S179D prolactin: antagonistic agony!

[Prolactin and its receptor: From animal models to pituitary pathophysiology]

Prolactin (PRL) is a polypeptide hormone that is mainly synthesized and secreted by lactotroph cells of the anterior pituitary gland. The actions of prolactin are mediated by its transmembrane receptor, PRLR. The principal role attributed to PRL is to stimulate the proliferation and differentiation of the mammary cells required for lactation, but studies of animal models have assigned more than 300 separate actions to this hormone in various species. Hyperprolactinaemia is the prototypical pathological state associated with this hormone. Indeed, hyperprolactinaemia is the most common cause of amenorrhoea due to hypogonadotropic anovulation and is one of the most prevalent endocrine causes of infertility in women. In recent years, the study of conditional or complete Prlr ^-/- mouse models had improved the understanding concerning the regulation of gonadotroph and lactotroph axes. It is now demonstrated that prolactin exerts autocrine or paracrine actions on lactotroph cells in vivo. One of the major advances was to better understand, using mouse models, the impact of hyperprolactinemia on gonadotroph axis. It is now accepted that hypogonadotropic hypogonadism in patients with hyperprolactinemia is mediated by a decrease of hypothalamic kisspeptin secretion. Gonadotroph axis can be restored by intravenous administration of kisspeptin. However, the mechanisms of lactotroph tumorigenesis in Prlr ^-/- animals remain incompletely understood and transposable to the human species, since the only patient with biallelic PRLR loss-of-function mutation leading to complete prolactin resistance that has been described so far did not have pituitary adenoma visible on MRI.

Recent findings on hyperprolactinemia and its pathological implications: a literature review

The prolactin hormone (PRL) is often secreted by lactotrophic cells of the anterior pituitary and has been shown to play a role in various biological processes, including breast feeding and reproduction. The predominant form of this hormone is the 23 kDa form and acts through its receptor (PRLR) on the cell membrane. This receptor is a member of the superfamily of hematopoietic/cytokine receptors. PRL also has a 16 kDa subunit with anti-angiogenic, proapoptotic, and anti-inflammatory effects which is produced by the proteolytic breakdown of this hormone under oxidative stress. Although the common side effects of hyperprolactinemia are exerted on the reproductive system, new studies have shown that hyperprolactinemia has a wide variety of effects, including playing a role in the development of autoimmune diseases and increasing the risk of cardiovascular disease, peripartum cardiomyopathy, and diabetes among others. The range of PRL functions is increasing with the discovery of multiple sites of PRL secretion as well as PRLR expression in various tissues. This review summarizes current knowledge of the biology of PRL and its receptor, as well as the role of PRL in human pathophysiology.

Estrogen induces phosphorylation of prolactin through p21-activated kinase 2 activation in the mouse pituitary gland

A phosphorylated prolactin is a kind of modified prolactin, which is produced through phosphorylation of native prolactin (PRL) by p21-activated kinase 2 (PAK2) in secretory granules in lactotrophs. Phosphorylated prolactin is involved in the regulation of the estrous cycle and in apoptosis in cancer cells, which seem to have important physiological and pathological roles, respectively. In previous research, it has been reported that estrogen induced the phosphorylation of prolactin in the mouse pituitary gland. However, the relationship between estrogen and PAK2 in the production of phosphorylated PRL has not been clarified yet. In order to examine whether PAK2 is involved in PRL phosphorylation by estrogen, we analyzed PAK2 protein levels in mice and phosphorylated prolactin levels in mouse pituitary cells by western blot analysis. The ratio of phosphorylated PAK2/total PAK2 was increased in estrogen implanted mice, but PAK2 protein and gene expression levels were decreased. In addition, the ratio of phosphorylated prolactin/non-phosphorylated prolactin was decreased in primary pituitary cells with introduced siPAK2. These findings suggest that estrogen could induce the phosphorylation of PRL through PAK2 activation. Therefore, this study contributes to better understanding of the mechanism of phosphorylated PRL production in physiological and pathological conditions associated with estrogen.

A novel type of prolactin expressed in the bullfrog pituitary specifically during the larval period

Prolactin (PRL) is one of the major hormones that control amphibian metamorphosis. Recently, a PRL (PRL1B) gene that is different from the known PRL (PRL1A) gene has been found in the genomes of several amphibian species. In order to ascertain whether the PRL1B gene is expressed in the bullfrog (Rana catesbeiana) pituitary, cloning of cDNA encoding PRL1B in the pituitary of the premetamorphic bullfrog tadpole was attempted. The bullfrog PRL1B amino acid sequence predicted from the obtained cDNA showed 62% identity with those of Xenopus PRL1Bs that have been presumed from the genome sequences, whereas the sequence identity between bullfrog PRL1A and PRL1B was 48%. A molecular phylogenetic tree showed that bullfrog PRL1B is most appropriately grouped with amphibian PRL1Bs. Quantitative PCR analysis revealed that the mRNA expression levels of bullfrog PRL1B in the pituitary were high during pre- and prometamorphosis, sharply declined at metamorphic climax and became undetectable after metamorphosis. In contrast, PRL1A mRNA levels were relatively low during pre- and prometamorphosis, rose at climax and remained high after metamorphosis. Immunohistochemical study using antibodies against partial peptides of PRL1A and PRL1B revealed that most of the PRL1A- and PRL1B-immunoreactive cells in the larval pituitary were distributed separately, but that some of the cells immunoreactive with both antibodies were also present. Western blot analysis with the larval pituitary extract indicated that PRL1B-immunoreactive band appeared at the position of molecular weight ca. 22.1 kDa and PRL1A-immunoreactive band at the position of ca. 22.8 kDa. The results obtained in this experiment suggest the possibility that PRL1B plays as-yet-unknown role(s) during the pre-climactic period of metamorphosis. This is the first report on the existence of PRL1B as a protein in the amphibian larval pituitary.

Prolactin Biology and Laboratory Measurement: An Update on Physiology and Current Analytical Issues

Prolactin is a 23 kDa single chain protein of 199 amino acids synthesised and released principally by lactotrophs in the anterior pituitary gland. The secretion is mainly under inhibitory control by hypothalamic dopamine and regulated in a negative feedback manner, with prolactin itself providing the afferent signal: short-loop feedback. The main function of prolactin is during pregnancy and lactation in the development of mammary glands, milk synthesis and maintenance of milk secretion. Serum prolactin levels rise rapidly during pregnancy with increase in the size and number of lactotrophs. During lactation suckling induces rapid secretion of prolactin via a neuroendocrine reflex pathway. In the absence of pregnancy, hyperprolactinaemia may present with symptoms of hypogonadotropic hypogonadism including menstrual disturbance and infertility or visual symptoms from a pituitary mass effect by a prolactinoma, the most common pituitary tumour. Hyperprolactinaemia is diagnosed by laboratory measurement of serum prolactin. There is considerable variability in routinely available prolactin immunoassays as a result of differing reactivity towards monomeric prolactin and macroprolactin and lack of commutability of the WHO 3^rd International Standard between routine methods. Macroprolactinaemia is a relatively common cause of interference in the prolactin assay that may lead to incorrect diagnosis and unnecessary investigations. Measurement of prolactin post polyethylene glycol precipitation (PEG) when prolactin levels are above the reference interval is routinely used to identify macroprolactin, however harmonisation of PEG precipitation process and reporting may improve clinical care.

BMAL1 facilitates trophoblast migration and invasion via SP1-DNMT1/DAB2IP pathway in recurrent spontaneous abortion

The underlying mechanism about rhythms and epigenetics leading to aberrant trophoblast migration and invasion in recurrent spontaneous abortion (RSA) remains unknown. Brain and muscle ARNT-like protein 1 (BMAL1) is considered as a crucial role in fertility, and polymorphism of BMAL1 gene has been reported to be associated with risk of miscarriage. However, the functional role of BMAL1 in RSA is not fully understood. Previous study shows the descended expression of DNA 5′-cytosine-methyltransferases 1 (DNMT1) in the villous of early pregnancy loss. Thus, understanding of the regulation of DNMT1 expression may be of significance for the elucidation of the process of RSA. Using HTR-8/SVneo and JEG-3 cell lines, we certified the induction of specificity protein 1 (SP1) to DNMT1 and DAB2 interaction protein (DAB2IP), respectively, both of which further activated matrix metallo-proteinase 2/9 (MMP2/9), bringing out changes in trophoblast migration and invasion. Notably, BMAL1 functioned as a positive upstream factor of SP1 only in HTR-8/SVneo cells but not in JEG-3 cells, inducing SP1-DNMT1/DAB2IP pathway and facilitating migration and invasion of trophoblasts. In addition, progesterone might restore the down-regulation of BMAL1 and downstream pathway in a dose-dependent manner. Last but not least, the decreased abundance of BMAL1 was correlated positively with that of SP1, DNMT1, DAB2IP, MMP2 and MMP9 in human villous specimens of RSA. Our results demonstrate that the induction of BMAL1 to SP1 contributes to the expression of DNMT1 and DAB2IP, respectively, activating trophoblast migration and invasion. The deregulation of the BMAL1-mediated pathway in RSA can be rescued by progesterone.

Targeting Prolactin Receptor (PRLR) Signaling in PRLR-Positive Breast and Prostate Cancer

In this issue of The Oncologist, Agarwal et al. report negative results from a phase I trial of LFA102. Although “negative” in terms of antitumor activity, the study provides useful pharmacokinetic and pharmacodynamic information. Future trials evaluating PRLR blockers alone and in combination with other agents may still be warranted in patients with breast and prostate cancer.

The prolactin receptor as a therapeutic target in human diseases: browsing new potential indications

INTRODUCTION

Prolactin (PRL) signaling has emerged as a relevant target in breast and prostate cancers. This has encouraged various laboratories to develop compounds targeting the PRL receptor (PRLR). As the latter is widely distributed, it is timely to address whether other conditions could also benefit from such inhibitors.

AREAS COVERED

The authors briefly overview the two classes of PRLR blockers, which involve: i) PRL-core based analogs that have been validated as competitive antagonists in various preclinical models, and ii) anti-PRLR neutralizing antibodies that are currently in clinical Phase I for advanced breast and prostate cancers. The main purpose of this review is to discuss the multiple organs/diseases that may be considered as potential targets/indications for such inhibitors. This is done in light of reports suggesting that PRLR expression/signaling is increased in disease, and/or that systemic or locally elevated PRL levels correlate with (or promote) organ pathogenesis.

EXPERT OPINION

The two immediate challenges in the field are i) to provide the scientific community with potent anti-prolactin receptor antibodies to map prolactin receptor expression in target organs, and ii) to take advantage of the availability of functionally validated PRLR blockers to establish the relevance of these potential indications in humans.

Proteomics of the human pituitary tissue: bioanalytical methods and applications

The pituitary is the central endocrine gland that plays complex regulatory roles in growth, reproduction and metabolism of the body. The human pituitary tissue proteome has been the target of a number of investigations that applied various combinations of advanced separation techniques, mass spectrometry, and bioinformatics tools. This review describes the main features of the bioanalytical workflows used in pituitary proteomics, and summarizes major applications in pituitary proteome mapping, differential protein expression profiling in health and disease, and discovery of post-translational modifications in pituitary proteins.

New insights in prolactin: pathological implications

Prolactin is a hormone that is mainly secreted by lactotroph cells of the anterior pituitary gland, and is involved in many biological processes including lactation and reproduction. Animal models have provided insights into the biology of prolactin proteins and offer compelling evidence that the different prolactin isoforms each have independent biological functions. The major isoform, 23 kDa prolactin, acts via its membrane receptor, the prolactin receptor (PRL-R), which is a member of the haematopoietic cytokine superfamily and for which the mechanism of activation has been deciphered. The 16 kDa prolactin isoform is a cleavage product derived from native prolactin, which has received particular attention as a result of its newly described inhibitory effects on angiogenesis and tumorigenesis. The discovery of multiple extrapituitary sites of prolactin secretion also increases the range of known functions of this hormone. This Review summarizes current knowledge of the biology of prolactin and its receptor, as well as its physiological and pathological roles. We focus on the role of prolactin in human pathophysiology, particularly the discovery of the mechanism underlying infertility associated with hyperprolactinaemia and the identification of the first mutation in human PRLR.

Blockade of estrogen-stimulated proliferation by a constitutively-active prolactin receptor having lower expression in invasive ductal carcinoma

A comprehensive understanding of prolactin's (PRL's) role in breast cancer is complicated by disparate roles for alternatively-spliced PRL receptors (PRLR) and crosstalk between PRL and estrogen signaling. Among PRLRs, the short form 1b (SF1b) inhibits PRL-stimulated cell proliferation. In addition to ligand-dependent PRLRs, constitutively-active varieties, missing the S2 region of the extracellular domain (ΔS2), naturally occur. Expression analysis of the ΔS2 version of SF1b (ΔS2SF1b) showed higher expression in histologically-normal contiguous tissue versus invasive ductal carcinoma. To determine the function of ΔS2SF1b, a T47D breast cancer line with inducible expression was produced. Induction of ΔS2SF1b blocked estrogen-stimulated cell proliferation. Unlike intact SF1b, induction of ΔS2SF1b had no effect on PRL-mediated activation of Stat5a. However induction inhibited estrogen's stimulatory effects on serine-118 phosphorylation of estrogen receptor α, serine-473 phosphorylation of Akt, serine-9 phosphorylation of GSK3β, and c-myc expression. In addition, induction of ΔS2SF1b increased expression of the cell cycle-inhibiting protein, p21. Thus, increased expression of ΔS2SF1b, such as we demonstrate occurs with the selective PRLR modulator, S179D PRL, would create a physiological state in which estrogen-stimulated proliferation was inhibited, but differentiative responses to PRL were maintained.

Prolactin actions

Molecular genetics and other contemporary approaches have contributed to a better understanding of prolactin (PRL) actions at the cellular and organismal levels. In this review, several advances in knowledge of PRL actions are highlighted. Special emphasis is paid to areas of progress with consequences for understanding of human PRL actions. The impacts of these advances on future research priorities are analyzed.

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.

A molecular mimic of phosphorylated prolactin (S179D PRL) secreted by eukaryotic cells has a conformation with an increased positive surface charge compared to that of unmodified prolactin

S179D prolactin (S179D PRL) is a pseudophosphorylated form of human PRL which has potent antitumor and anti-angiogenic activities in vivo. This molecule binds to the same forms of the PRL receptor (PRLR) as unmodified PRL, yet this binding results in different intracellular signaling and biological end points. Since it is now clear that PRLRs are predimerized and therefore that ligand binding must initiate signaling by inducing a conformational change in the receptor dimer, we hypothesized that S179D PRL had an altered conformation compared to unmodified PRL. The conformation of the ligand-receptor ternary complex would therefore also have an altered conformation, and thus, different signaling molecules would be activated. Here we present evidence in support of this hypothesis by demonstrating, in contrast to unmodified PRL, that S179D PRL has reduced nickel and zinc binding capacity and a higher affinity for heparin and DEAE. Conformational changes have occurred since these features are counterintuitive on the basis of the simple substitution of a serine with a negatively charged aspartate residue. To demonstrate that these particular properties of S179D PRL were not due to misfolding of the molecule during production, S179D PRL was expressed in two different mammalian cell lines. Also investigated was the potential for production of S179D PRL as a soluble cytoplasmic, or secreted periplasmic, protein in Escherichia coli.

Prolactin blocks nuclear translocation of VDR by regulating its interaction with BRCA1 in osteosarcoma cells

Based on their content of prolactin receptors, osteosarcoma cells were predicted to be responsive to prolactin (PRL), but whether PRL would be beneficial or contribute to pathogenesis was unclear. 1,25(OH)(2) vitamin D(3) [1alpha,25(OH)(2)D(3)] has antiproliferative effects on osteosarcoma cells, and a complex interregulatory situation exists between PRL and 1alpha,25(OH)(2)D(3). Using osteosarcoma cells, Western blot, real time RT-PCR, and promoter-luciferase assays, we have examined the interaction between PRL and 1alpha,25(OH)(2)D(3) and demonstrated that physiological concentrations of PRL block increased osteocalcin and vitamin D receptor (VDR) expression in response to 1alpha,25(OH)(2)D(3.) This blockade was shown to be the result of lack of nuclear accumulation of the VDR in response to 1alpha,25(OH)(2)D(3). Although inhibition of proteasomic degradation with MG132 had no effect on the VDR itself in a 30-min time frame, it relieved the blockade by PRL. Analysis of ubiquitinated proteins brought down by immunoprecipitation with anti-VDR showed PRL regulation of a 250-kDa protein-VDR complex. P250 was identified as the breast cancer tumor suppressor gene product, BRCA1, by Western blot of the VDR immunoprecipitate and confirmed by immunoprecipitation with anti-BRCA1 and blotting for the VDR in the absence and presence of PRL. Knockdown of BRCA1 inhibited nuclear translocation of the VDR and the ability of 1alpha,25(OH)(2)D(3) to induce the VDR. This, to our knowledge, is the first demonstration of a role for BRCA1 in nuclear accumulation of a steroid hormone and the first demonstration that PRL has the potential to affect the cell cycle through effects on BRCA1.

From bench to bedside: future potential for the translation of prolactin inhibitors as breast cancer therapeutics

A role for prolactin (PRL) in the pathogenesis of breast cancer has been confirmed at the cellular level in vitro, with multiple transgenic and knockout models in vivo, and within sizable patient populations through epidemiologic analysis. It is the obvious "next step" that these findings are translated into meaningful therapies to block PRL/PRLr function in human breast cancer. Several broad categories of PRL/PRLr antagonists are discussed in their pre-clinical context, including inhibitors of endocrine PRL elaboration, mutant ligand antagonists, ligand chimeras, and inhibitors of PRL-induced signaling and transactivation. The clinical potential for GHr antagonists are also discussed. These varied approaches all have demonstrated as proof-of-principle that PRL/PRLr antagonism can inhibit the in vitro and in vivo growth of breast cancer. Further pre-clinical development is required for most, however, before translation to clinical trials in breast cancer patients can occur.

Paradigm-shifters: phosphorylated prolactin and short prolactin receptors

Since the discovery of physiologically-regulated prolactin (PRL) phosphorylation, one focus of the laboratory has been an examination of the different functions of the unmodified and phosphorylated hormone. In the mammary gland, unmodified PRL promotes growth activities, whereas phosphorylated or pseudophosphorylated PRL antagonizes this while also being a superior agonist for changes that favor differentiation. Phosphorylated PRL also increases expression of the short forms of the PRL receptor. These short forms of the receptor have functions beyond the accepted dominant negative and in mammary epithelial cells are capable of generating an intracellular signal leading to increased tight junction formation and beta-casein expression.

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.

Common and specific effects of the two major forms of prolactin in the rat testis

Prolactin (PRL) has both stimulatory and inhibitory effects on testicular function, a finding we hypothesized may be related in some part to the form of the hormone present or administered. In the analysis of the pituitary secretion profiles of early pubescent vs. mature male rats, we found PRL released from early pubescent pituitaries had about twice the degree of phosphorylation. Treatment of mature males with either unmodified PRL (U-PRL) or phosphorylated PRL (via the molecular mimic S179D PRL) for a period of 4 wk (circulating level of approximately 50 ng/ml) showed serum testosterone decreased by approximately 35% only by treatment with the phospho-mimic S179D PRL. Given the specificity of this effect, it was initially surprising that both forms of PRL decreased testicular expression of 3beta-hydroxysteroid dehydrogenase and steroidogenic acute regulatory protein. Both forms also increased expression of the luteinizing hormone receptor, but only S179D PRL increased the ratio of short to long PRL receptors. Endogenous PRL and luteinizing hormone levels were unchanged in all groups in this time frame, suggesting that effects on steroidogenic gene expression were directly on the testis. Terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end-labeling analysis combined with staining for 3beta-hydroxysteroid dehydrogenase and morphometric analysis showed that S179D PRL, but not U-PRL, increased apoptosis of Leydig cells, a finding supported by increased staining for Fas and Fas ligand in the testicular interstitium, providing an explanation for the specific effect on testosterone. S179D PRL, but not U-PRL, also increased apoptosis of primary spermatogonia, and U-PRL, but not S179D PRL, decreased apoptosis of elongating spermatids. Thus, in mature males, hyperprolactinemic levels of both forms of PRL have common effects on steroidogenic proteins, but specific effects on the apoptosis of Leydig and germ cells.