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

Determinants of sex hormone-binding globulin in normal postmenopausal women

OBJECTIVE

To examine the factors influencing the levels of sex hormone-binding globulin (SHBG) in normal postmenopausal women by assessing the relationship between SHBG and measured anthropometric, metabolic and hormonal variables.

DESIGN

Cross-sectional, observational study.

SUBJECTS AND METHODS

Seventy normal postmenopausal women aged 47-71 years (mean 58 years), participated in the study. Information was collected on medical, reproductive and smoking history, alcohol use, dietary intake and physical activity. Body composition measurements using dual-energy absorptiometry, and analyses of biochemical and hormonal indices were performed.

RESULTS

Bivariate correlation coefficients indicated that SHBG was inversely related to body weight (r = - 0.44), fat mass (r = - 0.35), and abdominal obesity (r = - 0.42). It was also inversely related to the glucose and insulin levels during an oral glucose tolerance test (- 0.24 < r < - 0.40), serum oestradiol (r = - 0.26), and physical activity (r = - 0.24). Multiple regression analysis indicated that significant independent correlates of SHBG concentration were fat mass, physical activity, alcohol intake, serum oestradiol, and insulin-like growth factor-1, all having a negative impact on SHBG.

CONCLUSIONS

From these observed associations, it is concluded that maintenance of body weight, moderate alcohol consumption, and physical activity will tend to reduce SHBG concentrations in postmenopausal women, thereby increasing the levels of free oestradiol. This mechanism could mediate the beneficial effects of these factors in preventing the development of osteoporosis and cardiovascular disease.

The sex steroid binding protein (SBP or SHBG) of human plasma: identification of Tyr-57 and Met-107 in the steroid binding site

Tyrosine-57 (Y57) and methionine-107 (M107) have been identified in the binding site of the sex steroid binding protein (SBP) (or sex hormone binding globulin) of human plasma by replacing the two amino acids with a number of residues of varying structure. Replacement of Y57 with phenylalanine resulted in a fourfold increase in the K(d) of 5 alpha-dihydrotestosterone but left the K(d) of 17 beta-estradiol unchanged. Except in two cases, no further loss in binding took place when replacing Y57 with other residues, suggesting that the phenolic group of Y57 may form a hydrogen bond with the ligand. Replacement of M107 with isoleucine increased the 5 alpha-dihydrotestosterone K(d) fourfold to a value equal to that of rabbit SBP, which contains isoleucine at the corresponding position; however, the K(d) of 17 beta-estradiol remained unchanged. Replacement of M107 with threonine resulted in a tenfold decrease in 5 alpha-dihydrotestosterone binding affinity, whereas replacement with leucine left the K(d) unchanged. These data indicate that substitutions on the beta-carbon of the amino acid side-chain at position 107 causes significant loss of binding affinity but, as in the case of Y57, the activity was not totally eliminated. We conclude that Y57 and M107 form part of a structural motif within the steroid binding site and specifically contribute binding energy to ring A of 5 alpha-dihydrotestosterone but not to ring A of 17 beta-estradiol. We also propose that the integrated contribution of several side chains may be required to optimize the ligand affinity of the steroid binding site. This proposal may fit a 'lock and key' model where little movement of the side chains occurs during binding as might be expected for a rigid structure like the steroid nucleus.

Steroid-binding specificity of human sex hormone-binding globulin is influenced by occupancy of a zinc-binding site

One calcium-binding site (site I) and a second poorly defined metal-binding site (site II) have been observed previously within the amino-terminal laminin G-like domain (G domain) of human sex hormone-binding globulin (SHBG). By soaking crystals of this structure in 2.5 mm ZnCl(2), site II and a new metal-binding site (site III) were found to bind Zn(2+). Site II is located close to the steroid-binding site, and Zn(2+) is coordinated by the side chains of His(83) and His(136) and the carboxylate group of Asp(65). In this site, Zn(2+) prevents Asp(65) from interacting with the steroid 17beta-hydroxy group and alters the conformations of His(83) and His(136), as well as a disordered region over the steroid-binding site. Site III is formed by the side chains of His(101) and the carboxylate group of Asp(117), and the distance between them (2.7 A) is increased to 3.7 A in the presence of Zn(2+). The affinity of SHBG for estradiol is reduced in the presence of 0. 1-1 mm Zn(2+), whereas its affinity for androgens is unchanged, and chemically-related metal ions (Cd(2+) and Hg(2+)) have similar but less pronounced effects. This is not observed when Zn(2+) coordination at site II is modified by substituting Gln for His(136). An alteration in the steroid-binding specificity of human SHBG by Zn(2+) occupancy of site II may be relevant in male reproductive tissues where zinc concentrations are very high.

Characterization of the oligosaccharides of plasma sex hormone binding globulin from noncirrhotic alcoholic patients

In previous reports we have demonstrated high plasma levels of sex hormone-binding globulin (SHBG) in asymptomatic alcoholic men. In the present work the physicochemical properties of SHBG from plasma of noncirrhotic alcoholic patients have been further compared with SHBG of control subjects. Steroid binding to SHBG was similar for the two groups: alcoholic men, K(d) of 0.62 +/- 0.07 nM and control individuals, K(d) of 0.70 +/- 0.10 nM. The structure of oligosaccharides attached to SHBG from controls and alcoholic men were determined by using serial chromatography. Our data indicated that 7% of SHBG of control individuals was not retarded by the Con-A column, whereas approximately 30% of SHBG of alcoholic men eluted in the void volume of Con A. Approximately 46% of SHBG of alcoholics applied to Con A, possessed biantennary complex oligosaccharides, as indicated by the fact that it could be eluted with methyl-alpha-D-glucopyranoside and by its retention on wheat germ agglutinin; in contrast, when SHBG from control men was analyzed, approximately 51% was eluted with methyl-alpha-D-glucopyranoside. Approximately 9% of the biantennary complex oligosaccharides on SHBG of control men and none of those on SHBG from alcoholic men were fucosylated on the chitobiose core, as determined by chromatography on Lenn culinaris lectin. Galactosylated oligosaccharides were also present on the SHBG fraction as indicated by its interaction with Ricinus communis-I. Approximately 24% of SHBG of alcoholic men and 39% of those on SHBG from control individuals applied to Con-A were retained and could be eluted with methyl-alpha-D-mannopyranoside. Evidence based on the binding on mannoside-eluted SHBG to Con-A, wheat germ agglutinin, and R. communis-I indicated that at least the SHBG in this fraction, from alcoholics or controls, contained two glycosylation sites and that the sites were differentially glycosylated.

The rabbit sex hormone-binding globulin gene: structural organization and characterization of its 5-flanking region

Sex hormone-binding globulin (SHBG) transports sex steroids in the blood. In humans and rabbits, the gene encoding SHBG (shbg) is expressed primarily in the liver and testis, whereas the testis is the major site of shbg expression in rodents postnatally. Sequence analysis has revealed that rabbit shbg (rbshbg) spans 2.5 kb and comprises eight exons with consensus splice sites at all exon-intron junctions. The major transcription start site ofrbshbg is located 52 bp upstream from the translation initiation codon for the rabbit SHBG precursor. Unlike the situation in humans and rats, rbshbg transcripts contain no alternative exon 1 sequences in the liver or testis, and this suggests that the rbshbg 5'-flanking region plays an equally important role in controlling transcription of this gene in these tissues. Like the human and rat shbg promoter sequences, the rbshbg proximal promoter lacks a typical TATA box. It also contains several transcription factor-binding sites, but deoxyribonuclease I footprinting experiments indicated that the human and rabbit shbg proximal promoters interact quite differently with proteins extracted from rabbit liver nuclei. However, the predominant footprint on the rbshbg promoter is conserved at the same position within the human shbg (hshbg) promoter and includes consensus binding sites for the transcription factor nuclear factor- 1. Transient transfection studies of the rbshbg 5'-flanking sequence (893 bp) revealed regions that actively enhance and repress its activity in human hepatoblastoma and mouse Sertoli cells, but not in Chinese hamster ovary cells. Like the rat shbg proximal promoter, the rbshbg 5'-flanking sequence lacks a region that corresponds to a cis-element, designated footprinted region 4 in the hshbg proximal promoter. Furthermore, the hshbg promoter footprinted region 3 sequence is poorly conserved in rbshbg, and when mutated to resemble the corresponding human sequence it increased the transcriptional activity of the rbshbg promoter by 7-fold in hepatoblastoma cells. Thus, the rabbit and hshbg promoters appear to be controlled by a different set of transcriptional regulators. Further comparisons of their functional activities may shed light on species-specific differences in the spatial and temporal expression of this gene, the products of which play important roles in regulating sex steroid access to target cells.

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.

The effect of short term confinement stress on binding characteristics of sex steroid binding protein (SBP) in female black bream (Acanthopagrus butcheri) and rainbow trout (Oncorhynchus mykiss)

The effect of short term confinement stress on sex steroid binding protein (SBP) binding characteristics was examined in female black bream (Acanthopagrus butcheri), and rainbow trout (Oncorhynchus mykiss). Black bream were sampled immediately after capture from the wild and again after 1, 6 or 24 h confinement. Rainbow trout were sampled before and after 5 h confinement. Confinement of black bream for 6 h after capture significantly reduced the binding capacity of SBP. Binding affinity also tended to be lower after confinement. There were no differences in binding affinity or capacity of black bream SBP after 1 or 24 h confinement, or rainbow trout SBP after 5 h confinement. Plasma from rainbow trout at 3 and 6 h after treatment with cortisol was compared to plasma from saline-injected controls. No significant differences in binding characteristics were detected, but there was a trend of decreased binding capacity in cortisol-injected fish compared to controls at 6 h post-injection. Relative binding studies indicated that plasma cortisol at concentrations 100x or more greater than plasma estradiol (E(2)) may displace E(2) from SBP in black bream, and act to reduce circulating levels of E(2) through increased clearance of free steroid. Physiological levels of cortisol did not displace E(2) from SBP in trout. The observed changes in SBP and the competition of physiological concentrations of cortisol for SBP binding sites may generate a component of the stress-induced falls in plasma levels of E(2) reported across a range of species.

Interactions of 16alpha-[18F]-fluoroestradiol (FES) with sex steroid binding protein (SBP)

Fluorine-18 16alpha-Fluoroestradiol ([18F]-FES) is a positron-emitting tracer for the estrogen receptor that is used for positron emission tomography (PET) studies of tumor tissues rich in the estrogen receptor. The role of the sex steroid binding protein (SBP or SHBG) in the transport of the [18F]-FES to the estrogen-receptor-rich tissue in breast cancer patients in vivo was investigated. To determine the extent to which [18F]-FES is bound to SBP in the blood, we performed a series of studies using blood samples obtained from patients undergoing [18F]-FES PET scans. The binding of [18F]-FES to the SBP was measured using a simple protein precipitation assay. The binding of [18F]-FES metabolites to SBP was also measured. These measurements showed that the tracer was distributed between albumin and SBP, and the binding capacity of SBP was sufficient to ensure that the protein was not saturated when the tracer was fully mixed with the plasma; however, local saturation of SBP may occur when [18F]-FES is administered intravenously. Typically about 45% of [18F]-FES in circulating plasma was bound to SBP, but this fraction was dependent on the concentration of SBP in plasma. The transfer of the tracer between the two proteins was rapid, complete in less than 20 s at 0 degrees C, suggesting that the equilibrium was maintained under most circumstances and that local saturation resolved quickly when blood from the injection site entered the central circulation. These data suggest that SBP binding of [18F]-FES is significant and will affect the input function of the tracer for any model that is used for the quantitative evaluation of [18F]-FES uptake in PET studies. Estimates of equilibrium binding in blood samples are sufficient to characterize [18F]-FES binding to SBP in the circulation.

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.

Expression and regulation of human sex hormone-binding globulin transgenes in mice during development

Human sex hormone-binding globulin (SHBG) is produced by hepatocytes and transports sex steroids in the blood. The rat gene encoding SHBG is expressed transiently in the liver during fetal life, but it is not expressed in the liver postnatally, and the small amounts of SHBG in rat blood are derived from gonadal sources. To study the biosynthesis and function of human SHBG in an in vivo context, we have produced several lines of transgenic mice that contain either 11 kb (shbg11) or 4.3 kb (shbg4) portions of the human shbg locus. The expression and regulation of these transgenes have now been studied during fetal and postnatal development. In situ hybridization of an shbg11 transgenic mouse fetus at 17.5 days postcoitus located human shbg transcripts only in duodenal epithelial cells and hepatocytes. Temporal differences in the hepatic expression of mouse shbg and human shbg transgenes during late fetal development were reflected in corresponding differences in mouse and human SHBG levels in fetal and neonatal mouse blood. Serum concentrations of human SHBG increased during the first weeks of life regardless of gender until about 20 days of age in shbg11 mice, but after this time they continued to increase only in the males. This sexual dimorphism was reflected in corresponding differences in human SHBG messenger RNA (mRNA) abundance in the livers of these animals. However, it was not observed in shbg4 mice, in which hepatic production of plasma SHBG continued to increase after puberty regardless of gender. Serum testosterone and SHBG levels correlated in all sexually mature shbg transgenic mice. Human shbg transcripts were detectable only in testes of shbg11 mice and increased progressively in abundance from 10 days of age until the animal reached sexual maturity at 30 days of age, with appreciable increases occurring well before any changes in serum testosterone concentration. In the kidney, SHBG mRNA levels accumulated earlier in shbg11 than in shbg4 mice, and the expression of both types of transgenes was sexually dimorphic, with much higher SHBG mRNA levels in the kidneys of male mice. As increases in SHBG mRNA in the male kidneys coincided with increases in serum testosterone during sexual maturation, we reasoned that shbg transgene expression is androgen dependent in the kidney. This was confirmed by demonstrating that a decrease in SHBG mRNA abundance in male mouse kidneys after castration could be reversed by 5alpha-dihydrotestosterone treatment. Moreover, exogenous androgen increased human SHBG mRNA levels in the kidneys of female mice. In summary, comparisons of how different human shbg transgenes are expressed in vivo provides information about the positions of potential regulatory sequences that may control the hormonal regulation and tissue-specific expression of this gene during development.

Estradiol increases the production of sex hormone-binding globulin but not insulin-like growth factor binding protein-1 in cultured human hepatoma cells

OBJECTIVE

To study the effects of E2 on insulin-like growth factor binding protein-1 (IGFBP-1) and sex hormone-binding globulin (SHBG) production with cultured human HepG2 hepatoma cells.

DESIGN

Experimental cell culture.

SETTING

Academic medical center.

PATIENT(S)

None.

INTERVENTION(S)

Addition of E2 to cell culture medium.

MAIN OUTCOME MEASURE(S)

Intracellular and released concentrations of IGFBP-1 and SHBG.

RESULT(S)

Estradiol did not affect the intracellular or extracellular IGFBP-1 concentration, whereas the intracellular SHBG concentration increased significantly in response to 0.5-2.5 microM of E2.

CONCLUSION(S)

Whereas the two binding proteins share a number of regulatory factors, their regulation by E2 is dissimilar in human hepatoma cells. Estradiol does not affect the intracellular or secreted IGFBP-1 concentration, but it does increase the production of SHBG.

Virilization of the male pouch young of the tammar wallaby does not appear to be mediated by plasma testosterone or dihydrotestosterone

Virilization of the male urogenital tract of all mammals, including marsupials, is mediated by androgenic hormones secreted by the testes. We have previously demonstrated profound sexual dimorphism in the concentrations of gonadal androgens in pouch young of the tammar wallaby Macropus eugenii during the interval when the urogenital sinus virilizes. To provide insight into the mechanisms by which androgens are transported from the testes to the target tissues, we measured testosterone and dihydrotestosterone in plasma pools from tammar pouch young from the day of birth to Day 150. Plasma testosterone levels were measurable (0.5-2 ng/ml) at all times studied, but there were no differences between males and females. These low concentrations of plasma testosterone appear to be derived from the adrenal glands and not the testes. Plasma dihydrotestosterone levels in plasma pools from these animals were also low and not sexually dimorphic. We conclude that virilization of the male urogenital tract cannot be explained by the usual transport of testosterone or dihydrotestosterone in plasma but may be mediated by the direct delivery of androgens to the urogenital tract via the Wolffian ducts. Alternatively, circulating prohormones may be converted to androgens in target tissues.

Xenoestrogen interaction with human sex hormone-binding globulin (hSHBG)

This study reports on some environmental chemicals with estrogenic activity (xenoestrogens) and their binding interaction for human plasma sex-hormone binding globulin (hSHBG). The binding affinity constant of these xenoestrogens was measured in equilibrium conditions by solid phase binding assay, and their ability to displace endogenous testosterone and estradiol from hSHBG binding sites was determined with an ammonium sulfate precipitation assay in native plasma from normal men and women. The data showed that some of these xenoestrogens bind hSHBG, with a reversible and competitive binding activity for both [3H]testosterone and [3H]17beta-estradiol and with no apparent decrease in the number of hSHBG binding sites. Their respective binding affinity constants were low, ranging from 0.02 to 7.8 10(5) 1 x mol(-1). However, in native plasma from normal men and women, they were able to dose-dependently increase concentrations of hSHBG-unbound testosterone and/or estradiol. In this study, 4-nonylphenol and 4-tertoctylphenol, two alkylphenols used as surfactants in many commercial products, and bisphenol A and O-hydroxybiphenyl, widely used in the plastics industry, were identified as potent hSHBG-ligands. Additionally, the flavonoid phytoestrogens genistein and naringenin were also identified as hSHBG ligands, whereas their glucoside derivatives, genistin and naringin, had no binding activity for hSHBG. From these data, it is suggested that hSHBG binding may transport some contaminant xenoestrogens into the plasma and modulate their bioavailability to cell tissues. On the other hand, xenoestrogens may also displace endogenous sex steroid hormones from hSHBG binding sites and disrupt the androgen-to-estrogen balance. Whether xenoestrogen SHBG ligands could reach high enough concentrations in the blood to expose humans to any such effect merits further investigation.

Gene expression in the epididymis

The epididymis is a tubular organ exhibiting vectorial functions of sperm concentration, maturation, transport, and storage. The molecular basis for these functions is poorly understood. However, it has become increasingly clear that regional differences along the length of the duct play a role in epididymal physiology and that region-specific gene expression is involved in the formation of these differences. Although not an overtly segmented organ, the epididymis consists of a series of highly coiled "zones," separated by connective tissue septulae and distinct by cell morphology and their pattern of gene expression. Thus, it constitutes an interesting mammalian model to study how pattern formation is achieved by differential gene activity. A large number of epididymis-expressed genes have been cloned and analyzed at the molecular level, most of them have been characterized by a distinct temporal and spatial expression pattern within the organ. Only recently have theories been developed about how and when during ontogenesis this pattern formation takes place and what its significance might be. This review summarizes the current knowledge on regionalized gene expression in the epididymis and presents hypotheses concerning its ontogenetic origin and regulation in the adult.

Serum prostate specific antigen, sex hormone binding globulin and free androgen index as markers of pubertal development in boys

OBJECTIVE

Prostate specific antigen (PSA) expression in the prostate gland is regulated by androgens. Serum levels of PSA are undetectable by routine assays in normal boys. Measurable values could serve as a marker for pubertal development. In order to explore this question, we measured serum PSA levels in normal boys throughout puberty and examined the interrelationships with various hormonal and physical developmental changes.

DESIGN

Sera from 77 normal boys in Tanner stages I to V (T-I to T-V) were analysed for PSA levels by a sensitive time-resolved fluoro-immunometric assay (sensitivity: 0.012 microgram/l). In addition, sex hormone binding globulin (SHBG), insulin like growth factor I (IGF-I), IGF binding protein 3(IGFBP-3) and testosterone were measured.

RESULTS

PSA was detectable in 0% of Stage T-I (n = 16), 33% of T-II (n = 18), 65% of T-III (n = 17) and 100% of T-IV (n = 10) and T-V (n = 16) boys. PSA levels rose significantly according to stage (P < 0.05). Also, there were significant (P < 0.05) increments in serum testosterone, IGF-I and IGFBP-3 levels from stages T-I to T-IV. PSA showed a positive correlation with testosterone (r = 0.86, P < 0.001), IGF-I (r = 0.66, P < 0.001), and IGFBP-3 (r = 0.34, P = 0.004) levels. Both PSA and these analytes, however, showed significant overlap between stages T-I and T-II with only 6/18 (33%) and 12/18 (66%) of T-II subjects having PSA and testosterone levels, respectively, above the T-I range. In contrast, serum SHBG levels decreased markedly from stages I to II (P < 0.001). At the calculated best cut-off point for SHBG of 50 nmol/l, 16/18 T-II subjects had values below the T-I range (sensitivity = 89%). Because of this decrement of SHBG and the increasing testosterone secretion in early puberty, the Free Androgen Index (FAI = Testosterone/SHBG) could even better differentiate the onset of puberty with all except one of the T-II subjects having FAI levels above the T-I range (sensitivity = 94.4%). The decrease of SHBG in T-II subjects coincided with an increase in total body weight (P = 0.001) and body mass index (BMI, P = 0.0003). Despite the continuing pubertal rise in testosterone, SHBG levels showed a rebound increment from T-II-T-III subjects (P = 0.02) with a concomitant decrease in BMI (P = 0.0014).

CONCLUSIONS

Prostate specific antigen closely reflects serum free androgen activity during puberty. However, it was unable to differentiate the earliest pubertal development. In comparison, SHBG levels and Free Androgen Index are more sensitive markers for the onset of puberty in boys. The inverse association between SHBG levels and BMI in pubertal stages Tanner stages, I to III suggests that body fatness, via its effect on insulin sensitivity, may play an important role in the regulation of SHBG production during early pubertal development.

Heterologous expression of wild type and deglycosylated human sex steroid-binding protein (SBP or SHBG) in the yeast, Pichia pastoris. Characterization of the recombinant proteins

Wild type, partially and fully-deglycosylated human sex steroid-binding protein (SBP or SHBG) cDNAs lacking the native cucaryotic signal sequence were cloned into a yeast expression vector containing the Saccharomyces cerevisiae alpha-factor for extracellular secretion. Following transformation into Pichia pastoris, the wild type and all constructed mutants were successfully expressed. The levels were lower for the deglycosylated mutants indicating that oligosaccharide side chains may play a role in SBP secretion. Under fermentation conditions, the wild type protein was expressed at a level of 4 mg/l while the fully-deglycosylated mutant T7A/N351Q/N367Q was expressed at about 1.5 mg/l. The latter was purified from several fermentation runs and was found to be completely deglycosylated, electrophoretically homogeneous and fully active. The aminoterminus was found to have the sequence NH2QSAHDPPAV- indicating that cleavage of the alpha-factor occurred at the A(+7)-Q(+8) peptide bond. The molecular mass of the subunit was determined to be 39,717.8 Da, which is in complete agreement with the amino acid sequence of the T7A/N351Q/N367/Q mutant. The equilibrium constants for the dissociation of 5alpha-dihydrotestosterone and steroid binding specificity were found to be identical to that of the human plasma protein indicating that the missing N-terminal segment NH2-LRPVLPT and the removal of oligosaccharide side chains do not affect the stability and active conformation of the protein. In conclusion, the data presented reveal that the SBP mutant T7A/N351Q/N367/Q is the protein of choice for solving the three-dimensional structure.

Osteopathy and resistance to vitamin D toxicity in mice null for vitamin D binding protein

A line of mice deficient in vitamin D binding protein (DBP) was generated by targeted mutagenesis to establish a model for analysis of DBP's biological functions in vitamin D metabolism and action. On vitamin D-replete diets, DBP-/- mice had low levels of total serum vitamin D metabolites but were otherwise normal. When maintained on vitamin D-deficient diets for a brief period, the DBP-/-, but not DBP+/+, mice developed secondary hyperparathyroidism and the accompanying bone changes associated with vitamin D deficiency. DBP markedly prolonged the serum half-life of 25(OH)D and less dramatically prolonged the half-life of vitamin D by slowing its hepatic uptake and increasing the efficiency of its conversion to 25(OH)D in the liver. After an overload of vitamin D, DBP-/- mice were unexpectedly less susceptible to hypercalcemia and its toxic effects. Peak steady-state mRNA levels of the vitamin D-dependent calbindin-D9K gene were induced by 1,25(OH)2D more rapidly in the DBP-/- mice. Thus, the role of DBP is to maintain stable serum stores of vitamin D metabolites and modulate the rates of its bioavailability, activation, and end-organ responsiveness. These properties may have evolved to stabilize and maintain serum levels of vitamin D in environments with variable vitamin D availability.

Hepatocyte nuclear factor-4 controls transcription from a TATA-less human sex hormone-binding globulin gene promoter

Hepatocytes are the major source of sex hormone-binding globulin (SHBG), a glycoprotein that transports sex steroids in the blood and regulates their access to target tissues. The human SHBG proximal promoter was analyzed by DNase I footprinting, and the functional significance of 6 footprinted regions (FP1-FP6) within the proximal promoter was studied in human HepG2 hepatoblastoma cells. Two footprinted regions (FP1 and FP3) contain binding sites for the chicken ovalbumin upstream promoter-transcription factor (COUP-TF) and hepatocyte nuclear factor-4 (HNF-4). In experiments where SHBG promoter-luciferase reporter gene constructs were co-transfected into HepG2 cells with COUP-TF and/or HNF-4 expression vectors, HNF-4 markedly increased transcription, whereas COUP-TF suppressed this probably by displacing HNF-4 from their common FP1-binding site. This COUP-TF/HNF-4-binding site within FP1 includes a TTTAA sequence, located at nucleotides -30/-26 upstream of the transcription start site, which fails to interact with human TFIID, TATA-binding protein in vitro. When this sequence was replaced with an idealized HNF-4-binding site, the transcriptional activity of the promoter increased in HepG2 cells. Taken together, these data imply that an interplay between COUP-TF and HNF-4 at a site within FP1 regulates human SHBG expression and that HNF-4 controls transcription from this TATA-less promoter by somehow substituting for TATA-binding protein in the recruitment of a transcription preinitiation complex.

Characterization of serum SHBG isoforms in prepubertal and pubertal girls

OBJECTIVE

SHBG is a circulating glycoprotein that binds dihydrotestosterone, testosterone and oestradiol with high affinity and low capacity. In girls, serum concentrations of SHBG gradually decrease with age due to a true fall in concentration and not to a change in the binding characteristics. The aim of our study was to determine the pattern of serum SHBG isoforms in normal girls in early childhood (ECh), late childhood (LCh) and puberty (P).

SUBJECTS

Fifteen normal girls were studied. They were divided into three groups according to their age: ECh: 3.7 +/- 0.9 years (mean +/- SD, n = 5); LCh: 6.4 +/- 0.5 years (n = 5); and P: 13.4 +/- 1.5 years (n = 5).

METHODS AND MEASUREMENTS

Preparative isoelectric focusing was used to isolate SHBG isoforms according to their isoelectric point (pI). Three groups of isoforms were isolated: SI: pI 5.2-5.4; SII: pI 5.4-5.6 and SIII: pI 5.6-5.8. Steroid levels in serum were determined by RIA.

RESULTS

The relative distribution of SHBG isoforms (% of the total SHBG recovered, mean +/- SD) in the three groups of girls studied was: ECh: SI: 25.8 +/- 9.9, SII: 53 +/- 10.5 and SIII: 21.2 +/- 1.6; LCh: SI: 8.8 +/- 3.1, SII: 58.8 +/- 12.2 and SIII: 31.8 +/- 8.6; P: SI: non-detectable; SII: 51.6 +/- 12.6 and SIII: 48.4 +/- 12.6.

CONCLUSION

These results indicate that serum SHBG is more heterogeneous before puberty. A considerable proportion of acidic isoforms are present early in life; they decrease during the prepubertal period and disappear when sexual development is completed. After puberty the glycoprotein is more homogeneous and an important proportion of more basic isoforms is present. At puberty serum SHBG not only falls in concentration but also has an altered sialic acid content which modulates its circulating half-life.

Sex hormone-binding globulin and insulin resistance in African-American women

Sex hormone-binding globulin (SHBG) binds testosterone, determining the level of free, biologically active hormone, and is a sensitive indicator of androgen status in women. SHBG is strongly correlated with high-density lipoprotein (HDL), central obesity, and insulin sensitivity in Caucasian and Mexican-American women, thereby acting as a biologic marker for cardiovascular disease risk. The purpose of this study was to determine if SHBG was a significant correlate of metabolic cardiovascular risk factors in African-American women. Eighty-one nondiabetic, normotensive African-American women were enrolled (mean age, 30 years). After excluding women on oral contraceptives (n = 19), 62 women were examined during the follicular phase of the menstrual cycle. All subjects underwent an oral glucose tolerance test (OGTT) and a euglycemic-hyperinsulinemic insulin clamp, and the lipid and sex hormone levels were measured. Correlation analyses showed a significant correlation between SHBG and the following variables in women: central obesity, body mass index (BMI), HDL cholesterol, apolipoprotein B (apoB), insulin sensitivity adjusted for lean mass (M'), and the sum of insulin during the OGTT. The strongest correlates of SHBG in women were measures of insulin resistance (r = .421, P < .001). SHBG appears to be a biologic marker for insulin resistance, which is linked to cardiovascular risk, in African-American women.

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.

Direct evidence for the localization of the steroid-binding site of the plasma sex steroid-binding protein (SBP or SHBG) at the interface between the subunits

Complete dissociation of dimeric plasma sex steroid-binding protein (SBP or SHBG) was obtained in 6 M urea at 10 degrees C. Removal of urea resulted in the refolding of monomers, followed by reformation of dimeric SBP, which migrates with the same mobility as the native protein. Dimerization does not require Ca+2 or steroid. Renatured monomers yield dimers with dissociation constants for 5 alpha-dihydrotesterone (DHT) and 17 beta-estradiol (E2) indistinguishable from those of native human SBP. This phenomenon was also demonstrated by mixing human and rabbit SBPs that, upon renaturation, form a hybrid dimer composed of one human subunit and one rabbit subunit. The hybrid binds both DHT and E2 in contrast to rSBP, which only binds the androgen. Therefore, we conclude that (1) docking of the two subunits creates an asymmetric steroid-binding site located at the interface between the subunits, and (2) only one face of the dimer defines the specificity for binding E2 by encompassing portion of a structural motif that recognizes the flat ring A of E2. The remaining portion, which recognizes the saturated ring A of DHT, is shared by both faces of the dimer. Because native monomers do not exist alone, the often-asked question of whether the SBP monomer binds steroid can be considered meaningless; steroid-binding activity is expressed only in the dimeric state. Finally, formation of the hybrid indicates that SBP dimerization represents a conserved event during the molecular evolution of SBP, suggesting that the structural elements responsible for dimerization will be homologous in SBPs from other species.

Sex hormone binding globulin mRNA in human breast cancer: detection in cell lines and tumor samples

Sex hormone binding globulin (SHBG) is a high affinity binding protein for estrogens and androgens. SHBG has been found in breast tissue and cell lines through immunostaining. The goal of this series of experiments was to determine whether mRNA for SHBG is expressed in breast cancer cell lines and tumor tissue. Reverse transcriptase-polymerase chain reaction (RT-PCR) was used to detect SHBG and beta-2 microglobulin (control for tissue extractions). Three breast cancer cell lines, ZR-75-1, MCF-7, and MDA-MB-231 and 56 breast tissue samples were collected and analysed for SHBG mRNA expression. mRNA was successfully extracted from 30 of these breast tissue samples. SHBG mRNA was detected in ZR-75-1, MCF-7 and MDA-MB-231 cells, and in 11 of the breast tissue samples. Two PCR products were routinely amplified from the breast cancer cell line RNA, one at approximately 500 bp and another at approximately 300 bp. The DNA sequence of the 300 bp PCR produce was consistent with alternate splicing of the SHBG mRNA, where exon 7 is deleted, and is accompanied by a point deletion at the beginning of exon 8. SHBG protein production from the three breast cancer cell lines was detected by immunoprecipitation using an affinity purified SHBG antibody. SHBG mRNA was found in 11 of 30 samples of breast tissue. Some samples expressed only the 500 bp or the 300 bp PCR product, whereas others expressed both PCR products. The presence of SHBG mRNA in these samples was not associated with either the presence or absence of steroid receptors. SHBG mRNA is thus expressed in breast cancer cell lines, and in some breast tissue samples.

Distribution of immunoreactive androgen-binding protein/sex hormone-binding globulin in tissues of the fetal rat

Androgen-binding protein/sex hormone-binding globulin (ABP/SHBG) is an extracellular carrier protein that binds androgens and estrogens with high affinity. In the adult, ABP/SHBG is thought to function in the male reproductive system and the general circulation in both sexes to modulate the actions of sex steroids. The ABP/SHBG gene is also expressed in the embryonic rat liver, where SHBG is secreted into the fetal blood of male and female rats. The embryo also expresses an alternative SHBG with a unique N-terminal sequence. In this study, the distribution of immunoreactive SHBG in the 17-day-old male fetal rat was determined with six antisera. In general, all of the antisera reacted with the same structures. Specific tissue immunoreactivity was mostly cytoplasmic and/or extracellular. By far the most prominent immunoreactive structures were the mesoderm-derived tissues: connective tissue, striated and cardiac muscle, cartilage, and the liver hematopoietic system. In addition, all regions of the fetal brain contained immunoreactive neurons. In the developing male reproductive system, there was minor reactivity in the testicular cords, whereas the connective tissue in the differentiating Wolffian duct stained with all of the antisera. The Wolffian duct epithelium and epithelia in other developing organs contained small amounts of immunoreactive SHBG, except for the lung, which stained in the epithelial extracellular matrix. An antibody raised against a unique N-terminal peptide specific for the alternative SHBG protein revealed that it was also present in many tissues. These data suggest that SHBG is important for the differentiation of mesodermal tissues. SHBG may modulate the action of androgens in embryonic stroma, thereby regulating development of the epithelium in hormone-dependent tissues.

Purification and characterization of the sex hormone-binding globulin in serum from Djungarian hamsters

A high-affinity sex hormone-binding globulin (SHBG) was purified from the serum of prepubertal Djungarian hamsters (Phodopus sungorus). A purification of more than 2000-fold with an overall yield of 23% was achieved without the use of androgen affinity chromatography. Two predominant variants (51 and 55 kDa) were resolved by denaturing polyacrylamide gel electrophoresis. Both variants participated in the binding of dihydrotestosterone (DHT) and had identical amino-terminal sequences. The sequences obtained for Djungarian hamster SHBG (dhSHBG) showed a high degree of identity with those of other mammals. The affinity of purified dhSHBG for DHT (2.5 x 10(9) M(-1) was similar to that measured in unfractionated serum. This protein was isolated as a dimer with a single calcium-dependent steroid-binding site and a major pI of 4.7. The described purification procedure yielded active dhSHBG from small volumes of prepubertal serum. These studies also provide the first direct structural evidence that a SHBG-like protein, not of testicular origin, is expressed by a rodent during prepubertal development.

Potential functions of plasma steroid-binding proteins

The plasma steroid-binding proteins, sex hormone-binding globulin (SHBG) and corticosteroid-binding globulin (CBG), transport steroid hormones in the blood and regulate their access to target tissues. Recent biochemical and molecular analyses of these proteins and their genes, and studies of their biosynthesis and localization in the liver and other tissues during development, have led to the realization that CBG and SHBG function in much more sophisticated ways. In particular, the presence of plasma membrane binding sites for both CBG and SHBG on steroid target cells, and evidence for interactions between CBG and specific proteinases at sites of inflammation or tissue remodeling, suggest that these proteins control steroid hormone bioavailability and/or action in a highly selective or targeted fashion. This new information should not only serve to extend our understanding of the basis of steroid-hormone dependent diseases, but may influence the design of steroid hormone agonists and antagonist of therapeutic potential.

Hepatic expression of sex hormone-binding globulin associated with the postnatal surge of serum androgen-binding activity in the Djungarian hamster

Serum androgen-binding capacity in Djungarian hamsters, as in many other mammals, increases within days after birth and remains elevated until puberty. This increased activity has been attributed to a hepatic glycoprotein, sex hormone-binding globulin (SHBG), but expression of SHBG by the postnatal liver has not been demonstrated. Therefore, a full-length SHBG cDNA was cloned from the liver of neonatal hamsters and the expression of SHBG during development was examined. Hepatic SHBG RNA levels, as measured by both competitive RT-PCR and Northern analysis, were very low in fetal animals but increased significantly within 24 h of birth. Maximal values were maintained for 1 week after parturition, and then declined to basal adult levels. The developmental pattern in hepatic SHBG immunoactivity, as determined by Western analysis, mirrored that of hepatic SHBG mRNA. However, changes in serum SHBG immunoactivity and steroid-binding activity occurred approximately 1 week later. There were no sex differences in the levels of hepatic SHBG mRNA or protein during development, but serum immunoactivity tended to be higher in females at puberty. Sex- and age-related differences in the relative abundance of SHBG isoforms were also noted. Results of these studies demonstrate that Djungarian hamsters express an authentic SHBG and indicate that the postnatal surge in serum androgen-binding activity is due to perinatal up-regulation of SHBG expression.

Sex hormone-binding globulin and male sexual development

The masculinization of the brain, reproductive tract and many other structures is critically dependent on the testicular hormone, testosterone (T). In many species, T circulates bound with high affinity to sex hormone-binding globulin (SHBG). This protein has a wide phylogenetic distribution and SHBG or SHBG-like proteins are produced by the liver, testes, placenta, brain and other tissues. SHBG activity is detectable during gestation and its expression is both stage- and tissue-dependent. Although SHBG binds circulating androgens, it is argued that the trapping of steroids in the circulation is not the principal function of this protein. The specific binding and uptake of SHBG by various tissues has been observed and suggests that SHBG may directly affect the delivery of androgen signals to target tissues. Effects of SHBG on androgen metabolism, tissue retention, cellular targeting, and action are reviewed. Evidence to date indicates that SHBG is able to enhance or inhibit the uptake of androgens in a cell- and tissue-specific manner. Future work will be necessary to demonstrate whether such actions of SHBG are important for normal male reproductive development.