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Event: 1046

Key Event Title

The KE title should describe a discrete biological change that can be measured. It should generally define the biological object or process being measured and whether it is increased, decreased, or otherwise definably altered relative to a control state. For example “enzyme activity, decreased”, “hormone concentration, increased”, or “growth rate, decreased”, where the specific enzyme or hormone being measured is defined. More help

Suppression, Estrogen receptor (ER) activity

Short name
The KE short name should be a reasonable abbreviation of the KE title and is used in labelling this object throughout the AOP-Wiki. The short name should be less than 80 characters in length. More help
Suppression, Estrogen receptor (ER) activity

Biological Context

Structured terms, selected from a drop-down menu, are used to identify the level of biological organization for each KE. Note, KEs should be defined within a particular level of biological organization. Only KERs should be used to transition from one level of organization to another. Selection of the level of biological organization defines which structured terms will be available to select when defining the Event Components (below). More help
Level of Biological Organization

Cell term

Further information on Event Components and Biological Context may be viewed on the attached pdf.The biological context describes the location/biological environment in which the event takes place.  For molecular/cellular events this would include the cellular context (if known), organ context, and species/life stage/sex for which the event is relevant. For tissue/organ events cellular context is not applicable.  For individual/population events, the organ context is not applicable. More help
Cell term

Organ term

Further information on Event Components and Biological Context may be viewed on the attached pdf.The biological context describes the location/biological environment in which the event takes place.  For molecular/cellular events this would include the cellular context (if known), organ context, and species/life stage/sex for which the event is relevant. For tissue/organ events cellular context is not applicable.  For individual/population events, the organ context is not applicable. More help
Organ term

Key Event Components

Further information on Event Components and Biological Context may be viewed on the attached pdf.Because one of the aims of the AOP-KB is to facilitate de facto construction of AOP networks through the use of shared KE and KER elements, authors are also asked to define their KEs using a set of structured ontology terms (Event Components). In the absence of structured terms, the same KE can readily be defined using a number of synonymous titles (read by a computer as character strings). In order to make these synonymous KEs more machine-readable, KEs should also be defined by one or more “event components” consisting of a biological process, object, and action with each term originating from one of 22 biological ontologies (Ives, et al., 2017; See List). Biological process describes dynamics of the underlying biological system (e.g., receptor signalling). The biological object is the subject of the perturbation (e.g., a specific biological receptor that is activated or inhibited). Action represents the direction of perturbation of this system (generally increased or decreased; e.g., ‘decreased’ in the case of a receptor that is inhibited to indicate a decrease in the signalling by that receptor).Note that when editing Event Components, clicking an existing Event Component from the Suggestions menu will autopopulate these fields, along with their source ID and description. To clear any fields before submitting the event component, use the 'Clear process,' 'Clear object,' or 'Clear action' buttons. If a desired term does not exist, a new term request may be made via Term Requests. Event components may not be edited; to edit an event component, remove the existing event component and create a new one using the terms that you wish to add. More help
Process Object Action
estrogen receptor activity estrogen receptor decreased

Key Event Overview

AOPs Including This Key Event

All of the AOPs that are linked to this KE will automatically be listed in this subsection. This table can be particularly useful for derivation of AOP networks including the KE. Clicking on the name of the AOP will bring you to the individual page for that AOP. More help
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


This is a structured field used to identify specific agents (generally chemicals) that can trigger the KE. Stressors identified in this field will be linked to the KE in a machine-readable manner, such that, for example, a stressor search would identify this as an event the stressor can trigger. NOTE: intermediate or downstream KEs in one AOP may function as MIEs in other AOPs, meaning that stressor information may be added to the KE description, even if it is a downstream KE in the pathway currently under development.Information concerning the stressors that may trigger an MIE can be defined using a combination of structured and unstructured (free-text) fields. For example, structured fields may be used to indicate specific chemicals for which there is evidence of an interaction relevant to this MIE. By linking the KE description to a structured chemical name, it will be increasingly possible to link the MIE to other sources of chemical data and information, enhancing searchability and inter-operability among different data-sources and knowledgebases. The free-text section “Evidence for perturbation of this MIE by stressor” can be used both to identify the supporting evidence for specific stressors triggering the MIE as well as to define broad chemical categories or other properties that classify the stressors able to trigger the MIE for which specific structured terms may not exist. More help

Taxonomic Applicability

Latin or common names of a species or broader taxonomic grouping (e.g., class, order, family) can be selected from an ontology. In many cases, individual species identified in these structured fields will be those for which the strongest evidence used in constructing the AOP was available in relation to this KE. More help
Term Scientific Term Evidence Link
human Homo sapiens High NCBI
rat Rattus norvegicus High NCBI
mice Mus sp. High NCBI

Life Stages

The structured ontology terms for life-stage are more comprehensive than those for taxa, but may still require further description/development and explanation in the free text section. More help
Life stage Evidence
Not Otherwise Specified High

Sex Applicability

The authors must select from one of the following: Male, female, mixed, asexual, third gender, hermaphrodite, or unspecific. More help
Term Evidence
Mixed High

Key Event Description

A description of the biological state being observed or measured, the biological compartment in which it is measured, and its general role in the biology should be provided. For example, the biological state being measured could be the activity of an enzyme, the expression of a gene or abundance of an mRNA transcript, the concentration of a hormone or protein, neuronal activity, heart rate, etc. The biological compartment may be a particular cell type, tissue, organ, fluid (e.g., plasma, cerebrospinal fluid), etc. The role in the biology could describe the reaction that an enzyme catalyses and the role of that reaction within a given metabolic pathway; the protein that a gene or mRNA transcript codes for and the function of that protein; the function of a hormone in a given target tissue, physiological function of an organ, etc. Careful attention should be taken to avoid reference to other KEs, KERs or AOPs. Only describe this KE as a single isolated measurable event/state. This will ensure that the KE is modular and can be used by other AOPs, thereby facilitating construction of AOP networks. More help

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

One of the primary considerations in evaluating AOPs is the relevance and reliability of the methods with which the KEs can be measured. The aim of this section of the KE description is not to provide detailed protocols, but rather to capture, in a sentence or two, per method, the type(s) of measurements that can be employed to evaluate the KE and the relative level of scientific confidence in those measurements. Methods that can be used to detect or measure the biological state represented in the KE should be briefly described and/or cited. These can range from citation of specific validated test guidelines, citation of specific methods published in the peer reviewed literature, or outlines of a general protocol or approach (e.g., a protein may be measured by ELISA).Key considerations regarding scientific confidence in the measurement approach include whether the assay is fit for purpose, whether it provides a direct or indirect measure of the biological state in question, whether it is repeatable and reproducible, and the extent to which it is accepted in the scientific and/or regulatory community. Information can be obtained from the OECD Test Guidelines website and the EURL ECVAM Database Service on Alternative Methods to Animal Experimentation (DB-ALM). ?

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

This free text section should be used to elaborate on the scientific basis for the indicated domains of applicability and the WoE calls (if provided). While structured terms may be selected to define the taxonomic, life stage and sex applicability (see structured applicability terms, above) of the KE, the structured terms may not adequately reflect or capture the overall biological applicability domain (particularly with regard to taxa). Likewise, the structured terms do not provide an explanation or rationale for the selection. The free-text section on evidence for taxonomic, life stage, and sex applicability can be used to elaborate on why the specific structured terms were selected, and provide supporting references and background information.  More help

Neuronal cell in Hypothalamus

Evidence for Perturbation by Stressor

Overview for Molecular Initiating Event

When a specific MIE can be defined (i.e., the molecular target and nature of interaction is known), in addition to describing the biological state associated with the MIE, how it can be measured, and its taxonomic, life stage, and sex applicability, it is useful to list stressors known to trigger the MIE and provide evidence supporting that initiation. This will often be a list of prototypical compounds demonstrated to interact with the target molecule in the manner detailed in the MIE description to initiate a given pathway (e.g., 2,3,7,8-TCDD as a prototypical AhR agonist; 17α-ethynyl estradiol as a prototypical ER agonist). Depending on the information available, this could also refer to chemical categories (i.e., groups of chemicals with defined structural features known to trigger the MIE). Known stressors should be included in the MIE description, but it is not expected to include a comprehensive list. Rather initially, stressors identified will be exemplary and the stressor list will be expanded over time. For more information on MIE, please see pages 32-33 in the User Handbook.

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).


List of the literature that was cited for this KE description. Ideally, the list of references, should conform, to the extent possible, with the OECD Style Guide ( (OECD, 2015). More help

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].

Kerin, J. F., Liu, J. H., Phillipou, G., & Yen, S. S. (1985). Evidence for a hypothalamic site of action of clomiphene citrate in women. J Clin Endocrinol Metab. , 61(2), 65-68.

Kettel, L. M., Roseff, S. J., Berga, S. L., Mortola, J. F., & Yen, S. S. (1993). Hypothalamic-pituitary-ovarian response to clomiphene citrate in women with polycystic ovary syndrome. Fertil Steril. , 59(3), 532-38.

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.