Activation of dihaloalkanes by thiol-dependent mechanisms

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.

Spontaneous Production of Glutathione-Conjugated Forms of 1,2-Dichloropropane: Comparative Study on Metabolic Activation Processes of Dihaloalkanes Associated with Occupational Cholangiocarcinoma

Recently, epidemiological studies revealed a positive relationship between an outbreak of occupational cholangiocarcinoma and exposure to organic solvents containing 1,2-dichloropropane (1,2-DCP). In 1,2-DCP-administered animal models, we previously found biliary excretion of potentially oncogenic metabolites consisting of glutathione- (GSH-) conjugated forms of 1,2-DCP (GS-DCPs); however, the GS-DCP production pathway remains unknown. To enhance the understanding of 1,2-DCP-related risks to human health, we examined the reactivity of GSH with 1,2-DCP in vitro and compared it to that with dichloromethane (DCM), the other putative substance responsible for occupational cholangiocarcinoma. Our results showed that 1,2-DCP was spontaneously conjugated with GSH, whereas this spontaneous reaction was hardly detected between DCM and GSH. Further analysis revealed that glutathione S-transferase theta 1 (GSTT1) exhibited less effect on the 1,2-DCP reaction as compared with that observed for DCM. Although GSTT1-mediated bioactivation of dihaloalkanes could be a plausible explanation for the production of reactive metabolites related to carcinogenesis based on previous studies, this catalytic pathway might not mainly contribute to 1,2-DCP-related occupational cholangiocarcinoma. Considering the higher catalytic activity of GSTT1 on DCM as compared with that on 1,2-DCP, our findings suggested differences in the activation processes associated with 1,2-DCP and DCM metabolism.

Modifying effects of 1,2-dichloropropane on N-nitrosobis(2-oxopropyl)amine-induced cholangiocarcinogenesis in male Syrian hamsters

Based on the findings of epidemiological studies in Japan that occupational exposure to 1,2-dichloropropane (1,2-DCP) was associated with increased cholangiocarcinomas, 1,2-DCP has recently been classified as being carcinogenic to humans (Group 1). However, the cholangiocarcinogenicity of 1,2-DCP has not been demonstrated experimentally, and it was negative for cholangiocarcinogenicity in rats and mice. The present study determined the effects of 1,2-DCP on N-nitrosobis(2-oxopropyl)amine (BOP)-induced cholangiocarcinogenesis in male hamsters. We found that 1,2-DCP did not enhance the development of BOP-induced atypical biliary hyperplasia and did not induce any lesions in liver bile duct when administered alone. Notably, 1,2-DCP had no effect on the proliferative activity of bile duct epithelial cells regardless of BOP-initiation. These results demonstrate that 1,2-DCP lacks promoting effects on BOP-induced cholangiocarcinogenesis and suggest the possibility that 1,2-DCP is not cholangiocarcinogenic to the hamster in the present model. In addition, 1,2-DCP also lacks promoting effects on pancreatic, lung, and renal carcinogenesis. As the occurrence of occupational cholangiocarcinomas in Japan might be attributed to exposure to multiple chemicals, the results of the present study indicate that it will be necessary to determine the cholangiocarcinogenic effects of concurrent exposure of 1,2-DCP and the other halogen solvents to which workers with cholangiocarcinomas were exposed.

Halogenated hydrocarbon solvent-related cholangiocarcinoma risk: biliary excretion of glutathione conjugates of 1,2-dichloropropane evidenced by untargeted metabolomics analysis

Recently, the International Agency for Research on Cancer issued a warning about the carcinogenicity of 1,2-dichloropropane (1,2-DCP) to humans based on an epidemiological study suggesting a relationship between the incidence of cholangiocarcinoma and occupational exposure to halogenated hydrocarbon solvent comprised mostly of 1,2-DCP. Although this dihaloalkane has been used in various industrial fields, there has been no biological evidence explaining the cholangiocarcinoma latency, as well as little understanding of general cholangiocarcinoma risk. In the present study, we explored the biliary excretion of 1,2-DCP metabolites by an untargeted metabolomics approach and the related molecular mechanism with in vitro and in vivo experiments. We hypothesized that the biliary excretion of carcinogens derived from 1,2-DCP contribute to the increased cholangiocarcinoma risk. We found that 1,2-DCP was conjugated with glutathione in the liver, and that the glutathione-conjugated forms of 1,2-DCP, including a potential carcinogen that contains a chloride atom, were excreted into bile by the bile canalicular membrane transporter, ABCC2. These results may reflect a risk in the backfiring of biliary excretion as a connatural detoxification systems for xenobiotics. Our findings would contribute to uncover the latent mechanism by which the chronic exposure to 1,2-DCP increases cholangiocarcinoma risk and future understanding of cholangiocarcinoma biology.

Determination of Hepatotoxicity and Its Underlying Metabolic Basis of 1,2-Dichloropropane in Male Syrian Hamsters and B6C3F1 Mice

1,2-Dichloropropane (1,2-DCP) has recently been reclassified from not classifiable as to its carcinogenicity to humans (Group 3) to carcinogenic to humans (Group 1) by the International Agency for Research on Cancer. This was based on the findings of epidemiological studies in Japan that occupational exposure to paint stripers containing 1,2-DCP was associated with increased cholangiocarcinomas. It is known that 1,2-DCP is negative for cholangiocarcinogenicity in rats and mice. However, its toxicity and carcinogenicity has not been examined in hamsters and little is known about the regulation of its metabolism in hamsters. The purpose of this study was to determine the hepatobiliary toxicity of 1,2-DCP in hamsters and to characterize and compare the altered patterns of hepatic xenometabolic enzymes in hamsters and mice. Male Syrian hamsters and male B6C3F1 mice were treated with various doses of 1,2-DCP for 4 h or 3 days or 4 weeks. These experiments demonstrated that a high dose of 1,2-DCP induced centrilobular hepatocellular necrosis in hamsters. CYP2E1 is possibly the key enzyme responsible for bioactivation and the consequent hepatocytotoxicity of 1,2-DCP, and GSH conjugation catalyzed by GST-T1 may exert a cytoprotective role in hamsters and mice. Notably, the expression pattern of GST-T1 in bile duct epithelial cells differed between hamsters and mice: GST-T1 was expressed in bile duct epithelial cells of mice but not hamsters. This indicates that responses to 1,2-DCP in the bile duct of hamsters might differ from that of mice, and suggests that long-term studies are necessary to clarify the chalangiocarcinogenicity of 1,2-DCP in hamsters, though no biliary toxicity was observed in the present short-term experiments.

Copy number variation of GSTT1 and GSTM1 and the risk of prostate cancer in a Caribbean population of African descent

Background

Deletions of the glutathione S-transferase genes M1 and T1 (GSTM1 and GSTT1) have been studied as potential risk factors for prostate cancer. Conflicting results have been obtained. Moreover, most such studies could not discriminate heterozygous from homozygous carriers of the non-deleted alleles.

Objective

We investigated whether copy number variation (CNV) of the GSTM1 and/or GSTT1 genes contribute to the risk of prostate cancer in the Caribbean population of African descent of Guadeloupe.

Methods

In a population-based case-control study, we compared 629 prostate cancer patients and 622 control subjects. Logistic regression was used to estimate adjusted odds ratios (OR) and 95% confidence intervals (CI). Exact copy numbers of GSTM1 and GSTT1 were determined by real-time PCR.

Results

A higher copy number of GSTM1 was marginally associated with prostate cancer risk. Men with 2 and 3 or more GSTT1 genes were at higher risk of prostate cancer (OR: 1.55, 95% CI: 1.11–2.16 and OR: 4.89, 95% CI: 1.71–13.99, respectively; P_trend<0.001). Men with 3, 4 and 5 or more copies of both GSTM1 and GSTT1 genes were at higher risk of prostate cancer (OR: 2.18, 95% CI: 1.21–3.91, OR: 3.24, 95% CI: 1.63–6.46, and OR: 5.77, 95% CI: 1.40–23.84, respectively; P_trend<0.001).

Conclusions

Copy number of GSTT1 and combined GSTM1/GSTT1 appear to be associated with prostate cancer risk in our population study with gene dose relationship. Our results support the hypothesis that variations in copy number of GSTT1 modulate the risk of prostate cancer.

Assessment of the genotoxicity of 1,2-dichloropropane and dichloromethane after individual and co-exposure by inhalation in mice

OBJECTIVE

Occurrence of cholangiocarcinoma was recently reported at a high incidence rate among the employees working for an offset printing company in Osaka, Japan. 1,2-Dichloropropane (1,2-DCP) and dichloromethane (DCM) are suspected to be the causes of the cancer, as they had been used as ink cleaners in large amounts. However, it is not clear whether these chlorinated organic solvents played a role in the occurrence of cholangiocarcinoma or why the incidence rate is so high among the workers in this industry. To provide possible evidence for this severe occupational problem, we investigated the genotoxic effects of 1,2-DCP and DCM.

METHODS

Male B6C3F1 and gpt Delta C57BL/6J mice were exposed by inhalation to the individual solvents or both solvents at multiple concentrations including the levels that were possibly present in the workplaces. The genotoxicity was analyzed by Pig-a gene mutation and micronuclei assays in peripheral blood and gpt mutation and comet assays in the livers of mice after repeated inhalation of 1,2-DCP or/and DCM.

RESULTS

The Pig-a mutant frequencies and micronuclei incidences were not significantly increased by exposure of either 1,2-DCP or/and DCM at any concentration, suggesting there was no genotoxic potential in bone marrow for both solvents. In the liver, DNA damage, as measured by the comet assay, was dose dependently increased by 1,2-DCP but not by DCM. The gpt mutant frequency was 2.6-fold that of the controls in the co-exposure group.

CONCLUSIONS

These results indicate that 1,2-DCP showed stronger genotoxicity in the liver and that the genotoxic effects were greatly enhanced by simultaneous exposure to DCM.

High-throughput metabolic toxicity screening using magnetic biocolloid reactors and LC-MS/MS

An inexpensive, high-throughput genotoxicity screening method was developed by using magnetic particles coated with cytosol/microsome/DNA films as biocolloid reactors in a 96-well plate format coupled with liquid chromatography-mass spectrometry. Incorporation of both microsomal and cytosolic enzymes in the films provides a broad spectrum of metabolic enzymes representing a range of metabolic pathways for bioactivation of chemicals. Reactive metabolites generated via this process are trapped by covalently binding to DNA in the film. The DNA is then hydrolyzed and nucleobase adducts are collected using filters in the bottom for the 96-well plate of analysis by capillary liquid chromatography-tandem mass spectrometry (LC-MS/MS). The magnetic particles facilitate simple and rapid sample preparation and workup. Major DNA adducts from ethylene dibromide, N-acetyl-2-aminofluorene and styrene were identified in proof-of-concept studies. Relative formation rates of DNA adducts correlated well with rodent genotoxicity metric TD(50) for the three compounds. This method has the potential for high-throughput genotoxicity screening, providing chemical structure information that is complementary to toxicity bioassays.

Oxidative stress after sulfur mustard intoxication and its reduction by melatonin: efficacy of antioxidant therapy during serious intoxication

Sulfur mustard (SM) is an important chemical warfare agent. The mechanism of SM toxicity still has not been fully recognized. However, oxidative stress and following the damaging of macromolecules in the human body is considered one of the crucial steps in SM toxicity. Rats intoxicated with pure (i.e., distilled) SM were used as a model organism. The doses, 0 (control), 5, 20, and 80 mg/kg of body weight, were applied intradermally. A hormone with strong antioxidant potency, melatonin, was applied (25 and 50 mg/kg, subcutaneously) into the other group of rats exposed with the same doses of SM. Total plasma protein, ferric-reducing antioxidant power (FRAP), thiobarbituric-acid-reactive substances (TBARS), and plasma protein carbonyls were assayed in blood plasma. A significant decrease of total plasma proteins was found for control, and the lowest dose of SM was treated with melatonin. Melatonin was also able to enhance the production of low-molecular-weight antioxidants, as the SM-intoxicated rats had significantly (P ≤ 0.01) increasing FRAP levels after intoxication with SM in doses of 20 and 80 mg/kg, when compared to the control treated with melatonin. Melatonin also decreased TBARS level, representing reduced lipid peroxidation (LPO). However, LPO seems to be of less importance for SM toxic impact. The more reliable parameter was the level of total plasma protein carbonyls. The carbonyl levels were significantly increased due to SM, and the carbonylation was slowed due to melatonin intake. In conclusion, melatonin seems to be a prospective compound in reducing SM toxicity impact in the rat.

Dose-response relationship, kinetics of formation, and persistence of S-[2-(N7-guanyl)-ethyl]glutathione-DNA adduct in livers of channel catfish (Ictalurus punctatus) exposed in vivo to ethylene dichloride

Formation of DNA adducts by reactive chemicals or their metabolites are often a precursor of mutagenesis and other adverse effects. Studies in juvenile channel catfish (Ictalurus punctatus) were conducted to determine the dose-response, kinetics of formation, and persistence of S-[2-(N7-guanyl)ethyl]glutathione hepatic-DNA adducts following a 4-h in vivo aqueous exposure to ethylene dichloride (EDC) at several dose levels. S-[2-(N7-guanyl)ethyl] glutathione adducts were detectable in liver tissue after 2 h of exposure and were still detectable three weeks after a single pulse exposure (detection limit=approximately 10 fmol, approximately 1 DNA adduct in 10(7) bases). Pretreatment of catfish with the glutathione-depleting agent diethylmaleate significantly reduced the level of tissue glutathione levels and, as a result, DNA adducts were not detected in pretreated fish. Catfish may serve as a useful sentinel species for detecting DNA-reactive chemicals in aquatic systems.

Genetic variations in human glutathione transferase enzymes: significance for pharmacology and toxicology

Glutathione transferase enzymes (GSTs) catalyze reactions in which electrophiles are conjugated to the tripeptide thiol glutathione. While many GST-catalyzed transformations result in the detoxication of xenobiotics, a few substrates, such as dihaloalkanes, undergo bioactivation to reactive intermediates. Many molecular epidemiological studies have tested associations between polymorphisms (especially, deletions) of human GST genes and disease susceptibility or response to therapy. This review presents a discussion of the biochemistry of GSTs, the sources-both genetic and environmental-of interindividual variation in GST activities, and their implications for pharmaco- and toxicogenetics; particular attention is paid to the Theta class GSTs.

Screening for reactive metabolites using electro-optical arrays featuring human liver cytosol and microsomal enzyme sources and DNA

We demonstrate for the first time the combination of human liver cytosol and microsomal enzyme sources into an electro-optical array to screen for reactive metabolites produced in multi-enzyme metabolic processes.

Reactions of glyceraldehyde 3-phosphate dehydrogenase sulfhydryl groups with bis-electrophiles produce DNA-protein cross-links but not mutations

The environmental contaminant 1,2-dibromoethane and diepoxybutane, an oxidation product of the important industrial chemical butadiene, are bis-functional electrophiles and are known to be mutagenic and carcinogenic. One mechanism by which bis-electrophiles can exert their toxic effects is through the induction of genotoxic and mutagenic DNA-peptide cross-links. This mechanism has been shown in systems overexpressing the DNA repair protein O6 -alkylguanine DNA-alkyltransferase (AGT) or glutathione S-transferase and involves reactions with nucleophilic cysteine residues. The hypothesis that DNA-protein cross-link formation is a more general mechanism for genotoxicity by bis-electrophiles was investigated by screening nuclear proteins for reactivity with model monofunctional electrophiles. Glyceraldehyde 3-phosphate dehydrogenase (GAPDH) was identified as a candidate because of the nucleophilicity of two cysteine residues (Cys152 and Cys246) in reaction screens with model electrophiles (Dennehy, M. K. et al. (2006) Chem. Res. Toxicol. 19, 20-29). Incubation of GAPDH with bis-electrophiles resulted in inhibition of its catalytic activity, but only at high concentrations of diepoxybutane. In vitro assays indicated DNA-GAPDH cross-link formation in the presence of diepoxybutane, and bis-electrophile reactivity at Cys246 was confirmed using mass spectral analysis. In contrast to AGT, overexpression of human GAPDH in Escherichia coli did not enhance mutagenesis by diepoxybutane. We propose that the lack of mutational enhancement is in part due to the inherently lower reactivity of GAPDH toward bis-electrophiles as well as the reduced DNA binding ability relative to AGT, preventing the in vivo formation of DNA-protein cross-links and enhanced mutagenesis.

Thiols and the chemoprevention of cancer

Thiols such as glutathione interfere with the complex carcinogenic process. Under stress conditions, they scavenge harmful molecules: Glutathione conjugation of electrophilic carcinogens may prevent tumor initiation, and reduced thiols may defend against oxidative stress. Thus, associated chemopreventive strategies involve enhancement of antioxidant or conjugating capacity by increasing the levels of, particularly, glutathione through precursor application or synthesis stimulation and by inducing the corresponding enzymes. The antioxidant potential of thiols is, however, a part of a more general capacity to regulate redox status even in the absence of unequivocal stress conditions. Redox status controls the activities of various cellular signalling proteins through oxidation or reduction of particular sensor structures that are also mostly thiols. The development of feasible chemotherapeutic strategies on the basis of this complex system of redox-sensitive messenger proteins is a goal in ongoing and future research.

Screening and characterization of variant Theta-class glutathione transferases catalyzing the activation of ethylene dibromide to a mutagen

Ethylene dibromide (EDB) is a widespread environmental pollutant and mutagen/carcinogen. Certain Theta-class glutathione transferases (GSTs), enzymes that catalyze the reaction of reduced glutathione (GSH) with electrophiles, activate EDB to a mutagen. Previous studies have shown that human GST T1-1, but not rat GST T2-2, activates EDB. We have constructed an E. coli lacZ reversion mutagenicity assay system in which expression of recombinant GST supports activation of EDB to a mutagen. Hexa-histidine N-terminal tagging of GST T1-1 results in greatly enhanced expression of the recombinant enzyme and gives a lacZ strain that shows a mutagenic response to EDB at extremely low levels (approximately 1 ng EDB per plate). The hexa-histidine-tagged enzyme was purified in one step by Ni(2+)-affinity chromatography. We applied the lacZ mutagenicity assay to the rapid screening of a library of variant GST Theta enzymes. Sequence variants with altered catalytic activities were identified, purified, and characterized.

Cytochrome P450s and other enzymes in drug metabolism and toxicity

The cytochrome P450 (P450) enzymes are the major catalysts involved in the metabolism of drugs. Bioavailability and toxicity are 2 of the most common barriers in drug development today, and P450 and the conjugation enzymes can influence these effects. The toxicity of drugs can be considered in 5 contexts: on-target toxicity, hypersensitivity and immunological reactions, off-target pharmacology, bioactivation to reactive intermediates, and idiosyncratic drug reactions. The chemistry of bioactivation is reasonably well understood, but the mechanisms underlying biological responses are not. In the article we consider what fraction of drug toxicity actually involves metabolism, and we examine how species and human interindividual variations affect pharmacokinetics and toxicity.

Activation of alkyl halides by glutathione transferases

Glutathione (GSH) transferases catalyze the conjugation of the tripeptide GSH with alkyl halides and related compounds. If a second leaving group is present, the substrate is at least a potential bis-electrophile and the initial conjugate may be susceptible to further attack by the sulfur atom. This process can yield potent electrophiles that modify DNA and are genotoxic. Much of the chemistry is understood in the context of the halide order and size of rings generated in reactive sulfonium ions. Similar chemistry has been demonstrated with the active site cysteine residue in the DNA repair protein O(6)-alkylguanine DNA-alkyltransferase.

Structural basis of the suppressed catalytic activity of wild-type human glutathione transferase T1-1 compared to its W234R mutant

The crystal structures of wild-type human theta class glutathione-S-transferase (GST) T1-1 and its W234R mutant, where Trp234 was replaced by Arg, were solved both in the presence and absence of S-hexyl-glutathione. The W234R mutant was of interest due to its previously observed enhanced catalytic activity compared to the wild-type enzyme. GST T1-1 from rat and mouse naturally contain Arg in position 234, with correspondingly high catalytic efficiency. The overall structure of GST T1-1 is similar to that of GST T2-2, as expected from their 53% sequence identity at the protein level. Wild-type GST T1-1 has the side-chain of Trp234 occupying a significant portion of the active site. This bulky residue prevents efficient binding of both glutathione and hydrophobic substrates through steric hindrance. The wild-type GST T1-1 crystal structure, obtained from co-crystallization experiments with glutathione and its derivatives, showed no electron density for the glutathione ligand. However, the structure of GST T1-1 mutant W234R showed clear electron density for S-hexyl-glutathione after co-crystallization. In contrast to Trp234 in the wild-type structure, the side-chain of Arg234 in the mutant does not occupy any part of the substrate-binding site. Instead, Arg234 is pointing in a different direction and, in addition, interacts with the carboxylate group of glutathione. These findings explain our earlier observation that the W234R mutant has a markedly improved catalytic activity with most substrates tested to date compared to the wild-type enzyme. GST T1-1 catalyzes detoxication reactions as well as reactions that result in toxic products, and our findings therefore suggest that humans have gained an evolutionary advantage by a partially disabled active site.

Glutathione‐dependent Bioactivation of Haloalkanes and Haloalkenes

Haloalkanes and haloalkenes constitute an important group of widely used chemicals that have the potential to induce toxicity and cancer. The toxicity of haloalkanes and haloalkenes may be associated with cytochromes P450- or glutathione transferase-dependent bioactivation. This review is concerned with the glutathione- and glutathione transferase-dependent bioactivation of dihalomethanes, 1,2-dihaloalkanes, and haloalkenes. Dihalomethanes, e.g., dichloromethane, and 1,2-dihaloethanes, e.g., 1,2-dichloroethane and 1,2-dibromoethane, undergo glutathione transferase-catalyzed bioactivation to give S-(halomethyl)glutathione or glutathione episulfonium ions, respectively, as reactive intermediates. Haloalkenes, e.g., trichloroethene, hexachlorobutadiene, chlorotrifluoroethene, and tetrafluoroethene, undergo cysteine conjugate beta-lyase-dependent bioactivation to thioacylating intermediates, including thioacyl halides, thioketenes, and 2,2,3-trihalothiiranes. With all of these compounds, the formation of reactive intermediates is associated with their observed toxicity.