Structure/function analyses of human sex hormone-binding globulin: effects of zinc on steroid-binding specificity

Mendelian randomization reveals interactions of the blood proteome and immunome in mitral valve prolapse

BACKGROUND

Mitral valve prolapse (MVP) is a common heart disorder characterized by an excessive production of proteoglycans and extracellular matrix in mitral valve leaflets. Large-scale genome-wide association study (GWAS) underlined that MVP is heritable. The molecular underpinnings of the disease remain largely unknown.

METHODS

We interrogated cross-modality data totaling more than 500,000 subjects including GWAS, 4809 molecules of the blood proteome, and genome-wide expression of mitral valves to identify candidate drivers of MVP. Data were investigated through Mendelian randomization, network analysis, ligand-receptor inference and digital cell quantification.

RESULTS

In this study, Mendelian randomization identify that 33 blood proteins, enriched in networks for immunity, are associated with the risk of MVP. MVP- associated blood proteins are enriched in ligands for which their cognate receptors are differentially expressed in mitral valve leaflets during MVP and enriched in cardiac endothelial cells and macrophages. MVP-associated blood proteins are involved in the renewal-polarization of macrophages and regulation of adaptive immune response. Cytokine activity profiling and digital cell quantification show in MVP a shift toward cytokine signature promoting M2 macrophage polarization. Assessment of druggability identify CSF1R, CX3CR1, CCR6, IL33, MMP8, ENPEP and angiotensin receptors as actionable targets in MVP.

CONCLUSIONS

Hence, integrative analysis identifies networks of candidate molecules and cells involved in immune control and remodeling of the extracellular matrix, which drive the risk of MVP.

Challenges and Recommendations in Assessing Potential Endocrine-Disrupting Properties of Metals in Aquatic Organisms

New tools and refined frameworks for identifying and regulating endocrine-disrupting chemicals (EDCs) are being developed as our scientific understanding of how they work advances. Although focus has largely been on organic chemicals, the potential for metals to act as EDCs in aquatic systems is receiving increasing attention. Metal interactions with the endocrine system are complicated because some metals are essential to physiological systems, including the endocrine system, and nonessential metals can have similar physiochemical attributes that allow substitution into or interference with these systems. Consequently, elevated metal exposure could potentially cause endocrine disruption (ED) but can also cause indirect effects on the endocrine system via multiple pathways or elicit physiologically appropriate compensatory endocrine-mediated responses (endocrine modulation). These latter two effects can be confused with, but are clearly not, ED. In the present study, we provide several case studies that exemplify the challenges encountered in evaluating the endocrine-disrupting (ED) potential of metals, followed by recommendations on how to meet them. Given that metals have multiple modes of action (MOAs), we recommend that assessments use metal-specific adverse outcome pathway networks to ensure that accurate causal links are made between MOAs and effects on the endocrine system. We recommend more focus on establishing molecular initiating events for chronic metal toxicity because these are poorly understood and would reduce uncertainty regarding the potential for metals to be EDCs. Finally, more generalized MOAs such as oxidative stress could be involved in metal interactions with the endocrine system, and we suggest it may be experimentally efficient to evaluate these MOAs when ED is inferred. These experiments, however, must provide explicit linkage to the ED endpoints of interest. Environ Toxicol Chem 2023;42:2564-2579. © 2023 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Transgenerational effects of zinc in zebrafish following early life stage exposure

Although toxic effects of zinc (Zn) have been well established in the different developmental stages in fish, long-lasting effects of Zn exposure during embryonic development have not been explored. Exposure to an environmentally relevant Zn concentration of 10 μM (650 μg/L) during the first five days after fertilization did not affect survival, body weight, malformations or overall hatching success of F0 and F1 larvae. Zn exposure did, however, result in delayed hatching in both the F0 and F1 generations and caused significant changes in homeostasis of Zn and selenium (Se) in F0 and F1 fish. This was especially pronounced when F1 embryos from Zn-exposed parents were treated with 30 μM (2000 μg/L) Zn. In the F0 generation, skewed sex ratio towards males and changes in homeostasis of Zn, Se and manganese (Mn) in the brain, gill, liver and gonad of adult fish were also observed. These changes were associated with altered expression of Zn- and Mn-regulatory genes and sex differentiation genes in F0 and F1 fish. The present study suggests that fish may carry memory from embryo-larval Zn exposure into adulthood and further to the next generation. The present study shows that ecotoxicological risk of an exposure to Zn during embryo-larval development may persist long after recovery and may also manifest in the F1 generation.

Characterization and comparison of recombinant full-length ursine and human sex hormone-binding globulin

Sex hormone‐binding globulin (SHBG) regulates the bioavailability of sex steroid hormones in the blood. Levels of SHBG increase markedly in brown bears (Ursus arctos) during hibernation, suggesting that a key regulatory role of this protein is to quench sex steroid bioavailability in hibernation physiology. To enable characterization of ursine SHBG and a cross species comparison, we established an insect cell‐based expression system for recombinant full‐length ursine and human SHBG. Compared with human SHBG, we observed markedly lower secretion levels of ursine SHBG, resulting in a 10‐fold difference in purified protein yield. Both human and ursine recombinant SHBG appeared as dimeric proteins in solution, with a single unfolding temperature of ~ 58 °C. The thermal stability of ursine and human SHBG increased 5.4 and 9.5 °C, respectively, in the presence of dihydrotestosterone (DHT), suggesting a difference in affinity. The dissociation constants for [³H]DHT were determined to 0.21 ± 0.04 nm for human and 1.32 ± 0.10 nm for ursine SHBG, confirming a lower affinity of ursine SHBG. A similarly reduced affinity, determined from competitive steroid binding, was observed for most steroids. Overall, we found that ursine SHBG had similar characteristics to human SHBG, specifically, being a homodimeric glycoprotein capable of binding steroids with high affinity. Therefore, ursine SHBG likely has similar biological functions to those known for human SHBG. The determined properties of ursine SHBG will contribute to elucidating its potential regulatory role in hibernation physiology.

Expression and correlation of sex hormone-binding globulin and insulin signal transduction and glucose transporter proteins in gestational diabetes mellitus placental tissue

OBJECTIVE

The aim of the present study was to investigate the probable pathogenesis of gestational diabetes mellitus (GDM) by analyzing the correlation between sex hormone-binding globulin (SHBG) secreted by the placenta during pregnancy and insulin signaling components and glucose transporter proteins (GLUTs) in the placental tissue.

DESIGN AND METHODS

Placental tissue was collected from full-term and non-obese [body mass index <25kg/m(2)] pregnant women; 10 diagnosed with GDM and 10 with normal pregnancy. We used real-time polymerase chain reaction (PCR), immunohistochemistry and western blotting to detect expression of protein and mRNA of SHBG and insulin signaling components and GLUTs in placental tissue.

RESULTS

In the placental tissue of non-obese women, there was a decrease in expression of SHBG protein and mRNA, with a concurrent decrease in expression of GLUT-4 protein and mRNA in women with GDM compared with normal controls. There was a decrease in GLUT-3 and insulin receptor substrate (IRS)-1 protein expression and lower IRS-2 mRNA expression was also observed in GDM placental tissue. Linear correlation analyses showed a positive correlation between SHBG and IRS-2 mRNA (P=0.038, R(2)=0.2178, y=0.249x+1.4208); positive correlation between SHBG and phosphatidylinositol 3-kinase (PI3K) p85α mRNA (P=0.035, R(2)=0.224, y=0.3506x+0.7433); positive correlation between SHBG and GLUT-4 mRNA (P=0.000, R(2)=0.5174, y=1.3822+1.7811x); positive correlation between IRS-2 and GLUT-4 mRNA (P=0.002, R(2)=0.4064, y=-0.8272+2.9592x); negative correlation between IRS-1 and PI3K p85α mRNA (P=0.005, R(2)=0.366, y=2.4492-0.1929x); negative correlation between IRS-1 and GLUT-3 mRNA (P=0.027, R(2)=0.243, y=0.9254-0.0714x); and positive correlation between IRS-2 and GLUT-1 mRNA (P=0.004, R(2)=0.3794, y=0.0225+0.6298x).

CONCLUSION

The results confirm that defective receptors for insulin signal transduction and GLUT proteins are present in GDM placental tissue. Decreasing expression of SHBG may participate in regulation of insulin signaling, leading to a concomitant decrease in expression of relevant insulin signaling components in placental tissue, implying insulin resistance and eventual development of GDM.

Androgen action in prostate cancer

Prostate cancer represents a major health problem in men worldwide. Androgens are required for the growth and maintenance of the prostate. Androgens act by binding to the androgen receptor (AR), a nuclear receptor transcription factor present in the prostate tissues. Most prostate tumors also retain their androgen dependence; therefore, androgen ablation is usually the preferred initial therapeutic approach for the treatment of advanced prostate cancer patients. This review summarizes the current information regarding the role of androgens in prostate cancer.

In silico identification of anthropogenic chemicals as ligands of zebrafish sex hormone binding globulin

Anthropogenic compounds with the capacity to interact with the steroid-binding site of sex hormone binding globulin (SHBG) pose health risks to humans and other vertebrates including fish. Building on studies of human SHBG, we have applied in silico drug discovery methods to identify potential binders for SHBG in zebrafish (Danio rerio) as a model aquatic organism. Computational methods, including; homology modeling, molecular dynamics simulations, virtual screening, and 3D QSAR analysis, successfully identified 6 non-steroidal substances from the ZINC chemical database that bind to zebrafish SHBG (zfSHBG) with low-micromolar to nanomolar affinities, as determined by a competitive ligand-binding assay. We also screened 80,000 commercial substances listed by the European Chemicals Bureau and Environment Canada, and 6 non-steroidal hits from this in silico screen were tested experimentally for zfSHBG binding. All 6 of these compounds displaced the [(3)H]5alpha-dihydrotestosterone used as labeled ligand in the zfSHBG screening assay when tested at a 33 microM concentration, and 3 of them (hexestrol, 4-tert-octylcatechol, and dihydrobenzo(a)pyren-7(8H)-one) bind to zfSHBG in the micromolar range. The study demonstrates the feasibility of large-scale in silico screening of anthropogenic compounds that may disrupt or highjack functionally important protein:ligand interactions. Such studies could increase the awareness of hazards posed by existing commercial chemicals at relatively low cost.

Progressive Docking: A Hybrid QSAR/Docking Approach for Accelerating In Silico High Throughput Screening

A combination of protein-ligand docking and ligand-based QSAR approaches has been elaborated, aiming to speed-up the process of virtual screening. In particular, this approach utilizes docking scores generated for already processed compounds to build predictive QSAR models that, in turn, assess hypothetical target binding affinities for yet undocked entries. The "progressive docking" has been tested on drug-like substances from the NCI database that have been docked into several unrelated targets, including human sex hormone binding globulin (SHBG), carbonic anhydrase, corticosteroid-binding globulin, SARS 3C-like protease, and HIV1 reverse transcriptase. We demonstrate that progressive docking can reduce the amount of computations 1.2- to 2.6-fold (when compared to traditional docking), while maintaining 80-99% hit recovery rates. This progressive-docking procedure, therefore, substantially accelerates high throughput screening, especially when using high accuracy (slower) docking approaches and large-sized datasets, and has allowed us to identify several novel potent nonsteroidal SHBG ligands.

Calculation of bioavailable and free testosterone in men: a comparison of 5 published algorithms

BACKGROUND

Estimation of serum concentrations of free testosterone (FT) and bioavailable testosterone (bioT) by calculation is an inexpensive and uncomplicated method. We compared results obtained with 5 different algorithms.

METHODS

We used 5 different published algorithms [described by Sodergard et al. (bioTS and FTS), Vermeulen et al. (bioTV and FTV), Emadi-Konjin et al. (bioTE), Morris et al. (bioTM), and Ly et al. (FTL)] to estimate bioT and FT concentrations in samples obtained from 399 independently living men (ages 40-80 years) participating in a cross-sectional, single-center study.

RESULTS

Mean bioT was highest for bioTS (10.4 nmol/L) and lowest for bioT(E) (3.87 nmol/L). Mean FT was highest for FTS (0.41 nmol/L), followed by FTV (0.35 nmol/L), and FTL (0.29 nmol/L). For bioT concentrations, the Pearson correlation coefficient was highest for the association between bioTS and bioTV (r = 0.98) and lowest between bioTM and bioTE (r = 0.66). FTL was significantly associated with both FTS (r = 0.96) and FTV (r = 0.88). The Pearson correlation coefficient for the association between FTL and bioTM almost reached 1.0. Bland-Altman analysis showed large differences between the results of different algorithms. BioTM, bioTE, bioTV, and FTL were all significantly associated with sex hormone binding globulin (SHBG) concentrations.

CONCLUSION

Algorithms to calculate FT and bioT must be revalidated in the local setting, otherwise over- or underestimation of FT and bioT concentrations can occur. Additionally, confounding of the results by SHBG concentrations may be introduced.

Regulation of androgen receptor signaling in prostate cancer

Prostate cancer is a significant cause of morbidity and mortality worldwide. Normal prostate tissue is regulated by androgens, which activate the androgen receptor, a nuclear receptor transcription factor. Most prostate tumors retain androgen dependence, therefore, current therapies for advanced prostate cancer either reduce androgen levels or prevent binding to the androgen receptor. Despite this regimen, prostate cancer invariably progresses to a fatal, androgen-refractory state. Although these relapsed tumors are androgen independent, they are still dependent on the androgen receptor for their growth and survival. The focus of this review will be to highlight our current understanding of the mechanisms of androgen receptor activation in androgen-refractory prostate cancer. How these mechanisms of androgen receptor activation could be targeted in this advanced stage of the disease is also discussed.

‘Inductive' Charges on Atoms in Proteins: Comparative Docking with the Extended Steroid Benchmark Set and Discovery of a Novel SHBG Ligand

We have developed a novel iterative approach for calculation of partial charges in proteins within the framework of the 'molecular capacitance' model. The method operates by an effective 'inductive' electronegativity scale derived from a number of the conventional charge systems including CHARMM, AMBER, MMFF, OPLS, and PEOE among others. Our novel 'inductive' electronegativity equalization procedure allows rapid and conformation sensitive computation of adequate partial charges in proteins. Accuracy of the 'inductive' values was confirmed by their correlation with DFT-computed partial charges in common amino acids. A comparative docking study with an extended steroid data set not only illustrated the adequacy of 'inductive' protein charges but also demonstrated their superior performance compared to several conventional protein charging systems. Subsequent docking with 'inductive' charges resulted in identification of five potential leads as human Sex Hormone Binding Globulin (SHBG) ligands from a commercial library of natural compounds. When the selected substances were evaluated for their ability to bind SHBG in vitro, three of them displaced testosterone from the SHBG steroid-binding site, and with one compound this was achieved at micromolar concentrations.

Successful in Silico Discovery of Novel Nonsteroidal Ligands for Human Sex Hormone Binding Globulin

Using "in silico" drug design methodologies, we have discovered several nonsteroidal compounds of natural origin that bind to human sex hormone binding globulin (SHBG) with affinity constants of 0.1 x 10(6) to 1.2 x 10(6) M(-1). The computational solutions we developed involved pharmacophore-aided database search, virtual protein-ligand docking, and structure-activity modeling with "inductive" QSAR descriptors. By screening 23 836 natural substance structures, we identified 29 potential SHBG ligands, and eight of these bound the protein in vitro. These nonsteroidal ligands belong to four classes of molecular scaffolds with several available substitution positions that could allow chemical modification to enhance SHBG-binding activity. Interestingly, one of these compounds is structurally similar to a dicyclohexane derivative that binds to rat SHBG and causes azospermia when administered to male rats. Taken together, the in silico strategy we have developed will aid in the discovery of nonsteroidal ligands of SHBG with novel pharmacological properties.

Expression of rabbit sex hormone-binding globulin during pregnancy and prenatal development and identification of a novel isoform

SHBG is a homodimeric plasma glycoprotein. It functions as a carrier for sex steroids in blood and regulates their access to target cells. In human and rabbit, SHBG is a single-copy gene comprised of eight exons and is expressed primarily in the liver and testis. In the present study, the ontogeny of rabbit SHBG (rbSHBG) gene expression was examined in both fetus and mothers. Trace amounts of rbSHBG mRNA were detected in fetal liver from d 11 to d 29 gestation. These levels increased dramatically at d 30 and remained high until parturition (d 33). In contrast, high levels of rbSHBG mRNA were detected in the maternal liver early during pregnancy, with maximal levels being attained by d 22 and declining markedly thereafter. A rbSHBG transcript lacking the exon 4 sequences was consistently expressed along with the rbSHBG mRNA. When expressed as a glutathione-S-transferase-fusion protein, this alternatively spliced rbSHBG transcript resulted in a product with almost no steroid binding activity, unlike the full-length rbSHBG-glutathione-S-transferase fusion protein, which bound 5alpha-dihydrotestosterone. Antibody specific to the novel rbSHBG isoform lacking the exon 4-encoding domain was raised, and a single immunoreactive protein of 33-35 kDa was detected by Western blot analysis in both fetal and maternal liver, and this indicates that the rbSHBG transcripts lacking exon 4 sequences are translated in vivo. An RT-PCR analysis further revealed that this alternatively spliced SHBG transcript is present in human HepG2 cells as well as human and mouse testes, indicating that exon 4 splicing in SHBG transcription is conserved among mammalian species. To our knowledge, this is the first report of the identification of a SHBG exon 4 splice variant that is translated. Because the SHBG isoform it encodes lacks appreciable steroid-binding activity, it may function beyond that of the widely accepted role of SHBG as a steroid-transport protein.

Empirical estimation of free testosterone from testosterone and sex hormone-binding globulin immunoassays

BACKGROUND

The growing interest in measuring blood free testosterone (FT) is constrained by the unsuitability of the laborious reference methods for wider adoption in routine diagnostic laboratories. Various alternative derived testosterone measures have been proposed to estimate FT from either additional assay steps or calculations using total testosterone (TT) and sex hormone-binding globulin (SHBG) measured in the same sample. However, none have been critically validated in large numbers of blood samples.

METHODS

We analyzed a large dataset comprising over 4000 consecutive blood samples in which FT as well as TT and SHBG were measured. Dividing the dataset into samples with blood TT above and below 5 nM, using a bootstrap regression modeling approach guided by Akaike Information Criterion for model selection to balance parsimony against reduction of residual error, empirical equations were developed for FT in terms of TT and SHBG.

RESULTS

Comparison between the empirical FT equations with the laboratory FT measurements as well as three widely used calculated FT methods showed the empirical FT formulae had superior fidelity with laboratory measurements while previous FT formulae overestimated and deviated systematically from the laboratory FT values.

CONCLUSION

We conclude that these simple, assumption-free empirical FT equations can estimate accurately blood FT from TT and SHBG measured in the same samples with the present assay methods and have suitable properties for wider application to evaluate the clinical utility of blood FT measurements.