by redox status.