API

Event: 944

Key Event Title

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dimerization, AHR/ARNT

Short name

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dimerization, AHR/ARNT

Key Event Component

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Process Object Action
protein dimerization activity aryl hydrocarbon receptor disrupted
protein dimerization activity aryl hydrocarbon receptor nuclear translocator disrupted

Key Event Overview


AOPs Including This Key Event

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Stressors

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Level of Biological Organization

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Biological Organization
Molecular

Cell term

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Cell term
eukaryotic cell


Organ term

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Taxonomic Applicability

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Term Scientific Term Evidence Link
chicken Gallus gallus Strong NCBI
zebrafish Danio rerio Strong NCBI
mouse Mus musculus Strong NCBI

Life Stage Applicability

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Life stage Evidence
Embryo Strong
Development Strong

Sex Applicability

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Term Evidence
Unspecific Strong

How This Key Event Works

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Structure and Function of ARNT

  • The aryl hydrocarbon receptor nuclear translocator (ARNT) is a member of the Per-Arnt-Sim (PAS) family of proteins (Gu et al 2000).
  • PAS proteins share highly conserved PAS domains (Gu et al 2000).
  • PAS proteins act as transcriptional regulators in response to environmental and physiological cues (Gu et al 2000).
  • ARNTs have numerous key roles in vertebrates related to responses to developmental and environmental cues.

Isoforms of ARNT:

  • Over time ARNT has undergone gene duplication and diversification in vertebrates, which has resulted in three clades of ARNT, namely ARNT1, ARNT2, and ARNT3.
  • Each clade can include multiple isoforms and splice variants (Hill et al 2009; Lee et al 2007; Lee et al 2011; Powel & Hahn 2000; Tanguay et al 2000).
  • ARNT1s have been demonstrated to function predominantly through heterodimerization with the aryl hydrocarbon receptor (AhR) and hypoxia inducible factor 1 α (HIF1α) (Prasch et al 2004; 2006; Wang et al 1995).
  • ARNT2s are believed to function predominantly through heterodimerization with Single Minded (SIM) (Hirose et al 1996).
  • ARNT3s, which are also known as ARNT-like (ARNTL), Brain and Muscle ARNT-like-1 (BMAL1), or Morphine Preference 3 (MOP3), are believed to function predominantly through heterodimerization with Circadian Locomotor Output Cycles Kaput (CLOCK) (Gekakis et al 1998).

Roles of ARNTs in mammals:

  • ARNT1 functions in normal vascular and hematopoietic development (Kozak et al 1997; Maltepe et al 1997; Abbott & Buckalew 2000).
  • ARNT2 functions in development of the hypothalamus and nervous system (Hosoya et al 2001; Keith et al 2001).
  • ARNT3 functions in biological rhythms (Gekakis et al 1998).

Roles of ARNTs in other taxa:

  • ARNTs have been demonstrated to have roles in development of the heart, brain, liver, and possibly the peripheral nervous system in zebrafish (Danio rerio) (Hill et al 2009).
  • Roles of ARNTs in other taxa have not been sufficiently investigated to date.

Interaction with AHR

  • Both ARNT1s and ARNT2s are able to heterodimerize with AhR and interact with dioxin-responsive elements on the DNA in in vitro systems (Hirose et al 1996; Lee et al 2007; Lee et al 2011; Prasch et al 2004).
  • Selective knockdown of ARNTs in zebrafish (Danio rerio) demonstrates that ARNT1s, but not ARNT2s, are required for activation of the AhR in vivo (Prasch et al 2004; 2006).
  • In limited investigations ARNT3 has not been demonstrated to interact with the AHR either in vivo or in vitro (Jain et al 1998). 

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). The AHR-ARNT complex then binds to a xenobiotic response element (XRE) found in the promoter of an AHR-regulated gene and recruits co-regulators such as CREB binding protein/p300, steroid receptor co-activator (SRC) 1, SRC-2, SRC-3 and nuclear receptor interacting protein 1, leading to induction or repression of gene expression (Fujii-Kuriyama and Kawajiri 2010). Expression levels of several genes, including phase I (e.g. cytochrome P450 (CYP) 1A, CYP1B, CYP2A) and phase II enzymes (e.g. uridine diphosphate glucuronosyl transferase (UDP-GT), glutathione S-transferases (GSTs)), as well as genes involved in cell proliferation (transforming growth factor-beta, interleukin-1 beta), cell cycle regulation (p27, jun-B) and apoptosis (Bax), are regulated through this mechanism (Fujii-Kuriyama and Kawajiri 2010; Giesy et al. 2006; Mimura and Fujii-Kuriyama 2003; Safe 1994).


How It Is Measured or Detected

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AhR/ARNT heterodimerization can be measured in several ways:

1) The active AHR complexed with ARNT can be measured using protein-DNA interaction assays. Two methods are described in detail by Perez-Romero and Imperiale (Perez-Romero and Imperiale 2007). Chromatin immunoprecipitation measures the interaction of proteins with specific genomic regions in vivo. It involves the treatment of cells with formaldehyde to crosslink neighboring protein-protein and protein-DNA molecules. Nuclear fractions are isolated, the genomic DNA is sheared, and nuclear lysates are used in immunoprecipitations with an antibody against the protein of interest. After reversal of the crosslinking, the associated DNA fragments are sequenced. Enrichment of specific DNA sequences represents regions on the genome that the protein of interest is associated with in vivo. Electrophoretic mobility shift assay (EMSA) provides a rapid method to study DNA-binding protein interactions in vitro. This relies on the fact that complexes of protein and DNA migrate through a non-denaturing polyacrylamide gel more slowly than free DNA fragments. The protein-DNA complex components are then identified with appropriate antibodies. The EMSA assay was found to be consistent with the luciferase reporter gene assay (in chicken hepatoma cells dosed with dioxin-like compounds (Heid et al. 2001).

2) Species-specific differences in dimerization and differences in dimerization between ARNT isoform and AhR isoform combinations have been assessed through luciferase reporter gene (LRG) assays utilizing COS-7 cells transfected with expression constructs of AhR and ARNT isoforms of mammals, birds, and fishes (Abnet et al 1999; Bak et al 2013; Doering et al 2014; 2015; Hansson & Hahn 2008; Hirose et al 1996; Karchner et al 1999; Lee et al 2007; Lee et al 2011; Prasch et al 2004; Wirgin et al 2011). However, this method is indirect as it also includes binding of a ligand to the AhR, and interaction of the AhR/ARNT heterodimer with dioxin-responsive elements on the DNA.


Evidence Supporting Taxonomic Applicability

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Taxonomic Presence of ARNT genes:

  • ARNTs have been identified in all tetrapods investigated to date (Drutel et al 1996; Hirose et al 1996; Hoffman et al 1991; Lee et al 2007; Lee et al 2011).
  • ARNTs have been identified in a great phylogenetic diversity of fishes, including early fishes (Doering et al 2014; 2016).
  • ARNT has been identified in investigated invertebrates (Powell-Coffman et al 1998).

Taxonomic Applicability of Heterodimerization of ARNT isoforms with AhR isoforms:

  •  In mouse (Mus mus) and chicken (Gallus gallus) both the ARNT1 and ARNT2 were capable of heterdimerizing with AHR and interacting with dioxin-responsive elements on the DNA in vitro (Hirose et al 1996; Lee et al 2007; Lee et al 2011; Prasch et al 2004). However, no studies have yet confirmed involvement of both ARNT1 and ARNT2 in vivo.
  • In zebrafish, all adverse effects of DLCs so far examined in vivo are mediated solely by ARNT1 based on knockdown studies, although ARNT2 is capable of heterodimerizing with AHR2 and interacting with dioxin-responsive elements on the DNA in vitro (Prasch et al 2004; Prasch et al 2006). In addition to AHRs of zebrafish, AHRs of Atlantic salmon (Salmo salar), Atlantic tomcod (Microgadus tomcod), mummichog, rainbow trout, and red seabream (Pagrus major) have been demonstrated to heterodimerize with ARNT1 in vitro (Abnet et al 1999; Bak et al 2013; Hansson & Hahn 2008; Karchner et al 1999; Wirgin et al 2011), while AHRs of white sturgeon (Acipenser transmontanus), and lake sturgeon (Acipenser fulvescens) have been demonstrated to heterodimerize with ARNT2 in vitro (Doering et al 2014b; 2015b; Prasch et al 2004; 2006). 

This mechanism is conserved across species. Mammals possess a single AHR, whereas birds and fish express multiple isoforms, and all three express multiple ARNT isoforms. Not all of the isoforms identified are functionally active. For example, killifish AHR1 and AHR2 are active and display different transcription profiles, whereas zebrafish AHR2 and ARNT2 are active in mediating xenobiotic-mediated toxicity and AHR1 is inactive (Hahn et al. 2006; Prasch et al. 2006).


Evidence for Perturbation by Stressor



References

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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.

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4. 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.

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. Perez-Romero, P., and Imperiale, M. J. (2007). Assaying protein-DNA interactions in vivo and in vitro using chromatin immunoprecipitation and electrophoretic mobility shift assays. Methods Mol. Med. 131, 123-139.

7. Prasch, A. L., Tanguay, R. L., Mehta, V., Heideman, W., and Peterson, R. E. (2006). Identification of zebrafish ARNT1 homologs: 2,3,7,8-tetrachlorodibenzo-p-dioxin toxicity in the developing zebrafish requires ARNT1. Mol. Pharmacol. 69(3), 776-787.

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Abnet, C.C.; Tanguay, R.L.; Heideman, W.; Peterson, R.E. 1999. Transactivation activity of human, zebrafish, and rainbow trout aryl hydrocarbon receptors expressed in COS-7 cells: Greater insight into species differences in toxic potency of polychlorinated dibenzo-p-dioxin, dibenzofuran, and biphenyl congeners. Toxicol. Appl. Pharmacol. 159, 41-51.

 

Andreasen, E.A.; Hahn, M.E.; Heideman, W.; Peterson, R.E.; Tanguay, R.L. 2002. The zebrafish (Danio rerio) aryl hydrocarbon receptor type 1 is a novel vertebrate receptor. Molec. Pharmacol. 62, 234-249.

 

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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.

 

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Doering, J.A.; Tang, S.; Peng, H.; Eisner, B.K.; Sun, J.; Giesy, J.P.; Wiseman, S.; Hecker, M. 2016. High conservation in transcriptomic and proteomic response of white sturgeon to equipotent concentrations of 2,3,7,8-TCDD, PCB 77, and benzo[a]pyrene. Enviro. Sci. Technol. 50 (9), 4826-4835.

 

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.

 

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.; Wiseman, S; Beitel, S.C.; Giesy, J.P.; Hecker, M. 2014b. Identification and expression of aryl hydrocarbon receptors (AhR1 and AhR2) provide insight in an evolutionary context regarding sensitivity of white sturgeon (Acipenser transmontanus) to dioxin-like compounds. Aquat. Toxicol. 150, 27-35.

 

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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.

 

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