Naturally occurring mutants inform SHBG structure and function

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

OBJECTIVES

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Role of sex hormone-binding globulin in the free hormone hypothesis and the relevance of free testosterone in androgen physiology

According to the free hormone hypothesis, biological activity of a certain hormone is best reflected by free rather than total hormone concentrations. A crucial element in this theory is the presence of binding proteins, which function as gatekeepers for steroid action. For testosterone, tissue exposure is governed by a delicate equilibrium between free and total testosterone which is determined through interaction with the binding proteins sex hormone-binding globulin and albumin. Ageing, genetics and various pathological conditions influence this equilibrium, hereby possibly modulating hormonal exposure to the target tissues. Despite ongoing controversy on the subject, strong evidence from recent in vitro, in vivo and human experiments emphasizes the relevance of free testosterone. Currently, however, clinical possibilities for free hormone diagnostics are limited. Direct immunoassays are inaccurate, while gold standard liquid chromatography with tandem mass spectrometry (LC-MS/MS) coupled equilibrium dialysis is not available for clinical routine. Calculation models for free testosterone, despite intrinsic limitations, provide a suitable alternative, of which the Vermeulen calculator is currently the preferred method. Calculated free testosterone is indeed associated with bone health, frailty and other clinical endpoints. Moreover, the added value of free testosterone in the clinical diagnosis of male hypogonadism is clearly evident. In suspected hypogonadal men in whom borderline low total testosterone and/or altered sex hormone-binding globulin levels are detected, the determination of free testosterone avoids under- and overdiagnosis, facilitating adequate prescription of hormonal replacement therapy. As such, free testosterone should be integrated as a standard biochemical parameter, on top of total testosterone, in the diagnostic workflow of male hypogonadism.

Accurate measurement of total and free testosterone levels for the diagnosis of androgen disorders

The circulating concentrations of total and free testosterone vary substantially in people over time due to biologic factors as well as due to measurement variation. Accurate measurement of total and free testosterone is essential for making the diagnosis of androgen disorders. Total testosterone should ideally be measured in a fasting state in the morning using a reliable assay, such as liquid chromatography tandem mass spectrometry, in a laboratory that is certified by an accuracy-based benchmark. Free testosterone levels should be measured in men in whom alterations in binding protein concentrations are suspected or in whom total testosterone levels are only slightly above or slightly below the lower limit of the normal male range for testosterone.

Homozygous SHBG Variant (rs6258) Linked to Gonadotropin-Independent Precocious Puberty in a Young Girl

Sex hormone-binding globulin (SHBG) in the blood is a major determinant of bioactivity for key sex steroids such as testosterone and estradiol. Low serum levels of SHBG have been associated with obesity, polycystic ovaries, and metabolic syndrome, and other states associated with hyperandrogenemia. A 9-year, 6-month-old girl presented with a history of peripheral precocious puberty and aggressive behavior. The patient's SHBG level was remarkably low for her age, at less than 5 nmol/L (reference range for a girl with a bone age of 10 years, 73 nmol/L [SEM = 10]) [1]. On genetic and protein analysis, the patient was found to have a homozygous missense potentially pathogenic variant in the SHBG gene (c.554C>T, p.P185L); her parents were asymptomatic heterozygote carriers. Laboratory investigations supported the possible involvement of this genetic alteration in the patient's phenotype. Various analyses of this variant support its pathogenicity, although the exact mechanism remains unclear. In conclusion, we present a genetic SHBG variant in the homozygote state that may have been associated with gonadotropin-independent precocious puberty in a young girl.

Hindlimb unloading-induced reproductive suppression via Downregulation of hypothalamic Kiss-1 expression in adult male rats

BACKGROUND

Spaceflights-induced microgravity can alter various physiological processes in human's body including the functional status of the reproductive system. Rodent model of tail-suspension hindlimb unloading is extensively used to stimulate the organs responses to the microgravity condition. This study explores the potential effects of hindlimb unloading on testicular functions and spermatogenesis in adult male rats and the underlying mechanism/s.

METHODS

Twenty Sprague-Dawley rats were allotted into two groups: normally loaded group (control; all arms were in touch with the grid floor) and hindlimb unloaded group (HU; only the forearms were in contact with the grid floor).

RESULTS

Following 30 days of exposure, the HU group saw a decline in body weight, testicular and epidydimal weights, and all semen parameters. The circulating concentrations of gonadotropin-releasing hormone (GnRH), follicle stimulating hormone (FSH), luteinizing hormone (LH) and testosterone significantly decreased, while levels of kisspeptin, corticosterone, inhibin, prolactin and estradiol (E2) increased in the HU group. Intratesticular levels of 5α-reductase enzyme and dihydrotestosterone (DHT) were suppressed, while the levels of aromatase and kisspeptin were significantly elevated in the HU group. Hypothalamic kisspeptin (Kiss1) mRNA expression levels were downregulated while its receptors (Kiss1R) were upregulated in the HU group. On the contrary, the mRNA expression levels of testicular Kiss1 were upregulated while Kiss1R were downregulated. The pituitary mRNA expression levels of FSHβ and LHβ decreased in the HU group. The levels of the antioxidant enzymes, superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx) and nitric oxide (NO) concentrations, and total antioxidant capacity (TAC) were elevated while malondialdehyde (MDA) concentrations declined in the testes of HU group. The testes of the HU rats showed positive immunostaining of caspase-3, heat shock protein 70 (HSP70) and Bcl2.

CONCLUSIONS

Altogether, these results revealed an inhibitory effect of hindlimb unloading on kisspeptin signaling in the hypothalamic-pituitary-testicular axis with impaired spermatogenesis and steroidogenesis.

Research progress on the relationship between sex hormone-binding globulin and male reproductive system diseases

Sex hormone-binding globulin, also known as testosterone-estradiol-binding globulin, is a multifunctional protein synthesised by hepatocytes. Sex hormone-binding globulin specifically binds and transports sex hormones to regulate plasma bioactive sex hormone levels and affects their bioavailability. As male sex hormone expression is dominated by testosterone, the binding of sex hormone-binding globulin with testosterone leads to the reduction in bioavailable testosterone, which cannot fulfil its physiological roles, thereby resulting in male infertility, erectile and gonadal dysfunction, prostate cancer and other male reproductive system diseases. Sex hormone-binding globulin may be involved in the pathogenesis of male reproductive system diseases, seriously affecting the quality of life of men. In this article, we review the association between sex hormone-binding globulin and male reproductive system diseases.

Association between human SHBG gene polymorphisms and risk of PCOS: a meta-analysis

Sex hormone-binding globulin (SHBG) is a plasma glycoprotein that binds androgens and oestrogens, and regulates their bioavailability to target tissues. To date, several human SHBG gene polymorphisms have been identified. Polycystic ovary syndrome (PCOS) is one of the most common endocrine disorders, and hyperandrogenism has been considered to be a hallmark of PCOS. Many studies have examined the association between SHBG gene polymorphisms and PCOS risk, but the results have been inconclusive or inconsistent. Therefore, the aim of this meta-analysis was to investigate whether SHBG gene polymorphisms are associated with risk of PCOS. Twelve studies were included, involving 4733 participants: 2271 patients with PCOS and 2462 control participants. The results revealed that SHBG polymorphism of eight or more (TAAAA)n pentanucleotide repeats (rs35785886) was associated with PCOS risk (odds ratio [OR] = 1.24, 95% confidence interval [CI] = 1.06, 1.44, Z = 2.77, P = 0.006) and low serum SHBG concentrations in women with PCOS (standardized mean difference = -0.83, 95% CI = -1.54, -0.12, Z = 2.30, P = 0.02). Other SHBG gene polymorphisms (rs6259, rs6257, rs727428 and rs1799941) were not significantly associated with either PCOS risk or serum SHBG concentrations. These findings suggest that the presence of a polymorphism of eight or more SHBG (TAAAA)_n may be a predictive factor for the risk of PCOS.

Correlation of Fibulin-2 expression with proliferation, migration and invasion of breast cancer cells

Expression level of Fibulin-2 gene in breast cancer cells was evaluated to explore the impact of Fibulin-2 gene on the proliferation, migration and invasion of breast cancer cells. MDA-MB-231, BT483, MCF-7 and SK-BR-3 breast cancer cells were cultured in vitro. Then, expression of Fibulin-2 in cells was upregulated and downregulated using ribonucleic acid interference (RNAi) and lentiviral transfection techniques, respectively. Thereafter, expression levels of Fibulin-2 messenger RNA (mRNA) and protein were measured via quantitative real-time reverse transcription-polymerase chain reaction and western blotting, respectively. Cell Counting Kit-8 assay was applied to detect the proliferation ability, and wound healing assay was performed to determine the effect of transfection on the metastatic capacity of cells. The influence of transfection on the invasive ability of breast cancer cells was detected through Transwell chamber assay. MDA-MB-231 and MCF-7 cells did not express Fibulin-2, while BT483 and SK-BR-3 cells expressed Fibulin-2. Expression of Fibulin-2 mRNA and protein in SK-BR-3 Fibulin-2 siRNA group was significantly lower than that in SK-BR-3 NC siRNA group 48 h after transfection (P<0.01), while the expression of Fibulin-2 mRNA and protein in MDA-MB-231 Fibulin-2 lentiviral transfection (LAP) group was significantly higher than that in MDA-MB-231 NC LAP group. Compared with the MDA-MB-231 NC LAP group, the cell proliferation, migration and invasion ability of MDA-MB-231 Fibulin-2 LAP group were weakened. The tumor volume and weight of the MDA-MB-231 Fibulin-2 LAP group were significantly lower than those of the MDA-MB-231 NC LAP group. Low expression of Fibulin-2 is able to promote proliferation, migration and invasion of breast cancer cells, and can reduce the rate of tumor growth in nude mice. Therefore, Fibulin-2 may be a potential therapeutic target and an indicator of prognosis for breast cancer.

Molecular insights into how SHBG dimerization exerts changes on ligand molecular recognition

Sex hormone binding globulin (SHBG) is a homodimeric glycoprotein and is the major carrier protein for sex steroids in plasma, regulating sex hormone availability in most vertebrate groups. Although it was initially thought that human dimeric SHBG bound a single ligand at the homodimer interface, studies demonstrated that dimeric SHBG binds a ligand to each subunit with similar affinity. In fact, the findings from recent experimental studies suggest that ligand binding to the SHBG dimer involves a complex allosteric mechanism involving conformational changes that limit observations of the presence of allosteric regulation. Therefore, we combined structural data with molecular dynamics simulations using Molecular Mechanics Generalized-Born Surface Area (MMGBSA) to dissect the structural and energetic basis for molecular recognition between five ligands whose affinities and binding positions on SHBG are known, i.e., 3β,17α-diol; 3β,17β-diol; DHT; norgestrel (NOG); and estradiol (E2), and monomeric and dimeric SHBG. Protein-ligand complexes, involving dimeric SHBG saturated with two ligands on each subunit, reproduce the experimental affinity tendency and allow the observation that dimerization exerts disparate effects on binding affinity, characteristic of negative cooperativity for E2, DHT, and NOG, whereas 3β-17α-diol and 3β-17β-diol lack allostery.

Molecular interactions between sex hormone-binding globulin and nonsteroidal ligands that enhance androgen activity

Sex hormone-binding globulin (SHBG) determines the equilibrium between free and protein-bound androgens and estrogens in the blood and regulates their access to target tissues. Using crystallographic approaches and radiolabeled competitive binding-capacity assays, we report here how two nonsteroidal compounds bind to human SHBG, and how they influence androgen activity in cell culture. We found that one of these compounds, (-)3,4-divanillyltetrahydrofuran (DVT), present in stinging nettle root extracts and used as a nutraceutical, binds SHBG with relatively low affinity. By contrast, a synthetic compound, 3-(1H-imidazol-1-ylmethyl)-2phenyl-1H-indole (IPI), bound SHBG with an affinity similar to that of testosterone and estradiol. Crystal structures of SHBG in complex with DVT or IPI at 1.71-1.80 Å resolutions revealed their unique orientations in the SHBG ligand-binding pocket and suggested opportunities for the design of other nonsteroidal ligands of SHBG. As observed for estradiol but not testosterone, IPI binding to SHBG was reduced by ∼20-fold in the presence of zinc, whereas DVT binding was almost completely lost. Estradiol-dependent fibulin-2 interactions with SHBG similarly occurred for IPI-bound SHBG, but not with DVT-bound SHBG. Both DVT and IPI increased the activity of testosterone in a cell culture androgen reporter system by competitively displacing testosterone from SHBG. These findings indicate how nonsteroidal ligands of SHBG maybe designed to modulate the bioavailability of sex steroids.

Testosterone levels and the genetic variation of sex hormone-binding globulin gene of Bubalus bubalis, bulls in Egypt

Testosterone is themain plasma androgen produced and secreted by male testis. To be bioactive, it binds to sex hormonebinding globulin (SHBG), i.e. the major transporter protein of sex steroids in the blood of mammals and other vertebrate species.Firstly, this study aimed to determine the levels of testosterone in different-age groups of buffalo bulls (Bubalus bubalis) in Egypt, and secondly to screen the genetic polymorphisms in their SHBG gene and to investigatewhether these polymorphisms are associated with the level of the circulating serum testosterone. Blood samples were collected from 67 Egyptian buffalo bulls representing four different age-groups and testosterone concentration was measured by radioimmunoassay. Selected DNA samples from different age-groups were sequenced for the full length of theirSHBGgene.Asignificant increase in the concentration of serum testosterone in buffalo bulls was recorded with the advancement of age from 12/18 to 24/36 months. Based on the number of polymorphisms in SHBG gene, the current study reveals the presence of three bull genotypes, of which one is likely to be associated with low testosterone concentration, while other is likely to be associated with an increased testosterone concentration. Such genetic associations can provide a good tool for the selection of bull genotypes with higher testosterone concentrations, which are indispensible for breeding purposes.

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

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

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

OBJECTIVE

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

DESIGN

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Effects of sex hormone-binding globulin (SHBG) on androgen bioactivity in vitro

Biochemical assessments of androgen status (hyper- or hypoandrogenism) are usually based on serum testosterone concentrations. According to the free hormone hypothesis, sex hormone-binding globulin (SHBG) determines free and bioavailable testosterone concentrations. Previous studies have suggested that in vitro androgen bioassay results may also be influenced by SHBG and correlate with free or bioavailable testosterone concentrations. To test this hypothesis, we established a stable HEK293 cell line with high expression of the human androgen receptor (AR) and a luciferase reporter downstream of a classical androgen response element. Importantly, we demonstrate that bioassay results are sensitive to dilution effects which increase apparent bioactivity in an SHBG-dependent manner. We therefore adopted a method using undiluted serum, which reduced cell proliferation but did not significantly affect the luciferase signal, cell viability or cytotoxicity. To correct for serum matrix effects, we applied signal correction based on internal controls with AR agonists or antagonists. Using this method, we provide direct evidence that in vitro androgen bioactivity reflects the inhibitory effects of SHBG, and correlates with free or bioavailable testosterone concentrations in adult hypogonadal men receiving androgen replacement therapy. In men receiving anti-androgens, serum bioactivity decreased tenfold while serum testosterone concentrations decreased only four-fold. Further pilot results in prostate cancer patients showed that androgen synthesis inhibitors result in more complete inhibition of androgen bioactivity than gonadorelin-based androgen deprivation therapy, even in patients whose testosterone concentrations were undetectable by mass spectrometry. We conclude that in vitro androgen reporter bioassays are useful tools to study how androgen bioactivity in serum is determined by androgens, anti-androgens as well as SHBG, provided that dilution and matrix effects are accounted for.

Sex hormone-binding globulin regulation of androgen bioactivity in vivo: validation of the free hormone hypothesis

Sex hormone-binding globulin (SHBG) is the high-affinity binding protein for androgens and estrogens. According to the free hormone hypothesis, SHBG modulates the bioactivity of sex steroids by limiting their diffusion into target tissues. Still, the in vivo physiological role of circulating SHBG remains unclear, especially since mice and rats lack circulating SHBG post-natally. To test the free hormone hypothesis in vivo, we examined total and free sex steroid concentrations and bioactivity on target organs in mice expressing a human SHBG transgene. SHBG increased total androgen and estrogen concentrations via hypothalamic-pituitary feedback regulation and prolonged ligand half-life. Despite markedly raised total sex steroid concentrations, free testosterone was unaffected while sex steroid bioactivity on male and female reproductive organs was attenuated. This occurred via a ligand-dependent, genotype-independent mechanism according to in vitro seminal vesicle organ cultures. These results provide compelling support for the determination of free or bioavailable sex steroid concentrations in medicine, and clarify important comparative differences between translational mouse models and human endocrinology.

Plasma steroid-binding proteins: primary gatekeepers of steroid hormone action

Biologically active steroids are transported in the blood by albumin, sex hormone-binding globulin (SHBG), and corticosteroid-binding globulin (CBG). These plasma proteins also regulate the non-protein-bound or 'free' fractions of circulating steroid hormones that are considered to be biologically active; as such, they can be viewed as the 'primary gatekeepers of steroid action'. Albumin binds steroids with limited specificity and low affinity, but its high concentration in blood buffers major fluctuations in steroid concentrations and their free fractions. By contrast, SHBG and CBG play much more dynamic roles in controlling steroid access to target tissues and cells. They bind steroids with high (~nM) affinity and specificity, with SHBG binding androgens and estrogens and CBG binding glucocorticoids and progesterone. Both are glycoproteins that are structurally unrelated, and they function in different ways that extend beyond their transportation or buffering functions in the blood. Plasma SHBG and CBG production by the liver varies during development and different physiological or pathophysiological conditions, and abnormalities in the plasma levels of SHBG and CBG or their abilities to bind steroids are associated with a variety of pathologies. Understanding how the unique structures of SHBG and CBG determine their specialized functions, how changes in their plasma levels are controlled, and how they function outside the blood circulation provides insight into how they control the freedom of steroids to act in health and disease.

Gene changes may minimize masculinizing and defeminizing influences of exposure to male cotwins in female callitrichine primates

BACKGROUND

Sexual differentiation in female mammals can be altered by the proximity of male littermates in utero, a phenomenon known as the intrauterine position effect (IUP). Among simian primates, callitrichines (marmosets and tamarins) are likely candidates for IUP, since they exhibit obligate dizygotic twinning and fetuses share extensive vascularization in utero. In this paper, we determined whether female reproductive parameters are altered by gestating with a male twin and evaluated changes in genes associated with anti-Müllerian and steroid hormones in twinning callitrichine primates.

METHODS

We assessed the impact of gestation with male cotwins on reproductive performance and survivorship in female marmosets (Callithrix) and lion tamarins (Leontopithecus), contrasting measures for females gestated with one or more littermates (M+) or no male littermates (0M). We compared targeted coding regions for genes involved in steroidal and anti-Müllerian hormone mediation of sexual differentiation for representatives of twinning callitrichines (Callithrix, Saguinus, and Leontopithecus) with closely related New World primates that produce single births (Saimiri and Callimico).

RESULTS

IUP effects in females were absent in female callitrichine primates: age at first ovulation, average litter size, and the proportion of stillborn infants, and lifetime survivorship did not differ between M+ and 0M females. We documented multiple nonsynonymous substitutions in genes associated with steroid synthesis, transport, and cellular action (SRD5A2, CYP19A1, SHBG, and AR) and with anti-Müllerian hormone (AMH and AMHR2) in callitrichines. In the only callitrichine to produce single infants (Callimico), two genes contained nonsynonymous substitutions relative to twinning callitrichines (CYP19A1 and AMRHR2); these substitutions were identical with nontwinning Saimiri and humans, suggesting a reversion to an ancestral sequence.

CONCLUSIONS

In spite of a shared placental vasculature with opposite-sex twins throughout embryonic and fetal development, female callitrichine primates gestated with a male cotwin exhibit no decrement in reproductive performance relative to females gestated with female cotwins. Hence, IUP effects on female reproduction in callitrichines are modest. We have identified mutations in candidate genes relevant for steroid hormone signaling and metabolism, and especially in AMH-related genes, that are likely to alter protein structure and function in the callitrichines. These mutations may confer protection for females from the masculinizing and defeminizing influences of gestating with a male cotwin.

Effects of sulfonylurea as initial treatment on testosterone of middle-aged men with type 2 diabetes: A 16-week, pilot study

AIMS/INTRODUCTION

To evaluate the effect of sulfonylurea (glimepiride)-based oral antidiabetic agents on testosterone levels in middle-aged men with type 2 diabetes.

MATERIALS AND METHODS

As a substudy, 15 participants from the phase IV clinical trial of glimepiride (GREAT study) of middle-aged men with type 2 diabetes were included in the current study. After enrolment, the initial dose of oral glimepiride was 1 mg/day. The dose was titrated according to blood glucose levels and the participants were treated for 16 weeks. Meanwhile, another 15 healthy age- and body mass index-matched male subjects were randomly selected as the healthy control group.

RESULTS

Compared with the healthy control group, the middle-aged men with type 2 diabetes had significantly decreased total testosterone levels and a lower testosterone secretion index. Blood glucose and lipid profile levels were significantly improved after 16 weeks of treatment with no significant differences in bodyweight and waist circumference compared with baseline values. Recorded changes in luteinizing hormone, follicle-stimulating hormone and sex hormone-binding globulin levels were not statistically significant. However, total testosterone levels were significantly increased and testosterone secretion index values were significant higher than those of the baseline.

CONCLUSIONS

It is highly possible that sulfonylurea as an initial treatment can recover the decreased total serum testosterone levels and testosterone secretion index values in middle-aged men with type 2 diabetes.

Estrogen: an emerging regulator of insulin action and mitochondrial function

Clinical trials and animal studies have revealed that loss of circulating estrogen induces rapid changes in whole body metabolism, fat distribution, and insulin action. The metabolic effects of estrogen are mediated primarily by its receptor, estrogen receptor-α; however, the detailed understanding of its mechanisms is incomplete. Recent investigations suggest that estrogen receptor-α elicits the metabolic effects of estrogen by genomic, nongenomic, and mitochondrial mechanisms that regulate insulin signaling, substrate oxidation, and energetics. This paper reviews clinical and experimental studies on the mechanisms of estrogen and the current state of knowledge regarding physiological and pathobiological influences of estrogen on metabolism.

Reproductive endocrinology: functional effects of sex hormone-binding globulin variants

A new study has found eight single nucleotide polymorphisms in sex hormone-binding globulin that functionally affect its affinity for androgens or estrogens and other biochemical properties. This finding adds to growing concern about the ‘one size fits all’ approach in formulas to calculate free or bioavailable concentrations of estradiol and testosterone.