Iodination of thyroglobulin molecules depends on their diffusion velocity in follicular colloid

Auto-Regulation of the Thyroid Gland Beyond Classical Pathways

This mini-review asks how self-regulation of the thyroid gland is realized at the cellular and molecular levels by canonical and non-canonical means. Canonical pathways of thyroid regulation comprise thyroid stimulating hormone-triggered receptor signaling. As part of non-canonical regulation, we hypothesized an interplay between protease-mediated thyroglobulin processing and thyroid hormone release into the circulation by means of thyroid hormone transporters like Mct8. We proposed a sensing mechanism by different thyroid hormone transporters, present in specific subcellular locations of thyroid epithelial cells, selectively monitoring individual steps of thyroglobulin processing, and thus, the cellular thyroid hormone status. Indeed, we found that proteases and thyroid hormone transporters are functionally inter-connected, however, in a counter-intuitive manner fostering self-thyrotoxicity in particular in Mct8- and/or Mct10-deficient mice. Furthermore, the possible role of the G protein-coupled receptor Taar1 is discussed, because we detected Taar1 at cilia of the apical plasma membrane of thyrocytes in vitro and in situ. Eventually, through pheno-typing Taar1-deficient mice, we identified a co-regulatory role of Taar1 and the thyroid stimulating hormone receptors. Recently, we showed that inhibition of thyroglobulin-processing enzymes results in disappearance of cilia from the apical pole of thyrocytes, while Taar1 is re-located to the endoplasmic reticulum. This pathway features a connection between thyrotropin-stimulated secretion of proteases into the thyroid follicle lumen and substrate-mediated self-assisted control of initially peri-cellular thyroglobulin processing, before its reinternalization by endocytosis, followed by extensive endo-lysosomal liberation of thyroid hormones, which are then released from thyroid follicles by means of thyroid hormone transporters.

Deactivation of TSH receptor signaling in filter-cultured pig thyroid epithelial cells

Thyrotropin [thyroid-stimulating hormone (TSH)] receptor on-off signaling was studied in polarized monolayers of pig thyrocytes cultured on permeable support. Transepithelial resistance (R) and potential difference (PD) were used as parameters to monitor the effect of altered TSH concentrations on vectorial electrolyte transport. TSH induced rapid but long-lasting changes in R (decrease) and PD (increase) that were cAMP-dependent and related to enhanced transcellular conductance of sodium and chloride. Withdrawal of TSH from cultures prestimulated with TSH (0.1 mU/ml) for 48 h resulted in restitution of R to control level within 30 min. Such deactivation was markedly accelerated by mild trypsinization, which degraded receptor-bound ligand without affecting TSH receptor responsiveness or ion transporting capacity. Small alterations in the TSH concentration (0.01-0.1 mU/ml) were followed almost instantaneously by adjustments of R. In contrast, the reversal of R after acute TSH stimulation (30 min) and subsequent TSH washout was delayed for several hours independently of cell surface trypsinization. The observations indicate that, during continuous exposure to physiological concentrations, TSH exerts a close minute-to-minute surveillance of thyroid function and the rate-limiting step of deactivation is the dissociation of ligand from the TSH receptor at the cell surface. TSH-deprived cells briefly exposed to TSH are refractory to rapid deactivation, probably because of altered metabolism downstream of TSH receptor signal transduction.

Colloidal aggregates of insoluble inclusions in human goiters

To shed some light on the physicochemical properties of the thyroid follicular colloid, we have screened retrospectively the autoradiographs of 60 human nodular goiters labeled 17 h preoperatively with 100 microCi 125I for evidence of colloid compartmentalization. In 87% (52/60) of all goiters examined, sporadic or multiple colloidal inclusions ('colloid stones') not mixing with newly labeled Tg were detected. The detailed analysis of 17 goiters revealed a mean incidence of 0.09+/-0.11 'colloid stones' of variable size per follicle ranging from 0.02+/-0.01 (10) to 0.43+/-0.09 (5) (mean values +/- S.D., number of sections examined in brackets). In this study we did not find a clear-cut association of incidence of 'colloid stones' with sex, age or nosologic group (hyperthyroid, preclinically hyperthyroid, euthyroid). The existence of different colloidal compartments as demonstrated in this and other studies is of considerable importance for thyroid function, interpretation of iodine kinetics, and studies on the role of iodine on growth and function of the thyrocytes. Different thyroidal iodine compartments could well be of functional relevance, for example in the adaptation of thyroid hormone secretion to antithyroid drugs or in severe and prolonged iodine deficiency, when very slow compartments become an important source of minimal quantities of iodine and thyroid hormone. 'Colloid stones', for example, may well explain the repeatedly observed, surprisingly large total iodine store in human endemic goiters, even in the presence of severe iodine deficiency. It is evident that the existence of multiple iodine compartments and, in particular, of particulate slow-turnover pools complicates the interpretation of total glandular iodine measurements with modern techniques such as X-ray fluorescence and positron emission tomography.

Thyroglobulin regulates follicular function and heterogeneity by suppressing thyroid-specific gene expression

Thyroglobulin (TG) is the primary synthetic product of the thyroid and the macromolecular precursor of thyroid hormones. TG synthesis, iodination, storage in follicles, and lysosomal degradation can each modulate thyroid hormone formation and secretion into the circulation. Thyrotropin (TSH), via its receptor (the TSHR), increases thyroid hormone levels by upregulating expression of the sodium iodide symporter (NIS), thyroid peroxidase (TPO), and TG genes. TSH does this by modulating the expression and activity of the thyroid-specific transcription factors, thyroid transcription factor (TTF)-1, TTF-2, and Pax-8, which coordinately regulate NIS, TPO, TG, and the TSHR. Major histocompatibility complex (MHC) class I gene expression, which is also regulated by TTF-1 and Pax-8 in the thyroid, is simultaneously decreased; this maintains self tolerance in the face of TSH-increased gene products necessary for thyroid hormone formation. We now show that follicular TG, 27S > 19S > 12S, counter-regulates TSH-increased thyroid-specific gene transcription by suppressing the expression of the TTF-1, TTF-2, and Pax-8 genes. This decreases expression of the TG, TPO, NIS and TSHR genes, but increases class I expression. TG action involves an apical membrane TG-binding protein; however, it acts transcriptionally, targeting, for example, a sequence within 1.15 kb of the start of TTF-1 transcription. TG does not affect ubiquitous transcription factors regulating TG, TPO, NIS and/or TSHR gene expression. TG activity is not duplicated by thyroid hormones or iodide. We hypothesize that TG-initiated, transcriptional regulation of thyroid-restricted genes is a normal, feedback, compensatory mechanism which regulates follicular function, regulates thyroid hormone secretion, and contributes to follicular heterogeneity.

Autoregulation of thyroid-specific gene transcription by thyroglobulin

Thyroglobulin (TG), the primary synthetic product of the thyroid, is the macromolecular precursor of thyroid hormones. TG synthesis, iodination, storage in follicles, and degradation control thyroid hormone formation and secretion into the circulation. Thyrotropin (TSH), via its receptor (TSHR), increases thyroid hormone levels by up-regulating expression of the sodium iodide symporter (NIS), thyroid peroxidase (TPO), and TG genes. TSH does this by modulating the expression and activity of several thyroid-specific transcription factors, thyroid transcription factor (TTF)-1, TTF-2, and Pax-8, which coordinately regulate NIS, TPO, TG, and the TSHR. Major histocompatibility complex class I gene expression, which also is regulated by TTF-1 and Pax-8 in the thyroid, is decreased simultaneously. This helps maintain self-tolerance in the face of TSH-increased gene products necessary for thyroid hormone formation. In this report we show that follicular TG counter-regulates TSH-increased, thyroid-specific gene transcription by suppressing expression of the TTF-1, TTF-2, and Pax-8 genes. This decreases expression of the TG, TPO, NIS, and TSHR genes, but increases class I expression. TG acts transcriptionally, targeting, for example, a sequence within 1.15 kb of the 5' flanking region of TTF-1. TG does not affect ubiquitous transcription factors regulating TG, TPO, NIS, and/or TSHR gene expression. The inhibitory effect of TG on gene expression is not duplicated by thyroid hormones or iodide and may be mediated by a TG-binding protein on the apical membrane. We hypothesize that TG-initiated, transcriptional regulation of thyroid-restricted genes is a normal, feedback, compensatory mechanism that limits follicular function and contributes to follicular heterogeneity.

Autonomous growth and function of cultured thyroid follicles from cats with spontaneous hyperthyroidism

Spontaneous feline hyperthyroidism is a unique experimental model of toxic nodular goiter. To determine whether feline toxic goiter is caused by extrathyroidal stimulating factors or by the intrinsic autonomy of follicular cells, primary cultures of enzymatically dissociated follicles from 15 hyperthyroid cat goiters and from 3 normal cat thyroid glands were embedded in collagen gels. Growth and function in chemically defined media were assessed by autoradiography after double labeling with 3H-thymidine and 131I-Na. Iodine organification in follicles from normal glands was TSH dependent, but intense radioiodine organification occurred in follicles from hyperfunctioning goiters even in the absence of TSH. Similarly, twice as many follicular cells of hyperfunctioning thyroid tissue, maintained without TSH in the medium, were labeled after exposure to 3H-thymidine than in follicles from normal glands. The results strongly suggest that intrinsic alterations of cell function lead to autonomy of follicular growth and function and subsequently to the development of hyperplastic nodules, causing thyrotoxicosis. The reason for the focal nature of the disease remains an unresolved challenge. Further investigation using this model may further understanding of the growth of autonomous endocrine tumors.

Intrathyroidal iodine heterogeneity of iodocompounds and kinetic compartmentalization

In the normal thyroid, but not necessarily in the goitrous gland, the bulk of iodine is bound to thyroglobulin. Even in the normal thyroid-and much more so in goiters-iodine is contained in many different compartments with widely differing kinetics, biochemical composition, localization, and physiologic significance. Any change of thyroid function profoundly affects intrathyroidal iodine kinetics and produces a redistribution of stored iodine. This must be taken into account whenever the impact of a global change in intrathyroidal iodine stores on thyroid function and growth is studied in vivo.

The effects of dietary iodine on thyroid ultrastructure

This is a morphological study of changes in thyroid cells following iodine deficiency and iodine excess. Fifteen young male Sprague-Dawley rats were divided into three groups and fed one of the following diets for 6 weeks: low iodine (LID), normal iodine (NID) and high iodine (HID). Then the thyroid glands were removed and processed for light and electron microscopy. Thyroid tissue from the NID group was normal in appearance. The most outstanding feature of HID thyroids was the presence of numerous cells which contained irregularly shaped and stained lysosomes. These displaced other cell organelles and caused the apical cell surfaces to project into the follicle lumen. Thyroids from the LID group were three times heavier than the other two groups. Their follicles were very small, contained very little colloid. They were surrounded by dilated capillaries. Mitoses were frequent. Cells were columnar and contained abundant dilated endoplasmic reticulum, numerous apical vesicles, long microvilli and many mitochondria. Mitochondria were especially abundant in greatly infolded lateral and basal cell membranes. These findings show that there is a redistribution of organelles in thyroid cells in response to iodine deficiency and iodine excess which can be related to alterations in intracellular iodine metabolism.

Direct fluorescence localization of an endogenous N-acetyl-glucosamine-specific lectin in the thyroid gland

A sugar-binding protein, or endogenous lectin, was localized on fixed and paraffin-embedded thyroid sections by means of fluorescein-labelled neoglycoproteins. Fluorescence microscopy showed the binding of this lectin to be dependent on calcium ions and acidic pH and indicated sugar specificity for GlcNAc moieties only. In human, porcine and murine thyrocytes, specific binding was observed mainly on subcellular organelles. Conversely, in rabbit thyrocytes, specific labelling was seen mostly at the apical cell surface facing the follicular lumen. The possibility that this novel endogenous lectin is involved in the Tg metabolism is considered.