Transforming growth factor-beta regulation of estradiol-induced prolactinomas

Revealing Sexual Dimorphism in Prolactin Regulation From Early Postnatal Development to Adulthood in Murine Models

Serum prolactin (PRL) levels exhibit a gradual rise both in male and female rats from birth to adulthood, with females consistently displaying higher levels compared to age-matched males. This pattern has traditionally been attributed to the development and maturation of endocrine and neuroendocrine networks responsible for regulating PRL synthesis and secretion. However, the effect of dopamine (DA), which acts as an inhibitory factor on lactotroph function, also increases from birth to puberty, particularly in females. Nonetheless, the secretion of PRL remains higher in females compared to males. On the other hand, the observed sex differences in serum PRL levels during early postnatal development cannot be attributed to the influence of estradiol (E2). While serum E2 levels gradually increase after birth, only after 45 days of life do the disparities in E2 levels between females and males become evident. These observations collectively suggest that neither the maturation of hypothalamic DA regulation nor the rise in E2 levels can account for the progressive and sustained elevation in serum PRL levels and the observed sexual dimorphism during postnatal development. This review highlights the importance of recent discoveries in animal models that shed light on inhibitory mechanisms in the control of PRL secretion within the pituitary gland itself, that is intrapituitary mechanisms, with a specific emphasis on the role of transforming growth factor β1 and activins in PRL secretion.

Sex differences in the pituitary TGFβ1 system: The role of TGFβ1 in prolactinoma development

Prolactinomas are the most frequent functioning pituitary adenomas, and sex differences in tumor size, behavior and incidence have been described. These differences have been associated with earlier diagnosis in woman, as well as with serum estradiol levels. Experimental models of prolactinomas in rodents also show a higher incidence in females, and recent findings suggest that gender differences in the transforming growth factor beta 1 (TGFβ1) system might be involved in the sex-specific development of prolactinomas in these models. The aim of this review is to summarize the literature supporting the important role of TGFβ1 as a local modulator of pituitary lactotroph function and to provide recent evidence for TGFβ1 involvement in the sex differences found in prolactinoma development in animal models.

The anterior pituitary gap junctions: potential targets for toxicants

The anterior pituitary regulates endocrine organs and physiological activities in the body. Environmental pollutants and drugs deleterious to the endocrine system may affect anterior pituitary activity through direct action on anterior pituitary cells. Within the gland, endocrine and folliculostellate cells are organized into and function as individual tridimensional networks, each network regulating its activity by coordinating the connected cells' responses to physiological or pathological cues. The gap junctions connecting endocrine cells and/or folliculostellate cells allow transmission of information among cells that is necessary for adequate network function. Toxicants may affect gap junctions as well as the physiology of the anterior pituitary. However, whether toxicants effects on anterior pituitary hormone secretion involve gap junctions is unknown. The folliculostellate cell gap junctions are sensitive to hormones, cytokines and growth factors. These cells may be an interesting experimental model for evaluating whether toxicants target anterior pituitary gap junctions.

A combined opiate agonist and antagonist treatment reduces prolactin secreting pituitary tumor growth

Prolactin secreting pituitary adenomas (prolactinomas) is the most common pituitary tumors in humans. Animal studies have identified aggressive prolactinoma development in fetal alcohol exposed rats. We have recently identified a combination treatment of a μ opioid receptor antagonist naltrexone and a δ opioid receptor agonist D-Ala2-,N-Me-Phe4,Gly-ol Enkephalin (DPDPE) increases innate immune function. In this study, we tested whether naltrexone and DPDPE combination therapy is useful to control pituitary tumor growth. Fetal alcohol exposed and control Fischer 344 female rats at 60 days of age were ovariectomized and received an estrogen implant to induce prolactinomas. Six weeks after the estrogen implant, these animals received treatments of naltrexone and DPDPE or saline. The growth of the pituitary tumor prior to and after opioidergic agent treatments was visualized using magnetic resonance imaging (MRI). At the end of the treatment, pituitary weights, plasma prolactin and splenic levels of cytotoxic factors were determined. Both imaging data and weight data indicated that the volume and the weight of the pituitary were increased more after estrogen treatment in animals exposed to fetal alcohol than control. Naltrexone and DPDPE treatment reduced the weight and volume of the pituitary gland and plasma levels of prolactin in both fetal alcohol exposed and control-fed animals. The treatment of opioidergic agents also increased the levels of cytotoxic factors in the spleen. These data provide a novel possibility in treating pituitary tumors using a combination therapy of naltrexone and DPDPE.

The pituitary TGFβ1 system as a novel target for the treatment of resistant prolactinomas

Prolactinomas are the most frequently observed pituitary adenomas and most of them respond well to conventional treatment with dopamine agonists (DAs). However, a subset of prolactinomas fails to respond to such therapies and is considered as DA-resistant prolactinomas (DARPs). New therapeutic approaches are necessary for these tumors. Transforming growth factor β1 (TGFβ1) is a known inhibitor of lactotroph cell proliferation and prolactin secretion, and it partly mediates dopamine inhibitory action. TGFβ1 is secreted to the extracellular matrix as an inactive latent complex, and its bioavailability is tightly regulated by different components of the TGFβ1 system including latent binding proteins, local activators (thrombospondin-1, matrix metalloproteases, integrins, among others), and TGFβ receptors. Pituitary TGFβ1 activity and the expression of different components of the TGFβ1 system are regulated by dopamine and estradiol. Prolactinomas (animal models and humans) present reduced TGFβ1 activity as well as reduced expression of several components of the TGFβ1 system. Therefore, restoration of TGFβ1 inhibitory activity represents a novel therapeutic approach to bypass dopamine action in DARPs. The aim of this review is to summarize the large literature supporting TGFβ1 important role as a local modulator of pituitary lactotroph function and to provide recent evidence of the restoration of TGFβ1 activity as an effective treatment in experimental prolactinomas.

Persistent ERK/MAPK activation promotes lactotrope differentiation and diminishes tumorigenic phenotype

The signaling pathways that govern the lactotrope-specific differentiated phenotype, and those that control lactotrope proliferation in both physiological and pathological lactotrope expansion, are poorly understood. Moreover, the specific role of MAPK signaling in lactotrope proliferation vs differentiation, whether activated phosphorylated MAPK is sufficient for prolactinoma tumor formation remain unknown. Given that oncogenic Ras mutations and persistently activated phosphorylated MAPK are found in human tumors, including prolactinomas and other pituitary tumors, a better understanding of the role of MAPK in lactotrope biology is required. Here we directly examined the role of persistent Ras/MAPK signaling in differentiation, proliferation, and tumorigenesis of rat pituitary somatolactotrope GH4 cells. We stimulated Ras/MAPK signaling in a persistent, long-term manner (over 6 d) in GH4 cells using two distinct approaches: 1) a doxycycline-inducible, oncogenic V12Ras expression system; and 2) continuous addition of exogenous epidermal growth factor. We find that long-term activation of the Ras/MAPK pathway over 6 days promotes differentiation of the bihormonal somatolactotrope GH4 precursor cell into a prolactin-secreting, lactotrope cell phenotype in vitro and in vivo with GH4 cell xenograft tumors. Furthermore, we show that persistent activation of the Ras/MAPK pathway not only fails to promote cell proliferation, but also diminishes tumorigenic characteristics in GH4 cells in vitro and in vivo. These data demonstrate that activated MAPK promotes differentiation and is not sufficient to drive tumorigenesis, suggesting that pituitary lactotrope tumor cells have the ability to evade the tumorigenic fate that is often associated with Ras/MAPK activation.

Involvement of MEK/ERK1/2 and PI3K/Akt pathways in the refractory behavior of GH3B6 pituitary tumor cells to the inhibitory effect of TGFβ1

Pituitary tumor cells have a poor response to the growth inhibitory effect of TGFβ1, possibly resulting from the cross talk of TGFβ/Smads signal with other signaling pathways, an undescribed mechanism in these tumoral cells. To address this hypothesis, we investigated whether the mitogen-activated extracellular signal-regulated kinase (MEK)/ERK1/2 and phosphoinositide-3 kinase/protein kinase B (PI3K/Akt) pathways were able to regulate the antimitogenic effect of TGFβ1 on GH3B6 cells. TGFβ1 treatment decreased the cell proliferation and induced an activation of mothers against decapentaplegic homolog 2/3 (Smad2/3), effects that were potentiated by MEK and PI3K inhibitors, thus indicating the existence of a cross talk between TGFβ1/Smad with the MEK/ERK1/2 or PI3K/Akt pathways. In addition, through immunoprecipitation assays, a direct interaction was observed between Smad2/3-ERK1/2 and Smad2/3-Akt, which decreased when the GH3B6 cells were incubated with TGFβ1 in the presence of MEK or PI3K inhibitors, thereby suggesting that the ERK1/2- and Akt-activated states were involved. These Smad2/3-ERK1/2 and Smad2/3-Akt associations were also confirmed by confocal and transmission electron microscopy. These findings indicate that the TGFβ1-antimitogenic effect in GH3B6 cells was attenuated by the MEK/ERK1/2 and PI3K/Akt pathways via modulating Smad2/3 phosphorylation. This molecular mechanism could explain in part the refractory behavior of pituitary tumor cells to the inhibitory effect of TGFβ1.

Signaling pathways regulating pituitary lactotrope homeostasis and tumorigenesis

Dysregulation of the signaling pathways that govern lactotrope biology contributes to tumorigenesis of prolactin (PRL)-secreting adenomas, or prolactinomas, leading to a state of pathological hyperprolactinemia. Prolactinomas cause hypogonadism, infertility, osteoporosis, and tumor mass effects, and are the most common type of neuroendocrine tumor. In this review, we highlight signaling pathways involved in lactotrope development, homeostasis, and physiology of pregnancy, as well as implications for signaling pathways in pathophysiology of prolactinoma. We also review mutations found in human prolactinoma and briefly discuss animal models that are useful in studying pituitary adenoma, many of which emphasize the fact that alterations in signaling pathways are common in prolactinomas. Although individual mutations have been proposed as possible driving forces for prolactinoma tumorigenesis in humans, no single mutation has been clinically identified as a causative factor for the majority of prolactinomas. A better understanding of lactotrope-specific responses to intracellular signaling pathways is needed to explain the mechanism of tumorigenesis in prolactinoma.

TGF-β stimulation in human and murine cells reveals commonly affected biological processes and pathways at transcription level

Background

The TGF-β signaling pathway is a fundamental pathway in the living cell, which plays a key role in many central cellular processes. The complex and sometimes contradicting mechanisms by which TGF-β yields phenotypic effects are not yet completely understood. In this study we investigated and compared the transcriptional response profile of TGF-β1 stimulation in different cell types. For this purpose, extensive experiments are performed and time-course microarray data are generated in human and mouse parenchymal liver cells, human mesenchymal stromal cells and mouse hematopoietic progenitor cells at different time points. We applied a panel of bioinformatics methods on our data to uncover common patterns in the dynamic gene expression response in respective cells.

Results

Our analysis revealed a quite variable and multifaceted transcriptional response profile of TGF-β1 stimulation, which goes far beyond the well-characterized classical TGF-β1 signaling pathway. Nonetheless, we could identify several commonly affected processes and signaling pathways across cell types and species. In addition our analysis suggested an important role of the transcription factor EGR1, which appeared to have a conserved influence across cell-types and species. Validation via an independent dataset on A549 lung adenocarcinoma cells largely confirmed our findings. Network analysis suggested explanations, how TGF-β1 stimulation could lead to the observed effects.

Conclusions

The analysis of dynamical transcriptional response to TGF-β treatment experiments in different human and murine cell systems revealed commonly affected biological processes and pathways, which could be linked to TGF-β1 via network analysis. This helps to gain insights about TGF-β pathway activities in these cell systems and its conserved interactions between the species and tissue types.

Establishment and characterization of dairy cow growth hormone secreting anterior pituitary cell model

A dairy cow anterior pituitary cell (DCAPC) model was established in vitro for the study of growth hormone (GH) synthesis and secretion in the anterior pituitary gland of the dairy cow. Pituitary glands were obtained from Holstein dairy cows' heads cut by electric saw, and the posterior pituitary glands were removed to obtain integrated anterior pituitary glands. Immunohistochemistry assay of GH in the anterior pituitary glands showed that most somatotrophs were located within the lateral wings of the anterior pituitary. Tissues of the lateral wings of the anterior pituitary were dispersed and cultured in Dulbecco's modified Eagle's medium supplemented with 10% fetal bovine serum. The DCAPCs displayed a monolayer, cobblestone, epithelial-like morphology which are the typical characteristics of the anterior pituitary cells. The DCAPCs were subcultured continuously over ten passages. GH immunoreactivity was present in DCAPCs at passage 10. The transcription of the bovine GH mRNA in DCAPCs at passage 10 was decreased to below 50% compared with the lateral wings of the anterior pituitary tissues. Thus, our DCAPCs model is effective for the in vitro examination of GH synthesis and secretion in the dairy cow anterior pituitary gland. The effects of transforming growth factor beta 1 (TGF-β1) and interferon-γ (IFN-γ) on the expression of GH mRNA in DCAPCs at passage 3 were also investigated. There were no obvious changes in transcription of the GH gene after treatment with TGF-β1 for 24 h, while IFN-γ increased transcription of the GH gene in a dose-dependent manner.

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.

Treatment for prolactinomas and hyperprolactinaemia: a lifetime approach

BACKGROUND

Prolactinomas are the most common hormone-secreting pituitary tumours and are amenable to medical therapy with dopamine agonists. Indication for treatment will most commonly result from hypogonadism, infertility or symptoms related to tumour size. Thus, both diagnosis and treatment will essentially depend on the patients' stage of life, namely prepubertal, reproductive or postreproductive stage. This review will focus on a lifespan-dependent diagnosis and treatment for prolactinoma and hyperprolactinaemia.

METHODS

PubMed, the Cochrane Library, the Web of Science and EMBASE were searched electronically. No restriction was made with respect to language. Relevant current articles will be included in this review.

RESULTS

Prevalence of prolactinomas and clinical symptoms are age group-specific, and treatment of first choice is dopamine agonists over the whole lifespan. Open questions in the treatment for hyperprolactinaemia include optimal choice and duration of pharmacological treatment. In addition, concerns have been raised on the safety of dopamine agonists since a reported association of valvular heart disease with dopaminergic treatment in patients with Parkinson's disease.

CONCLUSIONS

Clinical presentation and consequences of hyperprolactinaemia and prolactinoma will differ in the specific stages of reproductive life and require an adequate lifetime-dependent diagnostic and therapeutic approach.

Sexual dimorphism in the effect of concomitant progesterone administration on changes caused by long-term estrogen treatment in pituitary hormone immunoreactivities of rats

BACKGROUND

Since in clinical practice long-term estrogen (E) treatment is frequently applied, our aim was to study the effect of concomitant progesterone (P) administration on changes caused by long-term estrogen treatment in the secretion of LH, FSH, PRL and GH.

MATERIAL/METHODS

Diethylstilbestrol (DES), P or both in silastic capsules were implanted under the skin of prepubertal Sprague-Dawley male and female rats. Animals survived for two or five months. We have also studied whether the changed hormone secretion caused by DES can return to normal level 1 or 2 months after removing DES capsule.

RESULTS

1.) The males more rapidly responded than females with decreasing basal LH release upon treatments. The basal FSH release was decreased only in males. The effect of DES persisted in males; however, in females basal LH and FSH levels were upregulated after removal of DES capsule. 2.) The basal GH levels were low in each group. The body weight and length were depressed by DES in both genders and P little blunted this effect. The body weight and length in males remained low after removal of DES capsule, in females it was nearly similar to intact rats. 3.) There was no sexual dimorphism in the effect of steroids on PRL secretion. In both genders DES extremely enhanced the PRL levels, P prevented the effect of DES. PRL levels returned to intact value after removal of DES influence. 4.) Removal of DES capsule reversed the changes in the immunohistochemical appearance of hormone immunoreactivities.

CONCLUSIONS

There was sexual dimorphism in the change of basal gonadotropic hormone and GH secretion but not of PRL upon DES and DES+P treatments. P was basically protective and this role may be mediated by P receptors locally in the pituitary gland.

Fibroblast growth factor-2 autofeedback regulation in pituitary folliculostellate TtT/GF cells

To investigate paracrine regulation of pituitary cell growth, we tested fibroblast growth factor (FGF) regulation of TtT/GF folliculostellate (FS) cells. FGF-2, and FGF-4 markedly induced cell proliferation, evidenced by induction of pituitary tumor transforming gene-1 (Pttg1) mRNA expression and percentage of cells in S phase. Signaling for FGF-2-induced FS cell proliferation was explored by specific pharmacological inhibition. A potent inhibitory effect on FGF-2 action was observed by blocking of Src tyrosine kinase with 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d] pyrimidine (>or=0.1 microm), followed by protein kinase C (PKC) inhibition with GF109203X. Treatment with FGF-2 (30 ng/ml; 10 min) activated phosphorylation of signal transducer and activator of transcription-3, ERK, stress-activated protein kinase/c-Jun N-terminal kinase, Akt, and focal adhesion kinase. Src inhibition with 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d] pyrimidine suppressed FGF-2-induced Akt and focal adhesion kinase, indicating effects downstream of FGF-2-induced Src activation. FGF-2 also markedly induced its own mRNA expression, peaking at 2-4 h, and this effect was suppressed by Src tyrosine kinase inhibition. The PKC inhibitor GF109203X abolished FGF-2 autoinduction, indicating PKC as the primary pathway involved in FGF-2 autoregulation in these cells. In addition to pituitary FGF-2 paracrine activity on hormonally active cells, these results show an autofeedback mechanism for FGF-2 in non-hormone-secreting pituitary FS cells, inducing cell growth and its own gene expression, and mediated by Src/PKC signaling.

Evaluation of angiogenesis in 77 pituitary adenomas using endoglin as a marker

Angiogenesis, a fundamental process for the development and growth of a tumor, is less expressive in adenomas than in the normal pituitary tissue. There is controversy about the behavior of angiogenesis as a function of hormonal secretion or other characteristics of pituitary tumors. Endoglin (CD105) is a proliferation-associated antigen on endothelial cells, as well as an endothelial progenitor cell marker. We used the anti-endoglin antibody, a glycoprotein expressed in endothelial cells and conjunctive tissue, as a new marker particularly associated with neovascularization, in order to determine microvascular density (MVD) in pituitary adenomas. There were 77 samples, 31 males and 46 females, carriers of micro- (n = 24) or macroadenomas (n = 53). No significant difference was found in MVD concerning the variables of age, clinical presentation, and immunohistochemical phenotype or tumor size. MVD in males (median 5.4) was significantly higher (P = 0.001) than in females (median 3.0). Cell proliferation, as evaluated by the MIB-1 antibody (a cellular proliferation index [Ki-67 antigen], which is present in all stages of the cellular cycle except for the resting cells), ranged from 0% to 19.58%. No correlation was found between MIB-1 and MVD. It is possible to infer that the lower MVD found in pituitary adenomas in females reflects an inhibitory estrogen action on TGF-beta1, a protein involved in vascular remodeling. Because of its role as a TGF receptor ligand, endoglin proved to be sensitive in detecting this gender difference in pituitary tumor angiogenesis.

The characteristic change in the distribution of S-100 immunoreactive folliculostellate cells in rat anterior pituitary upon long-term estrogen treatment is prevented by concomitant progesterone treatment

The presence of folliculostellate cells in the anterior pituitary was described 49 years ago. These cells give about 10% of the whole cell population and through their long processes they provide intrahypophyseal communication. The folliculostellate cells contain S-100 protein. Its immunostaining was used to identify these cells. It was previously found that the diethylstilbestrol treatment basically influences the morphology and function of the trophic hormone secreting as well as the folliculostellate cells. In the present experiment, we have studied whether a concomitant progesterone treatment can prevent or attenuate changes caused by diethylstilbestrol treatment in the distribution of folliculostellate, prolactin, and GH cells. Diethylstilbestrol alone induced the appearance of prolactinomas. Inside the prolactinomas, folliculostellate cells were scattered but outside the prolactinomas they formed a demarcation line. Inside the prolactinomas, there were only a few growth hormone immunoreactive cells but they surrounded the prolactinomas in a ring-like pattern. When diethylstilbestrol was implanted with progesterone, the changes being characteristic for diethylstilbestrol treatment, could not develop. Concomitant progesterone influence prevented morphological changes in the anterior pituitary. Progesterone alone had no effect. In accordance with the formation of prolactinomas, the plasma prolactin level was very high in diethylstilbestrol treated rats. Concomitant progesterone treatment prevented the effect of diethylstilbestrol. Progesterone alone did not influence the prolactin level. GH levels did not significantly differ in any groups.

Immunohistochemical evaluation of the microvascular density through the expression of TGF-beta (CD 105/endoglin) and CD 34 receptors and expression of the vascular endothelial growth factor (VEGF) in oligodendrogliomas

Angiogenesis has been proposed as essential for the growth of solid tumors. The determinants of this process, the growth factors and the vascular endothelial receptors have brought a potential in the tumor prognostic determination as well as perspectives of "targets" for antiangiogenic therapy. In oligodendrogliomas (OL), angiogenesis is little known and/or has generated conflicting results. In order to clarify angiogenesis in OL, we have evaluated the immunohistochemical expression of vascular endothelial growth factor (VEGF) and the microvascular density (MVD) through the expression of TGF-beta (CD105/endoglin) (MVD-CD105) and CD34 (MVD-CD34) receptors using the Chalkley point method in 30 OL. No significant immune reaction was found for the VEGF. There was expression in <10% of tumor cells and/or staining of weak intensity in 15 (50.0%), >10% of cells and moderate intensity staining in 1 (3.33%), and negative expression in 14 (46.67%). If present, the expression was restricted to tumor and endothelial cells. Our findings suggest that VEGF has little influence on OL angiogenesis. All specimens showed CD105 and CD34 expression in the intratumor vascular endothelium, suggesting involvement of CD105 in OL angiogenesis. The mean +/- SD MVD-CD105 and MVD-CD34 were 10.83 +/- 2.24 and 11.00 +/- 2.76 in OL (P = 0.086; r = 0.319); 10.00 +/- 2.00 and 10.40 +/- 3.02 in OL grade II (n = 15) (P = 0.547; r = 0.105), and 11.67 +/- 2.22 and 11.53 +/- 2.45 in OL grade III (n = 15) (P = 0.817; r = 0.551), respectively. The absence of correlation between DMV-CD105, DMV-CD34 and tumor grades suggests that anti-CD105 and anti-CD34 antibodies have different vascular specificities. MVD-CD105 was greater in OL grade III than in OL grade II (P = 0.0032), indicating an increase in the vascular neoformation, something which must be evaluated as a possible prognostic factor in OL. Both TGF-beta and CD105 bring perspectives as "targets" for antiangiogenic treatments in OL.

What can we learn from rodents about prolactin in humans?

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

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.

Estrogen actions on lactotroph proliferation are independent of a paracrine interaction with other pituitary cell types: a study using lactotroph-enriched cells

The mitogenic action of estrogen on estrogen-responsive tissues is suggested to be mediated by paracrine growth factors secreted from neighboring estrogen receptor-positive cells. Using pituitary lactotrophs in primary culture, on which estrogen exerts both mitogenic and antimitogenic actions in a cell context-dependent manner, we investigated whether a paracrine cell-to-cell interaction with other pituitary cell types was required for estrogen action. In pituitary cells, enriched for lactotrophs by 85% using differential sedimentation on a discontinuous Percoll gradient, 17beta-estradiol (E2) showed an antimitogenic action on lactotrophs in the presence of IGF-I, which was similar to that in control unenriched cells. Mitogenic actions were also seen in lactotroph-enriched cells when E2 was administered alone, in combination with serum, or in combination with the adenylate cyclase activator forskolin. Similar results were obtained in 90% lactotroph-enriched cells collected by fluorescence-activated cell sorting from transgenic rats expressing enhanced green fluorescent protein under the control of the prolactin promoter. The putative role of basic fibroblast growth factor (bFGF) as a paracrine factor mediating the mitogenic action of estrogen was not supported by the results that: 1) bFGF inhibited lactotroph proliferation; 2) immunoneutralization of bFGF failed to block E2-induced proliferation; and 3) cellular bFGF levels were not altered by E2 treatment. These results suggest that the antimitogenic and mitogenic actions of estrogen on lactotrophs do not require paracrine signals from other pituitary cell types and that estrogen directly influences lactotroph proliferation.

Changes in thrombospondin-1 levels in the endothelial cells of the anterior pituitary during estrogen-induced prolactin-secreting pituitary tumors

Thrombospondin-1 (TSP-1), a multifunctional matrix glyco-protein, has been shown to control tumor growth by inhibiting angiogenesis in various tissues. However, the role of this glycoprotein in pituitary angiogenesis is not well studied. In this report, we determined the changes in the production and action of TSP-1 on endothelial cells in anterior pituitary following estradiol treatment, which is known to increase prolactin-secreting tumor growth and vascularization in this tissue. We showed that TSP-1 immunoreactive protein is distributed in the anterior pituitary, particularly in the endothelial cells. Estradiol treatment for 2 and 4 weeks decreased the total tissue immunoreactive level of TSP-1 as well as the endothelial cell-specific immunoreactive level of this protein in the anterior pituitary. The steroid treatment also decreased the protein levels of TSP-1 in anterior pituitary tissues and in purified pituitary endothelial cells in primary cultures. Determination of the effects of TSP-1 on proliferation and migration of pituitary-derived endothelial cells in primary cultures elucidated an inhibitory action of TSP-1 on these vascular cell functions. These results suggest that locally produced TSP-1 may regulate estrogen angiogenic action on the pituitary.

Global analysis of gene expression in the estrogen induced pituitary tumor of the F344 rat

The F344 rat rapidly forms large prolactinomas in response to chronic estrogen treatment. To identify genes expressed in the course of this estrogen induced pituitary tumor growth, we performed microarray analysis on the F344 rat pituitary after chronic estrogen treatment and on untreated controls. At a significance level set to minimize type I error, some 72 genes were found to be differentially expressed between estrogen treated and untreated. Of those genes, 70 have not been reported previously as being affected by estrogen in the F344 rat pituitary. Since many other investigators have studied the effect of estrogen on specific gene expression in rat pituitary, we also examined the mRNA expression of the 36 genes that have been previously reported as having their expression affected by estrogen in the rat pituitary. Of these, 13 were found to have their expression affected by estrogen treatment in the same direction as had been reported by others.

Isolation and characterization of rat pituitary endothelial cells

Most previous studies that determined the effect of estradiol on angiogenesis used endothelial cells from nonpituitary sources. Because pituitary tumor tissue receives its blood supply via portal and arterial circulation, it is important to use pituitary-derived endothelial cells in studying pituitary angiogenesis. We have developed a magnetic separation technique to isolate endothelial cells from pituitary tissues and have characterized these cells in primary cultures. Endothelial cells of the pituitary showed the existence of endothelial cell marker, CD31, and of von Willebrand factor protein. These cells in cultures also showed immunoreactivity of estrogen receptors alpha and beta. The angiogenic factors, vascular endothelial growth factor and basic fibroblast growth factor, significantly increased proliferation and migration of the pituitary-derived endothelial cells in primary cultures. These results suggest that a magnetic separation technique can be used for enrichment of pituitary-derived endothelial cells for determination of cellular mechanisms governing the vascularization in the pituitary.

Bone morphogenetic protein-4 inhibits corticotroph tumor cells: involvement in the retinoic acid inhibitory action

The molecular mechanisms governing the pathogenesis of ACTH-secreting pituitary adenomas are still obscure. Furthermore, the pharmacological treatment of these tumors is limited. In this study, we report that bone morphogenetic protein-4 (BMP-4) is expressed in the corticotrophs of human normal adenohypophysis and its expression is reduced in corticotrophinomas obtained from Cushing's patients compared with the normal pituitary. BMP-4 treatment of AtT-20 mouse corticotrophinoma cells has an inhibitory effect on ACTH secretion and cell proliferation. AtT-20 cells stably transfected with a dominant-negative form of the BMP-4 signal cotransducer Smad-4 or the BMP-4 inhibitor noggin have increased tumorigenicity in nude mice, showing that BMP-4 has an inhibitory role on corticotroph tumorigenesis in vivo. Because the activation of the retinoic acid receptor has an inhibitory action on Cushing's disease progression, we analyzed the putative interaction of these two pathways. Indeed, retinoic acid induces both BMP-4 transcription and expression and its antiproliferative action is blocked in Smad-4dn- and noggin-transfected Att-20 cells that do not respond to BMP-4. Therefore, retinoic acid induces BMP-4, which participates in the antiproliferative effects of retinoic acid. This new mechanism is a potential target for therapeutic approaches for Cushing's disease.

Putting the brakes on lactotrope hyperplasia

Although much is known about the mechanisms by which dopamine, the major regulator of lactotrope function, controls hormone production and secretion, its growth inhibiting actions have remained less well characterized. Recent research has uncovered an important role for endogenously produced transforming growth factor beta (TGFbeta) and TGFbeta receptors in dopamine's ability to put the brakes on lactotrope proliferation. These novel findings promise increased knowledge of the genesis and potential therapeutic control of human prolactinomas.

Gene therapy for pituitary tumors

Pituitary tumors are the most common primary intracranial neoplasms. Although most pituitary tumors are considered typically benign, others can cause severe and progressive disease. The principal aims of pituitary tumor treatment are the elimination or reduction of the tumor mass, normalization of hormone secretion and preservation of remaining pituitary function. In spite of major advances in the therapy of pituitary tumors, for some of the most difficult tumors, current therapies that include medical, surgical and radiotherapeutic methods are often unsatisfactory and there is a need to develop new treatment strategies. Gene therapy, which uses nucleic acids as drugs, has emerged as an attractive therapeutic option for the treatment of pituitary tumors that do not respond to classical treatment strategies if the patients become intolerant to the therapy. The development of animal models for pituitary tumors and hormone hypersecretion has proven to be critical for the implementation of novel treatment strategies and gene therapy approaches. Preclinical trials using several gene therapy approaches for the treatment of anterior pituitary diseases have been successfully implemented. Several issues need to be addressed before clinical implementation becomes a reality, including the development of more effective and safer viral vectors, uncovering novel therapeutic targets and development of targeted expression of therapeutic transgenes. With the development of efficient gene delivery vectors allowing long-term transgene expression with minimal toxicity, gene therapy will become one of the most promising approaches for treating pituitary adenomas.

Dopamine, dopamine D2 receptor short isoform, transforming growth factor (TGF)-beta1, and TGF-beta type II receptor interact to inhibit the growth of pituitary lactotropes

The neurotransmitter dopamine is known to inhibit prolactin secretion and the proliferation of lactotropes in the pituitary gland. In this study, we determined whether dopamine and TGFbeta1 interact to regulate lactotropic cell proliferation. We found that dopamine and the dopamine agonist bromocriptine stimulated TGFbeta1 secretion and TGFbeta1 mRNA expression but inhibited lactotropic cell proliferation both in vivo and in vitro. The dopamine's inhibitory action on lactotropic cell proliferation was blocked by a TGFbeta1-neutralizing antibody. We also found that PR1 cells, which express low amounts of the dopamine D2 receptor, demonstrated reduced expression of TGFbeta1 type II receptor and TGFbeta1 mRNA levels and had undetectable levels of TGFbeta1 protein. These cells showed a reduced TGFbeta1 growth-inhibitory response. Constitutive expression of the D2 receptor short isoform, but not the D2 receptor long isoform, induced TGFbeta1 and TGFbeta1 type II receptor gene expression and recovered dopamine- and TGFbeta1-induced growth inhibition in PR1 cells. The constitutive expression of D2 receptor short isoform also reduced the tumor cell growth rate. These data suggest that a TGFbeta1 system may mediate, in part, the growth-inhibitory action of dopamine on lactotropes.

Effects of estrogen replacement therapy on pituitary size, prolactin and thyroid-stimulating hormone concentrations in menopausal women

There are few studies in the literature investigating pituitary size at an age consistent with the menopause and the influence of estrogen replacement therapy on pituitary height. We therefore evaluated the effect of estrogen on pituitary size, prolactin and thyroid-stimulating hormone (TSH) levels in menopausal women. Sixty-nine women were evaluated, 47 using estrogen and 22 controls. The measure of pituitary height was obtained from magnetic resonance imaging. Hormone evaluation did not show a statistically significant difference in mean (+/-standard deviation) prolactin level between the group using estrogen (7.6 +/- 6.4 ng/ml) and controls (5.1 +/- 3.4 ng/ml; p = 0.15), yet mean TSH level was significantly higher in the treated group (1.9 +/- 1.5 vs. 1.2 +/- 0.9 microU/ml; p = 0.03). Mean pituitary height in the estrogen-treated group (5.2 +/- 1.4 mm) was greater than in the controls (4.4 +/- 1.4 mm; p = 0.04). However, when such potential confounders as age, prolactin and TSH levels, treatment and duration of estrogen exposure were considered, the magnitude of difference did not attain significance. In conclusion, estrogen may play a role in pituitary size, since a mean difference in pituitary height, estimated as 0.78 mm, was detected between the groups in favor of the estrogen-treated group. This suggestion can be investigated in further studies.

Role of protein kinase C-Ras-MAPK p44/42 in ethanol and transforming growth factor-beta3-induced basic fibroblast growth factor release from folliculostellate cells

In the present study, we determined the interactive effects of ethanol and transforming growth factor beta-3 (TGF-beta3) on basic fibroblast growth factor (bFGF) release from folliculostellate (FS) cells and the role of the mitogen-activated protein kinase (MAPK) pathway in this interaction. We found that TGF-beta3 and ethanol alone increased release of bFGF from FS cells, but together they showed markedly increased levels of bFGF compared with the individual effect. Ethanol and TGF-beta3 alone moderately increased activation of MAPK p44/42, but together they produced marked activation of MAPK p44/42. TGF-beta3 alone increased the activation of smad2. Ethanol did not activate smad2 or alter TGF-beta3 activation of smad2. Pretreatment of FS cells with a mitogen-activated protein kinase kinase 1/2 inhibitor or with a protein kinase C (PKC) inhibitor suppressed the TGF-beta3 and ethanol actions on MAPK p44/42 activation and bFGF release. Ethanol and TGF-beta3, either alone or in combination, increased the levels of active Ras. Furthermore, the MAPK p44/42 activation by TGF-beta3 and ethanol was blocked by overexpression of Ras N17, a dominant negative mutant of Ras p21. These data suggest that the PKC-activated Ras-dependent MAPK p44/42 pathway is involved in the cross talk between TGF-beta3 and ethanol to increase bFGF release from FS cells.

Transforming growth factor-beta3 increases gap-junctional communication among folliculostellate cells to release basic fibroblast growth factor

Folliculostellate (FS) cells are known to communicate with each other and with endocrine cells via gap junctions in the anterior pituitary. We investigated whether TGFbeta3 and estradiol, known to regulate FS cell production and secretion of basic fibroblast growth factor (bFGF), increases gap junctional communication to alter bFGF secretion from FS cells. FS cells in monolayer cultures were treated with TGFbeta3 or vehicle alone for 24 h and then microinjected with Lucifer Yellow and high-molecular-weight Texas Red dextran. Ten minutes later the transfer of dye among adjacent cells was recorded with a digital microscope. TGFbeta3 increased the transfer of dye. The TGFbeta3-neutralizing antibody and the gap junction inhibitor octanol reduced the effect of TGFbeta3 on the transfer of dye. The TGFbeta3-induced transfer of dye was unaltered by simultaneous treatment with estradiol. The steroid alone also had no effect. TGFbeta3 increased total and phosphorylated levels of connexin 43. Estradiol treatment did not produce any significant changes on basal or TGFbeta3-induced increases in connexin 43 levels. The gap-junction inhibitor octanol reduced TGFbeta3-increased levels of bFGF in FS cells. Taken together, these results suggest that TGFbeta3 may act on FS cells to increase gap-junctional communication to maximize its effect on bFGF secretion.

Mediation of basic fibroblast growth factor-induced lactotropic cell proliferation by Src-Ras-mitogen-activated protein kinase p44/42 signaling

Basic fibroblast growth factor (bFGF), which is secreted from folliculostellate cells in the anterior pituitary, is known to be involved in the communication between folliculostellate cells and lactotropes during estradiol-induced lactotropic cell proliferation. We studied the role of MAPK p44/42 in bFGF-regulated cell proliferation using enriched lactotropes and the lactotrope-derived PR1 cell line. In cell cultures, bFGF increased cell proliferation of PR1 cells and enriched lactotropes. In both of these cell populations, bFGF also increased phosphorylation of MAPK p44/42. U0126, an inhibitor of MAPK p44/42, blocked the bFGF-induced activation of MAPK p44/42 as well as the bFGF-induced cell proliferation of enriched lactotropes and PR1 cells. Treatment of PR1 cells with bFGF increased the activity of Ras p21, whereas overexpression of a dominant negative mutant of Ras p21 abrogated the bFGF-induced activation of MAPK p44/42 in these cells. Furthermore, the Src kinase inhibitor PP1 suppressed bFGF-induced activation of MAPK p44/42 in both enriched lactotropes and PR1 cells. The Src kinase inhibitor PP1 also reduced bFGF activation of Ras p21 and cell proliferation in PR1 cells. On the other hand, the bFGF-induced activation of MAPK p44/42 in enriched lactotropes and PR1 cells was not affected by protein kinase C inhibitors. These data suggest that bFGF induction of lactotropic cell proliferation is possibly mediated by activation of Src kinase, Ras p21, and MAPK p44/42.

Endocrinology of lactation

The endocrine control of lactation is one of the most complex physiologic mechanisms of human parturition. Mammogenesis, lactogenesis, galactopoiesis, and galactokinesis are all essential to assure proper lactation. Prolactin is the key hormone of lactation and seems to be the single most important galactopoietic hormone. Oxytocin, serotonin, opioids, histamine, substance P, and arginine-leucine modulate prolactin release by means of an autocrine/paracrine mechanism, whereas estrogen and progesterone hormones can act at the hypothalamic and adenohypophysial levels. Human placental lactogen and growth factors play an essential role to assure successful lactation during pregnancy. Oxytocin is the most powerful galactokinetic hormone.

Cross-talk between the estrogen receptor-, protein kinase A-, and mitogen-activated protein kinase-mediated signaling pathways in the regulation of lactotroph proliferation in primary culture

Using pharmacological means we investigated the functional interactions between the estrogen receptor (ER)-, protein kinase A (PKA)-, and mitogen-activating protein kinase (MAPK)-mediated pathways in the regulation of lactotroph proliferation in primary culture. Treatment of lactotrophs for 28 h with the PKA inhibitor H89 or KT5720, an effective inhibitor of forskolin-induced proliferation, inhibited both insulin- and estradiol-induced proliferation. Inhibition of the MAPK activity by PD98059 or U0126 suppressed not only insulin-induced proliferation but also forskolin- and estradiol-induced proliferation. However, treatment for 28 h with the antiestrogens 4-hydroxy tamoxifen and ICI182780 failed to antagonize estradiol-induced lactotroph proliferation but instead enhanced it. Prolonging the antiestrogen treatment time from 28 to 88 h was effective in antagonizing estradiol-induced proliferation with this long-term treatment also inhibiting insulin- and forskolin-induced proliferation. There was no decrease in these mitogen-induced proliferations following treatment with a progesterone antagonist or protein kinase C inhibitor. These results suggest that cross-talk occurs between the ER-, PKA-, and MAPK-mediated signaling pathways in the regulation of lactotroph proliferation, and that antiestrogens stimulate and inhibit estradiol-induced proliferation in a time-dependent manner.

Involvement of protein kinase C-dependent mitogen-activated protein kinase p44/42 signaling pathway for cross-talk between estradiol and transforming growth factor-beta3 in increasing basic fibroblast growth factor in folliculostellate cells

We have recently shown that TGF-beta3, in the presence of estradiol, increases the release of basic fibroblast growth factor (bFGF) from folliculostellate (FS) cells in the pituitary. We determined the interactive effects of TGF-beta3 and estradiol on bFGF production and release from FS cells, and the role of the MAPK pathway in TGF-beta3 and estradiol interaction. We found that TGF-beta3 and estradiol alone moderately increased cell content and release of bFGF from FS cells; but together, they markedly increased the peptide. Estradiol and TGF-beta3 alone moderately activated MAPK p44/42; together they produced marked activation of MAPK p44/42. Pretreatment of FS cells with an MAPK kinase 1/2 inhibitor or with protein kinase C inhibitors suppressed the activation of MAPK p44/42, bFGF release, and protein level increases, all of which were induced by TGF-beta3 and estradiol. Estradiol and TGF-beta3, either alone or in combination, increased the levels of active Ras. Furthermore, bFGF induction by TGF-beta3 and estradiol was blocked by overexpression of Ras N17, a dominant negative mutant of Ras p21. Estrogen receptor blocker ICI 182,780 failed to prevent estrogen's and TGF-beta3's effects on bFGF. These data suggest that an estradiol receptor-independent protein kinase C- activated Ras-dependent MAPK pathway is involved in the cross-talk between TGF-beta3 and estradiol to increase bFGF production and/or release from FS cells.

Cellular strategies of estrogen-mediated neuroprotection during brain development

The role of estrogen during brain development is well documented. Estrogen influences cell survival and differentiation and also controls the formation and maintenance of neural networks. Knowledge of trophic estrogen action in the central nervous system (CNS) was the basis for the establishment of research programs directed toward a potential function of estrogen as a neuroprotective factor in the adult brain. Considerable evidence has accumulated over the years supporting this hypothesis. Experimental and epidemiologic studies as well as clinical trials have demonstrated that estrogen is beneficial for the course of neurodegenerative disorders such as Parkinson and Alzheimer diseases but may also protect neurons from postischemic neuronal degeneration. In this article, we aim to unravel potential physiologic responses and cell survival strategies that allow a more detailed understanding of estrogen-mediated neuroprotection in the brain. In particular, we focus on the participation of estrogen in the regulation of apoptotic processes. Furthermore, we present data on reciprocal estrogen-growth factor interactions. Both of these mechanisms were found to operate during brain development and to conciliate estrogen effects on neurons. This makes them likely candidates for taking part in conveying estrogen-dependent neuroprotection in the adult CNS.

Angiogenesis in normal and neoplastic pituitary tissues

Angiogenesis, or the formation of new blood vessels, is a dynamic process needed for embryogenesis, post-natal growth, morphogenesis, tumorigenesis, and for other biological processes. Angiogenesis is very important for tumor development and progression. This review examines the activators and inhibitors of angiogenesis with emphasis on the pituitary gland and pituitary neoplasms. Some of the proteins regulating angiogenesis in pituitary tumors such as vascular endothelial growth factor (VEGF) and VEGF receptors, fibroblasts growth factors (FGF), transforming growth factor beta (TGFB), interleukins, interferons, and matrix metalloproteinases (MMPs) and inhibitors of MMPs have been examined in animal and human pituitary tumor models. However, many other significant regulators of angiogenesis including angiopoietins, angiostatin, and thrombospondins have not been studied extensively in pituitary tumors to date. Newer concepts and developments in angiogenesis such as vasculogenic mimicry and gene therapy approaches to angiogenesis in cancer treatment are also discussed.

The estrogen-occupied estrogen receptor functions as a negative regulator to inhibit cell proliferation induced by insulin/IGF-1: a cell context-specific antimitogenic action of estradiol on rat lactotrophs in culture

Estrogens stimulate cell proliferation in typical estrogen-responsive tissues including the anterior pituitary gland. Here we report that 17-beta estradiol (E2) has estrogen receptor-mediated mitogenic and antimitogenic actions on rat lactotrophs in primary culture, depending on the cell context. E2 did not affect basal proliferation at 2 d after treatment, but it increased it at 4 d. Insulin markedly increased proliferative activity, which was inhibited by simultaneous treatment with E2, even after only 2 d of treatment. This antimitogenic action on insulin-induced proliferation was also observed with other estrogens but not with nonestrogenic steroids. Treatment with antiestrogens in combination with E2 antagonized both the mitogenic and antimitogenic actions of E2. Antiestrogen treatment alone inhibited basal proliferation, and it mimicked the inhibitory action of E2 on insulin-induced proliferation with less potency. In parallel with cell proliferation, an insulin-induced increase in the cell number of cyclin D1-immunoreactive lactotrophs was inhibited by E2 treatment. Although the antimitogenic action of E2 was seen with a wide range of doses of insulin or IGF-1, proliferation was stimulated rather than inhibited by E2 when cells were treated with serum or forskolin/isobutylmethylxanthine instead of insulin, indicating a mitogen-specific, but not proliferative activity-dependent, inhibition by E2. The results of estrogen-occupied estrogen receptors as negative regulators of proliferation suggest a novel interaction between estrogen and growth factors in the regulation of proliferation in estrogen-responsive cells.