Polypeptide hormone regulation of gene expression. Thyrotropin-releasing hormone rapidly stimulates both transcription of the prolactin gene and the phosphorylation of a specific nuclear protein

From membrane to nucleus: A three-wave hypothesis of cAMP signaling

For many decades, our understanding of G protein-coupled receptor (GPCR) activity and cyclic AMP (cAMP) signaling was limited exclusively to the plasma membrane. However, a growing body of evidence has challenged this view by introducing the concept of endocytosis-dependent GPCR signaling. This emerging paradigm emphasizes not only the sustained production of cAMP but also its precise subcellular localization, thus transforming our understanding of the spatiotemporal organization of this process. Starting from this alternative point of view, our recent work sheds light on the role of an endocytosis-dependent calcium release from the endoplasmic reticulum in the control of nuclear cAMP levels. This is achieved through the activation of local soluble adenylyl cyclase, which in turn regulates the activation of local protein kinase A (PKA) and downstream transcriptional events. In this review, we explore the dynamic evolution of research on cyclic AMP signaling, including the findings that led us to formulate the novel three-wave hypothesis. We delve into how we abandoned the paradigm of cAMP generation limited to the plasma membrane and the changing perspectives on the rate-limiting step in nuclear PKA activation.

Differential regulation of thyroid hormone receptor-mediated function by endocrine disruptors

It is well known that endocrine disruptors (EDs) act as anti-estrogenic agents and affect the function of reproductive organ. EDs are also thought to affect thyroid hormone (TH) system which is important for biological functions such as growth, development and metabolism. However, it is still not clear how EDs are able to regulate TH receptor (TR)-mediated functions. In this study, therefore, the modulatory effects of representative EDs such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), polychlorinated biphenyl (Aroclor 1254) and bisphenol A (BPA) were examined using TR-expressing GH3 cells (a rat pituitary gland epithelial tumor cell line) activated by triiodothyronine (T3). EDs tested significantly blocked T3 binding to TR in a dose-dependent manner. Biochemical characterization by Scatchard and Lineweaver-Burk plot analyses indicated that TCDD and aroclor 1254 bound to TH receptors in a competitive inhibitory manner, whereas BPA bound to TH receptors in a non-competitive pattern. The different inhibitory mode of action by EDs was also found in regulating TR-mediated production of prolactin (PRL). Aroclor 1254 exposure for 48 h enhanced T3-mediated PRL production, but BPA down-regulated. These results suggest that the EDs (TCDD, Aroclor 1254 and BPA) could differentially bind to TR and distinctly regulate the action of TR function, even though EDs are structurally similar.

Pituitary cell lines and their endocrine applications

The pituitary gland is an important component of the endocrine system, and together with the hypothalamus, exerts considerable influence over the functions of other endocrine glands. The hypothalamus either positively or negatively regulates hormonal productions in the pituitary through its release of various trophic hormones which act on specific cell types in the pituitary to secrete a variety of pituitary hormones that are important for growth and development, metabolism, reproductive and nervous system functions. The pituitary is divided into three sections-the anterior lobe which constitute the majority of the pituitary mass and is composed primarily of five hormone-producing cell types (thyrotropes, lactotropes, corticotropes, somatotropes and gonadotropes) each secreting thyrotropin, prolactin, ACTH, growth hormone and gonadotropins (FSH and LH) respectively. There is also a sixth cell type in the anterior lobe-the non-endocrine, agranular, folliculostellate cells. The intermediate lobe produces melanocyte-stimulating hormone and endorphins, whereas the posterior lobe secretes anti-diuretic hormone (vasopressin) and oxytocin. Representative cell lines of all the six cell types of the anterior pituitary have been established and have provided valuable information on genealogy of the various cell lineages, endocrine feedback control of hormone synthesis and secretions, intrapituitary interactions between the various cell types, as well as the role of specific transcription factors that determine each differentiated cell phenotype. In this review, we will discuss the morphology and function of the cell types that make up the anterior pituitary, and the characteristics of the various functional anterior pituitary cell systems that have been established to be representative of each anterior pituitary cell lineage.

Role of specific protein kinase C isozymes in mediating epidermal growth factor, thyrotropin-releasing hormone, and phorbol ester regulation of the rat prolactin promoter in GH4/GH4C1 pituitary cells

Epidermal growth factor (EGF) and TRH both produce enhanced prolactin (PRL) gene transcription and PRL secretion in GH4 rat pituitary tumor cell lines. These agents also activate protein kinase C (PKC) in these cells. Previous studies have implicated the PKCepsilon isozyme in mediating TRH-induced PRL secretion. However, indirect studies using phorbol ester down-regulation to investigate the role of PKC in EGF- and TRH-induced PRL gene transcription have been inconclusive. In the present study, we examined the role of multiple PKC isozymes on EGF- and TRH-induced activation of the PRL promoter by utilizing general and selective PKC inhibitors and by expression of genes for wild-type and kinase-negative forms of the PKC isozymes. Multiple nonselective PKC inhibitors, including staurosporine, bisindolylmaleimide I, and Calphostin C, inhibited both EGF and TRH induced rat PRL promoter activity. TRH effects were more sensitive to Calphostin C, a competitive inhibitor of diacylglycerol, whereas Go 6976, a selective inhibitor of Ca(2+)-dependent PKCs, produced a modest inhibition of EGF but no inhibition of TRH effects. Rottlerin, a specific inhibitor of the novel nPKCdelta isozyme, significantly blocked both EGF and TRH effects. Overexpression of genes encoding PKCs alpha, betaI, betaII, delta, gamma, and lambda failed to enhance either EGF or TRH responses, whereas overexpression of nPKCeta enhanced the EGF response. Neither stable nor transient overexpression of nPKCepsilon produced enhancement of EGF- or TRH-induced PRL promoter activity, suggesting that different processes regulate PRL transcription and hormone secretion. Expression of a kinase inactive nPKCdelta construct produced modest inhibition of EGF-mediated rPRL promoter activity. Taken together, these data provide evidence for a role of multiple PKC isozymes in mediating both EGF and TRH stimulated PRL gene transcription. Both EGF and TRH responses appear to require the novel isozyme, nPKCdelta, whereas nPKCeta may also be able to transmit the EGF response. Inhibitor data suggest that the EGF response may also involve Ca(2+)-dependent isozymes, whereas the TRH response appears to be more dependent on diacylglycerol.

Developmental expression of prolactin releasing peptide in the rat brain: localization of messenger ribonucleic acid and immunoreactive neurons

Prolactin releasing peptide (PrRP) was recently identified as the stimulator of prolactin release from the anterior pituitary. PrRP mRNA is expressed in the medulla oblongata and the hypothalamus in the rat brain. The fibers containing PrRP are widely distributed in the brain, therefore, it was postulated that PrRP may act as a neurotransmitter or neuromodulator as well as an endocrine substance. To clarify the developmental changes in the expression of PrRP during brain development, we examined PrRP in rat fetuses and neonates using in situ hybridization and immunohistochemistry. The PrRP mRNA was expressed in the nucleus of the solitary tract (NTS) at embryonic day 18 (E18) and in the ventral and lateral reticular nucleus (VLRN) of the caudal medulla oblongata at E20. The PrRP mRNA in the hypothalamus was first expressed at postnatal day 13 (P13). Reverse transcription-polymerase chain reaction analysis (RT-PCR) for PrRP revealed that PCR product, a 268 bp band, was detected from either E18 in the medulla or P13 in the hypothalamus. Immunodetection with monoclonal antibody against prepro-PrRP revealed intensive staining of cells in the NTS at E18, in the VLRN at E20 or in the dorsomedial hypothalamus at P13. Immunohistochemistry using monoclonal antibody against mature PrRP at P6 showed PrRP fibers to be distributed in the paraventricular hypothalamic nucleus, periventricular hypothalamic nucleus, medial preoptic area, basolateral amygdaloid nucleus, dorsomedial hypothalamus, ventromedial hypothalamus, periventricular nucleus of the thalamus and bed nucleus of the stria terminalis as previously shown in the adult rat. PrRP fibers were also found in the optic chiasm, dorsal endopiriform nucleus, cingulum, intermediate reticular nucleus, and caudal ventrolateral reticular nucleus at P6 and P9. However, PrRP fibers were never found in the above regions in the adult animal. These findings suggest that PrRP fibers originating in the medulla oblongata have been widely distributed in the rat brain during the early postnatal day and PrRP may play various roles in the brain development.

Thyrotropin-releasing hormone stimulates phosphorylation of the epidermal growth factor receptor in GH3 pituitary cells

TRH has been found to stimulate tyrosine phosphorylation of the epidermal growth factor (EGF) receptor. A specific EGF receptor kinase inhibitor, tyrphostin AG1478, substantially reduced TRH-stimulated tyrosine phosphorylation of the EGF receptor. TRH-induced EGF receptor phosphorylation was found to lead to the recruitment of the adapter proteins Grb2 and Shc. TRH treatment also led to phosphorylation of the related receptor tyrosine kinase, HER2. HER2 activation likely contributes to downstream signaling events and enhances EGF receptor action. TRH-induced tyrosine phosphorylation of the EGF receptor was reduced by incubation with a protein kinase C (PKC) kinase inhibitor, GF109203X. EGF receptor phosphorylation was required for full TRH-induced activation of mitogen-activated protein kinase (MAPK) and stimulation of specific transcriptional responses.

Molecular cloning of the chicken prolactin gene and activation by Pit-1 and cAMP-induced factor in GH3 cells

Transcription of the prolactin (PRL) gene has been reported to be activated by a nuclear factor, Pit-1. However, the precise molecular mechanisms of the Pit-1-mediated PRL gene activation are still unclear. We have cloned the chicken PRL (cPRL) gene and its 5'-flanking region to analyze their structure and transcription-initiating mechanism. In luciferase assay, forskolin activated the proximal promoter region between -248 and -76 to transcribe the cPRL gene in GH3 cells, although there is no canonical cyclic AMP-responsive element in the promoter region. In gel mobility shift assay, a DNA fragment between -104 and -76 containing a putative Pit-1 binding site was bound by nuclear factors from the GH3 cells. Furthermore, it was observed that Pit-1 protein specifically bound to the DNA fragment in the supershift assay. These results indicate that both Pit-1 and cAMP-induced factor(s) associated with the cis element on the proximal promoter region to activate cPRL gene expression in GH3 cells.

A role for the mitogen-activated protein kinase in mediating the ability of thyrotropin-releasing hormone to stimulate the prolactin promoter

The hypothalamic hormone, TRH, stimulates PRL secretion and gene transcription. We have examined the possibility that the mitogen-activated protein kinase (MAPK) may play a role in mediating TRH effects on the PRL gene. TRH was found to stimulate sustained activation of MAPK in PRL-producing, GH3 cells, consistent with a possible role in transcriptional regulation. A kinase-defective, interfering MAPK kinase (MAPKK) mutant reduced TRH induction of the PRL promoter. Treatment with the MAPKK inhibitor, PD98059, blocked TRH-induced activation of MAPK and also reduced TRH induction of a PRL-luciferase reporter gene, confirming that MAPK activation is necessary for TRH effects on PRL gene expression. Previous studies have demonstrated that the PRL promoter contains binding sites for members of the Ets family of transcription factors, which are important for mediating MAPK responsiveness of the PRL promoter. Mutation of specific Ets sites within the PRL promoter reduced responsiveness to both TRH and MAPK. The finding that DNA elements required for MAPK responsiveness of the PRL gene colocalize with DNA elements required for TRH responsiveness further supports a role for MAPK in mediating TRH effects on the PRL gene. We also explored the signaling mechanisms that link the TRH receptor to MAPK induction. Occupancy of the TRH receptor results in activation of protein kinase C (PKC) as well as increases in the concentration of Ca2+ due to release from intracellular stores and entry of Ca2+ through Ca2+ channels. A PKC inhibitor, GF109203X, and an L-type Ca2+ channel blocker, nimodipine, both partially reduced TRH-induced MAPK activation and PRL promoter activity. The effects of the two inhibitors were additive. These studies are consistent with a signaling pathway involving PKC- and Ca2+-dependent activation of MAPK, which leads to phosphorylation of an Ets transcription factor and activation of the PRL promoter.

Interaction between estrogen receptor and Pit-1 protein is influenced by estrogen in pituitary cells

The estrogen responsiveness of the rat prolactin gene expression requires the presence of both the estrogen receptor (ER) and the tissue-specific transcription factor, Pit-1 protein. We performed protein interaction assays using anti-rat Pit-1 antiserum (a-rPit-1) to investigate the physical interactions which occur between ER and Pit-1 proteins following estrogen treatment. After fusing maltose binding protein (MBP) and Pit-1 protein, we used the resulting MBP Pit-1 fusion protein to prepare a-rPit-1. Our results show that the estrogen receptor readily co-precipitated with the Pit-1 protein drawn from the lysates of two prolactin-expressing pituitary cell lines GH3 and PR1. The rate of precipitation appears to be both estrogen- and time-dependent. Cellular levels of estrogen receptors and Pit-1 proteins did not show significant changes during the time of estrogen treatment. We therefore suggest that an estrogen-dependent physical interaction between ER and Pit-1 protein exists in vivo, and that this interaction may play an important role in the regulation of prolactin gene expression.

Characterization of DNA regions mediating the ability of Ca2+/calmodulin dependent protein kinase II to stimulate prolactin promoter activity

The ability of Ca2+/calmodulin-dependent protein kinases (CaMKs) to regulate transcription of the rat prolactin (PRL) gene has been examined. We found that KN-62, a potent inhibitor of CaM kinases, blunted the ability of TRH to activate the prolactin promoter. Transfection experiments using expression plasmids for constitutively active forms of CaMKI, CaMKII, or CaMKIV show that CaMKII is the most effective activator of prolactin promoter expression. Deletion studies demonstrated that the upstream boundary of sequences necessary to respond to CaMKII is located within the distal enhancer of the prolactin gene. Neither the distal enhancer alone nor the proximal region of the prolactin gene are sufficient to mediate a response to CaMKII. Mutational analysis suggests that several Pit-1 binding sites contribute to CaMKII responsiveness. These findings suggest that CaMKII responsiveness of the prolactin promoter requires multiple factor binding sites in both the distal and proximal regions of the gene.

The role of estrogen receptor in modulation of chromatin conformation in the 5' flanking region of the rat prolactin gene

To determine whether the estrogen receptor (ER) has a role in the modification of chromatin structure, we developed cell lines to model discrete stages in the estrogen response. Each cell line carries a population of stably expressed papillomavirus-based minichromosomes containing the 5' flanking region of the rat prolactin gene. We examined ER effects at the distal enhancer domain of the rat prolactin promoter, using DNaseI to probe for alterations of the nucleoprotein complex. Within 1 h after the start of estrogen treatment, modifications in the chromatin state of the distal enhancer region were detected in a pituitary-derived, permissive cell line (GH3G1J). In rat-1 fibroblast cell lines that maintain the same stably expressed papillomavirus-based minichromosomes in the absence of ER or pituitary-specific transcription factors (Rat-1.2A2; non-permissive), no estrogen-induced modifications in the chromatin state were detected at 1 or 24 h. In rat-1 fibroblast cell lines that also contained ectopically expressed, functional ER (Rat-1 + ER.8A1), no estrogen-induced modifications in the chromatin state were detected at 1 h, but a 24 h a specific modification in the local structure was induced. These data support a model in which the ER interacts with chromatin to modify local structure in such a way as to induce a permissive state for interactions of transcription factors necessary for hormone-induced activation of gene transcription.

A composite Ets/Pit-1 binding site in the prolactin gene can mediate transcriptional responses to multiple signal transduction pathways

Binding sites for the tissue-specific transcription factor, Pit-1, are required for basal and hormonally induced prolactin gene transcription. Although Pit-1 is phosphorylated in response to several signaling pathways, the mechanism by which Pit-1 contributes to hormonal induction of gene transcription has not been defined. Recent reports suggest that phosphorylation of Pit-1 may not be required for hormonal regulation of the prolactin promoter. Analysis of the contribution of individual Pit-1 binding sites has been complicated due to the fact that some of the elements appear to be redundant. To better understand the role of Pit-1 sites in mediating hormonal regulation of the prolactin gene, we have performed enhancer tests using the three most proximal Pit-1 binding sites of the rat prolactin gene which are designated the 1P, 2P, and 3P sites. The results demonstrate that multimers of the 3P Pit-1 binding site are much more responsive to several hormonal and intracellular signaling pathways than multimers of the 1P or 2P sites. The 3P DNA element was found to contain a consensus binding site for the Ets family of proteins. Mutation of the Ets binding site greatly decreased the ability of epidermal growth factor, phorbol esters, Ras, or the Raf kinase to induce reporter gene activity. Mutation of the Ets site had little effect on basal enhancer activity. In contrast, mutation of the consensus Pit-1 binding site in the 3P element essentially abolished all basal enhancer activity. Overexpression of Ets-1 in GH3 pituitary cells enhanced both basal and Ras induced activity from the 3P enhancer. These data describe a composite element in the prolactin gene containing binding sites for two different factors and the studies suggest a mechanism by which Ets proteins and Pit-1 functionally cooperate to permit transcriptional regulation by different signaling pathways.

Constitutively active Gq-alpha stimulates prolactin promoter activity via a pathway involving Raf activity

We have investigated the ability of a constitutively active Gq-alpha mutant, Q209L-alpha q, to regulate target gene expression. Transient expression in GH3 pituitary cells of a rat proximal prolactin promoter-chloramphenicol acetyltransferase construct (-187)PRL-CAT, was stimulated by co-expression of Q209L alpha q, but not by wild-type alpha q. Q209L-alpha q stimulated expression of constructs driven by promoters for either rat prolactin or growth hormone, but not of a control construct driven by the thymidine kinase promoter. Thus, transcriptional effects of alpha q are specific both for the activated state of this G-alpha subunit and the promoter examined. Since both the prolactin and growth hormone promoters are activated by the pituitary cell-specific transcription factor Pit-1, we examined whether a Pit-1 binding site could direct a response to Q209L-alpha q. Two copies of prolactin promoter Pit-1 binding site 1P conferred upon a heterologous metallothionein promoter a response to Q209L-alpha q, implying an involvement of this site in the transcriptional action of Q209L-alpha q on the prolactin promoter. The phorbol ester activator of protein kinase C, 12-O-tetradecanoylphorbol-13-acetate, stimulated (-187)PRL-CAT activity, but opposed the action of Q209L-alpha q on activity of this PRL-CAT construct. Q209L-alpha q stimulation of (-187)PRL-CAT activity was inhibited by co-expression of a dominant negative Raf mutant, Raf-C4, but not by a point mutant of Raf-C4 with reduced inhibitory properties. These results imply that activated alpha q subunits can stimulate prolactin promoter activity via a pathway that involves a Pit-1 DNA binding site(s), is opposed by protein kinase C, and is mediated by a pathway in which Raf-1 kinase plays a role.

Calcitonin inhibits prolactin gene transcription in rat pituitary cells

Our recent studies have shown that calcitonin (CT)-like immunoreactive peptide is synthesized and released from cultured rat anterior pituitary (AP) cells, and may serve as a paracrine inhibitor of PRL release. The present studies investigated effects of CT on basal and TRH-induced PRL mRNA levels in rat AP and rat pituitary tumor GH(3) cells. CT attenuated steady-state PRL mRNA levels in a dose-dependent fashion in primary rat AP and GH(3) cells. The kinetics of CT action suggests that 100NM: CT caused a significant decline after 3 h, and the inhibition was sustained at least until the longest tested incubation period of 30 h. Results from nuclear run-on assays suggest that 100 nM CT decreased the rate of PRL gene transcription by 80% after 30 min of incubation. CT did not affect PRL mRNA levels in Ca(2+)-depleted GH(3) cells but dramatically decreased them in Ca(2+)-repleted cells. Bay K 8644 induced increase in PRL mRNA levels of Ca(2+)-repleted GH(3) cells and CT did not affect this increase. These results suggest that CT rapidly and selectively inhibits PRL gene transcription in primary AP and GH(3) cells, and support a possibility that CT-induced attenuation of PRL mRNA may involve cytoplasmic Ca(2+).

CpG methylation represses the activity of the rat prolactin promoter in rat GH3 pituitary cell lines

In the present report, we have investigated the role of DNA methylation on the binding and trans-acting properties of transcription factors involved in the regulation of the rat prolactin (rPRL) gene, specifically Pit-1. To this aim we took advantage of a model system composed of three GH3 rat pituitary tumor cell lines that greatly differed in the extent of rPRL gene methylation and in the level of rPRL gene expression. Northern blot analyses indicated that identical species of Pit-1 mRNA were present to similar extent in the three GH3 cell lines. Electrophoretic mobility shift assays further demonstrated that Pit-1 was present in nuclear extracts and displayed equal affinities to bind the 1P responsive element encompassing the -65 to -38 region of the rPRL promoter, whatever the GH3 cell line tested. These data suggested that differential expression of the rPRL gene among cell lines did not result from variable amounts of Pit-1. By combining in vitro methylation and transient transfection experiments with a rPRL promoter-driven CAT construct, we showed that extensive methylation at CpG sites abolished the expression of the reporter gene. Furthermore, in vivo competition assays demonstrated that CpG methylation inhibited gene expression by preventing the binding of transcription factors We propose that related mechanisms linked to DNA methylation might alter the activity of the endogenous PRL gene in the low expressing cell line.

Multiple intracellular signallings are involved in thyrotropin-releasing hormone (TRH)-induced c-fos and jun B mRNA levels in clonal prolactin cells

In mammosomatotropes GH3B6 cells, one of the primary responses to thyrotropin-releasing hormone (TRH) is the parallel induction of two proto-oncogenes, c-fos and jun B, which code for constituents of AP1 transcription factor. To better understand the mode of action of TRH and to look for possible functions of c-fos and jun B in these cells, we have investigated the role of different intracellular signals in the induction of each proto-oncogene on the one hand, and on prolactin (PRL) release and PRL gene expression on the other hand. Northern and dot-blot analyses revealed that the activation of protein kinase C (PKC)-, Ca(2+)- or adenylyl cyclase-dependent pathways acutely increased both c-fos and jun B transcripts. However, a gene specific responsiveness was revealed using phorbol 12-myristate 13-acetate (TPA) and several combined treatments. The simultaneous activation of PKC and Ca(2+)-dependent pathways resulted in synergistic stimulations of c-fos mRNA levels only. Consistently, ionomycin plus low doses of TPA solely reproduced the potent effect of TRH on c-fos transcripts. Data collected from TRH and TPA down-regulated cells indicated that TRH probably recruits TPA-dependent PKC isoforms for stimulating c-fos but not jun B transcripts. On the contrary, the TRH-induced stimulation of either proto-oncogene likely involves Ca(2+)-dependent mechanisms because calcium agonists and the peptide exert non-additive effects. Finally, the synergistic stimulations observed in response to TRH combined with forskolin, indicate that adenylyl cyclase-dependent mechanisms are interconnected with TRH-induced proto-oncogene expression. The overall study also reveals that among the agonists tested, the dihydropyridine Bay K 8644 and forskolin only were capable to induce a long-lasting stimulation of c-fos and jun B mRNA levels, concomitant to increased levels of PRL transcripts, as does TRH. Considering that AP1 is assumed to be involved in signal transmission from the cell surface to the nucleus, it might be thus proposed that a common stimulation of c-fos and jun B gene expression is possibly involved in the activation of the PRL gene. On the other hand, the systematic coincidence between acute PRL release and proto-oncogenes expression suggest a role for c-fos and jun B in the control of genes involved in the secretory process.

Altered tachykinin expression by dorsal root ganglion neurons in a rat model of neuropathic pain

The experiments described in the present study approached nerve injury from both a biochemical and anatomical perspective by monitoring changes in expression of preprotachykinin (PPT) mRNA encoding the prototypic tachykinin substance P and related peptide species in neurons of the rat dorsal root ganglia (DRG) following unilateral chronic constriction injury of the sciatic nerve. In situ hybridization histochemistry (ISHH) analyses in conjunction with computer-assisted image processing were employed to quantify levels of PPT mRNA distributed in DRG neurons. Injury-induced changes in PPT mRNA expression by affected DRG neurons included: (1) at early postoperative times, generally increased levels of PPT mRNA associated with small and intermediate-size B cells exhibiting normal morphology, (2) at late postoperative times, markedly decreased levels of PPT mRNA associated with degenerating B cells, and (3) induction of PPT gene expression by large A cells which is highly correlated with degenerative morphological changes. The significant aspects of these changes are discussed with special emphasis on the contribution of altered transmitter expression by DRG neurons to the pathophysiology of causalgia. In particular, the induction of PPT gene expression by many of the large neurons undergoing degenerative changes may represent an important biochemical parameter which is associated with the development and persistence of experimental allodynia.

Internalization and nuclear localization of peptide hormones

The presence of neuropeptide receptors on the plasma membrane is well accepted, as is its internalization and down-regulation. The analysis of the fate of these peptides within their target-cells is difficult. Endogenous peptides or administered native peptides are visualized in these cells using immunocytology after cryoultramicrotomy. Labelled peptides can be injected and their internalization kinetics studied using ultrastructural autoradiography. The pituitary gland is a suitable model for the study of the neuropeptide mechanism, with the lactotroph function being taken as an example in the present case. Prolactin (PRL) release depends on two main neuropeptides: thyrotropin-releasing hormone (TRH) and somatostatin (SS). The TRH immunoreactivity obtained from endogenous as well as injected material was restricted to the plasma membrane, secretory granules, cytoplasmic matrix and nucleus. The internalization kinetics of exogenous native TRH showed an increase of immunoreactive material in all compartments including the nucleus. The endogenous SSs (SS14 and SS28) were detected in the same subcellular lactotroph compartments. Injection of 125I-SS showed a rapid binding of SS at the plasma membrane level before internalization. For 60 min of in vivo uptake, 125I-SS28, the large SS molecule, was detected in the cytoplasm only, while 125I-SS14 was found in the nuclear matrix. In vitro 125I-SS28 was restricted to the nuclear membrane. Under physiological conditions the endogenous neuropeptides were visualized in the nucleus, but after injection of labelled peptides only small molecules were found in the nucleus. The significance of the presence of these neuropeptides is discussed.

Epidermal growth factor induces prolactin mRNA in GH4C1 cells via a protein synthesis-dependent pathway

Prolactin (PRL) gene expression is regulated through a complex network of signal transduction pathways activated by various hormones and growth factors. Estrogens regulate PRL gene transcription in vivo through both direct and indirect, protein synthesis-dependent, mechanisms. Therefore, we hypothesized that other stimulators of PRL gene transcription might also act via protein synthesis-dependent mechanisms. To test this hypothesis, we examined, in GH4C1 rat pituitary tumor cells, the effects of protein synthesis inhibitors on the induction of PRL mRNA by known stimulators of PRL gene transcription. Whereas induction by epidermal growth factor (EGF) was abolished by cycloheximide and puromycin, increases in PRL mRNA caused by thyrotropin releasing hormone, 12-O-tetradecanoylphorbol 13-acetate, forskolin, or dibutyryl cyclic AMP were unaffected. These data suggest that the induction of PRL mRNA by EGF may require the induced synthesis of an intermediary regulatory protein.

Stimulation of adenovirus early gene expression by phorbol ester: its possible mechanism

Treatment of Hela cells infected with adenovirus 5 wild type (Ad5WT) with the tumor-promoting phorbol ester TPA (12-O-tetradecanoyl phorbol-13-acetate), accelerated as well as stimulated expression of viral early genes EII and EIII but not that of EIA. TPA treatment of HeLa cells infected with dl312, an Ad5 EIA deletion mutant, activated expression of EIII but not EII. Stimulation of EII and EIII expression was blocked by H7 (1-5-isoquinolinyl sulfonyl-2-methyl piperazine), a specific inhibitor of protein kinase c (PKc). Nuclear run off assays demonstrated that TPA exerted a stimulatory effect at the level of transcription. PKc inhibitor alone reduced transcription of early genes in the absence of TPA activation. Phosphorylation of EIA 35 kDa but not 40- to 45-kDa proteins was dramatically increased by TPA. Three cellular proteins of 200, 24, and 20 kDa which coprecipitated with EIA proteins underwent enhanced and preferential phosphorylation by activated PKc. Inhibitor of PKc blocked phosphorylation of cellular proteins and reduced phosphorylation of EIA 35 kDa but not EIA 40- to 45-kDa proteins. These results tend to indicate that TPA stimulates adenovirus early gene expression through activation of protein kinase c and further suggest but do not prove that this may be due to specific phosphorylation of EIA 35 kDa and cellular proteins of 200, 24, and 20 kDa.

Identification of neurons expressing thyrotropin releasing-hormone receptor mRNA in spinal cord and lower brainstem of rat

The distribution of mRNA coding for a pituitary thyrotropin releasing-hormone (TRH) receptor was examined on sections of spinal cord and lower brainstem of rat using in situ hybridization. Hybridization signals were observed over large neurons in the ventral horn in cervical, thoracic, and lumbar segments of spinal cord, and over neurons in the motor nuclei of the lower brainstem. Although significant thyrotropin-releasing hormone binding has been reported in the superficial dorsal horn, only background levels of hybridization were observed over neurons in this region. These findings suggest that mRNA coding for thyrotropin-releasing hormone receptor is expressed in some spinal and brainstem motor neurons. Since many of these neurons are innervated by TRH-containing afferents, TRH may exert a direct effect upon at least some of these cells.

Specific inhibition of pituitary prolactin production by energy restriction in C3H/SHN female mice

Pituitaries were excised from control (C; 95 kcal/week) or energy restricted (ER; 48 kcal/week) female mice of 2, 3, 7, and 18 months of age. The total RNA and relative actin mRNA amounts in the pituitary were significantly greater in C than in ER mice both at 7 and 18 months. Prolactin (PRL) mRNA, standardized with actin mRNA, was significantly less in ER mice of 7 (50%) and 18 (51%) months of age than in age-matched controls, suggestive of specific inhibition of PRL mRNA transcription. Pituitary RNA and actin mRNA increased from 7 to 18 months in C mice but not in ER mice. Similarly, mean pituitary volumes increased between 2 and 18 months in C mice but not in ER mice. PRL mRNA, standardized with actin mRNA, did not change in either C or ER mice 7-18 months of age. All examined C mice of 3, 7, and 18 months of age had estrous cycles but none of the ER mice of the same ages. After 1 month of ER, the pituitary volumes and serum insulin concentrations in 2-month-old female mice were reduced. Thus net reduction of PRL mRNA per pituitary by ER is attributable to decreases in pituitary size and specific inhibition of PRL production, both of which may be due to low estrogen and insulin levels.

Phosphorylation of nuclear proteins in myelinating oligodendrocytes and its control by cyclic AMP

Oligodendroglial nuclei isolated from rat brains at different stages of myelinogenesis (10, 18, and 30 days of age) were incubated with [gamma-32P]ATP and extracted with 0.75 M perchloric acid to yield a fraction of nonacidic chromatin proteins. The protein extracts were then analyzed by polyacrylamide gel electrophoresis. The phosphorylation pattern of these proteins was found to be different for different age groups. In 10-day-old rat oligodendrocytes the most extensive phosphorylation occurred in low molecular mass species (less than 30 kDa), in contrast to fractions obtained from 18- and 30-day-old rat oligodendrocytes which showed a significantly higher labeling of the proteins with molecular masses greater than 30 kDa. The phosphorylation of the latter species was greatly stimulated by the presence of cyclic AMP in the incubation media. The results suggest that the phosphorylation of specific nuclear proteins, which may play a regulatory role at different stages of oligodendroglial maturation and myelinogenesis, may be at least partially modulated by intracellular cyclic AMP.

Potentiation of thyrotropin-releasing hormone-stimulated prolactin mRNA levels in GH3 cells by acetylcholine

We have examined the effect of acetylcholine (ACh) pretreatment on the thyrotropin-releasing hormone (TRH) induced prolactin gene expression in GH3 cells, a rat pituitary tumor cell line. Prolonged exposure (greater than 6 h) to ACh enhanced the TRH-induced prolactin mRNA accumulation in a time- and concentration-dependent manner while ACh by itself did not affect the basal prolactin mRNA levels appreciably. Maximal augmentation of the TRH-induced prolactin mRNA accumulation was obtained when cells were pretreated with 10(-5) M ACh for 24 h. The activation was mimicked by carbachol and oxotremorine and was blocked by the simultaneous presence of atropine. Preincubation of GH3 cells with pertussis toxin abolished the augmenting effect of ACh. These results indicate that prolonged exposure to muscarinic receptor agonists may enhance the TRH-stimulated prolactin mRNA expression and a pertussis toxin sensitive G-protein may be involved.

The neuropeptide VIP modulates the neurotransmitter phenotype of cultured chick sympathetic neurons

The present report provides evidence for a novel function for the neuropeptide vasoactive intestinal peptide (VIP). We demonstrate that VIP increases the cholinergic and the noradrenergic properties of cultured chick sympathetic neurons without changing neuronal survival and metabolism. VIP induces a 10- to 15-fold increase in the activity of choline acetyltransferase and an approximately twofold increase in the activity of tyrosine hydroxylase. Forskolin, an activator of adenylate cyclase, mimics all the effects of VIP on these cells. In addition, the effects of forskolin and VIP at optimal concentrations are not additive. Furthermore, VIP induces a rapid increase in the intracellular cAMP levels. Thus VIP acts via a cAMP-dependent pathway to enhance the cholinergic and noradrenergic properties of cultured chick sympathetic neurons.

Secretogranin I (chromogranin B) mRNA accumulation is hormonally regulated in GH3B6 rat pituitary tumor cells

Secretogranin I (SgI; chromogranin B) belongs to a class of acidic tyrosine-sulfated secretory proteins believed to play a role in the secretory process of endocrine cells. Our aim here was to compare the levels of SgI mRNA to that of prolactin (PRL) and growth hormone (GH), using rat pituitary cell lines. As far as the constitutive expression is concerned, we found a positive correlation between SgI mRNA and PRL mRNA levels. However, the neuropeptide TRH (50 nM) inhibited the accumulation of SgI mRNA in GH3B6 cells whereas, as expected, it induced a rapid and sustained increase in PRL mRNA accumulation. By contrast, 17 beta-estradiol (1 nM) stimulated the accumulation of both SgI and PRL mRNAs, with the same EC50 (18-59 pM). Reciprocally, treatment with dexamethasone (100 nM) reduced the level of SgI and PRL mRNAs to 23% and 29% of control, respectively, but led to a 2.1-fold increase in the GH mRNA level. Altogether, the present work shows that SgI gene expression is subject to multiple hormonal regulations and occasionally parallels the regulation of the PRL gene but never that of the GH gene, under the conditions tested.

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

We have cloned DNA sequences extending up to 6000 bp upstream from the first exon of the human prolactin (hPRL) gene. 5000 bp of these upstream sequences were fused to a CAT reporter gene and shown to provide tissue-specific transient expression in rat pituitary GH3 cells. Multihormonal response was found in this transient expression assay, leading to significant 2- to 5-fold induction by addition of 8-chlorophenylthio-cyclic AMP, thyrotropin-releasing hormone, epidermal growth factor, basic fibroblast growth factor, phorbol myristate acetate, a calcium channel agonist (Bay K-8644) and triiodothyronine. A 3-fold inhibition was observed in the presence of the glucocorticoid agonist dexamethasone. The sequence of the hPRL promoter was determined up to coordinate -3470. Computer similarity search between the rat and human sequences showed two highly conserved regions corresponding to the proximal and distal tissue specific enhancers described in both PRL promoters.

Prolactin messenger ribonucleic acid concentrations throughout the ovine estrous cycle: assessment relative to prolactin serum and pituitary amounts

Prolactin (PRL) mRNA concentrations were assessed by nucleic acid hybridization assays in pituitaries of ewes representing the defined stages of the ovine estrous cycle. Concomitantly, pituitary and serum PRL concentrations were measured in these ewes using radioimmunoassays. It was observed that PRL serum, pituitary and mRNA concentrations tended to increase near the time of the gonadotropin preovulatory surge, particularly between 24 hrs before behavioral estrus to 5 hours after estrus. However, the changes in PRL mRNA, serum and pituitary concentrations were shown not to be statistically significant. These data suggest that PRL production during the sheep estrous cycle is maintained without dramatic changes in synthesis or secretion.

Specific antisense RNA inhibition of growth hormone production in differentiated rat pituitary tumour cells

An expression vector that carried an inverted 800 base pair insert of the rat growth hormone (rGH) cDNA downstream of the SV40 promotor was used to transfect two different growth hormone (GH) producing rat pituitary cell strains, GH12C1 and GH3. This resulted in a specific transient inhibition of growth hormone production up to 75 percent in the course of 72 hours. GH synthesis reduction occurred parallel to a decrease of GH cytoplasmic mRNA levels. Levels of beta-actin and guanine nucleotide-binding regulatory protein (G protein) mRNAs were unaltered, but PRL mRNA levels were increased. Transfection with a control vector did not affect GH production.

Induction of distinct phenotypes in clonal and variant GH4 pituitary cells

GH cells are a widely used cell strain for the investigation of mechanisms regulating hormone release and synthesis. This report identifies two inducible phenotypes of the GH4 clone (epithelioid and motile) which may extend studies of this well-characterized cell line to different stages of pituitary cell development. GH4C1 cells treated in suspension with epidermal growth factor plus tetradecanoylphorbol acetate aggregate to form large epithelioid colonies with extensive cell-to-cell and cell-to-substratum adhesion. These cells cease replicating within 48 h, increase 50% in cell volume, and synthesize 40-fold more prolactin. A GH4C1 variant with enhanced substratum adhesion and little or no cell-to-cell adhesion (GH4S1), responds differently to this treatment. These cells cease replicating immediately, show increased cell separation, develop leading lamellae, and display locomotory activity. Each phenotype coexists in mixed cultures of GH4C1 and GH4S1 cells. This indicates that the different inducible response of the variant does not result from autocrine secretion. A molecular basis for cell-to-cell adhesion in GH4 cells was investigated. GH4C1, but not the variant cells, express a 180 kDa immunoreactive protein indistinguishable from an isoform of the neural cell adhesion molecule. Therefore the absence of cell-to-cell adhesion and inability to develop extensive cell-to-cell adhesion characteristic of the epithelioid phenotype may result from altered expression of the neural cell adhesion molecule. These findings are important because they have defined an in vitro approach to investigate genetic and cellular changes associated with the development and progression of pituitary cell phenotype.

GH4 pituitary cell variants selected as nonresponsive to thyrotropin-releasing hormone-enhanced substratum adhesion are nonresponsive to epidermal growth factor: evidence for a common signaling defect

Thyrotropin-releasing hormone (TRH) and epidermal growth factor both enhance prolactin synthesis and substrate adhesion (a morphological change called stretching) of GH4 rat pituitary cells. We have examined TRH- and EGF-induced cell stretching using genetic and pharmacologic approaches. We selected and isolated a series of GH4 cell variants nonresponsive to TRH-induced cell stretching (str-). This selection yielded several variants that were nonresponsive to both TRH- and EGF-induced stretching but were still responsive to stretching induced by several other agents (tetradecanoylphorbol acetate [TPA], butyrate, and Neplanocin A). One of the str- variants (a14) was examined in detail. TRH, EGF, and TPA each enhanced prolactin synthesis in a14 cells, indicating that the a14 variant contained functional receptor binding sites for all 3 ligands as well as the capacity to generate those intracellular signals required for enhanced prolactin synthesis. Because the str- variants were isolated without selective pressure for EGF-induced stretching and because the possibility of more than one selectable mutation in all the variants is unlikely, we suggest that TRH and EGF share a common mechanism to induce cell stretching. We next examined whether the str- variants had a defect in a signaling pathway or in the biochemical endpoint for TRH- and EGF-induced cell stretching. A pharmacologic approach was utilized to investigate the biochemical basis for induced cell stretching. A synthetic Arg-Gly-Asp-Ser tetrapeptide (RGDS), specific for fibronectin and vitronectin adhesion receptors, inhibited TRH-, EGF-, and TPA-induced GH4 cell stretching and attachment to fibronectin- and vitronectin-coated dishes. These results suggest that the interaction between fibronectin and/or vitronectin and their receptor(s) may be a biochemical endpoint by which several agonists induced stretching of GH4 cells. Because the str- variant has RGDS-specific binding sites for fibronectin and vitronectin and responds to some agents that induce cell stretching via an RGDS receptor, we conclude that the a14 str- variant has a defect in an intracellular signaling pathway, shared by TRH and EGF, which induces cell stretching.

Phorbol ester stimulates prolactin release but reduces prolactin mRNA in the human B-lymphoblastoid cell line IM-9-P3

The phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA) stimulated prolactin (PRL) release from the PRL producing human B-lymphoblastoid cell line IM-9-P3 within 30 min with an EC50 of 5 x 10(-9) M. Increased release was entirely attributable to a loss from intracellular PRL pools. No change in hPRL mRNA was observed during 8 h of exposure to TPA. Prolonged exposure of the cells to 2 x 10(-7) M TPA, however, led to a maximal reduction of hPRL mRNA levels after 24 h and a subsequent recovery by 72 h. Secretory rates followed a corresponding kinetic. The relative abundance of c-myc mRNA was not affected, although a persistent inhibition of cellular proliferation occurred upon chronic exposure to TPA. The addition of dibutyryl cAMP caused a minor transient increase in hPRL secretion by 35% after 1 h.

Activity-dependent regulation of gene expression in muscle and neuronal cells

In both the central and the peripheral nervous systems, impulse activity regulates the expression of a vast number of genes that code for synaptic proteins, including neuropeptides, enzymes involved in neurotransmitter biosynthesis and degradation, and membrane receptors. In recent years, the mechanisms involved in these regulations became amenable to investigation by the methods of recombinant DNA technology. The first part of this review focuses on the activity-dependent control of nicotinic acetylcholine receptor biosynthesis in vertebrate muscle, a model case for the regulation of synaptic protein biosynthesis at the postsynaptic level. The second part summarizes some examples of neuronal proteins whose biosynthesis is under the control of transsynaptic impulse activity. The first, second, and third intracellular messengers involved in membrane-to-gene signaling are discussed, as are possible posttranscriptional control mechanisms. Finally, models are proposed for a role of neuronal activity in the genesis and stabilization of the synapse.

Thyroid hormone action and the erbA oncogene family

In this review, we discuss the biological action and biochemical function of the v-erbA oncogene product, and the role of c-erbA proto-oncogene products as thyroid hormone receptors, as related to the molecular structure and function of the nuclear hormone receptors at large.

Growth hormone release from chicken anterior pituitary cells in primary culture: TRH and hpGRF synergy, protein synthesis, and cyclic adenosine 3'5'-monophosphate

Our earlier work showed that the effects of thyrotropin-releasing hormone (TRH) and human pancreatic growth hormone-releasing factor (hpGRF) on growth hormone (GH) release are synergistic (greater than additive) in a primary culture of chicken adenohypophyseal cells. The purpose of the present studies was to investigate the possible participation of protein synthesis and cyclic adenosine 3'5'-monophosphate (cAMP) in GH release. Following culture (48 hr), cells were incubated for 2 hr with test agents. Cycloheximide (an inhibitor of protein synthesis) had no effect on basal (absence of test agent) GH release or hpGRF-induced GH release. However, cycloheximide abolished the synergy between TRH and hpGRF. Although neither TRH nor hpGRF alone stimulated GH production (intracellular GH plus GH release) during a 2-hr incubation period, in combination these secretagogues increased total GH. These findings suggest that GH release from the chicken somatotroph under conditions of TRH and hpGRF synergy requires protein synthesis. In other studies, cells were exposed to agents inducing the formation of cAMP and either TRH or hpGRF. 8 Br-cAMP (10(-3) M), forskolin (10(-6) M), or isobutylmethylxanthine (IBMX; 10(-3) M) alone stimulated GH release to values between 30 and 50% over the basal value. The combined effects of each of these agents and TRH on GH release were synergistic. Similarly, IBMX and hpGRF exerted synergistic effects on GH release. In contrast, no synergy was shown between hpGRF and either 8 Br-cAMP or forskolin; their combined actions were less than additive.