Thyroid Regulatory Factors

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Chapter 3

Thyroid Regulatory Factors

Four major biological variables are regulated in the thyrocyte as in any other cell type: function, cell size, cell number, and differentiation. The first three variables are quantitative, and the latter is qualitative. In this chapter, we consider the factors involved in these controls in physiology and in pathology, the main regulatory cascades through which these factors exert their effects, and the regulated processes, which are function, proliferation and cell death, gene expression, and differentiation. Whenever possible, we describe what is known in humans.

Thyroid Regulatory Factors

In Physiology

The two main factors that control the physiology of the thyroid after embryogenesis are the requirement for thyroid hormones and the supply of its main and specialized substrate iodide (Table 3-1). Thyroid hormone plasma levels and action are monitored by the hypothalamic supraoptic nuclei and by the thyrotrophs of the anterior lobe of the pituitary, where they exert a negative feedback through T3 receptor β. In normal rats, serum thyroid-stimulating hormone (TSH) levels are inversely related to thyrocyte sensitivity to TSH.1 The corresponding homeostatic control is expressed by TSH (thyrotropin). The TSH receptor is also stimulated by a new, different natural hormone cloned by homology, thyrostimuline. The physiologic role of this protein is unknown, but its level is not controlled by a thyroid hormone feedback, and it does not participate in the homeostatic control of the thyroid.2 Iodide supply is monitored in part indirectly as a substrate for the synthesis of thyroid hormones and therefore through its effects on the plasma level of thyroid hormones, but mainly in the thyroid itself, where it depresses various aspects of thyroid function and the response of the thyrocyte to TSH. These two major physiologic regulators control the function and size of the thyroid: TSH positively, iodide negatively.36 In mice embryo, other unknown factors control differentiation and organ growth which takes place normally until birth in the absence of TSH receptor.7,8 In humans, in whom birth occurs later in development, homozygous inactivating mutations of the TSH receptor in familial congenital hypothyroidism were found to be associated with a very hypoplastic thyroid gland.9 Although the thyroid contains receptors for thyroid hormones, and a direct effect of these hormones on thyrocytes would make sense,10 as yet little evidence has indicated that such control plays a role in physiology.11 However, expression of dominant-negative thyroid hormone receptors in mice represses PPARγ expression and induces thyroid tumors in thyroid.12 The role of increased TSH plasma level in this induction is not defined. Luteinizing hormone (LH) and human chorionic gonadotropin (hCG) at high levels activate the TSH receptor and thus directly stimulate the thyroid. This effect accounts for the depression of TSH levels and sometimes elevated thyroid activity at the beginning of pregnancy.1315

The thyroid gland is also influenced by various other nonspecific hormones.16 Hydrocortisone exerts a differentiating action in vitro.17 Estrogens affect the thyroid by unknown mechanisms, directly or indirectly, as exemplified in the menstrual cycle, in pregnancy, and in the higher prevalence of thyroid tumors and other pathologies in women. Growth hormone (GH) induces thyroid growth, but its effects are thought to be mediated by locally produced somatomedins (insulin-like growth factor 1 [IGF-1]). Nevertheless the presence of basal TSH levels might be a prerequisite for the growth-promoting action of IGF-1, because a GH replacement therapy did not increase thyroid size in patients deficient for both GH and TSH.18 Conversely, TSH does not induce the proliferation of human thyrocytes in the absence of IGF-1 or high levels of insulin. The anomalously low endemic goiter prevalence among pygmies living in iodine-deficient areas,19 a population genetically resistant to IGF-1, is also compatible with an in vivo permissive effect of IGF-1 and IGF-1 receptor on TSH mitogenic action. In dog and human thyroid primary cultures, the presence of insulin receptors strictly depends on TSH, suggesting that thyroid might be a more specific target of insulin than generally considered.20,21 Effects of locally secreted neurotransmitters and growth factors on thyrocytes have been demonstrated in vitro and sometimes in vivo, and the presence of some of these agents in the thyroid has been ascertained. The set of neurotransmitters acting on the thyrocyte and their effects vary from species to species.3,22 In human cells, well-defined, direct but short-lived responses to norepinephrine, ATP, adenosine, bradykinin, and thyrotropin-releasing hormone (TRH) have been observed.3,23,24 Growth-factor signaling cascades demonstrated in vitro can exert similar effects in vivo. In nude mice, the injection of epidermal growth factor (EGF) promotes DNA synthesis in thyroid and inhibits iodide uptake in xenotransplanted rat25 and human thyroid tissues.26 By contrast, the injection of fibroblast growth factor (FGF) induces a colloid goiter in mice, with no inhibition of iodide metabolism or thyroglobulin and thyroperoxidase mRNA accumulation.27 These effects are exact replicas of initial observations from the dog thyroid primary culture system28 and other thyroid primary culture systems.2932 EGF and FGF have since been reported to be locally synthesized in the thyroid gland, as a possible response to thyroxine and TSH,33 respectively. Their exact role as autocrine and/or paracrine agents in the development, function, and pathology of the thyroid gland of different species has yet to be clarified.34,35 Transforming growth factor beta (TGF-β) constitutes another category of cytokines that are growth factors produced locally by thyrocytes and influence their proliferation and the action of mitogenic factors.34,36 TGF-β inhibits proliferation and prevents most of the effects of TSH and cAMP in human thyrocytes in vitro.37,38 TGF-β is synthesized as an inactive precursor which can be activated by different proteases produced by thyrocytes. TGF-β expression is up-regulated during TSH-induced thyroid hyperplasia in rats, suggesting an autocrine mechanism limiting goiter size.39 Also present in the thyroid are activin A and bone morphogenic peptide (BMP), which are related to TGF-β and inhibit thyrocyte proliferation in vitro.40 Unlike TGF-β, they are directly synthesized in an active form. Elements of a Wnt/β catenin signaling pathway (Wnt factors, Frizzled receptors, and disheveled isoforms) have been identified in human thyroid and thyroid cancer cell lines.41 The eventual role in vivo in humans of most of these factors remains to be proved and clarified.

In Pathology

Mutated, constitutively active TSH receptors and Gs proteins cause thyroid autonomous adenomas.42,43 Mutations which confer to the TSH receptor a higher sensitivity to LH/hCG cause hyperthyroidism in pregnancy.44,45 Pathologic extracellular signals play an important role in autoimmune thyroid disease. Thyroid-stimulating antibodies (TSAbs), which bind to the TSH receptor and activate it, reproduce the stimulatory effects of TSH on the function and growth of the tissue. Their effects on cAMP accumulation are slower and much more persistent than those of TSH.46 Their abnormal generation is responsible for the hyperthyroidism and goiter of Graves’ disease. Thyroid-blocking antibodies (TBAbs) also bind to the TSH receptor but do not activate it and hence behave as competitive inhibitors of the hormone. Such antibodies are responsible for some cases of hypothyroidism in thyroiditis. Both stimulating and inhibitory antibodies from mothers with positive sera induce transient hyperthyroidism or hypothyroidism in newborns.4 The existence of thyroid growth immunoglobulins has been hypothesized to explain the existence of Graves’ disease with weak hyperthyroidism and prominent goiter. The thyroid specificity of such immunoglobulins would imply that they recognize thyroid-specific targets. This hypothesis is now abandoned.4749 Discrepancies between growth and functional stimulation may instead reflect kinetics or cell intrinsic factors. Local cytokines have been shown to influence, mostly negatively, the function, growth, and differentiation of thyrocytes in vitro and thyroid function in vivo. Because they are presumably secreted in loco in autoimmune thyroid diseases, these effects might play a role in the pathology of these diseases, but this notion has not yet been proved.3,16 Moreover, in selenium deficiency, secretion of TGF-β by macrophages has been implicated in the generation of thyroid fibrosis50 and the pathogenesis of thyroid failure in endemic cretinism. The overexpression of both FGF and FGF receptor 1 in thyrocytes from human multinodular goiter might explain their relative TSH independence.51 On the other hand, the subversion of tyrosine kinase pathways similar to those normally operated by local growth factors (i.e., the activation and thyrocyte expression of Ret52 and TRK,53 the overexpression of Met/HGF receptor, sometimes in association with hepatocyte growth factor (HGF),54 or erbB/EGF receptor in association with its ligand, TGF-α55) can be causally associated with TSH-independent thyroid papillary carcinomas. An autocrine loop involving IGF-2 and the insulin receptor isoform-A is also proposed to stimulate growth of some thyroid cancers.56 Thyroid cancer cells often escape growth inhibition by TGF-β.57

Regulatory Cascades

The great number of extracellular signals acting through specific receptors on cells in fact control a very limited number of regulatory cascades. We will first outline these cascades, along with the signals that control them, and then describe in more detail the specific thyroid cell features: controls by iodide and the TSH receptor.

The Cyclic Adenosine Monophosphate Cascade

The cyclic adenosine monophosphate (cAMP) cascade in the thyroid corresponds, as far as it has been studied, to the canonic model of the β-adrenergic receptor cascade4 (Fig. 3-1). It is activated in the human thyrocyte by the TSH and the β-adrenergic and prostaglandin E receptors. These receptors are classical seven-transmembrane receptors controlling transducing guanosine triphosphate (GTP)-binding proteins. Activated G proteins belong to the Gs class and activate adenylyl cyclase; they are composed of a distinct αs subunit and nonspecific β and γ monomers. Activation of a G protein corresponds to its release of guanosine diphosphate (GDP) and binding of GTP and to its dissociation into αGTP and βγ dimers; αsGTP directly binds to and activates adenylyl cyclase. Inactivation of the G protein follows the spontaneous, more or less rapid hydrolysis of GTP to GDP by αs GTPase activity and the reassociation of αGDP with βγ. The effect of stimulation of the receptor by agonist binding is to increase the rate of GDP release and GTP binding, thus shifting the equilibrium of the cycle toward the αGTP active form. Unless constrained in a multiprotein complex, one receptor can consecutively activate several G proteins (hit-and-run model). A similar system negatively controls adenylyl cyclase through Gi. It is stimulated in the human thyroid by norepinephrine through α2-receptors and moderately by the TSH receptor. Adenosine at high concentrations directly inhibits adenylyl cyclase. The thyroid contains mainly three isoforms of adenylyl cyclase: III, VI, and IX.58 The cAMP generated by adenylyl cyclase binds to the regulatory subunit of protein kinase A (PKA) that is blocking the catalytic subunit and releases this now-active unit. The activated, released catalytic unit of protein kinase phosphorylates serines and threonines in the set of proteins containing accessible specific peptides that it recognizes. These phosphorylations, through more or less complex cascades, lead to the observed effects of the cascade. Two isoenzymes (I, II) of cAMP-dependent kinases have been found, the first of which is more sensitive to cAMP; as yet no clear specificity of action of these kinases has been demonstrated. In the case of the thyroid, this cascade is activated through specific receptors, by TSH in all species, and by norepinephrine receptors and prostaglandin E in humans, with widely different kinetics: prolonged for TSH and short lived (minutes) for norepinephrine and prostaglandins.59 Other neurotransmitters have been reported to activate the cascade in thyroid tissue, but not necessarily in the thyrocytes of the tissue.24 Besides PKA, cAMP activates EPAC (exchange protein directly activated by cAMP) or Rap guanine nucleotide exchange factor 1 (GEF1) and the less abundant GEF2, which activates the small G protein Rap.60 It does so in the thyroid.61 However, despite high expression of EPAC1 in thyrocytes and its further increase in response to TSH, all the physiologically relevant cAMP-dependent functions of TSH studied in dog thyroid cells, including acute regulation of cell functions (including thyroid hormone secretion) and delayed stimulation of differentiation expression and mitogenesis, are mediated only by PKA activation.61 The role of the cAMP/EPAC/Rap cascade in thyroid thus remains largely unknown. Activation of PKA inactivates small G proteins of the Rho family (RhoA, Rac1, and Cdc42), which reorganizes the actin cytoskeleton and could play an important role in stimulation of thyroid hormone secretion and induction of thyroid differentiation genes.62 Of the other known possible effectors of cAMP, cyclic guanosine monophosphate (cGMP)-dependent protein kinases are present in the thyroid but cyclic nucleotide–activated channels have not been looked for.

In the thyrocyte, the cAMP cascade is controlled by several negative feedbacks, including the direct activation by phosphorylation of phosphodiesterases and the induction of several proteins inhibiting the cascade.63

The thyrocyte is very sensitive to small changes in cAMP concentrations; a mere doubling of this concentration is sufficient to cause maximal thyroglobulin endocytosis.64

The Calcium–Inositol 1,4,5-Triphosphate Cascade

The calcium (Ca2+)–inositol 1,4,5-triphosphate (IP3) cascade in the thyroid also corresponds, as far as has been studied, to the canonic model of the muscarinic or α1-adrenergic receptor–activated cascades. It is activated in the human thyrocyte by TSH—through the same receptors that stimulate adenylyl cyclase—and by ATP, bradykinin, and TRH through specific receptors. In this cascade, as in the cAMP pathway, the activated receptor causes the release of GDP and the binding of GTP by the GTP-binding transducing protein (Gq, G11, G12), and their dissociation into αq and βγ. In turn, αGTP then stimulates phospholipase C. The TSH receptor activates Gs and Gq, with a higher affinity for Gs.65,66 Phospholipase C hydrolyzes membrane phosphatidylinositol 4,5-bisphosphate (PIP2) into diacylglycerol and IP3. IP3 enhances calcium release from its intracellular stores, followed by an influx from the extracellular medium. The rise in free ionized intracellular Ca2+ leads to the activation of several proteins, including calmodulin. The latter protein in turn binds to target proteins and thus stimulates them (e.g., cyclic nucleotide phosphodiesterase and, most importantly, calmodulin-dependent protein kinases). These kinases phosphorylate a whole set of proteins exhibiting serines and threonines on their specific peptides and thus modulate them and cause many observable effects of this arm of the cascade. Calmodulin also activates constitutive nitric oxide (NO) synthase in thyrocytes. The generated NO itself enhances soluble guanylyl cyclase activity in thyrocytes and perhaps in other thyroid cells and thus increases cGMP accumulation.67 Nothing is yet known about the role of cGMP in the thyroid cell.

Diacylglycerol released from PIP2 activates protein kinase C, or rather the family of protein kinases C, which by phosphorylating serines or threonines in specific accessible peptides in target proteins causes the effects of the second arm of the cascade.68 It inhibits phospholipase C or its Gq, thus creating a negative feedback loop. In the human thyroid, the PIP2 cascade is stimulated through specific receptors by ATP, bradykinin, TRH, and TSH.24,66,69,70 The effects of bradykinin and TRH are very short lived. Acetylcholine, which is the main activator of this cascade in the dog thyrocyte, is inactive on the human cell, although it activates nonfollicular (presumably endothelial) cells in this tissue.24

Other Phospholipid-Linked Cascades

In dog thyroid cells and in a functional rat thyroid cell line (FRTL5), TSH activates PIP2 hydrolysis weakly and at concentrations several orders of magnitude higher than those required to enhance cAMP accumulation. Of course, these effects have little biological significance. However, at lower concentrations in dog cells, TSH increases the incorporation of labeled inositol and phosphate into phosphatidylinositol. Similar effects may exist in human cells, but they would be masked by the stimulation of the PIP2 cascade. They may reflect increased synthesis, perhaps coupled to and necessary for cell growth.71

Diacylglycerol can be generated by other cascades than the classic Ca2+-IP3 pathway. Activation of phosphatidylcholine phospholipase D takes place in dog thyroid cells stimulated by carbamylcholine. Because it is reproduced by phorbol esters (i.e., by stable analogs of diacylglycerol), it has been ascribed to phosphorylation of the enzyme by protein kinase C, which would represent a positive feedback loop.72 Although such mechanisms operate in many types of cells, their existence in human thyroid cells has not been demonstrated.73

Release of arachidonate from phosphatidylinositol by phospholipase A2 and the consequent generation of prostaglandins by a substrate-driven process are enhanced in various cell types through G protein–coupled receptors, by intracellular calcium, or by phosphorylation by protein kinase C. In dog thyroid cells, all agents enhancing intracellular calcium concentration, including acetylcholine, also enhance the release of arachidonate and the generation of prostaglandins. In this species, stimulation of the cAMP cascade by TSH inhibits this pathway. In pig thyrocytes, TSH has been reported to enhance arachidonate release. In human thyroid, TSH, by stimulating PIP2 hydrolysis and intracellular calcium accumulation, might be expected to enhance arachidonate release and prostaglandin generation, but such effects have not yet been proved.

Regulatory Cascades Controlled by Receptor Tyrosine Kinases

Many growth factors and hormones act on their target cells by receptors that contain one transmembrane segment. They interact with the extracellular domain and activate the intracellular domain, which phosphorylates proteins on their tyrosines. Receptor activation involves in some cases a dimerization and in others a conformational change of a preexisting dimer. The first step in activation is protein-tyrosine phosphorylation, followed by binding of various protein substrates on tyrosine phosphates containing segments of the receptor. Such binding through src homology domains (SH2) leads to phosphorylation of these proteins on their tyrosines. In turn, this phosphorylation causes sequential activation of the ras and raf proto-oncogenes, mitogen-activated protein (MAP) kinase kinase, MAP kinase, and so on, on the one hand, and phosphatidylinositol-3-kinase (PI-3-kinase), protein kinase B (PKB), and TOR (target of rapamycin) on the other hand. The set of proteins phosphorylated by a receptor defines the pattern of action of this receptor. In thyroids of various species, insulin, IGF-1, EGF, FGF, HGF, but not platelet-derived growth factor activate such cascades.74,75 In the human thyroid, effects of insulin, IGF-1, EGF, FGF, but not HGF have been demonstrated.3,21,51,7678 TGF-β, acting through the serine threonine kinase activity of its receptors and its phosphorylated protein targets (Smad), inhibits proliferation and specific gene expression in human thyroid cells.37,38,79 TSH and cAMP do not activate either the MAPK-ERK nor the JUNK and p38 phosphorylation pathways in dog and human thyroids.75

Cross-Signaling Between the Cascades

Calcium, the intracellular signal generated by the PIP2 cascade, activates calmodulin-dependent cyclic nucleotide phosphodiesterases and thus inhibits cAMP accumulation and its cascade.80 This activity represents a negative cross-control between the PIP2 and the cAMP cascades. Activation of protein kinase C enhances the cAMP response to TSH and inhibits the prostaglandin E response, which suggests opposite effects on the TSH and prostaglandin receptors.81 No important effect of cAMP on the PIP2 cascade has been detected. On the other hand, stimulation of protein kinase C by phorbol esters inhibits EGF action.

Cross-signaling between the cyclic AMP pathway and growth factor–activated cascades have been observed in various cell types.82,83 In ovarian granulosa cells, FSH (through cAMP) activates MAP kinases and the PI3 kinase pathway.84 In FRTL5, but not in WRT cell lines, TSH (through cAMP) activates MAP kinase; in WRT cells but not in PCCl3 cells, TSH and cAMP activate PKB.85,86 Such cross signalings have not been observed in human or dog thyroid cells. Ras, MAPK, p38, Jun kinase and ERK5, as well as PI3 kinase and PKB, are not modulated by cAMP.74,75,87,88

Other growth-activating cascades have been little investigated in the thyroid. In dog and human cells, TSH or cAMP have no effect on STAT phosphorylations (i.e., on the Jak-STAT pathways). The nuclear factor κB (NF-κB) pathway has not yet been investigated in thyroid cells.

Specific Control by Iodide

Iodide, the main substrate of the specific metabolism of the thyrocyte, is known to control the thyroid. Its main effects in vivo and in vitro are to decrease the response of the thyroid to TSH, to acutely inhibit its own oxidation (Wolff-Chaikoff effect), to reduce its trapping after a delay (adaptation to the Wolff-Chaikoff effect), and at high concentrations to inhibit thyroid hormone secretion (Fig. 3-2). The first effect is very sensitive inasmuch as small changes in iodine intake without any other changes (e.g., thyroid hormone levels) are sufficient to reset the thyroid system at different serum TSH levels, which suggests that in physiologic conditions, modulation of the thyroid response to TSH by iodide plays a major role in the negative feedback loop.5,89 Iodide in vitro has also been reported to inhibit a number of metabolic steps in thyroid cells.90,91 These actions might be direct or indirect as a result of an effect on an initial step of a regulatory cascade. Certainly, iodide inhibits the cAMP cascade at the level of Gs or cyclase and the Ca2+-PIP2 cascade at the level of Gq or phospholipase C; such effects can account for the inhibition of many metabolic steps controlled by these cascades.92,93 In one case in which this process has been studied in detail, the control of H2O2 generation (i.e., the limiting factor of iodide oxidation and thyroid hormone formation), iodide inhibited both the cAMP and the Ca2+-PIP2 cascades at their first step but also the effects of the generated intracellular signals cAMP, Ca2+, and diacylglycerol on H2O2 generation.94

The mechanism of action of iodide on all the metabolic steps besides secretion fits the “XI” paradigm of Van Sande.95 These inhibitions are relieved by agents that block the trapping of iodide (e.g., perchlorate) or its oxidation (e.g., methimazole)—the Van Sande criteria. The effects are therefore ascribed to one or several postulated intracellular iodinated inhibitors called XI. The identity of such signals is still unproved. At various times, several candidates have been proposed for this role, such as thyroxine, iodinated eicosanoids (iodolactone),96 and more plausibly iodohexadecanal.97 The latter, the predominant iodinated lipid in the thyroid, can certainly account for the inhibition of adenylyl cyclase and of H2O2 generation.9799 It should be emphasized that iodination of the various enzymes, as well as a catalytic role of iodide in the generation of O2 radicals (shown to be involved in the toxic effects of iodide), could account for the Van Sande criteria with no need for the XI paradigm.95,100 With regard to thyroid secretion, its inhibition by iodide in patients treated with antithyroid drugs suggests a direct, XI-independent effect.

Apart from its inhibitory effects, iodide also activates H2O2 generation and thus protein iodination in the thyroid of some species including humans. This effect is also inhibited by inhibitors of thyroperoxidase transport and can be classified as an XI effect. It has physiologic sense as it links the generation of H2O2 to the availability of the cosubstrate iodide.

In dogs in vivo, iodide at moderate doses decreases cell proliferation and the expression of TPO and NIS mRNA but not the synthesis or secretion of thyroid hormone. The down-regulation of NIS explains the adaptation to the Wolff-Chaikoff effect.101

The Thyroid-Stimulating Hormone Receptor

Structure of the Protein

The β subunits of glycoprotein hormones, to which TSH belongs, are encoded by paralogous genes displaying substantial sequence similarity (Fig. 3-3). The corresponding receptors, FSHR, LH/CGR and TSHR, are members of the rhodopsin-like G protein–coupled receptor family. As such, TSHR contains a “serpentine” portion with seven transmembrane helices with many (but not all) of the sequence signatures typical of this receptor family. In addition, and a hallmark of the subfamily of glycoprotein hormone receptors, it contains a large (about 400 residues) aminoterminal ectodomain responsible for the high affinity and selective binding of TSH.102104 The higher-sequence identity of the serpentine portions of glycoprotein hormone receptors (about 70%), when compared with the ectodomains (about 40%, see Fig. 3-3), suggested early that the former are interchangeable modules capable of activating the G proteins (mainly Gas) after specific binding of the individual hormones to the latter.4 Contrary to other rhodopsin-like GPCRs, binding of the hormones to their ectodomains can be observed with high affinity in the absence of the serpentine.105 The intramolecular transduction of the signal between these two portions of the receptors involves mechanisms specific to the glycoprotein hormone receptor family (see later). The relatively high sequence identity between the hormone-binding domains of the TSH and LH/CG receptors opens the possibility of spillover phenomena during normal or, even more so, molar or twin pregnancies, when hCG concentrations are several orders of magnitude higher than TSH. This provides an explanation to cases of pregnancy hyperthyroidism.15,15

The TSHR contains six sites for N-glycosylation, of which four have been shown to be effectively glycosylated.105 The functional role of the individual carbohydrate chains is still debated. It is likely that they contribute to the routing and stabilization of the receptor through the membrane system of the cell.

Alone among the glycoprotein hormone receptors, the TSHR undergoes cleavage of its ectodomain, severing it from the serpentine domain.106 This phenomenon has been related to the presence in the TSHR of a 50-amino-acid “insertion” for which there is no counterpart in the FSHR or LH/CGR. The initial cleavage step, by a metalloprotease, takes place around position 314 (within the 50-amino-acid insertion), followed by nibbling of the aminoterminal extremity of the serpentine-containing portion.107,108 The aminoterminal and serpentine portions of the resulting dimer remain bound to each other by disulphide bonds. The functional meaning of this TSHR-specific posttranslational modification remains unclear. Whereas all wild-type TSHR at the surface of thyrocytes seem to be in cleaved form, it has been shown that noncleavable mutant constructs are functionally undistinguishable from cleaved receptors when expressed in transfected cells.106 Noteworthy, when transiently or permanently transfected in nonthyroid cells, the wild-type human TSHR is present at the cell surface as a mixture of monomer and cleaved dimer. The cleavage and shedding of the ectodomain from a portion of the mature receptors has been related to the triggering or maintenance of autoimmunity in Graves’ disease.109,110

The TSH receptor is specifically inserted into the basolateral membrane of thyrocytes. This phenomenon involves signals encoded in the primary structure of the protein, as it is conserved when the receptor is expressed in the MDCK cell, a polarized cell of nonthyroid origin.111

The possibility that TSHR are present at the cell surface as “dimers of cleaved dimers” has recently been raised following demonstration that most rhodopsin-like GPCRs do dimerize112 (see following).

The TSH Receptor Gene

The gene coding for the human TSH receptor has been localized on the long arm of chromosome 14 (14q31).113,114 It spreads over more than 60 kb and is organized into 10 exons displaying an interesting correlation with the protein structure.104 The extracellular domain is encoded by a series of 9 exons, each of which corresponds to one or an integer number of leucine-rich repeat (LRR) motifs. The C-terminal half of the receptor containing the C-terminal part of the ectodomain and the serpentine domain is encoded by a single large exon. This finding is reminiscent of the fact that many G protein–coupled receptor genes are intronless. A likely evolutionary scenario derives from this gene organization115: the glycoprotein hormone receptor genes would have evolved from the condensation of an intronless classic G protein–coupled receptor with a mosaic gene encoding a protein with LRRs. Triplication of this ancestral gene and subsequent divergence led to the receptors for LH/CG, FSH, and TSH. The existence of 10 exons in both the TSH and FSH receptor genes (as opposed to the 11-exon LH/CG receptor), suggests the following phylogeny: one ancestral glycoprotein hormone receptor duplicating in the LH receptor and in the ancestor of TSH and FSH receptor, which lost one intron. The latter would later duplicate yielding the TSH and FSH receptors.

The gene promoter has been cloned and sequenced in humans and rats.115,116 It has characteristics of “housekeeping” genes in that it is GC-rich and devoid of TATA boxes; in the rat it was shown to drive transcription from multiple start sites.116

Expression of the TSH receptor gene is largely thyroid specific. Constructs made of a chloramphenicol acetyltransferase reporter gene under control of the 5′ flanking region of the rat gene show expression when transfected into FRTL5 cells and FRT cells but not into nonthyroid HeLa or a rat liver cell line (BRL) cells.116 However, TSH receptor mRNA has been clearly demonstrated in fat tissue of the guinea pig117 and following differentiation of preadipocytes into adipocytes.118,119 Functional significance in man of reports showing its presence in lymphocytes, extraocular tissue, and recently in cartilage and bone requires additional studies.109,120

Recently, functional expression of the TSH receptor has been demonstrated in pars tuberalis of the pituitary in quails and ependymal cells in mice, in relation with the control of photoperiodic behavior.118,119,121,122 No extrapolation of these studies to man has been attempted yet. Expression of the TSH receptor in thyroid cells is extremely robust. It is moderately up-regulated and down-regulated by TSH in vitro123 and down-regulated by iodide in vivo.101

Functional Aspects

Recognition of the Receptor by TSH

The three-dimensional structures are available for hCG and FSH,124-126 which allows accurate modelization of TSH on these templates. The crystal structure of the human FSHR-FSH complex127 has confirmed that the ectodomain of glycoprotein hormone receptors belongs to the family of proteins with LRRs, as was previously suggested by sequence analysis and homology modeling.128 The concave inner surface of the receptor (Fig. 3-4A) is an untwisted, noninclined β sheet formed by ten LRRs. Whereas the N-terminal portion of the β sheet (LRR1-7) is nearly flat, the C-terminal portion (LRR7-10) has the horseshoe-like curvature of LRR proteins. The crystal structure of part of the TSHR ectodomain in complex with a thyroid-stimulating autoantibody has recently been obtained.129 Notably, both the structure of the ectodomain of TSHR and the receptor-binding arrangements of the autoantibody are very similar to those reported for the FSHR-FSH complex. The ectodomain of glycoprotein hormone receptors also contains, downstream of the LRR region, a cysteine cluster domain of unknown structure (the hinge region), involved in receptor inhibition/activation and containing sites for tyrosine sulfation important for hormone binding (see later).

Extensive amino acid substitutions by site-directed mutagenesis of the Xi residues in the LRR portion of the TSHR for their counterparts in the LH/CGR have been performed.130 Exchanging eight amino acids of the TSHR for the corresponding LH/CGR residues resulted in a mutant displaying a sensitivity to hCG matching that of the wild-type LH/CGR. Surprisingly, while gaining sensitivity to hCG, the mutant kept a normal sensitivity to TSH, making it a dual-specificity receptor. It is necessary to exchange 12 additional residues to fully transform it into a bona fide LH/CGR.130 From an evolutionary point of view, these observations indicate that nature has built recognition specificity of hormone-receptor couples on both attractive and repulsive residues, and that residues at different homologous positions have been selected to this result in the different receptors.

Inspection of electrostatic surface maps of models of the wild-type TSH and LH/CG receptors and some of the mutants is revealing in this respect.130,131 The LH/CGR displays an acidic groove in the middle of its horseshoe, extending to the lower part of it (corresponding to the C-terminal ends of the β strands). Generation of a similar distribution of charges in the dual-specificity and reverse-specificity TSHR mutants suggests that this is important for hCG recognition. A detailed modelization of the interactions between TSH and the ectodomain of its receptor has been realized.131

In addition to the hormone-specific interactions genetically encoded in the primary structure of glycoprotein hormones receptors and their ligands, the importance of nonhormone-specific ionic interactions has been demonstrated, involving sulfated tyrosine residues present in the ectodomains of all three receptors.132,133 Sulfation in the TSHR takes place on both tyrosine residues of a conserved Tyr-Asp-Tyr motif located close to the border between the ectodomain and the first transmembrane helix (Fig. 3-5). However, only sulfation of the first tyrosine of the motif seems to be functionally important,132 contributing crucially to the binding affinity without interfering with specificity. The functional role of this posttranslational modification of the TSH receptor has been confirmed by demonstration of profound hypothyroidism due to resistance to TSH in mice with inactivation of Tpst2, one of the enzymes responsible for tyrosine sulfation.134,135

Activation of the Serpentine Portion of the TSH Receptor

Because it belongs to the rhodopsin-like GPCR family and displays many of the cognate signatures in primary structure, the serpentine portion of the TSH receptor is likely to share with rhodopsin common mechanisms of activation.136,137 However, sequence signatures characteristic of the serpentine portion of the glycoprotein hormone receptors suggest the existence of idiosyncrasies associated with specific mechanisms of activation (see Fig. 3-5). In addition, the TSHR has been found to be activated by a wide spectrum of gain-of-function mutations.138140 Compilations of available data identify more than 30 residues, the mutation of which cause constitutive activation. Since many somatic mutations affecting a given residue have been found repeatedly, it is likely that we are close to a saturation map for spontaneous gain-of-function mutations. Attempts to translate this map into mechanisms of transition between inactive and active conformations of the receptor have been made, in the light of rhodopsin structural data. Three sequence patterns affected by gain-of-function mutations deserve special mention and may help in understanding how the TSH receptor is activated.

The first is centered on an aspartate in position 6.44 (Asp633) at the cytoplasmic side of transmembrane helix VI (TM-VI). When mutated to a variety of amino acids, the result is constitutive activation.138,141,142 This suggested that the gain of function resulted from the breakage of (a) bond(s) rather than the creation of novel interaction(s) by the mutated residue, and the main partner of Asp6.44 was identified as Asn7.49 in transmembrane helix VII. From a series of site-directed mutagenesis studies,141,143 it was tentatively concluded that in the inactive conformation of the TSH receptor, the side chain of Asp7.49 is normally “sequestered” by both Thr6.43 and Asp6.44, and that the active conformation(s) require(s) establishment of novel interaction(s) of N7.49 involving most probably Asp in position 2.50.

The second: Glutamate 3.49 and arginine 3.50 of the highly conserved “D/ERY/W” motif at the bottom of TM-III form an ionic lock with aspartate 6.30 at the cytoplasmic end of TM-VI. Disruption of this ionic lock (e.g., by mutations affecting Asp6.30) leads to constitutively active mutant receptors.143 Thus the movements of TM-III and TM-VI at the cytoplasmic side of the membrane (i.e., presumably an opening of the receptor) is necessary for receptor activation.144 The existence of this ionic lock has, however, been questioned recently, since it is not found in the determined structure of the β2-adrenergic receptor.136

The third: Serine 281 belongs to a GPHR-specific “YPSHCCAF” sequence signature located at the carboxyl-terminal end of the LRR portion in the ectodomain of the receptors (see Fig. 3-5). After mutation of this serine residue had been shown to activate the TSH receptor constitutively,145 this segment, sometimes referred to as the hinge motif, was shown to play an important role in activation of all three glycoprotein hormone receptors.146 The functional effect of substitutions of S281 in the TSHR likely results in a “loss of structure” locally, since the more destructuring the substitutions, the stronger the activation.146,147 This observation, together with results showing that mutation of specific residues in the extracellular loops of the TSHR cause constitutive activation,148 and the previous demonstration of activation of TSH receptor by extracellular trypsinization,149,150 led to the hypothesis that activation of the receptor could involve the rupture of an inhibitory interaction between the ectodomain and the serpentine domain (see later).145

Interaction Between the Ectodomain and the Serpentine Domain

Mutant TSHR constructs devoid of the ectodomain displayed a phenotype compatible with partial activation, thus confirming the inhibitory effect of the ectodomain on the serpentine portion already suggested by the partial activation of the receptor by a trypsin treatment. However, activation of cAMP production in cells transfected with truncated constructs was much lower than after full stimulation of the wild-type receptor by TSH or by mutation of Ser281.151 These observations led to the following model for activation of the TSHR (Fig. 3-6).151,152 In the resting state, the ectodomain would exert an inhibitory effect on the activity of an inherently noisy rhodopsin-like serpentine, qualifying pharmacologically as an inverse agonist of the serpentine. Upon activation, by binding of the hormone or secondary to mutation of S281 in the hinge region, the ectodomain would switch from inverse agonist to full agonist of the serpentine portion. The ability of the strongest S281 mutants to fully activate the receptor in the absence of hormone suggests that the ultimate agonist of the serpentine domain would be the “activated” ectodomain, with no need for an interaction between the hormone and the serpentine domain. This model in which the “real” agonist of the serpentine domain would be the activated ectodomain is confirmed by the identification of a monoclonal antibody recognizing the ectodomain and endowed with inverse agonistic activity.153,154 Recent data suggest that the activation step might be achieved by interaction of charged residues of the α subunit of the hormone with acidic residues of the hinge region.155

Activation by Chorionic Gonadotropin

As indicated above, the sequence similarity between TSH and hCG, and between their receptors, allows for some degree of promiscuous stimulation of the TSH receptor by hCG during the first trimester of pregnancy, when hCG levels reach peak values. The inverse relation between TSH and hCG observed in most pregnant women is clear indication that their thyroid is subjected to the thyrotropic activity of hCG.15 Whereas this situation is usually associated with euthyroidism, thyrotoxicosis may develop in cases of excessive hCG production (as it occurs in twin pregnancies or hydatidiform moles), or in rare patients harboring a mutant TSH receptor with increased sensitivity to hCG.156

Of note, the bioactivity of human TSH (and of all glycoprotein hormones, including hCG) is lower than that of bovine TSH and of other nonprimate mammals. This is due to positive selection in higher primates of α subunits in which several key basic amino acids have been substituted.157 The observation that this phenomenon parallels the evolution of chorionic gonadotropin suggests that it may be related to protection against promiscuous stimulation of the TSH receptor by hCG during pregnancy.

Activation by Autoantibodies

Autoantibodies found in Graves’ disease and some types of idiopathic myxedema can stimulate (TSAb) or block (TSBAb) TSH receptor, respectively. Epitopes recognized by TSAbs are being identified from precise mapping of binding sites of murine or human monoclonal antibodies endowed with TSAb activity on the partial crystal structure of the ectodomain129 or models thereof.158 However, the actual mechanisms implicated in activation of the receptor by TSAbs (and by TSH) are still unknown. Although most TSAbs do compete with TSH for binding to the receptor,110,159 and despite similarity in interaction surfaces,129 the precise targets of the hormone and autoantibodies are likely to be different, at least in part. It has indeed been shown that sulfated tyrosine residues, which are important for TSH binding, are not implicated in recognition of TSH receptor by TSAbs.160 Also, contrary to TSH, most TSAbs from Graves’ patients display a delay in their ability to stimulate cAMP accumulation in transfected cells.46 The availability of TSAb preparations purified from individual patients should allow exploration of these issues in a direct fashion.161

Down-Regulation of the TSH Receptor

Desensitization of some G protein–coupled receptors has been shown to involve phosphorylation of specific residues by G protein–receptor kinases (homologous desensitization) or PKA (heterologous desensitization) enzymes.165 When compared with other G protein–coupled receptors, the TSH receptor contains few phosphorylatable serine or threonine residues in its intracellular loops and C-terminal tail, which probably accounts for the limited desensitization observed after stimulation by TSH. Acute desensitization of the receptor in the presence of TSH, presumably by phosphorylation, is weak and delayed.166 Its internalization is rapidly followed by recycling to the cell surfaces, while the hormone is degraded in lysosomes.167 Similarly, a weak down-regulation, confounded by the long life of both TSHR mRNA and protein, takes place but has little functional role.123 On the other hand, in the presence of constant stimulation, cAMP accumulation is down-regulated mostly as a consequence of phosphodiesterase induction.168 The persistence of hyperthyroidism in patients submitted to constant TSH stimulations in TSH-secreting pituitary adenomas or TSAb stimulation in Graves’ disease testifies to the weakness of these negative regulations.

Dimerization

As do most rhodopsin-like GPCRs, the glycoprotein hormone receptors have been demonstrated to dimerize/oligomerize by a variety of experimental approaches, including FRET, BRET, and functional complementation of mutants.169-171 The physiologic significance of this phenomenon remains unknown, but it has been shown to be associated with allosteric properties of the dimers/oligomers. TSH binding to the TSHR displays strong negative cooperativity,169 which is classically considered to account for a shallow concentration action curve extending over more than two orders of magnitude.

Control of Thyroid Function

Thyroid Hormone Synthesis

Thyroid hormone synthesis requires the uptake of iodide by active transport, thyroglobulin biosynthesis, oxidation and binding of iodide to thyroglobulin, and within the matrix of this protein, oxidative coupling of two iodotyrosines into iodothyronines. All these steps are regulated by the cascades just described.

Iodide Transport

Iodide is actively transported by the iodide Na+/I symporter (NIS) against an electrical gradient at the basal membrane of the thyrocyte and diffuses by a specialized channel (pendrin or another channel)172,173 from the cell to the lumen at the apical membrane. The opposite fluxes of iodide, from the lumen to the cell and from the cell to the outside, are generally considered to be passive and nonspecific. At least five types of control have been demonstrated.90,91,172

1. Rapid and transient stimulation of iodide efflux by TSH in vivo, which might reflect a general increase in membrane permeability. The cascade involved is not known.

2. Rapid activation of iodide apical efflux from the cell to the lumen by TSH. This effect, which contributes to the concentration of iodide at the site of its oxidation, is mediated—depending on the species—by Ca2+ and/or cAMP.172,174 In human cells, it is mainly controlled by Ca2+ and therefore by the TSH effect on phospholipase C.

3. Delayed increase in the capacity (Vmax) of the active iodide transport NIS in response to TSH. This effect is inhibited by inhibitors of RNA and protein synthesis and is due to activation of iodide transporter gene expression. This effect of TSH is reproduced by cAMP analogs in vitro and is therefore mediated by the cAMP cascade.6 Expression of mRNA is enhanced by TSH and cAMP and decreased by iodide.101,175 TSH enhancement of thyroid blood flow, more or less delayed depending on the species, also contributes to increase in the uptake of iodide.6 Blood flow is inversely related to iodine levels in the thyroid.176

4. Rapid inhibition by iodide of its own transport in vivo and in vitro. This inhibitory effect requires an intact transport and oxidation function—that is, it fulfills the criteria of an XI effect. After several hours, the capacity of the active transport mechanism is greatly impaired (adaptation to the Wolff-Chaikoff effect).91 The mechanism of the first effect is unknown but probably initially involves direct inhibition of the transport system itself (akin to the desensitization of a receptor), followed later by inhibition of NIS gene expression and NIS synthesis (akin to the down-regulation of a receptor).101

5. Inhibition by iodide of the thyroid blood flow. This effect does not fit the XI paradigm, since it takes place in patients treated with thyroperoxidase inhibitors.

Iodide Binding to Protein and Iodotyrosine Coupling

Iodide oxidation and binding to thyroglobulin and iodotyrosine coupling in iodothyronines are catalyzed by the same enzyme, thyroperoxidase, with H2O2 used as a substrate.177 The same regulations therefore apply to the two steps. H2O2 is generated by an NADPH-oxidase system constituted by two DUOX proteins.178,179 The system is very efficient in the basal state inasmuch as little of the iodide trapped can be chased by perchlorate in vivo. Also, in vitro the amount of iodine bound to proteins mainly depends on the iodide supply. Nevertheless, in human thyroid in vitro, stimulation of the iodination process takes place even at low concentrations of the anion, thus indicating that iodination is a second limiting step. Such stimulation is caused in all species by intracellular Ca2+ and is therefore a consequence of activation of the Ca2+-PIP2 cascade. In many species, phorbol esters and diacylglycerol, presumably through protein kinase C, also enhance iodination.180 It is striking that in a species such as the human, in which TSH activates the PIP2 cascade, cAMP inhibits H2O2 generation and iodination, whereas in a species (dog) in which TSH activates only the cAMP cascade, cAMP enhances iodination. Obviously, in the latter species, a supplementary cAMP control was necessary.180,181

Thyroperoxidase does not contain any obvious phosphorylation site in its intracellular tail. On the other hand, all the agents that activate iodination also activate H2O2 generation, and inhibition of H2O2 generation decreases iodination, which suggests that iodination is a substrate–driven process and that it is mainly controlled by iodide supply and H2O2 generation.180,182 Congruent with the relatively high Km of thyroperoxidase for H2O2, H2O2 is generated in disproportionate amounts with regard to the quantity of iodide oxidized. Negative control of iodination by iodide (the Wolff-Chaikoff effect) is accompanied and mostly explained by the inhibition of H2O2 generation. This effect of I is relieved by perchlorate and methimazole and thus pertains to the XI paradigm.95,180

Iodotyrosine coupling to iodotyrosines is catalyzed by the same system and is therefore subject to the same regulations as iodination. However, coupling requires that suitable tyrosyl groups in thyroglobulin be iodinated (i.e., that the level of iodination of the protein be sufficient). In the case of severe iodine deficiency or when thyroglobulin exceeds the iodine available, insufficient iodination of each thyroglobulin molecule will preclude iodothyronine formation, whatever the activity of the H2O2 generating system and thyroperoxidase. On the other hand, when the iodotyrosines involved in the coupling are present, coupling is controlled by the H2O2 concentration but independent of iodide.177 In this case, H2O2 control has significance even at very low iodide concentrations.

Generation of H2O2 requires the reduced form of nicotinamide adenine dinucleotide phosphate (NADPH) as a coenzyme and is thus accompanied by NADPH oxidation. Limitation of the activity of the pentose phosphate pathway by NADP+ insures that NADPH oxidation for H2O2 generation causes stimulation of this pathway. Also, excess H2O2 leaking back into the thyrocyte is reduced mainly by catalase but also by glutathione (GSH) peroxidase, and the oxidized GSH (GSSG) produced is reduced by NADPH-linked GSH reductase. Thus both the generation of H2O2 and to some extent the disposal of excess H2O2 by pulling NADP reduction and the pentose pathway lead to activation of this pathway—historically one of the earliest and unexplained effects of TSH.6,180

In the long term, in vivo or in vitro, the activity of the whole iodination system obviously also depends on the level of its constitutive enzymes. It is therefore not surprising that activation of thyrocytes by the cAMP cascade increases the thyroperoxidase gene expression, whereas dedifferentiating treatments with EGF and phorbol esters inhibit this expression and thus reduce the capacity and activity of the system. Apparent discrepancies in the literature about the effects of phorbol esters on iodination are mostly explained by the kinetics of these effects (acute stimulation of the system, delayed inhibition of expression of the involved genes).

Thyroid Hormone Secretion

Secretion of thyroid hormone requires endocytosis of human thyroglobulin, its hydrolysis, and the release of thyroid hormones from the cell. Thyroglobulin can be ingested by the thyrocyte by three mechanisms.6,183185

In macropinocytosis, which is the first, pseudopods engulf clumps of thyroglobulin. In all species this process is triggered by acute activation of the camp/PKA cascade and therefore by TSH.186 Inactivation of the Rho small G proteins, resulting in microfilament depolarization and stress-fiber disruption, is probably involved in the process.62 Stimulation of macropinocytosis is preceded and accompanied by an enhancement of thyroglobulin exocytosis and thus of membrane to the apical surface.182,187,188 By micropinocytosis, the second process, small amounts of colloid fluid are ingested. This process does not appear to be greatly influenced by acute modulation of the regulatory cascades. It is enhanced in chronically stimulated thyroids and thyroid cells with induction of vesicle transport proteins Rab 5 and 7.189,190 It probably accounts for most of basal secretion. A third (hypothesized) process is receptor-mediated endocytosis; it would be enhanced in chronically stimulated thyroid cells.191193 The protein involved could be megalin194 and/or asialoglycoprotein. This process probably accounts for transcytosis of thyroglobulin.195

Contrary to the last named, the first two processes are not specific for the protein. They can be distinguished by the fact that macropinocytosis is inhibited by microfilament and microtubule poisons and by lowering of the temperature (below 23° C). Whatever its mechanism, endocytosis is followed by lysosomal digestion, with complete hydrolysis of thyroglobulin. The main iodothyronine in thyroglobulin is thyroxine. However, during its secretion, a small fraction is deiodinated by type I 5-deiodinases (DIO1 and DIO2) to triiodothyronine (T3), thus increasing relative T3 (the active hormone) secretion.196

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