Lactotropic Cell Proliferation Research Articles

Pituitary Hyperplasia, Hormonal Changes and Prolactinoma Development in Males Exposed to Estrogens-An Insight From Translational Studies.

Estrogen signaling plays an important role in pituitary development and function. In sensitive rat or mice strains of both sexes, estrogen treatments promote lactotropic cell proliferation and induce the formation of pituitary adenomas (dominantly prolactin or growth-hormone-secreting ones). In male patients receiving estrogen, treatment does not necessarily result in pituitary hyperplasia, hyperprolactinemia or adenoma development. In this review, we comprehensively analyze the mechanisms of estrogen action upon their application in male animal models comparing it with available data in human subjects. Sex-specific molecular targets of estrogen action in lactotropic (PRL) cells are highlighted in the context of their proliferative and secretory activity. In addition, putative effects of estradiol on the cellular/tumor microenvironment and the contribution of postnatal pituitary progenitor/stem cells and transdifferentiation processes to prolactinoma development have been analyzed. Finally, estrogen-induced morphological and hormone-secreting changes in pituitary thyrotropic (TSH) and adrenocorticotropic (ACTH) cells are discussed, as well as the putative role of the thyroid and/or glucocorticoid hormones in prolactinoma development, based on the current scarce literature.

Roles of Dopamine 2 Receptor Isoforms and G Proteins in Ethanol Regulated Prolactin Synthesis and Lactotropic Cell Proliferation

Alcohol consumption has been shown to increase prolactin (PRL) production and cell proliferation of pituitary lactotropes. It also causes a reduction in the lactotrope's response to dopaminergic agents and a differential expression of dopamine 2 receptor short (D2S) and long (D2L) isoforms in the pituitary. However, the role of each of these D2 receptor isoforms and its coupled G protein in mediation of ethanol actions on lactotropes is not known. We have addressed this issue by comparing ethanol effects on the level of PRL production gene transcription rate cellular protein, G proteins and cell proliferation in enriched lactotropes and lactotrope-derived PR1 cells containing various D2 receptor isoforms. Additionally, we determined the effects of G protein blockade on ethanol-induced PRL production and cell proliferation in these cells. We show here that the D2 receptor, primarily the D2S isoform, is critically involved in the regulation of ethanol actions on PRL production and cell proliferation in lactotropes. We also present data to elucidate that the presence of the pertussis toxin (PTX)-sensitive D2S receptor is critical to mediate the ethanol stimulatory action on Gs and the ethanol's inhibitory action on Gi3 protein in lactotropes. Additionally, we provide evidence for the existence of an inhibitory action of Gi3 on Gs that is under the control of the D2S receptor and is inhibited by ethanol. These results suggest that ethanol via the inhibitory action on D2S receptor activity suppresses Gi3 repression of Gs expression resulting in stimulation of PRL synthesis and cell proliferation in lactotropes.

Ethanol Alters Production and Secretion of Estrogen‐Regulated Growth Factors That Control Prolactin‐Secreting Tumors in the Pituitary

Chronic administration of ethanol increases plasma prolactin levels and enhances estradiol's mitogenic action on the lactotropes of the pituitary gland. The present study was conducted to determine whether ethanol's lactotropic cell-proliferating action, like estradiol's, is associated with alteration in the production of 3 peptides that regulate cell growth: transforming growth factor beta 1 (TGF-beta1), TGF-beta3 and basic fibroblast growth factor (bFGF). Using ovariectomized Fischer-344 female rats, we determined ethanol's and estradiol's actions on lactotropic cell proliferation and growth-regulatory peptide production and release in the pituitary gland during tumorigenesis. Ethanol increased basal and estradiol-enhanced mitosis of lactotropes in the pituitary glands of ovariectomized rats. The level of growth-inhibitory TGF-beta1 was reduced in the pituitary following ethanol and/or estradiol treatment for 2 and 4 weeks. In contrast, ethanol and estradiol alone as well as together increased levels of growth-stimulatory TGF-beta3 and bFGF in the pituitary at 2 and 4 weeks. In primary cultures of pituitary cells, both ethanol and estradiol reduced TGF-beta1 release and increased TGF-beta3 and bFGF release at 24 hours. Ethanol's effect on growth factor levels in the pituitary or growth factor release from the pituitary cells was less than that of estradiol. When ethanol and estradiol were applied together, their individual effects on these growth factors were amplified. These results confirm estradiol's modulation of pituitary growth factor production and release, and provide evidence that ethanol, like estradiol, alters the production and secretion of growth-regulatory peptides controlling lactotropic cell proliferation.

Dopamine, Dopamine D2 Receptor Short Isoform, Transforming Growth Factor (TGF)-β1, and TGF-β 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.

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.

Folliculostellate cells determine the susceptibility of lactotropes to estradiol's mitogenic action.

Estradiol is known to increase lactotropic cell proliferation, but estradiol susceptibility varies among human populations and among various strains of rats. We had reported that folliculostellate (FS) cells regulate estradiol's mitogenic action on lactotropes; therefore, we studied their role in determining the susceptibility to estradiol in a high estradiol-responsive rat strain, Fischer 344 (F344), and in a low-responsive strain, Sprague Dawley (SD). Determination of total S-100-positive FS cells in the pituitary revealed that F344 rats have significantly more FS cells than do SD rats. Estradiol treatment did not change the number of FS cells in both F344 and SD rats. When cotransplanted with F344 pituitaries under the kidney capsule or cocultured with F344-derived lactotropes in vitro, FS cells derived from F344 rats increased estradiol's mitogenic action. They also increased estradiol's mitogenic action on SD-derived lactotropes in primary cultures. However, SD-derived FS cells failed to increase estrogen's action on F344- or SD-derived lactotropes. The levels of basic fibroblast growth factor production and secretion by TGF-beta 3 and estradiol were much higher in F344-derived FS cells than in SD-derived FS cells. However, the lactotropes' growth response to basic fibroblast growth factor was similar in both strains. These data suggest that cell-cell interaction between FS cells and lactotropes regulates estradiol's mitogenic action on lactotropes and also determines lactotrope susceptibility to the steroid.

Ethanol Induces Hyperprolactinemia by Increasing Prolactin Release and Lactotrope Growth in Female Rats

Background Alcohol drinking is known to cause hyperprolactinemia in both humans and laboratory animals. The mechanism by which alcoholism causes hyperprolactinemia is not known. This study investigated whether increased pituitary production of prolactin, which leads to alcohol‐induced hyperprolactinemia, results from an increase in cell number and/or cell production of prolactin in the pituitary.Methods The effects of ethanol on lactotropes were determined in vivo using female rats as an animal model and in vitro using primary cultures of mixed rat anterior pituitary cells and enriched lactotropes. In vivo experiments involved administration of ethanol for 2 and 4 weeks using a liquid diet containing 8.7% ethanol (v/v), which provides 37% of the calories in cyclic, ovariectomized, and estradiol‐17β–treated ovariectomized Fischer‐344 rats. The control group was pair‐fed an isocaloric diet minus the ethanol or fed a normal diet ad libitum. These animals were used to determine ethanol's effects on plasma prolactin levels, pituitary wet weights, pituitary total protein levels, and the number of mitotic lactotropes. In vitro studies determined ethanol's effects in the presence and absence of estradiol on prolactin release and lactotropic cell proliferation. Prolactin levels in plasma and media samples were measured using radioimmunoassay. Mitotic lactotropes were determined using bromodeoxyuridine incorporation assay.Results Ethanol treatment increased in a time‐dependent manner the plasma levels of prolactin in cyclic, ovariectomized, and estradiol‐treated ovariectomized rats. Ethanol treatment also increased pituitary wet weight and/or pituitary total protein levels and DNA synthesis in lactotropes. Determination of ethanol's action on lactotropic cell proliferation and hormone secretion in vitro using primary cultures of mixed pituitary cells revealed that ethanol stimulated both basal and estradiol‐induced prolactin secretion and lactotropic cell proliferation. When ethanol's actions were studied in isolated lactotropes, ethanol alone or in combination with estradiol stimulated prolactin secretion but failed to increase lactotropic cell proliferation.Conclusions These results suggest that ethanol causes hyperprolactinemia by elevating prolactin release from lactotropes and by increasing the number of lactotropes in the anterior pituitary gland. The mitotic action of ethanol requires cell–cell communication between lactotropes and other pituitary cells. Furthermore, ethanol's mode of action on prolactin release and lactotrope growth is similar to that observed for estradiol.

Ethanol induces hyperprolactinemia by increasing prolactin release and lactotrope growth in female rats.

Alcohol drinking is known to cause hyperprolactinemia in both humans and laboratory animals. The mechanism by which alcoholism causes hyperprolactinemia is not known. This study investigated whether increased pituitary production of prolactin, which leads to alcohol-induced hyperprolactinemia, results from an increase in cell number and/or cell production of prolactin in the pituitary. The effects of ethanol on lactotropes were determined in vivo using female rats as an animal model and in vitro using primary cultures of mixed rat anterior pituitary cells and enriched lactotropes. In vivo experiments involved administration of ethanol for 2 and 4 weeks using a liquid diet containing 8.7% ethanol (v/v), which provides 37% of the calories in cyclic, ovariectomized, and estradiol-17beta-treated ovariectomized Fischer-344 rats. The control group was pair-fed an isocaloric diet minus the ethanol or fed a normal diet ad libitum. These animals were used to determine ethanol's effects on plasma prolactin levels, pituitary wet weights, pituitary total protein levels, and the number of mitotic lactotropes. In vitro studies determined ethanol's effects in the presence and absence of estradiol on prolactin release and lactotropic cell proliferation. Prolactin levels in plasma and media samples were measured using radioimmunoassay. Mitotic lactotropes were determined using bromodeoxyuridine incorporation assay. Ethanol treatment increased in a time-dependent manner the plasma levels of prolactin in cyclic, ovariectomized, and estradiol-treated ovariectomized rats. Ethanol treatment also increased pituitary wet weight and/or pituitary total protein levels and DNA synthesis in lactotropes. Determination of ethanol's action on lactotropic cell proliferation and hormone secretion in vitro using primary cultures of mixed pituitary cells revealed that ethanol stimulated both basal and estradiol-induced prolactin secretion and lactotropic cell proliferation. When ethanol's actions were studied in isolated lactotropes, ethanol alone or in combination with estradiol stimulated prolactin secretion but failed to increase lactotropic cell proliferation. These results suggest that ethanol causes hyperprolactinemia by elevating prolactin release from lactotropes and by increasing the number of lactotropes in the anterior pituitary gland. The mitotic action of ethanol requires cell-cell communication between lactotropes and other pituitary cells. Furthermore, ethanol's mode of action on prolactin release and lactotrope growth is similar to that observed for estradiol.

Ethanol Induces Hyperprolactinemia by Increasing Prolactin Release and Lactotrope Growth in Female Rats

Background Alcohol drinking is known to cause hyperprolactinemia in both humans and laboratory animals. The mechanism by which alcoholism causes hyperprolactinemia is not known. This study investigated whether increased pituitary production of prolactin, which leads to alcohol-induced hyperprolactinemia, results from an increase in cell number and/or cell production of prolactin in the pituitary. Methods The effects of ethanol on lactotropes were determined in vivo using female rats as an animal model and in vitro using primary cultures of mixed rat anterior pituitary cells and enriched lactotropes. In vivo experiments involved administration of ethanol for 2 and 4 weeks using a liquid diet containing 8.7% ethanol (v/v), which provides 37% of the calories in cyclic, ovariectomized, and estradiol-17β–treated ovariectomized Fischer-344 rats. The control group was pair-fed an isocaloric diet minus the ethanol or fed a normal diet ad libitum. These animals were used to determine ethanol's effects on plasma prolactin levels, pituitary wet weights, pituitary total protein levels, and the number of mitotic lactotropes. In vitro studies determined ethanol's effects in the presence and absence of estradiol on prolactin release and lactotropic cell proliferation. Prolactin levels in plasma and media samples were measured using radioimmunoassay. Mitotic lactotropes were determined using bromodeoxyuridine incorporation assay. Results Ethanol treatment increased in a time-dependent manner the plasma levels of prolactin in cyclic, ovariectomized, and estradiol-treated ovariectomized rats. Ethanol treatment also increased pituitary wet weight and/or pituitary total protein levels and DNA synthesis in lactotropes. Determination of ethanol's action on lactotropic cell proliferation and hormone secretion in vitro using primary cultures of mixed pituitary cells revealed that ethanol stimulated both basal and estradiol-induced prolactin secretion and lactotropic cell proliferation. When ethanol's actions were studied in isolated lactotropes, ethanol alone or in combination with estradiol stimulated prolactin secretion but failed to increase lactotropic cell proliferation. Conclusions These results suggest that ethanol causes hyperprolactinemia by elevating prolactin release from lactotropes and by increasing the number of lactotropes in the anterior pituitary gland. The mitotic action of ethanol requires cell–cell communication between lactotropes and other pituitary cells. Furthermore, ethanol's mode of action on prolactin release and lactotrope growth is similar to that observed for estradiol.

Effect of antisense suppression of transforming growth factor-beta3 gene on lactotropic cell proliferation.

The mechanism by which oestrogen regulates lactotropic cell proliferation is not well understood. Recently it has been shown that a transforming growth factor beta (TGF-beta) gene-related peptide, TGF-beta3, is produced in the lactotropes and stimulates lactotropic cell proliferation in vitro. In this study, the role of this growth factor in oestrogen-induced lactotropic cell proliferation was determined in vitro using oligonucleotide designed to inhibit TGF-beta3 gene expression. We used the oligonucleotide in an antisense orientation, which is complementary to regions in the TGF-beta3 message. Oligonucleotides in sense and missense orientation were also used. We found that the antisense sequence that was effectively incorporated into pituitary cells produced a marked inhibition of the stimulatory action of oestrogen on lactotropic cell proliferation. Sense and missense oligonucleotides produced no significant effects on oestrogen-stimulated lactotropic cell proliferation. The growth-inhibitory effect of antisense oligonucleotide was blocked by TGF-beta3 peptide. These results suggest that TGF-beta3 may be involved in the processes that mediate oestrogen-regulated lactotropic cell proliferation.

Opposing actions of two transforming growth factor-beta isoforms on pituitary lactotropic cell proliferation.

Three transforming growth factor-beta protein isoforms (TGFbeta1, TGFbeta2, and TGFbeta3) have been identified in mammals. These isoforms appear to have similar actions on cell growth in various tissues. In rat pituitary tissue, TGFbeta1 is localized in PRL-secreting lactotropes and has been shown to act on lactotropes to inhibit estradiol-induced cell proliferation. The steroid inhibits the production and secretion of TGFbeta1. It is not known whether the other two isoforms are produced in and/or act on lactotropes. Using immunocytochemical detection techniques, we determined that, like TGFbeta1, TGFbeta3 is colocalized with PRL in the anterior pituitary of Fischer-344 female rats. Administration of estradiol increased TGFbeta3-immunoreactive cell numbers, TGFbeta3 protein, and TGFbeta3 messenger RNA levels in the pituitary. Determinations of TGFbeta3 actions in vitro in primary cultures of pituitary cells indicated that TGFbeta3 concentration dependently increases lactotropic cell proliferation. The growth-promoting action of TGFbeta3 was potentiated by estradiol. Immunoneutralization studies indicated that although TGFbeta1 antibody failed to prevent estradiol's mitogenic action, it potentiated the mitogenic action of TGFbeta3. In contrast, TGFbeta3-neutralizing antibody inhibited lactotropic cell proliferation by estradiol. These data indicate that unlike many other tissues, TGFbeta1 and TGFbeta3 have opposite actions on lactotropic proliferation in the pituitary. Furthermore, TGFbeta1 and TGFbeta3 may be involved in estradiol's mitogenic action on lactotropes.

Opposing Actions of Two Transforming Growth Factor- Isoforms on Pituitary Lactotropic Cell Proliferation

Three transforming growth factor-β protein isoforms (TGFβ1, TGFβ2, and TGFβ3) have been identified in mammals. These isoforms appear to have similar actions on cell growth in various tissues. In rat pituitary tissue, TGFβ1 is localized in PRL-secreting lactotropes and has been shown to act on lactotropes to inhibit estradiol-induced cell proliferation. The steroid inhibits the production and secretion of TGFβ1. It is not known whether the other two isoforms are produced in and/or act on lactotropes. Using immunocytochemical detection techniques, we determined that, like TGFβ1, TGFβ3 is colocalized with PRL in the anterior pituitary of Fischer-344 female rats. Administration of estradiol increased TGFβ3-immunoreactive cell numbers, TGFβ3 protein, and TGFβ3 messenger RNA levels in the pituitary. Determinations of TGFβ3 actions in vitro in primary cultures of pituitary cells indicated that TGFβ3 concentration dependently increases lactotropic cell proliferation. The growth-promoting action of TGFβ3 was potentiated by estradiol. Immunoneutralization studies indicated that although TGFβ1 antibody failed to prevent estradiol’s mitogenic action, it potentiated the mitogenic action of TGFβ3. In contrast, TGFβ3-neutralizing antibody inhibited lactotropic cell proliferation by estradiol. These data indicate that unlike many other tissues, TGFβ1 and TGFβ3 have opposite actions on lactotropic proliferation in the pituitary. Furthermore, TGFβ1 and TGFβ3 may be involved in estradiol’s mitogenic action on lactotropes.

TRANSFORMING GROWTH FACTOR-β1 INHIBITS PROLACTIN SECRETION AND LACTOTROPIC CELL PROLIFERATION IN THE PITUITARY OF OESTROGEN-TREATED FISCHER 344 RATS

Recently it has been shown that lactotropic cells in the anterior pituitary gland produce and secrete transforming growth factor-β1- (TGF-β1) like peptide. The in vivo action(s) of this peptide growth factor on lactotropic cells have not been studied. In this study we determined the effects of TGF-β1 on lactotropic cell function in estradiol-17β-treated ovariectomized rats. Intrapituitary administration of TGF-β1 significantly inhibited plasma levels of prolactin (PRL). In addition, TGF-β1 decreased pituitary weight and the DNA synthesis in lactotropes and reduced the PRL levels in the pituitary. These results suggest that TGF-β1 may be a physiological regulator of PRL secretion and lactotropic cell proliferation. © 1997 Elsevier Science Ltd. All rights reserved

POTENTIATION OF THE MITOGENIC EFFECT OF ESTROGEN ON THE PITUITARY-GLAND BY ALCOHOL-CONSUMPTION

The effect of alcohol on estrogen-regulated lactotropic cell proliferation was examined in Fisher 344 rats. Alcohol was administered for 2 weeks using liquid diet containing 8.7% ethanol (v/v) and 37% ethanol-derived calories. The control group was pair-fed with an isocaloric diet minus the ethanol or adlib-fed with normal diet. Ethanol-treated rats showed mean blood ethanol concentrations between 60-90 mg/dl. Alcohol treatment did not effect the body growth rate, but increased the DNA synthesis in lactotropes and reduced the levels of lactotropic growth inhibitory transforming growth factor-beta 1 (TGF-beta 1) protein and mRNA in the pituitary. These results suggest that alcohol promotes estrogen-induced lactotropic proliferation, possibly by down regulating the inhibitory TGF-beta 1 control of lactotropic function.