Human testicular androgen-binding protein shares immunodeterminants with serum testosterone-estradiol-binding globulin

Concanavalin-A induces granulosa cell death and inhibits FSH-mediated follicular growth and ovarian maturation in female rats

Reproductive success stems from a finely regulated balance between follicular maturation and atresia, in which the role of carbohydrate structure is poorly understood. Here, we describe for the first time a fraction of purified recombinant human FSH that is capable of bringing about the cell death of granulosa cells and preventing follicular maturation in a rat model. Further analysis by mass spectrometry revealed the presence of the lectin Concanavalin-A (Con-A) within this fraction of recombinant FSH. Using both the fractionated FSH and Con-A, the observed cell death was predominantly located to the granulosa cells. Ex vivo culture of rat follicles demonstrated that follicle degeneration occurred and resulted in the release of a denuded and deteriorated oocyte. Moreover, in vivo experiments confirmed an increase in atresia and a corresponding reduction confined to follicle in early antral stage. As a mechanism of action, Con-A reduces ovarian proliferation, Von Willebrand staining, and angiogenesis. Based on the observation that Con-A may induce granulosa cell death followed by follicle death, our results further demonstrate that follicular carbohydrate moiety is changing under the influence of FSH, which may allow a carbohydrate-binding lectin to increase granulosa cell death. The physiological consequences of circulating lectin-like molecules remain to be determined. However, our results suggest a potential exploitation of carbohydrate binding in fertility and ovarian cancer treatment. This work may shed light on a key role of carbohydrates in the still obscure physiological process of follicular selection and atresia.

Effect of sialylation and complexity of FSH oligosaccharides on inhibin production by granulosa cells

Granulosa cell (GC) inhibin A and B production is regulated by FSH and gonadal factors. This gonadotrophin is released as a mixture of glycoforms, which induce different biological responses in vivo and in vitro. Our aim was to determine the effect of recombinant human FSH (rhFSH) glycosylation variants on inhibin A and B production by rat GCs. Preparative isoelectro focusing was used to isolate more acidic/sialylated (pH <4.00) and less acidic/sialylated (pH >5.00) rhFSH charge analogues. Concanavalin A was used to isolate unbound and firmly bound rhFSH glycoforms on the basis of their oligosaccharide complexity. GCs, obtained from oestrogen-primed immature rats, were cultured with either native rhFSH or its glycosylation variants. Inhibin A and B were determined using specific ELISAs. Results were expressed as mean±s.e.m. Under basal conditions, inhibin A was the predominant dimer produced (inhibin A: 673±55; inhibin B: 80±4 pg/ml). More acidic/sialylated charge analogues stimulated inhibin B production when compared to inhibin A at all doses studied; by contrast, less acidic/sialylated charge analogues stimulated inhibin A production and elicited no effect on inhibin B. Glycoforms bearing complex oligosaccharides showed a potent stimulatory effect on inhibin B when compared to inhibin A production (i.e. dose 1 ng/ml: 4.9±0.5 vs 0.9±0.1-fold stimulation, P<0.001). Glycoforms bearing hybrid-type oligosaccharides favoured inhibin A production (i.e. dose 4 ng/ml 2.9±0.1 vs 1.6±0.1-fold stimulation, P<0.05). These results show that the sialylation degree as well as the complexity of oligosaccharides present in the rhFSH molecule may be considered additional factors that differentially regulate dimeric inhibin production by rat GCs.

Carbohydrate complexity and proportion of serum FSH isoforms reflect pituitary-ovarian activity in perimenopausal women and depot medroxyprogesterone acetate users

BACKGROUND

FSH is synthesized and secreted in multiple glycosylation variants with different oligosaccharide structures; the endocrine milieu regulates the composition of FSH carbohydrate moiety.

OBJECTIVES

To characterize serum FSH isoforms according to their sialic acid content and oligosaccharide complexity in regularly menstruating women and in depot medroxyprogesterone acetate (DMPA) users during the menopausal transition. Subjects and methods Ten regularly menstruating perimenopausal women aged 45-52, with mid-follicular phase FSH levels < or =10 IU/l and 10 regularly menstruating women, aged 20-39, were included. Blood samples were collected on the ninth day of the menstrual cycle. Twenty DMPA users were divided into two groups (n = 10) according to age: DMPA(1), age range 20-39 and DMPA(2), age range 45-52. Blood samples were collected 90 +/- 5 days after the last injection of DMPA. Oestradiol (E(2)), inhibin B (Inh B), Pro-alphaC levels and the relative abundance of FSH isoforms on the basis of charge (preparative isoelectric focusing) and carbohydrate complexity (Concanavalin A chromatography) were determined.

RESULTS

Decreased Inh B and moderately elevated E(2) levels were observed in perimenopausal women associated with an increase in FSH sialylation and a decrease in its oligosaccharide complexity. DMPA induced changes in the hormonal profile and FSH molecular microheterogeneity; the secreted hormone was more heterogeneous and its oligosaccharides were less complex under this condition.

CONCLUSION

Serum FSH glycoforms with increased sialylation and decreased oligosaccharide complexity reflect the decline of the gonadal activity induced either by age or by the use of a DMPA as a contraceptive.

Carbohydrate complexity and proportions of serum FSH isoforms in the male: lectin-based studies

FSH is synthesized and secreted in multiple molecular forms with different oligosaccharide structures which are needed for full expression of biological activity. GnRH and sex steroids modulate oligosaccharide structure and composition. In the present study we have assessed the carbohydrate complexity and proportion of circulating FSH isoforms during puberty, aging and after androgen administration to pubertal anorchid boys. Preparative isoelectrofocusing and lectin chromatography were used to isolate FSH isoforms on the basis of charge and internal carbohydrate complexity. Differences in sialic acid content and a progressive increase of isoforms bearing highly branched oligosaccharides were found during puberty. Less acidic, more bioactive FSH isoforms, secreted at mid-puberty may modulate important maturational events in the Sertoli cell population. Androgen administration to pubertal anorchid boys favoured the secretion of this type of isoforms. In adult men, the predominance of FSH isoforms bearing complex type oligosaccharides remained unchanged until very advanced age. These results show that the predominance of FSH isoforms bearing fully processed oligosaccharides in circulation may contribute to the development and maintenance of seminiferous epithelium function in men.

Pituitary–ovarian axis during lactational amenorrhoea. II. Longitudinal assessment of serum FSH polymorphism before and after recovery of menstrual cycles

BACKGROUND

The association of normal serum levels of immunoassayable gonadotrophins with anovulation during lactational amenorrhoea (LA) has not been fully explained.

METHODS

Serum FSH polymorphism was analysed in 10 women during LA between days 60 and 70 post-partum and again, in the mid-follicular phase (MFP), after resuming menstrual cyclicity. FSH microheterogeneity was characterized according to charge, using preparative isoelectric focusing, and according to the inner structure of carbohydrate chains, using lectin chromatography.

RESULTS

A significantly higher proportion of FSH charge isoforms isolated below pH 4.10 and a lower proportion of FSH isoforms bearing highly branched oligosaccharides were observed during LA when compared to MFP. Further analysis with higher resolution showed that FSH charge isoforms, isolated in the lower pH range in LA, corresponded to FSH molecules bearing highly branched and biantennary oligosaccharides. FSH isoforms bearing hybrid-type oligosaccharides were only present during LA. The circulating FSH isoform mix was significantly less bioactive in LA than in MFP. LA is characterized by a more acidic mix of FSH isoforms, containing hormone bearing less processed oligosaccharides, with decreased biopotency in comparison with the follicular phase.

CONCLUSIONS

This FSH microheterogeneity may be one of the critical factors contributing to incomplete follicular development and anovulation during LA.

Human FSH isoforms: carbohydrate complexity as determinant of in-vitro bioactivity

Differences in sialic acid content of the hormone have been considered the main determinant of FSH polymorphism. The aim of the present study was to investigate the effect of variations in the oligosaccharide structure of the intrapituitary human FSH (hFSH) glycosylation variants on their intrinsic biological activity. FSH charge isoforms obtained after chromatofocusing were further separated by lectin affinity chromatography [Concanavalin A (ConA), Wheat germ agglutinin (WGA), Lentil lectin (LcH)]. Isolated isoforms were separately tested for in-vitro bioactivity in a rat Sertoli cell aromatization bioassay. Our results show that: (1) FSH microheterogeneity is due not only to variations in the sialic acid content of the hormone but also to differences in the internal structure of the carbohydrate chains, and (2) variations in the sialic acid content as well as differences in the complexity of the glycans determine the full biological expression of FSH glycosylation variants.

Sequence and functional relationships between androgen-binding protein/sex hormone-binding globulin and its homologs protein S, Gas6, laminin, and agrin

Androgen-binding protein/sex hormone-binding globulin (ABP/SHBG) is an extracellular binding protein that regulates the bioavailability of sex steroids. ABP/SHBG is closely related to the globular (G) domain of vitamin K-dependent protein S family of proteins and more distantly related to the G domains of several extracellular matrix proteins. ABP/SHBG appears to have evolved from the fusion of two ancestral G domains. Expanding evidence suggests that ABP/SHBG has other functions that are mediated through membrane binding, including signal transduction; however, the types of binding proteins (receptors) have not been identified. Sequence comparisons of ABP/SHBG with G domains of its homologs protein S, Gas6, laminin, and agrin have identified regions of ABP/SHBG that may bind receptors related to homolog receptors. These membrane receptors include beta-integrins, alpha-dystroglycan, and receptor tyrosine kinases. The G domains of laminin and related proteins have clearly evolved from a common ancestor to interact with specific receptors and binding proteins. It remains to be determined if ABP/SHBG followed this evolutionary pathway.

Structure, function, and regulation of androgen-binding protein/sex hormone-binding globulin

Despite over 20 years of research, the functions of ABP and SHBG remain elusive. The major reason for this lack of knowledge has been the unavailability of natural mutants with clinical defects for study. There is strong evidence that these binding proteins do act to modulate the gene regulatory actions of nuclear sex steroid receptors by controlling the availability of androgens and estrogens. In plasma, SHBG controls the metabolic clearance rate of sex steroids. In addition there is strong evidence that they have a much broader function. The identification of plasma membrane receptors in target tissues and the finding of homologous domains in several developmental proteins support other functions. Moreover, other experiments suggest the proteins may actually be hormones or growth factors. These findings are not compatible with a model that has the proteins only regulating free steroid hormone levels. Obviously, much more experimentation will be necessary to reveal the functions of ABP and SHBG. The recent discoveries have offered several clues to their functions and open new routes for study. These experiments, coupled with newly developed techniques, such as gene knockout by homologous recombination, make one optimistic that the functions of these unique proteins will be deciphered in the near future.

Changes of immunoreactivity in alpha 1-antitrypsin in patients with autoimmune diseases

Recent studies from this laboratory have shown that a monoclonal antibody prepared against a specific epitope on alpha 1-antitrypsin is a valuable diagnostic marker for autoimmune conditions. In the present study we have further characterized this monoclonal antibody and reassessed its diagnostic value in screening samples from patients with various autoimmune conditions. alpha 1-Antitrypsin was micropurified from patients with selected autoimmune conditions and from normal donors. The purified alpha 1-antitrypsin isolated from patients with autoimmune conditions and normal donors was deglycosylated using both a mixture of exoglycosidases and endoglycosidase F. The immunoreactivity of the native and deglycosylated alpha 1-antitrypsin was examined using both a monoclonal antibody and a polyclonal antibody in enzyme linked immunosorbent assay (ELISA) and radioimmunoassay (RIA), respectively. It was noted that alpha 1-antitrypsin isolated from patients with autoimmune diseases generated a displacement curve dissimilar to alpha 1-antitrypsin purified from normal donors or alpha 1-antitrypsin from patients with autoimmune diseases subjected to deglycosylation when these samples were examined by ELISA using the monoclonal antibody. However, when the polyclonal antibody was used for these studies, no difference was found between the native and deglycosylated alpha 1-antitrypsin suggesting that the monoclonal antibody recognized an epitope not detectable by the polyclonal antibody.(ABSTRACT TRUNCATED AT 250 WORDS)

The plasma sex steroid binding protein (SBP or SHBG). A critical review of recent developments on the structure, molecular biology and function

Significant developments have taken place within the past five years on the characterization, molecular biology and function of the plasma sex steroid-binding protein, SBP (or sex hormone binding globulin, SHBG). During the span of that time, amino acid sequences of two SBPs have been established, amino acid residues in the steroid-binding site have been identified, the structure of the human SBP gene has been deduced and evidence for the possible existence of a SBP membrane receptor has been presented. This review covers the salient aspects of these and other developments including a critical analysis of the various proposed models and interpretations with regards to the structure, evolution, molecular biology and function of SBP.

The presence of a hitherto undefined high-capacity androgen binding macromolecule in human ovarian cancer tissue

Sex hormone binding globulin (SHBG) is known to interfere in the quantitation of androgen receptors (AR) if dihydrotestosterone (DHT) is used. We used a monoclonal antibody to remove SHBG from cytosol. In cytosol of benign prostatic hyperplastic (BPH) tissue low capacity binding for DHT, but not for R1881, was found after removal of SHBG. AR were detected in 18 of 20 ovarian cancer cytosols. In the two AR-negative cases, non-saturable binding for DHT, testosterone and R1881 was observed. Incubation with anti-SHBG did not change this. An hitherto undefined androgen binding macromolecule(s), with high-capacity binding for natural and synthetic androgens, but not for estrogen and progesterone, seems to be present in these ovarian cancer tissues. The functionality of these androgen binding macromolecules in ovarian cancer is yet to be demonstrated.

Sertoli cell proteins in the human seminiferous tubule

It is well known that rat Sertoli cells in culture secrete both testis-specific proteins, such as inhibin and androgen binding protein (ABP), and proteins which are very similar, if not identical, to serum proteins, such as transferrin (TF), ceruloplasmin, and IGF-I. It is also well known that very few data have been reported about the secretory activity and the hormonal regulation of the Sertoli cell in man, mainly because of the difficulties associated with the isolation of pure cell populations from human tissue. Using histoimmunochemical techniques we tried to localize, with specific antisera, Sertoli cell proteins and, when possible, their receptors in the human testis. The results obtained with our Light Microscopy studies suggest that: (1) human Sertoli cells produce and/or store transferrin (TF), IGF-I, an albumin-like protein and ABP; (2) TF receptors are localized in spermatocytes and early spermatids and are absent in spermatogonia, in the cytoplasm of Sertoli cells and in differentiated spermatids; (3) IGF-I type I receptors are localized in the same germ cells and in the cytoplasm of Sertoli cells. The results obtained with our Electron Microscopy studies suggest that TF and IGF-I are internalized through a receptor mediated endocytosis mechanism both in Sertoli cells (basal compartment) and in germ cells (spermatocytes and early spermatids).

The extracellular sex steroid-binding proteins of testis and liver. Structure-function studies

Though the existence of extracellular sex steroid-binding proteins has been known for a number of years, we are still only on the threshold of understanding their biological role. Through efforts such as those described above, we are beginning to examine the structure of these macromolecules and correlating them with present known functions. As our understanding of the function of these proteins evolves, we will be further able to ascribe structural domains.

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)

Corticosteroid binding globulin, testosterone-estradiol binding globulin, and androgen binding protein belong to protein families distinct from steroid receptors

The cDNA nucleotide sequences and the deduced amino acid sequences of human corticosteroid binding globulin (hCBG), human testosterone-estradiol binding globulin (hTeBG), and rat androgen binding protein (rABP) were determined. Studies of the steroid binding sites suggest they are toward the carboxy-terminus in hTeBG and rABP and more central in hCBG. hCBG has remarkable sequence homology with members of a superfamily whose functions have diverged; these include thyroxine-binding protein, serine protease inhibitors, egg white proteins, and angiotensinogen. hTeBG and rABP have a 68% amino acid sequence identity. Hybridization studies suggest that hTeBG is probably even more closely related, if not identical, to hABP. The carboxy-terminal sequences of hTeBG and rABP are also similar to that of protein S, a vitamin-K-dependent clotting factor. There were no nucleotide or amino acid sequence homologies between hCBG, hTeBG, or rABP and other steroid binding proteins such as steroid receptors, albumin, alpha-fetoprotein, and vitamin D binding protein. We conclude that the "extracellular steroid binding proteins" and steroid receptors do not appear to have descended from a common ancestor.

The cDNA-deduced primary structure of human sex hormone-binding globulin and location of its steroid-binding domain

We have sequenced a cDNA for sex hormone-binding globulin (SHBG) isolated from a phage lambda gt11 human liver cDNA library. The library was screened with a radiolabeled rat androgen-binding protein (ABP) cDNA, and the abundance of SHBG cDNAs was 1 in 750,000 plaques examined. The largest human SHBG cDNA (1194 base-pairs) contained a reading frame for 381 amino acids. This comprised 8 amino acids of a signal peptide followed by 373 residues starting with the known NH2-terminal sequence of human SHBG, and ending with a termination codon. The predicted polypeptide Mr of SHBG is 40,509, and sites of attachment of one O-linked (residue 7) and two N-linked oligosaccharide (residues 351 and 367) chains were identified. Purified SHBG was photoaffinity-labeled with delta 6-[3H]testosterone and cleaved with trypsin. The labeled tryptic fragment was isolated by reverse-phase HPLC, and its NH2-terminal sequence was determined. The results suggest that a portion of the steroid-binding domain of SHBG is located between residue 296 and the 35 predominantly hydrophilic residues at the C-terminus of the protein.

Cellular distribution and hormonal regulation of h-SBP in human hepatoma cells

The cellular distribution of human Sex Steroid Binding Plasma Protein (h-SBP) was studied in human cells and tissues by indirect immunofluorescence. h-SBP was detected in the cytoplasm of hepatocytes, of prostate and epididymis epithelial cells and in endometrium. Sexual and non-sexual skin, intestine epithelium, striated muscle and some rodent organs were not labelled. The intracellular localization of h-SBP indicate that h-SBP could be taken up from the extracellular compartment or synthesized in situ in sex steroid target organs, where it may play a role in hormone uptake. The hormonal regulation of h-SBP secretion by a human hepatoma cell line, H5A, showed that tri-iodothyronine was more potent than estradiol or tamoxifen, which acted as estrogen agonist, in increasing secreted h-SBP and the combined effect of both thyroid and estrogen hormones resulted in an additive stimulation of h-SBP secretion. As shown by Northern blot analysis, oligonucleotides synthesized from the known sequence of h-SBP hybridized with a RNA of approximately 2 kb which was more represented in H5A cells than in normal human liver, and was increased 2-3 times after hormonal stimulation of the cells. The presence of a poly(A+)RNA coding for h-SBP in the human liver indicated the hepatic synthesis of this protein.

The heterogeneity of rat androgen binding protein (rABP) in the vascular compartment differs from that in the testicular tubular lumen. Further evidence for bidirectional secretion of rABP

Fractionation of testicular extracts and serum on a Concanavalin A-Sepharose column resolved two peaks of immunoreactive rat androgen binding protein. The rat androgen binding protein in the first peak, designated Form I, was present in the void volume; the other, designated Form II rat androgen binding protein, was bound by the column and specifically eluted by alpha-methylmannoside. In the course of studying the heterogeneity of rat androgen binding protein on Concanavalin A-Sepharose, it was observed that the distribution of the two forms of this protein was similar in the fluid obtained by micropuncture from the seminiferous tubule and the rete testis, that is, the ratios of Form I to Form II were 1:1 and 1:1.8, respectively. By contrast, Form I rat androgen binding protein in blood, interstitial fluid, and thoracic duct lymph of adult rats was reduced relative to Form II; the ratios of Form I:Form II in these fluids were 1:4.4, 1:3.1, and 1:4.6, respectively. since previous studies indicated that the reduced amount of Form I relative to Form II observed in the blood of adult rats was not the result of more rapid clearance of Form I, these results suggest that Form I rat androgen binding protein is preferentially secreted into the lumen of the seminiferous tubule rather than into the interstitial fluid and blood. We conclude that Sertoli cells in adult rats may partition rat androgen binding protein between the interstitial and luminal compartments of the testis based on the carbohydrate composition of this protein.