2016, Vol. 25
MicroRNAs (miRNAs)是一类约由21~25个核苷酸组成的内源性非编码单链RNA，其主要通过与靶mRNA的3’端非转录区互补配对后，切割或抑制靶mRNA翻译，从而影响蛋白质的表达。近来许多miRNAs被证实可调节TLRs信号通路所涉及的分泌蛋白及调节分子，主要包括以下三个机制：(1)直接作用于TLRs信号通路中的组成元件；(2)直接激活TLRs；(3)其表达受TLRs调节[4]。
2 脓毒症中的LPS-TLRs信号通路Toll样受体(toll-like receptors, TLRs)是一类识别病原体相关分子模式(pathogen associatedmolecule patterns，PAMPs)的跨膜蛋白，在脓毒症抗炎免疫机制中起着重要作用。其中，TLR4是识别脂多糖(lipopolysaccharide，LPS)的关键识别受体，LPS释放入血后，迅速与LPS结合蛋白(LPS-binding protein, LBP)结合[5]，通过糖基磷脂酰肌醇锚定于细胞膜与TLR4结合，经髓样分化因子(myeloid differentiation factor88, MyD88)依赖性或非依赖性途径[6]激活细胞内信号级联反应, 产生炎性介质，进一步发展为脓毒症，最终导致全身多器官功能障碍[7]。
3 与脓毒症相关的miRNAsmiRNAs不仅直接参与调控抗炎/促炎动态平衡，还可通过直接调节TLRs信号通路中的某些关键分子来间接调节二者间的平衡。O’Connell等[8]发现，LPS处理单核细胞可刺激miR-155和miR-146的表达上调，miR-146a可作用于TLRS信号通路中的IRAK1激酶和TRAF6连接酶，抑制通路活化[9]。miR-155可作用于FADD, IKKβ, IKKε, 以及Ripk1等TLRs信号途径中的蛋白[10]等。由此可见，miRNAs在脓毒症发生发展过程中扮演的角色远比早前设想的更为重要。
3.1 miR-146amiR-146a是最早证实与炎症相关的miRNAs之一。LPS处理单核细胞后，miR-146a可经由NF-κB的信号通路应答各种免疫介质时诱导[10-11], 于2 h后持续升高[12-14]；而Wang等[15]发现脓毒症患者血清内miR-146a表达降低，这一差异现象有待进一步探讨。
miR-146可作用于心肌细胞内IRAK和TRAF6，抑制NF-κB活性以及炎症因子的产生，从而减轻脓毒症导致的心功能障碍[16]；在肺组织中上调的miR-146a可抑制TNF-α的释放，减轻脓毒症致肺损伤的炎症反应[17]。Yang等[18]发现miR-146a通过负性调节TLR4从而调节氧化低密度脂蛋白(oxLDL)的累积及炎症反应。
3.2 miRNA-1552006年Baltimore Lab首次记录到在LPS处理后的人单核细胞内miR-155表达水平明显升高[10]，而后发现TLR4信号通路和潜伏膜蛋白1(LMP1)均可诱导其表达[8, 19]。研究表明，富含肌醇-5’ -磷酸酶的SH2域(SHIP1)和细胞因子信号1抑制物(SOCS1)是miR-155的重要作用靶点[20-21]，当IL-10抑制miR-155转录时，SHIP1表达升高[22]。李芮等[23]发现在内毒素血症小鼠肺组织中miR-155表达升高，抑制miR-155能减轻小鼠肺损伤。
用LPS处理miR-155缺失的B细胞及miR-155基因敲除小鼠中, TNF-α并未升高，证实miR-155可有助于TNF-α mRNA的稳定，但目前尚未证实miR-155可直接与TNF-α mRNA结合。Tili等[24]发现miR-155通过作用于FADD和RIPK1的转录从而间接促进TNF-α的翻译。
3.3 miR-125bTili等[24]用LPS刺激小鼠巨噬细胞系RAW264.7后，发现miR-125b表达水平明显下调，与miR-155呈现相互拮抗的作用，二者对TNF-α调节作用相反。研究发现，miR-125b可直接作用于TNF-α mRNA的3’端非转录区，也可加速TNF-α mRNA的脱腺苷化[24-25]。因此，miR-125b可能通过两条不同的机制下调TNF-α的表达。Kim等[26]发现肌成束蛋白下调可反转LPS引起的miR-125 b减少，进一步通过TLR4/PKC通路调节TNF mRNA翻译水平，但此调节作用不如miR-155显著。
3.4 miR-21miR-21在多种炎性状态下表达都有升高，如LPS刺激引起的肺炎、皮癣性关节炎、溃疡性结肠炎、心肌损伤等[27]。LPS处理巨噬细胞后，miR-21表达较迟，是一个慢反应miRNA。miR-21在脓毒症前期表达升高，晚期维持稳定[28]。Sheedy等[29]用RAW264.7细胞实验证实，LPS刺激后miR-21表达上调，miR-21可与PDCD4 mRNA的3’端非转录区结合，从而降低PDCD4的表达。当PDCD4表达缺失时，体内IL-10的水平升高，而NF-κB的激活受到抑制，IL-6的表达水平下降，结果表明，miR-21通过调节PDCD4的表达，在上调抗炎因子表达的同时通过调节NF-κB通路来下调促炎因子的表达而发挥抗炎作用。
3.5 miR-223在粒细胞内，miR-223不仅可调节TLR4和TLR3的表达[30]，而且可作用于IKKα [31]和STAT3，促进前炎症因子IL-6和IL-1β的产生[32]，从而调节信号通路。Wang等[15]发现，发现脓毒症患者组血清miR-223明显低于SIRS组和健康组；另有研究发现在脓毒症患者中，死亡患者体内miR-223水平明显低于存活患者[33]。在miR-223基因敲除小鼠模型中，脓毒症所致心功能障碍，炎性反应以及病死率均增高[34]。因此，低水平miR-223将导致脓毒性死亡，miR-223可能是病情严重程度预测工具[35]。然而，Benz等[36]发现miR-223在LPS注射小鼠体内有所升高，但在CLP小鼠以及ICU脓毒症患者体内并没有升高，miR-223血清水平并不能反应脓毒症的存在。因此，miR-223能否作为脓毒症的有效生物标志物仍有待进一步研究。
4 小结与展望脓毒症可导致多器官功能障碍，如肝、肾、肺、心等，其临床表现和病理生理机制复杂。目前，miRNAs都被推荐用于诊断或预测许多疾病的生物标志物[37-38]。大量研究证实miRNAs可调节炎症反应，但是由于这些研究规模都较小，且考虑到脓毒症的发生发展的复杂性，仍需要更多的前瞻性随机大规模临床试验来证实这些miRNAs作为脓毒症生物标志物的可靠性及准确性。此外，尽管证实miRNAs可调节TLRs信号通路，但在脓毒症模型中没有得到广泛研究。所有的治疗措施旨在通过调控各种通路来抑制炎症反应，从而提高脓毒症的预后，继续检测脓毒症患者细胞内外miRNAs变化仍有重要意义，因为这不仅帮助我们了解脓毒症复杂的病理生理过程，而且给我们提供新的治疗思路。
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