劉艷 田秀標(biāo) 韓穎

·小論壇·

GLP-1受體激動(dòng)劑呈葡萄糖依賴性刺激胰島β細(xì)胞胰島素分泌的機(jī)制

劉艷 田秀標(biāo) 韓穎

2型糖尿病的主要治療目標(biāo)是控制高血糖，然而傳統(tǒng)的治療藥物如胰島素和磺脲類藥物在降低血糖的同時(shí)也可以出現(xiàn)低血糖。因此，應(yīng)在一定的血糖范圍內(nèi)自我調(diào)整用藥量，避免出現(xiàn)低血糖。低血糖會(huì)造成身心障礙，同時(shí)可引起心血管事件。因此臨床用藥應(yīng)最大限度的降低低血糖風(fēng)險(xiǎn)。胰高血糖素樣肽-1受體激動(dòng)劑呈葡萄糖依賴性的刺激胰島素分泌，在胰島素劑量很小或者缺乏，以及血糖濃度不高時(shí)，胰高血糖素樣肽-1受體激動(dòng)劑則不會(huì)刺激或很少刺激胰島素分泌。

β細(xì)胞；抗糖尿病藥物； GLP-1；胰島素分泌；2型糖尿病

胰島素和胰高血糖素共同維持血糖的穩(wěn)定。外源性胰島素或磺脲類藥物治療，雖然可降低高血糖，但是達(dá)到理想的血糖水平后會(huì)繼續(xù)發(fā)揮降糖作用，因此會(huì)增加低血糖風(fēng)險(xiǎn)[1]。中、重度低血糖可降低患者生活質(zhì)量[2]。同時(shí)，低血糖也與QT間期延長、心律失常和其他心血管不良事件有關(guān)[3-4]。因此，對于糖尿病患者來說，降低低血糖的風(fēng)險(xiǎn)非常關(guān)鍵。胰高血糖素樣肽(GLP)-1是由腸道L細(xì)胞分泌的腸促胰素。與GLP-1受體(GLP-1R) 結(jié)合后通過葡萄糖依賴方式誘導(dǎo)胰島素的分泌。 研究顯示，GLP-1R激動(dòng)劑改善胰島素分泌的能力取決于血糖水平。當(dāng)血糖水平降至正常時(shí)，GLP-1R激動(dòng)劑也不再發(fā)揮作用。本文對GLP-1R激動(dòng)劑呈葡萄糖依賴性促進(jìn)胰島素分泌的機(jī)制進(jìn)行綜述

1 GLP-1R激動(dòng)劑呈葡萄糖依賴性的促進(jìn)胰島素分泌

一項(xiàng)大鼠胰島素瘤細(xì)胞株的體外實(shí)驗(yàn)證實(shí)，GLP-1R激動(dòng)劑促進(jìn)胰島素分泌的作用是呈葡萄糖依賴性的。GLP-1 (10 nmol/L)誘導(dǎo)的胰島素分泌量和單獨(dú)10 mmol/L葡萄糖環(huán)境下所誘發(fā)的胰島素最大分泌量較基礎(chǔ)水平的胰島素分泌量分別增加1.5倍和2.5倍，而當(dāng)同時(shí)給予10 mmol/L葡萄糖和GLP-1 (10 nmol/L)所誘發(fā)的最大胰島素分泌量較基線水平增加了6倍[5]。同樣，在大鼠胰腺灌注實(shí)驗(yàn)中，基線葡萄糖濃度(2.8 mmol/L)時(shí)GLP-1 (25 nmol/L)介導(dǎo)的胰島素分泌量僅輕度增加，當(dāng)葡萄糖濃度增加到5 mmol/L時(shí)，GLP-1介導(dǎo)的胰島素分泌量較單獨(dú)的5 mmol/L葡萄糖所誘導(dǎo)的胰島素分泌量明顯增加[6]。另有研究顯示，給空腹的健康受試者靜脈注射藥理濃度的GLP-1(7-36 酰胺)，不會(huì)引起低血糖[7]。表明GLP-1的促胰島素分泌作用是葡萄糖依賴性的，并且需要一定濃度的葡萄糖才能發(fā)揮作用。

與天然的GLP-1相似，GLP-1R激動(dòng)劑(如exendin-4)促進(jìn)胰島素分泌的作用也需要一定水平的葡萄糖。給健康空腹的受試者分別連續(xù)靜脈輸注艾塞那肽或安慰劑，與安慰劑組相比，在正糖鉗夾試驗(yàn)中(5.0 mmol/L)，艾塞那肽組胰島素分泌更多。在同樣的受試者中，當(dāng)血漿葡萄糖濃度降低至低血糖水平(4.0 mmol/L)時(shí)，艾塞那肽組的胰島素分泌量迅速下降到安慰劑組水平[8]。臨床中，艾塞那肽治療引起低血糖的發(fā)生率很低[9-10]。同樣，艾塞那肽或利拉魯肽單藥治療或與口服降糖藥物聯(lián)合治療，在降低糖化血紅蛋白之外，重度或輕度低血糖的發(fā)生率與安慰劑相比，差異無統(tǒng)計(jì)學(xué)意義[11-12]。甘精胰島素和格列美脲與GLP-1R激動(dòng)劑相比，低血糖的發(fā)生率均增加。 另外，磺脲類藥物(如格列美脲)可以影響GLP-1R激動(dòng)劑的葡萄糖依賴性，因此GLP-1R激動(dòng)劑聯(lián)用磺脲類藥物時(shí)低血糖發(fā)生率增加[13-16]。

2 葡萄糖誘導(dǎo)的胰島素分泌

胰島素主要在致密核心囊泡(最終變成分泌顆粒)中被儲(chǔ)存和轉(zhuǎn)運(yùn)。胞吐過程包括修飾和分泌顆粒融合于細(xì)胞膜。該過程由可溶性的NSF附著蛋白受體復(fù)合蛋白調(diào)節(jié)。囊泡膜上的NSF附著蛋白受體即小突觸泡蛋白與細(xì)胞膜上的NSF附著蛋白受體，突觸融合蛋白-1和SNAP25結(jié)合，并在細(xì)胞膜附近形成一個(gè)穩(wěn)定的復(fù)合物，從而利于膜的融合定位，促進(jìn)胰島素從胰島β細(xì)胞的分泌[17]。

胰島素分泌的速率主要取決于含胰島素顆粒的數(shù)量和分泌能力。大部分胰島素顆粒(95%～99%)存在于儲(chǔ)存池，一小部分(1%～5%)存在于待釋放池，后者離細(xì)胞膜更近[18]。待釋放池可進(jìn)一步分化為立即釋放池。立即釋放池與電壓依賴性鈣離子通道(VDCC) 結(jié)合后鑲嵌在細(xì)胞膜上，當(dāng)VDCC開放，Ca2+內(nèi)流時(shí)迅速準(zhǔn)備釋放胰島素。待釋放池內(nèi)容物的釋放主要負(fù)責(zé)胰島素第一時(shí)相的分泌，而相對緩慢的第二時(shí)相則反映了待釋放池的再填充即顆粒從儲(chǔ)存池向待釋放池的移動(dòng)和裝填過程。研究顯示，胰島素顆粒的酸化有助于儲(chǔ)存池的顆粒移動(dòng)到待釋放池，而顆粒的低pH值可能誘導(dǎo)SNARE蛋白的結(jié)構(gòu)變化，從而促進(jìn)膜的融合及定位[19]。

3 GLP-1R信號轉(zhuǎn)導(dǎo)與葡萄糖介導(dǎo)的胰島素分泌

GLP-1R 是一個(gè)有7次跨膜區(qū)域的G蛋白耦聯(lián)受體。GLP-1R激活細(xì)胞膜結(jié)合的腺苷酸環(huán)化酶，從而迅速產(chǎn)生cAMP。cAMP的下游效應(yīng)器分別為蛋白激酶A (PKA)和cAMP 調(diào)節(jié)的鳥嘌呤核苷酸交換因子(cAMP-GEF)——Epac。研究顯示，PKA和Epac在GLP-1介導(dǎo)的胰島素釋放過程中起關(guān)鍵作用[20-22]。

研究顯示，肌醇三磷酸受體和蘭逆堿受體可控制Ca2+從胞內(nèi)釋放，這些受體的拮抗劑可抑制鈣誘導(dǎo)的鈣釋放[26]。肌醇三磷酸受體的調(diào)節(jié)性區(qū)域包括Ca2+的結(jié)合位點(diǎn)，依賴于游離 Ca2+的濃度，可以控制通道的活動(dòng)。蘭逆堿受體同樣受Ca2+調(diào)節(jié)。GLP-1R激動(dòng)劑通過Ca2+依賴的方式促進(jìn)鈣誘導(dǎo)的鈣釋放，該過程需要細(xì)胞內(nèi)游離Ca2+的濃度迅速增加，從而調(diào)節(jié)Ca2+從胞內(nèi)釋放[27]。

GLP-1R活化PKA后不僅促進(jìn)胞吐顆粒中含有的胰島素分泌，而且可以調(diào)節(jié)待釋放池的重填，而這兩個(gè)過程對于第二相胰島素分泌至關(guān)重要。采用穿孔膜片鉗全細(xì)胞記錄法可以讓小鼠β細(xì)胞膜產(chǎn)生強(qiáng)烈而重復(fù)的去極化，從而誘導(dǎo)Ca2+內(nèi)流，促進(jìn)胞吐作用。而用forskolin持續(xù)增加cAMP濃度，使得Ca2+內(nèi)流，細(xì)胞去極化，通過這種方式產(chǎn)生的胞吐作用持續(xù)的時(shí)間顯著延長。提示cAMP可促進(jìn)囊泡從儲(chǔ)存池流動(dòng)到待釋放池。應(yīng)用PKA抑制劑僅可抑制部分胞吐作用， 表明cAMP促進(jìn)胞吐作用是通過依賴和不依賴PKA兩種途徑來進(jìn)行的。進(jìn)一步的研究證實(shí)，依賴PKA的cAMP的調(diào)節(jié)作用是Epac依賴性的。給小鼠使用選擇性的Epac激動(dòng)劑(8CPT-2Me-cAMP)，結(jié)果小鼠β細(xì)胞去極化，胞吐作用明顯增加。該效應(yīng)是在胞吐作用的早期，提示Epac可能增加了待釋放池的大小，從而可以更迅速的應(yīng)答Ca2+內(nèi)流[28]。Epac和PKA的結(jié)合效應(yīng)與GLP-1R活化過程時(shí)一、二相胰島素分泌增加一致。除促進(jìn)已經(jīng)儲(chǔ)存的胰島素釋放，GLP-1也可誘導(dǎo)胰島素基因的轉(zhuǎn)錄[29]。

總之，GLP-1R激動(dòng)劑在血糖升高時(shí)促進(jìn)胰島素的分泌，當(dāng)血糖降至正常水平以下時(shí)又能停止促泌作用。因此，其除了更好的降低糖化血紅蛋白外，低血糖的發(fā)生率顯著降低。然而GLP-1R激動(dòng)劑呈葡萄糖依賴性的降糖機(jī)制還有待進(jìn)一步研究。

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MechanismofGLP-1receptoragonistsinthestimulationofinsulinsecretionofisletβcellinaglucose-dependentmanner

LiuYan*,TianXiubiao,HanYing.

*GraduateSchoolofTianjinMedicalUniversity,Tianjin300070,China

The aim to treat type 2 diabetes is to control the high blood glucose. However, the traditional drugs such as insulin and sulfonylureas can not only lower the blood glucose, but also cause hypoglycemia.Therefore, the drug dosage should be adjusted frequently in order to maintain the blood glucose in a certain range and to avoid hypoglycemia. Hypoglycemia can cause physical and mental disorders, as well as cardiovascular events. Therefore, great efforts should be made in clinical medication to reduce the risk of hypoglycemia. Glucagon-like peptide -1 receptor agonists can stimulate the secretion of insulin in a blood glucose-dependent manner. When the insulin dose is very low or deficient, or the concentration of blood glucose is not high, glucagon like peptide-1 receptor agonists will not or rarely stimulate insulin secretion.

β cells;Anti diabetic drugs;GLP-1;Insulin secretion;Type 2 diabetes mellitus

(IntJEndocrinolMetab，2015，35：66-69)

10.3760/cma.j.issn.1673-4157.2015.01.017

300070 天津醫(yī)科大學(xué)研究生院(劉艷)；300280 天津海濱人民醫(yī)院內(nèi)分泌科(劉艷，田秀標(biāo)， 韓穎)

2014-10-10)