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At least 19 recordsLinked to original sources

PubMed2000

DARPP-32 and CREB are present in type 2 iodothyronine deiodinase-producing tanycytes: implications for the regulation of type 2 deiodinase activity.

Type 2 iodothyronine deiodinase, an enzyme involved in the conversion of thyroxin to the biologically active 3,5, 3'-triiodothyronine, is highly concentrated in a group of specialized ependymal cells, tanycytes, lining the wall and floor of the third ventricle. As this distribution is highly reminiscent of the distribution of cells containing the phosphatase inhibitor, DARPP-32, we raised the possibility that these two proteins may coexist in tanycytes and that DARPP-32 may modulate type 2 deiodinase activity by regulating the phosphorylation state of the cAMP regulatory factor, CREB. To address this question, double-labeling histochemical studies were performed for type 2 deiodinase mRNA and DARPP-32 immunoreactivity (IR), or DARPP-32- and CREB-IR in the same tissue sections. Type 2 deiodinase mRNA was found in the cell bodies of all DARPP-32-immunolabeled tanycytes. Both type 2 deiodinase mRNA and DARPP-32-IR also extended into tanycyte processes that ramified in the arcuate nucleus and median eminence, in close association with blood vessels and portal capillaries. In contrast, type 2 deiodinase mRNA was not present in the same cells that contained DARPP-32-IR in the pituitary gland. All tanycytes containing DARPP-32-IR also contained CREB-IR in their nucleus. Since type 2 deiodinase activity can be induced by substances that increase cAMP, we hypothesize that DARPP-32 may regulate the activity of type 2 deiodinase by prolonging the activation of CREB. Selectivity for the colocalization of these factors to tanycytes but not the pituitary gland, may explain the heterogeneous response of type 2 deiodinase activity in these two loci in response to specific stimuli such as fasting.

Animals↗

PubMed2005

Molecular cloning, expression and radiation hybrid mapping of the bovine deiodinase type II (DIO2) and deiodinase type III (DIO3) genes.

Thyroid hormones play a critical role in mammalian development and metabolism. Their activity is regulated in a complex, tissue-specific manner by three isoforms of deiodinases. The goal of this study was to sequence the full-length bovine type II deiodinase (DIO2) and type III deiodinase (DIO3) cDNAs and characterize mRNA expression levels of each of the three deiodinase isoforms in several bovine tissues. Sequencing of bovine DIO2 and DIO3 cDNAs revealed a high degree of predicted amino acid sequence identity with their orthologs in other mammalian species and demonstrated the conservation of selenocysteine residues within the catalytic domains of both bovine proteins. Bovine DIO2 and DIO3 were positioned on chromosomes 10 and 21, respectively, by radiation hybrid mapping. Expression patterns of the three deiodinase isoforms were similar for deiodinase type I (DIO1) and DIO2 to those observed in other species. Expression level of DIO3 transcripts was greatest in mammary gland and kidney, although low-level expression was detected in most tissues sampled. Results of this work will aid in the study of deiodinase gene expression and thyroid hormone regulation in cattle.

Amino Acid Sequence↗

PubMed1990

Iodothyronine deiodinase activities in FRTL5 cells: predominance of type I 5'-deiodinase.

FRTL5 cells, a thyroid follicular cell line derived from normal rat thyroid, has been extensively used as a model system to study various aspects of the physiology of the thyroid epithelium. The capacity of these cells to metabolize iodothyronines and to generate T3 from T4 has not been previously examined. Here we studied the deiodination of T4, T3, and rT3 in homogenates of FRTL5 cells. By far, these homogenates were more potent catalyzing the 5'-deiodination (outer ring) of T4 and rT3 than the inner ring deiodination of T4 or T3. Both the production of rT3 and the degradation of newly formed T3 from T4 were very limited. Thus, when T4 was used as a substrate, T3 and iodide accumulated in a linear fashion with time, and initially the amounts of iodide and T3 were approximately equal. rT3 and 3,3'-diiodothyronine were rapidly deiodinated by these homogenates, with the 3'-deiodination of 3,3'-diiodothyronine occurring at a slower rate than the 5'-deiodination of rT3. The iodothyronine 5'-deiodinase activity corresponded to type I, as indicated by the following: the Km for T4 and T3 was in the micromolar range; rT3 was a better substrate than T4 (maximum velocity = 101 vs. 19 pmol/min.mg protein; Km = 0.83 vs. 3.1 microM, respectively); and the kinetics of inhibition by 6n-propyl-2-thiouracil were uncompetitive and substrate dependent, suggesting ping-pong kinetics. The type I 5'-deiodinase activity of FRTL5 cells was distinctly stimulated by TSH. This stimulation seems to be mediated by cAMP and requires serum as a permissive factor. In conclusion, 1) FRTL5 cells exhibit both inner and outer ring iodothyronine-deiodinating activities; 2) iodothyronine 5'-deiodination is by far more active; 3) the 5'-deiodination has been characterized as type I deiodinase based on substrate preference, enzyme kinetics, and inhibitors; 4) in all respect iodothyronine deiodination by FRTL5 cell homogenates proceeded with marked similarity to that in homogenates or microsomes of thyroid glands from several species; and 5) the FRTL5 type I deiodinase is more active than that reported in thyroid tissue and as active as that reported in liver and kidney, the prototype of type I deiodinase-containing tissues. The present studies indicate that FRTL5 cells are an excellent model system to study cellular and biochemical aspects of the regulation of this enzyme as well as its regulation by TSH and putative serum factors.

Animals↗

PubMed1997

Rainbow trout liver expresses two iodothyronine phenolic ring deiodinase pathways with the characteristics of mammalian types I and II 5'-deiodinases.

Deiodinases are major determinants of thyroid hormone tissue availability and disposal. The knowledge of the expression of these enzymes in lower species is important to understand evolutionary and ontogenetic aspects of thyroid hormone action and metabolism. Here we have studied outer ring deiodination in the trout liver using both reverse T3 (rT3) and T4 as substrates. The use of rT3 disclosed two enzymatic components with the characteristics of mammalian types I and II 5'-deiodinases. The high rT3-K(m) type I 5'-deiodinase activity (180 nM) has a low cofactor requirement (5 mM dithiothreitol) and is relatively sensitive to propylthiouracil inhibition, whereas the low rT3-K(m) activity was akin to the outer ring deiodination of T4 in these regards. The use of T4 exhibited only a single type of activity with a low K(m) (0.63 nM), a relatively high cofactor requirement (25 mM dithiothreitol), and propylthiouracil-resistance. Teleosts constitute a unique example of type II activity expression in the liver of an adult vertebrate. Furthermore, the Vmax of this enzyme is as high as that found in comparable homogenates from hypothyroid mammalian tissues, whereas the Vmax of the type I activity is lower than that of mammalian liver. These findings are in consonance with the peculiar kinetics of T3 in trout liver, kinetics remarkably similar to those of the mammalian pituitary, cerebral cortex, and brown adipose tissue, which also preferentially express type II deiodinase.

Animals↗

PubMed1996

Decreased liver type I 5'-deiodinase and increased brown adipose tissue type II 5'-deiodinase activity in 2,3,7,8-tetrachlorobibenzo-p-dioxin (TCDD)-treated Long-Evans rats.

The effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was studied on the activity of type I 5'-deiodinase (5' DI) in liver and of type II 5'-deiodinase (5' DII) in brown adipose tissue (BAT) microsomes of Long-Evans rats. TCDD decreased the activity of liver 5' DI and increased that of BAT 5' DII. It also reduced the concentration of serum total thyroxine (T4) without affecting triiodothyronine (T3). These effects of TCDD on 5'-deiodinases and serum thyroid hormones are compatible with partial hypothyroidism. Based on our results, it is not possible to decide unequivocally whether TCDD affects thyroid hormone status and 5'-deiodination by a direct action or a feedback mechanism. However, these data are important because they demonstrate that TCDD affects thyroid status in a tissue-specific manner and that its effects on thyroid hormone metabolism are much more complex than currently assumed.

Adipose Tissue, Brown↗

PubMed1981

High affinity of triiodothyronine (T3) for nonphenolic ring deiodinase and high affinity of tetraiodothyroacetic acid (TETRAC) for phenolic ring deiodinase in cultured monkey hepatocarcinoma cells and in rat liver homogenates.

The metabolism of 3, 5-[3'-125I]triiodothyronin (T3) and 3-[3', 5'-125I]triiodothyronine (rT3) was studied in cultured monkey hepatocarcinoma cells (NCLP-6E), and the deiodinations of these iodothyronines were also investigated in cultured cell homogenates and in rat liver homogenates. The metabolites were analyzed by ion exchange column chromatography. For nonphenolic ring deiodination of 3, 5-[3'-125I]triiodothyronine, the order of the inhibitory effect of excess unlabeled iodothyronine or its analog was as follows: 3,3',5-triiodothyroinine greater than triiodothyroacetic acid greater than tetraiodothyroacetic acid greater than thyroxine. This order did not differ between in the intact cells (NCLP-6E) and their homogenates. The order of effectiveness of the excess unlabeled compounds on phenolic ring deiodination of 3-[3', 5'-125I]triiodothyronine in the intact cells was as follows: tetraiodothyroacetic acid greater than triiodothyroacetic acid, 3, 3', 5-triiodothyronine greater than thyroxine. This order was the same among monkey hepatocarcinoma cell homogenates, rat hepatoma cell homogenates and rat liver homogenates, and triiodothyroacetic acid was obviously more effective than 3, 3', 5-triiodothyronine. It was concluded that 3, 3', 5-triiodothyronine had the highest affinity for nonphenolic ring deiodinase among iodothyronines and their analogs used in the present study and that tetraiodothyroacetic acid and the highest affinity for phenolic ring deiodinase. It seems, therefore, that the metabolites derived from the thyroid hormones might contribute to deiodinations which involve activation and inactivation of the hormones.

Animals↗

PubMed1999

Pretranslational regulation of rhythmic type II iodothyronine deiodinase expression by beta-adrenergic mechanism in the rat pineal gland.

It has been demonstrated that type II iodothyronine deiodinase is present in rat pineal gland, and the deiodinase activity markedly increases during the hours of darkness, primarily through beta-adrenergic mechanism. We have studied the relationship between pineal type II iodothyronine deiodinase messenger RNA (mRNA) and the deiodinase activity to elucidate the mechanisms involved in the nocturnal rise in pineal deiodinase activity. Northern analysis has demonstrated that type II iodothyronine deiodinase mRNA is expressed in rat pineal gland, and the mRNA markedly increases during the hours of darkness. The nocturnal increase in pineal type II iodothyronine deiodinase activity is preceded by the increase in its mRNA. Daytime isoproterenol administration resulted in a rapid increase in pineal type II iodothyronine deiodinase mRNA followed by the increase in deiodinase activity. Propranolol treatment, bilateral superior cervical ganglionectomy, or constant light exposure significantly suppressed the nocturnal rise in type II iodothyronine deiodinase mRNA as well as the deiodinase activity. Moreover, isoproterenol or (Bu)2AMP stimulated type II iodothyronine deiodinase mRNA and the deiodinase activity in cultured rat pineal glands. These results suggest that the rhythmic change in pineal type II iodothyronine deiodinase activity is regulated at least in part at the pretranslational level by a beta-adrenergic mechanism transmitted through superior cervical ganglia.

Adrenergic beta-Agonists↗

PubMed1996

Thyroid hormone deiodinases--a selenoenzyme family acting as gate keepers to thyroid hormone action.

Development and tissue-specific deiodination of thyroid hormone leads to both activation of the prohormone thyroxine to the thyromimetically active T3 as well as to inactivation of T3 and its conjugates or inactivation of T4 to yield potential regulatory active rT3. At least three deiodinase isoenzymes have so far been characterized and cloned, and the deiodinase isozymes represent a new family of eukaryotic selenoproteins for which an enzyme function could be assigned. Selenium status apparently regulates the expression of these deiodinase isozymes to different extent indicating that a hierarchy of selenium incorporation exists for those enzymes. Currently, it appears that selenium deficiency does not affect expression of type II 5'-deiodinase or 5-deiodinase to a marked extent in vivo whereas type I 5'-deiodinase at least in liver and kidney is reduced in severe selenium deficiency. However, daily selenium intake in normal mideuropeans already saturates the requirement for the expression of the deiodinase isoenzymes. So far, only reduced expression of 5'-D I and decreased T 3 production has been observed in specific diets such as for PKU or in cystic fibrosis, where transport of ions (iodide, selenite?) might be affected. Further alterations of T3 production by 5'-D I activity are observed under the conditions of the low T3 syndrome, which comprise a broad spectrum of clinical disorders from carbohydrate withdrawal to intensive care patients. It is not yet clear if selenium supplementation or T3 treatment is beneficial to these patients. The marked tissue-specificity of expression of the deiodinases requires more detailed examinations on the relation between these enzymes and the expression of thyroid hormone action, which is mediated by the nuclear T3 receptor family or receptors and signal transduction molecules in the mitochondria, plasma membrane, or cytoskeleton. The location of the deiodinase enzymes either at the inner side of the plasma membrane or the cytosolic side of the endoplasmic reticulum positions these enzymes to a strategically important location enabling them to act as gate-keepers to the nuclear receptors. Similar to other enzymes involved in the activation or inactivation of compounds with hormone or signalling function, the deiodinases are key elements in the intracrine regulation of hormone activation in target tissues or inactivation in non-target tissues. Therefore, a detailed molecular, cell biological and physiological analysis of the function, regulation and gene structure of these enzymes is required before a development of tissue- or enzyme-specific pharmacological intervention is possible. Nevertheless, first data indicate that reduced 5'-deiodinase type I expression in tumor tissues can be re-induced by treatment with retinoids at least in follicular thyroid carcinoma. Further studies are needed to prove that retinoids might be a useful therapeutic tool for re-differentiation therapy of thyroid carcinoma which are inaccessible to surgical intervention or lack radio-iodide uptake and storage. The important function and regio- and cell-specific expression of deiodinase isozymes in the central nervous system is far from being understood. Current first evidence suggests a close interaction between thyroid hormone deiodination, thyroid hormone concentration, and expression of thyroid hormone responsive genes in the adult brain as well as tight regulation and interaction between thyroid hormone metabolism and neurotransmitter synthesis release and action.

Animals↗

PubMed1998

Northern analysis of type II iodothyronine deiodinase mRNA in rat Harderian gland.

It has been known that type II iodothyronine deiodinase activity is present in rat Harderian gland and the activity is significantly increased by isoproterenol administration. We have performed Northern analyses to study whether the transcript for type II iodothyronine deiodinase is expressed in rat Harderian gland and whether the isoproterenol stimulation of type II iodothyronine deiodinase activity in rat Harderian gland is due to the change in its mRNA level. Northern analyses have demonstrated that type II iodothyronine deiodinase mRNA, approximately 7.5 kb in size, is expressed in rat Harderian gland, and the mRNA levels as well as the deiodinase activities are greater in hypothyroid rats than those in euthyroid rats. Type II iodothyronine deiodinase mRNA levels and the deiodinase activities in Harderian gland were increased by isoproterenol administration, and the increase in the mRNA levels preceded that in the deiodinase activities. These results indicate that 7.5 kb transcript for type II iodothyronine deiodinase is expressed in rat Harderian gland and beta-adrenergic stimulation of type II iodothyronine deiodinase activity is due at least in part to the increase in its mRNA level.

Actins↗

PubMed1999

Local activation and inactivation of thyroid hormones: the deiodinase family.

Tissue-specific activation and inactivation of ligands of nuclear receptors which belong to the steroid retinoid-thyroid hormone superfamily of transcription factors represents an important principle of development- and tissue-specific local modulation of hormone action. Recently, several enzyme families have been identified which act as 'guardians of the gate' of ligand-activated transcription modulation. Three monodeiodinase isoenzymes which are involved in activation the 'prohormone' L-thyroxine (T4), the main secretory product of the thyroid gland, have been identified, characterized, and cloned. Both, type I and type II 5'-deiodinase generate the thyromimetically active hormone 3,3',5-triiodothyronine (T3) by reductive deiodination of the phenolic ring of T4. Inactivation of T4 and its product T3 occurs by deiodination of iodothyronines at the tyrosyl ring. This reaction is catalyzed both the type III 5-deiodinase and also by the type I enzyme, which has a broader substrate specificity. The three deiodinases appear to constitute a newly discovered family of selenocysteine-containing proteins and the presence of selenocysteine in the protein is critical for enzyme activity. Whereas the selenoenzyme characteristics of the type I and type III deiodinases are definitively established some controversy still exists for the type II 5'-deiodinase in mammals. The mRNA probably encoding the type II 5'-deiodinase subunit is markedly longer than those of the two other deiodinases and its selenocysteine-insertion element is located more than 5 kB downstream of the UGA-codon in the 3'-untranslated region. The three deiodinase isoenzymes show a distinct development- and tissue-specific pattern of expression, operate at individual optimal substrate levels, are differently regulated and modulated by hormones, cytokines, signaling pathways, natural factors, and pharmaceuticals. Whereas circulating T3 mainly originates from hepatic production via the type I 5'-deiodinase, the local cellular thyroid hormone concentration in various tissues including the central nervous system is controlled by complex para-, auto-, and intracrine interactions of all three deiodinases. Local thyroid hormone availability is further modulated by conjugation reactions of the phenolic 4'-OH-group of iodothyronines, which also inactivate the thyroid hormones.

Animals↗

PubMed1983

Somatostatin inhibits rat hepatic T4-5'-deiodinase. The effect is independent of the associated hypoinsulinemia.

Somatostatin decreases the serum 3,5,3'-triiodothyronine (T3) concentration in athyreotic subjects treated with L-thyroxine (T4). The present study was performed to determine the effect of somatostatin on T4-5'-deiodinase activity in rat tissue homogenate preparations. This enzyme is an important regulator of T3 production. Continuous somatostatin infusion at high dose (4 micrograms/kg per min subcutaneously) and low dose (0.8 micrograms/kg per min subcutaneously) for 48-72 h significantly increased (P less than 0.001) the mean aorta plasma somatostatin-like immunoreactivity concentration to 786 +/- 65 and 448 +/- 58 pg/ml, respectively compared with the normal mean of 69 +/- 17 pg/ml in the carbohydrate-fed rat (20% glucose in water ad lib.). The mean hepatic T4-5'-deiodinase activity at both 48 h (100 +/- 5 pmol/min per 100 mg protein) and 72 h (90 +/- 7 pmol/min per 100 mg protein) was significantly reduced in the high-dose group (P less than 0.005), compared with the mean enzyme activity in the glucose-fed control group (138 +/- 6 pmol/min per 100 mg protein). There was a negative correlation (r = -0.9, P less than 0.01) between the alterations in the peripheral plasma somatostatin-like immunoreactivity concentration and hepatic T4-5'-deiodinase activity. High-dose somatostatin did not consistently lower the serum T3 concentration in the glucose-fed rat. Somatostatin had no effect on pituitary T4-5'-deiodinase activity in the glucose-fed rat. High-dose somatostatin also significantly inhibited (P less than 0.01) the glucose-refeeding reactivation of hepatic T4-5'-deiodinase in the 72-h-fasted rat. The mean enzyme activity after 96 h was 96 +/- 8 pmol/min per 100 mg protein compared with 127 +/- 4 pmol/min per 100 mg protein in the refed control group. Somatostatin had a similar inhibitory effect on serum T3. There was a positive correlation (r = 0.5, P less than 0.01) between the somatostatin-induced alterations in serum T3 and hepatic T4-5'-deiodinase during refeeding. A significant positive correlation (r = 07, P less than 0.005) was noted between the somatostatin effect on hepatic T4-5'-deiodinase activity and the induced hypoinsulinemia in the fed group. In addition, a significant negative correlation (r = -0.9, P less than 0.001) was noted between the suppressed enzyme activity and the serum glucose/insulin ratio in the refed group. However, although low-dose somatostatin also induced the same degree of hypoinsulinemia (P less than 0.05) in the fed and refed groups it had no effect on hepatic T4-5'-deiodinase activity. Furthermore, despite the induction of hyperinsulinemia during refeeding, the high dose somatostatin inhibitory effect on enzyme activity persisted. Thus, somatostatin inhibited hepatic T4-5'-deiodinase activity in the carbohydrate-fed rat and prevented the carbohydrate-refeeding normalization of enzyme activity in the 72-h-fasted rat. The effect of somatostatin on enzyme activity was independent of the associated hypoinsulinemia. In the carbohydrate-fed animal the somatostatin effect was selective, as the hormone had no effect on pituitary T4-5'-deiodinase activity. These data suggest that somatostatin could play a role in the peripheral metabolism of thyroid hormones.

Animals↗

PubMed2003

An ascidian homolog of vertebrate iodothyronine deiodinases.

In all classes of vertebrates, the deiodination of the prohormone T(4) to T(3) represents an essential activation step in thyroid hormone action. The possible presence of iodothyronine deiodinase activity in protochordates has been demonstrated in vivo. Recent molecular cloning of the genomes and transcripts of several ascidian species allows further investigation into thyroid-related processes in ascidians. A cDNA clone from Halocynthia roretzi (hrDx) was found to have significant homology (30% amino acid identity) with the iodothyronine deiodinase gene sequences from vertebrates, including the presence of an in-frame UGA codon that might encode a selenocysteine (SeC) in the active site. Because it was not certain that the 3' untranslated region (UTR) contained a SeC insertion sequence (SECIS) element essential for SeC incorporation, a chimeric expression vector of the hrDx coding sequence and the rat deiodinase SECIS element was produced, as well as an expression vector containing the intact hrDx cDNA. COS, CHO, and HEK cells were transfected with these vectors, and deiodinase activity was measured in cell homogenates. Outer-ring deiodinase activity was detected using both T(4) and reverse T(3) as substrates, and activity was enhanced by the presence of the reductive cofactor dithiothreitol. The enzyme activity was optimal during incubation between 20 and 30 C (pH 6-7) and was strongly inhibited by gold-thioglucose. The Halocynthia deiodinase appears to be a high Michaelis-Menten constant (K(m)) enzyme (K(m) reverse T(3), 2 microM; and K(m) T(4), 4 microM). Deiodinase activity was completely lost upon the substitution of the SeC residue in the putative catalytic center by either cysteine or alanine. Transfection of the full-length hrDx cDNA produced deiodinase activity confirming the presence of a SECIS element in the 3'UTR, as revealed by the SECISearch program. In conclusion, our results show, for the first time, the existence of an ascidian iodothyronine outer-ring deiodinase. This raises the hypothesis that, in protochordates, the prohormone T(4) is activated by enzymatic outer-ring deiodination to T(3).

3' Untranslated Regions↗

PubMed2001

Sex steroids modulate rat anterior pituitary and liver iodothyronine deiodinase activities.

In this study, we investigated the sex hormone regulation of 5'-iodothyronine deiodinase activity, which is responsible for enzymatic conversion of thyroxine into the bioactive form, triiodothyronine. Pituitary homogenates and liver microsomes from: 1) ovariectomized rats injected with 17-beta-estradiol benzoate and/or progesterone (0.7 and 250 microg/100 g body weight, respectively, subcutaneously, over 10 days); 2) male castrated rats treated or not with 0.4 mg/100 g body weight testosterone propionate, intramuscular, over 7 days, were assayed for type 1 and type 2 deiodinase activity in the pituitary. Enzyme activities were measured by release of (125)I from deiodination of (125)I reverse triiodothyronine under varying assay conditions. Estrogen stimulated anterior pituitary and liver type 1 deiodinase activity in ovariectomized rats (45 and 30 %, p < 0.05). Progesterone inhibited the liver enzyme (40 %, p < 0.05), and had no effect on the pituitary, but in both tissues, blocked estrogen stimulatory effect on type 1 deiodinase. In males, testosterone normalized the reduced liver type 1 deiodinase of castrated rats. However, in the pituitary, castration increased (50 %) type 1 deiodinase independent of testosterone treatment, suggesting the existence of a inhibitory testicular regulator of pituitary type 1 enzyme. Treatments did not alter pituitary type 2 deiodinase activity. In conclusion, gonads and sex steroids differentially modulate type 1 deiodinase activity in rat pituitary and liver.

Animals↗

PubMed1995

Cloning of a cDNA for the type II iodothyronine deiodinase.

Three types of iodothyronine deiodinase have been identified in vertebrate tissues. cDNAs for the types I and III have been cloned and shown to contain an inframe TGA that codes for selenocysteine at the active site of the enzyme. We now report the cloning of a cDNA for a type II deiodinase using a reverse transcription/polymerase chain reaction strategy and RNA obtained from Rana catesbeiana tissues. This cDNA (RC5'DII) manifests limited but significant homology with other deiodinase cDNAs and contains a conserved in-frame TGA codon. Injection of capped in vitro synthesized transcripts of the cDNA into Xenopus laevis oocytes results in the induction of deiodinase activity with characteristics typical of a type II deiodinase. The levels of RC5'DII transcripts in R. catesbeiana tadpole tail and liver mRNA at stages XII and XXIII correspond well with that of type II deiodinase activity but not that of the type III activity in these tissues. These findings indicate that the amphibian type II 5'-deiodinase is a structurally unique member of the family of selenocysteine-containing deiodinases.

Amino Acid Sequence↗

PubMed2006

Deiodinase activity is present in Xenopus laevis during early embryogenesis.

Thyroid hormones orchestrate amphibian metamorphosis. The type 2 and type 3 deiodinases make vital contributions to this process by controlling levels of the thyroid hormones T(4) and T(3) available to different tissues. Because the tadpole thyroid gland is not functional until stage NF44, it has been widely assumed that thyroid signaling is absent during amphibian early development, thyroid hormone only becoming a major regulator during premetamorphic stages. Similarly, in mammals, thyroid function is known to be essential to neuronal development, especially during the perinatal stages, but again little is known about early stages of development. Here we demonstrate that key elements of thyroid hormone signaling are present during early development of Xenopus. In particular, we find functional thyroid hormone-activating deiodinases and significant levels of their substrates, T(4) and T(3), during early embryogenesis. Furthermore, we have further characterized a recently identified deiodinase in amphibians, homologous to mammalian type 1 deiodinase (D1). This enzyme is expressed in marked, spatially defined patterns during embryogenesis. The patterns of expression of type 1 deiodinase are distinct from those of type 2 and type 3 deiodinases. Deiodinase expression is found in neurogenic areas from stage NF30 onward, both in the central and peripheral nervous systems. We conclude that both activating and inactivating deiodinases show dynamic patterns of expression during early embryogenesis in amphibians, particularly in neurogenic areas. These findings suggest that thyroid hormone signaling is a key component of early neuronal development in vertebrates.

Animals↗

PubMed1985

Alterations in the pituitary-thyroid axis and 5'-deiodinase activity in mice with muscular dystrophy.

Mice with hereditary muscular dystrophy have reduced levels of serum T3. To determine possible causes of T3 deficits, we evaluated pituitary thyrotroph ultrastructure by electron microscopy, thyroid gland morphology by light microscopy, and T4 to T3 conversion by measuring iodothyronine 5'-deiodinase activity. Differences were not evident between dystrophic and normal littermates in either the structure of pituitary thyrotrophs or thyroid tissues. Dystrophic mice, however, had only 50% the normal hepatic 5'-deiodinase activity. Cerebral 5'-deiodinase, which does not appear to contribute significantly to serum T3, was similar in normal and dystrophic mice. Submandibular gland concentrations of nerve growth factor and epidermal growth factor are reduced in dystrophic mice but can be increased by T4 treatment. To distinguish whether growth factor deficits are due to reductions in serum T3 or to deficiencies in T4 5'-deiodinase activity and subsequent T3 utilization within the salivary gland, we measured submandibular deiodinase activity. Gland homogenates were active in the deiodinase assay, but no differences were detected between normal and dystrophic mice. In order to evaluate tissue responses to reductions in circulating T4, we treated mice with methimazole. Structural analyses revealed that thyrotrophs in dystrophic mice were less stimulated than thyrotrophs in similarly treated normal littermates. Likewise, thyroid follicular cells appeared less active, and thyroid weights increased only 40-50% as much as in normals. Liver 5'-deiodinase activity decreased in both normal and dystrophic mice. Cerebral 5'-deiodinase activity increased more than 4-fold in normal females but only 2-fold in dystrophic females; 2- to 3-fold increases occurred in both normal and dystrophic males. In summary, the structure of pituitary and thyroid glands in dystrophic mice is similar to that of tissues from normal littermates, but hepatic conversion of T4 to T3 is reduced. When challenged by methimazole-induced reductions in serum T4, pituitary and cerebral tissues in dystrophic mice respond abnormally.

Animals↗

PubMed1986

A new class of propylthiouracil analogs: comparison of 5'-deiodinase inhibition and antithyroid activity.

After in vivo administration, propylthiouracil (PTU) inhibits not only thyroid iodide uptake and organification, but also T4 5'-deiodinase activity in most peripheral organs. The present report describes the effects of some previously untested 6-substituted 2-thiouracil derivatives on in vivo and in vitro iodide uptake and organification, and on T4 5'-deiodinase activity in liver and pituitary homogenates. When added to homogenates, many analogs were as potent or more potent than PTU in inhibiting hepatic T4 5'-deiodinase activity. Three derivatives, 6-anilino-2-thiouracil (A compound), 6-(p-ethylanilino)2-thiouracil (B compound), and 6-(p-n-butylanilino) 2-thiouracil (C compound), which were among the most potent inhibitors of hepatic T4 5'-deiodinase, when added in vitro inhibited T4 5'-deiodinase activity in liver homogenates after in vivo administration. When added to pituitary homogenates prepared from hypothyroid rats, these compounds also significantly inhibited pituitary T4 5'-deiodinase activity. In a concentration of 1 mM in the presence of 20 mM dithiothreitol, the percent inhibition of pituitary T4 5'-deiodinase activity was 19.7 +/- 7.4 (mean +/- SE), 34.0 +/- 3.2, 47.3 +/- 3.1, and 89.0 +/- 1.0 for PTU and the A, B, and C compounds, respectively (P less than 0.05 for all groups vs. one another and vehicle). Despite their ability to inhibit hepatic T4 5'-deiodinase activity, none of the 13 analogs tested altered thyroid iodide uptake or organification after administration of 0.1 mg/rat. PTU, in the same dose, inhibited thyroid iodide uptake by 78.2 +/- 2.4% (P less than 0.001) and thyroid iodide organification by 36.4 +/- 7.3% (P less than 0.01). Furthermore, the A, B, and C compounds did not inhibit thyroid iodide uptake or iodide organification when administered in higher doses of 5, 5, and 1 mg/rat, respectively. In contrast to these in vivo results, the A, B, and C compounds were more potent than PTU in inhibiting iodide organification in a purified thyroid peroxidase system and in porcine thyroid slices. The concentrations causing 50% inhibition of iodide organification in the purified thyroid peroxidase system were 30, 7, 8, and 14 microM for PTU and the A, B, and C compounds, respectively. However, PTU was far more potent in inhibiting iodide organification in intact incubated thyroid lobes compared to the A, B, and C compounds.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1992

Dissociation of actin polymerization and enzyme inactivation in the hormonal regulation of type II iodothyronine 5'-deiodinase activity in astrocytes.

T4 dynamically regulates the levels of type II iodothyronine 5'-deiodinase in the brain. Using an astrocyte cell culture model, we have shown that thyroxine increases inactivation of this enzyme through a mechanism using the actin cytoskeleton. In the absence of T4, the filamentous actin (F-actin) stress fibers are absent, and deiodinase inactivation is relatively slow. T4 increases inactivation of type II 5'-deiodinase by 1) restoring the F-actin stress fibers, 2) promoting the binding of the enzyme to F-actin, and 3) stimulating enzyme internalization. To determine whether inactivation of the deiodinase is due solely to the restoration of stress fibers by T4 or also involves direct thyroxine-mediated enzyme-F-actin interactions, we examined the effects of retinoids on both actin polymerization and type II 5'-deiodinase activity in cultured astrocytes, as these hormones have been shown to alter cytoskeletal organization in other tissues. In thyroid hormone-deficient astrocytes, retinoic acid increased F-actin levels, with no change in total cell actin. The F-actin content increased approximately 40% within 30 min after the addition of retinoic acid. After a plateau of 6-8 h, the F-actin content increased further to approximately 90% of the total cell actin and was associated with the reappearance of stress fibers. Only this latter retinoid-stimulated increase in F-actin content was blocked by actinomycin-D. Restoration of the F-actin stress fibers by retinoids did not increase the turnover of the type II 5'-deiodinase (t1/2, 1.99 h-1) or promote binding of the enzyme to F-actin in the absence of T4. Similarly, retinoids did not affect the rapid T4-mediated turnover (t1/2, 0.18 h-1) of type II 5'-deiodinase. These data show that an intact F-actin cytoskeleton in the absence of T4 is inadequate to alter the inactivation of type II 5'-deiodinase and that specific T4-enzyme-F-actin interactions are necessary to initiate the rapid inactivation/internalization of this enzyme.

Actins↗