Delayed activation of phospholipase D by gonadotropin-releasing hormone in a clonal pituitary gonadotrope cell line (alpha T3-1)

Regulation of prostate cancer by hormone-responsive G protein-coupled receptors

Regulation of prostate cancer by androgen and androgen receptor (AR), and blockade of AR signaling by AR antagonists and steroidogenic enzyme inhibitors have been extensively studied. G protein-coupled receptors (GPCRs) are a family of membrane receptors that regulate almost all physiological processes. Nearly 40% of FDA-approved drugs in the market target GPCRs. A variety of GPCRs that mediate reproductive function have been demonstrated to be involved in the regulation of prostate cancer. These GPCRs include gonadotropin-releasing hormone receptor, luteinizing hormone receptor, follicle-stimulating hormone receptor, relaxin receptor, ghrelin receptor, and kisspeptin receptor. We highlight here GPCR regulation of prostate cancer by these GPCRs. Further therapeutic approaches targeting these GPCRs for the treatment of prostate cancer are summarized.

Different expression patterns of sperm motility-related genes in testis of diploid and tetraploid cyprinid fish†

Sperm motility is an important standard to measure the fertility of male. In our previous study, we found that the diploid spermatozoa from allotetraploid hybrid (4nAT) had longer durations of rapid and slow progressive motility than haploid spermatozoa from common carp (COC). In this study, to explore sperm motility-related molecular mechanisms, we compared the testis tissues transcriptomes from 2-year-old male COC and 4nAT. The RNA-seq data revealed that 2985 genes were differentially expressed between COC and 4nAT, including 2216 upregulated and 769 downregulated genes in 4nAT. Some differentially expressed genes, such as tubulin genes, dynein, axonemal, heavy chain(dnah) genes, mitogen-activated protein kinase(mapk) genes, tektin 4, FOX transcription factors, proteasome genes, and ubiquitin carboxyl-terminal hydrolase(uchl) genes, are involved in the regulation of cell division, flagellar and ciliary motility, gene transcription, cytoskeleton, energy metabolism, and the ubiquitin–proteasome system, suggesting that these genes were related to sperm motility of the 4nAT. We confirmed the differential expression of 12 such genes in 4nAT by quantitative PCR. By western blotting, we also confirmed increased expression of Uchl3 in 4nAT testis. In addition, we identified 1915 and 2551 predicted long noncoding RNA (lncRNA) transcripts from testis tissue transcriptomes of COC and 4nAT, respectively. Of these, 1575 lncRNAs were specifically expressed in 4nAT and 939 were specifically expressed in COC. This study provides insights into the transcriptome profile of testis tissues from diploid and tetraploid, which are useful for research on regulatory mechanisms behind sperm motility in male polyploidy.

Differential roles of PKC isoforms (PKCs) in GnRH stimulation of MAPK phosphorylation in gonadotrope derived cells

The role of protein kinase C (PKC) isoforms (PKCs) in GnRH-stimulated MAPK [ERK1/2, JNK1/2 and p38) phosphorylation was examined in gonadotrope derived cells. GnRH induced a protracted activation of ERK1/2 and a slower and more transient activation of JNK1/2 and p38MAPK. Gonadotropes express conventional PKCα and PKCβII, novel PKCδ, PKCε and PKCθ, and atypical PKC-ι/λ. The use of green fluorescent protein (GFP)-PKCs constructs revealed that GnRH induced rapid translocation of PKCα and PKCβII to the plasma membrane, followed by their redistribution to the cytosol. PKCδ and PKCε localized to the cytoplasm and Golgi, followed by the rapid redistribution by GnRH of PKCδ to the perinuclear zone and of PKCε to the plasma membrane. The use of dominant negatives for PKCs and peptide inhibitors for the receptors for activated C kinase (RACKs) has revealed differential role for PKCα, PKCβII, PKCδ and PKCε in ERK1/2, JNK1/2 and p38MAPK phosphorylation in a ligand-and cell context-dependent manner. The paradoxical findings that PKCs activated by GnRH and PMA play a differential role in MAPKs phosphorylation may be explained by persistent vs. transient redistribution of selected PKCs or redistribution of a given PKC to the perinuclear zone vs. the plasma membrane. Thus, we have identified the PKCs involved in GnRH stimulated MAPKs phosphorylation in gonadotrope derived cells. Once activated, the MAPKs will mediate the transcription of the gonadotropin subunits and GnRH receptor genes.

Differential roles of PKC isoforms (PKCs) and Ca2+ in GnRH and phorbol 12-myristate 13-acetate (PMA) stimulation of p38MAPK phosphorylation in immortalized gonadotrope cells

We examined the role of PKCs and Ca²⁺ in GnRH-stimulated p38MAPK phosphorylation in the gonadotrope derived αT3-1 and LβT2 cell lines. GnRH induced a slow and rapid increase in p38MAPK phosphorylation in αT3-1 and LβT2 cells respectively, while PMA gave a slow response. The use of dominant negatives for PKCs and peptide inhibitors for the receptors for activated C kinase (RACKs), has revealed differential role for PKCα, PKCβII, PKCδ and PKCε in p38MAPK phosphorylation in a ligand-and cell context-dependent manner. The paradoxical findings that PKCs activated by GnRH and PMA play a differential role in p38MAPK phosphorylation may be explained by differential localization of the PKCs. Basal, GnRH- and PMA- stimulation of p38MAPK phosphorylation in αT3-1 cells is mediated by Ca²⁺ influx via voltage-gated Ca²⁺ channels and Ca²⁺ mobilization, while in the differentiated LβT2 gonadotrope cells it is mediated only by Ca²⁺ mobilization. p38MAPK resides in the cell membrane and is relocated to the nucleus by GnRH (∼5 min). Thus, we have identified the PKCs and the Ca²⁺ pools involved in GnRH stimulated p38MAPK phosphorylation.

Ion channels and signaling in the pituitary gland

Endocrine pituitary cells are neuronlike; they express numerous voltage-gated sodium, calcium, potassium, and chloride channels and fire action potentials spontaneously, accompanied by a rise in intracellular calcium. In some cells, spontaneous electrical activity is sufficient to drive the intracellular calcium concentration above the threshold for stimulus-secretion and stimulus-transcription coupling. In others, the function of these action potentials is to maintain the cells in a responsive state with cytosolic calcium near, but below, the threshold level. Some pituitary cells also express gap junction channels, which could be used for intercellular Ca(2+) signaling in these cells. Endocrine cells also express extracellular ligand-gated ion channels, and their activation by hypothalamic and intrapituitary hormones leads to amplification of the pacemaking activity and facilitation of calcium influx and hormone release. These cells also express numerous G protein-coupled receptors, which can stimulate or silence electrical activity and action potential-dependent calcium influx and hormone release. Other members of this receptor family can activate calcium channels in the endoplasmic reticulum, leading to a cell type-specific modulation of electrical activity. This review summarizes recent findings in this field and our current understanding of the complex relationship between voltage-gated ion channels, ligand-gated ion channels, gap junction channels, and G protein-coupled receptors in pituitary cells.

The role of Mitogen activated protein kinase (MAPK) in sperm functions

The generation of mature spermatozoa in the epididymis includes the activation of the MAPK cascade in a complex manner. MAPKs are thought to be involved in the regulation of transcription and ectoplasmic specialization (ES) in the testis. MAPKs also regulate mature spermatozoa flagellar motility, hyperactivation and the acrosome reaction. Here we review the current data regarding the functions of MAPKs in spermatogenesis and in mature spermatozoa.

A preformed signaling complex mediates GnRH-activated ERK phosphorylation of paxillin and FAK at focal adhesions in L beta T2 gonadotrope cells

Most receptor tyrosine kinases and G protein-coupled receptors (GPCRs) operate via a limited number of MAPK cascades but still exert diverse functions, and therefore signal specificity remains an enigma. Also, most GPCR ligands utilize families of receptors for mediation of diverse biological actions; however, the mammalian type I GnRH receptor (GnRHR) seems to be the sole receptor mediating GnRH-induced gonadotropin synthesis and release. Signaling complexes associated with GPCRs may thus provide the means for signal specificity. Here we describe a signaling complex associated with the GnRHR, which is a unique GPCR lacking a C-terminal tail. Unlike other GPCRs, this signaling complex is preformed, and exposure of L beta T2 gonadotropes to GnRH induces its dynamic rearrangement. The signaling complex includes c-Src, protein kinase C delta, -epsilon, and -alpha, Ras, MAPK kinase 1/2, ERK1/2, tubulin, focal adhesion kinase (FAK), paxillin, vinculin, caveolin-1, kinase suppressor of Ras-1, and the GnRHR. Exposure to GnRH (5 min) causes MAPK kinase 1/2, ERK1/2, tubulin, vinculin, and the GnRHR to detach from c-Src, but they reassociate within 30 min. On the other hand, FAK, paxillin, the protein kinase Cs, and caveolin-1 stay bound to c-Src, whereas kinase suppressor of Ras-1 appears in the complex only 30 min after GnRH stimulation. GnRH was found to activate ERK1/2 in the complex in a c-Src-dependent manner, and the activated ERK1/2 subsequently phosphorylates FAK and paxillin. In parallel, caveolin-1, FAK, vinculin, and paxillin are phosphorylated on Tyr residues apparently by GnRH-activated c-Src. Receptor tyrosine kinases and GPCRs translocate ERK1/2 to the nucleus to phosphorylate and activate transcription factors. We therefore propose that the role of the multiprotein signaling complex is to sequester a cytosolic pool of activated ERK1/2 to phosphorylate FAK and paxillin at focal adhesions.

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.

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.

Mechanism of GnRH receptor signaling on gonadotropin release and gene expression in pituitary gonadotrophs

Gonadotropin releasing hormone (GnRH), the first key hormone of reproduction, is synthesized and secreted from the hypothalamus in a pulsatile manner and stimulates pituitary gonadotrophs (5-10% of the pituitary cells) to synthesize and release gonadotropin luteinizing hormone (LH) and follicle stimulating hormone (FSH). Gonadotrophs consist of 60% multihormonal cells (LH+FSH) and 18% LH- and 22% FSH-containing cells. LH and FSH, members of the glycoprotein hormone family, stimulate spermatogenesis, folliculogenesis, and ovulation. Although GnRH plays a pivotal role in gonadotropin synthesis and release, other factors such as gonadal steroids and gonadal peptides exert positive and negative feedback mechanisms, which affect GnRH actions. GnRH actions include activation of phosphoinositide turnover as well as phospholipase D and A2, mobilization and influx of Ca2+, activation of protein kinase C (PKC) and mitogen-activated protein kinase (MAPK). A complex crosstalk between the above messenger molecules mediates the diverse actions of GnRH. Understanding the signaling mechanisms involved in GnRH actions is the basis for our understanding of basic reproductive functions in general and gonadotropin synthesis and release in particular.

Transcriptional down-regulation of human gonadotropin-releasing hormone (GnRH) receptor gene by GnRH: role of protein kinase C and activating protein 1

Clinical applications of GnRH agonists (GnRHa) are based primarily on the decrease in gonadotropin release after down-regulation of the GnRH receptor (GnRHR) by continuous GnRHa administration. However, the molecular mechanisms underlying the transcriptional regulation of the human GnRHR gene after prolonged GnRH treatment remain poorly understood. In the present study GnRHa-mediated regulation of human GnRHR gene transcription was studied by transiently transfecting the mouse gonadotrope-derived (alphaT3-1) cells with a 2297-bp human GnRHR promoter-luciferase construct (p2300-LucF). A dose- and time-dependent decrease in human GnRHR promoter activity was observed after GnRHa treatment. An average 71% decrease in promoter activity was observed after 24-h treatment with 0.1 microM GnRHa, which was blocked by cotreatment of the GnRH antagonist, antide. This effect was mimicked by phorbol 12-myristate 13-acetate (TPA) administration. In addition, the GnRHa- and TPA-mediated decrease in the human GnRHR promoter activity was reversed by a specific protein kinase C (PKC) inhibitor, GF109203X, or depletion of PKC by TPA pretreatment. These findings indicate that the activation of the PKC pathway is important in regulating the human GnRHR gene expression. By progressive 5'-deletion studies, we have identified a 248-bp DNA fragment (-1018 to -771, relative to the translation start site) at the 5'-flanking region of the human GnRHR gene that is responsible for the GnRHa-mediated down-regulation of human GnRHR promoter activity. Analysis of this sequence reveals the existence of two putative activating protein-1 (AP-1) sites with 87% homology to the consensus sequence (5'-TGA(G/C)T(C/A)A-3'), located at -1000 to -994 (5'-TTAGACA-3', in complementary orientation) and -943 to 937 (5'-TGAATAA-3'). Using competitive gel mobility shift assays, AP-1 binding was observed within this 248-bp region. Site-directed mutation of the putative AP-1-binding site located at -1000 to -994 abolished the GnRHa-induced inhibition. Further competitive GMSA and supershift experiments confirmed the identity of AP-1 binding in this region. By the use of Western blot analysis, a significant increase in c-Jun (100%; P < 0.05) and c-Fos (50%; P < 0.05) protein levels was observed after GnRHa treatment in alphaT3-1 cells. In addition, our data suggested that a change in AP-1 composition, particularly c-Fos, was important in mediating GnRHa-induced inhibition of human GnRHR gene expression. We conclude that activation of the PKC pathway by GnRH is important in controlling human GnRHR gene expression. In addition, the putative AP-1-binding site located at -1000 to -994 of the human GnRHR5'-flanking region has been functionally identified to be involved in mediating this down-regulatory effect.

Functional uncoupling between intracellular calcium dynamics and secretion in the alphaT3-1 gonadotropic cell line

Gonadotropin releasing hormone (GnRH) stimulates both transcription and secretion of the alpha subunit of the gonadotropins in a Ca2+-dependent fashion. In this study, we examined the role of Ca2+ as the signal coupling agonist occupancy of GnRH receptors to hormone secretion using the gonadotropic cell line alphaT3-1. Treatment of alphaT3-1 cells for 60 min with GnRH (0.1-100 nM), veratridine (50 microM) or high K+ (56 mM) was completely ineffective in stimulating secretion. The lack of effect occurred in spite of a robust, specific, and dose-dependent biphasic [Ca2+]i response consisting of a rapid peak sensitive to thapsigargin (200 nM) followed by a smaller plateau sensitive to the extracellular application of EGTA (5 mM). On the other hand, treatment of alphaT3-1 cells with the Ca2+ ionophore ionomycin resulted in a significant dose-dependent stimulation of secretion and [Ca2+]i responses comparable to those elicited by GnRH. Binding assays revealed the presence of Ins(1,4,5)P3 receptors (Kd = 3.2 nM, Bmax = 50.5 fmol/mg protein) but not ryanodine receptors in alphaT3-1 cell membranes. Together, these results show a functional uncoupling between the [Ca2+]i response and secretion in this cell line, suggesting that the increase in [Ca2+]i triggered by GnRH and depolarization may be necessary but not sufficient to stimulate exocytosis.

Mechanism of GnRH receptor signaling: combinatorial cross-talk of Ca2+ and protein kinase C

Gonadotropin-releasing hormone (GnRH), the first key hormone of reproduction, is synthesized in the hypothalamus and is released in a pulsatile manner to stimulate pituitary gonadotrope-luteinizing hormone (LH) and follicle-stimulating hormone (FSH) synthesis and release. Gonadotropes represent only about 10% of pituitary cells and are divided into monohormonal cells (18% LH and 22% FSH cells) and 60% multihormonal (LH + FSH) cells. GnRH binds to a specific seven transmembrane domain receptor which is coupled to Gq and activates sequentially different phospholipases to provide Ca2+ and lipid-derived messenger molecules. Initially, phospholipase C is activated, followed by activation of both phospholipase A2 (PLA2) and phospholipase D (PLD). Generation of the second messengers inositol 1,4,5-trisphosphate and diacylglycerol (DAG) lead to mobilization of intracellular pools of Ca2+ and activation of protein kinase C (PKC). Early DAG and Ca2+, derived via enhanced phosphoinositide turnover, might be involved in rapid activation of selective Ca(2+)-dependent, conventional PKC isoforms (cPKC). On the other hand, late DAG, derived from phosphatidic acid (PA) via PLD, may activate Ca(2+)-independent novel PKC isoforms (nPKC). In addition, arachidonic acid (AA) which is liberated by activated PLA2, might also support selective activation of PKC isoforms (PKCs) with or without other cofactors. Differential cross-talk of Ca2+, AA, and selective PKCs might generate a compartmentalized signal transduction cascade to downstream elements which are activated during the neurohormone action. Among those elements is the mitogen-activated protein kinase (MAPK) cascade which is activated by GnRH in a PKC-, Ca(2+)-, and protein tyrosine kinase (PTK)-dependent fashion. Transcriptional regulation can be mediated by the activation of transcription factors such as c-fos by MAPK. Indeed, GnRH activates the expression of both c-jun and c-fos which might participate in gene regulation via the formation of AP-1. The signaling cascade leading to gonadotropin (LH and FSH) gene regulation by GnRH is still not known and might involve the above-mentioned cascades. AA and selective lipoxygenase products such as leukotriene C4 also participate in GnRH action, possibly by cross-talk with PKCs, or by an autocrine/paracrine amplification cycle. A complex combinatorial, spatial and temporal cross-talk of the above messenger molecules seems to mediate the diverse effects elicited by GnRH, the first key hormone of the reproductive cycle.

Dependence of intracellular signaling and neurosecretion on phospholipase D activation in immortalized gonadotropin-releasing hormone neurons

The excitability of gonadotropin-releasing hormone (GnRH) neurons is essential for episodic neuropeptide release, but the mechanism by which electrical activity controls GnRH secretion is not well characterized. The role of phospholipase D (PLD) in mediating the activity-dependent secretory pathway was investigated in immortalized GT1 neurons, which both secrete GnRH and express GnRH receptors. Activation of these Ca2+-mobilizing receptors was associated with transient hyperpolarization of GT1 cells, followed by sustained firing of action potentials. This was accompanied by an increase in PLD activity, as indicated by elevated phosphatidylethanol (PEt) production. GnRH-induced PEt production was reduced by inhibition of phospholipase C-dependent phosphoinositide hydrolysis by U73122 and neomycin, suggesting that signaling from phospholipase C led to activation of PLD. The intermediate role of protein kinase C (PKC) in this process was indicated by the ability of phorbol 12-myristate 13-acetate to induce time- and dose-dependent increases in PEt and diacylglycerol, but not inositol trisphosphate, and by reduction of GnRH-induced PEt accumulation in PKC-depleted cells. Consistent with the role of action potential-driven Ca2+ entry in this process, agonist-induced PLD activity was also reduced by nifedipine and low extracellular Ca2+. Inhibition of the PLD pathway by ethanol and propranolol reduced diacylglycerol production and caused a concomitant fall in GnRH release. These data indicate that voltage-gated Ca2+ entry and PKC act in an independent but cooperative manner to regulate PLD activity, which contributes to the secretory response in GT1 cells. Thus, the electrical activity of the GnRH-secreting neuron participates in the functional coupling between GnRH receptors and PLD pathway.

Effect of phorbol 12, 13-dibutyrate on ligand binding, enzyme activity and translocation of protein kinase C isoforms in the alpha T3-1 gonadotrope-derived cell line

The effect of incubating alpha T3-1 cells with phorbol 12,13-dibutyrate (PDBu) on the protein kinase C (PKC) isoform content (predominantly alpha, epsilon and zeta isoforms) was assessed by immunoblotting, enzyme activity assay and [3H]PDBu binding. After exposure to PDBu for 17 h the immunoreactivity detected for both PKC alpha and PKC epsilon had disappeared from cytosol and had increased slightly in membranes. Immunoreactivity for PKC zeta was present as two bands in cytosol; after PDBu treatment both bands decreased in intensity, the higher molecular weight band more than the lower. The lower molecular weight band corresponded with a component of constitutive PKC activity eluting from DEAE cellulose that was defined by inhibition of basal activity with GF 109203X or H7. Investigation of very short treatment times with PDBu using binding, immunoblot and activity measurements (in the presence/absence of Ca2+) indicated that translocation of PKC alpha and epsilon was very rapid-detectable by 10 sec, maximal within minutes. Reduction of these isoforms in membranes took much longer, and was not apparent up to 150 min. The immunoblot data for PKC zeta in cytosol showed no detectable effect of PDBu treatment on the low molecular weight band up to 150 min although it was reduced at 17 h. Translocation of the upper band was detectable at 10 sec but this band may have resulted from cross-reaction with other PKC isoforms. The constitutive activity and low molecular weight ("authentic') PKC zeta immunoreactivity were partially affected after long exposure only, suggesting an action of PDBu on PKC zeta secondary to activation of the other PKC isoforms. An endogenous receptor agonist, luteinising hormone-releasing hormone (LHRH), was also used to assess by immunoblotting, translocation of the PKC isoforms. Although all the isoforms did translocate from cytosol to membrane fractions, they did so with distinctly different time courses: PKC epsilon moved more rapidly than PKC zeta which appeared to translocate more quickly than PKC alpha. After downregulation of the responsive PKC isoforms with PDBu, the remaining PKC zeta was not translocated by LHRH.

Differential involvement of calcium channels and protein kinase-C activity in GnRH-induced phospholipase-C, -A2 and -D activation in a gonadotrope cell line (alpha T3-1)

The mode of action of GnRH on pituitary gonadotropes involves metabolism of phospholipids, protein kinase-C (PKC) and voltage sensitive Ca2+ channels (VSCC) activation. We have studied the differential role of PKC and VSCC on the coupling of the GnRH receptor with phospholipases-C (PLC), -A2 (PLA2) and -D (PLD) activities in a gonadotrope cell line (alpha T3-1), by measuring the production of inositol phosphates (IPs), arachidonic acid (AA) and phosphatidylethanol (PEt) respectively. We demonstrated that in these cells GnRH stimulated through a specific receptor, IPs formation, a rapid and sustained diacylglycerol generation, consequently AA release and a delayed PEt production in a dose-dependent manner. In contrast to GnRH-induced PLC activity, the PLA2 and PLD stimulation by the neuropeptide involved Ca2+ mobilization via VSCC activation. BAY-K8644 a VSCC agonist significantly potentiated, while the VSCC antagonist nitrendipine markedly inhibited GnRH-induced AA release and PEt production. TPA, a phorbol ester which induced a rapid and important redistribution of PKC, although unable to elicit PLC or PLA2 stimulation, specifically provoked PLD activation in a PKC-dependent but Ca(2+)-independent manner. The PKC stimulation by TPA significantly inhibited the GnRH-stimulated IPs and AA formation, while it potentiated the GnRH-evoked PEt production. This negative feed-back of PKC on GnRH-Induced PLC and PLA2 activities was reversed when PKC was either down regulated after long TPA treatments or inhibited by the PKC inhibitors, staurosporine or GF109203X. The GnRH-induced PEt formation was markedly diminished in PKC depleted cells or after PKC inhibition. Under such conditions, both agonist and antagonist of VSCC became less effective in modulating the remaining GnRH-evoked PEt formation. These results suggest that PKC, in coordination with Ca2+, plays a key role in regulating the cross-talk between the multiple phospholipases implicated in the GnRH signal transduction.

Signal transduction of the gonadotropin releasing hormone (GnRH) receptor: cross-talk of calcium, protein kinase C (PKC), and arachidonic acid

1. The decapeptide neurohormone gonadotropin releasing hormone (GnRH) is the first key hormone of the reproductive system. Produced in the hypothalamus, GnRH is released in a pulsatile manner into the hypophysial portal system to reach the anterior pituitary and stimulates the release and synthesis of the gonadotropin hormones LH and FSH. GnRH, a Ca2+ mobilizing ligand, binds to its respective binding protein, which is a member of the seven transmembrane domain receptor family and activates a G-protein (Gq). 2. The alpha subunit of Gq triggers enhanced phosphoinositide turnover and the elevation of multiple second messengers required for gonadotropin release and biosynthesis. 3. The messenger molecules IP3, diacylglycerol, Ca2+, protein kinase C, arachidonic acid and leukotriene C4 cross-talk in a complex networks of signaling, culminating in gonadotropin release and gene expression.

Dependence of stimulus-transcription coupling on phospholipase D in agonist-stimulated pituitary cells

Stimulation of phospholipase D activity is frequently observed during agonist activation of Ca(2+)-mobilizing receptors, but the cellular functions of this signaling pathway are not well defined. Pituitary gonadotrophs express Ca(2+)-mobilizing receptors for gonadotropin-releasing hormone (GnRH) and endothelin (ET), activation of which stimulates luteinizing hormone secretion and transient expression of c-fos. In pituitary cells and alpha T3-1 gonadotrophs, GnRH action was associated with both initial and sustained diacylglycerol (DG) production, whereas ET-1 induced only a transient DG response. Also, phospholipase D activity, estimated by the production of phosphatidylethanol from phosphatidylcholine in the presence of ethanol, was stimulated by GnRH but not ET-1. Such formation of phosphatidylethanol at the expense of phosphatidic acid (PA) during GnRH-induced activation of phospholipase D significantly reduced the production of PA, DG, and cytidine diphosphate diacylglycerol. Inhibition of PA-phosphohydrolase activity by propranolol also decreased GnRH-induced DG production and, in contrast to ethanol, increased PA and cytidine diphosphate diacylglycerol levels. The fall in DG production caused by ethanol and propranolol was accompanied by inhibition of GnRH-induced c-fos expression, whereas agonist-induced luteinizing hormone release was not affected. In contrast to their inhibitory actions on GnRH-induced early gene expression, neither ethanol nor propranolol affected ET-1-induced c-fos expression, or GnRH- and ET-1-induced inositol trisphosphate/Ca2+ signaling. These findings demonstrate that phospholipase D participates in stimulus-transcription but not stimulus-secretion coupling, and indicate that DG is the primary signal for this action.

Phospholipase D: regulation and functional significance

PLD is a major route for hydrolysis of PC in most tissues, consistent with it playing an important role in signal transduction. The enzyme appears to be activated by a variety of different mechanisms in different tissues, suggesting there might be several different isoforms. Little, however, is known at present about its enzymology and molecular biology. There is little direct evidence to indicate the functional significance of PLD activation but an accumulation of indirect evidence links PLD with prolonged changes in cell function. In particular, two areas where there is strong evidence for a role for PLD are mitogenesis and leukocyte hyperresponsiveness. An important area for future work will be the investigation of how products from the PLD pathway exert these effects. Current evidence suggests an important role for Ca(2+)-independent PKC isoforms and probably also for novel cellular targets for the putative second messenger PA.

Dissociation between release and gene expression of gonadotropin alpha-subunit in gonadotropin-releasing hormone-stimulated alpha T3-1 cell line

The alpha T3-1 cell line which was derived by targeted tumorigenesis in transgenic mice [Windle et al. (1990) Mol. Endocrinol. 4, 597-603] possesses high-affinity binding sites for GnRH analogs coupled to enhanced phosphoinositide turnover and phospholipase D activity. Incubation of alpha T3-1 cells with [D-Trp6]-GnRH analog (GnRH-A) resulted in a rapid increase in gonadotropin alpha-subunit mRNA levels which was detected already at 30 min of incubation (0.1 nM GnRH-A, 3-fold, p < 0.01). The effect diminished with time to reach basal levels at about 12 h of incubation, with a secondary rise in alpha mRNA levels between 12 and 24 h of incubation. Addition of the protein kinase C activator 12-O-tetradecanoylphorbol 13-acetate (TPA, 100 ng/mL) or the Ca2+ ionophore ionomycin (1 microM) to alpha T3-1 cells also resulted in a rapid increase in alpha-subunit mRNA levels. Surprisingly, GnRH-induced alpha-subunit release was detected only after a lag of 4 h of incubation. Thus, dissociation between exocytosis and gene expression can be demonstrated in GnRH-stimulated alpha T3-1 cell line.

Paradoxical signal transduction mechanism of endothelins and sarafotoxins in cultured pituitary cells: stimulation of phosphoinositide turnover and inhibition of prolactin release

Endothelins (ET-1, ET-2, ET-3 and vasoactive intestinal contractor, VIC) and sarafotoxins (SRTX-b and SRTX-c) appear to bind with high affinity to a homogeneous class of binding sites in cultured rat pituitary cells. All of these ligands seem to interact with the same receptor (ETA-R), except for SRTX-c which apparently binds to a separate receptor. Binding was followed by phosphodiesteric cleavage of phosphoinositides, resulting in the formation of inositol phosphates. No consistent effect on basal or gonadotropin-releasing hormone (GnRH)-induced release of luteinizing hormone (LH) was exerted by ET or SRTX during 2 h of static incubation. On the other hand, both groups of vasoactive peptides inhibited basal and thyrotropin-releasing hormone (TRH)-induced prolactin secretion. Surprisingly, activation of phosphoinositide turnover by TRH in pituitary mammotrophs led to stimulation of prolactin secretion, whereas activation of the same pathway by ET or SRTX resulted in inhibition of prolactin secretion. ET and SRTX stimulated inositol phosphate formation in GH3 cell line and in the gonadotroph-like cell line alpha T-3 (which is capable of producing the alpha subunit of the gonadotrophins), indicating that the peptides interact with both pituitary mammotrophs and gonadotrophs. The very low concentrations (nM range) needed to stimulate phosphoinositide turnover and to inhibit prolactin secretion, as well as the recent finding that ETs are present in the hypothalamo-pituitary axis suggest that ET might participate in the neuroendocrine modulation of pituitary functions. One such possibility is that ETs might be members of the prolactin inhibiting factors (PIFs) family.