胡金萌,王健
腫瘤的發(fā)生、發(fā)展不是一個(gè)單一的過程,而是多因素、多步驟共同作用的結(jié)果。已有研究結(jié)果表明基質(zhì)相互作用蛋白分子1(STIM1)參與腫瘤的發(fā)生及轉(zhuǎn)移過程[1]。STIM1是位于內(nèi)質(zhì)網(wǎng)(ER)上的鈣離子(Ca2+)傳感器,是調(diào)控鈣池操縱性鈣內(nèi)流(storeoperated calcium entry,SOCE)的重要組成部分[2-3]。而Ca2+信號(hào)是細(xì)胞代謝中重要的第二信使[4],參與腫瘤細(xì)胞的增殖、凋亡和遷移[4-7]。抑制腫瘤細(xì)胞中STIM1的活化,可以抑制STIM1依賴性Ca2+信號(hào)的傳導(dǎo),進(jìn)而抑制多種腫瘤的發(fā)生發(fā)展,如乳腺癌[8]、宮頸癌[9]、結(jié)直腸癌[10-11]、前列腺癌[12]、肝癌[13]、頭頸部鱗癌[14]、肺癌[15]和子宮內(nèi)膜癌[16]。本文就STIM1結(jié)構(gòu)、其與Ca2+的相互作用機(jī)制及促進(jìn)腫瘤侵襲轉(zhuǎn)移機(jī)制作一綜述。
STIM1是一種分子質(zhì)量為90 ku的Ⅰ型跨膜蛋白,其基因定位于人染色體11p15.5[17]。STIM1包含3個(gè)區(qū)域,內(nèi)質(zhì)網(wǎng)區(qū)域、跨膜區(qū)域(TM)和胞漿區(qū)域。內(nèi)質(zhì)網(wǎng)區(qū)域包含信號(hào)肽、N-末端、EF-手形結(jié)構(gòu)(EF-hand)和SAM(Sterile α Motif)結(jié)構(gòu)域,其中EF-hand和SAM區(qū)(EF-SAM)是Ca2+結(jié)合域??缒^(qū)域則主要將內(nèi)質(zhì)網(wǎng)內(nèi)鈣庫(kù)清空的信號(hào)以構(gòu)象變化方式傳導(dǎo)到STIM1蛋白的胞質(zhì)部分。胞漿區(qū)域含有多個(gè)螺旋卷曲結(jié)構(gòu)域(Coiled-coil 1~3,CC1~3),參與STIM1構(gòu)象變化,并介導(dǎo)依賴性失活的ID區(qū)、富含絲氨酸和脯氨酸的S/P結(jié)構(gòu)域以及C-末端賴氨酸富集的K區(qū),見圖1[6,18-19]。EF-SAM主要由10個(gè)α螺旋和2個(gè)β折疊組成(圖2a);EF-SAM與Ca2+結(jié)合后形成一個(gè)緊湊的實(shí)體(圖2b);EF-hand與疏水性SAM殘基(F108、L199、L195)相互作用,該位點(diǎn)突變可致Ca2+與EF-hand脫離(圖2c);Ca2+耗盡的EF-SAM呈分散的低聚物(圖2d)[18]。
Fig.1 Schematic representation of structures of STIM1[18]圖1 STIM1結(jié)構(gòu)示意圖[18]
Fig.2 Structural characteristics of EF-SAM[18]圖2 EF-SAM結(jié)構(gòu)示意圖[18]
Ca2+是細(xì)胞增殖、分化、基因轉(zhuǎn)錄等生命活動(dòng)的重要信使分子,ER鈣池排空會(huì)激活細(xì)胞膜表面的鈣池操縱性鈣通道(SOCs)開放,所形成的SOCE是非興奮細(xì)胞調(diào)節(jié)Ca2+穩(wěn)態(tài)平衡的重要途徑[20]。SOCE即細(xì)胞內(nèi)Ca2+損耗后引發(fā)的細(xì)胞外Ca2+內(nèi)流的過程。SOCE由SOCs介導(dǎo)完成,存在于各種類型的細(xì)胞中,在腫瘤的生長(zhǎng)、抗凋亡、遷移及免疫逃逸中起到重要作用[21-23]。當(dāng)ER腔內(nèi)Ca2+存儲(chǔ)充盈時(shí),STIM1分子以單聚體形式均勻分布于ER;隨著ER腔內(nèi)Ca2+的耗盡,EF-SAM結(jié)構(gòu)域構(gòu)象發(fā)生變化,疏水性增加,EF-SAM由單聚體轉(zhuǎn)變?yōu)椴环€(wěn)定的低聚物,導(dǎo)致STIM1分解,SOCE停止[17]。其大致過程為:細(xì)胞外信號(hào)分子激活細(xì)胞膜表面受體,活化磷脂酶C(PLC),使2,4,5-二磷酸磷脂酰肌醇(PIP2)裂解為1,4,5-三磷酸肌醇(IP3)和二酰基甘油(DAG)。IP3與ER上的IP3-受體結(jié)合,激活ER鈣池釋放Ca2+,ER中Ca2+濃度降低致使Ca2+從EF-hand脫離,Ca2+排空信號(hào)激活STIM1,有活性的STIM1二聚體激活鈣釋放激活鈣通道(CRAC),細(xì)胞外的Ca2+進(jìn)入細(xì)胞內(nèi)。當(dāng)ER再充滿Ca2+,Ca2+重新結(jié)合EF-hand,STIM1失活,Ca2+內(nèi)流停止[9]。STIM1作為SOCE重要的組成分子,是細(xì)胞主動(dòng)吸收Ca2+的重要分子[24]。
3.1 STIM1與肝癌 缺氧和細(xì)胞內(nèi)Ca2+瞬變是癌癥的基本特征。STIM1與缺氧誘導(dǎo)因子(HIF)-1α相互作用促進(jìn)肝癌細(xì)胞的形成[13]。Yang等[13]研究發(fā)現(xiàn),肝癌細(xì)胞中HIF-1α和STIM1的表達(dá)水平明顯高于正常肝細(xì)胞:將人類肝癌HepG2和Huh7細(xì)胞暴露于不同濃度氧氣中24 h后觀察發(fā)現(xiàn),缺氧暴露會(huì)導(dǎo)致HIF-1α和STIM1表達(dá)上調(diào),腫瘤細(xì)胞增殖增加,且HIF-1α和STIM1的表達(dá)呈正相關(guān)。HIF-1α通過與STIM1啟動(dòng)子直接結(jié)合,促進(jìn)肝癌細(xì)胞中STIM1的轉(zhuǎn)錄和表達(dá),并促進(jìn)SOCE發(fā)生[25],而SOCE是人類肝癌細(xì)胞系增殖過程中Ca2+內(nèi)流的主要類型[26]??梢奡TIM1在肝癌細(xì)胞形成中起到重要作用。
3.2 STIM1與前列腺癌 STIM1在前列腺癌進(jìn)展中起雙重作用。一方面,STIM1過表達(dá)可促進(jìn)上皮-間質(zhì)轉(zhuǎn)化(EMT),從而增強(qiáng)腫瘤細(xì)胞侵襲轉(zhuǎn)移;另一方面,STIM1過表達(dá)會(huì)加速腫瘤細(xì)胞衰老[27]。EMT是腫瘤細(xì)胞侵襲轉(zhuǎn)移的重要機(jī)制,在EMT過程中,腫瘤細(xì)胞失去上皮特征并獲得間質(zhì)表型特征,上皮標(biāo)記蛋白如E-鈣黏蛋白表達(dá)下降,間質(zhì)標(biāo)記蛋白如波形蛋白和N-鈣黏蛋白的表達(dá)增加,促進(jìn)EMT現(xiàn)象發(fā)生[28-31]。Xu 等[27]研究了 BPH-1、LNCaP、DU145 和PC3這4種細(xì)胞系,BPH-1是一種增生細(xì)胞,后3種均為前列腺癌細(xì)胞。該研究發(fā)現(xiàn),STIM1在前列腺癌細(xì)胞中的表達(dá)明顯高于BPH-1;且在過表達(dá)STIM1的DU145細(xì)胞中,E-鈣黏蛋白表達(dá)降低,N-鈣黏蛋白等表達(dá)顯著增加,提示EMT發(fā)生,細(xì)胞發(fā)生侵襲轉(zhuǎn)移;該研究還發(fā)現(xiàn)在過表達(dá)STIM1的前列腺癌DU145-STIM1-YFP、PC3-STIM1-YFP細(xì)胞中,大而長(zhǎng)的低密度紡錘體(細(xì)胞衰老的重要標(biāo)志)增加,癌細(xì)胞凋亡增加;而當(dāng)敲低STIM1基因表達(dá)時(shí),DU145和PC3細(xì)胞系中的衰老細(xì)胞顯著減少。
3.3 STIM1與結(jié)直腸癌 微小RNA(miRNAs)是一類非編碼的小RNA,與癌細(xì)胞的轉(zhuǎn)移密切相關(guān)[32-33]。研究表明,STIM1是miR-185的直接靶點(diǎn),miR-185可上調(diào)結(jié)直腸癌細(xì)胞中STIM1的表達(dá);STIM1過表達(dá)會(huì)促進(jìn)結(jié)直腸癌細(xì)胞的轉(zhuǎn)移增加,而沉默STIM1基因表達(dá)后,直腸癌細(xì)胞的轉(zhuǎn)移減少[1]。Zhang等[1]研究了SW480、SW620等4種結(jié)直腸癌細(xì)胞系中miR-185與STIM1表達(dá)的關(guān)系,發(fā)現(xiàn)SW620中miR-185表達(dá)上調(diào)可增加E-鈣黏蛋白和β-連環(huán)蛋白的表達(dá),降低波形蛋白和纖連蛋白的表達(dá),且癌細(xì)胞的侵襲轉(zhuǎn)移減少;證實(shí)miR-185是調(diào)控STIM1的上游分子,且兩者之間呈負(fù)調(diào)控關(guān)系。
3.4 STIM1與宮頸癌 Chen等[9]研究發(fā)現(xiàn),在宮頸癌SiHa細(xì)胞系,STIM1敲低可顯著抑制細(xì)胞增殖;同時(shí)流式細(xì)胞術(shù)(FACS)檢測(cè)發(fā)現(xiàn)STIM1敲低可顯著增加細(xì)胞周期中G1/S和G2/M期的細(xì)胞比例。研究發(fā)現(xiàn),STIM1可調(diào)節(jié)血管內(nèi)皮生長(zhǎng)因子(VEGF)在宮頸癌細(xì)胞中的表達(dá),在SCID小鼠皮下成瘤模型中,通過酶聯(lián)免疫吸附測(cè)定(ELISA)定量分析VEGF-A的表達(dá)水平,結(jié)果顯示STIM1與VEGF-A表達(dá)水平成正比;上調(diào)STIM1表達(dá)可促進(jìn)宮頸腫瘤細(xì)胞生長(zhǎng)、局部傳播和血管生成,并促進(jìn)宮頸癌細(xì)胞遷移[9]。
3.5 STIM1與頭頸部癌 STIM1在口腔癌細(xì)胞系TSCCA和喉癌細(xì)胞系Hep2中呈高表達(dá),在該兩種細(xì)胞系中沉默STIM1,會(huì)導(dǎo)致細(xì)胞周期阻滯于G0/G1期,顯著抑制口腔癌細(xì)胞的增殖及轉(zhuǎn)移[14]。STIM1亦可促進(jìn)人類舌鱗狀細(xì)胞癌細(xì)胞的生成,Cui等[34]研究發(fā)現(xiàn)在舌鱗癌細(xì)胞系Tca-8113中,STIM1敲低可明顯減少Tca-8113細(xì)胞總數(shù),且細(xì)胞生長(zhǎng)速率下降約60%,細(xì)胞凋亡數(shù)量也顯著增加。
3.6 STIM1與其他腫瘤 STIM1促進(jìn)乳腺癌的發(fā)生發(fā)展,且與乳腺癌患者總體生存率(OS)呈負(fù)相關(guān);STIM1過表達(dá)致SOCs通路Ca2+內(nèi)流增加,誘導(dǎo)EMT發(fā)生,促進(jìn)乳腺癌細(xì)胞的侵襲和轉(zhuǎn)移[35]。Casas-Rua等[16]研究也證實(shí)STIM1過表達(dá)會(huì)誘導(dǎo)子宮內(nèi)膜腺癌細(xì)胞EMT發(fā)生和細(xì)胞遷移增加。在胃癌細(xì)胞中,STIM1敲低可促進(jìn)細(xì)胞黏附,抑制侵襲遷移,但對(duì)胃癌細(xì)胞的增殖和凋亡無明顯影響[36]。
STIM1在胰腺癌細(xì)胞系中發(fā)揮促生存抗凋亡作用,吉西他濱可抑制STIM1表達(dá),進(jìn)而促使胰腺癌細(xì)胞凋亡增加[37]。此外,五氟尿嘧啶(5-FU)作為治療胰腺癌的化療藥物之一,也是通過抑制STIM1表達(dá)來促進(jìn)胰腺癌Panc1細(xì)胞凋亡[37]。順鉑通過誘導(dǎo)STIM1凋亡,對(duì)晚期非小細(xì)胞肺癌的治療效果顯著;順鉑會(huì)形成交聯(lián)的DNA復(fù)合物,其細(xì)胞毒性通過DNA損傷識(shí)別信號(hào)的逐級(jí)下傳來誘導(dǎo)STIM1凋亡的發(fā)生[38]。STIM1凋亡增加,使染色體在有絲分裂和微管細(xì)胞骨架極化期間分離的準(zhǔn)確性降低,進(jìn)而導(dǎo)致肺癌細(xì)胞死亡增加[38]。白藜蘆醇可誘導(dǎo)細(xì)胞周期阻滯,導(dǎo)致癌細(xì)胞死亡;在雄激素依賴性前列腺癌中,白藜蘆醇可抑制STIM1的表達(dá),阻斷SOCE激活,致使細(xì)胞中Ca2+內(nèi)流減少,進(jìn)而抑制前列腺癌細(xì)胞增殖和存活[9]。2-氨基乙氧基二苯基硼酸鹽(2-APB)可導(dǎo)致STIM1與Ca2+的結(jié)合功能喪失,抑制Ca2+進(jìn)入Orai1通道,阻斷SOCE激活,進(jìn)而抑制前列腺癌細(xì)胞增殖[39]。STIM1敲低還可以直接抑制人表皮樣癌A431細(xì)胞DNA的合成,控制癌細(xì)胞增殖[40];亦可通過下調(diào)SOCE及特異性細(xì)胞周期蛋白G1、D1的表達(dá),抑制肝癌細(xì)胞的生長(zhǎng)[41]。因此,STIM1將為腫瘤的治療提供了新的靶點(diǎn)及臨床藥物研究方向。
STIM1的臨床診斷和預(yù)后意義已在多種癌癥中得到證實(shí)[42],是腫瘤領(lǐng)域研究的熱點(diǎn)。進(jìn)一步深入研究STIM1在腫瘤細(xì)胞中的作用機(jī)制,將為其在腫瘤治療中的應(yīng)用提供更為廣闊的前景。
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