贾蓉蓉,赵 严,邱 磊,孙瑞青
1.上海市第十人民医院同济大学附属第十人民医院消化科,上海200072;2.苏州大学附属第一人民医院消化内科
在全球范围内,非酒精性脂肪性肝病(NAFLD)被认为是引起肝功能异常、慢性肝脏疾病最常见的原因[1-2],与肥胖、胰岛素抵抗、高血压、高甘油三酯血症紧密联系在一起,构成代谢综合征,胰岛素抵抗被视为其重要的病理生理特点[3-4]。非酒精性脂肪肝的诊断需要排除其他原因在肝脏中的脂质浸润(无显著的酒精摄入量、嗜肝病毒感染,或其他特殊原因的肝脏疾病)[5-6]。目前推荐的临床诊断之一为:肝脏影像学表现符合弥慢性脂肪肝的诊断标准且无其他原因可供解释[7],和(或)有代谢综合征相关组分的患者出现不明原因的血清ALT和(或)AST、GGT持续增高半年以上[7-9]。减肥和改善IR后,异常酶谱和影像学脂肪肝改善甚至恢复正常者可明确NAFLD的诊断[7-9]。近年来,很多学者都认为PPARα参与了脂肪肝的发病机制,并起到了显著作用。本文就两者之间的最新进展,作一概述。
2.1 PPARα的生理机能 过氧化物酶体增殖物激活受体(PPAR)属于核激素受体超家族的一员,是一类由配体激活的转录因子。它是由英国学家Issemann等[10]于1990年首先发现的。PPARs按其结构及功能可分为 PPARα、PPARβ及 PPARγ三种亚型 。其中PPARα主要表达在脂代谢能力强的组织中,例如肝脏、骨骼肌、棕色脂肪组织[11],其他线粒体含量的组织也有表达,例如肾脏、心脏、肠黏膜。PPARα主要调节人体的能量平衡(葡萄糖和脂类代谢)、炎症、细胞增生、分化、凋亡、衰老及免疫调节[12],同时也参与了调节血管张力,氧化应激及纤维化[13],这些与非酒精性脂肪肝的发病机制密切相关。PPARα一旦与细胞质内的脂肪酸配体激活后,与视黄酸类受体(RXR)结合形成异二聚体,并作为一个特殊的转录因子[14],与靶基因启动子上游的过氧化物酶体增殖物反应元件(PPRE)结合,使靶基因活化,从而调节脂质的合成与氧化、葡萄糖摄取、炎症及免疫基因的表达。PPARα对人体内脂质的调节结果为:降低极低密度脂蛋白(VLDL)甘油三酯水平,增加载脂蛋白AI和HDL-C的水平,上调参与促进胆固醇流出和胆固醇逆向转运的转运蛋白,增强脂肪酸的摄取、氧化和抗炎[15],维持人体的动态平衡,从而减少非酒精性脂肪肝的发生。
2.2 PPARα的激动剂 由于PPARα对脂质平衡的潜在影响,为临床治疗非酒精性脂肪肝开辟了新的思路,近来研究者对PPARα的激动剂颇感兴趣。以往文献报道中,其激动剂包括长链脂肪酸尤其是多聚不饱和脂肪酸如棕榈酸、油酸、亚油酸、花生四烯酸、聚合和氧化型脂肪酸、二十烷类化合物、非甾体消炎药、胰岛素、炎 症 介 质白 三烯 B4(LTB4)、L-FABP、GW501516、Wy14643、Wy214643、GW9578[16]、bezafibrate(苯扎贝特)、L2796449,同时也可被少数饱和脂肪酸激动,其中不饱和脂肪酸与PPARα的亲和力最高。纤维酸衍生物是弱的PPARα的激动剂,在临床试验中已被证明有显著影响[17]。近年来,多种天然的PPARα激动剂陆续被研究者报道,如梓甙(Catalposide)[18]、L-肉碱(L-carnitine)[19]、9-氧代-10,12-十八碳二烯酸(9-oxo-ODA)等可激活PPARα,增强脂肪酸氧化,从而抑制甘油三酯在肝细胞内积累[20]。
目前,NAFLD确切的发病机制仍不是很明确。根据“二次打击”学说[21],肝脏脂肪的积累是第一次打击,使肝脏脂质代谢紊乱,更易遭受第二次打击,导致肝脏损伤、炎症和纤维化。多数学者认为NAFLD与脂质代谢异常、反应性氧化系(ROS)生成增多、肝脂质过氧化增加、肝星状细胞活化及细胞因子产生异常有关。然而,胰岛素抵抗是NAFLD的主要病理生理基础,是代谢综合征发病的核心因素。Sanyal等[22]用高胰岛素正常葡萄糖钳夹实验证实:NAFLD存在胰岛素抵抗,然而 PPARα可改善胰岛素抵抗,延缓或控制NAFLD的发生。Everett等[23]用 PPARα 基因敲除鼠模型研究发现,其肝脏中游离脂肪酸含量明显增多,脂质代谢明显紊乱,肝脏出现明显的脂肪变性,在空腹状态时表现更明显,进一步表明PPARα参与肝脏脂肪代谢,保护NAFLD的发生和发展。
PPARα活化后促进脂质代谢的主要机制:①PPARα活化后通过诱导CD36[24]和脂肪酸结合蛋白1(FABP1)基因的编码,两者可促进脂肪酸从细胞膜转运到细胞质[25];PPARα可增强靶酶:肉毒碱脂酰辅酶A转移酶-1(carnitine palmitoyl transferase-1,CPT-1)、肉毒碱脂酰辅酶A转移酶-2(carnitine palmitoyl transferase-2,CPT-2)等脂肪酸氧化酶的表达,可促使脂酰肉毒碱(acyl-carnitine)转化为脂肪酰辅酶 A(acyl-CoA)[26],促进脂质氧化。② PPARα 激活后,诱导线粒体内有关脂质β-氧化的酶的基因编码,而且,在小鼠体内,激活的PPARα可促进小鼠肝脏肉毒碱(carnitine)的合成[27-28]。在 PPARα 基因敲除的小鼠中,血浆中游离肉碱含量下降,是由于合成肉毒碱的主要器官例如肝脏合成肉碱受抑制。在早先研究中Van Vlies和他的同事等已证实指出PPARα在小鼠体内肉碱代谢中占据的主要位置。③ PPARα可促进长链、支链脂肪酸氧化。此外,PPARα可增加CYP4A的表达,催化不饱和脂肪酸在ω和ω1位点羟基化,加速脂肪酸氧化[29]。④PPARα活化后调节脂蛋白的合成和组装[30]。此外,PPARα活化后调节极低密度脂蛋白的量(very low-density lipoprotein,VLDL),其通过调节脂蛋白脂酶(ipoprotein lipase,LPL)来介导VLDL的脂质分解,减少VLDL的量。⑤PPARα活化后间接调节胆汁酸的合成,促进胆固醇排泄。在小鼠体内PPARα活化后可上调胆固醇7α-羟化酶,特别是在空腹情况下此作用更显著[31]。⑥PPARα活化后可抑制炎症基因的表达,PPARα配体可通过抑制核因子-κB(NF-κB)信号通路,诱导血清TNF-α激活的吞噬细胞凋亡,减少肝脏炎症的发生和发展。⑦PPARα调控肝脏编码过氧化物酶体、线粒体和细胞色素P450中某些脂肪酸代谢酶的基因的转录,促进脂肪酸分解,进而进行β-氧化[32]。⑧ PPARα 可改善胰岛素抵抗,PPARα 活化后,可调节脂联素改善胰岛素敏感性[33],进而增强肝脏中编码脂肪酸氧化酶mRNA的表达水平,促进脂质分解代谢。
另外,PPARα的活化可受到雌激素的影响[34],女性雌激素可抑制PPARα活化促进脂质代谢,提示在治疗NAFLD的过程中,需考虑性别差异。PPARα还可联合肝脏X受体(LXR)了解人体脂质代谢和自身免疫反应[35]。在肝硬化的小鼠体内,PPARα活化后可改善肝血管内皮功能紊乱,降低门静脉压力,改善肝脏纤维化[36]。目前,普遍认为 PPARα 活化对控制NAFLD发生和发展是有益的,但也有相反报道。Chitturi等[37]认为,PPARα活化后会导致脂肪性肝炎的发生;Kurokawa等[38]也报道过PPARα的mRNA和靶基因的mRNA在肝脏癌组织中表达水平升高,可能是由于PPARα的致癌作用和能量代谢的特点有关。
总之,不是所有非酒精性脂肪肝的患者都有高脂血症,高脂血症是一个独立的危险因素,PPARα激活后调节脂质代谢血糖异常、炎症,除此之外还有细胞增生、分化、凋亡、衰竭,进一步降低甘油三酯和总胆固醇,从而提升高密度脂蛋白胆固醇,但是,也有报道称人体内肝脏中PPARα水平比啮齿类动物低[39]。这就导致了临床上以PPARα为靶基因治疗NAFLD会产生偏倚。在一般人群中,NAFLD的患病率波动在15%~30%,随着肥胖和糖尿病的日益普遍,NAFLD的发病率可能更高[40],这就需要我们更深入地研究PPARα及其激动剂,为临床提供更好的治疗效果。
[1]Law K,Brunt EM.Nonalcoholic fatty liver disease[J].Clin Liver Dis,2010,14(4):591-604.
[2]Oh MK,Winn J,Poordad F.Review article:diagnosis and treatment of non-alcoholic fatty liver disease [J].Aliment Pharmacol Ther,2008,28(5):503-522.
[3]Krawczyk M,Bonfrate L,Portincasa P.Nonalcoholic fatty liver disease[J].Best Pract Res Clin Gastroenterol,2010,24(5):695-708.
[4]Lonardo A,Bellentani S,Ratziu V,et al.Insulin resistance in nonalcoholic steatohepatitis:necessary but not sufficient-death of a dogma from analysis of therapeutic studies?[J].Expert Rev Gastroenterol Hepatol,2011,5(4):279-289.
[5]Dowman JK,Tomlinson JW,Newsome PN.Systematic review:the diagnosis and staging of non-alcoholic fatty liver disease and non-alcoholic steatohepatitis [J].Aliment Pharmacol Ther,2011,33(5):525-540.
[6]Sanyal AJ,Brunt EM,Kleiner DE,et al.Endpoints and clinical trial design for nonalcoholic steatohepatitis [J].Hepatology,2011,54(1):344-353.
[7]Farsell GC,Chitturi S,Lau GK,et al.Guidelines for the assessment and management of non-alcoholic fatty liver disease in the Asia-Pacific region:executive summary [J].J Gastroenterol Hepatol,2007,22(6):775-777.
[8]American Gastroenterological Association.American Gastroenterological Association medical position statement:nonalcoholic fatty liver disease[J].Gastroenterology,2002,123(5):1702-1704.
[9]Vuppalanchi R,Chalasani N.Nonalcoholic fatty liver disease and notudcoholic steatohepatitis:selected practical issues in their evaluation and management[J].Hepatology,2009,49(1):306-317.
[10]Issemann I,Green S,Braissant O,et al.Activation of a member of the steroid hormone receptor superfamily by peroxisome proliferators[J].Nature,1990,347(18):645-647.
[11]Auboeuf D,Rieusset J,Fajas L,et al.Tissue distribution and quantification of the expression of mRNAs of peroxisome proliferator-activated receptors and liver X receptor-alpha in humans:no alteration in adipose tissue of obese and NIDDM patients[J].Diabetes,1997,46(8):1319-1327.
[12]Mandard S,Muller M,Kersten S.Peroxisome proliferator-activated receptor alpha target genes[J].Cell Mol Life Sci,2004,61(4):393-416.
[13]Kersten S,Desvergne B,Wahli W.Roles of PPARs in health and disease[J].Nature,2000,405(6785):421-424.
[14]Adeghate E,Adem A,Hasan MY,et al.Medicinal chemistry and actions of dual and pan PPAR modulators[J].Open Med Chem J,2011,5(Suppl 2):93-98.
[15]Hiukka A,Maranghi M,Matikainen N,et al.PPARalpha:An emerging therapeutic target in diabetic microvascular damage[J].Nat Rev Endocrinol,2010,6(8):454-463.
[16]Brown PJ,Winegar DA,Plunket KD,et al.A ureido 2 thioisobutyric acid(GW9578)is a subtype 2selective PPARalpha agonist with potent lipid 2 lowering activity [J]. J Med Chem,1999,42(19):3785.
[17]Tenenbaum A,Motro M,Fisman EZ,et al.Bezafibrate for the secondary prevention of myocardial infarction in patients with metabolic syndrome[J].Arch Intern Med,2005,165(10):1154-1160.
[18]Lee JH,Jun HJ,Hoang MH,et al.Catalposide is a natural agonistic ligand of peroxisome proliferator-activated receptor-α [J].Biochem Biophys Res Commun,2012,422(4):568-572.
[19]Li JL,Wang QY,Luan HY,et al.Effects of L-carnitine against oxidative stress in human hepatocytes:involvement of peroxisome proliferator-activated receptor alpha[J].J Biomed Sci,2012,19:32.
[20]Kim YI,Hirai S,Takahashi H,et al.9-oxo-10(E),12(E)-octadecadienoic acid derived from tomato is a potent peroxisome proliferator-activated receptoraagonist to decrease triglyceride accumulation in mouse primary hepatocytes[J].Mol Nutr Food Res,2011,55(4):585-593.
[21]Tuyama AC,Chang CY.Non-alcoholic fatty liver disease[J].J Diabetes,2012,4(3):266-280.
[22]Sanyal AJ,Campbell-Sargent C,Mirshahi F,et al.Nonalcoholic steatohepatitis:association of insulin resistance and mitochondrial abnormalities[J].Gastroenterology,2001,120(5):1183-1192.
[23]Everett L,Galli A,Crabb D.The role of hepatic peroxisome proliferator-activated receptors(PPARs)in health and disease[J].Liver,2000,20(3):191-199.
[24]Motojima K,Passilly P,Peters JM,et al.Expression of putative fatty acid transporter genes are regulated by peroxisome proliferator-activated receptor α and γ activators in a tissue-and inducer-specific manner[J].J Biol Chem,1998,273(27):16710-16714.
[25]Poirier H,Niot I,Monnot MC,et al.Differential involvement of peroxisome-proliferator-activated receptors alpha and delta in fibrate and fatty-acid-mediated inductions of the gene encoding liver fatty-acidbinding protein in the liver and the small intestine [J].Biochem J,2001,355(Pt 2):481-488.
[26]Aoyama T,Peters JM,Iritani N,et al.Altered constitutive expression of fatty acid-metabolizing enzymes in mice lacking the peroxisome proliferator-activated receptor alpha(PPAR alpha)[J].J Biol Chem,1998,273(10):5678-5684.
[27]Gloerich J,van Vlies N,Jansen GA,et al.A phytol-enriched diet induces changes in fatty acid metabolism in mice both via PPARalphadependent and-independent pathways[J].J Lipid Res,2005,46(4):716-726.
[28]McGarry JD,Brown NF.The mitochondrial carnitine palmitoyltransferase system.From concept to molecular analysis[J].Eur J Biochem,1997,244(1):1-14.
[29]Ip E,Farrell GC,Robertson G,et al.Central role of PPARalpha-dependent hepatic lipid turnover in dietary steatohepatitis in mice[J].Hepatology,2003,38(1):123-132.
[30]Watts GF,Barrett PH,Ji J,et al.Differential regulation of lipoprotein kinetics by atorvastatin and fenofibrate in subjects with the metabolic syndrome[J].Diabetes,2003,52(3):803-811.
[31]Li F,Patterson AD,Krausz KW.Metabolomics reveals an essential role for peroxisome proliferator-activated receptor α in bile acid homeostasis[J].J Lipid Res,2012,53(8):1625-1635.
[32]Savas U,Machemer DE,Hsu MH,et al.Opposing roles of peroxisome proliferator-activated receptor alpha and growth hormone in the regulation of CYP4A11 expression in a transgenic mouse model[J].J Biol Chem,2009,284(24):16541-16552.
[33]McPherson R,Jones PH.The metabolic syndrome and type 2 diabetes:role of the adipocyte [J].Curr Opin Lipidol,2003,14(6):549-553.
[34]Leuenberger N,Pradervand S,Wahli W.Sumoylated PPARalpha mediates sex-specific gene repression and protects the liver from estrogeninduced toxicity in mice [J].J Clin Invest,2009,119(10):3138-3148.
[35]Kidani Y,Bensinger SJ.Liver X receptor and peroxisome proliferatoractivated receptor as integrators of lipid homeostasis and immunity[J].Immunol Rev,2012,249(1):72-83.
[36]Rodríguez-Vilarrupla A,Lavi¨na B,García-Calderó H,et al.PPARα activation improves endothelial dysfunction and reduces fibrosis and portal pressure in cirrhotic rats[J].J Hepatol,2012,56(5):1033-1039.
[37]C hitturi S,Farrell GC.Etiopathogenesis of nonalcoholic steatohepatitis[J].Semin Liver Dis,2001,21(1):27-41.
[38]K urokawa T,Shimomura Y,Bajotto G,et al.Peroxisome proliferatoractivated receptor α (PPARα)mRNA expression in human hepatocellular carcinoma tissue and non-cancerous liver tissue[J].World J Surg Oncol,2011,9:167.
[39]L uci S,Giemsa B,Kluge H,et al.Clofibrate causes an upregulation of PPAR-{alpha}target genes but does not alter expression of SREBP target genes in liver and adipose tissue of pigs[J].Am J Physiol Regul Integr Comp Physiol,2007,293(1):R70-R77.
[40]R atziu V,Bellentani S,Cortez-Pinto H,et al.A position statement on NAFLD/NASH based on the EASL 2009 special conference[J].J Hepatol,2010,53(2):372-384.