The antiprogestins RU486 and ZK98299 affect follicle-stimulating hormone secretion differentially on estrus, but not on proestrus

Altered GnRH neuron and ovarian innervation characterize reproductive dysfunction linked to the Fragile X messenger ribonucleoprotein (Fmr1) gene mutation

Introduction

Mutations in the Fragile X Messenger Ribonucleoprotein 1 (FMR1) gene cause Fragile X Syndrome, the most common monogenic cause of intellectual disability. Mutations of FMR1 are also associated with reproductive disorders, such as early cessation of reproductive function in females. While progress has been made in understanding the mechanisms of mental impairment, the causes of reproductive disorders are not clear. FMR1-associated reproductive disorders were studied exclusively from the endocrine perspective, while the FMR1 role in neurons that control reproduction was not addressed.

Results

Here, we demonstrate that similar to women with FMR1 mutations, female Fmr1 null mice stop reproducing early. However, young null females display larger litters, more corpora lutea in the ovaries, increased inhibin, progesterone, testosterone, and gonadotropin hormones in the circulation. Ovariectomy reveals both hypothalamic and ovarian contribution to elevated gonadotropins. Altered mRNA and protein levels of several synaptic molecules in the hypothalamus are identified, indicating reasons for hypothalamic dysregulation. Increased vascularization of corpora lutea, higher sympathetic innervation of growing follicles in the ovaries of Fmr1 nulls, and higher numbers of synaptic GABAA receptors in GnRH neurons, which are excitatory for GnRH neurons, contribute to increased FSH and LH, respectively. Unmodified and ovariectomized Fmr1 nulls have increased LH pulse frequency, suggesting that Fmr1 nulls exhibit hyperactive GnRH neurons, regardless of the ovarian feedback.

Conclusion

These results reveal Fmr1 function in the regulation of GnRH neuron secretion, and point to the role of GnRH neurons, in addition to the ovarian innervation, in the etiology of Fmr1-mediated reproductive disorders.

Neuroendocrine control of gonadotropin-releasing hormone: Pulsatile and surge modes of secretion

The concept that different systems control episodic and surge secretion of gonadotropin-releasing hormone (GnRH) was well established by the time that GnRH was identified and formed the framework for studies of the physiological roles of GnRH, and later kisspeptin. Here, we focus on recent studies identifying the neural mechanisms underlying these two modes of secretion, with an emphasis on their core components. There is now compelling data that kisspeptin neurons in the arcuate nucleus that also contain neurokinin B (NKB) and dynorphin (i.e., KNDy cells) and their projections to GnRH dendrons constitute the GnRH pulse generator in mice and rats. There is also strong evidence for a similar role for KNDy neurons in sheep and goats, and weaker data in monkeys and humans. However, whether KNDy neurons act on GnRH dendrons and/or GnRH soma and dendrites that are found in the mediobasal hypothalamus (MBH) of these species remains unclear. The core components of the GnRH/luteinising hormone surge consist of an endocrine signal that initiates the process and a neural trigger that drives GnRH secretion during the surge. In all spontaneous ovulators, the core endocrine signal is a rise in estradiol secretion from the maturing follicle(s), with the site of estrogen positive feedback being the rostral periventricular kisspeptin neurons in rodents and neurons in the MBH of sheep and primates. There is considerable species variations in the neural trigger, with three major classes. First, in reflex ovulators, this trigger is initiated by coitus and carried to the hypothalamus by neural or vascular pathways. Second, in rodents, there is a time of day signal that originates in the suprachiasmatic nucleus and activates rostral periventricular kisspeptin neurons and GnRH soma and dendrites. Finally, in sheep nitric oxide-producing neurons in the ventromedial nucleus, KNDy neurons and rostral kisspeptin neurons all appear to participate in driving GnRH release during the surge.

Molecular regulation of follicle-stimulating hormone synthesis, secretion and action

Follicle-stimulating hormone (FSH) plays fundamental roles in male and female fertility. FSH is a heterodimeric glycoprotein expressed by gonadotrophs in the anterior pituitary. The hormone-specific FSHβ-subunit is non-covalently associated with the common α-subunit that is also present in the luteinizing hormone (LH), another gonadotrophic hormone secreted by gonadotrophs and thyroid-stimulating hormone (TSH) secreted by thyrotrophs. Several decades of research led to the purification, structural characterization and physiological regulation of FSH in a variety of species including humans. With the advent of molecular tools, availability of immortalized gonadotroph cell lines and genetically modified mouse models, our knowledge on molecular mechanisms of FSH regulation has tremendously expanded. Several key players that regulate FSH synthesis, sorting, secretion and action in gonads and extragonadal tissues have been identified in a physiological setting. Novel post-transcriptional and post-translational regulatory mechanisms have also been identified that provide additional layers of regulation mediating FSH homeostasis. Recombinant human FSH analogs hold promise for a variety of clinical applications, whereas blocking antibodies against FSH may prove efficacious for preventing age-dependent bone loss and adiposity. It is anticipated that several exciting new discoveries uncovering all aspects of FSH biology will soon be forthcoming.

Regulation of reproduction via tight control of gonadotropin hormone levels

Mammalian reproduction is controlled by the hypothalamic-pituitary-gonadal axis. GnRH from the hypothalamus regulates synthesis and secretion of gonadotropins, LH and FSH, which then control steroidogenesis and gametogenesis. In females, serum LH and FSH levels exhibit rhythmic changes throughout the menstrual or estrous cycle that are correlated with pulse frequency of GnRH. Lack of gonadotropins leads to infertility or amenorrhea. Dysfunctions in the tightly controlled ratio due to levels slightly outside the normal range occur in a larger number of women and are correlated with polycystic ovaries and premature ovarian failure. Since the etiology of these disorders is largely unknown, studies in cell and mouse models may provide novel candidates for investigations in human population. Hence, understanding the mechanisms whereby GnRH regulates gonadotropin hormone levels will provide insight into the physiology and pathophysiology of the reproductive system. This review discusses recent advances in our understanding of GnRH regulation of gonadotropin synthesis.

Hormones in synergy: regulation of the pituitary gonadotropin genes

The precise interplay of hormonal influences that governs gonadotropin hormone production by the pituitary includes endocrine, paracrine and autocrine actions of hypothalamic gonadotropin-releasing hormone (GnRH), activin and steroids. However, most studies of hormonal regulation of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) in the pituitary gonadotrope have been limited to analyses of the isolated actions of individual hormones. LHbeta and FSHbeta subunits have distinct patterns of expression during the menstrual/estrous cycle as a result of the integration of activin, GnRH, and steroid hormone action. In this review, we focus on studies that delineate the interplay among these hormones in the regulation of LHbeta and FSHbeta gene expression in gonadotrope cells and discuss how signaling cross-talk contributes to differential expression. We also discuss how recent technological advances will help identify additional factors involved in the differential hormonal regulation of LH and FSH.

Facilitation or inhibition of the oestradiol-induced gonadotrophin surge in the immature female rat by progesterone: effects on pituitary responsiveness to gonadotrophin-releasing hormone (GnRH), GnRH self-priming and pituitary mRNAs for the progesterone receptor A and B isoforms

Progesterone can either facilitate or inhibit the oestradiol (E(2))-induced gonadotrophin surge. We have previously developed immature female rat models to characterise and investigate the mechanisms of progesterone inhibition or facilitation. The aim of the present study was to determine the role of pituitary responsiveness to gonadotrophin-releasing hormone (GnRH) and GnRH self-priming under conditions of progesterone-facilitation and progesterone-inhibition, and whether the underlying mechanisms reflect changes in mRNAs encoding the A and B isoforms of the progesterone receptor (PR) in the pituitary gland. Pituitary responsiveness to GnRH, determined by measuring the luteinising hormone (LH) response to one i.v. injection of GnRH, was decreased by 60-80% (P < 0.001) in the progesterone-inhibition model. GnRH self-priming, estimated as the increment in the LH response to the second of two injections of GnRH separated by 60 min, was also significantly reduced (P < 0.05) in this model. In the progesterone-facilitation model, the LH response to GnRH injection was increased 2.5-3-fold (P < 0.05), an effect suppressed by the progesterone receptor antagonist, mifepristone. Progesterone-facilitation of LH release and increased pituitary responsiveness to GnRH were blocked by sheep anti-GnRH serum injected i.v. immediately after insertion of progesterone implants. The PR-B mRNA isoform, measured by solution hybridisation/RNase protection assay, was the predominant form in the pituitary of the immature female rat. PR-B was increased by E(2) and decreased by progesterone in both models. Thus, in immature female rats, progesterone-inhibition of the E(2)-induced LH surge is due to significant reduction in pituitary responsiveness to GnRH as well as in the magnitude of GnRH self-priming. Progesterone-facilitation of the E(2)-induced LH surge is due to increased pituitary responsiveness to GnRH, which is mediated by PR, and depends on endogenous GnRH release. The differences between progesterone-facilitation and progesterone-inhibition are not due to differences in regulation of pituitary PR-B mRNA.

Androgens, progestins, and glucocorticoids induce follicle-stimulating hormone beta-subunit gene expression at the level of the gonadotrope

FSH is produced by the pituitary gonadotrope to regulate gametogenesis. Steroid hormones, including androgens, progestins, and glucocorticoids, have all been shown to stimulate expression of the FSHbeta subunit in primary pituitary cells and rodent models. Understanding the molecular mechanisms of steroid induction of FSHbeta has been difficult due to the heterogeneity of the anterior pituitary. Immortalized LbetaT2 cells are a model of a mature gonadotrope cell and express the endogenous steroid receptor for each of the three hormones. Transient transfection of each receptor, along with ligand treatment, stimulates the mouse FSHbeta promoter, but induction is severely diminished using receptors that lack the ability to bind DNA, indicating that induction is likely through direct DNA binding. All three steroid hormones act within the first 500 bp of the FSHbeta promoter where six putative hormone response elements exist. The -381 site is critical for FSHbeta induction by all three steroid hormones, whereas the -197 and -139 sites contribute to maximal induction. Interestingly, the -273 and -230 sites are also necessary for androgen and progestin induction of FSHbeta, but not for glucocorticoid induction. Additionally, we find that all three receptors bind the endogenous FSHbeta promoter, in vivo, and specifically bind the -381 site in vitro, suggesting that the binding of the receptors to this element is critical for the induction of FSHbeta by these 3-keto steroid hormones. Our data indicate that androgens, glucocorticoids, and progestins act via their receptors to directly activate FSHbeta gene expression in the pituitary gonadotrope.

Luteinizing hormone secretion from wild-type and progesterone receptor knockout mouse anterior pituitary cells

The progesterone receptor (PR) has a central role in the hypothalamo-pituitary events culminating in the preovulatory LH surge, and mice with genetically ablated PR provide a model for dissecting cellular pathways subserving this role. The aims of this study were to determine 1) whether the GnRH self-priming response and acute progesterone augmentation of secretagogue-stimulated LH secretion are present in cultured wild-type (WT) mouse pituitary cells, and 2) whether the PR is essential for self-priming by comparing the responses in PR knockout (PRKO) cells. Pituitary cells from ovariectomized WT or PRKO mice cultured +/- 17beta-estradiol (E(2)) for 3 days were challenged with hourly pulses of 1 nM GnRH or 54 mM K(+). A background of E(2) had no effect on the initial LH secretory response for either WT or PRKO cells. However, for subsequent GnRH pulses, E(2) was permissive for the GnRH self-priming response in WT cells. PRKO cells exhibited a blunted GnRH self-priming response. Exposure to progesterone for 90 min before secretagogue stimulation resulted in a modest (1.5-fold) augmentation of the LH response to GnRH but not K(+) pulses in WT cells; progesterone had no effect in PRKO cells. Unlike in the rat, the PR antagonists RU486 or ZK98299 failed to prevent potentiation of LH secretory responses to multiple GnRH pulses in WT cells. Although RU486 blocked progesterone augmentation of the initial GnRH pulse, it was ineffective in blocking progesterone's action after multiple GnRH pulses. In WT cells, 8- bromo-cAMP (8-Br-cAMP) was able to substitute for the GnRH priming pulse; 8-Br-cAMP also augmented GnRH-stimulated secretion in PRKO cells but less effectively. 8-Br-cAMP augmented K(+)-stimulated LH secretion in WT and PRKO cells equally. These results suggest that, although mouse gonadotropes show GnRH self-priming, they have adapted strategies different than rat cells for amplifying the GnRH signal as shown by the residual self-priming in PRKO cells, the modest or absent augmentation by acute progesterone of GnRH- or K(+)-stimulated secretion in WT cells, and the reduced ability of PR antagonists to interfere with GnRH self-priming and progesterone augmentation. We speculate that the adaptations could involve, at least in part, differences in the ratio of PR isoforms.

Progesterone regulation of the progesterone receptor in rat gonadotropes

For rat pituitary cells, progesterone receptor (PR) protein localizes to gonadotropes and PR messenger RNA is induced by E2 and rapidly but transiently down-regulated by progesterone. Here we quantitatively establish the down-regulatory effect of progesterone on PR protein and evaluate possible mechanisms. Nuclear PR-immunoreactivity (PR-IR) in gonadotropes, identified by dual immunofluorescence, was analyzed by quantitative confocal microscopy. Pituitary cells from female rats were cultured +/- 0.2 nM E2 for 3 days. We confirmed the E2 requirement for PR induction in gonadotropes and determined that the increase in PR-IR required about 24 h. After removal of E2, PR-IR decreases were not found until 24-36 h. Addition of progesterone (40 nM) to E2-treated cells led to a dramatic loss in PR-IR by 9 h (26% of control); by 24 h, PR-IR was barely detectable. Reappearance of nuclear PR-IR required progesterone removal (8-fold increase by 12 h after progesterone removal) and protein synthesis (cycloheximide inhibited the reappearance of PR-IR). Although progesterone decreased PR-IR whether or not E2 was present concurrent with progesterone, the recovery of PR-IR required E2. RU486 completely blocked progesterone-induced PR down-regulation. Because the sustained progesterone-induced loss of PR protein did not correlate with previously reported temporal changes in PR messenger RNA levels, we examined a role for protein degradation. When cells were coincubated with progesterone and the proteasome inhibitor, MG132 (1 microM), the expected decrease in PR protein was abrogated. In summary, progesterone leads to a rapid and extensive reduction in nuclear PR protein in gonadotropes. The progesterone-dependent down-regulation of PR occurs, at least in part, by a proteasome-mediated pathway. Recovery of PR protein requires removal of progesterone, the presence of E2, and protein synthesis. These dynamic changes in nuclear PR levels coincide with the temporal extent of the preovulatory LH surge in rats and could provide a basis for progesterone's biphasic action on LH secretion.

Comparison of the effects of antiprogestins RU38486, ZK98299 and ORG31710 on periovulatory hypophysial, ovarian and adrenal hormone secretion in the rat

The antiprogestin (AP) RU38486 (RU) blocks progesterone (P) and glucocorticoid (G) actions. Administration of 4 mg RU on proestrous morning to cyclic rats dissociates LH and FSH secretion on proestrous afternoon, early estrus and on estrous afternoon. In order to ascertain which action blocked by RU is predominant in the control of periovulatory LH and FSH secretion, a study was made on the effects of: a) 1 or 4 mg of ZK98299 (ZK) (type I P antagonist; Schering), b) 2 or 8 mg of Org31710 (OR) (type II P antagonist lacking anti-G actions; Organon) or c) 1 or 4 mg of RU (type II P antagonist; Exelgyn) to 4-day cyclic rats on proestrous morning on serum concentrations of LH, FSH, inhibin-alpha (I), estradiol-17beta (E), progesterone (P) and corticosterone (B) at 18:30 h on proestrus and at 02:00 and 18:30 h on estrus. Controls, receiving 0.2 ml oil, had elevated serum concentrations of all six hormones on proestrous afternoon; at early estrus, only serum concentrations of FSH and P remained elevated, and, on estrous afternoon, all hormones but I and B, that peaked again, had reached their lowest serum levels. All AP treatments except 1 mg ZK had the same effects. On proestrous afternoon serum LH concentrations were reduced and serum FSH concentrations were suppressed whereas serum levels of I, E, P and B were unaffected. At early estrus, basal serum concentrations of LH and E increased while FSH secretion was abolished. Serum levels of I, P and B did not differ from controls. AP treatments increased basal LH concentration, hyperstimulated FSH secretion and reduced serum I concentration on the afternoon of estrus. E, P and B serum levels did not differ from controls at this stage. Treatment with 1 mg ZK was less effective in reducing serum FSH on proestrous afternoon and at early estrus, and had no effect on serum concentrations of any hormone on estrous afternoon. These results indicate that blockade of P receptor activation by P is, predominantly, the mechanism of AP action on periovulatory gonadotropin secretion in rats.

Progesterone receptor A and B messenger ribonucleic acid levels in the anterior pituitary of rats are regulated by estrogen

In target tissues of most mammalian and avian species, progesterone receptors (PR) are expressed as structurally related, but functionally distinct, isoforms A and B, and they are regulated by estrogen (E) as well as by their cognate ligand, progesterone (P(4)). The objectives of the present work were to identify mRNA expression for the A and B isoforms of PR in the anterior pituitary of the rat, to examine its regulation by gonadal steroids, and to compare this regulation with that in the primary target organ, the uterus. Messenger RNAs for the PR isoforms, determined by two separate reverse transcription-polymerase chain reaction protocols, one that detects PR A and PR B equally and the other specific for PR B, were identified in anterior pituitary of female and male rats. In anterior pituitary of cycling female rats, steady-state mRNA levels for both PR A+B and PR B were highest at 0900 h on proestrus, declined rapidly to nadir values at 0900 h on metestrus (PR A+B) or 0900 h on estrus (PR B), and remained below proestrous values through 2100 h on diestrus. Administration of E to intact proestrous female rats caused significant increases in mRNA for both PR A+B and PR B on estrus and metestrus. Blockade of P(4) action by administration of the antiprogestins RU-486 and ZK-98299 on proestrus had no effect on PR mRNA levels on the morning of estrus. Ovariectomy two and ten days after surgery markedly reduced mRNA levels for both PR A+B and PR B. Whereas treatment of 10-day-ovariectomized rats with E led to marked induction of mRNA for PR A+B and PR B two days later, treatment with P(4) one day after treatment had no effect on basal or E-stimulated PR mRNA. Regulation of PR mRNA expression in the pituitary differed from that in the uterus, in which P(4) treatment of ovariectomized rats antagonized the E-induced rise in mRNA for PR B, and antiprogestins increased mRNA for both isoforms. In addition to induction of PR mRNA in the pituitary of female rats by E in vivo, we also demonstrated induction by E in primary culture of anterior pituitary cells in vitro. We conclude that in the anterior pituitary of female rats, both the A and B isoforms of PR are expressed and regulated by E.

Absence of gonadotropin surges and gonadotropin-releasing hormone self-priming in ovariectomized (OVX), estrogen (E2)-treated, progesterone receptor knockout (PRKO) mice

It is well known that estrogen (E2) stimulates expression of progesterone receptors (PRs), thereby inducing responsiveness of several tissues to the actions of progesterone (P). Recent studies have also suggested, however, that biological actions previously ascribed to E2 alone may also be mediated by activation of E2-induced PRs, even independently of signal changes in P concentrations. In the present experiments, the progesterone receptor knockout (PRKO) mice were used to assess the role of PR activation in the positive feedback actions of E2 on gonadotropin release. Ovariectomized (OVX) PRKO mice were tested for their capacity to mount primary gonadotropin surges in response to exogenous E2, and to exhibit a GnRH self-priming effect in response to sequential injections of the decapeptide. Wild-type (WT) and PRKO mice were OVX, treated with both 17beta-estradiol and estradiol benzoate (EB), and then killed at 1900 h on day 7 postOVX. Plasma LH RIA revealed that WT mice exhibited surges in response to the E2 treatment; the PRKO mice, however, showed no elevation in plasma LH above untreated controls. Instead, plasma LH levels in E2-treated, OVX PRKO mice decreased significantly in comparison to untreated OVX PRKO mice, suggesting that E2 can exert a negative feedback influence on LH release in PRKO mice, despite the absence of positive feedback effects. A slight but significant rise in plasma FSH was observed in E2-treated OVX WT mice in comparison to untreated controls: an effect not seen in E2-treated OVX PRKO mice, reinforcing the observation that estrogen's positive feedback effects are compromised in PRKO mice. In a second experiment, E2-treated OVX WT and PRKO mice were given either one or two pulses of GnRH 60 min apart, and killed 10 min later. The WT mice were found to exhibit a robust GnRH self-priming effect, as WT mice receiving two GnRH pulses displayed LH responses approximately 2-fold greater than those receiving only one pulse. By contrast, PRKO mice receiving two GnRH pulses exhibited no additional increase in plasma LH levels. We conclude that PR activation is obligatory for expression of the GnRH self-priming effect as well as for generation of E2-induced LH and FSH surges. The extent to which failure of LH surge secretion in PRKO mice is due to the absence of GnRH self-priming, lack of hypothalamic GnRH surges, and/or defects in other processes remains to be determined. These observations clearly demonstrate, however, that the presence of PR is an absolute requirement for the transmission of E2-induced signals leading to gonadotropin surges.

Steroid regulation of progesterone receptor expression in cultured rat gonadotropes

During the preovulatory period, the pituitary action of progesterone is biphasic, moving from a severalfold augmentation of the gonadotropin release action of GnRH to a suppression of GnRH efficacy, which occurs in rats over a period of about 12 h, but the extent to which these biphasic effects are dependent on alterations in progesterone receptor (PR) expression is not known. To address this, as well as the localization of PR in cultured rat pituitary cells, we used cells from ovariectomized rats cultured +/- 0.2 nM E2 with acute progesterone treatment on day 3. Northern blot of poly(A+) RNA extracts showed multiple PR messenger RNA (mRNA) transcripts between 4.8-10.2 kb; E2 treatment led to a 5- to 6-fold increase in the predominant PR mRNA transcripts (5.1 and 10.1 kb). In the presence of E2, 200 nM progesterone resulted in a decrease in steady-state PR mRNA levels by 3 h of exposure, with the greatest decrease around 6 h (50% of E2 control) and recovery by 12 h. Similarly treated pituitary cultures were subjected to dual immunofluorescence staining for LH and PR. In the absence of E2, PR was undetectable. In the presence of E2, essentially all LH-positive cells were positive for PR and only 1-2% of PR-immunopositive cells were negative for LH, possibly reflecting FSH-exclusive gonadotropes. PR staining was predominantly nuclear, but 20 nM progesterone led to a gradual increase in cytoplasmic staining, with the nuclear-to-cytoplasmic ratio decreasing to near unity by 9-12 h of exposure. In summary, we show for the first time, that PR colocalizes with LH in cultured female rat pituitary cells and that E2 induces expression of PR mRNA, as well as PR protein, in rat gonadotropes. In the presence of E2, progesterone causes a rapid but transient down-regulation of PR message; recovery of PR mRNA is accompanied by an increase in cytoplasmic PR, suggestive of an increase in synthesis. These dynamic changes implicate the gonadotrope PR as having a significant role within the temporal context of the rat preovulatory period.

Both prolactin and progesterone in proestrus are necessary for the induction of apoptosis in the regressing corpus luteum of the rat

This study was conducted to analyze the roles of prolactin (PRL) and progesterone in the induction of luteal cell apoptosis and accumulation of macrophages in the regressing corpus luteum. We studied the number of apoptotic cells and macrophages in regressing corpora lutea in estrus 1) in cycling rats or after blocking PRL secretion with the dopaminergic agonist CB154, and 2) after blocking progesterone actions with the progesterone receptor antagonists RU-486 or ZK98299. Cells showing the morphological features characteristic of apoptosis contained fragmented DNA as indicated by in situ 3' end labeling. In cycling rats, a 100-fold increase in the number of apoptotic cells and a 4-fold increase in the number of macrophages was found from the evening (1600 h) of proestrus to the morning (1100 h) of estrus. Both increases were blocked by PRL suppression with CB154. Furthermore, blocking progesterone actions with progesterone receptor antagonists RU-486 or ZK98299 without affecting PRL secretion inhibited apoptosis but did not affect the accumulation of macrophages, whether treatment was started on the morning of metestrus (blocking diestrous and proestrous progesterone) or on proestrus (blocking only proestrous progesterone). Otherwise, exogenous progesterone was not effective in inducing apoptosis in the absence of PRL. These results indicate that both PRL and progesterone in proestrus are necessary for the induction of apoptosis in the regressing corpora lutea, whereas the accumulation of macrophages seemed to be dependent exclusively on the PRL surge.