饥饿素(ghrelin)在非酒精性脂肪肝病(NAFLD)中的研究进展
2014-04-18毛雨晴综述樊晓明审校
毛雨晴(综述) 樊晓明(审校)
(复旦大学附属金山医院消化科 上海 201508)
饥饿素(ghrelin)在非酒精性脂肪肝病(NAFLD)中的研究进展
毛雨晴(综述) 樊晓明△(审校)
(复旦大学附属金山医院消化科 上海 201508)
非酒精性脂肪肝病(non-alcoholic fatty liver disease,NAFLD)在肝脏疾病谱中占据日益重要的地位,其与胰岛素抵抗和氧化应激密切相关。饥饿素(ghrelin)作为一种新发现的从胃内提取的激素,是第一种生长激素促分泌素的内源性配体,能促进生长激素分泌,同时增强食欲,减少脂肪利用,维持能量正平衡,并在改善胰岛素抵抗、抑制炎性反应、抗纤维化和肿瘤等方面有一定作用,可能对NAFLD起到治疗作用。本文就目前国内外关于饥饿素在NAFLD中的研究作一综述。
饥饿素(ghrelin); 非酒精性脂肪肝病(NAFLD); 胰岛素抵抗; 炎性反应; 纤维化; 肿瘤
随着生活方式和饮食结构的改变,非酒精性脂 肪肝病(non-alcoholic fatty liver disease,NAFLD)在肝脏疾病谱中占据日益重要的地位。NAFLD是排除过量饮酒、使用促脂肪变性药物、遗传异常等引起肝脂肪变性的继发性因素后,经影像学或组织学证据表明肝脏脂肪变性的肝病综合征,经历由单纯脂肪性变发展为肝炎、肝硬化甚至肝癌的发展过程[1],在西方国家患病率高达20%~30%[2],近20年来亚洲人群患病率也不断增高[3]。饥饿素(ghrelin)作为一种新发现的主要在胃内合成的生长激素促分泌素内源性配体,在NAFLD的发生、演变和治疗中可能起到重要作用。
饥饿素概述饥饿素是日本科学家Kojima等[4]从大鼠胃内提取的第一个生长激素促分泌素受体(growth hormone secretagogue receptor,GHSR)的内源性配体,由28个氨基酸残基组成,主要由胃底泌酸腺的X/A细胞合成,在脑、心肺、小肠、肾脏、脂肪组织、胰腺等均有发现。根据分泌形式不同可分为乙酰化(acyl ghrelin,AG)和去乙酰化(desacyl ghrelin,DAG),后者第3位丝氨酸残基可被正辛酸修饰成为AG饥饿素[5]。以往认为DAG饥饿素无活性,但近年来发现其也有一定作用。GHS-R是一种G蛋白偶联受体,主要分布于下丘脑-垂体,在心肺、胰腺、胃肠及多种免疫细胞中也有分布[6]。GHS-R分为GHS-R1a和GHS-R1b两种形式,目前认为前者是功能性受体。饥饿素与功能性受体结合后可发挥多种生物学效应,如促进GH释放后刺激胃酸分泌和胃肠蠕动、增加食欲等。饥饿素在抑制胰岛素抵抗、抗炎反应、抗纤维化和肿瘤中可能发挥一定作用。
饥饿素-GOAT系统与代谢综合征代谢综合征是以胰岛素抵抗为中心环节,包含不少于3个如下特征的全身综合征:腹型肥胖、高血糖、高血压、高三酰甘油血症和低高密度脂蛋白血症[7]。NAFLD是代谢综合征在肝脏的主要表现。
饥饿素与胰岛素抵抗 在“二次打击”学说中,导致NAFLD产生的第一次打击为胰岛素抵抗。胰岛素抵抗程度越严重,肝脏的损伤程度也越大,所以胰岛素增敏剂可能成NAFLD的治疗方法[8]。目前关于饥饿素与胰岛素的关系仍存在争议。Granata等[9]在胰岛β细胞系HIT-T15细胞株中发现饥饿素通过Galpha(s)蛋白促进HIT-T15增殖,抑制其凋亡,而饥饿素抗体可促进其凋亡,并可能通过c AMP蛋白激酶 A(c AMP/protein kinase A,c AMP/PKA)、磷脂酰丝氨酸 3 激酶-Akt(phosphatidylinositide 3-kinase,PI3K/Akt)、细胞外信号调节酶(extracellular regulated kinase1/2,ERK1/2)等信号通路抑制IFN-γ/TNF-a等炎性因子介导的胰岛β细胞凋亡作用。但也有研究认为饥饿素对胰岛素主要为抑制作用,并可能通过增加1型糖尿病患者胰岛β细胞自身抗原2β(IA-2β)表达[10]以及腺苷酸活化蛋白激酶-解偶联蛋白2(AMP-activated protein kinase-uncoupling protein 2,AMPK-UCP2)信号途径达到抑制作用[11]。有学者提出胰岛素能降低饥饿素浓度,且该作用与血糖影响相独立[12]。但Longo等[13]发现敲掉GHS-R基因能改善小鼠胰岛素敏感性。也有研究表明,低饥饿素水平与胰岛素抵抗指数(HOMAIR)呈正相关,高水平饥饿素可以降低患NAFLD的风险[14-15]。Qader等[16]在小鼠胰岛中发现低剂量AG抑制胰岛素分泌而高剂量则促进胰岛素分泌,大鼠胰岛素分泌与饥饿素水平呈浓度依赖性。饥饿素与胰岛素分泌及其敏感性的关系还有待深入研究。
饥饿素与脂质代谢 NAFLD患者中常见脂质代谢紊乱的现象。脂质摄入增加、高脂血症及脂肪组织动员增加,使肝脏摄取脂肪酸增加,线粒体氧化超载。高胰岛素血症使基因转录水平上调,促进脂肪在肝脏的重新合成;极低密度脂蛋白合成和分泌减少,致三酰甘油转运障碍[17]。饥饿素在肥胖人群和NAFLD患者中的水平下降,是否为减少能量摄入的自我保护机制还有待深入研究。饥饿素有促进摄食作用,可增加脂肪权重及增加脂肪酸合成酶(fatty acid synthase,FASN)、硬脂酰辅酶A脱氢酶(stearoyl-Co A desaturase 1,SCD1)等脂肪合成指标的表达[18],并通过丝裂原激活蛋白激酶(mitogen-activated protein kinase,MAPK)和 PI3/Akt途径促进脂肪细胞增殖和分化,抑制凋亡[19],尤其增加腹部白色脂肪储存,该作用与中枢刺激食欲作用相独立,而DAG对脂肪无影响[20]。
饥饿素与心血管风险 研究表明,NAFLD增加冠状动脉粥样硬化(coronary atherosclerosis,CA)的发生风险,可能为CA的独立危险因素[21],其机制可能与脂质过氧化产生活性氧自由基(reacetive oxygen species,ROS)、刺激炎性因子及C反应蛋白产生有关。研究发现,在糖尿病患者中,低水平饥饿素与CA的严重程度密切相关,可能对评估CA预后有重要价值[22]。Tesauro等[23]认为饥饿素通过增加NO生物活性来逆转代谢综合征患者体内的内皮细胞损伤。
GOAT与代谢综合征 饥饿素-O-乙酰转移酶(ghrelin-O-acyltransferase,GOAT)是一种能将饥饿素第3位丝氨酸残基乙酰化的、多面体的膜结合酶,主要表达于胃和肠腺[24],是唯一能够乙酰化饥饿素的酶。研究发现,GOAT基因敲除小鼠限制能量喂养后比普通小鼠体重下降更多、血糖更低,而该作用可以通过外源性乙酰化饥饿素逆转。GOAT抑制剂还能预防高脂饲料喂养的小鼠体重增加,刺激胰岛素释放[25]。GOAT能减少饥饿素乙酰化,增加DAG水平,饥饿素/Goat/GHS-R系统已成为治疗肥胖的新方向[26]。Romero等[27]认为,GOAT可能成为调节饥饿素活性的新靶向,对代谢综合征的治疗有潜在价值。
饥饿素与炎性反应肝细胞堆积的脂质会损伤线粒体抗氧化能力,刺激过氧化物酶和微粒体途径增加对脂质的氧化,ROS增加,激发炎性细胞因子产生。研究发现,单纯脂肪肝发生炎性反应与肝活化核因子κB(hepatic activities of nuclear factorκB,NF-κB)信号通路、库普弗细胞激活及肝细胞产生炎性介质有关,而游离脂肪酸会通过溶酶体和组织蛋白酶相关机制直接激活IKK-β/NF-κB信号通路。细胞凋亡也是氧化应激后肝细胞损伤的重要机制,因此抑制TNF-a对于抑制细胞凋亡至关重要[28]。
研究发现,饥饿素可能通过抑制交感神经系统活性和增加迷走神经系统活性来达到抗炎作用[29]。目前已在大脑多个部位发现饥饿素受体,推测饥饿素可能在中枢发挥作用。Sato等[30]发现,长期使用GHS-R拮抗剂的啮齿动物血浆肾上腺素和去甲肾上腺素水平上升。研究表明,大脑中的孤束核(nucleus tractus solitarii,NTS)能直接或间接兴奋延髓腹外侧区尾端(caudal ventrolateral medulla,CVLM),导致抑制性神经递质γ-氨基丁酸(gammaaminobutyric acid,GABA)输向延髓腹外侧区头端(rostral ventrolateral medulla,RVLM),同时刺激脊髓交感节前神经元和节后神经元,刺激靶器官(如肠道)释放去甲肾上腺素,与肝库普弗细胞相应受体结合,激活P38 MPAK途径,继而协同 NF-κB途径,释放TNF-a和IL-6等炎性因子[29],导致肝损伤。饥饿素能通过上调MAP磷酸激酶-1抑制P38 MAPK途径,从而改善炎性反应[31]。饥饿素抗炎的神经机制还与迷走神经激活有关。Shimizu等[32]证明,中枢注入饥饿素能激活心迷走神经,使心脏释放乙酰胆碱增加。外周乙酰胆碱释放增加能激活a7-亚基胆碱能受体,发挥胆碱能抗炎作用,抑制IL-1β、IL-6、IL-18及 TNF等炎性介质释放[33]。
Xia等[34]发现饥饿素能通过抗CD3抗体刺激抑制小鼠脾脏T细胞增生,并抑制IL-4和IL-10等细胞因子的m RNA表达。Dixit等[35]用小分子干扰RNA(siRNA)敲除人T细胞表面饥饿素表达后发现,IFN-γ、IL-17及其他促炎因子增加,提示饥饿素可能通过T细胞发挥抗炎作用。Li等[36]发现,饥饿素处理的NAFLD大鼠血浆ALT、AST、TNF-a水平及其mRNA表达较对照组明显下降,并可能通过LKB1/AMPK和PI3/Akt途径减轻氧化应激和炎性损伤,抑制肝细胞凋亡,可能成为治疗NAFLD的新药物[37]。饥饿素是一种特异的神经内分泌激素,对机体免疫反应有调节作用[38]。
饥饿素与肝纤维化肝脏中纤维发生细胞主要来自肝星状细胞[39],肝脏炎性损伤会激活肝星状细胞等纤维细胞迁移聚集在组织损伤部位并分泌大量细胞外基质,这是肝脏正常的修复反应之一。胰岛素抵抗、氧化应激、促炎因子释放及固有免疫对肝纤维化都有促进作用[40]。Moreno等[41]研究发现,对结扎胆管导致的慢性肝损伤小鼠模型予重组饥饿素处理后,能减少肝损伤和成肌纤维细胞、肝星状细胞积累,降低肝纤维化反应,同时降低肝氧化应激和炎性反应程度,而饥饿素缺陷小鼠的纤维化和肝损伤程度更严重。饥饿素能减轻肝细胞炎性坏死,减少肝氧化应激、转变生长因子β1(主要的肝促纤维细胞因子)表达,减少肝星状细胞积累和基质蛋白合成,这些可能解释饥饿素抑制肝纤维化的机制。饥饿素的抗纤维作用使其及GHS-R激动剂或成为治疗肝纤维化的药物,但饥饿素拮抗剂可以治疗肥胖和糖尿病,长期应用是否会促进NAFLD的纤维化进展,同时长期予饥饿素抗纤维治疗是否会导致肥胖和脂肪肝也值得思考[42]。
饥饿素与肝癌少数肝硬化患者发展为肝癌的机制尚不明确。饥饿素在多种上皮细胞起源的肿瘤组织及细胞中均有表达,包括乳腺癌、胰腺癌、前列腺癌和垂体瘤等[43]。Murata等[44]在肝癌细胞中发现GHS-R,并认为肝癌细胞暴露在饥饿素当中会引起胰岛素受体底物蛋白-1(insulin receptor substrate-1,IRS-1)酪氨酸磷酸化,调节小分子生长因素受体结合蛋白-2,促进有丝分裂蛋白激酶活性及细胞增殖。但Thielemans等[45]却未在人肝癌细胞株中检测到GHS-R1a表达。饥饿素在体外能明显抑制乳腺癌细胞增殖,但并未在乳腺癌细胞上发现GHS-R1a受体,提示乳腺癌细胞上还存在GHSR的其他亚型[46]。饥饿素能促进还是抑制肿瘤细胞生长,体内或体外效果是否一致,对乳腺癌细胞的抑制作用是否具有特异性。这些问题还需要进一步研究解决。饥饿素能促进肿瘤晚期患者的食欲,增加体重,改善恶液质状态,但需警惕长期使用是否会促进肿瘤细胞增殖[47]。饥饿素可以减轻肿瘤化疗带来的消化道不良反应,或成为改善化疗胃肠道反应的新药物[48]。
结语NAFLD已经成为威胁人类健康的第二大肝病,经历“二次打击”从单纯非酒精性脂肪肝到脂肪性肝炎、肝硬化甚至肝癌的演变。饥饿素作为一种新发现的内源性促生长激素释放肽,不仅能促进生长激素释放、增加食欲,对改善胰岛素抵抗,抑制炎症反应、抗肝纤维化和肿瘤等可能有促进作用,因此具有巨大的临床研究价值和应用前景。饥饿素通过抑制交感神经活性、促进迷走神经活性及激活LKB1/AMPK和PI3/Akt等信号通路抑制炎性反应,能否成为治疗NAFLD的新靶向;GOAT酶激动剂或抑制剂能否通过对饥饿素的调节作用来发挥对代谢综合征的治疗作用;饥饿素与胰岛素释放敏感性的关系如何;饥饿素增加细胞有丝分裂和癌变的风险在体内和体外试验是否有区别;在饥饿素促进食欲、增加脂肪合成、治疗NAFLD的同时如何权衡肥胖问题。我们期待对饥饿素的深入了解,以便充分挖掘出其潜在的临床价值。
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Research advances of ghrelin on nonalcoholic fatty liver disease(NAFLD)
MAO Yu-qing,FAN Xiao-ming△
(Department of Digestive,Jinshan Hospital,Fudan University,Shanghai201508,China)
ghrelin; non-alcoholic fatty liver disease (NAFLD); insulin resistance;inflammation; fibrosis; tumor
R 575.5
B
10.3969/j.issn.1672-8467.2014.02.022
2013-04-24;编辑:段佳)
上海市卫生局资助项目(2010075);上海市自然科学基金(11ZR1405700);上海市医学重点专科建设计划项目(ZK2012B20)
△Corresponding author E-mail:xiaomingfan57@hotmail.com
【Ahstract】Non-alcoholic fatty liver disease(NAFLD)has been playing an increasingly important role in liver disease which is closely related to insulin resistance and oxidative stress.As a newly found hormone purified from stomach,ghrelin is the first endogenous ligand for the growth hormone secretagogue receptor.It can promote the secretion of growth hormone,enhance appetitie,suppress lipometabolism and maintain energy balance,which also cintribute to improvement of insulin resistance,suppression of inflammation reaction,limitation of the development of fibrosis and tumor,etc.Ghrelin probably stands a chance of becoming a new drug treating NAFLD.This review outlines the recent studies on NAFLD and ghrelin both nationally and internationally.
*This work was supported hy Shanghai Municipal Health Bureau(2010075),the Science and Technology Commission of Shanghai Municipality(11ZR1405700)and the Key Clinical Disciplines Construction of Shanghai Municipality(ZK2012B20).