The role of protein kinases A and C pathways in the regulation of mitogen-activated protein kinase activation in response to gonadotropin-releasing hormone receptor activation

C-Src is Activated by the EGF Receptor in a Pathway that Mediates JNK and ERK Activation by Gonadotropin-Releasing Hormone in COS7 Cells

The key participants in G-protein-coupled receptor (GPCR) signaling are the mitogen-activated protein kinase (MAPK) signaling cascades. The mechanisms involved in the activation of the above cascades by GPCRs are not fully elucidated. The prototypical GPCR is the receptor for gonadotropin-releasing hormone (GnRHR), which serves as a key regulator of the reproductive system. Here, we expressed GnRHR in COS7 cells and found that GnRHR transmits its signals to MAPKs mainly via Gαi and the EGF receptor, without the involvement of Hb-EGF or PKCs. The main pathway that leads to JNK activation downstream of the EGF receptor involves a sequential activation of c-Src and PI3K. ERK activation by GnRHR is mediated by the EGF receptor, which activates Ras either directly or via c-Src. Beside the main pathway, the dissociated Gβγ and β-arrestin may initiate additional (albeit minor) pathways that lead to MAPK activation in the transfected COS7 cells. The pathways detected are significantly different from those in other GnRHR-bearing cells, indicating that GnRH can utilize various signaling mechanisms for MAPK activation. The unique pathway elucidated here, in which c-Src and PI3K are sequentially activated downstream of the EGF receptor, may serve as a prototype of signaling mechanisms by GnRHR and additional GPCRs in various cell types.

Molecular Mechanisms of Gonadotropin-Inhibitory Hormone (GnIH) Actions in Target Cells and Regulation of GnIH Expression

Since gonadotropin-inhibitory hormone (GnIH) was discovered in 2000 as the first hypothalamic neuropeptide that actively inhibits gonadotropin release, researches conducted for the last 18 years have demonstrated that GnIH acts as a pronounced negative regulator of reproduction. Inhibitory effect of GnIH on reproduction is mainly accomplished at hypothalamic-pituitary levels; gonadotropin-releasing hormone (GnRH) neurons and gonadotropes are major targets of GnIH action based on the morphological interaction with GnIH neuronal fibers and the distribution of GnIH receptor. Here, we review molecular studies mainly focusing on the signal transduction pathway of GnIH in target cells, GnRH neurons, and gonadotropes. The use of well-defined cellular model systems allows the mechanistic study of signaling pathway occurring in target cells by demonstrating the direct cause-and-effect relationship. The insights gained through studying molecular mechanism of GnIH action contribute to deeper understanding of the mechanism of how GnIH communicates with other neuronal signaling systems to control our reproductive function. Reproductive axis closely interacts with other endocrine systems, thus GnIH expression levels would be changed by adrenal and thyroid status. We also briefly review molecular studies investigating the regulatory mechanisms of GnIH expression to understand the role of GnIH as a mediator between adrenal, thyroid and gonadal axes.

A randomised, placebo-controlled, multicentre, Phase 2 clinical trial to evaluate the efficacy and safety of GV1001 in patients with benign prostatic hyperplasia

OBJECTIVES

To evaluate the efficacy and safety of three dosing schemes of GV1001 in patients with benign prostatic hyperplasia (BPH).

PATIENTS AND METHODS

Eligible patients were men aged ≥50 years, with an International Prostate Symptom Score (IPSS) of ≥13, maximum urinary flow rate (Q_max ) of 5-15 mL/s, post-void residual urine volume (PVR) of ≤200 mL, and prostate volume of ≥30 mL. After a 4 week run-in period, patients were randomly assigned to one of three treatment schedules: Group 1, GV1001 0.4 mg, 2-week interval; Group 2, GV1001 0.56 mg, 2-week interval; Group 3, GV1001 0.56 mg, 4-week interval) or placebo (Group 4). The eligible patients were administered GV1001 or placebo, for a total of seven intradermal injections that were administered at 2-week intervals at weeks 0, 2, 4, 6, 8, 10, and 12. Treatment continued for 12 weeks, and efficacy was evaluated at weeks 4, 8, 12, 13, and 16. Safety was evaluated throughout the 16-week period. The primary efficacy variable was change from baseline (CFB) in total IPSS. Secondary endpoints were CFB in Q_max , PVR, prostate volume, International Index of Erectile Function score, plasma testosterone level, dihydrotestosterone level, and prostate-specific antigen level.

RESULTS

A total of 161 patients were included (Group 1, n = 41; Groups 2-4, n = 40). Most patients (88.8%) received all planned doses of the study treatment. At week 13, a statistically significant difference in the mean CFB in IPSS was seen in GV1001 treatment Groups 1 and 2 vs the control group for the full analysis population (-3.5 [control] vs -7.2 and -6.8 in Groups 1 and 2, respectively; both P < 0.05). There were also statistically significant differences in CFB at weeks 8, 12, 13, and 16 in treatment Groups 1 and 2 vs control in the per-protocol population. There was a statistically significant reduction in prostate gland volume at week 16 vs control in all treatment groups (0.8 [control] vs -4.6, -2.5, and -4.2 mL in Groups 1-3, respectively; all P < 0.05). There were no statistically significant differences found in other secondary outcome measures. Adverse event (AE) reporting was similar across all four groups. No treatment-emergent AEs were considered to be related to the study drug.

CONCLUSIONS

The results indicate that GV1001 was effective and well tolerated, and may provide potential beneficial effects in patients with BPH. Compared with medical therapies that require daily dosing, the convenient dosing regimen of GV1001 may provide greater patient adherence. Further investigation of these observations will require large-scale clinical evaluation.

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.

Clinical Applications of Gonadotropins in the Male

The pituitary gonadotropins, luteinizing hormone (LH), and follicle-stimulating hormone (FSH) play a pivotal role in reproduction. The synthesis and secretion of gonadotropins are regulated by complex interactions among several endocrine, paracrine, and autocrine factors of diverse chemical structure. In men, LH regulates the synthesis of androgens by the Leydig cells, whereas FSH promotes Sertoli cell function and thereby influences spermatogenesis. Gonadotropins are complex molecules composed of two subunits, the α- and β-subunit, that are noncovalently associated. Gonadotropins are decorated with glycans that regulate several functions of the protein including folding, heterodimerization, stability, transport, conformational maturation, efficiency of heterodimer secretion, metabolic fate, interaction with their cognate receptor, and selective activation of signaling pathways. A number of congenital and acquired abnormalities lead to gonadotropin deficiency and hypogonadotropic hypogonadism, a condition amenable to treatment with exogenous gonadotropins. Several natural and recombinant preparations of gonadotropins are currently available for therapeutic purposes. The difference between natural and the currently available recombinant preparations (which are massively produced in Chinese hamster ovary cells for commercial purposes) mainly lies in the abundance of some of the carbohydrates that conform the complex glycans attached to the protein core. Whereas administration of exogenous gonadotropins in patients with isolated congenital hypogonadotropic hypogonadism is a well recognized therapeutic approach, their role in treating men with normogonadotropic idiopathic infertility is still controversial. This chapter concentrates on the main structural and functional features of the gonadotropin hormones and how basic concepts have been translated into the clinical arena to guide therapy for gonadotropin deficit in males.

GnRH pulse frequency-dependent differential regulation of LH and FSH gene expression

The pituitary gonadotropin hormones, FSH and LH, are essential for fertility. Containing an identical α-subunit (CGA), they are comprised of unique β-subunits, FSHβ and LHβ, respectively. These two hormones are regulated by the hypothalamic decapeptide, GnRH, which is released in a pulsatile manner from GnRH neurons located in the hypothalamus. Varying frequencies of pulsatile GnRH stimulate distinct signaling pathways and transcriptional machinery after binding to the receptor, GnRHR, on the cell surface of anterior pituitary gonadotropes. This ligand-receptor binding and activation orchestrates the synthesis and release of FSH and LH, in synergy with other effectors of gonadotropin production, such as activin, inhibin and steroids. Current research efforts aim to discover the mechanisms responsible for the decoding of the GnRH pulse signal by the gonadotrope. Modulating the response to GnRH has the potential to lead to new therapies for patients with altered gonadotropin secretion, such as those with hypothalamic amenorrhea or polycystic ovarian syndrome.

Disruption of parathyroid hormone and parathyroid hormone-related peptide receptor phosphorylation prolongs ERK1/2 MAPK activation and enhances c-fos expression

Previous studies have demonstrated that parathyroid hormone (PTH) binding to the PTH/PTH-related peptide receptor (PPR) stimulates G protein coupling, receptor phosphorylation, β-arrestin translocation, and internalization of the ligand/receptor complex. The extracellular signal-regulated mitogen-activated protein kinases 1/2 (ERK1/2 MAPK) are downstream effectors of PPR. In the current study, we investigated the role of PPR phosphorylation in the PTH regulation of the ERK1/2 MAPK pathway. Short treatment with PTH (0-40 min) of LLCP-K(1) cells stably expressing a wild-type (WT) or a phosphorylation-deficient (PD) PPR (WT-PPR or PD-PPR cells, respectively) results in similar activation of ERK1/2. Interestingly, PTH stimulation of ERK1/2 in the WT-PPR cells then decreases as a result of longer PTH (60 min) treatment, and inhibition of ERK1/2 by PTH is observed at 90 min. Strikingly, the PD-PPR cells exhibit prolonged ERK1/2 activation up to 90 min of PTH treatment. An ERK1/2-dependent increase in c-fos expression is observed in the PD-PPR cells. Subsequently, c-fos expression in the WT-PPR and PD-PPR cells was markedly attenuated by a specific ERK1/2 pathway inhibitor. Further investigations revealed that PTH treatment causes a robust recruitment of a green fluorescent protein-tagged β-arrestin2 (β-arrestin2-GFP) in the WT-PPR cells. In contrast, β-arrestin2 recruitment was reduced in the PD-PPR cells. Importantly, expression of a receptor phosphorylation-independent β-arrestin2 (R169E) in the PD-PPR cells restored the biphasic effect of PTH on ERK1/2 as in the WT-PPR cells. The study reports a novel role for receptor phosphorylation and β-arrestin2 in the subsequent inhibition of the ERK1/2 pathway and in control of gene expression.

Gonadotropin-inhibitory hormone inhibits GnRH-induced gonadotropin subunit gene transcriptions by inhibiting AC/cAMP/PKA-dependent ERK pathway in LβT2 cells

A neuropeptide that directly inhibits gonadotropin secretion from the pituitary was discovered in quail and named gonadotropin-inhibitory hormone (GnIH). The presence and functional roles of GnIH orthologs, RF-amide-related peptides (RFRP), that possess a common C-terminal LPXRF-amide (X = L or Q) motif have also been demonstrated in mammals. GnIH orthologs inhibit gonadotropin synthesis and release by acting on pituitary gonadotropes and GnRH neurons in the hypothalamus via its receptor (GnIH receptor). It is becoming increasingly clear that GnIH is an important hypothalamic neuropeptide controlling reproduction, but the detailed signaling pathway mediating the inhibitory effect of GnIH on target cells is still unknown. In the present study, we investigated the pathway of GnIH cell signaling and its possible interaction with GnRH signaling using a mouse gonadotrope cell line, LβT2. First, we demonstrated the expression of GnIH receptor mRNA in LβT2 cells by RT-PCR. We then examined the inhibitory effects of mouse GnIH orthologs [mouse RFRP (mRFRP)] on GnRH-induced cell signaling events. We showed that mRFRP effectively inhibited GnRH-induced cAMP signaling by using a cAMP-sensitive reporter system and measuring cAMP levels, indicating that mRFRP function as an inhibitor of adenylate cyclase. We further showed that mRFRP inhibited GnRH-stimulated ERK phosphorylation, and this effect was mediated by the inhibition of the protein kinase A pathway. Finally, we demonstrated that mRFRP inhibited GnRH-stimulated gonadotropin subunit gene transcriptions and also LH release. Taken together, the results indicate that mRFRP function as GnIH to inhibit GnRH-induced gonadotropin subunit gene transcriptions by inhibiting adenylate cyclase/cAMP/protein kinase A-dependent ERK activation in LβT2 cells.

Mechanisms underlying the tissue-specific and regulated activity of the Gnrhr promoter in mammals

The GnRH receptor (GnRHR) plays a central role in the development and maintenance of reproductive function in mammals. Following stimulation by GnRH originating from the hypothalamus, GnRHR triggers multiple signaling events that ultimately stimulate the synthesis and the periodic release of the gonadotropins, luteinizing-stimulating hormone (LH) and follicle-stimulating hormones (FSH) which, in turn, regulate gonadal functions including steroidogenesis and gametogenesis. The concentration of GnRHR at the cell surface is essential for the amplitude and the specificity of gonadotrope responsiveness. The number of GnRHR is submitted to strong regulatory control during pituitary development, estrous cycle, pregnancy, lactation, or after gonadectomy. These modulations take place, at least in part, at the transcriptional level. To analyze this facet of the reproductive function, the 5' regulatory sequences of the gene encoding the GnRHR have been isolated and characterized through in vitro and in vivo approaches. This review summarizes results obtained with the mouse, rat, human, and ovine promoters either by transient transfection assays or by means of transgenic mice.

Signaling responses to pulsatile gonadotropin-releasing hormone in LbetaT2 gonadotrope cells

The hypothalamic neuropeptide gonadotropin-releasing hormone (GnRH) is secreted in a pulsatile fashion by hypothalamic neurons, and alterations in pulse frequency and amplitude differentially regulate gonadotropin synthesis and release. In this study, we investigated the kinetics of G(s) and G(q) signaling in response to continuous or pulsatile GnRH using fluorescence resonance energy transfer reporters in live mouse LbetaT2 gonadotrope cells. cAMP and protein kinase A-dependent reporters showed a rapid but transient increase in fluorescence resonance energy transfer signal with increasing doses of constant GnRH, and in contrast diacylglycerol (DAG) and calcium reporters showed a rapid and sustained signal. Multiple pulses of GnRH caused multiple pulses of cAMP and protein kinase A activation without desensitization, but the DAG and calcium reporters were rapidly desensitized resulting in inhibition of calcium and DAG responses. At the transcriptional level, both a cAMP-dependent cAMP-response element reporter and a DAG/calcium-dependent AP-1 reporter showed a pulse frequency-dependent increase in luciferase activity. However, constant GnRH stimulation gave very little cAMP-response element activation but very strong AP-1 activation. Based on these data, we propose that both the GnRH-R-G(s) and G(q) pathways are responsive to pulses of GnRH, but only the G(q) pathway is responsive to constant GnRH. Furthermore, the G(q) pathway is subject to desensitization with multiple GnRH pulses, but the G(s) pathway is not.

Extracellular signal-regulated kinase mediates gonadotropin subunit gene expression and LH release responses to endogenous gonadotropin-releasing hormones in goldfish

The possible involvement of extracellular signal-regulated kinase (ERK) in mediating the stimulatory actions of two endogenous goldfish gonadotropin-releasing hormones (salmon (s)GnRH and chicken (c)GnRH-II) on gonadotropin synthesis and secretion was examined. Western blot analysis revealed the presence of ERK and phosphorylated (p)ERK in goldfish brain, pituitary, liver, ovary, testis and muscle tissue extracts, as well as extracts of dispersed goldfish pituitary cells and HeLa cells. Interestingly, a third ERK-like immunoreactive band of higher molecular mass was detected in goldfish tissue and pituitary cell extracts in addition to the ERK1-p44- and ERK2-p42-like immunoreactive bands. Incubation of primary cultures of goldfish pituitary cells with either a PKC-activating 4beta-phorbol ester (TPA) or a synthetic diacylglycerol, but not a 4alpha-phorbol ester, elevated the ratio of pERK/total (t)ERK for all three ERK isoforms. The stimulatory effects of TPA were attenuated by the PKC inhibitor GF109203X and the MEK inhibitor PD98059. sGnRH and cGnRH-II also elevated the ratio of pERK/tERK for all three ERK isoforms, in a time-, dose- and PD98059-dependent manner. In addition, treatment with PD98059 reduced the sGnRH-, cGnRH-II- and TPA-induced increases in gonadotropin subunit mRNA levels in Northern blot studies and sGnRH- and cGnRH-II-elicited LH release in cell column perifusion studies with goldfish pituitary cells. These results indicate that GnRH and PKC can activate ERK through MEK in goldfish pituitary cells. More importantly, the present study suggests that GnRH-induced gonadotropin subunit gene expression and LH release involve MEK/ERK signaling in goldfish.

Welcoming beta-catenin to the gonadotropin-releasing hormone transcriptional network in gonadotropes

GnRH binds its G-coupled protein receptor, GnRHR, on pituitary gonadotropes and stimulates transcription of Cga, Lhb, and Fshb. These three genes encode two heterodimeric glycoprotein hormones, LH and FSH, that act as gonadotropins by regulating gametogenesis and steroidogenesis in both the testes and ovary. GnRH also regulates transcription of Gnrhr. Thus, regulated expression of Cga, Lhb, Fshb, and Gnrhr provides a genomic signature unique to functional gonadotropes. Steadily increasing evidence now indicates that GnRH regulates transcription of its four signature genes indirectly through a hierarchical transcriptional network that includes distinct subclasses of DNA-binding proteins that comprise the immediate early gene (IEG) family. These IEGs, in turn, confer hormonal responsiveness to the four signature genes. Although the IEGs confer responsiveness to GnRH, they cannot act alone. Instead, additional DNA-binding proteins, including the orphan nuclear receptor steroidogenic factor 1, act permissively to allow the four signature genes to respond to GnRH-induced changes in IEG levels. Emerging new findings now indicate that beta-catenin, a transcriptional coactivator and member of the canonical WNT signaling pathway, also plays an essential role in transducing the GnRH signal by interacting with multiple DNA-binding proteins in gonadotropes. Herein we propose that these interactions with beta-catenin define a multicomponent transcriptional network required for regulated expression of the four signature genes of the gonadotrope, Cga, Lhb, Fshb, and Gnrhr.

Differential neurotrophic regulation of sodium and calcium channels in an adult sympathetic neuron

Adult neuronal phenotype is maintained, at least in part, by the sensitivity of individual neurons to a specific selection of neurotrophic factors and the availability of such factors in the neurons' environment. Nerve growth factor (NGF) increases the functional expression of Na(+) channel currents (I(Na)) and both N- and L-type Ca(2+) currents (I(Ca,N) and I(Ca,L)) in adult bullfrog sympathetic ganglion (BFSG) B-neurons. The effects of NGF on I(Ca) involve the mitogen-activated protein kinase (MAPK) pathway. Prolonged exposure to the ganglionic neurotransmitter luteinizing hormone releasing hormone (LHRH) also increases I(Ca,N) but the transduction mechanism remains to be elucidated as does the transduction mechanism for NGF regulation of Na(+) channels. We therefore exposed cultured BFSG B-neurons to chicken II LHRH (0.45 microM; 6-9 days) or to NGF (200 ng/ml; 9-10 days) and used whole cell recording, immunoblot analysis, and ras or rap-1 pulldown assays to study effects of various inhibitors and activators of transduction pathways. We found that 1) LHRH signals via ras-MAPK to increase I(Ca,N), 2) this effect is mediated via protein kinase C-beta (PKC-beta-IotaIota), 3) protein kinase A (PKA) is necessary but not sufficient to effect transduction, 4) NGF signals via phosphatidylinositol 3-kinase (PI3K) to increase I(Na), and 5) long-term exposure to LHRH fails to affect I(Na). Thus downstream signaling from LHRH has access to the ras-MAPK pathway but not to the PI3K pathway. This allows for differential retrograde and anterograde neurotrophic regulation of sodium and calcium channels in an adult sympathetic neuron.

Dependence of GnRH-induced phosphorylation of CREB and BAD on EGF receptor transactivation in GT1-7 neuronal cells

The hypothalamic neuropeptide, gonadotropin releasing hormone (GnRH), is a primary regulatory factor in the neuroendocrine control of reproduction. The GnRH decapeptide is released in an episodic manner from hypothalamic GnRH neurons, which are known to express GnRH receptors. Here we examined the signaling pathways by which autocrine GnRH stimulation generates cell survival and proliferative signals in hypothalamic GT1-7 cells. Both GnRH and epidermal growth factor (EGF) caused rapid phosphorylation of cyclic AMP response element binding protein (CREB) and BAD. The selective epidermal growth factor receptor (EGF-R) antagonist, AG1478, attenuates the phosphorylation of these proteins by GnRH and EGF. Inhibition of PKC and Src abolished the stimulatory effects of GnRH, but not that of EGF, consistent with a critical role of these signaling molecules upstream of the EGF-R. All of these effects of GnRH were mimicked by phorbol 12 myristate 13-acetate (PMA). Consistent with the prosurvival and mitogenic effects of phosphoinositide 3-kinase/Akt (P13-K/Akt) downstream of the EGF-R, inhibition of P13-K diminished the activation of these proteins following stimulation with GnRH, EGF, and PMA. Overexpression of dominant negative Akt attenuated agonist-induced phosphorylation of BAD, but not that of ERK1/2 and CREB. Moreover, overexpression of wild-type RSK-1 resulted in enhanced basal as well as agonist-induced phosphorylation of CREB and BAD, indicating a critical role of RSK-1 in activating cytosolic as well as nuclear proteins. These data reveal novel signaling mechanisms of GnRH-induced phosphorylation of CREB and BAD in GT1-7 neurons through transactivation of the EGF-R.

TCDD causes suppression of growth and differentiation of MCF10A, human mammary epithelial cells by interfering with their insulin receptor signaling through c-Src kinase and ERK activation

One of the proposed mechanisms of carcinogenic action of TCDD (=dioxin) on breast cells is that it causes significant inhibition of proper differentiation of mammary duct epithelial cells and thereby increases the number of terminal end buds, which are susceptible to other carcinogens (Fenton et al., Toxicol Sci 2002;67:63-74; Brown et al., Carcinogenesis 1998; 19:1623-1629; Lamartiniere, J Mammary Gland Biol Neoplasia 2002;7:67-76). To address this topic, we selected MCF10A, a line of immortalized normal human breast epithelial cells as an in vitro model. An initial effort was made to optimize the cultural condition of MCF10A cells to promote the cell differentiation effect of insulin. Under this condition, TCDD clearly antagonized the action of insulin only in the presence of cholera toxin that is known to promote the differentiation of normal human breast epithelial cells. To test the hypothesis that TCDD-induced c-Src kinase activation is casually related to this compound's antagonistic action against insulin, we treated MCF10A cells with two c-Src blocking agents, an anti-Src antisense oligonucleotides blocker and a known specific inhibitor of c-Src kinase, PP-2 and studied the effect of insulin and TCDD on cell proliferation. The results showed that, in cells treated with either of these two c-Src blocking agents, the antagonistic effect of TCDD disappeared. It was also found that agents which specifically block the activation of ERK could also abrogate the action of TCDD to suppress insulin signaling. Together, these results indicate that the mechanism of the antagonistic action of TCDD on insulin signaling is mainly mediated through c-Src signaling through activation of ERK.

Gonadotropin-releasing hormone initiates multiple signaling pathways in human GH-secreting adenomas

Abnormal GH responses to GnRH test, observed in about 15% of patients with acromegaly, have been reported exclusively in patients bearing tumors without gsp mutation. The absence of responsiveness to GnRH in gsp+ tumors was not predicted on the basis of the mechanism of GnRH action that mainly involves the activation of calcium and protein kinase C dependent pathways. The aim of the present study was to investigate in detail the transduction of GnRH signaling in these tumors. GH-secreting adenomas removed from patients in vivo responsive to GnRH test were studied. Tumor DNA was screened for Gsalpha and GnRH receptor gene sequences. Intracellular calcium ([Ca2+]i) and cAMP levels were measured in dispersed cells and adenylyl cyclase (AC) activity in membrane preparations. DNA analysis showed wild sequence of both Gsalpha and GnRH receptor genes. GnRH caused a significant increase in intracellular Ca2+ that was associated with a significant stimulation of cAMP accumulation. In these cells neither TRH nor GHRP-6 were effective in causing significant modifications of cAMP levels, despite their ability to increase [Ca2+]i. Finally, GnRH was able to directly stimulate AC from 11.1 +/- 3.3 pmol/mg prot/min to 26.9 +/- 5.4 (p<0.005). We report that GnRH was effective in increasing both [Ca2+]i and AC in GH-secreting adenomas removed from responsive patients. The ability of GnRH to signal through Gsalpha protein may account for the lack of GH responses to GnRH observed in acromegalic patients with tumors carrying gsp mutation.

Neurotrophic regulation of calcium channels by the peptide neurotransmitter luteinizing hormone releasing hormone

We exploited the simple organization of bullfrog paravertebral sympathetic ganglia (BFSG) to test whether the neurotransmitter peptide luteinizing hormone releasing hormone (LHRH), which generates the late slow EPSP, could also exert long-term neurotrophic control of ion channel expression. Whole-cell recordings from B-cells in BFSG showed that removal of all of the sources of ganglionic LHRH for 10 d by cutting preganglionic C-fibers in vivo caused a 28% reduction in Ca2+ current density. When BFSG B-neurons were dissociated from adult bullfrogs and maintained in a defined-medium, neuron-enriched, low-density, serum-free culture, the ICa density was increased by 49% after 6-7 d in the presence of 0.45 microm LHRH. This increase was not associated with alterations in the voltage dependence of Ca2+ current activation or inactivation and reflected a selective increase in N-type Ca2+ channel current. The increase in ICa density induced by LHRH was blocked by the transcription inhibitor actinomycin D. These results suggest that chronic exposure to a neurotransmitter that acts through G-protein-coupled receptors exerts long-term control of ion channel expression in a fully differentiated, adult sympathetic neuron in vitro or in vivo.

c-Src is activated by the epidermal growth factor receptor in a pathway that mediates JNK and ERK activation by gonadotropin-releasing hormone in COS7 cells

Key participants in G protein-coupled receptor (GPCR) signaling are the mitogen-activated protein kinase (MAPK) signaling cascades. The mechanisms involved in the activation of the above cascades by GPCRs are not fully elucidated. A prototypic GPCR that has been widely used to study these signaling mechanisms is the receptor for gonadotropin-releasing hormone (GnRHR), which serves as a key regulator of the reproductive system. Here we expressed GnRHR in COS7 cells and found that GnRHR transmits its signals to MAPKs mainly via G alpha i, EGF receptor without the involvement of Hb-EGF, and c-Src, but independently of PKCs. The main pathway that leads to JNK activation downstream of the EGF receptor involves a sequential activation of c-Src and phosphatidylinositol 3-kinase (PI3K). ERK activation by GnRHR is mediated by the EGF receptor, which activates Ras either directly or via c-Src. Besides the main pathway, the dissociated G beta gamma and beta-arrestin may initiate additional, albeit minor, pathways that lead to MAPK activation in the transfected COS7 cells. The pathways detected are significantly different from those in other cell lines bearing GnRHR, indicating that GnRH can utilize various signaling mechanisms for the activation of MAPK cascades. The unique pathway elucidated here in which c-Src and PI3K are sequentially activated downstream of the EGF receptor may serve as a prototype of signaling mechanisms by GnRHR and by additional GPCRs in various cell types.

Activation and nuclear translocation of PKCdelta, Pyk2 and ERK1/2 by gonadotropin releasing hormone in HEK293 cells

The mechanism of agonist-induced activation of Pyk2 and its relationship with ERK1/2 phosphorylation was analyzed in HEK293 cells stably expressing the gonadotropin releasing hormone (GnRH) receptor. GnRH stimulation caused rapid and sustained phosphorylation of ERK1/2 and Pyk2 that was accompanied by their nuclear translocation. Pyk2 was also localized on cell membranes and at focal adhesions. Dominant negative Pyk2 (PKM) had no effect on GnRH-induced ERK1/2 phosphorylation and c-fos expression. These actions of GnRH on ERK1/2 and Pyk2 were mimicked by activation of protein kinase C (PKC) and were abolished by its inhibition. GnRH caused translocation of PKCalpha and delta, but not of epsilon, iota and lambda, to the cell membrane, as well as phosphorylation of Raf at Ser338, a major site in the activation of MEK/ERK1/2. Stimulation of HEK293 cells by EGF caused marked ERK1/2 phosphorylation that was attenuated by the selective EGFR receptor (EGF-R) kinase inhibitor, AG1478. However, GnRH-induced ERK1/2 activation was independent of EGF-R activation. These results indicate that activation of PKC is responsible for GnRH-induced phosphorylation of both ERK1/2 and Pyk2, and that Pyk2 activation does not contribute to GnRH signaling. Moreover, GnRH-induced phosphorylation of ERK1/2 and expression of c-fos in HEK293 cells is independent of Src and EGF-R transactivation, and is mediated through the PKC/Raf/MEK cascade.

Identification of reciprocally regulated gene modules in regenerating dorsal root ganglion neurons and activated peripheral or central nervous system glia

Differential gene expression in the rat after injury of dorsal root ganglion neurons in vivo, and simulation injury of Schwann cells and oligodendrocytes in vitro was analyzed using high-density cDNA microarrays. The analyses were carried out to study the genetic basis of peripheral nerve regeneration, and to compare gene regulation in glia of the central (oligodendrocyte) and peripheral (Schwann cell) nervous systems. The genes showing significant differential regulation in the three study groups represented all aspects of cellular metabolism. However, two unexpected observations were made. Firstly, a number of identical genes were differentially regulated in activated Schwann cells, activated oligodendrocytes and regenerating DRG neurons. Specifically, a group of 113 out of 210 genes that were down-regulated in Schwann cells upon lipopolysaccharide (LPS) treatment, were identical to genes up-regulated in the injured, regenerating DRG. Furthermore, a group of 53 out of 71 genes that were down-regulated in interferon gamma (IFN-gamma)/LPS-activated oligodendrocytes, were identical to genes up-regulated in the DRG neurons. Finally, 22 genes were common to these three groups, i.e., down-regulated in activated oligodendrocytes, down-regulated in activated Schwann cells, and up-regulated in regenerating DRG neurons. Secondly, a group of 16 cell-cycle and proliferation-related genes were up-regulated in the DRG following sciatic nerve crush, despite the absence of cells undergoing mitosis in the DRG, or any significant presence of apoptosis-related gene expression. Therefore, it appears that in these three cell types, large sets of genes are reciprocally regulated upon injury and/or activation. This suggests that the activation of the injury-related gene expression program in cell derivatives of the neuroectoderm involves, in part, highly conserved genetic elements.

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

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

Ghrelin and growth hormone secretagogue receptor are expressed in the rat adrenal cortex: Evidence that ghrelin stimulates the growth, but not the secretory activity of adrenal cells

Ghrelin is an endogenous ligand of the growth hormone secretagogue receptor (GHS-R), which has been originally isolated from rat stomach. Evidence has been previously provided that adrenal gland possesses abundant ghrelin-displaceable GHS-Rs, but nothing is known about the possible role of ghrelin in the regulation of adrenocortical function. Reverse transcription-polymerase chain reaction demonstrated the expression of ghrelin and GHS-R in the rat adrenal cortex, and high adrenal concentrations of immunoreactive ghrelin were detected by radioimmune assay (RIA). Autoradiography localized abundant [(125)I]ghrelin binding sites in the adrenal zona glomerulosa (ZG) and outer zona fasciculata (ZF). Ghrelin (from 10(-10) to 10(-8) M) did not affect either basal steroid hormone (pregnenolone, progesterone, 11-deoxycorticosterone, corticosterone, 18-hydroxycorticosterone and aldosterone) secretion from dispersed ZG and zona fasciculata/reticularis (ZF/R) cells (as evaluated by quantitative high pressure liquid chromatography), or basal and agonist-stimulated aldosterone and corticosterone production from cultured ZG and ZF/R cells, respectively (as measured by RIA). Ghrelin (10(-8) and 10(-6) M) raised basal, but not agonist-stimulated, proliferation rate of cultured ZG cells (percent of cells able to incorporate 5-bromo-2'-deoxyuridine), without affecting apoptotic deletion rate (percent of cells able to incorporate biotinylated nucleosides into apoptotic DNA fragments). The tyrosine kinase (TK) inhibitor tyrphostin-23 and the p42/p44 mitogen-activated protein kinase (MAPK) inhibitor PD-98059 abolished the proliferogenic effect of 10(-8) M ghrelin, while the protein kinase A and C inhibitors H-89 and calphostin-C were ineffective. Ghrelin (10(-8) M) stimulated TK and MAPK activity of dispersed ZG cells, and the effect was abolished by preincubation with tyrphostin-23 and PD-98059, respectively. Tyrphostin-23 annulled ghrelin-induced activation of MAPK activity. Taken together, the present findings indicate that (i) ghrelin and GHS-R are both expressed in the rat adrenal cortex, ghrelin binding sites being very abundant in the ZG; (ii) ghrelin does not affect the secretory activity of rat adrenocortical cells, but significantly enhances the proliferation rate of cultured ZG cells, without affecting apoptotic deletion rate; and (iii) the ZG proliferogenic action of ghrelin involves the TK-dependent activation of the p42/p44 MAPK cascade.

Dependence of gonadotropin-releasing hormone-induced neuronal MAPK signaling on epidermal growth factor receptor transactivation

The hypothalamic decapeptide, gonadotropin-releasing hormone (GnRH), utilizes multiple signaling pathways to activate extracellularly regulated mitogen-activated protein kinases (ERK1/2) in normal and immortalized pituitary gonadotrophs and transfected cells expressing the GnRH receptor. In immortalized hypothalamic GnRH neurons (GT1-7 cells), which also express GnRH receptors, GnRH, epidermal growth factor (EGF), and phorbol 12-myristate 13-acetate (PMA) caused marked phosphorylation of ERK1/2. This action of GnRH and PMA, but not that of EGF, was primarily dependent on activation of protein kinase C (PKC), and the ERK1/2 responses to all three agents were abolished by the selective EGF receptor kinase inhibitor, AG1478. Consistent with this, both GnRH and EGF increased tyrosine phosphorylation of the EGF receptor. GnRH and PMA, but not EGF, caused rapid phosphorylation of the proline-rich tyrosine kinase, Pyk2, at Tyr(402). This was reduced by Ca(2+) chelation and inhibition of PKC, but not by AG1478. GnRH stimulation caused translocation of PKC alpha and -epsilon to the cell membrane and enhanced the association of Src with PKC alpha and PKC epsilon, Pyk2, and the EGF receptor. The Src inhibitor, PP2, the C-terminal Src kinase (Csk), and dominant-negative Pyk2 attenuated ERK1/2 activation by GnRH and PMA but not by EGF. These findings indicate that Src and Pyk2 act upstream of the EGF receptor to mediate its transactivation, which is essential for GnRH-induced ERK1/2 phosphorylation in hypothalamic GnRH neurons.

Inhibition of the MEK/ERK signaling pathway by the novel antimetastatic agent NAMI-A down regulates c-myc gene expression and endothelial cell proliferation

Imidazolium trans-imidazoledimethyl sulfoxide-tetrachlororuthenate (NAMI-A) is a novel ruthenium-containing experimental antimetastatic agent. Compelling evidence ascribes a pivotal role to endothelial cells in the orchestration of tumor angiogenesis and metastatic growth, suggesting antiangiogenic therapy as an attractive approach for anticancer treatment. In this context, activation of the mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) signaling pathway has been found fundamental in transducing extracellular stimuli that modulate a number of cellular process including cell proliferation, migration and invasion. Here we show that exposure of the transformed endothelial cell line ECV304 to NAMI-A significantly inhibited DNA synthesis, as well as the expression of the proliferating cell nuclear antigene (PCNA). These responses were associated with a marked down-regulation of ERK phosphorylation in serum-cultured cells. In addition, NAMI-A markedly reduced serum stimulated- and completely suppressed phorbol 12-myristate 13-acetate (PMA)-triggered MAPK/ERK kinase activity. NAMI-A was also able to inhibit the phosphorylation of MEK, the upstream activator of ERK, and, similar to both the protein kinase C (PKC) inhibitor GF109203X and the MAPK/ERK (MEK) inhibitor PD98059, it completely counteracted PMA-induced ERK phosphorylation. Finally, NAMI-A and PD98059 down regulated c-myc gene expression to the same extent in serum-cultured cells and dose-dependently counteracted, and ultimately abolished, the increase in c-myc gene expression elicited by PMA in serum-free cells. These results suggest that inhibition of MEK/ERK signaling by NAMI-A may have an important role in modulating c-myc gene expression and ECV304 proliferation.

Therapeutic potential of GnRH antagonists in the treatment of precocious puberty

Pituitary-gonadal axis activation depends upon pulsatile hypothalamic gonadotropin-releasing hormone (GnRH) secretion. This phenomenon has led to clinical use of GnRH agonists in the treatment of central precocious puberty. GnRH analogues contain substitutions of the native decapeptide. Depending upon the substitutions, the analogues have GnRH agonistic or antagonistic properties. The pharmacokinetics of GnRH agonists, the established treatment of precocious puberty, includes an initial 'flare-up' of the pituitary-gonadal axis, followed by a reduced luteinising hormone secretion by desensitisation of pituitary GnRH receptors. Antagonistic GnRH analogues act by competitive binding to the pituitary GnRH receptors, thereby preventing the action of endogenous GnRH - theoretically offering a more direct and dose-dependent treatment alternative. The antagonist available today in Germany is a concomitant in assisted reproduction with only 1 - 3 days duration. However, long-acting depot preparations of other GnRH antagonists are in primate-testing phase. Our animal tests indicate strong potential for the development and testing of long-acting depot preparations of GnRH antagonists in treating precocious puberty.

Involvement of both G(q/11) and G(s) proteins in gonadotropin-releasing hormone receptor-mediated signaling in L beta T2 cells

The hypothalamic hormone gonadotropin-releasing hormone (GnRH) stimulates the synthesis and release of the pituitary gonadotropins. GnRH acts through a plasma membrane receptor that is a member of the G protein-coupled receptor (GPCR) family. These receptors interact with heterotrimeric G proteins to initiate downstream signaling. In this study, we have investigated which G proteins are involved in GnRH receptor-mediated signaling in L beta T2 pituitary gonadotrope cells. We have shown previously that GnRH activates ERK and induces the c-fos and LH beta genes in these cells. Signaling via the G(i) subfamily of G proteins was excluded, as neither ERK activation nor c-Fos and LH beta induction was impaired by treatment with pertussis toxin or a cell-permeable peptide that sequesters G beta gamma-subunits. GnRH signaling was partially mimicked by adenoviral expression of a constitutively active mutant of G alpha(q) (Q209L) and was blocked by a cell-permeable peptide that uncouples G alpha(q) from GPCRs. Furthermore, chronic activation of G alpha(q) signaling induced a state of GnRH resistance. A cell-permeable peptide that uncouples G alpha(s) from receptors was also able to inhibit ERK, c-Fos, and LH beta, indicating that both G(q/11) and G(s) proteins are involved in signaling. Consistent with this, GnRH caused GTP loading on G(s) and G(q/11) and increased intracellular cAMP. Artificial elevation of cAMP with forskolin activated ERK and caused a partial induction of c-Fos. Finally, treatment of G alpha(q) (Q209L)-infected cells with forskolin enhanced the induction of c-Fos showing that the two pathways are independent and additive. Taken together, these results indicate that the GnRH receptor activates both G(q) and G(s) signaling to regulate gene expression in L beta T2 cells.

Adrenomedullin in the adrenal

Adrenomedullin (AM) was originally characterized in extracts of an adrenal medullary tumor. Since this original finding the peptide and its mRNA have also been found in the adrenal cortex, specifically, in the cells of the aldosterone-secreting zona glomerulosa. It is clear that the synthesis of AM is actively regulated in both cortex and medulla. Much research effort has been focused on identifying a role for AM in the adrenal gland. To date, no consistent effect on medullary catecholamine biosynthesis has been demonstrated. In the cortex the actions of AM are controversial and appear to depend on both the tissue preparation used and on the specific receptor population expressed in the individual gland. The results of further studies on the long-term actions of AM on adrenal growth and differentiation are awaited with interest.

The nuclear receptor coactivators p300/CBP/cointegrator-associated protein (p/CIP) and transcription intermediary factor 2 (TIF2) differentially regulate PKA-stimulated transcriptional activity of steroidogenic factor 1

Steroidogenic factor-1 (SF-1) is a member of the nuclear receptor superfamily that plays essential roles in the development of endocrine organs. Steroid receptor coactivator 1 and transcription intermediary factor 2 (TIF2) belong to the p160 coactivator family that mediates transcriptional activation by several nuclear receptors, including SF-1. Here, it is reported that another of the p160 coactivators, p/CIP, interacts with SF-1 through the activation function-2 domain. Both p300/CBP/cointegrator-associated protein (p/CIP) and TIF2 potentiated SF-1-mediated transcription from two reporter gene constructs in transfected nonsteroidogenic COS-1 cells and in adrenocortical Y1 cells. PKA was shown to stimulate SF-1 transcriptional activity, and coexpression of p/CIP together with the PKA catalytic subunit stimulated SF-1-mediated transactivation even further. In contrast, PKA catalytic subunit overexpression impaired the ability of TIF2 to potentiate SF-1-dependent transcription. Activation of PKA also inhibited the TIF2-mediated coactivation of other nuclear receptors such as PPAR alpha/-gamma and liver X receptor-alpha. The TIF2 mRNA levels were not affected by PKA, but instead we found that PKA activation led to a decrease in the levels of TIF2 protein. Moreover, the C-terminal activation domain 2 of TIF2 was required for the inhibitory effect of PKA, suggesting that this region is the target for the PKA-mediated down-regulation. Thus, in contrast to the regulation of p/CIP and steroid receptor coactivator 1, we suggest that activation of PKA leads to selective down-regulation of TIF2 and subsequently repression of TIF2 coactivator function.

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.

Mitogenic G(i) protein-MAP kinase signaling cascade in MC3T3-E1 osteogenic cells: activation by C-terminal pentapeptide of osteogenic growth peptide [OGP(10-14)] and attenuation of activation by cAMP

In osteogenic and other cells the mitogen-activated protein (MAP) kinases have a key role in regulating proliferation and differentiated functions. The osteogenic growth peptide (OGP) is a 14 mer mitogen of osteogenic and fibroblastic cells that regulates bone turnover, fracture healing, and hematopoiesis, including the engraftment of bone marrow transplants. It is present in the serum and extracellular fluid either free or complexed to OGP-binding proteins (OGPBPs). The free immunoreactive OGP consists of the full length peptide and its C-terminal pentapeptide OGP(10-14). In the present study, designed to probe the signaling pathways triggered by OGP, we demonstrate in osteogenic MC3T3 E1 cells that mitogenic doses of OGP(10-14), but not OGP, enhance MAP kinase activity in a time-dependent manner. The OGP(10-14)-induced stimulation of both MAP kinase activity and DNA synthesis were abrogated by pertusis toxin, a G(i) protein inhibitor. These data offer direct evidence for the occurrence in osteogenic cells of a peptide-activated, mitogenic Gi protein-MAP kinase-signaling cascade. Forskolin and dBu(2)-cAMP abrogated the OGP(10-14)-stimulated proliferation, but induced only 50% inhibition of the OGP(10-14)-mediated MAP kinase activation, suggesting additional MAP kinase-dependent, OGP(10-14)-regulated, cellular functions. Finally, it is demonstrated that OGP(10-14) is the active form of OGP, apparently generated proteolytically in the extracellular milieu upon dissociation of OGP-OGPBP complexes.

Activation of extracellular signal-regulated kinases (ERK 44/42) by chlorpyrifos oxon in Chinese hamster ovary cells

Acetylcholinesterase inhibition explains most but not all of the toxicological manifestations of exposure to the major organophosphorus insecticide chlorpyrifos (CP) and its metabolically activated form chlorpyrifos oxon (CPO); CPO is also reported to interact with muscarinic acetylcholine receptors and alter secondary messenger status. We find that CP and CPO activate extracellular signal-regulated kinases (ERK 44/42) in both wild-type (CHOK1) and human muscarinic receptor-expressing Chinese hamster ovary cells (CHO-M2). The degree of ERK 44/42 activation on treatment with 50 microM CPO for 40 minutes is 2- to 3-fold compared with control cells and is both concentration- and time-dependent. CP is at least 2-fold less potent than CPO as an activator of ERK 44/42 and the hydrolysis products 3,5,6-trichloropyridinol and diethyl phosphate are not activators. ERK 44/42 activation by CPO is insensitive to the protein kinase A inhibitor H-89, but is completely abolished by the phosphatidylinositol 3-kinase (P13-K) inhibitor wortmannin, the protein kinase C (PKC) inhibitor GF-109203X, and the mitogen-activated extracellular signal-regulated protein kinase kinase (MEK) inhibitor PD 098059. Therefore, CPO activates the ERK 44/42 signaling cascade in CHOK1 cells via a pathway dependent on P13-K, PKC, and MEK but not requiring PKA or the human M2 muscarinic receptor. In summary we find that CPO activates a mammalian signal transduction cascade involved in cell growth and differentiation. This occurs through a pathway common to growth factors and mitogens, consistent with a receptor-mediated event. However, CPO may also inhibit an enzyme involved in signal transduction. The specific target of CPO leading to the activation of ERK 44/42 and the potential effects of this activation on cell function remain to be determined.

Signal transduction pathways and transcription factors involved in the gonadotropin-releasing hormone-stimulated gonadotropin subunit gene expression

Gonadotropin-releasing hormone (GnRH) stimulates gonadotropin (GTH) subunit gene expression via G protein-coupled membrane receptors. GnRH-stimulated GTH subunit gene expression is mediated by protein kinase C (PKC) and Ca(2+) signaling pathways. Recent numerous reports on signal transduction pathways which are involved in GnRH stimulation of mammalian GTH subunit genes showed differential sensitivity of GTH subunit genes to the two signaling pathways. Our recent studies on salmon GTH (sGTH) IIbeta subunit gene showed that its stimulation by GnRH is dependent on the PKC pathway. Furthermore, gel retardation and mutagenesis studies suggested that pituitary homeo box 1 (Ptx1) and Sp1 mediate the GnRH-induced PKC signaling on the sGTHIIbeta gene. However, both PKC and Ca(2+) pathways are involved in the GnRH-stimulated GTH alpha and LHbeta genes. Different preference to the pathways were often reported in a certain GTH subunit gene in different circumstances, suggesting that molecular targets of the two signaling pathways are different. Ets-related factor and cAMP response element binding protein have been proposed as targets of GnRH signaling on GTH alpha genes. Sp1 and early growth response protein 1 play pivotal roles in GnRH-stimulated LHbeta gene expression in synergism with steroidogenic factor-1 and Ptx1. Activating protein-1 mediates GnRH-induced PKC signaling to stimulate FSHbeta gene expression. Therefore, divergent transcription factors are involved in GnRH stimulation of GTH subunit gene expression, and molecular mechanisms of GnRH stimulation may be partially conserved between sGTH IIbeta and mammalian LHbeta genes.

Regulation of gonadotropin subunit genes in tilapia

A steroidogenic tilapia gonadotropin (taGtH=LH) was purified from pituitaries of hybrid tilapia (Oreochromis niloticus x O. aureus) and a homologous RIA was established. This RIA enabled the study of the endocrine regulation of GtH release, the transduction pathways involved in its secretion and its profile during the spawning cycle. Discrepancies between steroid and taGtH peaks during the cycle led to the conclusion that an additional gonadotropin similar to salmonid FSH operates early in the cycle. In order to identify this hormone and to study the endocrine control of synthesis of all gonadotropin (GtH) subunits, a molecular approach was taken. The cDNA sequences and the entire gene sequences encoding the FSHbeta and LHbeta subunits, as well as an incomplete sequence of the glycoprotein hormone alpha subunit (GPalpha), were cloned. Salmon gonadotropin-releasing hormone (sGnRH) elevated mRNA steady-state levels of all three GtH subunits in cultured pituitary cells. Pituitary adenylate cyclase-activating polypeptide (PACAP) and neuropeptide Y (NPY) also stimulated the expression of these subunits and potentiated the effect of GnRH, except that NPY did not affect FSHbeta. The GnRH and NPY effects were found to be mediated mainly through protein kinase C (PKC), while protein kinase A (PKA) cascade was involved to a lesser extent. Mitogen-activated protein kinase (MAPK) cascade takes part in mediating GnRH effects, possibly via PKC. Testosterone (T) and estradiol (E2), but not 11-ketotestosterone (KT), are able to elevate GPalpha and LHbeta mRNAs in pituitary cells of early maturing or regressing males. Low levels of T exposure are associated with elevated FSHbeta mRNA in cells of mature fish, while higher levels suppress it, but elevate LHbeta mRNA. In vivo observations also showed the association of low T levels with increased FSHbeta mRNA and high T levels with elevated LHbeta mRNA. In accordance with these findings, analysis of LHbeta and FSHbeta 5' gene-flanking regions revealed on both gene promoters a GtH-specific element (GSE), half site estrogen response elements (ERE), cAMP response element (CRE) and AP1. In vitro experiments showed that recombinant human activin-A leads to higher levels of GPalpha, FSHbeta and LHbeta mRNAs in pituitary cell culture. Porcine inhibin marginally decreased the mRNA levels of GPalpha and FSHbeta, but at a low level (1 ng/ml) it stimulated that of LHbeta. These results shed some light on certain hypothalamic and gonadal hormones regulating the expression of GtH subunit genes in tilapia. In addition, they provide evidence for their differential regulation, and insight into their mode of action.

GnRH receptor signaling in tilapia pituitary cells: role of mitogen-activated protein kinase (MAPK)

The role of mitogen-activated protein kinase (MAPK, also known as extracellular signal regulated kinase; ERK) stimulation in gonadotropin-releasing hormone (GnRH) signaling was investigated in cultured pituitary cells of tilapia hybrids (Oreochromis niloticus x O. aureus). Exposure of the cells to salmon GnRH (sGnRH) resulted in a dose- and time-dependent elevation in ERK levels. The PKC activator, 1-O-tetradecanoyl phorbol-13-acetate (TPA) increased kinase levels, while addition of GnRH had no further effect. However, chronic exposure to TPA resulted in reduction of basal and GnRH-induced ERK elevation. When PKC was inhibited by GF109203X, the GnRH-elevated ERK levels were totally abolished. The role of MAPK activation on GPalpha, FSHbeta and LHbeta gene expression was determined by administration of MAPK-kinase (MEK) inhibitor (PD98059; PD). This inhibitor completely blocked GnRH-induced increases in ERK activity. Furthermore, it suppressed GPalpha and LHbeta mRNA responses to GnRH, but had no effect on FSHbeta transcript levels. PD also decreased basal LHbeta mRNA levels. These results indicate that in tilapia pituitary cells, GnRH activates MAPK cascade in a PKC-dependent manner. ERK is involved in GnRH elevation of GPalpha and LHbeta, but not in FSHbeta genes transcription.

Stimulation of mitogen-activated protein kinase by gonadotropin-releasing hormone in human granulosa-luteal cells

The present study investigated the activation of mitogen-activated protein kinases (MAPKs) by a GnRH agonist (GnRHa) in human granulosa-luteal cells (hGLCs). The phosphorylation state of p44 and p42 MAPK was examined using antibodies that distinguish phospho-p44/42 MAPK (Thr(202)/Tyr(204)) from total p44/42 MAPK (activated plus inactivated). Activation of MAPK by GnRHa was observed within 5 min and was sustained for 60 min after treatment. GnRHa stimulated MAPK activation in a dose-dependent manner, with maximum stimulation (6.7-fold over basal levels) at 10(-7) M. Pretreatment with a protein kinase C (PKC) inhibitor, GF109203X, completely blocked GnRHa-induced MAPK activation. In addition, pretreatment with a PKC activator, phorbol-12-myristate 13-acetate, potentiated GnRH-induced MAPK activation. These results indicate that GnRHa stimulates MAPK activation through a PKC-dependent pathway in hGLCs, possibly coupled to G(q)alpha protein. MAPK activation was also observed in response to 8-bromo-cAMP or cholera toxin, but not pertussis toxin. Forskolin (50 microM) substantially stimulated a rapid cAMP accumulation, whereas GnRHa (10(-7) M) or pertussis toxin (100 mg/ml) did not affect basal intracellular cAMP levels. Cotreatment of GnRHa (10(-7) M) did not attenuate forskolin- or hCG-stimulated cAMP accumulation. These results suggest that the GnRH receptor is probably not coupled to G(s)alpha or G(i)alpha in hGLCs. Finally, GnRHa (10(-7) M) stimulated a significant increase in Elk-1 phosphorylation and c-fos messenger RNA expression, as revealed by an in vitro kinase assay and Northern blot analysis, respectively. These results clearly demonstrate that GnRH activates the MAPK cascade through a PKC-dependent pathway in the human ovary.

Endothelin-1[1-31], acting as an ETA-receptor selective agonist, stimulates proliferation of cultured rat zona glomerulosa cells

Endothelin-1 (ET-1)[1-31] is a novel hypertensive peptide that mimics many of the vascular effects of the classic 21 amino acid peptide ET-1[1-21]. However, at variance with ET-1[1-21] that enhances aldosterone secretion from cultured rat zona glomerulosa (ZG) cells by acting via ETB receptors, ET-1[1-31] did not elicit such effect. Both ET-1[1-21] and ET-1[1-31] raised the proliferation rate of cultured ZG cells, the maximal effective concentration being 10(-8) M. This effect was blocked by the ETA-receptor antagonist BQ-123 and unaffected by the ETB-receptor antagonist BQ-788. Quantitative autoradiography showed that ET-1[1-21] displaced both [(125)I]PD-151242 binding to ETA receptors and [(125)I]BQ-3020 binding to ETB receptors in both rat ZG and adrenal medulla, while ET-1[1-31] displaced only [(125)I]BQ-3020 binding. The tyrosine kinase (TK) inhibitor tyrphostin-23 and the p42/p44 mitogen-activated protein kinase (MAPK) inhibitor PD-98059 abolished the proliferogenic effect of ET-1[1-31], while the protein kinase-C (PKC) inhibitor calphostin-C significantly reduced it. ET-1[1-31] (10(-8) M) stimulated TK and MAPK activity of dispersed ZG cells, an effect that was blocked by BQ-123. The stimulatory action of ET-1[1-31] on TK activity was annulled by tyrphostin-23, while that on MAPK activity was reduced by calphostin-C and abolished by either tyrphostin-23 and PD-98059. These data suggest that ET-1[1-31] is a selective agonist of the ETA-receptor subtype, and enhances proliferation of cultured rat ZG cells through the PKC- and TK-dependent activation of p42/p44 MAPK cascade.

Thyroxine promotes association of mitogen-activated protein kinase and nuclear thyroid hormone receptor (TR) and causes serine phosphorylation of TR

Activated nongenomically by l-thyroxine (T(4)), mitogen-activated protein kinase (MAPK) complexed in 10-20 min with endogenous nuclear thyroid hormone receptor (TRbeta1 or TR) in nuclear fractions of 293T cells, resulting in serine phosphorylation of TR. Treatment of cells with the MAPK kinase inhibitor, PD 98059, prevented both T(4)-induced nuclear MAPK-TR co-immunoprecipitation and serine phosphorylation of TR. T(4) treatment caused dissociation of TR and SMRT (silencing mediator of retinoid and thyroid hormone receptor), an effect also inhibited by PD 98059 and presumptively a result of association of nuclear MAPK with TR. Transfection into CV-1 cells of TR gene constructs in which one or both zinc fingers in the TR DNA-binding domain were replaced with those from the glucocorticoid receptor localized the site of TR phosphorylation by T(4)-activated MAPK to a serine in the second zinc finger of the TR DNA-binding domain. In an in vitro cell- and hormone-free system, purified activated MAPK phosphorylated recombinant human TRbeta1 (). Thus, T(4) activates MAPK and causes MAPK-mediated serine phosphorylation of TRbeta1 and dissociation of TR and the co-repressor SMRT.

The expression, regulation and signal transduction pathways of the mammalian gonadotropin-releasing hormone receptor

Normal mammalian sexual maturation and reproductive functions require the integration and precise coordination of hormones at the hypothalamic, pituitary, and gonadal levels. Hypothalamic gonadotropin-releasing hormone (GnRH) is a key regulator in this system; after binding to its receptor (GnRHR), it stimulates de novo synthesis and release of gonadotropins in anterior pituitary gonadotropes. Since the isolation of the GnRHR cDNA, the expression of GnRHR mRNA has been detected not only in the pituitary, but also in extrapituitary tissues, including the ovary and placenta. It has been shown that change in GnRHR mRNA is one of the mechanisms for regulating the expression of the GnRHR. To help understand the molecular mechanism(s) involved in transcriptional regulation of the GnRHR gene, the 5' flanking region of the GnRHR gene has recently been isolated. Initial characterization studies have identified several DNA regions in the GnRHR 5' flanking region which are responsible for both basal expression and GnRH-mediated homologous regulation of this gene in pituitary cells. The mammalian GnRHR lacks a C-terminus and possesses a relatively short third intracellular loop; both features are important in desensitization of many others G-protein coupled receptors (GPCRs), Homologous desensitization of GnRHR has been shown to be regulated by various serine-threonine protein kinases including protein kinase A (PKA) and protein kinase C (PKC), as well as by G-protein coupled receptor kinases (GRKs). Furthermore, GnRHR was demonstrated to couple with multiple G proteins (Gq/11, Gs, and Gi), and to activate cascades that involved the PKC, PKA, and mitogen-activator protein kinases. These results suggest the diversity of GnRHR-G protein coupling and signal transduction systems. The identification of second form of GnRH (GnRH-II) in mammals adds to the complexity of the GnRH-GnRHR system. This review summaries our recent progress in understanding the regulation of GnRHR gene expression and the GnRHR signal transduction pathways.Key words: gonadotropin-releasing hormone receptor, transcriptional regulation, desensitization, signal transduction.

8-Bromo-cyclic AMP induces phosphorylation of two sites in SRC-1 that facilitate ligand-independent activation of the chicken progesterone receptor and are critical for functional cooperation between SRC-1 and CREB binding protein

Elevation of intracellular 8-bromo-cyclic AMP (cAMP) can activate certain steroid receptors and enhance the ligand-dependent activation of most receptors. During ligand-independent activation of the chicken progesterone receptor (cPR(A)) with the protein kinase A (PKA) activator, 8-bromo-cAMP, we found no alteration in cPR(A) phosphorylation (W. Bai, B. G. Rowan, V. E. Allgood, B. W. O'Malley, and N. L. Weigel, J. Biol. Chem. 272:10457-10463, 1997). To determine if other receptor-associated cofactors were targets of cAMP-dependent signaling pathways, we examined the phosphorylation of steroid receptor coactivator 1 (SRC-1). We detected a 1.8-fold increase in SRC-1 phosphorylation in transfected COS-1 cells incubated with 8-bromo-cAMP. Phosphorylation was increased on two mitogen-activated protein kinase (MAPK) sites, threonine 1179 and serine 1185. PKA did not phosphorylate these sites in vitro. However, blockage of PKA activity in COS-1 cells with the PKA inhibitor (PKI) prevented the 8-bromo-cAMP-mediated phosphorylation of these sites. Incubation of COS-1 cells with 8-bromo-cAMP resulted in activation of the MAPK pathway, as determined by Western blotting with antibodies to the phosphorylated (active) form of Erk-1/2, suggesting an indirect pathway to SRC-1 phosphorylation. Mutation of threonine 1179 and serine 1185 to alanine in COS-1 cells coexpressing cPR(A) and the GRE(2)E1bCAT reporter resulted in up to a 50% decrease in coactivation during both ligand-independent activation and ligand-dependent activation. This was due, in part, to loss of functional cooperation between SRC-1 and CREB binding protein for coactivation of cPR(A). This is the first demonstration of cross talk between a signaling pathway and specific phosphorylation sites in a nuclear receptor coactivator that can regulate steroid receptor activation.

Epidermal growth factor induces oxidative neuronal injury in cortical culture

Recently, we have demonstrated that certain neurotrophic factors can induce oxidative neuronal necrosis by acting at the cognate tyrosine kinase-linked receptors. Epidermal growth factor (EGF) has neurotrophic effects via the tyrosine kinase-linked EGF receptor (EGFR), but its neurotoxic potential has not been studied. Here, we examined this possibility in mouse cortical culture. Exposure of cortical cultures to 1-100 ng/ml EGF induced gradually developing neuronal death, which was complete in 48-72 h; no injury to astrocytes was noted. Electron microscopic findings of EGF-induced neuronal death were consistent with necrosis; severe mitochondrial swelling and disruption of cytoplasmic membrane occurred, whereas nuclei appeared relatively intact. The EGF-induced neuronal death was accompanied by increased free radical generation and blocked by the anti-oxidant Trolox. Suggesting mediation by the EGFR, an EGFR tyrosine kinase-specific inhibitor, C56, attenuated EGF-induced neuronal death. In addition, inhibitors of extracellular signal-regulated protein kinase 1/2 (Erk-1/2) (PD98056), protein kinase A (H89), and protein kinase C (GF109203X) blocked EGF-induced neuronal death. A p38 mitogen-activated protein kinase inhibitor (SB203580) or glutamate antagonists (MK-801 and 6-cyano-7-nitroquinoxaline-2,3-dione) showed no protective effect. The present results suggest that prolonged activation of the EGFR may trigger oxidative neuronal injury in central neurons.

Adrenomedullin enhances cell proliferation and deoxyribonucleic acid synthesis in rat adrenal zona glomerulosa: receptor subtype involved and signaling mechanism

The effect of adrenomedullin (ADM) on the proliferative activity of the rat adrenal cortex has been investigated in vivo, using an in situ perfusion technique of the intact left gland. ADM and other chemicals were dissolved in the perfusion medium, and the perfusion was continued for 180 min. ADM infusion concentration dependently increased the mitotic index and [3H]thymidine incorporation into DNA in the zona glomerulosa (ZG; the maximal effective concentration was 10(-8) M), but not in inner adrenocortical layers, where basal proliferative activity was negligible. The effect of 10(-8) M ADM was equipotently counteracted by both the calcitonin gene-related peptide (CGRP) type 1 receptor antagonist CGRP-(8-37) and ADM-(22-52). The adenylate cyclase inhibitor SQ-22536 (10(-4) M), the cAMP blocker Rp-cAMP-S (10(-3) M), and the protein kinase A inhibitor H-89 (10(-5) M), although counteracting the ZG proliferogenic action of 10(-9) M ACTH, did not affect the 10(-8) M ADM-elicited increase in ZG DNA synthesis. Similar results were obtained using the phospholipase C inhibitor U-73122 (10(-5) M), the inositol-1,4,5-trisphosphate antagonist D,L-myo-inositol-1,4,5-trisphosphothiate (10(-4) M), and the protein kinase C inhibitor calphostin C (10(-5) M), which, however, significantly inhibited the ZG proliferogenic effect of 10(-9) M angiotensin II. The growth-promoting action of 10(-8) M ADM was not affected by the phospholipase A2 inhibitor AACOCF3 (10(-5) M), the cyclooxygenase (COX) inhibitor indomethacin (10(-5) M), or the mixed COX/lipoxygenase inhibitor phenidone (10(-5) M). In contrast, the ZG proliferogenic effect of 10(-8) M ADM was abolished by either the tyrosine kinase (TK) inhibitor tyrphostin-23 (10(-5) M) or the mitogen-activated protein kinase (MAPK) antagonists PD-98059 and U0216 (10(-4) M). ADM (10(-8) M) stimulated TK and p42/p44 MAPK activity in dispersed ZG, but not ZF, cells, and the effect was reversed by either 10(-6) M CGRP-(8-37) and ADM-(22-52) or preincubation with 10(-5) M tyrphostin-23. Collectively, our findings indicate that 1) ADM stimulates cell proliferation in the rat ZG, through CGRP-(8-37)- and ADM-(22-52)-sensitive receptors, probably of the CGRP1 subtype; and 2) the mitogenic effect of ADM is mediated by activation of the TK-MAPK cascade, without any involvement of the adenylate cyclase/protein kinase A-, phospholipase C/protein kinase C-, and COX- or lipoxygenase-dependent signaling pathways.

Activation of MAPK cascades by G-protein-coupled receptors: the case of gonadotropin-releasing hormone receptor

G-protein-coupled receptors (GPCRs) are a large group of integral membrane receptors that transmit signals from a diverse array of external stimuli, including neurotransmitters, hormones, phospholipids, photons, odorants and taste ligands. In response to ligand binding, the GPCRs initiate diverse downstream signaling pathways through four groups of G proteins and other interacting proteins. Key components in GPCR-induced intracellular signaling are four groups of mitogen-activated protein kinase (MAPK) cascades: extracellular signal-related kinase (ERK), Jun N-terminal kinase (JNK), p38MAPK and big MAPK (BMK). The hallmark of MAPK signaling is the stimulation-dependent nuclear translocation of the involved kinases, which regulate gene expression and the cytoplasmic acute response to mitogenic, stress-related, apoptotic and survival stimuli. A special type of GPCR is the gonadotropin-releasing hormone (GnRH) receptor, which uses primarily the Gq protein for its downstream signaling. GnRH activates all four MAPK cascades by a PKC-dependent mechanism. Common signaling molecules, including the tyrosine kinase c-SRC and the small GTPases CDC42, RAC and RAS, are implicated in various aspects of the GnRH-MAPK pathways. Thus, the activation of MAPK cascades by GnRH opens a new vista in the understanding of the transcriptional regulation of genes encoding gonadotropins. However, additional studies on cell lines and whole animals are required to understand GnRH signaling in the context of other hormones during the reproductive cycle of mouse and human.