Gonadotropin releasing hormone (GnRH) and estradiol (E(2)) regulation of cell cycle in gonadotrophs

The direct effects of gonadotropin-releasing hormone on proliferation of granulosa cells and development of follicles in goose

1. The study objectives were to determine the direct effects of gonadotropin-releasing hormone (GnRH) on the proliferation of ovarian granulosa cells (GCs) and the development of follicles in geese (Anser cygnoides) by colorimetry and ethynyl-2'-deoxyuridine (EdU) cell proliferation assays, in which primary GCs were treated with different concentrations of GnRH agonist (alarelin acetate) and an antagonist (cetrorelix acetate). Differently expressed genes (DEGs) were identified by RNA-sequencing and validated by quantitative reverse transcription polymerase chain reaction (RT-qPCR) and Western blotting. 2. The EdU assays showed that the proliferation of GCs was affected by the GnRH agonist and antagonist in a dose-dependent manner. The effect of treatment on cell proliferation was statistically significant at the concentrations of 10^-5 mol/l alarelin and 1 mg/l cetrorelix acetate. A total of 134 DEGs (76 downregulated and 58 upregulated for alarelin treatment) and 226 DEGs (90 downregulated and 136 upregulated for cetrorelix) were identified by RNA-sequencing analysis, respectively. Enrichment analysis indicated that DEGs were enriched in the GO terms of cell-cell signalling and cell junctions. The pathways that regulate the development of follicles were identified, including the biological progress of cAMP accumulation, ovulation cycle and vasculature that are essential to follicular selection. 3. The results suggested that GnRH might directly regulate GC proliferation via autocrine or paracrine pathways related to cell junctions. In particular, it was confirmed that the mRNA and protein expression levels of the oestrogen receptor 2 (ESR2) gene, a negative transcription factor involved in follicular maturation and ovulation, were affected by GnRH agonist or antagonist in GCs. 4. In conclusion, GnRH might play an important role in follicular development by changing the expression of genes that participate in cAMP accumulation, ovulation cycle and cell junctions in ovarian GCs.

Gonadotrope plasticity at cellular, population and structural levels: A comparison between fishes and mammals

Often referred to as "the master gland", the pituitary is a key organ controlling growth, maturation, and homeostasis in vertebrates. The anterior pituitary, which contains several hormone-producing cell types, is highly plastic and thereby able to adjust the production of the hormones governing these key physiological processes according to the changing needs over the life of the animal. Hypothalamic neuroendocrine control and feedback from peripheral tissues modulate pituitary cell activity, adjusting levels of hormone production and release according to different functional or environmental requirements. However, in some physiological processes (e.g. growth, puberty, or metamorphosis), changes in cell activity may be not sufficient to meet the needs and a general reorganization of cell composition and pituitary structure may occur. Focusing on gonadotropes, this review examines plasticity at the cellular level, which allows precise and rapid control of hormone production and secretion, as well as plasticity at the population and structural levels, which allows more substantial changes in hormone production. Further, we compare current knowledge of the anterior pituitary plasticity in fishes and mammals in order to assess what has been conserved or not throughout evolution, and highlight important remaining questions.

Pituitary Aromatase P450 May Be Involved in Maintenance of the Population of Luteinizing Hormone-Positive Pituitary Cells in Mice

As aromatase P450 is located in several pituitary cells, testosterone can be transformed into 17β-estradiol in the gland by the enzyme. The possible role of this transformation in pituitary function remains to be elucidated, but some evidence suggests a physiological and pathophysiological role for pituitary aromatase. To determine its relevance in the modulation of pituitary function, mainly associated with reproduction, luteinizing hormone (LH)-positive cells in the hypophysis of female and male aromatase knockout (ArKO) mice were studied. In all LH-positive cells, significant increases in the cellular (p < 0.01) and nuclear (p < 0.05) areas were found in the ArKO mice compared to the wild-type mice. In the ArKO mice, LH-positive cells were more abundant (p < 0.01); they were characterized by a stronger cytoplasmic reaction and the cells were more polygonal and exhibited more short, thick cytoplasmic prolongations than those in the wild-type mice. Moreover, LH-positive cells showed a greater proliferation rate in the ArKO mice compared to the wild-type mice (p < 0.01). These findings suggest that the local production of estradiol mediated by pituitary aromatase is necessary for the regulation of LH-positive gonadotropic cells, exerting an autoparacrine inhibitory regulation. These results could underlie the higher pituitary aromatase expression observed in male versus female mice. Similar effects were found in ArKO male and female mice, suggesting that in both sexes the effects of estrogens on maintenance of the LH-positive pituitary cell population could be related to the local aromatization of testosterone to estradiol inside the hypophysis. Therefore, aromatase could modulate pituitary LH-positive cells in males through local estradiol synthesis.

GnRH Pulse Frequency Control of Fshb Gene Expression Is Mediated via ERK1/2 Regulation of ICER

The pulsatile release of GnRH regulates the synthesis and secretion of pituitary FSH and LH. Two transcription factors, cAMP-response element-binding protein (CREB) and inducible cAMP early repressor (ICER), have been implicated in the regulation of rat Fshb gene expression. We previously showed that the protein kinase A pathway mediates GnRH-stimulated CREB activation. We hypothesized that CREB and ICER are activated by distinct signaling pathways in response to pulsatile GnRH to modulate Fshb gene expression, which is preferentially stimulated at low vs high pulse frequencies. In the LβT2 gonadotrope-derived cell line, GnRH stimulation increased ICER mRNA and protein. Blockade of ERK activation with mitogen-activated protein kinase kinase I/II (MEKI/II) inhibitors significantly attenuated GnRH induction of ICER mRNA and protein, whereas protein kinase C, calcium/calmodulin-dependent protein kinase II, and protein kinase A inhibitors had minimal effects. GnRH also stimulated ICER in primary mouse pituitary cultures, attenuated similarly by a MEKI/II inhibitor. In a perifusion paradigm, MEKI/II inhibition in LβT2 cells stimulated with pulsatile GnRH abrogated ICER induction at high GnRH pulse frequencies, with minimal effect at low frequencies. MEKI/II inhibition reduced GnRH stimulation of Fshb at high and low pulse frequencies, suggesting that the ERK pathway has additional effects on GnRH regulation of Fshb, beyond those mediated by ICER. Indeed, induction of the activating protein 1 proteins, cFos and cJun, positive modulators of Fshb transcription, by pulsatile GnRH was also abrogated by inhibition of the MEK/ERK signaling pathway. Collectively, these studies indicate that the signaling pathways mediating GnRH activation of CREB and ICER are distinct, contributing to the decoding of the pulsatile GnRH to regulate FSHβ expression.

Gonadotropin-releasing hormone is prerequisite for the constitutive expression of pituitary annexin A5

Annexin A5 (ANXA5), a member of the structurally related family of annexin proteins, is expressed in pituitary gonadotropes. We previously reported that ANXA5 expression is stimulated by gonadotropin-releasing hormone (GnRH). In the present study, we investigated ANXA5 expression in the anterior pituitary gland of GnRH-deficient mutant hypogonadal (hpg) mice. RT-PCR demonstrated that luteinizing hormone β subunit (LHβ) and ANXA5 mRNA levels were both lower in the pituitary gland of hpg mice than in wild-type mice. Immunohistochemistry showed that ANXA5 expression throughout the pituitary gland was very low in hpg mice, suggesting that ANXA5 is diminished in gonadotropes and also in other cell types. Subcutaneous administration of a GnRH analogue, des-gly10 (Pro9)-GnRH ethylamide (1 μg/day for 7 days), augmented the expression of LHβ and ANXA5 in the pituitary gland in hpg mice. However, LHβ- and ANXA5-positive cells did not show exactly matched spatial distributions. These findings suggest that GnRH is necessary for constitutive ANXA5 expression in the pituitary gland, not only in gonadotropes but also in other pituitary gland cell types. A close relationship between ANXA5 and LHβ expression was confirmed. It is suggested that a significant role of ANXA5 in the physiologic secretion of LH.

Gonadotrophin-releasing hormone signalling downstream of calmodulin

Gonadotrophin-releasing hormone (GnRH) regulates reproduction via binding a G-protein coupled receptor on the surface of the gonadotroph, through which it transmits signals, mostly via the mitogen-activated protein (MAPK) cascade, to increase synthesis of the gonadotrophin hormones: luteinising hormone (LH) and follicle-stimulating hormone (FSH). Activation of the MAPK cascade requires an elevation in cytosolic Ca(2+) levels, which is a result of both calcium influx and mobilisation from intracellular stores. However, Ca(2+) also transmits signals via an MAPK-independent pathway, through binding calmodulin (CaM), which is then able to bind a number of proteins to impart diverse downstream effects. Although the ability of GnRH to activate CaM was recognised over 20 years ago, only recently have some of the downstream effects been elucidated. GnRH was shown to activate the CaM-dependent phosphatase, calcineurin, which targets gonadotrophin gene expression both directly and indirectly via transcription factors such as nuclear factor of activated T-cells and Nur77, the Transducer of Regulated CREB (TORC) co-activators and also the prolyl isomerase, Pin1. Gonadotrophin gene expression is also regulated by GnRH-induced CaM-dependent kinases (CaMKs); CaMKI is able to derepress the histone deacetylase-inhibition of β-subunit gene expression, whereas CaMKII appears to be essential for the GnRH-activation of all three subunit genes. Asides from activating gonadotrophin gene expression, GnRH also exerts additional effects on gonadotroph function, some of which clearly occur via CaM, including the proliferation of immature gonadotrophs, which is dependent on calcineurin. In this review, we summarise these pathways, and discuss the additional functions that have been proposed for CaM with respect to modifying GnRH-induced signalling pathways via the regulation of the small GTP-binding protein, Gem, and/or the regulator of G-protein signalling protein 2.

Neonatal exposure to bisphenol a alters reproductive parameters and gonadotropin releasing hormone signaling in female rats

BACKGROUND

Bisphenol A (BPA) is a component of polycarbonate plastics, epoxy resins, and polystyrene and is found in many products. Several reports have revealed potent in vivo effects, because BPA acts as an estrogen agonist and/or antagonist and as an androgen and thyroid hormone antagonist.

OBJECTIVES

We analyzed the effects of neonatal exposure to BPA on the reproductive axis of female Sprague-Dawley rats.

METHODS

Female rats were injected subcutaneously, daily, from postnatal day 1 (PND1) to PND10 with BPA [500 microg/50 microL (high) or 50 microg/50 microL (low)] in castor oil or with castor oil vehicle alone. We studied body weight and age at vaginal opening, estrous cycles, and pituitary hormone release in vivo and in vitro, as well as gonadotropin-releasing hormone (GnRH) pulsatility at PND13 and in adults. We also analyzed two GnRH-activated signaling pathways in the adults: inositol-triphosphate (IP(3)), and extracellular signal-regulated kinase(1/2) (ERK(1/2)).

RESULTS

Exposure to BPA altered pituitary function in infantile rats, lowering basal and GnRH-induced luteinizing hormone (LH) and increasing GnRH pulsatility. BPA dose-dependently accelerated puberty onset and altered estrous cyclicity, with the high dose causing permanent estrus. In adults treated neonatally with BPA, GnRH-induced LH secretion in vivo was decreased and GnRH pulsatility remained disrupted. In vitro, pituitary cells from animals treated with BPA showed lower basal LH and dose-dependently affected GnRH-induced IP(3) formation; the high dose also impaired GnRH-induced LH secretion. Both doses altered ERK(1/2) activation.

CONCLUSIONS

Neonatal exposure to BPA altered reproductive parameters and hypothalamic-pituitary function in female rats. To our knowledge, these results demonstrate for the first time that neonatal in vivo BPA permanently affects GnRH pulsatility and pituitary GnRH signaling.

Transcriptional regulation of follistatin expression by GnRH in mouse gonadotroph cell lines: evidence for a role for cAMP signaling

GnRH applied continuously or in pulses of high frequency increases follistatin, and thereby differentially regulates FSH and LH. This study was conducted in alphaT3-1 and LbetaT2 gonadotroph cells to begin to understand the signaling pathways through which GnRH stimulates follistatin synthesis. GnRH increased follistatin expression and stimulated a follistatin-LUC reporter in LbetaT2 cells, but was inactive in alphaT3-1 cells. GnRH also increased cAMP levels and stimulated a cAMP-responsive promoter only in LbetaT2 cells. Forskolin stimulated follistatin in both cell lines. GnRH activation of follistatin was blocked by the PKA inhibitor H89 and by over-expression of a dominant-negative inhibitor of CREB (A-CREB). Activation was also suppressed by PKC depletion, and was reduced by the PKC inhibitor bisindolylmaleimide. The MEK inhibitor PD98059 blocked activation by GnRH or forskolin implying that MAPK contributes to cAMP/PKA-mediated activation of follistatin. When LbetaT2 cells were transfected with follistatin-LUC together with A-CREB, and perifused with GnRH, activation was blocked during continuous GnRH, but stimulation by hourly GnRH pulses was unaffected. These experiments provide evidence that GnRH stimulates follistatin through multiple signaling pathways, and that cAMP-CREB activation is obligatory when GnRH is applied continuously. The finding that follistatin transcription was CREB-dependent with continuous but not pulsatile GnRH implies that the mode of ligand activation of GnRH receptors modifies the transcriptional response by changing the signaling network. These results provide a mechanism linking GnRH pulsatility to the differential control of FSH-beta and LH-beta gene expression through follistatin.

Differential gonadotropin releasing hormone (GnRH) expression, autoregulation and effects in two models of rat luteinized ovarian cells

GnRH has been suggested to participate in corpus luteum function. Here we studied the expression of GnRH mRNA and peptide in two models of rat luteinized tissues: ovarian cells from PMSG-hCG treated prepubertal rats (SPO) and from intrasplenic ovarian tumors (Luteoma). A GnRH autoregulatory effect was evaluated as well as its action on cell proliferation and apoptosis. GnRH mRNA was present in SPO, isolated corpora lutea from SPO and Luteoma from 1 week to 7 months of development. In vitro cultures of Luteoma cells expressed 2-fold higher GnRH mRNA and 10-fold higher GnRH peptide than SPO cells. Buserelin (GnRH analog) increased GnRH mRNA and peptide expression in SPO but not in Luteoma cells. While basal proliferation was very low in Luteoma cells, SPO cells showed a significant increase in cell number by both the thymidine and the MTS methods after 72 h in culture. Buserelin induced a decrease in cell number in both cell types to a similar degree. Although basal apoptosis levels were higher in SPO than in Luteoma cells, Buserelin-induced apoptosis was only detected in Luteoma cells after 48 h treatment. These results show that the two types of rat, luteinized tissues, Luteoma and SPO, markedly differed in some intrinsic properties and in their local GnRH systems. Luteoma cells proliferate very weakly, express and secrete high amounts of GnRH, do not show an autoregulatory effect and respond to the decapeptide with apoptosis stimulation. In contrast SPO cells proliferate significantly, secrete low levels of GnRH but possess a positive, autoregulatory mechanism and respond to GnRH stimulation with impairment of proliferation.

The luteinizing hormone-releasing hormone inhibits the anti-apoptotic activity of insulin-like growth factor-1 in pituitary alphaT3 cells by protein kinase Calpha-mediated negative regulation of Akt

The luteinizing hormone-releasing hormone (LHRH) receptor is a G protein-coupled receptor involved in the synthesis and release of pituitary gonadotropins and in the proliferation and apoptosis of pituitary cells. Insulin-like growth factor-1 receptor (IGF-1R) is a tyrosine kinase receptor that has a mitogenic effect on pituitary cells. In this study, we used the alphaT3 gonadotrope cell line as a model to characterize the IGF-1R signaling pathways and to investigate whether this receptor interacts with the LHRH cascade. We found that IGF-1 activated the IGF-1R, insulin receptor substrate (IRS)-1, phosphatidylinositol 3-kinase, and Akt in a time-dependent manner in alphaT3 cells. The MAPK (ERK1/2, p38, and JNK) pathways were only weakly activated by IGF-1. In contrast, LHRH strongly stimulated the MAPK pathways but had no effect on Akt activation. Cotreatment with IGF-1 and LHRH had various effects on these signaling pathways. 1) It strongly increased IGF-1-induced tyrosine phosphorylation of IRS-1 and IRS-1-associated phosphatidylinositol 3-kinase through activation of the epidermal growth factor receptor. 2) It had an additive effect on ERK1/2 activation without modifying the phosphorylation of p38 and JNK1/2. 3) It strongly reduced IGF-1 activation of Akt. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assays and cell cycle analysis revealed that, in addition to having an additive effect on ERK1/2 activation, cotreatment with IGF-1 and LHRH also had an additive effect on cell proliferation. The LHRH-induced inhibition of Akt stimulated by IGF-1 was completely blocked by Safingol, a protein kinase C (PKC) alpha-specific inhibitor, and by a dominant negative form of PKCalpha. Finally, we showed that the inhibitory effect of LHRH on IGF-1-induced PKCalpha-mediated Akt activation was associated with a marked reduction in Bad phosphorylation and a substantial decrease in the ability of IGF-1 to rescue alphaT3 cells from apoptosis induced by serum starvation. Our results demonstrate for the first time that several interactions take place between IGF-1 and LHRH receptors in gonadotrope cells.