Exploring the mechanism of interaction between TBG and halogenated thiophenols: Insights from fluorescence analysis and molecular simulation

Thyroxine-binding globulin (TBG) plays a vital role in regulating metabolism, growth, organ differentiation, and energy homeostasis, exerting significant effects in various key metabolic pathways. Halogenated thiophenols (HTPs) exhibit high toxicity and harmfulness to organisms, and numerous studies have demonstrated their thyroid-disrupting effects. To understand the mechanism of action of HTPs on TBG, a combination of competitive binding experiments, multiple fluorescence spectroscopy techniques, molecular docking, and molecular simulations was employed to investigate the binding mechanism and identify the binding site. The competition binding assay between HTPs and ANS confirmed the competition of HTPs with thyroid hormone T4 for the active site of TBG, resulting in changes in the TBG microenvironment upon the binding of HTPs to the active site. Key amino acid residues involved in the binding process of HTPs and TBG were further investigated through residue energy decomposition. The distribution of high-energy contributing residues was determined. Analysis of root-mean-square deviation (RMSD) demonstrated the stability of the HTPs-TBG complex. These findings confirm the toxic mechanism of HTPs in thyroid disruption, providing a fundamental reference for accurately assessing the ecological risk of pollutants and human health. Providing mechanistic insights into how HTPS causes thyroid diseases.

Exploring thyroxine binding globulin structural changes and its release from human hepatoblastoma cells upon interaction with silica particles: A prelude to unrevealing the mechanism of thyroid hormone dysregulation

Endocrine dysregulation in the presence of environmental chemical risk factors is a global adverse health concern. The aim of this investigation was to explore the structural changes and binding affinity of thyroxine (T4) binding protein (TBG) upon interaction with SiO2 particles as the second largest mineral in the Earth's crust and one of the most important constituents of rock, soil, and dust. Therefore, the interaction of TBG with SiO2 particles was assessed by fluorescence quenching, molecular docking, ANS and synchronous fluorescence, and far-UV CD analyses. Also, the release of TBG from human hepatoblastoma cell line, Hep G2, was assessed by ELISA assay. The results displayed that the value of stoichiometry of binding site (n) of TBG for T4 was approximately equal to one, which was reduced to 0.36 in the presence of SiO2 particles. Also, the binding affinity (Kb) values revealed that the binding affinity between T4 and TBG was strong (97.90 × 105 L/mol), while the presence of SiO2 particles resulted in the calculation of a Kb around 0.00159 × 105 L/mol, which was significantly lower than that of the absence of SiO2 particles. This data was also verified by molecular docking analyses which indicated that SiO2 particles interacted with the T4 binding pocket of TBG. Moreover, further studies exhibited that although the equimolar concentration of T4 to TBG resulted in the superior stability of TBG-T4 complex relative to free TBG, the presence of SiO2 particles with the same concentration led to denaturation of the secondary structure of TBG. Furthermore, it was seen that the amount of released TBG in the cell culture medium of Hep G2 was about 2.21 ng/mL protein, whereas this amount in SiO2 particles-treated cell group was significantly reduced to 1.71 ng/mL protein (*P < 0.05). In conclusion, this study implies that SiO2 particles show the potential to result in inhibition of TBG release, TBG denaturation, and interfere with TBG binding affinity which may lead to dysregulation of the thyroid hormone transport and associated signaling pathways.

Identification of Mutations in the Thyroxine-Binding Globulin (TBG) Gene in Patients with TBG Deficiency in Korea

BACKGRUOUND

Thyroxine-binding globulin (TBG) is a major transporter protein for thyroid hormones. The serpin family A member 7 (SERPINA7) gene codes for TBG, and mutations of the SERPINA7 gene result in TBG deficiency. Although more than 40 mutations have been reported in several countries, only a few studies of TBG deficiency and SERPINA7 gene mutation have been performed in Korea. The aim of this study is to review the clinical presentations and laboratory findings of patients with TBG deficiency and to investigate the types of SERPINA7 gene mutation.

METHODS

Five unrelated Korean adults with TBG deficiency attending endocrinology clinic underwent SERPINA7 gene sequencing. Four patients harbored a SERPINA7 gene mutation. Serum thyroid hormones, anti-microsomal antibodies, and TBG were measured. Genomic DNA was extracted from whole blood. All exons and intron-exon boundaries of the TBG gene were amplified and sequencing was performed.

RESULTS

Two patients were heterozygous females, and the other two were hemizygous males. One heterozygous female had coexisting hypothyroidism. The other heterozygous female was erroneously prescribed levothyroxine at a local clinic. One hemizygous male harbored a novel mutation, p.Phe269Cysfs*18, which caused TBG partial deficiency. Three patients had the p.Leu372Phefs*23 mutation, which is known as TBG-complete deficiency Japan (TBG-CDJ) and was also presented in previous mutation analyses in Korea.

CONCLUSION

This study presents four patients diagnosed with TBG deficiency and provides the results of SERPINA7 gene sequencing. One novel mutation, p.Phe269Cysfs*18, causing TBD-partial deficiency and three cases of TBG-CDJ were demonstrated. It is necessary to identify TBG deficiency to prevent improper treatment. Also, sequencing of the SERPINA7 gene would provide valuable information about the TBG variants in Korea.

Sex Difference Leads to Differential Gene Expression Patterns and Therapeutic Efficacy in Mucopolysaccharidosis IVA Murine Model Receiving AAV8 Gene Therapy

Adeno-associated virus (AAV) vector-based therapies can effectively correct some disease pathology in murine models with mucopolysaccharidoses. However, immunogenicity can limit therapeutic effect as immune responses target capsid proteins, transduced cells, and gene therapy products, ultimately resulting in loss of enzyme activity. Inherent differences in male versus female immune response can significantly impact AAV gene transfer. We aim to investigate sex differences in the immune response to AAV gene therapies in mice with mucopolysaccharidosis IVA (MPS IVA). MPS IVA mice, treated with different AAV vectors expressing human N-acetylgalactosamine 6-sulfate sulfatase (GALNS), demonstrated a more robust antibody response in female mice resulting in subsequent decreased GALNS enzyme activity and less therapeutic efficacy in tissue pathology relative to male mice. Under thyroxine-binding globulin promoter, neutralizing antibody titers in female mice were approximately 4.6-fold higher than in male mice, with GALNS enzyme activity levels approximately 6.8-fold lower. Overall, male mice treated with AAV-based gene therapy showed pathological improvement in the femur and tibial growth plates, ligaments, and articular cartilage as determined by contrasting differences in pathology scores compared to females. Cardiac histology revealed a failure to normalize vacuolation in females, in contrast, to complete correction in male mice. These findings promote the need for further determination of sex-based differences in response to AAV-mediated gene therapy related to developing treatments for MPS IVA.

Binding and Activity of Tetrabromobisphenol A Mono-Ether Structural Analogs to Thyroid Hormone Transport Proteins and Receptors

BACKGROUND

Tetrabromobisphenol A (TBBPA) mono-ether structural analogs, identified as the by-products or transformation products of commercial TBBPA bis-ether derivatives, have been identified as emerging widespread pollutants. However, there is very little information regarding their toxicological effects.

OBJECTIVE

We aimed to explore the potential thyroid hormone (TH) system-disrupting effect of TBBPA mono-ether structural analogs.

METHODS

The binding potencies of chemicals toward human TH transport proteins [transthyretin (TTR) and thyroxine-binding globulin (TBG)] and receptors [TRα ligand-binding domain (LBD) and TRβ-LBD] were determined by fluorescence competitive binding assays. Molecular docking was used to simulate the binding modes of the chemicals with the proteins. The cellular TR-disrupting potencies of chemicals were assessed by a GH3 cell proliferation assay. The intracellular concentrations of the chemicals were measured by high-performance liquid chromatography and mass spectrometry.

RESULTS

TBBPA mono-ether structural analogs bound to TTR with half maximal inhibitory concentrations ranging from 0.1μM to 1.0μM but did not bind to TBG. They also bound to both subtypes of TR-LBDs with 20% maximal inhibitory concentrations ranging from 4.0μM to 50.0μM. The docking results showed that the analogs fit into the ligand-binding pockets of TTR and TR-LBDs with binding modes similar to that of TBBPA. These compounds likely induced GH3 cell proliferation via TR [with the lowest effective concentrations (LOECs) ranging from 0.3μM to 2.5μM] and further enhanced TH-induced GH3 cell proliferation (with LOECs ranging from 0.3μM to 1.2μM). Compared with TBBPA, TBBPA-mono(2,3-dibromopropyl ether) showed a 4.18-fold higher GH3 cell proliferation effect and 105-fold higher cell membrane transportation ability.

CONCLUSION

This study provided a possible mechanism underlying the difference in TTR or TR binding by novel TBBPA structural analogs. These compounds might exert TH system-disrupting effects by disrupting TH transport in circulation and TR activity in TH-responsive cells. https://doi.org/10.1289/EHP6498.

Disruption of thyroid hormone regulated proteins and gene expression by polychlorinated biphenyls, polybrominated diphenyl ethers and new flame retardants in residents of an e-waste region

Polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs) and new flame retardants (NFRs) are known thyroid hormone (TH) disruptors, but their disrupting mechanisms in humans are not completely understood. In this study, we aimed to explore the disrupting mechanisms of the aforementioned chemicals via examining TH-regulated proteins and gene expression in human serum. Adult participants from an e-waste dismantling (exposed group) and a control region (control group) in South China provided blood samples for the research. Some compounds of PCBs, PBDEs, and NFRs showed strong binding affinity to the thyroid-stimulating hormone (TSH), thyroglobulin, thyroxine-binding globulin (TBG), gene expression of TH receptor α (TRα) and β, and iodothyronine deiodinase I (ID1). The highly exposed individuals had lower levels of TBG, TSH, and expression of TRα, but higher expression of ID1 than those of the control group. The disruption of TH-regulated proteins and gene expression suggested the exertion of different and, at times, even contradictory effects on TH disruption. However, no statistically significant difference was found in the TH levels between the exposed and the control group, implying that the TH disruption induced by these chemicals depends on the combined influence of multiple mechanisms. Gene expression appears to be an effective approach for investigations of TH disruption and the potential health effects.

Binding and activity of polybrominated diphenyl ether sulfates to thyroid hormone transport proteins and nuclear receptors

Polybrominated diphenyl ethers (PBDEs) can be metabolized to hydroxylated PBDEs (OH-PBDEs), which play important roles in their disruption effects on the thyroid hormone (TH) system. Recently, multiple in vitro studies suggested that OH-PBDEs might be further metabolically transformed to PBDE sulfates. However, information about the bioactivity of PBDE sulfate metabolites is limited. In the present study, we explored the possible disruption effects of PBDE sulfates to the TH system by studying their binding and activity towards TH transport proteins and nuclear receptors. We found PBDE sulfates could bind to two major TH transport proteins (thyroxine-binding globulin and transthyretin). Besides, PBDE sulfates could also bind to two subtypes of TH nuclear receptors (TRα and TRβ) and showed agonistic activity towards the subsequent signaling pathway. Moreover, the PBDE sulfates showed higher binding potency to TH transport proteins and TRs compared with their corresponding OH-PBDE precursors. Molecular docking results showed that replacement of hydroxyl groups with sulfate groups might lead to more hydrogen bond interactions with these proteins. Overall, our study suggested that PBDE sulfates might disturb the TH system by binding to TH transport proteins or TRs. Our finding indicated a possible mechanism for the TH system disruption effects of PBDEs through their sulfate metabolites.

Longitudinal assessment of prenatal phthalate exposure on serum and cord thyroid hormones homeostasis during pregnancy - Tainan birth cohort study (TBCS)

An increasing number of studies have revealed that phthalate exposure alters thyroid hormone homeostasis in the general population, but there is insufficient evidence of the effect of longitudinal maternal phthalate exposure on maternal and fetal thyroid hormones during pregnancy. We longitudinally assessed the effect of prenatal phthalate exposure in pregnant women on umbilical cord and maternal thyroid hormones at three trimesters during pregnancy. We recruited 98 pregnant women and collected urine and blood samples at three trimesters in an obstetrics clinic in Southern Taiwan from 2013 to 2014. We analyzed the concentrations of 11 urinary phthalate metabolites, including monoethylhexyl phthalate, mono-(2-ethyl-5-oxo-hexyl) phthalate (MEOHP), mono-(2-ethyl-5-hydroxyhexyl) phthalate, mono-(2-ethyl-5-carboxypentyl) phthalate (MECPP), mono-n-butyl phthalate, monoisobutyl phthalate (MiBP), monoethyl phthalate (MEP), using online liquid chromatography-tandem mass spectrometry. The cord and maternal serum levels of thyroxine (T₄), free T₄, triiodothyronine (T₃), thyroid-stimulating hormone (TSH), and thyroxine-binding globulin were measured using an electrochemiluminescence immunoassay. A mixed-model analysis was utilized to assess the effect of longitudinal phthalate exposure on thyroid hormones and adjusted for significant covariates. We found that urinary MiBP (β=-0.065, 95% confidence interval (CI): -0.124, -0.005), and MEOHP (β=-0.083, 95% CI: -0.157, -0.009) were significantly negatively associated with serum TSH. Urinary MECPP was inversely related to serum T₃ (β=-0.027, 95% CI: -0.047, -0.006). Urinary MEP (β=0.014, 95% CI: -0.001, 0.028) and MiBP (β=0.033, 95% CI: 0.018, 0.049) were positively related to free T₄. We found that cord serum T₃ (β=0.067, 95% CI: 0.003, 0.131) and free T₄ (β=0.031, 95% CI: 0.001, 0.062) levels had significant positive associations with maternal ΣDBPm levels at the second trimester. We concluded that different phthalates exposure windows during gestation may alter cord and serum thyroid hormone homoeostasis.

TSH and FT4 Concentrations in Congenital Central Hypothyroidism and Mild Congenital Thyroidal Hypothyroidism

CONTEXT

In central hypothyroidism (CeH), free thyroxine (FT4) concentrations are low, whereas thyrotropin (TSH) concentrations may be low, normal, or even slightly elevated due to reduced bioactivity. Congenital CeH (CCeH) may be isolated or part of multiple pituitary hormone deficiencies (MPHD).

OBJECTIVE

We tested our hypotheses that (1) TSH concentrations have a more U-shaped distribution in children with CCeH compared with children with a normally functioning hypothalamic-pituitary-thyroid axis and (2) TSH concentrations in children with CCeH with MPHD are higher compared with children with isolated CCeH. We also studied whether FT4 levels are helpful in distinguishing CCeH from mild congenital hypothyroidism of thyroidal origin (CH-T).

METHODS

Dutch neonatal screening TSH and first diagnostic TSH and FT4 were analyzed in all children diagnosed with permanent CCeH between 1995 and 2012. Controls were children with T4-binding globulin deficiency. FT4 concentrations in CCeH were compared with those in CH-T with TSH values in the same range as those of CCeH.

RESULTS

We studied 120 children with CCeH (isolated CCeH, n = 50; MPHD, n = 70) and 350 control subjects. Screening TSH concentrations were not significantly different (P = 0.055), but diagnostic TSH values were significantly different between the CCeH group and the control group (P = 0.037). TSH was significantly higher in MPHD compared with isolated CCeH (P = 0.004). FT4 concentrations were significantly lower in CCeH compared with mild CH-T (P < 0.0005).

CONCLUSION

TSH values in CCeH have a more U-shaped distribution compared with controls with the highest TSH concentrations in MPHD. FT4 levels were significantly lower in CCeH compared with CH-T.

Evolution of thyroid hormone distributor proteins

Thyroid hormones (THs) are evolutionarily old hormones, having effects on metabolism in bacteria, invertebrates and vertebrates. THs bind specific distributor proteins (THDPs) to ensure their efficient distribution through the blood and cerebrospinal fluid in vertebrates. Albumin is a THDP in the blood of all studied species of vertebrates, so may be the original vertebrate THDP. However, albumin has weak affinity for THs. Transthyretin (TTR) has been identified in the blood across different lineages in adults vs juveniles. TTR has intermediate affinity for THs. Thyroxine-binding globulin has only been identified in mammals and has high affinity for THs. Of these THDPs, TTR is the only one known to be synthesised in the brain and is involved in moving THs from the blood into the cerebrospinal fluid. We analysed the rates of evolution of these three THDPs: TTR has been most highly conserved and albumin has had the highest rate of divergence.

Directional thyroid hormone distribution via the blood stream to target sites

Thyroid hormones are bound to three major serum transport proteins, thyroxin-binding globulin (TBG), transthyretin (TTR) and human serum albumin (HSA). TBG has the strongest affinity for thyroid hormones, TTR is also found in the cerebrospinal fluid and HSA is the most abundant protein in plasma. Combination defects of either a high affinity TTR or HSA variant do not compensate TBG deficiency, underscoring the dominant role of TBG among the thyroid hormone transport proteins. On the other hand, coexistence of raised affinity TTR and HSA variants causes an augmented hyperthyroxinemia. Variations in thyroid hormone transport proteins may alter thyroid function tests to mimic hypo- or hyperthyroidism. As affected individuals are clinically euthyroid and do not require treatment, identification of thyroid hormone transport protein defects is important to avoid unnecessary diagnostic and therapeutic interventions. Mammals share the multilayered system of thyroid hormone binding proteins with humans. Some of them, especially carnivores, do not express TBG. In dogs, this defect has been shown to be caused by a defective hepatocyte nuclear factor-1 binding site in the TBG promoter, preventing TBG synthesis in the liver. The major endogenous thyroid hormone metabolite 3-iodothyronamine (3-T1AM) exerts marked cryogenic, metabolic, cardiac and central nervous system actions. It is bound to apolipoproteinB-100 (ApoB100), possibly facilitating its cellular uptake via interaction with the low density lipoprotein-receptor. This review summarizes the handling of hydrophobic charged thyroid hormone signaling molecules and their metabolite 3-T1AM in aqueous body fluids and the advantages and limits of their serum distributor proteins.

Improving lipoprotein profiles by liver-directed gene transfer of low density lipoprotein receptor gene in hypercholesterolaemia mice

The defect of low density lipoprotein receptor disturbs cholesterol metabolism and causes familial hypercholesterolaemia (FH). In this study, we directly delivered exogenous Ldlr gene into the liver of FH model mice (Ldlr(-/-)) by lentiviral gene transfer system. The results showed that the Ldlr gene controlled by hepatocyte-specific human thyroxine-binding globulin (TBG) promoter successfully and exclusively expressed in livers.We found that, although, the content of high density lipoprotein in serum was not significantly affected by the Ldlr gene expression, the serum low density lipoprotein level was reduced by 46%, associated with a 30% and 28% decrease in triglyceride and total cholesterol, respectively, compared to uninjected Ldlr(-/-) mice. Moreover, the TBG directed expression of Ldlr significantly decreased the lipid accumulation in liver and reduced plaque burden in aorta (32%). Our results indicated that the hepatocyte-specific expression of Ldlr gene strikingly lowered serum lipid levels and resulted in amelioration of hypercholesterolaemia.

Conservation in the phylum of the local homology of apolipoproteins with the thyroid hormone plasma carriers

Thyroxine-binding globulin (TBG), transthyretin (TTR), albumin (HSA), plus other plasmatic proteins, which include apolipoproteins, can bind and transport thyroid hormones (TH). In 1994, a 5-residue motif (Y, L/I/M, X, X, V/L/I) conserved in human TBG, TTR, HSA, and human and animal apolipoproteins was identified. Recently, we noticed that a number of residues upstream and downstream that motif are also conserved.We tested in silico the conservation of this larger motif in the many additional animal sequences of TH plasma carriers discovered after 1994. To this aim, we searched for the occurrence of the "new" motif in human and animal apolipoprotein and non-apolipoprotein TH-binding plasmatic proteins, and in a group of randomly selected proteins (2918 sequences from 56 species) not known as TH binders.Our results confirm the conservation of the "new" motif, associated with TH binding, in a total of 426 sequences analyzed (220 belonging to 169 apolipoproteins from 69 species, 206 belonging to 123 nonapolipoproteins from 54 species). Additionally, we found that within such conserved segments some differences between groups of TH plasma carriers exist. Interestingly, number and type of differences appear related to the affinity of each carrier for thyroid hormones. No occurrence of the motif was found in control proteins (alpha- and beta-tubulin, eosinophil cationic protein, endothelin-1, -2 and -3, IgG receptor, tropomyosin, Wnt inhibitory factor 1, erythropoietin, insulin and haptoglobin).Maintenance of a TH-binding domain in apolipoproteins throughout the phylum should be not less important that maintenance of the lipid binding domain.

Challenges in interpretation of thyroid hormone test results

INTRODUCTION

In interpreting thyroid hormones results it is preferable to think of interference and changes in concentration of their carrier proteins.

OUTLINE OF CASES

We present two patients with discrepancy between the results of thyroid function tests and clinical status. The first case presents a 62-year-old patient with a nodular goiter and Hashimoto thyroiditis. Thyroid function test showed low thyroid-stimulating hormone (TSH) and normal to low fT4. By determining thyroid status (TSH, T4, fT4,T3, fT3) in two laboratories, basal and after dilution, as well as thyroxine-binding globulin (TBG), it was concluded that the thyroid hormone levels were normal. The results were influenced by heterophile antibodies leading to a false lower TSH level and suspected secondary hypothyroidism.The second case, a 40-year-old patient, was examined and followed because of the variable size thyroid nodule and initially borderline elevated TSH, after which thyroid status showed low level of total thyroid hormones and normal TSH. Based on additional analysis it was concluded that low T4 and T3 were a result of low TBG. It is a hereditary genetic disorder with no clinical significance.

CONCLUSION

Erroneous diagnosis of thyroid disorders and potentially harmful treatment could be avoided by proving the interference or TBG deficiency whenever there is a discrepancy between the thyroid function results and the clinical picture.

Chronic Kidney Disease Distinctly Affects Relationship Between Selenoprotein P Status and Serum Thyroid Hormone Parameters

INTRODUCTION

Chronic kidney disease (CKD) impairs thyroid hormone (TH) metabolism and is associated with low serum triiodothyronine (T3) concentrations in patients with a low glomerular filtration rate (GFR). Whether this results from decreased T3 formation from thyroxine (T4) by impaired 5'-deiodinase (DIO) activity and/or enhanced degradation of T3 and increased reverse triiodothyronine (rT3) formation from T4 by elevated 5-DIO activity remains unclear. Both activating 5'- and the inactivating 5-deiodination of TH are catalyzed by three selenium (Se)-dependent DIO isoenzymes. Selenoprotein P (SePP) is the major constituent of serum selenium, and functions as Se transport protein from liver to kidney and several other organs. This study tested the hypothesis that serum SePP and TH status are associated with the degree of renal impairment in patients with CKD.

PATIENTS AND METHODS

A total of 180 CKD patients (stages 1-5) and 70 chronic hemodialysis (CHD) patients undergoing hemodialysis three times per week for at least two years were prospectively investigated for clinical data, parameters of renal function, serum TH profile (thyrotropin, T4, free thyroxine [fT4], T3, free triiodothyronine (fT3), rT3, thyroxine-binding globulin [TBG]), C-reactive protein (CRP), and serum SePP.

RESULTS

In CKD patients, renal function was negatively associated with SePP concentration (standardized β = -0.17, p = 0.029); that is, SePP concentrations increased in more advanced CKD stages. In contrast, significantly lower SePP concentrations were found in patients on hemodialysis compared with CKD patients (M ± SD = 2.7 ± 0.8 mg/L vs. 3.3 ± .9 mg/L; p < 0.001). Notably, in CKD patients, the SePP concentration was negatively associated with T4 (standardized β = -0.16, p = 0.039) and fT4 (standardized β = -0.16, p = 0.039) concentrations, but no association was found with T3, fT3, rT3, T3/T4, rT3/T3, rT3/T4, or TBG concentrations. The SePP concentration was also negatively associated with CRP levels (standardized β = -0.17, p = 0.029). In the CHD group, no association was detected between SePP and the investigated TH parameters.

SUMMARY AND CONCLUSION

Impaired renal function is positively correlated with serum concentrations of SePP. In patients undergoing CHD treatment, SePP concentrations were significantly reduced, but the TH profile remained unaffected. These findings indicate an important contribution of kidney function on serum SePP homeostasis, and consequently on Se status.

A novel mechanism of inherited TBG deficiency: mutation in a liver-specific enhancer

CONTEXT

T4-binding globulin (TBG), a protein secreted by the liver, is the main thyroid hormone (TH) transporter in human serum. TBG deficiency is characterized by reduced serum TH levels, but normal free TH and TSH and absent clinical manifestations. The inherited form of TBG deficiency is usually due to a mutation in the TBG gene located on the X-chromosome.

OBJECTIVE

Among the 75 families with X-chromosome-linked TBG deficiency identified in our laboratory, no mutations in the TBG gene were found in four families. The aim of the study was to identify the mechanism of TBG deficiency in these four families using biochemical and genetic studies.

DESIGN

Observational cohort, prospective.

SETTING

University research center.

PATIENTS

Four families with inherited TBG deficiency and no mutations in the TBG gene.

INTERVENTION

Clinical evaluation, thyroid function tests, and targeted resequencing of 1 Mb of the X-chromosome.

RESULTS

Next-generation sequencing identified a novel G to A variant 20 kb downstream of the TBG gene in all four families. In silico analysis predicted that the variant resides within a liver-specific enhancer. In vitro studies confirmed the enhancer activity of a 2.2-kb fragment of genomic DNA containing the novel variant and showed that the mutation reduces the activity of this enhancer. The affected subjects share a haplotype of 8 Mb surrounding the mutation, and the most recent common ancestor among the four families was estimated to be 19.5 generations ago (95% confidence intervals, 10.4-37).

CONCLUSIONS

To our knowledge, the present study is the first report of an inherited endocrine disorder caused by a mutation in an enhancer region.

Determination of free thyroid hormones

Timely diagnosis and treatment of thyroid dysfunction is compelling given the prevalence and severity of the disease. It requires reliance on adequate laboratory testing of serum TSH as a hallmark in combination with free thyroxine/triiodothyronine. Free hormone methods have to accommodate variations in the concentration and binding capacity of binding proteins. This is a challenge because none of the methodologies developed so far measures the actual unbound hormone in serum. The indirect methods provide an approximation while the direct ones estimate the free hormone concentration either in the presence of the protein-bound counterpart, or after physical separation of the free from bound fraction. The ongoing controversy on the validity and lack of comparability of methodologies points to their imperfectness to reflect real in-vivo free hormone concentrations. Therefore, laboratories and clinicians should know the window of validity and limitations of their methods. The recently developed reference measurement system is a key advance towards improved standardization and clinical validity of free thyroid hormone measurements.

The allosteric modulation of thyroxine-binding globulin affinity is entropy driven

BACKGROUND

Thyroxine-binding globulin (TBG) is a non-inhibitory member of the serpin family of proteins whose main structural element is the reactive center loop (RCL), that, upon cleavage by proteases, is inserted into the protein core adopting a β-strand conformation (stressed to relaxed transition, S-to-R). After S-to-R transition thyroxine (T4) affinity decreases. However, crystallographic studies in the presence or absence of the hormone in different states are unable to show significant differences in the structure and interactions of the binding site. Experimental results also suggest the existence of several S states (differing in the number of inserted RCL residues), associated with a differential affinity.

METHODS

To shed light into the molecular basis that regulates T4 affinity according to the degree of RCL insertion in TBG, we performed extended molecular dynamics simulations combined with several thermodynamic analysis of the T4 binding to TBG in three different S states, and in the R state.

RESULTS

Our results show that, despite T4 binding in the protein by similar interactions in all states, a good correlation between the degree of RCL insertion and the binding affinity, driven by a change in TBG conformational entropy, was observed.

CONCLUSION

TBG allosteric regulation is entropy driven. The presence of multiple S states may allow more efficient T4 release due to protease activity.

GENERAL SIGNIFICANCE

The presented results are clear examples of how computer simulation methods can reveal the thermodynamic basis of allosteric effects, and provide a general framework for understanding serpin allosteric affinity regulation.

Measurement of total rather than free thyroxine in pregnancy: the diagnostic implications

BACKGROUND

In light of several recent recommendations to use total thyroxine (T4) measurements in the diagnosis of thyroid function in pregnancy (in particular, "Clinical Practice Guidelines for Hypothyroidism in Adults," cosponsored by the American Thyroid Association and the American Association of Clinical Endocrinologists, which promote the use of T4 over free T4 [FT4 ]), we have examined the implications of employing T4 for diagnostic discrimination in both pregnant and nonpregnant patient panels. Use of T4 assays has significant drawbacks in this regard, and we believe that the suggestion is a retrograde step in thyroid function testing.

SUMMARY AND CONCLUSIONS

Analysis of the interplay between the concentrations of T4 and thyroxine-binding globulin (TBG), typifying their respective reference ranges in either the nonpregnant or pregnant euthyroid state, shows that the effective T4 range is widened significantly by the accompanying hidden variation in TBG levels. Accordingly FT4 assays that fully compensate for serum T4-binding protein concentrations should discriminate dysfunctionality from normality more efficiently than total hormone measurements, whether in pregnant or nonpregnant states. The euthyroid FT4 reference ranges typical of late pregnancy should also be more compact than those for the total hormone, because of the increased dominance of higher, though equivalently variable TBG concentrations on T4 levels. Parallel effects on T4 from similarly variable, though lower concentrations of TBG are indicated in the nonpregnant group. While acknowledging the difficulties in FT4 measurement arising from inconsistent calibration of present-day commercial assays, this finding questions the recommendation that total hormone assays should supersede the former in pregnancy.

Thyroid disturbance related to chronic hepatitis C infection: role of CXCL10

Association between autoimmune thyroid diseases (AITD) and hepatitis C is controversial, but may occur or worsen during alpha-interferon treatment. The mechanism responsible for autoimmune diseases in infected patients has not been fully elucidated. This study aims to evaluate the frequency of AITD in chronic hepatitis C and the association of chemokine (CXC motif) ligand 10 (CXCL10) and AITD. One hundred and three patients with chronic hepatitis C and 96 controls were prospectively selected to clinical, hormonal, thyroid autoimmunity and ultrasound exams, besides thyroxine-binding globulin (TBG) and CXCL10 measurements and hepatic biopsies. The frequency of AITD among infected subjects was similar to controls. TT3 and TT4 distributions were right shifted, as was TBG, which correlated to both of them. Thyroid heterogeneity and hypoechogenicity were associated with AITD. Increased vascularization was more prevalent in chronic hepatitis C.CXCL10 was higher in infected patients (p=0.007) but was not related to thyroid dysfunction. Increase in CXCL10 levels were consistent with hepatic necroinflammatory activity (p=0.011). In summary, no association was found between chronic hepatitis C and AITD. Infected subjects had higher TT3 and TT4 which were correlated to TBG. Increased CXCL10 was not associated to thyroid dysfunction in HCV-infected population.

[Thyroid hormones and their precursors I. Biochemical properties]

This paper and the following one (see the next issue of Acta Pharmaceutica Hungarica) survey the biological roles and the related site-specific physico-chemical parameters (basicity and lipophilicity) of the presently known thyroid hormones (thyroxine, liothyronine and reverse liothyronine) and their biological precursors (monoiodotyrosine and diiodotyrosine). Here the literature of the thyroid hormone biochemistry, biosynthesis, plasma- and membrane transport is summarized, focusing on the pH-dependent processes. Biosyntheses of the thyroid hormones take place by oxidative coupling of two iodotyrosine residues catalyzed by thyreoperoxidase in thyreoglobulin. The protonation state of the precursors, especially that of the phenolic OH is crucial for the biosynthesis, since anionic iodotyrosine residues can only be coupled in the thyroid hormone biosyntheses. In the blood more than 99% of the circulating thyroid hormone is bound to plasma proteins among which the thyroxine-binding globulin and transthyretin are crucial. The amphiphilic character of the hormones is assumed to be the reason why their membrane transport is an energy-dependent, transport-mediated process, in which the organic anion transporter family, mainly OATP1C1, and the amino acid transporters, such as MCT8 play important roles. Liothyronine is the biologically active hormone; it binds the thyroid hormone receptor, a type of nuclear receptor. There are two major thyroid hormone receptor (TR) isoforms, alfa (TRalpha) and beta (TRbeta). The activation of the TRalpha is associated with modifications in cardiac behavior, while activation of the TRbeta is associated with increasing metabolic rates, resulting in weight loss and reduction of blood plasma lipid levels. The affinity of the thyroid hormones for different proteins depends on the ionization state of the ligands. The site-specific physico-chemical characterization of the thyroid hormones is of fundamental importance to understand their (patho)physiological behavior and also, to influence their therapeutic properties at the molecular level.

Assessment of the binding of hydroxylated polybrominated diphenyl ethers to thyroid hormone transport proteins using a site-specific fluorescence probe

Polybrominated diphenyl ethers (PBDEs) have been shown to disrupt thyroid hormone (TH) functions on experimental animals, and one of the proposed disruption mechanisms is the competitive binding of PBDE metabolites to TH transport proteins. In this report, a nonradioactive, site-specific fluorescein-thyroxine (F-T4) conjugate was designed and synthesized as a fluorescence probe to study the binding interaction of hydroxylated PBDEs to thyroxine-binding globulin (TBG) and transthyretin (TTR), two major TH transport proteins in human plasma. Compared with free F-T4, the fluorescence intensity of TTR-bound conjugate was enhanced by as much as 2-fold, and the fluorescence polarization value of TBG-bound conjugate increased by more than 20-fold. These changes provide signal modulation mechanisms for F-T4 as a fluorescence probe. Based on fluorescence quantum yield and lifetime measurements, the fluorescence intensity enhancement was likely due to the elimination of intramolecular fluorescence quenching of fluorescein by T4 after F-T4 was bound to TTR. In circular dichroism and intrinsic tryptophan fluorescence measurements, F-T4 induced similar spectroscopic changes of the proteins as T4 did, suggesting that F-T4 bound to the proteins at the T4 binding site. By using F-T4 as the fluorescence probe in competitive binding assays, 11 OH-PBDEs with different levels of bromination and different hydroxylation positions were assessed for their binding affinity with TBG and TTR, respectively. The results indicate that the binding affinity generally increased with bromine number and OH position also played an important role. 3-OH-BDE-47 and 3'-OH-BDE-154 bound to TTR and TBG even stronger, respectively, than T4. With rising environmental level and high bioaccumulation capability, PBDEs have the potential to disrupt thyroid homeostasis by competitive binding with TH transport proteins.

Thyroid dysfunction as a mediator of organochlorine neurotoxicity in preschool children

BACKGROUND

Exposure to organochlorine compounds (OCs) can alter thyroid function in humans, and hypothyroidism during early life can adversely affect a child's neurodevelopment.

OBJECTIVES

In this study we aimed to assess the relationship between developmental organochlorine exposures and thyroid function and the relationship between thyroid function and subsequent neurodevelopment.

METHODS

A population-based birth cohort of 182 children was followed annually up to 5.5 years of age. The assessments included OC concentrations in maternal pregnancy serum and milk, clinical thyroid parameters in maternal and cord serum, and subsequent neuropsychological outcomes of the child, along with sociodemographic cofactors. Resin triiodothyronine uptake ratio (T3RU) was also assessed as an estimate of the amount of thyroxine-binding globulin (TBG) sites unsaturated by thyroxine. The T3RU is high in hyperthyroidism and low in hypothyroidism.

RESULTS

The findings showed consistent inverse and monotonic associations between organochlorine exposure and T3RU after covariate adjustments. We observed no associations with other thyroid parameters. T3RU was positively associated with improved performance on most of the neuropsychological tests. For other thyroid parameters, the findings were less consistent.

CONCLUSIONS

The results suggest that OC exposures may decrease the T3RU during early life, which is a proxy measure of the binding capacity of TBG. In addition, minor decreases of the thyroid function may be inversely associated with a child's neurodevelopment.

Abnormal serum thyroid hormones concentration with healthy functional gland: a review on the metabolic role of thyroid hormones transporter proteins

Laboratory findings can definitely help the patients not to enter into status, where the damage might be happen due to a miss-diagnosis based on clinical assessment alone. The secondary disease accompanied with thyroid patients should also carefully check out due to the interference which some diseases can cause in the amount of serum thyroid hormone, particularly the free thyroxin. The dilemma over thyroid clinical diagnosis occur due to variation on serum thyroid hormone which initiated by other non-thyroidal disorders which can play an important roles in metabolic disorders of thyroid hormone due to the alteration which occur on the serum level of thyroid hormone transporter proteins. The majority of serum thyroid hormones of up to 95-99% are bound to the carrier proteins mainly to Thyroxin-Binding Globulins (TBG), some transthyretin already known as pre-albumin and albumin which are all synthesis in the liver and any modification which alter their production may alter the status of thyroid hormones. It seems TBG, transthyretin and albumin carries 75, 20, 5% of thyroid hormones within blood circulation, respectively. The dilemma facing the thyroid hormones following disruption of thyroid hormone transporter protein synthesis originate from this fact that any alteration of these protein contribute to the alteration of total thyroid and free serum thyroid hormones which are in fact the biologically active form of thyroid hormones. The subsequent of latter implication result in miss-understanding and miss-diagnosis of thyroid function tests, with possible wrongly thyroid clinical care, followed by undesired therapy of otherwise healthy thyroid.

Ghrelin affects the hypothalamus-pituitary-thyroid axis in humans by increasing free thyroxine and decreasing TSH in plasma

OBJECTIVE

Ghrelin promotes a positive energy balance, e.g. by increasing food intake. Stimulation of the activity of the hypothalamus-pituitary-thyroid (HPT) axis promotes a negative energy balance, e.g. by increasing energy expenditure. We therefore hypothesized that ghrelin suppresses the HPT axis in humans, counteracting its energy-saving effect.

DESIGN AND METHODS

In this single-blind, randomized, cross-over study, we determined secretion patterns of free triiodothyronine (fT(3)), free thyroxine (fT(4)), TSH, and thyroid-binding globulin (TBG) between 2000 and 0700 h in 20 healthy adults (10 males and 10 females, 25.3+/-2.7 years) receiving 50 microg ghrelin or placebo at 2200, 2300, 0000, and 0100 h.

RESULTS

FT(4) plasma levels were significantly higher after ghrelin administration than after placebo administration from 0000 h until 0620 h except for the time points at 0100, 0520, and 0600 h. TSH plasma levels were significantly lower from 0200 until the end of the study at 0700 h except for the time points at 0540, 0600, and 0620 h. The relative increase of fT(4) (area under the curve (AUC) 0130-0700 h (ng/dl x min): placebo: 1.31+/-0.03; ghrelin: 1.39+/-0.03; P=0.001) was much weaker than the relative decrease of TSH (AUC 0130-0700 h (mIU/ml x min): placebo: 1.74+/-0.12; ghrelin: 1.32+/-0.12; P=0.007). FT(3) and TBG were not affected.

CONCLUSIONS

This is the first study to report that ghrelin affects the HPT axis in humans. The early fT(4) increase was possibly induced by direct ghrelin action on the thyroid where ghrelin receptors have been identified. The TSH decrease might have been caused by ghrelin-mediated inhibition at hypothalamic level by feedback inhibition through fT(4), or both.

Molecular and structural basis of steroid hormone binding and release from corticosteroid-binding globulin

Corticosteroid-binding globulin (CBG), a non-inhibitory member of the serine proteinase inhibitor (serpin) super-family, is the high-affinity transport protein for glucocorticoids in vertebrate blood. Plasma CBG is a glycoprotein with 30% of its mass represented by N-linked oligosaccharide chains. Its well-characterized steroid-binding properties represent a "bench-mark data set" used extensively for in silico studies of protein-ligand interactions and drug design. Recent crystal structure analyses of intact rat CBG and cleaved human CBG have revealed the precise topography of the steroid-binding site, and shown that cortisol-bound CBG displays a typical stressed (S) serpin conformation with the reactive center loop (RCL) fully exposed from the central beta-sheet A, while proteolytic cleavage of the RCL results in CBG adopting a relaxed (R) conformation with the cleaved RCL fully inserted within the protein core. These crystal structures have set the stage for mechanistic studies of CBG function which have so far shown that helix D plays a key role in coupling RCL movement and steroid-binding site integrity, and provided evidence for an allosteric mechanism that modulates steroid binding and release from CBG. These studies have also revealed how the irreversible release of steroids occurs after proteolysis and re-orientation of the RCL within the R conformation. This recent insight into the structure and function of CBG reveals how naturally occurring genetic CBG mutations affect steroid binding, and helps understand how proteolysis of CBG enhances the targeted delivery of biologically active steroids to their sites of action.

Thyroid hormones, their carrier proteins, and thyroid antibodies in the pleural effusion of two patients with graves' disease-induced thyrotoxicosis

INTRODUCTION

Concentrations of thyroid hormones, their carrier proteins, and thyroid antibodies in plasma have been extensively investigated, but those in pleural effusion have not. PATIENTS AND MEDTHODS: In the present study, we report, for the first time, the concentrations of thyroid hormones, their carrier proteins, and thyroid antibodies in the pleural effusion of two thyrotoxicosis patients with Graves' disease.

RESULTS

The pleural effusions were transudates. The concentrations of thyroid hormone carrier proteins, such as thyroxine-binding globulin (TBG), thyroxine-binding prealbumin (TBPA), and albumin (Alb) were approximately 30-50% of the plasma. The concentrations of total triiodothyronine (TT3), total tetraiodothyronine (TT4), free triiodothyronine (FT3), and free tetraiodothyronine (FT4) were approximately 15-40%, 45-55%, 45-75%, and 80-85% of the plasma, respectively. The concentration of thyroid stimulating hormone receptor antibody (TRAb) (equal to TSH-binding inhibitory immunoglobulins%; TBII%) was approximately 90% of the plasma.

CONCLUSION

If the pleural effusions were treated with diuretics, substantial quantity of thyroid hormones and thyroid antibodies in the pleural effusion may have returned to the plasma, and might exacerbate thyrotoxicosis. For patients with thyrotoxicosis and pleural effusion, thoracentesis should be considered. The present findings will contribute to the understanding and treatment of hyperthyroidism-induced pleural effusion.

Sex steroids and the thyroid

Thyroid function is modulated by genetic and environmental causes as well as other illnesses and medications such as gonadal or sex steroids. The latter class of drugs (sex steroids) modulates thyroid function. Gonadal steroids exert their influence on thyroid function primarily by altering the clearance of thyroxine-binding globulin (TBG). While oestrogen administration causes an increase in serum TBG concentration, androgen therapy results in a decrease in this binding protein. These effects of gonadal steroids on TBG clearance and concentration are modulated by the chemical structure of the steroid being used, its dose and the route of administration. Despite the gonadal steroids-induced changes in serum TBG concentrations, subjects with normal thyroid glands maintain clinical and biochemical euthyroidism without changes in their serum free thyroxine (T4) or thyroid-stimulating hormone (TSH) levels. In contrast, the administration of gonadal steroids to patients with thyroid diseases causes significant biochemical and clinical alterations requiring changes in the doses of thyroid medications. Similarly, gonadal steroid therapy might unmask thyroid illness in previously undiagnosed subjects. It would be prudent to assess thyroid function in subjects with thyroid disease 6-8 weeks after gonadal steroid administration or withdrawal.