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Event: 1888

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

Increased (ectopic) all-trans retinoic acid concentration

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
increased atRA concentration

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

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

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
Ectopic ATRA in fetal testis leads to reduced spem count MolecularInitiatingEvent Arthur Author (send email) Under development: Not open for comment. Do not cite


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
Vertebrates Vertebrates NCBI
mammals mammals 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
Fetal Moderate

Sex Applicability

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

Retinoic acid (RA) function and metabolism

RA and retinoid signaling is central for numerous physiological processes, and has important roles within reproduction, vision, development and the immune system (O'Byrne & Blaner, 2013). As an important morphogen, the levels of RA is tightly controlled within tissues both spatially and temporally. Both insufficient and excess RA has proven to cause severe adverse effects (Kedishvili, 2013).

RA homeostasis is maintained by tissue-specific enzymes controlling a 2-step biosynthesis pathway: the precursor retinol is converted into retinaldehyde via retinol dehydrogenases (RDHs). Retinaldehyde dehydrogenases (RALDHs) then irreversibly oxidize retinaldehyde into biologically active RA (reviewed by (Shannon et al, 2017)). RA is removed via degradation to polar inactive metabolites by cytochrome P450 (CYP) family hydroxylases; chiefly CYP26A1, B1 and C1 (Topletz et al, 2015).

RA signals through the nuclear Retinoic Acid Receptors (RARs) and Retinoid X Receptors (RXRs) thereby regulating transcription of target genes (Cunningham & Duester, 2015).

Ectopic RA as Key Event

Inhibition or disruption of any of the enzymes in the RA degradation pathway, including the Cyp26 family, lead to increased concentrations of biologically active RA in target cells.

Equally, application of RA for medical treatments, including for acute promyelocytic leukemia and cystic acne, lead to increased concentrations of biologically active RA in target cells.

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). ?

In vitro

Indirect measurement of activity and potency of RXRs and RARs is possible via luciferase assays in cell lines, for example (Chassot et al, 2020; Jurutka & Wagner, 2019).

In vivo

Direct measurements of atRA in serum (humans, animals) can be performed by various chromatographic methods (Gundersen et al, 2007), for instance by high performance liquid chromatography (HPLC) (De Leenheer et al, 1982) or liquid chromatography-tandem mass spectrometry (LC-MS) (Morgenstern et al, 2021).

Indirect measurements in animal models can be performed with various reporter assays with RAR-RXR-RARE or RXR-RXR-RARE promoter elements driving expression of reporter proteins. These reporter assays can detect the presence of ATRA in tissues in a semi-quantitative manner. Examples include reporter mouse lines (Carlsen et al, 2021; Rossant et al, 1991; Solomin et al, 1998).

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

The retinoid signaling pathway is highly evolutionary conserved between vertebrates. This KE is applicable for both mammalian sexes, across developmental stages into adulthood, in numerous cells and tissues and across taxa.

Evidence for Perturbation by Stressor

Overview for Molecular Initiating Event

When a specific MIE can be defined (i.e., the molecular target and nature of interaction is known), in addition to describing the biological state associated with the MIE, how it can be measured, and its taxonomic, life stage, and sex applicability, it is useful to list stressors known to trigger the MIE and provide evidence supporting that initiation. This will often be a list of prototypical compounds demonstrated to interact with the target molecule in the manner detailed in the MIE description to initiate a given pathway (e.g., 2,3,7,8-TCDD as a prototypical AhR agonist; 17α-ethynyl estradiol as a prototypical ER agonist). Depending on the information available, this could also refer to chemical categories (i.e., groups of chemicals with defined structural features known to trigger the MIE). Known stressors should be included in the MIE description, but it is not expected to include a comprehensive list. Rather initially, stressors identified will be exemplary and the stressor list will be expanded over time. For more information on MIE, please see pages 32-33 in the User Handbook.


Ketoconazole, an azole antifungal that functions by binding and disrupting the action of fungal cytochrome P-450 to disrupt fungal membranes. In mouse testis it has been shown to increase levels of endogenous RA (Bowles et al, 2006).


Liarozole, Talarozole, imidazole-containing compounds that inhibit the CYP26 enzymes, used as treatments for acne, psoriasis and other keratinization disorders as well as some cancers. In mice, treatment increased testicular RA levels (Stevison et al, 2017).

Bisphenol A

Bisphenol A, an organic synthetic compound used to make polycarbonate plastic. In mice, BPA exposure increased levels of atRA and expression of RA biosynthesis enzymes (incl Adh1 and Cyp1a2) (Esteban et al, 2019).


R115866, a triazole-containing molecule acts as a CYP26 inhibitor. In rats, treatment leads to increased endogenous levels of RA (Stoppie et al, 2000).


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

Bowles J, Knight D, Smith C, Wilhelm D, Richman J, Mamiya S, Yashiro K, Chawengsaksophak K, Wilson MJ, Rossant J, Hamada H, Koopman P (2006) Retinoid signaling determines germ cell fate in mice. Science 312: 596-600

Carlsen H, Ebihara K, Kuwata NH, Kuwata K, Aydemir G, Rühl R, Blomhoff R (2021) A transgenic reporter mouse model for in vivo assessment of retinoic acid receptor transcriptional activation International Journal for Vitamin and Nutrition Research In Press

Chassot AA, Le Rolle M, Jolivet G, Stevant I, Guigonis JM, Da Silva F, Nef S, Pailhoux E, Schedl A, Ghyselinck NB, Chaboissier MC (2020) Retinoic acid synthesis by ALDH1A proteins is dispensable for meiosis initiation in the mouse fetal ovary. Sci Adv 6: eaaz1261

Cunningham TJ, Duester G (2015) Mechanisms of retinoic acid signalling and its roles in organ and limb development Nat Rev Mol Cell Biol 16: 110-123

De Leenheer AP, Lambert WE, Claeys I (1982) All-trans-retinoic acid: measurement of reference values in human serum by high performance liquid chromatography. J Lipid Res 23: 1362-1367

Esteban J, Serrano-Maciá M, Sánchez-Pérez i, Alonso-Magdalena P, Pellín MC, García-Arévalo M, Nadal A, Barril J (2019) In utero exposure to bisphenol-A disrupts key elements of retinoid system in male mice offspring. Food Chem Toxicol 126: 142-151

Gundersen TE, Bastani NE, Blomhoff R (2007) Quantitative high-throughput determination of endogenous retinoids in human plasma using triple-stage liquid chromatography/tandem mass spectrometry. Rapid Commun Mass Spectrom 21: 1176-1186

Jurutka PW, Wagner CE (2019) Methods to Assess Activity and Potency of Rexinoids Using Rapid Luciferase-Based Assays: A Case Study with NEt-TMN. Methods Mol Biol 2019

Kedishvili NY (2013) Enzymology of retinoic acid biosynthesis and degradation. J Lipid Res 54: 1744-1760

Morgenstern J, Fleming T, Kliemank E, Brune M, Nawroth P, Fischer A (2021) Quantification of All-Trans Retinoic Acid by Liquid Chromatography-Tandem Mass Spectrometry and Association with Lipid Profile in Patients with Type 2 Diabetes. Metabolites 11: 60

O'Byrne S, Blaner WS (2013) Retinol and retinyl esters: biochemistry and physiology. J Lipid Res 54: 1731-1743

Rossant J, Zirngibl R, Cado D, Shago M, Giguère V (1991) Expression of a retinoic acid response element-hsplacZ transgene defines specific domains of transcriptional activity during mouse embryogenesis. Genes Dev 5: 1333-1344

Shannon SR, Moise AR, Trainor PA (2017) New insights and changing paradigms in the regulation of vitamin A metabolism in development. Wiley Interdiscip Rev Dev Biol 6: 10.1002/wdev.1264

Solomin L, Johansson CB, Zetterström RH, Bissonnette RP, Heyman RA, Olson L, Lendahl U, Frisén J, Perlmann T (1998) Retinoid-X receptor signalling in the developing spinal cord. Nature 395: 398-402

Stevison F, Hogarth CA, Tripathy S, Kent T, Isoherranen N (2017) Inhibition of the all-trans Retinoic Acid ( at RA) Hydroxylases CYP26A1 and CYP26B1 Results in Dynamic, Tissue-Specific Changes in Endogenous at RA Signaling. Drug Metab Dispos 45

Stoppie P, Borgers M, Borghgraef P, Dillen L, Goossens J, Sanz G, Szel G, Van Hove C, Van Nyen G, Nobels G, Vanden Bossche H, Venet M, Willemsens G, Van Wauwe J (2000) R115866 inhibits all-trans-retinoic acid metabolism and exerts retinoidal effects in rodents. J Pharmacol Exp Ther 293: 304-312

Topletz AR, Tripathy S, Foti RS, Shimshoni JA, Nelson WL, Isoherranen N (2015) Induction of CYP26A1 by metabolites of retinoic acid: evidence that CYP26A1 is an important enzyme in the elimination of active retinoids. Mol Pharmacol 87: 430-441