Posted on 2019年6月6日

血管生成素受体作用在婴幼儿血管瘤中的探讨

  关键词 血管生成素; 婴幼儿; 血管瘤 
  中图分类号 R732.2 文献标识码 A 文章编号 1674-685(213)22-149-3 
  血管生长存在两种最主的方式即血管发生(vasculogenesis)和血管新生(angiogenesis)1-2。血管发生是指来自中胚层的血管干细胞或内皮前体细胞经过分化、丛集和反复重塑形成成熟血管网的过程。血管新生是指从己存在的血管上以发芽或嵌入的方式形成新的毛细血管的过程3。血管生长的整个过程中均需血管内皮细胞生长因子(vascular endothelial gowth factor,VEGF)及其受体家族的参与,而血管新生的后期尤其是血管的构建、成熟与稳定需血管生成素Angiopoietin/Tie2(简称Ang/Tie2)家族的参与4。胎儿的血管生长同时存在上述两种方式,而成人体内生理性的血管生长只有血管新生方式,而无血管发生方式5。SALIMATH等6认为,某些幼稚的原始血管内皮细胞在妊娠晚期发生变异,增殖能力过度增强,协同细胞生长因子的作用,内皮细胞过度增殖,形成大量的毛细血管,毛细血管反复交织、成网,最后形成血管瘤。本文主对Ang/Tie结构及其在婴幼儿血管瘤方面的作用加以概括和总结。 
  1 血管生成素家族基因结构特点 
  血管生成素(angiopoietin,Ang)是一分泌性单链碱性蛋白,由123个氨基酸残基组成。Ang属于胰腺核糖核酸酶超家族的一员,是该家族中唯一含受体抑制剂和激动剂的促血管生成因子,也是目前已知的所有血管生成因子中独具核糖核酸酶活性的因子7-1。结构分析发现,Ang蛋白晶体由三个结构域组成C-端纤维蛋白原样结构域、N-端螺旋-螺旋结构域和疏水性分泌信号肽。C-端纤维蛋白原样结构域包含Ang受体结合区,决定某种Ang亚型是激动剂或是拮抗剂,该结构域是Ang家族中最保守的序列。N-端螺旋-螺旋结构域的氨基酸序列与肌球蛋白及其相关蛋白有较大的同源性,该结构域与血管生成素的二聚化/多聚化有关。迄今为止发现的Ang家族存在四种形式,即Ang1、Ang2、Ang3和Ang4。这四种Ang均可与内皮细胞特异性的酪氨酸激酶受体Tie2结合。其中Ang1和Ang4能够诱导Tie2的磷酸化,从而激活Tie2;而Ang2和Ang3则抑制Tie2的磷酸化,从而抑制Tie2的激活11-15。Angl基因定位于染色体8q22.3q23部位,编码498个氨基酸,分子量约为7 kD16-2。C-端纤维蛋白原样结构域大约由2个氨基酸组成,N-端螺旋-螺旋结构域大约由18个氨基酸组成,两者之间的连接肽是 
  Ang1与细胞外基质连接的结构域21。Ang1可形成四种同分异构体,分别编码498、367、255、154个氨基酸,分子量分别为 
  1.5 kD、1.3 kD、.9 kD、.7 kD 22-24。Ang2基因定位于8p23.1部位,编码496个氨基酸,分子量约为7 kD。Ang2与Ang1同源性高达6%,其N-端和C-端同样存在螺旋-结构域和纤维蛋白原样结构域。Ang1的9个半胱氨酸中的8个在Ang2中是保守的,而Ang2的纤维蛋白原样结构域与螺旋-螺旋结构域的交界处比Ang1少一个半胱氨酸25。Ang3基因定位于2p13部位,编码523个氨基酸,与Ang1和Ang2同源性分别为45.1%和44.7%的26。Ang4基因定位于2p13部位,编码53个氨基酸27。 
  2 血管生成素家族组织分布特点 
  Ang主表达于成人富含血管的组织器官中,在多种病理条件下如创伤、炎症、肿瘤等也可表达28。Ang1主在富含血管的组织中如胚胎期的间质细胞、成人肌肉、胎盘、前列腺、卵巢、子宫等表达。Ang2主在血管重塑活跃的部位如胚胎期的大血管平滑肌、胎肝血管壁,成人卵巢、子宫、胎盘等表达29,其表达受多种因子如肿瘤坏死因子-α(tumor necrosis factor-α,TNF-α)、VEGF、EGF等调节,缺氧可以促进Ang2的表达,而Ang1和TGF-β则下调其表达3。Ang3表达于肺和人脐静脉内皮细胞上,VEGF可诱导人脐静脉内皮细胞中Ang3的高表达,并下调Ang2的表达31。Ang4主表达于肺组织内,人胶质瘤中亦可见到Ang4的表达,而在其它组织细胞内表达较低32。 
  3 血管生成素受体的结构 
  Tie是血管内皮细胞特异性的酪氨酸激酶(receptor tyrosine kinase,RTK)受体家族成员,包括Tie1和Tie2两个亚型。Tie1来源于K562细胞株,主发挥毛细血管间的液体交换和血管对血流动力学的应答等作用,并与内皮细胞的分化和结构完整性有关。目前尚未发现Tie1的配体,故大多数研究主集中在Tie2上33-34。Tie2为Ang的受体,结构与Tie1相似。其基因定位于9p21部位,编码1124个氨基酸组成的蛋白质。在胚胎时期,Tie2均匀表达于血管内皮上,Tie2的细胞内含有1个酪氨酸激酶结构域,该结构域含有1个激酶插入结构域,细胞外由8个功能区组成,临近细胞膜部分为由3个连续的重复序列组成的纤维蛋白,3个表皮生长因子基序分隔2个免疫球蛋白样结构域。Tie2的折叠结构与丝氨酸/苏氨酸和酪氨酸激酶结构相似,其催化核心在其羧基末端,羟基端去磷酸化形成激活的构象。Tie2的主在促进内皮细胞的存活以及维持血管的稳定性和完整性等方面发挥重的作用,其受体后途径可能是通过作用于PI3、AKT等激酶,激活转录因子STAT,促进p21转录等对血管生成起调节作用35。 
  4 婴幼儿血管瘤与Ang及其受体Tie2途径 
  婴幼儿血管瘤是小儿最常见的良性肿瘤之一,其发病率约在5%~1%36。性别和种族的差异可影响其发病率,调查显示男女发病率比例约为1∶3,高加索人的发病率高达1%。血管瘤的特殊性不仅仅是它较高的发病率,而且它可以在无任何干预治疗下自行消退。临床通常分为增殖期(~1岁),消退期(1~5岁),消退完全期(5~1岁)。婴幼儿血管瘤好发于面部,最初发现时间约在出生后2周~1个月间,随后的半年内肿瘤生长迅速,大部分血管瘤不需特殊治疗,1年后便可慢慢自然消退,消退率可达8%。但是,有部分婴幼儿血管瘤在发生发展过程中可能发生各种并发症,如出血、溃疡、感染等,严重者可危及生命。约有3%的患儿在瘤体消退后会遗留色素沉着、皮肤萎缩、皮肤瘢痕、畸形等后遗症,给患儿带来严重的肌肤损害和心理创伤37-38。根据临床传统分类方法,将血管瘤分为毛细血管瘤、海绵状血管瘤、蔓状血管瘤、混合型血管瘤。血管瘤的病理学特点是血管内皮细胞的异常过度增殖和血管异常生成。目前国内外对于血管瘤中Ang及其受体Tie2家族的表达情况研究和报道的较少。Nasser等39研究发现婴幼儿血管瘤组织中Ang2及其受体Tie2及bFGF、VEGF等生长因子表达均增高,而Ang1表达则与此相反。体外培养血管瘤内皮细胞和正常血管内皮细胞时发现,血管瘤内皮细胞较正常血管内皮细胞对Ang1和VEGF细胞应答及Ang2/Tie2的表达均升高。原位杂交方法显示,与消退期血管瘤相比,增生期血管瘤的Ang2及Tie2基因的mRNA水平增高4。以上均表明Ang/Tie2体系可能在血管瘤的血管生成过程中起重作用。正常情况下,Ang1起维持血管稳定的作用,由于Ang1表达降低,而Ang2表达增高,血管正常的稳定状态被削弱,血管内皮细胞与外周细胞之间的相互联系也被破坏,导致血管基底膜与细胞外基质裂解,血管内皮与基质处于不稳定状态,协同VEGF等生长因子的作用,内皮细胞迁移、增殖活跃,内皮细胞大量堆积,最终导致血管瘤的发生。故推测,失衡表达的Ang/Tie2体系,自身被激活,在众多生长因子的协同作用下,血管内皮细胞异常增殖并大量堆积,导致脉管系统形成紊乱,最终诱导婴幼儿血管瘤的形成。Wang等41报道显示人类肌间血管瘤上发现内皮细胞有突变的Tie2过表达,并且该内皮细胞增殖增加。Boye等42研究发现原代和传代的血管瘤内皮细胞较人类皮肤毛细血管内皮细胞增殖率和迁移率均增高。以上研究结果均表明内皮细胞自身出现异常是血管瘤发生的根本原因。由于基因突变,Ang/Tie2通路被激活,导致内皮细胞增殖紊乱,最终诱导血管瘤形成。但具体的突变基因及突变位点目前还尚未被证实。
  综上所述,血管瘤治疗方法繁多,目前国内、外仍没有一种药物或治疗手段能根治各类型的血管瘤,多数药物的机制尚不明确。怎样在治疗中做到治疗确切、副作用小是目前急需解决的一大难题,还有待于医学工作者的不断探索。选择效果更好,副作用更小的药物将会是以后治疗研究的方向。分子治疗、基因治疗是医学治疗的新的方向。婴幼儿血管瘤的病理生理学机制与Ang及其受体Tie2系统密切相关。目前血管瘤的治疗都存在一定的局限性,随着基因及蛋白组学的进步,可能通过基因调控和特异性抗体对Ang及其受体的作用,为肿瘤患者的治疗供了新希望和新的突破。 
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