谢甲琦
【摘 要】 adiponectin,是一种仅由脂肪细胞分泌的蛋白质,在肥胖者、糖尿病及冠状动脉疾病患者,循环adiponectin 水平低于正常,研究提示adiponectin 对糖、脂代谢有影响,与胰岛素抵抗、动脉粥样硬化有密切联系。本文就adiponectin的结构、功能及作用机制作一综述。
【关键词】 adiponectin;脂肪因子; 胰岛素抵抗; 动脉粥样硬化
adiponectin 是脂肪组织特有基因apM1 (adipose most abundant gene transcript 1) 的产物,是一种含247个氨基酸的可溶性的防御性胶原蛋白,血液循环中有相对较高的浓度(μg/mL)。人和鼠类的adiponectin由若干团体独立的分离出来,因而对这一化合物冠以不同的描述:Acrp30、Adipo Q、GBP28。
1 adiponectin的分子结构
Scherer等[1]早在1995年就对cDNA编码的adiponectin进行了描述:adiponectin由四个结构域组成,包括氨基末端的信号序列、一个可变区、胶原样结构域和羧基末端的球状结构域。根据它的主要氨基酸序列和它的亚单位域结构来看,adiponectin 与 C1q 最为相似,后者是补体系统的成员。然而,X光结晶学显示adiponectin的球状结构与TNF-α有着惊人的同源结构,暗示TNF-α的家族成员和 adiponectin 之间有进化上的的相关性[2]。
哺乳动物的adiponectin一旦合成,便要经历翻译后的羟基化和糖基化修饰而产生八种亚型[3]。其中六种为糖基化产物。O-联糖基化位置定位于胶原结构域内,包括第68、71、80和104四个赖氨酸残基,和第94位的一个脯氨酸残基[3]。功能分析显示糖基化的哺乳动物全长adiponectin比未经糖基化的重组细菌产物有更强的胰岛素增敏作用。这些发现表明 adiponectin 的翻译后修饰对其发挥最佳的生物学活性是必要的。
adiponectin的基本构件是紧密相联的三聚体,后者由三个单体在球状结构域相互联接而成。adiponectin单体(30-kDa)没有在循环中观察到并且似乎只限于脂肪细胞。四到六个三聚体通过他们的胶原样结构域形成更高级结构或低聚体,参加血液循环水平为5-30[mu]g/ml[1,4,2]。若没有胶原样结构域,adiponectin球状结构域仍可形成三聚体,但是不能连接成更高级结构[2]。虽然adiponectin三聚体间联结的精确分子的机制还不清楚,但是其球状结构和胶原结构的相互作用对确保其高级结构稳定性和活性的意义是确定的。
adiponectin的药理学作用已经通过其多种重组产物在动物个体、组织和细胞水平上进行研究。对adiponectin全长和单独球状区的生物活性的研究产生了混杂的结果。对饮食诱导和遗传而致的肥胖鼠类,adiponectin 的头部的球状域显示出比其全长形式更强的改善高血糖和高胰岛素血症的能力[5]。高脂饮食或静脉内注射甘油的而致血浆游离脂肪酸升高的鼠类,球状域也显示出更强的降脂作用[6]。而Berg等人的研究结果相反[7],细菌产生的球状adiponectin在注入1和2型糖尿病鼠模型体内后没有如同全长adiponectin那样引起血糖的降低。这可能是因为adiponectin作为一可变的蛋白质复合物在不同组织发挥不同的作用。
2 生理功能
adiponectin在人类和动物的胰岛素敏感组织葡萄糖和脂质新陈代谢的调节过程中发挥重要作用。研究发现与饮食诱导的肥胖人群一致[4],循环adiponectin在遗传性和饮食诱导的鼠类肥胖模型体内也减少[5]。在肥胖和脂肪萎缩的老鼠模型中,低的adiponectin水平与胰岛素抵抗的发展有关[5]。在人类胰岛素抵抗个体,包括2型糖尿病患者,血浆adiponectin 的水平明显较低[8],并且可通过注入能使胰岛素增敏的噻唑烷二酮类(TZD)药物而增 加[9,10,11,12]。糖尿并冠状动脉疾病(CAD)的患者较无CAD的糖尿病患者,其血浆adiponectin水平也较低,暗示adiponectin有抗动脉粥样硬化的效应[13]。在人类的大动脉内皮细胞上进行的研究中显示:adiponectin呈剂量依赖性地减少血管表面粘附分子的表达,后者调节内皮的炎症反应[14]。adiponectin也可抑制血管平滑肌的增殖[15],并向受损的血管内膜的聚集[16]。在临床实验中,低的adiponectin水平与致动脉粥样硬化的脂质有关[17]。adiponectin与肥胖、胰岛素抵抗、CAD和血脂障碍的关系表明这种蛋白在体内存在的重要意义,其含量或许可以作为代谢综合征的重要标志。
3 作用机制
adiponectin发挥它作用的机制在很大程度上还是未知和充满争议的。在鼠体内已经显示,由胰岛素诱导的骨骼肌胰岛素受体的酪氨酸磷酸化,在注射adiponectin后其酪氨酸磷酸化的增强与全身胰岛素敏感性增加有关[5]。这也在人类身上进行的研究中得到验证[18]。
adiponectin对骨骼肌及肝脏的葡萄糖利用和脂肪酸氧化的刺激作用,也可能通过5'-AMP激酶的激活进行。后者激活的蛋白激酶(AMPK)在参加运动[19]和服用二甲双胍[20]的人群提高胰岛素敏感性和降低葡萄糖过程中有重要的作用,而且还可能参与噻唑烷二酮类药物在肌组织中的作用[21]。在老鼠实验中证明adiponectin对5'-AMP激酶的作用具有组织特异性。在这些研究中,球状部和全长形式的adiponectin对骨骼肌5'-AMP激酶均有激活作用,但是只有全长形式的刺激了肝脏5'-AMP激酶活化和磷酸化[22]。此外,adiponectin可能增加过氧化物酶体增殖物激活受体α的活性。这些途径的激活作用使得肝脏的葡萄糖产生减少[23],肌肉的葡萄糖摄取和脂肪酸氧化作用增加[24,25,26],而且可能调节心血管的效应[27]。
在血管内皮细胞层中,adiponectin减少了单核细胞对内皮的粘附,抑制了巨噬细胞向泡沫细胞的转化过程,而且阻止血管平滑肌细胞增殖和移行。
对鼠类骨骼肌中编码参与脂肪酸转运和和氧化蛋白质的基因,adiponectin对其表达有上调作用,如 CD36,acyl-CoA氧化酶和解偶联蛋白,以致提高脂肪燃烧和能量消耗[5]。在肝脏中,低剂量的adiponectin减少了如 CD36这样的参与脂肪酸转运的蛋白质的表达,导致脂肪酸流入肝脏减少和肝甘油三酯降低[5]。肝脏胰岛素敏感性改善,以致研究人员认为adiponectin对肌肉的主要作用是增加,摄取和燃烧游离脂肪酸(FFAs),而肝脏甘油三脂的减少起因于继发的血浆FFA 和甘油三酯水平减少。该团体进行的另一实验[28],胰岛素抵抗的改善,beta-细胞脱粒,球状adiponectin转基因的ob/ob型、leptin缺陷的老鼠发生糖尿病。再一次,这些结果与增加的骨骼肌肉脂肪酸氧化有关。然而这一发现与另一个团体的报告相反:在基础情况下,adiponectin抑制肝葡萄糖排出量而不会持久地弱化该组织聚集甘油三酯,通过这样发挥对肝细胞的胰岛素增敏作用[7]。
最近,通过与球状的adiponectin的结合,adiponectin的两相关而不同的受体(AdipoR1 和 AdipoR2)被识别,而且其基因从人类骨骼肌肉表达文库中克隆得到[29],分别位于1q32 和12p13。该受体包含七个跨膜区但是在结构和功能上又与经典的G-蛋白质耦联受体不同。AdipoR1在体内的表达无处不在,但以骨骼肌最为丰富,而AdipoR2主要在肝脏中表达。两个受体间只有67%的同源性,而且显示出对全长或球状adiponectin不同的亲和力。球状adiponectin结构在AdipoR1上有较强的效果,而球状和全长的adiponectin在AdipoR2上的作用则较为轻微。受体分布的组织特异性和配体交互作用的差异使相关而有差别的生理途径的精确调节成为可能。人和鼠类的cDNAs显示出两种受体亚型(96.8% 为 AdipoR1 和 95.2% 对于 AdipoR2)有高的同源性,而从酵母到人类的的这种保守性进一步支持新陈代谢信号途径的基本相关性。最近发现,AdipoR1 和 AdipoR2 大量地表达于人和鼠类胰脏的 [β]细胞[30],而在此的表达可因暴露于游离脂肪酸油酸盐而增加[30]。
4 adiponectin分泌的调节
adiponectin作为一种脂肪细胞特异分泌的激素其分泌受多种因素影响。激素及细胞因子:Fasshauer等发现在体外向3T3-L1脂肪细胞注入β肾上腺素能激动剂异丙肾上腺素使adiponectin mRNA的水平降低了75%。对这些细胞用β肾上腺素能拮抗剂普萘洛尔和蛋白激酶A阻断剂H-89预处理,几乎完全逆转异丙肾上腺素的这种抑制作用。由此作者得出结论:儿茶酚胺类至少可以藉着对adiponectin基因表达的下调作用,参与胰岛素抵抗的诱导,而且这种抑制作用由β肾上腺素受体通过鸟苷酸结合蛋白(stimulatory guanine nucleotide binding)-PKA依赖的途径调节[31]。此外,体外实验显示IL-6 和TNF-α可下调脂肪组织adiponectin mRNA[32]。对人和鼠进行的高血胰岛素-正常血糖钳研究显示出在注射胰岛素后循环adiponectin降 低[33],而睾丸酮也可降低循环adiponectin含量,这可能与男性胰岛素抵抗和动脉粥样硬化的高发率有关[34]。
过氧化物酶体增殖物激活受体-γ(PPAR-γ) 是一个配体激活的转录因子,主要调节脂肪细胞的分化和多种基因的表达。噻唑烷二酮类药物(TZDs)是PPAR-γ的特定人工配体,对2型糖尿病患者和因各种不明机制而致的胰岛素抵抗的动物模型,它可改善葡萄糖耐受性和胰岛素敏感性。对胰岛素抵抗的人群、啮齿类和2型糖尿病患者注入TZDs,增高了血浆adiponectin水平[9,10,11,12]。adiponectin基因的启动子活性可被TZDs明显提高[9],尽管adiponectin基因中是否存在过氧化物增殖物激活受体γ的有效反应元件存在争议[35,36]。或许这种作用源于TZDs可诱导的其他基因激活后的继发效用,而不是特异地活化PPAR反应元件[37]。最近的研究还发现,适度的饮酒可增加血浆adiponectin水 平[38]。
此外,晚期肾病患者体内血浆adiponectin水平显著升高但其基因表达却明显下调,这提示adiponectin也许存在负性反馈调节[39]。
总之,adiponectin 作为脂肪细胞分泌的一种激素蛋白,具降低血脂,降低血糖,改善胰岛素敏感性的作用,同时可拮抗动脉粥样硬化的形成,不会使体重增加;到目前为止,在肥胖及2 型糖尿病患者未发现对adiponectin 的抵抗,由于这些优点,adiponectin 有望成为一种新的治疗糖尿病和抗动脉粥样硬化的药物。但是肥胖及2 型糖尿病患者循环adiponectin 水平的降低是胰岛素抵抗的原因还是结果,AdipoR下游信号传递通路是怎样的,这些疑问仍有待于进一步研究。
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