Relationship: 972



Activation, AhR leads to dimerization, AHR/ARNT

Upstream event


Activation, AhR

Downstream event


dimerization, AHR/ARNT

Key Event Relationship Overview


AOPs Referencing Relationship


Taxonomic Applicability


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


Sex Evidence
Unspecific High

Life Stage Applicability


Term Evidence
All life stages High

Key Event Relationship Description


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 Supporting this KER


Biological Plausibility


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).

Empirical Evidence


ARNT is a necessary dimerization partner for the transcriptional activation of AHR regulated genes (Hoffman et al. 1991; Poland et al. 1976). The AHR/ARNT complex was confirmed following in vitro exposure to halogenated aromatic hydrocarbons using an electrophoretic mobility shift assay; a dose-dependent supershift in DNA-binding was observed using specific antibodies in chicken and human cell lines (Heid et al. 2001).

  • Unliganded AhR exists as a cytosolic 9S form, while in the presence of a ligand the AhR exists as a nuclear 6S form. ARNT exists as a nuclear 6S form (Okey 2007).
  • The 6S form of AhR is approximately 210 kDa. Ligated AhR is approximately 100 kDa and ARNT is approximately 110 kDa (Elferink et al 1990; Swanson et al 1993).
  • Dimerization of AhRs with ARNTs has been demonstrated in all invertebrate and vertebrate species so far investigated (Butler et al 2001; Emmons et al 1999; Hahn et al 2002; Powell-Coffman et al 1998).
  • Heterodimers are not formed on response elements in gel shift assays in the absence of AhR and/or ARNT (Tanguay et al 2000). 


Uncertainties and Inconsistencies


  • 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.

Quantitative Understanding of the Linkage


  • Strong quantitative relationships are known for exposure to ligands and interaction with DREs on the DNA by use of transfected COS-7 cells and gel shift assays (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; Manning et al 2012; Oka et al 2016; Shoots et al 2015; Tanguay et al 1999; 2000; Wirgin et al 2011).
  • Specifically, greater concentrations of ligands or greater potency ligands cause greater interaction with DREs on the DNA.
  • Numerous ligands of the AhR are rapidly metabolized and only cause transient activation of the AhR. These ligands do not result in sustained interaction with DREs and do not cause downstream effects (Farmhin et al 2016).
  • However, no studies specifically investigate AHR/ARNT dimerization quantitatively despite considerable indirect quantitative information.


  • Because ARNT is a necessary dimerization partner for the transcriptional activation of AHR, it can be assumed that AHR interaction with DREs correlates with AHR/ARNT dimerization, which provides some insight into the quantitative understanding of this key event relationship.  However, it is not clear as to whether AHR interaction with DREs is directly proportional to AHR/ARNT dimerization.  Therefore, the quantitative understanding of this link is based solely on indirect evidence.

Response-response Relationship




Known modulating factors


Known Feedforward/Feedback loops influencing this KER


Domain of Applicability


  • 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).



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.