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

Bioactivation of 1,1-Dichloroethylene by CYP2E1 and CYP2F2 in Murine Lung

1,1-Dichloroethylene (DCE) exposure evokes lung toxicity with selective damage to bronchiolar Clara cells. Recent in vitro studies have implicated CYP2E1 and CYP2F2 in the bioactivation of DCE to 2-S-glutathionyl acetate [C], a putative conjugate of DCE epoxide with glutathione. An objective of this study was to test the hypothesis that bioactivation of DCE is catalyzed by both CYP2E1 and CYP2F2 in murine lung. Western blot analysis of lung microsomal proteins from DCE-treated CD-1 mice showed time-dependent loss of immunodetectable CYP2F2 and CYP2E1 protein. Dose-dependent formation of conjugate [C] was observed in the lungs of CD-1 mice treated with DCE (75-225 mg/kg), but it was not detected after pretreatment with 5-phenyl-1-pentyne (5-PIP). Treatment of mice with 5-PIP and also with diallyl sulfone (DASO2) significantly inhibited hydroxylation of p-nitrophenol (PNP) and chlorzoxazone (CHZX). Incubation of recombinant CYP2F3 (a surrogate for CYP2F2) and recombinant CYP2E1 with PNP and CHZX confirmed that they are substrates for both of the recombinant enzymes. Incubation of the recombinant enzymes with DASO2 or 5-PIP significantly inhibited hydroxylation of both PNP and CHZX. Bronchiolar injury was elicited in CD-1 mice treated with DCE (75 mg/kg), but it was abrogated with 5-PIP pretreatment. Bronchiolar toxicity also was manifested in the lungs of CYP2E1-null and wild-type mice treated with DCE (75 mg/kg), but protection ensued after pretreatment with 5-PIP or DASO2. These results showed that bioactivation of DCE in murine lung occurred via the catalytic activities of both CYP2E1 and CYP2F2 and that bioactivation by these enzymes mediated the lung toxicity.

Early events in naphthalene-induced acute Clara cell toxicity. II. Comparison of glutathione depletion and histopathology by airway location

One of the presumed roles of intracellular glutathione (GSH) is the protection of cells from injury by reactive intermediates produced by the metabolism of xenobiotics. To establish whether GSH depletion is a critical step in the initiation of events that lead to cytotoxicity by P450-activated cytotoxicants, naphthalene, a well-defined Clara cell cytotoxicant, was administered to mice (200 mg/kg) by intraperitoneal injection. Shortly after injection (1, 2, and 3 h), intracellular GSH content was assessed by high performance liquid chromatography or quantitative epifluorescent imaging microscopy and compared with the degree of cytotoxicity as assessed by high resolution histopathology. In highly susceptible airways (distal bronchioles), GSH decreased by 50% in 1 h. Cytoplasmic vacuolization was not visible until 2 h, when GSH had decreased by an additional 50%. By 3 h, cytoplasmic blebbing was extensive. In minimally susceptible airways (lobar and proximal bronchi), GSH depletion varied widely within the population; a small proportion of the cells lost greater than 50% of their GSH by 2 h and a significant percentage of the cells retained most of their GSH throughout the entire 3 h. Cytoplasmic vacuolization was apparent in some of the cells at 2 h but not visible in any cells at 3 h. We conclude that (1) loss of intracellular GSH is an early event that precedes initial signs of cellular damage in Clara cell cytotoxicity; (2) this pattern of loss in relation to early injury is found both in highly susceptible and minimally susceptible airway sites; (3) there is wide cell-to-cell heterogeneity in the response; (4) the heterogeneity in the response profile varies between populations in highly susceptible and minimally susceptible sites; and (5) once the intracellular GSH concentration within the entire cell population drops below a certain threshold, the initial phase of injury becomes irreversible.

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.

Heterogeneity of clara cell glutathione. A possible basis for differences in cellular responses to pulmonary cytotoxicants

Clara-cell populations show a high degree of variation in susceptibility to injury by bioactivated cytotoxicants. Because glutathione (GSH) is critical for detoxification of electrophilic metabolites, heterogeneity in Clara cell GSH levels may lead to a wide range of cytotoxic responses. This study was designed to define the distinct GSH pools within Clara cells, characterize heterogeneity within the population, and examine whether heterogeneity contributes to susceptibility. Using fluorescent imaging combined with high-performance liquid chromatography analysis, semiquantitative measurements were obtained by evaluation of GSH using monochlorobimane and monobromobimane. In steady-state conditions, the GSH measured in isolated cells was in the femtomole range, but varied 4-fold between individual cells. Clara cells analyzed in situ and in vitro confirmed this heterogeneity. The response of these cells to compounds that modulate GSH was also variable. Diethylmaleate depleted GSH, whereas GSH monoethylester augmented it. However, both acted nonuniformly in isolated Clara cells. The depletion of intracellular GSH caused a striking decrease in cell viability upon incubation with naphthalene (NA). The sulfhydryl-binding fluorochrome BODIPY, which colocalized with tetramethylrosamine, a mitochondrial dye, demonstrated by confocal microscopy that cellular sulfhydryls are highest in the mitochondria, next-highest in cytoplasm, and lowest in the nucleus. These pools responded differently to modulators of GSH. We concluded that the steady-state intracellular GSH of Clara cells exists in distinct pools and is highly heterogeneous within the population, and that the heterogeneity of GSH levels corresponds closely to the response of Clara cells to injury by NA.

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.

Biodegradation of individual and multiple chlorinated aliphatic hydrocarbons by methane-oxidizing cultures

The microbial degradation of chlorinated and nonchlorinated methanes, ethanes, and ethanes by a mixed methane-oxidizing culture grown under chemostat and batch conditions is evaluated and compared with that by two pure methanotrophic strains: CAC1 (isolated from the mixed culture) and Methylosinus trichosporium OB3b. With the exception of 1,1-dichloroethylene, the transformation capacity (Tc) for each chlorinated aliphatic hydrocarbon was generally found to be in inverse proportion to its chlorine content within each aliphatic group (i.e., methanes, ethanes, and ethenes), whereas similar trends were not observed for degradation rate constants. Tc trends were similar for all methane-oxidizing cultures tested. None of the cultures were able to degrade the fully chlorinated aliphatics such as perchloroethylene and carbon tetrachloride. Of the four cultures tested, the chemostat-grown mixed culture exhibited the highest Tc for trichloroethylene, cis-1,2-dichloroethylene, tetrachloroethane, 1,1,1-trichloroethane, and 1,2-dichloroethane, whereas the pure batch-grown OB3b culture exhibited the highest Tc for all other compounds tested. The product toxicity of chlorinated aliphatic hydrocarbons in a mixture containing multiple compounds was cumulative and predictable when using parameters measured from the degradation of individual compounds. The Tc for each chlorinated aliphatic hydrocarbon in a mixture (Tcmix) and the total Tc for the mixture (sigma Tcmix) are functions of the individual Tc, the initial substrate concentration (S0), and the first-order rate constant (k/Ks) of each compound in the mixture, indicating the importance of identifying the properties and compositions of all potentially degradable compounds in a contaminant mixture.

Nephrotoxicity mechanism of 1,1-dichloroethylene in mice

Male Swiss OF1 mice were administered orally with a single dose (200 mg/kg) of 1,1-dichloroethylene (DCE). Examination of cryostat kidney sections stained for alkaline phosphatase (APP) revealed damage to about 50% of the proximal tubules at 8 h following DCE administration. Pretreatment with the anionic transport inhibitor probenecid by i.p., (0.75 mmol/kg, 30 min prior to and 10 min and 5 h following DCE administration) and with the gamma-glutamyltranspeptidase (GGT) inactivator acivicin by gavage and i.p. (50 mg/kg, 1 h and 30 min prior to DCE administration) failed to prevent DCE-induced renal toxicity. Pretreatment with the beta-lyase inactivator amino-oxyacetic acid (AOAA) by gavage (100 mg/kg, 30 min prior to and 10 min and 5 h following DCE administration), and with the renal cysteine conjugate S-oxidase inhibitor methimazole by i.p. (40 mg/kg, 30 min prior to DCE administration) reduced the number of damaged tubules by approximately 50 and 60%, respectively in mice treated with DCE. The results suggest that the DCE undergoes biotransformation by NADPH-cytochrome P450 to several reactive species which conjugate with glutathione (GSH). After arriving in the kidneys, the resulting conjugates reach the renal cells by a mechanism which depends on neither GGT, nor on an anionic transport system which is sensitive to probenecid. Once in the cells, the presumed GSH conjugates and/or their derivatives undergo secondary modification by beta-lyase and cysteine conjugate S-oxidase to reactive metabolite(s).

Reaction of glutathione with the electrophilic metabolites of 1,1-dichloroethylene

1,1-Dichloroethylene (DCE) requires cytochrome P450-catalyzed bioactivation to electrophilic metabolites (1,1-dichloroethylene oxide, 2-chloroacetyl chloride and 2,2-dichloroacetaldehyde) to exert its cytotoxic effects. In this investigation, we examined the reactions of these metabolites with glutathione by spectroscopic and chromatographic techniques. In view of the extreme reactivity of 2-chloroacetyl chloride, primary reactions are likely to include alkylation of cytochrome P450, conjugation with GSH to give S-(2-chloroacetyl)-glutathione, or hydrolysis to give 2-chloroacetic acid. Our results showed conjugation of GSH with 1,1-dichloroethylene oxide, through formation of the mono- and di-glutathione adducts, 2-S-glutathionyl acetate and 2-(S-glutathionyl) acetyl glutathione, respectively. The observed equilibrium constant between the hydrate of 2,2-dichloroacetaldehyde and S-(2,2-dichloro-1-hydroxy)ethylglutathione was estimated from 1H-NMR experiments to be 14 +/- 2 M-1. Thus, 2,2-dichloroacetaldehyde is unlikely to make a significant contribution to GSH depletion as GSH concentrations above normal physiological levels would be necessary to form significant amounts of S-(2,2-dichloro-1-hydroxy)ethylglutathione. We also compared the formation of the glutathione conjugates in rat and mouse liver microsomes using 14C-DCE. The results demonstrated a species difference; the total metabolite production was 6-fold higher in microsomes from mice, compared with samples from rat. Production of DCE metabolites in hepatic microsomes from acetone-pretreated mice was 3-fold higher than those from untreated mice suggesting a role for P450 2E1 in DCE bioactivation. These results indicate that the epoxide is the major metabolite of DCE that is responsible for GSH depletion, suggesting that it may be involved in the hepatotoxicity evoked by DCE. Furthermore, this metabolite is formed to a greater extent in mouse than in rat liver microsomes and this difference may underlie the enhanced susceptibility found in the former species.

Methylene chloride: an inhalation study to investigate toxicity in the mouse lung using morphological, biochemical and Clara cell culture techniques

Single exposures of mice to methylene chloride (MC) cause vacuolation and necrosis of the bronchiolar Clara cells which subsequently recover normal morphology on continued exposure. Both cytochrome P-450 (CYP)- and glutathione S-transferase (GST)-dependent metabolism of MC are known to occur. The current studies have investigated the metabolism of MC in mouse lung using inhibitors of both GST and CYP-dependent routes of metabolism, the consequences of metabolic inhibition on the Clara cell vacuolation, and any changes in cell proliferation, assessed in vitro, in Clara cells cultured from exposed individuals. Vacuolated bronchiolar cells were seen in mice exposed to 2000 and 4000 ppm MC but were not seen at lower concentrations, while addition of the CYP inhibitor, piperonyl butoxide, significantly reduced the bronchiolar cell vacuolation seen following exposure to 2000 ppm MC. Treatment of mice with the glutathione depletor, buthionine sulphoximine, had no effect on the number of vacuolated bronchiolar cells following MC. Exposure of mice to 1000 ppm MC and above for 6 h caused a burst of DNA synthesis in bronchiolar Clara cells cultured in vitro from the lungs of exposed animals. The results suggest that the Clara cell vacuolation following MC exposure is mediated via CYP metabolism, that depression of the CYP metabolic pathway occurs following exposure, and that Clara cell vacuolation may have a priming role in stimulating cell proliferation in the unaffected cell population.