A mesoderm-inducing factor produced by WEHI-3 murine myelomonocytic leukemia cells is activin A

The first inductive interaction in amphibian development is mesoderm induction, during which a signal from the vegetal hemisphere of the blastula-staged embryo induces mesoderm from overlying equatorial cells. Recently, a number of ‘mesoderm-inducing factors’ (MIFs), which may be responsible for this interaction, have been discovered. Examples of these MIFs include members of the fibroblast growth factor family as well as members of the TGF-beta superfamily such as TGF-beta 2. In addition to these purified factors, several new sources of mesoderm-inducing activity have been described. One of the most potent of these is the murine myelomonocytic leukemia cell line WEHI-3. Even at high dilutions, conditioned medium from WEHI-3 cells induces isolated Xenopus animal pole regions to form a variety of mesodermal cell types. In this paper we show by several criteria, including N-terminal amino acid sequencing, Northern blotting and various functional assays, that the WEHI-MIF is activin A. Activins are known to modulate the release of follicle-stimulating hormone from cultured anterior pituitary cells and to cause the differentiation of two erythroleukemia cell lines. Our results, along with recent data from other laboratories, indicate that these molecules may also act in early development in the formation of the mesoderm.

Novel ovarian regulatory peptides: inhibin, activin, and follistatin

Based on the extensive amount of research on inhibin and related polypeptides accomplished during the past 5 years, the inhibin concept put forth more than 50 years ago has not only become well established but also more complex than originally imagined. The closed-loop feedback mechanism of ovarian inhibin and pituitary FSH has been joined by possible "inhibin-like" actions of follistatin and FSH-stimulatory effects of activin. In addition, in vitro experiments suggest possible autocrine and paracrine functions for the gonadal polypeptide hormones. Figure 3 shows a simplistic diagram summarizing our current understanding of inhibin/activin and follistatin action along the hypothalamic-pituitary-gonadal axis. Hopefully, research in the coming years will allow us to remove the many question marks still remaining but will undoubtedly add others.

An improved method of purifying inhibin radioligand for radioimmunoassay

Polyacrylamide gel electrophoresis (PAGE) separation of bovine inhibin from free iodine after iodination is described. Previous methods of separation typically relied upon a G25 gel filtration and Matrix gel Red A affinity column chromatography protocol. When compared to column chromatography, PAGE-purified radiolabelled inhibin resulted in significantly increased binding (13.1% vs. 7.5%) and enhanced sensitivity (ED50 = 92 microliters inhibin standard vs. ED50 = 198 microliters inhibin standard) to the inhibin antibody #1989. Our results demonstrate an advantageous approach to purifying 31,000 Mr bovine inhibin radioligand after iodination for RIA.

Short-term stimulatory effect of Sertoli cell conditioned medium on Leydig cell steroidogenesis is not mediated by inhibin

Addition of concentrated rat Sertoli cell conditioned medium (rSCCM) to isolated Leydig cells from immature rats stimulated steroid production more than 13-fold within 4 h. LH-stimulated steroidogenesis was not enhanced by addition of rSCCM. The biological activity of the concentrated rSCCM was higher after incubation of Sertoli cells with FSH, whereas FSH alone did not stimulate steroid production. This effect of rSCCM was not due to inhibin, since highly purified 32 kDa rat inhibin, in doses equivalent to those present in rSCCM, had no effect on steroidogenesis during the 4 h incubation period. Furthermore, inhibin could be separated from the Leydig cell stimulating factor by anion-exchange chromatography. These results indicate a short-term paracrine control of Leydig cell steroidogenesis by Sertoli cell derived factors, which differ from inhibin.

Isolation of bovine ovarian inhibin, its immunoneutralization in vitro and immunolocalization in bovine ovary

A purification scheme involving gel permeation chromatography, anion exchange chromatography and reversed-phase high performance liquid chromatography (RP-HPLC) was used to isolate from bovine follicular fluid (FF) biologically-active inhibin of molecular weight 32 kDa. Chromatographic fractions were monitored for inhibin-like biological activity (ILA) using a simplified bioassay procedure in which a suppression of total basal FSH production by rat pituitary cells in monolayer culture indicates the presence of ILA. Approximately 3 mg protein having an ILA potency (ED50 value in in vitro bioassay) of 1.7 ng/ml was obtained from 4 1 crude bovine FF (260 g protein; ILA potency 3750 ng/ml) reflecting an approximate 2200-fold purification factor with an overall recovery of about 3%. The isolated material appeared as a single major UV absorbance peak on RP-HPLC and as a single band (32 kDa) when subjected to SDS-PAGE (15% gel) under non-reducing conditions. Under reducing conditions the molecule dissociated into 2 subunits of apparent molecular weight 22 and 14 kDa confirming that it is probably identical to the 31/32 kDa form of bovine ovarian inhibin previously reported by two other independent research groups. An antiserum raised in a chicken against the isolated material completely neutralized the suppressive effects of both 32 kDa inhibin and bovine FF on basal production of FSH by rat pituitary cells in vitro but only partially reversed the suppressive effects of both porcine and human FF. Immunohistochemical staining of sections of bovine ovary and of isolated preparations of bovine granulosa cells using this antiserum confirmed that granulosa cells are a major source of inhibin. The observation that specific immunostaining was not confined to these cells, however, suggests that they may not be the exclusive source of immunoreactive inhibin in the bovine ovary.

Identification of a potent Xenopus mesoderm-inducing factor as a homologue of activin A

The first inductive interaction in amphibian development is mesoderm induction, when a signal from the vegetal hemisphere of the blastula induces mesoderm from overlying equatorial cells. Recently, several 'mesoderm-inducing factors' (MIFs) have been discovered. These cause isolated Xenopus animal caps to form mesodermal cell types such as muscle, instead of their normal fate of epidermis. The MIFs fall into two classes. One comprises members of the fibroblast growth factor (FGF) family, and the other members of the transforming growth factor type beta (TGF-beta) family. Of the latter group, the most potent is XTC-MIF, a protein produced by Xenopus XTC cells. Here we show that XTC-MIF is the homologue of mammalian activin A. Activins modulate the release of follicle-stimulating hormone from cultured anterior pituitary cells and cause the differentiation of two erythroleukaemia cell lines. Our results indicate that these molecules may also act in early development during formation of the mesoderm.

Purification and characterization of recombinant human activin B

Recombinant human activin B has been isolated to more than 95% purity from a mammalian kidney cell line. Activin B is a covalently-linked homodimer with an apparent molecular mass of 25.9 kDa (unreduced) and 15.2 kDa (reduced) as determined by SDS-polyacrylamide-gel electrophoresis. On gel filtration in 6 M guanidine hydrochloride, activin B chromatographs with an apparent molecular mass of 11 kDa, whether reduced or not. The amino-terminal sequence of the purified protein is consistent with the expected sequence derived from the beta subunit of inhibin B. The amino acid composition of the purified molecule agrees with the expected theoretical composition of the beta subunit of inhibin B. Activin B has an apparent pI of 4.6 as determined by isoelectric focusing in 6 M urea and 4.7 as determined by chromatofocusing in 6 M urea. The extinction coefficient is 1.8.

Current perspectives of inhibin biology

Inhibin is generally considered to be a feedback regulator of FSH secretion, but significant progress in the field of inhibin biology over the past four years has widened our perspective of inhibin and its production, regulation and action. Firstly, inhibin is a member of a larger family of hormones, secondly it is produced by organs other than the gonads, and thirdly its action is not confined to the pituitary gland. In this review we will outline the isolation and characterization of inhibin related peptides and their production by the Sertoli and granulosa cells of the testis and ovary. With this information in mind, more recent studies concerning the measurement, production, regulation and action of inhibin and inhibin-related proteins will be discussed, in an effort to alert the reader to the contemporary and controversial issues concerning inhibin biology.

Activin is a nerve cell survival molecule

The structures of five neurotrophic molecules have so far been published. Nerve growth factor, fibroblast growth factor and purpurin, have been identified as nerve-cell survival molecules. More recently, brain-derived neurotrophic factor (BDNF) and ciliary neurotrophic factor have been cloned and sequenced. As all these proteins stimulate the survival of ciliary or sensory neurons, a new cell survival assay is required if novel neurotrophic molecules are to be discovered. P19 teratoma cells differentiate to nerve-like cells in the presence of 5 x 10(-7) M retinoic acid (RA). But when P19 cells are plated in N2 synthetic medium without being exposed to RA, they die within 48 h. In an attempt to identify a molecule(s) that can substitute for RA in promoting P19 survival, we assayed serum-free growth-conditioned media for their ability to promote P19 survival. One cell line from the rat eye secreted a molecule that promoted the survival of P19 cells and some types of nerve cell. We identified this molecule as activin, better known for its role in hormone secretion.

Inhibin in immature rat Sertoli cell conditioned medium: a 32 kDa alpha beta-B dimer

Conditioned medium of cultured Sertoli cells from 21-day-old rats was used as starting material for the isolation of inhibin. Inhibin activity was monitored by the dose dependent suppression of the follicle-stimulating hormone release of cultured rat pituitary cells. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis analysis of the highly purified inhibin preparation revealed a 32 kDa protein after silver staining, which could be separated in subunits of 18 kDa and 12 kDa after reduction. Western blot analysis with an antibody recognizing the 22 N-terminal amino acids of the alpha-subunit of 32 kDa bovine inhibin confirmed the presence of a 32 kDa inhibin molecule under non-reducing conditions, whereas an 18 kDa alpha-subunit was found after reduction. An antibody recognizing the beta-A subunit of inhibin did not yield a signal after Western blotting. N-terminal amino acid sequence analysis of two highly purified preparations of inhibin obtained using different methods yielded the sequence predicted for a 32 kDa alpha beta-B dimer on basis of cDNA nucleotide sequence. This result is in agreement with the large excess of beta-B over beta-A mRNA in the rat testis.

Activin-binding protein from rat ovary is follistatin

Activin, a member of the transforming growth factor beta protein family, was originally isolated from gonadal fluids and stimulates the release of pituitary follicle-stimulating hormone (FSH). Activin has numerous functions in both normal and neoplastic cells. Various cells synthesize activin and have a specific binding site for this peptide. However, the molecular basis for its actions is unknown. A binding protein for activin was purified from rat ovary and was identical to follistatin, a specific inhibitor of FSH release. It is likely that the binding protein participates in the diverse regulatory actions of activin.

Inhibins, activins and follistatins

Inhibins are proteins consisting of two subunits, 18 K alpha- and 14 K beta-subunits, linked by disulfide bonds. Two forms of inhibins A and B consisting of a common alpha-subunit and a similar but distinguishable beta-subunit specifically suppress the secretion and cell content of FSH in a delayed action. The production of inhibin is regulated mainly by FSH; therefore, a reciprocal relation between FSH and inhibin is established. Each subunit (alpha-, beta A- or beta B-) is encoded by a different mRNA species. Inhibin secreted in response to FSH from the pituitary originates primarily in the granulosa cells of the ovary and the Sertoli cells or testes. Two beta-subunits, beta A beta A, beta A beta B, or beta B beta B form a new molecule, activin, that has opposite endocrine function to inhibin, but in the presence of inhibin, the activity of inhibin overrode that of activin. Follistatin, a single-chain polypeptide with no structurally similarity to inhibin, also specifically inhibits the release of FSH, approximately one third potency of inhibin. This probably is due to the fact that inhibin suppresses both FSH secretion and cell content of FSH whereas follistatin primarily inhibits the release of FSH. In addition to the endocrine function, these gonadal proteins also exert paracrine function on gonadotropin-mediated steroidogenesis. Outside the reproductive system, inhibin and activin also play a role in hemoglobin production, erythroid cell differentiation; all three proteins, together with TGF beta, are involved in immunosuppresion.

Evidence that the bovine ovary secretes large amounts of monomeric inhibin alpha subunit and its isolation from bovine follicular fluid

Analysis of bovine follicular fluid (FF) using sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) coupled with a sensitive immunoblotting procedure resolved several components that were immunoreactive with an antiserum directed against the n-terminus of the alpha subunit of human inhibin (hI alpha(1-32]. Under non-reducing conditions, three intensely stained bands having apparent Mr values of 116,000, 44,000 and 25,000 were present, whilst under reducing conditions only two intensely stained bands (Mr 43,000 and 21,000) were detected. The Mr 44,000 and 25,000 immunoreactive forms (non-reducing conditions) were also demonstrated in bovine utero-ovarian vein and peripheral venous plasma after subjecting samples (40 ml) to immunoaffinity concentration using Sepharose beads coupled to anti-hI alpha(1-32), SDS-PAGE and immunoblotting. The same approach revealed the presence of the smaller (Mr 25,000) form in bovine granulosa cell-conditioned culture medium (GCCM). Gel-permeation chromatography (Sephacryl S-200), immunoaffinity chromatography (Sepharose-anti-hI alpha(1-32] and reversed-phase high-performance liquid chromatography (RP-HPLC; C18 and C8 columns) were employed to isolate from bFF (30 ml, 19.5 g protein) 750 micrograms protein which appeared essentially homogeneous by RP-HPLC and SDS-PAGE and had an Mr of 25,000 (non-reducing conditions)/21,000 (reducing conditions), identical to that of the immunoreactive component of lowest Mr found in bovine FF, utero-ovarian vein plasma, peripheral plasma and GCCM. The isolated material was highly immunoreactive with antisera against both hI alpha(1-32) and purified Mr 32,000 bovine inhibin but was devoid of biological activity when tested in a rat pituitary cell inhibin bioassay. Amino-terminal analysis revealed an amino acid sequence (residues 1-14) identical to that reported elsewhere for the alpha subunit (Mr 20,000/21,000) of bovine inhibin. In conclusion, the present study has revealed that the bovine ovary secretes considerable quantities of monomeric inhibin alpha subunit. The unexpected presence of this material in peripheral blood is likely to hinder attempts to obtain physiologically relevant data on circulating levels of inhibin in cattle using conventional radioimmunoassays.

Inhibin alpha-subunit monomer is present in bovine follicular fluid

The 26 kDa form of the inhibin alpha-subunit monomer was isolated from bovine follicular fluid. Both sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotting analysis revealed that the 26 kDa form of the alpha-subunit monomer is composed of 6 kDa and 20 kDa fragments linked together by a disulfide bond(s). The NH2-terminal sequence and amino acid analyses of each fragment showed that those fragments are derived from residues 18 to 60 and 227 to 360, respectively, of the inhibin alpha-subunit precursor (residues 1 to 360) as deduced from the cDNA sequence. Thus, it is concluded that removal of the putative signal peptide (residues 1 to 17) and the middle portion (residues 61 to 226) from the alpha-subunit precursor generates the 26 kDa form of the alpha-subunit. In the in vitro rat pituitary assay, the 26 kDa alpha-subunit exhibited neither inhibin-like nor FSH-release stimulating activity.

The isolation and physiology of inhibin and related proteins

Inhibin, a glycoprotein that preferentially suppresses follicle-stimulating hormone (FSH) secretion, has been isolated from follicular fluid as a heterodimer of two dissimilar subunits linked by disulphide bonds. The larger subunit is termed alpha and the smaller is designated beta. Two forms of inhibin termed A and B have been isolated, the differences being due to variations in the amino acid sequence of the beta-subunit; Inhibin A consists of alpha-beta and Inhibin B of alpha-beta B. Dimers of the beta-subunit, termed activins, have also been found in follicular fluid; these stimulate pituitary FSH secretion. Inhibin is produced in the female by the granulosa cell and corpus luteum under the control of FSH and luteinizing hormone (LH), respectively. The levels in serum rise to peak at mid-cycle and in the mid-luteal phase of the human menstrual cycle, and decline prior to menstruation. In pregnancy, the late-luteal phase decline in inhibin does not occur and the levels increase slowly. Studies suggest that the levels in pregnancy arise from an embryonic source, particularly the placenta. In the male, inhibin is produced by the Sertoli cells under the control of FSH by mechanisms involving cyclic adenosine 3', 5'-monophosphate. Testosterone exerts a minor inhibitory control at supraphysiological levels (10(-5) M), but human chorionic gonadotropin stimulation results paradoxically in a rise in serum inhibin levels. Disruption of spermatogenesis in the rat by cryptorchidism, heat treatment, or efferent duct ligation results in a decline in inhibin levels and a rise in FSH levels, findings consistent with the negative feedback action of inhibin on FSH secretion. As well as their roles in the reproductive system, inhibin and activin have more widespread actions in the haemopoietic, immune and nervous systems as evidenced by the finding of mRNA for its subunits in a range of tissues. Other studies have shown actions on erythroid differentiation and on mitotic activity in thymocytes. These actions suggest that inhibin and activin may function as growth factors as well as regulators of FSH.

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.

[Control mechanism of FSH secretion from the pituitary]

FSH secretion from the pituitary is now generally believed to be controlled only by LH-RH. However, since 1971 I have been publishing lines of evidence suggesting that FSH is controlled by both LH-RH and FSH-RH distinct from LH-RH. Recent accumulated reports suggest that lots of data cannot be explained by the one-RH theory. The one-RH theory means that both LH and FSH are regulated by a sole releasing factor, LH-RH. In 1985 we succeeded in purification of porcine and bovine follicular inhibin for the first time in the world. Its molecular weight was 32,000, which consisted of alpha-subunit (mw 20,000) and beta-subunit (mw 13,000). The site of production of inhibin was confirmed to be granulosa cell by immuno-histochemical technique and to be corpus luteum by RIA. Secretion of inhibin was stimulated by FSH, insulin, platelet extract, dibutyryl c-AMP, cholera toxin, forskolin, and inhibited by cortisol. Testosterone augmented FSH action. In the pituitary cells, inhibin inhibited synthesis of FSH, so cycloheximide could mimicked inhibin action in vitro. Inhibin did not inhibit LH release from the pituitary, but LH release stimulated by LH-RH was also inhibited by inhibin in vitro. Using anti-porcine inhibin antibody and anti bovine inhibin antibody prepared by us, we could measure blood inhibin levels during rat, pig, cow and goat sexual cycles, human menstrual cycle, human pregnancy, and clomiphene-HMG stimulated menstrual cycle. We found maximum blood inhibin levels in the luteal phase and during pregnancy. These high levels of inhibin were demonstrated to be derived from the corpus luteum, pregnant corpus luteum and placenta.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibins, activins, and follistatins: gonadal proteins modulating the secretion of follicle-stimulating hormone

The endocrine system displays highly complex interactions among its components. Excesses or deficiencies of hormone production in one gland may alter the production of hormones by others. Several physiological functions are affected by a balance among hormones acting either together or in sequence. For example, FSH secretion has been demonstrated to be affected by hypothalamic influences upon the anterior pituitary through a specific releasing factor, the decapeptide LRF. This decapeptide stimulates the release of both LH and FSH by the pituitary, and these gonadotropins cause the production of steroids by the testes and the ovaries. Gonadal steroids in the blood act directly upon the anterior pituitary to regulate the output of gonadotropins as originally proposed by Moore and Price in 1932 (3), or act indirectly upon the hypothalamus to adjust the output of pituitary hormones in accordance with the needs of the reproductive system. However, such a simple negative feedback of steroids on the hypothalamic-hypophysial axis cannot account for the differential secretion of FSH observed during the estrus cycle. Therefore, the concept that a gonadal protein, inhibin, specifically regulates FSH secretion was proposed. This concept has now been validated by the isolation and characterization of two forms of inhibin that exert their effects on the pituitary to suppress FSH secretion both in vitro and probably in vivo. Furthermore, the production of inhibin is stimulated by FSH, thus establishing a reciprocal relationship between the release of FSH and inhibin. Since hormones in the body are controlled through interlocking complexes of factors, a variety of secondary factors, in one way or another, may also exert influence on the regulation of FSH secretion. As an example, TGF beta, a protein growth factor found in all tissues, promotes the basal secretion of FSH by the pituitary and enhances FSH-mediated estrogen production by the granulosa cells. It is therefore not surprising that two forms of a novel protein, activin and activin A, isolated from the same FF from which inhibins were isolated, show bioactivities similar to those of TGF beta. These activins are formed as dimers of the two beta-subunits of inhibin, probably as a result of the rearrangement of the gene products. This novel observation that different arrangements of gene products can result in opposite biological activities may thus reflect a wholly different level of control of FSH secretion. If such a phenomenon occurs in other biosystems, it would represent an important form of homeostatic mechanism for controlling biologically active substances.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibin

Inhibin has 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 as early as 1923 (250). However, only after reliable and sensitive bioassay systems for the detection and estimation of inhibin had been developed and an ample source of inhibin was found in the form of ovarian follicular fluid has progress been made in 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, which also includes Müllerian-inhibiting substance, transforming growth factor-beta, erythroid differentiation factor, an insect protein that plays an important role in differentiation, and the dimer of two inhibin beta-subunits, activin. The latter substance was reported to counteract the effects of inhibin in pituitary cells. Interactions between these regulatory substances will certainly be a field of major interest in the near future. Inhibin molecules, similar to those in the gonads, have been detected in placental tissue as well. The postulate of inhibin in seminal plasma, prostate, and gastric juice is apparently due to detection of substances that differ from gonadal inhibin. Most likely these substances, the amino acid sequences of which have been elucidated, have a biological function that differs from the suppression of peripheral levels of FSH in vivo. 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 presently a lot of controversy exists about other factors that might play a role in the regulation of the production of inhibin. Because of the lack of reliable methods for estimation of peripheral levels of inhibin in humans and experimental animals, almost all evidence for the physiological importance of inhibin in the regulation of reproductive processes is derived from circumstantial evidence. From these indirect results, 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.(ABSTRACT TRUNCATED AT 400 WORDS)

Beta-microseminoprotein (beta-MSP) is not an inhibin

Beta-microseminoprotein (beta-MSP), a sperm-coating antigen isolated from human seminal plasma, has apparent structural identity with "beta-inhibin" isolated from the same source. Publication of the amino acid sequence of beta-MSP revealed a greater than 96% homology with "beta-inhibin," with only a proline-threonine substitution at positions 39 and 40, and the omission of a glycine at position 93. Due to the nearly identical sequences of "beta-inhibin" and beta-MSP, we examined the ability of beta-MSP and its tryptic peptides to inhibit basal follicle-stimulating hormone (FSH) secretion from rat pituitary cells in culture, the inhibin bioassay. Whole pituitaries collected from 250- to 300-g male rats were dispersed enzymatically and plated onto 24-well culture dishes for 3 days. beta-MSP and its tryptic peptides were dissolved in cell culture medium, applied to the pituitary monolayer cell cultures, and incubated for an additional 3 days. FSH levels in the medium were determined by radioimmunoassay. A partially purified preparation of inhibin and our in-house inhibin standard, both prepared from porcine follicular fluid (pFFl), were included in the same assay. Whereas the partially purified inhibin from pFFl showed a dose-dependent inhibition of FSH secretion, with a 50% inhibitory dose (ID50) of 50 ng, which paralleled that of the standard, beta-MSP and its tryptic peptides failed to depress FSH levels in the medium at any of the doses tested (10-10,000 ng/ml). We conclude that beta-MSP is not an inhibin under our assay conditions.

Isolation of inhibin from ovine follicular fluid

Two forms of inhibin with molecular weights of 65,000 and 30,000 (65 and 30 kD) were isolated from ovine follicular fluid using a combination of gel permeation chromatography, reversed-phase high-performance liquid chromatography and preparative polyacrylamide gel electrophoresis. The 65 kD form was partially purified approximately 315-fold whilst the 30 kD form was isolated as two isoforms (29 and 30 kD) of similar biological activity and in greater than 95% purity (1210-fold purification and 4.2% recoveries). On reduction the 30 kD form resolved into four components of 36, 31, 20-21 and 16 kD of which the 20-21 and 16 kD components were similar to the corresponding inhibin subunits isolated from porcine and bovine follicular fluid. The 36 kD component was established as a non-reducible inhibin-like material, based on its binding to antiserum raised against bovine 58 kD inhibin. The nature of the remaining non-reducible 31 kD component is unknown. Two NH2-terminal amino acid sequences (first 13 amino acids) identified in purified 30 kD inhibin were identical to the corresponding subunit amino acid sequences of bovine 31 kD inhibin.

Purification and characterization of erythroid differentiation factor (EDF) isolated from human leukemia cell line THP-1

We isolated a protein, from a cell line of human origin, which exhibits extensive differentiation inducing activity toward Friend leukemia cells. The protein, called Erythroid Differentiation Factor (EDF), was found in a 4 day culture of THP-1 cells performed in the presence of 4 beta-phorbol 12-myristate 13-acetate(PMA). EDF is a homodimer of a molecular weight of 25,000, with an NH2-terminal sequence identical to that of the beta A-chain of porcine Inhibin. It was suggested that a single protein species is responsible for the activities of both EDF and FRP, a FSH releasing protein isolated from porcine ovarian follicular fluid.

Immunological study of ovarian inhibin

Antisera to purified porcine follicular fluid inhibin of 32 K protein (pFF 32 K inhibin) were raised in rabbits. Increasing doses of an antiserum with high titer could neutralize the maximal suppression of FSH secretion caused by 10 ng of pFF 32 K inhibin from rat anterior pituitary cells in culture. A radioimmunoassay was developed using the antiserum and 125I-labelled pFF 32 K inhibin. Specificity of the antiserum was examined by comparing immunological and biological potencies of various inhibin preparations. Cross-reactivity tests revealed that the antiserum almost recognizes rat ovarian inhibin preparations. The antiserum also recognizes purified bovine follicular fluid inhibin of 32 K protein (bFF 32 K inhibin), but with a cross-reactivity of approximately 20%. Cross-reactivity of human follicular fluid to the antiserum was less than 10%. The antiserum also recognizes inhibin forms of higher molecular weights, 100 K, 80 K, and 55 K proteins, which have previously been identified by gel filtration or SDS-PAGE of crude pFF inhibin preparations under protein-dissociation conditions, indicating that these inhibin forms have common or closely related immunodetermining sites.

A homodimer of the beta-subunits of inhibin A stimulates the secretion of pituitary follicle stimulating hormone

A 24,000 Dalton protein with follicle stimulating hormone (FSH)-releasing activity, named activin, has been characterized previously from porcine follicular fluid as a heterodimer composed of the beta-subunits of inhibins A and B linked by disulfide bond(s) [Ling et al. (1986) Nature, in press]. In this paper we report the isolation of another 24,000 Dalton protein with FSH-releasing activity from porcine follicular fluid, using successive steps of heparin-Sepharose affinity chromatography, gel filtration on Sephacryl S-200, and four steps of reversed-phase HPLC, followed by preparative sodium dodecyl-sulfate-polyacrylamide gel electrophoresis chromatography. Based on the molecular weight of the isolated molecule and its deduced NH2-terminal sequence, we propose that this second FSH-releasing substance present in porcine follicular fluid is a homodimeric protein composed of two beta-subunits of inhibin A joined together by disulfide bond(s). The name homo-activin-A is proposed for this substance.

Inhibin fractionation: a comparison of human and porcine follicular fluid, with particular reference to protease activation

A novel treatment has been devised in our studies of the purification of inhibin from porcine and human follicular fluids (pFFl and hFFl, respectively). Both pFFl and hFFl were precipitated with acetone and extracted with acetic acid to provide a starting material for subsequent gel filtration and reverse-phase high-pressure liquid chromatography (HPLC). Inhibin from pFFl was purified 4200-fold using this methodology. Inhibin from hFFl could not be purified to this degree since recoveries were relatively poorer than for pFFl and yielded too little material for the HPLC step. In our fractionation scheme, protease activities were assessed with a gel electrophoresis assay system. Protease activity at approximately 90 kDa was observed in raw pFFl. When inhibin was fractionated by extraction or chromatography, additional bands of protease activity appeared near 150 kDa, 66 kDa and at less than 45 kDa. In raw hFFl, only faint bands of protease activity were observed at approximately 90 kDa and at 85-90 kDa. Upon further fractionation of hFFl, protease activity was reduced below the ability of this method to detect it. Our results suggest that, with our treatment of follicular fluid, protease activity is present in pFFl and additional protease activity appears upon fractionation; proteases, although present, do not eliminate the possibility of obtaining a highly purified inhibin preparation with acceptable recoveries of inhibin activity during purification; and although protease activity could be reduced or eliminated from hFFl, the low yields of inhibin activity from this method mandate a different approach to purification of inhibin from hFFl.

The effects of inhibin purified from bovine follicular fluid in several in vitro pituitary cell culture systems

The effects of inhibin purified from bovine follicular fluid (bFF) and a charcoal-treated bFF preparation were investigated in several inhibin in vitro systems based on the use of pituitary cells in culture. FSH, LH, TSH and PRL were determined in the medium and cell extracts before and after a 4 h LHRH stimulation test. Both pure inhibin and bFF preparations markedly inhibited the basal release, cell content and LHRH-stimulated release of FSH in a parallel dose-dependent manner with minor or negligible effects on LH, TSH and PRL. Using parallel line bioassay statistics the inhibin activities of the purified inhibin preparations in the various in vitro systems were calculated with the charcoal-treated bFF as standard. Significantly higher inhibin values were obtained using the basal release procedure than with the cell content or LHRH-stimulated release procedures. This difference was influenced by the length of time the inhibin preparations were in culture. The highly purified preparations showed no signs of cytotoxicity in culture as assessed by a 51Cr release test. It is concluded that purified bFF inhibin is specific in suppressing pituitary FSH and not LH, TSH or PRL. The observation that purified bFF inhibin is more active in the basal release procedure is attributed to a loss of activity of the bFF standard in this system in contrast to that observed in either the cell content or LHRH-stimulated release procedure.

Peptides of postulated inhibin activity. Lack of in vitro inhibin activity of a 94-residue peptide isolated from human seminal plasma, and of a synthetic replicate of its C-terminal 28-residue segment

A 94-residue polypeptide isolated from human seminal plasma and its chemically synthesized C-terminal 28-residue segment were studied in an in vitro inhibin bioassay utilizing rat pituitary cell cultures. Both peptides have previously been claimed to have inhibin activities, and the effects on the secretion and cellular content of gonadotrophins (FSH and LH) were now assessed in the in vitro assay. No inhibition was found. After 72 h of culture, both the cellular content and the spontaneous as well as the LHRH-stimulated release of bioactive or immunoactive FSH and LH remained unaffected. Similarly, no effects were found on the storage and/or release of prolactin, growth hormone, or thyrotropin. We conclude that both the native 94-residue peptide and the synthetic replicate of its C-terminal 28-residue segment, do not influence the pituitary FSH secretion when assessed in this in vitro system.

Isolation of a 31 kDa form of inhibin from bovine follicular fluid

The introduction of a pH 4.75 precipitation step to a previously described purification procedure from bovine follicular fluid (bFF) resulted in the isolation of a 31 kDa form of inhibin, in addition to 58 kDa inhibin. The procedure was monitored by an in vitro bioassay based on the suppression of the FSH cell content by pituitary cells in culture. The 31 kDa form was purified 5550-fold with approximately 5% recovery. On SDS-polyacrylamide gel electrophoresis a single band was detected with a molecular weight of 31 000 +/- 1500 (mean +/- SD) which upon reduction gave 2 subunits of 20 200 +/- 300 and 14 800 +/- 600. The biological activity expressed on mg protein basis was similar for both 31 kDa and 58 kDa inhibin although on a molar basis the 58 kDa inhibin was 2-3 times higher. A high degree of cross-reaction was observed between both forms in a radioimmunoassay of bovine inhibin using an antiserum raised against the larger form with either iodinated 31 kDa or 58 kDa inhibin as tracer. Based on the subunit composition of the 31 kDa and 58 kDa inhibin, their similar cross-reaction in a radioimmunoassay system and the apparent generation of the 31 kDa inhibin following a pH precipitation step, it is concluded that 31 kDa inhibin is a smaller form of the 58 kDa inhibin resulting from a shortening of the 43 kDa subunit to a 20 kDa subunit.(ABSTRACT TRUNCATED AT 250 WORDS)

Isolation of bovine follicular fluid inhibin of about 32 kDa

Inhibin of about 32 kDa from bovine follicular fluid (bFF) was purified by using chromatographies operated under protein-dissociating conditions, which we have established for our previous purification of porcine follicular fluid (pFF) inhibin. On a gel filtration, bFF inhibin activity was eluted at 3 distinct regions corresponding to apparent molecular weight of 96, 55 and 32 kDa, representing 17%, 27% and 24% of the total inhibin activity in the follicular fluid, respectively. The smallest inhibin, named 32 kDa bFF inhibin, that evidently suppressed the basal secretion of FSH from the pituitary cells, was purified to homogeneity with a 5330-fold purification factor in a recovery yield of ca. 11%. 32 kDa bFF inhibin thus purified consists of 2 polypeptide chains (A-chain: 20 kDa and B-chain: 13 kDa), linked by disulfide bridges. N-Terminal sequences were Ser-Thr-Pro-Pro- for the A-chain and Gly-Leu-Glu-Cys- for the B-chain. The identical N-terminal sequences were also found in 32 kDa pFF inhibin, except that Pro-3 of the bFF A-chain is substituted by Ala in the 20 kDa chain of pFF inhibin.

Purification and partial characterization of inhibin from porcine follicular fluid

Inhibin, a protein of gonadal origin that suppresses the basal secretion of follicle stimulating hormone by anterior pituitary cells has been purified from porcine follicular fluid. Using several RP-HPLC steps and gel filtration under denaturing conditions, we obtained a fraction approximately ten thousand fold purified which showed one band on SDS PAGE and in the same experiment two bands after reduction (MW ca 14K and ca 18K) suggesting a molecular weight of 32K for inhibin. Edman degradation of isolated inhibin and carboxymethylated chain A indicated that the first 6 residues were H-Ser-Thr-Ala-Pro-Leu-Pro-; by subtraction, the first 3 residues of chain B could be deduced to be H-Gly-Leu-Glu-. EC50 was ca 0.3 ng/ml or 10 pM in our in vitro pituitary cell culture assay. Antibodies to residues 1-6 were raised which could immunoneutralize purified inhibin activity in an in vitro assay.

Isolation and partial characterization of a Mr 32,000 protein with inhibin activity from porcine follicular fluid

A Mr 32,000 protein with inhibin activity was isolated from porcine follicular fluid by heparin-Sepharose affinity chromatography, gel filtration on Sephacryl S-200, and four reversed-phase HPLC steps. The isolated molecule is composed of two chains having molecular weights of 18,000 and 14,000, respectively, and bound together by disulfide bonds. Amino acid sequence analysis revealed the 10 NH2-terminal residues of the Mr 18,000 chain to be Ser-Thr-Ala-Pro-Leu-Pro-Trp-Pro-Trp-Ser- and those of the Mr 14,000 chain to be Gly-Leu-Glu-Xaa-Asp-Gly-Arg-Thr-Asn-Leu-. This Mr 32,000 protein specifically inhibits the basal secretion of FSH, but not that of LH, in the rat anterior pituitary monolayer culture system, with a half-maximal effective dose of 450 pg/ml.

Inhibin: 1985 update on action and purification

Inhibin is a gonadal hormone, which exerts a specific negative feedback action on the pituitary secretion of follitropin (FSH) in male and female animals. The existence of inhibin was postulated over 60 years ago. Yet, until recently, little progress has been made in its isolation and characterisation. This lack of progress may be attributed to several factors: first, the use of a variety of assay systems of ill-defined specificity, secondly, the use of a variety of sources for unpurified inhibin, and thirdly, the inability of investigators to purify inhibin using classical purification procedures. During the last few years, and in particular during the last year, several publications on the isolation and characterisation of inhibin have appeared. This review attempts to place the various reports on the nature of inhibin into perspective. It is concluded that there are at least 2 classes of proteins with inhibin-like activity: a relatively large molecular weight material with apparent molecular mass between 40 and 70 kDa found in gonadal extracts and fluids, and a smaller material, with molecular mass between 5 and 20 kDa, found in seminal plasma. However, the observations that various purified seminal plasma inhibin preparations are either inactive in in vitro assays used to characterize gonadal inhibin or have been shown to be prostatic in origin suggest that they are unlikely to be involved in the gonadal regulation of FSH secretion. It has yet to be established if the purified gonadal inhibin preparations are the biological active forms involved in controlling FSH secretion in vivo.

Regulators of steroid secretion and inhibin activity in human ovarian follicular fluid

Inhibin activity from human follicular fluid was purified by successive chromatographies on Sephadex G-50, DEAE-Biogel A and orange A dye matrix. Inhibin activity was associated only with the protein(s) that bound to orange A (OrA-2). Daily injection of OrA-2, 1 h prior to hMG into 10-day-old female rats for 4 days caused a significant inhibition of hMG-induced estradiol secretion. In vitro, OrA-2 dose-dependently inhibited the amounts of estradiol secreted by porcine granulosa cells during a 3-h incubation. Orange A-unbound proteins, on the other hand, induced a dose-dependent increase in estradiol as well as progesterone secretion by porcine granulosa cells in vitro. Separation of stimulator from the inhibitor by orange A chromatography led to an increase in the relative inhibin activity (25-50-fold) as well as aromatisation-suppressing activity (60-fold). The results indicate a possible local action of hFF inhibin to regulate aromatisation activity.

On the identity of bovine seminal plasma inhibin

In light of current discussions on multiple forms of inhibin, it was thought of interest to ascertain the identity of the postulated 'iso-hormones' of bull seminal plasma inhibin (Chari et al., 1978). By subjecting the biologically active fraction, obtained by Sephadex G-100 gel filtration of bull seminal plasma acetone powder, to extensive dialysis in distilled water adjusted to pH 5.8, it was possible to remove the bulk of inert protein as a precipitate. The resulting active preparation could be readily fractionated by preparative iso-electric focusing in the pH range 4.0-6.5 yielding 2 distinct homogeneous peptides, alpha and beta, capable of suppressing hCG-induced uterine weight increase in immature mice, in a 'reversed Steelman-Pohley' assay design. However, of these, alpha alone was able to suppress post-castrational serum gonadotropin rise in appropriate animal models. This peptide is highly acidic in nature (iso-electric point congruent to 2.2) and has a molecular weight (Mr) of 18200 and a Stokes radius of 1.90 nm. On the basis of currently available evidence, it is concluded that the molecule consists of a single peptide chain.

Isolation of porcine follicular fluid inhibin of 32K daltons

Purification of ovarian inhibin from porcine follicular fluid was performed by using an bioassay based upon the suppression of spontaneous FSH release from cultured cells of rat anterior pituitary. The presence in the follicular fluid of four molecular forms of inhibin activity corresponding to Mr 100K, 80K, 55K and 32K was revealed by SDS-gel electrophoresis under non-reducing conditions. The smallest inhibin amongst them, named 32K inhibin, eliciting about 70% of the total activity in the follicular fluid, was separated by gel filtration in the presence of 8 M urea. By subsequent ion-exchange chromatography, followed by RP-HPLC, 32K inhibin was purified to homogeneity with a 8,000 fold purification factor in a yield of 12%. The purified 32K inhibin was found to comprise two polypeptide subunits (Mr 20K and 13K), linked by disulfide bridges and to specifically suppress the secretion of FSH, but not of LH from the pituitary cells.

Human seminal alpha inhibins: isolation, characterization, and structure

Two additional peptides with inhibin-like activity have been isolated from human seminal plasma. One consists of 52 amino acids and the other, 92 amino acids. They are designated alpha-inhibin-52 and alpha-inhibin-92. Sequence analyses show that the NH2-terminal 31 amino acids of alpha-inhibin-52 are identical to the structure of the inhibin-like peptide previously reported [ILP-(1-31), now designated alpha-inhibin-31], and the COOH-terminal 52 amino acids of alpha-inhibin-92 are identical to the structure of alpha-inhibin-52. The amino acid sequence of alpha-inhibin-92 is: (sequence in text) Bioassay data in mouse pituitaries in vitro show that alpha-inhibin-52 is 3.4 times more active and alpha-inhibin-92 is greater than 40 times more active than alpha-inhibin-31 in suppressing follitropin-release. Radioimmunoassay data indicate that alpha-inhibin-52 and alpha-inhibin-92 have only 60% immunoreactivity.

Isolation of inhibin from bovine follicular fluid

Bovine follicular fluid was used as a source for the isolation of gonadal inhibin, the activity of which was monitored by the dose dependent suppression of the FSH content of cultured pituitary cells. The procedures presented result in over 3000-fold purification of the starting material and the purified inhibin has an apparent molecular weight of 56000. The purified inhibin can be dissociated under reducing conditions into two subunits with molecular weights of 44000 and 14000 daltons.

Variability in the apparent molecular weight of inhibin

An in vitro bioassay based on suppression of GnRH-stimulated FSH secretion by pituitary cells in culture was used to monitor inhibin activity after dialysis, gel filtration or polyacrylamide gel electrophoresis of protein preparations from a variety of gonadal secretions and extracts under native and dissociating conditions. The suggestion that inhibin is a peptide of molecular weight less than 5000 was not confirmed. Although some fractions of low molecular weight suppressed FSH secretion, the amount of activity was low and the dose response curves were not parallel with a standard preparation of inhibin. Under most conditions, inhibin eluted with an apparent molecular weight of about 90 000. However, gel filtration of rete testis fluid protein in 1 M acetic acid resulted in elution of inhibin activity with a lower apparent molecular weight and with polyacrylamide gel electrophoresis in 0.1% (w/v) sodium dodecylsulfate, the apparent molecular weight was 30 000. It is concluded that inhibin is a protein which tends to aggregate and coelute with larger molecules.

Use of fast protein liquid chromatography in the purification of inhibin from bovine follicular fluid

Inhibin from bovine follicular fluid was partly purified using affinity chromatography on immobilized Procion Red 3B, gel filtration on Sephadex G-25 and ion-exchange chromatography on the fast protein liquid chromatography system. Inhibin was subsequently characterized using preparative sodium dodecyl sulfate polyacrylamide gel electrophoresis and electroelution. Biological activity was associated with a protein with an apparent molecular weight of approximately 65 kD.

Complete amino acid sequence of human seminal plasma beta-inhibin. Prediction of post Gln-Arg cleavage as a maturation site

The complete sequence of a 94 amino acid human seminal plasma polypeptide exhibiting inhibin-like activity is presented. This molecule, called beta-inhibin, selectively and specifically suppresses the release of pituitary FSH in vivo as well as in vitro. It does not affect the secretion of LH. Such a novel acidic protein contains a very basic C-terminal segment which is easily cleaved by mild tryptic digestion. It is predicted that the FSH inhibiting activity may reside within this region of the molecule. This would imply a post Gln-Arg cleavage to release the basic C-terminal active moiety.

[An inhibin-like factor in the testes of fetal and newborn rats]

A factor of protein nature that inhibits the proliferation of gonocytes and, probably, their ingress into meiosis and stimulates the development of some components of the blood-testicular barrier was isolated from the testis of 19-20-day-old fetuses and young rats of the first two weeks of life. It is assumed that the inhibin-like factor is formed in the testicular network. The time at which the receptors for the factor under discussion may occur in the cells of the testis of Wistar rat fetuses is discussed.

Isolation, structure, and synthesis of a human seminal plasma peptide with inhibin-like activity

A basic peptide isolated from pooled human seminal plasma exhibited inhibin-like activity by suppressing pituitary follicle-stimulating hormone secretion in vitro and in vivo. The peptide has been characterized and sequenced, and a 31-amino-acid synthetic replicate showed full biological activity in vitro.