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Decrease, DHT level leads to Decrease, AR activation
Key Event Relationship Overview
AOPs Referencing Relationship
|AOP Name||Adjacency||Weight of Evidence||Quantitative Understanding||Point of Contact||Author Status||OECD Status|
|Inhibition of 17α-hydrolase/C 10,20-lyase (Cyp17A1) activity leads to birth reproductive defects (cryptorchidism) in male (mammals)||adjacent||High||High||Bérénice COLLET (send email)||Open for citation & comment|
|Decreased testosterone synthesis leading to short anogenital distance (AGD) in male (mammalian) offspring||adjacent||High||Moderate||Terje Svingen (send email)||Under development: Not open for comment. Do not cite||Under Development|
|5α-reductase inhibition leading to short anogenital distance (AGD) in male (mammalian) offspring||adjacent||Terje Svingen (send email)||Under development: Not open for comment. Do not cite||Under Development|
Life Stage Applicability
|During development and at adulthood||High|
Key Event Relationship Description
Dihydrotestosterone (DHT) is a primary ligand for the Androgen receptor (AR), a nuclear receptor and transcription factor. DHT is an endogenous sex hormone that is synthesized from e.g. testosterone by the enzyme 5α-reductase in different tissues and organs (Davey & Grossmann, 2016; Marks, 2004). In the absence of ligand (e.g. DHT) the AR is localized in the cytoplasm in complex with molecular chaperones. Upon ligand binding, AR is activated, translocated into the nucleus, and dimerizes to carry out its ‘genomic function’ (Davey & Grossmann, 2016). Hence, AR transcriptional function is directly dependent on the presence of ligands, with DHT being a more potent AR activator than testosterone (Grino et al, 1990). Reduced levels of DHT may thus lead to reduced AR activation. Besides its genomic actions, the AR can also mediate rapid, non-genomic second messenger signaling (Davey and Grossmann, 2016). Decreased DHT levels that lead to reduced AR activation can thus entail downstream effects on both genomic and non-genomic signaling.
Evidence Collection Strategy
This KER is considered canonical knowledge and supporting literature was mainly sourced from key review articles from the open literature.
Evidence Supporting this KER
The biological plausibility of KER1935 is considered high.
The activation of AR is dependent on binding of ligands (though a few cases of ligand-independent AR activation has been shown, see uncertainties and inconsistencies), primarily testosterone and DHT in most vertebrates and 11-ketotestosterone in teleost fishes (Schuppe et al., 2020). Without ligand activation, the AR will remain in the cytoplasm associated with heat-shock and other chaperones and not be able to carry out its canonical (‘genomic’) function. Upon androgen binding, the AR undergoes a conformational change, chaperones dissociate, and a nuclear localization signal is exposed. The androgen/AR complex can now translocate to the nucleus, dimerize and bind AR response elements to regulate target gene expression (Davey and Grossmann, 2016; Eder et al., 2001).
The requirement for androgens binding to the AR for transcriptional activity has been extensively studied and proven and is generally considered textbook knowledge. The OECD test guideline no. 458 uses DHT as the reference chemical for testing androgen receptor activation in vitro (OECD, 2020). In the absence of DHT during development caused by 5α-reductase deficiency (i.e. still in the presence of testosterone) male fetuses fail to masculinize properly. This is evidenced by, for instance, individuals with congenital 5α-reductase deficiency conditions (Costa et al., 2012); conditions not limited to humans (Robitaille and Langlois, 2020), testifying to the importance of specifically DHT for AR activation and subsequent masculinization of certain reproductive tissues.
Binding of testosterone or DHT has differential effects in different tissues. E.g. in the developing mammalian male; testosterone is required for development of the internal sex organs (epididymis, vas deferens and the seminal vesicles), whereas DHT is crucial for development of the external sex organs (Keller et al., 1996; Robitaille and Langlois, 2020).
Uncertainties and Inconsistencies
Ligand-independent actions of the AR have been identified. To what extent and of which biological consequences is not well defined (Bennesch and Picard, 2015).
It should be noted, that in tissues, that are not DHT-dependent but rather respond to T, a decrease in DHT level may not influence AR activation significantly in that specific tissue.
Known modulating factors
|Modulating Factor (MF)||MF Specification||Effect(s) on the KER||Reference(s)|
|Age||AR expression changes with aging||Tissue-specific alterations in AR activity with aging||(Supakar et al., 1993; Wu et al., 2009)|
|Genotype||Number of CAG repeats in the first exon of AR||Decreased AR activation with increased number of CAGs||(Chamberlain et al., 1994; Tut et al., 1997)|
|Androgen deficiency syndrome||Low circulating testosterone levels due to primary (testicular) or secondary (pituitary-hypothalamic) hypogonadism||Reduced levels of circulating testosterone, precurser of DHT||(Bhasin et al., 2010)|
|Castration||Removal of testicles||Reduced levels of circulating testosterone, precurser of DHT||(Krotkiewski et al., 1980)|
There is a positive dose-response relationship between increasing concentrations of DHT and AR activation (Dalton et al., 1998; OECD, 2020). However, there is not enough data, or overview of the data, to define a quantitative linkage in vivo, and such a relationship will differ between biological systems (species, tissue, cell type).
Upon DHT binding to the AR, a conformational change that brings the amino (N) and carboxy (C) termini into close proximity occurs with a t1/2 of approximately 3.5 minutes, around 6 minutes later the AR dimerizes as shown in transfected HeLa cells (Schaufele et al., 2005). Addition of 5 nM DHT to the culture medium of ‘AR-resistant’ transfected prostatic cancer cells resulted in a rapid (from 15 minutes, maximal at 30 minutes) nuclear translocation of the AR with minimal residual cytosolic expression (Nightingale et al., 2003). AR and promoter interactions occur within 15 minutes of ligand binding, and RNA polymerase II and coactivator recruitment are then proposed to occur transiently with cycles of approximately 90 minutes (Kang et al., 2002).
Known Feedforward/Feedback loops influencing this KER
Androgens can upregulate and downregulate AR expression as well as 5α-reductase expression, but for 5α-reductase, each isoform in each tissue is differently regulated by androgens and can display sexual dimorphism (Lee and Chang, 2003; Robitaille and Langlois, 2020).
Domain of Applicability
KER1935 is assessed applicable to vertebrates, as DHT and AR activation are known to be related in these species.
KER1935 is assessed applicable to both sexes, as DHT activates AR in both males and females.
KER1935 is considered applicable to developmental and adult life stages, as DHT-mediated AR activation is relevant from the AR is expressed.
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