Sex hormone-binding globulin/androgen-binding protein: steroid-binding and dimerization domains

An antibody-free LC-MS/MS method for the quantification of sex hormone binding globulin in human serum and plasma

OBJECTIVES

Sex hormone binding globulin (SHBG) is a hormone binding protein which plays an important role in regulating the transport and availability of biologically active androgens and estradiol to target cells and used to calculate free testosterone concentrations.

METHODS

A liquid chromatography-tandem mass spectrometry (LC-MS/MS) method was developed, featuring an albumin removal step followed by a tryptic digestion. After a reduction step with dithiothreitol and alkylation with iodoacetamide three signature peptides were used for the quantification of SHBG.

RESULTS

The method enables the quantification of serum and plasma SHBG over the clinically relevant range of 200-20,000 ng/mL and was validated according to the most recent guidelines. The LC-MS/MS method correlates well with the Abbott Alinity immunoassay (R2>0.95), but the LC-MS/MS results are on average 16-17% lower than the immunoassay results, which is consistent for all three signature peptides.

CONCLUSIONS

The LC-MS/MS method which includes an albumin depletion step allows quantification of SHBG in serum and plasma without an immunocapture step at clinically relevant SHBG levels, thus contributing to better lab-to-lab consistency of results.

Trends of SHBG and ABP levels in male farmers: Influences of environmental fluoride exposure and ESR alpha gene polymorphisms

A number of epidemiological studies have reported that chronic exposure to high concentrations of fluoride not only causes dental and skeletal fluorosis but additionally affects serum levels of reproductive hormones. However, possible interaction between fluoride exposure and estrogen receptor alpha (ESRα) gene polymorphisms on sex hormone-binding globulin (SHBG) and androgen binding protein (ABP) of male farmers has not been detailed. Here, we conducted a cross-sectional study including 348 male farmers with different fluoride exposure levels from drinking water in Henan province of China to explore effects of fluoride exposure and ESRα genetic variation on serum SHBG and ABP levels. We found serum SHBG levels in male farmers from the high exposure group to be lower than those of the low exposure group. We also found that concentrations of SHBG affected ABP levels. Furthermore, fluoride exposure and single nucleotide polymorphisms at the XbaI and rs3798577 loci of the ESRα gene affected serum ABP levels. Our findings suggest that chronic fluoride exposure from drinking water is associated with alterations of serum SHBG and ABP concentrations in local male farmers and that the effect of fluoride exposure on ABP levels vary depending on ESRα gene polymorphisms.

Estrogen receptors and sex hormone binding globulin in neuronal cells and tissue

Estrogens exert a critical influence on neuronal tissues and cells. As demonstrated in many clinical studies, estrogens are neuroprotective to the extent that they improve prognosis for women with neurodegenerative diseases. Unfortunately, we still do not know exactly how these effects are mediated. Fifty years ago the first estrogen receptor was found, but since then many other new pathways of estrogen action have been identified. This review describes several of these pathways of estrogen effects and provides some conclusions and correlations about these as determined by recent studies with nerve growth factor differentiated rat pheochromocytoma cell line.

Sex hormone-binding globulin provides a novel entry pathway for estradiol and influences subsequent signaling in lymphocytes via membrane receptor

The complex effects of estradiol on non-reproductive tissues/cells, including lymphoid tissues and immunocytes, have increasingly been explored. However, the role of sex hormone binding globulin (SHBG) in the regulation of these genomic and non-genomic actions of estradiol is controversial. Moreover, the expression of SHBG and its internalization by potential receptors, as well as the influence of SHBG on estradiol uptake and signaling in lymphocytes has remained unexplored. Here, we found that human and mouse T cells expressed SHBG intrinsically. In addition, B lymphoid cell lines as well as both primary B and T lymphocytes bound and internalized external SHBG, and the amount of plasma membrane-bound SHBG decreased in B cells of pregnant compared to non-pregnant women. As potential mediators of this process, SHBG receptor candidates expressed by lymphocytes were identified in silico, including estrogen receptor (ER) alpha. Furthermore, cell surface-bound SHBG was detected in close proximity to membrane ERs while highly colocalizing with lipid rafts. The SHBG-membrane ER interaction was found functional since SHBG promoted estradiol uptake by lymphocytes and subsequently influenced Erk1/2 phosphorylation. In conclusion, the SHBG-SHBG receptor-membrane ER complex participates in the rapid estradiol signaling in lymphocytes, and this pathway may be altered in B cells in pregnant women.

Oxytocin and Steroid Actions

Biosynthesis and secretion of the hypothalamic nonapeptide oxytocin largely depends on steroid hormones. Estradiol, corticosterone, and vitamin D seem to be the most prominent actors. Due to their lipophilic nature, systemic steroids are thought to be capable of crossing the blood-brain barrier, thus mediating central functions including neuroendocrine and behavioral control. The actual mode of action of steroids in hypothalamic circuitry is still unknown: Most of the oxytocinergic perikarya lack nuclear steroid receptors but express proteins suspected to be membrane receptors for steroids. Oxytocin expressing neurons contain enzymes important for intrinsic steroid metabolism. Furthermore, they produce and probably liberate specific steroid-binding globulins. Rapid responses to steroid hormones may involve these binding proteins and membrane-associated receptors, rather than classic nuclear receptors and genomic pathways. Neuroendocrine regulation, reproductive behaviors, and stress response seem to depend on these mechanisms.

A Reappraisal of Testosterone's Binding in Circulation: Physiological and Clinical Implications

In the circulation, testosterone and other sex hormones are bound to binding proteins, which play an important role in regulating their transport, distribution, metabolism, and biological activity. According to the free hormone hypothesis, which has been debated extensively, only the unbound or free fraction is biologically active in target tissues. Consequently, accurate determination of the partitioning of testosterone between bound and free fractions is central to our understanding of how its delivery to the target tissues and biological activity are regulated and consequently to the diagnosis and treatment of androgen disorders in men and women. Here, we present a historical perspective on the evolution of our understanding of the binding of testosterone to circulating binding proteins. On the basis of an appraisal of the literature as well as experimental data, we show that the assumptions of stoichiometry, binding dynamics, and the affinity of the prevailing models of testosterone binding to sex hormone-binding globulin and human serum albumin are not supported by published experimental data and are most likely inaccurate. This review offers some guiding principles for the application of free testosterone measurements in the diagnosis and treatment of patients with androgen disorders. The growing number of testosterone prescriptions and widely recognized problems with the direct measurement as well as the computation of free testosterone concentrations render this critical review timely and clinically relevant.

Modulation of SHBG binding to testosterone and estradiol by sex and morbid obesity

OBJECTIVE

Sex hormone-binding globulin (SHBG) binds and transports testosterone and estradiol in plasma. The possibility that SHBG is a mixture of transporting proteins has been postulated. We analyzed in parallel the effects of obesity status on the levels and binding capacity of circulating SHBG and their relationship with testosterone and estradiol.

DESIGN

Anthropometric measures and plasma were obtained from apparently healthy young (i.e. 35 ± 7 years) premenopausal women (n = 32) and men (n = 30), with normal weight and obesity (BMI >30 kg/m²).

METHODS

SHBG protein (Western blot), as well as the plasma levels of testosterone, estradiol, cortisol and insulin (ELISA) were measured. Specific binding of estradiol and testosterone to plasma SHBG was analyzed using tritium-labeled hormones.

RESULTS

Significant differences in SHBG were observed within the obesity status and gender, with discordant patterns of change in testosterone and estradiol. In men, testosterone occupied most of the binding sites. Estrogen binding was much lower in all subjects. Lower SHBG of morbidly obese (BMI >40 kg/m²) subjects affected testosterone but not estradiol. The ratio of binding sites to SHBG protein levels was constant for testosterone, but not for estradiol. The influence of gender was maximal in morbid obesity, with men showing the highest binding/SHBG ratios.

CONCLUSIONS

The results reported here are compatible with SHBG being a mixture of at least two functionally different hormone-binding globulins, being affected by obesity and gender and showing different structure, affinities for testosterone and estradiol and also different immunoreactivity.

Analysis of Hormone-Protein Binding in Solution by Ultrafast Affinity Extraction: Interactions of Testosterone with Human Serum Albumin and Sex Hormone Binding Globulin

Ultrafast affinity extraction was used to study hormone-protein interactions in solution, using testosterone and its transport proteins human serum albumin (HSA) and sex hormone binding globulin (SHBG) as models. Both single column and two-dimensional systems based on HSA microcolumns were utilized to measure the free fraction of testosterone in hormone/protein mixtures at equilibrium or that were allowed to dissociate for various lengths of time. These data were used to determine the association equilibrium constants (Ka) or global affinities (nKa') and dissociation rate constants (kd) for testosterone with soluble HSA and SHBG. This method was also used to measure simultaneously the free fraction of testosterone and its equilibrium constants with both these proteins in physiological mixtures of these agents. The kd and Ka values obtained for HSA were 2.1-2.2 s(-1) and 3.2-3.5 × 10(4) M(-1) at pH 7.4 and 37 °C. The corresponding constants for SHBG were 0.053-0.058 s(-1) and 0.7-1.2 × 10(9) M(-1). All of these results gave good agreement with literature values, indicating that this approach could provide information on a wide range of rate constants and binding strengths for hormone-protein interactions in solution and at clinically relevant concentrations. The same method could be extended to alternative hormone-protein systems or other solutes and binding agents.

Sex hormone-binding globulins characterization and gonadal gene expression during sex differentiation in the rainbow trout, Oncorhynchus mykiss

Sex hormone-binding globulin (SHBG) binds androgens and estrogens in the blood of many vertebrates, including teleost fish. In mammals, SHBG is synthetized in the liver and secreted into the blood. In fish, shbga also exhibits a hepatic expression. In salmonids, in which the gene has been duplicated, the recently discovered shbgb gene exhibits a predominantly ovarian expression. The present work aimed at gaining new insight into shbgb gene structure and expression during gonadal sex differentiation, a steroid-sensitive process, and Shbgb protein structure and binding characteristics; specifically, rainbow trout (Oncorhynchus mykiss) shbgb was analyzed. shbgb structure was analyzed in silico while expression was characterized during gonadal sex differentiation using all-male and all-female populations. We observed that shbgb gene and cognate-protein structures are similar to homologs previously described in zebrafish and mammals. The shbgb gene is predominantly expressed in differentiating female gonads, with increased expression around the end of ovarian differentiation. In the ovary, shbgb mRNA was detected in a subset of somatic cells surrounding the ovarian lamellae. Furthermore, Shbgb binds steroids with a higher selectivity than Shbga, exhibiting a higher affinity for estradiol compared to Shbga. In conclusion, Shbgb binding characteristics are clearly different from those of Shbga. Shbgb is expressed in the differentiating ovary during a period when the synthesis and action of testosterone and estradiol must be tightly regulated. This strongly suggests that Shbgb participates in the regulation of steroid metabolism and/or mediation, that is, needed during early gonadal development in rainbow trout.

The ovarian response to standard gonadotrophin stimulation depends on FSHR, SHBG and CYP19 gene synergism

PURPOSE

Follicle stimulating hormone, sex hormone-binding globulin and cytochrome P450 aromatase play crucial roles in the regulation of mammalian reproduction. The synergistic effect of FSHR 307(T/A)/FSHR 680(N/S), SHBG(TAAAA) ( n ) and CYP19(TTTA) ( n ) genotypes on ovarian response to standard gonadotrophin stimulation of women undergoing medically assisted reproduction (IVF/ICSI) was explored.

METHODS

The study population consisted of 300 women under IVF/ICSI treatment and 300 women with at least with at least one successful child birth as controls. The polymorphisms were genotyped while the follicular size, the follicle and oocyte numbers were recorded during oocyte retrieval.

RESULTS

The genotype analysis, excluding heterozygotes for each particular polymorphism, revealed eight combined homozygotic FSHR/SHBG/CYP19 genotypes. A gradual reduction in the number of follicles and oocytes from FSHR 307Thr/680Asn allele/long SHBG allele/long CYP19 allele homozygotes to FSHR 307Ala/680Ser allele/short SHBG allele/short CYP19 allele homozygotes was observed (20.36 ± 6.74 vs. 8.05 ± 2.47, p < 0.008 and 13 ± 4.63 vs. 6.1 ± 2.32, p < 0.02, respectively).

CONCLUSIONS

FSHR/SHBG/CYP19 combined genotypes are associated with ovarian response to standard gonadotrophin stimulation of women undergoing medically assisted reproduction.

Selective cleavage of human sex hormone-binding globulin by kallikrein-related peptidases and effects on androgen action in LNCaP prostate cancer cells

Stimulation of the androgen receptor via bioavailable androgens, including testosterone and testosterone metabolites, is a key driver of prostate development and the early stages of prostate cancer. Androgens are hydrophobic and as such require carrier proteins, including sex hormone-binding globulin (SHBG), to enable efficient distribution from sites of biosynthesis to target tissues. The similarly hydrophobic corticosteroids also require a carrier protein whose affinity for steroid is modulated by proteolysis. However, proteolytic mechanisms regulating the SHBG/androgen complex have not been reported. Here, we show that the cancer-associated serine proteases, kallikrein-related peptidase (KLK)4 and KLK14, bind strongly to SHBG in glutathione S-transferase interaction analyses. Further, we demonstrate that active KLK4 and KLK14 cleave human SHBG at unique sites and in an androgen-dependent manner. KLK4 separated androgen-free SHBG into its two laminin G-like (LG) domains that were subsequently proteolytically stable even after prolonged digestion, whereas a catalytically equivalent amount of KLK14 reduced SHBG to small peptide fragments over the same period. Conversely, proteolysis of 5α-dihydrotestosterone (DHT)-bound SHBG was similar for both KLKs and left the steroid binding LG4 domain intact. Characterization of this proteolysis fragment by [(3)H]-labeled DHT binding assays revealed that it retained identical affinity for androgen compared with full-length SHBG (dissociation constant = 1.92 nM). Consistent with this, both full-length SHBG and SHBG-LG4 significantly increased DHT-mediated transcriptional activity of the androgen receptor compared with DHT delivered without carrier protein. Collectively, these data provide the first evidence that SHBG is a target for proteolysis and demonstrate that a stable fragment derived from proteolysis of steroid-bound SHBG retains binding function in vitro.

Association of serum and follicular fluid SHBG levels and SHBG (TAAAA)n polymorphism with follicle size in women undergoing ovarian stimulation

OBJECTIVE

Sex hormone-binding globulin (SHBG) is the main transport protein of sex steroids. Recently, it has been found to be produced by granulosa lutein cells, suggesting a local role of SHBG in the ovary. The aim of this study was to investigate whether serum and follicular fluid SHBG levels and SHBG (TAAAA)(n) polymorphism are related to follicle size and pregnancy rate in women undergoing in vitro fertilisation.

METHODS

The study population consisted of 154 women with tubal and/or male-factor infertility undergoing IVF/ICSI and follicular fluid with oocytes from small (diameter ≤12 mm) and large (diameter ≥18 mm) follicles were studied. Genotyping of SHBG (TAAAA)(n) polymorphism was performed in peripheral blood samples. Serum and follicular fluids were used for hormones determination.

RESULTS

Women with short allele genotypes (with less than 8 TAAAA repeats) had higher number of small follicles compared to women with long allele genotypes (5.6 ± 3.9 vs. 3.5 ± 3.2 small follicles, p < 0.003). Follicular fluid SHBG levels correlated positively with serum SHBG levels (p < 0.001) and with the total number of follicles (p < 0.02). Furthermore, small follicles had higher follicular fluid SHBG concentration compared to large follicles (102.9 ± 35.0 nmol/l vs. 85.85 ± 34.88 nmol/l, p < 0.028).

CONCLUSION

SHBG levels and the SHBG (TAAAA)(n) polymorphism are associated with follicle size.

Genetic variants of sex hormone-binding globulin and their biological consequences

Several hormonal and metabolic factors have been found to influence the production of sex hormone-binding globulin (SHBG). In addition, twin studies have suggested that genetic factors may also contribute to variation in SHBG levels. Given the clinical significance of SHBG in regulating bioavailable sex steroid hormones, a number of studies examined the potential association between polymorphisms of SHBG gene and serum SHBG levels as well as their possible contribution in the pathogenesis of common diseases. Thus, polymorphisms of SHBG, altering either the production or the metabolism of the protein, may represent part of the genetic background of sex steroid hormone activity in humans. There is considerable heterogeneity in the results of these studies indicating the multiplicity of the factors influencing SHBG variation. However, the weight of evidence suggests that some common genetic variants of SHBG may influence SHBG levels and in part contribute to the phenotypic expression of human diseases.

Structural analyses of sex hormone-binding globulin reveal novel ligands and function

Plasma sex hormone-binding globulin (SHBG) regulates the access of androgens and estrogens to their target tissues and cell types. An SHBG homologue, known as the androgen-binding protein, is expressed in Sertoli cells of many mammalians, but testicular expression of human SHBG is restricted to germ cells. The primary structure of SHBG comprises tandem laminin G-like (LG) domains. The amino-terminal LG-domain includes the steroid-binding site and dimerization interface, and its tertiary structure, resolved in complex with natural and synthetic sex steroids, has revealed unanticipated mechanisms of steroid binding at the atomic level. This LG-domain interacts with fibulin-1D and fibulin-2 in a ligand-specific manner, and this is attributed to the unique way estrogens reside within the steroid-binding site, and the ordering of an otherwise flexible loop structure covering the entrance of the steroid-binding pocket. This mechanism enables estradiol to enhance the sequestration of plasma SHBG by the stroma of some tissues through binding to these extra-cellular matrix-associated proteins. The human SHBG amino-terminal LG-domain also contains several cation-binding sites, and occupancy of a zinc-binding site influences its affinity for estradiol. The complete quaternary structure of SHBG remains unresolved but structural predictions suggest that the carboxy-terminal LG-domains extend laterally from the dimerized amino-terminal LG-domains. The carboxy-terminal LG-domain contains two N-glycosylation sites, but their biological significance remains obscure. Knowledge of the SHBG tertiary structure has helped develop computational techniques based on the use of a "bench-mark data set" of steroid ligands, and created novel drug discovery and toxicology risk assessment platforms.

Cellular uptake of steroid carrier proteins--mechanisms and implications

Steroid hormones are believed to enter cells solely by free diffusion through the plasma membrane. However, recent studies suggest the existence of cellular uptake pathways for carrier-bound steroids. Similar to the clearance of cholesterol via lipoproteins, these pathways involve the recognition of carrier proteins by endocytic receptors on the surface of target cells, followed by internalization and cellular delivery of the bound sterols. Here, we discuss the emerging concept that steroid hormones can selectively enter steroidogenic tissues by receptor-mediated endocytosis, and we discuss the implications of these uptake pathways for steroid hormone metabolism and action in vivo.

A novel spreadsheet method for calculating the free serum concentrations of testosterone, dihydrotestosterone, estradiol, estrone and cortisol: with illustrative examples from male and female populations

In humans, testosterone (T), dihydrotestosterone (DHT), estradiol (E2), estrone (E1) and cortisol (C) bind to the serum proteins sex hormone-binding globulin (SHBG), albumin (Alb) and corticosteroid-binding globulin (CBG). Equilibrium dialysis is considered to be the "gold standard" for measuring the free concentrations of these steroids but is technically difficult and not widely available. Based on a mathematical model of the 5-ligand/3-protein binding equilibria, we developed a novel spreadsheet method for calculating the free and bioavailable (free+Alb-bound) concentrations of each steroid in terms of the total steroid and protein concentrations. The model uses 15 association constants K(SHBG-X), K(Alb-X), and K(CBG-X) (X=T, DHT, E2, E1 and C) that have been estimated from a systematic review of published binding studies. The computation of the free and bioavailable concentrations uses an iterative numerical method that can be readily programmed on a spreadsheet. The method is illustrated with six examples corresponding to young men (YM), old men (OM), obese men (Ob M), young women (YM), pregnant women in the 3rd trimester (Preg T3) and oophorectomized women on oral conjugated equine estrogens (CEE). The resulting free hormone concentrations for YM and YW fall within the normal references ranges obtained by equilibrium dialysis for all five hormones. The model also accounts for the competitive binding effects of high estrogen levels on the free T levels in Preg T3. This novel spreadsheet method provides a "user-friendly" approach for estimating the free concentrations of circulating sex hormones and cortisol in men and women.

Development of an indirect enzyme immunoassay using monoclonal antibodies for the measurement of 17alpha-hydroxyprogesterone in human serum

An indirect enzyme immunoassay for the measurement of total 17alpha-hydroxyprogesterone (17OHP) in serum using monoclonal antibodies generated in our laboratory was developed. Here, (a) instead of extraction with solvents, serum was heated to free protein-bound 17OHP and assay was performed at pH 9.6, (b) to ensure uniform assay conditions for both standards and samples, buffer for standards contained charcoal-stripped pre-heated pooled cord serum. Assays were done in 96-well EIA microplates pre-coated with 17alpha-hydroxyprogesterone-3-(o-carboxymethyl)oxime: bovine serum albumin. Secondary antibody was horseradish peroxidase-linked sheep anti-mouse IgG polyclonal antibody. The method was accurate and suitable for screening for congenital adrenal hyperplasia.

Steroid conjugates of dichloro(6-aminomethylnicotinate)platinum(II): effects on DNA, sex hormone binding globulin, the estrogen receptor, and various breast cancer cell lines

Esters of 6-aminomethylnicotinic acid with various steroidal alcohols were treated with K(2)PtCl(4) to give the N,N-chelated dichloroplatinum(II) complex conjugates 4. Their interaction with plasmid DNA was monitored by electrophoretic mobility measurements. Their affinities towards sex hormone binding globulin (SHBG) and towards the nuclear estrogen receptor ER(alpha) were assessed by competitive displacement radioassays. The inhibitory effect of 4 on breast tumour cells MCF-7 ER(+)/ER(-) and MDA-MB-231 was investigated in vitro. Conjugates with 3-O-linked estrogens 4 a,b or 17-O-linked androgens 4 g bound strongly to SHBG. The conjugate complex 4 b, featuring a 3-O-linked estradiol, also bound strongly and agonistically to the estrogen receptor. It also elicited distinct growth retardation of MCF-7 (ER(+)) cells, presumably by a mechanism different from that of cisplatin.

Scavenger receptor of class B expressed by osteoblastic cells are implicated in the uptake of cholesteryl ester and estradiol from LDL and HDL3

Lipoproteins transport many vitamins and hormones that have been shown to be necessary for bone formation. However, the metabolism of LDL and HDL3 by bone-forming osteoblastic cells remains unknown. Here we report that osteoblastic cells express scavenger receptors of class B that are implicated in the uptake of cholesterol and estradiol from LDL and HDL3.

INTRODUCTION

The bone tissue is continuously remodeled, and its integrity requires a balance between osteoclastic bone resorption and osteoblastic bone formation. Recent studies have reported the importance of triglyceride-rich lipoproteins for the delivery of lipophilic vitamins necessary for normal bone metabolism. However, the ability of osteoblastic cells to process low- and high-density lipoproteins (LDL and HDL3) and the receptors involved remain unknown.

MATERIALS AND METHODS

Binding, competition, degradation, and selective uptake assays with LDL and HDL3 radiolabeled in their protein and lipid moieties or with [3H]estradiol were conducted on human osteoblasts (MG-63 cell line and primary cultures of human osteoblasts [hOB cells]) and on mouse osteoblasts (MC3T3-E1 cell line and primary cultures of murine osteoblasts [mOB cells]). The expression of scavenger receptors (SRs) by osteoblastic cells was determined by RT-PCR and Western immunoblotting, and cellular localization was assessed by sucrose gradient fractionation.

RESULTS

Osteoblastic cells were able to bind, internalize, and degrade HDL3 and LDL and are capable of selectively taking up cholesteryl esters (CEs) from these lipoproteins. Also, we provide evidence that osteoblastic cells express SR-BI, SR-BII, and CD36 (SR-Bs receptors) and that these receptors are localized in membrane lipid rafts or caveolin-rich membranes. The selective uptake of CE from LDL and HDL3 by osteoblastic cells was strongly inhibited by the known SR-B ligand oxidized LDL, indicating that SR-B receptors are responsible for the selective uptake. Finally, estradiol carried by LDL and HDL3 was selectively transferred to the osteoblastic cells also through SR-B receptors.

CONCLUSIONS

Overall, our results suggest a novel mechanism for the routing of cholesterol and estradiol to osteoblasts involving the metabolism of LDL and HDL3 by SR-B receptors.

Estrogen and the inner ear: megalin knockout mice suffer progressive hearing loss

Megalin, the largest member of the low-density lipoprotein receptor-related protein family, functions as an endocytic receptor for a variety of essential lipophilic metabolites, including the steroid hormone estrogen. In the cochlea, megalin is strongly expressed within the marginal cells of the stria vascularis, and previous studies demonstrated that beta-estrogen receptors are also expressed in megalin-expressing marginal cells. In the present study, we demonstrate that homozygous megalin mutant mice exhibit profound hearing loss at 3 months of age associated with features of presbycusis, enrichment of lipofuscin granules, and a reduced number of microvilli in marginal cells of the stria vascularis. FITC-labeled beta-estrogen is taken up into the strial marginal cells; however, in megalin-deficient mice the uptake of FITC-labeled beta-estrogen is reduced. This highlights beta-estrogen as a possible carrier-bound candidate ligand for megalin and supports the concept that estrogen may function via megalin within the inner ear. A crucial role of megalin in hearing should be considered and the megalin/estrogen interaction needs to be discussed in the context of early presbycusis in estrogen-deficient humans and mice.

Internalization of sex hormone-binding globulin into neurons and brain cells in vitro and in vivo

BACKGROUND

Sex hormone-binding globulin (SHBG) is a 94-kDa homodimer that binds steroids and is made in the hypothalamus. We have demonstrated that infusions of SHBG into the hypothalami of rats increase their female sexual receptivity except when SHBG is coupled to dihydrotestosterone (DHT) suggesting that SHBG has an active function in behavioral neuroendocrinology.

METHODS

This study examines the possibility that SHBG is internalized by neuronal and/or non-neuronal brain cells as one possible mode of action using in vitro and in vivo techniques.

RESULTS

First, analysis of the uptake of radiolabeled SHBG ((125)I-SHBG) found (125)I-SHBG uptake in HT22 hippocampal cells stably transfected with cDNA for ER beta (HT22-ER beta). The addition of DHT to (125)I-SHBG significantly inhibited (125)I-SHBG uptake in HT22-ER beta cells but not in HT22-ER alpha or HT22 wild-type cells. SHBG internalization was specific as it did not occur in either the human neuroblastoma cell line SK-N-SH or the glioma cell line C6. Second, SHBG was labeled with a fluor (Alexa-555), and infused into the lateral cerebroventricles of ovariectomized rats. Optimal SHBG uptake was seen 10 min after these infusions. SHBG uptake was seen in specific parts of the choroid plexus and periventricular cells as well as into cells in the paraventricular nucleus, the medial forebrain bundle, and the habenula.

CONCLUSIONS

These studies suggest that SHBG is internalized by brain cells, which may be affected by the presence of ER beta. The gonadal steroids have numerous effects in brain and the discovery that the steroid-binding protein SHBG is taken up into neurons and brain cells may demand a change in thinking about how steroids are delivered to brain cells to affect neurophysiology.

Evidence that fibulin family members contribute to the steroid-dependent extravascular sequestration of sex hormone-binding globulin

Sex hormone-binding globulin (SHBG) binds steroids in the blood but is also present in the extravascular compartments of some tissues. Mice expressing a human SHBG transgene in the liver have human SHBG in their blood. In these animals, human SHBG accumulates within the stromal matrix of the endometrium and epididymis. This is remarkable because these tissues do not express the transgene. Human SHBG administered intravenously to wild-type mice in the presence of estradiol is rapidly sequestered within the endometrial stroma, and this prompted us to search for SHBG interacting proteins. Yeast two-hybrid screens revealed that fibulin-1D and fibulin-2 interact with the amino-terminal laminin G domain of SHBG. These interactions were verified in GST-pull down assays in which human SHBG bound the carboxyl-terminal domains of fibulin-1D and fibulin-2 in a steroid-dependent manner, with estradiol being the most effective ligand, and were enhanced by reducing the N-glycosylation of human SHBG. Like human SHBG, fibulin-1 and fibulin-2 concentrate within the endometrial stroma. In addition, SHBG co-immunoprecipitates with these fibulins in a proestrus uterine extract. These matrix-associated proteins may therefore sequester plasma SHBG within uterine stroma where it can control sex-steroid access to target cells. Given the interplay between fibulins and numerous proteins within the basal lamina, interactions between SHBG and matrix proteins may exert novel biological effects.

Role of endocytosis in cellular uptake of sex steroids

Androgens and estrogens are transported bound to the sex hormone binding globulin (SHBG). SHBG is believed to keep sex steroids inactive and to control the amount of free hormones that enter cells by passive diffusion. Contrary to the free hormone hypothesis, we demonstrate that megalin, an endocytic receptor in reproductive tissues, acts as a pathway for cellular uptake of biologically active androgens and estrogens bound to SHBG. In line with this function, lack of receptor expression in megalin knockout mice results in impaired descent of the testes into the scrotum in males and blockade of vagina opening in females. Both processes are critically dependent on sex-steroid signaling, and similar defects are seen in animals treated with androgen- or estrogen-receptor antagonists. Thus, our findings uncover the existence of endocytic pathways for protein bound androgens and estrogens and their crucial role in development of the reproductive organs.

Molecular and functional characterization of sex hormone binding globulin in zebrafish

SHBG (sex hormone binding globulin) transports androgens and estrogens in the blood of vertebrates including fish. Orthologs of SHBG in fish are poorly defined, and we have now obtained a zebrafish SHBG cDNA and characterized the zebrafish SHBG gene and protein through molecular biological, biochemical, and informatics approaches. Amino-terminal analysis of zebrafish SHBG indicated that its deduced precursor sequence includes a 25-residue secretion polypeptide and exhibits 22-27% homology with mammalian SHBG sequences and 41% with a deduced fugufish SHBG sequence. The 356-residue mature zebrafish SHBG (39,243 Da) sequence comprises a tandem repeat of laminin G-like domains typical of SHBG sequences; contains three N-glycosylation sites; and exists as a 105,000 +/- 8700 Da homodimer. Zebrafish SHBG exhibits a high affinity and specificity for sex steroids. An RT-PCR indicated that SHBG mRNA first appears in zebrafish larva, and SHBG mRNA was localized within the liver and gut at this stage of development by whole-mount in situ hybridization. In adult fish, SHBG mRNA was found in liver, testis, and gut. In the liver, immunoreactive SHBG was present in hepatocytes and concentrated in intrahepatic bile duct cells, whereas in the testis it was confined to cells surrounding the seminiferous tubule cysts. In the intestine, immunoreactive SHBG was present in the stroma and epithelial cells of the villous projections and the surrounding muscle. The production and presence of SHBG in the gut of developing and adult zebrafish suggests a novel role for this protein in regulating sex steroid action at this site.

Binding Analysis of 1α- and 17α-Dihydrotestosterone Derivatives to Homodimeric Sex Hormone-Binding Globulin

Binding studies of the interaction of immobilized 1alpha- and 17alpha-aminoalkyl derivatives of 5alpha-dihydrotestosterone (DHT) with purified N-deglycosylated homodimeric human sex hormone-binding globulin (SHBG) were performed using a surface plasmon resonance biosensor. These 1alpha- and 17alpha-derivatives with spacers of appropriate lengths between the amine function and the steroid ring skeleton enabled privileged, sterically undisturbed, interactions of either the 17- or 3-characteristic functional groups of DHT with SHBG. The association constants (K(a)1) for the binding of these immobilized DHT derivatives to the first binding site of SHBG, determined by SPR measurements, were 0.16 x 10(7) M(-1) for 17alpha-aminopropyl-17beta-hydroxy-5alpha-androstan-3-one (1), 1.64 x 10(7) M(-1) for 17alpha-aminocaproyl-17beta-hydroxy-5alpha-androstan-3-one (2), and 1.2 x 10(8) M(-1) for 1alpha-aminohexyl-17beta-hydroxy-5alpha-androstan-3-one (3). These values were compared with global K(a) data for the corresponding nonimmobilized DHT derivatives from equilibrium measurements using competitions with a tritiated testosterone tracer: the K(a) values were 1.25 x 10(7) M(-1) for 1, 1.50 x 10(7) M(-1) for 2, and 140 x 10(7) M(-1) for 3, confirming a remarkably high binding affinity of this latter compound for SHBG. A global fitting analysis of the biosensor data revealed that the interaction of the three immobilized steroids with SHBG was best described by a kinetic model assuming two structurally independent binding sites. This hypothesis of a bivalent binding model was also directly suggested by a dual fluorescent signal observed by the flow cytometry analysis of SHBG immobilized as a hybrid complex binding simultaneously two 1alpha-aminohexyl DHT ligands, one formed by 3, covalently coupled to phycoerythrin-labeled latex microspheres, and the other by the same DHT derivative, coupled to a fluorescein derivative (4).

Syntheses and ligand-binding studies of 1 alpha- and 17 alpha-aminoalkyl dihydrotestosterone derivatives to human sex hormone-binding globulin

We report on the syntheses of 1 alpha- and 17 alpha-aminoalkyl dihydrotestosterone (DHT) derivatives and the particularly high binding affinity of the 1 alpha-aminohexyl ligand for human sex hormone-binding globulin (SHBG). The two 17 alpha-aminopropyl-17 beta-hydroxy-5 alpha-androstan-3-one (1) and 17 alpha-aminocaproylamidoethyl-17 beta-hydroxy-5 alpha-androstan-3-one (2) derivatives were synthesized via a 17beta-spirooxirane intermediate in high yields. The 1 alpha-aminohexyl-17 beta-hydroxy-5 alpha-androstan-3-one compound (3) was obtained in a seven step synthesis using a copper-catalyzed conjugate addition of a omega-silyloxyhexyl Grignard reagent to 17 beta-benzoyloxy-5 alpha-androst-1-en-3-one. All structures were elucidated based on 1H NMR spectroscopy and mass spectral analyses. The three aminosteroid derivatives were tested as ligands for SHBG by competition experiments with tritiated testosterone as tracer under equilibrium conditions. The association constants of the two 17 alpha-DHT derivatives were approximately 1 x 10(7) M(-1), whereas the 1 alpha-DHT derivative showed a remarkably high binding affinity to SHBG with an association constant of 1.40 x 10(9) M(-1). These aminoalkyl derivatives, substituted either at the D-ring or the A-ring of the steroid skeleton, can be easily coupled onto a carboxymethylated solid state surface of a biosensor. Such a device lends itself to kinetic and thermodynamic studies aimed to provide a better understanding of the biospecific interaction of steroids with SHBG.

Dynamics of testicular germ cell apoptosis in normal mice and transgenic mice overexpressing rat androgen-binding protein

The number and type of testicular germ cells undergoing apoptosis in different age groups of mice (from 7 to 360 days of age) was determined and compared in age-matched wild type (WT) control and in a transgenic (TG) mice homozygous to rat androgen binding protein (ABP) using flow cytometry. Flow cytometric quantification revealed that the total number of germ cells undergoing apoptosis did not differ significantly in WT and TG mice up to Day 14. From Day 21 to Day 60, the number of germ cells undergoing apoptosis was consistently higher in TG than in WT mice. Starting from Day 90, the number of germ cells undergoing apoptosis in TG mice was lower than controls until Day 360. In 21-60 days old TG mice, spermatogonia, S-Phase cells, and primary spermatocytes are the cell types undergoing apoptosis at significantly greater numbers than those in WT mice. However, starting from day 60, the total number of spermatids undergoing apoptosis was significantly lower in TG mice than in age-matched WT controls. TdT-mediated dUTP-biotin nick end labeling (TUNEL) in testicular sections from TG mice of 21 and 30 days of age confirmed the presence of increased numbers of apoptotic germ cells compared to their age matched controls. These data indicate that the continuous presence of greater than physiological concentrations of ABP in the mouse testis has a biphasic effect on the frequency of apoptosis in germ cells. The initial pre-pubertal increase in testicular germ cell apoptosis may result from direct or indirect actions of ABP and is likely to determine the subsequent life-death balance of germ cell populations in TG mice, whereas the subsequent reduction may result from maturation depletion. A wave of apoptosis during the pre-pubertal period is required for normal spermatogenesis to develop, and our data indicate that this apoptotic wave may be regulated by ABP and/or androgens.

Characterization of sex hormone-binding globulin isoforms in hypothyroid women

Liver sex hormone-binding globulin (SHBG) biosynthesis is regulated by triiodothyronine (T3). This regulation is responsible for increased serum SHBG concentrations in hyperthyroid states. However, in hypothyroidism, normal SHBG levels are frequently found. To understand this we have characterized circulating SHBG isoforms according to their sialic acid content, which determines its half-life, in euthyroid and hypothyroid women. Six euthyroid (aged 56 +/- 8 years) and five hypothyroid women (51 +/- 13 years) were studied. Their body mass index (BMI) range was 20-25. Hypothyroidism diagnosis was based on clinical findings, elevated basal thyrotropin (TSH) and decreased T3 and thyroxine (T4) values. Total SHBG was measured by radioimmunoassay (RIA) and SHBG isoforms were isolated using preparative isoelectrofocusing. For comparisons, two-tailed t test was applied. No statistical difference was found between the total SHBG levels of hypothyroid and euthyroid postmenopausal women. Three groups of SHBG isoforms were isolated in the euthyroid group: S(I): pl: 5.0-5.2: 20% +/- 4%, S(II) : pl 5.2-5.4: 50% +/- 3% and S(III): pl 5.4-5.6: 29% +/- 4%. In hypothyroid patients, although the three groups of isoforms were isolated in the same pH range, S(I) and S(II) proportions were different (p < 0.001) when compared to normal women: S(I): 34% +/- 4%, S(II): 33% +/- 9.9% and S(III): 29% +/- 5.7%. These results show that hypothyroid patients have a higher proportion of more acidic SHBG isoforms. This variation may explain the normal levels of serum SHBG observed in hypothyroidism.

Efficient eukaryotic expression system for authentic human sex hormone-binding globulin

Sex hormone-binding globulin (SHBG) is the main carrier for androgens and oestrogens in humans. It mediates the transport of steroid hormones in the circulation and testicular fluid, and regulates their bioavailability to steroid-responsive tissues. In addition, the protein interacts with membrane receptors expressed in target tissues. Binding to the receptors is suspected to facilitate the uptake of steroid hormones and/or elicit cellular signal transduction. The identity of the SHBG receptor has not yet been resolved, in part due to a lack of sufficient quantities of authentic SHBG for receptor purification and molecular characterization. We have successfully addressed this problem by establishing an episomal expression system in human embryonic kidney cells that produces 5 mg of fully active human SHBG per litre. The recombinant protein resembles native SHBG in terms of structure, glycosylation pattern and steroid-binding activity. Moreover, the protein interacts with plasma membranes in steroid target tissues, an activity not observed with SHBG from other recombinant expression systems. Thus our studies have removed an important obstacle to the further elucidation of the role SHBG plays in steroid hormone action.

Partial characterization of a sex steroid-binding protein in plasma from arctic charr (Salvelinus alpinus L.)

A sex steroid-binding protein (SBP) that binds 17beta-estradiol with high affinity and moderate capacity was identified in the plasma from Arctic charr (Salvelinus alpinus L.) sampled during the early stage of gonadal maturation in June and prior to spawning in October. Maximum specific binding (B(max)) and equilibrium dissociation constant (K(d)) of males (B(max) = 2122 fmol E(2)/mg protein, K(d) = 1.9 nM), females (B(max) = 4115 fmol E(2)/mg protein, K(d) = 3.0 nM), and juveniles (B(max) = 4355 fmol E(2)/mg protein, K(d) = 1.8 nM) resembled binding characteristics of SBP from related species. The early-maturing females displayed both B(max) and K(d) values significantly higher than those of males (June samples). No significant differences in binding characteristics between fully matured males or females and immature juveniles were observed in the October samples. Interestingly, despite large individual variations there was a strong correlation between SBP levels and affinity. The association rate for 17beta-estradiol was rapid (t(1/2) approximately 1-2 min) compared with the dissociation rate (t(1/2) approximately 3 h). Several native hormones (estrogens, androgens, and progesterone) were able to compete with tritiated 17beta-estradiol for the binding site. Gel filtration chromatography demonstrated a peak of estradiol binding at approximately 60 kDa, when eluted on a Sephadex S-200 HR column.

Concepts for the syntheses of biotinylated steroids. Part I: testosterone derivatives as immunochemical probes

We describe synthetic strategies for the biotinylation of testosterone (T) at positions 3, 7alpha, 17alpha, and 19. These T probes are able to mimic ligand binding and may provide for a better understanding of the biospecific interaction with steroid-binding proteins such as the androgen receptor, anti-steroid antibodies, or steroid-binding serum globulins. For the 7alpha- and 17alpha-derivatives, biotinyl-N-hydroxy-succinimide esters with different types of spacer chains were used. The 3-biotin hydrazone derivative was produced using N-(epsilon-biotinyl)-caproyl hydrazide, whereas for the 19-biotinylation, a biotinyl-1-N-diamino-3, 6-dioxaoctane-amide was applied. Key reaction for the biotinylation at position 3 is the oximation of the 3-oxo function. The 17alpha-position is accessible by the reaction of the 3-protected 4-androsten-17-epoxide with oxygen in the beta-position, followed by nucleophilic ring opening with cyanide which provides the 17alpha-cyanomethyl derivative. The key step is the regioselective ketal protection of the 3-oxo function of androst-4-ene-3,17-dione using a stannoxane catalyst. An alternative pathway for the insertion of biotin at the 19-position was established by the synthesis of 17beta-hydroxy-androst-4-en-3-one-19-yl carboxymethyl ether. After activation by the carbodiimide method, the compound reacts with aminoterminal biotin derivatives. The copper(I)-catalyzed 1,6 Michael addition of 17-acetoxy-6,7-dehydro-T leads to 7alpha-derivatives by use of omega-silyl protected hydroxylalkyl-modified Grignard reagents. A functional group interconversion using the Staudinger reaction transforms the azide function into a primary omega-amino group. The absolute configurations of the different biotinylated derivatives were investigated by (1)H NMR studies. For the 7alpha-biotinylated T series, additionally, an X-ray analysis proved the axial position of the spacer group. This results in a vertical orientation of the biotin moiety toward the alpha-face of the planar tetracyclic backbone. Thus, a negligible alteration of the original structure of the upper beta-face offers the feasibility of applying the 7alpha-derivatives as optimal immunochemical tracers in competitive immunoassays. Biotinylated T derivatives should be also suitable for ligand-binding studies to the androgen receptor or to sex hormone-binding globulin.

Crystal structure of human sex hormone-binding globulin: steroid transport by a laminin G-like domain

Human sex hormone-binding globulin (SHBG) transports sex steroids in blood and regulates their access to target tissues. In biological fluids, SHBG exists as a homodimer and each monomer comprises two laminin G-like domains (G domains). The crystal structure of the N-terminal G domain of SHBG in complex with 5alpha-dihydrotestosterone at 1.55 A resolution reveals both the architecture of the steroid-binding site and the quaternary structure of the dimer. We also show that G domains have jellyroll topology and are structurally related to pentraxin. In each SHBG monomer, the steroid intercalates into a hydrophobic pocket within the beta-sheet sandwich. The steroid and a 20 A distant calcium ion are not located at the dimer interface. Instead, two separate steroid-binding pockets and calcium-binding sites exist per dimer. The structure displays intriguing disorder for loop segment Pro130-Arg135. In all other jellyroll proteins, this loop is well ordered. If modelled accordingly, it covers the steroid-binding site and could thereby regulate access of ligands to the binding pocket.

Influence of glycosylation on the clearance of recombinant human sex hormone-binding globulin from rabbit blood

Human sex hormone-binding globulin (hSHBG) is a plasma glycoprotein that binds sex steroids with high affinity. Variations in hSHBG glycosylation contribute to its electrophoretic microheterogeneity, but the functional significance of different SHBG glycoforms is unknown. Carbohydrates may influence the biological activities and half-lives of glycoproteins and we have examined how oligosaccharides at specific sites influence the plasma clearance of hSHBG in vivo. To accomplish this, fully-glycosylated hSHBG, or hSHBG mutants lacking specific oligosaccharides chains, were expressed in Chinese hamster ovary (CHO) cells and purified by immunoaffinity chromatography. The purified recombinant proteins were then biotinylated to study their plasma half-lives after intravenous injection into rabbits. When compared to hSHBG isolated from serum, recombinant hSHBG migrates with a slightly larger average molecular size during denaturing polyacrylamide gel electrophoresis. This is due to a greater proportion (33-39% vs. 3%) of more highly branched N-linked oligosaccharides on the recombinant proteins. When injected into rabbits, the disappearance of recombinant hSHBG showed two exponential components, as previously shown for natural hSHBG in the same animal model. The mean +/- S.E.M. plasma half-lives of recombinant hSHBG (t 1/2alpha 0.11+/-0.03 h and t 1/2beta 18.94+/-1.65 h) are shorter than previously measured for natural hSHBG (t 1/2alpha 3.43+/-0.72 h and t 1/2beta 38.18+/-7.22 h) and this is likely due to differences in the composition of their N-linked oligosaccharides. An O-linked chain at Thr7 does not influence the plasma clearance of hSHBG in the presence or absence of N-linked carbohydrates at Asn351 and Asn367. However, a 1.5-1.6 fold (p<0.03) increase in plasma half-life of variants lacking both N-glycosylation sites was observed and this is probably due to the fact these variants are not recognized by the asialoglycoprotein receptor-mediated clearance system. Removal of either N-glycosylation consensus site also increased (p<0.0001) the plasma half-life of hSHBG by 2.3 2.4 fold. Thus, the metabolic clearance of hSHBG appears to be determined by the number of N-linked oligosaccharides rather than their location.

Monoclonal antibodies to human sex hormone-binding globulin (SHBG): characterization and use in a simple enzyme-linked immunosorbent assay (ELISA) of SHBG in plasma

Four monoclonal antibodies to human sex hormone-binding globulin were raised and characterized. Three of the four antibodies recognised different antigenic determinants on SHBG. Two of the distinct antibodies were useful for Western blotting and recognized a major 48 kDa band in human plasma as well as a 46 kDa minor component. Carbohydrate residues do not form part of the antigenic determinants of these two antibodies, although one of these showed increased signal following removal of N-linked oligosaccharides. Some of the antibodies were selected to form a basis of a same-day, non-competitive, enzyme-linked immunosorbent assay (ELISA) for SHBG in plasma. The assay employs a purified IgG2a SHBG monoclonal antibody adsorbed to the wells of a microtitre plate. After blocking any further adsorption to the plate, standards or diluted patient samples were added for a 5-h incubation at room temperature, after which the plate was washed and antibody-bound SHBG was detected with an anti-SHBG IgG1 monoclonal antibody followed by peroxidase-labeled antimouse-IgG1 and o-phenylenediamine substrate. The assay correlated well with an existing 2-day ELISA for SHBG in plasma using polyclonal antibodies and also correlated with a dihydrosterone (DHT) ligand-binding assay. The monoclonal antibody-based ELISA shows excellent performance characteristics and is unaffected by added testosterone or estradiol.

Dihydrotestosterone: a rationale for its use as a non-aromatizable androgen replacement therapeutic agent

Testosterone therapy is commonly used to treat male hypogonadism, androgen deficiency of severe illness, androgen deficiency of ageing and microphallus in infancy. The effects of testosterone are mediated directly as testosterone or after conversion to either dihydrotestosterone (DHT) or oestradiol. DHT is a potent androgen and cannot be aromatized to oestrogens, therefore acting as a pure androgen. DHT has been proposed as an androgen replacement therapy, with possible advantages over testosterone in certain circumstances in the ageing population as well as in patients with gynaecomastia and microphallus. A potential advantage of DHT over testosterone as an androgen replacement therapy is the reported and seemingly paradoxically muted effects of DHT on prostate growth. The decreased effect of DHT compared with testosterone on the prostate gland of humans may be due to the decrease in intraprostatic oestradiol levels. The potential beneficial effect of less prostate growth after DHT requires substantiation and, if true, must be balanced against any negative effects that might occur on bone, lipids and sexuality when a pure androgen replaces treatment with an aromatizable androgen.

Lignans interfering with 5 alpha-dihydrotestosterone binding to human sex hormone-binding globulin

The natural lignans (-)-3,4-divanillyltetrahydrofuran (1), (-)-matairesinol (2), (-)-secoisolariciresinol (3), (+/-)-enterolactone (4), (+/-)-enterodiol (5), and nordihydroguaiaretic acid (NDGA) (6) reduce the binding of 3H-labeled 5 alpha-dihydrotestosterone (DHT) to human sex hormone-binding globulin (SHBG). (-)-3,4-Divanillyltetrahydrofuran (1) has the highest binding affinity (Ka = 3.2 +/- 1.7 x 10(6)M-1) of all lignans investigated so far; the reversibility of its binding and a double reciprocal plot suggest a competitive inhibition of the SHBG-DHT interaction. Increasing hydrophobity in the aliphatic part of the lignans (butane-1,4-diol-butanolide-tetrahydrofuran structures) leads to higher binding affinity. In the aromatic part, a 3-methoxy-4-hydroxy substitution pattern is most effective for binding to SHBG.