Microsomal glutathione transferase: lipid-derived substrates and lipid dependence

Quantitative proteome analysis of LAP1-deficient human fibroblasts: A pilot approach for predicting the signaling pathways deregulated in LAP1-associated diseases

Lamina-associated polypeptide 1 (LAP1), a ubiquitously expressed nuclear envelope protein, appears to be essential for the maintenance of cell homeostasis. Although rare, mutations in the human LAP1-encoding TOR1AIP1 gene cause severe diseases and can culminate in the premature death of affected individuals. Despite there is increasing evidence of the pathogenicity of TOR1AIP1 mutations, the current knowledge on LAP1's physiological roles in humans is limited; hence, investigation is required to elucidate the critical functions of this protein, which can be achieved by uncovering the molecular consequences of LAP1 depletion, a topic that remains largely unexplored. In this work, the proteome of patient-derived LAP1-deficient fibroblasts carrying a pathological TOR1AIP1 mutation (LAP1 E482A) was quantitatively analyzed to identify global changes in protein abundance levels relatively to control fibroblasts. An in silico functional enrichment analysis of the mass spectrometry-identified differentially expressed proteins was also performed, along with additional in vitro functional assays, to unveil the biological processes that are potentially dysfunctional in LAP1 E482A fibroblasts. Collectively, our findings suggest that LAP1 deficiency may induce significant alterations in various cellular activities, including DNA repair, messenger RNA degradation/translation, proteostasis and glutathione metabolism/antioxidant response. This study sheds light on possible new functions of human LAP1 and could set the basis for subsequent in-depth mechanistic investigations. Moreover, by identifying deregulated signaling pathways in LAP1-deficient cells, our work may offer valuable molecular targets for future disease-modifying therapies for TOR1AIP1-associated nuclear envelopathies.

Kinetic Behavior of Glutathione Transferases: Understanding Cellular Protection from Reactive Intermediates

Glutathione transferases (GSTs) are the primary catalysts protecting from reactive electrophile attack. In this review, the quantitative levels and distribution of glutathione transferases in relation to physiological function are discussed. The catalytic properties (random sequential) tell us that these enzymes have evolved to intercept reactive intermediates. High concentrations of enzymes (up to several hundred micromolar) ensure efficient protection. Individual enzyme molecules, however, turn over only rarely (estimated as low as once daily). The protection of intracellular protein and DNA targets is linearly proportional to enzyme levels. Any lowering of enzyme concentration, or inhibition, would thus result in diminished protection. It is well established that GSTs also function as binding proteins, potentially resulting in enzyme inhibition. Here the relevance of ligand inhibition and catalytic mechanisms, such as negative co-operativity, is discussed. There is a lack of knowledge pertaining to relevant ligand levels in vivo, be they exogenous or endogenous (e.g., bile acids and bilirubin). The stoichiometry of active sites in GSTs is well established, cytosolic enzyme dimers have two sites. It is puzzling that a third of the site's reactivity is observed in trimeric microsomal glutathione transferases (MGSTs). From a physiological point of view, such sub-stoichiometric behavior would appear to be wasteful. Over the years, a substantial amount of detailed knowledge on the structure, distribution, and mechanism of purified GSTs has been gathered. We still lack knowledge on exact cell type distribution and levels in vivo however, especially in relation to ligand levels, which need to be determined. Such knowledge must be gathered in order to allow mathematical modeling to be employed in the future, to generate a holistic understanding of reactive intermediate protection.

Microsomal glutathione transferase 1 in cancer and the regulation of ferroptosis

Microsomal glutathione transferase 1 (MGST1) is a member of the MAPEG family (membrane associated proteins in eicosanoid and glutathione metabolism), defined according to enzymatic activities, sequence motifs, and structural properties. MGST1 is a homotrimer which can bind three molecules of glutathione (GSH), with one modified to a thiolate anion displaying one-third-of-sites-reactivity. MGST1 has both glutathione transferase and peroxidase activities. Each is based on stabilizing the GSH thiolate in the same active site. MGST1 is abundant in the liver and displays a broad subcellular distribution with high levels in endoplasmic reticulum and mitochondrial membranes, consistent with a physiological role in protection from reactive electrophilic intermediates and oxidative stress. In this review paper, we particularly focus on recent advances made in understanding MGST1 activation, induction, broad subcellular distribution, and the role of MGST1 in apoptosis, ferroptosis, cancer progression, and therapeutic responses.

Enzymology of reactive intermediate protection: kinetic analysis and temperature dependence of the mesophilic membrane protein catalyst MGST1

Glutathione transferases (GSTs) are a class of phase II detoxifying enzymes catalysing the conjugation of glutathione (GSH) to endogenous and exogenous electrophilic molecules, with microsomal glutathione transferase 1 (MGST1) being one of its key members. MGST1 forms a homotrimer displaying third-of-the-sites-reactivity and up to 30-fold activation through modification of its Cys-49 residue. It has been shown that the steady-state behaviour of the enzyme at 5 °C can be accounted for by its pre-steady-state behaviour if the presence of a natively activated subpopulation (~ 10%) is assumed. Low temperature was used as the ligand-free enzyme is unstable at higher temperatures. Here, we overcame enzyme lability through stop-flow limited turnover analysis, whereby kinetic parameters at 30 °C were obtained. The acquired data are more physiologically relevant and enable confirmation of the previously established enzyme mechanism (at 5 °C), yielding parameters relevant for in vivo modelling. Interestingly, the kinetic parameter defining toxicant metabolism, k_cat /K_M , is strongly dependent on substrate reactivity (Hammett value 4.2), underscoring that glutathione transferases function as efficient and responsive interception catalysts. The temperature behaviour of the enzyme was also analysed. Both the K_M and K_D values decreased with increasing temperature, while the chemical step k₃ displayed modest temperature dependence (Q₁₀ : 1.1-1.2), mirrored in that of the nonenzymatic reaction (Q₁₀ : 1.1-1.7). Unusually high Q₁₀ values for GSH thiolate anion formation (k₂ : 3.9), k_cat (2.7-5.6) and k_cat /K_M (3.4-5.9) support that large structural transitions govern GSH binding and deprotonation, which limits steady-state catalysis.

Genome-Wide Identification, Evolution and Expression Analysis of the Glutathione S-Transferase Supergene Family in Euphorbiaceae

Euphorbiaceae, a family of plants mainly grown in the tropics and subtropics, is also widely distributed all over the world and is well known for being rich in rubber, oil, medicinal materials, starch, wood and other economically important plant products. Glutathione S-transferases (GSTs) constitute a family of proteins encoded by a large supergene family and are widely expressed in animals, bacteria, fungi and plants, but with few reports of them in Euphorbiaceae plants. These proteins participate in and regulate the detoxification and oxidative stress response of heterogeneous organisms, resistance to stress, growth and development, signal transduction and other related processes. In this study, we identified and analyzed the whole genomes of four species of Euphorbiaceae, namely Ricinus communis, Jatropha curcas, Hevea brasiliensis, and Manihot esculenta, which have high economic and practical value. A total of 244 GST genes were identified. Based on their sequence characteristics and conserved domain types, the GST supergene family in Euphorbiaceae was classified into 10 subfamilies. The GST supergene families of Euphorbiaceae and Arabidopsis have been found to be highly conserved in evolution, and tandem repeats and translocations in these genes have made the greatest contributions to gene amplification here and have experienced strong purification selection. An evolutionary analysis showed that Euphorbiaceae GST genes have also evolved into new subtribes (GSTO, EF1BG, MAPEG), which may play a specific role in Euphorbiaceae. An analysis of expression patterns of the GST supergene family in Euphorbiaceae revealed the functions of these GSTs in different tissues, including resistance to stress and participation in herbicide detoxification. In addition, an interaction analysis was performed to determine the GST gene regulatory mechanism. The results of this study have laid a foundation for further analysis of the functions of the GST supergene family in Euphorbiaceae, especially in stress and herbicide detoxification. The results have also provided new ideas for the study of the regulatory mechanism of the GST supergene family, and have provided a reference for follow-up genetics and breeding work.

Integral Membrane Enzymes in Eicosanoid Metabolism: Structures, Mechanisms and Inhibitor Design

Eicosanoids are potent lipid mediators involved in central physiological processes such as hemostasis, renal function and parturition. When formed in excess, eicosanoids become critical players in a range of pathological conditions, in particular pain, fever, arthritis, asthma, cardiovascular disease and cancer. Eicosanoids are generated via oxidative metabolism of arachidonic acid along the cyclooxygenase (COX) and lipoxygenase (LOX) pathways. Specific lipid species are formed downstream of COX and LOX by specialized synthases, some of which reside on the nuclear and endoplasmic reticulum, including mPGES-1, FLAP, LTC4 synthase, and MGST2. These integral membrane proteins are members of the family "membrane-associated proteins in eicosanoid and glutathione metabolism" (MAPEG). Here we focus on this enzyme family, which encompasses six human members typically catalyzing glutathione dependent transformations of lipophilic substrates. Enzymes of this family have evolved to combat the topographical challenge and unfavorable energetics of bringing together two chemically different substrates, from cytosol and lipid bilayer, for catalysis within a membrane environment. Thus, structural understanding of these enzymes are of utmost importance to unravel their molecular mechanisms, mode of substrate entry and product release, in order to facilitate novel drug design against severe human diseases.

The mercapturic acid pathway

The mercapturic acid pathway is a major route for the biotransformation of xenobiotic and endobiotic electrophilic compounds and their metabolites. Mercapturic acids (N-acetyl-l-cysteine S-conjugates) are formed by the sequential action of the glutathione transferases, γ-glutamyltransferases, dipeptidases, and cysteine S-conjugate N-acetyltransferase to yield glutathione S-conjugates, l-cysteinylglycine S-conjugates, l-cysteine S-conjugates, and mercapturic acids; these metabolites constitute a "mercapturomic" profile. Aminoacylases catalyze the hydrolysis of mercapturic acids to form cysteine S-conjugates. Several renal transport systems facilitate the urinary elimination of mercapturic acids; urinary mercapturic acids may serve as biomarkers for exposure to chemicals. Although mercapturic acid formation and elimination is a detoxication reaction, l-cysteine S-conjugates may undergo bioactivation by cysteine S-conjugate β-lyase. Moreover, some l-cysteine S-conjugates, particularly l-cysteinyl-leukotrienes, exert significant pathophysiological effects. Finally, some enzymes of the mercapturic acid pathway are described as the so-called "moonlighting proteins," catalytic proteins that exert multiple biochemical or biophysical functions apart from catalysis.

Genome-Wide Association Study for Milk Fatty Acids in Holstein Cattle Accounting for the DGAT1 Gene Effect

The identification of genomic regions and candidate genes associated with milk fatty acids contributes to better understand the underlying biology of these traits and enables breeders to modify milk fat composition through genetic selection. The main objectives of this study were: (1) to perform genome-wide association analyses for five groups of milk fatty acids in Holstein cattle using a high-density (777K) SNP panel; and (2) to compare the results of GWAS accounting (or not) for the DGAT1 gene effect as a covariate in the statistical model. The five groups of milk fatty acids analyzed were: (1) saturated (SFA); (2) unsaturated (UFA); (3) short-chain (SCFA); (4) medium-chain (MCFA); and (5) long-chain (LCFA) fatty acids. When DGAT1 was not fitted as a covariate in the model, significant SNPs and candidate genes were identified on BTA5, BTA6, BTA14, BTA16, and BTA19. When fitting the DGAT1 gene in the model, only the MGST1 and PLBD1 genes were identified. Thus, this study suggests that the DGAT1 gene accounts for most of the variability in milk fatty acid composition and the PLBD1 and MGST1 genes are important additional candidate genes in Holstein cattle.

From Cyanobacteria to Human, MAPEG-Type Glutathione-S-Transferases Operate in Cell Tolerance to Heat, Cold, and Lipid Peroxidation

The MAPEG2 sub-family of glutathione-S-transferase proteins (GST) has been poorly investigated in vivo, even in prokaryotes such as cyanobacteria the organisms that are regarded as having developed glutathione-dependent enzymes to protect themselves against the reactive oxygen species (ROS) often produced by their powerful photosynthesis. We report the first in vivo analysis of a cyanobacterial MAPEG2-like protein (Sll1147) in the model cyanobacterium Synechocystis PCC 6803. While Sll1147 is dispensable to cell growth in standard photo-autotrophic conditions, it plays an important role in the resistance to heat and cold, and to n-tertbutyl hydroperoxide (n-tBOOH) that induces lipid peroxidation. These findings suggest that Sll1147 could be involved in membrane fluidity, which is critical for photosynthesis. Attesting its sensitivity to these stresses, the Δsll1147 mutant lacking Sll1147 challenged by heat, cold, or n-tBOOH undergoes transient accumulation of peroxidized lipids and then of reduced and oxidized glutathione. These results are welcome because little is known concerning the signaling and/or protection mechanisms used by cyanobacteria to cope with heat and cold, two inevitable environmental stresses that limit their growth, and thus their production of biomass for our food chain and of biotechnologically interesting chemicals. Also interestingly, the decreased resistance to heat, cold and n-tBOOH of the Δsll1147 mutant could be rescued back to normal (wild-type) levels upon the expression of synthetic MAPEG2-encoding human genes adapted to the cyanobacterial codon usage. These synthetic hmGST2 and hmGST3 genes were also able to increase the Escherichia coli tolerance to heat and n-tBOOH. Collectively, these finding indicate that the activity of the MAPEG2 proteins have been conserved, at least in part, during evolution from (cyano)bacteria to human.

Role of AccMGST1 in oxidative stress resistance in Apis cerana cerana

As detoxification enzymes, proteins in the glutathione S-transferase (GST) superfamily are reported to participate in oxidative stress resistance. Nevertheless, microsomal GSTs (MGSTs), a unique subclass of the GST superfamily associated with membranes, are rarely studied in insects. Here, we isolated an MGST gene in Apis cerana cerana (AccMGST1) and verified its role in oxidative stress response. We found higher expression of AccMGST1 in protective or defensive tissue, that is, the epidermis, which indicated its role in stress resistance. Real-time quantitative PCR (qRT-PCR) analysis indicated that AccMGST1 was upregulated by oxidative stresses at the transcriptional level. In contrast, AccMGST1 expression was inhibited when the antioxidant vitamin C (VC) was fed to experimental bees. Through western blotting, we found that the protein level of AccMGST1 under oxidative stress corresponded to the transcript level. Disc diffusion and mixed-function oxidation (MFO) assays suggested that AccMGST1 can protect not only cells but also DNA against oxidative damage. Furthermore, we discovered that the expression patterns of known antioxidant genes were changed in A. cerana cerana after AccMGST1 was silenced by RNA interference (RNAi). Thus, we concluded that the gene AccMGST1 exerts a significant role in the antioxidant mechanism.

Differences of RNA Expression in the Tendon According to Anatomic Outcomes in Rotator Cuff Repair

BACKGROUND

Despite increased understanding of the pathophysiology of rotator cuff tears and the evolution of rotator cuff repair, healing failure remains a substantial problem. The critical roles played by biological factors have been emphasized, but little is known of the implications of gene expression profile differences at the time of repair.

PURPOSE

To document the relationship between the perioperative gene expression of healed and unhealed rotator cuffs by RNA microarray analysis.

STUDY DESIGN

Case-control study; Level of evidence, 3.

METHODS

Superior (supraspinatus involvement) and posterosuperior (supraspinatus and infraspinatus involvement) tears were included in the study. Samples of rotator cuff tendons were prospectively collected during rotator cuff surgery. Three samples were harvested at the tendon ends of tears from the anterior, middle (apex), and posterior parts using an arthroscopic punch. Seven patients with an unhealed rotator cuff were matched one-to-one with patients with a healed rotator cuff by sex, age, tear size, and fatty degeneration of rotator cuff muscles. mRNA microarray analysis was used to identify genetic differences between healed and unhealed rotator cuff tendons. Gene ontology and gene association files were obtained from the Gene Ontology Consortium, and the Gene Ontology system in DAVID was used to identify enhanced biological processes.

RESULTS

Microarray analyses identified 262 genes that were differentially expressed by at least 1.5-fold between the healed and unhealed groups. Overall, in the healed group, 103 genes were significantly downregulated, and 159 were significantly upregulated. DAVID Functional Annotation Cluster analysis showed that in the healed group, the genes most upregulated were related to the G protein-coupled receptor protein signaling pathway and to the neurological system. On the other hand, the genes most downregulated were related to immune and inflammatory responses. BMP5 was the gene most upregulated in the healed group, and the majority of downregulated genes were involved in the immune/inflammatory response.

CONCLUSION

The downregulation of inflammatory response genes and the upregulation of cell differentiation genes in torn rotator cuffs at the time of surgery are related to rotator cuff healing. These results provide useful baseline information for future biological studies on rotator cuff healing.

Catalytic Conversion of Lipophilic Substrates by Phase constrained Enzymes in the Aqueous or in the Membrane Phase

Both soluble and membrane-bound enzymes can catalyze the conversion of lipophilic substrates. The precise substrate access path, with regard to phase, has however, until now relied on conjecture from enzyme structural data only (certainly giving credible and valuable hypotheses). Alternative methods have been missing. To obtain the first experimental evidence directly determining the access paths (of lipophilic substrates) to phase constrained enzymes we here describe the application of a BODIPY-derived substrate (PS1). Using this tool, which is not accessible to cytosolic enzymes in the presence of detergent and, by contrast, not accessible to membrane embedded enzymes in the absence of detergent, we demonstrate that cytosolic and microsomal glutathione transferases (GSTs), both catalyzing the activation of PS1, do so only within their respective phases. This approach can serve as a guideline to experimentally validate substrate access paths, a fundamental property of phase restricted enzymes. Examples of other enzyme classes with members in both phases are xenobiotic-metabolizing sulphotransferases/UDP-glucuronosyl transferases or epoxide hydrolases. Since specific GSTs have been suggested to contribute to tumor drug resistance, PS1 can also be utilized as a tool to discriminate between phase constrained members of these enzymes by analyzing samples in the absence and presence of Triton X-100.

Genome-wide association study using high-density single nucleotide polymorphism arrays and whole-genome sequences for clinical mastitis traits in dairy cattle

Mastitis is a mammary disease that frequently affects dairy cattle. Despite considerable research on the development of effective prevention and treatment strategies, mastitis continues to be a significant issue in bovine veterinary medicine. To identify major genes that affect mastitis in dairy cattle, 6 chromosomal regions on Bos taurus autosome (BTA) 6, 13, 16, 19, and 20 were selected from a genome scan for 9 mastitis phenotypes using imputed high-density single nucleotide polymorphism arrays. Association analyses using sequence-level variants for the 6 targeted regions were carried out to map causal variants using whole-genome sequence data from 3 breeds. The quantitative trait loci (QTL) discovery population comprised 4,992 progeny-tested Holstein bulls, and QTL were confirmed in 4,442 Nordic Red and 1,126 Jersey cattle. The targeted regions were imputed to the sequence level. The highest association signal for clinical mastitis was observed on BTA 6 at 88.97 Mb in Holstein cattle and was confirmed in Nordic Red cattle. The peak association region on BTA 6 contained 2 genes: vitamin D-binding protein precursor (GC) and neuropeptide FF receptor 2 (NPFFR2), which, based on known biological functions, are good candidates for affecting mastitis. However, strong linkage disequilibrium in this region prevented conclusive determination of the causal gene. A different QTL on BTA 6 located at 88.32 Mb in Holstein cattle affected mastitis. In addition, QTL on BTA 13 and 19 were confirmed to segregate in Nordic Red cattle and QTL on BTA 16 and 20 were confirmed in Jersey cattle. Although several candidate genes were identified in these targeted regions, it was not possible to identify a gene or polymorphism as the causal factor for any of these regions.

Microsomal glutathione transferase 1 protects against toxicity induced by silica nanoparticles but not by zinc oxide nanoparticles

Microsomal glutathione transferase 1 (MGST1) is an antioxidant enzyme located predominantly in the mitochondrial outer membrane and endoplasmic reticulum and has been shown to protect cells from lipid peroxidation induced by a variety of cytostatic drugs and pro-oxidant stimuli. We hypothesized that MGST1 may also protect against nanomaterial-induced cytotoxicity through a specific effect on lipid peroxidation. We evaluated the induction of cytotoxicity and oxidative stress by TiO(2), CeO(2), SiO(2), and ZnO in the human MCF-7 cell line with or without overexpression of MGST1. SiO(2) and ZnO nanoparticles caused dose- and time-dependent toxicity, whereas no obvious cytotoxic effects were induced by nanoparticles of TiO(2) and CeO(2). We also noted pronounced cytotoxicity for three out of four additional SiO(2) nanoparticles tested. Overexpression of MGST1 reversed the cytotoxicity of the main SiO(2) nanoparticles tested and for one of the supplementary SiO(2) nanoparticles but did not protect cells against ZnO-induced cytotoxic effects. The data point toward a role of lipid peroxidation in SiO(2) nanoparticle-induced cell death. For ZnO nanoparticles, rapid dissolution was observed, and the subsequent interaction of Zn(2+) with cellular targets is likely to contribute to the cytotoxic effects. A direct inhibition of MGST1 by Zn(2+) could provide a possible explanation for the lack of protection against ZnO nanoparticles in this model. Our data also showed that SiO(2) nanoparticle-induced cytotoxicity is mitigated in the presence of serum, potentially through masking of reactive surface groups by serum proteins, whereas ZnO nanoparticles were cytotoxic both in the presence and in the absence of serum.

Cloning of a novel glutathione S-transferase 3 (GST3) gene and expressionanalysis in pearl oyster, Pinctada martensii

Microsomal glutathione S-transferase (MGST) functions in cellular defense against xenobiotics and provides protection against the action of lipid hydroperoxides produced as a consequence of oxidative stress. In this study, a full-length cDNA encoding MGST3 (referred to as PmMGST3) was identified from the pearl oyster, Pinctada martensii by a combination of expressed sequence tag (EST) analysis and rapid amplification of cDNA ends (RACE). The full-length cDNA of PmMGST3 is 971 bp and contains a 5' UTR of 39 bp, a 3' UTR of 491 bp with a canonical polyadenylation signal sequence (AATAAA), and an open reading frame (ORF) of 447 bp encoding a polypeptide of 146 residues. The deduced polypeptide contains a conserved motif (FNCx(1)QRx(2)H) characteristic of the MGST3 subfamily. The PmMGST3 transcript could be detected in all tissues tested, with highest transcript level seen in hepatopancreas. Cadmium treatment significantly increased PmMGST3 mRNA levels in gill and hepatopancreas, while bacterial challenge initially depressed mRNA levels and then increased its level in haemocytes, gill and hepatopancreas in a time-dependent manner. In an assay using cumene hydroperoxide as a substrate, we demonstrated that PmMGST3 possesses glutathione-dependent peroxidase activity. These results suggest that PmMGST3 plays an important role in cellular defense against oxidative stress caused by cadmium and bacteria.

Microsomal glutathione transferase 1: mechanism and functional roles

Microsomal glutathione transferase 1 (MGST1) belongs to a superfamily named MAPEG (membrane-associated proteins in eicosanoid and glutathione metabolism). This family is represented in all life forms, except archae. Of the six human members, three are specialized in the synthesis of leukotrienes and prostaglandin E, whereas the others (MGST1-3) have potential roles in drug metabolism. MGST1 has a well-established role in the conjugation of electrophiles and oxidative stress protection, whereas MGST2 and 3 have been less studied. Here, we review the recent advances regarding the structure, mechanism, and functional roles of MGST1. Emerging data show that the enzyme is overexpressed in certain tumors and support a role for the enzyme in protecting cells from cytostatic drugs.

Role of MGST1 in reactive intermediate-induced injury

Microsomal glutathione transferase (MGST1, EC 2.5.1.18) is a membrane bound glutathione transferase extensively studied for its ability to detoxify reactive intermediates, including metabolic electrophile intermediates and lipophilic hydroperoxides through its glutathione dependent transferase and peroxidase activities. It is expressed in high amounts in the liver, located both in the endoplasmic reticulum and the inner and outer mitochondrial membranes. This enzyme is activated by oxidative stress. Binding of GSH and modification of cysteine 49 (the oxidative stress sensor) has been shown to increase activation and induce conformational changes in the enzyme. These changes have either been shown to enhance the protective effect ascribed to this enzyme or have been shown to contribute to cell death through mitochondrial permeability transition pore formation. The purpose of this review is to elucidate how one enzyme found in two places in the cell subjected to the same conditions of oxidative stress could both help protect against and contribute to reactive oxygen species-induced liver injury.

UVB light regulates expression of antioxidants and inflammatory mediators in human corneal epithelial cells

The cornea is highly sensitive to ultraviolet B (UVB) light-induced oxidative stress, a process that results in the production of inflammatory mediators which have been implicated in tissue injury. In the present studies, we characterized the inflammatory response of human corneal epithelial cells to UVB (2.5-25mJ/cm(2)). UVB caused a dose-dependent increase in the generation of reactive oxygen species in the cells. This was associated with increases in mRNA expression of the antioxidants Cu,Zn superoxide dismutase (SOD), Mn-SOD, catalase and heme oxygenase-1 (HO-1), as well as the glutathione S-transferases (GST), GSTA1-2, GSTA3, GSTA4, GSTM1, and mGST2. UVB also upregulated expression of the proinflammatory cytokines, IFNγ, IL-1β, TGFβ and TNFα, and enzymes important in prostaglandin (PG) biosynthesis including cyclooxygenase-2 (COX-2) and the PG synthases mPGES-2, PGDS, PGFS and thromboxane synthase, and in leukotriene biosynthesis including 5-lipoxygenase (5-LOX), 15-LOX-2, and the epidermal and platelet forms of 12-LOX. UVB was found to activate JNK and p38 MAP kinases in corneal epithelial cells; ERK1/2 MAP kinase was found to be constitutively active, and its activity increased following UVB treatment. Inhibition of p38 blocked UVB-induced expression of TNFα, COX-2, PGDS and 15-LOX-2, while JNK inhibition suppressed TNFα and HO-1. These data indicate that UVB modulates corneal epithelial cell expression of antioxidants and proinflammatory mediators by distinct mechanisms. Alterations in expression of these mediators are likely to be important in regulating inflammation and protecting the cornea from UVB-induced oxidative stress.

Identification of liver proteins and their roles associated with carbon tetrachloride-induced hepatotoxicity

Carbon tetrachloride (CCl(4)) is a common hepatotoxin used in experimental models to elicit liver injury. To identify the proteins involved in CCl(4)-induced hepatotoxicity, two-dimensional gel electrophoresis was employed followed by mass spectrometry - mass spectrometry (MS/MS) to study the differentially expressed proteins during CCl(4) exposure in the Fischer 344 rat liver proteome for 5 weeks. Ten spots with notable changes between the Control and CCl(4) groups were successfully identified. Among them, four proteins with significant up-regulation, namely calcium-binding protein 1, protein disulfide isomerase, mitochondrial aldehyde dehydrogenase precursor, and, glutathione-S-transferase mu1 and six proteins with significant down-regulation, namely catechol-O-methyltransferase, hemoglobin-alpha-2-chain, hemopexin precursor, methionine sulfoxide reductase A, catalase and carbonic anhydrase 3, were identified. The data indicates that CCl(4) causes hepatotoxicity by depleting oxygen radical scavengers in the hepatocytes. In this rat model, we profiled hepatic proteome alterations in response to CCl(4) intoxication. The findings should facilitate understanding of the mechanism of CCl(4)-induced liver injury.

Multiple roles of microsomal glutathione transferase 1 in cellular protection: a mechanistic study

The aim of this study was to investigate the involvement of membrane-bound microsomal glutathione transferase 1 (MGST1) in cellular resistance against oxidative stress as well as its mechanism of protection. MGST1 is ubiquitously expressed and predominantly located in the endoplasmic reticulum and outer mitochondrial membrane. Utilizing MCF7 cells overexpressing MGST1 we show significant protection against agents that are known to induce lipid peroxidation (e.g., cumene hydroperoxide and tert-butylhydroperoxide) and an end-product of lipid peroxidation (e.g., 4-hydroxy-2-nonenal). Furthermore, our results demonstrate that MGST1 protection can be enhanced by vitamin E when toxicity depends on oxidative stress, but not when direct alkylation is the dominant mechanism. Mitochondria in MGST1-overexpressing cells were shown to be protected from oxidative insult as measured by calcium loading capacity and respiration. MGST1 induces cellular resistance against cisplatin. Here we used vitamin E to elucidate whether oxidative stress caused by cisplatin is significant for cell toxicity. The results indicate that oxidative stress and induction of lipid peroxidation are not the most prominent toxic mechanism of cisplatin in our cell system. We thus conclude that MGST1 protects cells (and mitochondria) by both conjugation and glutathione peroxidase functions. A new protective mechanism against cisplatin is also indicated.

Induction of radical scavenging ability and suppression of lipid peroxidation in rat liver microsomes following whole-body, low-dose X-irradiation

PURPOSE

To investigate changes in radical scavenging ability and lipid peroxidation in liver microsomal membranes and cooperative suppression of lipid peroxidation by microsomal and cytosolic radical scavengers, 24 h after whole-body, low-dose X-irradiation of rats.

MATERIALS AND METHODS

Male Wistar rats were irradiated with 1-50 cGy of X-rays. Liver microsomal radical scavenging ability was determined using the trapping ability of 1,1-diphenyl-2-picrylhydrazyl (DPPH), a stable free radical. Microsomal alpha-tocopherol (Vit.E) content was determined using an electrochemical detector. Microsomal glutathione peroxidase (GPx) activity was determined as the consuming rate of NADPH. Microsomal lipid peroxidation was determined by the thiobarbituric acid method.

RESULTS

Low molecular weight radical scavenging ability of rat liver microsomes increased 24 h after whole-body, low-dose X-irradiation when alpha-tocopherol was included, showing a maximum level at 5-10 cGy. Microsomal GPx activity also increased 24 h after 5 cGy irradiation. The lipid peroxidation level in microsomes decreased, showing a maximal suppression at 5 cGy. High-dose irradiation-induced microsomal lipid peroxidation was strongly suppressed cooperatively by microsomal and cytosolic antioxidants induced by low-dose irradiation.

CONCLUSION

Low doses of radiation induce increases in liver microsomal antioxidants, which in turn result in enhanced suppression of microsomal lipid peroxidation cooperatively with cytosolic antioxidants induced by low-dose irradiation.

Microsomal glutathione transferase 1 exhibits one-third-of-the-sites-reactivity towards glutathione

The trimeric membrane protein microsomal glutathione transferase 1 (MGST1) possesses glutathione transferase and peroxidase activity. Previous data indicated one active site/trimer whereas structural data suggests three GSH-binding sites. Here we have determined ligand interactions of MGST1 by several techniques. Nanoelectrospray mass spectrometry of native MGST1 revealed binding of three GSH molecules/trimer and equilibrium dialysis showed three product molecules/trimer (K(d)=320+/-50 microM). All three product molecules could be competed out with GSH. Reinvestigation of GSH-binding showed one high affinity site per trimer, consistent with earlier data. Using single turnover stopped flow kinetic measurements, K(d) could be determined for a low affinity GSH-binding site (2.5+/-0.5 mM). Thus we can reconcile previous observations and show here that MGST1 contains three active sites with different affinities for GSH and that only the high affinity site is catalytically competent.

Increased oxidative stress and antioxidant expression in mouse keratinocytes following exposure to paraquat

Paraquat (1,1'-dimethyl-4,4'-bipyridinium) is a widely used herbicide known to induce skin toxicity. This is thought to be due to oxidative stress resulting from the generation of cytotoxic reactive oxygen intermediates (ROI) during paraquat redox cycling. The skin contains a diverse array of antioxidant enzymes which protect against oxidative stress including superoxide dismutase (SOD), catalase, glutathione peroxidase-1 (GPx-1), heme oxygenase-1 (HO-1), metallothionein-2 (MT-2), and glutathione-S-transferases (GST). In the present studies we compared paraquat redox cycling in primary cultures of undifferentiated and differentiated mouse keratinocytes and determined if this was associated with oxidative stress and altered expression of antioxidant enzymes. We found that paraquat readily undergoes redox cycling in both undifferentiated and differentiated keratinocytes, generating superoxide anion and hydrogen peroxide as well as increased protein oxidation which was greater in differentiated cells. Paraquat treatment also resulted in increased expression of HO-1, Cu,Zn-SOD, catalase, GSTP1, GSTA3 and GSTA4. However, no major differences in expression of these enzymes were evident between undifferentiated and differentiated cells. In contrast, expression of GSTA1-2 was significantly greater in differentiated relative to undifferentiated cells after paraquat treatment. No changes in expression of MT-2, Mn-SOD, GPx-1, GSTM1 or the microsomal GST's mGST1, mGST2 and mGST3, were observed in response to paraquat. These data demonstrate that paraquat induces oxidative stress in keratinocytes leading to increased expression of antioxidant genes. These intracellular proteins may be important in protecting the skin from paraquat-mediated cytotoxicity.

Distinct effects of ultraviolet B light on antioxidant expression in undifferentiated and differentiated mouse keratinocytes

Ultraviolet (UV) B causes oxidative stress, which has been implicated in carcinogenesis. We determined if the sensitivity of keratinocytes to UVB-induced oxidative stress is dependent on their differentiation state. In primary cultures of undifferentiated and differentiated mouse keratinocytes, UVB (25 mJ/cm(2)) stimulated production of reactive oxygen intermediates. This was associated with increased messenger RNA (mRNA) expression of the antioxidant enzymes glutathione peroxidase, heme oxygenase-1 (HO-1) and the glutathione S-transferase (GST), GSTA1-2. The effects of UVB on GSTA1-2 were greater in undifferentiated when compared with differentiated cells. UVB also induced GSTM1, but only in undifferentiated cells. In contrast, UVB reduced expression of manganese superoxide dismutase, metallothionein-2, GSTA3 and microsomal glutathione S-transferase (mGST)3 in both cell types, whereas it had no major effects on catalase, copper-zinc superoxide dismutase, GSTP1, mGST1 or mGST2. Of note, levels of GSTA4 mRNA were 4- to 5-fold greater in differentiated relative to undifferentiated cells. Moreover, whereas GSTA4 was induced by UVB in undifferentiated cells, it was inhibited in differentiated cells. UVB activated p38 and c-jun N-terminal kinase mitogen-activated protein (MAP) kinases in both undifferentiated and differentiated keratinocytes. Whereas inhibition of these kinases blocked UVB-induced HO-1 in both cell types, GSTA1-2 and GST-4 were only suppressed in undifferentiated cells. In differentiated keratinocytes, p38 inhibition also suppressed GSTA1-2. In contrast, MAP kinase inhibition had no major effects on UVB-induced suppression of GSTA4 in differentiated cells. These data indicate that UVB-induced alterations in antioxidant expression are differentiation dependent. Moreover, MAP kinases are critical regulators of this response. Alterations in antioxidants are likely to be important mechanisms for protecting the skin from UVB-induced oxidative stress.

Characterization of Taenia solium cysticerci microsomal glutathione S-transferase activity

Glutathione S-transferase activity has been shown to be associated with the microsomal fraction of Taenia solium. Electron microscopy and subcellular enzyme markers indicate the purity of the microsomal fraction that contains the glutathione S-transferase activity. T. solium microsomes were solubilized under conditions used to solubilize integral microsomal proteins. This procedure proved necessary to obtain enzymatic activity. To characterize this parasite enzyme activity, several substrates and inhibitors were used. The optimum activity for microsomal glutathione S-transferase was found to be pH 6.6, with a specific enzyme activity of 0.9, 0.1, 0.067, 0.03, and 0.05 micromol min(-1) mg(-1) with the substrates 1-chloro-2,4-dinitrobenzene (CDNB), 1,2-dichloro-4-nitrobenzene, 4-hydroxynonenal, 2,4-hexadienal, and trans-2-nonenal, respectively. No activity of glutathione peroxidase was observed. T. solium microsomes had an appKm (GSH)=0.161 microM, appKm (CDNB)=14.5 microM, and appVmax of 0.15 and 27.9 micromol min(-1) mg(-1) for GSH and CDNB, respectively. T. solium microsomes were inhibited by several glutathione S-transferase enzyme inhibitors, and it was possible to establish a simple inhibition system as well as corresponding Ki's for each inhibitor. These results indicate that the T. solium microsomal glutathione S-transferase is different from the parasite cytoplasmic enzymes that catalyze similar reactions.

Protection of cells from oxidative stress by microsomal glutathione transferase 1

Rat liver microsomal glutathione transferase 1 (MGST1) is a membrane-bound enzyme that displays both glutathione transferase and glutathione peroxidase activities. We hypothesized that physiologically relevant levels of MGST1 is able to protect cells from oxidative damage by lowering intracellular hydroperoxide levels. Such a role of MGST1 was studied in human MCF7 cell line transfected with rat liver mgst1 (sense cell) and with antisense mgst1 (antisense cell). Cytotoxicities of two hydroperoxides (cumene hydroperoxide (CuOOH) and hydrogen peroxide) were determined in both cell types using short-term and long-term cytotoxicity assays. MGST1 significantly protected against CuOOH and against hydrogen peroxide (although less pronounced and only in short-term tests). These results demonstrate that MGST1 can protect cells from both lipophilic and hydrophilic hydroperoxides, of which only the former is a substrate. After CuOOH exposure MGST1 significantly lowered intracellular ROS as determined by FACS analysis.

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.

Glutathione transferases in the genomics era: new insights and perspectives

In the last decade the tumultuous development of "omics" greatly improved our ability to understand protein structure, function and evolution, and to define their roles and networks in complex biological processes. This fast accumulating knowledge holds great potential for biotechnological applications, from the development of biomolecules with novel properties of industrial and medical importance, to the creation of transgenic organisms with new, favorable characteristics. This review focuses on glutathione transferases (GSTs), an ancient protein superfamily with multiple roles in all eukaryotic organisms, and attempts to give an overview of the new insights and perspectives provided by omics into the biology of these proteins. Among the aspects considered are the redefinition of GST subfamilies, their evolution in connection with structurally related families, present and future biotechnological outcomes.

Microsomal Glutathione Transferase 1

Microsomal glutathione transferase 1 (MGST1) is an abundant membrane-bound glutathione transferase and peroxidase constituting 3% of the endoplasmic reticulum protein in rat liver (and 5% of the outer mitochondrial membrane). The enzyme is most well studied in mammals and belongs to a large and widely distributed superfamily. Cellular and organelle protection versus oxidative stress has been demonstrated. The enzyme displays activity to a multitude of reactive substrates ranging from products of lipid peroxidation to cytostatic drugs. The methods developed for the study of MGST1 by necessity differs from that of cytosolic glutathione transferases, because detergents or lipids are included. Here, purification, assay, and preparation procedures that maintain the enzyme in its native functional state during isolation and characterization are described. Microsomal glutathione transferase 1 is activated by sulfhydryl reagents (and proteolysis), and procedures for activation and study of the activated enzyme are described. In new developments, the enzyme is studied by pre-steady state methods, as well as mass spectrometry involving direct observation of the native enzyme.

Structural organization of the murine microsomal glutathione S-transferase gene (MGST1) from the 129/SvJ strain: identification of the promoter region and a comprehensive examination of tissue expression

The structure and regulation of the murine microsomal glutathione transferase gene (MGST1) from the 129/SvJ strain is described and demonstrates considerable difference in nucleotide sequence and consequently in restriction enzyme sites as compared to other mouse strains. A comparison of the amino acid sequence for MGST1 revealed one difference in exon 2 between the 129/SvJ strain (arginine at position 5) and the sequence previously reported for the Balb/c strain (lysine). The promoter region immediately upstream of the dominant first exon is functional, transcriptionally responds to oxidative stress, and is highly homologous to the human region. Oxidative stress also induced the production of endogenous MGST1 mRNA. The tissue-specific expression of MGST1 mRNA was studied, and as anticipated, was indeed highest in liver. There was, however, marked mRNA expression in several tissues not previously studied including smooth muscle, epidymus, ovaries, and endocrine glands in which the expression of various peroxidases is also very high (salivary and thyroid). Overall, there was a good agreement between the mRNA content detected and previous reports of MGST1 activity with the exception of brain tissue.

Characterization of a class alpha glutathione-S-transferase with glutathione peroxidase activity in human liver microsomes

A 25.5kDa class alpha glutathione S-transferase (GST) designated as microsomal Ya-GST or M-GSTA has been purified to electrophoretic homogeneity from human liver microsomes. Limited proteolysis, gel filtration chromatography followed by EDTA, and alkaline Na(2)CO(3) treatments of microsomes indicate that the M-GSTA is intrinsic to the microsomes. Western immunoblot analysis revealed that human liver M-GSTA and the previously reported 17-kDa microsomal GST (FEBS Lett. 315 (1993) 77) did not have immunological cross reactivity. The enzyme showed conjugation activity towards substrates like 1-chloro-2,4-nitrobenzene (CDNB) and 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole, and 4-hydroxy-2-nonenal (4-HNE), a genotoxic alpha,beta-unsaturated aldehyde product of lipid peroxidation. In addition, the M-GSTA exhibited significant glutathione peroxidase activity towards physiologically relevant fatty acid hydroperoxides as well as phosphatidylcholine hydroperoxide, but not with H(2)O(2). C-terminal amino acid sequence analysis revealed a high homology with the human liver cytosolic GST-A1 and A3 isozymes. Western immunoblot analyses of the microsomes prepared from human hepatoblastoma (HepG2) showed that the expression of this M-GSTA was induced upon treatment with such prooxidants as H(2)O(2), suggesting that it may play an important role in the protection of cellular membranes from peroxidative damage.

Expression of the murine glutathione S-transferase α3 (GSTA3) subunit is markedly induced during adipocyte differentiation: activation of the GSTA3 gene promoter by the pro-adipogenic eicosanoid 15-deoxy-Δ12,14-prostaglandin J2

The expression of class alpha, micro, and pi glutathione S-transferases (GSTs) has been examined during the adipose conversion of mouse 3T3-L1 cells. The GSTA4, GSTM1, and GSTP1/2 subunits are expressed constitutively in confluent 3T3-L1 cells, and their levels remain essentially unchanged during adipocyte differentiation. By contrast, the GSTA3 subunit is virtually undetectable in confluent 3T3-L1 cells under basal conditions, but its expression is markedly induced during adipose conversion. Inhibition of the 3T3-L1 adipogenic program demonstrated that GSTA3 expression is associated specifically with acquisition of the adipocytic phenotype. Reporter gene assays demonstrated that the mouse GSTA3 5(')-upstream region is transcriptionally activated by 15-deoxy-Delta(12,14)-prostaglandin J(2) through an antioxidant response element, suggesting that this pro-adipogenic eicosanoid may be involved in regulating GSTA3 expression during adipogenesis. These data suggest a previously unrecognised role for GSTs in mouse adipocytes.

Molecular basis of basal cell carcinoma: analysis of differential gene expression by differential display PCR and expression array

Basal cell carcinoma (BCC) is the most common tumor in the Caucasian population. Although BCC rarely metastasize and cause death, they are problematic due to their destructive growth and the frequent localization on the face. Until now the knowledge of genes differentially expressed in BCC has been incomplete. To elucidate the complex alterations in BCC-associated gene expression, we took advantage of 2 techniques: the differential display RT-PCR (DD-PCR) and the differential hybridization of cDNA arrays. Using DD-PCR, we showed differential expression of genes known from other biological contexts (e.g., rac, ubiquitin hydrolase), which could now be associated with BCC. In addition, we detected unknown genes possibly contributing to the carcinogenesis of BCC. Of the 588 genes screened by differential hybridization of the Atlas human cDNA array, differences in the expression levels of BCC were observed for 10 genes. These data were obtained with RNA probes pooled from several BCC of different donors and were subsequently confirmed by semiquantitative RT-PCR for Janus protein tyrosine kinase 3 (Jak3), microsomal glutathione S-transferase 1 (GST 12), teratocarcinoma-derived growth factor cripto, glutaredoxin and the monocyte chemoattractant protein 1 (MCP-1) in 10 individual BCC specimens, 2 squamous cell carcinoma (SCC), the cell line HaCaT and cultured normal human keratinocytes (NHK) in comparison to normal skin. These genes are candidates from gene families with known association to tumors, but they have not been reported in the carcinogenesis of BCC yet. In summary, both approaches allow the detection of differentially expressed genes possibly involved in the carcinogenesis of BCC.

Reactive intermediates and the dynamics of glutathione transferases

Reactive intermediates are a continuous burden in biology and several defense mechanisms have evolved. Here we focus on the functions of glutathione transferases (GSTs) with the aim to discuss the quantitative aspects of defense against reactive intermediates. Humans excrete approximately 0.1 mmol of thioether conjugates per day. As the amount of GST active sites in liver is approximately 0.5 mmol, it appears that glutathione transferase catalysts are present in tremendous excess. In fact, the known catalytic properties of GSTs reveal that the enzymes can empty the liver glutathione (GSH) pool in a matter of seconds when provided with a suitable substrate. However, based on the urinary output of conjugates (or derivatives thereof), individual GSTs turn over (i.e., catalyze a single reaction) only once every few days. Glutathione transferase overcapacity reflects the fact that there is a linear relation between GST enzyme amount and protection level (provided that GSH is not depleted). Put in a different perspective, a few reactive molecules will always escape conjugation and reach cellular targets. It is therefore not surprising that signaling systems sensing reactive intermediates have evolved resulting in the increase of GSH and GST levels. Precisely for this reason, more moderately reactive electrophiles (Michael acceptors) are receiving growing interest due to their anticarcinogenic properties. Another putative regulatory mechanism involves direct activation of microsomal GST1 by thiol-reactive electrophiles through cysteine 49. The toxicological significance of low levels of reactive intermediates are of interest also in drug development, and here we discuss the use of microsomal GST1 activation as a surrogate detection marker.

Protective effect of carnosol on CCl(4)-induced acute liver damage in rats

BACKGROUND

We recently reported that (Lamiaceae) may alleviate CCl(4)-induced acute hepatotoxicity in rats, possibly blocking the formation of free radicals generated during CCl(4) metabolism. Carnosol, one of the main constituents of Rosmarinus, has been shown to have antioxidant and scavenging activities. Therefore, it is plausible to expect that carnosol may mediate some of the effects of Rosmarinus on oxidative stress consequences induced by CCl(4) in the liver.

DESIGN

We evaluated the effectiveness of carnosol to normalize biochemical and histological parameters of CCl(4)-induced acute liver injury.

METHODS

Male Sprague Dawley rats (n = 5) injured by CCl(4) (oral dose 4 g/kg of body weight) were treated with a single intraperitoneal dose (5 mg/kg) of carnosol. Twenty-four hours later, the rats were anaesthetized deeply to obtain the liver and blood, and biochemical and histological parameters of liver injury were evaluated.

RESULTS

Carnosol normalized bilirubin plasma levels, reduced malondialdehyde (MDA) content in the liver by 69%, reduced alanine aminotransferase (ALT) activity in plasma by 50%, and partially prevented the fall of liver glycogen content and distortion of the liver parenchyma.

CONCLUSIONS

Carnosol prevents acute liver damage, possibly by improving the structural integrity of the hepatocytes. To achieve this, carnosol could scavenge free radicals induced by CCl(4), consequently avoiding the propagation of lipid peroxides. It is suggested that at least some of the beneficial properties of Rosmarinus officinalis are due to carnosol.

Transcript profiling of enzymes involved in detoxification of xenobiotics and reactive oxygen in human normal and simian virus 40 T antigen-immortalized oral keratinocytes

The metabolic detoxification capacity may critically regulate the susceptibility of human tissues to cancer development. We used standardized and quantitative, reverse transcription-polymerase chain reaction (StaRT-PCR) and microarray chip techniques to analyze transcript levels of multiple detoxification enzymes in cultured normal human oral keratinocytes (NOK) and the Siman virus 40 T antigen-immortalized oral keratinocyte line SVpgC2a, viewing the latter as a model of a benign tumor state. With good agreement between the 2 methodologies, NOK and SVpgC2a were found to express transcripts for cytochrome P450 enzymes (CYPs), factors related to CYP induction and enzymes involved in conjugation reactions or detoxification of reactive oxygen. The cell types expressed similar levels of CYP 2B6/7, CYP 2E1, P450 oxidoreductase, the aryl hydrocarbon receptor nuclear translocator, sulfotransferase 1A1, sulfotransferase 1A3, epoxide hydrolase, glutathione S-transferase M3, glutathione S-transferase pi and catalase, superoxide dismutase 1, glutathione peroxidase 1 and glutathione peroxidase 3. In contrast, SVpgC2a exhibited comparatively higher levels of CYP1A1, 1B1, aryl hydrocarbon receptor, glutathione S-transferase M1, 2, 4, 5, glutathione S-transferase theta 1 and superoxide dismutase 2 and comparatively lower levels of UDP glycosyltransferase 2 and microsomal glutathione S-transferase 1. Some transcripts, e.g., CYP 2A6/7, were not detected by either standard, non quantitative RT-PCR or the above methods, whereas others were barely quantifiable by StaRT-PCR, i.e., were present at 1-10 molecules/10(6) molecules of actin. Overall, the expression analysis demonstrated presence of mRNA for multiple enzymes involved in foreign compound metabolism and detoxification pathways, including several enzymes not previously reported for oral epithelium. Generally, the comparison of NOK from 2 individuals indicated relatively similar transcript levels of these enzymes. In contrast, differences between NOK and SVpgC2a, e.g., for CYP1B1, may reflect alteration caused by immortalization and aid identification of early stage tumor markers in oral epithelium.

Catalytic function ofDrosophila melanogasterglutathioneS-transferase DmGSTS1-1 (GST-2) in conjugation of lipid peroxidation end products

Drosophila melanogaster glutathione S-transferase DmGSTS1-1 (earlier designated as GST-2) is related to sigma class GSTs and was previously described as an indirect flight muscle-associated protein with no known catalytic properties. We now report that DmGSTS1-1 isolated from Drosophila or expressed in Escherichia coli is essentially inactive toward the commonly used synthetic substrate 1-chloro-2,4-dinitrobenzene (CDNB), but has relatively high glutathione-conjugating activity for 4-hydroxynonenal (4-HNE), an electrophilic aldehyde derived from lipid peroxidation. 4-HNE is thought to have signaling functions and, at higher concentrations, has been shown to be cytotoxic and involved in the etiology of various degenerative diseases. Drosophila strains carrying P-element insertions in the GstS1 gene have a reduced capacity for glutathione conjugation of 4-HNE. In flies with both, one, or none of the GstS1 alleles disrupted by P-element insertion, there is a linear correlation between DmGSTS1-1 protein content and 4-HNE-conjugating activity. This correlation indicates that in adult Drosophila 70 +/- 6% of the capacity to conjugate 4-HNE is attributable to DmGSTS1-1. The high abundance of DmGSTS1-1 (approximately 2% of the soluble protein in adult flies) and its previously reported localization in tissues that are either highly aerobic (indirect flight muscle) or especially sensitive to oxidative damage (neuronal tissue) suggest that the enzyme may have a protective role against deleterious effects of oxidative stress. Such function in insects would be analogous to that carried out in mammals by specialized alpha class glutathione S-transferases (e.g. GSTA4-4). The independent emergence of 4-HNE-conjugating activity in more than one branch of the glutathione S-transferase superfamily suggests that 4-HNE catabolism may be essential for aerobic life.

Microsomal glutathione S-transferase A1-1 with glutathione peroxidase activity from sheep liver: molecular cloning, expression and characterization

A 25 kDa subunit of glutathione S-transferase (GST) from sheep liver microsomes (microsomal GSTA1-1) with a significant selenium-independent glutathione peroxidase activity has been isolated and characterized. Several analytical criteria, including EDTA stripping, protease protection assay and extraction with alkaline Na(2)CO(3), indicate that the microsomal GSTA1-1 is associated with the inner microsomal membrane. The specific cDNA nucleotide sequence reveals that the enzyme is made up of 222 amino acid residues and shares approx. 73-83% sequence similarity to Alpha-class GSTs from different species. The molecular mass, as determined by electrospray mass ionization, is 25611.3 Da. The enzyme is distinct from the previously reported rat liver microsomal GST in both amino acid sequence and catalytic properties [Morgenstern, Guthenberg and DePierre (1982) Eur. J. Biochem. 128, 243-248]. The microsomal GSTA1-1 differs from the sheep liver cytosolic GSTs, reported previously from this laboratory, in its substrate specificity profile and molecular mass [Reddy, Burgess, Gong, Massaro and Tu (1983) Arch. Biochem. Biophys. 224, 87-101]. In addition to catalysing the conjugation of 4-hydroxynonenal with GSH, the enzyme also exhibits significant glutathione peroxidase activity towards physiologically relevant fatty acid hydroperoxides, such as linoleic and arachidonic acid hydroperoxides, as well as phosphatidylcholine hydroperoxide, but not with H(2)O(2). Thus the microsomal GSTA1-1 isoenzyme might have an important role in the protection of biological membranes against oxidative damage.

Oxidative stress, human genetic variation, and disease

Oxidative stress has been implicated in numerous pathophysiological conditions and also aging. The tools for studying oxidative stress are now expanding as a result of the human genome effort and, in particular, expanding knowledge on human genetic variation. A few genetic variants, mostly in the form of single nucleotide polymorphisms of relevance to oxidative stress are already studied by a molecular epidemiologic approach. A review of the current knowledge on variant human genes that are directly implicated in human protection against oxidative stress is presented.

Disruption of the microsomal glutathione S-transferase-like gene reduces life span of Drosophila melanogaster

Microsomal glutathione S-transferase-I (MGST-I) has been thought to be important for protecting the cell from oxidative damages and/or xenobiotics. We have previously identified the Microsomal glutathione S-transferase-like (Mgstl) gene, a Drosophila homologue of human MGST-I. To investigate the function of the enzyme using Drosophila as a model system, we examined the expression pattern of Mgstl during development, and generated loss-of-function mutants to assess its in-vivo function. Mgstl was expressed in all developmental stages. It is expressed ubiquitously with the highest expression in the larval fat body, an insect organ thought to be functionally corresponding to mammalian liver, while relatively low in the central nervous system. This tissue distribution is consistent with that of MGST-I in humans or Rats. Mgstl null mutants generated from a P element insertion line showed no obvious defects in morphology, indicating that it is not essential for the development. However, their life span was significantly reduced compared to control flies, suggesting that the MGSTL protein is involved in processes somehow contributing to aging. We found an Mgstl pseudogene, which is apparently derived through the reverse transcription of Mgstl mRNA and subsequent integration into the genome.

Low yield of polymorphisms from EST blast searching: analysis of genes related to oxidative stress and verification of the P197L polymorphism in GPX1

To determine new polymorphisms in the antioxidant enzymes superoxide dismutase, glutathione peroxidases, catalase, and microsomal glutathione transferase 1, a search of the human expressed sequence tags (EST) database was performed (with BLAST 2.0). When any mutation, indicated by the BLAST search, gave rise to a nonconservative amino acid change we performed polymerase chain reaction (PCR) restriction analysis and/or sequence analysis of genomic DNA from human subjects in order to verify these potential polymorphisms. Of nine indicated polymorphisms from the EST analysis found in four different antioxidant enzymes, we could verify one, an amino acid substitution Pro-Leu at amino acid position 197 (P197L), in the glutathione peroxidase 1 gene. The corresponding allele frequencies were approximately 70/30%. In addition, a silent mutation (1167T/C) in the catalase gene indicated by the BLAST search could also be verified. Six to nine individuals were analyzed per indicated polymorphism, so that only common polymorphisms would be found. The indicated mutations not verified by direct analysis thus cannot be excluded as allelic variation in the human population. These results show that the EST database can be used to search for polymorphisms in genes with high abundance in the human EST database. In addition to the EST analysis, PCR/single-strand conformation polymorphism (SSCP) was employed for the analysis of the microsomal glutathione transferase 1 gene. No polymorphism in the coding sequence could be detected in the gene by either method. The high degree of conservation of the microsomal glutathione transferase 1 gene indicates an important physiological function for this enzyme.

Structural organization of the microsomal glutathione S-transferase gene (MGST1) on chromosome 12p13.1-13.2. Identification of the correct promoter region and demonstration of transcriptional regulation in response to oxidative stress

The structure and regulation of the microsomal glutathione S-transferase gene (MGST1) are considerably more complex than originally perceived to be. The MGST1 gene has two alternative first exons and is located in the 12p13.1-13.2 region. Two other potential first exons were determined to be nonfunctional. The region between the functional first exons cannot direct transcription. Thus, one common promoter element directing transcription exists, and RNA splicing occurs such that only one of the first exons (containing only untranslated mRNA) is incorporated into each mRNA species with common downstream exons. MGST1 expression and regulation are therefore similar to those of other hepatic xenobiotic handling enzymes, which also produce mRNA species differing only in the 5'-untranslated regions to yield identical proteins. MGST1 was previously considered a "housekeeping" gene, as non-oxidant inducers had little effect on activity. However, the promoter region immediately upstream of the dominant first exon transcriptionally responds to oxidative stress. In this respect, MGST1 is similar to glutathione peroxidases that also transcriptionally respond to oxidative stress. The discovery that MGST1 utilizes alternative first exon splicing eliminates a problem with the first description of MGST1 cDNA in that it appeared that MGST1 expression was in violation of the ribosomal scanning model. The identification that the first exon originally noted is in fact a minor alternative first exon far downstream of the primary first exon eliminates this conundrum.

Membrane-associated proteins in eicosanoid and glutathione metabolism (MAPEG). A widespread protein superfamily

The members of the MAPEG superfamily have been aligned and found to be distantly related, with a common pattern of hydropathy. Figure 2A shows the multiple sequence alignments of the human members and Figure 2B the corresponding superimposed hydropathy profiles. The alignment in Figure 2A demonstrates a total of six strictly conserved residues. The Arg-51 in LTC4 synthase has been suggested to function as proton donor for the opening of the LTA4 epoxide. This arginine is found in all but the FLAP sequences in accordance with the observation that FLAP has no known enzyme activity. Also the Tyr-93 in LTC4 synthase has been suggested to function as a base for the formation of the thiolate anion of glutathione. This tyrosine is not conserved in MGST1 or MGST1-L1. Table 1 summarizes some other properties of the individual human proteins. They are all of the same size, ranging from 147 to 161 amino acids. Only FLAP differs in that its isoelectric point is more neutral than that of the other, more basic proteins. The genes encoding these proteins all reside on different chromosomes (when known) (Table 1). In addition to the human proteins, MAPEG members have been identified in plants, fungi, and bacteria. It is clearly a challenge to elucidate their role in these different phyla in relation to their defined physiological functions in humans.

Glutathione and glutathione-dependent enzymes represent a co-ordinately regulated defence against oxidative stress

Increases in the intracellular levels of reactive oxygen species (ROS), frequently referred to as oxidative stress, represents a potentially toxic insult which if not counteracted will lead to membrane dysfunction, DNA damage and inactivation of proteins. Chronic oxidative stress has numerous pathological consequences including cancer, arthritis and neurodegenerative disease. Glutathione-associated metabolism is a major mechanism for cellular protection against agents which generate oxidative stress. It is becoming increasingly apparent that the glutathione tripeptide is central to a complex multifaceted detoxification system, where there is substantial inter-dependence between separate component members. Glutathione participates in detoxification at several different levels, and may scavenge free radicals, reduce peroxides or be conjugated with electrophilic compounds. Thus, glutathione provides the cell with multiple defences not only against ROS but also against their toxic products. This article discusses how glutathione biosynthesis, glutathione peroxidases, glutathione S-transferases and glutathione S-conjugate efflux pumps function in an integrated fashion to allow cellular adaption to oxidative stress. Co-ordination of this response is achieved, at least in part, through the antioxidant responsive element (ARE) which is found in the promoters of many of the genes that are inducible by oxidative and chemical stress. Transcriptional activation through this enhancer appears to be mediated by basic leucine zipper transcription factors such as Nrf and small Maf proteins. The nature of the intracellular sensor(s) for ROS and thiol-active chemicals which induce genes through the ARE is described. Gene activation through the ARE appears to account for the enhanced antioxidant and detoxification capacity of normal cells effected by many cancer chemopreventive agents. In certain instances it may also account for acquired resistance of tumours to cancer chemotherapeutic drugs. It is therefore clear that determining the mechanisms involved in regulation of ARE-driven gene expression has enormous medical implications.

The projection structure of the membrane protein microsomal glutathione transferase at 3 A resolution as determined from two-dimensional hexagonal crystals

The formation of two-dimensional crystals of the membrane-bound enzyme microsomal glutathione transferase is sensitive to fractional changes in the lipid-to-protein ratio. Variation of this parameter results in crystal polymorphism. The projection structure of a p6 crystal form of the enzyme has been determined by the use of electron crystallography. The unit cell at 3 A resolution is comprised of two trimers. The hexagonal p6 and the orthorhombic p21212 crystal types have common elements in the packing arrangement which imply dominant crystal contacts. An overall structural similarity between the protein molecules in the two crystal forms is suggested by the projection maps. Furthermore, a comparison of the p6 and p21212 projection maps identifies additional corresponding protein densities which could not be assigned to the microsomal glutathione transferase trimer previously. Surprisingly, an ambiguity of the rotational orientation was found for trimers interspersed at certain positions within the crystal lattice.

Distribution of microsomal glutathione transferase 1 in mammalian tissues. A predominant alternate first exon in human tissues

An extensive Northern blot analysis of microsomal glutathione transferase 1 in human and rat tissues was performed. When normalized against the glyceraldehyde-3-phosphate dehydrogenase or actin expression it was evident that the predominant expression occurs in liver and pancreas. An ontogenetic, as well as a functional, basis for the high levels in these two organs is possible. The relative expression levels in man ranged from: liver and pancreas (100%), to kidney, prostate, colon (30-40%), heart, brain, lung, testis, ovary, small intestine (10-20%), placenta, skeletal muscle, spleen, thymus and peripheral blood leucocytes (1-10%). Liver-enriched expression was detected in human fetal tissues with lung and kidney displaying lower levels (10-20%). No transcripts could be detected in fetal brain or heart. When comparing the expression levels between rat and man it is apparent that human extrahepatic mRNA levels are much higher relative to liver. Rat microsomal glutathione transferase mRNA expression ranges from 0.2 to 10% that of liver, with adrenal, uterus, ovary and stomach displaying the highest levels of the organs tested. Based on these observations, and the fact that the enzyme is encoded by a highly conserved single-copy gene, it is suggested that microsomal glutathione transferase 1 performs essential functions vital to most mammalian cell types. We suggest that protection against oxidative stress constitutes one such function. Human expressed sequence tag (EST) characterization yielded four alternate mRNA transcripts with different 5'-ends (four alternate noncoding exons 1). The predominant exon (based on the observed EST frequency) revealed a tissue distribution similar to that obtained using the reading frame as probe. Thus, it appears that one exon preferentially gives rise to mature mRNA in the human tissues examined. This exon is different from the one reported in the original cDNA characterized.