Estradiol

Sexual development

The development of secondary sex characteristics in women is driven by estrogens, to be specific, estradiol. These changes are initiated at the time of puberty, most are enhanced during the reproductive years, and become less pronounced with declining estradiol support after the menopause. Thus, estradiol produces breast development, and is responsible for changes in the body shape, affecting bones, joints, and fat deposition. Fat structure and skin composition are modified by estradiol.

Female reproduction

In the female, estradiol acts as a growth hormone for tissue of the reproductive organs, supporting the lining of the vagina, the cervical glands, the endometrium, and the lining of the fallopian tubes. It enhances growth of the myometrium. Estradiol appears necessary to maintain oocytes in the ovary. During the menstrual cycle, estradiol produced by the growing follicle triggers, via a positive feedback system, the hypothalamic-pituitary events that lead to the luteinizing hormone surge, inducing ovulation. In the luteal phase, estradiol, in conjunction with progesterone, prepares the endometrium for implantation. During pregnancy, estradiol increases due to placental production. The effect of estradiol, together with estrone and estriol, in pregnancy is less clear. They may promote uterine blood flow, myometrial growth, stimulate breast growth and at term, promote cervical softening and expression of myometrial oxytocin receptors. In baboons, blocking of estrogen production leads to pregnancy loss, suggesting estradiol has a role in the maintenance of pregnancy. Research is investigating the role of estrogens in the process of initiation of labor. Actions of estradiol are required before the exposure of progesterone in the luteal phase.

Male reproduction

The effect of estradiol (and estrogens in general) upon male reproduction is complex. Estradiol is produced by action of aromatase mainly in the Leydig cells of the mammalian testis, but also by some germ cells and the Sertoli cells of immature mammals. It functions (in vitro) to prevent apoptosis of male sperm cells. While some studies in the early 1990s claimed a connection between globally declining sperm counts and estrogen exposure in the environment, later studies found no such connection, nor evidence of a general decline in sperm counts. Suppression of estradiol production in a subpopulation of subfertile men may improve the semen analysis.

Males with certain sex chromosome genetic conditions, such as Klinefelter's syndrome, will have a higher level of estradiol.

Bone

Estradiol has a profound effect on bone. Individuals without it (or other estrogens) will become tall and eunuchoid, as epiphyseal closure is delayed or may not take place. Bone structure is affected also, resulting in early osteopenia and osteoporosis. Also, women past menopause experience an accelerated loss of bone mass due to a relative estrogen deficiency.

Brain

Estrogens can be produced in the brain from steroid precursors. As antioxidants, they have been found to have neuroprotective function.

The positive and negative feedback loops of the menstrual cycle involve ovarian estradiol as the link to the hypothalamic-pituitary system to regulate gonadotropins. (See Hypothalamic–pituitary–gonadal axis.)

Estrogen is considered to play a significant role in women’s mental health, with links suggested between the hormone level, mood and well-being. Sudden drops or fluctuations in, or long periods of sustained low levels of estrogen may be correlated with significant mood-lowering. Clinical recovery from depression postpartum, perimenopause, and postmenopause was shown to be effective after levels of estrogen were stabilized and/or restored.

Recently, the volumes of sexually dimorphic brain structures in transgender women were found to change and approximate typical female brain structures when exposed to estrogen concomitantly with androgen deprivation over a period of months, suggesting that estrogen and/or androgens have a significant part to play in sex differentiation of the brain, both prenatally and later in life.

There is also evidence the programming of adult male sexual behavior in many vertebrates is largely dependent on estradiol produced during prenatal life and early infancy. It is not yet known whether this process plays a significant role in human sexual behavior, although evidence from other mammals tends to indicate a connection.

Estrogen has been found to increase the secretion of oxytocin and to increase the expression of its receptor, the oxytocin receptor, in the brain. In women, a single dose of estradiol has been found to be sufficient to increase circulating oxytocin concentrations.

Gynecological cancers

Estradiol has been tied to the development and progression of cancers such as breast cancer, ovarian cancer and endometrial cancer. Estradiol affects target tissues mainly by interacting with two nuclear receptors called estrogen receptor α (ERα) and estrogen receptor β (ERβ). One of the functions of these estrogen receptors is the modulation of gene expression. Once estradiol binds to the ERs, the receptor complexes then bind to specific DNA sequences, possibly causing damage to the DNA and an increase in cell division and DNA replication. Eukaryotic cells respond to damaged DNA by stimulating or impairing G1, S, or G2 phases of the cell cycle to initiate DNA repair. As a result, cellular transformation and cancer cell proliferation occurs.

Skin health

The estrogen receptor, as well as the progesterone receptor, have been detected in the skin, including in keratinocytes and fibroblasts. At menopause and thereafter, decreased levels of female sex hormones result in atrophy, thinning, and increased wrinkling of the skin and a reduction in skin elasticity, firmness, and strength. These skin changes constitute an acceleration in skin aging and are the result of decreased collagen content, irregularities in the morphology of epidermal skin cells, decreased ground substance between skin fibers, and reduced capillaries and blood flow. The skin also becomes more dry during menopause, which is due to reduced skin hydration and surface lipids (sebum production). Along with chronological aging and photoaging, estrogen deficiency in menopause is one of the three main factors that predominantly influences skin aging.

HRT, consisting of systemic treatment with estrogen alone or in combination with a progestogen, has well-documented and considerable beneficial effects on the skin of postmenopausal women. These benefits include increased skin collagen content, skin thickness and elasticity, and skin hydration and surface lipids. Topical estrogen has been found to have similar beneficial effects on the skin. In addition, a study has found that topical 2% progesterone cream significantly increases skin elasticity and firmness and observably decreases wrinkles in peri- and postmenopausal women. Skin hydration and surface lipids, on the other hand, did not significantly change with topical progesterone. These findings suggest that progesterone, like estrogen, also has beneficial effects on the skin, and may be independently protective against skin aging.

Other effects

Estradiol has complex effects on the liver. It affects the production of multiple proteins, including lipoproteins, binding proteins, and proteins responsible for blood clotting. In high amounts, it can lead to cholestasis.

Estrogen affects certain blood vessels. Improvement in arterial blood flow has been demonstrated in coronary arteries.

Several gynecologic conditions are dependent on estrogen, such as endometriosis, leiomyomata uteri, and uterine bleeding.

Biological activity

Estradiol acts primarily as an agonist of the estrogen receptor (ER), a nuclear steroid hormone receptor. There are two subtypes of the ER, ERα and ERβ, and estradiol potently binds to and activates both of these receptors. The result of ER activation is a modulation of gene transcription and expression in ER-expressing cells, which is the predominant mechanism by which estradiol mediates its biological effects in the body. Estradiol also acts as an agonist of membrane estrogen receptors (mERs), such as GPER (GPR30), a recently discovered non-nuclear receptor for estradiol, via which it can mediate a variety of rapid, non-genomic effects. Unlike the case of the ER, GPER appears to be selective for estradiol, and shows very low affinities for other endogenous estrogens, such as estrone and estriol. Additional mERs besides GPER include ER-X, ERx, and G_q-mER.

In the E2 classical pathway or estrogen classical pathway, estradiol enters the cytoplasm, where it causes dissociation of heat-shock protein (HSP). Estradiol then binds to HSP and can homodimerise (form structures of two HSP and two estradiol molecules) and then bind to specific domains on the nucleus (estrogen response element, ERE), allowing for gene transcription which can take place over hours and days.

Estradiol is about 10 times as potent as estrone and about 80 times as potent as estriol in its estrogenic effect.

Biosynthesis

Estradiol, like other steroids, is derived from cholesterol. After side chain cleavage and using the Δ⁵ or the Δ⁴- pathway, Δ⁴-androstenedione is the key intermediary. A portion of the Δ⁴-androstenedione is converted to testosterone, which in turn undergoes conversion to estradiol by aromatase. In an alternative pathway, Δ⁴-androstenedione is aromatized to estrone, which is subsequently converted to estradiol.

During the reproductive years, most estradiol in women is produced by the granulosa cells of the ovaries by the aromatization of Δ⁴-androstenedione (produced in the theca folliculi cells) to estrone, followed by conversion of estrone to estradiol by 17β-hydroxysteroid dehydrogenase. Smaller amounts of estradiol are also produced by the adrenal cortex, and, in men, by the testes.

Estradiol is not produced in the gonads only, in particular, fat cells produce active precursors to estradiol, and will continue to do so even after menopause. Estradiol is also produced in the brain and in arterial walls.

The biosynthesis of estradiol has been observed in various other species, as indicated above, but also in such species as Phaseolus vulgaris. More often referred to as "beans", consumption may equate to unintentional ingestion of estradiol. In light of this, consumption can be counterproductive to patients undergoing treatment for breast cancer, which usually includes depriving the cancer cells of estrogens. Soybeans are another bean that contains chemicals that act similarly to estrogen in the human body and also cause such interactions.

Distribution

In plasma, estradiol is largely bound to SHBG, and also to albumin. Only a fraction of 2.21% (± 0.04%) is free and biologically active, the percentage remaining constant throughout the menstrual cycle.

Metabolism

Inactivation of estradiol includes conversion to less-active estrogens, such as estrone and estriol. Estriol is the major urinary metabolite. Estradiol is conjugated in the liver to form estrogen conjugates like estradiol sulfate, estradiol glucuronide and, as such, excreted via the kidneys. Some of the water-soluble conjugates are excreted via the bile duct, and partly reabsorbed after hydrolysis from the intestinal tract. This enterohepatic circulation contributes to maintaining estradiol levels.

Estradiol is also metabolized via hydroxylation into catechol estrogens. In the liver, it is non-specifically metabolized by CYP1A2, CYP3A4, and CYP2C9 via 2-hydroxylation into 2-hydroxyestradiol, and by CYP2C9, CYP2C19, and CYP2C8 via 17β-hydroxy dehydrogenation into estrone, with various other cytochrome P450 (CYP) enzymes and metabolic transformations also being involved.

Estradiol is also esterified into lipoidal estradiol forms like estradiol palmitate and estradiol stearate, which are stored in adipose tissue and may act as a very long-lasting reservoir of estradiol.

2-Hydroxylation

Addition of a hydroxyl group at C2 represents the major hepatic pathway for estradiol metabolism, as mediated by CYP1A2, CYP2C8, CYP2C9, and CYP3A4. Extrahepatic 2-hydroxylation is chiefly mediated by CYP1A1 and CYP3A4.

2-Hydroxyestradiol (2-OHE₂) can experience three metabolic fates: methylation to yield 2-meOHE₂, oxidation to form quinones, or dehydrogenation to yield 2-OHE₁.

2-OHE₂ can bind to estrogen receptors but with markedly lower affinity. This metabolite has several physiological consequences: the ability to influence intracellular signalling, adenohypophyseal hormone secretion, radical and quinone formation, and inhibition of tumor formation. Weak carcinogenic activity has been shown, likely due to radical formation and induction of single-strand DNA breaks.

Inactivation of 2-OHE₂ is catalysed by catechol-O-methyltransferase (COMT), with COMT exhibiting a faster rate for the methylation of 2-OHE₂ versus 4-OH-E₂. COMT, a blood-borne enzyme, mediates the most common form of 2- or 4-hydroxyestradiol inactivation, in addition to glucuronidation and sulfation. However, this inactivation can allow for the accumulation of 4-OHE₂, as 2-OHE₂ inhibits 4-OHE₂ methylation by COMT, but 4-OHE₂ does not inhibit 2-OH-E₂ methylation in return.

Antitumor activity of 2-meOE₂ is thought to be mediated by antiproliferative and antimetastatic effects. Inhibition of cellular proliferation and metastasis appears to be via induction of caspase-8, followed by caspase-3 and eventually DNA fragmentation. Induction of apoptosis by 2-meOE₂ may be p53 dependent or independent. 2-meOE₂ has also been found to inhibit aromatase activity, thereby lowering the in situ synthesis of E₂ in cancer tissue. 2-meOE₂ has a higher binding affinity for sex hormone-binding globulin (SHBG) than E₂ and 2-OH-E₂ and has no affinity for the estrogen receptor.

2-meOE₂ is also a potent inhibitor of angiogenesis in tumor tissues. Administration of this estradiol metabolite prevents vascular smooth muscle growth. This inhibition of angiogenesis is eliminated by co-administration with cytochrome P450 and COMT inhibitors, thereby confirming the involvement of cytochrome P450 enzymes in the blockade of tumor blood supply.

Further antitumor activity of 2-meOE₂ has been identified through immunomodulation. The cytokines IL-6 and TNFα, as well the prostaglandin PGE₂, are capable of stimulating aromatase activity. Since macrophages and lymphocytes are present in breast tissue, this provides a concerning means of upregulating in situ estradiol biosynthesis. 2-meOE₂ appeared to be able to halve the basal aromatase activity in mammary fibroblasts, possibly through destabilisation of the microtubules that mediate translocation of the cytokine receptors to the plasma membrane. Inhibition of cytokine receptor synthesis and blockade of the autocrine and paracrine actions of cytokines and PGE₂ were also observed.

4-Hydroxylation

The enzyme most responsible for estradiol 4-hydroxylation is CYP1B1. In humans, CYP1B1 mRNA and protein exhibit constitutive expression in the lung and kidney, as well as estrogen-regulated tissues such as breast, ovary and uterus. Whereas 4-hydroxylation constitutes the minor pathway in the liver, the greater proportion of CYP1B1 expression in extrahepatic tissues shifts the balance in favor of 4-OH-E₂ formation. 4-OH-E₂ is thought to be the most carcinogenic of all the estradiol metabolites, especially considering that CYP1B1 exhibits overexpression in breast cancer tumors.

4-OH-E₂, like 2-OH-E₂, can be physiologically active as well as tumorigenic. 4-OH-E₂ is capable of binding ER with a reduced dissociation rate and prolonged activation, thereby inducing cellular growth and proliferation, adenohypophyseal hormone secretion, and prostaglandin production.

Das et al. implicated 4-OH-E₂ in the induction of estrogen-responsive genes, a response that exhibited partial or no abrogation by coadministration with an antiestrogen, providing evidence for the ability of 4-OH-E₂ to carry out genetic upregulation via a pathway independent of ER signalling. Effects independent of ER binding include breakage of single-stranded DNA, especially when interacting synergistically with nitric oxide in human breast cancer cells and the production of quinones and free radicals.

CYP1B1 can be induced by E₂. ERα, after binding to estradiol, interacts with the CYP1B1 ERE to stimulate CYP1B1 expression. Thus, although E₂ causes genetic changes conducive to its own inactivation, the decrease in estrogenic activity yields a toxicologically active metabolite that constitutes an additional pathway of estradiol-dependent carcinogenesis.

4-OH-E₂ shares the metabolic scheme of 2-OH-E₂: methylation to 4-methoxyestradiol (4-meOE₂), oxidation to quinones, or dehydrogenation to 4-OH-E₁. Conjugation by the ubiquitously present COMT represents the most common extrahepatic pathway of 4-OH-E₂ inactivation. However, if estrogen homeostasis is imbalanced by an increase in CYP1B1 and a decrease in COMT, a greater degree of genotoxic quinone formation from 4-OH-E₂ will occur. 4-OHE₂ can be oxidized by microsomal CYPs or peroxidases to yield estradiol-3,4-semiquinone. This semiquinone can undergo redox cycling with oxygen to form estradiol-3,4-quinone (E₂-3,4-Q) and superoxide. E₂-3,4-Q can be converted back to 4-OHE₂ in a single step by quinone reductase, or in two sequential steps catalysed by P450 reductase via the semiquinone intermediate. GSH / S-transferase activity can abrogate E₂-3,4-Q levels via formation of glutathione conjugates.

E₂-3,4-Q is a potent nucleophile, and will readily react with electrophilic DNA. This yields the formation of the DNA adducts 4-OHE₂-1-N7Gua and 4-OHE₂-1-N3Ade via a Michael addition. Destabilization of the glycosyl bond between the nitrogenous base and ribose sugar creates apurinic sites as the unstable adducts are lost from DNA. 4-OHE₂-1-N7Gua has a relatively slow depurination half-life of approx. 3 hours, allowing enough time for base excision repair mechanisms to correct the change. However, 4-OHE₂-1-N3Ade exhibits instantaneous depurination, leading to error-prone repair and the induction of mutations. Indeed, E₂-3,4-Q has been shown to cause A-to-G mutations in the gene coding for H¬-ras, ras being vital to the correct regulation of the cellular response to growth factors. Though 2- and 4-OHE₂ have similar redox potentials and thus similar redox cycling activity, the greater carcinogenic capacity of 4-OHE₂ can be attributed to its increased reactivity with DNA. Another harmful effect of estrogen redox cycling is the production of superoxide and hydroxyl radicals. P450 reductase catalysis produces superoxide radicals, which can, in the presence of superoxide dismutase and Fe³⁺, form highly reactive hydroxyl radicals capable of damaging virtually all macromolecules.

16α-Hydroxylation

Through the action of CYP1A1, CYP1A2, CYP2C8, and the CYP3A isoforms, 16α-hydroxyestradiol (16α-OHE₂), also known as estriol, is produced in abundance during pregnancy. 16α-OHE₂ can be dehydrogenated to 16α-hydroxyestrone (16α-OHE₁), a metabolite that has been shown to bind covalently to the estrogen receptor via Schiff base formation. This covalent linkage occurs between the steroid carbonyl and the ε-amino group of lysine. In theory, 16α-OHE₁ could also bind DNA, although this has not been observed. 16α-OHE₂ is a potent ER agonist, capable of levels of cellular proliferation stimulation that near those obtained with E₂. Though studies in hamster kidney tumor models showed weak carcinogenicity, the carcinogenic potential of 16α-OHE₂ in humans remains unknown.

Other hydroxylations

The function of the remainder of the hydroxylated E₂ metabolites (6α-, 6β-, 7α-, 12β-, 15α-, 15β-, and 16β-OHE₂) remain to be elucidated. Some of these metabolites, such as 15α-OHE₂, are excreted in relatively large amounts in pregnant women, possibly serving as an indicator of good fetal health.

Levels

Levels of estradiol in premenopausal women are highly variable throughout the menstrual cycle and reference ranges widely vary from source to source. Estradiol levels are minimal and according to most laboratories range from 20 to 80 pg/mL during the early to mid follicular phase (or the first week of the menstrual cycle, also known as menses). Levels of estradiol gradually increase during this time and through the mid to late follicular phase (or the second week of the menstrual cycle) until the pre-ovulatory phase. At the time of pre-ovulation (a period of about 24 to 48 hours), estradiol levels briefly surge and reach their highest concentrations of any other time during the menstrual cycle. Circulating levels are typically between 130 and 200 pg/mL at this time, but in some women may be as high as 300 to 400 pg/mL, and the upper limit of the reference range of some laboratories are even greater (for instance, 750 pg/mL). Following ovulation (or mid-cycle) and during the latter half of the menstrual cycle or the luteal phase, estradiol levels plateau and fluctuate between around 100 and 150 pg/mL during the early and mid luteal phase, and at the time of the late luteal phase, or a few days before menstruation, reach a low of around 40 pg/mL. The mean integrated levels of estradiol during a full menstrual cycle have variously been reported by different sources as 80, 120, and 150 pg/mL. Although contradictory reports exist, one study found mean integrated estradiol levels of 150 pg/mL in younger women whereas mean integrated levels ranged from 50 to 120 pg/mL in older women.

During the reproductive years of the human female, levels of estradiol are somewhat higher than that of estrone, except during the early follicular phase of the menstrual cycle; thus, estradiol may be considered the predominant estrogen during human female reproductive years in terms of absolute serum levels and estrogenic activity. During pregnancy, estriol becomes the predominant circulating estrogen, and this is the only time at which estetrol occurs in the body, while during menopause, estrone predominates (both based on serum levels). The estradiol produced by male humans, from testosterone, is present at serum levels roughly comparable to those of postmenopausal women (14-55 versus <35 pg/mL, respectively). It has also been reported that if concentrations of estradiol in a 70-year-old man are compared to those of a 70-year-old woman, levels are approximately 2- to 4-fold higher in the man.

Measurement

In women, serum estradiol is measured in a clinical laboratory and reflects primarily the activity of the ovaries. As such, they are useful in the detection of baseline estrogen in women with amenorrhea or menstrual dysfunction, and to detect the state of hypoestrogenicity and menopause. Furthermore, estrogen monitoring during fertility therapy assesses follicular growth and is useful in monitoring the treatment. Estrogen-producing tumors will demonstrate persistent high levels of estradiol and other estrogens. In precocious puberty, estradiol levels are inappropriately increased.

Ranges

Individual laboratory results should always been interpreted using the ranges provided by the laboratory that performed the test.

In the normal menstrual cycle, estradiol levels measure typically <50 pg/ml at menstruation, rise with follicular development (peak: 200 pg/ml), drop briefly at ovulation, and rise again during the luteal phase for a second peak. At the end of the luteal phase, estradiol levels drop to their menstrual levels unless there is a pregnancy.

During pregnancy, estrogen levels, including estradiol, rise steadily toward term. The source of these estrogens is the placenta, which aromatizes prohormones produced in the fetal adrenal gland.

Medical uses

Estradiol is used as a medication, mainly in hormone replacement therapy.

Chemistry

Estradiol is an estrane (C18) steroid. It is also known as 17β-estradiol (to distinguish it from 17α-estradiol) or as estra-1,3,5(10)-triene-3,17β-diol. It has two hydroxyl groups, one at the C3 position and the other at the 17β position, as well as three double bonds in the A ring. Due to its two hydroxyl groups, estradiol is often abbreviated as E2. The structurally related estrogens, estrone (E1), estriol (E3), and estetrol (E4) have one, three, and four hydroxyl groups, respectively.

History

Estradiol was first isolated in 1935. It was also originally known as dihydroxyestrin or alpha-estradiol.

Etymology

The name estradiol derives from estra-, Gk. οἶστρος (oistros, literally meaning "verve or inspiration"), which refers to the estrane steroid ring system, and -diol, a chemical term and suffix indicating that the compound is a type of alcohol bearing two hydroxyl groups.