The inner ring (5-) monodeiodination of thyroxine (T4) in cerebral cortex during fetal, neonatal, and adult life

DIO3 protects against thyrotoxicosis-derived cranio-encephalic and cardiac congenital abnormalities

Maternal hyperthyroidism is associated with an increased incidence of congenital abnormalities at birth, but it is not clear which of these defects arise from a transient developmental excess of thyroid hormone and which depend on pregnancy stage, antithyroid drug choice, or unwanted subsequent fetal hypothyroidism. To address this issue, we studied a mouse model of comprehensive developmental thyrotoxicosis secondary to a lack of type 3 deiodinase (DIO3). Dio3-/- mice exhibited reduced neonatal viability on most genetic backgrounds and perinatal lethality on a C57BL/6 background. Dio3-/- mice exhibited severe growth retardation during the neonatal period and cartilage loss. Mice surviving after birth manifested brain and cranial dysmorphisms, severe hydrocephalus, choanal atresia, and cleft palate. These abnormalities were noticeable in C57BL/6J Dio3-/- mice at fetal stages, in addition to a thyrotoxic heart with septal defects and thin ventricular walls. Our findings stress the protecting role of DIO3 during development and support the hypothesis that human congenital abnormalities associated with hyperthyroidism during pregnancy are caused by transient thyrotoxicosis before clinical intervention. Our results also suggest thyroid hormone involvement in the etiology of idiopathic pathologies including cleft palate, choanal atresia, Chiari malformations, Kaschin-Beck disease, and Temple and other cranio-encephalic and heart syndromes.

The Type 3 Deiodinase: Epigenetic Control of Brain Thyroid Hormone Action and Neurological Function

Thyroid hormones (THs) influence multiple processes in the developing and adult central nervous system, and their local availability needs to be maintained at levels that are tailored to the requirements of their biological targets. The local complement of TH transporters, deiodinase enzymes, and receptors is critical to ensure specific levels of TH action in neural cells. The type 3 iodothyronine deiodinase (DIO3) inactivates THs and is highly present in the developing and adult brain, where it limits their availability and action. DIO3 deficiency in mice results in a host of neurodevelopmental and behavioral abnormalities, demonstrating the deleterious effects of TH excess, and revealing the critical role of DIO3 in the regulation of TH action in the brain. The fact the Dio3 is an imprinted gene and that its allelic expression pattern varies across brain regions and during development introduces an additional level of control to deliver specific levels of hormone action in the central nervous system (CNS). The sensitive epigenetic nature of the mechanisms controlling the genomic imprinting of Dio3 renders brain TH action particularly susceptible to disruption due to exogenous treatments and environmental exposures, with potential implications for the etiology of human neurodevelopmental disorders.

Genomic imprinting of the type 3 thyroid hormone deiodinase gene: regulation and developmental implications

BACKGROUND

In recent years, findings in a number of animal and human models have ignited renewed interest in the type 3 deiodinase (D3), the main enzyme responsible for the inactivation of thyroid hormones. The induction of D3 in models of illness and injury has raised critical questions about the physiological significance of reduced thyroid hormone availability in those states. Phenotypes in transgenic mice lacking this enzyme also point to important developmental roles for D3. A critical determinant of D3 expression is genomic imprinting, an epigenetic phenomenon that regulates a small number of dosage-critical genes in the mammalian genome. The D3 gene (Dio3) is imprinted and preferentially expressed from one of the alleles in most tissues.

SCOPE OF REVIEW

In the context of the physiological significance of D3 and the characteristics and purported origins of genomic imprinting, we review the current knowledge about the epigenetic mechanisms specifying gene dosage in the Dio3 locus.

MAJOR CONCLUSIONS

Altered Dio3 dosage is detrimental to development, suggesting that the level of thyroid hormone action needs to be exquisitely tailored in a timely fashion to the requirements of particular tissues. An appropriate Dio3 dosage is the result of the coordinated action of certain genomic elements and epigenetic marks in the Dlk1-Dio3 domain.

GENERAL SIGNIFICANCE

The imprinting of Dio3 prompts intriguing questions about why the level of thyroid hormone signaling should be regulated in this rare epigenetic manner, and to what extent altered Dio3 expression due to aberrant imprinting may be implicated in human conditions. This article is part of a Special Issue entitled Thyroid hormone signalling.

Type 3 deiodinase deficiency results in functional abnormalities at multiple levels of the thyroid axis

The type 3 deiodinase (D3) is a selenoenzyme that inactivates thyroid hormones and is highly expressed during development and in the adult central nervous system. We have recently observed that mice lacking D3 activity (D3KO mice) develop perinatal thyrotoxicosis followed in adulthood by a pattern of hormonal levels that is suggestive of central hypothyroidism. In this report we describe the results of additional studies designed to investigate the regulation of the thyroid axis in this unique animal model. Our results demonstrate that the thyroid and pituitary glands of D3KO mice do not respond appropriately to TSH and TRH stimulation, respectively. Furthermore, after induction of severe hypothyroidism by antithyroid treatment, the rise in serum TSH in D3KO mice is only 15% of that observed in wild-type mice. In addition, D3KO animals rendered severely hypothyroid fail to show the expected increase in prepro-TRH mRNA in the paraventricular nucleus of the hypothalamus. Finally, treatment with T(3) results in a serum T(3) level in D3KO mice that is much higher than that in wild-type mice. This is accompanied by significant weight loss and lethality in mutant animals. In conclusion, the absence of D3 activity results in impaired clearance of T(3) and significant defects in the mechanisms regulating the thyroid axis at all levels: hypothalamus, pituitary, and thyroid.

Gene expression from the imprinted Dio3 locus is associated with cell proliferation of cultured brown adipocytes

Active thyroid hormones are critical for the differentiation and function of brown adipose tissue. However, we have observed high basal and induced levels of type 3 deiodinase (D3), an enzyme that inactivates thyroid hormones and is coded by the imprinted gene Dio3, in differentiating brown preadipocytes in primary culture. We find that D3 activity and mRNA expression strongly correlate with the rate of proliferation of undifferentiated precursor cells under various conditions. Furthermore, differentiation of precursor cells to adipocytes is associated with decreased levels of D3 expression, and only very low levels of D3 mRNA are found in mature adipocytes. Dlk1, an inhibitor of adipocyte differentiation and a paternally expressed gene located in the same imprinted domain as Dio3, displayed changes in expression that parallel those of Dio3. In contrast, a 4-kb transcript for Dio3os, an antisense gene also located in the same imprinted domain, is markedly up-regulated in differentiated adipocytes. We conclude that D3 expression in differentiating preadipocytes is primarily linked to proliferating cells, whereas Dio3os expression is associated with mature adipocytes. Our results suggest that genomic imprinting and gene expression at the Dlk1/Dio3 imprinted domain may play a role in the regulation of adipocyte proliferation and differentiation.

Exposure to PCB 77 induces tissue-dependent changes in iodothyronine deiodinase activity patterns in the embryonic chicken

PCB 77 is a dioxin-like PCB that has been shown to reduce circulating thyroid hormone (TH) levels. This may be an important factor contributing to its neurotoxicity, since THs are essential for normal brain development. In this study, we investigated the changes in TH activating and inactivating iodothyronine deiodinase (D) activities in liver, telencephalon and cerebellum of chicken embryos during the final stages of embryonic development and hatching. We combined these results with measurements of plasma TH levels and intracellular TH availability in the tissues mentioned above, to find out whether D activity was a factor contributing to the PCB 77-induced decrease in peripheral TH levels and/or whether it was capable of reducing the adverse effects on brain via compensatory mechanisms. PCB 77 reduced both T(4) and T(3) levels in plasma and brain. Its effect on hepatic D1 and D3 activity was limited and rebuts a causative role of hepatic Ds in the drop of plasma TH levels. In cerebellum, D2 increased and D3 decreased, indicating a compensatory mechanism in this brain part, mainly during the stages of pipping and hatching. The changes in telencephalon occurred at the earlier stages and included an increase in both D2 and D3 activity.

Type 3 deiodinase is critical for the maturation and function of the thyroid axis

Developmental exposure to appropriate levels of thyroid hormones (THs) in a timely manner is critical to normal development in vertebrates. Among the factors potentially affecting perinatal exposure of tissues to THs is type 3 deiodinase (D3). This enzyme degrades THs and is highly expressed in the pregnant uterus, placenta, and fetal and neonatal tissues. To determine the physiological role of D3, we have generated a mouse D3 knockout model (D3KO) by a targeted inactivating mutation of the Dio3 gene in mouse ES cells. Early in life, D3KO mice exhibit delayed 3,5,3'-triiodothyronine (T3) clearance, a markedly elevated serum T3 level, and overexpression of T3-inducible genes in the brain. From postnatal day 15 to adulthood, D3KO mice demonstrate central hypothyroidism, with low serum levels of 3,5,3',5'-tetraiodothyronine (T4) and T3, and modest or no increase in thyroid-stimulating hormone (TSH) concentration. Peripheral tissues are also hypothyroid. Hypothalamic T3 content is decreased while thyrotropin-releasing hormone (TRH) expression is elevated. Our results demonstrate that the lack of D3 function results in neonatal thyrotoxicosis followed later by central hypothyroidism that persists throughout life. These mice provide a new model of central hypothyroidism and reveal a critical role for D3 in the maturation and function of the thyroid axis.

The roles of the iodothyronine deiodinases in mammalian development

The thyroid hormones (TH) are essential for normal development in vertebrate species. This review considers the roles that the three deiodinases, types 1, 2 and 3 (D1, D2, and D3), play in regulating intracellular levels of TH during this critical period. The focus is on rodents and humans with emphasis on brain development. There is little evidence to suggest that the D1 plays a significant role in development and this is substantiated by the absence of any obvious developmental impairment in a D1-deficient mouse model. There is, however, compelling indirect evidence pertaining to the importance of the D2 in development, particularly with respect to that of the brain. However, surprisingly, a D2-deficient mouse model exhibits a very mild phenotype. This, together with the fact that D2 activity is increased in hypothyroidism, suggests that this deiodinase may be of greater importance in development when supplies of thyroxine are limited. The D3 is clearly essential for development in the euthyroid mammal. Information, both indirect and that obtained from a D3-deficient mouse model, strongly suggests that its presence in placenta, uterus and some fetal tissues are critical for limiting exposure of fetal tissues to inappropriate levels of TH.

Complex organization and structure of sense and antisense transcripts expressed from the DIO3 gene imprinted locus

The human DIO3 gene and its mouse homolog, Dio3, map to chromosomes 14q32 and 12F1, respectively, and code for the type 3 deiodinase, an enzyme that inactivates thyroid hormones and is highly expressed during pregnancy and development. Mouse Dio3 is imprinted and preferentially expressed from the paternal allele in the fetus. We analyzed the human DIO3 genomic region and identified a gene (DIO3OS) that is transcribed in the antisense orientation. Multiple DIO3OS transcripts are expressed in most tissues. The structure of several DIO3OS cDNAs obtained by RT-PCR-based techniques reveals the occurrence of numerous splice variants. The exon-intron structures of DIO3OS are similar in mouse and human, but the homology of the exonic sequence is very low, except for the first exon, and no conserved open reading frame is present. We also detected DIO3 transcripts containing additional 5' untranslated sequence and a potential alternative upstream promoter for mouse Dio3. Exonic sequence of a Dio3os cDNA overlaps with the Dio3 promoter and strong promoter activity in the antisense orientation is detected in a genomic fragment located 3' of mouse and human DIO3 but not in the DIO3 promoter region. These results suggest that the DIO3 gene may lie within the structure of the antisense gene, a complex arrangement often observed in imprinted loci.

Type I iodothyronine deiodinase in euthyroid and hypothyroid chicken cerebellum

Immunocytochemistry using polyclonal anti-type I deiodinase (D1) led to the localization of D1 protein in the internal granule cells of the cerebellum in 1-day-old chicks, which was confirmed by the presence of in vitro D1 activity. Western blot analysis of hepatic and cerebellar extracts revealed a band of 27 kDa. In hypothyroid embryos D1 was expressed in both the internal and external granule cell layer and the signal diminished with more severe hypothyroidism, which is in agreement with the expected downregulation of D1 activity during hypothyroidism. In accordance with the protein data, hypothyroidism resulted in the downregulation of cerebellar D1 mRNA. Finally, histological stainings confirmed that the lack of staining in the external germinal layer of 1-day-old euthyroid chicks was due to the fact that migration of immature granule cells from the external towards the internal layer was completed at this stage while cell migration was retarded in hypothyroid animals.

Dexamethasone inhibits growth factor-induced type 3 deiodinase activity and mRNA expression in a cultured cell line derived from rat neonatal brown fat vascular-stromal cells

Studies examining the regulation of the type 3 deiodinase (D3) have been hampered by the lack of cell lines that constitutively express this enzyme. To address this issue, a new cell line, designated brown fat vascular-stromal (BVS-1), was generated by continuous subculturing of precursor cells derived from the vascular-stromal fraction of rat neonatal brown fat. BVS-1 cells did not differentiate into adipocytes when cultured for 5 d in DMEM supplemented with 2% newborn calf serum, 4 nM insulin, 2 nM T(3), and 10 nM dexamethasone (DEX). However, when cultured in regular medium, the cells expressed high levels of D3 activity (1-5 pmol/h per milligram protein) and mRNA. D3 mRNA was markedly induced by treatment for 6 h with epidermal growth factor, acid or basic fibroblast growth factors (10 ng/ml), or a 3-h treatment with a phorbol ester [12-O-tetradecanoylphorbol-13-acetate (TPA), 1 microM] or 10% fetal bovine serum. However, preincubation of cells overnight with 50 nM DEX completely blocked the D3-inducing effects of basic fibroblast growth factor. The DEX effect was partially blocked when a glucocorticoid receptor antagonist was present. Overnight DEX treatment (50 nM) also decreased basal D3 activity by 80%. In summary, we have established BVS-1 cells as a continuous cell line useful for studying the regulation of D3 expression. Furthermore, we have shown that DEX inhibits growth factor-induced D3 expression in these cells.

Pregnant rat uterus expresses high levels of the type 3 iodothyronine deiodinase

Although thyroid hormones are critically important for the coordination of morphogenic processes in the fetus and neonate, premature exposure of the embryo to levels of the hormones present in the adult is detrimental and can result in growth retardation, malformations, and even death. We report here that the pregnant rat uterus expresses extremely high levels of the type 3 iodothyronine deiodinase (D3), which inactivates thyroxine and 3,3', 5-triiodothyronine by 5-deiodination. Both D3 mRNA and activity were present at the implantation site as early as gestational day 9 (E9), when expression was localized using in situ hybridization to uterine mesometrial and antimesometrial decidual tissue. At later stages of gestation, uterine D3 activity remained very high, and the levels exceeded those observed in the placenta and in fetal tissues. After days E12 and E13, as decidual tissues regressed, D3 expression became localized to the epithelial cells lining the recanalized uterine lumen that surrounds the fetal cavity. These findings strongly suggest that the pregnant uterus, in addition to the placenta, plays a critical role in determining the level of exposure of the fetus to maternal thyroid hormones.

Iodothyronine deiodinase activities in fetal rat tissues at several levels of iodine deficiency: a role for the skin in 3,5,3'-triiodothyronine economy?

Iodothyronine deiodinases, types I, II, and III (D1, D2, and D3) activities were measured in tissues of fetal rats, at 18 and 21 days of gestation, at several levels of iodine deficiency (ID): mild ID diet (MID) and moderately severe ID, MID + 0.005% perchlorate (MID+P). D2 was present in fetal skin, increased between days 18 and 21, and also in MID and MID+P. In skin, D3 increased during ID at day 18, whereas there was a decrease at day 21. Skin T4 decreased in MID and MID+P, showing an inverse relationship with D2. Skin T3 decreased at day 18 in MID and MID+P but increased at day 21, probably because of the increased D2 and decreased D3, maintaining T3 concentrations. No effect of ID was observed on hepatic D1. D2 increased in brain and brown adipose tissue at day 21 in MID+P. No changes were found in maternal placental D2 and D3, but D2 and D3 increased in the fetal placenta at day 18 in MID+P. A higher level of D2 is present in fetal skin than in the brain. As the activity is increased, in even mild ID (and already at 18 days) it can be concluded that skin D2 is likely to be of considerable physiological importance, at least for fetal thyroid hormone economy, by contributing to the intracellular T3 content of the skin and, possibly, to the plasma T3.

Transcriptional activation of type III inner ring deiodinase by growth factors in cultured rat brown adipocytes

The activity of the type III inner ring deiodinase (DIII), which converts T4 and T3 to inactive metabolites, is induced by serum and growth factors in primary cultures of rat brown adipocytes. The contribution of pretranslational mechanisms to this increase in DIII activity was examined in the present studies. DIII mRNA is undetectable in differentiated brown adipocytes when cultured in serum-free medium. However, exposure to epidermal growth factor (EGF), acidic or basic fibroblast growth factors (aFGF or bFGF) increase DIII transcript levels. Lesser inductions are found with platelet-derived growth factor, and insulin-like growth factor I has no effect. Maximal induction of DIII mRNA is obtained after 9 h of exposure to EGF, bFGF, or aFGF at a concentration of 10 ng/ml. The increase in DIII mRNA in response to aFGF, bFGF, and EGF requires gene transcription and protein synthesis, as the inductive effect on mRNA is completely blocked by actinomycin D or cycloheximide. The DIII mRNA half-life is 4 h when stimulated with bFGF and increases to 12 h when 10% serum, EGF, or aFGF is present. In conclusion, EGF, aFGF, and bFGF increase DIII mRNA expression in differentiated brown adipocytes. This effect appears to be exerted at the level of both enhanced transcription and mRNA stabilization.

Induction of 5-deiodinase activity in astroglial cells by 12-O-tetradecanoylphorbol 13-acetate and fibroblast growth factors

In the brain, 5'-deiodinase (5'-D) is responsible for the metabolic activation of thyroxine (T4) into 3,5,3'-triiodothyronine (T3) and 5-deiodinase (5-D) deiodinates T4 and T3 into inactive metabolites. This study examines the effects of factors known to induce astroglial 5'-D activity on the 5-D activity in cultured rat astroglial cells. The potencies of these factors were compared after 8 h of incubation, when stimulations by these factors near their maximal effects. 12-O-Tetradecanoylphorbol 13-acetate (TPA) at 10(-7) M was a potent inducer of 5-D activity, producing a 30- to 80-fold increase after 8 h. The maximal effect of TPA was observed after about 14 h. The TPA stimulation of 5-D activity was not dependent on glucocorticoids, unlike 5'-D activity. In comparison with TPA, 8-bromo-cyclic AMP (10(-3) M) was a poor inducer of 5-D activity whereas it is an excellent inducer of 5'-D activity. It produced a 2- to 20-fold increase in 5-D activity after 8 h. Natural acidic fibroblast growth factor (20 ng/ml) produced a degree of stimulation similar to that of TPA after 8 h. The maximal effect of acidic fibroblast growth factor was observed after about 16 h (until a 120-fold increase). Recombinant acidic fibroblast growth factor also induced 5-D activity. Basic fibroblast growth factor was less potent than acidic fibroblast growth factor for increasing 5-D activity (maximal increase by 40- to 50-fold after 8 h).(ABSTRACT TRUNCATED AT 250 WORDS)

Phenolic and tyrosyl ring iodothyronine deiodination by the Caco-2 human colon carcinoma cell line

Thyroid hormone metabolism was studied in the human Caco-2 colon carcinoma cell line, which at confluence exhibits several functions of differentiated enterocytes. Cells were harvested two to 17 days after reaching confluence. Intact cells and homogenates were tested for deiodination of [125I]-labeled substrates. Small amounts of thyroxine (T4) were converted by homogenates to 3,3',5'-triiodothyronine (rT3), 3,3'-diiodothyronine (3,3'-T2), and 1-, with no detectable production of 3,5,3'-triiodothyronine (T3) by homogenates or cells. rT3 was converted to 3,3'-T2 and 1- with an apparent Michaelis constant (Km) for rT3 of 24 nmol/L; 6-n-propyl-2-thiouracil (PTU) had a 50% inhibitory concentration of 30 nmol/L and abolished rT3 5'-deiodination at 1 mmol/L in the presence of 20 mmol/L dithiothreitol (DTT). T3 was deiodinated to 3,3'-T2 and 3'-monoiodothyronine (3'-T1) with an apparent Michaelis constant (Km) for T3 of 5.7 nmol/L; this reaction was not inhibited by 1 mmol/L PTU. Phenolic and tyrosyl ring deiodinating activities were maximal four and six days, respectively, after the cells reached confluence. Homogenates of cells grown in standard medium containing fetal calf serum had fivefold higher rT3 5'-deiodinating activity than cells grown in a serum-free defined culture medium, reflecting a fivefold difference in the apparent Vmax with no difference in the apparent Km for rT3. There was no difference in T3 5-deiodination rates in homogenates of Caco-2 cells grown in the two media until 12 days postconfluence, when cells grown in standard medium had higher activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence against benefit from replacement doses of thyroid hormones in nonthyroidal illness (NTI): studies using turpentine oil-injected rat

Sprague-Dawley rats were treated with saline or turpentine oil (5 mu 1/g bw sc at 3-day intervals x3) with or without replacement doses of T4 (0.8 g/100 g bw/day ip) or T3(0.3 microgram/100 g bw/day ip). Injection of turpentine oil to the rat consistently caused a significant reduction in serum total T4, total T3, free T4 index and TSH. Despite marked changes in thyroidal economy in experimental rat, iodothyronine 5'-monodeiodinating activity (MA) in the liver, the kidney and the hearth and the hepatic alpha-glycerophosphate dehydrogenase activity were decreased inconsistently and when decreased, the various enzyme activities were not influenced appreciably by treatment with replacement doses of T4 or T3. Cerebral cortical T4 5-MA was normal or increased in the turpentine oil-injected rat. Dermal T4 5-MA was decreased in the turpentine oil-injected rat and replacement doses of thyroid hormones did not normalize it. Urinary excretion of urea nitrogen was normal in the turpentine oil-injected rat and did not change appreciably after treatment with thyroid hormones. Our data suggest that replacement doses of thyroid hormones are not beneficial to a host with altered thyroid economy during a systemic illness.