Up-regulation of glutathione synthesis in rat kidney by methyl mercury. Relationship to mercury-induced oxidative stress

Effects of low doses of methylmercury (MeHg) exposure on definitive endoderm cell differentiation in human embryonic stem cells

Fetal development is one of the most sensitive windows to methylmercury (MeHg) toxicity. Laboratory and epidemiological studies have shown a dose-response relationship between fetal MeHg exposure and neuro performance in different life stages from infants to adults. In addition, MeHg exposure has been reported to be associated with disorders in endoderm-derived organs, such as morphological changes in liver cells and pancreatic cell dysfunctions. However, the mechanisms of the effects of MeHg on non-neuronal organs or systems, especially during the early development of endoderm-derived organs, remain unclear. Here we determined the effects of low concentrations of MeHg exposure during the differentiation of definitive endoderm (DE) cells from human embryonic stem cells (hESCs). hESCs were exposed to MeHg (0, 10, 100, and 200 nM) that covers the range of Hg concentrations typically found in human maternal blood during DE cell induction. Transcriptomic analysis showed that sub-lethal doses of MeHg exposure could alter global gene expression patterns during hESC to DE cell differentiation, leading to increased expression of endodermal genes/proteins and the over-promotion of endodermal fate, mainly through disrupting calcium homeostasis and generating ROS. Bioinformatic analysis results suggested that MeHg exerts its developmental toxicity mainly by disrupting ribosome biogenesis during early cell lineage differentiation. This disruption could lead to aberrant growth or dysfunctions of the developing endoderm-derived organs, and it may be the underlying mechanism for the observed congenital diseases later in life. Based on the results, we proposed an adverse outcome pathway for the effects of MeHg exposure during human embryonic stem cells to definitive endoderm differentiation.

Mitochondrial Oxidative Stress Is the General Reason for Apoptosis Induced by Different-Valence Heavy Metals in Cells and Mitochondria

This review analyzes the causes and consequences of apoptosis resulting from oxidative stress that occurs in mitochondria and cells exposed to the toxic effects of different-valence heavy metals (Ag+, Tl+, Hg2+, Cd2+, Pb2+, Al3+, Ga3+, In3+, As3+, Sb3+, Cr6+, and U6+). The problems of the relationship between the integration of these toxic metals into molecular mechanisms with the subsequent development of pathophysiological processes and the appearance of diseases caused by the accumulation of these metals in the body are also addressed in this review. Such apoptosis is characterized by a reduction in cell viability, the activation of caspase-3 and caspase-9, the expression of pro-apoptotic genes (Bax and Bcl-2), and the activation of protein kinases (ERK, JNK, p53, and p38) by mitogens. Moreover, the oxidative stress manifests as the mitochondrial permeability transition pore (MPTP) opening, mitochondrial swelling, an increase in the production of reactive oxygen species (ROS) and H2O2, lipid peroxidation, cytochrome c release, a decline in the inner mitochondrial membrane potential (ΔΨmito), a decrease in ATP synthesis, and reduced glutathione and oxygen consumption as well as cytoplasm and matrix calcium overload due to Ca2+ release from the endoplasmic reticulum (ER). The apoptosis and respiratory dysfunction induced by these metals are discussed regarding their interaction with cellular and mitochondrial thiol groups and Fe2+ metabolism disturbance. Similarities and differences in the toxic effects of Tl+ from those of other heavy metals under review are discussed. Similarities may be due to the increase in the cytoplasmic calcium concentration induced by Tl+ and these metals. One difference discussed is the failure to decrease Tl+ toxicity through metallothionein-dependent mechanisms. Another difference could be the decrease in reduced glutathione in the matrix due to the reversible oxidation of Tl+ to Tl3+ near the centers of ROS generation in the respiratory chain. The latter may explain why thallium toxicity to humans turned out to be higher than the toxicity of mercury, lead, cadmium, copper, and zinc.

Cellular Compartmentalization, Glutathione Transport and Its Relevance in Some Pathologies

Reduced glutathione (GSH) is the most abundant non-protein endogenous thiol. It is a ubiquitous molecule produced in most organs, but its synthesis is predominantly in the liver, the tissue in charge of storing and distributing it. GSH is involved in the detoxification of free radicals, peroxides and xenobiotics (drugs, pollutants, carcinogens, etc.), protects biological membranes from lipid peroxidation, and is an important regulator of cell homeostasis, since it participates in signaling redox, regulation of the synthesis and degradation of proteins (S-glutathionylation), signal transduction, various apoptotic processes, gene expression, cell proliferation, DNA and RNA synthesis, etc. GSH transport is a vital step in cellular homeostasis supported by the liver through providing extrahepatic organs (such as the kidney, lung, intestine, and brain, among others) with the said antioxidant. The wide range of functions within the cell in which glutathione is involved shows that glutathione’s role in cellular homeostasis goes beyond being a simple antioxidant agent; therefore, the importance of this tripeptide needs to be reassessed from a broader metabolic perspective.

Protective Effect of Natural Antioxidant Compounds on Methimazole Induced Oxidative Stress in a Feline Kidney Epithelial Cell Line (CRFK)

The treatment of choice for feline hyperthyroidism is the administration of the antithyroid drug methimazole. Both the endocrinopathy and the drug adverse reactions (e.g., hepatotoxicosis, gastrointestinal disorders, and renal injury) are partly due to oxidative stress and redox unbalance. This study investigated the free radical production and the impairment of the antioxidant barrier induced by methimazole in an in vitro model of feline renal epithelium. The protective effects of quercetin and resveratrol were also explored. CRFK cells were incubated with a methimazole concentration equivalent to the maximum plasma levels in orally treated cats (4 µM), in the presence or absence of either one of the two selected antioxidants at different time-points (up to 72 h). Cell viability, ROS production, GSH levels, and mRNA expression of antioxidant enzymes (i.e., CAT, SOD, GPx, and GST) were assessed. Methimazole impaired cell viability and increased ROS levels in a time-dependent manner. Similarly, GSH content and CAT, SOD, and GPx3 expression were higher compared with control cells. Such effects were significantly counteracted by quercetin. These results provide new insights about the mechanisms underlying the methimazole-related side effects frequently observed in hyperthyroid cats. They also support the use of quercetin in the management of feline hyperthyroidism.

Stimulation of de novo glutathione synthesis by nitrofurantoin for enhanced resilience of hepatocytes

Toxicity is not only a function of damage mechanisms, but is also determined by cellular resilience factors. Glutathione has been reported as essential element to counteract negative influences. The present work hence pursued the question how intracellular glutathione can be elevated transiently to render cells more resistant toward harmful conditions. The antibiotic nitrofurantoin (NFT) was identified to stimulate de novo synthesis of glutathione in the human hepatoma cell line, HepG2, and in primary human hepatocytes. In intact cells, activation of NFT yielded a radical anion, which subsequently initiated nuclear-factor-erythroid 2-related-factor-2 (Nrf2)-dependent induction of glutamate cysteine ligase (GCL). Application of siRNA-based intervention approaches confirmed the involvement of the Nrf2-GCL axis in the observed elevation of intracellular glutathione levels. Quantitative activation of Nrf2 by NFT, and the subsequent rise in glutathione, were similar as observed with the potent experimental Nrf2 activator diethyl maleate. The elevation of glutathione levels, observed even 48 h after withdrawal of NFT, rendered cells resistant to different stressors such as the mitochondrial inhibitor rotenone, the redox cycler paraquat, the proteasome inhibitors MG-132 or bortezomib, or high concentrations of NFT. Repurpose of the antibiotic NFT as activator of Nrf2 could thus be a promising strategy for a transient and targeted activation of the endogenous antioxidant machinery. Graphical abstract.

Mercury interactions with selenium and sulfur and the relevance of the Se:Hg molar ratio to fish consumption advice

Eating fish is often recommended as part of a healthful diet. However, fish, particularly large predatory fish, can contain significant levels of the highly toxic methylmercury (MeHg). Ocean fish in general also contain high levels of selenium (Se), which is reported to confer protection against toxicity of various metals including mercury (Hg). Se and Hg have a high mutual binding affinity, and each can reduce the toxicity of the other. This is an evolving area of extensive research and controversy with variable results in the animal and epidemiologic literature. MeHg is toxic to many organ systems through high affinity for -SH (thiol) ligands on enzymes and microtubules. Hg toxicity also causes oxidative damage particularly to neurons in the brain. Hg is a potent and apparently irreversible inhibitor of the selenoenzymes, glutathione peroxidases (GPX), and thioredoxin reductases (TXNRD) that are important antioxidants, each with a selenocysteine (SeCys) at the active site. Hg binding to the SeCys inhibits these enzymes, accounting in part for the oxidative damage that is an important manifestation of Hg toxicity, particularly if there is not a pool of excess Se to synthesize new enzymes. A molar excess of Se reflected in an Se:Hg molar ratio > 1 is often invoked as evidence that the Hg content can be discounted. Some recent papers now suggest that if the Se:Hg molar ratio exceeds 1:1, the fish is safe and the mercury concentration can be ignored. Such papers suggested that the molar ratio rather than the Hg concentration should be emphasized in fish advisories. This paper examines some of the limitations of current understanding of the Se:Hg molar ratio in guiding fish consumption advice; Se is certainly an important part of the Hg toxicity story, but it is not the whole story. We examine how Hg toxicity relates also to thiol binding. We suggest that a 1:1 molar ratio cannot be relied on because not all of the Se in fish or in the fish eater is available to interact with Hg. Moreover, in some fish, Se levels are sufficiently high to warrant concern about Se toxicity.

The influence of gold(i) on the mechanism of thiolate, disulfide exchange

The mechanism of gold(i)-thiolate, disulfide exchange was investigated by using initial-rate kinetic studies, 2D ((1)H-(1)H) ROESY NMR spectroscopy, and electrochemical/chemical techniques. The rate law for exchange is overall second order, first order in gold(i)-thiolate and disulfide. 2D NMR experiments show evidence of association between gold(i)-thiolate and disulfide. Electrochemical/chemical investigations do not show evidence of free thiolate and are consistent with a mechanism involving formation of a [Au-S, S-S], four-centered metallacycle intermediate during gold(i)-thiolate, disulfide exchange.

Mercury exposure and a shift toward oxidative stress in avid seafood consumers

Mechanisms of mercury (Hg) toxicity at low doses from seafood consumption, the most common exposure route, are not well understood. We tested the hypothesis that seafood Hg exposure is related to a shift in redox status, indicated by a decrease in the ratio of reduced to oxidized glutathione (GSH:GSSG) in blood, or increase in redox potential (Eh). We also examined whether key seafood nutrients (selenium (Se), omega-3 fatty acids) confound or modify this shift. We measured blood concentrations of total Hg, Se, GSH, GSSG, and the Omega-3 Index (% omega-3s of total fatty acids in red blood cell membranes) in seafood consumers in Long Island, NY. We examined relationships between Hg, GSH:GSSG ratio and Eh. Elevated blood Hg (>5.8µgL(-1)) was associated with lower GSH:GSSG (β=-116.73, p=0.01), with no evidence of confounding by Se or Omega-3 Index. However, in models stratified by Omega-3 Index levels, Hg-GSH:GSSG associations were weakened among those with high Omega-3 Index levels (>6% of fatty acids, β=-63.46, p=0.28), and heightened among those with low Omega-3 Index (β=-182.53, p<0.01). We observed comparable patterns for Eh in relation to Hg. These results support the hypothesis that Hg exposure from seafood is linked to a shift in redox status toward oxidative stress, modified by omega-3 fatty acids in this population. Further work should examine the role of different seafood nutrients and Hg-induced shifts in redox status in the diverse health effects associated with elevated Hg exposure.

Glutathione status and the renal elimination of inorganic mercury in the Mrp2(-/-) mouse

Multidrug resistance-associated proteins (MRP) 2 and 4 are localized in proximal tubular epithelial cells and participate in the renal elimination of xenobiotics. MRP2 has also been implicated in the renal and hepatic elimination of mercury. The current study tested the hypothesis that MRP2 and MRP4 are involved in renal and hepatic handling of inorganic mercury (Hg²⁺). We examined the disposition of Hg²⁺ in Mrp2^−/− mice and assessed the transport of mercuric conjugates in inside-out membrane vesicles containing human MRP4. Since MRP2 has been shown to utilize glutathione (GSH) for transport of select substrates, we examined renal concentrations of GSH and cysteine and the expression of glutamate cysteine ligase (GCL) in Mrp2^−/− and FVB mice. The effect of Hg²⁺ exposure on renal GSH levels was also assessed in these mice. Our data suggest that MRP2, but not MRP4, is involved in proximal tubular export of Hg²⁺. In addition, GSH levels are greater in Mrp2^−/− mice and exposure to Hg²⁺ reduced renal levels of GSH. Expression of GCL was also altered in Mrp2^−/− mice under normal conditions and following exposure to HgCl₂. This study provides important novel data regarding the transport of Hg²⁺ and the effect of Hg²⁺ exposure on GSH levels.

Effect of garlic-derived organosulfur compounds on mitochondrial function and integrity in isolated mouse liver mitochondria

The objectives of this work were to evaluate the direct effects of diallysulfide (DAS) and diallyldisulfide (DADS), two major organosulfur compounds of garlic oil, on mitochondrial function and integrity, by using isolated mouse liver mitochondria in a cell-free system. DADS produced concentration-dependent mitochondrial swelling over the range 125-1000μM, while DAS was ineffective. Swelling experiments performed with de-energized or energized mitochondria showed similar maximal swelling amplitudes. Cyclosporin A (1μM), or ethylene glycol-bis(2-aminoethylether)-N,N,N',N'-tetraacetic acid (EGTA, 1mM) were ineffective in inhibiting DADS-induced mitochondrial swelling. DADS produced a minor (12%) decrease in mitochondrial membrane protein thiols, but did not induce clustering of mitochondrial membrane proteins. Incubation of mitochondria with DADS (but not DAS) produced an increase in the oxidation rate of 2',7' dichlorofluorescein diacetate (DCFH-DA), together with depletion of reduced glutathione (GSH) and increased lipid peroxidation. DADS (but not DAS) produced a concentration-dependent dissipation of the mitochondrial membrane potential, but did not induce cytochrome c release. DADS-dependent effects, including mitochondrial swelling, DCFH-DA oxidation, lipid peroxidation and loss of mitochondrial membrane potential, were inhibited by antioxidants and iron chelators. These results suggest that DADS causes direct impairment of mitochondrial function as the result of oxidation of the membrane lipid phase initiated by the GSH- and iron-dependent generation of oxidants.

The plausibility of a role for mercury in the etiology of autism: a cellular perspective

Autism is defined by a behavioral set of stereotypic and repetitious behavioral patterns in combination with social and communication deficits. There is emerging evidence supporting the hypothesis that autism may result from a combination of genetic susceptibility and exposure to environmental toxins at critical moments in development. Mercury (Hg) is recognized as a ubiquitous environmental neurotoxin and there is mounting evidence linking it to neurodevelopmental disorders, including autism. Of course, the evidence is not derived from experimental trials with humans but rather from methods focusing on biomarkers of Hg damage, measurements of Hg exposure, epidemiological data, and animal studies. For ethical reasons, controlled Hg exposure in humans will never be conducted. Therefore, to properly evaluate the Hg-autism etiological hypothesis, it is essential to first establish the biological plausibility of the hypothesis. This review examines the plausibility of Hg as the primary etiological agent driving the cellular mechanisms by which Hg-induced neurotoxicity may result in the physiological attributes of autism. Key areas of focus include: (1) route and cellular mechanisms of Hg exposure in autism; (2) current research and examples of possible genetic variables that are linked to both Hg sensitivity and autism; (3) the role Hg may play as an environmental toxin fueling the oxidative stress found in autism; (4) role of mitochondrial dysfunction; and (5) possible role of Hg in abnormal neuroexcitory and excitotoxity that may play a role in the immune dysregulation found in autism. Future research directions that would assist in addressing the gaps in our knowledge are proposed.

Sensitivity of dopaminergic neuron differentiation from stem cells to chronic low-dose methylmercury exposure

Perinatal exposure to low doses of methylmercury (MeHg) can cause adult neurological symptoms. Rather than leading to a net cell loss, the toxicant is assumed to alter the differentiation and neuronal functions such as catecholaminergic transmission. We used neuronally differentiating murine embryonic stem cells (mESC) to explore such subtle toxicity. The mixed neuronal cultures that formed within 20 days contained a small subpopulation of tyrosine hydroxylase (TH)-positive neurons with specific dopaminergic functions such as dopamine transport (DAT) activity. The last 6 days of differentiation were associated with the functional maturation of already preformed neuronal precursors. Exposure to MeHg during this period downregulated several neuronal transcripts, without affecting housekeeping genes or causing measurable cell loss. Profiling of mRNAs relevant for neurotransmitter systems showed that dopamine receptors were coordinately downregulated, whereas known counterregulatory systems such as galanin receptor 2 were upregulated. The chronic (6 days) exposure to MeHg, but not shorter incubation periods, attenuated the expression levels of endogenous neurotrophic factors required for the maturation of TH cells. Accordingly, the size of this cell population was diminished, and DAT activity as its signature function was lost. When mixed lineage kinase activity was blocked during MeHg exposure, DAT activity was restored, and the reduction of TH levels was prevented. Thus, transcriptional profiling in differentiating mESC identified a subpopulation of neurons affected by MeHg, and a pharmacological intervention was identified that specifically protected these cells.

Glutathione-indole-3-acetonitrile is required for camalexin biosynthesis in Arabidopsis thaliana

Camalexin, a major phytoalexin in Arabidopsis thaliana, consists of an indole ring and a thiazole ring. The indole ring is produced from Trp, which is converted to indole-3-acetonitrile (IAN) by CYP79B2/CYP79B3 and CYP71A13. Conversion of Cys(IAN) to dihydrocamalexic acid and subsequently to camalexin is catalyzed by CYP71B15. Recent studies proposed that Cys derivative, not Cys itself, is the precursor of the thiazole ring that conjugates with IAN. The nature of the Cys derivative and how it conjugates to IAN and subsequently forms Cys(IAN) remain obscure. We found that protein accumulation of multiple glutathione S-transferases (GSTs), elevation of GST activity, and consumption of glutathione (GSH) coincided with camalexin production. GSTF6 overexpression increased and GSTF6-knockout reduced camalexin production. Arabidopsis GSTF6 expressed in yeast cells catalyzed GSH(IAN) formation. GSH(IAN), (IAN)CysGly, and γGluCys(IAN) were determined to be intermediates within the camalexin biosynthetic pathway. Inhibitor treatments and mutant analyses revealed the involvement of γ-glutamyl transpeptidases (GGTs) and phytochelatin synthase (PCS) in the catabolism of GSH(IAN). The expression of GSTF6, GGT1, GGT2, and PCS1 was coordinately upregulated during camalexin biosynthesis. These results suggest that GSH is the Cys derivative used during camalexin biosynthesis, that the conjugation of GSH with IAN is catalyzed by GSTF6, and that GGTs and PCS are involved in camalexin biosynthesis.

The relevance of the individual genetic background for the toxicokinetics of two significant neurodevelopmental toxicants: mercury and lead

The heavy metals mercury and lead are well-known and significant developmental neurotoxicants. This review summarizes the genetic factors that modify their toxicokinetics. Understanding toxicokinetics (uptake, biotransformation, distribution, and elimination processes) is a key precondition to understanding the individual health risks associated with exposure. We selected candidate susceptibility genes when evidence was available for (1) genes/proteins playing a significant role in mercury and lead toxicokinetics, (2) gene expression/protein activity being induced by these metals, and (3) mercury and lead toxicokinetics being affected by gene knockout/knockdown or (4) by functional gene polymorphisms. The genetic background is far better known for mercury than for lead toxicokinetics. Involved are genes encoding L-type amino acid transporters, organic anion transporters, glutathione (GSH)-related enzymes, metallothioneins, and transporters of the ABC family. Certain gene variants can influence mercury toxicokinetics, potentially explaining part of the variable susceptibility to mercury toxicity. Delta-aminolevulinic acid dehydratase (ALAD), vitamin D receptor (VDR) and hemochromatosis (HFE) gene variants are the only well-established susceptibility markers of lead toxicity in humans. Many gaps remain in our knowledge about the functional genomics of this issue. This calls for studies to detect functional gene polymorphisms related to mercury- and lead-associated disease phenotypes, to demonstrate the impact of functional polymorphisms and gene knockout/knockdown in relation to toxicity, to confirm the in vivo relevance of genetic variation, and to examine gene-gene interactions on the respective toxicokinetics. Another crucial aspect is knowledge on the maternal-fetal genetic background, which modulates fetal exposure to these neurotoxicants. To completely define the genetically susceptible risk groups, research is also needed on the genes/proteins involved in the toxicodynamics, i.e., in the mechanisms causing adverse effects in the brain. Studies relating the toxicogenetics to neurodevelopmental disorders are lacking (mercury) or very scarce (lead). Thus, the extent of variability in susceptibility to heavy metal-associated neurological outcomes is poorly characterized.

A system-based comparison of gene expression reveals alterations in oxidative stress, disruption of ubiquitin-proteasome system and altered cell cycle regulation after exposure to cadmium and methylmercury in mouse embryonic fibroblast

Environmental and occupational exposures to heavy metals such as methylmercury (MeHg) and cadmium (Cd) pose significant health risks to humans, including neurotoxicity. The underlying mechanisms of their toxicity, however, remain to be fully characterized. Our previous studies with Cd and MeHg have demonstrated that the perturbation of the ubiquitin-proteasome system (UPS) was associated with metal-induced cytotoxicity and apoptosis. We conducted a microarray-based gene expression analysis to compare metal-altered gene expression patterns with a classical proteasome inhibitor, MG132 (0.5 microM), to determine whether the disruption of the UPS is a critical mechanism of metal-induced toxicity. We treated mouse embryonic fibroblast cells at doses of MeHg (2.5 microM) and Cd (5.0 microM) for 24 h. The doses selected were based on the neutral red-based cell viability assay where initial statistically significant decreases in variability were detected. Following normalization of the array data, we employed multilevel analysis tools to explore the data, including group comparisons, cluster analysis, gene annotations analysis (gene ontology analysis), and pathway analysis using GenMAPP and Ingenuity Pathway Analysis (IPA). Using these integrated approaches, we identified significant gene expression changes across treatments within the UPS (Uchl1 and Ube2c), antioxidant and phase II enzymes (Gsta2, Gsta4, and Noq1), and genes involved in cell cycle regulation pathways (ccnb1, cdc2a, and cdc25c). Furthermore, pathway analysis revealed significant alterations in genes implicated in Parkinson's disease pathogenesis following metal exposure. This study suggests that these pathways play a critical role in the development of adverse effects associated with metal exposures.

Methylated Metal(loid) Species in Humans

While the metal(loid)s arsenic, bismuth, and selenium (probably also tellurium) have been shown to be enzymatically methylated in the human body, this has not yet been demonstrated for antimony, cadmium, germanium, indium, lead, mercury, thallium, and tin, although the latter elements can be biomethylated in the environment. Methylated metal(loid)s exhibit increased mobility, thus leading to a more efficient metal(loid) transport within the body and, in particular, opening chances for passing membrane barriers (blood-brain barrier, placental barrier). As a consequence human health may be affected. In this review, relevant data from the literature are compiled, and are discussed with respect to the evaluation of assumed and proven health effects caused by alkylated metal(loid) species.

How can a chemical element elicit complex immunopathology? Lessons from mercury-induced autoimmunity

Although most autoimmune diseases develop without a manifest cause, epidemiological studies indicate that external factors play an important role in triggering or aggravating autoimmune processes in genetically predisposed individuals. Nevertheless, most autoimmune disease-promoting environmental agents are unknown because their relationships to immune function are not understood. Thus, the study of animal models of chemically-induced autoimmunity should shed light on the pathways involved and allow us to identify these agents. The rodent model of heavy metal-induced autoimmunity is one of the most intriguing experimental systems available to address such questions. Although the ultimate pathophysiology of this model remains mysterious, recent studies have started to elucidate the mechanisms by which heavy metal exposure leads to immune activation and loss of self-tolerance.

Immunology of mercury

The heavy metal mercury is ubiquitously distributed in the environment resulting in permanent low-level exposure in human populations. Mercury can be encountered in three main chemical forms (elemental, inorganic, and organic) which can affect the immune system in different ways. In this review, we describe the effects of these various forms of mercury exposure on immune cells in humans and animals. In genetically susceptible mice or rats, subtoxic doses of mercury induce the production of highly specific autoantibodies as well as a generalized activation of the immune system. We review studies performed in this model and discuss their implications for the role of environmental chemicals in human autoimmunity.

A protein from Cajanus indicus Spreng protects liver and kidney against mercuric chloride-induced oxidative stress

Mercuric chloride (HgCl(2)) is a widespread environmental toxin that affects mainly liver and kidney. The present study has been carried out to investigate the protective action of a protein (the CI protein) isolated from the herb, Cajanus indicus Spreng against HgCl(2) induced renal and hepatic toxicities in mice. Intraperitoneal administration of HgCl(2) at a dose of 5 mg/kg body weight for 1 d significantly reduced the activities of antioxidant enzymes like superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPx). Moreover, it also depleted the glutathione to oxidized glutathione (GSH/GSSG) ratio. In addition, HgCl(2) increased the activities of serum marker enzymes (namely, glutamate pyruvate transaminase, GPT and alkaline phosphatase, ALP), creatinine, blood urea nitrogen and serum tumor necrosis factor alpha (TNF-alpha) level along with hepatic and renal lipid peroxidation. Besides, application of HgCl(2) to hepatocytes increased reactive oxygen species production and reduced the total antioxidant activity of the treated hepatocytes. Treatment with the CI protein intraperitoneally at a dose of 2 mg/kg body weight before or after HgCl(2) administration showed that it could scavenge free radicals in vitro and protect the alterations of the antioxidant molecules and the other parameters used in this particular study. Histological studies also revealed a milder lesion in kidney and liver samples of the CI protein treated mice compared to mice treated with HgCl(2) alone. Effects of a known antioxidant N-acetylcysteine have been used to compare its action to that of the CI protein.

Antioxidant and biotransformation responses in Liza aurata under environmental mercury exposure - relationship with mercury accumulation and implications for public health

This study was carried out in the Laranjo basin (Ria de Aveiro, Portugal), an area impacted by mercury discharges. Liza aurata oxidative stress and biotransformation responses were assessed in the liver and related to total mercury (Hgt) concentrations. A seasonal fish survey revealed a sporadic increase in total glutathione (GSHt) and elevated muscle Hgt levels, although Hg levels did not exceed the EU regulatory limit. As a complement study, fish were caged for three days both close to the bottom and on the water surface at three locations, and displayed higher Hgt levels accompanied by increased GSHt content and catalase activity as well as EROD activity inhibition. The bottom group displayed higher hepatic Hgt and GSHt contents compared with the surface group. Globally, both wild and caged fish revealed that the liver accumulates higher Hgt concentrations than muscle and, thus, better reflects environmental contamination levels. The absence of peroxidative damage in the liver can be attributed to effective detoxification and antioxidant defense.

Toxicological effects of methylmercury on walleye (Sander vitreus) and perch (Perca flavescens) from lakes of the boreal forest

Biochemical and physiological responses of walleye (Sander vitreus) and perch (Perca flavescens) were studied in four Canadian boreal forest lakes representing a mercury (Hg) exposure gradient. The aim of this study was to assess the effects of Hg and methylmercury (MeHg) on the general physiological condition of fish as well as to gauge the relationship between MeHg and the glutathione (GSH) system in metal-contaminated and reference sites using a series of biomarkers. Walleye from Lake Malartic had the highest liver MeHg concentrations, exhibited lower hepatosomatic indices (HSI) and lower glutathione S-transferase (GST) activity. HSI was negatively related to liver total Hg concentrations in walleye (R2=0.33, n=108, P<0.0001). Glutathione reductase (GR) and GST activity for walleye from Lake Malartic were related to HSI (R2=0.38, n=25, P=0.0010; R2=0.46, n=27, P<0.0001, respectively). In Lake Desjardins-East, where perch had the highest liver MeHg concentrations, glutathione peroxidase selenium dependent activity (GSH-Px SD) and GST activity were negatively related to liver MeHg concentrations (R2=0.39, n=21, P=0.0026; R2=0.22, n=21, P=0.0298, respectively). This study suggests that Hg may induce adverse effects on the physiology and cellular metabolism of walleye and perch at environmentally relevant concentrations.

Interactive toxicity of inorganic mercury and trichloroethylene in rat and human proximal tubules: effects on apoptosis, necrosis, and glutathione status

Simultaneous or prior exposure to one chemical may alter the concurrent or subsequent response to another chemical, often in unexpected ways. This is particularly true when the two chemicals share common mechanisms of action. The present study uses the paradigm of prior exposure to study the interactive toxicity between inorganic mercury (Hg(2+)) and trichloroethylene (TRI) or its metabolite S-(1,2-dichlorovinyl)-l-cysteine (DCVC) in rat and human proximal tubule. Pretreatment of rats with a subtoxic dose of Hg(2+) increased expression of glutathione S-transferase-alpha1 (GSTalpha1) but decreased expression of GSTalpha2, increased activities of several GSH-dependent enzymes, and increased GSH conjugation of TRI. Primary cultures of rat proximal tubular (rPT) cells exhibited both necrosis and apoptosis after incubation with Hg(2+). Pretreatment of human proximal tubular (hPT) cells with Hg(2+) caused little or no changes in GST expression or activities of GSH-dependent enzymes, decreased apoptosis induced by TRI or DCVC, but increased necrosis induced by DCVC. In contrast, pretreatment of hPT cells with TRI or DCVC protected from Hg(2+) by decreasing necrosis and increasing apoptosis. Thus, whereas pretreatment of hPT cells with Hg(2+) exacerbated cellular injury due to TRI or DCVC by shifting the response from apoptosis to necrosis, pretreatment of hPT cells with either TRI or DCVC protected from Hg(2+)-induced cytotoxicity by shifting the response from necrosis to apoptosis. These results demonstrate that by altering processes related to GSH status, susceptibilities of rPT and hPT cells to acute injury from Hg(2+), TRI, or DCVC are markedly altered by prior exposures.

Ginkgo biloba extract protects against mercury(II)-induced oxidative tissue damage in rats

Mercury(II) is a highly toxic metal which induces oxidative stress in the body. In this study we aimed to investigate the possible protective effect of Ginkgo biloba (EGb), an antioxidant agent, against experimental mercury toxicity in rat model. Following a single dose of 5mg/kg mercuric chloride (HgCl(2); Hg group) either saline or EGb (150mg/kg) was administered for 5days. After decapitation of the rats trunk blood was obtained and the tissue samples from the brain, lung, liver, and kidney were taken for the determination of malondialdehyde (MDA) and glutathione (GSH) levels, myeloperoxidase (MPO) activity and collagen contents. Formation of reactive oxygen species in the tissue samples was monitored by chemiluminescence (CL) technique. BUN, creatinin, ALT, and AST levels and tumor necrosis factor-alpha (TNF-alpha) and lactate dehydrogenase (LDH) activity were assayed in serum samples. The results revealed that HgCl(2) induced oxidative damage caused significant decrease in GSH level, significant increase in MDA level, MPO activity and collagen content of the tissues. Treatment of rats with EGb significantly increased the GSH level and decreased the MDA level, MPO activity, and collagen contents. Similarly, serum ALT, AST and BUN levels, as well as LDH and TNF-alpha, were elevated in the Hg group as compared to control group. On the other hand, EGb treatment reversed all these biochemical indices. Our results implicate that mercury-induced oxidative damage in brain, lung, liver, and kidney tissues protected by G. biloba extract, with its antioxidant effects.

Oxidative stress, chronic disease, and muscle wasting

Underlying the pathogenesis of chronic disease is the state of oxidative stress. Oxidative stress is an imbalance in oxidant and antioxidant levels. If an overproduction of oxidants overwhelms the antioxidant defenses, oxidative damage of cells, tissues, and organs ensues. In some cases, oxidative stress is assigned a causal role in disease pathogenesis, whereas in others the link is less certain. Along with underlying oxidative stress, chronic disease is often accompanied by muscle wasting. It has been hypothesized that catabolic programs leading to muscle wasting are mediated by oxidative stress. In cases where disease is localized to the muscle, this concept is easy to appreciate. Transmission of oxidative stress from diseased remote organs to skeletal muscle is thought to be mediated by humoral factors such as inflammatory cytokines. This review examines the relationship between oxidative stress, chronic disease, and muscle wasting, and the mechanisms by which oxidative stress acts as a catabolic signal.

Protective effects of Ginkgo biloba extract against mercury(II)-induced cardiovascular oxidative damage in rats

This study was designed to determine the possible protective effect of Ginkgo biloba extract (EGb) against Hg II-induced oxidative damage and also thromboplastic activity in the aorta and heart tissues. Wistar albino rats of either sex (200-250 g) were divided into four groups. Rats were injected intraperitoneally with (1) control (C) group: 0.9% NaCl; (2) EGb group: Ginkgo biloba extract (Abdi Ibrahim Pharmaceutical Company, Istanbul, Turkey) at a dose of 50 mg/kg/day; (3) Hg group: a single dose of 5 mg/kg mercuric chloride (HgCl(2)); and (4) Hg + EGb group: First day EGb at a dose of 50 mg/kg/day, i.p., 1 hour after HgCl(2) (5 mg/kg) injection; following four days EGb at a dose 50 mg/kg/day, i.p. After decapitation of the rats, trunk blood was obtained and serum tumor necrosis factor-alpha (TNF-alpha), lactate dehydrogenase (LDH) activity, and malondialdehyde (MDA) and glutathione (GSH) levels were analysed. In the aorta and heart tissues total protein, MDA, GSH levels and thromboplastic activity were determined. The results revealed that HgCl(2) induced oxidative tissue damage, as evidenced by increases in MDA levels and decreased GSH levels both in serum and tissue samples. Thromboplastic activity was increased significantly following Hg administration, which verifies the cardiotoxic effects of HgCl(2). Serum LDH and TNF-alpha were elevated in the Hg group compared with the control group. Since EGb treatment reversed these responses, it seems likely that Ginkgo biloba extract can protect the cardiovascular tissues against HgCl(2)-induced oxidative damage.

In vitro and in vivo effects of mercuric chloride on thymic endocrine activity, NK and NKT cell cytotoxicity, cytokine profiles (IL-2, IFN-gamma, IL-6): role of the nitric oxide-L-arginine pathway

Mercury (Hg2+) affects cell-mediated immunity, including thymulin production. Thymulin, a zinc-dependent thymic hormone synthesized by thymic epithelial cells (TECs), is involved in NK cell cytotoxicity and Th1 cytokine production (IL-2 and IFN-gamma), which in turn affect both NKT and classic NK spleen cell cytotoxicity. High doses of Hg2+ induce an inflammatory status, increased production of IL-6 and consequent Th1/Th2 imbalance as well as cell-mediated immune depression. The mechanisms by which Hg+ affects the cell-mediated immune response are still unclear. The nitric oxide (NO) pathway may be implicated. The aim of this work was to further explore its noxious role in innate and adaptive immunity and to study the possible role played by the NO pathway. Young Balb/c mice treated in vivo for 1 month with 1.0 mg HgCl2/kg b.w. showed low thymulin activity, depressed NO production (as measured by nitrite and nitrate plasma levels), impaired classic NK spleen cell cytotoxicity, decreased Th1 (IL-2 and IFN-gamma) cytokine profiles, and increased IL-6 production. In vitro, 10(-6) M of HgCl2 inhibited active thymulin kinetics, TEC proliferation, NKT cell cytotoxicity and Th1 cytokine production, whereas IL-6 increased. L-arginine restored thymulin activity, TEC proliferation, NKT cytotoxicity, cytokine profiles and nitrite and nitrate plasma levels both in vivo and in vitro. Since L-arginine is the substrate for NO production, it may compensate for the cell-mediated immune defect induced by HgCl2, via the arginine-NO-pathway. L-arginine is also able to reduce glomerular kidney IgG antibodies deposits induced by higher dose of HgCl2 administration.

Glutamate cysteine ligase catalysis: dependence on ATP and modifier subunit for regulation of tissue glutathione levels

Glutamate cysteine ligase (GCL), which synthesizes gamma-glutamyl-cysteine (gamma-GC), is the rate-limiting enzyme in GSH biosynthesis. gamma-GC may be produced by the catalytic subunit GCLC or by the holoenzyme (GCLholo), which comprises GCLC and the modifier subunit GCLM. The Gclm(-/-) knock-out mouse shows tissue levels of GSH that are between 9 and 40% of the Gclm(+/+) wild-type mouse. In the present study, we used recombinant GCLC and GCLM and Gclm(-/-) mice to examine the role of GCLM on gamma-GC synthesis by GCLholo. GCLM decreased the Km for ATP by approximately 6-fold and, similar to other species, decreased the Km for glutamate and increased the Ki for feedback inhibition by GSH. Furthermore, GCLM increased by 4.4-fold the Kcat for gamma-GC synthesis; this difference in catalytic efficiency of GCLholo versus GCLC allowed us to derive a mathematical relationship for gamma-GC production and to determine the relative levels of GCLholo and GCLC; in homogenates of brain, liver, and lung, the ratio of GCLC to GCLholo was 7.0, 2.0, and 3.5, respectively. In kidney, however, the relationship between GCLC and GCLholo was complicated. Kidney contains GCLholo, free GCLC, and free GCLM, and free GCLC in kidney cannot interact with GCLM. Taken together, we conclude that, in most tissues, GCLM is limiting, suggesting that an increase in GCLM alone would increase gamma-GC synthesis. On the other hand, our results from kidney suggest that gamma-GC synthesis may be controlled post-translationally.

Regulation of glutathione in inflammation and chronic lung diseases

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

Inhibition of nitric oxide synthase aggravates cisplatin-induced nephrotoxicity: effect of 2-amino-4-methylpyridine

BACKGROUND

Nitric oxide (NO) has been shown to play a role in maintaining normal renal function. However, the role of NO in cisplatin (CDDP)-induced nephrotoxicity is still unclear. The aim of the present work was to examine the effect of the NO synthase (NOS) inhibitor, 2-amino-4-methylpyridine, on the severity of CDDP-induced nephrotoxicity.

METHODS

Male Wistar rats were divided into six groups. Three control groups received plain drinking water or water containing 1.5% L-arginine. One of the two groups receiving plain water was treated with an intraperitoneal injection of 2-amino-4-methylpyridine (1 mg/kg in normal saline), and the other two control groups were injected intraperitoneally with normal saline. Another three groups were treated in the same manner and injected with CDDP (6 mg/kg, i.p.). CDDP was injected 1 h after 2-amino-4-methylpyridine treatment. Rats were sacrificed 7 days after CDDP treatment, and serum as well as kidneys were isolated and analysed.

RESULTS

CDDP-treated rats showed increases in the kidney weight as a percentage of the total body weight and serum creatinine and urea levels and decreases in serum albumin and calcium levels. Also, CDDP treatment induced reductions in the kidney total nitrate/nitrite (NO(x)), reduced glutathione (GSH) and glutathione peroxidase activity (GSH-Px) levels and an increase in the kidney malondialdehyde (MDA) production level. In contrast, 2-amino-4-methylpyridine treatment 1 h prior to CDDP injection induced marked exacerbation of CDDP-induced nephrotoxicity, as manifested by severe aggravation of the indices of nephrotoxicity. Also, 2-amino-4-methylpyridine plus CDDP-treated rats showed exaggeration of the reduction in the kidney total NO(x) content and GSH-Px activity and elevation of the kidney platinum accumulation level with normalization of the kidney MDA production level and rebound in the kidney GSH content. Histopathologically, CDDP-treated rats showed marked interstitial nephritis, tubular atrophy and tubular necrosis. However, treatment with 2-amino-4-methylpyridine 1 h prior to CDDP injection revealed marked exacerbation of CDDP-induced histopathological changes.

CONCLUSIONS

The present findings suggest that NO plays a role in CDDP-induced nephrotoxicity. Administration of 2-amino-4-methylpyridine, an NOS inhibitor, exacerbates CDDP-induced nephrotoxicity.

Melatonin Protects Against Mercury(II)-Induced Oxidative Tissue Damage in Rats

Mercury exerts a variety of toxic effects in the body. Lipid peroxidation, DNA damage and depletion of reduced glutathione by Hg(II) suggest an oxidative stress-like mechanism for Hg(II) toxicity. Melatonin, the main secretory product of the pineal gland, was recently found to be a potent free radical scavenger and antioxidant. N-Acetylcysteine, a precursor of reduced glutathione and an antioxidant, is used in the therapy of acute heavy metal poisoning. In this study the protective effects of melatonin in comparison to that of N-acetylcysteine against Hg-induced oxidative damage in the kidney, liver, lung and brain tissues were investigated. Wistar albino rats of either sex (200-250 g) were divided into six groups, each consisting of 8 animals. Rats were intraperitoneally injected with 1) 0.9% NaCl, control (C) group; 2) a single dose of 5 mg/kg mercuric chloride (HgCl2), Hg group; 3) melatonin in a dose of 10 mg/kg, 1 hr after HgCl2 injection, Hg-melatonin group; 4) melatonin in a dose of 10 mg/kg one day before and 1 hr after HgCl2 injection, melatonin-Hg-melatonin group; 5) N-acetylcysteine in a dose of 150 mg/kg, 1 hr after HgCl2 injection, Hg-N-acetylcysteine group, and 6) N-acetylcysteine in a dose of 150 mg/kg one day before and 1 hr after HgCl2 injection, N-acetylcysteine-Hg-N-acetylcysteine group. Animals were killed by decapitation 24 hr after the injection of HgCl2. Tissue samples were taken for determination of malondialdehyde, an end-product of lipid peroxidation; glutathione (GSH), a key antioxidant, and myeloperoxidase activity, an index of neutrophil infiltration. The results revealed that HgCl2 induced oxidative tissue damage, as evidenced by increases in malondialdehyde levels. Myeloperoxidase activity was also increased, and GSH levels were decreased in the liver, kidney and the lungs. All of these effects were reversed by melatonin or N-acetylcysteine treatment. Since melatonin or N-acetylcysteine administration reversed these responses, it seems likely that melatonin or N-acetylcysteine can protect all these tissues against HgCl2-induced oxidative damage.

Protective effect of arabic gum against cardiotoxicity induced by doxorubicin in mice: a possible mechanism of protection

Arabic gum (AG) is a naturally occurring compound that has been proposed to possess potent antioxidant activity. In this study, the possible effects whereby AG could protect against cardiotoxicity induced by doxorubicin (DOX) in mice were carried out. Administration of single dose of DOX (15 mg/kg, i.p.) induced cardiotoxicity 72 h, manifested biochemically by a significant elevation of serum creatine kinase (CK) (EC 2.7.3.2). In addition, cardiotoxicity was further confirmed by the significant increase in lipid peroxides measured as malondialdehyde (MDA). Administration of AG (25 g/kg) orally for 5 days before and 72 h after DOX injection produced a significant protection against cardiotoxicity induced by DOX. This was evidenced by significant reductions in serum CK and cardiac lipid peroxides. The effect of AG was examined on the superoxide anion radical generated by enzymatic and nonenzymatic methods. The results indicate that AG is a potent superoxide scavenger. The superoxide scavenging effect of AG may explain, at least in part, the protective effect of AG against cardiotoxicity induced by DOX.

Effect of vitamin E on glutathione-dependent enzymes

Reactive oxygen species and various electrophiles are involved in the etiology of diseases varying from cancer to cardiovascular and pulmonary disorders. The human body is protected against damaging effects of these compounds by a wide variety of systems. An important line of defense is formed by antioxidants. Vitamin E (consisting of various forms of tocopherols and tocotrienols) is an important fat-soluble, chain-breaking antioxidant. Besides working as an antioxidant, this compound possesses other functions with possible physiological relevance. The glutathione-dependent enzymes form another line of defense. Two important enzymes in this class are the free radical reductase and glutathione S-transferases (GSTs). The GSTs are a family of phase II detoxification enzymes. They can catalyze glutathione conjugation with various electrophiles. In most cases the electrophiles are detoxified by this conjugation, but in some cases the electrophiles are activated. Antioxidants do not act in isolation but form an intricate network. It is, for instance, known that vitamin E, together with glutathione (GSH) and a membrane-bound heat labile GSH-dependent factor, presumably an enzyme, can prevent damaging effects of reactive oxygen species on polyunsaturated fatty acids in biomembranes (lipid peroxidation). This manuscript reviews the interaction between the two defense systems, vitamin E and glutathione-dependent enzymes. On the simplest level, antioxidants such as vitamin E have protective effects on glutathione-dependent enzymes; however, we will see that reality is somewhat more complicated.

Cytoprotection against oxidative stress and the regulation of glutathione synthesis

Adaptation to oxidative and nitrosative stress occurs in cells first exposed to a nontoxic stress, resulting in the ability to tolerate a toxic challenge of the same or a related oxidant. Adaptation is observed in a wide variety of cells including endothelial cells on exposure to nitric oxide or oxidized lipids, and lung epithelial cells exposed to air-borne pollutants and toxicants. This acquired characteristic has been related to the regulation of a family of stress responding proteins including those that control the synthesis of the intracellular antioxidant glutathione. The focus of this article, which includes a review of recent results along with new data, is the regulation and signaling of glutathione biosynthesis, especially those relating to adaptive mechanisms. These concepts are illustrated with examples using nitric oxide and oxidized low density lipoprotein mediated adaptation to oxidative stress. These data are discussed in the context of other adaptive mechanisms relating to glutathione synthesis including those from dietary constituents such as curcumin.

Influence of amalgam fillings on Hg levels and total antioxidant activity in plasma of healthy donors

In order to evaluate the influence of specific factors on mercury (P-Hg) levels and antioxidant power (P-FRAP) in human plasma, 26 healthy donors were examined by a dentist, their plasma analyzed for Hg by atomic absorption spectrometry and for total antioxidant activity by FRAP method. Hg plasma concentration was found to be correlated with the number of amalgam fillings, suggesting that Hg released from fillings is a source of Hg in non-occupational exposed subjects. P-FRAP correlated negatively with P-Hg suggesting a pro-oxidant role of the Hg released from amalgam fillings. Though age by itself was not significantly correlated with P-FRAP, when considered together with P-Hg in multivariate analysis, it was found to be a major related cofactor. Multivariate analysis showed no influence of fish consumption or cigarette smoking on P-FRAP.

Antioxidant effect of hydroxytyrosol (DPE) and Mn2+ in liver of cadmium-intoxicated rats

Liver TBARS formation in cadmium-intoxicated rats was completely reduced by administering a low amount of MnCl(2) (2 mg/kg b.w.) 1 h before intoxication. A similar antioxidant effect was first shown by hydroxytyrosol (2-(3,4-dihydroxyphenyl)ethanol, (DPE), a phenolic compound present in olive oil, given twice to rats (9 mg/kg b.w.) after cadmium administration. The antioxidant properties shown in vivo by both Mn(2+) and DPE were also active in vitro when rat liver microsomes were subjected to lipid peroxidation by cadmium or other prooxidant systems. The increase in liver glutathione concentrations occurring in cadmium-intoxicated rats, was also found, for the first time, 24 h after MnCl(2) administration. Unlike cadmium intoxication, which caused a higher formation of both glutathione and TBARS, Mn(2+) induced glutathione synthesis without any TBARS formation. The same situation was also observed when cadmium plus Mn(2+) or cadmium plus DPE was given to rats. Our data show that: (a). both DPE and low Mn(2+) concentrations may have an antioxidant effect in the livers of cadmium-intoxicated rats and (b). Mn(2+), like cadmium, induces liver glutathione synthesis and this effect is probably independent of TBARS formation.

Pretreatment of rats with the inducing agents phenobarbitone and 3-methylcholanthrene ameliorates the toxicity of chromium (VI) in hepatocytes

To exert cytotoxicity chromium VI (Cr(VI)) has to be reduced inside cells. This is achieved through both enzymatic and non-enzymatic mechanisms. Enzymatic mechanisms include DT-diaphorase, cytochrome P450, and NADPH cytochrome c reductase, and non-enzymatic mechanisms involve reduced glutathione (GSH) and ascorbic acid. The extent of cytotoxicity of Cr(VI) may thus be influenced by the availability of non-enzymatic reductants, and by the activities of the reductase enzymes. In the present paper we have investigated the effect of pretreatment with the inducing agents, phenobarbitone (PB) and 3-methylcholanthrene (3-MC), on the response of rat hepatocytes to Cr(VI). Pretreatment with PB increased the activity of NADPH cytochrome c reductase, and 3-MC increased DT-diaphorase activity in hepatocytes. Both inducers increased cytochrome P450 content, while neither influenced intracellular GSH content or the activity of glutathione reductase. Pretreatment with either PB or 3-MC resulted in amelioration of Cr(VI) toxicity both in terms of hepatocyte viability, and to a greater extent, in terms of Cr(VI) induced GSH loss. We propose that the inducing agents increase the amount of enzymatic reduction of Cr(VI) relative to non-enzymatic reduction. Thus, less GSH is used in the reduction of Cr(VI), and intracellular GSH does not fall as rapidly as in cells from control animals therefore cell integrity is better maintained. Exposure to environmental inducing agents in vivo may also alter the response of human tissues to Cr(VI).

Cellular glutathione and thiols metabolism

Low molecular weight thiol-containing compounds have an essential role in many biochemical and pharmacological reactions due to the ease with each they are oxidized, and the rapidity with which they can be regenerated. Thioredoxin and glutathione (GSH) are two of the major small molecular weight thiol-containing compounds synthesized de novo in mammalian cells that participate in those functions. Understanding the mechanisms of thiol metabolism has special relevance to understanding the cell's defense against toxicant exposure and as the focal point in redox signaling. This commentary will, however, focus on GSH consumption and synthesis, and the role of thiols in signaling. The chemical reactions of GSH, including conjugation reactions mediated by glutathione S-transferases (GST) and oxidation reactions mediated by glutathione peroxidases will be described. The regulation of GSH synthesis will be illustrated from a compilation of studies designed to understand the various levels at which enzymatic GSH biosynthesis is controlled, and the signaling pathways that mediate them. The response of the cell to 4-hydroxynonenal (4HNE), a reactive aldehyde produced physiologically in response to inflammation and various air pollutants, will be explored in detail. Finally, the direct role of thiols as signaling molecules will be addressed, with particular attention given to "redox state." It is our aim that this commentary will lead the reader to appreciate that studies investigating the signaling for and regulation of thiol metabolism must never be generalized, and that perturbations in any of step of thiol metabolism may have etiological roles in genetically, virally, and environmentally borne pathologies.

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.

Effects of p,p'-DDE on immature cells in culture at concentrations relevant to the Alaskan environment

Arctic Alaskan Natives who maintain a traditional lifestyle have a disease profile that is significantly different from the general US population. There is concern that food sources containing environmental pollutants may contribute to this profile. In a preliminary study, umbilical cord blood was examined for the presence of several environmental contaminants. All cord blood samples analyzed thus far contain p,p'-DDE (1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene) with an average concentration of 0.33 microg/l. This study was undertaken to ascertain if this concentration of p,p'-DDE had detectable effects on immature cells in culture. NIH 3T3 (embryonic mouse fibroblast) and WS1 (human fetal fibroblast) cultures were exposed to media containing either 1 or 10 times the average cord blood concentration of p,p'-DDE. Initial experiments indicated that exposure to p,p'-DDE resulted in a decrease in the cell number of both cell types. Subsequent analysis revealed that the decrease in cell number was due to cell death in NIH 3T3 cells and to cell-cycle arrest in WS1 cells. Furthermore, p,p'-DDE decreased the long-term survival of NIH 3T3 but not WS1 cells. This study has demonstrated that p,p'-DDE, at relevant environmental concentrations, has significant effects on two immature mammalian cell types in culture. In addition, these results highlight the necessity for further studies to address the specific effects of p,p'-DDE on developing fetal systems.

The use of biomarkers in Daphnia magna toxicity testing V. In vivo alterations in the carbohydrate metabolism of Daphnia magna exposed to sublethal concentrations of mercury and lindane

Aspects of the carbohydrate metabolism of Daphnia magna exposed for 48 and 96 h to sublethal concentrations of mercury and lindane were investigated. General as well as toxicant-specific perturbations in the intermediary metabolism were observed. Both model toxicants caused an increase in glycolytic and hexose-monophosphate shunt activity. Mercury exposure increased lactate dehydrogenase and isocitrate activity (only after 96 h), while lindane exposure, on the contrary, inhibited the cellular lactate formation and increased the Krebs' cycle activity (only after 48 h). Daphnids exposed to sublethal mercury concentrations clearly exhibited increased glycogenolytic activity, while in lindane-exposed organisms mainly glycogen phosphorylase inhibition was detected. The short-term enzyme-based effect levels (48--96 h LOEC and EC(10) values) were compared with the effects on the population dynamics. This evaluation for both model toxicants suggests that threshold levels (LOEC or EC(10) values) based on pyruvate kinase activity after 48 and 96 h of exposure could be potential early warning signals for long-term effects. A set of enzymatic endpoints, based on the intermediary metabolism, is suggested to characterize the metabolic state of the daphnids.

Inhibition of ERK and p38 MAP kinases inhibits binding of Nrf2 and induction of GCS genes

Genes encoding the catalytic (GCS(h)) and regulatory (GCS(l)) subunits of human gamma-glutamylcysteine synthetase (gammaGCS), which catalyzes the rate limiting step in glutathione synthesis, are up-regulated in response to xenobiotics through Electrophile Response Elements (EpREs). Exposure of HepG2 cells to the GCS-inducing agent, Pyrrolidine dithiocarbamate (PDTC), results in ERK and p38 MAP kinase activation. Inhibition of ERK or p38 kinases by PD98059 or SB202190, respectively, results in approximately 50% reduction in GCS gene induction, while simultaneous inhibition completely eliminates induction. Induction of GCS expression is associated with an increase in Nrf2 and JunD binding to GCS EpREs. Pretreatment with the MAPK inhibitors significantly reduces binding of both transcription factors. These studies indicate that ERK and p38 contribute to the transcriptional up-regulation of the GCS subunit genes following PDTC treatment. Furthermore, supershift analyses suggest that binding of Nrf2 and JunD to the EpRE is a downstream consequence of ERK and p38 phosphorylation events.

CYP2E1 overexpression in HepG2 cells induces glutathione synthesis by transcriptional activation of gamma-glutamylcysteine synthetase

Induction of CYP2E1 (cytochrome P450 2E1) by ethanol appears to be one of the central pathways by which ethanol generates a state of oxidative stress. CYP2E1 is a loosely coupled enzyme; formation of reactive oxygen species occurs even in the absence of added substrate. GSH is critical for preserving the proper cellular redox balance and for its role as a cellular protectant. Since cells must maintain optimal GSH levels to cope with a variety of stresses, the goal of this study was to characterize the GSH homeostasis in human hepatocarcinoma cells (HepG2) that overexpress CYP2E1. This study was prompted by the finding that toxicity in CYP2E1-overexpressing cells was markedly enhanced after GSH depletion by buthionine sulfoximine treatment. CYP2E1-overexpressing cells showed a 40-50% increase in intracellular H(2)O(2); a 30% increase in total GSH levels; a 50% increase in the GSH synthesis rate; and a 2-fold increase in gamma-glutamylcysteine synthetase heavy subunit (GCS-HS) mRNA, the rate-limiting enzyme in GSH synthesis. This GCS-HS mRNA increase was due to increased synthesis since nuclear run-on assays showed increased transcription in CYP2E1-expressing cells, and the GCS-HS mRNA decay after actinomycin D treatment was similar in CYP2E1-expressing cells and empty vector-transfected cells. The facts that treatment with GSH ethyl ester almost completely prevented the increase in GCS-HS mRNA and decreased H(2)O(2) levels and that transient transfection with catalase (but not manganese-superoxide dismutase) produced a decrease in GCS-HS mRNA only in CYP2E1-expressing cells suggest a possible role for H(2)O(2) in the induction of GCS-HS gene transcription. In contrast to results with HepG2 cells expressing CYP2E1, no increase in GCS-HS mRNA was found with a HepG2 cell line engineered to express human cytochrome P450 3A4. In summary, CYP2E1 overexpression in HepG2 cells up-regulates the levels of reduced GSH by transcriptional activation of GCS-HS; this may reflect an adaptive mechanism to remove CYP2E1-derived oxidants such as H(2)O(2).

Regulation of redox glutathione levels and gene transcription in lung inflammation: therapeutic approaches

Glutathione (L-gamma-glutamyl-L-cysteinylglycine, GSH), is a vital intra- and extracellular protective antioxidant. Glutathione is synthesized from its constituent amino acids by the sequential action of gamma-glutamylcysteine synthetase (gamma-GCS) and GSH synthetase. The rate-limiting enzyme in GSH synthesis is gamma-GCS. Gamma-GCS expression is modulated by oxidants, phenolic antioxidants, and inflammatory and anti-inflammatory agents in various mammalian cells. The intracellular GSH redox homeostasis is strictly regulated to govern cell metabolism and protect cells against oxidative stress. Growing evidence has suggested that cellular oxidative processes have a fundamental role in inflammation through the activation of stress kinases (JNK, MAPK, p38) and redox-sensitive transcription factors such as NF-kappaB and AP-1, which differentially regulate the genes for proinflammatory mediators and protective antioxidant genes such as gamma-GCS, Mn-SOD, and heme oxygenase-1. The critical balance between the induction of proinflammatory mediators and antioxidant genes and the regulation of the levels of GSH in response to oxidative stress at the site of inflammation is not known. Knowledge of the mechanisms of redox GSH regulation and gene transcription in inflammation could lead to the development of novel therapies based on the pharmacological manipulation of the production of this important antioxidant in inflammation and injury. This FORUM article features the role of GSH levels in the regulation of transcription factors, whose activation and DNA binding leads to proinflammatory and antioxidant gene transcription. The potential role of thiol antioxidants as a therapeutic approach in inflammatory lung diseases is also discussed.

Regulation of gamma-glutamylcysteine synthetase subunit gene expression: insights into transcriptional control of antioxidant defenses

Gamma-glutamylcysteine synthetase (GCS; also referred to as glutamate-cysteine ligase, GLCL) catalyzes the rate-limiting reaction in glutathione (GSH) biosynthesis. The GCS holoenzyme is composed of a catalytic and regulatory subunit, each encoded by a unique gene. In addition to some conditions which specifically upregulate the catalytic subunit gene, expression of both genes is increased in response to many Phase II enzyme inducers including oxidants, heavy metals, phenolic antioxidants and GSH-conjugating agents. Electrophile Response Elements (EpREs), located in 5'-flanking sequences of both the GCSh and GCSl subunit genes, are hypothesized to at least partially mediate gene induction following xenobiotic exposure. Recent experiments indicate that the bZip transcription factor Nrf2 participates in EpRE-mediated GCS subunit gene activation in combination with other bZip proteins. An AP-1-like binding sequence and an NF-kappaB site have also been implicated in regulation of the catalytic subunit gene following exposure to certain pro-oxidants. Potential signaling mechanisms mediating GCS gene induction by the diverse families of Phase II enzyme inducers include thiol modification of critical regulatory sensor protein(s) and the generation of the reactive oxygen species. This review summarizes recent progress in defining the molecular mechanisms operative in transcriptional control of the genes encoding the two GCS subunits, identifying areas of agreement and controversy. The mechanisms involved in GCS regulation might also be relevant to the transcriptional control of other components of the antioxidant defense battery.