Estrogens

There are three endogenous nineteen-carbon steroids in humans that have estrogenic activity. The principal ovarian estrogens are 17β-estradiol, which is the primary circulating form, and its metabolite, estrone, which the primary postmenopausal estrogen. During pregnancy the placenta synthesizes estriol. Estrogens coordinate systemic responses during the ovulatory cycle, including regulation of the reproductive tract, pituitary, breasts, and other tissues. Also, some forms of cancer are estrogen-dependent for growth. The hypothalamic-pituitary-ovarian axis and target organs for the actions of estrogens are shown in Figure 40-1. Estrogens are also responsible for mediating development of secondary sex characteristics when females enter puberty, including progressive maturation of the fallopian tubes, uterus, vagina, and external genitalia. Upon estrogenic stimulation, more fat is deposited in the breast, buttocks, and thighs, leading to the normal adult female habitus. The following are characteristics promoted by estrogens.

FIGURE 40–1 Feedback loops and target tissues. A, Negative and positive feedback action of estrogens and progesterone on the hypothalamic-pituitary-ovarian axis. B, Other target tissues for these steroid hormones.

Estrogens may play a direct role in the progression of some endometrial tumors, and lifetime exposure to estrogens is associated with the greatest risk for development of breast cancer. Exposure of the uterus to estrogen without exposure to progesterone is associated with endometrial hyperplasia, episodes of breakthrough bleeding, and an approximate sevenfold increased risk of endometrial cancer.

Progestins

The important endogenous progestin is progesterone, although 17α-, 20α-, and 20β-hydroxyprogesterones have weak progestational activities. Estrogen priming is necessary for progesterone receptor (PGR) expression in almost all progesterone-responsive tissues, including the uterus. Progesterone concentrations rise rapidly in the luteal phase of the menstrual cycle, resulting in a modulation of the action of estrogen on the uterus. Progesterone antagonizes estrogen-induced proliferation in the uterus and initiates secretory changes in preparation for embryo implantation. In the absence of pregnancy, plasma progesterone concentrations decrease, resulting in sloughing of the endometrial lining. Progesterone is responsible for causing the increased basal body temperature observed in the luteal phase. Progesterone is important in mammary glandular development and, unlike the uterus, probably stimulates breast cell proliferation. During pregnancy, progesterone can promote maintenance of pregnancy, inhibit uterine contraction, alter carbohydrate metabolism, decrease HDL, increase LDL, and increase Na⁺ and water elimination by competitive antagonism of aldosterone interaction with mineralocorticoid receptors. A variety of menstrual cycle disorders are treated with progestins, estrogens, or both.

Combined Effects

Progesterone and estrogen coordinate the events associated with the luteal phase of the ovulatory cycle and pregnancy. In females with primary ovarian failure, estrogens and progestins are administered to optimize normal development of secondary sex characteristics. An important pharmacological use of estrogens and progestins is as contraceptives. In this regard estrogens and progestins act

predominantly at the pituitary-hypothalamic axis to decrease production of the gonadotropins, follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Inhibition of the mid-cycle LH surge prevents ovulation. A combination oral contraceptive formulation is also approved for the treatment of severe acne in females over age 15 years (acne vulgaris). Antiestrogens have been developed to aid in treatment of infertility by inducing an increase in circulating FSH, which leads to ovulation. Estrogen and progestin replacement therapy have been used extensively in treatment of symptoms arising at menopause.

The major therapeutic uses of estrogens, progestins, their synthetic agonists and antagonists, and inhibitors of estrogen biosynthesis are summarized in the Therapeutic Overview Box.

Biosynthesis of Estrogens and Progestins

Estrogens and progestins are produced by steroidogenesis in various tissues (see Fig. 38-1). The ovary is the predominant source of these steroids in nonpregnant, premenopausal women. A significant amount of estrogenic activity is also produced by skeletal muscle, liver, and adipose tissue through the conversion of circulating androgens to estrone. Certain brain areas in males and females may also produce estrogens through the action of aromatase on circulating androgens. Small amounts of estradiol can be produced in the male testes.

During the menstrual cycle, the pituitary gonadotropins FSH and LH regulate the synthesis and release of estrogen and progesterone from the ovary. The pulsatile release of hypothalamic gonadotropin-releasing hormone (GnRH), in turn, regulates FSH and LH synthesis and release. GnRH concentrations are regulated through negative and positive feedback by the steroid hormones. Estrogens and progestins also act directly on the pituitary gonadotrophs to decrease FSH and LH concentrations. In addition, an ovarian protein, inhibin, negatively affects FSH synthesis. The pathways for the integrated control of hormone regulation are shown in Figure 40-1, A.

The ovulatory-menstrual cycle normally spans 25 to 35 days. The steps in the ovarian and endometrial cycles are shown in Figure 40-2. The ovarian cycle is divided into the follicular (preovulatory) phase, when ova maturation and estrogen release occurs, ovulation, when follicular rupture leads to ova release, and the postovulatory phase, when the corpus luteum maximally releases progesterone and stimulates growth of the endometrial lining. The follicle is the basic reproductive unit of the ovary and consists of an oocyte surrounded by granulosa cells. At the onset of a menstrual cycle, FSH accelerates maturation of several follicles. Through interactions with its receptor, FSH increases aromatase activity, which stimulates conversion of androgens to estradiol. By days 8 to 10, FSH decreases, and the dominant follicle becomes more sensitive to circulating gonadotropin because of an increased number of FSH receptors. In the late follicular phase, estradiol levels increase rapidly and initiate a mid-cycle LH surge (16 to 24 hours before ovulation). Increased LH levels promote follicular production of progesterone, prostaglandin F_2α, and proteolytic enzymes, and ultimately, follicular rupture and ovulation occur. After ovulation, the granulosa and theca cells become the corpus luteum, which produces and releases progesterone throughout the first half of the luteal phase (10 to 20 ng/mL). The suppression of FSH and LH release promotes the decline of progesterone and estrogen, luteolysis, initiation of menses, and ultimately a new cycle.

FIGURE 40–2 Ovulatory and menstrual cycle. Ovarian and uterine changes that occur with the cyclical hormonal changes during the normal human menstrual cycle. Note the increase in LH, FSH, and estradiol concentrations before ovulation during the follicular phase. Progesterone rises and peaks in the midluteal phase, concomitant with reductions in LH, FSH, and estradiol.

During pregnancy the placenta secretes chorionic gonadotropin into the maternal circulation. The chorionic gonadotropin concentration rises rapidly after implantation and peaks in approximately 6 to 8 weeks. Chorionic gonadotropin maintains the corpus luteum and stimulates progesterone production, which initially maintains placental implantation and pregnancy. Sometime after the fifth week of pregnancy, the fetal-placental unit becomes the major source of circulating progesterone and estrogens, especially estriol.

As women age, the number of follicles in the ovaries diminish, predominantly as a result of atresia. Eventually, the normal menstrual cycles cease (menopause). Without estrogen and progesterone to suppress the hypothalamic-pituitary axis, FSH and LH levels increase. Although adrenal androgens, predominantly androstenedione, can be converted to estrone by peripheral tissues with aromatase activity, circulating estrogen concentrations decrease to extremely low levels. This is associated with symptomology of estrogen deficiency, which can occur rapidly, whereas other symptoms (osteopenia) are delayed. The major acute symptoms include vasomotor instability (hot flashes and sweats) and vaginal atrophy, resulting in discomfort, dyspareunia, and urethral syndrome. Other symptoms, possibly related to decreased estrogen levels, include loss of concentration, loss of libido, weight gain, depression, thinning hair, joint discomfort, and sleep disruption.

Ligand Structure

Compounds with estrogenic activity can be classified as either steroidal or nonsteroidal. Steroidal estrogens can be subdivided into natural and synthetic forms. The structures of progesterone, the three endogenous human estrogens (β-estradiol, estrone, and estriol), and their biosynthetic pathways are shown in Figures 38-1 and 38-2. Estradiol is the most potent of the three estrogens. Synthetic hormones that are used therapeutically generally have a heterocyclic structure resembling endogenous steroids.

Endogenous human estrogens have a low potency if administered orally as a result of poor absorption and rapid inactivation by first-pass hepatic metabolism. Estrogen conjugates are formed by enzymatic addition of sulfate at C3 or glucuronidation, which confers inactivation and solubility, enhancing their renal excretion. The endogenous pool of estrogenic steroids represents a balance among the three naturally occurring estrogens and loss by conjugation and excretion. Estradiol predominates before menopause, estrone sulfate predominates in postmenopausal women, and estriol levels predominate during pregnancy.

The synthetic estrogens, ethinyl estradiol and mestranol, are used predominantly in combination oral contraceptives. These compounds have an ethinyl group at C17, which retards hepatic inactivation. Mestranol requires activation by hepatic conversion to ethinyl estradiol. More recently, attention has focused on the nonsteroidal synthetic compounds, selective estrogen receptor modulators (SERMs), that interact with estrogen receptors (ER). The first available SERM, tamoxifen, is a triphenylethylene derivative that acts as an estrogen antagonist in breast; clinical applications include treatment, and recently prevention, of breast cancer (see Chapter 55). A clinically important benefit of tamoxifen is its agonist activity in bone, which antagonizes osteopenia. A concern with tamoxifen is the significantly increased risk of endometrial cancer and venous thrombosis related to its estrogenic activity. Raloxifene is a SERM developed to delay osteoporosis in postmenopausal women who are not candidates for estrogen treatment. It has agonist activity in bone but displays little estrogen-like activity in breast or uterus. Clomiphene citrate is a racemic mixture of two stereoisomers that has both agonist and antagonist properties and is used to treat infertility by inducing ovulation. Considerable research is directed at identifying new SERMS with tissue-specific agonist and antagonist properties for each therapeutic goal.

The progestin derivatives are classified on the basis of their structure at positions C21 or C19 (19-nortestosterone). The C21 derivatives include the natural progestins, progesterone and 17α-hydroxyprogesterone, which use the same carbon backbone as pregnenolone, from which they are derived. The synthetic C21 compounds are derivatives of 17α-hydroxyprogesterone and include medroxyprogesterone acetate, megestrol acetate, and hydroxyprogesterone caproate. The presence of an acetate ester in medroxyprogesterone acetate and megestrol acetate helps protect these compounds from inactivation in the liver and allows their oral use.

The synthetic 19-nortestosterone derivatives are similar to testosterone but lack the C19 methyl group and have an ethinyl group at C17α. The ethinyl group present at C17 retards hepatic inactivation, which allows oral administration to attain effective blood levels. These compounds are divided into the estranes and gonanes. The estranes are 19-nortestosterone analogs that include norethindrone, norethindrone acetate, norethynodrel, and ethynodiol diacetate. The estranes exhibit relatively greater androgenic activity, less progestin activity than progesterone analogs, and relatively little estrogenic activity. Norethindrone acetate and norethynodrel are metabolized to the active progestin norethindrone. The gonanes are norgestrel analogs that include levonorgestrel, desogestrel, and norgestimate. These compounds are less androgenic and estrogenic than norethindrone.

Two additional important ligands of the PGR include danazol and mifepristone (formerly called RU486). Danazol is a steroid derivative that has significant agonist activity at both progesterone and androgen receptors and is used in treating endometriosis. Mifepristone is a steroid derivative that binds to both progesterone and glucocorticoid receptors and displays progestin antagonist activity in most target tissues.

Transport of Hormones in the Blood

Steroid hormones are highly hydrophobic molecules that must be transported by serum proteins to their target tissues. Circulating estrogens are specifically bound by SHBG, and progesterone by corticosteroid-binding globulin (CBG). These are relatively high-affinity, low-capacity interactions compared with those of albumin. The concentration of these binding globulins relative to hormone concentrations determines free hormone concentrations. Free hormone concentrations represents hormone availability to target tissues. The concentrations of the binding globulins are hormonally regulated, and the synthesis of both globulins increase in response to estrogen administration; serum albumin concentrations are unaffected. Synthetic ligands show variable affinities for these serum proteins.

Receptor Mechanisms

The molecular basis for hormone action, including estrogen and progesterone, is reviewed in Chapter 1. Free steroid passively diffuses into any cell but accumulates only in cells expressing the specific cytoplasmic steroid-binding proteins. Both estrogen and progesterone receptors are members of the nuclear receptor superfamily. The two distinct ER subtypes, ERα and ERβ, are products of different genes, and estrogen binds with high affinity to both receptor subtypes. ERα is expressed in the reproductive tract and breast and mediates many of the effects of estrogen on sexual development and reproductive function. ERβ is highly expressed in ovary and brain. All currently available drugs that target estrogen receptors can bind to both receptor dimers, suggesting that selectivity of action is not simply dependent on the presence of receptor subtypes. Because the ligand-binding domains of these two ER subtypes are different, specific ligands for each receptor are likely to be developed. Only one gene encodes the PGR, but two protein isoforms are produced, PGR-A and PGR-B. These proteins display some functional differences in experimental systems, and their ratio shows some variability. However, they have the same ligand-binding domain, and pharmacological effects result from activation of both isoforms.

The classical mechanism of action of nuclear steroid hormones is that the steroid-hormone complex can act as a steroid-activated transcription factor (see Chapter 1). The response of a tissue to a specific ligand is highly regulated at multiple levels:

The antihormones and SERMs competitively antagonize hormone receptor binding. These agents induce a distinct conformational change in the receptor, allowing it to bind to the HRE in target genes. Also, the effect of the multiprotein complex on gene activity depends on the ligand. These effects provide a rationale for SERMs to act either as agonist or antagonist in a tissue-specific manner.

Receptor concentrations also influence tissue responses and are strongly affected by the hormonal environment. PGRs are expressed in response to estrogen exposure, and high concentrations of progesterone decrease ER concentrations, which, in turn, leads to decreased PGR concentrations. Furthermore each hormone can directly regulate its own receptor concentration (down or up). The half-life for estrogen receptors is 2 to 4 hours, and this may be reduced by binding ligand.

Estrogens

Estrogens are rapidly absorbed from the gastrointestinal (GI) tract, skin, and mucous membranes, and after parenteral injection. Because the unconjugated natural estrogens are rapidly inactivated in the GI tract and liver, if taken orally, their delivery by other routes (transdermally, vaginally, nasally, or intramuscularly) is warranted. Micronized estradiols, steroidal estrogens that contain an ethinyl group at C17, the conjugated estrogens, and nonsteroidal estrogens are active orally. Once absorbed, they are rapidly metabolized in the liver.