In VivoExpression of Thyroid Transcription Factor-1 RNA and Its Relation to Thyroid Function and Follicular Heterogeneity: Identification of Follicular Thyroglobulin as a Feedback Suppressor of Thyroid Transcription Factor-1 RNA Levels and Thyroglobulin Synthesis

Abstract

We used in situ hybridization to evaluate thyroid transcription factor-1 (TTF-1) RNA expression in individual follicles and related this to thyroglobulin (Tg) synthesis in vivo, as estimated by immunohistochemical analysis. We studied the thyroids of Wistar rats treated with thyroxine (T4) or propylthiouracil (PTU), each of which modulates TSH levels, but affects follicular function and Tg accumulation in the follicular lumen very differently. We show that TTF-1 RNA levels in vivo correlate directly with an increase in the cytoplasmic accumulation of Tg within the cells of individual follicles. Because TTF-1 increases Tg gene expression, RNA levels, and protein synthesis in thyroid cell cultures and because there is no correlation with TSH-increased Tg degradation within the follicular lumen, the increased cytoplasmic accumulation of Tg in vivo is interpreted to reflect TTF-1-increased Tg synthesis. Increases in serum TSH levels in the PTU or T4 treated animals did not always correlate with increases in this measure of increased Tg synthesis; and TSH levels did not always correlate with changes in TTF-1 RNA levels that would be expected to accompany increased Tg synthesis. As one possibility, this suggested there might be a hitherto unrecognized suppressor of TTF-1 RNA levels and TSH-induced Tg synthesis in individual follicles. The immunohistochemical data suggested that this suppressor might be follicular Tg itself. Supporting this possibility, we show that physiological concentrations of highly purified 19S follicular Tg decrease TTF-1 RNA levels in rat FRTL-5 thyroid cells and inhibit the action of TSH to increase Tg synthesis. We therefore suggest that follicular Tg is a feedback autoregulator of thyroid function that can counterregulate TSH actions on thyroid function in vivo and in thyroid cells in culture. We suggest this phenomenon contributes to follicular heterogeneity in vivo.

Regulation of Dual Oxidase Expression and H2O2Production by Thyroglobulin

“…DUOX2 protein levels and the resultant H 2 O 2 generation were also reduced by Tg. Although the expression and function of most thyroid-specific genes that are necessary for hormone synthesis are tightly regulated by TSH and the Tg stored in each thyroid follicle (28,30,32,33), the regulation of Duoxs and Duoxas by Tg has not been investigated earlier.…”

“…Although overall thyroid function is tightly controlled by TSH, follicular function is remarkably heterogeneous (27,32,47,48). The function of each follicle is regulated by the amount of Tg stored in the lumen, the action of which counteracts the action of TSH (25,28,30).…”

“…The function of each follicle is regulated by the amount of Tg stored in the lumen, the action of which counteracts the action of TSH (25,28,30). Thus, the Tg stored in individual thyroid follicles is a potent negative feedback regulator of follicular function that mediates the transcriptional suppression of genes which are necessary for iodide transport and hormone biosynthesis (25,28,29,32). We, therefore, proposed that this is the basis of the follicular heterogeneity under constant TSH stimulation for every follicle (24)(25)(26)(27)(28)(29)31).…”

“…The stored Tg in the thyroid follicle is a potent negative feedback regulator of follicular function (24)(25)(26)(27)(28)(29)(30)(31). We have shown that physiological concentrations of follicular Tg suppressed not only thyroid-specific genes such as Slc5a5, Tpo, and Tg but also iodide uptake in the thyroid (28,30,32). In addition to suppressing gene expression, low concentrations of Tg also induce expression of the Pendred syndrome gene (Slc26a4) that mediates iodide transportation to the follicular lumen, which enhances hormone synthesis until enough Tg is accumulated, and the process is terminated by Tg-mediated suppression (25,33).…”

Follicular thyroglobulin enhances gene expression necessary for thyroid hormone secretion

“…Rat thyroid FRTL-5 cells were grown in Coon's modified Ham's F-12 medium supplemented with 5% bovine serum (Invitrogen, Waltham, MA, USA) and a six-hormone mixture (1 mU/mL TSH, 10 μg/mL insulin, 10 ng/mL somatostatin, 0.36 ng/mL hydrocortisone, 5 μg/mL transferrin, and 2 ng/mL glycyl-L-histidyl-L-lysine acetate) as described [2,6,15]. Culture medium that did not contain TSH was also used in some experiments.…”