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

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.

Exposure to a specific time-varying electromagnetic field inhibits cell proliferation via cAMP and ERK signaling in cancer cells

Exposure to specific electromagnetic field (EMF) patterns can affect a variety of biological systems. We have shown that exposure to Thomas-EMF, a low-intensity, frequency-modulated (25-6 Hz) EMF pattern, inhibited growth and altered cell signaling in malignant cells. Exposure to Thomas-EMF for 1 h/day inhibited the growth of malignant cells including B16-BL6 mouse melanoma cells, MDA-MB-231, MDA-MB-468, BT-20, and MCF-7 human breast cancer and HeLa cervical cancer cells but did not affect non-malignant cells. The Thomas-EMF-dependent changes in cell proliferation were mediated by adenosine 3',5'-cyclic monophosphate (cAMP) and extracellular-signal-regulated kinase (ERK) signaling pathways. Exposure of malignant cells to Thomas-EMF transiently changed the level of cellular cAMP and promoted ERK phosphorylation. Pharmacologic inhibitors (SQ22536) and activators (forskolin) of cAMP production both blocked the ability of Thomas-EMF to inhibit cell proliferation, and an inhibitor of the MAP kinase pathway (PD98059) was able to partially block Thomas-EMF-dependent inhibition of cell proliferation. Genetic modulation of protein kinase A (PKA) in B16-BL6 cells also altered the effect of Thomas-EMF on cell proliferation. Cells transfected with the constitutively active form of PKA (PKA-CA), which interfered with ERK phosphorylation, also interfered with the Thomas-EMF effect on cell proliferation. The non-malignant cells did not show any EMF-dependent changes in cAMP levels, ERK phosphorylation, or cell growth. These data indicate that exposure to the specific Thomas-EMF pattern can inhibit the growth of malignant cells in a manner dependent on contributions from the cAMP and MAP kinase pathways. Bioelectromagnetics. 39;217-230, 2018. © 2017 Wiley Periodicals, Inc.

Sex hormone-binding globulin (SHBG) expression in ovarian carcinomas and its clinicopathological associations

Sex hormone-binding globulin (SHBG) is known as a carrier protein. It is classically thought to be mainly synthesized in the liver and then secreted into the circulating system, where it binds to sex steroids with a high affinity and modulates the bio-availability of the hormones. Other organs known to produce SHBG include brain, uterus, testis, prostate, breast and ovary, and the local expressed SHBG may play an important role in tumor development. However, SHBG expression status and its clinicopathological significance in ovarian cancer cells are not reported yet. In our present study, we examined and found the variable SHBG expression in four ovarian cancer cell lines (OV-90, OVCAR-3, SKOV-3 and ES-2) by immunocytochemistry and Western blotting. We then extended our study to 248 ovarian carcinoma samples, which were collected at The Norwegian Radium Hospital, Oslo University Hospital with complete clinical information, and discovered that SHBG was variably expressed in these ovarian carcinomas. Higher level of SHBG expression was significantly associated with more aggressive histological subtype (p = 0.022), higher FIGO stage (p = 0.018) and higher histological grade (grade of differentiation, p = 0.020), although association between SHBG expression and OS/PFS was not observed. Our results demonstrate that ovarian cancer cells produce SHBG and higher SHBG expression in ovarian carcinoma is associated with unfavorable clinicopathological features.

SHBG is an important factor in stemness induction of cells by DHT in vitro and associated with poor clinical features of prostate carcinomas

Androgen plays a vital role in prostate cancer development. However, it is not clear whether androgens influence stem-like properties of prostate cancer, a feature important for prostate cancer progression. In this study, we show that upon DHT treatment in vitro, prostate cancer cell lines LNCaP and PC-3 were revealed with higher clonogenic potential and higher expression levels of stemness related factors CD44, CD90, Oct3/4 and Nanog. Moreover, sex hormone binding globulin (SHBG) was also simultaneously upregulated in these cells. When the SHBG gene was blocked by SHBG siRNA knock-down, the induction of Oct3/4, Nanog, CD44 and CD90 by DHT was also correspondingly blocked in these cells. Immunohistochemical evaluation of clinical samples disclosed weakly positive, and areas negative for SHBG expression in the benign prostate tissues, while most of the prostate carcinomas were strongly positive for SHBG. In addition, higher levels of SHBG expression were significantly associated with higher Gleason score, more seminal vesicle invasions and lymph node metastases. Collectively, our results show a role of SHBG in upregulating stemness of prostate cancer cells upon DHT exposure in vitro, and SHBG expression in prostate cancer samples is significantly associated with poor clinicopathological features, indicating a role of SHBG in prostate cancer progression.

Insulinotropic and antidiabetic effects of 17β-estradiol and the GPR30 agonist G-1 on human pancreatic islets

We have recently shown that 17β-estradiol (E2) and the synthetic G protein-coupled receptor 30 (GPR30) ligand G-1 have antiapoptotic actions in mouse pancreatic islets, raising the prospect that they might exert beneficial effects also in human islets. The objective of the present study was to identify the expression of GPR30 in human islets and clarify the role of GPR30 in islet hormone secretion and β-cell survival. GPR30 expression was analyzed by confocal microscopy, Western blot, and quantitative PCR in islets from female and male donors. Hormone secretion, phosphatidylinositol hydrolysis, cAMP content, and caspase-3 activity in female islets were determined with conventional methods and apoptosis with the annexin-V method. Confocal microscopy revealed GPR30 expression in islet insulin, glucagon, and somatostatin cells. GPR30 mRNA and protein expression was markedly higher in female vs. male islets. An amplifying effect of G-1 or E2 on cAMP content and insulin secretion from isolated female islets was not influenced by the E2 genomic receptor (ERα and ERβ) antagonists ICI 182,780 and EM-652. Cytokine-induced (IL-1β plus TNFα plus interferon-γ) apoptosis in islets cultured for 24 h at 5 mmol/liter glucose was almost abolished by G-1 or E2 treatment and was not affected by the nuclear estrogen receptor antagonists. Concentration-response studies on female islets from healthy controls and type 2 diabetic subjects showed that both E2 and G-1 displayed important antidiabetic actions by improving glucose-stimulated insulin release while suppressing glucagon and somatostatin secretion. In view of these findings, we propose that small molecules activating GPR30 could be promising in the therapy of diabetes mellitus.

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.

Sex Hormone-Binding Globulin (SHBG), estradiol and breast cancer

The human serum Sex Hormone-Binding Globulin (SHBG) plays an important role in breast cancer pathophysiology and risk definition, since it regulates the bioavailable fraction of circulating estradiol. We here summarize data reported over the years concerning the involvement of SHBG and SHBG polymorphisms in the definition of breast cancer risk. We also report what is known about the direct action of SHBG in breast cancer cells, illustrating its interaction with these cells and the subsequent initiation of a specific intracellular pathway leading to cross-talk with the estradiol-activated pathway and, finally, to the inhibition of several effects of estradiol in breast cancer cells. In conclusion, as a result of its unique property of regulating the estrogen free fraction and cross-talking with the estradiol pathways, by inhibiting estradiol-induced breast cancer cell growth and proliferation, SHBG is associated with a reduced risk of developing the neoplasm after estrogen exposure.

17 Beta-estradiol (E2) plus tumor necrosis factor-alpha induces a distorted maturation of human monocyte-derived dendritic cells and promotes their capacity to initiate T-helper 2 responses

There is growing evidence that 17 beta-estradiol (E2) modulates immune function. Recent studies indicated that certain effects of E2 on in vivo immune function are not a result of a direct action on T cells, but rather an indirect action on antigen-presenting cells. This study demonstrates that the pregnancy-associated doses of E2 plus tumor necrosis factor-alpha (TauNuF alpha) induce distorted maturation of human dendritic cells (DCs) that result in an increased capacity to induce T helper (Th) 2 responses. E2 did not affect the expression of human leukocyte antigen class II and costimulatory molecules by DCs, but elicited the ability of DC to produce CC chemokine ligand 1, which can attract CCR8-expressing Th2 cells and regulatory T cells. In addition, E2/TNF alpha-matured DCs increased the production of IL-10 relative to the IL-12p70 on CD40 ligation, thereby inducing naive T-cell differentiation into a Th2. Moreover, the increased concentration of E2 in the route of maturation did indeed further enhance Th2 deviation. The dominant Th2 deviation was induced at a high E2 concentration typical during pregnancy. These findings demonstrate that the high physiological levels of E2 may be an important endogenous component for regulating the DC function and skewing the immune response toward Th2.

Androgen-binding protein is co-expressed with oxytocin in the male reproductive tract

Androgen-binding protein (ABP) and the posterior lobe hormone oxytocin (OT) were co-localized in male rat reproductive organs. Immunostaining of serial semi-thin sections revealed a high rate of coexistence of both antigens in Sertoli cells and in the epithelial cells of the prostate. There was a considerably less co-localization of OT and ABP in epithelial cells of the epididymis, and in the different tissues of the ductus deferens. In situ hybridization with synthetic oligonucleotides complementary to a fragment of ABP mRNA showed specific staining in the same sites that were immunostained for ABP. ABP was isolated by affinity chromatography from homogenates of testis, epididymis, prostate and the content of the prostate lumen. Identical protein patterns could be shown with surface-enhanced laser desorption/ionization time-of-flight mass spectrometry in all samples except for the epididymis indicating that ABP structure is similar in all these tissues. ABP seems to be expressed in specified cells throughout the male rat reproductive tract. Most of these cells appear to be oxytocinergic. ABP and OT have previously been detected in the ejaculate. The observed epithelial cells are likely to be their source.

Sex hormone-binding globulin antagonizes the anti-apoptotic effect of estradiol in breast cancer cells

Sex hormone-binding globulin, the plasma carrier for sex steroids, inhibits the estradiol-induced proliferation of breast cancer cells. Estradiol induces cell proliferation triggering multiple mechanisms. Besides regulating growth factors, it activates Erk-1/-2, thus inhibiting apoptosis. In the present study, we investigated the effect of SHBG on estradiol-mediated anti-apoptotic effect in MCF-7 breast cancer cells. As expected, estradiol reduced the number of cells undergoing apoptosis. Although no modification of estradiol action was observed in cells treated contemporarily with estradiol and SHBG, pre-incubation with SHBG before estradiol treatment contrasted the anti-apoptotic effect completely. A mutant form of SHBG, lacking the O-linked oligosaccharide in Thr(7), displayed no such effect. Moreover, SHBG prevented the estradiol-induced phosphorylation of Erk-1/-2, whereas it had no effect on estradiol-induced transcription. Taken together, our observations suggest that the interaction of SHBG with MCF-7 cell membranes causes inhibition of the anti-apoptotic effect of estradiol which might account for SHBG's inhibitory effect on breast cancer cell growth.

Immunoassay for sex hormone-binding globulin in undiluted serum is influenced by high-molecular-mass aggregates

BACKGROUND

The new Elecsys chemiluminescence assay for measurement of homodimeric sex hormone-binding globulin (SHBG) was designed for use with undiluted serum, in contrast to other methods that require predilution. During assay development, unexpected calibration difficulties were observed that were attributable to particular biochemical properties of the highly concentrated SHBG in solution.

METHODS

We used a surface plasmon resonance (SPR) biosensor, which enables biomolecular interaction analysis of SHBG, and size-exclusion chromatography for this investigation. The immunoassay was evaluated for imprecision, linearity, and suitability of the dilution medium, and the method was compared with an IRMA for SHBG.

RESULTS

The SPR biosensor characterized the special protein properties of SHBG in various concentrations. Above 200 nmol/L there was a strong tendency toward formation of high-molecular-mass aggregates. This was also detectable by size-exclusion chromatography and could be reversed by simple dilution of the sample. On the basis of these results, the dynamic measuring range of the SHBG assay is restricted to 0.350-200 nmol/L. Assay evaluation on a 2010 analyzer revealed excellent precision (CV <or=2.5%). Mean recoveries were 84.2-98.8%. Intermethod comparison with an IRMA yielded a satisfactory concordance of the two assays with a Spearman correlation coefficient of 0.8807.

CONCLUSIONS

Aggregates of human SHBG may have a detrimental impact on the accurate measurement of the protein if measurements are performed with undiluted serum samples. Further work is needed to clarify whether these high-molecular-mass aggregates influence the free fraction of steroid hormones in vivo.

17beta-estradiol stimulates the growth of human keratinocytes by inducing cyclin D2 expression

Estrogen is reported to prevent age-associated epidermal thinning in the skin. We examined if 17beta-estradiol (E2) may enhance the growth of human keratinocytes, focusing on its effects on the expression of cell cycle-regulatory proteins. E2 enhanced proliferation, bromodeoxyuridine incorporation of keratinocytes, and increased the proportion of cells in the S phase. The E2-induced stimulation of proliferation and bromodeoxyuridine incorporation was suppressed by antisense oligonucleotide against cyclin D2, which induces G1 to S phase progression. E2 increased protein and mRNA levels of cyclin D2, and resultantly enhanced assembly and kinase activities of cyclin D2-cyclin-dependent kinases 4 or 6 complexes. E2 enhanced cyclin D2 promoter activity, and the element homologous to cAMP response element (CRE) on the promoter was responsible for the effect. Cyclin D2 expression was enhanced by antiestrogens, ICI 182,780 and 4-hydroxytamoxifen, and membrane-impermeable bovine serum albumin-conjugated E2, indicating the effects via membrane E2-binding sites. E2 increased the enhancer activity of CRE-like element and the amount of phosphorylated cAMP response element binding protein (CREB) binding this element, and the increases were suppressed by H-89, an inhibitor of cAMP-dependent protein kinase A. H-89 also suppressed E2-induced cyclin D2 expression, proliferation, and bromodeoxyuridine incorporation in keratinocytes. Antisense oligonucleotide against G-protein-coupled receptor GPR30 suppressed the E2-induced increases of phosphorylated CREB, cyclin D2 level, proliferation, and bromodeoxyuridine incorporation in keratinocytes. These results suggest that E2 may stimulate the growth of keratinocytes by inducing cyclin D2 expression via CREB phosphorylation by protein kinase A, dependent on cAMP. These effects of E2 may be mediated via cell surface GPR30.

17beta-estradiol inhibits oxidative stress-induced apoptosis in keratinocytes by promoting Bcl-2 expression

We examined in vitro effects of 17beta-estradiol on H2O2-induced apoptosis in human keratinocytes. 17beta-estradiol prevented the H2O2-induced apoptosis. H2O2 decreased, whereas 17beta-estradiol increased Bcl-2 protein and mRNA levels in keratinocytes, and H2O2 plus 17beta-estradiol led to basal levels. Overexpression of Bcl-2 protected keratinocytes against H2O2-induced apoptosis, indicating the anti-apoptotic effect of Bcl-2. H2O2 suppressed, whereas 17beta-estradiol enhanced bcl-2 promoter activity, and H2O2 plus 17beta-estradiol led to basal activity. Cyclic adenosine monophosphate (cAMP) response element on bcl-2 promoter was responsible for the effects of 17beta-estradiol and H2O2. Bcl-2 expression was enhanced by membrane-impermeable bovine serum albumin-conjugated 17beta-estradiol, indicating the effects via membrane 17beta-estradiol-binding sites. H2O2 decreased, whereas 17beta-estradiol increased the amount of phosphorylated cAMP response element-binding protein and cAMP response element-dependent transcriptional activity, and H2O2 plus 17beta-estradiol led to basal levels. H-89, an inhibitor of cAMP-dependent protein kinase A, suppressed basal and 17beta-estradiol-induced cAMP response element-binding protein phosphorylation, cAMP response element-dependent transcriptional activity, Bcl-2 expression, and apoptosis resistance. The cAMP analog, dibutyryl cAMP, enhanced cAMP response element-binding protein phosphorylation, cAMP response element-dependent transcriptional activity, Bcl-2 expression, and apoptosis resistance. 17Beta-estradiol increased intracellular cAMP level and protein kinase A activity, whereas these were not altered by H2O2. Keratinocytes expressed mRNA for estrogen receptor beta and guanine nucleotide-binding protein-coupled receptor, GPR30. GPR30 anti-sense oligonucleotide did, but anti-sense estrogen receptor beta did not suppress 17beta-estradiol-induced cAMP signal, cAMP response element-binding protein phosphorylation, Bcl-2 expression, and apoptosis resistance. These results suggest that 17beta-estradiol may enhance Bcl-2 expression and prevent H2O2-induced apoptosis by phosphorylating cAMP response element-binding protein via cAMP/protein kinase A pathway in keratinocytes. These effects of 17beta-estradiol may be mediated via membrane GPR30.

17Beta-estradiol enhances the production of nerve growth factor in THP-1-derived macrophages or peripheral blood monocyte-derived macrophages

We examined in vitro effects of 17beta-estradiol (E2) on nerve growth factor production by macrophages derived from monocytic cell line THP-1-or periphereal blood monocytes. E2 and membrane-impermeable bovine serum albumin-conjugated E2 (E2-BSA) enhanced nerve growth factor secretion and mRNA expression in both types of macrophages E2 enhanced nerve growth factor promotor activity in THP-1-derived macrophages and two activator protein-1 binding sites on the promoter were responsible for the enhancement. E2 and E2-BSA enhanced transcriptional activity and DNA binding of activator protein-1. E2 and E2-BSA shifted the activator protein-1 composition from c-Jun homodimers to c-Fos/c-Jun heterodimers. E2 and E2-BSA transiently induced c-Fos mRNA, which was constitutively undetectable in both types of macrophages. Adenylate cyclase inhibitor SQ22536 suppressed E2-induced nerve growth factor production and c-Fos expression. E2 and E2-BSA increased intracellular cyclic adenosine monophosphate level in both types of macrophages. Antisense oligonucleotide against guanine nucleotide-binding protein-coupled receptor, GPR30 suppressed the E2-induced cyclic adenosine monophosphate signal, c-Fos expression, and nerve growth factor secretion in both types of macrophages. These results suggest that E2 may enhance nerve growth factor production by inducing c-Fos expression via cyclic adenosine monophosphate signal in macrophages. These effects may be mediated via GPR30.

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.

The inhibitory effect of androgen and sex-hormone-binding globulin on the intracellular cAMP level and tyrosinase activity of normal human melanocytes

The effect of androgens on human melanocytes has not been well clarified. We studied the effects of androgens on normal human melanocytes in the presence or absence of sex-hormone-binding globulin (SHBG), which complexes with those hormones. Immunohistochemically, testosterone and SHBG co-localized on the cell membrane. Androgens such as testosterone, 5alpha-dihydrotestosterone, and methyltrienolone (R1881, a potent synthetic androgen), reduced intracellular cAMP levels after treatment with SHBG, but hydrocortisone had no effect. We also found that testosterone and R1881 slightly suppressed tyrosinase activity in melanocytes when treated with SHBG, although they had no effect on the expression of tyrosinase at the transcriptional or translational level, as measured by semi-quantitative reverse transcriptase-polymerase chain reaction and by Western blot analysis, respectively. Our results suggest that androgens may modulate tyrosinase activity at the posttranslational level through the cell membrane signaling pathway.

17beta-estradiol inhibits the production of interferon-induced protein of 10 kDa by human keratinocytes

The natural course of psoriasis is often modulated during pregnancy, indicating the regulatory effect of estrogen or progesterone on psoriasis. Interferon-induced protein of 10 kDa chemoattracts T helper 1 cells, and interferon-induced protein of 10 kDa production by keratinocytes is enhanced in psoriatic skin lesions. We examined in vitro effects of sex hormones on the interferon-induced protein of 10 kDa production by human keratinocytes. 17beta-estradiol inhibited interferon-gamma-induced interferon-induced protein of 10 kDa secretion, mRNA expression, and promoter activity. Interferon-stimulated response element on the promoter was responsible for the inhibition by 17beta-estradiol. Interferon-gamma-induced protein of 10 kDa production was also inhibited by anti-estrogens, ICI 182 780 and tamoxifen, and membrane-impermeable bovine serum albumin-conjugated 17beta-estradiol, suggesting the effects via membrane estrogen receptor, whereas 17alpha-estradiol, progesterone, and dihydrotestosterone had no effects. 17beta-estradiol and bovine serum albumin-conjugated 17beta-estradiol suppressed interferon-gamma-induced transcription through the interferon-stimulated response element and signal transducer and activator of transcription 1alpha binding to interferon-stimulated response element. 17beta-estradiol and bovine serum albumin-conjugated 17beta-estradiol suppressed interferon-gamma-induced tyrosine phosphorylation of signal transducer and activator of transcription 1alpha, and Janus tyrosine kinase 1 and 2. 17beta-estradiol-mediated suppression on the interferon-gamma-induced signal transducer and activator of transcription 1alpha activation and interferon-induced protein of 10 kDa synthesis was counteracted by adenylate cyclase inhibitor SQ22536. 17beta-estradiol, bovine serum albumin-conjugated 17beta-estradiol, ICI 182 780, and tamoxifen increased intracellular 3',5'-adenosine cyclic monophosphate level by activating adenylate cyclase in keratinocytes. Fluorescein isothiocyanate-labeled bovine serum albumin-conjugated 17beta-estradiol bound to the surface of keratinocytes, and mRNA for estrogen receptor beta but not for estrogen receptor alpha was detected in keratinocytes. These results suggest that 17beta-estradiol may interact with the membrane receptor on keratinocytes and generate 3',5'-adenosine cyclic monophosphate by activating adenylate cyclase, which may lead to the inhibition of interferon-gamma-induced signal transducer and activator of transcription 1alpha activation and interferon-induced protein of 10 kDa synthesis.

Phytoestrogens as modulators of steroid action in target cells

Although numerous reports exist on the potential beneficial role of nutritional phytoestrogens in human health, their molecular mechanism in target cells is still not completely understood. Phytoestrogens promote estrogen and antiestrogen effects by interacting with numerous molecules, carrier proteins, enzymes and membrane and nuclear receptors, directly or indirectly involved in the transfer of estrogen signals. The hypothesis that the ER beta subtype plays a key role in antiproliferative effect of phytoestrogens, especially in breast cancer, is examined here. This review focus on the effects of phytoestrogens in developmental processes such as those linked to reproductive function, tumorigenesis and angiogenesis.

Sex hormone-binding globulin in the human prostate is locally synthesized and may act as an autocrine/paracrine effector

Sex hormone-binding globulin (SHBG) is a plasma protein synthesized and secreted by the liver. Its initial description stemmed from its ability to bind estrogens and androgens and its capacity to regulate the free concentration of the steroids that bind to it. Additionally, it participates in signal transduction for certain steroid hormones at the cell membrane. It binds with high affinity to a specific membrane receptor (R(SHBG)) in prostate stromal and epithelial cells, wherein the SHBG.R(SHBG) complex forms. An appropriate steroid binds to this complex and results in increases of intracellular cAMP. These two disparate functions of SHBG, regulation of the concentration of free steroids in plasma and signal transduction in selected tissues, raise the question of how its synthesis and secretion might be regulated so as to best perform these two disparate functions. In this paper we demonstrate that SHBG is produced in human prostate cancer cell lines (LNCaP, DU 145, and PC 3) as well as in cultured human prostate epithelial and stromal cells. In addition, in tissue sections of human prostate, we demonstrate the presence of SHBG (immunocytochemistry) and SHBG mRNA (in situ hybridization). These observations are consistent with the hypothesis that SHBG, destined to participate in signaling at the cell membrane, is locally regulated and produced.

A sexual arousability model involving steroid effects at the plasma membrane

This review will discuss the status of research related to sexual arousability. It will also present a model for sexual arousability based on current knowledge of steroids effects at the membranes of cells. Steroids have multiple rapid actions that are suggested to result from actions at membrane-associated receptors. When stimulated by steroids these receptors alter G-protein coupling in a manner unique to this complex. Initial stimulation of the receptors by steroids alters the coupling pattern of G-proteins and of other binding sites associated with the complex. This change in G-protein coupling is a stable alteration and thus may serve as a long-term change in the system, which is a requirement of sexual arousability. Stimulation of this receptor system by a surge of oxytocin at ejaculation or orgasm then decouples the G-protein and reduces arousability. Sex hormone binding globulin may be an important ligand at this complex. This model suggests completely new relationships among steroids and their receptors that may complement or diverge from actions at known intracellular receptors.

Arginine-140 and isoleucine-141 determine the 17beta-estradiol-binding specificity of the sex-steroid-binding protein (SBP, or SHBG) of human plasma

Arginine-140 and isoleucine-141 were identified as key determinants of 17beta-estradiol (E(2)) binding affinity of the sex-steroid-binding protein (SBP, or SHBG) of human plasma. Amino acid residues that differ between human and rabbit SBP sequences were replaced in the human protein and the products tested for lowered E(2)binding activity as are seen in the rabbit protein. Only mutants containing either R140K or I141L replacements display an E(2) equilibrium dissociation constant (Kd) higher than the wild type, reaching a value of 30 nM when both were present. The 5alpha-dihydrotestosterone (DHT) equilibrium dissociation constant of these mutants was unaffected. The quadruple mutant M107I/I138V/R140K/I141L yielded an E(2) Kd of 65 nM, significantly closer to the 80 nM rabbit SBP E(2) Kd value. Although mutants containing the M107I and I138V replacements in the absence of R140K and I141L had normal E(2) Kds, the presence of the M107I replacement in the quadruple mutant was necessary to obtain an accurate E(2) Kd value by competitive Scatchard analysis. Molecular modeling using coordinates for the recently determined N-terminal domain of human SBP revealed a significant shift of the F56 phenyl ring away from ring A of E(2) in mutant models containing the R140K and I141L replacements. We conclude that R140 and I141 are required for sustaining the right proximity of the phenyl ring of F56 to ring A of 17beta-estradiol, thus optimizing the E(2)-binding affinity of human SBP.

The control of progesterone receptor expression in MCF-7 breast cancer cells: effects of estradiol and sex hormone-binding globulin (SHBG)

Estradiol controls the gene transcription and expression of many proteins in breast cancer cells, like the progesterone receptor, PR, that is up-regulated by the hormone. Moreover, estradiol is one of the crucial factors inducing the proliferation of breast cancer cells. Sex Hormone-Binding Globulin (SHBG), the plasma carrier for both estradiol and androgens, inhibits the estradiol-induced growth of MCF-7 cells (estrogen-dependent breast cancer cells), through its membrane receptor (SHBG-R), cAMP and PKA. The anti-estrogenic effect of SHBG, which has been described only as far as cell proliferation is concerned, could also play a meaningful role in the estradiol control of other factors in breast cancer cells. In the present study, the effect of SHBG on the estradiol control of PR expression (both mRNA and protein) and function (receptor binding capacity) in MCF-7 cells was examined. SHBG inhibited the estradiol-induced up-regulation of PR mRNA as well as protein level and function. Moreover, the effect of SHBG on estradiol control of PR expression and function was showed to be specific and mediated by PKA. The intermediacy of PKA in the PR expression control, together with the observation that it is effective in the condition in which the SHBG receptor is activated, supports the hypothesis that the anti-estrogenic effect of SHBG could be receptor-mediated. The ability of SHBG to inhibit estradiol action in a specific way in estrogen-dependent breast cancer cells has, therefore, to be taken into account for the development of future therapeutic strategies.

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.

The shbg Gene and Hormone Dependence of Breast Cancer: A Novel Mechanism of Hormone Dependence of MCF-7 Human Breast Cancer Cells Based upon SHBG

BACKGROUND: SHBG (sex hormone binding globulin) is a 45 kDa glycoprotein thatbinds sex steroid with high specificity and affinity. SHBG is produced in various tissues including breast, liver, endometrium, and prostate via activated ER alpha and is secreted into plasma. SHBG regulates the activity of bioavailable sex steroid in plasma and in cells and also modulates cell growth regulation. METHODS: The predictive value of SHBG on the efficacy of hormone therapy against human breast cancer was determined. To evaluate the role of shbg gene expression in estrogen-dependent cell growth of MCF-7 breast cancer, cDNA cloning and determination of the expression of the shbg gene of MCF-7 cells was performed using PCR, RT-PCR Southern blotting. RESULTS: The SHBG titer (17 beta -estradiol binding capacity of SHBG) showed high predictability for the hormone dependence of breast cancer. Tumors of patientswith high SHBG titers showed a 91.8% response rate (N = 49). In contrast, tumors of patients with low SHBG titers showed only an 8.2% response rate (N = 61). >From our experimental results using MCF-7 cells, it is suggested that the SHBG titer includes SHBG secreted from liver and breast cancer cells. MCF-7 cells showed high expression of the wild type shbg gene, hybridized with Hammond's SHBG probe, which represents the 3'portion of SHBG-cDNA cloned from hepatocytes. E2 (17 beta-estradiol) induced the expression of the wild type shbg gene. However, the exon VII splicing variant of the shbg gene did not respond to E2 induction. CONCLUSIONS: From our results and the reports of other investigators, it is suggested that loss of hormone dependence in breast cancer may be caused by the loss of wild type shbg gene and the appearance of the exon VII splicing variant.The shbg-E2 complex binds to SHBG receptor (SHBGR) in cell membrane and internalizes through SHBGR mediated endocytosis causing the production of intracellularcAMP and E2-responsive second messenger. SHBG functions as a nuclear protein. From these data, we prepared a model of a novel mechanism of hormone dependence of breast cancer based upon SHBG and the shbg gene.

Estradiol induction of cAMP in breast cancer cells is mediated by foetal calf serum (FCS) and sex hormone-binding globulin (SHBG)

Plasma sex hormone-binding globulin (SHBG or SBP), the specific carrier for estradiol and androgens, after binding to its membrane receptor (SHBG-R), causes a significant increase of cAMP in the presence of estradiol, in both breast (MCF-7) and prostate (LNCaP) cancer cells maintained in serum-free medium. On the other hand, it has been proposed that estrogens, in addition to the well-known nuclear receptor pathway, exert their biological effect inducing cAMP, as a consequence of a direct membrane action, in breast cancer and uterine cells. The aim of the present study was to clarify this controversial issue by verifying if the cAMP increase in MCF-7 cells was a direct effect of estradiol, or if it was mediated by FCS proteins, such as bovine sex hormone-binding globulin; and to reevaluate the effect of human SHBG on cAMP induction in the presence of FCS. MCF-7 cells were maintained in DCC-FCS (treated with DCC to remove steroids), in SHBG-FREE/DCC-FCS (treated with DCC and with a specific affinity chromatography to remove bovine sex hormone-binding globulin), or in serum-free medium (SFM). It was observed that estradiol determined a significant time-dependent increase of cAMP only in MCF-7 cells maintained in 10% DCC-FCS. When cells were maintained in 10% SHBG-FREE/DCC-FCS, estradiol had no detectable effect. However, its ability to increase cAMP was observed again after the addition of human SHBG, in doses ranging from 5 to 50 nM. Moreover, in the presence of 10% SHBG-FREE/DCC-FCS, SHBG, even in the absence of estradiol, caused a significant increase of cAMP. In conclusion, the data reported in the present study suggest that the ability of estradiol to induce cAMP in MCF-7 cells is not due to a direct membrane effect of the hormone, but rather it is mediated by FCS. SHBG is one of the serum factors mediating estradiol action. Lastly, it was proven that SHBG triggers the cAMP pathway in MCF-7 cells in a physiologic culture condition and at physiologic concentrations.

Sex hormone-binding globulin mediates steroid hormone signal transduction at the plasma membrane

Sex hormone-binding globulin is a plasma glycoprotein that binds certain estrogens and androgens with high affinity. Over the past several years it has been shown that, in addition to functioning as a regulator of the free concentration of a number of steroid hormones, SHBG plays a central role in permitting certain steroid hormones to act without entering the cell. The system is complex. SHBG interacts with a specific, high affinity receptor (R(SHBG)) on cell membranes that appears to transduce its signal via a G protein. The SHBG-R(SHBG) complex causes the activation of adenylyl cyclase and the generation of cAMP within a matter of minutes after exposure to an appropriate steroid. Only steroids that bind to SHBG can activate SHBG-R(SHBG), but not all steroids that bind have this function, e.g. are agonists. All steroids that bind to SHBG but do not activate adenylyl cyclase are antagonists. The signals generated by the steroid-SHBG-R(SHBG) complex generate messages that have effects on the transcriptional activity of classic, intracellular receptors for steroid hormones. These and other downstream effects of this system are reviewed.

Sex hormone-binding globulin, its membrane receptor, and breast cancer: a new approach to the modulation of estradiol action in neoplastic cells

The role of human Sex Hormone-Binding Globulin (SHBG), the plasma carrier of sex steroids, and its membrane receptor, SHBG-R, in estrogen-dependent breast cancer has been investigated in our laboratory in the past few years. SHBG-R is expressed in MCF-10 A cells (not neoplastic mammary cells), MCF-7 cells (breast cancer, ER positive) and in tissue samples from patients affected with ER positive breast cancer, but not in estrogen-insensitive MDA-MB 231 cells. The SHBG/SHBG-R interaction, followed by the binding of estradiol to the complex protein/receptor, causes a significant increase of the intracellular levels of cAMP, but does not modify the amount of estradiol entering MCF-7 cells. The estradiol-induced proliferation of MCF-7 cells is inhibited by SHBG, through SHBG-R, cAMP and PKA. Similarly, the proliferation rate of tissue samples positive for SHBG-R was significantly lower than the proliferation rate of negative samples. SHBG and SHBG-R could thus trigger a 'biologic' anti-estrogenic pathway. In order to get a more detailed knowledge of this system, we first examined the frequence of the reported mutated form of SHBG in 255 breast cancer patients. The mutated SHBG is characterized by a point mutation (Asp 327 --> Asn) causing an additional N-glycosylation site, which does not affect the binding of steroids to SHBG. The frequence of the mutation was significantly higher (24.5%) in estrogen-dependent breast cancers than in healthy control subjects (11.6%). This observation confirms the close relationship between SHBG and estrogen-dependent breast cancer and suggests that the mutation could modify SHBG activity at cell site. Lastly, the possibility of using SHBG to modulate the estradiol action in breast cancer was further studied by transfecting MCF-7 cells with an expression vector carrying the SHBG cDNA (study in collaboration with G.L. Hammond). Transfected cells are able to produce significant amount of SHBG in their medium, but their SHBG-R is reduced to undetectable levels. The SHBG produced by transfected MCF-7 cells is, however, able to inhibit estradiol-induced proliferation of MCF-7 cells expressing a functional receptor. Thus, the local production of SHBG obtained with transfection could be a useful tool to control cell growth in estrogen-dependent breast cancer.

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.

Membrane receptors for steroid hormones: a case for specific cell surface binding sites for vitamin D metabolites and estrogens

Steroid hormones, including vitamin D metabolites and estrogens, activate target cells through specific receptors that discriminate among ligands based upon recognition of distinct structural features. For both classes of ligands, cell surface and nuclear receptors co-exist in many target cells. Upon ligand binding, these receptors generate both rapid and long lasting responses. While the structure of the nuclear receptors and their function as transcriptional activators of specific target genes is generally understood, the identity of the membrane receptors remains elusive. Using pharmacological, functional and biochemical approaches, new insights are being gained into nature of the cell surface receptors for both vitamin D metabolites and estrogens.

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.

Secondary structure and shape of plasma sex steroid-binding protein--comparison with domain G of laminin results in a structural model of plasma sex steroid-binding protein

We have analyzed the secondary structure, shape and dimensions of plasma sex steroid-binding protein (SBP) by CD, size-exclusion chromatography and electron microscopy. CD spectra show extrema at 186 nm and 216 nm characteristic for beta-sheet structures. Analysis with different algorithms indicates 15% alpha-helix, 43% beta-sheet and 10-16% beta-turn structures. An irreversible structural change is observed upon heating above 60 degrees C, which correlates with the loss of steroid-binding activity. As the SBP sequence shows similarity with domains of several multidomain proteins, including laminins, we evaluated the structure of domain G of laminin-1. The CD spectrum shows extrema at 200 nm and 216 nm. Deconvolution results in 13% alpha-helix, 32% beta-sheet and 15% beta-turn structures. Steroid-binding assays indicate that laminin and fragments thereof have no activity. Size-exclusion chromatography reveals that SBP has an extended shape and can be modeled as a cylinder with a length and diameter of 23 nm and 3 nm, respectively. This shape and the dimensions are in agreement with the appearance on electron micrographs. We propose a model for the structure of SBP in which two monomers assemble head to head with the steroid-binding site located in the center of the rod-like particle.

Estradiol activates the prostate androgen receptor and prostate-specific antigen secretion through the intermediacy of sex hormone-binding globulin

These experiments were designed to examine the relationship between the effects of steroid hormones mediated by classic intracellular steroid hormone receptors and those mediated by a signaling system subserved at the plasma membrane by a receptor for sex hormone-binding globulin. It is known that unliganded sex hormone-binding globulin (SHBG) binds to a receptor (RSHBG) on prostate membranes. The RSHBG.SHBG complex is rapidly activated by estradiol to stimulate adenylate cyclase, with a resultant increase in intracellular cAMP. In this paper we examine the effect of this system on a prostate gene product known to be activated by androgens, prostate-specific antigen. In serum-free organ culture of human prostates, dihydrotestosterone caused an increase in prostate specific antigen secretion. This event was blocked by the anti-androgens cyproterone acetate and hydroxyflutamide. In the absence of androgens, estradiol added to prostate tissue, whose RSHBG was occupied by SHBG, reproduced the results seen with dihydrotestosterone. Neither estradiol alone nor SHBG alone duplicated these effects. The estradiol.SHBG-induced increase in prostate-specific antigen was not blocked by anti-estrogens, but was blocked both by anti-androgens and a steroid (2-methoxyestradiol) that prevents the binding of estradiol to SHBG. Furthermore, an inhibitor of protein kinase A prevented the estradiol.SHBG-induced increase in prostate-specific antigen but not that which followed dihydrotestosterone. These data indicate that there is a signaling system that amalgamates steroid-initiated intracellular events with steroid-dependent occurrences generated at the cell membrane and that the latter signaling system proceeds by a pathway that involves protein kinase A.

Sex steroid binding protein receptor (SBP-R) is related to a reduced proliferation rate in human breast cancer

In the last years, an increasing amount of studies described a membrane receptor for the Sex Steroid Binding Protein (SBP) on several androgen-estrogen dependent tissues. One of the suggested biological roles of the interaction between SBP and its receptor seems to be a negative control of the E2 induced proliferation of human breast cancer cells through the cAMP pathway. In the present work, SBP membrane receptor was evaluated on human breast cancer specimens with a radio-binding assay. Each tissue sample was also evaluated for ER and PGR status. Cytosol Thymidine Kinase levels were measured in tissue samples in order to evaluate cell proliferation rate. SBP binding to membranes of ER +/PGR + samples was time and temperature dependent, specific and at high affinity. In addition, SBP recognized on breast cancer membranes two sites at different affinity, as previously described for other human tissues and cultured cells. Membrane SBP-R was detected in a significantly higher number of samples positive for both ER and PGR than in negative samples. SBP-R positive samples showed a significantly lower proliferation rate than SBP-R negative samples as demonstrated by TK activity. The present study contains evidences for the existence of a specific membrane receptor for SBP in breast cancer sample membranes and the presence of SBP-R seems to be strictly related to a lower proliferation rate of the sample.

Predictive value of serum sex hormone binding globulin for the occurrence of hepatocellular carcinoma in male patients with cirrhosis

BACKGROUND/AIMS

The incidence of hepatocellular carcinoma is higher in males, presumably due to the influence of sex steroids. Therefore, to further assess the role of sex steroids in the occurrence of hepatocellular carcinoma, this study investigated the predictive value of different sex hormones and their binding protein.

METHODS

Among 101 male patients with cirrhosis included in a prospective screening study, 29 developed hepatocellular carcinoma. We assessed the predictive value of 17 clinico-biological and 4 serum hormonal variables collected at enrollment, by the log-rank test and the Cox model.

RESULTS

Age (p = 0.003), bilirubin (p = 0.04), sex-hormone-binding-globulin (p = 0.006) and albumin (p = 0.08) were predictive using the log-rank test, while estradiol and total and free testosterone were not. The Cox model showed age (p = 0.0003; relative risk = 7.52), sex-hormone-binding globulin (p = 0.001, relative risk = 3.37) and albumin (p = 0.02, relative risk = 2.94) as the most predictive parameters.

CONCLUSION

We conclude that high serum sex-hormone-binding-globulin levels have an independent predictive value for the occurrence of hepatocellular carcinoma. Serum sex-hormone-binding-globulin could be used to define patients at high risk for hepatocellular carcinoma and could hypothetically play a role in hepatocarcinogenesis.

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.

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.

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.