Distinct dysregulation of lipid metabolism by unliganded thyroid hormone receptor isoforms

Understanding the Roles of Selenium on Thyroid Hormone-Induced Thermogenesis in Adipose Tissue

Brown adipose tissue (BAT) and white adipose tissue (WAT) are known to regulate lipid metabolism. A lower amount of BAT compared to WAT, along with adipose tissue dysfunction, can result in obesity. Studies have shown that selenium supplementation protects against adipocyte dysfunction, decreases WAT triglycerides, and increases BAT triiodothyronine (T3). In this review, we discuss the relationship between selenium and lipid metabolism regulation through selenoprotein deiodinases and the role of deiodinases and thyroid hormones in the induction of adipose tissue thermogenesis. Upon 22 studies included in our review, we found that studies investigating the relationship between selenium and deiodinases demonstrated that selenium supplementation affects the iodothyronine deiodinase 2 (DIO2) protein and the expression of its associated gene, DIO2, proportionally. However, its effect on DIO1 is inconsistent while its effect on DIO3 activity is not detected. Studies have shown that the activity of deiodinases especially DIO2 protein and DIO2 gene expression is increased along with other browning markers upon white adipose tissue browning induction. Studies showed that thermogenesis is stimulated by the thyroid hormone T3 as its activity is correlated to the expression of other thermogenesis markers. A proposed mechanism of thermogenesis induction in selenium supplementation is by autophagy control. However, more studies are needed to establish the role of T3 and autophagy in adipose tissue thermogenesis, especially, since some studies have shown that thermogenesis can function even when T3 activity is lacking and studies related to autophagy in adipose tissue thermogenesis have contradictory results.

Intrahepatic hypothyroidism in MASLD: Role of liver-specific thyromimetics including resmetirom

BACKGROUND AND AIMS

Thyroid hormones are important regulators of hepatic lipid homeostasis and whole-body energy expenditure. Recent evidence suggests that euthyroid individuals with metabolic dysfunction-associated steatohepatitis (MASH) develop intrahepatic hypothyroidism that promotes progression of MASH.

METHODS

A literature search was performed with Medline (PubMed), Scopus and Google Scholar electronic databases from inception till March 2024, using the following keywords: hypothyroidism and nonalcoholic fatty liver disease; MASLD and thyroid function; intrahepatic hypothyroidism; TRβ agonists; and resmetirom. Relevant studies were extracted that described pathogenesis of MASH in the context of thyroid functions.

RESULTS

In euthyroid individuals with MASH, there is decreased conversion of prohormone thyroxine (T4) to bioactive tri-iodothyronine (T3) and increased conversion of T4 to inactive metabolite reverse T3 (rT3). Consequently, reduced levels of T3 results in impaired intrahepatic TRβ signaling, a state of intrahepatic hypothyroidism, which promotes progression of MASH. Hepatic TRβ activation leads to metabolically beneficial effects in the liver including mitochondrial fatty acid uptake and β-oxidation, mitochondrial biogenesis, increasing surface low-density lipoprotein (LDL) receptor density and lowering of circulatory LDL-cholesterol. In recent years, selective thyroid hormone mimetics that exhibit TRβ-selective binding and liver-selective uptake have been designed. Resmetirom, a liver-specific thyromimetic, improves intrahepatic TRβ signaling and in clinical trials significantly improved liver inflammation, fibrosis and lipid profile in patients with MASH.

CONCLUSIONS

In euthyroid individuals with MASH, development of intrahepatic hypothyroidism results in further progression of the disease. In clinical trials, resmetirom treatment results in a significant improvement in steatosis, inflammation and fibrosis and is the first drug approved by the US Food and Drug Administration (FDA) for the treatment of noncirrhotic MASH with moderate to advanced fibrosis.

Cross-Talk Between Thyroid Disorders and Nonalcoholic Fatty Liver Disease: From Pathophysiology to Therapeutics

The medical community acknowledges the presence of thyroid disorders and nonalcoholic fatty liver disease (NAFLD). Nevertheless, the interconnection between these two circumstances is complex. Thyroid hormones (THs), including triiodothyronine (T3) and thyroxine (T4), and thyroid-stimulating hormone (TSH), are essential for maintaining metabolic balance and controlling the metabolism of lipids and carbohydrates. The therapeutic potential of THs, especially those that target the TRβ receptor isoform, is generating increasing interest. The review explores the pathophysiology of these disorders, specifically examining the impact of THs on the metabolism of lipids in the liver. The purpose of this review is to offer a thorough analysis of the correlation between thyroid disorders and NAFLD, as well as suggest potential therapeutic approaches for the future.

The first MASH drug therapy on the horizon: Current perspectives of resmetirom

The rising prevalence of metabolic dysfunction-associated steatotic liver disease (MASLD) poses a significant global health challenge, affecting over 30% of adults worldwide. MASLD is linked to increased mortality rates and substantial healthcare costs, primarily driven by its progression to metabolic dysfunction-associated steatohepatitis (MASH), which can lead to severe liver complications including cirrhosis and hepatocellular carcinoma. Despite its growing burden, effective pharmacotherapy for MASLD/MASH has been lacking until the recent conditional approval of resmetirom by the FDA. Resmetirom, a liver-targeted thyroid hormone receptor-β selective drug, has shown promise in clinical trials for treating non-cirrhotic MASH with moderate to advanced fibrosis. It has demonstrated efficacy in reducing hepatic fat content, improving liver histology (both MASH resolution and fibrosis improvement), and ameliorating biomarkers of liver damage without significant effects on body weight or glucose metabolism. Notably, resmetirom also exhibits favourable effects on circulating lipids, potentially reducing cardiovascular risk in MASLD/MASH patients. The safety profile of resmetirom appears acceptable, with gastrointestinal adverse events being the most common, though generally mild or moderate. However, long-term surveillance is warranted to monitor for potential risks related to thyroid, gonadal, or bone diseases. Clinical implementation of resmetirom faces challenges in patient selection and monitoring treatment response, and will heavily rely on non-invasive tests for liver fibrosis assessment. Nonetheless, resmetirom represents a landmark breakthrough in MASLD/MASH treatment, paving the way for future therapeutic strategies aiming to mitigate the multifaceted risks associated with this complex metabolic liver disease.

Hormone-based pharmacotherapy for metabolic dysfunction-associated fatty liver disease

Metabolic dysfunction-associated fatty liver disease (MAFLD) has reached epidemic proportions globally in parallel to the rising prevalence of obesity. Despite its significant burden, there is no approved pharmacotherapy specifically tailored for this disease. Many potential drug candidates for MAFLD have encountered setbacks in clinical trials, due to safety concerns or/and insufficient therapeutic efficacy. Nonetheless, several investigational drugs that mimic the actions of endogenous metabolic hormones, including thyroid hormone receptor β (THRβ) agonists, fibroblast growth factor 21 (FGF21) analogues, and glucagon-like peptide-1 receptor agonists (GLP-1RAs), showed promising therapeutic efficacy and excellent safety profiles. Among them, resmetirom, a liver-targeted THRβ-selective agonist, has met the primary outcomes in alleviation of metabolic dysfunction-associated steatohepatitis (MASH), the advanced form of MAFLD, and liver fibrosis in phase-3 clinical trials. These hormone-based pharmacotherapies not only exhibit varied degrees of therapeutic efficacy in mitigating hepatic steatosis, inflammation and fibrosis, but also improve metabolic profiles. Furthermore, these three hormonal agonists/analogues act in a complementary manner to exert their pharmacological effects, suggesting their combined therapies may yield synergistic therapeutic benefits. Further in-depth studies on the intricate interplay among these metabolic hormones are imperative for the development of more efficacious combination therapies, enabling precision management of MAFLD and its associated comorbidities.

Targeting nuclear receptors for NASH/MASH: From bench to bedside

The onset of metabolic dysfunction-associated steatohepatitis (MASH) or non-alcoholic steatohepatitis (NASH) represents a tipping point leading to liver injury and subsequent hepatic complications in the natural progression of what is now termed metabolic dysfunction-associated steatotic liver diseases (MASLD), formerly known as non-alcoholic fatty liver disease (NAFLD). With no pharmacological treatment currently available for MASH/NASH, the race is on to develop drugs targeting multiple facets of hepatic metabolism, inflammation, and pro-fibrotic events, which are major drivers of MASH. Nuclear receptors (NRs) regulate genomic transcription upon binding to lipophilic ligands and govern multiple aspects of liver metabolism and inflammation. Ligands of NRs may include hormones, lipids, bile acids, and synthetic ligands, which upon binding to NRs regulate the transcriptional activities of target genes. NR ligands are presently the most promising drug candidates expected to receive approval from the United States Food and Drug Administration as a pharmacological treatment for MASH. This review aims to cover the current understanding of NRs, including nuclear hormone receptors, non-steroid hormone receptors, circadian NRs, and orphan NRs, which are currently undergoing clinical trials for MASH treatment, along with NRs that have shown promising results in preclinical studies.

The roles of nuclear receptors in cholesterol metabolism and reverse cholesterol transport in nonalcoholic fatty liver disease

As the most prevalent chronic liver disease globally, NAFLD encompasses a pathological process that ranges from simple steatosis to NASH, fibrosis, cirrhosis, and HCC, closely associated with numerous extrahepatic diseases. While the initial etiology was believed to be hepatocyte injury caused by lipid toxicity from accumulated triglycerides, recent studies suggest that an imbalance of cholesterol homeostasis is of greater significance. The role of nuclear receptors in regulating liver cholesterol homeostasis has been demonstrated to be crucial. This review summarizes the roles and regulatory mechanisms of nuclear receptors in the 3 main aspects of cholesterol production, excretion, and storage in the liver, as well as their cross talk in reverse cholesterol transport. It is hoped that this review will offer new insights and theoretical foundations for the study of the pathogenesis and progression of NAFLD and provide new research directions for extrahepatic diseases associated with NAFLD.

Differential gene expression and developmental pathologies associated with persistent organic pollutants in sentinel fish in Troutman Lake, Sivuqaq, Alaska

Persistent organic pollutants (POPs) are lipophilic compounds that bioaccumulate in animals and biomagnify within food webs. Many POPs are endocrine disrupting compounds that impact vertebrate development. POPs accumulate in the Arctic via global distillation and thereby impact high trophic level vertebrates as well as people who live a subsistence lifestyle. The Arctic also contains thousands of point sources of pollution, such as formerly used defense (FUD) sites. Sivuqaq (St. Lawrence Island), Alaska was used by the U.S. military during the Cold War and FUD sites on the island remain point sources of POP contamination. We examined the effects of POP exposure on ninespine stickleback (Pungitius pungitius) collected from Troutman Lake in the village of Gambell as a model for human exposure and disease. During the Cold War, Troutman Lake was used as a dump site by the U.S. military. We found that PCB concentrations in stickleback exceeded the U.S. Environmental Protection Agency's guideline for unlimited consumption despite these fish being low trophic level organisms. We examined effects at three levels of biological organization: gene expression, endocrinology, and histomorphology. We found that ninespine stickleback from Troutman Lake exhibited suppressed gonadal development compared to threespine stickleback (Gasterosteus aculeatus) studied elsewhere. Troutman Lake stickleback also displayed two distinct hepatic phenotypes, one with lipid accumulation and one with glycogen-type vacuolation. We compared the transcriptomic profiles of these liver phenotypes using RNA sequencing and found significant upregulation of genes involved in ribosomal and metabolic pathways in the lipid accumulation group. Additionally, stickleback displaying liver lipid accumulation had significantly fewer thyroid follicles than the vacuolated phenotype. Our study and previous work highlight health concerns for people and wildlife due to pollution hotspots in the Arctic, and the need for health-protective remediation.

International Union of Basic and Clinical Pharmacology CXIII: Nuclear Receptor Superfamily-Update 2023

The NR superfamily comprises 48 transcription factors in humans that control a plethora of gene network programs involved in a wide range of physiologic processes. This review will summarize and discuss recent progress in NR biology and drug development derived from integrating various approaches, including biophysical techniques, structural studies, and translational investigation. We also highlight how defective NR signaling results in various diseases and disorders and how NRs can be targeted for therapeutic intervention via modulation via binding to synthetic lipophilic ligands. Furthermore, we also review recent studies that improved our understanding of NR structure and signaling. SIGNIFICANCE STATEMENT: Nuclear receptors (NRs) are ligand-regulated transcription factors that are critical regulators of myriad physiological processes. NRs serve as receptors for an array of drugs, and in this review, we provide an update on recent research into the roles of these drug targets.

Lack of thyroid hormone receptor beta is not detrimental for non-alcoholic steatohepatitis progression

Agonists for thyroid hormone receptor β (TRβ) show promise in preclinical studies and clinical trials to improve non-alcoholic fatty liver disease. A recent study on human livers, however, revealed reduced TRβ expression in non-alcoholic steatohepatitis (NASH), indicating a developing thyroid hormone resistance, which could constitute a major obstacle for those agonists. Using a rapid NASH paradigm combining choline-deficient high-fat diet and thermoneutrality, we confirm that TRβ declines during disease progression in mice similar to humans. Contrary to expectations, mice lacking TRβ showed less liver fibrosis, and NASH marker genes were not elevated. Conversely, increasing TRβ expression in wild-type NASH mice using liver-targeted gene therapy did not improve histology, gene expression, or metabolic parameters, indicating that TRβ receptor levels are of minor relevance for NASH development and progression in our model, and suggest that liver-rather than isoform-specificity might be more relevant for NASH treatment with thyroid hormone receptor agonists.

Thyroid Dysfunction Induced by Fungicide Famoxadone Exposure Contributes to Nonalcoholic Fatty Liver Disease in Male Mice: In Vivo, In Vitro, and In Silico Studies

Thyroid dysfunction has become a serious public health problem, which is considered a trigger of nonalcoholic fatty liver disease (NAFLD). Pesticide exposure could contribute to thyroid dysfunction and NAFLD, but the relationship between these factors remains unclear. In this study, the effects of subchronic famoxadone exposure on thyroid and liver at no observed adverse effect level (NOEL) related concentrations were investigated using in vivo, in vitro, and in silico models. Famoxadone caused hepatic steatosis, lipid metabolism disorder, and liver oxidative stress and induced NAFLD in male mice. The suppression of hepatic fatty acid β-oxidation was the key factor of NAFLD, which was highly associated with hypothalamic-pituitary-thyroid (HPT) axis hormones disorder. Famoxadone disrupted thyroid hormone biosynthesis by causing thyroid follicle aberrations and abnormal HPT axis-related gene expression. In vitro studies confirmed that famoxadone inhibited the transport of thyroxine (T4) into hepatocytes and the conversion of T4 to triiodothyronine (T3). In silico studies verified that famoxadone interfered with the binding of thyroid hormones to proteins mediating thyroid hormone transport, conversion, and activation. This study comprehensively reported the association between NAFLD and thyroid dysfunction caused by famoxadone, providing new perspectives for the health risk evaluation of pesticides with a similar structure in mammals.

Thyroid hormone action and liver disease, a complex interplay

Thyroid hormone action is involved in virtually all physiological processes. It is well known that the liver and thyroid are intimately linked, with thyroid hormone playing important roles in de novo lipogenesis, beta-oxidation (fatty acid oxidation), cholesterol metabolism, and carbohydrate metabolism. Clinical and mechanistic research studies have shown that thyroid hormone can be involved in chronic liver diseases, including alcohol-associated or NAFLD and HCC. Thyroid hormone action and synthetic thyroid hormone analogs can exert beneficial actions in terms of lowering lipids, preventing chronic liver disease and as liver anticancer agents. More recently, preclinical and clinical studies have indicated that some analogs of thyroid hormone could also play a role in the treatment of liver disease. These synthetic molecules, thyromimetics, can modulate lipid metabolism, particularly in NAFLD/NASH. In this review, we first summarize the thyroid hormone signaling axis in the context of liver biology, then we describe the changes in thyroid hormone signaling in liver disease and how liver diseases affect the thyroid hormone homeostasis, and finally we discuss the use of thyroid hormone-analog for the treatment of liver disease.

Thyroid hormone Beta receptor agonists for treatment of kidney disease: A promising agent?

BACKGROUND

Chronic kidney disease is a common disorder affecting a significant portion of the adult population with high mortality and morbidity. Obesity and hyperlipidemia are prevalent in chronic kidney disease, and they may trigger fat accumulation in renal parenchyma and eventually fatty kidney. Chronic kidney disease and fatty kidney are also strongly associated with nonalcoholic fatty liver disease. Because they both lead to detrimental effects on organ function, they both need to be treated effectively to improve the outcome.

AIM

In this narrative review, we have hypothesized that thyroid hormone beta receptor agonists, a novel drug group, may potentially be beneficial in the management of chronic kidney disease due to its promising outcomes among patients with nonalcoholic fatty liver disease, a condition sharing multiple common underlying pathophysiological mechanisms.

RESULTS AND CONCLUSION

Thyroid hormone beta receptors are abundantly expressed in liver and kidney tissues, while both nonalcoholic fatty liver disease and chronic kidney disease share various similar pathophysiological mechanisms and triggers. Therefore, thyroid hormone beta receptor agonists may become a promising tool in the management of patients with chronic kidney disease.

An adipocentric perspective on the development and progression of non-alcoholic fatty liver disease

Alongside the liver, white adipose tissue (WAT) is critical in regulating systemic energy homeostasis. Although each organ has its specialised functions, they must work coordinately to regulate whole-body metabolism. Adipose tissues and the liver are relatively resilient and can adapt to an energy surplus by facilitating triglyceride (TG) storage up to a certain threshold level without significant metabolic disturbances. However, lipid storage in WAT beyond a "personalised" adiposity threshold becomes dysfunctional, leading to metabolic inflexibility, progressive inflammation, and aberrant adipokine secretion. Moreover, the failure of adipose tissue to store and mobilise lipids results in systemic knock-on lipid overload, particularly in the liver. Factors contributing to hepatic lipid overload include lipids released from WAT, dietary fat intake, and enhanced de novo lipogenesis. In contrast, extrahepatic mechanisms counteracting toxic hepatic lipid overload entail coordinated compensation through oxidation of surplus fatty acids in brown adipose tissue and storage of fatty acids as TGs in WAT. Failure of these integrated homeostatic mechanisms leads to quantitative increases and qualitative alterations to the lipidome of the liver. Initially, hepatocytes preferentially accumulate TG species leading to a relatively "benign" non-alcoholic fatty liver. However, with time, inflammatory responses ensue, progressing into more severe conditions such as non-alcoholic steatohepatitis, cirrhosis, and hepatocellular carcinoma, in some individuals (often without an early prognostic clue). Herein, we highlight the pathogenic importance of obesity-induced "adipose tissue failure", resulting in decreased adipose tissue functionality (i.e. fat storage capacity and metabolic flexibility), in the development and progression of NAFL/NASH.

Cell-Specific Transport and Thyroid Hormone Receptor Isoform Selectivity Account for Hepatocyte-Targeted Thyromimetic Action of MGL-3196

Thyroid hormones (THs) and TH receptor-beta (TRβ) reduce hepatic triglycerides, indicating a therapeutic potential for TH analogs in liver steatosis. To avoid adverse extrahepatic, especially TRα-mediated effects such as tachycardia and bone loss, TH analogs with combined TRβ and hepatocyte specificity are desired. MGL-3196 is a new TH analog that supposedly meets these criteria. Here, we characterize the thyromimetic potential of MGL-3196 in cell-based assays and address its cellular uptake requirements. We studied the contribution of liver-specific organic anion transporters (OATP)1B1 and 1B3 to MGL-3196 action. The TR isoform-specific efficacy of MGL-3196 compared with 3,5,3'-triiodothyronine (T₃) was determined with luciferase assays and gene expression analysis in OATP1B1 and OATP1B3 and TRα- or TRβ-expressing cells and in primary murine hepatocytes (PMHs) from wild-type and TRβ knockout mice. We measured the oxygen consumption rate to compare the effects of MGL-3196 and T₃ on mitochondrial respiration. We identified OATP1B1 as the primary transporter for MGL-3196. MGL-3196 had a high efficacy (90% that of T₃) in activating TRβ, while the activation of TRα was only 25%. The treatment of PMHs with T₃ and MGL-3196 at EC₅₀ resulted in a similar induction of Dio1 and repression of Serpina7. In HEK293 cells stably expressing OATP1B1, MGL-3196 had comparable effects on mitochondrial respiration as T₃. These data indicate that MGL-3196's hepatic thyromimetic action, the basis for its therapeutic use, results from a combination of hepatocyte-specific transport by OATP1B1 and the selective activation of TRβ over TRα.

NAFLD and thyroid function: pathophysiological and therapeutic considerations

Nonalcoholic fatty liver disease (NAFLD) is a worldwide rising challenge because of hepatic, but also extrahepatic, complications. Thyroid hormones are master regulators of energy and lipid homeostasis, and the presence of abnormal thyroid function in NAFLD suggests pathogenic relationships. Specifically, persons with hypothyroidism feature dyslipidemia and lower hepatic β-oxidation, which favors accumulation of triglycerides and lipotoxins, insulin resistance, and subsequently de novo lipogenesis. Recent studies indicate that liver-specific thyroid hormone receptor β agonists are effective for the treatment of NAFLD, likely due to improved lipid homeostasis and mitochondrial respiration, which, in turn, may contribute to a reduced risk of NAFLD progression. Taken together, the possible coexistence of thyroid disease and NAFLD calls for increased awareness and optimized strategies for mutual screening and management.

Important Hormones Regulating Lipid Metabolism

There is a wide variety of kinds of lipids, and complex structures which determine the diversity and complexity of their functions. With the basic characteristic of water insolubility, lipid molecules are independent of the genetic information composed by genes to proteins, which determine the particularity of lipids in the human body, with water as the basic environment and genes to proteins as the genetic system. In this review, we have summarized the current landscape on hormone regulation of lipid metabolism. After the well-studied PI3K-AKT pathway, insulin affects fat synthesis by controlling the activity and production of various transcription factors. New mechanisms of thyroid hormone regulation are discussed, receptor α and β may mediate different procedures, the effect of thyroid hormone on mitochondria provides a new insight for hormones regulating lipid metabolism. Physiological concentration of adrenaline induces the expression of extrapituitary prolactin in adipose tissue macrophages, which promotes fat weight loss. Manipulation of hormonal action has the potential to offer a new therapeutic horizon for the global burden of obesity and its associated complications such as morbidity and mortality.

Resistance to Thyroid Hormones: A Case-Series Study

The aim of the study is to describe the clinical features of two unrelated patients with resistance to thyroid hormones (RTH), the first, a total thyroidectomized patient, and the second, a pregnant woman. We report the features found in her newborn who also showed RTH. Patient 1 is a 38-year-old man with total thyroidectomy managed for excessive thyroid stimulating hormone (TSH) production, which poorly responded to the replacement therapy. He was found with a THRβ c.1378G>A p.(Glu460Lys) heterozygous mutation, which was also present in other members of his family (son, brother, and father). Interestingly, Patient 1 had hypertension, dyslipidemia, and hepatic steatosis, which have been recently suggested as RTH-related comorbidities. Patient 2 is a 32-year-old pregnant woman with multinodular goiter, and the THRβ heterozygous variant c.959G>C, that, to the best of our knowledge, has been reported in literature only once. Her newborn had tachycardia and increased thyroid hormone levels, and showed the same mutation. After delivery, high parathyroid hormone (PTH) and calcium serum levels were found in Patient 2 and the scintigraphy showed the presence of adenoma of a parathyroid gland. This case-series study provides a practical example of the management of RTH in a thyroidectomized patient, a pregnant woman, and a newborn. A novel RTH pathogenic mutation is described for the second time in literature. Furthermore, the importance of metabolic assessment in patients with RTHβ has been highlighted and the possible correlation between RTH and primary hyperparathyroidism is discussed.

TRα2—An Untuned Second Fiddle or Fine-Tuning Thyroid Hormone Action?

Thyroid hormones (THs) control a wide range of physiological functions essential for metabolism, growth, and differentiation. On a molecular level, TH action is exerted by nuclear receptors (TRs), which function as ligand-dependent transcription factors. Among several TR isoforms, the function of TRα2 remains poorly understood as it is a splice variant of TRα with an altered C-terminus that is unable to bind T3. This review highlights the molecular characteristics of TRα2, proposed mechanisms that regulate alternative splicing and indications pointing towards an antagonistic function of this TR isoform in vitro and in vivo. Moreover, remaining knowledge gaps and major challenges that complicate TRα2 characterization, as well as future strategies to fully uncover its physiological relevance, are discussed.

Effects of Thyroid Hormones on Lipid Metabolism Pathologies in Non-Alcoholic Fatty Liver Disease

The typical modern lifestyle contributes to the development of many metabolic-related disorders, as exemplified by metabolic syndrome. How to prevent, resolve, or avoid subsequent deterioration of metabolic disturbances and the development of more serious diseases has become an important and much-discussed health issue. Thus, the question of the physiological and pathological roles of thyroid hormones (THs) in metabolism has never gone out of fashion. Although THs influence almost all organs, the liver is one of the most important targets as well as the hub of metabolic homeostasis. When this homeostasis is out of balance, diseases may result. In the current review, we summarize the common features and actions of THs, first focusing on their effects on lipid metabolism in the liver. In the second half of the review, we turn to a consideration of non-alcoholic fatty liver disease (NAFLD), a disease characterized by excessive accumulation of fat in the liver that is independent of heavy alcohol consumption. NAFLD is a growing health problem that currently affects ~25% of the world’s population. Unfortunately, there are currently no approved therapies specific for NAFLD, which, if left uncontrolled, may progress to more serious diseases, such as cirrhosis or liver cancer. This absence of effective treatment can also result in the development of non-alcoholic steatohepatitis (NASH), an aggressive form of NAFLD that is the leading cause of liver transplantation in the United States. Because THs play a clear role in hepatic fat metabolism, their potential application in the prevention and treatment of NAFLD has attracted considerable research attention. Studies that have investigated the use of TH-related compounds in the management of NAFLD are also summarized in the latter part of this review. An important take-home point of this review is that a comprehensive understanding of the physiological and pathological roles of THs in liver fat metabolism is possible, despite the complexities of this regulatory axis—an understanding that has clinical value for the specific management of NAFLD.

Gut microbiota production of trimethyl-5-aminovaleric acid reduces fatty acid oxidation and accelerates cardiac hypertrophy

Numerous studies found intestinal microbiota alterations which are thought to affect the development of various diseases through the production of gut-derived metabolites. However, the specific metabolites and their pathophysiological contribution to cardiac hypertrophy or heart failure progression still remain unclear. N,N,N-trimethyl-5-aminovaleric acid (TMAVA), derived from trimethyllysine through the gut microbiota, was elevated with gradually increased risk of cardiac mortality and transplantation in a prospective heart failure cohort (n = 1647). TMAVA treatment aggravated cardiac hypertrophy and dysfunction in high-fat diet-fed mice. Decreased fatty acid oxidation (FAO) is a hallmark of metabolic reprogramming in the diseased heart and contributes to impaired myocardial energetics and contractile dysfunction. Proteomics uncovered that TMAVA disturbed cardiac energy metabolism, leading to inhibition of FAO and myocardial lipid accumulation. TMAVA treatment altered mitochondrial ultrastructure, respiration and FAO and inhibited carnitine metabolism. Mice with γ-butyrobetaine hydroxylase (BBOX) deficiency displayed a similar cardiac hypertrophy phenotype, indicating that TMAVA functions through BBOX. Finally, exogenous carnitine supplementation reversed TMAVA induced cardiac hypertrophy. These data suggest that the gut microbiota-derived TMAVA is a key determinant for the development of cardiac hypertrophy through inhibition of carnitine synthesis and subsequent FAO.

Intestinal microbiota alterations may affect heart function through the production of gut-derived metabolites. Here the authors found that gut microbiota-derived TMAVA is a key determinant for the development of cardiac hypertrophy through inhibition of carnitine synthesis and subsequent fatty acid oxidation.

Orally Induced Hyperthyroidism Regulates Hypothalamic AMP-Activated Protein Kinase

Besides their direct effects on peripheral metabolic tissues, thyroid hormones (TH) act on the hypothalamus to modulate energy homeostasis. However, since most of the hypothalamic actions of TH have been addressed in studies with direct central administration, the estimation of the relative contribution of the central vs. peripheral effects in physiologic conditions of peripheral release (or administration) of TH remains unclear. In this study we used two different models of peripherally induced hyperthyroidism (i.e., T4 and T3 oral administration) to assess and compare the serum and hypothalamic TH status and relate them to the metabolic effects of the treatment. Peripheral TH treatment affected feeding behavior, overall growth, core body temperature, body composition, brown adipose tissue (BAT) morphology and uncoupling protein 1 (UCP1) levels and metabolic activity, white adipose tissue (WAT) browning and liver metabolism. This resulted in an increased overall uncoupling capacity and a shift of the lipid metabolism from WAT accumulation to BAT fueling. Both peripheral treatment protocols induced significant changes in TH concentrations within the hypothalamus, with T3 eliciting a downregulation of hypothalamic AMP-activated protein kinase (AMPK), supporting the existence of a central action of peripheral TH. Altogether, these data suggest that peripherally administered TH modulate energy balance by various mechanisms; they also provide a unifying vision of the centrally mediated and the direct local metabolic effect of TH in the context of hyperthyroidism.

The Role and Mechanism of Oxidative Stress and Nuclear Receptors in the Development of NAFLD

The overproduction of reactive oxygen species (ROS) and consequent oxidative stress contribute to the pathogenesis of acute and chronic liver diseases. It is now acknowledged that nonalcoholic fatty liver disease (NAFLD) is characterized as a redox-centered disease due to the role of ROS in hepatic metabolism. However, the underlying mechanisms accounting for these alternations are not completely understood. Several nuclear receptors (NRs) are dysregulated in NAFLD, and have a direct influence on the expression of a set of genes relating to the progress of hepatic lipid homeostasis and ROS generation. Meanwhile, the NRs act as redox sensors in response to metabolic stress. Therefore, targeting NRs may represent a promising strategy for improving oxidation damage and treating NAFLD. This review summarizes the link between impaired lipid metabolism and oxidative stress and highlights some NRs involved in regulating oxidant/antioxidant turnover in the context of NAFLD, shedding light on potential therapies based on NR-mediated modulation of ROS generation and lipid accumulation.

Increased Hepatic Fat Content in Patients with Resistance to Thyroid Hormone Beta

Background: Thyroid hormone (TH) has important functions in controlling hepatic lipid metabolism. Individuals with resistance to thyroid hormone beta (RTHβ) who harbor mutations in the THRB gene experience loss-of-function of thyroid hormone receptor beta (TRβ), which is the predominant TR isoform expressed in the liver. We hypothesized that individuals with RTHβ may have increased hepatic steatosis. Methods: Controlled attenuation parameter (CAP) was assessed in individuals harboring the R243Q mutation of the THRB gene (n = 21) and in their wild-type (WT) first-degree relatives (n = 22) using the ultrasound-based transient elastography (TE) device (FibroScan). All participants belonged to the same family, lived on the same small island, and were therefore exposed to similar environmental conditions. CAP measurements and blood samples were obtained after an overnight fast. The observers were blinded to the status of the patients. Results: The hepatic fat content was increased in RTHβ individuals compared with their WT relatives (CAP values of 263 ± 21 and 218.7 ± 43 dB/m, respectively, p = 0.007). The CAP values correlated with age and body mass index (BMI) (age: r = 0.55, p = 0.011; BMI: r = 0.51, p = 0.022) in the WT first-degree relatives but not in RTHβ individuals, suggesting that the defect in TRβ signaling was predominant over the effects of age and obesity. Circulating free fatty acid levels were significantly higher in RTHβ individuals (0.29 ± 0.033 vs. 0.17 ± 0.025 mmol/L, p = 0.02). There was no evidence of insulin resistance evaluated by the homeostatic model assessment of insulin resistance in both groups studied. Conclusions: Our findings provide evidence that impairments in intrahepatic TRβ signaling due to mutations of the THRB gene can lead to hepatic steatosis, which emphasizes the influence of TH in the liver metabolism of lipids and provides a rationale for the development TRβ-selective thyromimetics. Consequently, new molecules with a very high TRβ affinity and hepatic selectivity have been developed for the treatment of lipid-associated hepatic disorders, particularly nonalcoholic fatty liver disease.

Early induction of hepatic deiodinase type 1 inhibits hepatosteatosis during NAFLD progression

OBJECTIVE

Nonalcoholic fatty liver disease (NAFLD) comprises a spectrum ranging from hepatosteatosis to progressive nonalcoholic steatohepatitis that can lead to cirrhosis. Humans with low levels of prohormone thyroxine (T₄) have a higher incidence of NAFLD, and thyroid hormone treatment is very promising in all patients with NAFLD. Deiodinase type 1 (Dio1) is a hepatic enzyme that converts T₄ to the bioactive T₃ and therefore regulates thyroid hormone availability within hepatocytes. We investigated the role of this intrahepatic regulation during the progression of NAFLD.

METHODS

We investigated hepatic thyroid hormone metabolism in two NAFLD models: wild-type mice fed a Western diet with fructose and Lepr^db mice fed a methionine- and choline-deficient diet. AAV8-mediated liver-specific Dio1 knockdown was employed to investigate the role of Dio1 during the progression of NAFLD. Intrahepatic thyroid hormone levels, deiodinase activity, and metabolic parameters were measured.

RESULTS

Dio1 expression and activity were increased in the early stages of NAFLD and were associated with an increased T₃/T₄ ratio. Prevention of this increase by AAV8-mediated liver-specific Dio1 knockdown increased hepatic triglycerides and cholesterol and decreased the pACC/ACC ratio and acylcarnitine levels, suggesting there was lower β-oxidation. Dio1 siRNA KD in hepatic cells treated with fatty acids showed increased lipid accumulation and decreased oxidative phosphorylation.

CONCLUSION

Hepatic Dio1 gene expression was modulated by dietary conditions, was increased during hepatosteatosis and early NASH, and regulated hepatic triglyceride content. These early adaptations likely represent compensatory mechanisms that reduce hepatosteatosis and prevent NASH progression.

Dyslipidemia, Insulin Resistance, Ectopic Lipid Accumulation, and Vascular Function in Resistance to Thyroid Hormone β

PURPOSE

In resistance to thyroid hormone due to mutations in thyroid hormone receptor β, peripheral tissues are variably refractory to the action of circulating thyroid hormones. We evaluated parameters contributing to atherosclerotic risk in this disorder.

METHODS

We measured low-density lipoprotein cholesterol (LDL-C), triglyceride (TG), high-density lipoprotein cholesterol (HDL-C), nonesterified fatty acids (NEFA), intrahepatic lipid (IHL) and intramyocellular lipid (IMCL), Homeostasis-model assessment of insulin resistance (HOMA-IR), augmentation index (AIx) and pulse wave velocity (PWV), flow-mediated dilatation, and carotid intima-media thickness (cIMT) in an unselected, genetically confirmed cohort of adult RTHβ patients (n = 27-77) and compared these with measurements in healthy subjects (up to n = 100) and thyrotoxic patients (n = 40).

RESULTS

Resistance to thyroid hormone beta (RTHβ) patients exhibited higher LDL-C (P = 0.008) and TG (P = 0.002) and lower HDL-C concentrations (P = 0.015 × 10-2) than control subjects, with LDL-C being higher than in thyrotoxic patients with comparable hyperthyroxinemia. Proprotein convertase subtilisin/kexin 9 (P = 0.002) and apolipoprotein B (P = 0.0009) levels were reduced in thyrotoxic patients but not lower in RTHβ patients or control subjects. Intrahepatic lipid (P = 0.02 × 10-4), IMCL (P = 0.002), HOMA-IR (P = 0.01 × 10-2), and NEFA (P = 0.04 × 10-6) were significantly higher in RTHβ patients than control subjects. Flow-mediated dilatation was increased (P = 0.04) but cIMT (P = 0.71), PWV P = 0.81), and AIx (P = 0.95) were unaltered in RTHβ patients.

CONCLUSIONS

We have documented mixed dyslipidemia with hepatic and IMCL accumulation in RTHβ, suggesting that surveillance for these metabolic abnormalities is warranted. How they combine with enhanced endothelial function and unaltered vessel wall thickness and compliance to determine overall cardiometabolic risk in this disorder remains to be defined.

Liver macrophages and inflammation in physiology and physiopathology of non-alcoholic fatty liver disease

Non‐alcoholic fatty liver disease (NAFLD) is the hepatic manifestation of metabolic syndrome, being a common comorbidity of type 2 diabetes and with important links to inflammation and insulin resistance. NAFLD represents a spectrum of liver conditions ranging from steatosis in the form of ectopic lipid storage, to inflammation and fibrosis in nonalcoholic steatohepatitis (NASH). Macrophages that populate the liver play important roles in maintaining liver homeostasis under normal physiology and in promoting inflammation and mediating fibrosis in the progression of NAFLD toward to NASH. Liver macrophages are a heterogenous group of innate immune cells, originating from the yolk sac or from circulating monocytes, that are required to maintain immune tolerance while being exposed portal and pancreatic blood flow rich in nutrients and hormones. Yet, liver macrophages retain a limited capacity to raise the alarm in response to danger signals. We now know that macrophages in the liver play both inflammatory and noninflammatory roles throughout the progression of NAFLD. Macrophage responses are mediated first at the level of cell surface receptors that integrate environmental stimuli, signals are transduced through multiple levels of regulation in the cell, and specific transcriptional programmes dictate effector functions. These effector functions play paramount roles in determining the course of disease in NAFLD and even more so in the progression towards NASH. The current review covers recent reports in the physiological and pathophysiological roles of liver macrophages in NAFLD. We emphasise the responses of liver macrophages to insulin resistance and the transcriptional machinery that dictates liver macrophage function.

Risk of Liver Fibrosis According to TSH Levels in Euthyroid Subjects

Alterations in thyroid function may contribute to the development of liver fibrosis especially in subjects with non-alcoholic fatty liver disease. This study aimed to investigate the risk of liver fibrosis according to low-normal thyroid function in the general population. We performed a descriptive cross-sectional study in subjects from 18–75 years randomly selected from 16 primary health care centers from 2017–2019. Each subject underwent clinical evaluation, physical examination, blood analysis and transient hepatic elastography. Descriptive and multivariate logistic regression analyses were used to identify factors associated with fibrosis. We included 1096 subjects (60 ± 11 years; 61% women); 70% had strict-normal thyroid function and 30% had low-normal thyroid function. Low-normal thyroid function was associated with a higher liver stiffness (LS) values: 5.2 vs. 4.8 kPa (p = 0.001) and a greater prevalence of fibrosis: 6.1 vs. 3% (p = 0.016) and 4.3 vs. 2.1% (p = 0.044) for the cut-off points of ≥8.0 kPa and ≥9.2 kPa, respectively. After adjustment for potential confounding factors, the risk of fibrosis in subjects with low-normal thyroid function was OR 1.54 (p = 0.213). In conclusion, low-normal thyroid function is associated with higher LS values and a greater risk of liver fibrosis in the general population, being dependent on other metabolic factors.

Interplay between Thyroid Hormones and Stearoyl-CoA Desaturase 1 in the Regulation of Lipid Metabolism in the Heart

Stearoyl-CoA desaturase 1 (SCD1), an enzyme that is involved in the biosynthesis of monounsaturated fatty acids, induces the reprogramming of cardiomyocyte metabolism. Thyroid hormones (THs) activate both lipolysis and lipogenesis. Many genes that are involved in lipid metabolism, including Scd1, are regulated by THs. The present study used SCD1 knockout (SCD1^−/−) mice to test the hypothesis that THs are important factors that mediate the anti-steatotic effect of SCD1 downregulation in the heart. SCD1 deficiency decreased plasma levels of thyroid-stimulating hormone and thyroxine and the expression of genes that regulate intracellular TH levels (i.e., Slc16a2 and Dio1-3) in cardiomyocytes. Both hypothyroidism and SCD1 deficiency affected genomic and non-genomic TH pathways in the heart. SCD1 deficiency is known to protect mice from genetic- or diet-induced obesity and decrease lipid content in the heart. Interestingly, hypothyroidism increased body adiposity and triglyceride and diacylglycerol levels in the heart in SCD1^−/− mice. The accumulation of triglycerides in cardiomyocytes in SCD1^−/− hypothyroid mice was caused by the activation of lipogenesis, which likely exceeded the upregulation of lipolysis and fatty acid oxidation. Lipid accumulation was also observed in the heart in wildtype hypothyroid mice compared with wildtype control mice, but this process was related to a reduction of triglyceride lipolysis and fatty acid oxidation. We also found that simultaneous SCD1 and deiodinase inhibition increased triglyceride content in HL-1 cardiomyocytes, and this process was related to the downregulation of lipolysis. Altogether, the present results suggest that THs are an important part of the mechanism of SCD1 in cardiac lipid utilization and may be involved in the upregulation of energetic metabolism that is associated with SCD1 deficiency.

Nuclear receptors and transcriptional regulation in non-alcoholic fatty liver disease

BACKGROUND

As a result of a sedentary lifestyle and excess food consumption in modern society, non-alcoholic fatty liver disease (NAFLD) characterized by fat accumulation in the liver is becoming a major disease burden. Non-alcoholic steatohepatitis (NASH) is an advanced form of NAFLD characterized by inflammation and fibrosis that can lead to hepatocellular carcinoma and liver failure. Nuclear receptors (NRs) are a family of ligand-regulated transcription factors that closely control multiple aspects of metabolism. Their transcriptional activity is modulated by various ligands, including hormones and lipids. NRs serve as potential pharmacological targets for NAFLD/NASH and other metabolic diseases.

SCOPE OF REVIEW

In this review, we provide a comprehensive overview of NRs that have been studied in the context of NAFLD/NASH with a focus on their transcriptional regulation, function in preclinical models, and studies of their clinical utility.

MAJOR CONCLUSIONS

The transcriptional regulation of NRs is context-dependent. During the dynamic progression of NAFLD/NASH, NRs play diverse roles in multiple organs and different cell types in the liver, which highlights the necessity of targeting NRs in a stage-specific and cell-type-specific manner to enhance the efficacy and safety of treatment methods.

Specific ablation of the NCoR corepressor δ splice variant reveals alternative RNA splicing as a key regulator of hepatic metabolism

The NCoR corepressor plays critical roles in mediating transcriptional repression by both nuclear receptors and non-receptor transcription factors. Alternative mRNA splicing of NCoR produces a series of variants with differing molecular and biological properties. The NCoRω splice-variant inhibits adipogenesis whereas the NCoRδ splice-variant promotes it, and mice bearing a splice-specific knockout of NCoRω display enhanced hepatic steatosis and overall weight gain on a high fat diet as well as a greatly increased resistance to diet-induced glucose intolerance. We report here that the reciprocal NCoRδ splice-specific knock-out mice display the contrary phenotypes of reduced hepatic steatosis and reduced weight gain relative to the NCoRω-/- mice. The NCoRδ-/- mice also fail to demonstrate the strong resistance to diet-induced glucose intolerance exhibited by the NCoRω-/- animals. The NCoR δ and ω variants possess both unique and shared transcriptional targets, with expression of certain hepatic genes affected in opposite directions in the two mutants, others altered in one but not the other genotype, and yet others changed in parallel in both NCoRδ-/- and NCoRω-/- animals versus WT. Gene set expression analysis (GSEA) identified a series of lipid, carbohydrate, and amino acid metabolic pathways that are likely to contribute to their distinct steatosis and glucose tolerance phenotypes. We conclude that alternative-splicing of the NCoR corepressor plays a key role in the regulation of hepatic energy storage and utilization, with the NCoRδ and NCoRω variants exerting both opposing and shared functions in many aspects of this phenomenon and in the organism as a whole.

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

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

Acrylamide induces a thyroid allostasis-adaptive response in prepubertal exposed rats

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Acrylamide acts as endocrine disruptor for the thyroid gland function.

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Acrylamide increases the transcript expression of proteins related to THs synthesis.

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Exposure to acrylamide alters the hypothalamus-pituitary-thyroid axis homeostasis.

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Acrylamide induces allostatic regulation of the hypothalamus-pituitary-thyroid axis.

Some endocrine-disrupting chemicals (EDCs) can affect the endocrine system through covalent interactions with specific sites, leading to deregulation of physiological homeostasis. The acrylamide (AA) present in some fried or baked foods is an example of an electrophile molecule that is able to form adducts with nucleophilic regions of nervous system proteins leading to neurological defects. A positive correlation between increased urinary AA metabolite concentration and reduced levels of thyroid hormones (TH) was described in adolescents and young adults. Thus, this study aimed to evaluate whether AA affects the physiology of the hypothalamus-pituitary-thyroid (HPT) axis and the possible repercussions in peripheral TH-target systems. For this, male Wistar rats were exposed to doses of 2.5 or 5.0 mg AA/Kg/day, based on the LOAEL (Lowest Observed Adverse Effect Level) during prepubertal development. The expression of molecular markers of HPT functionality was investigated in the hypothalamus, pituitary, thyroid, heart and liver, as well as the hormonal and lipid profiles in blood samples. Herein, we showed that AA acts as EDCs for thyroid gland function, increasing the transcript expression of several proteins related to TH synthesis and altering hypothalamus-pituitary-thyroid axis homeostasis, an effect evidenced by the higher levels of THs in the serum. Compensatory mechanisms were observed in TH-target tissues, such as an increase in Dio3 mRNA expression in the liver and a reduction in Mct8 transcript content in the hearts of AA-treated rats. Together, these results pointed out an allostatic regulation of the HPT axis induced by AA and suggest that chronic exposure to it, mainly associated with food consumption, might be related to the higher prevalence of thyroid dysfunctions.

TMAVA, a Metabolite of Intestinal Microbes, Is Increased in Plasma From Patients With Liver Steatosis, Inhibits γ-Butyrobetaine Hydroxylase, and Exacerbates Fatty Liver in Mice

BACKGROUND & AIMS

Nonalcoholic fatty liver disease is characterized by excessive hepatic accumulation of triglycerides. We aimed to identify metabolites that differ in plasma of patients with liver steatosis vs healthy individuals (controls) and investigate the mechanisms by which these might contribute to fatty liver in mice.

METHODS

We obtained blood samples from 15 patients with liver steatosis and 15 controls from a single center in China (discovery cohort). We performed untargeted liquid chromatography with mass spectrometry analysis of plasma to identify analytes associated with liver steatosis. We then performed targeted metabolomic analysis of blood samples from 2 independent cohorts of individuals who underwent annual health examinations in China (1157 subjects with or without diabetes and 767 subjects with or without liver steatosis; replication cohorts). We performed mass spectrometry analysis of plasma from C57BL/6J mice, germ-free, and mice given antibiotics. C57BL/6J mice were given 0.325% (m/v) N,N,N-trimethyl-5-aminovaleric acid (TMAVA) in their drinking water and placed on a 45% high-fat diet (HFD) for 2 months. Plasma, liver tissues, and fecal samples were collected; fecal samples were analyzed by 16S ribosomal RNA gene sequencing. C57BL/6J mice with CRISPR-mediated disruption of the gene encoding γ-butyrobetaine hydroxylase (BBOX-knockout mice) were also placed on a 45% HFD for 2 months. Hepatic fatty acid oxidation (FAO) in liver tissues was determined by measuring liberation of ³H₂O from [³H] palmitic acid. Liver tissues were analyzed by electron microscopy, to view mitochondria, and proteomic analyses. We used surface plasmon resonance analysis to quantify the affinity of TMAVA for BBOX.

RESULTS

Levels of TMAVA, believed to be a metabolite of intestinal microbes, were increased in plasma from subjects with liver steatosis compared with controls, in the discovery and replication cohorts. In 1 replication cohort, the odds ratio for fatty liver in subjects with increased liver plasma levels of TMAVA was 1.82 (95% confidence interval [CI], 1.14-2.90; P = .012). Plasma from mice given antibiotics or germ-free mice had significant reductions in TMAVA compared with control mice. We found the intestinal bacteria Enterococcus faecalis and Pseudomonas aeruginosa to metabolize trimethyllysine to TMAVA; levels of trimethyllysine were significantly higher in plasma from patients with steatosis than controls. We found TMAVA to bind and inhibit BBOX, reducing synthesis of carnitine. Mice given TMAVA had alterations in their fecal microbiomes and reduced cold tolerance; their plasma and liver tissue had significant reductions in levels of carnitine and acyl-carnitine and their hepatocytes had reduced mitochondrial FAO compared with mice given only an HFD. Mice given TMAVA on an HFD developed liver steatosis, which was reduced by carnitine supplementation. BBOX-knockout mice had carnitine deficiency and decreased FAO, increasing uptake and liver accumulation of free fatty acids and exacerbating HFD-induced fatty liver.

CONCLUSIONS

Levels of TMAVA are increased in plasma from subjects with liver steatosis. In mice, intestinal microbes metabolize trimethyllysine to TMAVA, which reduces carnitine synthesis and FAO to promote steatosis.

Noncanonical Action of Thyroid Hormone Receptors α and β

Thyroid hormone (TH) is essential for the regulation of many physiological processes, especially growth, organ development, energy metabolism and cardiovascular effects. TH acts via the TH receptors (TR) α and β. By binding to thyroid hormone responsive elements (TREs) on the DNA, TRs regulate expression of TH target genes. Thus, TRs are mainly characterized as ligand dependent transcription factors and regulation of gene expression and protein synthesis is considered the canonical mode of TH/TR action. The demonstration that the ligand-bound TRs α and β also mediate activation of the phosphatidylinositol-3-kinase (PI3K) pathway established noncanonical TH/TR action as an additional mode of TH signaling. Recently, TR mutant mouse models allowed to determine the underlying mode of TH/TR action, either canonical or noncanonical TH/TR signaling, for several physiological TH effects in vivo: Regulation of the hypothalamic-pituitary-thyroid axis requires DNA-binding of TRβ, whereas hepatic triglyceride content appears to be regulated by noncanonical TRβ signaling. TRα mediated effects in bone development are dependent on DNA-binding, whereas several cardiovascular TRα effects are rapid and independent from DNA-binding. Therefore, noncanonical TH/TR action contributes to the overall effects of TH in physiology.

Nonalcoholic Fatty Liver Disease

Nonalcoholic fatty liver disease (NAFLD) is considered the hepatic manifestation of the metabolic syndrome (MetS) and comprises one of the largest health threats of the twenty-first century. In this chapter, we review the current state of knowledge of NAFLD and underline the striking similarities with atherosclerosis. We first describe current epidemiological data showing the staggering increase of NAFLD numbers and its related clinical and economic costs. We then provide an overview of pathophysiological hepatic processes in NAFLD and highlight the systemic aspects of NAFLD that point toward metabolic crosstalk between organs as an important cause of metabolic disease. Finally, we end by highlighting the currently investigated therapeutic approaches for NAFLD, which also show strong similarities with a range of treatment options for atherosclerosis.

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.

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.

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.

Gangjihwan, a polyherbal composition, inhibits fat accumulation through the modulation of lipogenic transcription factors SREBP1C, PPARγ and C/EBPα

ETHNOPHARMACOLOGICAL RELEVANCE

Gangjihwan (DF) which is composed of Ephedra intermedia, Lithospermum erythrorhizon, and Rheum palmatum has been used for the treatment of obesity in traditional medical clinics in Korea.

AIM OF THE STUDY

This study was conducted to standardize DF and elucidate its mechanism of action for inhibiting fat accumulation in adipocytes and adipose tissues.

MATERIALS AND METHODS

The herbal ingredients of DF were extracted in water, 30% ethanol or 70% ethanol and freeze-dried followed by HPLC analyses. 3T3-L1 adipocytes and high-fat diet-induced obese mice were treated with each of the three DF preparations. Messenger RNA and protein expression levels were measured by real-time qPCR and Western blotting. RNA-Seq analyses were conducted to examine the effects of DF treatment on whole transcriptome of adipocyte.

RESULTS

(-)-Ephedrine and (+)-pseudoephedrine from E. intermedia, aloe-emodin and chrysophanol from R. palmatum and shikonin from L. erythrorhizon were identified as phytochemical components of DF. DF caused dose-dependent inhibition of fat accumulation in 3T3-L1 adipocytes. It also significantly reduced adipose tissue mass and adipocyte size in high-fat diet-induced obese mice. DF was found to down-regulate the expressions of the lipogenic transcription factors such as sterol regulatory element binding protein 1C (SREBP1C), peroxisome proliferator activated receptor gamma (PPARγ), and CCAAT/enhancer binding protein alpha (C/EBPα). Among the three preparations of DF, the 70% ethanol extract was the most effective. RNA-Seq analyses showed that DF treatment decreased the expression levels of up-regulators and increased those of down-regulators of lipogenic transcription factors.

CONCLUSIONS

DF preparations, among which 70% ethanol extract was the most effective, reduced fat accumulation in 3T3-L1 adipocytes and high-fat diet-induced obese mice through the down-regulation of lipogenic transcription factors SREBP1C, PPARγ and C/EBPα.

Noncanonical thyroid hormone signaling mediates cardiometabolic effects in vivo

Thyroid hormone (TH) and TH receptors (TRs) α and β act by binding to TH response elements (TREs) in regulatory regions of target genes. This nuclear signaling is established as the canonical or type 1 pathway for TH action. Nevertheless, TRs also rapidly activate intracellular second-messenger signaling pathways independently of gene expression (noncanonical or type 3 TR signaling). To test the physiological relevance of noncanonical TR signaling, we generated knockin mice with a mutation in the TR DNA-binding domain that abrogates binding to DNA and leads to complete loss of canonical TH action. We show that several important physiological TH effects are preserved despite the disruption of DNA binding of TRα and TRβ, most notably heart rate, body temperature, blood glucose, and triglyceride concentration, all of which were regulated by noncanonical TR signaling. Additionally, we confirm that TRE-binding-defective TRβ leads to disruption of the hypothalamic-pituitary-thyroid axis with resistance to TH, while mutation of TRα causes a severe delay in skeletal development, thus demonstrating tissue- and TR isoform-specific canonical signaling. These findings provide in vivo evidence that noncanonical TR signaling exerts physiologically important cardiometabolic effects that are distinct from canonical actions. These data challenge the current paradigm that in vivo physiological TH action is mediated exclusively via regulation of gene transcription at the nuclear level.

Thyroid Hormone Stimulation of Adult Brain Fatty Acid Oxidation

Thyroid hormone is a critical modulator of brain metabolism, and it is highly controlled in the central nervous system. Recent research has uncovered an important role of thyroid hormone in the regulation of fatty acid oxidation (FAO), an energetic process essential for neurodevelopment that continues to support brain metabolism during adulthood. Thyroid hormone stimulation of FAO has been shown to be protective in astrocytes and mouse models of brain injury, yet a clear mechanism of this relationship has not been elucidated. Thyroid hormone interacts with multiple receptors located in the nucleus and the mitochondria, initiating rapid and long-term effects via both genomic and nongenomic pathways. This has complicated efforts to isolate and study-specific interactions. This chapter presents the primary signaling pathways that have been identified to play a role in the thyroid hormone-mediated increase in FAO. Investigation of the impact of thyroid hormone on FAO in the adult brain has challenged classical models of brain metabolism and widened the window of potential neuroprotective strategies. A detailed understanding of these pathways is essential for any researchers aiming to expand the field of neuroenergetics.

Ubiquitin Ligases and Posttranslational Regulation of Energy in the Heart: The Hand that Feeds

Heart failure (HF) is a costly and deadly syndrome characterized by the reduced capacity of the heart to adequately provide systemic blood flow. Mounting evidence implicates pathological changes in cardiac energy metabolism as a contributing factor in the development of HF. While the main source of fuel in the healthy heart is the oxidation of fatty acids, in the failing heart the less energy efficient glucose and glycogen metabolism are upregulated. The ubiquitin proteasome system plays a key role in regulating metabolism via protein-degradation/regulation of autophagy and regulating metabolism-related transcription and cell signaling processes. In this review, we discuss recent research that describes the role of the ubiquitin-proteasome system (UPS) in regulating metabolism in the context of HF. We focus on ubiquitin ligases (E3s), the component of the UPS that confers substrate specificity, and detail the current understanding of how these E3s contribute to cardiac pathology and metabolism. © 2017 American Physiological Society. Compr Physiol 7:841-862, 2017.

TRα receptor mutations extend the spectrum of syndromes of reduced sensitivity to thyroid hormone

Since 2012, eight different abnormalities have been described in the THRA gene (encoding the TRα1 thyroid hormone receptor) of 14 patients from 9 families. These mutations induce a clinical phenotype (resistance to thyroid hormone type α) associating symptoms of untreated mild congenital hypothyroidism and a near-normal range of free and total thyroid hormones and TSH (the T4/T3 ratio is nevertheless usually low). The phenotype can diversely include short stature (due to growth retardation), dysmorphic syndrome (face and limb extremities), psychoneuromotor disorders, constipation and bradycardia. The identified genetic abnormalities are located within the ligand-binding domain and result in defective T3 binding, an abnormally strong interaction with corepressors and a dominant negative activity against still functional receptors. The identification of patients with consistent phenotypes and the underlying mutations are warranted to better delineate the spectrum of the syndromes of reduced sensitivity to thyroid hormone.

Thyroid hormone receptor sumoylation is required for preadipocyte differentiation and proliferation

Thyroid hormone and thyroid hormone receptor (TR) play an essential role in metabolic regulation. However, the role of TR in adipogenesis has not been established. We reported previously that TR sumoylation is essential for TR-mediated gene regulation and that mutation of either of the two sites in TRα or any of the three sites in TRβ reduces TR sumoylation. Here, we transfected TR sumoylation site mutants into human primary preadiocytes and the mouse 3T3L1 preadipocyte cell line to determine the role of TR sumoylation in adipogenesis. Reduced sumoylation of TRα or TRβ resulted in fewer and smaller lipid droplets and reduced proliferation of preadipocytes. TR sumoylation mutations, compared with wild-type TR, results in reduced C/EBP expression and reduced PPARγ2 mRNA and protein levels. TR sumoylation mutants recruited NCoR and disrupted PPARγ-mediated perilipin1 (Plin1) gene expression, associated with impaired lipid droplet formation. Expression of NCoRΔID, a mutant NCoR lacking the TR interaction domain, partially "rescued" the delayed adipogenesis and restored Plin1 gene expression and adipogenesis. TR sumoylation site mutants impaired Wnt/β-catenin signaling pathways and the proliferation of primary human preadipocytes. Expression of the TRβ K146Q sumoylation site mutant down-regulated the essential genes required for canonical Wnt signal-mediated proliferation, including Wnt ligands, Fzds, β-catenin, LEF1, and CCND1. Additionally, the TRβ K146Q mutant enhanced the canonical Wnt signaling inhibitor Dickkopf-related protein 1 (DKK1). Our data demonstrate that TR sumoylation is required for activation of the Wnt canonical signaling pathway during preadipocyte proliferation and enhances the PPARγ signaling that promotes differentiation.

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.

Enzymatic intracrine regulation of white adipose tissue

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

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.