Local activation and inactivation of thyroid hormones: the deiodinase family

The evolution of transthyretin synthesis in the choroid plexus

Choroid plexus has the highest concentration of transthyretin (TTR) mRNA in the body, 4.4 microg TTR mRNA/g wet weight tissue, compared with 0.39 microg in the liver. The proportion of TTR to total protein synthesis in choroid plexus is 12%. All newly synthesized TTR is secreted towards the ventricles. Net transfer of T4 occurs only towards the ventricle and depends on ongoing protein synthesis. Thyroxine-binding globulin (TBG), TTR and albumin form a "buffering" system for plasma [T4] because of their overlapping affinities and on/off rates for L-thyroxine (T4)-binding. The individual components of this network determining T4 distribution are functionally highly redundant. Absence of TBG (humans), or TTR (mice), or albumin (humans, rats) is not associated with hypothyroidism. Natural selection is based on small, inheritable alterations improving function. The study of these alterations can identify function. TTR genes were cloned and sequenced for a large number of vertebrate species. Systematic, stepwise changes during evolution occurred only in the N-terminal region, which became shorter and more hydrophilic. Simultaneously, a change in function occurred: TTR affinities for T4 are higher in mammals than in reptiles and birds. L-triiodothyronine (T3) affinities show the opposite trend. This favors site-specific regulation of thyroid hormones by tissue-specific deiodinases in the brain.

Central stimulatory effect of leptin on T3 production is mediated by brown adipose tissue type II deiodinase

To assess whether intracerebroventricular leptin administration affects monodeiodinase type II (D2) activity in the tissues where it is expressed [cerebral cortex, hypothalamus, pituitary, and brown adipose tissue (BAT)], hepatic monodeiodinase type I (D1) activity was inhibited with propylthiouracil (PTU), and small doses of thyroxine (T4; 0.6 nmol. 100 g body wt(-1). day(-1)) were supplemented to compensate for the PTU-induced hypothyroidism. Two groups of rats were infused with leptin for 6 days, one of them being additionally treated with reverse triiodothyronine (rT3), an inhibitor of D2. Control rats were infused with vehicle and pair-fed the amount of food consumed by leptin-infused animals. Central leptin administration produced marked increases in D2 mRNA expression and activity in BAT, changes that were likely responsible for increased plasma T3 and decreased plasma T4 levels. Indeed, plasma T3 and T4 concentrations were unaltered by central leptin administration in the presence of rT3. The additional observation of a leptin-induced increased mRNA expression of BAT uncoupling protein-1 suggested that the effect on BAT D2 may be mediated by the sympathetic nervous system.

Early expression of thyroid hormone deiodinases and receptors in human fetal cerebral cortex

Thyroid hormones are known to be important for optimal development of the human central nervous system. Classically, maternal thyroid hormones have not been thought to have a major role in defining central nervous system development. However, recent epidemiological evidence has indicated that subtle deficiencies in circulating maternal thyroid hormones in the first trimester of pregnancy are associated with adverse neurodevelopment. We have used real time PCR to quantitate the expression of mRNAs encoding the thyroid receptor isoforms (TR alpha1, alpha2, beta1 and beta2) and thyronine deiodinase subtypes (5'-DI, 5'-DII and 5-DIII) in human fetal cerebral cortex from the first and second trimesters of pregnancy. Deiodinase subtype activities have also been determined in these tissues and compared to 'normal' adult human cerebral cortex. Iodothyronine deiodinase mRNAs were expressed in human fetal cerebral cortex from 7 to 8 weeks of gestation. The expression of 5'-DI mRNA was variable in fetal life but increased relative to adult cortex (P<0.05), whereas the activity of this enzyme was below the level of assay detection. 5'-DII mRNA and activity in fetal cerebral cortex was detectable from as early as 7-8 weeks but not significantly different from that in adult life except at 15-16 weeks when mRNA expression increased (P<0.05). Fetal cortex 5-DIII mRNA expression was present from the early first trimester but less abundant than in adult tissue (P<0.01) and 5-DIII activity appeared greater in fetal cortex (P<0.01) as compared to adults. Only TR alpha1 mRNA was more abundantly expressed in fetal cortex than adult tissues (P<0.01). In contrast, the TR isoforms (alpha2 and beta1) were expressed significantly less than in adult tissues (P<0.05). Only 26% of fetal cerebral cortex samples expressed TR beta 1. There is evidence that the developing fetal brain, as early as the first trimester, expresses TRs and exhibits the mechanisms of pre-receptor control of thyroid hormone supply.

Genomic imprinting contributes to thyroid hormone metabolism in the mouse embryo

Many genes subject to genomic imprinting function in a number of endocrine/paracrine pathways that are important for normal mammalian development. Here, we show that an endocrine/paracrine pathway involving thyroid hormone metabolism is also regulated by imprinting. Thyroid hormone action depends on thyroid hormone receptors and their predominant ligand, 3,5,3'-triiodothyronine (T3). In vivo, thyroid hormone levels are maintained within the physiological range through the interaction of three iodothyronine deiodinases, D1, D2, and D3. D3 inactivates thyroxine (T4) and T3 by 5-deiodination, and the gene for this enzyme, Dio3, lies in the imprinted domain on human chromosome 14q32/distal mouse chromosome 12. Here, we report the imprinting of Dio3, which is expressed preferentially from the paternal allele. No differentially methylated region was identified in the CpG-island promoter, which is completely unmethylated. Localization of transcripts suggests that Dio3 may be exerting its function in both endocrine and autocrine/paracrine manners. An assay was developed for T3, and we show that its levels in maternal and paternal uniparental disomy (UPD) 12 fetuses are reciprocally affected. These results demonstrate that disruption of the imprinting status of Dio3 results in abnormal thyroid hormone levels and may contribute to the phenotypic abnormalities in UPD12 mice and UPD14 humans.

Substitution of cysteine for a conserved alanine residue in the catalytic center of type II iodothyronine deiodinase alters interaction with reducing cofactor

Human type II iodothyronine deiodinase (D2) catalyzes the activation of T(4) to T(3). The D2 enzyme, like the type I (D1) and type III (D3) deiodinases, contains a selenocysteine (SeC) residue (residue 133 in D2) in the highly conserved catalytic center. Remarkably, all of the D2 proteins cloned so far have an alanine two residue-amino terminal to the SeC, whereas all D1 and D3 proteins contain a cysteine at this position. A cysteine residue in the catalytic center could assist in enzymatic action by providing a nucleophilic sulfide or by participating in redox reactions with a cofactor or enzyme residues. We have investigated whether D2 mutants with a cysteine (A131C) or serine (A131S) two-residue amino terminal to the SeC are enzymatically active and have characterized these mutants with regard to substrate affinity, reducing cofactor interaction and inhibitor profile. COS cells were transfected with expression vectors encoding wild-type (wt) D2, D2 A131C, or D2 A131S proteins. Kinetic analysis was performed on homogenates with dithiothreitol (DTT) as reducing cofactor. The D2 A131C and A131S mutants displayed similar Michaelis-Menten constant values for T(4) (5 nM) and reverse T(3) (9 nM) as the wt D2 enzyme. The limiting Michaelis-Menten constant for DTT of the D2 A131C enzyme was 3-fold lower than that of the wt D2 enzyme. The wt and mutant D2 enzymes are essentially insensitive to propylthiouracil [concentration inhibiting 50% of activity (IC(50)) > 2 mM] in the presence of 20 mM DTT, but when tested in the presence of 0.2 mM DTT the IC(50) value for propylthiouracil is reduced to about 0.1 mM. During incubations of intact COS cells expressing wt D2, D2 A131C, or D2 A131S, addition of increasing amounts of unlabeled T(4) resulted in the saturation of [(125)I]T(4) deiodination, as reflected in a decrease of [(125)I]T(3) release into the medium. Saturation first appeared at medium T(4) concentrations between 1 and 10 nM.

IN CONCLUSION

substitution of cysteine for a conserved alanine residue in the catalytic center of the D2 protein does not inactivate the enzyme in vitro and in situ, but rather improves the interaction with the reducing cofactor DTT in vitro.

A novel interaction between thyroid hormones and 1,25(OH)(2)D(3) in osteoclast formation

Thyroid hormones enhance osteoclast formation and their excess is an important cause of secondary osteoporosis. 3,5,3' -Triiodo-L-thyronine (T3) induced the mRNA expression of receptor activator of nuclear factor-kappa B ligand (RANKL), which is a key molecule in osteoclast formation, in primary osteoblastic cells (POB). This effect was amplified in the copresence of 1 alpha,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)). Although T3 alone did not induce octeoclasts in coculture of bone marrow cells with POB, T3 enhanced 1,25(OH)(2)D(3)-induced osteoclast formation. Thyroxine (T4) also enhanced 1,25(OH)(2)D(3)-induced osteoclast formation. These data suggested that T4 was locally metabolized to T3 for its action, since T4 is a prohormone with little hormonal activity. The mRNA expression of type-2 iodothyronine deiodinase (D2), which is responsible for maintaining local T3 concentration, was induced by 1,25(OH)(2)D(3) dose- and time-dependently. Our data would facilitate our understanding of the mechanism of osteoclast formation by thyroid hormones and suggest a novel interaction between thyroid hormones and 1,25(OH)(2)D(3).

Thyroid hormones promote chondrocyte differentiation in mouse ATDC5 cells and stimulate endochondral ossification in fetal mouse tibias through iodothyronine deiodinases in the growth plate

Thyroid hormones (THs), 3,3',5-triiodo-L-thyronine (T3) and L-thyroxine (T4), are important for the normal development of the growth plate (GP); congenital TH deficiency leads to severe dwarfism. In mouse chondrogenic cell line, ATDC5, T3 enhanced differentiation and increased Alizarin red staining, but did not affect Alcian blue staining. In organ-cultured mouse tibias, THs stimulated the cartilage growth, especially in the hypertrophic zone. Interestingly, T4 was as equally potent as T3 in organ-cultured tibias, which suggests that T4 is metabolized locally to T3, because T4 is a prohormone and must be converted to T3 for its activity. Two enzymes catalyze the conversion; type I deiodinase (D1) and type II deiodinase (D2). D1 has a ubiquitous distribution and D2, with a high affinity for T4, is present where the maintenance of intracellular T3 concentration is critical. Messenger RNAs (mRNAs) for D1 and D2 were detected in neonatal mouse tibias and ATDC5 cells. The enzyme activity was unaffected by the D1 inhibitor 6-propyl-2-thiouracil, suggesting that D2 mainly catalyzes the reaction. D2 mRNA was detected in differentiated ATDC5 cells. In organ-cultured mouse tibias, D2 activity was greater at later stages. In contrast, thyroid hormone receptors (TRs) were expressed in neonatal mouse tibias and ATDC5 cells, but their expression levels in ATDC5 cells were stable throughout the culture periods. Therefore, increased T3 production at later stages by D2 is likely to contribute to the preferential effects of THs in the terminal differentiation of GP. This article is the first to show that T4 is activated locally in GP and enhances the understanding of TH effects in GP.

Association between a novel variant of the human type 2 deiodinase gene Thr92Ala and insulin resistance: evidence of interaction with the Trp64Arg variant of the beta-3-adrenergic receptor

Thyroid hormone action is an important determinant of energy and glucose metabolism. T4 metabolism is regulated by the deiodinases of which type 2 is expressed in humans in skeletal muscle and brown adipose tissue, where its transcription is stimulated by the beta-3 adrenergic pathway. We performed molecular scanning of the human type 2 deiodinase (DIO2) gene and evaluated a novel variant for associations with obesity and insulin resistance, assessing both the main effect and interaction with the Trp64Arg beta-3--adrenergic receptor (ADRB3) variant. Molecular scanning of DIO2 in 50 obese Caucasians demonstrated a Thr92Ala variant. Association studies in 972 nondiabetic patients, 135 of whom underwent euglycemic-hyperinsulinemic clamps, showed that subjects with the Thr92Ala variant had lower glucose disposal rate (0.54 plus minus 0.02 mg center dot min(-1) center dot kg(-1) fat-free mass Ala92 homozygotes vs. 0.44 plus minus 0.02 Ala92 heterozygotes vs. 0.42 plus minus 0.04 Thr92 homozygotes, P = 0.0088). Association analysis of the entire group showed significant evidence for a synergistic effect between the Thr92Ala DIO2 and Trp64Arg ADRB3 variants on BMI (both variants 34.3 plus minus 0.9 kg/m(2) vs. neither variant 33.1 plus minus 0.4 kg/m(2), P = 0.04 for interaction). To our knowledge, Thr92Ala is the first description of a missense mutation of DIO2. This variant strongly associates with insulin resistance and, in subjects with the Trp64Arg ADRB3 variant, an increased BMI, suggesting an interaction between these two common gene variants.

Neurodevelopmental control by thyroid hormone receptors

Recent studies have provided insights into the neurodevelopmental functions of thyroid hormone signaling. The nuclear thyroid hormone receptors (TRs) are ligand-activated transcription factors and a variety of TR isotypes, generated by two genes, mediate distinct processes. In addition, deiodinase enzymes that regulate levels of the main active form of thyroid hormone, T3, are likely to cooperate closely with TRs in specifying a localized and timely response to thyroid hormones in target tissues. Some of the most sensitive processes controlled by these pathways are in the auditory and visual sensory systems.

Environmental salinity selectively modifies the outer-ring deiodinating activity of liver, kidney and gill in the rainbow trout

We here analyzed the effect of a mild hyperosmotic challenge on the activities of deiodinases type I (D1) and II (D2) in the trout liver, and D1 in kidney and gill, two organs involved in osmoregulation. FW-adapted immature rainbow trout were transferred to 5 per thousand SW and killed 0.5, 1, 2, 4, 8 12, 24 and 48 h post-transfer (PT). Fish maintained in FW served as controls. Hepatic, renal and branchial D1 and hepatic D2 activities were assessed as well as circulating levels of T(3), T(4) and cortisol. Hyperosmotic challenge elicited significant and sustained decreases in kidney D1 and liver D2 activities at 8 h PT, which returned to control values at 48 h PT. In contrast, liver and gill D1 activities exhibited no significant change throughout the study. Also, significant increases in circulating T(4) at 2-4 and 48 h PT were observed. Circulating T(3) remained unmodified until 24-48 h PT, when it rose sharply. Simultaneously, cortisol showed a trend towards increase during the initial 4 h PT, which attained significance at 48 h PT. The present findings demonstrate that a mild hypertonic challenge is sufficient to elicit responses in the trout thyroidal axis. Hormonal changes in the circulatory compartment are in accordance with those previously described for migratory salmonids. A novel aspect of our findings is the organ-specific differential response exhibited by ORD-enzymes when trout are exposed to a mildly different osmotic environment. Our findings further establish the uniqueness of fish thyroid physiology, and can be of value in further understanding the evolutionary aspects of this ORD family of deiodinases.

Biochemistry, cellular and molecular biology, and physiological roles of the iodothyronine selenodeiodinases

The goal of this review is to place the exciting advances that have occurred in our understanding of the molecular biology of the types 1, 2, and 3 (D1, D2, and D3, respectively) iodothyronine deiodinases into a biochemical and physiological context. We review new data regarding the mechanism of selenoprotein synthesis, the molecular and cellular biological properties of the individual deiodinases, including gene structure, mRNA and protein characteristics, tissue distribution, subcellular localization and topology, enzymatic properties, structure-activity relationships, and regulation of synthesis, inactivation, and degradation. These provide the background for a discussion of their role in thyroid physiology in humans and other vertebrates, including evidence that D2 plays a significant role in human plasma T(3) production. We discuss the pathological role of D3 overexpression causing "consumptive hypothyroidism" as well as our current understanding of the pathophysiology of iodothyronine deiodination during illness and amiodarone therapy. Finally, we review the new insights from analysis of mice with targeted disruption of the Dio2 gene and overexpression of D2 in the myocardium.

Inhibition of type 2,5'-deiodinase by tumor necrosis factor alpha, interleukin-6 and interferon gamma in human thyroid tissue

The effects of tumor necrosis factor alpha (TNFalpha), interleukin-6 (IL-6) and interferon gamma (IFNgamma) were studied on the activity of type 2,5'-deiodinase and on the binding of [125I] T(4) to proteins in human thyroid cytosolic (supernatant) and membrane (pellet) fractions. The activity of thyroid type 2,5'-deiodinase was measured by iodothyronine outer ring deiodinase assay. The binding of [125I] T(4) to the proteins of thyroid cytosolic and membrane fractions was determined by autoradiography. The results showed that thyroid type 2,5'-deiodinase activity could be detected also in the cytosolic fraction, not only in the membrane. TNFalpha, IL-6 and IFNgamma could inhibit the type 2 deiodinase activity in vitro. The dose-dependent binding of [125I] T(4) to the proteins of 29, 66 and 200 kDa could be observed both in the cytosolic and membrane fractions. TNFalpha and IFNgamma inhibited the binding of [125I] T(4) to the two characteristic proteins of type 2,5'-deiodinase, to proteins of 29 and/or 200 kDa, both in the cytosolic and membrane fractions. We conclude that TNFalpha, IL-6 and IFNgamma can inhibit the activity of type 2,5'-deiodinase. Furthermore, TNFalpha and IFNgamma can still also decrease the binding of [125I] T(4) to the enzyme in vitro. It can be suggested that the increased level of these cytokines can be one of the reasons in the induction of the clinical symptoms in nonthyroidal illness associated with low levels of T(3).

Iodine in human milk: perspectives for infant health

Iodine is essential for normal growth, mental development, and survival of infants. The main source of iodine for breastfeeding infants is the iodine found in human milk. Despite the importance of iodine for infant health, there have been limited studies addressing human milk iodine concentrations. The newly recommended Adequate Intake of iodine for infants is 110 microg/day for infants 0-6 months and 130 microg/day for infants 7-12 months. Further studies of human milk iodine are needed to ensure that iodine prophylaxis is providing sufficient iodine for mothers and infants worldwide.

Hydroxypropyl-beta-cyclodextrin as delivery system for thyroid hormones, regulating glutathione S-transferase expression in rat hepatocyte co-cultures

Thyroid hormones play a role in the regulation of glutathione S-transferase (GST) expression. Here, co-cultures of rat hepatocytes with bile duct epithelial cells have been used to study the direct effects of both triiodothyronine (T3) and thyroxine (T4) on GST activities and proteins. Because T3 and T4 are poorly water soluble and organic solvents used to dissolve them often interfere with biotransformation pathways, an alternative delivery system namely hydroxypropyl-beta-cyclodextrin (HPBC) has been applied. Appropriate control cultures contained either 0.02 or 0.10% (w/v) HPBC, the concentrations necessary to supply T3 and T4 (10(-9) to 10(-5) M) to the cells, respectively. No effect of the vehicle HPBC on the different GST isoenzyme activities and proteins could be observed. On the contrary, after 10 days of co-culture, T3 and T4 decreased GST protein concentrations as well as GST activities measured by 1-chloro-2,4-dinitrobenzene (broad spectrum), 1,2-dichloro-4-nitrobenzene (Mu class M1/M2-specific) and 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole (Alpha class A1/2-specific) in a concentration-dependent manner. The Alpha class subunits A1/2 and A3, and the Mu class subunit M2 were mostly affected. No effect was observed on the Pi class enzyme. These findings indicate that a combination of co-cultured hepatocytes with an HPBC-based delivery system for hydrophobic compounds represents a powerful in vitro tool in drug development.

Identification of two novel splicing variants of human type II iodothyronine deiodinase mRNA

Human type II iodothyronine deiodinase (hDII) belongs to a family of selenoproteins. It catalyzes 5'-deiodination of thyroxine to generate an active thyroid hormone, 3,3',5-triiodothyronine. Two novel splice variants of hDII gene transcript; namely hDII-b and hDII-c, were identified. Three distinct DNA fragments (hDII-a-c) were amplified by a reverse transcription-polymerase chain reaction (RT-PCR) with hDII intron-spanning primers using total-RNA from human umbilical vein endothelial cell line, ECV304. The sequence of hDII-a was identical to that of previously reported hDII. hDII-b and hDII-c had an additional insertion of 108 and 242 bp, respectively. The insertion sequences were found in the intron of hDII gene, therefore, two novel exons exist between exons 1 and 2 of hDII gene. The splice sites of new exons (b and c) conserved the consensus sequences of splice acceptor and donor sites. hDII-b contains exon b with an in-frame TGA codon that may encode an extra selenocysteine. hDII-c contained exons b and c and the predicted open reading frame is interrupted by a stop codon (TAA) produced by a frame shift. RT-PCR analysis showed that hDII-a and hDII-b mRNAs are expressed in human tissues including brain, kidney, lung and trachea. The mRNA abundance of hDII-c was lower than that of hDII-a or hDII-b. Thus, the new variants of hDII transcripts suggest the presence of two short exons between exons 1 and 2 of hDII gene, and of functional variant of hDII.

Sulfation of iodothyronines by human sulfotransferase 1C1 (SULT1C1)*

Sulfation is an important component of human thyroid hormone metabolism. The role of the human sulfotransferase 1C1 (SULT1C1) is not known. Because SULT1C1 is present in the adult thyroid, intra-thyroidal sulfation of thyroid hormones and their metabolites might occur. We tested this hypothesis by determining the ability of recombinant human SULT1C1 to catalyze iodothyronine sulfation. Apparent K(m) values for 3,3',5-triiodothyronine (T(3)), 3, 3'-diiodothyronine (3,3'-T(2)), 3',5',3-triiodothyronine (rT(3)), and 3,3',5,5'-tetraiodothyronine (T(4)) with SULT1C1 were 28.7, 10.3, 10.2, and 59.3 microM, respectively. Thermal stability and responses to inhibitors also were tested with T(3) as the substrate. Enzyme aliquots were measured simultaneously to determine SULT1C1 substrate preferences at optimal iodothyronine concentrations. SULT1C1 activity obtained with T(3) was used as 100%, and the activities with 3,3'-T(2), rT(3), T(4), and 3,5-diiodothyronine (3, 5-T(2)) were 614, 314, 25, and 4%, respectively. We report for the first time the characterization of human SULT1C1 with T(3) and the preferences of the enzyme for various iodothyronines. The presence of SULT1C1 in the adult thyroid gland raises the possibilities that the enzyme can contribute to intraglandular thyroid hormone processing and iodide reutilization.

Selenium in biology: facts and medical perspectives

Several decades after the discovery of selenium as an essential trace element in vertebrates approximately 20 eukaryotic and more than 15 prokaryotic selenoproteins containing the 21st proteinogenic amino acid, selenocysteine, have been identified, partially characterized or cloned from several species. Many of these proteins are involved in redox reactions with selenocysteine acting as an essential component of the catalytic cycle. Enzyme activities have been assigned to the glutathione peroxidase family, to the thioredoxin reductases, which were recently identified as selenoproteins, to the iodothyronine deiodinases, which metabolize thyroid hormones, and to the selenophosphate synthetase 2, which is involved in selenoprotein biosynthesis. Prokaryotic selenoproteins catalyze redox reactions and formation of selenoethers in (stress-induced) metabolism and energy production of E. coli, of the clostridial cluster XI and of other prokaryotes. Apart from the specific and complex biosynthesis of selenocysteine, selenium also reversibly binds to proteins, is incorporated into selenomethionine in bacteria, yeast and higher plants, or posttranslationally modifies a catalytically essential cysteine residue of CO dehydrogenase. Expression of individual eukaryotic selenoproteins exhibits high tissue specificity, depends on selenium availability, in some cases is regulated by hormones, and if impaired contributes to several pathological conditions. Disturbance of selenoprotein expression or function is associated with deficiency syndromes (Keshan and Kashin-Beck disease), might contribute to tumorigenesis and atherosclerosis, is altered in several bacterial and viral infections, and leads to infertility in male rodents.

Retinoic acid redifferentiation therapy for thyroid cancer

For the treatment of differentiated thyroid cancer, surgery, radioiodide therapy, and thyrotropin-suppressive thyroxine application represent established therapeutic measures of proven efficiency, affording a good prognosis for this disease. However, in up to 30% of the cases, dedifferentiation is observed, giving rise to tumors that are refractory to conventional treatment. Eventually, this may lead to the most malignant human tumor, anaplastic thyroid carcinoma, with a life expectancy of only a few months after diagnosis. Among novel approaches for the treatment of dedifferentiated thyroid carcinomas, retinoic acid redifferentiation therapy was evaluated in several in vitro and in vivo studies. Cell culture experiments in thyroid carcinoma lines show that RA treatment affects thyroid specific functions (type I 5'-deiodinase, sodium/iodide-symporter), cell-cell or cell-matrix interaction (intercellular adhesion molecule-1, E-cadherin), differentiation markers (alkaline phosphatase, CD97), growth, and tumorigenicity. The observed changes, which involve multiple parameters that characterize a mature, functional thyrocyte, may be interpreted as partial redifferentiation. In clinical pilot studies, about 40% of the patients responded to RA application with an increased radioiodide uptake. In an evaluation of 20 RA-treated patients with well-documented data sets, 8 exhibited a decrease (4) or stabilization (4) in tumor size and/or in serum thyroglobulin levels in addition to enhanced radioiodide transport. This indicates that these patients with a long history of unresponsiveness to other treatment may have experienced an actual therapeutic benefit. These data suggest that RA redifferentiation therapy, considering especially its comparatively mild side effects, may soon represent an alternative therapeutic approach to otherwise untreatable thyroid tumors.

Functions of thyroid hormone receptors in mice

Thyroid hormone receptors (TRs) play a central role in mediating the actions of thyroid hormone in development and homeostasis in vertebrate species. The TRs are nuclear receptors that act as ligand-regulated transcription factors. There are two TR genes (TRalpha and TRbeta), each capable of generating different variant products, suggesting a potentially complex array of TR pathways. Targeted mutagenesis in the mouse has indicated that there are specific individual functions for the TR genes in vivo. The deletion of combinations of TRalpha and TRbeta variants has revealed that additional functions are convergently regulated by both TR genes and indicates that control of an extended range of functions is facilitated by a network of specific and common TR pathways. The TR-deficient mouse models have allowed investigation of the TR pathways underlying many functions of thyroid hormone and provide a unique perspective on receptor-mediated mechanisms of biological control.

Tissue-specific functions of individual glutathione peroxidases

The family of glutathione peroxidases comprises four distinct mammalian selenoproteins. The classical enzyme (cGPx) is ubiquitously distributed. According to animal, cell culture and inverse genetic studies, its primary function is to counteract oxidative attack. It is dispensible in unstressed animals, and accordingly ranks low in the hierarchy of glutathione peroxidases. The gastrointestinal isoenzyme (GI-GPx) is most related to cGPx and is exclusively expressed in the gastrointestinal tract. It might provide a barrier against hydroperoxides derived from the diet or from metabolism of ingested xenobiotics. The extreme stability in selenium deficiency ranks this glutathione peroxidase highest in the hierarchy of selenoproteins and points to a more vital function than that of cGPx. Plasma GPx (pGPx) behaves similar to cGPx in selenium deficiency. It is directed to extracellular compartments and is expressed in various tissues in contact with body fluids, e.g., kidney, ciliary body, and maternal/fetal interfaces. It has to be rated as an efficient extracellular antioxidant device, though with low capacity because of the limited extracellular content of potential thiol substrates. Phospholipid hydroperoxide glutathione peroxidase (PHGPx), originally presumed to be a universal antioxidant enzyme protecting membrane lipids, appears to have adopted a variety of specific roles like silencing lipoxygenases and becoming an enzymatically inactive structural component of the mitochondrial capsule during sperm maturation. Thus, all individual isoenzymes are efficient peroxidases in principle, but beyond their mere antioxidant potential may exert cell- and tissue-specific roles in metabolic regulation, as is evident for PHGPx and may be expected for others.

[Expression of selenoproteins in monocytes and macrophages--implications for the immune system]

Monocytes differentiate from myeloid precursors towards the macrophage state of differentiation under the influence of 1,25-dihydroxy vitamins D3 (1,25 [OH]2 vitamin D3) and other factors and this is further propagated by colony stimulating factors (MCSF and GMCSF). Macrophage activation and phagocytosis of foreign particles are regularly accompanied by a so called "respiratory burst", an increase in the production of reactive oxygen species (ROS), exerted by the enzyme complex NADPH oxidase. A number of antioxidant enzymes is expressed at the same time to protect the cells from the cytotoxic effects of ROS directed against engulfed microorganisms. The selenium-dependent glutathione peroxidases and thioredoxin reductases are important examples. The cytosolic GPx isoenzyme (cGPx) and thioredoxin reductase alpha (TrxR alpha) are upregulated during the process of differentiation and under the influence of 1.25 (OH)2 vitamin D3. GPx isoenzymes neutralize H2O2. TrxR reduce sulfhydryl-groups like in cysteins either directly or via their cofactor thioredoxin and thus are involved in protein folding and critical protein-protein and protein-DNA interactions, e.g. modulation of dimerization and/or DNA-binding and ligand binding of transcription factors (glucocorticoid receptor and other steroid receptors, NF kappa B). In addition, the antibiotic peptide NK-lysin was shown to be a substrate for TrxR alpha, suggesting that TrxR protects the cell itself from the cytotoxic effects of NK-lysin. Selenium is incorporated into selenocysteine (Secys) in a regulated fashion in the presence of a hairpin structure (Secis element) in the 3'UTR of selenoprotein genes. Secis elements direct the insertion of Secys at UGA codons, which function as opal stop codons in the absence of a suitable Secis element and in selenium deficiency. The above mentioned processes might therefore be altered in relative selenium deficiency or vice versa be upregulated through selenium supplementation. We have shown that TrxR alpha is a 1.25 (OH)2 vitamin D3-responsive early gene in monocytic cells and that TrxR activity as well as GPx activity in these cells can be upregulated by the addition of selenium in vitro and ex vivo. Recent work demonstrates that thioredoxin rapidly enters the cell nucleus upon treatment of cells with H2O2, but little is known about the compartimentalization of the respiratory burst and the intracellular localization of antioxidant enzymes during that process. Macrophage function is insufficient if the generation of a respiratory burst is altered like in hereditary chronic granulomatous disease on one hand, but on the other hand is as well disturbed, if there is a lack in antioxidant enzyme activity. Thioredoxin has been identified as a lymphocyte growth factor and might therefore be involved in the crosstalk between macrophages and lymphocytes. The relevance of the above mentioned and other yet undefined monocytic selenoproteins remains to be elucidated in detail as well as the relevance of selenium supplementation in nutrition in general and in situations of critical infectious disease and autoimmunity.