腸道微生態(tài)失衡與肝性腦病發(fā)病的研究進(jìn)展

張 柳， 田德安

1.華中科技大學(xué)同濟(jì)醫(yī)學(xué)院，湖北 武漢 430000；2.華中科技大學(xué)同濟(jì)醫(yī)學(xué)院附屬同濟(jì)醫(yī)院消化內(nèi)科

肝性腦病(hepatic encephalopathy，HE)是由嚴(yán)重肝病引起的、以代謝紊亂為基礎(chǔ)的中樞神經(jīng)系統(tǒng)功能失調(diào)綜合病征，表現(xiàn)為亞臨床改變到昏迷的各種神經(jīng)或精神異常，分為輕型肝性腦病(mild hepatic encephalopathy，MHE)和顯性肝性腦病(overt hepatic encephalopathy，OHE)[1]。過去幾十年有關(guān)HE的發(fā)病機(jī)理研究中主要有氨中毒學(xué)說、氨基酸失衡學(xué)說、假性神經(jīng)遞質(zhì)學(xué)說、GABA-BZ復(fù)合體學(xué)說、毒物的協(xié)同作用學(xué)說。其中高血氨仍是HE發(fā)生的重要因素。最開始人們普遍認(rèn)為，HE是由肝臟解毒功能下降和/或門靜脈-體循環(huán)分流導(dǎo)致循環(huán)系統(tǒng)毒物入腦引起的腦功能障礙[2]。而近年來研究[3-4]發(fā)現(xiàn)，腸道微生物在導(dǎo)致HE發(fā)生的高血氨、炎癥發(fā)生中扮演重要角色。肝硬化狀態(tài)下，腸道黏膜屏障破壞、腸道菌群失調(diào)，導(dǎo)致腸道菌群易位、小腸細(xì)菌過度生長(small intestine bacterial overgrowth，SIBO)、代謝產(chǎn)物異常、腸道局部及全身免疫激活、炎癥反應(yīng)等，最終導(dǎo)致循環(huán)系統(tǒng)血氨及炎癥因子水平升高，導(dǎo)致血腦屏障破壞、腦細(xì)胞水腫、代謝異常，引發(fā)HE[5]。因此，具有糾正腸道微生態(tài)失調(diào)功能的益生菌將有望成為治療HE的新方法。

1 腸道正常微生物

正常人腸道內(nèi)微生物數(shù)量達(dá)1010個(gè),是人體細(xì)胞數(shù)量的10倍[6]。腸道微生物種類和數(shù)量受年齡、性別、種族，甚至分娩方式、懷孕年齡、抗生素應(yīng)用、飲食、衛(wèi)生條件的影響[7-8]，由于其多樣、多態(tài)且影響因素多、檢測技術(shù)問題等，研究其對人類病理生理狀態(tài)的影響十分復(fù)雜。近年隨著新的微生物檢測技術(shù)(焦磷酸測序技術(shù))誕生和新的菌群研究為解決這一問題帶來了突破。研究發(fā)現(xiàn)，微生物絕非隨機(jī)組合，2011年ARUMUGAM等[9]發(fā)現(xiàn)人類腸道微生物大致分3種菌屬，即擬桿屬、普氏屬、瘤胃球菌屬，正常情況下這3種菌屬在腸道中占優(yōu)勢，且處于穩(wěn)態(tài)。一旦人體發(fā)育成熟，這樣的腸道菌群組合不受年齡、種族、體質(zhì)量指數(shù)、性別的影響，但機(jī)體消化代謝、功能狀態(tài)會(huì)使微生物比例發(fā)生改變[10]。

2 微生態(tài)失衡與肝硬化、HE

菌群失調(diào)是指機(jī)體某部位正常菌群間各菌種比例發(fā)生較大幅度改變并超過正常幅度的狀態(tài)，臨床常用CDR(腸道有益菌與有害菌的比值)評估菌群失調(diào)的嚴(yán)重程度[11]。研究[12]發(fā)現(xiàn)，在肝硬化、HE患者的腸道中存在嚴(yán)重的菌群失調(diào)。門靜脈高壓引起的腸道淤血、水腫、缺氧，一方面影響腸道動(dòng)力學(xué)，導(dǎo)致腸道自主清除能力下降，使過路菌接觸、黏附黏膜的概率增加；另一方面破壞黏膜屏障，腸壁局部抵抗力下降，使得各種致病菌大量繁殖，導(dǎo)致SIBO，多種因素綜合作用致使腸腔內(nèi)微生態(tài)環(huán)境遭到嚴(yán)重破壞、菌群比例失調(diào)。研究[13]發(fā)現(xiàn)，CDR在不同生理病理狀態(tài)下有顯著性差異，HE組顯著低于肝硬化組和正常對照組，且CDR與血清內(nèi)毒素水平呈負(fù)相關(guān)。BAJAJ等[12-14]多項(xiàng)研究表明，MHE與正常對照組之間存在腸道菌群的差異，前者糞便標(biāo)本擬桿菌、疣微菌科減少，腸桿菌、梭桿菌、韋榮球菌、鏈球菌、葡萄球菌、變形桿菌、梭狀芽胞桿菌、普氏菌科增加，并且另有研究[15]顯示，腸道內(nèi)存在不同程度的SIBO，其發(fā)生率為35%～61%。進(jìn)一步研究發(fā)現(xiàn)，細(xì)菌比例失衡與肝功能及認(rèn)知受損無單純的相關(guān)性，CHEN等[13]發(fā)現(xiàn),Child-Turcotte-Pugh (CTP)評分與鏈球菌比例呈正相關(guān)，與毛螺旋菌呈負(fù)相關(guān)。終末期肝病模型(MELD)評分與腸桿菌科比例呈正相關(guān)，與疣微菌科比例呈負(fù)相關(guān)。此外,SIBO與HE嚴(yán)重程度相關(guān)，CTP評分越高的患者SIBO發(fā)生率越高[16-17]。由此說明肝硬化、HE患者腸道確實(shí)存在不同程度的菌群紊亂。目前調(diào)節(jié)腸道菌群的藥物成功治療HE更證實(shí)了該觀點(diǎn)。MARLICZ等[18-19]采用益生菌、合生元、利福昔明治療后發(fā)現(xiàn)，腸道菌群比例發(fā)生改變，腸桿菌科豐度減少，乳酸桿菌比例增加，同時(shí)伴隨血氨水平的降低，MHE的逆轉(zhuǎn)、OHE發(fā)生率顯著降低。也有研究[20]表明，肝移植能通過調(diào)節(jié)腸道菌群改善大腦的認(rèn)知功能。因此，我們猜測菌群失調(diào)與HE的發(fā)生之間可能存在關(guān)聯(lián)性。

腸道菌群紊亂如何導(dǎo)致HE的發(fā)生、發(fā)展，涉及到腸道代謝產(chǎn)物的異常、黏膜屏障功能受損、SIBO的背景下引發(fā)高血氨癥、炎癥激活等多個(gè)環(huán)節(jié)。

3 腸道微生物、高血氨與HE

4 腸道微生物、炎癥與HE

雖然高血氨癥在HE發(fā)病機(jī)制中占有重要地位，但近年有研究[11,22,30-31]發(fā)現(xiàn)，由腸道菌群失調(diào)所致的炎癥在HE中發(fā)揮了作用。在部分認(rèn)知功能嚴(yán)重受損的HE患者中無明顯高氨血癥，但炎癥因子水平顯著升高，包括IL-10、IL-6、IL-2和TNF-α[30]，且發(fā)現(xiàn)炎癥能協(xié)同氨加重大腦認(rèn)知功能受損[21,27,31-32]。研究發(fā)現(xiàn),肝硬化時(shí)存在嚴(yán)重的菌群失調(diào)、SIBO，導(dǎo)致毒性物質(zhì)(脂多糖、肽糖和微生物核酸[14])產(chǎn)生增多、排泄減少，大量積聚在腸腔，加之肝硬化時(shí)腸道黏膜屏障受到嚴(yán)重破壞，這些毒性物質(zhì)能與腸道上皮細(xì)胞、肝細(xì)胞上多種模式識(shí)別受體(如TLR2)相互作用[33-34]。這樣的相互作用通過MyD88-NF-κB-依賴通路激活下游的信號(hào)通路，促炎因子增加，同時(shí)下調(diào)TGF-β使抑炎反應(yīng)減弱，引起炎癥瀑布級(jí)聯(lián)反應(yīng)[18]，最終導(dǎo)致HE的發(fā)生。炎癥因子導(dǎo)致大腦認(rèn)知功能受損的機(jī)制尚不明確,可能與炎癥因子導(dǎo)致腦血流量改變、血腦屏障的通透性增加、腦細(xì)胞水腫、谷氨酰胺合成減少有關(guān)[5]。

動(dòng)物模型研究證實(shí)，微生物失調(diào)是肝硬化發(fā)生大腦炎癥的關(guān)鍵，研究發(fā)現(xiàn)，與GF(腸道無微生物)肝硬化組比較，在腸道菌群失調(diào)肝硬化小鼠中有明顯的小膠質(zhì)細(xì)胞、星形膠質(zhì)細(xì)胞的激活，小腦IL-1b、MCP-1和皮層IL-1b的mRNA表達(dá)顯著增加，而抗炎因子IL-10的水平下降[35]。多項(xiàng)臨床研究[14,19]也發(fā)現(xiàn)了腸道微生物紊亂與炎癥的發(fā)生存在關(guān)聯(lián)性。腸桿菌科、韋榮球菌和梭桿菌科細(xì)菌、腸球菌、產(chǎn)堿桿菌、紫單胞菌比例增加和疣微菌科比例減少的患者血液炎癥水平更高，且認(rèn)知功能更差。由此可見，腸道菌群異常是導(dǎo)致HE炎癥因子升高的關(guān)鍵。此外，炎癥的存在確實(shí)能導(dǎo)致大腦認(rèn)知功能的損傷，小鼠模型研究發(fā)現(xiàn)，腹腔注射脂多糖所導(dǎo)致的炎癥反應(yīng)會(huì)誘導(dǎo)肝硬化前驅(qū)昏迷的出現(xiàn)，進(jìn)一步研究發(fā)現(xiàn)，肝硬化小鼠存在小膠質(zhì)細(xì)胞的激活和原位促炎細(xì)胞因子TNF-α、IL-1β和IL-6的合成增加[21]。WRIGHT等[36]發(fā)現(xiàn)，促炎因子主要是通過導(dǎo)致腦細(xì)胞水腫從而誘發(fā)昏迷。臨床研究[21,37]也發(fā)現(xiàn)，炎癥的存在會(huì)加重HE認(rèn)知功能的受損。LOREN AGUSTI等[38-39]的多項(xiàng)動(dòng)物研究采用西地那非、布洛芬等抗炎治療，發(fā)現(xiàn)其通過抑制小膠質(zhì)細(xì)胞和星形膠質(zhì)細(xì)胞的活化、降低炎癥因子水平后能降低小鼠門靜脈的壓力、改善認(rèn)知功能。同樣BAJAJ等[11,17,40]臨床試驗(yàn)發(fā)現(xiàn)，采用益生菌治療能降低內(nèi)毒素血癥和TNF的水平從而改善HE認(rèn)知功能。這些都提示腸道微生物所誘導(dǎo)的炎癥在HE發(fā)病中發(fā)揮作用。

5 益生菌治療HE

腸道微生物失衡在HE發(fā)病中占據(jù)重要地位，因此調(diào)節(jié)腸道菌群，從而增強(qiáng)黏膜屏障、減少細(xì)菌易位、SIBO、炎癥等是治療HE的新方案。近年益生菌在改善微生態(tài)失衡從而改善HE認(rèn)知功能上備受關(guān)注。DHIMAN等[40]的多項(xiàng)臨床研究表明，與安慰劑對照組相比，益生菌能改善肝功能和大腦認(rèn)知功能，發(fā)現(xiàn)治療后CTP評分、末期肝病評分和數(shù)字測試試驗(yàn)顯著好轉(zhuǎn)，在逆轉(zhuǎn)MHE、減少OHE的發(fā)生、降低HE的住院風(fēng)險(xiǎn)上療效顯著[29,40-41]。SAAB等[41-44]比較了益生菌與目前治療HE的推薦用藥如利福昔明、乳果糖的療效，結(jié)果發(fā)現(xiàn)其在降低血氨及炎癥因子水平、改善大腦認(rèn)知功能上療效相仿，并且在增加有益菌群、減少致病性細(xì)菌和長期耐受性上更勝一籌[26,42]，但益生菌使用并不能降低HE的死亡率[41]。益生菌在治療HE上療效確切，目前尚無報(bào)道嚴(yán)重不良反應(yīng)，安全性得到認(rèn)可。

HE嚴(yán)重威脅人類健康，它是由高血氨、炎癥、代謝毒性物質(zhì)等多種因素綜合作用所導(dǎo)致的大腦認(rèn)知功能損傷。近年發(fā)現(xiàn)腸道微生態(tài)失衡在其發(fā)病中扮演重要角色，腸道菌群失調(diào)、SIBO、腸黏膜受損等誘導(dǎo)高血氨、系統(tǒng)神經(jīng)炎癥，最終引發(fā)HE。而益生菌因其調(diào)節(jié)腸道菌群比例、改善腸道微生態(tài)，對HE有確切的治療效果，且不良反應(yīng)小，長期耐受性好，有望成為治療HE的一線藥物。但其劑量劑型、適應(yīng)證等尚無統(tǒng)一推薦標(biāo)準(zhǔn)，未來需更多大規(guī)模、多中心臨床試驗(yàn)研究其療效及用法。此外多項(xiàng)研究發(fā)現(xiàn)，炎癥在HE發(fā)病中發(fā)揮作用，但目前尚無直接抗炎制劑治療HE的臨床試驗(yàn)，未來可在抗炎制劑治療HE上進(jìn)一步研究，有望發(fā)現(xiàn)更多的HE治療方案，使HE患者獲益。

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