Estrogen Kinetics for Clinicians

Knowledge of estrogen kinetics is essential to clinicians providing gynecologic care. During the past decade, concepts involving estrogen kinetics have been key elements in the design of oral contraceptives, in the development of menopausal replacement therapy, in the tailoring of reproductive hormone support for transplanted embryos, and in hormone therapy of malignancies. In addition to affecting the female reproductive system, estrogens modulate insulin resistance, bone maintenance, muscle physiology, the cardiovascular system, and neoplastic processes. This chapter examines the process by which estrogens are delivered to the circulation and the intermediate metabolic processes that modulate their clearance and presentation to the microcirculation of target organs. It then integrates these principles into the clinical pharmacology of therapeutic estrogens.

Estrone (E₁) and estradiol (E₂) are classic estrogens that are heuristically the starting products in the expression of estrogen effect. Both steroids originate from ovarian secretion and from extraglandular aromatization of androgen precursors.^1,2,3 The sites of E_l and E₂ production depend on the stage of the menstrual cycle or on menopausal status.^1,2,3 In reviewing this topic, it is necessary to define key terms frequently used to describe steroid kinetic interrelationships. These terms and the techniques used to measure them have been reviewed previously.^1,2

Secretion rate (S_organ^steroid): Rate at which a steroid is secreted into the circulation by a specific steroid-synthesizing gland. The term secretion rate may be used in some circumstances to describe the total of secretion rates from all steroid-synthesizing glands.

(Blood) Production rate (P_BB^steroid): Rate of steroid entry into the blood from all sources. It is the total amount derived from both glandular secretion and extraglandular conversion from other active hormones (e.g., E₂ to E_l) or relatively inactive “prohormones” (e.g., androstenedione to [⁴A] to E₁).

Metabolic clearance rate (MCR): Volume of plasma or blood from which a hormone is irreversibly removed per unit of time. The MCR is usually expressed in liters per 24 hours (liters/24 hr) or liters/24-hr/m^2.

Transfer constant (p_BBAB): Percentage of hormone “A” entering the blood per unit of time that is cleared and returned to the circulation as hormone “B” over an equal unit of time. For example: p_BB^E₂^E₁ is 0.15; this means that 15% of E₂ produced per unit of time is cleared and returned to the circulation as E₁.

Estrogen Kinetics in the Premenopause

In premenopausal women, estrogen production and clearance are profoundly affected by the stage of the menstrual cycle.² Accordingly, blood production rates (P_B^E₂) and ovarian secretion rates (S_ov^E₂₎ for E₂ are shown as a function of menstrual cycle days in Figure 1.² For all phases of the cycle, approximately 95% or more of circulating E₂ originates by direct secretion from the ovary containing the preovulatory follicle or corpus luteum.² During menstruation, P_B^E₂ is approximately 60 μg/24 hr, reaches approximately 400 μg/24 hr immediately before ovulation, and stabilizes at approximately 330 μg/24 hr during the midluteal phase.² Mean MCR^E₂ is approximately 1300 liters/24 hr in menstruating women (Table 1).^3,4 The P_BB^E₁^E₂ is approximately 15%.^3,4 Blood production rates (P_B^E₂) and ovarian secretion rates (S_ov^E₂) for E₁ are shown as a function of menstrual cycle days in Figure 2. As contrasted to E₂, less than 50% of P_B^E₁ is derived from ovarian secretion; during menses and at midcycle, the percent P_B^E₁ derived from S_ov^E₂ is well under 30%.² The major sources of E_l additive to S_ov^E₁ are conversion of secreted E₂ to E₁, ⁴A to E_l, and possibly direct adrenal secretion of E₁. The substantial percentages of P_B^E₁ derived from “other sources” at midcycle may represent additional peripherally converted ⁴A, which is known to peak at midcycle and is probably predominantly of ovarian origin.³ During menstruation, P_B^E₁ is approximately 60 μg/24 hr, increases to 200 μg/24 hr immediately before ovulation, and reaches 150 to 380 μg/24 hr during the luteal phase, the time of maximum P_B^EE₁ being shortly after ovulation.² Mean MCR^E₁ is approximately 2200 liters/24 hr in menstruating women (see Table 1).^1,2 The P_BB^E₂^E₁ is approximately 5%.^1,2

TABLE 1. Metabolic Clearance Rates, Plasma Binding and Concentrations, Blood Production Rates, and Ovarian Secretion Rates for the Three Classic Estrogens and Estrone Sulfate

(Modified from Lipsett MP: Steroid hormones. In Yen SSC, Jaffe RB (eds): Reproductive Endocrinology, Physiology, Pathophysiology, and Clinical Management, p 80. Philadelphia, WB Sauders, 1978)

Estrogen Kinetics After Menopause

In postmenopause life, estrogen production and clearance become acylic and decline sharply.⁵ Nearly all E₂ and E_l in the circulation of postmenopausal women is derived from extraglandular aromatization of testosterone (T) and ⁴A, respectively (see Table 1). P_B^E₂ is approximately 12 μg/24 hr; MCR^E₂ decreases to 900 liters/24 hr; p_BB^TE₂ has not been established.⁵ P_B^E₁ is approximately 45 μg/24 hr; MCR^E₂ is 1600 liters/24 hr, and P_BB^4AE₁, is about 2.5%.^5,6 If ⁴AP_B^4A is approximately 2000 μg/24 hr and P_BB^4AE₁ is approximately 2.7%, this pathway can account for the bulk of the P_B^E₁ of 45 μg/24 hr in postmenopausal women.^5,6

Postmenopausal estrogen production is thus far less than what is observed during the menstrual cycle. Furthermore, estrogen does not enter the circulation in significant amounts from any of the endocrine glands. Increased body fat has been associated with increased extraglandular aromatization and increased circulating E₂ and E_l concentrations.⁷ P_B^E₁ exceeding the 40 μg/24 hr range has been associated with postmenopausal bleeding and endometrial hyperplasia.⁸ Thus, it is not surprising that obesity is associated with increasing risk of endometrial carcinoma.⁹

E₂ availability to intracellular receptors is modulated by plasma-protein binding; estrone sulfate (E₁S) conjugation; formation of metabolites, such as catechol estrogens, which may modulate estrogen-receptor interaction by acting as antiestrogens in some tissues, such as the endometrium, and estrogens in others, such as the nervous system; estrogen conjugation; microvascular extraction in target organs; and autoregulation in target cells. The roles of these metabolic regulators are reviewed next.

Plasma Protein Binding

Steroid hormones cross target tissues and microvascular membranes by passive diffusion.¹⁰ Their rate of transfer is regulated by the availability of nonprotein-bound steroid, which must compete between estrogen target tissue receptor proteins and plasma-binding proteins.^10,11 E₂ interaction with sex hormone binding globulin (SHBG), a high-affinity plasma glycoprotein, is about 38% of the total E₂. Circulating E₂ is also bound to albumin for another 60%. Of the total, only 3% is unbound and fully available.⁹ E₁ is not strongly bound to plasma proteins and has a much higher MCR than E₂. E₁S is bound with high affinity to albumin, with 90% of E₁S circulating in bound form.¹² MCR of E_l and E₂ is greatly influenced by protein binding. Whenever serum-binding capacity is increased, tissue extraction and MCR are proportionally decreased. Although SHBG is probably regulated primarily by growth hormone, which promotes its synthesis in the liver, and somatomedin C, which stimulates its extravasation and uptake in tissues, sex steroids are believed to have a modulating influence that is indirect and quantitatively somewhat less important.¹³ Accordingly, androgens decrease SHBG production in the liver such that the binding capacity in men is lower than in women. Estrogens and thyroid hormone increase SHBG production.¹³ Binding capacity is further increased in women with hyperthyroidism. in pregnancy. by exogenous estrogens. and by smoking.¹³ Thus, in patients with polycystic ovarian disease, SHBG is suppressed by excessive androgens, resulting in elevated free estrogen as well as elevated free testosterone levels, all of which act in concert to aggravate further the underlying pathophysiology of this disorder.¹⁴ Excess E₂ bioavailability may be important in the pathogenesis of breast carcinoma because women who develop this disease have higher levels of unbound E₂ and lower levels of SHBG than non-cancer-afflicted controls.¹⁵ E₁S is also affected by protein binding. Very high binding affinities between E₁S and albumin are responsible for the slow turnover of the E₁S pool, low E₁S MCR, and low E₁S glomerular filtration.⁸

Estrone Sulfate Conjugation and the E₁ S Pool

The interconversions between E₂, E_l, and E₁S are depicted in Figure 3. Transfer constants are given for each of these reversible interconversions. Although E₂ is the biologically active hormone, E₁ is the first step to biologic inactivation of E₂. It is an obligatory intermediate leading to the formation of either E₁S or to ring A (catechol) and ring D (estriol and epiestriol) estrogen metabolites (see Fig. 3) through nonreversible transformations.⁸ Although E₁ binds with estrogen receptors, much of its biologic activity may involve intercellular conversion to E₂.¹⁶ E₁S becomes hormonally active after cleavage of the sulfate radical by the liver or other peripheral sites and in target tissues, which may have sulfatase activity that permits autoregulated conversion to E_l and E₂ from E₁S. Such autoregulated activity, occurring at the cell level in the endometrium, breasts, and neoplastic tissues, may vary with the time of the menstrual cycle.¹⁶

E₁S is quantitatively the single most important plasma estrogen metabolite. Its relative importance is described by the respective transfer constants (p_BB^E₁^-E₁^S 0.54; p_BB^E₂^-E₁^S 0.65), which means that 54%. of E₁ and 65% of E₂ entering the blood is converted to E₁S.⁸ Although these transformations are reversible, the transfer constants describing the opposite reaction (p_BB^E₁^S-E₁ 0.21; p_BB^E₁^S-E₂ 0.014) indicate that a smaller percentage of E₁ is formed from E₁S than E_l to E₁S, and an almost insignificant amount of E₂ is formed from E₁S. E₁S apparently is not secreted by any endocrine tissue because virtually all of its blood production can be accounted for by the conversions E_l-E_lS and E₂-E_lS.⁸ The E₁S pool is conceptualized as a large, slowly metabolized estrogen reservoir with 90% of its mass albumin bound with consequently low MCR (~150 liters/24 hr) and low renal clearance.^8,12 P_B^E₁^S fluctuates with the menstrual cycle much as do its major precursors, E_l and E₂; follicular phase P_B^E₁^S is ~95 μg/24 hr, whereas luteal phase P_B^E₁^S is ~ 180 μg/24 hr.⁸ E_lS plasma concentrations are higher than any other known estrogen, with representative follicular phase values of 1000 pg/ml and luteal phase values of 1800 pg/ml.^17,18,19 When plasma E_lS concentrations (Fig. 4) are compared with E₁ and E₂ during the menstrual cycle, E_lS concentrations appear to lag slightly behind the large rises and decreases of E₂ by about 24 hours, an observation that probably results from the large size of the E_lS pool, its low MCR, and its slow turnover.¹⁹

Even though E_lS is not itself biologically active, E₁ and E₂ formed at the target tissue from E₁S may be of major importance. Endometrium, for example, converts E_lS to E₁ and E₂.¹¹ Target tissue metabolic activity of this type cannot be measured by in vivo isotope dilution techniques, and as such the actual contribution of E₁S to overall estrogen effect is difficult to assess.

Substantial quantities of E_lS and estradiol sulfate are found in breast tissues of patients with mammary carcinoma (Fig. 5).²⁰ E_lS is converted at high percentage to E₂ in different hormone-dependent mammary cancer cell lines, with very little or no conversion in hormone-independent mammary cancers.²⁰ Progesterone provokes a decrease in E₂ uptake in vitro.²⁰ Accordingly, the control of the sulfatase and 17β-hydroxysteroid dehydrogenase activity may be important regulators in estrogen-sensitive neoplastic tissues.²⁰

Estrogen Metabolites: Ring A (Catechol) and Ring D (Estriol and Epiestriol) Estrogens

Virtually all estrogen metabolites flow through E_l (Fig. 6) as the parent precursor. After formation of E_l, further metabolic transformation takes place through one of two major pathways: the ring A hydroxylated compounds (catechol estrogens)²¹ or ring D hydroxylated compounds (estriol and the epiestriols).²² The relationships are shown schematically in Figure 6. Quantitatively, these pathways are of approximately equal significance. They are mutually exclusive--that is, they do not convert into one another, and there are relatively few compounds metabolized concurrently as both ring A and ring D compounds. The hormonal effects of the ring A compounds are limited primarily to the central nervous system (CNS) and are antiestrogenic in other estrogen-sensitive tissues.²¹ The ring D compounds retain their estrogenic attributes but are less active than E₂.²³

CATECHOL ESTROGENS.

The parent catechol estrogen compound is 2-hydroxyestrone, and its principal metabolite is 2-methoxyestrone. The term catechol is derived from the presence of two hydroxyl groups on the aromatic A ring, a structural feature in common with the catecholamines.²¹ A group of 4-hydroxyestrogens, also catechol estrogens, has been identified but is of comparatively minor quantitative importance compared with 2-hydroxyestrones. Although the conversion of E_l to 2-hydroxyestrone takes place primarily in the liver, it also occurs in the hypothalamus and other CNS tissues.^24,25 The 2-hydroxylation of the parent classic estrogens in the CNS is of major importance because the derived 2-hydroxy estrogens compete with catecholamines for the enzyme catechol-O-methyltransferase.^24,26 As highly efficient competitive inhibitors of catechol-O-methyltransferase, catechol estrogens interfere with metabolic degradation of catecholamines at specific CNS foci, resulting in prolonged catecholamine activity with the consequent release of hypothalamic releasing and inhibiting factors that are catecholamine modulated.²⁶ An alternative CNS activity for the catechol estrogens is competition for CNS E₂ receptors; in this role, the catechol estrogens act as antiestrogens.²¹

The catechol estrogens behave as antiestrogens in peripheral estrogen-sensitive tissues. The 2-hydroxy compounds have virtually no uterotropic activity yet have 20% to 50% the affinity for uterine estrogen receptors as does E₂ itself.²¹ Thus, direction of E₂ metabolism through the catechol pathway may greatly suppress the peripheral estrogenic activity of secreted E₂.²¹

Plasma concentrations of 2-hydroxyestrone are slightly lower than E₂, with a range of approximately 50 to 95 pg/ml.²⁷ Urinary excretion of 2-hydroxyestrone observed over the menstrual cycle ranged from approximately 15 μg/24 hr, with highest values observed at midcycle and during the luteal phase (Fig. 7).²⁸

A shift of estrogen metabolites to the catechol pathway occurs in patients with hyperthyroidism or anorexia nervosa, female athletes, and in women who smoke.^{29,30,31,32,33} A shift to ring D metabolites occurs in hypothyroidism³⁰ and in obesity.³¹ Increased metabolism through catechol estrogens has been believed to be important in breast cancer pathogenesis,³⁴ uterotropic functions,³⁵ cigarette smoking,³³ and exercise-induced amenorrhea.³²

In competitive female athletes, serum levels of catechol estrogens are increased during strenuous exercise, suggesting that conversion to this non-uterotropic steroid could be a factor in the amenorrhea of these women.³² Premenopausal female smokers show significantly increased estrogen 2-hydroxylation expressed as increased 2-hydroxyestrone urinary excretion.³³ This shunting of estrogen metabolites through the catechol pathway may be an important mechanism for their association with the antiestrogen effects of cigarette smoking.³³ Thus, female smokers have an early natural menopause, a lower rate of cancer of the endometrium, and an increased risk of osteoporotic fractures.^33,36 They have a reduced risk of benign breast disease, endometriosis, and uterine fibroids.^37,38

RING D PATHWAY (ESTRIOL AND THE EPIESTRIOLS).

The principal ring D compound, estriol (E₃), is derived by 16α-hydroxylation of E₁ to form the intermediate, 16α-hydroxyestrone. The epiestriols comprise various 16- and 17-hydroxylated estrogens arrayed into an assortment of α and β configurations (see Fig. 6).²² Although E₃ is the most abundant urinary estrogen, unconjugated E₃ circulates in plasma at very low concentrations (7 to 11 pg/ml) because much of it is conjugated to glucuronides.³⁹ Although unconjugated E₃ is believed to have an estrogen potency comparable to E₁, its very low concentration indicates that its hormonal effect is not substantial.

Estrogen Conjugates and Their Excretion

Estrogen metabolites formed from either the catechol pathway or ring D pathway are conjugated in the liver and kidneys with a glucosiduronate radical at various hydroxyl groups to form highly polar (water-soluble) estrogen glucuronides, which are excreted into the urine. E₁ and E₂ can also be conjugated directly and excreted by the same process. Some estrogen is excreted in feces.^40,41 The details of these reactions have been reviewed extensively.^40,41

Microvascular Extraction

Bioavailability of biologically active estrogen is affected by the distribution and permeability properties of the microvascular circulation in estrogen-responsive organs. Extraction efficiency in estrogen-sensitive tissues may thus be much higher than in non-estrogen-sensitive tissues. Thus, when human uteri are perfused in vitro with a lactated Ringer's solution containing physiologic concentrations of E₁, E₂, and E₁S and compared with perfusion with human female serum, endometrial and myometrial estrogen extraction is much higher from the Ringer's solution because it is free of binding proteins. Also, endometrial extraction of unbound E₁S and E₁ greatly exceeds myometrial extraction.¹⁰ Extraction is most efficient in the luteal phase and in specimens with endometrial hyperplasia. ^10,42 This is thought to occur because influxes of steroids are related to the surface area of the microcirculation, which changes during the menstrual cycle.

Cellular Autoregulation

The final common pathway of estrogen hormone action is incorporation of E₂ into the nuclear receptor complex. Target tissues (e.g., myometrium, endometrium, and CNS structures) modulate their response to circulating estrogen at the cell level by self-regulated reversible conversion of biologically active estrogens to relatively inactive ones before receptor interaction. Thus, human endometrial cells convert E₂ to E_l by progesterone-stimulated 17β-estradiol dehydrogenase, which increases markedly in activity during the progesterone-dominant luteal phase.⁴³ Conversely, endometrial sulfotransferase augments conversion of unconjugated E₁ and E₂ to their respective and inactive 3-sulfates.⁴⁴ Like 17β-dehydrogenase, sulfotransferase activity increases during the luteal phase, when it produces increasing E₁S, which does not bind to intracellular estrogen receptors. E₁S is excreted back into the circulation and is associated with decreased nuclear estrogen-receptor complex formation during the luteal phase.⁴³ A similar activity occurs in myometrial tissues.⁴⁴ In breast carcinoma tissue, this interconversion may modulate the estrogen dependency of this disease.²⁰

Analogous autoregulatory functions involving the catechol estrogens modulate catecholamine metabolism in the CNS. Although catecholamine formation is modified by various physiological and pathological conditions, the regulatory impact of catechol estrogens is not well understood.²¹

The regulation of estrogen hormonal expression is complex far beyond the kinetics of secretion and target tissue receptor steroid interaction. Plasma binding, the pathway of metabolite formation, target organ microcirculation, and autoregulated target tissue responsiveness at the intracellular level are additional regulatory foci. The full clinical, diagnostic, and therapeutic impact of these mechanisms is far better understood today than when the previous edition of this chapter was published, but much remains to be learned.