高正睿 吳之濤 龔永福 楊克澤 馬金慧 魏玉杰

[摘要]非酒精性脂肪肝(NAFLD)是全球最常見的肝臟疾病,通常與代謝綜合征有關。自噬,一種由溶酶體介導的細胞內成分的降解過程。自噬在調控肝臟脂質代謝的過程中,轉錄因子EB(TFEB)、腺苷酸活化蛋白激酶(AMPK)/哺乳動物雷帕霉素靶蛋白(mTOR)、過氧化物酶體增殖劑激活受體α(PPARα)和法尼酯衍生物X受體(FXR)、細胞外信號調節激酶1/2(ERK1/2)、沉默信息調節因子3(SIRT3)等信號通路發揮了重要作用。近年來的研究表明,許多中藥及其活性成分可以通過誘導自噬減輕肝細胞脂肪變性。本文對自噬調控NAFLD的分子機制及中藥有效成分的干預作用進行了綜述。
[關鍵詞]自噬;非酒精性脂肪肝;中藥;信號通路
[中圖分類號] R575? ? ? ? ? [文獻標識碼] A? ? ? ? ? [文章編號] 1674-4721(2019)11(a)-0021-06
Research progress of molecular mechanism of autophagy in regulating non-alcoholic fatty liver disease and analysis of the effect of active ingredients of traditional Chinese medicine on liver autophagy
GAO Zheng-rui1,2,3? ?WU Zhi-tao1,2,3? ?GONG Yong-fu1,2,3? ?YANG Ke-ze1,2,3? ?MA Jin-hui1,2,3? ?WEI Yu-jie1,2,3
1. Gansu Academy of Agri-engineering Technology, Gansu Province, Wuwei? ?733006, China; 2. Key Laboratory of the Special Medicine Source Plant for Germplasm Innovation and Safety Utilization in Gansu Province, Gansu Province, Wuwei? ?733006, China; 3. Hexi Comprehensive Experimental Station of Industrial System for Chinese Herbal Medicine, Gansu Province, Wuwei? ?733006, China
[Abstract] Non-alcoholic fatty liver disease (NAFLD) is the most common liver disease in the world, which is usually associated with metabolic syndrome. Autophagy, a lysosome-mediated degradation process of intracellular components. During the process of regulating liver lipid metabolism by autophagy, transcription factor EB (TFEB), adenosine monophosphate activated protein kinase (AMPK), mammalian target of rapamycin (mTOR), peroxisome proliferator-activated receptor α (PPARα), farnesoid X receptor (FXR), extracellular signal-regulated kinase 1/2 (ERK1/2) and silent information regulation 3 (SIRT3) play the important roles. Recent studies have shown that many traditional Chinese medicines and their active ingredients can alleviate hepatocyte steatosis by inducing autophagy. This article would review the molecular mechanism of autophagy regulating NAFLD and the intervention effect of active ingredients of traditional Chinese medicine.
[Key words] Autophagy; Non-alcoholic fatty liver disease; Traditional Chinese medicine; Signal pathway
非酒精性脂肪肝(non-alcoholic fatty liver disease,NAFLD)是一種與肥胖等代謝疾病密切相關的臨床綜合征,包括簡單的脂肪變性、非酒精性脂肪性肝炎(nonalcoholic steatohepatitis,NASH)、肝纖維化和肝硬化[1]。自噬是一種高度保守的作用機制,在維持細胞、組織和機體穩態的過程中起著重要作用。許多證據表明,自噬功能缺陷與多種代謝疾病有關,包括肥胖、糖尿病、NAFLD等,自噬水平的降低,增加了NAFLD的患病風險[2],NAFLD患者的肝自噬受損。自噬能有效地降解正常細胞的代謝產物,這些產物在積累后會產生細胞毒性,如受損的線粒體和具有氧化還原活性的蛋白聚集體[3]。研究表明,自噬參與了細胞質脂滴的選擇性降解,所以調節肝細胞自噬可以作為一種肝臟的保護機制來治療NAFLD[4]。近年來,隨著中藥現代化步伐的加快,中藥有效成分調控自噬的研究已取得了初步進展。本文旨在探討自噬調節肝臟脂質的分子機制,以及中藥有效成分對NAFLD的影響。
1自噬與NAFLD
1.1自噬的類型和過程
自噬是一種動態的分解過程,為維持細胞內物質的能量平衡提供了保障。根據細胞內含物運送方式的不同,自噬可分為三類:巨自噬(macroautophagy)、微自噬(microautophagy)、分子伴侶介導的自噬(chaperon-mediated autophagy,CMA),其中最常見的是巨自噬[5],巨自噬也是調節NAFLD的主要自噬類型。
保守的代謝傳感器哺乳動物雷帕霉素靶蛋白(mammalian target of rapamycin,mTOR)和腺苷酸活化蛋白激酶(adenosine monophosphate activated protein kinase,AMPK)是自噬的主要調控因子,mTOR為抑制因子,AMPK為激活因子。自噬的過程分為5個連續的步驟:分隔膜的形成、分隔膜的延伸、自噬體的形成、自噬溶酶體的形成、自噬溶酶體的降解。這些步驟相應的受到自噬相關蛋白(autophagy-related proteins,ATGs)的調控,ATGs組裝成了多種復合物:UNC-51樣激酶1(UNC-51-like kinase1,ULK1)復合物、磷脂酰肌醇3-激酶(phosphatidylinositol 3-kinase,PI3K)復合物和磷脂酰肌醇-3-磷酸(phosphatidylinositol-3-phosphate,PI3P)復合物,這些復合物誘導了自噬體、ATG12和微管相關蛋白1輕鏈3(microtubule associated protein 1 light chain 3,LC3)共軛系統的形成。在ATG12共軛系統中,ATG12-ATG5-ATG16L1復合物促進了LC3共軛系統的形成,LC3在蛋白酶ATG4的作用下形成LC3-Ⅰ,LC3-Ⅰ與磷脂酰乙醇胺(phosphatidylethanolamine,PE)結合形成LC3-Ⅱ[6]。隨著自噬體被密封,隔離的胞漿物質被自噬溶酶體所降解[7]。降解后,生成的氨基酸、脂類和碳水化合物通過轉運蛋白和滲透酶到達細胞質,實現了細胞內物質的循環利用[8](圖1)。
1.2脂質自噬與NAFLD
NAFLD的特點是肝臟內異常聚集了大量的脂滴,這可導致肝臟炎癥和代謝紊亂[9]。脂滴可以儲存細胞內的游離脂肪酸和三酰甘油,在營養缺乏時期提供了一種快速獲取能量的途徑[10]。肝臟脂肪變性會通過氧化過多的脂質對肝臟造成損傷,限制脂滴的存儲可以防止NAFLD引起的肝細胞損傷[11]。脂質自噬是細胞質內脂滴的降解過程,在營養限制的情況下,脂滴被自噬體吞噬,在溶酶體中被酸性脂肪酶降解[12],脂質自噬所分解的脂質數量隨著細胞外營養物質的供應情況而變化[13]。在體外培養的肝細胞中,對自噬的抑制并不影響脂質的形成和分泌,但會導致細胞內脂質的顯著堆積,并使β-氧化受到抑制。敲除小鼠肝臟中一個重要的自噬基因ATG7,表現出三酰甘油和膽固醇的大量積累[14],表明自噬功能發生障礙會使肝臟脂質過度堆積,從而導致NAFLD的發生[15]。
2自噬調控NAFLD的作用機制
2.1 TFEB信號途徑
轉錄因子TFEB(transcription factor EB)是螺旋-環-螺旋亮氨酸拉鏈類轉錄因子中的MiTF/TFE(microphthalmia-transcription factor E)家族成員之一,在細胞器的起源和細胞代謝中起著關鍵作用[16]。mTOR是細胞生長和代謝的關鍵調節器,在營養缺乏時mTOR被迅速抑制,從而激活自噬[17]。TFEB通過調節溶酶體和自噬相關基因的表達,在誘導自噬的過程中發揮了重要作用。TFEB的亞細胞定位由mTOR激酶復合物介導的磷酸化調節,飲食限制可以使TFEB從細胞質轉移到細胞核,從而發揮其功能[18]。研究表明,在高脂飲食誘導的小鼠模型中,過表達肝細胞中的TFEB可以激活自噬,增強脂質分解的能力,提高肝臟中溶酶體酶的活性[19-20]。Kim等[21]發現,在NAFLD患者的肝臟中,肝細胞核中的TFEB表達水平降低。依澤替米貝(Ezetimibe)通過激活AMPK使TFEB轉移到細胞核,這條途徑與獨立于mTOR信號的MAPK/ERK通路有關,TFEB進入細胞核增強了自噬標志蛋白LC3的表達,減輕了小鼠肝臟的脂肪變性。這些結果說明,增強肝臟TFEB的水平可以誘導肝細胞自噬,有效改善肝臟的脂肪變性。
2.2 AMPK/mTOR信號途徑
AMPK是細胞和生物體代謝的一個主要調節因子,在能量水平下降時的適應性響應中起著至關重要的作用[22],AMPK會隨著細胞內ATP水平的降低而被激活[23]。AMPK可以通過下調mTOR的表達水平,從而激活自噬。He等[24]發現,胰高血糖素樣肽-1 (glucagon-likepeptide1,GLP-1)類似物利拉魯肽通過增強肝臟中P-AMPK的表達,降低P-mTOR的表達,增強了自噬蛋白LC3的表達,改善了肝細胞脂肪變性。有研究表明,在營養限制的情況下,轉化生長因子-β活化激酶1(TGF-β activated kinase 1,TAK1)通過激活AMPK并抑制mTOR誘導自噬,以防止肝細胞中脂質過多的積累[25]。Liu等[26]發現Ⅲ型纖連蛋白組件包含蛋白5(fibronectin type Ⅲ domaincontaining protein 5,FNDC5)可以通過AMPK/mTOR信號誘導自噬和脂肪酸氧化,從而使肝臟脂質積累減少。這些研究結果說明調控AMPK/mTOR信號可以誘導肝細胞自噬,減輕肝臟脂肪變性。
2.3 PPARα和FXR信號途徑
過氧化物酶體增殖物激活受體α(peroxisome proliferator-activated receptor α,PPARα)是一種配體激活的轉錄因子,屬于核受體亞家族成員,在肝臟等脂代謝活躍的組織中高度表達。在NAFLD的臨床前模型中,激活PPARα可改善肝臟的脂肪變性、炎癥和纖維化,因此被作為潛在的治療靶點[27]。法尼酯衍生物X受體(farnesoid X receptor,FXR)也是肝臟代謝中重要的轉錄調節因子。Lee等[28]發現,PPARα和FXR共同調控小鼠肝臟自噬過程。在禁食和正常喂食的小鼠肝臟中,PPARα和FXR分別被激活。藥理性激活PPARα解除了正常喂食對自噬的抑制作用,誘導了脂質的降解,FXR的藥理激活強烈抑制了禁食誘導的自噬現象,進一步研究發現,PPARa和FXR競爭性的與自噬基因啟動子位點結合,產生不同的調節結果。Seok等[29]發現,在肝細胞中,FXR通過與環磷酸腺苷反應元件結合蛋白(cAMP response element binding protein,CREB)結合,破壞了CREB-CREB轉錄共激活因子2(CREB regulated transcription coactivator 2,CRTC2)復合物,導致細胞核中CRTC2水平降低,抑制了自噬。這些研究表明,激活PPARα和抑制FXR的活性可以促進肝細胞自噬,緩解肝臟脂肪變性。
2.4 ERK1/2信號途徑
細胞外信號調節激酶1/2(extracellular signal-regulated kinase 1/2,ERK1/2)參與調控多種細胞代謝進程,研究表明,ERK信號可以調節自噬和溶酶體相關基因的表達[30]。Xiao等[31]發現,ERK1/2在瘦素受體缺乏(db/db)小鼠的肝臟中表達降低。腺病毒激活ERK1/2上游的調節器蛋白激酶1(mitogen extracellular kinase1,MEK1),增強了脂肪酸氧化和三酰甘油相關基因的表達,明顯改善了db/db小鼠的肝臟脂肪變性,進一步研究發現,ERK1/2通過ATG7激活肝細胞自噬從而減輕db/db小鼠肝臟的脂肪沉積,ERK1/2對ATG7的調控依賴于p38信號途徑。這些研究結果說明,激活肝臟ERK1/2通路可以誘導肝細胞自噬,改善NAFLD的表型。
2.5 SIRT3信號途徑
沉默信息調節因子3(silent information regulation 3,SIRT3)是一種煙酰胺腺嘌呤二核苷酸(nicotinamide adenine dinucleotide,NAD+)依賴的去乙酰化酶,屬于沉默信息調節因子2(SIRT2)相關酶(silent information regulator 2 related enzymes,Sirtuin)家族成員之一。在肝臟中,SIRT3可以調節線粒體功能并特異性調控參與脂肪酸氧化、氧化磷酸化、酮體合成和尿素循環的蛋白質的酶活性[32]。研究發現,在NAFLD小鼠模型中,自噬水平在SIRT3敲除小鼠的肝臟中明顯增強,而在肝臟中過表達SIRT3降低了自噬的水平,進一步研究發現,SIRT3過表達導致錳超氧化物歧化酶(manganese superoxide dismutase,MnSOD)的去乙酰化和活化,耗盡了細胞內的超氧化物,使AMPK被抑制,從而激活mTOR,最終導致自噬被抑制,這表明抑制SIRT3過度活化是治療NAFLD等代謝疾病的潛在靶點[33]。
3 中藥有效成分對肝臟自噬的干預作用
一些研究表明,許多中藥活性成分通過激活自噬改善了NAFLD的癥狀。小檗堿(berberine)是從黃連(Coptis chinensis)中提取的藥用生物堿,具有改善葡萄糖耐受性、降低血脂等作用[34],研究表明,給野生型小鼠每天5 mg/kg注射小檗堿共5周,肝細胞內甘油三酯的水平顯著降低,進一步實驗發現,小檗堿是通過增加肝臟沉默信息調節因子1(silent information regulation 1,SIRT1)的乙酰化活性,并以一種依賴ATG5的方式誘導自噬,改善肝臟脂肪變性[35]。He等[36]發現,小檗堿通過ERK介導的mTOR信號途徑誘導肝細胞自噬,減少了肝臟中脂質的積累。
白藜蘆醇(resveratrol)是一種多元酚類物質,存在于多種藥用植物中[37]。Zhang等[38]發現,白藜蘆醇可以通過cAMP-PRKA-AMPK-SIRT1信號通路誘導肝細胞發生自噬,在體內和體外實驗中都減輕了肝臟脂肪變性。Ji等[39]發現,對于蛋氨酸-膽堿缺乏(MCD)飼料誘導的NASH小鼠模型,白藜蘆醇能明顯增加肝臟細胞AML12中LC3-Ⅱ的表達,增強細胞自噬水平,緩解肝臟脂質沉積和炎癥。
芒果苷(mangiferin)主要存在于芒果(Mangifera indica)葉和知母(Anemarrhena asphodeloides)根當中,具有調節脂質代謝的作用[40]。Wang等[41]發現,對于高脂飲食誘導的NAFLD小鼠模型,在小鼠腹腔注射芒果苷12周后,降低了小鼠體重和肝臟中三酰甘油和總膽固醇的水平,進一步研究發現,芒果苷通過AMPK/mTOR信號途徑激活了自噬,緩解了肝臟中脂質的累積。
橄欖苦苷(oleuropein)是從橄欖葉中提取的一種酚類化合物[42],給C57BL/6J肥胖模型小鼠灌胃3%橄欖苦苷共8周后,橄欖苦苷上調了Ser555位點ULK1的磷酸化水平,從而誘導了肝細胞自噬,減輕了肝臟脂肪變性,這表明橄欖苦苷是一種通過靶向激活自噬來改善肝脂肪變性的潛在藥物[43]。
Huang等[44]發現,給瘦素受體缺乏(db/db)的小鼠每天10 mg/kg注射人參皂苷Rb2(ginsenoside Rb2)共4周,誘導了SIRT1和AMPK表達上調,恢復了肝臟自噬,顯著提高了肥胖db/db小鼠的葡萄糖耐受能力,減少了脂質的累積。
Zhong等[45]發現,黃蘭(Michelia champaca)活性成分木香內酯(micheliolide)通過上調PPARγ的表達水平,抑制核因子Kappa B(nuclear factor-kappa B,NF-κB)介導的炎癥,激活AMPK/mTOR介導的自噬,改善了肝臟脂肪變性。
澤瀉醇A-24-醋酸酯(Alisol A-24-acetate,AA)是從中藥澤瀉(Rhizoma Alismatis)中提取的一種三萜類化合物,Wu等[46]發現,對于MCD飲食誘導的NASH小鼠模型,AA通過AMPK/mTOR途徑抑制了氧化應激損傷,增強了自噬蛋白LC3-Ⅱ的水平,降低了自噬底物蛋白P62的水平,改善了肝臟脂質累積和炎癥。
當歸多糖(Angelica sinensis polysaccharide,ASP)是從當歸根中分離出的藥用成分,Wang等[47]發現,ASP降低了肥胖小鼠肝臟中脂質的積累,減輕了肝臟脂肪變性。此外,ASP可以顯著增加自噬蛋白LC3-Ⅱ的表達水平,誘導肝細胞發生自噬,這一降脂作用與激活SIRT1-AMPK信號途徑有關[48]。
木通皂苷D(Akebia Saponin D,ASD)是從川續斷(Dipsacus asper Wall)根莖中提取的三萜皂苷類化合物,Gong等[49]發現,對于瘦素受體缺乏的肥胖小鼠,經過ASD干預后,肝臟中血糖水平降低,自噬蛋白LC3-Ⅱ的表達水平升高,從而減輕了肝臟中脂質的累積,有效緩解了肝臟脂肪變性。
降脂顆粒是一種臨床常用的治療NAFLD的中藥配方,由絞股藍、丹參、虎杖、茵陳蒿、荷葉組成[50]。降脂顆粒能顯著緩解棕櫚酸誘導的肝細胞功能障礙和脂滴積累,進一步研究發現,降脂顆粒通過抑制mTOR信號誘導自噬,改善了NAFLD的相關癥狀[51]。
4小結
在自噬調節肝臟脂質變性的過程中,TFEB、AMPK/mTOR、PPARs和FXR、ERK1/2、SIRT3等信號途徑發揮了關鍵作用。近年來的研究表明,自噬可以作為治療NAFLD的有效靶點,一些中藥及其活性成分可以通過誘導自噬減輕NAFLD的相關癥狀,體現了中藥在改善NAFLD等代謝疾病方面的優勢。隨著對自噬調節機制的深入研究,進一步闡明中藥誘導自噬的信號通路,將為NAFLD等代謝疾病的防治提供新的思路。
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(收稿日期:2019-04-30? 本文編輯:孟慶卿)