Progestogens (also sometimes spelled progestagens or gestagens) are a class of steroid hormones that bind to and activate the progesterone receptor (PR). The most important progestogen in the body is progesterone (P4). Other endogenous progestogens include 17-hydroxyprogesterone, 5α-dihydroprogesterone, and 11-deoxycorticosterone. Synthetic progestogens are generally referred to as progestins. However, the terms progesterone, progestogen, and progestin are frequently used interchangeably both in the scientific literature and in clinical settings.
The progestogens are one of the five major classes of steroid hormones, in addition to the androgens, estrogens, glucocorticoids, and mineralocorticoids, as well as the neurosteroids. All progestogens are characterized by their basic 21-carbon skeleton, called a pregnane skeleton (C21). In similar manner, the estrogens possess an estrane skeleton (C18), and androgens, an andrane skeleton (C19).
In the first step in the steroidogenic pathway, cholesterol is converted into pregnenolone (P5), which serves as the precursor to the progestogens progesterone and 17-hydroxyprogesterone. These progestogens, along with another steroid, 17-hydroxypregnenolone, are the precursors of all other endogenous steroids, including the androgens, estrogens, glucocorticoids, mineralocorticoids, and neurosteroids. Thus, many tissues producing steroids, including the adrenal glands, testes, and ovaries, produce progestogens.
In some tissues, the enzymes required for the final product are not all located in a single cell. For example, in ovarian follicles, cholesterol is converted to androstenedione, an androgen, in the theca cells, which is then further converted into estrogen in the granulosa cells. Fetal adrenal glands also produce pregnenolone in some species, which is converted into progesterone and estrogens by the placenta (see below). In the human, the fetal adrenals produce dehydroepiandrosterone (DHEA) via the pregnenolone pathway.
Production by the ovary
Progesterone is the major progestogen produced by the corpus luteum of the ovary in all mammalian species. Luteal cells possess the necessary enzymes to convert cholesterol to pregnenolone, which is subsequently converted into progesterone. Progesterone is highest in the diestrus phase of the estrous cycle.
Production by the placenta
The role of the placenta in progestogen production varies by species. In the sheep, horse, and human, the placenta takes over the majority of progestogen production, whereas in other species the corpus luteum remains the primary source of progestogens. In the sheep and human, progesterone is the major placental progestogen.
The equine placenta produces a variety of progestogens, primarily 5α-dihydroprogesterone and 5α,20α-tetrahydroprogesterone, beginning on day 60. A complete luteo-placental shift occurs by day 120–150.
In women, progestogens are commonly used to prevent endometrial hyperplasia from unopposed estrogen during hormone replacement therapy. They also used to treat secondary amenorrhea, dysfunctional uterine bleeding and endometriosis.
In a normal menstrual cycle, declining levels of progesterone triggers menstruation. Norethindrone acetate and medroxyprogesterone acetate may be used to artificially induce progestogen-associated breakthrough bleeding.
In addition to their progestogen properties, some progestins are antagonists of the androgen receptor, and can be used clinically as antiandrogens. Examples include chlormadinone acetate, cyproterone acetate, dienogest, and drospirenone. Care must be taken as to which progestin is used however, as various others, such as levonorgestrel and norethindrone acetate, conversely have androgenic properties.
Progestogens, similarly to the androgens and estrogens through their own respective receptors, inhibit the secretion of the gonadotropins follicle-stimulating hormone (FSH) and luteinizing hormone (LH) via activation of the progesterone receptor. This effect is form of negative feedback on the hypothalamic-pituitary-gonadal (HPG) axis that the body uses to prevent sex hormone levels from becoming too elevated. Accordingly, progestogens, both endogenous and exogenous (i.e., progestins), have antigonadotropic effects, and progestins in sufficient amounts can markedly suppress the body's normal production of progestogens, androgens, and estrogens, as well as, in theory, neurosteroids. As such, some of the more potent progestins, including chlormadinone acetate, medroxyprogesterone acetate, megestrol acetate, nomegestrol acetate, and norethisterone acetate are sometimes used to suppress sex hormone levels in a variety of androgen and estrogen-associated conditions. Examples of indications include treating sex hormone-sensitive cancers (e.g., breast cancer), suppressing precocious puberty and puberty in transgender youth, and reducing sex drive in sex offenders.
Progesterone itself is not employed as an antigonadotropin, as it can function as a prohormone to androgens and estrogens, especially at the relatively high doses that would be necessary to achieve sufficient antigonadotropic action.
In many people suffering from solid malignancy, especially gastric and pancreatic cancer, high doses of certain progestins can be employed to improve appetite and reduce wasting. In general, they are used in combination with certain other steroids such as dexamethasone. Their effects take several weeks to become apparent, but are relatively long-lived when compared to those of corticosteroids. Furthermore, they are recognized as being the only drugs to increase lean body mass. Megestrol acetate is the lead drug of this class for the management of cachexia, and medroxyprogesterone acetate is also used.
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