Follitropin (FSH) stimulation of inhibin biological and immunological activities by seminiferous tubules and Sertoli cell cultures from immature rats

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.

Constitutive expression of prostaglandin-endoperoxide synthase 2 by somatic and spermatogenic cells is responsible for prostaglandin E2 production in the adult rat testis

Prostaglandins (PGs), particularly PGE(2), have been implicated in the control of testicular steroidogenesis, spermatogenesis, and local immunity. However, virtually nothing is known about the expression or activity of the prostaglandin-endoperoxide synthases (PTGSs; also referred to as the cyclooxygenases), the specific rate-limiting enzymes responsible for PG production, in the adult testis. This activity was investigated in rats under normal conditions and during lipopolysaccharide-induced inflammation using quantitative real-time PCR, in situ hybridization, Western blotting, and PGE(2) measurements by ELISA. The mRNA for both the "constitutive" Ptgs1 and the "inducible" Ptgs2 forms was detected in multiple testicular cell types. Testicular Ptgs2 expression was substantially higher than that of Ptgs1, and testicular production of PGE(2) in vitro was found to be suppressed by a specific PTGS2 inhibitor (NS-398), but not by an inhibitor of PTGS1. Further investigation indicated that 1) PGE(2) production in the adult testis is attributable to constitutive expression of PTGS2 by somatic (Leydig cells and Sertoli cells) and spermatogenic cells; 2) testicular macrophages constitutively produce relatively low levels of PTGS2 and PGE(2) but are the only cell type to respond significantly to an inflammatory stimulus by increasing production of PGE(2); and 3) testicular PTGS2 expression and intratesticular PGE(2) levels are only marginally affected by acute inflammation. These data point toward a previously unanticipated maintenance role for the "inducible" PTGS2 enzyme in normal testicular function, as well as an anomalous response of testicular PTGS2 to inflammatory stimuli. Both observations are consistent with the reduced capacity of the testis to initiate and support inflammatory reactions.

17 Alpha-oestradiol and 17 beta-oestradiol do not affect basal and follicle-stimulating hormone-stimulated inhibin B secretion by highly purified rat Sertoli cells

The effects of 17 alpha-oestradiol and 17 beta-oestradiol on basal and follicle-stimulating hormone (FSH)-stimulated inhibin B secretion by rat Sertoli cells were studied. Sertoli cells were isolated and cultivated from testes of 18-day-old Wistar rats in the presence and absence of FSH and different doses of oestrogens. On day 4 of culture, secreted inhibin was measured by enzyme-linked immunosorbent assay. Neither 17 alpha-oestradiol nor 17 beta-oestradiol had any effect on the secreted inhibin level in either the presence or absence of FSH. It is concluded that these oestradiols do not play an essential role in regulatory processes involving inhibin or FSH.

The mammalian homologues of frog Bv8 are mainly expressed in spermatocytes

Bv8, a protein from skin secretions of Bombina variegata, reacts with receptors present in mammalian brain and intestine (Mollay et al. (1999) Eur. J. Pharmacol. 374, 189-196). As deduced from cloned cDNAs, the murine and human Bv8 homologues have identical amino-terminal sequences and also contain 10 cysteines. From mouse testes, two forms of Bv8 mRNA have been characterized, of which one contains an additional exon which codes for 21 mostly basic amino acids. The mouse Bv8 gene is most active in mid-late pachytene spermatocytes. In mouse testes, Bv8 mRNA can first be detected at the end of the second week post partum.

Expression of stem cell factor in the postnatal rat testis

The expression of stem cell factor (SCF) mRNA and protein was examined in the postnatal rat testis. Northern blot analysis of total RNA from 1-4 days postpartum (dpp) testes showed none or barely detectable levels of the approximately 6.5 kb SCF transcript. At 5 dpp, there was a striking elevation in this mRNA, a timing that coincides with the onset of spermatogonial proliferation. Immunohistochemical staining of testes showed that SCF protein was readily detected within Sertoli cells at 1-7 dpp and in the adult. Immunoreactive material was also detected within gonocytes and within Leydig cells at 1-7 dpp and in Leydig cells in the adult. Given the intense staining of Leydig cells by two different antibodies to SCF, the potential synthesis of SCF mRNA by these cells was investigated. Northern blot and reverse transcription/polymerase chain reaction analysis indicated that adult Leydig cells do not synthesize SCF mRNA, and Sertoli cells do. The significance of these findings is discussed in the context of germ cell-Sertoli cell interactions and Leydig cell function.

Platelet-derived growth factor ligand and receptor subunit mRNA in the Sertoli and Leydig cells of the rat testis

The expression of mRNAs encoding the platelet-derived growth factor (PDGF) subunits (PDGF A and PDGF B) and the PDGF receptor subunits (PDGF-R alpha and PDGF-R beta) was studied in cells of the rat testis. Leydig cells, primary spermatocytes, round spermatids, and cytoplasts were isolated from adult animals, and enriched preparations of Sertoli cells were obtained from 20-day-old animals. RNA from these cells was examined using Northern blots. Messenger RNA encoding both the PDGF-R alpha and PDGF-R beta subunits was observed in Leydig and Sertoli cell preparations but not in any of the germ cell samples. The Leydig cells contained predominantly mRNA encoding PDGF B subunits, while the Sertoli cells contained abundant mRNAs encoding both ligand subunits. Neither PDGF subunit mRNA was detected in the germ cells. This paper has not examined the expression of protein corresponding to the mRNAs detected but the data suggest that both paracrine and autocrine roles for PDGF are likely in the testis, and that PDGF may mediate communication between the interstitial compartment and the seminiferous epithelium.

Identification of receptor tyrosine kinases in the rat testis

Evidence of receptor/ligand interactions that regulate testis cell function was sought in order to broaden the current understanding of the molecular basis of testis cell function. Using reverse transcription and the polymerase chain reaction, we have obtained novel evidence for the expression of three mRNAs encoding receptor tyrosine kinases in the adult rat testis: the platelet-derived growth factor type A receptor (PDGF-RA), the basic fibroblast growth factor receptor (flg), and fetal liver kinase 1 (Flk-1). A 6.8 kb transcript encoding the PDGF-RA was observed in RNA prepared from testes of rats aged day 5 through adult, with a decline in relative abundance with increasing age after day 17. Analysis of mRNA from isolated cell preparations (day 21 Sertoli cells, adult Leydig cells, round spermatids, and primary spermatocytes) and testes depleted of specific cell types [ethane dimethane sulfonate (EDS)-treated and cryptorchid] indicated that the Leydig cell was the predominant source of this mRNA in the adult testis. The addition of PDGF-BB to cultures of highly purified adult rat Leydig cell preparations resulted in a 40% increase in LH-stimulated testosterone production, confirming a role for this growth factor in regulation of Leydig cell function. These data indicate that the Leydig cell is a principal site of action of PDGF in the testis.

Characterisation of adult Sertoli cell cultures from cryptorchid rats: inhibin secretion in response to follicle-stimulating hormone stimulation

Testes from adult (90-120-day-old) rats, which had been made cryptorchid 28 days previously, were dispersed by successive treatment with trypsin, collagenase and hyaluronidase. The resulting crude cell suspension was fractionated on discontinuous Percoll density gradients to yield five distinct cell bands (1-5), at the interface between successive layers of Percoll. Crude cells and purified fractions were cultured for up to 7 days, and inhibin was subsequently measured in the media by radioimmunoassay and in vitro bioassay. Sertoli cells from density gradient bands 2 (1.03-1.04 g/ml) and 3 (1.04-1.05 g/ml) showed minimal germ cell or peritubular cell contamination, as determined by morphological and histochemical techniques. Cells from these bands secreted significantly higher levels of immunoactive inhibin/microgram DNA/48 h under both basal and either follicle-stimulating hormone (FSH)- (100 ng/ml) or dibutyryl cAMP-stimulated (100 micrograms/ml) conditions than did cells from the other bands. While there was a decline in basal secretion of inhibin with increasing duration of culture, the capacity of the purified Sertoli cells (bands 2 and 3) to respond to both FSH and dibutyryl cAMP increased over the culture period. The addition of dibutyryl cAMP (31.25-500 micrograms/ml) to the purified Sertoli cells also caused a stimulation of bioactive inhibin. Immunoactive inhibin production by purified Sertoli cells was unaffected by the addition of either rat LH (8 ng/ml) or testosterone (10(-6) M). The data describe a method for the isolation of adult Sertoli cells from cryptorchid testes, and demonstrate their responsiveness to both FSH and dibutyryl cAMP in vitro using the measurement of immunoactive inhibin as a marker of Sertoli cell function.

Regulation of inhibin beta B-subunit mRNA expression in rat Sertoli cells: consequences for the production of bioactive and immunoreactive inhibin

In Sertoli cells from 21-day-old rats, the expression of the mRNA encoding the alpha-subunit of inhibin, and the production of immunoreactive inhibin are stimulated by follicle-stimulating hormone (FSH). In contrast, the amount of beta B-subunit mRNA is not increased after FSH treatment of the cells, and the ratio between bioactive and immunoactive inhibin decreases after stimulation with FSH. These data suggest that the beta B-subunit is the limiting factor in the production of bioactive inhibin. The aim of the present experiments was to investigate the effect of changes in the amount of beta B-subunit mRNA on the production of bioactive and immunoreactive inhibin. During early postnatal testicular development, the relative amounts of the 4.2 kb and 3.5 kb mRNAs encoding the beta B-subunit of inhibin changed markedly. The meaning of this changing ratio between beta B-subunit mRNAs is not clear, since both mRNAs are actively translated, as demonstrated by polysomal analysis. The total amount of beta B-subunit mRNA correlated with the in vitro production of bioactive inhibin as published earlier. Prolonged stimulation of cultured Sertoli cells from 14-day-old rats with 4 beta-phorbol 12-myristate 13-acetate (PMA) caused a decreased expression of the beta B-subunit mRNAs, presumably by down-regulation of protein kinase C. A similar effect was obtained after addition of the calcium ionophore A23187. Concomitantly, a decreased production of bioactive inhibin was observed. Furthermore, Western blotting revealed that secretion of the 32 kDa inhibin alpha beta-dimer was decreased, whereas secretion of the combination of the C-terminal part with the pro-region of the alpha-subunit was increased. It is concluded that the level of the beta B-subunit of inhibin is rate-limiting for the production of bioactive inhibin in cultured Sertoli cells, and that its expression can be influenced by modulation of protein kinase C, and/or intracellular calcium levels.

The physiology of testicular inhibin and related proteins

The roles of inhibin and related proteins in the male remain unclear, although it is becoming increasingly evident that they play a part in FSH regulation and testicular function, including spermatogenesis. The difficulties associated with these questions have been the limited availability of inhibin and related proteins for in vivo studies, the absence of specific assay methods, and the unclear relationship of inhibin and testosterone in the regulation of FSH secretion. With the availability of human recombinant preparations and the current development of new assays, it is anticipated that many of these questions will be resolved.

Evaluation of the effect of peritubular cell secretions and the testicular paracrine factor P-Mod-S on Leydig cell steroidogenesis and immunoactive inhibin production

Testicular peritubular cells have been shown to produce a paracrine factor, termed P-Mod-S, under androgen control that has dramatic effects on Sertoli cell function and may provide an important mode of androgen action in the testis. Therefore, the current study was designed to investigate the possibility that peritubular cell secretory products could feedback and regulate Leydig cell function. The Leydig cell functional parameters that were examined included testosterone production and inhibin secretion. Purified forms of P-Mod-S (P-Mod-S(A) and P-Mod-S(B) shown to be biologically active on Sertoli cells) had no effect on basal or gonadotrophin-stimulated production of testosterone or inhibin by Leydig cells. A preparation of peritubular cell-secreted proteins (PSP) with molecular weights greater than 3 kDa did not influence testosterone production by Leydig cells. PSP, however, did influence cultured Leydig cell morphology and improved cell viability. PSP also had no effect on the ability of LH to stimulate Leydig cell testosterone production. Whilst determining the effect of PSP on Leydig cell inhibin production, PSP was found to contain endogenous levels of inhibin apparently due to 2% contamination of the peritubular cell cultures with Sertoli cells. When this endogenous inhibin level was considered, PSP was found to have no influence on basal or hormone-stimulated production of inhibin by Leydig cells. Results of the current study indicate that peritubular cell secretory products, including the paracrine factor P-Mod-S, do not appear to play a major role in the regulation of Leydig cell function. Therefore, the regulation of Leydig cell function by the seminiferous tubule will primarily be due to Sertoli cell secretory products.

Stage-specific cellular regulation of inhibin alpha-subunit mRNA expression in the rat seminiferous epithelium

To find out the local regulation of inhibin production and its possible paracrine role in the seminiferous epithelium, inhibin alpha mRNA levels were measured in sequential 1 mm segments of rat seminiferous tubules accurately staged by transillumination technique. Highest levels were found at stages XIV-I-IV of the cycle, and lowest at stages VI-VIIb of the cycle. When dividing spermatogonia were selectively destroyed by 3 Gy of high-energy X-irradiation, stage-specific inhibin alpha mRNA levels remained unchanged until 26 and 38 days after irradiation when stages VII and VIII of the cycle showed 6- and 4-fold increases during a selective reduction of pachytene spermatocyte and round spermatid numbers, respectively. The results suggest that these cells at a strictly stage-specific fashion have a paracrine inhibitory effect on Sertoli cell inhibin alpha gene expression. Inhibin alpha mRNA level also correlates closely to the follicle-stimulating hormone-stimulated cAMP production during the cycle of the seminiferous epithelium, but does not seem to have a correlation to spermatogonial DNA synthesis.

Follicle-stimulating hormone-stimulated secretion of an immunoreactive 29 kDa inhibin alpha-subunit complex, but not of 32 kDa bioactive inhibin, from cultured immature rat Sertoli cells

The medium of cultured Sertoli cells from immature rat testes contains 29 and 32 kDa proteins, which are recognized by an antiserum against the 22 N-terminal amino acids of the inhibin alpha-subunit. These proteins were detected by immunoprecipitation of labelled proteins after incubation of Sertoli cells with [35S]methionine, and by Western blotting. The amount of the 32 kDa protein was not affected by the addition of follicle-stimulating hormone (FSH) to the culture medium of the Sertoli cells, whereas FSH induced a large increase of the amount of the 29 kDa protein. Finally, the 29 and 32 kDa proteins in the medium from control and FSH-stimulated Sertoli cells were separated by sodium dodecyl sulfate-polyacrylamide electrophoresis, and inhibin bio- and immunoactivity were determined in eluates of the slices of the gel. Equal amounts of bioactivity were found in control and FSH-stimulated samples at 32 kDa, while the amount of immunoactivity at 29 kDa was increased; no bioactivity was detected in the eluates of these slices. It is concluded that FSH stimulates the secretion of a 29 kDa inhibin-like protein, which does not contain inhibin bioactivity. This indicates that results of experiments, in which antibodies against N-terminal peptides of the inhibin alpha-subunit are used to detect inhibin, do not necessarily reflect the amount of bioactive inhibin produced.

Immuno- and bioactive inhibin and inhibin alpha-subunit expression in rat Leydig cell cultures

The rise in serum immunoactive inhibin levels in male rats following hCG stimulation raised the possibility that Leydig cells may produce inhibin. This study therefore evaluated whether Percoll-purified Leydig cells from adult male rats synthesized and secreted inhibin in vitro as measured by Northern blot analysis, radioimmunoassay and in vitro bioassay. Northern blot analysis demonstrated the presence of alpha-inhibin subunit mRNA in the Leydig cell and inhibin bioactivity was detected in Leydig cell culture media. Levels of immunoactive inhibin increased in culture over 20 h and were directly dependent on the number of Leydig cells in culture. rLH (NIADDK-rLH-I-6) and not rFSH (NIADDK-rFSH-I-6) stimulated immunoactive inhibin levels in a dose-dependent manner. This study demonstrates that Leydig cells express mRNA for the alpha-subunit of inhibin and produce inhibin which is biologically and immunologically active prompting a re-evaluation of our concepts of testicular inhibin production.