Progesterone and some of its metabolites, such as 5β-dihydroprogesterone, are agonists of the pregnane X receptor (PXR),[30] albeit weakly so (EC50 >10 μM).[31] In accordance, progesterone induces several hepaticcytochrome P450enzymes,[32] such as CYP3A4,[33][34] especially during pregnancy when concentrations are much higher than usual.[35] Perimenopausal women have been found to have greater CYP3A4 activity relative to men and postmenopausal women, and it has been inferred that this may be due to the higher progesterone levels present in perimenopausal women.[33]
Progesterone modulates the activity of CatSper (cation channels of sperm) voltage-gated Ca2+ channels. Since eggs release progesterone, sperm may use progesterone as a homing signal to swim toward eggs (chemotaxis). As a result, it has been suggested that substances that block the progesterone binding site on CatSper channels could potentially be used in male contraception.[36][37]
Elevated levels of progesterone potently reduce the sodium-retaining activity of aldosterone, resulting in natriuresis and a reduction in extracellular fluid volume. Progesterone withdrawal, on the other hand, is associated with a temporary increase in sodium retention (reduced natriuresis, with an increase in extracellular fluid volume) due to the compensatory increase in aldosterone production, which combats the blockade of the mineralocorticoid receptor by the previously elevated level of progesterone.[43]
Early sexual differentiation
Progesterone plays a role in early human sexual differentiation.[44]Placental progesterone is the feedstock for the 5α-dihydrotestosterone (DHT) produced via the backdoor pathway found operating in multiple non-gonadal tissues of the fetus,[45] whereas deficiencies in this pathway lead to undervirilization of the male fetus, resulting in incomplete development of the male genitalia.[46][47] DHT is a potent androgen that is responsible for the development of male genitalia, including the penis and scrotum.
During early fetal development, the undifferentiated gonads can develop into either testes or ovaries. The presence of the Y chromosome leads to the development of testes. The testes then produce testosterone, which is converted to DHT via the enzyme 5α-reductase. DHT is a potent androgen that is responsible for the masculinization of the external genitalia and the development of the prostate gland. Progesterone, produced by the placenta during pregnancy, plays a role in fetal sexual differentiation by serving as a precursor molecule for the synthesis of DHT via the backdoor pathway. In the absence of adequate levels of steroidogenic enzymes during fetal development, the backdoor pathway for DHT synthesis can become deficient, leading to undermasculinization of the male fetus. This can result in the development of ambiguous genitalia or even female genitalia in some cases. Therefore, both DHT and progesterone play crucial roles in early fetal sexual differentiation, with progesterone acting as a precursor molecule for DHT synthesis and DHT promoting the development of male genitalia.[44]
Reproductive system
Progesterone has key effects via non-genomic signalling on human sperm as they migrate through the female reproductive tract before fertilization occurs, though the receptor(s) as yet remain unidentified.[48] Detailed characterisation of the events occurring in sperm in response to progesterone has elucidated certain events including intracellular calcium transients and maintained changes,[49] slow calcium oscillations,[50] now thought to possibly regulate motility.[51] It is produced by the ovaries.[52] Progesterone has also been shown to demonstrate effects on octopus spermatozoa.[53]
Progesterone is sometimes called the "hormone of pregnancy",[54] and it has many roles relating to the development of the fetus:
Progesterone converts the endometrium to its secretory stage to prepare the uterus for implantation. At the same time progesterone affects the vaginal epithelium and cervical mucus, making it thick and impenetrable to sperm. Progesterone is anti-mitogenic in endometrial epithelial cells, and as such, mitigates the tropic effects of estrogen.[55] If pregnancy does not occur, progesterone levels will decrease, leading to menstruation. Normal menstrual bleeding is progesterone-withdrawal bleeding. If ovulation does not occur and the corpus luteum does not develop, levels of progesterone may be low, leading to anovulatory dysfunctional uterine bleeding.
During implantation and gestation, progesterone appears to decrease the maternal immune response to allow for the acceptance of the pregnancy.[56]
Progesterone decreases contractility of the uterine smooth muscle.[54] This effect contributes to prevention of preterm labor.[56] Studies have shown that in individuals who are pregnant with a single fetus, asymptomatic in the prenatal stage, and at a high risk of giving pre-term birth spontaneously, vaginal progesterone medication has been found to be effective in preventing spontaneous pre-term birth. Individuals who are at a high risk of giving pre-term birth spontaneously are those who have a short cervix of less than 25 mm or have previously given pre-term birth spontaneously. Although pre-term births are generally considered to be less than 37 weeks, these studies found that vaginal progesterone is associated with fewer pre-term births of less than 34 weeks.[57]
A drop in progesterone levels is possibly one step that facilitates the onset of labor.
In addition, progesterone inhibits lactation during pregnancy. The fall in progesterone levels following delivery is one of the triggers for milk production.
Progesterone plays an important role in breast development. In conjunction with prolactin, it mediates lobuloalveolar maturation of the mammary glands during pregnancy to allow for milk production and thus lactation and breastfeeding of offspring following parturition (childbirth).[58]Estrogen induces expression of the PR in breast tissue and hence progesterone is dependent on estrogen to mediate lobuloalveolar development.[40][41][42] It has been found that RANKLTooltip Receptor activator of nuclear factor kappa-B ligand is a critical downstream mediator of progesterone-induced lobuloalveolar maturation.[59] RANKL knockout mice show an almost identical mammary phenotype to PR knockout mice, including normal mammary ductal development but complete failure of the development of lobuloalveolar structures.[59]
Ductal development
Though to a far lesser extent than estrogen, which is the major mediator of mammary ductal development (via the ERα),[60][61] progesterone may be involved in ductal development of the mammary glands to some extent as well.[62] PR knockout mice or mice treated with the PR antagonistmifepristone show delayed although otherwise normal mammary ductal development at puberty.[62] In addition, mice modified to have overexpression of PRA display ductal hyperplasia,[59] and progesterone induces ductal growth in the mouse mammary gland.[62] Progesterone mediates ductal development mainly via induction of the expression of amphiregulin, the same growth factor that estrogen primarily induces the expression of to mediate ductal development.[62] These animal findings suggest that, while not essential for full mammary ductal development, progesterone seems to play a potentiating or accelerating role in estrogen-mediated mammary ductal development.[62]
Breast cancer risk
Progesterone also appears to be involved in the pathophysiology of breast cancer, though its role, and whether it is a promoter or inhibitor of breast cancer risk, has not been fully elucidated.[63][64] Most progestins, or synthetic progestogens, like medroxyprogesterone acetate, have been found to increase the risk of breast cancer in postmenopausal people in combination with estrogen as a component of menopausal hormone therapy.[65][64] The combination of natural oral progesterone or the atypical progestin dydrogesterone with estrogen has been associated with less risk of breast cancer than progestins plus estrogen.[66][67][68] However, this may simply be an artifact of the low progesterone levels produced with oral progesterone.[63][69] More research is needed on the role of progesterone in breast cancer.[64]
Hormone replacement therapy, 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 people.[70][71] These benefits include increased skin collagen content, skin thickness and elasticity, and skin hydration and surface lipids.[70][71] Topical estrogen has been found to have similar beneficial effects on the skin.[70] 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 people.[71] Skin hydration and surface lipids, on the other hand, did not significantly change with topical progesterone.[71]
These findings suggest that progesterone, like estrogen, also has beneficial effects on the skin, and may be independently protective against skin aging.[71]
Dr. Diana Fleischman, of the University of Portsmouth, and colleagues looked for a relationship between progesterone and sexual attitudes in 92 women. Their research, published in the Archives of Sexual Behavior found that women who had higher levels of progesterone scored higher on a questionnaire measuring homoerotic motivation. They also found that men who had high levels of progesterone were more likely to have higher homoerotic motivation scores after affiliative priming compared to men with low levels of progesterone.[73][74][75][76]
Neurosteroids are neuromodulators, and are neuroprotective, neurogenic, and regulate neurotransmission and myelination.[78] The effects of progesterone as a neurosteroid are mediated predominantly through its interactions with non-nuclear PRs, namely the mPRs and PGRMC1, as well as certain other receptors, such as the σ1 and nACh receptors.[79]
Previous studies have shown that progesterone supports the normal development of neurons in the brain, and that the hormone has a protective effect on damaged brain tissue. It has been observed in animal models that females have reduced susceptibility to traumatic brain injury and this protective effect has been hypothesized to be caused by increased circulating levels of estrogen and progesterone in females.[80]
Proposed mechanism
The mechanism of progesterone protective effects may be the reduction of inflammation that follows brain trauma and hemorrhage.[81][82]
Damage incurred by traumatic brain injury is believed to be caused in part by mass depolarization leading to excitotoxicity. One way in which progesterone helps to alleviate some of this excitotoxicity is by blocking the voltage-dependent calcium channels that trigger neurotransmitter release.[83] It does so by manipulating the signaling pathways of transcription factors involved in this release. Another method for reducing the excitotoxicity is by up-regulating the GABAA, a widespread inhibitory neurotransmitter receptor.[84]
Progesterone has also been shown to prevent apoptosis in neurons, a common consequence of brain injury.[85] It does so by inhibiting enzymes involved in the apoptosis pathway specifically concerning the mitochondria, such as activated caspase 3 and cytochrome c.
Not only does progesterone help prevent further damage, it has also been shown to aid in neuroregeneration.[86] One of the serious effects of traumatic brain injury includes edema. Animal studies show that progesterone treatment leads to a decrease in edema levels by increasing the concentration of macrophages and microglia sent to the injured tissue.[83][87] This was observed in the form of reduced leakage from the blood brain barrier in secondary recovery in progesterone treated rats. In addition, progesterone was observed to have antioxidant properties, reducing the concentration of oxygen free radicals faster than without.[84] There is also evidence that the addition of progesterone can also help remyelinate damaged axons due to trauma, restoring some lost neural signal conduction.[84] Another way progesterone aids in regeneration includes increasing the circulation of endothelial progenitor cells in the brain.[88] This helps new vasculature to grow around scar tissue which helps repair the area of insult.
Addiction
Progesterone enhances the function of serotonin receptors in the brain, so an excess or deficit of progesterone has the potential to result in significant neurochemical issues. This provides an explanation for why some people resort to substances that enhance serotonin activity such as nicotine, alcohol, and cannabis when their progesterone levels fall below optimal levels.[89]
Sex differences in hormone levels may induce women to respond differently than men to nicotine. When women undergo cyclic changes or different hormonal transition phases (menopause, pregnancy, adolescence), there are changes in their progesterone levels.[90] Therefore, females have an increased biological vulnerability to nicotine's reinforcing effects compared to males and progesterone may be used to counter this enhanced vulnerability. This information supports the idea that progesterone can affect behavior.[89]
Similar to nicotine, cocaine also increases the release of dopamine in the brain. The neurotransmitter is involved in the reward center and is one of the main neurotransmitters involved with substance abuse and reliance. In a study of cocaine users, it was reported that progesterone reduced craving and the feeling of being stimulated by cocaine. Thus, progesterone was suggested as an agent that decreases cocaine craving by reducing the dopaminergic properties of the drug.[91]
Societal
In a 2012 University of Amsterdam study of 120 women, women's luteal phase (higher levels of progesterone, and increasing levels of estrogen) was correlated with a lower level of competitive behavior in gambling and math contest scenarios, while their premenstrual phase (sharply-decreasing levels of progesterone, and decreasing levels of estrogen) was correlated with a higher level of competitive behavior.[92]
Other effects
Progesterone also has a role in skin elasticity and bone strength, in respiration, in nerve tissue and in female sexuality, and the presence of progesterone receptors in certain muscle and fat tissue may hint at a role in sexually dimorphic proportions of those.[93]
During pregnancy, progesterone is said to decrease uterine irritability.[94]
During pregnancy, progesterone helps to suppress immune responses of the mother to fetal antigens, which prevents rejection of the fetus.[94]
Progesterone normalizes blood clotting and vascular tone, zinc and copper levels, celloxygen levels, and use of fat stores for energy.
Progesterone may affect gum health, increasing risk of gingivitis (gum inflammation).[99]
Progesterone appears to prevent endometrial cancer (involving the uterine lining) by regulating the effects of estrogen.
Progesterone plays an important role in the signaling of insulin release and pancreatic function, and may affect the susceptibility to diabetes or gestational diabetes.[100][101]
Progesterone levels in the blood were found to be lower in those who had higher weight and higher BMI among those who became pregnant through in vitro fertilization.[102]
Current data shows that micronized progesterone, which is chemically identical to the progesterone produced in people's bodies, in combination with estrogen in menopausal hormone therapy does not seem to have significant effects on venous thromboembolism (blood clots in veins) and ischemic stroke (lack of blood flow to the brain due to blockage of a blood vessel that supplies the brain). However, more studies need to be conducted to see whether or not micronized progesterone alone or in combined menopausal hormone therapy changes the risk of myocardial infarctions (heart attacks).[103]
There have not been any studies done yet on the effects of micronized progesterone on hair loss due to menopause.[104]
Despite suggestions for using hormone therapy to prevent loss of muscle mass in post-menopausal individuals (50 and older), menopausal hormone therapy involving either estrogen alone or estrogen and progesterone has not been found to preserve muscle mass.[105] Menopausal hormone therapy also does not result in body weight reduction, BMI reduction, or change in glucose metabolism.[106]
Notes: "The concentration of a steroid in the circulation is determined by the rate at which it is secreted from glands, the rate of metabolism of precursor or prehormones into the steroid, and the rate at which it is extracted by tissues and metabolized. The secretion rate of a steroid refers to the total secretion of the compound from a gland per unit time. Secretion rates have been assessed by sampling the venous effluent from a gland over time and subtracting out the arterial and peripheral venous hormone concentration. The metabolic clearance rate of a steroid is defined as the volume of blood that has been completely cleared of the hormone per unit time. The production rate of a steroid hormone refers to entry into the blood of the compound from all possible sources, including secretion from glands and conversion of prohormones into the steroid of interest. At steady state, the amount of hormone entering the blood from all sources will be equal to the rate at which it is being cleared (metabolic clearance rate) multiplied by blood concentration (production rate = metabolic clearance rate × concentration). If there is little contribution of prohormone metabolism to the circulating pool of steroid, then the production rate will approximate the secretion rate." Sources: See template.
Distribution
Progesterone binds extensively to plasma proteins, including albumin (50–54%) and transcortin (43–48%).[111] It has similar affinity for albumin relative to the PR.[17]
Relatively small portions of progesterone are hydroxylated via 17α-hydroxylase (CYP17A1) and 21-hydroxylase (CYP21A2) into 17α-hydroxyprogesterone and 11-deoxycorticosterone (21-hydroxyprogesterone), respectively,[116] and pregnanetriols are formed secondarily to 17α-hydroxylation.[123][124] Even smaller amounts of progesterone may be also hydroxylated via 11β-hydroxylase (CYP11B1) and to a lesser extent via aldosterone synthase (CYP11B2) into 11β-hydroxyprogesterone.[125][126][44] In addition, progesterone can be hydroxylated in the liver by other cytochrome P450 enzymes which are not steroid-specific.[127] 6β-Hydroxylation, which is catalyzed mainly by CYP3A4, is the major transformation, and is responsible for approximately 70% of cytochrome P450-mediated progesterone metabolism.[127] Other routes include 6α-, 16α-, and 16β-hydroxylation.[114] However, treatment of women with ketoconazole, a strong CYP3A4 inhibitor, had minimal effects on progesterone levels, producing only a slight and non-significant increase, and this suggests that cytochrome P450 enzymes play only a small role in progesterone metabolism.[128]
Progesterone levels are relatively low during the preovulatory phase of the menstrual cycle, rise after ovulation, and are elevated during the luteal phase, as shown in the diagram above. Progesterone levels tend to be less than 2 ng/mL prior to ovulation and greater than 5 ng/mL after ovulation. If pregnancy occurs, human chorionic gonadotropin is released, maintaining the corpus luteum and allowing it to maintain levels of progesterone. Between 7 and 9 weeks, the placenta begins to produce progesterone in place of the corpus luteum in a process called the luteal-placental shift.[131]
After the luteal-placental shift, progesterone levels start to rise further and may reach 100 to 200 ng/mL at term. Whether a decrease in progesterone levels is critical for the initiation of labor has been argued and may be species-specific. After delivery of the placenta and during lactation, progesterone levels are very low.
Progesterone levels are low in children and postmenopausal people.[132] Adult males have levels similar to those in women during the follicular phase of the menstrual cycle.
Endogenous progesterone production rates and plasma progesterone levels
Blood test results should always be interpreted using the reference ranges provided by the laboratory that performed the results. Example reference ranges are listed below.
During human pregnancy, progesterone is produced in increasingly high amounts by the ovaries and placenta. At first, the source is the corpus luteum that has been "rescued" by the presence of human chorionic gonadotropin (hCG) from the conceptus. However, after the 8th week, production of progesterone shifts to the placenta. The placenta utilizes maternal cholesterol as the initial substrate, and most of the produced progesterone enters the maternal circulation, but some is picked up by the fetal circulation and used as substrate for fetal corticosteroids. At term the placenta produces about 250 mg progesterone per day.
An additional animal source of progesterone is milk products. After consumption of milk products the level of bioavailable progesterone goes up.[144]
Plants
In at least one plant, Juglans regia, progesterone has been detected.[145] In addition, progesterone-like steroids are found in Dioscorea mexicana. Dioscorea mexicana is a plant that is part of the yam family native to Mexico.[146] It contains a steroid called diosgenin that is taken from the plant and is converted into progesterone.[147] Diosgenin and progesterone are also found in other Dioscorea species, as well as in other plants that are not closely related, such as fenugreek.
Another plant that contains substances readily convertible to progesterone is Dioscorea pseudojaponica native to Taiwan. Research has shown that the Taiwanese yam contains saponins — steroids that can be converted to diosgenin and thence to progesterone.[148]
Many other Dioscorea species of the yam family contain steroidal substances from which progesterone can be produced. Among the more notable of these are Dioscorea villosa and Dioscorea polygonoides. One study showed that the Dioscorea villosa contains 3.5% diosgenin.[149]Dioscorea polygonoides has been found to contain 2.64% diosgenin as shown by gas chromatography-mass spectrometry.[150] Many of the Dioscorea species that originate from the yam family grow in countries that have tropical and subtropical climates.[151]
Progesterone is used as a medication. It is used in combination with estrogens mainly in hormone therapy for menopausalsymptoms and low sex hormone levels.[116][152] It may also be used alone to treat menopausal symptoms. Studies have shown that transdermal progesterone (skin patch) and oral micronized progesterone are effective treatments for certain symptoms of menopause such as hot flashes and night sweats, which are otherwise referred to as vasomotor symptoms or VMS.[153]
Progesterone is commercially produced by semisynthesis. Two main routes are used: one from yam diosgenin first pioneered by Marker in 1940, and one based on soy phytosterols scaled up in the 1970s. Additional (not necessarily economical) semisyntheses of progesterone have also been reported starting from a variety of steroids. For the example, cortisone can be simultaneously deoxygenated at the C-17 and C-21 position by treatment with iodotrimethylsilane in chloroform to produce 11-keto-progesterone (ketogestin), which in turn can be reduced at position-11 to yield progesterone.[161]
The 16-DPA intermediate is important to the synthesis of many other medically important steroids. A very similar approach can produce 16-DPA from solanine.[163]
A total synthesis of progesterone was reported in 1971 by W.S. Johnson.[169] The synthesis begins with reacting the phosphonium salt7 with phenyl lithium to produce the phosphonium ylide8. The ylide 8 is reacted with an aldehyde to produce the alkene9. The ketalprotecting groups of 9 are hydrolyzed to produce the diketone 10, which in turn is cyclized to form the cyclopentenone 11. The ketone of 11 is reacted with methyl lithium to yield the tertiary alcohol 12, which in turn is treated with acid to produce the tertiary cation 13. The key step of the synthesis is the π-cation cyclization of 13 in which the B-, C-, and D-rings of the steroid are simultaneously formed to produce 14. This step resembles the cationic cyclization reaction used in the biosynthesis of steroids and hence is referred to as biomimetic. In the next step the enolorthoester is hydrolyzed to produce the ketone 15. The cyclopentene A-ring is then opened by oxidizing with ozone to produce 16. Finally, the diketone 17 undergoes an intramolecular aldol condensation by treating with aqueous potassium hydroxide to produce progesterone.[169]
Chemical synthesis of progesterone from stigmasterol and pregnanediol was accomplished later that year.[171][174] Up to this point, progesterone, known generically as corpus luteum hormone, had been being referred to by several groups by different names, including corporin, lutein, luteosterone, and progestin.[17][175] In 1935, at the time of the Second International Conference on the Standardization of Sex Hormones in London, England, a compromise was made between the groups and the name progesterone (progestational steroidal ketone) was created.[17][176]
Veterinary use
The use of progesterone tests in dog breeding to pinpoint ovulation is becoming more widely used. There are several tests available but the most reliable test is a blood test with blood drawn by a veterinarian and sent to a lab for processing. Results can usually be obtained with 24 to 72 hours. The rationale for using progesterone tests is that increased numbers begin in close proximity to preovulatory surge in gonadotrophins and continue through ovulation and estrus. When progesterone levels reach certain levels they can signal the stage of estrus the female is. Prediction of birth date of the pending litter can be very accurate if ovulation date is known. Puppies deliver with a day or two of 9 weeks gestation in most cases. It is not possible to determine pregnancy using progesterone tests once a breeding has taken place, however. This is due to the fact that, in dogs, progesterone levels remain elevated throughout the estrus period.[177]
Pricing
Pricing for progesterone can vary depending location, insurance coverage, discount coupons, quantity, shortages, manufacturers, brand or generic versions, different pharmacies, and so on. As of currently, 30 capsules of 100 mg of the generic version, Prometrium, from CVS Pharmacy is around $40 without any discounts or insurance applied. The brand version, Progesterone, is around $450 for 30 capsules without any discounts or insurance applied.[178] In comparison, Walgreens offers 30 capsules of 100 mg in the generic version for $51 without insurance or coupons applied. The brand name costs around $431 for 30 capsules of 100 mg.[179]
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