許菊青 蔣峰

CYP3A4、CYP3A5多態(tài)性與腫瘤的關(guān)系

許菊青 蔣峰

CYP3A家族(cytochrome P450, subfamily ⅢA)是細(xì)胞色素酶CYP450(cytochrome P450 enzyme system)重要成員,其含量豐富,底物覆蓋廣泛,主要功能是參與許多內(nèi)、外源性物質(zhì)的代謝,是藥物代謝反應(yīng)中最主要的限速酶[1-2]。CYP3A亞家族包含4個(gè)成員:CYP3A4、CYP3A5、CYP3A7和CYP3A43。CYP3A4主要分布在肝臟和小腸,是肝內(nèi)最常見最豐富的細(xì)胞色素亞家族;CYP3A5主要分布于一些肝外組織如腸壁、腎和前列腺等,是CYP3A亞家族最重要的肝外分布形式。

CYP3A4和CYP3A5基因位于人類第7號染色體q21.1-22.1,基因全長分別為27.6 kb和31.8 kb,它們都有13個(gè)外顯子,編碼CYP3A4和CYP3A5的氨基酸有84%的相似性[3-4]。CYP3A蛋白受CYP3A基因編碼調(diào)控。CYP3A基因多態(tài)性決定了CYP3A酶表達(dá)含量的多寡及功能的強(qiáng)弱。

在已經(jīng)發(fā)現(xiàn)的CYP3A4基因單核苷酸多態(tài)性(single nucleotide polymorphisms，SNPs)中,CYP3A4*1B基因變異最常見,CYP3A4*1B基因變異(rs2740574)位于第5′端側(cè)翼區(qū)的A-392G轉(zhuǎn)換,可下調(diào)CYP3A4酶的表達(dá)及活性[5]。

CYP3A5基因型呈高度多態(tài)性,具有廣泛的個(gè)體及種族差異。Kuehl等[6]發(fā)現(xiàn)攜帶至少1個(gè)CYP3A5*1等位基因的人,肝臟和小腸CYP3A5酶的含量顯著增加,可達(dá)CYP3A酶總量的50%以上,提示攜帶CYP3A5*1等位基因可產(chǎn)生高水平的CYP3A5 mRNA和蛋白。CYP3A5*1基因的種族差異性分布顯示:非裔美國人占45%,高加索人占15%,中國人占35%,日本人占15%。CYP3A5另一常見基因型是CYP3A5*3,其第3內(nèi)含子(A6986G)突變可引起可變剪接,產(chǎn)生不穩(wěn)定蛋白從而導(dǎo)致部分人組織中CYP3A5表達(dá)缺失。在中國人群中,CYP3A5*3的發(fā)生頻率高達(dá)71%～76%[7]。

目前,CYP3A4、CYP3A5在腫瘤方面的研究主要集中在流行病學(xué)方面,研究表明CYP3A SNPs與部分腫瘤發(fā)生風(fēng)險(xiǎn)及化療藥物代謝具有相關(guān)性。本文對CYP3A4、CYP3A5基因多態(tài)性與常見腫瘤發(fā)生風(fēng)險(xiǎn)之間的關(guān)聯(lián)做一詳細(xì)闡述。

1 前列腺癌

CYP3A4主要表達(dá)于成人肝臟,在肝外組織表達(dá)含量較低,在前列腺中表達(dá)僅占0～14%,目前沒有發(fā)現(xiàn)CYP3A4促進(jìn)或抑制前列腺癌發(fā)生發(fā)展的直接證據(jù);但一系列研究表明,細(xì)胞色素酶P450家族中CYP17、CYP19和CYP11A1酶可影響腎上腺及性腺中類固醇的生物合成,與激素依賴性腫瘤的癌癥易感性相關(guān)。同樣,作為P450家族的重要成員之一,CYP3A4酶可參與內(nèi)源性睪酮的氧化反應(yīng),使其代謝成無活性的2β,6β,15β-羥睪酮[8];另外,作為CYP3A4基因最常見的SNPs,CYP3A4*1B可下調(diào)CYP3A4酶的表達(dá)及酶活性,導(dǎo)致CYP3A4表達(dá)下降或缺失。關(guān)于CYP3A4*1B與前列腺癌風(fēng)險(xiǎn)關(guān)聯(lián)的研究發(fā)現(xiàn),前列腺癌高發(fā)的歐美地區(qū),CYP3A4*1B突變率明顯增加,且CYP3A4*1B基因表達(dá)與前列腺癌發(fā)生風(fēng)險(xiǎn)顯著相關(guān)[9];回顧性研究表明，前列腺癌病人CYP3A4*1B等位基因突變率明顯高于健康人群[10-11];相反,在前列腺癌發(fā)生率較低的亞洲人中并沒有發(fā)現(xiàn)CYP3A4*1B基因突變[12]。綜上,CYP3A4*1B基因突變與前列腺癌發(fā)生發(fā)展密切相關(guān),具有促癌作用,其可能的機(jī)制是CYP3A4*1B突變下調(diào)CYP3A4蛋白表達(dá),導(dǎo)致內(nèi)源性雄激素滅活減少，促進(jìn)了激素依賴性前列腺癌的發(fā)生。

CYP3A5在正常前列腺組織及腫瘤組織中均有表達(dá),目前研究普遍認(rèn)為CYP3A5 SNP與腫瘤的發(fā)生發(fā)展密切相關(guān),但存在較大爭議。Plummer等[11]發(fā)現(xiàn)表達(dá)CYP3A4*1B/CYP3A5*3單體型基因人群前列腺癌發(fā)生率明顯增加,而存在CYP3A5*1基因突變的人群,前列腺癌的發(fā)生率則顯著降低。Vaarala等[13]研究也發(fā)現(xiàn)CYP3A5*3(6986A>G)基因型的表達(dá)與前列腺癌臨床病理參數(shù)呈正相關(guān);已知的CYP3A5基因多態(tài)性中,CYP3A5*3基因突變可使CYP3A5低表達(dá)或表達(dá)缺失,而CYP3A5*1基因則可產(chǎn)生高水平CYP3A5 mRNA和蛋白。早期研究也證明CYP3A5活性酶能催化6β-羥基睪酮,誘導(dǎo)睪酮失活,從而抑制激素依賴性的前列腺細(xì)胞的惡性增殖[14]。以上研究表明,CYP3A5*1基因型可作為CYP3A5功能性酶的活化基因，可阻止前列腺癌的發(fā)生。但也有研究提出了相反的結(jié)論:CYP3A5基因多態(tài)性與前列腺癌發(fā)生風(fēng)險(xiǎn)及臨床病理特征之間無顯著關(guān)聯(lián)[15-16]。因此,CYP3A5基因多態(tài)性與激素依賴性腫瘤的相關(guān)性仍需大量研究證實(shí)。

2 乳腺癌

在乳腺癌的研究中發(fā)現(xiàn),腫瘤中CYP3A4表達(dá)水平與乳腺癌臨床病理特征及不良預(yù)后相關(guān)[17-18],CYP3A4是雌激素16α-羥化代謝反應(yīng)中的重要酶,CYP3A4可催化雌激素16α-羥化反應(yīng)產(chǎn)生16α-羥雌酮,后者與雌激素受體結(jié)合,轉(zhuǎn)換細(xì)胞核DNA合成,可誘導(dǎo)乳腺異常細(xì)胞增殖、有潛在的致癌作用[19],提示CYP3A4通過影響雌激素水平間接促進(jìn)了乳腺癌的發(fā)生發(fā)展。另外,Mitra等[17]通過體外實(shí)驗(yàn)發(fā)現(xiàn)CYP3A4促進(jìn)激素受體陽性乳腺癌細(xì)胞惡性增殖的信號通路,CYP3A4促進(jìn)環(huán)氧二十碳三烯酸(EET)生物合成,抑制了激活因子3(STAT-3)磷酸化作用和核轉(zhuǎn)運(yùn),從而導(dǎo)致惡性細(xì)胞的無限增殖。

流行病學(xué)研究已經(jīng)提供了雌激素水平與乳腺癌風(fēng)險(xiǎn)之間關(guān)聯(lián)的直接和間接證據(jù)。女性體內(nèi)性激素的差異受遺傳因素影響,CYP3A位點(diǎn)SNPs與乳腺癌罹患風(fēng)險(xiǎn)關(guān)系的研究證明常見遺傳變異CYP3A4*1B與激素水平或乳腺癌風(fēng)險(xiǎn)之間無明顯相關(guān)性[20-21]。雖然CYP3A4*1B與乳腺癌風(fēng)險(xiǎn)無直接關(guān)聯(lián),但研究顯示CYP3A4*1B變異與性早熟和他莫昔芬誘發(fā)的子宮內(nèi)膜癌相關(guān)[22-23],而早熟是乳腺癌的高危因素之一。美國放射技師協(xié)會進(jìn)行雌激素代謝相關(guān)基因的常見異常的巢式病例對照研究報(bào)告顯示,罕見的非同義CYP3A4的SNP(rs4986910,M445T)可降低乳腺癌風(fēng)險(xiǎn)[24],然而這一結(jié)果并沒有在Johnson等[25]研究中得到復(fù)制,但該研究發(fā)現(xiàn)了距離3′CYP3A5約50 kb堿基的標(biāo)簽SNP(rs10273424)與絕經(jīng)前女性尿中較低的雌酮葡萄苷酸(E1G)水平存在顯著關(guān)聯(lián),后者作為雌激素降解產(chǎn)物反映了體內(nèi)雌二醇(E2)的水平;這一發(fā)現(xiàn)表明攜帶罕見基因CYP3A5(rs10273424)可降低乳腺癌風(fēng)險(xiǎn)。

3 肝癌

研究表明,CYP3A酶參與了黃曲霉素B1代謝活化，生成具有致癌作用的AFB1-外-8,9-環(huán)氧化物(AFBO)，誘導(dǎo)肝細(xì)胞肝癌發(fā)生[26]。一項(xiàng)關(guān)于CYP3A亞家族與肝癌侵襲轉(zhuǎn)移關(guān)聯(lián)的研究發(fā)現(xiàn),CYP3A4可能具有促進(jìn)腫瘤細(xì)胞增殖的“癌基因”生物學(xué)功能;機(jī)制可能是通過造成肝癌細(xì)胞低氧環(huán)境來誘導(dǎo)低氧反應(yīng)元件(HRE)啟動子活性的增強(qiáng),促進(jìn)促紅細(xì)胞生成素(EPO)和血管內(nèi)皮生長因子(VEGF)mRNA表達(dá)含量的增加,繼而誘導(dǎo)磷酸肌醇3激酶/蛋白激酶B(PI3K/Akt)信號通路的活化[27],PI3K/Akt參與調(diào)節(jié)腫瘤細(xì)胞的增殖遷移。而Jiang等[28]發(fā)現(xiàn)CYP3A5可作為一個(gè)保護(hù)性因子抑制肝細(xì)胞肝癌侵襲轉(zhuǎn)移,其抑制肝癌細(xì)胞侵襲轉(zhuǎn)移的分子機(jī)制是CYP3A5通過細(xì)胞內(nèi)活性氧簇(ROS)單純抑制雷帕霉素靶點(diǎn)白復(fù)合物2(mTORC2)的活性,進(jìn)而選擇性抑制AKT(Ser473)活化,引起AKT信號通路下調(diào),從而抑制肝細(xì)胞肝癌侵襲轉(zhuǎn)移。提示CYP3A5具有區(qū)別于傳統(tǒng)代謝功能的“抑癌基因”功能。

4 肺癌

研究發(fā)現(xiàn),CYP1A1、CYP2C9等可參與煙草中前致癌物質(zhì)多環(huán)芳烴(PAH)的代謝活化,合成DNA加合物,從而誘導(dǎo)肺癌的發(fā)生[29]。研究發(fā)現(xiàn)吸煙病人PAH-DNA加合物的水平與肺泡巨噬細(xì)胞CYP3A5的含量呈正相關(guān)[30],提示CYP3A5可能通過參與PAH-DNA合成代謝誘導(dǎo)了肺癌的發(fā)生。而對CYP3A SNP研究顯示,CYP3A5*3純合子基因型使肺癌的發(fā)生風(fēng)險(xiǎn)增加了4.3倍[31],另一項(xiàng)包含了801例肺癌的病例對照研究顯示，攜帶CYP3A4*1B基因型的病人小細(xì)胞肺癌發(fā)生風(fēng)險(xiǎn)明顯增加,亞組分析顯示,攜帶CYP3A4*1B基因型的人群中吸煙>20包/年肺癌的發(fā)病風(fēng)險(xiǎn)將進(jìn)一步增加,可能與CYP3A4*1B基因型增加煙草中致癌物質(zhì)的代謝活化從而誘導(dǎo)癌癥發(fā)生有關(guān)[32]。但Timofeeva等[33]研究卻并未發(fā)現(xiàn)CYP3A4或CYP3A5基因多態(tài)性與肺癌發(fā)生風(fēng)險(xiǎn)之間存在顯著關(guān)聯(lián)。CYP3A4、CYP3A5在肺癌中的作用機(jī)制仍需大量研究。

5 其他腫瘤

研究顯示骨肉瘤[34]、尤文肉瘤[35]CYP3A4或CYP3A5高表達(dá)的病人較易出現(xiàn)遠(yuǎn)處轉(zhuǎn)移,提示CYP3A4或CYP3A5可能參與了腫瘤細(xì)胞的增殖遷徙過程。Hyland等[36]研究發(fā)現(xiàn)中國人群CYP3A5基因多態(tài)性與食管癌風(fēng)險(xiǎn)相關(guān),其中攜帶至少1個(gè)CYP3A5*1等位基因的人食管癌發(fā)生風(fēng)險(xiǎn)明顯升高。

綜上所述,CYP3A4、CYP3A5在體內(nèi)的差異性表達(dá)受遺傳因素影響,CYP3A SNP是造成個(gè)體、種族差異的基礎(chǔ),并影響了癌癥的易感性。目前,CYP3A SNP影響癌癥發(fā)生的具體機(jī)制尚未明確,有待進(jìn)一步研究。尋找可以預(yù)測癌癥風(fēng)險(xiǎn)的候選等位基因,并探索它們與腫瘤風(fēng)險(xiǎn)或藥物代謝的關(guān)系對預(yù)防癌癥發(fā)生及指導(dǎo)治療具有重要意義。

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210029江蘇省南京市，南京醫(yī)科大學(xué)第四臨床醫(yī)學(xué)院(許菊青)；210009江蘇省南京市，南京醫(yī)科大學(xué)附屬腫瘤醫(yī)院胸外科(蔣峰)

蔣峰，Email：jiangfeng174@sohu.com

R 730.2

A

10.3969/j.issn.1003-9198.2017.01.023

2016-11-23)