党永辉,刘仲伟,陈 峰,郭 坤,王家蓓

1西安交通大学医学部法医学院 卫生部法医学重点实验室 教育部环境与疾病相关基因重点实验室,西安710061

三联组氨酸核苷结合蛋白1概况

含三联组氨酸 (histidine triad,HIT)域的蛋白构成酶类超家族,共享组氨酸-x-组氨酸-x-组氨酸-xx基序 (x为疏水性氨基酸)。按酶活性分类,HIT蛋白分为核苷氨基磷酸酯水解酶、二核苷酸水解酶、核苷转移酶。HIT蛋白以组氨酸-x-组氨酸-x-组氨酸-xx活性位点正对底物的α-磷酸结合核苷酸。HIT蛋白在进化中较为保守,其超家族中超过35个成员已在包括细菌、古细菌、酵母、植物、线虫、果蝇和哺乳动物的29个物种中得以发现,表明它行使着基本而重要的生理功能[1]。人类基因组编码7个HIT蛋白,可被分为5类,即三联组氨酸核苷结合蛋白 (histidine triad nucleotide-binding protein,HINT)、半乳糖-1-磷酸尿苷酰基转移酶、Aprataxin、DcpS/DCS-1以及脆性组氨酸三联体蛋白,主要发挥核苷转移酶和水解酶的作用[2-5]。

HINT(包括人类HINT和非人类HINT)是HIT超家族中的第1类。已经发现在得到全长测序的所有基因组中至少存在1个HINT/Hint。人类基因组包括3个独立的基因,分别编码Hint1、Hint2及Hint3基因产物。HINT1蛋白编码基因位于人类染色体5q31.2,其基因序列全长6 160 bp,含有3个外显子,mRNA序列由782个碱基组成,编码产物是含126个氨基酸的细胞溶质蛋白,相对分子质量约 14 ×103[2,6]。晶体学和核磁共振波谱法结构研究结果表明,HINT1属嘌呤核苷酸结合蛋白[7-9]。HINT1形成同源二聚体,每一个亚单位结合1个核苷酸。1990年HINT1被首度发现是蛋白激酶抑制剂[10],在早期文献中一直被当作蛋白激酶C抑制剂-1[11-12]。然而其蛋白激酶C抑制作用现在被认为是存疑的,也正因为如此,蛋白激酶C抑制剂-1被重新命名为HINT1[7]。

HINT1的酶活性

研究显示HINT1能够水解在很多真核生物由AMP-SO4和NH3合成的细胞内溶质AMP-NH2[13-15]。而组氨酸—丙氨酸突变使该酶的活性丧失,因此其单胺磷酸酯酶活性依赖于HIT域中的第2个组氨酸残基[13]。AMP-NH2的生理作用未知,可能不是HINT1的天然底物。HINT1能够水解结合于腺苷酸化蛋白赖氨酸残基的AMP加成化合物,因而可能调控蛋白底物的核苷酸化[2]。Chou等[16]应用敏感、连续的荧光方法测定氨基磷酸酯的水解活性以研究HINT1的底物特异性,HINT1表现出对嘧啶氨基磷酸酯上嘌呤的优先结合特性。

使用纯化的赖氨酰-tRNA合成酶,Chou和Wagner[17]证实赖氨酰-AMP是HINT1的底物,这产生了酶的腺苷酸化形式。这一发现使人们注意到HINT1参与一系列细胞功能。氨酰-tRNA合成酶是联合氨基酸与其相应同工tRNA形成氨酰-tRNAs的重要酶,这一反应通过形成氨酰-AMP中间产物而进行。氨酰-tRNA合成酶在调控转录、翻译、剪切、炎症、血管发生和凋亡中具有多种生理功能[18]。几种氨酰-tRNA合成酶 (E、I、L、R、Q、M、K、D)与3种氨酰-tRNA合成酶相互作用多功能蛋白 (aminoacyl-tRNA synthetase-interacting multifunctional proteins,AIMP)1/p43、 AIMP2/p38以及AIMP3/p18关联而形成大分子复合物。AIMP1是内皮单核细胞活化多肽Ⅱ的前体,是一种致炎性细胞因子,可以增加成纤维细胞的增殖和胶原生成,诱导内皮细胞的迁移,并可激活巨噬细胞[19]。AIMP2是一种与p53相互作用的致凋亡因子[20]。AIMP3则是一种肿瘤抑制因子,可以激活对受损DNA的修复[18]。赖氨酰-tRNA合成酶对于增加前述复合物的稳定性是必需的[21]。研究显示人类赖氨酰-tRNA合成酶可由多种细胞系分泌,并与细胞表面趋化因子受体相互作用刺激肿瘤坏死因子的生成。色氨酰基-和酪氨酰基-tRNA合成酶也可产生细胞因子,但与以上复合物作用无关[21-22]。

HINT1与肿瘤

自HINT1发现以来,首先发现其与肿瘤具有相关性,因而该领域的研究相对较为深入。

HINT1的抑癌基因作用 Su等[23]报道给予致癌物,Hint1基因敲除小鼠更容易罹患肿瘤。不只是Hint1基因敲除 (KO,-/-)小鼠,杂合 (+/-)小鼠同样比野生型 (WT,+/+)小鼠更容易被诱导出乳腺和卵巢肿瘤,这意味着HINT1是单倍剂量不足肿瘤抑制因子[24]。

有研究表明在HINT1、赖氨酰-tRNA合成酶、黑色素瘤癌基因小眼畸形转录因子 (microphthalmia transcription factor,MITF)(MITF表达可以转化人类原初黑素细胞,并增加其化学抗性[25])以及在真核细胞广泛表达的癌基因上游刺激因子2(upstream stimulatory factor 2,USF2)[26-27]之间存在着一种功能性的多种蛋白交互作用。赖氨酰-tRNA合成酶除了在tRNA氨酰化中发挥作用,同时还能产生信号分子二腺苷四磷酸盐[28]。HINT1与MITF和USF2相互作用并抑制其活性,进而发挥抑制肿瘤发生的作用,而二腺苷四磷酸盐结合于HINT1,使HINT1从MITF、USF2上解离进而被激活。

HINT1的启动子在肝细胞癌中通常高度甲基化,保持HINT1在肿瘤组织中的低表达状态[29]。因此HINT1名列易受DNA启动子区甲基化状态影响 (使转录失活)的肿瘤抑制基因名单[30]。HINT1启动子区高甲基化的程度与肝细胞癌的预后相关[31]。HINT1基因的甲基化程度越高,HINT1蛋白的表达水平越低,肝细胞癌的预后越差。Calvisi等[31]的研究表明HINT1影响S期激酶相关蛋白2的活性,进而促进细胞周期调节蛋白p27KIP1降解,这些都是肝细胞癌预后不良的特征。另外,HINT1参与DNA双链断裂修复[32],而这一发现很可能会改变HINT1肿瘤抑制因子的属性。在电离辐射诱导DNA受损后小鼠Hint1与γ-组蛋白2AX和运动失调性毛细血管扩张症变异蛋白激酶复合物相互作用,导致两种蛋白被活化并募集于DNA断裂点参与DNA的损伤修复。Hint1敲除使得胚胎成纤维细胞对于电离辐射造成的细胞毒性和细胞凋亡效应更加耐受[23],这可能与Hint1对γ-组蛋白2AX和运动失调性毛细血管扩张症变异的调控作用缺失有关[32]。

HINT1介导的信号转导通路及对肿瘤细胞周期机制的影响 肿瘤是一类多步骤发生、多基因突变所致的细胞克隆性、进化性疾病,同时也是一类细胞周期疾病。Hint1的抑癌机制与许多细胞内的信号转导通路以及肿瘤细胞周期机制有关。有研究显示Hint1结合于富含SH3结构域蛋白 (protein“plenty of SH3 domains”,POSH),POSH是一种支架蛋白,参与由Rac-1、混合谱系激酶3(mixed lineage kinase 3,MLK3)、促分裂原活化蛋白激酶激酶4/7和Jun氨基末端激酶1/2形成的复合物[33-34]。POSH能够激活JNK并引起细胞凋亡,同时它的异位表达可以激活细胞核因子-κB信号通路,从而促进p65往核内转运,增强目的基因的表达[35]。另有研究表明,POSH的过度表达能够促进神经元的凋亡[36],而用反义寡核苷酸和siRNA沉默POSH能抑制JNK活性和由撤回神经生长因子介导的神经元细胞凋亡[36],同时可通过抑制混合谱系激酶3-促分裂原活化蛋白激酶激酶4-Jun氨基末端激酶信号通路和细胞凋亡蛋白酶3活性来预防局部缺血性损伤[37]。从另一个方面看,凋亡也可以增加POSH和MLKs在细胞内的表达[38]。Hint1/POSH的交互作用显着降低JNK2磷酸化活化蛋白1(癌细胞中的一种重要转录因子[33])的能力从而抑制癌细胞的增殖。除了抑制活化蛋白-1的转录活性之外,HINT1也可以与S期激酶相关蛋白2-SCF泛素连接酶复合物相互作用,并抑制这一复合物的形成,调控细胞周期调节蛋白p27KIP1的泛素化水平影响细胞周期[39]。

HINT1通过与细胞周期调节蛋白依赖激酶Cdk7相互作用而调控基础转录因子TFIIH与靶基因的结合[13,40]。利用酵母杂交实验也证实了 Hint1和 Kin28之间 (在哺乳动物分别是HINT1和Cdk7)相互作用,同时破坏Hint1和Kin28之间的相互作用会导致细胞周期延长和菌落形成减少[40]。但是,Hint1-/-小鼠未能表现出与Hint1调控Cdk7活性作用相一致的表型[41]。

Weiske 和 Huber[42]证实 HINT1 与 β-连环蛋白配体Pontin和Reptin之间存在相互作用,从而抑制三元复合物因子 (ternary complex factor,TCF)-β-连环蛋白的转录活性,进而抑制Wnt信号通路靶基因,如axin2和细胞周期蛋白D1的表达。免疫沉淀实验结果显示,HINT1不能直接与β-连环蛋白或者LEF-1结合,不影响LEF-1/β-连环蛋白的相互作用,也不干扰Pontin或 Reptin与 β-连环蛋白的结合,确切来说,HINT1干扰Pontin/Reptin复合物,能够结合于组蛋白乙酰转移酶Tip60[43]。这个结果表明HINT1/Tip60复合物至少对一部分TCF/β-连环蛋白靶基因具有抑制作用。

将HINT1瞬时转染于SW480和MCF-7细胞,可以引起p53、Bax表达增加以及Bcl-2表达减少,促进细胞凋亡,而HINT1蛋白的下调则降低p53和Bax的表达,进一步研究表明HINT1与Tip60形成复合物可以与Bax的启动子结合而激活其转录活性[44]。

HINT1与神经精神疾病

HINT1在中枢神经系统广泛表达[45-47],提示它在正常神经元生理功能或在神经精神疾病病理条件下可能发挥重要作用。HINT1基因位于5q31.2遗传位点内,这一区域与精神分裂症关联[48-49]。Vawter等[50-51]采用芯片技术筛选显示HINT1 mRNA在精神分裂症患者背外侧前额皮质 (dorsolateral prefrontal cortex,DLPFC)表达水平较低,这一发现为其随后的RT-PCR和原位杂交实验所进一步证实[52]。另一项对染色体5q22-33区域进行的精细定位研究显示SPEC2/PDZ-GEF2/ACSL6区域单体型与精神分裂症关联,而Hint1基因位于这一区域中[53]。此后利用患者群体和患者死后脑组织的多项研究均提示这种关联的存在[6,54-55],而这种关联呈性别特异性,仅与男性患者有关[6,52-53]。缺乏Hint1的小鼠对甲基苯丙胺和多巴胺受体激动剂阿扑吗啡的自主活动反应增强,表明缺乏Hint1导致突触后水平多巴胺传递的异常[56],这也是精神分裂症可能的发病原因之一[57]。

最近,人类Hint功能丧失已被认为是遗传性周围神经病的原因。一项对33个无关核心家庭50例患者的全基因组单核苷酸多态性分析表明,人类Hint1活性的丧失与具有神经肌强直表现的常染色体隐性轴突神经病变这一遗传性周围神经病存在强烈关联,成为第1个被发现与人类Hint1酶活性相关的疾病表型[45]。此外还发现HINT1调制精神活性物质的效应。HINT1与μ阿片受体特异性相互作用,缓和蛋白激酶C调节的μ阿片受体的磷酸化和脱敏[58]。基因关联研究表明Hint1基因变异与尼古丁依赖有关[59]。缺乏HINT1的小鼠基础痛阈增加,吗啡诱导的镇痛效应增强,对吗啡的耐受增加[58]。

HINT1在调制情绪行为中也发挥重要作用。HINT1在双相障碍患者DLPFC脑组织中表达下降[60],在抑郁症患者DLPFC表达上升[61];而HINT1-/-小鼠则存在躁狂样表现[62],且焦虑样行为增加,在不良环境条件下,情绪唤醒程度升高[62-64]。另外,HINT1蛋白还被发现在Down’s综合征胎儿脑中下调[65]。

HINT1与其他疾病

糖尿病 Chu等[66]采用弱阳离子交换磁珠法纯化样本,基质辅助激光解吸电离飞行时间质谱方法检测来自28份2型糖尿病患者和29份健康个体随机尿样本,结果显示与健康对照相比,3种不同表达的肽在2型糖尿病中减少,其中一种被鉴定为HINT1,研究人员据此认为HINT1可被视为区分2型糖尿病患者和健康对照的潜在生物标志物。

肝脏缺血/再灌注 在医学研究中,寻求能够降低缺血/再灌注 (ischemia/reperfusion,I/R)损伤的细胞通路一直是个前沿问题,其临床需求十分迫切。Martin等[67]对 Hint1 KO以及 WT小鼠 (外购)进行70%肝脏缺血后再灌注3或24 h,结果显示与对照组相比,I/R后,Hint1 KO小鼠血清转氨酶、肝实质坏死、肝细胞凋亡显着减少,细胞凋亡蛋白Bax表达减少2倍以上;而WT小鼠活性氧和血红素加氧酶-1表达增加,KO小鼠未见增加;KO小鼠磷酸化Src和p65核转运增加,磷酸化c-Jun核表达减少。Hint1 KO小鼠保护性细胞因子白细胞介素6和白细胞介素-10水平增加,增加KO小鼠I/R后的存活率,KO小鼠Kupffer细胞受脂多糖类刺激后活化细胞数比对照细胞少,这一实验说明Hint1蛋白能够影响I/R损伤过程,在治疗过程中降低Kupffer细胞中Hint1的表达可能会发挥限制损伤程度的作用。

肝纤维化 转化生长因子β(transforming growth factor-β,TGF-β)/Smad是参与肝纤维化的主要信号通路,而Wnt信号通路在肝纤维化的发展中也发挥重要作用。因此,Wu等[68]利用人类Hint1重组蛋白对四氯化碳诱发的大鼠肝纤维化进行干预并探讨其机制:实验中大鼠被随机分为正常对照组、肝纤维化模型组以及重组人类Hint1(50、100 μg/kg)干预组,4周干预后,对重组人类Hint1干预组大鼠的组织病理学分析表明肝纤维化显着减少,羟脯氨酸水平较低,其潜在机制可能与重组人类Hint1抑制肝组织α-平滑肌肌动蛋白表达,降低TGF-β1/Smad3和β-连环蛋白/细胞周期蛋白D1信号通路活性有关;然而,研究也显示重组人类Hint1能够激活TGF-β1信号通路Smad7的表达,可能是一种代偿机制,这一研究结果表明Hint1或可作为治疗肝纤维化的靶点分子。

综上,HINT1广泛参与肿瘤、神经精神疾病等人类疾病的病理生理过程。其中,探究HINT1酶活性、肿瘤抑制、神经病理之间的内在联系是一个重要的研究方向。

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