Activation of the aryl hydrocarbon receptor induces hepatic steatosis via the upregulation of fatty acid transport

doi:10.1016/j.abb.2010.09.001

Archives of Biochemistry and Biophysics

Volume 504, Issue 2, 15 December 2010, Pages 221-227

https://doi.org/10.1016/j.abb.2010.09.001 Get rights and content

Abstract

The aryl hydrocarbon receptor (AHR) is a basic helix-loop-helix/Per-ARNT-Sim domain transcription factor, which is activated by various xenobiotic ligands. AHR is known to be abundant in liver tissue and to be associated with hepatic steatosis. However, it has not yet been elucidated how the activation of AHR promotes hepatic steatosis. The aim of this study is to clarify the role of AHR in hepatic steatosis. The intraperitoneal injection of 3-methylcholanthrene (3MC), a potent AHR ligand, into C57BL/6J mice significantly increased the levels of triglycerides and six long-chain monounsaturated fatty acids in the livers of mice, resulting in hepatic microvesicular steatosis. 3MC significantly enhanced the expression level of fatty acid translocase (FAT), a factor regulating the uptake of long-chain fatty acids into hepatocytes, in the liver. In an in vitro experiment using human hepatoma HepG2 cells, 3MC increased the expression level of FAT, and the downregulation of AHR by AHR siRNA led to the suppression of 3MC-induced FAT expression. In addition, the mRNA level of peroxisome proliferator-activated receptor (PPAR) α, an upstream factor of FAT, was increased in the livers of 3MC-treated mice. Taking together, AHR activation induces hepatic microvesicular steatosis by increasing the expression level of FAT.

Research highlights

► Activation of aryl hydrocarbon receptor (AHR) induces steatosis in the liver. ► Activation of AHR enhances the hepatic expression level of fatty acid translocase. ► Hepatic steatosis induced by AHR depends on fatty acid translocase.

Introduction

The aryl hydrocarbon receptor (AHR¹) is a ligand-dependent basic helix-loop-helix/Per-ARNT-Sim domain transcription factor [1]. AHR is activated by various exogenous agonists, such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), polychlorinated biphenyl (PCB), benzo[a]pyrene (BaP), and 3-methylcholanthrene (3MC) [2], [3]. In addition, some endogenous ligands, for example, indigo and indirubin, are known to bind to AHR [4]. These ligands bind to cytosolic AHR, and then the activated AHR translocates into the nucleus, heterodimerizes with the AHR nuclear translocator (ARNT) and binds to specific enhancer sequences adjacent to target promoters termed “dioxin responsive elements” (DREs) [5]. Its activation leads to various toxic responses including tumor-promotion, teratogenicity, immunotoxicity, and lethality [6]. In previous studies, it was reported that AHR is abundant in liver tissues [7], which is one of the major target organs for AHR-related toxicity, such as fat accumulation, inflammation, and fibrosis, in laboratory animals [8], [9]. Boverhof et al. demonstrated that C57BL/6 mice gavaged with TCDD exhibited lipid accumulation in their liver tissues 7 days after exposure [9]. Korenaga et al. also revealed that rhesus monkeys that were subcutaneously administered TCDD for 4 years exhibited intrahepatic fatty changes [10]. However, the molecular mechanisms underlying fat accumulation induced by AHR activation have not been elucidated.

A variety of lipid metabolic pathways occur in the liver. The accumulation of lipids in the liver is also induced through several signaling pathways, and excessive lipid accumulation may lead to fatty liver. The liver is the major tissue of de novo lipogenesis, which is regulated by fatty acid synthase (FAS), and the upregulation of FAS causes increased triglyceride synthesis [11]. The enhanced uptake of fatty acids also leads to the accumulation of lipids in the liver. Free fatty acids enter cells via fatty acid translocase (FAT) and then bind to liver fatty acid binding protein (L-FABP), before transporting free fatty acids to mitochondria, where they will undergo metabolic conversion or be used to synthesize triglycerides [12], [13]. In addition, the accumulation of lipids in the liver is induced through the downregulation of lipid catabolism, for example, via β-oxidation. Carnitine palmitoyltransferase (CPT) 1 controls the transport of long-chain acyl-CoA into mitochondria, in which β-oxidation occurs, and reduced CPT1 activity leads to lipid accumulation accompanied by the downregulation of β-oxidation [14]. Triglycerides bind to apolipoprotein B to construct mature lipoprotein particles for secretion, and microsomal triglyceride transfer protein (MTP) plays a critical role in the assembly and secretion of very low density lipoproteins (VLDL) [15]. Therefore, decreased hepatic export of triglycerides as VLDL leads to lipid accumulation in the liver.

Nuclear factors are also important for the regulation of lipid metabolism. Fatty acid metabolism is transcriptionally regulated by two main systems; i.e., the liver X receptor (LXR) and peroxisome proliferator-activated receptor (PPAR)-related pathways. LXR activates the expression of sterol regulatory element binding proteins (SREBP)-1c, a dominant lipogenic gene regulator. On the other hand, peroxisomal, microsomal, and mitochondrial fatty acid metabolizing enzymes in the liver are transcriptionally regulated by PPARα [16]. However, the relationship between AHR activation and fatty acid metabolism has not been elucidated. Therefore, the aim of this study is to examine the role of AHR in hepatic steatosis.

Section snippets

Animal treatment

All animal treatments in this study were approved by the Institutional Animal Care and Use Committee and carried out according to the Kobe University Animal Experimentation Regulations. Male C57BL/6J mice (15 weeks old, CLEA Japan, Tokyo, Japan) were housed in a temperature controlled (23–25 °C) room at 60 ± 5% humidity under a 12-h light–dark cycle and acclimatized for 7 days with a commercial chow and distilled water. The mice fed the standard chow were randomly divided into two groups of five

Effects of 3MC on lipid levels in plasma and the liver

To investigate the involvement of AHR in hepatic steatosis, 3MC, an agonist of AHR, was used, and C57BL/6J mice were intraperitoneally given 3MC at 100 mg/kg BW. In liver histology assessed by Oil Red O staining, microvesicular fat accumulation was observed around the central vein in the 3MC-treated mice (Fig. 1A). Next, triglycerides were extracted from the liver, and the quantitative measurement of triglycerides was performed. The triglyceride level in the livers of the 3MC-treated mice was

Discussion

This study was designed to examine the role of AHR in intrahepatic fat accumulation. We used 3MC as an agonist of AHR. 3MC is a synthetic compound and a highly potent member of the polycyclic aromatic hydrocarbon family of chemicals [3], which are found in cigarette smoke [18]. In a previous study, it was reported that TCDD induced hepatic steatosis as a chronic toxicity [9], [10]. However, to the best of our knowledge, this is the first study to demonstrate that an AHR agonist induces hepatic

Acknowledgments

This study was supported by a grant from the Research Fellows of the Global COE Program “Global Center of Excellence for Education and Research on Signal Transduction Medicine in the Coming Generation” from the Ministry of Education, Culture, Sports, Science, and Technology of Japan [F031 to Y.K., M.Y. and T.A.]. This study was also supported, in part, by a grant for the Japan Society for the Promotion of Science [21780125 to S.N.].

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Activation of the aryl hydrocarbon receptor induces hepatic steatosis via the upregulation of fatty acid transport

Abstract

Research highlights

Introduction

Section snippets

Animal treatment

Effects of 3MC on lipid levels in plasma and the liver

Discussion

Acknowledgments

J. Biol. Chem.

Arch. Biochem. Biophys.

Arch. Biochem. Biophys.

J. Biol. Chem.

Arch. Biochem. Biophys.

Toxicology

Prog. Lipid Res.

J. Biol. Chem.

Nutr. Metab. Cardiovasc. Dis.

J. Biol. Chem.

J. Hazard Mater.

Toxicol. Appl. Pharmacol.

J. Biol. Chem.

Prostag. Leukot. Essent. Fatty Acids

J. Biol. Chem.