Dynamic changes in inhibin messenger RNAs in rat ovarian follicles during the reproductive cycle

Inhibin, activin, and follistatin. Potential roles in ovarian physiology

The physiological significance of these results will not become clear until patterns of activin and inhibin protein production and the expression of their receptors have been more thoroughly characterized in relation to follicular development. Meanwhile, in situ hybridization studies on rat and monkey ovaries suggest that inhibin/activin beta-subunit mRNA (favoring synthesis of activin) is relatively abundant in granulosa cells of immature antral follicles, whereas alpha-subunit mRNA (favoring synthesis of inhibin) predominates in Graafian follicles. The increased production of follistatin associated with advanced preovulatory development would serve to further reduce the activin "tone" relative to inhibin (Fig. 1). At the level of protein action in vitro, the pattern emerging is that inhibin minimally affects granulosa cell steroidogenesis at any stage of follicular development, whereas activin has pronounced modulatory effects that alter with follicular maturity. As suggested previously,60 the ability of activin to enhance gonadotropin-responsive aromatase activity and simultaneously suppress progesterone production by mature granulosa cells has physiological implications in that it hints at a mechanism for promoting estrogen synthesis and simultaneously suppressing progesterone synthesis, which is precisely what occurs in the preovulatory follicle. The effects of inhibin and activin on human thecal androgen synthesis observed in vitro suggest how these proteins might act locally to modulate preovulatory follicular growth and estrogen synthesis in vivo (Fig. 2).57 In essence, we propose that activin acting at early stages of antral follicular development plays a role in follicular recruitment through sensitizing immature granulosa cells to the cytodifferentiative action of FSH. On the other hand, inhibin is more likely to play a role in preovulatory follicular selection and maintenance of follicular dominance. Studies of follicular fluid levels of androgen and estrogen in relation to granulosa cell aromatase activity indicate that the capacity of the theca interna to generate aromatase substrate (androstenedione) increases hand in hand with aromatase activity in the human preovulatory follicle. It has therefore been suggested that a positive feedback loop (granulosa on theca) exists that promotes thecal androgen synthesis and hence estrogen synthesis in this follicle.64 The discovery that inhibin production in vitro is greatest by granulosa cells isolated from preovulatory follicles strongly implicates inhibin as a component of this feedback loop.

Physical mapping of inhibin beta-A in domestic cattle

The gene for the beta-A subunit of inhibin (INHBA) was assigned to bovine syntenic group U13 by bovine x rodent hybrid somatic cells and the polymerase chain reaction (PCR). A 482-bp PCR fragment was used to clone a 37-kb cosmid. This cosmid was assigned to bovine Chromosome (Chr) 4 (BTA 4) by fluorescence in situ hybridization (FISH). This is the first assignment of a U13 marker to a bovine chromosome. A restriction fragment length polymorphism (RFLP) was detected with PstI within the INHBA cosmid.

The measurement of activin/EDF in mouse serum: evidence for extragonadal production

Many studies have shown that activin/EDF mediates local physiological events at various sites. In this study, the authors confirmed the presence of activin in mouse serum by high performance liquid chromatography (HPLC) monitored by a specific bioassay. The retention time of the active fraction in HPLC was identical to that of authentic activin A, and the activity was neutralized by follistatin. That the serum activin levels in ovariectomized and aged mice were decreased suggests that the serum activin was generated partly by ovary (35%), but also by extragonadal organs. Activin and inhibin are structurally closely related, and both are involved in many physiological processes including control of follicle stimulating hormone secretion by the pituitary. The regulation of serum activin, however, appeared to differ from that of inhibin.

Activin-A as an intraovarian modulator: actions, localization, and regulation of the intact dimer in human ovarian cells

The actions, localization, and regulation of activin in the human ovary are unknown. Therefore, the aims of this study were (a) to define the effects of recombinant activin-A and its structural homologue, inhibin-A, on mitogenesis and steroidogenesis (progesterone secretion and aromatase activity) in human preovulatory follicular cells; (b) to localize the activin-A dimer in the human ovary by immunohistochemistry; and (c) to examine regulation of intracellular activin-A production in cultured human follicular cells. In addition to stimulating mitogenic activity, activin-A causes a dose- and time-dependent inhibition of basal and gonadotropin-stimulated progesterone secretion and aromatase activity in human luteinizing follicular cells on day 2 and day 4 of culture. Inhibin-A exerts no effects on mitogenesis, basal or gonadotropin-stimulated progesterone secretion and aromatase activity, and does not alter effects observed with activin-A alone. Immunostaining for dimeric activin-A occurs in granulosa and cumulus cells of human ovarian follicles and in granulosa-lutein cells of the human corpus luteum. cAMP, and to a lesser degree human chorionic gonadotropin and follicle-stimulating hormone, but not inhibin-A, activin-A, or phorbol 12-myristate 13-acetate, increased the immunostaining for activin-A in cultured granulosa cells. These results indicate that activin-A may function as an autocrine or paracrine regulator of follicular function in the human ovary.

Expression of activins and TGF beta 1 and beta 2 RNAs in early postimplantation mouse embryos and uterine decidua

The expression of the mesoderm inducing factors, activins and TGF beta s, was characterized in 5 1/2-9 1/2 day mouse embryos and implantation sites by in situ hybridization. Activin beta A RNA was not detected within the embryo, but is expressed in nearby decidual cells from 5 to 7 days. Thus activin A could play a role within the embyro during gastrulation. Activin beta A is also expressed in more mesometrially located decidual cells from 6 to 9 1/2 days. Activin beta B and inhibin alpha RNAs were not detected, while a control tissue was highly positive. TGF beta 1 is expressed in the secondary decidual zone and in developing endothelial cells in the decidua and embryo. TGF beta 2 is expressed in the mesometrial decidua at 6 1/2 days and in the midline of the cranial neural plate.

Control of immunoactive inhibin production by human granulosa cells

OBJECTIVE

The aim was to determine the relation between stage of antral follicular development and granulosa cell production of immunoactive inhibin.

DESIGN

Primary granulosa cell cultures in serum-free Medium 199 were incubated at 37 degrees C for 96 hours with a change of medium at 48 hours. Inhibin and steroid levels in culture medium were determined by radioimmunoassay. The inhibin assay was based on the N-terminal 1-26 amino acid sequence of the alpha-chain of porcine 32 kDa inhibin using pl alpha 1-26-GLY27-TYR28 as the immunogen, tracer and standard.

PATIENTS

Granulosa cells were obtained from the ovaries of women with regular menstrual cycles undergoing hysterectomy with unilateral or bilateral oophorectomy to treat non-malignant gynaecological disease.

RESULTS

Basal production of immunoactive inhibin by granulosa cells from presumptive preovulatory follicles (greater than 15 mm diameter) was 5-13 times higher than that by granulosa cells from immature (less than 10 mm diameter) or intermediately mature (10-15 mm diameter) follicles. Basal production of progesterone and oestradiol followed a qualitatively similar pattern, establishing a positive relation between functional granulosa cell maturity and inhibin production. Treatment of granulosa cell cultures from immature follicles with follicle-stimulating hormone (FSH), but not luteinizing hormone (LH), increased inhibin production, time and dose dependently. FSH, but not LH, also brought about similar increases in steroid hormone synthesis by granulosa cells from immature follicles. The stimulatory effect of FSH on granulosa cell inhibin production was augmented at least twofold by the presence of testosterone or 5 alpha-dihydrotestosterone (1.0 mumol/l) but was unaffected by oestradiol. Granulosa cells from intermediately mature follicles undertook variable degrees of both FSH and LH-responsive inhibin production which generally corresponded with gonadotrophin-responsive steroid production. Granulosa cells from presumptive preovulatory follicles showed inconsistent inhibin responses to FSH. However, LH caused marked (at least twofold) increases in inhibin production, paralleling LH-responsive steroid production.

CONCLUSION

These results show that for human beings, granulosa cell capacity to produce immunoactive inhibin in vitro increases with follicular maturity. FSH, but not LH, stimulates inhibin production by immature granulosa cells and this response to FSH is subject to modulation by androgen. During preovulatory follicular development, production of inhibin, like steroids, becomes increasingly responsive to LH. Such a development-related pattern of granulosa cell inhibin production helps explain how, post-ovulation, the corpus luteum is able to secrete inhibin as well as steroids. It is also compatible with the concept that locally produced inhibin could participate in the paracrine control of follicular development during the human menstrual cycle.

Radioimmunoassay for activin A/EDF. Method and measurement of immunoreactive activin A/EDF levels in various biological materials

A radioimmunoassay (RIA) for the measurement of activin A, which is identical to erythroid differentiation factor (EDF), has been developed. A specific antiserum against activin A/EDF was raised in rabbits using a mixture of recombinant EDF and polyvinyl pyrrolidone. Of the compounds tested this polyclonal antibody cross-reacted only with bovine inhibin (3.2%) and human TGF-beta (4.2%). The least detectable value in this assay was 0.06 ng/tube. The within- and between-assay coefficients of variation at three different concentrations were 3.6-9.8% and 3.4-7.7%, respectively. Using this RIA, immunoreactive activin A/EDF levels in various biological fluids and tissues were examined. The dose-response curves of porcine follicular fluid and ovarian extract were parallel to the standard curve, and porcine follicular fluid contained high activin A/EDF immunoreactivity (1050 ng/ml). On gel chromatography of porcine follicular fluid, the major immunoreactivity was eluted in the same position as authentic activin A/EDF. Human placental extract and amniotic fluid had relatively high immunoreactive activin A/EDF levels (174 ng/g wet wt. and 63.9 ng/ml, respectively), but the dose-response curve of amniotic fluid was not parallel to the standard curve. Among rat tissues, the ovary showed the highest activin A/EDF immunoreactivity (163 ng/g wet wt.) much lower than that of porcine ovary (1020 ng/g wet wt.). Low immunoreactive activin A/EDF levels were detected in most parts of rat brain (8.7-14.2 ng/g wet wt.), except for the pituitary gland (70.0 ng/g wet wt.). The initial plasma half clearance time (t1/2) of exogenous activin A/EDF was 14 min in the rat and the plasma FSH concentration did not change significantly during this period. These results suggest that this RIA system has sufficient sensitivity and specificity to measure activin A/EDF concentrations in biological materials, and that the reproductive tissues are the main sources of activin A/EDF.

Effect of recombinant inhibin on androgen synthesis in cultured human thecal cells

Effects of inhibin (recombinant human inhibin-A) on ovarian androgen synthesis were tested in vitro using serum-free monolayer cultures of human thecal cells. Treatment for 4 days with inhibin alone at doses between 10 and 100 ng/ml caused modest (approximately 2-fold) increases in production of androgen (androstenedione and dehydroepiandrosterone): similar to the maximal level of stimulation caused by luteinizing hormone (LH) (10 ng/ml) alone but only about one-third of that caused by insulin-like growth factor I (IGF-I) (30 ng/ml) alone. Combined treatment with LH and inhibin elicited additive effects on androgen production whereas LH and IGF-I were synergistic, giving rise to androgen production rates at least 40 times greater than control. Additional presence of inhibin caused up to 10-fold augmentation of the response to LH + IGF-I. Activin (recombinant human activin-A) was previously shown to inhibit LH + IGF-I-induced androgen synthesis in this human thecal cell culture system. In the present study we found that the additional presence of inhibin (greater than 1 ng/ml) completely neutralized this inhibitory action of activin (10 ng/ml). These effects of inhibin were dose-dependent (ED50 1-10 ng/ml) and maximal at approximately 100 ng/ml. Inhibin stimulation of androgen synthesis occurred in the absence of measurable effects on progesterone production, and cell numbers in cultured cell monolayers were unaltered by the protein. It is concluded that inhibin exerts potent and selective stimulation of human thecal cell androgen synthesis in vitro. These results a paracrine role for inhibin(s) in modulating follicular androgen biosynthesis in the human ovary.

Expression of inhibin alpha-subunit gene during mouse gametogenesis

Mammalian gametogenesis is regulated through complex interactions between germ and somatic cells. To investigate the mechanism underlying the differentiation of functional gametes, some genes specifically expressed during gametogenesis have been isolated and characterized. In a search for further examples of such genes, we have isolated from a newborn mouse testis cDNA library, a clone corresponding to mouse inhibin alpha-subunit. Although it is known that the inhibin alpha-subunit molecule is abundantly produced in ovarian follicle and in testicular Sertoli cells, the spatial and temporal patterns of expression of this gene remain to be elucidated. In this study, the patterns of expression of inhibin alpha-subunit mRNA during mouse gametogenesis were examined by RNA blot, cytoplasmic dot and in situ hybridization techniques. In the testis, the concentration of inhibin alpha-subunit mRNA increased from about 16 dpc (days post coitum), peaked at birth and then gradually decreased, paralleling testicular development. Inhibin alpha-subunit mRNA was localized in Sertoli cells of wild type as well as W/Wv testes. In adult testis, mRNA was restricted to the perinuclear cytoplasm of Sertoli cells. Inhibin alpha-subunit mRNA was expressed in follicle cells of adult ovary more abundantly than in adult testis. Analysis of expression during folliculogenesis showed that the accumulation of this mRNA began in preantrum follicles and the level of expression reached a maximum in Graafian follicles.

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.

Inhibin secretion during the rat estrous cycle: relationships to FSH secretion and FSH beta subunit mRNA concentrations

Serum inhibin and FSH and FSH beta subunit mRNA levels were measured at 3h intervals throughout the 4 day estrous cycle in female rats and hourly between 1000 and 2400 h of proestrus. On proestrus, serum inhibin concentrations fell during the late morning-early afternoon, then increased transiently during the late afternoon gonadotropin surges. Inhibin levels decreased during the late evening of proestrus, coincident with the FSH surge-related rise in FSH beta mRNA levels. Serum inhibin remained relatively stable during estrus and early metestrus, but rose during the late evening of metestrus and remained elevated until early diestrus. FSH beta mRNA levels were elevated on late estrus and early metestrus and declined during the evening of metestrus as serum inhibin levels increased. These data show that concentrations of serum inhibin change during the estrous cycle and that a general inverse relationship exists between serum inhibin and FSH levels and FSH beta mRNA concentrations in the pituitary. This suggests that inhibin may inhibit FSH beta gene expression and FSH secretion during the 4 day cycle in female rats.

Endocrine and superovulatory responses in heifers pretreated with FSH or bovine follicular fluid

This study examined the effects of altered serum FSH concentration on subsequent ovarian response to superovulation. Synchronized heifers were assigned randomly on Day 1 of the cycle (estrus = Day 0) to three pretreatment groups that consisted of 6-d of saline (7ml, s.c., b.i.d.; Group I), FSH-P (0.5 mg, i.m., b.i.d.; Group II) or charcoal-extracted bovine follicular fluid (BFF; 7 ml, s.c., b.i.d.; Group III) injections. Superovulation was initiated on Day 7 and consisted of FSH-P in decreasing dosages over 4 d (4,3,2,1 mg; i.m., b.i.d.), with cloprostenol (500 mug) on the morning of the third day. A second replicate with 14 heifers was conducted using the same protocol but twice the pretreatment dosage of FSH-P (1 mg) and BFF (14 ml). Endogenous plasma FSH decreased during BFF and FSH-P pretreatments compared to controls (P < 0.02). Endogenous FSH concentrations in both primed groups (II and III) were similar to control values (Group I) 12 h after the start of superovulation. Basal LH concentrations were not different between pretreatment groups. The interval from cloprostenol treatment to the preovulatory LH surge in Group III was 21.3 and 23.9 h longer (P < 0.0001) than it was in Groups I and II. The postovulation progesterone rise was delayed in Group III. The number of corpora lutea (CL) was lowest in the BFF-primed group (4.2 +/- 0.8) compared with the FSH-primed (7.4 +/- 1.3) and the control (12.0 +/- 1.8; P < 0.003) groups. In the FSH-primed group (0.68 +/- 0.06 cm(3)), CL volumes were larger than in the control group (0.45 +/- 0.03 cm(3)), whereas in the BFF-primed group (0.27 +/- 0.02 cm(3)) CL volumes were smaller compared with the control group (P < 0.0001). Mean FSH concentrations for 48 h preceding superovulation and the number of CL per cow were positively correlated (r = 0.55; P < 0.004; n = 26). We concluded that both FSH-P and BFF pretreatments decreased the superovulatory response of heifers to FSH-P. The mechanism for this would appear to be associated with reduced endogenous FSH prior to the start of superovulation.

Inhibin mRNAs in ovine and bovine ovarian follicles and corpora lutea throughout the estrous cycle and gestation

Follicles and corpora lutea were dissected from ovine and bovine ovaries and the RNA extracted and subjected to Northern blot analyses for alpha- and beta A-inhibin mRNAs, using bovine cDNA and cRNA probes. A cDNA probe detecting mRNA for cholesterol side-chain cleavage cytochrome P-450 (P-450scc) was used as a positive control. In cattle, alpha- and beta A-inhibin mRNAs were not detected in ovarian stroma, which could potentially have contained follicles up to 0.5 mm in diameter. Inhibin-alpha and -beta A mRNAs were detected in bovine antral follicles but after ovulation, the relative levels of alpha- and beta A-inhibin mRNAs declined and were undetectable in mature fully developed cyclic corpora lutea and in pregnancy corpora lutea from early to late gestation of the cow. In sheep, alpha- and beta A-inhibin mRNAs were detected in a pool of antral follicles but not in cyclic or pregnancy corpora lutea, which did contain P-450scc mRNA. It is concluded that in cattle and sheep, follicles and not mature corpora lutea are the ovarian source of inhibin.

The effect of hypophysectomy on inhibin production by adult rat ovaries: changes in ovarian inhibin gene expression and serum inhibin

We have investigated the effect of hypophysectomy on adult rat ovarian inhibin alpha- and beta A-subunit mRNA, serum inhibin, and circulating gonadotrophin levels. Immediate declines in ovarian inhibin gene expression and serum inhibin were observed following hypophysectomy, with a nadir on day 4. However, inhibin synthesis determined by these parameters was still apparent at 16 days post-hypophysectomy. Gonadotropin levels were suppressed following hypophysectomy and the fall in inhibin production was highly correlated with disappearance of FSH from the circulation. This study demonstrates that removal of pituitary gonadotropin stimulation results in diminished ovarian inhibin synthesis and provides further evidence indicating that FSH stimulates granulosa cell inhibin synthesis by enhancing inhibin gene transcription.