MUC17：粘蛋白“冷”靶點，探索胃癌新療法！
日期：2023-01-04 09:50:31
近來越來越多的研究證實，粘蛋白多樣化的生物學(xué)功能在腫瘤形成、細(xì)胞粘附、免疫應(yīng)答以及細(xì)胞信號傳導(dǎo)中起著至關(guān)重要的作用。當(dāng)前，多個粘蛋白已成為腫瘤免疫治療的火熱靶點，如MUC1，MUC16。此外，根據(jù)ClinicalTrials數(shù)據(jù)顯示，新近一項基于MUC17靶點（AMG199）的雙抗臨床項目，正在美國開展I期研究（NCT04117958），用于轉(zhuǎn)移性胃癌和胃食管交界癌（G/GEJ）患者，這是第一個將MUC17作為潛在抗腫瘤靶點的臨床試驗。
在腫瘤組織中，粘蛋白家族靶點多出現(xiàn)異常表達(dá)，與腫瘤的浸潤、轉(zhuǎn)移及預(yù)后相關(guān)。近年來，多個粘蛋白被認(rèn)為是某些癌癥的重要生物標(biāo)志物，以及非常具有前景的治療靶點。相比其它火熱的粘蛋白靶點，MUC17作為粘蛋白“冷”靶點，雖尚未經(jīng)廣泛推廣、暫無藥物上市，然而陸續(xù)的研究表明，MUC17是潛力非常不錯的免疫靶點！或許能為胃癌和胃食管交界癌（G/GEJ）或其它疾病治療帶來新曙光！
1. 什么是粘蛋白（Mucin）？
粘蛋白（Mucin，MUC）是上皮細(xì)胞產(chǎn)生、分泌的以粘液為主要成分的高分子糖蛋白。迄今為止，已發(fā)現(xiàn)有22種Mucin蛋白，根據(jù)黏蛋白的功能將其分為兩類，膜結(jié)合型粘蛋白（MUC1、MUC3A、MUC3B、MUC4、MUC12、MUC13、MUC14、MUC15、MUC16、MUC17、MUC20、MUC21、MUC22）和分泌型粘蛋白（MUC2、MUC5AC、MUC5B、MUC6、MUC7、MUC8、MUC9、MUC19） [1-3]。分泌型粘蛋白可以形成物理屏障，作為一種黏液凝膠，為呼吸道和胃腸道上皮細(xì)胞提供保護?？缒ば驼车鞍淄ㄟ^它們胞外域的O糖基化串聯(lián)重復(fù)序列形成棒狀結(jié)構(gòu)，構(gòu)建保護性的黏液凝膠層 [3]。大量炎癥細(xì)胞因子，比如白介素-1β（interleukin-1β，IL-1β）、IL-4、IL-6、TGF-β、IL-9、IL-13、IFN-γ和TNF-α，已經(jīng)被證明能促進體外培養(yǎng)的上皮細(xì)胞內(nèi)黏蛋白的表達(dá) [4-5]。
黏蛋白廣泛分布于機體多種器官和組織的黏膜表面，如消化道、呼吸道、生殖道、角結(jié)膜，以及肝臟、胰腺、膽囊、腎、唾液腺和淚腺上皮細(xì)胞 [3-4]。近年發(fā)現(xiàn)黏蛋白參與上皮細(xì)胞的分化、更新、調(diào)節(jié)細(xì)胞黏附、細(xì)胞信號轉(zhuǎn)導(dǎo)等過程 [1]。有研究提示，粘蛋白可作為預(yù)測COVID-19易感性的潛在工具 [5]。此外，大量研究已證實，黏蛋白基因在腫瘤的發(fā)生發(fā)展中發(fā)揮重要作用，且因其具有組織特異性和細(xì)胞特異性，目前越來越受到國內(nèi)外學(xué)者的重視（圖1）[6-10]。
圖1. 黏蛋白在腫瘤發(fā)生發(fā)展中發(fā)揮重要作用 [6]
2. 什么是MUC17？
MUC17（Mucin-17，又稱MUC3）是一種跨膜粘蛋白，由位于7號染色體的q22位點的MUC17基因編碼。MUC17于2002年發(fā)現(xiàn)并被鑒定，由跨膜段、胞內(nèi)段和胞外段三部分組成（圖2） [11, 14]。從氨基末端依次為粘蛋白樣結(jié)構(gòu)（串聯(lián)重復(fù)序列），EGF（表皮生長因子）樣區(qū)域，N-糖基化區(qū)域，第二個EGF樣區(qū)域，疏水的跨膜段，及胞漿內(nèi)部的梭基末端 [12]。MUC17依靠羧基端附近跨膜區(qū)域與細(xì)胞膜結(jié)合，可參與細(xì)胞內(nèi)信號傳遞。胞外段決定著MUC特異性的空間結(jié)構(gòu)和免疫原性 [14]。MUC17的表達(dá)可增強腸道粘液屏障，促進炎癥愈合。在上皮細(xì)胞中，MUC17起到參與信號轉(zhuǎn)導(dǎo)，維持管腔結(jié)構(gòu)，提供細(xì)胞保護，并給予失去極性的癌細(xì)胞抗黏附性能，從而使腫瘤細(xì)胞失去極性 [15]。
MUC17主要表達(dá)于腸道，是腸道黏液的主要組成成分。MUC17表達(dá)可在胎兒小腸和結(jié)腸內(nèi)被檢測。多數(shù)研究表明，正常胃粘膜無表達(dá)或少表達(dá) [16-18]。研究發(fā)現(xiàn)，從正常胃黏膜、腸化黏膜、進展期胃癌的發(fā)展過程中，MUC17的表達(dá)逐漸明顯增高，其低表達(dá)與胃癌預(yù)后不良相關(guān)。進一步研究顯示MUC17的表達(dá)與腫瘤的分化程度呈負(fù)相關(guān)，提示MUC17可作為胃癌的診斷和治療靶標(biāo) [19]。目前，基于MUC17的雙抗正在進行臨床研究，用于陽性轉(zhuǎn)移性胃癌和胃食管交界癌患者 [20]。
圖2. MUC17的結(jié)構(gòu) [14]
3. MUC17介導(dǎo)的信號轉(zhuǎn)導(dǎo)通路
MUC17作為具重要生物學(xué)功能的膜結(jié)合型粘蛋白，根據(jù)國內(nèi)外文獻報道，MUC17表達(dá)水平與多種炎癥、腫瘤細(xì)胞增殖相關(guān)，這為MUC17相關(guān)性疾病的防治提供了重要線索，但其分子機制不清。
在結(jié)腸腺癌細(xì)胞中，用促炎性細(xì)胞因子TNFα的長期刺激，誘導(dǎo)炎性狀態(tài)，可導(dǎo)致MUC17的增加，進而保護結(jié)腸腺癌細(xì)胞免受腸病原性大腸桿菌的黏附，其結(jié)果說明MUC17在炎癥中可起到防御作用（圖3）[21]。另有研究提示，MUC17的EGF樣結(jié)構(gòu)域可通過與MYH9相互作用，活化Rho/Rock通路，從而抑制炎癥因子對NFκB通路的活化，最終致使MUC17發(fā)揮了其抗炎的功能 [22]。還有一些關(guān)于MUC17的報道揭示，MUC17的C端與支架蛋白PDZK1結(jié)合，使其穩(wěn)定定位于小腸的腸細(xì)胞頂膜 [23]；DNA甲基化和組蛋白H3K9修飾有助于MUC17表達(dá) [24]。
同樣在結(jié)腸癌中，有研究揭示MUC17可與EGFR家族的ERBB2結(jié)合并使其磷酸化，激活Wnt信號，調(diào)控EMT靶基因表達(dá)進而啟動EMT [25-26]。此外，還有研究表明，初乳來源的、含TFLK基序的合成肽，能刺激結(jié)腸細(xì)胞株HT29細(xì)胞MUC17表達(dá)增加。TFLK基序是MSAA3蛋白的特征結(jié)構(gòu)序列，而MSAA3蛋白對新生兒預(yù)防病原菌侵襲、保護新生兒胃腸道有重要作用。該機制對增強腸道粘液屏障，防治腸道多種炎癥性疾病將提供新的思路 [27-28]。
圖3. MUC17在炎癥中可起到防御作用 [21]
4. MUC17在腫瘤中的作用
MUC17作為一個典型的跨膜黏蛋白，MUC17在抵御細(xì)菌侵襲、保護腸黏膜、促進上皮增生及炎癥愈合中發(fā)揮著重要作用，其表達(dá)減少是多種腸道疾病如新生兒腸炎、炎癥性腸病發(fā)生發(fā)展的重要原因 [29, 30]。目前，大量的研究集中在MUC17與胃腸道腫瘤和其它腫瘤，如結(jié)腸癌、胃癌、胰腺癌、食管癌、乳腺癌 [31-40]。
在結(jié)腸癌中，采用免疫組織化學(xué)法，檢測MUC17在結(jié)直腸癌及癌旁相對正常黏膜組織中的表達(dá)，MUC17在正常黏膜組織也呈100%陽性表達(dá)，結(jié)直腸癌標(biāo)本中MUC17陽性表達(dá)率為53.3%，表達(dá)呈下調(diào)趨勢，且MUC17的表達(dá)與結(jié)直腸癌的淋巴結(jié)轉(zhuǎn)移、浸潤深度和分期密切相關(guān) [16, 31]。
在胰腺癌中，與正常胰腺組織比較，MUC17在胰腺癌組織中過表達(dá)，用免疫組化檢測胰腺導(dǎo)管腺癌組織發(fā)現(xiàn)，MUC17表達(dá)顯著增強，提示MUC17可作為獨立的預(yù)后因素之一 [32-33]。另有研究提示，MUC17的表達(dá)變化與Barrett食管、食管腺癌、賁門腸化和賁門癌的發(fā)展呈正相關(guān) [35-37]。在乳腺癌中，研究發(fā)現(xiàn)MUC17敲低與患者藥物敏感性有關(guān)，進一步揭示MUC17和PCNX1可作為乳腺癌化療反應(yīng)的潛在生物標(biāo)志物，調(diào)節(jié)其活性可以影響化療耐藥性 [25, 38-39]。
在胃癌及胃食管交界處癌（G/GEJ）中，有研究證實MUC17在G/GEJ細(xì)胞膜上過表達(dá)，屬于腫瘤相關(guān)抗原（TAA）。胃癌與胃粘膜屏障功能障礙有關(guān)，而胃粘膜屏障的損害通常會導(dǎo)致慢性炎癥，這是胃癌發(fā)生的主要因素。進一步研究發(fā)現(xiàn)，當(dāng)MUC17表達(dá)降低，腫瘤侵襲細(xì)胞個數(shù)減少，細(xì)胞劃痕愈合率及細(xì)胞增殖率降低，克隆形成能力減弱。敲低MUC17表達(dá)可抑制胃癌細(xì)胞的侵襲和增殖能力，這提示MUC17或?qū)⒖赡艹蔀槲赴┲委煹男掳袠?biāo) [40]。
5. MUC17的臨床應(yīng)用前景
目前已有一款靶向MUC17的雙特異性抗體（AMG199；MUC17 x CD3）獲臨床許可，來自安進生物，用于MUC17陽性的實體腫瘤，包括胃腸癌、胃食管交界處、結(jié)腸直腸癌和胰腺癌。2020年，F(xiàn)DA授予AMG199孤兒藥資格認(rèn)定。2021年ASCO大會上，安進公布了首次在人體的I期開放標(biāo)簽劑量遞增研究，評估AMG199治療MUC17陽性胃/胃食管交界癌癥患者的劑量限制性毒性及有效率，并確定最大耐受劑量（MTD）和/或推薦的2期劑量（RP2D），預(yù)計在2024年3月完成70名受試者臨床I期研究。
大量研究提示粘蛋白在不同惡性腫瘤中過表達(dá)，粘蛋白與腫瘤的密切關(guān)系及誘發(fā)的免疫反應(yīng)為基于粘蛋白的抗腫瘤藥物設(shè)計提供了重要線索，許多新突破，新的藥物、新的治療方案正在路上。與此同時，隨著對粘蛋白MUC17結(jié)構(gòu)和功能研究的進一步深入，MUC17在腫瘤中的研究展示了良好的應(yīng)用前景，我們也期待MUC17靶點在腫瘤免疫治療方面帶來創(chuàng)新性或突破性進展！
為鼎力協(xié)助各藥企針對MUC17在腫瘤等疾病治療領(lǐng)域上的研發(fā)，CUSABIO推出MUC17活性蛋白產(chǎn)品（Code: CSB-MP727848HU，助力您在MUC17機制方面的研究或其潛在臨床價值的探索。
Recombinant Human Mucin-17(MUC17), partial (Active)
The purity was greater than 95% as determined by SDS-PAGE. (Tris-Glycine gel) Discontinuous SDS-PAGE (reduced) with 5% enrichment gel and 15% separation gel.
Immobilized Human MUC17 at 2 μg/mL can bind Anti-MUC17 recombinant antibody (CSB-RA727848MA1HU), the EC50 is 0.9057-1.259 ng/mL.
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