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Event: 2251

Key Event Title

A descriptive phrase which defines a discrete biological change that can be measured. More help

Estradiol availability, increased

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. More help
Increased E2 availability
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Biological Context

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Level of Biological Organization
Tissue

Organ term

The location/biological environment in which the event takes place.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.  Further information on Event Components and Biological Context may be viewed on the attached pdf. More help
Organ term
uterus

Key Event Components

The KE, as defined by a set structured ontology terms consisting of a biological process, object, and action with each term originating from one of 14 biological ontologies (Ives, et al., 2017; https://aopwiki.org/info_pages/2/info_linked_pages/7#List). Biological process describes dynamics of the underlying biological system (e.g., receptor signalling).Biological process describes dynamics of the underlying biological system (e.g., receptor signaling).  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 signaling 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.  Further information on Event Components and Biological Context may be viewed on the attached pdf. More help
Process Object Action
estradiol increased
hormone activity tissue increased

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
SULT1E1 inhibition and increased oestradiol availability KeyEvent Ana-Andreea Cioca (send email) Under development: Not open for comment. Do not cite
Aromatase induction leading to estrogen receptor alpha activation via increased estradiol KeyEvent Ana-Andreea Cioca (send email) Under development: Not open for comment. Do not cite
HSD17B2 inhibition leading to Activation, ER alpha KeyEvent Ana-Andreea Cioca (send email) Under development: Not open for comment. Do not cite
Decreased GnRH release leading to increased E2 KeyEvent Travis Karschnik (send email) Under development: Not open for comment. Do not cite

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 KE.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
mammals mammals NCBI

Life Stages

An indication of the the relevant life stage(s) for this KE. More help
Life stage Evidence
Adult
Adult, reproductively mature
Old Age

Sex Applicability

An indication of the the relevant sex for this KE. More help
Term Evidence
Male
Female

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. More help

Currently, there is an existing key event in the AOP Wiki (event number 1973) entitled Increased, estrogens which has a role of KE in one AOP (under development) (number 440 - Hypothalamic estrogen receptors inhibition leading to ovarian cancer). Some sections of the KE description were adapted from that event.

In the current AOP, “increased E2 availability” is intended as the increased availability of E2 to its intended biological destination, that is the estrogenic signalling pathways in target tissues (estrogen-sensitive tissues).

Biological state

The three major forms of endogenous estrogens are estrone (E1), oestradiol (E2, or 17β-oestradiol), and estriol (E3).  Estrogen metabolism is complex and multifactorial (Fig.6).

Figure 6. Schematic representation of estrogen metabolism. Dotted circles represent the enzyme system that would influence the bioavailability of oestradiol (Wikoff et al., 2016)

Although circulating estrogens exist in a dynamic equilibrium of metabolic interconversions, E2 is the principal intracellular human estrogen and is substantially more potent than its metabolites, E1 and E3 at the receptor level (NCI Thesaurus (NCIt) in PubChem, available at: https://ncit.nci.nih.gov/ncitbrowser/, version 22.11d).

Oestradiol (E2) is principally produced in the ovaries by follicular thecal and granulosa cells under the regulation of follicle-stimulating hormone (FSH) in the reproductive phase in women (Simpson 2003). Before puberty and after menopause E2 is mainly produced in extragonadal tissues, including kidney, breast, brain, liver and fat (Secky et al., 2013). In males it is mainly produced by the Leydig cells in the testis.

Vehiculated via the circulatory system to estrogen-sensitive tissues (female reproductive organs, breasts, hypothalamus and pituitary), E2 becomes available to specific estrogen receptors (subtypes alpha (ERα) and beta (ERβ), triggering the estrogenic signalling pathway: the receptor-ligand complex enters the nucleus of the target and promotes the gene expression necessary for the maintenance of fertility and secondary sexual characteristics in females and other effects, such as mild anabolic and metabolic properties, and increased blood coagulability.  E2 also exerts potent agonism of G Protein-coupled estrogen receptor (GPER), which is recognized an important regulator of E2 rapid effects.  

The three major forms of endogenous estrogens are estrone (E1), oestradiol (E2, or 17β-oestradiol), and estriol (E3). Although circulating estrogens exist in a dynamic equilibrium of metabolic interconversions, oestradiol is the principal intracellular human estrogen and is substantially more potent than its metabolites, estrone and estriol at the receptor level (NCI Thesaurus (NCIt) in PubChem).

In estrogen-responsive organs (uterus, breast, prostate), E2 availability to the estrogen signalling pathway is fine-tuned at cellular level; E2 activation/deactivation is controlled by a local machinery composed by enzymes analogous to those in gonadal tissue (Secky et al., 2013). This “intracrinological” pathway actively contributes to the modification of intracellular levels of E2, modulating its local effects and playing a major role in physiological and pathological conditions in premenopausal and menopausal women, in men and in animal models (Konings et al., 2018). Huhtinen et al., 2012 showed that E2 concentration in the human uterus (endometrium) is up to 10-fold higher in the proliferative phase compared with the secretory phase of the menstrual cycle, and this is accompanied by cyclic changes in intracrine enzyme levels, indicating that steroid exposure is locally cyclically regulated to support endometrial physiology.

An important reaction in the intracrine steroidogenesis is the interconversion of 17-keto and 17b-hydroxysteroids controlled by HSD17Bs. 17b-hydroxysteroids (testosterone and E2) have higher affinity for the receptors than the keto-steroids (A4 and E1). This balance determines the final androgenic/estrogenic activity at target tissue level (Konings et al., 2018). In addition, of relevance is the intracellular balance between unconjugated (free, active) and the sulfo-conjugated (inactive) E2. This is controlled by the “sulfatase pathway”, which is based on the interplay between SULT1E1 and STS and contributes to the modulation of E2 effects and protection versus its excess in target tissues (Cui et al., 2013; Cornel, 2018).

Biological compartment

Gonadal E2 is produced under the control of the hypothalamus-pituitary-ovary (HPO) axis and mainly released into the bloodstream to reach target tissues. In the female reproductive years E2 levels are physiologically subject to cyclic variations in the blood, reaching the highest level immediately before ovulation. Levels of circulating E2 during the follicular phase, pre-ovulatory phase, and luteal phase are 19–140 pg/ml, 110–410 pg/ml, and 19–160 pg/ml, respectively. During the menopause transition, E2 and E1 levels decrease by 85–90% and 65–75% respectively as compared to mean pre-menopausal levels, and in postmenopausal women they are below 35 pg/ml.

The above described “intracrinological” pathway actively contributes to the modification of intracellular levels of E2, modulating its local effects and playing a major role in physiological and pathological conditions in premenopausal and menopausal women, in men and in animal models (Konings et al., 2018). As a consequence, cellular E2 levels do not reflect the blood levels (Cornel et al., 2018); in the endometrium they can be up to five-times higher than in serum during the proliferative phase and 1.5-fold higher in the luteal period (Huhtinen et al., 2012, 2014).

Levels of estrogens (E2) in both the circulatory (plasma) and tissue compartments should both be considered and could offer a complementary information.

Among enzymes relevant in intracrinology, the sulfatase and sulfotransferase (SULT1E1), combined as the sulfatase pathway, represent a major route of estrogen supply and removal in endometrial cells. In physiological conditions the balance of the pathway is shifted towards the formation of free estrone, as indicated by the STS activity, that is few magnitudes higher than that of SULT1E1. Other enzymes relevant for the intracrine regulation and ultimately tissue level of E2 include aromatase CYP19A1 (converting testosterone to oestradiol), hydroxysteroid-dehydrogenase-17B (HSD17B, interconverting estrone and oestradiol)) (reviewed in see Wikoff et al 2015). In addition, other hormones can also contribute to the regulation of E2 at endometrial level. For example, progesterone diminishes estrogenic action in the endometrium by stimulating the local synthesis of 17-hydroxysteroid dehydrogenase and estrogen sulfotransferase, with the effect peaking in the luteal phase. The up regulation of these enzymes decreases intra-tissue estrogen levels and is one of the mechanisms of the uterine antiestrogenic effects of progesterone (Huntinhen et al., 2012).

General role in biology

Endogenous estrogens are largely responsible for the development and maintenance of the female reproductive system and secondary sexual characteristics. E2 is the principal intracellular human estrogen and is substantially more potent than its metabolites, estrone and estriol at the receptor level (HSD17B in PubChem). In the reproductive phase E2 together with progesterone controls the menstrual cycle and the reproductive functions; it induces endometrial cell proliferation in premenopausal women.

E2 effects are mediated by a complex estrogenic signalling, mainly via two nuclear estrogen receptors (ERα and β) and one membrane receptor (GPER); activating genomic and non-genomic actions upon ligand binding (Cornel et al., 2017)

Availability of E2 to its receptors is key to trigger the estrogenic signalling in estrogen-sensitive cells. It is related to the levels of circulating estrogens and it is locally fine-tuned by a set of intracellular enzymes (intracrinology).

Disruption of estrogen homeostasis resulting in prolonged increased E2 levels, associated with relative decreased progesterone (P4) would be among the leading risk factors for the development of pathological conditions such as endometrial cancer. An estrogen imbalance is associated with endometrial carcinomas in rats (Hilliard and Norris, 1979; Fox, 1984), spontaneous endometrial adenocarcinomas in the Donryu rat (Nagaoka et al., 1990), and in F344 rats (Tang et al., 1984). Epidemiological studies showed increased endometrial cancer risks among postmenopausal women who have increased blood levels of oestradiol (reviewed in Kaaks et al., 2002) as well as of steroid precursors of E2 (testosterone, androstenedione, DHEA, DHEA-S, estrone and estrone-S) compared with healthy controls (Cornel et al., 2017). This can be interpreted in the light of the “unopposed estrogen” hypothesis, which proposes that endometrial cancer may develop as a result of the mitogenic effects of estrogens, when these are insufficiently counterbalanced by progesterone.  Since the majority of the endometrial cancer patients are postmenopausal women, local formation of E2 from circulating precursors either from circulating androgens via the aromatase pathway or from E1S via the sulfatase pathway becomes important.

How It Is Measured or Detected

A description of the type(s) of measurements that can be employed to evaluate the KE and the relative level of scientific confidence in those measurements.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). Do not provide detailed protocols. More help

This KE refers to increased E2 availability in the uterus, where availability is intended as the extent E2 becoming completely available to its intended biological destination ER.

Circulating E2 levels are relevant in determining E2 levels available to endometrial cells in the uterus. These are measurable by a variety of standardised routine methods in humans and animals. Circulating levels are intracellularly subject to a complex intracrine control mechanism, activating and deactivating E2 available to receptors. The translation of circulating E2 levels into E2 availability in uterus needs further exploration and it is not fully addressed in this Scientific Opinion.

It has been acknowledged that availability of E2 in uterus is strongly associated with estrogenic activity. Therefore, E2 availability in target tissue can be measured with standardised methods evaluating estrogenicity, that are listed below.

In vivo

  • OECD TG 440 Uterotrophic bioassay in rodents

Others

Overall, no high resolution and/or standardised methods exist to quantify steroids within target tissues. However, in recent decades it has been recognized that steroid concentration within tissues is modulated independently from circulating levels and therefore investigations in this field are initiated (Cobice et al., 2013).

Uterine response to estrogens involves the activation of a large pattern of estrogen-sensitive genes:

  • Expression of Calbindin-D9k (CaBP-9k) gene and protein. The 9 kilodalton vitamin D-dependent calcium binding protein (CaBP9k), calbindin-D9k, is expressed in the intestine and uterus of mammals (bgee.org; L’ Horset et al., 1990) Different studies demonstrated that in the mammals uterus, the expression of CaBP-9k is regulated by hormones such as E2 and P4.  L’Horset et al., 1993; review by Choi et al., 2005).
  • Complement 3 (C3). It has been demonstrated that C3 could be regarded as an estrogen sensitive marker in rat uterus (Diel et al., 2000; Sundstrom et al., 1989)

In addition to this, it has been reported that blood glutamate levels are inversely related to plasma estrogen and progesterone level in plasma (Zlotnik et al., 2011).

In the regulatory area standard methods are available for serum estrogen analysis include radioimmunoassay (RIA), enzyme-linked immunosorbent assay (ELISA), and multiplex immunoassay. Liquid chromatography/mass spectrometry (LC/MS)-based methods are also becoming more widely used (as cost and sample size requirements decrease), particularly for measurement of estrogens and estrogen metabolites. For E2, rodent-specific immunoassays are commercially available (Andersson, 2013). In the OECD TG 422 dedicated to repeated dose toxicity and reproduction, sex hormones data are not routine endpoints. In OECD TG 408, measurement of sexual hormones is optional and should be considered on a case-by-case basis. However, it is not recommended to include female reproductive hormonal measurements in first-tier toxicity studies of standard design. Indeed, due to the limited standard number of animals per group the average number of each animal in each stage of the cycle is generally too few to permit conclusions (Stanislaus, 2012). Specifically designed and statistically powered investigative studies (with appropriate animal numbers) are best suited to measure serum hormones in female rodents (Andersson, 2013).

Domain of Applicability

A description of the scientific basis for the indicated domains of applicability and the WoE calls (if provided).  More help

Taxonomic Applicability: mammals. Endocrine systems with respect to hormone structure, receptors, synthesis pathways, hormonal axes and degradation pathways are well conserved across vertebrate taxa especially in the case of estrogen, androgen and thyroid hormones and steroidogenesis (OECD TG 150)

Life Stage Applicability: This KE is applicable to adulthood - reproductive and post reproductive (menopausal, aging) phases.

Sex Applicability: males, females

References

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

Andersson H, Rehm S, Stanislaus D and Wood CE, 2013. Scientific and Regulatory Policy Committee (SRPC) Paper:Assessment of Circulating Hormones in Nonclinical Toxicity Studies III. Female Reproductive Hormones. Toxicologic Pathology, 41:921-934. doi: 10.1177/0192623312466959

Choi J-Y, Lee K-M, Park SK, Noh D-Y, Ahn S-H, Chung H-W, Han W, Kim JS, Shin SG, Jang I-J, Yoo K-Y, Hirvonen A and Kang D, 2005. Genetic Polymorphisms of SULT1A1 and SULT1E1 and the Risk and Survival of Breast Cancer. Cancer Epidemiology, Biomarkers & Prevention, 14:1090-1095. doi: 10.1158/1055-9965.Epi-04-0688

Cobice DF, Mackay CL, Goodwin RJA, McBride A, Langridge-Smith PR, Webster SP, Walker BR and Andrew R, 2013. Mass Spectrometry Imaging for Dissecting Steroid Intracrinology within Target Tissues. Analytical Chemistry, 85:11576-11584. doi: 10.1021/ac402777k

Cornel KM, Krakstad C, Delvoux B, Xanthoulea S, Jori B, Bongers MY, Konings GF, Kooreman LF, Kruitwagen RF, Salvesen HB and Romano A, 2017. High mRNA levels of 17β-hydroxysteroid dehydrogenase type 1 correlate with poor prognosis in endometrial cancer. Mol Cell Endocrinol, 442:51-57. doi: 10.1016/j.mce.2016.11.030

Cornel KMC, Delvoux B, Saya T, Xanthoulea S, Konings GFJ, Kruitwagen RPFM, Bongers MY, Kooreman L and Romano A, 2018. The sulfatase pathway as estrogen supply in endometrial cancer. Steroids, 139:45-52. doi: https://doi.org/10.1016/j.steroids.2018.09.002

Cui J, Shen Y and Li R, 2013. Estrogen synthesis and signaling pathways during aging: from periphery to brain. Trends Mol Med, 19:197-209. doi: 10.1016/j.molmed.2012.12.007

Diel P, Schulz T, Smolnikar K, Strunck E, Vollmer G and Michna H, 2000. Ability of xeno- and phytoestrogens to modulate expression of estrogen-sensitive genes in rat uterus: estrogenicity profiles and uterotropic activity. The Journal of Steroid Biochemistry and Molecular Biology, 73:1-10. doi: https://doi.org/10.1016/S0960-0760(00)00051-0

Hilliard GD and Norris HJ, 1979. Pathologic effects of oral contraceptives. Recent Results Cancer Res, 66:49-71. doi: 10.1007/978-3-642-81267-5_2

Huhtinen K, Desai R, Ståhle M, Salminen A, Handelsman DJ, Perheentupa A and Poutanen M, 2012. Endometrial and endometriotic concentrations of estrone and estradiol are determined by local metabolism rather than circulating levels. J Clin Endocrinol Metab, 97:4228-4235. doi: 10.1210/jc.2012-1154

Kaaks R, Lukanova A and Kurzer MS, 2002. Obesity, endogenous hormones, and endometrial cancer risk: a synthetic review. Cancer Epidemiol Biomarkers Prev, 11:1531-1543

Konings G, Brentjens L, Delvoux B, Linnanen T, Cornel K, Koskimies P, Bongers M, Kruitwagen R, Xanthoulea S and Romano A, 2018. Intracrine Regulation of Estrogen and Other Sex Steroid Levels in Endometrium and Non-gynecological Tissues; Pathology, Physiology, and Drug Discovery. Frontiers in pharmacology, 9:940. doi: 10.3389/fphar.2018.00940 Available online: http://europepmc.org/abstract/MED/30283331

L'Horset F, Blin C, Brehier A, Thomasset M and Perret C, 1993. Estrogen-induced calbindin-D 9k gene expression in the rat uterus during the estrous cycle: late antagonistic effect of progesterone. Endocrinology, 132:489-495. doi: 10.1210/endo.132.2.8425470

Nagaoka T, Onodera H, Matsushima Y, Todate A, Shibutani M, Ogasawara H and Maekawa A, 1990. Spontaneous uterine adenocarcinomas in aged rats and their relation to endocrine imbalance. J Cancer Res Clin Oncol, 116:623-628. doi: 10.1007/bf01637084

OECD, 2007. Test No. 440: Uterotrophic Bioassay in Rodents.

OECD, 2016. Test No. 422: Combined Repeated Dose Toxicity Study with the Reproduction/Developmental Toxicity Screening Test.

OECD, 2018a. Revised Guidance Document 150 on Standardised Test Guidelines for Evaluating Chemicals for Endocrine Disruption.

OECD, 2018b. Test No. 408: Repeated Dose 90-Day Oral Toxicity Study in Rodents.

Secky L, Svoboda M, Klameth L, Bajna E, Hamilton G, Zeillinger R, Jäger W and Thalhammer T, 2013. The sulfatase pathway for estrogen formation: targets for the treatment and diagnosis of hormone-associated tumors. J Drug Deliv, 2013:957605. doi: 10.1155/2013/957605

Simpson ER, 2003. Sources of estrogen and their importance. The Journal of Steroid Biochemistry and Molecular Biology, 86:225-230. doi: https://doi.org/10.1016/S0960-0760(03)00360-1

Stanislaus D, Andersson H, Chapin R, Creasy D, Ferguson D, Gilbert M, Rosol TJ, Boyce RW and Wood CE, 2012. Society of toxicologic pathology position paper: review series: assessment of circulating hormones in nonclinical toxicity studies: general concepts and considerations. Toxicol Pathol, 40:943-950. doi: 10.1177/0192623312444622

Sundstrom SA, Komm BS, Ponce-de-Leon H, Yi Z, Teuscher C and Lyttle CR, 1989. Estrogen regulation of tissue-specific expression of complement C3. J Biol Chem, 264:16941-16947

Tang FY, Bonfiglio TA and Tang LK, 1984. Effect of estrogen and progesterone on the development of endometrial hyperplasia in the Fischer rat. Biol Reprod, 31:399-413. doi: 10.1095/biolreprod31.2.399

Wikoff DS, Rager JE, Haws LC and Borghoff SJ, 2016. A high dose mode of action for tetrabromobisphenol A-induced uterine adenocarcinomas in Wistar Han rats: A critical evaluation of key events in an adverse outcome pathway framework. Regul Toxicol Pharmacol, 77:143-159. doi: 10.1016/j.yrtph.2016.01.018

Zlotnik A, Gruenbaum BF, Mohar B, Kuts R, Gruenbaum SE, Ohayon S, Boyko M, Klin Y, Sheiner E, Shaked G, Shapira Y and Teichberg VI, 2011. The effects of estrogen and progesterone on blood glutamate levels: evidence from changes of blood glutamate levels during the menstrual cycle in women. Biol Reprod, 84:581-586. doi: 10.1095/biolreprod.110.088120