泌乳素298UIU mI正常吗(泌乳素298uiuml正常值)

编者按：GLP-1自发现以来，已然成为一种“多面手”激素——其接二连三的代谢功能被人们发现，远远超出了作为肠促胰素的经典定义。GLP-1众多有益的作用使其受体激动剂逐渐成为更多新兴的治疗领域如脂肪肝、肥胖和神经退行性疾病等的冉冉之“星”药物。时值利拉鲁肽在我国上市10周年、司美格鲁肽新上市之际，我们邀请一众专家，讲述一系列关于GLP-1的故事。本期上海交通大学医学院附属新华医院苏青教授将带您探寻胰高糖素如何通过胰高糖素原转录、调控、翻译、加工成GLP-1，以及GLP-1与GLP-1R结合与解离的一系列途径。

一、前胰高糖素原的转录调控

胰高糖素（Gcg）是胰岛α-细胞分泌的一种具有升糖作用的激素。GCG基因原始翻译产物为前胰高糖素原，前胰高糖素原切除信号肽后变成胰高糖素原。除胰岛α-细胞外，回肠远端和结肠L-细胞以及脑干孤束核(NTS)的神经元亦可表达胰高糖素原[1-7]。

激素原转化酶可将胰高糖素原分子切成不同大小的肽段，包括含肠高血糖素相关胰多肽(GRPP; 氨基酸片段1 - 30)、胰高糖素(氨基酸片段33 - 61)、胃泌酸调节素(OXM;氨基酸片段 33 - 69)的肠高血糖素(氨基酸片段1 - 69)和含胰高糖素样肽1 (GLP-1;氨基酸片段 72-107/108)、胰高糖素样肽2 (GLP-2;氨基酸片段126 - 158)的主要胰高糖素原片段(MPGF，氨基酸片段72 - 158)(图1)[1,8-10]。这些不同位点切割后形成的肽统称为胰高糖素原衍生肽（PGDPs），对全身代谢具有重要的调节作用。可见，Gcg和GLP-1、GLP-2来源于同一个基因。

图1. 胰高糖素原的组织选择性加工示意图

PCSK1：激素前转化酶1/3；PCSK2:激素前转化酶2；NTS：孤束核；GRPP：肠高血糖素相关肽；IP-1：中间肽-1；IP-2：中间肽-2；MPGF:主要胰高糖素原片段；GLP-1：胰高糖素样肽-1；GLP-2：胰高糖素样肽-2。

Gcg基因在胰腺、小肠和大脑的表达均受同一个启动子的调控，从相同的转录起始密码子开始启动。在α-细胞中，TATA盒以及相邻的G1和G4元件是Gcg表达所必需的最小启动子，而G5、G2、G3和CRE元件则代表位于更远端的增强子区域[11-14](图2)。

图2.胰岛中前胰高糖素原的组织选择性加工示意图

Pax6：配对盒6；CDX2/3：尾型同源盒2/3；MafB: MAF bZIP转录因子B；cMaf: c-Maf诱导蛋白；NKX2.1：NK2同源盒1；PDX1：胰腺和十二指肠同源盒1；Pax4：配对盒4；CRE：cAMP反应元件；CREB：cAMP反应元件结合蛋白；PPG:前胰高糖素原；HNF3：肝细胞核因子3；Isl1：ISL LIM同源盒1；Preb：催乳素元件结合。

α-细胞中刺激Gcg表达的信号事件包括转录因子配对盒蛋白6 (Pax6)与细胞肌肉腱膜纤维肉瘤(c-Maf)、MAF bZIP转录因子B (MafB)或尾型同源盒2/3 (Cdx2/3)的异二聚化，随后这些异二聚体与G1元件结合(图2)[15-18]。Pax6也可以与G3元件结合[19]，在调节Gcg表达和α-细胞发育中发挥关键作用[20,21]。

其他调节Gcg在α-细胞/L细胞中表达的转录机制包括Cdx2/3(小鼠中为Cdx2，仓鼠中为Cdx3)[15,22]、POU结构域转录因子脑4 (Brn-4)、LIM同源盒蛋白1(Isl1)[23-27]、肝细胞核因子3α(HNF3α；又名Foxa1)、肝细胞核因子3β(HNF3β；又名Foxa2)、配对盒蛋白2 (Pax2)，神经元分化因子1/β 2 (NeuroD/β 2)和带有G1、G2、G3或G4元件的基础螺旋-环-螺旋转录因子E47[19,13,16,18,22,23]。研究发现缺乏Foxa1或Foxa2的小鼠在出生后不久就会因严重低血糖和胰腺中Gcg mRNA水平显著降低而死亡[28,29]。值得注意的是，缺乏Foxa2的小鼠α-细胞发育受损，但缺乏Foxa1的小鼠α-细胞可以正常发育[28,29]，这表明Foxa1通过对Gcg启动子的作用影响胰高糖素水平，而Foxa2除了调节Gcg启动子的活性外，还影响α-细胞的分化。

在α-细胞和肠道L-细胞中，Gcg的表达受某些同源结构域蛋白[11,15,19]和cAMP活化蛋白激酶A(PKA)的调控[30-34]。其他促进肠Gcg表达的因素包括蛋白水解物[12]和胰岛素[35]。胰岛素刺激肠道Gcg启动子活性和GLP-1分泌[35]值得关注，因为胰岛素抑制离体大鼠胰岛[36]和仓鼠胰岛InR1-G9细胞系的胰高糖素的产生和分泌[37]。胰岛素抑制胰高糖素产生的机制通常被认为是通过α-细胞Gcg启动子中的胰岛素反应元件(IRE)的转录机制实现的[38]，这个机制还有待确认。

Gcg在α-细胞中稳定表达，但它在β-细胞中的表达却受到抑制。因为其结合了与G1元件结合的胰腺和十二指肠同源序列1 (Pdx1)、Pax4和同源序列蛋白Nkx6.1，竞争性地阻止Pax6/Maf异二聚体与G1元件结合(图2)[19,39,40]。

综上所述，Gcg的细胞性选择性表达是通过十几种转录因子在Gcg启动子和增强子区域选择性结合顺式作用元件来调控，从而刺激或抑制Gcg的表达。除此之外PKA也可以刺激Gcg的表达，以应对cAMP水平的增加[11,12,33]。胰岛素刺激肠道Gcg表达[35]，同时抑制α-细胞中Gcg的表达[7,37,38]。

二、前胰高糖素原的翻译后加工

绝大多数胰高糖素是在胰腺α-细胞中产生的，但是在某些条件下，在肠道L-细胞中、在NTS的某些Gcg阳性神经元中也检测到少量胰高糖素[2-5]。胰高糖素原的组织特异性切割是由激素原转化酶(PC)协调的。PC1(又名PCSK1或PC1/3)在脑和肠GCG+细胞中表达[41-44]，PC2(又名PCSK2)在胰腺中高表达[44]，PCSK1和PCSK2分别在大脑、肠道和胰腺中将胰高糖素原切割成相应的小分子片段(图1)[41-45]。

在生理条件下，α-细胞中PCSK2是主要的激素原转化酶，而在代谢应激的啮齿动物模型中，α-细胞PCSK1免疫反应性增加。α-细胞PCSK1活性和/或表达在胚胎和新生儿小鼠、妊娠以及糖尿病前期和糖尿病模型中均有发现[46-49]。在培养的α-细胞系或分离的胰岛中，高浓度葡萄糖可增加PCSK1表达和细胞GLP-1含量[50,51]。在链脲菌素诱导的β-细胞破坏后，胰岛PCSK1和Gcg表达急剧增加[47]。腺病毒在α-细胞中过表达PCSK1会增加胰岛GLP-1的产生和分泌[52]。胰高糖素受体敲除小鼠α-细胞GLP-1也有代偿性增加，GLP-1受体(GLP-1R)信号参与链脲菌素处理后葡萄糖反应的保留[53,54]。综上，在胰岛素抵抗、妊娠和细胞应激条件下，α-细胞通过分泌胰岛内GLP-1来补偿β-细胞功能需求的增加[55,56]。另外证据表明，胰腺GLP-1生产和分泌在某些情况下也通过旁分泌作用调节胰岛素分泌[57-59]。从β-细胞特异性GCGR敲除小鼠分离出来的胰岛中保留了胰高糖素刺激胰岛素分泌的作用，但用exendin(9-39)处理这些胰岛后其作用减弱[60]。另外从β-细胞特异性GLP-1R敲除小鼠分离的胰岛中胰高糖素刺激胰岛素分泌减少[60]。这些均强调了α-细胞与β-细胞之间的联系，并表明此时胰高糖素可能是主要的PGDP，通过GLP-1R来刺激胰岛素分泌[60]。

GLP-1的各种形式均由胰高糖素原加工而成，不同形式的GLP-1增强胰岛素分泌的能力各不相同。这些形式包括GLP-1(1-37)(或1-36酰胺)和GLP-1(7-36酰胺，酰胺化GLP-1)、GLP1(7-37，甘氨酸延伸的GLP-1)两种片段(图1)[61]。在人体循环中几乎约80%的GLP-1为GLP-1(7-36酰胺)，约20%为甘氨酸延伸GLP-1(7-37)[62]。各片段的相对丰度因物种而异[63-65]。

三 胰高糖素受体家族

GLP-1、GLP-2、胰高糖素、GIP、分泌素和生长激素释放激素(GHRH)属于一组结构相关的肽，通过与结构相似的B类家族G蛋白耦联受体(GPCRs)结合来促进其生物学作用[66,67]。这个家族的成员都是7个跨膜Gαs-耦联受体，均通过激活腺苷酸环化酶来增加cAMP[67]。这个家族的每一个受体都是根据其唯一且独特的内源性配体(GLP-1R、GLP-2R、GCGR、GIPR、SCTR和GHRHR)而命名。在生理条件下，该家族不同配体与受体之间无交叉反应[68,69]。

GLP-1R的组织分布

GLP-1R存在于胰岛素瘤细胞系[69-71]、生长抑素分泌细胞[70,71]、大鼠[72]和人[73]的离体胰岛中。在大鼠肺[74]、人脑、肾、胃、心脏中也有表达，肝脏、甲状腺、骨骼肌中没有表达[75,76]。

最近的研究确定了GLP-1R在脂肪细胞中（分化小鼠3T3-L1前脂肪细胞[77]、人骨髓源性间充质干细胞形成的脂肪细胞[78]，人心外膜脂肪[79]和人内脏白色脂肪组织[80]）表达。

在人类和非人类灵长类动物组织的胰腺中，证实GLP-1R在β-细胞和δ-细胞中稳定表达，在腺泡细胞中较弱，在腺管细胞中不表达[81]，在α-细胞中不表达或表达受限[82]。在肾和肺的动脉壁、窦房结的心肌细胞和十二指肠的Brunner腺中进一步检测到GLP-1R[81]。GLP-1R在胃壁细胞、平滑肌细胞和肠道肌肠丛神经元中低表达。在非人灵长类动物中，GLP-1表达部位在下丘脑的各个核团、后叶区、NTS和迷走神经背侧运动核[83]。而在人类大脑中。GLP-1R mRNA在大脑皮层、下丘脑、海马、丘脑、尾状核-壳核、苍白球等部位表达[84]。进一步转基因动物研究中发现，GLP-1R启动子在胰腺β-细胞和δ-细胞、血管平滑肌、心房、胃窦和幽门、肠神经元、迷走神经和背根神经节中都有活性[82]。在大脑中，GLP- 1R阳性的主要区域包括脑后区、弓状核（ARC）、腹膜内侧核（VMH）和髓质腹外侧区，而室旁核（PVH）和（孤束核）NTS中表达较低[82,85]。

配体诱导GLP-1R活化

大鼠和人类GLP-1R蛋白由463个氨基酸残基组成，物种之间的序列同源性高达90%[67,68]。GLP-1R属于B类G蛋白耦联受体（GPCR），由7个跨膜片段及细胞内、外襻，以及C-端胞内结构域和约120个氨基酸残基的N-端胞外结构域(ECD)组成[86]。GLP-1R在内质网(ER)生物合成时，其(以及其他B类GPCRs)N-端ECD包含一个编码信号肽的前导短序列。这个信号肽对于受体通过内质网的转位以及将受体运输到细胞表面至关重要[67,86,87]。通过位点定向诱变阻断信号肽会导致受体在ER中滞留[87]。受体通过内质网转位后，信号肽被切除，在ECD起点留下N-端完整的GLP-1R和四个β-片层，通过六个半胱氨酸(Cys)残基之间的三个二硫键连接[86]。Cys1和Cys3之间的二硫键连接受体的N-端α-螺旋到第一个β-片层，而Cys2和Cys5之间的二硫键连接两个β-片层，Cys4和Cys6之间的二硫键连接靠近受体C-端结构域的中心β-片层[86,88]。

对于大多数胰高糖素受体家族成员来说，受体的激活需要配体的N-端残基与受体的跨膜螺旋和细胞外襻相互作用[86,89]。研究提出了双结构域模型，即GLP-1通过其α-螺旋和C-端序列与GLP-1R的N-端ECD结合，然后通过GLP-1的N-端残基与受体的跨膜螺旋和细胞外襻结合激活GLP-1R[86,90]。

另有研究提出了配体诱导受体构象模型[91]。基于此模型，负变构调节剂通过结合GLP-1R螺旋VI与VIl之间的缝隙，结合GLP-1R并使其失活，从而将螺旋VI推向非活性状态，阻止了受体与G蛋白的结合[91]。正变构调节剂则通过结合螺旋V和VI，结合并激活GLP-1R，从而为G蛋白创造一个细胞间结合位点[91]。

配体诱导的GLP-1R激动存在偏向

GLP-1R的激活通过Gs信号刺激cAMP的形成以及通过Gq/11通路增加细胞内Ca2+并募集β-抑制蛋白促进ERK1/2信号[92,93]。尽管与相同的受体结合，不同的GLP-1R配体可以通过选择不同途径引发不同的细胞反应[92,93]。这种不同配体的选择性偏向作用被认为可能会强化特定信号通路而优化治疗效果。通过评估cAMP的形成、Ca2+的积累和ERK1/2的磷酸化，人们发现exendin-4和胃泌酸调节素相对于GLP-1更偏向β-抑制蛋白信号[92,93]。β-抑制蛋白的下调减弱了GLP-1对胰岛素分泌的刺激，降低了ERK1/2和CREB的激活[94]。β-抑制蛋白募集则激活ERK1/2等机制促进细胞增殖和存活[95]。相对于GLP-1和exendin-4，GLP-1R激动剂exendin P5更倾向于G蛋白耦联信号介导CAMP信号的增强从而有效改善葡萄糖，而非β-抑制蛋白的募集的缺失[96]。GLP-1信号的功能研究结合配体-受体复合物的三维建模确定GLP-1R ECD是偏向激动的关键触发因子。

GLP-1R失敏和再敏

在某些情况下，配体诱导的受体激活必须终止，或者受体被配体激活的敏感性必须恢复。有关GLP-1R失敏和随后GLP-1R再敏的分子机制的解释有待进一步探究。

GPCRs的失敏通常是由两个家族的丝氨酸/苏氨酸激酶实现的，即第二信使依赖蛋白激酶和受体特异性G蛋白耦联受体激酶(GRKs)[97,98]。配体诱导的受体激活后，这些激酶使受体磷酸化，诱导细胞内抑制素的募集，然后抑制素与GPCR结合，并将受体与其异三聚体G蛋白解耦联[97,98]。受体失敏的另一种潜在模式是配体-受体复合物转位到细胞内核体，使得受体与细胞膜分离[97,98]。虽然这种配体诱导的受体内化对受体失活的重要性仍存在争议，可能存在受体特异性，但受体内化对受体再敏至关重要。一个普遍认同的模型是磷酸化和抑制素结合受体与之前的膜结合G蛋白耦联受体磷酸酶(GRP)一起，被吞入成熟的核小体。核小体的酸性(pH值约5-6)会导致受体构象的改变，使GRP能够结合受体并使其去磷酸化。随着配体和抑制素的解离，再敏的受体或返回质膜或在溶酶体中被降解[98]。证据表明，配体诱导的GLP-1R信号在受体内吞作用后部分被保留，配体与受体相互作用的停留时间也影响内化受体继续发出信号的持续时间[99]。

GLP-1R缺陷的动物会怎样？

GLP-1R缺乏的小鼠在自由喂食标准饮食时仍可存活，且无明显的代谢表型[100]。这些小鼠体重和摄食正常[100]，但出现糖耐量受损[100]。长期喂食高脂饮食(HFD)后，GLP-1R敲除小鼠反而比野生型小鼠更瘦，出现了矛盾的保护作用。外周[101]或中枢[100]给予GLP-1R激动剂均不能影响GLP-1R缺乏的小鼠的血糖控制、体重和食物摄入。

专家寄语：

苏青教授 上海交通大学医学院附属新华医院

GLP-1和胰高糖素来自同一个基因。胰高糖素基因原始翻译产物在细胞内切除信号肽后，成为胰高糖素原。胰高糖素原在细胞内进一步加工切割成不同的肽段。在胰岛α-细胞，胰高糖素原最主要的加工产物是胰高糖素。在肠道L细胞，胰高糖素原最主要的加工产物是GLP-1。GLP-1和GIP都是重要的肠道激素，具有重要的代谢调节作用。GLP-1受体激动剂除降糖作用外，还有减轻体重、心血管及肾脏保护作用，已在临床广泛应用。GLP-1受体和GIP受体双激动剂、GLP-1受体和胰高糖素受体双激动剂也已开发成功，初步的临床研究显示它们具有良好的降糖和减重作用。此外，某些肽类激素受体或共受体（如FGFR1–β- klotho）的单克隆抗体、多受体激动剂也在研发当中。这些新兴药物的独特作用已预示其广阔的临床前景。相信在不久的将来这些新兴药物定会造福广大的患者，助力“健康中国”宏伟目标的实现。

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