Effects of Propyithiouracil (PTU) Administration on the Synthesis and Secretion of Thyroglobulin in the Rat Thyroid Gland: A Quantitative Immuno-electron Microscopic Study Using Immunogold Technique

Changes in thyroid histomorphology and thyroglobulin immunostaining upon exposure to thiourea in Triturus newts

Amphibians are useful bioindicators for monitoring aquatic health and the influence of xenobiotics such as endocrine disrupting chemicals. Because aquatic ecosystems experience the majority of global pollution, aquatic organisms are most exposed and vulnerable to endocrine disruptors. Furthermore, penetration of endocrine disruptors into aquatic organisms especially in amphibians is even easier because of more permeable skin, resulting in high bioavailability and bioaccumulation of chemicals. One of the most potent endocrine disruptors is thiourea, which chemically blocks the synthesis of thyroid hormones and prevents metamorphosis in amphibians. We investigated the influence of thiourea on histomorphology of the thyroid gland in Triturus newts at the metamorphic stage, when thyroid hormone concentrations should reach their maximum level. Chronic exposure to thiourea induced hypertrophy and hyperplasia of follicular cells as well as a significant reduction of interstitial tissue. The intensity of the thyroglobulin immunostaining signal significantly decreases upon chronic exposure to thiourea. Successful cross-reactivity of human primary antibody in immunochemical detection of thyroglobulin in Urodela confirms potential homology in thyroglobulin structure throughout the vertebrates.

Investigation of the effect of boron on thyroid functions and biochemical parameters in hypothyroid induced-rats

In the study investigating the effects of boron on thyroid hormones and some biochemical parameters in hypothyroid rats, 49 Wistar Albino male rats were divided into seven groups; (Control (C), Hypothyroidism (H), boron groups (B10, and B20), hypothyroid + boron groups (HB10 and HB20), and Treatment (T). Four groups (H, HB10, HB20, and T) were administered 10 mg/kg (B10 and HB10), 20 mg/kg (B20 and HB20) boron for 3 weeks, respectively after hypothyroidism was induced using Propycil® containing propylthiouracil (PTU). Thyroid hormone analyses and biochemical measurements were made from the serum and thyroid gland tissue was examined histopathologically. According to the findings, the fT3 level increased in the B10 group compared to the control group (p < 0.05). While AST, ALT, and ALP activities were found to be higher in the hypothyroid group than in the control group, AST and ALP activities in the HB10 and HB20 groups decreased to values close to the control group. Total cholesterol levels were found to be lower in boron-given groups compared to control and hypothyroid groups (p < 0.05). Sodium iodide symporter (NIS) immunoreactivity was found to be high in hypothyroid rat groups. As a result, it was observed that the increased AST and ALP activities in rats decreased with boron administration. The serum hormone levels measured in the study are not sufficient to understand the effect of boron on the thyroid gland, and it was concluded that further studies at the molecular level are needed to understand the effects of boron on the thyroid gland.

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.

Intrinsic regulation of thyroid function by thyroglobulin

BACKGROUND

The established paradigm for thyroglobulin (Tg) function is that of a high molecular weight precursor of the much smaller thyroid hormones, triiodothyronine (T3) and thyroxine (T4). However, speculation regarding the cause of the functional and morphologic heterogeneity of the follicles that make up the thyroid gland has given rise to the proposition that Tg is not only a precursor of thyroid hormones, but that it also functions as an important signal molecule in regulating thyroid hormone biosynthesis.

SUMMARY

Evidence supporting this alternative paradigm of Tg function, including the up- or downregulation by colloidal Tg of the transcription of Tg, iodide transporters, and enzymes employed in Tg iodination, and also the effects of Tg on the proliferation of thyroid and nonthyroid cells, is examined in the present review. Also discussed in detail are potential mechanisms of Tg signaling in follicular cells.

CONCLUSIONS

Finally, we propose a mechanism, based on experimental observations of Tg effects on thyroid cell behavior, that could account for the phenomenon of follicular heterogeneity as a highly regulated cycle of increasing and decreasing colloidal Tg concentration that functions to optimize thyroid hormone production through the transcriptional activation or suppression of specific genes.

Impact of experimental hypothyroidism on monoamines level in discrete brain regions and other peripheral tissues of young and adult male rats

The levels of dopamine (DA), norepinephrine (NE) and serotonin (5-HT) in different brain regions as well as in blood plasma, cardiac muscle and adrenal gland of young and adult male albino rats were measured following experimentally induced hypothyroidism. Hypothyroidism induced by daily oral administration of propylthiouracil (PTU, 5mg/kg body wt) caused a significant reduction in DA levels in most of the tissues examined of both young and adult rats after 21 and 28 days, in NE levels after all the time intervals studied in young rats, and after 21 and 28 days in adult rats. 5-HT exhibited a significant reduction in the selected brain regions and blood plasma after 21 and 28 days and in cardiac muscle after all the time intervals in the two age groups of animals. It may be suggested that the changes in monoamine levels induced by hypothyroidism may be due to disturbance in the synthesis and release of these amines through the neurons impairment or may be due to an alteration pattern of their synthesizing and/or degradative enzymes.

Mode of Action: Developmental Thyroid Hormone Insufficiency—Neurological Abnormalities Resulting From Exposure to Propylthiouracil

Because thyroid hormone is essential for normal brain development before and after birth, environmental chemicals that interfere with thyroid hormone signaling can adversely affect brain development. Adverse consequences of thyroid hormone insufficiency depend both on severity and developmental timing, indicating that environmental antithyroid factors may produce different effects at different developmental windows of exposure. Mechanistic studies can provide important insight into the potential impact of chemicals on human thyroid function, but relevance to humans must be systematically evaluated. This kind of analysis depends on data sets that include information about animals and humans. The drug 6-n-propyl-2-thiouracil (PTU) is used in animals to experimentally manipulate serum thyroid hormone levels, and in humans to treat patients, including pregnant women, with Graves' disease. A systematic analysis of the mode of action (MOA) of PTU in rats and in humans discloses similar modes of action. While the analysis predicts that PTU doses that produce thyroid hormone insufficiency in humans would adversely affect the developing brain, careful monitoring of PTU administration in pregnant and lactating humans keeps infant serum thyroid hormone levels within the normal range.

Follicular thyroglobulin suppresses iodide uptake by suppressing expression of the sodium/iodide symporter gene

A major function of the thyrocyte is to take up and concentrate iodide. This is needed for thyroid hormone synthesis and is accomplished by the sodium iodide symporter (NIS), whose expression and activity are up-regulated by TSH. Recently, we reported that follicular thyroglobulin (TG) is a potent suppressor ofthyroid-specific gene expression and can overcome TSH-increased gene expression. We suggested this might be a negative feedback, autoregulatory mechanism that counterbalanced TSH stimulation of follicular function. In this report, we support this hypothesis by coordinately evaluating TG regulation of NIS gene expression and iodide transport. We show that physiological concentrations of TG similarly and significantly suppress TSH-increased NIS promoter activity, NIS protein, and NIS-dependent iodide uptake as well as RNA levels. We show, in vivo, that TG accumulation at the apical membrane of a thyrocyte facing the follicular lumen is associated with decreased uptake ofradioiodide. It is likely, therefore, that TG suppresses NIS-dependent iodide uptake and NIS gene expression in vivo, as is the case in vitro. RNA levels of NIS and vascular endothelial growth factor/vascular permeability factor, which has been reported to be TSH regulated and possibly associated with TSH-increased iodide uptake, are coordinately decreased by follicular TG as a function of concentration and time. Also, removal of follicular TG from the medium, but not TSH, coordinately returns NIS and vascular endothelial growth factor/vascular permeability factor RNA levels to their TSH-stimulated state. TG accumulated in the follicular lumen appears, therefore, to be a negative feedback regulator of critical TSH-increased follicular functions, iodide uptake, and vascular permeability.

In vivo expression 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

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.