Abstract
增生性瘢痕是受损皮肤的细胞外基质过度堆积导致的病理性修复结果, 不同程度地影响患者的外观和功能。瘢痕形成的程度与创面愈合过程中炎症反应的强弱直接相关, 过度或延长的炎症反应会增加增生性瘢痕的发生率。白细胞介素6(IL-6)是一种多效性细胞因子, 可参与调控由成纤维细胞、巨噬细胞、角质形成细胞和血管内皮细胞组成的促纤维化网络, 与增生性瘢痕的形成密切相关。该文就IL-6及其信号通路在增生性瘢痕形成中的作用进行综述。
Keywords: 白细胞介素6, 炎症, 纤维化, 皮肤, 增生性瘢痕
Abstract
Hypertrophic scar is a pathological repair result of excessive accumulation of extracellular matrix after skin damage, which affects the appearance and function of patients with varying degrees. The degree of scar formation is directly related to the strength of inflammatory reaction during wound healing, and excessive or prolonged inflammatory response increases the incidence of hypertrophic scars. Interleukin-6 (IL-6) is a pleiotropic cytokine that is involved in regulating the fibrotic network composed of fibroblasts, macrophages, keratinocytes, and vascular endothelial cells, and is closely related to the formation of hypertrophic scars. This article reviews the role of IL-6 and its signaling pathway in hypertrophic scar formation.
Keywords: Interleukin-6, Inflammation, Fibro-sis, Skin, Hypertrophic scar
当成人的皮肤损伤累及深层真皮时, 创面则不能以再生方式填补缺损, 此时的伤口愈合过程可分为4个存在重叠的阶段, 即止血、炎症、增殖和组织重塑。增生性瘢痕被定义为凸起于皮肤表面, 但局限于原有皮损范围的病理性修复结果。增生性瘢痕具有多个特征, 如ECM大量堆积、Fb增殖分泌能力增强、微血管密度上升等, 且伴瘙痒、疼痛、外观改变和功能障碍, 可不同程度降低患者生活质量[1]。
损伤皮肤的炎症反应与瘢痕的形成程度呈正相关。在创面修复的早期, 一方面炎症反应能够清洁伤口、清除异物并预防感染;另一方面, 炎症细胞产生的多种炎症介质可以促进Fb的增殖, 促进瘢痕组织的产生从而影响修复的最终结果[2]。在创面愈合的任何阶段, 细胞因子的失衡均可导致异常瘢痕形成。目前普遍认为, 过度或延长的炎症反应会增加增生性瘢痕的发生率, 而抗炎治疗可以有效抑制增生性瘢痕形成。
IL-6是一种存在于免疫系统中的多效性细胞因子, 在创面愈合中发挥着重要作用。IL-6也能够通过调节细胞的生物学行为影响瘢痕形成[3]。近年来, 研究者们逐渐将目光聚焦于IL-6与增生性瘢痕发生的关系及其治疗策略[4], 本文就IL-6及其信号通路在增生性瘢痕形成中的作用进行综述。
1. 炎症反应与增生性瘢痕的关系
当皮肤创面较大、较深时, 炎症反应加重、持续时间延长。临床数据显示, 随着创面再上皮化时间的延长, 患者罹患增生性瘢痕的风险呈逐渐上升趋势[5]。伤口愈合过程中, 增加或延长局部网状真皮的炎症反应的因素包括:(1)机械力。局部机械力是导致病理性瘢痕形成的重要因素之一, 如瘢痕疙瘩常出现在张力较大的胸壁、肩胛等部位。有限元分析显示, 人瘢痕生长方向、严重程度常与创面周围皮肤张力方向一致[6]。Wong等[7]观察到, 小鼠体内的机械刺激强度与Th2细胞因子和趋化因子表达程度密切相关, 而IL-6的高表达将促进T细胞向Th2表型分化;T细胞缺陷小鼠由于不能响应机械力而募集全身性炎症细胞导致其增生性瘢痕面积仅为野生型小鼠的1/9。(2)感染。创面感染时细菌分泌的毒素可加重组织损伤并延长创面炎症反应阶段, 导致创面再上皮化延迟及瘢痕增生, 如人体毛囊炎、痤疮和烧伤后感染会增加增生性瘢痕风险[4, 8]。(3)重复损伤。受到二次损伤的创面组织受损程度会加重, 细胞可通过损伤相关分子模式(DAMP)激活免疫细胞。除前述局部因素外, 高血压等全身性因素也可加剧增生性瘢痕的形成[9]。
为探究愈合早期过度炎症反应与增生性瘢痕的关系, Qian等[10]使用热灭活的铜绿假单胞菌和真皮匀浆刺激兔耳腹侧创面模拟病原相关分子模式(PAMP)和DAMP, 结果显示兔瘢痕组织中有多形核白细胞浸润伴组织髓过氧化物酶活性升高, 提示早期过度炎症反应可延迟创面愈合、加重瘢痕形成;同时, 经前述刺激的兔组织中仅有IL-6表达明显升高。有研究显示, 受到相同刺激时病理性瘢痕患者真皮中的促炎基因表达明显高于生理性瘢痕患者, 而过度炎症反应导致的高增生性瘢痕风险可能主要由IL-6介导[11]。
2. IL-6在增生性瘢痕中的作用
在众多炎症因子中, IL-6促进人增生性瘢痕形成的效应最为明确, 在增生性瘢痕中IL-6含量显著高于生理性瘢痕[12-13]。妊娠早期, 胎儿皮肤损伤时以无瘢痕愈合为主。Liechty等[14]观察到, 将成人和胎儿游离皮肤移植在免疫缺陷小鼠的全层皮肤缺损创面中, 移植的胎儿皮肤在术后4 h表达IL-6, 术后12~72 h均未检出IL-6, 而移植的成人皮肤在术后0~72 h均持续高表达IL-6;此外, 将外源性IL-6重组蛋白注射在移植胎儿皮肤的小鼠全层皮肤缺损创面可导致瘢痕形成。刘娜等[15]构建了机械张力诱导的小鼠增生性瘢痕模型, 经尾静脉注射IL-6单克隆抗体后, 小鼠瘢痕组织中胶原含量明显降低, 瘢痕形成减轻。目前瘢痕的诸多治疗方法, 如激光治疗、局部注射糖皮质激素和5-氟尿嘧啶等药物治疗的作用机制均与IL-6表达下调有关[16]。上述研究均证实IL-6在瘢痕过度增生过程中起重要作用。此外, 增生性瘢痕的核心是大量肌Fb表型的出现及ECM过度堆积, IL-6在由Fb、巨噬细胞、KC和血管内皮细胞组成的皮肤促纤维化网络中占据中心地位(图 1)。
图 1.
IL-6在由成纤维细胞、巨噬细胞、角质形成细胞和血管内皮细胞组成的皮肤促纤维化网络中占据中心地位
注:IL为白细胞介素, TNF-α为肿瘤坏死因子α, TGF-β为转化生长因子β, PDGF为血小板衍生生长因子, MCP-1为单核趋化蛋白1, VEGF为血管内皮生长因子, KGF为角质形成细胞生长因子;↑表示上调或升高
3. IL-6信号通路及分子机制
IL-6与其受体结合可诱导六聚体复合物[IL-6、IL-6受体(IL-6R)和糖蛋白130, 各2个分子]的形成, 复合物中的糖蛋白130可激活Janus激酶/信号转导及转录激活因子(STAT)信号通路和促分裂原活化的MAPK信号通路, 进而诱发多种生物学效应[17]。IL-6与膜受体结合激活的通路被称作经典信号通路, 与血液中的可溶性IL-6R(sIL-6R)结合激活的通路则被称作反式信号通路, 这种反式信号通路可增加IL-6的生物利用度[18]。人体中的sIL-6R主要由解整合素金属蛋白酶(ADAM)17/10蛋白水解后经胞膜脱落, 较少由mRNA选择性剪接产生。研究表明, 趋化因子IL-8及多种炎症因子可促进人ADAM17分泌和sIL-6R产生, 而局部促炎刺激可增强IL-6反式信号通路[19]。尽管IL-6与sIL-6R及IL-6R有同等的亲和力, 且经典信号通路和反式信号通路具有相同的信号转导途径, 但IL-6与sIL-6R或IL-6R结合所介导的反应不尽相同。有研究表明, 经典信号通路主要介导IL-6的抗炎活性, 而反式信号通路表现为促炎活性且与机体的多种病理改变有关[20-21]。IL-6高表达与瘢痕疙瘩、系统性硬化症、肺纤维化等疾病相关, 抑制IL-6及其下游信号的药物已被应用于部分纤维化疾病的临床治疗中[21]。人体肝、肾、肺等多种器官纤维化疾病与IL-6反式信号通路密切相关, 通过阻断IL-6/STAT3信号通路可以显著减少这些器官纤维化的发生[22-23]。此外, 近期研究者观察到一种名为“集群信号通路”的新型通路, IL-6与树突状细胞IL-6R结合后可将信号转导给抗原特异性T细胞胞膜上的糖蛋白130, 以促进CD4+T细胞向Th2或Th17表型分化并参与自身免疫反应[24]。
组织受到创伤后, DAMP以及可能存在的PAMP可刺激模式识别受体, 如巨噬细胞、中性粒细胞胞膜上的Toll样受体, 进而启动核因子κB、MAPK等促炎通路[25]。伤口周围巨噬细胞分泌的IL-6可诱导CD4+T细胞向Th2和Th17表型分化, 而Th2分泌的IL-4和IL-13可导致巨噬细胞向M2表型分化, M2型巨噬细胞可通过促进血管内皮细胞分泌单核细胞趋化蛋白1(MCP-1)诱导炎症急性期反应蛋白的产生, 即IL-6最终在急、慢性炎症中均起到重要作用[26]。
4. IL-6与创面愈合相关细胞的相互作用
伤口愈合过程的关键步骤如再上皮化、胶原合成、组织重塑等都与多种细胞密切相关。炎症期, 炎症细胞趋化、浸润可清洁伤口, 预防感染;增殖期, 大量Fb与ECM形成的支架中有大量新血管生成, 可为新生组织提供足量氧气和营养物质, 同时KC的迁移促进了伤口愈合;重塑期, 炎症细胞逐渐减少以及胶原酶分泌增加, 使ECM逐渐降解, 此时的肉芽组织成熟且可转变为纤维结缔组织以维持创面的力学强度, 皮肤原有缺损逐渐被瘢痕组织取代[27]。高水平的IL-6可通过改变各时期创面愈合相关细胞的生物学行为, 破坏ECM的合成-降解平衡, 促进增生性瘢痕形成。
增生性瘢痕形成的关键事件是激活Fb并使其具有过强的增殖、迁移和分泌能力。真皮Fb中缺乏IL-6R, 因而瘢痕组织Fb中STAT3的激活主要通过IL-6反式信号通路实现, 如系统性硬化症患者血清中的IL-6和sIL-6R含量明显上升[28]。Ray等[29]观察到, 人增生性瘢痕Fb中的糖蛋白130及磷酸化STAT3含量上升, 磷酸化STAT3可促进增生性瘢痕Fb发挥过度增殖作用, 而IL-6/STAT3反式信号通路可以抑制Fb的过度增殖和迁移, 使其下调到正常皮肤Fb的水平, 以减少瘢痕组织中ECM的产生。伤口愈合过程中的伤口收缩功能主要由肌Fb执行, 肌Fb胞质中含有α-平滑肌肌动蛋白并具有类似于平滑肌的特性, IL-6能够通过促使Fb旁分泌TGF-β来刺激Fb向肌Fb分化[30]。
在伤口愈合的炎症阶段, 伤口周围组织存在大量M1型巨噬细胞, 而M1型巨噬细胞可产生促炎性细胞因子以清除伤口中的病原体及其碎片;而在伤口愈合的增殖、重塑阶段, 组织中的巨噬细胞则以介导抗炎作用的M2型为主, M2型巨噬细胞可以产生TGF-β等多种生长因子, 促进血管生成和胶原蛋白沉积, 被认为是具有促纤维化作用的细胞表型[31]。IL-6可诱导内皮细胞分泌MCP-1, MCP-1导致的巨噬细胞浸润将加重增生性瘢痕形成, 而在伤口愈合的炎症期或增殖期通过消耗巨噬细胞均可减轻瘢痕形成[31-32]。IL-6可促使巨噬细胞向M2型极化, M2型巨噬细胞分泌的以TGF-β为主的促纤维化分子可提高Fb的增殖、分泌能力, 最终导致ECM堆积和瘢痕增厚[33]。
KC与伤口愈合密切相关, 而IL-6可通过促进间质细胞分泌生长因子使KC增殖、迁移能力增强及制瘤素M分泌增多, 进而激活Fb的STAT3促纤维化信号通路, 最终导致增生性瘢痕的形成[34-35]。此外, 过量血管生成也与增生性瘢痕形成密切相关, IL-6可使血管内皮细胞高表达MCP-1、IL-8和IL-6以增强炎症反应;IL-6还可以增强巨噬细胞、KC和Fb中的VEGF表达, 最终导致瘢痕增生[26, 36]。
5. 总结及展望
IL-6可通过调控参与创面愈合的相关细胞的生物学行为来影响瘢痕形成, IL-6的过表达可导致瘢痕增生, 因此通过减少局部IL-6含量可减轻瘢痕形成。IL-6基因的敲除也可使创面愈合延迟, 但IL-6介导的部分信号转导通路在增生性瘢痕形成中的机制尚不明确, 这对创面愈合过程中选择干预时机及干预何种IL-6通路的抑制剂提出了挑战[37]。IL-6作为一种促炎、促纤维化的细胞因子, 在皮肤创面愈合过程中的作用及其机制较为复杂, 值得深入研究。
Funding Statement
武汉大学中南医院科技创新培育基金(znpy2019083)
Science and Technology Innovation Cultivation Fund of Zhongnan Hospital of Wuhan University (znpy2019083)
Footnotes
利益冲突 所有作者均声明不存在利益冲突
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