Modulation of gonadotropin-releasing hormone receptor concentration in cultured female rat pituitary cells by estradiol treatment

Estrogen-Induced Hypothalamic Synaptic Plasticity and Pituitary Sensitization in the Control of the Estrogen-Induced Gonadotrophin Surge

Proper gonadal function requires coordinated (feedback) interactions between the gonads, adenohypophysis, and brain: the gonads elaborate sex steroids (progestins, androgens, and estrogens) and proteins (inhibin-activin family) during gamete development. In both sexes, the brain-pituitary gonadotrophin-regulating interaction is coordinated by estradiol through its opposing actions on pituitary gonadotrophs (sensitization of the response to gonadotrophin-releasing hormone [GnRH]) versus hypothalamic neurons (inhibition of GnRH secretion). This dynamic tension between the gonadotrophs and the GnRH cells in the brain regulates the circulating gonadotrophins and is termed reciprocal/negative feedback. In females, reciprocal/negative feedback dominates approximately 90% of the ovarian cycle. In a spectacular exception, the dynamic tension is broken during the surge of circulating estrogen that marks follicle and oocyte(s) maturation. The cause is an estradiol-induced disinhibition of the GnRH neurons that releases GnRH secretion to the highly sensitized pituitary gonadotrophs that in turn release the gonadotrophin surge (the estrogen-induced gonadotrophin surge [EIGS], also known as positive feedback). Studies during the past 4 decades have shown this disinhibition to result from estrogen-induced synaptic plasticity (EISP), including a reversible approximately 50% loss in arcuate nucleus synapses. The disinhibited GnRH secretion occurs during maximal gonadotroph sensitization and results in the EIGS. Specific immunoneutralization of estradiol blocks the EISP and EIGS. The EISP is accompanied by increases in insulinlike growth factor 1, polysialylated neural cell adhesion molecule, and ezrin, 3 proteins that the authors believe are the links between estrogen-induced astroglial extension and the EISP that releases GnRH secretion at the moment of maximal sensitization of the pituitary gonadotrophs. The result is the paradoxical surge of gonadotrophins at the peak of ovarian estrogen secretion and the triggering of ovulation. This enhanced understanding of the mechanics of gonadotrophin control clarifies elements of the involved feedback loops and opens the way to a better understanding of the neurobiology of reproduction.

Regulation of GnRH I receptor gene expression by the GnRH agonist triptorelin, estradiol, and progesterone in the gonadotroph-derived cell line alphaT3-1

The secretion of luteinizing hormone (LH) and the GnRH receptor (GnRH-R) concentration are modulated by ovarian steroids and GnRH. To elucidate whether this regulation is due to alterations at the transcriptional level, we examined the GnRH I-R mRNA expression in the gonadotroph-derived cell line alphaT3-1 treated with different estradiol and progesterone paradigms and the GnRH I agonist triptorelin. alphaT3-1 cells were treated with different steroid paradigms: 1 nM estradiol or 100 nM progesterone for 48 h alone or in combination. Cells were exposed to 10 nM or 100 pM triptorelin for 30 min, 3 h, 9 h, or, in pulsatile way, with a 5-min pulse per hour. The GnRH I-R mRNA was determined by Northern blot analysis. GnRH I-R mRNA from cells treated with continuous triptorelin decreased in a time- and concentration-dependent manner. Pulsatile triptorelin increased GnRH I-R gene expression. Progesterone alone further enhanced this effect, whereas estradiol and its combination with progesterone diminished it. Continuous combined treatment with estradiol and progesterone lead to a significant decrease of GnRH I-R mRNA by 30% and by 35% for estradiol alone. The addition of 10 nM triptorelin for 30 min or 3 h could not influence that steroid effect. In conclusion, estradiol and progesterone exclusively decreased GnRH I-R mRNA in alphaT3-1 cells no matter whether they are treated additionally with the GnRH I agonist triptorelin. The enhanced sensitivity of gonadotrophs and GnRH I-R upregulation by estradiol is not due to increased GnRH I gene expression because GnRH I-R mRNA is downregulated by estradiol and progesterone. Other pathways of the GnRH I-R signal transduction might be involved.

Molecular biology of gonadotropin-releasing hormone (GnRH)-I, GnRH-II, and their receptors in humans

In human beings, two forms of GnRH, termed GnRH-I and GnRH-II, encoded by separate genes have been identified. Although these hormones share comparable cDNA and genomic structures, their tissue distribution and regulation of gene expression are significantly dissimilar. The actions of GnRH are mediated by the GnRH receptor, which belongs to a member of the rhodopsin-like G protein-coupled receptor superfamily. However, to date, only one conventional GnRH receptor subtype (type I GnRH receptor) uniquely lacking a carboxyl-terminal tail has been found in the human body. Studies on the transcriptional regulation of the human GnRH receptor gene have indicated that tissue-specific gene expression is mediated by differential promoter usage in various cell types. Functionally, there is growing evidence showing that both GnRH-I and GnRH-II are potentially important autocrine and/or paracrine regulators in some extrapituitary compartments. Recent cloning of a second GnRH receptor subtype (type II GnRH receptor) in nonhuman primates revealed that it is structurally and functionally distinct from the mammalian type I receptor. However, the human type II receptor gene homolog carries a frameshift and a premature stop codon, suggesting that a full-length type II receptor does not exist in humans.

Effects of changing gonadotropin-releasing hormone pulse frequency and estrogen treatment on levels of estradiol receptor-alpha and induction of Fos and phosphorylated cyclic adenosine monophosphate response element binding protein in pituitary gonadotropes: studies in hypothalamo-pituitary disconnected ewes

Estrogen receptor-alpha (ER alpha) levels in gonadotropes are increased during the follicular phase of the ovine estrous cycle, a time of increased frequency of pulsatile secretion of GnRH and elevated plasma estrogen levels. In the present study, our first aim was to determine which of these factors causes the rise in the number of gonadotropes with ER alpha. Ovariectomized hypothalamo-pituitary disconnected ewes (n = 4-6) received the following treatments: 1) no treatment, 2) injection (im) of 50 microg estradiol benzoate (EB), 3) pulses (300 ng iv) of GnRH every 3 h, 4) GnRH treatment as in group 3 and EB treatment as in group 2, 5) increased frequency of GnRH pulses commencing 20 h before termination, and 6) GnRH treatment as in group 5 with EB treatment. These treatments had predictable effects on plasma LH levels. The number of gonadotropes in which ER alpha was present (by immunohistochemistry) was increased by either GnRH treatment or EB injection, but combined treatment had the greatest effect. Immunohistochemistry was also performed to detect phosphorylated cAMP response element binding protein (pCREB) and Fos protein in gonadotropes. The number of gonadotropes with Fos and with pCREB was increased only in group 6. We conclude that either estrogen or GnRH can up-regulate ER alpha in pituitary gonadotropes. On the other hand, during the period of positive feedback action of estrogen, the appearance of pCREB and Fos in gonadotropes requires the combined action of estrogen and increased frequency of GnRH input. This suggests convergence of signaling for GnRH and estrogen.

Intra-pituitary regulation of gonadotrophs in male rodents and primates

Paracrine and autocrine regulation is well established in many organs including the gonads, but the notion of communication among pituitary cells is a relatively new concept. The FSH-beta and GnRH-receptor genes are up-regulated by pituitary activin and down-regulated by pituitary follistatin, and circulating inhibin disrupts this local regulation by functioning as an endogenous competitor of the activin receptor. Activin and follistatin production by folliculostellate cells may play a central role in these responses. alpha-Subunit expression is maintained at high levels in the absence of GnRH through unknown mechanisms. There is evidence that the intra-pituitary regulation of FSH-beta and GnRH-receptor gene expression may activate pubertal maturation in male rats. Finally, there are marked differences in follistatin expression and its regulation by GnRH and androgens in male primates and rats that appear to explain species differences in the differential secretion of FSH and LH, although the physiological significance of these differences is not yet known.

The pathophysiology of polycystic ovary syndrome: trying to understand PCOS and its endocrinology

The pathophysiology of the polycystic ovary syndrome (PCOS) encompasses inherent ovarian dysfunction that is strongly influenced by external factors, such as disturbances of the hypothalamic-pituitary-ovarian axis and hyperinsulinaemia. Exaggerated gonadotrophin releasing hormone (GnRH) pulsatility results in hypersecretion of luteinising hormone (LH), which has effects both on ovarian androgen production and oocyte development. Disturbed ovarian-pituitary and hypothalamic feedback accentuates the 0gonadotrophin abnormalities. Hyperinsulinaemia is secondary both to insulin resistance at the periphery and to abnormal pancreatic beta cell function. PCOS runs in families and a number of genetic abnormalities appear to result in features of the syndrome and account for the heterogeneity of the symptoms. Environmental influences, such as nutrition and lifestyle, further influence expression of the syndrome.

Phytoestrogens and gonadotropin-releasing hormone pulse generator activity and pituitary luteinizing hormone release in the rat

Phytoestrogens can produce inhibitory effects on gonadotropin secretion in both animals and humans. The aims of this study were 2-fold: 1) to determine in vivo whether genistein and coumestrol act on the GnRH pulse generator to suppress hypothalamic multiunit electrical activity volleys and associated LH pulses and/or on the pituitary to suppress the LH response to GnRH; and 2) to examine the effect of these phytoestrogens on GnRH-induced pituitary LH release in vitro and to determine whether estrogen receptors are involved. Wistar rats were ovariectomized and chronically implanted with recording electrodes and/or indwelling cardiac catheters, and blood samples were taken every 5 min for 7--11 h. Intravenous infusion of coumestrol (1.6-mg bolus followed by 2.4 mg/h for 8.5 h) resulted in a profound inhibition of pulsatile LH secretion, a 50% reduction in the frequency of hypothalamic multiunit electrical activity volleys, and a complete suppression of the LH response to exogenous GnRH. In contrast, both genistein (1.6-mg bolus followed by 2.4 mg/h for 8.5 h) and vehicle were without effect on pulsatile LH secretion. Coumestrol (10(-5) M; over 2 or 4 h) suppressed GnRH-induced pituitary LH release in vitro, an effect blocked by the antiestrogen ICI 182,780. It is concluded that coumestrol acts centrally to reduce the frequency of the hypothalamic GnRH pulse generator. In addition, the inhibitory effects of coumestrol on LH pulses occur at the level of the pituitary by reducing responsiveness to GnRH via an estrogen receptor-mediated process.

The expression, regulation and signal transduction pathways of the mammalian gonadotropin-releasing hormone receptor

Normal mammalian sexual maturation and reproductive functions require the integration and precise coordination of hormones at the hypothalamic, pituitary, and gonadal levels. Hypothalamic gonadotropin-releasing hormone (GnRH) is a key regulator in this system; after binding to its receptor (GnRHR), it stimulates de novo synthesis and release of gonadotropins in anterior pituitary gonadotropes. Since the isolation of the GnRHR cDNA, the expression of GnRHR mRNA has been detected not only in the pituitary, but also in extrapituitary tissues, including the ovary and placenta. It has been shown that change in GnRHR mRNA is one of the mechanisms for regulating the expression of the GnRHR. To help understand the molecular mechanism(s) involved in transcriptional regulation of the GnRHR gene, the 5' flanking region of the GnRHR gene has recently been isolated. Initial characterization studies have identified several DNA regions in the GnRHR 5' flanking region which are responsible for both basal expression and GnRH-mediated homologous regulation of this gene in pituitary cells. The mammalian GnRHR lacks a C-terminus and possesses a relatively short third intracellular loop; both features are important in desensitization of many others G-protein coupled receptors (GPCRs), Homologous desensitization of GnRHR has been shown to be regulated by various serine-threonine protein kinases including protein kinase A (PKA) and protein kinase C (PKC), as well as by G-protein coupled receptor kinases (GRKs). Furthermore, GnRHR was demonstrated to couple with multiple G proteins (Gq/11, Gs, and Gi), and to activate cascades that involved the PKC, PKA, and mitogen-activator protein kinases. These results suggest the diversity of GnRHR-G protein coupling and signal transduction systems. The identification of second form of GnRH (GnRH-II) in mammals adds to the complexity of the GnRH-GnRHR system. This review summaries our recent progress in understanding the regulation of GnRHR gene expression and the GnRHR signal transduction pathways.Key words: gonadotropin-releasing hormone receptor, transcriptional regulation, desensitization, signal transduction.

Regulation of lutenizing hormone secretion and subunit messenger ribonucleic acid expression by gonadal steroids in perifused pituitary cells from male monkeys and rats

The mechanisms by which gonadal steroids regulate gonadotropin secretion remain incompletely understood. As previous studies suggest that the pituitary actions of testosterone (T) and estradiol (E) differ in male primates and rodents, we compared the effects of 10 nM T, 0.1 nM E, and 10 nM dihydrotestosterone (DHT) on the LH response to hourly pulses of GnRH as well as the GnRH receptor (GnRH-R) and LH subunit messenger RNA (mRNA) levels in dispersed pituitary cells from intact male monkeys and rats. T suppressed (P < 0.01) and E increased (P < 0.05) GnRH-stimulated LH secretion by rat pituitary cells. With monkey pituitary cells, on the other hand, there was no significant effect of either T or DHT on GnRH-stimulated LH secretion. In E-treated monkey cells, a period of initial enhancement (P < 0.05) was followed by significant suppression (P < 0.05) of LH secretion. GnRH-R mRNA was unchanged by T or E in either rat or monkey cells. T suppressed LHbeta (P < 0.01) and alpha-subunit (P < 0.01) mRNAs, whereas E increased alpha-subunit (P < 0.01), but did not alter LHbeta mRNA levels in rat cells. In monkey cells, however, neither T nor E affected LHbeta or alpha-subunit mRNA levels significantly. Our results identify different regulatory mechanisms by which testicular steroid hormones control LH secretion by the pituitary in male primates and rodents. We propose that the primary site of androgen negative feedback in the male primate is to restrain GnRH pulsatile secretion, whereas in the male rat T also decreases gonadotropin synthesis and secretion by directly affecting the pituitary. E suppresses GnRH-stimulated LH secretion in the primate pituitary, but amplifies the action of GnRH in the rat. Our data also reveal that the action of T to suppress LH secretion and subunit mRNA in male rats is not through decreased GnRH-R gene expression.

Regulation of the preovulatory gonadotropin surge by endogenous steroids

Estradiol secreted by growing ovarian follicle(s) has been considered classically to be the neural trigger for the preovulatory surge of gonadotropins. The observation that the estradiol-induced gonadotropin surge in ovariectomized rats is of lesser magnitude and duration than that found in the cycling rat at proestrus has resulted in a search for other steroid regulators. Progesterone is a major regulator of the preovulatory gonadotropin surge. It can only act in the presence of an estrogen background, which is necessary for the synthesis of progesterone receptors. In the estrogen-primed ovariectomized rat, progesterone is able to initiate and enhance the gonadotropin surge to the magnitude observed on the day of proestrus and limit it to 1 day. The physiological role of progresterone in the induction of the preovulatory gonadotropin surge has been demonstrated by the attenuation of the progesterone-induced surge and the endogenous proestrus surge by progesterone receptor antagonist RU486 and the progesterone synthesis inhibitor trilostane. The promoter region of the follicle-stimulating hormone (FHS)-beta gene contains multiple progesterone response elements and progesterone brings about FSH release as well. The reduction of progesterone in the 5 alpha-position appears to be important for the regulation of progesterone secretion. Corticosteroids appear to play a significant role in the secondary FSH surge on late proestrus and early estrus.

Evidence for differential regulation of multiple transcripts of the gonadotropin releasing hormone receptor in the ovine pituitary gland; effect of estrogen

The number of pituitary gonadotropin-releasing hormone receptors (GnRH-R) varies across the estrous cycle. We report that there is variable expression of the differently-sized GnRH-R transcripts in cyclic ewes and in an experimental model. During the follicular phase of the cycle, and compared to the luteal phase, there was increased expression of the 1.5, 2.3 and 3.7 kilobase (kb) transcripts with no change in the levels of the 5.6 or the 1.2 kb transcripts. Steady state levels of mRNA for luteinising hormone beta and common alpha subunit were also increased in the follicular phase of the cycle. In hypothalamo-pituitary disconnected ovariectomised ewes given pulsatile GnRH replacement, injection of estrogen increased the 1.5, 2.3 and 3.7 kb, while the levels of the 5.6 and 1.2 kb transcripts were not altered. We conclude that the differential regulation of GnRH-R mRNA occurs through a direct effect of E on the pituitary.

Temporal effects of estradiol (E) on luteinizing hormone-releasing hormone (LHRH) and LH release in castrated male sheep

We tested the hypothesis that rapidly expressed inhibitory effects of estradiol (E) on luteinizing hormone (LH) release in the male are attributable, in part, to suppression of luteinizing hormone-releasing hormone (LHRH) release. Hypophyseal-portal cannulated, castrated male sheep were infused with E (15 ng/kg/hr) or vehicle. Portal and jugular blood samples were collected at 10-min intervals for 4 hr before, and for either 12 hr (E, n = 4; vehicle, n = 4) or 24 hr (E, n = 8; vehicle, n = 3) after the start of infusion. In animals sampled for 16 hr, temporal changes in both LHRH and LH were assessed. In animals sampled for 28 hr, only LH data were analyzed. Before either the 12-hr or 24-hr infusion, LHRH and/or LH mean concentrations, pulse amplitude and interpulse interval (IPI) did not differ between E- and vehicle-infused animals. In animals sampled for 16 hr, no effects of time or steroid x time interactions were detected for mean LHRH and LHRH pulse amplitude; however, both were greater (P < 0.01) in vehicle-infused than in E-infused males. LHRH IPI was unaffected by infusion. In contrast, both mean LH and LH pulse amplitude declined (P < 0.01) within 4-8 hr after the start of E infusion, whereas mean LH IPI was unaffected. In animals sampled for 28 hr, an effect of time (P < 0.01) and a steroid x time interaction (P < 0.01) was detected for mean LH, and there was an effect of time (P < 0.01) on LH pulse amplitude. Mean LH IPI was not affected. Our results show that in male sheep E rapidly reduces LH release in the absence of a detectable change in LHRH release.

Modulation of stimulus-secretion coupling in single rat gonadotrophs

1. Exocytosis and intracellular [Ca2+] were determined simultaneously in single anterior pituitary gonadotrophs from ovariectomized female rats. Dispersed cells were cultured for 2-4 days with or without 0.2 nM oestradiol-17 beta (E2) before use. Cells were stimulated with either gonadotrophin releasing hormone (GnRH) or by membrane depolarization. Exocytosis was determined from the change in membrane capacitance (Cm) using the perforated-patch whole-cell recording technique. Intracellular [Ca2+] was measured using fura-2 fluorescence. 2. The exocytotic response to 1 nM GnRH was characterized by a wide spectrum of responses, ranging from exocytotic bursts to relatively slow, graded increases that were dependent on the evoked intracellular Ca2+ pattern. A kinetic model is presented that incorporates the observed steep dependence of exocytosis on measured intracellular [Ca2+]; simulated exocytosis reasonably approximated observed exocytotic responses, both kinetically and quantitatively. The model also suggests that the modulatory effects of E2 are brought about either by a change in the Ca2+ sensitivity of exocytosis or by a preferential clustering of docked-secretory granules close to sites of Ca2+ release. The results suggest that in gonadotrophs an oscillatory Ca2+ signal is sensed by the exocytotic apparatus in a modified form of digital encoding. 3. Exocytosis in E2-treated cells was 3-fold greater than in non-treated cells for GnRH-evoked secretion, and 38% greater for depolarization; however, there was no effect of E2 on the intracellular Ca2+ response to either stimulus. The results show that maximum expression of the effect of E2 on exocytosis requires activation of GnRH-dependent pathways.

Comparative effects of short term treatment with norethisterone and sex steroids on gonadotropin secretion in rat pituitary cell cultures

The short term effects of norethisterone (NET), progesterone (P), estradiol (E2) and dihydrotestosterone (DHT) on the gonadotropin secretion of pituitary cells, from both male and female rats, in primary culture primed with E2 were studied. In female cells, NET only increased the GnRH-induced secretion of LH, while P increased both LH and FSH. Male pituitary cells showed an increased response to GnRH after P pretreatment only if the E2 concentration was augmented. However with the same E2 conditions pretreatment with NET decreased the stimulated LH, but not FSH secretion. Pretreatment with E2 inhibited LH stimulated secretion from pituitary cells of male but not female rats. Furthermore DHT treatment diminished the GnRH response for both LH and FSH in pituitary cells from both sexes. Androgen pretreatment increased basal gonadotropin secretion in male but not in female cells. Basal FSH secretion was increased by NET pretreatment in male cells. This suggests that NET is metabolized by cultured pituitary cells to A-ring reduced compounds during the 4 h incubation period. The formation of NET metabolites, particularly the 3 beta, 5 alpha and 5 alpha-NET might be responsible for the estrogenic and androgenic effects observed when NET was administered to the cultured pituitary cells.

Modulatory actions of estradiol and progesterone on phorbol ester-stimulated LH secretion from cultured rat pituitary cells

We compared the ability of estradiol and progesterone to modulate gonadotropin-releasing hormone (GnRH) and protein kinase C (PKC)-mediated luteinizing hormone (LH) secretion. Long-term (48 h) treatment of rat pituitary cells with 1 nM estradiol enhanced GnRH and phorbol ester (TPA)-stimulated LH secretion. This positive effect was facilitated by additional short-term (4 h) treatment with progesterone (100 nM). However, long-term progesterone treatment, which inhibited GnRH-stimulated LH secretion, did not influence TPA-stimulated gonadotropin release. These steroid actions occurred without an effect on the total amount of LH in the cell cultures (total LH = LH secreted + LH remaining in the cell) and neither the secretagogues nor the steroids altered total LH. Since GnRH or TPA-induced LH secretion depends on Ca2+ influx into the gonadotroph, we also analyzed the effects of estradiol and progesterone under physiological extracellular Ca2+ concentrations and in the absence of extracellular Ca2+. The steroids were able to influence GnRH or TPA-induced LH secretion under both conditions. However, when TPA was used as stimulus in Ca(2+)-deficient medium the relative changes induced by estradiol and progesterone were more pronounced, possibly indicating that the extracellular Ca(2+)-independent component of PKC-mediated LH secretion is more important for the regulation of the steroid effects. It is concluded that estradiol and progesterone might mediate their modulatory actions on GnRH-stimulated LH secretion via an influence on PKC. This effect can occur independently from de novo synthesis of LH and Ca2+ influx into gonadotrophs.

Effects of progesterone on gonadotropin-releasing hormone receptor concentration in cultured estrogen-primed female rat pituitary cells

Acute (0.5-4 h) treatment of estradiol (E)-primed female rat pituitary cells with progesterone (P) augments gonadotropin-releasing hormone (GnRH)-induced LH release, whereas chronic (48 h) P-treatment reduces pituitary responsiveness to the hypothalamic decapeptide. Dispersed E-primed (48 h, 1 nM) rat pituitary cells were cultured for 4 or 48 h in the presence of 100 nM P to assess the effects of the progestagen on GnRH receptors and on gonadotrope responsiveness to the decapeptide. P-treatment (4 h) significantly augmented GnRH-receptor concentrations (4.44 +/- 0.6 fmol/10(6) cells) as compared to cells treated only with E (2.6 +/- 0.5 fmol/10(6) cells). Parallel significant changes in GnRH-induced LH secretion were observed. The acute increase in GnRH-receptor number was nearly maximal (180% of receptor number in cells treated with E alone) within 30 min of P addition. Chronic P-treatment (48 h) significantly reduced pituitary responsiveness to GnRH as compared to E-treatment. The GnRH-receptor concentrations (3.9 +/- 0.6 fmol/10(6) cells), however, remained elevated above those in E-primed cells. GnRH-receptor affinity was not influenced by any of the different treatments. These results indicate that the acute facilitatory P-effect on GnRH-induced LH release is at least chronologically closely related to an increase in GnRH-receptor concentration. The chronic negative P-effect on pituitary responsiveness to GnRH, however, shows no relation to changes in available GnRH receptors.

Estradiol regulates gonadotropin-releasing hormone receptor number, growth and inositol phosphate production in alpha T3-1 cells

Gonadal steroids act at the pituitary to regulate gonadotropin-releasing hormone (GnRH) receptor number and the responsiveness of gonadotropes to GnRH and can act at post-receptor sites to modulate Ca(2+)-mediated and protein kinase C-mediated signal-transducing pathways. However, such effects have been seen in the mixed cell population of primary cell cultures and may involve indirect effects on cells other than gonadotropes. Here, steroid effects on a recently described gonadotrope-derived cell line (alpha T3-1 cells) have been assessed. In these cells estradiol, progesterone, testosterone and corticosterone all exerted trophic effects. Estradiol increased [3H]thymidine incorporation with an EC50 of 10(-12) to 10(-11) M and this effect was blocked by keoxifene, an estrogen receptor antagonist. Estradiol also reduced binding of [125I]buserelin (EC50 approximately 10(-11) M), an effect which appears to reflect a reduction in GnRH receptor number rather than a change in Kd. Estradiol also shifted the dose-response curve for GnRH-stimulated inositol phosphate (IP) accumulation rightward, increasing the EC50 for this GnRH effect by approximately 20-fold. Accordingly estradiol acts directly upon alpha T3-1 cells not only to reduce GnRH receptor number, but also to reduce the efficiency of coupling of residual GnRH receptors to second messenger generation.

Specific low affinity binding sites for gonadotropin-releasing hormone in human endometrial carcinomata

Specific low-affinity high-capacity binding sites for gonadotropin-releasing hormone (GnRH) have recently been discovered in human breast and ovarian carcinomata. We checked whether similar binding sites are present in human endometrial cancer. Plasma membrane preparations were incubated with [125I,D-Ala6-desGly10]-GnRH-ethylamide in the presence or absence of unlabelled GnRH agonists or other peptides. GnRH-binding could be demonstrated in all 12 tumor samples tested. The mathematical analysis of the binding data was consistent with a single class of low affinity (Ka = (0.8-1.4) x 10(5) M-1) and high-capacity (Bmax = (134-142) x 10(-12) M/mg membrane protein) binding sites. Native GnRH had a similar affinity to the binding sites as the GnRH agonist used. Other peptides such as oxytocin, somatostatin and thyrotropin-releasing hormone did not crossreact with the binding sites. A photolabelled derivative of [D-Lys6]-GnRH was prepared with the bifunctional photolabile reagent (4-azidobenzyl)-N-hydroxysuccinimide. Photoaffinity labelling of endometrial carcinoma membranes and subsequent sodium dodecyl sulfate electrophoresis in 10% polyacrylamide gel revealed the presence of a single molecular mass component of 62 +/- 1.9 kDa. The appearance of this photolabelled binding site could be largely suppressed by the addition of unlabelled GnRH-agonist (10(-4) M) and thus represents the specific binding site for GnRH in endometrial cancer.