王 颖，韩秀萍

0 引言

甘草酸苷(Glycyrrhizin，GL)又称甘草酸、甘草皂苷或甘草甜素，是甘草的主要活性成分，具有保护肝细胞、抗炎、抗病毒、免疫调节以及抗肿瘤等多种药理作用和生物学功能。甘草酸苷的疗效虽已在临床应用中得到广泛认可，但其作用机制仍不完全清楚，现就近年来甘草酸苷作用机制的研究进展做一简要综述。

1 甘草酸苷的结构

甘草酸苷是从甘草根部提取的三萜苷(图1)[1]，占甘草总提取物的3.63%～13.06%，是甘草的主要活性成分。甘草酸苷可被肠内 β-D-葡萄糖醛酸糖苷酶水解为一分子18β-甘草次酸和两分子葡萄糖醛酸[2-3]。18β-甘草次酸具有类皮质醇样结构[4]，可通过抑制11β-羟基固醇脱氢酶的活性模拟皮质醇的作用[5-6]。

图1 甘草酸苷的化学结构

2 甘草酸苷的作用机制

2.1 抗炎作用 目前已知的甘草酸苷抗炎作用机制主要有2种，一种为通过抑制花生四烯酸的代谢水平，抑制细胞色素C(CytC)的释放，从而抑制磷脂酶A2(PLA2)的活性，抑制补体激活；一种为非特异性的抗炎及保护细胞膜的作用，抑制热休克蛋白90(HSP-90)的表达，从而降低糖皮质激素受体与配体的亲和力[7-10]。

2.1.1 肝细胞保护作用 肝脏是机体最大的代谢器官和重要的免疫器官，其解毒和吞噬功能对机体有重要保护作用。各种致病因子作用于肝组织后，可直接导致肝细胞不同程度的损伤，还可通过自分泌和旁分泌作用引起细胞因子网络的激活，引起肝细胞的损伤、肝纤维化等，晚期则发展为肝硬化和肝癌。甘草酸苷被广泛应用于治疗肝细胞损伤和慢性肝炎[11-13]。

肝脏发生炎症时，会伴有环氧化酶-2(COX-2)、肿瘤坏死因子-α(TNF-α)、白介素-6(IL-6)以及诱生型一氧化氮合酶(iNOS)水平的升高，进而引起肝细胞炎症和凋亡。甘草酸苷可降低这些炎症因子的表达，抑制核因子-κB(NF-κB)的活性，减轻TNF-α-MAPK-NF-κB 途径所致的肝细胞损伤和凋亡[14-16]。另有研究发现，长时间肠外营养可引起严重的肝脏疾病，预防性的甘草酸苷治疗，可下调内质网应激因子，如c-Jun氨基末端激酶(JNK)、p38丝裂原活化蛋白激酶(p38 MAPK) 和C/EBP环磷酸腺苷反应元件结合转录因子同源蛋白(CHOP)的水平，减轻肝损伤[17]。动物实验发现，在代谢综合征中存在肝功能异常，包括丙氨酸氨基转移酶(ALT)、天冬氨酸氨基转移酶(AST)和谷氨酰转肽酶(GGT)的水平升高，给予甘草酸苷后，这些指标可恢复正常。对肝脏组织检查发现，用药后肝细胞线粒体中活性氧簇(ROS)减少，说明甘草酸苷可以改善线粒体的氧化性应激[18]。这种保护机制同样发生于丙型肝炎病毒感染的肝脏组织[19]。

2.1.2 其他抗炎作用 近年来，甘草酸苷作为高迁移率族蛋白-1(HMGB1)的天然抑制剂，其抗炎作用越来越受到关注。HMGB1是核内高度保守的非组蛋白染色体蛋白，主要稳定核小体的结构并调节基因转录。HMGBl可以主动或被动从细胞中释放，作为细胞因子和内源性信号分子与多种细胞表面不同的受体如高级糖化终产物(RAGE)[20]，Toll样受体2(TLR2)[21]、TLR4[22]结合，活化下游信号，如p38MAPK[23-24]、pJNK[25]和NF-κB，进而上调半胱天冬酶-3(Caspase-3)[26-27]、TNF-α、IL-1、IL-6的水平，引起炎症反应，加剧细胞损伤和凋亡。甘草酸苷可直接与HMGBl结合，抑制其细胞因子的活性[28]，在感染、烧伤、组织器官缺血性损伤、胰腺炎和溃疡性结肠炎等炎性疾病中发挥治疗作用。

另有研究发现，甘草酸苷可以降低细胞膜脂质筏胆固醇含量，并活化ATP结合盒转运体A-1(ABCA1)降低细胞膜脂质筏的胆固醇外流，从而干扰TLR4向脂质筏转移，最终减少NF-κB和干扰素调节因子-3(IRF3)表达，下调炎性因子的水平，减轻炎症反应[29-30]。甘草酸苷还可以使单核细胞趋化蛋白-1(MCP-1)、巨噬细胞炎性蛋白-2(MIP-2)和中性粒细胞趋化因子-2(CINC-2)表达下降，减轻炎性细胞浸润，改善预后[31-32]。

2.2 抗病毒作用 早在1979年，Pompei等[33]发现甘草酸有抗病毒活性，能抑制很多不同的DNA、RNA病毒的生长，并且不影响正常细胞的活性，还可以不可逆钝化单纯性疱疹病毒颗粒。另有研究通过免疫荧光和电子显微镜观察发现，甘草酸苷作用于感染丙型肝炎病毒(HCV)的Huh7细胞后，细胞外病毒量下降而细胞内的病毒量上升(HCV核心抗原和电子致密颗粒在内质网聚集)，说明甘草酸苷虽然不能阻断HCV的入侵，但是可以抑制受感染的细胞释放病毒颗粒[34]，并且在一定程度上可以抑制HCV的复制[35]。甘草酸苷虽然没有清除病毒的作用，但是可以通过阻断NF-κB抑制剂IκBκ的降解，下调炎性因子TNF-α、IL-1β和IL-6的水平，明显改善柯萨奇病毒B3(CVB3)所致的病毒性心肌炎[36]，还可通过减少趋化相关因子CXCL10、CCL5[37]、CXCL2和CCL2的表达，减轻炎症反应和细胞凋亡[38-39]。

2.3 免疫调节作用 甘草酸苷具有非特异性免疫调节作用，可以通过对T细胞亚群的调节，促进胸腺外T淋巴细胞分化[40]，活化NK细胞[41]，诱导干扰素-γ(IFN-γ)及调节细胞因子水平等发挥作用。

甘草酸苷可以剂量依赖性地下调过敏性鼻炎小鼠模型IgE、IL-4、IL-5、IL-6、NO、TNF-α和P物质的水平，上调IL-2、IL-12的分泌，增强乙酰胆碱酯酶(AchE)的活性，改善过敏反应[42-43]。Bordbar等[44]和Hua等[45]研究了甘草酸苷对小鼠不同来源树突状细胞成熟及功能的影响，发现甘草酸苷作用后，树突状细胞表达CD40、CD86和MHC-Ⅱ增加，分泌IL-10和IL-12增多，分泌TNF-α减少，说明甘草酸苷对树突状细胞有正向的促成熟作用。这种树突状细胞与同源的T细胞共培养后，T细胞分泌IFN-γ、IL-10水平增加，而分泌IL-4的水平下降，说明甘草酸苷可以增强1型辅助性T细胞(Th1)为主的免疫反应。在刀豆球蛋白A(Con.A)诱导的肝纤维化小鼠模型中，甘草酸苷通过调节Th1/Th2和调节性T细胞(Treg)/Th17的平衡，使免疫反应向Th1及 Treg细胞偏斜，显著上调IFN-γ和IL-10的水平。进一步的研究发现，这种作用可能是通过抑制JNK、细胞外调节蛋白激酶(ERK)和磷脂酰肌醇3-激酶/蛋白激酶B(PI3K/AKT)的磷酸化实现的[46]。

2.4 抗肿瘤作用 甘草酸苷抗炎、抗氧化应激及免疫调节等作用，使其可以用于预防和治疗各种慢性炎症所诱发的癌变。1项10年的甘草酸苷治疗丙型肝炎的临床研究显示，甘草酸苷不仅可以改善患者肝功能，还能够降低肝癌的发生率。进一步研究显示，甘草酸苷可上调抑癌基因jun-B的表达，减轻慢性肝损伤[47]，促进肝癌细胞线粒体膜去极化，上调p53、CytC、B淋巴细胞瘤-2基因(Bcl-2)相关X蛋白(Bax)的表达，促进肝癌细胞凋亡[48-49]。肺腺癌细胞系A549和NCI-H23高表达血栓素合酶，甘草酸苷可抑制其表达和活性，从而抑制肺腺癌细胞的生长[50]。动物实验证实，甘草酸苷通过下调由幽门螺杆菌及高盐饮食导致的COX-2、iNOS、血管内皮生长因子(VEGF)和IL-8水平升高，减轻慢性萎缩性胃炎症状，降低胃癌的发生率[51]。甘草次酸可增加细胞间质中酪氨酸mRNA的水平，通过激活转录机制诱导黑色素细胞B16中黑色素生成，加速肿瘤细胞老化，起到抗黑色素瘤作用[52]。另有研究显示，甘草酸苷可使人宫颈癌HeLa细胞核中对硫氧还蛋白(Trx)表达下降，降低肿瘤细胞的耐药性[53]。

癌细胞在抗癌药物的作用下，在发生坏死的同时释放HMGB1。HMGB1作为前炎症因子，可以促进血管内皮细胞增生和癌细胞迁移，上调TNF-α水平，造成对正常组织和细胞的损伤，甘草酸苷与HMGB1特异性地结合，抑制其细胞因子活性，可改善由抗癌药物引起的炎症反应[54]。

2.5 不良反应 众所周知，假性醛固酮增多症，主要表现为钠潴留、低钾血症和高血压等，是甘草酸苷的主要不良反应。Stewart等[55]研究表明，此现象是由18β-甘草次酸引起的，后者可抑制11β-羟基固醇脱氢酶在肾脏的代谢。11β-羟基固醇脱氢酶有2种亚型，肾型11β-脱氢酶和肝型11β-氧还原酶。肾型11β-脱氢酶可将皮质醇转化为肾上腺激素，肝型11β-氧还原酶则催化相反的反应。18β-甘草次酸抑制肾型11β-脱氢酶的代谢，导致肾脏皮质醇水平增加。又由于皮质醇和醛固酮与盐皮质激素受体有相同的亲和力，必然导致高盐皮质激素效应，进而抑制肾素-血管紧张素-醛固酮系统(RAAS)，引起肾素血管紧张素Ⅰ和醛固酮水平下降、钠潴留及尿钾增多。

然而，一项甘草酸苷应用于对干扰素联合利巴韦林治疗无效的丙型肝炎患者的有效性及安全性的Ⅲ期临床研究显示，经静脉给药200 mg/d，共10周后，患者ALT明显下降，肝细胞坏死及肝脏纤维化明显减轻，同时，有5.5%的患者出现不同程度的不良反应，主要为高血压和/或低钾血症[56]。另一项应用甘草酸苷治疗急性发作性自身免疫性肝炎的研究发现，甘草酸苷经静脉给药100 mg/d，在发病早期即可控制转氨酶的水平，并且与甘草酸苷联合皮质醇治疗组相比有更好的预后，而使用该剂量共4周的患者未出现明显的不良反应[57]。可见，甘草酸苷的不良反应是剂量和时间依赖性的，合理和适当地应用是比较安全和可耐受的。

3 展望

甘草因其和百药解百毒，在中药处方中最为常用。甘草酸苷是从甘草根部提取的主要活性成分。多年来，甘草酸苷制剂，如：复方甘草酸苷片、复方甘草酸苷注射剂等，在临床上广泛用于治疗各种急、慢性炎症和免疫性疾病，如病毒性肝炎、自身免疫性肝炎、肝硬化、湿疹、慢性荨麻疹、银屑病、斑秃、溃疡性结肠炎等，其疗效已经得到肯定。随着对其临床应用和作用机制的不断深入研究，人们对甘草酸苷的认识和使用将更为科学、有效和多样。

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