Potentiation of occupational carbon tetrachloride toxicity by ethanol abuse

Evaluation of the carcinogenicity of carbon tetrachloride

Carbon tetrachloride (CCl4) has been extensively used and reported to produce toxicity, most notably involving the liver. Carbon tetrachloride metabolism involves CYP450-mediated bioactivation to trichloromethyl and trichloromethyl peroxy radicals, which are capable of macromolecular interaction with cell components including lipids and proteins. Radical interaction with lipids produces lipid peroxidation which can mediate cellular damage leading to cell death. Chronic exposure with CCl4 a rodent hepatic carcinogen with a mode of action (MOA) exhibits the following key events: 1) metabolic activation; 2) hepatocellular toxicity and cell death; 3) consequent regenerative increased cell proliferation; and 4) hepatocellular proliferative lesions (foci, adenomas, carcinomas). The induction of rodent hepatic tumors is dependent upon the dose (concentration and exposure duration) of CCl4, with tumors only occurring at cytotoxic exposure levels. Adrenal benign pheochromocytomas were also increased in mice at high CCl4 exposures; however, these tumors are not of relevant importance to human cancer risk. Few epidemiology studies that have been performed on CCl4, do not provide credible evidence of enhanced risk of occurrence of liver or adrenal cancers, but these studies have serious flaws limiting their usefulness for risk assessment. This manuscript summarizes the toxicity and carcinogenicity attributed to CCl4, specifically addressing MOA, dose-response, and human relevance.

Antioxidant Potential of Parsley Leaf (Petroselinum crispum) Essential Oil on Hypothyroidism and Testicular Injury in Mice Intoxicated by Carbon Tetrachloride

The aim of the present study was to investigate the ameliorative potential of parsley (Petroselinum crispum) leaf essential oil (PO) against the detrimental effects of carbon tetrachloride (CCl₄) on the thyroid gland and testes of mice. Twenty-four adult male mice were divided into four groups and treated for 4 weeks. The 1^st control group received 3 mL/kg olive oil intraperitoneally, twice a week followed by 0.5 mL/kg saline intragastrically daily. The 2^nd CCl₄ group received CCl₄ (3 mL/kg intraperitoneally, twice a week). The 3^rd PO group received PO (0.5 mL/kg intragastrically daily), while the 4^th CCl₄+PO group received CCl₄ coadministrated with PO at the aforementioned doses. CCl₄ group recorded significant (p < 0.05) reduction in the activities of antioxidant enzyme catalase (CAT) and superoxide dismutase (SOD) and significant (p < 0.05) increase in the lipid peroxidation end product's level malondialdehyde (MDA) in the testes and thyroid glands. Meanwhile, serum levels of testosterone, follicle-stimulating hormone (FSH), luteinizing hormone (LH), and thyroid hormones (thyroid-stimulating hormone (TSH), total triiodothyronine (T3), free triiodothyronine (fT3), total thyroxine (T4), and free thyroxine (fT4)) significantly decreased. Also, histopathologically, the testicular tissue showed hypospermatogenesis within irregular-shaped seminiferous tubules with prominent edema in the interstitial spaces confirming the aforementioned biochemical alterations. Treatment with PO significantly reduced the testicular and thyroid oxidative stress (p < 0.05) and elevated the testosterone (73.47%), FSH (92.11%), LH (33.33%), T3 (23.47%), fT3 (39.13%), T4 (27.91%), and fT4 (75%) as compared to that of CCl₄-treated group corresponding values. The PO GC/MS analysis indicated bioactive monoterpenes (major component is 1,3,8-mentha triene 34.48%) and phenylpropenes (major component is myristicin 21.04%). Results suggested the ameliorative effect of PO against CCl₄-induced hypogonadism in mice by suppressing oxidative stress and maintaining thyroid gland function.

Diminished CCl4 -induced hepatocellular carcinoma, oxidative stress, and apoptosis by co-administration of curcumin or selenium in mice

Hepatocellular carcinoma (HCC) is a lethal disease, and in HCC advanced stages, there is limited therapeutic efficacy. HCC results in a complication of fibrosis or cirrhosis. In this study, the protective effect of curcumin and selenium versus hepatocellular carcinoma caused by CCl₄ in experimental animals was investigated. In all, 70 mice were divided into seven groups to study the effect of curcumin and selenium on CCl₄ -induced hepatocellular carcinoma. After treatment time, different animal groups were sacrificed, serum and liver samples were collected and processed for assay of biochemical and molecular parameters. Our results showed that CCl₄ administration induced various alterations such as significant elevation in the serum levels of ALT, AST, and hepatic contents of malondialdehyde (MDA), and depletion in the levels of antioxidant parameters. CCl₄ induced apoptosis in the hepatic cells indicated by an increased level of p53, CD4, CD8, Bax, and Annexin V/PI in addition to significant decrease in the level of Bcl-2. Administration of curcumin and selenium restored this abnormal variation in these biochemical parameters to normal values. Our study addressed that curcumin or selenium may be helpful in the protection against liver damage induced by CCl₄ . The hepatoprotective impact of curcumin or selenium might be mediated primarily by its potent antioxidant activity. PRACTICAL APPLICATIONS: Hepatocellular carcinoma (HCC) ranked third common cause of death, primary liver cancer. Exposure to CCl₄ was found to induce significant hepatotoxicity, characterized by fibrosis, bile duct proliferation, cirrhosis, and reduced hepatic function The work was prepared to investigate the protecting capacity of curcumin, selenium alone, and in combination against HCC induced by CCl₄ in the experimental animal model. This study proved the protective effect of curcumin and selenium, alone and in combination with each other, where curcumin showed multiple pharmacological activities, including anti-inflammation and antioxidant, and have an essential role in inhibiting the progression of HCC.

EASL Clinical Practice Guideline: Occupational liver diseases

A variety of chemicals have been linked to occupational liver diseases, including several solvents and mixtures thereof, pesticides, and metals. Workplace exposures have been associated with virtually the entire spectrum of acute and chronic liver diseases. However, their prevalence is inadequately quantified and their epidemiology limited. Occupational liver diseases may result from high accidental or from prolonged lower level exposures. Whereas the former is uncommon and easily recognised, the latter are relatively more frequent but often overlooked because they may display normal values of conventional markers, have an insidious onset and be asymptomatic or be obfuscated and confounded by concurrent conditions. In addition, specific tests of toxicity are not available, histopathology may not be revealing and the assessment of internal dose of chemicals is usually not decisive. Given these circumstances, the diagnosis of these liver disorders is challenging, one of exclusion and often requires an interdisciplinary approach. These recommendations offer a classification of the type of liver injuries associated with occupational exposures - based in part on the criteria for drug-induced liver injury - a grading of their severity, and the diagnostic and preventive criteria for chemically induced occupational liver disease.

Toxic effects of combined treatment of 1,2-dichloroethane and ethanol on mouse brain and the related mechanisms

The aim of this study was to explore the mechanisms of brain damage induced by the combined treatment of mice with 1,2-dichloroethane (1,2-DCE) and ethanol. Mice were divided into control group; 1,2-DCE-intoxicated group; ethanol-treated group; and low-, medium-, and high-dose combined treatment groups. Histological observations along with brain organ coefficients and water content were used to measure the brain damage directly and indirectly. The levels of nonprotein sulfhydryls, malondialdehyde (MDA), and superoxide dismutase activity were used as parameters to evaluate oxidative stress in the brain. Protein and messenger RNA (mRNA) levels of cytochrome P450 2E1 (CYP2E1), zonula occludens-1 (occludin and zo-1), aquaporin-4 (AQP4), nuclear factor erythroid 2-related factor 2 (Nrf2), heme oxygenase (HO)-1, and the γ-glutamyl cysteine synthetase catalytic and modulatory subunits (γ-GCSc, GR, and γ-GCSm) in the brain were examined by Western blot analysis and quantitative polymerase chain reaction analysis, respectively. Effects of the combined treatment of 1,2-DCE and ethanol were evaluated by analysis of variance with a factorial design. The results suggested that combined exposure to ethanol and 1,2-DCE synergistically increased CYP2E1 protein and mRNA levels, accelerated the metabolism of ethanol and 1,2-DCE in the brain tissue, induced high production of reactive oxygen species (ROS), and increased MDA levels, thereby damaging the blood-brain barrier and causing obvious pathological changes in brain tissue. However, the increased level of ROS activated the Nrf2 signal transduction pathway, promoting the expression of HO-1 and glutathione-related antioxidant enzymes in the brain to protect the cells from oxidative damage.

Carbon tetrachloride-induced hepatic and renal damages in rat: inhibitory effects of cacao polyphenol

Here, we investigated the protective effect of cacao polyphenol extract (CPE) on carbon tetrachloride (CCl4)-induced hepato-renal oxidative stress in rats. Rats were administered CPE for 7 days and then received intraperitoneal injection of CCl4. Two hours after injection, we found that CCl4 treatment significantly increased biochemical injury markers, lipid peroxides (phosphatidylcholine hydroperoxide (PCOOH) and malondialdehyde (MDA)) and decreased glutathione peroxidase activity in kidney rather than liver, suggesting that kidney is more vulnerable to oxidative stress under the present experimental conditions. CPE supplementation significantly reduced these changes, indicating that this compound has antioxidant properties against CCl4-induced oxidative stress. An inhibitory effect of CPE on CCl4-induced CYP2E1 mRNA degradation may provide an explanation for CPE antioxidant property. Together, these results provide quantitative evidence of the in vivo antioxidant properties of CPE, especially in terms of PCOOH and MDA levels in the kidneys of CCl4-treated rats.

Trichloroethylene risk assessment: a review and commentary

Trichloroethylene (TCE) is a widespread environmental contaminant that is carcinogenic when given in high, chronic doses to certain strains of mice and rats. The capacity of TCE to cause cancer in humans is less clear. The current maximum contaminant level (MCL) of 5 ppb (microg/L) is based on an US Environment Protection Agency (USEPA) policy decision rather than the underlying science. In view of major advances in understanding the etiology and mechanisms of chemically induced cancer, USEPA began in the late 1990s to revise its guidelines for cancer risk assessment. TCE was chosen as the pilot chemical. The USEPA (2005) final guidelines emphasized a "weight-of-evidence" approach with consideration of dose-response relationships, modes of action, and metabolic/toxicokinetic processes. Where adequate data are available to support reversible binding of the carcinogenic moiety to biological receptors as the initiating event (i.e., a threshold exists), a nonlinear approach is to be used. Otherwise, the default assumption of a linear (i.e., nonthreshold) dose-response is utilized. When validated physiologically based pharmacokinetic (PBPK) models are available, they are to be used to predict internal dosimetry as the basis for species and dose extrapolations. The present article reviews pertinent literature and discusses areas where research may resolve some outstanding issues and facilitate the reassessment process. Key research needs are proposed, including role of dichloroacetic acid (DCA) in TCE-induced liver tumorigenesis in humans; extension of current PBPK models to predict target organ deposition of trichloroacetic acid (TCA) and DCA in humans ingesting TCE in drinking water; use of human hepatocytes to ascertain metabolic rate constants for use in PBPK models that incorporate variability in metabolism of TCE by potentially sensitive subpopulations; measurement of the efficiency of first-pass elimination of trace levels of TCE in drinking water; and assessment of exogenous factors' (e.g., alcohol, drugs) ability to alter metabolic activation and risks at such low-level exposure.

Inhibition and induction of human cytochrome P450 enzymes: current status

Variability of drug metabolism, especially that of the most important phase I enzymes or cytochrome P450 (CYP) enzymes, is an important complicating factor in many areas of pharmacology and toxicology, in drug development, preclinical toxicity studies, clinical trials, drug therapy, environmental exposures and risk assessment. These frequently enormous consequences in mind, predictive and pre-emptying measures have been a top priority in both pharmacology and toxicology. This means the development of predictive in vitro approaches. The sound prediction is always based on the firm background of basic research on the phenomena of inhibition and induction and their underlying mechanisms; consequently the description of these aspects is the purpose of this review. We cover both inhibition and induction of CYP enzymes, always keeping in mind the basic mechanisms on which to build predictive and preventive in vitro approaches. Just because validation is an essential part of any in vitro-in vivo extrapolation scenario, we cover also necessary in vivo research and findings in order to provide a proper view to justify in vitro approaches and observations.

Rat hepatic CYP2E1 is induced by very low nicotine doses: an investigation of induction, time course, dose response, and mechanism

CYP2E1 is an ethanol- and drug-metabolizing enzyme that can also activate procarcinogens and hepatotoxicants and generate reactive oxygen species; it has been implicated in the pathogenesis of liver diseases and cancer. Cigarette smoke increases CYP2E1 activity in rodents and in humans and we have shown that nicotine (0.1-1.0 mg/kg s.c. x 7 days) increases CYP2E1 protein and activity in the rat liver. In the current study, we have shown that the induction peaks at 4 h postnicotine (1 mg/kg s.c. x 7 days) treatment and recovers within 24 h. No induction was observed after a single injection, and 18 days of treatment did not increase the levels beyond that found at 7 days. We found that CYP2E1 is induced by very low doses of chronic (x 7 days) nicotine with an ED50 value of 0.01 mg/kg s.c.; 0.01 mg/kg in a rat model results in peak cotinine levels (nicotine metabolite) similar to those found in people exposed to environmental tobacco smoke (passive smokers; 2-7 ng/ml). Previously, we have shown no change in CYP2E1 mRNA, and our current mechanistic study indicates that nicotine does not regulate CYP2E1 expression by protein stabilization. We postulated that a nicotine metabolite could be causing the induction but found that cotinine (1 mg/kg x 7 days) did not increase CYP2E1. Our findings indicate that nicotine increases CYP2E1 at very low doses and may enhance CYP2E1-related toxicity in smokers, passive smokers, and people treated with nicotine (e.g., smokers, patients with Alzheimer's disease, ulcerative colitis or Parkinson's disease).

The use of gene knockout mice to unravel the mechanisms of toxicity and chemical carcinogenesis

Metabolism of toxins and carcinogens is carried out by large groups of xenobiotic-metabolizing enzymes. These enzymes are generally considered to be required for elimination of xenobiotics such as drugs, dietary chemicals and environmental pollutants, and to be required for chemical toxicity and carcinogenicity. An important role for these enzymes in metabolism of endogenous chemicals has not been established. Mouse lines in which the genes encoding several xenobiotic-metabolizing enzymes were knocked out were produced and are being used to determine the role of metabolism in carcinogenesis, and acute and chronic toxicities in vivo. Mouse lines lacking the P450s CYP1A1, CYP1A2, CYP1B1 and CYP2E1, microsomal epoxide hydrolase (mEH), NADPH:quinone oxidoreductase and the glutathione S-transferase P1 have no deleterious phenotypes, indicating that these enzymes are not required for mammalian development and physiological homeostasis. However, when challenged with toxins and carcinogens, they respond differently from their wild-type (WT) counterparts. For example, mice lacking CYP1A2 and CYP2E1 are totally resistant to acetaminophen-induced hepatotoxicity. Mice lacking CYP1B1 or mEH are less responsive to tumorigenesis by 7,12-dimethybenz[a]anthracene. However, CYP1A2-null mice do not significantly differ from WT mice in their response to the hepatocarcinogen 4-aminobiphenyl. These and other studies indicate that the xenobiotic-metabolism null mice are of great value in the study of the mechanisms of chemical injury.

Identification of a possible association between carbon tetrachloride-induced hepatotoxicity and interleukin-8 expression

Hepatotoxicants can elicit liver damage by various mechanisms that can result in cell necrosis and death. The changes induced by these compounds can vary from gross alterations in DNA repair mechanisms, protein synthesis, and apoptosis, to more discrete changes in oxidative damage and lipid peroxidation. However, little is known of the changes in gene expression that are fundamental to the mechanisms of hepatotoxicity. We have used DNA microarray technology to identify gene transcription associated with the toxicity caused by the hepatotoxicant carbon tetrachloride. Labeled poly A+ RNA from cultured human hepatoma cells (HepG2) exposed to carbon tetrachloride for 8 hours was hybridized to a human microarray filter. We found that 47 different genes were either upregulated or downregulated more than 2-fold by the hepatotoxicant compared with dimethyl formamide, a chemical that does not cause liver cell damage. The proinflammatory cytokine interleukin-8 (IL-8) was upregulated over 7-fold compared with control on the array, and this was subsequently confirmed at 1 hour and 8 hours by Northern blot analyses. We also found that carbon tetrachloride caused a time-dependent increase in interleukin-8 protein release in HepG2 cells, which was paralleled by a decrease in cell viability. These data demonstrate that carbon tetrachloride causes a rapid increase in IL-8 mRNA expression in HepG2 cells and that this increase correlates with a later and significant increase in the levels of interleukin-8 protein. These results illustrate the potential of microarray technology in the identification of novel gene changes associated with toxic processes.

Cytochrome P450 2E1 is the primary enzyme responsible for low-dose carbon tetrachloride metabolism in human liver microsomes

We examined which human CYP450 forms contribute to carbon tetrachloride (CCl(4)) bioactivation using hepatic microsomes, heterologously expressed enzymes, inhibitory antibodies and selective chemical inhibitors. CCl(4) metabolism was determined by measuring chloroform formation under anaerobic conditions. Pooled human microsomes metabolized CCl(4) with a K(m) of 57 microM and a V(max) of 2.3 nmol CHCl(3)/min/mg protein. Expressed CYP2E1 metabolized CCl(4) with a K(m) of 1.9 microM and a V(max) of 8.9 nmol CHCl(3)/min/nmol CYP2E1. At 17 microM CCl(4), a monoclonal CYP2E1 antibody inhibited 64, 74 and 83% of the total CCl(4) metabolism in three separate human microsomal samples, indicating that at low CCl(4) concentrations, CYP2E1 was the primary enzyme responsible for CCl(4) metabolism. At 530 microM CCl(4), anti-CYP2E1 inhibited 36, 51 and 75% of the total CCl(4) metabolism, suggesting that other CYP450s may have a significant role in CCl(4) metabolism at this concentration. Tests with expressed CYP2B6 and inhibitory CYP2B6 antibodies suggested that this form did not contribute significantly to CCl(4) metabolism. Effects of the CYP450 inhibitors alpha-naphthoflavone (CYP1A), sulfaphenazole (CYP2C9) and clotrimazole (CYP3A) were examined in the liver microsome sample that was inhibited only 36% by anti-CYP2E1 at 530 microM CCl(4). Clotrimazole inhibited CCl(4) metabolism by 23% but the other chemical inhibitors were without significant effect. Overall, these data suggest that CYP2E1 is the major human enzyme responsible for CCl(4) bioactivation at lower, environmentally relevant levels. At higher CCl(4) levels, CYP3A and possibly other CYP450 forms may contribute to CCl(4) metabolism.

Resistance to carbon tetrachloride-induced hepatotoxicity in mice which lack CYP2E1 expression

CYP2E1 knockout mice (cyp2e1-/-) were used to investigate the involvement of CYP2E1 in the development of carbon tetrachloride (CCl4)-induced hepatotoxicity. Male cyp2e1-/- and wild-type (cyp2e1+/+) mice were given a single i.p. injection of 1 ml/kg (= 1.59 g/kg) CCl4 and 24 h later liver injury was assessed by elevations of serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST) activities and histopathology. No significant increases in serum ALT and AST activities were observed in cyp2e1-/- mice when compared to wild-type counterparts after CCl4 exposure. No detectable abnormality in liver histology was found in cyp2e1-/- mice after CCl4 exposure. In contrast, CCl4 treatment resulted in 442- and 125-fold increases in serum ALT and AST activities, respectively, in wild-type mice. Consistent with the results of serum ALT and AST activities, severe hepatic damage was noted in livers of wild-type mice, indicating the importance of CYP2E1 in mediating the hepatic damage following CCl4 exposure in these mice. In addition, a dramatic decrease in CYP2E1-catalyzed p-nitrophenol activity and complete loss of immunoreactive CYP2E1 were observed in wild-type mice after CCl4 treatment, suggesting that CYP2E1 was degraded during the process of CCl4-induced hepatotoxicity. These studies conclusively demonstrate that CYP2E1 is the major factor involved in the CCl4-induced hepatotoxicity in mice.