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Relationship: 2126

Title

A descriptive phrase which clearly defines the two KEs being considered and the sequential relationship between them (i.e., which is upstream, and which is downstream). More help

Decrease, testosterone levels leads to Decrease, DHT level

Upstream event

The causing Key Event (KE) in a Key Event Relationship (KER). More help

Decrease, testosterone levels

Downstream event

The responding Key Event (KE) in a Key Event Relationship (KER). More help

Decrease, DHT level

Key Event Relationship Overview

The utility of AOPs for regulatory application is defined, to a large extent, by the confidence and precision with which they facilitate extrapolation of data measured at low levels of biological organisation to predicted outcomes at higher levels of organisation and the extent to which they can link biological effect measurements to their specific causes.Within the AOP framework, the predictive relationships that facilitate extrapolation are represented by the KERs. Consequently, the overall WoE for an AOP is a reflection in part, of the level of confidence in the underlying series of KERs it encompasses. Therefore, describing the KERs in an AOP involves assembling and organising the types of information and evidence that defines the scientific basis for inferring the probable change in, or state of, a downstream KE from the known or measured state of an upstream KE. More help

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

Taxonomic Applicability

Latin or common names of a species or broader taxonomic grouping (e.g., class, order, family) that help to define the biological applicability domain of the KER.In general, this will be dictated by the more restrictive of the two KEs being linked together by the KER. More help

Term	Scientific Term	Evidence	Link
Vertebrates	Vertebrates	High	NCBI

Sex Applicability

An indication of the the relevant sex for this KER. More help

Sex	Evidence
Mixed	High

Life Stage Applicability

An indication of the the relevant life stage(s) for this KER. More help

Term	Evidence
During development and at adulthood	High

Key Event Relationship Description

Provides a concise overview of the information given below as well as addressing details that aren’t inherent in the description of the KEs themselves. More help

Testosterone (T) and dihydrotestosterone (DHT) are androgens that are involved in numerous developmental and functional processes across animal taxa. In vertebrates, testosterone can be aromatized into estrogens catalyzed by the enzyme aromatase (CYP19) or be metabolized to DHT by the enzyme 5α-reductase (Azzouni et al., 2012; Naamneh Elzenaty et al., 2022; Swerdloff et al., 2017). Both T and DHT binds to the androgen receptor (AR), but with different affinities. DHT has a higher affinity for the AR than T. DHT also has a longer half-life and slower dissociation rate than T and cannot be aromatized into estrogens (Gerald & Raj, 2022; Naamneh Elzenaty et al., 2022; Swerdloff et al., 2017).

During mammalian development, T is primarily produced by the fetal testes and is needed for differentiation of the Wolffian ducts, the epididymis, and the ejaculatory duct. In pubertal and adult mammals, T is produced by the testes, the ovaries (although at a much lower level), and the adrenal glands (Ogino et al., 2011; Rey, 2021). In peripheral tissues (i.e. relative to the testes), DHT is converted from T by 5α-reductase to induce the differentiation of the urogenital sinus and genital tubercle to form the prostate, penis, scrotum and urethra (Swerdloff et al., 2017). Both androgens are essential for masculinization, sexual development, and fertility.

Evidence Collection Strategy

Include a description of the approach for identification and assembly of the evidence base for the KER. For evidence identification, include, for example, a description of the sources and dates of information consulted including expert knowledge, databases searched and associated search terms/strings. Include also a description of study screening criteria and methodology, study quality assessment considerations, the data extraction strategy and links to any repositories/databases of relevant references.Tabular summaries and links to relevant supporting documentation are encouraged, wherever possible. More help

This KER is considered canonical knowledge and supporting literature was mainly sourced from key review articles from the open literature.

Evidence Supporting this KER

Addresses the scientific evidence supporting KERs in an AOP setting the stage for overall assessment of the AOP. More help

Biological Plausibility

Addresses the biological rationale for a connection between KEupstream and KEdownstream. This field can also incorporate additional mechanistic details that help inform the relationship between KEs, this is useful when it is not practical/pragmatic to represent these details as separate KEs due to the difficulty or relative infrequency with which it is likely to be measured. More help

The biological plausibility for this KER is considered high

It is well established that DHT is synthesized from circulating T. 5α-reductase is the enzyme responsible for the conversion of T into DHT. Multiple isoforms of this enzyme are expressed in different tissues. Expression of 5α-reductase in peripheral tissues dictates where DHT will be formed from circulating T (Azzouni et al., 2012; Swerdloff et al., 2017).

Since T can be converted to DHT, it stands to reason that a lack of T can lead to a lack of DHT. Therefore, if there is a marked reduction in the availability of T, it can be surmised that DHT levels are consequently affected. However, to what extent T needs to be diminished in tissues before there is a functionally relevant decrease in DHT is largely unknown. In addition, the quantitative relationship between substrate (T) availability and levels of synthesized DHT is not well characterized in tissues in vivo. Notably, DHT can be produced via other steroid intermediates through the ‘backdoor pathway’ in mammals such as marsupials and humans (Renfree & Shaw 2023).

Empirical Evidence

Provides specific (citable) evidence that supports the idea of a change in the upstream KE (KEupstream) leading to, or being associated with, a subsequent change in the downstream KE (KEdownstream), assuming the perturbation of KEupstream is sufficient. More help

The empirical evidence for this KER is considered moderate

As per Table 1, empirical data exists for effects on both T and DHT following chemical exposures, but it is not always possible to deduce if the reduction in DHT is a direct consequence of reduced T or because of other mechanisms such as e.g. interference with 5α-reductase. However, some studies do include 5α-reductase mRNA expression or measure the ratio of T/DHT which if unchanged, indicates that the decrease would most likely be due to decrease in T availability.

Table 1

Compound	Species	Effect level	KE: testosterone, decrease	KE: DHT, decrease	Details	References
DEHP	rat	LOEL = 117 mg/kg/day	Significant decrease day 1, 2, 3: from 4 to 2 ng/testis	Significant decrease day 1 from 2.5 to 1 ng/testis	In utero exposure, fetal testes ex vivo from GD20 rats.	(Culty et al., 2008)
Ibuprofen	human	One concentration tested: 10^-5M	48h: significant decrease -36.8%	48h: significant decrease -70.2%	Human fetal testes explants, measurements were done using the exposure media. No effect on SRD5A3 mRNA levels (5α-R3)	(Ben Maamar et al., 2017)
Rosiglitazone	human	One concentration tested: 8mg/day	significant decrease of production rates 318±62 µg/h to 272±72 µg/h	significant decrease of production rates 21±6 µg/h to 17±5 µg/h	Serum levels after 7 days of treatment in healthy men: “Calculated from the product of the known infusion rate (Rt) and the ratio of tracer infusate enrichment (Et) to tracer dilution in the plasma” Ratio T/DHT remained unchanged.	(Vierhapper et al., 2003)
PTU	rats	One concentration tested: 240 mg/kg/day	significant decrease ˜2ng/ml to 0.15ng/ml	significant decrease ˜0.5ng/ml to 0.17ng/ml	Oral exposure of 14day old rats treated until day 51. Serum testosterone and DHT measured	(Marty et al., 2001)(
Dibutyltin	Carp fish	One concentration tested: 100µM	significant decrease -16%	significant decrease -24%	Gonad microsomes. Dibultyltin inhibited 5α-reductase, whichdecreases possibility that this is solely due to decrease of testosterone	(Thibaut & Porte, 2004)
TCDD	rats	Effects observed at 15µg/kg	significant decrease -90%	significant decrease -75%	Oral exposure of 66-68 day old rats. Serum or plasma measurements. Dose dependent decrease of both was observed. Ratio T/DHT indicates effect is due to reduced testosterone.	(Moore et al., 1985)

Dose concordance:

All the exposure data shown above indicates dose-concordance, since the same concentration tested affects both the upstream and downstream key event.

Other evidence

One study focused on the condition Leydig cell hypoplasia (LCH) in one patient. This patient had mutations in the LHCGR, and when measuring the levels of testosterone and DHT before and after hCG stimulation a decrease in both levels under the normal range were observed, even with hCG stimulation (Xu et al., 2018).

Uncertainties and Inconsistencies

Addresses inconsistencies or uncertainties in the relationship including the identification of experimental details that may explain apparent deviations from the expected patterns of concordance. More help

The levels of T do not always reflect the levels of DHT. T is also converted to estradiol (Naamneh Elzenaty et al., 2022). Therefore, the decrease in T may lead to a decrease in estradiol while DHT levels remain unchanged.

Several studies have shown the existence of an alternative (‘backdoor’) pathway for DHT synthesis that is independent of T in marsupials and humans, but not in rodents (Marilyn B. Renfree et al., 1995). Instead of proceeding through the canonical pathway, progesterone or 17-OH progesterone, can be converted into allopregnanolone and 17OH-allopregnanolone. 17-OH allopregnanolone is then converted into androsterone leading to androstanediol that can finally be oxidized to produce DHT. Therefore, through this pathway, DHT can be synthesized without the presence of T (Auchus, 2004; Miller & Auchus, 2019).

Known modulating factors

This table captures specific information on the MF, its properties, how it affects the KER and respective references.1.) What is the modulating factor? Name the factor for which solid evidence exists that it influences this KER. Examples: age, sex, genotype, diet 2.) Details of this modulating factor. Specify which features of this MF are relevant for this KER. Examples: a specific age range or a specific biological age (defined by...); a specific gene mutation or variant, a specific nutrient (deficit or surplus); a sex-specific homone; a certain threshold value (e.g. serum levels of a chemical above...) 3.) Description of how this modulating factor affects this KER. Describe the provable modification of the KER (also quantitatively, if known). Examples: increase or decrease of the magnitude of effect (by a factor of...); change of the time-course of the effect (onset delay by...); alteration of the probability of the effect; increase or decrease of the sensitivity of the downstream effect (by a factor of...) 4.) Provision of supporting scientific evidence for an effect of this MF on this KER. Give a list of references. More help

Quantitative Understanding of the Linkage

Captures information that helps to define how much change in the upstream KE, and/or for how long, is needed to elicit a detectable and defined change in the downstream KE. More help

Response-response Relationship

Provides sources of data that define the response-response relationships between the KEs. More help

The response-response relationship is not clearly established.

Time-scale

Information regarding the approximate time-scale of the changes in KEdownstream relative to changes in KEupstream (i.e., do effects on KEdownstream lag those on KEupstream by seconds, minutes, hours, or days?). More help

Different time scales have been observed in the studies above, the shortest one found being 48h. With Ibuprofen treatment, a decrease in both testosterone and DHT was observed after 48h in human fetal explant’s exposure media (Ben Maamar et al., 2017). However, it is not evident that this effect is direct and only due to a decrease in T.

Known Feedforward/Feedback loops influencing this KER

Define whether there are known positive or negative feedback mechanisms involved and what is understood about their time-course and homeostatic limits. More help

Activity of 5α-reductase type 1 and 2: The activity of this enzyme determines how much T is converted into DHT. There are two isomers, with type 2 being the primary isomer expressed in DHT target organs. In deficiencies of this enzyme, there are studies that observe maintained DHT levels. This indicates that the type 1 enzyme can take over if needed (Azzouni et al., 2012).

Conversion of T to estradiol (E2): Aromatase can convert T into estrogens. The activity of this enzyme may push towards a decrease of T levels and an increase in estrogen levels without necessarily affecting DHT levels (Naamneh Elzenaty et al., 2022).

Hypothalamus-pituitary-gonadal (HPG) axis: Like most sex steroids, T production is controlled by the HPG axis during puberty and adulthood, but also during certain periods of development. For humans, the HPG axis is active following birth between 1-3 months in both males and females. Increase of LH and FSH are observed in infants up to 4-6months old. This stage is also known as the minipuberty (Lanciotti et al., 2018; Renault et al., 2020). Once GnRH is released from the hypothalamus, the pituitary gland secretes LH in pulses, which then stimulates the cells in the testes to produce T. A negative feedback loop can then occur, where testosterone then inhibits the release of GnRH and LH, in turn slowing down T production (Gerald & Raj, 2022; Naamneh Elzenaty et al., 2022; Nef & Parada, 2000).

Domain of Applicability

A free-text section of the KER description that the developers can use to explain their rationale for the taxonomic, life stage, or sex applicability structured terms. More help

T and DHT are androgens present in all vertebrates. They play a role in development and fertility in both males and females (Ogino et al., 2011; Prizant et al., 2014; Rey, 2021; Swerdloff et al., 2017). All tissues expressing 5α-reductase are applicable to this KER (Azzouni et al., 2012).

References

List of the literature that was cited for this KER description. More help

Auchus, R. J. (2004). The backdoor pathway to dihydrotestosterone. Trends in Endocrinology & Metabolism, 15(9), 432–438. https://doi.org/10.1016/j.tem.2004.09.004

Azzouni, F., Godoy, A., Li, Y., & Mohler, J. (2012). The 5 Alpha-Reductase Isozyme Family: A Review of Basic Biology and Their Role in Human Diseases. Advances in Urology, 2012, 1–18. https://doi.org/10.1155/2012/530121

Ben Maamar, M., Lesné, L., Hennig, K., Desdoits-Lethimonier, C., Kilcoyne, K. R., Coiffec, I., Rolland, A. D., Chevrier, C., Kristensen, D. M., Lavoué, V., Antignac, J.-P., Le Bizec, B., Dejucq-Rainsford, N., Mitchell, R. T., Mazaud-Guittot, S., & Jégou, B. (2017). Ibuprofen results in alterations of human fetal testis development. Scientific Reports, 7(1), 44184. https://doi.org/10.1038/srep44184

Culty, M., Thuillier, R., Li, W., Wang, Y., Martinez-Arguelles, D. B., Benjamin, C. G., Triantafilou, K. M., Zirkin, B. R., & Papadopoulos, V. (2008). In Utero Exposure to Di-(2-ethylhexyl) Phthalate Exerts Both Short-Term and Long-Lasting Suppressive Effects on Testosterone Production in the Rat1. Biology of Reproduction, 78(6), 1018–1028. https://doi.org/10.1095/biolreprod.107.065649

Gerald, T., & Raj, G. (2022). Testosterone and the Androgen Receptor. Urologic Clinics of North America, 49(4), 603–614. https://doi.org/10.1016/j.ucl.2022.07.004

Lanciotti, L., Cofini, M., Leonardi, A., Penta, L., & Esposito, S. (2018). Up-To-Date Review About Minipuberty and Overview on Hypothalamic-Pituitary-Gonadal Axis Activation in Fetal and Neonatal Life. Frontiers in Endocrinology, 9. https://doi.org/10.3389/fendo.2018.00410

Marilyn B. Renfree, Jenny L. Harry, & Geoffrey Shaw. (1995). The marsupial male: a role model for sexual development. Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences, 350(1333), 243–251. https://doi.org/10.1098/rstb.1995.0158

Marty, M. S., Crissman, J. W., & Carney, E. W. (2001). Evaluation of the Male Pubertal Assay’s Ability to Detect Thyroid Inhibitors and Dopaminergic Agents. Toxicological Sciences, 60(1), 63–76. https://doi.org/10.1093/toxsci/60.1.63

Miller, W. L., & Auchus, R. J. (2019). The “backdoor pathway” of androgen synthesis in human male sexual development. PLOS Biology, 17(4), e3000198. https://doi.org/10.1371/journal.pbio.3000198

Moore, R. W., Potter, C. L., Theobald, H. M., Robinson, J. A., & Peterson, R. E. (1985). Androgenic deficiency in male rats treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin. Toxicology and Applied Pharmacology, 79(1), 99–111. https://doi.org/10.1016/0041-008X(85)90372-2

Naamneh Elzenaty, R., du Toit, T., & Flück, C. E. (2022). Basics of androgen synthesis and action. Best Practice & Research Clinical Endocrinology & Metabolism, 36(4), 101665. https://doi.org/10.1016/j.beem.2022.101665

Nef, S., & Parada, L. F. (2000). Hormones in male sexual development. Genes & Development, 14(24), 3075–3086. https://doi.org/10.1101/gad.843800

Ogino, Y., Miyagawa, S., Katoh, H., Prins, G. S., Iguchi, T., & Yamada, G. (2011). Essential functions of androgen signaling emerged through the developmental analysis of vertebrate sex characteristics. Evolution & Development, 13(3), 315–325. https://doi.org/10.1111/j.1525-142X.2011.00482.x

Prizant, H., Gleicher, N., & Sen, A. (2014). Androgen actions in the ovary: balance is key. Journal of Endocrinology, 222(3), R141–R151. https://doi.org/10.1530/JOE-14-0296

Renault, C. H., Aksglaede, L., Wøjdemann, D., Hansen, A. B., Jensen, R. B., & Juul, A. (2020). Minipuberty of human infancy – A window of opportunity to evaluate hypogonadism and differences of sex development? Annals of Pediatric Endocrinology & Metabolism, 25(2), 84–91. https://doi.org/10.6065/apem.2040094.047

Renfree, M., & Shaw G. (2023). The alternate pathway of androgen metabolism and window of sensitivity. Journal of Endocrinology, JOE-22-0296, https://doi.org/10.1530/JOE-22-0296

Rey, R. A. (2021). The Role of Androgen Signaling in Male Sexual Development at Puberty. Endocrinology, 162(2). https://doi.org/10.1210/endocr/bqaa215

Swerdloff, R. S., Dudley, R. E., Page, S. T., Wang, C., & Salameh, W. A. (2017). Dihydrotestosterone: Biochemistry, Physiology, and Clinical Implications of Elevated Blood Levels. Endocrine Reviews, 38(3), 220–254. https://doi.org/10.1210/er.2016-1067

Thibaut, R., & Porte, C. (2004). Effects of endocrine disrupters on sex steroid synthesis and metabolism pathways in fish. The Journal of Steroid Biochemistry and Molecular Biology, 92(5), 485–494. https://doi.org/10.1016/j.jsbmb.2004.10.008

Vierhapper, H., Nowotny, P., & Waldhäusl, W. (2003). Reduced production rates of testosterone and dihydrotestosterone in healthy men treated with rosiglitazone. Metabolism, 52(2), 230–232. https://doi.org/10.1053/meta.2003.50028

Xu, Y., Chen, Y., Li, N., Hu, X., Li, G., Ding, Y., Li, J., Shen, Y., Wang, X., & Wang, J. (2018). Novel compound heterozygous variants in the LHCGR gene identified in a subject with Leydig cell hypoplasia type 1. Journal of Pediatric Endocrinology and Metabolism, 31(2), 239–245. https://doi.org/10.1515/jpem-2016-0445