Targeting thyroid hormone receptor-beta agonists to the liver reduces cholesterol and triglycerides and improves the therapeutic index

The Race to Bash NASH: Emerging Targets and Drug Development in a Complex Liver Disease

Nonalcoholic steatohepatitis (NASH) is a severe form of nonalcoholic fatty liver disease (NAFLD) characterized by liver steatosis, inflammation, and hepatocellular damage. NASH is a serious condition that can progress to cirrhosis, liver failure, and hepatocellular carcinoma. The association of NASH with obesity, type 2 diabetes mellitus, and dyslipidemia has led to an emerging picture of NASH as the liver manifestation of metabolic syndrome. Although diet and exercise can dramatically improve NASH outcomes, significant lifestyle changes can be challenging to sustain. Pharmaceutical therapies could be an important addition to care, but currently none are approved for NASH. Here, we review the most promising targets for NASH treatment, along with the most advanced therapeutics in development. These include targets involved in metabolism (e.g., sugar, lipid, and cholesterol metabolism), inflammation, and fibrosis. Ultimately, combination therapies addressing multiple aspects of NASH pathogenesis are expected to provide benefit for patients.

Thyroid Function and the Risk of Non-Alcoholic Fatty Liver Disease in Morbid Obesity

BACKGROUND

An association between hypothyroidism and the risk of Non-alcoholic Fatty Liver Disease (NAFLD) has been suggested. This association remains to be elucidated in patients with morbid obesity.

AIM

To evaluate the association between thyroid function and parameters of liver function and hepatic scores in patients with morbid obesity.

METHODS

Patients with morbid obesity followed in our center between January 2010 and July 2018 were included. The ones without evaluation of liver and thyroid functions were excluded. Fatty Liver Index (FLI) and BARD scores were used as predictors of hepatic steatosis and fibrosis, respectively.

RESULTS

We observed a positive association between TSH and both BARD (OR 1.14; p = 0.035) and FLI (OR 1.19; p = 0.010) in the unadjusted analysis. We found a negative association between free triiodothyronine levels and BARD (OR 0.70; p<0.01) and a positive association between free triiodothyronine levels and FLI (OR 1.48; p = 0.022). Concerning liver function, we found a positive association between total bilirubin and free thyroxine levels (β = 0.18 [0.02 to 0.35]; p = 0.033) and a negative association between total bilirubin and free triiodothyronine levels (β = -0.07 [-0.14 to -0.002]; p = 0.042).

CONCLUSION

Higher levels of TSH and free triiodothyronine may be associated with a higher risk of NAFLD, particularly steatosis, in patients with morbid obesity.

Design, synthesis and biological evaluation of novel TRβ selective agonists sustained by ADME-toxicity analysis

Although triiodothyronine (T3) induces several beneficial effects on lipid metabolism, its use is hampered by toxic side-effects, such as tachycardia, arrhythmia, heart failure, bone and muscle catabolism and mood disturbances. Since the α isoform of thyroid hormone receptors (TRs) is the main cause of T3-related harmful effects, several efforts have been made to develop selective agonists of the β isoform that could induce some beneficial effects (i.e. lowering triglyceride and cholesterol levels reducing obesity and improving metabolic syndrome), while overcoming most of the adverse T3-dependent side effects. Herein, we describe the drug discovery process sustained by ADME-Toxicity analysis that led us to identify novel agonists with selectivity for the isoform TRβ and an acceptable off-target and absorption, distribution metabolism, excretion and toxicity (ADME-Tox) profile. Within the small series of compounds synthesized, derivatives 1 and 3, emerge from this analysis as "potentially safe" to be engaged in preclinical studies. In in vitro investigation proved that both compounds were able to reduce lipid accumulation in HepG2 and promote lipolysis with comparable effects to those elicited by T3, used as reference drug. Moreover, a preliminary in vivo study confirmed the apparent lack of toxicity, thus suggesting compounds 1 and 3 as new potential TRβ-selective thyromimetics.

Nonalcoholic Fatty Liver Disease and Hypercholesterolemia: Roles of Thyroid Hormones, Metabolites, and Agonists

Background: Thyroid hormones (THs) exert a strong influence on mammalian lipid metabolism at the systemic and hepatic levels by virtue of their roles in regulating circulating lipoprotein, triglyceride (TAG), and cholesterol levels, as well as hepatic TAG storage and metabolism. These effects are mediated by intricate sensing and feedback systems that function at the physiological, metabolic, molecular, and transcriptional levels in the liver. Dysfunction in the pathways involved in lipid metabolism disrupts hepatic lipid homeostasis and contributes to the pathogenesis of metabolic diseases, such as nonalcoholic fatty liver disease (NAFLD) and hypercholesterolemia. There has been strong interest in understanding and employing THs, TH metabolites, and TH mimetics as lipid-modifying drugs. Summary: THs regulate many processes involved in hepatic TAG and cholesterol metabolism to decrease serum cholesterol and intrahepatic lipid content. TH receptor β analogs designed to have less side effects than the natural hormone are currently being tested in phase II clinical studies for NAFLD and hypercholesterolemia. The TH metabolites, 3,5-diiodo-l-thyronine (T2) and T1AM (3-iodothyronamine), have different beneficial effects on lipid metabolism compared with triiodothyronine (T3), although their clinical application is still under investigation. Also, prodrugs and glucagon/T3 conjugates have been developed that direct TH to the liver. Conclusions: TH-based therapies show clinical promise for the treatment of NAFLD and hypercholesterolemia. Strategies for limiting side effects of TH are being developed and may enable TH metabolites and analogs to have specific effects in the liver for treatments of these conditions. These liver-specific effects and potential suppression of the hypothalamic/pituitary/thyroid axis raise the issue of monitoring liver-specific markers of TH action to assess clinical efficacy and dosing of these compounds.

A Liver-Specific Thyromimetic, VK2809, Decreases Hepatosteatosis in Glycogen Storage Disease Type Ia

Background: Glycogen storage disease type Ia (GSD Ia), also known as von Gierke disease, is the most common glycogen storage disorder. It is caused by the deficiency of glucose-6-phosphatase, the enzyme that catalyzes the final step of gluconeogenesis and glycogenolysis. The accumulation of glucose-6-phosphate leads to increased glycogen and triglyceride levels in the liver. Patients with GSD Ia can develop steatohepatitis, cirrhosis, and increased risk for hepatocellular adenomas and carcinomas. We previously showed that animal models of GSD Ia had defective autophagy and dysfunctional mitochondria. In this study, we examined the effect of VK2809, a liver-specific thyroid hormone receptor β agonist, on hepatic steatosis, autophagy, and mitochondrial biogenesis in a mouse model of GSD Ia. Methods:G6pc^-/--deficient (GSD Ia) mice were treated with VK2809 or vehicle control by daily intraperitoneal injection for four days. The hepatic triglyceride and glycogen were determined by biochemical assays. Autophagy and mitochondrial biogenesis were measured by Western blotting for key autophagy and mitochondrial markers. Results: VK2809 treatment decreased hepatic mass and triglyceride content in GSD Ia mice. VK2809 stimulated hepatic autophagic flux as evidenced by increased microtubule-associated protein light chain 3-II (LC3B-II), decreased p62 protein levels, activation of AMP-activated protein kinase (AMPK), inhibition of the mammalian target of rapamycin (mTOR) signaling, enhancement of protein levels of ATG5-ATG12, and increased lysosomal protein expression. VK2809 also increased the expression of carnitine palmitoyltransferase 1a (CPT1α) and fibroblast growth factor 21 (FGF21), as well as mitochondrial biogenesis to promote mitochondrial β-oxidation. Conclusions: In summary, VK2809 treatment decreased hepatic triglyceride levels in GSD Ia mice through its simultaneous restoration of autophagy, mitochondrial biogenesis, and β-oxidation of fatty acids. Liver-specific thyromimetics represent a potential therapy for hepatosteatosis in GSD Ia as well as nonalcoholic fatty liver disease.

Phosphonate prodrugs: an overview and recent advances

Phosphonates, often used as isosteric replacements for phosphates, can provide important interactions with an enzyme. Due to their high charge at physiological pH, however, permeation into cells can be a challenge. Protecting phosphonates as prodrugs has shown promise in drug delivery. Thus, a variety of structures and cleavage/activation mechanisms exist, enabling release of the active compound. This review describes the structural diversity of these pro-moieties, relevant cleavage mechanisms and recent advances in the design of phosphonate prodrugs.

Effects of exercise on reverse cholesterol transport: A systemized narrative review of animal studies

AIMS

Reverse Cholesterol Transport (RCTr) is the mechanism by which excess cholesterol from peripheral tissues is transported to the liver for hepatobiliary excretion, thereby inhibiting foam cell formation and the development of atherosclerosis. Exercise affects RCTr, by influencing high-density lipoprotein cholesterol (HDL) through remodeling and by promoting hepatobiliary sterol excretion. The objectives of this systematized review of animal studies is to summarize the literature and provide an overview of the effects of chronic exercise (at least two weeks) on apolipoproteins (Apo A-I, Apo-E), Paraoxonase-1 (PON1), ATP-binding cassette transporters (ABCA1, ABCG1, ABCG4, ABCG5, ABCG8), scavenger receptor class B type I (SR-BI), cholesteryl ester transfer protein (CETP), low-density lipoprotein receptor (LDLr) and cholesterol 7 alpha-hydroxylase (CYP7A1) and Niemann-Pick C1-like 1 (NPC1L1).

MATERIALS AND METHODS

Three electronic databases (PubMed, Science Direct and Google Scholar) were searched for eligible studies conducted from the earliest available date to August 2018.

KEY FINDINGS

Most of studies investigate the effects of low to moderate intensity aerobic training on RCTr elements. The majority were on exercised rats undertaking moderate intensity aerobic training.

SIGNIFICANCE

This review highlights that moderate intensity and longer-term training has a greater effect on RCTr elements than low intensity training. There a few studies examining high intensity training which warrants further investigation.

Molecular functions and clinical impact of thyroid hormone-triggered autophagy in liver-related diseases

The liver is controlled by several metabolic hormones, including thyroid hormone, and characteristically displays high lysosomal activity as well as metabolic stress-triggered autophagy, which is stringently regulated by the levels of hormones and metabolites. Hepatic autophagy provides energy through catabolism of glucose, amino acids and free fatty acids for starved cells, facilitating the generation of new macromolecules and maintenance of the quantity and quality of cellular organelles, such as mitochondria. Dysregulation of autophagy and defective mitochondrial homeostasis contribute to hepatocyte injury and liver-related diseases, such as non-alcoholic fatty liver disease (NAFLD) and liver cancer.

Thyroid hormones (TH) mediate several critical physiological processes including organ development, cell differentiation, metabolism and cell growth and maintenance. Accumulating evidence has revealed dysregulation of cellular TH activity as the underlying cause of several liver-related diseases, including alcoholic or non-alcoholic fatty liver disease and liver cancer. Data from epidemiologic, animal and clinical studies collectively support preventive functions of THs in liver-related diseases, highlighting the therapeutic potential of TH analogs. Elucidation of the molecular mechanisms and downstream targets of TH should thus facilitate the development of therapeutic strategies for a number of major public health issues.

Here, we have reviewed recent studies focusing on the involvement of THs in hepatic homeostasis through induction of autophagy and their implications in liver-related diseases. Additionally, the potential underlying molecular pathways and therapeutic applications of THs in NAFLD and HCC are discussed.

Anti-Obesity Therapy: from Rainbow Pills to Polyagonists

With their ever-growing prevalence, obesity and diabetes represent major health threats of our society. Based on estimations by the World Health Organization, approximately 300 million people will be obese in 2035. In 2015 alone there were more than 1.6 million fatalities attributable to hyperglycemia and diabetes. In addition, treatment of these diseases places an enormous burden on our health care system. As a result, the development of pharmacotherapies to tackle this life-threatening pandemic is of utmost importance. Since the beginning of the 19th century, a variety of drugs have been evaluated for their ability to decrease body weight and/or to improve deranged glycemic control. The list of evaluated drugs includes, among many others, sheep-derived thyroid extracts, mitochondrial uncouplers, amphetamines, serotonergics, lipase inhibitors, and a variety of hormones produced and secreted by the gastrointestinal tract or adipose tissue. Unfortunately, when used as a single hormone therapy, most of these drugs are underwhelming in their efficacy or safety, and placebo-subtracted weight loss attributed to such therapy is typically not more than 10%. In 2009, the generation of a single molecule with agonism at the receptors for glucagon and the glucagon-like peptide 1 broke new ground in obesity pharmacology. This molecule combined the beneficial anorectic and glycemic effects of glucagon-like peptide 1 with the thermogenic effect of glucagon into a single molecule with enhanced potency and sustained action. Several other unimolecular dual agonists have subsequently been developed, and, based on their preclinical success, these molecules illuminate the path to a new and more fruitful era in obesity pharmacology. In this review, we focus on the historical pharmacological approaches to treat obesity and glucose intolerance and describe how the knowledge obtained by these studies led to the discovery of unimolecular polypharmacology.

Potential Role of Thyroid Receptor β Agonists in the Treatment of Hyperlipidemia

Thyroid hormones have important effects on cellular development, growth, and metabolism and are necessary for the healthy function of almost all tissues. Hyperthyroid patients with excess thyroid hormone levels experience tachycardia, fatigue, muscle wasting, and osteoporosis. However, although high thyroid hormone levels have adverse effects, efforts have been made to harness the beneficial effects, such as reduced serum low-density lipoprotein (LDL) cholesterol levels, elevated basal metabolic rate, and weight loss. Thyroid hormones interact with nuclear thyroid hormone receptors (TRs), and cholesterol levels are reduced through TRβ, whereas extrahepatic adverse actions are primarily connected to TRα. Thus, to develop a useful compound for clinical use, efforts have been focusing on developing compounds with isomer-specific functions based on the structure of thyroid hormones, i.e., thyromimetics that are liver and/or TRβ specific. In this short review, we discuss the development of the early thyromimetics that enabled, through modern molecular techniques, the progress towards improved design of TRβ-selective thyromimetics. We also address the early promise shown in human clinical trials and the current status of these drugs and other emerging compounds.

The Role of Thyroid Hormones in Hepatocyte Proliferation and Liver Cancer

Thyroid hormones T3 and T4 (thyroxine) control a wide variety of effects related to development, differentiation, growth and metabolism, through their interaction with nuclear receptors. But thyroid hormones also produce non-genomic effects that typically start at the plasma membrane and are mediated mainly by integrin αvβ3, although other receptors such as TRα and TRβ are also able to elicit non-genomic responses. In the liver, the effects of thyroid hormones appear to be particularly important. The liver is able to regenerate, but it is subject to pathologies that may lead to cancer, such as fibrosis, cirrhosis, and non-alcoholic fatty liver disease. In addition, cancer cells undergo a reprogramming of their metabolism, resulting in drastic changes such as aerobic glycolysis instead of oxidative phosphorylation. As a consequence, the pyruvate kinase isoform M2, the rate-limiting enzyme of glycolysis, is dysregulated, and this is considered an important factor in tumorigenesis. Redox equilibrium is also important, in fact cancer cells give rise to the production of more reactive oxygen species (ROS) than normal cells. This increase may favor the survival and propagation of cancer cells. We evaluate the possible mechanisms involving the plasma membrane receptor integrin αvβ3 that may lead to cancer progression. Studying diseases that affect the liver and their experimental models may help to unravel the cellular pathways mediated by integrin αvβ3 that can lead to liver cancer. Inhibitors of integrin αvβ3 might represent a future therapeutic tool against liver cancer. We also include information on the possible role of exosomes in liver cancer, as well as on recent strategies such as organoids and spheroids, which may provide a new tool for research, drug discovery, and personalized medicine.

Thyroid hormone: a resurgent treatment for an emergent concern

The story of thyroid hormone in human physiology is one of mixed emotions. Studying past literature on its use leads one to believe that it serves only a few functions in a handful of diseases. In reality, the pathophysiological role of thyroid hormone is an uncharted expanse. Over the past few decades, research on thyroid hormone has been understandably monopolized by studies of hypo- and hyperthyroidism and cancers. However, in our focused pursuit, we have neglected to observe its role in systems that are not so easily relatable. Recent evidence in lung disease suggests that the thyroid hormone is capable of preserving mitochondria in an indirect manner. This is an exciting revelation given the profound implications of mitochondrial dysfunction in several lung diseases. When paired with known links between thyroid hormone and fibrotic pathways, thyroid hormone-based therapies become more enticing for research. In this article, we inspect the sudden awareness surrounding thyroid hormone and discuss why it is of paramount importance that further studies scrutinize the potential of thyroid hormone, and/or thyromimetics, as therapies for lung diseases.

Direct effects of thyroid hormones on hepatic lipid metabolism

It has been known for a long time that thyroid hormones have prominent effects on hepatic fatty acid and cholesterol synthesis and metabolism. Indeed, hypothyroidism has been associated with increased serum levels of triglycerides and cholesterol as well as non-alcoholic fatty liver disease (NAFLD). Advances in areas such as cell imaging, autophagy and metabolomics have generated a more detailed and comprehensive picture of thyroid-hormone-mediated regulation of hepatic lipid metabolism at the molecular level. In this Review, we describe and summarize the key features of direct thyroid hormone regulation of lipogenesis, fatty acid β-oxidation, cholesterol synthesis and the reverse cholesterol transport pathway in normal and altered thyroid hormone states. Thyroid hormone mediates these effects at the transcriptional and post-translational levels and via autophagy. Given these potentially beneficial effects on lipid metabolism, it is possible that thyroid hormone analogues and/or mimetics might be useful for the treatment of metabolic diseases involving the liver, such as hypercholesterolaemia and NAFLD.

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.

Quantification of Thyromimetic Sobetirome Concentration in Biological Tissue Samples

Thyroid hormone is a principal regulator of essential processes in vertebrate physiology and homeostasis. Synthetic derivatives of thyroid hormone, known as thyromimetics, display desirable therapeutic properties. Thoroughly understanding how thyromimetics distribute throughout the body is crucial for their development and this requires appropriate bioanalytical techniques to quantify drug levels in different tissues. Here, we describe a detailed protocol for the quantification of the thyromimetic sobetirome using liquid chromatography tandem-mass spectrometry (LC-MS/MS).

Novel thyroid hormone analogues, enzyme inhibitors and mimetics, and their action

Thyroid hormones (THs) play key roles in modulating the overall metabolism of the body, protein synthesis, fat metabolism, neuronal and bone growth, and cardiovascular as well as renal functions. In this review, we discuss on the thyroid hormone synthesis and activation, thyroid hormone receptors (TRs) and mechanism of action, applications of thyroid hormone analogues, particularly the compounds that are selective ligands for TRβ receptors, or enzyme inhibitors for the treatment of thyroidal disorders with a specific focus on thyroid peroxidase and iodothyronine deiodinases. We also discuss on the development of small-molecule deiodinase mimetics and their mechanism of deiodination, as these compounds have the potential to regulate the thyroid hormone levels.

GC-1: A Thyromimetic With Multiple Therapeutic Applications in Liver Disease

Thyroid hormones (THs), namely, 3,5,3'-triiodo-l-thyronine (T3) and 3,5,3',5'-tetraiodo-l-thyronine (thyroxine or T4), influence a variety of physiological processes that have important implications in fetal development, metabolism, cell growth, and proliferation. While THs elicit several beneficial effects on lipid metabolism and improve myocardial contractility, these therapeutically desirable effects are associated to a thyrotoxic state that severely limits the possible use of THs as therapeutic agents. Therefore, several efforts have been made to develop T3 analogs that could retain the beneficial actions (triglyceride, cholesterol, obesity, and body mass lowering) without the adverse TH-dependent side effects. This goal was achieved by the synthesis of TRβ-selective agonists. In this review, we summarize the current knowledge on the effects of one of the best characterized TH analogs, the TRβ1-selective thyromimetic, GC-1. In particular, we review some of the effects of GC-1 on different liver disorders, with reference to its possible clinical application. A brief comment on the possible therapeutic use of GC-1 in extrahepatic disorders is also included.

Thyroid hormone analogs for the treatment of dyslipidemia: past, present, and future

OBJECTIVE

Treatment of dyslipidemia is a major burden for public health. Thyroid hormone regulates lipid metabolism by binding the thyroid hormone receptor (TR), but the use of thyroid hormone to treat dyslipidemia is not indicated due to its deleterious effects on heart, bone, and muscle. Thyroid hormone analogs have been conceived to selectively activate TR in the liver, thus reducing potential side-effects.

METHODS

The authors searched the PubMed database to review TR and the action of thyromimetics in vitro and in animal models. Then, all double-blind, placebo controlled trials that analyzed the use of thyroid hormone analog for the treatment of dyslipidemia in humans were included. Finally, the ongoing research on the use of TR agonists was searched, searching the US National Institutes of Health Registry and the WHO International Clinical Trial Registry Platform (ICTRP).

RESULTS

Thyromimetics were tested in humans for the treatment of dyslipidemia, as a single therapeutic agent or as an add-on therapy to the traditional lipid-lowering drugs. In most trials, thyromimetics lowered total cholesterol, low-density lipoprotein cholesterol, and triglycerides, but their use has been associated with adverse side-effects, both in pre-clinical studies and in humans.

CONCLUSIONS

The use of thyromimetics for the treatment of dyslipidemia is not presently recommended. Future possible clinical applications might include their use to promote weight reduction. Thyromimetics might also represent an interesting alternative, both for the treatment of non-alcoholic steatohepatitis, and type 2 diabetes due to their positive effects on insulin sensitivity. Finally, additional experimental and clinical studies are needed for a better comprehension of the effect(s) of a long-term therapy.

Effects of Supra-Physiological Levothyroxine Dosages on Liver Parameters, Lipids and Lipoproteins in Healthy Volunteers: A Randomized Controlled Crossover Study

Eprotirome, a liver specific thyroid hormone agonist, was shown to induce significant increases in markers of liver injury along with a modest decrease in atherogenic lipids and lipoproteins. To get more insight into whether these effects on liver parameters were compound specific or the effect of mimicking thyrotoxicosis, we studied the effects of supra-physiological levothyroxine dosages on liver parameters, lipids and lipoproteins. We used data of a single-blinded, randomized controlled crossover trial. Herein, healthy volunteers received levothyroxine or no medication for 14 days. Thyroid hormone excess did not induce clinically relevant changes in liver parameters, while significant reductions in total cholesterol, low-density lipoprotein-cholesterol as well as apolipoprotein-B levels were observed in the intervention periods compared with the control periods. Supra-physiological thyroid hormone levels did not induce clinically relevant increases in markers of liver injury after 2 weeks of exposure, while it reduced total cholesterol, low-density lipoprotein cholesterol and apolipoprotein B levels. This suggests that the effects of eprotirome on liver parameters in previous studies were either off-target and compound specific or due to drug-drug interaction at the level of the liver. The results of our study are relevant for the development of novel thyroid hormone agonists to reduce atherogenic lipoproteins.

[Pharmacological approaches for correction of thyroid dysfunctions in diabetes mellitus]

Thyroid diseases are closely associated with the development of types 1 and 2 diabetes mellitus (DM), and as a consequence, the development of effective approaches for their treatment is one of the urgent problems of endocrinology. Traditionally, thyroid hormones (TH) are used to correct functions of the thyroid system. However, they are characterized by many side effects, such as their negative effect on the cardiovascular system as well as the ability of TH to enhance insulin resistance and to disturb insulin-producing function of pancreas, exacerbating thereby diabetic pathology. Therefore, the analogues of TH, selective for certain types of TH receptors, that do not have these side effects, are being developed. The peptide and low-molecular weight regulators of thyroid-stimulating hormone receptor, which regulate the activity of the thyroid axis at the stage of TH synthesis and secretion in thyrocytes, are being created. Systemic and intranasal administration of insulin, metformin therapy and drugs with antioxidant activity are effective for the treatment of thyroid pathology in types 1 and 2 DM. In the review, the literature data and the results of own investigations on pharmacological approaches for the treatment and prevention of thyroid diseases in patients with types 1 and 2 DM are summarized and analyzed.

The Thyromimetic KB2115 (Eprotirome) Induces Rat Hepatocyte Proliferation

Although the hepatomitogenic activity of T3 is well established, the wide range of harmful effects exerted by this hormone precludes its use in regenerative therapy. The aim of this study was to investigate whether an agonist of TRβ, KB2115 (Eprotirome), could exert a mitogenic effect in the liver, without most of the adverse T3/TRα-dependent side effects. F-344 rats treated with KB2115 for 1 week displayed a massive increase in bromodeoxyuridine incorporation (from 20% to 40% vs. 5% of controls), which was associated with increased mitotic activity in the absence of significant signs of liver toxicity. Noteworthy, while cardiac hypertrophy typical of T3 was not observed, beneficial effects, such as lowering blood cholesterol levels, were associated to KB2115 administration. Following a single dose of KB2115, hepatocyte proliferation was evident as early as 18 h, demonstrating its direct mitogenic effect. No increase in serum transaminase levels or apoptosis was observed prior to or concomitantly with the S phase. While KB2115-induced mitogenesis was not associated to enhance expression of c-fos, c-jun, and c-myc, cyclin D1 levels rapidly increased. In conclusion, KB2115 induces hepatocyte proliferation without overt toxicity. Hence, this agent may be useful for regenerative therapies in liver transplantation or other surgical settings.

Hypothalamic AMPK-ER Stress-JNK1 Axis Mediates the Central Actions of Thyroid Hormones on Energy Balance

Thyroid hormones (THs) act in the brain to modulate energy balance. We show that central triiodothyronine (T3) regulates de novo lipogenesis in liver and lipid oxidation in brown adipose tissue (BAT) through the parasympathetic (PSNS) and sympathetic nervous system (SNS), respectively. Central T3 promotes hepatic lipogenesis with parallel stimulation of the thermogenic program in BAT. The action of T3 depends on AMP-activated protein kinase (AMPK)-induced regulation of two signaling pathways in the ventromedial nucleus of the hypothalamus (VMH): decreased ceramide-induced endoplasmic reticulum (ER) stress, which promotes BAT thermogenesis, and increased c-Jun N-terminal kinase (JNK) activation, which controls hepatic lipid metabolism. Of note, ablation of AMPKα1 in steroidogenic factor 1 (SF1) neurons of the VMH fully recapitulated the effect of central T3, pointing to this population in mediating the effect of central THs on metabolism. Overall, these findings uncover the underlying pathways through which central T3 modulates peripheral metabolism.

Effects of age and soybean isoflavones on hepatic cholesterol metabolism and thyroid hormone availability in acyclic female rats

Soy-food and its isoflavones, genistein (G) and daidzein (D), were reported to exert mild cholesterol-lowering effect, but the underlying mechanism is still unclear. In this research, first we studied age-related alterations in hepatic cholesterol metabolism of acyclic middle-aged (MA) female rats. Then we tested if purified isoflavones may prevent or reverse these changes, and whether putative changes in hepatic thyroid hormone availability may be associated with this effect. Serum and hepatic total cholesterol (TChol), bile acid and cholesterol precursors, as well as serum TSH and T4 concentrations, hepatic deiodinase (Dio) 1 enzyme activity and MCT8 protein expression were determined by comparing data obtained for MA with young adult (YA) intact (IC) females. Effects of subcutaneously administered G or D (35mg/kg) to MA rats were evaluated versus vehicle-treated MA females. MA IC females were characterized by: higher (p<0.05) serum TChol, lower (p<0.05) hepatic TChol and its biosynthetic precursors, lower (p<0.05) hepatic 7α-hydroxycholesterol but elevated (p<0.05) 27- and 24-hydroxycholesterol in comparison to YA IC. Both isoflavone treatments decreased (p<0.05) hepatic 27-hydroxycholesterol, G being more effective than D, without affecting any other parameter of Chol metabolism. Only G elevated hepatic Dio1 activity (p<0.05). In conclusion, age-related hypercholesteremia was associated with lower hepatic Chol synthesis and shift from main neutral (lower 7α-hydroxycholesterol) to alternative acidic pathway (higher 27-hydroxycholesterol) of Chol degradation to bile acid. Both isoflavones lowered hepatic 27-hydroxycholesterol, which may be considered beneficial. Only G treatment increased hepatic Dio1 activity, thus indicating local increase in thyroid hormones, obviously insufficient to induce prominent cholesterol-lowering effect.

Relationship between Hypothyroidism and Non-Alcoholic Fatty Liver Disease: A Systematic Review and Meta-analysis

BACKGROUND

Previous studies propose that hypothyroidism might play a crucial role in the pathogenesis of non-alcoholic fatty liver disease (NAFLD), but findings from published studies on the relationship between hypothyroidism and NAFLD are still controversial. Our study aimed to make a comprehensive evaluation of the relationship between hypothyroidism and NAFLD through a meta-analysis.

METHODS

PubMed, China Dissertation Database, and EMBASE databases were searched to find observational studies assessing the relationship between hypothyroidism and NAFLD. The pooled odds ratios (ORs) with 95% confidence intervals (95% CIs) were calculated to evaluate the strength of the relationship between hypothyroidism and NAFLD through meta-analysis.

RESULTS

Thirteen articles were ultimately included in our meta-analysis. Meta-analysis of the 13 studies found a high correlation between hypothyroidism and NAFLD (OR = 1.52, 95% CI 1.24-1.87, P < 0.001). Meta-analysis of 9 studies providing adjusted ORs found that hypothyroidism was independently correlated with NAFLD (OR = 1.72, 95% CI 1.32-2.23, P < 0.001). Subgroup analysis found that both overt hypothyroidism and subclinical hypothyroidism were significantly correlated with NAFLD, and the pooled ORs were 1.70 (95% CI 1.23-2.36, P = 0.002) and 1.40 (95% CI 1.10-1.77, P = 0.006), respectively. Besides, meta-analysis of studies providing adjusted ORs also found that both overt hypothyroidism and subclinical hypothyroidism were independently correlated with NAFLD, and the pooled ORs were 1.81 (95% CI 1.30-2.52, P < 0.001) and 1.63 (95% CI 1.19-2.24, P = 0.002), respectively.

CONCLUSION

The meta-analysis provides strong epidemiological evidence for the relationship between hypothyroidism and NAFLD. Both individuals with subclinical and overt hypothyroidism are at higher risk for NAFLD than euthyroid subjects.

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.

Thyroid Hormone Mimetics: the Past, Current Status and Future Challenges

The association between thyroid hormone status and plasma levels of low-density lipoprotein cholesterol has raised the awareness for the development of thyroid hormone mimetics as lipid-lowering agents. The discovery of the two main types of thyroid hormone receptors (α and β) as well as the development of novel combinatorial chemistry providing organ specificity has drastically improved the selectivity of these compounds. In the past decades, several thyroid hormone mimetics have been investigated with the purpose of lowering low-density lipoprotein cholesterol levels. However, until now, none of the thyromimetics reached the stage of completing a phase III clinical trial without deleterious side effects. Here, we review the currently available literature on thyromimetics investigated for the treatment of dyslipidemia, their rise, their downfall and the challenges for the development of novel agents.

T3/TRs axis in hepatocellular carcinoma: new concepts for an old pair

Hepatocellular carcinoma (HCC) is a leading cause of cancer-related death worldwide, and its burden is expected to further increase in the next years. Chronic inflammation, induced by multiple viruses or metabolic alterations, and epigenetic and genetic modifications, cooperate in cancer development via a combination of common and distinct aetiology-specific pathways. In spite of the advances of classical therapies, the prognosis of this neoplasm has not considerably improved over the past few years. The advent of targeted therapies and the approval of the systemic treatment of advanced HCC with the kinase inhibitor sorafenib have provided some hope for the future. However, the benefits obtained from this treatment are still disappointing, as it extends the median life expectancy of patients by only few months. It is thus mandatory to find alternative effective treatments. Although the role played by thyroid hormones (THs) and their nuclear receptors (TRs) in human cancer is still unclear, mounting evidence indicates that they behave as oncosuppressors in HCC. However, the molecular mechanisms by which they exert this effect and the consequence of their activation following ligand binding on HCC progression remain elusive. In this review, we re-evaluate the existing evidence of the role of TH/TRs in HCC development; we will also discuss how TR alterations could affect fundamental biological processes, such as hepatocyte proliferation and differentiation, and consequently HCC progression. Finally, we will discuss if and how TRs can be foreseen as therapeutic targets in HCC and whether selective TR modulation by TH analogues may hold promise for HCC treatment.

Hepatocellular carcinoma: thyroid hormone promotes tumorigenicity through inducing cancer stem-like cell self-renewal

Cancer stem-like cells (CSCs) play a key role in maintaining the aggressiveness of hepatocellular carcinoma (HCC), but the cell-biological regulation of CSCs is unclear. In the study, we report that thyroid hormone (TH) promotes cell self-renewal in HCC cells. TH also increases the percentage of CD90 + HCC cells and promotes drug resistance of HCC cells. By analyzing primary human HCC samples, we found that TRα transcript level is significantly elevated in primary liver cancer and portal vein metastatic tumor, compared to that of adjacent normal liver tissue. Knocking down TRα not only inhibits HCC self-renewal in vitro but also suppresses HCC tumor growth in vivo. Interestingly, treatment of TH leads to activation of NF-κB, which is required for the function of TH on inducing HCC cell self-renewal. We also found TRα and p65 cooperatively drive the expression of BMI1 by co-binding to the promoter region of BMI1 gene. In summary, our study uncovers a novel function of TH signaling in regulating the CSCs of HCC, and these findings might be useful for developing novel therapies by targeting TH function in HCC cells.

MicroRNA: a connecting road between apoptosis and cholesterol metabolism

Resistance to apoptosis leads to tumorigenesis and failure of anti-cancer therapy. Recent studies also highlight abrogated lipid/cholesterol metabolism as one of the root causes of cancer that can lead to metastatic transformations. Cancer cells are dependent on tremendous supply of cellular cholesterol for the formation of new membranes and continuation of cell signaling. Cholesterol homeostasis network tightly regulates this metabolic need of cancer cells on cholesterol and other lipids. Genetic landscape is also shared between apoptosis and cholesterol metabolism. MicroRNAs (miRNAs) are the new fine tuners of signaling pathways and cellular processes and are known for their ability to post-transcriptionally repress gene expression in a targeted manner. This review summarizes the current knowledge about the cross talk between apoptosis and cholesterol metabolism via miRNAs. In addition, we also emphasize herein recent therapeutic modulations of specific miRNAs and their promising potential for the treatment of deadly diseases including cancer and cholesterol related pathologies. Understanding of the impact of miRNA-based regulation of apoptosis and metabolic processes is still at its dawn and needs further research for the development of future miRNA-based therapies. As both these physiological processes affect cellular homeostasis, we believe that this comprehensive summary of miRNAs modulating both apoptosis and cholesterol metabolism will open uncharted territory for scientific exploration and will provide the foundation for discovering novel drug targets for cancer and metabolic diseases.

3,5 Diiodo-L-Thyronine (T2) Does Not Prevent Hepatic Steatosis or Insulin Resistance in Fat-Fed Sprague Dawley Rats

Thyroid hormone mimetics are alluring potential therapies for diseases like dyslipidemia, nonalcoholic fatty liver disease (NAFLD), and insulin resistance. Though diiodothyronines are thought inactive, pharmacologic treatment with 3,5- Diiodo-L-Thyronine (T2) reportedly reduces hepatic lipid content and improves glucose tolerance in fat-fed male rats. To test this, male Sprague Dawley rats fed a safflower-oil based high-fat diet were treated with T2 (0.25 mg/kg-d) or vehicle. Neither 10 nor 30 days of T2 treatment had an effect on weight, adiposity, plasma fatty acids, or hepatic steatosis. Insulin action was quantified in vivo by a hyperinsulinemic-euglycemic clamp. T2 did not alter fasting plasma glucose or insulin concentration. Basal endogenous glucose production (EGP) rate was unchanged. During the clamp, there was no difference in insulin stimulated whole body glucose disposal. Insulin suppressed EGP by 60% ± 10 in T2-treated rats as compared with 47% ± 4 suppression in the vehicle group (p = 0.32). This was associated with an improvement in hepatic insulin signaling; insulin stimulated Akt phosphorylation was ~2.5 fold greater in the T2-treated group as compared with the vehicle-treated group (p = 0.003). There was no change in expression of genes thought to mediate the effect of T2 on hepatic metabolism, including genes that regulate hepatic lipid oxidation (ppara, carnitine palmitoyltransferase 1a), genes that regulate hepatic fatty acid synthesis (srebp1c, acetyl coa carboxylase, fatty acid synthase), and genes involved in glycolysis and gluconeogenesis (L-pyruvate kinase, glucose 6 phosphatase). Therefore, in contrast with previous reports, in Sprague Dawley rats fed an unsaturated fat diet, T2 administration failed to improve NAFLD or whole body insulin sensitivity. Though there was a modest improvement in hepatic insulin signaling, this was not associated with significant differences in hepatic insulin action. Further study will be necessary before diiodothyronines can be considered an effective treatment for NAFLD and dyslipidemia.

Characterisation of liver-specific distribution of a novel 1-benzyl-4-aminoindole-based thyroid hormone receptor β agonist, SKL-13784: comparison with GC-1

1. SKL-13784, a novel series of 1-benzyl-4-aminoindole-based thyroid hormone receptor β (TRβ)-selective agonists, showed higher liver selectivity than GC-1 and was poorly distributed in the heart and brain. We aimed to clarify the mechanism of liver selectivity of SKL-13784 through a comparative study with GC-1. 2. Media-loss assays using fresh rat hepatocytes showed that the Oatp family may have been involved in liver uptake for both compounds and that SKL-13784 was more efficiently taken up than GC-1. 3. In addition, the media-loss assay results showed that hepatic uptake was important in eliminating both compounds in rats. 4. The low passive permeability of SKL-13784 on the parallel artificial membrane permeability assay (PAMPA) contributed to the limited distribution of SKL-13784 into extrahepatocytes. 5. Biliary extraction was a major route of SKL-13784 and GC-1 disposition. SKL-13784 was excreted into bile unchanged and in its glucuronide form, whereas almost all GC-1 in bile was in its glucuronide form. In bile duct-cannulated rats, a 4.3-fold decrease in t1/2 of SKL-13784 was observed, implicating enterohepatic biliary recirculation. 6. The selective distribution of SKL-13784 in the liver was largely due to efficient uptake via hepatic transporters.

The amelioration of hepatic steatosis by thyroid hormone receptor agonists is insufficient to restore insulin sensitivity in ob/ob mice

Thyroid hormone receptor (TR) agonists have been proposed as therapeutic agents to treat non-alcoholic fatty liver disease (NAFLD) and insulin resistance. We investigated the ability of the TR agonists GC-1 and KB2115 to reduce hepatic steatosis in ob/ob mice. Both compounds markedly reduced hepatic triglyceride levels and ameliorated hepatic steatosis. However, the amelioration of fatty liver was not sufficient to improve insulin sensitivity in these mice and reductions in hepatic triglycerides did not correlate with improvements in insulin sensitivity or glycemic control. Instead, the effects of TR activation on glycemia varied widely and were found to depend upon the time of treatment as well as the compound and dosage used. Lower doses of GC-1 were found to further impair glycemic control, while a higher dose of the same compound resulted in substantially improved glucose tolerance and insulin sensitivity, despite all doses being equally effective at reducing hepatic triglyceride levels. Improvements in glycemic control and insulin sensitivity were observed only in treatments that also increased body temperature, suggesting that the induction of thermogenesis may play a role in mediating these beneficial effects. These data illustrate that the relationship between TR activation and insulin sensitivity is complex and suggests that although TR agonists may have value in treating NAFLD, their effect on insulin sensitivity must also be considered.

Reductions in serum levels of LDL cholesterol, apolipoprotein B, triglycerides and lipoprotein(a) in hypercholesterolaemic patients treated with the liver-selective thyroid hormone receptor agonist eprotirome

BACKGROUND

Liver-selective thyromimetic agents could provide a new approach for treating dyslipidaemia.

METHODS

We performed a multicentre, randomized, placebo-controlled, double-blind study to evaluate the efficacy and safety of eprotirome, a liver-selective thyroid hormone receptor agonist, in 98 patients with primary hypercholesterolaemia. After previous drug wash-out and dietary run-in, patients received 100 or 200 μg day(-1) eprotirome or placebo for 12 weeks. The primary end-point was change in serum LDL cholesterol; secondary end-points included changes in other lipid parameters and safety measures.

RESULTS

Eprotirome treatment at 100 and 200 μg daily reduced serum LDL cholesterol levels by 23 ± 5% and 31 ± 4%, respectively, compared with 2 ± 6% for placebo (P < 0.0001). Similar reductions were seen in non-HDL cholesterol and apolipoprotein (apo) B, whereas serum levels of HDL cholesterol and apo A-I were unchanged. There were also considerable reductions in serum triglycerides and lipoprotein(a), in particular in patients with elevated levels at baseline. There was no evidence of adverse effects on heart or bone and no changes in serum thyrotropin or triiodothyronine, although the thyroxine level decreased. Low-grade increases in liver enzymes were evident in most patients.

CONCLUSION

In hypercholesterolaemic patients, the liver-selective thyromimetic eprotirome decreased serum levels of atherogenic lipoproteins without signs of extra-hepatic side effects. Selective stimulation of hepatic thyroid hormone receptors may be an attractive way to modulate lipid metabolism in hyperlipidaemia.

Prodrugs of phosphonates and phosphates: crossing the membrane barrier

A substantial portion of metabolism involves transformation of phosphate esters, including pathways leading to nucleotides and oligonucleotides, carbohydrates, isoprenoids and steroids, and phosphorylated proteins. Because the natural substrates bear one or more negative charges, drugs that target these enzymes generally must be charged as well, but small charged molecules can have difficulty traversing the cell membrane by means other than endocytosis. The resulting dichotomy has stimulated a great deal of effort to develop effective prodrugs, compounds that carry little or no charge to enable them to transit biological membranes, but able to release the parent drug once inside the target cell. This chapter presents recent studies on advances in prodrug forms, along with representative examples of their application to marketed and developmental drugs.

New avenues for regulation of lipid metabolism by thyroid hormones and analogs

Weight loss due to negative energy balance is a goal in counteracting obesity and type 2 diabetes mellitus. The thyroid is known to be an important regulator of energy metabolism through the action of thyroid hormones (THs). The classic, active TH, 3,5,3'-triiodo-L-thyronine (T3) acts predominantly by binding to nuclear receptors termed TH receptors (TRs), that recognize TH response elements (TREs) on the DNA, and so regulate transcription. T3 also acts through "non-genomic" pathways that do not necessarily involve TRs. Lipid-lowering therapies have been suggested to have potential benefits, however, the establishment of comprehensive therapeutic strategies is still awaited. One drawback of using T3 in counteracting obesity has been the occurrence of heart rhythm disturbances. These are mediated through one TR, termed TRα. The end of the previous century saw the exploration of TH mimetics that specifically bind to TR beta in order to prevent cardiac disturbances, and TH derivatives such as 3,5-diiodo-L-thyronine (T2), that possess interesting biological activities. Several TH derivatives and functional analogs have low affinity for the TRs, and are suggested to act predominantly through non-genomic pathways. All this has opened new perspectives in thyroid physiology and TH derivative usage as anti-obesity therapies. This review addresses the pros and cons of these compounds, in light of their effects on energy balance regulation and on lipid/cholesterol metabolism.

Thyroid hormone regulation of hepatic lipid and carbohydrate metabolism

Thyroid hormone (TH) has important roles in regulating hepatic lipid, cholesterol, and glucose metabolism. Recent findings suggest that clinical conditions such as non-alcoholic fatty liver disease and type 2 diabetes mellitus, which are associated with dysregulated hepatic metabolism, may involve altered intracellular TH action. In addition, TH has key roles in lipophagy in lipid metabolism, mitochondrial quality control, and the regulation of metabolic genes. In this review, we discuss recent findings regarding the functions of TH in hepatic metabolism, the relationship between TH and metabolic disorders, and the potential therapeutic use of thyromimetics to treat metabolic dysfunction in the liver.

Regulation of fatty acid composition and lipid storage by thyroid hormone in mouse liver

Background

Thyroid hormones (THs) are potent hormones modulating liver lipid homeostasis. The perturbation of lipid homeostasis is a hallmark of non-alcoholic fatty liver disease (NAFLD), a very common liver disorder. It was reported that NAFLD patients were associated with higher incidence of hypothyroidism. However, whether abnormal thyroid function contributes to the pathogenesis of NAFLD remains unclear.

Results

We used in vivo models to investigate the influence of hypothyroidism and TH on hepatic lipid homeostasis. We did not observe hepatic triglyceride accumulation in the liver of hypothyroid mice, although the liver was enlarged. We then characterized the hepatic fatty acid composition with gas chromatography–mass spectrometry in mice under different thyroid states. We found that hypothyroidism decreased saturated fatty acid (SFA) content while TH treatment restored the level of SFA. In agreement with this finding, we observed that the expression of acetyl-CoA carboxylase 1 and fatty acid synthase, the rate-limit enzymes for de novo lipogenesis (DNL), decreased in hypothyroid mice while increased after TH treatment. We also found that the ratio of C18:1n-9/C18:0 and C16:1n-7/C16:0 was decreased by TH treatment, suggesting the activity of stearoyl-CoA desaturase-1 was suppressed. This finding indicated that TH is able to suppress triglyceride accumulation by reducing fatty acid desaturation. Additionally, we found that hepatic glycogen content was substantially influenced by TH status, which was associated with glycogen synthase expression. The increased glycogen storage might explain the enlarged liver we observed in hypothyroid mice.

Conclusions

Taken together, our study here suggested that hypothyroidism in mice might not lead to the development of NAFLD although the liver became enlarged. However, disturbed TH levels led to altered hepatic fatty acid composition and glycogen accumulation.

In vivo selective expression of thyroid hormone receptor α1 in endothelial cells attenuates myocardial injury in experimental myocardial infarction in mice

Ischemic heart disease (IHD) is the single most common cause of death. New approaches to enhance myocardial perfusion are needed to improve outcomes for patients with IHD. Thyroid hormones (TH) are known to increase blood flow; however, their usefulness for increasing perfusion in IHD is limited because TH accelerates heart rate, which can be detrimental. Therefore, selective activation of TH effects is desirable. We hypothesized that cell-type-specific TH receptor (TR) expression can increase TH action in the heart, while avoiding the negative consequences of TH treatment. We generated a binary transgenic (BTG) mouse that selectively expresses TRα1 in endothelial cells in a tetracycline-inducible fashion. In BTG mice, endothelial TRα1 protein expression was increased by twofold, which, in turn, increased coronary blood flow by 77%, coronary conductance by 60%, and coronary reserve by 47% compared with wild-type mice. Systemic blood pressure was decreased by 20% in BTG mice after TRα1 expression. No effects on heart rate were observed. Endothelial TRα1 expression activated AKT/endothelial nitric oxide synthase pathway and increased A2AR adenosine receptor. Furthermore, hearts from BTG mice overexpressing TRα1 that were submitted to 20 min ischemia and 20 min reperfusion showed a 20% decline in left ventricular pressure (LVP) compared with control mice where LVP was decreased by 42%. Studies using an infarction mouse model demonstrated that endothelial overexpression of TRα1 decreased infarct size by 45%. In conclusion, selective expression of TRα1 in endothelial cells protects the heart against injury after an ischemic insult and does not result in adverse cardiac or systemic effects.

Discovery of 2-[3,5-dichloro-4-(5-isopropyl-6-oxo-1,6-dihydropyridazin-3-yloxy)phenyl]-3,5-dioxo-2,3,4,5-tetrahydro[1,2,4]triazine-6-carbonitrile (MGL-3196), a Highly Selective Thyroid Hormone Receptor β agonist in clinical trials for the treatment of dyslipidemia

The beneficial effects of thyroid hormone (TH) on lipid levels are primarily due to its action at the thyroid hormone receptor β (THR-β) in the liver, while adverse effects, including cardiac effects, are mediated by thyroid hormone receptor α (THR-α). A pyridazinone series has been identified that is significantly more THR-β selective than earlier analogues. Optimization of this series by the addition of a cyanoazauracil substituent improved both the potency and selectivity and led to MGL-3196 (53), which is 28-fold selective for THR-β over THR-α in a functional assay. Compound 53 showed outstanding safety in a rat heart model and was efficacious in a preclinical model at doses that showed no impact on the central thyroid axis. In reported studies in healthy volunteers, 53 exhibited an excellent safety profile and decreased LDL cholesterol (LDL-C) and triglycerides (TG) at once daily oral doses of 50 mg or higher given for 2 weeks.

Hepatic nuclear corepressor 1 regulates cholesterol absorption through a TRβ1-governed pathway

Transcriptional coregulators are important components of nuclear receptor (NR) signaling machinery and provide additional mechanisms for modulation of NR activity. Expression of a mutated nuclear corepressor 1 (NCoR1) that lacks 2 NR interacting domains (NCoRΔID) in the liver leads to elevated expression of genes regulated by thyroid hormone receptor (TR) and liver X receptor (LXR), both of which control hepatic cholesterol metabolism. Here, we demonstrate that expression of NCoRΔID in mouse liver improves dietary cholesterol tolerance in an LXRα-independent manner. NCoRΔID-associated cholesterol tolerance was primarily due to diminished intestinal cholesterol absorption as the result of changes in the composition and hydrophobicity of the bile salt pool. Alterations of the bile salt pool were mediated by increased expression of genes encoding the bile acid metabolism enzymes CYP27A1 and CYP3A11 as well as canalicular bile salt pump ABCB11. We have determined that these genes are regulated by thyroid hormone and that TRβ1 is recruited to their regulatory regions. Together, these data indicate that interactions between NCoR1 and TR control a specific pathway involved in regulation of cholesterol metabolism and clearance.

Thyroid hormone, thyromimetics, and metabolic efficiency

Thyroid hormone (TH) has long been recognized as a major modulator of metabolic efficiency, energy expenditure, and thermogenesis. TH effects in regulating metabolic efficiency are transduced by controlling the coupling of mitochondrial oxidative phosphorylation and the cycling of extramitochondrial substrate/futile cycles. However, despite our present understanding of the genomic and nongenomic modes of action of TH, its control of mitochondrial coupling still remains elusive. This review summarizes historical and up-to-date findings concerned with TH regulation of metabolic energetics, while integrating its genomic and mitochondrial activities. It underscores the role played by TH-induced gating of the mitochondrial permeability transition pore (PTP) in controlling metabolic efficiency. PTP gating may offer a unified target for some TH pleiotropic activities and may serve as a novel target for synthetic functional thyromimetics designed to modulate metabolic efficiency. PTP gating by long-chain fatty acid analogs may serve as a model for such strategy.

Thyroid hormone status regulates the expression of secretory phospholipases

Thyroid hormone (T3) stimulates various metabolic pathways and the hepatic actions of T3 are mediated primarily through the thyroid hormone receptor beta (TRβ). Hypothyroidism has been linked with low grade inflammation, elevated risk of hepatic steatosis and atherosclerosis. Secretory phospholipases (sPLA2) are associated with inflammation, hyperlipidemia and atherosclerosis. Due to potential linkage between thyroid hormone and sPLA2, we investigated the effect of thyroid hormone status on the regulation of secretory phospholipases in mice, rats and human liver. T3 suppressed the expression of the sPLA2 group IIa (PLA2g2a) gene in the liver of BALB/c mice and C57BL/6 transgenic mice expressing the human PLA2g2a. PLA2g2a was elevated with hypothyroidism and high fat diets which may contribute to the low grade inflammation associated with hypothyroidism and diet induced obesity. We also examined the effects of the TRβ agonist eprotirome on hepatic gene regulation. We observed that eprotirome inhibited the expression of selected sPLA2 genes and furthermore the cytokine mediated induction PLA2g2a was suppressed. In addition, eprotirome induced genes involved in fatty acid oxidation and cholesterol clearance while inhibiting lipogenic genes. Our results indicate that in vivo thyroid hormone status regulates the abundance of sPLA2 and the inhibition of PLA2g2a by T3 is conserved across species. By regulating sPLA2 genes, T3 may impact processes associated with atherosclerosis and inflammation and TRβ agonists may ameliorate inflammation and hyperlipidemia.

American Thyroid Association Guide to investigating thyroid hormone economy and action in rodent and cell models

BACKGROUND

An in-depth understanding of the fundamental principles that regulate thyroid hormone homeostasis is critical for the development of new diagnostic and treatment approaches for patients with thyroid disease.

SUMMARY

Important clinical practices in use today for the treatment of patients with hypothyroidism, hyperthyroidism, or thyroid cancer are the result of laboratory discoveries made by scientists investigating the most basic aspects of thyroid structure and molecular biology. In this document, a panel of experts commissioned by the American Thyroid Association makes a series of recommendations related to the study of thyroid hormone economy and action. These recommendations are intended to promote standardization of study design, which should in turn increase the comparability and reproducibility of experimental findings.

CONCLUSIONS

It is expected that adherence to these recommendations by investigators in the field will facilitate progress towards a better understanding of the thyroid gland and thyroid hormone dependent processes.