A Nongenomic Action of 17β-Estradiol as the Mechanism Underlying the Acute Suppression of Secretion of Luteinizing Hormone1

Kisspeptin has an independent and direct effect on the pituitary gland in the mare

To more clearly understand the equine gonadotrope response to kisspeptin and gonadotropin releasing hormone (GnRH), peripheral LH and FSH were quantified in diestrous mares after treatment with either equine kisspeptide (eKp-10, 0.5 mg iv), GnRH (25 μg iv), or a combination thereof every 4 h for 3 days. The following observations were made: 1) a diminished LH and FSH response to eKp-10 and GnRH was observed by Day 3, but was not different by treatment, 2) a decrease in basal LH concentration was observed from Day 1 to Day 3 for the eKp-10, but not the GnRH treated mares, 3) there was no change in basal FSH with either treatment. Additionally, pre-treatment with GnRH antagonist (antide 1.0 mg iv) eliminated any measurable change in LH after eKp-10 (1.0 mg iv) treatment. Both GnRH and kisspeptin are Gα_q/11 coupled receptors, therefore quantifying the rise in intracellular calcium following treatment with cognate ligand allows simultaneous assessment of receptor activation. Direct stimulation of equine primary pituitary cells with GnRH and/or eKp-10 demonstrates three distinct populations of pituitary cells: one population responded to both eKp-10 and GnRH, a second, independent population, responded to only eKp-10, and a third population responded only to GnRH. These populations were confirmed using co-immunofluorescence of hemipituitaries from mares in diestrus. Although the rise in peripheral LH concentration elicited by eKp-10 is dependent on GnRH, this work suggests that kisspeptin also has a specific and direct effect on the equine gonadotrope, independent of GnRH.

Discovery of new receptors regulating luteinizing hormone and follicle-stimulating hormone secretion by bovine gonadotrophs to explore a new paradigm for mechanisms regulating reproduction

Previous studies in the 1960s and 1970s have reported that both gonadotropin-releasing hormone (GnRH) and estradiol-activated nuclear estrogen receptors regulate gonadotropin secretion in women. However, I had previously reported that gonadotroph function is regulated by complex crosstalk between several membrane receptors. RNA-seq had previously revealed 259 different receptor genes expressed in the anterior pituitary of heifers. However, the biological roles of most of these receptors remain unknown. I identified four new receptors of interest: G protein-coupled receptor 30 (GPR30), anti-Mullerian hormone (AMH) receptor type 2 (AMHR2), and G protein-coupled receptors 61 and 153 (GPR61 and GPR153). GPR30 rapidly (within a few minutes) mediates picomolar, but not nanomolar, levels of estradiol to suppress GnRH-induced luteinizing hormone (LH) secretion from bovine gonadotrophs, without decreasing mRNA expressions of the LHα, LHβ, or follicle-stimulating hormone (FSH) β subunits. GPR30 is activated by other endogenous estrogens, estrone and estriol. Moreover, GPR30 activation by zearalenone, a nonsteroidal mycoestrogen, suppresses LH secretion. AMHR2, activated by AMH, stimulates LH and FSH secretion, thus regulating gonadotrophs, where other TGF-β family members, including inhibin and activin, potentially affect FSH secretion. I also show that GPR61, activated by its ligand (recently discovered) significantly alters LH and FSH secretion. GPR61, GPR153, and AMHR2 co-localize with the GnRH receptor in unevenly dispersed areas of the bovine gonadotroph cell surface, probably lipid rafts. The findings summarized in this review reveal a new paradigm regarding the mechanisms regulating reproduction via novel receptors expressed on bovine 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.

Reconsidering the roles of endogenous estrogens and xenoestrogens: the membrane estradiol receptor G protein-coupled receptor 30 (GPR30) mediates the effects of various estrogens

Estrone (E1) and estriol (E3) are considered “weak” estrogens, which exert suppressive effects through estrogen receptors α and β. However, recent studies have demonstrated that E1 and E3, as well as estradiol (E2), suppress gonadotropin-releasing hormone-induced luteinizing hormone secretion from bovine gonadotrophs via G-protein-coupled receptor 30, which is expressed in various reproductive organs. Currently, there is a lack of fundamental knowledge regarding E1 and E3, including their blood levels. In addition, xenoestrogens may remain in the body over long time periods because of enterohepatic circulation. Therefore, it is time to reconsider the roles of endogenous estrogens and xenoestrogens for reproduction.

GPR30 mediates estrone, estriol, and estradiol to suppress gonadotropin-releasing hormone-induced luteinizing hormone secretion in the anterior pituitary of heifers

Recent studies demonstrated that G-protein-coupled receptor 30 (GPR30) on the plasma membrane of gonadotroph cells mediates picomolar, but not nanomolar, levels of estradiol (E2) to rapidly suppress gonadotropin-releasing hormone (GnRH)-induced luteinizing hormone (LH) secretion in the anterior pituitary (AP). While estrone (E1) and estriol (E3) are considered "weak" estrogens that exert suppressive effects through estrogen receptors α and β, it is conceivable that they also strongly suppress GnRH-induced LH secretion via GPR30. Both E1 and E3 are likely present within the blood at picomolar or nanomolar concentrations, indicating that such concentrations are sufficient to suppress GnRH-induced LH secretion. To evaluate this possibility, bovine AP cells were cultured under steroid-free conditions and then incubated with various concentrations (0.01 pM to 10 nM) of E2, E1, or E3, prior to stimulation with GnRH. Notably, GnRH-induced LH secretion from AP cells was inhibited by 1-100 pM E2, 1-10 pM E1, and 1-100 pM E3. GnRH-induced LH secretion from AP cells was not inhibited by lower (0.01-0.1 pM) or higher (1-10 nM) concentrations of E2, E1, and E3. These suppressive effects were inhibited by pre-treatment of AP cells with the GPR30 antagonist G36, but not with the estrogen receptor alpha antagonist. Treatment with E1 or E3 also yielded decreased cytoplasmic cAMP levels in cultured AP cells pre-treated with dopamine and phosphodiesterase inhibitors. Therefore, these results suggest that GPR30 mediates the suppressive effects of E1, E3, and E2 on GnRH-induced LH secretion from bovine AP.

Cytoplasmic kinases downstream of GPR30 suppress gonadotropin-releasing hormone (GnRH)-induced luteinizing hormone secretion from bovine anterior pituitary cells

GPR30 is known as a membrane receptor for picomolar concentrations of estradiol. The GPR30-specific agonist G1 causes a rapid, non-genomic suppression of gonadotropin-releasing hormone (GnRH)-induced luteinizing hormone (LH) secretion from bovine anterior pituitary (AP) cells. A few studies have recently clarified that protein kinase A (PKA) and phosphorylated extracellular signal-regulated kinase (pERK) might be involved in cytoplasmic signaling pathways of GPR30 in other cells. Therefore, we tested the hypothesis that PKA and ERK kinase (MEK) are important cytoplasmic mediators for GPR30-associated non-genomic suppression of GnRH-induced LH secretion from bovine AP cells. Bovine AP cells (n = 8) were cultured for 3 days under steroid-free conditions. The AP cells were previously treated for 30 min with one of the following: 5000 nM of PKA inhibitor (H89), 1000 nM of MEK inhibitor (U0126), or a combination of H89 and U0126. Next, the AP cells were treated with 0.01 nM estradiol for 5 min before GnRH stimulation. Estradiol treatment without inhibitor pretreatment significantly suppressed GnRH-induced LH secretion (P < 0.01). In contrast, estradiol treatment after pretreatment with H89, U0126 or their combination had no suppressive effect on GnRH-induced LH secretion. The inhibitors also inhibited the G1 suppression of GnRH-induced LH secretion. Therefore, these data supported the hypothesis that PKA and MEK (thus, also pERK) are the intracellular mediators downstream of GPR30 that induce the non-genomic suppression of GnRH-induced LH secretion from bovine AP cells by estradiol or G1.

The non-steroidal mycoestrogen zeranol suppresses luteinizing hormone secretion from the anterior pituitary of cattle via the estradiol receptor GPR30 in a rapid, non-genomic manner

Picomolar concentrations of estradiol produce rapid suppression of GnRH-induced luteinizing hormone (LH) secretion from the anterior pituitary (AP) of cattle via G-protein-coupled receptor 30 (GPR30). Zeranol is a strong estrogenic metabolite derived from zearalenone, a non-steroidal mycoestrogen produced by Fusarium that induces reproductive disorders in domestic animals. The hypothesis was tested that zeranol suppresses GnRH-induced LH release from the AP of cattle via GPR30 in a rapid, non-genomic manner. The AP cells (n=15) were cultured for 3 days in steroid-free conditions and then treated them with estradiol (0.001-10nM) or zeranol (0.001-100nM) for 5min before GnRH stimulation. Pre-treatment with 0.001-0.1nM estradiol suppressed GnRH-stimulated LH secretion. Pre-treatment with zeranol at concentrations of 0.001nM (P<0.01), 0.01nM (P<0.01), 0.1nM (P<0.05), and 1nM (P<0.05), but not at concentrations of 10 and 100nM, also inhibited GnRH-stimulated LH secretion from AP cells. Pre-treatment for 5min with a GPR30-specific antagonist, G36, inhibited estradiol or zeranol suppression of LH secretion from cultured AP cells. Cyclic AMP measurements and quantitative PCR analyses revealed that pre-treatment with small amounts of estradiol (P<0.05) or zeranol (P<0.01) decreased cAMP, but not gene expressions of the LHα, LHβ, or FSHβ subunits in the AP cells. Hence, zeranol may suppress luteinizing hormone secretion from the AP of cattle via GPR30 in a rapid, non-genomic manner.

Effects of STX, a novel estrogen membrane receptor agonist, on GnRH-induced luteinizing hormone secretion from cultured bovine anterior pituitary cells

STX is an agonist for a recently characterized membrane estrogen receptor whose structure has not been identified. We evaluated whether STX suppresses gonadotropin-releasing hormone (GnRH)-induced luteinizing hormone (LH) release from bovine anterior pituitary (AP) cells. We cultured AP cells (n=12) for 3 days in steroid-free conditions, followed by increasing concentrations (0.001, 0.01, 0.1, 1 and 10 nM) of 17β-estradiol or STX for 5 min before GnRH stimulation until the end of the experiment. Estradiol (0.001 to 0.1 nM) significantly suppressed GnRH-stimulated LH secretion, whereas STX did not affect GnRH-stimulated LH secretion at any of the tested concentrations. In conclusion, STX, unlike estradiol, possesses no suppressive effect on GnRH-induced LH release from bovine AP cells.

Expression of estradiol receptor, GPR30, in bovine anterior pituitary and effects of GPR30 agonist on GnRH-induced LH secretion

G-protein - coupled receptor 30 (GPR30) is an estradiol receptor located on the plasma membrane, and it initiates several rapid, non-genomic signaling events. GPR30 has recently been identified in rat anterior pituitary (AP); however, little is known about the role of GPR30 in controlling luteinizing hormone (LH) secretion from gonadotropes in animals. To fill this research gap, we hypothesized that GPR30 is expressed in bovine AP and mediates estradiol inhibition of gonadotropin-releasing hormone (GnRH)-induced LH release. We confirmed the expressions of GPR30 mRNA and protein by RT-PCR, western blotting, and immunohistochemistry. We cultured bovine AP cells (n=8) for 3 days in steroid-free conditions and then treated them with increasing concentrations (0.001nM, 0.01nM, 0.1nM, 1nM, and 10nM) of estradiol or a GPR30-specific agonist, G1, for 5min before GnRH stimulation. As expected, estradiol at 0.001-0.1nM inhibited the GnRH-stimulated LH secretion. However, we found also that G1 at 0.001nM was able to inhibit this secretion (P<0.05). In contrast, both estradiol and G1 at higher doses were less efficient in suppressing the GnRH-stimulated LH secretion. Neither estradiol nor G1 suppressed GnRH-stimulated follicle-stimulating hormone secretion. In separate experiments, fluorescent immunohistochemistry and immunocytochemistry revealed that approximately 50% of GPR30-positive cells express LH, and about 30% of LH-positive cells express GPR30. In conclusion, GPR30 is expressed in bovine gonadotropes and other AP cells and may partially contribute to rapid negative estradiol feedback of GnRH-induced LH secretion.

Ovulation-inducing factor (OIF) induces LH secretion from pituitary cells

A substance in the seminal plasma of llamas and alpacas has been discovered that induces ovulation and growth of the corpus luteum (CL) in the female of the same species. The ovarian effects of the ovulation-inducing factor (OIF) are associated with a surge release of LH into circulation. We hypothesize that OIF stimulates LH release from gonadotroph cells in the anterior pituitary gland. Four experiments were done to determine if purified OIF isolated from llama seminal plasma stimulates LH secretion in pituitary cells using tissue from an induced ovulator (llama) and spontaneous ovulator (cattle). Anterior pituitary cells were cultured in vitro for two days, and on the third day, wells were incubated for 2 h with media containing no treatment (control), GnRH or OIF. Concentrations of LH in the culture medium were measured using radioimmunoassay and compared among groups by analysis of variance. In all experiments, GnRH and OIF treatments induced more LH secretion than untreated controls (P<0.05). A dose-related effect was evident in the llama pituitary cell cultures in that mean LH concentrations were greater (P<0.05) in wells treated with a higher dose of OIF (5.41 ± 0.28 ng/mL) compared to wells treated with a lower dose (2.70 ± 0.50 ng/mL), both of which were higher (P<0.05) than in wells with no treatment (0.87 ± 0.18 ng/mL). Although OIF stimulated LH release in bovine cell cultures, a dose-related effect was not detected. We conclude that OIF stimulates LH secretion from pituitary gonadotrophs in vitro.

Does a nonclassical signaling mechanism underlie an increase of estradiol-mediated gonadotropin-releasing hormone receptor binding in ovine pituitary cells?

Estradiol-17beta (E2) is the major regulator of GnRH receptor (GnRHR) gene expression and number during the periovulatory period; however, the mechanisms underlying E2 regulation of the GNRHR gene remain undefined. Herein, we find that E2 conjugated to BSA (E2-BSA) mimics the stimulatory effect of E2 on GnRH binding in primary cultures of ovine pituitary cells. The time course for maximal GnRH analog binding was similar for both E2 and E2-BSA. The ability of E2 and E2-BSA to increase GnRH analog binding was blocked by the estrogen receptor (ER) antagonist ICI 182,780. Also, increased GnRH analog binding in response to E2 and the selective ESR1 agonist propylpyrazole triol was blocked by expression of a dominant-negative form of ESR1 (L540Q). Thus, membrane-associated ESR1 is the likely candidate for mediating E2 activation of the GNRHR gene. As cAMP response element binding protein (CREB) is an established target for E2 activation in gonadotrophs, we next explored a potential role for this protein as an intracellular mediator of the E2 signal. Consistent with this possibility, adenoviral-mediated expression of a dominant-negative form of CREB (A-CREB) completely abolished the ability of E2 to increase GnRH analog binding in primary cultures of ovine pituitary cells. Finally, the presence of membrane-associated E2 binding sites on ovine pituitary cells was demonstrated using a fluorescein isothiocyanate conjugate of E2-BSA. We suggest that E2 regulation of GnRHR number during the preovulatory period reflects a membrane site of action and may proceed through a nonclassical signaling mechanism, specifically a CREB-dependent pathway.

Physiological consequences of membrane-initiated estrogen signaling in the brain

Many of the actions of 17beta-estradiol (E2) in the central nervous system (CNS) are mediated via the classical nuclear steroid receptors, ER(alpha) and ERbeta, which interact with the estrogen response element to modulate gene expression. In addition to the nuclear-initiated estrogen signaling, E2 signaling in the brain can occur rapidly within minutes prior to any sufficient effects on transcription of relevant genes. These rapid, membrane-initiated E2 signaling mechanisms have now been characterized in many brain regions, most importantly in neurons of the hypothalamus and hippocampus. Furthermore, our understanding of the physiological effects of membrane-initiated pathways is now a major field of interest in the hypothalamic control of reproduction, energy balance, thermoregulation and other homeostatic functions as well as the effects of E2 on physiological and pathophysiological functions of the hippocampus. Membrane signaling pathways impact neuronal excitability, signal transduction, cell death, neurotransmitter release and gene expression. This review will summarize recent findings on membrane-initiated E2 signaling in the hypothalamus and hippocampus and its contribution to the control of physiological and behavioral functions.

Intracellular, not membrane, estrogen receptors control vitellogenin synthesis in the rainbow trout

The synthesis of vitellogenin, via estrogens, by the liver of female oviparous vertebrates provides the precursor for yolk proteins in developing oocytes. There are two distinct estrogenic transduction pathways in vertebrates that could control vitellogenin synthesis. One provides direct genomic (i.e., nuclear) control in which estrogens bind to estrogen receptors (ERs) that function as transcription factors within the cell nucleus. The other involves a non-genomic pathway initiated by estrogens binding to membrane-bound ERs at the cell surface. The objective of this paper was to determine which ER transduction pathway regulates hepatic vitellogenin synthesis in rainbow trout. For this study an estrogenic molecule, 17alpha-ethynylestradiol (EE2), was conjugated to a peptide moiety (PEP) to make 17alpha-ethynylestradiol-peptide (EE2-PEP) to bind to membrane-bound ERs. This was compared with EE2 that is capable of crossing the cell membrane and binding to intracellular ERs. An in vivo experiment using male rainbow trout injected with either EE2-PEP or EE2 demonstrated that only EE2 stimulated a significant increase in plasma vitellogenin concentrations. This was further confirmed by treating male rainbow trout hepatocytes in primary culture for 24h with PEP, EE2-PEP or EE2. Only the EE2 treatment resulted in significantly higher vitellogenin expression in trout hepatocytes. These results demonstrate that estrogens must gain entry into hepatocytes to bind to intracellular ERs in order to stimulate vitellogenin synthesis. While this study cannot conclude that a membrane ER system is absent in the rainbow trout liver, it has established that the liver synthesis of vitellogenin in rainbow trout is regulated by intracellular ERs.

Membrane-initiated actions of estradiol (E2) in the regulation of LH secretion in ovariectomized (OVX) ewes

BACKGROUND

We demonstrated that E2 conjugated to BSA (E2BSA) induces a rapid membrane-initiated inhibition of LH secretion followed hours later by a slight increase in LH secretion. Whether these actions of E2BSA are restricted to the pituitary gland and whether the membrane-initiated pathway of E2BSA contributes to the up-regulation of the number of GnRH receptors during the positive feedback effect of E2 were evaluated here. We have shown that the suppression of LH secretion induced by E2 and E2BSA is the result of a decreased responsiveness of the pituitary gland to GnRH. In this study we further tested the ability of E2BSA to decrease the responsiveness of the pituitary gland to GnRH under the paradigm of the preovulatory surge of LH induced by E2.

METHODS

For the first experiment GnRH and LH secretions were determined in samples of pituitary portal and jugular blood, respectively, in ewes treated with 12 mg E2BSA. In the second experiment, the number of GnRH receptors was quantified in ewes 12 h after administration of 25 micrograms E2 (the expected time for the increase in the number of GnRH receptors and the positive feedback effect of E2 in LH secretion) or 12 mg E2BSA. In the third experiment, the preovulatory-like surge of LH was characterized in ewes injected with 25 micrograms E2 alone or followed 8 h later (before the beginning of the LH surge) with 60 mg E2BSA.

RESULTS

a) the decrease in LH secretion induced by E2BSA was not accompanied by changes in the pulsatile pattern of GnRH, b) E2BSA increased the number of GnRH receptors, and c) the presence of E2BSA in E2-treated ewes delayed the onset, reduced the length, and decreased the amount of LH released during the preovulatory surge of LH.

CONCLUSIONS

a) the rapid suppression of LH secretion induced by E2BSA is mediated only via a direct action on the pituitary gland, b) E2 acting via a membrane-initiated pathway contributes to increase the number of GnRH receptors and, c) administration of E2BSA near the beginning of the pre-ovulatory surge of LH delays and reduces the magnitude of the surge.

Estradiol-17beta inhibits gonadotropin-releasing hormone-induced Ca2+ in gonadotropes to regulate negative feedback on luteinizing hormone release

In pituitary gonadotropes, estrogens have biphasic actions to cause an initial negative feedback followed by a positive feedback on LH secretion, but the mechanisms involved are not clearly understood. To investigate the feedback effects of estrogen, we used mixed ovine pituitary cell cultures (48-72 h), which were treated with 10(-9) M estradiol-17beta (E(2)) or vehicle followed by a pulse of 10(-9) M GnRH. Medium was collected for LH assay and cells extracted to determine activation of MAPK (phosphorylated ERK-1/2). E(2) treatment for 5 min reduced GnRH-induced LH release and caused phosphorylation of ERK-1/2. E(2) alone also caused phosphorylation of ERK-1/2, similar to the response evoked by GnRH alone. GnRH increased cytoplasmic intracellular free calcium concentration ([Ca(2+)](i)) and this was abolished by 2 min pretreatment with E(2) or E-bovine serum albumen conjugate. Blockade of Ca(2+) channels with nifedipine had no effect on the initial peak of GnRH-induced increase in [Ca(2+)](i) but reduced its duration by 27 +/- 6%. Depletion of intracellular Ca(2+) stores with thapsigargin prevented GnRH-induced increase in [Ca(2+)](i). Thapsigargin (10(-7) M) or nifedipine (10(-5) M) pretreatment (15 min) of cells lowered GnRH-induced LH secretion by 30 +/- 6 and 50% +/- 4%, respectively. We conclude that inhibition of the GnRH-induced increase in [Ca(2+)](i) in gonadotropes by E(2) is a likely mechanism for the negative feedback effect of E(2) on LH secretion involving a rapid nongenomic effect of E(2). Activation of the MAPK pathway by E(2) may be the mechanism for the time-delayed positive feedback effect on LH secretion at the level of the gonadotrope.

Non-genomic actions of progesterone and estrogens in regulating reproductive events in domestic animals

It has been established that nuclear receptors mediate the action of estrogens and progestins in regulating gene expression in the hypothalamic-hypophyseal-gonadal axis of domestic animals during various reproductive states. Results of recent in vitro studies suggest that estradiol-17beta and progesterone can act non-genomically to affect signal transduction responses in target cells by binding to receptors in the plasma membrane. The genomic action of steroids is generally detectable in hours to days whereas non-genomic responses of cells occur in seconds to minutes. The nature of the plasma membrane receptors for estrogens and progesterone has been explored but has not been conclusively established for all cell types studied. In the ewe, estradiol-17beta or estradiol-bovine serum albumin conjugate has been shown by in vitro and in vivo approaches to act non-genomically to suppress luteinizing hormone secretion by gonadotropes and stimulate production of nitric oxide by uterine arterial endothelial cells. Progesterone has been shown to inhibit oxytocin (OT) binding to its receptor in isolated ovine endometrial plasma membranes. This non-genomic action of progesterone blocks OT activation of the phosphoinositide cascade and production of prostaglandin F(2alpha) by ovine and bovine endometrium. The acrosome reaction of caprine and porcine spermatozoa is activated by the non-genomic action of progesterone. Further research is required to define the biological significances of the non-genomic actions of estrogens and progestins.

Insight into the neuroendocrine site and cellular mechanism by which cortisol suppresses pituitary responsiveness to gonadotropin-releasing hormone

Stress-like elevations in plasma glucocorticoids rapidly inhibit pulsatile LH secretion in ovariectomized sheep by reducing pituitary responsiveness to GnRH. This effect can be blocked by a nonspecific antagonist of the type II glucocorticoid receptor (GR) RU486. A series of experiments was conducted to strengthen the evidence for a mediatory role of the type II GR and to investigate the neuroendocrine site and cellular mechanism underlying this inhibitory effect of cortisol. First, we demonstrated that a specific agonist of the type II GR, dexamethasone, mimics the suppressive action of cortisol on pituitary responsiveness to GnRH pulses in ovariectomized ewes. This effect, which became evident within 30 min, documents mediation via the type II GR. We next determined that exposure of cultured ovine pituitary cells to cortisol reduced the LH response to pulse-like delivery of GnRH by 50% within 30 min, indicating a pituitary site of action. Finally, we tested the hypothesis that suppression of pituitary responsiveness to GnRH in ovariectomized ewes is due to reduced tissue concentrations of GnRH receptor. Although cortisol blunted the amplitude of GnRH-induced LH pulses within 1-2 h, the amount of GnRH receptor mRNA or protein was not affected over this time frame. Collectively, these observations provide evidence that cortisol acts via the type II GR within the pituitary gland to elicit a rapid decrease in responsiveness to GnRH, independent of changes in expression of the GnRH receptor.

Differential Modulation of Gonadotropin Secretion by Selective Estrogen Receptor 1 and Estrogen Receptor 2 Agonists in Ovariectomized Ewes1

The objectives of this study were to determine whether activation of estrogen receptor 1 (ESR1; also known as ERalpha), or estrogen receptor 2 (ESR2; also known as ERbeta), or both are required to: 1) acutely inhibit secretion of LH, 2) induce the preovulatory-like surge of LH, and 3) inhibit secretion of FSH in ovariectomized (OVX) ewes. OVX ewes (n = 6) were administered intramuscularly 25 micrograms estradiol (E2), 12 mg propylpyrazoletriol (PPT; a subtype-selective ESR1 agonist), 21 mg diaprylpropionitrile (DPN; a subtype-selective ESR2 agonist), or PPT + DPN. Like E2, administration of PPT, DPN, or combination of the two rapidly decreased (P < 0.05) secretion of LH. Each agonist induced a gradual, prolonged rise in secretion of LH after the initial inhibition, but neither agonist alone nor the combined agonists was able to induce a "normal" preovulatory-like surge of LH similar to that induced by E2. Compared with E2-treated ewes, the beginning of the increase in secretion of LH occurred earlier (P < 0.01) in DPN-treated ewes, later (P < 0.05) in PPT-treated ewes, and at a similar interval in ewes receiving the combined agonist treatment. Like E2, PPT decreased (P < 0.05) secretion of FSH, but the duration of suppression was much longer in PPT-treated ewes. DPN did not alter secretion of FSH in this study. Modulation of the number of GnRH receptors by PPT and DPN was examined in primary cultures of ovine pituitary cells. In our hands, both PPT and DPN increased the number of GnRH receptors, but the dose of DPN required to stimulate synthesis of GnRH receptors was 10 times higher than that of PPT. We conclude that in OVX ewes: 1) ESR1 and ESR2 mediate the negative feedback of E2 on secretion of LH at the level of the pituitary gland, 2) ESR1 and ESR2 do not synergize or antagonize the effects of each other; however, they do interact to synchronize the beginning of the stimulatory effect of E2 on secretion of LH, 3) ESR1 and ESR2 may mediate at least partially the positive feedback of E2 on LH secretion by increasing the number of GnRH receptors, and 4) only ESR1 appears to be involved in the negative feedback of E2 on secretion of FSH.

Equol and para-ethyl-phenol stimulate prostaglandin F(2alpha) secretion in bovine corpus luteum: intracellular mechanisms of action

Corpus luteum (CL) is a reproductive gland that plays a crucial endocrine role in the regulation of the estrous cycle, fertility, and pregnancy in cattle. The main function of CL is secretion of progesterone (P4), an important hormone for establishment a successful pregnancy, whereas prostaglandin F(2alpha) (PGF(2alpha)), 17beta-estradiol (E(2)) and testosterone (T) are implicated in the regulation of luteolysis. It has been shown that phytoestrogens may disrupt numerous reproductive functions on several levels of regulation and via different intracellular mechanisms. Using a cell-culture system of steroidogenic cells of the bovine CL, we determined effects of active phytoestrogen metabolites (equol and para-ethyl-phenol) on PGF(2alpha), P4, and T synthesis in steroidogenic CL cells. Moreover, we examined the intracellular mechanisms of phytoestrogen metabolite actions. Phytoestrogen metabolites did not affect P4 production in steroidogenic CL cells. However, PGF(2alpha) and T were significantly stimulated by metabolites of phytoestrogens in the bovine steroidogenic CL cells. To study the intracellular mechanism of endogenous E(2) and phytoestrogen metabolites action, steroidogenic cells were preincubated with a phospholipase C inhibitor (U73122), a protein kinase C inhibitor (staurosporine), an estrogen receptor antagonist (ICI) and a transcription inhibitor (actinomycin D) for 0.5h, and then stimulated with para-ethyl-phenol, equol or E(2). Only U73122 and staurosporine totally reduced the stimulatory effect of E(2) on PGF(2alpha) production by the cells. ICI and actinomycin D only partially reduced E(2) action on CL cells. In contrast, the stimulatory effect of phytoestrogen metabolites was totally inhibited by ICI and actinomycin D. Moreover, in contrast to E(2) action, phytoestrogen metabolites did not cause intracellular calcium mobilization in the cells. The present study demonstrated that phytoestrogen metabolites stimulate PGF(2alpha) secretion in steroidogenic cells of the bovine CL via the estrogen receptor-dependent, genomic pathway.

A nongenomic action of estradiol as the mechanism underlying the acute suppression of secretion of luteinizing hormone in ovariectomized ewes

The objective of the present study was to determine how rapidly estradiol (E2) was able to suppress the secretion of LH in ovariectomized (OVX) ewes and to evaluate the ability of conjugated forms of E2 (E2 conjugated to BSA [1,3,5(10)-estratrien-3,17beta-diol-6-one-6-carboxymethyloxime:BSA [E2-BSA] and a novel conjugate, E2 conjugated to a small peptide [E2-PEP]) to mimic the actions of E2 on secretion of LH and FSH. Animals (n = 5-6 per group) were given infusions for 4 h of 50 microg of E2 or equimolar concentrations of E2-BSA or E2-PEP. Treatments with E2, E2-BSA, and E2-PEP each induced an acute suppression of LH secretion (<20 min, P < 0.01). In contrast, E2, but not E2-BSA or E2-PEP, induced the characteristic preovulatory-like surge of LH (at 10 h after priming treatment) and decreased secretion of FSH (at 4 h after priming treatment). In conclusion, the acute inhibition of LH secretion induced by E2 in OVX ewes supports the concept of a nongenomic action as the mechanism underlying the sudden suppression in secretion of LH. In addition, the fact that conjugated forms of E2 mimicked only the acute suppression of secretion of LH, without inducing the putative genomic actions on secretion of LH or FSH (i.e., a preovulatory-like surge), suggests that the acute effect of E2 may be mediated via the plasma membrane.