Metabolic activation of 1,1-dichloroethylene by mouse lung and liver microsomes

Mitochondrial dysfunction is an early manifestation of 1,1-dichloroethylene-induced hepatotoxicity in mice

Hepatotoxicity induced by 1,1-dichloroethylene (DCE) is mediated by cytochrome P450-dependent metabolism to reactive intermediates, including the epoxide. We have tested the hypothesis that mitochondria are a primary target of toxicity by investigating dose- and time-dependent effects of DCE on mitochondrial respiration. Hepatotoxicity, as assessed by serum alanine aminotransferase (ALT) activity, was evaluated. We have also determined the effectiveness of N-acetyl-L-cysteine (NAC) in protecting against respiratory perturbations and hepatotoxicity. Liver mitochondria were isolated 2 h after DCE (50, 75, 100, 125, and 150 mg/kg) treatment. Glutamate (complex I)- and succinate (complex II)-supported mitochondrial respiration was assessed by measurement of state 3 (ADP-stimulated) and state 4 (resting) rates of oxygen consumption. The corresponding respiratory control ratios (RCRs, state 3/state 4) and ADP:O ratios were then calculated. A DCE dose of 125 mg/kg significantly inhibited glutamate- and succinate-supported state 3 respiration, leading to a significant reduction in corresponding RCRs and ADP:O ratios. In time-dependent studies, state 3 respiration rates and RCRs for glutamate-supported respiration were significantly decreased as early as 20 min after DCE (125 mg/kg) treatment, whereas those for succinate-supported respiration were significantly decreased at 90 min. Additionally, ADP:O ratios for glutamate-supported respiration were significantly decreased starting at 60 min, and those for succinate-supported respiration at 90 min. Alterations in mitochondrial function preceded significant increases in ALT activity, which was first manifested at 2 h. Pretreatment with NAC (1200 mg/kg) abrogated DCE-induced GSH depletion and inhibited disturbances in mitochondrial respiration. Moreover, NAC protected against increased ALT activity, suggesting that the protective effect of NAC is due to increased GSH for conjugation reactions and/or its antioxidant property. These results showed that DCE-mediated mitochondrial dysfunction is an early event that preceded the onset of hepatotoxicity.

Mechanisms of 1,1-dichloroethylene-induced cytotoxicity in lung and liver

Exposure to 1,1-dichloroethylene (DCE) causes lung and liver toxicities in mice. The lesions are characterized by damage preferentially to bronchiolar Clara cells in the lung and necrosis of centrilobular hepatocytes in the liver. The primary metabolites formed from DCE in lung and liver microsomal incubations are the epoxide, 2,2-dichloroacetaldehyde and 2-chloroacetyl chloride, which undergo hydrolysis and/or conjugation with glutathione (GSH). The major products formed are the epoxide-derived GSH conjugates 2-(S-glutathionyl) acetyl glutathione [B] and 2-S-glutathionyl acetate [C]. The hydrate of 2,2-dichloroacetaldehyde (acetal) is also detected. These metabolites are detected in vivo in murine lung and liver cytosol and in bile, and importantly, also in human lung and liver microsomal incubations. Formation of the epoxide is mediated mainly by CYP2E1. Immunohistochemical studies localized the epoxide-derived GSH conjugate [C] and cysteine-containing proteins in Clara cells and centrilobular hepatocytes. These findings are consistent with the premise that the lung and liver cytotoxicities induced by DCE are associated with in situ formation of the epoxide within the target cells.

1,1-Dichloroethylene-induced Clara cell damage is associated with in situ formation of the reactive epoxide. Immunohistochemical detection of its glutathione conjugate

Pulmonary Clara cells are selectively damaged in mice given 1, 1-dichloroethylene (DCE), a chemical used in the plastics industry. The cytotoxicity is attributed to formation of a reactive metabolite believed to be the DCE-epoxide, which was detected in vitro. We have undertaken in vivo studies to test the hypothesis that in situ formation of the DCE-epoxide within Clara cells mediates the cell-specific injury manifested after DCE exposure. Formation of the epoxide was estimated by trapping of the metabolite with glutathione (GSH) and identifying the conjugated products as 2-(S-glutathionyl) acetyl glutathione ([B]) and 2-S-glutathionyl acetate ([C]). High-pressure liquid chromatographic analyses showed that conjugates [B] and [C] were both detected in lung cytosol isolated from mice treated in vivo with [14C]DCE. Epoxide levels in the cytosol, as estimated by the total amount of conjugates formed, were dose-dependent at DCE doses ranging from 25 to 225 mg/kg. Pretreatment of mice with buthionine sulfoximine (BSO) decreased sulfhydryl levels and significantly inhibited the formation of the GSH conjugates. Epoxide levels were also reduced by pretreatment with diallyl sulfone (DASO2), an inhibitor of the P450 isozyme CYP2E1. A polyclonal antibody was developed that is specific for conjugate [C] and that recognizes an antigen consisting of the conjugate epoxide-GSH-glutaraldehyde-bovine serum albumin. Immunohistochemical studies with this antibody revealed staining in Clara cells of mice treated with DCE. Staining was also present in Clara cells of mice treated with both BSO and DCE, but at slightly reduced levels. Reduction of this staining was more pronounced in Clara cells of mice treated with both DASO2 and DCE. These results show that the DCE-epoxide is formed in vivo, is localized in Clara cells, and correlates with the cytotoxicity manifested in this cell type.

Immunohistochemical detection of CYP2E1 and 2-S-glutathionyl acetate in murine lung tumors: diminished formation of reactive intermediates of 1,1-dichloroethylene

This study investigates the potential of urethane-induced lung tumors to activate 1,1-dichloroethylene (DCE), a chemical that causes Clara cell damage in mice. Metabolism of DCE is catalyzed by the cytochrome P450 isozyme CYP2E1 to the DCE-epoxide, as assessed by formation of 2-S-glutathionyl acetate (GTA), the glutathione (GSH)-conjugated product of the epoxide. Immunohistochemical studies were performed in normal non-tumor- and tumor-bearing mice to determine lung cells that contained CYP2E1 available for DCE metabolism. The site of GTA formation was also determined. The results showed that most of the CYP2E1 were expressed in Clara cells from normal lung and in uninvolved tissue of tumor-bearing lung. In contrast, CYP2E1 was minimally expressed in neoplastic tissue, including hyperplasias, adenomas, and carcinomas. Parallel studies of adjacent lung sections revealed that GTA immunostaining was most intense in Clara cells of normal lung tissue and in uninvolved tissue of tumor-bearing lungs from DCE-treated mice. However, GTA staining was negligible at all tumor sites. These results demonstrated that the cellular sites where CYP2E1 was expressed were the same as those in which the GTA metabolite was identified. They further showed that expression of both CYP2E1 and GTA was markedly reduced in hyperplasias, adenomas, and carcinomas. These observations suggest that these tissue types are defective in their capability for bioactivation of DCE.

Conjugation of glutathione with the reactive metabolites of 1, 1-dichloroethylene in murine lung and liver

Exposure to 1,1-dichloroethylene (DCE) elicits lung and liver cytotoxicities that are manifested in bronchiolar Clara cell injury and centrilobular necrosis, respectively. The tissue damage is associated with cytochrome P450-dependent bioactivation of DCE to reactive intermediates, and is consistent with the finding that the target cells coincided with the sites of high concentrations of cytochrome P450 enzymes. The metabolites formed from DCE bind covalently to cellular macromolecules, and the extent of binding and cell damage are inversely related to the content of intracellular glutathione (GSH). Histochemical studies showed that staining for GSH in the lung is localized in the bronchiolar epithelial and alveolar septal cells, with relatively strong staining in the Clara cells. In the liver, staining is observed rather uniformly in hepatocytes distributed across the hepatic lobule. Depletion of GSH correlates with the Clara cell damage and centrilobular necrosis observed in the lung and liver, respectively. The primary metabolites of DCE formed in lung and liver microsomal incubations have been identified as DCE-epoxide, 2,2-dichloroacetaldehyde and 2-chloroacetyl chloride. All are electrophilic metabolites that form secondary reactions including conjugation with GSH. Results of our studies indicated that the DCE-epoxide is the major metabolite forming conjugates with GSH, and this reaction is likely responsible for the depletion of GSH observed in vivo. Our findings support the premise that, following depletion of intracellular GSH, metabolites of DCE including the DCE-epoxide bind to cellular proteins, a process which leads to cell damage and suggests that conjugation with the thiol nucleophile represents a-detoxification mechanism.

Nephrotoxicity and covalent binding of 1,1-dichloroethylene in buthionine sulphoximine-treated mice

Autoradiography of mice injected i.p. with 14C-labelled 1,1-dichloroethylene (vinylidene chloride, VDC) in C57B1/6 mice revealed a selective covalent binding of radioactivity in the proximal tubules, in the midzonal parts of the liver lobules and in the mucosa of the upper and lower respiratory tract. Since VDC is a renal carcinogen in male mice the effects of compounds modulating biotransformation and glutathione (GSH) levels on the renal covalent binding were examined following a single i.p. dose of 14C-VDC. Most pretreatments did not influence the level of binding but treatment with buthionine sulphoximine (BSO), an irreversible inhibitor of gamma-glutamylcysteine synthetase and glutathione (GSH)-depleting agent, increased the renal covalent binding of VDC three-fold. Histopathological examination of kidneys in BSO-pretreated male mice given single i.p. injections of subtoxic doses of VDC (25 and 50 mg/kg) showed necrosis in the proximal tubules (S1 and S2 segments) 24 h following administration. In mice given VDC only, no significant lesions in the kidneys were observed. The severe renal toxicity of VDC in BSO-pretreated mice is suggested to be related to metabolic activation of VDC in the proximal tubules, resulting in further GSH depletion and covalent binding.

1,1-Dichloroethylene-induced alterations in glutathione and covalent binding in murine lung: morphological, histochemical, and biochemical studies

Non-ciliated Clara cells of the pulmonary bronchiolar epithelium are preferentially damaged by administration of 1,1-dichloroethylene (1,1-DCE) to mice. In this study, an in vivo system was utilized to investigate the dose-dependent effects of 1,1-DCE (75, 125, 175, and 225 mg/kg) on covalent binding and on reduced glutathione (GSH) in murine lung. Treatment of mice with each dose level of 1,1-DCE elicited significant decreases in GSH content and resulted in covalent binding of [14C]1,1-DCE in a dose-dependent manner. Histochemical staining for GSH in lungs of control mice revealed positive cellular sites in alveolar septa and bronchiolar epithelium, with the highest staining intensities in Clara cells. Staining was reduced after exposure to 75 and 125 mg/kg 1,1-DCE, and at higher doses it was abolished in alveolar septa and retained in bronchiolar epithelium, albeit at considerably reduced intensities. Heterogeneity with respect to staining intensities was consistently observed in the Clara cell population in both control and 1,1-DCE-treated mice. Progressive increases in covalent binding and decreases in GSH content correlated with increasing severities of Clara cell injury. These results show a dose dependence in regard to the magnitudes of [14C]1,1-DCE binding, the alterations in cellular GSH, and the severities of Clara cell necrosis.

Methanol suppression of trichloroethylene degradation by Methylosinus trichosporium (OB3b) and methane-oxidizing mixed cultures

The effect of methanol on trichloroethylene (TCE) degradation by mixed and pure methylotrophic cultures was examined in batch culture experiments. Methanol was found to relieve growth inhibition of Methylosinus trichosporium (OB3b) at high (14 mg/L) TCE concentrations. Degradation of TCE was determined by both radiolabeling and gas chromatography techniques. When cultures were grown on methanol over 10 to 14 d with 0.3 mg/L TCE, OB3b degraded 16.89 +/- 0.82% (mean +/- SD) of the TCE, and a mixed culture (DT type II) degraded 4.55 +/- 0.11%. Mixed culture (JS type I) degraded 4.34 +/- 0.06% of the TCE. When grown on methane with 0.3 mg/L TCE, 32.93 +/- 2.01% of the TCE was degraded by OB3b, whereas the JS culture degraded 24.3 +/- 1.38% of the TCE, and the DT culture degraded 34.3 +/- 2.97% of the TCE. The addition of methanol to cultures grown on methane reduced TCE degradation to 16.21 +/- 1.17% for OB3b and to 5.08 +/- 0.56% for JS. Although methanol reduces the toxicity of TCE to the cultures, biodegradation of TCE cannot be sustained in methanol-grown cultures. Since high TCE concentrations appear to inhibit methane uptake and growth, we suggest the primary toxicity of TCE is directed towards the methane monooxygenase.