**2. Thyroid hormone**

#### **2.1. Thyroid hormone and thyroid hormone receptors**

Thyroid hormones (THs) include thyroxine (T4) and triiodothyronine (T3). They are synthesized and secreted by the thyroid gland. T4 is the major secreted hormone, while T3 has a higher affinity for TH receptors (TRs). T3 is considered as the active and more potent TH. T4 could be converted to T3 through a deiodination process catalyzed by deiodinases. TH regulates a number of biological functions including growth, development and metabolism in almost all tissues [1]. TH exerts these effects through binding to TRs which are expressed on different cells and tissues. TRs have two isoforms, TRα and TRβ, which are encoded by the THRA and THRB genes, respectively, in humans. Each TR isoform has several splice products, TRα1 (α2) and TRβ1 (β2). TRα1 and TRβ1 are ubiquitously expressed, while TRβ1 is the major TR existed in the liver. TRβ2 is expressed in the hypothalamus, the pituitary gland and the developing brain [2]. TRs are ligand-activated transcription factors, belonging to the family of nuclear receptors (NRs). It can bind to DNA sequences called TH-responsive elements (TREs) together with the retinoid X receptor alpha (RXR-α). In the absence of TH, TRs bind with corepressors, e.g., nuclear receptor corepressor and silencing mediator for retinoid and thyroid hormone receptor (NCOR2), suppressing the transcriptional activity. In the presence of TH, the binding induces a conformational change of TRs, releasing the corepressors and recruiting several co-activators to enhance the transcriptional activity. Since TRs associate with corepressors without ligand binding, it could decrease the transcriptional activity of the target genes. Therefore, it should be cautious to compare the data from animal models in which TRs are genetically deleted with the models with low levels of circulating THs, such as hypothyroidism or thyroidectomy [3].

#### **2.2. Role of TH in cholesterol metabolism**

There is substantial evidence linking TH status with cholesterol or lipid metabolism. Thyroid dysfunction exerts an important effect on the cholesterol level. Hypothyroidism patients typically have elevated plasma cholesterol and increased lipid accumulation in the liver. TH supplement can normalize this lipid dysregulation. THs promote cholesterol synthesis through inducing HMG-CoA reductase and farnesyl pyrophosphate gene expression [1]. THs markedly decrease the expression of apoB-100, the major protein of LDL, while increasing the expression of apo A-I, the major protein of HDL. In addition, THs increase LDLr gene expression. LDLr mediates the uptake of LDL from blood to the liver. Rat LDLr promoter contains two functional TREs. THs could directly bind to the TRE and upregulate the LDLr gene expression [4]. THs may also regulate the clearance of circulating remnant lipoproteins. Hepatic low-density lipoprotein receptor-related protein 1 (LRP1) is a receptor for remnant lipoproteins. Hepatic LRP1 protein expression and function are reduced in the hypothyroidism mouse model. T3 supplement partially normalizes its protein expression level [5]. THs also promote the cholesterol elimination by increasing conversion and secretion of cholesterol into bile acids. In this process, cholesterol 7α-hydroxylase (Cyp7A1), the enzyme in the cytochrome P450 family, is responsible for catalyzing the rate-limiting reaction in the degradation of cholesterol. Cyp7A1 is a direct TR target gene with TREs in its promoter region [6]. ATP-binding cassette (ABC) transporters G5 (ABCG5) and G8 (ABCG8) form a heterodimer that limits intestinal absorption and facilitates biliary secretion of cholesterol. Mice homozygous for disruption of Abcg5 demonstrate a significant reduction in basal biliary cholesterol secretion. T3 treatment does not increase the cholesterol secretion in Abcg5−/− mice as in the wild-type control mice. This observation suggests that THs induce secretion of cholesterol, largely dependent on the ABCG5/G8 transporter complex [7]. THs also modulate gene expression via micro-RNAs. In a human hepatic cell line, THs decrease sterol O-acyltransferase 2 (SOAT2 or ACAT2), the enzyme crucial for the hepatic secretion of cholesterol esters, via miR-181d [8].

T3 also upregulates LDLr gene expression by activating the expression of the sterol regulatory element-binding protein-2 (SREBP-2) and scavenger receptor class B1 (SR-B1) [9]. Cholesteryl ester transfer protein (CETP) mediates the exchange of cholesteryl esters from HDL to the VLDL and from total triglyceride (TG) to the opposite direction. THs could increase the activity of CETP to influence HDL metabolism [10]. In addition, THs stimulate the lipoprotein lipase (LPL) and hepatic lipase (HL) levels, catabolizing the TG-rich lipoproteins.

## **2.3. Interaction with other transcription factors**

of cholesterol by stimulating the transcription of LDL and HMG-CoA. LDL receptor (LDLr) is responsible for importing LDL from extracellular to intracellular environment for metabolism. Cholesterol is the primary source for biogenesis of steroid hormones. In turn, many hormones exert critical effects on cholesterol synthesis or metabolism. This occurs through the direct effect of these hormones on regulation of the expression or activity of HMG-CoA reductase, SREBP-1c or LDLr. In this chapter, we will discuss the regulatory role of several

Thyroid hormones (THs) include thyroxine (T4) and triiodothyronine (T3). They are synthesized and secreted by the thyroid gland. T4 is the major secreted hormone, while T3 has a higher affinity for TH receptors (TRs). T3 is considered as the active and more potent TH. T4 could be converted to T3 through a deiodination process catalyzed by deiodinases. TH regulates a number of biological functions including growth, development and metabolism in almost all tissues [1]. TH exerts these effects through binding to TRs which are expressed on different cells and tissues. TRs have two isoforms, TRα and TRβ, which are encoded by the THRA and THRB genes, respectively, in humans. Each TR isoform has several splice products, TRα1 (α2) and TRβ1 (β2). TRα1 and TRβ1 are ubiquitously expressed, while TRβ1 is the major TR existed in the liver. TRβ2 is expressed in the hypothalamus, the pituitary gland and the developing brain [2]. TRs are ligand-activated transcription factors, belonging to the family of nuclear receptors (NRs). It can bind to DNA sequences called TH-responsive elements (TREs) together with the retinoid X receptor alpha (RXR-α). In the absence of TH, TRs bind with corepressors, e.g., nuclear receptor corepressor and silencing mediator for retinoid and thyroid hormone receptor (NCOR2), suppressing the transcriptional activity. In the presence of TH, the binding induces a conformational change of TRs, releasing the corepressors and recruiting several co-activators to enhance the transcriptional activity. Since TRs associate with corepressors without ligand binding, it could decrease the transcriptional activity of the target genes. Therefore, it should be cautious to compare the data from animal models in which TRs are genetically deleted with the models with low levels of circulating THs, such as hypothyroidism or thyroidectomy [3].

There is substantial evidence linking TH status with cholesterol or lipid metabolism. Thyroid dysfunction exerts an important effect on the cholesterol level. Hypothyroidism patients typically have elevated plasma cholesterol and increased lipid accumulation in the liver. TH supplement can normalize this lipid dysregulation. THs promote cholesterol synthesis through inducing HMG-CoA reductase and farnesyl pyrophosphate gene expression [1]. THs markedly decrease the expression of apoB-100, the major protein of LDL, while increasing the expression of apo A-I, the major protein of HDL. In addition, THs increase LDLr gene expression. LDLr mediates the uptake of LDL from blood to the liver. Rat LDLr promoter contains two functional TREs. THs could directly bind to the TRE and upregulate

interesting hormones in cholesterol metabolism.

**2.2. Role of TH in cholesterol metabolism**

**2.1. Thyroid hormone and thyroid hormone receptors**

**2. Thyroid hormone**

20 Cholesterol - Good, Bad and the Heart

In addition to the direct action on the cholesterol-related genes, TRs also cross talk with many nuclear receptors to regulate their transcriptions. It shares the same DNA-binding site (direct repeat 4) with liver X receptor (LXR). Activation of TRβ1 by T3 upregulates mouse LXRα, but not LXRβ, mRNA expression in the liver at the transcriptional level [11]. TRβ1 is the major TR mediating the TH effects on plasma cholesterol. ATP-binding cassette transporter A1 (ABCA1) is important for HDL assembly and transporting cholesterol back to the liver for excretion. TR forms a heterodimer with retinoid X receptor (RXR) and binds to the DR-4 element of ABCA1 promoter, suppressing its transcription [12]. The apolipoprotein AV gene (APOA5) is a key determinant of the plasma triglyceride level. It affects the plasma TG level through promoting lipolysis of TG-rich lipoproteins and removal of their remnants [13]. TR-β mediates the effects of THs on the activation of APOA5 gene. Administration of TR-β-selective agonist increases apoAV and diminishes triglyceride levels [14]. In addition, TR-β may compete with LXR/ RXR heterodimers for binding to the DR-4 element in the CYP7A1 promoter [15]. TR-β but not TR-α KO mice completely lost the induction effects of T3 on Cyp7a1 gene, confirming the critical role of TR-β in mediating the TH effect on cholesterol metabolism [16].

Taken together, TH regulates the serum cholesterol level in multiple crucial steps including stimulating its hepatic synthesis, serum uptake and the intrahepatic conversion to bile acids. The physiological level of TH is essential for maintaining the cholesterol homeostasis.
