Four major classes of steroids are derived from cholesterol: the progestogens, the androgens, the estrogens and the corticosteroids. The ovary is involved in the synthesis and secretion of the first three (Figure Principal pathways of steroid hormone biosynthesis in the human ovary.).

Natural progestogens are characterized by possessing 21 carbons (C-21 steroids), androgens by being comprised of 19 carbons (C-19 steroids), and natural estrogens have 18 carbons (C-18 steroids) in their structure. Ovarian steroids can exert feedback on both the hypothalamus and the pituitary. Whether estrogens and progestogens stimulate or inhibit gonadotropin release depends upon the plasma level and the duration of exposure. The plasma concentrations, production rates and secretion rates of the main ovarian steroids are given in Table Concentration, production rates and ovarian secretion rates of steroids in blood.

Over 97-98% of the steroids secreted by the ovary are bound to plasma proteins. Testosterone is mainly bound to sex hormone-binding globulin.

Table Properties of human luteinizing hormone, luteinizing hormone-releasing hormone, follicle-stimulating hormone and prolactin

Hormone Secreted from Acts upon Composition Distribution half-life f in blood (min) Levels in human blood (U/l)
luteinizing hormone-releasing hormone Hypothalam us (preoptic area and arcuate nucleus) Anterior pituitary Decapeptide 2-A N/A
luteinizing hormone Anterior pituitary gonadotrophs (basophilic) Thecal cells; granulosa cells; lutealcells; interstitial cells Glycoprotein , a chain 89 aminoacids;в chain 115 amino acids, 1 carbohydrate chain 30-60** Male > 12 years 5-12 U/ 1
FemaleEarly follicular 0. 4-15 IU/1 Midcycle 20-70 IU/1 Luteal 0.4-15 IU/1 Menopause 20-70 IU/1
follicle-stimulating hormone As luteinizing hormone Granulosa cells Glycoprotein    identical to luteinizing hormone; p chai 115 amino acids, 2 carbohydrate  chains 120-150** Male (age, a chain 13-70) 1.2-16IU/1Female Early follicular 2-8 IU/1 Midcycle 2. 7-27 IU/1 Luteal 1.2-7.3 IU/1 Menopause 18-93 IU/1
Prolactin Anterior pituitary lactotrophs (acidophilic) Ovarian follicles; luteal cells; mammary glands Polypeptide single chain of 198 amino acids 10-20 Male and female 60-450 U/l

Values vary from one laboratory to another

Elimination half-life: luteinizing hormone, 10-12 h; follicle-stimulating hormone, 17 ± 3h f Distribution half-life and biological half-life are different concepts. For the reproductive hormones the biological half-life is greater than the distribution half-life Estradiol is bound to albumin (60%) and sex hormone-binding globulin в8%). sex hormone-binding globulin is a в-globulin formed in the liver with a molecular weight of about 95 000. The level of sex hormone-binding globulin, and thus the level of free hormone, can be affected by a number of conditions. Levels are increased by estradiol, combined oral contraceptives and thyroid hormones, and are decreased by androgens, hypothyroidism and obesity.

There are number of naturally occurringgonadal steroids, all of them with different potency. The ones that are most important for clinical practice and their principal actions on the reproductive system are outlined in Table Relative potency and principal actions of some naturally occurring sex steroids in females. In our discussion we are going to use the terms progesterone and estrogen or estradiol to denote all naturally occurring progestogens and estrogens.

Mechanism of action of steroid hormones

All ovarian steroids have the same basic mechanism of action. For clarity and because the estrogen activity has been widely studied, the mode of action described here uses estrogen as the example (Figure A schematic representation of the subcellular effects of estrogen (estrogen) in estrogen target tissue).

Table Concentration, production rates and ovarian secretion rates of steroids in blood

Compound Menstrual cycle phase Representative concentration in plasma (nmol/1) PR(mg/day) SR** by both ovaries (mg/ day)
Estradiol Early follicular 0.2 (200 pmol/ 1) 0.08 0.07
Late follicular 1.2-2.6 0.5-1.5 0.4-0.8
Midluteal 0.7 (700pmol/ 1) 0.270 0.250
Menopause < 0.11 (110 pmol/1)
Progesterone Follicular 3.0 2.1 1.5
Luteal 30-100 25 24
Testosterone 1.3(0.5-2.8) 0.25-0.5 0.2-0.5
Androstenedione 5.6 3.2 0.8-1.6
Dehydroepiand rosterone 17 8.0 0.3-3

*PR, production rate, consisting of the sum of secretion rate and amount contributed by interconversion of precursor steroids; 1″SR, secretion rate, being the secretion of ovarian steroids in units per day

Table Relative potency and principal actions of some naturally occurring sex steroids in females

Type of steroid and relative potency Properties
Estrogens
17в-Estradiol (100%) Stimulate secondary sexual characteristics
Estrone (10%) Prepare the genital tract for spermatozoal
Estriol (l%) transport
Stimulate growth and the activity of mammary glands
Stimulate the growth of the endometrium and prepare the endometrium for progesterone action
Associated with sexual behavior
Regulate secretion of gonadotropins
Progestogens
Progesterone (100%) Prepare uterus to receive embryo
17a-Hydroxyprogesterone (40-70%) Maintain uterus during early pregnancy
Stimulate growth of mammary glands but suppress the secretion of milk
Regulate secretion of gonadotropins
Androgens
5a-Dihydrotestosterone (100%) Testosterone (50%) Induce growth of androgen-dependent body hair
Dehydroepiandrosterone (4%) Influence sexual and aggressive behavior
? Regulate secretion of gonadotropins

*The relative potencies are approximations only. They vary with (1) the assay used; (2) the affinity of the steroid for the steroid receptor in different tissues; в) the local enzymatic conversion of the steroids in the target tissues; and (4) the differences in systemic metabolism

Free steroids are thought to diffuse passively to all cells because there is no evidence as yet of an active transport mechanism. Steroids are preferentially retained in target cells as stable complexes bound to intracellular receptor proteins (i.e., estrogen receptor), which are steroid- and tissue specific. The receptor is thought to be a hormone- or ligand-activated transcription factor. The terms are used interchangeably.

The estrogen receptor has six structural domains (protein regions having some distinct feature or role), A to F, but the important ones are the steroid-binding domain and the DNA-binding domain. The receptor binds the hormone, i.e., estrogen, through its steroid-binding domain. The binding of the steroid by the receptor results in the activation of the receptor molecules, which leads to conformational changes in the hormone-receptor complex, including its DNA-binding domain. This activation allows the hormone-receptor complex to bind to specific sites in the DNA, termed nuclear acceptor sites. Once bound to the DNA, the activated steroid-receptor complex acts as a transcription factor, which ‘switches on’genes, coding for the production of new proteins. The newly synthesized proteins change the metabolism of the target cell in a steroid-specific manner. The transfer of the steroid in the cell and nuclear binding of the steroid-receptor complex is rapid, occurring within minutes. Nuclear binding affects messenger RNA levels and synthesis within several hours, and finally protein synthesis and turnover happens within 12-24 h. The major physiologic effects of steroids in cells are seen in 12-36 h.

There are two estrogen receptors so far described: ERa (classic estrogen receptor) and estrogen receptor в (recently described). Classic estrogen receptor was cloned and sequenced from human breast cancer cells in 1986. The ERa consists of 595 aminoacids with a molecular weight of 66 kDa. The estrogen receptor в was cloned in 1996 from rat prostate and ovary. It consists of 485 amino acids and has a molecular weight of 54.2 kDa. estrogen receptorв is 95%

Figure A schematic representation of the subcellular effects of estrogen in estrogen target tissue. Estrogens dissociate from plasma proteins, bind to the estrogen receptor and the complex becomes activated (estrogen receptor*). Activated estrogen receptor complex interacts with the nuclear acceptor site on the DNA (A). This results in the activation of DNA polymerase and RNA polymerase to initiate subsequent cell proliferation and protein synthesis, respectively. The receptor is then destroyed (processed), in which case a new cytoplasmic receptor is synthesized or recycled for subsequent ligand binding. Whether the binding of estrogen and the receptor occurs on the cytoplasm in the nucleus has been debated. It is currently thought that the interaction happens in the nucleus.

Homologous with ERa in the DNA-binding domain and 55% in the hormone-binding domain. ERa resides no chromosome 6 and estrogen receptor в on chromosome 14. ERa has a higher affinity for short-acting estrogens such as 17a-estradiol. Tissue distribution of ERa and estrogen receptor в varies and is under intense scientific investigation. Most of the work has been done on rodents, so-called estrogen receptor knockout (ERKO) mice. A knockout mouse is a genetically engineered animal in which the genome has been altered by site-directed recombination so that a particular gene is deleted. The reported findings may not be directly applicable to humans. The results depend on the sensitivity of the assays and are sometimes conflicting. Recent reports describe ERa predominance in the vagina, uterus, ovarian stroma, breast, cardiovascular system, liver, skeletal muscle, pituitary and epididymis; in contrast, estrogen receptor в is predominantly found in ovarian granulosa cells and the prostate. Both receptors are well represented in the brain and bone, but in different structural and functional parts. The levels of ERa and estrogen receptorв may vary depending on the age of the animal. The physiologic role of the different receptors is currently being studied. For example, ERa knockout mice develop to maturity, but are infertile, do not exhibit female sexual behavior and do not respond to estradiol.

Physiologic functions of steroid hormones

The main function of the ovarian steroids is related to reproduction. They are instrumental in developing the secondary sexual characteristics, establishing the menstrual cycle and in maintaining pregnancy. However, as our methods for studying the steroid hormones have developed, so has our understanding of their wider functions.

Estrogen

Female maturation Estrogen stimulates the growth of the vagina, uterus and fallopian tubes and the secondary sexual characteristics during puberty. It stimulates fat deposition, stromal development and ductal growth of the breast and is responsible for the accelerated growth phase and the closing of the epiphyses of the long bones that occurs at puberty. Estrogen contributes to the growth of axillary and pubic hair and alters the distribution of the body fat so as to produce the typical female body habitus. It stimulates the pigmentation of the skin, most prominent in the region of the nipples and areolae and in the genital region.

Other biological effects of estrogen Estrogen exerts effects on the cardiovascular system, connective tissue and numerous aspects of the metabolism such as lipids and carbohydrate metabolism. Some of those effects are well established and important and some are less well studied and/or less significant. Some estrogenic effects are summarized in Table Biological effects of estrogen.

The main sources of estrogen in women are the granulosa cells and the luteinied granulosa and theca cells of the ovaries. Estrogen is also produced by fat tissue and, in smaller amounts, by muscle and nervous tissue. Estrone and estriol are mostly formed from estradiol in the liver.

Progesterone

The chief function of progesterone is to prepare the endometrium for acceptance and maintenance of pregnancy, and the stimulation of alveolar growth of the mammary glands. Some of the effects of progesterone are listed in Table Biological effects of progesterone. Progesterone is produced by theca and granulosa lutein cells and the corpus luteum.

Androgens

Androgen production in the female is greater than is widely appreciated. The role of androgens in the female includes acting as precursors for estrogen production, anabolic effects, stimulation of axillary and pubic hair growth, sebum production, stimulation of bone formation, and stimulation of production of erythropoietin from the kidneys (Table Biological effects of androgens).

Table Biological effects of estrogen

Reproductive system
Gonadotropin regulation
Stimulation of secondary sexual characteristics
Increasing cervical mucus production
Breast development (stromal and ductal tissue)
Modulation of sexual behavior
Endometrial stimulation
Cardiovascular system
Increased cardiac output Vasodilatation
Endothelial effects
Suppression of appetite
Stimulates skin growth and wound healing
Reduces motility of the bowel
Mild anabolic effect
Metabolic effects
Higher levels of corticosteroid-binding globulin, thyroxin-binding globulin, sex hormone-binding globulin, renin
Reduction of cholesterol
Reduction of bone resorption
Reduction of capillary fragility
Promotion of coagulation

Androgens are produced from the ovaries, the adrenal glands and from peripheral conversion in adipose tissue. During reproductive life, the relative contribution from these sources varies. The ovaries and adrenals produce androstenedione, testosterone and dehydroepiandrosterone, and the adrenals also produce dehydroepiandrosterone sulfate. Androstenedione, dehydroepiandrosterone and dehydroepiandrosterone sulfate are converted peripherally to testosterone, dihydrotestosterone and estrogen. Only 1-2% of the total circulating testosterone is free or biologically active, the rest being bound to sex hormone-binding globulin and albumin. In women, there are alterations in the level because sex hormone-binding globulin has a dramatic effect on the free levels in plasma, binding 66% of total circulating testosterone. Sex hormone-binding globulin is increased by increased levels of estradiol and thyroxine, and suppressed by testosterone, glucocorticoids, excessive growth hormone, high insulin levels and obesity. The daily androstenedione and testosterone production in premenopausal women is thought to be about 3.2 mg and 0.26 mg, respectively.

In premenopausal women, 25% of testosterone is produced by the ovaries, 25% by the adrenals and 50% by peripheral conversion. In postmenopausal women, 50% of testosterone is produced by the ovaries, 10% by the adrenals and 40% by peripheral conversion, and the overall androgen production decreases with age. The age-related decrease in androgen production starts premenopausally and testosterone levels fall by approximately 50% between the ages of 20 and 40, and then level off. After the menopause, the process continues and the age-related decline is particularly noticeable for dehydroepiandrosterone and dehydroepiandrosterone sulfate. Following natural menopause, the level of androstenedione is 50% of the premenopausal value. After oophorectomy, the levels of testosterone and androstenedione fall by 50% in previously premenopausal women and by 50% and 21% respectively in previously post-menopausal women. Some androgenic effects are listed in Table Biological effects of androgens.

Table  Biological effects of progesterone

Reproductive system
gonadotropin regulation
endometrial decidualization
maintenance of early pregnancy
breast development (alveolar tissue)
Increase of appetite
Mild catabolic effect
Increase of basal body temperature via thermoregulatory centre of the hypothalamus
Binding to the aldosterone receptor in the kidney and promoting natriuresis
Contributes to premenstrual symptoms such as bloatedness, heavy tender breasts
Slows peristalsis in the gastrointestinal tract, which may cause constipation
Depressant and hypnotic effects on the brain
Alters the function of the respiratory centre (increases respiratory drive)

Table Biological effects of androgens

Reproductive system
libido, sexual behavior
growth of androgen-dependent body hair
? regulation of gonadotropins
Anabolic effect
nitrogen retention
muscle growth
Stimulate bone formation
Increase serum production
Increase erythropoietin production
Decrease HDL cholesterol
Contribute to general well-being

 

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