肾素血管紧张素系统(renin-angiotensin systerm,RAS)是机体内一种重要体液调节系统,对维持正常生理功能发挥重要作用。肝纤维化是由肝脏损伤时机体过度修复反应造成肝脏内细胞外基质(extracellular matrix,ECM)的合成与降解失衡所致[ 1 ]。研究发现Ang Ⅱ与肝纤维化的发生、发展有密切联系[ 2,3 ],Ang Ⅱ效应阻滞可能成为抗纤维化治疗的一个有效靶点。 1 RAS及AngⅡ 1.1 RAS的组成
经典的RAS由血管紧张素Ⅱ(angiotensin Ⅱ,Ang Ⅱ)、血管紧张素转换酶(angiotensin converting enzyme,ACE)及血管紧张素受体1(angiotensin receptor 1,AT1R)组成,不仅能够调节控制血管收缩及水电解质的平衡,在炎症、细胞因子分泌、ECM代谢及细胞增殖等方面也具有十分重要的作用[ 4 ]。近年来,RAS的另外1条作用轴ACE2-Ang(1-7)-MasR逐渐被大家所认识,它代表了ACE-Ang Ⅱ-AT1R的主要负性调节机制,在RAS中平衡Ang Ⅱ的生物效应。Zhang等[ 5 ]发现ACE、ACE2、Ang Ⅱ、AT1R、Ang(1-7)和Mas在四氯化碳(CCl4)诱导的纤维化大鼠肝脏内的表达均上调,ACE2将Ang Ⅱ转化为Ang(1-7)从而减弱Ang Ⅱ对肝脏纤维化的促进作用。 1.2 Ang Ⅱ及其受体AT1R
Ang Ⅱ是一个八肽结构,作为RAS系统的主要效应物,具有广泛的生物学效,是一种强效的促肝纤维化因子[ 4 ]。肝脏中RAS系统的生物作用由一系列的后续酶促反应调控:肝脏中的血管紧张素原(angiotensinogen,AGT)被肾素裂解形成血管紧张素Ⅰ(angiotensin Ⅰ,Ang Ⅰ),Ang Ⅰ继而被ACE加工处理生成Ang Ⅱ,Ang Ⅱ的绝大多数作用主要通过多数细胞质膜上都含有的G蛋白偶联受体——AT1R介导,形成经典的ACE-Ang Ⅱ-AT1R活性轴。肝脏损伤后,RAS的一些成分,包括ACE、Ang Ⅱ及AT1R的水平显著增加,并在肝脏纤维化活跃部位聚集发挥效应[ 2 ] 。
2 Ang Ⅱ/AT1R在肝纤维化中的作用 2.1 Ang Ⅱ/AT1R与HSCs
肝星状细胞(hepatic stellate cells,HSCs)的激活在肝纤维化中起关键作用,活化的HSCs是ECM的主要来源。在正常肝脏中,静息态HSCs中的RAS成分水平很低,也不能分泌Ang Ⅱ,肝脏损伤时HSCs激活,活化的HSCs表达ACE和AT1R并产生Ang Ⅱ[ 6 ]。活化的HSCs是肝纤维化的主要效应细胞,Ang Ⅱ能够剂量依赖性地促进活化型HSCs增殖、DNA合成及细胞收缩,诱导HSCs产生一系列的促纤维化反应,包括加速细胞增殖、迁移,合成Ⅰ型和Ⅲ型胶原等ECM成分及促纤维化因子转化生长因子β(Transforming growth factor β1,TGF-β)的表达[ 7,8 ] 。经Ang Ⅱ处理后的HSC-LI90细胞内,α平滑肌肌动蛋白(smooth muscle actin α,α-SMA)、胶质纤维酸性蛋白(glial fibrillary acidic protein,GFAP)和Ⅰ型胶原这些HSCs活化的特异性标志物的表达明显上调[ 9 ] 。
此外,Ang Ⅱ还可以与内皮因子1(endothelin-1,ET-1)共同调节HSCs引起肝纤维化。Ang Ⅱ与AT1R结合后能够通过磷脂酰肌醇-3-激酶(Phosphatidyl inositol-3-kinase,PI3K)/Akt信号通路增加ET-1的表达水平,而ET-1/ET受体A轴能够调节Ang Ⅱ对HSCs向肌成纤维样细胞(myofibroblast-like cells,MFB)的转化[ 10 ]。Okamoto等[ 9 ] 研究认为,肝内胆管癌(intrahepatic cholangiocarcinoma,ICC)组织中高水平的Ang Ⅱ还可以通过Ang Ⅱ/AT1R轴促进癌组织中的ICC细胞和HSCs活化和增殖,使得这两种细胞之间产生协同作用加速间质纤维化及肿瘤进展,引起了ICC的高度恶性。 2.2 Ang Ⅱ/AT1R与NADPH氧化酶
NADPH氧化酶(nicotinamide adenine dinucleotide phosphate oxidase,NADPH oxidase,NOX)介导的氧化应激是肝纤维化的重要调节器,Ang Ⅱ的促纤维化效应正需要NADPH氧化酶及其产生的活性氧簇(reactive oxygen species,ROS)来实现[ 11 ]。研究发现,NOX亚基p47phox基因缺失小鼠实验性肝损伤及肝纤维化程度比正常野生型小鼠明显要低,同时从p47phox基因缺失小鼠体内分离的HSCs对Ang Ⅱ刺激反应也更迟钝[ 11 ]。Ang Ⅱ能够诱导静息和活化型HSCs内细胞外信号调节激酶1/2(extracellular signal-regulate kinases 1/2,ERK1/2)磷酸化,并通过NOX增加活化型HSC内ROS的产生[ 8 ]。随后,Aoyama等[ 12 ]研究表明Ang Ⅱ刺激增加HSC内ROS的产生后,肝脏内胶原蛋白的沉积增加,HSCs内TIMP-1、胶原蛋白1a及NOX4的mRNA表达水平均上调,而它的这些促纤维化效应都依赖于NOX1。有研究人员指出,Ang Ⅱ刺激是通过非吞噬型NOX诱导产生氧化应激而促进肝脏纤维化发生的,他们研究发现丙肝患者,口服洛沙坦治疗后体内主要的非吞噬型NOX(NOXO-1、NOXA-1和Rac1)明显降低,同时Ⅰ型和Ⅳ型前胶原,这两个纤维化肝脏中主要的胞外蛋白的表达也明显减少[ 13 ]。 2.3 Ang Ⅱ/AT1R与 肝纤维化
早期研究发现,Ang Ⅱ是肝纤维化的一个重要调控介质,Ang Ⅱ水平的增加对肝脏炎症、氧化应激及纤维化具有明显促进作用[ 3 ] 。非酒精性脂肪肝(non-alcoholic fatty liver disease,NAFLD)是常见的致肝脏纤维化的肝损伤之一,临床研究显示,在活检证实患有NAFLD的高血压患者中,接受Ang Ⅱ效应抑制剂治疗的患者与不接受Ang Ⅱ效应抑制剂治疗的患者相比,肝脏纤维化分期前者显著更低,这提示肝纤维化需要Ang Ⅱ的参与[ 14 ]。研究表明,Ang Ⅱ/AT1R刺激能够增加JAK2的表达及磷酸化,磷酸化的JAK2随后磷酸化激活下游的RhoA/Rho激酶通路和膜突蛋白,引起HSC活化和肝脏纤维化的发生[ 15 ]。
同时,有观点提出脂多糖(lipopolysaccharide ,LPS) 和TGF-β促纤维化效应依赖于Ang Ⅱ,Ang Ⅱ与它们在肝脏纤维化中发挥协同作用。他们认为,Ang Ⅱ增加活化HSC内TLR4的表达,通过加强LPS-TLR4信号通路表达下调HSCs细胞膜上TGF-β1的抑制性受体类似物BAMBI的表达从而促进TGF-β发挥致纤维化效应[ 16 ]。与此同时,Goto等[ 17 ]研究发现在常染色体隐性多囊肾病/先天性肝纤维化模型大鼠体内ACE-Ang Ⅱ-AT1R轴成分及下游促纤维化介质TGF-β的表达均增加,这提示Ang Ⅱ/AT1R也可能是先天性肝纤维化病情发展中的一个重要调控介质。
3 Ang Ⅱ/AT1R抑制的抗纤维化效应
鉴于Ang Ⅱ在肝纤维化中的重要作用,抑制Ang Ⅱ的生物效应或生成可能将有效减轻肝脏纤维化。研究发现,患有慢性丙肝及高血压的患者接受Ang Ⅱ阻滞剂治疗后肝纤维化程度明显低于没有接受Ang Ⅱ阻滞剂的患者[ 18 ]。利用基因沉默技术降低Ang Ⅱ和AT1R的基因表达后HSC内TGF-β的mRNA水平及与肝脏内胶原的合成相关的前胶原肽Ⅲ、透明质酸、层粘连蛋白的分泌均明显降低[ 19,20 ],并且在AT1R缺失小鼠,JAK2的促HSCs活化及肝纤维化的效应降低[ 15 ]。
越来越多的证据显示,药理性阻滞剂AT1R能有效减弱肝脏纤维化,并且在肝纤维化早期阶段使用效果可能更好[ 7, 10, 21, 22, 23 ]。研究证明,ARB可以抑制HSC的增殖、激活转化为MFB并降低ROS生成 [ 9,24 ]。ARB对各种急慢性血吸虫性肝纤维化均具有良好抗纤维化效应,它能显著降低血吸虫性肝纤维化小鼠MMP-2、TGF-β1 和羟脯氨酸的血清和组织表达水平 [ 22,23 ]。此外,ARB还能有效阻止非酒精性脂肪肝向肝癌发展[ 25 ]。Koganti等[ 26 ]研究发现甲氧雌二醇就是通过抑制AT1R基因转录下调肝脏上皮细胞内AT1R的水平发挥对机体的保护作用。
综上所述,肝纤维化已经成为一个严重影响人们健康的疾病,但是目前仍未有能够显著逆转或治愈肝维化的有效药物。Ang Ⅱ及其受体AT1R在肝纤维化发生、发展中的重要作用与地位,为我们提供了一个潜在有效治疗靶点。然而值得一提的是,Karimian等[ 27 ] 最新研究发现Ang Ⅱ/AT1R能够对胆酸诱导的肝细胞凋亡发挥保护作用,而药理性抑制AT1R后这种保护作用会被减弱,可能给肝实质细胞带来损伤。因此,肝纤维化治疗的仍需要我们进一步的探索研究。
4 利益冲突
所有作者声明本文不涉及任何利益冲突。
[1] | Ghosh A K,Quaggin S E,Vaughan D E.Molecular basis of organ fibrosis: potentialtherapeutic approaches[J].Exp Biol Med,2013,238:461-481. |
[2] | Paizis G,Cooper M E,Schembri J M,Tikellis C,Burrell L M,Angus P W.Up-regulation of components of the renin-angiotensin system in the bile duct-ligated rat liver[J].Gastroenterology,2002,123:1667-1676. |
[3] | Bataller R,Gäbele E,Parsons C J,Morris T,Yang L,Schoonhoven R,et al.Systemic infusion of angiotensin Ⅱ exacerbates liver fibrosis in bile ductligated rats[J].Hepatology,2005,41:1046-1055. |
[4] | Hasegawa H,Komuro I.The progress of the study of RAAS[J].Nihon Rinsho,2009,67:655-661. |
[5] | Zhang W,Miao J,Li P,Wang Y,Zhang Y.Up-regulation of components of the renin-angiotensin systerm in liver fibrosis in the rat induced by CCL[J].Res Vet Sci,2013,95:54-58. |
[6] | Bataller R,Sancho-Bru P,Gines P,Lora J M,Al-Garawi A,Solé M,et al.Activated human hepatic stellate cells express the renin-angiotensin system and synthesize angiotensin Ⅱ[J].Gastroenterology,2003,125:117-125. |
[7] | Sancho-Bru P,Bataller R,Gasull X,Colmenero J, Khurdayan V,Gual A,et al.Genomic and functional characterization of stellate cells isolated from human cirrhotic livers[J].J Hepatol,2005,43:272-282. |
[8] | Moreno-Alvarez P,Sosa-Garrocho M,Briones-Orta M A, González-Espinosa C,Medina-Tamayo J,Molina-Jijón E,et al.Angiotensin Ⅱ increases mRNA levels of all TGF-beta isoforms in quiescent and activated rat hepatic stellate cells[J].Cell Biol Int,2010,34:969-978. |
[9] | Okamoto K,Tajima H,Ohta T,Nakanuma S,Hayashi H,Nakagawara H,et al.Angiotensin Ⅱ induces tumor progression and fibrosis in intrahepatic cholangiocarcinoma through an interaction with hepatic stellate cells[J].Int J Oncol,2010,37:1251-1259. |
[10] | He C,Miao X,Li J,Qi H.Angiotensin Ⅱ induces endothelin-1 expression in human hepatic Stellatecells[J].Dig Dis Sci,2013,58:2542-2549. |
[11] | Bataller R,Schwabe R F,Choi Y H,Yang L,Paik Y H,Lindquist J,et al.NADPH oxidase signal transduces angiotensin Ⅱ in hepatic stellate cells and is critical in hepatic fibrosis[J].J Clin Invest,2003,112:1383-1394. |
[12] | Aoyama T,Paik Y H,Watanabe S,Laleu B,Gaggini F,Fioraso-Cartier L,et al.Nicotinamide adenine dinucleotide phosphate oxidase in experimental liver fibrosis:GKT137831 as a novel potential therapeutic agent[J].Hepatology,2012,56:2316-2327. |
[13] | Colmenero J,Bataller R,Sancho-Bru P,Domínguez M,Moreno M,Forns X,et al.Effects of losartan on hepatic expression of nonphagocytic NADPH oxidase and fibrogenic genes in patients with chronic hepatitis C[J].Am J Physiol Gastrointest Liver Physiol,2009,297:G726-G734. |
[14] | Goh G B,Pagadala M R,Dasarathy J,Unalp-Arida A,Sargent R,Hawkins C,et.al.Renin-angiotensin system and fibrosis in non-alcoholic fatty liver disease[J].Liver Int,2014. |
[15] | Granzow M,Schierwagen R,Klein S,Kowallick B,Huss S,Linhart M,et al.AT1 receptor-mediated Janus-kinase-2 activation induces liver fibrosis[J].Hepatology,2014,60:334-348. |
[16] | Li Y S,Ni S Y,Meng Y,Shi X L,Zhao X W,Luo H H,et al.Angiotensin Ⅱ facilitates fibrogenic effect of TGF-b1 through enhancing the down-regulation of BAMBI caused by LPS:a new pro-fibrotic mechanism of angiotensin II[J].PLoS ONE,2013,8:e76289. |
[17] | Goto M,Hoxha N,Osman R,Wen J,Wells R G,Dell K M.Renin-angiotensin system activation in congenital hepatic fibrosis in the PCK rat model of autosomal recessive polycystic kidney disease[J].J Pediatr Gastroenterol Nutr,2010,50:639-644. |
[18] | Corey K E,Shah N,Misdraji J,Abu Dayyeh B K,Zheng H,Bhan A K,et al.The effect of angiotensin- blocking agents on liver fibrosis in patients with hepatitis C[J].Liver Int,2009,29:748-753. |
[19] | Fu J Y,Pei H D,Xu F F,Lin Z,Chen Y P,Ji L.Down-regulation of angiotensin Ⅱ by shRNA reducescollagen synthesis in hepatic stellate cells[J].Int J Mol Med,2010,25:801-806. |
[20] | Dong P,Yu F,Fan X,Lin Z,Chen Y,Li J.Inhibition of ATIR by shRNA prevents collagen synthesis in hepatic stellate cells[J].Mol Cell Biochem,2010,344:195-202. |
[21] | Moreno M,Gonzalo T,Kok R J,Sancho-Bru P,van Beuge M,Swart J,et al.Reduction of advanced liver fibrosis by short-term targeted delivery of an angiotensin receptor blocker to hepatic stellate cells in rats[J].Hepatology,2010,51:942-952. |
[22] | El-Lakkany N M,El-Maadawy W,Ain-Shoka A,Badawy A,Hammam O,Ebeid F.Potential antifibrotic effects of AT1 receptor antagonist,losartan,and/or praziquantel on acute and chronic experimental liver fibrosis induced by Schistosoma mansoni[J].Clin Exp Pharmacol Physiol,2011,38:695-704. |
[23] | Attia Y M,Elalkamy E F,Hammam O A,Mahmoud S S,El-Khatib A S.Telmisartan,an AT1 receptor blocker and a PPAR gamma activator,alleviates liver fibrosis induced experimentally by Schistosoma mansoni infection[J].Parasites Vectors,2013,6:199. |
[24] | Yaguchi S,Ogawa Y,Shimmura S,Kawakita T,Hatou S,Satofuka S,et al.Angiotensin Ⅱ type 1 receptor antagonist attenuates lacrimal gland,lung,and liver fibrosis in a murine model of chronic graft-versus-host disease[J].PLoS ONE,2013,8:e64724. |
[25] | Tamaki Y,Nakade Y,Yamauchi T,Makino Y,Yokohama S,Okada M,et al.Angiotensin Ⅱ type 1 receptor antagonist prevents hepatic carcinoma in rats with nonalcoholic steatohepatitis[J].J Gastroenterol,2013,48:491-503. |
[26] | Koganti S,Snyder R,Thekkumkara T.Pharmacologic effects of 2-methoxyestradiol on angiotensin type 1 receptor down-regulation in rat liver epithelial and aortic smooth muscle cells[J].Gend Med,2012,9:76-93. |
[27] | Karimian G,Buist-Homan M,Mikus B,Henning R H,Faber K N,Moshage H.Angiotensin Ⅱ protects primary rat hepatocytes against bile salt-induced apoptosis[J].PLoS ONE,2012,7:e52647. |