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Key Event Title
|Level of Biological Organization|
Key Event Components
Key Event Overview
AOPs Including This Key Event
|AOP Name||Role of event in AOP||Point of Contact||Author Status||OECD Status|
|5α-reductase,female fish||MolecularInitiatingEvent||Young Jun Kim (send email)||Open for citation & comment||Under Development|
|5α-reductase inhibition leading to short AGD||MolecularInitiatingEvent||Terje Svingen (send email)||Under development: Not open for comment. Do not cite||Under Development|
|During development and at adulthood||High|
Key Event Description
This KE describes the inhibition of 5α-reductases (3-oxo-5α-steroid 4-dehydrogenases). These enzymes are widely expressed in tissues of both sexes and responsible for conversion of steroid hormones.
There are three isozymes: 5α-reductase type 1, 2, and 3. The substrates for 5α-reductases are 3-oxo (3-keto), Δ4,5 C19/C21 steroids such as testosterone, progesterone, androstenedione, epi-testosterone, cortisol, aldosterone, and deoxycorticosterone. The enzymatic reaction leads to an irreversible breakage of the double bond between carbon 4 and 5 and subsequent insertion of a hydride anion at carbon 5 and insertion of a proton at carbon 4. The reaction is aided by the cofactor NADPH. The substrate affinity and reaction velocity differ depending on the combination of substrate and enzyme isoform, for instance 5α-reductase type 2 has a higher substrate affinity for testosterone than the type 1 isoform of the enzyme, and the enzymatic reaction occurs at a higher velocity under optimal conditions. Likewise, inhibitors of 5α-reductase may exhibit differential effects depending on isoforms (Azzouni et al., 2012).
How It Is Measured or Detected
There is currently (as of 2023) no OECD test guideline for the measurement of 5α-reductase inhibition.
Inhibition of 5α-reductase can be assessed using transfected cell lines. This has been demonstrated in HEK-293 cells stably transfected with human 5α-reductase type 1, 2, and 3 (Yamana et al., 2010), in CHO cells stably transfected with human 5α-reductase type 1 and 2 (Thigpens et al., 1993), and COS cells transfected with human and rat 5α-reductase with unspecified isoforms (Andersson & Russell, 1990). The transfected cells are typically used as intact cells or cell homogenates. Further, 5α-reductase 1 and 2 has been successfully expressed and isolated from Escherichia coli with subsequent functionality allowing for examination of enzyme inhibition (Peng et al., 2020).
The output of the above methods could be decreased dihydrotestosterone (DHT) with increasing test chemical concentrations. Other substrates exist for the different isoforms that could be used to assess the enzymatic inhibition (Peng et al., 2020). The use of radiolabeled steroids has historic and continued use for 5α-reductase inhibition examination (Andersson & Russell, 1990; Peng et al., 2020; Thigpens et al., 1993; Yamana et al., 2010); however, alternative methods are available, such as conventional ELISA kits or advanced analytical methods such as liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS).
Domain of Applicability
This KE is applicable to both sexes, across developmental stages into adulthood, in many different tissues and across taxa.
Essentially the reaction performed by the isozymes is the same, but the enzyme is differentially expressed in the body. 5α-reductase type 1 is mainly linked to the production of neurosteroids, 5α-reductase type 2 is mainly involved in production of 5α-DHT, whereas 5α-reductase type 3 is involved in N-glycosylation (Robitaille & Langlois, 2020).
The expression profile of the three 5α-reductase isoforms depends on the developmental stage, the tissue of interest, and the disease state of the tissue. The enzymes have been identified in, for instance, non-genital and genital skin, scalp, prostate, liver, seminal vesicle, epididymis, testis, ovary, kidney, exocrine pancreas, and brain (Azzouni, 2012, Uhlen 2015).
5α-reductase is well-conserved, all primary species in Eukaryota contain all three isoforms (from plant, amoeba, yeast to vertebrates) (Azzouni, 2012) and the enzymes are expressed in both males and females (Langlois, 2010, Uhlen 2015).
Andersson, S., & Russell, D. W. (1990). Structural and biochemical properties of cloned and expressed human and rat steroid 5a-reductases. Proc. Natl. Acad. Sci. USA, 87, 3640–3644. https://www.pnas.org
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. In Advances in Urology. https://doi.org/10.1155/2012/530121
Peng, H. M., Valentin-Goyco, J., Im, S. C., Han, B., Liu, J., Qiao, J., & Auchus, R. J. (2020). Expression in escherichia coli, purification, and functional reconstitution of human steroid 5α-reductases. Endocrinology (United States), 161(8), 1–11. https://doi.org/10.1210/ENDOCR/BQAA117
Robitaille, J., & Langlois, V. S. (2020). Consequences of steroid-5α-reductase deficiency and inhibition in vertebrates. In General and Comparative Endocrinology (Vol. 290). Academic Press Inc. https://doi.org/10.1016/j.ygcen.2020.113400
Thigpens, A. E., Cala, K. M., & Russell, D. W. (1993). Characterization of Chinese Hamster Ovary Cell Lines Expressing Human Steroid 5a-Reductase Isozymes. The Journal of Biological Chemistry, 268(23), 17404–17412.
Yamana, K., Fernand, L., Luu-The, V., & Luu-The, V. (2010). Human type 3 5α-reductase is expressed in peripheral tissues at higher levels than types 1 and 2 and its activity is potently inhibited by finasteride and dutasteride. Hormone Molecular Biology and Clinical Investigation, 2(3), 293–299. https://doi.org/10.1515/HMBCI.2010.035