microRNA-26a與腫瘤

鄧靜靜 鄭建明

microRNA-26a與腫瘤

鄧靜靜 鄭建明

microRNA(miRNA)是一類(lèi)長(zhǎng)度為18～25個(gè)核苷酸(nt)的非編碼小RNA，通過(guò)與靶mRNA的互補(bǔ)配對(duì)在轉(zhuǎn)錄后水平調(diào)控基因表達(dá)，導(dǎo)致mRNA的降解或翻譯抑制，控制哺乳類(lèi)動(dòng)物約30%的蛋白質(zhì)編碼基因活性[1-2]。miRNA與其靶mRNA分子組成了一個(gè)復(fù)雜的調(diào)控網(wǎng)絡(luò)，參與包括細(xì)胞增殖、分化、凋亡、發(fā)育等多種生物學(xué)過(guò)程。研究顯示，在多種腫瘤中miRNA表達(dá)異常，可能與腫瘤的形成、細(xì)胞分化及凋亡有關(guān)，它們有的起癌基因作用，有的則起抑癌基因作用。

一、miR-26a的結(jié)構(gòu)和作用靶點(diǎn)

miR-26a含有21個(gè)核苷酸(nt),位于人3號(hào)染色體3P21.3,在人的大多數(shù)組織中表達(dá)，序列分析顯示miR-26a位于染色體的不穩(wěn)定位點(diǎn)[3]。通過(guò)基因數(shù)據(jù)分析確定，miR-26a與CDK4和CENTG1共同組成一個(gè)擴(kuò)增子，而CDK4和CENTG1作為癌基因分別調(diào)控RB1和PI3/AKT通路[4]。Zcchc11(zinc-finger, CCHC domain-containing protein 11)通過(guò)其尿苷化酶的作用維持miR-26族miRNA的3′末端的A尾，從而影響miR-26族miRNA生物學(xué)功能[5]。另有文獻(xiàn)報(bào)道[6-7]，miR-26a的活性受到myc的調(diào)控。目前已證實(shí),在腫瘤中miR-26a的直接作用靶點(diǎn)有Ezh2、PTEN、cyclinD2、cyclinE2、GSK-3β；功能相關(guān)靶點(diǎn)有Rb1、MAP3K2/MEKK1和SMAD1。miR-26a通過(guò)作用于這些靶點(diǎn)發(fā)揮其調(diào)控腫瘤細(xì)胞生物學(xué)行為的作用。

二、miR-26a起抑癌基因調(diào)控作用

1．miR-26a與肝細(xì)胞癌：miR-26a在肝細(xì)胞癌(HCC)中過(guò)表達(dá)，它能顯著下調(diào)cyclinE2蛋白的表達(dá)，抑制肝癌細(xì)胞株HepG2的增殖[8]。大宗HCC病例統(tǒng)計(jì)資料顯示，miR-26a表達(dá)量低的HCC患者總生存期短于miR-26a表達(dá)量高的，但對(duì)干擾素的治療更為敏感[9]。Kota 等[10]研究顯示，miR-26a能抑制小鼠肝癌模型的形成，在體外實(shí)驗(yàn)中，miR-26a可引起肝癌細(xì)胞周期阻滯，這與其靶蛋白cyclinD2和cyclinE2表達(dá)下調(diào)有關(guān)，miR-26a能抑制肝癌細(xì)胞的增殖，導(dǎo)致特異性凋亡，抑制肝癌進(jìn)展。

2．miR-26a與乳腺癌：miRNA參與乳腺癌細(xì)胞生長(zhǎng)的調(diào)節(jié)，發(fā)揮廣泛的雌激素依賴(lài)性抑制作用。研究顯示，miR-26a和miR-181a抑制cyclin E2的表達(dá)，調(diào)節(jié)許多與細(xì)胞增殖和生長(zhǎng)有關(guān)的基因，包括在雌激素信號(hào)效應(yīng)中起關(guān)鍵作用的孕激素受體基因[11]。另有報(bào)道顯示，miR-26a在乳腺癌細(xì)胞系中表達(dá)下降，短暫轉(zhuǎn)染miR-26a可引起乳腺癌細(xì)胞系McF7細(xì)胞凋亡。應(yīng)用逆轉(zhuǎn)錄病毒載體轉(zhuǎn)染miR-26a，可抑制乳腺癌細(xì)胞克隆形成，并且在動(dòng)物體內(nèi)成瘤能力減弱。miR-26a的兩個(gè)作用靶標(biāo)MTDH和Ezh2在乳腺癌中顯著上調(diào)。轉(zhuǎn)染表達(dá)miR-26a載體，乳腺癌McF7細(xì)胞的MTDH和Ezh2表達(dá)下降，細(xì)胞凋亡增加。miR-26a主要通過(guò)作用于MTDH和Ezh2抑制乳腺癌的形成[12]。

3．miR-26a 與鼻咽癌：miR-26a在鼻咽癌(nosopharyngeal cancer, NPC)組織及細(xì)胞系中普遍下調(diào)，在NPC的形成中發(fā)揮重要作用。轉(zhuǎn)染miR-26a可引起細(xì)胞G1期阻滯，抑制NPC細(xì)胞增殖和克隆形成。實(shí)驗(yàn)表明，miR-26a能下調(diào)NPC的癌基因Ezh2的表達(dá)，Ezh2低表達(dá)又能抑制miR-26a的作用。NPC組織mRNA高表達(dá)Ezh2，與miR-26a的表達(dá)呈負(fù)相關(guān)。因此，miR-26a作為NPC中的一個(gè)生長(zhǎng)抑制性miRNA，主要通過(guò)抑制Ezh2的表達(dá)發(fā)揮其抑制作用[13]。

4．miR-26a 與淋巴瘤：Sander等[14]發(fā)現(xiàn)，miR-26a在原發(fā)性Burkitt淋巴瘤組織中表達(dá)下調(diào)，是myc的靶分子。他們運(yùn)用基因芯片技術(shù)分別檢測(cè)myc誘導(dǎo)和myc抑制后的miRNA基因表達(dá)譜，結(jié)果顯示myc在多種腫瘤中持續(xù)抑制miR-26a的表達(dá)。在myc誘導(dǎo)的淋巴瘤細(xì)胞中轉(zhuǎn)染miR-26a，可導(dǎo)致細(xì)胞增殖能力下降，周期進(jìn)程受損，并且證實(shí)Ezh2為miR-26a的直接作用靶點(diǎn)。myc通過(guò)抑制miR-26a表達(dá)，從而減弱對(duì)Ezh2表達(dá)的抑制，提示miR-26a在myc誘導(dǎo)的淋巴瘤中發(fā)揮很強(qiáng)的抑癌基因作用[15]。另外，Di Lisio等[16]檢測(cè)了套細(xì)胞淋巴瘤miRNA表達(dá)譜，通過(guò)生物信息學(xué)方法分析miRNA可能的作用靶點(diǎn)，并用功能實(shí)驗(yàn)證實(shí)miR-26a調(diào)節(jié)套細(xì)胞淋巴瘤中NF-κB亞單位的核轉(zhuǎn)移，從而影響套細(xì)胞的生物學(xué)行為。

5．miR-26a與胰腺癌：胰腺癌miRNA表達(dá)譜芯片結(jié)果顯示，miR-26a在胰腺癌中表達(dá)[17]。在研究治療糖尿病的藥物二甲雙胍抑制胰腺癌細(xì)胞生長(zhǎng)的機(jī)制中發(fā)現(xiàn)，二甲雙胍通過(guò)上調(diào)胰腺癌細(xì)胞中包括miR-26a在內(nèi)的數(shù)種miRNA的表達(dá)發(fā)揮作用[18]。二甲雙胍呈劑量依賴(lài)性上調(diào)胰腺癌miR-26a的表達(dá)，并引起細(xì)胞生長(zhǎng)抑制，侵襲、轉(zhuǎn)移能力下降，凋亡增加，其機(jī)制可能是通過(guò)HMGA1發(fā)揮其作用[19]。此外，經(jīng)CDF(抗癌藥物姜黃素的類(lèi)似物)處理的胰腺癌細(xì)胞生長(zhǎng)抑制，侵襲能力下降，Ezh2表達(dá)下調(diào)，miR-26a表達(dá)上調(diào)[20]，認(rèn)為miR-26a可能參與了胰腺癌的發(fā)生發(fā)展過(guò)程，并在抗癌藥物的作用機(jī)制中發(fā)揮重要作用。

此外，miR-26a在肺原發(fā)性鱗狀細(xì)胞癌[21]、口腔鱗癌[22]等腫瘤中也發(fā)揮著抑癌基因作用。研究發(fā)現(xiàn)，miR-26a在良惡性胸水中表達(dá)存在差異[23]；在高侵襲的轉(zhuǎn)移性腎透明細(xì)胞癌和橫紋肌肉瘤中呈低表達(dá)[24-25]；在甲狀腺未分化癌(ATC)中表達(dá)下調(diào)，外源性給予miR-26a后，癌細(xì)胞的生長(zhǎng)受到抑制[26]。

三、miR-26a起癌基因調(diào)控作用

1．miR-26a與膠質(zhì)瘤：膠質(zhì)瘤的miR-26a常常是擴(kuò)增的，miR-26a作為擴(kuò)增子的組成部分發(fā)揮作用，該擴(kuò)增子還包括CDK4和CENTG1。CDK4和CENTG1作為癌基因，分別調(diào)控RB1和PI3/AKT通路。有人通過(guò)調(diào)查DNA的拷貝數(shù)、mRNA和miRNA及DNA甲基化數(shù)據(jù)庫(kù)，確定膠質(zhì)瘤中與miR-26a功能相關(guān)的靶基因有PTEN、Rb1和MAP3K2/MEKK1。實(shí)驗(yàn)證實(shí)，miR-26a能夠轉(zhuǎn)化細(xì)胞，在體外促進(jìn)膠質(zhì)瘤細(xì)胞增殖，在鼠腦內(nèi)通過(guò)降低PTEN、Rb1和MAP3K2/MEKK1蛋白表達(dá)，從而增加AKT的活性，促進(jìn)增殖，降低c-JUN癌基因N端激酶依賴(lài)的凋亡。在PTEN高表達(dá)以及PTEN表達(dá)缺失的膠質(zhì)瘤細(xì)胞中過(guò)表達(dá)miR-26a，在體外均能促進(jìn)腫瘤生長(zhǎng)，也能促進(jìn)CDK4或GENTG1過(guò)表達(dá)的細(xì)胞增殖，含該擴(kuò)增子的膠質(zhì)瘤患者生存率明顯下降。因此has-miR-26a、CDK4和GENTG1功能相關(guān)Oncomir/Oncogene簇通過(guò)協(xié)同靶作用于RB1、PI3K/AKT及JNK通路促進(jìn)癌的侵襲[4]。

2．miR-26a與前列腺癌：美國(guó)非裔的前列腺癌的發(fā)病率較其他種族高，檢測(cè)非裔和白種人前列腺癌發(fā)病階段中miR-26a的表達(dá)量，發(fā)現(xiàn)非裔的前列腺癌miR-26a較白種人在該癌相同的發(fā)展階段(良性、惡性、轉(zhuǎn)移)分別高出2.25、13.3、2.38倍，而且侵襲力高的腫瘤miR-26a升高[27]。表明miR-26a表達(dá)升高可能與前列腺癌的侵襲性有關(guān)。

四、miR-26a和細(xì)胞分化

Kang等[28]用基于分子熒光信號(hào)的生物影像學(xué)證實(shí)，miR-26a僅在高分化的肌母細(xì)胞C2C12中表達(dá)。Wong等[29]報(bào)道，miR-26a的表達(dá)在肌母細(xì)胞增殖到分化成肌小管過(guò)程中逐步上調(diào)，miR-26a表達(dá)升高促進(jìn)肌形成，并且確定骨骼肌細(xì)胞分化抑制子Ezh2作為miR-26a作用靶點(diǎn)，通過(guò)下調(diào)Ezh2的表達(dá)促進(jìn)肌細(xì)胞的分化。Luzi等[30]認(rèn)為，miR-26a通過(guò)作用于轉(zhuǎn)錄因子Smad1調(diào)節(jié)晚期骨母細(xì)胞的分化。當(dāng)抑制骨母細(xì)胞miR-26a表達(dá)，并增加其靶蛋白Smad1表達(dá)時(shí)，骨特異基因表達(dá)上調(diào)，骨母細(xì)胞分化明顯增多。此外，通過(guò)體內(nèi)外實(shí)驗(yàn)證實(shí)，miR-26a與人類(lèi)角化細(xì)胞的分化有關(guān)[31]。

五、miR-26a和細(xì)胞凋亡

Havelange等[32]通過(guò)對(duì)急性髓系白血病(AML)的mRNA和miRNA表達(dá)譜進(jìn)行相關(guān)性、基因本位(gene ontology)和作用網(wǎng)絡(luò)(network)分析，認(rèn)為miR-26a與促凋亡基因BIM和PTEN成正相關(guān)，并用實(shí)驗(yàn)證實(shí)miR-26a與AML細(xì)胞凋亡有關(guān)。Alajez等[33]用Luciferase實(shí)驗(yàn)證實(shí)，miR-26a調(diào)控NPC中Ezh2的表達(dá)，進(jìn)而調(diào)控細(xì)胞凋亡。Kota等[10]的研究結(jié)果顯示，miR-26a在肝細(xì)胞癌模型中引起肝癌細(xì)胞特異性凋亡。Zhang等[12]將miR-26a瞬時(shí)轉(zhuǎn)染乳腺癌細(xì)胞株MCF7從而啟動(dòng)癌細(xì)胞凋亡。但也有文獻(xiàn)報(bào)道，miR-26a降低HeLa、前列腺癌(DU145)和結(jié)腸癌細(xì)胞(SW480)對(duì)Trail誘導(dǎo)凋亡的敏感性[34]，降低膠質(zhì)瘤細(xì)胞中C-JUN-terminal Kinase依賴(lài)性凋亡[4]。故miR-26a在凋亡中的作用機(jī)制還有待于進(jìn)一步的研究。

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10.3760/cma.j.issn.1674-1935.2012.04.025

200433 上海，第二軍醫(yī)大學(xué)長(zhǎng)海醫(yī)院病理科

鄭建明，Email:jmzheng1962@163.com

2011-05-04)

(本文編輯：呂芳萍)