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瘢痕疙瘩发病的分子机制研究进展

来源:用户上传      作者: 刘畅 陈敏亮

  瘢痕是创伤愈合的必然结果,可一旦增生过度,尤其是在面颈部及四肢等关节部位不仅影响外观而且会引起功能障碍,给患者身心健康带来严重危害。而瘢痕疙瘩(keloid,KD)为病理性瘢痕的一种,是由于胶原过度沉积而导致的纤维增生性疾病,常超过损伤边界的正常皮肤,伴瘙痒和疼痛,由于KD的发病机制十分复杂,目前对它的治疗仍无突破性进展。近年来,大量的与KD相关的基因数据被报道,而本文就与KD相关的信号通路及功能基因的研究进展综述如下。
  1 凋亡和促蛋白激酶(Mitogen-activated protein kinase,MAPK)信号通路
  细胞凋亡是KD发生的重要机制之一。Nassiri 等[1]选择了三条相互联系的信号通路,涉及凋亡,细胞因子和核因子kB (Nuclear factor-kappa B, NFkB) 通路,将KD组织与正常皮肤组织进行微阵列分析观察基因表达情况,发现了9个在KD中高表达的基因,其中4个基因涉及细胞因子或细胞因子间的交互,4个基因参与了MAPK信号通路,2个参与了与细胞凋亡重要相关的caspase信号通路。边曦等[2]通过研究发现共有34个相关基因参与调控KD的细胞凋亡,其中包括上调的p53、caspase-14和下调的caspase-6。Xia等[3]和Kuo[4]均提出MAPK信号通路参与了KD的发病机制,但具体机制仍未清晰。Xia等[3]发现通过体外血清刺激,KD成纤维细胞中p38 MAPK磷酸化而正常皮肤成纤维细胞并未出现p38 MAPK的磷酸化,而p38 MAPK的高表达又促进了KD中TGF-β2的表达,并且发现阻断p38 MAPK 的表达亦会阻断TGF-β2的表达。Kuo等[4]发现p38 MAPK抑制剂可以下调TGF-β1的表达。
  2 转化生长因子-β(TGF-β,transforming growth factor-β)信号通路
  TGF-β主要参与调节细胞增殖和分化,参与胚胎发育调节,促进细胞外基质形成和抑制免疫反应等,基质金属蛋白酶-2( matrix metallo- proteinase, MMPs),MMP-9[5],纤溶酶[6]被认为是TGF-β的激活剂。TGF-β在哺乳动物中有3个亚型,分别是TGF-β1,TGF-β2和TGF-β3,不同亚型在参与创伤愈合过程中所需不同的受体和具有不同的生物作用。TGF-β1和TGF-β2在KD中表达增高[7]且被认为促进纤维化及瘢痕形成,而TGF-β3被发现在小鼠线性瘢痕模型中减少瘢痕形成[8]以及在胎兔外伤模型中与增加伤口周围组织挛缩有关[9]。Seifert等[10]提出在KD中造成TGF-β表达增加促进细胞外基质沉积增加的原因,可能是缺乏SMAD6和SMAD7的抑制剂并与SMAD信号通路相关。有大量研究试图建立KD与TGF-β1,TGF-β2,TGF-β3,TGF-βRⅠ, TGF-βRⅡ, TGF-βRⅢ和SMAD3,SMAD6,SMAD7之间的关系[11-15]。Fujiwara[16]和Lee[17]的研究均发现KD中TGF-β1的基因和蛋白表达水平较正常皮肤中表达增高。而Bayat[11]却发现KD患者与对照组血液中TGF-β1表达量无统计学差异。同样在KD中TGF-β2,TGF-βRⅠ和TGF-βRⅡ的蛋白表达水平亦较对照组增高[17]。通过微阵列研究发现TGF-βRⅢ在KD组织中低表达[18]。孙慧娟等[19]发现miRNA-200c明显抑制经TGF-β1诱导的人KD成纤维细胞的增殖和胶原合成;亦能明显降低磷酸化SMAD2和SMAD3的蛋白表达水平及抑制博来霉素诱导的TGF-β1的分泌,故而推测miR-200c抑制人KD成纤维细胞增殖及胶原合成可能是通过靶向抑制TGF-β/SMAD通路上的关键因子实现的。除了关注TGF-β信号通路在KD组织及细胞中的表达情况,亦有研究提出TGF-β信号通路与KD异常的表皮-间质相互作用的发病机制相关[20]。另有研究关注于KD成纤维细胞中TGF-β与Fas基因相关的细胞调亡[21],以及TGF-β与细胞外基质沉积的关系[22]。
  3 细胞外基质蛋白及其降解
  KD的一大特点是细胞外基质的过度沉积,主要是大量细胞外基质蛋白的过表达,包括Ⅰ型胶原(type I collagen),纤连蛋白和弹性蛋白等[7]。PAI-1是与KD发生过程中细胞外基质产生及其降解的重要信号通路。在PAI-1基因敲除小鼠中发现其较野生型小鼠肺纤维化明显减少[23]。PAI-1抑制尿激酶纤维蛋白溶酶原( urokinase-mediat- edplasminogenactivation, uPA)的激活,uPA参与纤维蛋白降解,在组织塑形、细胞迁移和血管化过程中起重要作用。通过抑制MMP的表达PAI-1的表达降低从而影响KD细胞外基质的产生。Tuan等[24]在无血清培养条件下,观察到正常皮肤成纤维细胞纤维蛋白发生降解,而KD成纤维细胞并无这一现象。与此同时正常皮肤成纤维细胞有高水平的uPA和低水平的PAI-1表达,而瘢痕疙瘩成纤维细胞中这两个蛋白出现相反的表达水平[24-25]。此外,在培养基中加入TGF-β1,正常成纤维细胞中纤维蛋白降解被抑制[24]。Tuan等[26]通过小RNA抑制过表达PAI-1的正常成纤维细胞及瘢痕疙瘩成纤维细胞中PAI-1的表达,发现抑制PAI-1的表达可以降低正常或瘢痕疙瘩成纤维细胞中胶原蛋白的过度沉积积聚。在瘢痕疙瘩中异常表达的众多基因如TGF-β,胰岛素生长因子-1(insulin growth factor, IGF)和血小板衍生生长因子(PDGF)均可调节PAI-1基因的表达。IGF-1受体亦在KD中过表达并有增强瘢痕疙瘩成纤维细胞侵袭的作用[27]。除了生长因子及细胞因子等对于PAI-1表达的促进作用,缺氧同样促进PAI-1的表达[28]。在KD患者自身病变组织与正常组织比较发现中PAI-1和HIF-1A在病变组织中表达增高[18]。抑制PAI-1和HIF-1A的表达可以减少瘢痕疙瘩成纤维细胞中的胶原沉积[25]。研究发现有众多信号通路参与到HIF-1A的激活,包括ERK1/2,PI3-K/Akt和PTKs[29]。Wu等[30]发现血管内皮生长因子(endothelial growth factor ,VEGF)是HIF-1A的一个靶基因,VEGF不仅在瘢痕疙瘩成纤维细胞中蛋白和基因水平均增高,且其促进PAI-1的表达与剂量-时间成正比相关。   4 脂质代谢及机械力
  脂质是细胞膜的必要组成部分,脂质及其内源性的代谢产物可作为抗炎因子及二级信使,从而推断在KD的发病过程中,脂质代谢通过影响细胞内信号的传导从而致病[31]。KD是一种纤维过度增生的皮肤疾病,且其与局部的机械张力密切相关,而力传导的过程是机械外力转化为生化信号,进而参与细胞反应。在KD的发生过程中,参与力传导过程的信号通路有TGF-β/Smad,MAPK,integrin,RhoA/ROCK,Wnt/β-catenin和TNF-α/NF-κB 信号通路[32]。
  目前,随着分子生物学、遗传生物学和细胞生物学的发展,使得我们对于KD的发病机制有了较深刻的理解,但多种生物学及遗传学因素影响着瘢痕疙瘩的发生过程,各种生物活性因子间的相互作用相互影响,形成错综复杂的网络关系。对于KD发病过程中多条信号通路及重要分子功能的深入研究,进一步分析有助于揭示KD的发病机制,并为临床治疗提供新的思路和方向。
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