Thyroglobulin suppresses thyroid-specific gene expression in cultures of normal but not neoplastic human thyroid follicular cells

Influencing Factors of Urinary Iodine Concentration Before and After Radioiodine Therapy for Differentiated Thyroid Cancer: An Initial Exploration of the Relationship With Therapeutic Efficacy

OBJECTIVE

To investigate the impact of urinary iodine concentration (UIC) and post-stimulatory thyroglobulin (ps-Tg) levels on the therapeutic efficacy of differentiated thyroid cancer (DTC) patients after initial radioiodine therapy, and to analyze the validity of these indicators as prognostic factors.

METHODS

A total of 213 DTC patients received initial radioiodine therapy from June 2022 to September 2023. Demographic data and UIC were collected before and after therapy. Thyrotropin, thyroglobulin (Tg), and thyroglobulin antibody levels were assessed. Iodine uptake rate was measured, and therapeutic efficacy was evaluated 6 months post-therapy. Statistical tests were used for data comparison, and logistic regression analysis for response factors.

RESULTS

Post-therapy UIC and pre-post UIC difference were significantly correlated with Tg levels but not with reaching excellent response (ER) indicated by suppression of Tg levels below 0.2 ug/L. Ps-Tg levels related to therapeutic efficacy, while UIC did not correlate with outcomes. ROC curve analysis found optimal ps-Tg cut-off points for the low-intermediate and high-risk groups classified by primary tumor size, invasion, metastasis, and pathological type.

CONCLUSION

Post-treatment UIC and pre-post UIC difference correlate with ps-Tg levels. Ps-Tg levels are an associated factor for DTC, but UIC changes, despite correlation with ps-Tg, are not significantly related to outcomes and cannot be used as a prognostic factor.

Primary human thyrocytes maintained the function of thyroid hormone production and secretion in vitro

PURPOSE

Thyroid cell lines are useful tools to study the physiology and pathology of the thyroid, however, they do not produce or secrete hormones in vitro. On the other hand, the detection of endogenous thyroid hormones in primary thyrocytes was often hindered by the dedifferentiation of thyrocytes ex vivo and the presence of large amounts of exogenous hormones in the culture medium. This study aimed to create a culture system that could maintain the function of thyrocytes to produce and secrete thyroid hormones in vitro.

METHODS

We established a Transwell culture system of primary human thyrocytes. Thyrocytes were seeded on a porous membrane in the inner chamber of the Transwell with top and bottom surfaces exposed to different culture components, mimicking the 'lumen-capillary' structure of the thyroid follicle. Moreover, to eliminate exogenous thyroid hormones from the culture medium, two alternatives were tried: a culture recipe using hormone-reduced serum and a serum-free culture recipe.

RESULTS

The results showed that primary human thyrocytes expressed thyroid-specific genes at higher levels in the Transwell system than in the monolayer culture. Hormones were detected in the Transwell system even in the absence of serum. The age of the donor was negatively related to the hormone production of thyrocytes in vitro. Intriguingly, primary human thyrocytes cultured without serum secreted higher levels of free triiodothyronine (FT3) than free thyroxine (FT4).

CONCLUSION

This study confirmed that primary human thyrocytes could maintain the function of hormone production and secretion in the Transwell system, thus providing a useful tool to study thyroid function in vitro.

Thyroglobulin regulates the expression and localization of the novel iodide transporter solute carrier family 26 member 7 (SLC26A7) in thyrocytes

Solute carrier family 26 member 7 (SLC26A7), identified as a causative gene for congenital hypothyroidism, was found to be a novel iodide transporter expressed on the apical side of the follicular epithelium of the thyroid. We recently showed that TSH suppressed the expression of SLC26A7 and induces its localization to the plasma membrane, where it functions. We also showed that the ability of TSH to induce thyroid hormone synthesis is completely reversed by an autocrine negative-feedback action of thyroglobulin (Tg) stored in the follicular lumen. In the present study, we investigated the potential effect of follicular Tg on SLC26A7 expression and found that follicular Tg significantly suppressed the promoter activity, mRNA level, and protein level of SLC26A7 in rat thyroid FRTL-5 cells. In addition, follicular Tg inhibited the ability of TSH to induce the membrane localization of SLC26A7. In rat thyroid sections, the expression of SLC26A7 was weaker in follicles with a higher concentration of Tg, as evidenced by immunofluorescence staining. These results indicate that Tg stored in the follicular lumen is a feedback suppressor of the expression and membrane localization of SLC26A7, thereby downregulating the transport of iodide into the follicular lumen.

Thyroid stimulating hormone suppresses the expression and activity of cytosolic sulfotransferase 1a1 in thyrocytes

Sulfonation is an important step in the metabolism of dopamine, estrogens, dehydroepiandrosterone, as well as thyroid hormones. However, the regulation of cytosolic sulfotransferases in the thyroid is not well understood. In a DNA microarray analysis of rat thyroid FRTL-5 cells, we found that the mRNA expression of 10 of 48 sulfotransferases was significantly altered by thyroid stimulating hormone (TSH), with that of sulfotransferase family 1A member 1 (SULT1A1) being the most significantly affected. Real-time PCR and Western blot analyses revealed that TSH, forskolin and dibutyryl cyclic AMP significantly suppressed SULT1A1 mRNA and protein levels in a time- and concentration-dependent manner. Moreover, immunofluorescence staining of FRTL-5 cells showed that SULT1A1 is localized in the perinuclear area in the absence of TSH but is spread throughout the cytoplasm with reduced fluorescence intensity in the presence of TSH. Sulfotransferase activity in FRTL-5 cells, measured using 3'-phosphoadenosine-5'-phosphosulfate as a donner and p-nitrophenol as an acceptor substrate, was significantly reduced by TSH. These findings suggest that the expression and activity of SULT1A1 are modulated by TSH in thyrocytes.

Intrathyroidal feedforward and feedback network regulating thyroid hormone synthesis and secretion

Thyroid hormones (THs), including T4 and T3, are produced and released by the thyroid gland under the stimulation of thyroid-stimulating hormone (TSH). The homeostasis of THs is regulated via the coordination of the hypothalamic-pituitary-thyroid axis, plasma binding proteins, and local metabolism in tissues. TH synthesis and secretion in the thyrocytes-containing thyroid follicles are exquisitely regulated by an elaborate molecular network comprising enzymes, transporters, signal transduction machineries, and transcription factors. In this article, we synthesized the relevant literature, organized and dissected the complex intrathyroidal regulatory network into structures amenable to functional interpretation and systems-level modeling. Multiple intertwined feedforward and feedback motifs were identified and described, centering around the transcriptional and posttranslational regulations involved in TH synthesis and secretion, including those underpinning the Wolff-Chaikoff and Plummer effects and thyroglobulin-mediated feedback regulation. A more thorough characterization of the intrathyroidal network from a systems biology perspective, including its topology, constituent network motifs, and nonlinear quantitative properties, can help us to better understand and predict the thyroidal dynamics in response to physiological signals, therapeutic interventions, and environmental disruptions.

Regulation of solute carrier family 26 member 7 (Slc26a7) by thyroid stimulating hormone in thyrocytes

Iodine transportation is an important step in thyroid hormone biosynthesis. Uptake of iodine into the thyroid follicle is mediated mainly by the basolateral sodium-iodide symporter (NIS or solute carrier family 5 member 5: SLC5A5), and iodine efflux across the apical membrane into the follicular lumen is mediated by pendrin (SLC26A4). In addition to these transporters, SLC26A7, which has recently been identified as a causative gene for congenital hypothyroidism, was found to encode a novel apical iodine transporter in the thyroid. Although SLC5A5 and SLC26A4 have been well-characterized, little is known about SLC26A7, including its regulation by TSH, the central hormone regulator of thyroid function. Using rat thyroid FRTL-5 cells, we showed that the mRNA levels of Slc26a7 and Slc26a4, two apical iodine transporters responsible for iodine efflux, were suppressed by TSH, whereas the mRNA level of Slc5a5 was induced. Forskolin and dibutyryl cAMP (dbcAMP) had the same effect as that of TSH on the mRNA levels of these transporters. TSH, forskolin and dbcAMP also had suppressive effects on SLC26A7 promoter activity, as assessed by luciferase reporter gene assays, and protein levels, as determined by Western blot analysis. TSH, forskolin and dbcAMP also induced strong localization of Slc26a7 to the cell membrane according to immunofluorescence staining and confocal laser scanning microscopy. Together, these results suggest that TSH suppresses the expression level of Slc26a7 but induces its accumulation at the cell membrane, where it functions as an iodine transporter.

A matrix-assisted laser desorption/ionization imaging mass spectrometric approach to study weight-related changes within thyroid tissue

Obesity is associated with numerous comorbidities along with abnormalities of the endocrine system, more commonly manifesting as dysfunctions of the thyroid gland such as goiter. Changes in weight, especially an increase, could lead to an increase in the incidence of thyroid dysfunction; however, its pathophysiology remains to be elucidated. In the present study, we aimed to interrogate the changes in the protein distribution and abundance between the lean patients and patients with obesity thyroid tissue sections through utilizing this technique. The FFPE-fixed thyroid tissue blocks from the selected cases and controls were identified and targeted for matrix-assisted laser desorption/ionization (MALDI) imaging mass spectrometry (IMS) analysis. Patients in the 30 to 75 years age group and undergoing total thyroidectomy for benign thyroid disease were recruited. Patients with thyroid cancers, autoimmune disorders, and other inflammatory conditions were excluded from the study. The selected patients were divided into two groups according to their BMIs: lean (BMI < 25) and obese (BMI > 35). An initial trial set was used as a pilot study for the optimization of the MALDI IMS protocol that was next applied to the selected thyroid tissues. MALDI IMS data from all the samples were aligned and statistical analysis carried out by k-means and linear discriminant analysis (LDA) classification model using principle component analysis (PCA) results were evaluated between the two groups: controls (lean) and cases (obese). Receiver operator characteristic (ROC) curves were alternatively used to calculate the variability of the identified peptides. The discriminating peptides were also independently identified and related to their corresponding proteins by using liquid chromatography and tandem mass spectrometry (LC-MS/MS) analyses. Eight peptides mainly from thyroglobulin were found to be upregulated whereas 10 others were found to be downregulated in the lean compared to the obese group. Through this technique, we will be able to better understand the relationship between the disease entity and obesity.

Pathological role of excessive DNA as a trigger of keratinocyte proliferation in psoriasis

Psoriasis is characterized by excessive growth and aberrant differentiation of epidermal keratinocytes due to persistent inflammation. However, the underlying mechanism that triggers immune activation in psoriasis is not clear. In this study, we explored excessive DNA as a potential trigger of psoriasis using cultured human keratinocytes and psoriatic skin tissues. We demonstrated that human genomic DNA fragments induced tumour necrosis factor (TNF)-α expression, hyperproliferation and over-expression of heparin-binding epidermal-like growth factor (HB-EGF) and transforming growth factor (TGF)-α, accompanied by defective expression of keratins 1 and 10 in cultured normal human epidermal keratinocytes, which have a similar phenotype to that of keratinocytes in psoriatic skin lesions. In psoriatic lesions, we found high levels of double-stranded (ds)DNA fragments, accompanying keratinocytes expressing Ki-67, HB-EGF and TNF-α. In addition, we showed that 1,25-dihydroxyvitamin D₃ inhibited genomic DNA fragment-induced TNFA and interleukin-1β (IFNB) expression in human keratinocytes, and an intact function of cathelicidin anti-microbial peptide (CAMP) was required for this effect. These results suggest that excessive dsDNA fragments probably act as a risk factor for immune activation in psoriasis, and the active form of vitamin D can prevent genomic DNA-mediated skin inflammation via CAMP.

[Progress in the research of negative feedback effect of thyroglobulin]

Thyroglobulin is the most important and abundant protein in thyroid follicles and has been widely studied as a tumor marker of thyroid cancer recurrence and persistence. Tg is considered the material basis of thyroid hormone synthesis and does not participate in the regulation of thyroid hormone synthesis and secretion. This review summarizes the recent progress in the research of thyroid hormone synthesis and secretion regulation via a negative feedback regulation mechanism by the thyroid-hypothalamus-pituitary axis. Thyroglobulin can negatively regulate the synthesis of thyroid hormone by thyroid follicular cells and antagonize the positive regulation of thyrotropin TSH. The function of thyroid follicular cells is presumably a result of Tg and TSH interaction, and a follicular cycle model is proposed to explain the causes of follicular heterogeneity in glands. We also discuss the prospects and clinical significance of studies into the negative feedback regulation mechanism of the thyroid-hypothalamus-pituitary axis and compare two theories for this mechanism.

Comparative Efficacy and Toxicity of Different Species of Sargassum in Haizao Yuhu Decoction in PTU-Induced Goiter Rats

BACKGROUND

Haizao Yuhu Decoction has been widely used to treat thyroid-related diseases especially goiter with few side effects in traditional Chinese medicine (TCM), including herb pair Sargassum (HZ) and Glycyrrhizae Radix et Rhizoma (GC), as one of "eighteen antagonistic medicaments." The two different species of Sargassum, Sargassum fusiforme (Sf) and Sargassum pallidum (Sp), are not clearly differentiated in clinical use, so that herb pair Sf-GC and Sp-GC could show different effect and toxicity.

METHODS

We investigated the antigoitrous effect and toxicity and clarified the potential underlying mechanism of the two different species of Sargassum in HYD (HYDf and HYDp) in PTU-reduced goiter rats.

RESULTS

The results demonstrated that both HYDf and HYDp could exhibit antigoitrous effect through alterations in hypothalamus-pituitary-thyroid (HPT) axis and inhibition of the TPO gene expression; there is no difference in the antigoitrous effects between the two different species of Sargassum application in HYD.

CONCLUSION

This study evaluated the safety and efficacy of herb pair HZ-GC applied in HYD in goiter rats at molecular, cellular, and whole level and compared the two species of Sargassum further. We provide a reliable way to clarify the possible mechanism of the antagonistic medicament herb pair HZ-GC for its application.

Follicular thyroglobulin induces cathepsin H expression and activity in thyrocytes

Thyroglobulin (Tg) stored in thyroid follicles exerts a potent negative-feedback effect on each step of pre-hormone biosynthesis, including Tg gene transcription and iodine uptake and organification, by suppressing the expression of specific transcription factors that regulate these steps. Pre-hormones are stored in the follicular colloid before being reabsorbed. Following lysosomal proteolysis of its precursor, thyroid hormone (TH) is released from thyroid follicles. Although the suppressive effects of follicular Tg on each step of pre-hormone biosynthesis have been extensively characterized, whether follicular Tg accumulation also affects hormone reabsorption, proteolysis, and secretion is unclear. In this study we explored whether follicular Tg can regulate the expression and function of the lysosomal endopeptidases cathepsins. We found that in the rat thyroid cell line FRTL-5 follicular Tg induced cathepsin H mRNA and protein expression, as well as cathepsin H enzyme activity. Double immunofluorescence staining showed that Tg endocytosis promoted cathepsin H translocalization into lysosomes where it co-localized with internalized Tg. These results suggest that cathepsin H is an active participant in lysosome-mediated pre-hormone degradation, and that follicular Tg stimulates mobilization of pre-hormones by activating cathepsin H-associated proteolysis pathways.

A Novel Role for Flotillin-Containing Lipid Rafts in Negative-Feedback Regulation of Thyroid-Specific Gene Expression by Thyroglobulin

BACKGROUND

Thyroglobulin (Tg) stored in thyroid follicles regulates follicular function in thyroid hormone (TH) synthesis by suppressing thyroid-specific gene expression in a concentration-dependent manner. Thus, Tg is an intrinsic negative-feedback regulator that can restrain the effect of thyrotropin (TSH) in the follicle. However, the underlying mechanisms by which Tg exerts its prominent autoregulatory effect following recognition by thyrocytes remains unclear.

METHODS

In order to identify potential proteins that recognize and interact with Tg, mass spectrometry was used to analyze immunoprecipitated Tg-bound proteins derived from Tg-treated rat thyroid FRTL-5 cells.

RESULTS

Flotillin 1 and flotillin 2, two homologs that are integral membrane proteins in lipid rafts, were identified as novel Tg-binding proteins with high confidence. Further studies revealed that flotillins physically interact with endocytosed Tg, and together these proteins redistribute from the cell membrane to cytoplasmic vesicles. Treatment with the lipid raft disrupter methyl-β-cyclodextrin abolished both the endocytosis and the negative-feedback effect of Tg on thyroid-specific gene expression. Meanwhile, siRNA-mediated knockdown of flotillin 1 or flotillin 2 also significantly inhibited Tg effects on gene expression.

CONCLUSION

Together these results indicate that flotillin-containing lipid rafts are essential for follicular Tg to be recognized by thyrocytes and exert its negative-feedback effects in the thyroid.

Iodide transport: implications for health and disease

Disorders of the thyroid gland are among the most common conditions diagnosed and managed by pediatric endocrinologists. Thyroid hormone synthesis depends on normal iodide transport and knowledge of its regulation is fundamental to understand the etiology and management of congenital and acquired thyroid conditions such as hypothyroidism and hyperthyroidism. The ability of the thyroid to concentrate iodine is also widely used as a tool for the diagnosis of thyroid diseases and in the management and follow up of the most common type of endocrine cancers: papillary and follicular thyroid cancer. More recently, the regulation of iodide transport has also been the center of attention to improve the management of poorly differentiated thyroid cancer. Iodine deficiency disorders (goiter, impaired mental development) due to insufficient nutritional intake remain a universal public health problem. Thyroid function can also be influenced by medications that contain iodide or interfere with iodide metabolism such as iodinated contrast agents, povidone, lithium and amiodarone. In addition, some environmental pollutants such as perchlorate, thiocyanate and nitrates may affect iodide transport. Furthermore, nuclear accidents increase the risk of developing thyroid cancer and the therapy used to prevent exposure to these isotopes relies on the ability of the thyroid to concentrate iodine. The array of disorders involving iodide transport affect individuals during the whole life span and, if undiagnosed or improperly managed, they can have a profound impact on growth, metabolism, cognitive development and quality of life.

The Emerging Roles of Thyroglobulin

Thyroglobulin (Tg), the most important and abundant protein in thyroid follicles, is well known for its essential role in thyroid hormone synthesis. In addition to its conventional role as the precursor of thyroid hormones, we have uncovered a novel function of Tg as an endogenous regulator of follicular function over the past decade. The newly discovered negative feedback effect of Tg on follicular function observed in the rat and human thyroid provides an alternative explanation for the observation of follicle heterogeneity. Given the essential role of the regulatory effects of Tg, we consider that dysregulation of normal Tg function is associated with multiple human thyroid diseases including autoimmune thyroid disease and thyroid cancer. Additionally, extrathyroid Tg may serve a regulatory function in other organs. Further exploration of Tg action, especially at the molecular level, is needed to obtain a better understanding of both the physiological and pathological roles of Tg.