Synthesis and apical and basolateral secretion of thyroglobulin by thyroid cell monolayers on permeable substrate: modulation by thyrotropin

Clinical use of thyroglobulin: not only thyroid cancer

Thyroglobulin (TG) is a dimeric glycoprotein produced exclusively by mature thyroid tissue and stored within the follicular lumen. It is essential for the organification of iodine and the production of thyroid hormones. The concentration of TG in the bloodstream varies between individuals and depends on factors such as thyroid mass, stimulation of the gland by thyrotropin or autoantibodies, and tissue destruction. TG is essential to monitor patients with differentiated thyroid cancer; however, its use is not limited only to this clinical entity. Measurement of circulating TG can provide better insight into numerous clinical scenarios, such as destructive thyroiditis, presence of ectopic thyroid tissue, thyroid trauma, factitious thyrotoxicosis, or iodine nutrition. Lately, TG has found its new clinical use in immune checkpoint-related thyroid dysfunction. TG measurement should be performed carefully in patients with antithyroglobulin antibodies due to possible laboratory interferences. In this review, we offer a summary of current knowledge about the clinical use of TG and the implications it brings to daily practice.

Serum thyroglobulin is associated with orbitopathy in Graves' disease

PURPOSE

Serum thyroglobulin levels are often elevated in Graves' disease (GD) and in most cases decrease during treatment. Its relation to Graves' orbitopathy (GO) has not been clarified. Previously, a risk of GO has been linked to smoking, TSH receptor stimulation, high TSH-receptor antibodies (TRAb), low thyroid peroxidase and thyroglobulin antibodies (TPOAb, TgAb).

METHODS

We examined Tg levels in 30 consecutive patients with GD were given drug therapy (methimazole + thyroxine) for up to 24 months. GO was identified by clinical signs and symptoms. 17 patients had GO, 11 of whom had it at diagnosis while 6 developed GO during treatment. During the study, 5 subjects were referred to radioiodine treatment, 3 to surgery. The remaining 22 subjects (GO n = 12, non-GO n = 10) completed the drug regimen.

RESULTS

At diagnosis, Tg levels in GO patients (n = 11) were higher (84, 30-555 µg/L, median, range) than in non-GO patients (n = 19) (38, 3.5-287 µg/L), p = 0.042. Adding the 6 subjects who developed eye symptoms during treatment to the GO group (n = 17), yielded p = 0.001 vs. non-GO (n = 13). TRAb tended to be higher, while TPOAb and TgAb tended to be lower in the GO group. For the 22 patients who completed the drug regimen, Tg levels were higher in GO (n = 12) vs. non-GO (n = 10), p = 0.004, whereas TRAb levels did not differ.

CONCLUSION

The data may suggest that evaluation of thyroglobulin levels in GD could contribute to identify patients at increased risk of developing GO. Possibly, thyroidal release of Tg in GD reflects a disturbance that also impacts retroorbital tissues.

The plasminogen-like molecule apically secreted by epithelial thyroid cells is sulfated

Plasminogen (Pl), a circulating protease synthesized in the liver, is also present in several tissues. In the thyroid gland a Pl-like protease was found in the apical lumen where it is involved, through its proteolytic activity, in luminal degradation of thyroglobulin (Tg). Here, we showed for the first time that the Pl-like protease apically secreted by epithelial thyroid cells is sulfated, both on tyrosine residue(s) and on oligosaccharide side chains. The Pl molecule is composed of a large N-terminal moiety made of five distinct Kringle domains (K1-K5) separated by small peptidic fragments, and of a C-terminal domain with serine protease activity. Using a software tool able to predict tyrosine sulfation sites in protein sequences we localized the potential tyrosine sulfation sites of Pl. Then, we became aware that, whatever the species considered, at least three of the four potential tyrosine sulfation sites of Pl were located on Kringle sites, and more precisely, for K1, on the highly conserved binding domain of K1. We determined with the same software tool which potential sulfation sites were the most likely to be really sulfated. We hypothesize that the sulfation of these sites modulates the binding properties of Pl.

Role of sulfated tyrosines of thyroglobulin in thyroid hormonosynthesis

Our previous studies showed that sulfated tyrosines (Tyr-S) are involved in thyroid hormone synthesis and that Tyr(5), the main hormonogenic site of thyroglobulin (Tg), is sulfated. In the present paper, we studied the role of Tyr-S in the formation and activity of the peroxidase-Tg complex. Results show that noniodinated (35)SO(3)-Tg specifically binds (Kd=1.758 microM) to immobilized lactoperoxidase (LPO) via Tyr-S linkage by using saturation binding and competition experiments. We found that NIFEY-S, a 15-amino acid peptide corresponding to the NH2-end sequence of Tg and containing the hormonogenic acceptor Tyr5-S, was a better competitor than cholecystokinin and Tyr-S. 35SO3-Tg, iodinated without peroxidase, bound to LPO with a Kd (1.668 microM) similar to that of noniodinated Tg, suggesting that 1) its binding occurs via Tyr-S linkage and 2) Tyr-S requires peroxidase to be iodinated, whereas nonsulfated Tyr does not. Iodination of NIFEY-S with [125I]iodide showed that Tyr5-S iodination increased with LPO concentration, whereas iodination of a nonsulfated peptide containing the donor Tyr130 was barely dependent on LPO concentration. Enzymatic hydrolysis of iodinated Tg or NIFEY-S showed that the amounts of sulfated iodotyrosines also depended on LPO amount. Sulfated iodotyrosines were detectable in the enzyme-substrate complex, suggesting they have a short life before the coupling reaction occurs. Our data suggest that after Tyr-S binding to peroxidase where it is iodinated, the sulfate group is removed, releasing an iodophenoxy anion available for coupling with an iodotyrosine donor.

A plasminogen-like protein, present in the apical extracellular environment of thyroid epithelial cells, degrades thyroglobulin in vitro

The prothyroid hormone, thyroglobulin (Tg), is stored at high concentrations in the thyroid follicular lumen as a soluble 19S homo-dimer and as heavier soluble (27S and 37S) and insoluble (Tgm) forms. Follicular degradation of Tg may contribute to maintaining Tg concentrations compatible with follicle integrity. Here, we report on the presence of a plasminogen-like protein in the follicular lumen of normal human thyroids and its synthesis and apical secretion by cultured epithelial thyroid cells. Since all the main luminal forms of Tg are cleaved by this plasminogen-like protein, we suggest that it contributes to Tg degradation in the follicular lumen.

Influence of TSH on uptake of [18F]fluorodeoxyglucose in human thyroid cells in vitro

Recent clinical evidence suggests that positron emission tomography with fluorine-18 fluorodeoxyglucose (FDG-PET) is more accurate in detecting thyroid carcinomatous tissue at high than at low TSH levels. The aim of this study was to determine the influence of TSH on FDG uptake in human thyroid cells in vitro. Monolayers of human thyroid tissue were cultured after mechanical disintegration and enzymatic digestion of samples from patients undergoing surgery for nodular goitre. The purity of thyroid cell preparations was ascertained by immunohistochemical staining for the epithelial antigen KL-1, and their viability by measuring the synthesis of thyroglobulin in vitro. The cells were incubated with 0.8-1.5 MBq FDG/ml uptake medium for 1 h. FDG uptake in thyroid cells was quantified as percent of whole FDG activity per well (% ID) or as % ID in relation to total protein mass. This experimental protocol was subsequently varied to study the effect of incubation time, glucose dependency and TSH. Furthermore, radio-thin layer chromatography was used to identify intracellular FDG metabolites. FDG accumulated in the thyroid cells linearly with time, doubling roughly every 20 min. Uptake was competitively inhibited by unlabelled glucose and decreased to approximately 70% at 100 mg/dl glucose compared to the value measured in glucose-free medium. FDG was intracellularly trapped as FDG-6 phosphate and FDG-1,6-diphosphate. TSH significantly increased FDG uptake in vitro in a time- and concentration-dependent manner: Cells cultured at a TSH concentration of 50 micro U/ ml doubled FDG uptake compared to TSH-free conditions, and uptake after 72 h of TSH pre-incubation was approximately 300% of that without TSH pre-incubation. TSH stimulates FDG uptake by benign thyroid cells in a time- and concentration-dependent manner. This supports the clinical evidence that in well-differentiated thyroid carcinomas, most of which are still TSH-sensitive, FDG-PET is more accurate at high levels of TSH.

Thyrotropin and iodide regulate sulfate concentration in thyroid cells. Relationship to thyroglobulin sulfation

Thyroglobulin (Tg), the thyroid hormone precursor, is sulfated both on tyrosines and on carbohydrates. We showed recently that sulfated tyrosines were involved in thyroid hormone synthesis. Moreover, we also reported that Tg sulfation is downregulated by thyrotropin (TSH), especially on tyrosines. This control may occur at each step in the sulfation process. In this paper, we studied the regulation of the concentration of cytosolic inorganic sulfate, the first substrate, in porcine thyroid cells stimulated by TSH with or without iodide. The amounts of cytosolic sulfate and the cytosolic volumes measured showed that the sulfate concentration depends only on cytosolic volume changes in response to TSH and iodide treatment. After the cells were labelled with [35S]-sulfate, the specific radioactivity (SRA) of cytosolic sulfate was determined. When cells were treated with only TSH, the concentration and SRA of cytosolic sulfate decreased by 30%, and by about 15% when cells were incubated with both TSH and iodide. TSH decreased more conspicuously the rate of [35S]-sulfate incorporation into Tg (by 57% without iodide, by 43% with iodide) than the concentration and SRA of cytosolic sulfate, while iodide altered these parameters to the same extent (15%). These findings suggest that TSH regulates other steps in the sulfation process, such as specific substrate and enzyme levels, while iodide controls mainly the sulfate concentration.Key words: cytosolic inorganic sulfate measurement, capillary electrophoresis, intracellular sulfate concentration, thyroglobulin sulfation, primary culture thyroid cells.

The hormonogenic tyrosine 5 of porcine thyroglobulin is sulfated

Our previous results showed that sulfated tyrosines of thyroglobulin (Tg), the molecular support of thyroid hormonosynthesis, are involved in the hormonogenic process. Moreover, the consensus sequence required for tyrosine sulfation is present in most of the hormonogenic sites. These observations suggest that tyrosine sulfation might play a critical role in the hormonogenic process. In this paper we studied the putative sulfation of tyrosine 5 contained in the preferential hormonogenic site. Porcine thyrocytes were cultured with thyrotropin but without iodide to preserve the sulfation state of tyrosine 5 and then incubated or not with [35S]sulfate. Secreted Tg was purified and submitted to peptide sequence analysis which confirmed the known peptide sequence of the NH(2) extremity of Tg:NIFEYQV. The treatment of [35S]sulfate-labeled Tg by leucine aminopeptidase, which sequentially digested its amino-terminal extremity, released the same amino acids and further analysis by thin layer chromatography showed that the tyrosine was sulfated. We concluded that tyrosine 5 is sulfated but the role of sulfate group in the hormonogenic process remains to be elucidated.

Polarized trafficking of thyrocyte proteins in MDCK cells

Recent studies suggest striking similarities between polarized protein sorting in thyrocytes and MDCK epithelial cells, including apical trafficking of thyroglobulin (Tg), thyroid peroxidase, and aminopeptidase N; as well as basolateral targeting of heparan sulfate proteoglycans, thrombospondin 1 (TSP1), type 1 5'-deiodinase, sodium-potassium ATPase, and the thyrotropin receptor. In this report, we have firstly expressed in stably transfected MDCK II cells a range of truncation mutants lacking up to 78% of the C-terminus of TSP1; these studies indicate that the N-terminal region containing the heparin binding domain is sufficient for basolateral targeting of TSP1. Secondly, we have stably transfected MDCK II cells with both Tg and sodium-iodide symporter (NIS) cDNAs, obtaining clones that simultaneously express both thyroid-specific proteins at the apical and basolateral cell surfaces, respectively. These studies represent promising early steps towards designing artificial thyrocytes by thyroid gene transfer into MDCK cells.

Sulfate transport in porcine thyroid cells. Effects of thyrotropin and iodide

In porcine thyroid cells, thyroglobulin sulfation is controlled by thyrotropin (TSH) and iodide, which contribute to regulating the intracellular sulfate concentration, as we previously established. Here, we studied the transport of sulfate and its regulation by these two effectors. Kinetic studies were performed after [(35)S]sulfate was added to either the basal or apical medium of cell monolayers cultured without any effectors, or with TSH with or without iodide. The basolateral uptake rates were about tenfold higher than the apical uptake rates. TSH increased the basolateral and apical uptake values (by 24 and 9%, respectively, compared with unstimulated cells), and iodide inhibited these effects of TSH. On the basis of results of the pulse-chase experiments, the basolateral and apical effluxes appeared to be well balanced in unstimulated cells and in cells stimulated by both TSH and iodide: approximately 40-50% of the intracellular radioactivity was released into each medium, whereas in the absence of iodide, 70% of the intracellular radioactivity was released on the basolateral side. The rates of transepithelial sulfate transport were increased by TSH compared with unstimulated cells, and these effects decreased in response to iodide. These results suggest that TSH and iodide may each control the sulfate transport process on two sides of the polarized cells, and that the absence of iodide in the TSH-stimulated cells probably results in an unbalanced state of sulfate transport.

Regulation of thyroid cell volumes and fluid transport: opposite effects of TSH and iodide on cultured cells

Cell volume regulation by thyrotropin (TSH) and iodide, the main effectors involved in thyroid function, was studied in cultured thyroid cells. The mean cell volume, determined by performing 3-D reconstitution on confocal microscopy optical slices from living octadecylrhodamine-labeled cells cultured with both TSH and iodide (control cells), was 3.73 +/- 0.06 pl. The absence of iodide resulted in cell hypertrophy (136% of control value) and the absence of TSH in cell shrinkage (81%). These changes mainly affected the cell heights. The effect of TSH on cell volume was mediated by cAMP. The proportion of cytosolic volume (3-O-methyl-D-glucose space vs. total volume) decreased in the absence of iodide (85% of control value) and increased in the absence of TSH (139%), whereas protein content showed the opposite changes (121 and 58%, respectively). The net apical-to-basal fluid transport was also inversely controlled by the two effectors. Iodide thus antagonizes TSH effects on cell volumes and fluid transport, probably via adenylylcyclase downregulation mechanisms. The absence of either iodide or TSH may mimic the imbalance occurring in pathological thyroids.

Sulfated tyrosines of thyroglobulin are involved in thyroid hormone synthesis

Thyroid hormone synthesis is under the control of thyrotropin (TSH), which also regulates the sulfation of tyrosines in thyroglobulin (Tg). We hypothesized that sulfated tyrosine (Tyr[S]) might be involved in the hormonogenic process, since the consensus sequence required for tyrosine sulfation to occur was observed at the hormonogenic sites. Porcine thyrocytes, cultured with TSH but without iodide in the presence of [(35)S]sulfate, secreted Tg which was subjected to in vitro hormonosynthesis with increasing concentrations of iodide. A 63% consumption of Tyr[S] (1 residue) was observed at 40 atoms of iodine incorporated into Tg, corresponding to a 40% hormonosynthesis efficiency. In addition, hyposulfated Tg secreted by cells incubated with sodium chlorate was subjected to in vitro hormonosynthesis. With 0.5 Tyr[S] residue (31% of the initial content), the efficiency of the hormonosynthesis was 29%. In comparison, when hormonosynthesis was performed by cells, with only 0.25 Tyr[S] residue (16% of the initial content), the hormonosynthesis efficiency fell to 18%. These results show that there exists a close correlation between the sulfated tyrosine content of Tg and the production of thyroid hormones.

Polarized targeting of epithelial cell proteins in thyrocytes and MDCK cells

Polarized trafficking signals may be interpreted differently in different cell types. In this study, we have compared the polarized trafficking of different proteins expressed endogenously in primary porcine thyroid epithelial cells to similar proteins expressed in MDCK cells. As in MDCK cells, NH4Cl treatment of filter-grown thyrocytes caused mis-sorted soluble proteins to exhibit enhanced secretion to the apical medium. In independent studies, thrombospondin 1 (a thyroid basolaterally secreted protein) was secreted basolaterally from MDCK cells. Likewise, the 5′-deiodinase (a thyroid basolateral membrane protein) encoded by the DIO1 gene was also distributed basolaterally in transfected MDCK cells. Consistent with previous reports, when the secretion of human growth hormone (an unglycosylated regulated secretory protein) was examined from transfected MDCK cells, the release was nonpolarized. However, transfected thyrocytes secreted growth hormone apically in a manner dependent upon zinc addition. Moreover, two additional regulated secretory proteins expressed in thyrocytes, thyroglobulin (the major endogenous glycoprotein) and parathyroid hormone (an unglycosylated protein expressed transiently), were secreted apically even in the absence of zinc. We hypothesize that while cellular mechanisms for interpreting polarity signals are generally similar between thyrocytes and MDCK cells, thyrocytes allow for specialized packaging of regulated secretory proteins for apical delivery, which does not require glycosylation but may involve availability of certain ions as well as appropriate intracellular compartmentation.

DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid) increases iodide trapping, inhibits thyroperoxidase and antagonizes the TSH-induced apical iodide efflux in porcine thyroid cells

4,4'-Di-isothiocyanatostilbene-2,2'-disulfonic acid (DIDS), an inhibitor of several anionic channels and transporters including the band 3 protein of the red blood cell membrane was tested on iodide metabolism in cultured porcine thyroid cells. We used three experimental cell culture models: (i) forskolin-stimulated correctly inside-in polarized follicle-associated thyroid cells cultured onto plastic support (ii) suspensions of isolated cells derived from such cultures (iii) polarized monolayers in bicameral chambers. DIDS was observed to increase free-iodide trapping in all conditions. Organification of iodide by follicle-associated cell cultures incubated for 6 h decreased as a function of DIDS concentration with an IC50 of 5 x 10(-5) M. This block in organification is accounted for a block in thyroperoxidase activity as in vitro both purified lactoperoxidase and purified porcine thyroperoxidase were inhibited by DIDS with a similar dose-dependency the IC50 being also of 5 x 10(-5) M. Both control and DIDS-treated cells in suspension, actively trapped iodide and reached a steady concentration in about 50 min; however the plateau was 4.4-fold higher in (10(-3) M) DIDS-treated cells. Acute TSH-stimulation at this plateau of 125I-preloaded cells in suspension in the presence of 2 mM methimazole (MMI) induced a fast release of iodide from these cells as expected (first step of the TSH-biphasic effect). This TSH-induced iodide efflux was however completely inhibited by DIDS (10(-3) M). Furthermore, addition of DIDS to the apical compartment of TSH-prestimulated cell monolayers in bicameral chambers resulted in an increase in intracellular-iodide concentration and in an inhibition of iodide efflux into the apical medium. Taken together, the present results demonstrate that DIDS mainly interacts with two main components of the thyroid apical cell membrane: thyroperoxidase and a cAMP-sensitive iodide channel.

Cytokines and thyroid epithelial integrity: interleukin-1alpha induces dissociation of the junctional complex and paracellular leakage in filter-cultured human thyrocytes

Locally produced proinflammatory cytokines are likely to play a pathophysiological role in autoimmune thyroid disease. An important feature of the thyroid, not previously considered in cytokine actions, is the barrier created by the follicular epithelium, which secludes two lumenal autoantigens [thyroglobulin (Tg) and thyroperoxidase] from the extrafollicular space. We examined the influence of recombinant cytokines on the barrier function of human thyrocytes cultured as a tight and polarized monolayer in bicameral chambers. Whereas interleukin (IL)-6 (100 U/mL), interferon-gamma (100 U/mL), tumor necrosis factor-alpha (10 ng/mL), and transforming growth factor-beta1 (10 ng/mL) had no effects, exposure to IL-1alpha for 24-48 h reduced the transepithelial resistance from >1000 to <50 omega x cm2 and increased the paracellular flux of [3H]inulin and exogenous 125I-Tg. This response to IL-1alpha, which was dose dependent (1-1000 U/mL) and reversible, was accompanied by dramatic morphological changes of the epithelial junction complex, including aberrant localization of the tight junction protein zonula occludens-1. At the same time, IL-1alpha decreased the apical secretion of endogenous Tg and stimulated the basolateral release of a novel high-molecular-mass protein. We conclude that IL-1alpha reduces the thyroid epithelial barrier without signs of general cytotoxicity. The observation suggests a mechanism by which IL-1alpha may promote the exposure of hidden autoantigens to the immune system in thyroid autoimmunity.

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.

Calcium is transported into the lumen of pig thyroid follicles by fluid phase basolateral to apical transcytosis

The lumen of thyroid follicles contains a high concentration of thyroglobulin, the thyroid prohormone and a high concentration of calcium (Ca2+). As thyroglobulin binds Ca2+, intraluminal Ca2+ is expected to be in free and protein-bound forms. In the present work, we have investigated the mechanism(s) by which Ca2+ could enter the lumen of thyroid follicles. 45Ca2+ uptake studies were carried out on reconstituted pig thyroid follicles (RTF) and pig thyroid cell monolayers (TCM) in primary culture, representing experimental systems with two compartments (cells + lumina) and one compartment, respectively. 45Ca2+ accumulation in RTF was rapid during the first hour of incubation and then slowly increased. Analysis of the uptake data with a "two compartments" model gave two kinetic constant values: k = 1.71 +/- 0.28 hr(-1) and k(-2) = 0.20 +/- 0.05 hr(-1) (n = 10). The slow uptake process accounted for 20-50% of the total RTF-associated Ca2+ after 24 hr. 45Ca2+ uptake by TCM was rapid and reached a stable level within 1-2 hr. Experimental data fitted with a "single compartment" model and gave a k(-1) value of 1.64 +/- 0.15 hr(-1) (n = 10) which was not statistically different from the k(-1) obtained for 45Ca2+ uptake by RTF. We then compared the kinetics of 45Ca2+ uptake by RTF with the kinetics of transport of fluid phase markers: [14C]-sucrose and Lucifer Yellow from the medium to the lumen of RTF. [14C]-sucrose and Lucifer Yellow uptakes by RTF appeared as slow processes compatible with the entry in a single compartment with k values of 0.32 +/- 0.06 hr(-1) (n = 3) and 0.23 +/- 0.015 hr(-1) (n = 3), respectively. These values were not significantly different from the k(-2) value obtained for 45Ca2+ uptake by RTF. These data suggest that thyroid follicles would possess two independent Ca2+ compartments: cells and lumen, and that the entry of Ca2+ into the lumen of follicles probably could take place by fluid phase basolateral to apical transcytosis.

Intracellular protein transport to the thyrocyte plasma membrane: potential implications for thyroid physiology

We present a snapshot of developments in epithelial biology that may prove helpful in understanding cellular aspects of the machinery designed for the synthesis of thyroid hormones on the thyroglobulin precursor. The functional unit of the thyroid gland is the follicle, delimited by a monolayer of thyrocytes. Like the cells of most simple epithelia, thyrocytes exhibit specialization of the cell surface that confronts two different extracellular environments-apical and basolateral, which are separated by tight junctions. Specifically, the basolateral domain faces the interstitium/bloodstream, while the apical domain is in contact with the lumen that is the primary target for newly synthesized thyroglobulin secretion and also serves as a storage depot for previously secreted protein. Thyrocytes use their polarity in several important ways, such as for maintaining basolaterally located iodide uptake and T4 deiodination, as well apically located iodide efflux and iodination machinery. The mechanisms by which this organization is established, fall in large part under the more general cell biological problem of intracellular sorting and trafficking of different proteins en route to the cell surface. Nearly all exportable proteins begin their biological life after synthesis in an intracellular compartment known as the endoplasmic reticulum (ER), upon which different degrees of difficulty may be encountered during nascent polypeptide folding and initial export to the Golgi complex. In these initial stages, ER molecular chaperones can assist in monitoring protein folding and export while themselves remaining as resident proteins of the thyroid ER. After export from the ER, most subsequent sorting for protein delivery to apical or basolateral surfaces of thyrocytes occurs within another specialized intracellular compartment known as the trans-Golgi network. Targeting information encoded in secretory proteins and plasma membrane proteins can be exposed or buried at different stages along the export pathway, which is likely to account for sorting and specific delivery of different newly-synthesized proteins. Defects in either burying or exposing these structural signals, and consequent abnormalities in protein transport, may contribute to different thyroid pathologies.

[Hexamethylene bisacetamide (HMBA) prevents the loss of porcine thyroid monolayer cells to thyrotropin sensitivity]

The polar compound hexamethylenebisacetamide (HMBA) is a differentiating agent in the murine erythroleukemia cell system (MELC). It induces, like dimethylsulfoxide, the commitment to terminal differentiation leading to a recovery in the expression of several genes like the globin gene. This molecule which also induces differentiation in other cellular types is a growth agent for human, ovine and porcine thyroid cells. Forty-eight hours after the onset of culture, porcine thyroid monolayer cells do not respond to thyrotropin (TSH). We demonstrate that a pretreatment from the onset of culture with HMBA of porcine thyroid cells prevents the loss of TSH-sensitivity. The TSH-sensitivity is concentration-dependent in HMBA and leads to the reorganization of cells into follicles, even in the presence of HMBA. However, the withdrawal of HMBA when TSH is added is absolutely required to obtain a total recovery in iodide trapping and organification. If HMBA is present during TSH-stimulation, it inhibits iodide trapping partially but iodide organification completely Cells remain sensitive to TSH for at least 12 days if HMBA treated, and their sensitivity is totally restored after 3, 6 or 9 days of TSH-stimulation. HMBA, which is, like TSH, a growth agent for the thyroid cell and an agent that maintains some of the specialized functions, could be a putative candidate to obtain normal human thyroid cell lines.

Thyrotropin controls dolichol-linked sugar pools and oligosaccharyltransferase activity in thyroid cells

We previously showed that thyroglobulin (Tg) glycosylation is enhanced 1.5-fold under thyrotropin (TSH) stimulation, corresponding to an increased number of oligosaccharide chains per molecule of Tg. Now the steps involving dolichol components and oligosaccharyltransferase activity have been studied. Porcine thyroid cells were cultured on porous bottom filters with or without TSH and incubated with [14C]mevalonate. Under TSH regulation, the level of the whole of dolichol components was increased 1.25-fold without modifying their distribution. Dolichol, and free and monosaccharide-linked dolichyl-phosphate, represented respectively 40% and 45% of total dolichol components while dolichyl-pyrophosphate-oligosaccharide represented 3% only. A marked enhancement (4.2-fold) of oligosaccharyltransferase activity occurred in stimulated cells, which could correspond to the addition of the two TSH effects: stimulation of Tg synthesis (3-fold) and of Tg glycosylation (1.5-fold). The amount of lipid carriers appeared to be insufficiently increased but no component is a limiting step, suggesting that the turnover of dolichol derivatives may be increased under TSH control through their use by higher amounts of Tg.

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.

Polarized secretion of tissue-plasminogen activator in cultured thyroid cells

We studied the polarized secretion of tissue-type plasminogen activator in porcine thyroid cells cultured as a monolayer on porous bottom chambers. The presence of tissue-type plasminogen activator was detected by zymographic analysis on two independent media that were in contact either with the apical surface or with the basolateral membrane. The amount of tissue-type plasminogen activator was determined in both media by ELISA and enzyme assay. Measurable tissue-type plasminogen activator activity was found in the basal but not in the apical medium. However, on zymogram, a lytic zone corresponding to tissue-type plasminogen activator was visible in both media. In addition, a lytic band at 130 kDa suggested presence of a complex formed by tissue-type plasminogen activator and an inhibitor. Preferential basolateral tissue-type plasminogen activator antigen secretion (70%) has been observed, showing the possible relation between tissue-type plasminogen activator and extracellular matrix components. Neither tissue-type plasminogen activator level nor polarized secretion seemed to be regulated by thyrotropin (0.1 mU/ml).

Hormonogenesis in thyroid cells cultured on porous bottom chambers

Different processes implied in thyroid hormonogenesis (thyroglobulin, thyroperoxidase and hydrogen peroxide generating system expressions) and their regulation by TSH and iodide have been studied using porcine thyroid cells cultured in porous bottomed chambers. This system allowed to reproduce the functional bipolarity. Cells form a tight and polarized monolayer. Both apical and basolateral poles of epithelial cells were independently accessible and the cell layer separated two compartments which can contain different media. A major polarized secretion of thyroglobulin into the apical compartment was observed; it was increased in the presence of TSH as well as the thyroglobulin synthesis and mRNA level. These TSH effects were the consequence of adenylcyclase stimulation. Active transport of iodide, iodination of thyroglobulin and hormonosynthesis took place only in the presence of TSH. These steps occurred at the apical pole of cells. In the culture chamber system, thyroglobulin was weakly iodinated (6 atoms of iodide per mole of thyroglobulin; in vivo up to 40 atoms per mole) but hormonogenesis efficiency was close to this one observed in vivo (40%). Iodide concentrations higher than 0.5 microM daily added to the basal medium inhibited iodination of thyroglobulin and hormonosynthesis. Some components contained in culture media were inhibitors for iodination when they were present in the apical medium such as vitamins, amino acids and phenol red. The culture system appears to be interesting for pharmacological and toxicological studies.

Accessibility and function of human thyroid epithelial cells in monolayer culture

Measurement of thyroid stimulating immunoglobulins (TSI) has not yet found widespread application in the diagnosis and management of Graves' disease. One of the problems is the poor and variable sensitivity of the different assays. We therefore studied the influence of the accessibility of the basal part of human thyroid epithelial cells in monolayer culture on their cAMP response to thyroid stimulating hormone (TSH) and TSI. Test media containing either ammoniumsulphate-precipitated globulin fractions of sera from normal controls and treated or untreated patients with Graves' disease or TSH were used to stimulate cAMP production by cryopreserved human thyroid epithelial cells in monolayer culture. Incubations with and without the use of porous membrane inserts as culture surface were compared by univariate parametric and non-parametric testing. It appears that more TSH-receptors, probably on the basal part of the thyrocyte, can be exposed to the medium by using a porous membrane as culture surface as demonstrated by the increased analytical sensitivity of the semi-bioassay of TSH and TSI.

Regulation of thyroid follicular volume by bidirectional transepithelial ion transport

Previous studies have shown that thyroid cells in monolayer culture exhibit bidirectional ion transport comprising apical-to-basal amiloride-sensitive Na+ transport and oppositely directed bumetanide-sensitive Cl- transport. We have now investigated the role of ion transport in the regulation of thyroid follicular size using follicular primary porcine thyroid cell cultures. Bumetanide (10 microM) added at the beginning of culture inhibited the formation of follicular lumina and caused a fall in follicle height when added to 3-day-old cultures. In contrast, phenamil (1 microM; an amiloride analog) increased follicle size both in freshly isolated and 3-day-old cultures. The effect of bumetanide was prevented by the prior addition of phenamil. Micropuncture studies showed that follicles had a lumen-negative, basal-positive transepithelial potential difference which was progressively reduced in magnitude by the serial addition of bumetanide (10 microM) and phenamil (1 microM). We conclude that thyroid follicles possess a bidirectional ion transport system which transports Na+ in an apical-to-basal direction and Cl- in the opposite direction. The balance between these two processes determines net solute flux and hence follicular size. A physiological role of ion transport in the thyroid may be to regulate follicular volume suggesting that abnormalities of ion transport may be responsible for disorders of follicular size.

Phenol red: an inhibitor of thyroglobulin iodination in cultured porcine thyroid cells

Phenol red, commonly used as a pH indicator in tissue culture media, is known to possess estrogenic properties. We investigated the effect of phenol red on the process of thyroglobulin iodination which occurs only at the apical surface of porcine thyroid cells when cultured in porous bottom chambers. When phenol red was added simultaneously to both compartments (apical and basolateral), separated by the polarized monolayer, thyroglobulin iodination was inhibited by about 86% without any effect on thyroglobulin secretion and apical iodine concentration. When phenol red was added separately to either the apical or basal media, inhibition was 68% and 43%, respectively. A large amount of phenol red which was introduced into the basal medium crossed through the monolayer. Thus, inhibition was dependent upon the concentration of phenol red present in the apical compartment. A maximal inhibition was observed from 30 microM apical concentration. Phenol red acts as a substrate for thyroperoxidase in the iodination reaction.

Regulated and constitutive protein targeting can be distinguished by secretory polarity in thyroid epithelial cells

We have studied concurrent apical/basolateral and regulated/constitutive secretory targeting in filter-grown thyroid epithelial monolayers in vitro, by following the exocytotic routes of two newly synthesized endogenous secretory proteins, thyroglobulin (Tg) and p500. Tg is a regulated secretory protein as indicated by its acute secretory response to secretagogues. Without stimulation, pulse-labeled Tg exhibits primarily two kinetically distinct routes: less than or equal to 80% is released in an apical secretory phase which is largely complete by 6-10 h, with most of the remaining Tg retained in intracellular storage from which delayed apical discharge is seen. The rapid export observed for most Tg is unlikely to be because of default secretion, since its apical polarity is preserved even during the period (less than or equal to 10 h) when p500 is released basolaterally by a constitutive pathway unresponsive to secretagogues. p500 also exhibits a second, kinetically distinct secretory route: at chase times greater than 10 h, a residual fraction (less than or equal to 8%) of p500 is secreted with an apical preponderance similar to that of Tg. It appears that this fraction of p500 has failed to be excluded from the regulated pathway, which has a predetermined apical polarity. From these data we hypothesize that a targeting hierarchy may exist in thyroid epithelial cells such that initial sorting to the regulated pathway may be a way of insuring apical surface delivery from one of two possible exocytotic routes originating in the immature storage compartment.

Production of lipocortin-like proteins by cultured human tracheal submucosal gland cells

Evidence is obtained for the presence of lipocortin-like proteins in human tracheal gland cells in culture. Using polyclonal antibodies to lipocortin I, indirect immunofluorescence studies demonstrate that lipocortin I is mainly confined to the tracheal gland cell surface. From cell membranes, four Ca2(+)-dependent proteins (35, 40, 45 and 67 kDa) were identified as lipocortin related proteins by using immunoblotting and fluorography following [35S]methionine metabolic labeling experiments. A strong immunoreactivity for the 35 kDa protein was observed. In addition, lipocortin-like proteins with apparent Mr33, 35, 37 and 67 kDa, respectively, were released in the apical culture medium by tracheal gland cells cultured on microporous membrane of a double chamber culture system.

Differentiation expression during proliferative activity induced through different pathways: in situ hybridization study of thyroglobulin gene expression in thyroid epithelial cells

In canine thyrocytes in primary culture, our previous studies have identified three mitogenic agents and pathways: thyrotropin (TSH) acting through cyclic AMP (cAMP), EGF and its receptor tyrosine protein kinase, and the phorbol esters that stimulate protein kinase C. TSH enhances, while EGF and phorbol esters inhibit, the expression of differentiation. Given that growth and differentiation expression are often considered as mutually exclusive activities of the cells, it was conceivable that the differentiating action of TSH was restricted to noncycling (Go) cells, while the inhibition of the differentiation expression by EGF and phorbol esters only concerned proliferating cells. Therefore, the capacity to express the thyroglobulin (Tg) gene, the most prominent marker of differentiation in thyrocytes, was studied in proliferative cells (with insulin) and in quiescent cells (without insulin). Using cRNA in situ hybridization, we observed that TSH (and, to a lesser extent, insulin and insulin-like growth factor I) restored or maintained the expression of the Tg gene. Without these hormones, the Tg mRNA content became undetectable in most of the cells. EGF and 12-0-tetradecanoyl phorbol-13-acetate (TPA) inhibited the Tg mRNA accumulation induced by TSH (and/or insulin). Most of the cells (up to 90%) responded to both TSH and EGF. Nevertheless, the range of individual response was quite variable. The effects of TSH and EGF on differentiation expression were not dependent on insulin and can therefore be dissociated from their mitogenic effects. Cell cycling did not affect the induction of Tg gene. Indeed, the same cell distribution of Tg mRNA content was observed in quiescent cells stimulated by TSH alone, or in cells approximately 50% of which had performed one mitotic cycle in response to TSH + insulin. Moreover, after proliferation in "dedifferentiating" conditions (EGF + serum + insulin), thyrocytes had acquired a fusiform fibroblast-like morphology, and responded to TSH by regaining a characteristic epithelial shape and high Tg mRNA content. 32 h after the replacement of EGF by TSH, cells in mitosis presented the same distribution of the Tg mRNA content as the rest of the cell population. This implies that cell cycling (at least 27 h, as previously shown) did not affect the induction of the Tg gene which is clearly detectable after a time lag of at least 24 h. The data unequivocally show that the reexpression of differentiation and proliferative activity are separate but fully compatible processes when induced by cAMP in thyrocytes.(ABSTRACT TRUNCATED AT 400 WORDS)

Small conductance chloride channels in the apical membrane of thyroid cells

A small conductance chloride channel has been identified on the apical membrane of porcine thyroid cells using the patch-clamp technique. In cell attached membrane patches with NaCl in the pipette, the single channel conductance is 5.5 pS. The channel is highly selective for chloride over gluconate and iodide, and is impermeable to Na+, K+ and tetraethylammonium ions. The open state probability of the channel is not affected by voltage. The channel activity disappears after excision of the patch. The Cl- channel blocker 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB) did not affect the activity of the thyroid Cl- channels. Treatment of thyroid cells with 8-(4-chlorophenylthio)adenosine-3',5'-cyclic monophosphate (8-chloro-cAMP) (0.5 mM) prior to giga-seal formation increased Cl- channel activity in the apical membrane of thyroid cells.

The thyroid cell monolayer in culture. A tight sodium absorbing epithelium

When cultured on collagen coated nitrocellulose filters, thyroid epithelial cells form morphologically and functionally polarized monolayers. The bioelectric parameters of these monolayers were measured after mounting in Ussing chambers; transepithelial potential (Vab), short circuit current (Isc) and transepithelial resistance were respectively 12 +/- 1 mV (apical side negative), 3.8 +/- 0.2 microA cm-2 and 3250 +/- 214 omega cm2 (mean +/- SEM, n = 75). Eighty two percent of the short circuit current was related to sodium absorption as shown by inhibition by apical amiloride (Km = 0.2 microM) and by basal ouabain (K1/2 = 0.3 microM). Amphotericin B (5-25 micrograms/ml) added to the apical bath increased Isc suggesting an apical rate-limiting step. Step by step replacement of choline by Na+ in a Na+-free medium resulted in a progressive increase in Vab and Isc with half maximal effect at 20 +/- 1 mM Na+. Thyrotropin (TSH) increased Isc and Vab in a biphasic way with a transient maximum after 5 min and a plateau after 20 min (about four times the basal level at 100 microU/ml TSH). This increase in sodium transport was also inhibited by apical amiloride. Thus, in culture, the thyroid cell monolayer behaves as a tight sodium absorbing epithelium controlled by TSH, with a rate limiting apical sodium channel as the entry mechanism and a basolateral Na+, K+-ATPase as the electromotive force.

In vitro effects of estrogen on tgb and c-myc gene expression in normal and neoplastic human thyroids

The authors investigated the effects of 17 beta-estradiol (E) and thyroid-stimulating hormone (TSH) on tgb (coding for thyroglobulin), c-myc RNA levels, and [3H]thymidine (thy) incorporation in suspension cultures of normal, adenomatous and carcinomatous human thyroid follicles. The cultured follicles showed decreased tgb RNA and enhanced c-myc RNA levels. In the culture of normal and adenomatous samples E caused a significant increase of [3H]thy incorporation and tgb RNA levels, with no effect on c-myc RNA levels. No effect of E was observed in the carcinomatous thyroid culture. TSH induced a significant increase of [3H]thy incorporation and c-myc expression only in adenoma cultures and a significant increase of tgb RNA levels in both normal and adenomatous samples. TSH had no effect on the carcinoma. The results show that E, like TSH, stimulates in vitro the expression of the tgb gene in differentiated cells, without stimulating the expression of the c-myc proto-oncogene, suggesting a possible action of E on normal thyroid function and perhaps growth, even if not associated with increased c-myc expression.

Thyroglobulin structure and function: recent advances

Thyroglobulin is a large-size iodoglycoprotein specific to thyroid tissue and is the substrate for the synthesis of thyroid hormones, thyroxine and 3,5,3'-triiodothyronine. Recent studies, which greatly benefited from recombinant DNA methodologies, improved the knowledge of several structural features of this dimeric protein and permitted insights into some structure-function relationships. Analysis-function of the primary structure of the human thyroglobulin monomer revealed several main characteristics: 1) 3 types of internal homologies; 2) extensive homology with the bovine thyroglobulin monomer and known partial sequences in the thyroglobulins of other mammalian species; 3) significant homologies with 2 other non-thyroid proteins (acetylcholinesterase and the invariant chain of the Ia class II histocompatibility antigen); 4) a terminal localization of the hormonogenic sites at both ends of the monomer. Current studies aim at determining conformational characteristics, understanding the molecular mechanisms of thyroid hormone formation and unraveling those interactions which in the thyroid cell and the thyroid follicle will permit this large pro-hormone to synthesize and release a few small thyroid hormone molecules. A more precise knowledge of this molecule in higher vertebrates and during evolution would impart valuable information concerning thyroid pathology, since thyroglobulin has been implicated in some genetic and in autoimmune thyroid diseases.