李騁+何金枝+周學東+徐欣

[摘要] 胰島素抵抗是肝臟、肌肉和脂肪組織等周圍靶組織細胞對胰島素的敏感性降低導致葡萄糖攝取和利用效率下降而產生的一系列臨床表現,是2型糖尿病的重要發病機制之一。黃連素提取自天然植物,安全性高、毒副反應小,具有顯著的降血糖、改善胰島素抵抗的作用,對2型糖尿病及其并發癥有較好療效。該文對黃連素調控胰島素抵抗相關2型糖尿病研究進展進行綜述,探討黃連素對胰島素抵抗及2型糖尿病防治的相關機制。黃連素的生物利用度極低,提示其可能通過調節腸道菌群來發揮降脂、降糖的作用。腸道微生物可能成為黃連素治療胰島素抵抗相關2型糖尿病的新靶點。
[關鍵詞] 黃連素; 胰島素抵抗; 2型糖尿病; 腸道菌群
[Abstract] Insulin resistance (IR) is defined as a series of clinical manifestations for diminished effectiveness of insulin in lowering blood sugar levels caused by decreased sensitivity to insulin of liver, muscle and adipose tissue. IR is the major contributor to the etiology and pathogenesis of type 2 diabetes mellitus (T2DM). Berberine, a traditional Chinese herb extract, has been shown to be safe and effective in lowering blood sugar, alleviating insulin resistance and moderating type 2 diabetes mellitus and its complications. The bioavailability of berberine is extremely low, suggesting that it may play a role in lowering blood sugar and lipid by regulating intestinal flora. Intestinal microbiota may serve as a new potential target for berberine treatment of type 2 diabetes mellitus.
[Key words] berberine; insulin resistance; type 2 diabetes mellitus; intestinal microbiota
糖尿病是一組以高血糖為特征的代謝性疾病,由胰島素分泌缺陷和/或其生物作用受損引起。高血糖可導致機體各種組織,特別是眼、腎、心臟、血管、神經的慢性損害、功能障礙。世界衛生組織(world health organization,WHO)2014年統計結果顯示,全世界糖尿病患者已達34.7億[1],嚴重危害人類健康。根據其發病機制,糖尿病主要分為1型糖尿病(type 1 diabetes mellitus,T1DM)和2型糖尿病(type 2 diabetes mellitus,T2DM)。T2DM特指因胰島素抵抗和胰島素分泌不足而產生的葡萄糖和脂肪代謝紊亂綜合征。胰島素抵抗(insulin resistance,IR)指肝臟、肌肉和脂肪組織等周圍靶組織細胞對胰島素的敏感性降低、胰島素促進的葡萄糖攝取和利用效率下降,從而產生高血糖癥、高胰島素血癥、血脂紊亂等一系列臨床表現。T2DM多在35~40歲之后發病,占糖尿病患者90%以上。研發有效、低毒的T2DM治療藥物對于糖尿病的控制具有重要意義。
黃連素又名鹽酸小檗堿(berberine),是黃連的主要成分,毒副反應小,安全價廉。目前,黃連素調節、改善糖脂代謝的作用已被證實[2-3]。大量研究結果表明黃連素具有顯著的降血糖、降血脂、改善IR及抗氧化作用,對T2DM及其各種并發癥具有較好療效,具有很大的開發價值。近期研究表明,腸道微生物與機體胰島素抵抗及2型糖尿病的調控密切相關,提示腸道微生物可能成為黃連素治療胰島素抵抗相關2型糖尿病的新靶點,但目前對此方面研究的綜述十分缺乏。本文對黃連素調節腸道菌群參與調控胰島素抵抗相關2型糖尿病研究進展進行綜述,為進一步研究提供思路。
1 黃連素抗T2DM的體內研究
1.1 黃連素抗T2DM的臨床研究
迄今為止,已有數個臨床試驗證實黃連素對T2DM的治療效果。Yin等[4]對36例T2DM患者給予黃連素治療(每日3次,每次0.5 g),發現黃連素可明顯降低患者空腹血糖、餐后血糖、糖化血紅蛋白和甘油三酯。Cannillo等[5]給予112例糖尿病患者黃連素每日3次,每次1.5~3.0 g,觀察到黃連素具有顯著的降血糖作用,總有效率高達90%。Xia等[6]用黃連素治療肝源性糖尿病患者,研究發現黃連素可降低患者空腹血糖、改善葡萄糖受損耐量和肝功能。其他臨床研究均顯示黃連素可有效調節血糖,體現在治療組空腹血糖(FBG)、空腹血清胰島素(FINS)、穩態模型評估胰島素抵抗指數(HOMA-IR)水平較治療前及對照組均降低[7-8]。
1.2 黃連素抗T2DM的動物實驗研究
Wu等[9]學者通過飲食誘導肥胖大鼠模型研究,發現黃連素可改善IR,與二甲雙胍具有相似療效。與二甲雙胍相比,經黃連素治療的糖耐量受損大鼠其游離脂肪、載脂蛋白B、血漿甘油三酯和總膽固醇顯著降低,并且黃連素可導致胰島素抵抗大鼠的胰島素敏感指數(ISI)、體重及內臟脂肪細胞面積數值顯著降低,提示黃連素有增加胰島素敏感性、改善IR作用[10]。Wang等[11]通過高脂飲食與注射STZ建立糖尿病大鼠模型,研究結果顯示黃連素可降低模型大鼠空腹血糖。代謝組學分析表明,高脂飲食誘導的Wistar大鼠經黃連素治療可增加血清中丙酮酸、生酮氨基酸和糖原水平[12]。黃連素還可降低高脂誘導肥胖小鼠的體重、減少能量攝入和血糖血脂水平[13]、降低血清脂多糖結合蛋白(lipopolysaccharide-binding protein,LBP)水平、核細胞趨化蛋白-1、脂聯素,升高瘦素水平[14]。Lee等[15]研究發現,黃連素可減輕胰島素抵抗模型大鼠體重,改善IR;Wu等[16]也觀察到胰島素抵抗模型大鼠ISI降低。
2 黃連素抗糖尿病作用機制的研究
2.1 保護胰島細胞
黃連素對胰島細胞具有保護作用。黃連素可修復糖尿病大鼠受損的胰腺組織[17]。黃連素還可通過增加胰島細胞內肝細胞核因子4α(hepatocyte nuclear factor 4α,HNF4α)的轉錄及表達水平,保護胰島細胞[18]。此外,黃連素可下調 Bax/Bcl-2比值,從而抑制小鼠胰島細胞的凋亡[19]。
2.2 促胰島素分泌
Zhou [20]通過STZ誘導的糖尿病大鼠模型研究,發現黃連素能增加胰島素分泌、減少胰島β細胞氧化應激、增加抗氧化酶的活性、降低脂類的氧化、促進 β 細胞的再生。Ko等[21]研究顯示黃連素可引起MIN6胰島β細胞株葡萄糖刺激后的胰島素分泌,同時促進胰島B細胞增生。Lee等[15]研究表明,黃連素能夠改善高脂飲食Wistar大鼠的胰島素分泌,恢復胰島功能。Shen 等[22]同樣證實黃連素可促進胰島β細胞釋放胰島素。Zhou等[23]研究發現黃連素可顯著增加胰島細胞AMPK活性,通過環腺苷酸(cAMP)信號通路調節胰島素分泌。值得注意的是,黃連素促胰島素分泌作用尚具有爭議。Yin等[4]體外實驗發現,黃連素對β-TC3細胞胰島素分泌無顯著促進作用。
2.3 降低炎癥反應
機體炎癥反應與IR以及胰島β細胞損傷密切相關。炎癥因子可導致細胞凋亡并影響胰腺β細胞功能。脂多糖結合蛋白LBP是外源性抗原負荷的生物標志,單核細胞趨化蛋白-1(MCP-1)可作為系統性炎癥水平的評價指標,研究發現黃連素能降低高脂喂養大鼠血漿中增加的LBP和MCP-1水平,改善系統性炎癥[24-25]。腫瘤壞死因子(TNF-α)可干擾外周組織胰島素功能,導致IR。動物研究發現,黃連素具有明顯的抗炎作用,可降低胰島素抵抗大鼠模型血清TNF-α,IL-6水平,上調抗炎癥因子如IL-10的表達,增加胰島素敏感性[26-27]。
2.4 改善血脂紊亂
脂毒性是肥胖導致胰島β細胞功能受損的病理機制之一。黃連素可顯著降低血清總膽固醇TC、甘油三酯TG和低密度脂蛋白LDL-c水平,增加高密度脂蛋白HDL-c及一氧化氮NO濃度[17]。Brusq等[28]臨床研究表明,黃連素可顯著降低血清TG,可能與黃連素通過激活AMPK信號通路抑制人肝細胞內脂質合成有關。Lee等[15]的研究表明黃連素可減輕高脂飼養大鼠血漿中甘油三酯含量,改善胰島功能。Zhou等[29]研究結果顯示,黃連素能降低糖尿病大鼠肝臟質量及肝臟指數,促進血糖、TC和TG恢復正常,并降低LDL-c和載脂蛋白Al水平。
2.5 促外周組織葡萄糖吸收利用
葡萄糖吸收入血后,依賴葡萄糖轉運體(GLUTs)進入細胞。GLUT1在人類所有組織中均存在表達,GLUT4主要表達于胰島素敏感的骨骼肌、脂肪細胞和心肌中。黃連素可激活GLUT1[30]并上調GLUT1表達水平[23]、增加GLUT4的表達和轉位活性[31],從而增加機體組織對葡萄糖的攝取。Xu[32]和Ko[21]等學者的研究均證實在3T3-L1脂肪細胞上,黃連素通過激活IRS-1-PI3K-Akt-GLUT-4通路,增加脂肪細胞葡萄糖攝取及對胰島素的敏感性,改善游離脂肪酸誘導的IR。此外,蛋白激酶AMPK信號通路被激活,可抑制合成代謝,促進分解代謝,與IR的改善密切相關。黃連素可明顯增加3T3-L1脂肪細胞中的AMP水平,增加AMPK磷酸化程度[15,23],上調線粒體解偶聯蛋白 UCP2,增加能量消耗,促進葡萄糖分解代謝。
2.6 其他功能
Kong等[33]研究顯示,黃連素可上調Hep G2肝細胞胰島素受體水平,增加胰島素敏感性。在高脂飲食加STZ注射誘導的糖尿病倉鼠模型中,黃連素可提高肝臟X受體α(LXRα)、過氧化物酶體增殖因子活化受體α/δ(PPARα/δ)以及PPARα的mRNA水平,減少PPARγ表達,降低SREBPs的mRNA水平,進而降低肝臟質量以及肝臟與血漿的甘油三酯和膽固醇水平[34-35]。黃連素還可提高大鼠HNF4α表達,改善高胰島素血癥,降低胰島素抵抗指數[36]。此外,Xia等[6]提出黃連素在肝臟中對胰島素信號通路沒有影響,黃連素可下調糖元合成基因PEPCK和G6Pase,以及轉錄因子FoxO1,SREBP1,ChREBP的表達,從而發揮降糖作用。
3 黃連素治療T2DM的新靶點——腸道微生物
腸道是人體巨大的微生態系統,定植于腸道的微生物集合統稱為腸道菌群[37]。Eckburg等[38]通過宏基因組學研究發現,人體腸道菌群主要由厚壁菌門Firmicutes、擬桿菌門Bacteroides、放線菌門Actinobacteria、變形菌門Proteobacteria、梭桿菌門Fusobacteria及疣微菌門Verrucomicrobia構成。其中,硬壁菌門、擬桿菌門比例占90%以上。腸道菌群具有多樣性和特異性,受宿主遺傳基因、年齡、病理、飲食等多種因素影響[39-42]。目前認為,腸道菌群直接參與營養吸收、生物屏障、免疫調節、脂肪代謝、抗腫瘤等諸多生理過程,對維持宿主正常生理功能不可或缺[43];與肥胖、脂肪肝、糖尿病等疾病關系密切[44]。
3.1 腸道菌群與T2DM關系密切
Yazigi和Turnbaugh等[45-46]發現消瘦無菌小鼠腸道內移植肥胖小鼠的腸道菌群后,消瘦小鼠的體重和脂肪含量均有增加,提示腸道菌群與肥胖之間的潛在關系。代謝綜合征患者十二指腸移植正常人腸道菌群6周后,胰島素敏感性增加、IR癥狀改善;進一步研究發現,接受菌群移植后,代謝綜合征患者十二指腸定植菌群中產丁酸鹽菌-霍氏真桿菌Eubacterium hallii豐度增加,提示改變腸道菌群可改善IR,其機制可能與十二指腸菌群產丁酸鹽能力上調有關[47]。
現有研究表明,促糖尿病發生相關腸道細菌主要包括: 糞擬桿菌、變異梭狀芽胞桿菌、大腸埃希菌、脫硫弧菌屬、加氏乳桿菌、變形鏈球菌和副流感嗜血桿菌等;具有抗糖尿病作用的細菌主要有:梭狀芽胞桿菌、直腸真桿菌、羅斯氏菌、疣微菌科、Akkermansia muciniphila菌和普氏糞桿菌等[48-49]。Zhang等[50]通過對腸道細菌16S rRNA的高通量測序,發現菌群多樣性與機體代謝參數明顯相關。腸道內低濃度的柔嫩梭菌屬Faecalibacterium prausnitzii,羅氏菌屬Roseburia,擬桿菌屬與肥胖及糖尿病密切相關[51]。研究顯示,2型糖尿病患者腸道艱難梭菌Clostridium difficile的豐度增高,擬桿菌豐度降低,但產氣菌Dore屬、普氏菌Prevotella,柯林斯氏菌Collinsella豐度更高[14]。對十二指腸菌群的測序研究發現,肥胖組較健康組十二指腸厭氧菌比例、乙酰輔酶A脫氫酶基因增加,而需氧菌和蔗糖磷酸酶和1-4α葡萄糖支鏈酶減少[52]。另有研究發現,糖尿病患者腸道的有益菌-雙歧桿菌Bifidobacterium數量降低,而糞腸球菌Enterococcus faecalis數目增高[53];在糖耐量損傷的小鼠中,雙歧桿菌減少,產硫酸鹽、內毒素的細菌增多[54]。目前認為腸道乳酸菌Lactobacillus,雙歧桿菌等益生菌的減少與糖耐量異常密切相關。將含有嗜酸乳桿菌Lactobacillus acidophilus和干酪乳桿菌L. casei的酸奶喂養高果糖飲食大鼠,具有明顯抗糖尿病效果[55]。有研究發現添加益生菌飲食可使肥胖小鼠小腸硬壁菌門和梭狀芽孢桿菌群ⅩⅣab Clostridium clusterⅩⅣab豐度降低,硬壁菌門/擬桿菌門比值減低,減緩小鼠體重增長,降低肥胖相關代謝指標[56]。
近期研究發現,Akkermansia muciniphila菌與胰島素、血糖、甘油三酯水平、肥胖、代謝性內毒素血癥等糖尿病危險因子呈負相關,該細菌通過降解腸黏膜中的黏蛋白,產生游離脂肪酸,影響人體代謝平衡和免疫功能[54]。有研究通過聯用綠茶和植物乳桿菌L. plantarum治療高脂小鼠,發現處理后Akkermansia菌數量無顯著改變,但小腸細菌群落多樣性明顯增加、乳酸菌生長增多,高脂飲食誘導的炎癥反應得以緩解[57]。另有學者將蔓越莓提取物喂養髙脂飲食小鼠后,發現其可增加腸道中Akkermansia菌群數量、降低體重和內臟脂肪、循環血液中的LPS、減少胰島素抵抗、增加胰島素敏感性[58]。
3.2 黃連素通過調節腸道菌群緩解T2DM
研究表明黃連素具有抗炎、抑制葡萄糖吸收、調節腸道菌群、保護腸黏膜屏障等作用[59]。黃連素的生物利用度極低,在腸道吸收率僅5%~10%[60]。Li等[61]研究發現,在STZ誘導的糖尿病小鼠中,黃連素幾乎不被腸道吸收,但可顯著降低腸道二糖酶和β-葡糖苷酸酶的活力,從而減少葡萄糖吸收,降低餐后高血糖。因此,目前認為黃連素并不主要經消化道吸收進入血液循環后發揮生理學作用,可能通過調節腸道菌群結構,間接起到降脂、降糖的作用[14,62]。黃連素可通過調控腸內菌群生長,影響腸內糖脂成分的吸收及體內糖脂代謝,起到間接的降血糖、調血脂作用[14]。Zhang等[63]研究發現,黃連素不僅能改變高脂飲食大鼠腸道菌群結構,抑制細菌生長,還能改善大鼠的肥胖和IR。肥胖小鼠或肥胖人群與非肥胖者相比,微生物多樣性明顯降低,被稱為“胖菌”的厚壁菌門及放線菌門比例增高,而被稱為“瘦菌”的擬桿菌門比例減少[64-65]。擬桿菌/厚壁菌門的比值增加可顯著抑制糞腸球菌的生長,增加乳酸菌和雙歧桿菌的生長[66]。Li等[12]通過16S rDNA測序發現黃連素治療可導致包括擬桿菌,布勞特氏菌Blautia,埃希氏菌屬Escherichia等在內的腸道益生菌豐度增加。Guo等學者研究顯示,黃連素治療組的小鼠回腸末端和大腸的擬桿菌屬豐度增加[67]。Cao等[68]發現,高脂飲食小鼠經黃連素治療后,雙歧桿菌相對豐度及擬桿菌門/厚壁菌門的比值回升。Aronsson等[69]研究顯示,黃連素上調髙脂飲食小鼠腸道乳酸菌屬豐度,增加內臟和腸道脂肪組織的禁食誘導脂肪細胞因子(fasting-induced adipose factor,Fiaf)基因表達,減少脂肪儲存。Fiaf是一種負性調節腸道微生物的關鍵蛋白,Xie等[70]的研究發現,黃連素可能通過上升Fiaf的表達,上升PGC1α,UCP2,CPT1α,Hadhb等線粒體能量代謝相關的mRNA的表達,進而調節機體的能量代謝。
3.3 黃連素調節腸道菌群參與T2DM的3種假說
3.3.1 內毒素學說 Cani[71]首次提出“代謝性內毒素血癥”假說:高脂飲食誘導腸道菌群改變,有益菌數量下降,機會致病菌數量增加并產生內毒素破壞腸道黏膜,腸道通透性增強,入血內毒素量增加,導致機體長期處于低水平全身性炎癥反應狀態,進而產生肥胖等代謝失調。糖代謝異常的肥胖小鼠經抗生素處理后,血清LPS濃度及隨機血糖水平均顯著下降[72]。代謝性內毒素血癥引起的慢性系統性炎癥與肥胖、胰島素抵抗及脂肪肝密切相關[73]。內毒素血癥模型小鼠結腸緊密連接(tight junction,TJ)蛋白表達量減少,且從黏膜表面向隱窩移位,黃連素預先灌胃能改善這種不良改變及對緊密連接的損傷[74],影響 TJ 蛋白的表達、分布及結構,抑制腸道通透性增加,改善內毒素血癥[75]。Zhang等[76]的研究表明,黃連素能通過促進回腸超氧化物歧化酶(superoxide dismutase,SOD)和谷胱甘肽過氧化物酶(glutathione peroxidase,GSH-Px)的表達,抑制TLR4(toll-like receptor 4)和核轉錄因子NF-κB的表達,減少腸道損傷,改善回腸和系統性炎癥。
3.3.2 短鏈脂肪酸學說 短鏈脂肪酸(SCFAs)作為能量調節的信號分子,可通過免疫和神經內分泌機制調節宿主能量攝入和代謝[77]。SCFAs可作為能源被消耗,維護腸道上皮細胞的完整性和杯狀細胞的分泌功能,增強腸屏障功能,還可促進小腸糖異生,增加固有層淋巴細胞(lamina propria lymphocytes,LPL)活性[78-79]。SCFAs還可提高結腸內的酸性環境,抑制有害菌的生長,減少炎癥因子生成,有利于減輕黏膜炎癥[80]。有臨床研究通過在高脂飲食中適當補充丁酸鹽 (SCFAs的主要成分),證明了SCFAs改善IR的作用[81]。Qin等[50]基于鳥槍法測序技術,通過比較受試者的腸道微生物DNA,發現2型糖尿病患者腸道微生物菌群失調、各種條件致病菌數增加、產丁酸鹽細菌數下降、SCFAs合成減少,提示2型糖尿病、腸道微生物及SCFAs間存在密切聯系。Zhang等[14]的研究顯示,黃連素可富集腸道內產SCFAs細菌Blautia和Allobaculum,增加腸道SCFAs,進而改善IR。
3.3.3 生長因子學說 遠端腸道的L細胞可分泌胰高血糖素樣肽-1(glucagon like peptide-1,GLP-1)和胰高血糖素樣肽-2(GLP-2)。GLP-1可刺激胰島β細胞的增殖分化,抑制胰島β細胞凋亡,促進胰島素基因的轉錄、胰島素的合成和分泌;作用于胰島α細胞,抑制胰高血糖素的釋放;延緩胃排空,延緩食物吸收,進而發揮降糖作用;GLP-2可調節腸道屏障功能[82]。腸道有益菌還可發酵膳食纖維,促進結腸L細胞分化,增加內分泌調節肽(PYY)、胰高血糖素樣肽GLP-1和GLP-2的分泌[83]。Yu等[84]研究發現黃連素可提高正常小鼠糖負荷后腸道GLP-1分泌,抑制PKC及 MAPK途徑。Zhang等[85]的研究也提示黃連素的降糖機制可能與腸道MAPK和GnRh-Glp-1通路有關。Sun等[86]的研究表明黃連素不僅可以調節腸道微生物的結構和多樣性、促進腸道GLP-1的分泌,還能升高血清GLP-1和神經肽Y(neuropeptide Y,NPY)水平、降低的食欲素A(orexin A)水平、上調GLP-1受體mRNA水平、改善下丘腦的超微結構,進而提出黃連素可通過調節微生物-腸-腦軸(microbiota-gut-brain axis),提高高脂飲食喂養小鼠的代謝水平。
4 展望
黃連素作為一種抗感染、抗炎癥的傳統中藥,常用于胃腸道疾病治療。近期研究表明黃連素具有調控糖脂代謝、改善胰島素抵抗的作用。黃連素可通過抑制腸道有害菌、促進腸道益生菌的生長,起到調控糖脂代謝,緩解T2DM癥狀的作用(圖1)。黃連素通過調控腸道菌群,進而改善糖尿病患者代謝功能紊亂的機制有待進一步研究。另外,亟待大規模臨床研究,確定黃連素用于T2DM治療的安全劑量、最適劑量和劑型,研究特征人群間的差異及個性化治療方案,最終為糖尿病的防治提供新的思路與模式。
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