microRNA-26a與腫瘤

鄧靜靜 鄭建明

microRNA-26a與腫瘤

鄧靜靜 鄭建明

microRNA(miRNA)是一類長度為18～25個核苷酸(nt)的非編碼小RNA，通過與靶mRNA的互補配對在轉錄后水平調控基因表達，導致mRNA的降解或翻譯抑制，控制哺乳類動物約30%的蛋白質編碼基因活性[1-2]。miRNA與其靶mRNA分子組成了一個復雜的調控網絡，參與包括細胞增殖、分化、凋亡、發育等多種生物學過程。研究顯示，在多種腫瘤中miRNA表達異常，可能與腫瘤的形成、細胞分化及凋亡有關，它們有的起癌基因作用，有的則起抑癌基因作用。

一、miR-26a的結構和作用靶點

miR-26a含有21個核苷酸(nt),位于人3號染色體3P21.3,在人的大多數組織中表達，序列分析顯示miR-26a位于染色體的不穩定位點[3]。通過基因數據分析確定，miR-26a與CDK4和CENTG1共同組成一個擴增子，而CDK4和CENTG1作為癌基因分別調控RB1和PI3/AKT通路[4]。Zcchc11(zinc-finger, CCHC domain-containing protein 11)通過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生物學功能[5]。另有文獻報道[6-7]，miR-26a的活性受到myc的調控。目前已證實,在腫瘤中miR-26a的直接作用靶點有Ezh2、PTEN、cyclinD2、cyclinE2、GSK-3β；功能相關靶點有Rb1、MAP3K2/MEKK1和SMAD1。miR-26a通過作用于這些靶點發揮其調控腫瘤細胞生物學行為的作用。

二、miR-26a起抑癌基因調控作用

1．miR-26a與肝細胞癌：miR-26a在肝細胞癌(HCC)中過表達，它能顯著下調cyclinE2蛋白的表達，抑制肝癌細胞株HepG2的增殖[8]。大宗HCC病例統計資料顯示，miR-26a表達量低的HCC患者總生存期短于miR-26a表達量高的，但對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，在體外實驗中，miR-26a可引起肝癌細胞周期阻滯，這與其靶蛋白cyclinD2和cyclinE2表達下調有關，miR-26a能抑制肝癌細胞的增殖，導致特異性凋亡，抑制肝癌進展。

2．miR-26a與乳腺癌：miRNA參與乳腺癌細胞生長的調節，發揮廣泛的雌激素依賴性抑制作用。研究顯示，miR-26a和miR-181a抑制cyclin E2的表達，調節許多與細胞增殖和生長有關的基因，包括在雌激素信號效應中起關鍵作用的孕激素受體基因[11]。另有報道顯示，miR-26a在乳腺癌細胞系中表達下降，短暫轉染miR-26a可引起乳腺癌細胞系McF7細胞凋亡。應用逆轉錄病毒載體轉染miR-26a，可抑制乳腺癌細胞克隆形成，并且在動物體內成瘤能力減弱。miR-26a的兩個作用靶標MTDH和Ezh2在乳腺癌中顯著上調。轉染表達miR-26a載體，乳腺癌McF7細胞的MTDH和Ezh2表達下降，細胞凋亡增加。miR-26a主要通過作用于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)組織及細胞系中普遍下調，在NPC的形成中發揮重要作用。轉染miR-26a可引起細胞G1期阻滯，抑制NPC細胞增殖和克隆形成。實驗表明，miR-26a能下調NPC的癌基因Ezh2的表達，Ezh2低表達又能抑制miR-26a的作用。NPC組織mRNA高表達Ezh2，與miR-26a的表達呈負相關。因此，miR-26a作為NPC中的一個生長抑制性miRNA，主要通過抑制Ezh2的表達發揮其抑制作用[13]。

4．miR-26a 與淋巴瘤：Sander等[14]發現，miR-26a在原發性Burkitt淋巴瘤組織中表達下調，是myc的靶分子。他們運用基因芯片技術分別檢測myc誘導和myc抑制后的miRNA基因表達譜，結果顯示myc在多種腫瘤中持續抑制miR-26a的表達。在myc誘導的淋巴瘤細胞中轉染miR-26a，可導致細胞增殖能力下降，周期進程受損，并且證實Ezh2為miR-26a的直接作用靶點。myc通過抑制miR-26a表達，從而減弱對Ezh2表達的抑制，提示miR-26a在myc誘導的淋巴瘤中發揮很強的抑癌基因作用[15]。另外，Di Lisio等[16]檢測了套細胞淋巴瘤miRNA表達譜，通過生物信息學方法分析miRNA可能的作用靶點，并用功能實驗證實miR-26a調節套細胞淋巴瘤中NF-κB亞單位的核轉移，從而影響套細胞的生物學行為。

5．miR-26a與胰腺癌：胰腺癌miRNA表達譜芯片結果顯示，miR-26a在胰腺癌中表達[17]。在研究治療糖尿病的藥物二甲雙胍抑制胰腺癌細胞生長的機制中發現，二甲雙胍通過上調胰腺癌細胞中包括miR-26a在內的數種miRNA的表達發揮作用[18]。二甲雙胍呈劑量依賴性上調胰腺癌miR-26a的表達，并引起細胞生長抑制，侵襲、轉移能力下降，凋亡增加，其機制可能是通過HMGA1發揮其作用[19]。此外，經CDF(抗癌藥物姜黃素的類似物)處理的胰腺癌細胞生長抑制，侵襲能力下降，Ezh2表達下調，miR-26a表達上調[20]，認為miR-26a可能參與了胰腺癌的發生發展過程，并在抗癌藥物的作用機制中發揮重要作用。

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

三、miR-26a起癌基因調控作用

1．miR-26a與膠質瘤：膠質瘤的miR-26a常常是擴增的，miR-26a作為擴增子的組成部分發揮作用，該擴增子還包括CDK4和CENTG1。CDK4和CENTG1作為癌基因，分別調控RB1和PI3/AKT通路。有人通過調查DNA的拷貝數、mRNA和miRNA及DNA甲基化數據庫，確定膠質瘤中與miR-26a功能相關的靶基因有PTEN、Rb1和MAP3K2/MEKK1。實驗證實，miR-26a能夠轉化細胞，在體外促進膠質瘤細胞增殖，在鼠腦內通過降低PTEN、Rb1和MAP3K2/MEKK1蛋白表達，從而增加AKT的活性，促進增殖，降低c-JUN癌基因N端激酶依賴的凋亡。在PTEN高表達以及PTEN表達缺失的膠質瘤細胞中過表達miR-26a，在體外均能促進腫瘤生長，也能促進CDK4或GENTG1過表達的細胞增殖，含該擴增子的膠質瘤患者生存率明顯下降。因此has-miR-26a、CDK4和GENTG1功能相關Oncomir/Oncogene簇通過協同靶作用于RB1、PI3K/AKT及JNK通路促進癌的侵襲[4]。

2．miR-26a與前列腺癌：美國非裔的前列腺癌的發病率較其他種族高，檢測非裔和白種人前列腺癌發病階段中miR-26a的表達量，發現非裔的前列腺癌miR-26a較白種人在該癌相同的發展階段(良性、惡性、轉移)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表達升高可能與前列腺癌的侵襲性有關。

四、miR-26a和細胞分化

Kang等[28]用基于分子熒光信號的生物影像學證實，miR-26a僅在高分化的肌母細胞C2C12中表達。Wong等[29]報道，miR-26a的表達在肌母細胞增殖到分化成肌小管過程中逐步上調，miR-26a表達升高促進肌形成，并且確定骨骼肌細胞分化抑制子Ezh2作為miR-26a作用靶點，通過下調Ezh2的表達促進肌細胞的分化。Luzi等[30]認為，miR-26a通過作用于轉錄因子Smad1調節晚期骨母細胞的分化。當抑制骨母細胞miR-26a表達，并增加其靶蛋白Smad1表達時，骨特異基因表達上調，骨母細胞分化明顯增多。此外，通過體內外實驗證實，miR-26a與人類角化細胞的分化有關[31]。

五、miR-26a和細胞凋亡

Havelange等[32]通過對急性髓系白血病(AML)的mRNA和miRNA表達譜進行相關性、基因本位(gene ontology)和作用網絡(network)分析，認為miR-26a與促凋亡基因BIM和PTEN成正相關，并用實驗證實miR-26a與AML細胞凋亡有關。Alajez等[33]用Luciferase實驗證實，miR-26a調控NPC中Ezh2的表達，進而調控細胞凋亡。Kota等[10]的研究結果顯示，miR-26a在肝細胞癌模型中引起肝癌細胞特異性凋亡。Zhang等[12]將miR-26a瞬時轉染乳腺癌細胞株MCF7從而啟動癌細胞凋亡。但也有文獻報道，miR-26a降低HeLa、前列腺癌(DU145)和結腸癌細胞(SW480)對Trail誘導凋亡的敏感性[34]，降低膠質瘤細胞中C-JUN-terminal Kinase依賴性凋亡[4]。故miR-26a在凋亡中的作用機制還有待于進一步的研究。

[1] Rachagani S,Kumar S, batra SK, et al. MicroRNA in pancreatic cancer: Pathological, diagnostic and therapeutic implications. Cancer Lett, 2010,292: 8-16.

[2] Sun W, Julie Li YS, Huang HD, et al.microRNA: a master regulator of cellular processes for bioeng-ineering systems. Annu Rev Biomed Eng, 2010,12:1-27.

[3] Calin GA,Sevignani C,Dumitru CD,et al.Human microRNA genes are frequently located at fragile sites and genomic regions involved in cancers.Proc Natl Acad Sci USA,2004,101:2999-3004.

[4] Kim H, Huang W, Jiang X, et al.Integrature genome analysis reveals an oncomir/ aocogene claster regulating glioblastoma survivorship.Proc Natl Acad Sci USA, 2010,107:2183-2188.

[5] Jones MR, Quinton LJ,Blahna MT, et al.Zcchc11-dependent uridylation of microRNA directs cytokine expression. Nat Cell Biol,2009,11:1157-1163.

[6] Sudbery I, Enright AJ, Fraser AG, et al.Systematic analysis of off-target effects in an RNAi screen reveals microRNAs affecting sensitivity to TRAIL-induced apoptosis. BMC Genomics, 2010,11:175.

[7] Sander S, Bullinger L, Klapproth K,et al.MYC stimulates EzH2 expression by repression of its negative regulator miR-26a. Blood,2008,112:4202-4212.

[8] Chen X,Murad M,Cui YY,et al.miRNA regulation of liver growth after 50% partial hepatectomy and small size grafts in rats. Transplantation, 2011,91:293-299.

[9] Ji J,Shi J,Budhu A, et al. MicroRNA expression, survival, and response to interferon in liver cancer. N Engl J Med, 2009,361:1437-1447.

[10] Kota J, Chivukula RR, O′Donnell KA, et al. Therapeutic microRNA delivery suppresses tumorigenesis in a murine liver cancer model. Cell, 2009,137:1005-1017.

[11] Maillot G, Lacroin-Triki M, Pierredon S, et al. Widespread estrogen-dependent repression of micrornas involved in breast tumor cell growth. Cancer Res, 2009,69: 8332-8340.

[12] Zhang B, Liu XX,He JR,et al. Pathologically decreased miR-26a antagonizes apoptosis and facilitates carcinogenesis by targeting MTDH and Ezh2 in breast cancer. Carcinogenesis, 2011,32:2-9.

[13] Lu J, He ML,Wang L,et al. MiR-26a inhibits cell growth and tumorigenesis of nasopharyngeal carcinoma through repression of EZH2. Cancer Res, 2011,71:225-233.

[14] Sander S, Bullinger L,Klapproth K,et al.MYC stimulates EZH2 expression by repression of its negative regulator miR-26a. Blood,2008,112:4202-4212.

[15] Sander S, Bullinger L, Wirth T.Repressing the repressor: a new mode of myc action in lymphomagenesis. Cell Cycle, 2009,8:556-559.

[16] Di Lisio L, Gómez-López G,Sánchez-Beato M, et al.Mantle cell lymphoma: transcriptional regulation by microRNAs. Leukemia,2010,24:1335-1342.

[17] Zhang Y, Li M, Wang H, et al. Profiling of 95 MicroRNAs in pancreatic cancer cell Lines and surgical specimens by Real-time PCR analysis. World J Surg,2009,33:698-709.

[18] Bao B, Wang Z, Ali S, et al. Metformin inhibits cell proliferation, migration and invasion by attenuating CSC function mediated by deregulating miRNAs in pancreatic cancer cells. Cancer Prev Res (Phila), 2012,5:355-364.

[19] Li W, Yuan Y, Huang L, et al.Metformin alters the expression profiles of microRNAs in human pancreatic cancer cells. Diabetes Res Clin Pract, 2012,96:187-195.

[20] Bao B,Ali S,Baneriee S,et al.Curcumin analogue CDF inhibits pancreatic tumor growth by switching on suppressor microRNAs and attenuating EZH2 expression.Cancer Res,2012,72:335-345.

[21] Gao W,Shen H, Liu L, et al. MiR-21 overexpression in human primary squamous cell lung carcinoma is associated with poor patient prognosis. J Cancer Res Clin Oncol, 2011,137:557-566.

[22] Yu T, Wang XY, Gong RG, et al.The expression profile of microRNAs in a model of7,12-dimethyl-benz[a] anthrance-induced oral carcinogenesis in Syrian hamster. J Exp Clin Cancer Res, 2009,13;28:64.

[23] Xie L, Chen X, Wang L, et al. Cell-free miRNAs may indicate diagnosis and docetaxel sensitivity of tumor cells in malignant effusions. BMC Cancer, 2010,10:591.

[24] Heinzelmann J, Henning B, Sanjmyatav J, et al. Specific miRNA signatures are associated with metastasis and poor prognosis in clear cell renal cell carcinoma. World J Urol, 2011,29:367-373.

[25] Ciarapica R, Russo G, Verginelli F, et al.Deregulated expression of miR-26a and Ezh2 in rhabdomyosarcoma. Cell Cycle,2009,8:172-175.

[26] Visone R, Pallante P, Vecchione A, et al. Specific microRNAs are downregulated in human thyroid anaplastic carcinomas. Oncogene, 2007,26:7590-7595.

[27] Huse JT,Brennan C,Hambardzumyan D,et al.The PTEN-regulating miR-26a is amplified in high-grade glioma and facilitates gliomagenesis in vivo.Genes Dev,2009,23:1327-1337.

[28] Kang WJ, Cho YL, Chae JR, et al. Molecular beacon-based bioimaging of multiple microRNAs during myogenesis.Biomaterials,2011,32:1915-1922.

[29] Wong CF, Tellam RL.MicroRNA-26a targets the histone methyltransferase Enhancer of Zeste homolog 2 during myogenesis. J Biol Chem,2008,283:9836-9843.

[30] Luzi E,Marini F, Sala SC, et al. Osteogenic differentiation of human adipose tissue-derived stem cells is modulated by the miR-26a targeting of the SMAD1 transcription factor. J Bone Miner Res, 2008,23:287-295.

[31] Hildebrand J, Rütze M, Walz N, et al.A comprehensive analysis of microRNA expression during human keratinocyte differentiation in vitro and in vivo. J Invest Dermatol, 2011,131:20-29.

[32] Havelange V, Stauffer N, Heaphy CC, et al.Functional implications of microRNAs in acute myeloid leukemia by integrating microRNA and messenger RNA expression profiling. Cancer, 2011, 10.1002/cncr.26096.[Epub ahead of print].

[33] Alajez NM, Shi W, Hui AB, et al. Enhancer of Zeste homolog 2 (EZH2) is overexpressed in recurrent nasopharyngeal carcinoma and is regulated by miR-26a, miR-101, and miR-98. Cell Death Dis, 2010,1:e85.

[34] Sudbery I,Enright AJ,Fraser AG,et al.Systematic analysis of off-target effects in an RNAi screen reveals microRNAs affecting sensitivity to TRAIL-induced apoptosis.BMC Genomics,2010,11:175.

10.3760/cma.j.issn.1674-1935.2012.04.025

200433 上海，第二軍醫大學長海醫院病理科

鄭建明，Email:jmzheng1962@163.com

2011-05-04)

(本文編輯：呂芳萍)