Characterization of nascent and secreted thyroxine-binding globulin in cultured human hepatoma (Hep G2) cells

Identification of a novel mutation in thyroxine-binding globulin (TBG) gene associated with TBG-deficiency and its effect on the thyroid function

PURPOSE

This study presents a case of familial transmission of thyroxine-binding globulin (TBG) deficiency. The SERPINA7-gene which codes for TBG is located on the X-chromosome (Xq21-22). More than 45 mutations have been reported to cause TBG- deficiency from various countries, but none from India so far. Genetic analysis of SERPINA7 gene was carried out to determine the cause of low TBG levels in one family.

METHODS

DNA samples of the propositus and the family members were subjected to Polymerase Chain Reaction (PCR) followed by direct sequencing. Allele-specific PCR and Next-gen sequencing (NGS) were employed to confirm the site of the mutation. Thyroid function tests were estimated by Radioimmunoassay (RIA) and Immunoradiometric assay (IRMA) kits. X-chromosomal inactivation status was analyzed in the female members harboring the mutation.

RESULTS

A mutational screening in this family revealed a novel frame-shift mutation S353Q, 354fs3X in the exon 4 of the SERPINA7 gene which will be referred to as TBG-complete deficiency-India (TBG-CD-Ind). One out of four female family members harboring the mutation showed selective X-chromosomal inactivation. The affected family members were clinically euthyroid initially, showed changes in the thyroid function when tested after a long time span. However, the changes in the thyroid function in the affected family members had an autoimmune etiology.

CONCLUSION

This study presents the first report of TBG-CD from India wherein a novel frameshift mutation referred to as TBG-CD-Ind (S353Q, 354fs3X) in the SERPINA7 gene was detected. No apparent association was identified between thyroid function and the TBG-mutation in the affected subjects. A detailed biochemical and genomic testing to determine the exact cause of discordant TFT in the patients would certainly aid in the unequivocal diagnosis of the thyroid function and for the precise individualized treatment.

Qualitative and semiquantitative assessment of thyroid hormone binding proteins in greyhounds and other dog breeds

Total thyroxine (T4) concentrations are lower in healthy greyhounds compared to most other non-sighthound breeds. In humans, variations in the structure or concentration of the major thyroid hormone binding proteins are responsible for most reported differences between total T4 concentrations in healthy individuals from different ethnic groups or other subpopulations. The aim of this study was to determine if such variations are also responsible for the lower total T4 concentrations in greyhounds. The predicted protein sequences of thyroxine-binding globulin (TBG), transthyretin and albumin were determined in liver tissue from a euthyroid greyhound with decreased T4 concentration and a Jack Russell terrier using reverse-transcriptase PCR. Sequences were compared to each other and online reference sequences. Serum proteins from 21 greyhounds and 21 non-sighthound dogs were separated by denaturing electrophoresis and immunoblots probed with polyclonal antibodies to human TBG and transthyretin. Reactive bands were quantified by densitrometry, expressed relative to the mean of reference samples included in each gel. Serum albumin concentrations were measured using a commercially-available assay. Several SNPs were identified but none was thought likely to explain the lower total T4 concentrations in greyhounds. There was no significant difference between the quantity of any of the binding proteins in serum from greyhounds and non-sighthound dogs. However, total T4 and transthyretin concentrations were highly correlated in the greyhound group (r = 0.73, P = 0.0002). Variation in the sequence of thyroid hormone binding proteins is not responsible for low greyhound total T4 concentrations. Further evaluation of the role of transthyretin is warranted.

Clinical recognition and evaluation of patients with inherited serum thyroid hormone-binding protein mutations

There are three important thyroid hormone-binding proteins in human serum, thyroxine-binding globulin, transthyretin, and albumin. Genetic variation in these proteins can lead to altered thyroid hormone binding and abnormalities in serum tests of thyroid hormone. Importantly, patients harboring these mutations are euthyroid; thus, the recognition of these conditions is crucial to prevent unnecessary repeated testing and treatment. This article provides an updated overview of serum thyroid hormone transport biology and reviews the underlying genetic alterations, clinical presentation, and appropriate evaluation of patients with suspected mutations in serum thyroid hormone-binding proteins.

Increased serum reverse triiodothyronine levels at diagnosis of hepatocellular carcinoma in patients with compensated HCV-related liver cirrhosis

OBJECTIVE

The aim of this study was to investigate changes in thyroid hormone metabolism in relation to the development of hepatocellular carcinoma (HCC) in patients with HCV-related liver cirrhosis.

MATERIALS AND METHODS

The study group (Group A) comprised 31 patients (25 M, 6 F; median age 62.1 years, range 54.0-81.5 years) affected by HCV-related liver cirrhosis with superimposed HCC. Acute and chronic systemic disease, other than cirrhosis, inducing 'euthyroid sick syndrome' was excluded in all patients. Serum TSH, FT4, FT3, rT3, and thyroxine-binding globulin (TBG) levels were retrospectively evaluated in frozen aliquots drawn at the time of tumour diagnosis and every 6 months for 3-7 years before HCC diagnosis. The control group (Group B) comprised 29 patients affected by HCV-related liver cirrhosis without HCC, matched for sex, age and grade of liver dysfunction.

RESULTS

At the time of HCC diagnosis, all patients in Group A were euthyroid with serum TSH, FT4, FT3 and TBG values not significantly different from those of cirrhotic patients of Group B. However, at diagnosis Group A patients had serum rT3 values that were significantly higher than those in Group B (35.0 ng/dl, range 12.0-162.0 vs. 19.0 ng/dl, range 10.0-51.0; Group A vs. Group B; P < 0.001). Serum rT3 values above the normal range were found in 12 patients in Group A (38.7%) but in only one of the patients from Group B (3.4%) (chi2 10.2; P = 0.001). The serum rT3 levels were not significantly correlated to the Child grade of liver cirrhosis (rho 0.1; P = 0.5). The intrasubject analysis demonstrated that a significant increase in serum rT3 levels occurred at the time of HCC diagnosis but serum FT4, FT3 and TSH values did not change significantly. A receiver operating curve (ROC) demonstrated that a 6-monthly increase in serum rT3 levels of at least +22.5% identified patients with HCC with a diagnostic accuracy of 81.7%.

CONCLUSIONS

Our study has demonstrated that development of hepatocellular carcinoma is accompanied by a significant increase in serum rT3 levels in patients with low-grade HCV-related liver cirrhosis who had no other illness causing the 'euthyroid sick syndrome'.

Increased serum interleukin-6 concentration in patients with subacute thyroiditis: relationship with concomitant changes in serum T4-binding globulin concentration

Interleukin-6 (IL-6) is the main mediator of the acute phase response. Increased serum concentrations of the cytokine have been found in patients with nonthyroidal inflammatory disorders and infections. In 18 patients with subacute thyroiditis (SAT) evaluated within 1-2 weeks after the onset of the disease, serum IL-6 values, as assessed by an ELISA method having a limit of detection of 25 fmol/L, ranged 139.2-543.9 fmol/L (mean +/- SE, 287.2 +/- 28.2 fmol/L). These values were significantly higher than those of 25 normal healthy controls (mean +/- SE, 26.2 +/- 5.5 fmol/L, range < 25-99.4), 18 of whom had serum IL-6 values below the detection limit. The increase in serum IL-6 levels in SAT patients appeared to be related to the inflammatory disorder and not to thyrotoxicosis, because 18 Graves' disease patients and 13 patients with toxic adenoma or toxic multinodular goiter had significantly lower serum IL-6 concentrations (101.7 +/- 35.2 fmol/L, range < 25-251, for Graves' disease, 79.6 +/- 41.4 fmol/L, range < 25-168.5, for toxic adenoma, p < 0.001 vs SAT for both groups) despite the markedly higher levels of total and free thyroid hormones. Neither free T4 nor free T3 values were correlated with serum IL-6 levels both in SAT and Graves' patients. Twelve SAT patients were reevaluated 3-4 months later, after remission of the disease and at least one month after glucocorticoid withdrawal. At the final observation, all SAT patients showed a normalization of IL-6 concentration, which was undetectable in 8/12 (mean +/- SE, 22.8 +/- 5.4 fmol/L, p < 0.001 vs acute phase values).(ABSTRACT TRUNCATED AT 250 WORDS)

Variations in thyroid hormone transport proteins and their clinical implications

Variations in major thyroid hormone transport proteins may be inherited or acquired and may be associated with changes in serum concentration of the proteins or their affinity for thyroid hormones. These variations most frequently involve thyroxine-binding globulin (TBG), but changes in transthyretin and albumin are also observed. The consequent alteration of thyroid hormone-binding capacity in serum is associated with variations in total thyroid hormone concentration. Increased serum total thyroid hormone levels are found in subjects with TBG excess, familial dysalbuminemic hyperthyroxinemia, and transthyretin-associated hyperthyroxinemia. Conversely, diminished serum thyroid hormone values are observed in subjects with TBG deficiency, and decreased concentration or affinity of transthyretin and albumin is not associated with variations in serum concentrations of thyroid hormones. The transport protein-associated variations in serum total thyroid hormone concentrations do not reflect a change in thyroid status. Euthyroidism can be easily established in subjects with transport protein abnormalities by the normal free thyroid hormone and TSH concentrations. It is, however, crucial to select methods for free thyroid hormone measurement that are not affected by abnormalities of transport proteins. Some assays, such as the analog method, often provide artifactual and misleading results, which may lead to inappropriate and even detrimental treatments. The evolutionary advantage of TBG (and albumin) in terms of thyroid homeostasis still remains to be elucidated.

The differentiation-inducing agent sodium butyrate produces divergent effects on albumin and thyroxine-binding globulin synthesis by human hepatoblastoma-derived (Hep G2) cells

The addition of sodium butyrate, a differentiation-inducing agent, to the culture medium of human hepatoblastoma-derived (Hep G2) cells, produced a dose-dependent and time-dependent increase in albumin (ALB) and decrease in T4-binding globulin (TBG) synthesis and secretion. In the presence of 0.01 to 2.0 mM sodium butyrate, newly synthesized [35S]ALB progressively increased up to 139% of control cultures grown in the absence of sodium butyrate, whereas TBG synthesis was already slightly inhibited using the lowest concentrations of this agent and further diminished thereafter. The use of 5 mM and 10 mM sodium butyrate inhibited the synthesis of both proteins, probably as a consequence of toxic effects on cell cultures. The addition of 1 mM sodium butyrate for variable time intervals caused an increase in the amount of ALB recovered in the medium up to 146% after 72 h, and a decrease of TBG up to 44% of controls. These different effects on ALB and TBG occurred concomitantly with an inhibition of cell growth, as shown by the reduction in the cell number/flask compared to control cultures. At the highest sodium butyrate concentrations, a relevant impairment in the secretion of newly synthesized TBG, but not of ALB, also occurred. These divergent effects on ALB and TBG synthesis by Hep G2 cells might be related to biochemical differentiation induced by sodium butyrate in this tumoral cell system, suggesting that TBG synthesis is increased in Hep G2 cells because of their neoplastic nature.

Effects of the antileukemic drug L-asparaginase on sex hormone-binding globulin: studies in vivo and in vitro

L-asparaginase, an antineoplastic drug used in the treatment of acute lymphoblastic leukemia (ALL), has been previously shown to inhibit the hepatic synthesis of thyroxine-binding globulin (TBG). In two children treated by this drug for ALL, a dramatic decrease in serum sex hormone-binding globulin (SHBG) concentrations was also observed. Serum SHBG levels were still below normal 10 days after L-asparaginase withdrawal. To ascertain whether this reduction was due to the inhibition of SHBG synthesis, SHBG was measured by an immunoradiometric assay (IRMA) in the medium from human hepatoblastoma-derived cells, Hep G2 cells, grown in the absence or presence of graded amounts of the drug from 0.1 nM to 0.1 mM. The results showed a dose-dependent inhibition of SHBG synthesis, with a 50% reduction of SHBG in the medium, assayed by IRMA, using 250 nM L-asparaginase. Furthermore, a time-dependent inhibition was observed using a fixed concentration of the drug (50 nM) added for variable time intervals (1-4 days). These data suggest that the changes observed in vivo are likely due to the inhibitory effect exerted by the drug on SHBG synthesis. This action is not specific, but is part of a general effect at the hepatic level.

Thyroxine uptake by human hepatoma cells from serum of patients submitted to long-term thyroxine suppressive therapy

The significance of thyroxine (T4) uptake from serum in the assessment of thyroid status was evaluated, using human hepatoma (Hep G2) cells, in 30 euthyroid subjects, 6 hypothyroid and 19 hyperthyroid patients, and in 23 athyreotic cancer patients under T4 suppressive therapy. Cellular thyroxine (CT4) was determined according to Sarne and Refetoff, J. Clin. Endocrinol. Metab. 61: 1046, 1985. CT4 averaged 9.9 +/- 2.8 pg/well (mean +/- SD, range 5.7-15.3) in euthyroid subjects, 1.5 +/- 1.0 pg/well (range 0.05-4.2) in hypothyroid patients, 40.5 +/- 18.8 pg/well (range 18.3 +/- 104.7) in hyperthyroid patients, and 23.7 +/- 7.2 pg/well (range 14.2-40.2) in T4-treated patients. In eu-, hypo- and hyperthyroid patients, a significant correlation was observed between CT4 and free T4 index (FT4I), free T4 (FT4) or Sex Hormone Binding Globulin (SHBG) values. In T4-treated patients, CT4 values were correlated with FT4I values, but not with FT4 or SHBG levels. All T4-treated patients with elevated SHBG levels had elevated FT4, FT4I and CT4 values. In contrast, of the 16 T4-treated subjects with normal serum SHBG concentrations, all but one had normal FT3, 3 (19%) had elevated FT4, 10 (62%) elevated FT4I and 13 (81%) elevated CT4, but all (100%) had undetectable TSH levels. Thus, considering serum SHBG concentrations as a parameter of hepatic tissue response to thyroid hormone, CT4 values, at least in T4-treated patients, do not accurately reflect the liver responsiveness to thyroid hormone action.

Cellular distribution and hormonal regulation of h-SBP in human hepatoma cells

The cellular distribution of human Sex Steroid Binding Plasma Protein (h-SBP) was studied in human cells and tissues by indirect immunofluorescence. h-SBP was detected in the cytoplasm of hepatocytes, of prostate and epididymis epithelial cells and in endometrium. Sexual and non-sexual skin, intestine epithelium, striated muscle and some rodent organs were not labelled. The intracellular localization of h-SBP indicate that h-SBP could be taken up from the extracellular compartment or synthesized in situ in sex steroid target organs, where it may play a role in hormone uptake. The hormonal regulation of h-SBP secretion by a human hepatoma cell line, H5A, showed that tri-iodothyronine was more potent than estradiol or tamoxifen, which acted as estrogen agonist, in increasing secreted h-SBP and the combined effect of both thyroid and estrogen hormones resulted in an additive stimulation of h-SBP secretion. As shown by Northern blot analysis, oligonucleotides synthesized from the known sequence of h-SBP hybridized with a RNA of approximately 2 kb which was more represented in H5A cells than in normal human liver, and was increased 2-3 times after hormonal stimulation of the cells. The presence of a poly(A+)RNA coding for h-SBP in the human liver indicated the hepatic synthesis of this protein.

Direct application of radioiodinated aminoacyl tRNA for radiolabeling nascent proteins

A two-step procedure to incorporate 125I-iodotyrosine into protein synthesized in a reticulocyte lysate is described. In the first step, the iodination of tyrosyl tRNA was catalyzed by a solid-state glycouril compound. More than one-third of 200 microCi of radioiodine became bound to 70 micrograms of aminoacyl tRNA after 15 min at 0 degrees C. The isotope was distributed in a three-to-one ratio of monoiodotyrosine to di-iodotyrosine. In the second step, the soluble product of the radioiodination was transferred directly into a nuclease-treated reticulocyte lysate coded with RNA isolated from the human hepatoma cell line Hep G2. Fractional recovery of radioiodine in nascent protein was maximally 7.6%. Reaction of the product of translation with antibody against alpha-antitrypsin separated an 125I-containing protein having a molecular weight estimated as 47,000. The synthesis of unprocessed alpha-antitrypsin was confirmed by cleavage of the labeled protein with leader peptidase and by its displacement from immunocomplex formation with purified alpha-antitrypsin. The amount of 125I incorporated into alpha-antitrypsin was proportionate to iodinated tRNA additions up to a concentration of 70 micrograms/ml. The synthesis of alpha-antitrypsin as detected in radioautograms after gel electrophoresis was more than twice as sensitive using radioiodinated aminoacyl tRNA as compared with [35S]methionine. Iodine labeling of thyroxine-binding globulin was also demonstrated in the translation product of Hep G2 RNA. Since the specific activity of the radioiodine is high and the means for detection of the isotope efficient, the method described can facilitate the demonstration of quantitatively minor translation products.