Production of leukotrienes in gonadotropin-releasing hormone-stimulated pituitary cells: potential role in luteinizing hormone release

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.

Sensory system-predominant distribution of leukotriene A4 hydrolase and its colocalization with calretinin in the mouse nervous system

Leukotriene B4 is a potent lipid mediator, which has been identified as a potent proinflammatory and immunomodulatory compound. Although there has been robust evidence indicating that leukotriene B4 is synthesized in the normal brain, detailed distribution and its functions in the nervous system have been unclear. To obtain insight into the possible neural function of leukotriene B4, we examined the immunohistochemical distribution of leukotriene A4 hydrolase, an enzyme catalyzing the final and committed step in leukotriene B4 biosynthesis, in the mouse nervous system. Immunoreactivity for leukotriene A4 hydrolase showed widespread distribution with preference to the sensory-associated structures; i.e. neurons in the olfactory epithelium and vomeronasal organ, olfactory glomeruli, possibly amacrine cells, neurons in the ganglion cell layer and three bands in the inner plexiform layer of the retina, axons in the optic nerve and tract up to the superior colliculus, inner and outer hair cells and the spiral ganglion cells in the cochlea, vestibulocochlear nerve bundle, spinal trigeminal tract, and lamina II of the spinal cord. Double immunofluorescence staining demonstrated that most of the leukotriene A4-hydrolase-immunopositive neurons coexpressed calretinin, a calcium-binding protein in neurons. The ubiquitous distribution of leukotriene A4 hydrolase was in sharp contrast with the distribution of leukotriene C4 synthase [Shimada A, Satoh M, Chiba Y, Saitoh Y, Kawamura N, Keino H, Hosokawa M, Shimizu T (2005) Highly selective localization of leukotriene C4 synthase in hypothalamic and extrahypothalamic vasopressin systems of mouse brain. Neuroscience 131:683-689] which was confined to the hypothalamic and extrahypothalamic vasopressinergic neurons. These results suggest that leukotriene B4 may exert some neuromodulatory function mainly in the sensory nervous system, in concert with calretinin.

Glutathione, S-substituted glutathiones, and leukotriene C4 as substrates for peptidylglycine alpha-amidating monooxygenase

The C-terminal alpha-amide moiety of most peptide hormones arises by the posttranslational cleavage of a glycine-extended precursor in a reaction catalyzed by bifunctional peptidylglycine alpha-amidating monooxygenase (PAM). Glutathione and the S-alkylated glutathiones have a C-terminal glycine and are, thus, potential substrates for PAM. The addition of PAM to glutathione, a series of S-alkylated glutathiones, and leukotriene C(4) results in the consumption of O(2) and the production of the corresponding amidated peptide and glyoxylate. This reaction proceeds in two steps with the intermediate formation of a C-terminal alpha-hydroxyglycine-extended peptide. Amidated glutathione (gammaGlu-Cys-amide) is a relatively poor substrate for glutathione S-transferase with a V/K value that is 1.3% of that for glutathione. Peptide substrates containing a penultimate hydrophobic or sulfur-containing amino acid exhibit the highest (V/K)(app) values for PAM-catalyzed amidation. The S-alkylated glutathiones incorporate both features in the penultimate position with S-decylglutathione having the highest (V/K)(app) of the substrates described in this report.

Involvement of the 5-lipoxygenase pathway in the neurotoxicity of the prion peptide PrP106-126

Transmissible spongiform encephalopathies are characterised by the transformation of the normal cellular prion protein (PrP(C)) into an abnormal isoform (PrP(TSE)). Previous studies have shown that N-methyl-D-aspartate (NMDA) receptor antagonists can inhibit glutathione depletion and neurotoxicity induced by PrP(TSE) and a toxic prion protein peptide, PrP106-126, in vitro. NMDA receptor activation is known to increase intracellular accumulation of Ca(2+), resulting in up-regulation of arachidonic acid (AA) metabolism. This can stimulate the lipoxygenase pathways that may generate a number of potentially neurotoxic metabolites. Because of the putative relationship between AA breakdown and PrP106-126 neurotoxicity, we investigated AA metabolism in primary cerebellar granule neuron cultures treated with PrP106-126. Our studies revealed that PrP106-126 exposure for 30 min significantly up-regulated AA release from cerebellar granule neurons. PrP106-126 neurotoxicity was mediated through the 5-lipoxygenase (5-LOX) pathway, as shown by abrogation of neuronal death with the 5-LOX inhibitors quinacrine, nordihydroguaiaretic acid, and caffeic acid. These inhibitors also prevented PrP106-126-induced caspase 3 activation and annexin V binding, indicating a central role for the 5-LOX pathway in PrP106-126-mediated proapoptosis. Interestingly, inhibitors of the 12-lipoxygenase pathway had no effect on PrP106-126 neurotoxicity or proapoptosis. These studies clearly demonstrate that AA metabolism through the 5-LOX pathway is an important early event in PrP106-126 neurotoxicity and consequently may have a critical role in PrP(TSE)-mediated cell loss in vivo. If this is so, therapeutic intervention with 5-LOX inhibitors may prove beneficial in the treatment of prion disorders.

Production of leukotrienes in a model of focal cerebral ischaemia in the rat

1. The aim of this work was to evaluate the role of leukotrienes in brain damage in vivo in a model of focal cerebral ischaemia in the rat, obtained by permanent occlusion of middle cerebral artery. 2. A significant (P < 0.01) elevation of LTC(4), LTD(4) and LTE(4) (cysteinyl-leukotrienes) levels occurred 4 h after ischaemia induction in the ipsilateral cortices of ischaemic compared to sham-operated animals (3998 +/- 475 and 897 +/- 170 fmol g(-1) tissue, respectively, P < 0.01). 3. The NMDA receptor antagonist MK-801 and the adenosine A(2A) receptor antagonist SCH 58261 were administered in vivo at doses known to reduce infarct size and compared with the leukotriene biosynthesis inhibitor MK-886. 4. MK-886 (0.3 and 2 mg kg(-1) i.v.) and MK-801 (3 mg kg(-1) i.p.) decreased cysteinyl-leukotriene levels (-78%, P < 0.05; -100%, P < 0.01; -92%, P < 0.01, respectively) 4 h after permanent occlusion of the middle cerebral artery, whereas SCH 58261 (0.01 mg kg(-1) i.v.) had no significant effects. 5. MK-886 (2 mg kg(-1) i.v.) was also able to significantly reduce the cortical infarct size by 30% (P < 0.05). 6. We conclude that cysteinyl-leukotriene formation is associated with NMDA receptor activation, and that it represents a neurotoxic event, the inhibition of which is able to reduce brain infarct area in a focal ischaemic event.

Analysis of leukotrienes in cerebrospinal fluid of a reference population and patients with inborn errors of metabolism: further evidence for a pathognomonic profile in LTC(4)-synthesis deficiency

Cysteinyl leukotrienes (LTC(4), LTD(4), LTE(4)) are potent lipid mediators derived from arachidonate in the 5-lipoxygenase pathway. Recently, the first inborn error of leukotriene synthesis, LTC(4)-synthesis deficiency, has been identified in association with a fatal developmental syndrome. The absence of leukotrienes in cerebrospinal fluid was one of the most striking biochemical findings in this disorder. We analysed leukotrienes in cerebrospinal fluid of patients with a broad spectrum of other well-defined inborn errors of metabolism, including glutathione synthetase deficiency (n=2), Zellweger syndrome (n=3), mitochondrial disorders (n=8), fatty acid oxidation defects (n=7), organic acidurias (n=7), neurotransmitter defects (n=5) and patients with non-specific neurological symptoms, as a reference population (n=120). The concentrations of leukotrienes were not related to age. Representative percentiles were calculated as reference intervals of each leukotriene. In all patients with an inborn error of metabolism concentration of cysteinyl leukotrienes and LTB(4) did not differ from the reference group. Our results indicate that absence of cysteinyl leukotrienes (<5 pg/ml) in association with normal or increased LTB(4) (50.0-67.3 pg/ml) is pathognomonic for LTC(4)-synthesis deficiency. The unique profile of leukotrienes in cerebrospinal fluid in this new disorder is primarily related to the defect and represents a new diagnostic approach.

Delayed luteolysis after intra-uterine infusions of nordihydroguaiaretic acid in the ewe

Intrauterine administration of nordihydroguaiaretic acid (5 mg, bid. NDGA), an inhibitor of the enzyme 5-lipoxygenase, on days 10-14 of the oestrous cycle, maintained luteal function and delayed oestrus in the ewe. The duration (mean +/- SD) of the oestrous cycle in the treatment group (n = 4) was 24 +/- 1 days, which was significantly (P < 0.001) longer than that of 16 +/- 1 days in vehicle-treated controls (n = 4); plasma progesterone concentrations were also significantly (P < 0.01) higher in the treatment group. On days 13 and 14 of the cycle (oestrus = Day 0) in the control group large pulses of 13,14-dihydro-15-keto prostaglandin F2alpha (PGFM) were evident, with mean (+/- SD) maximum concentrations of 232.5 +/- 66 and 415 +/- 309 pg ml(-1), respectively. In the treatment group, however, concentrations of PGFM were below detection level (< 50 pg ml(-1)). Similarly, in the control group, oxytocin release was highly pulsatile, with mean (+/- SD) peak concentrations of 21.8 +/- 5 and 18.5 +/- 6 pg ml(-1) on days 13 and 14, respectively; these were significantly (P < 0.01) higher than values of 7.6 +/- 3 and 6.1 +/- 3 pg ml(-1) in NDGA-treated ewes, where pulses were of relatively low amplitude. These results suggest that 5-lipoxygenase products of arachidonic acid metabolism may be involved in the positive feedback mechanism between luteal oxytocin and uterine PGF2alpha during luteolysis in the ewe.

Inhibitory effect of leukotrienes on luteinizing hormone release

The aim of this work was to study the effect of high concentrations of leukotrienes on luteinizing hormone (LH) secretion in rat anterior pituitary cells. We also investigated the effect of leukotrienes in parallel with gonadotropin-releasing hormone (GnRH) action. Experiments were on cells gained from trypsinized pituitaries of female rats. Tests were performed by superfusion of the cells attached to cytodex-1 carrier beads. The LH content in samples of perfusate was estimated by radioimmunoassay. This work reports 48% inhibition of basic LH release by action of leukotriene C4 in superfused cells when applied continuously at a concentration of 100 nmol/l. Moreover, we have shown that leukotrienes suppressed GnRH-induced LH secretion in rat pituitary cells when applied in parallel to GnRH (1 nmol/l) as a 4-min pulse at a concentration of 0.1 nmol/l. GnRH-induced LH release was reduced to 66% of its value by leukotriene (LT) B4 (0.1 nmol/l) action; also to 54% by LTC4, 66% by LTD4 and 74% by LTE4 action. In contrast, arachidonic acid (50 pmol/l) and its other 5-lipoxygenase metabolites: 5-hydroperoxyeicosatetraenoic acid (5-HPETE) (50 pmol/l), or 5-hydroxyeicosatetraenoic acid (5-HETE) (50 pmol/l), had no inhibitory effect on GnRH-induced LH release. Arachidonic acid and 5-HETE potentiated GnRH-induced LH release up to 249% and 429%, respectively, when applied in parallel with GnRH (1 nmol/l) as a 4-min pulse at a concentration of 10 pmol/l. In our earlier work we have shown that several leukotrienes are potent stimulants of LH release. The present report documents the finding that the 5-lipoxygenase pathway is also involved in the inhibitory regulation of hormone release in anterior pituitary cells.

Mechanism of mitogen-activated protein kinase activation by gonadotropin-releasing hormone in the pituitary of alphaT3-1 cell line: differential roles of calcium and protein kinase C

The mechanism of mitogen-activated protein kinase (MAPK, ERK) stimulation by the GnRH analog [D-Trp6]GnRH (GnRH-a) was investigated in the gonadotroph-derived alphaT3-1 cell line. GnRH-a as well as the protein kinase C (PKC) activator 12-O-tetradecanoyl phorbol-13-acetate (TPA) stimulated a sustained response of MAPK activity, whereas epidermal growth factor (EGF) stimulated a transient response. MAPK kinase (MEK) is also activated by GnRH-a, but in a transient manner. GnRH-a and TPA apparently activated mainly the MAPK isoform ERK1, as revealed by Mono-Q fast protein liquid chromatography followed by Western blotting as well as by gel kinase assay. GnRH-a and TPA stimulated the tyrosine phosphorylation of several proteins, and this effect as well as the stimulation of MAPK activity were inhibited by the PKC inhibitor GF 109203X. Similarly, down-regulation of TPA-sensitive PKC subspecies nearly abolished the effect of GnRH-a and TPA on MAPK activity. Furthermore, the protein tyrosine kinase (PTK) inhibitor genistein inhibited protein tyrosine phosphorylation and reduced GnRH-a-stimulated MAPK activity by 50%, suggesting the participation of genistein-sensitive and insensitive pathways in GnRH-a action. Although Ca2+ ionophores have only a marginal stimulatory effect, the removal of Ca2+ markedly reduced MAPK activation by GnRH-a and TPA, but had no effect on GnRH-a and TPA stimulation of protein tyrosine phosphorylation. Interestingly, the removal of Ca2+ also partly inhibited the activation of MAPK by EGF and vanadate/H2O2. Thus, a calcium-dependent component(s) downstream of PKC and PTK might also participate in MAPK activation. Elevation of cAMP by forskolin exerted partial inhibition on EGF, but not on TPA or GnRH-a action, suggesting that MEK activators other than Raf-1 might be involved in GnRH action. We conclude that Ca2+, PTK, and PKC participate in the activation of MAPK by GnRH-a, with Ca2+ being necessary downstream to PKC and PTK.

Thrombin stimulates activation of the cerebral 5-lipoxygenase pathway during blood-brain cell contact

The purpose of this study was to identity the trigger mechanism activating the 5-lipoxygenase pathway during blood-brain cell contact and to estimate the contribution of blood and brain cells to the cysteinyl-leukotriene (LT) biosynthesis observed under these conditions. Incubation of dissociated rat brain cells in Krebs-Henseleit solution for up to 60 min did not stimulate any detectable cysteinyl-LT biosynthesis. Incubation of recalcified rat whole blood in vitro for up to 60 min led to release of only small amounts of cysteinyl-LT into the serum samples. However, coincubation of dissociated rat brain cells with physiologically recalcified autologous whole blood triggered a time-dependent release of large amounts of immunoreactive cysteinyl-LT into the serum samples. By reverse-phase HPLC, immunoreactive cysteinyl-LT was identified as a mixture of LTC4, LTD4, and LTE4. The extent of the 5-lipoxygenase stimulation depended on the amount of autologous blood coincubated with the dissociated brain cells. Activation of the 5-lipoxygenase pathway also occurred with coincubation of dissociated rat brain cells with recalcified autologous plasma. Stimulation of cysteinyl-LT biosynthesis during blood-brain cell contact remained unaffected by aprotinin, but concentration-dependent inhibition by the structurally and functionally unrelated thrombin inhibitors D-Phe-Pro-Arg-CH2Cl and recombinant hirudin was seen. Finally, when dissociated rat brain cells were incubated in Krebs-Henseleit solution in the presence of human alpha-thrombin, a concentration-dependent release of cysteinyl-LT into the buffer samples was observed. These data demonstrate that, in rats, during blood-brain cell contact, stimulation of the 5-lipoxygenase pathway in brain cells proceeds via alpha-thrombin as effector molecule.

Arachidonic acid and lipoxygenase products stimulate protein kinase C beta mRNA levels in pituitary alpha T3-1 cell line: role in gonadotropin-releasing hormone action

The cross-talk of arachidonic acid (AA) and its lipoxygenase products with protein kinase C beta (PKC beta) mRNA levels during the action of gonadotropin-releasing hormone (GnRH) was investigated in the pituitary alpha T3-1 cell line. The addition of AA or its 5-lipoxygenase products 5-hydroxyeicosatetraenoic acid (5-HETE) or leukotriene C4 (LTC4) for 30 or 60 min stimulated PCK beta, but not PKC alpha mRNA levels (3-5-fold); PCK gamma is not expressed by the cells. Other HETEs or leukotrienes tested showed no significant effect. The range of effective concentration for LTC4 and 5-HETE (around 10(-10) M) is the range found in GnRH-stimulated pituitary cells. Although PKC beta mRNA levels were preferentially elevated by LTC4 and 5-HETE at early time points, PKC alpha mRNA levels were elevated at 6-12 h of incubation when PKC beta mRNA levels returned to basal levels. The addition of the phospholipase A2 inhibitor 4-bromophenacyl bromide or the selective 5-lipoxygenase inhibitor L-656,224 abolished [D-Trp6]GnRH (GnRH-A) elevation of PKC beta mRNA levels, whereas PKC alpha mRNA levels were not increased by this neurohormone. The cyclo-oxygenase inhibitor indomethacin elevated basal PKC beta mRNA levels and potentiated the GnRH-A response. Cross-talk exists between AA and some of its lipoxygenase products and PKC beta gene expression during cell signalling. AA, 5-HETE and LTC4 participate in the rapid stimulation of PKC beta mRNA levels by GnRH.

Role of leukotriene C4 in follicle-stimulating hormone (FSH) secretion in female rat pituitary

Leukotriene C4, at doses of 0.01 and 0.1 nmol/l added to superfused cells in pulse of 4-min duration, evoked follicle-stimulating hormone (FSH) release up to 12- to 26-fold of basal secretion. Higher and lower concentrations of leukotriene C4 were not able to induce FSH secretion. Gonadotropin-releasing hormone (GnRH)-induced FSH release was reduced by 38-57% by the leukotriene receptor antagonist FPL 55712 (10 mumol/l). Moreover, we have shown that FSH release occurs parallel to leukotriene C4 synthesis in rat anterior pituitary cells. Mellitin (100 nmol/l), an activator of phospholipase A2, induced FSH and radioactivity secretion in rat anterior pituitary cells previously preincubated for 24 h with [3H]arachidonic acid (AA).

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.

Arachidonic acid and lipoxygenase products stimulate gonadotropin alpha-subunit mRNA levels in pituitary alpha T3-1 cell line: role in gonadotropin releasing hormone action

The role of arachidonic acid (AA) and its lipoxygenase metabolites in gonadotropin releasing hormone (GnRH) induced alpha-subunit gene expression was investigated in the transformed gonadotroph cell line alpha T3-1. The stable analog [D-Trp6]GnRH (GnRHa) stimulated [3H]AA release from prelabeled cells after a lag of 1-2 min. Addition of AA stimulated alpha-subunit mRNA levels in a dose-dependent manner, a significant effect being detected at 5 microM AA. Among various lipoxygenase metabolites of AA, only the 5-lipoxygenase products 5-hydroxyeicosatetraenoic acid (5-HETE) and leukotriene C4 (LTC4) stimulated alpha-subunit mRNA levels. However, while 5-HETE and LTC4 (0.1 nM each) were active already after 30 min of incubation, similar to GnRHa, AA (20 microM) stimulated alpha-mRNA levels after 1 h of incubation. Addition of the phospholipase A2 inhibitor 4-bromophenacyl bromide (BPB) or the selective 5-lipoxygenase inhibitor L-656,224 inhibited GnRHa elevation of alpha-subunit mRNA by 65%, while the cyclooxygenase inhibitor indomethacin had no effect. Addition of AA (20 microM) or LTC4 (0.1 nM) to normal cultured rat pituitary cells mimicked the rapid (30 min) stimulatory effect of GnRH (1 nM) upon alpha-subunit, LH beta, and FSH beta mRNA levels, while 5-HETE (0.1 nM) stimulated only FSH beta mRNA levels at this time point. Thus AA and selected 5-lipoxygenase products, in particular LTC4, participate in GnRHa-induced alpha-subunit mRNA elevation.

The arachidonic acid cascade, eicosanoids, and signal transduction

Eicosanoid biosynthesis in animal cells either results from agonist-stimulated phospholipase activation (endogenous pathway) or from lipoprotein receptor-mediated uptake and lysosomal lipid hydrolase-dependent release of AA (exogenous pathway) (see Fig. 1 for schematic representation). LDL stimulates eicosanoid formation through delivery of substrate AA to enzymes of oxidative AA metabolism. The classical LDL receptor is a control point of the effects of LDL AA on eicosanoid formation in different tissues: LDL AA metabolism occurs in several cell types of mesenchymal and epithelial origin and generates the formation of distinct eicosanoid patterns in each case. The LDL AA pathway does appear to couple directly to the PGH synthase reaction, whereas it does not couple directly to the 5-lipoxygenase reaction. We expect that a more complete characterization of the LDL unsaturated fatty acid pathway in different tissue will yield additional information on the biochemistry of lipoproteins, AA, and eicosanoids.

Intracellular mediation of GnRH action on GTH release in tilapia

The objective of the present study was to confirm previous results on the mediation of GnRH signal in tilapia by providing evidence from experiments in cultured pituitary cells and from perifusion experiments using a GnRH-antagonist. After 4 days in culture under identical conditions, cells taken from pituitaries of fish maintained at 26°C were more sensitive to GnRHa ([D-Ala(6), Pro(9)-NEt]-LHRH) than those taken from fish maintained at 19°C. Cells from female pituitaries were more responsive than those from males. taGTH release in culture was augmented by Ca(2+) ionophore (A23187; 1-100 μM) or ionomycin (0.02-10 μM). The response of perifused pituitary to GnRH was reduced by nimodipine (1-10 μM) indicating that Ca(2+) influx via voltage-sensitive Ca(2+) channels is involved in the stimulation of GTH release. Activation of protein kinase C by OAG (1-oleyl-2-acetyl glycerol; 0.16-160 μM) or TPA (1-O-tetra-decanoyl phorbol-13-acetate; 1.25-125 nM) resulted in a dose-dependent stimulation of taGTH release from cultured cells. Arachidonic acid (0.33-330 μM) also augmented the release of taGTH from the culture. Four sequential pulses of sGnRH (100 nM) at 2h intervals resulted in surges of taGTH release from perifused pituitary fragments; the surges were similar in magnitude with no signs of desensitization. Sequential stimulation with graded doses of sGnRH (0.1 nM to 1 μM) in the presence of GnRH-antagonist ([Pro(2,6), Trp(3)]-GnRH) resulted in an attenuation of taGTH release. However, the GnRH-antagonist did not alter the pattern of forskolin-stimulated GTH release, indicating that forskolin stimulation is exerted at the level of the adenohypophyseal cells. It is concluded that, as in other vertebrates, the transduction of GnRH stimulation of GTH release involves Ca(2+) influx through voltage-sensitive Ca(2+) channels, mobilization of the ion from intracellular sources, arachidonic acid and activation of PKC. Adenylate cyclase-cAMP system us also involved in the mediation but its relationship with other transduction cascades requires further investigations.

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.

Gonadotropin releasing hormone activates the lipoxygenase pathway in cultured pituitary cells: role in gonadotropin secretion and evidence for a novel autocrine/paracrine loop

The formation and role of arachidonic acid (AA) and its metabolites during gonadotropin releasing hormone- (GnRH-) induced gonadotropin secretion were investigated in primary cultures of rat pituitary cells. Prelabeled cells ([3H]AA) responded to GnRH challenge with increased formation (about 2-fold) of the leukotrienes LTC4, LTD4, and LTE4 as well as 5- and 15-eicosatetraenoic acids (5- and 15-HETE) as identified by HPLC. Formation of leukotrienes and 15-HETE was further verified by specific radioimmunoassays. No significant increase in the formation of 12-HETE or of the cyclooxygenase products prostaglandin E (PGE) and thromboxane A2 by GnRH was noticed. Addition of physiological concentrations of LTC4 enhanced basal LH release, while subphysiological concentrations of LTC4 (10(-15)-10(-12) M) inhibited GnRH-induced LH release by about 35% (p less than 0.02). Using specific lipoxygenase inhibitors L-656,224 and MK 886, we found inhibition of GnRH-induced LH release by about 40% at concentrations known to specifically inhibit the 5-lipoxygenase pathway. The peptidoleukotriene receptor antagonist ICI 198,615 inhibited LTC4- and LTE4-induced LH release and surprisingly also the effect of GnRH on LH release by 40%. The data strongly suggest a role for AA and its lipoxygenase metabolites in the on/off reactions of GnRH upon LH release. The data also present a novel amplification cycle in which newly formed leukotrienes become first messengers and establish an autocrine/paracrine loop.