LOXL2
——具有潛在臨床應(yīng)用價值的新標(biāo)志物

魏楓林

(三門峽市中心醫(yī)院心胸外科，河南 三門峽 472000)

LOXL2
——具有潛在臨床應(yīng)用價值的新標(biāo)志物

魏楓林

(三門峽市中心醫(yī)院心胸外科，河南 三門峽 472000)

賴氨酰氧化酶樣蛋白2(Lysyl oxidase-like 2，LOXL2)是賴氨酰氧化酶家族成員之一，其在細(xì)胞外主要參與基質(zhì)膠原蛋白和彈性蛋白的交聯(lián)產(chǎn)物的形成，在細(xì)胞內(nèi)主要通過Snail途徑調(diào)節(jié)上皮細(xì)胞-間葉樣細(xì)胞轉(zhuǎn)化(Epithelial-mesenchymal transition，EMT)過程，LOXL2正在作為一種新的治療方法的靶點研究。但是在不同腫瘤中或不同腫瘤亞型中的表達(dá)情況差異明顯，需要進(jìn)一步研究。

賴氨酰氧化酶樣蛋白2；侵襲；轉(zhuǎn)移

賴氨酰氧化酶樣蛋白2是近年來逐漸受到重視的一種胺氧化酶。它與組織纖維化和腫瘤的侵襲、轉(zhuǎn)移都有密切的關(guān)系，但是在不同類型的腫瘤中，發(fā)揮的作用是不同的，甚至是矛盾的。需要我們進(jìn)一步的深入研究，以了解其中的分子生物學(xué)機制，為臨床治療或預(yù)測提供一種新的途徑或指標(biāo)。

1 賴氨酰氧化酶家族

賴氨酰氧化酶樣蛋白2(Lysyl oxidase-like 2，LOXL2)是賴氨酰氧化酶家族成員之一，這個家族具有代表性成員，分別是LOX、LOXL1、LOXL2、LOXL3、LOXL4。它們的-COOH端具有高度保守的序列，這決定了它們的同源性和相似性[1]。然而，LOX家族各亞型的N-端具有更大的差異性，這可能決定了這一組同工酶各自不同的功能角色和組織分布[2]。賴氨酸-酪氨酸輔酶因子和四組氨酸銅結(jié)合域是賴氨酰氧化酶家族成員獨有的[3-4]，也是LOX、LOXL2、LOXL4發(fā)揮胺氧化酶活性的必要結(jié)構(gòu)[5-8]。賴氨酰氧化酶家族中最具代表性的LOX和LOXL一直被看做是與基質(zhì)膠原蛋白和彈性蛋白的交聯(lián)產(chǎn)物形成密切相關(guān)的細(xì)胞核外酶[4，9]。然而，Hayashi等[9]和Csiszar等[10]的研究發(fā)現(xiàn)，在正常壯年小鼠的各種組織的不同區(qū)域細(xì)胞的核內(nèi)和核外都有LOX和LOXL存在，發(fā)揮各種對腫瘤的發(fā)生和進(jìn)展密切相關(guān)的生物學(xué)功能，包括：細(xì)胞生長調(diào)控[11]、粘附、細(xì)胞運動和侵襲[3，12]。

2 LOXL2與腫瘤

1997年，Saito等[13]首次研究發(fā)現(xiàn)，相對于粘附性腫瘤細(xì)胞株，在各種非粘附腫瘤細(xì)胞株中LOXL2表達(dá)均下調(diào)。在RAS轉(zhuǎn)化小鼠的纖維母細(xì)胞中LOXL2 mRNA的表達(dá)減少近60%[14]。還有報道，在頭頸部鱗狀細(xì)胞癌(HNSCC)和漿液性卵巢癌中發(fā)現(xiàn)LOXL2表達(dá)的下調(diào)[15-17]。Saux等[18]在1998年將LOXL2基因定位于8p21.2-p21.3，這個區(qū)域的染色體丟失現(xiàn)象在多種惡性腫瘤中被發(fā)現(xiàn)，包括卵巢癌[19]、頭頸部鱗狀細(xì)癌[20]、腸癌[21-22]、食管癌[23-24]和乳腺癌[25-26]。

但是，在新近的研究中卻發(fā)現(xiàn)LOXL2的高表達(dá)和腫瘤的侵襲行為有關(guān)。首先在高致瘤性、轉(zhuǎn)移性鼠鱗狀細(xì)胞癌和梭狀細(xì)胞癌中發(fā)現(xiàn)LOXL2 mRNA的表達(dá)，而同時在無致瘤性的角質(zhì)細(xì)胞株中未發(fā)現(xiàn)LOXL2 mRNA的表達(dá)[27]。而且同樣的情況也出現(xiàn)在高侵襲性、轉(zhuǎn)移性乳腺癌細(xì)胞株與低侵襲性、轉(zhuǎn)移性乳腺癌細(xì)胞株的對比研究中[3]。有關(guān)LOXL2的表達(dá)與乳腺癌的臨床腫瘤分級的研究資料也支持上述結(jié)果[29]。而且進(jìn)一步研究發(fā)現(xiàn)，原本無侵襲性的MCF-7乳腺癌細(xì)胞株被轉(zhuǎn)染了LOXL2基因后即出現(xiàn)了侵襲性[29]，支持了LOXL2是一種致癌基因的觀點。隨后，有多家分別報道在乳腺癌、腸癌、食管癌、膽管癌、胰腺癌和胃癌細(xì)胞和組織中LOXL2的表達(dá)升高，而且這種高表達(dá)與更差的分化、更高的N分期和臨床TNM分期以及較差的生存期有關(guān)[29-33]。

在RAS轉(zhuǎn)化小鼠的纖維母細(xì)胞、頭頸部鱗狀細(xì)胞癌、肺腺癌和漿液性卵巢癌中發(fā)現(xiàn)LOXL2表達(dá)的下調(diào)。但是，在乳腺癌、腸癌、食管癌、膽管癌、胰腺癌和胃癌中LOXL2的表達(dá)上調(diào)，這說明了LOXL2在腫瘤發(fā)展過程中的角色是復(fù)雜的。而且在Zhan等[34]的報道中，在NSCLC中LOXL2表達(dá)的顯著下調(diào)，而且和更高的N分期和臨床TNM分期有密切聯(lián)系，但這種現(xiàn)象僅僅出現(xiàn)在肺腺癌患者，而在肺鱗癌患者中卻沒有觀察到這種聯(lián)系。這更進(jìn)一步說明了LOXL2的復(fù)雜性。

3 LOXL2與EMT

進(jìn)一步研究發(fā)現(xiàn)，在上皮細(xì)胞-間葉樣細(xì)胞轉(zhuǎn)化(Epithelial-mesenchymal transition，EMT)過 程 中LOXL2的表達(dá)上調(diào)，其中包括上皮表型的丟失和間葉細(xì)胞的活動性表型的獲得[35]。SNAIL是EMT過程中至關(guān)重要的一種轉(zhuǎn)錄因子[27]。Fong等[29]研究顯示LOXL2能夠與SNAIL相互作用并提高SNAIL蛋白的穩(wěn)定性，誘導(dǎo)EMT。當(dāng)SNAIL表達(dá)的轉(zhuǎn)移性癌細(xì)胞中LOXL2下調(diào)，引起E-cadherin的合成，減少了間葉細(xì)胞轉(zhuǎn)化和促血管生成物質(zhì)的生成，減低了侵襲性。雖然目前還不確定LOXL2是否通過抑制SNAIL活性的促進(jìn)侵襲、轉(zhuǎn)移，但重要的是發(fā)現(xiàn)了在EMT過程中LOXL2與NAIL相互作用的重要性，這對于腫瘤的進(jìn)展，特別是侵襲性和遠(yuǎn)處轉(zhuǎn)移非常重要[29]。LOXL2抗體可以明顯抑制腫瘤的生長和轉(zhuǎn)移[32]。在上皮細(xì)胞株T47D、MCF-7、HCT-116、HCT-15和DLD-1中，LOXL2的表達(dá)缺失，而在高侵襲性和轉(zhuǎn)移性的乳腺癌細(xì)胞株MDAMB-231和Hs578T中表達(dá)[29]，MDAMB-231和Hs578T具有間葉細(xì)胞表型[3]，這與前面的結(jié)果是一致的。在一些組織中，例如乳腺和卵巢，早期可能出現(xiàn)LOXL2的表達(dá)缺失，但在腫瘤的演化過程中出現(xiàn)LOXL2的再表達(dá)，這種現(xiàn)象可能與微環(huán)境的變化誘因有關(guān)。事實上，MCF-7細(xì)胞與纖維母細(xì)胞在特定的培養(yǎng)基中共同培養(yǎng)，能夠被促發(fā)LOXL2的表達(dá)[2]。這和家族中的另一個成員LOX是相近的。LOX已經(jīng)被證明，根據(jù)細(xì)胞類型和轉(zhuǎn)化狀態(tài)的不同，可以表現(xiàn)出腫瘤抑制或促腫瘤兩種截然不同的功能[3，36-37]。此外，Peng等[32]研究顯示分泌的LOXL2能夠同時激活Snail/E-cadherin and Src kinase/Focal adhesion kinase(Src/FAK)途徑。然而，LOXL2誘導(dǎo)胃癌細(xì)胞侵襲和遠(yuǎn)處轉(zhuǎn)移僅僅通過Src/ FAK途徑。

4 LOXL2的調(diào)節(jié)

Kaneda等[38]提出，是否象LOX一樣，LOXL2表達(dá)調(diào)節(jié)存在一種外在轉(zhuǎn)錄調(diào)節(jié)機制。Fong等[29]用脫甲基因子處理腸癌細(xì)胞株，結(jié)果發(fā)現(xiàn)LOXL2表達(dá)的增加。Lind等[39]報道，在腸癌細(xì)胞株與原位癌細(xì)胞中，LOXL2表達(dá)和甲基化率是相同的。然而，但在隨后的腸癌細(xì)胞的進(jìn)展演化中，隨著啟動子的去甲基化，獲得了LOXL2的表達(dá)。表達(dá)的調(diào)控也可能通過腫瘤演化和EMT過程中的轉(zhuǎn)錄因子完成，如AP1、NFjB和WT1[40-42]。這意味著遺傳和外在環(huán)境因素都可能參與了癌癥進(jìn)展過程中LOXL2基因表達(dá)的調(diào)節(jié)。

綜上所述，LOXL2可能會成為一種預(yù)測腫瘤侵襲性和遠(yuǎn)處轉(zhuǎn)移風(fēng)險的指標(biāo)，或者成為一種新的治療方法的靶點，但仍需要進(jìn)一步的研究來證明在體內(nèi)腫瘤轉(zhuǎn)移過程中LOXL2的功能作用，探索如何把LOXL2阻斷發(fā)展成為一個新的、具有發(fā)展?jié)摿Φ闹委煵呗浴?/p>

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R33

A

1003—6350(2014)15—2258—03

10.3969/j.issn.1003-6350.2014.15.0877

2014-01-08)

魏楓林。E-mail：hhyywfl@163.com