腸道菌群在非酒精性脂肪性肝病中作用機(jī)制的研究進(jìn)展

楊 釗，汪芳裕，楊妙芳

南方醫(yī)科大學(xué)金陵醫(yī)院(南京軍區(qū)南京總醫(yī)院)消化內(nèi)科，江蘇 南京 210002

腸道菌群在非酒精性脂肪性肝病中作用機(jī)制的研究進(jìn)展

楊 釗，汪芳裕，楊妙芳

南方醫(yī)科大學(xué)金陵醫(yī)院(南京軍區(qū)南京總醫(yī)院)消化內(nèi)科，江蘇 南京 210002

非酒精性脂肪性肝病(non-alcoholic fatty liver disease，NAFLD)已經(jīng)成為最常見的慢性肝臟疾病之一。近年來研究發(fā)現(xiàn)，腸-肝軸在NAFLD的發(fā)生、發(fā)展中起著重要作用。作為腸-肝軸重要組成部分，腸道菌群通過增加宿主能量攝入、調(diào)節(jié)膽堿及膽汁酸代謝、激活模式識(shí)別受體(pattern recognition receptors,PRRs)而促發(fā)炎癥反應(yīng)等機(jī)制，促使NAFLD的發(fā)生、發(fā)展；益生菌對(duì)NAFLD有一定療效進(jìn)一步證實(shí)腸道菌群在NAFLD中發(fā)揮著重要作用。本文對(duì)腸道菌群在NAFLD發(fā)病中可能的作用機(jī)制作一概述。

非酒精性脂肪性肝?。荒c道菌群；腸-肝軸；模式識(shí)別受體；益生菌

非酒精性脂肪性肝病(non-alcoholic fatty liver disease，NAFLD)是指除酒精和其他明確肝損傷因素外所致的，以彌漫性肝細(xì)胞大泡性脂肪變?yōu)橹饕卣鞯呐R床病理綜合征[1]。在美國(guó)，非酒精性脂肪性肝炎(non-alcoholic steatohepatitis，NASH)已經(jīng)成為第2大肝移植的病因[2]，然而目前對(duì)其發(fā)病機(jī)制尚未完全闡明。經(jīng)典的“二次打擊”已被擴(kuò)展為“多次平行打擊”[3]，其中，腸道菌群是腸道來源的重要致病因子，參與調(diào)控腸道免疫功能[4]。近年來研究表明，腸道菌群與NAFLD密切相關(guān)[5-6]，腸道中某些細(xì)菌甚至與NAFLD炎癥及纖維化程度相關(guān)[7]。益生菌、益生元在治療NAFLD中有一定的效果[8-9]。本文主要概述近年來腸道菌群在NAFLD發(fā)生、發(fā)展中所起到的作用。

1 腸道菌群與腸-肝軸

近年來，腸道菌群受到研究者的普遍關(guān)注。Qin等[10]研究發(fā)現(xiàn)，人體共含有1 000～1 150種細(xì)菌，腸道細(xì)菌基因量多達(dá)330萬，約為人體基因的150倍；Le Chatelier等[11]研究發(fā)現(xiàn)，與腸道菌群豐度較高組相比，腸道菌群豐度較低組表現(xiàn)為胰島素抵抗(insulin resistance,IR)、血脂異常及更明顯的炎癥表型等。多項(xiàng)研究[5，12]發(fā)現(xiàn)，腸道菌群與NAFLD密切相關(guān)，如與正常對(duì)照組相比，NAFLD患者腸道中厚壁菌門與擬桿菌門之比增大；此外，研究[13-14]發(fā)現(xiàn)，雙歧桿菌及乳酸桿菌的含量在NAFLD患者中低于正常對(duì)照，而雙歧桿菌及乳酸桿菌等益生菌的增加可改善NAFLD的病情[9]。

目前認(rèn)為,腸-肝軸是腸道菌群作用于NAFLD的重要途徑。腸-肝軸是指腸道與肝臟在解剖及功能上緊密聯(lián)系的系統(tǒng)，在人體解剖上，肝臟接收來自腸系膜上靜脈及脾靜脈匯合成的門靜脈血；在功能上，腸道及肝臟在物質(zhì)營(yíng)養(yǎng)的吸收及代謝中共同發(fā)揮重要作用。門靜脈血不僅含有各種營(yíng)養(yǎng)物質(zhì)，還可含有各種信號(hào)分子及細(xì)菌代謝產(chǎn)物，如脂多糖(LPS)、細(xì)菌DNA等[15]。當(dāng)小腸細(xì)菌過度生長(zhǎng)時(shí)，腸道中有害物質(zhì)增多，腸道通透性增加，內(nèi)毒素血癥形成，而LPS等通過門靜脈進(jìn)入肝臟，激活模式識(shí)別受體，促發(fā)肝臟發(fā)生炎癥反應(yīng)[15-16]。

2 腸道菌群相關(guān)的NAFLD發(fā)病機(jī)制

2.1模式識(shí)別受體(patternrecognitionreceptors，PRRs)及炎癥反應(yīng)PRRs是一類可識(shí)別入侵細(xì)胞的微生物信號(hào)或無菌性信號(hào)，引起細(xì)胞因子轉(zhuǎn)錄表達(dá)進(jìn)而促發(fā)一系列炎癥反應(yīng)的受體，可分為TLRs、CLRs、RLRs及NLRs四大類，其中TLRs及NLRs近年來頗受研究者重視。TLRs可通過MyD88依賴途徑及TRIF依賴途徑激活MAPK及NF-κB或激活I(lǐng)RF3，進(jìn)而使腫瘤壞死因子及白介素等細(xì)胞因子釋放，誘發(fā)一系列炎癥反應(yīng)，如TLR4、TLR9等與LPS、細(xì)菌DNA等結(jié)合，激活下游信號(hào)，最終使肝臟發(fā)生炎癥反應(yīng)[17]。此外，最新研究[18]發(fā)現(xiàn)，當(dāng)細(xì)菌、病毒等被TLR7識(shí)別后，肝臟釋放IGF-1，促發(fā)細(xì)胞發(fā)生自噬，及時(shí)清除病原菌，從而在NAFLD中起保護(hù)作用。NLRs又稱炎癥小體，其通過炎癥小體裝配及激活Caspase-1，進(jìn)而使IL-1β成熟、釋放[19]。在眾多炎癥小體中，NLRP3最受研究者關(guān)注，在與NAFLD相關(guān)的研究[20]中發(fā)現(xiàn)，NLRP3識(shí)別模式是NAFLD發(fā)生脂肪性肝炎、纖維化的重要信號(hào)通路。另外，NLRP6也與腸道菌群密切相關(guān)，其主要在小腸上皮細(xì)胞表達(dá)，參與維持正常的腸道屏障功能，從而在腸-肝對(duì)話中發(fā)揮重要作用，因此與NAFLD的發(fā)生、發(fā)展密切相關(guān)[21]。Henao-Mejia等[5]發(fā)現(xiàn)，普雷沃氏菌科及紫單胞菌科可能與NAFLD的發(fā)病密切相關(guān)，更重要的是，研究中發(fā)現(xiàn)是IL-18而非普遍認(rèn)識(shí)的IL-1β介導(dǎo)腸道菌群在NAFLD中發(fā)揮重要作用。

2.2增加能量攝入NAFLD與能量攝入增加相關(guān)。高熱量飲食，如富含反式/飽和脂肪酸、膽固醇食物及果糖飲料等，可增加肝臟內(nèi)脂質(zhì)的積累[22]。早期研究[23]發(fā)現(xiàn)，無菌小鼠接觸正常小鼠腸道細(xì)菌2周后，即使飲食量減少，它們體內(nèi)脂肪含量卻比對(duì)照的無菌小鼠增多60%，并出現(xiàn)IR，其作用機(jī)制為：腸道菌群促進(jìn)腸道對(duì)單糖的吸收，激活碳水化合物反應(yīng)元件結(jié)合蛋白及固醇調(diào)節(jié)元件結(jié)合蛋白，或通過抑制小腸上皮產(chǎn)生空腹誘導(dǎo)脂肪細(xì)胞因子，進(jìn)而激活脂蛋白脂酶活性，從而增加甘油三酯的儲(chǔ)存。Turnbaugh等[24]發(fā)現(xiàn)，肥胖小鼠腸道中厚壁菌門增多，后者可將食物分解為短鏈脂肪酸，一方面促進(jìn)糖及脂肪合成，使能量產(chǎn)生增加，另一方面抑制AMPK活性，減少脂肪酸β氧化，從而使游離脂肪酸在肝臟中累積。

2.3調(diào)節(jié)膽汁酸代謝研究發(fā)現(xiàn)，初級(jí)膽汁酸與NAFLD脂肪變性程度相關(guān)，甚至與NASH的纖維化程度相關(guān)[25]，其作用機(jī)制可能為初級(jí)膽汁酸可增加腸黏膜通透性，產(chǎn)生內(nèi)毒素血癥，進(jìn)而引起IR使NAFLD病情加重[26]。另外，膽汁酸還調(diào)控多種受體，包括法尼酯X受體(FXR)、TGR5等。Jiang等[27]研究發(fā)現(xiàn)，在小鼠高脂模型中，肝臟合成?；铅率竽懰?tauro-β-muricholic acid，T-β-MCA)增多，T-β-MCA在回腸中轉(zhuǎn)化為鼠膽酸(MAC)，后者激活FXR后促使神經(jīng)酰胺釋放，進(jìn)而通過激活SREBP-1C使脂肪從頭合成增加，而在抗生素處理后的小鼠中，其肝臟脂肪變性可發(fā)生逆轉(zhuǎn)。最近研究[25]發(fā)現(xiàn)，NASH患者膽汁酸合成量比正常對(duì)照者增多，初級(jí)膽汁酸與次級(jí)膽汁酸之比高于正常組，并發(fā)現(xiàn)柔嫩梭菌(C.leptum)與?；悄懰岢收龋c膽酸及鵝脫氧膽酸呈反比，提示C.leptum可能有助于將初級(jí)膽汁酸轉(zhuǎn)化為次級(jí)膽汁酸，從而減輕初級(jí)膽汁酸對(duì)肝臟的損傷。

2.4調(diào)節(jié)膽堿代謝膽堿缺乏可引起NAFLD，其機(jī)制包括：增加脂肪合成及儲(chǔ)存，減少甘油三酯輸出等[28]。此外，近年來，腸道菌群與膽堿代謝的關(guān)系在NAFLD發(fā)病中作用受到研究者的關(guān)注。腸道中廣泛存在代謝膽堿的細(xì)菌，主要包括變形菌門、放線菌門及厚壁菌門，將可代謝膽堿的細(xì)菌植入無菌小鼠腸道中，發(fā)現(xiàn)膽堿生物利用度下降，其效果相當(dāng)于膽堿缺乏飲食而引起NAFLD[29]。一項(xiàng)前瞻性臨床研究[30]發(fā)現(xiàn)，經(jīng)過42 d膽堿缺乏的飲食后，腸道中γ-變形菌綱明顯降低，可能機(jī)制是γ-變形菌綱含有大量的代謝磷脂酰膽堿的酶，故該菌豐度越高，膽堿消耗量越大。研究者認(rèn)為,結(jié)合基因多態(tài)性因素，腸道中γ-變形菌豐度升高者對(duì)膽堿缺乏更敏感，發(fā)生NAFLD的風(fēng)險(xiǎn)更大。

2.5其他相關(guān)機(jī)制內(nèi)源性酒精增加腸道菌群失調(diào)，如大腸桿菌(Escherichiacoli)豐度升高可引起腸道內(nèi)源性酒精增多，腸道黏膜通透性增大，內(nèi)毒素、酒精等通過門靜脈直接對(duì)肝臟進(jìn)行打擊，或進(jìn)一步激活PRRs，使NAFLD向NASH發(fā)展[13]。

腸道動(dòng)力降低腸道菌群失調(diào)，促使腸道釋放出酪酪肽(PYY)，后者可使胃排空延遲及小腸蠕動(dòng)減慢，而腸道動(dòng)力降低，可使?fàn)I養(yǎng)物質(zhì)吸收不良，進(jìn)一步促進(jìn)腸道菌群失調(diào)，形成惡性循環(huán)，促使NAFLD發(fā)生、發(fā)展[16]。

3 益生菌與益生元

益生菌是一類定植于人體腸道、生殖系統(tǒng)內(nèi)，能產(chǎn)生確切健康功效、改善宿主微生態(tài)平衡的活性微生物的總稱；益生元?jiǎng)t是不易消化的碳水化合物，可改變腸道菌群的組成及活性[31]。臨床研究[9，32-33]表明，益生菌和/或益生元可以使NAFLD患者血清轉(zhuǎn)氨酶下降，HOMA-IR、血清甘油三酯及膽固醇等降低；在動(dòng)物實(shí)驗(yàn)[8]中發(fā)現(xiàn)，補(bǔ)充益生菌可使肝臟脂肪沉積減少，肝臟組織炎性改變程度減輕。益生菌和/或益生元作用的機(jī)制主要包括：(1)調(diào)節(jié)腸道菌群，如增加雙歧桿菌、乳酸桿菌等[34]；(2)降低腸道通透性，減輕內(nèi)毒素血癥[8]；(3)調(diào)節(jié)血糖，改善IR[9]；(4)調(diào)節(jié)脂質(zhì)代謝[35]；(5)產(chǎn)生短鏈脂肪酸，發(fā)揮抗炎作用[8]。以上療效及作用機(jī)制從另一角度反映了腸道菌群在NAFLD發(fā)病中的重要作用。然而，目前益生菌/益生元在NAFLD中的療效多基于動(dòng)物實(shí)驗(yàn)，各臨床試驗(yàn)之間的療效也不一致，可能與樣本量不足、研究設(shè)計(jì)、納入人群不同等有關(guān)，迫切需要高質(zhì)量的多中心臨床隨機(jī)試驗(yàn)來提供更可靠的證據(jù)。

目前，腸道菌群在NAFLD中的作用主要基于腸-肝軸的作用機(jī)制，即通過改變腸道中菌群組成成分，參與炎癥反應(yīng)，調(diào)控宿主能量代謝，參與調(diào)節(jié)膽汁酸及膽堿代謝，以及產(chǎn)生內(nèi)源性酒精等，從而促進(jìn)NAFLD發(fā)生、發(fā)展。腸道菌群是一個(gè)復(fù)雜的生態(tài)系統(tǒng)，其作用與功能尚未完全明確，未來需要多種檢測(cè)技術(shù)如蛋白組學(xué)、代謝組學(xué)、轉(zhuǎn)錄組學(xué)、宏基因組學(xué)等相互配合使用以闡明其作用機(jī)制。同時(shí)，為更加清楚地了解腸道菌群的功能，傳統(tǒng)細(xì)菌培養(yǎng)方法不能忽視，將培養(yǎng)法及分子生態(tài)學(xué)檢測(cè)法相結(jié)合，可以更加全面了解腸道菌群在NAFLD中的作用。

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(責(zé)任編輯：王全楚)

Theroleofgutmicrobiotainnon-alcoholicfattyliverdisease

YANG Zhao, WANG Fangyu, YANG Miaofang

Department of Gastroenterology, Jinling Hospital, Southern Medical University/Nanjing General Hospital of Nanjing Military Region, PLA, Nanjing 210002, China

Non-alcoholic fatty liver disease (NAFLD) has become one of the most common chronic liver diseases. Recent years, many studies have found that gut-liver axis plays an important role in the pathogenesis of NAFLD. As an important part of the gut-liver axis, gut microbiota contributes to the development of NAFLD by enhancing energy intake, regulating the choline and bile acid metabolism, and activating the pattern recognition receptors (PRRs) which promote inflammation. Probiotics are beneficial for NAFLD, which support that gut microbiota plays an important role in NAFLD. This article reviewed the possible role of gut microbiota in the pathogenesis of NAFLD.

Non-alcoholic fatty liver disease; Gut microbiota; Gut-liver axis; Pattern recognition receptors; Probiotics

R575.5

A

1006-5709(2017)10-1100-03

2016-11-06

國(guó)家自然科學(xué)基金(81370546)

楊釗，碩士，研究方向：腸道微生態(tài)與脂肪肝。E-mail：zhaoyangjinrui@hotmail.com

楊妙芳，博士，副主任醫(yī)師，研究方向：腸道微生態(tài)與脂肪肝、肝癌的發(fā)病機(jī)制。E-mail：miaofangyang@hotmail.com

10.3969/j.issn.1006-5709.2017.10.007