Selective damage to nonciliated bronchiolar epithelial cells in relation to impairment of pulmonary monooxygenase activities by 1,1-dichloroethylene in mice

1,1-Dichloroethylene-Induced Mitochondrial Damage Precedes Apoptotic Cell Death of Bronchiolar Epithelial Cells in Murine Lung

1,1-Dichloroethylene (DCE) causes pulmonary injury that is characterized by necrosis of bronchiolar Clara cells. Mitochondria have been identified as an early target in the toxic response. Because mitochondria have been implicated in both necrotic and apoptotic cell death, we have undertaken studies to test the hypothesis that DCE induces apoptosis, in addition to necrosis, in murine lung. A primary objective is to identify the biochemical events associated with pulmonary apoptosis. Groups of female CD-1 mice were treated with DCE (75 mg/kg i.p.) or corn oil. Using an antibody directed against DCE-cysteine conjugates, adducts were detected primarily in association with mitochondria in the apices of bronchiolar Clara cells. Furthermore, morphological studies demonstrated early mitochondrial alterations in Clara cells that included severe swelling and disruption of cristae. Western blotting of lung cytosolic proteins showed greater immunoreactivity for cytochrome c in fractions from mice treated with DCE for 4 h than in controls. Immunohistochemical studies with an antibody to activated caspase-3 and terminal deoxynucleotidyl transferase dUTP nick-end labeling were used to detect apoptotic cells. In both experiments, positive reactivities were observed in the bronchiolar epithelium at 12 and 24 h after DCE treatment, whereas reactivities were absent in tissues from control animals. Finally, bronchiolar epithelial cells showing morphological criteria of apoptosis (chromatin condensation and margination) were observed at 24 h after 75 and 125 mg/kg DCE. Apoptotic-like cells were more abundant in larger bronchioles. These data suggested that DCE produces pulmonary bronchiolar apoptosis by inducing mitochondrial perturbations, causing release of cytochrome c into the cytosol and caspase activation.

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.

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.

Ultrastructural changes in rat Clara cells induced by bis(4-amino-3-methylcyclohexyl)methane

Repeated oral administration of bis(4-amino-3-methylcyclohexyl)methane to rats induced unusual ultrastructural changes in Clara cells of the bronchiolar epithelium, involving marked accumulation of electron-dense inclusion bodies with a lamellar structure in the cytoplasm. The appearance of the inclusion bodies was speculated to result from accumulation of complexes formed between the agent and phospholipids within lysosomes, a condition known as drug-induced lipidosis. This finding appeared noteworthy, since it might reflect high potential for metabolism of xenobiotic compounds in Clara cells.

In vitro studies on the metabolism and covalent binding of [14C]1,1-dichloroethylene by mouse liver, kidney and lung

The metabolism and covalent binding of 1,1-dichloro[1,2-14C]ethylene (DCE) to subcellular fractions of liver, kidney and lung of C57BL/6N mice have been investigated in vitro. Covalent binding was NADPH- and cytochrome P-450-dependent. The microsomal fraction bound more radiolabel than any other subcellular fraction, and the levels of covalent binding in cell fractions correlated well with their cytochrome P-450 content. Covalent binding by mouse liver and lung microsomes also reflected their cytochrome P-450 content. However, although mouse kidney microsomes contained twice as much total cytochrome P-450 as the lung, no detectable covalent binding of DCE-derived radioactivity occurred in kidney. Omission of NADPH, heat inactivation of microsomes, carbon monoxide, addition of SKF-525A, piperonyl butoxide or reduced glutathione (GSH), all inhibited (40-90%) covalent binding of radiolabel to liver and lung microsomes. The absence of O2 (incubation under N2) did not greatly affect the metabolism and covalent binding. Pretreatment of mice with various inducers, phenobarbital (PB), beta-naphthoflavone (beta-NF), pregnenolone 16 alpha-carbonitrile (PCN) and 3-methylcholanthrene (3-MC), evoked increases in total liver microsomal cytochrome P-450 content (2-fold) and corresponding increases in covalent binding (3-fold). However, microsomes from PCN-treated mice showed only a 50% increase in DCE binding. Kidney microsomes from control, PB-, and beta-NF-pretreated mice were incapable of covalent binding of radiolabel but those from PCN- and 3-MC-pretreated mice showed levels of binding similar to untreated mouse lung microsomes. It is proposed that the nephrotoxicity of DCE may be due to translocation of reactive metabolites from the liver to the kidney.

Effects of administration of metabolic inducers and inhibitors on pulmonary toxicity and covalent binding by 1,1-dichloroethylene in CD-1 mice

The administration of 1,1-dichloroethylene (1,1-DCE, 125 mg/kg ip) to CD-1 mice caused bronchiolar necrosis, which was accompanied by substantial covalent binding of radiolabeled compound and/or metabolite to lung. Lung injury and covalent binding were not modified by phenobarbital pretreatment. However, 3-methylcholanthrene provided a protective influence but failed to alter covalent binding to lung macromolecules. Prior administration with the metabolic inhibitors, piperonyl butoxide and SKF 525-A, produced differential effects. While piperonyl butoxide exacerbated bronchiolar injury by 1,1-DCE, covalent binding remained unaltered. In contrast, SKF 525-A protected from lung damage and significantly decreased covalent binding. Hepatic necrosis was relatively mild, and was not observed in all animals treated with 1,1-DCE. Although the hepatic lesion was not modified by phenobarbital, liver injury was slightly diminished by 3-methylcholanthrene. The inducers, piperonyl butoxide and SKF 525-A, enhanced liver necrosis, with the latter eliciting more severe effects than the former agent. Covalent binding to liver tissues was not significantly changed by pretreatment with either inducers or inhibitors. These results indicate that lack of an unequivocal correlation of cellular injury with covalent binding, but suggest that metabolism may be involved in the pneumotoxicity by 1,1-DCE. The influence and modification of lung injury by the liver, however, remain to be further elucidated.

Comparative cytotoxicity of naphthalene and its monomethyl- and mononitro-derivatives in the mouse lung

Male Swiss-Webster mice were exposed to naphthalene, 1-methyl-, 2-methyl-, 1-nitro- and 2-nitronaphthalene by intraperitoneal injection of peanut oil solutions over a dose range of 0.5-3.0 mmol kg-1 body weight. Treated mice were killed at times from 6 hours to 14 days post-treatment. Tissues were analyzed for cytotoxic effects by optical and electron microscopy, and for cell proliferation by autoradiography following in vitro labeling of lung slices with 3H-thymidine. The naphthalene derivatives varied widely in their cytotoxic effects. The most toxic was 1-nitronaphthalene with no mice surviving doses greater than 1 mmol kg-1. Naphthalene and 2-methylnaphthalene were about equally toxic, followed by 2-nitro- and 1-methylnaphthalene, in decreasing order of toxicity. In all cases the first evidence of cytotoxic effects was seen in the Clara cells of the bronchiolar epithelium, and, at the highest doses, toxic effects were found in the adjacent ciliated cells. Changes could be detected at the ultrastructural level at all doses, and within 6 hours after treatment. Only slight effects were seen in other cell types. Increased cell proliferation following chemical treatment was seen only in the bronchiolar epithelium, among cells tentatively identified as Clara cells or their precursors. Cytotoxic effects of naphthalene and its 1- and 2-methyl derivatives were confined to the lung, with minimal evidence of toxicity in the liver and kidney. The mononitronaphthalenes both produce small areas of centrizonal necrosis in the liver, but no discernible effects in the kidney. The experiments demonstrate the effect of small structural differences on the cytotoxicity of this group of environmental pollutants and also illustrate the sensitivity of the Clara cell as a target for xenobiotics.

Protection by methylprednisolone against butylated hydroxytoluene-induced pulmonary damage and impairment of microsomal monooxygenase activities in the mouse: lack of effect on fibrosis

The effects of the synthetic corticosteroid methylprednisolone (MP; 30 mg/kg, s.c. given twice daily for 3 days), on the pneumotoxic effects of a single dose of butylated hydroxytoluene (BHT; 400 mg/kg, i.p.) over a 10 day experimental period was investigated in male C57BL/6N mice. BHT alone caused time-dependent alveolar hypercellularity, inflammatory infiltration, alveolar septal thickening and hypercellularity of the bronchiolar epithelium, reaching a maximum by day 5 with some degree of recovery by day 10. The pulmonary monooxygenase activities reflected the degree of alveolar damage and Clara cell abnormality with time; reductions in monooxygenase activities occurred and minimum levels (7-15% of control) were reached by day 5 and again a trend towards recovery by day 10. MP administered 0, 24 and 48 hr after BHT treatment partially protected mice from these effects of BHT in a distinctly time-dependent fashion; the degree of protection decreased as the time between BHT challenge and MP treatment increased. Although MP alone did not morphologically affect Clara and alveolar cells, it increased, decreased or had no effect on the monooxygenase activities. About 25% of the mice that received BHT alone died by day 5 and 50% by day 10. MP completely blocked the lethal effects of BHT by day 5 and reduced the deaths to between 15% and 25% by day 10. Interestingly, MP did not protect against the BHT-induced pulmonary fibrosis, measured as total lung hydroxyproline content, irrespective of the time between BHT challenge and MP treatment. MP alone did not cause any deaths nor increase lung hydroxyproline content.

Studies on the distribution and covalent binding of 1,1-dichloroethylene in the mouse. Effect of various pretreatments on covalent binding in vivo

The distribution and covalent binding of a single dose of [1,2-14C] 1,1-dichloroethylene (DCE; 125 mg/kg, i.p.) was studied in male C57Bl/6N mice. Total radioactivity was distributed in whole homogenates of all tissues studied, with peak levels occurring within 6 hr. Covalent binding of radioactive material peaked at 6-12 hr in all tissues, and highest levels were found in kidney, liver, and lung with smaller amounts in skeletal muscle, heart, spleen, and gut. Covalent binding in kidney, liver, and lung fell to 50% of peak levels in about 4 days. Between 12 hr and 4 days after DCE administration, 70-100% of total radioactivity present in homogenates of kidney, liver, and lung was covalently bound. The three tissues showed a similar spread in total radioactivity in subcellular fractions 24 hr after exposure to DCE; most of the radioactivity was covalently bound (60-100%) and distributed fairly uniformly with a slight tendency to concentrate in the mitochondrial fraction. Phenobarbital (PB) and 3-methylcholanthrene (3-MC) pretreatments increased the covalent binding in the liver and lung but had no effect in the kidney. Piperonyl butoxide and SKF-525A decreased the covalent binding in liver and lung, but the latter increased binding in the kidney while the former decreased it. Diethylmaleate administration increased the covalent binding (2- to 3-fold) in all three tissues as well as increasing lethal toxicity. These results are consistent with the view that DCE is metabolized to some reactive intermediate(s) which may be detoxified by conjugation with glutathione.

Differences in naphthalene-induced toxicity in the mouse and rat

Following the intraperitoneal administration of naphthalene (200 mg/kg) to mice, the lung, in comparison with other organs, was selectively damaged. Histological examination of the lungs showed that it was the non-ciliated, bronchiolar epithelial cells (Clara cells) which were damaged. At higher doses (400 mg/kg and 600 mg/kg, i.p.), there was also damage to the cells in the proximal tubules of the kidney. In contrast to the effect in mice, doses of naphthalene as high as 1600 mg/kg (i.p.) caused no detectable pulmonary or renal damage in the rat. This difference in toxicity between the mouse and rat was reflected by the ability of naphthalene to more severely deplete the non-protein sulphydryls in the mouse lung and kidney than in those organs in the rat. In order to investigate the species difference in toxicity, the metabolism of naphthalene by lung and liver microsomes of the mouse and rat was studied. In all cases, naphthalene was metabolised to a covalently bound product(s) and to two major methanol-soluble products, which co-chromatographed with 1-naphthol and 1,2-dihydro-1,2-dihydroxynaphthalene. However, both the covalent binding and metabolism were approximately 10-fold greater in microsomes prepared from mouse lung compared with those from the rat. This observation may in part explain the difference in toxicity of naphthalene to the mouse and rat lung. As 1-naphthol is a major metabolite of naphthalene and previous work had suggested that most of the microsomal catalysed binding of naphthalene was due to further oxidation of 1-naphthol, the role of 1-naphthol in mediating the naphthalene-induced toxicity was investigated. In neither the mouse nor the rat did 1-naphthol cause a depletion of non-protein sulphydryl levels or tissue damage in the liver, lung or kidney. Thus the toxicity of naphthalene does not appear to be mediated via 1-naphthol.

Lung injury induced by trichloroethylene

Trichloroethylene (TCE) produced bronchiolar damage when administered to mice. Administration of 2000 mg/kg caused injury in Clara cells of the bronchiolar epithelium, which was observed at 24 h following TCE treatment; increase of the dosage to 2500 mg/kg induced additionally, alterations in alveolar Type II cells of the parenchyma. Specifically, lamellar bodies were reduced in number and microvilli displayed distorted protrusions. The increase in severity of cellular injury with higher dosages of TCE coincided with increased accumulation of pulmonary calcium and lengthened anesthesia recovery times following TCE-induced anesthesia. Time-course studies conducted with 2000 mg/kg demonstrated rapid and marked reduction in pulmonary microsomal cytochrome P-450 content and aryl hydrocarbon hydroxylase activity. Significant decreases were observed as early as 1 h, and the levels were still depressed at 24 h following TCE treatment. Hepatic necrosis was relatively mild at the dosages of TCE examined. These results demonstrate that TCE is pneumotoxic and affects Clara and alveolar Type II cells.