This Key Event Relationship is licensed under the Creative Commons BY-SA license. This license allows reusers to distribute, remix, adapt, and build upon the material in any medium or format, so long as attribution is given to the creator. The license allows for commercial use. If you remix, adapt, or build upon the material, you must license the modified material under identical terms.

Relationship: 1515

Title

A descriptive phrase which clearly defines the two KEs being considered and the sequential relationship between them (i.e., which is upstream, and which is downstream). More help

Activation of Cyp2E1 leads to Hepatocytotoxicity

Upstream event

The causing Key Event (KE) in a Key Event Relationship (KER). More help

Activation of Cyp2E1

Downstream event

The responding Key Event (KE) in a Key Event Relationship (KER). More help

Hepatocytotoxicity

Key Event Relationship Overview

The utility of AOPs for regulatory application is defined, to a large extent, by the confidence and precision with which they facilitate extrapolation of data measured at low levels of biological organisation to predicted outcomes at higher levels of organisation and the extent to which they can link biological effect measurements to their specific causes.Within the AOP framework, the predictive relationships that facilitate extrapolation are represented by the KERs. Consequently, the overall WoE for an AOP is a reflection in part, of the level of confidence in the underlying series of KERs it encompasses. Therefore, describing the KERs in an AOP involves assembling and organising the types of information and evidence that defines the scientific basis for inferring the probable change in, or state of, a downstream KE from the known or measured state of an upstream KE. More help

AOPs Referencing Relationship

AOP Name	Adjacency	Weight of Evidence	Quantitative Understanding	Point of Contact	Author Status	OECD Status
Cyp2E1 Activation Leading to Liver Cancer	non-adjacent	High	Not Specified	Agnes Aggy (send email)	Open for citation & comment	WPHA/WNT Endorsed

Taxonomic Applicability

Latin or common names of a species or broader taxonomic grouping (e.g., class, order, family) that help to define the biological applicability domain of the KER.In general, this will be dictated by the more restrictive of the two KEs being linked together by the KER. More help

Term	Scientific Term	Evidence	Link
Rodentia sp.	Rodentia sp.	High	NCBI

Sex Applicability

An indication of the the relevant sex for this KER. More help

Sex	Evidence
Mixed	High

Life Stage Applicability

An indication of the the relevant life stage(s) for this KER. More help

Term	Evidence
All life stages	High

Key Event Relationship Description

Provides a concise overview of the information given below as well as addressing details that aren’t inherent in the description of the KEs themselves. More help

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 hepatotoxicity. 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.

Evidence Collection Strategy

Include a description of the approach for identification and assembly of the evidence base for the KER. For evidence identification, include, for example, a description of the sources and dates of information consulted including expert knowledge, databases searched and associated search terms/strings. Include also a description of study screening criteria and methodology, study quality assessment considerations, the data extraction strategy and links to any repositories/databases of relevant references.Tabular summaries and links to relevant supporting documentation are encouraged, wherever possible. More help

Evidence Supporting this KER

Addresses the scientific evidence supporting KERs in an AOP setting the stage for overall assessment of the AOP. More help

Biological Plausibility

Addresses the biological rationale for a connection between KEupstream and KEdownstream. This field can also incorporate additional mechanistic details that help inform the relationship between KEs, this is useful when it is not practical/pragmatic to represent these details as separate KEs due to the difficulty or relative infrequency with which it is likely to be measured. More help

Strong. Metabolite-dependent toxicity and adduct formation are well known side-effects of cytochrome P450 mono-oxygenase metabolism of xenobiotics in the liver. Because primary metabolites are more reactive than the parent compound, they often create adducts to cellular proteins or DNA. In both cases, this prevents the normal functioning of the molecules. In extreme cases this will lead to hepatocytotoxicity due to: (1) the large number of adducts, (2) the loss of function of important cellular proteins and the related cellular processes, and (3) the loss of function of important genes due to DNA damage and mutation.

Empirical Evidence

Provides specific (citable) evidence that supports the idea of a change in the upstream KE (KEupstream) leading to, or being associated with, a subsequent change in the downstream KE (KEdownstream), assuming the perturbation of KEupstream is sufficient. More help

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 and Inconsistencies

Addresses inconsistencies or uncertainties in the relationship including the identification of experimental details that may explain apparent deviations from the expected patterns of concordance. More help

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.

Known modulating factors

This table captures specific information on the MF, its properties, how it affects the KER and respective references.1.) What is the modulating factor? Name the factor for which solid evidence exists that it influences this KER. Examples: age, sex, genotype, diet 2.) Details of this modulating factor. Specify which features of this MF are relevant for this KER. Examples: a specific age range or a specific biological age (defined by...); a specific gene mutation or variant, a specific nutrient (deficit or surplus); a sex-specific homone; a certain threshold value (e.g. serum levels of a chemical above...) 3.) Description of how this modulating factor affects this KER. Describe the provable modification of the KER (also quantitatively, if known). Examples: increase or decrease of the magnitude of effect (by a factor of...); change of the time-course of the effect (onset delay by...); alteration of the probability of the effect; increase or decrease of the sensitivity of the downstream effect (by a factor of...) 4.) Provision of supporting scientific evidence for an effect of this MF on this KER. Give a list of references. More help

Quantitative Understanding of the Linkage

Captures information that helps to define how much change in the upstream KE, and/or for how long, is needed to elicit a detectable and defined change in the downstream KE. More help

Unable to determine.

Response-response Relationship

Provides sources of data that define the response-response relationships between the KEs. More help

Time-scale

Information regarding the approximate time-scale of the changes in KEdownstream relative to changes in KEupstream (i.e., do effects on KEdownstream lag those on KEupstream by seconds, minutes, hours, or days?). More help

Known Feedforward/Feedback loops influencing this KER

Define whether there are known positive or negative feedback mechanisms involved and what is understood about their time-course and homeostatic limits. More help

Domain of Applicability

A free-text section of the KER description that the developers can use to explain their rationale for the taxonomic, life stage, or sex applicability structured terms. More help

References

List of the literature that was cited for this KER description. More help

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.