Multihormonal regulation of the human prolactin gene expression from 5000 bp of its upstream sequence

Genetic polymorphism of prolactin and nitric oxide synthase in Holstein cattle

Background and Aim

Bacterial and viral infections affect the welfare of animals and lead to large economic losses in dairy cattle breeding due to decreased productive indicators and increased culling rates. In modern dairy farming, farmers are looking for effective solutions to prevent and minimize infectious disease risks. To this end, the most relevant study field is the search for gene sites that impact production and health. This study aimed to determine the nature of the distribution of the relative frequencies of alleles and genotypes of polymorphic prolactin (PRL) and nitric oxide synthase (NOS2) in Holstein cows and identify the relationship of these genes with resistance to mastitis and bovine leukemia.

Materials and Methods

For this study, we chose cows because infectious diseases affect the amount of lactation and milk quality. Holstein cattle with mastitis and bovine leukemia were selected. Animal genotypes were determined by restriction fragment length polymorphism (RFLP) of polymerase chain reaction (PCR) products. The results were analyzed using a nonparametric statistical method using Microsoft Excel 2010 and Statistica 6.0.

Results

In healthy animals, 94 genotypes were identified for both genes under study. For bPRL, bPRL-RsaI^AA (72) was the most common genotype and bPRL-RsaI^BB (4) the least; for NOS2, bNOS2 -HinfI^AB (47) was the most common genotype and bNOS2 -HinfI^AA the least (21). In animals with leukemia, 34 genotypes were identified. For PRL, bPRL-RsaI^AA (25) was the most common genotype and bPRL-RsaI^BB (2) the least; for NOS2, bNOS2 -HinfI^BB (17) was the most common genotype and bNOS2 -HinfI^AA (3) the least. In animals with mastitis, 67 genotypes were identified. For PRL, bPRL-RsaI^AA (43) was the most common genotype and bPRL-RsaI^BB (6) the least; for NOS2, bNOS2 -HinfI^BB (31) was the most common genotype and bNOS2-HinfI^AA (7) the least. The distribution of genotypes of polymorphic bPRL and bNOS2 generally coincides, and bPRL-RsaI^BB is the most common genotype. In groups of sick animals, the number of bNOS2 -HinfI^AA homozygotes was lower than that of the control group. In particular, the proportion of animals with the bNOS2 -HinfI^AA genotype with bovine leukemia was 8.7% and with mastitis was 10.3% compared with 22.4% in healthy animals. These data support the possible association of the bNOS2 -HinfI^AA genotype with resistance to infection. The frequency of the bPRL-RsaI^B allele was higher in groups of sick animals. This allele is associated with increased milk productivity, suggesting that highly productive animals are less resistant to the incidence of viral bovine leukemia and mastitis of bacterial etiology.

Conclusion

DNA amplification of Holstein cattle for the polymorphic regions of PRL and NOS2 using the PCR-RFLP method revealed a possible connection between the distribution of relative allele frequencies of bPRL and bNOS2 and resistance to viral and bacterial infections. Thus, in groups of sick animals, the frequency of bPRL-RsaI^BB, associated with increased milk production compared with the theoretically calculated equilibrium value was higher and the number of homozygotes bNOS2 -HinfI^AA was lower than in the control group. In conclusion, animals with increased milk production were more prone to diseases, such as mastitis and bovine leukemia.

Prolactin and its significance in the placenta

Prolactin, a pituitary hormone that was discovered about 80 years ago and is primarily known for its functions in mammary gland development and lactation, is now known to participate in numerous functions across different phylogenetic groups. Fundamentally known for its secretion from lactotroph cells in adenohypophysis region of pituitary gland, newer studies have demonstrated a number of extrapituitary sites which secrete prolactin, where it acts in an autocrine, paracrine, and endocrine manner to regulate essential physiological and biochemical processes. These sites include lymphocytes, epithelial cells of lactating mammary glands, breast cancer cells of epithelial origin, and the placenta. The placenta is one of the most important organs secreting prolactin; however, its role in placental biology has not to date been reviewed comprehensively. This review elaborates upon the various facets of prolactin hormone, including prolactin production and its post-translational modifications and signaling. Major emphasis is placed on placental prolactin and its potential roles, ranging from the role of prolactin in angiogenesis, preeclampsia, maternal diabetes, and anti-apoptosis, among others.

Transcription Factor Pit-1 Affects Transcriptional Timing in the Dual-Promoter Human Prolactin Gene

Gene transcription occurs in short bursts interspersed with silent periods, and these kinetics can be altered by promoter structure. The effect of alternate promoter architecture on transcription bursting is not known. We studied the human prolactin (hPRL) gene that contains 2 promoters, a pituitary-specific promoter that requires the transcription factor Pit-1 and displays dramatic transcriptional bursting activity and an alternate upstream promoter that is active in nonpituitary tissues. We studied large hPRL genomic fragments with luciferase reporters, and used bacterial artificial chromosome recombineering to manipulate critical promoter regions. Stochastic switch mathematical modelling of single-cell time-lapse luminescence image data revealed that the Pit-1-dependent promoter showed longer, higher-amplitude transcriptional bursts. Knockdown studies confirmed that the presence of Pit-1 stabilized and prolonged periods of active transcription. Pit-1 therefore plays an active role in establishing the timing of transcription cycles, in addition to its cell-specific functions.

Differential effects of prenatal stress in female 5-HTT-deficient mice: towards molecular mechanisms of resilience

Prenatal stress (PS) exposure is known to increase the risk of developing emotional disorders like major depression in later life. However, some individuals do not succumb to adversity following developmental stress exposure, a phenomenon referred to as resilience. To date, the molecular mechanisms explaining why some subjects are vulnerable and others more resilient to PS are far from understood. Recently, we have shown that the serotonin transporter (5-HTT) gene may play a modulating role in rendering individuals susceptible or resilient to PS. However, it is not clear which molecular players are mediating the interaction between PS and the 5-Htt genotype in the context of vulnerability and resilience to PS. For this purpose, we performed a microarray study with the help of Affymetrix GeneChip® Mouse Genome 430 2.0 Array, in which we separated wild-type and heterozygous 5-Htt-deficient (5-Htt+/-) PS offspring into susceptible and resilient offspring according to their performance in the forced swim test. Performance-oriented LIMMA analysis on the mRNA expression microarray data was followed by subsequent Spearman's correlation analysis linking the individual qRT-PCR mRNA expression data to various anxiety- and depression-related behavioral and neuroendocrine measures. Results indicate that, amongst others, Fos-induced growth factor (Figf), galanin receptor 3 (Galr3), growth hormone (Gh) and prolactin (Prl) were differentially expressed specifically in resilient offspring when compared to controls, and that the hippocampal expression of these genes showed several strong correlations with various measures of the hypothalamus-pituitary-adrenal axis (re)activity. In conclusion, there seems to be an intricate interplay between the expression of Figf, Galr3, Gh and Prl and neuroendocrine regulation, which may be critical in mediating resilience to PS exposure. More insight into the exact role of these molecular players may significantly enhance the development of new treatment strategies for stress-related emotional disorders.

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.

Luman/CREB3 recruitment factor regulates glucocorticoid receptor activity and is essential for prolactin-mediated maternal instinct

The hypothalamic-pituitary-adrenal (HPA) axis is a major part of the neuroendocrine system in animal responses to stress. It is known that the HPA axis is attenuated at parturition to prevent detrimental effects of glucocorticoid secretion including inhibition of lactation and maternal responsiveness. Luman/CREB3 recruitment factor (LRF) was identified as a negative regulator of CREB3 which is involved in the endoplasmic reticulum stress response. Here, we report a LRF gene knockout mouse line that has a severe maternal behavioral defect. LRF(-/-) females lacked the instinct to tend pups; 80% of their litters died within 24 h, while most pups survived if cross-fostered. Prolactin levels were significantly repressed in lactating LRF(-/-) dams, with glucocorticoid receptor (GR) signaling markedly augmented. In cell culture, LRF repressed transcriptional activity of GR and promoted its protein degradation. LRF was found to colocalize with the known GR repressor, RIP140/NRIP1, which inhibits the activity by GR within specific nuclear punctates that are similar to LRF nuclear bodies. Furthermore, administration of prolactin or the GR antagonist RU486 restored maternal responses in mutant females. We thus postulate that LRF plays a critical role in the attenuation of the HPA axis through repression of glucocorticoid stress signaling during parturition and the postpartum period.

The prolactin gene: a paradigm of tissue-specific gene regulation with complex temporal transcription dynamics

Transcription of numerous mammalian genes is highly pulsatile, with bursts of expression occurring with variable duration and frequency. The presence of this stochastic or 'noisy' expression pattern has been relatively unexplored in tissue systems. The prolactin gene provides a model of tissue-specific gene regulation resulting in pulsatile transcription dynamics in both cell lines and endocrine tissues. In most cell culture models, prolactin transcription appears to be highly variable between cells, with differences in transcription pulse duration and frequency. This apparently stochastic transcription is constrained by a transcriptional refractory period, which may be related to cycles of chromatin remodelling. We propose that prolactin transcription dynamics result from the summation of oscillatory cellular inputs and by regulation through chromatin remodelling cycles. Observations of transcription dynamics in cells within pituitary tissue show reduced transcriptional heterogeneity and can be grouped into a small number of distinct patterns. Thus, it appears that the tissue environment is able to reduce transcriptional noise to enable coordinated tissue responses to environmental change. We review the current knowledge on the complex tissue-specific regulation of the prolactin gene in pituitary and extra-pituitary sites, highlighting differences between humans and rodent experimental animal models. Within this context, we describe the transcription dynamics of prolactin gene expression and how this may relate to specific processes occurring within the cell.

Cellular and molecular specificity of pituitary gland physiology

The anterior pituitary gland has the ability to respond to complex signals derived from central and peripheral systems. Perception of these signals and their integration are mediated by cell interactions and cross-talk of multiple signaling transduction pathways and transcriptional regulatory networks that cooperate for hormone secretion, cell plasticity, and ultimately specific pituitary responses that are essential for an appropriate physiological response. We discuss the physiopathological and molecular mechanisms related to this integrative regulatory system of the anterior pituitary gland and how it contributes to modulate the gland functions and impacts on body homeostasis.

Growth hormone and prolactin regulate human neural stem cell regenerative activity

We have previously shown that the growth hormone (GH)/prolactin (PRL) axis has a significant role in regulating neuroprotective and/or neurorestorative mechanisms in the brain and that these effects are mediated, at least partly, via actions on neural stem cells (NSCs). Here, using NSCs with properties of neurogenic radial glia derived from fetal human forebrains, we show that exogenously applied GH and PRL promote the proliferation of NSCs in the absence of epidermal growth factor or basic fibroblast growth factor. When applied to differentiating NSCs, they both induce neuronal progenitor proliferation, but only PRL has proliferative effects on glial progenitors. Both GH and PRL also promote NSC migration, particularly at higher concentrations. Since human GH activates both GH and PRL receptors, we hypothesized that at least some of these effects may be mediated via the latter. Migration studies using receptor-specific antagonists confirmed that GH signals via the PRL receptor promote migration. Mechanisms of receptor signaling in NSC proliferation, however, remain to be elucidated. In summary, GH and PRL have complex stimulatory and modulatory effects on NSC activity and as such may have a role in injury-related recovery processes in the brain.

Molecular interaction of BMP-4, TGF-beta, and estrogens in lactotrophs: impact on the PRL promoter

The regulatory role of estrogen, bone morphogenetic protein-4 (BMP-4), and TGF-beta has a strong impact on hormone secretion, gene transcription, and cellular growth of prolactin (PRL)-producing cells. In contrast to TGF-beta, BMP-4 induces the secretion of PRL in GH3 cells. Therefore, we studied the mechanism of their transcriptional regulation. Both BMP-4 and TGF-beta inhibited the transcriptional activity of the estrogen receptor (ER). Estrogens had no effect on TGF-beta-specific Smad protein transcriptional activity but presented a stimulatory action on the transcriptional activity of the BMP-4-specific Smads. BMP-4/estrogen cross talk was observed both on PRL hormone secretion and on the PRL promoter. This cross talk was abolished by the expression of a dominant-negative form for Smad-1 and treatment with ICI 182780 but not by point mutagenesis of the estrogen response element site within the promoter, suggesting that Smad/ER interaction might be dependent on the ER and a Smad binding element. By serial deletions of the PRL promoter, we observed that indeed a region responsive to BMP-4 is located between -2000 and -1500 bp upstream of the transcriptional start site. Chromatin immunoprecipitation confirmed Smad-4 binding to this region, and by specific mutation and gel shift assay, a Smad binding element responsible site was characterized. These results demonstrate that the different transcriptional factors involved in the Smad/ER complexes regulate their transcriptional activity in differential ways and may account for the different regulatory roles of BMP-4, TGF-beta, and estrogens in PRL-producing cells.

Hormonal regulation of prolactin cell development in the fetal pituitary gland of the mouse

The developmental process of prolactin (PRL) cells in the fetal pituitary gland was studied in mice. Although PRL cells were hardly detectable in the pituitary gland of intact fetuses, a treatment with 17beta-estradiol (E(2)) in vitro induced a number of PRL cells that varied drastically in number depending on the stage of gestation with a peak at embryonic d 15. This effect was specific to E(2), with epidermal growth factor, insulin, and forskolin failing to induce PRL cells. Although both estrogen receptor (ER)alpha and ERbeta were expressed in the fetal pituitary gland, the results from ER knockout models showed that only ERalpha mediates E(2) action on PRL cells. A few PRL cells were observed in ERalpha-deficient mice as well as in their control littermates, suggesting that estrogen is not required for the phenotype determination of PRL cells. Unexpectedly, the effect of E(2) on the induction of PRL cells in vitro was diminished after embryonic d 15. Present results suggest that the exposure of fetal PRL cells to glucocorticoids (GCs) results in a reduction of sensitivity to E(2). The mechanism underlying the down-regulation of estrogen sensitivity by GCs was found not to be down-regulation of ER levels, induction of annexin 1, a GC-inducible inhibitor of PRL secretion, or a decrease in the number of PRL precursors by apoptosis. The effect of GCs appeared within 2 h and did not require a de novo protein synthesis. GCs are considered to be involved in the mechanisms of silencing pituitary PRL in gestation possibly through a novel mechanism.

Hyperprolactinaemia

Hyperprolactinaemia is a frequent cause of reproductive problems encountered in clinical practice. A variety of pathophysiological conditions can lead to hyperprolactinaemia; therefore, pregnancy, drug effects, hypothyroidism and polycystic ovary syndrome should be excluded before investigating for prolactin-secreting pituitary tumours. Prolactinomas are mainly diagnosed in women aged 20-40 years. They present with clinical features of hyperprolactinaemia (galactorrhoea, gonadal dysfunction), and more rarely with large tumours, headache and visual field loss due to optic chiasm compression. Medical therapy with dopamine agonists is the treatment of choice for both micro- and macroprolactinomas. Tumour shrinkage and restoration of gonadal function are achieved in the majority of cases with dopamine agonists. A trial of withdrawal of medical therapy may be considered in many patients with close follow-up. Pituitary surgery and radiotherapy currently have very limited indications. Pregnancies in patients with prolactinomas need careful planning and close monitoring.

Prolactin and breast cancer etiology: an epidemiologic perspective

A number of epidemiologic studies of prolactin and breast cancer etiology have recently become available. Retrospective case-control studies have suggested a modest positive or null relationship between circulating prolactin concentrations and risk of breast cancer. However these studies are limited by small sample sizes and the collection of blood after case diagnosis. Several large prospective studies, in which blood was collected prior to diagnosis, have observed modest positive associations between prolactin and risk. In a pooled analysis of approximately 80% of the world's prospective data, the relative risk (RR) comparing women in the top vs bottom quartile of prolactin levels was 1.3 (95% confidence interval (CI): 1.1, 1.6, p-trend = 0.002). The results were similar for premenopausal and postmenopausal women. Most notably, high prolactin levels were associated with a 60% increased risk of estrogen receptor (ER) positive tumors, but not with ER negative tumors. Limited genetic data suggest a role of polymorphisms in the prolactin and prolactin receptor genes in risk of breast cancer. Studies of survival have suggested that high pretreatment prolactin levels were associated with treatment failure, earlier recurrence, and worse overall survival. Parity and certain medications are the only confirmed factors associated with prolactin levels in women. Overall, epidemiologic data suggest that prolactin is involved in breast cancer etiology. Further research to better elucidate these associations and their underlying mechanisms is warranted.

Human prolactin gene promoter regulation by estrogen: convergence with tumor necrosis factor-alpha signaling

Estrogens have been implicated in the regulation of prolactin gene expression in man, although previous studies have not defined the molecular mechanism whereby estradiol activates the human prolactin gene promoter (hPrl). We found that estradiol induced a reproducible 1.8-fold activation of the hPrl gene promoter, using pituitary GH3 cells stably transfected with a 5000-bp hPrl promoter fragment linked to luciferase reporter gene. This activation was blocked by treatment with estrogen receptor (ER) antagonists 4-hydroxytamoxifen and ICI-182,780. Promoter deletion and mutagenesis experiments identified a functional estrogen response element (ERE) sequence 1189 bp upstream of the transcription start site that was responsible for estrogen-mediated promoter activation. This site differed from the consensus ERE sequence by two base pairs, one in each half-site. This ERE was identified to be functional through binding ERalpha in EMSAs. Chromatin immunoprecipitation assays confirmed ERalpha binding to this sequence in vivo in the absence of ligand, with increased recruitment when cells were cultured in the presence of estradiol. When cells were treated with both estradiol and TNFalpha, we observed synergistic activation of the hPrl promoter, which was mediated by the -1189-bp ERE. Mutagenesis of this ERE abolished the promoter-activating effect not only of estradiol but also of TNFalpha. These data suggest a novel, promoter-specific signaling interaction between estrogen and TNFalpha signaling, which is likely to be important for prolactin regulation in vivo.

Corticosterone impairs the mRNA expression and activity of 3beta- and 17beta-hydroxysteroid dehydrogenases in adult rat Leydig cells

Clinical and experimental studies, including our own observations, have shown the adverse effects of excess glucocorticoids on testicular steroid hormone production. The present study was designed to gain insight into the molecular mechanisms by which excess corticosterone impairs Leydig cell steroidogenesis. To achieve this, adult rats were administered with corticosterone-21-acetate (2 mg/100 g body weight) twice daily for 15 days. After the treatment period, rats were killed by decapitation. The testes were removed, decapsulated aseptically and used for the isolation of Leydig cells. Purified Leydig cells were used for assessing the activity of 3beta- and 17beta-hydroxysteroid dehydrogenases (HSDs) and total RNA isolation. For in vitro studies, purified Leydig cells (7.5 x 10(6) cells) of control rats were plated in culture flasks and exposed to different concentrations (50, 100, 200, 400, and 800 nmol/L) of corticosterone for 24 h. At the end of incubation, total RNA was isolated from cultured Leydig cells, and the mRNA of 3beta- and 17beta-HSDs was quantified by RT-PCR. A significant reduction in the activities and levels of 3beta-HSD type-I and 17beta-HSD type-III mRNAs in Leydig cells were observed. In vitro studies demonstrated a dose-dependent significant impairment in both the activity and mRNA expression of these enzymes. These results suggest that corticosterone might have a direct effect on the transcription of the genes of 3beta- and 17beta-HSD. It is inferred from the present in vivo and in vitro studies that one of the molecular mechanisms by which excess corticosterone decreases the steroidogenic potency of Leydig cells is by suppressing the mRNA expression of 3beta-HSD type-I and 17beta-HSD type-III enzymes.

Prolactin is involved in glial responses following a focal injury to the juvenile rat brain

A cerebral growth hormone axis is activated following brain injury in the rat and treatment with growth hormone is neuroprotective. We have now investigated whether the closely related prolactin axis has similar properties following injury to the developing rat brain. From one day following a unilateral hypoxic ischemic injury, prolactin immunoreactivity was increased in the affected cortex parallel to the development of the injury (P<0.001). Initial prolactin and prolactin receptor staining on penumbral neurons progressively decreased whereas astrocytes remained strongly immunopositive. Reactive microglia also became strongly prolactin immunoreactive. Unlike growth hormone, central treatment with prolactin failed to rescue neurons in this paradigm. This was confirmed in vitro; rat prolactin failed to protect neurons under conditions for which growth hormone was neuroprotective. However, prolactin had trophic and pro-proliferative effects on glia (P<0.001). We confirmed the expression of the prolactin receptor in vitro by reverse transcriptase polymerase chain reaction, and show its strong association with astrocytes as compared with neurons by immunocytochemistry. In summary, we show for the first time that hypoxia ischemia induces a robust activation of the prolactin axis in regions of the cerebral cortex affected by injury. The lack of neuroprotective properties in vivo and in vitro indicates that, unlike growth hormone, prolactin is not directly involved in neuronal rescue in the injured brain. Its strong relation to glial reactions and its gliatrophic effects suggest that the prolactin axis is primarily involved in a gliogenic response during recovery from cerebral injury.

Tumor necrosis factor-alpha activates the human prolactin gene promoter via nuclear factor-kappaB signaling

Pituitary function has been shown to be regulated by an increasing number of intrapituitary factors, including cytokines. Here we show that the important cytokine TNF-alpha activates prolactin gene transcription in pituitary GH3 cells stably expressing luciferase under control of 5 kb of the human prolactin promoter. Similar regulation of the endogenous rat prolactin gene by TNF-alpha in GH3 cells was confirmed using real-time PCR. Luminescence microscopy revealed heterogeneous dynamic response patterns of promoter activity in individual cells. In GH3 cells treated with TNF-alpha, Western blot analysis showed rapid inhibitory protein kappaB (IkappaBalpha) degradation and phosphorylation of p65. Confocal microscopy of cells expressing fluorescence-labeled p65 and IkappaBalpha fusion proteins showed transient cytoplasmic-nuclear translocation and subsequent oscillations in p65 localization and confirmed IkappaBalpha degradation. This was associated with increased nuclear factor kappaB (NF-kappaB)-mediated transcription from an NF-kappaB-responsive luciferase reporter construct. Disruption of NF-kappaB signaling by expression of dominant-negative variants of IkappaB kinases or truncated IkappaBalpha abolished TNF-alpha activation of the prolactin promoter, suggesting that this effect was mediated by NF-kappaB. TNF-alpha signaling was found to interact with other endocrine signals to regulate prolactin gene expression and is likely to be a major paracrine modulator of lactotroph function.

Prolactin in the brushtail possum (Trichosurus vulpecula): development of homologous radioimmunoassay using recombinant possum prolactin

We report the production of recombinant possum prolactin (posPrl), and its use in the development and validation of a highly specific homologous radioimmunoassay for the measurement of prolactin (Prl) in brushtail possums. This enabled the subsequent investigation of some basic mechanisms involved in the regulation of Prl secretion in this species. Recombinant posPrl spanning the entire coding region was expressed in Escherichia coli, resulting in a 199 amino acid protein with a molecular weight approximately 23 kDa. The potency of posPrl was 45.3 +/- 4.8% that of ovine Prl in a radioreceptor assay using possum mammary gland receptors and induced a 3.4 +/- 0.8-fold increase in progesterone secretion in primary possum granulosa cells. Antiserum (G27) was raised against recombinant posPrl and was highly specific for possum Prl (approximately 30% binding at 1:60,000 final dilution), and exhibited negligible cross-reactivity (<0.0001%) with possum growth hormone. Serial dilutions of pituitary gland extracts, and plasma samples from male and female possums gave parallel inhibition curves to recombinant posPrl standards in the assay. Biological validation of the RIA included treating possums with drugs known to alter Prl secretion in other mammals. In seasonally anoestrous female possums, administration of 20 microg thyrotropin-releasing hormone (TRH) resulted in a 15-fold increase (P < 0.01) in plasma Prl concentrations. In mid-late lactating female possums, a bolus of cabergoline (dopamine agonist; 75 microg) reduced (P < 0.05) plasma Prl levels to baseline for 24 h, while repeated administration (6 x 75 microg at 12 h intervals) suppressed (P < 0.01) plasma Prl concentrations until 24h after the last injection. Prolonged inhibition of Prl levels subsequently caused marked (P < 0.01) attenuation in rate of bodyweight increase of pouch young. The amplitude of the Prl surge in response to a bolus of TRH (15 microg) was 5-fold lower in cabergoline-treated, compared to control mid-late lactating possums. In conclusion, we report the development and validation of a robust and sensitive RIA for measuring Prl concentrations in the plasma of brushtail possums.

EGF stimulates Pit-1 independent transcription of the human prolactin pituitary promoter in human breast cancer SK-BR-3 cells through its proximal AP-1 response element

Normal and neoplastic human mammary gland cells are targets for the proliferative action of prolactin. These cells also synthesize prolactin, thereby inducing an autocrine/paracrine proliferative loop. We present the first extensive analysis of the transcriptional regulation of the human prolactin gene (hPRL) in human mammary tumor cells, SK-BR-3. We show that the pituitary promoter is functional in these cells in the absence of the pituitary-specific factor Pit-1. Expression of exogenous Pit-1 or epidermal growth factor (EGF) treatment stimulates the transfected hPRL pituitary promoter and the endogenous hPRL expression. EGF stimulation is mediated by increased synthesis of c-fos and c-jun, resulting in AP-1 binding to the proximal hPRL pituitary promoter. This regulation involves the EGF receptor, possibly ErbB2 that is highly expressed in SK-BR-3 cells, and a PI3K/JNK pathway. The stimulation of hPRL gene transcription by EGF in mammary cells may include hPRL in a complex regulatory network controlling growth of human mammary cells.

Calcium dynamics and resting transcriptional activity regulates prolactin gene expression

Research on the regulation of hormone gene expression by calcium signaling is hampered by the difficulty of monitoring both parameters within the same individual, living cells. Here we achieved concurrent, dynamic measurements of both intracellular Ca(2+) concentration ([Ca(2+)](i)) and prolactin (PRL) gene promoter activity in single, living pituitary cells. Cells were transfected with the luciferase reporter gene under control of the PRL promoter and subjected to bioluminescence and fluorescence imaging before and after presentation of TSH-releasing hormone (TRH), a prototypic regulator of PRL secretion and gene expression that induces a transient Ca(2+) release, followed by sustained Ca(2+) influx. We found that cells displaying specific photonic emissions (i.e. mammotropes) showed heterogeneous calcium and transcriptional responses to TRH. Transcriptionally responsive cells always exhibited a TRH-induced [Ca(2+)](i) increase. In addition, transcriptional responses were related to the rate of Ca(2+) entry but not Ca(2+) release. Finally, cells lacking transcriptional responses (but showing [Ca(2+)](i) rises) exhibited larger levels of resting PRL promoter activity than transcriptionally responsive cells. Thus, our results suggest that the sustained entry of Ca(2+) induced by TRH (but not the Ca(2+) release) regulates transcriptional responsiveness. Superimposed on this regulation, the previous, resting PRL promoter activity also controls transcriptional responses.

Characterization of a prolactin gene polymorphism and its associations with systemic lupus erythematosus

OBJECTIVE

Hyperprolactinemia is associated with systemic lupus erythematosus (SLE), but the mechanism is unknown. Prolactin is expressed in T lymphocytes and is under the control of an alternative promoter region. We characterized a G/T single-nucleotide polymorphism (SNP) at position -1149 of this promoter and assessed its prevalence in patients with SLE.

METHODS

Electrophoretic mobility shift assays (EMSAs) were performed to determine DNA protein complex formation in the prolactin promoter. Transient transfection of reporter gene constructs containing the G/T promoter alleles into the Jurkat T cell line were used to determine transcription activity. Peripheral blood lymphocytes (PBLs) were treated in vitro with phytohemagglutinin (PHA) to determine levels of prolactin messenger RNA (mRNA).

RESULTS

EMSAs indicated that binding of a GATA-related transcription factor was altered by the G/T SNP at position -1149. Transient transfection studies in Jurkat cells showed that the G allele consistently produced higher promoter activity. PHA treatment of PBLs in vitro induced a greater increment of prolactin mRNA from patients with the GG(-1149) genotype than from those with the TT(-1149) genotype. Disease association studies in a cohort of SLE patients demonstrated an increased frequency of the prolactin -1149 G allele compared with control subjects.

CONCLUSION

We found a functionally significant polymorphism that alters prolactin promoter activity and mRNA levels in the lymphocytes. Altered local prolactin production by immune cells may contribute to disease progression by affecting T cell function.

Estrogen receptor interaction with estrogen response elements

The estrogen receptor (ER) is a ligand-activated enhancer protein that is a member of the steroid/nuclear receptor superfamily. Two genes encode mammalian ER: ERalpha and ERbeta. ER binds to specific DNA sequences called estrogen response elements (EREs) with high affinity and transactivates gene expression in response to estradiol (E(2)). The purpose of this review is to summarize how natural and synthetic variations in the ERE sequence impact the affinity of ER-ERE binding and E(2)-induced transcriptional activity. Surprisingly, although the consensus ERE sequence was delineated in 1989, there are only seven natural EREs for which both ERalpha binding affinity and transcriptional activation have been examined. Even less information is available regarding how variations in ERE sequence impact ERbeta binding and transcriptional activity. Review of data from our own laboratory and those in the literature indicate that ERalpha binding affinity does not relate linearly with E(2)-induced transcriptional activation. We suggest that the reasons for this discord include cellular amounts of coactivators and adaptor proteins that play roles both in ER binding and transcriptional activation; phosphorylation of ER and other proteins involved in transcriptional activation; and sequence-specific and protein-induced alterations in chromatin architecture.

Cell type-specific adenoviral transgene expression in the intact ovine pituitary gland after stereotaxic delivery: an in vivo system for long-term multiple parameter evaluation of human pituitary gene therapy

Ablative therapies for pituitary tumors commonly cause irreversible damage to normal pituitary cells. Toxin gene therapy should therefore ideally be targeted to specific cell types to avoid collateral cell damage. To evaluate cell-type-specific adenoviral gene transfer in the intact pituitary gland we have used stereotaxic transcranial delivery of recombinant adenoviruses in the sheep with continuous assessment of endocrine function. Adenoviral ss-galactosidase expression was driven either by the human cytomegalovirus (hCMV) promoter or the human PRL gene promoter. The hCMV promoter directed adenoviral ss-galactosidase expression in all pituitary cell types, but the PRL promoter restricted this exclusively to lactotropic cells, indicating that this promoter conferred appropriate cell type specificity in the context of adenoviral transduction in vivo. Serial measurements of plasma hormones showed no disruption of endocrine function over 7 days after intrapituitary injection. In summary, this work shows cell type-specific expression of an adenoviral transgene in the mixed cell population of the intact pituitary gland in vivo in a large animal model and indicates that stereotaxic intrapituitary delivery does not disrupt normal endocrine function.

Prolactin: structure, function, and regulation of secretion

Prolactin is a protein hormone of the anterior pituitary gland that was originally named for its ability to promote lactation in response to the suckling stimulus of hungry young mammals. We now know that prolactin is not as simple as originally described. Indeed, chemically, prolactin appears in a multiplicity of posttranslational forms ranging from size variants to chemical modifications such as phosphorylation or glycosylation. It is not only synthesized in the pituitary gland, as originally described, but also within the central nervous system, the immune system, the uterus and its associated tissues of conception, and even the mammary gland itself. Moreover, its biological actions are not limited solely to reproduction because it has been shown to control a variety of behaviors and even play a role in homeostasis. Prolactin-releasing stimuli not only include the nursing stimulus, but light, audition, olfaction, and stress can serve a stimulatory role. Finally, although it is well known that dopamine of hypothalamic origin provides inhibitory control over the secretion of prolactin, other factors within the brain, pituitary gland, and peripheral organs have been shown to inhibit or stimulate prolactin secretion as well. It is the purpose of this review to provide a comprehensive survey of our current understanding of prolactin's function and its regulation and to expose some of the controversies still existing.

Transcriptional targeting to anterior pituitary lactotrophic cells using recombinant adenovirus vectors in vitro and in vivo in normal and estrogen/sulpiride-induced hyperplastic anterior pituitaries

The use of pituitary cell type-specific promoters is a powerful molecular tool to achieve pituitary cell type-specific transcriptional targeting of transgenes encoded by viral vectors. It has recently been proposed that transcriptional targeting of therapeutic genes could be harnessed as a gene therapy strategy for the treatment of pituitary disease. We describe the successful use of the human PRL promoter (hPrl) encoded within recombinant adenovirus vectors to target transgene expression of Herpes Simplex Virus Type 1-Thymidine Kinase (HSV1-TK) or beta-galactosidase to lactotrophic cells in vitro and in vivo. Functionally, the restriction of expression of HSV1-TK to lactotrophic tumor cells, using the hPrl promoter, resulted in the cell type-specific induction of apoptosis in the lactotrophic GH3 tumor cell line, in the presence of ganciclovir (GCV). In the corticotrophic AtT20 cell line, we detected neither HSV1-TK expression, nor apoptosis in the presence of GCV. The hPrl promoter encoded within a recombinant adenoviral vector also restricted transgene expression to lactotrophic cells in primary anterior pituitary (AP) cultures, and importantly, within the anterior pituitary gland in vivo. When the HSV1-TK driven by hPrl promoter was used in an in vivo model ofestrogen/sulpiride lactotroph induced hyperplasia within the AP in situ, the treatment was not effective in either reducing the weight of the gland, the number of lactotrophic cells within the transduced area in vivo, or the circulating PRL levels. This is in contrast to the human cytomegalovirus promoter (hCMV) driving expression of HSV1-TK in the same experimental paradigm, which was effective in reducing pituitary weight and circulating PRL levels. Our results have important implications in the design of gene therapy strategies for pituitary tumors. We demonstrate that both the choice of the in vivo animal model, i.e. adenoma in the AP gland in situ, and the particular gene therapy strategy chosen, i.e. use of strong ubiquitous promoters vs. weaker but cell type-specific promoters, determine the experimental therapeutic outcome.

Glucocorticoids repress transcription from a negative glucocorticoid response element recognized by two homeodomain-containing proteins, Pbx and Oct-1

Several studies have established that the prolactin (PRL) gene is expressed not only in lactotrophs and somatotrophs of the anterior pituitary but, albeit to a lesser extent, in non-pituitary cells like human thymocytes, decidualized endometrium, mammary glands during lactation, and some human non-pituitary cell lines. Despite the requirement in the pituitary for the pituitary-specific transcription factor Pit-1/GHF-1 for PRL expression, the expression in non-pituitary cells occurs in the absence of Pit-1/GHF-1 and can be repressed by glucocorticoids. This prompted us to investigate the transcription factors in non-pituitary cells which are involved in controlling expression and glucocorticoid repression of a previously characterized negative glucocorticoid response element from the bovine prolactin gene (PRL3 nGRE). Here we have demonstrated that non-pituitary cells (COS-7 and mouse hepatoma Hepa1c1c7 cells) conferred increased expression via the PRL3 nGRE mainly because of the binding of the ubiquitously expressed POU-homeodomain-containing octamer transcription factor-1 (Oct-1) to an AT-rich sequence present in the PRL3 sequence. However, full transcriptional activity required the binding of a second ubiquitously expressed homeodomain-containing protein, Pbx, previously shown to bind cooperatively with several homeotic selector proteins. The Pbx binding site in the PRL3 nGRE, located just upstream of the Oct-1 binding site, showed a strong sequence similarity with known Pbx binding sites and bound Pbx with an affinity similar to that of other established Pbx target sequences. Interestingly, both Oct-1 and Pbx binding to the PRL3 nGRE were found to be required for glucocorticoid repression. Addition of in vitro translated glucocorticoid receptor DNA binding domain to the nuclear extract prevented Oct-1 and Pbx from binding to the PRL element. The involvement of the homeobox protein Pbx in glucocorticoid repression via an nGRE identifies a new role for this protein.

Dynamic changes in prolactin promoter activation in individual living lactotrophic cells

The firefly luciferase gene has become widely used as a convenient reporter for studies of gene promoter regulation. Very recently, the development of ultralow-light imaging cameras has enabled the quantitative digital imaging of light signals resulting from luciferase activation in the presence of luciferin substrate. We have applied this technology to the study of PRL promoter activation in individual pituitary tumor cells to study the temporal and spatial characteristics of the expression of a well-characterized pituitary hormone gene. Rat pituitary GH3 cells were transfected by lipofection with a luciferase reporter gene linked to 5000 bp from the human PRL gene 5'-flanking region. A series of stably transfected cell clones were generated, and one of these was chosen for detailed study on the basis of appropriate regulation of high-level luciferase expression by a series of known stimuli including TRH, forskolin, the calcium channel agonist Bay K8644, and basic fibroblast growth factor (bFGF). These cells were subjected to direct imaging of luciferase activity using a Hamamatsu photon-counting camera linked to a Zeiss Axiovert microscope with an Argus-50 image processor. Cells were exposed to 1 mM luciferin, and images were integrated over 30-min periods for up to 72 h. The total photon count over a given field settled to steady levels within 10 h and then remained constant for over 55 h. Addition of forskolin, TRH, or bFGF increased the total photon count of fields of 20-100 cells by 2- to 4-fold consistent with previous data from transient expression assays using the human PRL promoter. Individual cells, on the other hand, showed marked marked temporal and spatial heterogeneity and variability of luciferase expression when studied at 3-h intervals. Unstimulated cells showed variable luciferase expression with up to 40-fold excursions in photon counts per single cell area within 12-h periods. Stimulation of cells with either TRH, forskolin, or bFGF resulted in smooth increases in photon output over fields of 20-100 cells, but again individual cell responses differed widely, with some cells showing slow progressive rises in photon output, others showing phasic or transient responses, and yet others showing no response. In conclusion, we found a surprising degree of heterogeneity and temporal variability in the level of gene expression in individual living pituitary tumor cells over long periods of time, with markedly divergent responses to hormonal or intracellular stimulation. The use of stably transfected clonal cell lines with extended periods of reporter gene imaging offers a valuable insight into control of gene expression in living cells in real time.

The glucocorticoid receptor inhibits the human prolactin gene expression by interference with Pit-1 activity

Glucocorticoids have been shown to inhibit the activity of the human prolactin (hPRL) promoter. Using transient expression experiments in rat pituitary cells, we located the sequence conferring glucocorticoid inhibition to a region which contains Pit-1 binding sites, responsible for pituitary-specific expression, but does not seem to contain a glucocorticoid receptor (GR) binding site. Co-transfection experiments in non-pituitary cell lines, using expression vectors for Pit-1 and different mutants of the human GR show that inhibition of the hPRL gene is seen only in the presence of Pit-1 and GR, and that the DNA binding function of the receptor is not required. Immunoprecipitation studies show that either anti-GR or anti-Pit-1 antibodies are able to co-precipitate GR and Pit-1, suggesting an interaction between these factors. We conclude that the activated GR functionally interferes with the pituitary specific factor Pit-1, thereby leading to the observed transcriptional repression.

Thyroid hormone inhibits the human prolactin gene promoter by interfering with activating protein-1 and estrogen stimulations

Transcription of the human PRL (hPRL) gene in the pituitary is subject to tissue-specific and multihormonal regulation involving two main regulatory regions, a proximal promoter and a distal enhancer. In this report we show that thyroid hormone inhibits the expression of the hPRL gene in rat pituitary cells. Transient expression experiments show that thyroid hormone regulation involves a strong inhibitory element, located in the proximal (-164/-35) promoter, which is modulated by a more distal stimulatory response control region. Gel retardation experiments reveal that the thyroid hormone receptor does not bind to the proximal negative element. We show the existence of an activating protein-1 (AP-1) response element located at positions -61 to -54 of the proximal promoter, conferring AP-1 stimulation to the hPRL promoter. This AP-1 induction is abolished when hormone-bound thyroid hormone receptor is present, indicating that there is an interference between the thyroid hormone receptor and AP-1 regulatory pathways. Furthermore, using the complete hPRL upstream region, we show that estrogen induction is abolished by simultaneous thyroid hormone treatment.

Studies on the mechanism of glucocorticoid-mediated repression from a negative glucocorticoid response element from the bovine prolactin gene

Several models for repression of transcription by glucocorticoid hormone, some of which involve so-called negative glucocorticoid response elements (nGRE), have been suggested. In the cases where nGREs are required, the glucocorticoid receptor (GR) is thought to bind to the nGRE and interfere with transcriptional activation by positively acting transactivating factors. We have studied an nGRE from the bovine prolactin gene promoter (PRL3), which increases basal expression from a heterologous promoter in rat pituitary cells (GH3) and is repressed by glucocorticoids. Two proteins in addition to the GR were identified in pituitary cells to bind specifically to the PRL3 nGRE, one of which was the pituitary-specific transcription factor Pit-1/GHF-1. A mutation in the PRL3 nGRE, which destroyed Pit-1/GHF-1 binding, totally abolished the increased basal expression as well as glucocorticoid repression in transfected GH3 cells. A mutation in the binding site for the second protein, termed XTF, partially impaired basal activity but totally abrogated glucocorticoid repression. The same mutation had no effect on GR binding to the PRL3 nGRE. Mixing experiments with whole-cell extracts containing overexpressed GR from COS cells decreased the binding of both Pit-1/GHF-1 and XTF to the PRL3 element. However, Pit-1/GHF-1 displacement from the PRL3 element by the GR required XTF binding. Furthermore, GR binding to the PRL3 nGRE was required for glucocorticoid repression to occur, because a mutation of the GR binding site abolished the glucocorticoid effect. Moreover, the PRL3 nGRE was found to contain only half a palindromic GRE, allowing only one GR moiety to contact the DNA. These data demonstrate that the PRL3 nGRE is composite in nature and that the ability of the GR to repress transactivation by displacement requires an intermediary factor, XTF.

Prolactin as an autocrine/paracrine factor in breast tissue

The neuroendocrine hormone prolactin (PRL) stimulates breast growth and differentiation during puberty, pregnancy, and lactation. Despite extensive and convincing data indicating that PRL significantly contributes to the pathogenesis and progression of rodent mammary carcinoma, parallel observations for human breast cancer have not been concordant. In particular, the therapeutic alteration of somatolactogenic hormone levels has not consistently altered the course of human breast cancer. Recent data, however, suggest that extra-pituitary tissues are capable of elaborating PRL; indeed, the observation of sustained serum levels of PRL in post-hypophysectomy patients supports this hypothesis. Proof of an autocrine/paracrine loop for PRL within normal and malignant human breast tissues requires that the following three criteria be met: (1) PRL must be synthesized and secreted within mammary tissues; (2) the receptor for PRL (PRLR) must be present within these tissues; and, (3) proliferative responses to autocrine/paracrine PRL must be demonstrated. These criteria have now been fulfilled in several laboratories. With the demonstration of a PRL autocrine/paracrine loop in mammary glands, the basis for the ineffective treatment of human breast cancer by prior endocrine-based anti-somatolactogenic therapies is evident. These findings provide the precedent for novel therapeutic strategies aimed at interrupting the stimulation of breast cancer growth by PRL at both endocrine and autocrine/paracrine levels.

Heparin-binding secretory transforming gene (hst) facilitates rat lactotrope cell tumorigenesis and induces prolactin gene transcription

We have shown previously that human prolactinomas express transforming sequences of the heparin-binding secretory transforming gene (hst) which encodes fibroblast growth factor-4 (FGF-4). To elucidate the role of hst in pituitary tumorigenesis we treated primary rat pituitary and pituitary tumor cell cultures with recombinant FGF-4 and also stably transfected pituitary cell lines with full-length human hst cDNA. Transfectants were screened for hst mRNA expression and FGF-4 production. FGF-4 (0.1-50 ng/ml) caused a dose-dependent 2.5-fold increase of prolactin (PRL) secretion (P < 0.001) in GH4 cells and up to 60% (P < 0.05) in primary cultures, while decreasing growth hormone release (P < 0.001). GH4 hst transfectants displayed markedly enhanced basal PRL secretion (threefold, P < 0.001) and also proliferated faster (P < 0.001). FGF-4 treatment of wild-type GH4 cells, transiently transfected with an expression construct (rPRL.luc) containing a luciferase reporter driven by the rPRL promoter, resulted in a dose-dependent increase of up to 3.3-fold in PRL transcriptional activity. Tumors derived from in vivo subcutaneous injection of GH4 hst-transfected cells strongly expressing FGF-4 grew more aggressively as assessed by histologic invasiveness and proliferating cell nuclear antigen staining (P < 0.01). The results indicate that hst overexpression mediates lactotrope tumor growth and potently stimulates PRL synthesis. Thus, hst may directly facilitate prolactinoma development via paracrine or autocrine action of its secreted protein, FGF-4.

Cyclosporin A, rapamycin and FK506 decrease prolactin release from rat pituitary cells in primary culture

It is at present well established that prolactin exerts a non-specific immunoactivating function. In this work we tested whether the immunosuppressant drugs cyclosporin A, FK506 and rapamycin influence prolactin release from rat pituitary cells in primary culture. The tested drugs had no effect on the prolactin release measured during a 2h incubation period, indicating that they do not influence the secretion of prolactin from intracellular stores into the culture medium. During longer incubation times (48h), however, prolactin release was diminished to 56% +/- 18 (10 microM cyclosporin A), 64% +/- 14 (1 microM rapamycin) or 64% +/- 7 (1 microM FK506), suggesting an effect on prolactin production. At these drug concentrations no toxic effects were observed. The data indicate that inhibition of pituitary prolactin synthesis might contribute to the immunosuppressant action of cyclosporin A, rapamycin and FK506.

Rapamycin, FK506 and cyclosporin A inhibit human prolactin gene expression

In this work we demonstrate that transcription of the human prolactin gene is inhibited by the immunosuppressants FK506 (IC50 = 25 nM), cyclosporin A (IC50 = 190 nM) and rapamycin (IC50 = 25 nM). Whereas the effect of FK506 and cyclosporin A is specific for prolactin gene transcription, rapamycin has a more general effect on transcription and/or translation in pituitary cells. In view of recent work demonstrating the immunoactivating role of prolactin, these results suggest that inhibition of prolactin gene expression in the pituitary may contribute to the mechanism of action of immunosuppressants.

Differential impact of flanking sequences on estradiol- vs 4-hydroxytamoxifen-liganded estrogen receptor binding to estrogen responsive element DNA

The mechanism by which antiestrogens antagonize the ability of estrogen receptor (ER) to induce the transcription of estrogen-regulated genes is only partially understood. To examine the effect of estrogen responsive element (ERE) stereoalignment and flanking sequences on estradiol-liganded ER (E2-ER)-ERE and antiestrogen-liganded ER (4-hydroxytamoxifen-liganded ER or 4-OHT-ER)-ERE binding, several dimeric EREs, containing a perfect inverted repeat (5'-GGTCAgagTGACC-3') but lacking the AT-rich flanking sequences typical of highly estrogen-responsive promoters, were cloned into a plasmid vector. The ERE centers of symmetry were spaced 1.5, 2.0, 3.0, 6.4 and 6.7 helical turns apart. E2-ER and 4-OHT-ER binding to these constructs was specific and saturable, but orientation-independent and, in contrast to our earlier work with E2-ER binding to AT-rich EREs, not cooperative. The affinity of E2-ER binding decreased as the distance between adjacent EREs was increased, suggesting that E2-ER binding to closely spaced EREs is more stable (Kd = 0.38, 0.58, 0.83, 1.23, and 0.96 nM, respectively, for the above spacings). In contrast, the affinity of 4-OHT-ER binding increased with increased ERE spacing (Kd = 2.90, 4.79, 1.39, 1.77, and 0.92 nM, respectively). The presence of AT-rich sequences flanking the ERE increased the binding affinity of E2-ER and 4-OHT-ER, an increase reflected in slower dissociation rates of ER from these EREs. The AT-rich sequence also enhanced the binding capacity of E2-ER but not 4-OHT-ER. Since the binding capacity of 4-OHT-ER is identical with or without an AT-rich region, we suggest that flanking sequences are more important in stabilizing E2-ER binding and may be critical for cooperative binding to stereoaligned EREs.

Dopamine D2 receptor mRNA in the pituitary during the oestrous cycle, pregnancy and lactation in the rat

We have used the technique of quantitative in situ hybridization in order to study the changes in the levels of expression of D2 receptor mRNA in the anterior pituitary gland of female rats at different stages of the reproductive cycle. Plasma prolactin levels in the same animals were determined by radioimmunoassay. Rats in the prooestrous, oestrous, dioestrous 1, dioestrous 2 phases of the oestrous cycle and in pregnant, lactating, ovariectomised and ovariectomised animals treated with diethylstilbestrol (DES) have been examined. Our results show that expression of D2 receptor mRNA in the anterior pituitary gland varies during the oestrous cycle, with the lowest expression measured during oestrus. Expression levels increased during dioestrus 1, reaching the highest values in dioestrus 2 and declining again in prooestrus. Expression of D2 receptor mRNA was reduced during pregnancy when compared to lactating animals. In ovariectomised animals, the level of D2 receptor mRNA was similar to that observed in intact animals during oestrus. Ovariectomised animals treated with DES showed dramatically increased prolactin levels, while D2 receptor mRNA remained low. Prolactin secretion might be controlled not only by variations in the release and plasma concentrations of dopamine itself, but also by modulation of D2 receptor expression in pituitary cells. Our results suggest that the variations in D2 receptor density in the anterior pituitary cells at different physiological states are, at least to some extent, regulated at the level of gene expression.

Thyrotropin-releasing hormone and epidermal growth factor induce human prolactin expression via identical multiple cis elements

Pituitary GH3 cells were transfected with different deletion mutants of the human prolactin (hPRL) promoter fused to the CAT reporter gene. The proximal region (-250 to -42) was sufficient to confer stimulation by both thyrotropin-releasing hormone (TRH) and epidermal growth factor (EGF). Further deletion analyses demonstrated the importance of the three proximal Pit-1 binding sites in this response. However, Pit-1 binding oligonucleotides confer neither TRH nor EGF induction to a linked neutral promoter, suggesting that other elements might be involved. We have previously shown that sequence A (-115 to -85) is needed together with Pit-1 binding sites for full cyclic AMP response of hPRL-CAT. Mutation of this sequence strongly affects TRH and EGF induction. On the other hand, three copies of sequence A confer both TRH and EGF response to a linked neutral promoter. In conclusion, although TRH and EGF activate mostly different intracellular pathways, they mediate transcriptional induction of the hPRL promoter via identical cis elements.

Binding of a 100-kDa ubiquitous factor to the human prolactin promoter is required for its basal and hormone-regulated activity

cAMP strongly stimulates the activity of the human prolactin (hPRL) promoter. We have previously shown that two types of cis-element are required for this cAMP regulation; binding sites for the pituitary-specific factor Pit-1, and the sequence spanning nucleotides -115 to -85 (named sequence A). Sequence A contains the TGACG motif found in the consensus sequence of the cAMP-responsive element (CRE). In this study, we show that a mutation in the TGACG motif of sequence A strongly reduces not only the cAMP regulation but also the Ca2+ regulation and basal activity of the hPRL promoter. Furthermore, gel-shift assays indicate that the mutation prevents binding of a ubiquitous factor which is not the CRE-binding protein. Southwestern experiments suggest that this ubiquitous factor's molecular mass is approximately 100 kDa. We conclude that binding of a 100-kDa ubiquitous factor to sequence A is required for full basal and hormonal regulation of hPRL-promoter activity.

Negative regulation by dexamethasone of the potentiation of neuromedin C-induced growth hormone and prolactin release by estradiol in anterior pituitary cell aggregates

The bombesin-like peptide neuromedin C (NMC) stimulated the release of growth hormone (GH) and prolactin (PRL) from adult male rat anterior pituitary cell aggregates cultured in serum-free medium. This effect, particularly on GH release, was significantly increased when the cells were cultured in the presence of estradiol (E2). The GH response to NMC was not affected by 4 nM dexamethasone (Dex) but was enhanced in the presence of 80 nM Dex. However, the strong stimulatory action of E2 on the GH response to NMC was completely abolished by 4 nM Dex. In contrast, the PRL response to NMC was reduced by 4 nM Dex and this to a proportionally similar extent in the presence or absence of E2. The present data show that glucocorticoids can be both positive and negative regulators of the same response system.