The comparative endocrinology of prolactin

Age-Dependent Decline in Salinity Tolerance in a Euryhaline Fish

Euryhaline teleost fish are characterized by their ability to tolerate a wide range of environmental salinities by modifying the function of osmoregulatory cells and tissues. In this study, we experimentally addressed the age-related decline in the sensitivity of osmoregulatory transcripts associated with a transfer from fresh water (FW) to seawater (SW) in the euryhaline teleost, Mozambique tilapia, Oreochromis mossambicus. The survival rates of tilapia transferred from FW to SW were inversely related with age, indicating that older fish require a longer acclimation period during a salinity challenge. The relative expression of Na⁺/K⁺/2Cl⁻ cotransporter 1a (nkcc1a), which plays an important role in hyposmoregulation, was significantly upregulated in younger fish after SW transfer, indicating a clear effect of age in the sensitivity of branchial ionocytes. Prolactin (Prl), a hyperosmoregulatory hormone in O. mossambicus, is released in direct response to a fall in extracellular osmolality. Prl cells of 4-month-old tilapia were sensitive to hyposmotic stimuli, while those of >24-month-old fish did not respond. Moreover, the responsiveness of branchial ionocytes to Prl was more robust in younger fish. Taken together, multiple aspects of osmotic homeostasis, from osmoreception to hormonal and environmental control of osmoregulation, declined in older fish. This decline appears to undermine the ability of older fish to survive transfer to hyperosmotic environments.

Prolactin receptor gene transcriptional control, regulatory modalities relevant to breast cancer resistance and invasiveness

The prolactin receptor (PRLR) is a member of the lactogen/cytokine receptor family, which mediates multiple actions of prolactin (PRL). PRL is a major hormone in the proliferation/differentiation of breast epithelium that is essential for lactation. It is also involved in breast cancer development, tumor growth and chemoresistance. Human PRLR expression is controlled at the transcriptional level by multiple promoters. Each promoter directs transcription/expression of a specific non-coding exon 1, a common non-coding exon 2 and coding exons E3-11. The identification of exon 11 of PRLR led to finding of alternative spliced products and two novel short forms (SF) that can inhibit the long form (LF) of PRLR activity with relevance in physiological regulation and breast cancer. Homo and heterodimers of LF and SF are formed in the absence of PRL that acts as a conformational modifier. Heterodimerization of SF with LF is a major mechanism through which SF inhibits some signaling pathways originating at the LF. Biochemical/molecular modeling approaches demonstrated that the human PRLR conformation stabilized by extracellular intramolecular S-S bonds and several amino acids in the extracellular D1 domain of PRLR SF are required for its inhibitory actions on PRLR LF-mediated functions. Studies in breast cancer cells demonstrated that the transcription of PRLR was directed by the preferentially utilized PIII promoter, which lacks an estrogen responsive element. Complex formation of non-DNA bound ERα dimer with Sp1 and C/EBPβ dimers bound to their sites at the PRLR promoter is required for basal activity. Estradiol induces transcriptional activation/expression of the PRLR gene, and subsequent studies revealed the essential role of autocrine PRL released by breast cancer cells and CDK7 in estradiol-induced PRLR promoter activation and upregulation. Other studies revealed stimulation of the PRLR promoter activity and PRLR LF protein by PRL in the absence of estrogen via the STAT5/phospho-ERα activation loop. Additionally, EGF/ERBB1 can induce the transcription of PRLR independent of estrogen and prolactin. The various regulatory modalities contributing to the upregulation of PRLR provide options for the development of therapeutic approaches to mitigate its participation in breast cancer progression and resistance.

Secretion and Function of Pituitary Prolactin in Evolutionary Perspective

The hypothalamo-pituitary system developed in early vertebrates. Prolactin is an ancient vertebrate hormone released from the pituitary that exerts particularly diverse functions. The purpose of the review is to take a comparative approach in the description of prolactin, its secretion from pituitary lactotrophs, and hormonal functions. Since the reproductive and osmoregulatory roles of prolactin are best established in a variety of species, these functions are the primary subjects of discussion. Different types of prolactin and prolactin receptors developed during vertebrate evolution, which will be described in this review. The signal transduction of prolactin receptors is well conserved among vertebrates enabling us to describe the whole subphylum. Then, the review focuses on the regulation of prolactin release in mammals as we have the most knowledge on this class of vertebrates. Prolactin secretion in response to different reproductive stimuli, such as estrogen-induced release, mating, pregnancy and suckling is detailed. Reproduction in birds is different from that in mammals in several aspects. Prolactin is released during incubation in avian species whose regulation and functional significance are discussed. Little information is available on prolactin in reptiles and amphibians; therefore, they are mentioned only in specific cases to explain certain evolutionary aspects. In turn, the osmoregulatory function of prolactin is well established in fish. The different types of pituitary prolactin in fish play particularly important roles in the adaptation of eutherian species to fresh water environments. To achieve this function, prolactin is released from lactotrophs in hyposmolarity, as they are directly osmosensitive in fish. In turn, the released prolactin acts on branchial epithelia, especially ionocytes of the gill to retain salt and excrete water. This review will highlight the points where comparative data give new ideas or suggest new approaches for investigation in other taxa.

Olfactory marker protein regulates prolactin secretion and production by modulating Ca2+ and TRH signaling in lactotrophs

Olfactory marker protein (OMP) is a marker of olfactory receptor-mediated chemoreception, even outside the olfactory system. Here, we report that OMP expression in the pituitary gland plays a role in basal and thyrotropin-releasing hormone (TRH)-induced prolactin (PRL) production and secretion. We found that OMP was expressed in human and rodent pituitary glands, especially in PRL-secreting lactotrophs. OMP knockdown in GH4 rat pituitary cells increased PRL production and secretion via extracellular signal-regulated kinase (ERK)1/2 signaling. Real-time PCR analysis and the Ca²⁺ influx assay revealed that OMP was critical for TRH-induced PRL secretion. OMP-knockout mice showed lower fertility than control mice, which was associated with increased basal PRL production via activation of ERK1/2 signaling and reduced TRH-induced PRL secretion. However, both in vitro and in vivo results indicated that OMP was only required for hormone production and secretion because ERK1/2 activation failed to stimulate cell proliferation. Additionally, patients with prolactinoma lacked OMP expression in tumor tissues with hyperactivated ERK1/2 signaling. These findings indicate that OMP plays a role in PRL production and secretion in lactotrophs through the modulation of Ca²⁺ and TRH signaling.

Uncovering the regulatory mechanism behind production of the prolactin hormone may help tackle reproductive health problems. As well as triggering milk production in female mammals, prolactin is critical for healthy reproduction in both sexes. An excess of prolactin secreted by cells called lactotrophs in the pituitary gland can cause infertility. While scientists know which hormones stimulate prolactin release, how prolactin levels are regulated is unclear. Eun Jig Lee and Cheol Ryong Ku at Yonsei University in Seoul, Korea, and co-workers demonstrated that the olfactory marker protein (OMP) plays a central role in regulating prolactin production. They found that OMP specifically and highly expressed in lactotrophs. Eliminating OMP expression in mice left a key signalling pathway and calcium ion levels upregulated, resulting in increased prolactin and reduced fertility.

Neuroendocrine control of photoperiodic changes in immune function

Seasonal variation in immune function putatively maximizes survival and reproductive success. Day length (photoperiod) is the most potent signal for time of year. Animals typically organize breeding, growth, and behavior to adapt to spatial and temporal niches. Outside the tropics individuals monitor photoperiod to support adaptations favoring survival and reproductive success. Changes in day length allow anticipation of seasonal changes in temperature and food availability that are critical for reproductive success. Immune function is typically bolstered during winter, whereas reproduction and growth are favored during summer. We provide an overview of how photoperiod influences neuronal function and melatonin secretion, how melatonin acts directly and indirectly to govern seasonal changes in immune function, and the manner by which other neuroendocrine effectors such as glucocorticoids, prolactin, thyroid, and sex steroid hormones modulate seasonal variations in immune function. Potential future research avenues include commensal gut microbiota and light pollution influences on photoperiodic responses.

Decidual cells produce a heparin-binding prolactin family cytokine with putative intrauterine regulatory actions

Pregnancy in mice and rats is associated with the production of a large family of hormones/cytokines related to prolactin (PRL). The hormones/cytokines are hypothesized to coordinate maternal and fetal adaptations to pregnancy. In this study, PRL-like protein-J (PLP-J, also known as PRL family 3, subfamily c, member 1 (Prl3c1)) is shown to be a product of the uterine decidua and a regulator of postimplantation intrauterine events. PLP-J-specific antibodies and a series of recombinant PLP-J proteins were generated and used to investigate PLP-J expression and as ligands for investigating biological targets. Decidual PLP-J migrates as a 29-kDa protein and localizes to a band of decidual cells surrounding the trophoblast cell layer on gestation day 8.5. PLP-J ligands specifically bound in situ to the surrounding uterine stromal cells and vasculature within the decidua of gestation day 8.5 implantation sites. We then investigated the in vitro actions of PLP-J on uterine stromal cells and endothelial cells. PLP-J specifically interacted with both cell populations. PLP-J promoted uterine stromal cell proliferation and inhibited endothelial cell proliferation. We determined that PLP-J does not interact with PRL receptors. Instead, PLP-J interacts with heparin-containing molecules, including syndecan-1, which is expressed in gestation day 8.5 pregnant uteri, as well as in uterine stromal cells and endothelial cells. The restricted expression of PLP-J and its specific interactions with uterine stromal cells and endothelial cells suggests that it acts locally and regulates decidual cell development and the endometrial vasculature.

A prolactin family paralog regulates reproductive adaptations to a physiological stressor

Successful species develop strategies to optimize their reproductive performance. This optimization likely includes the evolution of genes that specifically permit reproduction in physiologically challenging conditions. The prolactin (PRL) family gene cluster is one of 25 mouse-specific gene clusters, the majority of which are associated with reproduction. A prevailing theme characterizing the PRL family is its connection with pregnancy and mechanisms controlling viviparity. PRL-like protein A (PLP-A) is one of 26 genes located within the PRL family locus. It is a nonclassical member of the PRL family (e.g., PLP-A does not use the PRL receptor) produced by trophoblast cells of the chorioallantoic placenta and acts on uterine natural killer cells. In this report, the biology of PLP-A has been investigated by generating mice with a PLP-A null mutation. Under standardized animal husbandry conditions, PLP-A possesses modest effects on reproductive performance. However, this same gene is critical for reproduction when mice are exposed to a physiological stressor. Wild-type mice exposed to hypobaric hypoxia during gestation readily adapt and maintain their pregnancies, whereas PLP-A null mutant mice fail to adapt, resulting in pregnancy failure. PLP-A contributes to hypoxia-induced adaptations critical to hemochorial placentation and thus nutrient flow to extraembryonic and embryonic tissues. The findings provide insights into species-specific reproductive adaptations.

Anterior pituitary hormones: development of a bioassay leading to the discovery of prolactin

This essay looks at the historical significance of an APS classic paper that is freely available online:

Riddle O, Bates RW, and Dykshorn SW. The preparation, identification and assay of prolactin–a hormone of the anterior pituitary. Am J Physiol 105: 191—216, 1933 ( http://ajplegacy.physiology.org/cgi/reprint/105/1/191 ).

Are dopamine antagonists a risk factor for breast cancer? An answer from Parkinson's disease

Women undergoing chemotherapy for breast cancer are often administered dopamine antagonist adjuvant medications that may increase levels of prolactin potentially increasing the risk of cancer. Using nationwide computerized registers of death data for the years 1991 through 1996 we examined 12,430,473 deaths of persons over 40 years of age and extracted 144,364 cases with Parkinson's disease (PD), a dopamine deficient population. Patients with PD had lower rates of breast and other types of malignancies, even in the presence of depression and suicide.

Micromethod for the determination of free and total prolactin receptors: measurement of receptor levels in normal and malignant mammary and prostate tissues

A sensitive micromethod for the determination of free and total prolactin receptors in normal or malignant tissues has been developed. Positive and negative quality controls are incorporated in the procedure. Either whole tissue or the pellet fraction remaining from tissue that had undergone processing for estrogen receptors can be used. Crude microsomal and plasma membrane fractions obtained by homogenization and differential centrifugation are incubated with labeled prolactin in the presence or absence of increasing amounts of unlabeled hormone. The labeled ligand is prepared by a stoichiometric iodination procedure in which one atom of iodine-125 is incorporated into one molecule of the hormone, resulting in an intact labeled prolactin with a high specific activity of 170-186 muCi/micrograms (1 Ci = 37 GBq). Human prolactin labeled by this procedure has much greater specific binding capacity to various rat tissues than does iodinated rat prolactin. This technique permits an accurate measurement of prolactin receptors in as little as 50 micrograms of membrane protein. Highest levels of free and total prolactin receptors were found in the liver of 60-day-old female rats that served as a positive control. Liver of immature 21-day-old male rats, devoid of prolactin receptors, was used as a negative control. The amount of detectable free receptors was dependent on the level of circulating plasma prolactin. In 3-day postpartum lactating rats with high prolactin levels in plasma, all prolactin receptors in the mammary glands were found to be occupied, and no free receptors could be detected. When these receptors were desaturated from the endogenous prolactin by exposure to 3 M MgCl2, one class of receptors in a high quantity (1.75 nmol/mg of protein) and with a moderate affinity (Kd = 6.41 X 10(-9) M) was detected. A similar type of receptor was found in the mammary glands of rats at midpregnancy and of cycling adult female rats. In malignant rat mammary tissue, however, fewer receptors (27 pmol/mg of protein) but with a very high affinity (Kd = 6.8 X 10(-14) M) were detected. Normal ventral and dorsolateral rat prostate contained two classes of prolactin receptors (Kd = 3.46 X 10(-10) M and 1.93 X 10(-8) M). In the cancerous rat prostate, however, only one of these two classes of receptors was detected, and the number was smaller.

In vitro studies of the influence of prolactin on tail regeneration in the adult newtNotophthalmus viridescens

In vitro experiments were carried out to determine the effects of prolactin, and prolactin in combination with other hormones on the regeneration of adult newt tail blastemata. A total of 271 blastemata were explanted 13 days postamputation and were organ cultured for 96 h at 20 (±1)°C. Treatment with prolactin alone resulted in an increase in the blastema cell density of the tail regenerates. Cell accumulation and cell alignment were observed ventral to the reconstituted spinal cord. Prolactin and thyroxine, in combination, improved development of tail regenerates as compared with treatment with prolactin or thyroxine singly, supporting the results of earlier in vivo studies. Optimal development was obtained only when prolactin, insulin, thyroxine and hydrocortisone were added to the culture medium. Regeneration of tail explants maintained in medium augmented with the four hormones closely resembles that of in vivo tail blastemata 17 days post-amputation.

The effects of prolactin and oxytocin on milk secretion and on the permeability of the mammary epithelium in the rabbit

The effects of prolactin or oxytocin on milk secretion and the permeability of the mammary epithelium have been investigated in rabbits. 2. Milk yield was increased by prolactin treatment in late (25-28 days) but not in established (11-14 days) lactation. 3. Prolactin treatment increased milk [lactose] and [K] and decreased [Na] and [Cl] in late lactation, and thus reversed the normal changes in late lactation, but had no significant effect in established lactation. 4. [14C]sucrose movements from blood to milk were significantly decreased to levels characteristic of established lactation, following prolactin treatment in late lactation. No significant effect was evident with treatment in established lactation. Na and Cl movements showed similar trends. 5. It is suggested that prolactin in some way affects paracellular movements of ions and small molecules like lactose across the mammary epithelium, and that this mechanism is responsible for the changes in the composition of the aqueous phase of milk. 6. Immediately following a single dose of 100 m-u. oxytocin no significant effects on milk composition were evident but after 1 u. milk [Na] and [cl] were significantly increased. 7. Twenty-four hr after 1 u. oxytocin, milk [Na] and [cl] were decreased while [K], [lactose], [fat] and [protein] were increased. 8. During an I.V. infusion of oxytocin milk [Na] and [Cl] increased while [K] and [lactose] decreased. The passage of [(14)C]sucrose, 24Na and (36)Cl from blood to milk also increased. 9. These effects of oxytocin are discussed in relation to the permeability of the mammary epithelium and the pathways for ion movements, and to other studies on milk composition in the rabbit involving the administration of oxytocin to aid in the evacuation of milk.

The ontogenesis of human fetal hormones. III. Prolactin

The synthesis and release of human prolactin (hPRL) in the human fetus was assessed by radioimmunoassay analysis of the content and concentration of hPRL in 82 pituitary glands and the concentration of serum hPRL in 47 fetuses of gestational age 68 days to term. Fetal hPRL exhibited parallelism with the reference standard (Lewis 203-1). hPRL was detected by 68 days of gestation (10 wk), the earliest fetal pituitary gland studied; 8 out of 33 pituitaries had a prolactin (PRL) content above 2.0 ng between 10-15 wk gestation. The mean ocntent of PRL in the pituitary gland increased sharply from 14.8 plus or minus 4.6 ng at 15-19 wk to 405 plus or minus 142 ng at 20-24 wk and 542 plus or minus ng at 25-29 wk gestation. By term, the mean content was 2,039 plus or minus 459 (range 493-3,689) and the mean concentration 15.9 plus or minus 2.4 ng/mg (range 7-20). There was a significant positive correlation (P less than 0.001) between the hPRL and human growth hormone (hGH) content of fetal pituitary glands; at term the hPRL/hGH ratio was 1/290. The concentration of serum hPRL between 12 and 24 wk ranged from 2.9 to 67 ng/ml, mean 19.5 plus or minus 2.5 ng/ml )n = 21); by 26 wk fetal serum hPRL increased sharply and attained levels of 300-500 ng/ml in late gestation. At delivery, the mean plasma concentration of hPRL was 167 plus or minus 14.2 ng/ml in 36 umbilical venous specimens and 111.8 plus or minus 12.3 ng/ml in the matched maternal venous specimens. No correlation between serum hPRL and the pituitary content or concentration of hPRL was demonstrable in 12 matched fetal specimens. In five anencephalic infants, umbilical venous hPRL levels were between 65 and 283 ng/ml. In two anencephalic infants, thyrotropin releasing factor (TRF) (200 mug IV) evoked a rise in serum hPRL in one patient from 43 to 156 ng/ml at 30 min, and in the other from 65 to 404 ng/ml at 120 min. In both patients, plasma thyroid-stimulating hormone (TSH) rose from undetectable base-line levels to peak levels of 97 and 380 muU/ml, respectively. The pattern of change in serum hPRL in the human fetus contrasts sharply with that of serum hGH, luteinizing hormone, or follicle-stimulating hormone. These observations in the fetus and in anencephalic infants suggest that the striking elevation of serum PRL in the fetus is neither mediated by a putative PRL releasing factor or by TRF, nor is a consequence of suppression or absence of PRL release inhibiting factor alone, as a functional hypothalamus is not required to attain the high PRL concentration at term. Several lines of evidence support the view that high plasma estrogen levels characteristic of gestation act directly on the fetal anterior hypophysis to stimulate PRL secretion or to sensitize the secretory mechanism of the lactotrope, increasing its responsiveness to other stimuli.

Rapid inactivation of ovalbumin messenger ribonucleic acid after acute withdrawal of estrogen

Synthesis of ovalbumin mRNA is induced and maintained in the avian oviduct by estrogen. When estrogen is rapidly removed from circulation, there is a general involution of the oviduct and an unusually rapid decay of ovalbumin mRNA activity. The kinetics of ovalbumin mRNA decay were not first order; instead, the rate of degradation increased about 10-fold over a 20-hr period after removal of estrogen. These results are in contrast with first-order decay kinetics observed for ovalbumin mRNA in estrogen-stimulated chicks (t(1/2) = about 24 hr) and in cell-free extracts. The degradative response triggered by hormonal withdrawal becomes more rapid between 1 and 4 days of estrogen-stimulated growth. We conclude that in the process of inducing egg-white protein synthesis, estrogen produces a cellular environment in which the egg-white protein mRNAs are relatively stable; removal of estrogen initiates cellular catabolism in a manner that is not understood.

The time course of cardiovascular changes in lactation in the rat

1. Cardiovascular changes in lactating rats have been traced from the first day post-partum to the end of the third week of lactation. The pattern of changes showed three phases.2. Between days 1 and 5 of lactation there were sharp rises in both cardiac output and in the blood flow/g tissue for most organs, but little change in the distribution of the cardiac output.3. Between days 5 and 15 of lactation cardiac output remained steady. The blood flow to tissues actively involved in the body's response to lactation (mammary glands, liver, gastrointestinal tract) also remained at high steady levels, but the blood flow to other tissues declined due to a redistribution of the cardiac output away from them and towards the growing mammary glands and splanchnic organs.4. Between days 15 and 22 of lactation there were further rises in both cardiac output and in the blood flow/g tissue for most organs.5. It is suggested that the increases in organ blood flows that occurred in the first few days after parturition (days 1-5) and at the end of lactation (days 15-22) were largely dependent on increases in cardiac output and may represent the maternal response to rapidly rising demands from the young at these times.

Human prolactin: measurement in plasma by in vitro bioassay

Prolactin has been measured in unextracted human plasma by a sensitive and specific in vitro bioassay. Secretory activity of breast tissue fragments from mid-pregnant mice, incubated in organ culture with human plasma, serves as the histologic end point. Sensitivity is 5 ng/ml (0.14 mU/ml) or somewhat better for ovine prolactin, and approximately 0.42 mU/ml for prolactin activity of human plasma at the dilutions used in the assay. Human growth hormone as it circulates in blood, like the material extracted from pituitary glands, is strongly lactogenic. Antisera to human growth hormone are capable of completely neutralizing the prolactin effect of large amounts (600 ng/ml) of human growth hormone added to the system. Plasma prolactin activity is less than 0.42 mU/ml in normal men and women. Of 26 patients with nonpuerperal galactorrhea, 14 had elevated prolactin activities ranging from 0.42 to 3.5 mU/ml. Growth hormone levels by radioimmunoassay were far too low, in general, to account for the observed prolactin activity. All of 14 nursing mothers, 1-30 days post partum, had elevated prolactin activity with a mean of 2.29 and a total range of 0.56-4.5 mU/ml. Growth hormone was in the low normal range in all of these subjects. Seven patients on psychoactive drugs of the phenothiazine series similarly had elevated prolactin activity with low growth hormone. Antiserum to human growth hormone, when preincubated with plasma samples from each of these three groups of subjects, produced no significant inhibition of prolactin activity. In two acromegalic patients with markedly elevated growth hormone levels, antiserum to growth hormone produced complete inhibition of prolactin activity in one and partial inhibition in the other. These studies indicate that human growth hormone and human prolactin are separate molecules, with little if any immunologic cross-reactivity, at least as demonstrated by the antisera used in this study, and that their release is governed by different physiologic mechanisms.