Upstream eventAgonism, Androgen receptor
Reduction, Testosterone synthesis by ovarian theca cells
Key Event Relationship Overview
AOPs Referencing Relationship
|AOP Name||Adjacency||Weight of Evidence||Quantitative Understanding|
|Androgen receptor agonism leading to reproductive dysfunction||non-adjacent||Moderate||Low|
|fathead minnow||Pimephales promelas||High||NCBI|
|Fundulus heteroclitus||Fundulus heteroclitus||Moderate||NCBI|
Life Stage Applicability
|Adult, reproductively mature||High|
Key Event Relationship Description
At present, a direct structural/functional linkage between androgen receptor agonism and reduced testosterone production by ovarian theca cells is not known. This linkage is thought to operate indirectly through endocrine feedback along the hypothalamic-pituitary-gonadal axis. Consequently, the relationship is supported primarily via association/correlation.
Evidence Supporting this KER
Synthesis of the steroidogenic enzymes that catalyze the formation of testosterone from cholesterol as a precursor is stimulated by gonadotropins, particulary luteinizing hormone, whose synthesis and secretion are in turn regulated by gonadotropin releasing hormone (GnRH) released from the hypothalamus (Payne and Hales 2004; Norris 2007; Miller 1988). Negative feedback of circulating androgens (e.g., testosterone) on GnRH release from the hypothalamus and/or gonadotropin release from the pituitary is a well established physiological phenomenon in vertebrate endocrinology (Norris 2007). While similar processes of negative feedback of sex steroids on gonadotropin expression and release have been established in fish (Levavi-Sivan et al. 2010), there are many remaining uncertainties about the exact mechanisms through which feedback takes place in fish as well as other vertebrates. For example, feedback is thought to involve a complex interplay of neurotransmitter signaling, kisspeptins, and the follistatin/inhibin/activin system (Trudeau et al. 2000; Trudeau 1997; Oakley et al. 2009; Cheng et al. 2007). In addition, the nature of the feedback produced by androgens is dependent on the concentration, form of the androgen (e.g., aromatizable versus non-aromatizable), life-stage and likely species (Habibi and Huggard 1998; Trudeau et al. 2000; Gopurappilly et al. 2013). At present, such negative feedback responses in vivo provide a biologically plausible connection between androgen receptor agonism and reducted testosterone production in ovarian theca cells, but uncertainty regarding the details of the underlying biology and the relevant applicability domain remain.
Direct evidence based on measurements of T production following exposure to established AR agonists:
- Ekman et al. (2011) reported significant reductions in ex vivo T production by ovary tissue following 24 or 48 h of in vivo exposure to the synthetic, non-aromatizable, AR agonist 17ß-trenbolone.
- Glinka et al. (2015) reported significant reductions in T production by ovary tissue collected from Fundulus heteroclitus that had been exposed to the model (non-aromatizable) androgen 5alpha-dihyrotestosterone.
- Sharpe et al. (2004) reported that testosterone biosynthetic capacity was inhibited in female Fundulus heteroclitus following 7 d of in vivo exposure to 0.25 ug methyl testosterone/L or 14 d of exposure to 0.1 ug methyl testosterone/L. Note - methyl testostosterone is an aromatizable androgen.
Indirect evidence based on measurements of circulating T following exposure to established AR agonists:
- Ankley et al. (2003) showed that 17ß-trenbolone binds the Pimephales promelas androgen receptor, induced turbercle formation (an androgen regulated male secondary sex characteristic in females), and reduced circulating concentrations of T and 11-ketotestosterone following 21 d of continuous exposure.
- Jensen et al. (2006) reported tubercle formation in females and reduced circulating concentrations of T in female Pimephales promelas exposed to 17alpha-trenbolone for 21 d.
- LaLone et al. (2013) reported tubercle formation in female Pimephales promelas and papillary complexes on the anal fin (a male secondary sex characteristic) of female Oryzias latipes following 21 d of exposure to spironolactone, along with significant reductions in plasma T measured in Pimephales promelas (that endpoint was not measured in medaka due to limited plasma volumes).
Uncertainties and Inconsistencies
See biological plausibility section above regarding current uncertainties in the mechanisms through which AR agonists may reduce gonadotropin secretion.
- Rutherford et al. (2015) reported an increase in plasma T concentrations in female and no change in gonadal T production in Fundulus heteroclitus following 14 d of exposure to 100 ug/L 5alpha-dihydrotestosterone.
Quantitative Understanding of the Linkage
- Li et al. (2011) describe a computational model of the female fathead minnow (Pimephales promelas) hypothalamic-pituitary-gonadal axis that can be used to simulate impacts on plasma T, plasma E2, and plasma vitellogenin concentrations following exposure to 17ß-trenbolone. However, to date, that model has not been robustly tested to determine applicability to other species, or other types of AR agonists.
- At present, the scope of data for associating AR-activation potency with decreased T production is not sufficient to describe a quantitative response-response relationship.
Known modulating factors
Known Feedforward/Feedback loops influencing this KER
Domain of Applicability
This KER is potentially relevant to sexually mature female vertebrates and amphioxus. It is not relevant to invertebrates.
- Androgen receptor orthologs are primarily limited to vertebrates (Baker 1997; Thornton 2001; Eick and Thornton 2011; Markov and Laudet 2011).
- Cytochrome P45011a (Cyp11a), a rate limiting enzyme for the production of testosterone, is specific to vertebrates and amphioxus (Markov et al. 2009; Baker et al. 2011; Payne and Hales, 2004).
- Cyp11a does not occur in invertebrates, as a result, they do not synthesize testosterone, nor other steroid intermediates required for testosterone synthesis (Markov et al. 2009; Payne and Hales, 2004).
- Baker ME. 1997. Steroid receptor phylogeny and vertebrate origins. Molecular and cellular endocrinology 135(2): 101-107.
- Baker ME. 2011. Origin and diversification of steroids: Co-evolution of enzymes and nuclear receptors. Mol Cell Endocrinol 334: 14-20.
- Cheng GF, Yuen CW, Ge W. 2007. Evidence for the existence of a local activin follistatin negative feedback loop in the goldfish pituitary and its regulation by activin and gonadal steroids. The Journal of endocrinology 195(3): 373-384.
- Eick GN, Thornton JW. 2011. Evolution of steroid receptors from an estrogen-sensitive ancestral receptor. Molecular and cellular endocrinology 334(1-2): 31-38.
- Ekman DR, Villeneuve DL, Teng Q, Ralston-Hooper KJ, Martinović-Weigelt D, Kahl MD, Jensen KM, Durhan EJ, Makynen EA, Ankley GT, Collette TW. Use of gene expression, biochemical and metabolite profiles to enhance exposure and effects assessment of the model androgen 17β-trenbolone in fish. Environ Toxicol Chem. 2011 Feb;30(2):319-29. doi: 10.1002/etc.406.
- Ankley GT, Jensen KM, Makynen EA, Kahl MD, Korte JJ, Hornung MW, Henry TR, Denny JS, Leino RL, Wilson VS, Cardon MC, Hartig PC, Gray LE. Effects of the androgenic growth promoter 17-beta-trenbolone on fecundity and reproductive endocrinology of the fathead minnow. Environ Toxicol Chem. 2003 Jun;22(6):1350-60.
- Glinka CO, Frasca S Jr, Provatas AA, Lama T, DeGuise S, Bosker T. The effects of model androgen 5α-dihydrotestosterone on mummichog (Fundulus heteroclitus) reproduction under different salinities. Aquat Toxicol. 2015 Aug;165:266-76. doi: 10.1016/j.aquatox.2015.05.019.
- Gopurappilly R, Ogawa S, Parhar IS. 2013. Functional significance of GnRH and kisspeptin, and their cognate receptors in teleost reproduction. Frontiers in endocrinology 4: 24.
- Habibi HR, Huggard DL. 1998. Testosterone regulation of gonadotropin production in goldfish. Comparative biochemistry and physiology Part C, Pharmacology, toxicology & endocrinology 119(3): 339-344.
- Jensen KM, Makynen EA, Kahl MD, Ankley GT. Effects of the feedlot contaminant 17alpha-trenbolone on reproductive endocrinology of the fathead minnow. Environ Sci Technol. 2006 May 1;40(9):3112-7.
- LaLone CA, Villeneuve DL, Cavallin JE, Kahl MD, Durhan EJ, Makynen EA, Jensen KM, Stevens KE, Severson MN, Blanksma CA, Flynn KM, Hartig PC, Woodard JS, Berninger JP, Norberg-King TJ, Johnson RD, Ankley GT. Cross-species sensitivity to a novel androgen receptor agonist of potential environmental concern, spironolactone. Environ Toxicol Chem. 2013 Nov;32(11):2528-41. doi: 10.1002/etc.2330.
- Levavi-Sivan B, Bogerd J, Mananos EL, Gomez A, Lareyre JJ. 2010. Perspectives on fish gonadotropins and their receptors. General and comparative endocrinology 165(3): 412-437.
- Li Z, Kroll KJ, Jensen KM, Villeneuve DL, Ankley GT, Brian JV, Sepúlveda MS, Orlando EF, Lazorchak JM, Kostich M, Armstrong B, Denslow ND, Watanabe KH. A computational model of the hypothalamic: pituitary: gonadal axis in female fathead minnows (Pimephales promelas) exposed to 17α-ethynylestradiol and 17β-trenbolone. BMC Syst Biol. 2011 May 5;5:63. doi: 10.1186/1752-0509-5-63.
- Markov GV, Laudet V. 2011. Origin and evolution of the ligand-binding ability of nuclear receptors. Molecular and cellular endocrinology 334(1-2): 21-30.
- Markov GV, Tavares R, Dauphin-Villemant C, Demeneix BA, Baker ME, Laudet V. Independent elaboration of steroid hormone signaling pathways in metazoans. Proc Natl Acad Sci U S A. 2009 Jul 21;106(29):11913-8. doi: 10.1073/pnas.0812138106.
- Miller WL. 1988. Molecular biology of steroid hormone synthesis. Endocrine reviews 9(3): 295-318.
- Norris DO. 2007. Vertebrate Endocrinology. Fourth ed. New York: Academic Press.
- Oakley AE, Clifton DK, Steiner RA. 2009. Kisspeptin signaling in the brain. Endocrine reviews 30(6): 713-743.
- Payne AH, Hales DB. 2004. Overview of steroidogenic enzymes in the pathway from cholesterol to active steroid hormones. Endocrine reviews 25(6): 947-970.
- Rutherford R, Lister A, Hewitt LM, MacLatchy D. Effects of model aromatizable (17α-methyltestosterone) and non-aromatizable (5α-dihydrotestosterone) androgens on the adult mummichog (Fundulus heteroclitus) in a short-term reproductive endocrine bioassay. Comp Biochem Physiol C Toxicol Pharmacol. 2015 Apr;170:8-18. doi: 10.1016/j.cbpc.2015.01.004.
- Sharpe RL, MacLatchy DL, Courtenay SC, Van Der Kraak GJ. Effects of a model androgen (methyl testosterone) and a model anti-androgen (cyproterone acetate) on reproductive endocrine endpoints in a short-term adult mummichog (Fundulus heteroclitus) bioassay. Aquat Toxicol. 2004 Apr 28;67(3):203-15.
- Thornton JW. 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.
- Trudeau VL, Spanswick D, Fraser EJ, Lariviére K, Crump D, Chiu S, et al. 2000. The role of amino acid neurotransmitters in the regulation of pituitary gonadotropin release in fish. Biochemistry and Cell Biology 78: 241-259.
- Trudeau VL. 1997. Neuroendocrine regulation of gonadotropin II release and gonadal growth in the goldfish, Carassius auratus. Reviews of Reproduction 2: 55-68.