Progesterone modulation of gonadotropin secretion by dispersed rat pituitary cells in culture. III. A23187, cAMP, phorbol ester and DiC8-stimulated luteinizing hormone release

Activity and localization of 3beta-hydroxysteroid dehydrogenase/ Delta5-Delta4-isomerase in the zebrafish central nervous system

Little information is available for neurosteroidogenesis in the central nervous system (CNS) of lower vertebrates. Therefore, in the present study, we examined the enzymatic activity and localization of 3beta-hydroxysteroid dehydrogenase/Delta5-Delta4-isomerase (3betaHSD), a key steroidogenic enzyme, in the CNS of adult male zebrafish to clarify central progesterone biosynthesis. Biochemical studies together with HPLC analysis revealed that the zebrafish brain converted pregnenolone to progesterone, suggesting the enzymatic activity of 3betaHSD. This conversion was significantly reduced by trilostane, a specific inhibitor of 3betaHSD. By using Western immunoblotting with the polyclonal antiserum directed against purified bovine adrenal 3betaHSD, a 3betaHSD-like substance was found in homogenates of the zebrafish brain. Immunocytochemical analysis was then undertaken to investigate the localization of the 3betaHSD-like substance in the zebrafish brain and spinal cord. Clusters of immunoreactive cell bodies were localized in the dorsal telencephalic areas (D), central posterior thalamic nucleus (CP), preoptic nuclei (NPO), posterior tuberal nucleus (PTN), paraventricular organ (PVO), and nucleus of medial longitudinal fascicle (NMLF). 3betaHSD-like immunoreactivity was also observed in somata of cerebellar Purkinje neurons. A widespread distribution of immunoreactive fibers was found throughout the brain and spinal cord. In addition, positively stained cells were restricted to other organs, such as the pituitary and retina. Preabsorbing the antiserum with purified bovine adrenal microsome resulted in a complete absence of 3betaHSD-like immunoreactivity. These results suggest that the fish CNS possesses steroidogenic enzyme 3betaHSD and produces progesterone. The present study further provides the first immunocytochemical mapping of the site of 3betaHSD expression in the fish CNS.

Genomic and membrane actions of progesterone: implications for reproductive physiology and behavior

Progesterone, produced by the ovaries and adrenal glands, regulates reproductive behavior and the surge of luteinizing hormone which precedes ovulation by acting on neurons located in different parts of the hypothalamus. The study of the activation of these reproductive functions in female rats has allowed to explore the different mechanisms of progesterone action in the brain. It has allowed to demonstrate that new actions of the hormone, which have been observed in particular in vitro systems, are also operational in vivo, and may thus be biologically relevant. This mainly concerns the direct actions of progesterone on receptors of neurotransmitters such as oxytocin and GABA. Activation of the progesterone receptor in the absence of ligand by phosphorylation may also play a role.

Androgen modulation of luteinizing hormone secretion by female rat gonadotropes

In the female, androgens can have negative and positive actions in the regulation of LH, but it is not clear how they may function during the reproductive cycle. Toward resolving these potentially conflicting roles for androgen, we used an in vitro model of preovulatory gonadotropes to examine the effect of proestrous levels of testosterone (1.7 nM) or dihydrotestosterone (DHT; 0.7 nM) on LH secretion in response to pulsatile GnRH (1 nM) or elevated extracellular K+ (54 mM). For female rat pituitary cells cultured in 17beta-estradiol (E2)-containing medium, androgen treatment for 16 h, but not for 4 h, inhibited the LH secretory response to a pulse of either GnRH or K+ by about 60% and suppressed the acute augmentation action of 20 nM progesterone on GnRH- or K+-induced LH secretion. In the absence of E2, DHT also decreased LH secretion induced by a pulse of GnRH. DHT's suppressive effect on progesterone could be partially overcome with increased progesterone (200 nM) or by removal of DHT during progesterone exposure. For pituitary cells transfected with a reporter plasmid containing three progesterone response elements, DHT only partially suppressed progesterone-stimulated transcriptional activity. The positive action of androgen (16 h) on LH secretion was elicited by multiple GnRH pulses with a latency of about 2 h after the first pulse; this facilitatory action of androgen did not require an E2 background and, therefore, is distinct from GnRH self priming. In summary, these data demonstrate both facilitatory and inhibitory actions of androgen on LH secretion function in female gonadotropes in vitro in the absence or presence of E2; these actions occur with a time course suggestive of a role for androgen in shaping the preovulatory LH surge. Androgen also markedly suppresses progesterone augmentation of stimulated LH secretion, which could be due in part to interference with the trans-activation function of the progesterone receptor.

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.

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

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

Synthesis, metabolism and levels of the neuroactive steroid, 3alpha-hydroxy-4-pregnen-20-one (3alphaHP), in rat pituitaries

The neuroactive steroid, 3a-hydroxy-4-pregnen-20-one (3alphaHP), is a metabolite of progesterone and a precursor of 3alpha-hydroxy-5alpha-pregnan-20-one (5alphaP3alpha; allopregnanolone). In addition to analgesic and anxiolytic effects by interaction with the GABA(A) receptor complex, 3alphaHP regulates pituitary FSH secretion by rapid non-genomic interaction with the Ca2+-driven cell signaling mechanisms. Since gonadectomy and adrenalectomy do not result in elimination of 3alphaHP, and since there is the possibility of paracrine and/or autocrine regulation of FSH release, the capacity of pituitary cells to regulate levels (by synthesis, metabolism, and storage) of 3alphaHP was examined. Anterior pituitaries from random cycling female rats were incubated, either as fragments or as cultured cells, for 1, 4 or 8 h with 3H- or 14C-labeled progesterone. The steroid metabolites were identified by thin-layer chromatography, autoradiography, high pressure liquid chromatography (HPLC), derivatization and GC/MS. Pituitary cells actively converted progesterone to 3alphaHP along with 5alphaP3alpha, 5alpha-pregnane-3,20-dione, 20alpha-hydroxy-5alpha-pregnan-3-one, 3beta-hydroxy-5alpha-pregnan-20-one, 5alpha-pregnane-3alpha(beta), 20alpha-diols, 20alpha-hydroxy-4-pregnen-3-one, and 4-pregnene-3alpha(beta), 20alpha-diols. The results indicate the presence of the following steroidogenic enzymes in anterior pituitary cells: 3alpha-hydroxysteroid oxidoreductase (3alpha-HSO), 20alpha-HSO, 3beta-HSO, and 5alpha-reductase. The activities of 5alpha-reductase and 3alpha-HSO were approximately equal and greatly exceeded those of the other enzymes. After 8 h of incubation with 100 ng progesterone per pituitary, about 20% of the progesterone was metabolized and 3.18 ng of 3alphaHP had been formed. The accumulation of 3alphaHP increased approximately linearly with the time of incubation. Metabolism studies using [1,2,6,7-(3)H]3alphaHP showed that pituitary cells convert about 29% and 8% of the 3alphaHP to progesterone and 5alphaP3alpha, respectively, in 2 h. Specific radioimmunoassays determined 11.6 and 7.5 ng of 3alphaHP per pituitary, respectively, in 25- and 40-day-old non-cycling female rats; these concentrations of 3alphaHP were about 2-3-fold greater than those of progesterone in the same pituitaries. In older (80-100 days old) cycling rats, the levels of 3alphaHP were about 9.4 and 18.6 ng/pituitary at 13.00 h and 22.00 h, respectively, on the day of proestrus, while the concomitant circulating levels were 13.7 and 5.4 ng/ml. The results indicate a marked capacity of rat pituitary cells to synthesize the neuroactive and FSH regulating steroid, 3alphaHP, from progesterone, and in turn to metabolize 3alphaHP to the neurosteroid, allopregnanolone, and to progesterone. The studies suggest cyclic biosynthetic and metabolic pathways for 3alphaHP and other steroids in the pituitary. They also indicate that the regulation of FSH secretion by 3alphaHP may be (in part, or in whole) via paracrine or autocrine mechanisms.

Acute, nongenomic actions of the neuroactive gonadal steroid, 3 alpha-hydroxy-4-pregnen-20-one (3 alpha HP), on FSH release in perifused rat anterior pituitary cells

We have previously shown that the gonadal and neurosteroid, 3 alpha-hydroxy-4-pregnen-20-one (3 alpha HP), can selectively suppress gonadotrophin-releasing hormone (GnRH) induced follicle-stimulating hormone (FSH) release from static cultures of anterior pituitary cells during a 4-h incubation period. The actions appeared to be at the level of the gonadotroph membrane and the cell signaling pathway involving Ca2+ and protein kinase C (PKC). In order to investigate further if the effects of 3 alpha HP on FSH release are generated by nongenomic mechanisms, we monitored the short-term effects of 3 alpha HP using dispersed anterior pituitary cells in a low dead-volume perifusion system with short (< or = 5 min) exposures to the steroid. Pulses of GnRH (10(-8) or 10(-7) M) lasting 2-5 min resulted in marked peaks of FSH release, and the variation in FSH amounts released from the cells in a particular column were minimal if the interval between successive GnRH pulses was at least 3-4 h. A 5-min pulse of 3 alpha HP (10(-9) M) administered simultaneously with the GnRH pulse suppressed GnRH-induced FSH release. On the other hand, similar treatment with the stereoisomer 3 beta-hydroxy-4-pregnen-20-one (3 beta HP), had no effect, but progesterone and estradiol pulses augmented the GnRH-induced FSH release. Pretreatment of cells with a 5-min pulse of 3 alpha HP, at 120, 60, or 30 min prior to a GnRH pulse suppressed the GnRH-induced FSH release. The suppression of GnRH-induced FSH release by 3 alpha HP was only partial if the start of the 3 alpha HP pulse occurred 0.5 or 1.0 min after the start of the GnRH pulse, and no suppression occurred if the start of the 3 alpha HP pulse was delayed by 2-5 min. The FSH release elicited by 5-min pulses of the Ca2+ ionophore A23187, the Ca2+ agonist BAY K8644, the PKC activator phorbol 12-myristate 13-acetate (PMA), or phospholipase C (PLC) was suppressed by simultaneous pulses of 3 alpha HP. The suppression of FSH release by 3 alpha HP appeared to be stereospecific, since no suppression was observed with 5 alpha-pregnane-3,20-dione (5 alpha P) or 3 alpha-hydroxy-5 alpha-pregnan-20-one (5 alpha P3 alpha). In separate experiments, cells were treated with pulses of BSA conjugates of 3 alpha HP, 3 beta HP, or progesterone; the 3 alpha HP-BSA, but not the 3 beta HP-BSA or the progesterone-BSA, suppressed the GnRH-induced release of FSH. The results of this study provide the first evidence that 3 alpha HP exerts immediate (nongenomic) and direct effects on GnRH-induced FSH release by interacting at the level of the pituitary gonadotroph membrane and the phosphoinositol cell signaling cascade involving Ca2+.

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

The ovarian steroids estradiol and progesterone act as important modulators of GnRH-induced luteinizing hormone (LH) secretion from anterior pituitary cells. Recently, we demonstrated that the steroids are able to influence GnRH-stimulated Ca2+ mobilization from extra- and intracellular sources. Here we investigated the actions of estradiol and progesterone on GnRH-induced biphasic LH secretory responses in the model of perifused female rat pituitary cells. A 20 min GnRH stimulus elicited biphasic LH responses composed of an initial peak followed by a prolonged plateau phase. Both phases were equally enhanced by long-term (48 h) estradiol treatment. This action was facilitated by subsequent short-term progesterone treatment. In contrast, combined treatment with estradiol and progesterone for 48 h led to inhibited LH secretory profiles. To determine the steroid actions on the extracellular Ca2+ independent component of LH secretion we performed experiments using cells that were perifused with Ca2+ deficient medium. Under these conditions the cells responded exclusively with a single peak phase of LH secretion, which was augmented or inhibited by estradiol and progesterone treatment as described above. To test the hypothesis that an effect of estradiol and progesterone on GnRH-induced polyphophoinositide hydrolysis is responsible for their modulatory actions on Ca2+ signals and LH secretion we measured inositol phosphate (IP) accumulation after different steroid treatment paradigms in rat pituitary cells and alpha T3-1 immortalized gonadotrophs. GnRH-induced IP production was enhanced by long-term estradiol treatment. Short-term exposure of estradiol-primed cells to progesterone did not lead to significant changes of IP production. The long-term progesterone treatment paradigm enhanced GnRH-induced IP formation, while it decreased Ca2+ signals and LH secretion. Alpha T3-1 cells were used to perform more detailed analysis of IP formation. The actions of estradiol and progesterone on the production of inositol mono-, bis-, and trisphosphates were similar to those observed in the mixed cell population. It is concluded that estradiol and progesterone modulate both peak and plateau phases of GnRH-stimulated LH secretory responses, effects which are associated with their impact on Ca2+ signals. Our findings argue against a role of IP modulation in the mechanism of progesterone actions on Ca2+ signaling and LH secretion in gonadotrophs. Such a mechanism might be involved in the positive effects of estradiol in these cells.

Progesterone modulation of gonadotropin secretion by dispersed rat pituitary cells in culture. IV. Follicle-stimulating hormone synthesis and release

Estradiol-treated, rat pituitary cells were studied to examine the effects of progesterone (P) on follicle-stimulating hormone (FSH) synthesis and secretion. Progesterone was administered prior to or concurrent with 3 h secretory challenges with either gonadotropin-releasing hormone (GnRH), the iontophore A23187, the protein kinase C activator phorbol 12,13-myristate (PMA), or no secretagogue. Medium FSH levels and cell FSH stores were quantified by radioimmunoassay and bioassay. Acute (< 6 h) exposures to P increased medium levels of immunoreactive and bioactive FSH following GnRH challenge without influencing total (cell + medium) values whereas chronic (9-24 h) treatments increased both parameters. Chronic P elevated total FSH levels even when no secretagogue was present. Studies with antiprogestins, 5 alpha-dihydroprogesterone and 5 alpha-reductase inhibitors revealed that this direct action of P depended on progestin receptor occupation but not on 5 alpha-reduction. These studies indicate that P selectively increases bioactive and immunoactive FSH levels, presumably by increasing FSH synthesis, and characterize the time course and cellular mechanisms of this response. To accommodate for P modulation of total FSH levels, FSH secretion was standardized as the percentage of cellular stores available for release. Progesterone modulation of GnRH-stimulated FSH secretion was multiphasic, i.e. increased at 0-6 h, unchanged at 9 h and suppressed at 24 h. Acute and chronic exposures to P similarly modulated A23187-stimulated FSH release, whereas both P treatments increased PMA-stimulated FSH secretion. In these experiments P modulated luteinizing hormone secretion in parallel fashion, suggesting that common cellular mechanisms underlie peptidergic and steroidal regulation of the secretion of both gonadotropins.

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.

Functional cross-talk between receptors for peptide and steroid hormones

Communication between a cell surface peptide hormone receptor and an intracellular steroid hormone receptor can take various routes, as dictated by the physiology of a particular cell type. There is increasing evidence for a novel route which requires that a peptide hormone receptor pathway converge on a steroid hormone receptor, leading to its activation. One consequence of such a process can be signal amplification for the peptide hormone receptor agonist. This is exemplified by the self-potentiating action of GnRH, which is a critical component in events leading to a surge in LH secretion and ovulation. One signaling pathway stimulated by the GnRH receptor may entail a phosphorylation cascade resulting in progesterone-independent modulation of progesterone receptor activity.

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.

PLC-alpha: a common mediator of the action of estrogen and other hormones?

The phosphatidyl inositol (PI) second messenger pathway may mediate diverse effects of estrogen, including its potentiation of the effects of other hormones. Both estradiol (E2) and luteinizing hormone-releasing hormone (LHRH) induce a putative isoform of PI-specific phospholipase C-alpha (PLC-alpha). PLC-alpha catalyzes PI hydrolysis, which in turn can increase protein kinase C (PKC) activation, Ca2+ mobilization, and arachidonic acid metabolism. Estrogen activates the PI pathway, and components of the PI pathway can mimic or enhance some effects of estrogen. Furthermore, estrogen potentiates effects of several hormones (e.g., LHRH, prolactin, and insulin) which can also act through the PI system. PLC-alpha may therefore provide a common second messenger pathway mediating the potentiation by E2 of the effects of other hormones; in addition it may also mediate some or all of the many actions of E2, since components of the PI pathway can have secretory, trophic, toxic, and neuromodulatory effects.