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

Prevention of age-related truncation of γ-glutamylcysteine ligase catalytic subunit (GCLC) delays cataract formation

A sharp drop in lenticular glutathione (GSH) plays a pivotal role in age-related cataract (ARC) formation. Despite recognizing GSH's importance in lens defense for decades, its decline with age remains puzzling. Our recent study revealed an age-related truncation affecting the essential GSH biosynthesis enzyme, the γ-glutamylcysteine ligase catalytic subunit (GCLC), at aspartate residue 499. Intriguingly, these truncated GCLC fragments compete with full-length GCLC in forming a heterocomplex with the modifier subunit (GCLM) but exhibit markedly reduced enzymatic activity. Crucially, using an aspartate-to-glutamate mutation knock-in (D499E-KI) mouse model that blocks GCLC truncation, we observed a notable delay in ARC formation compared to WT mice: Nearly 50% of D499E-KI mice remained cataract-free versus ~20% of the WT mice at their age of 20 months. Our findings concerning age-related GCLC truncation might be the key to understanding the profound reduction in lens GSH with age. By halting GCLC truncation, we can rejuvenate lens GSH levels and considerably postpone cataract onset.

Nrf2/ARE axis signalling in hepatocyte cellular death

The Nrf2-ARE transcriptional pathway plays an important role amongst cellular defence systems regulating and ensuring adequacy of redox responses and oxidant signalling factors. Hepatocyte cellular death and injury is a prominent feature underlying liver pathologies. Diverse endogenous molecules and targets contribute to the outcome of cell survival and the consequent mode of cell death. Several research efforts focused on the confirmation of Nrf2 presence in cell death and its vital necessity against cell compromise, however, little they comprehend of such participation. Hepatocyte cell death modes discussed in this review including autophagy, apoptosis, necrosis, ferroptosis, pyroptosis, fibrosis and others, vary in response of the stimuli burdened. The current review presents a handful of highlights and crosstalk involved in the communication of Nrf2 signalling network with the "up to date" reported hepatocyte cell death modes and their underling mechanisms, and addressing key cellular networks of hepatocyte fate, through a perspective of Nrf2 as a critical transcriptional factor. Collectively, labelling the cross-transduction of Nrf2-ARE axis with key cell execution pathways could provide insights to therapeutic interventions and better research outcomes.

Lipoperoxide Nanoemulsion as Adjuvant in Cisplatin Cancer Therapy: In Vitro Study on Human Colon Adenocarcinoma DLD-1 Cells

Cisplatin is a first-choice chemotherapeutic agent used to treat solid tumors even though the onset of multi-drug resistance and the time-dose side-effects impair its mono-therapeutic application. Therefore, new drug-delivery approaches, based on nanomedicine strategies, are needed to enhance its therapeutic potential in favor of a dose-reduction of cisplatin. Polyunsaturated fatty acids and their metabolism-derived intermediates, as well as lipid peroxidation end-products, are used as adjuvants to improve the effectiveness of chemotherapy. Lipid hydroperoxides, derived from the oxidation of edible oils, can contribute to cell death, generating breakdown products (e.g., reactive aldehydes). In this regard, the aim of this present study was to evaluate an invitro combinatory strategy between a lecithin-based nanoemulsion system of K600, a patented mixture of peroxidated oil and peroxidated cholesterol, and cisplatin on DLD1 human adenocarcinoma cells. Our findings showed that nanoemulsions, acting in synergy with cisplatin, improve cisplatin bioactivity, in terms of enhancing its anti-cancer activity, towards DLD1 cells. Indeed, this combination approach, whilst maintaining cisplatin at low concentrations, induces a significant reduction in DLD1 cell viability, an increase in pro-apoptotic markers, and genotoxic damage. Therefore, K600 nanoemulsions as an efficient targeted delivery system of cisplatin allow for the reduction in the chemotherapeutic agent doses.

Aging lens epithelium is susceptible to ferroptosis

Age-related cataracts (ARC) are the primary cause of blindness worldwide, and oxidative stress is considered the central pathogenesis of age-related cataractogenesis. Interestingly, ample evidence suggests that there is no remarkable apoptosis present in aged and cataractous human lenses despite the profound disruption of redox homeostasis, raising an essential question regarding the existence of other cell death mechanisms. Here we sought to explore the lens epithelial cell's (LEC) susceptibility to ferroptosis after documentation has concluded that aged and cataractous human lenses manifest with increased reactive oxygen species (ROS) formation, elevated lipid peroxidation, and accumulative intracellular redox-active iron, constituting the three hallmarks of ferroptosis during aging and cataractogenesis. Here we show that very low concentrations of system Xc- inhibitor Erastin (0.5 μM) and glutathione peroxidase 4 (GPX4) inhibitor RSL3 (0.1 μM) can drastically induce human LEC (FHL124) ferroptosis in vitro and mouse lens epithelium ferroptosis ex vivo. Depletion of intracellular glutathione (GSH) in human LECs and mouse lens epithelium significantly sensitizes ferroptosis, particularly under RSL3 challenge. Intriguingly, both human LECs and the mouse lens epithelium demonstrate an age-related sensitization of ferroptosis. Transcriptome analysis indicates that clusters of genes are up-or down-regulated in aged LECs, impacting cellular redox and iron homeostases, such as downregulation of both cystine/glutamate antiporter subunits SLC7A11 and SLC3A2 and iron exporter ferroportin (SLC40A1). Here, for the first time, we are suggesting that LECs are highly susceptible to ferroptosis. Moreover, aged and cataractous human lenses may possess more pro-ferroptotic criteria than any other organ in the human body.

The protective role of glutathione in osteoarthritis

It is currently understood that osteoarthritis (OA) is a major chronic inflammatory musculoskeletal disease. While this disease has long been attributed to biomechanical trauma, recent evidence establishes a significant correlation between osteoarthritic progression and unbridled oxidative stress, responsible for prolonged inflammation. Research describes this as a disturbance in the balanced production of reactive oxygen species (ROS) and antioxidant defenses, generating macromolecular damage and disrupted redox signaling and control. Since ROS pathways are being considered new targets for OA treatment, the development of antioxidant therapy to counteract exacerbated oxidative stress is being continuously researched and enhanced in order to fortify the cellular defenses. Experiments with glutathione and its precursor molecule, N-acetylcysteine (NAC), have shown interesting results in the literature for the management of OA, where they have demonstrated efficacy in reducing cartilage degradation and inflammation markers as well as significant improvements in pain and functional outcomes. Glutathione remains a safe, effective and overall cheap treatment alternative in comparison to other current therapeutic solutions and, for these reasons, it may prove to be comparably superior under particular circumstances.

METHODS

Literature was reviewed using PubMed and Google Scholar in order to bring up significant evidence and illustrate the defensive mechanisms of antioxidant compounds against oxidative damage in the onset of musculoskeletal diseases. The investigation included a combination of keywords such as: oxidative stress, oxidative damage, inflammation, osteoarthritis, antioxidant, glutathione, n-acetylcysteine, redox, and cell signaling.

CONCLUSION

Based on the numerous studies included in this literature review, glutathione and its precursor N-acetylcysteine have demonstrated significant protective effects in events of prolonged, exacerbated oxidative stress as seen in chronic inflammatory musculoskeletal disorders such as osteoarthritis.

Malformations and mortality in zebrafish early stages associated with elevated caspase activity after 24 h exposure to MS-222

Tricaine methanesulfonate (MS-222) is a commonly used anaesthetic agent for immobilization of aquatic species. However, delayed development and malformations have been observed in 24 hpf (hours post-fertilization) zebrafish embryos after long-term immobilization. Still, no comprehensive study has been described regarding zebrafish exposure to MS-222 during the first hours of development, which are one of the most sensitive life stages to toxicants. Therefore, this research aimed to assess the toxicity of a 24 h exposure to MS-222 on zebrafish embryonic development. Based on the MS-222 LC₅₀, early blastula stage embryos (~2 hpf) were exposed to 0, 12.5, 25 and 50 mg L^-1 for 24 h and then allowed to develop up to 144 hpf. The chromatographic analysis showed that this anaesthetic agent bioaccumulates in 26 hpf zebrafish larvae in a concentration-dependent manner. In addition, increased mortalities and skeletal abnormalities were observed at 144 hpf, namely in the highest tested concentration. Yet, no craniofacial anomalies were observed either by alcian blue or calcein staining methods. Independently of the tested concentration, decreased speed and distance travelled were perceived in 144 hpf larvae. At the biochemical level, decreased in vivo reactive oxygen species (ROS) generation and apoptosis was observed. Additionally, catalase activity was increased at 26 hpf while results of mRNA expression showed a decreased gclc transcript content at the same time-point. Overall, data obtained highlight the toxicological risk of MS-222 and support ROS-mediated cell death signalling changes through the elevation of catalase activity as an adaptative or protective response.

Down regulation of glutathione and glutamate cysteine ligase in the inflammatory response of macrophages

Glutathione (GSH) plays critical roles in the inflammatory response by acting as the master substrate for antioxidant enzymes and an important anti-inflammatory agent. In the early phase of the inflammatory response of macrophages, GSH content is decreased due to the down regulation of the catalytic subunit of glutamate cysteine ligase (GCLC). In the current study we investigated the underlying mechanism for this phenomenon. In human THP1-differentiated macrophages, GCLC mRNA had a half-life of 4 h under basal conditions, and it was significantly reduced to less than 2 h upon exposure to lipopolysaccharide (LPS), suggesting an increased decay of GCLC mRNA in the inflammatory response. The half-life of GCLC protein was >10 h under basal conditions, and upon LPS exposure the degradation rate of GCLC protein was significantly increased. The pan-caspase inhibitor Z-VAD-FMK but not the proteasome inhibitor MG132, prevented the down regulation of GCLC protein caused by LPS. Both caspase inhibitor Z-LEVD-FMK and siRNA of caspase-5 abrogated LPS-induced degradation of GCLC protein. In addition, supplement with γ-GC, the GCLC product, efficiently restored GSH content and suppressed the induction of NF-κB activity by LPS. In conclusion, these data suggest that GCLC down-regulation in the inflammatory response of macrophages is mediated through both increased mRNA decay and caspase-5-mediated GCLC protein degradation, and γ-GC is an efficient agent to restore GSH and regulate the inflammatory response.

Dual disruption of aldehyde dehydrogenases 1 and 3 promotes functional changes in the glutathione redox system and enhances chemosensitivity in nonsmall cell lung cancer

Aldehyde dehydrogenases (ALDHs) are multifunctional enzymes that oxidize diverse endogenous and exogenous aldehydes. We conducted a meta-analysis based on The Cancer Genome Atlas and Gene Expression Omnibus data and detected genetic alterations in ALDH1A1, ALDH1A3, or ALDH3A1, 86% of which were gene amplification or mRNA upregulation, in 31% of nonsmall cell lung cancers (NSCLCs). The expression of these isoenzymes impacted chemoresistance and shortened survival times in patients. We hypothesized that these enzymes provide an oxidative advantage for the persistence of NSCLC. To test this hypothesis, we used genetic and pharmacological approaches with DIMATE, an irreversible inhibitor of ALDH1/3. DIMATE showed cytotoxicity in 73% of NSCLC cell lines tested and demonstrated antitumor activity in orthotopic xenografts via hydroxynonenal-protein adduct accumulation, GSTO1-mediated depletion of glutathione and increased H₂O₂. Consistent with this result, ALDH1/3 disruption synergized with ROS-inducing agents or glutathione synthesis inhibitors to trigger cell death. In lung cancer xenografts with high to moderate cisplatin resistance, combination treatment with DIMATE promoted strong synergistic responses with tumor regression. These results indicate that NSCLCs with increased expression of ALDH1A1, ALDH1A3, or ALDH3A1 may be targeted by strategies involving inhibitors of these isoenzymes as monotherapy or in combination with chemotherapy to overcome patient-specific drug resistance.

γ-Glutamyltransferase enzyme activity of cancer cells modulates L-γ-glutamyl-p-nitroanilide (GPNA) cytotoxicity

L-γ-Glutamyl-p-nitroanilide (GPNA) is widely used to inhibit the glutamine (Gln) transporter ASCT2, but recent studies have demonstrated that it is also able to inhibit other sodium-dependent and independent amino acid transporters. Moreover, GPNA is a well known substrate of the enzyme γ-glutamyltransferase (GGT). Our aim was to evaluate the effect of GGT-mediated GPNA catabolism on cell viability and Gln transport. The GGT-catalyzed hydrolysis of GPNA produced cytotoxic effects in lung cancer A549 cells, resulting from the release of metabolite p-nitroaniline (PNA) rather than from the inhibition of Gln uptake. Interestingly, compounds like valproic acid, verapamil and reversan were able to increase the cytotoxicity of GPNA and PNA, suggesting a key role of intracellular detoxification mechanisms. Our data indicate that the mechanism of action of GPNA is more complex than believed, and further confirm the poor specificity of GPNA as an inhibitor of Gln transport. Different factors may modulate the final effects of GPNA, ranging from GGT and ASCT2 expression to intracellular defenses against xenobiotics. Thus, other strategies - such as a genetic suppression of ASCT2 or the identification of new specific inhibitors - should be preferred when inhibition of ASCT2 function is required.

Changes in Oxidant Defense, Apoptosis, and Cognitive Abilities During Treatment for Childhood Leukemia

Aggressive central nervous system (CNS)-directed treatment for acute lymphoblastic leukemia (ALL), the most prevalent cancer among children and adolescents, prevents metastasis of leukemia cells into the brain. Up to 60% of survivors experience cognitive problems, but knowledge about risk factors for and mechanisms of neurologic injury is lacking. Objectives of the present study were to (1) quantify changes in oxidant defense and apoptosis over the course of ALL therapy and (2) elucidate risk factors for long-term cognitive problems. The sample included 71 children with ALL. Cerebrospinal fluid (CSF) samples were collected at diagnosis and during intrathecal chemotherapy administration. Oxidant defense was measured by reduced glutathione (GSH), oxidized glutathione (GSSG), and the ratio of GSH:GSSG. Apoptosis was measured by activity of several cysteine-dependent aspartate-specific protease (abbreviated as caspase) enzymes that initiate (caspases 8 and 9) or execute (caspases 3/7) apoptosis. Cognitive abilities were assessed by standardized measures of short-term memory, visual-motor integration, and attention 3 years after ALL diagnosis. GSH and GSSG concentration increased significantly during ALL therapy, and a low GSH:GSSG ratio was indicative of an oxidized extracellular environment. Caspase enzyme activity increased significantly, and caspases 3/7 activity was significantly and negatively associated with performance on measures of cognitive abilities. Younger age at time of ALL diagnosis was associated with some measures of attention. Efflux of glutathione into CSF maintains oxidant defense by scavenging free radicals and other reactive oxygen species and is an early event in apoptosis. These mechanisms may be involved in neurologic injury associated with CNS-directed treatment and subsequent cognitive problems.

Multifaceted Roles of Glutathione and Glutathione-Based Systems in Carcinogenesis and Anticancer Drug Resistance

SIGNIFICANCE

Glutathione is the most abundant antioxidant molecule in living organisms and has multiple functions. Intracellular glutathione homeostasis, through its synthesis, consumption, and degradation, is an intricately balanced process. Glutathione levels are often high in tumor cells before treatment, and there is a strong correlation between elevated levels of intracellular glutathione/sustained glutathione-mediated redox activity and resistance to pro-oxidant anticancer therapy. Recent Advances: Ample evidence demonstrates that glutathione and glutathione-based systems are particularly relevant in cancer initiation, progression, and the development of anticancer drug resistance.

CRITICAL ISSUES

This review highlights the multifaceted roles of glutathione and glutathione-based systems in carcinogenesis, anticancer drug resistance, and clinical applications.

FUTURE DIRECTIONS

The evidence summarized here underscores the important role played by glutathione and the glutathione-based systems in carcinogenesis and anticancer drug resistance. Future studies should address mechanistic questions regarding the distinct roles of glutathione in different stages of cancer development and cancer cell death. It will be important to study how metabolic alterations in cancer cells can influence glutathione homeostasis. Sensitive approaches to monitor glutathione dynamics in subcellular compartments will be an indispensible step. Therapeutic perspectives should focus on mechanism-based rational drug combinations that are directed against multiple redox targets using effective, specific, and clinically safe inhibitors. This new strategy is expected to produce a synergistic effect, prevent drug resistance, and diminish doses of single drugs. Antioxid. Redox Signal. 27, 1217-1234.

Sex-specific molecular genetic response to UVB exposure in Xiphophorus maculatus skin

In both Xiphophorus fishes and humans, males are reported to have a higher incidence of melanoma than females. To better understand sex-specific differences in the molecular genetic response to UVB, we performed RNA-Seq experiments in skin of female and male Xiphophorus maculatus Jp 163 B following UVB doses of 8 or 16kJ/m(2) exposure. Male X. maculatus differentially express a significantly larger number of transcripts following exposure to 16kJ/m(2) UVB (1293 genes) compared to 8kJ/m(2) UVB (324 genes). Female skin showed differential gene expression in a larger number of transcripts following 8kJ/m(2) UVB (765) than did males; however, both females and males showed similar numbers of differentially expressed genes at 16kJ/m(2) UVB (1167 and1293, respectively). Although most modulated transcripts after UVB exposure represented the same dominant pathways in both females and males (e.g., DNA repair, circadian rhythm, and fatty acid biosynthesis), we identified genes in several pathways that exhibited opposite modulation in female vs. male skin (e.g., synaptic development, cell differentiation, wound healing, and glucose metabolism). The oppositely modulated genes appear related through uncoupling protein 3 (UCP3) that is involved with the regulation of fatty acid oxidation and serves to balance glucose and lipid metabolism. Overall, these results identify gender-specific differences in UVB-induced genetic profiles in the skin of females and males and show female and male X. maculatus respond to UVB differently through pathways involved in reactive oxygen species, wound healing, and energy homeostasis.

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.

Alteration in glutathione homeostasis and oxidative stress during the sequelae of trimethyltin syndrome in rat brain

Trimethyltin (TMT), a by-product of tin, is used in a wide variety of industrial and agricultural purposes which serves as a model neurotoxicant in hippocampal neurodegeneration, and this could, in turn, be exploited for various therapeutic compounds essential for hippocampal neurodegeneration. Therefore, the present investigation explores the sequential changes in behavior, oxidative burden, and apoptosis following TMT administration in rat hippocampus. Male SD rats weighing 250 g were given single dose of 8.5 mg/kg TMT (i.p.) that resulted in "TMT syndrome" which begins at the third post-TMT exposure and continued till 21 days posttreatment. This resulted in behavioral alteration (aggression and spontaneous seizures), cognitive impairment as assessed by plus maze, and passive avoidance resulting in short-term memory deficits. These behavioral alterations were associated with an increase in oxidative stress. The levels of malondialdehyde, reactive oxygen species, and protein carbonyl were significantly increased (p < 0.001) in the TMT-treated rats after the third day of exposure and were maximum at day 14 postexposure. The glutathione system was not able to adapt rapidly in response to oxidative stress which resulted in imbalance in redox status. The imbalance in the redox state resulted in the death of neurons as seen by a significant increase in caspase activation at gene as well as protein level after TMT exposure on day 14, quoting an extent of changes. Therefore, it is proposed that behavioral deficits could be accounted by the impairment of endogenous glutathione homeostasis which resulted in death of neurons in the hippocampal region.

Low-level domoic acid protects mouse cerebellar granule neurons from acute neurotoxicity: role of glutathione

Domoic acid (DomA) is a potent marine neurotoxin. By activating α-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid/kainate receptors, DomA induces oxidative stress-mediated apoptotic cell death in neurons. The effect of prolonged (10 days) exposure to a low, nontoxic concentration (5nM) of DomA on acute (intermediate concentration) neurotoxicity of this toxin was investigated in cerebellar granule neurons (CGNs) from wild-type mice and mice lacking the glutamate cysteine ligase (GCL) modifier subunit (Gclm (/)). CGNs from Gclm (/) mice have very low glutathione (GSH) levels and are very sensitive to DomA toxicity. In CGNs from wild-type mice, prolonged exposure to 5nM DomA did not cause any overt toxicity but reduced oxidative stress-mediated apoptotic cell death induced by exposure to an intermediate concentration (100nM for 24h) of DomA. This protection was not observed in CGNs from Gclm (/) mice. Prolonged DomA exposure increased GSH levels in CGNs of wild-type but not Gclm (/) mice. Levels of GCLC (the catalytic subunit of GCL) protein and mRNA were increased in CGNs of both mouse strains, whereas levels of GCLM protein and mRNA, activity of GCL, and levels of GCL holoenzyme were only increased in CGNs of wild-type mice. Chronic DomA exposure also protected wild-type CGNs from acute toxicity of other oxidants. The results indicate that CGNs from Gclm (/) mice, which are already more sensitive to DomA toxicity, are unable to upregulate their GSH levels. As Gclm (/) mice may represent a model for a common human polymorphism in GCLM, such individuals may be at particular risk for DomA-induced neurotoxicity.

Decreased zinc availability affects glutathione metabolism in neuronal cells and in the developing brain

A deficit in zinc (Zn) availability can increase cell oxidant production, affect the antioxidant defense system, and trigger oxidant-sensitive signals in neuronal cells. This work tested the hypothesis that a decreased Zn availability can affect glutathione (GSH) metabolism in the developing rat brain and in neuronal cells in culture, as well as the capacity of human neuroblastoma IMR-32 cells to upregulate GSH when challenged with dopamine (DA). GSH levels were low in the brain of gestation day 19 (GD19) fetuses from dams fed marginal Zn diets throughout gestation and in Zn-deficient IMR-32 cells. γ-Glutamylcysteine synthetase (GCL), the first enzyme in the GSH synthetic pathway, was altered by Zn deficiency (ZD). The protein and mRNA levels of the GCL modifier (GCLM) and catalytic (GCLC) subunits were lower in the Zn-deficient GD19 fetal brain and in IMR-32 cells compared with controls. The nuclear translocation of transcription factor nuclear factor (erythroid-derived 2)-like 2, which controls GCL transcription, was impaired by ZD. Posttranslationally, the caspase-3-dependent GCLC cleavage was high in Zn-deficient IMR-32 cells. Cells challenged with DA showed an increase in GCLM and GCLC protein and mRNA levels and a consequent increase in GSH concentration. Although Zn-deficient cells partially upregulated GCL subunits after exposure to DA, GSH content remained low. In summary, results show that a low Zn availability affects the GSH synthetic pathway in neuronal cells and fetal brain both at transcriptional and posttranslational levels. This can in part underlie the GSH depletion associated with ZD and the high sensitivity of Zn-deficient neurons to pro-oxidative stressors.

Glutathione efflux and cell death

SIGNIFICANCE

Glutathione (GSH) depletion is a central signaling event that regulates the activation of cell death pathways. GSH depletion is often taken as a marker of oxidative stress and thus, as a consequence of its antioxidant properties scavenging reactive species of both oxygen and nitrogen (ROS/RNS).

RECENT ADVANCES

There is increasing evidence demonstrating that GSH loss is an active phenomenon regulating the redox signaling events modulating cell death activation and progression.

CRITICAL ISSUES

In this work, we review the role of GSH depletion by its efflux, as an important event regulating alterations in the cellular redox balance during cell death independent from oxidative stress and ROS/RNS formation. We discuss the mechanisms involved in GSH efflux during cell death progression and the redox signaling events by which GSH depletion regulates the activation of the cell death machinery.

FUTURE DIRECTIONS

The evidence summarized here clearly places GSH transport as a central mechanism mediating redox signaling during cell death progression. Future studies should be directed toward identifying the molecular identity of GSH transporters mediating GSH extrusion during cell death, and addressing the lack of sensitive approaches to quantify GSH efflux.

Regulation of cell survival and death by pyridine nucleotides

Pyridine nucleotides (PNs), such as NAD(H) and NADP(H), mediate electron transfer in many catabolic and anabolic processes. In general, NAD(+) and NADP(+) receive electrons to become NADH and NADPH by coupling with catabolic processes. These electrons are utilized for biologically essential reactions such as ATP production, anabolism and cellular oxidation-reduction (redox) regulation. Thus, in addition to ATP, NADH and NADPH could be defined as high-energy intermediates and "molecular units of currency" in energy transfer. We discuss the significance of PNs as energy/electron transporters and signal transducers, in regulating cell death and/or survival processes. In the first part of this review, we describe the role of NADH and NADPH as electron donors for NADPH oxidases (Noxs), glutathione (GSH), and thioredoxin (Trx) systems in cellular redox regulation. Noxs produce superoxide/hydrogen peroxide yielding oxidative environment, whereas GSH and Trx systems protect against oxidative stress. We then describe the role of NAD(+) and NADH as signal transducers through NAD(+)-dependent enzymes such as PARP-1 and Sirt1. PARP-1 is activated by damaged DNA in order to repair the DNA, which attenuates energy production through NAD(+) consumption; Sirt1 is activated by an increased NAD(+)/NADH ratio to facilitate signal transduction for metabolic adaption as well as stress responses. We conclude that PNs serve as an important interface for distinct cellular responses, including stress response, energy metabolism, and cell survival/death.

Glutathione synthesis

BACKGROUND

Glutathione (GSH) is present in all mammalian tissues as the most abundant non-protein thiol that defends against oxidative stress. GSH is also a key determinant of redox signaling, vital in detoxification of xenobiotics, and regulates cell proliferation, apoptosis, immune function, and fibrogenesis. Biosynthesis of GSH occurs in the cytosol in a tightly regulated manner. Key determinants of GSH synthesis are the availability of the sulfur amino acid precursor, cysteine, and the activity of the rate-limiting enzyme, glutamate cysteine ligase (GCL), which is composed of a catalytic (GCLC) and a modifier (GCLM) subunit. The second enzyme of GSH synthesis is GSH synthetase (GS).

SCOPE OF REVIEW

This review summarizes key functions of GSH and focuses on factors that regulate the biosynthesis of GSH, including pathological conditions where GSH synthesis is dysregulated.

MAJOR CONCLUSIONS

GCL subunits and GS are regulated at multiple levels and often in a coordinated manner. Key transcription factors that regulate the expression of these genes include NF-E2 related factor 2 (Nrf2) via the antioxidant response element (ARE), AP-1, and nuclear factor kappa B (NFκB). There is increasing evidence that dysregulation of GSH synthesis contributes to the pathogenesis of many pathological conditions. These include diabetes mellitus, pulmonary and liver fibrosis, alcoholic liver disease, cholestatic liver injury, endotoxemia and drug-resistant tumor cells.

GENERAL SIGNIFICANCE

GSH is a key antioxidant that also modulates diverse cellular processes. A better understanding of how its synthesis is regulated and dysregulated in disease states may lead to improvement in the treatment of these disorders. This article is part of a Special Issue entitled Cellular functions of glutathione.

Mechanism and significance of changes in glutamate-cysteine ligase expression during hepatic fibrogenesis

GSH is synthesized sequentially by glutamate-cysteine ligase (GCL) and GSH synthase and defends against oxidative stress, which promotes hepatic stellate cell (HSC) activation. Changes in GSH synthesis during HSC activation are poorly characterized. Here, we examined the expression of GSH synthetic enzymes in rat HSC activation and reversion to quiescence. Expression of the GCL catalytic subunit (GCLC) fell during HSC activation and increased when activated HSCs revert back to quiescence. Blocking the increase in GCLC expression kept HSCs in an activated state. Activated HSCs have higher nuclear levels and binding activity of MafG to the antioxidant response element (ARE) of GCLC but lower Nrf2/MafG heterodimer binding to the ARE. Quiescent HSCs have a lower nuclear MafG level but higher Nrf2/MafG heterodimer binding to ARE. This occurred because of enhanced sumoylation of Nrf2 and MafG by SUMO-1, which promoted Nrf2 binding to ARE and heterodimerization with MafG. In vivo, knockdown of GCLC exacerbated bile duct ligation-induced liver injury and fibrosis. Ursodeoxycholic acid and S-adenosylmethionine are anti-fibrotic in bile duct ligation, but this effect was nearly lost if GCLC induction was blocked. In conclusion, sumoylation of Nrf2 and MafG enhances heterodimerization and increases GCLC expression, which keeps HSCs in a quiescent state. Antifibrotic agents require activation of GCLC to fully exert their protective effect.

Interaction of GAG trinucleotide repeat and C-129T polymorphisms impairs expression of the glutamate-cysteine ligase catalytic subunit gene

Glutamate cysteine ligase (GCL) catalyzes the rate-limiting step in the de novo synthesis of glutathione (GSH). The catalytic subunit (GCLC) of GCL contains a GAG trinucleotide-repeat (TNR) polymorphism within the 5'-untranslated region (5'-UTR) that has been associated with various human disorders. Although several studies suggest that this variation influences GSH content, its implication for GCLC expression remains unknown. To better characterize its functional significance, we performed reporter gene assays with constructs containing the complete GCLC 5'-UTR upstream of a luciferase gene. Transfection of these vectors into various human cell lines did not reveal any significant differences between 7, 8, 9, or 10 GAG repeats, under either basal or oxidative stress conditions. To correlate these results with the previously described down-regulation induced by the C-129T GCLC promoter polymorphism, combinations of both variations were tested. Interestingly, the -129T allele down-regulates gene expression when combined with 7 GAG but not with 8, 9, or 10 GAG TNRs. This observation was confirmed in primary fibroblast cells, in which the combination of GAG TNR 7/7 and -129C/T genotypes decreased the GCLC protein level. These results provide evidence that interaction of the two variations can efficiently impair GCLC expression and thus suggest its involvement in the pathogenesis of diseases related to GSH metabolism.

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.

Stereoselective effects of 4-hydroxynonenal in cultured mouse hepatocytes

4-Hydroxynonenal (HNE) is produced from arachidonic acid or linoleic acid during oxidative stress. Although HNE is formed in tissues as a racemate, enantiospecific HNE effects have not been widely documented, nor considered. Therefore, a panel of cellular responses was compared after treatment with (R)-HNE, (S)-HNE, or racemic HNE. The phosphorylation status of Jun kinase (JNK) or Akt increased 28-fold or 2-3-fold, respectively, after treatment with 100 μM (S)-HNE and racemic HNE compared to (R)-HNE. In contrast, the increase in phosphorylation of MAPK was greatest for (R)-HNE. Caspase-3-dependent cleavage of the glutamate cysteine ligase (GCL) catalytic subunit and focal adhesion kinase (FAK) were greater in cells treated with (S)-HNE at 48 h. (S)-HNE also caused a greater number of subG1 nuclei, a hallmark of apoptosis, at 30 h after treatment. Together, the results demonstrate different dose- and time-dependent responses to (R)-HNE and (S)-HNE. The results further suggest that HNE enantiomers could differentially contribute to the progression of different diseases or contribute by different mechanisms.

An ethnic-specific polymorphism in the catalytic subunit of glutamate-cysteine ligase impairs the production of glutathione intermediates in vitro

Glutathione plays a crucial role in free radical scavenging, oxidative injury, and cellular homeostasis. Previously, we identified a non-synonymous polymorphism (P462S) in the gene encoding the catalytic subunit of glutamate-cysteine ligase (GCLC), the rate-limiting enzyme in glutathione biosynthesis. This polymorphism is present only in individuals of African descent. Presently, we report that this ethnic-specific polymorphism (462S) encodes an enzyme with significantly decreased in vitro activity when expressed by either a bacterial or mammalian cell expression system. In addition, overexpression of the 462P wild-type GCLC enzyme results in higher intracellular glutathione concentrations than overexpression of the 462S isoform. We also demonstrate that apoptotically stimulated mammalian cells overexpressing the 462S enzyme have increased caspase activation and increased DNA laddering compared to cells overexpressing the wild-type 462P enzyme. Finally, we genotyped several African and African-descent populations and demonstrate that the 462S polymorphism is in Hardy-Weinberg disequilibrium, with no individuals homozygous for the 462S polymorphism identified. These findings describe a glutathione production pathway polymorphism present in individuals of African descent with significantly decreased in vitro activity.

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.

Enhanced glutathione biosynthetic capacity promotes resistance to As3+-induced apoptosis

Trivalent arsenite (As(3+)) is a known human carcinogen capable of inducing both cellular transformation and apoptotic cell death by mechanisms involving the production of reactive oxygen species. The tripeptide antioxidant glutathione (GSH) constitutes a vital cellular defense mechanism against oxidative stress. While intracellular levels of GSH are an important determinant of cellular susceptibility to undergo apoptotic cell death, it is not known whether cellular GSH biosynthetic capacity per se regulates As(3+)-induced apoptosis. The rate-limiting enzyme in GSH biosynthesis is glutamate cysteine ligase (GCL), a heterodimeric holoenzyme composed of a catalytic (GCLC) and a modifier (GCLM) subunit. To determine whether increased GSH biosynthetic capacity enhanced cellular resistance to As(3+)-induced apoptotic cell death, we utilized a mouse liver hepatoma (Hepa-1c1c7) cell line stably overexpressing both GCLC and GCLM. Overexpression of the GCL subunits increased GCL holoenzyme formation and activity and inhibited As(3+)-induced apoptosis. This cytoprotective effect was associated with a decrease in As(3+)-induced caspase activation, cleavage of caspase substrates and translocation of cytochrome c to the cytoplasm. In aggregate, these findings demonstrate that enhanced GSH biosynthetic capacity promotes resistance to As(3+)-induced apoptosis by preventing mitochondrial dysfunction and cytochrome c release and highlight the role of the GSH antioxidant defense system in dictating hepatocyte sensitivity to As(3+)-induced apoptotic cell death.

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.

Apoptosis and glutathione: beyond an antioxidant

Apoptosis is a conserved homeostatic process critical for organ and tissue morphogenesis, development, and senescence. This form of programmed cell death also participates in the etiology of several human diseases including cancer, neurodegenerative, and autoimmune disorders. Although the signaling pathways leading to the progression of apoptosis have been extensively characterized, recent studies highlight the regulatory role of changes in the intracellular milieu (permissive apoptotic environment) in the efficient activation of the cell death machinery. In particular, glutathione (GSH) depletion is a common feature of apoptotic cell death triggered by a wide variety of stimuli including activation of death receptors, stress, environmental agents, and cytotoxic drugs. Although initial studies suggested that GSH depletion was only a byproduct of oxidative stress generated during cell death, recent discoveries suggest that GSH depletion and post-translational modifications of proteins through glutathionylation are critical regulators of apoptosis. Here, we reformulate these emerging paradigms into our current understanding of cell death mechanisms.

Distinct Nrf1/2-independent mechanisms mediate As 3+-induced glutamate-cysteine ligase subunit gene expression in murine hepatocytes

Trivalent arsenite (As(3+)) is a known human carcinogen that is also capable of inducing apoptotic cell death. Increased production of reactive oxygen species is thought to contribute to both the carcinogenic and the cytotoxic effects of As(3+). Glutathione (GSH) constitutes a vital cellular defense mechanism against oxidative stress. The rate-limiting enzyme in GSH biosynthesis is glutamate-cysteine ligase (GCL), a heterodimeric holoenzyme composed of a catalytic (GCLC) and a modifier (GCLM) subunit. In this study, we demonstrate that As(3+) coordinately upregulates Gclc and Gclm mRNA levels in a murine hepatocyte cell line resulting in increased GCL subunit protein expression, holoenzyme formation, and activity. As(3+) increased the rate of transcription of both the Gclm and the Gclc genes and induced the posttranscriptional stabilization of Gclm mRNA. The antioxidant N-acetylcysteine abolished As(3+)-induced Gclc expression and attenuated induction of Gclm. As(3+) induction of Gclc and Gclm was also differentially regulated by the MAPK signaling pathways and occurred independent of the Nrf1/2 transcription factors. These findings demonstrate that distinct transcriptional and posttranscriptional mechanisms mediate the coordinate induction of the Gclc and Gclm subunits of GCL in response to As(3+) and highlight the potential importance of the GSH antioxidant defense system in regulating As(3+)-induced responses in hepatocytes.

Follicle-stimulating hormone and estradiol interact to stimulate glutathione synthesis in rat ovarian follicles and granulosa cells

Glutathione (GSH), the most abundant intracellular nonprotein thiol, is critical for many cellular functions. The rate-limiting step in GSH synthesis is catalyzed by glutamate cysteine ligase (GCL), a heterodimer composed of a catalytic (GCLC) and a modifier (GCLM) subunit. The tissue-specific regulation of GSH synthesis is poorly understood. We showed previously that gonadotropin hormones regulate ovarian GSH synthesis. In the present study, we sought to clarify the ovarian cell type-specific effects of follicle-stimulating hormone (FSH) and estradiol on GSH synthesis. Immature female rats were treated with estradiol to stimulate development of small antral follicles. Granulosa cells (GCs) from these follicles or whole follicles were cultured in serum-free media, with or without FSH and 17beta-estradiol. The GSH and GCLC protein and mRNA levels increased in GCs treated with FSH alone. The effects of FSH on GCLC and GCLM protein and mRNA levels, GCL enzymatic activity, and GSH concentrations in GCs were significantly enhanced by the addition of estradiol. Estradiol alone had no effects on GSH. Dibromo-cAMP mimicked and protein kinase A (PKA) inhibitors prevented FSH stimulation of GCL subunit protein levels. In cultured small antral follicles, FSH stimulated estradiol synthesis and robustly increased GCL subunit mRNA and protein levels and GSH concentrations. The GCL subunit mRNA expression increased in both the granulosa cells and theca cells of follicles with FSH stimulation. These data demonstrate that maximal stimulation of GSH synthesis by FSH in granulosa cells and follicles requires estradiol. Without estradiol, FSH causes lesser increases in GCL subunit expression via a PKA-dependent pathway.

Differences in lung glutathione metabolism may account for rodent susceptibility in elastase-induced emphysema development

Syrian Golden hamsters develop more severe emphysema than Sprague-Dawley rats after intratracheal instillation of the same dose of elastase/body weight. Although species variations in antielastase defenses may largely explain these results, other variables, such as differences in lung antioxidants, cannot be overlooked since oxidative stress modulates antiprotease activity. We propose that elastase instillation might affect lung glutathione (GSH) metabolism differently in these species. Our aim was to study in hamsters and rats, lung glutathione metabolism at different times, from the stage of diffuse alveolar damage to advanced emphysema. We measured total and oxidized glutathione content as well as activity and expression of enzymes related to GSH synthesis and redox cycling: γ-glutamylcysteine synthetase, glutathione peroxidase, and glutathione reductase. Whereas rats showed no significant changes in these measurements, hamsters showed significant derangement in GSH metabolism early after elastase instillation: 25% fall in total GSH ( P < 0.05) with no increase in oxidized glutathione associated with reduced enzyme activities 24 h after elastase [60% for γ-glutamylcysteine synthetase ( P < 0.01), 30% for glutathione peroxidase ( P < 0.01), and 75% for glutathione reductase ( P < 0.001)]. GSH homeostasis was restored at the end of the first week, involving transient increased expression of these enzymes. We conclude that elastase induces significant alterations in GSH metabolism of hamster lungs and no overall change in rat lungs. Although differences in disease severity may account for our findings, the hamster becomes vulnerable to functional inhibition of α1-antitrypsin by oxidants and thus, even more susceptible to injury than it would be, considering only its low α1-antitrypsin level.

Effect of polyI:C cotreatment on halothane-induced liver injury in mice

Drug-induced liver injury (DILI) is a challenging problem in drug development and clinical practice. Patient susceptibility to DILI is multifactorial, making these reactions difficult to predict and prevent. Clinical observations have suggested that concurrent bacterial and viral infections represent an important risk factor in determining patient susceptibility to developing adverse drug reactions, although the underlying mechanism is not clear. In the present study, we employed the viral RNA mimetic (polyinosinic-polycytidylic acid [polyI:C]) to emulate viral infection and examined its effect on halothane-induced liver injury. Although pretreatment of mice with polyI:C attenuated halothane hepatotoxicity due to its inhibitory effect on halothane metabolism, posttreatment significantly exacerbated liver injury with hepatocellular apoptosis being significantly higher than that in mice treated with polyI:C alone or halothane alone. The pan-caspase inhibitor z-VAD-fmk suppressed liver injury induced by polyI:C/posthalothane cotreatment, suggesting that the increased hepatocyte apoptosis contributes to the exacerbation of liver injury. Posttreatment with polyI:C also caused activation of hepatic Kupffer cells (KCs) and natural killer (NK) cells and upregulated multiple proapoptotic factors, including tumor necrosis factor-alpha (TNF-alpha), NK receptor group 2, member D (NKG2D), and Fas ligand (FasL). These factors may play important roles in mediating polyI:C-induced hepatocyte apoptosis.

CONCLUSION

This is the first study to provide evidence that concurrent viral infection can inhibit cytochrome (CYP)450 activities and activate the hepatic innate immune system to proapoptotic factors. DILI may be attenuated or exacerbated by pathogens depending on the time of infection.

Glutathione depletion and disruption of intracellular ionic homeostasis regulate lymphoid cell apoptosis

Intracellular glutathione (GSH) depletion is an important hallmark of apoptosis. We have recently shown that GSH depletion by its extrusion regulates apoptosis independently of excessive reactive oxygen species accumulation. However, the mechanisms by which GSH depletion regulates apoptosis are still unclear. Because disruption of intracellular ionic homeostasis, associated with apoptotic volume decrease (AVD), is necessary for the progression of apoptotic cell death, we sought to evaluate the relationship between GSH transport and ionic homeostasis during Fas ligand (FasL)-induced apoptosis in Jurkat cells. GSH depletion in response to FasL was paralleled by distinct degrees of AVD identified by differences in cellular forward scatter and electronic impedance analysis. Inhibition of GSH efflux prevented AVD, K+ loss, and the activation of two distinct ionic conductances, mediated by Kv1.3 and outward rectifying Cl- channels. Reciprocally, stimulation of GSH loss accelerated the loss of K+, AVD, and consequently the progression of the execution phase of apoptosis. Although high extracellular K+ inhibited FasL-induced apoptosis, GSH depletion was largely independent of K+ loss. These results suggest that deregulation of GSH and ionic homeostasis converge in the regulation of apoptosis in lymphoid cells.

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.

Regulation of glutathione synthesis

Glutathione (GSH) is a ubiquitous intracellular peptide with diverse functions that include detoxification, antioxidant defense, maintenance of thiol status, and modulation of cell proliferation. GSH is synthesized in the cytosol of all mammalian cells in a tightly regulated manner. The major determinants of GSH synthesis are the availability of cysteine, the sulfur amino acid precursor, and the activity of the rate-limiting enzyme, glutamate cysteine ligase (GCL). GCL is composed for a catalytic (GCLC) and modifier (GCLM) subunit and they are regulated at multiple levels and at times differentially. The second enzyme of GSH synthesis, GSH synthase (GS) is also regulated in a coordinated manner as GCL subunits and its up-regulation can further enhance the capacity of the cell to synthesize GSH. Oxidative stress is well known to induce the expression of GSH synthetic enzymes. Key transcription factors identified thus far include Nrf2/Nrf1 via the antioxidant response element (ARE), activator protein-1 (AP-1) and nuclear factor kappa B (NFkappaB). Dysregulation of GSH synthesis is increasingly being recognized as contributing to the pathogenesis of many pathological conditions. These include diabetes mellitus, pulmonary fibrosis, cholestatic liver injury, endotoxemia and drug-resistant tumor cells. Manipulation of the GSH synthetic capacity is an important target in the treatment of many of these disorders.

Defective DNA strand break repair causes chromosomal instability and accelerates liver carcinogenesis in mice

Chromosomal instability is a characteristic feature of hepatocellular carcinoma (HCC) but its origin and role in liver carcinogenesis are undefined. We tested whether a defect in the nonhomologous end-joining (NHEJ) DNA repair gene Ku70 was associated with chromosomal abnormalities and enhanced liver carcinogenesis. Male Ku70 NHEJ-deficient (Ku70-/-), heterozygote (Ku70 +/-), and wild-type (WT) mice were injected with diethylnitrosamine (DEN), a liver carcinogen, at age 15 days. Animals were killed at 3, 6, and 9 months for assessment of tumorigenesis and hepatocellular proliferation. For karyotype analysis, primary liver tumor cell cultures were prepared from HCCs arising in Ku70 mice of all genotypes. Compared to WT littermates, Ku70-/- mice injected with DEN displayed accelerated HCC development. Ku70-/- HCCs harbored clonal increases in numerical and structural aberrations of chromosomes 4, 5, 7, 8, 10, 14, and 19, many of which recapitulated the spectrum of equivalent chromosomal abnormalities observed in human HCC. Ku70-/- HCCs showed high proliferative activity with increased cyclin D1 and proliferating cell nuclear antigen expression, Aurora A kinase activity, enhanced ataxia telangiectasia mutated kinase and ubiquitination, and loss of p53 via proteasomal degradation, features which closely resemble those of human HCC.

CONCLUSION

These findings demonstrate that defects in the NHEJ DNA repair pathway may participate in the disruption of cell cycle checkpoints leading to chromosomal instability and accelerated development of HCC.

Fullerene C60 exposure elicits an oxidative stress response in embryonic zebrafish

Due to its unique physicochemical and optical properties, C60 has raised interest in commercialization for a variety of products. While several reports have determined this nanomaterial to act as a powerful antioxidant, many other studies have demonstrated a strong oxidative potential through photoactivation. To directly address the oxidative potential of C60, the effects of light and chemical supplementation and depletion of glutathione (GSH) on C60-induced toxicity were evaluated. Embryonic zebrafish were used as a model organism to examine the potential of C60 to elicit oxidative stress responses. Reduced light during C60 exposure significantly decreased mortality and the incidence of fin malformations and pericardial edema at 200 and 300 ppb C60. Embryos co-exposed to the glutathione precursor, N-acetylcysteine (NAC), also showed reduced mortality and pericardial edema; however, fin malformations were not reduced. Conversely, co-exposure to the GSH synthesis inhibitors, buthionine sulfoximine (BSO) and diethyl maleate (DEM), increased the sensitivity of zebrafish to C60 exposure. Co-exposure of C60 or its hydroxylated derivative, C60(OH)(24), with H2O2 resulted in increased mortality along the concentration gradient of H2O2 for both materials. Microarrays were used to examine the effects of C60 on the global gene expression at two time points, 36 and 48 h post fertilization (hpf). At both life stages there were alterations in the expression of several key stress response genes including glutathione-S-transferase, glutamate cysteine ligase, ferritin, alpha-tocopherol transport protein and heat shock protein 70. These results support the hypothesis that C60 induces oxidative stress in this model system.

Responses of nontransformed human hepatocytes to conditional expression of full-length hepatitis C virus open reading frame

Hepatitis C virus (HCV) is a major cause of chronic hepatitis that can lead to cirrhosis and hepatocellular carcinoma. To study the effects of HCV protein expression on host cells, we established conditional expression of the full-length open reading frame (ORF) of an infectious cDNA clone of HCV (genotype 1a, H77 strain) in the nontransformed human hepatocyte line cell HH4 using the ecdysone receptor regulatory system. Treatment with the ecdysone analog ponasterone-A induced tightly regulated and dose-dependent full-length HCV ORF expression and properly processed HCV proteins. HCV Core, NS3, and NS5A colocalized in perinuclear regions and associated with the early endosomal protein EEA1. HCV ORF expression caused marked growth inhibition, increased intracellular reactive oxygen species, up-regulation of glutamate-l-cysteine ligase activity, increased glutathione level, and activation of nuclear factor kappaB. Although it was not directly cytotoxic, HCV ORF expression sensitized HH4 cells to Fas at certain concentrations but not to tumor necrosis factor-related apoptosis-inducing ligand. HCV ORF expression in HH4 cells up-regulated genes involved in innate immune response/inflammation and oxidative stress responses and down-regulated cell growth-related genes. Expression of HCV ORF in host cells may contribute to HCV pathogenesis by producing oxidative stress and increasing the expression of genes related to the innate immune response and inflammation.

Tumor necrosis factor-associated protein 1 (TRAP-1) protects cells from oxidative stress and apoptosis

TRAP-1 is a mitochondrial heat shock protein (HSP), recently identified in Saos-2 osteosarcoma cells adapted to mild oxidative stress induced by diethylmaleate (DEM). TRAP-1 mRNA expression is increased in DEM-adapted cells as well as in tumor cells resistant to 5-fluorouracil and to platin derivatives. Since a strong decrease of TRAP-1 protein levels, upon cisplatin treatment, is observed only in controls but not in the DEM-adapted counterpart, a possible role for this protein in the development of resistant phenotypes could be hypothesized. To characterize the protective role of TRAP-1 against oxidative stress and apoptosis, stable transfectants were generated and characterized for their response to different stress types. These stable clones expressing constitutively high TRAP-1 levels: (i) are more resistant to H2O2-induced DNA damage and to apoptosis by cisplatin; (ii) contain higher reduced glutathione (GSH) levels than control cells; and (iii) do not release the apoptosis-inducing factor into the nucleus upon cisplatin treatment. Furthermore, high TRAP-1 levels interfere with caspase 3 activation. These results confirm the anti-apoptotic role of TRAP-1, and suggest that increased expression of this mitochondrial HSP in DEM-adapted and chemoresistant cells could be part of a pro-survival signaling pathway aimed to evade toxic effects of oxidants and anticancer drugs.

The central role of glutathione in the pathophysiology of human diseases

Reduced glutathione (L-gamma-glutamyl-L-cysteinyl-glycine, GSH) is the prevalent low-molecular-weight thiol in mammalian cells. It is formed in a two-step enzymatic process including, first, the formation of gamma-glutamylcysteine from glutamate and cysteine, by the activity of the gamma-glutamylcysteine synthetase; and second, the formation of GSH by the activity of GSH synthetase which uses gamma-glutamylcysteine and glycine as substrates. While its synthesis and metabolism occur intracellularly, its catabolism occurs extracellularly by a series of enzymatic and plasma membrane transport steps. Glutathione metabolism and transport participates in many cellular reactions including: antioxidant defense of the cell, drug detoxification and cell signaling (involved in the regulation of gene expression, apoptosis and cell proliferation). Alterations in its concentration have also been demonstrated to be a common feature of many pathological conditions including diabetes, cancer, AIDS, neurodegenerative and liver diseases. Additionally, GSH catabolism has been recently reported to modulate redox-sensitive components of signal transduction cascades. In this manuscript, we review the current state of knowledge on the role of GSH in the pathogenesis of human diseases with the aim to underscore its relevance in translational research for future therapeutic treatment design.

Cigarette smoke extract-induced suppression of caspase-3-like activity impairs human neutrophil phagocytosis

Neutrophils are the primary inflammatory cell in smokers’ lungs, but little is known about the ability of cigarette smoke to modulate neutrophil function. Neutrophils undergo caspase-3-dependent spontaneous, as well as phagocytosis-induced, apoptosis. This study investigated the ability of cigarette smoke extract (CSE) to alter neutrophil caspase-3 activity, apoptosis, and phagocytosis. CSE treatment resulted in a dramatic suppression of neutrophil caspase-3-like activity, which correlated with reduced cleavage of glutamate-l-cysteine ligase catalytic subunit, a known target of active caspase-3. CSE did not affect procaspase-3 processing to its active fragment, suggesting a direct effect of CSE on active caspase-3. Consistent with this, CSE inhibited active recombinant caspase-3 activity, which was abolished by dithiothreitol, suggesting a redox-sensitive mechanism. CSE-induced suppression of caspase-3 activity did not alter spontaneous apoptosis but did impair phagocytic activity. Since CSE treatment resulted in profound suppression of caspase-3 activity but did not alter apoptosis, the possibility of a threshold level of caspase-3 activity was investigated. CSE reduced caspase-3 activity in a concentration-dependent manner. Despite near complete suppression of caspase-3 activity, spontaneous apoptosis was not altered. Conversely, treatment with the pan-caspase inhibitor, Z-Val-Ala-Asp-fluoromethylketone, reduced spontaneous apoptosis. These data demonstrate that CSE does not suppress caspase-3 activity below a threshold level to prevent spontaneous apoptosis, but the level of inhibition is sufficient to impair neutrophil phagocytic activity. These divergent functions of caspase-3 may contribute to the persistence of neutrophils in the lungs of smokers, as well as be a factor in their higher incidence of community-acquired pneumonia.

Prostaglandin I(2) and E(2) mediate the protective effects of cyclooxygenase-2 in a mouse model of immune-mediated liver injury

Studies of the molecular and cellular mechanisms of concanavalin A (ConA)-induced liver injury have provided important knowledge on the pathogenesis of many liver diseases involving hepatic inflammation. However, studies identifying hepato-protective factors based on the mechanistic understanding of this model are lacking. Evidence suggests that certain prostaglandin (PG) products of cyclooxygenase (COX)-1 and COX-2 provide important anti-inflammatory and cytoprotective functions in some pathophysiological states. In the present study, we demonstrate a protective role of COX-2 derived PGs in ConA-induced liver injury. COX-2(-/-) mice developed much more severe liver damage upon ConA treatment compared with wild-type and COX-1(-/-) mice. Treatment of COX-2(-/-) mice with misoprostol (a PGE(1/2) analog) or beraprost (a PGI(2) analog) significantly decreased ConA-induced liver injury. Data from both in vivo and in vitro experiments demonstrated that misoprostol and beraprost acted directly on hepatic leukocytes, including natural killer (NK)T and T cells, and down-regulated their production of interferon (IFN)-gamma, which are critical in mediating ConA-induced tissue damage. Collectively, the results provide strong evidence that the protective effects of COX-2 within the liver are mediated through the production of PGE(2) and PGI(2), which exert anti-inflammatory functions. These findings suggest that COX-2-derived PGs may have great therapeutic potentials in treating patients with inflammatory liver diseases.

ICAM-1 cross-linking stimulates endothelial glutathione synthesis

What mechanisms regulate endothelial glutathione (GSH) during inflammation? Addressing this question is critical in understanding mechanisms leading to endothelial dysfunction and cardiovascular disease. Herein, the authors show data that support the hypothesis that the intercellular cell adhesion molecule-1 (ICAM-1) regulates GSH. Ligating either constitutive or induced ICAM-1 on the endothelial surface, or exposing endothelial cells to soluble ICAM-1, increases GSH concentrations. ICAM-1 is important in mediating leukocyte adhesion and modulates endothelial signaling pathways important in controlling transmigration. The present data underscore a novel function for ICAM-1 in modulating GSH metabolism and raise the hypothesis that this adhesion molecule controls endothelial redox status under basal and inflammatory conditions.