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Event Title

Androgen receptor, Agonism
Short name: Androgen receptor, Agonism

Key Event Overview

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AOPs Including This Key Event

AOP Name Event Type Essentiality
Androgen receptor agonism leading to reproductive dysfunction MIE Strong

Chemical Initiators

The following are chemical initiators that operate directly through this Event:

  1. 17beta-Trenbolone
  2. Spironolactone

Taxonomic Applicability

Name Scientific Name Evidence Links
fathead minnow Pimephales promelas Very Strong NCBI
medaka Oryzias latipes Strong NCBI

Level of Biological Organization

Biological Organization

How this Key Event works

Site of action: The molecular site of action is the ligand binding domain of the AR. The cellular site of action for the molecular initiating event is undefined.

Responses at the macromolecular level: Binding of a ligand, including xenobiotics that act as AR agonists, to the cytosolic AR mediates a conformational shift that facilitates dissociation from accompanying heat shock proteins and dimerization with another AR (Prescott and Coetzee 2006; Claessens et al. 2008; Centenera et al. 2008). Homodimerization unveils a nuclear localization sequence, allowing the AR-ligand complex to translocate to the nucleus and bind to androgen-response elements (AREs) (Claessens et al. 2008; Cutress et al. 2008). This elicits recruitment of additional transcription factors and transcriptional activation of androgen-responsive genes (Heemers and Tindall 2007).

How it is Measured or Detected

Methods that have been previously reviewed and approved by a recognized authority should be included in the Overview section above. All other methods, including those well established in the published literature, should be described here. Consider the following criteria when describing each method: 1. Is the assay fit for purpose? 2. Is the assay directly or indirectly (i.e. a surrogate) related to a key event relevant to the final adverse effect in question? 3. Is the assay repeatable? 4. Is the assay reproducible?

Measurement/detection: Binding to the androgen receptor can be directly measured in cell free systems based on displacement of a radio-labeled standard (generally testosterone or DHT) in a competitive binding assay (e.g., (Olsson et al. 2005; Sperry and Thomas 1999; Wilson et al. 2007; Tilley et al. 1989; Kim et al. 2010). However, cell based transcriptional activation assays are typically required to differentiate agonists from antagonists, in vitro. A number of reporter gene assays have been developed and used to screen chemicals for AR agonist and/or antagonist activity (e.g., (Wilson et al. 2002; van der Burg et al. 2010; Mak et al. 1999; Araki et al. 2005). Expression of androgen responsive proteins like spiggin in primary cell cultures has also been used to detect AR agonist activity (Jolly et al. 2006).

In fish, phenotypic masculinization of females has frequently been used as an indirect measurement of in vivo androgen receptor agonism. For example, development of nuptial tubercles, a dorsal fatpad, and a characteristic banding pattern has been observed in female fathead minnows exposed to androgen agonists (Ankley et al. 2003; Jensen et al. 2006; Ankley et al. 2010; LaLone et al. 2013). Likewise, anal fin elongation in female western mosquitofish (Gambusia affinis) has similarly been viewed as evidence of AR activation (Raut et al. 2011; Sone et al. 2005).

Measures of AR agonism have been included in high throughput screening programs, such as US EPA's Toxcast program. Toxcast assays relevant for screening chemicals for their ability to bind and/or activate the AR include:

  • ATG_AR_TRANS A cell based assay that can differentiate agonism from antagonism
  • NVS_NR_hAR A cell free assay using recombinant human AR. Can detect binding, but cannot distinguish agonism from antagonism.
  • NVS_NR_rAR A cell free assay using recombinant rat AR. Can detect binding, but cannot distinguish agonism from antagonism.
  • OT_AR_ARELUC_AG_1440 A cell based assay that measures expression of a reporter gene under control of androgen-responsive elements. Can distinguish agonism from antagonism.
  • Tox21_AR_BLA_Agonist_ratio A cell based assay with an inducible reporter. Can distinguish agonists from antagonists.
  • Tox21_AR_LUC_MDAKB2_agonist A cell based assay with an inducible reporter. Can distinguish agonists from antagonists.

Evidence Supporting Taxonomic Applicability

Taxonomic applicability: Androgen receptor orthologs are primarily limited to vertebrates (Baker 1997; Thornton 2001; Eick and Thornton 2011; Markov and Laudet 2011). Therefore, this MIE would generally be viewed as relevant to vertebrates, but not necessarily invertebrates.

Evidence for Chemical Initiation of this Molecular Initiating Event

Characterization of chemical properties: Androgen receptor binding chemicals can be grouped into two broad structural domains, steroidal and non-steroidal (Yin et al. 2003). Steroidal androgens consist primarily of testosterone and its derivatives (Yin et al. 2003). Many of the non-steroidal AR binding chemicals studied are derivatives of well known non-steroidal AR antagonists like bicalutamide, hydroxyflutamide, and nilutamide (Yin et al. 2003). Nonetheless, a number of QSARs and SARs that consider AR binding of both these pharmaceutical agents as well as environmental chemicals have been developed (Waller et al. 1996; Serafimova et al. 2002; Todorov et al. 2011; Hong et al. 2003; Bohl et al. 2004). However, it has been shown that very minor structural differences can dramatically impact function as either an agonist or antagonist (Yin et al. 2003; Bohl et al. 2004; Norris et al. 2009), making it difficult at present to predict agonist versus antagonist activity based on chemical structure alone.

In vivo considerations: A variety of steroidal androgens can be converted to estrogens in vitro through the action of cytochrome P450 19 (aromatase). Structures subject to aromatization may behave in vivo as estrogens despite exhibiting potent androgen receptor agonism in vitro.


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