Tissue-specific induction of murine glutathione transferase mRNAs by butylated hydroxyanisole

NRF2 and the Ambiguous Consequences of Its Activation during Initiation and the Subsequent Stages of Tumourigenesis

NF-E2 p45-related factor 2 (NRF2, encoded in the human by NFE2L2) mediates short-term adaptation to thiol-reactive stressors. In normal cells, activation of NRF2 by a thiol-reactive stressor helps prevent, for a limited period of time, the initiation of cancer by chemical carcinogens through induction of genes encoding drug-metabolising enzymes. However, in many tumour types, NRF2 is permanently upregulated. In such cases, its overexpressed target genes support the promotion and progression of cancer by suppressing oxidative stress, because they constitutively increase the capacity to scavenge reactive oxygen species (ROS), and they support cell proliferation by increasing ribonucleotide synthesis, serine biosynthesis and autophagy. Herein, we describe cancer chemoprevention and the discovery of the essential role played by NRF2 in orchestrating protection against chemical carcinogenesis. We similarly describe the discoveries of somatic mutations in NFE2L2 and the gene encoding the principal NRF2 repressor, Kelch-like ECH-associated protein 1 (KEAP1) along with that encoding a component of the E3 ubiquitin-ligase complex Cullin 3 (CUL3), which result in permanent activation of NRF2, and the recognition that such mutations occur frequently in many types of cancer. Notably, mutations in NFE2L2, KEAP1 and CUL3 that cause persistent upregulation of NRF2 often co-exist with mutations that activate KRAS and the PI3K-PKB/Akt pathway, suggesting NRF2 supports growth of tumours in which KRAS or PKB/Akt are hyperactive. Besides somatic mutations, NRF2 activation in human tumours can occur by other means, such as alternative splicing that results in a NRF2 protein which lacks the KEAP1-binding domain or overexpression of other KEAP1-binding partners that compete with NRF2. Lastly, as NRF2 upregulation is associated with resistance to cancer chemotherapy and radiotherapy, we describe strategies that might be employed to suppress growth and overcome drug resistance in tumours with overactive NRF2.

Acute effects of microcystins on the transcription of 14 glutathione S-transferase isoforms in Wistar rat

The glutathione S-transferases (GST) play important roles in the detoxification of microcystins (MCs). For better understanding of the responses of GST isforms to MCs exposure, informations about the effects of MCs on GSTs are necessary. In this experiment, we cloned the full length cDNA of 14 GST isoforms (GST alpha, kappa, mu, omega, pi, theta, zeta, and microsomal GST) from Wistar rat. The mRNA abundance of each rat GST isoform in the liver, kidney, and testis was analyzed by real time quantitative PCR. Multiple GST isoforms were constitutively expressed in all examined organs, but some isoforms were expressed at higher level in one organ than in others. The relative changes of the mRNA abundance in the liver, kidney, and testis of Wiatar rat i.v. injected with crude MCs extract at dose of 1LD(50) were also analyzed. Generally, the expression of most GSTs in the liver and testis was suppressed while that in kidney was induced after being injected with MCs. It is suggested that the transcription of GST isoforms varied in different ways within an organ and between organs of Wistar rat exposed to MCs.

Cancer chemoprevention mechanisms mediated through the Keap1-Nrf2 pathway

The cap'n'collar (CNC) bZIP transcription factor Nrf2 controls expression of genes for antioxidant enzymes, metal-binding proteins, drug-metabolising enzymes, drug transporters, and molecular chaperones. Many chemicals that protect against carcinogenesis induce Nrf2-target genes. These compounds are all thiol-reactive and stimulate an adaptive response to redox stress in cells. Such agents induce the expression of genes that posses an antioxidant response element (ARE) in their regulatory regions. Under normal homeostatic conditions, Nrf2 activity is restricted through a Keap1-dependent ubiquitylation by Cul3-Rbx1, which targets the CNC-bZIP transcription factor for proteasomal degradation. However, as the substrate adaptor function of Keap1 is redox-sensitive, Nrf2 protein evades ubiquitylation by Cul3-Rbx1 when cells are treated with chemopreventive agents. As a consequence, Nrf2 accumulates in the nucleus where it heterodimerizes with small Maf proteins and transactivates genes regulated through an ARE. In this review, we describe synthetic compounds and phytochemicals from edible plants that induce Nrf2-target genes. We also discuss evidence for the existence of different classes of ARE (a 16-bp 5'-TMAnnRTGABnnnGCR-3' versus an 11-bp 5'-RTGABnnnGCR-3', with or without the embedded activator protein 1-binding site 5'-TGASTCA-3'), species differences in the ARE-gene battery, and the identity of critical Cys residues in Keap1 required for de-repression of Nrf2 by chemopreventive agents.

Induction of glutathione S-transferase in biofilms and germinating spores of Mucor hiemalis strain EH5 from cold sulfidic spring waters

The occurrence and activation of glutathione S-transferase (GST) and the GST activities in biofilms in cold sulfidic spring waters were compared to the occurrence and activation of GST and the GST activities of the aquatic fungal strains EH5 and EH7 of Mucor hiemalis isolated for the first time from such waters. Using fluorescently labeled polyclonal anti-GST antibodies and GST activity measurements, we demonstrated that a high level of GST occurred in situ in natural biofilms and pure cultures of strain EH5. Measurement of microsomal and cytosolic soluble GST activities using different xenobiotic substrates, including 1-chloro-2,4-dinitrobenzene (CDNB), 1,2-dichloro-4-nitrobenzene, 1,2-epoxy-3-(4-nitrophenoxy)propane, 1-iodo-2,4-dinitrobenzene, and fluorodifen, showed that the overall biotransforming abilities of biofilms were at least sixfold greater than that of strain EH5 alone. Increasing the level of sodium thiosulfate (STS) in the medium stimulated the microsomal and cytosolic GST activities with CDNB of strain EH5 about 44- and 94-fold, respectively, compared to the activities in the control. The induction of microsomal GST activity with fluorodifen by STS was strongly linear, but the initial strong linear increase in cytosolic GST activity with fluorodifen showed saturation-like effects at STS concentrations higher than approximately 1 mM. Using laser scanning confocal and conventional fluorescence microscopy, abundant fluorescently labeled GST proteins were identified in germinating sporangiospores of strain EH5 after activation by STS. High-performance size exclusion chromatography and sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed the presence of at least two main GSTs ( approximately 27.8- and approximately 25.6-kDa subunits) in the cytosol of EH5, whereas the major 27.8-kDa subunit was the only GST in microsomes. We suggest that differential cellular GST expression takes place in strain EH5 depending on spore and hyphal development. Our results may contribute to our understanding of induction of GST by sulfurous compounds, as well as to the immunofluorescence visualization of GST in aquatic fungus and fungus-bacterium biofilms.

Glutathione transferases

This review describes the three mammalian glutathione transferase (GST) families, namely cytosolic, mitochondrial, and microsomal GST, the latter now designated MAPEG. Besides detoxifying electrophilic xenobiotics, such as chemical carcinogens, environmental pollutants, and antitumor agents, these transferases inactivate endogenous alpha,beta-unsaturated aldehydes, quinones, epoxides, and hydroperoxides formed as secondary metabolites during oxidative stress. These enzymes are also intimately involved in the biosynthesis of leukotrienes, prostaglandins, testosterone, and progesterone, as well as the degradation of tyrosine. Among their substrates, GSTs conjugate the signaling molecules 15-deoxy-delta(12,14)-prostaglandin J2 (15d-PGJ2) and 4-hydroxynonenal with glutathione, and consequently they antagonize expression of genes trans-activated by the peroxisome proliferator-activated receptor gamma (PPARgamma) and nuclear factor-erythroid 2 p45-related factor 2 (Nrf2). Through metabolism of 15d-PGJ2, GST may enhance gene expression driven by nuclear factor-kappaB (NF-kappaB). Cytosolic human GST exhibit genetic polymorphisms and this variation can increase susceptibility to carcinogenesis and inflammatory disease. Polymorphisms in human MAPEG are associated with alterations in lung function and increased risk of myocardial infarction and stroke. Targeted disruption of murine genes has demonstrated that cytosolic GST isoenzymes are broadly cytoprotective, whereas MAPEG proteins have proinflammatory activities. Furthermore, knockout of mouse GSTA4 and GSTZ1 leads to overexpression of transferases in the Alpha, Mu, and Pi classes, an observation suggesting they are part of an adaptive mechanism that responds to endogenous chemical cues such as 4-hydroxynonenal and tyrosine degradation products. Consistent with this hypothesis, the promoters of cytosolic GST and MAPEG genes contain antioxidant response elements through which they are transcriptionally activated during exposure to Michael reaction acceptors and oxidative stress.

Inhibition of cellular enzymes by equine catechol estrogens in human breast cancer cells: specificity for glutathione S-transferase P1-1

Glutathione S-transferases (GSTs) are a family of detoxification isozymes that protect cells by conjugating GSH to a variety of toxic compounds, and they may also play a role in the regulation of both cellular proliferation and apoptosis. We have previously shown that human GST P1-1, which is the most widely distributed extrahepatic isozyme, could be inactivated by the catechol estrogen metabolite 4-hydroxyequilenin (4-OHEN) in vitro [Chang, M., Shin, Y. G., van Breemen, R. B., Blond, S. Y., and Bolton, J. L. (2001) Biochemistry 40, 4811-4820]. In the present study, we found that 4-OHEN and another catechol estrogen, 4,17beta-hydroxyequilenin (4,17beta-OHEN), significantly decreased GSH levels and the activity of GST within minutes in both estrogen receptor (ER) negative (MDA-MB-231) and ER positive (S30) human breast cancer cells. In addition, 4-OHEN caused significant decreases in GST activity in nontransformed human breast epithelial cells (MCF-10A) but not in the human hepatoma HepG2 cells, which lack GST P1-1. We also showed that GSH partially protected the inactivation of GST P1-1 by 4-OHEN in vitro, and depletion of cellular GSH enhanced the 4-OHEN-induced inhibition of GST activity. In addition, 4-OHEN GSH conjugates contributed about 27% of the inactivation of GST P1-1 by 4-OEHN in vitro. Our in vitro kinetic inhibition experiments with 4-OHEN showed that GST P1-1 had a lower K(i) value (20.8 microM) compared to glyceraldehyde-3-phosphate dehydrogenase (GAPDH, 52.4 microM), P450 reductase (PR, 77.4 microM), pyruvate kinase (PK, 159 microM), glutathione reductase (GR, 230 microM), superoxide dismutase (SOD, 448 microM), catalase (562 microM), GST M1-1 (620 microM), thioredoxin reductase (TR, 694 microM), and glutathione peroxidase (GPX, 1410 microM). In contrast to the significant inhibition of total GST activity in these human breast cancer cells, 4-OHEN only slightly inhibited the cellular GAPDH activity, and other cellular enzymes including PR, PK, GR, SOD, catalase, TR, and GPX were resistant to 4-OHEN-induced inhibition. These data suggest that GST P1-1 may be a preferred protein target for equine catechol estrogens in vivo.

Nrf2 transactivator-independent GSTP1-1 expression in "GSTP1-1 positive" single cells inducible in female mouse liver by DEN: a preneoplastic character of possible initiated cells

Whether single cells immunohistochemically positive for glutathione S-transferase P1-1 (GSTP1-1) induced in the female mouse liver by DEN (Hatayama et al., Carcinogenesis, 14, 537-538, 1993) are precursor initiated cells of preneoplastic foci, is of importance in chemical hepatocarcinogenesis. Nrf2 transactivates a wide variety of ARE (anti-oxidant response element)-mediated enzymes including GSTP1-1. Quantitative examination revealed that the basal expression of hepatic GSTP1-1 was 60% lower in Nrf2 gene knock-out female mice(-/-) than in wild type females, and that treatment with butyrated hydroxyanisole (BHA) increased by 10-fold GSTP1-1 expression in the liver of wild type female mice but not in knockout female mice(-/-). Despite the lack of Nrf2, GSTP1-1-positive single cells were detected in livers of DEN-treated female(-/-) 3 months after treatment. Subsequent BHA feeding to the positive cell-bearing females for one more week clearly showed that the single cells were detectable with females(-/-) but not with females(+/+,+/-) due to the strong induction of GSTP1-1 in the surrounding hepatocytes. The sensitivity to DEN hepatocarcinogenesis was not significantly different among genotypes. These results demonstrate that Nrf2 is regulatory in normal hepatocytes but not in the single cells positive for GSTP1-1 inducible in the female mouse liver by DEN. The transcriptional distinction observed for the DEN-transformants is suggestive of a preneoplastic character of precursor initiated cells.

Functional characterization of transcription regulators that interact with the electrophile response element

The electrophile response element (EpRE), also referred to as the antioxidant responsive element (ARE), is found in the 5'-regulatory region of a number of genes encoding phase II, drug-metabolizing enzymes. Gene knockout studies have demonstrated the primary regulatory role that an Nrf2:Maf dimer plays by binding to nucleotides within the EpRE consensus sequence. Current models of transcription regulation have also shown the involvement of higher-order transcriptional coactivators, proteins that nucleate around DNA sequence-specific transcription factors, enhancing transcription of the target gene by interacting with components of the basal transcriptional apparatus and by enabling chromatin remodeling. Here, we hypothesized that multiple transcriptional regulators, including: (i) a primary Nrf2-Maf heterodimer, (ii) a proposed secondary, EpRE-specific, p160 family coactivator, ARE-binding protein-1, and (iii) a tertiary coactivator, CBP/p300, nucleate to form a complex at the EpRE that regulates transcription of the dependent gene. To test this hypothesis, we constructed a HepG2 cell line which contains a stably integrated green fluorescent protein (GFP) gene; its inducible expression is regulated by a synthetic TK promoter containing a linked EpRE. To identify the involvement of specific, primary and higher-order transcriptional regulators in the EpRE-mediated regulation of the GFP reporter gene, we microinjected antibodies directed against specific transcription factors into the HepG2/GFP cells and determined their effect upon tBHQ-induced expression of the GFP gene. The results demonstrate that microinjected antibodies directed against Nrf2, MafK, CBP and p300 could each, individually, significantly inhibit tBHQ-induced GFP expression. This directly demonstrates the role that the tertiary regulators, CBP or p300, play in mediating EpRE activation of phase II genes, and also implicates the involvement of secondary, p160 family coactivators. Moreover, we found that the same anti-MafK antibody that blocked induction of the EpRE-regulated GFP gene completely ablated the gel-shift complex that we hypothesize contains an Nrf2:Maf dimer, ARE-binding protein-1, and CBP or p300.

Polymorphic electrophile response elements in the mouse glutathione S-transferase GSTa1 gene that confer increased induction

Induced transcription of a battery of stress response genes in mammals, including several phase I and phase II drug-metabolizing enzymes, is regulated by the electrophile responsive element (EpRE). Because previous directed mutagenesis of nucleotide motifs within the large, composite EpRE were shown to affect transcription factor binding and associated induced expression of dependent genes, we hypothesized that naturally-occurring variation or polymorphism in the EpRE sequence, if found, could affect the induced expression of important protective genes like glutathione S-transferases, and that this could be an important determinant of cancer risk in humans and other mammals. To determine whether this occurred in nature, 32 strains and species of inbred mice were screened to examine the EpRE sequence present in the mGSTa1 promoter. Two species, Mus caroli and Mus spretus, showed TGAC-->TGGC mutations in the tandem TGAC motif. Inducibility (15-fold) of the variant Mus spretus EpRE sequence in a reporter gene construct in HepG2 cells was significantly increased versus the wild-type EpRE sequence (8-fold). A comparison of mGSTa1-induced expression in the livers of Mus spretus, Mus caroli, and BALB/cJ mice showed the highest level of mGSTa1 mRNA in livers from the Mus spretus and Mus carolimice. This naturally-occurring polymorphism within the EpRE domain is the first mutation with an associated phenotype to be reported within a promoter regulatory element of a drug metabolizing gene.

Induction of class alpha glutathione S-transferases by 4-methylthiazole in the rat liver: role of oxidative stress

The expression of glutathione S-transferase (GST) is a crucial factor in determining the sensitivity of cells and organs in response to a variety of toxicants. Expression of class alpha GST genes by methyl-substituted thiazoles was assessed in the rat liver. Northern blot analysis revealed that 4-methylthiazole (4-MT) elevated rGSTA2, A3, A5 and M1 mRNAs in the liver by 19-, 4-, 6- and 9-fold at 24 h after treatment, respectively, as compared to control. Consecutive 3-day treatment with 4-MT resulted in 4- to 7-fold increases in rGSTA and M1 mRNAs. Multiple treatments with 5-methylthiazole (5-MT) caused marginal increases in GST mRNAs in spite of the large increases in certain GST mRNAs at 24 h. Either 4, 5-dimethylthiazole (DT) or 2,4,5-trimethylthiazole (TT) minimally affected the rGSTA and rGSTM mRNA expression at 1-3 day(s). Western blot analysis showed that 4-MT induced rGSTA1/2, rGSTA3/5 and rGSTM1 proteins by 2.6-, 2.1- and 2.1-fold at 3 days, respectively, while other methylthiazoles failed to induce the GST subunits. Starving rats were treated with a lower dose of methylthiazoles to study the role of oxidative stress in the mRNA expression. The levels in rGSTA2/3/5 mRNAs were significantly enhanced by 4-MT in starving rats, whereas rGSTM1/2 mRNAs were not further increased. Other methylthiazoles were inactive in enhancing the mRNAs in starving animals. Pretreatment of starving rats with either cysteine or methionine completely prevented the increases in class alpha GST mRNAs by 4-MT. Data showed that 4-MT induces class alpha GSTs with the increases in the mRNAs, whereas 5-methyl-, dimethyl- and trimethyl-substituted thiazoles were minimally active. Increases in the class alpha GST mRNAs by 4-MT may be associated with the oxidative stress in hepatocytes, as supported by starvation and sulfur amino acid experiments.

Redox regulation of c-Ha-ras and osteopontin signaling in vascular smooth muscle cells: implications in chemical atherogenesis

Reduction/oxidation (redox) reactions play a central role in the regulation of vascular cell functions. Recent studies in this laboratory have identified c-Ha-ras and osteopontin genes as critical molecular targets during oxidant-induced atherogenesis. This review focuses on the deregulation of gene transcription by redox-activated trans-acting factors after benzo(a)pyrene challenge and the modulation of extracellular matrix signaling in vascular smooth muscle cells by allylamine-induced oxidative injury. The induction of atherogenic vascular smooth muscle cell phenotypes by chemical injury exhibits remarkable parallels with those seen in other forms of atherogenesis.

Human glutathione transferase A4-4 crystal structures and mutagenesis reveal the basis of high catalytic efficiency with toxic lipid peroxidation products

The oxidation of lipids and cell membranes generates cytotoxic compounds implicated in the etiology of aging, cancer, atherosclerosis, neurodegenerative diseases, and other illnesses. Glutathione transferase (GST) A4-4 is a key component in the defense against the products of this oxidative stress because, unlike other Alpha class GSTs, GST A4-4 shows high catalytic activity with lipid peroxidation products such as 4-hydroxynon-2-enal (HNE). The crystal structure of human apo GST A4-4 unexpectedly possesses an ordered C-terminal alpha-helix, despite the absence of any ligand. The structure of human GST A4-4 in complex with the inhibitor S-(2-iodobenzyl) glutathione reveals key features of the electrophilic substrate-binding pocket which confer specificity toward HNE. Three structural modules form the binding site for electrophilic substrates and thereby govern substrate selectivity: the beta1-alpha1 loop, the end of the alpha4 helix, and the C-terminal alpha9 helix. A few residue changes in GST A4-4 result in alpha9 taking over a predominant role in ligand specificity from the N-terminal loop region important for GST A1-1. Thus, the C-terminal helix alpha9 in GST A4-4 provides pre-existing ligand complementarity rather than acting as a flexible cap as observed in other GST structures. Hydrophobic residues in the alpha9 helix, differing from those in the closely related GST A1-1, delineate a hydrophobic specificity canyon for the binding of lipid peroxidation products. The role of residue Tyr212 as a key catalytic residue, suggested by the crystal structure of the inhibitor complex, is confirmed by mutagenesis results. Tyr212 is positioned to interact with the aldehyde group of the substrate and polarize it for reaction. Tyr212 also coopts part of the binding cleft ordinarily formed by the N-terminal substrate recognition region in the homologous enzyme GST A1-1 to reveal an evolutionary swapping of function between different recognition elements. A structural model of catalysis is presented based on these results.

Distinctive structure of the human GSTM3 gene-inverted orientation relative to the mu class glutathione transferase gene cluster

The sequence and exon-intron structure of the human class mu GSTM3 glutathione transferase gene and its orientation with respect to the remainder of the human class mu GSTM gene cluster were determined. The GSTM3 gene is 2847 bp long and is thus considerably shorter than the other class mu genes in the cluster, which range in size from 5325 to 7212 bp. Outside the protein-coding region, the GSTM3 gene does not share significant sequence similarity with other class mu glutathione transferase genes. Identification of overlapping cosmid clones that span the region between GSTM5, the next nearest glutathione transferase gene, and GSTM3 showed that the two genes are about 20,000 bp apart. PCR primers developed from sequences 3'-downstream from the GSTM5 gene were used to identify clones containing the GSTM3 gene. Amplification with these primers showed that the orientation of the GSTM3 gene is 5'-GSTM5-3'-3'-GSTM3-5'. Long-range PCR reactions confirmed this orientation both in the GSTM-YAC2 YAC clone, which contains the five class mu glutathione transferase genes on chromosome 1, and in human DNA. This tail-to-tail orientation is consistent with an evolutionary model of class mu glutathione transferase divergence from a pair of tail-to-tail "M1-like" and "M3-like" class mu glutathione transferase genes that was present at the mammalian radiation to the current organization of multiple head-to-tail M1-like genes tail-to-tail with a single M3-like gene with distinct structural properties and expression patterns.

Cloning, expression, and biochemical characterization of a functionally novel alpha class glutathione S-transferase with exceptional activity in the glutathione conjugation of (+)-anti-7,8-dihydroxy-9,10-oxy-7,8,9,10-tetrahydrobenzo(a)pyrene

The present study describes cDNA cloning, expression, and kinetic characterization of the two subunits of a murine alpha-class glutathione (GSH) S-transferase (GST) isoenzyme (previously designated as GST 9.5), which, unlike other alpha-class mammalian GSTs, is exceptionally efficient in the GSH conjugation of (+)-anti-7,8-dihydroxy-9,10-oxy-7,8,9,10-tetrahydrobenzo(a)pyrene [(+)-anti-BPDE] [X. Hu, S. K. Srivastava, H. Xia, Y. C. Awasthi, and S. V. Singh (1996) J. Biol. Chem. 271, 32684-32688]. The cDNAs for both subunits of GST 9.5 (GST 9.5-1 and GST 9.5-2) were cloned by RT-PCR. The deduced amino acid sequences of GST 9.5-1 and GST 9.5-2 clones were identical to those of mGSTA1 and mGSTA2, respectively. Both these subunits were expressed in Escherichia coli to determine the relationships between recombinant mGSTA1-1 and mGSTA2-2 and corresponding subunits of tissue-isolated GST 9.5. The pI values of recombinant mGSTA1-1 and mGSTA2-2 (9.49 and 9.45, respectively) were similar to that of the tissue-isolated isoenzyme (pI 9.5). The reverse-phase HPLC elution profiles and immunological cross-reactivities of recombinant mGSTA1-1 and mGSTA2-2 were also similar to those of the corresponding subunits of tissue-isolated GST 9.5. The catalytic efficiency of recombinant mGSTA1-1 toward (+)-anti-BPDE, 131 mM-1.s-1, was approximately 9.5-to 655-fold higher compared with tissue-isolated mGSTP1-1, mGSTA3-3, mGSTM1-1, and mGSTA4-4. Moreover, the catalytic efficiency of mGSTA1-1 toward (+)-anti-BPDE was about 3.3-fold higher compared with recombinant mGSTA2-2. The mGSTA1 and/or mGSTA2 subunits were expressed to varying degrees in female A/J mouse tissues. For example, mGSTA1, but not mGSTA2, subunit expression was observed in the skin, which is a target organ for benzo(a)pyrene (BP)-induced cancer in mice. On the other hand, the expression of either mGSTA1 or mGSTA2 subunit could not be detected in the lung, which is another target organ for BP-induced cancer in mice. Interestingly, relatively large amounts of both mGSTA1 and mGSTA2 subunits were detected in the kidney. In conclusion, the results of the present study clearly indicate that the A1-type subunit of GST 9.5 is responsible for its exceptional catalytic efficiency in the GSH conjugation of (+)-anti-BPDE, which is the ultimate carcinogen of widespread environmental pollutant BP.

Characterization of the human class Mu glutathione S-transferase gene cluster and the GSTM1 deletion

A partial physical map has been constructed of the human class Mu glutathione S-transferase genes on chromosome 1p13.3. The glutathione S-transferase genes in this cluster are spaced about 20 kilobase pairs (kb) apart, and arranged as 5'-GSTM4-GSTM2-GSTM1-GSTM5-3'. This map has been used to localize the end points of the polymorphic GSTM1 deletion. The left repeated region is 5 kb downstream from the 3'-end of the GSTM2 gene and 5 kb upstream from the beginning of the GSTM1 gene; the right repeated region is 5 kb downstream from the 3'-end of the GSTM1 and 10 kb upstream from the 5'-end of the GSTM5 gene. The GSTM1-0 deletion produces a novel 7.4-kb HindIII fragment with the loss of 10.3- and 11.4-kb HindIII fragments. The same novel fragment was seen in 13 unrelated individuals (20 null alleles), suggesting that most GSTM1-0 deletions involve recombinations between the same two regions. We have cloned and sequenced the deletion junction that is produced at the GSTM1-null locus; the 5'- and 3'-flanking regions are more than 99% identical to each other and to the deletion junction sequence over 2.3 kb. Because of the high sequence identity between the left repeat, right repeat, and deletion junction regions, the crossing over cannot be localized within the 2.3-kb region. The 2.3-kb repeated region contains a reverse class IV Alu repetitive element near one end of the repeat.

Comprehensive analysis of proteins which interact with the antioxidant responsive element: correlation of ARE-BP-1 with the chemoprotective induction response

Transcriptional activation of the mouse glutathione S-transferase Ya gene by chemoprotective molecules is mediated through the interaction of trans-acting factors with an antioxidant responsive element (ARE) in the promoter region of this gene. In a step toward identifying those factors which bind productively to the GST Ya ARE, all of the discernible, specific ARE-binding proteins (ARE-BP) in nuclear extracts from HepG2 cells were systematically characterized. By gel-mobility-shift analysis, seven specific ARE-BPs, termed ARE-BP-1 through 7 in order of increasing mobility, were observed that did not vary in concentration or migration between induced and uninduced cell extracts. The molecular weights of the individual ARE-BP subunits were determined by a two-dimensional electrophoresis protocol. Ferguson gel analysis of native protein size indicated that several of the ARE-BP-DNA complexes are composed of multiple protein subunits. Wild-type AREs and GST Ya ARE fragments and mutant sequences were evaluated for their ability to mediate induction in a reporter gene system in HepG2 cells. This same panel of sites was tested in an in vitro binding assay for the ability to compete for the ARE-BPs. A binding profile for each ARE-BP was compiled. Correlation between the ARE-BP binding profiles and induction results indicated that: (i) the ARE-BP-1 and ARE-BP-2 complexes formed only with AREs that supported induction, and (ii) the ARE-BP-4 complex formed with all inducible AREs, but it also bound to ARE mutants that failed to support induction. Based on the studies, an early composite regulatory element model for ARE-mediated expression is presented. ARE-BP-1 is proposed to be the mediator of the ARE's unique induction response to chemoprotective agents.

Preferential overexpression of a class MU glutathione S-transferase subunit in mouse liver by myristicin

The present studies were undertaken to elucidate the mechanism of induction of glutathione S-transferase (GST) in mouse liver by myristicin, an active constituent of parsley leaf. A/J albino mice, given 5 to 50 mg doses of myristicin, showed 4- to 14-fold increase in liver GST specific activity over the control. GST purified from equal amounts of control and myristicin-treated livers indicated a marked increase in the GST activity. A relatively higher increase in GST activity towards 2,4-dichloronitrobenzene and a profound increase in the levels of GST mu on Western blot analysis of the myristicin-treated mouse liver suggest a preferential induction of GST mu. Results of the study also indicate that out of the two mu class GST subunits (Mr. 26,500 and Mr. 25,000) expressed in liver only one (Mr. 26,500) is significantly elevated. Myristicin treatment caused a slight change in the GST pi levels while the levels of GST alpha showed a modest increase. These results suggest that myristicin could be an effective chemopreventive agent, particularly for carcinogens that are detoxified by the mu class GST.

An Nrf2/small Maf heterodimer mediates the induction of phase II detoxifying enzyme genes through antioxidant response elements

The induction of phase II detoxifying enzymes is an important defense mechanism against intake of xenobiotics. While this group of enzymes is believed to be under the transcriptional control of antioxidant response elements (AREs), this contention is experimentally unconfirmed. Since the ARE resembles the binding sequence of erythroid transcription factor NF-E2, we investigated the possibility that the phase II enzyme genes might be regulated by transcription factors that also bind to the NF-E2 sequence. The expression profiles of a number of transcription factors suggest that an Nrf2/small Maf heterodimer is the most likely candidate to fulfill this role in vivo. To directly test these questions, we disrupted the murine nrf2 gene in vivo. While the expression of phase II enzymes (e.g., glutathione S-transferase and NAD(P)H: quinone oxidoreductase) was markedly induced by a phenolic antioxidant in vivo in both wild type and heterozygous mutant mice, the induction was largely eliminated in the liver and intestine of homozygous nrf2-mutant mice. Nrf2 was found to bind to the ARE with high affinity only as a heterodimer with a small Maf protein, suggesting that Nrf2/small Maf activates gene expression directly through the ARE. These results demonstrate that Nrf2 is essential for the transcriptional induction of phase II enzymes and the presence of a coordinate transcriptional regulatory mechanism for phase II enzyme genes. The nrf2-deficient mice may prove to be a very useful model for the in vivo analysis of chemical carcinogenesis and resistance to anti-cancer drugs.

Functional antioxidant responsive elements

Exposure of human and rodent cells to a wide variety of chemoprotective compounds confers resistance against a broad set of carcinogens. For a subset of the chemoprotective compounds, protection is generated by an increase in the abundance of protective enzymes like glutathione S-transferases (GST). Antioxidant responsive elements (AREs) mediate the transcriptional induction of a battery of genes which comprise much of this chemoprotective response system. Past studies identified a necessary ARE "core" sequence of RTGACnnnGC, but this sequence alone is insufficient to mediate induction. In this study, the additional sequences necessary to define a sufficient, functional ARE are identified through systematic mutational analysis of the murine GST Ya ARE. Introduction of the newly identified necessary nucleotides into the regions flanking a nonresponsive, ARE-like, GST-Mu promoter sequence produced an inducible element. A screen of the GenBank database with the newly identified ARE consensus identified 16 genes which contained the functional ARE consensus sequence in their promoters. Included within this group was an ARE sequence from the murine ferritin-L promoter that mediated induction when tested. In an electrophoretic mobility-shift assay, the ferritin-L ARE was bound by ARE-binding protein 1, a protein previously identified as the likely mediator of the chemoprotective response. A three-level ARE classification system is presented to account for the distinct induction strengths observed in our mutagenesis studies. A model of the ARE as a composite regulatory site, where multiple transcription factors interact, is presented to account for the complex characteristics of ARE-mediated chemoprotective gene expression.

Expression of glutathione S-transferases Ya, Yb1, Yb2, Yc1 and Yc2 and microsomal epoxide hydrolase genes by thiazole, benzothiazole and benzothiadiazole

The effects of thiazole (TH), benzothiazole (BT) and benzothiadiazole (BZ) on the expression of hepatic glutathione S-transferases (GSTs) Ya, Yb1, Yb2, Yc1 and Yc2 and microsomal epoxide hydrolase (mEH) genes were compared in rats. TH treatment resulted in 4- to 24-fold increases in GST Ya mRNA levels at 24 hr posttreatment; the ED50 value was 70 mg/kg. GST Ya mRNA levels were elevated 13-, 20-, 20- and 9-fold at 12, 24, 48 and 72 hr following 100 mg/kg of TH treatment, respectively, as compared with the control. BT was a moderate inducer of GST Ya with a maximal 18-fold increase observed, whereas BZ treatment caused a transient increase in GST Ya mRNA at 12 hr posttreatment, followed by a return to a 4-fold relative increase at 24 hr or afterward. Treatment of rats with TH at the dose of 100 mg/kg resulted in an approximately 10-fold increase in either Yb1 or Yb2 mRNA levels at 24 hr posttreatment. BT-treated rats showed 7- and 3-fold increases in the GST subunit Yb1 and Yb2 mRNA levels at 24 hr posttreatment. BZ was the least effective in modulating either GST Yb1 or Yb2 mRNA, resulting in < 2-fold changes. GST Yc1 and Yc2 mRNA levels were increased approximately 8-fold at the dose of 200 mg/kg of TH. BT minimally affected GST subunit Yc1 and Yc2 mRNA levels, with a maximal 4-fold relative increase observed. BZ was the least effective in enhancing Yc1 and Yc2 mRNA levels. Protein levels for GST subunit Ya, Yb1, Yb2 and Yc were also elevated in response to TH by 3-, 2-, 2-, and 2-fold, respectively. Thus, TH was effective in modulating both constitutive and inducible GST gene expression. BT or BZ was much less effective in increasing the expression of GST subunits. These RNA and Western blot analyses revealed that the levels of major GST were differentially increased after treatment with these thiazoles, exhibiting a rank order of GST expression of TH > BT > BZ. mEH expression by these compounds appeared to be consistent with that of GST Ya. The mRNA levels for GST Ya, Yb1, Yb2, Yc1 and Yc2 and mEH were also determined after treatment with triazole (TR), imidazole (IM), benzoxazole (BX), benzotriazole (BTR) or benzimidazole (BIM). TR, IM, BX or BTR caused increases in Ya, Yb1, Yc1 and Yc2 mRNA levels by 2- to 3-fold, whereas the agents failed to modulate the expression of GST Yb2. The fact that benzene, cyclohexane or n-hexane minimally affected the major GST or mEH mRNA levels provided evidence that certain heterocyclic compounds are more capable of modulating GST or mEH gene expression than hydrocarbons. These results corroborate evidence that the thiazoles differentially stimulate GST or mEH genes, with TH being the most efficacious; that thiazoles with carbocyclic ring are much less effective in increasing GST or mEH levels than is TH; and that the changes in these GST and mEH levels are primarily associated with increases in mRNA levels.

The 30 kDa protein co-purified with chick liver glutathione S-transferases is a carbonyl reductase

An unidentified 30 kDa protein was co-purified with chick liver glutathione S-transferases from S-hexylglutathione affinity column. The protein was isolated to apparent homogeneity with chromatofocusing. The molecular mass of the protein was determined to be 30 277 +/- 3 dalton by mass spectrometry. The protein was digested with Achromobacter proteinase I. Amino-acid sequence analyses of the resulting peptides show a high degree of identity with those of human carbonyl reductase. The protein is active with menadione as substrate. Thus, it is identified as chick liver carbonyl reductase.

Pleotropic effects of dietary DHEA

We present data pertaining to some of the in vivo effects associated with dietary DHEA administration to mice and rats. Dietary DHEA leads to: (1) decrease in body weight gain; (2) relative increases in liver weight; (3) liver color change; (4) induction of hepatic peroxisomal enzymes; (5) proliferation of hepatic peroxisomes with increased cross-sectional area; (6) decreased hepatic mitochondrial cross-sectional area; (7) elevated levels of hepatic cytosolic malic enzyme; (8) slight decreases, significant decreases, or significant increases in serum triglyceride levels, depending on mouse strain; (9) increases in total serum cholesterol levels; (10) significant decreases in the hepatic rates of fatty acid synthesis; (11) significant increases in the hepatic rates of cholesterol synthesis; (12) decreases in both protein content and specific activity of hepatic mitochondrial carbamoyl phosphate synthetase-I without concomitant changes in serum urea nitrogen; (13) induction of glutathione S-transferase activity in liver; (14) decrease in hepatic endogenous protein phosphorylation; (15) increase in hepatic AMPase and GTPase activities; (16) formation of 5-androstene-3 beta,17 beta-diol as a major metabolite of DHEA by subcellular fractions of liver, which is reflected in serum and tissue levels; and (17) reduction in serum prolactin levels.

The glutathione S-transferase supergene family: regulation of GST and the contribution of the isoenzymes to cancer chemoprotection and drug resistance

The glutathione S-transferases (GST) represent a major group of detoxification enzymes. All eukaryotic species possess multiple cytosolic and membrane-bound GST isoenzymes, each of which displays distinct catalytic as well as noncatalytic binding properties: the cytosolic enzymes are encoded by at least five distantly related gene families (designated class alpha, mu, pi, sigma, and theta GST), whereas the membrane-bound enzymes, microsomal GST and leukotriene C4 synthetase, are encoded by single genes and both have arisen separately from the soluble GST. Evidence suggests that the level of expression of GST is a crucial factor in determining the sensitivity of cells to a broad spectrum of toxic chemicals. In this article the biochemical functions of GST are described to show how individual isoenzymes contribute to resistance to carcinogens, antitumor drugs, environmental pollutants, and products of oxidative stress. A description of the mechanisms of transcriptional and posttranscriptional regulation of GST isoenzymes is provided to allow identification of factors that may modulate resistance to specific noxious chemicals. The most abundant mammalian GST are the class alpha, mu, and pi enzymes and their regulation has been studied in detail. The biological control of these families is complex as they exhibit sex-, age-, tissue-, species-, and tumor-specific patterns of expression. In addition, GST are regulated by a structurally diverse range of xenobiotics and, to date, at least 100 chemicals have been identified that induce GST; a significant number of these chemical inducers occur naturally and, as they are found as nonnutrient components in vegetables and citrus fruits, it is apparent that humans are likely to be exposed regularly to such compounds. Many inducers, but not all, effect transcriptional activation of GST genes through either the antioxidant-responsive element (ARE), the xenobiotic-responsive element (XRE), the GST P enhancer 1(GPE), or the glucocorticoid-responsive element (GRE). Barbiturates may transcriptionally activate GST through a Barbie box element. The involvement of the Ah-receptor, Maf, Nrl, Jun, Fos, and NF-kappa B in GST induction is discussed. Many of the compounds that induce GST are themselves substrates for these enzymes, or are metabolized (by cytochrome P-450 monooxygenases) to compounds that can serve as GST substrates, suggesting that GST induction represents part of an adaptive response mechanism to chemical stress caused by electrophiles. It also appears probable that GST are regulated in vivo by reactive oxygen species (ROS), because not only are some of the most potent inducers capable of generating free radicals by redox-cycling, but H2O2 has been shown to induce GST in plant and mammalian cells: induction of GST by ROS would appear to represent an adaptive response as these enzymes detoxify some of the toxic carbonyl-, peroxide-, and epoxide-containing metabolites produced within the cell by oxidative stress. Class alpha, mu, and pi GST isoenzymes are overexpressed in rat hepatic preneoplastic nodules and the increased levels of these enzymes are believed to contribute to the multidrug-resistant phenotype observed in these lesions. The majority of human tumors and human tumor cell lines express significant amounts of class pi GST. Cell lines selected in vitro for resistance to anticancer drugs frequently overexpress class pi GST, although overexpression of class alpha and mu isoenzymes is also often observed. The mechanisms responsible for overexpression of GST include transcriptional activation, stabilization of either mRNA or protein, and gene amplification. In humans, marked interindividual differences exist in the expression of class alpha, mu, and theta GST. The molecular basis for the variation in class alpha GST is not known. (ABSTRACT TRUNCATED)

Contribution of amino acid residue 208 in the hydrophobic binding site to the catalytic mechanism of human glutathione transferase A1-1

Glutathione transferases (GSTs) catalyze the nucleophilic attack of the thiolate of glutathione on a variety of noxious, often hydrophobic, electrophiles. The interactions responsible for the binding of glutathione have been deduced in great detail from the 3-dimensional structures that have been solved for three different GSTs, each a member of a distinct structural class. However, the interactions of the electrophilic substrates with these enzymes are still largely unexplored. The contribution of the active-site Met208 to aromatic and benzylic chloride substitution reactions catalyzed by human class Alpha GST A1-1 has been evaluated by comparison of wild-type enzyme with variants mutated in position 208. The results show that the amino acid residue at position 208 primarily affects the aromatic substitution reaction, tested with 1-chloro-2,4-dinitrobenzene as substrate, possibly by interacting with the delocalized negative charge of the substituted ring structure in the transition state.

Induction of murine hepatic glutathione S-transferase by dietary dehydroepiandrosterone

The naturally occurring steroid dehydroepiandrosterone (DHEA), when administered as a supplement to the diet of mice and rats, produces alterations in the relative concentrations of specific liver proteins; among these, a protein of Mr approximately 28 K is markedly induced by DHEA action. In the present study we identified the murine hepatic approximately 28 kDa protein as glutathione S-transferase subtype GT-8.7. Glutathione S-transferases belong to a gene superfamily that encode closely related proteins which are induced in liver and other tissues by various chemicals, including carcinogens and chemoprotective agents such as dietary antioxidants. Based on the above finding, we evaluated glutathione S-transferase activity in cytosols and microsomes prepared from liver tissue of mice fed either a control diet or a DHEA-containing diet (0.45%, by weight). The specific activity of hepatic cytosolic glutathione S-transferase in mice treated with DHEA up to 7 days was either unchanged or slightly decreased when compared to controls; however, treatment for 14 days or longer resulted in significant increases in activity. The specific activity of microsomal glutathione S-transferase also was increased by long-term DHEA treatment; however, its activity was approximately one-tenth of that in corresponding cytosols.

Comparative studies on the effect of butylated hydroxyanisole on glutathione and glutathione S-transferases in the tissues of male and female CD-1 mice

1. Male CD-1 mice had about 1.6-fold higher glutathione (GSH), 2-fold higher glutathione S-transferase (GST) activity and 2.8-fold higher GST protein in their livers as compared to the female mice. 2. When mice were fed a diet containing 0.75% BHA for 2 weeks, a 1.8-fold increase was observed in GSH levels of female mice liver as opposed to only 1.2-fold increase in male mice. BHA caused 10-fold increase in GST activity and protein in livers of female mice as compared to only about 3-4-fold increase in livers of males. Differential induction of GSH and GST in males and females was also observed in other tissue besides liver but was not as remarkable. 3. Sex-related differences were also observed in the induction of the alpha- and mu- and pi-classes of GSTs by BHA; most noticeable being GST pi, which was induced to about 10-fold in female liver as opposed to only 3.4-fold in male liver.

Glutathione S-transferases: gene structure and regulation of expression

The current knowledge about the structure of GST genes and the molecular mechanisms involved in regulation of their expression are reviewed. Information derived from the study of rat and mouse GST Alpha-class, Ya genes, and a rat GST Pi-class gene seems to indicate that a single cis-regulatory element, composed of two adjacent AP-1-like binding sites in the 5'-flanking region of these GST genes, is responsible for their basal and xenobiotic-inducible activity. The identification of Fos/Jun (AP-1) complex as the trans-acting factor that binds to this element and mediates the basal and inducible expression of GST genes offers a basis for an understanding of the molecular processes involved in GST regulation. The induction of expression of Fos and Jun transcriptional regulatory proteins by a variety of extracellular stimuli is known to mediate the activation of target genes via the AP-1 binding sites. The modulation of the AP-1 activity may account for the changes induced by growth factors, hormones, chemical carcinogens, transforming oncogenes, and cellular stress-inducing agents in the pattern of GST expression. Recent observations implying reactive oxygen as the transduction signal that mediates activation of c-fos and c-jun genes are presently considered to provide an explanation for the induction of GST gene expression by chemical agents of diverse structure. The possibility that these agents may all induce conditions of oxidative stress by various pathways to activate expression of GST genes that are regulated by the AP-1 complex is discussed.

The electrophile counterattack response: protection against neoplasia and toxicity

Exposure of rodents or their cells in culture to low doses of a wide variety of chemical agents, many of which are electrophiles, evokes a coordinated metabolic response that protects these systems against the toxicity (including mutagenicity and carcinogenicity) of higher doses of the same or other electrophiles. This response involves enhanced transcription of Phase 2 enzymes: glutathione transferases, NAD(P)H:quinone reductase, UDP-glucuronsyltransferases, and epoxide hydrolase, as well as the elevation of intracellular levels of reduced glutathione. We suggest that this cellular adaptation, which occurs in the liver and many peripheral tissues, be designated as the "Electrophile Counterattack" response. Seven families of highly diverse chemical agents that elicit this response include: oxidatively labile diphenols and quinones; Michael reaction acceptors (olefins conjugated to electron-withdrawing groups); isothiocyanates; organic hydroperoxides; vicinal dimercaptans; trivalent arsenicals; heavy metals (HgCl2, CdCl2) as well as mercury derivatives with high affinities for sulfhydryl groups; and 1,2-dithiole-3-thiones. An analysis of the molecular mechanisms of these enzyme inductions was carried out by transient expression in hepatoma cells of a plasmid containing a 41-bp enhancer element derived from the 5'-upstream region of the mouse glutathione transferase Ya gene, and the promoter region of this gene, linked to a human growth hormone reporter gene. The concentrations of 28 inducers (belonging to the seven chemical classes) required to double growth hormone production in this system spanned a range of four orders of magnitude and were closely and linearly correlated with the concentrations of the same compounds required to double the specific activity of quinone reductase in murine hepatoma cells. We therefore conclude that the regulation of these Phase 2 enzymes (and possibly also that of glutathione synthesis) by all of these inducers is mediated by the same enhancer element that contains AP-1-like sites. Similar enhancer sequences are present in the rat glutathione transferase Ya gene, and in the upstream regulatory regions of the quinone reductase genes of rat and human liver.

A subgroup of class alpha glutathione S-transferases. Cloning of cDNA for mouse lung glutathione S-transferase GST 5.7

A full-length cDNA clone encoding the previously purified mouse glutathione S-transferase GST 5.7 [(1991), Biochem. J. 278, 793-799] has been isolated from a mouse lung cDNA library in lambda gt11. Sequencing of the clone revealed the presence of microheterogeneity in GST 5.7. Comparison of the deduced protein sequence with other glutathione S-transferases, together with previous information available on GST 5.7, indicates that the enzyme belongs to a novel subgroup within the alpha class of glutathione S-transferases. Members of the subgroup, which also include the rat GST 8-8 and perhaps chicken GST CL3, show high sequence homology with each other, but only moderate similarity to other alpha class enzymes. They share a substrate specificity profile that resembles pi-class enzymes, and are active in the conjugation of lipid peroxidation products.

Induction of rat hepatic and intestinal glutathione S-transferases by dietary butylated hydroxyanisole

To obtain insight into the protection mechanism of butylated hydroxyanisole (BHA), a widely used food preservative with anticarcinogenic properties, we investigated the effects of dietary BHA on rat hepatic and intestinal glutathione S-transferase (GST) enzyme activity, and GST isozyme levels. In the proximal small intestine and liver, BHA supplementation significantly increased GST enzyme activity as compared with controls (2.3- and 1.7-fold, respectively, P less than 0.05). GST class alpha and mu contents were significantly higher only in the small intestine (1.6-2.1-fold and 1.3-1.5-fold, respectively, P less than 0.05), whereas GST class pi was significantly induced in liver (4.6-fold, P less than 0.05).

Induction of glutathione S-transferase isoenzymes in mouse liver by 5-(2-pyrazynl)-4-methyl-1,2-dithiole-3-thione (oltipraz)

Treatment of mice with a single dose of oltipraz (OPZ) at 200 mg/kg led to a significant (P less than 0.05) increase in hepatic cytosolic glutathione S-transferase (GST) activity and content. GST activity monitored with 1,2-dichloro-4-nitrobenzene was increased 3.8-fold 3 days after treatment, suggesting the induction of mu class isoenzymes. Ethacrynic acid, a marker for pi class isoforms, showed only a slight increase in GST activity while no induction was observed with cumene hydroperoxide, an indicator for the alpha class. The increase in mu class isoenzymes was further confirmed by separation of the mouse liver affinity purified GST by chromatofocusing and also by resolving the GST subunits by reverse-phase high performance liquid chromatographic procedures. Therefore, OPZ induces mainly the mu class isoenzymes in mouse hepatic tissues.

Primary sequence heterogeneity and tissue expression of glutathione S-transferases of Fasciola hepatica

Glutathione S-transferases (GSTs) from Fasciola hepatica have been purified by glutathione affinity chromatography. Two closely migrating species of Mr 26,000 and 26,500 were identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and several species resolved by two-dimensional gel analysis, indicating substantial heterogeneity among the GSTs. N-terminal amino acid sequencing revealed one core sequence containing three polymorphisms, whereas the sequence of GST peptides implied a minimum of three different GSTs. The amino acid sequence data assigned the F. hepatica GSTs to the mu class of GSTs with high similarities to these proteins in other helminths and mammals. The native GSTs of F. hepatica appeared to behave as dimers as determined by molecular sieving chromatography. The observation that the GSTs of F. hepatica are heterogeneous in sequence and behave as dimers in the native state suggest that these isoenzymes may exhibit considerable functional heterogeneity which may be of importance to the parasite. Immunocytochemical studies suggest that the main source of GST in F. hepatica are the parenchymal cells and peripheral tissues of the parasite. Some extracellular GST is associated with the lamellae of the intestinal epithelium. The identification of an intestinal GST is unique among trematodes studied to date.

Enhanced expression, purification, and characterization of a novel class alpha glutathione S-transferase isozyme appearing in rabbit hepatic cytosol following treatment with 4-picoline

A novel class alpha glutathione S-transferase (GST) isozyme is expressed in the hepatic cytosol of rabbits treated with 4-picoline. SDS-PAGE analysis revealed the presence of a new 28-kDa band which cross-reacted with class alpha GST-specific IgG. This new GST isozyme was isolated from the hepatic cytosol of 4-picoline-treated rabbits and purified to homogeneity using S-hexylglutathione-agarose, CM-Sepharose, and PBE118 chromatofocusing chromatography. The isozyme was determined by SDS-PAGE and gel filtration analyses to be a homodimer of approximately 28 kDa with blocked N-terminus. A heterodimer consisting of 25 and 28 kDa subunits with activity toward the substrate 1-chloro-2,4-dinitrobenzene was also purified. Immunoblot analysis revealed that the 25, 26.5, and 28 kDa bands cross-reacted with class alpha GST-specific IgG and failed to react with either class mu or class pi GST-specific antibodies. The 28 kDa enzyme had a pI of 8.2 as determined by nonequilibrium pH gel electrophoresis. The purified 28 kDa enzyme exhibited activity toward 1-chloro-2,4-dinitrobenzene (Km = 1.60 mM and Vmax = 73.5 mumol/min/mg) and cumene hydroperoxide (Km = 1.02 mM and Vmax = 6.92 mumol/min/mg). Amino acid sequence analysis of several fragments resulting from cyanogen bromide cleavage of the 28 kDa GST isozyme revealed a class alpha GST consensus sequence. In addition, proteolytic digestion with alpha-chymotrypsin yielded peptide maps which showed distinct differences between the purified 28 kDa GST and another purified class alpha GST isozyme present in rabbit liver. These results provide evidence that class alpha GST isozymes containing a novel 28 kDa subunit are expressed following treatment with 4-picoline.

Glutathione transferases and cancer

The glutathione transferases, a family of multifunctional proteins, catalyze the glutathione conjugation reaction with electrophilic compounds biotransformed from xenobiotics, including carcinogens. In preneoplastic cells as well as neoplastic cells, specific molecular forms of glutathione transferase are known to be expressed and have been known to participate in the mechanisms of their resistance to drugs. In this article, following a brief description of recently identified molecular forms, we review new findings regarding the respective molecular forms involved in carcinogenesis and anticancer drug resistance, with particular emphasis on Pi class forms in preneoplastic tissues. The rat Pi class form, GST-P (GST 7-7), is strongly expressed not only in hepatic foci and hepatomas, but also in initiated cells that occur at the very early stages of chemical hepatocarcinogenesis, and is regarded as one of the most reliable markers for preneoplastic lesions in the rat liver. 12-O-Tetradecanoylphorbol-13-acetate (TPA)-responsive element-like sequences have been identified in upstream regions of the GST-P gene, and oncogene products c-jun and c-fos are suggested to activate the gene. The Pi-class forms possess unique enzymatic properties, including broad substrate specificity, glutathione peroxidase activity toward lipid hydroperoxides, low sensitivity to organic anion inhibitors, and high sensitivity to active oxygen species. The possible functions of Pi class glutathione transferases in neoplastic tissues and drug-resistant cells are discussed.

Nucleotide sequence of a class mu glutathione S-transferase from chicken liver

A clone coding for glutathione S-transferase (GST) CL2 was isolated from a chicken liver cDNA library. This clone (819 bp) encodes a polypeptide comprising 219 amino acids with a molecular weight of 25,717, excluding the initiator methionine. The primary amino acid sequence of the enzyme has 47% identical sequence with other class mu GSTs.

Butylated hydroxyanisole in perspective

Butylated hydroxyanisole (BHA) is a synthetic food antioxidant used to prevent oils, fats and shortenings from oxidative deterioration and rancidity. This review depicts the current knowledge on BHA. The physical and chemical characteristics of BHA are summarized and its function as a food antioxidant is made clear. The toxicological characteristics of BHA and its metabolic fate in man and animal are briefly reviewed. Special emphasis is laid on the carcinogenicity of BHA in the forestomach of rodents and to related events in the forestomach and other tissues in experimental animals. At present there is sufficient evidence for carcinogenicity of BHA, but there is hardly any indication that BHA is genotoxic. Therefore risk assessment for this epigenetic carcinogen is based on non-stochastic principles. However, the mechanism underlying the tumorigenicity of BHA is not known. In the last part of this review an attempt is made to unravel the unknown mechanism of carcinogenicity. It is hypothesized that BHA gives rise to tumor formation in rodent forestomach by inducing heritable changes in DNA. Evidence is being provided that reactive oxygen species, in particular hydroxylradicals, may play a crucial role. The key question with respect to risk assessment for BHA is whether or not the underlying mechanism is thresholded, which is important for the choice of the appropriate model to assess the risk, if any, for man and to manage any potential risk.

Mouse liver glutathione S-transferase isoenzyme activity toward aflatoxin B1-8,9-epoxide and benzo[a]pyrene-7,8-dihydrodiol-9,10-epoxide

As part of the studies of the biochemical basis for species differences in biotransformation of the carcinogen aflatoxin B1 (AFB1) and its modulation by phenolic antioxidants, we have investigated the role of mouse liver glutathione S-transferase (GST) isoenzymes in the conjugation of AFB1-8,9-epoxide. Isoenzymes of GST were purified to electrophoretic homogeneity from Swiss-Webster mouse liver cytosol by affinity chromatography and chromatofocusing. The isoenzyme fractions were characterized in terms of activity toward surrogate substrates and immunologic cross-reactivity with antisera to rat GSTs. The major isoenzymes were identified as SW 4-4, SW 3-3, and SW 1-1. The specific activity of SW 4-4 toward AFB1-8,9-epoxide was at least 50- and 150-fold greater than that of SW 3-3 and SW 1-1, respectively. Relatively high activity toward another epoxide carcinogen, benzo[a]pyrene-7,8-dihydrodiol-9,10-epoxide, was observed with both SW 4-4 and SW 3-3. SW 1-1 had the highest activity toward 1-chloro-2,4-dinitrobenzene (CDNB) whereas SW 4-4 had relatively low CDNB activity. Following pretreatment with 0.75% butylated hydroxyanisole in the diet, the fraction of total GST contributed by SW 1-1 appeared to increase dramatically, whereas in control mice SW 3-3 constituted the predominant isoenzyme. The high GST activity of mouse liver cytosol toward AFB1-8,9-epoxide is apparently due to an isoenzyme that contributes little to the overall cytosolic CDNB activity.