Control of the dog thyrocyte plasma membrane iodide permeability by the Ca(2+)-phosphatidylinositol and adenosine 3',5'-monophosphate cascades

Calcium Signaling in the Thyroid: Friend and Foe

Simple Summary

All cells in our body are activated by several different signals. The calcium ion is one of the most versatile signaling molecules, and regulates a multitude of different events in the cells. These range from activation of muscle contraction, to the regulation of cell movement, just to name a few. In normal thyroid cells, calcium signaling is of importance for the normal physiology of the cells. In thyroid pathologies, e.g., thyroid cancer, calcium is important for the regulation of proliferation and invasion, and may also activate gene transcription programs important for cancer cell survival. In this Commentary, we summarize what is known regarding calcium in the normal thyroid, and highlight the importance of calcium signaling in thyroid pathologies.

Abstract

Calcium signaling participates in a vast number of cellular processes, ranging from the regulation of muscle contraction, cell proliferation, and mitochondrial function, to the regulation of the membrane potential in cells. The actions of calcium signaling are, thus, of great physiological significance for the normal functioning of our cells. However, many of the processes that are regulated by calcium, including cell movement and proliferation, are important in the progression of cancer. In the normal thyroid, calcium signaling plays an important role, and evidence is also being gathered showing that calcium signaling participates in the progression of thyroid cancer. This review will summarize what we know in regard to calcium signaling in the normal thyroid as, well as in thyroid cancer.

TAS2R bitter taste receptors regulate thyroid function

Dysregulation of thyroid hormones triiodothyronine and thyroxine (T3/T4) can impact metabolism, body composition, and development. Thus, it is critical to identify novel mechanisms that impact T3/T4 production. We found that type 2 taste receptors (TAS2Rs), which are activated by bitter-tasting compounds such as those found in many foods and pharmaceuticals, negatively regulate thyroid-stimulating hormone (TSH)-dependent Ca(2+) increases and TSH-dependent iodide efflux in thyrocytes. Immunohistochemical Tas2r-dependent reporter expression and real-time PCR analyses reveal that human and mouse thyrocytes and the Nthy-Ori 3-1 human thyrocyte line express several TAS2Rs. Five different agonists for thyrocyte-expressed TAS2Rs reduced TSH-dependent Ca(2+) release in Nthy-Ori 3-1 cells, but not basal Ca(2+) levels, in a dose-dependent manner. Ca(2+) responses were unaffected by 6-n-propylthiouracil, consistent with the expression of an unresponsive variant of its cognate receptor, TAS2R38, in these cells. TAS2R agonists also inhibited basal and TSH-dependent iodide efflux. Furthermore, a common TAS2R42 polymorphism is associated with increased serum T4 levels in a human cohort. Our findings indicate that TAS2Rs couple the detection of bitter-tasting compounds to changes in thyrocyte function and T3/T4 production. Thus, TAS2Rs may mediate a protective response to overingestion of toxic materials and could serve as new druggable targets for therapeutic treatment of hypo- or hyperthyroidism.

Anoctamin-1/TMEM16A is the major apical iodide channel of the thyrocyte

Iodide is captured by thyrocytes through the Na(+)/I(-) symporter (NIS) before being released into the follicular lumen, where it is oxidized and incorporated into thyroglobulin for the production of thyroid hormones. Several reports point to pendrin as a candidate protein for iodide export from thyroid cells into the follicular lumen. Here, we show that a recently discovered Ca(2+)-activated anion channel, TMEM16A or anoctamin-1 (ANO1), also exports iodide from rat thyroid cell lines and from HEK 293T cells expressing human NIS and ANO1. The Ano1 mRNA is expressed in PCCl3 and FRTL-5 rat thyroid cell lines, and this expression is stimulated by thyrotropin (TSH) in rat in vivo, leading to the accumulation of the ANO1 protein at the apical membrane of thyroid follicles. Moreover, ANO1 properties, i.e., activation by intracellular calcium (i.e., by ionomycin or by ATP), low but positive affinity for pertechnetate, and nonrequirement for chloride, better fit with the iodide release characteristics of PCCl3 and FRTL-5 rat thyroid cell lines than the dissimilar properties of pendrin. Most importantly, iodide release by PCCl3 and FRTL-5 cells is efficiently blocked by T16Ainh-A01, an ANO1-specific inhibitor, and upon ANO1 knockdown by RNA interference. Finally, we show that the T16Ainh-A01 inhibitor efficiently blocks ATP-induced iodide efflux from in vitro-cultured human thyrocytes. In conclusion, our data strongly suggest that ANO1 is responsible for most of the iodide efflux across the apical membrane of thyroid cells.

Canonical transient receptor potential channel 2 (TRPC2): old name-new games. Importance in regulating of rat thyroid cell physiology

In addition to the TSH-cyclic AMP signalling pathway, calcium signalling is of crucial importance in thyroid cells. Although the importance of calcium signalling has been thoroughly investigated for several decades, the nature of the calcium channels involved in signalling is unknown. In a recent series of investigations using the well-studied rat thyroid FRTL-5 cell line, we showed that these cells exclusively express the transient receptor potential canonical 2 (TRPC2) channel. Our results suggested that the TRPC2 channel is of significant importance in regulating thyroid cell function. These investigations were the first to show that thyroid cells express a member of the TRPC family of ion channels. In this review, we will describe the importance of the TRPC2 channel in regulating TSH receptor expression, thyroglobulin maturation, intracellular calcium and iodide homeostasis and that the channel also regulates thyroid cell proliferation.

[Characterization of muscarinic receptors in undifferentiated thyroid cells in Fisher rats]

INTRODUCTION

The parasympathetic autonomous nervous system exerts control over thyroid function by activation of the muscarinic receptors in follicular cells. Various pharmacological and molecular subtypes of muscarinic receptors (M(1), M(2), M(3), M(4), M(5)) have been identified in central nervous system and peripheral tissues. Controversy surrounds receptor characterization in thyroid cells.

MATERIALS AND METHODS

Undifferentiated Fisher rat thyroid epithelial cells (FRT) were cultured. Association and dissociation kinetics assays and antagonist competition studies of the binding of (3)H-N-methylscopolamine ((3)H-NMS) to muscarinic receptors were performed to demonstrate the presence of muscarinic receptors.

RESULTS

Specific muscarinic receptors in the plasma membrane of FRT cells were observed with an equilibrium dissociation constant (K(d)) of 0.44 nmol. The order of affinities obtained fitting the data to one binding site model in competition experiments with the muscarinic receptor antagonist was: dicyclomine > hexahydrosiladifenidol (HHSD) = 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) > pirenzepine > himbacine = 11-[[2-[(diethylamino)methyl]- 1-piperidinyl]acetyl]-5,11-dihydro-6H-pyrido (414)benzodiazepine (AF-DX 116).

CONCLUSIONS

The results obtained indicate the existence of specific (3)H-NMS muscarinic binding sites located in the plasma membrane of FRT cells. The results obtained in competition experiments suggest that the receptors present in FRT cells belong to the M(3) subtype.

Effects of extracellular nucleotides in the thyroid: P2Y2 receptor-mediated ERK1/2 activation and c-Fos induction in PC Cl3 cells

Aim of the present paper was to investigate the signaling pathways of P2Y2 in rat thyroid PC Cl3 cell line and its effects on proliferation. This study demonstrates that P2Y2 activation provoked: (a) a cytosol-to-membrane translocation of PKC-alpha, -betaI and -epsilon; (b) the phosphorylation of the extra cellular signal-regulated kinases 1 and 2 (ERK1/2); (c) the expression of c-Fos protein; (d) no effects on the G1/S progression and overall cell proliferation. The P2Y2-stimulated ERK1/2 phosphorylation was: (a) completely blocked by PD098059, a mitogen-activated protein kinase (MEK) inhibitor or by W-7, a Ca2+-calmodulin (CaM) antagonist; (b) reduced by GF109203X, inhibitor of PKCs, or AG1478, inhibitor of EGFR tyrosine kinase, or LY294002/wortmannin, inhibitors of phosphoinositide 3-kinases, or cytochalasin D, inhibitor of actin microfilament bundles polymerization. The c-Fos induction was greatly diminished by Go6976 or PD098059, and completely abolished when combined. In conclusion, data indicate that the P2Y2-induced phosphorylation of ERK1/2 and the induction of c-Fos are due to the operation of CaM, with PKC, PI3K, EGFR and receptor endocytosis mechanisms endorsing the signalling. On the other hand, no mitogenic effects of P2Y2 are whatsoever noticed in PC Cl3 cells.

Mechanisms of P2 receptor-evoked DNA synthesis in thyroid FRTL-5 cells

The expression of the P2 receptors and their functional responses were studied in rat thyroid FRTL-5 cells. RT-PCR analysis revealed transcripts for the G protein-coupled P2Y(2), P2Y(4) and P2Y(6) receptors, and for the transmitter-gated ion channel P2X(3), P2X(4) and P2X(5) subunits. In Fura-2-loaded cells, UTP, ATP, ATPgammaS or UDP increased [Ca(2+)](i), and behaved as potent full agonists, while 2-Methylthio-ATP (2-MeSATP), alpha,beta-methylene-ATP (alpha,beta-meATP) and pure ADP were weak agonists. The agonist-mediated [Ca(2+) ](i) increases were diminished in Ca(2+) -free buffer, and by pertussis toxin (PTX) or suramin treatments. ATP, UTP, UDP and ATPgammaS increased (3)H-thymidine incorporation into DNA and expression of the protooncogenes c-Fos and c-Jun, while 2-MeSATP was ineffective, and alpha,beta-meATP gave a response only at 100-microM dose. The ATP-stimulated expression of c-Fos and c-Jun was dependent on Ca(2+), and protein kinase C, but not on calmodulin or Ca(2+)/calmodulin-dependent protein kinase II. Extracellular signal-regulated kinases (ERK1 and ERK2) are also involved as the MEK inhibitor, PD98059, reduced both ATP-evoked (3)H-thymidine incorporation and c-Fos and c-Jun expression. These results indicate that multiple P2Y receptor subtypes and at least the P2X(5) subtype are functionally expressed in FRTL-5 cells, and that nucleotides acting via P2 receptors are involved in the regulation of DNA-synthesis.

Hydrogen peroxide attenuates store-operated calcium entry and enhances calcium extrusion in thyroid FRTL-5 cells

Redox modulation participates in the regulation of intracellular free calcium concentration ([Ca(2+)](i)) in several cell types. In thyroid cells, including FRTL-5 cells, changes in [Ca(2+)](i) regulate several important functions, including the production of H(2)O(2) (hydrogen peroxide). As H(2)O(2) is of crucial importance for the production of thyroid hormones, we investigated the effects of H(2)O(2) on [Ca(2+)](i) in thyroid FRTL-5 cells. H(2)O(2) itself did not modulate basal [Ca(2+)](i). However, H(2)O(2) attenuated store-operated calcium entry evoked by thapsigargin, both in a sodium-containing buffer and in a sodium-free buffer. The effect of H(2)O(2) was abrogated by the reducing agent beta-mercaptoethanol. H(2)O(2) also attenuated the thapsigargin-evoked entry of barium and manganese. The effect of H(2)O(2) was, at least in part, mediated by activation of protein kinase C (PKC), as H(2)O(2) enhanced the binding of [(3)H]phorbol 12,13-dibutyrate. H(2)O(2) also stimulated the translocation of the isoenzyme PKCepsilon from the cytosolic fraction to the particulate fraction. Furthermore, H(2)O(2) did not attenuate store-operated calcium entry in cells treated with staurosporine or calphostin C, or in cells with down-regulated PKC. H(2)O(2) depolarized the membrane potential in bisoxonol-loaded cells and when patch-clamp in the whole-cell mode was used. The depolarization was attenuated in cells with down-regulated PKC. This depolarization, at least in part, explained the H(2)O(2)-evoked inhibition of calcium entry. In addition, H(2)O(2) enhanced the extrusion of calcium from cells stimulated with thapsigargin and this effect was abolished in cells with down-regulated PKC and after treatment of the cells with the reducing agent beta-mercaptoethanol. In conclusion H(2)O(2) attenuates an increase in [Ca(2+)](i). As H(2)O(2) is produced in thyroid cells in a calcium-dependent manner, our results suggest that H(2)O(2) may participate in the regulation of [Ca(2+)](i) in these cells via a negative-feedback mechanism involving activation of PKC.

Analysis of human sodium/iodide symporter, thyroid transcription factor-1, and paired-box-protein-8 gene expression in benign thyroid diseases

The ability to concentrate iodide, a fundamental property of normally functioning thyroid tissue, is altered in various thyroid diseases. Given the critical role of the Na+/I- symporter (NIS) in controlling iodide access to the thyroid gland, altered expression of NIS may be responsible, at least in part, for an enhanced or diminished capacity to concentrate iodide. In this study, we used Northern blot analysis, a newly established quantitative polymerase chain reaction (PCR) assay and in addition hNIS-directed immunohistochemical analysis to assess the levels of hNIS mRNA and protein expression in various localized and diffuse benign thyroid abnormalities, including Graves' disease (GD), scintigraphically cold solitary benign thyroid nodule (CBTN), nontoxic multinodular goiter (NMNG), solitary autonomously functioning thyroid nodule (AFTN), and mild diffuse iodine deficiency goiter (IDG). In addition, in view of the recent identification of putative binding sites for the transcription factors thyroid transcription factor-1 (TTF-1) and human paired-box-protein-8 (Pax-8) in the human NIS gene promoter, we used reverse transcriptase-polymerase chain reaction (RT-PCR) to assess in these same samples the levels of TTF-1 and Pax-8 gene expression. Northern blot analysis revealed high levels of hNIS gene expression in thyroid specimens derived from patients with GD and AFTN. In contrast, levels of hNIS mRNA expression were moderate in NMNG, low in diffuse IDG, and very low in CBTN. Quantitative RT-PCR analysis of hNIS mRNA transcripts revealed variable but generally low levels of hNIS gene expression in IDG and NMNG, and undetectable or very low levels of hNIS mRNA in all scintigraphically CBTN studied. In contrast, markedly elevated levels of hNIS mRNA transcripts were detected in active GD (up to 17-fold) and AFTN (up to 25-fold). Immunohistochemical analysis revealed abundant hNIS protein expression by thyroid follicular cells in GD, moderate and heterogeneous levels in NMNG, and very low levels in CBTN. hNIS mRNA levels were correlated with TTF-1 and Pax-8 gene expression in GD and, to a lesser degree, in AFTN, NMNG, and IDG, but not in CBTN. In general, hNIS gene expression was more closely correlated with TTF-1 as compared to Pax-8 gene expression. In conclusion, the abundance of hNIS mRNA and protein expression in a broad range of benign thyroid pathologies correlated well with their functional state as assessed by thyroid scintigraphy. In addition to TTF-1 and Pax-8, other transcription factors and enhancer elements may contribute to regulation of NIS gene promoter activity.