Mitogen-activated protein kinase activation by stimulation with thyrotropin-releasing hormone in rat pituitary GH3 cells

Regulation of phosphosignaling pathways involved in transcription of cell cycle target genes by TRH receptor activation in GH1 cells

Thyrotropin-releasing hormone (TRH) is known to activate several cellular signaling pathway, but the activation of the TRH receptor (TRH-R) has not been reported to regulate gene transcription. The aim of this study was to identify phosphosignaling pathways and phosphoprotein complexes associated with gene transcription in GH1 pituitary cells treated with TRH or its analog, taltirelin (TAL), using label-free bottom-up mass spectrometry-based proteomics. Our detailed analysis provided insight into the mechanism through which TRH-R activation may regulate the transcription of genes related to the cell cycle and proliferation. It involves control of the signaling pathways for β-catenin/Tcf, Notch/RBPJ, p53/p21/Rbl2/E2F, Myc, and YY1/Rb1/E2F through phosphorylation and dephosphorylation of their key components. In many instances, the phosphorylation patterns of differentially phosphorylated phosphoproteins in TRH- or TAL-treated cells were identical or displayed a similar trend in phosphorylation. However, some phosphoproteins, especially components of the Wnt/β-catenin/Tcf and YY1/Rb1/E2F pathways, exhibited different phosphorylation patterns in TRH- and TAL-treated cells. This supports the notion that TRH and TAL may act, at least in part, as biased agonists. Additionally, the deficiency of β-arrestin2 resulted in a reduced number of alterations in phosphorylation, highlighting the critical role of β-arrestin2 in the signal transduction from TRH-R in the plasma membrane to transcription factors in the nucleus.

Biochemical and physiological insights into TRH receptor-mediated signaling

Thyrotropin-releasing hormone (TRH) is an important endocrine agent that regulates the function of cells in the anterior pituitary and the central and peripheral nervous systems. By controlling the synthesis and release of thyroid hormones, TRH affects many physiological functions, including energy homeostasis. This hormone exerts its effects through G protein-coupled TRH receptors, which signal primarily through G_q/11 but may also utilize other G protein classes under certain conditions. Because of the potential therapeutic benefit, considerable attention has been devoted to the synthesis of new TRH analogs that may have some advantageous properties compared with TRH. In this context, it may be interesting to consider the phenomenon of biased agonism and signaling at the TRH receptor. This possibility is supported by some recent findings. Although knowledge about the mechanisms of TRH receptor-mediated signaling has increased steadily over the past decades, there are still many unanswered questions, particularly about the molecular details of post-receptor signaling. In this review, we summarize what has been learned to date about TRH receptor-mediated signaling, including some previously undiscussed information, and point to future directions in TRH research that may offer new insights into the molecular mechanisms of TRH receptor-triggered actions and possible ways to modulate TRH receptor-mediated signaling.

A single nucleotide polymorphism of the thyrotropin releasing hormone degrading ectoenzyme (TRHDE) gene is associated with post-partum anestrus in Murrah buffalo

Thyrotropin releasing hormone degrading enzyme (TRHDE) gene is implicated in Thyrotropin releasing hormone (TRH) mediated prolactin secretion. It has been shown that the prolactin secretion alters the Gonadotropin-releasinghormone(GnRH) mediated estrous cycle. Therefore, TRHDE may also regulate postpartum anestrus. Earlier studies reported the role of non-synonymous single nucleotide polymorphism (SNPs) in various pathophysiological conditions by altering the structure and function of the proteins. Hence, in the present study, we identified SNPs in the putative promoter, first exon, middle exon and 3'-UTR containing the last exon of TRHDE gene and determined their association with postpartum anestrus (PPA) in Murrah buffaloes. We found one non synonymous SNP (G > C at 118095875 bp on chromosome 4) in the first exon of TRHDE and performed its association analysis in a population sample of 50 extreme PPA (residual PPAI: 123.06 ± 12.98 days) and 50 normal (residual PPAI: -80.46 ± 3.19 days) buffaloes. The residual PPAI value was the observed PPAI adjusted for the effect of 38 non-genetic factors. The analysis showed a significant (P < 0.004167) association of this SNP with PPA in buffaloes. Molecular dynamics simulations (MDS) also supported that the C allele altering Glutamine to Histidine at the amino acid 148 of TRHDE could enhance the stability and rigidity of TRHDE protein, which may lower its activity, increase TRH and prolactin, and reduce GnRH in PPA buffaloes. The MDS analysis further strengthens the association of the SNP (G > C) in the TRHDE gene with PPA condition in Murrah buffaloes. However, further investigation is needed to prove the MDS observations.

Characterization of a novel thyrotropin-releasing hormone receptor, TRHR3, in chickens

The physiological roles of thyrotropin-releasing hormone (TRH) are proposed to be mediated by TRH receptors (TRHR), which have been divided into 3 subtypes, namely, TRHR1, TRHR2, and TRHR3, in vertebrates. Although 2 TRH receptors (TRHR1 and TRHR3) have been predicted to exist in birds, it remains unclear whether TRHR3 is a functional TRH receptor similar to TRHR1. Here, we reported the functionality and tissue expression of TRHR3 in chickens. The cloned chicken TRHR3 (cTRHR3) encodes a receptor of 387 amino acids, which shares high-amino-acid identities (63–80%) to TRHR3 of parrots, lizards, Xenopus tropicalis, and tilapia and comparatively lower sequence identities to chicken TRHR1 or mouse TRHR2. Using cell-based luciferase reporter assays and Western blot, we demonstrated that similar to chicken TRHR1 (cTRHR1), cTRHR3 expressed in HEK 293 cells can be potently activated by TRH and that its activation stimulates multiple signaling pathways, indicating both TRH receptors are functional. Quantitative real-time PCR revealed that cTRHR1 and cTRHR3 are widely, but differentially, expressed in chicken tissues, and their expression is likely controlled by promoters located upstream of exon 1, which display strong promoter activities in cultured DF-1 cells. cTRHR1 is highly expressed in the anterior pituitary and testes, while cTRHR3 is highly expressed in the muscle, testes, fat, pituitary, spinal cord, and many brain regions (including hypothalamus). These findings indicate that TRH actions are likely mediated by 2 TRH receptors in chickens. In conclusion, our data provide the first piece of evidence that both cTRHR3 and cTRHR1 are functional TRH receptors, which helps to elucidate the physiological roles of TRH in birds.

Role of pituitary adenylate cyclase-activating polypeptide in modulating hypothalamic-pituitary system

BACKGROUND

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a multifunctional peptide that is isolated and identified from the ovine hypothalamus, whose effects and mechanisms have been elucidated in numerous studies. The PACAP and its receptor are widely expressed, not only in the hypothalamus but also in peripheral organs.

METHODS

The studies on the role of PACAP in the hypothalamic-pituitary system, including those by the authors, were summarized.

RESULTS

In the pituitary gonadotrophs, PACAP increases the gonadotrophin α-, luteinizing hormoneβ-, and follicle-stimulating hormone β-subunit expression and the expression of gonadotropin-releasing hormone (GnRH) receptor and its own receptor, PAC1R. Moreover, a low-frequency GnRH pulse increases the expression of PACAP and PAC1R more than a high-frequency GnRH pulse in the gonadotrophs. The PACAP stimulates prolactin synthesis and secretion and increases PAC1R in the lactotrophs. In the hypothalamus, PACAP increases the expression of the GnRH receptors, although it is unable to increase the expression of GnRH in the GnRH-producing neurons.

CONCLUSION

The PACAP not only acts directly in each hormone-producing cell, it possibly might regulate hormone synthesis via the expression of its own receptors or those of other hormones.

Interactions between Two Different G Protein-Coupled Receptors in Reproductive Hormone-Producing Cells: The Role of PACAP and Its Receptor PAC1R

Gonadotropin-releasing hormone (GnRH) and gonadotropins are indispensable hormones for maintaining female reproductive functions. In a similar manner to other endocrine hormones, GnRH and gonadotropins are controlled by their principle regulators. Although it has been previously established that GnRH regulates the synthesis and secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH)—both gonadotropins—from pituitary gonadotrophs, it has recently become clear that hypothalamic GnRH is under the control of hypothalamic kisspeptin. Prolactin, which is also known as luteotropic hormone and is released from pituitary lactotrophs, stimulates milk production in mammals. Prolactin is also regulated by hypothalamic factors, and it is thought that prolactin synthesis and release are principally under inhibitory control by dopamine through the dopamine D2 receptor. In addition, although it remains unknown whether it is a physiological regulator, thyrotropin-releasing hormone (TRH) is a strong secretagogue for prolactin. Thus, GnRH, LH and FSH, and prolactin are mainly regulated by hypothalamic kisspeptin, GnRH, and TRH, respectively. However, the synthesis and release of these hormones is also modulated by other neuropeptides in the hypothalamus. Pituitary adenylate cyclase-activating polypeptide (PACAP) is a hypothalamic peptide that was first isolated from sheep hypothalamic extracts based on its ability to stimulate cAMP production in anterior pituitary cells. PACAP acts on GnRH neurons and pituitary gonadotrophs and lactotrophs, resulting in the modulation of their hormone producing/secreting functions. Furthermore, the presence of the PACAP type 1 receptor (PAC1R) has been demonstrated in these cells. We have examined how PACAP and PAC1R affect GnRH- and pituitary hormone-secreting cells and interact with their principle regulators. In this review, we describe our understanding of the role of PACAP and PAC1R in the regulation of GnRH neurons, gonadotrophs, and lactotrophs, which are regulated mainly by kisspeptin, GnRH, and TRH, respectively.

L-type calcium channels and MAP kinase contribute to thyrotropin-releasing hormone-induced depolarization in thalamic paraventricular nucleus neurons

In rat paraventricular thalamic nucleus (PVT) neurons, activation of thyrotropin-releasing hormone (TRH) receptors enhances neuronal excitability via concurrent decrease in a G protein-coupled inwardly rectifying K (GIRK)-like conductance and opening of a cannabinoid receptor-sensitive transient receptor potential canonical (TRPC)-like conductance. Here, we investigated the calcium (Ca(2+)) contribution to the components of this TRH-induced response. TRH-induced membrane depolarization was reduced in the presence of intracellular BAPTA, also in media containing nominally zero [Ca(2+)]o, suggesting a critical role for both intracellular Ca(2+) release and Ca(2+) influx. TRH-induced inward current was unchanged by T-type Ca(2+) channel blockade, but was decreased by blockade of high-voltage-activated Ca(2+) channels (HVACCs). Both the pharmacologically isolated GIRK-like and the TRPC-like components of the TRH-induced response were decreased by nifedipine and increased by BayK8644, implying Ca(2+) influx via L-type Ca(2+) channels. Only the TRPC-like conductance was reduced by either thapsigargin or dantrolene, suggesting a role for ryanodine receptors and Ca(2+)-induced Ca(2+) release in this component of the TRH-induced response. In pituitary and other cell lines, TRH stimulates MAPK. In PVT neurons, only the GIRK-like component of the TRH-induced current was selectively decreased in the presence of PD98059, a MAPK inhibitor. Collectively, the data imply that TRH-induced depolarization and inward current in PVT neurons involve both a dependency on extracellular Ca(2+) influx via opening of L-type Ca(2+) channels, a sensitivity of a TRPC-like component to intracellular Ca(2+) release via ryanodine channels, and a modulation by MAPK of a GIRK-like conductance component.

Role of thyrotropin-releasing hormone in prolactin-producing cell models

Thyrotropin-releasing hormone (TRH) is a hypothalamic hypophysiotropic neuropeptide that was named for its ability to stimulate the release of thyroid-stimulating hormone in mammals. It later became apparent that it exerts a number of species-dependent hypophysiotropic activities that regulate other pituitary hormones. TRH also regulates the synthesis and release of prolactin, although whether it is a physiological regulator of prolactin that remains unclear. Occupation of the Gq protein-coupled TRH receptor in the prolactin-producing lactotroph increases the turnover of inositol, which in turn activates the protein kinase C pathway and the release of Ca(2+) from storage sites. TRH-induced signaling events also include the activation of extracellular signal-regulated kinase (ERK) and induction of MAP kinase phosphatase, an inactivator of activated ERK. TRH stimulates prolactin synthesis through the activation of ERK, whereas prolactin release occurs via elevation of intracellular Ca(2+). We have been investigating the role of TRH in a pituitary prolactin-producing cell model. Rat pituitary somatolactotroph GH3 cells, which produce and release both prolactin and growth hormone (GH), are widely used as a model for the study of prolactin- and GH-secreting cells. In this review, we describe the general action of TRH as a hypophysiotropic factor in vertebrates and focus on the role of TRH in prolactin synthesis using GH3 cells.

Prolonged stimulation with thyrotropin-releasing hormone and pituitary adenylate cyclase-activating polypeptide desensitize their receptor functions in prolactin-producing GH3 cells

We used somatolactotroph GH3 cells to examine changes in response to stimulation with thyrotropin-releasing hormone (TRH) and pituitary adenylate cyclase-activating polypeptide (PACAP) after sustained treatment with these peptides. TRH and PACAP increased prolactin promoter activity in mock- and PACAP type 1 receptor (PAC1R)-transfected cells. When the cells were pretreated with TRH for 48 h, the response of the prolactin promoter to both TRH and PACAP was diminished. Similarly, in PAC1R-transfected GH3 cells pretreated with PACAP, the effects of TRH and PACAP on the prolactin promoter were eliminated. The stimulation of prolactin mRNA expression by TRH and PACAP was eliminated by prolonged pretreatment with these peptides in PAC1R-transfected cells. Both the serum response element (SRE) promoters and cAMP response element (CRE) promoters were activated by TRH and PACAP in either mock- or PAC1R-transfected cells. Pretreatment for 48 h with TRH also eliminated the effects of TRH and PACAP on the SRE and CRE promoters, and pretreatment of PAC1R-transfected cells with PACAP for 48 h reduced the responses of the SRE and CRE promoters to TRH and PACAP. These observations demonstrated that sustained stimulation with TRH and PACAP desensitizes their own and each other's receptors.

Angiotensins inhibit cell growth in GH3 lactosomatotroph pituitary tumor cell culture: a possible involvement of the p44/42 and p38 MAPK pathways

The local renin-angiotensin system is present in the pituitary. We investigated the effects of angiotensins on GH3 lactosomatotroph cells proliferation in vitro and the involvement of p44/42 and p38 MAPK inhibitors in the growth-regulatory effects of angiotensins. Materials and Methods. Cell viability using the Mosmann method and proliferation by the measurement of BrdU incorporation during DNA synthesis were estimated. Results. Ang II and ang IV decreased the viability and proliferation of GH3 cells. Inhibitor of p44/42 MAPK attenuated the effects of ang II on cell viability and proliferation but did not affect the ang 5-8-dependent actions. Inhibitor of p38 MAPK prevented the decrease in the number of GH3 cells in ang-II- and ang-IV-treated groups. Conclusions. The growth-inhibitory effect of ang II is possibly mediated by the p44/42 MAPK. The p38 MAPK appears to mediate the inhibitory effects of both ang II and ang 5-8 upon cell survival.

Effects of estradiol and progesterone on prolactin transcriptional activity in somatolactotrophic cells

We examined the effects of sex steroids on prolactin promoter activity in rat somatolactotrophic GH3 cells. Both estradiol (E2) and progesterone (P4) were found to inhibit basal prolactin promoter activity, but to potentiate Thyrotropin-releasing hormone (TRH)-induced prolactin promoter activity. P4 had a greater inhibitory effect on basal prolactin promoter activity than E2, and P4 also potentiated TRH-induced prolactin promoter more potently than E2. Combined treatment with E2 and P4 further increased TRH-induced prolactin promoter activity. E2 and P4 also both reduced basal serum response element (SRE) promoter activity, and increased TRH-induced SRE promoter activity. Combination treatment with E2 and P4 reduced basal activity of SRE promoter and increased TRH-induced SRE activity more potently than E2 or P4 alone. In contrast, basal cAMP response element (CRE) promoter activity was not influenced by either E2 or P4, although TRH-induced CRE promoter was potentiated by each of these steroids, and was further increased by E2 and P4 combination treatment. Both E2 and P4 increased TRH-induced extracellular signal-regulated kinase (ERK) phosphorylation; however, intracellular cAMP levels was not influenced by E2 or P4. TRH-induced CRE promoter was inhibited by mitogen-activated protein kinase/ERK kinase (MEK) inhibitor and was increased by overexpression of MEK kinase (MEKK). This study showed that ERK and SRE transcriptional pathways, but not the cAMP/CRE pathway, may be involved in the suppression of basal prolactin promoter activity, whereas both the ERK/SRE and MAP kinase-mediated CRE pathways appear to be involved in the increased transcriptional efficiency of the prolactin promoter induced by TRH stimulation.

Role of Neurokinin B and Dynorphin A in pituitary gonadotroph and somatolactotroph cell lines

The role of Neurokinin B (NKB) and Dynorphin A (Dyn) in the regulation of the hypothalamic pituitary axis is an important area of recent investigation. These peptides are critical for the rhythmic release of GnRH, which subsequently stimulates the secretion of the gonadotropins, luteinizing hormone (LH) and follicle-stimulating hormone (FSH). The present study utilized the gonadotroph cell line LβT2 and the somatolactotroph GH3 cell line to examine the possible role of these peptides in pituitary hormone secretion. The NKB receptor (NK3R) and the Dyn receptor (the κ-opiate receptor (KOR)) were both detected in LβT2 cells and GH3 cells. NKB, by itself, failed to increase gonadotropin LHβ and FSHβ promoter activities and did not modulate the effects of GnRH on gonadotropin promoter activity. In GH3 cells, NKB significantly increased TRH-induced PRL promoter activity although NKB alone did not have an effect on basal PRL promoter activity. Dyn had no effect on gonadotropin promoters alone or in combination with GnRH stimulation. PRL promoters stimulated by TRH were not significantly changed by Dyn. TRH-induced PRL promoter activity was further increased in the presence of higher concentrations of NKB, whereas Dyn did not have a significant effect on the PRL promoter even at a high concentration. In addition, TRH-induced ERK (Extracelluar signal-regulated kinase) activation was enhanced in the presence of NKB. Our current study demonstrated that NKB had a stimulatory effect on PRL expression in a PRL-producing cell, but had no effect on gonadotropin secretion from a gonadotroph cell line.

Secondary amenorrhea in a woman with spinocerebellar degeneration treated with thyrotropin-releasing hormone: a case report and in vitro analysis

Introduction

While thyrotropin-releasing hormone is known to be a prolactin-release stimulating factor, thyrotropin-releasing hormone-tartrate and its derivative, taltirelin hydrate, are used for the treatment of spinocerebellar degeneration, a degenerative disease characterized mainly by motor ataxia. We report the case of a patient being treated with a thyrotropin-releasing hormone preparation for spinocerebellar degeneration who developed amenorrhea after a dose increase. Her hormonal background was analyzed and the effect of prolonged exposure to thyrotropin-releasing hormone on pituitary prolactin-producing cells was examined in vitro.

Case presentation

Our patient was a 36-year-old Japanese woman who experienced worsening of gait disturbance at around 23 years of age, and was subsequently diagnosed as having spinocerebellar degeneration. She had been treated with thyrotropin-releasing hormone-tartrate for four years. Taltirelin hydrate was added to the treatment seven months prior to her presentation, followed by an improvement in gait disturbance. Around the same period, she started lactating and subsequently developed amenorrhea three months later. Taltirelin hydrate was discontinued and she was referred to our hospital. She was found to have normal sex hormone levels. A thyrotropin-releasing hormone provocation test showed a normal response of thyroid-stimulating hormone level and an over-response of prolactin at 30 minutes (142.7 ng/mL). Resumption of menstruation was noted three months after dose reduction of thyrotropin-releasing hormone. In our in vitro study, following long-term exposure to thyrotropin-releasing hormone, cells from the rat pituitary prolactin-producing cell line GH3 exhibited an increased basal prolactin promoter activity but showed a marked decrease in responsiveness to thyrotropin-releasing hormone.

Conclusions

Physicians should be aware of hyperprolactinemia-associated side effects in patients receiving thyrotropin-releasing hormone treatment. Long-term treatment with a thyrotropin-releasing hormone preparation might cause a large amount of prolactin to accumulate in prolactin-producing cells and be released in response to exogenous thyrotropin-releasing hormone stimulation.

Stimulatory effect of pituitary adenylate-cyclase activating polypeptide (PACAP) and its PACAP type I receptor (PAC1R) on prolactin synthesis in rat pituitary somatolactotroph GH3 cells

In this present study, we investigated the role of pituitary adenylate cyclase-activating polypeptide (PACAP) and its receptor, PACAP type I receptor (PAC1R) on prolactin synthesis in pituitary somatolactotroph GH3 cells. PACAP increased prolactin promoter activity up to 1.3 ± 0.1-fold. This increase, while significant, was less than the increase resulting from thyrotropin-releasing hormone (TRH) stimulation. By transfection of a PAC1R expression vector to the cells, the response to PACAP on prolactin promoter activity was dramatically potentiated to a degree proportional to the amount of PAC1R transfected. In the PAC1R expressing GH3 cells, TRH and PACAP alone increased prolactin promoter up to 3.3 ± 0.3-fold and 4.9 ± 0.2-fold, respectively, and combined treatment with TRH and PACAP further increased prolactin promoters up to 6.8 ± 0.6-fold. PACAP binds both Gs- and Gq-coupled receptors and stimulates adenylate cyclase/cAMP and protein kinase C/extracellular signal-regulated kinase (ERK) signaling pathways. PACAP increased ERK phosphorylation in PAC1R expressing cells to the same degree as TRH. Combined treatment with TRH and PACAP had a synergistic effect on ERK activation. GH3 cells produce both prolactin and growth hormone. Stimulation of GH3 cells with TRH significantly increased the mRNA level of prolactin and attenuated growth hormone mRNA expression. PACAP increased both prolactin and growth hormone mRNA levels, particularly in PAC1R expressing cells. In addition, increasing amount of PAC1R in GH3 cells potentiated the action of TRH on prolactin promoter activity, as well as on ERK phosphorylation. PAC1R was induced by PACAP itself, but not by TRH. Our current study demonstrates that PACAP and its PAC1R, functions as a stimulator of prolactin alone or with TRH in prolactin producing cells.

Knockdown of IGF-binding protein 7 inhibits transformation of the endometrial gland in an in vitro model

Uterine endometrial glands and their secretory products are critical for the implantation and survival of the peri-implantation embryo, and for the establishment of uterine receptivity. We previously reported that insulin-like growth factor binding protein 7 (IGFBP7) is abundantly expressed in uterine glandular epithelial cells during the secretory phase of the menstrual cycle. In the present study, we used a cultured glandular epithelial cell line of human (EM1) to investigate the significance of IGFBP7 in the function of endometrial glands. EM1 cells formed a mesh-like structure on Matrigel, which was accompanied by elevated levels of intracellular cyclic AMP. However, these morphological changes were blocked by treatment with protein kinase A (PKA) inhibitor (H89). IGFBP7 knockdown using specific short interference RNA (siRNA) inhibited the formation of the mesh-like structure on Matrigel. Cyclic AMP analogs, dibutyryl-cAMP, and N(6)-phenyl-cAMP induced the expression of leukemia inhibitory factor (LIF) which is essential for the onset of implantation. Enhanced LIF expression was suppressed by IGFBP7 siRNA treatment. Western blot analysis revealed that IGFBP7 knockdown results in the aberrant, constitutive expression of the MAPK signaling pathway. These results suggest that IGFBP7 regulates morphological changes of glandular cells by interfering with the normal PKA and MAPK signaling pathways that are associated with the transformation and/or differentiation of endometrial glands.

Follistatin, induced by thyrotropin-releasing hormone (TRH), plays no role in prolactin expression but affects gonadotropin FSHbeta expression as a paracrine factor in pituitary somatolactotroph GH3 cells

Follistatin regulates FSHbeta gene expression by binding to and bioneutralizing activin effects. In this study, we found that thyrotropin-releasing hormone (TRH) increased follistatin gene expression in pituitary somatolactotroph GH3 cells. Treatment of GH3 with 100 nM TRH significantly increased follistatin mRNA expression as determined by real time PCR. TRH-induced follistatin expression was significantly abrogated in the presence of MEK inhibitor, U0126. Overexpression of constitutive active MEKK in GH3 cells dramatically increased follistatin expressions. Transfection of GH3 cells with follistatin siRNA reduced endogenous follistatin mRNA expression, but failed to modulate prolactin promoter activity. Prolactin mRNA levels were not affected by increasing the dose of follistatin, and TRH-induced prolactin promoter activity was not modulated in the presence of follistatin. In other experiments using pituitary gonadotroph LbetaT2 cells, activin increased FSHbeta promoter activity and mRNA expression, and follistatin completely inhibited this activin-increased FSHbeta gene expression. Treatment of GH3 cells with activin reduced the basal activity of prolactin promoter and follistatin prevented this effect. GH3 cells were co-cultured with LbetaT2 cells, which had been transfected with FSHbeta promoter-linked luciferase vectors and treated with activin in the presence of TRH. Activin-induced FSHbeta promoter activity was completely inhibited in the presence of TRH. In addition to that, FSHbeta mRNA was not detected from LbetaT2 cells which were co-cultured with GH3 cells. Our current results suggest the possibility that TRH increases follistatin gene expression in prolactin-producing cells in association with ERK pathways. Somatolactotroph-derived follistatin affects gonadotrophs by countering activin-induced FSHbeta gene expression in a paracrine fashion.

Activation of extracellular signal-regulated kinase (ERK) and induction of mitogen-activated protein kinase phosphatase 1 (MKP-1) by perifused thyrotropin-releasing hormone (TRH) stimulation in rat pituitary GH3 cells

We investigated the pattern of extracellular signal-regulated kinase (ERK) phosphorylation and the induction of mitogen-activated protein kinase phosphatase 1 (MKP-1) by thyrotropin-releasing hormone (TRH) under various stimulation conditions in pituitary GH3 cells. In static culture, ERK activation by continuous TRH was maximal at 10 min and persisted for up to 60 min, with a return to the basal level by 2h. Stimulation with continuous TRH in perifused cells resulted in a similar level of ERK phosphorylation. MKP-1 was expressed 60 min following either static or perifused, continuous TRH stimulation. When cells were stimulated with pulsatile TRH every 30 min, ERK activation was maximal at 10 min and returned to its baseline level by 30 min. ERK was phosphorylated again with each subsequent pulse. Pulsatile TRH did not induce MKP-1. Prolactin promoter activity following continuous, static TRH stimulation was higher than that following perifused TRH stimulation. TRH at a frequency of one pulse every 30 min increased prolactin promoter activity similar to that of perifused, continuous TRH stimulation. Additionally, changes in pulse frequency resulted in alterations in the level of prolactin promoter. Following static stimulation, a 10 min exposure to TRH was sufficient to obtain full activation of the prolactin promoter. Additionally, a 5-10 min exposure of TRH was sufficient to maintain ERK activation. A single 5-min pulse of TRH stimulation resulted in low activation of the prolactin promoter. ERK activation was necessary for prolactin gene transcription; however, prolactin gene transcription is not entirely determined by the strength or duration of TRH-induced ERK activation.

Involvement of cAMP response element-binding protein in the regulation of cell proliferation and the prolactin promoter of lactotrophs in primary culture

Hypothalamic hormones, including dopamine, regulate critical functions of pituitary cells via the cAMP-protein kinase A (PKA) pathway. The PKA-downstream transcription factor cAMP response element (CRE)-binding protein (CREB) is an integrating molecule that is also activated by many other protein kinase pathways. We investigated the involvement of CREB in the regulation of cell proliferation and the PRL promoter of rat lactotrophs in primary cell culture. Recombinant adenoviruses were used for efficient gene delivery into pituitary cells. Bromocriptine, a dopaminergic agonist known to decrease intracellular cAMP concentrations, caused inhibition of PRL promoter activity and lactotroph proliferation, which was accompanied by decreases in CRE-mediated transcription and CREB phosphorylation in lactotrophs. Expression of a dominant-negative form of CREB (MCREB), which was effective in suppressing CRE-mediated transcription induced by the adenylate cyclase activator forskolin, inhibited basal and forskolin-induced PRL promoter activity and PRL mRNA expression. MCREB expression lowered basal proliferative levels and blocked forskolin-induced proliferation of lactotrophs. Insulin-like growth factor I (IGF-I), a potent mitogen in lactotrophs, did not affect intracellular cAMP concentrations but transiently increased lactotroph CREB phosphorylation. MCREB expression also inhibited IGF-I-induced lactotroph proliferation. These results suggest that CREB is involved in the regulation of cell proliferation and the PRL promoter in normal lactotrophs and that dopamine inhibition of these lactotroph functions is at least in part due to inhibition of the cAMP-PKA-CREB pathway.

Cyclic adenosine 3',5'monophosphate/protein kinase A and mitogen-activated protein kinase 3/1 pathways are involved in adenylate cyclase-activating polypeptide 1-induced common alpha-glycoprotein subunit gene (Cga) expression in mouse pituitary gonadotroph LbetaT2 cells

Adenylate cyclase-activating polypeptide 1 (ADCYAP1) binds both Gs- and Gq-coupled receptors and stimulates adenylate cyclase/cAMP and protein kinase C/mitogen-activated protein kinase 3/1 (MAPK3/1) signaling pathways in pituitary gonadotrophs. In this study, we investigated the cAMP and MAPK3/1 signaling pathways induced by ADCYAP1 stimulation and examined the effects of ADCYAP1 on the expression of gonadotropin subunit genes using a clonal gonadotroph cell line, LbetaT2. ADCYAP1 increased intracellular cAMP accumulation up to 19-fold in LbetaT2 cells. Common alpha-glycoprotein subunit gene (Cga) promoter activity was strongly activated by both ADCYAP1 and the cyclic-AMP analog, 8-(4-chlorophenylthio) adenosine 3',5'-cyclic monophosphate (CPT-cAMP). Both had little effect on luteinizing hormone beta (Lhb) and follicle-stimulating hormone beta (Fshb) promoter activities. Cga promoter activity was significantly increased by transfection with constitutively active cAMP-dependent protein kinase (PKA). Activities of the Lhb and Fshb promoters were only modestly increased. Both ADCYAP1 and CPT-cAMP induced MAPK3/1 activation in LbetaT2 cells. The MEK inhibitor, U0126, and the PKA inhibitors, H89 and cAMP-dependent protein kinase peptide inhibitor (PKI), completely inhibited MAPK3/1 activation by either ADCYAP1 or CPT-cAMP. Using luciferase reporter constructs containing cis-elements, the cAMP response element (Cre) promoter was stimulated about 4-fold by ADCYAP1. ADCYAP1-induced Cre promoter activity was completely inhibited by H89, but not by U0126. ADCYAP1 also increased the activity of the serum response element (Sre) promoter, a target for MAPK3/1, and treatment of the cells with U0126 completely inhibited ADCYAP1-induced Sre promoter activity. ADCYAP1-increased Cga promoter activity was inhibited partially by both H89 and U0126. Although combining the inhibitors showed an additive inhibition effect, it did not result in complete inhibition. These results suggest that in LbetaT2 cells, ADCYAP1 mainly increases Cga through activation of PKA and MAPK3/1, as well as through an additional unknown pathway.

Dopamine D(2) receptor expression and regulation of gonadotropin alpha-subunit gene in clonal gonadotroph LbetaT2 cells

This study investigated the role of dopamine on the regulation of gonadotropin secretion at the gonadotroph cell line. We examined the function of the dopamine D(2) receptor in the regulation of pituitary gonadotropin gene expression using LbetaT2 cells, a mature, well differentiated clonal gonadotroph cell line. The presence of the dopamine D(2) receptor in the LbetaT2 cells was confirmed by both RT-PCR and Western blot. Gonadotropin releasing hormone (GnRH) stimulation resulted in gonadotropin LHbeta, FSHbeta and alpha-subunit promoter activation, and none were inhibited by quinpirol, a specific dopamine D(2) receptor agonist. Pituitary adenylate cyclase-activating polypeptide (PACAP) increased gonadotropin alpha-subunit promoter activity, but not LHbeta and FSHbeta promoter activity. The activity of PACAP was significantly inhibited in the presence of quinpirol. The protein kinase A inhibitor, H89, also inhibited PACAP-induced alpha-subunit gene expression. PACAP increased intracellular cAMP more than GnRH did in LbetaT2 cells, and the elevation of cAMP was strongly inhibited in the presence of various dopamine D(2) agonists. These results suggest that in pituitary gonadotrophs, the dopamine D(2) receptor is a negative regulator of gonadotropin alpha-subunit gene expression which is induced by cAMP-elevating factors in a cAMP-dependent pathway.

Molecular Mechanism of Neuronal Plasticity: Induction and Maintenance of Long-Term Potentiation in the Hippocampus

Recent studies have demonstrated that activation of enzymes can be observed in living cells in response to stimulation with neurotransmitters, hormones, growth factors, and so forth. Thus, the activation of enzymes was shown to be closely related to the dynamic states of various cell functions. The development of new experimental methodologies has enabled researchers to study the molecular basis of neuronal plasticity in living cells. In 1973, Bliss and his associates identified the phenomena of long-term potentiation (LTP). Since it was thought to be a model for neuronal plasticity such as learning and memory, its molecular mechanism has been extensively investigated. The mechanism was found to involve a signal transduction cascade that includes release of glutamate, activation of the NMDA glutamate receptors, Ca(2+) entry, and activations of Ca(2+)/calmodulin-dependent protein kinases (CaM kinases) II and IV and mitogen-activated protein kinase (MAPK). Consequently, AMPA glutamate receptors were activated by phosphorylation by CaM kinase II, resulting in an increase of Ca(2+) entry into postsynaptic neurons. Furthermore, activation of CaM kinase IV and MAPK increased phosphorylation of CREB (cyclic AMP response element binding protein) and expression of c-Fos by stimulation of gene expression. These results suggest that LTP induction and maintenance would be models of short- and long-term memory, respectively.

PRDM5 is silenced in human cancers and has growth suppressive activities

Several genes that contain the PR (PRDI-BF1 and RIZ) domain have been linked with human cancers. We describe here a new PR-domain-containing gene designated as PRDM5 (PFM2). A PRDM5 cDNA was isolated based on its homology to the PR domain of RIZ1 (PRDM2). The gene encodes an open reading frame of 630 amino acids and contains a PR domain in the NH-terminal region followed by 16 zinc finger motifs. Radiation hybrid analysis mapped PRDM5 to human chromosome 4q26, a region thought to harbor tumor suppressor genes for breast, ovarian, liver, lung, colon, and other cancers. The gene has a CpG island promoter and is silenced in human breast, ovarian, and liver cancers. A recombinant adenovirus expressing PRDM5 caused G2/M arrest and apoptosis upon infection of tumor cells. These results suggest that inactivation of PRDM5 may play a role in carcinogenesis.

Differential Activation of the Luteinizing Hormone β-Subunit Promoter by Activin and Gonadotropin-Releasing Hormone: A Role for the Mitogen-Activated Protein Kinase Signaling Pathway in LβT2 Gonadotrophs1

LH consists of alpha- and beta-subunits, and synthesis of the beta-subunit has been reported to be the rate-limiting step in LH production. In this study, we found that activin A increased both the LHbeta mRNA level and LH content in cells of the gonadotroph cell line, LbetaT2. We next examined the effects of activin A and GnRH on LHbeta promoter activity by reporter gene assay and compared the signal transduction pathways. Activin A and GnRH activated the LHbeta promoter, and the response to a combination of activin A and GnRH was higher than that to activin A or GnRH alone. The effects of activin A and GnRH were specifically inhibited by inhibin-like peptide and antide, a GnRH antagonist, respectively. The activation of the LHbeta promoter by GnRH was inhibited by PD098059 and U0126, MAP kinase kinase (MEK) inhibitors. In contrast, these protein kinase inhibitors did not inhibit the activin A-induced activation. GnRH, but not activin A, activated MAP kinase in LbetaT2 cells. Overexpression of constitutively active MEK1 or MEK kinase activated both MAP kinase and the LHbeta promoter. Furthermore, GnRH, but not activin A, strongly induced SRE-mediated transcription, a known target of the MAP kinase pathway. These results suggest that GnRH activates the LHbeta promoter via the MAP kinase pathway and that activin A-induced activation of the LHbeta promoter is independent of the MAP kinase pathway.

Characterization of alphaT3-1 cells stably transfected with luteininzing hormone beta-subunit complementary deoxyribonucleic acid

Luteinizing hormone (LH) consists of alpha- and beta-subunits, and synthesis and secretion of LH are regulated by gonadotropin-releasing hormone (GnRH). In order to examine the molecular mechanisms by which GnRH regulates LH secretion, we transfected alphaT3-1 cells with rat LHbeta-subunit cDNA under the control of a constitutive promoter and established a stable cell line of LH2 cells which secreted LH in response to GnRH. Pulsatile and continuous GnRH pretreatments increased gene expression of the alpha-subunit and synthesis of LH, and enhanced the LH secretion by brief treatments with GnRH and 56 mM KCl. The LH secretions were partially blocked by elimination of extracellular Ca2+. GnRH-induced LH secretion was completely inhibited by calphostin C (a protein kinase C inhibitor) and 1 microM wortmannin. In contrast to the GnRH induction, high K+-induced LH secretion was inhibited by KN93, a Ca2+/calmodulin-dependent protein kinase II inhibitor, as well as by 1 microM wortmannin. We also confirmed that activation of cAMP-pathway induced LH secretion, but activation of mitogen-activated protein (MAP) kinase pathway was not involved in LH secretion. These results suggest that GnRH directly regulates LH secretion as well as LHbeta-subunit synthesis, and that LH2 cells are a useful model for the study of LH secretion induced by several secretagogues.

Regulation of gonadotropin alpha subunit gene expression by dopamine D(2) receptor agonist in clonal mouse gonadotroph alphaT3-1 cells

Pituitary prolactin biosynthesis is negatively regulated by hypothalamic dopamine through D(2) receptors in pituitary lactotrophs, but little is known about the direct effect of dopamine on gonadotrophs. In this study, the clonal gonadotroph-derived cell line, alphaT3-1, was used to examine whether gene expression of the pituitary gonadotropin alpha subunit, stimulated with GnRH or pituitary adenylate cyclase-activating polypeptide (PACAP), was controlled by dopamine D(2) receptor. Western blotting and reverse transcription-polymerase chain reaction analysis demonstrated the presence of dopamine D(2) receptors in alphaT3-1 cells. Both GnRH and PACAP increased alpha subunit gene expression. GnRH-induced alpha subunit gene expression was not affected by quinpirol, a specific dopamine D(2) receptor agonist. In contrast, PACAP-induced gene expression was significantly lower in the presence of quinpirol. The roles of extracellular signal-regulated kinase (ERK) and cAMP in the expression of the alpha subunit gene were examined. GnRH activated ERK, but PACAP did not, and the activation was not inhibited by quinpirol. GnRH-induced alpha subunit gene expression was completely inhibited by an ERK inhibitor, PD098059. Cyclic AMP accumulation in alphaT3-1 cells was increased by treatment with PACAP, and quinpirol inhibited this effect. GnRH did not affect cAMP production in these cells. These results suggest that in alphaT3-1 cells, dopamine D(2) receptors negatively regulate pituitary alpha subunit gene expression in association with the cAMP-dependent pathway, but not with the ERK pathway.

Dopamine-D2S receptor inhibition of calcium influx, adenylyl cyclase, and mitogen-activated protein kinase in pituitary cells: distinct Galpha and Gbetagamma requirements

The G protein specificity of multiple signaling pathways of the dopamine-D2S (short form) receptor was investigated in GH4ZR7 lactotroph cells. Activation of the dopamine-D2S receptor inhibited forskolin-induced cAMP production, reduced BayK8644- activated calcium influx, and blocked TRH-mediated p42/p44 MAPK phosphorylation. These actions were blocked by pretreatment with pertussis toxin (PTX), indicating mediation by G(i/o) proteins. D2S stimulation also decreased TRH-induced MAPK/ERK kinase phosphorylation. TRH induced c-Raf but not B-Raf activation, and the D2S receptor inhibited both TRH-induced c-Raf and basal B-Raf kinase activity. After PTX treatment, D2S receptor signaling was rescued in cells stably transfected with individual PTX-insensitive Galpha mutants. Inhibition of adenylyl cyclase was partly rescued by Galpha(i)2 or Galpha(i)3, but Galpha(o) alone completely reconstituted D2S-mediated inhibition of BayK8644-induced L-type calcium channel activation. Galpha(o) and Galpha(i)3 were the main components involved in D2S-mediated p42/44 MAPK inhibition. In cells transfected with the carboxyl-terminal domain of G protein receptor kinase to inhibit Gbetagamma signaling, only D2S-mediated inhibition of calcium influx was blocked, but not inhibition of adenylyl cyclase or MAPK. These results indicate that the dopamine-D2S receptor couples to distinct G(i/o) proteins, depending on the pathway addressed, and suggest a novel Galpha(i)3/Galpha(o)-dependent inhibition of MAPK mediated by c-Raf and B-Raf-dependent inhibition of MAPK/ERK kinase.

Differential regulation of pituitary hormone secretion and gene expression by thyrotropin-releasing hormone. A role for mitogen-activated protein kinase signaling cascade in rat pituitary GH3 cells

We examined the possible involvement of mitogen-activated protein (MAP) kinase activation in the secretory process and gene expression of prolactin and growth hormone. Thyrotropin-releasing hormone (TRH) rapidly stimulated the secretion of both prolactin and growth hormone from GH3 cells. Secretion induced by TRH was not inhibited by 50 microM PD098059, but was completely inhibited by 1 microM wortmannin and 10 microM KN93, suggesting that MAP kinase does not mediate the secretory process. Stimulation of GH3 cells with TRH significantly increased the mRNA level of prolactin, whereas expression of growth hormone mRNA was largely attenuated. The increase in prolactin mRNA stimulated by TRH was inhibited by addition of PD098059, and the decrease in growth hormone mRNA was also inhibited by PD098059. Transfection of the cells with a pFC-MEKK vector (a constitutively active MAP kinase kinase kinase), significantly increased the synthesis of prolactin and decreased the synthesis of growth hormone. These data taken together indicate that MAP kinase mediates TRH-induced regulation of prolactin and growth hormone gene expression. Reporter gene assays showed that prolactin promoter activity was increased by TRH and was completely inhibited by addition of PD098059, but that the promoter activity of growth hormone was unchanged by TRH. These results suggest that TRH stimulates both prolactin and growth hormone secretion, but that the gene expressions of prolactin and growth hormone are differentially regulated by TRH and are mediated by different mechanisms.

Insulin, insulin-like growth factor-I, and platelet-derived growth factor activate extracellular signal-regulated kinase by distinct pathways in muscle cells

We have investigated the signaling pathways initiated by insulin, insulin-like growth factor-1 (IGF-I), and platelet-derived growth factor (PDGF) leading to activation of the extracellular signal-regulated kinase (ERK) in L6 myotubes. Insulin but not IGF-I or PDGF-induced ERK activation was abrogated by Ras inhibition, either by treatment with the farnesyl transferase inhibitor FTP III, or by actin disassembly by cytochalasin D, previously shown to inhibit Ras activation. The protein kinase C (PKC) inhibitor bisindolylmaleimide abolished PDGF but not IGF-I or insulin-induced ERK activation. ERK activation by insulin, IGF-I, or PDGF was unaffected by the phosphatidylinositol 3-kinase inhibitor wortmannin but was abolished by the MEK inhibitor PD98059. In contrast, activation of the pathway involving phosphatidylinositol 3-kinase (PI3k), protein kinase B, and glycogen synthase kinase 3 (GSK3) was mediated similarly by all three receptors, through a PI 3-kinase-dependent but Ras- and actin-independent pathway. We conclude that ERK activation is mediated by distinct pathways including: (i) a cytoskeleton- and Ras-dependent, PKC-independent, pathway utilized by insulin, (ii) a PKC-dependent, cytoskeleton- and Ras-independent pathway used by PDGF, and (iii) a cytoskeleton-, Ras-, and PKC-independent pathway utilized by IGF-I.

Involvement of mitogen-activated protein kinase in cyclic adenosine 3',5'-monophosphate-induced hormone gene expression in rat pituitary GH(3) cells

We examined whether mitogen-activated protein (MAP) kinase was activated by stimulation of the cAMP pathway and whether MAP kinase activation was involved in synthesis of PRL and GH in GH(3) cells. Treatment of the cells with a cAMP analog, 8-(4-chlorophenylthio)cAMP (CPT-cAMP), activated MAP kinase and increased PRL at both the protein and messenger RNA levels. The protein and messenger RNA of GH were decreased by the treatment. We constructed the luciferase reporter genes after the promoters of PRL and GH and found the activation of both promoters by the CPT-cAMP treatment. We confirmed that overexpression of the catalytic subunit of cAMP-dependent protein kinase had essentially the same effects on MAP kinase activation and synthesis of PRL and GH as the CPT-cAMP treatment. Furthermore, treatment of the cells with pituitary adenylate cyclase-activating polypeptide 27 activated MAP kinase. The activation of PRL promoter by CPT-cAMP and pituitary adenylate cyclase-activating polypeptide 27 was abolished by pretreatment with PD098059 and H89. Although the increase in PRL and GH secretion by CPT-cAMP was inhibited by H89, PD098059 had no effect on secretion. These results suggest that cAMP-induced MAP kinase activation is essential for PRL gene expression, but not for secretion of PRL and GH.

Restoration of growth hormone-releasing hormone (GHRH) responsiveness in pituitary GH3 cells by adenovirus-directed expression of the human GHRH receptor

GH-secreting GH3 cells lack GH-releasing hormone (GHRH) receptors. In this study we used adenoviral vectors to transfer the human GHRH receptor to GH3 cells in an effort to restore GHRH responsiveness. A replication-deficient recombinant adenovirus (AdGHRH-R) was designed to allow cytomegalovirus promoter-driven expression of the GHRH receptor messenger RNA. COS-7 cells and GH-producing GH3 cells infected with AdGHRH-R showed GHRH receptor expression on their membranes and exhibited specific GHRH binding. The addition of GHRH to GH3 cells infected with AdGHRH-R increased cAMP levels, induced cAMP response element-binding protein phosphorylation and restored GH secretory responsiveness. GHRH treatment also caused activation of mitogen-activated-protein kinase, induction of c-fos, stimulation of GH promotor activity, and increased cellular proliferation. These findings indicate that adenoviral vectors carrying human GHRH receptor are useful for in vitro studies of GHRH receptor biology and represent a first step toward the development of gene therapy for dwarfism caused by GHRH receptor mutations.

Thyrotropin-releasing hormone and epidermal growth factor regulate iron-regulatory protein binding in pituitary cells via protein kinase C-dependent and -independent signaling pathways

Intracellular iron homeostasis is regulated, in part, by interactions between iron-regulatory proteins (IRP1 and IRP2) and iron-responsive elements (IREs) in ferritin and transferrin receptor mRNAs. In addition to iron, cellular oxidative stress induced by H(2)O(2), nitric oxide, and hypoxia, and hormonal activation by thyroid hormone and erythropoeitin have each been shown to regulate IRP binding to IREs. Hormonal signals, in particular mediated through protein kinase C (PKC), play a central role in the modulation of IRP/IRE interactions since phorbol esters were shown to activate IRP binding (Eisenstein, R. S., Tuazon, P. T., Schalinske, K. L., Anderson, S. A., and Traugh, J. A. (1993) J. Biol. Chem. 268, 27363-27370). In pituitary thyrotrophs (TtT97), we found that thyrotropin releasing hormone (TRH) and epidermal growth factor (EGF) increased IRP binding to a ferritin IRE, dependent on PKC and mitogen-activated protein kinase (MAPK) activity. In contrast, TRH and EGF decreased IRP binding in pituitary lactotrophs (GH3), despite activation of PKC and MAPK. IRP1 and IRP2 levels remained constant and IRP2 binding was predominant throughout. TRH and EGF markedly decreased IRP binding in MAPK kinase inhibitor-treated GH3 cells, whereas, they increased IRP binding in phosphatase inhibitor-treated GH3 cells. IRE-dependent CAT reporter translational expression closely reflected IRP binding to the ferritin IRE in both GH3 and TtT97 cells. Interestingly, ferritin protein levels were regulated similarly by TRH in both cell lines. These data link two different cell receptor systems to common signaling pathways that regulate IRP binding and ferritin expression. Remarkably, for TRH and EGF, these effects may be PKC-dependent or -independent determined by the cell type.

Expression of cyclooxygenase 2 by prostaglandin E(2) in human endometrial adenocarcinoma cell line HEC-1B

The regulation of expression of cyclooxygenase 2 (COX-2) was investigated by treatment with PGE(2) in human endometrial adenocarcinoma cell line HEC-1B. One microM PGE(2) could stimulate the expression of COX-2 approximately twofold in this cell line. The same concentration of PGE(2) also stimulated activation of mitogen-activated protein kinase (MAP kinase) and protein kinase B (PKB). PGE(2)-induced MAP kinase activation was sensitive to a MAP kinase kinase (MEK) inhibitor, PD098059, and a protein kinase A inhibitor, H-89. PD098059 and H-89 also partially inhibited the expression of COX-2 stimulated by PGE(2). PGE(2) could stimulate the activation of PKB, which was sensitive to phosphatidylinositol-3-OH kinase (PI3K) inhibitor, wortmannin. Whereas wortmannin alone partially inhibited the expression of COX-2, a combination of wortmannin and PD098059 totally inhibited PGE(2)-mediated COX-2 expression. These results suggest that MAP kinase and PI3K pathways are stimulated with PGE(2), and that both of these pathways are involved in the expression of COX-2. In addition, they also suggest that protein kinase A remains upstream of PGE(2)-induced activation of MAP kinase in HEC-1B cells.

Involvement of p38 mitogen-activated protein kinase activation in bromocriptine-induced apoptosis in rat pituitary GH3 cells

Bromocriptine, a dopamine D(2) receptor agonist, is a therapeutic agent for patients with prolactinoma and hyperprolactinemia. In this study we demonstrated that bromocriptine induced activation of p38 mitogen-activated protein (MAP) kinase, with concomitant induction of apoptosis in rat pituitary adenoma cell line GH3 cells. Treatment of GH3 cells for 48 h with bromocriptine increased the p38 MAP kinase activity up to 3- to 5-fold and simultaneously increased the number of apoptotic cells. Inclusion in the medium of SB212090 or SB203580, specific p38 MAP kinase inhibitors, completely abolished the bromocriptine-induced activation of p38 MAP kinase and significantly reduced the number of apoptotic cells. The bromocriptine-induced p38 MAP kinase activation was not prevented by S(-)-eticropride hydrochloride, a specific D(2) receptor antagonist. Treatment with either epidermal growth factor (EGF) or thyrotropin-releasing hormone (TRH), which stimulates p44/42 MAP kinase, rescued cells from the bromocriptine-induced apoptosis, with concomitant inhibition of the bromocriptine-induced p38 MAP kinase activation. These results suggest that bromocriptine induces apoptosis in association with p38 MAP kinase activation, and that the p44/42 MAP kinase signaling through EGF and TRH receptors has an opposing effect on p38 MAP kinase activation as well as on apoptosis induced with bromocriptine in GH3 cells.