Thyrotropin-releasing hormone stimulates MAP kinase activity in GH3 cells by divergent pathways. Evidence of a role for early tyrosine phosphorylation

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.

Impact of growth hormone-releasing hormone (GHRH) antagonist on Decidual stromal cell growth and apoptosis in vitro

Endometrial stromal cells remodeling is critical during human pregnancy. GHRH and its functional receptor have been shown to be expressed in gynecological cancer cells and eutopic endometrial stromal cells. Recent studies have demonstrated the potential clinical uses of antagonists of GHRH as effective antitumor agents because of its directly antagonistic effect on the locally produced GHRH in gynecological tumors. However, the impact of GHRH antagonists on normal endometrial stromal cell growth remained to be elucidated. The aim of this study was to investigate the effect of a GHRH antagonist (JMR-132) on cell proliferation and apoptosis of human decidual stromal cells and the underlying molecular mechanisms. Our results showed that GHRH and the splice variant 1 (SV1) of GHRH receptor (GHRH-R SV1) are expressed in human decidual stromal cells isolated from the decidual tissues of early pregnant women receiving surgical abortion. In addition, treatment of stroma cells with JMR-132 induced cell apoptosis with increasing cleaved caspase-3 and caspase-9 activities, and decrease cell viability in a time- and dose-dependent manner. Using a dual inhibition approach (pharmacological inhibitors and siRNA-mediated knockdown), we showed that JMR-132-induced activation of apoptotic signals are mediated by the activation of ERK1/2 and JNK signaling pathways and the subsequent upregulation of GADD45α. Taken together, JMR-132 suppresses cell survival of decidual stromal cells by inducing apoptosis through the activation of ERK1/2- and JNK-mediated upregulation of GADD45α in human endometrial stromal cells. Our findings provide new insights into the potential impact of GHRH antagonist on the decidual programming in humans.

Effect of L-thyroxine administration on long-term potentiation and accompanying mitogen-activated protein kinases in rats

The present study investigated the differences in the activation of c-Jun NH2-terminal kinases (JNK), p38 mitogen-activated protein kinases (p38^MAPK ), and extracellular signal-regulated kinases 1/2 (Erk1/2) 1 hr after the induction of long-term potentiation (LTP) between rats with hyperthyroidism that was produced at two different stages of development. Hyperthyroidism was produced in rats by daily injections of L-thyroxine (T4, ip., 0.2 mg/kg) to their dams for lactation period or to the rats itself during the young adult period. LTP was induced by application of high-frequency stimulation protocol. Five-min averages of the excitatory postsynaptic potential (EPSP) slopes and population spike (PS) amplitudes at the end of recording were averaged to measure the magnitude of LTP. Total and phosphorylated levels of Erk1/2, JNK, and P38-MAPK were assessed via western blotting in these hippocampi. LTP was found to be impaired in both groups of hyperthyroidisms, but this impairment observed together with increased expression and phosphorylation of ERK1/2, and increased phosphorylation of JNK in rats treated maternally with T4 compared to those treated adultly. These results suggest that excessiveness of thyroid hormone has longstanding effects on hippocampal function and may account for failed LTP in both early and relatively late stage of development depending on various molecular pathways, such as ERK1/2 and JNK.

Mechanisms of PAR-1 mediated kinase receptor transactivation: Smad linker region phosphorylation

Protease activated receptors (PARs) transactivate both epidermal growth factor receptors (EGFR) and transforming growth factor (TGF)-β receptors (TGFBR1) in vascular smooth muscle leading to the increased expression of genes (CHST11 and CHSY1) which are rate limiting for the enzymes that mediate hyperelongation of glycosaminoglycan (GAG) chains on the lipid-binding proteoglycan, biglycan. This is an excellent model to investigate mechanisms of transactivation as the processes are biochemically distinct. EGFR transactivation is dependent on the classical matrix metalloprotease (MMP) based triple membrane bypass mechanism and TGFBR1 transactivation is dependent on Rho/ROCK signalling and integrins. We have shown that all kinase receptor signalling is targeted towards phosphorylation of the linker region of the transcription factor, Smad2. We investigated the mechanisms of thrombin mediated kinase receptor transactivation signalling using anti-phospho antibodies and Western blotting and gene expression by RT-PCR. Thrombin stimulation of phospho-Smad2 (Ser 245/250/255) and of phospho-Smad2(Thr220) via EGFR transactivation commences quickly and extends out to at least 4 h whereas transactivation via TGFBR1 is delayed for 120 min but also persists for at least 4 h. Signalling of thrombin stimulated Smad linker region phosphorylation is approximately equally inhibited by the MMP inhibitor, GM6001 and the ROCK inhibitor, Y27632, and similarly expression of CHST11 and CHSY1 is approximately equally inhibited by GM6001 and Y27632. The data establishes Smad linker region phosphorylation as a central target of all transactivation signalling of GAG gene expression and thus an upstream kinase may be a target to prevent all transactivation signalling and its pathophysiological consequences.

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.

Hypothalamus-Pituitary-Thyroid Axis

The hypothalamus-pituitary-thyroid (HPT) axis determines the set point of thyroid hormone (TH) production. Hypothalamic thyrotropin-releasing hormone (TRH) stimulates the synthesis and secretion of pituitary thyrotropin (thyroid-stimulating hormone, TSH), which acts at the thyroid to stimulate all steps of TH biosynthesis and secretion. The THs thyroxine (T4) and triiodothyronine (T3) control the secretion of TRH and TSH by negative feedback to maintain physiological levels of the main hormones of the HPT axis. Reduction of circulating TH levels due to primary thyroid failure results in increased TRH and TSH production, whereas the opposite occurs when circulating THs are in excess. Other neural, humoral, and local factors modulate the HPT axis and, in specific situations, determine alterations in the physiological function of the axis. The roles of THs are vital to nervous system development, linear growth, energetic metabolism, and thermogenesis. THs also regulate the hepatic metabolism of nutrients, fluid balance and the cardiovascular system. In cells, TH actions are mediated mainly by nuclear TH receptors (210), which modify gene expression. T3 is the preferred ligand of THR, whereas T4, the serum concentration of which is 100-fold higher than that of T3, undergoes extra-thyroidal conversion to T3. This conversion is catalyzed by 5'-deiodinases (D1 and D2), which are TH-activating enzymes. T4 can also be inactivated by conversion to reverse T3, which has very low affinity for THR, by 5-deiodinase (D3). The regulation of deiodinases, particularly D2, and TH transporters at the cell membrane control T3 availability, which is fundamental for TH action. © 2016 American Physiological Society. Compr Physiol 6:1387-1428, 2016.

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.

Biological and Pharmacological Aspects of the NK1-Receptor

The neurokinin 1 receptor (NK-1R) is the main receptor for the tachykinin family of peptides. Substance P (SP) is the major mammalian ligand and the one with the highest affinity. SP is associated with multiple processes: hematopoiesis, wound healing, microvasculature permeability, neurogenic inflammation, leukocyte trafficking, and cell survival. It is also considered a mitogen, and it has been associated with tumorigenesis and metastasis. Tachykinins and their receptors are widely expressed in various human systems such as the nervous, cardiovascular, genitourinary, and immune system. Particularly, NK-1R is found in the nervous system and in peripheral tissues and are involved in cellular responses such as pain transmission, endocrine and paracrine secretion, vasodilation, and modulation of cell proliferation. It also acts as a neuromodulator contributing to brain homeostasis and to sensory neuronal transmission associated with depression, stress, anxiety, and emesis. NK-1R and SP are present in brain regions involved in the vomiting reflex (the nucleus tractus solitarius and the area postrema). This anatomical localization has led to the successful clinical development of antagonists against NK-1R in the treatment of chemotherapy-induced nausea and vomiting (CINV). The first of these antagonists, aprepitant (oral administration) and fosaprepitant (intravenous administration), are prescribed for high and moderate emesis.

ERK and RSK are necessary for TRH-induced inhibition of r-ERG potassium currents in rat pituitary GH3 cells

The transduction pathway mediating the inhibitory effect that TRH exerts on r-ERG channels has been thoroughly studied in GH3 rat pituitary cells but some elements have yet to be discovered, including those involved in a phosphorylation event(s). Using a quantitative phosphoproteomic approach we studied the changes in phosphorylation caused by treatment with 1μM TRH for 5min in GH3 cells. The activating residues of Erk2 and Erk1 undergo phosphorylation increases of 5.26 and 4.87 fold, respectively, in agreement with previous reports of ERK activation by TRH in GH3 cells. Thus, we studied the possible involvement of ERK pathway in the signal transduction from TRH receptor to r-ERG channels. The MEK inhibitor U0126 at 0.5μM caused no major blockade of the basal r-ERG current, but impaired the TRH inhibitory effect on r-ERG. Indeed, the TRH effect on r-ERG was also reduced when GH3 cells were transfected with siRNAs against either Erk1 or Erk2. Using antibodies, we found that TRH treatment also causes activating phosphorylation of Rsk. The TRH effect on r-ERG current was also impaired when cells were transfected with any of two different siRNAs mixtures against Rsk1. However, treatment of GH3 cells with 20nM EGF for 5min, which causes ERK and RSK activation, had no effect on the r-ERG currents. Therefore, we conclude that in the native GH3 cell system, ERK and RSK are involved in the pathway linking TRH receptor to r-ERG channel inhibition, but additional components must participate to cause such inhibition.

NR4A2 is regulated by gastrin and influences cellular responses of gastric adenocarcinoma cells

The peptide hormone gastrin is known to play a role in differentiation, growth and apoptosis of cells in the gastric mucosa. In this study we demonstrate that gastrin induces Nuclear Receptor 4A2 (NR4A2) expression in the adenocarcinoma cell lines AR42J and AGS-G_R, which both possess the gastrin/CCK2 receptor. In vivo, NR4A2 is strongly expressed in the gastrin responsive neuroendocrine ECL cells in normal mucosa, whereas gastric adenocarcinoma tissue reveals a more diffuse and variable expression in tumor cells. We show that NR4A2 is a primary early transient gastrin induced gene in adenocarcinoma cell lines, and that NR4A2 expression is negatively regulated by inducible cAMP early repressor (ICER) and zinc finger protein 36, C3H1 type-like 1 (Zfp36l1), suggesting that these gastrin regulated proteins exert a negative feedback control of NR4A2 activated responses. FRAP analyses indicate that gastrin also modifies the nucleus-cytosol shuttling of NR4A2, with more NR4A2 localized to cytoplasm upon gastrin treatment. Knock-down experiments with siRNA targeting NR4A2 increase migration of gastrin treated adenocarcinoma AGS-G_R cells, while ectopically expressed NR4A2 increases apoptosis and hampers gastrin induced invasion, indicating a tumor suppressor function of NR4A2. Collectively, our results uncover a role of NR4A2 in gastric adenocarcinoma cells, and suggest that both the level and the localization of NR4A2 protein are of importance regarding the cellular responses of these cells.

Gonadotropin-releasing hormone type II (GnRH-II) agonist regulates the invasiveness of endometrial cancer cells through the GnRH-I receptor and mitogen-activated protein kinase (MAPK)-dependent activation of matrix metalloproteinase (MMP)-2

Background

More than 25% of patients diagnosed with endometrial carcinoma have an invasive primary cancer accompanied by metastases. Gonadotropin-releasing hormone (GnRH) plays an important role in reproduction. In mammals, expression of GnRH-II is higher than GnRH-I in reproductive tissues. Here, we examined the effect of a GnRH-II agonist on the motility of endometrial cancer cells and its mechanism of action in endometrial cancer therapy.

Methods

Immunoblotting and immunohistochemistry (IHC) were used to determine the expression of the GnRH-I receptor protein in human endometrial cancer. The activity of MMP-2 in the conditioned medium was determined by gelatin zymography. Cell motility was assessed by invasion and migration assay. GnRH-I receptor si-RNA was applied to knockdown GnRH-I receptor.

Results

The GnRH-I receptor was expressed in the endometrial cancer cells. The GnRH-II agonist promoted cell motility in a dose-dependent manner. The GnRH-II agonist induced the phosphorylation of ERK1/2 and JNK, and the phosphorylation was abolished by ERK1/2 inhibitor (U0126) and the JNK inhibitor (SP600125). Cell motility promoted by GnRH-II agonist was suppressed in cells that were pretreated with U0126 and SP600125. Moreover, U0126 and SP600125 abolished the GnRH-II agonist-induced activation of MMP-2. The inhibition of MMP-2 with MMP-2 inhibitor (OA-Hy) suppressed the increase in cell motility in response to the GnRH-II agonist. Enhanced cell motility mediated by GnRH-II agonist was also suppressed by the knockdown of the endogenous GnRH-I receptor using siRNA.

Conclusion

Our study indicates that GnRH-II agonist promoted cell motility of endometrial cancer cells through the GnRH-I receptor via the phosphorylation of ERK1/2 and JNK, and the subsequent, MAPK-dependent activation of MMP-2. Our findings represent a new concept regarding the mechanism of GnRH-II-induced cell motility in endometrial cancer cells and suggest the possibility of exploring GnRH-II as a potential therapeutic target for the treatment of human endometrial cancer.

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.

NR4A1 (Nur77) mediates thyrotropin-releasing hormone-induced stimulation of transcription of the thyrotropin β gene: analysis of TRH knockout mice

Thyrotropin-releasing hormone (TRH) is a major stimulator of thyrotropin-stimulating hormone (TSH) synthesis in the anterior pituitary, though precisely how TRH stimulates the TSHβ gene remains unclear. Analysis of TRH-deficient mice differing in thyroid hormone status demonstrated that TRH was critical for the basal activity and responsiveness to thyroid hormone of the TSHβ gene. cDNA microarray and K-means cluster analyses with pituitaries from wild-type mice, TRH-deficient mice and TRH-deficient mice with thyroid hormone replacement revealed that the largest and most consistent decrease in expression in the absence of TRH and on supplementation with thyroid hormone was shown by the TSHβ gene, and the NR4A1 gene belonged to the same cluster as and showed a similar expression profile to the TSHβ gene. Immunohistochemical analysis demonstrated that NR4A1 was expressed not only in ACTH- and FSH- producing cells but also in thyrotrophs and the expression was remarkably reduced in TRH-deficient pituitary. Furthermore, experiments in vitro demonstrated that incubation with TRH in GH4C1 cells increased the endogenous NR4A1 mRNA level by approximately 50-fold within one hour, and this stimulation was inhibited by inhibitors for PKC and ERK1/2. Western blot analysis confirmed that TRH increased NR4A1 expression within 2 h. A series of deletions of the promoter demonstrated that the region between bp -138 and +37 of the TSHβ gene was responsible for the TRH-induced stimulation, and Chip analysis revealed that NR4A1 was recruited to this region. Conversely, knockdown of NR4A1 by siRNA led to a significant reduction in TRH-induced TSHβ promoter activity. Furthermore, TRH stimulated NR4A1 promoter activity through the TRH receptor. These findings demonstrated that 1) TRH is a highly specific regulator of the TSHβ gene, and 2) TRH mediated induction of the TSHβ gene, at least in part by sequential stimulation of the NR4A1-TSHβ genes through a PKC and ERK1/2 pathway.

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.

Physiopathology of somatolactotroph cells: from transduction mechanisms to cotargeting therapy

In pituitary somatolactotroph cells, G protein-coupled receptors and receptor tyrosine kinases binding their specific ligands trigger an enzymatic cascade that converges to MAP kinase activation in the subcellular compartment. Different signaling pathways, such as AC/cAMP/PKA and PI3K/Akt pathways, interact with MAP kinase to regulate key physiological functions, such as hormonal secretion and cell proliferation. Abnormalities affecting these signaling pathways have been identified as preponderant factors of pituitary tumorigenesis. In addition to trans-sphenoidal surgery, somatostatin analogs are used to control hormonal hypersecretion in GH-secreting adenomas. However, a subset of these tumors remains uncontrolled with these treatFments, calling for new therapeutic approaches. In these cases, novel multivalent somatostatin analogs or new somatostatin-dopamine chimeric molecules could be of interest. Another attractive therapeutic approach may be to use one or several inhibitors acting downstream in the signaling pathway, such as mammalian target of rapamycin inhibitor. Cotargeting therapy and gene therapy are promising tools for these problematic pituitary tumors.

Thyrotropin-releasing hormone (TRH) in the cerebellum

Thyrotropin-releasing hormone (TRH) was originally isolated from the hypothalamus. Besides controlling the secretion of TSH from the anterior pituitary, this tripeptide is widely distributed in the central nervous system and regarded as a neurotransmitter or modulator of neuronal activities in extrahypothalamic regions, including the cerebellum. TRH has an important role in the regulation of energy homeostasis, feeding behavior, thermogenesis, and autonomic regulation. TRH controls energy homeostasis mainly through its hypophysiotropic actions to regulate circulating thyroid hormone levels. Recent investigations have revealed that TRH production is regulated directly at the transcriptional level by leptin, one of the adipocytokines that plays a critical role in feeding and energy expenditure. The improvement of ataxic gait is one of the important pharmacological properties of TRH. In the cerebellum, cyclic GMP has been shown to be involved in the effects of TRH. TRH knockout mice show characteristic phenotypes of tertiary hypothyroidism, but no morphological changes in their cerebellum. Further analysis of TRH-deficient mice revealed that the expression of PFTAIRE protein kinase1 (PFTK1), a cdc2-related kinase, in the cerebellum was induced by TRH through the NO-cGMP pathway. The antiataxic effect of TRH and TRH analogs has been investigated in rolling mouse Nagoya (RMN) or 3-acetylpyridine treated rats, which are regarded as a model of human cerebellar degenerative disease. TRH and TRH analogs are promising clinical therapeutic agents for inducing arousal effects, amelioration of mental depression, and improvement of cerebellar ataxia.

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.

Gonadotropin-releasing hormone type II induces apoptosis of human endometrial cancer cells by activating GADD45alpha

Gonadotropin-releasing hormone type II (GnRH-II) has an antiproliferative effect on human endometrial cancer cells. Apoptosis in cancer cells may play a critical role in regulating cell proliferation. However, more studies are necessary to elucidate the underlying molecular mechanisms and develop potential applications of GnRH-II. Therefore, we explored the mechanisms of GnRH-II-induced apoptosis and the effects of GnRH-II on GADD45alpha activation in human endometrial cancer cell lines. GnRH-II decreased cell viability in a dose- and time-dependent manner. Apoptosis was induced with increased terminal deoxyribonucleotidyl transferase-mediated dUTP nick end labeling apoptotic cells after GnRH-II treatment. Knockdown of the endogenous GnRH-I receptor with small interfering RNA (siRNA) rescued the cells from GnRH-II-mediated cell growth inhibition and abolished the induction of apoptosis. GnRH-II activated extracellular signal-regulated kinase (ERK)-1/2 and p38 mitogen-activated protein kinase (MAPK) in a time-dependent manner, and the activation was abolished by GnRH-I receptor siRNA and MAPK inhibitors. Cells pretreated with MAPK inhibitors were rescued from GnRH-II-mediated cell growth inhibition. Moreover, both inhibitors abolished GnRH-II-induced apoptosis. GnRH-II induced GADD45alpha expression, which was abolished by knockdown of endogenous GnRH-I receptors and MAPK inhibitors. GnRH-II-stimulated cell growth inhibition was rescued by knockdown of endogenous GADD45alpha with siRNA. Cells treated with GADD45alpha siRNA were refractory to GnRH-II-induced apoptosis. Thus, GnRH-II inhibits cell growth by inducing apoptosis through binding of the GnRH-I receptor, activation of the ERK1/2 and p38 MAPK pathways, and induction of GADD45alpha signaling. This finding may provide a new concept relating to the mechanism of GnRH-II-induced antiproliferation and apoptosis in endometrial cancer cells, indicating the possibility of GnRH-II as a promising therapeutic intervention for human endometrial cancer.

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.

Conditional overexpression of the wild-type Gs alpha as the gsp oncogene initiates chronic extracellularly regulated kinase 1/2 activation and hormone hypersecretion in pituitary cell lines

In pituitary cells, activation of the cAMP pathway by specific G protein-coupled receptors controls differentiative functions and proliferation. Constitutively active forms of the alpha subunit of the heterotrimeric G(s) protein resulting from mutations at codon 201 or 227 (gsp oncogene) were first identified in 30-40% of human GH-secreting pituitary adenomas. This rate of occurrence suggests that the gsp oncogene is not responsible for initiating the majority of these tumors. Moreover, there is a large overlap between the clinical phenotypes observed in patients with tumors bearing the gsp oncogene and those devoid of this oncogene. To explore the role of G(s)alpha in GH-secreting adenomas, we obtained somatolactotroph GH4C1 cell lines by performing doxycycline-dependent conditional overexpression of the wild-type G(s)alpha protein and expression of the gsp oncogene. Although the resulting adenylyl cyclase and cAMP levels were 10-fold lower in the wild-type G(s)alpha-overexpressing cell line, a sustained MAPK ERK1/2 activation was observed in both cell lines. Overexpression of the wild-type G(s)alpha protein as the gsp oncogene initiated chronic activation of endogenous prolactin synthesis and release, as well as chronic activation of ERK1/2-sensitive human prolactin and GH promoters.

Regulation of the RAP1/RAF-1/extracellularly regulated kinase-1/2 cascade and prolactin release by the phosphoinositide 3-kinase/AKT pathway in pituitary cells

In pituitary cells, prolactin (PRL) synthesis and release are controlled by multiple transduction pathways. In the GH4C1 somatolactotroph cell line, we previously reported that MAPK ERK-1/2 are a point of convergence between the pathways involved in the PRL gene regulation. In the present study, we focused on the involvement of the phosphoinositide 3-kinase (PI3K)/Akt pathway in the MAPK ERK-1/2 regulation and PRL secretion in pituitary cells. Either specific pharmacological PI3K and Akt inhibitors (LY294002, Akt I, and phosphoinositide analog-6) or Akt dominant-negative mutant (K179M) enhanced ERK-1/2 phosphorylation in unstimulated GH4C1 cells. Under the same conditions, PI3K and Akt inhibition also both increased Raf-1 kinase activity and the levels of GTP-bound (active form) monomeric G protein Rap1, which suggests that a down-regulation of the ERK-1/2 cascade is induced by the PI3K/Akt signaling pathway in unstimulated cells. On the contrary, ERK-1/2 phosphorylation, Raf-1 activity, and Rap1 activation were almost completely blocked in IGF-I-stimulated cells previously subjected to PI3K or Akt inhibition. Although the PRL promoter was not affected by either PI3K/Akt inhibition or activation, PRL release increased in response to the pharmacological PI3K/Akt inhibitors in unstimulated GH4C1 and rat pituitary primary cells. The IGF-I-stimulated PRL secretion was diminished, on the contrary, by the pharmacological PI3K/Akt inhibitors. Taken together, these findings indicate that the PI3K/Akt pathway exerts dual regulatory effects on both the Rap1/Raf-1/ERK-1/2 cascade and PRL release in pituitary cells, i.e. negative effects in unstimulated cells and positive ones in IGF-I-stimulated cells.

Endocrine signaling in ovarian surface epithelium and cancer

Ovarian cancer is the sixth most common cancer and the fifth leading cause of cancer-related death among women in developed countries. Greater than 85% of human ovarian cancer arises within the ovarian surface epithelium (OSE), with the remainder derived from granulosa cells or, rarely, stroma or germ cells. The pathophysiology of ovarian cancer is the least understood among all major human malignancies because of a poor understanding of the aetiological factors and mechanisms of ovarian cancer progression. There is increasing evidence suggesting that several key reproductive hormones, such as GnRH, gonadotrophins and sex steroids, regulate the growth of normal OSE and ovarian cancer cells. The objective of this review was to highlight the effects of these endocrine factors on ovarian cancer cell growth and to summarize the signalling mechanisms involved in normal human OSE and its neoplastic counterparts.

In vivo pharmacokinetics, activation of MAPK signaling and induction of phase II/III drug metabolizing enzymes/transporters by cancer chemopreventive compound BHA in the mice

Phenolic antioxidant butylated hydroxyanisole (BHA) is a commonly used food preservative with broad biological activities, including protection against chemical-induced carcinogenesis, acute toxicity of chemicals, modulation of macromolecule synthesis and immune response, induction of phase II detoxifying enzymes, as well as its undesirable potential tumor-promoting activities. Understanding the molecular basis underlying these diverse biological actions of BHA is thus of great importance. Here we studied the pharmacokinetics, activation of signaling kinases and induction of phase II/III drug metabolizing enzymes/transporter gene expression by BHA in the mice. The peak plasma concentration of BHA achieved in our current study after oral administration of 200 mg/kg BHA was around 10 microM. This in vivo concentration might offer some insights for the many in vitro cell culture studies on signal transduction and induction of phase II genes using similar concentrations. The oral bioavailability (F) of BHA was about 43% in the mice. In the mouse liver, BHA induced the expression of phase II genes including NQO-1, HO-1, gamma-GCS, GST-pi and UGT 1A6, as well as some of the phase III transporter genes, such as MRP1 and Slcolb2. In addition, BHA activated distinct mitogen-activated protein kinases (MAPKs), c-Jun N-terminal kinase (JNK), extracellular signal-regulated protein kinase (ERK), as well as p38, suggesting that the MAPK pathways may play an important role in early signaling events leading to the regulation of gene expression including phase II drug metabolizing and some phase III drug transporter genes. This is the first study to demonstrate the in vivo pharmacokinetics of BHA, the in vivo activation of MAPK signaling proteins, as well as the in vivo induction of Phase II/III drug metabolizing enzymes/transporters in the mouse livers.

Thyrotropin-releasing hormone increases GABA release in rat hippocampus

Thyrotropin-releasing hormone (TRH) is a tripeptide that is widely distributed in the brain including the hippocampus where TRH receptors are also expressed. TRH has anti-epileptic effects and regulates arousal, sleep, cognition, locomotion and mood. However, the cellular mechanisms underlying such effects remain to be determined. We examined the effects of TRH on GABAergic transmission in the hippocampus and found that TRH increased the frequency of GABAA receptor-mediated spontaneous IPSCs in each region of the hippocampus but had no effects on miniature IPSCs or evoked IPSCs. TRH increased the action potential firing frequency recorded from GABAergic interneurons in CA1 stratum radiatum and induced membrane depolarization suggesting that TRH increases the excitability of interneurons to facilitate GABA release. TRH-induced inward current had a reversal potential close to the K+ reversal potential suggesting that TRH inhibits resting K+ channels. The involved K+ channels were sensitive to Ba2+ but resistant to other classical K+ channel blockers, suggesting that TRH inhibits the two-pore domain K+ channels. Because the effects of TRH were mediated via Galphaq/11, but were independent of its known downstream effectors, a direct coupling may exist between Galphaq/11 and K+ channels. Inhibition of the function of dynamin slowed the desensitization of TRH responses. TRH inhibited seizure activity induced by Mg2+ deprivation, but not that generated by picrotoxin, suggesting that TRH-mediated increase in GABA release contributes to its anti-epileptic effects. Our results demonstrate a novel mechanism to explain some of the hippocampal actions of TRH.

The molecular mechanism of EGF receptor activation in pancreatic beta-cells by thyrotropin-releasing hormone

Thyrotropin-releasing hormone (TRH) and its receptor subtype TRH receptor-1 (TRHR1) are found in pancreatic beta-cells, and it has been shown that TRH might have potential for autocrine/paracrine regulation through the TRHR1 receptor. In this paper, TRHR1 is studied to find whether it can initiate multiple signal transduction pathways to activate the epidermal growth factor (EGF) receptor in pancreatic beta-cells. By initiating TRHR1 G protein-coupled receptor (GPCR) and dissociated alphabetagamma-complex, TRH (200 nM) activates tyrosine residues at Tyr845 (a known target for Src) and Tyr1068 in the EGF receptor complex of an immortalized mouse beta-cell line, betaTC-6. Through manipulating the activation of Src, PKC, and heparin-binding EGF-like growth factor (HB-EGF), with corresponding individual inhibitors and activators, multiple signal transduction pathways linking TRH to EGF receptors in betaTC-6 cell line have been revealed. The pathways include the activation of Src kinase and the release of HB-EGF as a consequence of matrix metalloproteinase (MMP)-3 activation. Alternatively, TRH inhibited PKC activity by reducing the EGF receptor serine/threonine phosphorylation, thereby enhancing tyrosine phosphorylation. TRH receptor activation of Src may have a central role in mediating the effects of TRH on the EGF receptor. The activation of the EGF receptor by TRH in multiple circumstances may have important implications for pancreatic beta-cell biology.

Thyrotropin-releasing hormone (TRH) specific interaction between amino terminus of P-Lim and CREB binding protein (CBP)

P-Lim (Lhx3a) is a LIM homeodomain transcription factor essential for pituitary development and motor neuron specification in mice. The Lhx3 gene encodes two isoforms, which differ in their amino (N) termini, Lhx3a and 3b. The P-Lim DNA binding site on the glycoprotein hormone alpha subunit (alpha-GSU) gene promoter is conserved in mammals. P-Lim plays a pivotal role in mediating thyrotropin-releasing hormone (TRH) signaling by binding CREB binding protein (CBP), as we have reported previously. Here, we demonstrate that P-Lim (Lhx3a) but not Lhx3b can mediate TRH signaling and bind to CBP. Moreover, TRH specifically induces P-Lim-CBP binding through the N-termini of P-Lim. We also found that the protein kinase C (PKC) phosphorylation site within the N-terminus of P-Lim is responsible for the P-Lim-CBP binding suggesting that the TRH signaling pathway phosphorylates P-Lim. These studies have elucidated the molecular mechanism by which TRH stimulates alpha-GSU gene expression.

Congenital hypothyroidism alters the phosphorylation of ERK1/2 and p38MAPK in the hippocampus of neonatal rats

Thyroid hormone deficiency during the critical period of neural differentiation produces permanent and severe alterations in the morphology and function of the nervous system leading to cretinism. Perinatal hypothyroidism results in permanent alterations of hippocampal synaptic functions in adult rats consequently causing learning and memory impairment. Mitogen-activated protein kinases (MAPKs) are a family of protein kinases that regulate essential cellular activities ranging from gene expression, mitosis, programmed cell death to plasticity and memory formation, but their involvement in perinatal hypothyroidism is not determined. The present work was designed to investigate MAPKs phosphorylation in hippocampus of congenital neonatal hypothyroid rats. Congenital hypothyroidism promotes an increase in extracellular signal-regulated kinases 1/2 (ERK 1/2) phosphorylation (+50%) and a decrease in p38(MAPK) phosphorylation (-50%) without changing in Jun N-terminal kinases (JNK) phosphorylation. Therefore, the congenital hypothyroidism model disturbs ERK 1/2 and p38(MAPK) phosphorylation pathways causing an important molecular alteration in the hippocampus. This event might be related, at least partially, to the deficits in hippocampal development and cognitive functions due neonatal congenital hypothyroidism.

Not so strange bedfellows: G-protein-coupled receptors and Src family kinases

Src family nonreceptor tyrosine kinases are an integral component of the signal transduction apparatus employed by growth factor receptor tyrosine kinases. As such, their role in cellular growth control and malignant transformation has been the subject of intensive investigation. In contrast, classical G-protein-coupled receptor (GPCR) signaling involves activation of second messenger-regulated serine/threonine kinases or ion channels, and is primarily involved in neurotransmission and the short-term regulation of intermediary metabolism. Over the past decade, this strictly dichotomous model of transmembrane signaling has been challenged by the discovery that GPCRs also exert control over cellular growth, proliferation, and differentiation, and do so by stimulating tyrosine phosphorylation cascades. Several mechanisms, from the direct association of Src family kinases with GPCRs or receptor-associated proteins, to the transactivation of receptor tyrosine kinases and focal adhesion complexes by G-protein-mediated signals, permit GPCRs to activate Src family kinases. Conversely, Src activity plays a central role in controlling GPCR trafficking and effects on cell proliferation and cytoskeletal rearrangement. It is now clear that GPCRs and Src family kinases do not belong to separate, exclusive clubs. Rather, these strange bedfellows are intimately involved in multilayered forms of crosstalk that influence a host of cellular processes.

Differential involvement of the Ras and Rap1 small GTPases in vasoactive intestinal and pituitary adenylyl cyclase activating polypeptides control of the prolactin gene

In pituitary cells, transcriptional regulation of the prolactin (PRL) gene and prolactin secretion are controlled by multiple transduction pathways through the activation of G protein coupled receptors and receptor tyrosine kinases. In the somatolactotrope GH4C1 cell line, we have previously identified crosstalk between the MAPKinase cascade ERK1/2 and the cAMP/protein kinase A pathway after the activation of the VPAC2 receptor by vasoactive intestinal polypeptide (VIP) or pituitary adenylyl cyclase-activating polypeptide (PACAP38). In the present study, we focus on the involvement of the GTPases Ras and Rap1 as downstream components of signal transmission initiated by activation of the VPAC2 receptor. By using pull-down experiments, we show that VIP and PACAP38 preferentially activate Rap1, whereas thyrotropin releasing hormone (TRH) and epidermal growth factor (EGF) mainly activate Ras GTPase. Experiments involving the expression of the dominant-negative mutants of Ras and Rap1 signaling (RasN17 or Rap1N17) indicate that both GTPases Ras and Rap1 are recruited for the ERK activation by VIP and PACAP38, whereas Rap1 is poorly involved in TRH or EGF-induced ERK activation. The use of U0126, a selective inhibitor of MAPKinase kinase, provides evidence that MAPKinase contributes to the regulation of the PRL gene. Moreover, cotransfection of RasN17 or Rap1N17 with the PRL proximal promoter luciferase reporter construct indicates that Rap1 may be responsible for VIP/PACAP-induced activation of the PRL promoter. Interestingly, Ras would be involved as a negative regulator of VIP/PACAP-induced PRL gene activation, in contrast to its stimulatory role in the regulation of the PRL promoter by TRH and EGF.

Rapid activation of Ras by fluid flow is mediated by Galpha(q) and Gbetagamma subunits of heterotrimeric G proteins in human endothelial cells

OBJECTIVE

Temporal gradients in fluid shear stress have been shown to induce a proatherogenic phenotype in endothelial cells. The biomechanical mechanism(s) that enables the endothelium to respond to fluid shear stress requires rapid activation and signal transduction. The small G protein Ras has been identified as an early link between rapid mechanotransduction events and the effects of shear stress on downstream signal-transduction cascades. The aim of this study was to elucidate the upstream mechanotransduction signaling events mediating the rapid activation of Ras by fluid shear stress in human endothelial cells.

METHODS AND RESULTS

Direct measurement of Ras-bound GTP and GDP showed that fluid-flow activation of Ras was rapid (10-fold within 5 seconds) and dose dependent on shear stress magnitude. Treatment with protein tyrosine kinase inhibitors or pertussis toxin did not significantly affect flow-induced Ras activation. However, activation was inhibited by transient transfection with antisense to Galpha(q) or the Gbetagamma scavenger beta-adrenergic receptor kinase carboxy terminus. Transfection with several Gbetagamma subunit isoforms revealed flow-induced Ras activation was most effectively enhanced by Gbeta1gamma2.

CONCLUSIONS

These results suggest that the rapid, shear-induced activation of Ras is mediated by Galpha(q) through the activity of Gbetagamma subunits in human vascular endothelial cells.

Adenosine triphosphate activates mitogen-activated protein kinase in pre-neoplastic and neoplastic ovarian surface epithelial cells

To investigate the role of ATP in ovarian tumorigenesis, the present study examined the expression of the P2U purinoceptor (P2U-R) and effect of ATP on growth stimulation in pre-neoplastic and neoplastic ovarian surface epithelial (OSE) cells. The immortalized OSE (IOSE) cell lines, including IOSE-29 (pre-neoplastic), IOSE-29EC (neoplastic), and OVCAR-3 (ovarian adenocarcinoma cell line) were used. Our results indicated that P2U-R mRNA was expressed and that ATP exerted a growth-stimulatory effect in IOSE-29, IOSE-29EC, and OVCAR-3. To investigate the mechanism of the growth-stimulatory effect, the activation of mitogen-activated protein kinases (MAPKs) by ATP was examined. Treatment with ATP resulted in MAPK activation in IOSE-29 and IOSE-29EC cells, whereas the stimulatory effect of ATP in cellular proliferation and MAPK activation was completely abolished in the presence of PD98059 (an MAPK/ERK kinase inhibitor) and staurosporin (a protein kinase C inhibitor), suggesting that the growth stimulatory effect of ATP is mediated via protein kinase C-dependent MAPK activation in pre-neoplastic and neoplastic OSE cells. In a time-dependent study, ATP significantly increased MAPK activity at 5-20 min in IOSE-29 cells. Activated MAPK declined to control levels after 20 min in these cells. Treatment with ATP significantly induced MAPK activation after 5 min and was sustained for 60 min in IOSE-29EC cells. In addition, treatment with ATP resulted in substantial phosphorylation of Elk-1, the Ets family transcriptional factor, confirming that ATP action is mediated by activation of MAPK. In conclusion, we have demonstrated that P2U-R was expressed and that ATP induced growth stimulation in IOSE and OVCAR-3 cells. Furthermore, treatment with ATP resulted in the activation of an MAPK cascade and phosphorylation of Elk-1 in IOSE-29 and IOSE-29EC cells. These results suggest that the MAPK cascade may be involved in growth stimulation in response to ATP in pre-neoplastic and neoplastic OSE cells.

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.

G protein preferences for dopamine D2 inhibition of prolactin secretion and DNA synthesis in GH4 pituitary cells

Dopamine is the primary inhibitory regulator of lactotroph proliferation and prolactin (PRL) secretion in vivo, acting via dopamine D2 receptors (short D2S and long D2L forms). In GH4C1 pituitary cells transfected with D2S or D2L receptor cDNA, dopamine inhibits PRL secretion and DNA synthesis. These actions were blocked by pertussis toxin, implicating G(i)/G(o) proteins. To address roles of specific G(i)/G(o)4 proteins in these actions a series of GH4C1 cell lines specifically depleted of individual Galpha subunits was examined. D2S-mediated inhibition of BayK8644-stimulated PRL secretion was primarily dependent on G(o) over G(i), as observed for BayK8644-induced calcium influx. By contrast, inhibitory coupling of the D2S receptor to TRH-induced PRL secretion was partially impaired by depletion of any single G protein, but especially G(i)3. Inhibitory coupling of D2L receptors to PRL secretion required G(o), but not G(i)2, muscarinic receptor coupling was resistant to depletion of any G(i)/G(o) protein, whereas the 5-HT1A and somatostatin receptors required G(i)2 or G(i)3 for coupling. The various receptors also demonstrated distinct G protein requirements for inhibition of DNA synthesis: depletion of any G(i)/G(o) subunit completely uncoupled the D2S receptor, the D2L receptor was uncoupled by depletion of G(i)2, and muscarinic and somatostatin receptors were resistant to depletion of G(i)2 only. These results demonstrate distinct receptor-G protein preferences for inhibition of TRH-induced PRL secretion and DNA synthesis.

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.

Follicle-stimulating hormone activates mitogen-activated protein kinase in preneoplastic and neoplastic ovarian surface epithelial cells

To investigate the role of FSH in ovarian cancer development, the present study examined the expression of FSH receptor (FSH-R) and the effect of FSH on proliferation of normal, preneoplastic, and neoplastic ovarian surface epithelium (OSE) cells. Recently, immortalized OSE (IOSE) cell lines, including IOSE-29 (preneoplastic) and IOSE-29EC (neoplastic), were used. Our results indicated that FSH-R mRNA was expressed and that FSH exerted a growth stimulatory effect in normal, preneoplastic, and neoplastic OSE cells. To investigate the mechanism of the growth stimulatory effect, the activation of MAPKs by FSH was examined in preneoplastic and neoplastic OSE cells. Treatment with FSH resulted in MAPK activation of IOSE-29 and IOSE-29EC cells, whereas the stimulatory effect of FSH on cellular proliferation and MAPK activation was completely abolished in the presence of PD98059, a MAPK kinase inhibitor, suggesting that the growth stimulatory effect of FSH is mediated through MAPK activation in these OSE cells. In a time-dependent study, FSH significantly increased MAPK activity at 5-10 min in IOSE-29 cells. The activated MAPK declined to the control level after 20 min in these cells. Similarly, treatment with FSH significantly induced MAPK activation after 5 min and sustained it for 60 min in IOSE-29EC cells. In addition, treatment with FSH resulted in substantial phosphorylation of Elk-1, confirming that FSH action is mediated via activation of MAPK. In conclusion, we have demonstrated that FSH-R was expressed, and FSH induced growth stimulation in normal, preneoplastic, and neoplastic OSE cells. Furthermore, treatment with FSH stimulated activation of the MAPK cascade and phosphorylated Elk-1 in neoplastic OSE cells. These results suggest that the MAPK cascade may be involved in cellular functions such as growth stimulation in response to FSH in preneoplastic and neoplastic OSE cells.

Activation and targeting of mitogen-activated protein kinases by G-protein-coupled receptors

Over the past decade, it has become apparent that many G-protein-coupled receptors (GPCRs) generate signals that control cellular differentiation and growth, including stimulation of Ras family GTPases and activation of mitogen-activated protein (MAP) kinase pathways. The mechanisms that GPCRs use to control the activity of MAP kinases vary between receptor and cell type but fall broadly into one of three categories: signals initiated by classical G protein effectors, e.g., protein kinase (PK)A and PKC, signals initiated by cross-talk between GPCRs and classical receptor tyrosine kinases, e.g., "transactivation" of epidermal growth factor (EGF) receptors, and signals initiated by direct interaction between beta-arrestins and components of the MAP kinase cascade, e.g., beta-arrestin "scaffolds". While each of these pathways results in increased cellular MAP kinase activity, emerging data suggest that they are not functionally redundant. MAP kinase activation occurring via PKC-dependent pathways and EGF receptor transactivation leads to nuclear translocation of the kinase and stimulates cell proliferation, while MAP kinase activation via beta-arrestin scaffolds primarily increases cytosolic kinase activity. By controlling the spatial and temporal distribution of MAP kinase activity within the cell, the consequences of GPCR-stimulated MAP kinase activation may be determined by the mechanism by which they are activated.

Neuropeptide Y inhibits spontaneous alpha-melanocyte-stimulating hormone (alpha-MSH) release via a Y(5) receptor and suppresses thyrotropin-releasing hormone-induced alpha-MSH secretion via a Y(1) receptor in frog melanotrope cells

In amphibians, the secretion of alpha-MSH by melanotrope cells is stimulated by TRH and inhibited by NPY. We have previously shown that NPY abrogates the stimulatory effect of TRH on alpha-MSH secretion. The aim of the present study was to characterize the receptor subtypes mediating the action of NPY and to investigate the intracellular mechanisms involved in the inhibitory effect of NPY on basal and TRH-induced alpha-MSH secretion. Y(1) and Y(5) receptor mRNAs were detected by RT-PCR and visualized by in situ hybridization histochemistry in the intermediate lobe of the pituitary. Various NPY analogs inhibited in a dose-dependent manner the spontaneous secretion of alpha-MSH from perifused frog neurointermediate lobes with the following order of potency porcine peptide YY (pPYY) > frog NPY (fNPY) > porcine NPY (pNPY)-2-36) > pNPY-(13-36) > [D-Trp(32)]pNPY > [Leu(31),Pro(34)]pNPY. The stimulatory effect of TRH (10(-8)6 M) on alpha-MSH release was inhibited by fNPY, pPYY, and [Leu(31),Pro(34)]pNPY, but not by pNPY-(13-36) and [D-Trp(32)]pNPY. These data indicate that the inhibitory effect of fNPY on spontaneous alpha-MSH release is preferentially mediated through Y(5) receptors, whereas the suppression of TRH-induced alpha-MSH secretion by fNPY probably involves Y(1) receptors. Pretreatment of neurointermediate lobes with pertussis toxin (PTX; 1 microg/ml; 12 h) did not abolish the inhibitory effect of fNPY on cAMP formation and spontaneous alpha-MSH release, but restored the stimulatory effect of TRH on alpha-MSH secretion, indicating that the adenylyl cyclase pathway is not involved in the action of fNPY on TRH-evoked alpha-MSH secretion. In the majority of melanotrope cells, TRH induces a sustained and biphasic increase in cytosolic Ca(2+) concentration. Preincubation of cultured cells with fNPY (10(-7) M) or omega-conotoxin GVIA (10(-7) M) suppressed the plateau phase of the Ca(2+) response induced by TRH. However, although fNPY abrogated TRH-evoked alpha-MSH secretion, omega-conotoxin did not, showing dissociation between the cytosolic Ca(2+) concentration increase and the secretory response. Collectively, these data indicate that in frog melanotrope cells NPY inhibits spontaneous alpha-MSH release and cAMP formation through activation of a Y(5) receptor coupled to PTX- insensitive G protein, whereas NPY suppresses the stimulatory effect of TRH on alpha-MSH secretion through a Y(1) receptor coupled to a PTX-sensitive G protein-coupled receptor.

Molecular mechanisms involved in gastrin-mediated regulation of cAMP-responsive promoter elements

In the present study, we explore the role of cAMP-responsive (CRE) promoter elements in gastrin-mediated gene activation. By using the minimal CRE promoter reporter plasmid, pCRELuc, we show that gastrin can activate CRE. This activation is blocked by H-89 and GF 109203x, which inhibit protein kinases A and C, respectively. Moreover, Ca(2+)-activated pathways seem to be involved, because the calmodulin inhibitor W-7 reduced gastrin-mediated activation of pCRELuc. Deletion of CRE from the c-fos promoter rendered this promoter completely unresponsive to gastrin, indicating that CRE plays a central role in c-fos transactivation. Interestingly, gastrin-induced expression of the inducible cAMP early repressor (ICER), a gene that is known to be regulated by CRE promoter elements, was not reduced by H-89, W-7, or GF 109203x. Furthermore, bandshift analyses indicated that the region of the ICER promoter containing the CRE-like elements CARE 3-4 binds transcription factors that are not members of the CRE-binding protein-CRE modulator protein-activating transcription factor, or CREB/CREM/ATF-1, family. Our results underline the significance of the CRE promoter element in gastrin-mediated gene regulation and indicate that a variety of signaling mechanisms are involved, depending on the CRE promoter context.

Fibroblast growth factor activation of the rat PRL promoter is mediated by PKCdelta

Fibroblast growth factors play a critical role in cell growth, development, and differentiation and are also implicated in the formation and progression of tumors in a variety of tissues including pituitary. We have previously shown that fibroblast growth factor activation of the rat PRL promoter in GH4T2 pituitary tumor cells is mediated via MAP kinase in a Ras/Raf-1-independent manner. Herein we show using biochemical, molecular, and pharmacological approaches that PKCdelta is a critical component of the fibroblast growth factor signaling pathway. PKC inhibitors, or down-regulation of PKC, rendered the rat PRL promoter refractory to subsequent stimulation by fibroblast growth factors, implying a role for PKC in fibroblast growth factor signal transduction. FGFs caused specific translocation of PKCdelta from cytosolic to membrane fractions, consistent with enzyme activation. In contrast, other PKCs expressed in GH4T2 cells (alpha, betaI, betaII, and epsilon) did not translocate in response to fibroblast growth factors. The PKCdelta subtype-selective inhibitor, rottlerin, or expression of a dominant negative PKCdelta adenoviral construct also blocked fibroblast growth factor induction of rat PRL promoter activity, confirming a role for the novel PKCdelta isoform. PKC inhibitors selective for the conventional alpha and beta isoforms or dominant negative PKCalpha adenoviral expression constructs had no effect. Induction of the endogenous PRL gene was also blocked by adenoviral dominant negative PKCdelta expression but not by an analogous dominant negative PKCalpha construct. Finally, rottlerin significantly attenuated FGF-induced MAP kinase phosphorylation. Together, these results indicate that MAP kinase-dependent fibroblast growth factor stimulation of the rat PRL promoter in pituitary cells is mediated by PKCdelta.

Cyclic adenosine 3',5'-monophosphate (cAMP) and cAMP responsive element-binding protein are involved in the transcriptional regulation of gonadotropin-releasing hormone (GnRH) receptor by GnRH and mitogen-activated protein kinase signal transduction pathway in GGH(3) cells

Stimulation of mouse GnRH receptor promoter by a GnRH agonist (Buserelin), or by a cAMP analogue, significantly increased reporter (luciferase) activity. Overexpression of Raf-1, ERK1, or ERK2 partially blocked Buserelin-stimulated luciferase activity. In contrast, treatment with a mitogen-activated protein kinase (MAPK) kinase inhibitor (PD 98059) activated basal and Buserelin-stimulated luciferase activity in a dose-dependent manner. Transient transfection of the deleted cAMP response element expression vector followed by pretreatment with PD98059 prior to Buserelin stimulation showed that the transcriptional response was decreased compared to wild-type promoter. A gel-mobility shift assay using a probe containing the cAMP response element showed the presence of two specific protein-DNA complexes that contain one or more members of the cAMP responsive element-binding (CREB) protein family. These results suggest that cAMP and CREB participate in the GnRH activation of GnRH receptor promoter activity and that the MAPK cascade is involved in the negative regulation of basal and GnRH-stimulated GnRH receptor transcriptional activity.

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.

Modulation of rat erg1, erg2, erg3 and HERG K+ currents by thyrotropin-releasing hormone in anterior pituitary cells via the native signal cascade

The mechanism of thyrotropin-releasing hormone (TRH)-induced ether-a-go-go-related gene (erg) K+ current modulation was investigated with the perforated-patch whole-cell technique in clonal somatomammotroph GH3/B6 cells. These cells express a small endogenous erg current known to be reduced by TRH. GH3/B6 cells were injected with cDNA coding for rat erg1, erg2, erg3 and HERG K+ channels. The corresponding erg currents were isolated with the help of the specific erg channel blockers E-4031 and dofetilide and their biophysical properties were determined. TRH (1 M) was able to significantly reduce the different erg currents. The voltage dependence of activation was shifted by 15 mV (erg1), 10 mV (erg2) and 6 mV (erg3) to more positive potentials without strongly affecting erg inactivation. TRH reduced the maximal available erg current amplitude by 12% (erg1), 13% (erg2) and 39% (erg3) and accelerated the time course of erg1 and erg2 channel deactivation, whereas erg3 deactivation kinetics were not significantly altered. The effects of TRH on HERG currents did not differ from those on its rat homologue erg1. In addition, coinjection of rat MiRP1 with HERG cDNA did not influence the TRH-induced modulation of HERG channels. Rat erg1 currents recorded in the cell-attached configuration were reduced by application of TRH to the extra-patch membrane in the majority of the experiments, confirming the involvement of a diffusible second messenger. Application of the phorbol ester phorbol 12-myristate 13-acetate (PMA; 1 M) shifted the voltage dependence of erg1 activation in the depolarizing direction, but it did not reduce the maximal current amplitude. The voltage shift could not be explained by a selective effect on protein kinase C (PKC) since the PKC inhibitor bisindolylmaleimide I did not block the effects of TRH and PMA on erg1. In addition, cholecystokinin, known to activate the phosphoinositol pathway similarly to TRH, did not significantly affect the erg1 current. Various agents interfering with different known TRH-elicited cellular responses were not able to completely mimic or inhibit the TRH effects on erg1. Tested substances included modulators of the cAMP-protein kinase A pathway, arachidonic acid, inhibitors of tyrosine kinase and mitogen-activated protein kinase, sodium nitroprusside and cytochalasin D. The results demonstrate that all three members of the erg channel subfamily are modulated by TRH in GH3/B6 cells. In agreement with previous studies on the TRH-induced modulation of the endogenous erg current in prolactin-secreting anterior pituitary cells, the TRH effects on overexpressed erg1 channels are not mediated by any of the tested signalling pathways.