肠源性尿毒症毒素硫酸对甲酚和硫酸吲哚酚的研究进展

·综述·

肠源性尿毒症毒素硫酸对甲酚和硫酸吲哚酚的研究进展

成云曹学森邹建洲

(复旦大学附属中山医院肾内科，上海市肾病与透析研究所， 上海200032)

关键词

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

DepartmentofNephrology，ZhongshanHospital,FudanUniversity，Shanghai200032，China

尿毒症毒素是指终末期肾病(end-stage renal disease，ESRD)时不能经尿液清除、潴留在体内且有毒性作用的物质。据欧洲尿毒症毒素协作组(EUTox)统计，至2011年4月已发现160种尿毒症毒素[1]。尿毒症毒素根据其理化性质可分为3类。(1)不能与蛋白质结合的水溶性小分子物质：相对分子质量通常小于500，较易经血液透析清除，如尿素、肌酐；(2)蛋白质结合物质：大多数相对分子质量较小，很难通过血液透析清除，如硫酸对甲酚(p-cresyl sulfate，PCS)、硫酸吲哚酚(indoxyl sulfate，IS)；(3)中分子物质：相对分子质量通常大于500，常规血液透析效果不理想，如甲状旁腺素、β2微球蛋白。尿毒症毒素根据其来源分类也可分为3类，(1)内源性代谢产物：自身代谢产生，如非对称性二甲基精氨酸(asymmetric dimethylarginine,ADMA)[2]；(2)微生物代谢产物：主要是肠道菌群代谢物质，如吲哚类、酚类；(3)外源性摄入物质：如草酸盐[3]。研究[4- 5]表明，慢性肾脏病(chronic kidney disease，CKD)患者肠道菌群的种类和数量与健康人群显著不同，其毒性产物与CKD及其并发症的进展密切相关。其中，PCS和IS是当前研究最多的肠源性尿毒症毒素，本文对其研究进展作一综述。

1PCS、IS的产生和代谢

PCS主要在肠道产生，相对分子质量188，与血浆白蛋白结合率为94%[6]。肠道厌氧菌将食物中的苯丙氨酸和酪氨酸转变为4-羟基苯乙酸。4-羟基苯乙酸脱羧为对甲酚，大部分对甲酚经肠道黏膜吸收，在肠道上皮细胞磺基转移酶的作用下转化为PCS[7]。PCS主要通过肾小管基底膜侧的有机阴离子转运体(organic anion transporter，OAT)分泌到肾小管管腔，经尿液排出[8]。

IS主要在肠道产生，相对分子质量251[9]，蛋白结合率达90%以上。食物中的色氨酸经大肠埃希菌分解产生吲哚，吲哚经门静脉进入肝脏经羟化、硫酸化，最终生成IS。IS主要通过肾小管OAT分泌、排泄[10]。

2PCS、IS的肾脏毒性

PCS可通过促进肾脏纤维化加快肾脏病进展、肾功能下降[11]。PCS主要通过以下机制促进肾脏纤维化：(1)PCS可显著增加肾组织肾素、血管紧张素Ⅱ1型受体(AT1R)表达，激活肾素-血管紧张素-醛固酮系统(renin-angiotensin-aldosterone system，RAAS)，进而促进肾间质成纤维细胞的增殖与分化，加重肾组织纤维化[12]；(2)PCS具有促炎作用，可促进肾间质单核细胞/巨噬细胞浸润[11]，上调促炎因子表达[13]，引起肾间质纤维化；(3)体外实验证实，PCS可促进小鼠近端肾小管上皮细胞炎性相关基因的表达，如转化生长因子-β(transforming growth factor-β，TGF-β)、白介素-6(interleukin-6，IL-6)等[14]，而TGF-β可促进肾小管间质纤维化[15]，IL-6可通过诱导肾脏纤维化相关基因及内皮素-1基因的表达加速CKD的进展[16]；(4)Klotho基因可编码一种参与成纤维细胞生长因子受体构成的跨膜蛋白，这种跨膜蛋白可延缓肾脏纤维化进程，发挥肾脏保护作用[17-18]，而PCS通过促进DNA甲基转移酶表达，使Klotho基因超甲基化，进而抑制Klotho基因表达[19]，使Klotho基因产物的肾脏保护作用下降或消失，促进肾脏纤维化，加速肾脏病进展。

IS促进肾脏纤维化的机制主要有：(1)IS促进肾小管上皮细胞活性氧簇(reactive oxygen species，ROS)的产生，激活核转录因子κB(nuclear factor-κB，NF-κB)、p53、 细胞外信号调节激酶(extracellular signal-regulated kinase，ERK)等调节因子，使单核细胞趋化蛋白-1(monocyte chemotactic protein-1，MCP-1)、细胞间黏附分子-1(intercellular adhesion molecule-1，ICAM-1)的表达上调，引起单核细胞/巨噬细胞在小管间质聚集，进而促进肾脏纤维化[20-21]；(2)IS使肾组织肾素、血管紧张素原、AT1R表达增加，AT2R表达减少，进而通过激活RAAS及促进TGF-β表达，使肾间质细胞向成纤维细胞转化，引起肾脏纤维化[22]；(3)IS也可通过促进Klotho基因超甲基化而促进肾脏纤维化[19]。

3PCS、IS的心血管毒性

大量研究[7，23-24]证实，PCS水平与CKD患者心血管疾病的发生及全因死亡独立相关。Schepers等[13]研究发现，PCS可诱导白细胞产生自由基，进而引起ESRD患者的血管损伤。Watanabe等[25]研究发现，PCS可使人脐静脉内皮细胞及人主动脉平滑肌细胞内NADPH氧化酶(NAPDH oxidase，NOX)的表达显著增加，促进细胞内产生ROS，进而损害血管内皮细胞及平滑肌细胞。Han等[26]研究发现，PCS可通过增强NOX活性、增加ROS，促进心肌细胞凋亡。

研究[27]显示，IS可提高ESRD患者全因死亡率及心血管事件发病率，其机制主要有以下两方面。(1)IS可促进血管损伤，研究[28]发现，IS可致循环中内皮损伤标志物内皮微粒(endothelial microparticles，EMPs)产生增加，提示其有致血管内皮损伤作用。IS引起内皮损伤主要是通过促氧化应激作用实现的。IS可促进NOX活化、使内皮细胞产生的ROS增多[29]，升高的ROS可通过激活NF-κB，增加MCP-1及ICAM-1的表达[30]，导致血管内皮损伤。此外，IS可以通过激活丝裂原活化蛋白激酶(mitogen-activated protein kinase ，MAPK)途径促进血管平滑肌细胞(vascular smooth muscle cell ，VSMC)增殖[31]；并可通过促进骨母细胞特异性蛋白表达增加而加重动脉钙化、使动脉壁增厚[32]。近年来研究[33]证实，IS可促进大鼠主动脉细胞衰老相关蛋白，如p16INK4a、p21WAF1/CIP1的表达，提示IS有加速动脉衰老作用。(2)IS可加速心肌损伤，研究[34]发现，IS可通过促氧化应激、削弱抗氧化屏障作用促进心肌纤维化及心肌细胞肥大。此外，IS可通过抑制单磷酸腺苷活化蛋白激酶/解偶联蛋白2(AMP-activated protein kinase/uncoupling protein 2，AMPK/UCP2)途径促进心肌肥大[35]。

4PCS、IS的其他作用

近年研究提示，PCS可能与CKD相关的胰岛素抵抗有关。PCS通过激活胰岛素信号转导通路中的ERK1/2诱导小鼠出现胰岛素抵抗，使其脂肪含量减少，脂肪在肝脏及肌肉重新分布[36]。骨代谢方面，Tanaka等[37]研究发现，PCS通过激活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)信号转导途径导致成骨细胞功能障碍，引起肾性骨病。

Kim等[38]研究发现，IS可通过抑制成骨细胞的分化、诱导成骨细胞凋亡，从而引起骨骼病变。此外，研究[39]发现，IS可导致体外培养的成骨细胞抵抗甲状旁腺激素，从而导致肾性骨病的发生。

5PCS和IS的清除

PCS和IS均为蛋白质高亲和力毒素，常规透析方法难以清除。Meert等[40]研究发现，透析中增加对流量也利于PCS及IS的清除。不同材质的透析膜对这两种毒素的清除率无差异[41]。Meijers等[42]发现，血浆分离吸附技术对PCS的清除效果显著优于高通量透析，但血浆分离吸附技术成本高昂，目前无法在临床推广。

此外，由于PCS和IS主要由肠道产生，理论上可以通过改变肠道菌群降低PCS和IS的浓度，但目前尚无相关研究。目前研究较多的肠道吸附剂，如AST-120，Owada 等[43]的研究显示，AST-120可清除部分肠源性毒素，并可延缓尿毒症大鼠的肾功能恶化。

6展望

目前对PCS及IS 作用机制的了解已较深入，但是仍无有效的、适合临床应用的清除PCS及IS的透析方式或药物，需要进一步探索。

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中图分类号R692.5

文献标识码A

通讯作者邹建洲，E-mail：jianzzou@163.com

基金项目：上海市科学技术委员会基金项目(编号：15DZ0503402)