PDB rendering based on 1n9d.
Prolactin (PRL), also known as luteotropic hormone or luteotropin, is a protein that in humans is best known for its role in enabling female mammals to produce milk, however, it is influential over a large number of functions with over 300 separate actions of PRL having been reported in various vertebrates. Prolactin is secreted from the pituitary gland in response to eating, mating, estrogen treatment, ovulation, and nursing. Prolactin is secreted in a pulsatile fashion in between these events. Prolactin also plays an essential role in metabolism, regulation of the immune system, and pancreatic development.
Discovered in non-human animals around 1930 by Oscar Riddle  at Cold Spring Harbor Laboratory on Long Island, New York, and confirmed in humans in 1970 by Henry Friesen  prolactin is a peptide hormone, encoded by the PRL gene. 
Although often associated with human milk production, prolactin plays a wide range of other roles in both humans and other vertebrates. (For example, in fish—the oldest known vertebrates—an important function is probably related to control of water and salt balance.) Prolactin also acts in a cytokine-like manner and as an important regulator of the immune system. It has important cell cycle related functions as a growth-, differentiating- and anti-apoptotic factor. As a growth factor, binding to cytokine like receptors, it also has profound influence on hematopoiesis, angiogenesis and is involved in the regulation of blood clotting through several pathways. The hormone acts in endocrine, autocrine, and paracrine manner through the prolactin receptor and a large number of cytokine receptors.
Pituitary prolactin secretion is regulated by endocrine neurons in the hypothalamus, the most important ones being the neurosecretory tuberoinfundibulum (TIDA) neurons of the arcuate nucleus, which secrete dopamine (aka Prolactin Inhibitory Hormone) to act on the D2 receptors of lactotrophs, causing inhibition of prolactin secretion. Thyrotropin-releasing factor (thyrotropin-releasing hormone) has a stimulatory effect on prolactin release.
Several variants and forms are known per species. Many fish have variants prolactin A and prolactin B. Most vertebrates including humans also have the closely related somatolactin. In humans, three smaller (4, 16, and 22 kDa) and several larger (so called big and big-big) variants exist.[not verified in body]
- 1 Effects
- 2 Production and regulation
- 3 Variance in levels
- 4 Structure and isoforms
- 5 Prolactin receptor
- 6 Diagnostic use
- 7 Units and unit conversions
- 8 Reference ranges
- 9 Conditions associated with elevated levels
- 10 Conditions associated with decreased levels
- 11 See also
- 12 References
- 13 External links
Prolactin has a wide range of effects. It stimulates the mammary glands to produce milk (lactation): increased serum concentrations of prolactin during pregnancy cause enlargement of the mammary glands of the breasts and prepare for the production of milk. Milk production normally starts when the levels of progesterone fall by the end of pregnancy and a suckling stimulus is present. Sometimes, newborn babies (males as well as females) secrete a milky substance from their nipples known as witch's milk. This is in part caused by maternal prolactin and other hormones.
Prolactin provides the body with sexual gratification after sexual acts: The hormone counteracts the effect of dopamine, which is responsible for sexual arousal. This is thought to cause the sexual refractory period. The amount of prolactin can be an indicator for the amount of sexual satisfaction and relaxation. Unusually high amounts are suspected to be responsible for impotence and loss of libido (see hyperprolactinemia symptoms).
Highly elevated levels of prolactin decrease the levels of sex hormones — estrogen in women and testosterone in men. The effects of mildly elevated levels of prolactin are much more variable, in women both substantial increase or decrease of estrogen levels may result.
Prolactin is sometimes classified as a gonadotropin although in humans it has only a weak luteotropic effect while the effect of suppressing classical gonadotropic hormones is more important. Prolactin within the normal reference ranges can act as a weak gonadotropin but at the same time suppresses GnRH secretion. The exact mechanism by which it inhibits GnRH is poorly understood although expression of prolactin receptors (PRL-R) have been demonstrated in rat's hypothalmus, the same has not been observed in GnRH neurons. Physiologic levels of prolactin in males enhance luteinizing hormone-receptors in Leydig cells, resulting in testosterone secretion, which leads to spermatogenesis.
Prolactin also stimulates proliferation of oligodendrocyte precursor cells. These cells differentiate into oligodendrocytes, the cells responsible for the formation of myelin coatings on axons in the central nervous system.
Prolactin also has a number of other effects including contributing to pulmonary surfactant synthesis of the fetal lungs at the end of the pregnancy and immune tolerance of the fetus by the maternal organism during pregnancy.
Prolactin delays hair regrowth in mice.
Production and regulation
Pituitary PRL is controlled by the Pit-1 transcription factor and ultimately dopamine, extrapituitary PRL is controlled by a superdistal promoter and apparently unaffected by dopamine. The thyrotropin-releasing hormone and the vasoactive intestinal peptide stimulate the secretion of prolactin in experimental setting, however their physiological influence is unclear. The main stimulus for prolactin secretion is suckling, the effect of which is neuronally mediated.
In decidual cells and in lymphocytes the distal promoter and thus prolactin expression is stimulated by cAMP. Responsivness to cAMP is mediated by an imperfect cAMP–responsive element and two CAAT/enhancer binding proteins (C/EBP). Progesterone has been observed to upregulate prolactin synthesis in the endometrium but decreases it in myometrium and breast glandular tissue. However breast and other tissues may also express the Pit-1 promoter in addition to the distal promoter.
Extrapituitary production of prolactin is thought to be special to humans and primates and may serve mostly tissue specific paracrine and autocrine purposes. It has been hypothesized that in other vertebrates such as mice a similar tissue specific effect is achieved by a large family of prolactin like proteins controlled by at least 26 paralogous PRL genes not present in primates.
Variance in levels
During pregnancy, high circulating concentrations of progesterone increase prolactin levels by 10- to 20-fold. However, at the same time, estrogen, as well as progesterone, inhibit the stimulatory effects of prolactin on milk production. It is the abrupt drop of estrogen and progesterone levels following delivery that allows prolactin — which temporarily remains high — to induce lactation.[verification needed]
After childbirth, prolactin levels fall as the internal stimulus for them is removed. Sucking by the baby on the nipple then promotes further prolactin release, maintaining the ability to lactate. The sucking activates mechanoreceptors in and around the nipple. These signals are carried by nerve fibers through the spinal cord to the hypothalamus, where changes in the electrical activity of neurons that regulate the pituitary gland cause increased prolactin secretion. The suckling stimulus also triggers the release of oxytocin from the posterior pituitary gland, which triggers milk let-down: Prolactin controls milk production (lactogenesis) but not the milk-ejection reflex; the rise in prolactin fills the breast with milk in preparation for the next feed.
It has also been found that compared to un-mated males, fathers and expectant fathers have increased prolactin concentrations.
High prolactin levels can also contribute to mental health issues.
Prolactin levels peak during REM sleep, and in the early morning. Levels can rise after exercise, high-protein meals, sexual intercourse, breast examination, minor surgical procedures, following epileptic seizures or (occasionally) due to physical or emotional stress, In a study on female volunteers under hypnosis, prolactin surges resulted from the evocation, with rage, of humiliating experiences, but not from the fantasy of nursing.
Hypersecretion of prolactin is more common than hyposecretion. Hyperprolactinemia is the most frequent abnormality of the anterior pituitary tumors, termed prolactinomas. Prolactinomas may disrupt the hypothalamic-pituitary-gonadal axis as prolactin tends to suppress the secretion of GnRH from the hypothalamus and in turn decreases the secretion of FSH and LH from the anterior pituitary, therefore disrupting the ovulatory cycle in females. Such hormonal changes may manifest as amenorrhea and infertility in females as well as impotence in males. Inappropriate lactation is another important clinical sign of prolactinomas.
Structure and isoforms
The structure of prolactin is similar to that of growth hormone and placental lactogen. The molecule is folded due to the activity of three disulfide bonds. Significant heterogeneity of the molecule has been described, thus bioassays and immunoassays can give different results due to differing glycosylation, phosphorylation, sulfation, as well as degradation. The non-glycosylated form of prolactin is the dominant form of prolactin that is secreted by the pituitary gland.
There are mainly three different forms of prolactin in regard to size:
- Little prolactin is the predominant form. It has a molecular weight of appxoximately 22-kDa. It is a single-chain polypeptide of 198 amino acids, and is apparently the result of removal of some amino acids.
- Big prolactin of approximately 48 kDa. It may be the product of interaction of several prolactin molecules. It appears to have little, if any, biological activity.
- Big big prolactin of approximately 150 kDa. It appears to have a low biological activity.
The levels of larger ones are somewhat higher during the early postpartum period.
Pit-1 is a transcription factor that binds to the prolactin gene at several sites to allow for the production of prolactin in the pituitary gland. A key regulator of prolactin production is estrogens that enhance growth of prolactin-producing cells and stimulate prolactin production directly, as well as suppressing dopamine.
Human prolactin receptors are insensitive to mouse prolactin.
Prolactin receptors are present in the mamillary glands, ovaries, pituitary glands, heart, lung, thymus, spleen, liver, pancreas, kidney, adrenal gland, uterus, skeletal muscle, skin, and areas of the central nervous system. When prolactin binds to the receptor, it causes it to dimerize with another prolactin receptor. This results in the activation of Janus kinase 2, a tyrosine kinase that initiates the JAK-STAT pathway. The activation of the prolactin receptor also results in the activation of mitogen-activated protein kinases and Src kinase.
Prolactin levels may be checked as part of a sex hormone workup, as elevated prolactin secretion can suppress the secretion of FSH and GnRH, leading to hypogonadism, and sometimes causing erectile dysfunction in men.
Units and unit conversions
The serum concentration of prolactin can be given in mass concentration (µg/L or ng/mL), molar concentration (nmol/L or pmol/L) or in international units (typically mIU/L). The current IU is calibrated against the third International Standard for Prolactin, IS 84/500. Reference ampoules of IS 84/500 contain 2.5 µg of lyophilized human prolactin, and have been assigned an activity of .053 International Units of prolactin. Measurements that are calibrated against the current international standard can be converted into mass units using this ratio of grams to IUs; prolactin concentrations expressed in mIU/L can be converted to µg/L by dividing by 21.2. Previous standards use other ratios.
The first International Reference Preparation (or IRP) of human Prolactin for Immunoassay was established in 1978 (75/504 1st IRP for human Prolactin) at a time when purified human prolactin was in short supply. Previous standards relied on prolactin from animal sources. Purified human prolactin was scarce, heterogeneous, unstable, and difficult to characterize. A preparation labelled 81/541 was distributed by the WHO Expert Committee on Biological Standardization without official status and given the assigned value of 50 mIU/ampoule based on an earlier collaborative study. It was determined that this preparation behaved anomalously in certain immunoassays and was not suitable as an IS. However, in the absence of an alternative, it was used. Three different human pituitary extracts containing prolactin were subsequently obtained as candidates for an IS. These were distributed into ampoules coded 83/562, 83/573, and 84/500. On the basis of collaborative studies involving 20 different laboratories, it was concluded that there was little difference between these three preparations. 83/562 appeared to be the most stable. This preparation was largely free of dimers and polymers of prolactin. On the basis of these investigations 83/562 was established as the Second IS for human Prolactin. Once stocks of these ampoules were depleted, 84/500 was established as the Third IS for human Prolactin.
General guidelines for diagnosing prolactin excess (hyperprolactinemia) define the upper threshold of normal prolactin at 25 µg/L for women, and 20 µg/L for men. Similarly, guidelines for diagnosing prolactin deficiency (hypoprolactinemia) are defined as prolactin levels below 3 µg/L in women, and 5 µg/L in men. However, different assays and methods for measuring prolactin are employed by different laboratories, and as such the serum reference range for prolactin is often determined by the laboratory performing the measurement. Furthermore, prolactin levels also vary with, for example, age, sex, menstrual cycle stage, and pregnancy. The circumstances surrounding a given prolactin measurement (assay, patient condition, etc.) must therefore be considered before the measurement can be accurately interpreted.
The following chart illustrates the variations seen in normal prolactin measurements across different populations. Prolactin values were obtained from specific control groups of varying sizes using the IMMULITE assay.
|women, follicular phase (n = 803)||
|women, luteal phase (n = 699)||
|women, mid-cycle (n = 53)||
|women, whole cycle (n = 1555)||
|women, pregnant, 1st trimester (n = 39)||
|women, pregnant, 2nd trimester (n = 52)||
|women, pregnant, 3rd trimester (n = 54)||
|Men, 21–30 (n = 50)||
|Men, 31–40 (n = 50)||
|Men, 41–50 (n = 50)||
|Men, 51–60 (n = 50)||
|Men, 61–70 (n = 50)||
The following table illustrates variability in reference ranges of serum prolactin between some commonly used assay methods (as of 2008), using a control group of healthy health care professionals (53 males, age 20–64 years, median 28 years; 97 females, age 19–59 years, median 29 years) in Essex, England:
An example usage of table above is, if using the Centaur assay to estimate prolactin values in µg/L for females, the mean is 7.92 µg/L, and the reference range is 3.35–16.4 µg/L.
Conditions associated with elevated levels
Hyperprolactinaemia, or excess serum prolactin, is associated with hypoestrogenism, anovulatory infertility, oligomenorrhoea, amenorrhoea, unexpected lactation, and loss of libido in women, and erectile dysfunction and loss of libido in men.
Conditions associated with decreased levels
Hypoprolactinaemia, or serum prolactin deficiency, is associated with ovarian dysfunction in women, and metabolic syndrome, anxiety, arteriogenic erectile dysfunction, premature ejaculation, oligozoospermia, asthenospermia, hypofunction of seminal vesicles, and hypoandrogenism in men. In one study, normal sperm characteristics were restored when prolactin levels were brought up to normal values in hypoprolactinemic men.
Hypoprolactinemia can result from:
- Excess tuberoinfundibular pathway dopamine
- Anti-Parkinson drugs that bind to postsynaptic D2 receptors
- Epileptic seizure
- Male lactation
- Prolactin receptor
- Prolactin-releasing hormone
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