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

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.

Prolactin Attenuates Neuroinflammation in LPS-Activated SIM-A9 Microglial Cells by Inhibiting NF-κB Pathways Via ERK1/2

Prolactin (PRL) is a pleiotropic hormone with multiple functions in several tissues and organs, including the brain. PRL decreases lesion-induced microgliosis and modifies gene expression related to microglial functions in the hippocampus, thereby providing a possible mechanism through which it might participate in neuroimmune modulatory responses and prevent neuronal cell damage. However, the direct contribution of microglial cells to PRL-mediated neuroprotection is still unclear and no studies have yet documented whether PRL can directly activate cellular pathways in microglial cells. The aim of this study is to elucidate in vitro actions of PRL on the immortalized SIM-A9 microglia cell line in basal and LPS-stimulated conditions. PRL alone induced a time-dependent extracellular signal-regulated kinase 1/2 (ERK1/2) activation. Pretreatment with PRL attenuated LPS (200 ng/ml) stimulated pro-inflammatory markers: nitric oxide (NO) levels, inducible nitric oxide synthase (iNOS), interleukins (IL)-6, -1β and tumor necrosis factor (TNF-α) expression at 20 nM dosage. PRL suppressed LPS-induced nuclear factor (NF)-κappaB (NF-κB) p65 subunit phosphorylation and its upstream p-ERK1/2 activity. In conclusion, PRL exhibits anti-inflammatory effects in LPS-stimulated SIM-A9 microglia by downregulating pro-inflammatory mediators corresponding to suppression of LPS-activated ERK1/2 and NF-κB phosphorylation.

The short isoform of PRLR suppresses the pentose phosphate pathway and nucleotide synthesis through the NEK9-Hippo axis in pancreatic cancer

Prolactin binding to the prolactin receptor exerts pleiotropic biological effects in vertebrates. The prolactin receptor (PRLR) has multiple isoforms due to alternative splicing. The biological roles and related signaling of the long isoform (PRLR-LF) have been fully elucidated. However, little is known about the short isoform (PRLR-SF), particularly in cancer development and metabolic reprogramming, a core hallmark of cancer. Here, we reveal the role and underlying mechanism of PRLR-SF in pancreatic ductal adenocarcinoma (PDAC). Methods: A human PDAC tissue array was used to investigate the clinical relevance of PRLR in PDAC. The in vivo implications of PRLR-SF in PDAC were examined in a subcutaneous xenograft model and an orthotopic xenograft model. Immunohistochemistry was performed on tumor tissue obtained from genetically engineered KPC (KrasG12D/+; Trp53R172H/+; Pdx1-Cre) mice with spontaneous tumors. 13C-labeled metabolite measures, LC-MS, EdU incorporation assays and seahorse analyses were used to identify the effects of PRLR-SF on the pentose phosphate pathway and glycolysis. We identified the molecular mechanisms by immunofluorescence, coimmunoprecipitation, proximity ligation assays, chromatin immunoprecipitation and promoter luciferase activity. Public databases (TCGA, GEO and GTEx) were used to analyze the expression and survival correlations of the related genes. Results: We demonstrated that PRLR-SF is predominantly expressed in spontaneously forming pancreatic tumors of genetically engineered KPC mice and human PDAC cell lines. PRLR-SF inhibits the proliferation of PDAC cells (AsPC-1 and BxPC-3) in vitro and tumor growth in vivo. We showed that PRLR-SF reduces the expression of genes in the pentose phosphate pathway (PPP) and nucleotide biosynthesis by activating Hippo signaling. TEAD1, a downstream transcription factor of Hippo signaling, directly regulates the expression of G6PD and TKT, which are PPP rate-limiting enzymes. Moreover, NEK9 directly interacts with PRLR-SF and is the intermediator between PRLR and the Hippo pathway. The PRLR expression level is negatively correlated with overall survival and TNM stage in PDAC patients. Additionally, pregnancy and lactation increase the ratio of PRLR-SF:PRLR-LF in the pancreas of wild-type mice and subcutaneous PDAC xenograft tumors. Conclusion: Our characterization of the relationship between PRLR-SF signaling, the NEK9-Hippo pathway, PPP and nucleotide synthesis explains a mechanism for the correlation between PRLR-SF and metabolic reprogramming in PDAC progression. Strategies to alter this pathway might be developed for the treatment or prevention of pancreatic cancer.

Prolactin Receptor Isoforms as the Basis of Tissue-Specific Action of Prolactin in the Norm and Pathology

The review describes functional and structural features of different isoforms of prolactin receptor, mechanisms of signaling pathway activation, and molecular messengers involved in the transmission and termination of signal from the prolactin receptor isoforms. Changes in the ratio between prolactin receptor isoforms, key mediators of prolactin signal transduction and termination in various organs and tissues, are analyzed. Special attention is given to the role of molecular mediators and the ratio between the isoforms in normal physiological functions and pathologies. Approaches for therapeutic correction of prolactin signaling impairments are discussed.

Involvement of miR-106b in tumorigenic actions of both prolactin and estradiol

Prolactin promotes a variety of cancers by an array of different mechanisms. Here, we have investigated prolactin's inhibitory effect on expression of the cell cycle-regulating protein, p21. Using a miRNA array, we identified a number of miRNAs upregulated by prolactin treatment, but one in particular that was strongly induced by prolactin and predicted to bind to the 3′UTR of p21 mRNA, miR-106b. By creating a p21 mRNA 3′UTR-luciferase mRNA construct, we demonstrated degradation of the construct in response to prolactin in human breast, prostate and ovarian cancer cell lines. Increased expression of miR-106b replicated, and anti-miR-106b counteracted, the effects of prolactin on degradation of the 3′UTR construct, p21 mRNA levels, and cell proliferation in breast (T47D) and prostate (PC3) cancer cells. Increased expression of miR-106b also stimulated migration of the very epithelioid T47D cell line. By contrast, anti-miR-106b dramatically decreased expression of the mesenchymal markers, SNAIL-2, TWIST-2, VIMENTIN, and FIBRONECTIN. Using signaling pathway inhibitors and the 3′UTR construct, induction of miR-106b by prolactin was determined to be mediated through the MAPK/ERK and PI3K/Akt pathways and not through Jak2/Stat5 in both T47D and PC3 cells. Prolactin activation of MAPK/ERK and PI3K/Akt also activates ERα in the absence of an ERα ligand. 17β-estradiol promoted degradation of the construct in both cell lines and pre-incubation in the estrogen antagonist, Fulvestrant, blocked the ability of both prolactin and 17β-estradiol to induce the construct-degrading activity. Together, these data support a convergence of the prolactin and 17β-estradiol miR-106b-elevating signaling pathways at ERα.

Prolactin receptor in breast cancer: marker for metastatic risk

Prolactin and prolactin receptor signaling and function are complex in nature and intricate in function. Basic, pre-clinical and translational research has opened up our eyes to the understanding that prolactin and prolactin receptor signaling function differently within different cellular contexts and microenvironmental conditions. Its multiple roles in normal physiology are subverted in cancer initiation and progression, and gradually we are teasing out the intricacies of function and therapeutic value. Recently, we observed that prolactin has a role in accelerating the time to bone metastasis in breast cancer patients and identified the mechanism by which prolactin stimulated breast cancer cell-mediated lytic osteoclast formation. The possibility that the prolactin receptor is a marker for metastasis, and specifically bone metastasis, is one that may have to be put into the context of the different variants of prolactin, different prolactin receptor isoforms and intricate signaling pathways that are regulated by the microenvironment. The more complete the picture, the better one can test biomarker identity and design clinical trials to test therapeutic intervention. This review will cover the recent advances and highlight the complexity of prolactin receptor biology.

Anti-metastatic outcome of isoform-specific prolactin receptor targeting in breast cancer

Controversy exists concerning the role of the long prolactin receptor (PRLR) in the progression of breast cancer. By targeting pre-mRNA splicing, we succeeded in knocking down only the long PRLR in vivo, leaving the short forms unaffected. Using two orthotopic and highly-metastatic models of breast cancer, one of which was syngeneic (mouse 4T1) to allow assessment of tumor-immune interactions and one of which was endocrinologically humanized (human BT-474) to activate human PRLRs, we examined the effect of long PRLR knockdown on disease progression. In both models, knockdown dramatically inhibited metastatic spread to the lungs and liver and resulted in increased central death in the primary tumor. In the syngeneic model, immune infiltrates in metastatic sites were changed from innate inflammatory cells to lymphocytes, with an increase in the incidence of tumor-specific cytotoxic T cells. Long PRLR knockdown in three-dimensional culture induced apoptosis of tumor-initiating/cancer stem cells (death of 95% of cells displaying stem cell markers in 15 days). We conclude that the long PRLR plays an important role in breast cancer metastasis.

Residue 146 regulates prolactin receptor folding, basal activity and ligand-responsiveness: potential implications in breast tumorigenesis

PRLR(I146L) is the first identified gain-of-function variant of the prolactin receptor (PRLR) that was proposed to be associated with benign breast tumorigenesis. Structural investigations suggested this hydrophobic core position in the extracellular D2 domain to be linked to receptor dimerization. Here, we used a mutational approach to address how the conservative I-to-L substitution induced constitutive activity. Using cell-based assays of different I146-PRLR variants in combination with spectroscopic/nuclear magnetic resonance analyses we found that chemical manipulation of position 146 profoundly altered folding, PRL-responsiveness, and ligand-independent activity of the receptor in a mutation-specific manner. Together, these data further add to the critical role of position 146, showing it to also be crucial to structural integrity thereby imposing on the biological PRLR properties. When stably introduced in MCF-7 (luminal) and MDA-MB231 (mesenchymal) breast cancer cells, the most potent of the PRL-insensitive mutants (PRLR(I146D)) had minimal impact on cell proliferation and cell differentiation status.

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.

Reproductive actions of prolactin mediated through short and long receptor isoforms

Prolactin (PRL) is a polypeptide hormone with a wide range of physiological functions, and is critical for female reproduction. PRL exerts its action by binding to membrane bound receptor isoforms broadly classified as the long form and the short form receptors. Both receptor isoforms are highly expressed in the ovary as well as in the uterus. Although signaling through the long form is believed to be more predominant, it remains unclear whether activation of this isoform alone is sufficient to support reproductive functions or whether both types of receptor are required. The generation of transgenic mice selectively expressing either the short or the long form of PRL receptor has provided insight into the differential signaling mechanisms and physiological functions of these receptors. This review describes the essential finding that both long and short receptor isoforms are crucial for ovarian functions and female fertility, and highlights novel mechanisms of action for these receptors.

An N-terminal splice variant of human Stat5a that interacts with different transcription factors is the dominant form expressed in invasive ductal carcinoma

We have identified a new variant of human Stat5a, found at higher ratios to full-length Stat5a in invasive ductal carcinoma versus contiguous normal tissue. The variant, missing exon 5, inhibits p21 and Bax production and increases cell number. After prolactin stimulation, only full-length Stat5a interacts with the vitamin D and retinoid X receptors, whereas only Δ5 Stat5a interacts with activating protein 1-2 and specificity protein 1. Prolactin also oppositely regulates interaction of the two Stat5a forms with β-catenin. We propose that a change in splicing leading to upregulation of this new isoform is a pathogenic aspect of invasive ductal carcinoma.

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 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.

Investigation of prolactin receptor activation and blockade using time-resolved fluorescence resonance energy transfer

The prolactin receptor (PRLR) is emerging as a therapeutic target in oncology. Knowledge-based drug design led to the development of a pure PRLR antagonist (Del1-9-G129R-hPRL) that was recently shown to prevent PRL-induced mouse prostate tumorogenesis. In humans, the first gain-of-function mutation of the PRLR (PRLR(I146L)) was recently identified in breast tumor patients. At the molecular level, the actual mechanism of action of these two novel players in the PRL system remains elusive. In this study, we addressed whether constitutive PRLR activation (PRLR(I146L)) or PRLR blockade (antagonist) involved alteration of receptor oligomerization and/or of inter-chain distances compared to unstimulated and PRL-stimulated PRLR. Using a combination of various biochemical and spectroscopic approaches (co-IP, blue native electrophoresis, BRET(1)), we demonstrated that preformed PRLR homodimers are altered neither by PRL- or I146L-induced receptor triggering, nor by antagonist-mediated blockade. These findings were confirmed using a novel time-resolved fluorescence resonance energy transfer (TR-FRET) technology that allows monitoring distance changes between cell surface tagged receptors. This technology revealed that PRLR blockade or activation did not involve detectable distance changes between extracellular domains of receptor chains within the dimer. This study merges with our previous structural investigations suggesting that the mechanism of PRLR activation solely involves intermolecular contact adaptations leading to subtle intramolecular rearrangements.

Characterization of ductal and lobular breast carcinomas using novel prolactin receptor isoform specific antibodies

BACKGROUND

Prolactin is a polypeptide hormone responsible for proliferation and differentiation of the mammary gland. More recently, prolactin's role in mammary carcinogenesis has been studied with greater interest. Studies from our laboratory and from others have demonstrated that three specific isoforms of the prolactin receptor (PRLR) are expressed in both normal and cancerous breast cells and tissues. Until now, reliable isoform specific antibodies have been lacking. We have prepared and characterized polyclonal antibodies against each of the human PRLR isoforms that can effectively be used to characterize human breast cancers.

METHODS

Rabbits were immunized with synthetic peptides of isoform unique regions and immune sera affinity purified prior to validation by Western blot and immunohistochemical analyses. Sections of ductal and lobular carcinomas were stained with each affinity purified isoform specific antibody to determine expression patterns in breast cancer subclasses.

RESULTS

We show that the rabbit antibodies have high titer and could specifically recognize each isoform of PRLR. Differences in PRLR isoform expression levels were observed and quantified using histosections from xenografts of established human breast cancer cells lines, and ductal and lobular carcinoma human biopsy specimens. In addition, these results were verified by real-time PCR with isoform specific primers. While nearly all tumors contained LF and SF1b, the majority (76%) of ductal carcinoma biopsies expressed SF1a while the majority of lobular carcinomas lacked SF1a staining (72%) and 27% had only low levels of expression.

CONCLUSIONS

Differences in the receptor isoform expression profiles may be critical to understanding the role of PRL in mammary tumorigenesis. Since these antibodies are specifically directed against each PRLR isoform, they are valuable tools for the evaluation of breast cancer PRLR content and have potential clinical importance in treatment of this disease by providing new reagents to study the protein expression of the human PRLR.

Effects of prolactin on TSC2-null Eker rat cells and in pulmonary lymphangioleiomyomatosis

RATIONALE

Lymphangioleiomyomatosis, a cystic lung disease of women, is characterized by proliferation of smooth muscle-like lymphangioleiomyomatosis cells, which possess mutations in the tuberous sclerosis complex genes, TSC1/TSC2. Growth factors involved in lymphangioleiomyomatosis cell proliferation are unknown. Prolactin, an important reproductive hormone in women, is known to promote cell proliferation and survival in other tissues.

OBJECTIVES

To determine the role of prolactin in signaling and proliferation in lymphangioleiomyomatosis.

METHODS

Prolactin levels in the sera of patients with lymphangioleiomyomatosis were correlated with clinical status. Components of prolactin signal transduction pathways were assessed in lymphangioleiomyomatosis lesions from human lung explants by real-time reverse transcription-polymerase chain reaction (RT-PCR) and immunohistochemistry. Prolactin effects on proliferation and signaling were quantified in tuberin-deficient and tuberin-expressing rat cells in vitro.

MEASUREMENTS AND MAIN RESULTS

Higher prolactin levels in the sera of patients with lymphangioleiomyomatosis were associated with a faster rate of decline in FEV(1) and an increased history of pneumothorax (P < 0.01). Higher levels of prolactin and prolactin receptor mRNA and immunoreactivity were found in lymphangioleiomyomatosis lesions when compared with vascular smooth muscle cells in the same region of tissue. This was accompanied by evidence of activation of signal transducer and activator of transcription-1 (STAT1), STAT3, p44/42, and p38 mitogen-activated protein kinase. Tsc2(-/-) Eker rat embryonic fibroblasts expressed more prolactin receptor than did Tsc2(+/+) cells, and responded to prolactin with increased proliferation and activation of the same signaling pathways seen in vivo.

CONCLUSIONS

Prolactin may be an important growth factor in the pathogenesis of lymphangioleiomyomatosis.

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.

Long term increased expression of the short form 1b prolactin receptor in PC-3 human prostate cancer cells decreases cell growth and migration, and causes multiple changes in gene expression consistent with reduced invasive capacity

BACKGROUND

We have shown that treatment of human prostate cancer cells with the selective prolactin (PRL) receptor modulator, S179D PRL, inhibits growth in vitro, and the initiation and growth of xenografts in vivo. S179D PRL treatment also upregulates expression of the short form 1b (SF1b) PRL receptor, activation of which upregulates expression of the cell cycle-regulating protein, p21.

METHODS

We examined the consequences of long term increased expression and activation of SF1b, at levels comparable to those resulting from treatment with S179D PRL, by creating PC-3-derived stable cell lines expressing a constitutively active form of SF1b, DeltaS2 SF1b.

RESULTS

Increased expression of DeltaS2 SF1b decreased growth and migration of the cells. This was accompanied by an increase in cell-matrix interactions, and cell-cell aggregation when cells were plated on basement membrane components. Real-time PCR evaluated the expression of genes related to invasive capacity. Of particular interest was decreased expression of the protease, urokinase-type plaminogen activator, and its receptor, uPAR, and increased expression of its inhibitors, PAI-1 and 2. Also decreased in cells with increased expression of DeltaS2 SF1b was expression of basic fibroblast growth factor and vascular endothelial growth factor.

CONCLUSION

We conclude that at least part of the beneficial effects of S179D PRL is the result of increased expression of SF1b, and that the effects of increased expression and activation of SF1b continue to be of potential benefit in the long term.

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.

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.

Prolactin and estradiol utilize distinct mechanisms to increase serine-118 phosphorylation and decrease levels of estrogen receptor alpha in T47D breast cancer cells

Potential interactions between prolactin (PRL) and estradiol (E2) in breast cancer cells were explored by examining the effect of PRL on estrogen receptor (ER) serine-118 phosphorylation, ER down-regulation, and E2-stimulated cell proliferation. Both E2 and PRL resulted in prolonged ERalpha serine-118 phosphorylation, but used different signaling pathways to achieve this end. Both hormones also decreased the amount of ERalpha, but the mechanisms were different: for E2, the decrease was rapid and resulted from proteasomic degradation, whereas for PRL the decrease was slow and resulted from an effect on levels of ERalpha mRNA. PRL alone had no effect on cell number, but enhanced the increase in number in response to E2. These results are the first to demonstrate similar effects of PRL and E2 on parameters considered key to E2's effects. This suggests heretofore unrecognized and potentially important interactions between these two hormones in the natural history of breast cancer.

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.

Rational design of competitive prolactin/growth hormone receptor antagonists

There is increasing evidence that prolactin (PRL) and growth hormone (GH) act as growth-promoters of breast tumors. Recent arguments have accumulated to suggest that when they are locally-produced within the mammary tissue, these hormones, acting by an autocrine-paracrine mechanism may have enhanced, or even specific functions compared to endocrine PRL and GH. Classical drugs blocking pituitary hormone production (dopamine and somatostatin analogs) are ineffective on extrapituitary expression of PRL/GH genes, therefore the undesirable effects of these locally-produced hormones remain a target of interest for alternative strategies. This has encouraged the development of competitive PRL and/or GH receptor antagonists, which involve engineered variants of natural receptor ligands (PRL or GH) aimed at blocking receptor activation rather than hormone production in peripheral tissues. This article overviews the rational design of this new class of molecules, their specific molecular features (receptor specificity, biological properties, etc.) and whenever available, the data that have been obtained in cell or animal models of breast cancer.

S2 deletion variants of human PRL receptors demonstrate that extracellular domain conformation can alter conformation of the intracellular signaling domain

Using spacers between the C-termini of the long (LF) or short (SF) human prolactin receptors and luciferase/GFP such that bioluminescence resonance energy transfer (BRET) occurred minimally in intact versions of these receptors in the absence of ligand, we have monitored the BRET signal after deletion of regions of the extracellular domain (ECD). Deletion of S2 produced ligand-independent BRET for only those pairings normally occurring in the presence of ligand with the intact receptor. Deletion of the similarly sized S1, or S1 plus S2, produced no ligand-independent or -dependent BRET. When deleted receptors were transfected into human breast (T47D) or prostate (DU145) cancer cells incubated in the absence of added prolactin (PRL) and presence of anti-PRL, expression of the DeltaS2LF resulted in increased cell number, whereas expression of the intact receptor did not. When endogenous beta-casein expression was examined in T47D cells, the DeltaS2LF and DeltaS2F1a both showed ligand-independent activation of transcription, again not duplicated by the intact receptor. Paired with evidence in the literature for predimerization of PRLRs, these results demonstrate that altered ECD conformation, and not just a change in bulk, produces altered conformation of the intracellular signaling region of the receptors, supporting the concept that ligand binding to the ECD of intact predimerized receptors could initiate signaling. In addition, the current work supports a dual proliferative and differentiative role for the LF receptor, but only a differentiative role for the SF1a receptor. Naturally occurring DeltaS2 PRL receptors (PRLR) were also found in normal and cancerous human cells. This additionally suggests a heretofore unappreciated ligand-independent role for PRLRs.

Reproductive role of prolactin

The biological actions of prolactin (PRL), a polypeptide hormone, are mostly related to lactation and reproduction. These actions have been clarified by studies of PRL and PRL-deficient receptor mice, which have a clear phenotype of reproductive failure at multiple sites. This review aims to summarize current knowledge about PRL and its receptor, role in reproductive axis and presents information of hyperprolactinemia in reproductive medicine. Our understanding of the physiology and transduction pathway of PRL has largely increased in the past 20 years with the cloning of PRL and its receptor gene.

S179D prolactin: antagonistic agony!

The aims of this review are three-fold: first, to collate what is known about the production and activities of phosphorylated prolactin (PRL), the latter largely, but not exclusively, as illustrated through the use of the molecular mimic, S179D PRL; second, to apply this and related knowledge to produce an updated model of prolactin-receptor interactions that may apply to other members of this cytokine super-family; and third, to promote a shift in the current paradigm for the development of clinically important growth antagonists. This third aim explains the title since, based on results with S179D PRL, it is proposed that agents which signal to antagonistic ends may be better therapeutics than pure antagonists-hence antagonistic agony. Since S179D PRL is not a pure antagonist, we have proposed the term selective prolactin receptor modulator (SPeRM) for this and like molecules.

Drug Insight: prolactin-receptor antagonists, a novel approach to treatment of unresolved systemic and local hyperprolactinemia?

Prolactin is a polypeptide hormone whose major biological actions are related to normal lactation and reproduction. Abnormally high prolactin levels, referred to as hyperprolactinemia, can result in various reproductive disorders. Currently, therapeutic management of hyperprolactinemia relies on dopamine agonists, since dopamine is the primary physiological suppressor of pituitary prolactin production. Epidemiologic studies have shown that prolactin levels in the high-normal range, as well as medications that interfere with dopamine action (e.g. certain antipsychotic drugs), might correlate with increased breast cancer risk. In addition to circulating prolactin, it is now well established that prolactin is also produced locally within various tissues, including breast and prostate. Increasing evidence, mainly from animal studies at present, suggests that excess locally produced prolactin may promote the growth of breast and prostate tumors via an autocrine or paracrine mechanism. These findings have renewed the interest in finding alternative strategies to suppress prolactin actions when dopamine agonists are ineffective. Our studies of the relationship between prolactin structure and function have resulted in the development of pure prolactin-receptor antagonists. These molecules prevent endogenous prolactin from exerting its actions via a competitive mechanism for receptor binding. In this review, we discuss the possible future therapeutic utility of this novel class of compounds.

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.

Two wrongs can make a right: dimers of prolactin and growth hormone receptor antagonists behave as agonists

Prolactin (PRL) and GH have two distinct binding sites (site 1 with high affinity; site 2 with low affinity) that each interact with a PRL receptor (PRLR) to form a functional receptor dimer that activates signal transduction. The G129R mutation in PRL and the G120R mutation in GH disrupt the structural integrity of site 2 such that the ligands retain the ability to bind to the first receptor with high affinity, but act as receptor antagonists. In this study, we examined the ability of monomeric and dimeric forms of these ligands, human (h) PRL and hGH, and their antagonists (hPRL-G129R and hGH-G120R) to 1) bind to PRLRs; 2) induce conformational changes in PRLRs; 3) activate signaling pathways associated with the PRLR; and 4) mediate cell proliferation in vitro. In contrast to monomeric hPRL-G129R, homodimeric hPRL-G129R induced PRLR dimerization; activated Janus family of tyrosine kinases 2/signal transducer and activator of transcription 5, Ras/Raf/MAPK kinase/Erk, and phosphatidylinositol 3-kinase/Akt signaling; and stimulated Nb2 cell proliferation. Similarly, homodimeric hGH-G120R was able to mediate signaling via the PRLR and to stimulate Nb2 cell proliferation. These experiments demonstrate that a ligand must have two functional binding sites, but that these may be site 1 plus site 2 or two site 1's, to elicit receptor-mediated signal transduction. The size of the ligand plays less of a role in receptor activation, suggesting that the extracellular portion of the PRLR (and possibly the GH receptor) is rather flexible and can accommodate larger ligands. These findings may have implications for designing multifunctional therapeutics that target this class of cytokine receptors.

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.

Abrogation of delayed type hypersensitivity response to Candida albicans produced by a molecular mimic of phosphorylated prolactin

The effects of two major forms of prolactin (PRL) were examined on delayed type hypersensitivity (DTH) responses to Candida albicans. Unmodified PRL (U-PRL) had no effect on the DTH response, whereas a molecular mimic of phosphorylated PRL (S179D PRL) significantly inhibited immune responses to this robust antigen. This effect of S179D PRL was not accompanied by gross alterations in splenic T cell numbers, CD4/CD8 ratios, or T and B cell activation markers, but did produce a decrease in splenocyte apoptosis. Using gld animals, Fas ligand (FasL) was implicated in the suppressive effects of S179D PRL. Circulating IgG1 and IgG2 antibody levels were increased in response to treatment with both forms of PRL, but the effects of S179D PRL were most pronounced. Cytokine changes in the popliteal lymph nodes specific to S179D PRL treatment showed an inhibition of pro-inflammatory cytokines. In conclusion, mice treated with a molecular mimic of phosphorylated prolactin showed a profound inhibition of DTH responses to Candida correlating with an absence of GM-CSF, IL-4, and IL-13 production and a marked reduction in IL-12p70 synthesis.