Assessment of the contribution of Leydig cells to the secretion of inhibin by the rat testis

Regulation of activin and inhibin in the adult testis and the evidence for functional roles in spermatogenesis and immunoregulation

Activin A provides a unique link between reproduction and immunity, which is especially significant in the adult testis. This cytokine, together with inhibin B and follistatin acting as regulators of activin A activity, is fundamentally involved in the regulation of spermatogenesis and testicular steroidogenesis. However, activin A also has a much broader role in control of inflammation, fibrosis and immunity. In the Sertoli cell, activin A is regulated by signalling pathways that normally regulate stress and inflammation, signalling pathways that intersect with the classical hormonal regulatory pathways mediated by FSH. Modulation of activin A production and activity during spermatogenesis is implicated in the fine control of the cycle of the seminiferous epithelium. The immunoregulatory properties of activin A also suggest that it may be involved in maintaining testicular immune privilege. Consequently, elevated activin A production within the testis during inflammation and infection may contribute to spermatogenic failure, fibrosis and testicular damage.

Immunolocalization of Inhibin/Activin Subunits in the Shiba Goat Fetal, Neonatal, and Adult Testes

The objective of this study was to investigate the cellular immunolocalization of inhibin alpha and inhibin/activin (betaA and betaB) subunits in the fetal, neonatal and adult testes of Shiba goats. The testes were obtained from a fetus at 90 days, a neonate at 15 days, and two adult Shiba goats (both of 3 years old). The sections of testes were immunostained by the avidin-biotin-peroxidase complex method (ABC) using polyclonal antisera raised against porcine inhibin alpha, inhibin/activin betaA, and inhibin/activin betaB. Inhibin alpha and inhibin/activin (betaA and betaB) subunits were expressed in Leydig cells, but not in the Sertoli cells of the fetus with a weak immunostaining. An increase in the number of positive cells and a more intense immunohistochemical signal for inhibin alpha and inhibin/activin (betaA and betaB) subunits were observed in the Leydig cells of neonatal testes. Moreover, inhibin alpha, betaA, and betaB subunits were expressed in the Sertoli cells and Leydig cells of adult testes, respectively. These results suggest that Shiba goats testes have the ability to synthesize inhibins in the fetus, neonate, and adult, and the cellular localization of inhibin/activin subunits showed age-related changes in fetal, neonatal, and adult testes of Shiba goats.

Altered serum profile of inhibin B, Pro-alphaC and anti-Müllerian hormone in prepubertal and pubertal boys with varicocele

OBJECTIVE

Anti-Müllerian hormone (AMH) and inhibin B are reliable markers of Sertoli cell function. The aim of the present study was to assess the functional state of Sertoli cells in order to detect early changes in the testicular function of prepubertal and pubertal patients with untreated grade II or III varicocele.

DESIGN AND PATIENTS

Seven prepubertal and 55 pubertal boys with untreated grade II or III varicocele were studied. Seven prepubertal and 43 pubertal normal boys were considered as controls.

MEASUREMENTS

Serum levels of gonadotrophins, testosterone, inhibin B and Pro-alphaC and AMH were determined by time-resolved immunofluorometric assays, radioimmunoassay (RIA) and specific enzyme-linked immunosorbent assays (ELISAs), respectively.

RESULTS

Inhibin B and Pro-alphaC serum levels were higher in prepubertal patients with varicocele than in controls (P < 0.001). No further increment in inhibin B and Pro-alphaC levels was observed in pubertal patients with varicocele. Higher levels of AMH were found in patients in Tanner stages I, III, IV and V when compared to normal boys by Tanner stage (P < 0.05, P < 0.01, P < 0.01, P < 0.001, respectively). The direct correlation found in normal boys between inhibin B levels and LH, testosterone and testicular volume was not observed in patients with varicocele.

CONCLUSIONS

The altered serum profile of gonadal hormones observed in untreated prepubertal and pubertal patients with varicocele may indicate an early abnormal regulation of the seminiferous epithelium function.

Serum inhibin B levels reflect contralateral testicular damage following unilateral testicular trauma

INTRODUCTION

The aim of this study was to evaluate contralateral testicular damage (CTD) following unilateral blunt testicular trauma (BTT) and testicular capsule laceration (TCL) by the serum inhibin B level which is an accepted marker of spermatogenesis.

METHODS

Fifty peripubertal male Wistar albino rats were divided into 5 groups each containing 10 rats. Group 1 was the control group. Group 2 was the BTT group in which the right testicle was placed on a firm surface and a metal rod weighing 215 g was dropped onto the testicle from a height of 5.5 cm. Group 3 was the TCL group in which right testicular tunica albuginea was lacerated using the needle of 4/0 silk suture. Group 4 had right orchiectomy initially. Group 5 was the sham group. In all groups, 3-ml blood samples were taken and bilateral orchiectomies were performed 6 weeks after initial manipulations.

RESULTS

Groups 2 and 3 had decreased inhibin B levels (p < 0.001), although the orchiectomy group had normal levels. Histological analyses showed lower Johnsen scores for both trauma groups in the ipsilateral and contralateral testes (p < 0.05).

CONCLUSIONS

Serum inhibin B levels decrease following unilateral testicular trauma reflecting CTD.

Immunohistochemical localization of inhibin subunits in the testis of the bull

The differential localization of the inhibin beta subunits betaA and betaB in the testis of adult bull was studied using specific monoclonal and polyclonal primary antibodies. Inhibin betaA- and betaB-subunits were localized only in the Sertoli cells. The inhibin betaA-subunit was observed in the cytoplasm while the betaB-subunit was localized in the nucleus. No specific findings depending on spermatogenic stages were observed among the seminiferous tubules. Moreover, the inhibin alpha-subunit was not detected in the testis of the bulls. In addition, no inhibin subunits were detected in the Leydig cells and spermatogenic cells. These findings indicate the presence of betaA- and betaB-subunits in the bull, which may suggest a possibility that activin is produced and/or stored in the Sertoli cells and regulates spermatogenesis in an autocrine/paracrine manner. Moreover, the inhibin betaB-subunit may be produced in the nucleus but the functional meaning of this is not yet clear.

Inhibins in the male Göttingen miniature pig: Leydig cells are the predominant source of inhibin B

The expression of inhibin subunits in the testes of the Göttingen miniature pig was examined by in situ hybridization and immunohistochemistry. In addition, the major forms were determined by enzyme-linked immunosorbent assay (ELISA). Strong positive immunostaining for the inhibin alpha subunit was observed in Sertoli and late-stage germ cells, but it was weak in Leydig cells. However, Leydig cells showed strong positive staining for the betaA subunit, but Sertoli cells and spermatogonia showed a weak reaction. Strong positive immunostaining for the betaB subunit was observed in Leydig cells but spermatogonia showed weak staining for it. In contrast to the staining specificity of inhibin alpha and betaA subunits, the betaB subunit did not exhibit positive staining in Sertoli cells. In situ hybridization revealed that although the a subunit mRNA signal was highly expressed in all cell types, the reaction appeared to be stronger in Sertoli cells and spermatogonia than in Leydig cells. betaA subunit mRNA expression was somewhat identical to that of the alpha subunit, however, germ cells showed a weak stain for it. A strong, positive mRNA signal for the betaB subunit was confined to Leydig cells and late-stage germ cells. ELISA results showed that concentrations of inhibin B and inhibin pro-alphaC were high in the circulation and testes. In contrast, inhibin A levels in both plasma and testes were undetectable. The present results strongly suggest that inhibin B is the major form of circulating inhibin and that Leydig cells are the predominant source of this dimeric inhibin in male Göttingen miniature pigs. Furthermore, the germ cells also appear to be an important source of circulating inhibins.

Low serum inhibin B concentrations in male survivors of childhood malignancy

The aim of this study was to assess the value of serum inhibin B in detecting male gonadal dysfunction in childhood cancer survivors. 27 male postpubertal (Tanner's pubertal stage G5 or P6) and 12 pubertal (> or = G2) patients were drawn from the endocrine follow-up protocol of childhood cancer patients at the Paediatric Clinic of Turku University Hospital, Turku, Finland. The average time (mean +/- S.D.) between the diagnosis and this study was 11.7 +/- 4.5 years in the postpubertal and 7.0 +/- 3.9 years in the pubertal group. Serum samples for the determination of follicle-stimulating hormone (FSH), luteinising hormone (LH), oestradiol, testosterone, and inhibin A and B dimers were collected. The demographic factors, pubertal stage and testicular size of the patient were measured at the same routine outpatient visit. Serum inhibin concentrations were correlated to testicular volume and gonadotrophin concentrations. Strong correlations were observed between testicular size (r = 0.80, P < 0.001) or FSH (r = -0.58, P = 0.002) and inhibin B concentration in the postpubertal group. Inhibin A was not detectable (< 2 pg/ml). Testicular volume measurement was accurately documented in 21 postpubertal subjects. Patients with small testicles (< 10 ml) had inhibin B concentrations under 42 pg/ml and those whose testicular size was over 13 ml had inhibin B concentrations exceeding 100 pg/ml. In all 12 pubertal survivors, serum inhibin B levels were > or = 94 pg/ml, except in one case of testicular cancer where inhibin B was 8.1 pg/ml and the FSH concentration was elevated. Inhibin B seems to be an indicator of male gonadal function in postpubertal childhood cancer survivors and could be used in the estimation of gonadal function of male survivors earlier than testicular volume or semen analyses would be routinely possible. However, the correct cut-off level of serum inhibin B, as well as the details of inhibin B physiology during puberty, remain to be determined before semen analysis can be replaced by the measurement of inhibin B.

A study of the relative roles of follicle-stimulating hormone and luteinizing hormone in the regulation of testicular inhibin secretion in the rhesus monkey (Macaca mulatta)

The purpose of this study was to examine the relative roles of FSH and LH in stimulating testicular inhibin secretion in the male rhesus monkey. Recombinant human (rh) FSH and rhCG were used as the gonadotropic stimuli, and juvenile rhesus monkeys, in which the endocrine activity of the pituitary-testicular axis was being driven in an adult manner with an intermittent i.v. GnRH infusion, were studied. Immunoactive inhibin levels were measured by the Monash RIA. Initiation of an intermittent i.v. infusion of rhFSH (10 IU every 3 h) resulted, after a delay of 5-6 h, in a progressive increase in the concentrations of immunoactive inhibin, which achieved, after 48 h of stimulation, a value twice that observed during vehicle treatment. Gel filtration chromatography revealed that the FSH-induced elevation in immunoactive inhibin was the result of an increase in three distinct mol wt fractions: peak I (100 kDa), peak II (50-60 kDa), and peak III (31 kDa). Although peak III accounted for most of the inhibin immunoactivity in vehicle-treated animals, peaks I and II were most responsive to FSH stimulation. Application of recently developed enzyme-linked immunosorbent assays for inhibin B and pro-alpha-C-related peptides provided additional insights into the nature of the FSH-sensitive forms of circulating immunoactive inhibin. Most notably, the 31-kDa fraction (peak III) was comprised of inhibin B and pro-alpha-C. In contrast to FSH stimulation, an intermittent infusion of rhCG (40 IU every 3 h), which markedly elevated testicular testosterone secretion, failed to increase immunoactive inhibin concentrations. These findings indicate that various forms of immunoactive inhibin are present in the circulation of the rhesus monkey, and that in this species, FSH is the principal stimulus of the secretion of testicular inhibins, including inhibin B. Additionally, they further underline the importance of the FSH-inhibin feedback loop in governing testicular function in primates.

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.

Testicular Leydig cells in vitro secrete only inhibin alpha-subunits, whereas Leydig cell tumors can secrete bioactive inhibin

The secretion of inhibin and inhibin-related proteins by testicular Leydig cells was studied by estimation of inhibin immunoreactivity and bioactivity in spent media of preparations of immature and mature rat Leydig cells and of tumor Leydig cells. Immature and mature rat Leydig cells expressed inhibin alpha-subunit mRNA and secreted immunoreactive inhibin. The immunoreactive material did not contain inhibin bioactivity as measured by an in vitro rat pituitary bioassay system. Results of pulse labeling with [35S]methionine followed by immunoprecipitation indicated that the inhibin-related proteins secreted by the immature Leydig cell preparations are 26 kDa and 44 kDa molecules. Mature rat Leydig cells only secreted the 44 kDa inhibin-related protein. Tumor Leydig cells (rat H540 and mouse MA10) secreted immunoreactive and bioactive inhibin, which could be immunoneutralized by an antibody against inhibin. In the culture medium of some H540 tumor Leydig cells 26 kDa and 42 kDa inhibin-related proteins and 30 kDa inhibin were detected. In culture medium of other H540 tumor Leydig cells, not secreting bioactive inhibin, only 26 kDa and 42 kDa inhibin-related proteins were found. No activin bioactivity was detected in culture media of immature rat Leydig cells, H540 and MA10 tumor Leydig cells. It is concluded that normal Leydig cells secrete inhibin alpha-subunits, while Leydig cell tumors can also secrete bioactive inhibin. Neither normal Leydig cells nor Leydig cell tumors produce activin.

The role of calcium in luteinizing hormone/human chorionic gonadotrophin stimulation of Leydig cell immunoactive inhibin secretion in vitro

The mechanism by which luteinizing hormone (LH) stimulates Leydig cell immunoactive inhibin (I-inhibin) secretion was investigated using Percoll-purified adult rat Leydig cells. Using a maximally stimulating dose of LH (16 ng/ml). Leydig cell I-inhibin secretion was non-detectable at 1-2 h of incubation, but subsequently increased at all time points during a 25 h incubation period. LH stimulated both Leydig cell content and release of I-inhibin. Increasing concentrations of LH stimulated both inhibin and testosterone immunoactivity in the incubation media over a similar dose-response range, with a 2- to 4-fold rise in I-inhibin secretion at maximal doses of LH. Dibutyryl cAMP stimulated testosterone secretion in a manner similar to that of LH, but I-inhibin secretion was less sensitive than testosterone and a significant stimulation was observed only at the highest doses (200-1000 micrograms/ml). LH-stimulated I-inhibin secretion was significantly decreased when Leydig cells were incubated in calcium-depleted (0.15 mM Ca2+ + 1 mM EGTA) or low [Ca2+] media (0.15 mM) as compared to normal (1.15 mM) or high [Ca2+] (2-5 mM) media. In contrast, LH-stimulated testosterone secretion remained unchanged by altering extracellular [Ca2+], and although decreased in the presence of EGTA, testosterone secretion remained significantly greater than basal levels. Furthermore both diltiazem and verapamil completely blocked the LH and dibutyryl cAMP-stimulated increase in Leydig cell I-inhibin, but did not reduce either LH or dibutyryl cAMP-stimulated testosterone production to basal levels. We conclude that LH stimulates both I-inhibin synthesis and release by adult rat Leydig cells in culture, by mechanisms involving calcium.

Immunohistochemical localization of inhibin-alpha in the testes of normal men and in men with testicular disorders

Testicular biopsies from normal men and from men with testicular disorders were examined by immunohistochemistry for the presence of the inhibin-alpha subunit using two different antisera. Immunoreactive inhibin-alpha (irI-alpha) was found in Leydig cells in normal, oligospermic, and azoospermic men and in men with Klinefelter's syndrome, and it was also found in a Leydig cell tumour. hCG-treatment apparently increased the amount of immunoreactive inhibin-alpha, particularly in Leydig cells. Sertoli cells also contained irI-alpha but the staining intensity was considerably stronger in testes with impaired spermatogenesis or Sertoli-cell-only syndrome than in normal testes. It is suggested that the serum concentration of irI-alpha and inhibin in humans may, in a complex way, be related to both Leydig and Sertoli cell function, and that the relative contribution from these cells may change in cases of testicular malfunction.