PROLACTIN 1533 (1268).What may be truly surprising is that the receptor ent ways:it controls ductal side branching and terminal and its mRNA are also found in numerous parts of the end bud regression in virgin animals via indirect mecha- CNS. The distribution of mRNA for the long form of the nisms.but A un( 2231 nuclous of the stria torminalis amyadala the central arat since hyn oophysectomy during p regnancy prevents subse of the midbrain,thalamus,hypothalamus,cerebral cortex. quent lactation(1306).Due to impairment of mammogen- and olfactory bulb(586,587).Recently,prolactin binding esis,both prolactin knockout (790)and prolactin receptor of the 10 ce mil the blood-brain barrier (2).Prolactin receptors are uts have normal mammoge esis (790).the F.ger also present in a wide range of peripheral organs like the heterozygous prolactin receptor knockouts are unable to pituitary gland,heart,lung,thymus, lactate r their first litter to the 58).However,milk is deli ty no s se With further reproductive cvcles mamr V.BIOLOGICAL ACTIONS OF PROLACTIN the F and F2 generations proceeds sufficiently to support lactati on,indic ating that the defect in heterozygotes is a ng the mary gland dev opmen Our goal in this section is to summarize the most n the mamn body.Ar alleles of the prolactin receptor are required for tebrate tation and c found in the Alt differences betweer recent review written by Kelly et al (184). ough there are dramatic mals in the A.Reproduction tin (1093)initiate lactation in pseudopregnant rabbits Prolactin is best kne (1694).Replacement of prolactin to hypophysectomizec exerts on the rab .fully restore lactatio 36 othe effects on other targets important to the reproduction of tatio ysectomy of ra tail sho the mammalian species.In some mammals,particularly consist of prolactin and glucocorticoid or adrenocortico rodents,prolacti important for th trophin to maintain sufficient nurturing of the pups(173) Add I growth ormone permits the maintenance of maternal behaviors. t sh actions is solely 1.Lactation due to prolactin,but the hormone is merely a plaver in an orchestra of hormones and growth factors that affect the mammary gland.A great deal dence from hypoph that the en maintenance of milk secretion (galactopoie ent in vivo requires olactin estrogen pro Although it has long been accepted that prolactin is gesterone,and glucocorticoids(837).During pregnancy. the sive branching of the ducts and development o the alv al lac of proge eron tin knockout)(790)or knockout of the prolactinr ntor wth hormone thyroid hor mone narathyroid hormone (1358)results in abnormal mammogenesis characterized calcitonin.several growth factors.and even oxvtocin may by complete absence of lobuloalveolar units in adult ho also play a role in galactopoiesis in varous mammals ola 1779 of loo tinguishable from wild type (790)Transnlantation of uptake of some aino acids the synthesis of the milk mammary epithelium from prolactin receptor knockouts proteins casein and a-lactalbumin,uptake of glucose,and into mammary fat pads of wild-type mice revealed that synthesis of the milk sugar lactose as well as milk fats prolactin affects mammary morphogenesis in two differ (118,1779)
(1268). What may be truly surprising is that the receptor and its mRNA are also found in numerous parts of the CNS. The distribution of mRNA for the long form of the prolactin-R has been characterized in the rat brain (112). Abundant message is found in the choroid plexus, bed nucleus of the stria terminalis, amygdala, the central gray of the midbrain, thalamus, hypothalamus, cerebral cortex, and olfactory bulb (586, 587). Recently, prolactin binding sites have been described in the area postrema, which is one of the main chemosensitive areas of the brain lacking the blood-brain barrier (1112). Prolactin receptors are also present in a wide range of peripheral organs like the pituitary gland, heart, lung, thymus, spleen, liver, pancreas, kidney, adrenal gland, uterus, skeletal muscle, and skin (184, 1268). V. BIOLOGICAL ACTIONS OF PROLACTIN Our goal in this section is to summarize the most relevant actions of prolactin in the mammalian body. An extensive summary of prolactin’s effects in different vertebrate species and organ systems can be found in the recent review written by Kelly et al. (184). A. Reproduction Prolactin is best known for the multiple effects it exerts on the mammary gland. However, it also exerts effects on other targets important to the reproduction of the mammalian species. In some mammals, particularly rodents, prolactin is also important for the maintenance and secretory activity of the corpus luteum. It also affects other actions related to reproduction such as mating and maternal behaviors. 1. Lactation The varied effects of prolactin on the mammary gland include growth and development of the mammary gland (mammogenesis), synthesis of milk (lactogenesis), and maintenance of milk secretion (galactopoiesis). Although it has long been accepted that prolactin is involved in the development of the mammary gland (154), recent elegant techniques have confirmed such findings. Indeed, targeted disruption of the prolactin gene (prolactin knockout) (790) or knockout of the prolactin receptor (1358) results in abnormal mammogenesis characterized by complete absence of lobuloalveolar units in adult homozygous females. Prolactin knockout heterozygotes appear to have nearly normal mammogenesis that is indistinguishable from wild type (790). Transplantation of mammary epithelium from prolactin receptor knockouts into mammary fat pads of wild-type mice revealed that prolactin affects mammary morphogenesis in two different ways: it controls ductal side branching and terminal end bud regression in virgin animals via indirect mechanisms, but acts directly on the mammary epithelium to produce lobuloalveolar development during pregnancy (223). Lactogenesis clearly requires pituitary prolactin, since hypophysectomy during pregnancy prevents subsequent lactation (1306). Due to impairment of mammogenesis, both prolactin knockout (790) and prolactin receptor knockout (1358) homozygous mice fail to produce milk. Interestingly, although the heterozygous prolactin knockouts have normal mammogenesis (790), the F1 generation heterozygous prolactin receptor knockouts are unable to lactate for their first litter (1358). However, milk is delivered perfectly normally to the F1’s second litter (1358). The phenotype of the F2 generation was similar (1358). With further reproductive cycles, mammogenesis in both the F1 and F2 generations proceeds sufficiently to support lactation, indicating that the defect in heterozygotes is one affecting the rate of mammary gland development. These experiments further indicate that two functional alleles of the prolactin receptor are required for full lactation. Although there are dramatic differences between mammals in the hormonal requirements for galactopoiesis, the common absolute requirement is prolactin. Aqueous extracts of anterior pituitary gland containing prolactin (1093) initiate lactation in pseudopregnant rabbits (1694). Replacement of prolactin to hypophysectomized rabbits will fully restore lactation (364). On the other hand, while hypophysectomy of rats and mice stops lactation (1587), the replacement cocktail should minimally consist of prolactin and glucocorticoid or adrenocorticotrophin to maintain sufficient nurturing of the pups (173). Addition of growth hormone permits the maintenance of maximal lactation (1094). It should be noted that none of these actions is solely due to prolactin, but the hormone is merely a player in an orchestra of hormones and growth factors that affect the mammary gland. A great deal of evidence from hypophysectomized-ovariectomized-adrenalectomized rodents suggests that the mammary gland’s lobuloalveolar growth and development in vivo requires prolactin, estrogen, progesterone, and glucocorticoids (837). During pregnancy, the extensive branching of the ducts and development of the alveoli is a function of progesterone and prolactin or placental lactogen (837). There is evidence that insulin, growth hormone, thyroid hormone, parathyroid hormone, calcitonin, several growth factors, and even oxytocin may also play a role in galactopoiesis in various mammals (1779). In the process of lactogenesis, prolactin stimulates uptake of some amino acids, the synthesis of the milk proteins casein and a-lactalbumin, uptake of glucose, and synthesis of the milk sugar lactose as well as milk fats (118, 1779). October 2000 PROLACTIN 1533
1534 FREEMAN,KANYICSKA,LERANT,AND NAGY Volume 80 to d the long form of prolactin-R (1268). The activation of tion failure (184).Prolactin is essential for progesterone promote Il involves binding of C/ERBB (CCAAT-en biosynthesis and luteal cell hypertrophy during preg and Spl (recognizing cy.In additio futo lut function,the tin-R me nd ae ation of a d as well (184) ment resembling the consensus ap2 binding site (812)As Aside from its luteotrophic role.there is evidence in in vitro studies indicate(750)and in vivo studies confirm th rat that prolactin may be lute as well (1111 (4),prolactir bindi gland 8) for hoth ediated by CD3-p lactogenesis.STAT5 (e specially STAT5a)activated by the the exn on of the membrane form of the Fas ligand cription of milk known o mediate luteal cell death through the Fas recep genes the tor (98 I the rat,as man y as thre ns o t of t cionby cing the structural regre sion of the oldest of these.I transductior pathways over which prolactin induces should be emphasized that the corpora lutea are nonfunc mammary gla grow th and dev nt have been ex tional at th e that prola erts u tensively s recently (750 2.Luteal function ritical time between periods of exposure to pr lactin during the estrous cycle,the corpora lutea of the rat depend on express monocyte moattra eoie 200173 lytic in the absence of a mati no stimulus Both short and long isoforms of the p In most rodents,prolactin acts as a luteotrophic hor- present in the ovaries (337,1621).Transcription of the mone by maintar ning the structural and funct onal prolact deve um I and ho Regul best described in the rat.is characterized by enhance is ac ad-snecifen noter I and the progesterone secretion (580).Progesterone is essential genericpromoter I.Essential transcriptional activator ctin-R's promoter I is steroic dogenic factor-I (S s elem (58OY I (810 i steroid produced by the corpus luteum of the rat is 20 tein also kmown as Ad4Bp (1089) whose synthesis from progester- Recently. sion of a prolactin-R associated 20a-hydroxysteroid dehydrogenase pho in (PRAP)has beer scribed in luteal cells 1508 oI prog terone secretion two ways prolactin potentiates the ste sion of PRAp is u roidogenic effects of luteinizing hor ne (LH)in granu- (469)Structurally PRAP shows 8906 homology with a losa-luteal cells (1471 and inhibits the 200 newly characterized form(type 7)of 17B-hydroxysteroid hydroxyste h ina -Ke s (17-HS ),sug talyzing t stradiol (1324) LH,follicle stimulating hormone (ESH and prolactin (672).There is some evidence that prolactin may also be 3.Reproductive behavior part 14 ophi (13 ulos inization (o 1170)and steroido (1017).Further evidence of luteal dependence on prolac- mans,high prolactin levels are associated with psychoso- tin is found in prolactin receptor knockouts who lack matic reactions including pseudopregnancy (1653).There
Controlled exclusively by promoter III of the rat prolactin-R gene (812), the mammary gland expresses mainly the long form of prolactin-R (1268). The activation of promoter III involves binding of C/ERBb (CCAAT-enhancer binding protein) and Sp1 (recognizing GC boxes of DNA) transcription factors to their respective binding elements and activation of a downstream sequence element resembling the consensus AP2 binding site (812). As in vitro studies indicate (750) and in vivo studies confirm (854), prolactin-R in the mammary gland is phosphorylated upon prolactin binding (1868) and activates the Jak2/STAT5 pathway responsible for both mammo- and lactogenesis. STAT5 (especially STAT5a) activated by the long form of the prolactin-R induces transcription of milk protein genes (151). Null mutation of the STAT5a or STAT5b gene is detrimental to tubuloalveolar development of the mammary gland and results in inability to lactate in homozygous (2/2) females (184). The signal transduction pathways over which prolactin induces mammary gland growth and development have been extensively studied in vitro and reviewed recently (750). 2. Luteal function Actions of prolactin on luteal function depend on species and the stage of the estrous cycle. In rodents, prolactin can either be luteotrophic after mating or luteolytic in the absence of a mating stimulus. In most rodents, prolactin acts as a luteotrophic hormone by maintaining the structural and functional integrity of the corpus luteum for 6 days after mating (1232). This “luteotrophic” action of prolactin, which has been best described in the rat, is characterized by enhanced progesterone secretion (580). Progesterone is essential for the implantation of the fertilized ovum (along with estrogen), maintenance of pregnancy, and inhibition of ovulation (580). In the absence of prolactin, the dominant steroid produced by the corpus luteum of the rat is 20ahydroxyprogesterone, whose synthesis from progesterone is catalyzed by 20a-hydroxysteroid dehydrogenase (1508). This metabolite of progesterone is “inactive” in most progesterone bioassays. Prolactin enhances progesterone secretion two ways: prolactin potentiates the steroidogenic effects of luteinizing hormone (LH) in granulosa-luteal cells (1471) and inhibits the 20ahydroxysteroid dehydrogenase enzyme, which inactivates progesterone (580). In other rodents such as hamsters, prolactin is part of a “luteotrophic complex” consisting of LH, follicle stimulating hormone (FSH), and prolactin (672). There is some evidence that prolactin may also be part of a luteotrophic complex in dogs (1322) and primates (1472). In humans, high levels of prolactin inhibit granulosa cell luteinization (10, 1170) and steroidogenesis (1017). Further evidence of luteal dependence on prolactin is found in prolactin receptor knockouts who lack normal luteal function and thus are sterile due to decreased ovulation rate, aberrant oogenesis, and implantation failure (184). Prolactin is essential for progesterone biosynthesis and luteal cell hypertrophy during pregnancy. In addition to luteal function, the prolactin-R mediates numerous functions in granulosa cells and oocytes as well (184). Aside from its luteotrophic role, there is evidence in the rat that prolactin may be luteolytic as well (1111, 1887) by inducing programmed cell death in the corpora lutea (901, 1156). Prolactin’s luteolytic effect seems to be mediated by CD3-positive lymphocytes, which increase the expression of the membrane form of the Fas ligand, known to mediate luteal cell death through the Fas receptor (989). In the rat, as many as three generations of corpora lutea may appear on the ovary. There is evidence that prolactin may perform a “housekeeping function” by inducing the structural regression of the oldest of these. It should be emphasized that the corpora lutea are nonfunctional at the time that prolactin exerts this effect. The mechanism by which prolactin can be both luteotrophic and luteolytic is still uncertain. One suggestion is that at some critical time between periods of exposure to prolactin during the estrous cycle, the corpora lutea of the rat acquire the capacity to express monocyte chemoattractant protein-1, which subsequently interacts with prolactin on proestrus to induce luteal cell death (200, 1773). Both short and long isoforms of the prolactin-R are present in the ovaries (337, 1621). Transcription of the prolactin-R in the ovaries is controlled by intricate developmental and hormonal regulation (340, 1730). Regulation of transcription of the prolactin-R gene in the ovaries is accomplished by the gonad-specific promoter I and the “generic” promoter III. Essential transcriptional activator of prolactin-R’s promoter I is steroidogenic factor-1 (SF- 1)-binding consensus element, which is activated by SF-1 (810, 811). SF-1 is a specific zinc finger DNA binding protein, also known as Ad4BP (1089). Recently, expression of a prolactin-R associated phosphoprotein (PRAP) has been described in luteal cells (468). PRAP binds to the intracellular domain of the long form of the prolactin-R, but not to the short form. Expression of PRAP is upregulated by estrogen and prolactin (469). Structurally, PRAP shows 89% homology with a newly characterized form (type 7) of 17b-hydroxysteroid dehydrogenases/17-ketosteroid reductases (17-HSD), suggesting that PRAP may be an enzyme catalyzing the conversion of estrone to estradiol (1324). 3. Reproductive behavior A) FEMALE RECEPTIVITY. There are data suggesting that prolactin influences reproductive behavior (476). In humans, high prolactin levels are associated with psychosomatic reactions including pseudopregnancy (1653). There 1534 FREEMAN, KANYICSKA, LERANT, AND NAGY Volume 80
PROLACTIN 1535 are prolactin-R in the ventromedial nucleus of the hypd ment regimen.dramatically advanced the onset of mater thalamus (375),an area which controls female sexual nal behaviors(218).Suppression of endogenous prolactin behavior. of prolacti with bromocryptine prevents the onset of mater vity (743 supe t932 founded by the multiplicity of experimental designs.For acting within the medial preoptic area of the hypothala example,when given in the third ventricle of estrogen and mus220.17011 progesterone-primed ovariectomized rats,prolactin di- up contact has been shown to induce transcriptior of the n of femal 470.Tm ctomized rats.prolactin enhances sexual is nr ented by ovariectomy and hy opbysectomy (1701) receptivity (738).Enhancement of endogenous prolactin It seems that the effect of pup contact is not sex specific secretion in response to dopamine antagonism has been the same induc tion of brain prolactin-R long-form ml re) to ha ct on ma oig be ved i se to the nurs ing stimulus diminishe ual behe female contact supp ses.the effect of pu ior(1655).In contrast,when the rat is sexually receptive contact in males (1530)In mice carrving a germ line nul in the afternoon of proestrus,suppression of the sponta mutation of the prolactin receptor gene,homozygous mu a dopa h gh a null mutation of the olactin-R in the Mor ous heter fomalos mouse produces most of the defects associated with a profound deficit in maternal care when challenged with deficiency of prolacti such receptor-deficien foster pups.Such data suggest that pup contact is re appea for t 1358 d-Lyp of the prol ctin recep h l be whos has is fo well-de 1086 for prolactin in female sexual behavior.In contrast,it is Although not as widely studied,prolactin may have a clear that prolactin suppresses stereotypical male sexual role in paternal care as well.The data for this role are behavior in rats (451 90 and (629 cing in fish ut somewha ntal is an old" d be that t mammals.maternal behavior is the most extensively stud. recent ancestors has become somewhat redundant This ied(218,221,1086,1530).These include nest building as is emphasized by the significant stereotypical patemal well as gathering,gr ng,cleaning ing over role of vertebrates (e.g.,the male ator)an als the effects of prola ctin on the induetion and mainter ance CPROLACTIN-R IN THE HYPOTHALAMU Altho ough the brain of these mate al behaviors in mice.rabbit.hamsters.and contains mainly the long isoform of the prolactin-R.the sheep (219, ,1329).It should be empha that prolactin. hyp oth contains both the I by its the ae Withi Intracerebroventricular infusion of prolactin the mediobasal hypothalamus (23-325).Also.the mRNA the latency to initiation of maternal behavior in steroid of the long form of prolactin-R is found within the primed rats )The basic vation was made that arcuate,and fer reg of dats ort these observations by showing that the Superimposition of prolactin treatment on the varian tha amic areas contain prolactin-R protein as well(1028. steroid regime es the latency of maternal be 1415,149 Expression olactin-R in the brain in ays (2 age (2 and by re onse to the rential steroid treatment.On the other on i ontact (1700) hand. prolactin-hypersecreting pituitary transplants Few studies have examined the signal transduction placed beneath the kidney capsule of hypophysectomized path vays specincall activated upon binding of prolacti female rats kept on a maternal ovarian steroid replace to its receptor in the CNS.Preliminary data from our
are prolactin-R in the ventromedial nucleus of the hypothalamus (375), an area which controls female sexual behavior. Coincidentally, iontophoresis of prolactin to this area increases local neuronal electrical activity (743). However, in rats, prolactin’s precise action has been confounded by the multiplicity of experimental designs. For example, when given in the third ventricle of estrogen and progesterone-primed ovariectomized rats, prolactin diminishes lordosis frequency, an index of sexual receptivity (470). Though, when given in the midbrain of estradioltreated ovariectomized rats, prolactin enhances sexual receptivity (738). Enhancement of endogenous prolactin secretion in response to dopamine antagonism has been reported to have no effect on mating behavior in females (1654), whereas elevation of prolactin secretion in response to the nursing stimulus diminishes sexual behavior (1655). In contrast, when the rat is sexually receptive in the afternoon of proestrus, suppression of the spontaneous release of prolactin with a dopamine agonist dramatically attenuates sexual receptivity (1884). Finally, although a null mutation of the prolactin-R gene in the mouse produces most of the defects associated with a deficiency of prolactin, such receptor-deficient females appear to mate normally with heterozygote or wild-type males (1358, 1680). Thus these data, taken together, do not provide a firm basis for assigning a well-defined role for prolactin in female sexual behavior. In contrast, it is clear that prolactin suppresses stereotypical male sexual behavior in rats (451, 896) and sheep (629). B) PARENTAL BEHAVIOR. Probably the best-characterized prolactin-driven behaviors are the parental behaviors. In mammals, maternal behavior is the most extensively studied (218, 221, 1086, 1530). These include nest building as well as gathering, grouping, cleaning, crouching over, and nursing of the young by the mother. Although most widely described in rats, there is also an extensive literature on the effects of prolactin on the induction and maintenance of these maternal behaviors in mice, rabbit, hamsters, and sheep (219, 1329). It should be emphasized that prolactin, by itself, does not initiate maternal behavior, but merely decreases the latency to the onset of maternal behavior. Intracerebroventricular infusion of prolactin decreases the latency to initiation of maternal behavior in steroidprimed rats (220). The basic observation was made that nulliparous female rats treated with a pregnancy-like regimen of estrogen and progesterone for 10 days showed maternal behaviors with a mean latency of 5–6 days. Superimposition of prolactin treatment on the ovarian steroid regimen reduces the latency of maternal behavior to 1–2 days (217). In addition, hypophysectomized rats failed to display a facilitation of maternal behavior in response to the sequential steroid treatment. On the other hand, prolactin-hypersecreting pituitary transplants placed beneath the kidney capsule of hypophysectomized female rats kept on a maternal ovarian steroid replacement regimen, dramatically advanced the onset of maternal behaviors (218). Suppression of endogenous prolactin release with bromocryptine prevents the onset of maternal behavior, whereas superimposition of prolactin promotes it (222). Prolactin may be exerting this effect by acting within the medial preoptic area of the hypothalamus (220, 1701). Pup contact has been shown to induce transcription of the long-form prolactin-R mRNA in the brain of female rats. The effect of pup contact on prolactin-R expression is prevented by ovariectomy and hypophysectomy (1701). It seems that the effect of pup contact is not sex specific; the same induction of brain prolactin-R long-form mRNA expression and maternal behavior can be observed in pup-contacted male rats. Administration of prolactin promotes, while female contact suppresses, the effect of pup contact in males (1530). In mice carrying a germ line null mutation of the prolactin receptor gene, homozygous mutant and heterozygous mutant nulliparous females show a deficiency in pup-induced maternal behavior (1086). Moreover, primiparous heterozygous females exhibit a profound deficit in maternal care when challenged with foster pups. Such data suggest that pup contact is required for transcription of the prolactin receptor whose stimulation by prolactin eventuates in maternal behavior (1086). Although not as widely studied, prolactin may have a role in paternal care as well. The data for this role are most convincing in fish and birds but somewhat less convincing in mammals (1574). Indeed, because prolactin is an “old” hormone, it could be that this role in our most recent ancestors has become somewhat redundant. This is emphasized by the significant stereotypical paternal role of nonmammalian vertebrates (e.g., the male sea horse is the incubator) and the almost nonexistent role in most mammals. C) PROLACTIN-R IN THE HYPOTHALAMUS. Although the brain contains mainly the long isoform of the prolactin-R, the hypothalamus contains both the long and short forms (323). Within the hypothalamus, prolactin-R mRNA-containing neurons have been found in the anterior as well as the mediobasal hypothalamus (323–325). Also, the mRNA of the long form of prolactin-R is found within the periventricular, paraventricular, supraoptic, arcuate, and ventromedial nuclei of the hypothalamus as well as the medial preoptic area (112). Immunocytochemical data support these observations by showing that these hypothalamic areas contain prolactin-R protein as well (1028, 1415, 1495). Expression of prolactin-R in the brain increases with age (325, 993, 1241), exposure to estrogens (1264, 1598), elevation in serum prolactin levels, and by pup contact (1700). Few studies have examined the signal transduction pathways specifically activated upon binding of prolactin to its receptor in the CNS. Preliminary data from our October 2000 PROLACTIN 1535
1536 FREEMAN,KANYICSKA,LERANT,AND NAGY Volume 80 indicate that lactin ge ane leads to don rons of the mediobasal hypothalamus.This suggests that tween homozygotes and heterozygotes in the frequency of the signal trans sduction pathways coupled to prolactin-R and T-cell a gen expression (790).Such results argue s are similar tha play an I or its and e in ont ean ho sated by tions of cNs neuron s,among them the tuberoinfundibular The role of prolactin in the immune response of the (TIDA)neurons of the arcuate nucleus (1524). organism is a matter of continuing concer It appears respons s in vivo ar anced b B.Homeostasis ule (1270)or e-induced Aside from its actions on reproductive processes actin (1272)restores dinitrochlorobenz contact dermatitis impaired by hypophyse omy.On the prolactin plays a role in maintaining the constancy of the by regulatio of the immune system, a tin(725). kin allograft transplants elevate ctin levels induced by skin allografts 1.bnmune response elevated serum pro can be suppres ed by either bromocryptine o r an antilvm (1505 However, antilymph data lactin lays a significant role in regulation of the humoral ific role in skin graft reiection and ma y play a role in and cellular immune responses in physiological as well as other transplantation respo nses as well (764) m demons prolactin-F RNA immune response was the demonstration in 1972 that forms in the thym mph odes and hone exogenous prolactin enhanced thymic function in prolac rrow of both rats and mice (1020).Expression of pro dwar mice (314).Shortly thereafter,Nagy lactin- was more extensiv mapped 12731e to attenuation of humoral or cell-mediated immunity that lactation (Prolactin's functions are the ten could be reversed by treatment with exogenous prolactin. sively des scribed and reviewed in the Nb2 cell line(1920) A large number of immune perturbation Thi cell line also expres be .I tin stimulates mitoge sis in both normal T lyn of STAT5a STAT5h (1828)and the Nb2 Iymphoma cell line (1622).It should and STAT3:2)rapid and selective formation of not be surpris sing that prolactin at cts lymphocytes STAT5a/t eterodimers;3) narked Ser,but not Thr phos 1517-1519 a on RNA perp 4) appea ulated by prolactin itself (442)Mor that discriminate lactin on lymphocytes may involve interleukin (IL)-2 since onding to the prolactin response elements of the T-lymphocyte activatio by IL-2 requires prola B-caser and interferon regulatory factor-1 gene promot 2).mt site of es(945 nueleus (343)Prolactin is a uired for mit 2.Osmoregulation ulated proliferation of Iymphocytes (741 757 758)Nh2 cells,der ed from immature T lymphocytes One of the least understood actions of prolactin is t on h m ogen of solute and water tra across mamma cific bioas say for ctin Hov motivated by the finding in lower is not uniform agreement on the role of prolactin in tin stimulates solute transport across cell membranes and hematopoiesis.Although targeted disruption of the pro- thus could be an osmoregulatory hormone(153).Some of
laboratory indicate that systemic administration of prolactin results in nuclear translocation of STAT5 in neurons of the mediobasal hypothalamus. This suggests that the signal transduction pathways coupled to prolactin-R in CNS neurons are similar to those described in peripheral tissues. Prolactin also increases the expression of NGFI-A, NGFI-B, c-fos, and c-jun in numerous populations of CNS neurons, among them the tuberoinfundibular (TIDA) neurons of the arcuate nucleus (1524). B. Homeostasis Aside from its actions on reproductive processes, prolactin plays a role in maintaining the constancy of the internal environment by regulation of the immune system, osmotic balance, and angiogenesis. 1. Immune response Prolactin is a common mediator of the immunoneuroendocrine network, where nervous, endocrine, and immune systems communicate with each other (631). Prolactin plays a significant role in regulation of the humoral and cellular immune responses in physiological as well as pathological states, such as autoimmune diseases (253, 1295, 1850). The earliest evidence that prolactin plays a role in the immune response was the demonstration in 1972 that exogenous prolactin enhanced thymic function in prolactin-deficient dwarf mice (314). Shortly thereafter, Nagy and Berczi (1269) found that hypophysectomy or suppression of prolactin secretion with bromocryptine (1273) led to attenuation of humoral or cell-mediated immunity that could be reversed by treatment with exogenous prolactin. A large number of immune perturbations were found to be associated with prolactin deficiency (148, 1269–1273). As noted in two recent reviews (1145, 1871), prolactin stimulates mitogenesis in both normal T lymphocytes (1828) and the Nb2 lymphoma cell line (1622). It should not be surprising that prolactin affects lymphocytes since prolactin-R has been detected on human peripheral lymphocytes (1517–1519) and their mRNA expression is regulated by prolactin itself (442). Moreover, effects of prolactin on lymphocytes may involve interleukin (IL)-2 since T-lymphocyte activation by IL-2 requires prolactin (344, 712). Interestingly, prolactin’s site of action for modifying the effects of IL-2 on lymphocytes appears to be the nucleus (343). Prolactin is also required for mitogen-stimulated proliferation of lymphocytes (741, 757, 758). Nb2 cells, derived from immature T lymphocytes, are dependent on the mitogenic activity of prolactin (1622, 1713). Indeed, this property has served as the basis for a highly sensitive, specific bioassay for prolactin. However, there is not uniform agreement on the role of prolactin in hematopoiesis. Although targeted disruption of the prolactin gene leads to numerous defects in prolactin-dependent events such as lactation, there is no difference between homozygotes and heterozygotes in the frequency of B- and T-cell antigen expression (790). Such results argue that prolactin does not play an indispensable role in primary lymphocyte differentiation or its absence during development can be compensated by other factors. The role of prolactin in the immune response of the organism is a matter of continuing concern. It appears that immune responses in vivo are enhanced by prolactin. For example, prolactin-secreting pituitary grafts placed beneath the kidney capsule (1270) or administration of prolactin (1272) restores dinitrochlorobenzene-induced contact dermatitis impaired by hypophysectomy. On the other hand, skin allograft transplants elevate serum prolactin (725). During graft rejection, lymphocytic prolactin gene expression is also upregulated (1601). Moreover, elevated serum prolactin levels induced by skin allografts can be suppressed by either bromocryptine or an antilymphocytic serum (1505). However, only antilymphocytic serum prolongs the survival time of the graft (1505). These data suggest that lymphocytic prolactin plays a specific role in skin graft rejection and may play a role in other transplantation responses as well (764). Immunocytochemical demonstration of prolactin-R on T and B lymphocytes (1769) was followed by detection of mRNA encoding the short and long prolactin-R isoforms in the thymus, spleen, lymph nodes, and bone marrow of both rats and mice (1020). Expression of prolactin-R isoforms was more extensively mapped in rat splenocytes and thymocytes from birth to adulthood (703), as well as during the estrous cycle, pregnancy, and lactation (704). Prolactin’s functions are the most extensively described and reviewed in the Nb2 cell line (1920). This cell line also expresses an intermediate (393 amino acid) isoform of prolactin-R (184). In Nb2 lymphocytes, activation of the prolactin-R is associated with (945) 1) rapid tyrosine phosphorylation of STAT5a, STAT5b, STAT1a, and STAT3; 2) rapid and selective formation of STAT5a/b heterodimers; 3) marked Ser, but not Thr phosphorylation of STAT5a and STAT5b; and 4) the appearance of two qualitatively distinct STAT5 protein complexes that discriminate between oligonucleotides corresponding to the prolactin response elements of the b-casein and interferon regulatory factor-1 gene promoters (945). 2. Osmoregulation One of the least understood actions of prolactin is regulation of solute and water transport across mammalian cell membranes (1602). Studies in this area were motivated by the finding in lower vertebrates that prolactin stimulates solute transport across cell membranes and thus could be an osmoregulatory hormone (153). Some of 1536 FREEMAN, KANYICSKA, LERANT, AND NAGY Volume 80
PROLACTIN 1537 the actions in mammals are easier to envision in a phys VL.PATTERNS OF PITUITARY iological perspective than others.For example,prolactin PROLACTIN RELEASE exerts a host of activities on transport of solute across mammary epithelial cell membranes.In keeping with its when the release of prolactin is assessed at the sin gle-cell level,the pattern of prolactin sec retion of individ lactogenic prop ong t earliest es wa on that ctin decre te trans ual lactotrophs shows sexual dimorphism.In general. slightly mo han half the lactotrophs of female rats e prol treated rabbits (534,535).Similarly,prolactin stimulates the uptake of amino acids by the rat mammary gland (297).In the following sections we summarize the pat (1825)as well as the uptake of the nonmetabolizable tems of prolactin secretion at the level of the whole org amino acid a-aminoisobutyric acid by mouse mammary explants(1480).Th n prolactin-dr Prolactin lact on ha O A.Circadian Rhythm of Prolactin Secretion Plasma concentrations of prolactin are the highest oig and she amnion (1114)but inhibits it in human amnion (1026)Prolactin is responsible for fluid (1447) during sleep and the lowest during the waking hours in sodium,chloride (1102-1104),and calcium (1365)trans cent an vo exper port across intestinal epithelial membranes.Correlation on is maintained in a co ont indepe sweat chloride and prolactin dent of the rhythm of sleep (1848).although with 92m sis one of the po derably larger amplitude in women than men.These ata i e that the m y prolactin release in seems likely that the enhanced solute tra ort dur the clei of the late pregnaney (205)might be a mechanism whereby (1848) prolactin contributes to the preparation by the pregnant There is ample chronobiological evidence that the mother for subsequent lactation.a similar teleological tempor organ ton ol prola is controlle argument 67 act on the convolute (16 promo and abolished by lesion of the suprachiasmatic nuclei in rats (167,168).In contrast to humans,there is a tight relat onship betweer sleep patterns an ctin level solute transport in a physiological context. oConidamd natters In rats using either inieetion of prolactir 3.Angiogenesis (1496)or implantation of anterior pituitary grafts to ren der the animal hyperprolactine nic(1332),high prolactin the develo of native p REM th idea of (333).This antiangiogenic activity is inherent to the 16- functional relationship between noctumal kDa fragment(334).In fact,there are specific,high-affin- prolactin and REMS.There is some evidence that vasoac tive intes ity,saturable binding sites for the 16-kDa al polypeptide (VIP),a poter fragment of g pepude,may als prolactin on capillary endothelia al cells (336 ne 14-KD 16-kD temically administered VIP-enhan ed REMS (981 1331 835.1n re However,immunoneutralization of endogenous circula nla wth hormone have nic activities (1695).Al. ing prolactin only slightly attenuates spontaneous though a physiological significance has not been ascribed to these opposing effects,it seems likely that there may lactin ciated with REMS was alse deseribed in hu be a therapeutic use for prolactin fragments as local mans (1556).However,slow-wave sleep (SWS)(1848) inhibitors of tumorigenesis,or conversely,a role as patho- also appears to be ass ciated with nocturnal prolactin logical effector through its antiangiogenic actions secretion in humans (1055)
the actions in mammals are easier to envision in a physiological perspective than others. For example, prolactin exerts a host of activities on transport of solute across mammary epithelial cell membranes. In keeping with its lactogenic properties, among the earliest discoveries was the observation that prolactin decreases the transport of sodium and increases the transport of potassium across mammary epithelial cells taken from bromocryptinetreated rabbits (534, 535). Similarly, prolactin stimulates the uptake of amino acids by the rat mammary gland (1825) as well as the uptake of the nonmetabolizable amino acid a-aminoisobutyric acid by mouse mammary explants (1480). The requirement of such prolactin-driven solute movement for lactation has not been described. Prolactin also affects water transport across amniotic membranes. It stimulates water transport across guinea pig and sheep amnion (1114) but inhibits it in human amnion (1026). Prolactin is responsible for fluid (1447), sodium, chloride (1102–1104), and calcium (1365) transport across intestinal epithelial membranes. Correlation between sweat chloride and prolactin concentrations (1492) may implicate prolactin as one of the possible pathogenic factors in cystic fibrosis (968). Although not examined in a systematic manner, it seems likely that the enhanced solute transport during late pregnancy (205) might be a mechanism whereby prolactin contributes to the preparation by the pregnant mother for subsequent lactation. A similar teleological argument can be made for the observation that prolactin acts on the proximal convoluted tubule of the renal nephron to promote sodium, potassium, and water retention (1685). These data, taken together, argue for the need for systematic studies on the role of prolactin on fluid and solute transport in a physiological context. 3. Angiogenesis Angiogenesis, the development of blood vessels, is inhibited by proteolytic fragments of native prolactin (333). This antiangiogenic activity is inherent to the 16- kDa fragment (334). In fact, there are specific, high-affinity, saturable binding sites for the 16-kDa fragment of prolactin on capillary endothelial cells (336). The 14-kDa fragment shares the antiangiogenic activity of the 16-kDa fragment (335). In contrast, it has recently been found that intact human prolactin, placental lactogen, and growth hormone have angiogenic activities (1695). Although a physiological significance has not been ascribed to these opposing effects, it seems likely that there may be a therapeutic use for prolactin fragments as local inhibitors of tumorigenesis, or conversely, a role as pathological effector through its antiangiogenic actions. VI. PATTERNS OF PITUITARY PROLACTIN RELEASE When the release of prolactin is assessed at the single-cell level, the pattern of prolactin secretion of individual lactotrophs shows sexual dimorphism. In general, slightly more than half the lactotrophs of female rats secrete prolactin in a continuous pattern, whereas those of males secrete in a discontinuous or intermittent pattern (297). In the following sections we summarize the patterns of prolactin secretion at the level of the whole organism under different physiological and experimental conditions. A. Circadian Rhythm of Prolactin Secretion Plasma concentrations of prolactin are the highest during sleep and the lowest during the waking hours in humans (1380, 1555). Recent human volunteer experiments prove, however, that this rhythm of prolactin secretion is maintained in a constant environment independent of the rhythm of sleep (1848), although with considerably larger amplitude in women than men. These data indicate that the rhythm of daily prolactin release in humans is a true circadian rhythm that may be generated by the suprachiasmatic nuclei of the hypothalamus (1848). There is ample chronobiological evidence that the temporal organization of prolactin secretion is controlled by circadian input in rats as well (167, 951). The rhythm of prolactin release is maintained in constant environment and abolished by lesion of the suprachiasmatic nuclei in rats (167, 168). In contrast to humans, there is a tight relationship between sleep patterns and prolactin levels in rats. Experimental data suggest that high prolactin levels may be the cause rather than the result of change in sleep patterns. In rats, using either injection of prolactin (1496) or implantation of anterior pituitary grafts to render the animal hyperprolactinemic (1332), high prolactin levels increase the duration and frequency of rapid-eyemovement sleep (REMS), thus leading to the idea of a functional relationship between nocturnal elevations of prolactin and REMS. There is some evidence that vasoactive intestinal polypeptide (VIP), a potent prolactin-releasing peptide, may also be involved in REMS (863, 1333). Immunoneutralization of circulating prolactin blocks systemically administered VIP-enhanced REMS (981, 1331). However, immunoneutralization of endogenous circulating prolactin only slightly attenuates spontaneous REMS, suggesting that central rather than systemic prolactin may be the physiological effector (981, 1331). Release of prolactin associated with REMS was also described in humans (1556). However, slow-wave sleep (SWS) (1848) also appears to be associated with nocturnal prolactin secretion in humans (1055). October 2000 PROLACTIN 1537