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Agonism, Androgen receptor leads to Increased, Differentiation to Testis
Key Event Relationship Overview
AOPs Referencing Relationship
|AOP Name||Adjacency||Weight of Evidence||Quantitative Understanding||Point of Contact||Author Status||OECD Status|
|Androgen receptor agonism leading to male-biased sex ratio||adjacent||Dan Villeneuve (send email)||Open for citation & comment|
Life Stage Applicability
Key Event Relationship Description
This key event relationship links androgen receptor agonism in teleost fish during gonadogenesis to increased differentiation to testis. Sex determination in teleost fishes is highly plastic; it can be genetically or environmentally influenced. Species with environmentally-based sex determination in particular can be very sensitive to exogenous chemicals during the period of differentiation. Exogenous hormones are of ecological concern because they have the potential to alter gonad development and sex differentiation. Activation of the androgen receptor (AR) by endogenous androgens plays a crucial role in normal sex differentiation, sexual maturation, and spermatogenesis in vertebrates and inappropriate signaling by exogenous AR agonists can disrupt theses processes. For example, studies have shown that during early development in some teleost species, exposure to androgenic steroids can induce complete gonadal sex inversion, resuting in increased differentiation to testis.
Evidence Collection Strategy
Evidence Supporting this KER
The biological plausibility linking AR activation to increased differentiation to testis is very strong. Actions of androgens are mediated by the AR, a ligand-dependent transcription factors (Hossain et al., 2008). Steroidal androgens act by entering the cell and forming a complex with the AR, resulting in conformational change (Bohen et al., 1995; Pratt and Toft, 1997). The ligand-AR complex is translocated to the nucleus where it binds to specific short DNA sequences thereby activating transcripton of androgen regulated genes (Harbott et al., 2009). During sexual development, endogenous androgen can therefore induce the upregulation of many genes involved in the male developmental pathway, including gonad development/differentiation.
Uncertainties and Inconsistencies
As noted below, it is uncertain as to the full range of species this key event relationship might be applicable due to susbtantial taxonomic variation in the role that steroid signaling plays in gonadal differentiation.
Known modulating factors
There are almost certainly many factors that could modulate this KER, but a systematic description of these is not currently possible.
|Modulating Factor (MF)||MF Specification||Effect(s) on the KER||Reference(s)|
The timeframe for differentiation of the bipotential gonad is species-dependent occurring, for example, over the course of days to weeks in most fishes. However, this period of time could be substantially longer in long-lived species.
Known Feedforward/Feedback loops influencing this KER
Domain of Applicability
The life stage applicable to this KER is developing embryos and juveniles prior to- or during the gonadal developmental stage. This KER is not applicable to sexually differentiated adults.
The molecular initiating event for this KER occurs prior to gonad differentiation. Therefore, this AOP is only applicable to sexually undifferentiated individuals.
Most evidence for this KER is derived from fish in the class Osteichthyes. Both phylogenetic analysis and evaluation of protein sequence conservation via SeqAPASS (https://seqapass.epa.gov/seqapass/) has shown that the structure of the AR is well conserved among most jawed vertebrates (LaLone et al. 2018). This KER is not expected to apply to mammals, birds, or other jawed vertebrates with genetic sex determination. However, it may be applicable to fishes, amphibians, and reptiles with environmentally-dependent sex determination, although outcomes may differ across physiologically different taxa. The present KER is not considered relevant to Agnathans since the AR appears not to be present in jawless fishes (Thornton 2001; Hossain et al 2008).
Baumann, L., Knörr, S., Keiter, S., Nagel, T., Rehberger, K., Volz, S., Oberrauch, S., Schiller, V., Fenske, M., Holbech, H., Segner, H., & Braunbeck, T. (2014). Persistence of endocrine disruption in zebrafish (Danio rerio) after discontinued exposure to the androgen 17β-trenbolone. Environmental toxicology and chemistry, 33(11), 2488–2496. https://doi.org/10.1002/etc.2698
Bohen, S. P., Kralli, A., & Yamamoto, K. R. (1995). Hold 'em and fold 'em: chaperones and signal transduction. Science (New York, N.Y.), 268(5215), 1303–1304. https://doi.org/10.1126/science.7761850
Harbott, L. K., Burmeister, S. S., White, R. B., Vagell, M., & Fernald, R. D. (2007). Androgen receptors in a cichlid fish, Astatotilapia burtoni: structure, localization, and expression levels. The Journal of comparative neurology, 504(1), 57–73. https://doi.org/10.1002/cne.21435
Hossain, M. S., Larsson, A., Scherbak, N., Olsson, P. E., & Orban, L. (2008). Zebrafish androgen receptor: isolation, molecular, and biochemical characterization. Biology of reproduction, 78(2), 361–369. https://doi.org/10.1095/biolreprod.107.062018
LaLone, C.A., D.L. Villeneuve, J.A. Doering, B.R. Blackwell, T.R. Transue, C.W. Simmons, J. Swintek, S.J. Degitz, A.J. Williams and G.T. Ankley. 2018. Evidence for cross-species extrapolation of mammalian-based high-throughput screening assay results. Environ. Sci. Technol. 52, 13960-13971.
Morthorst, J. E., Holbech, H., & Bjerregaard, P. (2010). Trenbolone causes irreversible masculinization of zebrafish at environmentally relevant concentrations. Aquatic toxicology (Amsterdam, Netherlands), 98(4), 336–343. https://doi.org/10.1016/j.aquatox.2010.03.008
Orn, S., Yamani, S., & Norrgren, L. (2006). Comparison of vitellogenin induction, sex ratio, and gonad morphology between zebrafish and Japanese medaka after exposure to 17alpha-ethinylestradiol and 17beta-trenbolone. Archives of environmental contamination and toxicology, 51(2), 237–243. https://doi.org/10.1007/s00244-005-0103-y
Pratt, W. B., & Toft, D. O. (1997). Steroid receptor interactions with heat shock protein and immunophilin chaperones. Endocrine reviews, 18(3), 306–360. https://doi.org/10.1210/edrv.18.3.0303
Seki, M., Yokota, H., Matsubara, H., Maeda, M., Tadokoro, H., & Kobayashi, K. (2004). Fish full life-cycle testing for androgen methyltestosterone on medaka (Oryzias latipes). Environmental toxicology and chemistry, 23(3), 774–781. https://doi.org/10.1897/03-26
Shi, W. J., Jiang, Y. X., Huang, G. Y., Zhao, J. L., Zhang, J. N., Liu, Y. S., Xie, L. T., & Ying, G. G. (2018). Dydrogesterone Causes Male Bias and Accelerates Sperm Maturation in Zebrafish ( Danio rerio). Environmental science & technology, 52(15), 8903–8911. https://doi.org/10.1021/acs.est.8b02556
Thornton J. W. (2001). Evolution of vertebrate steroid receptors from an ancestral estrogen receptor by ligand exploitation and serial genome expansions. Proceedings of the National Academy of Sciences of the United States of America, 98(10), 5671–5676. https://doi.org/10.1073/pnas.091553298