Complementary deoxyribonucleic acid (cDNA) cloning and DNA sequence analysis of rat ovarian inhibins

Complementary DNA cloning and sequencing of rat ovarian basic fibroblast growth factor and tissue distribution study of its mRNA

Three cDNA clones encoding rat basic fibroblast growth factor (FGF) were isolated from 10(6) independent clones prepared from a pregnant mare serum gonadotropin (PMSG)-stimulated rat ovarian cDNA library. One of the cDNA clones contained the entire coding sequence for basic FGF. The other two possessed the sequence coding the carboxy terminal 61 amino acids of rat basic FGF, the putative upstream intron sequence, and a 3'-noncoding region. The cDNAs encoding rat basic FGF predict a molecule consisting of 154 amino acid residues, which is one amino acid shorter than the human and bovine basic FGF. Otherwise, there are only 5 conservative amino acid substitutions between the rat and the human/bovine sequences. Poly A+ RNA from brain cortex and hypothalamus show a single 6.0 kb band that hybridizes to the cloned cDNA probe by Northern analyses. The observation that basic FGF mRNA is below the limits of detection in adrenal, spleen, heart, lung, kidney, liver, stomach, small intestine, large intestine, testis, and ovary support the notion that the that the high levels of the protein found in these tissues is due to storage of the mitogen in the extracellular matrix and not continuous gene expression. The significance of the abundance of mRNA in tissues which are not undergoing either active angiogenesis or cell proliferation (hypothalamus and brain cortex) is unclear but emphasizes the potential neuronotrophic function of basic FGF.

Induction of follicle stimulating hormone receptor by erythroid differentiation factor on rat granulosa cell

Erythroid differentiation factor (EDF), inhibin beta A-homodimer, induced expression of follicle stimulating hormone receptors on rat granulosa cells prepared from diethylstilbestrol primed immature female rats. After 3 day incubation with EDF, the number of FSH receptors on the granulosa cells was increased to about 3.5 times of the control value in a dose dependent manner with an ED50 value of 61 ng/ml. On the other hand, EDF related peptides, i.e., bovine 32K Da inhibin A and TGF beta, had no effect on the FSH receptor induction. The present observation suggests that EDF may play a role in the initiation of the cytodifferentiation of ovarian granulosa cells.

Adenosine receptor-dependent modulation of inhibin secretion in cultured immature rat Sertoli cells

Inhibitory (A1) adenosine receptors that attenuate adenylate cyclase activity are present in cultured Sertoli cells. To investigate the possible effect of activating these receptors on the secretion of inhibin by the Sertoli cell, immature rat Sertoli cells were incubated for 24 h with follicle-stimulating hormone (FSH) in the absence or presence of the non-metabolizable, adenosine agonist phenyl-isopropyl-adenosine (PIA), and the accumulation of alpha-inhibin immunoreactivity was measured in the medium. Although devoid of effects when added alone, PIA inhibited the FSH-dependent secretion of alpha-inhibin in a concentration-dependent manner (ED50 = 1-1.5 nM). PIA treatment of the Sertoli cells also rendered the cells less sensitive to FSH in terms of alpha-inhibin secretion. The concentration-response curve to FSH was shifted to the right when cells were incubated in the presence of 100-1000 nM PIA. In contrast, dibutyryl cAMP stimulation of alpha-inhibin accumulation was unaffected by treatment with PIA, indicating that the site of PIA action is at the level of cAMP synthesis. These data provide experimental evidence of adenosine modulation of inhibin secretion by the Sertoli cell and suggest that adenosine may act as a local modulator within the pituitary-testicular axis.

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.

Inhibin beta in central neural pathways involved in the control of oxytocin secretion

Inhibin (I) a gonadal hormone glycoprotein which suppresses follicle-stimulating hormone (FSH) secretion from the anterior pituitary, is a heterodimer consisting of an alpha subunit and one of two distinct beta subunits. S1 nuclease analysis has revealed that RNAs encoding all three subunits (alpha, beta A and beta B) are expressed in rat brain. We report here on the localization, and a potential function, of inhibin beta in the rat brain. A cell group centred in the nucleus of the solitary tract (NTS), a major recipient of visceral sensory information, was stained immunohistochemically with antisera against synthetic fragments of I beta, but not I alpha. The distribution of I beta-stained fibres is consistent with known NTS projections, and includes a prominent projection to oxytocinergic aspects of the magnocellular neurosecretory system.

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.

Radioimmunoassay of rat serum inhibin: changes after PMSG stimulation and gonadectomy

A heterologous inhibin radioimmunoassay (RIA) method has been developed which is highly specific and of sufficient sensitivity to detect inhibin in female and male rat serum. Purified bovine 31 kDa inhibin was used in the generation of the antiserum and following iodination as tracer in the assay. Parallel logit-log dose-response lines were observed between a rat ovarian extract used as standard and serial dilutions of female and male serum and testicular interstitial fluid. The within-assay variation based on index of precision was 0.049 (n = 5) and the between-assay variation (n = 4) was 9.8%. The specificity of the assay was assessed from: (a) the failure of a number of structurally related proteins (activin-A, transforming growth factor-beta (TGF-beta), Müllerian inhibitory substance) as well as inhibin subunits to crossreact (less than 0.5%) in the assay relative to bovine 31 kDa inhibin; (b) nondetectable levels of immunoactivity in the serum of gonadectomised rats; and (c) a close correlation (r = 0.96) between serum levels of in vitro biological and immunological activities from rats following pregnant mare serum gonadotropin (PMSG) stimulation. Similar initial t 1/2 values (14-15 min) of serum inhibin following gonadectomy were obtained in both sexes. This RIA method will be useful in the study of the physiology of inhibin in the female and male rat.

Erythroid differentiation factor can modulate follicular granulosa cell functions

The action of human erythroid differentiation factor (EDF) on the functions of rat granulosa cells cultured in a chemically defined medium was investigated. In the presence of FSH that induced LH receptor expression and progesterone synthesis during culture of the cells, EDF augmented both responses in a dose- and time-dependent manner. Unlike FSH, EDF itself did not have such an inducing effect at all. Furthermore, in the absence of FSH, EDF was found to strongly enhance the ability of granulosa cells to produce inhibin. Thus, EDF may play an important role in the regulation of granulosa cell function and differentiation during follicle development.

Porcine follistatin gene structure supports two forms of mature follistatin produced by alternative splicing

Follistatin (FS), a novel gonadal protein which inhibits specifically the secretion of pituitary follicle stimulating hormone (FSH), has recently been isolated from porcine follicular fluid. cDNA cloning of the porcine ovarian FS precursor revealed two populations of cDNAs which differed at the 3'-region of the open reading frames; one population encodes a precursor of 317 amino acids while the other encodes another precursor having the same 317 amino acids, but with an additional 27 amino acids at the carboxy-terminal. Herein, we report the cloning of the porcine FS gene whose DNA structure reveals that the two populations of mRNA are generated by alternative splicing. In addition, restriction endonuclease mapping and DNA sequencing show that the FS gene is approximately 6 Kb long and consists of six exons separated by five introns. The first exon encodes the putative signal sequence, followed by four exons which encode the four domains of FS, three of which are highly homologous to each other. The last exon encodes the extra 27-amino acid carboxy-terminal domain of the 344-residued precursor.

Erythroid differentiation factor is encoded by the same mRNA as that of the inhibin beta A chain

We have isolated a protein that exhibits a potent differentiation-inducing activity toward mouse Friend erythroleukemia (MEL) cells and human K-562 cells. The protein, designated erythroid differentiation factor (EDF), was found in the culture fluid of human THP-1 cells that had been treated with phorbol 12-myristate 13-acetate. EDF is a homodimer with a Mr of 25,000; the Mr of the monomer is 15,500. cDNA clones encoding the Mr 15,500 subunit of EDF from THP-1 libraries were isolated and sequenced. Surprisingly, the sequence of EDF mRNA is identical to that for the beta A subunit of inhibin, a gonadal protein that suppresses the secretion of pituitary follicle-stimulating hormone. Southern blot analysis indicates that only one gene for EDF/inhibin beta A exists in the human genome. When the EDF subunit cDNA was linked to a simian virus 40 expression vector containing the dihydrofolate reductase gene and transfected into Chinese hamster ovary dihydrofolate reductase negative cells, the transformants began to secrete EDF, demonstrating that the cDNA actually encoded the EDF subunit.

Activin stimulates secretion of follicle-stimulating hormone from pituitary cells desensitized to gonadotropin-releasing hormone

The secretion of follicle-stimulating hormone (FSH) by pituitary cells is stimulated by activin and gonadotropin-releasing hormone, GnRH. To examine the possible interrelationships between the intracellular actions of these secretagogues, responsiveness to activin was tested following pretreatment with 0, 0.1, or 10 nM GnRH. In cells pretreated with 0 or 0.1 nM GnRH, FSH secretion was increased approximately 2-fold during a subsequent challenge with either activin or GnRH. In contrast, in cells pretreated with 10 nM GnRH, FSH secretion became unresponsive to GnRH but could still be stimulated 2-fold by activin. These results demonstrate that activin is able to stimulate FSH secretion in cells that have undergone desensitization to GnRH.

Gonadal and extragonadal expression of inhibin alpha, beta A, and beta B subunits in various tissues predicts diverse functions

The S1-nuclease analysis was used to investigate the pattern of inhibin expression in the rat. In a first series of experiments, expression of the alpha, beta A, and beta B subunits of inhibin were monitored in various tissues from male and female rats. Two observations emerge from these studies. First, expression of inhibin subunits was found in gonadal and extragonadal tissues. In addition to the ovary and testis, inhibin alpha, beta A, and beta B RNAs were detected in the placenta, pituitary, adrenal, bone marrow, kidney, spinal cord, and brain. Detection of inhibin RNAs in the brain and spinal cord suggested that these subunits may exert neuroregulatory functions in the central and peripheral nervous systems. Furthermore, the presence of inhibin alpha and beta subunits in the placenta and the pituitary gland, two cell types that have clearly been shown to be regulated by exogenous inhibin, may reflect existing paracrine and/or autocrine processes active in these tissues. The second observation is that expression of inhibin subunit RNAs may vary by severalfold in a tissue-specific fashion. for example, alpha-subunit RNA levels are abundant in the gonads, whereas beta A-subunit RNA is predominant in the placenta and bone marrow. Finally, it is noted that expression of testicular inhibin RNA subunits decreases during sexual maturation. We conclude that the dimers comprised of inhibin subunits possess diverse functions and may act as growth/differentiation factors as well as a hormone.

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.