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Relationship: 972


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, AhR leads to dimerization, AHR/ARNT

Upstream event
The causing Key Event (KE) in a Key Event Relationship (KER). More help
Downstream event
The responding Key Event (KE) in a Key Event Relationship (KER). More help

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
Aryl hydrocarbon receptor activation leading to early life stage mortality, via reduced VEGF adjacent High Moderate Arthur Author (send email) Open for citation & comment WPHA/WNT Endorsed
Aryl hydrocarbon receptor activation leading to early life stage mortality, via increased COX-2 adjacent High Moderate Allie Always (send email) Open for citation & comment WPHA/WNT Endorsed
Embryonic Activation of the AHR leading to Reproductive failure, via epigenetic down-regulation of GnRHR adjacent High Moderate Arthur Author (send email) Under development: Not open for comment. Do not cite
AhR activation leading to preeclampsia adjacent Agnes Aggy (send email) Under development: Not open for comment. Do not cite Under Development
Aryl hydrocarbon receptor activation leading to early stage mortality via impeded craniofacial development adjacent High High Agnes Aggy (send email) Under development: Not open for comment. Do not cite
Aryl hydrocarbon receptor activation leading to early stage mortality via cardiovascular toxicity adjacent High High Allie Always (send email) Under development: Not open for comment. Do not cite

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
Mus musculus Mus musculus High NCBI
Danio rerio Danio rerio High NCBI
rainbow trout Oncorhynchus mykiss High NCBI
Pagrus major Pagrus major High NCBI
Acipenser fulvescens Acipenser fulvescens High NCBI
Salmo salar Salmo salar High NCBI
Acipenser transmontanus Acipenser transmontanus High NCBI
Xenopus laevis Xenopus laevis High NCBI
Ambystoma mexicanum Ambystoma mexicanum High NCBI
Microgadus tomcod Microgadus tomcod High NCBI
human Homo sapiens High NCBI
Gallus gallus Gallus gallus High NCBI
Phasianus colchicus Phasianus colchicus High NCBI
Coturnix japonica Coturnix japonica High NCBI

Sex Applicability

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

In its unliganded form, the AHR is part of a cytosolic complex containing heat shock protein 90 (HSP90), the HSP90 co-chaperone p23 and AHR-interacting protein (AIP) (Fujii-Kuriyama et al. 2010).  Upon ligand binding, the aryl hydrocarbon receptor (AHR) migrates to the nucleus where it dissociates from the cytosolic complex and forms a heterodimer with AHR nuclear translocator (ARNT) (Mimura and Fujii-Kuriyama 2003).

AhRs can heterodimerize with ARNT1 and ARNT2 isoforms in order to activate reporter constructs in transfected cells and recognize response elements in gel shift assays in all investigated vertebrates, including birds, fishes, and reptiles (Abnet et al 1999; Andreasen et al 2002a; 2002b; Bak et al 2013; Doering et al 2014; Doering et al 2015; Farmahin et al 2012; 2013; Hansson & Hahn 2008; Karchner et al 1999; 2006; Lavine et al 2005; Shoots et al 2015; Tanguay et al 1999; 2000; Wirgin et al 2011). 

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

The mechanism of AHR-mediated transcriptional regulation is well understood (Fujii-Kuriyama and Kawajiri 2010).

Numerous PAS proteins are known to interact with each other in response to environmental and developmental cues through dimerization at their PAS domains (Pohjanvirta 2012).

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 are uncertainties in the precise physiological and toxicological roles of different AhR clades (AhR1, AhR2, AhR3) and isoforms (α, β, δ, γ).
  • There are uncertainties in the precise physiological and toxicological roles of different ARNT clades (ARNT1, ARNT2, ARNT3) and isoforms (a, b, c).
  • Nothing is known about differences in binding affinity of AhR for ARNT and of the AhR/ARNT heterodimer for DNA among species and taxa.
  • There is uncertainty in whether anthropogenic contaminants that act as ligands of the AhR and lead to dimerization of AhR with ARNT in vertebrates also act as ligands in invertebrates.

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
Response-response Relationship
Provides sources of data that define the response-response relationships between the KEs.  More help
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
  • The aryl hydrocarbon receptor (AhR) and aryl hydrocarbon receptor nuclear translocator (ARNT) are highly conserved and ancient proteins with homologs having been identified in most major animal groups, apart from the most ancient lineages, such as sponges (Porifera) (Hahn et al 2002). 
  • In vitro dimerization of AhRs and ARNTs have been demonstrated in mammals, birds, reptiles, amphibians, teleost and non-teleost fishes, and some invertebrates (Butler et al 2001; Emmons et al 1999; Hahn et al 2002; Powell-Coffman et al 1998).


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

1. Fujii-Kuriyama, Y., and Kawajiri, K. (2010). Molecular mechanisms of the physiological functions of the aryl hydrocarbon (dioxin) receptor, a multifunctional regulator that senses and responds to environmental stimuli. Proc. Jpn. Acad. Ser. B Phys. Biol. Sci. 86(1), 40-53.

2. Giesy, J. P., Kannan, K., Blankenship, A. L., Jones, P. D., and Newsted, J. L. (2006). Toxicology of PCBs and related compounds. In Endocrine Disruption Biological Bases for Health Effects in Wildlife and Humans (D.O.Norris and J.A.Carr, Eds.), pp. 245-331. Oxford University Press, New York.

3. Heid, S. E., Walker, M. K., and Swanson, H. I. (2001). Correlation of cardiotoxicity mediated by halogenated aromatic hydrocarbons to aryl hydrocarbon receptor activation. Toxicol. Sci 61(1), 187-196.

4. Hoffman, E. C., Reyes, H., Chu, F. F., Sander, F., Conley, L. H., Brooks, B. A., and Hankinson, O. (1991). Cloning of a factor required for activity of the Ah (dioxin) receptor. Science 252(5008), 954-958.

5. Mimura, J., and Fujii-Kuriyama, Y. (2003). Functional role of AhR in the expression of toxic effects by TCDD. Biochimica et Biophysica Acta - General Subjects 1619(3), 263-268.

6. Poland, A., Glover, E., and Kende, A. S. (1976). Stereospecific, high affinity binding of 2,3,7,8-tetrachlorodibenzo-p-dioxin by hepatic cytosol. Evidence that the binding species is receptor for induction of aryl hydrocarbon hydroxylase. J. Biol. Chem. 251(16), 4936-4946.

7. Safe, S. (1994). Polychlorinated biphenyls (PCBs): Environmental impact, biochemical and toxic responses, and implications for risk assessment. Critical Reviews in Toxicology 24(2), 87-149.

Andreasen, E.A.; Tanguay, R.L.; Peterson, R.E.; Heideman, W. 2002. Identification of a critical amino acid in the aryl hydrocarbon receptor. J. Biol. Chem. 277 (15), 13210-13218.

Bak, S.M.; Lida, M.; Hirano, M.; Iwata, H.; Kim, E.Y. 2013. Potencies of red seabream AHR1- and AHR2-mediated transactivation by dioxins: implications of both AHRs in dioxin toxicity. Environ. Sci. Technol. 47 (6), 2877-2885.

Butler, R.A.; Kelley, M.L.; Powell, W.H.; Hahn, M.E.; Van Beneden, R.J. (2001). An aryl hydrocarbon receptor (AHR) homologue from the soft-shelled clam, Mya arenaria: evidence that invertebrate AHR homologues lack 2,3,7,8-tetrachlorodibenzo-p-dioxin and beta-naphthoflavone binding. Gene. 278, 223-234.

Doering, J.A.; Farmahin, R.; Wiseman, S.; Beitel, S.C.; Kennedy, S.W.; Giesy, J.P.; Hecker, M. 2015. Differences in activation of aryl hydrocarbon receptors of white sturgeon relative to lake sturgeon are predicted by identities of key amino acids in the ligand binding domain. Enviro. Sci. Technol. 49, 4681-4689.

Doering, J.A.; Farmahin, R.; Wiseman, S.; Kennedy, S.; Giesy J.P.; Hecker, M. 2014. Functionality of aryl hydrocarbon receptors (AhR1 and AhR2) of white sturgeon (Acipenser transmontanus) and implications for the risk assessment of dioxin-like compounds. Enviro. Sci. Technol. 48, 8219-8226.

Elfrink, C.; Gasiewicz, T.; Whitlock, J. (1990). Protein-DNA interactions at a dioxin-responsive enhancer. Evidence that the transformed Ah receptor is heteromeric. J. Biol. Chem. 265, 20708-20712.

Emmons, R.B.; Duncan, D.; Estes, P.A.; Kiefel, P.; Mosher, J.T.; Sonnenfeld, M.; Ward, M.P.; Duncan, I.; Crews, S.T. (1999). The spineless-aristapedia and tango bHLH-PAS proteins interact and control antennal and tarsal development in Drosophilia. Dev. 126, 3937-3945.

Farmahin, R.; Manning, G.E.; Crump, D.; Wu, D.; Mundy, L.J.; Jones, S.P.; Hahn, M.E.; Karchner, S.I.; Giesy, J.P.; Bursian, S.J.; Zwiernik, M.J.; Fredricks, T.B.; Kennedy, S.W. 2013. Amino acid sequence of the ligand-binding domain of the aryl hydrocarbon receptor 1 predicts sensitivity of wild birds to effects of dioxin-like compounds. Toxicol. Sci. 131 (1), 139-152.

Farmahin, R.; Wu, D.; Crump, D.; Herve, J.C.; Jones, S.P.; Hahn, M.E.; Karchner, S.I.; Giesy, J.P.; Bursian, S.J.; Zwiernik, M.J.; Kennedy, S.W. 2012. Sequence and in vitro function of chicken, ring-necked pheasant, and Japanese quail AHR1 predict in vivo sensitivity to dioxins. Enviro. Sci. Toxicol. 46 (5), 2967-2975.

Farmahin, R.; Crump, D.; O’Brien, J.M.; Jones, S.P.; Kennedy, S.W. (2016). Time-dependent transcriptomic and biochemical responses of 6-formylindolo[3,2-b]carbazole (FICZ) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) are explained by AHR activation time. Biochem. Pharmacol. 115 (1), 134-143.

Hahn, M.E. 2002. Aryl hydrocarbon receptors: diversity and evolution. Chemico-Biol. Interact. 141, 131-160.

Hansson, M.C.; Hahn, M.E. 2008. Functional properties of the four Atlantic salmon (Salmo salar) aryl hydrocarbon receptor type 2 (AHR2) isoforms. Aquat. Toxicol. 86, 121-130.

Karchner, S.I.; Franks, D.G.; Kennedy, S.W.; Hahn, M.E. 2006. The molecular basis for differential dioxin sensitivity in birds: Role of the aryl hydrocarbon receptor. Proc. Natl. Acad. Sci. USA. 103, 6252-6257.

Karchner, S.I.; Powell, W.H.; Hahn, M.E. 1999. Identification and functional characterization of two highly divergent aryl hydrocarbon receptors (AHR1 and AHR2) in the Teleost Fundulus heteroclitus. Evidence for a novel subfamily of ligand-binding basic helix loop helix-Per-ARNT-Sim (bHLH-PAS) factors. J. Biol. Chem. 274, 33814-33824.

Lavine, J.A.; Rowatt, A.J.; Klimova, T.; Whitington, A.J.; Dengler, E.; Beck, C.; Powell, W.H. 2005. Aryl hydrocarbon receptors in the frog Xenopus laevis: two AhR1 paralogs exhibit low affinity for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Toxicol. Sci. 88 (1), 60-72.

Manning G.E.; Farmahin, R.; Crump, D.; Jones, S.P.; Klein, J.; Konstantinov, A.; Potter, D.; Kennedy, S.W. 2012. A luciferase reporter gene assay and aryl hydrocarbon receptor 1 genotype predict the LD50 of polychlorinated biphenyls in avian species. Toxicol. Appl. Pharm. 263, 390-401.

Ohi, H.; Fujita, Y.; Miyao, M.; Saguchi, K.; Murayama, N.; Higuchi, S. 2003. Molecular cloning and expression analysis of the aryl hydrocarbon receptor of Xenopus laevis. Biochem. Biophysic. Res. Comm. 307 (3), 595-599.

Powell-Coffman, J.A.; Bradfield, C.A.; Wood, W.B. (1998). Caenorhabditis elgans orthologs of the aryl hydrocarbon receptor and its dimerization partner the aryl hydrocarbon receptor nuclear translocator. Proceedings of the National Academy of Sciences of the United States of America. 95, 2844-2449.

Shoots, J.; Fraccalvieri, D.; Franks, D.G.; Denison, M.S.; Hahn, M.E.; Bonati, L.; Powell, W.H. 2015. An aryl hydrocarbon receptor from the salamander Ambystoma mexicanum exhibits low sensitivity to 2,3,7,8-tetrachlorodibenzo-p-dioxin. Enviro. Sci. Technol. 49, 6993-7001.

Swanson, H.; Tullis, K.; Denison, M. (1993). Binding of transformed Ah receptor complex to a dioxin responsive transcriptional enhancer: evidence for two distinct heterodimeric DNA-binding forms. Biochem. 32, 12841-12849.

Tanguay, R.L.; Abnet, C.C.; Heideman, W. Peterson, R.E. (1999). Cloning and characterization of the zebrafish (Danio rerio) aryl hydrocarbon receptor1. Biochimica et Biophysica Act 1444, 35-48.

Tanguay, R.L.; Andreasen, E.; Heideman, W.; Peterson, R.E. (2000). Identification and expression of alternatively spliced aryl hydrocarbon nuclear translocator 2 (ARNT2) cDNAs from zebrafish with distinct functions. BBA. 1494 (1-2), 117-128.

Okey, A. (2007). An aryl hydrocarbon receptor odyssey to the shores of toxicology: the deichmann Lecture, International Congress of Toxicology-XI. Toxicol. Sci. 98, 5-38.

Wirgin, I.; Roy, N.K.; Loftus, M.; Chambers, R.C.; Franks, D.G.; Hahn, M.E. 2011. Mechanistic basis of resistance to PCBs in Atlantic tomcod from the Hudson River. Science. 331, 1322-1324.