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脂联素与胰岛素抵抗研究进展

2017-04-11安平王安平母义明

生物技术通讯 2017年3期
关键词:信号转导脂联素抵抗

安平,王安平,母义明

解放军总医院 内分泌科,北京 100853

脂联素与胰岛素抵抗研究进展

安平,王安平,母义明

解放军总医院 内分泌科,北京 100853

脂联素是脂肪组织分泌一种脂肪因子,与胰岛素抵抗和肥胖密切相关,在2型糖尿病和肥胖人群中,脂联素的血浆浓度下降。脂联素信号通路通过激活AMPK和PPAR-α与胰岛素信号通路相联系。研究表明,上调脂联素信号通路的活性可以有效缓解胰岛素抵抗。因此,脂联素信号转导通路机制是以胰岛素抵抗为病理生理基础的2型糖尿病的研究热点。本文简要介绍脂联素的生物学特征、脂联素的信号通路机制、脂联素与胰岛素抵抗的现有研究成果及临床价值。

脂联素;胰岛素抵抗;肥胖

肥胖已成为严重威胁人类健康的公共卫生问题,在全世界范围内造成了严重的社会经济负担。肥胖是包括2型糖尿病在内许多慢性非传染性疾病的重要致病因素,导致全世界包括发展中国家在内的死亡率逐年升高[1]。80%的2型糖尿病被认为与肥胖相关[2],而2型糖尿病又以外周组织如肝脏、骨骼肌、脂肪的胰岛素抵抗为特征[3]。肥胖的本质是脂肪细胞数量及质量增加的状态,现在认为肥胖和胰岛素抵抗通过脂肪组织相互联系[4-5]。脂肪组织不仅是多余能量的存储仓库,更是一个内分泌器官,它可以分泌多种被称为脂肪因子的生物活性分子[6]。其中脂联素就是一种在体内丰富表达的脂肪因子,与其受体结合后具有显著的胰岛素增敏作用。在肥胖引起的胰岛素抵抗中,脂联素及其受体的表达均下调,改变了正常代谢信号通路[7-9]。

1 脂联素概述

1.1 脂联素的结构

人脂联素是相对分子质量(Mr)为30×103的蛋白质分子,由244个氨基酸残基构成,N端为胶原结构域,C端为球状结构域[10]。脂联素在人体循环中以多聚体的形式存在,90%的脂联素结合成一个高分子量多聚体(HMW)和低分子量多聚体(LMW)。HMW的Mr为360×103~540×103,由12个或18个脂联素分子组成;LMW的Mr为180×103,由6个脂联素分子组成。人体循环中其余10%的脂联素以三聚体的形式存在,被称为全长型脂联素,相对分子质量为90×103。

高分子量多聚体、低分子量多聚体、全长型脂联素在体内较为稳定,意味着某种亚型脂联素含量下降并不能通过改变其他亚型脂联素的构象而恢复[11]。但是在体外实验中发现,HMW和LMW在低pH值等还原条件时分子内二硫键断裂,产生了无N端胶原结构域的全长型脂联素三聚体和球状脂联素三聚体。

脂联素的多聚复合物构象被认为是调节其生物功能的重要机制[12]。体内及体外实验证实,HMW是脂联素中具有生物学活性的形式,具有抗动脉粥样硬化、抗糖尿病、抗炎的作用,可以阻止糖尿病和心血管疾病的进展[13]。

1.2 脂联素受体

不同亚型的脂联素与不同的脂联素受体相结合。与HMW和LMW结合的受体是T型钙黏蛋白[14]。T型钙黏蛋白是一种由Ca2+介导细胞间信号传递的细胞表面蛋白[15],它没有细胞内结构,由糖原锚定于细胞膜表面[16]。脂联素分子C端球状结构域含有一个Ca2+结合位点,因而可以通过Ca2+介导与T型钙黏蛋白结合[11]。T型钙黏蛋白在受伤的血管内皮细胞和粥样硬化的血管平滑肌细胞中大量表达[17-18],有研究认为其具有降低心脏负荷的作用19。但是,T型钙黏蛋白与脂联素结合后的信号转导以及相关功能的分子机制仍不清楚,需要进一步研究来证实。

AdipoR1和AdipoR2是另外2种主要的脂联素受体,由25个氨基酸残基构成,蛋白质分子N端位于胞质中,而C端位于胞膜外[20-21]。2种受体均具有七跨膜结构,但在结构和功能上都与G蛋白偶联受体有所区别。AdipoR1在全身组织中均有表达,而AdipoR2在肝脏中特异性表达[21]。有体外研究表明,全长型脂联素主要在肝脏中与AdipoR2结合,而球状脂联素与AdipoR1的亲和力较高[22]。

参与脂联素受体下游信号通路中分子包括含有PH结构的衔接蛋白(APPL1)、激活蛋白激酶受体C1(RACK1)、蛋白激酶CK2β、Mr为46×103的内质网蛋白(ERp46)[23-27]。

此外,脂联素作为一种凝集素可以与巨噬细胞表面的钙网蛋白结合,促进死亡细胞的吸收,因而可以在一定程度上减轻全身性炎症[28-29]。

1.3 脂联素的信号转导通路

脂联素基因启动子包含多个转录因子结合位点,可以通过多种因子调节其活性[30-31]。能够上调脂联素表达的转录因子有过氧化物酶增殖物激活受体γ(PPAR-γ)、C/EBPa、叉头转录因子FoxO1[32-33]。在肥胖引起的慢性低度炎症、氧化应激等状态下,脂联素的表达下调[31]。氧化应激时通过影响Akt和JAK/STAT信号通路,炎症因子TNFα通过影响蛋白激酶C和JNK信号通路、炎症因子IL-6通过影响p44/42MAPK均能下调脂联素的表达[34-37]。另外,在内质网应激时,脂联素mRNA表达水平下调[38-39]。转录因子CREB可以被β肾上腺素能信号通路激活,进而通过ATF3、NFAT、蛋白激酶A下调脂联素的表达[30-31]。

2 脂联素与胰岛素抵抗

脂联素与其受体AdipoR结合后,主要通过AMP活化的蛋白激酶(AMPK)和过氧化物酶增殖物激活受体α(PPAR-α)来调节糖代谢[21]。脂联素抑制肝脏糖异生产生降糖作用的具体机制包括以下几点:增加乙酰辅酶A羧化酶的磷酸化、增加脂肪酸消耗、增加葡萄糖摄取率、增加肌细胞产生乳酸[40]。

AMPK是依赖AMP的蛋白激酶,在细胞代谢中起传感器作用。脂联素通过AMPK参与葡萄糖的摄取和β氧化[22]。研究发现,APPL1在脂联素激活AMPK的信号转导通路中发挥重要作用[41-42]。骨骼肌中的全长型脂联素和球状脂联素可以激活AMPK,而肝脏中的全长型脂联素可以激活AMPK[43-44]。从肥胖的2型糖尿病患者中分离出的骨骼肌细胞中,AMPK的磷酸化水平下降,说明脂联素受体下游的信号通路受损可以引起胰岛素抵抗[45]。

脂联素通过PPAR-α增加了机体对脂肪酸和能量的消耗,从而降低了甘油三酯的水平,同样增加了肝脏和骨骼肌的胰岛素敏感性[40]。较高的游离脂肪酸导致葡萄糖摄取率下降,其机制可能是高甘油三酯血症引起线粒体内蓄积较多的NADH和乙酰辅酶A,抑制了己糖激酶2和6-磷酸果糖激酶-1的活性,从而增加了细胞内葡萄糖含量,降低了细胞对葡萄糖的摄取率。但是也有学者认为较高的游离脂肪酸会首先影响葡萄糖转运和磷酸化,再导致糖原合成和葡萄糖有氧氧化的减少。在骨骼肌细胞中,脂联素可以通过上调PPAR-α信号通路下游蛋白CD36、乙酰辅酶A氧化酶的表达增加脂肪酸的转运,从而缓解胰岛素抵抗[46]。另一方面,高甘油三酯血症可以通过影响由胰岛素刺激下的磷脂酰肌醇3激酶(PI3K)和葡萄糖转运体4(GLUT-4)的活化来影响细胞对葡萄糖的摄取率。细胞内脂肪酸代谢产物的堆积,可能会增加丝氨酸/苏氨酸级联反应的活性,通过蛋白激酶C增加胰岛素受体底物1(IRS-1)丝氨酸/苏氨酸的磷酸化。磷酸化的IRS-1丝氨酸不能与PI3K结合并将其活化,导致葡萄糖转运降低并产生胰岛素抵抗。游离脂肪酸也可以直接影响GLUT-4的表达、转运、趋化。此外,游离脂肪酸能够降低肝脏对胰岛素的清除率,并增加肝糖输出,进一步促进了胰岛素抵抗的发展[47]。

但是,体内脂联素信号通路对胰岛素敏感性的影响存在一些争议。体内脂联素受体AdipoR1主要存在于骨骼肌中,其与球状脂联素的亲和力远高于全长型脂联素。大多数体外实验的研究对象为球状脂联素,而体内全长型脂联素的含量远高于球状脂联素48。研究发现,小鼠模型中AdipoR1基因敲除可以抑制由脂联素激活的AMPK信号通路活性;抑制AdipoR2的表达可以抑制PPAR-α信号通路活性;同时敲除AdipoR1和AdipoR2可以阻止脂联素与其受体结合并激活,进而产生胰岛素抵抗[49]。也有研究发现,AdipoR1基因敲除小鼠表现出显著的肥胖,而AdipoR2基因敲除小鼠则较为消瘦,并且具有阻止高脂高糖饮食条件下产生肥胖的效应,这表明AdipoR1和AdipoR2具有相反的作用[50]。

3 脂联素信号通路在临床中的价值

在临床上以脂联素信号通路为靶点改善胰岛素抵抗的方法主要有以下2种,其一是提高脂联素血浆浓度,其二是增加脂联素信号转导通路的活性。

提高脂联素的血浆浓度可以通过2种方式实现,其一是直接应用外源性脂联素,其二是通过药物治疗增加内源性脂联素的表达。由于脂联素在体内以多聚体的形式存在,故直接应用外源性脂联素较为困难。因此,增加脂联素体内含量的可行方法是通过药物以及改善生活方式来促进内源性脂联素表达。能够有效提高脂联素水平的药物有PPAR-α受体激动剂噻唑烷二酮(TZDs)[51-52]、血管紧张素转换酶抑制剂(ACEI)和血管紧张素Ⅱ受体阻断剂(ARBs)[53-54];近来的研究发现,大麻素-1受体阻断剂,如taranabant,可以调节体内脂联素的水平,但这种药物尚不能应用于临床,需要评估其治疗价值以及对中枢神经系统的影响[55]。此外,已发现某些日常食物如鱼油、亚油酸、菜籽提取物、绿茶提取物、多酚白藜芦醇等可以促进体内脂联素的表达[56]。对于肥胖和糖尿病人群,减重和体育活动也能够有效提高体内脂联素水平[57]。

可以通过上调体内脂联素受体的表达来增加体内脂联素信号转导通路的活性。PPAR-α激动剂、PPAR-γ激动剂、ARBs等都可以上调脂联素受体的表达[58]。之前发现一种名为osmotin的蛋白可能具有脂联素受体激动剂的效应[59]。最近新研制出一种可以口服的合成活性小分子脂联素受体激动剂AdipoRon。在动物模型中,AdipoRon具有改善肥胖相关疾病的效应,如胰岛素抵抗、糖耐量异常、2型糖尿病等,并且发现能够延长动物模型寿命[60-61]。

4 结语

脂联素是一种重要的脂肪因子,在体内参与多种代谢的调节。因脂联素对胰岛素抵抗具有正向调节作用,脂联素信号转导通路机制可能成为糖尿病新药研究的方向,这些机制包括脂联素的生物合成、脂联素多聚化过程以增加活化亚型HMW的比例、脂联素在基因水平的调控、脂联素受体下游的信号通路机制,以及脂联素在细胞内的活性。

因此,还需要对脂联素的信号转导通路机制进行更深入的研究以应用于临床。而前述改善脂联素表达和脂联素通路活性的药物还需要大样本量的临床试验来证实其对脂联素信号通路的调控作用。

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Progress of the Relationship Between Adiponectin and Insulin Resistance

AN Ping,WANG An-Ping,MU Yi-Ming*

Department of Endocrinology,General Hospital of Chinese PLA,Beijing 100853,China *Corresponding author,E-mail:hgx0536@163.com

s]Adiponectin,a protein secreted by adipose tissue,is considered that associated with insulin resistance and obesity.The reduction of adiponectin level was observed in people with insulin resistance.The insulin-sensitizing efforts of adiponectin resulting from the activation of AMPK and PPAR-αand the decrease of triglyceride level.Up-regulation of adiponectin signaling may be the potential treatment strategy of insulin resistance.This review aims to discuss the mechanisms of adiponectin related to insulin resistance and its perspective of clinical value.

adiponectin;insulin resistance;obesity

R335.6

A

1009-0002(2017)03-0360-06

10.3969/j.issn.1009-0002.2017.03.024

2016-10-21

安平(1990-),男,硕士研究生,(E-mail)anping6666@sina.com

母义明,(E-mail)hgx0536@163.com

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