A Novel Role of the Aryl Hydrocarbon Receptor in Hepatic Steatosis

描述(申请人提供)：肝脏脂肪变性，或脂肪肝，与代谢综合征密切相关。了解肝脏脂肪变性的发病机制将有助于预防和治疗这一常见的医学问题。芳香烃受体(AhR)在肝脏中高度表达，是一种转录因子，最初被克隆为“异种受体”。AHR通过与靶基因启动子中存在的二恶英反应元件(DRE)结合来调节异源酶的表达。后来的研究表明，AhR也可能通过影响生理而具有内生功能，但AhR内生功能的分子机制在很大程度上仍不清楚。我们的初步结果表明：(1)表达有成分激活的AhR(CA-AhR)的转基因小鼠在饲料中保持时出现肝脏脂肪变性，这一表型已在用AhR激动剂处理的野生型小鼠中概括；(2)在转基因小鼠中，AhR的激活诱导了CD36/脂肪的表达，这是一种对肝脏脂肪酸摄取和脂肪变性重要的脂肪酸转运体；(3)在用AhR激动剂处理的野生型小鼠中，CD36基因的表达也被激活，而这种激活在AhR-/-小鼠中被取消；(4)AhR激动剂处理的人肝癌细胞诱导CD36的表达并增加对游离脂肪酸的摄取；(5)AhR激活小鼠和人CD36基因启动子；(6)AhR激活抑制极低密度脂蛋白(VLDL)-甘油三酯的分泌；(7)AhR激活抑制过氧化体脂肪酸2-氧化；(8)CA-AhR转基因小鼠在与C57BL/6J回交的第三代出现自发性脂肪变性和糖耐量下降的迹象。根据我们的初步数据，我们假设AhR的激活通过多种机制促进肝脏脂肪变性，包括激活脂肪酸转运体CD36，抑制脂肪酸氧化，以及抑制甘油三酯的输出。通过使用“功能获得”CA-AhR转基因、“功能丧失”AhR-/-和CD36-/-以及AhR配体处理的野生型小鼠，我们提出了四个具体目标：(1)确定AhR的激活是否充分和必要地诱导肝脏脂肪变性；(2)表征AhR诱导的肝脏脂肪变性；(3)确定脂肪酸转运体CD36是否对AhR的脂肪变性作用是必需的；以及(4)确定AhR调节CD36表达的分子机制。据我们所知，目前的研究是首次尝试确定AhR在肝脏脂肪变性和相关代谢异常中的病理生理学作用。四环素诱导的AhR转基因小鼠即使在饮食中保持也会表现出脂肪肝，代表了一种新的、方便的和可逆的非酒精性脂肪性肝病(NAFLD)模型。希望本研究结果有助于建立AhR及其靶向脂肪酸转运体作为治疗人类脂肪肝的新靶点。 与公共健康相关：芳香烃受体(AhR)是一种转录因子，最初被克隆为“异生受体”，用于感知异生毒物。这项研究的目的是确定AhR是否在促进肝脏脂肪变性方面具有内生作用。肝脏脂肪变性，或脂肪肝，是一种与代谢综合征密切相关的常见医学问题。希望本研究结果有助于将AhR作为治疗人类脂肪肝的新靶点。

DESCRIPTION (provided by applicant): Hepatic steatosis, or fatty liver, is strongly associated with metabolic syndrome. Understanding the mechanism of hepatic steatosis will help to prevent and treat this common medical problem. The aryl hydrocarbon receptor (AhR), highly expressed in the liver, is a transcriptional factor originally cloned as a "xenobiotic receptor." AhR regulates the expression of xenobiotic enzymes by binding to the dioxin response elements (DREs) present in target gene promoters. Subsequent studies suggest that AhR may also have endobiotic functions by affecting physiology, but the molecular mechanism for the endobiotic function of AhR remains largely unknown. Our preliminary results showed that: (1) Transgenic mice expressing the constitutively activated AhR (CA-AhR) exhibited hepatic steatosis when maintained in chow diet, a phenotype that has been recapitulated in wild type mice treated with the AhR agonist; (2) Activation of AhR in transgenic mice induced the expression of CD36/FAT, a fatty acid transporter important for hepatic fatty acid uptake and steatosis; (3) Activation of CD36 gene expression was also seen in wild type mice treated with the AhR agonist and this activation was abolished in AhR-/- mice; (4) Treatment of human hepatoma cells with AhR agonist induced the expression of CD36 and increased the uptake of free fatty acids; (5) The mouse and human CD36 gene promoters were activated by AhR; (6) Activation of AhR inhibited very-low density lipoprotein (VLDL)-triglyceride secretion; (7) Activation of AhR suppressed peroxisomal fatty acid 2-oxidation; and 8) CA-AhR transgenic mice in third generation of backcross to C57BL/6J showed spontaneous steatosis and signs of compromised glucose tolerance. Based on our preliminary data, we hypothesize that activation of AhR promotes hepatic steatosis through multiple mechanisms, including the activation of fatty acid transporter CD36, suppression of fatty acid oxidation, and inhibition of export of triglycerides. By using the "gain-of-function" CA-AhR transgenic, "loss-of-function" AhR-/- and CD36-/-, and AhR ligand-treated wild type mice, we propose four specific aims: (1) To determine whether activation of AhR is sufficient and necessary to induce hepatic steatosis; (2) To characterize AhR-induced hepatic steatosis; (3) To determine whether the fatty acid transporter CD36 is necessary for the steatotic effect of AhR; and (4) To determine the molecular mechanism by which AhR regulates the expression of CD36. To our knowledge, the current study represents the first attempt to determine the pathophysiological role of AhR in hepatic steatosis and associated metabolic abnormalities. The tetracycline inducible AhR transgenic mice, exhibiting fatty liver even when maintained in chow diet, represent a novel, convenient and reversible model of nonalcoholic fatty liver disease (NAFLD). It is hoped that results from this study may help to establish AhR and its target fatty acid transporter as novel therapeutic targets for fatty liver in human patients. PUBLIC HEALTH RELEVANCE: The aryl hydrocarbon receptor (AhR) is a transcriptional factor originally cloned as a "xenobiotic receptor" to sense xenobiotic toxicants. The goal of this study is to determine whether AhR has an endobiotic role in promoting hepatic steatosis. Hepatic steatosis, or fatty liver, is a common medical problem strongly associated with metabolic syndrome. It is hoped that results from this study may help to establish AhR as a novel therapeutic target for fatty liver in human patients.