中华急诊医学杂志  2019, Vol. 28 Issue (6): 803-804   DOI: 10.3760/cma.j.issn.1671-0282.2019.06.033
百草枯致肺纤维化的上皮-间质转化的机制研究进展
符红娜 , 聂时南     
南方医科大学南京临床学院 (中国人民解放军东部战区总医院) 急救医学科 210002

百草枯(paraquat, PQ)是一种有效且广泛使用的除草剂,在人体经口或经皮肤吸收后可引起肺纤维化[1]。PQ中毒主要损害肺、肝、肾及心脏等多脏器[2],其引起的肺毒性作用分为两个阶段:损伤破坏性阶段及增殖纤维化阶段[3]。肺内成纤维前体细胞浸润并快速分化为成纤维细胞,进而发展成纤维化。上皮-间质转化(epithelial-mesenchymal transition,EMT)是上皮细胞转分化为间质细胞的过程,是由肺上皮细胞转分化为间质型肌成纤维细胞进而形成肺纤维化的重要机制[4]。EMT的分子特征在于上皮标记物E-cadherin的丧失和活化的成纤维细胞标记物α-SMA的从头表达。另外,上皮细胞获得间质特征表现出细胞外基质(ECM)沉积增加[5],因此基质金属肽酶家族在其中也起着至关重要的作用。然而,肺纤维化中EMT调控的确切分子机制尚不清楚,本综述回顾和总结了与百草枯诱导的肺纤维化的EMT相关分子机制的文献。

1 TGF-β1/Smad信号通路

TGF-β1/Smad信号通路不仅参与了急性肺损伤的活动性早期阶段,而且与晚期肺纤维化相关[6]。PQ能够诱导AT2,向肌成纤维细胞转变,该过程通过TGF-β1依赖性机制进行[7]。Kan等[8]研究证明百草枯使得大鼠肺组织TGF-β1mRNA表达明显增加,唐艳等[9]则证实抑制TGF-β1可减轻百草枯所致的肺纤维化程度。而TGF-β1诱导的EMT主要通过Smads信号介导:通过TGF-β1激活的受体触发Smad2和Smad3磷酸化,而后与Smad4形成三聚体进入细胞核以激活转录因子或编码ECM的靶基因[10]。而Smad7可竞争性结合TGF-β1受体抑制肺纤维化发生[11]。Smad3缺陷减轻了PQ诱导的肺纤维化[12];使用siRNA抑制Smad2表达逆转了由TGF-β1处理诱导的EMT[13]。在肺损伤和纤维化过程中,AT2是TGF-β1的主要来源。Xie等[7]观察到,PQ不仅诱导总TGF-β1的分泌增加,而且诱导来自A549细胞的内源性自分泌TGF-β1的激活。目前的研究证实,PQ处理的A549细胞不仅具有肌成纤维细胞样特征,而且具有合成Co1Ⅰ的能力,这表明PQ诱导的EMT促进了肺纤维化过程中的ECM沉积。另外,PQ诱导的肺纤维化动物模型表现出逐渐增加的基质金属蛋白酶,其能够激活潜伏的TGF-β1并且诱导EMT[14]

TGF-β1的过度表达可导致肺纤维化的发生,阻断TGF-β1/Smad则可使之逆转。这确定了PQ诱导的EMT的新途径,可能为今后PQ致肺纤维化的治疗提供新的靶点。

2 MAPK通路

MAPK家族主要包括JNK、Erk和p38,是调控EMT的重要信号通路。EMT的引发通常伴随着Erk1/2、Erk5、p38和(或)JNK以及它们的上游激酶的活化[15],并且MAPK和Smad信号传导途径在PQ诱导的EMT中具有协同的作用[16]

Huang等[17]研究数据显示PQ处理RLE-6NT细胞后,细胞可获得充质细胞特征,具有EMT样表型,并且p38,Erk1/2和Smad2的磷酸化形式增加,而在Erk和(或)MAPK抑制剂存在时,上述磷酸化显著减弱,表明PQ诱导的EMT过程可被MAPK/Erk通路抑制剂完全阻断。此外研究表明,Erk5通过失活GSK3β从而稳定Snail1,促进EMT[15];相反,沉默Erk表达后Snail1表达下调,EMT减轻[18]。另外,有人用Erk、p38两种特定的MAPK通路抑制剂研究MAPK参与肺纤维化过程。结果显示,p38抑制剂处理后Erk的磷酸化增强,相反,Erk抑制剂处理增加了p38的磷酸化,意味着在PQ诱导的肺毒性中存在Erk和p38磷酸化之间的平衡[19]。除此之外,JNK可能通过磷酸化作用促进EMT,从而促进Smad1和Smad3的磷酸化,导致Smad介导的应答增强[20]

上述研究结果表明,MAPK及其亚族在调控PQ诱导的的EMT中发挥了重要作用,这意味着MAPK通路是有望成为可行的治疗靶点。

3 miRNA

miRNA(微小核糖核酸)是一种具有调控功能的非编码单链小分子RNA,它们与靶基因的3’UTR互补结合,导致靶mRNA降解或转录后翻译抑制,从而调节靶基因表达。miRNA在PQ致肺纤维化的EMT进程中发挥重要作用。Zhu等[21]检测了体内和体外miR-210的表达来研究其在PQ诱导的EMT中的作用,发现PQ处理后miR-210的表达水平随着时间的延长显著增加;而在A549细胞中过表达miR-210后,E-cadherin表达明显降低而α-SMA表达显著增加,在过表达miR-210的RLE-6TN细胞中也出现了相似的结果,而下调miR-210的表达后,α-SMA表达显著降低而E-cadherin表达升高,表明抑制miR-210可减轻PQ诱导的EMT。另外,Liu等[22]对小鼠进行miR-21敲除后建立PQ诱导的肺纤维化模型,PQ处理14 d后,小鼠肺中miR-21基因表达水平增加,且α-SMA在肺泡上皮沉积明显增多,提示miR-21与PQ诱导的肺纤维化发生存在正性相关,且可能促进PQ所致的EMT。Yamada等[23]报道,抑制miR-21可减弱小鼠原代肺泡Ⅱ型上皮细胞中TGF-β1诱导的EMT。除此之外,研究发现let-7d[24],miR-98[25]、miR-200[24]、miR-29b[26]、miR-326[25]、miR-27b[27]、miR-154[28]能够减轻人和(或)动物的EMT过程,而miR-424[29]可促进EMT发生,然而,这些miRNA对百草枯诱导的EMT进程的影响仍有待进一步研究验证。

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