The liver of Fundulus heteroclitus expresses deiodinase type 1 mRNA

Cross-species applicability of an adverse outcome pathway network for thyroid hormone system disruption

Thyroid hormone system disrupting compounds are considered potential threats for human and environmental health. Multiple adverse outcome pathways (AOPs) for thyroid hormone system disruption (THSD) are being developed in different taxa. Combining these AOPs results in a cross-species AOP network for THSD which may provide an evidence-based foundation for extrapolating THSD data across vertebrate species and bridging the gap between human and environmental health. This review aimed to advance the description of the taxonomic domain of applicability (tDOA) in the network to improve its utility for cross-species extrapolation. We focused on the molecular initiating events (MIEs) and adverse outcomes (AOs) and evaluated both their plausible domain of applicability (taxa they are likely applicable to) and empirical domain of applicability (where evidence for applicability to various taxa exists) in a THSD context. The evaluation showed that all MIEs in the AOP network are applicable to mammals. With some exceptions, there was evidence of structural conservation across vertebrate taxa and especially for fish and amphibians, and to a lesser extent for birds, empirical evidence was found. Current evidence supports the applicability of impaired neurodevelopment, neurosensory development (eg, vision) and reproduction across vertebrate taxa. The results of this tDOA evaluation are summarized in a conceptual AOP network that helps prioritize (parts of) AOPs for a more detailed evaluation. In conclusion, this review advances the tDOA description of an existing THSD AOP network and serves as a catalog summarizing plausible and empirical evidence on which future cross-species AOP development and tDOA assessment could build.

Deiodinases: How Nonmammalian Research Helped Shape Our Present View

Iodothyronine deiodinases are enzymes capable of activating and inactivating thyroid hormones (THs) and have an important role in regulating TH action in tissues throughout the body. Three types of deiodinases (D1, D2, and D3) were originally defined based on their biochemical characteristics. Cloning of the first complementary DNAs in the 1990s (Dio1 in rat and dio2 and dio3 in frog) allowed to confirm the existence of 3 distinct enzymes. Over the years, increasing genomic information revealed that deiodinases are present in all chordates, vertebrates, and nonvertebrates and that they can even be found in some mollusks and annelids, pointing to an ancient origin. Research in nonmammalian models has substantially broadened our understanding of deiodinases. In relation to their structure, we discovered for instance that biochemical properties such as inhibition by 6-propyl-2-thiouracil, stimulation by dithiothreitol, and temperature optimum are subject to variation. Data from fish, amphibians, and birds were key in shifting our view on the relative importance of activating and inactivating deiodination pathways and in showing the impact of D2 and D3 not only in local but also whole body T3 availability. They also led to the discovery of new local functions such as the acute reciprocal changes in D2 and D3 in hypothalamic tanycytes upon photostimulation, involved in seasonal rhythmicity. With the present possibilities for rapid and precise gene silencing in any species of interest, comparative research will certainly further contribute to a better understanding of the importance of deiodinases for adequate TH action, also in humans.

An AOP-based alternative testing strategy to predict the impact of thyroid hormone disruption on swim bladder inflation in zebrafish

The adverse outcome pathway (AOP) framework can be used to help support the development of alternative testing strategies aimed at predicting adverse outcomes caused by triggering specific toxicity pathways. In this paper, we present a case-study demonstrating the selection of alternative in chemico assays targeting the molecular initiating events of established AOPs, and evaluate use of the resulting data to predict higher level biological endpoints. Based on two AOPs linking inhibition of the deiodinase (DIO) enzymes to impaired posterior swim bladder inflation in fish, we used in chemico enzyme inhibition assays to measure the molecular initiating events for an array of 51 chemicals. Zebrafish embryos were then exposed to 14 compounds with different measured inhibition potentials. Effects on posterior swim bladder inflation, predicted based on the information captured by the AOPs, were evaluated. By linking the two datasets and setting thresholds, we were able to demonstrate that the in chemico dataset can be used to predict biological effects on posterior chamber inflation, with only two outliers out of the 14 tested compounds. Our results show how information organized using the AOP framework can be employed to develop or select alternative assays, and successfully forecast downstream key events along the AOP. In general, such in chemico assays could serve as a first-tier high-throughput system to screen and prioritize chemicals for subsequent acute and chronic fish testing, potentially reducing the need for long-term and costly toxicity tests requiring large numbers of animals.

Duplication of Dio3 genes in teleost fish and their divergent expression in skin during flatfish metamorphosis

Deiodinase 3 (Dio3) plays an essential role during early development in vertebrates by controlling tissue thyroid hormone (TH) availability. The Atlantic halibut (Hippoglossus hippoglossus) possesses duplicate dio3 genes (dio3a and dio3b). Expression analysis indicates that dio3b levels change in abocular skin during metamorphosis and this suggests that this enzyme is associated with the divergent development of larval skin to the juvenile phenotype. In larvae exposed to MMI, a chemical that inhibits TH production, expression of dio3b in ocular skin is significantly up-regulated suggesting that THs normally modulate this genes expression during this developmental event. The molecular basis for divergent dio3a and dio3b expression and responsiveness to MMI treatment is explained by the multiple conserved TREs in the proximal promoter region of teleost dio3b and their absence from the promoter of dio3a. We propose that the divergent expression of dio3 in ocular and abocular skin during halibut metamorphosis contributes to the asymmetric pigment development in response to THs.

Microcystin-LR Alters the Gene Transcription and Activities of Iodothyronine Deiodinases in the Hepatic Cells of Grass Carp (Ctenopharyngodon Idella)

This study investigated the gene transcription and activities of iodothyronine deiodinases in the hepatic cell line of grass carp (Ctenopharyngodon idella) exposed in vitro to 1, 10, 100, and 1000 μg /L microcystin-LR (MC-LR) for either 24 or 48 h. The cell viabilities were not significantly affected by MC-LR exposure. The mRNA expressions of type I iodothyronine deiodinase (ID1) and type Ⅱ iodothyronine deiodinase (ID2) reduced after the exposure to MC-LR. However, MC-LR exposure led to the increase in the mRNA expression of type Ⅲ iodothyronine deiodinase (ID3). Moreover, significant ID1 and ID2 activities decline were also observed in the hepatic cell line of grass carp exposed to MC-LR, and the activity of ID3 increased significantly in the MC-LR treated groups. The results suggested that MC-LR could alter the gene transcription or activities of IDs in the hepatic cell line of grass carp.

The variable region of iodothyronine deiodinases directs their catalytic properties and subcellular localization

The stereospecific removal of iodine from thyroid hormones is an essential first step for T3 action and is catalyzed by three different deiodinases: D2 and D3 remove iodine only from the outer or inner ring, respectively, whereas D1 catalyzes both pathways. We used in silico predictions from vertebrate deiodinase sequences to identify two domains: the N-terminal variable region (VR) containing the transmembrane, hinge and linker domains, and the conserved or globular region (CR). Given the high sequence and structural identity of the CR among paralogs as well as of the VR among orthologs but not paralogs, we hypothesized that both the catalytic properties and the subcellular localization rely on the VR. We used shark D2 and D3 as templates to build the chimeric enzymes D2VR/D3CR and D3VR/D2CR. Biochemical characterization revealed that D3VR/D2CR has inner-ring deiodination activity and T3 as preferred substrate, whereas D2VR/D3CR showed no deiodinating activity. Also, D2VR/D3CR and D3VR/D2CR reside in the endoplasmic reticulum and plasmatic membrane, respectively, as do their D2 and D3 wild-type counterparts. We conclude that the VR determines the subcellular localization and is critical in defining the catalytic properties and activity of thyroid hormone deiodinases.

Zebrafish as a model to study peripheral thyroid hormone metabolism in vertebrate development

To unravel the role of thyroid hormones (THs) in vertebrate development it is important to have suitable animal models to study the mechanisms regulating TH availability and activity. Zebrafish (Danio rerio), with its rapidly and externally developing transparent embryo has been a widely used model in developmental biology for some time. To date many of the components of the zebrafish thyroid axis have been identified, including the TH transporters MCT8, MCT10 and OATP1C1, the deiodinases D1, D2 and D3, and the receptors TRα and TRβ. Their structure and function closely resemble those of higher vertebrates. Interestingly, due to a whole genome duplication in the early evolution of ray-finned fishes, zebrafish possess two genes for D3 (dio3 and dio3a) and for TRα (thraa and thrab). Transcripts of all identified genes are present during embryonic development and several of them show dynamic spatio-temporal distribution patterns. Transient morpholino-knockdown of D2, D3 or MCT8 expression clearly disturbs embryonic development, confirming the importance of each of these regulators during early life stages. The recently available tools for targeted stable gene knockout will further increase the value of zebrafish to study the role of peripheral TH metabolism in pre- and post-hatch/post-natal vertebrate development.

Iodothyronine deiodinases: a functional and evolutionary perspective

From an evolutionary perspective, deiodinases may be considered pivotal players in the emergence and functional diversification of both thyroidal systems (TS) and their iodinated messengers. To better understand the evolutionary pathway and the concomitant functional diversification of vertebrate deiodinases, in the present review we summarized the highlights of the available information regarding this ubiquitous enzymatic component that represents the final, common physiological link of TS. The information reviewed here suggests that deiodination of tyrosine metabolites is an ancient feature of all chordates studied to date and consequently, that it precedes the integration of the TS that characterize vertebrates. Phylogenetic analysis presented here points to D1 as the oldest vertebrate deiodinase and to D2 as the most recent deiodinase gene, a hypothesis that agrees with the notion that D2 is the most specialized and finely regulated member of the family and plays a key role in vertebrate neurogenesis. Thus, deiodinases seem to be major participants in the evolution and functional expansion of the complex regulatory network of TS found in vertebrates.

Iodothyronine deiodinase structure and function: from ascidians to humans

Iodothyronine deiodinases are important mediators of thyroid hormone (TH) action. They are present in tissues throughout the body where they catalyse 3,5,3'-triiodothyronine (T(3)) production and degradation via, respectively, outer and inner ring deiodination. Three different types of iodothyronine deiodinases (D1, D2 and D3) have been identified in vertebrates from fish to mammals. They share several common characteristics, including a selenocysteine residue in their catalytic centre, but show also some type-specific differences. These specific characteristics seem very well conserved for D2 and D3, while D1 shows more evolutionary diversity related to its Km, 6-n-propyl-2-thiouracil sensitivity and dependence on dithiothreitol as a cofactor in vitro. The three deiodinase types have an impact on systemic T(3) levels and they all contribute directly or indirectly to intracellular T(3) availability in different tissues. The relative contribution of each of them, however, varies amongst species, developmental stages and tissues. This is especially true for amphibians, where the impact of D1 may be minimal. D2 and D3 expression and activity respond to thyroid status in an opposite and conserved way, while the response of D1 is variable, especially in fish. Recently, a number of deiodinases have been cloned from lower chordates. Both urochordates and cephalochordates possess selenodeiodinases, although they cannot be classified in one of the three vertebrate types. In addition, the cephalochordate amphioxus also expresses a non-selenodeiodinase. Finally, deiodinase-like sequences have been identified in the genome of non-deuterostome organisms, suggesting that deiodination of externally derived THs may even be functionally relevant in a wide variety of invertebrates.

Alterations along the Hypothalamic-Pituitary-Thyroid Axis of the Zebrafish (Danio rerio) after Exposure to Propylthiouracil

In the past, various approaches have been developed to detect adverse effects of pollutants on the thyroid of vertebrates, most of these with special emphasis on the South African clawed frog, Xenopus laevis. Although fish are primarily affected by thyroid-disrupting chemicals, studies into alterations of the thyroid of fish are scarce. Therefore, effects of the reference compound propylthiouracil on histopathology of the thyroid axis were analyzed in a modified early life-stage test with zebrafish (Danio rerio) exposed to propylthiouracil. The test substance induced dose-dependent alterations of thyroidal tissue concomitant with increases in the number of surrounding blood vessels. Despite this massive proliferation of the thyroid, zebrafish were not able to maintain thyroxin concentrations. The pituitary was affected displaying significant alterations in thyroid-stimulating hormone cell counts. Quantitative evaluation of pituitary surface areas revealed a dose-dependent increase of adenohypophyseal tissue. Distinct histopathological effects may contribute to a more easy identification and interpretation of alterations induced by thyroid-disrupting chemicals.

Coordination of deiodinase and thyroid hormone receptor expression during the larval to juvenile transition in sea bream (Sparus aurata, Linnaeus)

To test the hypothesis that THs play an important role in the larval to juvenile transition in the marine teleost model, sea bream (Sparus auratus), key elements of the thyroid axis were analysed during development. Specific RT-PCR and Taqman quantitative RT-PCR were established and used to measure sea bream iodothyronine deiodinases and thyroid hormone receptor (TR) genes, respectively. Expression of deiodinases genes (D1 and D2) which encode enzymes producing T3, TRs and T4 levels start to increase at 20-30 days post-hatch (dph; beginning of metamorphosis), peak at about 45 dph (climax) and decline to early larval levels after 90-100 dph (end of metamorphosis) when fish are fully formed juveniles. The profile of these different TH elements during sea bream development is strikingly similar to that observed during the TH driven metamorphosis of flatfish and suggests that THs play an analogous role in the larval to juvenile transition in this species and probably also in other pelagic teleosts. However, the effect of T3 treatment on deiodinases and TR transcript abundance in sea bream is not as clear cut as in larval flatfish and tadpoles indicating divergence in the responsiveness of TH axis elements and highlighting the need for further studies of this axis during development of fish.

Xenopus laevis as a model for studying thyroid hormone signalling: from development to metamorphosis

Amphibian metamorphosis is a well-established model for dissecting the mechanisms underlying thyroid hormone (TH) action. How the pro-hormone, T(4), the active form, T(3), the deiodinases and the nuclear receptors (TRs) contribute to metamorphosis in Xenopus has been extensively investigated. Our recent work has concentrated on two key ideas in TH signalling in Xenopus: first, that there could be active roles for both liganded and unliganded receptors, and second, that ligand availability is a determining factor orchestrating these actions and is tightly controlled in target tissues. Recently, we addressed these questions at stages preceding metamorphosis, i.e. during embryogenesis, before differentiation of a functional thyroid gland. We show that repression by unliganded TR is essential to craniofacial and eye development during early development and that at these stages all three deiodinases are active. These results open new perspectives on the potential roles of TH signalling during embryogenesis.

Thyroid disruption in walleye (Sander vitreus) exposed to environmental contaminants: cloning and use of iodothyronine deiodinases as molecular biomarkers

Thyroid hormones play a role in the initiation of ovarian maturation in fish. Thus, reports of delayed sexual maturation in female walleye (Sander vitreus) exposed to contaminants in the Ottawa River suggest the presence of endocrine disrupting chemicals. The objectives of this study were to assess the effects of environmental contaminants in the Ottawa River on thyroid hormones of immature walleye and to develop a molecular biomarker of thyroid status. Walleye were sampled in the Ottawa River at Deep River (reference site), at Rivière Blanche (downstream from the Ottawa and Gatineau municipal wastewater treatment plants outflows), and at Plaisance (downstream from a pulp and paper mill). Plasma thyroid hormone levels were measured by radioimmunoassay. Walleye at Plaisance had two-fold elevated levels of thyroxine (T(4)) and 1.5-fold elevated levels triiodothyronine (T(3)), whereas the molar ratio of T(3):T(4) was reduced by over 50% compared to Deep River. Plasma T(3) levels were also elevated by approximately 1.5-fold at Rivière Blanche. Three iodothyronine deiodinases, a family of enzymes responsible for converting the prohormone T(4) to biologically active T(3), as well as for inactivating these two hormones, were partially cloned in walleye. A real-time PCR assay of deiodinase expression indicated that hepatic mRNA levels of type I and type III deiodinase were not modified between sites, whereas they were increased for type II deiodinase at Rivière Blanche as compared to the other sites. The response of this novel molecular transcript indicates a divergence with that expected based on the effects of experimentally induced hyperthyroidism on fish deiodinase expression; additional endpoints are therefore necessary to interpret changes in thyroid hormones levels in fish exposed to environmental contaminants.

The influence of stress on thyroid hormone production and peripheral deiodination in the Nile tilapia (Oreochromis niloticus)

The existence of an interaction between the adrenal/interrenal axis and the thyroidal axis has since long been established in vertebrates, including fish. However, in contrast to mammals, birds and amphibians, no effort was made in fish to expand these studies beyond the level of measuring plasma thyroid hormones. We therefore set out to examine the acute effects of a single dose of dexamethasone (DEX) on plasma thyroxine (T(4)) and 3,5,3'-triiodothyronine (T(3)) levels, as well as on the activity and mRNA expression of the different iodothyronine deiodinases in liver, gills, kidney and brain in Nile tilapia. To take into account the effect of handling stress, this treatment was compared both to a non-treated and to a saline injected group. In general, the observed changes were acute (3 and 6h) while values had returned to control levels by 24h post-injection. Only DEX administration caused an acute drop in circulating T(3) levels compared to non-treated animals, while none of the treatments affected plasma T(4) levels. This indicates that the DEX induced decrease in plasma T(3) levels was not due to a lowered thyroidal hormone production and secretion. DEX injection provoked a decrease in peripheral T(3) production capacity via a decrease in hepatic outer ring deiodination activity (both D1 and D2), whereas T(3) clearance increased by induction of the inner ring deiodinating D3 pathway in liver and in gills. Deiodination activities in kidney and brain were not affected. Effects of saline injection were only observed in liver, where D1 activity decreased and D3 activity increased as in the DEX group, but to a lesser extent. Real-time PCR showed that the changes in hepatic D3 were clearly regulated at the pretranslational level, while this was not confirmed for the other changes. Our results show that both handling stress and DEX injection acutely disturb peripheral deiodination activity in Nile tilapia. However, the effects of the long acting glucocorticoid analogue are more pronounced and result in a decrease in circulating T(3) availability.

Characterization of recombinant Xenopus laevis type I iodothyronine deiodinase: substitution of a proline residue in the catalytic center by serine (Pro132Ser) restores sensitivity to 6-propyl-2-thiouracil

In frogs such as Rana and Xenopus, metamorphosis does not occur in the absence of a functional thyroid gland. Previous studies indicated that coordinated development in frogs requires tissue and stage-dependent type II and type III iodothyronine deiodinase expression patterns to obtain requisite levels of intracellular T(3) in tissues at the appropriate stages of metamorphosis. No type I iodothyronine deiodinase (D1), defined as T(4) or reverse T(3) (rT3) outer-ring deiodinase (ORD) activity with Michaelis constant (K(m)) values in the micromolar range and sensitivity to 6-propyl-2-thiouracil (6-PTU), could be detected in tadpoles so far. We obtained a X. laevis D1 cDNA clone from brain tissue. The complete sequence of this clone (1.1 kb, including poly A tail) encodes an ORF of 252 amino acid residues with high homology to other vertebrate D1 enzymes. The core catalytic center includes a UGA-encoded selenocysteine residue, and the 3' untranslated region (about 300 nt) contains a selenocysteine insertion sequence element. Transfection of cells with an expression vector containing the full-length cDNA resulted in generation of significant deiodinase activity in the homogenates. The enzyme displayed ORD activity with T(4) (K(m) 0.5 microm) and rT3 (K(m) 0.5 microm) and inner-ring deiodinase activity with T(4) (K(m) 0.4 microm). Recombinant Xenopus D1 was essentially insensitive to inhibition by 6-PTU (IC(50) > 1 mm) but was sensitive to gold thioglucose (IC(50) 0.1 mum) and iodoacetate (IC(50) 10 microm). Because the residue 2 positions downstream from the selenocysteine is Pro in Xenopus D1 but Ser in all cloned PTU-sensitive D1 enzymes, we prepared the Pro132Ser mutant of Xenopus D1. The mutant enzyme showed strongly increased ORD activity with T(4) and rT3 (K(m) about 4 microm) and was highly sensitive to 6-PTU (IC(50) 2 microm). Little native D1 activity could be detected in Xenopus liver, kidney, brain, and gut, but significant D1 mRNA expression was observed in juvenile brain and adult liver and kidney. These results indicate the existence of a 6-PTU-insensitive D1 enzyme in X. laevis tissues, but its role during tadpole metamorphosis remains to be defined.

A cDNA for a putative type III deiodinase in the trout (Oncorhynchus mykiss): influence of holding conditions and thyroid hormone treatment on its hepatic expression

A putative rainbow trout type III deiodinase (D3) cDNA was amplified by PCR, using primers to evolutionarily conserved sequences. The RACE-derived complete cDNA was then identified by sequence comparison to that in tilapia and other vertebrates. The cDNA coded for a predicted 31,500 kDa protein of 278 amino acids, with a hydrophobic trans-membrane segment and with 80% similarity to tilapia D3 and 39% similarity to rainbow trout type II deiodinase (D2). It also showed a selenocysteine codon at position 141 and a putative SECIS element in the 3' untranslated end. In the liver, a second form of D3 was found that differed only at this 3' untranslated region; the coding region was identical in both forms. The D3 mRNA, measured by RT-PCR using primers located within the common, translated portion of the cDNA, was expressed in the brain and, depending on thyroidal status, in liver and kidney. Holding trout for 7 days in static water as opposed to flowing water caused increased plasma T4 levels, decreased hepatic D2 mRNA levels and T4 outer-ring deiodination (ORD) activity and increased D3 mRNA levels and T3 inner-ring (IRD) activity. Trout held in flowing water and fed T3 for 7 days showed increased plasma T3 levels and hepatic D3 mRNA levels and T3 IRD activity but decreased D2 mRNA levels and T4ORD activity. Trout held in static water and exposed to ambient T4 for 7 days showed increased plasma T4 levels and hepatic T3IRD activity but with no significant change in D2 or D3 mRNA levels. We conclude that hepatic D3 mRNA levels and T3IRD activity are enhanced and D2 mRNA levels and T4ORD activity are suppressed by adverse holding conditions or T3 treatment suggesting that the putative D3 cDNA and D2 cDNA represent respectively the genes determining T3IRD and T4ORD activities. However, there were changes in the ratios of mRNA levels to enzyme activity, raising the potential for post-transcriptional regulation and showing that mRNA levels alone may be unreliable indices of deiodinase activity. Post-transcriptional regulation of D3 enzyme activity may be influenced by the observed alternative 3' and 5' untranslated regions of the D3 mRNA.

Biochemical mechanisms of thyroid hormone deiodination

Deiodination is the foremost pathway of thyroid hormone metabolism not only in quantitative terms but also because thyroxine (T(4)) is activated by outer ring deiodination (ORD) to 3,3',5-triiodothyronine (T(3)), whereas both T(4) and T(3) are inactivated by inner ring deiodination (IRD) to 3,3',5-triiodothyronine and 3,3'-diiodothyronine, respectively. These reactions are catalyzed by three iodothyronine deiodinases, D1-3. Although they are homologous selenoproteins, they differ in important respects such as catalysis of ORD and/or IRD, deiodination of sulfated iodothyronines, inhibition by the thyrostatic drug propylthiouracil, and regulation during fetal and neonatal development, by thyroid state, and during illness. In this review we will briefly discuss recent developments in these different areas. These have resulted in the emerging view that the biological activity of thyroid hormone is regulated locally by tissue-specific regulation of the different deiodinases.

Regulatory role of the 3' untranslated region (3'UTR) of rat 5' deiodinase (D1). effects on messenger RNA translation and stability

The previous findings that both a long and a short type 1 deiodinase (D1) mRNA are present in different tissues and that the D1 gene contains two potential polyA signals suggest that the two mRNAs result from differential polyA signal usage. In this study, we examined the properties of the two D1 mRNAs generated in HEK 293 cells by the alternative use of each of the poly A signals in order to ascertain the potential regulatory role of the 3'UTR of this gene. Our results showed that the long mRNA is less stable, but that it is translated more efficiently than the short mRNA. The net result of these differences is a higher D1 activity with the long message. These data suggest that the D1 3'UTR may play an important role in regulating the stability and translational efficiency of the D1 mRNA, both of which could be physiologically relevant when the demand for D1 activity is high.

Thyroid hormone deiodination in fish

We review the experimental evidence accumulated within the past decade regarding the physiologic, biochemical, and molecular characterization of iodothyronine deiodinases (IDs) in piscine species. Agnathans, chondrichthyes, and teleosts express the three isotypes of IDs: ID1, ID2, and ID3, which are responsible for the peripheral fine-tuning of thyroid hormone (TH) bioactivity. At the molecular and operational level, fish IDs share properties with their corresponding vertebrate counterparts. However, fish IDs also exhibit discrete features that seem to be distinctive for piscine species. Indeed, teleostean ID1 is conspicuously resistant to propylthiouracil (PTU) inhibition, and its response to thyroidal status differs from that exhibited by other ID1s. Moreover, both the high level of ID2 activity and its expression in the liver of teleosts are unique among vertebrates. The physiologic role of iodothyronine deiodination in functions regulated by TH in fish is not entirely clear. Nevertheless, current experimental evidence suggests that IDs may coordinate and facilitate, in a tissue-specific fashion, the action of iodothyronines and other hormones involved in such processes.

Circulating thyroid hormone levels and iodothyronine deiodinase activities in Nile tilapia (Oreochromis niloticus) following dietary exposure to Endosulfan and Aroclor 1254

We evaluated the effects of two organochlorinated environmental contaminants, Endosulfan and Aroclor 1254 on peripheral thyroid hormone metabolism and thyroid hormone plasma levels in Nile tilapia (Oreochromis niloticus). Tilapia were exposed through diet to 0.1 and 0.5 microg g(-1) of Endosulfan and 0.5 microg g(-1) of Aroclor 1254 for 21 and 35 days. Decreased plasma T4 and rT3 levels were observed in tilapia exposed to the lower dose of Endosulfan, while treatment with a higher dose and Aroclor 1254 produced no changes. Plasma T3 levels were not affected by these compounds. Hepatic type I deiodinase (D1) activity was depressed by a lower dose of Endosulfan and hepatic type III (D3) activity was increased following 35 days of exposure to the lower dose of Endosulfan and following 21 and 35 days of exposure to Aroclor 1254; while type II (D2) remained unchanged in liver as well as in all other organs analysed. Apart from hepatic D3 activity, Endosulfan and Aroclor 1254 also increased D3 activity in gill, but not in other tested organs. It is concluded that dietary exposure of tilapia to Endosulfan or Aroclor 1254 can lead to changes in circulating thyroid hormone levels and/or in peripheral thyroid hormone metabolism. The changes in hormone metabolism differ between tissues, eventually reflecting tissue-specific differences in adaptation.

Halometabolites and cellular dehalogenase systems: an evolutionary perspective

We review the role of iodothyronine deiodinases (IDs) in the evolution of vertebrate thyroidal systems within the larger context of biological metabolism of halogens. Since the beginning of life, the ubiquity of organohalogens in the biosphere has provided a major selective pressure for the evolution and conservation of cellular mechanisms specialized in halogen metabolism. Among naturally available halogens, iodine emerged as a critical component of unique developmental and metabolic messengers. Metabolism of iodinated compounds occurs in the three major domains of life, and invertebrate deuterostomes possess several biochemical traits and molecular homologs of vertebrate thyroidal systems, including ancestral homologs of IDs identified in urochordates. The finely tuned cellular regulation of iodometabolite uptake and disposal is a remarkable event in evolution and might have been decisive for the explosive diversification of ontogenetic strategies in vertebrates.

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).

Effects of iodothyronines on the hepatic outer-ring deiodinating pathway in killifish

Substrate availability has been thought to be a major regulator of the outer-ring deiodinating pathway (ORD) in fish. However, current information strongly suggests that while fish iodothyronine deiodinase type 2 (D2) responds to iodothyronines in the same manner as its mammalian counterpart, fish deiodinase type 1 (D1) exhibits a distinct response. Furthermore, 3,5-T2, generally considered to be an inactive product of iodothyronine metabolism, has recently been described as bioactive, but its effects upon D1 and D2 are not yet known. We examined the effect that short-term immersion in T4, T3, and 3,5-T2 (0.1 microM; 12 or 24 h) exerts on both D1 and D2 activities and on the levels of expression of D1 and D2 mRNAs in killifish liver. In agreement with previous reports in teleosts, no iodothyronine exerted a significant effect on D1 enzymatic activity. However, all three iodothyronines significantly decreased D2 activity. Furthermore, at 24 h post-immersion T4, T3, and 3,5-T2 inhibited both D1 and D2 transcription. Together, the present results confirm the differential effect of iodothyronines upon the hepatic ORD pathway in fish and show that this effect can occur at a transcriptional level. Furthermore, we provide the first evidence that 3,5-T2 can affect both activity and transcription of hepatic deiodinases in teleosts.

Debromination of the flame retardant decabromodiphenyl ether by juvenile carp (Cyprinus carpio) following dietary exposure

The congener 2,2',3,3',4,4',5,5',6,6'-decabromodiphenyl ether (BDE 209) is the primary component in a commonly used flame retardant known as decaBDE. This flame retardant constitutes approximately 80% of the world market demand for polybrominated diphenyl ethers (PBDEs). Because this compound is very hydrophobic (log K(ow) approximately 10), it has been suggested that BDE 209 has very low bioavailability, although debromination to more bioavailable metabolites has also been suggested to occur in fish tissues. In the present study, juvenile carp were exposed to BDE 209 amended food on a daily basis for 60 days, followed by a 40-day depuration period in which the fate of BDE 209 was monitored in whole fish and liver tissues separately. No net accumulation of BDE 209 was observed throughout the experiment despite an exposure concentration of 940 ng/day/fish. However, seven apparent debrominated products of BDE 209 accumulated in whole fish and liver tissues over the exposure period. These debrominated metabolites of BDE 209 were identified as penta- to octaBDEs using both GC/ECNI-MS and GC/HRMS. Using estimation methods for relative retention times of phenyl substitution patterns, we have identified possible structures for the hexa- and heptabromodiphenyl ethers identified in the carp tissues. Although exposure of carp to BDE 209 did not result in the accumulation of BDE 209 in carp tissues, our results indicate evidence of limited BDE 209 bioavailability from food in the form of lower brominated metabolites.