表1(Table 1)
2. 分子靶标新药研究协同创新中心, 湖南 长沙 410078;
3. 中南大学临床药理研究所, 遗传药理湖南省重点实验室, 湖南 长沙 410078
2. Hunan Province Cooperative Innovation Center for Molecular Target New Drug Study, Changsha 410078, China;
3. Institute of Clinical Pharmacology, Central South University, Hunan Key Laboratory of Pharmacogenetics, Changsha 410078, China
随着社会经济的发展和生活方式的改变，心脑血管疾病的患病人数逐年攀升[1]。由于我国人口结构老龄化程度持续加重，心脑血管疾病的发病形势已经非常严峻，心脑血管疾病模型的开发研究也日益受到重视[2, 3]。据2013年中国心脏大会《中国心血管病报告》显示，我国目前约1/5的成年人患有心脑血管疾病，且近5年来，心脑血管疾病死亡人数居各种死因之首。心脑血管疾病已经成为我国乃至全球面临的重大公共卫生问题之一，深入探究心脑血管疾病的发病机制、提高心脑血管疾病的防治水平和改善心脑血管疾病患者的预后刻不容缓。
1 二甲基精氨酸(dimethylarginine，DMA)与心脑血管疾病气体分子一氧化氮(NO) 具有强大的舒血管功能，可促进内皮细胞的增殖和迁移，抑制脂多糖诱导的内皮细胞凋亡、抑制血小板聚集及其与内皮细胞的黏附，并可在血管受损时抑制血管的畸形发展，在维持血管内皮功能和心脑血管系统稳态中起重要作用[4, 5]。动脉粥样硬化是心力衰竭和中风等多种心脑血管疾病的病理基础，而内皮功能的紊乱则是动脉粥样硬化的早期表现。人体内NO由L-精氨酸在NO合酶(nitric oxide synthase，NOS)的催化下生成，当NOS活性受到抑制时，NO的生成减少，血管内皮功能发生紊乱，进而导致动脉粥样硬化等一系列的心脑血管疾病的发生发展[6, 7]。
二甲基精氨酸是体内含精氨酸的蛋白质被蛋白精氨酸甲基转移酶(protein-arginine methyltransferase，PRMT)甲基化的产物，分为非对称性二甲基精氨酸(asymmetric dimethylarginine，ADMA)和对称性二甲基精氨酸(symmetric dimethylarginine，SDMA)两种(Fig1)，均在调节体内NO的合成中发挥重要作用。
1.1 ADMA与心脑血管疾病ADMA具有与L-精氨酸相似的结构，可竞争结合NOS并抑制其活性，减少NO的生成，进而引起炎症反应和内皮功能紊乱，促进多种心脑血管疾病的发生与发展[8]。有学者测定了151例平均年龄为57岁无糖尿病并发症的冠心病患者血浆ADMA的含量，并对冠状动脉的狭窄程度进行分级，发现冠脉狭窄程度较重的患者血浆中ADMA的水平高于狭窄程度较轻的患者，且ADMA水平与Sullivan评分呈正相关[9]。随机双盲安慰剂对照试验结果表明，低剂量ADMA可升高健康志愿者的血压[10]。未经治疗的原发性高血压患者血浆中ADMA水平也明显升高，且ADMA与原发性高血压患者内皮功能紊乱相关[11]。Sonmez等[12]也发现了类似的现象。除高血压、冠心病以外，心力衰竭患者血浆中ADMA水平也较健康对照人群明显升高[13]。研究表明，慢性心衰患者血浆 ADMA 水平与纽约心脏协会(NYHA)心功能分级和血浆N端-B型利钠肽的前体(NT-proBNP)密切相关[14]，而后者是心衰的一个重要诊断和预后指标[15]。此外，ADMA还与参与心律失常[16]、心肌梗死[17]和高胆固醇血症[18]等多种心脑血管疾病的发生发展。血浆ADMA水平升高已成为多种心脑血管疾病及其并发症心脑血管事件的独立预测因子。
1.2 SDMA与心脑血管疾病SDMA虽然不抑制NOS的活性，但却可与介导L-精氨酸跨膜转运的阳离子转运体hCAT-2B竞争性结合，抑制细胞摄取L-精氨酸，从源头间接抑制NO的合成[19]。Bode-Boger等[20]首次发现SDMA与肾小球滤过率及冠心病的严重程度相关。Schulze等[21]对394例急性缺血性脑卒中患者进行了长达7.5年的随访，发现SDMA是急性缺血性脑卒中患者全因死亡的独立预测因子，风险比为2.41。此外，有报道表明，SDMA也可预测冠状动脉疾病及其主要不良心脑血管事件的发生风险[22]。对健康志愿者进行的研究发现，血浆ADMA和SDMA水平呈明显的正相关，两者水平升高的个体5年观测期内发生心脑血管疾病的风险比分别达3.86和7.91[23]。可见，SDMA和ADMA都可作为心脑血管疾病及心脑血管事件的预测因子[24]。
2 DMA的体内代谢人体内ADMA和SDMA的代谢途径存在差别，SDMA主要以原型形式经肾脏直接排泄，而ADMA则主要经过一系列的酶促反应代谢消除[25]。二甲基精氨酸二甲胺水解酶(dimethylarginine dimethylaminohydrolase，DDAH)是体内ADMA的主要水解酶，可将ADMA代谢为瓜氨酸和二甲胺[26]。DDAH有两种亚型，分别是DDAH1和DDAH2。DDAH1主要表达在中枢神经系统、肝脏、肾脏、肺和骨骼肌等组织和器官的血管内皮细胞中，DDAH2则主要在心脏、肾脏、内皮细胞和血管平滑肌细胞等组织和细胞中表达[27]。研究证实，DDAH1是体内代谢灭活ADMA的主要酶[28, 29]。我们的前期研究发现，人类DDAH1存在3个不同的转录本，而在原代培养的人脐静脉内皮细胞中，只有DDAH1-V1转录本的表达水平与ADMA代谢活性相关[30]。此外，近年来的研究发现，位于线粒体的丙氨酸-乙醛酸转氨酶2(alanine-glyoxylate aminotransferase 2,AGXT2)也参与ADMA的体内代谢，该酶以ADMA作为氨基供体，催化其分解为α-酮-δ-(N，N-二甲基胍)戊酸[α-keto-δ-(N,N-dimethyl-guanidino) valeric acid，DMGV][31, 32]，在短期内降低体内ADMA水平中发挥重要的作用[33](Fig1)
|
| Fig 1 Synthetic and metabolic pathways of DMA *CAT: Cationic amino acid transporters. |
AGXT是一种依赖磷酸吡哆醛(pyridoxal 5-phosphate)的氨基转移酶，以L-丙氨酸为氨基供体，不可逆地将乙醛酸盐转化成甘氨酸[34]。多种哺乳动物肝脏或肾脏中均能检测到AGXT的表达[35]。AGXT存在两种亚型，分别是AGXT1和AGXT2(Fig2)，两者在细胞内分布、结构和功能上均存在明显的差异。AGXT1主要分布于过氧化物酶体，兼具丙氨酸-乙醛酸和丝氨酸-丙酮酸两种氨基转移酶的活性[36]，其基因突变导致活性降低或表达缺失时可引发Ⅰ型原发性高草酸尿症(primary hyperoxaluria，PH1)[37, 38]。AGXT2经核糖体翻译过后运送至线粒体中发挥生理效应，不具有丝氨酸-丙酮酸转氨酶的活性，但参与ADMA的代谢。
|
| Fig 2 Expression and function of AGXT *PLP：Pyridoxal 5'-phosphate |
Rodionov等[31]发现过表达人AGXT2的小鼠肝脏和血浆中ADMA水平均明显降低，从而间接促进血管内皮细胞NO的合成。Caplin等[39]的研究发现，AGXT2基因敲除小鼠血浆ADMA的水平明显升高，而其代谢产物DMGV的血浆浓度水平则明显降低。AGXT2的底物β-氨基异丁酸(β-aminoisobutyrate，BAIB)可与ADMA竞争AGXT2，并在有丙酮酸盐(pyruvate)的情况下被AGXT2代谢为氧代丙酸甲酯(2-methyl-3-oxopropanoate )和丙氨酸(alanine)[40]。通过微型泵向C57/BL6小鼠腹腔内灌注BAIB 后发现，小鼠血浆ADMA和SDMA的水平均明显升高，而DMGV的水平明显下降[41]。这些研究均提示，AGXT2在ADMA代谢中的重要作用。
3.2 AGXT2基因多态性与心脑血管疾病的临床相关研究由于AGXT2在内源性活性物质ADMA和BAIB等代谢中的重要作用，近年来有关AGXT2基因遗传变异的临床相关性的研究也相继展开。Suhre等[40]对人体尿液中59种代谢产物进行的全基因组关联研究(genome wide association study，GWAS)发现，AGXT2的非同义多态性rs37369(Val140Ile)与尿液BAIB水平间呈很强的关联(P=3.17×10-75)。Rhee等[42]随后发现AGXT2 rs37370位点与血浆BAIB的水平间存在非常明显的关联性。也有GWAS研究发现rs37369位点T等位基因可升高舒张压的水平(P=0.0052)[39]。此外，有学者发现，AGXT2基因位点rs37369和rs16899974多态性与血清SDMA水及心率变异性(heart rate variability，HRV)明显相关，rs37369多态性同时还与ADMA/SDMA的比值相关联[43]。在一项纳入了394例中风患者进行的研究发现，AGXT2基因的rs28305、rs40200和rs37369多态位点均与体内SDMA的水平相关联性，其中rs40200多态性可在一定程度上预测中风患者的预后[44]。我们的前期研究发现，rs37369多态性与中国汉族吸烟人群冠心病的发病风险，该位点GG基因型(Val140Val)可升高吸烟且同时伴有糖尿病的个体冠心病的发病风险，其机制可能与吸烟状态下血浆中ADMA水平升高有关[45]。
4 总结与展望大量研究业已证实，ADMA能竞争性抑制NOS的活性，而SDMA也可通过干扰L-精氨酸的摄取从而间接抑制NO的生成。随着对AGXT2在ADMA和SDMA代谢中作用认识的深入，AGXT2表达缺失或者活性降低可能是导致心脑血管疾病患者体内ADMA水平改变和内皮功能紊乱的重要机制[46, 47, 48, 49]。近年来已发现，AGXT2基因的多个单核苷酸多态性位点与ADMA和SDMA及其他内源性物质的代谢存在明显的关联(Tab1)，提示AGXT2基因位点可能存在导致其表达或酶活性发生改变的功能性遗传变异，目前关于这些位点多态性的功能研究相对较少，且这些多态位点与心脑血管疾病易感性关系的研究也鲜见报道。进一步深入探究AGXT2及其遗传变异在心脑血管发病机制中的作用，可为开发靶点为AGXT2的新型心脑血管保护药物奠定基础，进而改善我国心脑血管疾病高发病率、高患病率、高死亡率的现状。
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