Identification of an important cysteine residue in human glutamate-cysteine ligase catalytic subunit by site-directed mutagenesis

Click chemistry-based thiol redox proteomics reveals significant cysteine reduction induced by chronic ethanol consumption

In the U.S., alcohol-associated liver disease (ALD) impacts millions of people and is a major healthcare burden. While the pathology of ALD is unmistakable, the molecular mechanisms underlying ethanol hepatotoxicity are not fully understood. Hepatic ethanol metabolism is intimately linked with alterations in extracellular and intracellular metabolic processes, specifically oxidation/reduction reactions. The xenobiotic detoxification of ethanol leads to significant disruptions in glycolysis, β-oxidation, and the TCA cycle, as well as oxidative stress. Perturbation of these regulatory networks impacts the redox status of critical regulatory protein thiols throughout the cell. Integrating these key concepts, our goal was to apply a cutting-edge approach toward understanding mechanisms of ethanol metabolism in disrupting hepatic thiol redox signaling. Utilizing a chronic murine model of ALD, we applied a cysteine targeted click chemistry enrichment coupled with quantitative nano HPLC-MS/MS to assess the thiol redox proteome. Our strategy reveals that ethanol metabolism largely reduces the cysteine proteome, with 593 cysteine residues significantly reduced and 8 significantly oxidized cysteines. Ingenuity Pathway Analysis demonstrates that ethanol metabolism reduces specific cysteines throughout ethanol metabolism (Adh1, Cat, Aldh2), antioxidant pathways (Prx1, Mgst1, Gsr), as well as many other biochemical pathways. Interestingly, a sequence motif analysis of reduced cysteines showed a correlation for hydrophilic, charged amino acids lysine or glutamic acid nearby. Further research is needed to determine how a reduced cysteine proteome impacts individual protein activity across these protein targets and pathways. Additionally, understanding how a complex array of cysteine-targeted post-translational modifications (e.g., S-NO, S-GSH, S-OH) are integrated to regulate redox signaling and control throughout the cell is key to the development of redox-centric therapeutic agents targeted to ameliorate the progression of ALD.

CtrA activates the expression of glutathione S-transferase conferring oxidative stress resistance to Ehrlichia chaffeensis

Ehrlichia chaffeensis, the causative agent of human monocytic ehrlichiosis (HME), is a Gram-negative obligatory intracellular bacterium, which infects and multiplies in human monocytes and macrophages. Host immune cells produce reactive oxygen species (ROS) to eliminate E. chaffeensis upon infection. E. chaffeensis global transcriptional regulator CtrA activates the expression of GshA and GshB to synthesize glutathione (GSH), the most potent natural antioxidant, upon oxidative stress to combat ROS damage. However, the mechanisms exploited by E. chaffeensis to utilize GSH are still unknown. Here, we found that in E. chaffeensis CtrA activated the expression of glutathione S-transferase (GST) upon oxidative stress, and E. chaffeensis GST utilizes GSH to eliminate ROS and confers the oxidative stress resistance to E. chaffeensis. We found that CtrA bound to the promoter regions of 211 genes, including gst, in E. chaffeensis using chromatin immunoprecipitation coupled to deep sequencing (ChIP-seq). Recombinant E. chaffeensis CtrA directly bound to the gst promoter region determined with electrophoretic mobility shift assay (EMSA), and activated the gst expression determined with reporter assay. Recombinant GST showed GSH conjugation activity towards its typical substrate 2,4-dinitrochlorobenzene (CDNB) in vitro and peptide nucleic acid (PNA) transfection of E. chaffeensis, which can knock down the gst transcription level, reduced bacterial survival upon oxidative stress. Our results demonstrate that E. chaffeensis CtrA regulates GSH utilization, which plays a critical role in resistance to oxidative stress, and aid in the development of new therapeutics for HME.

Exploring Idiopathic Pulmonary Fibrosis Biomarker by Simultaneous Two-Photon Fluorescence Imaging of Cysteine and Peroxynitrite

Idiopathic pulmonary fibrosis (IPF) has been characterized as a chronic inflammatory disease that leads to irreversible damage to pulmonary function. However, there is no specific IPF biomarker that can be used to distinguish IPF and not pneumonia. Endoplasmic reticulum (ER) stress is prominent in IPF. To search for a specific biomarker of IPF, we developed two ER-targeting two-photon (TP) fluorescent probes, TP^ER-ONOO and TP^ER-Cys, for peroxynitrite (ONOO^-) and cysteine (Cys) imaging, respectively. A significant increase in Cys levels in the lungs was discovered only in mice with IPF, which implied that Cys might be an IPF biomarker candidate. Furthermore, we uncovered the mechanism of glutathione (GSH) deficiency in IPF, which was not due to Cys shortage but instead was attributable to impaired glutamate cysteine ligase and glutathione synthetase activities via ONOO^--induced post-transcriptional modification. This work has potential to provide a new method for IPF early diagnosis and drug efficacy evaluation.

Ligaza γ-glutamylocysteiny – od molekularnych mechanizmów regulacji aktywności enzymatycznej do implikacji terapeutycznych

Glutation (γ-glutamylocysteinyloglicyna, GSH) jest najbardziej rozpowszechnionym tiolowym antyoksydantem wytwarzanym w cytozolu wszystkich komórek ssaków, który pełni ważną rolę ochronną przed stresem oksydacyjnym. GSH jest syntetyzowany de novo przez sekwencyjne działanie dwóch enzymów: ligazy γ-glutamylocysteiny (GCL) i syntetazy glutationowej (GS). GCL katalizuje pierwszy etap biosyntezy GSH, którego produktem jest γ-glutamylocysteina (γ-GC). GCL jest heterodimerycznym enzymem zbudowanym z podjednostki katalitycznej (GCLc) i modulatorowej (GCLm), kodowanych przez dwa różne geny. Podjednostki GCL podlegają złożonej regulacji zarówno na poziomie przed-, jak i potranslacyjnym. Zmiany w ekspresji i aktywności GCL mogą zaburzać poziom GSH i homeostazy redoks. Przyczyną wielu przewlekłych schorzeń związanych ze stresem oksydacyjnym jest upośledzenie aktywności katalitycznej GCL oraz spadek stężenia GSH. Badania przedkliniczne sugerują, że podawanie egzogennej γ-GC podwyższa wewnątrzkomórkowe GSH przez dostarczenie brakującego substratu i może wykazywać potencjał jako terapia uzupełniająca w chorobach związanych z deplecją GSH.

The Central Role of Amino Acids in Cancer Redox Homeostasis: Vulnerability Points of the Cancer Redox Code

A fine balance in reactive oxygen species (ROS) production and removal is of utmost importance for homeostasis of all cells and especially in highly proliferating cells that encounter increased ROS production due to enhanced metabolism. Consequently, increased production of these highly reactive molecules requires coupling with increased antioxidant defense production within cells. This coupling is observed in cancer cells that allocate significant energy reserves to maintain their intracellular redox balance. Glutathione (GSH), as a first line of defense, represents the most important, non-enzymatic antioxidant component together with the NADPH/NADP⁺ couple, which ensures the maintenance of the pool of reduced GSH. In this review, the central role of amino acids (AAs) in the maintenance of redox homeostasis in cancer, through GSH synthesis (cysteine, glutamate, and glycine), and nicotinamide adenine dinucleotide (phosphate) production (serine, and glutamine/glutamate) are illustrated. Special emphasis is placed on the importance of AA transporters known to be upregulated in cancers (such as system x_c-light chain and alanine-serine-cysteine transporter 2) in the maintenance of AA homeostasis, and thus indirectly, the redox homeostasis of cancer cells. The role of the ROS varies (often described as a "two-edged sword") during the processes of carcinogenesis, metastasis, and cancer treatment. Therefore, the context-dependent role of specific AAs in the initiation, progression, and dissemination of cancer, as well as in the redox-dependent sensitivity/resistance of the neoplastic cells to chemotherapy are highlighted.

Adaptive Posttranslational Control in Cellular Stress Response Pathways and Its Relationship to Toxicity Testing and Safety Assessment

Although transcriptional induction of stress genes constitutes a major cellular defense program against a variety of stressors, posttranslational control directly regulating the activities of preexisting stress proteins provides a faster-acting alternative response. We propose that posttranslational control is a general adaptive mechanism operating in many stress pathways. Here with the aid of computational models, we first show that posttranslational control fulfills two roles: (1) handling small, transient stresses quickly and (2) stabilizing the negative feedback transcriptional network. We then review the posttranslational control pathways for major stress responses-oxidative stress, metal stress, hyperosmotic stress, DNA damage, heat shock, and hypoxia. Posttranslational regulation of stress protein activities occurs by reversible covalent modifications, allosteric or non-allosteric enzymatic regulations, and physically induced protein structural changes. Acting in feedback or feedforward networks, posttranslational control may establish a threshold level of cellular stress. Sub-threshold stresses are handled adequately by posttranslational control without invoking gene transcription. With supra-threshold stress levels, cellular homeostasis cannot be maintained and transcriptional induction of stress genes and other gene programs, eg, those regulating cell metabolism, proliferation, and apoptosis, takes place. The loss of homeostasis with consequent changes in cellular function may lead to adverse cellular outcomes. Overall, posttranslational and transcriptional control pathways constitute a stratified cellular defense system, handling stresses coherently across time and intensity. As cell-based assays become a focus for chemical testing anchored on toxicity pathways, examination of proteomic and metabolomic changes as a result of posttranslational control occurring in the absence of transcriptomic alterations deserves more attention.

Emerging regulatory paradigms in glutathione metabolism

One of the hallmarks of cancer is the ability to generate and withstand unusual levels of oxidative stress. In part, this property of tumor cells is conferred by elevation of the cellular redox buffer glutathione. Though enzymes of the glutathione synthesis and salvage pathways have been characterized for several decades, we still lack a comprehensive understanding of their independent and coordinate regulatory mechanisms. Recent studies have further revealed that overall central metabolic pathways are frequently altered in various tumor types, resulting in significant increases in biosynthetic capacity and feeding into glutathione synthesis. In this review, we will discuss the enzymes and pathways affecting glutathione flux in cancer and summarize current models for regulating cellular glutathione through both de novo synthesis and efficient salvage. In addition, we examine the integration of glutathione metabolism with other altered fates of intermediary metabolites and highlight remaining questions about molecular details of the accepted regulatory modes.

Toxicity of Glutathione-Binding Metals: A Review of Targets and Mechanisms

Mercury, cadmium, arsenic and lead are among priority metals for toxicological studies due to the frequent human exposure and to the significant burden of disease following acute and chronic intoxication. Among their common characteristics is chemical affinity to proteins and non-protein thiols and their ability to generate cellular oxidative stress by the best-known Fenton mechanism. Their health effects are however diverse: kidney and liver damage, cancer at specific sites, irreversible neurological damages with metal-specific features. Mechanisms for the induction of oxidative stress by interaction with the cell thiolome will be presented, based on literature evidence and of experimental findings.

Targeting aberrant glutathione metabolism to eradicate human acute myelogenous leukemia cells

The development of strategies to eradicate primary human acute myelogenous leukemia (AML) cells is a major challenge to the leukemia research field. In particular, primitive leukemia cells, often termed leukemia stem cells, are typically refractory to many forms of therapy. To investigate improved strategies for targeting of human AML cells we compared the molecular mechanisms regulating oxidative state in primitive (CD34(+)) leukemic versus normal specimens. Our data indicate that CD34(+) AML cells have elevated expression of multiple glutathione pathway regulatory proteins, presumably as a mechanism to compensate for increased oxidative stress in leukemic cells. Consistent with this observation, CD34(+) AML cells have lower levels of reduced glutathione and increased levels of oxidized glutathione compared with normal CD34(+) cells. These findings led us to hypothesize that AML cells will be hypersensitive to inhibition of glutathione metabolism. To test this premise, we identified compounds such as parthenolide (PTL) or piperlongumine that induce almost complete glutathione depletion and severe cell death in CD34(+) AML cells. Importantly, these compounds only induce limited and transient glutathione depletion as well as significantly less toxicity in normal CD34(+) cells. We further determined that PTL perturbs glutathione homeostasis by a multifactorial mechanism, which includes inhibiting key glutathione metabolic enzymes (GCLC and GPX1), as well as direct depletion of glutathione. These findings demonstrate that primitive leukemia cells are uniquely sensitive to agents that target aberrant glutathione metabolism, an intrinsic property of primary human AML cells.

Transcriptional regulation of glutathione biosynthesis genes, γ-glutamyl-cysteine ligase and glutathione synthetase in response to cadmium and nonylphenol in Chironomus riparius

We characterized Chironomus riparius glutathione (GSH) biosynthesis genes, γ-glutamyl-cysteine ligase catalytic subunit (cr-gcl) and glutathione synthetase (cr-gs) and studied their expression after cadmium (Cd) and nonylphenol (NP) exposure. The full length cDNA of the Cr-GCL catalytic subunit was 2185 base pair (bp) in length containing an open reading frame of 1905bp, a 13bp 5' and 267bp 3' untranslated regions. The theoretical molecular mass of the deduced amino acid sequence (633) was 72.65kDa with an estimated pI of 5.42. The partial cDNA of Cr-GS was 739bp in length consisting 221 amino acids. The deduced amino acid sequence of Cr-GCL and Cr-GS cDNAs showed high conservation with homologs from other species. In phylogenetic analysis Cr-GCL and Cr-GS were grouped with equivalent genes from insects belonging to the dipteran order. The expression of cr-gcl and cr-gs was measured using quantitative real-time PCR after exposure to sub lethal concentrations of Cd (2, 10 and 20mg/L) and NP (10, 50 and 100μg/L) for 12, 24, 48 and 72h using real-time PCR methods. The mRNA expression of Cr-GCL and Cr-GS was significantly modulated after exposure to different concentrations of Cd and NP for different time periods. Total GSH levels showed a non-significant decrease after exposure to Cd for 24h. However, no change in GSH levels was observed after exposure to NP for 24h. These results suggest that Cr-GS and Cr-GCL expression is modulated by Cd and NP stress and may play an important role in detoxification of xenobiotics and antioxidant defense. We conclude that Cr-GS and Cr-GCL could be used as biomarkers of Cd and NP stress in aquatic environment for the studied species.

Redox-dependent stability of the γ-glutamylcysteine synthetase enzyme of Escherichia coli: a novel means of redox regulation

Glutathione is a thiol-containing tripeptide that plays important roles in redox-related processes. The first step in glutathione biosynthesis is catalysed by γ-GCS (γ-glutamylcysteine synthetase). The crystal structure of Escherichia coli γ-GCS has revealed the presence of a disulfide bond. As the disulfide-bonding cysteine residues Cys372 and Cys395 are not well conserved among γ-GCS enzymes in this lineage, we have initiated a biochemical genetic strategy to investigate the functional importance of these and other cysteine residues. In a cysteine-free γ-GCS that was non-functional, suppressor analysis yielded combinations of cysteine and aromatic residues at the position of the disulfide bond, and one mutant that lacked any cysteine residues. Kinetic analysis of the wild-type and mutant enzymes revealed that the disulfide bond was not involved in determining the affinity of the enzyme towards its substrate, but had an important role in determining the stability of the protein, and its catalytic efficiency. We show that in vivo the γ-GCS enzyme can also exist in a reduced form and that the mutants lacking the disulfide bond show a decreased half-life. These results demonstrate a novel means of regulation of γ-GCS by the redox environment that works by an alteration in its stability.

Transcription of genes involved in glutathione biosynthesis in the solitary tunicate Ciona intestinalis exposed to metals

Exposure to metals is known to generate oxidative stress risk in living organisms, which are able to respond with the induction of antioxidant defenses, both enzymatic and non-enzymatic. Glutathione (GSH) is considered to be an important cellular component involved in protecting cells, both as metal chelating agent and oxygen radical scavenger. In this work we used molecular techniques to analyze the nucleotide and predicted amino acid sequences of genes involved in GSH biosynthesis, γ-glutamyl-cysteine ligase catalytic subunit (ci-gclc), γ-glutamyl-cysteine ligase modifier subunit (ci-gclm) and GSH synthase (ci-gs) in the solitary tunicate Ciona intestinalis. We also studied the transcription of the above genes after in vivo exposure to Cd, Cu and Zn by semiquantitativ RT-PCR to improve our knowledge about the relationship between metal-induced oxidative stress and GSH production and locate mRNA expression by in situ hybridization (ISH). These genes exhibit a good level of sequence conservation with metazoan homologs generally, especially for residues important for the activity of the enzymes. Phylogenetic analyses indicate that the three enzymes evolved in different ways, Ci-GCLC and Ci-GS being mostly correlated with invertebrate proteins, Ci-GCLM being as sister group of vertebrate GCLMs. Our in silico analyses of the ci-gs and ci-gclc promoter regions revealed putative consensus sequences similar to mammalian metal-responsive elements (MRE) and antioxidant response elements (ARE), indicating that the transcription of these genes may directly depend on metals and/or reactive oxygen species. Results highlight a statistically significant increase in gene transcription, demonstrating that metal treatments have inducible effects on these genes. They can modulate gene transcription not only through MREs but also through AREs, as a consequence of metal-dependent ROS formation. The ISH location of Ci-GS and Ci-GCLC mRNAs shows that the cells most involved in glutathione biosynthesis are circulating hemocytes. The data presented here emphasize the importance of complex metal regulation of ci-gclc, ci-gclm and ci-gs transcription, which can create an efficient detoxification pathway allowing C. intestinalis to survive in continued elevated presence of metals in the environment.

Enzymatic defects underlying hereditary glutamate cysteine ligase deficiency are mitigated by association of the catalytic and regulatory subunits

Glutamate cysteine ligase (GCL) deficiency is a rare autosomal recessive trait that compromises production of glutathione, a critical redox buffer and enzymatic cofactor. Patients have markedly reduced levels of erythrocyte glutathione, leading to hemolytic anemia and, in some cases, impaired neurological function. Human glutamate cysteine ligase is a heterodimer comprised of a catalytic subunit (GCLC) and a regulatory subunit (GCLM), which catalyzes the initial rate-limiting step in glutathione production. Four clinical missense mutations have been identified within GCLC: Arg127Cys, Pro158Leu, His370Leu, and Pro414Leu. Here, we have evaluated the impacts of these mutations on enzymatic function in vivo and in vitro to gain further insight into the pathology. Embryonic fibroblasts from GCLC null mice were transiently transfected with wild-type or mutant GCLC, and cellular glutathione levels were determined. The four mutant transfectants each had significantly lower levels of glutathione relative to that of the wild type, with the Pro414Leu mutant being most compromised. The contributions of the regulatory subunit to GCL activity were investigated using a Saccharomyces cerevisiae model system. Mutant GCLC alone could not complement a glutathione deficient strain and required the concurrent addition of GCLM to restore growth. Kinetic characterizations of the recombinant GCLC mutants indicated that the Arg127Cys, His370Leu, and Pro414Leu mutants have compromised enzymatic activity that can largely be rescued by the addition of GCLM. Interestingly, the Pro158Leu mutant has kinetic constants comparable to those of wild-type GCLC, suggesting that heterodimer formation is needed for stability in vivo. Strategies that promote heterodimer formation and persistence would be effective therapeutics for the treatment of GCL deficiency.

Posttranslational modification and regulation of glutamate-cysteine ligase by the α,β-unsaturated aldehyde 4-hydroxy-2-nonenal

4-Hydroxy-2-nonenal (4-HNE) is a lipid peroxidation product formed during oxidative stress that can alter protein function via adduction of nucleophilic amino acid residues. 4-HNE detoxification occurs mainly via glutathione (GSH) conjugation and transporter-mediated efflux. This results in a net loss of cellular GSH, and restoration of GSH homeostasis requires de novo GSH biosynthesis. The rate-limiting step in GSH biosynthesis is catalyzed by glutamate-cysteine ligase (GCL), a heterodimeric holoenzyme composed of a catalytic (GCLC) and a modulatory (GCLM) subunit. The relative levels of the GCL subunits are a major determinant of cellular GSH biosynthetic capacity and 4-HNE induces the expression of both GCL subunits. In this study, we demonstrate that 4-HNE can alter GCL holoenzyme formation and activity via direct posttranslational modification of the GCL subunits in vitro. 4-HNE directly modified Cys553 of GCLC and Cys35 of GCLM in vitro, which significantly increased monomeric GCLC enzymatic activity, but reduced GCL holoenzyme activity and formation of the GCL holoenzyme complex. In silico molecular modeling studies also indicate these residues are likely to be functionally relevant. Within a cellular context, this novel posttranslational regulation of GCL activity could significantly affect cellular GSH homeostasis and GSH-dependent detoxification during periods of oxidative stress.

Rapid activation of glutamate cysteine ligase following oxidative stress

Glutamate cysteine ligase (GCL) catalyzes the rate-limiting step in the formation of the cellular antioxidant glutathione (GSH). The GCL holoenzyme consists of two separately coded proteins, a catalytic subunit (GCLC) and a modifier subunit (GCLM). Both GCLC and GLCM are controlled transcriptionally by a variety of cellular stimuli, including oxidative stress. This study addresses post-translational control of GCL activity, which increased rapidly in human lymphocytes following oxidative stress. Activation of GCL occurred within minutes of treatment and without any change in GCL protein levels and coincided with an increase in the proportion of GCLC in the holoenzyme form. Likewise, GCLM shifted from the monomeric form to holoenzyme and higher molecular weight species. Normal rat tissues also showed a distribution of monomeric and higher molecular weight forms. Neither GCL activation, nor the formation of holoenzyme, required a covalent intermolecular disulfide bridge between GCLC and GCLM. However, in immunoprecipitation studies, a neutralizing epitope associated with enzymatic activity was protected following cellular oxidative stress. Thus, the N-terminal portion of GCLC may undergo a change that stabilizes the GCL holoenzyme. Our results suggest that a dynamic equilibrium exists between low and high activity forms of GCL and is altered by transient oxidative stress. This provides a mechanism for the rapid post-translational activation of GCL and maintenance of cellular GSH homeostasis.

Sensing sulfur conditions: simple to complex protein regulatory mechanisms in plant thiol metabolism

Sulfur is essential for plant growth and development, and the molecular systems for maintaining sulfur and thiol metabolism are tightly controlled. From a biochemical perspective, the regulation of plant thiol metabolism highlights nature's ability to engineer pathways that respond to multiple inputs and cellular demands under a range of conditions. In this review, we focus on the regulatory mechanisms that form the molecular basis of biochemical sulfur sensing in plants by translating the intracellular concentration of sulfur-containing compounds into control of key metabolic steps. These mechanisms range from the simple (substrate availability, thermodynamic properties of reactions, feedback inhibition, and organelle localization) to the elaborate (formation of multienzyme complexes and thiol-based redox switches). Ultimately, the dynamic interplay of these regulatory systems is critical for sensing and maintaining sulfur assimilation and thiol metabolism in plants.

Manipulation of cellular GSH biosynthetic capacity via TAT-mediated protein transduction of wild-type or a dominant-negative mutant of glutamate cysteine ligase alters cell sensitivity to oxidant-induced cytotoxicity

The glutathione (GSH) antioxidant defense system plays a central role in protecting mammalian cells against oxidative injury. Glutamate cysteine ligase (GCL) is the rate-limiting enzyme in GSH biosynthesis and is a heterodimeric holoenzyme composed of catalytic (GCLC) and modifier (GCLM) subunits. As a means of assessing the cytoprotective effects of enhanced GSH biosynthetic capacity, we have developed a protein transduction approach whereby recombinant GCL protein can be rapidly and directly transferred into cells when coupled to the HIV TAT protein transduction domain. Bacterial expression vectors encoding TAT fusion proteins of both GCL subunits were generated and recombinant fusion proteins were synthesized and purified to near homogeneity. The TAT-GCL fusion proteins were capable of heterodimerization and formation of functional GCL holoenzyme in vitro. Exposure of Hepa-1c1c7 cells to the TAT-GCL fusion proteins resulted in the time- and dose-dependent transduction of both GCL subunits and increased cellular GCL activity and GSH levels. A heterodimerization-competent, enzymatically deficient GCLC-TAT mutant was also generated in an attempt to create a dominant-negative suppressor of GCL. Transduction of cells with a catalytically inactive GCLC(E103A)-TAT mutant decreased cellular GCL activity in a dose-dependent manner. TAT-mediated manipulation of cellular GCL activity was also functionally relevant as transduction with wild-type GCLC(WT)-TAT or mutant GCLC(E103A)-TAT conferred protection or enhanced sensitivity to H(2)O(2)-induced cell death, respectively. These findings demonstrate that TAT-mediated transduction of wild-type or dominant-inhibitory mutants of the GCL subunits is a viable means of manipulating cellular GCL activity to assess the effects of altered GSH biosynthetic capacity.

Mechanistic details of glutathione biosynthesis revealed by crystal structures of Saccharomyces cerevisiae glutamate cysteine ligase

Glutathione is a thiol-disulfide exchange peptide critical for buffering oxidative or chemical stress, and an essential cofactor in several biosynthesis and detoxification pathways. The rate-limiting step in its de novo biosynthesis is catalyzed by glutamate cysteine ligase, a broadly expressed enzyme for which limited structural information is available in higher eukaryotic species. Structural data are critical to the understanding of clinical glutathione deficiency, as well as rational design of enzyme modulators that could impact human disease progression. Here, we have determined the structures of Saccharomyces cerevisiae glutamate cysteine ligase (ScGCL) in the presence of glutamate and MgCl(2) (2.1 A; R = 18.2%, R(free) = 21.9%), and in complex with glutamate, MgCl(2), and ADP (2.7 A; R = 19.0%, R(free) = 24.2%). Inspection of these structures reveals an unusual binding pocket for the alpha-carboxylate of the glutamate substrate and an ATP-independent Mg(2+) coordination site, clarifying the Mg(2+) dependence of the enzymatic reaction. The ScGCL structures were further used to generate a credible homology model of the catalytic subunit of human glutamate cysteine ligase (hGCLC). Examination of the hGCLC model suggests that post-translational modifications of cysteine residues may be involved in the regulation of enzymatic activity, and elucidates the molecular basis of glutathione deficiency associated with patient hGCLC mutations.

A systems biology perspective on Nrf2-mediated antioxidant response

Cells in vivo are constantly exposed to reactive oxygen species (ROS) generated endogenously and exogenously. To defend against the deleterious consequences of ROS, cells contain multiple antioxidant enzymes expressed in various cellular compartments to scavenge these toxic species. Under oxidative stresses, these antioxidant enzymes are upregulated to restore redox homeostasis. Such an adaptive response results from the activation of a redox-sensitive gene regulatory network mediated by nuclear factor E2-related factor 2. To more completely understand how the redox control system is designed by nature to meet homeostatic goals, we have examined the network from a systems perspective using engineering approaches. As with man-made control devices, the redox control system can be decomposed into distinct functional modules, including transducer, controller, actuator, and plant. Cells achieve specific performance objectives by utilizing nested feedback loops, feedforward control, and ultrasensitive signaling motifs, etc. Given that endogenously generated ROS are also used as signaling molecules, our analysis suggests a novel mode of action to explain oxidative stress-induced pathological conditions and diseases. Specifically, by adaptively upregulating antioxidant enzymes, oxidative stress may inadvertently attenuate ROS signals that mediate physiological processes, resulting in aberrations of cellular functions and adverse consequences. Lastly, by simultaneously considering the two competing cellular tasks-adaptive antioxidant defense and ROS signaling-we re-examine the premise that dietary antioxidant supplements is generally beneficial to human health. Our analysis highlights some possible adverse effects of these widely consumed antioxidants.

Genomic analyses reveal a conserved glutathione homeostasis pathway in the invertebrate chordate Ciona intestinalis

The major thiol redox buffer glutathione (l-gamma-glutamyl-l-cysteinylglycine, GSH) is central to cell fate determination, and thus, associated metabolic and regulatory pathways are exquisitely sensitive to a wide range of environmental cues. An imbalance of cellular redox homeostasis has emerged as a pathologic hallmark of a diverse range of human gene-environment disorders. Despite the central importance of GSH in cellular homeostasis, underlying genetic regulatory pathways remain poorly defined. This report describes the annotation and expression analysis of genes contributing to GSH homeostasis in the invertebrate chordate Ciona intestinalis. A core pathway comprising 19 genes contributing to the biosynthesis of GSH and its use as both a redox buffer and a conjugate in phase II detoxification as well as known transcriptional regulators were analyzed. These genes exhibit a high level of sequence conservation with corresponding human, rat, and mouse homologs and were expressed constitutively in tissues of adult animals. The GSH biosynthetic genes Gclc and Gclm were also responsive to the prototypical antioxidant tert-butylhydroquinone. The present evidence of a conserved GSH homeostasis pathway in C. intestinalis together with its phylogenetic position as a basal chordate and lifestyle as a filter feeder constantly exposed to natural marine toxins introduces this species as an important animal model for defining molecular mechanisms that potentially underlie genetic susceptibility to environmentally associated stress.

Bacterial ubiquitin-like modifier Pup is deamidated and conjugated to substrates by distinct but homologous enzymes

In analogy to ubiquitin in eukaryotes, the bacterial protein Pup is attached to lysine residues of substrate proteins, thereby targeting them for proteasomal degradation. It has been proposed that, before its attachment, Pup is modified by deamidation of its C-terminal glutamine to glutamate. Here we have identified Dop (locus tag Rv2112) as the specific deamidase of Pup in Mycobacterium tuberculosis. Deamidation requires ATP as a cofactor but not its hydrolysis. Furthermore, we provide experimental evidence that PafA (locus tag Rv2097) ligates deamidated Pup to the proteasomal substrate proteins FabD and PanB. This formation of an isopeptide bond requires hydrolysis of ATP to ADP, suggesting that deamidated Pup is activated for conjugation via phosphorylation of its C-terminal glutamate. By combining these enzymes, we have reconstituted the complete bacterial ubiquitin-like modification pathway in vitro, consisting of deamidation and ligation steps catalyzed by Pup deamidase (Dop) and Pup ligase (PafA).

Structure, function, and post-translational regulation of the catalytic and modifier subunits of glutamate cysteine ligase

Glutathione (GSH) is a tripeptide composed of glutamate, cysteine, and glycine. The first and rate-limiting step in GSH synthesis is catalyzed by glutamate cysteine ligase (GCL, previously known as gamma-glutamylcysteine synthetase). GCL is a heterodimeric protein composed of catalytic (GCLC) and modifier (GCLM) subunits that are expressed from different genes. GCLC catalyzes a unique gamma-carboxyl linkage from glutamate to cysteine and requires ATP and Mg(++) as cofactors in this reaction. GCLM increases the V(max) and K(cat) of GCLC, decreases the K(m) for glutamate and ATP, and increases the K(i) for GSH-mediated feedback inhibition of GCL. While post-translational modifications of GCLC (e.g. phosphorylation, myristoylation, caspase-mediated cleavage) have modest effects on GCL activity, oxidative stress dramatically affects GCL holoenzyme formation and activity. Pyridine nucleotides can also modulate GCL activity in some species. Variability in GCL expression is associated with several disease phenotypes and transgenic mouse and rat models promise to be highly useful for investigating the relationships between GCL activity, GSH synthesis, and disease in humans.

Age-associated perturbations in glutathione synthesis in mouse liver

The nature of the mechanisms underlying the age-related decline in glutathione (GSH) synthetic capacity is at present unclear. Steady-state kinetic parameters of mouse liver GCL (glutamate-cysteine ligase), the rate-limiting enzyme in GSH synthesis, and levels of hepatic GSH synthesis precursors from the trans-sulfuration pathway, such as homocysteine, cystathionine and cysteine, were compared between young and old C57BL/6 mice (6- and 24-month-old respectively). There were no agerelated differences in GCL V(max), but the apparent K(m) for its substrates, cysteine and glutamate, was higher in the old mice compared with the young mice (approximately 800 compared with approximately 300 microM, and approximately 710 compared with 450 microM, P<0.05 for cysteine and glutamate in young and old mice respectively). Amounts of cysteine, cystathionine and Cys-Gly increased with age by 91, 24 and 28% respectively. Glutathione (GSH) levels remained unchanged with age, whereas GSSG content showed an 84% increase, suggesting a significant pro-oxidizing shift in the 2GSH/GSSG ratio. The amount of the toxic trans-sulfuration/glutathione biosynthetic pathway intermediate, homocysteine, was 154% higher (P<0.005) in the liver of old mice compared with young mice. The conversion of homocysteine into cystathionine, a rate-limiting step in trans-sulfuration catalysed by cystathionine beta-synthase, was comparatively less efficient in the old mice, as indicated by cystathionine/homocysteine ratios. Incubation of tissue homogenates with physiological concentrations of homocysteine caused an up to 4.4-fold increase in the apparent K(m) of GCL for its glutamate substrate, but had no effect on V(max). The results suggest that perturbation of the catalytic efficiency of GCL and accumulation of homocysteine from the trans-sulfuration pathway may adversely affect de novo GSH synthesis during aging.

Thiol-based regulation of redox-active glutamate-cysteine ligase from Arabidopsis thaliana

Glutathione biosynthesis is a key component in the network of plant stress responses that counteract oxidative damage and maintain intracellular redox environment. Using a combination of mass spectrometry and site-directed mutagenesis, we examined the response of Arabidopsis thaliana glutamate-cysteine ligase (GCL) to changes in redox environment. Mass spectrometry identified two disulfide bonds (Cys186-Cys406 and Cys349-Cys364) in GCL. Mutation of either Cys-349 or Cys-364 to a Ser reduced reaction rate by twofold, but substitution of a Ser for either Cys-186 or Cys-406 decreased activity by 20-fold and abrogated the response to changes in redox environment. Redox titrations show that the regulatory disulfide bond has a midpoint potential comparable with other known redox-responsive plant proteins. Mutation of Cys-102, Cys-251, Cys-349, or Cys-364 did not alter the response to redox environment, indicating that modulation of activity depends on the Cys186-Cys406 disulfide bond. In vivo analysis of GCL in Arabidopsis root extracts revealed that multiple oxidative stresses altered the distribution of oxidized (active) and reduced (inactive) enzyme and that this change correlated with increased GCL activity. The thiol-based regulation of GCL provides a posttranslational mechanism for modulating enzyme activity in response to in vivo redox environment and suggests a role for oxidative signaling in the maintenance of glutathione homeostasis in plants.

Interaction between the catalytic and modifier subunits of glutamate-cysteine ligase

Glutamate-cysteine ligase (GCL) is the rate-limiting enzyme in the glutathione (GSH) biosynthesis pathway. This enzyme is a heterodimer, comprising a catalytic subunit (GCLC) and a regulatory subunit (GCLM). Although GCLC alone can catalyze the formation of l-gamma-glutamyl-l-cysteine, its binding with GCLM enhances the enzyme activity by lowering the K(m) for glutamate and ATP, and increasing the K(i) for GSH inhibition. To characterize the enzyme structure-function relationship, we investigated the heterodimer formation between GCLC and GCLM, in vivo using the yeast two-hybrid system, and in vitro using affinity chromatography. A strong and specific interaction between GCLC and GCLM was observed in both systems. Deletion analysis indicated that most regions, except a portion of the C-terminal region of GCLC and a portion of the N-terminal region of GCLM, are required for the interaction to occur. Point mutations of selected amino acids were also tested for the binding activity. The GCLC Cys248Ala/Cys249Ala and Pro158Leu mutations enzyme showed the same strength of binding to GCLM as did wild-type GCLC, yet the catalytic activity was dramatically decreased. The results suggest that the heterodimer formation may not be dependent on primary amino-acid sequence but, instead, involves a complex formation of the tertiary structure of both proteins.

Activation of the Nrf2 pathway, but decreased gamma-glutamylcysteine synthetase heavy subunit chain levels and caspase-3-dependent apoptosis during exposure of primary mouse hepatocytes to diphenylarsinic acid

Diphenylarsinic acid (DPAsV) is a degradation product of chemical warfare agents, over which there has been a public outcry in the Kamisu Area of Ibaraki Prefecture in Japan. In this study, we investigated the cytotoxicity of and cellular response to DPAsV in primary mouse hepatocytes. Exposure of the hepatocytes to DPAsV resulted in cell damage accompanied by cellular accumulation of DPAsV in a time-dependent manner. The cell death caused by DPAsV was attributable to apoptosis. DPAsV activated a basic leucine-zipper transcription factor Nrf2 as determined by the nuclear translocation of Nrf2, anti-oxidant response element (ARE)-dependent luciferase activity, and upregulation of downstream gene products. However, gamma-glutamylcysteine synthetase heavy subunit chain (gamma-GCS(H)), which is regulated by Nrf2, underwent cleavage by activated caspase-3 to a 17 kDa fragment, leading to a minimal level of constitutive gamma-GCS(H) expression 72 h following the exposure (25 microM). Experiments with cycloheximide revealed that the DPAsV-mediated reduction in gamma-GCS(H) was due to a post-translational modification. The results suggest that DPAsV causes caspase-3-dependent cleavage of gamma-GCS(H) regardless of Nrf2 activation in primary mouse hepatocytes.

Structural Basis for the Redox Control of Plant Glutamate Cysteine Ligase

Glutathione (GSH) plays a crucial role in plant metabolism and stress response. The rate-limiting step in the biosynthesis of GSH is catalyzed by glutamate cysteine ligase (GCL) the activity of which is tightly regulated. The regulation of plant GCLs is poorly understood. The crystal structure of substrate-bound GCL from Brassica juncea at 2.1-A resolution reveals a plant-unique regulatory mechanism based on two intramolecular redox-sensitive disulfide bonds. Reduction of one disulfide bond allows a beta-hairpin motif to shield the active site of B. juncea GCL, thereby preventing the access of substrates. Reduction of the second disulfide bond reversibly controls dimer to monomer transition of B. juncea GCL that is associated with a significant inactivation of the enzyme. These regulatory events provide a molecular link between high GSH levels in the plant cell and associated down-regulation of its biosynthesis. Furthermore, known mutations in the Arabidopsis GCL gene affect residues in the close proximity of the active site and thus explain the decreased GSH levels in mutant plants. In particular, the mutation in rax1-1 plants causes impaired binding of cysteine.

Differential regulation of glutamate-cysteine ligase subunit expression and increased holoenzyme formation in response to cysteine deprivation

GCL (glutamate-cysteine ligase) is a heterodimer of a GCLC (GCL catalytic subunit) that possesses all of the enzymatic activity and a GCLM (GCL modifier subunit) that alters the K(i) of GCLC for GSH. We hypothesized that the expression of GCLM and the association of GCLM with GCLC were responsible for the apparent increase in GCL activity state observed in the liver of rats fed low-protein diets or in hepatocytes cultured in low-sulphur amino acid-containing medium. Therefore we conducted a series of studies using rats and a human hepatoma (HepG2/C3A) cell line to assess the role of GCLM and holoenzyme formation in the regulation of GCL activity in response to sulphur amino acid intake or availability. Increases in GCL activity in rat liver, as well as in HepG2 cells, were due to the additive effects of changes in the amount of GCLC and the kcat for GCLC. The increase in the kcat for GCLC was associated with increased holoenzyme formation, which was associated with an increase in the molar ratio of GCLM to GCLC. Furthermore, our results indicate that the GCLM level in rat liver is always limiting and that up-regulation of the GCLM level results in increased holoenzyme formation and an increase in the kcat. This is the first report demonstrating that the catalytic efficiency of rat GCL is increased by holoenzyme formation and the first demonstration of differential up-regulation of the GCL subunits in response to cysteine deprivation.

Glutathione homeostasis and redox-regulation by sulfhydryl groups

Continuous control of metabolism and developmental processes is a key feature of live cells. Cysteine thiol residues of proteins are both exceptionally useful in terms of structural and regulatory aspects, but at the same time exceptionally vulnerable to oxidation. Conserved cysteines thus are highly important for the function of metabolic enzymes and for signaling processes underlying responses to environmental factors. The underlying mechanism for the central role of thiol-mediated redox control in cellular metabolism is the ability of the cysteine-thiols to reversibly change their redox state followed by changes of structural, catalytic or regulatory functions. The cellular glutathione/glutathione disulfide redox buffer is present in cells at millimolar concentrations and forms one major basis of redox homeostasis by which protein thiols can maintain their redox state or oxidized protein thiols can be reverted to their reduced state. Besides acting as redox buffer, glutathione also acts as an electron donor for both scavenging of reactive oxygen, e.g. from photosynthesis and respiration, and metabolic reactions such as reduction of hydroperoxides and lipidperoxides or sulfate assimilation. The central role of glutathione is further emphasized by its involvement in signaling processes and the crosstalk of redox signaling processes with other means of signaling including protein glutathionylation and control of transcription factors. The present review aims at highlighting the key functions of glutathione in thiol-mediated redox control and its interplay with other protein-thiol-based redox systems.

Regulation of glutathione in inflammation and chronic lung diseases

Oxidant/antioxidant imbalance, a major cause of cell damage, is the hallmark for lung inflammation. Glutathione (GSH), a ubiquitous tripeptide thiol, is a vital intra- and extra-cellular protective antioxidant against oxidative stress, which plays a key role in the control of signaling and pro-inflammatory processes in the lungs. The rate-limiting enzyme in GSH synthesis is glutamylcysteine ligase (GCL). GSH is essential for development as GCL knock-out mouse died from apoptotic cell death. The promoter (5'-flanking) region of human GCL is regulated by activator protein-1 (AP-1) and antioxidant response element (ARE), and are modulated by oxidants, phenolic antioxidants, growth factors, inflammatory and anti-inflammatory agents in various cells. Recent evidences have indicated that Nrf2 protein, which binds to the erythroid transcription factor (NF-E2) binding sites, and its interaction with other oncoproteins such as c-Jun, Jun D, Fra1 and Maf play a key role in the regulation of GCL. Alterations in alveolar and lung GSH metabolism are widely recognized as a central feature of many chronic inflammatory lung diseases. Knowledge of the mechanisms of GSH regulation could lead to the pharmacological manipulation of the production and/or gene transfer of this important antioxidant in lung inflammation and injury. This article describes the role of AP-1 and ARE in the regulation of cellular GSH biosynthesis and assesses the potential protective and therapeutic role of glutathione in oxidant-induced lung injury and inflammation.

Increase of intracellular glutathione by low-level NO mediated by transcription factor NF-κB in RAW 264.7 cells

The mechanism underlying the elevation of intracellular glutathione (GSH) in RAW 264.7 cells exposed to low concentrations of sodium nitroprusside (SNP), a well-known nitric oxide (NO) donor, was investigated. The peak of intracellular GSH was reached at 6 h after exposure of the cells to SNP (0.1-0.5 mM), and this was preceded by the induction of mRNA for gamma-glutamylcysteine synthetase (gamma-GCS; the rate-limiting enzyme of de novo GSH synthesis), which peaked at 3 h. N-alpha-Tosyl-L-phenylalanine chloromethyl ketone (TPCK) and caffeic acid phenethyl ester (CAPE), specific inhibitors of NF-kappaB, significantly suppressed the SNP-induced elevation of GSH protein and gamma-GCS mRNA, while curcumin, an inhibitor of AP-1, was less effective. Electrophoretic mobility shift assay (EMSA) showed that SNP exposure markedly increased the DNA binding of NF-kappaB, but not that of AP-1. Deletion or mutagenesis of the NF-kappaB site in the gamma-GCS gene promoter abolished the SNP-induced up-regulation of GSH protein and gamma-GCS mRNA. These results suggest that the elevation of intracellular GSH in RAW 264.7 cells exposed to low concentrations of SNP occurs through the operation of the de novo GSH pathway, and is mediated by transcriptional up-regulation of the gamma-GCS gene, predominantly at the NF-kappaB binding site in its promoter.

Glutathione Synthesis in Streptococcus agalactiae

Gamma-glutamylcysteine synthetase (gamma-GCS) and glutathione synthetase (GS), distinct enzymes that together account for glutathione (GSH) synthesis, have been isolated and characterized from several Gram-negative prokaryotes and from numerous eukaryotes including mammals, amphibians, plants, yeast, and protozoa. Glutathione synthesis is relatively uncommon among the Gram-positive bacteria, and, to date, neither the genes nor the proteins involved have been identified. In the present report, we show that crude extracts of Streptococcus agalactiae catalyze the gamma-GCS and GS reactions and can synthesize GSH from its constituent amino acids. The putative gene for S. agalactiae gamma-GCS was identified and cloned, and the corresponding protein was expressed and purified. Surprisingly, it was found that the isolated enzyme catalyzes both the ATP-dependent synthesis of L-gamma-glutamyl-L-cysteine from L-glutamate and L-cysteine and the ATP-dependent synthesis of GSH from L-gamma-glutamyl-L-cysteine and glycine. This novel bifunctional enzyme, referred to as gamma-GCS-GS, has been characterized in terms of catalytic activity, substrate specificity, and inhibition by GSH, cystamine, and transition state analog sulfoximines. The N-terminal 518 amino acids of gamma-GCS-GS (total M(r) 85,000) show 32% identity and 43% similarity with E. coli gamma-GCS (M(r) 58,000), but the C-terminal putative GS domain (remaining 202 amino acids) of gamma-GCS-GS shows no significant homology with known GS sequences. The C terminus (360 amino acids) is, however, homologous to D-Ala, D-Ala ligase (24% identity; 38% similarity), an enzyme having the same protein fold as known GS proteins. These results are discussed in terms of the evolution of GSH synthesis and the possible occurrence of a similar bifunctional GSH synthesis enzyme in other bacterial species.

Genetically altered mice to evaluate glutathione homeostasis in health and disease

The tripeptide glutathione (GSH) is part of an integrated antioxidant system that protects cells and tissues from oxidative damage. Oxidative stress can result from exposure to excessive amounts of endogenous and exogenous electrophiles. Until recently, animal and cell model systems used to investigate the role of GSH in disease processes had employed chemical agents that deplete cellular GSH by inhibiting GSH synthesis or by reacting chemically with GSH. Such models have proven useful, but questions concerning nonspecific effects of such chemicals remain. Recently, our laboratories and others have developed mouse models with genetic deficiencies in enzymes of the GSH biosynthetic pathway. This review focuses on the regulation of GSH homeostasis and, specifically, the new GSH-deficient mouse models that have been developed. These models will improve our understanding of the role of GSH in animal and human diseases.

Arabidopsis thaliana glutamate-cysteine ligase: functional properties, kinetic mechanism, and regulation of activity

In plants, glutathione accumulates in response to different stress stimuli as a protective mechanism, but only limited biochemical information is available on the plant enzymes that synthesize glutathione. Glutamatecysteine ligase (GCL) catalyzes the first step in glutathione biosynthesis and plays an important role in regulating the intracellular redox environment. Because the putative Arabidopsis thaliana GCL (AtGCL) displays no significant homology to the GCL from bacteria and other eukaryotes, the identity of this protein as a GCL has been debated. We have purified AtGCL from an Escherichia coli expression system and demonstrated that the recombinant enzyme catalyzes the ATP-dependent formation of gamma-glutamylcysteine from glutamate (Km = 9.1 mm) and cysteine (Km = 2.7 mm). Glutathione feedback inhibits AtGCL (Ki approximately 1.0 mm). As with other GCL, buthionine sulfoximine and cystamine inactivate the Arabidopsis enzyme but with inactivation rates much slower than those of the mammalian, bacterial, and nematode enzymes. The slower inactivation rates observed with AtGCL suggest that the active site differs structurally from that of other GCL. Global fitting analysis of initial velocity data indicates that a random terreactant mechanism with a preferred binding order best describes the kinetic mechanism of AtGCL. Unlike the mammalian GCL, which consists of a catalytic subunit and a regulatory subunit, AtGCL functions and is regulated as a monomeric protein. In response to redox environment, AtGCL undergoes a reversible conformational change that modulates the enzymatic activity of the monomer. These results explain the reported posttranslational change in AtGCL activity in response to oxidative stress.

The modifier subunit of Drosophila glutamate-cysteine ligase regulates catalytic activity by covalent and noncovalent interactions and influences glutathione homeostasis in vivo

Glutamate-cysteine ligase (GCL) has a key influence on glutathione homeostasis. It has been proposed that mammalian GCL is regulated by the redox environment, and we show here that cysteine residues in the Drosophila melanogaster GCL modifier subunit (DmGCLM) can form covalent interactions with the catalytic subunit (DmGCLC) and modify its activity. Candidate components of intersubunit disulfides (Cys213, Cys214, and Cys267) were identified using matrix-assisted laser desorption ionization time-of-flight spectroscopy of iodoacetamide-modified DmGCLM as well as examination of the evolutionary conservation of cysteines. Mutation of the 3 cysteine residues allowed DmGCLM to associate with DmGCLC, but inhibited the formation of intersubunit disulfides. This caused a 2-fold reduction in the catalytic efficiency of Drosophila GCL, although activity remained significantly higher than the catalytic subunit alone. The cysteine mutant was also more sensitive to inhibition by glutathione than the unmodified holoenzyme. Notably, human GCLM could substitute for DmGCLM in modification of DmGCLC activity. The role of DmGCLM in vivo was examined by analysis of a Drosophila mutant (l(3)L0580) containing a P-element insertion in Gclm. We found that the P-element is not responsible for the lethal phenotype and separated the recessive lethal mutation from the P-element by recombination. This yielded two fully viable and fertile recombinants bearing the P-element insertion, which Western and Northern blotting indicated is a severely hypomorphic allele of Gclm. Glutathione levels were approximately 2-fold lower in the GclmL0580 mutants than in control strains, demonstrating the importance of DmGCLM in the regulation of glutathione homeostasis in vivo.

Cell signalling and the glutathione redox system

The reduction/oxidation (redox) state of the cell is a consequence of the balance between the levels of oxidising and reducing equivalents. A reducing intracellular environment is often associated with cell survival; however, redox unbalance is necessary since it represents a regulatory sensor for several nuclear transcription factors. Activator protein 1 (AP-1), nuclear factor-kappaB (NF-kappaB) and protein tyrosine phosphatases 1-B (PTP-1B) are some of the well-known molecular factors for which a redox modulation of their activity has been demonstrated. The glutathione buffer system modulates cell response to redox changes induced by either external or intracellular stimuli. This paper summarises recent knowledge on the role played by several redox modulators in inducing signalling events that finally regulate cell cycle progression.

Caspase-3-Dependent Cleavage of the Glutamate-L-Cysteine Ligase Catalytic Subunit during Apoptotic Cell Death

Apoptotic cell death is usually accompanied by activation of a family of cysteine proteases termed caspases. Caspases mediate the selective proteolysis of multiple cellular targets often resulting in the disruption of survival pathways. Intracellular levels of the antioxidant glutathione (GSH) are an important determinant of cellular susceptibility to apoptosis. The rate-limiting step in GSH biosynthesis is mediated by glutamate-L-cysteine ligase (GCL), a heterodimeric enzyme consisting of a catalytic (GCLC) and a modifier (GCLM) subunit. In this report we demonstrate that GCLC is a direct target for caspase-mediated cleavage in multiple models of apoptotic cell death. Mutational analysis revealed that caspase-mediated cleavage of GCLC occurs at Asp(499) within the sequence AVVD(499)G. GCLC cleavage occurs upstream of Cys(553), which is thought to be important for association with GCLM. GCLC cleavage is accompanied by a rapid loss of intracellular GSH due to caspase-mediated extrusion of GSH from the cell. However, while GCLC cleavage is dependent on caspase-3, GSH extrusion occurs by a caspase-3-independent mechanism. Our identification of GCLC as a target for caspase-3-dependent cleavage during apoptotic cell death suggests that this post-translational modification may represent a novel mechanism for regulating GSH biosynthesis during apoptosis.

Aging and oxidation of reactive protein sulfhydryls

Protein sulfhydryls are potential sites of reversible oxidative modification by S-glutathiolation, and S-nitrosylation, but they are also susceptible to irreversible damage by oxidative conditions. In the absence of adequate antioxidant protection, these reactive sites may become useless because of this irreversible damage. It has recently become possible to directly access the nature and amount of irreversibly oxidized protein sulfhydryls by both gel-based methods and direct amino acid analysis. Results are in keeping with the concept that irreversible oxidation of protein sulfhydryls is more extensive in aged tissue samples. It is proposed that an adequate pool of glutathione is essential to prevent this increase in sulfhdryl oxidation. The increased amount of protein sulfhydryl damage may be critically important to the function of signal-transduction and transcription events that utilize proteins containing these reactive sites.

Hepatocellular response to chemical stress in CD-1 mice: induction of early genes and gamma-glutamylcysteine synthetase

Exposure of cells to toxic chemical species can result in reduced glutathione (GSH) depletion, generation of free radicals, and/or binding to critical cell determinants. Chemical stress is usually followed by a concerted cellular response aimed at restoring homeostasis, although the precise initial stimulus for the response is unclear. We have focused on one component of this stress response, the up-regulation of gamma-glutamylcysteine synthetase (gamma-GCS) and the preceding molecular events involved in its regulation in an in vivo mouse model. Male CD-1 mice received buthionine sulphoximine (BSO; 7.2 mmol/kg), diethyl maleate (DEM; 4.2 mmol/kg), paracetamol (APAP; 3.5 and 1.0 mmol/kg), or carbon tetrachloride (CCl(4); 1.0 and 0.2 mmol/kg). Biochemical (serum transaminase and hepatic GSH levels) and molecular (c-jun and c-fos messenger RNA [mRNA] levels and activator protein 1 [AP-1] DNA binding activity) parameters were measured, as well as the consequent effects on gamma-GCS levels and activity. All compounds produced GSH depletion, but only the higher doses of APAP and CCl(4) caused liver damage. DEM, APAP, and CCl(4) increased c-jun and c-fos mRNA levels, together with an increase in AP-1 binding; BSO failed to induce AP-1 despite an increase in c-fos. Interestingly, the effects on gamma-GCS varied markedly according to the compound: BSO and DEM increased gamma-GCS enzyme activity, although only DEM, but not BSO, resulted in an increase in gamma-GCS(h) mRNA and protein. In contrast, APAP and CCl(4) both increased gamma-GCS(h) mRNA and protein; however, there was a marked dose-dependent decrease in gamma-GCS activity. These data indicate that the effect of chemical stress on the liver is compound specific and is not merely dependent on depletion of GSH.

Multi-faceted regulation of gamma-glutamylcysteine synthetase

Glutathione is an important antioxidant that is involved in numerous cellular activities. gamma-Glutamylcysteine synthetase (gammaGCS) is a key regulatory enzyme in the synthesis of glutathione. It is a heterodimeric zinc metalloprotein that belongs to a unique class of proteins that gain activity due to formation of a reversible disulfide bond. The two subunits of gammaGCS exhibit differential and coordinate transcription regulation. In addition, the subunits are regulated at the posttranscriptional and posttranslational levels. These various levels of regulation allow numerous stimuli to induce or inhibit activity.

Lung glutathione and oxidative stress: implications in cigarette smoke-induced airway disease

Glutathione (GSH), a ubiquitous tripeptide thiol, is a vital intra- and extracellular protective antioxidant in the lungs. The rate-limiting enzyme in GSH synthesis is gamma-glutamylcysteine synthetase (gamma-GCS). The promoter (5'-flanking) region of the human gamma-GCS heavy and light subunits are regulated by activator protein-1 and antioxidant response elements. Both GSH and gamma-GCS expression are modulated by oxidants, phenolic antioxidants, and inflammatory and anti-inflammatory agents in lung cells. gamma-GCS is regulated at both the transcriptional and posttranscriptional levels. GSH plays a key role in maintaining oxidant-induced lung epithelial cell function and also in the control of proinflammatory processes. Alterations in alveolar and lung GSH metabolism are widely recognized as a central feature of many inflammatory lung diseases including chronic obstructive pulmonary disease (COPD). Cigarette smoking, the major factor in the pathogenesis of COPD, increases GSH in the lung epithelial lining fluid of chronic smokers, whereas in acute smoking, the levels are depleted. These changes in GSH may result from altered gene expression of gamma-GCS in the lungs. The mechanism of regulation of GSH in the epithelial lining fluid in the lungs of smokers and patients with COPD is not known. Knowledge of the mechanisms of GSH regulation in the lungs could lead to the development of novel therapies based on the pharmacological or genetic manipulation of the production of this important antioxidant in lung inflammation and injury. This review outlines 1) the regulation of cellular GSH levels and gamma-GCS expression under oxidative stress and 2) the evidence for lung oxidant stress and the potential role of GSH in the pathogenesis of COPD.

Inflammation and the regulation of glutathione level in lung epithelial cells

Inflammation is a highly complex biochemical protective response to cellular injury. If this process is continuously unchecked, it leads to chronic inflammation, a hallmark of various inflammatory lung diseases. Reactive oxygen intermediates generated by immune cells recruited to the sites of inflammation are a major cause of cell damage. Glutathione (GSH), is a vital intra- and extracellular protective antioxidant in the lungs. The rate-limiting enzyme in GSH synthesis is gamma-glutamylcysteine synthetase (gamma-GCS). Both GSH and gamma-GCS expression are modulated by oxidants, phenolic antioxidants, inflammatory, and anti-inflammatory agents in lung cells. GSH plays a key role in regulating oxidant-induced lung epithelial cell function and also in the control of pro-inflammatory processes. Alterations in the alveolar and lung GSH metabolism are widely recognized as a central feature of many inflammatory lung diseases. Oxidative processes have a fundamental role in lung inflammation through redox-sensitive transcription factors such as NF-kappaB and AP-1, which regulated the genes for pro-inflammatory mediators and protective antioxidant genes such as gamma-GCS. The critical balance between the induction of pro-inflammatory mediators and antioxidant genes in response to oxidative stress at the site of inflammation is not known. Knowledge of the mechanisms of GSH regulation in lung inflammation could lead to the development of novel therapies based on the pharmacological manipulation of the production of this important antioxidant in lung inflammation and injury. This review describes the potential role of GSH for lung oxidant stress, inflammation and injury.