Regulation of thyroperoxidase, thyroglobulin and iodide levels in sheep thyroid cells by TSH, tumor promoters and epidermal growth factor

Transcriptomic modifications of the thyroid gland upon exposure to phytosanitary-grade fipronil: Evidence for the activation of compensatory pathways

Fipronil is a phenylpyrazole insecticide used for the control of a variety of pest for domestic, veterinary and agricultural uses. Fipronil exposure is associated to thyroid disruption in the rat. It increases thyroid hormone (TH) hepatic clearance. The effect on thyroxine (T4) clearance is about four fold higher than the effect on T4 plasma concentrations suggesting that the thyroid gland might develop compensatory mechanisms. The aim of this study was to document the potential effects of fipronil treatment on the thyroid transcriptome together with its effects on TSH and TH blood levels under well characterized internal exposure to fipronil and its main metabolite fipronil sulfone. Fipronil (3 mg/kg/d by gavage for 14 days) clearance increased while its half-life decreased (about 10 fold) throughout treatment. Fipronil treatment in adult female rats significantly decreased total T4 and free triiodothyronine (T3) concentrations. Key genes related to thyroid hormone synthesis and/or cellular dynamic were modulated by fipronil exposure. RT-PCR confirmed that thyroglobulin gene expression was upregulated. A trend toward higher Na/I symporter expression was also noted, while sulfotransferase 1a1 gene expression was down-regulated. The expression of genes potentially involved in thyroid cell dynamic were upregulated (e.g. prostaglandin synthase 1, amphiregulin and Rhoa). Our results indicate that both pathways of TH synthesis and thyroid cell dynamics are transcriptional targets of fipronil and/or its main sulfone metabolite. The underlying mechanisms remain to be elucidated.

Progesterone Upregulates Gene Expression in Normal Human Thyroid Follicular Cells

Thyroid cancer and thyroid nodules are more prevalent in women than men, so female sex hormones may have an etiological role in these conditions. There are no data about direct effects of progesterone on thyroid cells, so the aim of the present study was to evaluate progesterone effects in the sodium-iodide symporter NIS, thyroglobulin TG, thyroperoxidase TPO, and KI-67 genes expression, in normal thyroid follicular cells, derived from human tissue. NIS, TG, TPO, and KI-67 mRNA expression increased significantly after TSH 20 μUI/mL, respectively: 2.08 times, P < 0.0001; 2.39 times, P = 0.01; 1.58 times, P = 0.0003; and 1.87 times, P < 0.0001. In thyroid cells treated with 20 μUI/mL TSH plus 10 nM progesterone, RNA expression of NIS, TG, and KI-67 genes increased, respectively: 1.78 times, P < 0.0001; 1.75 times, P = 0.037; and 1.95 times, P < 0.0001, and TPO mRNA expression also increased, though not significantly (1.77 times, P = 0.069). These effects were abolished by mifepristone, an antagonist of progesterone receptor, suggesting that genes involved in thyroid cell function and proliferation are upregulated by progesterone. This work provides evidence that progesterone has a direct effect on thyroid cells, upregulating genes involved in thyroid function and growth.

Nuclear localization of epidermal growth factor and epidermal growth factor receptors in human thyroid tissues

Epidermal growth factor (EGF) has widespread growth effects, and in some tissues proliferation is associated with the nuclear localization of EGF and epidermal growth factor receptor (EGFR). In the thyroid, EGF promotes growth but differs from thyrotropin (TSH) in inhibiting rather than stimulating functional parameters. We have therefore studied the occurrence and cellular distribution of EGF and EGFR in normal thyroid, in Graves' disease, where growth is mediated through the thyrotropin receptor (TSHR), and in a variety of human thyroid tumors. In the normal gland the staining was variable, but largely cytoplasmic, for both EGF and EGFR. In Graves' disease there was strong cytoplasmic staining for both EGF and EGFR, with frequent positive nuclei. Nuclear positivity for EGF and particularly for EGFR was also a feature of both follicular adenomas and follicular carcinomas. Interestingly, nuclear staining was almost absent in papillary carcinomas. These findings document for the first time the presence of nuclear EGF and EGFR in thyroid. Their predominant occurrence in tissues with increased growth (Graves' disease, follicular adenoma, and carcinoma) may indicate that nuclear EGF and EGFR play a role in growth regulation in these conditions. The absence of nuclear EGF and EGFR in papillary carcinomas would suggest that the role played by EGF in growth control differs between papillary carcinoma and follicular adenomas/carcinomas of the thyroid.

Thyrotropin chronically regulates the pool of thyroperoxidase and its intracellular distribution: a quantitative confocal microscopic study

The regulation of thyroperoxidase (TPO) expression and of its intracellular distribution was studied in porcine thyroid cells cultured on porous bottom filters. Cells were cultured for 18 days in the absence or in the presence of thyrotropin (TSH) and with or without iodide. Microsomes were purified and analyzed by electrophoresis. TPO was detected by immunoblotting with polyclonal anti-porcine TPO antibodies and quantified by scanning the bands. The amount of TPO was increased 2-fold by TSH. High concentrations of iodide (1-50 microM, added daily) decreased the level of TPO. Confocal microscopy served to determine the intracellular localization of TPO and its quantitative distribution. Intracellular and surface-located TPO was detected by fluorescein-labeled antibodies on saponin-treated cells. Quantitative confocal microscopy showed that TSH increased the total amount of TPO 2-fold as for immunoblotting. The highest amount of TPO was found in the perinuclear area and between the nucleus and the Golgi apparatus. Only 4% of TPO was present on the apical surface and about 1% on the basolateral membrane; the remainder (about 95%) was inside the cells. TSH did not change these relative contents. TSH modified the intracellular distribution of the enzyme, increasing the TPO pool from the perinuclear area to apical membrane. This domain could be a site of storage of TPO. Adding a physiological concentration of iodide (0.5 microM, daily) did not influence the intracellular distribution of TPO. We concluded that chronic TSH stimulation 1) increased 2-fold the pool of TPO but did not change the relative proportion of TPO inside the cells and on the apical surface, and 2) modified the intracellular distribution of vesicular TPO, the major part of which was accumulated in the perinuclear and cytoplasmic area under the subapical domain of the polarized cells.

Regulation of myo-inositol transport during the growth and differentiation of thyrocytes: a link with thyroid-stimulating hormone-induced phospholipase A2 activity

The Vmax of myo-inositol transport increased 3-fold during epidermal growth factor (EGF)-induced growth and thyroid-stimulating hormone. (TSH)-induced differentiation in primary cultures of sheep and human thyrocytes. The Km remained unaltered. This up-regulation required the presence of insulin. The TSH-induced rise in myo-inositol transport commenced 8 to 16 h after the initial stimulus and achieved a plateau at 24 h. In human thyrocytes the change in Vmax was accompanied by an increase in the steady-state levels of mRNA for the myo-inositol transporter following treatment with either ligand. Examination of the metabolites of myo-inositol showed few significant changes after treatment of sheep thyrocytes with EGF for 24 h. This is consistent with maintenance of the intracellular concentration of myo-inositol as the cells enlarge in preparation for cell division. In TSH-treated cells, however, up-regulation of myo-inositol transport was linked with increased myo-inositol cycling across the cell membrane, increased phospholipase A2-mediated turnover of phosphatidylinositol and a concomitant increase in arachidonic acid turnover. Increased levels of myo-inositol phosphates were also noted 24 h after TSH treatment. These results indicate the initiation of secondary signalling events many hours after the primary stimulus.

Actions of epidermal growth factor and its receptor in the thyroid

Since its discovery by Stanley Cohen (1962), epidermal growth factor (EGF) has been found to influence the growth and function of most mammalian cells. EGF is secreted, after cleavage of a large precursor molecule, as a 53-amino acid polypeptide that exerts its effects through the epidermal growth factor receptor (EGF-R), a single 170-kD transmembrane molecule exhibiting intrinsic tyrosine kinase activity of crucial importance to signal transduction (Hsuan et al 1989). Although generally mitogenic, EGF has a wide range of other effects, which vary considerably among organs, cell types, and species. [For a comprehensive update, see the review by Fisher and Lakshmanan (1990).] This article summarizes the present knowledge of EGF actions on thyroid follicular cells (thyrocytes), discusses the possible role of EGF in physiological and pathological conditions of the thyroid gland, and points out some issues that warrant further studies.

Effect of thyroid-stimulating hormone on cultured thyrocytes obtained from patients with Graves' disease and inhibitive effect by sodium iodide: a functional study

Thyrocytes obtained from patients with Graves' disease were cultured for 3 days. This was followed by culture with 10 mU/mL thyroid stimulating hormone (TSH) (TSH group), TSH and sodium iodide (Nal group), or without (control group) for 3 additional days. On the 8th culture day, the amounts of intra- and extra-cellular cyclic adenosine monophosphate (cAMP), extracellular cAMP and thyroglobulin (TG), peroxidase (PO) activity, and cell numbers were measured. The amounts of intra- and extra-cellular cAMP correlated well. TSH increased the values of cAMP, TG and PO to levels higher than those of the control group. As the amount of Nal added to the medium increased, these values decreased. Addition of 10(-5) mol/L Nal lowered the value of cAMP only. When 10(-4) mol/L Nal was added, these three levels were lower than those of the TSH group and the value of cAMP was almost equal to that of the control group. On cell number, no difference was found between the cells cultured with TSH, TSH and Nal, and without TSH or Nal. When the thyrocytes were cultured with 1 mmol/L dibutyryl cAMP sodium salt or 8-bromoadenosine 3',5'-cyclic monophosphate instead of TSH, 10(-4) mol/L Nal did not lower the values of thyroglobulin and peroxidase activity. These results suggest that the Nal blocks the intracellular signal transduction provoked by TSH, only at the cAMP production level.

Hormonal regulation of some steps of thyroglobulin synthesis and secretion in bicameral cell culture

Porcine thyroid cells were cultured for 15 days on porous bottom chambers with or without different mixtures of hormones added to serum-free basal medium. Assays with 10% serum were also performed for comparison with previously published results. The effects of the hormones, particularly insulin, TSH and hydrocortisone, were studied on total RNA content, thyroglobulin mRNA level, the amount of thyroglobulin secreted into the apical medium and on glycosylation. Insulin and TSH similarly increased the total RNA content, and their effects were additive. Thyroglobulin mRNA content was increased twofold by insulin and threefold by TSH. When they were added simultaneously, the maximal level of thyroglobulin mRNA was reached, showing that TSH and insulin effects on thyroglobulin gene expression were additive. Hydrocortisone alone did not modify total RNA or thyroglobulin mRNA content but the hormone amplified total RNA when insulin and TSH were present together. The basal level of thyroglobulin secreted into the apical medium was increased threefold by insulin and fourfold by TSH. The effects of these two hormones added together appeared to be additive. Hydrocortisone had no effect alone or even when combined with insulin or TSH. However, when the three hormones were added together, the hormonal response was amplified. TSH effect and insulin effect on the incorporation of 3H-mannose into thyroglobulin as well as on the anionic residue content of the molecule were additive.

Phorbol esters showing selective activation of PKC isozymes in vitro regulate thyroid function and insulin-like growth factor binding protein secretion

We have examined the effects of phorbol derivatives which show selective activation of protein kinase C (PKC) isozymes in vitro, on several parameters of thyroid function. Functions examined were iodide uptake and organification, iodocompound secretion and insulin-like growth factor binding protein (IGFBP) secretion, all of which have been shown previously to be modulated by 12-O-tetradecanoylphorbol 13-acetate (TPA), a pan activator of PKC isozymes. All of the agents examined, including DOPPA (12-deoxyphorbol-13-O-phenylacetate-20 acetate), which is specific for the beta 1 isozyme in vitro, were able to mimic the effects of TPA. These effects were evident by 2 h in the iodide uptake and organification assays, by 4 h in the secretion assays and by 8 h in the IGFBP secretion assays. The phorbol derivatives differed from TPA in their ability to down-regulate total PKC activity, DOPPA being weakly effective at 8 h (14.7% inhibition) when TPA had effected > 70% down-regulation of PKC. As the effects of DOPPA were detected by 8 h at the latest, these data indicate that the effects observed were due to PKC activation rather than down-regulation. Furthermore, the differences in down-regulation profiles between DOPPA and TPA suggest that in vivo, DOPPA may maintain its in vitro specificity. We conclude that inhibition of thyroid iodide uptake and its organification, stimulation of iodocompound secretion and stimulation of IGFBP-2 and IGFBP-3 secretion may be effected through the modulation of a limited number of PKC isozymes and possibly initially, only through PKC beta 1.

Trace element deficiencies in cattle

Deficiency of cobalt, copper, iron, iodine, manganese, selenium, or zinc can cause a reduction in production. Reduced production occurs most commonly when a deficiency corresponds to the phases of growth, reproduction, or lactation. Because of environmental, nutrient, disease, genetic, and drug interactions, deficiencies of single or multiple elements can occur even when the levels recommended by the National Research Council for these nutrients are being fed. Additionally, random supplementation of trace elements above National Research Council recommendations is not justified because of the negative interaction among nutrients and potential toxicosis. Evaluation of trace element status can be difficult because many disease states will alter blood analytes used to evaluate nutrient adequacy. Proper dietary and animal evaluation, as well as response to supplementation, are necessary before diagnosing a trace element deficiency.

The microsomal/peroxidase antigen: modulation of its expression in thyroid cells

Evidence has accumulated in the last few years that the expression of the microsomal/peroxidase antigen (M/TPO-Ag) in thyroid cells is induced by TSH, through pathways which involve intracellular cAMP accumulation and protein synthesis. These data have been found true in any thyroid system studied so far, both in terms of immunologic and enzymatic activity of TPO. TSH and cAMP also increase the levels of the specific mRNA for TPO in thyroid cells from different species. Whether this phenomenon is due to a direct transcriptional regulation of the TPO gene, as shown in dog thyroid cells, or to posttranscriptional effects, as it would appear in FRTL-5 cells, remains to be clarified by future experiments. Thyroid stimulating antibody (TSAb) of Graves' disease also stimulates the expression of M/TPO-Ag. This finding gives further support to the relevance of TSAb in the pathogenesis of hyperthyroidism and explains the well known observation that the "microsomal" antigen is particularly abundant in glands of Graves' patients. The modulation of M/TPO-Ag surface expression by TSH can explain the decrease of circulating anti-MAb observed during L-thyroxine therapy in hypothyroid patients with Hashimoto's thyroiditis. Other agents, such as methimazole and sodium iodide, which influence thyroid cell function, do not directly interfere with the expression of M/TPO-Ag. Cytokines, such as gamma-interferon, interleukin-1, and interleukin-6 have been shown to inhibit the TSH-induced increase of TPO mRNA, but further investigations are required to elucidate the exact role of cytokines in the regulation of M/TPO-Ag expression.

Thyrotropin inhibits while insulin, epidermal growth factor and tetradecanoyl phorbol acetate stimulate insulin-like growth factor binding protein secretion from sheep thyroid cells

Six insulin-like growth factor binding proteins (IGFBP) have been identified in the conditioned medium from sheep thyroid cells cultured under serum-free conditions. IGFBPs of 32, 28, 23 and 19 kDa were secreted by cells cultured for 14 days in serum-free and hormone-free medium. The constitutive secretion of IGFBP was inhibited by thyrotropin (TSH, 0.3 mU per mL). The effect was most marked on the secretion of the 28 kDa BP. High insulin concentrations stimulated the secretion of this IGFBP. The stimulatory effects of insulin were inhibited by TSH. Growth hormone treatment decreased the secretion of the 28 kDa protein. Tetradecanoylphorbol-13 acetate (TPA) and epidermal growth factor (EGF) both of which stimulate thyroid cell growth but inhibit differentiated function, markedly stimulated IGFBP secretion and induced the appearance of a 46 and a 150 kDa IGFBP. The effects of EGF and TPA were not identical. A rat IGFBP-2 cDNA reacted with sheep thyroid RNA of approximate size 1.6 kb. TPA treatment increased IGFBP-2 mRNA. Other hormones used to enhance differentiation and growth in thyroid cells in culture i.e. transferrin, somatostatin, cortisol and glycyl-histidyl-lysine acetate had no marked effects on IGFBP secretion nor on TSH-dependent, insulin-mediated iodide uptake and organification and cell growth. We show a correlation between secretion of high molecular weight IGFBP with enhanced growth but decreased function. Conversely, we find a correlation between decreased secretion of the 28 kDa BP and increased growth and function.

Interaction of TSH, insulin and insulin-like growth factors in regulating thyroid growth and function

Primary cultures of sheep thyroid cells have been used to study regulation of thyroid growth and function by growth factors and TSH. Cells were plated at low density to minimize contributions from the endogenously produced insulin-like growth factors and their binding proteins and other proteins or hormones secreted by thyroid cells in culture. Growth of the cells was followed for 7-11 days in medium without serum. We found that TSH by itself was unable to stimulate thyroid growth. However, the ability of insulin and IGF-I to stimulate thyroid cell growth was markedly potentiated by TSH. Thyroid function was assayed by measurement of uptake of pertechnetate and organification of iodide and also by synthesis of thyroglobulin mRNA. TSH alone was unable to stimulate thyroid function appreciably. Insulin and IGF-I were ineffective by themselves at stimulating thyroid differentiated function, but in the presence of TSH, all indices were stimulated markedly. We conclude that TSH by itself is not a growth factor for thyroid cells. However, in the presence of insulin or IGF-I, TSH potentiates the growth-stimulating properties of this hormone. Similarly, TSH by itself does not stimulate thyroid function but requires the presence of insulin or IGF-I. These data show the cooperative interactions between growth factors and TSH in regulating both thyroid growth and function.