周達，范建高

·導(dǎo)向與述評·

腸道菌群與非酒精性脂肪性肝病研究進展

周達，范建高

非酒精性脂肪性肝?。╪onalcoholic fatty liver disease,NAFLD）發(fā)病率不斷升高，對其發(fā)生、發(fā)展及防治研究變得極為迫切。近年來腸道菌群被認為是機體一個重要的“特殊器官”，參與機體代謝及相關(guān)疾病的發(fā)生、發(fā)展，與NAFLD關(guān)系密切。本文就腸道菌群與NAFLD的關(guān)系研究進行綜述。

脂肪肝；腸桿菌科；生態(tài)失調(diào)；內(nèi)毒素類；乙醇

非酒精性脂肪性肝?。╪onalcoholic fatty liver disease,NAFLD）發(fā)病率逐年增高，已成為我國第一大慢性肝病[1]。它是遺傳-環(huán)境-代謝應(yīng)激相關(guān)性疾病，以肝實質(zhì)細胞脂肪變性和脂肪蓄積為特征，其疾病譜包括單純性脂肪肝、非酒精性脂肪性肝炎（nonalcoholic steatohepatitis,NASH）、肝硬化甚至原發(fā)性肝癌[2]。NAFLD發(fā)病機制復(fù)雜[3]，近年研究表明機體腸道菌群與NAFLD關(guān)系密切，為NAFLD發(fā)病機制及防治研究提供了新的切入點。

機體腸道內(nèi)寄居著大量厭氧和兼性需氧細菌共生群體，可以看作是一個位于腸道內(nèi)的特殊微生物器官，受宿主基因型及環(huán)境雙重調(diào)控[4-5]。腸道菌群數(shù)量可達1014，其所含基因總量是人類的100倍之多，主要由厚壁菌門、擬桿菌門、放線菌門、疣微菌門、變形菌門等構(gòu)成，以前二種為主（約占總體的90%）[6]，影響著宿主發(fā)育、能量代謝和免疫功能，對機體各種生理病理活動發(fā)揮重要調(diào)節(jié)作用[7]。隨著研究深入，腸道菌群與代謝性疾病之間的復(fù)雜機制越來越多得以闡述[8]。本文就腸道菌群與NAFLD之間的復(fù)雜調(diào)控關(guān)系進行闡述。

1 腸道菌群與肥胖和能量代謝

肥胖和胰島素抵抗（insulin resistance,IR）是NAFLD發(fā)生發(fā)展的二大最重要因素[9]。大量人體和動物學(xué)研究均顯示肥胖與腸道菌群密切相關(guān)[10]。肥胖者體內(nèi)腸道菌群結(jié)構(gòu)失調(diào)，厚壁菌門和放線菌門含量比例顯著升高而擬桿菌門下降。通過移植肥胖者菌群至正常者，可引起后者明顯肥胖，促進能量代謝吸收增加[11-12]，而移植正常者菌群是否能夠減輕肥胖、減少能量攝入仍不十分明確，但前景依然樂觀。一項人體學(xué)研究提示，通過腸道菌群移植，增加了代謝綜合征患者的胰島素敏感性，同時改善其腸道菌群結(jié)構(gòu)[13]。值得一提的是，腸道菌群與肥胖的復(fù)雜關(guān)系不能簡單理解為厚壁菌門和擬桿菌門2種優(yōu)勢菌門比例失調(diào)造成，還包括內(nèi)部微生物群結(jié)構(gòu)的變化[14]。

Mouzaki等[15]研究顯示，NASH患者腸道內(nèi)擬桿菌含量比例比單純性脂肪肝患者和健康對照者低，其糞便中球形梭菌含量比單純性脂肪肝患者高，提示NASH的發(fā)病與糞便中擬桿菌的含量呈負相關(guān)，且不受飲食/體質(zhì)量指數(shù)影響的。另一項研究也表明，腸道菌群可以不依賴于肥胖而獨立促進NAFLD的發(fā)生，同時使機體對高脂飲食產(chǎn)生差異性反應(yīng)[16]。

由此可知，腸道菌群與代謝性疾病關(guān)系密切，與NAFLD之間并非僅存在間接作用，而且存在相互的直接作用，后者更值得進一步探索，以發(fā)現(xiàn)其中的奧秘。

2 腸道菌群與內(nèi)毒素

關(guān)于NAFLD的發(fā)病機制，較為認可的是“二次打擊”學(xué)說。第一次打擊主要是IR增加了肝細胞內(nèi)脂質(zhì)沉積；第二次打擊主要是內(nèi)質(zhì)網(wǎng)應(yīng)激、線粒體功能障礙、脂質(zhì)過氧化及多種炎癥因子的釋放[3]，上述機制均與腸道菌群密不可分。腸道菌群失調(diào)或過度生長可以引起肥胖及IR已無爭議[17-18]，此外還可以導(dǎo)致腸道內(nèi)細菌毒素（如脂多糖）產(chǎn)生增多，破壞腸黏膜結(jié)構(gòu)，使其通透性增加，促進毒素吸收和菌群移位，由門靜脈入肝而進一步促進NAFLD發(fā)展[19]。同時，人體學(xué)及動物學(xué)均證實患NAFLD的機體存在小腸細菌過度生長及腸道通透性增加[20-21]，二者互為促進，形成不良循環(huán)。

細菌毒素吸收增加，可通過門靜脈直接造成肝細胞損害，同時脂多糖可以作用于肝內(nèi)Kupffer細胞及星狀細胞表面多種Toll樣受體（Toll-like receptor, TLR），特別是TLR-4，激活受體下游MyD88和轉(zhuǎn)錄因子——核因子κB，最終促進其分泌各種促炎因子，如腫瘤壞死因子（tumor necrosis factor,TNF）-α以及白細胞介素（interleukin,IL）-1、IL-6、IL-18和IL-12等[3，22-23]。這些炎癥因子又進一步促進IR、肝內(nèi)脂質(zhì)過氧化、內(nèi)質(zhì)網(wǎng)應(yīng)激以及線粒體功能障礙，最終導(dǎo)致NAFLD的發(fā)展惡化[24-25]。但細菌毒素并非僅僅影響了促炎因子，而是造成促炎與抗炎因子失衡使疾病進展[22]。

此外，菌群異?？梢砸种颇c上皮細胞分泌饑餓誘導(dǎo)脂肪細胞因子，后者為脂肪酶抑制物，調(diào)節(jié)外周脂肪代謝，其被抑制后可以增加脂肪酶活性，促進脂肪細胞甘油三酯異位至肝臟，促進NAFLD的發(fā)展[19]。

3 腸道菌群與乙醇

近年來，眾多研究發(fā)現(xiàn)：排除體外乙醇攝入情況，糖尿病和NASH患者體內(nèi)乙醇含量均較健康對照者升高，意味著這部分患者自體產(chǎn)生過多乙醇，即內(nèi)生性乙醇，同時NASH患者肝內(nèi)代謝乙醇相關(guān)基因表達明顯升高[26-28]；肥胖患者或肥胖小鼠的血乙醇濃度較正常體重者增加[29-30]；Zhu等[26]研究還發(fā)現(xiàn)，NASH患者的腸道菌群與肥胖者和健康者的腸道菌群存在明顯的差異，NASH患者腸道內(nèi)大腸埃希菌明顯多于肥胖者。大腸埃希菌能夠發(fā)酵產(chǎn)生乙醇，是內(nèi)源性乙醇的主要來源[31]。內(nèi)源性乙醇會被肝臟乙醇脫氫酶、過氧化氫酶及微粒體乙醇氧化系統(tǒng)代謝清除，但當(dāng)乙醛脫氫酶活性過低時，造成乙醛代謝障礙，導(dǎo)致蓄積。乙醇和乙醛對組織和肝臟均可造成直接的氧化損傷，通過激活體內(nèi)相關(guān)酶活性，導(dǎo)致體內(nèi)氧自由基產(chǎn)生增多和脂質(zhì)過氧化。同時，乙醛可以損害腸黏膜，導(dǎo)致腸黏膜通透性增加，內(nèi)毒素吸收增多，促進NASH的發(fā)生及持續(xù)進展[32-33]。

在正常情況下，人體內(nèi)會不斷產(chǎn)生乙醇[34]，腸道菌群發(fā)酵是內(nèi)源性乙醇的主要來源，小腸細菌過度生長及腸道菌群紊亂均可導(dǎo)致內(nèi)源性乙醇增多。由此可見，腸道菌群體內(nèi)發(fā)酵不容忽視，其代謝發(fā)酵產(chǎn)物對人體病理生理至關(guān)重要[35]，除了存在對機體不利的產(chǎn)物，也有相當(dāng)多產(chǎn)物對機體有利，比如短鏈脂肪酸、維生素等[36]。因此如何高效、優(yōu)化利用菌群代謝，使其對人體產(chǎn)生更加有益的作用有待進一步探討。

4 腸道菌群與免疫

腸肝軸是肝腸對話的一個重要通路，除了傳輸各種營養(yǎng)、生理活性物質(zhì)、眾多病原性和免疫性物質(zhì)，甚至推測淋巴細胞也通過腸肝軸循環(huán)由腸道轉(zhuǎn)移至肝臟，參與NAFLD發(fā)生發(fā)展[37-39]。腸道菌群對機體免疫系統(tǒng)的發(fā)育成熟至關(guān)重要，同時機體免疫系統(tǒng)也影響宿主腸道菌群的最終結(jié)構(gòu)[40-42]。

腸道黏膜及黏膜下層存在數(shù)量和種類眾多的免疫細胞，與腸道菌群相互作用。腸道菌群可以調(diào)控腸道不同淋巴細胞亞群［如輔助性T細胞（T helper cells,Th）1/Th2、調(diào)節(jié)性T細胞/Th17］的分化，調(diào)節(jié)B細胞、自然殺傷T細胞、樹突狀細胞、巨噬細胞等免疫細胞功能活性，維持黏膜免疫系統(tǒng)中促炎與抗炎機制之間的動態(tài)平衡，是黏膜免疫穩(wěn)態(tài)的關(guān)鍵調(diào)節(jié)因素，避免自身免疫[43-45]；其失調(diào)可以導(dǎo)致免疫細胞比例失調(diào)，黏膜免疫穩(wěn)態(tài)被打破，誘發(fā)炎性因子分泌紊亂，可通過腸肝軸直接影響到肝臟[42]。但是，腸道淋巴細胞特異性歸巢入肝仍需要更多強有力的直接證據(jù)，而通過對腸道免疫干預(yù)是否可以防治NAFLD值得進一步探討。

5 腸道菌群與NAFLD治療

腸道菌群在NAFLD發(fā)生發(fā)展中的重要作用為NAFLD的干預(yù)和防治提供了嶄新的思路，調(diào)節(jié)腸道菌群以改善NAFLD值得深層次研究[46]。

益生菌對各種病因所致慢性肝病均有改善作用。針對NAFLD的益生菌療法不論在臨床上還是動物實驗中已得到支持[46-47]，其可以提高宿主免疫力，抑制有害菌生長及移位，保護腸黏膜屏障，減少毒素吸收，顯著降低肝內(nèi)促炎因子（如干擾素γ、TNF-α、IL-1β、IL-6等）的表達分泌，改善IR，減輕NAFLD患者體內(nèi)氧化/氮化應(yīng)激，明顯改善肝臟病理過程[46，48]。此外，可進行口服抗生素（如多粘菌素B）治療，直接清除過度繁殖的致病菌，但該種方法容易破壞腸道正常定植菌，造成細菌耐藥，存在一定的風(fēng)險[46]。雖然有較多實驗為益生菌治療NAFLD提供了依據(jù)，但還須進行大樣本臨床隨機對照試驗。益生菌的真實療效有待進一步明確。

我們在研究通過調(diào)節(jié)腸道菌群治療NAFLD時，目光不僅要聚焦腸道菌群本身，更要關(guān)注其代謝產(chǎn)物（如短鏈脂肪酸等）的作用。目前同樣有眾多研究提示菌群代謝產(chǎn)物在改善和調(diào)節(jié)機體代謝方面發(fā)揮不可或缺的作用[49]，也許二者聯(lián)合可以提供更完整有效的NAFLD防治策略。

6 結(jié)語

NAFLD已成為全球性的第一大慢性肝病，并在未來長時間內(nèi)將持續(xù)存在，但是至今沒有明確的理想防治策略。深入研究腸道菌群在NAFLD發(fā)病和進展中所起的作用，使得發(fā)現(xiàn)該類疾病嶄新的預(yù)防和治療策略成為可能，也進一步使探索腸道菌群或其代謝相關(guān)產(chǎn)物作為對NAFLD系列疾病預(yù)測評估指標(biāo)成為可能。

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（2015-04-21收稿2015-06-05修回）

（責(zé)任編委李軍本文編輯王姝）

Research progress of the correlation between intestinalm icrobiota and nonalcoholic fatty liver disease

ZHOU Da,FAN Jian-gao*
Department of Gastroenterology,Xinhua Hospital,Shanghai Jiaotong University School of Medicine,Shanghai200092,China
*Corresponding author,E-mail:fattyliver2004@126.com

Currently,themorbidity ofnonalcoholic fatty liver disease(NAFLD)is increasing,so it is extremely pivotal to study the development,progression,prevention and treatmentof NAFLD.Intestinalmicrobiota,as an importantand special organ of the body, intimately participates in the metabolism of the body,and the development and progression of NAFLD as well.In this article,the correlation between intestinalmicrobiota and NAFLD is reviewed.

fatty liver;Enterobacteriaceae;dysbiosis;endotoxins;ethanol

R575.5；R378.1

A

1007-8134(2015)04-0200-04

10.3969/j.issn.1007-8134.2015.04.003

國家重點基礎(chǔ)研究發(fā)展計劃（973計劃）項目（2012CB 517501）；國家自然基金（81070322、81270491、81470840）；上海市科委課題（0914090350、10411956300）；中國肝炎防治基金（XJS20120 501）；上海市衛(wèi)生局百人計劃（XBR 2011007）

200092，上海交通大學(xué)醫(yī)學(xué)院附屬新華醫(yī)院消化內(nèi)科（周達、范建高）

范建高，E-mail:fattyliver2004@126.com