TSH-like actions of dibutyryl-cAMP on isolated bovine thyroid cells

Progress in Thyroid Cancer Genomics: A 40-Year Journey

Background: Very little was known about the molecular pathogenesis of thyroid cancer until the late 1980s. As part of the Centennial celebration of the American Thyroid Association, we review the historical discoveries that contributed to our current understanding of the genetic underpinnings of thyroid cancer. Summary: The pace of discovery was heavily dependent on scientific breakthroughs in nucleic acid sequencing technology, cancer biology, thyroid development, thyroid cell signaling, and growth regulation. Accordingly, we attempt to link the primary observations on thyroid cancer molecular genetics with the methodological and scientific advances that made them possible. Conclusions: The major genetic drivers of the common forms of thyroid cancer are now quite well established and contribute to a significant extent to how we diagnose and treat the disease. However, many challenges remain. Future work will need to unravel the complexity of thyroid cancer ecosystems, which is likely to be a major determinant of their biological behavior and on how they respond to therapy.

Association of thyroid-stimulating hormone and cardiovascular risk factors

Thyroid hormone plays an important role in regulating the lipid and glucose metabolism. Previously, much attention has been drawn to define the pathophysiological relationship between thyroid dysfunction and the incidence of cardiovascular diseases (CVDs). While the conditions of overt hypothyroidism and subclinical hypothyroidism were both emphasized, the association between CVD risks and the deregulated circulating thyroid-stimulating hormone (TSH) level remains to be elucidated. Nevertheless, multiple TSH-mediated physiological adaptations, including alteration of the serum lipids, body mass index, blood pressure and insulin sensitivity, have led to the difficulty of clearly examining the association between the TSH level and CVD prevalence. The current review aims to 1) summarize the evidence for the role of thyroid dysfunction and TSH abnormality in CVD pathogenesis and 2) explore the possible underlying molecular mechanisms of TSH-mediated cardiovascular pathology in hopes of providing better therapeutic strategies for the patients with deregulated TSH.

Small-molecule thyrotropin receptor agonist activates naturally occurring thyrotropin-insensitive mutants and reveals their distinct cyclic adenosine monophosphate signal persistence

BACKGROUND

Subclinical hypothyroidism (SHT), characterized by normal thyroid hormone levels maintained by elevated thyrotropin (TSH), predisposes patients to health problems as they age. Some cases arise from mutations of the TSH receptor (TSHR) that confer TSH resistance. This resistance might be circumvented by TSHR agonists with different modes of binding compared with TSH. We hypothesized that the recently discovered small-molecule TSHR agonist C2, with its unique mode of receptor binding, would activate mutant TSHRs associated with SHT, facilitating their study.

MATERIALS AND METHODS

HEK-EM293 cells transiently expressing TSHR variants-wild-type TSHR or mutants C41S, L252P, L467P, or C600R-were analyzed for TSH or C2-induced cyclic adenosine monophosphate (cAMP) signaling to establish C2 as a mutant TSHR agonist. These cells were also pretreated with TSH or C2 to characterize each mutant receptor's ability to maintain and desensitize cAMP signaling.

RESULTS

We showed that C2 could activate the TSH-unresponsive TSHR ectodomain mutants C41S and L252P but had no effect on the serpentine mutant L467P. We found that TSH and C2 could acutely activate the serpentine mutant C600R. Preincubation with C2 caused persistent cAMP signaling and receptor desensitization in wild-type TSHR and cAMP signal persistence with no detectable desensitization in the cases of C41S and L252P.

CONCLUSIONS

The small-molecule agonist C2 is a useful pharmacological tool for the study of mutant TSHRs. It revealed that some naturally occurring TSH-insensitive mutants can mediate induction of cAMP elevation upon stimulation with C2 and that this signal is differentially maintained within cells.

Bioassays for TSH-receptor autoantibodies: an update

Immunoglobulins in patients with Graves' disease (GD) that modulate the thyroid stimulating hormone receptor (TSH-R) do so via stimulating cAMP dependent signals (TSI), blocking TSH or inhibition of TSH-receptor activation (TBI) or inducing apoptotic signals. These functional immunoglobulins represent powerful biomarkers of anti-self reactivity in the thyroid and systemic tissues that harbor TSH-R expressing target cells. TSI on thyrocytes induce hyperthyroidism, and TSI on TSH-R fibroblasts of orbital muscles, skin and heart provoke the release of cytokines and antigen-specific T-cell responses leading to systemic inflammation. Bioassays of anti-TSH-R autoantibodies provide decisive information on GD activity. This review examines the past and present bioassays in GD. The critical goal of cell-based anti-TSH-R autoantibody bioassays, to identify the pathogenic immunoglobulins in GD under robust and standardized conditions suitable for routine clinical laboratory practice, is discussed.

Functional expression of the thyrotropin receptor in C cells: new insights into their involvement in the hypothalamic-pituitary-thyroid axis

Thyroid C cells, or parafollicular cells, are mainly known for producing calcitonin, a hormone involved in calcium homeostasis with hypocalcemic and hypophosphatemic effects. Classically, the main endocrine activity of this cell population has been believed to be restricted to its roles in serum calcium and bone metabolism. Nonetheless, in the last few years evidence has been accumulating in the literature with regard to local regulatory peptides secreted by C cells, such as somatostatin, ghrelin, thyrotropin releasing hormone or the recently described cocaine- and amphetamine-related transcript, which could modify thyroid function. As thyrotropin is the main hormone controlling the hypothalamic-pituitary-thyroid axis and, accordingly, thyroid function, we have examined the functional expression of the thyrotropin receptor in C-cell lines and in thyroid tissues. We have found that rat and human C-cell lines express the thyrotropin receptor at both mRNA and protein levels. Furthermore, incubation of C cells with thyrotropin resulted in a 10-fold inhibition of thyrotropin-receptor expression, and a concomitant decrease of the steady-state mRNA levels for calcitonin and calcitonin gene-related peptide determined by quantitative real-time PCR was found. Finally, thyrotropin receptor expression by C cells was confirmed at protein level in both normal and pathological thyroid tissues by immunohistochemistry and immunofluorescence. These results confirm that C cells, under regulation by thyrotropin, are involved in the hypothalamic-pituitary-thyroid axis and suggest a putative role in local fine-tuning of follicular cell activity.

Modulation of TSHR signaling by posttranslational modifications

Posttranslational modifications of seven transmembrane receptors (7TMRs) affect their function to a large extent. Many studies of glycosylation or phosphorylation of 7TMRs have shown that these modifications influence the cell-surface expression or signaling of the receptor. Recently, other types of posttranslational modifications of the thyrotropin-stimulating hormone receptor (TSHR) have been characterized, including sialylation and dimerization. Increased TSHR sialylation results in increased TSHR cell-surface expression. Furthermore, TSHR oligomerization and the probable modification of TSHR signaling in lipid rafts require further clarification with regard to their functional consequences. In addition to its known coupling to Galphas and Galphaq, and possibly other G proteins, the TSHR also couples to further signaling pathways, such as the mitogen-activated protein kinase (MAPK) pathway, which involves G-protein-coupled receptor kinases (GRKs) and arrestins. We discuss these emerging new findings and their implications for signaling of the TSHR.

Molecular characterization and seasonal changes in gonadal expression of a thyrotropin receptor in the European sea bass

The thyroid stimulating hormone (TSH) is a glycoprotein synthesized and secreted from thyrotrophs of the anterior pituitary gland. It acts by binding to and activating its specific receptor, the TSHR, to induce the synthesis and secretion of thyroid hormones. Recent studies conducted in diverse fish species suggest a direct role of TSH on gonadal physiology. In this work, we describe the cloning of a cDNA encoding a TSHR which was isolated from the gonads of the European sea bass (Dicentrarchus labrax). The mature protein displays typical features of the members of the glycoprotein hormone receptor family and shows the highest amino acid sequence identity with the TSHRs of other fish species. An insertion of approximately 50 amino acids, specific for the TSHR subfamily is also present in the carboxyl end of the extracellular domain of the sbsTSHR. By RT-PCR analysis, we demonstrate the extrathyroidal expression of sbsTSHR in numerous tissues of the sea bass. Also, two transcripts that differ in the length of their 3' untranslated regions were found. They reflect the use of alternative polyadenylation cleavage sites. Seasonal changes in sbsTSHR mRNA levels in female and male sea bass during the first ovarian and testicular recrudescence suggest that in females the TSHR could participate in active vitellogenesis and in the regulation of gamete maturation and ovulation, whereas in males, the TSHR would be involved in the regulation of processes that occur during the early stages of the gonadal development and also of gamete maturation and spermiation. The results of this work indicate that a sbsTSHR has been cloned from the testis of the European sea bass and they provide the basis for future studies concerning the function of TSHR in this species.

Modulation of thyroidal radioiodine uptake by theophylline

Diagnostic and therapeutic use of radioiodine in the management of thyroid disorders depends on the ability of thyroid cells to concentrate radioiodine, a process regulated by thyrotropin and dependent on the intracellular increase in cAMP. We tested the ability of theophylline, a drug known to increase intracellular cAMP via inhibition of phosphodiesterase, to modulate the thyroidal radioiodine uptake in FRTL-5 cells, in mice and in humans. In FRTL-5 cells, theophylline increased the uptake of radioactive iodine and intracellular cAMP only at low concentrations (1 microM). In mice, theophylline increased slightly the radioiodine uptake, although this increase varied from 1.5- to 6.6-fold. In humans, theophylline decreased slightly the radioiodine uptake, a decrease that became more pronounced with time after radioiodine administration. These studies suggest that theophylline modulates the radioiodine uptake in a dose-dependent fashion, although the modulation is mild and probably not applicable to the clinical setting.

Theophylline increases the uptake of radioiodine by mouse thyroid

Diagnostic and therapeutic use of radioiodine in the management of thyroid disorders depends on the ability of thyroid cells to concentrate radioiodine, a process that is regulated by the intracellular increase in cAMP. We hypothesized that theophylline, a drug known to increase intracellular cAMP via inhibition of phosphodiesterase, could increase thyroidal radioiodine uptake. We tested this effect in vivo, using C57BL/6j mice, and in vitro, using Fisher rat thyroid (FRTL-5) cells. One mouse received 2.5mg theophylline i.p., whereas a control mouse received only saline. Twenty-hours after theophylline, mice were injected with 10 microCi Na125I in 0.1 mL saline through the tail vein. Mean thyroidal 125I activity was 3.3-fold higher in theophylline-treated mice than in their respective controls. Radioiodine uptake and intracellular cAMP production of FRTL-5 cells were increased by a relatively low concentration of theophylline (1 microM). Intracellular cAMP increased up to 30 min and then declined in response to 1 microM theophylline. Sera from theophylline-treated mice stimulated 125I uptake and intracellular cAMP production by FRTL-5 cells. These findings show that theophylline can enhance radioiodine uptake by thyrocytes in vivo and in vitro. The in vitro effects of theophylline on both radioiodine uptake and cAMP production in a dose-dependent manner are consistent with an action mediated by phosphodiesterase inhibition.

Extracellular adenosine increases Na+/I- symporter gene expression in rat thyroid FRTL-5 cells

We studied the effect of extracellular adenosine on iodide (I-) transport in FRTL-5 thyroid cells. I- accumulation increases after a 48 h exposure to adenosine in a concentration-dependent manner, reaching a maximum of 7.9-fold basal levels at 72 h after the addition of 300 microM adenosine. Neither I- efflux nor intracellular cyclic adenosine monophosphate accumulation is affected by the exposure to adenosine. The stimulation of I- transport by adenosine is partly as a result of an increase in Na+/I- symporter (NIS) mRNA and protein levels. Northern blot analysis revealed that adenosine increases NIS mRNA levels at 24 h, reaching a maximum at 36 h. Western blot analysis demonstrated that adenosine increases NIS protein levels at 36 h, reaching a maximum at 72 h, in parallel with the kinetics of adenosine-induced I- transport. Adenosine increased the promoter activity of a full-length NIS promoter-luciferase chimera, suggesting that the effect of adenosine on NIS mRNA levels is transcriptional. The stimulatory effect of adenosine on NIS mRNA levels, is mimicked by N6-(L-2-phenylisopropyl) adenosine (PIA), an A1 adenosine receptor agonist, and inhibited by 1,3-dipropyl-8-cyclopentylxanthine, an A1 adenosine receptor antagonist, suggesting that the effect is mediated via the A1 adenosine receptor stimulation in FRTL-5 cells. Incubating cells with islet-activating protein inhibited the adenosine-induced NIS mRNA levels. In sum, extracellular adenosine increases NIS gene expression and stimulates I- transport via the A1 adenosine receptor-Gi/Go protein signal transduction pathway.

A novel thyroid transcription factor is essential for thyrotropin-induced up-regulation of Na+/I- symporter gene expression

The stimulation of iodide (I-) transport by TSH in FRTL-5 thyroid cells is partly due to an increase in Na+/I- symporter (NIS) gene expression. The identification of a TSH-responsive element (TRE) in the NIS promoter and its relationship to the action of thyroid transcription factor-1 (TTF-1) on the promoter are the subjects of this report. By transfecting NIS promoter-luciferase chimeric plasmids into FRTL-5 cells in the presence or absence of TSH, we identify a TRE between -420 and -370 bp of the NIS 5'-flanking region. Nuclear extracts from FRTL-5 cells cultured in the absence of TSH form two groups of protein-DNA complexes, A and B, in gel mobility shift assays using an oligonucleotide having the sequence from -420 to -385 bp. Only the A complex is increased by exposure of FRTL-5 cells to TSH or forskolin. The addition of TSH to FRTL-5 cells can increase the A complex at 3-6 h, reaching a maximum at 12 h. FRTL-5, but not nonfunctioning FRT thyroid or Buffalo rat liver (BRL) cell nuclear extracts, form the A complex. The TSH-increased nuclear factor in FRTL-5 cells interacting with the NIS TRE is distinct from TTF-1, thyroid transcription factor-2, or Pax-8, as evidenced by the absence of competition using oligonucleotides specific for these factors in gel shift assays. Neither is it the nuclear protein interacting with cAMP response element. The TRE is in the upstream of a TTF-1-binding site, -245 to -230 bp. Mutation of the TRE causing a loss of TSH responsiveness also decreases TTF-1-induced promoter activity in a transfection experiment. The formation of the A complex between FRTL-5 nuclear extracts and the NIS TRE is redox-regulated. In sum, TSH/cAMP-induced up-regulation of the NIS requires a novel thyroid transcription factor, which also appears to be involved in TTF-1-mediated thyroid-specific NIS gene expression.

Regulation by thyroid-stimulating hormone of sodium/iodide symporter gene expression and protein levels in FRTL-5 cells

To investigate the mechanism of I- transport stimulation by TSH, we studied the effects of TSH on Na+/I- symporter (NIS) messenger RNA (mRNA) and protein levels in FRTL-5 cells and correlated these with I- transport activity. When 1 mU/ml TSH was added to quiescent FRTL-5 cells, a 12-h latency was observed before the onset of increased I- transport activity, which reached a maximum [approximately 27 times basal (5H medium) levels] at 72 h. In contrast, Northern blot analysis, using rat NIS complementary DNA as a probe, revealed that addition of TSH to these cells significantly increased NIS mRNA at 3-6 h, reaching a maximum after 24 h (approximately 5.9 times basal levels). Forskolin and (Bu)2cAMP mimicked this stimulatory effect on both the I- transport activity and mRNA levels. D-ribofranosylbenzimidazole, a transcription inhibitor, almost completely blocked TSH-induced stimulation of I- transport and NIS mRNA levels. Western blot analysis demonstrated that TSH increased NIS protein levels at 36 h, reaching a maximum at 72 h, in parallel with the kinetics of TSH-induced I- transport activity. However, it also showed that the amount of NIS protein already present in FRTL-5 cell membranes before the addition of TSH was about one third of the maximum level induced by TSH. These results indicate that stimulation of I- transport activity by TSH in thyrocytes is partly due to a rapid increase in NIS gene expression, followed by a relatively slow NIS protein synthesis. However, the existence of an abundant amount of protein in quiescent FRTL-5 cells with very low I- transport activity also suggests that this activity is controlled by another TSH-regulated factor(s).

Effects of iodide on thyroglobulin biosynthesis in FRTL-5 cells

Iodide inhibits several thyroid parameters through an organic intermediate, and this process has been related to thyroid autoregulation. The aim of this study was to determine the effect of iodine on thyroglobulin (Tg) synthesis in the rat thyroid cell line FRTL-5. TSH stimulated amino acid incorporation into the cells by 400% and iodine had no effect on this parameter. No effect of TSH or iodide on [35S] methionine incorporation into protein was found under our experimental conditions (approximately 80% of total [35S]methionine incorporated was found in TCA-precipitable material). TSH caused an increase in Tg synthesis, after 1 h, while iodide partially blocked the effect of TSH (control 6.4% of TCA precipitable radioactivity; TSH 10.7%; iodide 8.4%). After 24 h, the protein released into the medium was measured. TSH stimulated total protein liberation and iodide inhibited this parameter. TSH stimulated total RNA content, and iodide caused an inhibition. Northern analysis did not show inhibition by iodide of TSH-stimulated Tg mRNA levels. The present results show an inhibitory effect of excess iodide on TSH-stimulated thyroglobulin biosynthesis in FRTL-5 cells.

Inhibition of the Na+/I- symporter by harmaline and 3-amino-1-methyl-5H-pyrido(4,3-b)indole acetate in thyroid cells and membrane vesicles

Novel inhibitors of the Na+/I- symporter were identified using rat-thyroid-derived FRTL-5 cells and sealed vesicles from calf thyroid as model systems. Na(+)-dependent 125I- uptake was inhibited by the hallucinogenic drug harmaline and by a chemically related convulsive agent, 3-amino-1-methyl- 5H-pyrido(4,3-b)indole acetate (TRP-P-2). TRP-P-2 (Ki = 0.25 mM) was tenfold more effective as an inhibitor than harmaline (Ki = 4.0 mM). Inhibition by TRP-P-2 was competitive with respect to Na+ and was fully reversible. Although TRP-P-2 is a relatively low-affinity inhibitor, its affinity for the Na+ site of the Na+/I- symporter is over 100 times higher than that of Na+ (Km = 50 mM). 45Ca(2+)-efflux rates in calf thyroid membrane vesicles were not affected by TRP-P-2, indicating that membrane integrity is not disrupted by the drug. These findings show that TRP-P-2 may be a potentially useful tool for the identification and characterization of the Na+/I- symporter.

Forskolin and 12-0-tetradecanoylphorbol-13-acetate mimic thyrotropin-stimulated protein iodination in mouse thyroid

In the present study, the capacity of the diterpene, forskolin, and the phorbol ester, 12-0-tetradecanoylphorbol-13-acetate (TPA), to stimulate protein iodination in freshly dispersed mouse thyroid open follicles was assessed. Although both agents stimulated 125I incorporation into TCA precipitable material, dose response curves (0.1 - 25 microM) showed that maximal concentrations of either agonist alone failed to reproduce the stimulatory effect of a maximal concentration of thyrotropin (TSH; 50 mU/ml). When a maximal concentration of forskolin (20 microM) and TPA (10 microM) were added in combination, the stimulatory effect was additive and mimicked the effect of TSH. TPA had no significant effect on either basal or forskolin-stimulated cyclic-AMP production. We conclude that the regulation of protein iodination by TSH may involve both the adenylate cyclase-cyclic-AMP-dependent protein kinase system and the diacylglycerol-activated calcium/phospholipid-dependent protein kinase C pathway.

The effects of growth factors and serum on DNA synthesis and differentiation in thyroid cells in culture

The effects of three putative growth factors and serum on [Me-3H]thymidine and Na125I incorporation into thyroid cell cultures have been examined. We found that serum and EGF could stimulate radioactively labelled thymidine incorporation into confluent cultures. However, both factors completely inhibited iodine uptake and organification at low concentrations. Insulin also stimulated [Me-3H]thymidine incorporation but had no adverse effect on thyroid differentiated function. TSH examined under the same conditions was not a growth factor but was essential to maintain differentiated functions. We conclude that thyroid growth and differentiation are not mutually exclusive processes. However, EGF and serum inhibit thyroid differentiated function at very low concentrations. Elucidation of the physiological role of these factors and their mechanism of action may lead to a greater understanding of thyroid hormone biosynthesis.

Functional properties of porcine-thyroid follicles in suspension

Porcine thyroid follicle cells were isolated (about 10(7) cells per gram of tissue) and cultured in small aggregates in agarose-coated culture dishes. The aggregates became arranged into follicle-like structures capable of iodide uptake and organification. In the presence of TSH (0.2 mU/ml), the aggregation of follicles was enhanced, and iodide uptake as well as TSH-stimulated organification of iodide was increased compared with that in the control. In culture, the active iodide metabolism was gradually lost over a 7-day period. This was not due to a disappearance of the TSH-adenylate cyclase system, since cAMP production was retained and stimulated by TSH (half-maximal effect at about 1 mU/ml). Acutely TSH stimulated iodide efflux and iodide organification (half-maximal effect at about 20 microU/ml). The stimulatory effect on organification was transient: within an hour further organification proceeded as in the absence of hormone. The effects on efflux and organification were already maximal at low TSH concentrations, whereas cAMP production was stimulated with up to 50-fold higher TSH levels, i.e. the findings were typical of spare receptors. In the continued presence of epidermal growth factor, a potent mitogen for thyroid cells, the follicles increased in size and contained one single large lumen. Their capability to take up and organify iodide was reduced.

Reorganization of porcine thyroid cells into functional follicles in a chemically defined, serum- and thyrotropin-free medium

In the serum-free, chemically defined medium NCTC 109, freshly isolated porcine thyroid cells aggregate and form functional follicles in culture even in the absence of thyrotropin. The follicular pattern observed under light and electron microscopy express the main morphological characteristics of in vivo thyroid cells. Follicles are large, replete with dense colloid, and the apical pole of cells is characterized by well-developed microvilli and the presence of aminopeptidase N. The index of iodide transport activity (125I-C/M ratio) decreases vs. days of culture to a resting value of about 1 or 2 at day 2. Addition of thyrotropin (200 microU/ml final concentration) at day 4 is followed by a 10-fold increase in iodide transport activity within 24 h and a 40-fold increase 4 d later. Incorporation and organification of iodide are dose dependent between 0 and 250 microU/ml thyrotropin; highest concentrations (4,000--16,000 muU/ml) are significantly inhibitory. In the absence of thyrotropin each cell synthesizes 8.2 pg thyroglobulin/d. Acute stimulation by thyrotropin at day 4 resulted in a slight decrease in the quantity of thyroglobulin present in the cell layer but in an increase in the total amount of thyroglobulin recovered in both cells and medium, reaching 34.3 pg/cell/d. The protein exported into the medium is thyroglobulin, as shown by SDS PAGE and immunological properties. Here we demonstrate that porcine thyroid cells can be maintained in culture as resting, highly differentiated, follicular-associated cells, sensitive to acute stimulation by thyrotropin.

Iodination of thyroglobulin. An intracellular or extracellular process?

The location in the thyroid follicle of the iodination of thyroglobulin has been a matter of debate for several decades. This problem is not a question of mere academic interest. Knowledge of the locus--or loci--of iodination is necessary for a full understanding of the mechanisms involved in thyroid-hormone synthesis and release. In the discussion about this problem 3 fundamentally different views have been--and still are--advocated. The first view implies that the site of iodination is the follicle lumen, the second that iodination is an intracellular process restricted to some organelle(s) in the follicle cells and the third that iodination occurs at the interface between the follicle cells and the follicle lumen. Below I will survey the major observations on which these different opinions are based and discuss the validity of the interpretations.

Identification of an intracellular pathway of thyroxine synthesis by dispersed thyroid cells

This study deals with the identification of the biochemical events involved in the metabolic sequence leading from the synthesis to the release of thyroxine in the dispersed thyroid cell system. (1) Using an experimental model allowing the differentiation between intracellular and extracellular sites of iodination, it is shown that thyroxine is synthesized inside the cells by an iodinating system sensitive to thyrotropin stimulation. (2) The secretion of thyroxine synthesized inside the cells is not mediated by an exocytotic-endocytotic phenomenon. Colchicine, vinblastine, fluoride, propanolol and chlorpromazine, at concentrations equal to or 10--100-times higher than those required to inhibit hormone release in follicular-organized thyroid tissue have no effect on thyrotropin-stimulated thyroxine secretion. (3) The secretion involves the intracellular proteolysis of hormone-containing iodoprotein(s) which, in addition to free thyroxine, generates free mono- and diiodotyrosines. Free thyroxine is released into the incubation medium and iodotyrosines are deiodinated under normal conditions and accumulate in the presence of an inhibitor of iodotyrosine deiodinase: 3,5-dinitrotyrosine. This proteolysis is inhibited by 5 mM chlorpromazine. These data indicate that the complete metabolic sequence leading from the uptake of iodide to the release of free thyroxine into the incubation medium can be described as an 'intracellular metabolic sequence for thyroxine synthesis'.

Intracellular and extracellular sites of iodination in dispersed hog thyroid cells

Iodination and hormone synthesis has been studied in isolated hog thyroid cells in suspension. We characterized three iodination processes by use of pharmacological agents. (1) Intracellular iodination dependent on active iodide transport, which was inhibited by NaClO4 or ouabain, but not by catalase. This iodination was linear for 6h with no apparent Km for iodide of 1.5 muM, was stimulated by thyrotropin or N6O2'-dibutyryladenosine 3':5'-cyclic monophosphate, yielded mostly iodinated thyroglobulin and was efficient for tetraiodothyronine synthesis. (2) Extracellular iodination, which was sensitive to catalase, but not to NaClO4 or ouabain. This iodination plateaued after 2h and the apparent Km was 16.5 muM. This process was insensitive to thyrotropin and dibutyryl cyclic AMP. The major products were iodoprotein other then thyroglobulin and iodolipid and the yield of tetraiodothyronine was low. (3) Intracellular iodination from passively diffused iodide, which was not sensitive to inhibitors. Other characteristics of passive intracellular iodination were intermediate between active intracellular iodination and extracellular iodination. The fact that the three processes are inhibited by similar concentrations of methimazole, and their apparent Km values, when corrected for the concentrating effect of iodide trapping, are all of the same order as the Km of purified thyroid peroxidases, suggest that although their locations are different, the enzymic systems involved are identical. These results show that, besides an extracellular site of iodination, dispersed thyroid cells process an intracellular site of iodination with biochemical characteristics of physiological relevance.

Effect of dibutyryl cyclic adenosine 3'5'-monophosphate on mitotic activity in the thyroid of hypophysectomized rats

Effect of dibutyryl cyclic adenosine 3'5'-monophosphate (dbcAMP) on mitotic activity in the thyroid of hypophysectomized rats has been examined. It has been demonstrated that dbcAMP stimulates the incidence of mitoses in the thyroid follicular cells. It is therefore suggested that cAMP may be a mediator of the proliferogenic effect of TSH on the thyroid in vivo. Cyclic AMP could also release some unidentified growth-promoting factors for the thyroid. A direct stimulating effect of dbcAMP on the proliferation of the thyroid follicular cells is assumed to be possible as well.

Accumulation of 125I in the oocytes, thyroids, and plasma of laying Japanese quail treated with cyclic AMP, theophylline, or prostaglandins E1 or E2

Dibutyryl cyclic AMP, theophyline, or prostaglandins E1 or E2 were injected intraperitoneally into laying Japanese quail in an attempt to stimulate increased transference of iodide by the ovarian membranes. Theophyline induced increased 18-hr 125I accumulations (by 7 to 18% of the injected dose) of borderline significance in the growing oocytes of some test groups. Theophyline plus DBcAMP treatment lead to a slight increase in 125I accumulation in the thyroid. Other treatments had no apparent effect on 125I transference. The prostaglandins induced oviposition within minutes of injection, as has been reported for uterine application of prostaglandins by other investigators. Means for plasma 125I levels for treated quail were 1.7 to 12.5 times greater than control levels. Values for oocyte to plasma and thyroid to plasma 125I ratios, it appeared that DBcAMP, theophyline, and the prostaglandins inhibited rather than stimulated the accumulation of 125I by the ovary and thyroid of the quail.

Thyroxine secretion by isolated hog thyroid cells: a cyclic AMP independent pathway

The release of 131I-labeled thyroxine (T4) from isolated hog thyroid cells was increased 1.5--2-fold by thyrotropin (TSH). Dibutyryl cyclic AMP failed to reproduce this TSH action. In this in vitro system another cell activity, T4 synthesis, was stimulated in an essentially identical fashion by TSH and dibutyryl cyclic AMP (time course of action, dose-response relationship). 3-Isobutyl-1-methylxanthine (IBMX), 0.5 mM, did not alter the basal [131I]T4 release whereas it enhanced the [131I]T4 synthesis. TSH, 60 MU/ml, increased the intracellular cyclic AMP concentration 3-4-fold. Chlorpromazine (5 X 10(-4)M) abolished the TSH stimulation of cyclic AMP accumulation but did not alter the TSH-induced increase in [131I]T4 secretion. It is concluded that the TSH action on [131I]T4 secretion by isolated thyroid cells is not mediated by the adenylate cyclase-cylic AMP system.

Discontinuity of thyroid gland response to hormonal stimulation: effect of TSH and cAMP on iodide organification

The action of TSH on the process of iodide organification was studied in rat thyroid glands under different experimental in vitro incubation conditions. The effects on glands of both short and prolonged exposure to TSH were evaluated using two different procedures: continuous and pulse labelling of thyroids with radioactive iodide. It was demonstrated that during prolonged contact with thyroid cells, TSH stimulates iodide organification periodically. This periodic effect is cyclic and is composed of a stimulatory and an inhibitory phase. Each cycle lasts 30-45 min, and several cycles follow one another in a regular manner. Furthermore, it has been shown that the periodic effect of THS is due to an intrinsic property of the thyroid cell to respond in a cyclic manner to hormonal stimulation. TSH stimulated the accumulation of organic iodide only when introduced at a precise phase of the cycle. The same type of discontinuous thyroid cell response was obtained when TSH was replaced by its intracellular mediator, cAMP. This indicates that the initiation of the cyclic response to hormonal stimulation is localized at the steps after that of cAMP formation. It seems, therefore, that this cyclic response of thyroid glands is not related to the recently observed phenomenon of desensitization. This phenomenon, characterized by the development of resistance in many target organs and cells toward their respective hormonal stimulators, is due to modifications in steps preceding those of cAMP formation. The discontinuity of thyroid gland response to both TSH and cAMP described in this work seems to be a new phenomenon, whose physiological significance and possible molecular mechanisms are discussed.

Active transport of iodide in isolated porcine thyroid cells. Application to an in vitro bioassay of thyrotropin

Porcine thyroid cells were cultured for 2 days with or without dibutyryl cyclic AMP or thyrotropin (TSH). Then they were isolated post-culture by a gentle treatment with a calcium chelating agent. Some characteristics of the iodide transport system were studied in these thyroid cell suspensions. Iodide influx is a saturable, temperature- and energy-dependent phenomenon. It is blocked by ouabaïn, N-ethylmaleimide, dinitrophenol, cardiotoxin, low Na+ concentration and harmaline. Only 3% of the intracellular iodide is trichloracetic-acid-insoluble at equilibrium. The apparent Michaelis constant (Km) of the transport system is 30 micronM. For monolayer cells, the decrease of C/M ratio, increase of apparent Km, and decrease of Vmax between day 0 (freshly isolated cells) and day 6, indicate a loss of iodide-trapping ability up to passive diffusion. To the contrary, high values of C/M and normal Km (30 micronM) are observed in TSH follicles from reassociated cells. At iodide equilibrium, thryotropin, prostaglandins E1 and E2 and long-acting thyroid stimulator (LATS), induce a fast release of iodide. This release is dose-dependent in the first 5 min. It has been used to develop a bioassay of TSH and a fast detection of LATS.

Kinetic analysis of iodide transport in dog thyroid slices: perchlorate-induced discharge

Dog thyroid slices were incubated with methimazole (2 mM) and radioiodide. The medium radioactivity was continuously recorded for 8 h. Multiple data were collected for individual glands to calculate simultaneously by compartmental analysis the influx and efflux rates of iodide from the slices. A two-compartment thyroid model was necessary and sufficient to simulate the transport of inorganic iodide. The two compartments could be defined as the cellular and the luminal spaces, but interferences due to slice thickness, nonuniform follicle sizes, and open follicles were not excluded. Sodium perchlorate (1 mM) inhibited the influx of iodide into the follicles and discharged the trapped radioiode into the medium with increased efflux rates. Our data suggest that perchlorate decreases the inward influx rates by competition and enhances the outward efflux rates by countertransport and support the hypothesis of mobile iodide carriers at the basal and apical membranes of the thyroid cells.

Evidence for a secretion of thyroxine by isolated hog thyroid cells

Isolated thyroid cells prepared from hog thyroid glands by tryptic dispersion were incubated with 131I- for 1--6 h. Free [131I]thyroxine was identified in the incubation medium by three chromatographic methods. Neither [131I]iodotyrosines nor [131I]triiodothyronine were detected. The [131I]thyroxine released in the medium by 100 mul of cells (packed cell volume) after a 6-h incubation period amounted to 1.16% (S.E. = +/- 0.39) of the total radioactivity. The medium [131I]thyroxine represented 15--25% of the total [131I]thyroxine synthesized during the 6 h of incubation. Thyrotropin, 1--60 munits/ml, increased the medium [131I]thyroxine content 2-4 fold. Dibutyryl cyclic AMP mimicked the effect of thyrotropin. The amount of medium [131]thyroxine was strictly related to the amount of incubated cells but was independent of the volume of the incubation medium. When prelabeled cells were incubated in the presence of methimazole the increase in medium [131I]thyroxine was quantitatively related to a decrease in the intracellular [131I]thyroxine. Addition of dinitrotyrosine, an inhibitor of the deiodinase activity, induced the release of iodotyrosines in the incubation medium. That the incubation supernatant of isolated thyroid cells did contain free thyroxine but not iodotyrosines suggests that the normal mechanisms of proteolysis of thyroglobulin and deiodination of iodotyrosines inside the cells are preserved. From these data, it was concluded that the thyroxine release by isolated cells represents a real secretion.

Demonstration of iodide transport defect but normal iodide organification in nonfunctioning nodules of human thyroid glands

Benign and malignant nodules in human thyroid glands, which did not concentrate iodide in vivo, were also unable to accumulate iodide in vitro. The mean thyroid-to-medium ratio (T/M) in seven benign nodules was 0.8+/-0.2 compared with 7+/-2 in adjacent normal thyroid tissue. In four malignant thyroid nodules, the mean T/M was 0.5+/-0.1 compared with 11+/-4 in adjacent normal thyroid. Despite the inability of such nodules to concentrate iodide, iodide organification was present but was only one-half to one-third as active as in surrounding normal thyroid. Thyroid-stimulating hormone (TSH) increased iodide organification equally in both benign nodules and normal thyroid although it had no effect in three of the four malignant lesions. The reduction in organification is probably related to the absence of iodide transport, since incubation of normal thyroid slices with perchlorate caused similar diminution in iodide incorporation but no change in the response to TSH. Monoiodotyrosine (MIT) and di-iodotyrosine (DIT) accounted for most of the organic iodide in both the nodules and normal tissue. The MIT/DIT ratio was similar in normal and nodule tissue. The normal tissue contained much more inorganic iodide than the nodules, consistent with the absence of the iodide trap in the latter tissue. The thyroxine content of normal thyroid was 149+/-17 mug/g wet wt and 18+/-4 mug/g wet wt in the nodules. The transport defect in the nodules was not associated with any reduction in total, Na(+)-K(+)- or Mg(++)-activated ATPase activities or the concentration of ATP. Basal adenylate cyclase was higher in nodules than normal tissue. Although there was no difference between benign and malignant nodules, the response of adenylate cyclase to TSH was greater in the benign lesions. These studies demonstrate that nonfunctioning thyroid nodules, both benign and malignant, have a specific defect in iodide transport that accounts for their failure to accumulate radioactive iodide in vivo. In benign nodules, iodide organification was increased by TSH while no such effect was found in three of four malignant lesions, suggesting additional biochemical defects in thyroid carcinomas.