A Novel Role of the Aryl Hydrocarbon Receptor in Hepatic Steatosis

描述（由申请人提供）：肝脂肪变性或脂肪肝与代谢综合征密切相关。了解肝脂肪变性的发生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，最初克隆为“异生物质受体”。AhR通过与目标基因启动子中的二恶英反应元件（DRES）结合来调节异生素酶的表达。随后的研究表明，AhR也可能通过影响生理学而具有内源性功能，但AhR的内源性功能的分子机制在很大程度上仍然未知。我们的初步结果表明：（1）表达组成性激活AhR的转基因小鼠（CA-AhR）在普通饲料中表现出肝脂肪变性，这一表型在用AhR激动剂处理的野生型小鼠中重现：（2）AhR在转基因小鼠中的激活诱导了CD 36/FAT的表达，CD 36/FAT是一种对肝脂肪酸摄取和脂肪变性至关重要的脂肪酸转运蛋白;（3）野生型小鼠经AhR激动剂处理后，CD 36基因的表达也被激活，而AhR-/-小鼠则被抑制，（4）AhR激动剂处理人肝癌细胞后，CD 36基因的表达被激活，游离脂肪酸的摄取增加，（5）小鼠和人CD 36基因启动子被AhR激活，而人CD 36基因启动子被激活。（6）AhR的激活抑制极低密度脂蛋白（VLDL）-甘油三酯分泌;（7）AhR的激活抑制过氧化物酶体脂肪酸2-氧化;和（8）与C57 BL/6 J回交的第三代CA-AhR转基因小鼠显示自发性脂肪变性和葡萄糖耐量受损的迹象。基于我们的初步数据，我们假设AhR的激活通过多种机制促进肝脂肪变性，包括激活脂肪酸转运蛋白CD 36、抑制脂肪酸氧化和抑制甘油三酯输出。通过使用“功能获得型”CA-AhR转基因小鼠、“功能丧失型”AhR-/-和CD 36-/-小鼠以及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激活是否是诱导肝脂肪变性的充分和必要条件;（2）表征AhR诱导的肝脂肪变性;（3）确定AhR的脂肪化作用是否需要脂肪酸转运蛋白CD 36的参与;（4）确定AhR调控CD 36表达的分子机制。据我们所知，目前的研究代表了首次尝试确定AhR在肝脂肪变性和相关代谢异常中的病理生理作用。四环素诱导的AhR转基因小鼠，即使在普通饮食中也表现出脂肪肝，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。希望这项研究的结果可以帮助建立AhR及其靶向脂肪酸转运蛋白作为人类患者脂肪肝的新治疗靶点。 公共卫生相关性：芳烃受体（AhR）是一种转录因子，最初被克隆为“异生物质受体”，以感知异生物质毒物。本研究的目的是确定AhR是否在促进肝脂肪变性中具有内源性作用。肝脂肪变性，或脂肪肝，是一种常见的医学问题，与代谢综合征密切相关。希望本研究的结果有助于将AhR确定为人类患者脂肪肝的新治疗靶点。

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.