第二军医大学学报  2019, Vol. 40 Issue (2): 209-212   PDF    
上皮间质转化相关长链非编码RNA与胃癌的关系
李江, 袁士杰, 聂明明     
海军军医大学(第二军医大学)长海医院普通外科, 上海 200433
摘要: 胃癌是威胁人类健康的主要恶性肿瘤之一,其发生涉及多步骤、多基因的相互作用过程。上皮间质转化(EMT)是上皮细胞通过特定程序转化为有间质表型细胞的过程。研究表明长链非编码RNA(lncRNA)可以通过与靶蛋白结合、作为竞争性内源RNA竞争微RNA等方式促进或抑制EMT过程,且许多特异表达的EMT相关lncRNA与胃癌的侵袭、转移密切相关。本文主要就EMT相关lncRNA在胃癌侵袭和转移中的调控机制作一综述。
关键词: 上皮间质转化     长链非编码RNA     胃肿瘤     肿瘤侵袭     肿瘤转移    
Epithelial-mesenchymal transition-related long non-coding RNA and gastric cancer
LI Jiang, YUAN Shi-jie, NIE Ming-ming     
Department of General Surgery, Changhai Hospital, Naval Medical University(Second Military Medical University), Shanghai 200433, China
Supported by National Natural Science Foundation of China (81472277).
Abstract: Gastric cancer is one of the main malignancies threatening human health, and its occurrence involves multistep and multigene interactions. Epithelial-mesenchymal transition (EMT) is a biological process in which epithelial cells transform to mesenchymal phenotypic cells through specific procedures. Studies have shown that long non-coding RNAs (lncRNAs) can promote or inhibit EMT by binding to target proteins and competing microRNAs as competitive endogenous RNAs, and some specific EMT-related lncRNAs are deeply involved in the invasion and metastasis of gastric cancer. In this review, we summarized the recent mechanisms of EMT-related lncRNAs in the regulating the invasion and metastasis of gastric cancer.
Key words: epithelial-mesenchymal transition     long non-coding RNA     stomach neoplasms     neoplasm invasiveness     neoplasm metastasis    

胃癌是威胁人类健康的主要恶性肿瘤之一, 其发病率居恶性肿瘤的第6位, 病死率居第2位[1]。中晚期胃癌患者的预后很差, 其中绝大部分患者死因归于肿瘤的远处转移[2]。研究表明, 上皮间质转化(epithelial-mesenchymal transition, EMT)在肿瘤转移过程中发挥重要作用。EMT发生后细胞的黏附力下降, 侵袭和转移能力增强, 对凋亡的抗性提高, 在此过程中间质细胞相关标志物波形蛋白(vimentin)、N-钙黏蛋白(N-cadherin)等表达上调, 上皮细胞标志物E-钙黏蛋白(E-cadherin)表达下调[3]。长链非编码RNA (long non-coding RNA, lncRNA)在胃癌的发生、发展过程中发挥着促癌或抑癌作用, 可以通过调节E-钙黏蛋白等细胞连接蛋白及碱性的螺旋-环-螺旋蛋白(Twist1)、E盒结合锌指蛋白(ZEB1, ZEB2)、Snail超家族(Snail, Slug)等EMT诱导转录因子来调控EMT进程[4]。本文旨在结合新的研究及文献, 总结部分EMT相关lncRNA在胃癌中的调控作用和分子机制, 梳理和加深对EMT调控机制的认识。

1 LncRNA概述

LncRNA属于人类基因组的转录产物, 其RNA长度为200~100 000 nt。既往认为lncRNA没有蛋白质编码功能, 直接以RNA的形式参与基因调控, 但2016年Nelson等[5]研究证实lncRNA也可以编码生成短肽, 该短肽可竞争性抑制肌质网/内质网钙ATP酶抑制剂。目前人类基因中能确定的lncRNA约为25 000个[6], 但只有不到1%的lncRNA具有功能性[7]。LncRNA主要从以下4个层面参与基因表达调控[8-9]:(1)表观遗传学水平。LncRNA通常与DNA、异染色质蛋白、组蛋白修饰酶及转录因子等结合而参与基因转录前水平调节。(2)转录水平。LncRNA可以诱导染色质重塑或转录干扰影响邻近基因表达、调控转录因子活性、结合RNA结合蛋白而激活下游启动子区、调控RNA聚合酶Ⅱ活性。(3)转录后水平。LncRNA可以通过被剪切成非信使小RNA (small non-messenger RNA, snmRNA)、调控mRNA前体的可变剪切、调节mRNA表达丰度和稳定性等参与基因转录后水平调控。(4)与某些微RNA (microRNA, miRNA)形成竞争性内源RNA (competitive endogenous RNA, ceRNA)调控网络。通过以上方式, 一些lncRNA可以促进或抑制胃癌的EMT。因此, 与胃癌EMT相关的lncRNA成为近年胃癌研究的热点。

2 EMT相关lncRNA与胃癌 2.1 HOX转录反义RNA (HOX transcript antisense RNA, HOTAIR)

HOTAIR定位于人类染色体12q13.13区域, 来自HOX基因家族成员HOXC基因的反义链。HOTAIR是第1个被证实具有反式作用的lncRNA[10-11]。Endo等[12]应用实时荧光定量PCR技术发现胃癌组织中HOTAIR的表达水平明显高于癌旁组织。随后将HOTAIR表达上调和下调的肿瘤细胞分别注入裸鼠尾静脉及腹腔中, 结果显示注入HOTAIR表达上调的肿瘤细胞的裸鼠肝转移及腹膜转移的肿瘤数量和平均大小均明显大于对照组。Xu等[13]发现HOTAIR在胃癌组织中的表达显著上调, 且表达水平与肿瘤的淋巴结转移及分期明显相关, 同时统计学结果提示HOTAIR表达水平与患者术后生存率具有相关性; 抑制HOTAIR表达后可以减少波形蛋白和基质金属蛋白酶(matrix metallopeptidase, MMP)-9、MMP-1的表达, 同时上调E-钙黏蛋白和紧密连接蛋白1(zonula occludens 1, ZO-1)蛋白的表达, 逆转EMT过程, 从而抑制胃癌细胞的侵袭, 提示HOTAIR可能是一种参与EMT的多效性调节剂; 沉默HOTAIR的胃癌细胞AGS转染Snail蛋白后, 被逆转的EMT可以恢复, 提示HOTAIR可能是通过Snail通路促进EMT。另外Liu等[14]的研究也发现无论是体内还是体外的胃癌细胞, 在HOTAIR表达下调后肿瘤细胞的侵袭性和转移能力均显著下降, 并且EMT过程发生逆转, 原因是HOTAIR可以通过抑制miRNA如miR-34a, 激活HGF/C-Met/Snail信号通路促进EMT。

2.2 LncRNA-H19

印记基因H19定位于人类染色体11p15.5区域, 其编码的lncRNA-H19是第1个被发现与肿瘤相关的lncRNA。H19在胚胎形成时期广泛表达, 出生后却在大多数组织中失去活性; 而当肿瘤生成时, H19可以重新恢复其表达活性[15]。LncRNA-H19在肿瘤发生过程中的作用具有双面性, 如研究发现其在肝癌组织中呈低表达, 有抑癌作用[16]; 而在乳腺癌组织中却呈高表达状态, 可以促进肿瘤增殖[17]H19基因外显子还能编码1个miRNA分子, 即miR-675, 因此H19的过表达能增强miR-675的表达水平, 进而下调E-钙黏蛋白, 促进细胞迁移[18]。还有研究表明在胃癌中lncRNA-H19也可以起到促进细胞增殖及转移的作用, 因其与miR-141呈负性调控关系, 可以通过抑制miR-141及其靶基因ZEB1表达而促进EMT[19]

2.3 被转化生长因子β(transforming growth factor β, TGF-β)活化的长链非编码RNA (lncRNA activated by TGF-β, lncRNA-ATB)

LncRNA-ATB定位于人类14号染色体, 是第1个被证实能被TGF-β激活的lncRNA, 通过调节TGF-β竞争性结合miR-200家族并上调其靶基因ZEB1ZEB2来促进EMT。Saito等[20]检测了不同胃癌组织中lncRNA-ATB、miR-200b、miR-200c、ZEB1的表达水平, 发现lncRNA-ATB表达水平显著升高, 并且通过上调TGF-β/miR-200s/ZEB轴促进胃癌细胞的侵袭和转移; 多变量分析结果显示lncRNA-ATB是一个独立的预后因子, 可以作为一种新的生物标志物判断胃癌预后。另外lncRNA-ATB可以作为ceRNA直接结合miR-141-3p, 竞争性抑制其与下游TGF-β相关靶基因结合, 从而实现对TGF-β表达水平的调控, 最终通过miR-141-3p/TGF-β正反馈调控环路来维持自身持续高表达[21]

2.4 结肠癌相关转录子2(colon cancer-associated transcript 2, CCAT2)

CCAT2定位于人类染色体8q24区域, 由Ling等[22]于2013年首次报道, 研究发现其在结直肠癌组织中高表达, 可以作为Wnt通路的下游靶基因促进结直肠癌细胞的增殖与转移。Wang等[23]研究发现CCAT2表达水平与胃癌的肿瘤大小、淋巴结转移和TNM分期有关, 沉默CCAT2可以有效抑制胃癌细胞迁移和侵袭。进一步研究发现, CCAT2通过下调E-钙黏蛋白表达及上调ZEB2、波形蛋白和N-钙黏蛋白表达促进胃癌细胞发生EMT; CCAT2与zeste基因增强子同源物2(enhancer of zeste homolog 2, EZH2)相互作用并因此调节E-钙黏蛋白和大肿瘤抑制因子同源物2(large tumor suppressor homolog 2, LATS2)表达。结果表明CCAT2在胃癌细胞中具有促癌作用并参与了胃癌进展。

2.5 Linc00261

Linc00261定位于人类染色体20p11.21区域。Yu等[24]研究发现linc00261在胃癌组织和细胞系中表达减少, 而且低表达linc00261的胃癌患者预后相对较差。另外研究中发现Slug是linc00261的RNA结合蛋白, linc00261可以通过降低Slug蛋白的稳定性下调Slug表达, 还可以通过增强糖原合成酶激酶3β(glycogen synthase kinase 3β, GSK3β)和Slug之间的相互作用促进Slug的降解, 从而抑制EMT进程, 缓解肿瘤进展[24]。这提示我们或许可以通过上调linc00261的表达来抑制胃癌转移, 改善高侵袭性胃癌患者的预后。

2.6 LEIGC

Han等[25]在2014年报道了一种在胃癌组织中显著下调的lncRNA, 并将其命名为LEIGC。他们通过过表达和沉默LEIGC观察其对胃癌细胞生长、分化、转移的影响, 发现LEIGC的过表达可以抑制胃癌细胞的增殖, 抑制胃癌细胞的EMT, 并增强胃癌细胞对5-氟尿嘧啶(5-fluorouracil, 5-FU)的敏感性; 而沉默LEIGC后则出现相反结果。提示LEIGC可能作为肿瘤抑制因子在胃癌的发生和发展中起着重要作用。

2.7 其他EMT相关lncRNA

除了以上6种与胃癌EMT过程相关的lncRNA外, 近年还有一些lncRNA被陆续报道与胃癌的EMT过程有密切联系, 比如linc00152、SPRY4内含子转录本(SPRY4 intronic transcript 1, SPRY4-IT1)、锌指结构反义转录本1(zinc finger antisense 1, ZFAS1)等[26-28]。它们也可以通过各种途径调控胃癌的EMT。

3 小结

EMT是一个复杂的、多途径的过程, 在肿瘤转移中扮演了重要角色, 甚至还能导致肿瘤细胞同源物耐药的发生[29-30]。但其详细机制仍不十分清楚, 甚至有很多争议, 比如以前认为EMT是肿瘤转移的必需条件, 但Fischer等[31]发现在乳腺癌肺转移小鼠模型中只有小部分肿瘤细胞发生EMT, 而且肺转移灶主要由非EMT肿瘤细胞组成。LncRNA可以通过与靶蛋白结合、作为ceRNA竞争miRNA等方式构成调控网络促进或抑制EMT过程。因此, 研究EMT相关lncRNA在胃癌侵袭、转移中的作用和机制, 寻找新的针对EMT的靶向治疗策略, 对提高胃癌的治疗水平将产生深远影响。

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