microRNA-26a与肿瘤

邓静静 郑建明

microRNA-26a与肿瘤

邓静静 郑建明

microRNA(miRNA)是一类长度为18～25个核苷酸(nt)的非编码小RNA，通过与靶mRNA的互补配对在转录后水平调控基因表达，导致mRNA的降解或翻译抑制，控制哺乳类动物约30%的蛋白质编码基因活性[1-2]。miRNA与其靶mRNA分子组成了一个复杂的调控网络，参与包括细胞增殖、分化、凋亡、发育等多种生物学过程。研究显示，在多种肿瘤中miRNA表达异常，可能与肿瘤的形成、细胞分化及凋亡有关，它们有的起癌基因作用，有的则起抑癌基因作用。

一、miR-26a的结构和作用靶点

miR-26a含有21个核苷酸(nt),位于人3号染色体3P21.3,在人的大多数组织中表达，序列分析显示miR-26a位于染色体的不稳定位点[3]。通过基因数据分析确定，miR-26a与CDK4和CENTG1共同组成一个扩增子，而CDK4和CENTG1作为癌基因分别调控RB1和PI3/AKT通路[4]。Zcchc11(zinc-finger, CCHC domain-containing protein 11)通过其尿苷化酶的作用维持miR-26族miRNA的3′末端的A尾，从而影响miR-26族miRNA生物学功能[5]。另有文献报道[6-7]，miR-26a的活性受到myc的调控。目前已证实,在肿瘤中miR-26a的直接作用靶点有Ezh2、PTEN、cyclinD2、cyclinE2、GSK-3β；功能相关靶点有Rb1、MAP3K2/MEKK1和SMAD1。miR-26a通过作用于这些靶点发挥其调控肿瘤细胞生物学行为的作用。

二、miR-26a起抑癌基因调控作用

1．miR-26a与肝细胞癌：miR-26a在肝细胞癌(HCC)中过表达，它能显著下调cyclinE2蛋白的表达，抑制肝癌细胞株HepG2的增殖[8]。大宗HCC病例统计资料显示，miR-26a表达量低的HCC患者总生存期短于miR-26a表达量高的，但对干扰素的治疗更为敏感[9]。Kota 等[10]研究显示，miR-26a能抑制小鼠肝癌模型的形成，在体外实验中，miR-26a可引起肝癌细胞周期阻滞，这与其靶蛋白cyclinD2和cyclinE2表达下调有关，miR-26a能抑制肝癌细胞的增殖，导致特异性凋亡，抑制肝癌进展。

2．miR-26a与乳腺癌：miRNA参与乳腺癌细胞生长的调节，发挥广泛的雌激素依赖性抑制作用。研究显示，miR-26a和miR-181a抑制cyclin E2的表达，调节许多与细胞增殖和生长有关的基因，包括在雌激素信号效应中起关键作用的孕激素受体基因[11]。另有报道显示，miR-26a在乳腺癌细胞系中表达下降，短暂转染miR-26a可引起乳腺癌细胞系McF7细胞凋亡。应用逆转录病毒载体转染miR-26a，可抑制乳腺癌细胞克隆形成，并且在动物体内成瘤能力减弱。miR-26a的两个作用靶标MTDH和Ezh2在乳腺癌中显著上调。转染表达miR-26a载体，乳腺癌McF7细胞的MTDH和Ezh2表达下降，细胞凋亡增加。miR-26a主要通过作用于MTDH和Ezh2抑制乳腺癌的形成[12]。

3．miR-26a 与鼻咽癌：miR-26a在鼻咽癌(nosopharyngeal cancer, NPC)组织及细胞系中普遍下调，在NPC的形成中发挥重要作用。转染miR-26a可引起细胞G1期阻滞，抑制NPC细胞增殖和克隆形成。实验表明，miR-26a能下调NPC的癌基因Ezh2的表达，Ezh2低表达又能抑制miR-26a的作用。NPC组织mRNA高表达Ezh2，与miR-26a的表达呈负相关。因此，miR-26a作为NPC中的一个生长抑制性miRNA，主要通过抑制Ezh2的表达发挥其抑制作用[13]。

4．miR-26a 与淋巴瘤：Sander等[14]发现，miR-26a在原发性Burkitt淋巴瘤组织中表达下调，是myc的靶分子。他们运用基因芯片技术分别检测myc诱导和myc抑制后的miRNA基因表达谱，结果显示myc在多种肿瘤中持续抑制miR-26a的表达。在myc诱导的淋巴瘤细胞中转染miR-26a，可导致细胞增殖能力下降，周期进程受损，并且证实Ezh2为miR-26a的直接作用靶点。myc通过抑制miR-26a表达，从而减弱对Ezh2表达的抑制，提示miR-26a在myc诱导的淋巴瘤中发挥很强的抑癌基因作用[15]。另外，Di Lisio等[16]检测了套细胞淋巴瘤miRNA表达谱，通过生物信息学方法分析miRNA可能的作用靶点，并用功能实验证实miR-26a调节套细胞淋巴瘤中NF-κB亚单位的核转移，从而影响套细胞的生物学行为。

5．miR-26a与胰腺癌：胰腺癌miRNA表达谱芯片结果显示，miR-26a在胰腺癌中表达[17]。在研究治疗糖尿病的药物二甲双胍抑制胰腺癌细胞生长的机制中发现，二甲双胍通过上调胰腺癌细胞中包括miR-26a在内的数种miRNA的表达发挥作用[18]。二甲双胍呈剂量依赖性上调胰腺癌miR-26a的表达，并引起细胞生长抑制，侵袭、转移能力下降，凋亡增加，其机制可能是通过HMGA1发挥其作用[19]。此外，经CDF(抗癌药物姜黄素的类似物)处理的胰腺癌细胞生长抑制，侵袭能力下降，Ezh2表达下调，miR-26a表达上调[20]，认为miR-26a可能参与了胰腺癌的发生发展过程，并在抗癌药物的作用机制中发挥重要作用。

此外，miR-26a在肺原发性鳞状细胞癌[21]、口腔鳞癌[22]等肿瘤中也发挥着抑癌基因作用。研究发现，miR-26a在良恶性胸水中表达存在差异[23]；在高侵袭的转移性肾透明细胞癌和横纹肌肉瘤中呈低表达[24-25]；在甲状腺未分化癌(ATC)中表达下调，外源性给予miR-26a后，癌细胞的生长受到抑制[26]。

三、miR-26a起癌基因调控作用

1．miR-26a与胶质瘤：胶质瘤的miR-26a常常是扩增的，miR-26a作为扩增子的组成部分发挥作用，该扩增子还包括CDK4和CENTG1。CDK4和CENTG1作为癌基因，分别调控RB1和PI3/AKT通路。有人通过调查DNA的拷贝数、mRNA和miRNA及DNA甲基化数据库，确定胶质瘤中与miR-26a功能相关的靶基因有PTEN、Rb1和MAP3K2/MEKK1。实验证实，miR-26a能够转化细胞，在体外促进胶质瘤细胞增殖，在鼠脑内通过降低PTEN、Rb1和MAP3K2/MEKK1蛋白表达，从而增加AKT的活性，促进增殖，降低c-JUN癌基因N端激酶依赖的凋亡。在PTEN高表达以及PTEN表达缺失的胶质瘤细胞中过表达miR-26a，在体外均能促进肿瘤生长，也能促进CDK4或GENTG1过表达的细胞增殖，含该扩增子的胶质瘤患者生存率明显下降。因此has-miR-26a、CDK4和GENTG1功能相关Oncomir/Oncogene簇通过协同靶作用于RB1、PI3K/AKT及JNK通路促进癌的侵袭[4]。

2．miR-26a与前列腺癌：美国非裔的前列腺癌的发病率较其他种族高，检测非裔和白种人前列腺癌发病阶段中miR-26a的表达量，发现非裔的前列腺癌miR-26a较白种人在该癌相同的发展阶段(良性、恶性、转移)分别高出2.25、13.3、2.38倍，而且侵袭力高的肿瘤miR-26a升高[27]。表明miR-26a表达升高可能与前列腺癌的侵袭性有关。

四、miR-26a和细胞分化

Kang等[28]用基于分子荧光信号的生物影像学证实，miR-26a仅在高分化的肌母细胞C2C12中表达。Wong等[29]报道，miR-26a的表达在肌母细胞增殖到分化成肌小管过程中逐步上调，miR-26a表达升高促进肌形成，并且确定骨骼肌细胞分化抑制子Ezh2作为miR-26a作用靶点，通过下调Ezh2的表达促进肌细胞的分化。Luzi等[30]认为，miR-26a通过作用于转录因子Smad1调节晚期骨母细胞的分化。当抑制骨母细胞miR-26a表达，并增加其靶蛋白Smad1表达时，骨特异基因表达上调，骨母细胞分化明显增多。此外，通过体内外实验证实，miR-26a与人类角化细胞的分化有关[31]。

五、miR-26a和细胞凋亡

Havelange等[32]通过对急性髓系白血病(AML)的mRNA和miRNA表达谱进行相关性、基因本位(gene ontology)和作用网络(network)分析，认为miR-26a与促凋亡基因BIM和PTEN成正相关，并用实验证实miR-26a与AML细胞凋亡有关。Alajez等[33]用Luciferase实验证实，miR-26a调控NPC中Ezh2的表达，进而调控细胞凋亡。Kota等[10]的研究结果显示，miR-26a在肝细胞癌模型中引起肝癌细胞特异性凋亡。Zhang等[12]将miR-26a瞬时转染乳腺癌细胞株MCF7从而启动癌细胞凋亡。但也有文献报道，miR-26a降低HeLa、前列腺癌(DU145)和结肠癌细胞(SW480)对Trail诱导凋亡的敏感性[34]，降低胶质瘤细胞中C-JUN-terminal Kinase依赖性凋亡[4]。故miR-26a在凋亡中的作用机制还有待于进一步的研究。

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10.3760/cma.j.issn.1674-1935.2012.04.025

200433 上海，第二军医大学长海医院病理科

郑建明，Email:jmzheng1962@163.com

2011-05-04)

(本文编辑：吕芳萍)