Inhibin: from concept to reality

Inhibin

Inhibin is a glycoprotein hormone, consisting of two dissimilar, disulphide-linked subunits, termed α (MW 20kD) and β (MW 3-15kD), which inhibits the production and/or secretion of pituitary gonadotrophins, preferentially follicle stimulating hormone (FSH). The most widely studied inhibin molecule has a molecular weight of 31-32kD, as purified and cloned from bovine, porcine, ovine, rat and human sources. Higher molecular weight forms have been identified in ovarian follicular fluids and in culture media of granulosa and Sertoli cells, and generally differ from the 31kD form in having larger α-subunits, designated by their molecular weights, e.g. α44 in 58kD inhibin. There are two forms of the β-subunit, named βA and βB and the corresponding inhibin dimers have been named inhibin A and inhibin B. Dimers of the β-subunit, which have been shown to have FSH stimulating activity, are termed activins and are designated activin A, B or AB depending on whether the dimer is a homodimer of βA or βB or a heterodimer of βA and βB (Figure 1). The major gonadal source of inhibin is the granulosa cell in the female and the Sertoli cell in the male. Other potential cellular sites of origin will be discussed below.

Effect of passive immunization with buffalo follicular fluid antisera on ovarian activity in guinea pigs

Ovarian activity and follicular populations were studied in guinea pigs (Cavia porcellus) following administration of antisera against buffalo follicular fluid (buFF). Antibodies were raised in rabbits and the titre tested by immunodiffusion assay. Fourteen guinea pigs cycling normally were randomized into two groups. Animals in Group I (n = 8) were treated (i.p.) with 0.5 ml antisera and in Group II (control, n = 6) with the same volume of normal rabbit serum at 12 h intervals on the 10th and 11th day of their oestrous cycle. They were sacrificed 24 h after onset of estrus when ovulation points were counted and ovaries processed for microscopical examination. Treatment with buFF-antisera increased ovulation rate (3.6 vs. 2.0; p < 0.01) but had no significant effect on the total number of follicles. However, the treatment reduced the percentages of atretic follicles in all size classes. These results indicated that the administration of a buFF-antisera produced in the rabbits increased ovulation rate in guinea pigs by reducing the incidence of atresia.

Serum levels of inhibin in maternal and umbilical blood during pregnancy

Inhibin levels were measured by a double antibody heterologous radioimmunoassay in the peripheral serum of 75 pregnant women throughout gestation and in serum from the umbilical vein and artery, which was obtained at the time of delivery. For reference, samples were obtained from 20 nonpregnant women in the early (days 0 to 3), mid (days 4 to 8), and late (days 9 to 14) luteal or follicular phase. Maternal serum levels of inhibin (mean +/- SEM) in early (6 to 12 weeks) gestation (36.4 +/- 2.6 U/ml, n = 36) were significantly (p less than 0.01) higher than those in serum from nonpregnant women in the mid (23.9 +/- 2.5 U/ml, n = 19) or late (11.3 +/- 0.6 U/ml, n = 19) luteal phase. Inhibin levels in maternal serum fell to 15.9 +/- 1.4 U/ml (n = 24) in mid (14 to 20 weeks) gestation and then gradually increased during late (21 to 40 weeks) gestation to peak levels of 49.4 +/- 5.1 U/ml (n = 9) at 36 to 37 weeks. Inhibin levels declined in parallel with human chorionic gonadotropin concentrations during the first trimester (r = 0.587 at p less than 0.01). Significant positive correlations (p less than 0.001) were observed between serum levels of inhibin and 17 beta-estradiol (r = 0.560), progesterone (r = 0.648), and human placental lactogen (r = 0.715) during mid and late (20 to 40 weeks) gestation. Inhibin levels in umbilical vein serum (38.5 +/- 1.3 U/ml, n = 5) were not different from those in umbilical artery serum (39.4 +/- 3.6 U/ml) but were significantly (p less than 0.01) lower than those in maternal serum (50.9 +/- 5.3 U/ml), which was obtained at the time of delivery. By day 5 of puerperium, serum levels of inhibin in the maternal vein were extremely low (2.3 +/- 0.1 U/ml, n = 7); these levels were nearly one fifth lower than follicular phase levels of 10.9 +/- 3.4 U/ml (n = 38). We propose that maternal inhibin in early gestation is secreted from the corpus luteum of pregnancy but that increasing inhibin levels during mid and late gestation result from inhibin that is produced by the placenta. The lack of an umbilical arterial-venous gradient for inhibin and the higher levels of inhibin in maternal serum argue against a fetal source of inhibin in the maternal circulation. The physiologic function of inhibin that is produced by the corpus luteum and by the placenta remains to be determined.

Relationship between hormone profiles and the restoration of spermatogenesis in men treated with gossypol

Gossypol acetic acid was administered orally to 35 male volunteers at a dose of 20 mg once a day for 52-70 days in the loading phase and twice a week for 22 months in the maintenance phase. Sperm counts and the serum concentrations of LH, FSH, prolactin, testosterone and oestradiol were monitored regularly during treatment and for a follow-up period of 12 months. At around 90 days after treatment, all treated participants approached or attained azoospermia and remained at this level throughout the maintenance phase. By the end of the follow-up phase, eight treated men were still azoospermic, while sperm counts in the other 27 men were restored to normal levels. The only hormone that changed significantly during and after the treatment was FSH. From the 6th month of the treatment to the end of the follow-up phase, serum concentrations of FSH in the eight participants that reached irreversible azoospermia were significantly higher than in the other 27 men or in controls. It is suggested that monitoring of FSH levels might be of diagnostic use for identifying those participants with irreversible azoospermia during gossypol treatment.

In-vivo bioactivity of gonadotrophin surge attenuating factor (GnSAF)

Gonadotrophin surge attenuating factor (GnSAF) is a nonsteroidal ovarian factor responsible for the attenuation of the endogenous LH surge in superovulated cycles. To study the bioactivity of GnSAF in vivo, the pituitary response to exogenous GnRH was investigated in 13 normally ovulating women during spontaneous and FSH-treated cycles. GnRH experiments were performed at three stages of both spontaneous and FSH cycles, i.e. early (n = 8), mid- (n = 8) and late follicular phase (n = 8). LH response to GnRH increased significantly from the early to late follicular phase in the spontaneous cycles, while in the FSH cycles it decreased significantly in mid- and increased in late follicular phase. A marked attenuation of the LH response to GnRH during both the mid- and late follicular phase was seen in the FSH as compared to the corresponding spontaneous cycles. We conclude that in superovulated cycles small, growing follicles produce GnSAF.

Inhibin and related proteins: localization, regulation, and effects

Inhibin has originally been defined as a gonadal hormone that exerts a specific negative feedback action on the secretion of FSH from the gonadotropic cells of the pituitary gland. The existence of inhibin was postulated by Mottram and Cramer (15) as early as 1923. However, only after reliable and sensitive bioassay systems had been developed for detection and estimation of inhibin and an ample source of inhibin was found in the form of ovarian follicular fluid, was progress made on the isolation and characterization of the hormone. It is apparent now that inhibin, which itself consists of a dimer of two different subunits, alpha and beta, is a member of a much larger family of (glyco)protein hormones and growth factors that includes Müllerian inhibiting substance, transforming growth factor-beta, activin/erythroid differentiation factor, bone morphogenetic proteins, and an insect and a Xenopus protein. All play important roles in cell differentiation. Gonadal inhibin is produced in the Sertoli cells in the testis and in the granulosa cells in the ovary. The production of inhibin is stimulated by FSH, but controversy exists about other factors that might play a role in the regulation of the production of inhibin. It appears likely that inhibin plays an important role in the feedback regulation of peripheral concentrations of FSH during the period in which Sertoli cells and granulosa cells--the target cells for FSH--divide, i.e., during puberty in male animals and during the development of ovarian follicles in female animals. In this way, inhibin may be an important regulator of the number of developing Sertoli cells and of the length of the seminiferous tubuli in the testis and of the number of developing follicles in the ovary. Apart from its function in the pituitary-gonadal axis, inhibin and activin may be produced and act in a number of other organs such as the placenta, hypothalamus, adrenal, and bone marrow. Investigation of the role of the members of the inhibin family in these systems has only begun, but will certainly be a field of major interest in the near future.

Serum inhibin levels during the periovulatory interval in normal women: relationships with sex steroid and gonadotrophin levels

Inhibin is a gonadal glycoprotein believed to be important in the regulation of pituitary FSH secretion and/or to function as a paracrine factor within the ovary and testis. We studied serum levels of inhibin, oestradiol (E2), progesterone (P), FSH and LH during the periovulatory interval in order to determine whether there is differential control of sex steroid and inhibin secretion by the mature follicle and the emerging corpus luteum. Seven normal cyclic women were admitted 3-4 days prior to midcycle and blood samples drawn every 3 h for 5-7 days. Serum E2, P, FSH, LH and inhibin were measured by radioimmunoassay. Data were normalized around the peak LH value (0 h). Serum E2 and inhibin rose in parallel (r = 0.92, P less than 0.001) between -69 and -18 h, E2 reached a peak of 1296 +/- 154 (mean +/- SEM) pmol/l at -18 h, then fell to 1050 +/- 139 pmol/l at 0 h. Serum inhibin, on the other hand, continued to rise to a peak of 837 +/- 95 U/l at -6 h, fell to 455 +/- 48 U/l at +45 h, then rose again. On average, the peak inhibin level occurred 10.4 +/- 5.1 h after the peak E2 (P less than 0.05). Inhibin levels were positively correlated with both serum LH and FSH between -24 and +24 h (P less than 0.01). Serum E2 was negatively correlated with LH, FSH and inhibin between -24 and 0 h (P less than 0.01). Serum P levels increased from 1.8 +/- 0.3 nmol/l at -24 h to 14.3 +/- 1.0 nmol/l at +60 h. Serum inhibin was positively correlated with serum P from -24 to 0 h (P less than 0.01) and +45 to +60 h (P less than 0.01), but was inversely correlated from 0 to +45 h (P less than 0.01). We conclude that the maturing follicle secretes both E2 and inhibin in parallel until -18 h, at which time the process of luteinization is initiated by the onset of the midcycle LH surge, as evidenced by the rise in P. E2 secretion then falls while inhibin secretion rises, indicating different regulation of secretion of these two hormones by the maturing follicle. Furthermore, the close positive correlation between inhibin and gonadotrophin levels around midcycle suggests that FSH and/or LH stimulate inhibin secretion and that the presumed negative feedback effect of inhibin on FSH secretion is overcome at this time. After midcycle, inhibin secretion initially falls, then rises, while P rises progressively.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibin and related peptides: mechanisms of action and regulation of secretion

The structure of inhibin is known; it consists of a heterodimer composed of one alpha and one beta subunit. The homodimer of beta A (beta A-beta A) and the heterodimer beta A-beta B, called activin A and B, respectively, stimulate the release and synthesis of FSH by gonadotrophs. Inhibin exerts effects at the hypophyseal, hypothalamic, and gonadal levels. Produced by granulosa cells in the female and by Sertoli cells in the male, inhibin synthesis is stimulated by FSH and reduced by hypophysectomy and progesterone. At present, there is no evidence for a signal from germinal cells to modify inhibin production. Inhibin secretion evolves in parallel with follicular maturation and aromatase activity, whereas luteinization arrests its production. Nevertheless, important differences in the regulation of inhibin secretion seem to exist from one species to another. Sperm inhibin levels can be correlated with spermatozoa number. Administration of inhibin to sheep induces either anovulation or an increase in the rate of ovulation depending on the scheme of treatment.

Relative roles of follicle-stimulating hormone and luteinizing hormone in the control of inhibin secretion in normal men

The glycoprotein hormone inhibin is produced by the Sertoli cells of the testis under the influence of follicle-stimulating hormone (FSH) and is postulated in turn to inhibit FSH secretion. Luteinizing hormone (LH) is not recognized to have an important role in the control of inhibin secretion in any species. To determine the relative roles of FSH and LH in the control of inhibin secretion in man, we examined the effects of selective FSH and LH replacement on serum inhibin levels in normal men whose endogenous gonadotropins were suppressed by testosterone (T). After a 3-mo control period, nine men received 200 mg T enanthate i.m. weekly for 3-9 mo. During T treatment, serum LH and FSH levels were markedly suppressed and serum inhibin levels fell to 40% of control values. While continuing T, 3-5 mo of treatment with purified hFSH (n = 4) or hLH (n = 4) increased the respective serum gonadotropin level into the upper normal range and significantly increased inhibin levels back to 64 and 55% of control values, respectively. Supraphysiological LH replacement with high doses of human chorionic gonadotropin (n = 3) returned serum inhibin levels to 63% of control values. In no case did inhibin levels return fully to control levels. In conclusion, serum inhibin levels fell during gonadotropin suppression and were partially and approximately equally restored by either FSH or LH treatment. FSH presumably acts directly on the Sertoli cell to increase inhibin secretion whereas LH may act via increases in intratesticular T levels and/or other factor(s).

Altered pulsatile pattern of luteinizing hormone in men with idiopathic normogonadotropic oligospermia

Gonadotropin serum levels and pulsatile secretion of gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH) are regulated by sexual steroids and perhaps inhibin, but the relative rates of LH and follicle-stimulating hormone (FSH) secretion are modulated by the frequency of GnRH pulses. This study evaluated LH pulsatility in patients with idiopathic normogonadotropic oligospermia (INO) and normal men before and after clomiphene citrate (CC) administration. INO patients evidenced a lower mean LH levels (P less than 0.001), a higher mean pulse frequency (P less than 0.05) and similar pulse amplitude than normal men. CC induced in normal men a higher LH and testosterone (T) increments and increased pulse amplitude only in normal men. Estradiol (E2) showed no difference in either group. Patients with INO might evidence a hypothalamic disorder that may alter pulsatile GnRH secretion. A different response to CC in patients with INO seems to lend support to a primary hypothalamic lesion. A probable gonadotropin imbalance might alter intratesticular concentrations of T and E2 and be the cause of spermatogenic failure.

Passive immunoneutralization of endogenous inhibin: sex-related differences in the role of inhibin during development

The purpose of these studies was to ascertain the extent to which endogenous inhibin regulates follicle-stimulating hormone (FSH) secretion at different intervals during development in the male and female rat. This was determined by examining the changes in plasma FSH that resulted from immunoneutralizing endogenous inhibin in male and female rats at different ages during development and into adulthood. Passive immunoneutralization of endogenous inhibin was achieved using specific, high titer ovine antiserum, generated against the alpha-subunit of the recently described inhibin molecule. Optimal antiserum volumes and time after injection required to observe maximal changes in FSH secretion were determined in initial experiments. No clear effect of immunoneutralizing endogenous inhibin could be demonstrated on FSH secretion in female rats until 20 days of age, after the completion of the endogenous rise in FSH which occurs between days 5 and 20. Thereafter, injection of the anti-alpha-inhibin serum (anti-alpha IN) produced a progressively marked increase in plasma FSH as the age of the females increased. In male rats, injection of the anti-alpha IN serum caused an increase in FSH secretion as early as 5 days of age, although the response was more delayed at this age than at later times. The ability of the anti-alpha IN serum to increase plasma FSH was observed through 20 days of age. At 30 days of age, during the peak of the endogenous rise in plasma FSH, injection of the anti-alpha IN serum failed to further increase the already elevated levels of plasma FSH. As the endogenously high levels of FSH gradually decreased, the ability of anti-alpha IN serum to increase FSH secretion returned (40 days of age) but was diminished by 50 days of age and was completely lost by 60 days of age. The results of the present study indicate that inhibin plays an increasingly important role as a regulator of FSH secretion in the female from at least 20 days of age into adulthood. In the male, however, the role of inhibin in regulating FSH secretion, which is clearly present during early postnatal development, is apparently lost at the time of puberty.

Selective control of rat granulosa cell inhibin production by FSH and LH in vitro

We studied the role of luteinising hormone (LH) and human chorionic gonadotropin (hCG) in regulation of rat granulosa cell inhibin production. Whereas pregnant mare serum gonadotropin (PMSG) or purified rat follicle-stimulating hormone (FSH) stimulated inhibin accumulation in culture media up to 6-fold in a dose-dependent manner, hCG or LH alone were without significant effect. Concomitant addition of hCG or LH to cultures containing half maximal or maximal stimulating concentrations of PMSG did, however, result in a dose-dependent inhibition of PMSG/FSH-induced inhibin production. However, in the 2-step culture system a biphasic effect of hCG treatment on FSH-primed granulosa cell inhibin and progesterone production was evident. hCG was only inhibitory when the cells in the 2-step system were primed with PMSG. These data indicate that granulosa cell inhibin production is under direct control of both FSH and LH, and provide a possible explanation for alterations in inhibin activity around the time of ovulation in vivo.

Synthetic human seminal alpha-inhibin-92 selectively suppresses follicle-stimulating hormone release in vivo

A 92-amino acid polypeptide, alpha-inhibin-92 (alpha-IB-92), has been isolated and characterized from human seminal plasma and found to be active in suppressing follicle-stimulating hormone (FSH) release in vitro. In the present in vivo study, intravenous injection of synthetic alpha-IB-92 (4 and 20 micrograms) significantly suppressed FSH release (P less than 0.001), whereas this peptide had no effect on luteinizing hormone (LH) release in 1-day orchidectomized male rats. In contrast, third ventricular injection of alpha-IB-92 (0.02, 0.4, 4, or 20 micrograms) had no effect on FSH and LH release in 1- or 2-day orchidectomized rats. These results indicate that alpha-IB-92 exerts a FSH suppressing activity by direct action on the pituitary gland.

Immunocytochemical localization of inhibin in rat and human reproductive tissues

Recently, the structures of two forms of inhibin present in human follicular fluid were elucidated from the corresponding cDNA sequences. Using specific antisera generated against the alpha-chain common to both forms, we have examined the cellular localization of inhibin in the male and female rat gonads and in human placental tissue. Specific alpha-inhibin immunoreactivity was localized within the Sertoli cells of a number of tubules in each testes section. However, other adjacent tubules were unstained suggesting a stage-specific production of inhibin. Intense immunostaining was observed in the granulosa cells of ovarian follicles at various stages but not in the thecal cells. Immunostaining was present in the human placenta and limited to the cytotrophoblast cells, suggesting a role of inhibin during pregnancy. The present study demonstrates the probable site of production of inhibin in the gonads and placenta and further implicates this important factor as a key regulator of reproductive functions.

Isolation and partial characterization of follistatin: a single-chain Mr 35,000 monomeric protein that inhibits the release of follicle-stimulating hormone

A Mr 35,000 protein with follicle-stimulating hormone release-inhibitory activity was isolated from porcine ovarian follicular fluid by heparin-Sepharose affinity chromatography, gel filtration on Sephacryl S-200, and multiple steps of high-performance liquid chromatography. The isolated molecule is highly enriched in cysteines and is composed of a single polypeptide chain. In addition, it has no sequence homology with the previously characterized follicular fluid inhibins, which are heterodimeric proteins of Mr 32,000 with follicle-stimulating hormone release-inhibiting activity. This protein specifically inhibits the basal secretion of follicle-stimulating hormone, but not that of luteinizing hormone, in the rat anterior pituitary monolayer culture system with a half-maximal effective dose of 2.5-6.0 ng/ml. Another form of the molecule of Mr 32,000 present in much lower concentration in follicular fluid was also isolated. It may differ from the Mr 35,000 form in glycosylation or carboxyl-terminal truncation. We suggest that this compound be called "follistatin" to signify its structural difference from inhibin.

The pituitary-Leydig cell axis in men with severe damage to the germinal epithelium

Eighteen men (mean age 27, range 18-30 years) treated for Hodgkin's disease with 6-8 courses of MVPP (Mustine, Vinblastine, Procarbazine and Prednisolone) have had Leydig cell function assessed by their steroidogenic responses to stimulation by a single bolus dose of HCG (1000 units intramuscularly). Normal age-matched men (n = 16) acted as controls. Baseline immunoreactive FSH was markedly raised in the patients (mean 18.1 +/- SD 6.9 vs 2.0 +/- 1.5 IU/l, P less than 0.0001) reflecting damage to the germinal epithelium. Immunoreactive LH was also greater in patients (10.3 +/- 3.9 IU/l) than in controls (3.9 +/- 1.9 IU/l, P less than 0.0001). There were no differences between the baseline testosterone, androstenedione, oestradiol, oestrone and sex hormone binding globulin (SHBG) concentrations. The testosterone/SHBG ratios were similar in the two groups and there was no correlation between baseline LH and testosterone concentrations or testosterone/SHBG ratios. Testosterone, androstenedione, oestradiol and oestrone secretion in response to HCG stimulation were similar at 24 h and 96 h in both groups. In order to explain the paradox of elevated immunoreactive LH in the face of normal testicular steroidogenesis in such patients, LH biological activity (B) as well as LH immunoreactivity (I) and FSH and testosterone were estimated in a second similar group of patients (n = 17, mean age 27, range 17-43 years) and in a further age-matched control group (n = 17). Bioactive and immunoreactive LH levels were significantly increased (P less than 0.005 and P less than 0.001, respectively) in the patient group.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of inhibin production by rat granulosa cells

Inhibin production by cultured granulosa cells from immature diethylstilbestrol (DES)-primed rats was studied in relation to estradiol and progesterone production. The inhibin content in culture media was assayed with a specific radioimmunoassay (RIA) using an antibody to porcine 32 kDa inhibin that recognizes rat inhibin as well. Inhibin production was about 10 ng/ml/2 X 10(4) cells/72 h at the basal levels and was maximally stimulated with 25 ng/ml of follicle stimulating hormone (FSH) to 45 ng/ml which was 4.5 times the basal levels, with an ED50 value of 2.0 ng/ml. A cyclic AMP analog (dibutyryl cyclic AMP) or reagents that promote cAMP production were also effective in inhibin production, indicating that FSH stimulates inhibin production through a cAMP-dependent pathway. Luteinizing hormone (LH) was not effective in producing inhibin from freshly prepared granulosa cells, whereas granulosa cells pre-incubated with FSH for 48 h because responsive to LH regarding inhibin production. Testosterone sensitized the granulosa cells to the FSH stimulation, whereas hydrocortisone (4 ng/ml) decreased the sensitivity of granulosa cells by increasing the ED50 value for inhibin production by FSH about 10 times. A similar effect was observed regarding estradiol production, while progesterone production due to stimulation by FSH was enhanced by the hydrocortisone treatment. Insulin and platelet extract both stimulated inhibin production and enhanced the maximal response of inhibin production due to stimulation by FSH without altering, or even increasing the ED50 values. Epidermal growth factor (EGF), (D-Leu6)Des-Gly10-LHRH N-ethylamide (GnRH agonist) and 12-O-tetradecanoylphorbol-13-acetate (TPA), a potent protein kinase C activator, inhibited both inhibin production and estradiol or progesterone production. Consequently, the regulation of inhibin production was similar to that of estradiol production, but markedly different from that of progesterone. However, inhibin and estradiol production were modulated differently by various growth factors and hormones. These phenomena might account for possible discrete changes in the plasma levels of inhibin and estradiol in vivo.

The maternal ovary is not the source of circulating inhibin levels during human pregnancy

The concentration of immunoreactive inhibin in serum was measured in three pregnant women with premature ovarian failure involved in a donor oocyte in-vitro fertilization programme. Inhibin was not detectable in peripheral serum prior to conception but rose within 2-4 weeks of embryo transfer, whereafter levels rose gradually during pregnancy (less than 20 weeks 1.22 U/ml (0.85-1.76) versus greater than 20 weeks 2.28 U/ml (1.42-3.67), P less than 0.01; geometric mean +/- 67% confidence interval) and were similar to those observed in 24 normal pregnant women. hCG rose in parallel with inhibin during early gestation, but declined after 3 months. FSH levels were elevated before conception and were suppressed during pregnancy. In conclusion (i) immunoreactive inhibin is detectable from early gestation in women with no endogenous ovarian function indicating that the maternal ovary does not contribute significantly to inhibin secretion during pregnancy; (ii) the trophoblast is the likely source of inhibin during pregnancy; (iii) the regulation of hCG and inhibin secretion differs throughout gestation; and (iv) inhibin may have a role in FSH regulation during pregnancy and/or a local role within the feto-placental unit.

Inhibin production by Sertoli cells during testicular regression in the golden hamster

The objective of this study was to determine if testicular regression in the hamster results in changes in Sertoli cell function that reflect altered pituitary function. Inhibin-like activity was measured in spent media from Sertoli cells cultured from adult control and optically enucleated hamsters with regressed testes using a homologous hamster Sertoli cell/hamster pituitary cell bioassay. The inhibin activity resulted in a dose-related decrease in FSH release from both normal and "regressed" pituitary cultures but maximal inhibition occurred at a 3- to 4-fold lower dose with media from Sertoli cells obtained from regressed hamsters. When pituitary cells from control adult or adult hamsters with regressed testes were incubated with Sertoli cell spent media, pituitary cells from regressed hamsters were more sensitive than normal pituitary cells to both normal and "regressed" inhibin. This greater production of inhibin-like activity and/or an enhanced sensitivity to inhibin in the regressed hamster may contribute to the decline in FSH levels during testicular regression. This data lends further support to a physiologic role for inhibin.

Heterodimers and homodimers of inhibin subunits have different paracrine action in the modulation of luteinizing hormone-stimulated androgen biosynthesis

Inhibin, a gonadal hormone capable of preferential suppression of pituitary follicle-stimulating hormone (FSH) secretion, has recently been purified. The major form of this protein is an alpha beta heterodimer encoded by two separate genes. In contrast to the FSH-suppressing action of the alpha beta heterodimer, the beta beta homodimer stimulates FSH secretion. Luteinizing hormone (LH)-secreting pituitary cells and gonadal androgen-producing cells have long been shown to form a closed-loop feedback axis. Based on recent studies demonstrating the FSH stimulation of inhibin biosynthesis by ovarian granulosa and testis Sertoli cells, an additional closed-loop feedback axis exists between pituitary FSH- and gonadal inhibin-producing cells. Because uncharacterized Sertoli cell factors have been suggested to either stimulate or inhibit androgen production by testicular Leydig cells, we have tested the intragonadal paracrine actions of heterodimers and homodimers of inhibin subunits. In primary cultures of testis cells, the alpha beta heterodimer of inhibin enhances Leydig cell androgen biosynthesis stimulated by LH, whereas the beta beta homodimer suppresses androgen production. Furthermore, similar modulatory actions of inhibin-related proteins were found in cultured ovarian theca-interstitial cells and theca explants treated with LH. In contrast, treatment with the inhibin-related proteins alone did not affect gonadal steroidogenesis. Our data indicate that the inhibin-related gene products synthesized by Sertoli and granulosa cells may form heterodimers or homodimers to serve as intragonadal paracrine signals in the modulation of LH-stimulated androgen biosynthesis and allow cross-communication between the two feedback loops.

Characteristics of binding of human seminal alpha-inhibin-92 to human pituitary membranes

We investigated the binding of 125I-labeled alpha-inhibin-92 (a 92-residue peptide) to human pituitary membrane preparations. Unlabeled alpha-inhibin-92 competed effectively with the labeled peptide for binding to the membranes. Binding was also inhibited by both alpha-inhibin-52 and alpha-inhibin-31, but less effectively. Scatchard analysis of the alpha-inhibin-92 binding data indicated the presence of high-affinity binding sites (1.35 nM/mg of membrane protein) with an apparent Kd of 0.37 nM. When 125I-labeled alpha-inhibin-92 was covalently crosslinked to the pituitary membrane preparation with disuccinimidyl suberate and the solubilized labeled receptor complex was analyzed by NaDodSO4/PAGE under either reducing or nonreducing conditions, a single radioactive band at an apparent molecular weight of 90,000 +/- 5000 was observed. These data suggest that human pituitary has specific binding sites for alpha-inhibins.

Action mechanism of inhibin in vitro--cycloheximide mimics inhibin actions on pituitary cells

The effects of inhibin on gonadotropin secretion from cultured rat anterior pituitary cells were examined by using purified porcine follicular fluid (pFF) 32 kDa inhibin. pFF 32 kDa inhibin suppressed the basal FSH secretion as well as cell content of FSH with identical ED50 values (ED50 = 1.0 ng/ml) in a dose-dependent manner, but did not alter either basal secretion or cell content of LH. On the other hand, pretreatment of the pituitary cells with pFF 32 kDa inhibin during the first 3 days resulted in suppression of subsequent LH-RH-stimulated release of both FSH and LH in a dose-dependent manner, indicating that the suppression of LH-RH-stimulated release of LH is one of the intrinsic inhibin actions on pituitary cells. The marked difference between ED50 values for FSH (ED50 = 1.1 ng/ml) and LH (ED50 = 2.5 ng/ml) in the suppression of LH-RH-stimulated release of gonadotropins, together with the fact that the total amount of LH (cell content plus released) after LH-RH stimulation remained unchanged following inhibin treatment suggests that the suppression of LH-RH-stimulated release of LH by inhibin is quite different from that of FSH regarding the action mechanism. Similarly, cycloheximide, an inhibitor of protein biosynthesis, suppressed both basal secretion and cell content of FSH with almost the same ED50 values (ED50 = 22.5 ng/ml) but did not alter either basal secretion or cell content of LH. Cycloheximide also suppressed LH-RH-stimulated release of both FSH and LH, and the ED50 values were different from each other (ED50 = 25.0 ng/ml for FSH and 60.0 ng/ml for LH suppression, respectively). Our finding that cycloheximide completely mimicked the action of inhibin on gonadotropin secretion strongly suggests that LH is quite insensitive to biosynthetic inhibition, and that preferential effects of inhibin or cycloheximide on FSH in appearance may reflect the difference between LH and FSH in susceptibility to biosynthetic inhibition.

Inhibin--a non-steroidal regulator of pituitary follicle stimulating hormone

Inhibin is a gonadal glycoprotein produced by the granulosa and Sertoli cell under the influence of FSH and acts to specifically suppress pituitary FSH secretion. Recently, ovarian inhibin has been purified from several species and its amino acid sequence deduced using cloning techniques. Inhibin consists of two disulphide-linked heterologous subunits of which the smaller may exist in two different forms accounting for two different forms of inhibin in humans and pigs. Heterogeneity of inhibin also exists as a result of proteolytic processing of the molecule during its passage into the circulation. Significant homology exists between the subunits of inhibin and the dimeric peptides TGF-beta and Mullerian inhibitory substance (MIS), suggesting they are all derived from a common ancestral gene. Furthermore, dimers of the smaller subunit of inhibin (FSH-releasing protein (FRP) or activin) have now been found in follicular fluid (FF) and, along with TGF-beta, shown to be potent and specific stimulators of FSH secretion. These proteins may be involved in controlling FSH by another as yet unknown pathway and may prove to be the FSH-releasing factor, analogous to LHRH, which has been postulated to exist for some years. Inhibin can no longer be simply considered as an isolated FSH-suppressing protein. The physiological significance and relationship between inhibin and its related proteins represent one of the most challenging and interesting areas in reproductive endocrinology. Further studies, particularly with the development and use of sensitive assays for both the FSH releasing hormone and inhibin will clarify their role in reproduction and their usefulness in monitoring or treating fertility.

Molecular cloning and sequence of the cDNA for a 94-amino-acid seminal plasma protein secreted by the human prostate

The precursor to a seminal plasma protein reported to have inhibin-like activity was characterized through cDNA cloning and sequencing. It is a 114-amino-acid polypeptide which differs from its seminal plasma derivative mainly by the presence of a 20-residue amino-terminal extension, a putative signal sequence, carrying a possible N-glycosylation site. The protein is specified by a single gene per haploid genome. Its mRNA is detectable in the prostate but not in the testis, which suggests that it is primarily a prostatic secretory protein.

Immunohistochemical detection of inhibin in the gonad

Antiserum to inhibin was produced in rabbits by immunization with a synthetic [Tyr30]alpha-chain(1-30)NH2 fragment of porcine inhibin coupled to bovine serum albumin, and the elicited antiserum was used in conjunction with the avidin-biotin immunoperoxidase procedure to localize inhibin-reactive cells in various rat tissue preparations. In the testes, only the Sertoli cells revealed immunoreactivity with the antiserum. Intense staining was also observed in ovarian follicular granulosa cells but not in the theca layer outside the basement membrane. In addition, the luteal cells in the corpus luteum were also stained by the antiserum. The positive staining in the gonadal tissues could be blocked completely by pre-adsorbing the serum with either the synthetic peptide or native inhibin. Immunostaining was not detected in brain, pituitary, thymus, stomach, pancreas, kidney and adrenal section, thus confirming that inhibin is a polypeptide originating only from specific cells of the gonad.

Distribution of immunoreactive inhibin like material in different regions of human prostate

Distribution of immunoreactive inhibin material (IR-ILM) in different regions of human prostate was studied by using specific radio-immunoassay. Mean IR-ILM concentrations in the central, peripheral and periurethral zones were 27.3 +/- 9.3, 2.4 +/- 0.7 and 3.0 +/- 1.5 micrograms/mg protein respectively (P central vs periurethral and peripheral less than 0.05).

A simple in vivo bioassay for inhibin-like activity using ovariectomized ewes

Long term ovariectomized ewes were used in a bioassay for inhibin-like activity. The concentration of FSH 6 to 7 hr after injection of follicular fluid (a rich source of inhibin), as a percentage of pretreatment, regressed on the log of the dose had a slope of -26.0 +/- 7.6 (5 replications, mean +/- SD) and an index of precision of .32 +/- .04. This system was rapid, relatively easy and specific for in vivo inhibin-like activity. This bioassay was also used to determine the relative potency of an affinity-purified fraction of follicular fluid.

Lesch-Nyhan syndrome: growth delay, testicular atrophy and a partial failure of the 11 beta-hydroxylation of steroids

There is a failure of growth in hypoxanthine guanine phosphoribosyltransferase deficiency; slow weight gain is marked after the second year of age but is apparent in the birth weights of all eight of our patients for whom we have data. However, head growth and bone development are less affected than weight. A partial defect in the adrenocortical 11 beta-hydroxylation of steroids was demonstrated after ACTH stimulation in all four patients studied. This hydroxylation takes place in mitochondria the function of which is modulated by purine nucleotide concentrations; this may be the link with the enzyme defect. Testicular atrophy at autopsy was found in two pubertal age boys and seven patients aged 12-17 years had no signs of puberty. All five boys aged 3-11 years showed less than the normal mean response of plasma testosterone concentration to human chorionic gonadotrophin despite the normal histological appearance of the testes of one 6-year-old-boy. Follicle stimulating hormone responses to gonadotrophin releasing hormone are probably less than in normal in at least three of the seven prepubertal boys. The absence of the normally high activities of hypoxanthine guanine phosphoribosyltransferase in testes appears to inhibit their ability to respond to gonadotrophin.

Inhibin concentration in the culture media of human oocyte-cumulus-corona cell complexes is not related to subsequent embryo cleavage

We have demonstrated an in vitro secretion of inhibin by human OCCC, not related to subsequent egg cleavage. A putative signal, released by OCCC, that could modulate inhibin secretion by granulosa cells has not been discovered. Inhibin secretion by OCCC has no predictive value concerning the further fertilization and cleavage of the oocytes.

Human inhibin genes. Genomic characterisation and sequencing

Inhibin is a gonadal hormone involved in the non-steroidal regulation of follicle stimulating hormone (FSH) secretion. Using the cDNAs coding for bovine inhibin A and B subunits we have identified inhibin genes within the human genome using Southern blot hybridisation techniques. The genes are likely to be present as single copies. Cloning and sequencing inhibin genes obtained from lambda libraries of human genomic DNA provide structural and sequence data on the human A and B genes. Comparison of the known inhibin gene sequences showed, in particular, that the B subunits have identical sequences in man, pigs and cattle thus demonstrating a remarkable evolutionary conservation in these genes.

Absence of predictive value of follicular inhibin on the results of human in vitro fertilization

During the periovulatory period, inhibin did not have a discriminant role in oocyte fertilizability. A possible role in the gonadotropin surge was not revealed by analysis at the time of follicular rupture. Furthermore, inhibin activity appears to be very low. But inhibin could still have an effect upon growth and atresia of the follicles of the following cycles.

Stimulation of ornithine decarboxylase in the rat prostate by seminal plasma inhibin

The effect of seminal plasma inhibin on ornithine decarboxylase activity (ODC) in the rat prostate was studied. A single bolus injection of seminal plasma inhibin caused a 3- to 4-fold increase in ODC activity within 2 h whereas testicular ODC was unchanged. The ODC response to seminal plasma inhibin was neutralized by specific antibodies generated against the inhibin preparation. Another peptide, thyroid releasing hormone has no effect on prostatic ODC activity, but it blocked the increase in enzyme activity induced by treatment with seminal plasma inhibin.

Demonstration of high molecular weight forms of inhibin in bovine follicular fluid (bFF) by using monoclonal antibodies to bFF 32K inhibin

Monoclonal antibodies specifically recognizing each 20K and 13K subunit of bovine follicular fluid (bFF) 32K inhibin have been prepared. By immunoblotting procedures using these antibodies, we have demonstrated in bFF at least six inhibin forms, the apparent molecular weights of which are estimated to be 120K, 108K, 88K, 65K, 55K and 32K daltons, respectively. Amongst them 65K, 55K and 32K inhibin forms comprise two polypeptide subunits linked by disulfide bridge(s). In these inhibin forms a polypeptide of 13K daltons, a shorter subunit of bFF 32K inhibin, is attached by disulfide linkage(s) to polypeptides of 57K, 44K and 20K daltons, which are immunologically related to a larger subunit of the 32K inhibin, to yield 65K, 55K and 32K inhibins, respectively. On the other hand the higher molecular weight forms, 120K, 108K and 88K inhibins, are composed of three polypeptide subunits. In these forms a polypeptide of 62K daltons, immunologically related to the shorter subunit of bFF 32K inhibin, is attached by disulfide bridge(s) as the third component to the respective smaller inhibin forms which are composed of two subunits. These findings suggest that the complex formation of the subunit precursors may be extremely important and that restricted proteolytic cleavages following the complex formation may yield such divergent forms of inhibin in bFF.

Human seminal alpha-inhibins: detection in human pituitary, hypothalamus, and serum by immunoreactivity

An antiserum generated in rabbits against human seminal alpha-inhibin-52 has been used to develop a sensitive radioimmunoassay for the detection of alpha-inhibins. The alpha-inhibin-52 antiserum reacts with alpha-inhibin-92 and alpha-inhibin-31 with equal avidity. These peptides were found to be present in human pituitary, hypothalamus, and serum. In exclusion chromatography on Sephadex G-100, the immunoreactive material eluted in a large molecular size region. Immunoblot analysis of column-derived fractions of these extracts revealed the presence of alpha-inhibin-92. The mean concentrations of immunoreactive alpha-inhibin were found to be 7.2 ng/ml in normal adult male serum, 70.3 ng/g (wet weight) of pituitary, and 12.9 ng/g (wet weight) of hypothalamus. This communication reports on the evidence for the existence of gonadal peptides in the brain.

Detection of folliculostatin: a sensitive and specific bioassay using the female rat

To establish a sensitive and specific bioassay for the FSH-suppressing activity present in porcine follicular fluid (pFF), we examined the latency of pFF action when injected IV in the acutely ovariectomized (ovax) metestrous rat. By 2h post injection (5.5h after ovax), FSH was suppressed significantly in pFF vs. porcine serum-injected controls. LH was unaltered. In an experiment establishing a dose-response curve for pFF 4.5h after injection, 1.77 mg of pFF protein significantly suppressed FSH. The index of precision (-0.2188) and precision of slope (1.088) were well within acceptable limits for bioassays. We conclude that the ovax metestrous rat, injected 3.5h after surgery and sacrificed at 4.5 or 5.5h, is a sensitive and specific bioassay for folliculostatin.

Paracrine regulation of follicular maturation in primates

Taken together, the studies reviewed here suggest that although gonadotropins are necessary for follicular growth, they are insufficient by themselves to explain the dynamics of folliculogenesis. Indeed, the role of gonadotropins in follicular maturation must necessarily be permissive: that is LH and FSH initiate a synchronized cascade of follicular events directly mediated by paracrine and autocrine factors.

Isolation of bovine follicular fluid inhibin of about 32 kDa

Inhibin of about 32 kDa from bovine follicular fluid (bFF) was purified by using chromatographies operated under protein-dissociating conditions, which we have established for our previous purification of porcine follicular fluid (pFF) inhibin. On a gel filtration, bFF inhibin activity was eluted at 3 distinct regions corresponding to apparent molecular weight of 96, 55 and 32 kDa, representing 17%, 27% and 24% of the total inhibin activity in the follicular fluid, respectively. The smallest inhibin, named 32 kDa bFF inhibin, that evidently suppressed the basal secretion of FSH from the pituitary cells, was purified to homogeneity with a 5330-fold purification factor in a recovery yield of ca. 11%. 32 kDa bFF inhibin thus purified consists of 2 polypeptide chains (A-chain: 20 kDa and B-chain: 13 kDa), linked by disulfide bridges. N-Terminal sequences were Ser-Thr-Pro-Pro- for the A-chain and Gly-Leu-Glu-Cys- for the B-chain. The identical N-terminal sequences were also found in 32 kDa pFF inhibin, except that Pro-3 of the bFF A-chain is substituted by Ala in the 20 kDa chain of pFF inhibin.