膽汁酸的合成調(diào)控及其在生理與病理中的功能機(jī)制

劉笑，王琰

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膽汁酸的合成調(diào)控及其在生理與病理中的功能機(jī)制

劉笑，王琰

武漢大學(xué)生命科學(xué)學(xué)院，細(xì)胞穩(wěn)態(tài)湖北省重點(diǎn)實(shí)驗(yàn)室，武漢 430072

膽汁酸是一類膽固醇的代謝物，在機(jī)體膽固醇與能量代謝平衡和小腸營養(yǎng)物質(zhì)吸收等方面起著重要作用。肝臟是合成膽汁酸的主要場(chǎng)所。饑餓條件下，膽汁酸從肝臟分泌進(jìn)入膽管并被儲(chǔ)存到膽囊；進(jìn)食后膽囊收縮，貯存的膽汁酸被排出進(jìn)入小腸。在小腸中，95%的膽汁酸會(huì)被小腸重新吸收，通過肝門靜脈返回肝臟，這一過程被稱為膽汁酸的肝腸循環(huán)。膽汁酸一方面作為乳化劑促進(jìn)小腸中脂類等物質(zhì)的吸收及轉(zhuǎn)運(yùn)，同時(shí)也作為重要的信號(hào)分子與多種受體結(jié)合，包括核受體法呢醇X受體(farnesoid X receptor, FXR)、維生素D受體(vitamin D receptor, VDR)、孕烷X受體(pregnane X receptor, PXR)以及細(xì)胞膜表面受體G蛋白偶聯(lián)受體(cell membrane surface receptor-G protein coupled receptor, TGR5)等，在調(diào)節(jié)體內(nèi)膽汁酸的代謝平衡、糖脂代謝與能量代謝平衡等方面發(fā)揮重要作用。肝細(xì)胞生長因子(hepatocyte growth factor, HGF)、白介素1-b(interleukin-1b, IL-1b)及腫瘤壞死因子(tumor necrosis factora, TNF-a)等協(xié)同作用構(gòu)成了膽汁酸合成的精密調(diào)控網(wǎng)絡(luò)。本文主要綜述了膽汁酸的合成調(diào)控及其功能方面的最新研究進(jìn)展，旨在為膽汁酸代謝相關(guān)研究提供參考。

膽汁酸；膽固醇7a羥化酶；法呢醇X受體；代謝；調(diào)控

膽固醇是機(jī)體內(nèi)膜性結(jié)構(gòu)的重要組成物質(zhì)，其代謝紊亂會(huì)引發(fā)動(dòng)脈粥樣硬化和冠心病等一系列代謝性疾病[1]。體內(nèi)膽固醇的來源可以分為外源食物攝取和內(nèi)源機(jī)體合成。機(jī)體無法直接將膽固醇分解，但是可以利用膽固醇為原料，經(jīng)過一系列的酶促催化反應(yīng)將其轉(zhuǎn)化為膽汁酸。肝臟合成的膽汁酸以及部分游離膽固醇以膽汁的形式從肝臟分泌進(jìn)入膽管，并最終分泌至小腸。進(jìn)入小腸中的膽汁酸95%以上會(huì)被小腸重新吸收，然后通過肝臟門靜脈循環(huán)進(jìn)入肝臟，另外5%左右會(huì)以糞便的形式排出體外[2]。機(jī)體通過調(diào)節(jié)膽汁酸的合成、分泌及重吸收等過程精確調(diào)節(jié)體內(nèi)膽汁酸及膽固醇的穩(wěn)態(tài)平衡。而體內(nèi)的膽汁酸也是一種信號(hào)分子，能夠與其受體核受體法呢醇X受體(farnesoid X receptor, FXR)和細(xì)胞膜表面受體G蛋白偶聯(lián)受體(cell membrane surface receptor-G protein coupled receptor, TGR5)等相互作用，啟動(dòng)下游信號(hào)通路。本文將從膽汁酸的生物合成、肝腸循環(huán)及膽汁酸合成限速酶CYP7A1的表達(dá)調(diào)控等方面，總結(jié)近年來膽汁酸的合成調(diào)控及功能機(jī)制研究進(jìn)展，以期為膽汁酸代謝調(diào)控分子機(jī)制的研究提供參考。

1 膽汁酸的生物合成

體內(nèi)膽固醇水平的穩(wěn)態(tài)主要由膽固醇的外源攝取、膽固醇的體內(nèi)合成及膽固醇外排協(xié)調(diào)控制的，其中膽固醇經(jīng)過一系列的酶促反應(yīng)生成膽汁酸是膽固醇代謝的主要去路[3]。人體內(nèi)膽汁酸的合成通路包括經(jīng)典通路和非經(jīng)典通路[4]。經(jīng)典通路是在肝臟中，以定位于肝細(xì)胞內(nèi)質(zhì)網(wǎng)上的膽固醇7-a羥化酶(cholesterol 7a-hydroxylase, CYP7A1)為主要限速酶經(jīng)過一系列的催化反應(yīng)發(fā)生的，生成膽酸(cholic acid, CA)和鵝脫氧膽酸(chenodeoxycholic acid, CDCA)兩種疏水性初級(jí)膽汁酸。非經(jīng)典通路發(fā)生在多種組織及巨噬細(xì)胞中，以定位于線粒體的甾醇27A羥化酶(sterol 27A-hydroxylase, CYP27A1)和定位于內(nèi)質(zhì)網(wǎng)的氧甾醇和類固醇7a-羥化酶(oxysterol and steroid 7a-hydroxylase, CYP7B1)啟動(dòng)發(fā)生的[5,6]。Axelson等[7]的研究認(rèn)為，非經(jīng)典通路主要發(fā)生在一些病理狀態(tài)下，當(dāng)肝臟中CYP7A1的活性下降時(shí)，非經(jīng)典通路通過產(chǎn)生鵝脫氧膽酸調(diào)節(jié)體內(nèi)的代謝平衡。肝臟中生成的疏水性初級(jí)膽汁酸可以被甘氨酸或?；撬峁矁r(jià)修飾形成膽酸鹽。膽酸鹽較初級(jí)膽汁酸的水溶性增加，降低了膽汁酸的毒性[8]，使其可以被分泌到小腸。小腸中的腸道菌群可進(jìn)一步代謝膽酸鹽，使其脫去羥基，移去甘氨酸和?；撬嵝纬纱渭?jí)膽汁酸，即脫氧膽酸(deoxycholic acid, DCA)和石膽酸(lithocholic acid, LCA)。CA、DCA及CDCA可以被小腸的刷狀緣細(xì)胞重吸收經(jīng)過門靜脈循環(huán)被運(yùn)回到肝臟[9]。

2 膽汁酸的肝腸循環(huán)

肝臟中生成的膽酸鹽通過肝細(xì)胞表面的膽汁酸轉(zhuǎn)運(yùn)蛋白-膽鹽輸出泵(bile salt export pump, BSEP)被運(yùn)送到膽小管，并儲(chǔ)存在膽囊中。當(dāng)進(jìn)食后，膽囊收縮，將膽汁酸分泌到腸道[10]，少部分膽汁酸可通過被動(dòng)吸收的方式被十二指腸吸收。其中95%的膽汁酸在回腸中被主動(dòng)吸收[9]，這一過程依賴于小腸刷狀緣細(xì)胞表面的Na+依賴的膽汁酸轉(zhuǎn)運(yùn)體(apical sodium dependent bile acid transporter, ASBT)。進(jìn)入小腸細(xì)胞內(nèi)的膽汁酸可以從腸上皮細(xì)胞極性膜一側(cè)轉(zhuǎn)移到基底膜，通過基底膜上的有機(jī)溶質(zhì)轉(zhuǎn)運(yùn)體(organic solute transporteraandbheterdimer, OSTa/OSTb)異源二聚體排出細(xì)胞，進(jìn)而被轉(zhuǎn)運(yùn)到肝臟門靜脈。到達(dá)肝臟的膽汁酸被肝細(xì)胞細(xì)胞膜表面的Na+依賴的?；撬猁}協(xié)同轉(zhuǎn)運(yùn)肽(Na+-dependent taurocholate co-transport peptide, NTCP)吸收進(jìn)入肝細(xì)胞。這一過程被稱為膽汁酸的肝腸循環(huán)。人體內(nèi)的膽汁酸總量大約有3 g，每天可以進(jìn)行4~12次的肝腸循環(huán)。人體糞便中每天排出的膽汁酸大約有0.5 g，這部分膽汁酸通過肝臟中膽汁酸的從頭合成途徑生成，從而維持膽汁酸總量的動(dòng)態(tài)平衡[4,7]。

膽汁酸的肝腸循環(huán)促進(jìn)了脂類及維生素等營養(yǎng)物質(zhì)的乳化和吸收[11]，并且使肝臟內(nèi)膽汁酸的合成和小腸內(nèi)膽汁酸的重吸收協(xié)同作用，共同維持機(jī)體膽汁酸及膽固醇的代謝平衡。

3 以CYP7A1為靶標(biāo)的膽汁酸合成的代謝調(diào)控

CYP7A1作為膽汁酸合成的關(guān)鍵限速酶[3]，其表達(dá)調(diào)控對(duì)于維持機(jī)體膽汁酸的穩(wěn)態(tài)發(fā)揮了重要作用。研究發(fā)現(xiàn)，F(xiàn)XR作為膽汁酸的受體，在機(jī)體膽汁酸負(fù)反饋調(diào)節(jié)過程中發(fā)揮了重要作用[12,13]。FXR主要表達(dá)于肝臟和小腸[14]，響應(yīng)膽汁酸的刺激，在小腸和肝臟中分別通過不同的調(diào)控途徑負(fù)反饋抑制CYP7A1的表達(dá)。機(jī)體中CYP7A1的表達(dá)水平除了受FXR調(diào)節(jié)之外，還受到FXR非依賴途徑的調(diào)節(jié)，這一途徑受到多種細(xì)胞因子、激素和酶的調(diào)節(jié)。這些因子共同作用，確保機(jī)體能夠響應(yīng)不同環(huán)境刺激，維持機(jī)體的正常運(yùn)行。

3.1 FXR依賴的CYP7A1表達(dá)調(diào)控

3.1.1 肝臟中FXR參與的CYP7A1表達(dá)調(diào)控

FXR作為膽汁酸的感應(yīng)器負(fù)反饋抑制CYP7A1的表達(dá)[12,15]。FXR高表達(dá)于肝臟和小腸，在肝臟中CA及CDCA可以激活FXR核受體活性[5]。但是CYP7A1啟動(dòng)子區(qū)沒有FXR的結(jié)合域，F(xiàn)XR通過與其他基因相互作用間接抑制CYP7A1的表達(dá)。活化的FXR首先與視黃酸X受體a(retinoid X receptora, RXRa)結(jié)合形成異源二聚體，此異源二聚體可與目的基因啟動(dòng)子區(qū)的法呢醇X受體響應(yīng)元件(farnesoid X receptor response element, FXREs)結(jié)合，進(jìn)而上調(diào)或抑制基因的表達(dá)[16]。Goodwin等[15]經(jīng)過大量篩選找到了FXR特異性的激活劑GW4064，發(fā)現(xiàn)GW4064對(duì)FXR的激活作用是CDCA的1000倍。GW4064刺激人和大鼠肝細(xì)胞時(shí)，小異二聚體伴侶(small heterodimer partner-1, SHP-1)的mRNA含量明顯增加[17]。SHP-1是一個(gè)非典型的核孤兒受體家族成員，它缺乏DNA結(jié)合域，含有一個(gè)N端受體二聚化結(jié)合域，SHP-1通過其N端二聚化受體結(jié)合域招募其他受體，并通過抑制這些受體的轉(zhuǎn)錄來調(diào)節(jié)其他下游基因的表達(dá)。SHP-1在肝臟中低表達(dá)，在膽汁酸刺激的情況下表達(dá)量迅速升高。SHP-1 KO小鼠中，CYP7A1的表達(dá)水平明顯上調(diào)，膽汁酸池明顯增大。SHP-1過表達(dá)小鼠中膽汁酸池明顯減小，并伴隨甘油三酯的堆積[18,19]。SHP-1可以與核孤兒受體肝臟相關(guān)同系物1 (liver related homologue-1, LRH-1)結(jié)合并抑制此受體的活性。LRH-1是核受體家族的胞內(nèi)轉(zhuǎn)錄因子，可以與CYP7A1的啟動(dòng)子區(qū)結(jié)合，上調(diào)CYP7A1的轉(zhuǎn)錄[17,20]。另一方面，LRH-1可以通過促進(jìn)腸上皮細(xì)胞分泌成纖維細(xì)胞生長因子15 (fibroblast growth factor 15, FGF15)進(jìn)而抑制CYP7A1的合成[16,18]。

綜上所述，在肝臟中，CA及CDCA可以結(jié)合并活化FXR，活化的FXR首先與RXRa結(jié)合形成異源二聚體，此異源二聚體可以與SHP-1的啟動(dòng)子區(qū)結(jié)合并上調(diào)SHP-1的表達(dá)，SHP-1通過與LRH-1相互作用從而抑制CYP7A1的表達(dá)(圖1)[17,19]。

3.1.2 小腸中FXR參與的CYP7A1表達(dá)調(diào)控

小腸中的FXR通過調(diào)節(jié)內(nèi)分泌成纖維細(xì)胞生長因子15/19 (fibroblast growth factor 15/19, FGF15/19)的表達(dá)來抑制肝臟中CYP7A1的表達(dá)[20~22]。FGF19是成纖維細(xì)胞生長因子(fibroblast growth factor, FGFs)亞家族的成員之一，包括FGF19、FGF21和FGF23，是膽汁酸合成、葡糖糖吸收、脂代謝、維生素D和磷酸鹽穩(wěn)態(tài)的重要調(diào)控因子。小鼠中沒有FGF19，研究發(fā)現(xiàn)小鼠FGF15蛋白的氨基酸序列與人FGF19蛋白的氨基酸序列有51%的同源性，并發(fā)揮了相似作用[23]。FGF19高表達(dá)于十二指腸和回腸，低表達(dá)于肝臟[24]。當(dāng)小腸中的膽汁酸濃度增加時(shí)FXR被激活，活化的FXR進(jìn)而上調(diào)FGF19的表達(dá)，F(xiàn)GF19可以通過旁分泌和內(nèi)分泌途徑發(fā)揮作用(圖1)。FGF19分泌到血液后隨肝門靜脈循環(huán)被運(yùn)回到肝臟，與肝細(xì)胞表面的成纖維細(xì)胞生長因子受體4 (fibroblast growth factor receptors 4, FGFR4)結(jié)合并磷酸化激活FGFR4受體酪氨酸激酶活性，通過有絲分裂原活化蛋白激酶(mitogen-activated protein kinase, MAPK)級(jí)聯(lián)反應(yīng)抑制CYP7A1的表達(dá)[25,26]。FGF19與FGFR4的作用需要b-Klotho的輔助。b-Klotho是一個(gè)大小為130 kDa的1型跨膜蛋白，主要表達(dá)于肝臟、脂肪以及胰腺[27]。b-Klotho缺失小鼠與FGFR4基因敲除鼠的表型非常一致，都伴隨著CYP7A1轉(zhuǎn)錄水平的增加和膽汁酸池的增大[21]。b-Klotho可與FGFs作用并促進(jìn)FGFs與受體的結(jié)合[28]。研究發(fā)現(xiàn)FGFR4與b-Klotho的相互作用發(fā)生在內(nèi)質(zhì)網(wǎng)。FGFR4含有3個(gè)糖基化修飾位點(diǎn)(N112、N258和N290)，在內(nèi)質(zhì)網(wǎng)中形成核心糖基化FGFR4，核心糖基化FGFR4轉(zhuǎn)運(yùn)到高爾基體后經(jīng)過進(jìn)一步的修飾形成終端糖基化FGFR4。在b-Klotho存在時(shí)，F(xiàn)GFR4主要以終端糖基化形式存在。在內(nèi)質(zhì)網(wǎng)中b-Klotho與核心糖基化FGFR4相互結(jié)合，促進(jìn)了核心糖基化FGFR4的蛋白酶體降解，從而使細(xì)胞內(nèi)的終端糖基化FGFR4得到富集。研究還發(fā)現(xiàn)FGF19只與終端糖基化FGFR4結(jié)合并磷酸化激活FGFR4受體酪氨酸激酶活性，進(jìn)而抑制CYP7A1的表達(dá)[29,30]。小腸中的FXR對(duì)肝臟中膽汁酸的合成起了重要的負(fù)反饋調(diào)節(jié)作用[31]。

3.2 FXR非依賴的CYP7A1表達(dá)調(diào)控

3.2.1 組蛋白去乙酰化酶參與的膽汁酸代謝調(diào)控

膽固醇水平過高會(huì)引發(fā)動(dòng)脈粥樣硬化以及心腦血管等疾病。膽固醇轉(zhuǎn)變?yōu)槟懼崾悄懝檀即x的主要去路，在這個(gè)過程中發(fā)揮關(guān)鍵催化作用的酶是膽固醇7-a羥化酶CYP7A1[3]。研究發(fā)現(xiàn)，在HepG2細(xì)胞中，用CDCA刺激1 h后CYP7A1的表達(dá)有顯著下調(diào)，但此時(shí)FXR、SHP和FGF19的表達(dá)沒有變化，刺激16 h后SHP和FGFG19的mRNA含量明顯增加[32]。但是用FXR的特異性激活劑GW4064刺激1 h[33]，并未看到CYP7A1的下調(diào)。這說明起始CYP7A1的下調(diào)不依賴于FXR途徑[34]。研究還發(fā)現(xiàn)在CDCA刺激的初始階段，組蛋白去乙酰化酶7 (histone deacetylase 7, HDCA7)參與了CYP7A1的抑制調(diào)控。HDCA7正常情況下定位在細(xì)胞膜上，但是在BA刺激情況下會(huì)移位到細(xì)胞核內(nèi)，并被招募到CYP7A1的啟動(dòng)子區(qū)，與組蛋白去乙?；?(histone deacetylase 3, HDAC3)、視黃酸和甲狀腺受體沉默介質(zhì)(silencingmediator forretinoid andthyroid receptors, SMRT–a)以及核受體輔阻遏物(nuclear receptor co-repressor, N-COR)相互作用促進(jìn)RNA聚合酶2的解離，從而抑制CYP7A1的表達(dá)[32,35]。

圖1 膽汁酸的肝腸循環(huán)及代謝調(diào)控

肝臟中的膽固醇在CYP7A1等一系列酶的催化作用下可代謝為膽汁酸。一方面，肝臟中的膽汁酸通過激活FXR/SHP信號(hào)通路，抑制LRH-1的表達(dá)進(jìn)而抑制的CYP7A1的表達(dá)。另一方面，肝臟中合成的膽汁酸在BSEP和ASBT的作用下被吸收進(jìn)入小腸。在小腸中，膽汁酸激活FXR通路，刺激腸上皮細(xì)胞分泌FGF15/19，分泌的FGF15/19通過肝腸循環(huán)途徑進(jìn)入肝臟與肝細(xì)胞表面的FGFR4及b-Klotho相互作用抑制CYP7A1的表達(dá)。小腸中的膽汁酸通過OSTa/OSTb和NTCP的作用被吸收進(jìn)入肝細(xì)胞。GW4064是FXR的特異性激活劑。

3.2.2 胰高血糖素參與的膽汁酸代謝調(diào)控

在肝臟中胰高血糖素通過蛋白激酶A (protein kinase A, PKA)通路激活磷酸烯醇丙酮酸羧化激酶(phosphoenolpyruvate carboxykinase, PEPCK)并磷酸化肝細(xì)胞核因子(hepatocyte nuclear factor 4a, HNF4a)第304位的絲氨酸殘基，從而抑制CYP7A1的表達(dá)。HNF4a是一個(gè)核轉(zhuǎn)錄因子，可以與CYP7A1的DR-1序列結(jié)合，從而激活CYP7A1的轉(zhuǎn)錄[36]。但是當(dāng)HNF4a第304位絲氨酸殘基被磷酸化后就會(huì)降低其與CYP7A1的結(jié)合能力和反式激活能力。因CYP7A1特異性的在肝臟中表達(dá)，PKA依賴的CYP7A1的抑制是肝臟特異性的，但是PKA依賴的PEPCK的活化在肝臟和腎臟中都存在。胰高血糖素在饑餓時(shí)分泌增加，所以CYP7A1的表達(dá)受饑餓進(jìn)食影響[37]。胰高血糖素刺激下CYP7A1的轉(zhuǎn)錄被抑制，使體內(nèi)膽汁酸的合成被抑制，進(jìn)一步在膽汁酸乳化作用下脂的吸收水平下降[38]，從而確保糖異生最大程度的被活化以維持體內(nèi)的糖代謝和能量代謝。

3.2.3 細(xì)胞因子參與的膽汁酸代謝調(diào)控

肝細(xì)胞生長因子(hepatocyte growth factor, HGF)通過與酪氨酸激酶受體c-Met結(jié)合使其磷酸化被活化，進(jìn)而調(diào)控了下游包括Ras、MAPK、PIP3及PKC等信號(hào)通路，在促進(jìn)細(xì)胞生長、增殖、凋亡、創(chuàng)傷修復(fù)和組織再生等過程中發(fā)揮了重要作用[39]。肝臟部分切除后，在肝臟再生過程中膽汁酸的合成及CYP7A1的表達(dá)被抑制，同時(shí)血液中HGF的含量明顯上調(diào)，HGF在肝再生過程中對(duì)膽汁酸的代謝調(diào)控發(fā)揮了重要作用[40~42]。研究發(fā)現(xiàn)，在人的原代肝細(xì)胞中，HGF可以顯著抑制CYP7A1的表達(dá)。其作用機(jī)理是HGF通過與c-Met結(jié)合激活c-Met的磷酸酪氨酸激酶活性，通過磷酸化Erk1/2、JNK及PKC來抑制CYP7A1的表達(dá)。同時(shí)，HGF可以上調(diào)SHP-1的表達(dá)，進(jìn)而抑制CYP7A1的轉(zhuǎn)錄(圖2)[43]。HGF對(duì)CYP7A1的抑制，在肝損傷再生過程中發(fā)揮了重要作用，使肝細(xì)胞內(nèi)膽汁酸濃度維持在一個(gè)較低水平，防止高膽汁酸濃度對(duì)細(xì)胞的毒害作用。

腫瘤壞死因子a(tumor necrosis factora, TNF-a)通過激活MAPKs家族的促分裂原活化蛋白激酶1 (mitogen-activated protein kinase kinase kinase 1, MEKK-1)進(jìn)而磷酸化結(jié)合在CYP7A1膽汁酸響應(yīng)元件序列區(qū)的HNF4a，從而降低HNF4a與CYP7A1的結(jié)合，抑制轉(zhuǎn)錄因子HNF4a的反式激活能力，從而下調(diào)CTP7A1的表達(dá)(圖2)[34]。

膽汁酸可以激活肝臟巨噬細(xì)胞分泌炎癥因子IL-1b，同時(shí)IL-1b又可以抑制膽汁酸的合成[43]。研究發(fā)現(xiàn)，IL-1b可以激活c-Jun的轉(zhuǎn)錄，一方面c-Jun可以抑制HNF4a招募過氧化物酶體增生激活受體γ的共活化因子1a(peroxisome proliferator-activated receptor γ co-activator 1a, PGC1-a)；另一方面c-Jun可以被c-Jun-NH2末端激酶JNK磷酸化活化并抑制HNF4a與CYP7A1的結(jié)合，從而抑制CYP7A1的表達(dá)，以保護(hù)肝細(xì)胞免受炎癥介導(dǎo)的毒害作用(圖2)[44]。

4 其他膽汁酸受體參與的代謝調(diào)控

4.1 TGR5參與的代謝調(diào)控

膽汁酸作為細(xì)胞膜表面蛋白-G蛋白偶聯(lián)受體TGR5的配體在能量代謝過程中發(fā)揮了重要作用[5,45]。TGR5廣泛表達(dá)于小腸及膽囊的內(nèi)皮細(xì)胞、肝竇內(nèi)皮細(xì)胞和星狀巨噬細(xì)胞，但是在肝實(shí)質(zhì)細(xì)胞中不表達(dá)[46,47]。研究發(fā)現(xiàn)，在人和小鼠脂肪細(xì)胞中，DCA和LCA可以激活TGR5，活化的TGR5通過腺苷酸環(huán)化酶(cyclic adenosine monophosphate, cAMP)通路促進(jìn)了甘油三酯的分解、脂肪酸的b氧化及線粒體的分裂和形成，從而促進(jìn)了皮下白色脂肪組織的米色化和能量代謝[48]。在小鼠的棕色脂肪組織中，膽汁酸通過與TGR5結(jié)合，可激活腺苷酸環(huán)化酶，進(jìn)而激活2型碘化甲狀腺氨酸脫碘酶(type2 iodothyronine deiodinases, D2)，D2可以使抑制型的甲狀腺激素T4轉(zhuǎn)變?yōu)榛钴S型的甲狀腺激素T3[49]，從而促進(jìn)能量代謝，抑制肥胖的發(fā)生并提高胰島素敏感性[50]。在小鼠腸內(nèi)分泌細(xì)胞系中，膽汁酸通過激活TGR5進(jìn)而促進(jìn)了胰高血糖素樣肽-1 (glucagon-like peptide-1, GLP-1)的分泌。GLP-1可刺激胰島素的合成，促進(jìn)胰島素從胰島b細(xì)胞分泌，在調(diào)節(jié)血糖平衡，抑制糖尿病發(fā)生過程中發(fā)揮了重要作用[46,51]。最近的研究發(fā)現(xiàn)，當(dāng)用小腸FXR的特異性激動(dòng)劑Fexaramine (FEX)刺激小鼠時(shí)，可誘導(dǎo)腸道微生物分泌LCA，分泌的LCA可激活TGR5/GLP-1通路，進(jìn)而提高了胰島素的敏感性以及白色脂肪組織的棕色化，促進(jìn)了能量代謝[47]。這為治療肥胖、糖尿病和非酒精性脂肪肝的研究提供了靶標(biāo)。

圖2 FXR非依賴的CYP7A1表達(dá)調(diào)控

在肝臟中，F(xiàn)XR非依賴的CYP7A1表達(dá)調(diào)控主要有以下幾條途徑：（1）膽汁酸通過激活炎癥因子IL-1b，進(jìn)一步激活c-Jun的轉(zhuǎn)錄，通過磷酸化HNF4a來抑制CYP7A1的表達(dá)；（2）膽汁酸也可激活HDCA7，在HDAC3以及共軛抑制子SMRT–a和N-COR的相互作用下抑制CYP7A1的表達(dá)；（3）腫瘤壞死因子TNF-a通過激活MEKK1進(jìn)而磷酸化HNF4a來抑制CYP7A1的表達(dá)；（4）肝細(xì)胞生長因子HGF通過激活c-Met，進(jìn)而激活Erk1/2、PKA或JNK通路從而抑制CYP7A1的表達(dá)。

膽汁酸激活的TGR5代謝通路在天然免疫、細(xì)胞的增殖和遷移以及癌癥發(fā)生過程中也發(fā)揮重要的作用。最近的研究發(fā)現(xiàn)，病毒感染后誘導(dǎo)膽汁酸合成與轉(zhuǎn)運(yùn)通路的激活，胞內(nèi)的膽汁酸通過其受體TGR5激活RIG-1和MAVS通路，在抗病毒天然免疫過程中發(fā)揮重要作用[52]。也有文獻(xiàn)報(bào)道，TGR5被膽汁酸(包括LCA、DCA、CDCA和CA)活化后，通過Janus激酶2/信號(hào)轉(zhuǎn)導(dǎo)和轉(zhuǎn)錄活化因子3 (janus kinase 2/signal transducer and activator of transcription 3, JAK2/STAT3)通路可促進(jìn)非小細(xì)胞肺癌的增值和遷移[53,54]，促進(jìn)癌癥的發(fā)生，這為非小細(xì)胞肺癌的治療提供了靶點(diǎn)[55]。

4.2 孕烷X受體參與的代謝調(diào)控

研究發(fā)現(xiàn)，在慢性膽汁淤積性肝病患者體內(nèi)，LCA的濃度偏高[56]。膽汁淤積，即膽汁流動(dòng)停止或減少，會(huì)導(dǎo)致營養(yǎng)代謝失衡，脂質(zhì)吸收不良，并導(dǎo)致對(duì)肝臟具有毒性的膽汁酸的淤積，從而使肝臟遭受不可逆的損傷[57]。肝臟主要通過兩條途徑排出毒性膽汁酸：一是羥基化；二是氨基酸修飾。孕烷X受體(pregnane X receptor, PXR)可以與孕烷孕烯醇酮16a-碳腈以及LCA結(jié)合并被激活，活化的PXR通過激活細(xì)胞色素P450-3A (cytochrome P450-3A, CYP3A)的表達(dá)促進(jìn)LCA的6羥基化[58,59]，增加了疏水性LCA的水溶性，從而降低其毒性。研究還發(fā)現(xiàn)，丹參酮IIA (tanshinone IIA, Tan IIA)是PXR的有效激活劑，活化的PXR通過誘導(dǎo)CYP3A的表達(dá)降低LCA的毒性[60]。Tan IIA是從丹參根中提取的天然活性物質(zhì)，具有肝保護(hù)作用[61]。

PXR可以與HNF4a和PGC1-a相互作用調(diào)節(jié)CYP7A1的表達(dá)。半合成藥物利福平，可以激活PXR核受體活性，活化的PXR可以與HNF4a相互作用，抑制PGC1-a與HNF4a的相互作用，進(jìn)而抑制CYP7A1的轉(zhuǎn)錄[62]。

4.3 維生素D受體參與的代謝調(diào)控

維生素D受體(vitamin D receptor, VDR)在小腸中高表達(dá)，在人的肝細(xì)胞中表達(dá)量較低，在小鼠的肝臟中不表達(dá)。VDR作為LCA的受體，對(duì)于小腸中毒性膽汁酸LCA的代謝具有重要意義[63]。研究發(fā)現(xiàn)LCA對(duì)VDR的敏感性是PXR的10倍。在LCA或維生素D作用下VDR可被活化，活化的VDR通過CYP3A途徑對(duì)LCA進(jìn)行羥基化，從而降低毒性膽汁酸的濃度[64]。

小腸中VDR的缺失可降低小腸CYP3A的表達(dá)，抑制LCA的代謝，但同時(shí)可間接上調(diào)膽汁酸轉(zhuǎn)運(yùn)蛋白的表達(dá)，促進(jìn)膽汁酸的肝腸循環(huán)，使大量的毒性膽汁酸被轉(zhuǎn)運(yùn)到肝臟，造成肝臟膽汁淤積并產(chǎn)生肝毒性。小腸中的VDR對(duì)于維持小腸屏障具有重要意義，在小腸中過表達(dá)CYP3A可促進(jìn)小腸LCA的羥基化，保護(hù)小腸屏障[65]。

5 結(jié)語與展望

膽汁酸作為信號(hào)分子廣泛參與了體內(nèi)的糖脂代謝和能量代謝。近年來，對(duì)CYP7A1依賴的膽汁酸的合成調(diào)控已有深入的研究。形成了以CYP7A1為核心，核受體、細(xì)胞膜表面受體、細(xì)胞因子和酶等共同參與的代謝調(diào)控網(wǎng)絡(luò)。多基因共同調(diào)控以確保機(jī)體在不同生理及外界刺激條件下體內(nèi)代謝穩(wěn)態(tài)的維持。

然而，在該領(lǐng)域內(nèi)仍有一些問題尚未解決。膽汁酸受體FXR能夠響應(yīng)膽汁酸和GW4064的刺激，抑制膽汁酸合成限速酶CYP7A1的表達(dá)，GW4064激活的FXR通路主要通過SHP-1來抑制CYP7A1的表達(dá)，但是膽汁酸激活的FXR通路存在SHP-1依賴和SHP-1非依賴兩條途徑[18,19]。SHP-1非依賴的膽汁酸負(fù)反饋調(diào)節(jié)通路的分子機(jī)制尚不清楚。TGR5是已知的膽汁酸膜蛋白受體，該受體主要表達(dá)在膽囊等器官，在肝臟和小腸等膽汁酸代謝相關(guān)重要器官幾乎不表達(dá)。越來越多的證據(jù)表明，膽汁酸是體內(nèi)一類非常重要的信號(hào)分子，在肝臟和小腸等膽汁酸代謝重要組織中是否存在其他的膽汁酸的膜蛋白受體仍完全未知。這些問題的解決是全面認(rèn)識(shí)膽汁酸及膽固醇代謝的重要途徑。

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An overview of bile acid synthesis and its physiological and pathological functions

Xiao Liu, Yan Wang

Bile acids are a class of cholesterol derivatives that play important roles in cholesterol and energy homeostasis and intestinal nutrition absorption. Bile acids are mainly synthesized in the liver. During fasting, bile acids are secreted from the liver and stored in the gallbladder. After a meal, the stored bile acids are released into small intestines. In the intestine, about 95% of bile acids will be re-absorbed and travel back into the liver through port veins, which is called bile acid enterohepatic circulation. This enterohepatic circulation of bile acids plays important roles in the emulsification and intestinal absorption of lipids and other nutrition. On the other hand, bile acids function as ligands for a number of receptors, such as farnesoid X receptor (FXR), proterane X receptor (PXR), vitamin D receptor (VDR) and cell membrane surface receptor-G protein coupled receptor (TGR5), which play important roles from metabolic homeostasis to innate immunity. A number of cytokines such as hepatocyte growth factor (HGF), interleukin-1b(IL-1b) and tumor necrosis factora(TNF-a) regulate the homeostasis of bile acids. In the current review, we will summarize the recent progress in the regulation of bile acid synthesis and its physiological and pathological functions from energy homeostasis to innate immunity and cancer progression to provide a reference for the study of bile acid metabolism.

bile acid; CYP7A1; FXR; metabolism; regulation

2019-01-09;

2019-03-18

國家自然科學(xué)基金項(xiàng)目(編號(hào)：91754101)資助[Supported by the National Natural Science Foundation of China (No. 91754101)]

劉笑，碩士研究生，專業(yè)方向：脂類代謝。E-mail: 2016202040152@whu.edu.cn

王琰，博士，教授，博士生導(dǎo)師，研究方向：脂類代謝。E-mail: Wang.y@whu.edu.cn

10.16288/j.yczz.19-011

2019/3/26 9:24:44

URI: http://kns.cnki.net/kcms/detail/11.1913.R.20190326.0924.001.html

(責(zé)任編委: 陳雁)