Covalent binding and inhibition of cytochrome P4502E1 by trichloroethylene

Micronuclei Formation by Promutagens in Metabolism-Incompetent V79 Cells Interacting With Activation-Proficient Cells in Various Experimental Settings

The accessibility of reactive metabolites to test cells is critical for a genotoxic response. However, sulfo-conjugates formed outside may not readily enter cells, and some metabolites formed by cytochromes P450 (CYPs) may not endure transport. This topic was addressed in the present study, using V79 cells engineered for human CYPs and/or a sulfotransferase (SULT). First, 1-methylpyrene, 1-hydroxymethylpyrene, benzo[a]pyrene, and aflatoxin B1 significantly induced micronuclei in V79-hCYP1A2-hSULT1A1, V79-hSULT1A1, V79-hCYP1A1, and V79-hCYP1A2 cells, respectively. Subsequently, we used these cell lines as external activating systems in various experimental settings in combination with V79-derived target cells lacking critical enzymes. 1-Methylpyrene (activated by CYPs and SULTs sequentially) showed an activity similar to that in V79-hCYP1A2-hSULT1A1 cells, in each following model: a mixed V79-hCYP1A2:V79-hSULT1A1 (1:1) culture, exposure of V79-hCYP1A2 to 1-methylpyrene followed by transfer of medium to V79-hSULT1A1 target cells, and V79-hSULT1A1 communicating with V79-hCYP1A2 through 0.4-μm pores and over a 1-mm distance in a unique transwell system. These results suggest ready transfer of 1-hydroxymethylpyrene formed in V79-hCYP1A2 to V79-hSULT1A1 for further activation. In the last two models, with V79-hSULT1A1 for activation and V79-Mz as target, 1-hydroxymethylpyrene induced micronuclei mildly, suggesting limited intercellular transfer of the ultimate genotoxicant, 1-sulfooxymethylpyrene. Benzo[a]pyrene induced micronuclei in V79-Mz communicating with V79-hCYP1A1 via porous membranes, whereas aflatoxin B1 was inactive in V79-Mz communicating with V79-hCYP1A2. Our results suggest that the sulfo-conjugate tested may have difficulty entering cells for a genotoxic effect, and the reactive metabolite of aflatoxin B1, unlike that of benzo[a]pyrene, could not travel an adequate distance to enter cells. Environ. Mol. Mutagen. 61:224-234, 2020. © 2019 Wiley Periodicals, Inc.

Evidence for cellular protein covalent binding derived from styrene metabolite

Styrene is one of the most important industrial intermediates consumed in the world. Human exposure to styrene occurs mainly in the reinforced plastics industry, particularly in developing countries. Styrene has been found to be hepatotoxic and pneumotoxic in humans and animals. The biochemical mechanisms of styrene-induced toxicities remain unknown. Albumin and hemoglobin adduction derived from styrene oxide, a major reactive metabolite of styrene, has been reported in blood samples obtained from styrene-exposed workers. The objectives of the current study focused on cellular protein covalent binding of styrene metabolite and its correlation with cytotoxicity induced by styrene. We found that radioactivity was bound to cellular proteins obtained from mouse airway trees after incubation with (14)C-styrene. Microsomal incubation studies showed that the observed protein covalent binding required the metabolic activation of styrene. The observed radioactivity binding in protein samples obtained from the cultured airways and microsomal incubations was significantly suppressed by co-incubation with disulfiram, a CYP2E1 inhibitor, although disulfiram apparently did not show a protective effect against the cytotoxicity of styrene. A 2-fold increase in radioactivity bound to cellular proteins was detected in cells stably transfected with CYP2E1 compared to the wild-type cells after (14)C-styrene exposure. With the polyclonal antibody developed in our lab, we detected cellular protein adduction derived from styrene oxide at cysteinyl residues in cells treated with styrene. Competitive immunoblot studies confirmed the modification of cysteine residues by styrene oxide. Cell culture studies showed that the styrene-induced protein modification and cell death increased with the increasing concentration of styrene exposure. In conclusion, we detected cellular protein covalent modification by styrene oxide in microsomal incubations, cultured cells, and mouse airways after exposure to styrene and found a good correlation between styrene-induced cytotoxicity and styrene oxide-derived cellular protein adduction.

Chronic exposure to trichloroethene causes early onset of SLE-like disease in female MRL +/+ mice

Trichloroethene (TCE) exacerbates the development of autoimmune responses in autoimmune-prone MRL +/+ mice. Although TCE-mediated autoimmune responses are associated with an increase in serum immunoglobulins and autoantibodies, the underlying mechanism of autoimmunity is not known. To determine the progression of TCE-mediated immunotoxicity, female MRL +/+ mice were chronically exposed to TCE through the drinking water (0.5 mg/ml of TCE) for various periods of time. Serum concentrations of antinuclear antibodies increased after 36 and 48 weeks of TCE exposure. Histopathological analyses showed lymphocyte infiltration in the livers of MRL +/+ mice exposed to TCE for 36 or 48 weeks. Lymphocyte infiltration was also apparent in the pancreas, lungs, and kidneys of mice exposed to TCE for 48 weeks. Immunoglobulin deposits in kidney glomeruli were found after 48 weeks of exposure to TCE. Our results suggest that chronic exposure to TCE promotes inflammation in the liver, pancreas, lungs, and kidneys, which may lead to SLE-like disease in MRL +/+ mice.

Modulation of hepatic and renal metabolism and toxicity of trichloroethylene and perchloroethylene by alterations in status of cytochrome P450 and glutathione

The relative importance of metabolism of trichloroethylene (Tri) and perchloroethylene (Perc) by the cytochrome P450 (P450) and glutathione (GSH) conjugation pathways in their acute renal and hepatic toxicity was studied in isolated cells and microsomes from rat kidney and liver after various treatments to modulate P450 activity/expression or GSH status. Inhibitors of P450 stimulated GSH conjugation of Tri and, to a lesser extent, Perc, in both kidney cells and hepatocytes. Perc was a more potent, acute cytotoxic agent in isolated kidney cells than Tri but Perc-induced toxicity was less responsive than Tri-induced toxicity to modulation of P450 status. These observations are consistent with P450-dependent bioactivation being more important for Tri than for Perc. Incubation of isolated rat hepatocytes with Tri produced no acute cytotoxicity in isolated hepatocytes while Perc produced comparable cytotoxicity as in kidney cells. Modulation of P450 status in hepatocytes produced larger changes in Tri- and Perc-induced cytotoxicity than in kidney cells, with non-selective P450 inhibitors increasing toxicity. Induction of CYP2E1 with pyridine also markedly increased sensitivity of hepatocytes to Tri but had little effect on Perc-induced cytotoxicity. Increases in cellular GSH concentrations increased Tri- and Perc-induced cytotoxicity in kidney cells but not in hepatocytes, consistent with the role of GSH conjugation in Tri- and Perc-induced nephrotoxicity. In contrast, depletion of cellular GSH concentrations moderately decreased Tri- and Perc-induced cytotoxicity in kidney cells but increased cytotoxicity in hepatocytes, again pointing to the importance of different bioactivation pathways and modes of action in kidney and liver.

Autoimmune response in MRL+/+ mice following treatment with dichloroacetyl chloride or dichloroacetic anhydride

Dichloroacetyl chloride (DCAC) is formed from trichloroethene (TCE), which is implicated in inducing/accelerating autoimmune response. Due to its potent acylating activity, DCAC may convert proteins to neo-antigens and thus could induce autoimmune responses. Dichloroacetic anhydride (DCAA), which is a similar acylating agent, might also induce autoimmune responses. To evaluate if chloroacylation plays a role in the induction of autoimmunity, we have measured the autoimmune responses following treatment with DCAC or DCAA in autoimmune-prone MRL+/+ mice. Five-week-old female mice were injected intraperitoneally (twice weekly) with 0.2 mmol/kg of DCAC or DCAA in corn oil for 6 weeks. Total serum IgG, IgG1, and IgE levels were significantly increased in DCAC-treated mice as compared to controls. These increases corresponded with increases in DCAC-specific IgG and IgG1 levels. Total serum IgM was decreased in both DCAC- and DCAA-treated mice. Antinuclear antibodies, measured as an indication of systemic autoimmune responses, were increased in both DCAC- and DCAA-treated mice. Of eight Th1/Th2 cytokines measured in the serum, only IL-5 was significantly decreased in both treatment groups. The cytokine secretion patterns of splenic lymphocytes after stimulation with antibodies against CD3 (T cell receptor-mediated signal) and CD28 (costimulatory signal) differed between treatment and control groups. Levels of IL-1, IL-3, IL-6, IFN-gamma, G-CSF, and KC were higher in cultures of stimulated splenocytes from either DCAC- or DCAA-treated mice than from controls. The level of IL-17 was only increased in cultures from DCAC-treated mice. Increased lymphocytic populations were found in the red pulp of spleens following treatment with either DCAC or DCAA. In addition, thickening of the alveolar septa in the lungs of DCAC- or DCAA-treated mice was observed. The lung histopathology in exposed mice was consistent with the symptomology observed in welders exposed to DCAC/phosgene. Thickening was more pronounced in DCAC-treated mice. Our data suggest that DCAC and DCAA elicit autoimmune responses in MRL+/+ mice that might be reflective of their chloroacylation potential in vivo.

Evidence for trichloroethylene bioactivation and adduct formation in the rat epididymis and efferent ducts

Recent studies indicate that trichloroethylene (TCE) may be a male reproductive toxicant. It is metabolized by conjugation with glutathione and cytochrome p450-dependent oxidation. Reactive metabolites produced along both pathways are capable of forming protein adducts and are thought to be involved in TCE-induced liver and kidney damage. Similarly, in situ bioactivation of TCE and subsequent binding of metabolites may be one mechanism by which TCE acts as a reproductive toxicant. Cysteine-conjugate beta-lyase (beta-lyase) bioactivates the TCE metabolite dichlorovinyl cysteine (DCVC) to a reactive intermediate that is capable of binding cellular macromolecules. In the present study, Western blot analysis indicated that the soluble form of beta-lyase, but not the mitochondrial form, was present in the epididymis and efferent ducts. Both forms of beta-lyase were detected in the kidney. When rats were dosed with DCVC, no protein adducts were detected in the epididymis or efferent ducts, although adducts were present in the proximal tubule of the kidney. Trichloroethylene can also be metabolized and form protein adducts through a cytochrome p450-mediated pathway. Western blot analysis detected the presence of cytochrome p450 2E1 (CYP2E1) in the efferent ducts. Immunoreactive proteins were localized to efferent duct and corpus epididymis epithelia. Metabolism of TCE was demonstrated in vitro using microsomes prepared from untreated rats. Metabolism was inhibited 77% when efferent duct microsomes were preincubated with an antibody to CYP2E1. Dichloroacetyl adducts were detected in epididymal and efferent duct microsomes exposed in vitro to TCE. Results from the present study indicate that the cytochrome p450-dependent formation of reactive intermediates and the subsequent covalent binding of cellular proteins may be involved in the male reproductive toxicity of TCE.

Regulatory heme and trichloroethylene intoxication: A possible explanation of the case of "A Civil Action"

In 1998, a amovie entitled "A Civil Action" was released. The movie described the Woburn case, begun in 1982 and concluded in 1990, one of the most famous cases of trichloroethylene pollution. In a small town near Boston, twelve children died of leukemia, which seemed attributable to trichloroethylene contamination of the drinking water. The victims, however, could not win the case, since evidence that the identified chemicals could cause leukemia and other human illnesses was rather sketchy. There have been many cases of trichloroethylene pollution in industrial nations including Japan, therefore, we reconsidered the missing link. Our conclusion is that the disease occurred not by a direct effect of the chemical hazard on biological macromolecules but by an indirect effect through the physiological system such as signal transduction and transcriptional regulation. In 1984, we reported a marked reduction in the regulatory heme pool by trichloroethylene exposure, however, the biological significance was not well understood. Recently, we found that the DNA binding activity of Bach1, a negative regulator of genes, is controlled by heme, the regulation of which seems to explain how leukemia develops. The heterodimer of Bach1 with MafK recognizes Maf recognition elements (MAREs) competing with the erythroid type positive regulator, a complex of NF-E2 with MafK. Bach1/MafK occupies MAREs under lower heme conditions, whereas MAREs are open to NF-E2/MafK along with increasing heme concentration. Since the NF-E2/MafK function is closely related to normal erythroid differentiation, of which disorders such as sideroblastic anemia are often related to neoplasia; i.e., a clonal disorder that can progress to leukemia. Thus, a marked decline in regulatory heme by trichloroethylene intoxication could be one of the pathways to leukemia.

Potential biomarkers of trichloroethylene and toluene exposure in Corbicula fluminea

Freshwater clams Corbicula fluminea were exposed in aquariums to four doses of trichloroethylene-TCE-(1.56 up to 100 mg/1) or toluene-TOL-(7.5 up to 60 mg/1) for 5 days. At the end of exposure, components of (de)toxification metabolism of phases I and II, parameters related to oxidative stress and propionylcholinesterase activity were assayed. Determination of TCE and TOL concentrations in water revealed an important evaporative loss during the experiment, characteristic of acute and occasional contaminations by such products occurring in the environment. Appropriate statistical methods such as ANOVA, Tukey test and discriminant analysis underlined the relevance of cytochromes P450 and P418, NADH-cytochrome c reductase, catalase, peroxided and peroxidizable lipids and net peroxidation as biomarkers of exposure to these solvents in C. fluminea. This experiment emphasised the importance of a multi-biomarker approach in environmental surveys and will be completed further by mesocosm studies.

Trapping and identification of the dichloroacetate radical from the reductive dehalogenation of trichloroacetate by mouse and rat liver microsomes

A key question in the risk assessment of trichloroethylene (TRI) is the extent to which its carcinogenic effects might depend on the formation of dichloroacetate (DCA) as a metabolite. One of the metabolic pathways proposed for the formation of DCA from TRI is by the reductive dehalogenation of trichloroacetate (TCA), via a free radical intermediate. Although proof of this radical has been elusive, the detection of fully dechlorinated metabolites in the urine and the formation of lipid peroxidation by-products in microsomal incubations with TCA argue for its existence. We report here the trapping of the dichloroacetate radical with the spin-trapping agent PBN, and its identification by GC/MS. The PBN/dichloroacetate radical adduct was found to undergo an intramolecular rearrangement during its extraction into organic solvent. An internal condensation reaction between the acetate and the nitroxide radical moieties is hypothesized to form a cyclic adduct with the elimination of an OH radical. The PBN/dichloroacetate radical adduct has been identified by GC/MS in both a chemical Fenton system and in rodent microsomal incubations with TCA as substrate.

Contribution of direct solvent injury to the dose-dependent kinetics of trichloroethylene: portal vein administration to rats

Presystemic elimination of trichloroethylene (TCE), a common contaminant of drinking water, has been shown by Lee et al. (Toxicol. Appl. Pharmacol. 139, 262-271, 1996) to be inversely related to dose. When relatively high doses were administered to rats via the portal vein (PV), first-pass hepatic extraction became negligible. This phenomenon could result not only from metabolic saturation, but from suicidal destruction of cytochromes P450 and hepatocellular injury as well. The objectives of the current investigation were to: (a) clarify the relative roles of P450 depletion and hepatocellular toxicity in the apparent cessation of hepatic elimination of TCE in animals given relatively high doses of TCE via the PV; and (b) investigate mechanism(s) of hepatocellular injury under such exposure conditions. TCE (16 and 64 mg/kg body weight (bw) was incorporated into a 5% aqueous Alkamuls emulsion and injected via an indwelling jugular vein (JV) or PV cannula into male Sprague-Dawley rats. Some animals received 73.5 micromol/kg of p-nitrophenol (PNP), a competitive metabolic inhibitor of TCE, through the PV cannula 3 min before TCE. Administration of TCE via the PV resulted in deposition of relatively high levels of TCE in the liver. PV dosing resulted in lower total hepatic P450 levels than did JV dosing. PV dosing produced marked elevations of cytoplasmic enzymes in serum, but JV dosing did not. Decreases in hepatic P450 were not selective for cytochrome P4502E1. Histological examination of the liver of PV-dosed rats revealed periportal rather than centrilobular necrosis. PNP pretreatment failed to prevent the increase in serum enzymes, decrease in hepatic P450 content, and hepatic necrosis following PV TCE. It is concluded that PV injection of bolus doses of TCE >/= 16 mg/kg causes liver injury within minutes in rats, primarily through direct solvent action on hepatocellular membranes rather than by P450-mediated effects. This liver damage likely plays a modest role in reducing the liver's capacity to metabolize high PV doses of TCE.

Mechanisms of the dose-dependent kinetics of trichloroethylene: oral bolus dosing of rats

Trichloroethylene (TCE), a common contaminant of drinking water, is oxidized by high-affinity, low-capacity cytochrome P450 isozymes and subsequently converted to metabolites, some of which are carcinogenic in mice and rats. Although the initial oxidation step is known to be rate-limiting and saturable, the oral dosage-range over which saturation materializes is unclear. One objective of this study was to characterize the dose-dependency of gastrointestinal (GI) absorption of TCE and its kinetics over a wide range of oral bolus doses. A related objective was to investigate cause(s) of the apparent saturation kinetics observed. Cannulas were surgically implanted into a carotid artery and the stomach of male Sprague-Dawley rats. TCE was incorporated into a 5% aqueous Alkamuls emulsion and given in doses of 2 to 1200 mg/kg bw via the stomach tube. Serial blood samples were taken from the arterial cannula for up to 14 h postdosing and analyzed for TCE content by headspace gas chromatography. The rate of GI absorption of TCE diminished as the dosage increased. Pharmacokinetic analysis indicated that TCE was eliminated by capacity-limited hepatic metabolism, with incursion into nonlinear kinetics with bolus doses >/=8 to 16 mg/kg. Effects of p-nitrophenol, a competitive metabolic inhibitor, were manifest at a high, but not at a low TCE dose. Gavage bolus doses as high as 1200 mg/kg did not cause rapid elevation of serum enzyme levels, typical of the solvation of hepatocellular membranes observed after portal vein administration of TCE (Lee et al., Toxicol. Appl. Pharmacol. 163, 000-000, 2000). No evidence of cytochrome P4502E1 (CYP2E1) destruction was seen with oral doses up to 1000 mg/kg. Instead, CYP2E1 activity was induced as early as 1 h postdosing. Induction was maximal at 12 h, then returned toward controls during the next 12 h. Pretreatment with cycloheximide did not reduce CYP2E1 activity in rats given 432 or 1000 mg TCE/kg, suggesting that binding of TCE to CYP2E1 may stabilize the isozyme. Metabolic saturation, in concert with relatively slow GI absorption, are responsible for the prolonged elevation of blood TCE levels in rats given high TCE doses, while suicidal inactivation of CYP2E1 and hepatocellular injury apparently play little role.

Trichloroethylene accelerates an autoimmune response by Th1 T cell activation in MRL +/+ mice

Trichloroethylene (1,1,2-trichloroethene) is a major environmental contaminant. There is increasing evidence relating exposure to trichloroethylene with autoimmunity. To investigate potential mechanisms, we treated the autoimmune-prone MRL +/+ mice with trichloroethylene in the drinking water at 0, 2.5 or 5.0 mg/ml and sacrificed them at 4, 8 and 22 weeks. As early as 4 weeks of treatment, Western blot analysis showed a dose-dependent increase in the level of trichloroethylene-modified proteins, indicating that a reactive metabolite of trichloroethylene was formed. Significant increases in antinuclear antibodies (ANA) and total serum immunoglobulins were found following 4-8 weeks of trichloroethylene treatment, indicating that trichloroethylene was accelerating an autoimmune response. Investigation into possible mechanisms of this autoimmune response revealed that trichloroethylene treatment dramatically increased the expression of the activation marker CD44 on splenic CD4+ T cells at 4 weeks. In addition, splenic T cells from mice treated for 4 weeks with trichloroethylene secreted more IFN-gamma and less IL-4 than control T cells, consistent of a T-helper type 1 (Th1) type immune or inflammatory response. A specific immune response directed against dichloroacetylated proteins was found at 22 weeks of trichloroethylene treatment. Taken collectively, the results suggest that trichloroethylene treatment accelerated an autoimmune response characteristic of MRL +/+ mice in association with nonspecific activation of Th1 cells. In addition, long-term treatment with trichloroethylene led to the initiation of a trichloroethylene-specific immune response.

Toxicokinetics of organic solvents: a review of modifying factors

This article reviews, with an emphasis on human experimental data, factors known or suspected to cause changes in the toxicokinetics of organic solvents. Such changes in the toxicokinetic pattern alters the relation between external exposure and target dose and thus may explain some of the observed individual variability in susceptibility to toxic effects. Factors shown to modify the uptake, distribution, biotransformation, or excretion of solvent include physical activity (work load), body composition, age, sex, genetic polymorphism of the biotransformation, ethnicity, diet, smoking, drug treatment, and coexposure to ethanol and other solvents. A better understanding of modifying factors is needed for several reasons. First, it may help in identifying important potential confounders and eliminating negligible ones. Second, the risk assessment process may be improved if different sources of variability between external exposures and target doses can be quantitatively assessed. Third, biological exposure monitoring may be also improved for the same reason.

Covalent binding of trichloroethylene to proteins in human and rat hepatocytes

The environmental contaminant and occupational solvent trichloroethylene is metabolized to a reactive intermediate that covalently binds to specific hepatic proteins in exposed mice and rats. In order to compare covalent binding between humans and rodents, primary hepatocyte cultures were exposed to vaporized trichloroethylene at 0-10,000 parts per million for up to 2 h. Immunochemical detection of three major dose- and time-dependent trichloroethylene protein adducts at 50, 52 and 100 kDa was demonstrated in the rat hepatocytes, while a single, distinctively different 47 kDa adduct was detected in human hepatocytes. The 50 kDa adduct in rat hepatocytes was found to comigrate on SDS-PAGE with cytochrome P450 2E1 (CYP2E1), while the adduct found in humans did not comigrate with CYP2E1. These data show that reactive metabolites of trichloroethylene can be formed in human and rat hepatocytes and bind covalently to discrete hepatic proteins, and suggests that in rats, but not humans, that one of the targets is CYP2E1.

Detection of trichloroethylene-protein adducts in rat liver and plasma

Trichloroethylene is an industrial chemical with widespread occupational exposure and is a major environmental contaminant. In a Western blot using antiserum that recognizes trichloroethylene covalently bound to protein, a single 50 kDa microsomal adduct was detected in the livers of trichloroethylene-treated Sprague-Dawley rats. To determine if trichloroethylene-protein adducts could be detected in blood, plasma proteins were immunoaffinity purified using an antidichloroacetyl column. A single 50 kDa protein was detected in the affinity-purified fraction in a Western blot using dichloroacetyl antiserum. This protein was also immunochemically reactive with anti-cytochrome P450 2E1 antibodies. The 50 kDa trichloroethylene-protein adduct may be formed in the liver and released into the blood following exposure to trichloroethylene. The significance of adduct formation with respect to trichloroethylene toxicity remains to be established; however, the data suggest that this approach may be useful in the investigation of trichloroethylene-protein adducts and adverse effects following exposure.