Activation of mitogen-activated protein kinase phosphatase 2 by gonadotropin-releasing hormone

Dual-Specificity Phosphatase 4 Promotes Malignant Features in Colorectal Cancer Through Cyclic-AMP Response Element Binding Protein/Protein Kinase CAMP-Activated Catalytic Subunit Beta Activation

INTRODUCTION

Previous studies have demonstrated that Dual-specificity phosphatase 4 (DUSP4) plays an important role in the progression of different tumor types. However, the role and mechanism of DUSP4 in colorectal cancer (CRC) remain unclear.

AIMS

We investigate the role and mechanisms of DUSP4 in CRC.

METHODS

Immunohistochemistry was used to investigate DUSP4 expression in CRC tissues. Cell proliferation, apoptosis and migration assays were used to validate DUSP4 function in vitro and in vivo. RNA-sequence assay was used to identify the target genes of DUSP4. Human phosphokinase array and inhibitor assays were used to explore the downstream signaling of DUSP4.

RESULTS

DUSP4 expression was upregulated in CRC tissues relative to normal colorectal tissues, and DUSP4 expression showed a significant positive correlation with CRC stage. Consistently, we found that DUSP4 was highly expressed in colorectal cancer cells compared to normal cells. DUSP4 knockdown inhibits CRC cell proliferation, migration and promotes apoptosis. Furthermore, the ectopic expression of DUSP4 enhanced CRC cell proliferation, migration and diminished apoptosis in vitro and in vivo. Human phosphokinase array data showed that ectopic expression of DUSP4 promotes CREB activation. RNA-sequencing data showed that PRKACB acts as a downstream target gene of DUSP4/CREB and enhances CREB activation through PKA/cAMP signaling. In addition, xenograft model results demonstrated that DUSP4 promotes colorectal tumor progression via PRKACB/CREB activation in vivo.

CONCLUSION

These findings suggest that DUSP4 promotes CRC progression. Therefore, it may be a promising therapeutic target for CRC.

Therapeutic resistance in breast cancer cells can result from deregulated EGFR signaling

The epidermal growth factor receptor (EGFR) interacts with various downstream molecules including phospholipase C (PLC)/protein kinase C (PKC), Ras/Raf/MEK/ERK, PI3K/PTEN/Akt/GSK-3, Jak/STAT and others. Often these pathways are deregulated in human malignancies such as breast cancer. Various therapeutic approaches to inhibit the activity of EGFR family members including small molecule inhibitors and monoclonal antibodies (MoAb) have been developed. A common problem with cancer treatments is the development of drug-resistance. We examined the effects of a conditionally-activated EGFR (v-Erb-B:ER) on the resistance of breast cancer cells to commonly used chemotherapeutic drugs such as doxorubicin, daunorubicin, paclitaxel, cisplatin and 5-flurouracil as well as ionizing radiation (IR). v-Erb-B is similar to the EGFR-variant EGFRvIII, which is expressed in various cancers including breast, brain, prostate. Both v-Erb-B and EGFRvIII encode the EGFR kinase domain but lack key components present in the extracellular domain of EGFR which normally regulate its activity and ligand-dependence. The v-Erb-B oncogene was ligated to the hormone binding domain of the estrogen receptor (ER) which results in regulation of the activity of the v-Erb-ER construct by addition of either estrogen (E2) or 4-hydroxytamoxifen (4HT) to the culture media. Introduction of the v-Erb-B:ER construct into the MCF-7 breast cancer cell line increased the resistance to the cells to various chemotherapeutic drugs, hormonal-based therapeutics and IR. These results point to the important effects that aberrant expression of EGFR kinase domain can have on therapeutic resistance.

Estradiol inhibits fMLP-induced neutrophil migration and superoxide production by upregulating MKP-2 and dephosphorylating ERK

Estrogen has been reported to inhibit neutrophil infiltration related inflammation and suppress neutrophils migration in vitro, but the underlying mechanism is not fully understood. By using HL-60 differentiated neutrophil-like cells (dHL-60) and human neutrophils, we examined the effect of 17-β estradiol (E₂) on cell migration and superoxide production in response to chemotactic peptide N-formyl-methionyl-leucyl-phenylalanine (fMLP) and explored the mechanisms involved. We found that fMLP significantly induced dHL-60 cell and neutrophil migration and superoxide production, which was inhibited by ERK inhibitor PD98059. E₂ significantly inhibited fMLP-induced dHL-60 cell and neutrophil migration and superoxide production at both physiological and pharmacological concentrations. Mechanistic studies showed that pretreatment of these cells with E₂ rapidly elevated the protein level of mitogen-activated protein kinase phosphatase 2 (MKP-2) and inhibited fMLP-induced ERK phosphorylation. Pretreatment of these cells with estrogen receptor (ER) antagonist ICI 182780 reversed the inhibition of fMP-induced cell migration and superoxide production, and the induction of MKP-2 expression and the suppression of fMP-induced ERK phosphorylation by E₂. However, pretreatment of cells with G-protein coupled ER antagonist G15 had no such effect. Collectively, these results demonstrate that fMLP stimulates neutrophil chemotaxis and superoxide production through activating ERK, and indicate that ER-mediated upregulation of MKP-2 may dephosphorylate ERK and contribute to the inhibitory effect of E₂ on neutrophil activation by fMLP. Our study reveals new mechanisms involved in the anti-inflammatory activity of estrogen.

Up-regulation of DUSP5 and DUSP6 by gonadotropin-releasing hormone in cultured hypothalamic neurons, GT1-7 cells

Gonadotropin-releasing hormone (GnRH) is secreted from hypothalamic neurons (GnRH neurons) and stimulates anterior pituitary gonadotrophs to synthesize and secrete gonadotropins. In addition to gonadotrophs, GnRH neurons also express GnRH receptors, and the autocrine action of GnRH is reportedly involved in the regulation of functions of GnRH neurons. There is accumulating evidence that extracellular signal-regulated kinase (ERK), one of mitogen-activated protein kinases (MAPKs), is activated by GnRH and involved in various effects of GnRH in GnRH neurons. In the present study, we performed microarray analysis to examine the types of genes whose expression was regulated by GnRH in immortalized mouse GnRH neurons (GT1-7 cells). We found that 257 genes among 55,681 genes examined were up-regulated after 30-min treatment of GT1-7 cells with GnRH. These up-regulated genes included four dual-specificity MAPK phosphatases (DUSPs), DUSP1, DUSP2, DUSP5, and DUSP6. Reverse transcription-polymerase chain reaction analysis confirmed that the mRNA levels of DUSP5 and DUSP6 were robustly increased within 30 min. U0126, an inhibitor of ERK activation, completely inhibited the increases in the mRNA levels of DUSP5 and DUSP6. Immunoblotting analysis revealed that ERK activation peaked at 5 min and declined steeply at 60 min, whereas DUSP5 and DUSP6 proteins were increased from 60 min. It was notable that down-regulation of DUSP6 augmented GnRH-induced ERK activation approximately 1.7-fold at 60 min. These results suggested that the up-regulation of DUSP6 regulates the duration of ERK activation at least in part.

Changes in pituitary gene expression may underlie multiple domesticated traits in chickens

Domesticated animals share a unique set of morphological and behavioral traits, jointly referred to as the domesticated phenotype. Striking similarities amongst a range of unrelated domesticated species suggest that similar regulatory mechanisms may underlie the domesticated phenotype. These include color pattern, growth, reproduction, development and stress response. Although previous studies have focused on the brain to find mechanisms underlying domestication, the potential role of the pituitary gland as a target of domestication is highly overlooked. Here, we study gene expression in the pituitary gland of the domesticated White Leghorn chicken and its wild ancestor, the Red Junglefowl. By overlapping differentially expressed genes with a previously published list of functionally important genes in the pituitary gland, we narrowed down to 34 genes. Amongst them, expression levels of genes with inhibitory function on pigmentation (ASIP), main stimulators of metabolism and sexual maturity (TSHB and DIO2), and a potential inhibitor of broodiness (PRLR), were higher in the domesticated breed. Additionally, expression of 2 key inhibitors of the stress response (NR3C1, CRHR2) was higher in the domesticated breed. We suggest that changes in the transcription of important modulatory genes in the pituitary gland can account not only for domestication of the stress response in domestic chickens, but also for changes in pigmentation, development, and reproduction. Given the pivotal role of the pituitary gland in the regulation of multiple shared domesticated traits, we suggest that similar changes in pituitary transcriptome may contribute to the domesticated phenotype in other species as well.

Gonadotropin regulation by pulsatile GnRH: Signaling and gene expression

The precise orchestration of hormonal regulation at all levels of the hypothalamic-pituitary-gonadal axis is essential for normal reproductive function and fertility. The pulsatile secretion of hypothalamic gonadotropin-releasing hormone (GnRH) stimulates the synthesis and release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) by pituitary gonadotropes. GnRH acts by binding to its high affinity seven-transmembrane receptor (GnRHR) on the cell surface of anterior pituitary gonadotropes. Different signaling cascades and transcriptional mechanisms are activated, depending on the variation in GnRH pulse frequency, to stimulate the synthesis and release of FSH and LH. While changes in GnRH pulse frequency may explain some of the differential regulation of FSH and LH, other factors, such as activin, inhibin and sex steroids, also contribute to gonadotropin production. In this review, we focus on the transcriptional regulation of the gonadotropin subunit genes and the signaling pathways activated by pulsatile GnRH.

Consider the context: Ras/ERK and PI3K/AKT/mTOR signaling outcomes are pituitary cell type-specific

Conserved signaling pathways are critical regulators of pituitary homeostasis and, when dysregulated, contribute to adenoma formation. Pituitary adenomas are typically benign and rarely progress to malignant cancer. Pituitary and other neuroendocrine cell types often display non-proliferative responses to ERK and PI3K, in contrast to non-endocrine cell types which typically proliferate in response to ERK and PI3K activation. These differences likely contribute to the infrequent progression to malignancy in many endocrine tumors. In this review, we highlight the Ras/ERK and PI3K/AKT/mTOR signaling pathways in each pituitary cell type, as well as in other endocrine tissues. Furthermore, we provide evidence that a balance of ERK and PI3K signaling is required to maintain pituitary homeostasis. It is unlikely that one sole oncogene will be identified as being responsible for sporadic pituitary adenoma formation. This review emphasizes the necessity to consider endocrine cell-specific contexts and the interplay of signaling pathways to define the mechanisms underlying pituitary tumorigenesis.

The F0F1 ATP Synthase Complex Localizes to Membrane Rafts in Gonadotrope Cells

Fertility in mammals requires appropriate communication within the hypothalamic-pituitary-gonadal axis and the GnRH receptor (GnRHR) is a central conduit for this communication. The GnRHR resides in discrete membrane rafts and raft occupancy is required for signaling by GnRH. The present studies use immunoprecipitation and mass spectrometry to define peptides present within the raft associated with the GnRHR and flotillin-1, a key raft marker. These studies revealed peptides from the F0F1 ATP synthase complex. The catalytic subunits of the F1 domain were validated by immunoprecipitation, flow cytometry, and cell surface biotinylation studies demonstrating that this complex was present at the plasma membrane associated with the GnRHR. The F1 catalytic domain faces the extracellular space and catalyzes ATP synthesis when presented with ADP in normal mouse pituitary explants and a gonadotrope cell line. Steady-state extracellular ATP accumulation was blunted by coadministration of inhibitory factor 1, limiting inorganic phosphate in the media, and by chronic stimulation of the GnRHR. Steady-state extracellular ATP accumulation was enhanced by pharmacological inhibition of ecto-nucleoside triphosphate diphosphohydrolases. Kisspeptin administration induced coincident GnRH and ATP release from the median eminence into the hypophyseal-portal vasculature in ovariectomized sheep. Elevated levels of extracellular ATP augmented GnRH-induced secretion of LH from pituitary cells in primary culture, which was blocked in media containing low inorganic phosphate supporting the importance of extracellular ATP levels to gonadotrope cell function. These studies indicate that gonadotropes have intrinsic ability to metabolize ATP in the extracellular space and extracellular ATP may serve as a modulator of GnRH-induced LH secretion.

Regulation of angiopoietin-1/Tie-2 receptor signaling in endothelial cells by dual-specificity phosphatases 1, 4, and 5

Background

Angiopoietin‐1 (Ang‐1) promotes survival and migration of endothelial cells, in part through the activation of mitogen‐activated protein kinase (MAPK) pathways downstream of Tie‐2 receptors. Dual‐specificity phosphatases (DUSPs) dephosphorylate phosphotyrosine and phosphoserine/phosphothreonine residues on target MAPKs. The mechanisms by which DUSPs modulate MAPK activation in Ang‐1/Tie‐2 receptor signaling are unknown in endothelial cells.

Methods and Results

Expression of various DUSPs in human umbilical vein endothelial cells exposed to Ang‐1 was measured. The functional roles of DUSPs in Ang‐1‐induced regulation of MAPK activation, endothelial cell survival, migration, differentiation, and permeability were measured using selective siRNA oligos. Ang‐1 differentially induces DUSP1, DUSP4, and DUSP5 in human umbilical vein endothelial cells through activation of the PI‐3 kinase, ERK1/2, p38, and SAPK/JNK pathways. Lack‐of‐function siRNA screening revealed that DUSP1 preferentially dephosphorylates p38 protein and is involved in Ang‐1‐induced cell migration and differentiation. DUSP4 preferentially dephosphorylates ERK1/2, p38, and SAPK/JNK proteins and, under conditions of serum deprivation, is involved in Ang‐1‐induced cell migration, several antiapoptotic effects, and differentiation. DUSP5 preferentially dephosphorylates ERK1/2 proteins and is involved in cell survival and inhibition of permeability.

Conclusions

DUSP1, DUSP4, and DUSP5 differentially modulate MAPK signaling pathways downstream of Tie‐2 receptors, thus highlighting the importance of these phosphatases to Ang‐1‐induced angiogenesis.

MAPK phosphatase-2 (MKP-2) is induced by hCG and plays a role in the regulation of CYP11A1 expression in MA-10 Leydig cells

MAPKs such as ERK1/2 are dephosphorylated, and consequently inactivated, by dual specificity phosphatases (MKPs). In Leydig cells, LH triggers ERK1/2 phosphorylation through the action of protein kinase A. We demonstrate that, in MA-10 Leydig cells, LH receptor activation by human chorionic gonadotropin (hCG) up-regulates MKP-2, a phosphatase that dephosphorylates ERK1/2, among other MAPKs. After 2 hours, hCG and 8-bromo-cAMP (8Br-cAMP) significantly increased MKP-2 mRNA levels (3-fold), which declined to basal levels after 6 hours. MKP-2 protein accumulation exhibited a similar kinetic profile. In cells transiently expressing flag-MKP-2 protein, hCG/8Br-cAMP stimulation promoted the accumulation of the chimera (2.5-fold after 3 h of stimulation). Pharmacologic and biochemical approaches showed that the accumulation of flag-MKP-2 involves a posttranslational modification that increases MKP-2 half-life. MKP-2 down-regulation by a short hairpin RNA (MKP-2 shRNA) raised the levels of phosphorylated ERK1/2 reached by 8Br-cAMP stimulation. This effect was evident after 180 min of stimulation, which suggests that MKP-2 down-regulates the late phase of cAMP-induced ERK1/2 activity. Also, MKP-2 down-regulation by MKP-2 shRNA increased the stimulatory effect of 8Br-cAMP on both promoter activity and messenger levels of CYP11A1, which encodes for the steroidogenic enzyme P450scc and is induced by LH/hCG through protein kinase A and ERK1/2 activities. Our findings demonstrate, for the first time, that LH/hCG tightly regulates MKP-2 expression, which modulates the induction of CYP11A1 by 8Br-cAMP. MKP-2 up-regulation might control ERK1/2 activity in a specific temporal frame to modulate the expression of a finite repertory of ERK-dependent genes.

Gonadotrophin-releasing hormone signalling downstream of calmodulin

Gonadotrophin-releasing hormone (GnRH) regulates reproduction via binding a G-protein coupled receptor on the surface of the gonadotroph, through which it transmits signals, mostly via the mitogen-activated protein (MAPK) cascade, to increase synthesis of the gonadotrophin hormones: luteinising hormone (LH) and follicle-stimulating hormone (FSH). Activation of the MAPK cascade requires an elevation in cytosolic Ca(2+) levels, which is a result of both calcium influx and mobilisation from intracellular stores. However, Ca(2+) also transmits signals via an MAPK-independent pathway, through binding calmodulin (CaM), which is then able to bind a number of proteins to impart diverse downstream effects. Although the ability of GnRH to activate CaM was recognised over 20 years ago, only recently have some of the downstream effects been elucidated. GnRH was shown to activate the CaM-dependent phosphatase, calcineurin, which targets gonadotrophin gene expression both directly and indirectly via transcription factors such as nuclear factor of activated T-cells and Nur77, the Transducer of Regulated CREB (TORC) co-activators and also the prolyl isomerase, Pin1. Gonadotrophin gene expression is also regulated by GnRH-induced CaM-dependent kinases (CaMKs); CaMKI is able to derepress the histone deacetylase-inhibition of β-subunit gene expression, whereas CaMKII appears to be essential for the GnRH-activation of all three subunit genes. Asides from activating gonadotrophin gene expression, GnRH also exerts additional effects on gonadotroph function, some of which clearly occur via CaM, including the proliferation of immature gonadotrophs, which is dependent on calcineurin. In this review, we summarise these pathways, and discuss the additional functions that have been proposed for CaM with respect to modifying GnRH-induced signalling pathways via the regulation of the small GTP-binding protein, Gem, and/or the regulator of G-protein signalling protein 2.

ERK signaling, but not c-Raf, is required for gonadotropin-releasing hormone (GnRH)-induced regulation of Nur77 in pituitary gonadotropes

Stimulation of pituitary gonadotropes by hypothalamic GnRH leads to the rapid expression of several immediate early genes that play key roles in orchestrating the response of the gonadotrope to hypothalamic stimuli. Elucidation of the signaling mechanisms that couple the GnRH receptor to this immediate early gene repertoire is critical for understanding the molecular basis of GnRH action. Here we identify signaling mechanisms that underlie regulation of the orphan nuclear receptor Nur77 as a GnRH-responsive immediate early gene in αT3-1 cells and mouse gonadotropes in culture. Using a variety of approaches, we show that GnRH-induced transcriptional upregulation of Nur77 in αT3-1 cells is dependent on calcium, protein kinase C (PKC), and ERK signaling. Transcriptional activity of Nur77 within the gonadotrope is regulated posttranslationally by GnRH signaling via PKC but not ERK activity. Surprisingly, neither activation of the ERK pathway nor the transcriptional response of Nur77 to GnRH requires the activity of c-Raf kinase. In corroboration of these results, Nur77 responsiveness to GnRH was maintained in gonadotropes from mice with pituitary-targeted ablation of c-Raf kinase. In contrast, gonadotropes from mice with pituitary deficiency of ERK signaling failed to up-regulate Nur77 after GnRH stimulation. These results further clarify the role of ERK and PKC signaling in regulation of the GnRH-induced immediate early gene program as well as GnRH-induced transcription-stimulating activity of Nur77 in the gonadotrope and shed new light on the complex functional organization of this signaling pathway in the pituitary gonadotrope.

Extracellular Signal-Regulated Kinase (ERK) Activation and Mitogen-Activated Protein Kinase Phosphatase 1 Induction by Pulsatile Gonadotropin-Releasing Hormone in Pituitary Gonadotrophs

The frequency of gonadotropin-releasing hormone (GnRH) pulse secreted from the hypothalamus differently regulates the expressions of gonadotropin subunit genes, luteinizing hormone β (LHβ) and follicle-stimulating hormone β (FSHβ), in the pituitary gonadotrophs. FSHβ is preferentially stimulated at slower GnRH pulse frequencies, whereas LHβ is preferentially stimulated at more rapid pulse frequencies. Several signaling pathways are activated, including mitogen-activated protein kinase (MAPK), protein kinase C, calcium influx, and calcium-calmodulin kinases, and these may be preferentially regulated under certain conditions. Previous studies demonstrated that MAPK pathways, especially the extracellular signal-regulated kinase (ERK), play an essential role for induction of gonadotropin subunit gene expression by GnRH, whereas, MAPK phosphatases (MKPs) inactivate MAPKs through dephosphorylation of threonine and/or tyrosine residues. MKPs are also induced by GnRH, and potential feedback regulation between MAPK signaling and MKPs within the GnRH signaling pathway is evident in gonadotrophs. In this paper, we reviewed and mainly focused on our observations of the pattern of ERK activation and the induction of MKP by different frequencies of GnRH stimulation.

Using automated imaging to interrogate gonadotrophin-releasing hormone receptor trafficking and function

Gonadotrophin-releasing hormone (GnRH) acts via seven transmembrane receptors on gonadotrophs to stimulate gonadotrophin synthesis and secretion, and thereby mediates central control of reproduction. Type I mammalian GnRHR are unique, in that they lack C-terminal tails. This is thought to underlie their resistance to rapid homologous desensitisation as well as their slow rate of internalisation and inability to provoke G-protein-independent (arrestin-mediated) signalling. More recently it has been discovered that the vast majority of human GnRHR are actually intracellular, in spite of the fact that they are activated at the cell surface by a membrane impermeant peptide hormone. This apparently reflects inefficient exit from the endoplasmic reticulum and again, the absence of the C-tail likely contributes to their intracellular localisation. This review is intended to cover some of these novel aspects of GnRHR biology, focusing on ways that we have used automated fluorescence microscopy (high content imaging) to explore GnRHR localisation and trafficking as well as spatial and temporal aspects of GnRH signalling via the Ca(2+)/calmodulin/calcineurin/NFAT and Raf/MEK/ERK pathways.

Encoding and decoding mechanisms of pulsatile hormone secretion

Ultradian pulsatile hormone secretion underlies the activity of most neuroendocrine systems, including the hypothalamic-pituitary adrenal (HPA) and gonadal (HPG) axes, and this pulsatile mode of signalling permits the encoding of information through both amplitude and frequency modulation. In the HPA axis, glucocorticoid pulse amplitude increases in anticipation of waking, and, in the HPG axis, changing gonadotrophin-releasing hormone pulse frequency is the primary means by which the body alters its reproductive status during development (i.e. puberty). The prevalence of hormone pulsatility raises two crucial questions: how are ultradian pulses encoded (or generated) by these systems, and how are these pulses decoded (or interpreted) at their target sites? We have looked at mechanisms within the HPA axis responsible for encoding the pulsatile mode of glucocorticoid signalling that we observe in vivo. We review evidence regarding the 'hypothalamic pulse generator' hypothesis, and describe an alternative model for pulse generation, which involves steroid feedback-dependent endogenous rhythmic activity throughout the HPA axis. We consider the decoding of hormone pulsatility by taking the HPG axis as a model system and focussing on molecular mechanisms of frequency decoding by pituitary gonadotrophs.

Modulation of gonadotropin-releasing hormone-induced extracellular signal-regulated kinase activation by dual-specificity protein phosphatase 1 in LbetaT2 gonadotropes

As the regulator of pituitary reproductive hormone synthesis, the hypothalamic neuropeptide GnRH is the central regulator of reproduction. A hallmark of GnRH action is the differential control of gene expression in pituitary gonadotropes through varied pulsatile stimulation. Among other signaling events, GnRH activation of the ERK family of MAPKs plays a significant role in the transcriptional regulation of the luteinizing hormone β-subunit gene and regulation of cap-dependent translation. We evaluated the ERK response to different GnRH pulse amplitudes in the gonadotrope cell line LβT2. We found that low-amplitude stimulation with GnRH invokes a rapid and transient ERK activation, whereas high-amplitude stimulation invokes a prolonged activation specifically in the cytoplasm fraction of LβT2 cells. Nuclear and cytoplasmic targets of ERK, Ets-like gene 1, and eukaryotic initiation factor 4E, respectively, are similarly activated. Feedback control of ERK activation occurs mainly through the dual-specificity protein phosphatases (DUSPs). DUSP1 is localized to the nucleus in LβT2 cells but DUSP4, another member implicated in GnRH feedback, exists in both the nucleus and cytoplasm. Manipulation of nuclear DUSP activity through overexpression or knockdown of Dusp1 modulates the ERK response to low and high GnRH pulse amplitudes and activation of the Lhb promoter. Dusp1 overexpression abolishes sustained ERK activation and inhibits Lhb promoter activity induced by high amplitude pulses. Conversely, Dusp1 knockdown enhances ERK activation by low-amplitude stimulation and increases stimulation of Lhb promoter activity. We conclude that DUSP1 feedback activity modulates ERK activation and the transcriptional response to GnRH.

Pulsatile and sustained gonadotropin-releasing hormone (GnRH) receptor signaling: does the ERK signaling pathway decode GnRH pulse frequency?

Gonadotropin-releasing hormone (GnRH) acts via G-protein-coupled receptors on gonadotrophs to stimulate synthesis and secretion of luteinizing hormone and follicle-stimulating hormone. It is secreted in pulses, and its effects depend on pulse frequency, but decoding mechanisms are unknown. Here we have used an extracellular signal regulated kinase-green fluorescent protein (ERK2-GFP) reporter to monitor GnRH signaling. GnRH caused dose-dependent ERK2-GFP translocation to the nucleus, providing a live-cell readout for activation. Pulsatile GnRH caused dose- and frequency-dependent ERK2-GFP translocation. These responses were rapid and transient, showed only digital tracking, and did not desensitize under any condition tested (dose, frequency, and receptor number varied). We also tested for the effects of cycloheximide (to prevent induction of nuclear-inducible MAPK phosphatases) and used GFP fusions containing ERK mutations (D319N, which prevents docking domain-dependent binding to MAPK phosphatases, and K52R, which prevents catalytic activity). These manipulations had little or no effect on the translocation responses, arguing against a role for MAPK phosphatases or ERK-mediated feedback in shaping ERK activation during pulsatile stimulation. GnRH also caused dose- and frequency-dependent activation of the alpha-gonadotropin subunit-, luteinizing hormone beta-, and follicle-stimulating hormone beta- luciferase reporters, and the latter response was inhibited by ERK1/2 knockdown. Moreover, GnRH caused frequency-dependent activation of an Egr1-luciferase reporter, but the response was proportional to cumulative pulse duration. Our data suggest that frequency decoding is not due to negative feedback shaping ERK signaling in this model.

Mitogen-activated protein kinase phosphatase 2 regulates the inflammatory response in sepsis

Sepsis results from a dysregulation of the regulatory mechanisms of the pro- and anti-inflammatory response to invading pathogens. The mitogen-activated protein (MAP) kinase cascades are key signal transduction pathways involved in the cellular production of cytokines. The dual-specific phosphatase 1 (DUSP 1), mitogen-activated protein kinase phosphatase-1 (MKP-1), has been shown to be an important negative regulator of the inflammatory response by regulating the p38 and Jun N-terminal protein kinase (JNK) MAP kinase pathways to influence pro- and anti-inflammatory cytokine production. MKP-2, also a dual-specific phosphatase (DUSP 4), is a phosphatase highly homologous with MKP-1 and is known to regulate MAP kinase signaling; however, its role in regulating the inflammatory response is not known. We hypothesized a regulatory role for MKP-2 in the setting of sepsis. Mice lacking the MKP-2 gene had a survival advantage over wild-type mice when challenged with intraperitoneal lipopolysaccharide (LPS) or a polymicrobial infection via cecal ligation and puncture. The MKP-2(-/-) mice also exhibited decreased serum levels of both pro-inflammatory cytokines (tumor necrosis factor alpha [TNF-alpha], interleukin-1beta [IL-1beta], IL-6) and anti-inflammatory cytokines (IL-10) following endotoxin challenge. Isolated bone marrow-derived macrophages (BMDMs) from MKP-2(-/-) mice showed increased phosphorylation of the extracellular signal-regulated kinase (ERK), decreased phosphorylation of JNK and p38, and increased induction of MKP-1 following LPS stimulation. The capacity for cytokine production increased in MKP-2(-/-) BMDMs following MKP-1 knockdown. These data support a mechanism by which MKP-2 targets ERK deactivation, thereby decreasing MKP-1 and thus removing the negative inhibition of MKP-1 on cytokine production.

Induction of dual specificity phosphatase 1 (DUSP1) by gonadotropin-releasing hormone (GnRH) and the role for gonadotropin subunit gene expression in mouse pituitary gonadotroph L beta T2 cells

We examined the expression of dual specificity phosphatase 1 (DUSP1) by gonadotropin-releasing hormone (GnRH) stimulation and investigated the role of DUSP1 on gonadotropin gene expression using LbetaT2 gonadotroph cell line. DUSP1 expression was markedly increased 60 min after GnRH stimulation, and mitogen-activated protein kinase 3/1 (MAPK3/1) activation was gradually decreased after 60 min. GnRH-induced MAPK3/1 activation was completely inhibited by U0126, a MEK inhibitor, whereas GnRH-induced DUSP1 expression was partially inhibited by U0126. GnRH-induced DUSP1 induction was inhibited by triptolide, a diterpenoid triepoxide. In contrast, this compound potentiated MAPK3/1 activation. U0126 prevented GnRH-stimulated gonadotropin subunit promoter activation dose dependently, and 10 muM of U0126 reduced the effects of GnRH on the Lhb and Fshb promoters to 79.15% and 55.66%, respectively. GnRH-stimulated activation of Lhb and Fshb promoters as well as serum response factor (Srf) promoters were almost completely inhibited by triptolide, suggesting that this component had a nonspecific effect to the cells. Dusp1 siRNA reduced the expression of DUSP1 and augmented MAPK3/1 phosphorylation, but it did not increase of gonadotropin promoters. By overexpression of DUSP1, both GnRH-stimulated Lhb and Fshb promoters were significantly reduced. We have previously shown that insulin-like growth factor 1 (IGF1) increases MAPK3/1 but does not activate gonadotropin subunit promoters. IGF1 failed to induce DUSP1 expression. In addition, under pulsatile GnRH stimulation, DUSP1 expression was observed following high-frequency GnRH pulses but not following low-frequency pulses. Our study demonstrated that DUSP1, induced by GnRH, functions not only as an MAPK3/1-inactivating phosphatase but also as an important mediator in gonadotropin subunit gene expression regulation.

ERK signaling in the pituitary is required for female but not male fertility

Males and females require different patterns of pituitary gonadotropin secretion for fertility. The mechanisms underlying these gender-specific profiles of pituitary hormone production are unknown; however, they are fundamental to understanding the sexually dimorphic control of reproductive function at the molecular level. Several studies suggest that ERK1 and -2 are essential modulators of hypothalamic GnRH-mediated regulation of pituitary gonadotropin production and fertility. To test this hypothesis, we generated mice with a pituitary-specific depletion of ERK1 and 2 and examined a range of physiological parameters including fertility. We find that ERK signaling is required in females for ovulation and fertility, whereas male reproductive function is unaffected by this signaling deficiency. The effects of ERK pathway ablation on LH biosynthesis underlie this gender-specific phenotype, and the molecular mechanism involves a requirement for ERK-dependent up-regulation of the transcription factor Egr1, which is necessary for LHbeta expression. Together, these findings represent a significant advance in elucidating the molecular basis of gender-specific regulation of the hypothalamic-pituitary-gonadal axis and sexually dimorphic control of fertility.

Gonadotropin-releasing hormone and protein kinase C signaling to ERK: spatiotemporal regulation of ERK by docking domains and dual-specificity phosphatases

Activated ERK translocates to the nucleus to regulate transcription. Spatiotemporal aspects of this response dictate biological consequences and are influenced by dual-specificity phosphatases (DUSPs) that can scaffold and dephosphorylate ERK. In HeLa cells, GnRH causes transient and protein kinase C (PKC)-dependent ERK activation, but termination mechanisms are unknown. We now explore DUSP roles using short inhibitory RNA to knock down endogenous ERK, adenoviruses to express GnRH receptors and add-back ERK2-GFP, and automated microscopy to monitor ERK location and activation. GnRH caused rapid and transient increases in dual phosphorylated ERK2 (ppERK2) and nuclear to cytoplasmic ERK2-green fluorescent protein (GFP) ratio, whereas responses to a PKC-activating phorbol ester were more sustained. In cells expressing D319N ERK2-GFP (D319N mutation impairs docking-domain-dependent binding to DUSPs), GnRH caused more sustained increases in ppERK2 and nuclear to cytoplasmic ERK2-GFP ratio and also had more pronounced effects on Egr-1 luciferase (a transcriptional reporter for ERK activation). Cycloheximide caused more sustained effects of GnRH and phorbol ester on ppERK, suggesting termination by nuclear-inducible DUSPs. GnRH also increased expression of nuclear-inducible DUSP1 and -4, but their knockdown did not alter GnRH-mediated ERK signaling. Screening a short inhibitory RNA library targeting 16 DUSPs (nuclear-inducible DUSPs, cytoplasmic ERK MAPK phosphatases, c-Jun N-terminal kinase/p38 MAPK phosphatases, and atypical DUSPs) revealed GnRH effects to be influenced by DUSPs 5, 9, 10, 16, and 3 (i.e. by each DUSP class). Thus, GnRH-mediated ERK responses (like PKC-mediated ERK responses) are dependent on protein neosynthesis and docking-domain-dependent binding, but for GnRH activation (unlike PKC activation), this does not reflect dependence on nuclear-inducible DUSPs. Termination of these GnRH effects is apparently dependent upon a preexisting rapid turnover protein.

Signaling by G-protein-coupled receptor (GPCR): studies on the GnRH receptor

Gonadotropin-releasing hormone (GnRH) is the first key hormone of reproduction. GnRH analogs are extensively used in in vitro fertilization, and treatment of sex hormone-dependent cancers, due to their ability to bring about 'chemical castration'. The interaction of GnRH with its cognate type I receptor (GnRHR) in pituitary gonadotropes results in the activation of Gq/G(11), phospholipase Cbeta (PLCbetaI), PLA(2), and PLD. Sequential activation of the phospholipases generates the second messengers inositol 1, 4, 5-trisphosphate (IP(3)), diacylglycerol (DAG), and arachidonic acid (AA), which are required for Ca(2+) mobilization, the activation of various protein kinase C isoforms (PKCs), and the production of prostaglandin (PG) and other metabolites of AA, respectively. PKC isoforms are the major mediators of the downstream activation of a number of mitogen-activated protein kinase (MAPK) cascades by GnRH, namely: extracellular signal-regulated kinase (ERK), jun-N-terminal kinase (JNK), and p38MAPK. The activated MAPKs phosphorylate both cytosolic and nuclear proteins to initiate the transcriptional activation of the gonadotropin subunit genes and the GnRHR. While Ca(2+) mobilization has been found to initiate rapid gonadotropin secretion, Ca(2+), together with various PKC isoforms, MAPKs and AA metabolites also serve as key nodes, in the GnRH-stimulated signaling network that enables the gonadotropes to decode GnRH pulse frequencies and translating that into differential gonadotropin synthesis and release. Even though pulsatility of GnRH is recognized as a major determinant for differential gonadotropin subunit gene expression and gonadotropin secretion very little is yet known about the signaling circuits governing GnRH action at the 'Systems Biology' level. Direct apoptotic and metastatic effects of GnRH analogs in gonadal steroid-dependent cancers expressing the GnRHR also seem to be mediated by the activation of the PKC/MAPK pathways. However, the mechanisms dictating life (pituitary) vs. death (cancer) decisions made by the same GnRHR remain elusive. Understanding these molecular mechanisms triggered by the GnRHR through biochemical and 'Systems Biology' approaches would provide the basis for the construction of the dynamic connectivity maps, which operate in the various cell types (endocrine, cancer, and immune system) targeted by GnRH. The connectivity maps will open a new vista for exploring the direct effects of GnRH analogs in tumors and the design of novel combined therapies for fertility control, reproductive disorders and cancers.

Analysis of the calcium-dependent regulation of proline-rich tyrosine kinase 2 by gonadotropin-releasing hormone

Calcium influx through L-type voltage-gated calcium channels (VGCC) is required for ERK activation induced by GnRH in pituitary gonadotropes. The current studies investigate VGCC-sensitive catalytic activities that may lie upstream of ERKs within the GnRH signaling network. Ion exchange fractionation of alphaT3-1 cell lysates subjected to anti-phosphotyrosine Western blot analysis revealed a nifedipine-sensitive activity that colocalized with proline-rich tyrosine kinase (Pyk) 2 immunoreactivity. Phosphorylated Pyk2 was present in alphaT3-1 cells after GnRH agonist administration for a time course that lasted up to 4 h. Pyk2 phosphorylation was also evident in gonadotropes in vivo after administration of a bolus of GnRH. Knockdown of Pyk2 using specific small interfering RNAs revealed that Pyk2 contributed to modulation of GnRH-induced ERK but not c-Jun N-terminal kinase activation. Using pharmacological approaches, calmodulin (Cam) was also demonstrated to be required for the phosphorylation of Pyk2. Pyk2 was shown to bind specifically to a Cam agarose affinity column in a calcium-dependent manner, suggesting Cam and Pyk2 are capable of forming a complex. Specific mutation of a putative Cam binding motif within the catalytic domain of Pyk2 blocked association with Cam and uncoupled Pyk2's ability to activate ERK-dependent gene transcription. Thus, GnRH induces Pyk2 tyrosine phosphorylation dependent upon calcium flux within gonadotropes. Furthermore, association of Pyk2 and Cam may be required to mediate the effects of calcium on Pyk2 phosphorylation and subsequent activation of ERKs by GnRH.

3', 5'-cyclic adenosine 5'-monophosphate response element-dependent transcriptional regulation of the secretogranin II gene promoter depends on gonadotropin-releasing hormone-induced mitogen-activated protein kinase activation and the transactivator activating transcription factor 3

Previous studies demonstrated that GnRH-induced secretogranin II (SgII) promoter regulation required a consensus cAMP response element (CRE) and protein kinase A/CRE binding protein. The present studies examined the role of additional components of the GnRH signaling network on SgII promoter activity with particular attention devoted to CRE-dependent gene regulation. Disruption of the SgII CRE by mutagenesis resulted in inhibition of GnRH agonist (GnRHa) induction of this promoter in alphaT3-1 cells. Pharmacological and dominant-negative inhibition of the ERK and c-Jun N-terminal kinase (JNK) signaling pathways revealed that GnRHa-induced SgII promoter activity required functional JNK and ERK modules. Combined inhibition of both pathways nearly abolished GnRHa-induced SgII promoter activity. Specific induction of the ERK cascade alone using overexpression of Raf-CAAX was not sufficient to activate the SgII gene promoter. In contrast, overexpression of the catalytic domain of the more pleiotropic MAPK activator, MAPK/ERK kinase-1, was sufficient to induce SgII promoter activity. The effect(s) of mitogen-activated protein/ERK kinase-1 on SgII promoter activity was CRE dependent and was reversed by the combined pharmacological inhibition of both JNK and ERK modules. CRE DNA binding studies demonstrated the recruitment of activating transcription factor (ATF)-3 and c-Jun to the CRE after administration of GnRHa to alphaT3-1 cells. Specific small interfering RNA knockdown of ATF3 reduced ATF3 DNA binding and the effect of GnRHa on the SgII promoter. These studies support the conclusion that MAPK signaling and ATF3 action are essential for full SgII promoter activation by GnRHa through a canonical CRE. Moreover, we suggest that within the GnRH signaling network, CRE-dependent gene regulation in general may be mediated primarily through the immediate early response gene ATF3.

Regulation of MAP kinases by MAP kinase phosphatases

MAP kinase phosphatases (MKPs) catalyze dephosphorylation of activated MAP kinase (MAPK) molecules and deactivate them. Therefore, MKPs play an important role in determining the magnitude and duration of MAPK activities. MKPs constitute a structurally distinct family of dual-specificity phosphatases. The MKP family members share the sequence homology and the preference for MAPK molecules, but they are different in substrate specificity among MAPK molecules, tissue distribution, subcellular localization and inducibility by extracellular stimuli. Our understanding of their protein structure, substrate recognition mechanisms, and regulatory mechanisms of the enzymatic activity has greatly increased over the past few years. Furthermore, although there are a number of MKPs, that have similar substrate specificities, non-redundant roles of MKPs have begun to be identified. Here we focus on recent findings regarding regulation and function of the MKP family members as physiological regulators of MAPK signaling.

Signaling complexes associated with the type I gonadotropin-releasing hormone (GnRH) receptor: colocalization of extracellularly regulated kinase 2 and GnRH receptor within membrane rafts

Our previous work demonstrated that the type I GnRH receptor (GnRHR) resides exclusively and constitutively within membrane rafts in alphaT3-1 gonadotropes and that this association was necessary for the ability of the receptor to couple to the ERK signaling pathway. G(alphaq), c-raf, and calmodulin have also been shown to reside in this compartment, implicating a raft-associated multiprotein signaling complex as a functional link between the GnRHR and ERK signaling. In the studies reported here, we used subcellular fractionation and coimmunoprecipitation to analyze the behavior of ERKs with respect to this putative signaling platform. ERK 2 associated partially and constitutively with low-density membranes both in alphaT3-1 cells and in whole mouse pituitary. Cholesterol depletion of alphaT3-1 cells reversibly blocked the association of both the GnRHR and ERKs with low-density membranes and uncoupled the ability of GnRH to activate ERK. Analysis of the kinetics of recovery of ERK inducibility after cholesterol normalization supported the conclusion that reestablishment of the association of the GnRHR and ERKs with the membrane raft compartment was not sufficient for reconstitution of signaling activity. In alphaT3-1 cells, the GnRHR and ERK2 coimmunoprecipitated from low-density membrane fractions prepared either in the presence or absence of detergent. The GnRHR also partitioned into low-density, detergent-resistant membrane fractions in mouse pituitary and coimmunoprecipitated with ERK2 from these fractions. Collectively, these data support a model in which coupling of the GnRHR to the ERK pathway in gonadotropes involves the assembly of a multiprotein signaling complex in association with specialized microdomains of the plasma membrane.

Gonadotropin-releasing hormone pulse frequency-dependent activation of extracellular signal-regulated kinase pathways in perifused LbetaT2 cells

The pattern of GnRH release is associated with differential synthesis and release of LH and FSH. Using a perifusion system, we previously reported that stimulation of the LbetaT2 cell line with varying GnRH pulse frequencies resulted in differential stimulation of LHbeta and FSHbeta gene transcription, analogous to previous observations in primary gonadotropes. In the present study, we investigated the patterns of MAPK activation by GnRH and the role of MAPK in mediating the frequency-dependent effects. In static culture, ERK activation in LbetaT2 cells stimulated with continuous GnRH (10 nM) was maximal by 10 min and persisted for up to 6 h, with a return to basal levels by 20 h. In contrast, stimulation with continuous GnRH (10 nM) in perifused cells resulted in a more sustained activation of ERK. To investigate the effects of GnRH pulse frequency on ERK activation, perifused LbetaT2 cells were stimulated with pulsatile GnRH at a frequency of one pulse every 30 min or one pulse every 2 h for 20 h (10 nM, 5 min/pulse). After the final GnRH pulse, cells were lysed at frequent intervals and levels of ERK phosphorylation were measured. Under high-frequency conditions, ERK activation was maximal 10 min after the GnRH pulse and returned to baseline levels by 20 min. In contrast, under lower GnRH pulse frequency conditions, ERK activation occurred more rapidly and activation was more sustained, with a slower rate of ERK dephosphorylation. These changes resulted in different levels of nuclear phosphorylated ERK. Blockade of ERK activation abolished GnRH-dependent activation of LHbeta and FSHbeta transcription at both high and low pulse frequencies. These results demonstrate that in perifused LbetaT2 cells, distinct patterns of ERK activation/inactivation are regulated by GnRH pulse frequency, and the difference in ERK activation may be important for GnRH pulse frequency-dependent differential stimulation of LHbeta and FSHbeta gene expression.

Gonadotropin-releasing hormone induction of extracellular-signal regulated kinase is blocked by inhibition of calmodulin

Our previous studies demonstrate that GnRH-induced ERK activation required influx of extracellular Ca2+ in alphaT3-1 and rat pituitary cells. In the present studies, we examined the hypothesis that calmodulin (Cam) plays a fundamental role in mediating the effects of Ca2+ on ERK activation. Cam inhibition using W7 was sufficient to block GnRH-induced reporter gene activity for the c-Fos, murine glycoprotein hormone alpha-subunit, and MAPK phosphatase (MKP)-2 promoters, all shown to require ERK activation. Inhibition of Cam (using a dominant negative) was sufficient to block GnRH-induced ERK but not c-Jun N-terminal kinase activity activation. The Cam-dependent protein kinase (CamK) II inhibitor KN62 did not recapitulate these findings. GnRH-induced phosphorylation of MAPK/ERK kinase 1 and c-Raf kinase was blocked by Cam inhibition, whereas activity of phospholipase C was unaffected, suggesting that Ca2+/Cam modulation of the ERK cascade potentially at the level of c-Raf kinase. Enrichment of Cam-interacting proteins using a Cam agarose column revealed that c-Raf kinase forms a complex with Cam. Reconstitution studies reveal that recombinant c-Raf kinase can associate directly with Cam in a Ca2+-dependent manner and this interaction is reduced in vitro by addition of W7. Cam was localized in lipid rafts consistent with the formation of a Ca2+-sensitive signaling platform including the GnRH receptor and c-Raf kinase. These data support the conclusion that Cam may have a critical role as a Ca2+ sensor in specifically linking Ca2+ flux with ERK activation within the GnRH signaling pathway.

Pituitary cell lines and their endocrine applications

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

Gonadotropin-releasing hormone and TGF-beta activate MAP kinase and differentially regulate fibronectin expression in endometrial epithelial and stromal cells

Gonadotropin-releasing hormone analog (GnRHa) is used for medical management of endometriosis and premature luteinizing hormone surge during controlled ovarian stimulation. Human endometrium expresses GnRH receptors, and GnRHa alters the expression of transforming growth factor-beta (TGF-beta) and receptors in endometrial cells. Because the diverse biological actions of GnRHa and TGF-beta are mediated in part through the MAPK pathway, we determined whether utilization of MAPK/ERK and transcriptional activation of immediate early genes c-fos and c-jun result in differential regulation of fibronectin, known as key regulator of embryo implantation and endometriosis progression. Using endometrial stromal cells (ESC) and the endometrial epithelial cell line HES, we demonstrated that GnRHa and TGF-beta, in a dose-, time-, and cell-dependent manner, increased the level of phosphorylated ERK1/2 (pERK1/2). GnRH antagonist Antide also increased pERK1/2 induction in ESC and HES, whereas pretreatment reduced GnRHa-induced pERK2 in ESC but not in HES. Cotreatments with GnRHa plus TGF-beta1 did not have an additive or an inhibitory effect on pERK1/2 induction compared with GnRHa or TGF-beta1 action alone. TGF-beta1 and GnRHa increased ERK1/2 nuclear accumulation and inversely regulated the expression of c-fos and c-jun and that of fibronectin in a cell-specific manner. Pretreatment with U-0126, a MEK1/2 inhibitor, blocked basal, as well as GnRHa- and TGF-beta1-induced pERK1/2; however, it differentially affected c-fos, c-jun, and fibronectin expression. In conclusion, the results indicate that GnRHa and TGF-beta signaling through MAPK/ERK results in differential regulation of fibronectin expression in endometrial cells, a molecular mechanism where short- and long-term GnRHa therapy and locally expressed TGF-beta could influence embryo implantation and endometriosis implants, respectively.

Structure of the GnRH receptor-stimulated signaling network: insights from genomics

The GnRH receptor influences gene expression in the gonadotrope through activating signaling cascades that modulate transcription factor expression and activity. A longstanding question in neuroendocrinology is how instructions received at the membrane in the form of the pattern of receptor stimulation are processed into specific biosynthetic changes at each gonadotropin promoter. Signal transduction from the membrane to preformed transcription factors relies on recognition of altered conformations. Signal transduction through the layers of the gene network also requires the biosynthesis of new transcription factors. The signal processing of this system depends on its molecular connectivity map and its feedback and feed-forward loops. Review of signal transduction, gene control, and genomic studies provide evidence of key loops that cross between cellular and nuclear compartments. Genomic studies suggest that the signal transduction and gene network form a continuum. We propose that information transfer in the gonadotrope depends on robust signaling modules that serve to integrate events at different time scales across cytoplasmic and nuclear compartments.

Molecular cloning and characterization of a novel human protein phosphatase, LMW-DSP3

Reversible phosphorylation is recognized to be a major mechanism for the control of intracellular events in eukaryotic cells. From a human fetal brain cDNA library, we isolated a cDNA clone encoding a novel dual specificity protein phosphatase, which showed 88% identity with previously reported mouse LMW-DSP3 at the amino acid level. The deduced protein had a single dual-specificity phosphatase catalytic domain, and lacked a cdc25 homology domain. LMW-DSP3 was expressed in the heart, lung, liver, and pancreas, and the expression level in the pancreas was highest. The LMW-DSP3 gene was located in human chromosome 2q32, and consisted of five exons spanning 21kb of human genomic DNA. LMW-DSP3 fused to GST showed phosphatase activity towards p-nitrophenyl phosphate which was optimal at pH 7.0 and 40 degrees C, and the activity was enhanced by Ca(2+) and Mn(2+). The phosphatase activity of LMW-DSP3 was inhibited by orthovanate. LMW-DSP3 showed phosphatase activity toward oligopeptides containing pSer/Thr and pTyr, indicating that LMW-DSP3 is a protein phosphatase with dual substrate specificity.

The gonadotrophin-releasing hormone receptor: signalling, cycling and desensitisation

Sustained stimulation of G-protein coupled receptors (GPCRs) typically causes receptor desensitisation that is mediated by phosphorylation, often within the C-terminal tail of the receptor. The consequent binding of beta-arrestin not only prevents the receptor from activating its G-protein (causing desensitisation) but can also target it for internalisation via clathrin-coated vesicles and can mediate signalling to proteins regulating endocytosis and mitogen-activated protein kinase (MAPK) cascades. GnRH acts via phospholipase C coupled GPCRs on pituitary gonadotrophs. The type I GnRH-receptors (GnRH-Rs) found only in mammals, are unique in that they lack C-terminal tails and apparently do not undergo agonist-induced phosphorylation or bind beta-arrestin. They are therefore resistant to receptor desensitisation and internalise slowly. In contrast, the type II GnRH-Rs, found in numerous vertebrates, possess such tails and show rapid desensitisation and internalisation with concomitant receptor phosphorylation (within the C-terminal tails) and/or binding of beta-arrestin. The binding to beta-arrestin may also be important for association with dynamin, a GTPase that controls cleavage of endosomes from the plasma membrane. Using recombinant adenovirus to express GnRH-R, we have found that blockade of dynamin-dependent endocytosis inhibits internalisation of type II (Xenopus) GnRH-Rs but not type I (human) GnRH-Rs, revealing the existence of functionally distinct routes through which these receptors are internalised. Although type I GnRH-R do not rapidly desensitise, sustained activation of GnRH receptors does cause desensitisation of gonadotrophin secretion, an effect which must therefore involve adaptive responses distal to the receptor. One such response is the GnRH-induced down regulation of inositol 1, 4, 5 trisphosphate receptors that apparently underlies desensitisation of Ca2+ mobilisation in a gonadotroph-derived cell line. Although activation of other GPCRs can down-regulate inositol 1, 4, 5 trisphosphate receptors, the effect of GnRH is atypically rapid and pronounced, presumably because of the receptor's atypical resistance to desensitisation. GnRH-Rs are also expressed in several extra-pituitary sites and these may mediate direct inhibition of proliferation of hormone-dependent cancer cells. Infection with type I GnRH-R expressing adenovirus facilitated expression of high affinity, PLC-coupled GnRH-R in mammary and prostate cancer cells and these mediated pronounced antiproliferative effects of receptor agonists. No such effect was seen in cells transfected with a type II GnRH-R, implying that it is mediated most efficiently by a non-desensitising receptor. Thus it appears that the GnRH-Rs have undergone a period of rapidly accelerated molecular evolution that is of functional relevance to GnRH-R signalling in pituitary and extra-pituitary sites.

An early growth response protein (Egr) 1 cis-element is required for gonadotropin-releasing hormone-induced mitogen-activated protein kinase phosphatase 2 gene expression

In pituitary gonadotropes, gonadotropin-releasing hormone (GnRH) activates all three major mitogen-activated protein kinase (MAPK) cascades. The MAPKs play key roles in transcriptional activation of GnRH-responsive genes. MAPK phosphatases (MKPs) are dual specificity protein phosphatases involved in feedback regulation of MAPK activity. Previous studies indicate that GnRH activates MKP-2 expression in gonadotropes, dependent upon activation of multiple MAPKs and discrete Ca(2+) signals. To further understand the transcriptional mechanism(s) of MKP-2 induction by GnRH, we studied the activity of a 198-nucleotide MKP-2 proximal promoter region that supports GnRH responsiveness in reporter gene assays. Functional analysis of the MKP-2 promoter confirmed a requirement for the protein kinase C-extracellular signal-regulated kinase (ERK) pathway and VGCC-derived Ca(2+) signals in transcriptional activation of the MKP-2 gene. However, the inhibitory effect of thapsigargin on MKP-2 protein expression previously identified was not mediated at the level of promoter activation, suggesting a distinct mechanism for the action of thapsigargin-sensitive Ca(2+) signals. MGRE (MKP-2 GnRH response element) within the MKP-2 promoter mediated promoter activation through the protein kinase C-ERK pathway. The zinc finger transcription factor Egr-1 was identified in the MGRE-binding complex. Egr-1/MGRE binding was induced by GnRH in an ERK-dependent manner. Transcriptional activity of Egr-1 protein was enhanced by GnRH treatment. In addition, overexpression of the Egr-interacting protein, NAB1, resulted in increased GnRH-stimulated MKP-2 gene transcription. Consistent with the putative role of Egr-1 in MKP-2 promoter regulation, Egr-1 protein expression closely correlated with the expression of MKP-2 protein in alpha T3-1 cells. Together, these data suggest that Egr-1 may be a key factor in mediating GnRH-dependent transcriptional activation of the MKP-2 gene.

Structural organization of the rat mitogen-activated protein kinase phosphatase 2 gene

Mitogen-activated protein kinase (MAPK) phosphatases (MKPs) are dual specificity protein phosphatases that specifically inactivate MAPKs. Regulated expression of MKPs plays a key role in determining their physiological function. However, little is known about the molecular mechanism of the activation of MKP genes. In this study, we cloned the rat MKP-2 gene and characterized its structure. The MKP-2 gene has four exons and three introns. The organization of exons of the MKP-2 gene is very similar to that of the MKP-1 gene, suggesting that MKP-1 and MKP-2 are derived from the same ancestral gene. We identified multiple transcription start sites (TSSs) for the MKP-2 gene. There is no functional TATA motif in the 5' proximal region of the TSSs. Instead, this region is highly GC-rich and has two putative Sp1 sites. A 1.8 kb 5' flanking region of the MKP-2 gene is sufficient to mediate transcriptional activation of the luciferase reporter gene by phorbol ester in GH3 cells. These results provide essential information about structural organization and regulatory sequences of the MKP-2 gene for further investigation of the molecular mechanisms of MKP-2 induction by extracellular stimuli.