[摘要]胃癌是一種最常見(jiàn)的嚴(yán)重威脅人類健康的消化系統(tǒng)惡性腫瘤,其對(duì)化療藥物的耐藥性是臨床治療失敗的關(guān)鍵因素。微小RNA(miRNA)是一種內(nèi)源性單鏈非編碼RNA,通過(guò)與靶基因的信使RNA結(jié)合調(diào)控基因的轉(zhuǎn)錄后表達(dá),廣泛參與胃癌的多藥耐藥進(jìn)程。因此,探究miRNA參與的胃癌耐藥機(jī)制并實(shí)現(xiàn)其有效逆轉(zhuǎn)對(duì)胃癌的治療具有重要意義。本文從miRNA的概述、作用機(jī)制及常見(jiàn)抗腫瘤藥物監(jiān)測(cè)等方面進(jìn)行綜述。
[關(guān)鍵詞]微RNA;胃腫瘤;抗藥性,腫瘤;綜述
[中圖分類號(hào)]R735.2[文獻(xiàn)標(biāo)志碼]A[文章編號(hào)]2096-5532(2018)06-0739-05
胃癌是一種最常見(jiàn)的消化系統(tǒng)惡性腫瘤,已成為全球癌癥死亡的第二大原因[1]?;熓俏赴┩砥谥委煹闹匾侄沃?,但原發(fā)或繼發(fā)性耐藥是胃癌化療失敗的一個(gè)關(guān)鍵因素。微小RNA(miRNA)是一種長(zhǎng)度約22個(gè)核苷酸的非編碼RNA分子,主要由內(nèi)源基因編碼并在進(jìn)化中高度保守。miRNA主要和靶基因mRNA的3′非編碼區(qū)(3′UTR)結(jié)合抑制其翻譯或促進(jìn)其降解,參與胃癌的增殖、轉(zhuǎn)移和耐藥過(guò)程[2]。此外,不同miRNAs在胃癌中的表達(dá)是有差異的,有些miRNAs表達(dá)上調(diào),而有些miRNAs表達(dá)下調(diào),這些異常表達(dá)的miRNAs與胃癌的多重耐藥性關(guān)系密切,有望成為逆轉(zhuǎn)胃癌耐藥的生物分子標(biāo)志物[3]。因此,可以通過(guò)監(jiān)測(cè)miRNAs變化來(lái)探尋耐藥機(jī)制,為逆轉(zhuǎn)抗腫瘤藥物耐藥和胃癌治療提供有效的輔助治療方案。
1miRNA的概述
1993年,LEE等[4]在秀麗隱桿線蟲(chóng)中首先發(fā)現(xiàn)了一種非蛋白編碼RNA,稱為miRNA。后來(lái)證實(shí)miRNA是一類長(zhǎng)度約為22個(gè)核苷酸的內(nèi)源性非編碼小分子單鏈RNA,在各種真核生物中普遍存在,具有高度的保守性、時(shí)序性和特異性[5]。miRNA的合成過(guò)程復(fù)雜:首先在細(xì)胞核中由RNA聚合酶作用生成原始轉(zhuǎn)錄片段,繼而被Drosha酶剪切生成miRNA前體,并被原始轉(zhuǎn)錄片段轉(zhuǎn)運(yùn)體exportin-5特異性識(shí)別;隨后在細(xì)胞質(zhì)中被Dicer酶剪切生成成熟miRNA。成熟miRNA通過(guò)其第2~8位的核苷酸與其靶miRNA的3′UTR進(jìn)行特異性識(shí)別并發(fā)揮其轉(zhuǎn)錄翻譯抑制功能,從而從轉(zhuǎn)錄后水平調(diào)控靶基因[5-6]。目前研究結(jié)果顯示,miRNA對(duì)靶基因的調(diào)控是錯(cuò)綜復(fù)雜的多通路、多靶點(diǎn)的“多對(duì)多”的過(guò)程,起到癌基因或抑癌基因的作用,涉及包括胃癌在內(nèi)的多種腫瘤的發(fā)生發(fā)展。這些異常表達(dá)miRNA使靶基因功能異常,蛋白表達(dá)水平錯(cuò)亂,繼而影響相關(guān)的細(xì)胞信號(hào)通路,從而改變腫瘤細(xì)胞的自噬、分化、增殖、凋亡、耐藥等生物學(xué)過(guò)程[5-7]。
1.1miRNA與胃癌耐藥機(jī)制
胃癌的耐藥途徑及耐藥機(jī)制復(fù)雜多變,包括細(xì)胞內(nèi)藥物濃度的降低、藥物靶點(diǎn)的改變、細(xì)胞生存和死亡信號(hào)通路的失調(diào)、腫瘤細(xì)胞和腫瘤微環(huán)境之間的相互作用等。目前研究表明,miRNA參與胃癌耐藥途徑主要包括以下幾種[8-27]。
1.2耐藥相關(guān)蛋白的異常表達(dá)
最具代表性的是ATP結(jié)合盒(ABC)轉(zhuǎn)運(yùn)蛋白超家族,主要包括P-糖蛋白(P-gp)、多藥耐藥相關(guān)蛋白1、肺耐藥相關(guān)蛋白和乳癌耐藥蛋白等,它們能加速藥物泵出,使藥物在細(xì)胞內(nèi)的濃度降低,不能有效抑制腫瘤細(xì)胞增殖過(guò)程從而產(chǎn)生耐藥性。如miR-129-5p就是通過(guò)作用于ABC轉(zhuǎn)運(yùn)蛋白參與胃癌的多重耐藥過(guò)程[8]。miR-21通過(guò)作用于P-gp來(lái)調(diào)節(jié)對(duì)紫杉醇的敏感性[9]。miR-27a的下調(diào)可能會(huì)抑制胃癌細(xì)胞在體內(nèi)和體外的增殖,增加藥物的積累,減少阿霉素的釋放,從而提高對(duì)阿霉素的藥物敏感性;下調(diào)miR-27a可以顯著減少P-gp的表達(dá)和細(xì)胞周期蛋白D1的轉(zhuǎn)錄活動(dòng),并使p21的表達(dá)增加[10]。miR-129也會(huì)通過(guò)抑制P-gp的表達(dá)來(lái)逆轉(zhuǎn)胃癌細(xì)胞對(duì)順鉑(DDP)的耐藥[11]。此外,miR-30a在胃癌耐藥組織中表達(dá)降低,而增加其表達(dá)可以調(diào)控P-gp從而逆轉(zhuǎn)胃癌的多重耐藥[12]。
1.3細(xì)胞凋亡途徑介導(dǎo)胃癌耐藥
細(xì)胞凋亡途徑在胃癌耐藥進(jìn)程中發(fā)揮重要作用,促凋亡基因的缺失和抗凋亡基因蛋白的過(guò)度表達(dá)尤為關(guān)鍵。最常見(jiàn)的有BCL-2(B-cell lymphoma 2)基因的表達(dá)異常,如miR-497[13]、miR-181b[14]、miR-204[15]通過(guò)作用于靶標(biāo)BCL-2調(diào)節(jié)細(xì)胞凋亡來(lái)調(diào)控胃癌細(xì)胞系的多藥耐藥性。除此之外,P53、生存蛋白、葡萄糖神經(jīng)酰胺合成酶、熱休克蛋白等異常表達(dá)也與胃癌耐藥關(guān)系密切。
1.4上皮-間質(zhì)轉(zhuǎn)化(EMT)介導(dǎo)胃癌耐藥
EMT是上皮樣細(xì)胞轉(zhuǎn)化為間質(zhì)細(xì)胞的過(guò)程。在EMT過(guò)程中,一些黏附分子如E-鈣黏蛋白的消失使得細(xì)胞之間的極性被打亂,失去與基膜的連接,細(xì)胞之間的相互作用發(fā)生改變,導(dǎo)致間充質(zhì)表型變化,最終激活癌細(xì)胞的遷移和侵襲,導(dǎo)致耐藥的發(fā)生[16]。如在耐曲妥珠單抗的胃癌細(xì)胞中發(fā)生典型的EMT,通過(guò)活躍Notch信號(hào)通路,激活信號(hào)傳導(dǎo)轉(zhuǎn)錄因子3(STAT3),導(dǎo)致胃癌細(xì)胞侵襲及遷移能力增強(qiáng),加速胃癌耐藥進(jìn)程[17]。此外,miR-1274a通過(guò)EMT過(guò)程加速胃癌細(xì)胞增殖及遷徙,誘導(dǎo)耐藥發(fā)生[18],miR-30a會(huì)抑制EMT過(guò)程,使本來(lái)耐藥的胃癌細(xì)胞對(duì)DDP的治療變得敏感[19]。
1.5細(xì)胞微環(huán)境變化
全身正常的組織或細(xì)胞可以免疫、代謝和分泌等形式限制腫瘤細(xì)胞的發(fā)生發(fā)展,而在腫瘤微環(huán)境中,內(nèi)源性遞質(zhì)、免疫異常細(xì)胞及調(diào)控因子等形成免疫抑制網(wǎng)絡(luò)結(jié)構(gòu),可以對(duì)抗人體的抗腫瘤免疫,激活多種信號(hào)通路,加速細(xì)胞生長(zhǎng),在胃癌細(xì)胞的增殖、遷徙及轉(zhuǎn)移中發(fā)揮重要作用[20]。內(nèi)源性巨噬細(xì)胞是腫瘤微環(huán)境中較豐富的細(xì)胞成分之一,其代謝異常與胃癌耐藥關(guān)系密切,如miR-32的表達(dá)抑制使腫瘤微環(huán)境中的巨噬細(xì)胞和CD44表達(dá)增加,細(xì)胞對(duì)活性氧的耐受提高,促進(jìn)細(xì)胞生長(zhǎng),加速腫瘤惡化[21]。
1.6以外泌體的形式參與胃癌耐藥
外泌體作為一種大小為30~100 nm的囊泡狀小體,可以被多種細(xì)胞分泌并存在于體液中。胃癌來(lái)源的外泌體通過(guò)其內(nèi)容物的異常表達(dá),調(diào)控Wnt/β-catenin、PI3K/AKT及MAPK/ERK等信號(hào)通路,影響微環(huán)境變化,抑制免疫應(yīng)答,誘導(dǎo)胃癌的發(fā)生發(fā)展及耐藥進(jìn)程[22]。OHSHIMA等[23]研究顯示,轉(zhuǎn)移性胃癌細(xì)胞株AZ-P7a通過(guò)exosome將let-7 miRNA家族運(yùn)輸至胞外,減少其在腫瘤細(xì)胞內(nèi)的含量,從而降低其抗腫瘤作用,促進(jìn)腫瘤的發(fā)生與發(fā)展。
此外,DNA甲基化和組蛋白修飾錯(cuò)誤也會(huì)導(dǎo)致耐藥的發(fā)生。miR-34c-5p在胃癌細(xì)胞對(duì)紫杉醇的耐藥中,DNA甲基化發(fā)揮至關(guān)重要的作用[24]。而且,多種信號(hào)通路的調(diào)節(jié)異??蓪?dǎo)致相關(guān)細(xì)胞過(guò)度增殖、凋亡受阻,引起胃癌耐藥的發(fā)生。胃癌中mRNA參與的信號(hào)通路主要有:Wnt/β-catenin、PI3K/AKT、MAPK/ERK、mTOR、IGF1R/IRS1、Hippo、Notch等[25-26],但這些通路中的相互關(guān)系及激活時(shí)間需要進(jìn)一步研究[27]。
2miRNA與胃癌耐藥監(jiān)測(cè)
2.1與5-氟尿嘧啶(5-Fu)耐藥有關(guān)的miRNAs
5-Fu是胃癌多種化療方案中的一種基礎(chǔ)抗腫瘤藥物,因其耐受性好、價(jià)格低廉、療效肯定等原因廣泛應(yīng)用于臨床[28],但耐藥性的出現(xiàn)降低了其治療效果。如WANG等[3]建立了對(duì)5-Fu耐藥的胃癌細(xì)胞系并分析其中差異表達(dá)的miRNAs,結(jié)果顯示,9種miRNAs(miR-10b,-22,-31,-133b,-190,-501,-615,-501-5p,-615-5p)表達(dá)上調(diào),18種miRNAs(miR-32,-197,-210,-766,-1229,-1238,-3131,-3149,-1224-3p,-3162-3p,-532,-877,-4701-5p,-5096,-4728-3p,-1273d,-486-3p,-4763-3p)下調(diào),這些異常表達(dá)的miRNAs與胃癌的多重耐藥性關(guān)系密切,有望成為逆轉(zhuǎn)胃癌耐藥的生物分子新指標(biāo)。miR197在對(duì)5-Fu耐藥的SGC7901/5-Fu細(xì)胞中低表達(dá),上調(diào)miR197的表達(dá)會(huì)作用于靶標(biāo)MAPK1(mitogenactivated protein kinase 1),使SGC7901/5-FU細(xì)胞株保持對(duì)5-Fu治療敏感性[29]。此外,miR-19a/b[30]、miR-129-5p[8]、miR-23b-3p[31]等也與5-Fu的耐藥密切相關(guān)。CHOI等[32]最近的研究結(jié)果顯示,EBV(Epstein-Barr virus)中產(chǎn)生的一種miR-BART15-3p,通過(guò)下調(diào)TAX1BP1(Tax1-binding protein 1)基因表達(dá)來(lái)增強(qiáng)胃癌細(xì)胞的凋亡及對(duì)5-Fu的敏感性。最近研究發(fā)現(xiàn),miR-182-5p抑制劑與5-Fu合用,可顯著降低SGC7901/5-Fu細(xì)胞株的增殖能力,促進(jìn)細(xì)胞凋亡,抑制細(xì)胞遷移,顯著增加SGC7901/5-Fu細(xì)胞株對(duì)5-Fu化療的敏感性[33]。
2.2與DDP治療有關(guān)的miRNAs
化療及聯(lián)合化療仍然是治療胃癌的主要策略。YANG等[34]對(duì)SGC7901細(xì)胞系研究顯示,miR-21通過(guò)調(diào)節(jié)PTEN在SGC7901/DDP細(xì)胞中高表達(dá)發(fā)揮耐藥作用。最新研究發(fā)現(xiàn),miR-34a可以通過(guò)調(diào)控MET基因影響胃癌細(xì)胞的增殖和凋亡,從而影響胃癌細(xì)胞對(duì)DDP的敏感性,為胃癌的治療提供了新思路[35]。LI等[36]研究發(fā)現(xiàn),與正常的胃黏膜上皮細(xì)胞相比,miR-101在5種常見(jiàn)的胃癌細(xì)胞中時(shí)常低表達(dá),尤其是在SGC7901細(xì)胞系中,人為用miR-101模擬物提高miR-101的表達(dá),結(jié)果顯示高表達(dá)的miR-101可以抑制細(xì)胞增殖,加速細(xì)胞凋亡,促進(jìn)對(duì)DDP耐藥的SGC7901/DDP細(xì)胞系重新對(duì)DDP敏感;進(jìn)一步的研究顯示,miR-101通過(guò)作用于靶標(biāo)VEGF-C來(lái)影響SGC7901/DDP細(xì)胞系對(duì)DDP的敏感過(guò)程。miR-106a也會(huì)通過(guò)PTEN/Akt通路,在胃癌SGC7901/DDP細(xì)胞中表達(dá)增加;抑制miR-106a的表達(dá),會(huì)使DDP的細(xì)胞毒性增強(qiáng),增加SGC7901/DDP細(xì)胞對(duì)DDP的敏感性[37]。此外,miR-129可通過(guò)抑制P-gp的表達(dá)來(lái)逆轉(zhuǎn)胃癌細(xì)胞對(duì)DDP的耐藥[11]。ZHANG等[38]研究顯示,上調(diào)miR-218的表達(dá)會(huì)增加SGC7901細(xì)胞對(duì)DDP的敏感性,同時(shí)在裸鼠體內(nèi)也發(fā)現(xiàn)miR-218可抑制腫瘤細(xì)胞生長(zhǎng),這為抑制腫瘤進(jìn)展和逆轉(zhuǎn)耐藥藥物研究提供了新思路。進(jìn)一步研究顯示,與親本細(xì)胞相比,miR-218在多重耐藥細(xì)胞系中亦呈低表達(dá),過(guò)度表達(dá)的miR-218會(huì)使化療藥物DDP排出減少,從而使藥物積累而加速細(xì)胞凋亡,使胃癌細(xì)胞對(duì)DDP治療敏感[39]。另外,CHANG等[40]對(duì)27例胃癌組織研究發(fā)現(xiàn),與正常組織對(duì)比,miRNA-200c低表達(dá),RhoE高表達(dá);在胃癌細(xì)胞中也發(fā)現(xiàn),SGC7901/DDP細(xì)胞系中miRNA-200c表達(dá)降低,RhoE表達(dá)升高;進(jìn)一步研究顯示,miRNA-200c通過(guò)直接作用于RhoE的3′UTR來(lái)調(diào)控胃癌細(xì)胞對(duì)DDP的耐藥。此外,miR-503直接作用于靶標(biāo)IGF1R和BCL-2抑制細(xì)胞增殖,加速細(xì)胞的凋亡進(jìn)程,使本來(lái)對(duì)DDP耐藥的SGC7901細(xì)胞系對(duì)DDP變得敏感[41]。
2.3與阿霉素治療有關(guān)的miRNAs
miRNA-135a-5p模擬物增加了胃癌BGC-823細(xì)胞的凋亡率,使其對(duì)阿霉素的敏感性增加。其中miRNA-135a-5p-AP-2ɑ-BCL-2通路在誘導(dǎo)細(xì)胞凋亡中發(fā)揮重要的作用[42]。
ZHANG等[43]用微陣列分析技術(shù)的研究顯示,與親本細(xì)胞對(duì)比,耐多藥SGC7901(SGC7901/ADR)細(xì)胞中的miR-103/107表達(dá)水平明顯降低,而提高miR-103/107的表達(dá)則能夠使SGC7901/ADR細(xì)胞對(duì)阿霉素的敏感性增加;進(jìn)一步研究發(fā)現(xiàn),miR-103/107通過(guò)下調(diào)Cav-1蛋白增加細(xì)胞對(duì)阿霉素的敏感性。WANG等[44]的研究結(jié)果顯示,miR-126在耐藥的胃癌細(xì)胞系中低表達(dá),miR-126的異常高表達(dá)直接減少EZH2的表達(dá),重新致敏耐藥的胃癌細(xì)胞。也有研究顯示,阿霉素通過(guò)誘導(dǎo)microRNA-520h的表達(dá)可以使組蛋白去乙酰化酶1(HDAC1)蛋白表達(dá)減少;反之,增加HDAC1的表達(dá)會(huì)使胃癌細(xì)胞對(duì)阿霉素產(chǎn)生耐藥性[45]。
2.4與紫杉醇治療有關(guān)的miRNAs
馬筱秋等[46]利用miR-29c模擬物轉(zhuǎn)染BGC-823胃癌細(xì)胞,結(jié)果發(fā)現(xiàn)miR-29c過(guò)表達(dá)可以抑制細(xì)胞增殖過(guò)程,使細(xì)胞周期受阻于S期而促進(jìn)細(xì)胞凋亡,增加了腫瘤細(xì)胞對(duì)多西紫杉醇治療的敏感性;實(shí)時(shí)定量PCR和Western blot研究顯示,miR-29c可以直接調(diào)控Mcl-1的表達(dá)。此外,miR-34c-5p在對(duì)紫杉醇耐藥的細(xì)胞株中低表達(dá),如果上調(diào)miR-34c-5p的表達(dá),微管相關(guān)蛋白Tau (MAPT)表達(dá)減少,可使本來(lái)對(duì)紫杉醇耐藥的胃癌細(xì)胞株重新對(duì)紫杉醇敏感;DNA甲基化、miR-34c-5p和MAPT的異常表達(dá)在胃癌對(duì)紫杉醇的耐藥中發(fā)揮至關(guān)重要的作用[24]。另外,miR-21在對(duì)紫杉醇耐藥的SGC7901細(xì)胞系中高表達(dá),降低miR-21表達(dá),胃癌細(xì)胞株增殖能力減弱,凋亡能力增加;進(jìn)一步研究發(fā)現(xiàn),miR-21通過(guò)作用于P-gp調(diào)節(jié)對(duì)紫杉醇的敏感性[9]。
2.5在其他藥物治療胃癌中異常表達(dá)的miRNAs
曲妥珠單抗已成為晚期胃癌的關(guān)鍵治療藥物,但耐藥性的發(fā)生也是導(dǎo)致治療失敗的關(guān)鍵因素。研究顯示,miRNA-21可通過(guò)miR-21/PTEN和miR-223/FBXW7通路來(lái)調(diào)控細(xì)胞凋亡途徑,從而增加胃癌細(xì)胞株對(duì)曲妥珠單抗的敏感性[34]。miR-1284 在胃癌組織標(biāo)本以及對(duì)長(zhǎng)春新堿耐藥的SGC7901細(xì)胞(SGC7901/VCR)中表達(dá)均降低,SGC7901/VCR細(xì)胞中miR-1284 的過(guò)表達(dá)可以克服多重耐藥,阻止細(xì)胞進(jìn)入S期,加速藥物引起的細(xì)胞凋亡;miR-1284 的過(guò)表達(dá)也會(huì)降低腫瘤細(xì)胞的遷移和侵襲能力,進(jìn)一步的研究結(jié)果顯示,miR-1284 直接作用于靶標(biāo)EIF4A1而發(fā)揮作用[47]。隨著研究的深入,越來(lái)越多的miRNAs參與一種或多種抗腫瘤藥物的耐藥過(guò)程,調(diào)控途徑也越來(lái)越復(fù)雜,調(diào)節(jié)靶基因也越來(lái)越豐富。
3展望
miRNA是一類長(zhǎng)度約22 個(gè)核苷酸的內(nèi)源性非編碼小分子單鏈RNA,普遍并穩(wěn)定存在于組織標(biāo)本、血液及體液中,影響內(nèi)環(huán)境穩(wěn)態(tài),調(diào)節(jié)胃癌的發(fā)生發(fā)展過(guò)程。近年來(lái)研究人員已通過(guò)基因芯片、反義核酸、轉(zhuǎn)染等技術(shù)手段,證實(shí)了miRNA在腫瘤細(xì)胞耐藥途徑中發(fā)揮著重要調(diào)控作用。而且,不同 miRNAs在胃癌耐藥過(guò)程中的表達(dá)是有差異的,這些差異表達(dá)的 miRNAs有望成為逆轉(zhuǎn)胃癌耐藥新的生物標(biāo)志物[6-7]。因此,我們可以通過(guò)監(jiān)測(cè)miRNA的變化,并應(yīng)用miRNA的拮抗劑或模擬物抑制或上調(diào)miRNA表達(dá),調(diào)節(jié)胃癌細(xì)胞對(duì)化療藥物的敏感性,為胃癌治療提供一種潛在的治療方案。
miRNA作為一種新的腫瘤標(biāo)志物,在臨床診斷和靶向治療方面具有良好的應(yīng)用前景。但是也存在一些問(wèn)題:如同一種miRNA調(diào)控的多種基因中有些并不是我們所需,大部分關(guān)于miRNA與胃癌耐藥機(jī)制的研究都是體外實(shí)驗(yàn),缺乏相關(guān)的臨床試驗(yàn)研究成果支持;同時(shí)人類是個(gè)復(fù)雜統(tǒng)一體,將miRNA的功能和調(diào)節(jié)機(jī)制分離出來(lái)研究,無(wú)法系統(tǒng)地指導(dǎo)臨床用藥。因此,亟待從更廣的領(lǐng)域進(jìn)行更深入的研究。總之,隨著miRNA在腫瘤耐藥機(jī)制中作用的進(jìn)一步明確,對(duì)miRNA的研究也逐漸從實(shí)驗(yàn)室走向臨床。我們期待能夠利用miRNA類似物或抑制劑與抗癌藥物聯(lián)合使用,為病人個(gè)體化用藥提供新的輔助治療方案,為胃癌治療帶來(lái)新的進(jìn)展。
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