缺氧诱导因子-1与肿瘤
景绍武(综述)/王雅棣*(审校)
(河北医科大学第四医院放疗科,河北 石家庄 050011)
【摘要】 缺氧诱导因子-1(hypoxia inducible factor-1, HIF-1)是调节氧稳态的核心转录因子,在肿瘤的血管形成、能量代谢、侵袭转移等方面起着关键作用。HIF-1活性主要由HIF-1α亚基决定,而后者的DNA结合活性和转录活性又受到严格调控。本文对近年来有关HIF-1在肿瘤发生发展中作用的研究进行综述。
【关键词】 缺氧;缺氧诱导因子-1; 肿瘤
中图分类号:R730.231 文献标识码: A 文章编号: 1004-616X(2011)03-0235-04 doi: 10.3969/j.issn.1004-616x.2011.03.020
缺氧诱导因子-1α(hypoxia inducible factor -1 alpha, HIF-1α)最初是从缺氧诱导的Hep3B细胞核中发现的一种转录因子[1]。 已经证实,HIF-1可介导肿瘤细胞产生一系列的缺氧适应反应,同时又受到多种癌基因、抑癌基因的调控,与肿瘤的血管形成、能量代谢、侵袭转移等密切相关[2]。本文综合近年来对该基因与肿瘤发生、发展规律的研究进展做一综述。
1 HIF-1的基本结构
HIF-1是由120 kDa的α亚基和91~94 kDa的β亚基组成的一种异源二聚体。其中HIF-1α为氧调节蛋白,N-端具有基本的螺旋-套索-螺旋(basic helix-loop-helix, bHLH)区域和一个PAS(PER-ARNT-SIM)区域,这两个区域负责与HIF-1β形成异二聚体。C-端是反式激活区,包括两个反式激活域(transactivation domain, TAD)。这两个TAD之间为抑制结构域(inhibitory domain, ID),能抑制该分子在正常氧条件下的反式激活。HIF-1β又称芳香烃受体核转运蛋白,为基础表达蛋白。
人的HIF-1α基因位于14号染色体(14q21~24),HIF-1β基因则位于1号染色体(1q21)。HIF-1α是调节亚单位,决定HIF-1的活性,HIF-1β则与稳定HIF-1及其二聚化有关。HIF-1α主要存在于细胞核,而HIF-1β在细胞核和细胞质中均有表达[3]。以往多数研究认为在常氧下,HIF-1α由于受到泛素-蛋白酶体系统的作用被迅速降解[4],而缺氧可抑制其降解,HIF-1α大量集聚并转移至细胞核,与HIF-1β结合形成有活性的HIF-1。但最近的研究发现即使在常氧环境中,HIF-1α在某些恶性肿瘤中也有表达[5]。激活的HIF-1再结合到靶基因的启动子或增强子的缺氧反应元件上,从而启动靶基因的表达[6]。
2 HIF-1的靶基因及其表达调控
2.1 HIF-1的靶基因
HIF-1可调控数十种基因,其过表达与肿瘤的组织学特征、恶性表型密切相关[7]。HIF-1的靶基因主要包括:①血管内皮生长因子(vascular endothelial growth factor, VEGF)编码基因:VEGF是肿瘤血管生成的最重要的刺激因子,在多种实体肿瘤中高表达[8]。有研究发现HIF-1α可促进VEGF高表达,其作用机制为HIF-1α可与一种特异的DNA低氧应答产物结合,促进VEGF mRNA的转录,并使其稳定性增加,从而促进新生血管形成[9]。②糖酵解酶编码基因:HIF-1α与肿瘤细胞的糖酵解途径密切相关[10],使肿瘤细胞在相对缺氧状态下获得更多的能量供应,以维持存活、生长和分裂。HIF-1α可促使13种不同的糖酵解酶产生,可上调糖酵解过程中所需的醛缩酶、烯醇化酶、乳酸脱氢酶、磷酸果糖激酶-1和丙酮酸激酶等的表达[11],涉及该通路的全过程。③E-钙粘蛋白及基质金属蛋白酶(matrix metalloproteinase, MMP)编码基因:E-钙粘蛋白是最重要的细胞间粘附分子之一[12],MMP既可降解基膜和肿瘤基质,促进肿瘤侵袭转移,也可通过新生血管生成促进肿瘤生长和扩散[13],有研究显示HIF-1α通过下调E-钙粘蛋白[14]、上调MMP表达[15],促进肿瘤细胞的侵袭转移。④血红素加氧酶-1(heme oxygenase-1, HO-1)编码基因:缺氧时HIF-1α可诱导HO-1表达升高,HO-1继而催化产生CO。CO作为气体信号分子激活鸟苷酸环化酶,提高cGMP水平,使血管平滑肌松弛,抑制血小板凝聚,从而增加血流量和血管通透性,使缺氧组织得到充足的氧气供应。⑤诱导型NO合酶(inducible nitric oxide, iNOS)编码基因:HIF-1α可诱导iNOS合成,产生的NO作为血管舒张因子,可舒张血管、增加血流量和组织供氧。
2.2 HIF-1的表达调控
HIF-1的活性主要由HIF-1α亚基决定,而HIF-1α亚基的活性和表达又受到严格调控。这些调控因素主要有:①氧依赖的调节:HIF-1α在530~658位氨基酸残基部位形成一个氧依赖性降解区(oxygen dependent degradation domain,ODDD)。Maxwell等[16]证实林希病肿瘤因子 (product of Von Hippel-Lindau disease, pVHL)能直接与ODDD结合。pVHL具有E3泛素化蛋白酶活性,二者一旦结合,泛素化的HIF-1α可立即被降解。而pVHL与ODDD 区结合的首要条件是ODDD 区402和/或564位脯氨酸发生羟基化[17]。该过程是由脯氨酸羟基酶(proline hydroxylase, PHD)实现的。PHD也是由α、β亚基组成,其中α亚基具有脯氨酸羟化酶的活性,而缺氧时脯氨酸羟化酶失去活性,HIF-1α不能与pVHL结合,结果导致HIF-1α发生积聚。②金属离子置换:将Fe2+置换成Co2+或Ni2+能够稳定HIF-1α,并增加HIF-1活性及其下游基因的表达。其机制涉及两方面:一方面Co2+或Ni2+可取代与血红蛋白结合的Fe2+,使其处于去氧状态,从而模拟低氧环境。目前CoCl2已被广泛应用于细胞乏氧状态的模拟诱导,能在正常氧条件下抑制脯氨酸羟化酶的活性,从而抑制HIF-1α的降解[18]。另一方面Fe2+也是PHD的辅助因子,与PHD的两个组氨酸残基和一个羧基化残基协调作用参与HIF-1α的降解。Co2+将Fe2+置换后可抑制该酶活性,从而导致HIF-1α降解减少而发生积聚。③NO:NO在正常氧条件下可增加HIF-1α DNA的结合活性和转录活性[19],而在缺氧状态下可抑制HIF-1α DNA的活性及下游基因的表达[20],说明细胞本身的氧状态影响NO的作用。而且诱导型NO合酶又受到HIF-1α的调节,可见NO参与的HIF-1α调节机制复杂,确切机制还有待进一步研究。④活性氧类(reactive oxygen species,ROS):有研究显示低氧有利于ROS的产生。Bell等[21]发现缺氧环境增加ROS的生成,后者为HIF-1α的激活和稳定所必需,而ROS激活HIF-1α又有一个复杂的信号通路[22]。
此外,多种生长因子和细胞因子如纤维生长因子、胰岛素样生长因子、干扰素-β等可通过磷酸肌醇3-激酶(phosphoinositide3 - kinase,PI3K)/蛋白激酶B(AKT)和FRAP 信号途径参与HIF-1α的调节。
3 HIF-1与肿瘤
3.1 HIF-1与癌基因、抑癌基因
HIF-1α在肿瘤组织中表达增高,一方面与肿瘤组织增生过快造成局部组织缺氧有关,另一方面也与某些癌基因激活或抑癌基因失活有关。①p53:低氧诱导产生两种形式的HIF-1α,磷酸化HIF-1α和去磷酸化HIF-1α,前者倾向于与HIF-1β结合,形成异源二聚体;而去磷酸化HIF-1α则与p53关系密切,是同其结合的主要形式,在p53的稳定及诱导凋亡中发挥重要作用[23]。也有研究显示p53可使HIF-1α稳定性和活性下降[24]。②PTEN:该基因被认为是继p53之后另一个与肿瘤密切相关的抑癌基因。它通过PI3K/AKT信号传导途径负向调节HIF-1α,进而达到抑制VEGF表达的作用,因此PTEN的失活可加速肿瘤的进展[25]。③VHL:VHL是与林希病关系密切的抑癌基因,其产物pVHL为HIF-1α降解所必需。若VHL基因发生突变,pVHL不能形成或功能出现缺陷,可导致HIF-1α在正常氧分压下发生堆积。
3.2 HIF-1在肿瘤中的表达
HIF-1α是许多恶性肿瘤重要的生长调节因子[26],与预后密切关联[27]。Talks等[28]采用免疫组化发现大多数恶性肿瘤细胞中均有HIF-1α的表达,且在肿瘤坏死明显的区域和肿瘤浸润的区域,HIF-1α表达明显增多,而肿瘤组织内的基质细胞和邻近的正常组织则未见HIF-1α表达,提示HIF-1α可预测肿瘤的侵袭转移。有学者[29]还采用RNA干扰技术证实HIF-1α可调控肿瘤侵袭因子如组织蛋白酶D、转化生长因子-α、MMP等的表达。此外,Lee等[30]检测了89例非小细胞肺癌患者的术后标本,发现66.7%的鳞癌标本有HIF-1α表达,而腺癌中只有20.5%表达,提示HIF-1α的表达还与肿瘤组织分型有关。BOS等[31]发现在乳腺高分化导管原位癌、低分化导管原位癌和浸润性导管癌中HIF-1α的阳性率分别为55%、75%、100%,提示HIF-1α表达与组织分化程度呈负相关。该研究还指出HIF-1α增高与肿瘤细胞的增殖状态、VEGF的表达和平均微血管密度(microvessel density, MVD)相关。Takahashi等[32]则分析了53例胃恶性间质瘤标本,指出HIF-1α表达阳性者其预后较差。与之相反,Volm等[33]研究认为肿瘤细胞HIF-1α阳性染色的患者,由于凋亡的增加,其预后要明显好于阴性者。
3.3 HIF-1在肿瘤治疗中的作用
HIF-1α在肿瘤的血管形成、能量代谢、侵袭转移方面发挥着重要作用,因此有望成为肿瘤治疗的新靶点。①通过抑制HIF-1来抑制血管生成:由于VEGF的编码基因是HIF-1α的靶基因之一,而VEGF是目前已知的作用最强的血管形成促进因子[34],因此通过抑制HIF-1可抑制血管的生成。Buchler等[35]发现敲除HIF-1α基因或阻断HIF-1α转录可使肿瘤细胞无法生成VEGF,从而抑制肿瘤新生血管形成。Sun等[36]将HIF-1α的反义寡核苷酸转染胸腺淋巴瘤EL-4中发现,瘤组织中VEGF、MVD均明显下降,直径在0.1 cm以下肿瘤缩小甚至完全消失。②通过抑制HIF-1来增加凋亡: Zhang等[37]研究人舌鳞状细胞癌发现,用RNA干扰技术封闭HIF-1α表达后可引起肿瘤细胞凋亡增加,肿瘤生长变缓。③通过抑制HIF-1来辅助放化疗:众所周知,肿瘤缺氧的微环境是导致治疗效果差,产生放化疗耐受的重要原因,其机制与HIF-1α诱导DNA双链修复酶表达有关[38],有研究证实将HIF-1α 基因沉默后,提高了口腔鳞癌对化疗药物的敏感性[39]。因此,通过抑制HIF-1α有望提高肿瘤放化疗的临床疗效。
4 结 语
缺氧是实质性肿瘤微环境的基本特征之一,而HIF-1是调节肿瘤细胞适应缺氧的核心转录因子,参与多种基因的转录调控和肿瘤细胞对缺氧的适应过程,促进细胞的增殖和恶性转化,与疗效及预后密切相关[40]。因此,随着研究的不断深入,以HIF-1为靶点的研究可望为恶性肿瘤的防治提供新的重要思路。
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收稿日期: 2010-08-16; 修订日期:2010-09-26
基金项目: 教育部留学回国人员科研启动基金(教外司留[2008]101号),高等学校博士学科点专项科研基金(20091323110011)
作者简介: 景绍武(1982- ),男,河北省元氏县人,博士研究生,研究方向:恶性肿瘤的综合治疗。E-mail: jsw20062132@163.com
*Correspondence to: WANG Ya-di,E-mail: wangyadi @hot mail.com
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