A distinct thyroid hormone response element mediates repression of the human cholesterol 7alpha-hydroxylase (CYP7A1) gene promoter

Gallstone Disease and Its Correlation With Thyroid Disorders: A Narrative Review

Over the years, several studies have revealed an important link between thyroid disorders and gallstone disease. According to these studies, hypothyroidism and hyperthyroidism are associated with cholesterol gallstone disease. This association between thyroid hormone disorders and cholesterol gallstone disease is due to the importance of thyroid hormones on cholesterol synthesis, bile functioning and content, and gallbladder motility. Several genes and receptors have been found on the thyroid gland, liver, and gallbladder to verify this association. These genes affect thyroid hormone secretion, lipid metabolism, and bile secretion. Defects in these various gene expression and protein functions lead to bile duct diseases. Other causes that lead to cholesterol gallstone disease are supersaturation of the bile with cholesterol and impaired gallbladder motility, which leads to bile stasis. This article has discussed these factors in detail while highlighting the association between thyroid hormones and cholesterol gallstone disease.

Up to date on cholesterol 7 alpha-hydroxylase (CYP7A1) in bile acid synthesis

Cholesterol 7 alpha-hydroxylase (CYP7A1, EC1.14) is the first and rate-limiting enzyme in the classic bile acid synthesis pathway. Much progress has been made in understanding the transcriptional regulation of CYP7A1 gene expression and the underlying molecular mechanisms of bile acid feedback regulation of CYP7A1 and bile acid synthesis in the last three decades. Discovery of bile acid-activated receptors and their roles in the regulation of lipid, glucose and energy metabolism have been translated to the development of bile acid-based drug therapies for the treatment of liver-related metabolic diseases such as alcoholic and non-alcoholic fatty liver diseases, liver cirrhosis, diabetes, obesity and hepatocellular carcinoma. This review will provide an update on the advances in our understanding of the molecular biology and mechanistic insights of the regulation of CYP7A1 in bile acid synthesis in the last 40 years.

Thyroid Hormone Regulates the mRNA Expression of Small Heterodimer Partner through Liver Receptor Homolog-1

BACKGROUND

Expression of hepatic cholesterol 7α-hydroxylase (CYP7A1) is negatively regulated by orphan nuclear receptor small heterodimer partner (SHP). In this study, we aimed to find whether thyroid hormone regulates SHP expression by modulating the transcriptional activities of liver receptor homolog-1 (LRH-1).

METHODS

We injected thyroid hormone (triiodothyronine, T3) to C57BL/6J wild type. RNA was isolated from mouse liver and used for microarray analysis and quantitative real-time polymerase chain reaction (PCR). Human hepatoma cell and primary hepatocytes from mouse liver were used to confirm the effect of T3 in vitro. Promoter assay and electrophoretic mobility-shift assay (EMSA) were also performed using human hepatoma cell line.

RESULTS

Initial microarray results indicated that SHP expression is markedly decreased in livers of T3 treated mice. We confirmed that T3 repressed SHP expression in the liver of mice as well as in mouse primary hepatocytes and human hepatoma cells by real-time PCR analysis. LRH-1 increased the promoter activity of SHP; however, this increased activity was markedly decreased after thyroid hormone receptor β/retinoid X receptor α/T3 administration. EMSA revealed that T3 inhibits specific LRH-1 DNA binding.

CONCLUSION

We found that thyroid hormone regulates the expression of SHP mRNA through interference with the transcription factor, LRH-1.

Thyroid hormones and thyroid hormone receptors: Effects of thyromimetics on reverse cholesterol transport

Reverse cholesterol transport (RCT) is a complex process which transfers cholesterol from peripheral cells to the liver for subsequent elimination from the body via feces. Thyroid hormones (THs) affect growth, development, and metabolism in almost all tissues. THs exert their actions by binding to thyroid hormone receptors (TRs). There are two major subtypes of TRs, TRα and TRβ, and several isoforms (e.g. TRα1, TRα2, TRβ1, and TRβ2). Activation of TRα1 affects heart rate, whereas activation of TRβ1 has positive effects on lipid and lipoprotein metabolism. Consequently, particular interest has been focused on the development of thyromimetic compounds targeting TRβ1, not only because of their ability to lower plasma cholesterol but also due their ability to stimulate RCT, at least in pre-clinical models. In this review we focus on THs, TRs, and on the effects of TRβ1-modulating thyromimetics on RCT in various animal models and in humans.

Thyroid hormone crosstalk with nuclear receptor signaling in metabolic regulation

Thyroid hormone influences diverse metabolic pathways important in lipid and glucose metabolism, lipolysis and regulation of body weight. Recently, it has been recognized that thyroid hormone receptor interacts with transcription factors that predominantly respond to nutrient signals including the peroxisome proliferator-activated receptors, liver X receptor and others. Crosstalk between thyroid hormone signaling and these nutrient responsive factors occurs through a variety of mechanisms: competition for retinoid X receptor heterodimer partners, DNA binding sites and transcriptional cofactors. This review focuses on the mechanisms of interaction of thyroid hormone signaling with other metabolic pathways and the importance of understanding these interactions to develop therapeutic agents for treatment of metabolic disorders, such as dyslipidemias, obesity and diabetes.

Thyroid hormone mimetics: potential applications in atherosclerosis, obesity and type 2 diabetes

Thyroid hormones influence heart rate, serum lipids, metabolic rate, body weight and multiple aspects of lipid, carbohydrate, protein and mineral metabolism. Although increased thyroid hormone levels can improve serum lipid profiles and reduce fat, these positive effects are counterbalanced by harmful effects on the heart, muscle and bone. Thus, attempts to use thyroid hormones for cholesterol-lowering and weight loss purposes have so far been limited. However, over the past decade, thyroid hormone analogues that are capable of uncoupling beneficial effects from deleterious effects have been developed. Such drugs could serve as powerful new tools to address two of the largest medical problems in developed countries--atherosclerosis and obesity.

Two uniquely arranged thyroid hormone response elements in the far upstream 5' flanking region confer direct thyroid hormone regulation to the murine cholesterol 7alpha hydroxylase gene

Cholesterol 7α hydroxlyase (CYP7A1) is a key enzyme in cholesterol catabolism to bile acids and its activity is important for maintaining appropriate cholesterol levels. The murine CYP7A1 gene is highly inducible by thyroid hormone in vivo and there is an inverse relationship between thyroid hormone and serum cholesterol. Eventhough gene expression has been shown to be upregulated, whether the induction was mediated through a direct effect of thyroid hormone on the CYP7A1 promoter has never been established. Using gene targeted mice, we show that either of the two TR isoforms are sufficient to maintain normal hepatic CYP7A1 expression but a loss of both results in a significant decrease in expression. We also identified two new functional thyroid hormone receptor-binding sites in the CYP7A1 5′ flanking sequence located 3 kb upstream from the transcription start site. One site is a DR-0, which is an unusual type of TR response element, and the other consists of only a single recognizable half site that is required for TR/retinoid X receptor (RXR) binding. These two independent TR-binding sites are closely spaced and both are required for full induction of the CYP7A1 promoter by thyroid hormone, although the DR-0 site was more crucial.

Suppression of bile acid synthesis by thyroid hormone in primary human hepatocytes

AIM: It is known that thyroid hormones alter the bile acid metabolism in humans, however the effect on individual enzymes has been difficult to elucidate. This is mainly due to the lack of human liver cell lines producing bile acids. We used cultures of primary human hepatocytes to study the effects of triiodothyronine (T₃) on bile acid synthesis.

METHODS: Primary hepatocytes were isolated from liver tissue obtained from three different patients undergoing liver resection due to underlying malignancy. The hepatocytes were cultured under serum-free conditions and treated from d 1 to d 5 with culture containing 0.1 - 1000 nmol/L of T₃. Bile acid formation and mRNA levels of key enzymes were analysed.

RESULTS: The lowest concentration of T₃ decreased cholic acid (CA) formation to 43%-53% of controls and chenodeoxycholic acid (CDCA) to 52%-75% of controls on d 5. The highest dose further decreased CA formation to 16%-48% of controls while CDCA formation remained at 50%-117% of controls. Expression of mRNA levels of cholesterol 7α-hydroxylase (CYP7A1) and sterol 12α-hydroxylase (CYP8B1) dose-dependently decreased. Sterol 27-hydroxylase (CYP27A1) levels also decreased, but not to the same extent.

CONCLUSION: T₃ dose-dependently decreased total bile acid formation in parallel with decreased expression of CYP7A1 and CYP8B1. CA formation is inhibited to a higher degree than CDCA, resulting in a marked decrease in the CA /CDCA ratio.

Hormonal regulation of the human sterol 27-hydroxylase gene CYP27A1

The nucleotide sequence data reported in this paper will appear in EMBL Nucleotide Sequence Database under the accession number AJ 544720. The mitochondrial sterol 27-hydroxylase (CYP27A1) is a multifunctional cytochrome P450 enzyme that catalyses important hydroxylations in the biosynthesis of bile acids and bioactivation of vitamin D(3). Previous results [Babiker, Andersson, Lund, Xiu, Deeb, Reshef, Leitersdorf, Diczfalusy and Bj örkhem (1997) J. Biol. Chem. 272, 26253-26261] suggest that CYP27A1 plays an important role in cholesterol homoeostasis and affects atherogenesis. In the present study, the regulation of the human CYP27A1 gene by growth hormone (GH), insulin-like growth factor-1 (IGF-1), dexamethasone, thyroid hormones and PMA was studied. HepG2 cells were transfected transiently with luciferase reporter gene constructs containing DNA fragments flanking the 5'-region of the human CYP27A1 gene. GH, IGF-1 and dexamethasone increased the promoter activity by 2-3-fold, whereas thyroxine (T(4)) and PMA repressed the activity significantly when measured with luciferase activity expressed in the cells. The endogenous CYP27A1 enzyme activity in the cells was stimulated by GH, IGF-1 and dexamethasone, whereas T(4) and PMA inhibited the activity. Experiments with progressive deletion/luciferase reporter gene constructs indicated that the response elements for GH may be localized in a region upstream to position -1094 bp. The putative response elements for dexamethasone were mapped to positions between -792 and -1095 bp. The -451 bp fragment of the human CYP27A1 gene was found to confer the activation by IGF-1, and the inhibition by T(4) and PMA. Results of the present study suggest that CYP27A1 is regulated in human cells by hormones and signal-transduction pathways.