Differential signal transduction of the short, Nb2, and long prolactin receptors. Activation of interferon regulatory factor-1 and cell proliferation

Prolactin-induced and neuronal activation in the brain of mother mice

Nursing has important consequences on mothers. To separate the prolactin-mediated and the neuronally-mediated actions of nursing, neurons directly affected by prolactin were visualized using pSTAT5 immunohistochemistry in relation to Fos-expressing neurons in suckled mother mice. In response to pup exposure following 22-h pup deprivation, we found a markedly elevated number of pSTAT5-containing neurons in several brain regions, including the lateral septum, medial amygdaloid nucleus, subparafascicular area, caudal periaqueductal gray, dorsal raphe, lateral parabrachial nucleus, nucleus of the solitary tract, and the periventricular, medial preoptic, paraventricular, arcuate and ventromedial nuclei of the hypothalamus. Pup exposure also induced Fos expression in all of these brain regions except the arcuate and ventromedial hypothalamic nuclei. Bromocriptine treatment known to reduce prolactin levels eliminated pSTAT5 from most brain regions while it did not affect Fos activation following suckling. The degree of colocalization for pSTAT5 and Fos ranged from 8 to 80% in the different brain regions suggesting that most neurons responding to pup exposure in mother mice are driven either by prolactin or direct neuronal input from the pups, while the number of neurons affected by both types of inputs depends on the examined brain area. In addition, both pSTAT5 and Fos were also double-labeled with estrogen receptor alpha (ERα) in mother mice, which revealed a very high degree of colocalization between pSTAT5 and ERα with much less potential interaction between Fos- and ERα-containing neurons suggesting that estrogen-sensitive neurons are more likely to be affected by prolactin than by direct neuronal activation.

Inhibition of MAPK by prolactin signaling through the short form of its receptor in the ovary and decidua: involvement of a novel phosphatase

Prolactin (PRL) is essential for normal reproduction and signals through two types of receptors, the short (PRL-RS) and long (PRL-RL) form. We have previously shown that transgenic mice expressing only PRL-RS (PRLR(-/-)RS) display abnormal follicular development and premature ovarian failure. Here, we report that MAPK, essential for normal follicular development, is critically inhibited by PRL in reproductive tissues of PRLR(-/-)RS mice. Consequently, the phosphorylation of MAPK downstream targets are also markedly inhibited by PRL without affecting immediate upstream kinases, suggesting involvement of MAPK specific phosphatase(s) in this inhibition. Similar results are obtained in a PRL-responsive ovary-derived cell line (GG-CL) that expresses only PRL-RS. However, we found the expression/activation of several known MAPK phosphatases not to be affected by PRL, suggesting a role of unidentified phosphatase(s). We detected a 27-kDa protein that binds to the intracellular domain of PRL-RS and identified it as dual specific phosphatase DUPD1. PRL does not induce expression of DUDP1 but represses its phosphorylation on Thr-155. We also show a physical association of this phosphatase with ERK1/2 and p38 MAPK. Using an in vitro phosphatase assay and overexpression studies, we established that DUPD1 is a MAPK phosphatase. Dual specific phosphatase inhibitors as well as siRNA to DUPD1, completely prevent PRL-mediated MAPK inhibition in ovarian cells. Our results strongly suggest that deactivation of MAPK by PRL/PRL-RS contributes to the severe ovarian defect in PRLR(-/-)RS mice and demonstrate the novel association of PRL-RS with DUPD1 and a role for this phosphatase in MAPK deactivation.

Cloning and functional characterization of allelic variation in the porcine prolactin receptor

Prolactin (PRL) regulates various functions in pigs including reproduction, mammary development and lactation. We used 5'-rapid amplification of cDNA ends (5'-RACE) to clone three full-length alleles of the porcine PRL receptor (pPRLR) from Landrace (alleles LR2 and LR4) and Yucatan miniature (MP) pigs, corresponding to the A and B alleles previously reported to be associated with reproductive traits. When expressed in Chinese hamster ovary (CHO-K1) cells, all three pPRLRs transduced differentiation signals to a beta-casein promoter with the same effectiveness, where human growth hormone (hGH) and porcine PRL (pPRL) were more effective ligands than ovine PRL (oPRL). The pPRLR had a lower binding affinity for oPRL than pPRL while binding affinity for hGH was not different between the three pPRLR variants. The pPRLRs primarily localized to the cytoplasm with perinuclear concentration. In conclusion, we have cloned three allelic variants of the pPRLR and have functionally characterized these as different from the hPRLR. However, our data do not support the proposal that allelic variation of the pPRLR confers functional differences in vivo.

The short prolactin receptor predominates in endothelial cells of micro- and macrovascular origin

BACKGROUND

Controversial reports on prolactin receptors (PRL-R), the long and short form, on endothelial cells (EC) may be explained by the choice of EC derived from the micro- and macrovascular bed of either endocrine and non-endocrine organs.

METHODS

We studied here PRL-R expression in organs [bovine corpus luteum (CL), umbilical vein, aorta] and in organ-derived EC cultures.

RESULTS

In the intact CL, both PRL-R forms were present at mRNA and protein level throughout the oestrous cycle stages. The short form prevailed as protein. PRL-R-positive EC were noted by immunofluorescent staining in arterial blood vessels of CL septa, in the umbilical vein and the aorta. In EC cultures of micro- and macrovascular origin, transcripts of both PRL-R forms were shown; again the short-form protein prevailed. Blocking experiments with anti-prolactin (PRL) antibody led to a 60% decrease in cell growth. Treatment with PRL had no effect.

CONCLUSION

PRL-R expression in micro- and macrovascular EC is associated with the predominant short form.

Positive and negative regulatory elements in the late lactation protein-A gene promoter from the tammar wallaby (Macropus eugenii)

Little is known about the regulation of the marsupial-specific late lactation protein-A (LLP-A) gene, first expressed at mid-lactation in the mammary gland of the tammar wallaby. A genomic clone of LLP-A was sequenced and shown to include seven exons. The LLP-A promoter region of 1969 bp ligated to a secreted alkaline phosphatase (SEAP) gene reporter was co-transfected into CHO-K1 cells with prolactin (PRL) receptor cDNA. Transfected cells cultured with insulin, cortisol and PRL did not secrete SEAP into media. Similarly, this construct was not expressed in the mammary gland of eight lines of transgenic mice. In contrast, when the LLP-A promoter region was reduced to 850 bp, the expression of the SEAP reporter in CHO-K1 cells was constitutive and PRL-independent, despite the presence of two low affinity Stat5 binding sites. The 1969 bp promoter was analyzed using nine serial deletions ligated to the SEAP gene. The expression of these constructs was PRL-independent. Five putative inhibitory elements were identified between -1969 and -1796, -1404 and -1184, -1184 and -992, -992 and -757, and -591 and -425, and a putative enhancer or core transcription element between -425 and-239. These studies indicate that the complex temporal regulation of the LLP-A gene involves elements in its 5'-regulatory region.

Prolactin receptor mRNA expression in oocytes and preimplantation mouse embryos

Prolactin was first identified as an anterior pituitary lobe hormone, responsible for the regulation of mammary gland growth and development. Prolactin receptors have been localized in a number of peripheral tissues, including tissues involved in reproduction. Studies with knockout animals have shown that prolactin receptor deficient mice present reproductive defects, whereas prolactin promotes the developmental potential of preimplantation mouse and rat embryos in vitro. To better understand the role of prolactin in the process of reproduction and early embryo development in mice, the expression of the four transcript variants of prolactin receptor was examined in the first stages of mouse embryo development. Prolactin long receptor mRNA was expressed in all stages examined, that is in cumulus cells, oocytes, zygotes, 2-cell embryos, 4-cell embryos, morulae and blastocysts. Prolactin receptor type S1 mRNA was observed only in cumulus cells, while S2 mRNA was present in cumulus cells, oocytes, zygotes and 2-cell embryos. S3 mRNA was expressed only in cumulus cells and oocytes. These results indicate that different isoforms of prolactin receptors may be present in the various stages of mouse preimplantation embryo and may play an important role in the control of its growth and development.

Tissue-specific gene expression of prolactin receptor in the acute-phase response induced by lipopolysaccharides

Acute inflammation can elicit a defense reaction known as the acute-phase response (APR) that is crucial for reestablishing homeostasis in the host. The role for prolactin (PRL) as an immunomodulatory factor maintaining homeostasis under conditions of stress has been proposed; however, its function during the APR remains unclear. Previously, it was shown that proinflammatory cytokines characteristic of the APR (TNF-alpha, IL-1beta, and IFNgamma) induced the expression of the PRL receptor (PRLR) by pulmonary fibroblasts in vitro. Here, we investigated the in vivo expression of PRLR during lipopolysaccharide (LPS)-induced APR in various tissues of the mouse. We show that PRLR mRNA and protein levels were downregulated in hepatic tissues after intraperitoneal LPS injection. Downregulation of PRLR in the liver was confirmed by immunohistochemistry. A suppressive effect on mRNA expression was also observed in prostate, seminal vesicle, kidney, heart, and lung tissues. However, PRLR mRNA levels were increased in the thymus, and no changes were observed in the spleen. The proportion of transcripts for the different receptor isoforms (long, S1, S2, and S3) in liver and thymus was not altered by LPS injection. These findings suggest a complex tissue-specific regulation of PRLR expression in the context of the APR.

Endogenous human prolactin and not exogenous human prolactin induces estrogen receptor alpha and prolactin receptor expression and increases estrogen responsiveness in breast cancer cells

Prolactin (PRL) and estrogen act synergistically to increase mammary gland growth, development, and differentiation. Based on their roles in the normal gland, these hormones have been studied to determine their interactions in the development and progression of breast cancer. However, most studies have evaluated only endocrine PRL and did not take into account the recent discovery that PRL is synthesized by human mammary cells, permitting autocrine/paracrine activity. To examine the effects of this endogenous PRL, we engineered MCF7 cells to inducibly overexpress human prolactin (hPRL). Using this Tet-On MCF7hPRL cell line, we studied effects on cell growth, PRLR, ER alpha, and PgR levels, and estrogen target genes. Induced endogenous hPRL, but not exogenous hPRL, increased ER alpha levels as well as estrogen responsiveness in these cells, suggesting that effects on breast cancer development and progression by estrogen may be amplified by cross-regulation of ER alpha levels by endogenous hPRL. The long PRLR isoform was also upregulated by endogenous, but not exogenous PRL. This model will allow investigation of endogenous hPRL in mammary epithelial cells and will enable further dissection of PRL effects on other hormone signaling pathways to determine the role of PRL in breast cancer.

The distribution of mRNA for the short form of the prolactin receptor in the forebrain of the female rat

Prolactin exerts its diverse effects on peripheral tissue and on the brain via receptors that have two forms, a short form and a long form. The distribution of the mRNA for both forms of the receptor has been examined in brain and peripheral tissue regions using methods based on regional dissection. Although the cell-specific distribution of the long form of the prolactin receptor has been examined using in situ hybridization in the rat brain, the cell-specific distribution of the short form has not been described. In this study we mapped the distribution of neurons and other cells expressing the short from of the receptor transcript in the forebrain, ovary, and uterus of the female rat by using in situ hybridization with a 33P-labeled cRNA probe specific for the short form of the prolactin receptor mRNA (PRL-SR mRNA). Neurons expressing the PRL-SR mRNA were located predominantly in the preoptic area and hypothalamus as well as in certain limbic structures. Specific nuclei included the anteroventral periventricular nucleus, paraventricular and supraoptic nucleus, medial preoptic area, suprachiasmatic nucleus, and ventromedial and arcuate nuclei of the hypothalamus, as well as the bed nucleus of stria terminalis and the medial amygdala. Scattered neurons expressing PRL-SR mRNA were also found in the cortex, habenula, zona incerta, and thalamus. Cells in the choroid plexus expressed high levels of PRL-SR mRNA, as did the luteal cells of the corpus luteum and the epithelial cells of the uterine glands. These data confirm previous reports and extend our knowledge of the distribution of the short form of the receptor to the cellular level. The neuroanatomic distribution of neurons expressing PRL-SR mRNA suggests that they may influence the mediation and coordination of prolactin-regulated endocrine and behavioral events.

Prolactin induces ERalpha-positive and ERalpha-negative mammary cancer in transgenic mice

The role of prolactin in human breast cancer has been controversial. However, it is now apparent that human mammary epithelial cells can synthesize prolactin endogenously, permitting autocrine/paracrine actions within the mammary gland that are independent of pituitary prolactin. To model this local mammary production of prolactin (PRL), we have generated mice that overexpress prolactin within mammary epithelial cells under the control of a hormonally nonresponsive promoter, neu-related lipocalin (NRL). In each of the two examined NRL-PRL transgenic mouse lineages, female virgin mice display mammary developmental abnormalities, mammary intraepithelial neoplasias, and invasive neoplasms. Prolactin increases proliferation in morphologically normal alveoli and ducts, as well as in lesions. The tumors are of varied histotype, but papillary adenocarcinomas and adenosquamous neoplasms predominate. Neoplasms can be separated into two populations: one is estrogen receptor alpha (ERalpha) positive (greater than 15% of the cells stain for ERalpha), and the other is ERalpha- (<3%). ERalpha expression does not correlate with tumor histotype, or proliferative or apoptotic indices. These studies provide a mouse model of hormonally dependent breast cancer, and, perhaps most strikingly, a model in which some neoplasms retain ERalpha, as occurs in the human disease.

Cytokine induction of prolactin receptors mediates prolactin inhibition of nitric oxide synthesis in pulmonary fibroblasts

Prolactin (PRL) has been implicated as a modulator of immune function, and some of its actions may be linked to NO synthesis. Because NO acts as a mediator of inflammation, we speculated that an inflammatory milieu could unmask pathways by which PRL could affect NO synthesis. Here, we show that pro-inflammatory cytokines induce the expression of PRL receptors in pulmonary fibroblasts, allowing PRL to inhibit cytokine-induced NO production and the expression of the inducible nitric oxide synthase (iNOS). Inhibition of iNOS expression by PRL correlates with the phosphorylation of STAT-5b (signal transducer and activator of transcription 5b) and the suppression of expression of IRF-1 (interferon regulatory factor 1), a transcription factor for iNOS. These results reveal previously unrecognized mechanisms by which PRL and PRL receptors may play significant modulatory roles during immune-inflammatory processes.

Expression of prolactin receptors and regulation of cell proliferation by prolactin, corticotropin-releasing factor, and corticosterone in a neuroblastoma cell line

The aetiology of neuroblastoma remains obscure, although a number of neuropeptides have been implicated in its pathogenesis. Using the mouse neuroblastoma cell line Neuro2a as a model, we have investigated the mitogenic actions of prolactin (PRL) and two hypothalamo-pituitary-adrenal stress axis hormones, corticotropin-releasing factor (CRF) and corticosterone. Using established polyclonal PRL receptor antisera with immunofluorescence cytochemistry, we show that the Neuro2a cells possess immunoreactive forms of both the long and short forms of the receptor. PRL and CRF were effective as mitogens in Neuro2a cell cultures, where a 10(-7) M concentration of PRL or CRF elicited a two-fold increase in the numbers of cells after 72 h (p < 0.0001). Corticosterone, however, attenuated their proliferation. These data suggest that prolactin may act to increase the proliferation and regulation of neuroblastomas and that the effects of PRL may be modified by hypothalamo-pituitary-adrenal hormones.

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.

Stimulation of interferon regulatory factor-1 by prolactin

Prolactin (PRL), a pituitary peptide hormone, is known to regulate diverse cellular functions including proliferation, differentiation, angiogenesis and protection against apoptosis and inflammation. To understand the mechanism of PRL signaling in T cells, we have cloned both PRL and its receptor (PRL-R), one potent mediator of PRL signaling, Stat5b, and a panel of PRL-inducible immediate early genes from T cells. We are employing these genes as tools with which to understand how PRL regulates the expression of one target gene, the transcription factor interferon regulatory factor-1 (IRF-1), which is a multifunctional immune regulator gene. In investigating regulatory events along the PRL-R/JAK/Stat/IRF-1 signaling pathway, we show that Stat factors can activate as well as inhibit IRF-1 promoter activity and that cross-talk between Stat and NFkappaB signaling pathways also regulates IRF-1 promoter activity. These findings have much broader implications not only for T lymphocytes but also for other PRL responsive target cells and tissues.

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 actions of prolactin in the brain during pregnancy and lactation

The vital role played by prolactin during pregnancy and lactation is emphasized by the physiological adaptations that occur in the mother to maintain a prolonged state of hyperprolactinemia. In many species the placenta provides a source of lactogenic hormones in the circulation, ensuring the continued presence of a hormone capable of activating the prolactin receptor throughout pregnancy. In addition, the tuberoinfundibular dopamine neurons, which normally maintain a tonic inhibitory influence over prolactin secretion, show a reduced ability to respond to prolactin during late pregnancy and lactation, allowing high levels of prolactin to be maintained unopposed by a regulatory feedback mechanisms. There is clear evidence that systemic prolactin gains access to the cerebrospinal fluid, from where it can diffuse to numerous brain regions. Prolactin receptors are expressed in several hypothalamic nuclei, including the medial preoptic and arcuate nuclei, and we have observed marked increases in expression of prolactin receptors in these nuclei during lactation. Moreover, a number of hypothalamic nuclei, including the paraventricular, supraoptic and ventromedial nuclei, in which prolactin receptors were not detected in diestrous rats, were found to express significant amounts of prolactin receptor during lactation. These observations have important implications for the variety of documented actions of prolactin on the brain. Prolactin has been reported to influence numerous brain functions, including maternal behavior, feeding and appetite, oxytocin secretion, and ACTH secretion in response to stress. In light of the high circulating levels of prolactin during pregnancy and lactation and the increased expression of prolactin receptors in the hypothalamus, many of these effects of prolactin may be enhanced or exaggerated during lactation. Hence, prolactin may be a key player in the coordination of neuroendocrine and behavioral adaptations of the maternal brain.

Prolactin receptors in the brain during pregnancy and lactation: implications for behavior

Numerous studies have documented prolactin regulation of a variety of brain functions, including maternal behavior, regulation of oxytocin neurons, regulation of feeding and appetite, suppression of ACTH secretion in response to stress, and suppression of fertility. We have observed marked changes in expression of prolactin receptors in specific hypothalamic nuclei during pregnancy and lactation. This has important implications for neuronal functions regulated by prolactin. In light of the high circulating levels of prolactin during pregnancy and lactation and the increased expression of prolactin receptors in the hypothalamus, many of these functions may be enhanced or exaggerated in the maternal brain. The adaptations of the maternal brain allow the female to exhibit the appropriate behavior to feed and nurture her offspring, to adjust to the nutritional and metabolic demands of milk production, and to maintain appropriate hormone secretion to allow milk synthesis, secretion, and ejection. This review aims to summarize the evidence that prolactin plays a key role in regulating hypothalamic function during lactation and to discuss the hypothesis that the overall role of prolactin is to organize and coordinate this wide range of behavioral and neuroendocrine adaptations during pregnancy and lactation.

Prolactin stimulates activation of c-jun N-terminal kinase (JNK)

In recent years the mitogen-activated protein (MAP) kinase family has expanded to include both c-jun N-terminal kinases (JNKs), and the p38/HOG1 family in addition to the extracellular regulated kinase (ERK) family. These kinases are activated by a variety of growth factors, as well as extra- and intracellular insults such as osmotic stress, UV light, and chemotherapeutic agents. Stimulation of the PRL-dependent Nb2 cell line with PRL results in the rapid activation of JNK as determined by the glutathione-S-transferase (GST)-jun kinase assay. Activation was maximal 30 min after stimulation with 50 nM rat PRL (rPRL) and decreased after that time. Dose response studies indicated that concentrations as low as 10 nM rPRL resulted in maximal activation. The interleukin-3 (IL-3)-dependent myeloid progenitor cell line 32Dcl3 was transfected with the long, Nb2, and short forms of the rat PRL receptor (rPRLR), as well as the long form of the human PRLR (hPRLR). The long and Nb2 forms of the PRLR were able to stimulate activation of JNK; however, the short form of the rPRLR was not. This corresponds with the inability of the short form of the rPRLR to stimulate proliferation of 32Dcl3 cells. Activation of JNK in 32Dcl3 cells expressing the long form of the hPRLR was maximal at 30 min after stimulation with 100 nM ovine PRL (oPRL) and declined after that time. Dose response studies indicated that activation of JNK was maximal after 30 min at a concentration of 10 nM, and the amount of activated JNK declined at the highest concentration of oPRL, 100 nM. Immunoblot analysis with an antibody that recognizes the activated (phosphorylated) forms of JNK1 and JNK2 indicated that both JNK1 and JNK2 isoforms were activated in 32D/hPRLR cells stimulated with oPRL. A recombinant human adenovirus expressing a kinase-inactive mutant of JNK1 (APF mutant) was used to determine the biological effect of blocking JNK activity in Nb2 cells. Expression of the JNK1-APF mutant inhibited cellular proliferation and induced DNA fragmentation typical of cells undergoing apoptosis. These data suggest that activation of JNKs may be important in mitogenic signaling and/or suppression of apoptosis in Nb2 cells.

Sequence and functional characterisation of the marmoset monkey (Callithrix jacchus) prolactin receptor: comparative homology with the human long-form prolactin receptor

This study demonstrates the cloning and in-vitro characterisation of the marmoset monkey (Callithrix jacchus) prolactin receptor cDNA. The marmoset prolactin receptor cDNA was generated by reverse transcription-polymerase chain reaction using adrenal RNA and primers designed from prolactin receptor conserved regions. Sequence analysis predicts a mature protein of 598 amino acids exclusive of the 24 amino acid signal peptide. The marmoset prolactin receptor cDNA shares 93 and 61% base pair, and 89 and 61% amino acid sequence homologies with the long form human and rat prolactin receptor cDNA, respectively. The marmoset prolactin receptor cDNA sequence retains all the receptor sequences that have been shown previously to be essential for ligand binding, structural integrity and signal transduction. Transfection of human 293 fibroblast cells with the marmoset prolactin receptor cDNA (three independent experiments) confirmed the expression of a receptor that has high binding affinity to human growth hormone (K(a)=3.6+/-0.07 nM(-1) and B(max)=7.55+/-2.06x10(-11) M) and human prolactin (K(a)=3.1+/-0.12 nM(-1) and B(max)=2.87+/-0.66x10(-11) M). Functionality of the receptor was assessed by co-transfection of 293 fibroblast cells with marmoset prolactin receptor cDNA and the Jak2 cDNA, or marmoset prolactin receptor and a Stat5 responsive element linked to the luciferase coding sequence. Incubation of the cells with 18 nM ovine prolactin resulted in rapid phosphorylation of Jak2 as ascertained by Western blotting. In addition, the marmoset prolactin receptor cDNA led to 9.06+/-0.47-fold induction of luciferase gene activity. This was comparable with the induction observed following transfection with the human prolactin receptor cDNA (8.55+/-0. 5-fold). In-vivo prolactin receptor expression in the marmoset monkey was assessed by ribonuclease protection assay and detected in a number of tissues including female reproductive organs. These data confirm the cloning and functionality of the marmoset prolactin receptor cDNA. The marmoset prolactin receptor shares a high sequence homology with the long-form human prolactin receptor, and both receptors bind hormones with comparable affinity and confer a similar intracellular response. The marmoset monkey may provide a useful tool to investigate the role of prolactin in primate reproduction.

Prolactin (PRL)-PRL receptor system increases cell proliferation involving JNK (c-Jun amino terminal kinase) and AP-1 activation: inhibition by glucocorticoids

PRL receptor (PRLR) signal transduction supports PRL-induced growth/differentiation processes. While PRL is known to activate Jak2-Stat5 (signal transducer and activator of transcription 5) signaling pathway, the mechanism by which cell proliferation is stimulated is less known. We show that PRL induces proliferation of bovine mammary gland epithelial cells and AP-1 site activation. Using PRLR mutants and the PRLR short form, we have found that both homodimerization of PRLR wild type and the integrity of box-1 and C-distal tyrosine of PRLR intracellular domain are needed in PRL-induced proliferation and AP-1 activation. The effect of PRL has been assayed in the presence of dexamethasone (Dex), insulin, and alone. We found that Dex negatively regulates PRL-induced proliferation and AP-1 site activation. We demonstrate that PRL exerts activation of AP-1 transcriptional complex, and the mechanism by which this activation is produced is also studied. We show that PRL induces an increase in the c-Jun content of AP-1 transcriptional complexes. The PRL-induced c-Jun of AP-1 transcriptional complex diminishes in the presence of Dex in a dose-dependent manner. Dex inhibition was reversed by the higher concentration of PRL added to cells. Despite the fact that the regulation of the AP-1 site is complex, we found that PRL activates the c-Jun amino terminal kinase (JNK), while glucocorticoid prevents this JNK activation. These data support a regulation of cellular growth by PRL-PRLR system by increasing AP-1 transcriptional complex activity via JNK activation. JNK activation can be repressed by glucocorticoid in a DNA-binding-independent manner.

Association of 2',5'-oligoadenylate synthetase with the prolactin (PRL) receptor: alteration in PRL-inducible stat1 (signal transducer and activator of transcription 1) signaling to the IRF-1 (interferon-regulatory factor 1) promoter

The PRL receptor (PRL-R) signals through the Janus tyrosine kinases (JAK) and other non-JAK tyrosine kinases, some of which are preassociated with the PRL-R. To clone PRL-R interacting proteins, the intracellular domain (ICD) of the long form of the PRL-R was used in a yeast two-hybrid screen of a human B cell cDNA library. One PRL-R interacting protein was identified as the 42-kDa form of the enzyme 2',5'-oligoadenylate synthetase (OAS). The in vivo interactions in yeast were further confirmed by an in vitro interaction assay and by coimmunoprecipitation in transfected mammalian cells. Functionally, OAS reduced the basal activity of two types of promoters in transiently transfected COS-1 cells. In the presence of PRL, OAS inhibited PRL induction of the immediate early IRF-1 (interferon-regulatory factor 1) promoter, but not PRL induction of the differentiation-specific beta-casein promoter, suggesting that OAS exerts specific effects on immediate early gene promoters. The inhibitory effects of OAS were accompanied by a reduction in PRL-inducible Stat1 (signal transducer and activator of transcription 1) DNA binding activity at the IRF-1 GAS (interferon-gamma-activated sequence) element. These results demonstrate a novel interaction of OAS with the PRL-R and suggest a role for OAS in modulating Stat1-mediated signaling to an immediate early gene promoter. Although previously characterized as a regulator of ribonuclease (RNase) L antiviral responses, OAS may have additional effects on cytokine receptor signal transduction pathways.

Stat5b inhibits NFkappaB-mediated signaling

Signal transducers and activators of transcription (Stat) are latent transcription factors that participate in cytokine signaling by regulating the expression of early response genes. Our previous studies showed that Stat5 functions not only as a transcriptional activator but also as a transcriptional inhibitor, depending on the target promoter. This report further investigates the mechanism of Stat5b-mediated inhibition and demonstrates that PRL-inducible Stat5b inhibits nuclear factorkappaB (NFkappaB) signaling to both the interferon regulatory factor-1 promoter and to the thymidine kinase promoter containing multimerized NFkappaB elements (NFkappaB-TK). Further, PRL-inducible Stat5b inhibits tumor necrosis factor-alpha signaling presumably by inhibiting endogenous NFkappaB. This Stat5b-mediated inhibitory effect on NFkappaB signaling is independent of Stat5b-DNA interactions but requires the carboxyl terminus of Stat5b as well as Stat5b nuclear translocation and/or accumulation, suggesting that Stat5b is competing for a nuclear factor(s) necessary for NFkappaB-mediated activation of target promoters. Increasing concentrations of the coactivator p300/CBP reverses Stat5b inhibition at both the interferon-regulatory factor-1 and NFkappaB-TK promoters, suggesting that Stat5b may be squelching limiting coactivators via protein-protein interactions as one mechanism of promoter inhibition. These results further substantiate our observation that Stat factors can function as transcriptional inhibitors. Our studies reveal cross-talk between the Stat5b and NFkappaB signal transduction pathways and suggest that Stat5b-mediated inhibition of target promoters occurs at the level of protein-protein interactions and involves competition for limiting coactivators.

Autoregulation of the rat prolactin gene in lactotrophs

The autoregulation of prolactin (PRL) secretion in the rat has been demonstrated at both the hypothalamus and the pituitary levels. Studies on the direct negative feedback effect of PRL in the lactotrophs have concentrated on the acute effect on PRL secretion which does not involve change in PRL synthesis. In this study, we have developed a cotransfection assay in somatolactotrophs where we examine the effect of PRL on the transcription of its own gene. We found that oPRL, at physiological concentrations, exerts a strong and specific inhibition of the rPRL gene transcription in PRL-deficient GC cells. This effect is mediated by both the intermediate and the long forms of PRL receptor. The inhibition was also reproduced in GH3 cells, which secretes PRL, by adding exogenous oPRL in the presence of anti-rat PRL antiserum to neutralize endogenous rPRL. Cellular specificity was demonstrated by testing this regulation in non-pituitary cell types where no modulation of the PRL promoter reporter gene could be elicited by PRL, even with cotransfection with the Pit-1 expression vector. Finally, deletions of the rPRL promoter indicate that the full inhibitory effect of PRL requires the same regulatory domains (proximal and distal) that have been described for the other PRL gene regulators. These results strongly suggest the existence of the extra-short loop regulation of the rat PRL at the transcriptional level.

Functional characterization of the intermediate isoform of the human prolactin receptor

Prolactin-dependent signaling occurs as the result of ligand-induced dimerization of the prolactin receptor (PRLr). While three PRLr isoforms have been characterized in the rat, studies have suggested the existence of several human isoforms in breast carcinoma species and normal tissues. Reverse transcription polymerase chain reaction was performed on mRNA isolated from the breast carcinoma cell line T47D, revealing two predominant receptor isoforms: the previously described long PRLr and a novel human intermediate PRLr. The nucleotide sequence of the intermediate isoform was found to be identical to the long isoform except for a 573-base pair deletion occurring at a consensus splice site, resulting in a frameshift and truncated intracytoplasmic domain. Scatchard analysis of the intermediate PRLr revealed an affinity for PRL comparable with the long PRLr. While Ba/F3 transfectants expressing the long PRLr proliferated in response to PRL, intermediate PRLr transfectants exhibited modest incorporation of [(3)H]thymidine. Significantly, however, both the long and intermediate PRLr were equivalent in their inhibition of apoptosis of the Ba/F3 transfectants after PRL treatment. The activation of proximal signaling molecules also differed between isoforms. Upon ligand binding, Jak2 and Fyn were activated in CHO-K1 cells transiently transfected with the long PRLr. In contrast, the intermediate PRLr transfectants showed equivalent levels of Jak2 activation but only minimal activation of Fyn. Last, Northern analysis revealed variable tissue expression of intermediate PRLr transcript that differed from that of the long PRLr. Taken together, differences in signaling and tissue expression suggest that the human intermediate PRLr differs from the long PRLr in physiological function.

The 20-kilodalton (kDa) human growth hormone (hGH) differs from the 22-kDa hGH in the effect on the human prolactin receptor

Previously we have demonstrated that 20-kDa human GH (20K-hGH) is a full agonist for hGH receptor (hGHR) even though its complex formation with hGHR and hGH-binding protein differs from that of 22-kDa human GH (22K-hGH). In this study, we focused on the effect of 20K-hGH on human PRL receptor (hPRLR). To elucidate the effects of 20K-hGH on hPRLR and compare them with those of 22K-hGH, we prepared two cells stably expressing full-length hPRLR, Ba/F3-hPRLR and CHO-hPRLR. In the proliferation of Ba/F3-hPRLR cells, which can grow in a dose-response to lactogenic hormones, both 20K- and 22K-hGH exhibited bell-shaped curves in the absence of exogenous zinc ion (Zn2+); however, the curve of 20K-hGH was shifted to a 10-fold higher concentration than that of 22K-hGH in view of EC50 value (the EC50 of 20K- and 22K-hGH were 15 nM and 1.5 nM, respectively). Addition of Zn2+ up to 25 microM increased the activities of both 20K- and 22K-hGH; however, the enhancement by Zn2+ was greater in 20K-hGH than in 22K-hGH, thereby the activities of both hGH isoforms reached the same level at 25 microM Zn2+. Nevertheless, in the presence of 0.25-1 microM free Zn2+, which is equal in human serum, the activity of 20K-hGH was still lower than that of 22K-hGH. The modest effect of 20K-hGH on activating hPRLR in the absence of Zn2+ was confirmed in the rat serine protease inhibitor 2.1 (Spi2.1) gene promoter activation and JAK2/Stat5 tyrosine phosphorylation in CHO-hPRLR. In addition, in human breast cancer cell T-47D, 20K-hGH was proved to stimulate Stat5 tyrosine phosphorylation to much lower degree than 22K-hGH via not hGHR but hPRLR. Taken together, our data suggest that 20K-hGH may be a weaker agonist for hPRLR than 22K-hGH in the human body; therefore 20K-hGH may alleviate the hPRLR-mediated side-effects such as breast cancer when administered to human body.

Prolactin and neuroimmunomodulation: in vitro and in vivo observations

Prolactin (PL) is a mammotropic neuropeptide produced by the pituitary and extrapituitary cells existing as several isoforms and belongs to the growth and lactogenic hormone family, which includes growth hormone and placental lactogens. The secretion of pituitary PL is under hypothalamic control. The cytokines IL-1, IL-2, and IL-6 also stimulate production, while IFN-gamma and endothelin-3 are inhibitory. PL exerts its effects via PL receptors (PLr) which exist as three isoforms. PL regulates reproduction, osmoregulation, and behavior and has potent immunomodulatory effects. PL is structurally related to members of the cytokine/hematopoietic family such as erythropoietin, GM-CSF, growth hormone, and IL-2 to IL-7. The PLr is a member of the cytokine/hematopoietic receptor family. Interaction of PL with PLr activates the Jak kinases which phosphorylate latent STAT proteins resulting in the activation of transcription. PL counteracts the effects of corticosteroids by enhancing Th1 cellular responses. Perturbations of PL physiology have significant immunologic effects. Hypoprolactinemia impairs immune function while hyperprolactinemia enhances active systemic lupus erythematosus, Reiter's disease, juvenile and adult rheumatoid arthritis (RA), autoimmune thyroiditis, multiple sclerosis, and cardiac allograft rejection. PL gene polymorphisms have now been shown to be linked to RA. Thus, manipulation of PL may have significant clinical utility. Further study of the relationship of the PL/PLr complex, other hormones, and the immune system will provide further insights into the potential therapeutic utility of this complex in immune diseases.

Prolactin receptor expression in the developing human prostate and in hyperplastic, dysplastic, and neoplastic lesions

In situ hybridization and immunohistochemistry were used to localize and compare the expression of the long form of the human prolactin receptor in fetal, prepubertal, and adult prostate. Results were then compared with hyperplastic, dysplastic, and neoplastic lesions. Both receptor message and protein were predominately localized in epithelial cells of the fetal, neonatal, prepubertal, and normal adult prostate. In hyperplastic lesions the expression of the receptor was unchanged with respect to normal epithelial cells. Irrespective of grade, markedly enhanced expression of the receptor was evident in dysplastic lesions. In lower Gleason grade carcinomas the intensity of receptor signal at the message and protein levels approximated that found in normal prostatic epithelium. However, in foci within higher grade cancers, receptor expression appeared diminished. Results from our study suggest that prolactin action plays a role in the development and maintenance of the human prostate and may also participate in early neoplastic transformation of the gland. Diminution of receptor expression in high grade neoplasms could reflect the emergence of a population of cells that are no longer responsive to the peptide hormone.

Characterization of unique truncated prolactin receptor transcripts, corresponding to the intracellular domain, in the testis of the sexually mature chicken

We have examined expression of the chicken PRL receptor (cPRLR) gene in different tissues of the chicken by Northern blot analysis. Most tissues examined (ovary, testis, oviduct, kidney, and fat) possess a prominent full-length (4.6-kb) cPRLR transcript. A larger (11.7-kb) transcript is also detected in ovary, oviduct, testis, and kidney after longer exposure. A unique pattern of cPRLR expression was found in the testis of sexually mature chickens, which have an unusually high abundance of three small transcripts (1.2, 1.7, and 2 kb) in addition to the 4.6-kb transcript found in other tissues. Three domain-specific complementary DNA (cDNA) probes were constructed that correspond to the first and second ligand-binding regions in the extracellular domain and the transmembrane-intracellular domain. With these probes, Northern blot analysis of polyadenylated RNA prepared from the testes of a mature (22-week-old) chicken indicates that the highly abundant (1.2- and 1.7-kb) and less abundant (2.0-kb) cPRLR transcripts in testis hybridize only to the intracellular domain probe. Two types of truncated testis-specific cPRLR transcripts were identified using 5'-RACE (rapid amplification of cDNA ends) analysis of polyadenylated RNA from the testis of a 22-week-old chicken. The predominant truncated cDNA sequence contains the highly conserved box 1 motif [(+)box 1 cDNA] and diverges (at nucleotide 1396) from that of the cPRLR cDNA, just downstream of the transmembrane domain. The other truncated cDNA lacks the box 1 motif [(-)box 1 cDNA], which is replaced by 39 bases that could encode a hydrophobic N-terminus with a putative 5'-untranslated region of 131 bases. Young chickens predominately express the full-length cPRLR messenger RNA (4.6 kb) in the testis. At the onset of sexual maturity, there is a dramatic increase in abundance of the testis-specific (+)box 1 transcript, whereas expression of the full-length cPRLR is depressed. The presence of truncated [(+) or (-)box 1] cPRLR transcripts in the sexually mature chicken testis suggests a complex mechanism of PRL action on gonadal function.

Distinct cytoplasmic regions of the prolactin receptor are required for prolactin-induced calcium entry

Two cytoplasmic regions of the prolactin (PRL) receptor are well documented for their participation in PRL signal transduction, the membrane proximal box 1 and the COOH-terminal region. In order to study the role of these regions in PRL-induced Ca2+ increase, we use Chinese hamster ovary (CHO) cells stably transfected with mutated PRL receptor cDNA. These cells express the long form of PRL receptor deleted from box 1 (CHO Delta1 cells) or the 141 amino acids of the COOH-terminal region (CHO H3 cells). The patch-clamp technique in "whole-cell" configuration and microfluorimetric techniques were used singly or in combination. Data obtained for these cells were compared with those we have recently published using CHO cells expressing the wild-type long form of the PRL receptor (CHO TSE32). In contrast to CHO TSE32 cells, exposure of CHO Delta1 or H3 cells to PRL (0.05-50 nM) did not modify [Ca2+]i. We have previously shown that the PRL-induced calcium influx via voltage-insensitive, Ca2+ channels was due to the activation of tyrosine kinase-dependent K+ channels that hyperpolarize the CHO TSE32 cell membrane (hyperpolarization-driven Ca2+ influx). Therefore, two events are involved in PRL-induced Ca2+ changes (i) JAK2-activation of K+ channels and (ii) intracellular messenger-opening of Ca2+ channels. In CHO Delta1 cells, PRL (0.05-50 nM) neither hyperpolarized the membrane potential nor stimulated the JAK2-dependent K+ current, confirming the pivotal role played by box 1/JAK2 in the PRL-induced activation of K+ channels. However, when these cells were voltage-clamped below the resting membrane potential, application of 5 nM PRL resulted in an increase in Ca2+ influx. Therefore, box 1/JAK2 was not involved in the opening of these Ca2+ channels. In CHO H3 cells, 5 nM PRL activated the K+ current and hyperpolarized the membrane potential without any effect on [Ca2+]i. Moreover, PRL was also ineffective on CHO H3 cells voltage-clamped below the resting membrane potential. Therefore, the COOH-terminal region is involved in the production of the intracellular messenger that opens voltage-independent Ca2+ channels. We conclude from these findings that box 1 and COOH-terminal regions are both needed for PRL-induced Ca2+ changes.

Erythropoietin Receptor and STAT5-Specific Pathways Promote SKT6 Cell Hemoglobinization

Erythrocyte production in mammals is known to depend on the exposure of committed progenitor cells to the glycoprotein hormone erythropoietin (Epo). In chimeric mice, gene disruption experiments have demonstrated a critical role for Epo signaling in development beyond the erythroid colony-forming unit (CFU-e) stage. However, whether this might include the possible Epo-specific induction of red blood cell differentiation events is largely unresolved. To address this issue, mechanisms of induced globin expression in Epo-responsive SKT6 cells have been investigated. Chimeric receptors containing an epidermal growth factor (EGF) receptor extracellular domain and varied Epo receptor cytoplasmic domains first were expressed stably at physiological levels in SKT6 cells, and their activities in mediating induced hemoglobinization were assayed. While activity was exerted by a full-length chimera (EE483), truncation to remove 7 of 8 carboxyl-terminal tyrosine sites (EE372) markedly enhanced differentiation signaling. Moreover, mutation of a STAT5 binding site in this construct (EE372-Y343F) inhibited induced globin expression and SKT6 cell hemoglobinization, as did the ectopic expression of dominant-negative forms of STAT5 in parental SKT6 cells. As in normal CFU-e, SKT6 cells also were shown to express functional receptors for stem cell factor (SCF). To further define possible specific requirements for differentiation signaling, effects of SCF on SKT6 cell hemoglobinization were tested. Interestingly, SCF not only failed to promote globin expression but inhibited this Epo-induced event in a dose-dependent, STAT5-independent fashion. Thus, effects of Epo on globin expression may depend specifically on STAT5-dependent events, and SCF normally may function to attenuate terminal differentiation while promoting CFU-e expansion.

© 1998 by The American Society of Hematology.

Erythropoietin Receptor and STAT5-Specific Pathways Promote SKT6 Cell Hemoglobinization

Erythrocyte production in mammals is known to depend on the exposure of committed progenitor cells to the glycoprotein hormone erythropoietin (Epo). In chimeric mice, gene disruption experiments have demonstrated a critical role for Epo signaling in development beyond the erythroid colony-forming unit (CFU-e) stage. However, whether this might include the possible Epo-specific induction of red blood cell differentiation events is largely unresolved. To address this issue, mechanisms of induced globin expression in Epo-responsive SKT6 cells have been investigated. Chimeric receptors containing an epidermal growth factor (EGF) receptor extracellular domain and varied Epo receptor cytoplasmic domains first were expressed stably at physiological levels in SKT6 cells, and their activities in mediating induced hemoglobinization were assayed. While activity was exerted by a full-length chimera (EE483), truncation to remove 7 of 8 carboxyl-terminal tyrosine sites (EE372) markedly enhanced differentiation signaling. Moreover, mutation of a STAT5 binding site in this construct (EE372-Y343F) inhibited induced globin expression and SKT6 cell hemoglobinization, as did the ectopic expression of dominant-negative forms of STAT5 in parental SKT6 cells. As in normal CFU-e, SKT6 cells also were shown to express functional receptors for stem cell factor (SCF). To further define possible specific requirements for differentiation signaling, effects of SCF on SKT6 cell hemoglobinization were tested. Interestingly, SCF not only failed to promote globin expression but inhibited this Epo-induced event in a dose-dependent, STAT5-independent fashion. Thus, effects of Epo on globin expression may depend specifically on STAT5-dependent events, and SCF normally may function to attenuate terminal differentiation while promoting CFU-e expansion.

© 1998 by The American Society of Hematology.

Prolactin: the forgotten hormone of human breast cancer

Prolactin (PRL) is both a mitogen and a differentiating agent in the mammary gland. It has been shown to be involved in mammary cancer development in rodents, but in human breast cancer, its role has long been overlooked. Three criteria are applied to demonstrate PRL's involvement in this disease: (1) PRL receptors are present in human breast cancer cells, (2) human breast cancer cells in culture respond to PRL as a mitogen, and (3) PRL is synthesized by human breast cancer cells and inhibition of the binding of PRL to its receptors inhibits cell growth.

Expression cloning of a novel estrogenic mouse 17 beta-hydroxysteroid dehydrogenase/17-ketosteroid reductase (m17HSD7), previously described as a prolactin receptor-associated protein (PRAP) in rat

17 beta-Hydroxysteroid dehydrogenases/17-ketosteroid reductases (17HSDs) modulate the biological activity of certain estrogens and androgens by catalyzing reductase or dehydrogenase reactions between 17-keto- and 17 beta-hydroxysteroids. In the present study, we demonstrate expression cloning of a novel type of 17HSD, chronologically named 17HSD type 7, from the HC11 cell line derived from mouse mammary gland. The cloned cDNA, 1.7 kb in size, encodes a protein of 334 amino acids with a calculated molecular mass of 37,317 Da. The primary structure contains segments characteristic of enzymes belonging to the short-chain dehydrogenase/reductase superfamily. Strikingly, mouse 17HSD type 7 (m17HSD7) shows 89% identity with a recently cloned rat protein called PRL receptor-associated protein (PRAP). The function of PRAP has not yet been demonstrated. The enzymatic characteristics of m17HSD7 and RT-PCR-cloned rat PRAP (rPRAP) were analyzed in cultured HEK-293 cells, where both of the enzymes efficiently catalyzed conversion of estrone (E1) to estradiol (E2). With other substrates tested no detectable 17HSD or 20 alpha-hydroxysteroid dehydrogenase activities were found. Kinetic parameters for m17HSD7 further indicate that E1 is a preferred substrate for this enzyme. Relative catalytic efficiencies (Vmax/K(m) values) for E1 and E2 are 244 and 48, respectively. As it is the case with rPRAP, m17HSD7 is most abundantly expressed in the ovaries of pregnant animals. Further studies show that the rat enzyme is primarily expressed in the middle and second half of pregnancy, in parallel with E2 secretion from the corpus luteum. The mRNA for m17HSD7 is also apparent in the placenta, and a slight signal for m17HSD7 is found in the ovaries of adult nonpregnant mice, in the mammary gland, liver, kidney, and testis. Altogether, because of their similar primary structures, enzymatic characteristics, and the tissue distribution of m17HSD7 and rPRAP, we suggest that rPRAP is rat 17HSD type 7. Furthermore, the results indicate that 17HSD7 is an enzyme of E2 biosynthesis, which is predominantly expressed in the corpus luteum of the pregnant animal.

Prolactin (PRL) and its receptor: actions, signal transduction pathways and phenotypes observed in PRL receptor knockout mice

PRL is an anterior pituitary hormone that, along with GH and PLs, forms a family of hormones that probably resulted from the duplication of an ancestral gene. The PRLR is also a member of a larger family, known as the cytokine class-1 receptor superfamily, which currently has more than 20 different members. PRLRs or binding sites are widely distributed throughout the body. In fact, it is difficult to find a tissue that does not express any PRLR mRNA or protein. In agreement with this wide distribution of receptors is the fact that now more than 300 separate actions of PRL have been reported in various vertebrates, including effects on water and salt balance, growth and development, endocrinology and metabolism, brain and behavior, reproduction, and immune regulation and protection. Clearly, a large proportion of these actions are directly or indirectly associated with the process of reproduction, including many behavioral effects. PRL is also becoming well known as an important regulator of immune function. A number of disease states, including the growth of different forms of cancer as well as various autoimmune diseases, appear to be related to an overproduction of PRL, which may act in an endocrine, autocrine, or paracrine manner, or via an increased sensitivity to the hormone. The first step in the mechanism of action of PRL is the binding to a cell surface receptor. The ligand binds in a two-step process in which site 1 on PRL binds to one receptor molecule, after which a second receptor molecule binds to site 2 on the hormone, forming a homodimer consisting of one molecule of PRL and two molecules of receptor. The PRLR contains no intrinsic tyrosine kinase cytoplasmic domain but associates with a cytoplasmic tyrosine kinase, JAK2. Dimerization of the receptor induces tyrosine phosphorylation and activation of the JAK kinase followed by phosphorylation of the receptor. Other receptor-associated kinases of the Src family have also been shown to be activated by PRL. One major pathway of signaling involves phosphorylation of cytoplasmic State proteins, which themselves dimerize and translocate to nucleus and bind to specific promoter elements on PRL-responsive genes. In addition, the Ras/Raf/MAP kinase pathway is also activated by PRL and may be involved in the proliferative effects of the hormone. Finally, a number of other potential mediators have been identified, including IRS-1, PI-3 kinase, SHP-2, PLC gamma, PKC, and intracellular Ca2+. The technique of gene targeting in mice has been used to develop the first experimental model in which the effect of the complete absence of any lactogen or PRL-mediated effects can be studied. Heterozygous (+/-) females show almost complete failure to lactate after the first, but not subsequent, pregnancies. Homozygous (-/-) females are infertile due to multiple reproductive abnormalities, including ovulation of premeiotic oocytes, reduced fertilization of oocytes, reduced preimplantation oocyte development, lack of embryo implantation, and the absence of pseudopregnancy. Twenty per cent of the homozygous males showed delayed fertility. Other phenotypes, including effects on the immune system and bone, are currently being examined. It is clear that there are multiple actions associated with PRL. It will be important to correlate known effects with local production of PRL to differentiate classic endocrine from autocrine/paracrine effects. The fact that extrapituitary PRL can, under some circumstances, compensate for pituitary PRL raises the interesting possibility that there may be effects of PRL other than those originally observed in hypophysectomized rats. The PRLR knockout mouse model should be an interesting system by which to look for effects activated only by PRL or other lactogenic hormones. On the other hand, many of the effects reported in this review may be shared with other hormones, cytokines, or growth factors and thus will be more difficult to study. (ABSTRACT TRUNCATED)

Growth hormone stimulates interferon regulatory factor-1 gene expression in the liver

Interferon regulatory factor-1 (IRF-1) is a transcription factor first identified as part of the nuclear response to interferons. IRF-1 has been shown to be activated by many cytokines, including PRL, and has been thought to play a role in PRL-regulated gene expression in several experimental systems, including the Nb2 T lymphoma cell line, where it was first characterized as a PRL-responsive gene. We now find that IRF-1 gene expression is rapidly activated in vivo by both PRL and GH treatment. A single i.p. injection of rat PRL to hypophysectomized female rats caused a transient increase in nascent hepatic nuclear IRF-1 RNA within 15 min of hormone treatment. The rise in IRF-1 transcripts was accompanied by induction of nuclear protein binding to a DNA element from the proximal IRF-1 promoter, as assessed by gel mobility shift assays; this element was shown previously to mediate PRL-activated gene transcription. GH treatment stimulated a greater and more sustained increase in nascent IRF-1 RNA than PRL, leading to accumulation of IRF-1 transcripts for up to 16 h after a single hormone injection. GH also caused a pronounced induction of hepatic nuclear protein binding to the IRF-1 promoter element. Supershift experiments with specific antibodies showed that signal transducer and activator of transcription 1 (STAT1) and to a lesser extent STAT3 were components of the GH-activated protein-DNA complexes. By contrast, these two STATs were not induced in the liver by PRL. Protein binding to the IRF-1 DNA element and IRF-1 gene activation by GH were not blunted by pretreatment with the protein synthesis inhibitor, cycloheximide, indicating that these hormonal effects are primary consequences of GH-activated signal transduction pathways. Our results identify another component of the rapid nuclear response to GH, and support the idea that multiple primary and secondary signaling pathways contribute to the acute actions of GH on gene expression.

Stoichiometric structure-function analysis of the prolactin receptor signaling domain by receptor chimeras

The intracellular domain of the prolactin (PRL) receptor (PRLr) is required for PRL-induced signaling and proliferation. To identify and test the functional stoichiometry of those PRLr motifs required for transduction and growth, chimeras consisting of the extracellular domain of either the alpha or beta subunit of human granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor (GM-CSFr) and the intracellular domain of the rat PRLr were synthesized. Because the high-affinity binding of GM-CSF results from the specific pairing of one alpha- and one beta-GM-CSFr, use of GM-CSFr/PRLr chimera enabled targeted dimerization of the PRLr intracellular domain. To that end, the extracellular domains of the alpha- and beta-GM-CSFr were conjugated to one of the following mutations: (i) PRLr C-terminal truncations, termed alpha278, alpha294, alpha300, alpha322, or beta322; (ii) PRLr tyrosine replacements, termed Y309F, Y382F, or Y309+382F; or, (iii) PRLr wild-type short, intermediate, or long isoforms. These chimeras were cotransfected into the cytokine-responsive Ba/F3 line, and their expression was confirmed by ligand binding and Northern and Western blot analyses. Data from these studies revealed that heterodimeric complexes of the wild type with C-terminal truncation mutants of the PRLr intracellular domain were incapable of ligand-induced signaling or proliferation. Replacement of any single tyrosine residue (Y309F or Y382F) in the dimerized PRLr complex resulted in a moderate reduction of receptor-associated Jak2 activation and proliferation. In contrast, trans replacement of these residues (i.e., alphaY309F and betaY382F) markedly reduced ligand-driven Jak2 activation and proliferation, while cis replacement of both tyrosine residues in a single intracellular domain (i.e., alphaY309+382F) produced an inactive signaling complex. Analysis of these GM-CSFr-PRLr complexes revealed equivalent levels of Jak2 in association with the mutant receptor chains, suggesting that the tyrosine residues at 309 and 382 do not contribute to Jak association, but instead to its activation. Heterodimeric pairings of the intracellular domains from the known PRLr receptor isoforms (short-intermediate, short-long, and intermediate-long) also yielded inactive receptor complexes. These data demonstrate that the tyrosine residues at 309 and 382, as well as additional residues within the C terminus of the dimerized PRLr complex, contribute to PRL-driven signaling and proliferation. Furthermore, these findings indicate a functional requirement for the pairing of Y309 and Y382 in trans within the dimerized receptor complex.

Transcriptional inhibition by Stat5. Differential activities at growth-related versus differentiation-specific promoters

Prolactin (PRL) induces transcriptional activation of not only growth-related genes such as interferon regulatory factor-1 (IRF-1) but also differentiation-specific genes such as beta-casein through a signaling cascade consisting of Janus kinases and Stat (signal transducer and activator of transcription) factors. To understand better the role of Stats in PRL signaling, we cloned rat Stat5b from a PRL-responsive T cell line Nb2. A Stat5b-specific peptide antibody was generated. In PRL receptor reconstituted COS cells cotransfected with Stat5b or Stat5a, both Stat5 proteins become tyrosine phosphorylated and bind to the IRF-1 GAS (interferon-gamma activation sequence) element in a PRL-inducible manner. Unexpectedly, both Stat5b and Stat5a inhibit PRL induction of the IRF-1 promoter, but they mediate PRL stimulation of the beta-casein promoter. Stat5-mediated inhibition was observed only at the native IRF-1 promoter and not at the isolated IRF-1 GAS element linked to a heterologous thymidine kinase promoter. Mutational analyses showed that the DNA binding activity of Stat5b is not required, but the carboxyl-terminal transactivation domain is essential for Stat5b to inhibit PRL induction of the IRF-1 promoter. These results suggest that Stat5b mediates inhibition via protein-protein interactions. In contrast, both DNA binding and transactivation domains of Stat5b are required to mediate PRL induction of the beta-casein promoter. Furthermore, a carboxyl-terminal truncated dominant negative Stat5b can reverse Stat5b inhibition at the IRF-1 promoter. These studies suggest that Stat proteins can act as not only positive but also negative regulators of gene transcription. Further, Stat5 can modulate gene expression without binding to DNA but via protein-protein interactions.

The short form of the prolactin (PRL) receptor silences PRL induction of the beta-casein gene promoter

The PRL receptor (PRLR) is a member of the cytokine receptor superfamily. Rats and mice express two forms of PRLR, short (SPRLR) and long (LPRLR), which differ in the length and sequence of their cytoplasmic domains. We have analyzed the ability of each form of rat PRLR to transduce lactogenic signals in a bovine mammary gland epithelial cell line. The rat PRLR forms were expressed and detected by RT-PCR, indirect immunofluorescence, and cell surface ligand binding. When the biological activity of each form of PRLR was assessed by transient transfection, we found that the long form was able to activate the beta-casein gene promoter and that the short form was inactive. Interestingly, the coexpression of both forms of PRLR resulted in a block of PRL signal to the milk protein gene promoter as a function of the concentration of the SPRLR. Similar results were obtained when LPRLR was coexpressed with totally or partially inactive tyrosine mutants of either the Nb2 form or the LPRLR form. Thus, these results suggest that the SPRLR form has at least one clear biological function, i.e. to silence lactogenic signals and to contribute to a differential and acute PRL effect in rat tissues. Furthermore, the data derived from coexpression of LPRLR and PRLR mutants confirm a crucial role of the C-terminal tyrosine residue in lactogenic signaling and the dimerization of PRLRs.

Multiple prolactin (PRL) receptor cytoplasmic residues and Stat1 mediate PRL signaling to the interferon regulatory factor-1 promoter

The Nb2 PRL receptor (PRL-R) is known to mediate PRL signaling to the interferon (IFN) regulatory factor-1 (IRF-1) gene via the family of signal transducers and activators of transcription or Stats. To analyze the components of the PRL-R/Stat/IRF-1 signaling pathway, various PRL-R, Stat, and IRF-1-CAT reporter constructs were transiently cotransfected into COS cells. First, mutations in the IFNgamma-activated sequence (GAS), either multimerized or in the context of the 1.7-kb IRF-1 promoter, failed to mediate a PRL response, showing that the IRF-1 GAS is a target of PRL signaling. Next, pairwise alanine substitutions into conserved residues in the proline-rich motif or Box 1 region and two tyrosine mutations, Y308F and Y382F, in the PRL-R intracellular domain all impaired PRL signaling to multimerized GAS or to the 1.7-kb IRF-1 promoter. Furthermore, these PRL-R mutants mediated reduced Stat1 binding to the IRF-1 GAS. Transfection of Stat1 further enhanced PRL signaling to the IRF-1 promoter, suggesting that Stat1 is a positive mediator of PRL action. These studies show that both membrane proximal and distal residues of the PRL-R are involved in signaling to the IRF-1 gene. Further, Stat1 and the GAS element are important for PRL activation of the IRF-1 gene.

JAK2 and STAT5, but not JAK1 and STAT1, are required for prolactin-induced beta-lactoglobulin transcription

Several different Janus kinases (JAKs) and signal transducers and activation of transcription (STATs) have been implicated in mediating the biological responses induced by PRL, based on their ligand-dependent tyrosine phosphorylation and activation. However, these criteria alone do not prove that a particular JAK or STAT is essential for signal transduction. We have used mutant cell lines defective in JAK1, JAK2, or STAT1 to examine their roles in PRL-dependent signaling. JAK2 is absolutely required for PRL-dependent phosphorylation of the receptor, activation of STATs, and induction of beta-lactoglobulin. Wild type, but not kinase-negative JAK2, restores all responses to PRL in JAK2-defective cells, suggesting that JAK2 function, not merely the protein, is required. In contrast, JAK1, which is phosphorylated in response to PRL, is not required for any of these functions. Although STAT1 homodimers do form in response to PRL, no defect in PRL-dependent signaling is apparent when STAT1 is missing, suggesting that STAT5, which is strongly activated in response to PRL, is primarily responsible for driving the expression of PRL-responsive genes.

Specificity of transcription enhancement via the STAT responsive element in the serine protease inhibitor 2.1 promoter

The growth hormone regulated serine protease inhibitor (SPI) 2.1 and 2.2 gene promoters have been shown to contain a response element similar to the gamma-interferon activated sequence (GAS) family of signal transducer and activator of transcription (STAT) response elements. We have investigated the STAT and cytokine specificity of the SPI 2.1 STAT responsive element using a luciferase (LUC) reporter construct and a cDNA complementation strategy in the COS 7 cell line. Growth hormone was found to stimulate SPI-LUC reporter gene expression via activation of STAT 5, but not STATs 1 or 3, which indicates that the SPI 2.1 STAT responsive element is STAT 5 specific. In addition to the growth hormone receptor, the receptors for prolactin and erythropoietin enhanced gene transcription via the SPI 2.1 STAT responsive element, which indicates that this element is, on the other hand, not cytokine specific. Activation of STAT 5 was also observed after growth hormone treatment of cells transfected with cDNA expression plasmids for several different truncated growth hormone receptor mutants, although this activation was less efficient than with the wild type receptor. Point mutation of individual tyrosines in the growth hormone receptor intracellular domain to phenylalanines had no significant effect on signal transduction via STAT 5. These data, taken together with results from experiments using the phosphatase inhibitor sodium orthovanadate, suggest that STAT 5 may not have an absolute requirement for specific phosphorylated receptor tyrosine docking sites. That receptor tyrosine residues in a variety of amino acid contexts, or phosphorylated Janus kinase (JAK) 2 alone, can facilitate STAT 5 activation could explain the observed lack of cytokine specificity in STAT 5 activation.

Epo-induced hemoglobinization of SKT6 cells is mediated by minimal cytoplasmic domains of the Epo or prolactin receptors without modulation of GATA-1 or EKLF

Interaction of erythropoietin with its type 1 receptor is essential to the development of late erythroid progenitor cells. Through the ectopic expression of receptor mutants in lymphoid and myeloid cell lines, insight has been gained regarding effectors that regulate Epo-induced proliferation. In contrast, effectors that regulate Epo-induced differentiation events (e.g. globin gene expression) are largely undefined. For in vitro studies of this pathway, erythroleukemic SKT6 cell sublines have been isolated which stably and efficiently hemoglobinize in response to Epo. Epo rapidly activated Jak2, STAT5 and detectably STATs 1 and 3, while no effects on GATA-1, EKLF or STAT5 expression were observed. Finally, efficient hemoglobinization of SKT6 cells was shown to be mediated by chimeric receptors comprised of the EGF receptor extracellular domain and truncated cytoplasmic subdomains of either the Epo receptor or the prolactin Nb2 receptor. This work further establishes SKT6 cells as an important model for studies of Epo-stimulated differentiation, and shows that this signaling pathway is promoted by a limited set of membrane-proximal receptor domains and effectors.

Increased expression of prolactin receptor mRNA in adenomyotic uterus in mice

To investigate changes in uterine prolactin receptor (PRLR) expression in advanced uterine adenomyosis induced by ectopic pituitary isograft, the amount of PRLR mRNA was measured, by a quantitative competitive PCR, in both normal uteri and uteri with adenomyosis in mice at 20 days or 4, 5 and 6 months after pituitary grafting. There was no difference in the amount of PRLR mRNA between the normal uteri and uteri with pituitary grafts, at 20 days after grafting. At 4-6 months after the operation, however, uteri with adenomyosis contained 2-10 times more PRLR mRNA than did in intact uteri, and the increased amount accompanied the advance of the pathological disorder. The ratio of the short to the long form of PRLR mRNA was little affected by the progress of adenomyosis. These findings suggest that increases in the amount of functional PRLR are in fact a result of the progress of adenomyotic changes, rather than simply a result of elevated circulating prolactin levels.