凝集素样氧化型低密度脂蛋白受体1在动脉粥样硬化中的作用
2013-11-12冉小丽石京山陈修平
冉小丽,吴 芹,石京山,陈修平,3
(1.遵义医学院药理学教研室 贵州省基础药理重点实验室,贵州 遵义 563003;2.北京东燕郊冶金医院,河北 廊坊 065000;3.澳门大学中华医药研究院 中药质量研究国家重点实验室,澳门)
动脉粥样硬化(atherosclerosis,AS)是一种血管慢性炎症性病变,由低密度脂蛋白(low-density lipoprotein,LDL)氧化形成的氧化型低密度脂蛋白(oxidized LDL,ox-LDL)在AS发生、发展过程中起着重要作用。目前在AS发病相关细胞如血管内皮细胞、血管平滑肌细胞、单核细胞、巨噬细胞以及泡沫细胞上已经发现和鉴定了多种ox-LDL受体,主要是清道夫受体,如CD36,B族Ⅰ型清道夫受体,CD68,含成束蛋白的、表皮生长因子样、层连蛋白型表皮生长因子样和连接域清道夫受体(fascicin,EGF like,laminin-type EGF-like,and link domain-containing scavenger receptor,FEEL)-1/2,内皮清道夫和A族Ⅰ型清道夫受体等[1-2]。这些受体介导的ox-LDL毒性,可能在诱导AS过程中起重要作用,然而这些受体在内皮细胞却基本不表达或表达量很低。内皮细胞的激活、功能失调和损伤是AS早期的关键环节之一,近年来发现的凝集素样氧化型低密度脂蛋白受体1(lectinlike oxidized LDL receptor-1,LOX-1)是表达于内皮细胞的ox-LDL的主要受体,在AS的发生发展中起着重要作用。本文总结和评述了LOX-1在AS发病过程中的研究进展。
1 LOX-1简介
LOX-1由日本学者Sawamura等[3]从牛主动脉内皮细胞上发现并鉴定的。牛的LOX-1蛋白含有270个氨基酸残基,相对分子质量为30 872;人LOX-1蛋白含有273个氨基酸残基,相对分子质量为30 939。人和牛的LOX-1在氨基酸序列上有72%的同源性。LOX-1蛋白是一种Ⅱ型单链跨膜蛋白,属C型选择素家族,但其蛋白的结构和序列不同于任何已知的清道夫受体,由4个蛋白结构域组成:N端胞质结构域、跨膜结构域、颈结构域以及C端凝集素样结构域。LOX-1基因属于单拷贝基因,在基因组中延伸长度有19 kb,包含8个外显子。在启动子域有TATA盒、CAAT盒以及多个顺式作用元件位点如 NF-κB,转录因子AP-1和AP-2等[4]。已经证实,LOX-1受体是一个多配基受体,可结合ox-LDL、末端糖基化终产物[5-6]、C反应蛋白[7]、4-羟基-2-壬烯醛-组氨酸加成物(4-hydroxy-2-nonenal-histidine adducts)[8]、热激蛋白60、凋亡细胞、衰老的红细胞、激活的血小板和细菌等[9-10]。最近,LOX-1受体的配基结合域(ligand-binding domain,LBD)的结构也已阐明[11-12]。LOX-1对ox-LDL具有高度的配体特异性,由3个LOX-1受体共同结合一个ox-LDL分子[5]。LOX-1对可结合多种清道夫受体的乙酰化的LDL无明显结合。在内皮细胞,LOX-1可被多种因素如ox-LDL、脂多糖、肿瘤坏死因子α(tumor necrosis factor-alpha,TNF-α)、末端糖基化终产物、血管紧张素Ⅱ(angiotensinⅡ,AngⅡ)、干扰素、内皮素和高同型半胱氨酸等诱导表达。在高血压、糖尿病尤其是AS患者粥样硬化斑块以及血浆中、相关实验动物模型中LOX-1表达量亦明显增加[13-14]。LOX-1的基因多态性被认为与心血管疾病密切相关[15-16]。此外,LOX-1蛋白在血浆中可形成可溶性LOX-1,其浓度变化被认为可做为急性冠状动脉综合征等并预测其预后的生物学标志物[17-18]。
2 LOX-1在AS发生发展中的作用
目前研究表明,作为ox-LDL的主要受体,LOX-1在AS中的内皮细胞、血管平滑肌细胞、巨噬细胞、泡沫细胞等表达,介导和(或)参与了ox-LDL内吞、内皮激活、损伤、功能失调与凋亡、血管平滑肌细胞增殖与迁移、内皮-单核细胞黏附、泡沫细胞形成等诸多病理过程,通过多种途径促进AS的发生发展。
2.1 LOX-1与血管内皮细胞激活与损伤
各种内外源因素导致的血管内皮功能障碍被认为是AS的始动环节之一。内皮细胞是LOX-1的主要表达细胞,LOX-1在牛主动脉内皮细胞[3],以及人的冠状动脉内皮细胞、主动脉内皮细胞[19]、微血管内皮细胞[20]、脐静脉内皮细胞等都有较高的组成性表达,并受多种AS致病因素如ox-LDL、TNF-α、末端糖基化终产物、AngⅡ、高同型半胱氨酸和高葡萄糖等的调节[13-14,21]。作为内皮上 ox-LDL 的主要受体,LOX-1在介导ox-LDL诱导的内皮细胞激活、损伤和功能失调方面具有重要作用:ox-LDL可明显诱导LOX-1在内皮细胞的表达,并介导ox-LDL的内吞;ox-LDL与LOX-1结合,激活LOX-1,通过 NADPH 氧化酶、线粒体电子传递链等途径诱导内皮细胞内活性氧(reactive oxygen species,ROS)包括超氧阴离子和过氧化氢等生成,而ROS可快速直接地与一氧化氮(nitric oxide,NO)反应,生成过氧化亚硝酸,灭活 NO,影响内皮舒张功能[22-23];ROS亦作为第二信使,激活磷脂酰肌醇3激酶(phosphatidylinositol 3-kinase,PI3 K)、p38丝裂原激活蛋白激酶(mitogen activated protein kinase p38,p38 MAPK)、胞外信号调节激酶1/2(extracellular regulated kinase 1/2,ERK1/2)等通路以及下游的NF-κB激活基因转录,上调血管细胞黏附分子1(vascular cell adhesion molecule 1,VCAM-1)、细胞间黏附分子1(intercellular adhesion molecule 1,ICA M-1)、P-选择素和单核细胞趋化蛋白1(monocyte chemotactic protein-1,MCP-1)等分子的表达,引起内皮细胞的激活,促进内皮与单核细胞的黏附。同时,ox-LDL诱导的内皮细胞凋亡至少部分由LOX-1所介导,主要通过ROS/NF-κB 通 路[24-25]。高脂饲养的LOX-1高表达小鼠的内皮依赖性舒张功能受损,可能与过多的ROS产生灭活NO以及内皮型一氧化氮合酶(endothelial nitricoxide synthase,e NOS)表达降低有关[26]。在 AS患者的颈动脉内膜切除标本中,内皮LOX-1高表达,尤其是在不稳定斑块中,并伴有基质金属蛋白酶2和基质金属蛋白酶9的表达增加[27]。一些具有内皮细胞保护和(或)抗AS活性的药物能够明显抑制内皮LOX-1的表达及其介导的毒性作用,如他汀类(辛伐他汀和阿托伐他汀[28])通过调节 LOX-1抑制ox-LDL诱导的内皮细胞上黏附分子E-选择素、P-选择素,VCA M-1和ICA M-1的表达;钙通道阻滞剂(如马尼地平和氨氯地平[29])抑制AngⅡ诱导的内皮LOX-1表达;姜黄素通过抗氧化机制抑制TNF-α诱导的人脐静脉内皮细胞LOX-1表达[30];芹菜素抑制高糖和 TNF-α联合诱导的LOX-1的表达[31]、表没食子儿茶素没食子酸酯通过抑制LOX-1介导的信号通路(NADPH 氧化酶/ROS/p38 MAPK/NF-κB)保护ox-LDL诱导的内皮损伤[32]、柔花酸也抑制ox-LDL诱导内皮LOX-1表达及炎性反应[33]。在内皮LOX-1信号通路中,ROS可能是介导LOX-1作用的关键分子,LOX-1激活可直接诱导ROS产生外,LOX-1在内皮细胞的表达也多由ROS介导[34-35]。我们认为,ox-LDL-ROS-LOX-1之间可能存 在一种潜在的“恶性循环”:血液中的ox-LDL结合 LOX-1诱导ROS生成,生成的ROS一方面介导下游的信号诱导内皮损伤,又可氧化血液及内皮下更多的LDL形成ox-LDL,这又诱导LOX-1表达。增强的LOX-1表达将介导更多的ox-LDL内吞。这一循环可能在AS早期的内皮细胞激活、损伤和功能失调方面具有重要意义。
2.2 LOX-1与血管平滑肌细胞的增殖与迁移
血管平滑肌细胞是调节血管收缩与舒张的重要因素,是参与AS发病的主要细胞之一,其增殖与迁移在AS的早期、中期或斑块的形成中起着重要作用。与内皮细胞相似,血管平滑肌细胞尤其是AS斑块中LOX-1表达丰富[36]。ox-LDL可通过使ERK磷酸化诱导血管平滑肌细胞中LOX-1的表达[37],而且ox-LDL中的一个成分溶血磷脂酰胆碱亦可诱导血管平滑肌细胞中LOX-1表达并增强细胞对ox-LDL的吞噬[38]。促炎症细胞因子TNF-α,IL-1α,IL-1β等通过调节过氧化物酶增殖体激活受体γ[39-40]诱导血管平滑肌细胞中LOX-1表达;肝素结合性表皮生长因子样生长因子〔heparinbinding epider mal growt h factor(HB-EGF)-like growt h factor〕通过使EGF受体磷酸化,激活 ERK,p38 MAPK和PI3 K等通路上调血管平肌细胞中LOX-1表达[41]。在人血管平滑肌细胞,AngⅡ通过脂氧酶12和脂氧酶15依赖性诱导 LOX-1表达[42]。LOX-1介导了ox-LDL诱导的血管平滑肌细胞凋亡,其机制可能通过调节Bcl-2/Bax表达比例[37]。低浓度ox-LDL诱导的血管平滑肌细胞增殖可被LOX-1 mRNA的反义寡核苷酸所阻断,提示LOX-1参与介导ox-LDL诱导的血管平滑肌细胞增殖[43]。高糖可诱导血管平滑肌细胞LOX-1表达和胱天蛋白酶3依赖性凋亡,而这可被钙通道抑制剂地尔硫艹卓通过下调LOX-1表达和活性所抑制[44]。LOX-1介导牛血管平滑肌细胞对残粒样脂蛋白颗粒的吞噬,而残粒样脂蛋白颗粒可显著诱导LOX-1表达和细胞迁移。后两种作用可被EGF受体激酶、HB-EGF、MEK1和p38 MAPK的抑制剂所阻断[45]。笔者最近的研究也显示,ox-LDL可浓度和时间依赖性地诱导大鼠主动脉血管平滑肌细胞的LOX-1表达,并可被环氧化酶抑制剂阿司匹林、LOX抑制剂去甲二氢愈创木酸、NADPH氧化酶抑制剂夹竹桃麻素、NF-κB抑制剂吡咯烷二硫基甲酸盐、JNK抑制剂SP600125等所阻断,提示这一作用通过 ROS/NF-κB/JNK 所介导[46]。高脂饲养的LOX-1高表达小鼠保持了血管平滑肌的舒张性能,但在阻力动脉平滑肌的钙激活钾通道的电流幅度高度增加[26]。肺炎衣原体是AS发病的重要病原体之一,肺炎衣原体感染诱导血管平滑肌细胞中LOX-1表达并导致AS病变中脂质的积累,氟伐他汀可抑制LOX-1表达至基础水平[47]。ox-LDL诱导人脐动脉血管平滑肌细胞中LOX-1表达,而后者介导了基质金属蛋白酶9 mRNA的表达[48]。最近研究显示,ox-LDL和机械应力可共同增加血管平滑肌细胞中ERK1/2的激活和促进细胞增殖,这一作用是通过LOX-1所介导,并可被辛伐他汀所抑制[49]。
2.3 LOX-1与单核-巨噬细胞和泡沫细胞形成
单核细胞聚集、与内皮相互作用、黏附到血管内皮并迁移到内皮下是AS的早期主要病理变化之一。一些研究证实,内皮LOX-1表达可介导内皮与单核细胞的黏附[50-52]。在人外周血单核细胞、人单核细胞T HP-1细胞株,人单核细胞白血病HL-60细胞株,LOX-1基本不表达,但经佛波酯诱导形成的巨噬细胞及其形成过程中,LOX-1表达显著增强[36,53-56]。TNF-α[39,53]、组胺[57]、葡萄糖[58]等均可诱导单核-巨噬细胞LOX-1表达。组胺诱导THP-1细胞LOX-1表达通过H2受体介导的c AMP通路[57],而葡萄糖诱导人单核细胞源性巨噬细胞LOX-1表达通过ROS/蛋白激酶C/MAPK/NF-κB/AP-1通路[58]。LOX-1在 THP-1源性巨噬细胞上的表达可被川陈皮素及其去甲基化代谢物通过ERK/JNK/NF-κB/AP-1通路所抑制[56,59],花姜酮[55]也显示抑制作用,而非对称二甲基精氨酸和转化生长因子β1则增强其表达[54,60]。
泡沫细胞的形成是AS发展中的标志性事件之一,其主要由单核细胞源性巨噬细胞通过清道夫受体摄取大量ox-LDL后转变而成。巨噬细胞上介导ox-LDL内吞的清道夫受体有CD36,SRI,SR-A,CD68等。无论是人单核细胞源性巨噬细胞还是T HP-1,HL-60细胞源性巨噬细胞,均可大量吞噬 DiI-ox-LDL,其中部分是通过LOX-1介 导[54,48]。此外,在经Toll样受体9介导的RAW264.7细胞株和小鼠腹腔巨噬细胞源性泡沫细胞形成过程中,LOX-1和NADPH氧化酶一起通过p38 MAPK通路起重要作用[61]。
2.4 LOX-1与血小板激活、聚集
血小板的激活、聚集以及与内皮的黏附是动脉血栓形成的关键环节之一。静息条件下的血小板表达少量LOX-1 mRNA和蛋白,而经凝血酶刺激后,LOX-1表达显著增强,并伴P-选择素的表达增加[62]。ADP刺激亦可引起血小板LOX-1表达增加,且阻断LOX-1可抑制ADP诱导的血小板聚集[63]。内皮细胞表达的LOX-1是内皮结合血小板的黏附分子,其与血小板的结合可被磷酸化丝氨酸结合蛋白和膜联蛋白Ⅴ抑制,但却可以被血小板激动剂所增强,提示活化的血小板表面上带负电荷的磷脂是LOX-1的抗原决定簇。血小板与LOX-1的结合增加了内皮素1的释放[64],这可能是通过LOX-1与CD40的协同作用,但二者的作用机制有所不同[65]。经诱导的人血小板与牛内皮细胞及转染牛LOX-1表达的CHO细胞(BLOX-1-CHO)共孵育可快速诱导内皮细胞内ROS、超氧阴离子生成,降低基础的以及缓激肽诱导NO的增加。这些作用可被抗氧化剂抗坏血酸、抗LOX-1抗体所阻断。血小板与内皮黏附诱导的超氧阴离子不被阿司匹林、NOS抑制剂、黄嘌呤氧化酶抑制剂、电子传递链抑制剂所阻断,而可被NADPH氧化酶抑制剂所阻断。但二者的相互作用不影响e NOS的活性[66]。LOX-1可能通过活化蛋白激酶C在ADP介导的整合素激活中起重要作用。阿司匹林和普伐他汀通过影响激活的血小板ROS和NO的释放来抑制LOX-1表达,且二者具有协同作用[63]。在高胆固醇血症患者血小板上LOX-1高表达,血浆中ox-LDL和血小板相关ox-LDL(与血小板结合的ox-LDL)显著增高,而这可被降脂药阿托伐他汀所降低[67]。
LOX-1在AS发生过程中的主要作用总结如图1。LOX-1主要表达于内皮细胞,然多种AS发病相关因素如ox-LDL、肿瘤坏死因子α、内皮素、血管紧张素Ⅱ、高同型半胱氨酸等可诱导平滑肌细胞、巨噬细胞等表达LOX-1,介导ox-LDL的内吞、ROS生成、黏附分子的表达以及单核-内皮细胞的黏附、血小板激活与聚集、平滑肌细胞增殖与迁移、泡沫细胞形成等。其中,LOX-1介导的内皮细胞的激活与损伤是其参与AS的关键环节。LOX-1介导ROS生成可能是LOX-1信号通路中的主要枢纽。
图1 凝集素样氧化型低密度脂蛋白受体1(LOX-1)在动脉粥样硬化(AS)发生过程中的作用.P-selectin:P选择素;VCA M-1:血管细胞黏附分子1;ICA M-1:细胞间黏附分子1;MCP-1:单核细胞趋化蛋白1;ROS:活性氧;NADPH oxidase:NADPH氧化酶.
3 LOX-1对AS模型动物的作用
在整体动物水平研究LOX-1功能的主要策略和方法包括测定病理条件下LOX-1的表达、运用抗LOX-1抗体封闭LOX-1、敲除 LOX-1、LOX-1高表达等。高脂饲养的 Apo E敲除小鼠微冠状动脉存在内皮依赖性舒张障碍,给予抗LOX-1抗体能恢复NO介导的动脉扩张[68],这为后来的LOX-1敲除小鼠主动脉条实验所证实[69]。笔者研究也发现,抗LOX-1抗体可明显降低Apo E敲除的AS模型小鼠血浆LOX-1、LDL、总胆固醇的水平,并升高高密度脂蛋白、脂联素水平。同时降低血管壁ROS生成、降低血管内皮对ox-LDL的吞噬并改善血管反应性[70]。LOX-1敲除的小鼠经急性心缺血再灌注损伤后,左心室舒张压和左心室舒张末期压较野生型明显改善,并伴有氧化应激明显下降[71]。同一模型中,氧化应激敏感性激酶如p38 MAPK和PKB/Akt-1,硝基酪氨酸和诱导型NOS(inducible NOS,i NOS)的表达以及超氧化物歧化酶的活性,在LOX-1敲除小鼠比野生型都明显降低[72]。在左冠状动脉阻塞的慢性缺血模型中,LOX-1敲除小鼠心肌肥厚和胶原蛋白的积累明显降低,同时伴有氧化应激的降低和AT1R的上调[73]。与野生型相比,经高胆固醇饲养的LDL受体(LDLR)敲除小鼠的主动脉斑块面积高达61%,而LOX-1敲除小鼠的主动脉斑块面积明显减少。有趣的是,LDLR和LOX-1双敲除的小鼠斑块面积仅为36%,提示LOX-1敲除能抑制LDLR敲除引起的 AS[69]。与野生型相比,LOX-1敲除小鼠经高脂饲养后诱导的脂肪细胞炎症因子如 MCP-1,MIP-1α和IL-6的表达明显降低[74]。与上述LOX-1敲除的研究相对应,LOX-1过表达可引起明显的病理变化。利用 LOX-1高表达的 C57BL/6小鼠(LOX-1tg)以及 其与 Apo E敲除裸鼠的杂交鼠(LOX-1tg/Ao E敲除)模型研究显示,LOX-1tg小鼠LOX-1主要表达于冠状动脉和心肌细胞,心脏对ox-LDL的吞噬明显增高,心脏ICA M-1和VCA M-1表达明显增加。LOX-1高表达通过诱导内皮功能失调促进炎症性心肌内血管病变[75]。高脂饲养后LOX-1高表达小鼠的微抵抗动脉血管功能降低,虽然维持了血管平滑肌的舒张,但对NO的内皮细胞依赖性舒张受损;e NOS表达降低且由于ROS的过量生成导致血管NO浓度降低;阻力动脉的血管平滑肌细胞的钙激活钾通道电流幅度明显增大,内皮细胞源性超极化因子经P450代谢物介导的内皮舒张增加[26]。利用内皮细胞高表达LOX-1并转入Apo E敲除小鼠的模型研究发现,经高脂饲养后颈总动脉斑块覆盖面积明显增加,显示LOX-1高表达促进AS发生[76]。
4 基于LOX-1的抗AS药物研发
鉴于LOX-1在心血管疾病中的重要作用,普遍认为LOX-1及其介导的信号通路可能是心血管及相关疾病的潜在的药物靶点[13,77]。靶向 LOX-1通路可采取的策略有:抑制LOX-1的表达、阻断LOX-1受体与ox-LDL的结合、抑制LOX-1下游信号分子如ROS,PI3K和p38 MAPK等。反义LOX-1 mRNA可抑制内皮 LOX-1表达,抑制ox-LDL诱导的NF-κB的激活、MCP-1表达和单核细胞与内皮细胞的黏附、内皮细胞的凋亡[25,50]。最近一项研究显示,给予 Apo E敲除小鼠给予基于LOX-1裂褶的反义寡核苷酸〔反义硫代磷酸寡核苷酸靶向小鼠OLR1 mRNA的编码序列(外显子7)〕可显著降低小鼠主动脉LOX-1蛋白表达,但对血浆总胆固醇、甘油三酯水平无明显影响[78]。反义RNA存在分子量大、易于降解、稳定性差、跨膜困难和半衰期短等缺点,限制了其进一步的研究。此外,一些小分子肽如LSIPPKA,FQTPPQL和LTPATAI也显示出对LOX-1的较高亲和力[79]。
作为受体靶点,最可行的策略是研究抑制剂或阻断剂,尤其是小分子抑制剂。目前已经发现了一些LOX-1的抑制剂/配基,如多聚肌苷酸、角叉菜胶可分别抑制[125I]ox-LDL与BLOX-1-CHO结合60%和62%[80],可能是 LOX-1的抑制剂。但这些化合物也都存在稳定性差、分子量大等缺点,目前仅作为工具药使用。为寻找LOX-1的小分子配基,我们采用极化荧光的方法,建立了LOX-1配基的高通量筛选模型,找到了3个对 LOX-1有较高亲和力的配基[81-82]。由于目前对LOX-1的生物学特性、生理病理意义还有待深入的研究,LOX-1抑制剂的有效性、安全性等尚需后续大量研究。
5 结语与展望
目前研究显示,LOX-1与AS发病相关的内皮细胞、血管平滑肌细胞、血小板、单核-巨噬细胞以及泡沫细胞等密切相关。LOX-1参与了AS发病过程中的内皮细胞的激活与损伤、平滑肌细胞的增殖与迁移、血小板的激活与聚集、内皮-单核细胞的黏附以及泡沫细胞形成等。但目前的研究多集中于LOX-1在这些细胞中表达的调节方面,对LOX-1的具体功能与机制尚缺乏深入研究。LOX-1敲除和过表达均证实其对AS的发生发展具有重要影响,是一个潜在的抗AS药物靶点。ROS可能在LOX-1的表达以及LOX-1介导的信号通路中起重要作用,而NADPH氧化酶可能是参与LOX-1表达、介导ROS形成的重要来源。通过抑制或清除ROS可显著抑制LOX-1的表达,部分阻断LOX-1介导的信号通路及毒性作用。鉴于受体作为药物靶点的广泛性,LOX-1作为一个受体,其特异性的抑制剂将是靶向LOX-1药物研发的主要方向。鉴于目前缺乏小分子的LOX-1配基,LOX-1激动剂以及抑制剂的筛选和鉴定都将会促进对其信号通路、功能的研究以及其作为药物靶点的鉴定和确证。对LOX-1及其信号通路的深入研究可能会对AS的发病以及未来的治疗提供新的思路。
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