3,5-Diiodothyronine (T2) is on a role. A new hormone in search of recognition

Differential effects of 3,5-T2 and T3 on the gill regeneration and metamorphosis of the Ambystoma mexicanum (axolotl)

Thyroid hormones (THs) regulate tissue remodeling processes during early- and post-embryonic stages in vertebrates. The Mexican axolotl (Ambystoma mexicanum) is a neotenic species that has lost the ability to undergo metamorphosis; however, it can be artificially induced by exogenous administration of thyroxine (T4) and 3,3',5-triiodo-L-thyronine (T3). Another TH derivative with demonstrative biological effects in fish and mammals is 3,5-diiodo-L-thyronine (3,5-T2). Because the effects of this bioactive TH remains unexplored in other vertebrates, we hypothesized that it could be biologically active in amphibians and, therefore, could induce metamorphosis in axolotl. We performed a 3,5-T2 treatment by immersion and observed that the secondary gills were retracted, similar to the onset stage phenotype; however, tissue regeneration was observed after treatment withdrawal. In contrast, T4 and T3 immersion equimolar treatments as well as a four-fold increase in 3,5-T2 concentration triggered complete metamorphosis. To identify the possible molecular mechanisms that could explain the contrasting reversible or irreversible effects of 3,5-T2 and T3 upon gill retraction, we performed a transcriptomic analysis of differential expression genes in the gills of control, 3,5-T2-treated, and T3-treated axolotls. We found that both THs modify gene expression patterns. T3 regulates 10 times more genes than 3,5-T2, suggesting that the latter has a lower affinity for TH receptors (TRs) or that these hormones could act through different TR isoforms. However, both TH treatments regulated different gene sets known to participate in tissue development and cell cycle processes. In conclusion, 3,5-T2 is a bioactive iodothyronine that promoted partial gill retraction but induced full metamorphosis in higher concentrations. Differential effects on gill retraction after 3,5,-T2 or T3 treatment could be explained by the activation of different clusters of genes related with apoptosis, regeneration, and proliferation; in addition, these effects could be initially mediated by TRs that are expressed in gills. This study showed, for the first time, the 3,5,-T2 bioactivity in a neotenic amphibian.

The importance of thyroid hormone signaling during early development: Lessons from the zebrafish model

The zebrafish is an optimal experimental model to study thyroid hormone (TH) involvement in vertebrate development. The use of state-of-the-art zebrafish genetic tools available for the study of the effect of gene silencing, cell fate decisions and cell lineage differentiation have contributed to a more insightful comprehension of molecular, cellular, and tissue-specific TH actions. In contrast to intrauterine development, extrauterine embryogenesis observed in zebrafish has facilitated a more detailed study of the development of the hypothalamic-pituitary-thyroid axis. This model has also enabled a more insightful analysis of TH molecular actions upon the organization and function of the brain, the retina, the heart, and the immune system. Consequently, zebrafish has become a trendy model to address paradigms of TH-related functional and biomedical importance. We here compilate the available knowledge regarding zebrafish developmental events for which specific components of TH signaling are essential.

3,5-T2-an Endogenous Thyroid Hormone Metabolite as Promising Lead Substance in Anti-Steatotic Drug Development?

Thyroid hormones, their metabolites, and synthetic analogues are potential anti-steatotic drug candidates considering that subclinical and manifest hypothyroidism is associated with hepatic lipid accumulation, non-alcoholic fatty liver disease, and its pandemic sequelae. Thyromimetically active compounds stimulate hepatic lipogenesis, fatty acid beta-oxidation, cholesterol metabolism, and metabolic pathways of glucose homeostasis. Many of these effects are mediated by T3 receptor β1-dependent modulation of transcription. However, rapid non-canonical mitochondrial effects have also been reported, especially for the metabolite 3,5-diiodothyronine (3,5-T2), which does not elicit the full spectrum of “thyromimetic” actions inherent to T3. Most preclinical studies in rodent models of obesity and first human clinical trials are promising with respect to the antisteatotic hepatic effects, but potent agents exhibit unwanted thyromimetic effects on the heart and/or suppress feedback regulation of the hypothalamus-pituitary-thyroid-periphery axis and the fine-tuned thyroid hormone system. This narrative review focuses on 3,5-T2 effects on hepatic lipid and glucose metabolism and (non-)canonical mechanisms of action including its mitochondrial targets. Various high fat diet animal models with distinct thyroid hormone status indicate species- and dose-dependent efficiency of 3,5-T2 and its synthetic analogue TRC150094. No convincing evidence has been presented for their clinical use in the prevention or treatment of obesity and related metabolic conditions.

Bioenergetic Aspects of Mitochondrial Actions of Thyroid Hormones

Much is known, but there is also much more to discover, about the actions that thyroid hormones (TH) exert on metabolism. Indeed, despite the fact that thyroid hormones are recognized as one of the most important regulators of metabolic rate, much remains to be clarified on which mechanisms control/regulate these actions. Given their actions on energy metabolism and that mitochondria are the main cellular site where metabolic transformations take place, these organelles have been the subject of extensive investigations. In relatively recent times, new knowledge concerning both thyroid hormones (such as the mechanisms of action, the existence of metabolically active TH derivatives) and the mechanisms of energy transduction such as (among others) dynamics, respiratory chain organization in supercomplexes and cristes organization, have opened new pathways of investigation in the field of the control of energy metabolism and of the mechanisms of action of TH at cellular level. In this review, we highlight the knowledge and approaches about the complex relationship between TH, including some of their derivatives, and the mitochondrial respiratory chain.

Enrichment and Quantitative Determination of Free 3,5- Diiodothyronine, 3',5'-Diiodothyronine, and 3,5-Diiodothyronamine in Human Serum of Thyroid Cancer by Covalent Organic Hyper Cross-linked Poly-ionic Liquid

The incidence of thyroid cancer is increasing worldwide. So far, still no non-invasive clinical test biomarkers were developed for the diagnosis of thyroid cancer. The diiodothyronines (T2s) are precursors and metabolites of thyroid hormone (T4). Some reports predict that T2s may be associated with several thyroid diseases, especially the thyroid cancer. Detecting free T2s in human serum may help the diagnosis of thyroid cancer. However, few works have reported the detection of T2s due to their trace amounts. Here we developed a novel hyper organic cross-linked poly ionic liquid (PIL) material for the enrichment of three main compounds in T2s family, including 3,5- diiodothyronine (3,5-T2), 3',5'-diiodothyronine (3',5'-T2), and 3,5-diiodothyronamine (3,5-T2AM). This PIL material provided specific enrichment superiority for three T2s. After enrichment, the signal intensity of 3,5-T2, 3',5'-T2, and 3,5-T2AM increased 14, 132 and 1.6 folds, respectively, with LOQ of 76, 87, and 107 fM, respectively. Finally, we successfully applied PIL material coupled with HPLC-ESI-MS/MS in enrichment and quantitative determination of free 3,5-T2, 3',5'-T2, and 3,5-T2AM in human serum of 45 thyroid cancer patients and 15 healthy people. We also used free thyroid hormone (FT4) as the calibration reference to eliminate individual differences. We found that the levels of 3,5-T2 (P < 0.001), and 3',5'-T2 (P = 0.001) in patients with thyroid cancer were significantly higher than those in healthy people. Additionally, we further investigated the power of different T2 thyroid hormones divided FT4 to classify thyroid cancer patients and healthy people. And 3,5-T2/FT4 had the highest classification performance for discriminating thyroid cancer patients from healthy people at certain threshold, indicating that 3,5-T2/FT4 in human serum can act as potential biomarkers for "non-invasive" clinical diagnosis of thyroid cancer.

Hypothyroidism-Induced Nonalcoholic Fatty Liver Disease (HIN): Mechanisms and Emerging Therapeutic Options

Nonalcoholic fatty liver disease (NAFLD) is an emerging worldwide problem and its association with other metabolic pathologies has been one of the main research topics in the last decade. The aim of this review article is to provide an up-to-date correlation between hypothyroidism and NAFLD. We followed evidence regarding epidemiological impact, immunopathogenesis, thyroid hormone-liver axis, lipid and cholesterol metabolism, insulin resistance, oxidative stress, and inflammation. After evaluating the influence of thyroid hormone imbalance on liver structure and function, the latest studies have focused on developing new therapeutic strategies. Thyroid hormones (THs) along with their metabolites and thyroid hormone receptor β (THR-β) agonist are the main therapeutic targets. Other liver specific analogs and alternative treatments have been tested in the last few years as potential NAFLD therapy. Finally, we concluded that further research is necessary as well as the need for an extensive evaluation of thyroid function in NAFLD/NASH patients, aiming for better management and outcome.

Alternative ligands for thyroid hormone receptors

Thyroid hormone receptors (TRs) are ligand-dependent transcription factors that activate or repress gene transcription, resulting in the regulation of numerous physiological programs. While 3,3',5-L-triiodothyronine is the TR cognate ligand, these receptors can also be activated by various alternative ligands, including endogenous and synthetic molecules capable of inducing diverse active receptor conformations that influence thyroid hormone-dependent signaling pathways. This review mainly discusses current knowledge on 3,5-diiodo-L-thyronine and 3,5,3'-triiodothyroacetic acid, two endogenous molecules that bind to TRs and regulate gene expression; and the molecular interactions between TRs and ligands, like synthetic thyromimetics developed to target specific TR isoforms for tissue-specific regulation of thyroid-related disorders, or endocrine disruptors that have allowed the design of new analogues and revealed essential amino acids for thyroid hormone binding.

Knock-Down of Specific Thyroid Hormone Receptor Isoforms Impairs Body Plan Development in Zebrafish

The role of thyroid hormones (THs) in development has been extensively studied, however, the specific molecular mechanisms involved are far from being clear. THs act by binding to TH nuclear receptors (TR) that act as ligand-dependent transcription factors to regulate TH-dependent gene expression. Like vertebrates, zebrafish express different isoforms of functional Tr alpha and beta, some of which can bind alternative ligands like 3,5-T2. In this study, we first analyzed the effects of exogenous T3 and 3,5-T2 exposure during embryogenesis. The percentage of affected embryos was similar to those vehicle-injected, suggesting that the early exposure to low TH levels is not sufficient to elicit effects upon the phenotype of the embryo. We then generated crispants for four isoforms of thr to learn more about the role of these receptors in early development. We found that crispant larvae from thraa and a newly identified l-thrb+, but not thrab and canonical thrb1 showed profound deleterious effects upon symmetry and laterality, suggesting early novel roles for these Tr isoforms in the body plan developmental program. Since critical events that determine cell fate start in the late gastrula, we tested if some genes that are expressed during early developmental stages could indeed be TH targets. We identify early development genes, like sox10 and eve, that were specifically over-expressed in thraa and l-thrb+ crispants, suggesting that these specific thr isoforms function as transcription repressors for these genes, while transcription of zic and ets appear to be thraa and l-thrb+-mediated, respectively. Overall, present results show that TH signaling participates in early zebrafish development and identify Tr isoform-specific mediated regulation of early gene expression.

Thyroid hormone receptor localization in target tissues

The thyroid hormone receptors, TRα1, TRβ1 and other subtypes, are members of the nuclear receptor superfamily that mediate the action of thyroid hormone signaling in numerous tissues to regulate important physiological and developmental processes. Their most well-characterized role is as ligand-dependent transcription factors; TRs bind thyroid hormone response elements in the presence or absence of thyroid hormone to facilitate the expression of target genes. Although primarily residing in the nucleus, TRα1 and TRβ1 shuttle rapidly between the nucleus and cytoplasm. We have identified multiple nuclear localization signals and nuclear export signals within TRα1 and TRβ1 that interact with importins and exportins, respectively, to mediate translocation across the nuclear envelope. More recently, enigmatic cytoplasmic functions have been ascribed to other TR subtypes, expanding the diversity of the cellular response to thyroid hormone. By integrating data on localization signal motifs, this review provides an overview of the complex interplay between TR's dynamic transport pathways and thyroid hormone signaling activities. We examine the variation in TR subtype response to thyroid hormone signaling, and what is currently known about regulation of the variety of tissue-specific localization patterns, including targeting to the nucleus, the mitochondria and the inner surface of the plasma membrane.

3,5-Diiodo-L-Thyronine Affects Structural and Metabolic Features of Skeletal Muscle Mitochondria in High-Fat-Diet Fed Rats Producing a Co-adaptation to the Glycolytic Fiber Phenotype

Hyperlipidemic state-associated perturbations in the network of factors controlling mitochondrial functions, i. e., morphogenesis machinery and metabolic sensor proteins, produce metabolic inflexibility, insulin resistance and reduced oxidative capacity in skeletal muscle. Moreover, intramyocellular lipid (IMCL) accumulation leads to tissue damage and inflammation. The administration of the naturally occurring metabolite 3,5-diiodo-L-thyronine (T2) with thyromimetic actions to high fat diet (HFD)-fed rats exerts a systemic hypolipidemic effect, which produces a lack of IMCL accumulation, a shift toward glycolytic fibers and amelioration of insulin sensitivity in gastrocnemius muscle. In this study, an integrated approach combining large-scale expression profile and functional analyses was used to characterize the response of skeletal muscle mitochondria to T2 during a HFD regimen. Long-term T2 administration to HDF rats induced a glycolytic phenotype of gastrocnemius muscle as well as an adaptation of mitochondria to the fiber type, with a decreased representation of enzymes involved in mitochondrial oxidative metabolism. At the same time, T2 stimulated the activity of individual respiratory complex I, IV, and V. Moreover, T2 prevented the HFD-associated increase in the expression of peroxisome proliferative activated receptor γ coactivator-1α and dynamin-1-like protein as well as mitochondrial morphological aberrations, favoring the appearance of tubular and tethered organelles in the intermyofibrillar regions. Remarkably, T2 reverted the HDF-associated expression pattern of proinflammatory factors, such as p65 subunit of NF-kB, and increased the fiber-specific immunoreactivity of adipose differentiation–related protein in lipid droplets. All together, these results further support a role of T2 in counteracting in vivo some of the HFD-induced impairment in structural/metabolic features of skeletal muscle by impacting the mitochondrial phenotype.

Thyroid Hormones, Thyromimetics and Their Metabolites in the Treatment of Liver Disease

The signaling pathways activated by thyroid hormone receptors (THR) are of fundamental importance for organogenesis, growth and differentiation, and significantly influence energy metabolism, lipid utilization and glucose homeostasis. Pharmacological control of these pathways would likely impact the treatment of several human diseases characterized by altered metabolism, growth or differentiation. Not surprisingly, biomedical research has been trying for the past decades to pharmacologically target the 3,5,3'-triiodothyronine (T3)/THR axis. In vitro and in vivo studies have provided evidence of the potential utility of the activation of the T3-dependent pathways in metabolic syndrome, non-alcoholic fatty liver disease (NAFLD), and in the treatment of hepatocellular carcinoma (HCC). Unfortunately, supra-physiological doses of the THR agonist T3 cause severe thyrotoxicosis thus hampering its therapeutic use. However, the observation that most of the desired beneficial effects of T3 are mediated by the activation of the beta isoform of THR (THRβ) in metabolically active organs has led to the synthesis of a number of THRβ-selective thyromimetics. Among these drugs, GC-1, GC-24, KB141, KB2115, and MB07344 displayed a promising therapeutic strategy for liver diseases. However, although these drugs exhibited encouraging results when tested in the treatment of experimentally-induced obesity, dyslipidemia, and HCC, significant adverse effects limited their use in clinical trials. More recently, evidence has been provided that some metabolites of thyroid hormones (TH), mono and diiodothyronines, could also play a role in the treatment of liver disease. These molecules, for a long time considered inactive byproducts of the metabolism of thyroid hormones, have now been proposed to be able to modulate and control lipid and cell energy metabolism. In this review, we will summarize the current knowledge regarding T3, its metabolites and analogs with reference to their possible clinical application in the treatment of liver disease. In particular, we will focus our attention on NAFLD, non-alcoholic steatohepatitis (NASH) and HCC. In addition, the possible therapeutic use of mono- and diiodothyronines in metabolic and/or neoplastic liver disease will be discussed.

Lamprey metamorphosis: Thyroid hormone signaling in a basal vertebrate

As one of the most basal living vertebrates, lampreys represent an excellent model system to study the evolution of thyroid hormone (TH) signaling. The lamprey hypothalamic-pituitary-thyroid and reproductive axes overlap functionally. Lampreys have 3 gonadotropin-releasing hormones and a single glycoprotein hormone from the hypothalamus and pituitary, respectively, that regulate both the reproductive and thyroid axes. TH synthesis in larval lampreys takes place in an endostyle that transforms into typical vertebrate thyroid tissue during metamorphosis; both the endostyle and follicular tissue have all the typical TH synthetic components found in other vertebrates. Furthermore, lampreys also have the vertebrate suite of peripheral regulators including TH distributor proteins (THDPs), deiodinases and TH receptors (TRs). Although at the molecular level the components of the lamprey thyroid system are ancestral to other vertebrates, their functions have been largely conserved. TH signaling as it relates to lamprey metamorphosis represents a particularly interesting phenomenon. Unlike other metamorphosing vertebrates, lamprey THs increase throughout the larval period, peak prior to metamorphosis and decline rapidly at the onset of metamorphosis; patterns of deiodinase activity are consistent with these increases and declines. Moreover, goitrogens (which suppress TH levels) initiate precocious metamorphosis, and exogenous TH treatment blocks goitrogen-induced metamorphosis and disrupts natural metamorphosis. Despite this clear physiological difference, TH action via TRs is consistent with higher vertebrates. Based on observations that TRs are upregulated in a tissue-specific fashion during morphogenesis and the finding that lamprey TRs upregulate genes via THs in a fashion similar to higher vertebrates, we propose the following hypothesis for further testing. THs have a dual role in lampreys where high TH levels promote larval feeding and growth and then at the onset of metamorphosis TH levels decrease rapidly; at this time the relatively low TH levels function via TRs in a fashion similar to that of other metamorphosing vertebrates.

Evolution of ligands, receptors and metabolizing enzymes of thyroid signaling

Thyroid hormones (THs) play important roles in vertebrates such as the control of the metabolism, development and seasonality. Given the pleiotropic effects of thyroid disorders (developmental delay, mood disorder, tachycardia, etc), THs signaling is highly investigated, specially using mammalian models. In addition, the critical role of TH in controlling frog metamorphosis has led to the use of Xenopus as another prominent model to study THs action. Nevertheless, animals regarded as non-model species can also improve our understanding of THs signaling. For instance, studies in amphioxus highlighted the role of Triac as a bona fide thyroid hormone receptor (TR) ligand. In this review, we discuss our current understanding of the THs signaling in the different taxa forming the metazoans (multicellular animals) group. We mainly focus on three actors of the THs signaling: the ligand, the receptor and the deiodinases, enzymes playing a critical role in THs metabolism. By doing so, we also pinpoint many key questions that remain unanswered. How can THs accelerate metamorphosis in tunicates and echinoderms while their TRs have not been yet demonstrated as functional THs receptors in these species? Do THs have a biological effect in insects and cnidarians even though they do not have any TR? What is the basic function of THs in invertebrate protostomia? These questions can appear disconnected from pharmacological issues and human applications, but the investigation of THs signaling at the metazoans scale can greatly improve our understanding of this major endocrinological pathway.

Non-mammalian models reveal the role of alternative ligands for thyroid hormone receptors

Thyroid hormones, or THs, are well-known regulators of a wide range of biological processes that occur throughout the lifespan of all vertebrates. THs act through genomic mechanisms mediated by thyroid hormone receptors (TRs). The main product of the thyroid gland is thyroxine or T4, which can be further transformed by different biochemical pathways to produce at least 15 active or inactive molecules. T3, a product of T4 outer-ring deiodination, has been recognized as the main bioactive TH. However, growing evidence has shown that other TH derivatives are able to bind to, and/or activate TRs, to induce thyromimetic effects. The compiled data in this review points to at least two of these TR alternative ligands: TRIAC and T2. Taking this into account, non-mammalian models have proven to be advantageous to explore new TH derivatives with potential novel actions, prompting a re-evaluation of the role and mechanism of action of TR alternative ligands that were previously believed to be inactive. The functional implications of these ligands across different vertebrates may require us to reconsider current established notions of thyroid physiology.

Role of thyroid hormone in hepatic gene regulation, chromatin remodeling, and autophagy

Thyroid hormone (TH) actions on development and metabolism have been studied ever since the discovery of thyroxine almost a century ago. Initial studies focused on the physiological and biochemical actions of TH. Later, the cloning of the thyroid hormone receptor (THR) isoforms and the development of techniques enabled the study of TH regulation of complex cellular processes (such as gene transcription). Recently we found that TH activates secondary transcription factors such as FOXO1, to amplify gene transcription; and also is a potent inducer of autophagy that was critical for fatty acid β-oxidation in the liver. This review summarizes the recent advancements in our understanding of TH regulation of gene expression of metabolic genes (via co-regulators/transcription factors and epigenetic control) and autophagy in the liver. Our deeper understanding of TH action recently has led to the development of tissue- and THR isoform-specific TH mimetics that may be useful for the treatment of metabolic disorders.

The teleost head kidney: Integrating thyroid and immune signalling

The head kidney, analogous to the mammalian adrenal gland, is an organ unique for teleost fish. It comprises cytokine-producing lymphoid cells from the immune system and endocrine cells secreting cortisol, catecholamines, and thyroid hormones. The intimate organization of the immune system and endocrine system in one single organ makes bidirectional signalling between these possible. In this review we explore putative interactions between the thyroid and immune system in the head kidney. We give a short overview of the thyroid system, and consider the evidence for the presence of thyroid follicles in the head kidney as a normal, healthy trait in fishes. From mammalian studies we gather data on the effects of three important pro-inflammatory cytokines (TNFα, IL-1β, IL-6) on the thyroid system. A general picture that emerges is that pro-inflammatory cytokines inhibit the activity of the thyroid system at different targets. Extrapolating from these studies, we suggest that the interaction of the thyroid system by paracrine actions of cytokines in the head kidney is involved in fine-tuning the availability and redistribution of energy substrates during acclimation processes such as an immune response or stress response.

3,5,3'-Triiodo-L-Thyronine- and 3,5-Diiodo-L-Thyronine- Affected Metabolic Pathways in Liver of LDL Receptor Deficient Mice

3,5,3′-triiodo-L-thyronine (T3) and 3,5-diiodo-L-thyronine (T2), when administered to a model of familial hypercholesterolemia, i.e., low density lipoprotein receptor (LDLr)-knockout (Ldlr^−/−) mice fed with a Western type diet (WTD), dramatically reduce circulating total and very low-density lipoprotein/LDL cholesterol with decreased liver apolipoprotein B (ApoB) production. The aim of the study was to highlight putative molecular mechanisms to manage cholesterol levels in the absence of LDLr. A comprehensive comparative profiling of changes in expression of soluble proteins in livers from Ldlr^−/− mice treated with either T3 or T2 was performed. From a total proteome of 450 liver proteins, 25 identified proteins were affected by both T2 and T3, 18 only by T3 and 9 only by T2. Using in silico analyses, an overlap was observed with 11/14 pathways common to both iodothyronines, with T2 and T3 preferentially altering sub-networks centered around hepatocyte nuclear factor 4 α (HNF4α) and peroxisome proliferator-activated receptor α (PPARα), respectively. Both T2 and T3 administration significantly reduced nuclear HNF4α protein content, while T2, but not T3, decreased the expression levels of the HNFα transcriptional coactivator PGC-1α. Lower PPARα levels were found only following T3 treatment while both T3 and T2 lowered liver X receptor α (LXRα) nuclear content. Overall, this study, although it was not meant to investigate the use of T2 and T3 as a therapeutic agent, provides novel insights into the regulation of hepatic metabolic pathways involved in T3- and T2-driven cholesterol reduction in Ldlr^−/− mice.

Mitochondrial Actions of Thyroid Hormone

The hypermetabolic effects of thyroid hormones (THs), the major endocrine regulators of metabolic rate, are widely recognized. Although, the cellular mechanisms underlying these effects have been extensively investigated, much has yet to be learned about how TH regulates diverse cellular functions. THs have a profound impact on mitochondria, the organelles responsible for the majority of cellular energy production, and several studies have been devoted to understand the respective importance of the nuclear and mitochondrial pathways for organelle activity. During the last decades, several new aspects of both THs (i.e., metabolism, transport, mechanisms of action, and the existence of metabolically active TH derivatives) and mitochondria (i.e., dynamics, respiratory chain organization in supercomplexes, and the discovery of uncoupling proteins other than uncoupling protein 1) have emerged, thus opening new perspectives to the investigation of the complex relationship between thyroid and the mitochondrial compartment. In this review, in the light of an historical background, we attempt to point out the present findings regarding thyroid physiology and the emerging recognition that mitochondrial dynamics as well as the arrangement of the electron transport chain in mitochondrial cristae contribute to the mitochondrial activity. We unravel the genomic and nongenomic mechanisms so far studied as well as the effects of THs on mitochondrial energetics and, principally, uncoupling of oxidative phosphorylation via various mechanisms involving uncoupling proteins. The emergence of new approaches to the question as to what extent and how the action of TH can affect mitochondria is highlighted. © 2016 American Physiological Society. Compr Physiol 6:1591-1607, 2016.

3,5-Diiodothyronine-mediated transrepression of the thyroid hormone receptor beta gene in tilapia. Insights on cross-talk between the thyroid hormone and cortisol signaling pathways

T3 and cortisol activate or repress gene expression in virtually every vertebrate cell mainly by interacting with their nuclear hormone receptors. In contrast to the mechanisms for hormone gene activation, the mechanisms involved in gene repression remain elusive. In teleosts, the thyroid hormone receptor beta gene or thrb produces two isoforms of TRβ1 that differ by nine amino acids in the ligand-binding domain of the long-TRβ1, whereas the short-TRβ1 lacks the insert. Previous reports have shown that the genomic effects exerted by 3,5-T2, a product of T3 outer-ring deiodination, are mediated by the long-TRβ1. Furthermore, 3,5-T2 and T3 down-regulate the expression of long-TRβ1 and short-TRβ1, respectively. In contrast, cortisol has been shown to up-regulate the expression of thrb. To understand the molecular mechanisms for thrb modulation by thyroid hormones and cortisol, we used an in silico approach to identify thyroid- and cortisol-response elements within the proximal promoter of thrb from tilapia. We then characterized the identified response elements by EMSA and correlated our observations with the effects of THs and cortisol upon expression of thrb in tilapia. Our data show that 3,5-T2 represses thrb expression and impairs its up-regulation by cortisol possibly through a transrepression mechanism. We propose that for thrb down-regulation, ligands other than T3 are required to orchestrate the pleiotropic effects of thyroid hormones in vertebrates.

New insights into the structure and mechanism of iodothyronine deiodinases

Iodothyronine deiodinases are a family of enzymes that remove specific iodine atoms from one of the two aromatic rings in thyroid hormones (THs). They thereby fine-tune local TH concentrations and cellular TH signaling. Deiodinases catalyze a remarkable biochemical reaction, i.e., the reductive elimination of a halogenide from an aromatic ring. In metazoans, deiodinases depend on the rare amino acid selenocysteine. The recent solution of the first experimental structure of a deiodinase catalytic domain allowed for a reappraisal of the many mechanistic and mutagenesis data that had been accumulated over more than 30 years. Hence, the structure generates new impetus for research directed at understanding catalytic mechanism, substrate specificity, and regulation of deiodinases. This review will focus on structural and mechanistic aspects of iodothyronine deiodinases and briefly compare these enzymes with dehalogenases, which catalyze related reactions. A general mechanism for the selenium-dependent deiodinase reaction will be described, which integrates the mouse deiodinase 3 crystal structure and biochemical studies. We will summarize further, sometimes isoform-specific molecular features of deiodinase catalysis and regulation, and we will then discuss available compounds for modulating deiodinase activity for therapeutic purposes.

Quantitative Analysis of Thyroid Hormone Metabolites in Cell Culture Samples Using LC-MS/MS

A liquid-liquid extraction and liquid chromatography-electrospray ionization tandem mass spectrometry (LC-MS/MS) method to determine iodothyronines and thyronamines (TAM) from cell culture media was developed. Thyroid hormones (TH) are metabolized by sequential deiodination to eventually yield thyronine (T0), but can also be decarboxylated, resulting in TAM. The method presented here for extraction of DMEM/F12 cell culture media is a fundamental procedure for a precise determination of 9 TH and 6 TAM from a single LC run. Analytes and internal standards (IS) were extracted from DMEM/F12 (w/o phenol red) by liquid-liquid extraction using isopropanol-TBME (30:70 v/v). Measurement of TH and TAM was performed during a 10-min run time using (13)C6-T4, (13)C6-T3, (13)C6-rT3, (13)C6-3,3'T2 and (2)H4-T1AM as IS. Calibration curves covered 11 calibrators measured as triplicates each for the analysis of the 9 TH and 6 TAM metabolites, and the 5 IS were linear and reproducible in the range of 0.12-120 nM (R(2) 0.991-0.999) for all calibrators. The lower limit of quantification was 0.078-0.234 nM. Method validation and robustness were demonstrated by the analysis of precision, accuracy, process efficiency, matrix effects and recoveries, as well as intra- and interassay stability. These parameters were investigated for high, middle and low concentrations of quality controls of all 9 TH and 6 TAM metabolites. This validated, sensitive and interaction-free LC-MS/MS method allows rapid analysis and accurate determination of TH and TAM from DMEM/F12 (w/o phenol red) conditioned media and seems to be easily transferable and applied to commonly used buffers and cell culture media.

Proteomic approaches for the study of tissue specific effects of 3,5,3'-triiodo-L-thyronine and 3,5-diiodo-L-thyronine in conditions of altered energy metabolism

In vertebrates and, specifically, in mammals, energy homeostasis is achieved by the integration of metabolic and neuroendocrine signals linked to one another in an intricate network hierarchically responding to the tight modulating action of hormones among which thyroid hormones (THs) play a central role. At the cellular level, 3,5,3′-triiodo-L-thyronine (T3) acts mainly by binding to specific nuclear receptors (TRs) but actually it is becoming more and more evident that some T3- actions are independent of TRs and that other iodothyronines, such as 3,5-diiodo-L-thyronine (T2), affect energy metabolism and adiposity. In the postgenomic era, clinical and basic biological researches are increasingly benefiting from the recently developed new omics approaches including, among the others, proteomics. Considering the recognized value of proteins as excellent targets in physiology, the functional and simultaneous analysis of the expression level and the cellular localization of multiple proteins can actually be considered fundamental in the understanding of complex mechanisms such as those involved in thyroid control of metabolism. Here, we will discuss new leads (i.e., target proteins and metabolic pathways) emerging in applying proteomics to the actions of T3 and T2 in conditions of altered energy metabolism in animal tissues having a central role in the control of energy balance.

Enzymatic intracrine regulation of white adipose tissue

Abdominal fat formation has become a permanent risk factor for metabolic syndrome and various cancers in one-third of the world's population of obese and even lean patients. Formation of abdominal fat involves additional mechanisms beyond an imbalance in energy intake and expenditure, which explains systemic obesity. In this review, we briefly summarized autonomous regulatory circuits that locally produce hormones from inactive precursors or nutrients for intra-/auto-/paracrine signaling in white adipose depots. Enzymatic pathways activating steroid and thyroid hormones in adipose depots were compared with enzymatic production of retinoic acid from vitamin A. We discussed the role of intracrine circuits in fat-depot functions and strategies to reduce abdominal adiposity through thermogenic adipocytes with interrupted generation of retinoic acid.