Inhibin binding sites and proteins in pituitary, gonadal, adrenal and bone cells.

Activin signals via complexes of type I (50-55 kDa) and II (70-75 kDa) activin receptors, but the mechanism of inhibin action is unclear. Proposed models range from an anti-activin action at the type II activin receptor to independent actions involving putative inhibin receptors. Two membrane-embedded proteoglycans, betaglycan and p120, have recently been implicated in inhibin binding, but neither appears to be a signalling receptor. The present studies on primary cultures of rat pituitary and adrenal cells, and several murine and human cell lines were undertaken to characterise inhibin binding to its physiological targets. High affinity binding of inhibin to the primary cultures and several of the cell lines, like that previously described for ovine pituitary cells, was saturable and reversible. Scatchard analysis revealed two classes of binding sites (K(d) of 40-400 and 500-5000 pM, respectively). Affinity labelling identified [125I]inhibin binding proteins with apparent molecular weights of 41, 74, 114 and >170 kDa in all cell types that displayed high affinity, high capacity binding of inhibin. Additional labelling of a 124 kDa species was evident in gonadal TM3 and TM4 cell lines. In several cases, activin (> or =20 nM) competed poorly or not at all for binding to these proteins. The 74, 114 and >170 kDa inhibin binding proteins in TM3 and TM4 cells were immunoprecipitated by an anti-betaglycan antiserum. These three proteins correspond in size to the activin receptor type II and the core protein and glycosylated forms of betaglycan, respectively, that have been proposed to mediate anti-activin actions of inhibin, but the identity of the 74 kDa species is yet to be confirmed. Studies of [125I]inhibin binding kinetics and competition for affinity labelling of individual binding proteins in several cell lines suggest these three species and the 41 and 124 kDa proteins form a high affinity inhibin binding complex. In summary, common patterns of inhibin binding and affinity labelling were observed in inhibin target cells. Novel inhibin binding proteins of around 41 and 124 kDa were implicated in the high affinity binding of inhibin to cells from several sources.

Identification of specific inhibin A-binding proteins on mouse Leydig (TM3) and sertoli (TM4) cell lines.

The binding of human inhibin A to cell surface binding proteins of mouse Leydig (TM3) and Sertoli (TM4) cell lines was investigated. Scatchard analysis identified two classes of inhibin A-binding sites on TM3 (K(d(1)) = 85 pM and 4,160 sites/cell; K(d(2)) = 520 pM and 12,500 sites/cell) and TM4 (K(d(1)) = 61 pM and 2,620 sites/cell; K(d(2)) = 520 pM and 10,400 sites/cell) cells. Compared with inhibin A, inhibin B only partially competed [(125)I]inhibin A binding (6-8%), whereas activin A competed weakly (<0.01%). Chemical cross-linking of [(125)I]inhibin A to both cell lines identified five [(125)I]inhibin A binding complexes with apparent molecular masses of 70, 95, 145, 155, and more than 200 kDa. Inhibin A displacement of [(125)I]inhibin A from each of these cross-linked species (ED(50) = 60-110 pM) closely resembled displacement from intact TM3 (ED(50) = 97 +/- 32 pM) and TM4 (ED(50) = 75 +/- 28 pM) cells, suggesting that all of these proteins are involved in the high affinity inhibin A binding complex. Immunoprecipitation of iodinated inhibin A complexed to TM3 and TM4 cells with an antibody against human betaglycan identified protein complexes of more than 200, 145, and 95 kDa. It is concluded that the high affinity binding complex for inhibin A found in these cell lines consists of betaglycan and several proteins of unknown identity and may represent the putative inhibin receptor complex.

Is the action of inhibin mediated via a unique receptor?

The receptor system and the molecular mechanisms by which inhibin acts on its target cells are poorly understood, in contrast to the situation for the structurally related molecule, activin. On the basis of evidence that the biological action of inhibin in a number of systems resembles that of an activin antagonist, it has been contended that inhibin operates by competition for the activin receptor rather than through a specific inhibin receptor. However, mounting evidence indicates that inhibin also interacts with high affinity and specificity with membrane-binding proteins that are likely to be the putative inhibin receptor.

Identification of high affinity binding sites for inhibin on ovine pituitary cells in culture.

The aim of this study was to identify and characterize binding sites for inhibin in primary cultures of ovine anterior pituitary cells. Recombinant human 31-kDa inhibin A was iodinated by an optimized lactoperoxidase procedure. Fractionation of the labeled protein by gel filtration chromatography on Sephadex G-100 in 0.1 M HCl yielded two immunoactive peak regions, the second of which was bioactive as assessed by in vitro bioassay, with a ratio of bioactivity/immunoactivity of 0.62-0.77 and an iodine incorporation ratio of 1.7-2.0 mol 125I/mol inhibin. The specific binding of purified [125I]inhibin to cultured ovine pituitary cells varied with time, temperature, and cell number. Displacement of the tracer by unlabeled inhibin, as assessed by Scatchard analysis, revealed two binding sites with average Kd values of 0.28 and 3.9 nM and with approximately 250 and 3100 binding sites/anterior pituitary cell, respectively. There was little cross-reaction between inhibin and activin A (<2%), transforming growth factor-beta (<0.2%), or follistatin (<<0.1%). Examination of cell lines that were not expected to have inhibin receptors showed that there was no specific binding of inhibin to human leukemia (Jurkat) cells, whereas the binding to human embryonic kidney (293) cells was displaced by both inhibin and activin with a similar degree of cross-reaction, which suggests binding to an activin receptor. It is concluded that inhibin-binding sites with high affinity and specificity have been identified on ovine pituitary cells, consistent with both inhibin action on the pituitary and the presence of the putative inhibin receptor.

Growth hormone-releasing peptide-2 (GHRP-2) does not act via the human growth hormone-releasing factor receptor in GC cells.

Effect of growth hormone-releasing peptide-2 (GHRP-2) on ovine somatotrophs is abolished by a growth hormone-releasing factor (GRF) receptor antagonist, which raises the possibility that GHRP-2 may act on GRF receptors. In the present study, we used rat pituitary GC cells with or without stable transfection of cDNA coding for the human GRF receptor (GC/R+ or GC/R-) to determine whether or not GHRP-2 acts via the GRF receptor. Northern blot analysis indicated that GRF receptor mRNA was undetectable in GC/R-cells, whereas a high level of expression occurred in GC/R+ cells that were transfected by GRF receptor cDNA. In GC/R- cells, incubation with up to 10(-7)M of either hGRF or GHRP-2 did not alter the intracellular cAMP, [Ca2+]i, or GH secretion. In GC/R+ cells, hGRF (10(-11)-10(-7)M) increased cAMP levels in a concentration-dependent manner up to 20-fold. This increase in cAMP levels was blocked by a GRF receptor antagonist, [Ac-Tyr1, D-Arg2]-GRF 1-29, but not by a Ca2+ channel blocker, NiCl2 (0.5 mM). GH secretion and [Ca2+]i were, however, not increased by hGRF. Incubation of the transfected cells with 10(-1)-10(-8)MGH RP-2 did not modify intracellular cAMP levels. This result suggests that GHRP-2 does not act through the GRF receptor.

Ovine allantoic fluid contains high concentrations of activin A: partial dissociation of immunoactivity and bioactivity.

In a preliminary study, allantoic fluid collected from pregnant sheep across gestational ages of 20-124 days contained significantly higher levels of activin bioactivity (189 +/- 74 ng/ml, mean +/- SE) than did amniotic fluid (3.2 +/- 0.6 ng/ml). Using a combination of chromatography steps, we isolated from 5 L of allantoic fluid approximately 612 microg of immunoactive activin, which eluted over 10 fractions from a C8 reversed-phase column. When these fractions were assayed in a rat pituitary cell culture bioassay, in a specific RIA, and in an activin A two-site ELISA, the RIA activity was skewed to the less hydrophobic side of the activin profile, while the bioactivity was skewed to the more hydrophobic forms. The activity measured in the two-site ELISA more closely matched the mass of activin as determined by laser densitometry. Amino-terminal sequencing of fractions containing either peak immunoactivity or bioactivity showed each to be identical to activin A. This was confirmed by internal sequences from a fraction that eluted in the area of overlapping immunoactivity and bioactivity. A peptide containing at least 18 amino acids at its amino terminus, which were identical to the conserved region of the acute-phase protein serum amyloid A, was identified in the most immunoactive activin fractions.

Bombesin regulation of adrenocorticotropin release from ovine anterior pituitary cells.

Mammalian members of the bombesin-like peptide family (gastrin releasing peptides; GRP) have been localized in the ovine median eminence and in hypophysial-portal blood, suggesting a role in the regulation of anterior pituitary function. In this study we have shown that although bombesin cannot stimulate ACTH secretion alone, it potentiates release by ovine CRF, an effect blocked by the GRP receptor antagonist D-Tyr6bombesin (6-13) propylamide. Bombesin did not potentiate AVP-stimulated ACTH release; instead release was attenuated when bombesin was given at a 10-fold or greater molar excess over AVP, with no interaction seen at lower concentrations. We conclude that ovine corticotrophs express bombesin receptors, and that GRP may act in concert with other hypothalamic releasing factors to regulate ACTH secretion.

Characterization and determination of the biological activities of noncleavable high molecular weight forms of inhibin A and activin A.

Recombinant expression of human alpha- and beta A-inhibin subunit cDNAs in mammalian 293 cells results in the secretion of 20-53K free alpha-subunit-derived products, 30-105K alpha beta A-inhibin dimers, and 24-110K beta A-activin dimers. The present study verifies that the wide variation in the size of these products is due to incomplete cleavage of the proteolytic processing sites and the differential glycosylation of the N-linked glycosylation site at amino acid number 302 in the alpha C-subunit. The identity of each of these products was established by mutagenesis of proteolytic processing sites and N-linked glycosylation sites, combined with the analysis of transfection products by immunoprecipitation and one- and two-dimensional SDS-PAGE (SDS/SDS-beta-ME). Transient expression of processing site mutants of the alpha- and beta A-subunits in 293 cells was used to generate microgram quantities of noncleavable 55K and 65K inhibin dimers, and noncleavable 110K activin A dimers. The 55K and 65K inhibin A forms were purified and found to be fully biologically active in a rat pituitary cell bioassay. The 110K high molecular weight (HMW) form of human activin A failed to show any FSH-releasing activity in the pituitary assay. Since radioactively labeled 55K and 65K inhibin A and 110K activin A remained intact after incubation with rat pituitary cells for 72 h, there appears to be no conversion of these dimers to lower molecular weight forms by proteolytic cleavage at additional sites. These results show for the first time that 55K and 65K inhibit A are intrinsically biologically active and do not require cleavage to the 32K form for activation. In contrast, cleavage of the 110K activin A precursor to the 24K form would appear to be necessary for activity.

Ovine anterior pituitary production of follistatin in vitro.

Rat pituitary cells express messenger RNA for the activin-binding protein, follistatin (FS), and rat and bovine pituitary cell cultures secrete FS into the medium. In the present study, a previously validated, heterologous RIA for ovine FS was employed to investigate FS synthesis, secretion, and regulation in cultures of ovine anterior pituitary cells. The validity of the RIA was confirmed by the finding that FS immunoreactivity in ovine pituitary cell culture-conditioned medium diluted in parallel with purified bovine FS, and fractionation of the conditioned medium resulted in the coelution of activin-binding activity with the FS immunoactivity. The concentration of endogenous ovine FS achieved in the culture medium (0.08-0.6 nM) was in the range over which bovine FS suppresses FSH secretion in these cultures (IC50 = 0.5 nM). To characterize the relationship between endogenous FS and FSH secretion, dispersed ovine pituitary cells were preincubated with 10% fetal bovine serum for 2 days, then cultured between days 2-5 in the presence of a chemically defined serum substitute. Under these conditions, FS was continuously secreted at a rate of 12.1 +/- 1.8 ng/10(6) cells.day (mean +/- SEM; n = 18), whereas FSH was secreted at 64 +/- 13 ng/10(6) cells.day (n = 7). The secretion of FS and FSH changed in a reciprocal way as culture conditions were altered either by maintaining exposure of the cells to fetal bovine serum or by plating the cells at a 6- to 10-fold higher seeding density. Under the latter circumstance, for instance, FS secretion during the 3-day test period decreased to 47 +/- 14% (n = 10) and FSH secretion increased to 137 +/- 6% (n = 6) of the respective values in cultures of dispersed cells. FS secretion was increased nearly 3-fold (P < 0.05) in a dose-dependent manner by continuous exposure of ovine pituitary cells between days 2-5 to recombinant human activin A (1-10 nM), which concomitantly increased FSH secretion. Recombinant human inhibin A (0.003-10 nM); the synthetic glucocorticoids, RU28362 and dexamethasone (each 1-100 nM); the sex steroids, testosterone (1-100 nM), 17 beta-estradiol (0.001-5 nM), and progesterone (4-2500 nM); and the vitamin A derivative, retinoic acid (0.3-32 microM), each inhibited FSH secretion from these cultures, but only the last agent significantly (P < 0.05) increased FS secretion. Inhibin prevented the stimulation of FSH secretion by activin A without affecting its stimulation of FS secretion.(ABSTRACT TRUNCATED AT 400 WORDS)

Gonadotrophin secretion revisited. How many ways can FSH leave a gonadotroph?

In summary, gonadotrophs make and secrete many biologically active proteins, some of which participate in autocrine regulation of gonadotrophin, and particularly FSH, secretion. This essay focuses on different vehicle(s) for secretion of protein products as one way that the gonadotroph might control secretion of the two gonadotrophins. Gonadotrophs phasically secrete LH and (proportionately less) FSH via secretory granules in response to an increase in intracellular Ca2+, but also secrete FSH without LH by a distinct pathway. The latter is tonically regulated by not only activin, but also inhibin and follistatin, at least partly at the level of FSH synthesis. Thus GnRH-independent secretion of FSH may masquerade as a second form of 'regulated' secretion through its linkage with FSH synthesis. Three major types of vacuole that possibly mediate gonadotrophin secretion have been identified in gonadotrophs: the small, dense core secretory granule that is rich in LH; a larger, more diffuse granule that is rich in FSH; and the much smaller, synaptic vesicle-like vacuole that contains no identifiable gonadotrophin. Which of these subserves the tonically regulated, GnRH-independent pathway for secretion of FSH without LH has not yet been determined. It is unlikely to be the small granule, because of its preponderance of LH. It may be a form of synaptic vesicle-like vacuole, which is immunocytochemically 'silent' with respect to FSH, or the FSH-rich larger form of secretory granule, for which a specialized, constitutive-like secretory function has not yet been assigned.(ABSTRACT TRUNCATED AT 250 WORDS)

Synthesis of LHRH antagonists suitable for oral administration via the vitamin B12 uptake system.

Conjugates have been synthesized between vitamin B12 and two lysyl derivatives of the LHRH antagonist, ANTIDE. Lys6-ANTIDE and Lys8-ANTIDE were both found to have similar activities to the native analogue in the in vitro pituitary cell assay. The in vitro bioactivity of the VB12-ANTIDE conjugates was preserved following linkage using a number of spacers; however, the in vivo bioactivity was lost. In order to produce conjugates which had similar in vivo bioactivity to the native analogue, it was necessary to link the VB12 to the ANTIDE analogues using thiol cleavable spacers. The resultant conjugates had similar activity to ANTIDE both in vitro and in vivo and were also found to be much more water soluble than ANTIDE. These VB12-ANTIDE conjugates show potential utility as water soluble ANTIDE analogues for parenteral use and are protease resistant LHRH antagonist analogues suitable for uptake from the intestine via the VB12-transport system following oral administration.

Aspects of current and future inhibin research.

Inhibin was first isolated in 1985. Major progress has been made in defining various aspects of its structure and physiology, using a heterologous radioimmunoassay. Current research is aimed at characterizing the nature of the circulating forms of inhibin and is examining whether there are sex-specific roles for inhibins A and B. It has been recognized that various forms of epithelial and stromal ovarian cancer produce members of the inhibin peptide family but the precise nature of these products is not yet clear. The recognition that the inhibin subunits together with follistatin are expressed locally within the pituitary has lead to an investigation of their possible roles in intrapituitary regulation. It is clear that these peptides also have intragonadal roles. Of particular current interest is the nature of the signals that control the specificity of cellular peptide production and that determine whether a particular cell produces inhibin or activin. The inhibins are members of a complex family with many potential roles in physiology and pathophysiology. The role of the inhibins in feedback control of follicle stimulating hormone in the male, particularly, remains unclear. New applications for inhibin and related peptides are likely to be developed.

Ovine anterior pituitary proopiomelanocortin gene expression is not increased by ACTH secretagogues in vitro.

Recent reports have demonstrated that the secretion of ACTH from sheep anterior pituitary primary cultures is markedly stimulated by arginine vasopressin (AVP) but not by CRF, and that AVP-stimulated ACTH secretion is potentiated by CRF. It has also been reported that AVP increases total ACTH content (secreted plus intracellular ACTH), suggesting that AVP stimulates POMC biosynthesis in the ovine anterior pituitary. These observations differ from the rat, in which CRF is the most potent of the ACTH-releasing factors and the only ACTH secretagogue which stimulates POMC gene expression and biosynthesis. The second messenger pathways which mediate CRF- and AVP-stimulated ACTH release (protein kinase A and protein kinase C, respectively) are the same in sheep and rat corticotrophs. The present studies were undertaken to determine if ovine POMC gene expression, unlike the rat POMC gene, is stimulated by AVP via the protein kinase C pathway. A 295 base pair portion of the ovine POMC gene was isolated using polymerase chain reaction and sequenced. Ovine POMC messenger RNA (mRNA) levels were quantitated using this partial complementary DNA clone in a solution hybridization/nuclease protection assay with cytoplasmic RNA from sheep anterior pituitary primary cultures which had been treated with various combinations of ACTH secretagogues or with glucocorticoids for 18 h. Treatment with AVP, alone or with CRF, greatly increased total and secreted ACTH levels; however, the amount of POMC mRNA in these cells was not significantly increased. Treatments which stimulated secretion to a lesser extent and did not alter total ACTH levels (CRF alone, cAMP alone, or with phorbol ester) were associated with a decrease in POMC mRNA levels relative to untreated cells. Glucocorticoid treatment decreased both total ACTH and POMC mRNA levels. Taken together, the data demonstrate a lack of secretagogue-induced stimulation of POMC mRNA levels concomitant with increased total ACTH levels, an unexpected result given the close association between secretion of POMC-derived peptides and POMC gene expression in other mammalian corticotroph systems.

Interaction between activin and follicle-stimulating hormone-suppressing protein/follistatin in the regulation of basal inhibin production by cultured rat granulosa cells.

There is evidence that FSH-suppressing protein (FSP) antagonizes the action of activin on the differentiation of rat granulosa cells by binding activin in vitro. We tested the interaction of activin and FSP in this in vitro system by examining the effects of FSP on activin dose-related stimulation of immunoreactive inhibin release by rat granulosa cells. Granulosa cells (2 x 10(5) viable cells/well) from diethylstilbestrol-treated immature rats were cultured for 48 h in McCoy's 5a serum-free medium with additives and increasing doses of bovine FSP (0-30 nM) and human recombinant activin (0-20 nM). Inhibin was measured in the medium by RIA. Activin caused a dose-related increase in basal inhibin production, which was maximal between 4-10 nM activin (ED50, 0.6 nM). With the addition of FSP, an apparent increase in the ED50 of the activin dose-response curves was observed, but there were no changes in the maximum response. This pattern closely resembled that of chemical antagonism of an agonist by an agent that binds with relatively high affinity to form a biologically inactive complex. Based on this premise, apparent high affinity activin binding to FSP was determined by Scatchard analysis to have a Kd of 0.13 +/- 0.07 nM (mean +/- SD) and to occur in a 2:1 or greater FSP/activin molar ratio. These data support the proposition that the antagonistic effect of FSP on activin is due to the formation of an inactive complex.

Follistatin steady state messenger ribonucleic acid levels in confluent cultures of a rat renal mesangial cell line are regulated by multiple factors.

The regulation of steady state levels of follistatin (FS) messenger RNA (mRNA) was examined in a rat renal mesangial cell line in tissue culture. A specific 32P-radiolabeled antisense probe was used which corresponds to the 3' end of exon 5 together with the 5' end of exon 6 of the rat FS gene, and which distinguishes between the two different forms of FS mRNA. In addition, a specific 35S-radiolabeled probe for the ubiquitous protein cyclophilin was developed and used as an internal standard. Total RNA was harvested from confluent cell cultures to yield four independent samples per treatment/time point, and equal amounts of RNA from every sample in a given experiment were subjected to S1-nuclease analysis for the estimation of specific mRNA levels. Treatment of the cultured cells with epidermal growth factor (10 nM) caused an 8- to 9-fold increase in the FS mRNA level after 4 h, but no consistent change was observed after treatment with basic fibroblast growth factor (0.28 or 0.56 nM), somatostatin (3.7-73 nM), angiotensin II (0.1-2500 nM), or FS itself (0.29 nM) for between 4 and 48 h. Neither activin (0.5 or 1.2 nM) nor inhibin (0.64 nM) changed the FS mRNA level in the mesangial cell line during a 24-h treatment. FS mRNA levels in the cells also were not affected by a 48-h treatment with the steroids dihydrotestosterone (1-1000 nM), estradiol (1 and 100 nM), and the antiprogesterone RU 486 (1000 nM), whereas 100 nM RU 28362 (a synthetic glucocorticoid) caused a 5- to 6-fold increase and 1000 nM progesterone increased the FS mRNA level up to 3.5-fold above control. Retinoic acid, a vitamin A derivative, significantly increased the FS steady state mRNA level at 3 nM, and at 1000 nM stimulated FS mRNA up to 5-fold within 4 h, whereas incubation of the cells with 30 microM prostaglandin E2 for 4 h caused a 10-fold increase. The FS mRNA level increased 3- and 4-fold within 4 h during incubation of the cells with 100 nM phorbol 12-myristate, 13-acetate, and 25 microM forskolin, respectively, whereas the calcium ionophore A23187 (1-100 microM) caused no change within this timespan. None of the tested hormones had an obvious effect on the ratio of the two different forms of FS mRNA (FS 344:FS 317).(ABSTRACT TRUNCATED AT 400 WORDS)

Regulation of the synthetic rate of gonadotropin-releasing hormone receptors in rat pituitary cell cultures by inhibin.

Regulation of steady state levels of plasma membrane receptors for GnRH is the arithmetic result of processes that contribute to the appearance of receptors (synthesis, recycling, and unmasking) less those that contribute to the loss of receptors (degradation, internalization, and inactivation). We have adapted the density shift technique to evaluate specifically the rate of synthesis of GnRH receptors in rat pituitary cell cultures. Recently, it has been shown that inhibin can decrease the steady state levels of GnRH receptors in rat pituitary cell cultures and can block homologous up-regulation of GnRH receptors. In the present study we have evaluated the ability of purified inhibin to affect the synthesis rate of GnRH receptors under basal conditions and after exposure of cultured gonadotropes (from female weanling rats) to GnRH. Cells were exposed to inhibin alone (4 or 12 ng/ml) or to GnRH (10(-10) M) plus inhibin (0.4, 4, or 12 ng/ml) in the presence of densely labeled amino acids. GnRH was administered as a 20-min pulse, but inhibin treatment was continued for up to 2 days. After these treatments, GnRH receptors were covalently linked to a radio-labeled photoaffinity probe (125I- Tyr5-[azido-benzoyl-D-Lys6] GnRH) and solubilized with 1% sodium dodecyl sulfate. Newly synthesized GnRH receptors (those that had incorporated the dense amino acids) were separated from previously synthesized receptors (those containing normal amino acids) by velocity sedimentation through sucrose gradients (O-20% sucrose, 1% sodium dodecyl sulfate, and 10 mM Tris-HCl, pH 7.0; centrifuged at 156,000 x g for 24 h). After velocity sedimentation, gradients were fractionated, and the radioactivity in each fraction was quantified. Treatment with inhibin alone had no effect on the synthesis rate of GnRH receptors compared to that of control cultures (t1/2, 23.5 +/- 0.3 vs. 23.3 +/- 0.3 vs. 22.9 +/- 0.9 h for control, 4 ng/ml inhibin, and 12 ng/ml inhibin, respectively). In contrast, inhibin blocked the stimulation of homologous receptor synthesis by GnRH in a dose-dependent manner (t1/2, 12.2 +/- 0.7 vs. 14.0 +/- 0.7 vs. 19.2 +/- 1.5 vs. 20.0 +/- 2.9 h for GnRH alone and GnRH plus 0.4, 4, or 12 ng/ml inhibin, respectively). These data indicate that in rat pituitary cell cultures, inhibin does not decrease basal levels of GnRH receptors by affecting the synthesis rate of receptors, but prevents up-regulation of GnRH receptors by blocking stimulation of GnRH receptor synthesis by homologous hormone.

Chronic inhibitory effect of follicle-stimulating hormone (FSH)-suppressing protein (FSP) or follistatin on activin- and gonadotropin-releasing hormone-stimulated FSH synthesis and secretion in cultured rat anterior pituitary cells.

The effects of bovine FSH-suppressing protein (FSP) or follistatin on activin- and GnRH-stimulated FSH synthesis and secretion have been studied using cultured pituitary cells from adult male Sprague-Dawley rats. Exposure to FSP (0.001-10 nM) for 3 days dose-dependently suppressed basal FSH secretion (IC50 = 146 +/- 21 pM., mean +/- SE), cellular content (IC50 = 269 +/- 8 pM) and total FSH (IC50 = 181 +/- 25 pM), with no effect on LH. Activin (0.3 nM) increased FSH secretion 2.1-fold, cellular content 1.3-fold, and total FSH 1.9-fold during a 3-day incubation, but these increases were dose-dependently inhibited by concomitant treatment with 35-kDa bovine FSP (0.1-3 nM), with complete inhibition occurring at concentrations between 1 and 3 nM. The 31- and 39-kDa forms of bovine FSP also antagonized the actions of activin. GnRH (1 nM) increased FSH secretion 1.8-fold and total FSH 1.6-fold during a 3-day incubation, effects that were dose-dependently inhibited by concomitant treatment with 35-kDa bovine FSP. The highest tested concentration of FSP (3 nM) suppressed GnRH-stimulated FSH secretion and total FSH to 59 and 57%, respectively, of the levels found in untreated cultures. All three forms of bovine FSP produced a significant inhibition of FSH secretion and total FSH stimulated by GnRH. FSP also suppressed FSH secretion and total FSH in response to activators of protein kinase C including 100 nM phorbol 12-myristate 13-acetate (43 and 59%, respectively) and 100 nM mezerein (40 and 60%, respectively). Finally, treatment of cultured pituitary cells with 35-kDa FSP at 1 and 3 nM for 3 days resulted in 21 and 24% decreases in GnRH binding sites, respectively. It is concluded that (i) FSP inhibits not only the secretion but also the synthesis of FSH induced by activin and GnRH in long-term culture, and (ii) FSP may cause its inhibitory effects on GnRH by suppression of the protein kinase C system, and possibly by reduction of GnRH binding sites.

Acute inhibitory effect of follicle-stimulating hormone-suppressing protein (FSP) on gonadotropin-releasing hormone-stimulated gonadotropin secretion in cultured rat anterior pituitary cells.

Follicle-stimulating hormone (FSH)-suppressing protein (FSP) or follistatin, a novel gonadal glycoprotein hormone, has been shown to have chronic inhibitory effects on the secretion of both FSH and luteinizing hormone (LH) in response to gonadotropin-releasing hormone (GnRH) in vitro. The present study was designed to investigate the acute effects of bovine FSP on GnRH-stimulated gonadotropin secretion and to examine the potential subcellular sites of this action of FSP using cultured pituitary cells. Anterior pituitaries from adult male Sprague-Dawley rats were enzymatically dispersed and cultured for 48 h, after which the cells were treated with bovine FSP for 6 h, followed by a 4 h stimulation with secretagogues in the continued presence of FSP. Results showed that the 35 kDa form of bovine FSP (0.1-3 nM) dose-dependently suppressed GnRH-stimulated FSH and LH secretion, with inhibition of 38 and 25%, respectively, at 3 nM. In addition, FSP suppressed gonadotropin secretion in response to activators of protein kinase C (phorbol 12-myristate 13-acetate (PMA) and mezerein) and a calcium ionophore (A23187). However, FSP had no effect on gonadotropin secretion evoked by melittin, an activator of phospholipase A2. Furthermore, 35 kDa bovine FSP did not compete with GnRH for GnRH binding sites in a direct competition study and treatment of cultured pituitary cells with FSP (0.1-3 nM) for 10 h did not alter the number of GnRH binding sites on the cell membranes. Finally, similar inhibitory effects on gonadotropin secretion in response to GnRH, PMA and mezerein were obtained with the 31 and 39 kDa forms of bovine FSP, each at a concentration of 1 nM. We conclude from the present study that FSP acutely inhibits GnRH-stimulated gonadotropin secretion in cultured pituitary cells, and that FSP exerts its action beyond the GnRH receptor, possibly by affecting the protein kinase C and/or the calcium-calmodulin systems.