Identification and characterization of the prolactin-binding protein in human serum and milk

The prolactin receptor: A cross-species comparison of gene structure, transcriptional regulation, tissue-specificity, and genetic variation

The conserved and multifaceted functions of prolactin (PRL) are coordinated through varied distribution and expression of its cell-surface receptor (PRLR) across a range of tissues and physiological states. The resultant heterogeneous expression of PRLR mRNA and protein across different organs and cell types supports a wide range of PRL-regulated processes including reproduction, lactation, development, and homeostasis. Genetic variation within the PRLR gene also accounts for several phenotypes impacting agricultural production and human pathology. The goal of this review is to highlight the many elements that control differential expression of the PRLR across tissues, and the various phenotypes that exist across species due to variation in the PRLR gene.

The Choroid Plexus Is an Alternative Source of Prolactin to the Rat Brain

Among the more than 300 functions attributed to prolactin (PRL), this hormone has been associated with the induction of neurogenesis and differentiation of olfactory neurons especially during pregnancy, which are essential for maternal behavior. Despite the original hypothesis that PRL enters the central nervous system through a process mediated by PRL receptors (PRLR) at the choroid plexus (CP), recent data suggested that PRL transport into the brain is independent of its receptors. Based on transcriptomic data suggesting that PRL could be expressed in the CP, this work aimed to confirm PRL synthesis and secretion by CP epithelial cells (CPEC). The secretion of PRL and the distribution of PRLR in CPEC were further characterized using an in vitro model of the rat blood-cerebrospinal fluid barrier. RT-PCR analysis of PRL transcripts showed its presence in pregnant rat CP, in CPEC, and in the rat immortalized CP cell line, Z310. These observations were reinforced by immunocytochemistry staining of PRL in CPEC and Z310 cell cytoplasm. A 63-kDa immunoreactive PRL protein was detected by Western blot in CP protein extracts as well as in culture medium incubated with rat pituitary and samples of rat cerebrospinal fluid and serum. Positive immunocytochemistry staining of PRLR was present throughout the CPEC cytoplasm and in the apical and basal membrane of these cells. Altogether, our evidences suggest that CP is an alternative source of PRL to the brain, which might impact neurogenesis of olfactory neurons at the subventricular zone, given its proximity to the CP.

The human intermediate prolactin receptor is a mammary proto-oncogene

The hormone prolactin (PRL) and its receptor (hPRLr) are significantly involved in breast cancer pathogenesis. The intermediate hPRLr (hPRLrI) is an alternatively-spliced isoform, capable of stimulating cellular viability and proliferation. An analogous truncated mouse PRLr (mPRLr) was recently found to be oncogenic when co-expressed with wild-type mPRLr. The goal of this study was to determine if a similar transforming event occurs with the hPRLr in human breast epithelial cells and to better understand the mechanism behind such transformation. hPRLrL+I co-expression in MCF10AT cells resulted in robust in vivo and in vitro transformation, while hPRLrI knock-down in MCF7 cells significantly decreased in vitro malignant potential. hPRLrL+I heterodimers displayed greater stability than hPRLrL homodimers, and while being capable of activating Jak2, Ras, and MAPK, they were unable to induce Stat5a tyrosine phosphorylation. Both immunohistochemical breast cancer tissue microarray data and RNA sequencing analyses using The Cancer Genome Atlas (TCGA) identified that higher hPRLrI expression associates with triple-negative breast cancer. These studies indicate the hPRLrI, when expressed alongside hPRLrL, participates in mammary transformation, and represents a novel oncogenic mechanism.

Orchestration of signaling by structural disorder in class 1 cytokine receptors

BACKGROUND

Class 1 cytokine receptors (C1CRs) are single-pass transmembrane proteins responsible for transmitting signals between the outside and the inside of cells. Remarkably, they orchestrate key biological processes such as proliferation, differentiation, immunity and growth through long disordered intracellular domains (ICDs), but without having intrinsic kinase activity. Despite these key roles, their characteristics remain rudimentarily understood.

METHODS

The current paper asks the question of why disorder has evolved to govern signaling of C1CRs by reviewing the literature in combination with new sequence and biophysical analyses of chain properties across the family.

RESULTS

We uncover that the C1CR-ICDs are fully disordered and brimming with SLiMs. Many of these short linear motifs (SLiMs) are overlapping, jointly signifying a complex regulation of interactions, including network rewiring by isoforms. The C1CR-ICDs have unique properties that distinguish them from most IDPs and we forward the perception that the C1CR-ICDs are far from simple strings with constitutively bound kinases. Rather, they carry both organizational and operational features left uncovered within their disorder, including mechanisms and complexities of regulatory functions.

CONCLUSIONS

Critically, the understanding of the fascinating ability of these long, completely disordered chains to orchestrate complex cellular signaling pathways is still in its infancy, and we urge a perceptional shift away from the current simplistic view towards uncovering their full functionalities and potential. Video abstract.

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.

Variant Prolactin Receptor in Agalactia and Hyperprolactinemia

A loss-of-function variant in the gene encoding the prolactin receptor ( PRLR) was reported previously in a woman with persistent postpartum galactorrhea; however, this paradoxical phenotype is not completely understood. Here we describe a 35-year-old woman who presented with idiopathic hyperprolactinemia that was associated with a complete lack of lactation after each of her two deliveries. She is a compound heterozygote for loss-of-function variants of PRLR. Her unaffected parents are heterozygotes. These findings are consistent with previous work showing that mice deficient in functional Prlr do not lactate.

Other miscellaneous hormone binding proteins: attempt at an epilogue

An overview of the detection, mechanism of formation and clinical function of hormone binding proteins shedded from the membrane receptor and detected in the last twenty years is presented. The representatives of such binding proteins are restricted only to human soluble receptors that have been already detected in blood or other intravasal fluids such as soluble receptors for LH/hCG, prolactin, TSH, erythropoietin, insulin and IGF-I. The clinical or diagnostic significance of these putative-detectable or indeed circulating proteins often remains largely unclear.

Characterization of Δ7/11, a functional prolactin-binding protein

Prolactin is essential for normal mammary gland development and differentiation, and has been shown to promote tumor cell proliferation and chemotherapeutic resistance. Soluble isoforms of the prolactin receptor (PrlR) have been reported to regulate prolactin bioavailability by functioning as 'prolactin-binding proteins'. Included in this category is Δ7/11, a product of alternate splicing of the PrlR primary transcript. However, the direct interactions of prolactin with Δ7/11, and the resulting effect on cell behavior, have not been investigated. Herein, we demonstrate the ability of Δ7/11 to bind prolactin using a novel proximity ligation assay and traditional immunoprecipitation techniques. Biochemical analyses demonstrated that Δ7/11 was heavily glycosylated, similar to the extracellular domain of the primary PrlR, and that glycosylation regulated the cellular localization and secretion of Δ7/11. Low levels of Δ7/11 were detected in serum samples of healthy volunteers, but were undetectable in human milk samples. Expression of Δ7/11 was also detected in six of the 62 primary breast tumor biopsies analyzed; however, no correlation was found with Δ7/11 expression and tumor histotype or other patient demographics. Functional analysis demonstrated the ability of Δ7/11 to inhibit prolactin-induced cell proliferation as well as alter prolactin-induced rescue of cell cycle arrest/early senescence events in breast epithelial cells. Collectively, these data demonstrate that Δ7/11 is a novel regulatory mechanism of prolactin bioavailability and signaling.

Molecular mechanisms of prolactin and its receptor

Prolactin and the prolactin receptors are members of a family of hormone/receptor pairs which include GH, erythropoietin, and other ligand/receptor pairs. The mechanisms of these ligand/receptor pairs have broad similarities, including general structures, ligand/receptor stoichiometries, and activation of several common signaling pathways. But significant variations in the structural and mechanistic details are present among these hormones and their type 1 receptors. The prolactin receptor is particularly interesting because it can be activated by three sequence-diverse human hormones: prolactin, GH, and placental lactogen. This system offers a unique opportunity to compare the detailed molecular mechanisms of these related hormone/receptor pairs. This review critically evaluates selected literature that informs these mechanisms, compares the mechanisms of the three lactogenic hormones, compares the mechanism with those of other class 1 ligand/receptor pairs, and identifies information that will be required to resolve mechanistic ambiguities. The literature describes distinct mechanistic differences between the three lactogenic hormones and their interaction with the prolactin receptor and describes more significant differences between the mechanisms by which other related ligands interact with and activate their receptors.

Prolactin--to be reconsidered in canine mammary tumourigenesis?

Mammary tumours represent the most common neoplastic disease of the female dog, and the incidence in female dogs is much higher than in women. Whereas the influence of sexual steroids on breast cancer (BC) development in dogs has been studied, very little is known about the role of prolactin (PRL). New studies show that until recently, the importance of PRL in human BC development and progression has been highly underestimated. PRL plays a role in promoting benign as well as malignant neoplastic cell growth in BC in vitro and in vivo. Sporadic publications proposed a tumour promotor role in the dog. The goal of this review is to summarize our knowledge about PRL and human BC as well as canine mammary tumourigenesis, and propose future research in this area.

Prolactin signaling mechanisms in ovary

Prolactin is a hormone that is essential for normal reproduction and signals through two types of receptors. Not only is the classical long form of the prolactin receptor identified, but so are many short form receptors in rodents and human tissues. Mouse mutagenesis studies have offered insight into the biology of prolactin family, providing compelling evidence that the different isoforms have independent biological activity. The possibility that short forms mediate cell proliferation is important for a variety of tissues including mammary gland and ovarian follicles. This review summarizes our current knowledge about prolactin signaling and its role in reproduction through either long or short isoform receptors.

A humanized pattern of aromatase expression is associated with mammary hyperplasia in mice

Aromatase is essential for estrogen production and is the target of aromatase inhibitors, the most effective endocrine treatment for postmenopausal breast cancer. Peripheral tissues in women, including the breast, express aromatase via alternative promoters. Female mice lack the promoters that drive aromatase expression in peripheral tissues; thus, we generated a transgenic humanized aromatase (Arom(hum)) mouse line containing a single copy of the human aromatase gene to study the link between aromatase expression in mammary adipose tissue and breast pathology. Arom(hum) mice expressed human aromatase, driven by the proximal human promoters II and I.3 and the distal promoter I.4, in breast adipose fibroblasts and myoepithelial cells. Estrogen levels in the breast tissue of Arom(hum) mice were higher than in wild-type mice, whereas circulating levels were similar. Arom(hum) mice exhibited accelerated mammary duct elongation at puberty and an increased incidence of lobuloalveolar breast hyperplasia associated with increased signal transducer and activator of transcription-5 phosphorylation at 24 and 64 wk. Hyperplastic epithelial cells showed remarkably increased proliferative activity. Thus, we demonstrated that the human aromatase gene can be expressed via its native promoters in a wide variety of mouse tissues and in a distribution pattern nearly identical to that of humans. Locally increased tissue levels, but not circulating levels, of estrogen appeared to exert hyperplastic effects on the mammary gland. This novel mouse model will be valuable for developing tissue-specific aromatase inhibition strategies.

A major prolactin-binding complex on human milk fat globule membranes contains cyclophilins A and B: the complex is not the prolactin receptor

Prolactin (PRL) in milk influences maturation of gastrointestinal epithelium and development of both the hypothalamo-pituitary and immune systems of offspring. Here, we demonstrate that most PRL in human milk is part of a novel, high-affinity, multicomponent binding complex found on the milk fat globule membrane and not in whey. To examine properties of the complex, a sensitive ELISA was developed such that human PRL (hPRL) binding to the complex was measured by loss of hPRL detectability; thus, as much as 50 ng of hPRL was undetectable in the presence of 10 μl of human milk. Using the same methodology, no comparable complex formation was observed with human serum or amniotic fluid. hPRL complexation in milk was rapid, time dependent, and cooperative. Antibodies to or competitors of the hPRL receptor (placental lactogen and growth hormone) showed the hPRL receptor was not involved in the complex. However, hPRL complexation was antagonized by cyclosporine A and anti-cyclophilins. The complex was very stable, resisting dissociation in SDS, urea, and dithiothreitol. Western analysis revealed an ∼75-kDa complex that included hPRL, cyclophilins A and B, and a 16-kDa cyclophilin A. Compared with noncomplexed hPRL, complexed hPRL in whole milk showed similar activation of STAT5 but markedly delayed activation of ERK. Alteration of signaling suggests that complex formation may alter hPRL biological activity. This is the first report of a unique, multicomponent, high-capacity milk fat reservoir of hPRL; all other analyses of milk PRL have utilized defatted milk.

Alpha-lactalbumin in human milk alters the proteolytic degradation of soluble CD14 by forming a complex

Mother's milk represents a foundational step in the proper development of newborn immunity. This is achieved, in part, through the action of numerous regulatory proteins such as soluble cluster of differentiation 14 (sCD14) found in significant quantities in human milk (~25-50 μg/mL). In adults, CD14 stimulates cytokine production in response to lipopolysaccharide (LPS), the major lipid component found in the outer membrane of Gram-negative bacteria. However, the fate and function of sCD14 in the neonatal gastrointestinal (GI) tract are unknown and may function differently from adults. Therefore, we administered human sCD14 to experimental animals and observed that it persisted in the upper GI tract after feeding. In our search for potential proteolytic protectants, immunoprecipitation of sCD14 from human milk revealed a 15-kD novel protein that copurified with sCD14. Mass spectrometry analysis of the protein identified alpha-lactalbumin. CD14 was also identified by immunoblot after immunoprecipitation of alpha-lactalbumin from milk. In vitro digestion assays revealed that purified alpha-lactalbumin decreases the proteolytic degradation of human milk derived sCD14 in vitro, suggesting a mechanism by which this key LPS receptor may remain functional in the neonate gut.

Impact of prolactin receptor isoforms on reproduction

Prolactin is a hormone involved in growth, development, reproduction, metabolism, water and electrolyte balance, brain and behavior, and immunoregulation. Its actions on reproductive processes represent the largest group of functions identified for this hormone. Besides the classic long form of the prolactin receptor, many short form receptors have been identified in rodents and human tissues. Mouse mutagenesis studies have offered insight into the biology of the prolactin family, providing compelling evidence that different isoforms have independent biological activity. The possibility that short forms mediate cell proliferation is important for a variety of tissues including mammary glands and ovarian follicles. This review summarizes the current knowledge about prolactin signaling and its role in reproduction through either long or short isoform receptors.

New mechanisms for PRLr action in breast cancer

Prolactin (PRL) is a pleiotrophic hormone that contributes to the growth of normal and malignant breast tissues. PRL signals through its receptor (PRLr), a transmembrane receptor that belongs to the cytokine receptor family. The mechanism of how the PRL:PRLr interaction triggers activation of signaling networks remains enigmatic. This review examines the effect of ligand binding on PRLr and the processes that initiate receptor-associated signaling. Evidence for PRLr predimerization in the absence of ligand and the actions of the prolyl isomerase cyclophilin A in ligand-induced activation of PRLr-associated Jak2 kinase are discussed. These studies reveal that ligand-induced conformational change of the PRLr complex is necessary for its function and open avenues for therapies to inhibit PRLr action in breast cancer.

Changes in post-translational modifications of prolactin during development and reproductive cycles in the chicken

Changes in proportion of glycosylated prolactin in the anterior pituitary glands of chickens were assessed using one- and two-dimensional western blotting analysis during the perihatch stage of embryos and reproductive cycles. Multiple isoforms of prolactin were detected by one-dimensional analysis and glycosylated (G) and non-glycosylated (NG) isoforms were identified by N-glycosidase and neuraminidase treatment. Increases of ratio of G to NG isoforms were observed in both embryonic stages and reproductive cycles by the one-dimensional analysis. Although a similar tendency of increase of proportion of G prolactin was obtained, different values of proportion were observed between one-dimensional and two-dimensional analysis. Since two-dimensional analysis may better resolve isoforms differing slightly in molecular size of G prolactin, the results from two-dimensional analysis may reflect the actual proportion of prolactin isoforms. Furthermore, isoforms differing in isoelectric points were detected after N-glycosidase and neuraminidase treatment. These results indicate that prolactin may also be additionally post-translationally modified such as by phosphorylation. Thus function and biological activity of prolactin were, at least in part, regulated by post-translational modification in the various physiological stages.

Structure and function of a new class of human prolactin antagonists

Delta 41-52 hPRL (human prolactin with residues 41-52 removed) is a lead compound for a new class of hPRL antagonists. The deleted sequence contains residues that functionally couple sites 1 and 2, the two hormone surfaces that each bind receptors. Delta 41-52 hPRL retains 0.03% agonist activity in FDC-1 cell bioassays, a 3054-fold reduction in activity, and displays approximately 100-fold less agonist activity than G129R hPRL, an antagonist that reduces the binding of hPRL receptor at site 2 during the formation of the heterotrimeric hormone/receptor complex. Replacement of various numbers and types of residues into the gap created by the deletion of residues 41 through 52 created hPRLs with varying agonist activities, suggested that manipulation of the sequence connecting the C-terminal of helix 1 with the disulfide bond (cysteines 58 with 174) linking helices 1 and 4 modulates articulation of these helices and influences agonist activity. We have compared the antagonist activities of G129R and Delta 41-52 hPRLs to induce apoptosis in Jurkat cells, a human lymphoid cell line displaying an autocrine/paracrine hPRL/receptor system. Delta 41-52 hPRL induces apoptosis in a time and dose-dependent fashion. Under these same conditions G129R hPRL fails to induce apoptosis. We conclude Delta 41-52 hPRL is a lead compound of a new class of hPRL antagonists capable at low concentrations of inducing apoptosis in human cells expressing an autocrine/paracrine hPRL/receptor system.

SRC family kinases accelerate prolactin receptor internalization, modulating trafficking and signaling in breast cancer cells

Despite the growing body of evidence supporting prolactin (PRL) actions in human breast cancer, little is known regarding PRL regulation of its own receptor in these cells. Ligand-initiated endocytosis is a key process in the regulation of receptor availability and signaling cascades that may lead to oncogenic actions. Although exposure to exogenous PRL accelerates degradation of the long isoform of the PRL receptor (lPRLR), neither the signals initiated by PRL that lead to lPRLR internalization and subsequent down-regulation, nor the relationship to downstream pathways are understood in breast cancer cells. In this study, we showed that PRL-induced down-regulation of the lPRLR was reduced by inhibition of src family kinases (SFKs), but not Janus kinase 2, in MCF-7 cells. Inhibition of SFKs also resulted in accumulation of a PRL-induced PRLR fragment containing the extracellular domain, which appeared to be generated from newly synthesized PRLR. lPRLR was constitutively associated with SFKs in lipid rafts. PRL-induced SFK activation led to recruitment of the guanosine triphosphatase, dynamin-2, to an internalization complex, resulting in endocytosis. Inhibition of endocytosis by small interfering RNA-mediated knockdown of dynamin-2 blocked PRL-induced down-regulation of lPRLR, confirming that internalization is essential for this process. Endocytosis also was required for optimal phosphorylation of ERK1/2 and Akt, but not for Janus kinase 2 or signal transducer and activator of transcription 5, indicating that internalization selectively modulates signaling cascades. Together, these data indicate that SFKs are key mediators of ligand-initiated lPRLR internalization, down-regulation, and signal transduction in breast cancer cells, and underscore the importance of target cell context in receptor trafficking and signal transduction.

Prolactin release and response to vasoactive intestinal peptide in an opportunistic breeder, the zebra finch (Taeniopygia guttata)

Zebra finches in arid regions of Australia are opportunistic breeders that time their breeding cycles to coincide with nonseasonal rainfall. Hormonal profiles associated with reproductive behaviors may differ from those observed in seasonal breeders because these birds need to be reproductively competent on short notice. This study measured plasma prolactin (PRL) levels in nonbreeding and breeding zebra finches and in birds with and without prior reproductive experience. We also investigated the change in plasma PRL following injection with vasoactive intestinal peptide (VIP), the avian PRL-releasing hormone. PRL was lowest in non-paired birds, increased after pair bonds had formed, and was highest in incubating birds. No differences in PRL levels were found between males and females in these biparental care-givers. A single injection of VIP resulted in a rapid increase in plasma PRL in nonbreeding zebra finches, while PRL remained unchanged in incubating birds. When escalating doses of VIP were administered, nonbreeders responded with a maximal response in PRL release, but PRL levels in breeders remained unchanged following even the highest VIP dose. Among nonbreeders, inexperienced birds had significantly lower PRL levels than birds that had successfully reared a clutch, but both groups responded with an equally robust increase in PRL following a VIP challenge. This pattern differs from that observed in most photosensitive species in which only during a breeding cycle do birds secrete significant levels of PRL in response to exogenous VIP. Zebra finches, even when not actively breeding, must maintain competent pituitary lactotrophs that can secrete PRL at maximal rates. This is part of the suite of characters enabling these birds to respond to favorable breeding conditions at any time.

Regulation of prolactin receptor levels and activity in breast cancer

From its traditional identity as a hormone involved in growth and differentiation of mammary epithelium and in lactation, to having a pertinent role in the development of mammary carcinoma, the peptide hormone/cytokine prolactin (PRL) has emerged as a versatile signaling molecule. There has been significant progress in our understanding of the fine working of PRL in the past several years. Notably, much effort has been concentrated on the mediator of PRL action, namely, the prolactin receptor (PRLr). The causal link between increased PRLr expression and breast cancer is being increasingly appreciated. Considering that the level of the receptor on the surface is a critical determinant of signaling output in response to PRL, the uncovering of regulatory elements that control receptor expression becomes important. The principle focus of this review is on the regulation of PRLr expression and activity in breast cancer with a brief overview of different isoforms of PRLr, their expression, signaling capabilities and the biological outcomes of PRL/PRLr signaling.

What can we learn from rodents about prolactin in humans?

Prolactin (PRL) is a 23-kDa protein hormone that binds to a single-span membrane receptor, a member of the cytokine receptor superfamily, and exerts its action via several interacting signaling pathways. PRL is a multifunctional hormone that affects multiple reproductive and metabolic functions and is also involved in tumorigenicity. In addition to being a classical pituitary hormone, PRL in humans is produced by many tissues throughout the body where it acts as a cytokine. The objective of this review is to compare and contrast multiple aspects of PRL, from structure to regulation, and from physiology to pathology in rats, mice, and humans. At each juncture, questions are raised whether, or to what extent, data from rodents are relevant to PRL homeostasis in humans. Most current knowledge on PRL has been obtained from studies with rats and, more recently, from the use of transgenic mice. Although this information is indispensable for understanding PRL in human health and disease, there is sufficient disparity in the control of the production, distribution, and physiological functions of PRL among these species to warrant careful and judicial extrapolation to humans.

Insulin-like growth factor-I mRNA and peptide in the human anterior pituitary

The pituitary is the central organ regulating virtually all endocrine processes, and pathologies of the pituitary cause manifold adverse effects. Because insulin-like growth factor (IGF)-I appears to be involved in tumour pathogenesis, progression, and persistence, and only few data exist on the cellular synthesis sites of IGF-I, the present study aims to create a basis for further research on pituitary adenomas by investigating the presence of IGF-I in the human pituitary using reverse transcriptase-polymerase chain reaction, in situ hybridisation, immunohistochemistry and immunocytochemistry. IGF-I was expressed in the pituitary, and gene sequence analysis revealed a sequence identical to that found in human liver. The distribution pattern of IGF-I mRNA found by in situ hybridisation corresponded to that of IGF-I peptide in immunohistochemistry. In all pituitary samples investigated, IGF-I-immunoreactivity occurred in almost all adrenocorticotrophic hormone (ACTH)-immunoreactive cells. Occasionally, an interindividually varying number of growth hormone (GH) and, infrequently, follicle-stimulating hormone and luteinising hormone cells contained IGF-I-immunoreactivity but none was detected in supporting cells. At the ultrastructural level, IGF-I-immunoreactivity was confined to secretory granules in coexistence with ACTH- or GH-immunoreactivity, respectively, indicating a concomitant release of the hormones. Thus, in humans, IGF-I appears to be a constituent in ACTH cells whereas its production in GH-producing and gonadotrophic cells may depend on the physiological status (e.g. serum IGF-I level, age or reproductive phase). It is assumed that locally produced IGF-I plays a crucial role in the regulation of endocrine cells by autocrine/paracrine mechanisms in addition to the endocrine route.

Insulin-like growth factor I (IGF-I) and its receptor (IGF-1R) in the rat anterior pituitary

Few and controversial results exist on the cellular sites of insulin-like growth factor (IGF)-I synthesis and the type 1 IGF receptor (IGF-1R) in mammalian anterior pituitary. Thus, the present study analysed IGF-I and the IGF-1R in rat pituitary. Reverse transcription-polymerase chain reaction revealed IGF-I and IGF-1R mRNA expression in pituitary. The sequences of both were identical to the corresponding sequences in other rat organs. In situ hybridization localized IGF-I mRNA in endocrine cells. The majority of the growth hormone (GH) cells and numerous adrenocorticotropic hormone (ACTH) cells exhibited IGF-1R-immunoreactivity at the cell membrane. At lower densities, IGF-1 receptors were also present at the other hormone-producing cell types, indicating a physiological impact of IGF-I for all endocrine cells. IGF-I-immunoreactivity was located constantly in almost all ACTH-immunoreactive cells. At the ultrastructural level, IGF-I-immunoreactivity was confined to secretory granules in co-existence with ACTH-immunoreactivity, indicating a concomitant release of both hormones. Occasionally, IGF-I-immunoreactivity was detected in an interindividually varying number of GH cells. In some individuals, weak IGF-I-immunoreactions were also detected also in follicle-stimulating hormone and luteinizing hormone cells. Thus, IGF-I seems to be produced as a constituent in ACTH cells, possibly indicating its particular importance in stress response. Generally, IGF-I from the endocrine cells may regulate synthesis and/or release of hormones in an autocrine/paracrine manner as well as prevent apoptosis and stimulate proliferation. Production of IGF-I in GH cells may depend on the physiological status, most likely the serum IGF-I level. IGF-I released from GH cells may suppress GH synthesis and/or release by an autocrine feedback mechanism in addition to the endocrine route.

A prospective study of plasma prolactin concentrations and risk of premenopausal and postmenopausal breast cancer

PURPOSE

Epidemiologic studies suggest that prolactin is associated with breast cancer risk in older women. Because of limited prospective data, particularly in younger women, we examined whether prolactin concentrations were associated with breast cancer risk among women 42 to 55 years (68% premenopausal) from the Nurses' Health Study (NHS), and then conducted a pooled analysis of three studies.

PATIENTS AND METHODS

The analysis included 377 cases of breast cancer diagnosed after blood draw and before June 2000; two controls were matched per case on age, menopausal status at blood draw and diagnosis, fasting status, and time of day and month of blood collection. These data were pooled with two previously published data sets from the NHS and NHSII cohorts (n = 1,539 cases, 2,681 controls; ages 32 to 70 years).

RESULTS

Prolactin was modestly associated with an increased breast cancer risk (relative risk [RR], top v bottom quartile = 1.3; 95% CI, 0.9 to 1.9; P for trend = .12). Risk estimates did not vary by menopausal status, tumor invasiveness, or estrogen receptor (ER) status. In the pooled analysis, the overall RR was 1.3 (95% CI, 1.1 to 1.6; P for trend = .002), and did not vary by menopausal status (P for interaction = .95). The risk was strongest for women with ER+ tumors (RR = 1.6; 95% CI, 1.2 to 2.0; P for trend < .001). Correcting for within-person variability, the RR comparing the median of the top versus the bottom prolactin quartile increased from 1.3 to 1.7 for all women and from 1.5 to 2.1 for ER+ cases.

CONCLUSION

These data, in conjunction with experimental studies, indicate that prolactin likely is important in breast cancer etiology, particularly ER+ tumors, over a wide range of ages.

Detection of molecular variants of prolactin in human serum, evaluation of a method based on ultrafiltration

BACKGROUND

In human blood, there are several molecular variants of prolactin with different biological effects. There is a need for new methods to detect and quantify these variants in order to fully understand the pathophysiological role of prolactin.

METHODS

A method based on ultrafiltration was optimized, validated and compared to PEG precipitation. Serum samples from 84 patients were analyzed before and after pre treatment on two immunoassays, Elecsys (Roche) and Access (Beckman). Protein G precipitation was used to confirm presence of macroprolactin.

RESULTS

The recovery of prolactin after ultrafiltration was lower than after PEG precipitation. A limit of 40% recovery after PEG precipitation corresponded to 27% recovery after ultrafiltration. Using these limits there were total agreement regarding detection of macroprolactin (r(s)=0.96). In contrast, recovery of prolactin in samples without macroprolactin showed a considerable disagreement between ultrafiltration and PEG precipitation (r(s)=0.48). Within-run CV was 4% for the ultrafiltration method. The correlation coefficient (r) between the immunoassays was 0.96 after ultrafiltration.

CONCLUSIONS

Ultrafiltration can be used to compare different prolactin immunoassays and to detect macroprolactin in assays with interference from PEG. For samples without macroprolactin ultrafiltration may give additional information reflecting individual variations of other molecular variants of prolactin.

Determination of Ultrafilterable Prolactin: Elimination of Macroprolactin Interference With a Monomeric Prolactin-Selective Sample Pretreatment

Context.—Macroprolactin (macroPRL), present in as many as 25% of serum specimens with elevated serum prolactin concentrations, can cause apparent hyperprolactinemia in the absence of clinical features and lead to unnecessary clinical, laboratory, and neuroradiological workups.

Objective.—To develop an ultrafiltration method that eliminates macroPRL interference from PRL immunoassays.

Design.—The method involves centrifugation of undiluted serum in a Centricon-100 filter device followed by a PRL assay of the serum ultrafiltrate.

Results.—Ultrafiltrates prepared by this technique are devoid of gamma globulins and contain (mean ± SE) 19% ± 7% of the albumin concentration of the original serum. These ultrafiltrates contain 85% ± 7% of the total PRL immunoreactivity of serum spiked with 23 kd recombinant human prolactin (rHuPRL) and less than 2% of the 50 kd big PRL (bPRL) of whole serum. The fractional recovery of ultrafilterable PRL (uPRL) from serum samples of 54 female patients was 0.78 (confidence interval 0.73–0.83) of the total. The run-to-run coefficient of variation of the uPRL assay was 4.3%. The uPRL concentration (mean ± SD) in a group of healthy female controls was 8.0 ± 3.1 ng/mL.

Conclusions.—Ultrafiltration is a rapid and simple method for eliminating analytical interference by macroPRL. Ultrafiltrates can be analyzed by most, if not all, currently available PRL immunoassays and represent a practical and precise alternative to gel filtration chromatography for the estimation of the monomeric prolactin concentration of serum.

Ligand-Independent Dimerization of the Human Prolactin Receptor Isoforms: Functional Implications

Prolactin (PRL) contributes to the growth of normal and malignant breast tissues. PRL initiates signaling by engaging the PRL receptor (PRLr), a transmembrane (TM) receptor belonging to the cytokine receptor family. The accepted view has been that PRL activates the PRLr by inducing dimerization of the receptor, but recent reports show ligand-independent dimerization of other cytokine receptors. Using coimmunoprecipitation assays, we have confirmed ligand-independent dimerization of the PRLr in T47D breast cancer and HepG2 liver carcinoma cells. In addition, mammalian cells transfected with differentially epitope-tagged isoforms of the PRLr indicated that long, intermediate, and DeltaS1 PRLrs dimerized in a ligand-independent manner. To determine the domain(s) involved in PRLr ligand-independent dimerization, we generated PRLr constructs as follows: (1) the TM-ICD, which consisted of the TM domain and the intracellular domain (ICD) but lacked the extracellular domain (ECD), and (2) the ECD-TM, which consisted of the TM domain and the ECD but lacked the ICD. These constructs dimerized in a ligand-independent manner in mammalian cells, implicating a significant role for the TM domain in this process. These truncated PRLrs were functionally inert alone or in combination in cells lacking the PRLr. However, when introduced into cells containing endogenous PRLr, the ECD-TM inhibited human PRLr signaling, whereas the TM-ICD potentiated human PRLr signaling. These studies indicate that the ECD-TM and the TM-ICD are capable of modulating PRLr function. We also demonstrated an endogenous TM-ICD in T47D cells, suggesting that these findings are relevant to PRL-signaling pathways in breast cancer.

Therapeutic potential of S179D prolactin – from prostate cancer to angioproliferative disorders: the first selective prolactin receptor modulator

Increasing evidence suggests an important role for autocrine/paracrine prolactin in breast and prostate cancers and other disease states. Prolactin production in these extrapituitary sites is not governed by dopamine agonists, a finding that has spurred the production of prolactin receptor antagonists. This review focuses on one such antagonist, S179D prolactin, which was produced by mimicking a natural antagonist, phosphorylated prolactin. S179D prolactin is a very effective growth antagonist, partly because it inhibits signalling from unmodified prolactin and partly because it produces its own intracellular signal. This signal results in cell differentiation, cell-cycle arrest or apoptosis depending on dose, duration of treatment and cellular context. S179D prolactin is also a potent antiangiogenic and initial studies have shown it to be a potent anti-inflammatory agent. In light of these additional modes of action, it is suggested that S179D prolactin should now be more aptly referred to as a selective prolactin receptor modulator.

Association between Plasma Prolactin Concentrations and Risk of Breast Cancer among Predominately Premenopausal Women

Recent evidence suggests that prolactin may be positively associated with postmenopausal breast cancer risk; however, little data are available in younger women. Therefore, we conducted a prospective, nested case-control study to examine the relationship between plasma prolactin concentrations and breast cancer risk in predominately premenopausal women from the Nurses' Health Study II. Blood samples were collected from 1996 to 1999. The analysis includes 316 cases of breast cancer diagnosed after blood donation and before June 1, 2003, who had two controls matched on age, fasting status, time of day and month of blood collection, race/ethnicity, and timing of blood draw within the menstrual cycle. Sixty-three percent of participants provided a timed follicular and luteal menstrual phase blood sample; other women provided a single untimed sample. When including all women, we observed a positive association between prolactin and breast cancer risk [relative risk (RR), top quartile versus bottom quartile, 1.5; 95% confidence interval (95% CI), 1.0-2.3; P(trend) = 0.03] that was slightly stronger among estrogen receptor-positive/progesterone receptor-positive tumors (comparable RR, 1.9; 95% CI, 1.1-3.3; P(trend) = 0.04). Associations were similar among premenopausal women only. However, we did not find an association between prolactin and breast cancer risk among the subset of women who only provided timed samples (comparable RR, average of timed samples, 1.3; 95% CI, 0.8-2.3; P(trend) = 0.40). The association seemed stronger among women > or = 45 years old and for cases diagnosed within approximately 4 years of blood collection. Our data suggest a modest positive association between prolactin and breast cancer risk among predominately premenopausal women; however, further follow-up is needed to increase power for subgroup analyses.

S179D prolactin increases vitamin D receptor and p21 through up-regulation of short 1b prolactin receptor in human prostate cancer cells

In this study, we further investigated the mechanisms by which pseudophosphorylated prolactin (S179D PRL) inhibits the growth of human prostate cancer cells. When treated with S179D PRL for 3 days, LnCAP cells responded by increasing expression of the vitamin D receptor (VDR) and the cell cycle regulatory molecule, p21, whereas PC3 and DU145 cells did not. After 5 days of treatment, both PC3 and DU145 cells responded. Untreated LnCAP cells express the short 1b form (SF1b) of the human prolactin receptor, but DU145 and PC3 cells express only low amounts of this receptor until elevated by treatment with S179D PRL. DU145 and PC3 cells become sensitive to the negative effects of S179D PRL on cell number after induction of the SF1b. Transfection of either SF1b or SF1a into PC3 or DU145 cells made them sensitive to S179D PRL in the 3-day time frame, a finding that was not duplicated by transfection with the long form of the receptor. Treatment of LnCAP cells with S179D PRL increased long-term activation of extracellular signal-regulated kinase 1/2 (ERK1/2). This did not occur in PC3 and DU145 cells until transfection with SF1a/SF1b. Blockade of ERK signaling eliminated S179D PRL-stimulated expression of the VDR and p21 in LnCAP cells and transfected PC3 and DU145 cells. We conclude that initiation of alternative splicing to produce SF1b, and subsequent altered signaling, contribute to the growth inhibitory mechanisms of S179D PRL. This is the first indication of a role for short prolactin receptors in the regulation of cell proliferation.

Proteasomes mediate prolactin-induced receptor down-regulation and fragment generation in breast cancer cells

Prolactin regulates a variety of physiological processes, including mammary gland growth and differentiation, and recent findings support an important role in breast cancer development and progression. However, little is known about the trafficking of its receptor, a member of the cytokine receptor superfamily. In the present study, we examined the effect of ligand on the endogenous "long" isoform of the prolactin receptor in breast cancer cells. We found that prolactin caused rapid and prolonged down-regulation of this receptor. The prolactin-induced increase in degradation was blocked by inhibitors of both proteasomes and lysosomes. However, the ubiquitin-conjugating system was not required for internalization. Prolactin also resulted in the concomitant appearance of a cell-associated prolactin receptor fragment containing the extracellular domain. This latter process required proteasomal, but not metalloprotease, activity, distinguishing it from ectodomain "shedding" of other membrane receptors, which are secreted as binding proteins. The prolactin receptor fragment was labeled by surface biotinylation and independent of protein synthesis. Together, these data indicated that prolactin binding initiates limited proteasomal cleavage of its receptor, generating a cell-associated fragment containing the extracellular domain. Our findings described a new potential mediator of prolactin action and a novel mechanism whereby proteasomes modulate cellular processes.

Unmodified Prolactin (PRL) and S179D PRL-Initiated Bioluminescence Resonance Energy Transfer between Homo- and Hetero-Pairs of Long and Short Human PRL Receptors in Living Human Cells

We have used bioluminescence resonance energy transfer (BRET) to examine the interaction between human prolactins (PRLs) and the long (LF) and two short isoforms (SF1a and SF1b) of the human PRL receptor in living cells. cDNA sequences encoding the LF, SF1a, and SF1b were subcloned into codon-humanized vectors containing cDNAs for either Renilla reniformis luciferase (Rluc) or a green fluorescent protein (GFP2) with a 12- or 13-amino acid linker connecting the parts of the fusion proteins. Transfection into human embryonic kidney 293 cells demonstrated maintained function of Rluc and GFP2 when linked to the receptors, and confocal microscopy demonstrated the localization of tagged receptors in the plasma membrane by 48 h after transfection. All three tagged receptors transduced a signal, with the LF and SF1a stimulating, and SF1b inhibiting, promoter activity of an approximately 2.4-kb β-casein-luc construct. Both unmodified PRL (U-PRL) and the molecular mimic of phosphorylated PRL, S179D PRL, induced BRET with all combinations of long and short receptor isoforms except SF1a plus SF1b. No BRET was observed with the site two-inactive mutant, G129R PRL. This is the first demonstration, 1) that species homologous PRL promotes both homo- and hetero-interaction of most long and short PRLR pairs in living cells, 2) that both U-PRL and S179D PRL are active in this regard, and 3) that there is some aspect of SF1a-SF1b structure that prevents this particular hetero-receptor pairing. In addition, we conclude that preferential pairing of different receptor isoforms is not the explanation for the different signaling initiated by U-PRL and S179D PRL.

Development and potential clinical uses of human prolactin receptor antagonists

There is a large body of literature showing that prolactin (PRL) exerts growth-promoting activities in breast cancer, and possibly in prostate cancer and prostate hyperplasia. In addition, increasing evidence argues for the involvement of locally produced (autocrine) PRL, perhaps even more than pituitary-secreted (endocrine) PRL, in tumor growth. Because dopamine analogs are unable to inhibit PRL production in extrapituitary sites, alternative strategies need investigation. To that end, several PRL receptor antagonists have been developed by introducing various mutations into its natural ligands. For all but one of these analogs, the mechanism of action involves a competition with endogenous PRL for receptor binding. Such compounds are thus candidates to counteract the undesired actions of PRL, not only in tumors, but also in dopamine-resistant prolactinomas. In this review, we describe the different versions of antagonists that have been developed, with emphasis on the controversies regarding their characterization, and the limits for their potential development as a drug. The most recently developed antagonist, Delta1-9-G129R-hPRL, is the only one that is totally devoid of residual agonistic activity, meaning it acts as pure antagonist. We discuss to what extent this new molecule could be considered as a lead compound for inhibiting the actions of human PRL in the above-mentioned diseases. We also speculate on the multiple questions that could be addressed with respect to the therapeutic use of PRL receptor antagonists in patients.

Prolonged lymphopenia, lymphoid depletion, and hypoprolactinemia in children with nosocomial sepsis and multiple organ failure

Lymphopenia and lymphoid depletion occur in adults dying of sepsis. Prolactin increases Bcl-2 expression, suppresses stress-induced lymphocyte apoptosis, and improves survival from experimental sepsis. We hypothesized that prolonged lymphopenia, lymphoid depletion, and hypoprolactinemia occur in children dying with sepsis and multiple organ failure (MOF). Fifty-eight critically ill children with and 55 without MOF admitted to a university hospital pediatric intensive care unit were enrolled in a prospective, longitudinal, observational clinical study. Prolactin levels and absolute lymphocyte count were measured on days 1, 3, 7, 14, and 21. Lymph node, thymus, and spleen autopsy specimens were examined for lymphoid depletion, with immunohistochemical staining for CD4, CD20, and CD21 and for lymphoid apoptosis. Prolonged lymphopenia (absolute lymphocyte count < 1000 for >7 days) occurred only in children with MOF (29 vs 0%, p < 0.05) and was associated independently with nosocomial infection (odds ratio (OR), 5.5, 95% confidence interval (CI), 1.7-17, p < 0.05), death (OR, 6.8, 95% CI, 1.3-34, p < 0.05), and splenic and lymph node hypocellularity (OR, 42, 95% CI, 3.7-473, p < 0.05). Lymphocyte apoptosis and ante/postmortem infection were observed only in children with lymphoid depletion. Prolonged hypoprolactinemia (>7 days) was more common in children with MOF (17 vs 2%, p < 0.05) and was associated independently with prolonged lymphopenia (OR, 8.3, 95% CI, 2.1-33, p < 0.05) and lymphoid depletion (OR, 12.2, 95% CI, 2.2-65, p < 0.05). Prolonged lymphopenia and apoptosis-associated depletion of lymphoid organs play a role in nosocomial sepsis-related death in critically ill children. Prolonged hypoprolactinemia is a previously unrecognized risk factor for this syndrome.

Hyperprolactinaemia does not always mean ‘hyperprolactinaemia’!

The presence of macroprolactinaemia was investigated in the symptom-free hyperprolactinaemia cases to reveal its incidence. The serum prolactin (PRL) fractions in 21 female patients with hyperprolactinaemia without any clinical symptoms were analyzed with PEG (polyethylene glycol precipitation) procedure. In 14 of these 21 cases, hyperprolactinaemia was detected with a high fraction of macroprolactin. In cases with asymptomatic hyperprolactinaemia, it is more appropriate to employ the PEG precipitation test to detect the disorder. High levels of serum prolactin, do not essentially indicate the presence of a prolactinoma but may only indicate macroprolactinaemia.

Application of difference gel electrophoresis to the identification of inner medullary collecting duct proteins

In this study, we present a standardized approach to purification of native inner medullary collecting duct (IMCD) cells from rat kidney for proteomic analysis and apply the approach to identification of abundant proteins utilizing two-dimensional difference gel electrophoresis (DIGE) coupled with matrix-assisted laser desorption-ionization-time of flight mass spectrometry. Fractionation of inner medullary cell suspensions by low-speed centrifugation gave a highly purified IMCD cell fraction in which aquaporin-2 was enriched 10-fold. When DIGE was initially applied to rat inner medullas fractionated into IMCD cells (labeled with Cy3) and non-IMCD cells (labeled with Cy5), we identified 50 highly abundant proteins expressed in the IMCD cells. These proteins, identifiable without subcellular fractionation, included chiefly enzymes, structural proteins, and signaling intermediates. An additional 35 proteins were found predominantly in the non-IMCD cell types. Proteins that were highly enriched in the IMCD fraction included cytokeratin 8, cytokeratin 18, transglutaminase II, aminopeptidase B, T-plastin, heat shock protein (HSP) 27, HSP70, and lactate dehydrogenase A. Semiquantitative immunoblotting and immunohistochemistry confirmed relative expression levels and distribution of selected proteins. An additional 40 IMCD proteins were identified in separate experiments aimed at further enrichment of proteins through optimization of sample loading. These studies document the applicability of a standardized approach to purification of IMCD cells for proteomic analysis of IMCD proteins and demonstrate the feasibility of large scale identification of proteins in the native IMCD cell.

Prolactin and the prolactin receptor: new targets of an old hormone

Prolactin (PRL) is one of a family of related hormones including growth hormone (GH) and placental lactogen (PL) that are hypothesized to have arisen from a common ancestral gene about 500 million years ago. Over 300 different functions of PRL have been reported, highlighting the importance of this pituitary hormone. PRL is also synthesized by a number of extra-pituitary tissues including the mammary gland and the uterus. Most of PRL's actions are mediated by the unmodified 23 kDa peptide, however, PRL may be modified post-translation, thereby altering its biological effects. PRL exerts these effects by binding to its receptor, a member of the class I cytokine receptor super-family. This activates a number of signaling pathways resulting in the transcription of genes necessary for the tissue specific changes induced by PRL. Mouse knockout models of the major forms of the PRL receptor have confirmed the importance of PRLs role in reproduction. Further knockout models have provided insight into the importance of PRL signaling intermediates and the advent of transcript profiling has allowed the elucidation of a number of PRL target genes.

Macroprolactinemia: a new cause of hyperprolactinemia

Hyperprolactinemia is not only seen in pregnancy but also in several pathological conditions such as prolactin (PRL) secreting pituitary adenoma (prolactinoma), intracranial tumors compressing the pituitary stalk or hypothalamus, and PRL stimulative drugs. However, some patients with hyperprolactinemia are diagnosed as having idiopathic hyperprolactinemia because the causes are unknown. They are subjected to repeated radiological examinations to find a microadenoma, to a long-term treatment with bromocriptine, and even to a surgical intervention. There is accumulating evidence that macroprolactinemia, in which most circulating PRL forms large protein complexes (more than 150 kDa), is a major cause of idiopathic hyperprolactinemia. The patients with macroprolactinemia are clinically characterized by the lack of hyperprolactinemia-related symptoms such as amenorrhea and galactorrhea. We found that anti-PRL autoantibody is a leading cause of macroprolactinemia that might be heterogeneous in nature. Most patients with anti-PRL autoantibodies were symptom-free and pregnancy was possible despite a marked hyperprolactinemia. Identification of macroprolactinemia is clinically important to prevent unnecessary examinations and treatments in patients with idiopathic hyperprolactinemia.

The growth hormone-binding protein is a location-dependent cytokine receptor transcriptional enhancer

In the rat, a growth hormone-binding protein (GHBP) exists that is derived from the growth hormone (GH) receptor gene by an alternative mRNA splicing mechanism such that the transmembrane and intracellular domains of the GH receptor are replaced by a hydrophilic carboxyl terminus. In isolation, the GHBP is inactive, although it does compete with the receptor for ligand binding in the extracellular space and therefore inhibits the cellular response to GH. The GHBP is also located intracellularly and is translocated to the nucleus upon ligand stimulation. We show here that endogenously produced GHBP, in contrast to exogenous GHBP, was able to enhance the STAT5-mediated transcriptional response to GH. Interestingly, when the GHBP was targeted constitutively to the nucleus by the addition of the nuclear localization sequence of the SV40 large T antigen, greater enhancement of STAT5-mediated transcription was obtained. The transcriptional enhancement did not require GH per se and was not specific to the GH receptor, since similar enhancement of STAT5-mediated transcription by nuclear localized GHBP was obtained with specific ligand stimulation of both prolactin and erythropoietin receptors. Thus, the GHBP exerts divergent effects on STAT5-mediated transcription depending on its cellular location. The use of a soluble cytokine receptor as a location-dependent transcriptional enhancer and the ligand-independent involvement of the extracellular domain of a polypeptide ligand receptor in intracellular signal transduction provide additional novel mechanisms of transcriptional control.

The role of prolactin in mammary carcinoma

The contribution of prolactin (PRL) to the pathogenesis and progression of human breast cancer at the cellular, transgenic, and epidemiological levels is increasingly appreciated. Acting at the endocrine and autocrine/paracrine levels, PRL functions to stimulate the growth and motility of human breast cancer cells. The actions of this ligand are mediated by at least six recognized PRL receptor isoforms found on, or secreted by, human breast epithelium. The PRL/PRL receptor complex associates with and activates several signaling networks that are shared with other members of the cytokine receptor superfamily. Coupled with the recently identified intranuclear function of PRL, these networks are integrated into the in vitro and in vivo actions induced by ligand. These findings indicate that antagonists of PRL/PRL receptor interaction or PRL receptor-associated signal transduction may be of considerable utility in the treatment of human breast cancer.

Prolactin receptor signal transduction

Within the immune system, multiple isoforms of the human prolactin receptor (PRLr) serve to mediate the effects of its ligand (PRL). Now numbering four, these isoforms are structurally and functionally distinct, demonstrating significant differences in ligand affinities, kinetics of transduction and the transduction proteins activated. The proximal transduction pathways activated during PRLr-associated signaling include the tyrosine kinases Jak2, Fyn and Tec, the phosphatase SHP-2, the guanine nucleotide exchange factor Vav, and the signaling suppressor SOCS. Differential activation of these pathways may contribute to the pleiotropism of PRL action in tissues of the immune system.

The role of prolactin in fish osmoregulation: a review

The protein hormone prolactin (PRL) was first discovered as an anterior pituitary factor capable of stimulating milk production in mammals. We now know that PRL has over 300 different functions in vertebrates. In fish, PRL plays an important role in freshwater osmoregulation by preventing both the loss of ions and the uptake of water. This paper will review what is currently known about the structure and evolution of fish PRL and its mechanisms of action in relation to the maintenance of hydromineral balance. Historically, functional studies of fish PRL were carried out using heterologous PRLs and the results varied greatly between experiments and species. In some cases this variability was due to the ability of these PRLs to bind to both growth hormone and PRL receptors. In fact, a recurring theme in the literature is that the actions of PRL cannot be generalized to all fish due to marked differences between species. Many of the effects of PRL on hydromineral balance are specific to euryhaline fish, which is appropriate given that they frequently experience sudden changes in environmental salinity. Much of the recent work has focused on the isolation and characterization of fish PRLs and their receptors. These studies have provided the necessary tools to obtain a better understanding of the evolution of PRL and its role in osmoregulation.

A serum prolactin-binding protein: implications for growth hormone

Many membrane receptors have truncated soluble forms that circulate in the blood, and a protein in serum with characteristics of the extracellular domain of the human prolactin receptor has recently been identified. Because the extracellular domain of the prolactin receptor binds human growth hormone, does the prolactin-binding protein in serum affect the amount of bound circulating growth hormone? A more general question is whether this serum protein has a minor or a major effect on the biological activities of growth hormone and prolactin.