陳曦,朱蓉,趙逵

(遵義醫(yī)學(xué)院附屬醫(yī)院,貴州遵義563099)

·綜述·

Ascl2與結(jié)腸癌發(fā)生發(fā)展的關(guān)系研究進(jìn)展

陳曦,朱蓉,趙逵

(遵義醫(yī)學(xué)院附屬醫(yī)院,貴州遵義563099)

Ascl2是一個(gè)堿性/螺旋—環(huán)—螺旋轉(zhuǎn)錄因子,是Wnt信號(hào)通路的靶分子，僅表達(dá)于胎盤(pán)及小腸、大腸隱窩基底的Lgr5陽(yáng)性的腸隱窩基底柱細(xì)胞(CBC細(xì)胞)。近來(lái)研究證實(shí)Ascl2是CBC成體腸干細(xì)胞的一個(gè)重要標(biāo)記物,在維持結(jié)腸癌干/前體細(xì)胞的干細(xì)胞性方面發(fā)揮重要作用。本文對(duì)Ascl2與結(jié)腸癌的關(guān)系進(jìn)行了綜述。

干性標(biāo)志物；Ascl2；Wnt信號(hào)通路；結(jié)腸癌

據(jù)統(tǒng)計(jì)，結(jié)腸癌居全球腫瘤發(fā)病率的第3位[1]。近年來(lái)隨著生活水平的提高和飲食習(xí)慣的改變，結(jié)腸癌發(fā)病率逐年升高，且趨于年輕化[2，3]。腫瘤干細(xì)胞理論的提出為治療結(jié)腸癌帶來(lái)了新希望，尋找理想的結(jié)腸癌干細(xì)胞特異性表面分子標(biāo)志物是該領(lǐng)域目前最重要的研究方向之一。新近研究發(fā)現(xiàn)，Ascl2是腸隱窩基底柱細(xì)胞(CBC細(xì)胞)作為成體腸干細(xì)胞的一個(gè)重要標(biāo)記物[4]，是Wnt通路的直接靶基因[5]；在結(jié)腸癌發(fā)生發(fā)展的各個(gè)時(shí)期，結(jié)腸癌組織中Ascl2均表達(dá)上調(diào)[6，7]。近年來(lái)有關(guān)Ascl2與結(jié)腸癌發(fā)生發(fā)展關(guān)系的研究有較大進(jìn)展，現(xiàn)綜述如下。

1 腫瘤干細(xì)胞理論

Dontu等[8]首次證實(shí)腫瘤干細(xì)胞存在于乳腺癌等實(shí)體瘤中，隨后在腦腫瘤[9]、前列腺癌[10]、乳腺癌[11]、胰腺癌[12]、黑素瘤[13]及肺癌中也得到證實(shí)[14]，近年來(lái)O′Brien等[15]和Ricci-Vitiani等[16]陸續(xù)發(fā)現(xiàn)結(jié)直腸癌中腫瘤干細(xì)胞的存在。腫瘤干細(xì)胞理論認(rèn)為腫瘤發(fā)生的根源是腫瘤干細(xì)胞，腫瘤干細(xì)胞是根治腫瘤的靶點(diǎn)[17]。腫瘤干細(xì)胞具有自我更新和分化為祖細(xì)胞的能力，在所有腫瘤細(xì)胞中僅占極少數(shù)，其具有的不對(duì)稱(chēng)分裂和無(wú)限增殖能力導(dǎo)致惡性腫瘤難治療、易轉(zhuǎn)移和易復(fù)發(fā)[18]。Tang等[19]認(rèn)為正常干細(xì)胞和腫瘤干細(xì)胞在維持組織穩(wěn)態(tài)方面存在共性。Todaro等[20]認(rèn)為正常干細(xì)胞通過(guò)不對(duì)稱(chēng)分裂成一個(gè)新細(xì)胞，而腫瘤干細(xì)胞對(duì)稱(chēng)分裂成兩個(gè)細(xì)胞，并且只有一小部分細(xì)胞分化。除腫瘤干細(xì)胞以外，其余大部分腫瘤細(xì)胞不具有無(wú)限增殖能力，短暫的生存后即開(kāi)始凋亡,放化療時(shí)即可被殺死。傳統(tǒng)的腫瘤“隨機(jī)模型”認(rèn)為所有腫瘤細(xì)胞均具有同等增殖、轉(zhuǎn)移能力。腫瘤干細(xì)胞理論的提出顛覆了這一傳統(tǒng)認(rèn)識(shí)。腫瘤干細(xì)胞在整個(gè)腫瘤過(guò)程中起著至關(guān)重要的作用,是腫瘤發(fā)生、發(fā)展過(guò)程的起始細(xì)胞。從腫瘤干細(xì)胞角度分析結(jié)腸癌的生物學(xué)特性，可能為結(jié)腸癌提供新的治療思路[21]。

腸黏膜上皮是人體組織中更新速度最快的，正常情況下每4～5 d更新一次，不斷補(bǔ)充衰老脫落的上皮細(xì)胞，其細(xì)胞學(xué)基礎(chǔ)是黏膜干細(xì)胞的自我更新和定向分化[17]。腸隱窩干細(xì)胞是位于腸隱窩基底部的成體干細(xì)胞，一部分沿“隱窩—絨毛軸”方向分化為四種類(lèi)型細(xì)胞—杯狀細(xì)胞、潘氏細(xì)胞、腸吸收細(xì)胞、腸內(nèi)分泌細(xì)胞；另一部分細(xì)胞則向隱窩基底遷移分化為潘氏細(xì)胞，并在CBC細(xì)胞周?chē)纬蒀BC細(xì)胞的干細(xì)胞壁龕[27，28]，“干細(xì)胞壁龕”通過(guò)一系列信號(hào)通路(Notch、Wnt、BMP、Hedgehog和JAK/STAT等)調(diào)控腸干細(xì)胞的命運(yùn)，并提供干細(xì)胞增殖與分化必要的物質(zhì)基礎(chǔ),對(duì)細(xì)胞的增殖、分化起決定性的作用[29～32]。Zhang等[17]采用熒光標(biāo)記發(fā)現(xiàn)，位于基底部腸隱窩的是活躍的腸干細(xì)胞，在+4位置的腸干細(xì)胞并不活躍。有學(xué)者通過(guò)研究小鼠的基因過(guò)表達(dá)和基因敲除模型發(fā)現(xiàn)，Ascl2是控制CBC成體腸干細(xì)胞命運(yùn)非常重要的轉(zhuǎn)錄因子，是CBC成體腸干細(xì)胞的重要標(biāo)記物，小鼠腸上皮中Ascl2轉(zhuǎn)基因過(guò)表達(dá)可導(dǎo)致腸隱窩的過(guò)度增生以及腸絨毛上的異位隱窩出現(xiàn)，而Ascl2基因表達(dá)缺失可導(dǎo)致Lgr5陽(yáng)性的CBC細(xì)胞消失。上述均表明Ascl2在干細(xì)胞維持中具有重要作用。

2 Ascl2與結(jié)直腸癌

Ascl基因?qū)儆谝粋€(gè)保守的轉(zhuǎn)錄因子家族，被定義為基本的螺旋—環(huán)—螺旋結(jié)構(gòu)域[22]。Ascl2基因定位于染色體11p15.5，是一個(gè)堿性/螺旋—環(huán)—螺旋轉(zhuǎn)錄因子，負(fù)責(zé)人類(lèi)正常胎盤(pán)的滋養(yǎng)層細(xì)胞譜系的分化，其表達(dá)僅限于胎盤(pán)以及小腸、大腸隱窩基底的Lgr5陽(yáng)性CBC細(xì)胞[4]。Ascl2(Mash2/HASH2)基因與果蠅的Achaete-scute復(fù)合體基因同源[23]，編碼一個(gè)堿性/螺旋—環(huán)—螺旋轉(zhuǎn)錄因子，主要表達(dá)于胚外組織，具有部位特異性[24]。Jubb等[22]通過(guò)原位雜交實(shí)驗(yàn)證實(shí)其在正常組織中僅表達(dá)于胎盤(pán)及小腸、大腸的隱窩基底部，在其他正常組織中幾乎不表達(dá)。

目前鑒別和分離出的結(jié)腸癌干/前體細(xì)胞表面標(biāo)志物有許多，例如CD133[15，16]、CD44[33]、CD24[34]、和Lgr5[35]等，但尚無(wú)公認(rèn)的非常特異的表面標(biāo)志物，Ziskin[5]等研究認(rèn)為Ascl2和Lgr5在結(jié)腸癌中的表達(dá)分別為85%和74%，二者表達(dá)呈正相關(guān);且大部分腺癌來(lái)源于Lgr5+/ Ascl2+隱窩干細(xì)胞,依賴(lài)于Wnt/β-catenin信號(hào)通路[36]。隱窩干細(xì)胞與大腸癌的形成密切相關(guān)，對(duì)隱窩干細(xì)胞的生物學(xué)研究有助于深入了解大腸癌的生物學(xué)特性。因此，腸隱窩干細(xì)胞的準(zhǔn)確識(shí)別很重要[37]。Neal等[38]認(rèn)為每個(gè)隱窩通常含有約6個(gè)獨(dú)立的干細(xì)胞和活躍的腸干細(xì)胞(a-iscs)、沉默的腸干細(xì)胞(q-iscs)兩大派系,腸隱窩干細(xì)胞與損傷后的再生密切相關(guān)，參與結(jié)腸癌的發(fā)生。Papailiou等[39]也認(rèn)為結(jié)腸癌干細(xì)胞可能起源于正常成體腸隱窩干細(xì)胞。結(jié)腸癌的發(fā)生、發(fā)展是一個(gè)漸進(jìn)的過(guò)程，常因數(shù)年累積的突變導(dǎo)致，正常結(jié)腸隱窩干細(xì)胞突變可導(dǎo)致腸上皮組織結(jié)構(gòu)和功能異常，造成內(nèi)環(huán)境的穩(wěn)定失調(diào)，促使結(jié)腸癌發(fā)生[40]。有學(xué)者通過(guò)基因芯片篩選發(fā)現(xiàn)Ascl2可以調(diào)控Lgr5、Olfm4、Sox9、EphB3等腸干細(xì)胞相關(guān)的分子。并且，Ascl2在腸道隱窩基底僅表達(dá)在Lgr5+的CBC細(xì)胞，而Lgr5是目前被公眾認(rèn)可度較高的正常成體腸干細(xì)胞標(biāo)志物之一，因此認(rèn)為Ascl2也應(yīng)是腸隱窩干細(xì)胞標(biāo)志物[4]。Barker等[41]對(duì)小鼠小腸隱窩基底部的干細(xì)胞標(biāo)記Lgr5，證實(shí)Lgr5標(biāo)記的細(xì)胞能夠分化為上皮細(xì)胞譜系；且ascl2表達(dá)與Lgr5同步化，這提示Lgr5與Ascl2可能是腸道干細(xì)胞的標(biāo)記物。我們前期研究亦證實(shí)Ascl2與結(jié)腸癌干細(xì)胞相關(guān)[28]。

2.2 Ascl2與Wnt信號(hào)通路 目前已知的結(jié)腸癌干細(xì)胞相關(guān)信號(hào)調(diào)節(jié)通路有Wnt、Notch、BMP通路等[42]。Wnt是一個(gè)富含半胱氨酸的分泌型配體家族。Wnt信號(hào)通路對(duì)于正常腸隱窩結(jié)構(gòu)和穩(wěn)態(tài)的維持非常重要，整個(gè)腸黏膜隱窩—絨毛結(jié)構(gòu)都受到Wnt信號(hào)通路的調(diào)控；Wnt信號(hào)通路在正常細(xì)胞增殖以及干細(xì)胞的維持方面也發(fā)揮了重要作用[17]。其機(jī)制是胞質(zhì)內(nèi)游離的β-catenin積累，進(jìn)入核內(nèi)與TCF-4結(jié)合形成復(fù)合物，激活下游靶基因轉(zhuǎn)錄，啟動(dòng)腫瘤的生長(zhǎng)程序[43]。Kuhnert等[44]在在體實(shí)驗(yàn)中使用Wnt拮抗劑Dkkl抑制該信號(hào)通路，破壞了正常腸隱窩的結(jié)構(gòu)和穩(wěn)態(tài);而該信號(hào)通路的異常則與結(jié)腸癌的發(fā)生、發(fā)展有直接或間接關(guān)系。Radtke等[45]發(fā)現(xiàn)，抑癌基因Apc的截短將導(dǎo)致正常腸隱窩穩(wěn)態(tài)的破壞，這與Wnt/β-catenin信號(hào)通路的表達(dá)增強(qiáng)密切相關(guān)。近來(lái)Sousa等[46]研究發(fā)現(xiàn)，Wnt信號(hào)通路在結(jié)腸癌干細(xì)胞中活性明顯增高，而在已分化的結(jié)腸癌細(xì)胞中則明顯降低，說(shuō)明Wnt信號(hào)通路與結(jié)腸癌干細(xì)胞關(guān)系密切。有學(xué)者研究了在基底部表達(dá)的17個(gè)干細(xì)胞標(biāo)記物，發(fā)現(xiàn)與Wnt信號(hào)關(guān)系最緊密的是Lgr5和Ascl2。Ascl2在腸道黏膜上皮組織中的表達(dá)是Wnt通路依賴(lài)的，是Wnt信號(hào)的一個(gè)靶分子[20，47，48]。Sansom等[49]報(bào)道當(dāng)APC被截短，Ascl2的表達(dá)與對(duì)照組相比上調(diào)了22.2倍。Jubb等[6]發(fā)現(xiàn)，Ascl2在人類(lèi)結(jié)直腸腫瘤組織中高表達(dá)，Ascl2過(guò)度表達(dá)是早期腸腫瘤Wnt信號(hào)異常調(diào)節(jié)的一個(gè)結(jié)果，Wnt信號(hào)在正常腸道和腸道腫瘤中均調(diào)節(jié)Ascl2的轉(zhuǎn)錄。

3 結(jié)語(yǔ)

靶向治療是目前熱門(mén)研究方向，而靶向治療CSC還處于實(shí)驗(yàn)階段[50]。Ascl2在腸道的表達(dá)僅限于隱窩基底，是維持結(jié)腸癌干細(xì)胞"干性"的重要轉(zhuǎn)錄因子，為靶向治療結(jié)腸癌帶來(lái)新的希望,但具體功能和機(jī)制尚待進(jìn)一步研究。

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貴州省高層次人才科研條件特助基金資助項(xiàng)目(TZJF-2011-32)。

趙逵

10.3969/j.issn.1002-266X.2015.06.036

R735.3

A

1002-266X(2015)06-0091-04

2014-11-15)