To the extent possible under law, AOP-Wiki has waived all copyright and related or neighboring rights to KE:1394

Event: 1394

Key Event Title

The KE title should describe a discrete biological change that can be measured. It should generally define the biological object or process being measured and whether it is increased, decreased, or otherwise definably altered relative to a control state. For example “enzyme activity, decreased”, “hormone concentration, increased”, or “growth rate, decreased”, where the specific enzyme or hormone being measured is defined. More help

Induction, persistent proliferation/sustained proliferation

Short name
The KE short name should be a reasonable abbreviation of the KE title and is used in labelling this object throughout the AOP-Wiki. The short name should be less than 80 characters in length. More help
Sustained proliferation

Biological Context

Structured terms, selected from a drop-down menu, are used to identify the level of biological organization for each KE. Note, KEs should be defined within a particular level of biological organization. Only KERs should be used to transition from one level of organization to another. Selection of the level of biological organization defines which structured terms will be available to select when defining the Event Components (below). More help
Level of Biological Organization

Cell term

Further information on Event Components and Biological Context may be viewed on the attached pdf.The biological context describes the location/biological environment in which the event takes place.  For molecular/cellular events this would include the cellular context (if known), organ context, and species/life stage/sex for which the event is relevant. For tissue/organ events cellular context is not applicable.  For individual/population events, the organ context is not applicable. More help

Organ term

Further information on Event Components and Biological Context may be viewed on the attached pdf.The biological context describes the location/biological environment in which the event takes place.  For molecular/cellular events this would include the cellular context (if known), organ context, and species/life stage/sex for which the event is relevant. For tissue/organ events cellular context is not applicable.  For individual/population events, the organ context is not applicable. More help

Key Event Components

Further information on Event Components and Biological Context may be viewed on the attached pdf.Because one of the aims of the AOP-KB is to facilitate de facto construction of AOP networks through the use of shared KE and KER elements, authors are also asked to define their KEs using a set of structured ontology terms (Event Components). In the absence of structured terms, the same KE can readily be defined using a number of synonymous titles (read by a computer as character strings). In order to make these synonymous KEs more machine-readable, KEs should also be defined by one or more “event components” consisting of a biological process, object, and action with each term originating from one of 22 biological ontologies (Ives, et al., 2017; See List). Biological process describes dynamics of the underlying biological system (e.g., receptor signalling). The biological object is the subject of the perturbation (e.g., a specific biological receptor that is activated or inhibited). Action represents the direction of perturbation of this system (generally increased or decreased; e.g., ‘decreased’ in the case of a receptor that is inhibited to indicate a decrease in the signalling by that receptor).Note that when editing Event Components, clicking an existing Event Component from the Suggestions menu will autopopulate these fields, along with their source ID and description. To clear any fields before submitting the event component, use the 'Clear process,' 'Clear object,' or 'Clear action' buttons. If a desired term does not exist, a new term request may be made via Term Requests. Event components may not be edited; to edit an event component, remove the existing event component and create a new one using the terms that you wish to add. More help
Process Object Action
cellular response to oxidative stress increased
macrophage activation involved in immune response macrophage increased
hypoxia hypoxia-inducible factor 1-alpha decreased
hypoxia von Hippel-Lindau disease tumor suppressor decreased

Key Event Overview

AOPs Including This Key Event

All of the AOPs that are linked to this KE will automatically be listed in this subsection. This table can be particularly useful for derivation of AOP networks including the KE. Clicking on the name of the AOP will bring you to the individual page for that AOP. More help
AOP Name Role of event in AOP Point of Contact Author Status OECD Status
Cyp2E1 Activation Leading to Liver Cancer KeyEvent Francina Webster (send email) Open for citation & comment WPHA/WNT Endorsed


This is a structured field used to identify specific agents (generally chemicals) that can trigger the KE. Stressors identified in this field will be linked to the KE in a machine-readable manner, such that, for example, a stressor search would identify this as an event the stressor can trigger. NOTE: intermediate or downstream KEs in one AOP may function as MIEs in other AOPs, meaning that stressor information may be added to the KE description, even if it is a downstream KE in the pathway currently under development.Information concerning the stressors that may trigger an MIE can be defined using a combination of structured and unstructured (free-text) fields. For example, structured fields may be used to indicate specific chemicals for which there is evidence of an interaction relevant to this MIE. By linking the KE description to a structured chemical name, it will be increasingly possible to link the MIE to other sources of chemical data and information, enhancing searchability and inter-operability among different data-sources and knowledgebases. The free-text section “Evidence for perturbation of this MIE by stressor” can be used both to identify the supporting evidence for specific stressors triggering the MIE as well as to define broad chemical categories or other properties that classify the stressors able to trigger the MIE for which specific structured terms may not exist. More help

Taxonomic Applicability

Latin or common names of a species or broader taxonomic grouping (e.g., class, order, family) can be selected from an ontology. In many cases, individual species identified in these structured fields will be those for which the strongest evidence used in constructing the AOP was available in relation to this KE. More help
Term Scientific Term Evidence Link
rodents rodents High NCBI
Homo sapiens Homo sapiens High NCBI

Life Stages

The structured ontology terms for life-stage are more comprehensive than those for taxa, but may still require further description/development and explanation in the free text section. More help
Life stage Evidence
All life stages High

Sex Applicability

The authors must select from one of the following: Male, female, mixed, asexual, third gender, hermaphrodite, or unspecific. More help
Term Evidence
Mixed High

Key Event Description

A description of the biological state being observed or measured, the biological compartment in which it is measured, and its general role in the biology should be provided. For example, the biological state being measured could be the activity of an enzyme, the expression of a gene or abundance of an mRNA transcript, the concentration of a hormone or protein, neuronal activity, heart rate, etc. The biological compartment may be a particular cell type, tissue, organ, fluid (e.g., plasma, cerebrospinal fluid), etc. The role in the biology could describe the reaction that an enzyme catalyses and the role of that reaction within a given metabolic pathway; the protein that a gene or mRNA transcript codes for and the function of that protein; the function of a hormone in a given target tissue, physiological function of an organ, etc. Careful attention should be taken to avoid reference to other KEs, KERs or AOPs. Only describe this KE as a single isolated measurable event/state. This will ensure that the KE is modular and can be used by other AOPs, thereby facilitating construction of AOP networks. More help

Cellular proliferation rates increase in response to cell death in the liver in order to replace the dying cells. Cellular proliferation refers to the production of new cells to maintain a balance of cell division and cell loss. This key event describes conditions under which this process is persistent or sustained because of chronic exposure. This process is analogous to liver regenerative proliferation (e.g., regerating liver cells following partial hepatectomy), which is often used as a model. 

The liver has two modes of regenerating lost cells: (1) via cellular hypertrophy and division of existing cells; or (2) via proliferation of a population of facultative stem cells, called biliary epithelial cells (BECs), located at the Canals of Hering (in zone 1 where canaliculi join and drain into the main bile duct). Facultative stem cells are functional, differentiated cells that will dedifferentiate in response to tissue damage, thereby becoming a population of progenitor cells that can redifferentiate to replace multiple lost cell types.In a process known as ductal expansion, BECs dedifferentiate into oval cells, which then redifferentiate into hepatocytes or BECs in order to regenerate damaged liver tissue. Liver regeneration has been reviewed (Mao, et al. 2014, Stanger 2015, Yanger and Stanger 2011).

At the molecular level, two dimeric transcription factors, AP-1 (particularly the c-Jun monomer) and NF-kappaB, are key players during liver cellular proliferation and regeneration. While neither is expressed in normal liver tissue, they are upregulated during normal hepatic regeneration, and are required for regeneration (Alcorn, et al. 1990, Cressman, et al. 1994, FitzGerald, et al. 1995). Indeed, rodents lacking AP-1 or NF-kappaB display impaired liver regeneration, often leading to death (Behrens, et al. 2002, Schrum, et al. 2000). Both NF-kappaB and c-Jun (AP-1) are required for embryonic liver development, and a loss of either one is embryonic lethal due to widespread cell death and liver degeneration (Behrens, et al. 2002, Eferl, et al. 1999, Jochum, et al. 2001, Li, et al. 1999, Rudolph, et al. 2000).

A causal network for sustained proliferation and regenerative proliferation in liver can also occur via WNT signaling and the following pathways: the network begins with oxidative stress or other mechanisms causing liver tissue injury which in turn causes (2) activation of macrophages and wound repair (Boulter et al., 2012), (3) increased hypoxia through diminished blood supply or activity of reactive oxygen species (Ju et al., 2016, Gonzalez et al., 2018) and (4) increased expression of Wnt ligands (Okabe et al., 2016). The activation of macrophages causes (5) activation of Wnt proteins and Wnt signaling (Boulter et al., 2012, Vanella and Wynn 2017). The activation and/or increased expression of Wnt signaling ligands causes (6) binding of the Wnt ligand to the co-receptors Frizzled (FZD family) and (7) Low-density lipoprotein receptor-related proteins 5 and 6 (LRP5/6) which then (8) recruit and phosphorylate Dishevelled (DVL1) and the scaffold protein Axin (Takigawa and Brown 2008).

The phosphorylation and recruitment of Axin (AXIN1, AXIN2), (33) inhibits formation of the beta-catenin destruction complex, composed of AXIN1 or AXIN2, adenomatosis polyposis coli (APC), beta-catenin (CTNNB1) and glycogen synthase kinase 3 (GSK3), which (10) targets beta-catenin for degradation. Inhibiting formation of the destruction complex increases the amount of available beta-catenin to (11) interact and complex with the transcription factor 7 and lymphoid enhancer-binding factor (TCF/LEF) family of transcription factors (TF7, TCF7L1, TCF7L2, LEF1; Takigawa and Brown 2008).  The TCF/LEF:beta-catenin complex then (12) activates transcription of MYC proto-oncogene (MYC) and (13) cyclin D1 or CCND1 (Schuijers et al 2014;Katoh 2017). Activation of Wnt signaling (14) inhibits GSK3 phosphorylation activity which then (15) represses forkhead box M1 (FOXM1) activity, (34) causes increased turnover of CCND1 and (35) increases proteolysis of MYC (Katoh 2017; Gregory et al., 2003).

FOXM1 activates (16) transcription of MYC and (17) transcription of MAPK8, the mitogen-activated protein kinase (also known as JNK1; Wierstra and Alves 2007; Wang et al., 2008). Transcriptional activation of MAPK8 then leads to (36) transcriptional activation of CCND1 (Wang et al., 2008). Transcriptional activation of MYC causes (18) transcription of cyclin-dependent kinase 4 (CDK4) which leads to (19, 20) formation of a CDK4 and CCND1 complex (Wang et al 2011). The cyclin-CDK complex then (21) inhibits activity of the retinoblastoma (RB1) transcriptional corepressor 1 which (22) negatively regulates the cell cycle (Burkhart and Sage 2008). Dysregulation of G1/S transition by inhibition of RB1 and/or FOXM1 (23) leads to cell proliferation (Wierstra and Alves 2007; Burkhart and Sage 2008).

MYC can also be activated via hypoxia signaling where an increase in hypoxia (24) decreases the activity of oxygen sensor hypoxia-inducible factor 1 alpha inhibitor (HIF1AN) thereby reducing the ability of HIF1AN to (25) hydroxylate and inhibit hypoxia-inducible factor 1 alpha (HIF1A) activity (Whyte et al., 2012; Mahon et al., 2001). Hypoxia also can (26) inhibit activity of the von Hippel-Lindau (VHL) tumor suppressor protein which has been shown to (27) hydroxylate HIF1A in an O2 dependent manner marking HIF1A for degradation and inactivation in addition to inhibiting expression of HIF1A (Mahon et al., 2001).  In stem cells, activated HIF1A (28) increases expression of TCF/LEF leading to increased expression of genes including MYC (Whyte et al., 2012; Tiburcio et al., 2014).

The long WNT Signaling Pathway Activating noncoding RNA (WSPAR) is often highly expressed in hepatocellular carcinoma cells and has been found to (29) activate expression of members of the TCF/LEF family (Zhan et al 2017). TCF/LEF transcription factors (30) increase transcription of AXIN2 and increase destruction of beta-catenin in a Wnt signaling negative feedback loop (Jho et al., 2002). TCF/LEF transcription factors form a negative feedback loop that inhibits Wnt signaling by (31) activating transcription of the dickkopf Wnt signaling pathway inhibitor 1 (DKK1) which then (32) binds to the LRP co-receptor (Takigawa and Brown 2008). Finally, cellular G1/S transition can also be dysregulated by (35) phosphorylation of RB1 by the 26S proteasome non-ATPase regulatory subunit 10 (PSMD10) which results in an increase in proteosomal degradation of RB1 (Higashitsuji et al., 2005).

How It Is Measured or Detected

One of the primary considerations in evaluating AOPs is the relevance and reliability of the methods with which the KEs can be measured. The aim of this section of the KE description is not to provide detailed protocols, but rather to capture, in a sentence or two, per method, the type(s) of measurements that can be employed to evaluate the KE and the relative level of scientific confidence in those measurements. Methods that can be used to detect or measure the biological state represented in the KE should be briefly described and/or cited. These can range from citation of specific validated test guidelines, citation of specific methods published in the peer reviewed literature, or outlines of a general protocol or approach (e.g., a protein may be measured by ELISA).Key considerations regarding scientific confidence in the measurement approach include whether the assay is fit for purpose, whether it provides a direct or indirect measure of the biological state in question, whether it is repeatable and reproducible, and the extent to which it is accepted in the scientific and/or regulatory community. Information can be obtained from the OECD Test Guidelines website and the EURL ECVAM Database Service on Alternative Methods to Animal Experimentation (DB-ALM). ?
  • Proliferation. In vivo or in vitro cellular proliferation can be measured following a multiday 5-bromo-2'-deoxyuridine (BrdU) exposure and quantification of BrdU incorporation in DNA by immunohistochemistry. Alternatively, cells or tissue sections may be stained for Ki-67 or proliferating cell nuclear antigen (PCNA) for a snapshot of cellular proliferation. Use of BrdU, Ki-67, and PCNA in risk assessment has been described in detail (Wood, et al. 2015). A variety of commercial kits exist for this assay.
  • Regeneration. Liver regeneration can be observed following partial hepatectomy. Method for 2/3 partial hepatectomy have been described (Mitchell and Willenbring 2008, Mitchell and Willenbring 2014)
  • Gene expression analysis can be conducted to demonstrate increased expression of AP-1 or NF-kappaB monomers, or decreased expression of negative regulators, which can be used as an indicator that there is increased cellular proliferation in the liver.
  • A BrdU-ELISA incorporation in vivo test method is also described in OECD TG 442B (The Local Lymph Node Assay, which is for skin sensitization).

Domain of Applicability

This free text section should be used to elaborate on the scientific basis for the indicated domains of applicability and the WoE calls (if provided). While structured terms may be selected to define the taxonomic, life stage and sex applicability (see structured applicability terms, above) of the KE, the structured terms may not adequately reflect or capture the overall biological applicability domain (particularly with regard to taxa). Likewise, the structured terms do not provide an explanation or rationale for the selection. The free-text section on evidence for taxonomic, life stage, and sex applicability can be used to elaborate on why the specific structured terms were selected, and provide supporting references and background information.  More help

This key event has been well studied in mice, rats (Taub 2004), and zebrafish (Cox and Goessling 2015, Goessling and Sadler 2015), which are all systems that are thought to work in a similar way to human liver cell proliferation and regeneration (Kwon, et al. 2015).

Evidence for Perturbation by Stressor


List of the literature that was cited for this KE description. Ideally, the list of references, should conform, to the extent possible, with the OECD Style Guide ( (OECD, 2015). More help

Alcorn, J.A., Feitelberg, S.P., Brenner, D.A., 1990. Transient induction of c-jun during hepatic regeneration. Hepatology 11, 909-915.

Behrens, A., Sibilia, M., David, J.P., Möhle-Steinlein, U., Tronche, F., Schütz, G., Wagner, E.F., 2002. Impaired postnatal hepatocyte proliferation and liver regeneration in mice lacking c-jun in the liver. EMBO J. 21, 1782-1790.

Boulter L, Govaere O, Bird TG, Radulescu S, Ramachandran P, Pellicoro A, Ridgway RA, et al. Macrophage-derived Wnt opposes Notch signaling to specify hepatic progenitor cell fate in chronic liver disease. Nat Med. 2012 Mar 4;18(4):572-9. doi: 10.1038/nm.2667.

Burkhart DL, Sage J. Cellular mechanisms of tumour suppression by the retinoblastoma gene. Nat Rev Cancer. 2008 Sep;8(9):671-82. doi: 10.1038/nrc2399.

Cox, A.G., Goessling, W., 2015. The lure of zebrafish in liver research: regulation of hepatic growth in development and regeneration. Curr. Opin. Genet. Dev. 32, 153-161.

Cressman, D.E., Greenbaum, L.E., Haber, B.A., Taub, R., 1994. Rapid activation of post-hepatectomy factor/nuclear factor κB in hepatocytes, a primary response in the regenerating liver. J. Biol. Chem. 269, 30429-30435.

Eferl, R., Sibilia, M., Hilberg, F., Fuchsbichler, A., Kufferath, I., Guertl, B., Zenz, R., Wagner, E.F., Zatloukal, K., 1999. Functions of c-Jun in liver and heart development. J. Cell Biol. 145, 1049-1061.

FitzGerald, M.J., Webber, E.M., Donovan, J.R., Fausto, N., 1995. Rapid DNA binding by nuclear factor κB in hepatocytes at the start of liver regeneration. Cell Growth and Differentiation 6, 417-427.

Goessling, W., Sadler, K.C., 2015. Zebrafish: an important tool for liver disease research. Gastroenterology 149, 1361-1377.

Gregory MA, Qi Y, Hann SR. Phosphorylation by glycogen synthase kinase- controls c-myc proteolysis and subnuclear localization. J Biol Chem. 2003 Dec 19;278(51):51606-12. Epub 2003 Oct 16. PubMed PMID: 14563837.

Higashitsuji H, Liu Y, Mayer RJ, Fujita J. The oncoprotein gankyrin negatively regulates both p53 and RB by enhancing proteasomal degradation. Cell Cycle. 2005 Oct;4(10):1335-7. Epub 2005 Oct 17. PubMed PMID: 16177571. Jho EH, Zhang T, Domon C, Joo CK, Freund JN, Costantini F. Wnt/beta-catenin/Tcf signaling induces the transcription of Axin2, a negative regulator of the signaling pathway. Mol Cell Biol. 2002 Feb;22(4):1172-83 Katoh M. Canonical and non-canonical WNT signaling in cancer stem cells and their niches: Cellular heterogeneity, omics reprogramming, targeted therapy and tumor plasticity (Review). Int J Oncol. 2017 Nov;51(5):1357-1369. doi: 10.3892/ijo.2017.4129. Jochum, W., Passegué, E., Wagner, E.F., 2001. AP-1 in mouse development and tumorigenesis. Oncogene 20, 2401-2412.

Kwon, Y.J., Lee, K.G., Choi, D., 2015. Clinical implications of advances in liver regeneration. Clin. Mol. Hepatol. 21, 7-13.

Li, Q., Van Antwerp, D., Mercurio, F., Lee, K.F., Verma, I.M., 1999. Severe liver degeneration in mice lacking the IkappaB kinase 2 gene. Science 284, 321-325.

Mao, S.A., Glorioso, J.M., Nyberg, S.L., 2014. Liver regeneration. Transl. Res. 163, 352-362.

Mitchell, C., Willenbring, H., 2008. A reproducible and well-tolerated method for 2/3 partial hepatectomy in mice. Nat. Protoc. 3, 1167-1170.

Mitchell, C., Willenbring, H., 2014. Addendum: A reproducible and well-tolerated method for 2/3 partial hepatectomy in mice. Nat. Protoc. 9, 10.1038/nprot.2014.122.

OECD (2018), Test No. 442B: Skin Sensitization: Local Lymph Node Assay: BrdU-ELISA or –FCM, OECD Guidelines for the Testing of Chemicals, Section 4, OECD Publishing, Paris,

Rudolph, D., Yeh, W.-., Wakeham, A., Rudolph, B., Nallainathan, D., Potter, J., Elia, A.J., Mak, T.W., 2000. Severe liver degeneration and lack of NF-κB activation in NEMO/IKK γ- deficient mice. Genes and Development 14, 854-862.

Schrum, L.W., Black, D., Iimuro, Y., Rippe, R.A., Brenner, D.A., Behrns, K.E., 2000. c-Jun does not mediate hepatocyte apoptosis following NFκB inhibition and partial hepatectomy. J. Surg. Res. 88, 142-149.

Schuijers J, Mokry M, Hatzis P, Cuppen E, Clevers H. Wnt-induced transcriptional activation is exclusively mediated by TCF/LEF. EMBO J. 2014 Jan 13;33(2):146-56. doi: 10.1002/embj.201385358.

Stanger, B.Z., 2015. Cellular homeostasis and repair in the mammalian liver. Annu. Rev. Physiol. 77, 179-200.

Takigawa Y, Brown AM. Wnt signaling in liver cancer. Curr Drug Targets. 2008; 9:1013-24 ​ Tiburcio PD, Choi H, Huang LE. Complex role of HIF in cancer: the known, the unknown, and the unexpected. Hypoxia (Auckl). 2014 Jun 18;2:59-70.

Vannella KM, Wynn TA. Mechanisms of Organ Injury and Repair by Macrophages. Annu Rev Physiol. 2017 Feb 10;79:593-617. doi:10.1146/annurev-physiol-022516-034356.

Wang C, Lisanti MP, Liao DJ. Reviewing once more the c-myc and Ras collaboration: converging at the cyclin D1-CDK4 complex and challenging basic concepts of cancer biology. Cell Cycle. 2011 Jan 1;10(1):57-67.

Wang IC, Chen YJ, Hughes DE, Ackerson T, Major ML, Kalinichenko VV, Costa RH, Raychaudhuri P, Tyner AL, Lau LF. FoxM1 regulates transcription of JNK1 to promote the G1/S transition and tumor cell invasiveness. J Biol Chem. 2008 Jul 25;283(30):20770-8. doi: 10.1074/jbc.M709892200.

Whyte JL, Smith AA, Helms JA. Wnt signaling and injury repair. Cold Spring Harb Perspect Biol. 2012 Aug 1;4(8):a008078. doi: 10.1101/cshperspect.a008078. Wierstra I, Alves J. FOXM1, a typical proliferation-associated transcription factor. Biol Chem. 2007 Dec;388(12):1257-74.

Wierstra I, Alves J. FOXM1c is activated by cyclin E/Cdk2, cyclin A/Cdk2, and cyclin A/Cdk1, but repressed by GSK-3alpha. Biochem Biophys Res Commun. 2006 Sep15;348(1):99-108.

Wood, C.E., Hukkanen, R.R., Sura, R., Jacobson-Kram, D., Nolte, T., Odin, M., Cohen, S.M., 2015. Scientific and Regulatory Policy Committee (SRPC) Review: Interpretation and Use of Cell Proliferation Data in Cancer Risk Assessment. Toxicol. Pathol. 43, 760-775.

Yanger, K., Stanger, B.Z., 2011. Facultative stem cells in liver and pancreas: fact and fancy. Dev. Dyn. 240, 521-529.

Zhan T, Rindtorff N, Boutros M. Wnt signaling in cancer. Oncogene. 2017;36:146-1473