Upstream eventActivation of Cyp2E1 in the liver
Key Event Relationship Overview
AOPs Referencing Relationship
|AOP Name||Directness||Weight of Evidence||Quantitative Understanding|
|Chronic Cyp2E1 Activation Leading to Liver Cancer||indirectly leads to||Strong||Not Specified|
Life Stage Applicability
How Does This Key Event Relationship Work
Metabolism of xenobiotics by cytochrome P450 mono-oxygenases produces reactive metabolites. Under normal circumstances, these metabolites immediately become conjugated to molecules like glutathione or glucuronic acid, which facilitates their excretion. However, these metabolites can react with off-target cellular molecules, which in extreme cases (e.g., at toxic doses or following glutathione depletion during periods of oxidative stress) cause damage that results in cytotoxicity. Typically, the unmetabolised Cyp2E1 substrates are inert, whereas their metabolites are highly cytotoxic; e.g., furan and its metabolite cis-2-butene-1,4-dial (BDA); ethanol (EtOH) and acetaldehyde; carbon tetrachloride and trichloromethyl radical (which forms the trichloromethyl peroxy radical); and, chloroform and phosgene. Lipid peroxidation in the context of Cyp2E1 has been reviewed (Caro and Cederbaum 2004). Moreover, chronic exposure to Cyp2E1 agonists depletes of conjugating enzymes and diminishes capacity to deal with reactive metabolites in the cell.
Weight of Evidence
Strong. Metabolite-dependent toxicity and adduct formation are well known side-effects of cytochrome P450 mono-oxygenase metabolism of xenobiotics in the liver.
Empirical Support for Linkage
Strong. There is a large amount of published data that demonstrate the cytotoxic effects of Cyp2E1 substrates following metabolic activation.
Cyp2E1 converts furan to BDA. While furan can be tolerated by cells at extremely high doses, BDA is cytotoxic at much lower doses (Kellert, et al. 2008). Furan treatment causes dose-dependent increases in hepatotoxicity in mice (Moser, et al. 2009), which can be prevented by co-treatment with a Cyp2E1 inhibitor (Fransson-Steen, et al. 1997).
Cyp2E1 converts carbon tetrachloride to the trichloromethyl radical. Trichloromethyl radical reacts with oxygen to form the trichloromethyl peroxy radical. Carbon tetrachloride-dependent cytotoxicity is preventable by pre-treatment with cytochrome P450 inhibitors (Bechtold, et al. 1982, Letteron, et al. 1990, Martinez, et al. 1995), and is exacerbated in cells that over-express Cyp2E1 (Takahashi, et al. 2002). Carbon tetrachloride produces hepatotoxicity in wild-type mice, but not in Cyp2E1-null mice (Wong, et al. 1998).
Cyp2E1 converts chloroform to phosgene. Phosgene protein adducts are co-localized to regions of chloroform-dependent cytotoxicity (Fabrizi, et al. 2001, Ilett, et al. 1973) and protein adducts occur prior to hepatotoxicity (Stevens and Anders 1981). Chloroform-dependent hepatotoxicity increases with dose (Larson, et al. 1994) and levels of cytotoxicity are related to the rate of chloroform biotransformation by Cyp2E1 (Brown, et al. 1974).
Blocking Cyp2E1 gene transcription (using the drug Bortezomib) blocks acetaminophen-, carbon tetrachloride-, and thioacetamide-dependent hepatotoxicity in a dose and time dependent manner (Park, et al. 2016).
Uncertainties or Inconsistencies
While the prevailing opinion in the literature is that the toxicity of these metabolites is the result of non-genotoxic mechanisms, there are studies that argue in favour of direct genotoxic effects. It is widely thought that any observed genotoxicity is actually ‘indirect’ and is the product of oxidative stress.
Quantitative Understanding of the Linkage
Unable to determine.
Evidence Supporting Taxonomic Applicability
Bechtold, M.M., Gee, D.L., Bruenner, U., Tappel, A.L., 1982. Carbon tetrachloride-mediated expiration of pentane and chloroform by the intact rat: the effects of pretreatment with diethyl maleate, SKF-525A and phenobarbital. Toxicol. Lett. 11, 165-171.
Brown, B.R.,Jr, Sipes, I.G., Sagalyn, A.M., 1974. Mechanisms of acute hepatic toxicity: chloroform, halothane, and glutathione. Anesthesiology 41, 554-561.
Caro, A.A., Cederbaum, A.I., 2004. Oxidative stress, toxicology, and pharmacology of CYP2E1. Annu. Rev. Pharmacol. Toxicol. 44, 27-42.
Fabrizi, L., Taylor, G.W., Edwards, R.J., Boobis, A.R., 2001. Adducts of the chloroform metabolite phosgene. Adv. Exp. Med. Biol. 500, 129-132.
Fransson-Steen, R., Goldsworthy, T.L., Kedderis, G.L., Maronpot, R.R., 1997. Furan-induced liver cell proliferation and apoptosis in female B6C3F1 mice. Toxicology 118, 195-204.
Kellert, M., Brink, A., Richter, I., Schlatter, J., Lutz, W.K., 2008. Tests for genotoxicity and mutagenicity of furan and its metabolite cis-2-butene-1,4-dial in L5178Y tk+/- mouse lymphoma cells. Mutation research 657, 127-32.
Larson, J.L., Wolf, D.C., Butterworth, B.E., 1994. Induced cytolethality and regenerative cell proliferation in the livers and kidneys of male B6C3F1 mice given chloroform by gavage. Fundamental and applied toxicology : official journal of the Society of Toxicology 23, 537-43.
Letteron, P., Labbe, G., Degott, C., Berson, A., Fromenty, B., Delaforge, M., Larrey, D., Pessayre, D., 1990. Mechanism for the protective effects of silymarin against carbon tetrachloride-induced lipid peroxidation and hepatotoxicity in mice. Evidence that silymarin acts both as an inhibitor of metabolic activation and as a chain-breaking antioxidant. Biochem. Pharmacol. 39, 2027-2034.
Martinez, M., Mourelle, M., Muriel, P., 1995. Protective effect of colchicine on acute liver damage induced by CCl4. Role of cytochrome P-450. J. Appl. Toxicol. 15, 49-52.
Moser, G.J., Foley, J., Burnett, M., Goldsworthy, T.L., Maronpot, R., 2009. Furan-induced dose–response relationships for liver cytotoxicity, cell proliferation, and tumorigenicity (furan-induced liver tumorigenicity). Experimental and Toxicologic Pathology 61, 101-111.
Park, W.J., Kim, S.Y., Kim, Y.R., Park, J.W., 2016. Bortezomib alleviates drug-induced liver injury by regulating CYP2E1 gene transcription. Int. J. Mol. Med. 37, 613-622.
Stevens, J.L., Anders, M.W., 1981. Effect of cysteine, diethyl maleate, and phenobarbital treatments on the hepatotoxicity of [1H]chloroform. Chem. Biol. Interact. 37, 207-217.
Takahashi, S., Takahashi, T., Mizobuchi, S., Matsumi, M., Morita, K., Miyazaki, M., Namba, M., Akagi, R., Hirakawa, M., 2002. Increased cytotoxicity of carbon tetrachloride in a human hepatoma cell line overexpressing cytochrome P450 2E1. J. Int. Med. Res. 30, 400-405.