夏 珊,趙梓綱,解 方,賀 晶

Makino等最早提出腫瘤干細(xì)胞(cancer stem cells,CSCs)的假說(shuō),推測(cè)腫瘤產(chǎn)生可能是由于CSCs導(dǎo)致[1]。1997年Bonnet等首次實(shí)驗(yàn)證實(shí)急性髓細(xì)胞白血病患者體內(nèi)含有CSCs。隨后各項(xiàng)研究證實(shí)了CSCs存在于腫瘤實(shí)體中,與干細(xì)胞相似,具有以下特性:(1)失控的自我更新能力,CSCs能夠無(wú)限地分裂產(chǎn)生與自身相同的子細(xì)胞,并有自我更新的能力,維持腫瘤細(xì)胞的生長(zhǎng)；(2)高致瘤性,核型分析或畸胎瘤實(shí)驗(yàn)等表明,CSCs的致瘤性高于一般干細(xì)胞或其他細(xì)胞；(3)多向分化特性,CSCs在特定條件下能夠誘導(dǎo)分化成為異質(zhì)性子代瘤細(xì)胞,從而形成新的腫瘤；(4)化療過(guò)程的耐藥性,CSCs能夠表達(dá)ABC家族膜轉(zhuǎn)運(yùn)蛋白,該蛋白能夠?qū)C(jī)體代謝產(chǎn)物、藥物、毒性物質(zhì)等排出細(xì)胞外,從而致使機(jī)體對(duì)許多化療藥物產(chǎn)生多重耐藥性。腫瘤中CSCs數(shù)量很少,比例不足5%,但其對(duì)腫瘤的存活、增殖、轉(zhuǎn)移、復(fù)發(fā)和化療耐藥性有著重要作用,是腫瘤細(xì)胞群生命力的支柱[2-3]。近年來(lái),很多對(duì)CSCs的研究結(jié)果表明,CSCs的相關(guān)信號(hào)轉(zhuǎn)導(dǎo)途徑(如Wnt/β-catenin、JAK-STATs、Hh、PIP3等)對(duì)鱗狀細(xì)胞癌的發(fā)生和發(fā)展有著重要影響[4-6]。本文主要對(duì)影響鱗狀細(xì)胞癌發(fā)生、發(fā)展的CSCs內(nèi)信號(hào)轉(zhuǎn)導(dǎo)途徑進(jìn)行闡述,以期為鱗狀細(xì)胞癌的治療開(kāi)辟新的途徑。

1 鱗癌干細(xì)胞

鱗狀細(xì)胞癌屬于皮膚表皮細(xì)胞的一種皮膚腫瘤,臨床被稱(chēng)為皮樣癌[7]。在皮膚與結(jié)膜交界處多發(fā),其中比較常見(jiàn)的有頭頸部鱗狀細(xì)胞癌和食管鱗狀細(xì)胞癌等。目前臨床和研究?jī)A向于認(rèn)為成體組織干細(xì)胞是CSCs的起源細(xì)胞,機(jī)制可能為:腫瘤細(xì)胞是正常細(xì)胞至少4～7次突變后累積導(dǎo)致的,一般情況下,正常細(xì)胞不會(huì)有這么長(zhǎng)的壽命,而且不具有累計(jì)多次的突變,而干細(xì)胞很長(zhǎng)的壽命以及累計(jì)的突變最終形成CSCs,進(jìn)而生成腫瘤。腫瘤分化程度可能與腫瘤起源的干細(xì)胞的增殖分化階段也有關(guān)。目前研究提示,鱗狀細(xì)胞癌可能起源于鱗狀干細(xì)胞增殖分化過(guò)程的任何階段[8]。假如干細(xì)胞分化過(guò)程早期階段發(fā)生腫瘤,那么便呈現(xiàn)為低分化的特性；若起源于干細(xì)胞分化過(guò)程后期階段,則腫瘤呈現(xiàn)為高分化的特性；而良性腫瘤一般起源于干細(xì)胞分化鏈的晚期階段。干細(xì)胞分化成熟過(guò)程中的任何一個(gè)階段都可被阻斷,最終形成腫瘤分化程度各異性。目前研究者普遍認(rèn)為,干細(xì)胞定居于一個(gè)特異性的微環(huán)境中,稱(chēng)之為“壁龕(niche)”中。有研究發(fā)現(xiàn),在原發(fā)性頭頸部鱗狀細(xì)胞癌中有類(lèi)似情況的微環(huán)境,大多數(shù)CSCs在血管周?chē)欢ǚ秶鷥?nèi),通過(guò)血管內(nèi)皮細(xì)胞能夠分泌IL-6等相關(guān)因子,激活STAT3、ERK 和Akt等信號(hào)通路,使得腫瘤細(xì)胞遷移能力增強(qiáng),同時(shí)CSCs受到保護(hù)[9]。CSCs進(jìn)入細(xì)胞周期后,可轉(zhuǎn)變?yōu)槌墒旒?xì)胞,為維持干細(xì)胞池和產(chǎn)生有特定功能的子代細(xì)胞起到關(guān)鍵性的作用,而這一過(guò)程受到了Wnt,JAK-STATs和hedgehog等信號(hào)途徑的調(diào)控[10]。

2 Wnt信號(hào)轉(zhuǎn)導(dǎo)途徑

Wnt 蛋白是一種富含半胱氨酸的糖蛋白,其信號(hào)轉(zhuǎn)導(dǎo)途徑有兩種,包括經(jīng)典途徑和非經(jīng)典途徑,經(jīng)典途徑?jīng)Q定細(xì)胞命運(yùn),非經(jīng)典途徑控制細(xì)胞運(yùn)動(dòng)及組織極性[11]。當(dāng)Wnt蛋白與Frizzled(FZD)家族跨膜蛋白受體和低密度脂蛋白受體關(guān)聯(lián)蛋白輔助性受體結(jié)合后,可激活Wnt/β-連環(huán)蛋白(β-catenin 信號(hào)通路),為Wnt經(jīng)典信號(hào)轉(zhuǎn)導(dǎo)途徑。當(dāng)FZD家族受體和ROR2/RYK聯(lián)合受體結(jié)合可以引發(fā)相關(guān)反應(yīng),激活Wnt非經(jīng)典信號(hào)轉(zhuǎn)導(dǎo)途徑[12]。Wnt信號(hào)通路及其有關(guān)的其他通路在胚胎的早期發(fā)育、器官形成、組織再生等過(guò)程中發(fā)揮著重要作用,因此,Wnt 信號(hào)通路失調(diào)可導(dǎo)致多種惡性腫瘤的發(fā)生[13]。正常的成熟細(xì)胞β-catenin表達(dá)水平低,經(jīng)典Wnt信號(hào)通路處于關(guān)閉狀態(tài)；而CSCs處于活化狀態(tài)時(shí),β-catenin降解產(chǎn)生障礙,細(xì)胞質(zhì)中游離β-catenin逐漸增多,可與TCF/LEF-1結(jié)合進(jìn)入細(xì)胞核,進(jìn)而激活下游靶基因c-myc、cyclin D1進(jìn)行轉(zhuǎn)錄,從而致使腫瘤的發(fā)生和發(fā)展[14]。張衛(wèi)民等[15]利用免疫組織化學(xué)方法研究Wnt-1、APC和β-catenin在口腔黏膜鱗狀細(xì)胞癌中的表達(dá),初步揭示了Wnt信號(hào)傳導(dǎo)通路組成蛋白Wnt-1、APC和β-catenin在口腔鱗狀細(xì)胞癌分化和增殖中的作用。結(jié)果如表1所示,Wnt-1、APC和β-catenin在高分化鱗狀細(xì)胞癌細(xì)胞膜中呈高表達(dá),而在中、低分化的鱗狀細(xì)胞癌細(xì)胞膜中表達(dá)較低,揭示了Wnt信號(hào)轉(zhuǎn)導(dǎo)途徑對(duì)鱗狀細(xì)胞癌的增殖和分化起著重要作用。

表1 Wnt信號(hào)轉(zhuǎn)導(dǎo)組成蛋白與鱗狀細(xì)胞癌增殖和分化的關(guān)系

相關(guān)研究證實(shí)Wnt信號(hào)通路可能與鱗狀細(xì)胞癌預(yù)后及放化療的敏感程度有密切關(guān)系。有研究者通過(guò)免疫組織化學(xué)檢測(cè)121例鱗狀細(xì)胞癌患者標(biāo)本發(fā)現(xiàn),Axin蛋白表達(dá)的量與食管癌浸潤(rùn)深度呈負(fù)相關(guān)；單因素分析結(jié)果表明,食管癌預(yù)后相關(guān)不良因素與Axin蛋白表達(dá)水平降低有關(guān)[16]?，F(xiàn)階段研究表明,WISP-1、WIF-1、DKK1等的異常表達(dá)可能是根治性術(shù)后食管癌患者不良預(yù)后指標(biāo)。研究發(fā)現(xiàn),食管癌細(xì)胞株順鉑敏感性與干擾素誘導(dǎo)跨膜蛋白-1(interferon induced transmembrane protein 1,IFITM1)有關(guān),并且LEF-1和β-catenin的異常是導(dǎo)致食管癌細(xì)胞放療無(wú)效的重要因素[17]。

3 JAK-STATs信號(hào)轉(zhuǎn)導(dǎo)途徑

JAK-STATs通路是一條由細(xì)胞因子刺激的信號(hào)轉(zhuǎn)導(dǎo)通路,在細(xì)胞凋亡、增殖和分化過(guò)程中起著重要的調(diào)節(jié)作用,維持人類(lèi)胚胎干細(xì)胞的自我更新[18-19]。最新報(bào)道,STATs 蛋白中某些亞型與腫瘤形成、發(fā)展高度相關(guān)[20]。

激活的STAT1屬于STATs 蛋白一類(lèi),能夠抑制細(xì)胞生長(zhǎng),介導(dǎo)細(xì)胞凋亡[21]。JAK-STAT1信號(hào)轉(zhuǎn)導(dǎo)途徑可在分子水平調(diào)節(jié)細(xì)胞的增殖和分化以及凋亡過(guò)程,在腫瘤發(fā)展過(guò)程中具有重要作用。對(duì)剔除STAT1基因小鼠的研究表明,細(xì)胞能夠在IFN-γ誘導(dǎo)下,通過(guò)使STAT1磷酸化而凋亡。鱗狀細(xì)胞癌由于下調(diào)了抗原呈遞機(jī)制,逃避細(xì)胞毒性T細(xì)胞的捕獲,而STAT1信號(hào)通路受到IFN-γ激活,從而使腫瘤細(xì)胞恢復(fù)對(duì)CTL的敏感性[22]。

Onishi等[23]應(yīng)用免疫定位研究發(fā)現(xiàn),JAK、Src信號(hào)和EGFR信號(hào)在鱗狀細(xì)胞癌中起著重要作用,其介導(dǎo)新生的上皮鈣黏著蛋白,誘導(dǎo)STAT3磷酸化,形成STAT3-Y705,促進(jìn)上皮細(xì)胞的過(guò)度增殖和惡性轉(zhuǎn)化。此外,STAT3能激活Bcl-xl、Mcl-1、cyclin D1等抗凋亡基因,進(jìn)而延長(zhǎng)腫瘤細(xì)胞的生存周期。Boehm等[24]發(fā)現(xiàn),在頭頸部鱗狀細(xì)胞癌中,Bcl-XL抗凋亡蛋白通過(guò)對(duì)細(xì)胞色素C的釋放的影響,進(jìn)而影響細(xì)胞凋亡。

4 Hh信號(hào)轉(zhuǎn)導(dǎo)途徑

Hh基因編碼的一種高度保守的分泌性糖蛋白,具有3種不同亞型,分別編碼Shh、Ihh和Dhh蛋白。而跨膜蛋白PTCH1為Hh受體[25]。PTCH1負(fù)調(diào)控Hh途徑,與配體結(jié)合可抑制受體活性。Hh缺乏時(shí),PTCH1與SMO結(jié)合,抑制SMO活性；存在Hh信號(hào)時(shí),Hh與PTCH1結(jié)合,SMO釋放至胞漿中,作用于轉(zhuǎn)錄因子GlI,影響細(xì)胞增殖；而HHIP和SUFU復(fù)合體為該通路的抑制環(huán)節(jié)[26]。

研究發(fā)現(xiàn),Hh信號(hào)通路中的一些蛋白或下游信號(hào),如BMI1陽(yáng)性和GlI-1陽(yáng)性等與鱗狀細(xì)胞癌患者經(jīng)新輔助放射化療后的早期復(fù)發(fā)及預(yù)后不良的相關(guān)性顯著,其平均DFS和OS等均有顯著性差異,提示Hh通路的激活參與惡性腫瘤CRT后的再生和發(fā)展[27]。Zhu等[28]分別檢測(cè)100例新輔助放化療鱗狀細(xì)胞癌活檢標(biāo)本的PTCH1和GLI-1,結(jié)果發(fā)現(xiàn)大部分標(biāo)本中PTCH1和GLI-1表達(dá)量有了很大的升高,二者表達(dá)水平與腫瘤大小,局部進(jìn)展和腫瘤對(duì)放化療反應(yīng)程度具有顯著相關(guān)性。單因素分析結(jié)果表明,PTCH1和GLI-1高表達(dá)與腫瘤局部復(fù)發(fā)快、DFS和OS差異有很大的關(guān)系。

5 其他信號(hào)轉(zhuǎn)導(dǎo)途徑以及交叉作用

鱗狀細(xì)胞癌最常見(jiàn)遺傳病變機(jī)制是EGFR過(guò)表達(dá)和p53基因突變,Notch信號(hào)通路參與上述兩者的負(fù)調(diào)控,且影響Wnt通路,并受到Wnt通路的影響[29]。Isohata等指出,大多數(shù)鱗狀細(xì)胞癌共表達(dá)EMT信號(hào)和Hh信號(hào)基因,首次提出EMT和Hh信號(hào)相互交叉的作用。PTK6具有使GSK3和Akt磷酸化程度降低的能力,進(jìn)而激活β-catenin蛋白。TE-10細(xì)胞系和TE-1細(xì)胞系體外研究證實(shí),PIP3途徑在N-Shh、Gβγ及表皮生長(zhǎng)因子的刺激下,能夠在Hh信號(hào)轉(zhuǎn)導(dǎo)通路中發(fā)揮重大作用；MAPK和PIP3協(xié)同作用,可使Shh通路促進(jìn)鱗狀癌細(xì)胞存活、增殖。He等[30]還發(fā)現(xiàn),Wnt信號(hào)通路可被Hh通路經(jīng)sFRP-1作用而抑制。

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