Article in press - uncorrected proof Biol. Chem., Vol. 388, pp. 1053–1059, October 2007 • Copyright ᮊ by Walter de Gruyter • Berlin • New York. DOI 10.1515/BC.2007.122 Selenoproteins of the thyroid gland: expression, localization and possible function of glutathione peroxidase 3 Cornelia Schmutzler1,*, Birgit Mentrup1,a, Lutz Introduction: the thyroid gland, thyroid Schomburg1, Cuong Hoang-Vu2, Volker Herzog3 hormones and selenium and Josef Ko¨ hrle1 The main physiological function of the thyroid gland is 1 Institut fu¨ r Experimentelle Endokrinologie, Charite´– the synthesis of thyroid hormones (TH), for which it is the Universita¨ tsmedizin Berlin, Charite´ platz 1, exclusive source in the vertebrate body. THs are key D-10117 Berlin, Germany regulators of development, growth, and differentiation, 2 Experimentelle und Chirurgische Onkologie, Martin- particularly of the nervous system, as well as many phys- Luther-Universita¨ t Halle/Wittenberg, Magdeburger Str. iological processes in the adult, such as body tempera- 18, D-06097 Halle/Saale, Germany ture, heart activity and energy metabolism. THs are 3 Institut fu¨ r Zellbiologie, Rheinische Friedrich-Wilhelms- synthesized by the epithelial cells of the thyroid gland, Universita¨ t, Ulrich-Haberland-Str. 61a, D-53121 Bonn, the thyrocytes (Taurog, 2005). These cells line the lumen Germany of the thyroid follicles, which contain the so-called col- * Corresponding author loid, mainly consisting of thyroglobulin (Tg) and small e-mail: [email protected] amounts of other proteins. The three crucial steps of TH synthesis are all catalyzed by a single enzyme, the heme protein thyroid peroxidase (TPO) and comprise: (1) the oxidation of iodide; (2) the iodination of ‘hormonogenic’ tyrosine residues of Tg; and (3) the oxidative coupling of two iodinated tyrosine residues to give Tg-bound TH thy- Abstract roxine (T4) and triiodo-thyronine (T3). Tg is secreted in dimeric form (2=330 kDa, 19 S) towards the apical lumen The thyroid gland has an exceptionally high selenium of the thyroidal follicle. After iodination and coupling, the content, even during selenium deficiency. At least 11 colloidal Tg with the bound TH may be proteolyzed either selenoproteins are expressed, which may be involved in in the follicular space or within lysosomes after micro- the protection of the gland against the high amounts of pinocytotic uptake. This eventually leads to TH release into the circulation. Alternatively, Tg forms highly poly- H2O2 produced during thyroid hormone biosynthesis. As determined here by in situ hybridization and Northern merized ‘storage Tg’ deposited in the colloid lumen (see blotting experiments, glutathione peroxidases (GPx) 1 below). Thus, Tg is another central player in the reaction and 4 and selenoprotein P were moderately expressed, sequence of TH biosynthesis. occurring selectively in the follicular cells and in leuko- An essential co-substrate for TPO in this pathway is H O , which provides oxidative equivalents for TPO and cytes of germinal follicles of thyroids affected by Hashi- 2 2 is produced in high amounts by the NADPH-dependent moto’s thyroiditis. Selenoprotein 15 was only marginally flavoproteins thyroid oxidase DUOX (dual oxidases) 1 expressed and distributed over all cell types. GPx3 and 2. Thus, as a consequence of its normal physiolog- mRNA was exclusively localized to the thyrocytes, ical activity, the gland is continuously exposed to relevant showed the highest expression levels and was down- concentrations of H O (in addition to the ‘normal’ share regulated in 5 of 6 thyroid cancer samples as compared 2 2 of a cell, which is contributed by mitochondrial metabo- to matched normal controls. GPx3 could be extracted lism) and to H O -derived reactive oxygen species (ROS) from thyroidal colloid by incubation with 0.5% sodium 2 2 probably arising as by-products of these processes. dodecyl sulfate indicating that this enzyme is (i) secreted Although TH synthesis is compartmentalized to the into the follicular lumen and (ii) loosely attached to the lumen of the follicles, and both the DUOX enzymes and colloidal thyroglobulin. These findings are consistent with TPO are localized to the apical membrane of the thyro- a role of selenoproteins in the protection of the thyroid cyte, H O can freely diffuse into the cytoplasm and from possible damage by H O . Particularly, GPx3 might 2 2 2 2 nucleus, where it may lead to aberrant oxidation and iodi- use excess H2O2 and catalyze the polymerization of thyroglobulin to the highly cross-linked storage form nation of proteins and lipids, trigger apoptosis and induce DNA damage. However, the H O produced is present in the colloid. 2 2 consumed at least in part to form a special ‘storage Tg’. This form consists of highly polymeric Tg cross-linked by Keywords: deiodinase; glutathione peroxidase 1; intra- and intermolecular disulfide bonds and is depos- glutathione peroxidase 3; glutathione peroxidase 4; ited in the follicular lumen in protein concentrations of up selenoprotein 15; selenoprotein P; thyroglobulin. to 590 mg/ml as insoluble protein globules (Berndorfer et al., 1996). In the current model, it is assumed that the aPresent address: Max-Planck-Institut fu¨ r Molekulare Genetik, formation of these covalent bonds is also catalyzed by Fabeckstr. 60-62, D-14195 Berlin, Germany. TPO. Bereitgestellt von | Universitäts- und Landesbibliothek Bonn (Universitäts- und Landesbibliothek Bonn) 2007/218 Angemeldet | 172.16.1.226 Heruntergeladen am | 12.07.12 14:38 Article in press - uncorrected proof 1054 C. Schmutzler et al. The thyroid gland, together with the brain and several stimulation by TSH triggering of compensatory growth of other endocrine tissues, is among the organs with the the thyroid as well as – inadequately – increased H2O2 highest selenium (Se) content in vertebrates (Dickson and production for TH synthesis. However, the latter cannot Tomlinson, 1967; Behne et al., 1988). Furthermore, under be counter-regulated due to inadequate function of H2O2- conditions of Se deficiency, the gland retains or even scavenging selenoenzymes, resulting in a ‘vicious circle’ accumulates Se, as shown in rats exposed to nutritional of stimulation and increased tissue damage followed by Se shortage (Behne et al., 1988; Bermano et al., 1995). thyroid necrosis and, finally, fibrosis. Other factors, such These findings have been impressively verified in a as deficiency in further trace elements, may also contrib- genetic model for Se deficiency, the selenoprotein P ute to pathogenesis. In rats, Se deficiency aggravates the (SePP) knockout mouse, which lacks the plasma protein susceptibility to inflammation and necrosis of the thyroid SePP, the main Se distributor in the vertebrate organism. gland caused by iodide overload in iodine-deficient thy- SePP knockout mice display decreased serum Se levels, roid glands; TGFb seems to play a prominent role in this with manifest growth defects and neurological abnor- process (reviewed by Ko¨ hrle et al., 2005). However, con- malities indicating Se deficiency in the central nervous tradictory data have also been published (Colzani et al., system (Hill et al., 2003; Schomburg et al., 2003). Sur- 1999). prisingly, however, thyroid Se concentration, thyroid Thyroid carcinoma, although it comprises only 1% of glutathione peroxidase (GPx) activity, thyroid gland mor- all cancers diagnosed, is the most frequent malignant phology, and serum levels of T4, T3 and TSH were within disease of the endocrine system, and the anaplastic var- normal range. Thus, the thyroid gland occupies a partic- iant is one of the most deadly cancers, with a mean sur- ularly high rank in the Se hierarchy that regulates the pri- vival rate of only a few months after diagnosis (Pasieka, ority according to which various tissues obtain their share 2003). The Norwegian JANUS control study demonstrat- of a limited pool of Se. Accordingly, at least 11 seleno- ed an elevated risk of developing thyroid carcinoma proteins occur in the thyroid gland (Behne et al., 1988), associated with prediagnostically low plasma Se concen- including 59-deiodinases type I and type II (59DI, 59DII), trations (Glattre et al., 1989). Another study examined glutathione peroxidases GPx1, GPx3 and GPx4, thiore- concentrations of minerals in the blood and thyroid tissue doxin reductase (TrxR), the Se transport protein SePP of several patients with thyroid diseases and found that and selenoprotein 15 (SeP15), where they may catalyze patients with thyroid cancer had the lowest mean Se lev- redox reactions involved in various physiological pro- els (Kucharzewski et al., 2003). cesses such as the scavenging of ROS, TH metabolism Accordingly, in thyroid cancer, the expression of vari- and others (Ko¨ hrle, 2005; Ko¨ hrle et al., 2005). ous selenoproteins is dysregulated. 59DI expression and The observations summarized above indicate that an enzyme activity are decreased or absent in thyroid car- adequate Se supply is required for proper thyroid func- cinoma tissues and cell lines (Ko¨ hrle, 1997). DNA array tion. This conclusion is supported by additional data link- data indicate that 59DI is down-regulated in follicular thy- ing several pathological conditions of the thyroid gland roid adenoma compared to follicular thyroid carcinoma, to Se status. For example, patients with autoimmune thy- and the same is true for SePP (Barden et al., 2003). Sele- roiditis (AITD) seem to benefit from Se supplementation. nium binding protein 1 (SELENBP1; Chang et al.,