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赖氨酰氧化酶家族在恶性肿瘤中的矛盾角色

2013-01-22田孟鑫刘卫仁樊嘉史颖弘

中国临床医学 2013年4期
关键词:趋化靶向家族

田孟鑫 刘卫仁 樊嘉 史颖弘

(复旦大学肝癌研究所,上海 200032)

肿瘤的浸润和转移是恶性肿瘤最主要的生物学特征之一,是影响肿瘤患者预后的关键因素。研究已证实,肿瘤转移与缺氧密切相关,缺氧情况下的肿瘤细胞具有高侵袭性、转移性及耐药性[1]。赖氨酰氧化酶(lysyl oxidase,LOX)是一种铜依赖性单胺氧化酶,参与细胞外基质中胶原与弹性蛋白聚合起始阶段,并且是稳定细胞外基质(extracellular matrix,ECM)的关键酶,催化弹性蛋白和胶原蛋白发生交联,从而增加细胞外基质的稳定性。最新研究[2-5]发现,在食管、结直肠、膀胱及头颈等部位发生的恶性肿瘤组织中,缺氧诱导的LOX家族蛋白表达增加,并具有促进肿瘤转移的作用。相反,在前列腺、肺脏、乳腺以及胃等部位发生的恶性肿瘤中,LOX家族蛋白则具有抑制肿瘤生长的作用[6-9]。现针对LOX家族在恶性肿瘤中的矛盾作用作一综述。

1 LOX家族及其生物学特性

1.1 LOX家族的基因背景、结构和生物学功能LOX家族的基因位于5号染色体长臂(5q23.3-31.2),由7 个外显子组成,编码 417 个氨基酸[10]。LOX由成纤维细胞合成糖基化的前酶原(相对分子量50000),酶原分泌到细胞外后,即由溶胶原C-蛋白水解酶水解为LOX前肽(LOX propeptide,LOX-PP,相对分子量18000)和具有催化活性的成熟LOX(相对分子量32000),后者具有Cu2+和赖氨酰酪氨酰苯醌(lysine tyrosylquinone,LTQ)两个活性基团。与Cu2+结合后的LOX才具有活性,能够催化ECM中胶原蛋白和弹性蛋白形成共价交联,以保持ECM结构的完整性。LOX同时还具有其他重要的生物学功能,包括参与肿瘤的发生与发展、细胞的分化、运动以及黏附等。

1.2 LOX家族成员 LOX家族包括LOX和LOX样蛋白(LOX-like protein,LOXL),后者包括4个成员:LOXL1、LOXL2、LOXL3和 LOXL4[11]。4种LOXL的羧基端为保守序列,均含有205个氨基酸构成的LOX催化结构域,氨基端差异较大,决定了4种LOXL的不同生物学功能。Molnar等[12]研究发现,LOXL分别存在于不同细胞的胞质和胞核内,共同发挥调控细胞生长、肿瘤抑制、调控细胞衰老及趋化等作用。LOXL1、LOXL2、LOXL3和LOXL4还能促进多种肿瘤的侵袭和转移[13-18],而且LOX、LOX-PP、LOXL1和LOXL4等与肿瘤增殖的抑制有关[9,19]。

2 LOX家族在肿瘤中的双重作用

近年来,越来越多的研究证实,LOX家族在肿瘤进展中扮演双重角色:肿瘤抑制作用和转移促进作用。LOX家族的双重作用可能与LOX家族的细胞定位有关,通常情况下,肿瘤抑制作用由胞内LOX家族蛋白参与,而转移促进作用则由胞外LOX家族蛋白参与。同时,LOX家族的致瘤性和转移性主要是通过ECM的重建而实现的。以下将分别介绍LOX家族的双重作用。

2.1 肿瘤抑制作用及其机制 LOX最早被称为Ras消除基因(ras recision gene,Rrg),大鼠细胞LOX异常表达可抑制Hras诱导的肿瘤转化,表明LOX 具有肿瘤抑制功能[20-21]。近年来,研究[22-25]发现,由LOX前酶原裂解而来的相对分子量为18000的LOX前肽,既能够抑制NRKFs(normal rat kidney fibroblasts)的肿瘤转化,也能抑制相关的信号通路激活 NF-κB(Naclear Factor-κB),从而抑制哺乳动物上皮细胞转化和移植瘤形成,此外可下调BCL2(B-cell CLL/lymphoma 2)原 癌 基 因 的 转录[25]。LOX还可与受体型蛋白酪氨酸磷酸酶-kappa(receptor-type protein tyrosine phosphatase kappa,RPTP-kappa)相互作用,抑制β-连环蛋白的转录活性而发挥肿瘤抑制作用[24]。Palamakumbura等[6]的 研 究 显 示,LOX-PP 能 够 结 合 FGF(fibroblast grocoth factor)信号受体,抑制前列腺癌细胞的增殖。另外,LOX-PP还能够增强阿霉素对胰腺癌和乳腺癌细胞的毒性作用,诱导癌细胞凋亡[26]。LOX-PP 还 能 抑 制 Blimp1(B lymphocyte-induced maturoation protein 1)介导的 Ras/Raf/AP-1信号通路以下调转移表型,促进小鼠移植瘤的凋亡[7-8]。

2.2 肿瘤转移促进作用及其机制 LOX可表达于头颈部鳞状细胞癌、乳腺癌、结直肠癌和前列腺癌,在肿瘤转移过程中发挥重要作用[27-30]。人肾透明细胞癌中LOX表达增加,且其与患者总体生存率和无复发生存率降低有关[31-32]。LOX也可激活FAK/SRC信号通路,增加ECM硬度,促进结直肠癌的进展[29]。LOXL2高表达与肿瘤分期、预后不良和存活率下降有关[13,33-34]。在胃癌中的研究[35]发现,分泌性LOXL2作用于SRC-FAK通路,增强了肿瘤侵袭和转移能力,LOXL2特异性抗体可抑制肿瘤侵袭和转移能力。LOXL4作为头颈部原发、转移肿瘤分子标志物,也能促进肿瘤转移[36]。与头颈部正常组织相比,几乎所有的口腔黏膜样本中均有LOXL4表达增高,并伴有重度异型性增生和淋巴结转移[37]。不仅如此,LOXL4、LOX、LOXL2等还可以促进转移微环境的形成[38]。

2.2.1 缺氧 瘤内缺氧与肿瘤转移和不良预后密切相关[39]。缺氧既可以限制肿瘤细胞的分裂,还可以筛选出恶性程度更高的表型。缺氧促进肿瘤转移主要通过两条途径:(1)诱导 HIF-1α(Hypoxia induced factor-1α)表达的增加,HIF-1α可调节表皮生长因子、LOX、Snail等基因的表达,使E-cadherin表达下 调[40-44],而 E-cadherin 是 上 皮 细 胞 间 质 转 化(epithelial to mesenchymal transition,EMT)的关键步骤;(2)促进血管生成,即新生毛细血管的形成,这是肿瘤转移的另一个关键步骤。在结直肠癌的研究中[45]发现,缺氧环境下 HIF-1α可引起LOX的表达增加,随后血小板源性生长因子受体β(plateletderived growth factor receptor-β,PDGFR-β)激 活Akt,导致 VEGF(Vascular endothelial growth factor)表达的增加,促进肿瘤内血管的生成,该作用可被LOX靶向消除或被PDGFR-β抑制剂阻断。

2.2.2 ECM重建 LOX家族通过ECM重建和改变肿瘤微环境硬度进而改变肿瘤行为。实体肿瘤组织中致密结缔组织广泛形成,以纤维母细胞增多和(或)ECM沉积、紊乱为特征。临床发现,组织纤维化和肿瘤关系密切。乳腺X线检查剂量升高,不仅可增加乳腺癌发生的风险,而且该类肿瘤多以I型胶原沉积为特征[46]。胶原纤维在ECM中含量最丰富,对组织抗张强度和硬度影响最大,其合成增多、沉积过度导致ECM紊乱,与肿瘤进展密不可分[47]。一直以来,研究认为,肿瘤转移的先决条件是周围基质的分解,而组织病理研究指出,瘤内纤维化与患者不良预后有相关性,并强调ECM重建在肿瘤进展、转移过程中的关键作用。胶原纤维重组促进肿瘤的侵袭,并且该行为显著增加了胶原的密度[48]。在LOXL2合成增加的非侵袭性乳腺癌细胞中,侵袭和纤维化同时出现,暗示两者之间存在内源性联系[49]。硬化的肿瘤微环境中,肿瘤细胞借表面整合素受体接受外界机械刺激,更易增殖和侵袭周围基质[50]。

2.2.3 EMT EMT是肿瘤细胞扩散和转移的核心步骤[51-52]。在表达 LOX的肿瘤组织中,LOX或LOXL均可结合并激活转录因子Snail,从而直接诱导 EMT[53]。有研究[53]表明,MDCK(Madin Darby canine Kidney)细胞系中LOXL2的表达增加可促进上皮细胞向成纤维细胞或梭形细胞表型的转变,并伴随着波状蛋白的分布变化和E-cadherin的表达下调。

2.2.4 趋化作用 活化形式的LOX同时是一种趋化因子,它可以使多种细胞,如外周血单核细胞、血管平滑肌细胞等发生趋化运动[54]。β氨基丙腈(β-Aminopropionitrile,BAPN)作 为 LOX 的 抑 制剂,可以明显抑制LOX对血管平滑肌细胞趋化反应的强度和敏感性。LOX也能增强血小板源性生长 因 子 BB (plateletderived growth factor-BB,PDGF-BB)的趋化活性,其原因可能是分泌到细胞外的LOX可以氧化细胞膜蛋白,如PDGFR-β,从而增强了PDGF-BB诱导的趋化反应。此外,Erler等[2]的研究发现,缺氧诱导乳腺癌细胞分泌的LOX在转移前微环境(premetastatic niche)中积累,催化胶原纤维IV在基底膜处交联,同时募集CD11b+骨髓细胞,后者与胶原纤维IV相黏附,引起MMP-2合成增加,MMP-2(Matrix metallopeptidase 2)既可以促进胶原裂解,也可以募集骨髓源性细胞(bone marrow-derived cells,BMDC)和转移的肿瘤细胞,促进转移灶的形成。

3 LOX家族的靶向治疗

虽然LOX家族具有双重作用,但是针对LOX家族的靶向治疗仍然具有重要的意义。众多研究[27,33-55]表明,LOX 和 LOXL2抑制物能显著减少肿瘤的播散。转基因和移植瘤大鼠模型中,应用BAPN、功能性封闭抗体或者LOX特异性干扰RNA,可降低肺转移、肝转移发生率[27]。Barry-Hamilton等[33]的研究指出,在大鼠体内和体外BAPN也能抑制人重组LOXL2的表达。Payne等[56]的研究发现,低侵袭MCF-7细胞LOX表达上升可出现侵袭和黏附现象,添加BAPN后即消失。大鼠鳞癌模型中,LOXL2基因敲除可导致E-cadherin再表达、肿瘤细胞凋亡增加[53],而大鼠胃癌模型中,LOXL2抗体抑制LOXL2后对Snail无影响[35]。四 硫 钼 酸 盐 (tetrathiomolybdate,TM)是LOX家族靶向治疗的一种药物,临床试验[57]中转移性乳腺癌患者接受TM治疗后病情可稳定数月,甚至部分患者出现缓解。乳腺癌[58]和结直肠癌[59]大鼠模型研究发现,HIF1-α靶向治疗可抑制LOX家族在体外对缺氧的反应。另外,BMP1可激活LOX和LOXL1以前体酶原的形式分泌,抑制BMP1或者敲除BMP1可减少体外试验中活化的LOX水平[60]。总之,LOX家族在肿瘤进展中作用复杂,需要进行针对LOX家族的联合治疗或者针对诱导LOX家族合成的靶向治疗。

4 展 望

目前,LOX家族的肿瘤抑制能力是否与转移密切相关,LOX家族启动转移的活性在恶性转化中是否加强,都未得到深入研究。不仅如此,缺乏对LOX家族化学晶体结构的全面认识也阻碍了经典药物的发展和检查方法的改进,且当前应用的小分子抑制剂不具有选择性。令人欣慰的是,尽管挑战重重,但是LOX家族的靶向治疗可能会为抗肿瘤治疗带来新的希望。

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