Inhibition of de novo synthesis of FSH by the Sertoli cell factor (SCF)

The Sertoli cell: one hundred fifty years of beauty and plasticity

It has been one and a half centuries since Enrico Sertoli published the seminal discovery of the testicular 'nurse cell', not only a key cell in the testis, but indeed one of the most amazing cells in the vertebrate body. In this review, we begin by examining the three phases of morphological research that have occurred in the study of Sertoli cells, because microscopic anatomy was essentially the only scientific discipline available for about the first 75 years after the discovery. Biochemistry and molecular biology then changed all of biological sciences, including our understanding of the functions of Sertoli cells. Immunology and stem cell biology were not even topics of science in 1865, but they have now become major issues in our appreciation of Sertoli cell's role in spermatogenesis. We end with the universal importance and plasticity of function by comparing Sertoli cells in fish, amphibians, and mammals. In these various classes of vertebrates, Sertoli cells have quite different modes of proliferation and epithelial maintenance, cystic vs. tubular formation, yet accomplish essentially the same function but in strikingly different ways.

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.

Localization of inhibin in human testes by immunoperoxidase technique

Immunohistological studies on the localization of inhibin in human testes were carried out to investigate the cell types involved in inhibin synthesis/storage. The paraffin embedded testicular tissues were stained by an indirect immunoperoxidase technique. Specific antiserum to human testicular inhibin generated against the homogenous preparation of human testicular inhibin at our laboratory was used as the first antibody. Inhibin was found to be localized in Sertoli cells, spermatogonia, and primary spermatocytes. A weak positive reaction was also observed in spermatids. Interestingly, Leydig cells showed positive reaction indicating presence of inhibin in these cells too.

Protein secretions of Sertoli cells

As was stated in the introduction, many of the functions of the Sertoli cells are apparently carried out by the protein secretions of these cells. The use of Sertoli cell cultures and appropriate biochemical and immunological techniques has allowed the characterization of some of these secretion products. It is likely that many of the functions of the Sertoli cells are necessary because of the presence of the blood-testis barrier. Many growth and nutritive factors which are necessary for cell viability are available to most cells via the serum. The germinal cells within the adluminal compartment do not have access to serum factors and one of the functions of the Sertoli cells is to synthesize serum-like components and secrete them into the adluminal compartment. The historical description of Sertoli cells as "nurse cells" thus appears to have been accurate. The nurse-cell function is most clearly demonstrated by the proposed mechanism by which germinal cells obtain ferric ions. The Sertoli cells have developed a system to move serum-derived iron through their own cytoplasm and to secrete it bound to newly synthesized testicular transferrin molecules which can deliver it to specific receptors on the germinal cell surface (Huggenvik et al., 1984). Functionally, all of the secreted proteins from Sertoli cells which have been characterized or proposed fall into one of five basic classes. First, Sertoli cells secrete a number of transport proteins including transferrin, ceruloplasmin, and ABP. The proposed function of these proteins is the transport of Fe3+, Cu2+, and androgens to the germinal cells or to the epididymis (ABP). Second, Sertoli cells synthesize and secrete a number of proteins which have a hormone-like or growth factor-like activity. AMH is a clear and well-documented example of this type of product while the evidence for inhibin, somatomedin C, EGF-like growth factor, and seminiferous growth factor will require further corroboration. Third, Sertoli cells secrete proteins which have enzymatic activities. Plasminogen activator is the best characterized example of this class of products and the alpha-lactalbumin-like activity is of potential interest. The fourth class of Sertoli cell secretion products includes those proteins which contribute to the basement membrane, namely, type IV collagen and laminin. Finally, there is a very important group of Sertoli cell secretion products for which there is, as yet, no evidence for a defined function. This group includes SGP-1 and SGP-2 which are the major sertoli cell products in rats and which have been well-characterized biochemically.(ABSTRACT TRUNCATED AT 400 WORDS)

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-subunit cDNAs from porcine ovary and human placenta

Inhibin, a gonadal protein that preferentially suppresses the secretion of pituitary follicle-stimulating hormone, has been isolated from porcine follicular fluid and characterized as a 32-kDa protein composed of 18-kDa and 14-kDa subunits. In the present work, oligonucleotide probes predicted from amino-terminal inhibin amino acid sequences have been used to isolate, from a porcine ovarian lambda gt11 cDNA library, clones encoding the 18-kDa subunit, or A chain, of inhibin. DNA sequence analysis showed that the inhibin A chain is initially synthesized as a larger precursor protein and is predicted to be a glycopeptide. Inhibin A-chain mRNA is present specifically in the gonads, and its synthesis can be induced by treatment of the animal with gonadotropins. The porcine probe was used to isolate a human inhibin A-subunit cDNA from a placental cDNA library. The human precursor is highly homologous to its porcine counterpart and is predicted to generate an 18-kDa glycosylated inhibin A subunit.

Inhibin activity and secretion of gonadotropin during the period of follicular maturation

To evaluate the relative contributions of the ovarian inhibin and estradiol-17 beta (E) on the regulation of FSH secretion, inhibin and E in ovarian venous plasma (OVP) and FSH and LH in peripheral plasma were simultaneously measured using superovulating rats with special reference to follicular maturation. By the transplantation of a pituitary gland from adult male rats under the kidney capsule between 1100 and 1200 hr on diestrus-1 in cyclic rats, superovulation was successfully induced on the morning of the next estrus without any additional treatment with human chorionic gonadotropin (hCG). The number of maturing follicles capable of ovulating in response to hCG significantly increased at 12 hours after the grafting as compared with sham-operated controls and further increases occurred until the afternoon of proestrus. In the superovulating rat, first and second surges of FSH were completely blocked and an LH surge was also partially suppressed during the periovulatory period when surges of FSH and LH were normally observed in controls. Contents of FSH as well as LH in the animal's own pituitary gland were suppressed significantly after the grafting as compared with controls. A marked increase in inhibin activity in OVP of rats with a pituitary transplant occurred concomitantly with an increase in the number of follicles capable of ovulating whereas E levels in OVP did not so. Inhibin activity in OVP at each point was much higher in the pituitary grafted rats than in controls but this was not true for E levels. These results suggest that ovarian inhibin derived from the maturing follicles rather than E may be a primary factor for regulation of FSH secretion, and high levels of endogenous inhibin can suppress synthesis of LH as well as FSH in the pituitary gland of the female rat.

Possible significance of transferrin levels in seminal plasma of fertile and infertile men

Human seminal plasma contains large amounts of transferrin, which is a protein secreted mostly by Sertoli cells. It has been suggested that the concentration of transferrin may serve as a possible clinical marker of Sertoli cell function. Therefore the concentration of this protein in human seminal plasma from fertile and infertile men has been evaluated in order to find a relationship between transferrin concentrations and human semen parameters and plasma FSH levels. Findings show that seminal transferrin in subjects with oligozoospermia or azoospermia is significantly lower than in controls, and that it is strongly related to sperm count. Results also indicate that transferrin secretion can be impaired when plasma FSH levels are still normal, suggesting that seminal transferrin is an early and specific marker of Sertoli cell function. These results, however, do not clarify whether impairment of transferrin secretion by Sertoli cells is due to an organic dysfunction or to an organic secretory alteration.

Modulation of gonadotropin secretion by Sertoli cell inhibin, LHRH, and sex steroids

Sertoli cell culture media (SCM) from 10-, 20-, 30-, 35-, and 40-day-old male Wistar rats were assayed to determine the inhibin activity in anterior pituitary cells in culture. In agreement with previous data, SCM did not affect the luteinizing hormone (LH) spontaneous release at all ages studied, whereas it inhibited specifically follicle-stimulating hormone (FSH) spontaneous release by 40% for the 40-day-old rats. Younger animals (10-, 20-, and 30-day-old) showed a 60% inhibition of the FSH basal release. The inhibin activity was also different at all stages studied, the IC50 being markedly displaced to the right as the age increased, leading to a fivefold difference between 10- and 30- to 40-day-old rats. The same pattern was observed when the LH and FSH responses to 0.3 nM LH-releasing hormone (LHRH) were studied. SCM from 35-day-old rats did not alter total LH, whereas total FSH was markedly reduced, thus suggesting a reduced FSH synthesis in the presence of inhibin. SCM exerts an additive inhibitory effect with dihydrotestosterone on the LH response to LHRH, whereas it reverses the stimulatory effect of the androgen on spontaneous and LHRH-induced FSH release. Moreover, SCM reversed the stimulatory effect of 17 beta-estradiol on both spontaneous and LHRH-induced LH and FSH release, whereas the stimulatory effect of progesterone on FSH release was 50-80% inhibited. The present data show that inhibin activity of Sertoli cell origin can exert marked interactions with sex steroids in the control of gonadotropin secretion. These data also demonstrate that the inhibin component is an important factor in sexual maturation of the rat and that high FSH levels of 10-day-old rats could suggest a modulation by a nonandrogenic factor of gonadotropin secretion in developing rats.

An assessment of inhibin-like activity secreted by Sertoli cells in culture using castrated adult male rats

Sertoli cells isolated from testes of 16-18 days old male rats were maintained in culture. Incubation media from these culture were pooled on day 7 and tested for its inhibin-like activity either with (SCCM) or after charcoal treatment (CSCCM) in castrated adult male rats. The assay was based on the tacit assumption that SCCM or CSCCM would specifically lower circulating blood serum levels of FSH. Subcutaneous (sc)injections of CSCCM at a dose level of 1 mg protein per rat, per day, x 3 days caused a specific suppression of FSH levels, while lower dosages of CSCCM (Protein content of 300 micrograms or 600 micrograms/rat/day, x 3 days) were without any affect on basal levels of FSH and LH. SCCM was ineffective at all dose levels tested. Intracardiac injections of varying doses of LHRH (25 to 400 ng/rat) to CSCCM pre-treated rats (200 micrograms/rat/day, x 3 days) failed to increase the levels of LH and FSH. These results support the presence of inhibin like activity in SCCM by a bioassay procedure alternate to in vitro pituitary cell culture system used by other investigators.

Possible existence of a long-loop feedback system between FSH and inhibin in female rats

Experiments were designed in which peripheral plasma inhibin levels were presumably altered in an attempt to investigate an interdependency between pituitary follicle-stimulating hormone (FSH) and ovarian inhibin secretion. In the first study, unilateral ovariectomy (ULO) was performed on 4-day cycling female rats under ether anesthesia at 0800 h on diestrous day 1 (D1). Inhibin-like activity [FSH-inhibiting activity(FSH-IA)] in untreated ovarian venous plasma (OVP) collected from the remaining ovary was assessed by an in vitro pituitary bioassay system. Both plasma FSH levels and FSH-IA significantly increased between 4 and 12 h after ULO. Thereafter, plasma FSH declined between 12 and 32 h after ULO, whereas FSH-IA remained elevated during this same time interval. Compared to sham-operated rats, plasma FSH was significantly elevated 4, 12, and 24 h after ULO, whereas FSH-IA was statistically higher only at 32 h after ULO. In a second experiment, rats were injected with charcoal-treated porcine follicular fluid (PFF) on proestrus and estrus. Control rats received saline. The data indicate that increased plasma FSH levels on D1 in PFF-treated rats (FSH rebound) may be a consequence of reduced endogenous inhibin secretion on estrus. As well, return of FSH to control levels on D2 in PFF-treated rats may have resulted from an FSH-associated increase in FSH-IA on D1 and D2.

Specific FSH suppression in male rat by porcine follicular fluid

A variety of protocols using intact, castrated, and sham-castrated adult male rats was used to test the ability of porcine follicular fluid to selectively suppress serum follicle-stimulating hormone (FSH). A total dose of 500 microliters follicular fluid injected intraperitoneally will suppress FSH in the rat with testes within 5 h of injection. This is more than is necessary to suppress FSH in the intact or ovariectomized female. A total dose of 1 ml of follicular fluid is needed to suppress FSH reliably in the castrated male rat. Serum FSH is suppressed significantly beginning between 4 and 5.5 h after intraperitoneal injection of 1 ml of follicular fluid and 3 h after intravenous injection. The suppression continues until 10 h postinjection and is abated by 26 h postinjection. Serum luteinizing hormone (LH) is unaffected by follicular fluid in any protocol tested. It is concluded that 1) follicular fluid can suppress serum FSH in the male if large enough doses are given, 2) follicular fluid has no effect on serum LH, and 3) follicular fluid suppresses serum FSH levels in the male as well as previously tried testicular sources. These results suggest that the delay in demonstrating the existence of inhibin may be due to the use of males as both source and test subject.

Inhibin--fact or artifact

Peripheral concentrations of follicle-stimulating hormone (FSH) in male as well as in female animals appear to be partly regulated by inhibin, a protein which is secreted by the gonads. The molecular structure of this substance is still unknown, and the mechanism(s) of its action on the pituitary or hypothalamic level is not clear. Much of the confusion about inhibin stems from the fact that no generally accepted definition of inhibin exists and that fundamentally different biological assay systems have been used by different groups. Therefore this short review starts with a discussion of the definition of inhibin and the assay principles. From the available information on the site of origin of the hormone it appears likely that inhibin is produced in the Sertoli cells of the testis and the granulosa cells of the ovary. The available data on the chemical nature of inhibin suggest that different principles, acting on different sites of the hypothalamic--pituitary axis, might be present in preparations with inhibin-like activity. Finally, with respect to the biological significance of inhibin, it seems that inhibin could play a more important role in the feedback regulation of FSH in the adult female than in the adult male animal.