Molecular cloning and heterologous expression of a cDNA encoding a mouse glutathione S-transferase Yc subunit possessing high catalytic activity for aflatoxin B1-8,9-epoxide

The metabolism and biotransformation of AFB1: Key enzymes and pathways

Aflatoxins B₁ (AFB₁) is a hepatoxic compound produced by Aspergillus flavus and Aspergillus parasiticus, seriously threatening food safety and the health of humans and animals. Understanding the metabolism of AFB₁ is important for developing detoxification and intervention strategies. In this review, we summarize the AFB₁ metabolic fates in humans and animals and the key enzymes that metabolize AFB₁, including cytochrome P450s (CYP450s) for AFB₁ bioactivation, glutathione-S-transferases (GSTs) and aflatoxin-aldehyde reductases (AFARs) in detoxification. Furthermore, AFB₁ metabolism in microbes is also summarized. Microorganisms specifically and efficiently transform AFB₁ into less or non-toxic products in an environmental-friendly approach which could be the most desirable detoxification strategy in the future. This review provides a wholistic insight into the metabolism and biotransformation of AFB₁ in various organisms, which also benefits the development of protective strategies in humans and animals.

Protective effect of glutathione S-transferase enzyme activity against aflatoxin B1 in poultry species: relationship between glutathione S-transferase enzyme kinetic parameters, and resistance to aflatoxin B1

Comparative studies designed to investigate the role of glutathione S-transferase (GST) activity on the enzyme catalyzed trapping of aflatoxin B1-8,9-epoxide (AFBO) with glutathione, and the relationship with aflatoxin B1 (AFB1) resistance have not been conducted in poultry. Hepatic cytosolic fractions of chickens, quail, turkeys and ducks were used to measure in vitro the enzymatic parameters maximal velocity (Vmax), Michaelis-Menten constant (Km) and intrinsic clearance (CLint) for GST activity. AFB1 used ranged from 2.0 to 157.5 µM and the AFB1-GSH produced was identified and quantitated by HPLC. Significant differences were found in GST Vmax values, being the highest in chickens, followed by quail, ducks and turkeys. The Km values were also significantly different, with chickens < ducks < turkeys < quail. Chickens had the higher CLint value in contrast to ducks. Differences by sex showed that duck females had a higher CLint value than the turkey and quail, whereas duck males had a CLint close to that of turkey. The ratio "AFBO production /AFB1-GSH production" follows the order duck>turkey>quail>chicken, in agreement with the known poultry sensitivity. The extremely high "AFB1 epoxidation activity/ GST activity" ratio observed in ducks might be the explanation for the development of hepatocellular carcinoma in this species.

The effects of Nigella sativa seeds and thymoquinone on aflatoxin phase-2 detoxification through glutathione and glutathione-S-transferase alpha-3, and the relationship between aflatoxin B1-DNA adducts in broilers

The primary pathway of the detoxification of aflatoxin (AF) metabolites occurring at the end of phase-1 biotransformation is glutathione (GSH) conjugation via glutathione-S-transferase (GST) enzyme in phase-2. In this study, the activity of Nigella sativa seeds (NS) and thymoquinone (TQ) on phase-2 detoxification pathways of AF was investigated in light of GSH, GST alpha-3 (GSTA3), and AFB1-DNA adducts detected by the immunohistochemical method in broilers' livers. One-hundred twenty chickens at one-day-old were divided into six groups as the control (CNT), AF, TQ, NS, AF + TQ, and AF + NS groups and fed for 28 days. AF, TQ, and NS were added to the relevant diets at 2 mg/kg, 300 mg/kg, and 5%, respectively. The administration of AF alone strongly stimulated the formation of AFB1-DNA adduct and reduced GSH and GSTA3 levels in hepatocytes (p < 0,01). In contrast, TQ and NS were found to reduce significantly the amount of AFB1-DNA adducts in AF + TQ and AF + NS groups (p < 0,01). We concluded that NS and TQ achieved this by increasing GSH and GSTA3 levels (p < 0,01) thanks to their antioxidant properties, and hence detoxifying the reactive metabolites of AF. Also, we consider that the AFB1-DNA adduct constituted in 28 days can be used as a biomarker for exposure to AF in broilers.

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.

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.

Participation of liver stem cells in cholangiocarcinogenesis after aflatoxin B1 exposure of glutathione S-transferase A3 knockout mice

Aflatoxin B1, arguably the most potent human carcinogen, induces liver cancer in humans, rats, trout, ducks, and so on, but adult mice are totally resistant. This resistance is because of a detoxifying enzyme, mouse glutathione S-transferase A3, which binds to and inactivates aflatoxin B1 epoxide, preventing the epoxide from binding to DNA and causing mutations. Glutathione S-transferase A3 or its analog has not been detected in any of the sensitive species, including humans. The generation of a glutathione S-transferase A3 knockout (represented as KO or -/-) mice has allowed us to study the induction of liver cancer in mice by aflatoxin B1. In contrast to the induction of hepatocellular carcinomas in other species, aflatoxin B1 induces cholangiocarcinomas in GSTA3-/- mice. In other species and in knockout mice, the induction of liver cancer is preceded by extensive proliferation of small oval cells, providing additional evidence that oval cells are bipolar stem cells and may give rise to either hepatocellular carcinoma or cholangiocarcinoma depending on the nature of the hepatocarcinogen and the species of animal. The recent development of mouse oval cell lines in our laboratory from aflatoxin B1-treated GSTA3-/- mice should provide a new venue for study of the properties and potential of putative mouse liver stem cells.

Sensitivity of Arbor Acres broilers and chemoprevention of aflatoxin B1-induced liver injury by curcumin, a natural potent inducer of phase-II enzymes and Nrf2

In this study, we scrutinized the effects of curcumin and AFB₁ supplemented diet alone or in combination on phase-ӀӀ enzymes. Histopathological examination showed that after 28 days, AFB1 (5.0 mg/kg diet) induced liver injury in broilers, but curcumin supplementation partially ameliorated liver injury in a dose-dependent manner. RT-PCR data revealed that AFB₁ significantly (p < 0.01) down-regulated Nrf2 and its downstream genes mRNA expression level. Moreover, Western blot analysis showed that Nrf2, GSTM2, and GSTA3 protein expression level was markedly (p < 0.01) reduced in AFB₁-fed group. However, curcumin supplementation ameliorated AFB₁-induced liver injury via enhancing phase-ӀӀ enzymes expressions and activity. HPLC results showed that curcumin increased AFB₁-GSH conjugation in-vitro in liver cytosol. Surprisingly, similar trends were noted in mRNA, protein expression level of Nrf2 and its downstream genes at day 35, one week after the withdrawal of AFB₁ and curcumin from the diet, showing the preventive effects of curcumin.

Characterization of liver injury, oval cell proliferation and cholangiocarcinogenesis in glutathione S-transferase A3 knockout mice

We recently generated glutathione S-transferase (GST) A3 knockout (KO) mice as a novel model to study the risk factors for liver cancer. GSTA3 KO mice are sensitive to the acute cytotoxic and genotoxic effects of aflatoxin B1 (AFB1), confirming the crucial role of GSTA3 in resistance to AFB1. We now report histopathological changes, tumor formation, biochemical changes and gender response following AFB1 treatment as well as the contribution of oxidative stress. Using a protocol of weekly 0.5 mg AFB1/kg administration, we observed extensive oval (liver stem) cell (OC) proliferation within 1-3 weeks followed by microvesicular lipidosis, megahepatocytes, nuclear inclusions, cholangiomas and small nodules. Male and female GSTA3 KO mice treated with 12 and 24 weekly AFB1 injections followed by a rest period of 12 and 6 months, respectively, all had grossly distorted livers with macro- and microscopic cysts, hepatocellular nodules, cholangiomas and cholangiocarcinomas and OC proliferation. We postulate that the prolonged AFB1 treatment leads to inhibition of hepatocyte proliferation, which is compensated by OC proliferation and eventually formation of cholangiocarcinoma (CCA). At low-dose AFB1, male KO mice showed less extensive acute liver injury, OC proliferation and AFB1-DNA adducts than female KO mice. There were no significant compensatory changes in KO mice GST subunits, GST enzymatic activity, epoxide hydrolase, or CYP1A2 and CYP3A11 levels. Finally, there was a modest increase in F2-isoprostane and isofuran in KO mice that confirmed putative GSTA3 hydroperoxidase activity in vivo for the first time.

Hepatic Transcriptome Responses of Domesticated and Wild Turkey Embryos to Aflatoxin B₁

The mycotoxin, aflatoxin B₁ (AFB₁) is a hepatotoxic, immunotoxic, and mutagenic contaminant of food and animal feeds. In poultry, AFB₁ can be maternally transferred to embryonated eggs, affecting development, viability and performance after hatch. Domesticated turkeys (Meleagris gallopavo) are especially sensitive to aflatoxicosis, while Eastern wild turkeys (M. g. silvestris) are likely more resistant. In ovo exposure provided a controlled AFB₁ challenge and comparison of domesticated and wild turkeys. Gene expression responses to AFB₁ in the embryonic hepatic transcriptome were examined using RNA-sequencing (RNA-seq). Eggs were injected with AFB₁ (1 μg) or sham control and dissected for liver tissue after 1 day or 5 days of exposure. Libraries from domesticated turkey (n = 24) and wild turkey (n = 15) produced 89.2 Gb of sequence. Approximately 670 M reads were mapped to a turkey gene set. Differential expression analysis identified 1535 significant genes with |log₂ fold change| ≥ 1.0 in at least one pair-wise comparison. AFB₁ effects were dependent on exposure time and turkey type, occurred more rapidly in domesticated turkeys, and led to notable up-regulation in cell cycle regulators, NRF2-mediated response genes and coagulation factors. Further investigation of NRF2-response genes may identify targets to improve poultry resistance.

Mechanisms of activation of the transcription factor Nrf2 by redox stressors, nutrient cues, and energy status and the pathways through which it attenuates degenerative disease

Nuclear factor-erythroid 2 p45-related factor 2 (Nrf2) regulates the basal and stress-inducible expression of a battery of genes encoding key components of the glutathione-based and thioredoxin-based antioxidant systems, as well as aldo-keto reductase, glutathione S-transferase, and

NAD(P)H

quinone oxidoreductase-1 drug-metabolizing isoenzymes along with multidrug-resistance-associated efflux pumps. It therefore plays a pivotal role in both intrinsic resistance and cellular adaptation to reactive oxygen species (ROS) and xenobiotics. Activation of Nrf2 can, however, serve as a double-edged sword because some of the genes it induces may contribute to chemical carcinogenesis by promoting futile redox cycling of polycyclic aromatic hydrocarbon metabolites or confer resistance to chemotherapeutic drugs by increasing the expression of efflux pumps, suggesting its cytoprotective effects will vary in a context-specific fashion. In addition to cytoprotection, Nrf2 also controls genes involved in intermediary metabolism, positively regulating those involved in NADPH generation, purine biosynthesis, and the β-oxidation of fatty acids, while suppressing those involved in lipogenesis and gluconeogenesis. Nrf2 is subject to regulation at multiple levels. Its ability to orchestrate adaptation to oxidants and electrophiles is due principally to stress-stimulated modification of thiols within one of its repressors, the Kelch-like ECH-associated protein 1 (Keap1), which is present in the cullin-3 RING ubiquitin ligase (CRL) complex CRLKeap1. Thus modification of Cys residues in Keap1 blocks CRLKeap1 activity, allowing newly translated Nrf2 to accumulate rapidly and induce its target genes. The ability of Keap1 to repress Nrf2 can be attenuated by p62/sequestosome-1 in a mechanistic target of rapamycin complex 1 (mTORC1)-dependent manner, thereby allowing refeeding after fasting to increase Nrf2-target gene expression. In parallel with repression by Keap1, Nrf2 is also repressed by β-transducin repeat-containing protein (β-TrCP), present in the Skp1-cullin-1-F-box protein (SCF) ubiquitin ligase complex SCFβ-TrCP. The ability of SCFβ-TrCP to suppress Nrf2 activity is itself enhanced by prior phosphorylation of the transcription factor by glycogen synthase kinase-3 (GSK-3) through formation of a DSGIS-containing phosphodegron. However, formation of the phosphodegron in Nrf2 by GSK-3 is inhibited by stimuli that activate protein kinase B (PKB)/Akt. In particular, PKB/Akt activity can be increased by phosphoinositide 3-kinase and mTORC2, thereby providing an explanation of why antioxidant-responsive element-driven genes are induced by growth factors and nutrients. Thus Nrf2 activity is tightly controlled via CRLKeap1 and SCFβ-TrCP by oxidative stress and energy-based signals, allowing it to mediate adaptive responses that restore redox homeostasis and modulate intermediary metabolism. Based on the fact that Nrf2 influences multiple biochemical pathways in both positive and negative ways, it is likely its dose-response curve, in terms of susceptibility to certain degenerative disease, is U-shaped. Specifically, too little Nrf2 activity will lead to loss of cytoprotection, diminished antioxidant capacity, and lowered β-oxidation of fatty acids, while conversely also exhibiting heightened sensitivity to ROS-based signaling that involves receptor tyrosine kinases and apoptosis signal-regulating kinase-1. By contrast, too much Nrf2 activity disturbs the homeostatic balance in favor of reduction, and so may have deleterious consequences including overproduction of reduced glutathione and NADPH, the blunting of ROS-based signal transduction, epithelial cell hyperplasia, and failure of certain cell types to differentiate correctly. We discuss the basis of a putative U-shaped Nrf2 dose-response curve in terms of potentially competing processes relevant to different stages of tumorigenesis.

Gamma-glutamyl transpeptidase: redox regulation and drug resistance

The expression of gamma-glutamyl transpeptidase (GGT) is essential to maintaining cysteine levels in the body. GGT is a cell surface enzyme that hydrolyzes the gamma-glutamyl bond of extracellular reduced and oxidized glutathione, initiating their cleavage into glutamate, cysteine (cystine), and glycine. GGT is normally expressed on the apical surface of ducts and glands, salvaging the amino acids from glutathione in the ductal fluids. GGT in tumors is expressed over the entire cell membrane and provides tumors with access to additional cysteine and cystine from reduced and oxidized glutathione in the blood and interstitial fluid. Cysteine is rate-limiting for glutathione synthesis in cells under oxidative stress. The induction of GGT is observed in tumors with elevated levels of intracellular glutathione. Studies in models of hepatocarcinogenesis show that GGT expression in foci of preneoplastic hepatocytes provides a selective advantage to the cells during tumor promotion with agents that deplete intracellular glutathione. Similarly, expression of GGT in tumors enables cells to maintain elevated levels of intracellular glutathione and to rapidly replenish glutathione during treatment with prooxidant anticancer therapy. In the clinic, the expression of GGT in tumors is correlated with drug resistance. The inhibitors of GGT block GGT-positive tumors from accessing the cysteine in extracellular glutathione. They also inhibit GGT activity in the kidney, which results in the excretion of GSH in the urine and a rapid decrease in blood cysteine levels, leading to depletion of intracellular GSH in both GGT-positive and GGT-negative tumors. GGT inhibitors are being developed for clinical use to sensitize tumors to chemotherapy.

Effect of 8-oxoguanine glycosylase deficiency on aflatoxin B1 tumourigenicity in mice

The mycotoxin aflatoxin B1 (AFB1) may initiate cancer by causing oxidatively damaged DNA, specifically by causing 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) lesions. Base excision repair removes these lesions, with 8-oxoguanine glycosylase (OGG1) being the rate-limiting enzyme. The aim of this study was to determine the effect of ogg1 deficiency on AFB1-induced oxidatively damaged DNA and tumourigenesis. Female wild-type, heterozygous and homozygous ogg1 null mice were given a single dose of 50mg/kg AFB1 or 40 µl dimethyl sulfoxide (DMSO) ip. Neither ogg1 genotype nor AFB1 treatment affected levels of oxidised guanine in lung or liver 2h post-treatment. AFB1-treated ogg1 null mice showed exacerbated weight loss and mortality relative to DMSO-treated ogg1 null mice, but AFB1 treatment did not significantly increase lung or liver tumour incidence compared with controls, regardless of ogg1 genotype. Suspect lung masses from three of the AFB1-treated mice were adenomas, and masses from two of the mice were osteosarcomas. No osteosarcomas were observed in DMSO-treated mice. All liver masses from AFB1-treated mice were adenomas, and one also contained a hepatocellular carcinoma. In DNA from the lung tumours, the K-ras mutation pattern was inconsistent with initiation by AFB1. In conclusion, ogg1 status did not have a significant effect on AFB1-induced oxidatively damaged DNA or tumourigenesis, but deletion of one or both alleles of ogg1 did increase susceptibility to other aspects of AFB1 toxicity.

Prenatal exposure of mice to the human liver carcinogen aflatoxin B1 reveals a critical window of susceptibility to genetic change

It has become axiomatic that critical windows of susceptibility to genotoxins exist and that genetic damage in utero may be a trigger for later life cancers. Data supporting this critical window hypothesis are remarkably few. This study provides a quantitative bridge between DNA damage by the liver carcinogen aflatoxin B1 (AFB1 ) during prenatal development and the risk of later life genetic disease. AFB1 was given to pregnant C57BL/6J mice, carrying F1 gestation day 14 (GD14) embryos of the B6C3F1 genotype. Ultra-high performance liquid chromatography and mass spectrometry (UPLC-MS) using aflatoxin-(15) N5 -guanine adduct standards afforded measurement of the AFB1 -N(7) -Gua and AFB1 -FAPY adducts 6-hr post dosing in liver DNA of mothers and embryos. A parallel cohort gave birth and the livers of the F1 were analyzed for mutations in the gpt gene at 3 and 10 weeks of age. The data revealed mutational spectra dominated by G:C to T:A mutations in both the mother and offspring that are characteristic of AFB1 and distinct from background. It was shown that adducts in GD14 embryos were 20-fold more potent inducers of mutagenesis than adducts in parallel-dosed adults. This sensitivity enhancement correlated with Ki67 staining of the liver, reflecting the proliferative potential of the tissue. Taken together, these data provide insight into the relative genetic risks of prenatal and adult exposures to AFB1 . Early life exposure, especially during the embryonic period, is strikingly more mutagenic than treatment later in life. Moreover the data provide a baseline against which risk prevention strategies can be evaluated.

Trp266 determines the binding specificity of a porcine aflatoxin B₁ aldehyde reductase for aflatoxin B₁-dialdehyde

Aflatoxin B₁ (AFB₁) is a severe threat to human and animal health. The aflatoxin B₁ aldehyde reductase (AFAR) family specifically catalyzes AFB₁-dialdehyde, a toxic metabolic intermediate of AFB₁, producing a nontoxic dialcohol. Although several AFARs have been found and characterized, the binding specificity of the family for AFB₁-dialdehyde remains unclear. Herein, according to the published sequence, we cloned a porcine AFAR gene. Recombinant porcine AFAR was expressed and purified from Escherichia coli as hexa-histidine tagged fusion protein. Using the cloned porcine AFAR as a model, site-directed mutagenesis combined with high performance liquid chromatography studies revealed that the substitution of Trp266 with Ala resulted in almost complete loss of catalytic activity for AFB₁-dialdehyde. Interestingly, the substitution of Met86 with Ala exhibited an obviously increased activity to the dialdehyde. Based on these results and by using molecular docking simulations, this work provides a structural explanation for why the AFAR family exhibits high specificity for AFB₁-dialdehyde. The Trp266 residue in porcine AFAR plays a critical role in stabilizing the binding of AFB₁-dialdehyde in the active pocket through the hydrophobic interaction of the side-chain indole ring of Trp266 with the fused coumarin rings of the dialdehyde molecule. The enhanced activity of M86A may be attributed to the formed π-π stacking interaction between Trp266 and the dialdehyde. In addition, other hydrophobic residues (e.g. Phe and Trp) around the dialdehyde molecule also stabilize the substrate binding. The findings may contribute to understanding the substrate specificity of the AFAR family for AFB₁-dialdehyde.

Heterologous expression and functional characterization of avian mu-class glutathione S-transferases

Hepatic glutathione S-transferases (GSTs: EC2.5.1.1.8) catalyze the detoxification of reactive electrophilic compounds, many of which are toxic and carcinogenic intermediates, via conjugation with the endogenous tripeptide glutathione (GSH). Glutathione S-transferase (GST)-mediated detoxification is a critical determinant of species susceptibility to the toxic and carcinogenic mycotoxin aflatoxin B1 (AFB1), which in resistant animals efficiently detoxifies the toxic intermediate produced by hepatic cytochrome P450 bioactivation, the exo-AFB1-8,9-epoxide (AFBO). Domestic turkeys (Meleagris gallopavo) are one of the most sensitive animals known to AFB1, a condition associated with a deficiency of hepatic GST-mediated detoxification of AFBO. We have recently shown that unlike their domestic counterparts, wild turkeys (Meleagris gallopavo silvestris), which are relatively resistant, express hepatic GST-mediated detoxification activity toward AFBO. Because of the importance of GSTs in species susceptibility, and to explore possible GST classes involved in AFB1 detoxification, we amplified, cloned, expressed and functionally characterized the hepatic mu-class GSTs tGSTM3 (GenBank accession no. JF340152), tGSTM4 (JF340153) from domestic turkeys, and a GSTM4 variant (ewGSTM4, JF340154) from Eastern wild turkeys. Predicted molecular masses of tGSTM3 and two tGSTM4 variants were 25.6 and 25.8kDa, respectively. Multiple sequence comparisons revealed four GSTM motifs and the mu-loop in both proteins. tGSTM4 has 89% amino acid sequence identity to chicken GSTM2, while tGSTM3 has 73% sequence identity to human GSTM3 (hGSTM3). Specific activities of Escherichia coli-expressed tGSTM3 toward 1-chloro-2,4-dinitrobenzene (CDNB) and peroxidase activity toward cumene hydroperoxide were five-fold greater than tGSTM4 while tGSTM4 possessed more than three-fold greater activity toward 1,2-dichloro-4-nitrobenzene (DCNB). The two enzymes displayed equal activity toward ethacrynic acid (ECA). However, none of the GSTM proteins had AFBO detoxification capability, in contrast to recombinant alpha-class GSTs shown in our recent study to possess this important activity. In total, our data indicate that although turkey hepatic GSTMs may contribute to xenobiotic detoxification, they probably play no role in detoxification of AFBO in the liver.

Dual effects of phloretin on aflatoxin B1 metabolism: activation and detoxification of aflatoxin B1

Typically, chemopreventive agents involve either induction of phase II detoxifying enzymes and/or inhibition of cytochrome P450 enzymes (CYPs) that are required for the activation of procarcinogens. In this study, we investigated the protective effects of phloretin against aflatoxin B1 (AFB1) activation to the ultimate carcinogenic intermediate, AFB(1)-8, 9-epoxide (AFBO), and its subsequent detoxification. Phloretin markedly inhibited formation of the epoxide with human liver microsomes in a dose-dependent manner. Phloretin also inhibited the activities of nifedipine oxidation and ethoxyresorufin O-deethylase (EROD) in human liver microsomes. These data show that phloretin strongly inhibits CYP1A2 and CYP3A4 activities, which are involved in the activation of AFB1. Phloretin increased glutathione S-transferase (GST) activity of alpha mouse liver 12 (AML 12) cells in a dose-dependent manner. GST activity toward AFBO in cell lysates treated with 20 μM phloretin was 23-fold that of untreated control cell lysates. The expression of GSTA3, GSTA4, GSTM1, GSTP1 and GSTT1 was induced by phloretin in a dose-dependent manner in AML 12 cells. GSTP1, GSTM1, and GSTT1 were able to significantly increase the conjugation of AFBO with glutathione. Concurrently, induction of the GST isozyme genes was partially associated with the Nrf2/ARE pathway. Taken together, the results demonstrate that phloretin has a strong chemopreventive effect against AFB1 through its inhibitory effect on CYP1A2, CYP3A4, and its inductive effect on GST activity.

Functional characterization of alpha-class glutathione s-transferases from the Turkey (meleagris gallopavo)

Six Alpha-class glutathione S-transferase (GST) subunits were cloned from domestic turkey livers, which are one of the most susceptible animals known to the carcinogenic mycotoxin aflatoxin B₁. In most animals, GST dysfunction is a risk factor for susceptibility toward AFB₁, and we have shown that turkeys lack GSTs with affinity toward the carcinogenic intermediate exo-aflatoxin B(1)-8-9-epoxide (AFBO). Conversely, mice are resistant to AFB₁ carcinogenesis, due to high constitutive expression of mGSTA3 that has high affinity toward AFBO. When expressed in Escherichia coli, all six tGSTA subunits possessed conjugating activities toward substrates 1-chloro-2,4-dinitrobenzene (CDNB), 1,2-dichloro-4-nitrobenzene (DCNB), ethacrynic acid (ECA), and cumene hydroperoxide (CHP) with tGSTA1.2 appearing most active. Interestingly, tGSTA1.1, which lacks one of the four Alpha-class signature motifs, possessed enzymatic activities toward all substrates. All had comparable activities toward AFBO conjugation, an activity absent in turkey liver cytosols. E. coli-expressed mGSTA3 conjugated AFBO with more than 3-fold greater activity than that of tGSTAs and had higher activity toward GST prototype substrates. Mouse hepatic cytosols had approximately 900-fold higher catalytic activity toward AFBO compared with those from turkey. There was no apparent amino acid profile in tGSTAs that might correspond to specificity toward AFBO, although tGSTA1.2, which had slightly higher AFBO-trapping ability, shared Tyr¹⁰⁸ with mGSTA3, a residue postulated to be critical for AFBO trapping activity in mammalian systems. The observation that recombinant tGSTAs detoxify AFBO, whereas their hepatic forms do not, implies that the hepatic forms of these enzymes are silenced by one or more regulatory mechanisms.

Aflatoxin B1-DNA adduct formation and mutagenicity in livers of neonatal male and female B6C3F1 mice

Exposure to genotoxic chemicals at a young age increases cancer incidence later in life. Aflatoxin B(1) (AFB(1)) is a potent genotoxin that induces hepatocellular carcinoma (HCC) in many animal species and in humans. Whereas adult mice are insensitive to aflatoxin-induced carcinogenesis, mice treated with AFB(1) shortly after birth develop a high incidence of HCC in adulthood. Furthermore, the incidence of HCC in adult male mice treated as infants is much greater than in females, reasons for which are unclear. In this study, treatment with AFB(1) produced similar levels of DNA damage and mutations in the liver of newborn male and female gpt delta B6C3F1 mice. Twenty-four hours after dosing with AFB(1) (6 mg/kg), the highly mutagenic AFB(1)-FAPY adduct was present at twice the level of AFB(1)-N(7)-guanine in liver DNA of males and females. A multiple dose regimen (3 × 2 mg/kg), while delivering the same total dose, resulted in lower AFB(1) adduct levels. Mutation frequencies in the gpt transgene in liver were increased by 20- to 30-fold. The most prominent mutations in AFB(1)-treated mice were G:C to T:A transversions and G:C to A:T transitions. At this 21-day time point, no significant differences were found in mutation frequency or types of mutations between males and females. These results show that infant male and female B6C3F1 mice experience similar amounts of DNA damage and mutation from AFB(1) that may initiate the neoplastic process. The gender difference in the subsequent development of HCC highlights the importance of elucidating additional factors that modulate HCC development.

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.

Sulforaphane induces glutathione S-transferase isozymes which detoxify aflatoxin B(1)-8,9-epoxide in AML 12 cells

The aflatoxin B(1)-8,9-epoxide (AFBO) is hepatocarcinogenic intermediate of aflatoxin B(1) (AFB(1)) and is detoxified by glutathione S-transferases (GSTs). In this study, we investigated whether sulforaphane (SFN) could increase the rate of conjugation between AFBO and glutathione (GSH) as well as which of the GST isozymes were involved in the conjugation reaction. The conjugation potential was inhibited dose dependently with curcumin, an inhibitor of GSTs. SFN induced the expression of GST A3, GST A4, GST M1, GST P1, and GST T1 in alpha mouse line (AML) 12 cells. The cells treated with SFN (10 microM) for 12 h showed a 35-fold increase in conjugation potential of AFBO with GSH compared with the vehicle-treated cell. The conjugation potential was blocked partially by transfection of cells with siRNAs against each of the GST isozymes. The activity of GST A3 had the strongest effect on the conjugation potential. SFN treatment also increased total GST activity detected with 1-chloro-2,4-dinitrobenzene (CDNB) up to 4.3-fold. The induction fold was much lower than that detected with AFBO. These results suggest that the chemopreventive effect of SFN on the decomposition of AFBO is related to the upregulation of several GST isozymes genes. The increase of GST activity by SFN was extremely specific toward the conjugation reaction of AFBO compared with CDNB. Therefore, this system for detecting GST activity seems to be an excellent method for screening chemopreventive compounds toward AFB(1) toxicity.

Aflatoxin B1 in poultry: toxicology, metabolism and prevention

Aflatoxins (AF) are ubiquitous in corn-based animal feed and causes hepatotoxic and hepatocarcinogenic effects. The most important AF in terms of toxic potency and occurrence is aflatoxin B1 (AFB1). Poultry, especially turkeys, are extremely sensitive to the toxic and carcinogenic action of AFB1, resulting in millions of dollars in annual losses to producers due to reduced growth rate, increased susceptibility to disease, reduced egg production and other adverse effects. The extreme sensitivity of turkeys and other poultry to AFB1 is associated with efficient hepatic cytochrome P450-mediated bioactivation and deficient detoxification by glutathione S-transferases (GST). Discerning the biochemical and molecular mechanisms of this extreme sensitivity of poultry to AFB1, will contribute in the development of novel strategies to increase aflatoxin resistance. Since AFB1 is an unavoidable contaminant of corn-based poultry feed, chemoprevention strategies aimed at reducing AFB1 toxicity in poultry and in other animals have been the subject of numerous studies. This brief review summarizes many of the key recent findings regarding the action of aflatoxins in poultry.

Glutathione-S-transferase A3 knockout mice are sensitive to acute cytotoxic and genotoxic effects of aflatoxin B1

Aflatoxin B1 (AFB1) is a major risk factor for hepatocellular carcinoma (HCC) in humans. However, mice, a major animal model for the study of AFB1 carcinogenesis, are resistant, due to high constitutive expression, in the mouse liver, of glutathione S-transferase A3 subunit (mGSTA3) that is lacking in humans. Our objective was to establish that a mouse model for AFB1 toxicity could be used to study mechanisms of toxicity that are relevant for human disease, i.e., an mGSTA3 knockout (KO) mouse that responds to toxicants such as AFB1 in a manner similar to humans. Exons 3-6 of the mGSTA3 were replaced with a neomycin cassette by homologous recombination. Southern blotting, RT-PCR, Western blotting, and measurement of AFB1-N(7)-DNA adduct formation were used to evaluate the mGSTA3 KO mice. The KO mice have deletion of exons 3-6 of the mGSTA3 gene, as expected, as well as a lack of mGSTA3 expression at the mRNA and protein levels. Three hours after injection of 5 mg/kg AFB1, mGSTA3 KO mice have more than 100-fold more AFB1-N(7)-DNA adducts in their livers than do similarly treated wild-type (WT) mice. In addition, the mGSTA3 KO mice die of massive hepatic necrosis, at AFB1 doses that have minimal toxic effects in WT mice. We conclude that mGSTA3 KO mice are sensitive to the acute cytotoxic and genotoxic effects of AFB1, confirming the crucial role of GSTA3 subunit in protection of normal mice against AFB1 toxicity. We propose the mGSTA3 KO mouse as a useful model with which to study the interplay of risk factors leading to HCC development in humans, as well as for testing of additional possible functions of mGSTA3.

Naturally occurring chlorophyll derivatives inhibit aflatoxin B1-DNA adduct formation in hepatoma cells

The inhibitory effects of four chlorophyll derivatives (chlorophyllide [Chlide] a and b and pheophorbide [Pho] a and b) on aflatoxin B1 (AFB1)-DNA adduct formation, and on the modulation of hepatic glutathione S-transferase (GST) were evaluated in murine hepatoma (Hepa-1) cells. Enzyme-linked immunosorbent assay showed that pretreatment with Chlide or Pho significantly reduced the formation of AFB1-DNA adducts, and that Pho was the most potent inhibitor. However, wash-out prior to adding AFB1 totally eliminated inhibition by Childe and partially eliminated inhibition by Pho, indicating that the inhibitory effect of Chlide, and to some extent Pho, was mediated through direct trapping of AFB1. Furthermore, spectrophotometric analysis showed that Pho treatment could increase GST activity in Hepa-1 cells. These observations indicate that the chlorophyll derivatives studied may attenuate AFB1-induced DNA damage in the Hepa-1 cell by direct trapping of AFB1. Pho provided additional protection not only by direct trapping, but also by increasing GST activity against hepatic AFB1 metabolites.

Enhanced Spontaneous and Benzo(a)pyrene-Induced Mutations in the Lung of Nrf2-Deficient gpt Delta Mice

The lung is an organ that is sensitive to mutations induced by chemicals in ambient air, and transgenic mice harboring guanine phosphoribosyltransferase (gpt) gene as a target gene are a well-established model system for assessing genotoxicity in vivo. Transcription factor Nrf2 mediates inducible and constitutive expression of cytoprotective enzymes against xenobiotics and mutagens. To address whether Nrf2 is also involved in DNA protection, we generated nrf2+/−::gpt and nrf2−/−::gpt mice. The spontaneous mutation frequency of the gpt gene in the lung was approximately three times higher in nrf2-null (nrf2−/−) mice than nrf2 heterozygous (nrf2+/−) and wild-type (nrf2+/+) mice, whereas in the liver, the mutation frequency was higher in nrf2−/− and nrf2+/− mice than in nrf2+/+ wild-type mice. By contrast, no difference in mutation frequency was observed in testis among the three genotypes. A single intratracheal instillation of benzo(a)pyrene (BaP) increased the lung mutation frequency 3.1- and 6.1-fold in nrf2+/− and nrf2−/− mice, respectively, compared with BaP-untreated nrf2+/− mice, showing that nrf2−/− mice are more susceptible to genotoxic carcinogens. Surprisingly, mutation profiles of the gpt gene in BaP-treated nrf2+/− mice was substantially different from that in BaP-untreated nrf2−/− mice. In nrf2−/− mice, spontaneous and BaP-induced mutation hotspots were observed at nucleotides 64 and 140 of gpt, respectively. These results thus show that Nrf2 aids in the prevention of mutations in vivo and suggest that Nrf2 protects genomic DNA against certain types of mutations. [Cancer Res 2007;67(12):5643–8]

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.

Gene expression profile in liver of differing ages of rats after single oral administration of acetaminophen

In order to verify the influence of the rat age on hepatotoxicity, male Sprague-Dawley rats of 6 (young) and 12 (adult) weeks of age were orally administered acetaminophen (APAP), isoniazid (INH), or carbon tetrachloride (CCl4). Liver samples were obtained in a time-course manner, and changes in gene expression examined by an Affymetrix GeneChip. APAP caused more prominent hepatic injury with respect to pathology and blood biochemistry in adults than in young rats, whereas no obvious age-related differences were observed in INH- or CCl4-treated rats. Comparing gene expression in control rats, CYP3A13 was higher and GSTY2c was lower in adults, suggesting that production of the active metabolite of APAP is higher and its detoxification is lower in adults. The total amount of glutathione and total SH in rat liver was found to be higher in adult rats whereas the extent of its reduction by APAP was larger in adults. A detailed analysis of genes showing age-related differences revealed that some of them were different not in their extent but in their time course, i.e., the stress responses occurred earlier in the young than in the adult, resulting in a difference at 24 hr after dosing. These results suggest that the age-related difference in toxicity would be attributed to a higher expression of CYP3A13, producing the active metabolite of APAP as well as the lower expression of the detoxification enzyme, GSTY2c, in adult rats. Furthermore, these differences affect the time course of APAP toxicity. The present study clearly depicts the advantage of the multi-time, multi-dose protocol employed in our project for analyzing the mechanism of toxicity by gene expression profiling.

Changes in hepatic cytosolic glutathione S-transferase activity and expression of its class-P during prenatal and postnatal period in rats treated with aflatoxin B1

The effect of aflatoxin B1 (AFB1) on the expression of glutathione S-transferase-P (GST-P) which is the major isoform of GST in developmental stages has been investigated in rat liver during prenatal and postnatal stages. Following administration of AFB1 (0, 0.5, 1.0, 2.0, 3.0 or 4.0 mg/kg bw) injected I.P on day 8.5 of gestation the number of dead or reabsorbed fetuses and malformed embryos were recorded. Then the fetal livers were processed for measurement of total GST and GST-P activities, using 1-chloro-2,4-dinitrobenzene (CDNB) and ethacrynic acid (ETA) as substrates respectively. RT-PCR using rat GST-P specific primers was performed on mRNA extracted from livers. Besides, the effects of AFB1 on hepatic GST and GST-P were assessed in groups of suckling rats directly injected with the toxin. The results show that a single dose of AFB1 (1.0 or 2.0 mg/kg bw) caused approximately 50-60% depletion in fetal liver GST towards CDNB. Postnatal experiments revealed that liver GST (using CDNB as substrate) was significantly induced (approximately 40%) in suckling rats injected with a single dose of AFB1 (3.0 mg AFB1/kg) 24 h before killing. Liver GST-P expression was unaffected due to AFB1 exposures of rats before and after the birth. This finding was substantiated by western blotting and RT-PCR techniques. These data suggest that AFB1-related induction in rat liver total GST after birth may be implicated in protective mechanisms against AFB1. In contrast, inhibition of this enzyme in fetal liver following placental transfer of the carcinogen may explain high susceptibility of fetal cells to trans-plancental aflatoxins. Furthermore, lack of influence of AFB1 on GST-P expression in developmental stages can role out the involvement of this class of GST in AFB1 biotransformation.

Thyme (Thymus vulgaris L.) leaves and its constituents increase the activities of xenobiotic-metabolizing enzymes in mouse liver

The effects of thyme (Thymus vulgaris L.) leaves and its phenolic compounds, thymol and carvacrol, on the activities of xenobiotic-metabolizing enzymes, i.e., phase I enzymes such as 7-ethoxycoumarin O-deethylase (ECOD) and phase II enzymes such as glutathione S-transferase (GST) and quinone reductase (QR), were investigated. Mice were fed with a diet containing thyme (0.5% or 2.0%) or treated orally with thymol (50-200 mg/kg) or carvacrol (50-200 mg/kg) once a day for 7 successive days, and then the enzyme activities in the livers were analyzed. Dietary administration of 2% thyme caused slightly but significantly higher ECOD, GST, and QR activities by 1.1-1.4-fold. Thymol (200 mg/kg) treatment resulted in significantly higher ECOD, GST, and QR activities by 1.3-1.9-fold, and carvacrol (200 mg/kg) treatment caused significantly higher ECOD, GST, and QR activities by 1.3-1.7-fold. Thymol-treated animals had significantly higher protein levels of GST alpha and GST micro, and carvacrol-treated animals had significantly higher levels of GST micro. These results imply that thyme contains bifunctional inducers (i.e., substances capable of inducing both phase I and phase II enzymes) and that thymol and carvacrol may account for the effects of thyme.

A conserved N-capping motif contributes significantly to the stabilization and dynamics of the C-terminal region of class Alpha glutathione S-transferases

Helix 9, the major structural element in the C-terminal region of class Alpha glutathione transferases, forms part of the active site of these enzymes where its dynamic properties modulate both catalytic and ligandin functions. A conserved aspartic acid N-capping motif for helix 9 was identified by sequence alignments of the C-terminal regions of class Alpha glutathione S-transferases (GSTs) and an analysis by the helix-coil algorithm AGADIR. The contribution of the N-capping motif to the stability and dynamics of the region was investigated by replacing the N-cap residue Asp-209 with a glycine in human glutathione S-transferase A1-1 (hGST A1-1) and in a peptide corresponding to its C-terminal region. Far-UV circular dichroism and AGADIR analyses indicate that, in the absence of tertiary interactions, the wild-type peptide displays a low intrinsic tendency to form a helix and that this tendency is reduced significantly by the Asp-to-Gly mutation. Disruption of the N-capping motif of helix 9 in hGST A1-1 alters the conformational dynamics of the C-terminal region and, consequently, the features of the H-site to which hydrophobic substrates (e.g. 1-chloro-2,4-dinitrobenzene (CDNB)) and nonsubstrates (e.g. 8-anilino-1-naphthalene sulfonate (ANS)) bind. Isothermal calorimetric and fluorescence data for complex formation between ANS and protein suggest that the D209G-induced perturbation in the C-terminal region prevents normal ligand-induced localization of the region at the active site, resulting in a less hydrophobic and more solvent-exposed H-site. Therefore, the catalytic efficiency of the enzyme with CDNB is diminished due to a lowered affinity for the electrophilic substrate and a lower stabilization of the transition state.

Characterization of bromosulphophthalein binding to human glutathione S-transferase A1-1: thermodynamics and inhibition kinetics

In addition to their catalytic functions, GSTs (glutathione S-transferases) bind a wide variety of structurally diverse non-substrate ligands. This ligandin function is known to result in the inhibition of catalytic function. The interaction between hGSTA1-1 (human class Alpha GST with two type 1 subunits) and a non-substrate anionic ligand, BSP (bromosulphophthalein), was studied by isothermal titration calorimetry and inhibition kinetics. The binding isotherm is biphasic, best described by a set of two independent sites: a high-affinity site and a low-affinity site(s). The binding stoichiometries for these sites are 1 and 3 molecules of BSP respectively. BSP binds to the high-affinity site 80 times more tightly (K(d)=0.12 microM) than it does to the low-affinity site(s) (K(d)=9.1 microM). Binding at these sites is enthalpically and entropically favourable, with no linkage to protonation events. Temperature- and salt-dependent studies indicate the significance of hydrophobic interactions in the binding of BSP, and that the low-affinity site(s) displays low specificity towards the anion. Binding of BSP results in the release of ordered water molecules at these hydrophobic sites, which more than offsets unfavourable entropic changes during binding. BSP inhibition studies show that the binding of BSP to its high-affinity site does not inhibit hGSTA1-1. This site, located near Trp-20, may be related to the buffer-binding site observed in GSTP1-1. The low-affinity-binding site(s) for BSP is most probably located at or near the active site of hGSTA1-1. Binding to this site(s) results in non-competitive inhibition with respect to CDNB (1-chloro-2,4-dinitrobenzene) (K(i)(BSP)=16.8+/-1.9 microM). Given the properties of the H site and the relatively small size of the electrophilic substrate CDNB, it is plausible that the active site of the enzyme can simultaneously accommodate both BSP and CDNB. This would explain the non-competitive behaviour of certain inhibitors that bind the active site (e.g. BSP).

Expression of the aflatoxin B1-8,9-epoxide-metabolizing murine glutathione S-transferase A3 subunit is regulated by the Nrf2 transcription factor through an antioxidant response element

High expression of the aflatoxin B1 (AFB1)-8,9-epoxide-conjugating glutathione S-transferase A3 (mGSTA3) subunit in mouse liver confers intrinsic resistance to AFB1 hepatocarcinogenesis. It is not known how the gene encoding this protein is regulated. The murine mGSTA3 gene has been identified using bioinformatics. It localizes to mouse chromosome 1 (A3-4), spans approximately 24.6 kilobases (kb) of DNA, and comprises seven exons. High levels of mGSTA3 mRNA are present in organs associated with detoxification. Expression of mGSTA3 in Hepa1c1c7 mouse hepatoma cells was found to be inducible by sulforaphane, an organic isothiocyanate that can transcriptionally activate genes through the antioxidant response element (ARE). Sulforaphane also induced transcription of a luciferase reporter containing a 1.5 kb fragment of the mGSTA3 5'-upstream region. A putative ARE, with sequence 5'-TGACATTGC-3', was identified within this fragment, approximately 150 base pairs upstream of exon 1. Mutation of this sequence abrogated both basal and sulforaphane-inducible reporter activity. Overexpression of the basic-region leucine zipper Nrf2 transcription factor augmented activity of the mGSTA3-luciferase reporter through this ARE. Electrophoretic mobility shift assays demonstrated that Nrf2 binds the mGSTA3 ARE. Measurement of mGSTA3 mRNA levels in tissues isolated from both wild-type and nrf2-null mice revealed that loss of the Nrf2 transcription factor is associated with a reduction in basal expression of mGSTA3. Collectively, these data demonstrate a role for Nrf2 and the ARE in regulating transcription of mGSTA3.

Effects of dietary butylated hydroxytoluene on aflatoxin B1-relevant metabolic enzymes in turkeys

We have shown previously that the extreme sensitivity of turkeys to aflatoxin B(1) (AFB(1)) is due to a combination of efficient AFB(1) activation by cytochrome P450s (CYPs) 1A and deficient detoxification by glutathione S-transferases (GSTs). Phenolic antioxidants such as butylated hydroxytoluene (BHT) have been shown to be chemoprotective in some animal models due, in part, to modulation of AFB(1)-relevant phase I and/or phase II activities, and we wished to determine whether BHT has a similar effect in turkeys. Ten-day-old male turkeys were maintained on diets amended with 1000 or 4000 ppm of BHT for 10 days, then sampled. Hepatic microsomal CYP 1A activity as well as conversion of AFB(1) to the putative toxic metabolite, the exo-AFB(1)-8,9-epoxide (AFBO), were significantly lower compared with control. Conversely, dietary BHT significantly increased activities of several isoforms of hepatic cytosolic GST, as well quinone oxidoreductase (QOR). Western immunoblotting confirmed that dietary BHT increased expression of homologues to rodent GST isoforms Yc1, Yc2 and Ya. There was, however, no observable BHT-related increase in GST-mediated specific conjugation with microsomally-generated AFBO. In total, our data indicates that dietary BHT modulates a variety of AFB(1)-relevant phase I and phase II enzymes, while having no measurable effect towards specific AFB(1) detoxification by GST.

Mechanisms of protection against aflatoxin B(1) genotoxicity in rats treated by organosulfur compounds from garlic

Diallyl sulfide (DAS) and diallyl disulfide (DADS), two garlic constituents, were found previously to inhibit aflatoxin B(1) (AFB(1))-initiated carcinogenesis in rat liver, DADS being the most effective. In order to study the mechanisms involved in this protection, we have examined the ability of liver microsomes and cytosols from DAS- and DADS-treated rats to modulate the mutagenicity and the metabolism of AFB(1). We also examined the effects of these compounds on the expression of cytochromes P450 (CYP) and phase II enzymes known to be involved in AFB(1) metabolism. Administration of DAS (1 mmol/kg for 4 days) to rats resulted in significant inhibition of microsome-mediated mutagenicity of AFB(1), whereas DADS treatment did not alter AFB(1) mutagenicity. DAS treatment increased the metabolism of AFB(1) mainly towards the formation of AFQ(1) and AFM(1), which might account for the reduction of AFB(1) microsomal-mediated mutagenicity. DADS treatment slightly affected the oxidative metabolism of AFB(1). DAS and DADS induced CYP3A2, CYP2B1 and CYP2B2, DAS being more potent. Cytosols from DAS- and DADS-treated rats produced a significant inhibition of AFB(1)-8,9-epoxide (AFBO)-induced mutagenicity and significantly increased the cytosolic formation of AFB(1)-glutathione conjugates, DADS treatment being more effective. Western blot analysis showed that DADS is a potent inducer of glutathione S-transferase A5 (rGSTA5) and AFB(1) aldehyde reductase 1 (rAFAR1), while DAS is a weak inducer of these enzymes. Finally, we demonstrated that antibodies raised against rGSTA5 strongly reduced the antimutagenic activity of cytosols from DAS- and DADS-treated rats against AFBO. All together, these results demonstrate that DAS prevents AFB(1) mutagenicity through a dual mechanism, i.e. by modulating both the phase I and II metabolism of AFB(1), whereas DADS acts mainly by increasing the phase II metabolism of AFB(1). The induction of rGSTA5 and rAFAR1 is probably the main mechanism by which allyl sulfides give protection against AFB(1)-induced carcinogenesis.

Interindividual differences in response to chemoprotection against aflatoxin-induced hepatocarcinogenesis: implications for human biotransformation enzyme polymorphisms

It is now evident that most, if not all, of the remarkable species differences in susceptibility to AFB hepatocarcinogenesis is due in large part, if not exclusively, to differences in biotransformation. Certainly the relative rate of oxidative formation of the proximate carcinogen, AFB-8,9-exo-epoxide, is an important determinant of species and interindividual differences in susceptibility to AFB. However, mice produce relatively large amounts of exo-AFBO, yet are highly resistant to AFB-hepatocarcinogenesis because they express a particular form of GST with remarkably high catalytic activity toward the exo-epoxide of AFB. Rats, which are highly susceptible to AFB hepatocarcinogenesis,can be made resistant through dietary induction of an orthologous form of GST that is normally expressed in only very small amounts. Based on these findings in laboratory animal models, there is great interest in identifying chemicals and/or specific dietary constituents that could offer protection against AFB-hepatocarcinogenesis to humans. Current experimental strategies have focused on the antiparasitic drug, oltipraz, which induces protection in rats and has also shown some promise in humans. The mechanism of protection in rats appears to be via induction of an alpha class GST with high catalytic activity toward AFBO (rGSTA5-5). vet human alpha class GST proteins that are constitutively expressed in the liver (hGSTA1 and hGSTA2) have little, if any activity toward AFBO. Rather, it appears that mu class GSTs may be responsible for the very low, but potentially significant, detoxification activity toward AFBO. Oltipraz and certain dietary constituents may induce mu class GSTs in human liver, and this could afford some protection against the genotoxic effects of AFBO. However, it also appears that oltipraz, and perhaps certain dietary constituents, act as competitive inhibitors of human CYP1A2. As CYP1A2 appears to mediate most of the activation of AFB to exo-AFBO in human liver at low dietary concentrations of AFB encountered in the human diet, much of the putative protective effects of oltipraz could be mediated via inhibition of CYP1A2 rather than induction of GSTs. There is now evidence that human microsomal epoxide hydrolase (mEH) could play a role in protecting human DNA from the genotoxic effects of AFB, although the importance of this detoxification pathway, relative to mu class GSTs, remains to be elucidated. Oltipraz is an effective inducer of mEH in rats (Lamb Franklin, 2000), and thus induction of this pathway in humans could also potentially contribute to the protective effects of this drug toward AFB genotoxicity. Because the dihydrodiol of AFB may contribute indirectly to the carcinogenic effects of AFB via protein adduction and subsequent hepatotoxicity, the recently characterized human aflatoxin aldehyde reductase (AFAR) may also offer some protection against AFB-induced carcinogenicity in humans. Current and future dietary and/or chemointervention strategies aimed at reducing the carcinogenic effects of AFB in humans should consider all of the possible mechanistic approaches for modifying AFB-induced genotoxicity.

Characterisation of a novel mouse liver aldo-keto reductase AKR7A5

We have characterised a novel aldo-keto reductase (AKR7A5) from mouse liver that is 78% identical to rat aflatoxin dialdehyde reductase AKR7A1 and 89% identical to human succinic semialdehyde (SSA) reductase AKR7A2. AKR7A5 can reduce 2-carboxybenzaldehyde (2-CBA) and SSA as well as a range of aldehyde and diketone substrates. Western blots show that it is expressed in liver, kidney, testis and brain, and at lower levels in skeletal muscle, spleen heart and lung. The protein is not inducible in the liver by dietary ethoxyquin. Immunodepletion of AKR7A5 from liver extracts shows that it is one of the major liver 2-CBA reductases but that it is not the main SSA reductase in this tissue.

Loss of the Nrf2 transcription factor causes a marked reduction in constitutive and inducible expression of the glutathione S-transferase Gsta1, Gsta2, Gstm1, Gstm2, Gstm3 and Gstm4 genes in the livers of male and female mice

Mice that lack the Nrf2 basic-region leucine-zipper transcription factor are more sensitive than wild-type (WT) animals to the cytotoxic and genotoxic effects of foreign chemicals and oxidants. To determine the basis for the decrease in tolerance of the Nrf2 homozygous null mice to xenobiotics, enzyme assay, Western blotting and gene-specific real-time PCR (TaqMan) have been used to examine the extent to which hepatic expression of GSH-dependent enzymes is influenced by the transcription factor. The amounts of protein and mRNA for class Alpha, Mu and Pi glutathione S-transferases were compared between WT and Nrf2 knockout (KO) mice of both sexes under both constitutive and inducible conditions. Among the class Alpha and class Mu transferases, constitutive expression of Gsta1, Gsta2, Gstm1, Gstm2, Gstm3, Gstm4 and Gstm6 subunits was reduced in the livers of Nrf2 mutant mice to between 3% and 60% of that observed in WT mice. Induction of these subunits by butylated hydroxyanisole (BHA) was more marked in WT female mice than in WT male mice. TaqMan analyses showed the increase in transferase mRNA caused by BHA was attenuated in Nrf2(-/-) mice, with the effect being most apparent in the case of Gsta1, Gstm1 and Gstm3. Amongst class Pi transferase subunits, the constitutive hepatic level of mRNA for Gstp1 and Gstp2 was not substantially affected in the KO mice, but their induction by BHA was dependent on Nrf2; this was more obvious in female mutant mice than in male mice. Nrf2 KO mice exhibited reduced constitutive expression of the glutamate cysteine ligase catalytic subunit, and, to a lesser extent, the expression of glutamate cysteine ligase modifier subunit. Little variation was observed in the levels of glutathione synthase in the different mouse lines. Thus the increased sensitivity of Nrf2(-/-) mice to xenobiotics can be partly attributed to a loss in constitutive expression of multiple GSH-dependent enzymes, which causes a reduction in intrinsic detoxification capacity in the KO animal. These data also indicate that attenuated induction of GSH-dependent enzymes in Nrf2(-/-) mice probably accounts for their failure to adapt to chronic exposure to chemical and oxidative stress.

Dietary butylated hydroxytoluene protects against aflatoxicosis in Turkeys

Turkeys are among the most sensitive species to the toxic effects of the mycotoxin aflatoxin B(1) (AFB(1)). In mammals, dietary antioxidants, such as butylated hydroxytoluene (BHT), have been shown to lessen the toxic effects of AFB(1) by various mechanisms. To test whether BHT protects against aflatoxicosis in turkeys, we supplemented the feed of 10-day-old male white turkeys with low (1000 ppm) and high (4000 ppm) BHT for 20 days. AFB(1) (1 ppm) was then added to the diets and continued for another 10 days. Birds in the AFB(1)-only group had a lower weight gain, a condition that had returned to near control in groups fed diets containing AFB(1) + BHT. Significant elevations in serum aspartate transaminase, alanine aminotransferase, and lactate dehydrogenase, which were evident in the AFB(1) group, were reversed in the AFB(1) + BHT groups. Histopathology revealed hepatic submassive necrotic lesions and biliary hyperplasia, the severity of which was lessened in the AFB(1) + BHT-treated birds. Hepatocellular hydropic degeneration was observed in the BHT-only group, but not in the AFB(1) + BHT groups. This condition associated with BHT treatment was found in a separate study to be reversible and without any long-term adverse effects. These results indicate that BHT counteracts many of the deleterious effects caused by AFB(1) and that this antioxidant may prove to be a viable feed additive for the reduction of aflatoxicosis in turkeys.

Biochemical factors underlying the age-related sensitivity of turkeys to aflatoxin B(1)

Poultry are some of the most sensitive species to the toxic effects of aflatoxin B(1) (AFB(1)), and younger poultry are more sensitive to this mycotoxin. To elucidate the mechanisms for this age-related susceptibility, various enzyme activities relevant to AFB(1) were measured in liver microsomes prepared from male turkeys 9, 41 and 65 days of age. Hepatic microsomal o-dealkylation of methoxy- and pentoxyresorufin significantly increased, while that of ethoxyresorufin decreased with age. Microsomal AFB(1) activation to the reactive AFB(1)-8,9-epoxide (AFBO) was most efficient in the youngest birds, with apparent K(m) and V(max) values of 168 and 19, 110 and 6, and 116 microM and 10 nmol/mg/min for 9, 41 and 65-day-old birds, respectively. The activity of hepatic cytosolic glutathione S-transferases (GSTs) was deficient in the youngest age group, but were higher in the older groups. There was also an age-related increase in the expression of GST isoforms Yc, Yc(2), as well as AFB(1)-aldehyde reductase (AFAR). However, livers from all ages lacked specific GST-mediated conjugation of AFBO, indicating that turkeys are deficient in this key AFB(1)-detoxification pathway. Our data indicate that efficient activation may underlie the extreme sensitivity of young turkeys to the toxic effects of AFB(1).

Two dimensional non equilibrium pH gel electrophoresis mapping of cytosolic protein changes caused by dietary protein depletion in mouse liver

Two-dimensional non-equilibrium pH gel electrophoresis (2D-NEPHGE) analysis was used to evaluate the effects of dietary protein depletion on the protein composition of mouse liver cytosol. Analysing the cytosol from both normal and protein depleted liver, the position in gels of more than three hundred protein spots was determined. After 5 days of protein depletion, about 20% of the spots either increased or decreased more than 2 fold. Five spots of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) were recognised by specific antibodies. The glutathione S-transferase (GSTs) subunits Ybl, Yc and Yf were identified by the simultaneous analysis of both glutathione-binding cytosolic proteins and the corresponding standards. As estimated by internal optical density (IOD) of spots, the changes caused by protein depletion in GAPDH and GST subunit contents were similar to those obtained by other methods. By means of mass spectrometric analysis of tryptic peptides generated from spots and/or comparison of two-dimensional gel electrophoretic patterns, carbonic anhydrase III (CAIII), Cu, Zn superoxide dismutase (CuZnSOD) and a cytochrome P450 cytosolic protein (cyt P450) were identified. These three proteins, as well as GSTs, are related with intracellular detoxification and free radical scavenging systems. Their contents were regulated by dietary protein restriction in a manner indicative of diminished liver defence against oxidising agents.

Biochemical basis for the extreme sensitivity of turkeys to aflatoxin B(1)

Poultry are the most susceptible food animal species to the toxic effects of the mycotoxin aflatoxin B(1) (AFB(1)). Feed contaminated with even small amounts of AFB(1) results in significant adverse health effects in poultry. The purpose of this study was to explain the biochemical mechanism(s) for this extreme sensitivity. We measured microsomal activation of AFB(1) to the AFB(1)-8,9-epoxide (AFBO), the putative toxic intermediate, as well as cytosolic glutathione S-transferase (GST)-mediated detoxification of AFBO, in addition to other hepatic phase I and phase II enzyme activities, in 3-week-old male Oorlop strain turkeys. Liver microsomes prepared from these turkeys activated AFB(1) in vitro with an apparent K(m) of 109 microM and a V(max) of 1.25 nmol/mg/min. Preliminary evidence for the involvement of cytochromes P450 (CYP) 1A2 and, to a lesser extent, 3A4 for AFB(1) activation was assessed by the use of specific mammalian CYP inhibitors. The possible presence of avian orthologues of these CYPs was supported by activity toward ethoxyresorufin and nifedipine, as well as by Western immunoblotting using antibodies to human CYPs. Cytosol prepared from turkey livers exhibited GST-mediated conjugation of 1-chloro-2,4-dinitrobenzene (CDNB) and 3,4-dichloronitrobenzene (DCNB), but at a much lower rate than that observed in other species. Western immunoblotting indicated the presence of alpha and sigma class GSTs and another AFB(1)-detoxifying enzyme, AFB(1)-aldehyde reductase (AFAR). Turkey liver cytosol also had quinone oxidoreductase (QOR) activity. Importantly, cytosol exhibited no measurable GST-mediated detoxification of microsomally activated AFB(1), indicating that turkeys are deficient in the most crucial AFB(1)-detoxification pathway. In total, our data indicate that the extreme sensitivity of turkeys to AFB(1) may be attributed to a combination of efficient AFB(1) activation and deficient detoxification by phase II enzymes, such as GSTs.

Specificity of murine glutathione S-transferase isozymes in the glutathione conjugation of (-)-anti- and (+)-syn-stereoisomers of benzo[g]chrysene 11,12-diol 13,14-epoxide

Specificities of murine glutathione (GSH) S-transferase (GST) isozymes mGSTA1-1, mGSTA2-2, mGSTA3-3 and mGSTA4-4 (alpha class), mGSTP1-1 (pi class) and mGSTM1-1 (mu class) for GSH conjugation of (-)-anti- and (+)-syn-stereoisomers of benzo[g]chrysene 11, 12-diol 13,14-epoxide (B[g]CDE), the activated metabolites of the environmental pollutant benzo[g]chrysene (B[g]C), have been determined. When GST activity was determined as a function of varying (-)-anti- or (+)-syn-B[g]CDE concentration (10-320 microM) at a fixed saturating concentration of GSH (2 mM), each isozyme obeyed Michaelis-Menten kinetics. mGSTA1-1 was significantly more efficient than other murine GSTs in the GSH conjugation of not only (-)-anti-stereoisomer but also (+)-syn-B[g]CDE. For example, the catalytic efficiency (k(cat)/K(m)) of mGSTA1-1 towards (-)-anti-B[g]CDE was approximately 2.3- to 16.6-fold higher compared with other murine GSTs. Likewise, mGSTA1-1 was approximately 2.7-, 6.7-, 4.4- and 12.4-fold more efficient than mGSTA2-2, mGSTA3-3, mGSTP1-1 and mGSTM1-1, respectively, in catalyzing the GSH conjugation of (+)-syn-B[g]CDE. Interestingly, mGSTA4-4, which also belongs to class alpha, was virtually inactive towards both stereoisomers of B[g]CDE. The results of the present study indicate that murine GSTs, especially alpha class isozymes, significantly differ in their ability to detoxify B[g]CDE stereoisomers and that mGSTA1-1 plays a major role in the detoxification of both (-)-anti- and (+)-syn-B[g]CDE, which among four B[g]CDE stereoisomers are formed from the carcinogen B[g]C as major DNA binding metabolites.

Determinants of specificity for aflatoxin B1-8,9-epoxide in alpha-class glutathione S-transferases

We have used homology modelling, based on the crystal structure of the human glutathione S-transferase (GST) A1-1, to obtain the three-dimensional structures of rat GSTA3 and rat GSTA5 subunits bound to S-aflatoxinyl-glutathione. The resulting models highlight two residues, at positions 208 and 108, that could be important for determining, either directly or indirectly, substrate specificity for aflatoxin-exo-8,9-epoxide among the Alpha-class GSTs. Residues at these positions were mutated in human GSTA1-1 (Met-208, Leu-108), rat GSTA3-3 (Glu-208, His-108) and rat GSTA5-5 (Asp-208, Tyr-108): in the active rat GSTA5-5 to those in the inactive GSTA1-1; and in the inactive human GSTA1-1 and rat GSTA3-3 to those in the active rat GSTA5-5. These studies show clearly that, in all three GSTs, an aspartate residue at position 208 is a prerequisite for high activity in aflatoxin-exo-8,9-epoxide conjugation, although this alone is not sufficient; other residues in the vicinity, particularly residues 103-112, are important, perhaps for the optimal orientation of the aflatoxin-exo-8,9-epoxide in the active site for catalysis to occur.

Contribution of breakdown in the regulation of mouse liver glutathione S-transferase subunits during protein depletion and re-feeding

The in vivo radioactivity decay of glutathione S-transferase (GST) was observed in livers of normal-fed (N), protein-depleted (D), and re-fed mice (R), labelled with [35S]methionine. Half-lives in days at N, D and R, respectively, were: total GST, 1.7, 1.4, 4.3; Yb1-subunit, 2.0, 1.8, 3.3; Yc-subunit, 2.4, 1.0, 4.2; Yf-subunit, 1. 1, 2.1, 5.0. These findings, together with the fact that the proportions of GST-subunits depend on dietary protein, show that breakdown contributes differentially to control the content of each GST subunit. Data also indicate that GSTs are long-life proteins.