An in silico study on human carcinogenicity mechanism of polybrominated biphenyls exposure.

Polybrominated biphenyls (PBBs) are associated with an increased risk of thyroid cancer; however, relevant mechanistic studies are lacking. In this study, we investigated the mechanisms underlying PBB-induced human thyroid cancer. Molecular docking and molecular dynamics methods were employed to investigate the metabolism of PBBs by the cytochrome P450 enzyme under aryl hydrocarbon receptor mediation into mono- and di-hydroxylated metabolites. This was taken as the molecular initiation event. Subsequently, considering the interactions of PBBs and their metabolites with the thyroxine-binding globulin protein as key events, an adverse outcome pathway for thyroid cancer caused by PBBs exposure was constructed. Based on 2D quantitative structure activity relationship (2D-QSAR) models, the contribution of amino acid residues and binding energy were analyzed to understand the mechanism underlying human carcinogenicity (adverse effect) of PBBs. Hydrogen bond and van der Waals interactions were identified as key factors influencing the carcinogenic adverse outcome pathway of PBBs. Analysis of non-bonding forces revealed that PBBs and their hydroxylation products were predominantly bound to the thyroxine-binding globulin protein through hydrophobic and hydrogen bond interactions. The key amino acids involved in hydrophobic interactions were alanine 330, arginine 381 and lysine 270, and the key amino acids involved in hydrogen bond interactions were arginine 381 and lysine 270. This study provides valuable insights into the mechanisms underlying human health risk associated with PBBs exposure.

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.

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.

Structural binding perspectives of common plasticizers and a flame retardant, BDE-153, against thyroxine-binding globulin: potential for endocrine disruption.

The human exposure to diverse endocrine-disrupting chemicals (EDCs) has increased dramatically over several decades with very adverse health effects. Plasticizers and flame retardants constitute important classes of EDCs interfering in endocrine physiology including the thyroid function. Thyroxine (T4) is an important hormone regulating metabolism and playing key roles in developmental processes. In this study, six phthalate and nonphthalate plasticizers and one flame retardant (BDE-153) were subjected to structural binding against thyroxine-binding globulin (TBG). The aim was to understand their potential role in thyroid dysfunction using structural binding approach. The structural study was performed using Schrodinger's induced fit docking, followed by binding energy estimations of ligands and the molecular interaction analysis between the ligands and the amino acid residues in the TBG ligand-binding pocket. The results indicated that all the compounds packed tightly into the TBG ligand-binding pocket with similar binding pattern to that of TBG native ligand, T4. A high majority of TBG interacting amino acid residues for ligands showed commonality with native ligand, T4. The estimated binding energy values were highest for BDE-153 followed by nonphthalate plasticizer, DINCH, with values comparable with native ligand, T4. The estimated binding energy values of other plasticizers DEHP, DEHT, DEHA, ATBC, and TOTM were less than DINCH. In conclusion, the tight docking conformations, amino acid interactions, and binding energy values of the most of the indicated ligands were comparable with TBG native ligand, T4, suggesting their potential for thyroid dysfunction. The results revealed highest potential thyroid disruptive action for BDE-153 and DINCH.

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 TRß-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.

Endocrine disruption: Molecular interactions of environmental bisphenol contaminants with thyroid hormone receptor and thyroxine-binding globulin.

Many bisphenol A (BPA) analogs have been commercially used recently, such as 2,2-bis(4-hydroxyphenyl)butane (BPB), 4,4'-ethylidenebisphenol, 4,4'-methylenediphenol (BPF), 4,4'-(1,4-phenylenediisopropylidene)bisphenol (BPP), 4,4'-dihydroxydiphenyl sulfone (BPS), 4,4'-cyclohexylidenebisphenol (BPZ), 4,4'-(hexafluoroisopropylidene)diphenol (BPAF), 4,4'-(1-phenylethylidene)bisphenol (BPAP), and 2,2-bis(4-hydroxy-3,5-dimethylphenyl)propane (TMBPA), to circumvent adverse effects of BPA. However, their increasing use is also contaminating the environment, which is a potential cause of concern for human health. Thyroid hormone transport and signaling are potential targets for endocrine-disrupting activity of BPA analogs. Thyroxine-binding globulin (TBG) is the major carrier protein for thyroxine (T4) and triiodothyronine (T3) in blood. Thyroid hormones exert their action through thyroid hormone receptors (TRα and TRß). This report presents the thyroid-disrupting potential of indicated nine BPA analogs from structure-based studies with TBG and TRα. Each BPA analog formed important polar and hydrophobic interactions with a number of residues of TBG and TRα. Majority of TBG residues (77-100%) and TRα residues (70-91%) interacting with BPA analogs were common with those of native ligands T4 and T3, respectively. Majority of BPA analogs interacted with TBG forming a salt bridge interaction at Lys-270. The hydrogen-bonding interaction of T3 with TRα at His-381 was also shared by majority of analogs. The binding energy for BPP, BPB, BPZ, BPAP, and TMBPA with both proteins was closer to binding energy of respective native ligands. The similarity in structural binding characteristics suggested potential disrupting activity of thyroid hormone signaling and transport.

Molecular interactions of thyroxine binding globulin and thyroid hormone receptor with estrogenic compounds 4-nonylphenol, 4-tert-octylphenol and bisphenol A metabolite (MBP).

AIM: Endocrine disruption due to environmental chemical contaminants is a global human health issue. The aim of present study was to investigate the structural binding aspects of possible interference of commonly detected environmental contaminants on thyroid function. MATERIAL AND METHODS: Three compounds, 4-tert-octylphenol (4-tert-OP), 4-nonylphenol (4-NP), and 4-methyl-2,4-bis(4-hydroxypentyl)pent-1-ene (MBP) were subjected to induced fit docking (IFD) against thyroxine binding globulin (TBG) and thyroid hormone receptor (THR). Structural analysis included molecular interactions of the amino acid residues and binding energy estimation between the ligands and the target proteins. KEY RESULTS: All the ligands were successfully placed in the ligand binding pocket of TBG and THR using induced fit docking (IFD). The IFD results revealed high percentage of commonality in interacting amino acid residues between the aforementioned compounds and the native ligand for both TBG and THR. The results of our study further revealed that all the compounds have the potential to interfere with thyroid transport and signaling. However, MBP showed higher binding affinity for both TBG and THR, suggesting higher thyroid disruptive potential as compared to 4-t-OP and 4-NP. Furthermore, our results also suggest that the reported disruptive effects of BPA could actually be exerted through its metabolite; MBP. SIGNIFICANCE: This work implies that all the three compounds 4-NP, 4-t-OP and especially MBP have the potential to interfere with thyroid hormone transport and signaling. This potentially leads to disruption of thyroid hormone function.

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 ß, 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 TRß) 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 (ß=-0.065, 95% confidence interval (CI): -0.124, -0.005), and MEOHP (ß=-0.083, 95% CI: -0.157, -0.009) were significantly negatively associated with serum TSH. Urinary MECPP was inversely related to serum T3 (ß=-0.027, 95% CI: -0.047, -0.006). Urinary MEP (ß=0.014, 95% CI: -0.001, 0.028) and MiBP (ß=0.033, 95% CI: 0.018, 0.049) were positively related to free T4. We found that cord serum T3 (ß=0.067, 95% CI: 0.003, 0.131) and free T4 (ß=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.

Exposure to DBP and High Iodine Aggravates Autoimmune Thyroid Disease Through Increasing the Levels of IL-17 and Thyroid-Binding Globulin in Wistar Rats.

Autoimmune thyroid disease (AITD) is the most common autoimmune disease that causes hypothyroidism. High iodine is a well-known factor that can induce thyroid disorders, including Hashimoto's thyroiditis, one of the main types of AITD. Recent epidemiological studies have indicated that phthalates, especially di-n-butyl phthalate (DBP) may induce thyroid disease. In this study, we aim to determine the effects and underlying mechanisms of high iodine and/or DBP exposure on AITD. Female Wistar rats were modeled with thyroglobulin and exposed to high iodine and/or DBP. We investigated histopathological changes in the thyroid and measured thyroid hormone levels in serum to assess thyroid function. In the thyroid and liver, we detected oxidative stress, proinflammatory factors (IL-1ß, IL-6, and IL-17) and the activation of activator protein 1 (AP-1), a transcription factor that is related to the synthesis of the thyroxine-binding globulin (TBG) and the activation of Th17. After blocking AP-1 with SP600125, we detected TBG and the Th17 related cytokines (IL-6 and IL-17). The data showed that thyroid damage and the alteration of thyroid hormones were greater when the rats were exposed to both high iodine and DBP. Coexposure to DBP and high iodine enhanced the activation of AP-1 in the liver and thyroid, and induced an increase in the levels of TBG in serum and IL-17 in the thyroid. Blocking AP-1 activation prevented the increase of TBG and IL-17. The results indicate that high iodine and/or DBP exposure exacerbated AITD through altering TBG levels in serum and aggravating IL-17 in the thyroid.

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.

Identification of thyroxine-binding globulin as a candidate plasma marker of chronic obstructive pulmonary disease.

Biomarkers for the management of chronic obstructive pulmonary disease (COPD) are limited. The aim of this study was to explore new plasma biomarkers in patients with COPD. Thyroxine-binding globulin (THBG) was initially identified by proteomics in a discovery panel and subsequently verified by enzyme-linked immunosorbent assay in another verification panel with a 1-year follow-up. THBG levels were elevated in patients with COPD (9.2±2.3 µg/mL) compared to those of the controls (6.6±2.0 µg/mL). Receiver operating characteristic curves suggested that THBG was able to slightly differentiate between patients with COPD and controls (area under the curve [AUC]: 0.814) and performed better if combined with fibrinogen (AUC: 0.858). THBG was more capable of distinguishing Global Initiative for Obstructive Lung Disease stages I-III and IV (AUC: 0.851) compared with fibrinogen (AUC 0.582). THBG levels were negatively associated with predicted percentage forced expiratory volume in 1 s and positively related to predicted percentage residual volume, RV/percentage total lung capacity, and percentage low-attenuation area. COPD patients with higher baseline THBG levels had a greater risk of acute exacerbation (AE) than those with lower THBG levels (P=0.014, by Kaplan-Meier curve; hazard ratio: 4.229, by Cox proportional hazards model). In summary, THBG is a potential plasma biomarker of COPD and can assist in the management of stable stage and AEs in COPD patients.

Supplemental Iodide for Preterm Infants and Developmental Outcomes at 2 Years: An RCT.

BACKGROUND: The recommendation for enteral iodide intake for preterm infants is 30 to 40 µg/kg per day and 1 µg/kg per day for parenteral intake. Preterm infants are vulnerable to iodide insufficiency and thyroid dysfunction. The hypothesis tested whether, compared with placebo, iodide supplementation of preterm infants improves neurodevelopment. METHODS: A randomized controlled trial of iodide supplementation versus placebo in infants <31 weeks' gestation. Trial solutions (sodium iodide or sodium chloride; dose 30 µg/kg per day) were given within 42 hours of birth to the equivalent of 34 weeks' gestation. The only exclusion criterion was maternal iodide exposure during pregnancy or delivery. Whole blood levels of thyroxine, thyrotropin, and thyroid-binding globulin were measured on 4 specific postnatal days. The primary outcome was neurodevelopmental status at 2 years of age, measured by using the Bayley Scales of Infant Development-III. The primary analyses are by intention-to-treat, and data are presented also for survivors. RESULTS: One thousand two hundred seventy-three infants (637 intervention, 636 placebo) were recruited from 21 UK neonatal units. One hundred thirty-one infants died, and neurodevelopmental assessments were undertaken in 498 iodide and 499 placebo-supplemented infants. There were no significant differences between the intervention and placebo groups in the primary outcome: mean difference cognitive score, -0.34, 95% confidence interval (CI) -2.57 to 1.89; motor composite score, 0.21, 95% CI -2.23 to 2.65; and language composite score, -0.05, 95% CI -2.48 to 2.39. There was evidence of weak interaction between iodide supplementation and hypothyroxinemic status in the language composite score and 1 subtest score. CONCLUSIONS: Overall iodide supplementation provided no benefit to neurodevelopment measured at 2 years of age.

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.

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.

Binding interactions of perfluoroalkyl substances with thyroid hormone transport proteins and potential toxicological implications.

Perfluoroalkyl substances (PFASs) have been shown to cause abnormal levels of thyroid hormones (THs) in experimental animals, but the molecular mechanism is poorly understood. Here, a fluorescence displacement assay was used to determine the binding affinities of 16 PFASs with two major TH transport proteins, transthyretin (TTR) and thyroxine-binding globulin (TBG). Most of the tested PFASs bound TTR with relative potency (RP) values of 3×10(-4) to 0.24 when compared with that of the natural ligand thyroxine, whereas fluorotelomer alcohols did not bind. Only perfluorotridecanoic acid and perfluorotetradecanoic acid bound TBG, with RP values of 2×10(-4) when compared with that of thyroxine. Based on these results, it was estimated that displacement of T4 from TTR by perfluorooctane sulfonate and perfluorooctanoic acids would be significant for the occupationally exposed workers but not the general population. Structure-binding analysis revealed that PFASs with a medium chain length and a sulfonate acid group are optimal for TTR binding, and PFASs with lengths longer than 12 carbons are optimal for TBG binding. Three mutant proteins were prepared to examine crucial residues involved in the binding of PFASs to TH transport proteins. TTR with a K15G mutation and TBG with either a R378G or R381G mutation showed decreased binding affinity to PFASs, indicating that these residues play key roles in the interaction with the compounds. Molecular docking showed that the PFASs bind to TTR with their acid group forming a hydrogen bond with K15 and the hydrophobic chain towards the interior. PFASs were modeled to bind TBG with their acid group forming a hydrogen bond with R381 and the hydrophobic chain extending towards R378. The findings aid our understanding of the behavior and toxicity of PFASs on the thyroid hormone system.

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.