·綜述·

腸源性尿毒癥毒素硫酸對甲酚和硫酸吲哚酚的研究進(jìn)展

成云曹學(xué)森鄒建洲

(復(fù)旦大學(xué)附屬中山醫(yī)院腎內(nèi)科，上海市腎病與透析研究所， 上海200032)

關(guān)鍵詞

Research Development of Enterogenous Uremic Toxins: P-cresyl Sulfate and Indoxyl SulfateCHENGYunCAOXuesenZOUJianzhou

DepartmentofNephrology，ZhongshanHospital,FudanUniversity，Shanghai200032，China

尿毒癥毒素是指終末期腎病(end-stage renal disease，ESRD)時不能經(jīng)尿液清除、潴留在體內(nèi)且有毒性作用的物質(zhì)。據(jù)歐洲尿毒癥毒素協(xié)作組(EUTox)統(tǒng)計(jì)，至2011年4月已發(fā)現(xiàn)160種尿毒癥毒素[1]。尿毒癥毒素根據(jù)其理化性質(zhì)可分為3類。(1)不能與蛋白質(zhì)結(jié)合的水溶性小分子物質(zhì)：相對分子質(zhì)量通常小于500，較易經(jīng)血液透析清除，如尿素、肌酐；(2)蛋白質(zhì)結(jié)合物質(zhì)：大多數(shù)相對分子質(zhì)量較小，很難通過血液透析清除，如硫酸對甲酚(p-cresyl sulfate，PCS)、硫酸吲哚酚(indoxyl sulfate，IS)；(3)中分子物質(zhì)：相對分子質(zhì)量通常大于500，常規(guī)血液透析效果不理想，如甲狀旁腺素、β2微球蛋白。尿毒癥毒素根據(jù)其來源分類也可分為3類，(1)內(nèi)源性代謝產(chǎn)物：自身代謝產(chǎn)生，如非對稱性二甲基精氨酸(asymmetric dimethylarginine,ADMA)[2]；(2)微生物代謝產(chǎn)物：主要是腸道菌群代謝物質(zhì)，如吲哚類、酚類；(3)外源性攝入物質(zhì)：如草酸鹽[3]。研究[4- 5]表明，慢性腎臟病(chronic kidney disease，CKD)患者腸道菌群的種類和數(shù)量與健康人群顯著不同，其毒性產(chǎn)物與CKD及其并發(fā)癥的進(jìn)展密切相關(guān)。其中，PCS和IS是當(dāng)前研究最多的腸源性尿毒癥毒素，本文對其研究進(jìn)展作一綜述。

1PCS、IS的產(chǎn)生和代謝

PCS主要在腸道產(chǎn)生，相對分子質(zhì)量188，與血漿白蛋白結(jié)合率為94%[6]。腸道厭氧菌將食物中的苯丙氨酸和酪氨酸轉(zhuǎn)變?yōu)?-羥基苯乙酸。4-羥基苯乙酸脫羧為對甲酚，大部分對甲酚經(jīng)腸道黏膜吸收，在腸道上皮細(xì)胞磺基轉(zhuǎn)移酶的作用下轉(zhuǎn)化為PCS[7]。PCS主要通過腎小管基底膜側(cè)的有機(jī)陰離子轉(zhuǎn)運(yùn)體(organic anion transporter，OAT)分泌到腎小管管腔，經(jīng)尿液排出[8]。

IS主要在腸道產(chǎn)生，相對分子質(zhì)量251[9]，蛋白結(jié)合率達(dá)90%以上。食物中的色氨酸經(jīng)大腸埃希菌分解產(chǎn)生吲哚，吲哚經(jīng)門靜脈進(jìn)入肝臟經(jīng)羥化、硫酸化，最終生成IS。IS主要通過腎小管OAT分泌、排泄[10]。

2PCS、IS的腎臟毒性

PCS可通過促進(jìn)腎臟纖維化加快腎臟病進(jìn)展、腎功能下降[11]。PCS主要通過以下機(jī)制促進(jìn)腎臟纖維化：(1)PCS可顯著增加腎組織腎素、血管緊張素Ⅱ1型受體(AT1R)表達(dá)，激活腎素-血管緊張素-醛固酮系統(tǒng)(renin-angiotensin-aldosterone system，RAAS)，進(jìn)而促進(jìn)腎間質(zhì)成纖維細(xì)胞的增殖與分化，加重腎組織纖維化[12]；(2)PCS具有促炎作用，可促進(jìn)腎間質(zhì)單核細(xì)胞/巨噬細(xì)胞浸潤[11]，上調(diào)促炎因子表達(dá)[13]，引起腎間質(zhì)纖維化；(3)體外實(shí)驗(yàn)證實(shí)，PCS可促進(jìn)小鼠近端腎小管上皮細(xì)胞炎性相關(guān)基因的表達(dá)，如轉(zhuǎn)化生長因子-β(transforming growth factor-β，TGF-β)、白介素-6(interleukin-6，IL-6)等[14]，而TGF-β可促進(jìn)腎小管間質(zhì)纖維化[15]，IL-6可通過誘導(dǎo)腎臟纖維化相關(guān)基因及內(nèi)皮素-1基因的表達(dá)加速CKD的進(jìn)展[16]；(4)Klotho基因可編碼一種參與成纖維細(xì)胞生長因子受體構(gòu)成的跨膜蛋白，這種跨膜蛋白可延緩腎臟纖維化進(jìn)程，發(fā)揮腎臟保護(hù)作用[17-18]，而PCS通過促進(jìn)DNA甲基轉(zhuǎn)移酶表達(dá)，使Klotho基因超甲基化，進(jìn)而抑制Klotho基因表達(dá)[19]，使Klotho基因產(chǎn)物的腎臟保護(hù)作用下降或消失，促進(jìn)腎臟纖維化，加速腎臟病進(jìn)展。

IS促進(jìn)腎臟纖維化的機(jī)制主要有：(1)IS促進(jìn)腎小管上皮細(xì)胞活性氧簇(reactive oxygen species，ROS)的產(chǎn)生，激活核轉(zhuǎn)錄因子κB(nuclear factor-κB，NF-κB)、p53、 細(xì)胞外信號調(diào)節(jié)激酶(extracellular signal-regulated kinase，ERK)等調(diào)節(jié)因子，使單核細(xì)胞趨化蛋白-1(monocyte chemotactic protein-1，MCP-1)、細(xì)胞間黏附分子-1(intercellular adhesion molecule-1，ICAM-1)的表達(dá)上調(diào)，引起單核細(xì)胞/巨噬細(xì)胞在小管間質(zhì)聚集，進(jìn)而促進(jìn)腎臟纖維化[20-21]；(2)IS使腎組織腎素、血管緊張素原、AT1R表達(dá)增加，AT2R表達(dá)減少，進(jìn)而通過激活RAAS及促進(jìn)TGF-β表達(dá)，使腎間質(zhì)細(xì)胞向成纖維細(xì)胞轉(zhuǎn)化，引起腎臟纖維化[22]；(3)IS也可通過促進(jìn)Klotho基因超甲基化而促進(jìn)腎臟纖維化[19]。

3PCS、IS的心血管毒性

大量研究[7，23-24]證實(shí)，PCS水平與CKD患者心血管疾病的發(fā)生及全因死亡獨(dú)立相關(guān)。Schepers等[13]研究發(fā)現(xiàn)，PCS可誘導(dǎo)白細(xì)胞產(chǎn)生自由基，進(jìn)而引起ESRD患者的血管損傷。Watanabe等[25]研究發(fā)現(xiàn)，PCS可使人臍靜脈內(nèi)皮細(xì)胞及人主動脈平滑肌細(xì)胞內(nèi)NADPH氧化酶(NAPDH oxidase，NOX)的表達(dá)顯著增加，促進(jìn)細(xì)胞內(nèi)產(chǎn)生ROS，進(jìn)而損害血管內(nèi)皮細(xì)胞及平滑肌細(xì)胞。Han等[26]研究發(fā)現(xiàn)，PCS可通過增強(qiáng)NOX活性、增加ROS，促進(jìn)心肌細(xì)胞凋亡。

研究[27]顯示，IS可提高ESRD患者全因死亡率及心血管事件發(fā)病率，其機(jī)制主要有以下兩方面。(1)IS可促進(jìn)血管損傷，研究[28]發(fā)現(xiàn)，IS可致循環(huán)中內(nèi)皮損傷標(biāo)志物內(nèi)皮微粒(endothelial microparticles，EMPs)產(chǎn)生增加，提示其有致血管內(nèi)皮損傷作用。IS引起內(nèi)皮損傷主要是通過促氧化應(yīng)激作用實(shí)現(xiàn)的。IS可促進(jìn)NOX活化、使內(nèi)皮細(xì)胞產(chǎn)生的ROS增多[29]，升高的ROS可通過激活NF-κB，增加MCP-1及ICAM-1的表達(dá)[30]，導(dǎo)致血管內(nèi)皮損傷。此外，IS可以通過激活絲裂原活化蛋白激酶(mitogen-activated protein kinase ，MAPK)途徑促進(jìn)血管平滑肌細(xì)胞(vascular smooth muscle cell ，VSMC)增殖[31]；并可通過促進(jìn)骨母細(xì)胞特異性蛋白表達(dá)增加而加重動脈鈣化、使動脈壁增厚[32]。近年來研究[33]證實(shí)，IS可促進(jìn)大鼠主動脈細(xì)胞衰老相關(guān)蛋白，如p16INK4a、p21WAF1/CIP1的表達(dá)，提示IS有加速動脈衰老作用。(2)IS可加速心肌損傷，研究[34]發(fā)現(xiàn)，IS可通過促氧化應(yīng)激、削弱抗氧化屏障作用促進(jìn)心肌纖維化及心肌細(xì)胞肥大。此外，IS可通過抑制單磷酸腺苷活化蛋白激酶/解偶聯(lián)蛋白2(AMP-activated protein kinase/uncoupling protein 2，AMPK/UCP2)途徑促進(jìn)心肌肥大[35]。

4PCS、IS的其他作用

近年研究提示，PCS可能與CKD相關(guān)的胰島素抵抗有關(guān)。PCS通過激活胰島素信號轉(zhuǎn)導(dǎo)通路中的ERK1/2誘導(dǎo)小鼠出現(xiàn)胰島素抵抗，使其脂肪含量減少，脂肪在肝臟及肌肉重新分布[36]。骨代謝方面，Tanaka等[37]研究發(fā)現(xiàn)，PCS通過激活c-Jun 氨基末端激酶( c-Jun N-terminal kinase，JNK) 和p38分裂原激活蛋白激酶(p38 mitogen activated protein kinases，p38MAPK)信號轉(zhuǎn)導(dǎo)途徑導(dǎo)致成骨細(xì)胞功能障礙，引起腎性骨病。

Kim等[38]研究發(fā)現(xiàn)，IS可通過抑制成骨細(xì)胞的分化、誘導(dǎo)成骨細(xì)胞凋亡，從而引起骨骼病變。此外，研究[39]發(fā)現(xiàn)，IS可導(dǎo)致體外培養(yǎng)的成骨細(xì)胞抵抗甲狀旁腺激素，從而導(dǎo)致腎性骨病的發(fā)生。

5PCS和IS的清除

PCS和IS均為蛋白質(zhì)高親和力毒素，常規(guī)透析方法難以清除。Meert等[40]研究發(fā)現(xiàn)，透析中增加對流量也利于PCS及IS的清除。不同材質(zhì)的透析膜對這兩種毒素的清除率無差異[41]。Meijers等[42]發(fā)現(xiàn)，血漿分離吸附技術(shù)對PCS的清除效果顯著優(yōu)于高通量透析，但血漿分離吸附技術(shù)成本高昂，目前無法在臨床推廣。

此外，由于PCS和IS主要由腸道產(chǎn)生，理論上可以通過改變腸道菌群降低PCS和IS的濃度，但目前尚無相關(guān)研究。目前研究較多的腸道吸附劑，如AST-120，Owada 等[43]的研究顯示，AST-120可清除部分腸源性毒素，并可延緩尿毒癥大鼠的腎功能惡化。

6展望

目前對PCS及IS 作用機(jī)制的了解已較深入，但是仍無有效的、適合臨床應(yīng)用的清除PCS及IS的透析方式或藥物，需要進(jìn)一步探索。

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中圖分類號R692.5

文獻(xiàn)標(biāo)識碼A

通訊作者鄒建洲，E-mail：jianzzou@163.com

基金項(xiàng)目：上海市科學(xué)技術(shù)委員會基金項(xiàng)目(編號：15DZ0503402)