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Key Event Title
Suppression, Estrogen receptor (ER) activity
|Level of Biological Organization|
Key Event Components
|estrogen receptor activity||estrogen receptor||decreased|
Key Event Overview
AOPs Including This Key Event
|AOP Name||Role of event in AOP||Point of Contact||Author Status||OECD Status|
|Antiestrogens and ovarian adenomas/granular cell tumors||KeyEvent||Evgeniia Kazymova (send email)||Under Development: Contributions and Comments Welcome|
|Hypothalamic estrogen receptors inhibition leading to ovarian cancer||MolecularInitiatingEvent||Cataia Ives (send email)||Under development: Not open for comment. Do not cite|
|Not Otherwise Specified||High|
Key Event Description
Estrogen receptors are produced in all vertebrates and located in either the cell cytoplasm or nucleus(Bondesson et al., 2015; Eick and Thornton, 2011). Estrogen receptors are localized either in cytoplasm, or on the cell surface.
Site of action: Stressors (e.g., clomiphene) act on neuronal cell in the hypothalamus, where it inhibits hypothalamic Estrogen Receptors selectively.
Responses at the macromolecular level: Stressors activate the Estrogen Receptor α in the presence of lower level of estrogen and partially blocks the same for higher level of estrogen and works as antagonist for the Estrogen Receptor β(Trost and Khera, 2014). Stressors appear to act in the brain's pituitary gland to secrete an increased amount of gonadotropins hormone (GnRH) in hypothalamus leading towards increased GnRH level in blood.
Estrogen Receptor α: ERα (Estrogen Receptor α or NR3A1 or ESR1) - A nuclear receptor and it is activated by the estrogen (sex hormone). Estrogen located at chromosome number 6 ( 6q25.1)
Estrogen Receptor β: ERβ (Estrogen Receptor β or NR3A2 or ESR2) – This is also nuclear receptor and activated by the sex hormone estrogen which is located at chromosome number 14 (14q23.2). I ERβ has both N-terminal has DNA binding domain and C-terminal has ligand binding domain. This is localized to the nucleus, cytoplasm, and mitochondria. Selective estrogen receptor modulators (SERM) inhibits the ERβ. Drugs used as SERM are clomiphene, tamoxifen, raloxifene etc.
Biological compartments: Estrogen receptors (ER) are present in the plasma membrane. Both ERα and ERβ have diverse functions depending on cells and organs. ERs have also been loacated in cytoplasmic organelles including mitochondria and the endoplasmic reticulum(Levin, 2009).
General role in biology: Estrogen receptors (both estrogen receptor alpha (ERα) and estrogen receptor beta (ERβ) binds the estrogens to promote the the biological functions of estrogens. Depending upon a balance between ERα and ERβ activities in target organs, estrogen signaling is selectively stimulated or inhibited (Welboren et al., 2009). ERβ has a high degree of sequence homology with the classical estrogen receptor. Interestingly, ERβ is detected in many tissues, including those previously assumed to be estrogen insensitive. In tissues where both ERs are expressed, such as the hypothalamus, uterus, mammary glands, and immune system, ERα promotes proliferation whereas ERβ has pro-apoptotic and pro-differentiating functions(Morani et al., 2008). ERα is present mainly in ovary (thecal cells) where as ERβ is found mainly in ovary (granulosa cells)(Paterni et al., 2014). ERα and ERβ is identical approximately 97% in the DNA-binding domain and approximately 56% in the ligand-binding domain(Dahlman-Wright et al., 2006).
How It Is Measured or Detected
Radioreceptor assay/The estrogen receptor binding assay (using Rat Uterine Cytosol): This assay identifies chemicals that have the potential to interact with the estrogen receptor (ER) in vitro. Principle of this particular assay is based on the competitive protein-binding methods. A radiolabelled ligand and an unlabelled ligand are presented together to a specific receptor. The radioactivity measurement provides the quantitative estimation of the bound and unbound fraction of the ligand with the receptor. All cytosolic estrogen receptor subtypes that are expressed in the specific tissue, including ERα and ERβ are used for the determination of estrogen receptor binding. This assay is simple and rapid to perform when optimal conditions for binding are determined. Assay determines if a ligand/chemical can interact and displace the endogenous hormone 17β-estradiol (Freyberger et al., 2010).
Domain of Applicability
Neuronal cell in Hypothalamus
Evidence for Perturbation by Stressor
Overview for Molecular Initiating Event
Clomiphene citrate (a stressor) at 10-10 - 10-12 M concentrations exhibits approximately 30% of the estrogenic activity which is same from 17β-estradiol (at 10-10 M) in ERα-expressing cells where as no activity in ERβ cells.
Clomiphene citrate at the concentration of 10-10 M reveals weak estrogen agonist activity in the presence of 17 β -estradiol (E2) at the concentration of 10-14 M in ERα-expressing cells, and no activity was found in ERβ cells.
Clomiphene citrate at lower doses (10-10 - 10-12 M), but not higher doses (10-6 - 10-8 M) showed estrogenic activity via ERα. However, clomiphene citrate at concentrations between 10-6 M and 10-12 M did not reveal any estrogenic activity via ERβ. In the presence of E2, clomiphene citrate worked as either as an agonist or an antagonist through ERα depending on the concentrations of E2. Clomiphene citrate worked as antagonistic when it is combined with the higher E2 concentrations and worked as agonistic with the lower E2 concentrations. On the other hand, via ER β, clomiphene citrate acted as an estrogen antagonist irrespective of the concentration of E2. (Kurosawa et al., 2010).
Adashi, E. Y., Hsueh, A. J., & Yen, S. S. (1980). Alterations induced by clomiphene in the concentrations of oestrogen receptors in the uterus, pituitary gland and hypothalamus of female rats. J Endocrinol. , 87(3), 383-92.
Bharti, S., Misro, M., & Rai, U. (2013). Clomiphene citrate potentiates the adverse effects of estrogen on rat testis and down-regulates the expression of steroidogenic enzyme genes. Fertility and sterility, 99(1), 140-148. e5.
Bondesson, M., Hao, R., Lin, C.-Y., Williams, C., & Gustafsson, J.-Å. (2015). Estrogen receptor signaling during vertebrate development. Biochimica et Biophysica Acta (BBA)-Gene Regulatory Mechanisms, 1849(2), 142-151.
Bussenot, I., Parinaud, J., Clamagirand, C., Vieitez, G., & Pontonnier, G. (1990). Effect of clomiphene cirate on oestrogen secretion by human granulosa cells in culture. Human Reproduction, 5(5), 533-536.
Dahlman-Wright, K., Cavailles, V., Fuqua, S. A., Jordan, V. C., Katzenellenbogen, J. A., Korach, K. S., et al. (2006). International union of pharmacology. LXIV. Estrogen receptors. Pharmacological reviews, 58(4), 773-781.
Dominguez, R., & Micevych, P. (2010). Estradiol rapidly regulates membrane estrogen receptor alpha levels in hypothalamic neurons. J Neurosci, 30(38), 12589-96. doi:30/38/12589 [pii]10.1523/JNEUROSCI.1038-10.2010.
Eick, G. N., & Thornton, J. W. (2011). Evolution of steroid receptors from an estrogen-sensitive ancestral receptor. Molecular and cellular endocrinology, 334(1-2), 31-38.
Freyberger, A., Wilson, V., Weimer, M., Tan, S., Tran, H. S., & Ahr, H. J. (2010). Assessment of a robust model protocol with accelerated throughput for a human recombinant full length estrogen receptor-alpha binding assay: protocol optimization and intralaboratory assay performance as initial steps towards validation. Reprod Toxicol, 30(1), 50-9. doi:S0890-6238(10)00003-1 [pii].
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Koch, Y., Dikstein, S., Superstine, E., & Sulman, F. G. (1971). THE EFFECT OF PROMETHAZINE AND CLOMIPHENE ON GONADOTROPHIN SECRETION IN THE RAT. Journal of Endocrinology, 49(1), 13-17. doi:10.1677/joe.0.0490013.
Kurosawa, T., Hiroi, H., Momoeda, M., Inoue, S., & Taketani, Y. (2010). Clomiphene citrate elicits estrogen agonistic/antagonistic effects differentially via estrogen receptors αand β. Endocrine journal, 57(6), 517-521.
Levin, E. R. (2009). Plasma membrane estrogen receptors. Trends in Endocrinology & Metabolism, 20(10), 477-482.
Morani, A., Warner, M., & Gustafsson, J. Å. (2008). Biological functions and clinical implications of oestrogen receptors alfa and beta in epithelial tissues. Journal of internal medicine, 264(2), 128-142.
Oride, A., Kanasaki, H., Tumurbaatar, T., Zolzaya, T., Okada, H., Hara, T., et al. (2020). Effects of the Fertility Drugs Clomiphene Citrate and Letrozole on Kiss-1 Expression in Hypothalamic Kiss-1-Expressing Cell Models. Reproductive sciences (Thousand Oaks, Calif.), 27. doi:10.1007/s43032-020-00154-1.
Paterni, I., Granchi, C., Katzenellenbogen, J. A., & Minutolo, F. (2014). Estrogen receptors alpha (ERα) and beta (ERβ): subtype-selective ligands and clinical potential. Steroids, 90, 13-29.
Sutaria, U., Crooke, A., Bertrand, P., & Hodgson, C. (1980). Clomiphene citrate and human chorionic gonadotropin in the treatment of anovulatory infertility. International Journal of Gynecology & Obstetrics, 18(6), 435-437.
Taheripanah, R., Kabir-Salmani, M., Favayedi, M., Zamaniyan, M., Malih, N., & Taheripanah, A. (2020). Effects of clomiphene citrate plus estradiol or progesterone on endometrial ultrastructure: An RCT. International Journal of Reproductive BioMedicine, 18(3), 201.
Trost, L. W., & Khera, M. (2014). Alternative treatment modalities for the hypogonadal patient. Current urology reports, 15(7), 1-12.
Wahab, O. A., Princely, A. C., Oluwadamilare, A. A., Ore-Oluwapo, D. O., Blessing, A. O., & Alfred, E. F. (2019). Clomiphene citrate ameliorated lead acetate-induced reproductive toxicity in male Wistar rats. JBRA assisted reproduction, 23(4), 336-343. doi:10.5935/1518-0557.20190038.
Welboren, W.-J., Sweep, F. C., Span, P. N., & Stunnenberg, H. G. (2009). Genomic actions of estrogen receptor?: what are the targets and how are they regulated? Endocrine-related cancer, 16(4), 1073.