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Relationship: 1513

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

Oxidative Stress leads to Hepatocytotoxicity

Upstream event

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

Oxidative Stress

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	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

Sex Applicability

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

Life Stage Applicability

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

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

Oxidative stress leads directly to hepatotoxicity through lipid peroxidation. Lipid peroxidation occurs when ROS scavenge electrons from poly-unsaturated fatty acids (PUFA), including membrane phospholipids. Lipid peroxidation occurs in three steps: initiation (in which the PUFA radical is produced), propagation (in which PUFA radicals react with molecular oxygen and a non-radical molecule to produce a lipid peroxide and lipid radical), and termination (in which two radicals combine to form a non-radical). Left unchecked, the propagation chain reaction is highly damaging to cellular membranes. Lipid peroxidation of mitochondrial membranes has been shown to result in both necrosis and apoptosis. The former occurs due to decreased mitochondrial membrane potential leading to decreased ATP production. The latter is a result of mitochondrial permeability transition (MPT). MPT is a process that can lead to necrosis or apoptosis. It is an important cell death mechanism because it is sensitive to redox conditions. Accumulation of ROS and depletion of glutathione trigger the mitogen activated protein kinase (MAPK) cascade (ASK1-->MKK4-->JNK), which recruits Bax to the outer mitochondrial membrane (Youle and Strasser 2008). Bax triggers the opening of mitochondrial permeability transition pore (MTP), through which cytochrome c is released, which triggers the caspase cascade and apoptosis. Alternatively, when the MTP opens across the inner and outer mitochondrial membranes, mitochondrial swelling and decoupling of oxidative phosphorylation (i.e., loss of ATP generation) leads to cell death by necrosis (Pessayre, et al. 2010, Rasola and Bernardi 2007).

In parallel, oxidative stress triggers cytotoxicity indirectly by modifying redox sensitive cellular molecules. Proteins with neighboring cysteine residues sense ROS through the oxidation of adjacent thiol groups (2SH, reduced; S=S, oxidized). Examples of this include: (1) the cellular anti-oxidant glutathione (GSH), which acts to ‘mop up’ ROS (GSH oxidized to GS=SG), and its depletion is associated with elevated cytotoxicity because ROS levels remain elevated or increase; (2) the cellular anti-oxidant thioredoxin, which inhibits the apoptosis signaling kinase 1 (Ask1) in its reduced form, but not in its oxidized form (Liu, et al. 2000, Saitoh, et al. 1998); and, (3) the mitochondrial permeability transition pore, which opens when oxidized (Petronilli, et al. 1994). Oxidative stress can also produce cell death through the production of oxidative damage to DNA, which can lead to apoptosis through p53 signalling. Examples of types of oxidative DNA damage include: (Sharma, et al. 2012, Shukla, et al. 2013, Skipper, et al. 2016).

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. It is well known that cellular oxidative damage, especially by lipid peroxidation, is cytotoxic.

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. This is an extremely data-rich KER. A large number of studies have taken measures of both oxidative stress and cytotoxicity within the same study. Below, we summarize a few examples, in vitro and in vivo, to demonstrate some empirical data that provides strong support for this KER.

For example, exposure of HepG2 cells to the pesticide malathion demonstrates a relationship between ROS and cytotoxicity at higher levels of exposure (Moore, et al. 2010). After 48 h of exposure to 0, 6, 12, 18, and 24 mM malathion, lipid peroxidation increases from the first concentration, whereas cytoxicity (MTT assay) does not significantly increase until 18 and 24 mM.

Similarly, treatment of cultured premonocytic U937 cells with increasing concentrations of lipid peroxidation-inducing agents (tert-butylhydroperoxide and 2,2’-azobis (2-amindinopropane) hydrochloride) results in increased lipid peroxidation within 30 min, and subsequent declines in relative survival (trypan blue assay) six hours post-exposure (Park, et al. 2002). In parallel, the major anti-oxidant enzymes catalase, SOD, glucose-6-phosphate-dehydrogenase, and glutathione peroxidase are deactivated in a dose-dependent fashion that is concordant with declines in relative survival.

Studies on primary rat hepatocytes show that a dose-dependent increase in TBARS is associated with a concomitant increase in cytotoxicity following exposure to fumonisin B1 (FB1) (Abel and Gelderblom 1998). Moreover, addition of the antioxidant alpha-tocopherol significantly decreases cytotoxicity and decreases TBARS to basal levels, supporting that lipid peroxidation contributes to the cytotoxic effects of FB1.

In female Wistar rat hepatocyte cultures, exposure to chloroform causes a small dose-dependent increase in M(1)dG adducts (a marker of lipid perodixation; occurs at 4 mM and above), DNA strand breakage (8 mM and above) and lipid peroxidation (at 4 mM and above). GSH depletion occurs in association with cytotoxicity (20 mM; lactate dehydrogenase release). Carbon tetrachloride (1 and 4 mM) exposure produces a small elevation in M(1)dG adducts and increases in 8-oxodG occur at the threshold of, and concomitant with, cytotoxicity (4 mM). (Beddowes, et al. 2003).

Similarly, exposure of 12-week old male Long Evans rats to increasing doses of cadmium (Cd) (i.p. injection) reveals that lipid peroxidation (TBARS) occurs in liver at low to medium doses, below those inducing tissue necrosis (Manca, et al. 1991). In this study, liver injury occurred above 125 ug Cd/kg (increased serum ALT and SDH). Levels of TBARS were similar in both the 125 and the 250 ug/doses, though only reached statistical significance relative to controls at 250 ug/kg in the liver. However, subsequent time-series analysis revealed increased liver TBARS after 2 h exposure to both 25 and 500 ug Cd/kg, prior to any changes in serum ALT, SDH, and tissue ALP (at the low dose, ALP was the only marker that was slightly increased at 6 hour). Therefore, lipid peroxidation precedes liver damage in this study.

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

There exist some examples where measures of cytotoxicity could be observed below doses where assays for endpoints of oxidative stress were measured. However, it is difficult to compare endpoints measured using assays with different specificities and sensitivities. Quite generally, there is a high degree of association between measures of oxidative stress and cytotoxicity across tissues and species.

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

Abel, S., Gelderblom, W.C., 1998. Oxidative damage and fumonisin B1-induced toxicity in primary rat hepatocytes and rat liver in vivo. Toxicology 131, 121-131.

Beddowes, E.J., Faux, S.P., Chipman, J.K., 2003. Chloroform, carbon tetrachloride and glutathione depletion induce secondary genotoxicity in liver cells via oxidative stress. Toxicology 187, 101-115.

Liu, H., Nishitoh, H., Ichijo, H., Kyriakis, J.M., 2000. Activation of apoptosis signal-regulating kinase 1 (ASK1) by tumor necrosis factor receptor-associated factor 2 requires prior dissociation of the ASK1 inhibitor thioredoxin. Mol. Cell. Biol. 20, 2198-2208.

Manca, D., Ricard, A.C., Trottier, B., Chevalier, G., 1991. Studies on lipid peroxidation in rat tissues following administration of low and moderate doses of cadmium chloride. Toxicology 67, 303-323.

Moore, P.D., Yedjou, C.G., Tchounwou, P.B., 2010. Malathion-induced oxidative stress, cytotoxicity, and genotoxicity in human liver carcinoma (HepG2) cells. Environ. Toxicol. 25, 221-226.

Park, J.E., Yang, J.H., Yoon, S.J., Lee, J.H., Yang, E.S., Park, J.W., 2002. Lipid peroxidation-mediated cytotoxicity and DNA damage in U937 cells. Biochimie 84, 1199-1205.

Pessayre, D., Mansouri, A., Berson, A., Fromenty, B., 2010. Mitochondrial involvement in drug-induced liver injury. Handb. Exp. Pharmacol. (196):311-65. doi, 311-365.

Rasola, A., Bernardi, P., 2007. The mitochondrial permeability transition pore and its involvement in cell death and in disease pathogenesis. Apoptosis 12, 815-833.

Saitoh, M., Nishitoh, H., Fujii, M., Takeda, K., Tobiume, K., Sawada, Y., Kawabata, M., Miyazono, K., Ichijo, H., 1998. Mammalian thioredoxin is a direct inhibitor of apoptosis signal-regulating kinase (ASK) 1. EMBO Journal 17, 2596-2606.

Sharma, V., Singh, P., Pandey, A.K., Dhawan, A., 2012. Induction of oxidative stress, DNA damage and apoptosis in mouse liver after sub-acute oral exposure to zinc oxide nanoparticles. Mutat. Res. 745, 84-91.

Shukla, R.K., Kumar, A., Gurbani, D., Pandey, A.K., Singh, S., Dhawan, A., 2013. TiO(2) nanoparticles induce oxidative DNA damage and apoptosis in human liver cells. Nanotoxicology 7, 48-60.

Skipper, A., Sims, J.N., Yedjou, C.G., Tchounwou, P.B., 2016. Cadmium Chloride Induces DNA Damage and Apoptosis of Human Liver Carcinoma Cells via Oxidative Stress. Int. J. Environ. Res. Public. Health. 13, 10.3390/ijerph13010088.

Youle, R.J., Strasser, A., 2008. The BCL-2 protein family: opposing activities that mediate cell death. Nat. Rev. Mol. Cell Biol. 9, 47-59.