Receptor for sex steroid-binding protein of endometrium membranes: solubilization, partial characterization, and role of estradiol in steroid-binding protein-soluble receptor interaction

Classic and Novel Sex Hormone Binding Globulin Effects on the Cardiovascular System in Men

In men, 70% of circulating testosterone binds with high affinity to plasma sex hormone binding globulin (SHBG), which determines its bioavailability in their target cells. In recent years, a growing body of evidence has shown that circulating SHBG not only is a passive carrier for steroid hormones but also actively regulates testosterone signaling through putative plasma membrane receptors and by local expression of androgen-binding proteins apparently to reach local elevated testosterone concentrations in specific androgen target tissues. Circulating SHBG levels are influenced by metabolic and hormonal factors, and they are reduced in obesity and insulin resistance, suggesting that SHBG may have a broader clinical utility in assessing the risk for cardiovascular diseases. Importantly, plasma SHBG levels are strongly correlated with testosterone concentrations, and in men, low testosterone levels are associated with an adverse cardiometabolic profile. Although obesity and insulin resistance are associated with an increased incidence of cardiovascular disease, whether they lead to abnormal expression of circulating SHBG or its interaction with androgen signaling remains to be elucidated. SHBG is produced mainly in the liver, but it can also be expressed in several tissues including the brain, fat tissue, and myocardium. Expression of SHBG is controlled by peroxisome proliferator-activated receptor γ (PPARγ) and AMP-activated protein kinase (AMPK). AMPK/PPAR interaction is critical to regulate hepatocyte nuclear factor-4 (HNF4), a prerequisite for SHBG upregulation. In cardiomyocytes, testosterone activates AMPK and PPARs. Therefore, the description of local expression of cardiac SHBG and its circulating levels may shed new light to explain physiological and adverse cardiometabolic roles of androgens in different tissues. According to emerging clinical evidence, here, we will discuss the potential mechanisms with cardioprotective effects and SHBG levels to be used as an early metabolic and cardiovascular biomarker in men.

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.

Interactions of sex hormone-binding globulin with target cells

Sex hormone-binding globulin (SHBG) was initially described as a plasma protein synthesized in, and secreted by, the liver. It was discovered by its ability to bind certain androgens and estrogens and, for many years, was believed to serve as a transporter/reservoir for the steroids which it bound. Subsequently, it became clear that the cell membranes of selected tissues contained a receptor for SHBG (R(SHBG)). This review deals with what is known of that receptor - its anatomy, physiology and biochemistry.

Access of reproductive steroids to target tissues

The access of reproductive steroids to their target cells varies considerably between tissues, and is influenced to a great extent by their interactions with plasma steroid-binding proteins, and with SHBG and CBG in particular. An increased awareness of how SHBG and CBG function within the blood circulation, and within extravascular compartments of steroid-responsive target tissues, needs to be incorporated into the design and evaluation of therapies involving the administration of both natural and synthetic steroids, which influence female reproduction and healthy aging.

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.

Sex hormone-binding globulin: not only a transport protein. What news is around the corner?

The plasma Sex Hormone-Binding Globulin (SHBG) transports androgens and estradiol in the blood and regulates their bioavailable fraction and access to target cells. The recent advances in the knowledge of its structure and gene expression, and notabily the demonstration of a specific receptor (SHBG-R) located on membranes of sex steroid responsive cells, gave support to the thesis that SHBG has much more sophisticated functions at cell site. In particular, the receptor-mediated action of SHBG, which uses as a second messenger cAMP, has been linked to the effects of androgens and estradiol. It is conceivable that the SHBG/SHBG-R system works as an additional control mechanism which inhibits or amplifies the effects of DHT and estradiol in cells. In the prostate, it has been suggested that the estradiol-activated SHBG/SHBG-R complex cross-talks with the androgen receptor, and is able to activate AR even in the absence of DHT. Of great interest, for its potential clinical applications, is the observation that in estrogen-dependent breast cancer SHBG, through SHBG-R, cAMP and PKA, specifically inhibits the estradiol-induction of cell proliferation. This anti-proliferative, anti-estrogenic effect of human SHBG has not only increased and continues to increase our understanding of the molecular mechanisms involved in the biology of breast cancer, but could also be exploited as a future therapeutic strategy in the managing of estrogen-dependent tumours.

Control of the membrane sex hormone-binding globulin-receptor (SHBG-R) in MCF-7 cells: effect of locally produced SHBG

The interaction between plasma sex hormone-binding globulin (SHBG) and its receptor (SHBG-R) inhibits estradiol-induced proliferation of MCF-7 cells (human estrogen-dependent breast cancer) through cAMP and PKA. Thus, SHBG can modulate estradiol action in breast cancer, but the implications of this require a more detailed knowledge of the SHBG-R. To this end, we have transfected MCF-7 cells with an expression vector carrying the human SHBG cDNA (S-MCF-7) and studied the effects of this on both SHBG-R binding and cell proliferation. Control cells were parental MCF-7 (P-MCF-7) and MCF-7 cells transfected with the beta-galactosidase gene (B-MCF-7). Transfections were mediated by lipofectin followed by selection of transfected cells with G418. The amounts of SHBG in culture medium were evaluated by IRMA assay, with only S-MCF-7 cells shown to secrete SHBG; SHBG-R levels were evaluated by tracer binding technique. In P-MCF-7 and B-MCF-7 cells, SHBG-R was detectable as a two-binding site receptor, but no binding of SHBG was observed in S-MCF-7 cells. Proliferation of cells treated with estradiol was evaluated by [3H]thymidine incorporation in the three cell lines and in cells pretreated with SHBG (1 nM) purified from human serum or with conditioned medium from S-MCF-7 cells (medium S). In all three lines, cell proliferation increased after estradiol treatment. Preincubation with purified SHBG was effective in reducing estrogen-induced cell proliferation to basal levels in P-MCF-7 and B-MCF-7 but not in S-MCF-7 cells. The estradiol effect was also inhibited in P-MCF-7 cells treated with medium S. In conclusion, 1) SHBG inhibits estradiol-induced proliferation in cells containing a functional SHBG-R, whereas it has no detectable effect in cells in which the SHBG-R is either absent or not available to bind SHBG; and 2) S-MCF-7 cells are insensitive to SHBG (locally produced or exogenous) because their SHBG-R is occupied by SHBG.

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.

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.

Use of non-radioactive labels for half-life measurement of sex hormone-binding globulin in the rabbit

The purpose of this study was to investigate two methods for labeling rabbit sex hormone-binding globulin (rSHBG) with non-radioactive material, biotin (B) and europium (Eu3+), in order to obtain stable labeled SHBG and measure in vivo its metabolism and distribution. The obtained half-life values were compared with [125I]rSHBG half-lives. rSHBG was first isolated by immunoaffinity chromatography using an immobilized monoclonal anti-human SHBG (hSHBG) antibody that cross-reacts with rSHBG. This purified rSHBG was labeled by either biotin-X-N-hydroxysuccinimide ester (rSHBG-B), Eu3(+)-diethylenetriaminepentaacetic dianhydride, or Eu(3+)-isothiocyanatobenzyldiethylenetriamine-tetraacetic acid reagents (rSHBG-Eu3+) or by 125I using Bolton and Hunter reagent ([125I]rSHBG). The labeling procedure preserved the main properties of native SHBG: interaction with the lectine concanavaline A-Sepharose, recognition by anti-hSHBG monoclonal antibody, and, although lower than in native SHBG, the binding affinity for 5 alpha-dihydrotestosterone. These characteristics were the prerequisite for reliable measurement of the metabolism of labeled SHBG. Labeled rSHBG was injected into various rabbits with blood sampling at 2 min and at 1, 2, 4, 8, 12, 24, 48, 72, and 96 h after injection. rSHBG-B or desiaylated rSHBG-B and rSHBG-Eu3+ were captured from serum samples by tubes coated with anti-hSHBG antibody prior to the following detection procedure: biotin was detected by luminometry with the [streptavidin-alkaline phosphatase-dioxetane (AMPPD)] system and europium by time-resolved fluorimetry. [125I]rSHBG was detected by measurement of radioactivity either directly on serum or after fixation on concanavaline A-Sepharose.(ABSTRACT TRUNCATED AT 250 WORDS)

Interrelations between sex hormone-binding globulin (SHBG), plasma lipoproteins and cardiovascular risk

The incidence of coronary artery disease is significantly higher in men than in women, at least until menopause. This gender difference could be explained by the action of sex steroids on the lipoprotein profile. In prepubertal children, high-density lipoprotein (HDL) cholesterol and triglyceride levels are similar between sexes, while adult men have generally lower HDL cholesterol and higher triglyceride levels than premenopausal adult women. Most cross-sectional studies have reported that sex hormone binding globulin (SHBG) and testosterone levels correlate positively with HDL cholesterol levels between sexes. Thus SHBG by modulating the balance in the biodisposal of testosterone and estradiol, might have a profound effect on the risk of cardiovascular disease. However, adjustment for body weight and body fat distribution weakens the association between SHBG, testosterone and HDL cholesterol. The negative correlation of fasting insulin with SHBG and HDL cholesterol levels in both sexes, and some evidence that insulin is an inhibitor of SHBG production in vitro, has suggested that hyperinsulinism might negatively regulate SHBG and HDL levels. It remains to be determined whether the inverse relationship between SHBG and insulin levels is coincidental or has a causal effect on the increase of atherosclerosis. Decreased SHBG has been shown to be predictive of the incidence of non-insulin-dependent diabetes mellitus in women but not in men, and of subsequent development of cardiovascular disease and overall mortality in postmenopausal women. SHBG is an index of androgenism in women and of insulin-resistance in both sexes, and might be useful in epidemiological studies of cardiovascular risk. However, in men, SHBG is not predictive of the occurrence of cardiovascular disease. Whether SHBG might have an intrinsic protective effect on the arterial wall through SHBG-receptors is still highly speculative.

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.

Binding of sex steroid binding protein to plasma membranes of human testis

The existence of a specific binding site for sex steroid binding protein (SBP or SHBG) was detected on plasma membranes prepared from the testis of a patient affected by a variant form of testicular feminization. A binding technique using [125I]SBP as a tracer allowed us to identify a single set of binding sites, characterized by a Kd of 1.917 x 10(-11) M. The maximum number of binding sites was 5.2 fmol/mg membrane protein. Membranes were also prepared from a sample of genital skin from the same patient, but no binding for [125I]SBP was detectable. The evidence of the SBP membrane receptor in the testis of a patient affected by Morris syndrome extends our knowledge about the tissue distribution of the SBP receptor and suggests the possible implication of SBP and its recognition system in a disorder related to peripheral androgen insensitivity.

The receptor-mediated action of sex steroid binding protein (SBP, SHBG): accumulation of cAMP in MCF-7 cells under SBP and estradiol treatment

The interaction of sex steroid binding protein (SBP) with its specific receptor in MCF-7 cell (estrogen-sensitive human breast cancer cells), followed by the binding of estradiol (E2) to the complex SBP-receptor, induced a significant accumulation of intracellular cAMP. SBP alone as well as E2 alone did not elicit any modification of the nucleotide. The maximal increase in cAMP was observed with 1 nM SBP + 1 nM E2. Increasing doses of both SBP and E2, even raising cAMP levels with respect to basal, did not give any higher response. Both testosterone and dihydrotestosterone, used instead of E2, were not able to induce any significant modification of cAMP. E2-induced MCF-7 cell proliferation was significantly reduced by 8Br-cAMP. MDA-MB 231 cells (estrogen-insensitive breast cancer cells) were not shown to bind SBP, or to respond to SBP + E2 as far as both their proliferation and cAMP content are concerned. In summary, the present study provides evidence that the SBP receptor is part of the G-protein receptor family, and that SBP can act as modulator of E2 action at cell site through the second messenger cAMP.

Interaction of sex hormone-binding globulin with plasma membranes from the rat epididymis and other tissues

The binding of human sex hormone-binding globulin (hSHBG) to plasma membranes prepared from the adult rat epididymis and other potential target and non-target tissues was examined. Specific binding sites were detected in the epididymis, testis, prostate, skeletal muscle and liver. The first three organs exhibited a higher (KD approx. 0.1 nM; Bmax approx. 0.05-0.10 pmol/mg membrane protein, Site I) and a lower (KD approx. 5 nM; Bmax approx. 1.0-2.5 pmol/mg membrane protein, Site II) affinity binding site. Only Site I was detected in muscle membranes and only Site II was detected in membranes isolated from liver. Specific binding was not detectable in either spleen or brain. Regional distribution of hSHBG binding sites occurred in the epididymis. Both Site I and Site II were present in the proximal caput and distal cauda. The distal caput and proximal cauda contained only Site II; no specific binding was detected in the corpus. Binding of hSHBG to epididymal membranes was time- and temperature-dependent. The presence of Ca2+ did not affect binding. Non-liganded [125I]-labeled hSHBG can bind to both sites in epididymal membranes. The affinity of hSHBG for Site I increased 2-fold when it was complexed with 5 alpha-dihydrotestosterone, testosterone or estradiol. The hSHBG-androgen complex had little effect on Site II versus steroid-free SHBG. However, the affinity of the hSHBG-estradiol complex for these sites was increased 10-fold. Cortisol, which has a low affinity for hSHBG, did not influence its binding to either the higher or lower affinity membrane sites.

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

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

Biological relevance of the interaction between sex steroid binding protein and its specific receptor of MCF-7 cells: effect on the estradiol-induced cell proliferation

The human breast cancer cells MCF-7 were shown to bind sex steroid binding protein (SBP) at a receptor site. The binding to whole cells was specific, time-dependent, saturable, and at high affinity. Estradiol, bound to SBP, induced a significant inhibition of SBP-cell binding at a dose of 10(-9) M. The presence of SBP, bound either to estradiol, or to cells, did not alter the amount of estradiol entering cells, but it "captured" an additional quantity of the hormone at the outer surface of cells. Furthermore, the effect of SBP on estradiol-induced MCF-7 cell proliferation was evaluated. While estradiol is an effective proliferating agent on MCF-7 cells, SBP itself did not produce any significant cell proliferation; the growth of MCF-7 cells in the presence of the complex SBP-estradiol was not different from the growth in the presence of estradiol alone; SBP bound to its receptor produced a significant reduction of the estradiol-induced cell proliferation. In summary, the present study provides evidence that the interaction of SBP with its receptor on MCF-7 cells is not involved in the uptake of estradiol, but it can modify the effect of estradiol at target site by a mechanism which is not likely to be a simple sequestration of the hormone at the outer surface of cells.