PDCD4基因在腫瘤中表達的調(diào)控及其功能的研究進展

薛迪新 陳積賢

●綜 述

PDCD4基因在腫瘤中表達的調(diào)控及其功能的研究進展

薛迪新 陳積賢

PDCD4基因是一種新的抑癌基因，也是一種與細胞凋亡相關的基因。近年來研究表明在許多實體瘤中PDCD4基因的表達下降，甚至出現(xiàn)缺失。低表達的PDCD4基因促使腫瘤細胞增殖、侵襲和轉移。一般情況下，低表達的PDCD4提示腫瘤患者的不良預后，故PDCD4基因可能成為生物治療新的靶點。從分子水平闡述PDCD4基因的表達調(diào)控機制和功能，對恢復PDCD4基因的表達來治療腫瘤的意義重大，同時也可以從PDCD4基因的角度去闡述腫瘤的發(fā)生、侵襲和轉移的機制。本文主要是將近些年來PDCD4基因的功能與調(diào)控作一總結，并闡述PDCD4表達與腫瘤患者預后的關系。

1 PDCD4基因的發(fā)現(xiàn)及其編碼的蛋白結構

1995 年，為了闡明細胞凋亡的分子機制，Shibahara K等[1]采用差異顯示法在各種凋亡細胞系中分離出cDNA克隆，在這些cDNA克隆中，MA-3 mRNA的表達被誘導。之后陸續(xù)克隆出雞PDCD4基因[2]和鼠的DUG基因[3]等，統(tǒng)稱為PDCD4。人類PDCD4基因定位于染色體10q24[4]，其表達的蛋白經(jīng)序列分析表明由469個氨基酸組成，包括N末端結構域、C末端結構域和2個保守的α螺旋MA-3結構域[5]。N末端和C末端結構域可能是PDCD4蛋白核定位信號區(qū)，但是關于PDCD4蛋白亞細胞定位的報道存在爭議。有文獻報道PDCD4蛋白定位在正常細胞的胞核和腫瘤細胞的胞質中[6-7]，但也有文獻報道存在相反的分布[8]，這可能是由于PDCD4在細胞核與質之間的轉運引起的[9]。MA-3結構域通過與翻譯起始因子相互作用影響PDCD4的翻譯，包括eIF4GI、eIF4GII和eIF4A[10-11]。

2 PDCD4的功能

2.1 調(diào)節(jié)基因轉錄的功能 PDCD4蛋白能調(diào)節(jié)基因的轉錄從而發(fā)揮特定的生物學功能。用siRNA-Pdcd4轉染GEO細胞株，導致細胞u-PAR mRNA和蛋白的表達水平升高，促進腫瘤細胞的侵襲。進一步用染色質免疫共沉淀技術（chromatin immunoprecipitation，ChIP）分析表明轉錄因子SP1/SP3與u-PAR基因的啟動子元件-152/-135和-380/-354相互作用，調(diào)節(jié)u-PAR基因的轉錄[12]。而在乳腺癌細胞株中，PDCD4表達能增加TIMP2基因的轉錄，抑制乳腺癌細胞的轉移[13]。在GEO細胞株中，敲除PDCD4能上調(diào)Snail蛋白來抑制E-cadherin的表達，刺激β-catenin/Tcf轉錄復合物依賴的轉錄，從而促進u-PAR和c-myc基因的轉錄，進一步用ChIP分析表明β-catenin/Tcf與u-PAR和c-myc基因的啟動子結合來激活基因的轉錄，u-PAR和c-myc基因與腫瘤細胞轉移相關，當敲除u-PAR和c-myc時，能夠抑制GEO-shPdcd4細胞的轉移[14]。此外，PDCD4蛋白通過直接抑制c-Jun的磷酸化，抑制轉錄因子AP-1的表達，從而抑制了細胞的惡性轉化[15-16]。轉錄因子AP-1可以誘導血管生成素Ang-2的轉錄[17]，用siRNA-Pdcd4轉染細胞株Bon-1和HCT116，PDCD4表達下降，而Ang-2 mRNA及其蛋白質表達升高，進一步用血管生成分析法表明高表達Ang-2能刺激血管的形成[18]。PDCD4還能與轉錄因子Twist1相互作用，導致Twist1的靶基因YB-1表達下降，從而抑制腫瘤細胞的增殖[19]。

2.2 調(diào)節(jié)基因翻譯的功能 PDCD4蛋白不僅可以抑制帽子依賴的翻譯，還可以抑制內(nèi)核糖體進入位點（internal ribosome entry site，IRES）依賴的翻譯。真核生物翻譯起始因子eIF4A是依賴于ATP的RNA解旋酶，能展開5′mRNA的二級結構，酵母雙雜交技術分析表明PDCD4蛋白能與翻譯起始因子eIF4A結合，抑制帽子依賴的翻譯，進而抑制AP-1的反式激活，從而抑制細胞的惡性轉化[11]。PDCD4表達抑制JNK的活性，抑制c-jun磷酸化，進而抑制真核生物翻譯起始因子eIF4E的表達和磷酸化，從而抑制帽子依賴的翻譯，而用H2O2處理肝癌細胞株MHCC97L后，eIF4E的表達和磷酸化水平升高，促進金屬蛋白酶MMP-2和MMP-9表達水平升高，從而促進肝癌細胞的轉移[20]。PDCD4能對DNA的損傷產(chǎn)生應答反應。PDCD4蛋白與eIF4A結合后，抑制eIF4A與p53 mRNA 5′非編碼區(qū)結合，從而抑制p53 mRNA的翻譯，用DNA損傷劑處理細胞后，PDCD4的表達下降，促進p53 mRNA的翻譯[21]，有利于DNA損傷的修復。XIAP和Bcl-xL mRNA的5′非編碼區(qū)包含IRES元件，PDCD4蛋白直接與XIAP和Bcl-xL mRNA的IRES元件結合，抑制48S起始復合物的形成，從而抑制XIAP和Bcl-xL mRNA的翻譯[22]。除此之外，PDCD4蛋白還可以直接調(diào)節(jié)基因的翻譯。c-myb編碼區(qū)包含PDCD4的反應元件，此反應元件存在于c-myb編碼區(qū)的XmaI到SalI之間，PDCD4蛋白的N端結構域與c-myb mRNA中的PDCD4的反應區(qū)域直接結合抑制c-myb mRNA的翻譯[23]。

此外，PDCD4的表達還可以使腫瘤細胞對某些化學物質或射線的敏感性發(fā)生變化。在正常生長的條件下，功能缺陷的PDCD4基因的雞淋巴瘤細胞株DT40的增殖和細胞周期并沒有受影響，但是在敲除PDCD4基因后，細胞對一些DNA損傷劑的敏感性增強，包括紫外線，依托泊甙和甲磺酸乙酯[24]。在胃癌細胞中敲除PDCD4基因后，癌細胞對腫瘤壞死因子相關凋亡誘導配體（TRAIL）的敏感性減弱[25]。PDCD4基因的功能見圖1。

圖1 PDCD4基因的功能

3 PDCD4基因表達的調(diào)控

3.1 轉錄水平的調(diào)控 在轉錄水平調(diào)節(jié)PDCD4基因的表達，主要是通過一些轉錄因子與PDCD4基因的調(diào)控區(qū)相互作用，或甲基化5′CpG島來實現(xiàn)的。轉錄因子v-Myb可以誘導雞細胞中PDCD4基因的表達[26]，而敲除c-Myb基因，PDCD4基因的表達明顯降低[27]。此外還有轉錄因子ZBP-89單獨或與SP家族成員相互作用后通過與PDCD4基因的啟動子結合促進PDCD4基因的轉錄[28]。在神經(jīng)膠質瘤細胞株和組織中，PDCD4基因的5′CpG島的甲基化，抑制PDCD4 mRNA的轉錄，同時用DNA甲基化轉移酶抑制劑封閉5′CpG島的甲基化后，細胞中PDCD4基因的表達可以恢復[29]。

3.2 轉錄后水平的調(diào)控 最近發(fā)現(xiàn)一種約由20個核苷酸組成的非編碼的microRNA，它與靶基因轉錄的mRNA的3′非編碼區(qū)結合，通過抑制mRNA翻譯或直接降解mRNA來負性調(diào)節(jié)靶基因的功能[30-32]。許多實驗均表明PDCD4是miR-21的靶基因，miR-21在轉錄后水平負性調(diào)節(jié)PDCD4的表達，包括大腸癌[33-35]，胃癌[36-37]，胰腺癌[38-39]，惡性膠質瘤[40]，膀胱癌[41]，子宮頸癌[42]等。此外，其他miRNA也同樣調(diào)節(jié)PDCD4基因的表達。用miR-183轉染人肝癌細胞株Huh7后，PDCD4蛋白的表達降低，用qRT-PCR技術檢測25對臨床活體肝癌及其正常肝臟組織中PDCD4 mRNA和miR-183的表達情況，結果顯示miR-183的表達與PDCD4 mRNA的表達存在明顯的負相關，進一步用生物信息學和熒光素酶報告基因檢測證實miR-183是與PDCD4 mRNA 3′非編碼區(qū)結合來負性調(diào)節(jié)PDCD4靶基因的表達[43]，表明PDCD4也是miR-183的靶基因。在大腸癌中，用上述同樣的實驗方法檢測，結果顯示microRNA-499-5p也可在轉錄后水平負性調(diào)節(jié)靶基因PDCD4的表達[44]。以上研究表明PDCD4基因的表達在轉錄后水平受到多種miRNA的調(diào)控。

3.3 其他信號轉導通路對PDCD4基因表達的調(diào)控多種信號轉導通路能共同調(diào)節(jié)基因PDCD4的表達。TGF-β信號轉導通路可誘導肝癌Huh7細胞株PDCD4的表達，用信號通路抑制物Smad7轉染細胞株后，可抑制TGF-β通路誘導PDCD4基因的表達[45]，但是在血管平滑肌細胞中，TGF-β通過上調(diào)miR-21的表達，負性調(diào)節(jié)PDCD4基因的表達[46]，此外用PKCδ和PKCεsiRNA轉染肝癌細胞，可增強TGF-β誘導的PDCD4蛋白的表達[47]。當腫瘤細胞處在炎癥的微環(huán)境下，能激活PI3K-mTOR信號途徑，增強蛋白酶體降解PDCD4蛋白的作用，從而使腫瘤細胞中PDCD4蛋白的表達下降[48]。用PI3K的抑制劑處理胃癌和卵巢癌細胞株，抑制PI3KAkt信號轉導通路，可使PDCD4蛋白的表達增加[25,49]。此外，激活FGF-2-S6K2信號途徑，也可以使PDCD4蛋白磷酸化而被降解[22]。還有一些信號轉導途徑通過調(diào)節(jié)miR-21的表達，從而間接調(diào)節(jié)PDCD4基因的表達。在TLR4信號途徑中，配體LPS（細菌內(nèi)毒素）結合Toll樣受體4（Toll-like receptor 4，TLR4）可激活銜接蛋白MyD88和NF-kappaB誘導miR-21的表達，可使PDCD4的表達降低[50]。在乳腺癌細胞株MCF-7中，透明質烷（HA）能與受體CD44的結合激活PKCε,后者磷酸化干細胞標志物Nanog，磷酸化的Nanog從細胞質進入細胞核，與RNase III DROSHA和RNA解旋酶p68結合，誘導miR-21的表達，從而負性調(diào)節(jié)PDCD4蛋白的表達[51]。在人類膠質母細胞瘤U251細胞株中，熊果酸可抑制TGF-β信號轉導通路，進而抑制miR-21的表達，間接地促進了PDCD4蛋白的表達[52]。

此外，有一種稱為抗亞砷酸鹽蛋白-2（Ars2），它能夠沉默miRNA表達[53]，在人類膽管癌細胞和組織中，Ars2的缺失可引起miR-21的表達增加，上調(diào)的miR-21可抑制PDCD4蛋白的表達[54]。PDCD4表達的調(diào)節(jié)機制見圖2。

圖2 PDCD4基因表達的調(diào)控機制

4 PDCD4基因在臨床活體組織中的表達及預后判斷

在許多實體腫瘤中，PDCD4的表達與腫瘤的臨床預后密切相關。從正常、交界性到惡性卵巢癌組織中，PDCD4蛋白的表達逐漸下降，在卵巢癌患者中，PDCD4蛋白表達越低，患者生存率就越低，用免疫組化的方法檢測發(fā)現(xiàn)正常卵巢細胞中的PDCD4蛋白主要定位在細胞核，而卵巢癌細胞的PDCD4主要定位在細胞質[55]，這可能與pAkt在細胞核和細胞質中的表達有關[56]。從正常大腸黏膜、腺瘤到大腸癌的演變過程中，PDCD4蛋白的表達逐漸下降，在此過程中，細胞PDCD4蛋白核與質表達的比率逐漸降低，pAkt與PDCD4蛋白從細胞核到細胞質的轉移（即核與質的比率）之間存在負相關，表明PDCD4的轉移受pAkt的調(diào)控。用Kaplan-Meier分析表明PDCD4蛋白表達越低，大腸癌患者的總生存時間越短，疾病相關存活率越低。PDCD4蛋白的缺失，提示大腸癌患者的不良預后[56]。在腎臟細胞癌中，PDCD4蛋白表達降低，與腎臟細胞癌的分級、分期和轉移有關，即PDCD4蛋白表達越低，腎癌的分期越晚，越容易轉移，患者的平均總生存時間越短，PDCD4蛋白的低表達提示腎臟細胞癌的不良預后[57]。Motoyama等[37]分析PDCD4 mRNA在105例胃癌中的表達與臨床病理特征的關系，結果顯示低表達的PDCD4 mRNA與胃癌的大小、浸潤深度、淋巴結轉移、血管轉移和臨床分期有關，表明低表達的PDCD4 mRNA提示胃癌的不良預后。Horiuchi等[58]分析326例大腸癌患者，在大腸癌Dukes′B和C期患者中，低表達的PDCD4 mRNA患者的總生存時間短，并且無瘤生存率低，在Dukes′D期的大腸癌患者中，低表達的PDCD4 mRNA患者的總生存時間短，但在Dukes′A期的患者中，低表達和高表達的PDCD4 mRNA患者之間的總生存時間和無瘤生存率均無差別。表明在Dukes′B、C和D期的患者中，低表達的PDCD4 mRNA提示大腸癌的不良預后。Gao等[29]研究表明在神經(jīng)膠質瘤中，PDCD4基因5′CpG島的甲基化導致PDCD4表達的沉默，在84例神經(jīng)膠質瘤患者中，PDCD4表達的缺失提示患者的不良預后。

一般情況下，低表達PDCD4提示腫瘤患者的不良預后，但是有一部分高表達的PDCD4的腫瘤患者的預后也很差，經(jīng)Powers等[59]的研究表明這與PDCD4蛋白N端被蛋白精氨酸甲基轉移酶（PRMT5）甲基化，影響了PDCD4蛋白的功能有關。故PDCD4蛋白的功能對腫瘤患者預后的判斷也是極其重要的。

5 結論

PDCD4表達的調(diào)節(jié)機制極其復雜，不僅在轉錄水平，轉錄后水平，而且一些信號轉導通路也可以調(diào)節(jié)PDCD4的表達。PDCD4的表達也可以通過調(diào)節(jié)其他基因的轉錄和翻譯，從而影響腫瘤細胞的增殖、侵襲和轉移，形成復雜的調(diào)節(jié)網(wǎng)絡。同時，PDCD4表達的缺失提示部分腫瘤患者的不良預后，因此，PDCD4可成為腫瘤治療新的靶點，恢復PDCD4的表達有望成為腫瘤治療新的策略。

[1]Shibahara K,Asano M,Ishida Y,et al.Isolation of a novel mouse gene MA-3 that is induced upon programmed cell death[J]. Gene,1995,166(2):297-301.

[2]Schlichter U,Burk O,Worpenberg S,et al.The chicken Pdcd4 gene is regulated by v-Myb[J].Oncogene,2001,20(2):231-239.

[3]Go ke A,Goke R,Knolle A,et al.DUG is a novel homologue of translation initiation factor 4G that binds eIF4A[J].Biochem Biophys Res Commun,2002,297(1):78-82.

[4]Soejima H,Miyoshi O,Yoshinaga H,et al.Assignment of the programmed cell death 4 gene (PDCD4)to human chromosome band 10q24 by in situ hybridization[J].Cytogenet Cell Genet, 1999,87(1-2):113-114.

[5]Kang M J,Ahn H S,Lee J Y,et al.Up-regulation of PDCD4 in senescent human diploid fibroblasts[J].Biochem Biophys Res Commun,2002,293(1):617-621.

[6]Matsuhashi S,Narisawa Y,Ozaki I,et al.Expression patterns of programmed cell death 4 protein in normal human skin and some representative skin lesions[J].Exp Dermatol,2007,16(3):179-184.

[7]Mudduluru G,Medved F,Grobholz R,et al.Loss of programmed cell death 4 expression marks adenoma-carcinoma transition, correlates inversely with phosphorylated protein kinase B,and is an independent prognostic factor in resected colorectal cancer [J].Cancer,2007,110(8):1697-1707.

[8]Wen Y H,Shi X,Chiriboga L,et al.Alterations in the expression of PDCD4 in ductal carcinoma of the breast[J].Oncol Rep,2007,18(6): 1387-1393.

[9]Bohm M,Sawicka K,Siebrasse J P,et al.The transformation suppressor protein Pdcd4 shuttles between nucleus and cytoplasm and binds RNA[J].Oncogene,2003,22(31):4905-4910.

[10]Ponting C P.Novel eIF4G domain homologues linking mRNA translation with nonsense-mediated mRNA decay[J].Trends Biochem Sci,2000,25(9):423-426.

[11]Yang H S,Jansen A P,Komar A A,et al.The transformation suppressor Pdcd4 is a novel eukaryotic translation initiation factor 4A binding protein that inhibits translation[J].Mol Cell Biol,2003, 23(1):26-37.

[12]Leupold J H,Yang H S,Colburn N H,et al.Tumor suppressor Pdcd4 inhibits invasion/intravasation and regulates urokinase receptor(u-PAR)gene expression via Sp-transcription factors [J].Oncogene,2007,26(31):4550-4562.

[13]Nieves-Alicea R,Colburn N H,Simeone A M,et al.Programmed cell death 4 inhibits breast cancer cell invasion by increasing tissue inhibitor of metalloproteinases-2 expression[J].Breast Cancer Res Treat,2009,114(2):203-209.

[14]Wang Q,Sun Z X,Allgayer H,et al.Downregulation of E-cadherin is an essential event in activating beta-catenin/Tcf-dependent transcription and expression of its target genes in Pdcd4 knockdown cells[J].Oncogene,2010,29(1):128-138.

[15]Yang H S,Jansen A P,Nair R,et al.A novel transformation suppressor,Pdcd4,inhibits AP-1 transactivation but not NF-kappaB or ODC transactivation[J].Oncogene,2001,20(6):669-676.

[16]Bitomsky N,Bo hm M,Klempnauer K H.Transformation suppressor protein Pdcd4 interferes with JNK-mediated phosphorylation of c-Jun and recruitment of the coactivator p300 by c-Jun[J].Oncogene,2004,23(45):7484-7493.

[17]Ye F C,Blackbourn D J,Mengel M,et al.Kaposi's sarcoma-as-sociated herpesvirus promotes angiogenesis by inducing angiopoietin-2 expression via AP-1 and Ets1[J].J Virol,2007,81 (8):3980-3991.

[18]Krug S,Huth J,Go ke F,et al.Knock-down of Pdcd4 stimulates angiogenesis viaup-regulation of angiopoietin-2[J].Biochim Biophys Acta,2012,1823(4):789-799.

[19]Shiota M,Izumi H,Tanimoto A,et al.Programmed cell death protein 4 down-regulates Y-box binding protein-1 expression via a direct interaction with Twist1 tosuppress cancer cell growth[J].Cancer Res,2009,69(7):3148-3156.

[20]Jiang Y,Zhang S H,Han G Q,et al.Interaction of Pdcd4 with eIF4E inhibits the metastatic potential of hepatocellular carcinoma[J].Biomed Pharmacother,2010,64(6):424-429.

[21]Wedeken L,Singh P,Klempnauer K H.Tumor suppressor protein Pdcd4 inhibits translation of p53 Mrna[J].J Biol Chem, 2011,286(50):42855-42862.

[22]Liwak U,Thakor N,Jordan L E,et al.Tumor suppressor PDCD4 represses internal ribosome entry site-mediated translation of antiapoptotic proteins and is regulated by S6 kinase 2[J].Mol Cell Biol,2012,32(10):1818-1829.

[23]Singh P,Wedeken L,Waters L C,et al.Pdcd4 directly binds the coding region of c-myb mRNA and suppresses its translation [J].Oncogene,2011,30(49):4864-4873.

[24]Singh P,Marikkannu R,Bitomsky N,et al.Disruption of the Pdcd4 tumor suppresor gene in chicken DT40 cells reveals its role in the DNA-damage response[J].Oncogene,2009,28(42):3758-3764.

[25]Wang W Q,Zhang H,Wang H B,et al.Programmed cell death 4 (PDCD4)enhances the sensitivity of gastric cancer cells to TRAIL-induced apoptosis by inhibiting the PI3K/Akt signaling pathway[J].Mol Diagn Ther,2010,14(3):155-161.

[26]Schlichter U,Burk O,Worpenberg S,et al.The chicken Pdcd4 gene is regulated by v-Myb[J].Oncogene,2001,20(2):231-239.

[27]Appl H,Klempnauer K H.Targeted disruption of c-myb in the chicken pre B-cell line DT40[J].Oncogene,2002,21(19):3076-3081.

[28]Leupold J H,Asangani I A,Mudduluru G,et al.Promoter cloning and characteri-zation of the human programmed cell death protein 4(pdcd4)gene:evidence for ZBP-89 and Sp-binding motifs as essential Pdcd4 regulators[J].Biosci Rep,2012,32(3): 281-297.

[29]Gao F,Wang X,Zhu F,et al.PDCD4 gene silencing in gliomas is associated with 5'CpG island methylation and unfavourable prognosis[J].J Cell Mol Med,2009,13(10):4257-4267.

[30]Berezikov E,Guryev V,van de Belt J,et al.Phylogenetic shadowing and computational identification of human microRNA genes[J].Cell,2005,120(1):21-24.

[31]Zamore P D,Haley B.Ribo-gnome:the big world of small RNAs[J].Science,2005,309(5740):1519-1524.

[32]Esquela-Kerscher A,Slack F J.Oncomirs-microRNAs with a role in cancer[J].Nat Rev Cancer,2006,6(4):259-269.

[33]Asangani I A,Rasheed S A,Nikolova D A,et al.MicroRNA-21 (miR-21)post-transcriptionally downregulates tumor suppressor Pdcd4 and stimulates invasion,intr-avasation and metastasis in colorectal cancer[J].Oncogene,2008,27(15):2128-2136.

[34]Fassan M,Pizzi M,Giacomelli L,et al.PDCD4 nuclear loss inversely correlates with miR-21 levels in colon carcinogenesis [J].Virchows Arch,2011,458(4):413-419.

[35]Chang K H,Miller N,Kheirelseid E A,et al.MicroRNA-21 and PDCD4 expression in colorectal cancer[J].Eur J Surg Oncol, 2011,37(7):597-603.

[36]Cao Z,Yoon J H,Nam S W,et al.PDCD4 expression inversely correlated with miR-21 levels in gastric cancers[J].J Cancer Res Clin Oncol,2012,138(4):611-619.

[37]Motoyama K,Inoue H,Mimori K,et al.Clinicopathological and prognostic significance of PDCD4 and microRNA-21 in human gastric cancer[J].Int J Oncol,2010,36(5):1089-1095.

[38]Bhatti I,Lee A,James V,et al.Knockdown of microRNA-21 inhibits proliferation and increases cell death by targeting programmed cell death 4(PDCD4)in pancreatic ductal adenocarcinoma[J].J Gastrointest Surg,2011,15(1):199-208.

[39]Nagao Y,Hisaoka M,Matsuyama A,et al.Association of microRNA-21 expressi on with its targets,PDCD4 and TIMP3,in pancreatic ductal adenocarcinoma[J].Mod Pathol,2012,25(1): 112-121.

[40]Gaur A B,Holbeck S L,Colburn N H,et al.Downregulation of Pdcd4 by mir-21 facilitates glioblastoma proliferation in vivo[J]. Neuro Oncol,2011,13(6):580-590.

[41]Fischer N,Go ke F,Splittstosser V,et al.Expression of programmed cell death protein 4(PDCD4)and miR-21 in urothelial carcinoma[J].Biochem BiophysRes Commun,2012, 417(1):29-34.

[42]Yao Q,Xu H,Zhang Q Q,et al.MicroRNA-21 promotes cell proliferation and down-regulatesthe expression ofprogrammed cell death 4 (PDCD4)in HeLa cervical carcinoma cells[J].Biochem Biophys Res Commun,2009,388(3):539-542.

[43]Li J,Fu H,Xu C,et al.miR-183 inhibits TGF-beta1-induced apoptosis by down regulation of PDCD4 expression in human hepatocellular carcinomacells[J].BMC Cancer,2010,10:354.

[44]Liu X,Zhang Z,Sun L,et al.MicroRNA-499-5p promotes cellular invasion and tumor metastasis in colorectal cancer by targeting FOXO4 and PDCD4[J].Carcinogenesis,2011,32(12): 1798-1805.

[45]Zhang H,Ozaki I,Mizuta T,et al.Involvement of programmed cell death 4 in transforming growth factor-beta1-induced apoptosis in human hepatocellular carcinoma[J].Oncogene, 2006,25(45):6101-6112.

[46]Davis B N,Hilyard A C,Lagna G,et al.SMAD proteins control DROSHA-mediated microRNA maturation[J].Nature,2008,454 (7200):56-61.

[47]Nakashima M,Hamajima H,Xia J,et al.Regulation of tumor suppressor PDCD4 by novel protein kinase C isoforms[J].Biochim Biophys Acta,2010,1803(9):1020-1027.

[48]Schmid T,Bajer M M,Blees J S,et al.Inflammation-induced loss of Pdcd4 is mediated by phosphorylation-dependent degradation[J].Carcinogenesis,2011,32(10):1427-1433.

[49]Wei N,Liu S S,Chan K K,et al.Tumour suppressive function and modulation of programmed cell death 4(PDCD4)in ovarian cancer[J].PLoS One,2012,7(1):e30311.

[50]Sheedy F J,Palsson-McDermott E,Hennessy E J,et al.Negative regulation of TLR4 via targeting of the proinflammatory tumor suppressor PDCD4 by the microRNA miR-21[J].Nat Immunol,2010,11(2):141-147.

[51]Bourguignon L Y,Spevak C C,Wong G,et al.Hyaluronan-CD44 interaction with protein kinase C(epsilon)promotes oncogenic signaling by the stem cell marker Nanog and the Production of microRNA-21,leading to down-regulation of the tumor suppressor protein PDCD4,anti-apoptosis,and chemotherapy resistance in breast tumor cells[J].J Biol Chem,2009,284(39): 26533-26546.

[52]Wang J,Li Y,Wang X,et al.Ursolic Acid Inhibits Proliferation and Induces Apoosis in Human Glioblastoma Cell Lines U251 by Suppressing TGF-β1/miR-21/PDCD4 Pathway[J].Basic Clin Pharmacol Toxicol,2012,111(2):106-112.

[53]Sabin L R,Zhou R,Gruber J J,et al.Ars2 regulates both miRNA-and siRNA-dependent silencing and suppresses RNA virus infection in Drosophila[J].Cell,2009,138(2):340-351.

[54]He Q,Cai L,Shuai L,et al.Ars2 is overexpressed in human cholangiocarcinomas and its depletion increases PTEN and PDCD4 by decreasing MicroRNA-21[J].Mol Carcinog,2011. [Epub ahead of print].

[55]Wei N A,Liu S S,Leung T H,et al.Loss of Programmed cell death 4 (Pdcd4)associates with the progression of ovarian cancer[J].Mol Cancer,2009,8:70.

[56]Mudduluru G,Medved F,Grobholz R,et al.Loss of programmed cell death 4 expression marks adenoma-carcinoma transition,correlates inversely with phosphory-lated protein kinase B,and is an independent prognostic factor in resected colorectal cancer[J].Cancer,2007,110(8):1697-1707.

[57]Li X,Xin S,Yang D,et al.Down-regulation of PDCD4 expression is an independent predictor of poor prognosis in human renal cell carcinoma patients[J].J Cancer Res Clin Oncol,2012,138 (3):529-535.

[58]Horiuchi A,Iinuma H,Akahane T,et al.Prognostic significance of PDCD4 expression and association with microRNA-21 in each Dukes'stage of colorectal cancer patients[J].Oncol Rep, 2012,27(5):1384-1392.

[59]Powers M A,Fay M M,Factor R E,et al.Protein arginine methyltransferase 5 accelerates tumor growth by arginine methylation of the tumor suppressor programmed cell death 4[J].Cancer Res,2011,71(16):5579-5587.

2012-07-19）

（本文編輯：胥昀）

瑞安市科技計劃項目（201102039）

325200 瑞安，溫州醫(yī)學院附屬第三醫(yī)院腫瘤外科

陳積賢，E-mail:CJX1957@sohu.com.