Glutamate-cysteine ligase modifier subunit: mouse Gclm gene structure and regulation by agents that cause oxidative stress

Induction of glutathione synthesis in human hepatocytes by acute and chronic arsenic exposure: differential roles of mitogen-activated protein kinases

Glutathione (GSH) is a vital component of antioxidant defense which protects cells from toxic insults. Previously we found intracellular GSH was involved in cell resistance against arsenic-induced cytotoxicity. However, molecular mechanisms of GSH homeostasis during arsenic exposure are largely undefined. Here, we investigated roles of mitogen-activated protein kinases (MAPKs) in GSH synthesis pathway with two arsenic exposure strategies by using Chang human hepatocytes. In one strategy, acute arsenic exposure (20 μM, 24 h) was applied, as MAPK signaling is generally considered to be transient. In the other one, chronic arsenic exposure (500 nM, 20 weeks) was applied, which mimicked the general human exposure to arsenic. We found that acute arsenic exposure activated extracellular signal-regulated 1/2 kinases (ERK1/2) and c-Jun N-terminal kinase (JNK) in parallel with increased transcription and nuclear translocation of factor-erythroid 2-related factor 2 (NRF2) and enhanced expression of γ-glutamyl cysteine ligase catalytic subunit (GCLC), resulting in elevated intracellular GSH levels. Specific ERK inhibitor abolished arsenic-induced NRF2 nuclear translocation and GSH synthesis. During chronic arsenic exposure which induced a malignant cellular phenotype, continuous p38 activation and NRF2 nuclear translocation were observed with enhanced GSH synthesis. Specific p38 inhibitor attenuated arsenic-enhanced GSH synthesis without changing NRF2 nuclear translocation. Taken together, our results indicate MAPK pathways play an important role in cellular GSH homeostasis in response to arsenic. However, the specific activation of certain MAPK is different between acute and chronic arsenic exposure. Furthermore, it appears that during chronic arsenic exposure, GSH synthesis is regulated by p38 at least in part independent of NRF2.

Molecular cloning and analysis of Ancylostoma ceylanicum glutamate-cysteine ligase

Glutamate-cysteine ligase (GCL) is a heterodimer enzyme composed of a catalytic subunit (GCLC) and a modifier subunit (GCLM). This enzyme catalyses the synthesis of γ-glutamylcysteine, a precursor of glutathione. cDNAs of the putative glutamate-cysteine ligase catalytic (Ace-GCLC) and modifier subunits (Ace-GCLM) of Ancylostoma ceylanicum were cloned using the RACE-PCR amplification method. The Ace-gclc and Ace-gclm cDNAs encode proteins with 655 and 254 amino acids and calculated molecular masses of 74.76 and 28.51kDa, respectively. The Ace-GCLC amino acid sequence shares about 70% identity and 80% sequence similarity with orthologs in Loa loa, Onchocerca volvulus, Brugia malayi, and Ascaris suum, whereas the Ace-GCLM amino acid sequence has only about 30% sequence identity and 50% similarity to homologous proteins in those species. Real-time PCR analysis of mRNA expression in L3, serum stimulated L3 and adult stages of A. ceylanicum showed the highest level of Ace-GCLC and Ace-GCLM expression occurred in adult worms. No differences were detected among adult hookworms harvested 21 and 35dpi indicating expression of Ace-gclc and Ace-gclm in adult worms is constant during the course of infection. Positive interaction between two subunits of glutamate-cysteine ligase was detected using the yeast two-hybrid system, and by specific enzymatic reaction. Ace-GCL is an intracellular enzyme and is not exposed to the host immune system. Thus, as expected, we did not detect IgG antibodies against Ace-GCLC or Ace-GCLM on days 21, 60 and 120 of A. ceylanicum infection in hamsters. Furthermore, vaccination with one or both antigens did not reduce worm burdens, and resulted in no improvement of clinical parameters (hematocrit and hemoglobin) of infected hamsters. Therefore, due to the significant role of the enzyme in parasite metabolism, our analyses raises hope for the development of a successful new drug against ancylostomiasis based on the specific GCL inhibitor.

Significance of the rapid increase in GSH levels in the protective response to cadmium exposure through phosphorylated Nrf2 signaling in Jurkat T-cells

Although cadmium (Cd) is a redox system disruptor, the systematic defensive responses to Cd-induced oxidative stress remain unclear. In this study, we initially determined that when human T-cell-derived Jurkat cells were exposed to a low concentration of Cd, the glutathione (GSH) concentration rapidly increased via the transient nuclear accumulation of the transcription factor Nrf2. Therefore, we hypothesized that this increase in the GSH levels was a significant event that occurred in response to the Cd toxicity in the Jurkat T-cells. To test this hypothesis, the expression of Nrf2 in the cells was silenced using siRNA transfection. These restricted expression conditions demonstrated that the sensitivity of the Jurkat T-cells to Cd toxicity was significantly higher in the knockdown cells. Whereas we could not find differences in the metallothionein (MT) expression responses, accumulation of Nrf2 in the nuclei and the GSH increase after Cd exposure were clearly suppressed in the Nrf2 knockdown cells. These findings strongly suggest that the Cd-induced activation of GSH synthesis is initiated as an acute response for Cd detoxification. Furthermore, the Cd remaining in the Jurkat T-cells did not cause a significant inhibition of cell growth after the rapid and transient increase in the GSH concentration returned to its basal level. Additionally, we found that MT expression induced by Cd occurred much later, with the expression seen at least 12h or more after the Nrf2-dependent immediate responses were almost completed. These results indicate that the rapid increase in GSH is an essential defensive response, with the subsequent induction of MT potentially chelating the Cd retained in the cell, thereby leading to continued suppression of Cd toxicity.

Bystander effects as manifestation of intercellular communication of DNA damage and of the cellular oxidative status

It is becoming increasingly clear that cells exposed to ionizing radiation (IR) and other genotoxic agents (targeted cells) can communicate their DNA damage response (DDR) status to cells that have not been directly irradiated (bystander cells). The term radiation-induced bystander effects (RIBE) describes facets of this phenomenon, but its molecular underpinnings are incompletely characterized. Consequences of DDR in bystander cells have been extensively studied and include transformation and mutation induction; micronuclei, chromosome aberration and sister chromatid exchange formation; as well as modulations in gene expression, proliferation and differentiation patterns. A fundamental question arising from such observations is why targeted cells induce DNA damage in non-targeted, bystander cells threatening thus their genomic stability and risking the induction of cancer. Here, we review and synthesize available literature to gather support for a model according to which targeted cells modulate as part of DDR their redox status and use it as a source to generate signals for neighboring cells. Such signals can be either small molecules transported to adjacent non-targeted cells via gap-junction intercellular communication (GJIC), or secreted factors that can reach remote, non-targeted cells by diffusion or through the circulation. We review evidence that such signals can induce in the recipient cell modulations of redox status similar to those seen in the originating targeted cell - occasionally though self-amplifying feedback loops. The resulting increase of oxidative stress in bystander cells induces, often in conjunction with DNA replication, the observed DDR-like responses that are at times strong enough to cause apoptosis. We reason that RIBE reflect the function of intercellular communication mechanisms designed to spread within tissues, or the entire organism, information about DNA damage inflicted to individual, constituent cells. Such responses are thought to protect the organism by enhancing repair in a community of cells and by eliminating severely damaged cells.

Control of angiogenesis dictated by picomolar superoxide levels

Control of vascular insufficiencies due to various cardiovascular pathologies is important for developing specific and effective treatments. Fluctuations in oxidative stress significantly alter the progression of angiogenesis under physiological and pathological conditions. However, the precise amount of reactive oxygen species (ROS) required to influence subsequent signaling pathways for ischemic angiogenesis remains undefined. Here, we have determined the effect of ROS-mediated molecular mechanisms on angiogenesis in a murine model of peripheral artery disease using Gclm mutant mice (a model of compromised glutathione synthesis and therefore reduced antioxidant capacity). Left femoral artery ligation and excision were performed in Gclm WT (+/+), heterozygous (+/-), and null (-/-) mice. Blood flow (laser Doppler), angiogenic index (CD31/DAPI), and proliferation index (Ki67/DAPI) were significantly increased in Gclm(+/-) mice but not in Gclm(+/+) or Gclm(-/-) mice. Measurements of reactive oxygen species suggest that the amount of superoxide required to stimulate angiogenesis after the induction of ischemia is 9.82 pmol/mg of tissue. Protein carbonyl levels increased in a manner consistent with increasing oxidative stress. Superoxide and protein carbonyl levels were reduced by the addition of the nitroxide tempol, a known superoxide dismutase mimetic. Finally, restoration of blood flow in Gclm(+/-) mice was attenuated by a VEGF164 aptamer, verifying that slightly elevated levels of ROS restore blood flow by stimulating endothelial cell proliferation through a VEGF-dependent pathway. The results of this study reveal new information on the amount of ROS necessary for angiogenic activity and provide the foundation of critical redox parameters for vascular remodeling responses. The information obtained from this study on vascular ischemia, using a model of decreased antioxidant capacity, has provided insight into the control of revascularization and is a step forward in our ability to regulate angiogenic therapies.

Proteome-wide measurement of protein half-lives and translation rates in vasopressin-sensitive collecting duct cells

Vasopressin regulates water excretion, in part, by controlling the abundances of the water channel aquaporin-2 (AQP2) protein and regulatory proteins in the renal collecting duct. To determine whether vasopressin-induced alterations in protein abundance result from modulation of protein production, protein degradation, or both, we used protein mass spectrometry with dynamic stable isotope labeling in cell culture to achieve a proteome-wide determination of protein half-lives and relative translation rates in mpkCCD cells. Measurements were made at steady state in the absence or presence of the vasopressin analog, desmopressin (dDAVP). Desmopressin altered the translation rate rather than the stability of most responding proteins, but it significantly increased both the translation rate and the half-life of AQP2. In addition, proteins associated with vasopressin action, including Mal2, Akap12, gelsolin, myosin light chain kinase, annexin-2, and Hsp70, manifested altered translation rates. Interestingly, desmopressin increased the translation of seven glutathione S-transferase proteins and enhanced protein S-glutathionylation, uncovering a previously unexplored vasopressin-induced post-translational modification. Additional bioinformatic analysis of the mpkCCD proteome indicated a correlation between protein function and protein half-life. In particular, processes that are rapidly regulated, such as transcription, endocytosis, cell cycle regulation, and ubiquitylation are associated with proteins with especially short half-lives. These data extend our understanding of the mechanisms underlying vasopressin signaling and provide a broad resource for additional investigation of collecting duct function (http://helixweb.nih.gov/ESBL/Database/ProteinHalfLives/index.html).

Therapeutic Effect of Chung-Pae, an Experimental Herbal Formula, on Acute Lung Inflammation Is Associated with Suppression of NF- κ B and Activation of Nrf2

Acute lung injury (ALI) is an inflammatory disease with high mortality, but therapeutics against it is unavailable. Recently, we elaborated a formula, named Chung-pae (CP), that comprises four ethnic herbs commonly prescribed against various respiratory diseases in Asian traditional medicine. CP is being administered in aerosol to relieve various respiratory symptoms of patients in our clinic. Here, we sought to examine whether CP has a therapeutic effect on ALI and to uncover the mechanism behind it. Reporter assays show that CP suppressed the transcriptional activity of proinflammatory NF-κB and activated that of anti-inflammatory Nrf2. Similarly, CP suppressed the expression of NF-κB dependent, proinflammatory cytokines and induced that of Nrf2 dependent genes in RAW 264.7. An aerosol intratracheal administration of CP effectively reduced neutrophilic infiltration and the expression of pro-inflammatory cytokines, hallmarks of ALI, in the lungs of mice that received a prior intraperitoneal injection of lipopolysaccharide. The intratracheal CP administration concomitantly enhanced the expression of Nrf2 dependent genes in the lung. Therefore, our results evidenced a therapeutic effect of CP on ALI, in which differential regulation of the two key inflammatory factors, NF-κB and Nrf2, was involved. We propose that CP can be a new therapeutic formula against ALI.

Nuclear factor (erythroid-derived 2)-like 2 (NFE2L2) is a novel therapeutic target for diabetic complications

Diabetes is a leading cause of death and disability. In 2004, 3.4 million people worldwide died of symptoms relating to high blood sugar. Diabetic complications are caused by organ damage resulting from long-term exposure to high blood sugar, and include diseases such as heart failure, kidney failure, vision loss and neuropathy. The transcription factor nuclear factor (erythroid-derived 2)-like 2 (NFE2L2, also known as NRF2) is an important component of the intracellular antioxidant machinery and a target for treatment of diabetic complications. This article reviews the role of NFE2L2 in diabetic complications with a focus on diabetic nephropathy, cardiomyopathy, neuropathy and retinopathy. Activation of NFE2L2 protects against oxidative stress in vitro and in vivo, and represents an important target for prophylaxis and treatment of diabetic complications. NFE2L2 has potential clinical applications for diabetic patients in the near future.

The Keap1-Nrf2 system in cancers: stress response and anabolic metabolism

The Keap1–Nrf2 [Kelch-like ECH-associated protein 1–nuclear factor (erythroid-derived 2)-like 2] pathway plays a central role in the protection of cells against oxidative and xenobiotic stresses. Nrf2 is a potent transcription activator that recognizes a unique DNA sequence known as the antioxidant response element (ARE). Under normal conditions, Nrf2 binds to Keap1 in the cytoplasm, resulting in proteasomal degradation. Following exposure to electrophiles or reactive oxygen species, Nrf2 becomes stabilized, translocates into the nucleus, and activates the transcription of various cytoprotective genes. Increasing attention has been paid to the role of Nrf2 in cancer cells because the constitutive stabilization of Nrf2 has been observed in many human cancers with poor prognosis. Recent studies have shown that the antioxidant and detoxification activities of Nrf2 confer chemo- and radio-resistance to cancer cells. In this review, we provide an overview of the Keap1–Nrf2 system and discuss its role under physiological and pathological conditions, including cancers. We also introduce the results of our recent study describing Nrf2 function in the metabolism of cancer cells. Nrf2 likely confers a growth advantage to cancer cells through enhancing cytoprotection and anabolism. Finally, we discuss the possible impact of Nrf2 inhibitors on cancer therapy.

Glutathione synthesis

BACKGROUND

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

SCOPE OF REVIEW

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

MAJOR CONCLUSIONS

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

GENERAL SIGNIFICANCE

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

Nrf2 redirects glucose and glutamine into anabolic pathways in metabolic reprogramming

Cancer cells consume large quantities of nutrients and maintain high levels of anabolism. Recent studies revealed that various oncogenic pathways are involved in modulation of metabolism. Nrf2, a key regulator for the maintenance of redox homeostasis, has been shown to contribute to malignant phenotypes of cancers including aggressive proliferation. However, the mechanisms with which Nrf2 accelerates proliferation are not fully understood. Here, we show that Nrf2 redirects glucose and glutamine into anabolic pathways, especially under the sustained activation of PI3K-Akt signaling. The active PI3K-Akt pathway augments the nuclear accumulation of Nrf2 and enables Nrf2 to promote metabolic activities that support cell proliferation in addition to enhancing cytoprotection. The functional expansion of Nrf2 reinforces the metabolic reprogramming triggered by proliferative signals.

SHPS-1 deficiency induces robust neuroprotection against experimental stroke by attenuating oxidative stress

Src homology 2 domain-containing protein tyrosine phosphatase substrate-1 (SHPS-1), also known as Signal-regulatory protein alpha (SIRPα) or SIRPA is a transmembrane protein that is predominantly expressed in neurons, dendritic cells, and macrophages. This study was conducted to investigate the role of SHPS-1 in the oxidative stress and brain damage induced by acute focal cerebral ischemia. Wild-type (WT) and SHPS-1 mutant (MT) mice were subjected to middle cerebral artery occlusion (60 min) followed by reperfusion. SHPS-1 MT mice had significantly reduced infarct volumes and improved neurological function after brain ischemia. In addition, neural injury and oxidative stress were inhibited in SHPS-1 MT mice. The mRNA and protein levels of the antioxidant genes nuclear factor-E2-related factor 2 (Nrf2) and heme oxygenase 1 were up-regulated in SHPS-1 MT mice. The SHPS-1 mutation suppressed the phosphorylation of SHP-1 and SHP-2 and increased the phosphorylation of Akt and GSK3β. These results provide the first demonstration that SHPS-1 plays an important role in the oxidative stress and brain injury induced by acute cerebral ischemia. The activation of Akt signaling and the up-regulation of Nrf2 and heme oxygenase 1 likely account for the protective effects that were observed in the SHPS-1 MT mice.

Activated microglia decrease histone acetylation and Nrf2-inducible anti-oxidant defence in astrocytes: restoring effects of inhibitors of HDACs, p38 MAPK and GSK3β

Histone deacetylase (HDAC) inhibitors have promising neuroprotective and anti-inflammatory properties although the exact mechanisms are unclear. We have earlier showed that factors from lipopolysaccharide (LPS)-activated microglia can down-regulate the astroglial nuclear factor-erythroid 2-related factor 2 (Nrf2)-inducible anti-oxidant defence. Here we have evaluated whether histone modification and activation of GSK3β are involved in these negative effects of microglia. Microglia were cultured for 24 h in serum-free culture medium to achieve microglia-conditioned medium from non-activated cells (MCM(0)) or activated with 10 ng/mL of LPS to produce MCM(10). Astrocyte-rich cultures treated with MCM(10) showed a time-dependent (0-72 h) increase in astroglial HDAC activity that correlated with lower levels of acetylation of histones H3 and H4 and decreased levels of the transcription factor Nrf2 and γ-glutamyl cysteine ligase modulatory subunit (γGCL-M) protein levels. The HDAC inhibitors valproic acid (VPA) and trichostatin-A (TSA) elevated the histone acetylation levels, restored the Nrf2-inducible anti-oxidant defence and conferred protection from oxidative stress-induced (H(2)O(2)) death in astrocyte-rich cultures exposed to MCM(10). Inhibitors of GSK3β (lithium) and p38 MAPK (SB203580) signaling pathways restored the depressed histone acetylation and Nrf2-related transcription whereas an inhibitor of Akt (Ly294002) caused a further decrease in Nrf2-related transcription. In conclusion, the study shows that well tolerated drugs such as VPA and lithium can restore an inflammatory induced depression in the Nrf2-inducible antioxidant defence, possibly via normalised histone acetylation levels.

Molecular profiling of the human nasal epithelium: A proteomics approach

A comprehensive proteomic profiling of nasal epithelium (NE) is described. This study relies on simple subcellular fractionation used to obtain soluble- and membrane-enriched fractions followed by 2-dimensional liquid chromatography (2D-LC) separation and tandem mass spectrometry (MS/MS). The cells were collected using a brushing technique applied on NE of clinically evaluated volunteers. Subsequently, the soluble- and the membrane-protein enriched fractions were prepared and analyzed in parallel using 2D-LC-MS/MS. In a set of 1482 identified proteins, 947 (63.9%) proteins were found to be associated to membrane fraction. Grand average hydropathy value index (GRAVY) analysis, the transmembrane protein mapping and annotations of primary location deposited in the Human Protein Reference Database (HPRD) confirmed an enrichment of hydrophobic proteins on this dataset. Ingenuity Pathway Analysis (IPA) of soluble fraction revealed an enrichment of molecular and cellular functions associated with cell death, protein folding and drug metabolism while in membrane fraction showed an enrichment of functions associated with molecular transport, protein trafficking and cell-to-cell signaling and interaction. The IPA showed similar enrichment of functions associated with cellular growth and proliferation in both soluble and membrane subproteomes. This finding was in agreement with protein content analysis using exponentially modified protein abundance index (emPAI). A comparison of our data with previously published studies focusing on respiratory tract epithelium revealed similarities related to identification of proteins associated with physical barrier function and immunological defence. In summary, we extended the NE molecular profile by identifying and characterizing proteins associated to pivotal functions of a respiratory epithelium, including the control of fluid volume and ionic composition at the airways' surface, physical barrier maintenance, detoxification and immunological defence. The extent of similarities supports the applicability of a less invasive analysis of NE to assess prognosis and treatment response of lung diseases such as asthma, cystic fibrosis and chronic obstructive pulmonary disease.

The Nrf2-inducible antioxidant defense in astrocytes can be both up- and down-regulated by activated microglia:Involvement of p38 MAPK

The effects of microglia-conditioned medium (MCM) on the inducible Nrf2 system in astrocyte-rich cultures were investigated by determination of glutathione (GSH) levels, γglutamylcysteine ligase (γGCL) activity, the protein levels of Nrf2, Keap1, the modulatory subunit of γGCL (γGCL-M) and activated MAP kinases (ERK1/2, JNK and p38). Microglia were either cultured for 24 h in serum-free culture medium to achieve microglia-conditioned medium from non-activated cells (MCM(0) ), used as control condition, or activated with different concentrations (0.1-1,000 ng mL(-1) ) of lipopolysaccharide (LPS) to produce MCM(0.1-1,000) . Acute exposure (24 h) to MCM(100) increased GSH, γGCL activity, the protein levels of γGCL-M, Nrf2, and activated JNK and ERK1/2 in astrocyte-rich cultures. In contrast, treatment with MCM(10) for 24 h decreased components of the Nrf2 system in parallel with activation of p38 MAPK. Stimulation of the Nrf2 system by tBHQ was partly intact after 24 h but blocked after 72 h treatment with MCM(10) and MCM(100) . This down-regulation after 72 h correlated with activation of p38 MAPK and lack of ERK1/2 and JNK activation. The negative effects were partly reversed by an inhibitor of p38 which restored tBHQ mediated protection against oxidative stress. In conclusion, the study showed a negative effect of MCM(10) on the inducible anti-oxidant defense in astrocyte-rich cultures at both 24 and 72 h that correlated with activation of p38 and was partly reversed by a p38 inhibitor. A transient protective effect of MCM(100) on astrocyte-rich cultures against H(2)O(2) toxicity was observed at 24 h which coincided with activation of JNK and ERK1/2.

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

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

Tert-butylhydroquinone induces mitochondrial oxidative stress causing Nrf2 activation

Tert-butylhydroquinone (tBHQ), the major metabolite of butylated hydroxyanisole, induces an antioxidant response through the redox-sensitive transcription factor, nuclear factor-E2-related factor-2 (Nrf2). However, the mechanism by which tBHQ induces Nrf2 activity is not entirely understood. Here, we show that tBHQ preferentially alters the redox status in the mitochondrial compartment in HeLa cells. HeLa cells treated with tBHQ showed a preferential oxidation of mitochondrial thioredoxin-2 (Trx2), while cellular glutathione and cytosolic thioredoxin-1 were not affected. Preferential mitochondrial oxidation by tBHQ was supported by detection of reactive oxygen species (ROS) specific to this compartment. To determine the role of Trx2 in regulating downstream effects of tBHQ, HeLa cells were transiently transfected with an empty, Trx2, or C93S (Cys93Ser) Trx2 dominant-negative mutant expression vector. Overexpression of Trx2 decreased basal mitochondrial ROS production, whereas expression of C93S Trx2 enhanced it. In addition, under untreated conditions, expression of C93S Trx2 led to an increase in the basal activities of Nrf2. With tBHQ treatments, Trx2 overexpression suppressed Nrf2 accumulation and activity, whereas expression of C93S Trx2 had no effect on the degree of inducibility or Nrf2 accumulation but did increase the overall activity of Nrf2. Quantitative polymerase chain reaction analysis of Nrf2-regulated gene expression corroborate Trx2 control of tBHQ-mediated Nrf2 activation. These data show a compartment-specific effect where tBHQ-induced Nrf2 signaling is mediated by Trx2 and suggest that antioxidant status in various compartments would provide different levels of control of redox signaling.

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

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

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

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

Nrf2 activation enhances biliary excretion of sulfobromophthalein by inducing glutathione-S-transferase activity

Sulfobromophthalein (BSP) is used to study hepatobiliary excretory function. BSP is conjugated with glutathione (GSH), whereas its dibrominated analog disulfobromophthalein (DBSP) is not conjugated with GSH prior to biliary excretion. In addition, both BSP and DBSP are transported into hepatocytes via organic anion-transporting polypeptides and excreted into bile via multidrug resistance-associated protein 2 (Mrp2). Nuclear factor erythroid 2-related factor 2 (Nrf2) is a transcription factor that under basal conditions is targeted for proteasomal degradation in the cytosol by kelch-like ECH-associated protein 1 (Keap1). Electrophilic and oxidative stress facilitate Nrf2 nuclear translocation and subsequent induction of cytoprotective genes, including GSH synthetic enzymes, GSH-S-transferases (Gsts), and Mrp transporters. The current study determined whether varying the amount of Nrf2 activation would effect the elimination of BSP and DBSP. Male wild-type (WT), Nrf2-null, and Keap1-knockdown (Keap1-kd) mice were administered BSP or DBSP. Within 30 min, Nrf2-null mice excreted 25%, WT mice 52%, and Keap1-kd mice 80% of the injected BSP. Liver GSH content was not altered by BSP. The biliary excretion of GSH and messenger RNA (mRNA) expression of major Gsts were directly proportional to the amount of Nrf2. Moreover, BSP-GSH conjugation activity in the liver of Nrf2-null and Keap1-kd mice was 42% and 237% of WT mice, respectively. In contrast to BSP, there were no differences in biliary excretion or plasma disappearance of DBSP among the three genotypes, suggesting that the modest differences in Mrp2 mRNA expression among genotypes do not affect BSP or DBSP biliary excretion. Collectively, these results indicate that increased biliary excretion of BSP, and possibly other compounds, is due to Nrf2-induced Gst mRNA expression and enzyme activity.

Probiotics prevent intestinal barrier dysfunction in acute pancreatitis in rats via induction of ileal mucosal glutathione biosynthesis

Background

During acute pancreatitis (AP), oxidative stress contributes to intestinal barrier failure. We studied actions of multispecies probiotics on barrier dysfunction and oxidative stress in experimental AP.

Methodology/Principal Findings

Fifty-three male Spraque-Dawley rats were randomly allocated into five groups: 1) controls, non-operated, 2) sham-operated, 3) AP, 4) AP and probiotics and 5) AP and placebo. AP was induced by intraductal glycodeoxycholate infusion and intravenous cerulein (6 h). Daily probiotics or placebo were administered intragastrically, starting five days prior to AP. After cerulein infusion, ileal mucosa was collected for measurements of E. coli K12 and ⁵¹Cr-EDTA passage in Ussing chambers. Tight junction proteins were investigated by confocal immunofluorescence imaging. Ileal mucosal apoptosis, lipid peroxidation, and glutathione levels were determined and glutamate-cysteine-ligase activity and expression were quantified. AP-induced barrier dysfunction was characterized by epithelial cell apoptosis and alterations of tight junction proteins (i.e. disruption of occludin and claudin-1 and up-regulation of claudin-2) and correlated with lipid peroxidation (r>0.8). Probiotic pre-treatment diminished the AP-induced increase in E. coli passage (probiotics 57.4±33.5 vs. placebo 223.7±93.7 a.u.; P<0.001), ⁵¹Cr-EDTA flux (16.7±10.1 vs. 32.1±10.0 cm/s10⁻⁶; P<0.005), apoptosis, lipid peroxidation (0.42±0.13 vs. 1.62±0.53 pmol MDA/mg protein; P<0.001), and prevented tight junction protein disruption. AP-induced decline in glutathione was not only prevented (14.33±1.47 vs. 8.82±1.30 nmol/mg protein, P<0.001), but probiotics even increased mucosal glutathione compared with sham rats (14.33±1.47 vs. 10.70±1.74 nmol/mg protein, P<0.001). Glutamate-cysteine-ligase activity, which is rate-limiting in glutathione biosynthesis, was enhanced in probiotic pre-treated animals (probiotics 2.88±1.21 vs. placebo 1.94±0.55 nmol/min/mg protein; P<0.05) coinciding with an increase in mRNA expression of glutamate-cysteine-ligase catalytic (GCLc) and modifier (GCLm) subunits.

Conclusions

Probiotic pre-treatment diminished AP-induced intestinal barrier dysfunction and prevented oxidative stress via mechanisms mainly involving mucosal glutathione biosynthesis.

Behavioral, morphological, and biochemical changes after in ovo exposure to methylmercury in chicks

Methylmercury (MeHg) is an environmental pollutant known to induce neurotoxicity in several animal species, including humans. However, studies focusing the effects of MeHg poisoning in chicks were based on phenomenological approaches and did not delve into the molecular mechanisms. The purpose of this study was to evaluate the postnatal consequences of the in ovo exposure to MeHg on behavioral, morphological and biochemical parameters in chicks. At the fifth embryonic day (E5), Gallus domesticus eggs were submitted to a single injection of 0.1 microg MeHg/0.05 ml saline. After treatment, the eggs returned to the incubator until hatching (E21). From first to fifth postnatal days (PN 1-PN 5), the MeHg-treated chicks showed lower frequency of exploratory movements and a significantly higher frequency of wing and anomalous movements. Cerebellar glutathione (GSH) levels and the activities of the GSH-related enzymes GSH reductase and GSH peroxidase were significantly higher (70, 72, and 80%, respectively) in MeHg exposed chicks in comparison to controls. Mercury impregnation was densest in the granular layer, followed by the Purkinje and molecular layers of treated chicks. A significant reduction of the number of Purkinje cells, as well as a greater distance between these cells were observed in chicks of MeHg group. Our results disclose that the prehatching exposure to MeHg induced motor impairments, which were correlated to histological damage and alterations on the cerebellar GSH system's development from PN 1 to PN 5.

Regulation of glutathione synthesis

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

Comparative glycoproteomics based on lectins affinity capture of N-linked glycoproteins from human Chang liver cells and MHCC97-H cells

We present here an effective technique for the large-scale separation and identification of N-linked glycoproteins from Chang liver cells, the human normal liver cells. To enrich N-linked glycoproteins from the whole cells, a procedure containing the lysis of human liver cells, the solubilization of total proteins, lectin affinity chromatography including Concanavalin A and wheat germ agglutinin was established. Furthermore, captured N-linked glycoproteins were separated by 2-DE, and identified by MS and database searching. Finally, we found 63 N-glycoproteins in Chang liver cells. In addition, using the above method, we identified 7 remarkably up-regulated glycoproteins from MHCC97-H cells, highly metastatic liver cancer cells, compared to Chang liver cells. These up-regulated glycoproteins were associated with liver cancer and might be used as biomarkers for tumor diagnosis. Results showed that we established a high-throughput proteomic analysis for separating N-linked glycoproteins from human liver cells. This strategy greatly improved the glycoprotein analysis method associated with proteome-wide glycosylation changes related to liver cancer. Our work was part of the HUPO Human Liver Proteome Project (HLPP) studies and was supported by CHINA HUPO.

Hepatocyte-specific Gclc deletion leads to rapid onset of steatosis with mitochondrial injury and liver failure

Oxidative stress is considered to be a critical mediator in liver injury of various etiologies. Depletion of glutathione (GSH), the major antioxidant in liver, has been associated with numerous liver diseases. To explore the specific role of hepatic GSH in vivo, we targeted Gclc, a gene essential for GSH synthesis, so that it was flanked by loxP sites and used the albumin-cyclization recombination (Alb-Cre) transgene to disrupt the Gclc gene specifically in hepatocytes. Deletion within the Gclc gene neared completion by postnatal day (PND)14, and loss of GCLC protein was complete by PND21. Cellular GSH was progressively depleted between PND14 and PND28-although loss of mitochondrial GSH was less severe. Nevertheless, ultrastructural examination of liver revealed dramatic changes in mitochondrial morphology; these alterations were accompanied by striking decreases in mitochondrial function in vitro, cellular ATP, and a marked increase in lipid peroxidation. Plasma liver biochemistry tests from these mice were consistent with progressive severe parenchymal damage. Starting at PND21, livers from hepatocyte-specific Gclc knockout [Gclc(h/h)] mice showed histological features of hepatic steatosis; this included inflammation and hepatocyte death, which progressed in severity such that mice died at approximately 1 month of age due to complications from liver failure.

CONCLUSION

GSH is essential for hepatic function and loss of hepatocyte GSH synthesis leads to steatosis with mitochondrial injury and hepatic failure.

Tissue and species distribution of the glutathione pathway transcriptome

The goal of this study was to compare and contrast the basal gene expression levels of the various enzymes involved in glutathione metabolism among tissues and genders of the rat, mouse and canine. The approach taken was to use Affymetrix GeneChip microarray data for rat, mouse and canine tissues, comparing intensity levels for individual probes between tissues and genders. As was hypothesized, the relative expression in liver, lung, heart, kidney and testis varied from gene to gene, with differences of expression between tissues sometimes greater than a 1000-fold. The pattern of differential expression was usually similar between male and female animals, but varied greatly between the three species. Gstp1 appears to be expressed at high levels in male mouse liver, reasonable levels in canine liver, but very low levels in male rat liver. In all species examined, Gstp1 expression was below detectable levels in testis. Gsta3/Yc2 expression appeared high in rodent liver and female canine liver, but not male canine liver. Finally, Mgst1 and Gpx3 expression appeared to be lower in canine heart and testis than seen in rodents. Given the critical role of the glutathione pathway in the detoxification of many drugs and xenobiotics, the observed differences in basal tissue distribution among mouse, rat and canine has far-reaching implications in comparing responses of these species in safety testing.

Astrocytes survive chronic infection and cytopathic effects of the ts1 mutant of the retrovirus Moloney murine leukemia virus by upregulation of antioxidant defenses

The ts1 mutant of Moloney murine leukemia virus (MoMuLV) induces a neurodegenerative disease in mice, in which glial cells are infected by the retrovirus but neurons are not. ts1 infection of primary astrocytes, or of the immortalized astrocytic cell line C1, results in accumulation of the ts1 gPr80(env) envelope protein in the endoplasmic reticulum (ER), with ER and oxidative stress. Notably, only about half of the infected astrocytes die in these cultures, while the other half survive, continue to proliferate, and continue to produce virus. To determine how these astrocytes survive ts1 infection in culture, we established a chronically infected subline of the living cells remaining after the death of all acutely infected cells in an infected C1 cell culture (C1-ts1-S). We report here that C1-ts1-S cells proliferate more slowly, produce less virus, show reduced H2O2 levels, increase their uptake of cystine, and maintain higher levels of intracellular GSH and cysteine compared to acutely infected or uninfected C1 cells. C1-ts1-S cells also upregulate their thiol antioxidant defenses by activation of the transcription factor NF-E2-related factor 2 (Nrf2) and its target genes. Interestingly, despite maintenance of higher levels of intracellular reduced thiols, C1-ts1-S cells are more sensitive to cystine deprivation than uninfected C1 cells. We conclude that some ts1-infected astrocytes survive and adapt to virus-induced oxidative stress by successfully mobilizing their thiol redox defenses.

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.

Tumour necrosis factor alpha induces co-ordinated activation of rat GSH synthetic enzymes via nuclear factor kappaB and activator protein-1

GSH synthesis occurs via two enzymatic steps catalysed by GCL [glutamate-cysteine ligase, made up of GCLC (GCL catalytic subunit), and GCLM (GCL modifier subunit)] and GSS (GSH synthetase). Co-ordinated up-regulation of GCL and GSS further enhances GSH synthetic capacity. The present study examined whether TNFalpha (tumour necrosis factor alpha) influences the expression of rat GSH synthetic enzymes. To facilitate transcriptional studies of the rat GCLM, we cloned its 1.8 kb 5'-flanking region. TNFalpha induces the expression and recombinant promoter activities of GCLC, GCLM and GSS in H4IIE cells. TNFalpha induces NF-kappaB (nuclear factor kappaB) and AP-1 (activator protein 1) nuclear-binding activities. Blocking AP-1 with dominant negative c-Jun or NF-kappaB with IkappaBSR (IkappaB super-repressor, where IkappaB stands for inhibitory kappaB) lowered basal expression and inhibited the TNFalpha-mediated increase in mRNA levels of all three genes. While all three genes have multiple AP-1-binding sites, only GCLC has a NF-kappaB-binding site. Overexpression with p50 or p65 increased c-Jun mRNA levels, c-Jun-dependent promoter activity and the promoter activity of GCLM and GSS. Blocking NF-kappaB also lowered basal c-Jun expression and blunted the TNFalpha-mediated increase in c-Jun mRNA levels. TNFalpha treatment resulted in increased c-Jun and Nrf2 (nuclear factor erythroid 2-related factor 2) nuclear binding to the antioxidant response element of the rat GCLM and if this was prevented, TNFalpha no longer induced the GCLM promoter activity. In conclusion, both c-Jun and NF-kappaB are required for basal and TNFalpha-mediated induction of GSH synthetic enzymes in H4IIE cells. While NF-kappaB may exert a direct effect on the GCLC promoter, it induces the GCLM and GSS promoters indirectly via c-Jun.

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.

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.

Genetically altered mice to evaluate glutathione homeostasis in health and disease

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

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.

Pharmacological rescue of the 14CoS/14CoS mouse: hepatocyte apoptosis is likely caused by endogenous oxidative stress

Whereas ch/ch wild-type mice and ch/14CoS heterozygotes are viable, 14CoS/14CoS mice homozygous for a 3800 kb deletion on chromosome 7 die during the first day postpartum. Death is caused by disruption of the fumarylacetoacetate hydrolase (Fah) gene; absence of FAH, final enzyme in the tyrosine catabolism pathway, leads to accumulation of reactive electrophilic intermediates. In this study, we kept 14CoS/14CoS mice alive for 60 d with oral 2-(2-nitro-4-trifluoromethyl-benzyol)-1,3-cyclohexanedione (NTBC), an inhibitor of p-hydroxyphenylpyruvate dioxygenase, second enzyme in the tyrosine catabolic pathway. The 70% of NTBC-treated 14CoS/14CoS mice that survived 60 d showed poor growth and developed corneal opacities, compared with ch/14CoS littermates; NTBC-rescued Fah(-/-) knockout mice did not show growth retardation or ocular toxicity. NTBC-rescued 14CoS/14CoS mice also exhibited a striking oxidative stress response in liver and kidney, as measured by lower GSH levels and mRNA induction of four genes: glutamate cysteine ligase catalytic (Gclc) and modifier (Gclm) subunits, NAD(P)H:quinone oxidoreductase (Nqo1), and heme oxygenase-1 (Hmox1). Withdrawal of NTBC for 24-48 h from rescued adult 14CoS/14CoS mice resulted in severe apoptosis of the liver, detected histologically and by cytochrome c release from the mitochondria, increased caspase 3-like activity, and further decreases in GSH content. In kidney, proximal tubular epithelial cells were abnormal. Human hereditary tyrosinemia type I (HT1), caused by mutations in the FAH gene, is an autosomal recessive disorder in which the patient usually dies of liver fibrosis and cirrhosis during early childhood; NTBC treatment is known to prolong HT1 children's lives-although liver fibrosis, cirrhosis, hepatocarcinoma, and corneal opacities sometimes occur. The mouse data in the present study are consistent with the possibility that endogenous oxidative stress-induced apoptosis may be the underlying cause of liver pathology seen in NTBC-treated HT1 patients.

Initial characterization of the glutamate-cysteine ligase modifier subunit Gclm(-/-) knockout mouse. Novel model system for a severely compromised oxidative stress response

Glutamate-cysteine ligase (GCL) is the rate-limiting enzyme in the GSH biosynthesis pathway. In higher eukaryotes, this enzyme is a heterodimer comprising a catalytic subunit (GCLC) and a modifier subunit (GCLM), which change the catalytic characteristics of the holoenzyme. To define the cellular function of GCLM, we disrupted the mouse Gclm gene to create a null allele. Gclm(-/-) mice are viable and fertile and have no overt phenotype. In liver, lung, pancreas, erythrocytes, and plasma, however, GSH levels in Gclm(-/-) mice were 9-16% of that in Gclm(+/+) littermates. Cysteine levels in Gclm(-/-) mice were 9, 35, and 40% of that in Gclm(+/+) mice in kidney, pancreas, and plasma, respectively, but remained unchanged in the liver and erythrocytes. Comparing the hepatic GCL holoenzyme with GCLC in the genetic absence of GCLM, we found the latter had an approximately 2-fold increase in K(m) for glutamate and a dramatically enhanced sensitivity to GSH inhibition. The major decrease in GSH, combined with diminished GCL activity, rendered Gclm(-/-) fetal fibroblasts strikingly more sensitive to chemical oxidants such as H(2)O(2). We conclude that the Gclm(-/-) mouse represents a model of chronic GSH depletion that will be very useful in evaluating the role of the GCLM subunit and GSH in numerous pathophysiological conditions as well as in environmental toxicity associated with oxidant insult.