Ovarian steroids modulate gonadotropin-releasing hormone-induced biphasic luteinizing hormone secretory responses and inositol phosphate accumulation in rat anterior pituitary cells and alpha T3-1 gonadotrophs

Nonclassical actions of estradiol-17beta are not detectable in the alphaT3-1 and LbetaT2 immortalized gonadotrope cell lines†

The immortalized mouse gonadotrope cell lines alphaT3-1 and LbetaT2 cells have been a substitute model for primary gonadotropes. These cell lines have provided a homogeneous cell population, as compared to the dissociated anterior pituitaries, which contain a heterogeneous population of cells potentially responsive to estradiol-17beta (E2). Nonclassical actions of E2 assumed to occur through the plasma membrane estrogen receptor 1 (ESR1, also known as ERalpha). These actions have included inhibition of gonadotropin-releasing hormone (GnRH)-induced increases in intracellular calcium concentrations and phosphorylation of p44/42 mitogen-activated protein kinase (ERK-1/2) in ovine pituitaries including primary gonadotropes in vitro. The objective of the present experiment was to determine if alphaT3-1 and LbetaT2 are cell models with limitations to examine the nonclassical actions of E2 occurring in gonadotropes. Experiments were conducted to determine if the cells have ESR1 at the plasma membrane using biotinylation cell and isolation of surface protein and staining with a fluorescently labeled E2 conjugate. The alphaT3-1 cells contain ESR1 associated with but not enriched within lipid rafts of the plasma membrane and do not translocate to lipid rafts upon binding of E2. In contrast, LbetaT2 cells lack ESR1 associated with the plasma membrane. Pretreatment with E2 did not cause inhibition of GnRH-stimulated increases in intracellular concentrations of calcium for either cell type. Phosphorylation of ERK-1/2 was not stimulated by E2 in either cell type. Although these cells lines have been used extensively to study GnRH signaling, in vitro or in vivo effects of nonclassical actions of E2 cannot be replicated in either cell line.

Gonadotrophin-releasing hormone (GnRH) and GnRH agonists: mechanisms of action

The hypothalamic decapeptide gonadotrophin-releasing hormone (GnRH) binds to specific receptors on pituitary gonadotrophs. These receptors belong to the family of G protein-coupled receptors. Their activation leads to phosphoinositide breakdown with generation of inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) and diacylglycerol. These second messengers initiate Ca2+ release from intracellular stores and activation of protein kinase C, both of which are important for gonadotrophin secretion and synthesis. Prolonged activation of GnRH receptors by GnRH leads to desensitization and consequently to suppressed gonadotrophin secretion. This is the primary mechanism of action of agonistic GnRH analogues. By contrast, GnRH antagonists compete with GnRH for receptors on gonadotroph cell membranes, inhibit GnRH-induced signal transduction and consequently gonadotrophin secretion. These compounds are free of agonistic actions, which might be beneficial in certain clinical applications.

Identification of estradiol/ERα-regulated genes in the mouse pituitary

Estrogen acts to prime the pituitary prior to the GnRH-induced LH surge by undiscovered mechanisms. This study aimed to identify the key components that mediate estrogen action in priming the pituitary. RNA extracted from the pituitaries of metestrous (low estrogen) and proestrus (high estrogen) stage mice, as well as from ovariectomized wild-type and estrogen receptor α (ERα) knockout mice treated with 17β-estradiol (E(2)) or vehicle, was used for gene expression microarray. Microarray data were then aggregated, built into a functional electronic database, and used for further characterization of E(2)/ERα-regulated genes. These data were used to compile a list of genes representing diverse biological pathways that are regulated by E(2) via an ERα-mediated pathway in the pituitary. This approach substantiates ERα regulation of membrane potential regulators and intracellular vesicle transporters, among others, but not the basic components of secretory machinery. Subsequent characterization of six selected genes (Cacna1a, Cacna1g, Cited1, Abep1, Opn3, and Kcne2) confirmed not only ERα dependency for their pituitary expression but also the significance of their expression in regulating GnRH-induced LH secretion. In conclusion, findings from this study suggest that estrogen primes the pituitary via ERα by equipping pituitary cells with critical cellular components that potentiate LH release on subsequent GnRH stimulations.

Actions of gonadotropin-releasing hormone analogues in pituitary gonadotrophs and their modulation by ovarian steroids

Recently, GnRH antagonists (GnRHant) like cetrorelix and ganirelix have been introduced in protocols of controlled ovarian hyperstimulation for assisted reproductive techniques to prevent premature luteinizing hormone (LH) surges. Here we tested, whether the actions of cetrorelix and the GnRH agonist (GnRHag) triptorelin in gonadotrophs are dependent on the steroid milieu. Furthermore, we characterized the actions of cetrorelix and triptorelin on LH secretion and the total LH pool. Female rat pituitary cells were treated either with 0.1 nM triptorelin for 1, 2, 4 and 6 days or for 1, 3, 5 and 6 h or with 1, 10 or 100 nM cetrorelix for 1, 2, 3 and 5 h or for 10 min. Cells were stimulated for 3h with different concentrations of GnRH (10 pM-1 microM). For analysis of the total LH pool, which is composed of stored and released LH, cells were lysed with 0.1% Triton X-100 at -80 degrees C overnight. To test, whether the steroid milieu affects the actions of cetrorelix and triptorelin, cells were incubated for 52 h with 1 nM estradiol (E) alone or with combinations of 100 nM progesterone (P) for 4 or 52 h, respectively. Cells were then treated with 0.1 nM triptorelin for 9 h or 1 nM cetrorelix for 3 h and stimulated for 3 h with different concentrations of GnRH (10 pM-1 microM). The suppressive effect of triptorelin on LH secretion was fully accomplished after 3 h of treatment, for cetrorelix only 10 min were sufficient. The concentration of cetrorelix must be at least equimolar to GnRH to block LH secretion. Cetrorelix shifted the EC50s of the GnRH dose-response curve to the right. Triptorelin suppressed total LH significantly (from 137 to 36 ng/ml) after 1 h in a time-dependent manner. In contrast, only high concentrations of cetrorelix increased total LH. In steroid treated cells the suppressive effects of triptorelin were more distinct. One nanomolar cetrorelix suppressed GnRH-stimulated LH secretion of cells not treated with steroids from 10.1 to 3.5 ng/ml. In cells, additionally treated with estradiol alone or estradiol and short-term progesterone, LH levels were higher (from 3.5 to 5.4 or 4.5 ng/ml, respectively). In cells co-treated with estradiol and progesterone for 52 h LH secretion was only suppressed from 10.1 to 9.5 ng/ml. Steroid treatments diminished the suppressive effect of cetrorelix on LH secretion. In conclusion, the depletion of the total LH pool contributes to the desensitizing effect of triptorelin. The actions of cetrorelix and triptorelin are dependent on the steroid milieu.

Short-term effects of IGF-I and estradiol on LH secretion from female rat gonadotrophs

OBJECTIVE

Growth factors and ovarian steroids modulate LH-secretion from pituitary gonadotrophs. Our previous studies demonstrated that long-term IGF-I treatment enhanced LH-secretion from female rat pituitary cells and estradiol facilitated this effect. The effects of estradiol on LH secretion are time-dependent. Short-term treatment inhibited, long-term treatment enhanced GnRH-induced LH-secretion in serum-containing medium. Here we tested the short-term actions of IGF-I and its interaction with estradiol and whether IGF-I is a prerequisite for the negative effect of short-term estradiol treatment in female rat pituitary cells.

DESIGN

Pituitary cells were incubated with a series of increasing concentrations of estradiol (1 pM, 10 pM, 50 pM, 100 pM, 500 pM, 1 nM, 10 nM and 100 nM) for 4 h, IGF-I (10 pM, 100 pM, 1 nM and 10 nM) for 4 h and 14 h and their combinations for 4h in serum-free medium, and then stimulated with 1 nM GnRH during the last 3h of incubation. To clarify the role of IGF-I, cells were incubated simultaneously with estradiol, IGF-I and antibody against IGF-I. LH was measured by radioimmunoassay.

RESULTS

Short-term IGF-I treatment did not modify basal or GnRH-induced LH-secretion. Short-term treatment with estradiol did not affect basal or GnRH-induced LH-secretion in serum-free medium. The addition of 100 pM IGF-I to serum-free medium established the negative effect of estradiol short-term treatment on GnRH-induced LH-secretion. The addition of IGF-I antibody fully abolished the negative effect of estradiol.

CONCLUSIONS

In conclusion, effects of IGF-I on LH-secretion in female rat pituitary cells require long-term treatment. The negative effect of estradiol short-term treatment on GnRH-induced LH-secretion is dependent on serum-containing medium or the addition of 100 pM IGF-I to serum-free medium.

The effect of stimulators and blockers of adrenergic receptors on LH secretion and cyclic nucleotide (cAMP and cGMP) production by porcine pituitary cells in vitro

The direct effects of alpha- and beta-adrenergic agents on luteinizing hormone (LH) secretion in vitro by porcine pituitary cells and the participation of secondary messengers, adenosine 3'5'-monophosphate (cAMP) and guanosine 3'5'-monophospate (cGMP), in transduction of signals induced by adrenergic agents and gonadotropin-releasing hormone (GnRH) in these cells have been investigated. Pituitary glands were obtained from mature gilts, which were ovariectomized (OVX) 1 month before slaughter. OVX gilts, assigned to four groups, were primed with: (1) vehicle (OVX); (2 and 3) estradiol benzoate (EB; 2.5mg/100kg b.w.) at 30-36h (OVX+EB I) or 60-66h (OVX+EB II) before slaughter, respectively; (4) progesterone (P(4); 120mg/100kg b.w.) for 5 consecutive days before slaughter (OVX+P(4)). Anterior pituitaries were dispersed with trypsin and then pituitary cells were cultured (10(6) per well) in McCoy's 5a medium containing horse serum (10%) and fetal calf serum (2.5%) for 3 days, at 37 degrees C and under the atmosphere of 95% air and 5% CO(2). On day 4 of the culture, the cells were submitted to 3.5h incubation in the presence of GnRH (a positive control), alpha- and beta-adrenergic agonists (phenylephrine (PHEN) and isoproterenol (ISOP), respectively), and alpha- and beta-adrenergic blockers (phentolamine (PHENT) and propranolol (PROP), respectively). The culture media were assayed for LH (experiment I) and cyclic nucleotides (experiment II). In experiment I, addition of GnRH (100ng/ml) increased LH secretion by pituitary cells taken from gilts of all experimental groups. The effects of alpha- and beta-adrenergic agents on LH secretion by the cells depended on hormonal status of gilts. The LH secretion by pituitary cells of OVX gilts was potentiated in the presence of PHEN (10, 100nM, and 1microM) and PHENT (1microM), alone or in combination with PHEN (100nM) and by the cells derived from OVX+EB I and OVX+P(4) animals in response to PHEN (100nM) and ISOP (1microM). ISOP (1microM) also stimulated LH secretion by the cells taken from OVX+EB II gilts. In experiment II, GnRH (100ng/ml) increased cGMP production by pituitary cells obtained from all groups of gilts and cAMP secretion by the cells taken from OVX and OVX+P(4) animals. PHEN (100nM) decreased and PROP (1microM) enhanced cAMP production by pituitary cells derived from OVX+EB I and OVX gilts, respectively. Moreover, PHEN (100nM) reduced, while PHENT (1microM) stimulated the release of cGMP by pituitary cells taken from OVX+EB II animals. In turn, ISOP (100nM) decreased and increased cGMP production by the cells derived from OVX+EB II and OVX+P(4) gilts, respectively. PROP (1microM) potentiated cGMP accumulation by pituitary cells taken from OVX+EB I and OVX+P(4) animals. In conclusion, our results suggest that adrenergic agents can modulate LH release by porcine pituitary cells acting through guanyl and adenylyl cyclase and in a manner dependent on hormonal status of gilts.

Ligand-independent activation of pituitary ER: dependence on PKA-stimulated pathways

In pituitary and other target tissues, estrogen acts through ERs, which are ligand-activated nuclear transcription factors. ERs can also be activated by intracellular signaling pathways in a ligand-independent manner in some cells. Because the pituitary is the target of several cAMP-activating factors, we examined the ability of cAMP to activate ERs in the alphaT3 gonadotrope cell line. Forskolin, 8-bromo-cAMP, and pituitary adenylate cyclase-activating polypeptide all enhanced ER-dependent promoter activity, which was inhibited by antiestrogen or a pituitary-specific inhibitory ER variant. Activation was PKA dependent and was blocked by the PKA inhibitor H89 or cotransfection of the inhibitor PKI. Although cAMP activated MAPK in alphaT3 cells, inhibition of MAPK with the MEK inhibitor PD98059 did not prevent forskolin-induced ER activation. Similarly, epidermal growth factor did not stimulate ER activity, although it increased MAPK activation. Forskolin-induced activation of ER was enhanced by cotransfection of steroid receptor coactivator-1 and was inhibited by the repressor of ER action, suggesting that cAMP does not alter the normal interactions between ER and cofactors. In contrast to results with estrogen, cAMP treatment did not decrease ER protein levels. These results demonstrate that in the pituitary, cAMP activates ER in a ligand-independent manner exclusively through PKA.

Glue secretion in the Drosophila salivary gland: a model for steroid-regulated exocytosis

Small hydrophobic hormones like steroids control many tissue-specific physiological responses in higher organisms. Hormone response is characterized by changes in gene expression, but the molecular details connecting target-gene transcription to the physiology of responding cells remain elusive. The salivary glands of Drosophila provide an ideal model system to investigate gaps in our knowledge, because exposure to the steroid 20-hydroxyecdysone (20E) leads to a robust regulated secretion of glue granules after a stereotypical pattern of puffs (activated 20E-regulated genes) forms on the polytene chromosomes. Here, we describe a convenient bioassay for glue secretion and use it to analyze mutants in components of the puffing hierarchy. We show that 20E mediates secretion through the EcR/USP receptor, and two early-gene products, the rbp(+) function of BR-C and the Ca2+ binding protein E63-1, are involved. Furthermore, we demonstrate that 20E treatment of salivary glands leads to Ca2+ elevations by a genomic mechanism and that elevated Ca2+ levels are required for ectopically produced E63-1 to drive secretion. The results presented establish a connection between 20E exposure and changes in Ca2+ levels that are mediated by Ca2+ effector proteins, and thus establish a mechanistic framework for future studies.

In vitro application of testosterone potentiates gonadotropin-releasing hormone-stimulated gonadotropin-II secretion from cultured goldfish pituitary cells

The in vitro effects of overnight treatment with testosterone (T) on gonadotropin (GTH)-II secretion from primary cultures of dispersed female goldfish pituitary cells were examined. T (100 nM) did not affect basal GTH-II release, but increased GTH-II responses to initial applications of 0.5-h pulses of sGnRH or cGnRH-II in cells from females at sexually regressed, recrudescing, or mature (prespawning) stages. Pretreatment with 10 nM T was also effective, except in experiments with cells from sexually regressed females. Analysis of GTH-II response profiles to the first GnRH pulse revealed that T increased the size of the initial (first 15 min) and sustained (rest of response) release phases, and the duration of the total response to both GnRHs. These results indicate that direct positive influence of T on GnRH-stimulated GTH-II release is demonstrable in cells from female goldfish at all ovarian maturational stages; in addition, T affects both the initial and the sustained response phases. However, compared to the initial GnRH challenge, responses to a second 0.5-h GnRH pulse were decreased in T-treated but not in control cells, suggesting that T also enhanced desensitization. Ovarian maturational conditions modulated the effects of T on the GTH-II response kinetics. In cells prepared from sexually regressed females, T treatment changed the "monophasic" (initial phase only) GTH-II response to sGnRH to a "biphasic" one characteristic of cells prepared from fish at later stages of gonadal recrudescence. Advancing gonadal maturity increased the magnitude of both initial and sustained phases of the T-enhanced GTH-II response to sGnRH, but only elevated the initial phase of T-potentiated cGnRH-II-induced release. Direct actions of T on pituitary cells may play a role in ovarian steroid feedback regulation of GTH-II secretion during the seasonal reproductive cycle in goldfish.

Dual action of androgen on calcium signaling and luteinizing hormone secretion in pituitary gonadotrophs

An increase in serum androgen levels associated with a suppression of cyclic gonadotropin secretion is frequently observed in females with impaired ovarian function. Here, we addressed the hypotheses that androgens (testosterone and dihydrotestosterone) alter gonadotropin secretion by modulating agonist-induced Ca2+ signaling and/or Ca(2+)-controlled exocytosis. In mixed populations of pituitary cells from female rats, addition of testosterone reduced basal and agonist (GnRH)-induced gonadotropin secretion in a concentration- and time-dependent manner. The suppressive actions of this androgen on gonadotropin secretion were observed over the full GnRH concentration range. Reduction in agonist-induced gonadotropin secretion was also observed after addition of dihydrotestosterone, indicating that the inhibitory action of testosterone is not mediated by its conversion to estradiol. Both the extracellular Ca(2+)-independent spike phase and extracellular Ca(2+)-dependent sustained phase of GnRH-induced gonadotropin secretions were affected by testosterone. In part, the inhibitory action of testosterone was mediated by attenuation of GnRH-induced InsP3 production and InsP3-dependent Ca2+ mobilization. In addition, testosterone exhibited a Ca(2+)-independent action on gonadotropin secretion, as documented by attenuation of high potassium-induced secretion without an affect on depolarization-induced Ca2+ signals. These results suggest that androgen inhibition of gonadotropin secretion occurs at two distinct steps in the secretory pathway, one prior to and one after elevation in cytosolic Ca2+ concentration.

In vitro action of testosterone in potentiating gonadotropin-releasing hormone-stimulated gonadotropin-II secretion in goldfish pituitary cells: involvement of protein kinase C, calcium, and testosterone metabolites

Overnight preincubation of goldfish pituitary cell culture with testosterone (T) enhanced the gonadotropin (GTH)-II responses to GTH-releasing hormone (GnRH). In this study, the involvement of GnRH signal transduction components and the requirement for T metabolism in mediating this direct, pituitary cell action of T were examined using cultured pituitary cells from both male and female goldfish. Each sets of related experiments were done in at least two different stages of the gonadal reproductive cycle and similar effects were observed. Overnight treatment with 10 nM T increased GTH-II responses to maximal stimulatory doses (100 nM) of either salmon (s)GnRH or chicken (c)GnRH-II, but not the total cellular GTH-II contents measured prior to and after a 2-h GnRH challenge. T increased the efficacy and sensitivity of the GTH-II response to stimulation by a protein kinase C (PKC) activator, tetradecanoyl phorbol acetate (TPA) without altering the ED50 of the dose-response curve. In T-treated cells, addition of a PKC inhibitor attenuated GTH-II responses to 100 nM doses of sGnRH, cGnRH-II, or TPA. T did not affect the GTH-II release stimulated by high concentrations of the Ca2+ ionophore ionomycin (100 microM) and the voltage-sensitive Ca2+ channel (VSCC) agonist Bay K 8644 (10 microM); similarly, the sensitivity of the GTH-II response to ionomycin and Bay K 8644 was also unaltered. Taken together, these data suggest that T potentiates GnRH-stimulated GTH-II release by enhancing the effectiveness of PKC-dependent pathways, but not by increasing the total Ca2+-sensitive GTH-II pool, the sensitivity of the release response to increases in intracellular Ca2+, or the amount of available GTH-II. However, the VSCC agonist nifedipine reduced sGnRH- and cGnRH-II-elicited GTH-II release in T-treated as well as in non-T-treated cells, suggesting that VSCC dependence is still present in the GnRH-induced response following exposure to T. Since total cGnRH-II binding to pituitary cells was not increased by T, increases in GnRH receptor capacity are unlikely following T treatment. The ability of T to increase GnRH-stimulated GTH-II secretion was not mimicked by 11-ketotestosterone or dihydrotestosterone, but was abolished by coincubation with an aromatase inhibitor. When viewed together, these observations suggest that aromatization of T may be required for the pituitary action of T on GnRH-induced GTH-II release.

The regulation of gonadotropin-releasing hormone-induced calcium signals in male rat gonadotrophs by testosterone is mediated by dihydrotestosterone

The biological effects of testosterone (T) may be mediated directly by T or indirectly by its metabolites, dihydrotestosterone (DHT) and estradiol. The present study examined whether the metabolism of T is involved in the regulation of GnRH-induced Ca2+ signaling at the pituitary. In gonadotrophs from castrated rats, a significantly greater percentage of gonadotrophs demonstrated oscillatory Ca2+ responses to 100 nM GnRH than cells from intact rats (72% vs. 24%; P < 0.05). This increase was prevented by the administration of T propionate (0.1 mg/kg x day), DHT benzoate (2 mg/kg x day,), estradiol benzoate (EB; 5 microg/kg x day), or the combination of the above doses of DHT benzoate and EB. In all cases the proportion of gonadotrophs from the steroid-treated rats having oscillatory Ca2+ responses to 100 nM GnRH was between 21-25% (P > 0.05, compared with intact rats). To assess the importance of T metabolism, intact male rats were treated with the aromatase inhibitor letrozole (1 mg/kg x day), the 5alpha-reductase inhibitor finasteride (50 mg/kg x day), or their respective vehicles for 7 days. Letrozole had no effect on GnRH-induced Ca2+ signals, serum LH concentrations, or ventral prostate or testes weight. Finasteride treatment, however, mimicked the effects of castration, with significantly more gonadotrophs exhibiting Ca2+ oscillations in response to 100 nM GnRH than gonadotrophs from the vehicle-treated group (71% vs. 20% respectively; P < 0.05). Finasteride also caused a significant (P < 0.05) decrease in prostatic weight and DHT concentration, but had no significant effect on either prostatic T or serum LH concentrations. These findings suggest that in the intact male rat, the effects of T on GnRH-induced Ca2+ signaling are preferentially mediated via DHT. The results of this study also show that in the absence of androgens, estradiol may regulate GnRH-induced Ca2+ signaling in the male rat pituitary.