Development and validation of an LC-MS/MS methodology for the quantification of thyroid hormones in dko MCT8/OATP1C1 mouse brain

The Allan-Herndon Dudley Syndrome (AHDS) is a rare disease caused by the progressive loss of monocarboxylate transporter 8 (MCT8). In patients with AHDS, the absence of MCT8 impairs transport of thyroid hormones (TH) through the blood brain barrier, leading to a central state of TH deficiency. In mice, the AHDS is mimicked by simultaneous deletion of the TH transporters MCT8 and the solute carrier organic anion transporter family member 1c1 (OATP1C1). To support preclinical mouse studies, an analytical methodology was developed and successfully applied for quantifying selected thyroid hormones in mouse whole brain and in specific regions using liquid chromatography tandem mass-spectrometry (LC-MS/MS). An important requirement for the methodology was its high sensitivity since a very low concentration of THs was expected in MCT8/OATP1C1 double-knockout (dko) mouse brain. Seven THs were targeted: L-thyroxine (T4), 3,3,5-triiodo-L-thyronine-thy-ronine (T3), 3,3’,5’-triiodo-L-thyronine-thyronine (rT3), 3,3-diiodo-L-thyronine (3,3’-T2, T2), 3,5-diiodo-L-thyro-nine (rT2, 3,5-T2), 3-iodo-L-thyronine (T1), 3-iodothyronamine (T1AM). Isotope dilution liquid chromatography triple-quadrupole mass spectrometry methodology was applied for detection and quantification. The method was validated in wild-type animals for mouse whole brain and for five different brain regions (hypothalamus, hippocampus, prefrontal cortex, brainstem and cortex). Instrumental calibration curves ranged from 0.35 to 150 pg/μL with good linearity (r² >0.996). The limit of quantification was from 0.08 to 0.6 pg/mg, with an intra- and inter-day precision of 4.2−14.02% and 0.4−17.9% respectively, and accuracies between 84.9% and 114.8% when the methodology was validated for the whole brain. In smaller, distinct brain regions, intra- and inter-day precision were 0.6−20.7% and 2.5−15.6% respectively, and accuracies were 80.2−128.6%. The new methodology was highly sensitive and allowed for the following quantification in wild-type mice: (i) for the first time, four distinct thyroid hormones (T4, T3, rT3 and 3,3’-T2) in only approximately 100 mg of mouse brain were detected; (ii) the quantification of T4 and T3 for the first time in distinct mouse brain regions were reported. Further, application of our method to MCT8/OATP1C1 dko mice revealed the expected, relative lack of T3 and T4 uptake into the brain, and confirmed the utility of our analytical method to study TH transport across the blood brain barrier in a preclinical model of central TH deficiency.

T1AM Attenuates the Hypoxia/Reoxygenation-Induced Necroptosis of H9C2 Cardiomyocytes via RIPK1/RIPK3 Pathway

Purpose

To investigate the detailed mechanism of 3-iodothyronamine (T1AM) in cell apoptosis and programmed necrosis of hypoxia/reoxygenation- (H/R-) induced H9C2 injury.

Materials and Methods

Cardiomyocyte H9C2 cells were cultured in vitro for the establishment of cardiomyocyte H/R models. Cells were randomly divided into four groups: the control group, H/R group, T1AM pretreatment group, T1AM pretreatment and H/R (6 μm T1AM+H/R) group. The degree of myocardial injury was determined by the detection of the cardiomyocyte inhibition rate by CCK8 and the detection of lactic dehydrogenase (LDH) activity. Cell apoptosis was assessed through TUNEL assay and flow cytometry analysis. The protein level and mRNA level of RIPK1, RIPK3, and CAMKII were detected by western blotting and qRT-PCR.

Results

Compared with the control group, the cell inhibition rate was dramatically elevated in the H/R group. LDH release of cardiomyocytes was significantly increased. Protein and mRNA expressions of RIPK1, RIPK3, and CAMKII were significantly enhanced. Compared with the H/R group, the cell inhibition rate, LDH release, cardiomyocyte necroptosis rate, and protein and mRNA levels of RIPK1, RIPK3, and CAMKII of the T1AM+H/R group were significantly decreased.

Conclusion

Pretreatment with T1AM could alleviate cardiomyocytes' H/R injury and inhibit necroptosis of cardiomyocytes, which might exert a protective function upon activation of the RIPK1/RIPK3 pathway.

Association Between 3-Iodothyronamine (T1am) Concentrations and Left Ventricular Function in Chronic Heart Failure

CONTEXT

Thyroid hormone metabolites might affect the heart. The endogenous aminergic metabolite 3-iodothyronamine (T1am) reduces left ventricular ejection fraction (LVEF) in rodents.

OBJECTIVE

To investigate concentration of T1am and its association with LVEF and biomarkers of heart function in patients with chronic heart failure (CHF) without thyroid disease, including patients with cardiac cachexia (nonedematous weight loss >5% over 6 months).

METHODS

Cross-sectional study. CHF was characterized by LVEF <45% and symptoms. Three groups were included (n = 19 in each group, matched on age, sex, and kidney function): patients with cachexia (CAC), patients without (non-CAC), and control (C) patients with prior myocardial infarction and LVEF >45%. T1am was measured by a monoclonal antibody-based chemiluminescence immunoassay. N-amino terminal pro-BNP (NT-proBNP) concentrations were also analyzed.

RESULTS

Mean (SD) LVEF: CAC, 32 ± 9%; non-CAC, 38 ± 8%; and C, 60 ± 8% (P < 0.0001). TSH, T4, and T3 levels did not differ between groups and did not correlate to T1am. Serum T1am (nmol/L) concentrations were higher in CHF: CAC (mean ± SD), 12.4 ± 6.6; non-CAC, 9.1 ± 5; and C, 7.3 ± 2.9. A negative association between T1am and LVEF was present after adjusting for sex, age, T3, and estimated glomerular filtration rate (P = 0.03). Further, serum T1am levels tended to be associated with NT-proBNP (P = 0.053).

CONCLUSION

Serum T1am levels were increased in patients with CHF and numerically highest (although nonsignificant) in patients with cardiac cachexia. Increasing T1am concentrations were independently associated with reduced LVEF, suggesting a direct effect on the human heart.

3-Iodothyronamine-A Thyroid Hormone Metabolite With Distinct Target Profiles and Mode of Action

The rediscovery of the group of thyronamines (TAMs), especially the first detailed description of their most prominent congener 3-iodothyronamine (3T1AM) 14 years ago, boosted research on this thyroid hormone metabolite tremendously. TAMs exert actions partly opposite to and distinct from known functions of thyroid hormones. These fascinating metabolic, anapyrexic, cytoprotective, and brain effects quickly evoked the hope to use hormone-derived TAMs as a therapeutic option. The G protein-coupled receptor (GPCR) TAAR1, a member of the trace amine-associated receptor (TAAR) family, was identified as the first target and effector of TAM action. The initial enthusiasm on pharmacological actions of exogenous TAMs elicited many questions, such as sites of biosynthesis, analytics, modes of action, inactivation, and role of TAMs in (patho)physiology. Meanwhile, it became clear that TAMs not only interact with TAAR1 or other TAAR family members but also with several aminergic receptors and non-GPCR targets such as transient receptor potential channels, mitochondrial proteins, and the serum TAM-binding protein apolipoprotein B100, thus classifying 3T1AM as a multitarget ligand. The physiological mode of action of TAMs is still controversial because regulation of endogenous TAM production and the sites of its biosynthesis are not fully elucidated. Methods for 3T1AM analytics need further validation, as they revealed different blood and tissue concentrations depending on detection principles used such as monoclonal antibody-based immunoassay vs liquid chromatography- matrix-assisted laser desorption/ionization mass spectrometry or time-of-flight mass spectrometry. In this review, we comprehensively summarize and critically evaluate current basic, translational, and clinical knowledge on 3T1AM and its main metabolite 3-iodothyroacetic acid, focusing on endocrine-relevant aspects and open but highly challenging issues.

Trace Amines and Their Receptors

Trace amines are endogenous compounds classically regarded as comprising β-phenylethyalmine, p-tyramine, tryptamine, p-octopamine, and some of their metabolites. They are also abundant in common foodstuffs and can be produced and degraded by the constitutive microbiota. The ability to use trace amines has arisen at least twice during evolution, with distinct receptor families present in invertebrates and vertebrates. The term "trace amine" was coined to reflect the low tissue levels in mammals; however, invertebrates have relatively high levels where they function like mammalian adrenergic systems, involved in "fight-or-flight" responses. Vertebrates express a family of receptors termed trace amine-associated receptors (TAARs). Humans possess six functional isoforms (TAAR1, TAAR2, TAAR5, TAAR6, TAAR8, and TAAR9), whereas some fish species express over 100. With the exception of TAAR1, TAARs are expressed in olfactory epithelium neurons, where they detect diverse ethological signals including predators, spoiled food, migratory cues, and pheromones. Outside the olfactory system, TAAR1 is the most thoroughly studied and has both central and peripheral roles. In the brain, TAAR1 acts as a rheostat of dopaminergic, glutamatergic, and serotonergic neurotransmission and has been identified as a novel therapeutic target for schizophrenia, depression, and addiction. In the periphery, TAAR1 regulates nutrient-induced hormone secretion, suggesting its potential as a novel therapeutic target for diabetes and obesity. TAAR1 may also regulate immune responses by regulating leukocyte differentiation and activation. This article provides a comprehensive review of the current state of knowledge of the evolution, physiologic functions, pharmacology, molecular mechanisms, and therapeutic potential of trace amines and their receptors in vertebrates and invertebrates.

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.

Thyronamines and Analogues - The Route from Rediscovery to Translational Research on Thyronergic Amines

Thyronamines are a novel class of endogenous signaling compounds, structurally related to thyroid hormones (THs). Specific thyronamines, particularly 3-iodothyronamine (T1AM), stimulate with nanomolar affinity trace amine-associated receptor 1 (TAAR1), a G protein-coupled membrane receptor, and may also interact with other TAAR subtypes (particularly TAAR5), adrenergic receptors (particularly α2 receptors), amine transporters, and mitochondrial proteins. In addition to its structural similarities with THs, T1AM also contains the arylethylamine scaffold as in monoamine neurotransmitters, implicating an intriguing role for T1AM as both a neuromodulator and a hormone-like molecule constituting a part of thyroid hormone signaling. A large number of T1AM derivatives have already been synthesized. We discuss the different chemical strategies followed to obtain thyronamine analogues, their potency at TAAR1, and their structure-activity relationship. Preliminary characterization of the functional effects of these synthetic compounds is also provided.

Metabolic profiling reveals reprogramming of lipid metabolic pathways in treatment of polycystic ovary syndrome with 3-iodothyronamine

Complex diseases such as polycystic ovary syndrome (PCOS) are associated with intricate pathophysiological, hormonal, and metabolic feedbacks that make their early diagnosis challenging, thus increasing the prevalence risks for obesity, cardiovascular, and fatty liver diseases. To explore the crosstalk between endocrine and lipid metabolic pathways, we administered 3‐iodothyronamine (T1AM), a natural analog of thyroid hormone, in a mouse model of PCOS and analyzed plasma and tissue extracts using multidisciplinary omics and biochemical approaches. T1AM administration induces a profound tissue‐specific antilipogenic effect in liver and muscle by lowering gene expression of key regulators of lipid metabolism, PTP1B and PLIN2, significantly increasing metabolites (glucogenic, amino acids, carnitine, and citrate) levels, while enhancing protection against oxidative stress. In contrast, T1AM has an opposing effect on the regulation of estrogenic pathways in the ovary by upregulating STAR, CYP11A1, and CYP17A1. Biochemical measurements provide further evidence of significant reduction in liver cholesterol and triglycerides in post‐T1AM treatment. Our results shed light onto tissue‐specific metabolic vs. hormonal pathway interactions, thus illuminating the intricacies within the pathophysiology of PCOS. This study opens up new avenues to design drugs for targeted therapeutics to improve quality of life in complex metabolic diseases.

Pharmacological hypothermia: a potential for future stroke therapy?

Mild physical hypothermia after stroke has been associated with positive outcomes. Despite the well-studied beneficial effects of hypothermia in the treatment of stroke, lack of precise temperature control, intolerance for the patient, and immunosuppression are some of the reasons which limit its clinical translation. Pharmacologically induced hypothermia has been explored as a possible treatment option following stroke in animal models. Currently, there are eight classes of pharmacological agents/agonists with hypothermic effects affecting a multitude of systems including cannabinoid, opioid, transient receptor potential vanilloid 1 (TRPV1), neurotensin, thyroxine derivatives, dopamine, gas, and adenosine derivatives. Interestingly, drugs in the TRPV1, neurotensin, and thyroxine families have been shown to have effects in thermoregulatory control in decreasing the compensatory hypothermic response during cooling. This review will briefly present drugs in the eight classes by summarizing their proposed mechanisms of action as well as side effects. Reported thermoregulatory effects of the drugs will also be presented. This review offers the opinion that these agents may be useful in combination therapies with physical hypothermia to achieve faster and more stable temperature control in hypothermia.

Neuroprotective mechanisms and translational potential of therapeutic hypothermia in the treatment of ischemic stroke

Stroke is a leading cause of disability and death, yet effective treatments for acute stroke has been very limited. Thus far, tissue plasminogen activator has been the only FDA-approved drug for thrombolytic treatment of ischemic stroke patients, yet its application is only applicable to less than 4–5% of stroke patients due to the narrow therapeutic window (< 4.5 hours after the onset of stroke) and the high risk of hemorrhagic transformation. Emerging evidence from basic and clinical studies has shown that therapeutic hypothermia, also known as targeted temperature management, can be a promising therapy for patients with different types of stroke. Moreover, the success in animal models using pharmacologically induced hypothermia (PIH) has gained increasing momentum for clinical translation of hypothermic therapy. This review provides an updated overview of the mechanisms and protective effects of therapeutic hypothermia, as well as the recent development and findings behind PIH treatment. It is expected that a safe and effective hypothermic therapy has a high translational potential for clinical treatment of patients with stroke and other CNS injuries.

Thyronamines and Derivatives: Physiological Relevance, Pharmacological Actions, and Future Research Directions

Thyronamines (3-T₁AM, T₀AM) are endogenous compounds probably derived from L-thyroxine or its intermediate metabolites. Combined activities of intestinal deiodinases and ornithine decarboxylase generate 3-T₁AM in vitro. Alternatively, 3-T₁AM might be formed by the thyroid gland and secreted into the blood. 3-T₁AM and T₀AM concentrations have been determined by liquid chromatography-tandem mass spectrometry analysis (LC-MS/MS) in tissues, serum, and cell lines. However, large variations of 3-T₁AM concentrations in human serum were reported by LC-MS/MS compared with a monoclonal antibody-based immunoassay. These differences might be caused by strong binding of the highly hydrophobic 3-T₁AM to apolipoprotein B100. Pharmacological administration of 3-T₁AM results in dose-dependent reversible effects on body temperature, cardiac function, energy metabolism, and neurological functions. The physiological relevance of these actions is unclear, but may occur at tissue concentrations close to the estimated endogenous concentrations of 3-T₁AM or its metabolites T₀AM or thyroacetic acid (TA₁). A number of putative receptors, binding sites, and cellular target molecules mediating actions of the multi-target ligand 3-T₁AM have been proposed. Among those are members of the trace amine associated receptor family, the adrenergic receptor ADRα2a, and the thermosensitive transient receptor potential melastatin 8 channel. Preclinical studies employing various animal experimental models are in progress, and more stable receptor-selective agonistic and antagonistic analogues of 3-T₁AM are now available for testing. The potent endogenous thyroid hormone-derived biogenic amine 3-T₁AM exerts marked cryogenic, metabolic, cardiac and central actions and represents a valuable lead compound linking endocrine, metabolic, and neuroscience research to advance development of new drugs.

Biosynthesis of 3-Iodothyronamine From T4 in Murine Intestinal Tissue

The endogenous metabolite 3-iodothyronamine (3-T1AM) induces strong hypothermia and bradycardia at pharmacological doses. Although its biosynthesis from thyroid hormone precursors appears likely, the sequence and sites of reactions are still controversial: studies in T4-substituted thyroid cancer patients lacking functional thyroid tissue suggested extrathyroidal 3-T1AM production, whereas studies using labeled T4 in mice indicated intrathyroidal formation. However, because the patients received T4 orally, whereas the mice were injected ip, we hypothesized that 3-T1AM synthesis requires the intestinal passage of T4. Using the everted gut sac model in combination with mass spectrometry, we demonstrate 3-T1AM production from T4 in mouse intestine via several deiodination and decarboxylation steps. Gene expression analysis confirmed the expression of all 3 deiodinases as well as ornithine decarboxylase (ODC) in intestine. Subsequent experiments employing purified human ODC revealed that this enzyme can in fact mediate decarboxylation of 3,5-T2 and T4 to the respective thyronamines (TAMs), demonstrating that the intestine expresses the entire molecular machinery required for 3-T1AM biosynthesis. Interestingly, TAM production was strongly affected by the antithyroid treatment methimazole and perchlorate independently of thyroid status, limiting the validity of the respective mouse models in this context. Taken together, our data demonstrate intestinal 3-T1AM biosynthesis from T4 involving decarboxylation through ODC with subsequent deiodination, and explain the apparent discrepancy between 3-T1AM serum levels in patients substituted orally and mice injected ip with T4. Identifying ODC as the first enzyme capable of decarboxylating thyroid hormone, our findings open the path to further investigations of TAM metabolism on molecular and cellular levels.

Update on 3-iodothyronamine and its neurological and metabolic actions

3-iodothyronamine (T1AM) is an endogenous amine, that has been detected in many rodent tissues, and in human blood. It has been hypothesized to derive from thyroid hormone metabolism, but this hypothesis still requires validation. T1AM is not a ligand for nuclear thyroid hormone receptors, but stimulates with nanomolar affinity trace amine-associated receptor 1 (TAAR1), a G protein-coupled membrane receptor. With a lower affinity it interacts with alpha2A adrenergic receptors. Additional targets are represented by apolipoprotein B100, mitochondrial ATP synthase, and membrane monoamine transporters, but the functional relevance of these interactions is still uncertain. Among the effects reported after administration of exogenous T1AM to experimental animals, metabolic and neurological responses deserve special attention, because they were obtained at low dosages, which increased endogenous tissue concentration by about one order of magnitude. Systemic T1AM administration favored fatty acid over glucose catabolism, increased ketogenesis and increased blood glucose. Similar responses were elicited by intracerebral infusion, which inhibited insulin secretion and stimulated glucagon secretion. However, T1AM administration increased ketogenesis and gluconeogenesis also in hepatic cell lines and in perfused liver preparations, providing evidence for a peripheral action, as well. In the central nervous system, T1AM behaved as a neuromodulator, affecting adrenergic and/or histaminergic neurons. Intracerebral T1AM administration favored learning and memory, modulated sleep and feeding, and decreased the pain threshold. In conclusion T1AM should be considered as a component of thyroid hormone signaling and might play a significant physiological and/or pathophysiological role. T1AM analogs have already been synthetized and their therapeutical potential is currently under investigation. 3-iodothyronamine (T1AM) is a biogenic amine whose structure is closely related to that of thyroid hormone (3,5,3′-triiodothyronine, or T3). The differences with T3 are the absence of the carboxylate group and the substitution of iodine with hydrogen in 5 and 3′ positions (Figure 1). In this paper we will review the evidence supporting the hypothesis that T1AM is a chemical messenger, namely that it is an endogenous substance able to interact with specific receptors producing significant functional effects. Special emphasis will be placed on neurological and metabolic effects, which are likely to have physiological and pathophysiological importance.

The role of thyroid hormones as inductors of oxidative stress and neurodegeneration

Reactive oxygen species (ROS) are oxidizing agents amply implicated in tissue damage. ROS production is inevitably linked to ATP synthesis in most cells, and the rate of production is related to the rate of cell respiration. Multiple antioxidant mechanisms limit ROS dispersion and interaction with cell components, but, when the balance between ROS production and scavenging is lost, oxidative damage develops. Many traits of aging are related to oxidative damage by ROS, including neurodegenerative diseases. Thyroid hormones (THs) are a major factor controlling metabolic and respiratory rates in virtually all cell types in mammals. The general metabolic effect of THs is a relative acceleration of the basal metabolism that includes an increase of the rate of both catabolic and anabolic reactions. THs are related to oxidative stress not only by their stimulation of metabolism but also by their effects on antioxidant mechanisms. Thyroid dysfunction increases with age, so changes in THs levels in the elderly could be a factor affecting the development of neurodegenerative diseases. However, the relationship is not always clear. In this review, we analyze the participation of thyroid hormones on ROS production and oxidative stress, and the way the changes in thyroid status in aging are involved in neurodegenerative diseases.

NMR-based metabolomics and breath studies show lipid and protein catabolism during low dose chronic T(1)AM treatment

OBJECTIVE

3-Iodothyronamine (T1 AM), an analog of thyroid hormone, is a recently discovered fast-acting endogenous metabolite. Single high-dose treatments of T1 AM have produced rapid short-term effects, including a reduction of body temperature, bradycardia, and hyperglycemia in mice.

DESIGN AND METHODS

The effect of daily low doses of T1 AM (10 mg/kg) for 8 days on weight loss and metabolism in spontaneously overweight mice was monitored. The experiments were repeated twice (n = 4). Nuclear magnetic resonance (NMR) spectroscopy of plasma and real-time analysis of exhaled (13) CO2 in breath by cavity ring down spectroscopy (CRDS) were used to detect T1 AM-induced lipolysis.

RESULTS

CRDS detected increased lipolysis in breath shortly after T1 AM administration that was associated with a significant weight loss but independent of food consumption. NMR spectroscopy revealed alterations in key metabolites in serum: valine, glycine, and 3-hydroxybutyrate, suggesting that the subchronic effects of T1 AM include both lipolysis and protein breakdown. After discontinuation of T1 AM treatment, mice regained only 1.8% of the lost weight in the following 2 weeks, indicating lasting effects of T1 AM on weight maintenance.

CONCLUSIONS

CRDS in combination with NMR and (13) C-metabolic tracing constitute a powerful method of investigation in obesity studies for identifying in vivo biochemical pathway shifts and unanticipated debilitating side effects.

Thyroid hormone analogues: where do we stand in 2013?

Thyroid hormones (THs) are important in the development and maintenance of lipid and energy homeostasis. THs act through two closely related TH receptors (TRs α and β), which are conditional transcription factors. Recently, TH analogues or thyromimetics with varying degrees of TR subtype and liver uptake selectivity have been developed. These compounds exert beneficial effects of TH excess states without many undesirable TR-dependent side effects. Several selective TR modulators (STRMs) showed exceptionally promising results in lowering serum cholesterol in preclinical animal models and human clinical studies. Moreover, some first generation STRMs elicit other potentially beneficial effects on obesity, glucose metabolism, and nonalcoholic fatty liver disease (NAFLD). While it was initially thought that STRMs would be an effective long-term therapy to combat elevated cholesterol, possibly in conjunction with another cholesterol-lowering therapy, the statins, three major first generation STRMs failed to progress beyond early phase III human trials. The aim of this review is to discuss how STRMs work, their actions in preclinical animal models and human clinical trials, why they did not progress beyond clinical trials as cholesterol-lowering therapeutics, whether selective TR modulation continues to hold promise for dyslipidemias, and whether members of this drug class could be applied to the treatment of other aspects of metabolic syndrome and human genetic disease.

Mitochondrial F(0) F(1) -ATP synthase is a molecular target of 3-iodothyronamine, an endogenous metabolite of thyroid hormone

BACKGROUND AND PURPOSE

3-iodothyronamine (T1AM) is a metabolite of thyroid hormone acting as a signalling molecule via non-genomic effectors and can reach intracellular targets. Because of the importance of mitochondrial F(0) F(1) -ATP synthase as a drug target, here we evaluated interactions of T1AM with this enzyme.

EXPERIMENTAL APPROACH

Kinetic analyses were performed on F(0) F(1) -ATP synthase in sub-mitochondrial particles and soluble F(1) -ATPase. Activity assays and immunodetection of the inhibitor protein IF(1) were used and combined with molecular docking analyses. Effects of T1AM on H9c2 cardiomyocytes were measured by in situ respirometric analysis.

KEY RESULTS

T1AM was a non-competitive inhibitor of F(0) F(1) -ATP synthase whose binding was mutually exclusive with that of the inhibitors IF(1) and aurovertin B. Both kinetic and docking analyses were consistent with two different binding sites for T1AM. At low nanomolar concentrations, T1AM bound to a high-affinity region most likely located within the IF(1) binding site, causing IF(1) release. At higher concentrations, T1AM bound to a low affinity-region probably located within the aurovertin binding cavity and inhibited enzyme activity. Low nanomolar concentrations of T1AM increased ADP-stimulated mitochondrial respiration in cardiomyocytes, indicating activation of F(0) F(1) -ATP synthase consistent with displacement of endogenous IF(1,) , reinforcing the in vitro results.

CONCLUSIONS AND IMPLICATIONS

Effects of T1AM on F(0) F(1) -ATP synthase were twofold: IF(1) displacement and enzyme inhibition. By targeting F(0) F(1) -ATP synthase within mitochondria, T1AM might affect cell bioenergetics with a positive effect on mitochondrial energy production at low, endogenous, concentrations. T1AM putative binding locations overlapping with IF(1) and aurovertin binding sites are described.

LC-MS/MS detection of thyroid hormone metabolites in tissue samples

Evaluation of: Ackermans MT, Kettelarij-Haas Y, Boelen A, Endert E. Determination of thyroid hormones and their metabolites in tissue using SPE UPLC-tandem MS. Biomed. Chromatogr. 26(4), 485-490 (2012). Liquid chromatography-tandem mass spectrometry is used in research laboratories as a gold standard for endocrine analytics. This technology provides a precise tool for the measurement of serum and tissue thyroid hormones (TH) and their deiodinated metabolites. The 'inactive pro-hormone' 3,3´,5,5´-tetraiodo-l-thyronine (T₄) is synthesized in and secreted by the thyroid gland. Activation and inactivation of T₄ in the brain, liver and other tissues is controlled by the iodothyronine deiodinases through the sequential removal of iodine atoms resulting in eight TH derivatives: tri- (T₃, rT₃), di- (3,5-T₂, 3,3´-T₂, 3´,5´-T₂), monoiodothyronines (3-T₁, 3´-T₁) and the iodine-free l-thyronine (T₀). This methodical article fits very well into the current line of research on analytics and local metabolism of TH.

Distribution of exogenous [125I]-3-iodothyronamine in mouse in vivo: relationship with trace amine-associated receptors

3-Iodothyronamine (T1AM) is a novel chemical messenger, structurally related to thyroid hormone, able to interact with G protein-coupled receptors known as trace amine-associated receptors (TAARs). Little is known about the physiological role of T1AM. In this prospective, we synthesized [125I]-T1AM and explored its distribution in mouse after injecting in the tail vein at a physiological concentration (0.3 nM). The expression of the nine TAAR subtypes was evaluated by quantitative real-time PCR. [125I]-T1AM was taken up by each organ. A significant increase in tissue vs blood concentration occurred in gallbladder, stomach, intestine, liver, and kidney. Tissue radioactivity decreased exponentially over time, consistent with biliary and urinary excretion, and after 24 h, 75% of the residual radioactivity was detected in liver, muscle, and adipose tissue. TAARs were expressed only at trace amounts in most of the tissues, the exceptions being TAAR1 in stomach and testis and TAAR8 in intestine, spleen, and testis. Thus, while T1AM has a systemic distribution, TAARs are only expressed in certain tissues suggesting that other high-affinity molecular targets besides TAARs exist.

Detection of 3-iodothyronamine in human patients: a preliminary study

CONTEXT AND OBJECTIVE

The primary purpose of this study was to detect and quantify 3-iodothyronamine (T(1)AM), an endogenous biogenic amine related to thyroid hormone, in human blood.

DESIGN

T(1)AM, total T(3), and total T(4) were assayed in serum by a novel HPLC tandem mass spectrometry assay, which has already been validated in animal investigations, and the results were related to standard clinical and laboratory variables.

SETTING AND PATIENTS

The series included one healthy volunteer, 24 patients admitted to a cardiological ward, and 17 ambulatory patients suspected of thyroid disease, who underwent blood sampling at admission for routine diagnostic purposes. Seven patients were affected by type 2 diabetes, and six patients showed echocardiographic evidence of impaired left ventricular function.

INTERVENTIONS

No intervention or any patient selection was performed.

MAIN OUTCOME MEASURES

serum T(1)AM, total and free T(3) and T(4), routine chemistry, routine hematology, and echocardiographic parameters were measured.

RESULTS

T(1)AM was detected in all samples, and its concentration averaged 0.219 ± 0.012 pmol/ml. The T(1)AM concentration was significantly correlated to total T(4) (r = 0.654, P < 0.001), total T(3) (r = 0.705, P < 0.001), glycated hemoglobin (r = 0.508, P = 0.013), brain natriuretic peptide (r = 0.543, P = 0.016), and γ-glutamyl transpeptidase (r = 0.675, P < 0.001). In diabetic vs. nondiabetic patients T(1)AM concentration was significantly increased (0.232 ± 0.014 vs. 0.203 ± 0.006 pmol/ml, P = 0.044), whereas no significant difference was observed in patients with cardiac dysfunction.

CONCLUSIONS

T(1)AM is an endogenous messenger that can be assayed in human blood. Our results are consistent with the hypothesis that circulating T(1)AM is produced from thyroid hormones and encourage further investigations on the potential role of T(1)AM in insulin resistance and heart failure.

Differential modulation of Beta-adrenergic receptor signaling by trace amine-associated receptor 1 agonists

Trace amine-associated receptors (TAAR) are rhodopsin-like G-protein-coupled receptors (GPCR). TAAR are involved in modulation of neuronal, cardiac and vascular functions and they are potentially linked with neurological disorders like schizophrenia and Parkinson's disease. Subtype TAAR1, the best characterized TAAR so far, is promiscuous for a wide set of ligands and is activated by trace amines tyramine (TYR), phenylethylamine (PEA), octopamine (OA), but also by thyronamines, dopamine, and psycho-active drugs. Unfortunately, effects of trace amines on signaling of the two homologous β-adrenergic receptors 1 (ADRB1) and 2 (ADRB2) have not been clarified yet in detail. We, therefore, tested TAAR1 agonists TYR, PEA and OA regarding their effects on ADRB1/2 signaling by co-stimulation studies. Surprisingly, trace amines TYR and PEA are partial allosteric antagonists at ADRB1/2, whereas OA is a partial orthosteric ADRB2-antagonist and ADRB1-agonist. To specify molecular reasons for TAAR1 ligand promiscuity and for observed differences in signaling effects on particular aminergic receptors we compared TAAR, tyramine (TAR) octopamine (OAR), ADRB1/2 and dopamine receptors at the structural level. We found especially for TAAR1 that the remarkable ligand promiscuity is likely based on high amino acid similarity in the ligand-binding region compared with further aminergic receptors. On the other hand few TAAR specific properties in the ligand-binding site might determine differences in ligand-induced effects compared to ADRB1/2. Taken together, this study points to molecular details of TAAR1-ligand promiscuity and identified specific trace amines as allosteric or orthosteric ligands of particular β-adrenergic receptor subtypes.

Effects of the thyroid hormone derivatives 3-iodothyronamine and thyronamine on rat liver oxidative capacity

Thyronamines T(0)AM and T(1)AM are naturally occurring decarboxylated thyroid hormone derivatives. Their in vivo administration induces effects opposite to those induced by thyroid hormone, including lowering of body temperature. Since the mitochondrial energy-transduction apparatus is known to be a potential target of thyroid hormone and its derivatives, we investigated the in vitro effects of T(0)AM and T(1)AM on the rates of O(2) consumption and H(2)O(2) release by rat liver mitochondria. Hypothyroid animals were used because of the low levels of endogenous thyronamines. We found that both compounds are able to reduce mitochondrial O(2) consumption and increase H(2)O(2) release. The observed changes could be explained by a partial block, operated by thyronamines, at a site located near the site of action of antimycin A. This hypothesis was confirmed by the observation that thyronamines reduced the activity of Complex III where the site of antimycin action is located. Because thyronamines exerted their effects at concentrations comparable to those found in hepatic tissue, it is conceivable that they can affect in vivo mitochondrial O(2) consumption and H(2)O(2) production acting as modulators of thyroid hormone action.