Thyroid hormone upregulates zinc-α2-glycoprotein production in the liver but not in adipose tissue. PLoS One. 2014;9:e85753. [PMID: 24465683 DOI: 10.1371/journal.pone.0085753]

Levothyroxine and Non-alcoholic Fatty Liver Disease: A Mini Review

Levothyroxine or l-thyroxine is artificially manufactured thyroxine, which is used as a drug to treat underactive thyroid conditions in humans. The drug, levothyroxine, is consumed daily in a prescribed dose to replace the missing thyroid hormone thyroxine in an individual with an underactive thyroid, and it helps to maintain normal physiological conditions. Though it is a life-maintaining drug, it replaces the missing thyroid hormone and performs the necessary daily metabolic functions in our body. Like all other allopathic drugs, it comes with certain side effects, which include joint pain, cramps in muscle, weight gain/loss, hair loss, etc. The thyroid hormone, thyroxine, is known to mobilize fat in our body, including the ones from the hepatic system. An underactive thyroid may cause an accumulation of fat in the liver, leading to a fatty liver, which is clinically termed Non-Alcoholic Fatty Liver Disease (NAFLD). The correlation between hypothyroidism and NAFLD is now well-studied and recognized. As levothyroxine performs the functions of the missing thyroxine, it is anticipated, based on certain preliminary studies, that the drug helps to mobilize hepatic fat and thus may have a crucial role in mitigating the condition of NAFDL.

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.

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.

Role of Zinc in Zinc-α2-Glycoprotein Metabolism in Obesity: a Review of Literature

Excessive adipose tissue promotes the manifestation of endocrine disorders such as reduction of the secretion of zinc-α2-glycoprotein (ZAG), an adipokine with anti-inflammatory and lipid-mobilizing activity. The molecular structure of this adipokine includes binding sites for zinc, a trace element with important antioxidant and immunological proprieties that also participates in energy metabolism and stimulates the function of ZAG. The objective of this review is to highlight current data on the metabolism of ZAG in obesity and the role of zinc in this process. The identified studies show that subjects with obesity have low serum concentrations of zinc and ZAG, as well as low expression of the genes encoding this protein. Thus, zinc appears to be an important regulator of the homeostasis of ZAG in the body; however, alterations in the metabolism of zinc in obesity appear to compromise the functions of ZAG. Therefore, further studies are needed to clarify the relationship between zinc and ZAG metabolism and its repercussions in obesity.

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.

Serum zinc-α2-glycoprotein levels are elevated and correlated with thyroid hormone in newly diagnosed hyperthyroidism

BACKGROUND

Zinc-α2-glycoprotein (ZAG) is a recently novel lipolytic adipokine implicated in regulation of glucose and lipid metabolism in many metabolic disorders. In vitro and animal studies suggest that thyroid hormones (TH) up-regulates ZAG production in hepatocytes. However, there is no data evaluating the possible relationship between ZAG and TH in a human model of hyperthyroidism. The objective of the present study is to assess the association of serum ZAG levels with TH and lipid profile in patients with hyperthyroidism before and after methimazole treatment.

METHODS

A total of 120 newly diagnosed overt hyperthyroidism and 122 healthy control subjects were recruited. Of them, 39 hyperthyroidism patients were assigned to receive methimazole treatment as follow-up study for 2 months.

RESULTS

The clinical consequence showed that serum ZAG levels were elevated in patients with hyperthyroidism (P < 0.01). Adjust for age, gender and BMI, serum ZAG levels were positively related with serum free T3 (FT3), free T4 (FT4) levels and negatively correlated with serum total cholesterol (TC), low density lipoprotein cholesterol (LDLC) levels in hyperthyroidism subjects (all P < 0.01). After methimazole treatment, serum ZAG levels were decreased and the decline was associated with decreased FT3, FT4 and increased TC levels (all P < 0.001).

CONCLUSION

We conclude that ZAG may be involved in the pathogenesis of lipid metabolism disorder in patients with hyperthyroidism.

TRIAL REGISTRATION

ChiCTR-ROC-17012943 . Registered 11 October 2017, retrospectively registered.

Expression and Function of Zinc-α2-Glycoprotein

Zinc-α2-glycoprotein (ZAG), encoded by the AZGP1 gene, is a major histocompatibility complex I molecule and a lipid-mobilizing factor. ZAG has been demonstrated to promote lipid metabolism and glucose utilization, and to regulate insulin sensitivity. Apart from adipose tissue, skeletal muscle, liver, and kidney, ZAG also occurs in brain tissue, but its distribution in brain is debatable. Only a few studies have investigated ZAG in the brain. It has been found in the brains of patients with Krabbe disease and epilepsy, and in the cerebrospinal fluid of patients with Alzheimer disease, frontotemporal lobe dementia, and amyotrophic lateral sclerosis. Both ZAG protein and AZGP1 mRNA are decreased in epilepsy patients and animal models, while overexpression of ZAG suppresses seizure and epileptic discharges in animal models of epilepsy, but knowledge of the specific mechanism of ZAG in epilepsy is limited. In this review, we summarize the known roles and molecular mechanisms of ZAG in lipid metabolism and glucose metabolism, and in the regulation of insulin sensitivity, and discuss the possible mechanisms by which it suppresses epilepsy.

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.

Isoflavin-β modifies muscle oxidative stress and prevents a thyrotoxicosis-induced loss of muscle mass in rats

INTRODUCTION

We sought to verify whether isoflavin-beta (Iso-β), a mixture of isoflavones with antioxidant properties, could prevent thyrotoxicosis-induced loss of muscle mass and the participation of oxidative stress (OS) in the mechanisms of this prevention.

METHODS

Two experimental periods of thyrotoxicosis induction were used in Wistar rats: 3 and 5 days to assess Iso-β effects before and after thyrotoxicosis-induced muscle wasting. After euthanasia, peritoneal fat and gastrocnemius muscle were collected, weighed, and muscle OS was assessed.

RESULTS

Iso-β prevented the loss of gastrocnemius mass in thyrotoxic rats through the prevention of muscle OS generation during thyrotoxicosis, increasing muscle total antioxidant capacity and decreasing mitochondrial cytochrome c oxidase activity, lipid peroxidation, and protein carbonyl content.

CONCLUSION

Iso-β decreased oxidative modification of proteins, which is known to exert a major role during proteolysis induction and is present in thyrotoxic myopathy, highlighting the potential action of Iso-β in this complication of the disease. Muscle Nerve 56: 975-981, 2017.

Role of visfatin and zinc-α2-glycoprotein in pathogenesis of non-alcoholic fatty liver disease

Visfatin is a novel adipokine which was discovered in 2005 by Fukuhara. Visfatin is highly enriched in the visceral fat of humans and mice with obesity, which was previously called pre-B-cell colony-enhancing factor, and exhibits nicotinamide phosphoribosyl transferase enzymatic activity. Visfatin regulates insulin secretion in pancreatic β cells. Visfatin is not only an adipocyte-specific protein, and it has a close relation with non-alcoholic fatty liver disease (NAFLD). Zinc-α2-glycoprotein (ZAG), which is identical to the previously named lipid-mobilising factor, is a single-chain polypeptide. It is secreted by both brown and white adipocytes. ZAG possibly functions as a novel protein factor responsible for the decomposition of tissue lipids. ZAG also has a close relationship with NAFLD. This paper will review the advances in understanding the role of visfatin and ZAG in the pathogenesis of NAFLD.

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.