Induction of glutamate-cysteine ligase (gamma-glutamylcysteine synthetase) in the brains of adult female mice subchronically exposed to methylmercury

Methylmercury-Mediated Oxidative Stress and Activation of the Cellular Protective System

Methylmercury (MeHg) is a well-known neurotoxicant that causes severe intoxication in humans. In Japan, it is referred to as Minamata disease, which involves two characteristic clinical forms: fetal type and adult type depending on the exposed age. In addition to MeHg burden level, individual susceptibility to MeHg plays a role in the manifestation of MeHg toxicity. Research progress has pointed out the importance of oxidative stress in the pathogenesis of MeHg toxicity. MeHg has a high affinity for selenohydryl groups, sulfhydryl groups, and selenides. It has been clarified that such affinity characteristics cause the impairment of antioxidant enzymes and proteins, resulting in the disruption of antioxidant systems. Furthermore, MeHg-induced intracellular selenium deficiency due to the greater affinity of MeHg for selenohydryl groups and selenides leads to failure in the recoding of a UGA codon for selenocysteine and results in the degradation of antioxidant selenoenzyme mRNA by nonsense-mediated mRNA decay. The defect of antioxidant selenoenzyme replenishment exacerbates MeHg-mediated oxidative stress. On the other hand, it has also been revealed that MeHg can directly activate the antioxidant Keap1/Nrf2 signaling pathway. This review summarizes the incidence of MeHg-mediated oxidative stress from the viewpoint of the individual intracellular redox system interactions and the MeHg-mediated aforementioned intracellular events. In addition, the mechanisms of cellular stress pathways and neuronal cell death triggered by MeHg-mediated oxidative stress and direct interactions of MeHg with reactive residues of proteins are mentioned.

Sexual dimorphism in glutathione metabolism and glutathione-dependent responses

Glutathione is the most abundant intracellular low molecular weight thiol in cells and tissues, and plays an essential role in numerous cellular processes, including antioxidant defenses, the regulation of protein function, protein localization and stability, DNA synthesis, gene expression, cell proliferation, and cell signaling. Sexual dimorphisms in glutathione biology, metabolism and glutathione-dependent signaling have been reported for a broad range of biological processes, spanning the human lifespan from early development to aging. Sex-depended differences with regard to glutathione and its biology have also been reported for a number of human pathologies and diseases such as neurodegeneration, cardiovascular diseases and metabolic disorders. Here we review the latest literature in this field and discuss the potential impact of these sexual dimorphisms in glutathione biology on human health and diseases.

Neurodevelopmental Effects of Mercury

The toxicology of mercury (Hg) is of concern since this metal is ubiquitously distributed in the environment, and living organisms are routinely exposed to Hg at low to high levels. The toxic effects of Hg are well studied and it is known that they may differ depending on the Hg chemical species. In this chapter, we emphasize the neurotoxic effects of Hg during brain development. The immature brain is more susceptible to Hg exposure, since all the Hg chemical forms, not only the organic ones, can harm it. The possible consequences of Hg exposure during the early stages of development, the additive effects with other co-occurring neurotoxicants, and the known mechanisms of action and targets will be addressed in this chapter.

The pulmonary inflammatory response to multiwalled carbon nanotubes is influenced by gender and glutathione synthesis

Inhalation of multiwalled carbon nanotubes (MWCNTs) during their manufacture or incorporation into various commercial products may cause lung inflammation, fibrosis, and oxidative stress in exposed workers. Some workers may be more susceptible to these effects because of differences in their ability to synthesize the major antioxidant and immune system modulator glutathione (GSH). Accordingly, in this study we examined the influence of GSH synthesis and gender on MWCNT-induced lung inflammation in C57BL/6 mice. GSH synthesis was impaired through genetic manipulation of Gclm, the modifier subunit of glutamate cysteine ligase, the rate-limiting enzyme in GSH synthesis. Twenty-four hours after aspirating 25µg of MWCNTs, all male mice developed neutrophilia in their lungs, regardless of Gclm genotype. However, female mice with moderate (Gclm heterozygous) and severe (Gclm null) GSH deficiencies developed significantly less neutrophilia. We found no indications of MWCNT-induced oxidative stress as reflected in the GSH content of lung tissue and epithelial lining fluid, 3-nitrotyrosine formation, or altered mRNA or protein expression of several redox-responsive enzymes. Our results indicate that GSH-deficient female mice are rendered uniquely susceptible to an attenuated neutrophil response. If the same effects occur in humans, GSH-deficient women manufacturing MWCNTs may be at greater risk for impaired neutrophil-dependent clearance of MWCNTs from the lung. In contrast, men may have effective neutrophil-dependent clearance, but may be at risk for lung neutrophilia regardless of their GSH levels.

Role of Nrf2, HO-1 and GSH in Neuroblastoma Cell Resistance to Bortezomib

The activation of Nrf2 has been demonstrated to play a crucial role in cancer cell resistance to different anticancer therapies. The inhibition of proteasome activity has been proposed as a chemosensitizing therapy but the activation of Nrf2 could reduce its efficacy. Using the highly chemoresistant neuroblastoma cells HTLA-230, here we show that the strong reduction in proteasome activity, obtained by using low concentration of bortezomib (BTZ, 2.5 nM), fails in reducing cell viability. BTZ treatment favours the binding of Nrf2 to the ARE sequences in the promoter regions of target genes such as heme oxygenase 1 (HO-1), the modulatory subunit of γ-glutamylcysteine ligase (GCLM) and the transporter for cysteine (x-CT), enabling their transcription. GSH level is also increased after BTZ treatment. The up-regulation of Nrf2 target genes is responsible for cell resistance since HO-1 silencing and GSH depletion synergistically decrease BTZ-treated cell viability. Moreover, cell exposure to all-trans-Retinoic acid (ATRA, 3 μM) reduces the binding of Nrf2 to the ARE sequences, decreases HO-1 induction and lowers GSH level increasing the efficacy of bortezomib. These data suggest the role of Nrf2, HO-1 and GSH as molecular targets to improve the efficacy of low doses of bortezomib in the treatment of malignant neuroblastoma.

Effects of deletion of the transcription factor Nrf2 and benzo [a]pyrene treatment on ovarian follicles and ovarian surface epithelial cells in mice

Polycyclic aromatic hydrocarbons, like benzo[a]pyrene (BaP), are ubiquitous environmental pollutants and potent ovarian toxicants. The transcription factor NRF2 is an important regulator of the cellular response to electrophilic toxicants like BaP and to oxidative stress. NRF2 regulates transcription of genes involved in the detoxification of reactive metabolites of BaP and reactive oxygen species. We therefore hypothesized that Nrf2-/- mice have accelerated ovarian aging and increased sensitivity to the ovarian toxicity of BaP. A single injection of BaP dose-dependently depleted ovarian follicles in Nrf2+/+ and Nrf2-/- mice, but the effects of BaP were not enhanced in the absence of Nrf2. Similarly, Nrf2-/- mice did not have increased ovarian BaP DNA adduct formation compared to Nrf2+/+ mice. Ovarian follicle numbers did not differ between peripubertal Nrf2-/- and Nrf2+/+ mice, but by middle age, Nrf2-/- mice had significantly fewer primordial follicles than Nrf2+/+ mice, consistent with accelerated ovarian aging.

Silica nanoparticles induce oxidative stress, inflammation, and endothelial dysfunction in vitro via activation of the MAPK/Nrf2 pathway and nuclear factor-κB signaling

Despite the widespread application of silica nanoparticles (SiNPs) in industrial, commercial, and biomedical fields, their response to human cells has not been fully elucidated. Overall, little is known about the toxicological effects of SiNPs on the cardiovascular system. In this study, SiNPs with a 58 nm diameter were used to study their interaction with human umbilical vein endothelial cells (HUVECs). Dose- and time-dependent decrease in cell viability and damage on cell plasma-membrane integrity showed the cytotoxic potential of the SiNPs. SiNPs were found to induce oxidative stress, as evidenced by the significant elevation of reactive oxygen species generation and malondialdehyde production and downregulated activity in glutathione peroxidase. SiNPs also stimulated release of cytoprotective nitric oxide (NO) and upregulated inducible nitric oxide synthase (NOS) messenger ribonucleic acid, while downregulating endothelial NOS and ET-1 messenger ribonucleic acid, suggesting that SiNPs disturbed the NO/NOS system. SiNP-induced oxidative stress and NO/NOS imbalance resulted in endothelial dysfunction. SiNPs induced inflammation characterized by the upregulation of key inflammatory mediators, including IL-1β, IL-6, IL-8, TNFα, ICAM-1, VCAM-1, and MCP-1. In addition, SiNPs triggered the activation of the Nrf2-mediated antioxidant system, as evidenced by the induction of nuclear factor-κB and MAPK pathway activation. Our findings demonstrated that SiNPs could induce oxidative stress, inflammation, and NO/NOS system imbalance, and eventually lead to endothelial dysfunction via activation of the MAPK/Nrf2 pathway and nuclear factor-κB signaling. This study indicated a potential deleterious effect of SiNPs on the vascular endothelium, which warrants more careful assessment of SiNPs before their application.

Genomic and non-genomic regulation of PGC1 isoforms by estrogen to increase cerebral vascular mitochondrial biogenesis and reactive oxygen species protection

We previously found that estrogen exerts a novel protective effect on mitochondria in brain vasculature. Here we demonstrate in rat cerebral blood vessels that 17β-estradiol (estrogen), both in vivo and ex vivo, affects key transcriptional coactivators responsible for mitochondrial regulation. Treatment of ovariectomized rats with estrogen in vivo lowered mRNA levels of peroxisome proliferator-activated receptor-γ coactivator-1 alpha (PGC-1α) but increased levels of the other PGC-1 isoforms: PGC-1β and PGC-1 related coactivator (PRC). In vessels ex vivo, estrogen decreased protein levels of PGC-1α via activation of phosphatidylinositol 3-kinase (PI3K). Estrogen treatment also increased phosphorylation of forkhead transcription factor, FoxO1, a known pathway for PGC-1α downregulation. In contrast to the decrease in PGC-1α, estrogen increased protein levels of nuclear respiratory factor 1, a known PGC target and mediator of mitochondrial biogenesis. The latter effect of estrogen was independent of PI3K, suggesting a separate mechanism consistent with increased expression of PGC-1β and PRC. We demonstrated increased mitochondrial biogenesis following estrogen treatment in vivo; cerebrovascular levels of mitochondrial transcription factor A and electron transport chain subunits as well as the mitochondrial/nuclear DNA ratio were increased. We examined a downstream target of PGC-1β, glutamate-cysteine ligase (GCL), the rate-limiting enzyme for glutathione synthesis. In vivo estrogen increased protein levels of both GCL subunits and total glutathione levels. Together these data show estrogen differentially regulates PGC-1 isoforms in brain vasculature, underscoring the importance of these coactivators in adapting mitochondria in specific tissues. By upregulating PGC-1β and/or PRC, estrogen appears to enhance mitochondrial biogenesis, function and reactive oxygen species protection.

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.

Glutathione (GSH) and the GSH synthesis gene Gclm modulate plasma redox and vascular responses to acute diesel exhaust inhalation in mice

CONTEXT

Inhalation of fine particulate matter (PM₂.₅) is associated with acute pulmonary inflammation and impairments in cardiovascular function. In many regions, PM₂.₅ is largely derived from diesel exhaust (DE), and these pathophysiological effects may be due in part to oxidative stress resulting from DE inhalation. The antioxidant glutathione (GSH) is important in limiting oxidative stress-induced vascular dysfunction. The rate-limiting enzyme in GSH synthesis is glutamate cysteine ligase and polymorphisms in its catalytic and modifier subunits (GCLC and GCLM) have been shown to influence vascular function and risk of myocardial infarction in humans.

OBJECTIVE

We hypothesized that compromised de novo synthesis of GSH in Gclm⁻/⁺ mice would result in increased sensitivity to DE-induced lung inflammation and vascular effects.

MATERIALS AND METHODS

WT and Gclm⁻/⁺ mice were exposed to DE via inhalation (300 μg/m³) for 6 h. Neutrophil influx into the lungs, plasma GSH redox potential, vascular reactivity of aortic rings and aortic nitric oxide (NO•) were measured.

RESULTS

DE inhalation resulted in mild bronchoalveolar neutrophil influx in both genotypes. DE-induced effects on plasma GSH oxidation and acetylcholine (ACh)-relaxation of aortic rings were only observed in Gclm⁻/⁺ mice. Contrary to our hypothesis, DE exposure enhanced ACh-induced relaxation of aortic rings in Gclm⁻/⁺ mice.

DISCUSSION AND CONCLUSION

THESE data support the hypothesis that genetic determinants of antioxidant capacity influence the biological effects of acute inhalation of DE. However, the acute effects of DE on the vasculature may be dependent on the location and types of vessels involved. Polymorphisms in GSH synthesis genes are common in humans and further investigations into these potential gene-environment interactions are warranted.

The role of the Keap1/Nrf2 pathway in the cellular response to methylmercury

Methylmercury (MeHg) is an environmental electrophile that covalently modifies cellular proteins with reactive thiols, resulting in the formation of protein adducts. While such protein modifications, referred to as S-mercuration, are thought to be associated with the enzyme dysfunction and cellular damage caused by MeHg exposure, the current consensus is that (1) there is a cellular response to MeHg through the activation of NF-E2-related factor 2 (Nrf2) coupled to S-mercuration of its negative regulator, Kelch-like ECH-associated protein 1 (Keap1), and (2) the Keap1/Nrf2 pathway protects against MeHg toxicity. In this review, we introduce our findings and discuss the observations of other workers concerning the S-mercuration of cellular proteins by MeHg and the importance of the Keap1/Nrf2 pathway in protection against MeHg toxicity in cultured cells and mice.

Endogenous ovarian hormones affect mitochondrial efficiency in cerebral endothelium via distinct regulation of PGC-1 isoforms

Mitochondria support the energy-intensive functions of brain endothelium but also produce damaging-free radicals that lead to disease. Previously, we found that estrogen treatment protects cerebrovascular mitochondria, increasing capacity for ATP production while decreasing reactive oxygen species (ROS). To determine whether these effects occur specifically in endothelium in vivo and also explore underlying transcriptional mechanisms, we studied freshly isolated brain endothelial preparations from intact and ovariectomized female mice. This preparation reflects physiologic influences of circulating hormones, hemodynamic forces, and cell-cell interactions of the neurovascular unit. Loss of ovarian hormones affected endothelial expression of the key mitochondrial regulator family, peroxisome proliferator-activated receptor γ coactivator 1 (PGC-1), but in a unique way. Ovariectomy increased endothelial PGC-1α mRNA but decreased PGC-1β mRNA. The change in PGC-1β correlated with decreased mRNA for crucial downstream mitochondrial regulators, nuclear respiratory factor 1 and mitochondrial transcription factor A, as well as for ATP synthase and ROS protection enzymes, glutamate-cysteine ligase and manganese superoxide dismutase. Ovariectomy also decreased mitochondrial biogenesis (mitochondrial/nuclear DNA ratio). These results indicate ovarian hormones normally act through a distinctive regulatory pathway involving PGC-1β to support cerebral endothelial mitochondrial content and guide mitochondrial function to favor ATP coupling and ROS protection.

Glutathione (GSH) and the GSH synthesis gene Gclm modulate vascular reactivity in mice

Oxidative stress has been implicated in the development of vascular disease and in the promotion of endothelial dysfunction via the reduction in bioavailable nitric oxide (NO()). Glutathione (GSH) is a tripeptide thiol antioxidant that is utilized by glutathione peroxidase (GPx) to scavenge reactive oxygen species such as hydrogen peroxide and phospholipid hydroperoxides. Relatively frequent single-nucleotide polymorphisms (SNPs) within the 5' promoters of the GSH synthesis genes GCLC and GCLM are associated with impaired vasomotor function, as measured by decreased acetylcholine-stimulated coronary artery dilation, and with increased risk of myocardial infarction. Although the influence of genetic knockdown of GPx on vascular function has been investigated in mice, no work to date has been published on the role of genetic knockdown of GSH synthesis genes on vascular reactivity. We therefore investigated the effects of targeted disruption of Gclm in mice and the subsequent depletion of GSH on vascular reactivity, NO() production, aortic nitrotyrosine protein modification, and whole-genome transcriptional responses as measured by DNA microarray. Gclm(-/+) and Gclm(-/-) mice had 72 and 12%, respectively, of wild-type (WT) aortic GSH content. Gclm(-/+) mice had a significant impairment in acetylcholine (ACh)-induced relaxation in aortic rings as well as increased aortic nitrotyrosine protein modification. Surprisingly, Gclm(-/-) aortas showed enhanced relaxation compared to Gclm(-/+) aortas, as well as increased NO() production. Although aortic rings from Gclm(-/-) mice had enhanced ACh relaxation, they had a significantly increased sensitivity to phenylephrine (PE)-induced contraction. Alternatively, the PE response of Gclm(-/+) aortas was nearly identical to that of their WT littermates. To examine the role of NO() or other potential endothelium-derived factors in differentially regulating vasomotor activity, we incubated aortic rings with the NO() synthase inhibitor L-NAME or physically removed the endothelium before PE treatment. L-NAME treatment and endothelium removal enhanced PE-induced contraction in WT and Gclm(-/+) mice, but this effect was severely diminished in Gclm(-/-) mice, indicating a potentially unique role for GSH in mediating vessel contraction. Whole-genome assessment of aortic mRNA in Gclm(-/-) and WT mice revealed altered expression of genes within the canonical Ca(2+) signaling pathway, which may have a role in mediating these observed functional effects. These findings provide additional evidence that the de novo synthesis of GSH can influence vascular reactivity and provide insights regarding possible mechanisms by which SNPs within GCLM and GCLC influence the risk of developing vascular diseases in humans.

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.

Heterozygosity in the glutathione synthesis gene Gclm increases sensitivity to diesel exhaust particulate induced lung inflammation in mice

CONTEXT

Inhalation of ambient fine particulate matter (PM₂.₅) is associated with adverse respiratory and cardiovascular effects. A major fraction of PM₂.₅ in urban settings is diesel exhaust particulate (DEP), and DEP-induced lung inflammation is likely a critical event mediating many of its adverse health effects. Oxidative stress has been proposed to be an important factor in PM₂.₅-induced lung inflammation, and the balance between pro- and antioxidants is an important regulator of this inflammation. An important intracellular antioxidant is the tripeptide thiol glutathione (GSH). Glutamate cysteine ligase (GCL) carries out the first step in GSH synthesis. In humans, relatively common genetic polymorphisms in both the catalytic (Gclc) and modifier (Gclm) subunits of GCL have been associated with increased risk for lung and cardiovascular diseases.

OBJECTIVE

This study was aimed to determine the effects of Gclm expression on lung inflammation following DEP exposure in mice.

MATERIALS AND METHODS

We exposed Gclm wild type, heterozygous, and null mice to DEP via intranasal instillation and assessed lung inflammation as determined by neutrophils and inflammatory cytokines in lung lavage, inflammatory cytokine mRNA levels in lung tissue, as well as total lung GSH, Gclc, and Gclm protein levels.

RESULTS

The Gclm heterozygosity was associated with a significant increase in DEP-induced lung inflammation when compared to that of wild type mice.

DISCUSSION AND CONCLUSION

This finding indicates that GSH synthesis can mediate DEP-induced lung inflammation and suggests that polymorphisms in Gclm may be an important factor in determining adverse health outcomes in humans following inhalation of PM₂.₅.

DIESEL particulate exposed macrophages alter endothelial cell expression of eNOS, iNOS, MCP1, and glutathione synthesis genes

There is considerable debate regarding inhaled diesel exhaust particulate (DEP) causing impairments in vascular reactivity. Although there is evidence that inhaled particles can translocate from the lung into the systemic circulation, it has been suggested that inflammatory factors produced in the lung following macrophage particle engulfment also pass into the circulation. To investigate these differing hypotheses, we used in vitro systems to model each exposure. By using a direct exposure system and a macrophage-endothelial cell co-culture model, we compared the effects of direct DEP exposure and exposure to inflammatory factors produced by DEP-treated macrophages, on endothelial cell mRNA levels for eNOS, iNOS, endothelin-1, and endothelin-converting-enzyme-1. As markers of oxidative stress, we measured the effects of DEP treatment on glutathione (GSH) synthesis genes and on total GSH. In addition, we analyzed the effect of DEP treatment on monocyte chemo-attractant protein-1. Direct DEP exposure increased endothelial GCLC and GCLM as well as total GSH in addition to increased eNOS, iNOS, and Mcp1 mRNA. Alternatively, inflammatory factors released from DEP-exposed macrophages markedly up-regulated endothelial iNOS and Mcp1 while modestly down-regulating eNOS. These data support both direct exposure to DEP and the release of inflammatory cytokines as explanations for DEP-induced impairments in vascular reactivity.

Bisphenol A effect on glutathione synthesis and recycling in testicular Sertoli cells

BACKGROUND AND OBJECTIVE

Controversial effects of bisphenol A (BPA) have been reported on testicular function. These differences might reflect dissimilar exposure conditions. Dose responses to toxicants may be non-linear, e.g. U-shaped, with effects at low and at high levels of exposure and lower or inexistent effects at intermediate levels. Sertoli cells produce high levels of glutathione (GSH) as a cell defense mechanism. In this study, we addressed the question whether the exposure to different doses of BPA could influence Sertoli cell GSH synthesis and recycling.

MATERIALS AND METHODS

Primary Sertoli cell cultures were exposed to various doses of BPA (0.5 nM-100 μM). Cell viability was measured as an outcome of toxic effect. GSH cell content was determined to evaluate cell response to toxicant exposure. Glutamate-cysteine ligase catalytic (GCLC) and modulatory (GCLM) subunit expression were assessed to estimate GSH synthesis, and GSH reductase (GR) expression to estimate GSH recycling.

RESULTS

BPA 100 μM, but not lower doses, decreased cell viability. BPA 10 and 50 μM, but not lower doses, induced an increment in Sertoli cell GSH levels, due to a rapid upregulation of GCLC and GR and a slower upregulation of GCLM.

CONCLUSIONS

High doses of BPA are deleterious for Sertoli cells. Intermediate doses do not affect Sertoli cell viability and increase cell content of GSH owing to increased GSH synthesis and recycling enzyme expression. Lower doses of BPA are not capable of eliciting a cell defense response. These observations may explain a non-linear dose response of Sertoli cells to BPA.

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.

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.

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.

Overexpression of glutamate-cysteine ligase protects human COV434 granulosa tumour cells against oxidative and gamma-radiation-induced cell death

Ionizing radiation is toxic to ovarian follicles and can cause infertility. Generation of reactive oxygen species (ROS) has been implicated in the toxicity of ionizing radiation in several cell types. We have shown that depletion of the antioxidant glutathione (GSH) sensitizes follicles and granulosa cells to toxicant-induced apoptosis and that supplementation of GSH is protective. The rate-limiting reaction in GSH biosynthesis is catalysed by glutamate-cysteine ligase (GCL), which consists of a catalytic subunit (GCLC) and a regulatory subunit (GCLM). We hypothesized that overexpression of Gclc or Gclm to increase GSH synthesis would protect granulosa cells against oxidant- and radiation-induced cell death. The COV434 line of human granulosa tumour cells was stably transfected with vectors designed for the constitutive expression of Gclc, Gclm, both Gclc and Gclm or empty vector. GCL protein and enzymatic activity and total GSH levels were significantly increased in the GCL subunit-transfected cells. GCL-transfected cells were resistant to cell killing by treatment with hydrogen peroxide compared to control cells. Cell viability declined less in all the GCL subunit-transfected cell lines 1-8 h after 0.5 mM hydrogen peroxide treatment than in control cells. We next examined the effects of GCL overexpression on responses to ionizing radiation. ROS were measured using a redox-sensitive fluorogenic dye in cells irradiated with 0, 1 or 5 Gy of gamma-rays. There was a dose-dependent increase in ROS within 30 min in all cell lines, an effect that was significantly attenuated in Gcl-transfected cells. Apoptosis, assessed by terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labelling and activated caspase-3 immunoblotting, was significantly decreased in irradiated Gclc-transfected cells compared to irradiated control cells. Suppression of GSH synthesis in Gclc-transfected cells reversed resistance to radiation. These findings show that overexpression of GCL in granulosa cells can augment GSH synthesis and ameliorate various sequelae associated with exposure to oxidative stress and irradiation.

Decline in NRF2-regulated antioxidants in chronic obstructive pulmonary disease lungs due to loss of its positive regulator, DJ-1

RATIONALE

Oxidative stress is a key contributor in chronic obstructive pulmonary disease (COPD) pathogenesis caused by cigarette smoking. NRF2, a redox-sensitive transcription factor, dissociates from its inhibitor, KEAP1, to induce antioxidant expression that inhibits oxidative stress.

OBJECTIVES

To determine the link between severity of COPD, oxidative stress, and NRF2-dependent antioxidant levels in the peripheral lung tissue of patients with COPD.

METHODS

We assessed the expression of NRF2, NRF2-dependent antioxidants, regulators of NRF2 activity, and oxidative damage in non-COPD (smokers and former smokers) and smoker COPD lungs (mild and advanced). Cigarette smoke-exposed human lung epithelial cells (Beas2B) and mice were used to understand the mechanisms.

MEASUREMENTS AND MAIN RESULTS

When compared with non-COPD lungs, the COPD patient lungs showed (1) marked decline in NRF2-dependent antioxidants and glutathione levels, (2) increased oxidative stress markers, (3) significant decrease in NRF2 protein with no change in NRF2 mRNA levels, and (4) similar KEAP1 but significantly decreased DJ-1 levels (a protein that stabilizes NRF2 protein by impairing KEAP1-dependent proteasomal degradation of NRF2). Exposure of Bea2B cells to cigarette smoke caused oxidative modification and enhanced proteasomal degradation of DJ-1 protein. Disruption of DJ-1 in mouse lungs, mouse embryonic fibroblasts, and Beas2B cells lowered NRF2 protein stability and impaired antioxidant induction in response to cigarette smoke. Interestingly, targeting KEAP1 by siRNA or the small-molecule activator sulforaphane restored induction of NRF2-dependent antioxidants in DJ-1-disrupted cells in response to cigarette smoke.

CONCLUSIONS

NRF2-dependent antioxidants and DJ-1 expression was negatively associated with severity of COPD. Therapy directed toward enhancing NRF2-regulated antioxidants may be a novel strategy for attenuating the effects of oxidative stress in the pathogenesis of COPD.

N-acetylcysteine as a potential antidote and biomonitoring agent of methylmercury exposure

BACKGROUND

Many people, by means of consumption of seafood or other anthropogenic sources, are exposed to levels of methylmercury (MeHg) that are generally considered to be quite low, but that may nevertheless produce irreversible brain damage, particularly in unborn babies. The only way to prevent or ameliorate MeHg toxicity is to enhance its elimination from the body.

OBJECTIVES

Using N-acetylcysteine (NAC), we aimed to devise a monitoring protocol for early detection of acute exposure or relatively low MeHg levels in a rodent model, and to test whether NAC reduces MeHg levels in the developing embryo.

RESULTS

NAC produced a transient, dose-dependent acceleration of urinary MeHg excretion in rats of both sexes. Approximately 5% of various MeHg doses was excreted in urine 2 hr after injection of 1 mmol/kg NAC. In pregnant rats, NAC markedly reduced the body burden of MeHg, particularly in target tissues such as brain, placenta, and fetus. In contrast, NAC had no significant effect on urinary MeHg excretion in preweanling rats.

CONCLUSIONS

Because NAC causes a transient increase in urinary excretion of MeHg that is proportional to the body burden, it is promising as a biomonitoring agent for MeHg in adult animals. In view of this and because NAC is effective at enhancing MeHg excretion when given either orally or intravenously, can decrease brain and fetal levels of MeHg, has minimal side effects, and is widely available in clinical settings, NAC should be evaluated as a potential antidote and biomonitoring agent in humans.

Over expression of glutamate cysteine ligase increases cellular resistance to H2O2-induced DNA single-strand breaks

Hydrogen peroxide (H2O2) can cause single strand DNA breaks (ssDNA) in cells when the mechanisms normally in place to reduce it are overwhelmed. Such mechanisms include catalase, glutathione peroxidases (GPx), and peroxiredoxins. The relative importance of these enzymes in H2O2 reduction varies with cell and tissue type. The role of the GPx cofactor glutathione (GSH) in oxidative defense can be further understood by modulating its synthesis. The first and rate-limiting enzyme in GSH synthesis is glutamate-cysteine ligase (GCL), which has a catalytic subunit (Gclc) and a modifier subunit (Gclm). Using mouse hepatoma cells we evaluated the effects of GCL over expression on H2O2-induced changes in GSH and ssDNA break formation with the single cell gel electrophoresis assay (SCG or comet assay), and the acridine orange DNA unwinding flow cytometry assay (AO unwinding assay). Cells over expressing GCL had higher GSH content than control cells, and both SCG and AO unwinding assays revealed that cells over expressing GCL were significantly more resistant to H2O2-induced ssDNA break formation. Furthermore, using the AO unwinding assay, the prevalence of H2O2-induced breaks in different phases of the cell cycle was not different, and the degree of protection afforded by GCL over expression was also not cell cycle phase dependent. Our results support the hypothesis that GCL over expression enhanced GSH biosynthesis and protected cells from H2O2-induced DNA breaks. These results also suggest that genetic polymorphisms that affect GCL expression may be important determinants of oxidative DNA damage and cancer.

FSH and bFGF stimulate the production of glutathione in cultured rat Sertoli cells

Migration of developing germ cells from the basal to the adluminal compartment of the seminiferous epithelium requires extensive tissue restructuring, resulting in the production of reactive oxygen species. Sertoli cells are involved in this process. Glutathione (GSH), produced by Sertoli cells, has an essential role in cell protection against oxidative stress. Intracellular GSH content is maintained by de novo synthesis, involving glutamate-cysteine ligase catalytic (GCLC) and modulatory (GCLM) subunits, and by recycling from oxidized GSH, catalysed by glutathione reductase (GR). To assess whether follicle-stimulating hormone (FSH) and basic fibroblast growth factor (bFGF) modulate GSH production in Sertoli cells by regulating the expression of GCLC, GCLM and/or GR, we performed in vitro studies using rat Sertoli cells in primary culture. FSH and bFGF stimulation increased Sertoli cell GSH levels after 24 h incubation. The simultaneous addition of FSH and bFGF did not produce any further effect. GCLM expression was upregulated by FSH and bFGF 6 h. At 24 h, only the FSH-mediated effect was still observed. FSH and bFGF also upregulated GR expression. In conclusion, our results show that FSH and bFGF increase GSH levels in Sertoli cells through stimulation of the de novo synthesis and recycling by upregulating GCLM and GR expression respectively. Therefore, protection of germ cells against oxidative stress seems to be regulated by hormones and germ cell-released growth factors capable of influencing the production of Sertoli cell GSH.

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.

Glutamate Cysteine Ligase Modifier Subunit Deficiency and Gender as Determinants of Acetaminophen-Induced Hepatotoxicity in Mice

The analgesic and antipyretic drug acetaminophen (APAP) is bioactivated to the reactive intermediate N-acetyl-p-benzoquinoneimine, which is scavenged by glutathione (GSH). APAP overdose can deplete GSH leading to the accumulation of APAP-protein adducts and centrilobular necrosis in the liver. N-acetylcysteine (NAC), a cysteine prodrug and GSH precursor, is often given as a treatment for APAP overdose. The rate-limiting step in GSH biosynthesis is catalyzed by glutamate cysteine ligase (GCL) a heterodimer composed of catalytic and modifier (GCLM) subunits. Previous studies have indicated that GCL activity is likely to be an important determinant of APAP toxicity. In this study, we investigated APAP toxicity, and NAC or GSH ethyl ester (GSHee)-mediated rescue in mice with normal or compromised GCLM expression. Gclm wild-type, heterozygous, and null mice were administered APAP (500 mg/kg) alone, or immediately following NAC (800 mg/kg) or GSHee (168 mg/kg), and assessed for hepatotoxicity 6 h later. APAP caused GSH depletion in all mice. Gclm null and heterozygous mice exhibited more extensive hepatic damage compared to wild-type mice as assessed by serum alanine aminotransferase activity and histopathology. Additionally, male Gclm wild-type mice demonstrated greater APAP-induced hepatotoxicity than female wild-type mice. Cotreatment with either NAC or GSHee mitigated the effects of APAP in Gclm wild-type and heterozygous mice, but not in Gclm null mice. Collectively, these data reassert the importance of GSH in protection against APAP-induced hepatotoxicity, and indicate critical roles for GCL activity and gender in APAP-induced liver damage in mice.

cDNA cloning and expression analysis of the catalytic subunit of glutamate cysteine ligase gene in an annelid polychaete after cadmium exposure: a potential tool for pollution biomonitoring

The enzyme glutamate cysteine ligase (GCL) is the rate-limiting enzyme in glutathione (GSH) synthesis and is formed by a catalytic (GCLC) and a modulatory subunit (GCLM). Some studies have demonstrated that environmental pollutants can regulate the expression of these subunits. Despite the importance of these genes in toxicological responses, no sequences are available for the GCL subunits in annelids. The present study reports, for the first time, the cDNA sequence for the GCLC in an annelid species, the polychaete Laeonereis acuta (Nereididae). The deduced amino acid sequence of L. acuta GCLC showed homology with other animal species, and was used to infer a phylogenetic tree with GCLC amino acid sequences from other taxonomic groups. Exposure to cadmium (100 and 1000 microg Cd/L) during 14 days augmented the level of L. acuta GCLC transcripts in a dose-dependent manner. These gene expression results can be related to the known cadmium effect on GSH depletion. Since a number of contaminants can also exert their toxicity through this mechanism, GCLC gene expression might be applied not only for cadmium biomonitoring, but also for a wide range of contaminants that directly or indirectly promote the same effect in the cellular GSH content.

Acetaminophen-induced liver injury is attenuated in male glutamate-cysteine ligase transgenic mice

Acetaminophen overdose is a leading cause of drug-related acute liver failure in the United States. Glutathione, a tripeptide antioxidant protects cells against oxidative damage from reactive oxygen species and plays a crucial role in the detoxification of xenobiotics, including acetaminophen. Glutathione is synthesized in a two-step enzymatic reaction. Glutamate-cysteine ligase carries out the rate-limiting and first step in glutathione synthesis. We have generated C57Bl/6 mice that conditionally overexpress glutamate-cysteine ligase, and report here their resistance to acetaminophen-induced liver injury. Indices of liver injury included histopathology and serum alanine aminotransferase activity. Male transgenic mice induced to overexpress glutamate-cysteine ligase exhibited resistance to acetaminophen-induced liver injury when compared with acetaminophen-treated male mice carrying, but not expressing glutamate-cysteine ligase transgenes, or to female glutamate-cysteine ligase transgenic mice. We conclude that glutamate-cysteine ligase activity is an important factor in determining acetaminophen-induced liver injury in C57Bl/6 male mice. Because people are known to vary in their glutamate-cysteine ligase activity, this enzyme may also be an important determinant of sensitivity to acetaminophen-induced liver injury in humans.

Glutathione intensifies gliotoxin-induced cytotoxicity in human neuroblastoma SH-SY5Y cells

Gliotoxin is a fungal second metabolite produced by diverse species that can be found in compost, stored crops, moist animal feed and sawdust. The role of glutathione in gliotoxin-induced toxicity was studied in order to elucidate the toxic mechanisms leading to neurite degeneration and cell death in differentiated human neuroblastoma (SH-SY5Y) cells. After 72 h of exposure to gliotoxin, moderate cytotoxicity was induced at 0.1 micromol/L, which was more severe at higher concentrations. A reduction in the number of neurites per cell was also observed. By decreasing the level of intracellular glutathione with L: -buthionine-sulfoxamine (BSO) a specific inhibitor of glutathione synthesis, the cytotoxic effect of gliotoxin was significantly attenuated. The gliotoxin-induced cytotoxicity was also slightly reduced by the antioxidant vitamin C. However, the neurite degenerative effect was not altered by BSO, or by vitamin C. A concentration-dependent increase in the ratio between oxidized and reduced forms of glutathione, as well as the total intracellular glutathione levels, was noted after exposure to gliotoxin. The increase of glutathione was also reflected in western blot analyses showing a tendency for the regulatory subunit of gamma-glutamylcysteine synthetase to be upregulated. In addition, the activity of glutathione reductase was slightly increased in gliotoxin-exposed cells. These results indicate that glutathione promotes gliotoxin-induced cytotoxicity, probably by reducing the ETP (epipolythiodioxopiperazine) disulfide bridge to the dithiol form.

Minimal ovarian upregulation of glutamate cysteine ligase expression in response to suppression of glutathione by buthionine sulfoximine

The antioxidant tripeptide glutathione (GSH) protects ovarian follicles against oxidative damage that may lead to apoptotic death. The rate-limiting step in synthesis of GSH is catalyzed by glutamate cysteine ligase (GCL), a heterodimer composed of a catalytic subunit (GCLC), and a modifier subunit (GCLM). We hypothesized that GSH depletion in vivo or in vitro with buthionine sulfoximine (BSO), a specific inhibitor of GCL activity, would increase ovarian and granulosa cell GCL subunit expression. Ovarian glutathione levels are lowest on proestrous morning and increase to their highest levels on estrus and metestrus. Therefore, we treated rats on proestrous morning or on proestrous morning and again 12h later to prevent the normal increase in ovarian glutathione between proestrus and estrus. Ovarian Gclc and Gclm mRNA levels and GCLC protein levels increased transiently by 1.4-1.5-fold at 8 h, but not at 12 or 24 h, after a single dose of BSO administered to adult rats on the morning of proestrus. GCLC protein levels were also modestly increased 1.4-fold at 12 h after a second dose of BSO. GCLM protein levels increased 1.4-fold at 24 h after a single dose of BSO, but not at other time points. BSO treatment did not significantly alter ovarian GCL enzymatic activity or the intraovarian localization of either GCL subunit mRNA. Treatment of a human granulosa cell line or primary rat granulosa cells with BSO suppressed intracellular GSH; however, there was no compensatory upregulation of GCL subunit protein or mRNA levels. These results demonstrate that ovarian follicles and granulosa cells are minimally able to respond to acute GSH depletion by upregulating expression of GCL.

Gonadotropin regulation of glutamate cysteine ligase catalytic and modifier subunit expression in rat ovary is subunit and follicle stage specific

We have observed that levels of the antioxidant glutathione (GSH) and protein levels of the catalytic and modifier subunits of the rate-limiting enzyme in GSH synthesis, GCLc and GCLm, increase in immature rat ovaries after treatment with gonadotropin. The goals of the present studies were to delineate the time course and intraovarian localization of changes in GSH and GCL after pregnant mare's serum gonadotropin (PMSG) and after an ovulatory gonadotropin stimulus. Twenty-four hours after PMSG, there was a shift from predominantly granulosa cell expression of gclm mRNA, and to a lesser extent gclc, to predominantly theca cell expression. GCLc immunostaining increased in granulosa and theca cells and in interstitial cells. Next, prepubertal female rats were primed with PMSG, followed 48 h later by 10 IU of hCG. GCLm protein and mRNA levels increased dramatically from 0 to 4 h after hCG and then declined rapidly. There was minimal change in GCLc. The increase in gclm mRNA expression was localized mainly to granulosa and theca cells of preovulatory follicles. To verify that GCL responds similarly to an endogenous preovulatory gonadotropin surge, we quantified ovarian GCL mRNA levels during the periovulatory period in adult rats. gclm mRNA levels increased after the gonadotropin surge on proestrus and then declined rapidly. Finally, we assessed the effects of gonadotropin on ovarian GCL enzymatic activity. GCL enzymatic activity increased significantly at 48 h after PMSG injection and did not increase further after hCG. These results demonstrate that gonadotropins regulate follicular GCL expression in a follicle stage-dependent manner and in a GCL subunit-dependent manner.

Effects of Methylmercury on Primary Brain Cells in Mono- and Co-culture

We report on the uptake of MeHg in astrocytes and neurons, as well as specific indicators of neurotoxicity. Cerebellar granule neurons and astrocytes separately and in co-culture were cultured in the presence of MeHg and changes in 3-[4, 5-dimethylthiazol-2-yl]-2, 5 diphenyltetrazolium bromide (MTT)-reduction, lactate dehydrogenase (LDH) leakage, and cellular content of glutathione and amino acids were used as indicators of MeHg toxicity. Mitochondria in cortical astrocytes were slightly more sensitive than those in cerebellar astrocytes to the toxic effects of MeHg; furthermore, cellular integrity was better preserved in cerebellar astrocytes. When neurons and astrocytes from cerebellum were incubated in separable co-cultures using inserts, the astrocytes showed cellular damage at lower exposure to MeHg while neurons showed less changes compared to respective cell types in mono-cultures. Mercury uptake studies at 25 microM MeHg (10% serum present) showed that for neurons in co-culture the uptake was 1/3 compared to mono-cultures. In contrast, for astrocytes in co-culture, uptake was increased by 75%. A MeHg concentration-dependent increase of glutamate content in mono-cultures was noted. When MeHg concentration was increased to 10, 25, or 50 microM, neurons in co-cultures decreased their glutamate content, whereas astrocytes showed an increase. Other amino acids, such as glutamine, serine, valine, isoleucine, taurine, and phenylalanine were unaffected by MeHg. Glutathione content showed MeHg concentration-dependent changes in astrocytes and was increased in neurons in co-culture incubated with 5 microM MeHg. In conclusion, astrocytes appear to increase neuronal resistance, indicating a possible protective role for astrocytes in MeHg neurotoxicity.

Effect of methylmercury on glutamate-cysteine ligase expression in the placenta and yolk sac during mouse development

The placenta and the yolk sac play critical roles in fetal development, including protection from oxidative stress through the presence of detoxifying enzymes. Glutathione (GSH; gamma-glutamylcysteinylglycine), a crucial molecule in the maintenance of cellular redox status, plays a critical role in development, and it is also protective against methylmercury toxicity. Glutamate-cysteine ligase (GCL), the enzyme that catalyzes the rate-limiting step in GSH synthesis, is widely expressed in the mouse embryo and extraembryonic membranes throughout development. The aim of this study was to investigate the effect of low-level subchronic methylmercury exposure on GCL expression in the mouse placenta and yolk sac, after describing the basal developmental expression of the enzyme in these tissues. We found that basal mRNA expression levels increased dramatically in the placenta and the yolk sac at gd 18, whereas protein levels did not increase in parallel with the mRNA. We also found that methylmercury induced GCLc mRNA expression in the placenta at gd 18 in a dose-dependent manner, suggesting an important role for this enzyme in the response of the placenta to toxicants. These changes in expression may be useful as a biomarker of MeHg exposure during development.

Activation of transcription factor Nrf-2 and its downstream targets in response to moloney murine leukemia virus ts1-induced thiol depletion and oxidative stress in astrocytes

The neuroimmunodegenerative syndrome that develops in mice infected with ts1, a mutant of Moloney murine leukemia virus, resembles human AIDS. Both ts1 and human immunodeficiency virus type 1 infect astrocytes, microglia, and oligodendrocytes but do not infect neurons. Oxidative stress has been implicated in the neuropathology of AIDS dementia and other neurodegenerative diseases. We report here that ts1 infection of astrocytes (both transformed C1 cells and primary cultures) also induces thiol (i.e., glutathione and cysteine) depletion and reactive oxygen species (ROS) accumulation, events occurring in parallel with viral envelope precursor gPr80(env) accumulation and upregulated expression of endoplasmic reticulum chaperones GRP78 and GRP94. Furthermore, ts1-infected astrocytes mobilize their thiol redox defenses by upregulating levels of the Nrf-2 transcription factor, as well its targets, the xCT cystine/glutamate antiporter, gamma-glutamylcysteine ligase, and glutathione peroxidase. Depleting intracellular thiols by treating uninfected astrocytes with buthionine sulfoximine (BSO), a glutathione synthesis inhibitor, or by culturing in cystine-deficient medium, also induces ROS accumulation, activates Nrf-2, and upregulates Nrf-2 target gene expression in these astrocytes. Overexpression of Nrf-2 in astrocytes specifically increases expression of the above thiol synthesis-related proteins. Further treatment with BSO or N-acetylcysteine in transfected cells modulates this expression. Thiol depletion also accelerates cell death, while thiol supplementation promotes survival of ts1-infected cells. Together, our results indicate that ts1 infection of astrocytes, along with ts1-induced gPr80(env) accumulation, endoplasmic reticulum stress, thiol depletion, and oxidative stress, accelerates cell death; in response to the thiol depletion and oxidative stress, astrocytes activate their Nrf-2-mediated thiol antioxidant defenses, promoting cell survival.

Glutamate-cysteine ligase attenuates TNF-induced mitochondrial injury and apoptosis

Glutathione (GSH) is important in free radical scavenging, maintaining cellular redox status, and regulating cell survival in response to a wide variety of toxicants. The rate-limiting enzyme in GSH synthesis is glutamate-cysteine ligase (GCL), which is composed of catalytic (GCLC) and modifier (GCLM) subunits. To determine whether increased GSH biosynthetic capacity enhances cellular resistance to tumor necrosis factor-alpha- (TNF-alpha-) induced apoptotic cell death, we have established several mouse liver hepatoma (Hepa-1) cell lines overexpressing GCLC and/or GCLM. Cells overexpressing GCLC alone exhibit modest increases in GCL activity, while cells overexpressing both subunits have large increases in GCL activity. Importantly, cells overexpressing both GCL subunits exhibit increased resistance to TNF-induced apoptosis as judged by a loss of redox potential; mitochondrial membrane potential; translocation of cytochrome c to the cytoplasm; and activation of caspase-3, caspase-8, and caspase-9. Analysis of the effects of TNF on these parameters indicates that maintaining mitochondrial integrity mediates this protective effect in GCL-overexpressing cells.

Regulation of endothelial glutathione by ICAM‐1: implications for inflammation

The role of glutathione (GSH) in inflammation is largely discussed from the context of providing reducing equivalents to detoxify reactive oxygen and nitrogen species. Inflammation is now recognized to be an underlying cause of many vascular diseases including atherosclerosis, a disease in which endothelial GSH concentrations are decreased. However, mechanisms that control GSH levels are poorly understood. Key players in the inflammatory process are endothelial adhesion molecules, including intercellular adhesion molecule-1 (ICAM-1). This adhesion molecule is present constitutively and can be induced by a variety of inflammatory stimuli. In this study, using mouse aortic endothelial cells (MAEC) deficient in ICAM-1, we demonstrate a novel interplay between constitutive ICAM-1 and cellular GSH. Deficiency of ICAM-1 was associated with an approximately twofold increase in total GSH content. Inhibiting glutamate-cysteine ligase (GCL), the enzyme that catalyses the rate-limiting step in GSH biosynthesis, prevented the increase in GSH. In addition, the catalytic subunit of GCL was increased (approximately 1.6-fold) in ICAM-1 deficient relative to wild-type cells, suggesting that constitutive ICAM-1 represses GCL expression. Furthermore, the ratio of reduced (GSH) to oxidized (GSSG) glutathione was also increased suggesting a role for ICAM-1 in modulating cellular redox status. Interestingly, increasing cytosolic GSH in wild-type mouse endothelial cells decreased constitutive ICAM-1, suggesting the presence of an inverse and reciprocal pathway. To test the effects of inducible ICAM-1 on GSH, cells were stimulated with the proinflammatory cytokine TNF-alpha. TNF-alpha stimulated production of ICAM-1, which was however not associated with induction of GSH. In contrast, supplementation of endothelial cells with GSH before TNF-alpha addition, inhibited induction of ICAM-1. These data suggest a novel regulatory pathway between constitutive ICAM-1 and GSH synthesis in the endothelium and are discussed in the context of modulating the inflammatory response.

Brain antioxidant systems in human methamphetamine users

Animal data suggest that the widely abused psychostimulant methamphetamine can damage brain dopamine neurones by causing dopamine-dependent oxidative stress; however, the relevance to human methamphetamine users is unclear. We measured levels of key antioxidant defences [reduced (GSH) and oxidized (GSSG) glutathione, six major GSH system enzymes, copper-zinc superoxide dismutase (CuZnSOD), uric acid] that are often altered after exposure to oxidative stress, in autopsied brain of human methamphetamine users and matched controls. Changes in the total (n = 20) methamphetamine group were limited to the dopamine-rich caudate (the striatal subdivision with the most severe dopamine loss) in which only activity of CuZnSOD (+ 14%) and GSSG levels (+ 58%) were changed. In the six methamphetamine users with severe (- 72 to - 97%) caudate dopamine loss, caudate CuZnSOD activity (+ 20%) and uric acid levels (+ 63%) were increased with a trend for decreased (- 35%) GSH concentration. Our data suggest that brain levels of many antioxidant systems are preserved in methamphetamine users and that GSH depletion, commonly observed during severe oxidative stress, might occur only with severe dopamine loss. Increased CuZnSOD and uric acid might reflect compensatory responses to oxidative stress. Future studies are necessary to establish whether these changes are associated with oxidative brain damage in human methamphetamine users.

Antioxidant defenses in killifish (Fundulus heteroclitus) exposed to contaminated sediments and model prooxidants: short-term and heritable responses

A population of killifish (Fundulus heteroclitus) inhabiting a Superfund site on the Elizabeth River (VA, USA) is tolerant of the acute toxicity of the sediments from the site; previous work suggests that this tolerance is based both on genetic adaptation and physiological acclimation. In this study, larval first- and second-generation (F1 and F2) offspring of Elizabeth River killifish were more resistant to the toxicity of t-butyl hydroperoxide (a model prooxidant) than were King's Creek (reference site) offspring, indicating a heritable tolerance of exposure to oxidative stress. In laboratory experiments designed to elucidate the mechanistic basis for this increased tolerance, we exposed laboratory-raised F1 and F2 offspring from Elizabeth River and King's Creek killifish to Elizabeth River sediments, menadione, or t-butyl hydroperoxide, and measured the following antioxidant parameters: total oxyradical scavenging capacity (TOSC); glutathione content (total and disulfide); activities of glutathione reductase (GR); glutathione peroxidase (GPx); and glutamate cysteine ligase (GCL) activities and protein levels of copper-zinc superoxide dismutase (CuZnSOD); and protein levels of manganese superoxide dismutase (MnSOD). Exposure to Elizabeth River sediments lead to consistent increases in total glutathione concentrations, GR activities, and MnSOD protein levels, and in some cases increased GPx and GCL activities, in both populations. In addition, Elizabeth River offspring (larvae) showed higher basal TOSC values, glutathione concentrations, and MnSOD protein levels. These data suggest that upregulated antioxidant defenses play a role in both short-term (physiological) and heritable (multigenerational/evolutionary) tolerance of the toxicity of these Superfund sediments. The responses of specific antioxidant parameters, including sex-specific responses in the cases of glutathione concentrations and GR activity, are also discussed.

Antioxidant perturbations in the olfactory mucosa of alachlor-treated rats

The chloracetanilide herbicide alachlor (2-chloro-2',6'-diethyl-N-(methoxymethyl)acetanilide) induces olfactory mucosal tumors in rats following chronic dietary exposure. Previous reports demonstrated that alachlor exposure was associated with depletion of glutathione (GSH) in liver in vivo and in vitro, but did not address this issue in the target tissue for the carcinogenic response. In this study we investigated a potential oxidative stress pathway in olfactory tissue by examining perturbations in olfactory mucosal antioxidants. Male Long-Evans rats were fed alachlor for up to 10 days (10-126 mg/kg per day), and intracellular reduced GSH and ascorbate levels were measured in olfactory mucosa. Both GSH and ascorbate rapidly decreased in olfactory mucosa following alachlor exposure, with a subsequent increase in both antioxidants to approximately 160% of control levels in the high dose group, and recovery of GSH to control levels in all groups by 10 days. Using Western blot analysis, we found that the modifier subunit of the rate-limiting enzyme in GSH synthesis, glutamate-cysteine ligase, increased in olfactory mucosa and remained elevated (126 mg/kg per day group). Two ascorbate transporters were detected by RT-PCR in olfactory mucosa, but neither appeared to be upregulated by alachlor exposure, and ascorbate synthesis was not stimulated in olfactory mucosa by alachlor treatment. Dietary exposure to alachlor depletes olfactory mucosa antioxidants, which may contribute to DNA damage and tissue-specific tumor formation.

TGFbeta1-induced suppression of glutathione antioxidant defenses in hepatocytes: caspase-dependent post-translational and caspase-independent transcriptional regulatory mechanisms

TGFbeta1-induced hepatocyte apoptosis involves the production of reactive oxygen species. An effective cellular defense mechanism against oxidative stress is the tripeptide glutathione (GSH), and the rate-limiting step in GSH biosynthesis is catalyzed by the heterodimeric holoenzyme glutamate cysteine ligase (GCL). Here, we demonstrate that TGFbeta1-induced apoptosis in the TAMH murine hepatocyte cell line is accompanied by both the cleavage and loss of the catalytic subunit of GCL (GCLC) and the down-regulation of GCLC gene expression resulting in a reduction in GCL activity and depletion of intracellular GSH. TGFbeta1-induced apoptosis is also accompanied by a reduction in Bcl-XL, an effect that may facilitate TGFbeta1-induced apoptosis as Bcl-XL overexpression inhibits TGFbeta1-induced caspase activation and cell death. Interestingly, Bcl-XL overexpression prevents TGFbeta1-induced cleavage of GCLC protein but not down-regulation of GCLC mRNA. Furthermore, TGFbeta1-induced down-regulation of GCLC mRNA is prevented by inhibition of histone deacetylase activity, suggesting that this is an active repression of GCLC gene transcription. These findings suggest that the suppression of GSH antioxidant defenses associated with the caspase-dependent cleavage of GCLC protein, caspase-independent suppression of GCLC gene expression, and depletion of intracellular GSH may play a role in enhancing TGFbeta1-induced oxidative stress and potentiating apoptotic cell death.

Fluorescence-based microtiter plate assay for glutamate-cysteine ligase activity

Glutamate-cysteine ligase (GCL; also known as gamma-glutamylcysteine synthetase) is the rate-limiting enzyme in glutathione (GSH) synthesis. Traditional assays for the activity of this enzyme are based either on coupled reactions with other enzymes or on high-performance liquid chromatography (HPLC) assessment of gamma-glutamylcysteine (gamma-GC) product formation. We took advantage of the reaction of naphthalene dicarboxaldehyde (NDA) with GSH or gamma-GC to form cyclized products that are highly fluorescent. Hepa-1 cells which were designed to overexpress mouse GCL and mouse liver homogenates were used to evaluate and compare the utility of the NDA method with an assay based on monobromobimane derivatization and HPLC analysis with fluorescence detection. Excellent agreement was found between GCL activities measured by HPLC and NDA-microtiter plate analyses. This assay should be useful for high-throughput GCL activity analyses.

Neurotoxicity of organomercurial compounds

Mercury is a ubiquitous contaminant, and a range of chemical species is generated by human activity and natural environmental change. Elemental mercury and its inorganic and organic compounds have different toxic properties, but all them are considered hazardous in human exposure. In an equimolecular exposure basis, organomercurials with a short aliphatic chain are the most harmful compounds and they may cause irreversible damage to the nervous system. Methylmercury (CH(3)Hg(+)) is the most studied following the neurotoxic outbreaks identified as Minamata disease and the Iraq poisoning. The first description of the CNS pathology dates from 1954. Since then, the clinical neurology, the neuropathology and the mechanisms of neurotoxicity of organomercurials have been widely studied. The high thiol reactivity of CH(3)Hg(+), as well as all mercury compounds, has been suggested to be the basis of their harmful biological effects. However, there is clear selectivity of CH(3)Hg(+) for specific cell types and brain structures, which is not yet fully understood. The main mechanisms involved are inhibition of protein synthesis, microtubule disruption, increase of intracellular Ca(2+) with disturbance of neurotransmitter function, oxidative stress and triggering of excitotoxicity mechanisms. The effects are more damaging during CNS development, leading to alterations of the structure and functionality of the nervous system. The major source of CH(3)Hg(+) exposure is the consumption of fish and, therefore, its intake is practically unavoidable. The present concern is on the study of the effects of low level exposure to CH(3)Hg(+) on human neurodevelopment, with a view to establishing a safe daily intake. Recommendations are 0.4 micro g/kg body weight/day by the WHO and US FDA and, recently, 0.1 micro g/kg body weight/day by the US EPA. Unfortunately, these levels are easily attained with few meals of fish per week, depending on the source of the fish and its position in the food chain.

Cytochrome P450 2E1 responsiveness in the promoter of glutamate-cysteine ligase catalytic subunit

Previous studies have shown cytochrome P450 2E1 (CYP2E1)-dependent transcriptional up-regulation of glutamate-cysteine ligase (GCL). To identify sequences mediating constitutive and induced expression of the catalytic subunit of GCL (GCLC), a series of deletion mutants from the 5'-flanking region (-3,802 to +465) were transfected into control (C34) and CYP2E1-overexpressing (E47) HepG2 cells. Increased luciferase expression, both basal (2- to 3-fold) and following exposure to ethanol, arachidonic acid (AA), or AA plus iron, was detected in E47 cells with the full-length but not shorter reporter vectors. Basal induction was blocked by CYP2E1 inhibitors and catalase. Basal and inducible luciferase expression in E47 cells was blunted by the full-length construct mutated in the ARE4 site. Catalase and diallyl sulfide prevented basal and AA-induced messenger RNA (mRNA) levels of GCLC and the modulatory subunit of GCL (GCLM). Preincubation with low doses of AA increased glutathione (GSH) levels as well as GCLC and GCLM mRNAs, and this protected against H(2)O(2) and menadione toxicity. Primary hepatocytes from pyrazole-injected rats with high levels of CYP2E1 showed an increase in GSH levels as well as GCLC and GCLM mRNAs compared with saline controls, and this was prevented by diallyl sulfide. In conclusion, redox-sensitive elements directing constitutive and induced expression of the GCLC in CYP2E1-expressing cells are present in the ARE4 distal portion of the 5'-flanking region, between positions -3,802 and -2,752, perhaps a reflection of metabolic adaptation to CYP2E1-generated oxidative stress.

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.

Expression of glutamate-cysteine ligase during mouse development

The tripeptide glutathione (GSH), which plays a crucial role in protecting cells against oxidative stress, is synthesized in a two-step process. The rate-limiting step is the binding of glutamate and cysteine, which is catalyzed by the enzyme glutamate-cysteine ligase (GCL). This enzyme is composed of two subunits: a large catalytic subunit (GCLc) and a smaller modifying subunit (GCLm), originating from different genes. Control of cellular GSH levels is essential for normal development. In the current study, we investigated the tissue distribution of Gclc and Gclm transcripts, as well as GCLc protein, in the developing mouse embryo. We found that both mRNAs were highly expressed in the liver and CNS at gestational day 10 (gd 10) and gd 12, with Gclm being more abundant than Gclc in the liver relative to other tissues. Also, the expression of the two subunit mRNAs was not always parallel in the embryo, in that some tissues expressed one of the subunits preferentially, suggesting that the two genes are differentially expressed during mouse development. The GCLc protein was also widely expressed throughout the embryo, and, in general, it co-localized with the Gclc mRNA.

Organic mercury compounds: human exposure and its relevance to public health

Humans may be exposed to organic forms of mercury by either inhalation, oral, or dermal routes, and the effects of such exposure depend upon both the type of mercury to which exposed and the magnitude of the exposure. In general, the effects of exposure to organic mercury are primarily neurologic, while a host of other organ systems may also be involved, including gastrointestinal, respiratory, hepatic, immune, dermal, and renal. While the primary source of exposure to organic mercury for most populations is the consumption of methylmercury-contaminated fish and shellfish, there are a number of other organomercurials to which humans might be exposed. The antibacterial and antifungal properties of organomercurials have resulted in their long use as topical disinfectants (thimerosal and merbromin) and preservatives in medical preparations (thimerosal) and grain products (both methyl and ethyl mercurials). Phenylmercury has been used in the past in paints, and dialkyl mercurials are still used in some industrial processes and in the calibration of certain analytical laboratory equipment. The effects of exposure to different organic mercurials by different routes of exposure are summarized in this article.

Gonadotropin regulation of glutathione synthesis in the rat ovary

Glutathione (GSH), an antioxidant and conjugator of electrophilic toxicants, prevents toxicant-mediated destruction of ovarian follicles and oocytes. Ovarian GSH has previously been shown to change with estrous cycle stage in rats, suggesting that the gonadotropin hormones may regulate ovarian GSH synthesis. The present studies tested the hypotheses that [1] estrous cycle-related changes in ovarian GSH result from cyclic changes in protein and mRNA expression of the rate-limiting enzyme in GSH synthesis, glutamate cysteine ligase (GCL, also called gamma-glutamylcysteine synthetase), and [2] that these changes result from gonadotropin-mediated regulation of GCL subunit expression. In the first experiment, ovaries were harvested from cycling adult female rats on each stage of the estrous cycle. In the second experiment immature female rats were injected with pregnant mare's serum gonadotropin (PMSG) to stimulate follicular development or with vehicle and killed 8, 24, or 48 h later. In both experiments the ovaries were harvested for [1] total GSH assay, [2] Western analysis for GCL catalytic (GCLc) and regulatory (GCLm) subunit protein levels, or [3] Northern analysis for Gclc and Gclm mRNA levels. Ovarian GSH concentrations and Gclc and Gclm mRNA levels, but not GCL subunit protein levels, varied significantly with estrous cycle stage. PMSG administration significantly increased ovarian GSH concentrations 24 and 48 h later. GCLm protein levels increased significantly at 24 h and 48 h following PMSG. GCLc protein levels did not increase significantly following PMSG. Gcl subunit mRNA levels were not significantly increased at any time point by the planned ANOVA; however, an increase in Gelc at 48 h was identified by t-testing. These results support the hypothesis that gonadotropins regulate ovarian GSH synthesis by modulating GCL subunit expression.

Tissue specific changes in the expression of glutamate-cysteine ligase mRNAs in mice exposed to methylmercury

Glutamate-cysteine ligase (GLCL), the rate-limiting enzyme in glutathione (GSH) synthesis is composed of two subunits, a catalytic (GLCLc) and a regulatory subunit (GLCLr). These two subunits are known to be differentially regulated in vitro, in different cell types and in response to various xenobiotic exposures. In this study, we examined whether these two subunits can also be differentially regulated in vivo. We found that GLCLc and GLCLr are differentially regulated at the transcriptional level in a tissue-dependent manner in female mice treated with methylmercury (MeHg). MeHg caused a downregulation of both subunit mRNAs in the liver, upregulation of both subunit mRNAs in the kidney and upregulation of only the catalytic subunit mRNA in the small intestine of female mice treated with a single dose of MeHg (6 mg/kg) by intraperitoneal injection. These results suggest that GLCLc and GLCLr can be differentially regulated in vivo, and that this regulation is tissue dependent in the mouse.