TGF-β/Smad信號(hào)通路在胰腺癌上皮間質(zhì)轉(zhuǎn)化過(guò)程中的作用

姚瑤 林堃 黃浩杰 李兆申

·綜述與講座·

TGF-β/Smad信號(hào)通路在胰腺癌上皮間質(zhì)轉(zhuǎn)化過(guò)程中的作用

姚瑤 林堃 黃浩杰 李兆申

胰腺導(dǎo)管腺癌( pancreatic ductal adenocarcinoma，PDAC)發(fā)病率逐年升高，早期診斷困難，5年生存率低，死亡率高，目前的治療效果仍不理想[1-3]。PDAC具有間質(zhì)比重大，乏血供的特征，癌周間質(zhì)與癌細(xì)胞相互作用促進(jìn)胰腺癌進(jìn)展[4]，上皮-間質(zhì)轉(zhuǎn)化(epithelial-mesenchymal transition，EMT)在其中起著關(guān)鍵的作用。因此，眾多研究將腫瘤的治療針對(duì)誘導(dǎo)細(xì)胞發(fā)生EMT的關(guān)鍵因子轉(zhuǎn)化生長(zhǎng)因子-β(transforming growth factor beta，TGF-β)。本文就TGF-β/Smad信號(hào)通路在胰腺癌EMT過(guò)程中的作用進(jìn)行綜述。

一、EMT過(guò)程

EMT的概念最早于1982年由Greenburg和Hay[5]提出，EMT是指在某些特殊的生理或病理?xiàng)l件下，上皮細(xì)胞失去了與基底膜的連接等上皮表型，極性消失，獲得較高的遷移與侵襲、抗凋亡和降解細(xì)胞外基質(zhì)(extracellular matrix，ECM)的能力等間質(zhì)表型，即轉(zhuǎn)變?yōu)榫哂谢顒?dòng)能力、能在ECM中移動(dòng)的間質(zhì)細(xì)胞的過(guò)程。它分為3種類型[6]：第一種是原腸胚胎時(shí)期，原始的上皮細(xì)胞發(fā)生EMT,參與多細(xì)胞生物胚胎發(fā)育和器官形成；第二種是內(nèi)皮或上皮轉(zhuǎn)化為組織成纖維細(xì)胞，參與組織的傷口愈合和器官纖維化等過(guò)程；第三種是上皮來(lái)源的細(xì)胞失去極性轉(zhuǎn)化為具有侵襲和遷移運(yùn)動(dòng)能力的腫瘤細(xì)胞。有研究認(rèn)為發(fā)生EMT的細(xì)胞能夠在類似于人體內(nèi)基底膜的基質(zhì)膠上生長(zhǎng)并穿透基質(zhì)膠，提示EMT可能是腫瘤細(xì)胞突破基底膜浸潤(rùn)性生長(zhǎng)的一個(gè)重要機(jī)制[7-8]。

二、TGF-β/Smad信號(hào)通路的組成及其在EMT中的作用

目前發(fā)現(xiàn)EMT涉及的一些通路主要有TGF-β/Smad通路、Wnt/β-catenin通路、PI3K/AKT通路、Notch通路等。其中TGF-β在誘導(dǎo)EMT過(guò)程中的作用已經(jīng)比較明確，是誘導(dǎo)EMT的主要因子之一，主要通過(guò)Smad和非Smad信號(hào)通路誘導(dǎo)腫瘤細(xì)胞發(fā)生EMT，與腫瘤侵襲轉(zhuǎn)移密切相關(guān)[9-11]。

TGF-β是廣泛存在于細(xì)胞間且具有多種生物學(xué)效應(yīng)的細(xì)胞因子，對(duì)細(xì)胞及機(jī)體的發(fā)育、增殖、分化、凋亡、血管生成、間質(zhì)形成、代謝等功能起到關(guān)鍵作用[12]。機(jī)體多種細(xì)胞均可分泌非活性狀態(tài)的TGF-β。在體外，非活性狀態(tài)的TGF-β可通過(guò)酸處理活化；在體內(nèi)，酸性環(huán)境可存在于骨折附近和正在愈合的傷口，蛋白本身的裂解作用也可活化TGF-β。一般在細(xì)胞分化活躍的組織常含有較高水平的TGF-β，如成骨細(xì)胞、腎臟、骨髓和胎肝的造血細(xì)胞。TGF-β通過(guò)與3個(gè)高親和性受體Ⅰ型受體(TβRⅠ)、Ⅱ型受體(TβRⅡ)、Ⅲ型受體(TβRⅢ)相結(jié)合，才能啟動(dòng)跨膜信號(hào)，發(fā)揮生物學(xué)效應(yīng)。Smad蛋白是TGF-β信號(hào)轉(zhuǎn)導(dǎo)通路的始動(dòng)因子。目前已發(fā)現(xiàn)至少9種Smad蛋白,從結(jié)構(gòu)和功能上分為3個(gè)亞型：受體調(diào)節(jié)性Smad，包括Smad1、2、3、5、8；共同介導(dǎo)性Smad，如Smad4；抑制性Smad，如Smad6、7等。

在體內(nèi)具有生物活性的TGF-β蛋白同轉(zhuǎn)化生長(zhǎng)因子結(jié)合蛋白(laetnt TGF-β binding portein，LTBP)結(jié)合后轉(zhuǎn)變?yōu)闊o(wú)活性狀態(tài)，并分泌到ECM[13]。在ECM中通過(guò)蛋白酶分解等作用，TGF-β同LTBP解離恢復(fù)活性?；罨腡GF-β首先與TβRⅡ在細(xì)胞外結(jié)合，TβRⅡ通過(guò)調(diào)節(jié)TβRI胞質(zhì)區(qū)的GS結(jié)構(gòu)域的絲氨酸/蘇氨酸磷酸化激活TβRI,活化后的TβRI使下游的信號(hào)分子Smad2/3發(fā)生磷酸化，再進(jìn)一步與Smad4形成復(fù)合物進(jìn)入胞核，調(diào)節(jié)相應(yīng)靶基因轉(zhuǎn)錄，引起緊密連接蛋白、上皮生物標(biāo)志物E-cadherin等表達(dá)下調(diào)，間質(zhì)生物標(biāo)志物蛋白表達(dá)上調(diào)，使上皮來(lái)源的腫瘤細(xì)胞失去極性呈現(xiàn)纖維樣表型，黏附能力下降，細(xì)胞遷移和侵襲能力增強(qiáng)[14]。國(guó)內(nèi)外均有大量文獻(xiàn)報(bào)道，在肝臟、肺、腎臟、乳腺和胰腺等器官中，TGF-β/Smad信號(hào)通路可通過(guò)誘導(dǎo)EMT過(guò)程促進(jìn)腫瘤的發(fā)生和轉(zhuǎn)移[15-18]。

三、胰腺癌EMT與TGF-β/Smad信號(hào)通路的關(guān)系

1．TGF-β信號(hào)通路在胰腺癌發(fā)展過(guò)程中的作用：近年來(lái)遺傳學(xué)及表觀遺傳學(xué)研究發(fā)現(xiàn)，從正常胰腺導(dǎo)管上皮到上皮內(nèi)瘤變，以及最終進(jìn)展為胰腺癌的過(guò)程中存在一些表達(dá)顯著改變的基因如K-ras、p53、Smad4、p16等[19-21]。Smad4是TGF-β信號(hào)通路的關(guān)鍵分子，且TGF-β信號(hào)通路在胰腺癌的間質(zhì)形成、ECM沉積中起關(guān)鍵作用[22]。TGF-β1在腫瘤發(fā)生發(fā)展過(guò)程中具有兩面性，在胰腺癌早期TGF-β1可以使細(xì)胞停留在G1期抑制細(xì)胞增殖，發(fā)揮抑癌作用；當(dāng)胰腺癌進(jìn)展后，Smad4發(fā)生突變失活，TGF-β1依賴Smad4的抑癌作用被阻擋，此時(shí)腫瘤細(xì)胞自身分泌大量的TGF-β1使上皮細(xì)胞出現(xiàn)間質(zhì)化改變，進(jìn)而促進(jìn)胰腺癌生長(zhǎng)、侵襲及轉(zhuǎn)移[23-24]。

2．胰腺癌間質(zhì)的成分與特點(diǎn)：間質(zhì)是腫瘤細(xì)胞間及其周圍的組織，包含成纖維細(xì)胞、星狀細(xì)胞、血管神經(jīng)、炎癥細(xì)胞、免疫細(xì)胞等多種成分[25]，在胰腺癌發(fā)展過(guò)程中參與形成腫瘤細(xì)胞的微環(huán)境，間質(zhì)內(nèi)部各種成分之間及間質(zhì)與腫瘤細(xì)胞間均存在信號(hào)通路之間的相互作用，共同參與腫瘤發(fā)生、發(fā)展及藥物抵抗等過(guò)程[26-28]。多項(xiàng)研究證實(shí)上皮細(xì)胞是胰腺癌腫瘤間質(zhì)中纖維成分的重要來(lái)源之一，而TGF-β信號(hào)通路在EMT過(guò)程中起到了核心作用[29]。

3．TGF-β/Smad信號(hào)通路參與胰腺癌EMT并促進(jìn)腫瘤發(fā)展：在胰腺癌間質(zhì)形成過(guò)程中，間質(zhì)與胰腺癌細(xì)胞的相互作用稱為促結(jié)締組織增生反應(yīng)(desmoplastic reaction，DR)。腫瘤細(xì)胞通過(guò)旁分泌方式分泌的配體可以激活間質(zhì)中TGF-β信號(hào)通路，促進(jìn)ECM沉淀。國(guó)內(nèi)外均有文獻(xiàn)報(bào)道TGF-β信號(hào)通路參與間質(zhì)的起源[30-32]。目前認(rèn)為胰腺癌間質(zhì)有多種來(lái)源，主要包括已存在的間質(zhì)細(xì)胞、間充質(zhì)干細(xì)胞轉(zhuǎn)分化、EMT、腫瘤干細(xì)胞分化而來(lái)。體外實(shí)驗(yàn)發(fā)現(xiàn)TGF-β可以誘導(dǎo)纖維母細(xì)胞表達(dá)成纖維細(xì)胞標(biāo)志物并且使其具有成纖維細(xì)胞的形態(tài)特征，還可以通過(guò)活化癌周的間質(zhì)細(xì)胞，促進(jìn)腫瘤細(xì)胞募集間質(zhì)細(xì)胞形成腫瘤血管[33]。有學(xué)者報(bào)道在胰腺癌細(xì)胞株及手術(shù)切除的胰腺癌組織中發(fā)現(xiàn)EMT現(xiàn)象的存在[34]。大量的信號(hào)通路包括TGF-β、Wnt、Hh、PDGF等都參與胰腺癌間質(zhì)形成過(guò)程并促進(jìn)腫瘤的進(jìn)展。Sannino等[35]發(fā)現(xiàn)胰腺癌細(xì)胞中上皮細(xì)胞標(biāo)記物β-catenin的協(xié)同因子BCL9L表達(dá)水平升高，并通過(guò)TGF-β信號(hào)通路誘導(dǎo)EMT，從而導(dǎo)致胰腺癌的進(jìn)展，而敲除該基因可抑制腫瘤細(xì)胞的生長(zhǎng)和肝轉(zhuǎn)移。Thakur等[36]發(fā)現(xiàn)p53的一種同源蛋白Tap73在胰腺癌中過(guò)表達(dá)，并促進(jìn)胰腺癌的發(fā)展，在小鼠胰腺癌模型中發(fā)現(xiàn)Tap73缺失增加了EMT程度，進(jìn)一步證明了Tap73的缺失可通過(guò)上調(diào)Bgn、Sma及Smad等TGF-β信號(hào)通路的關(guān)鍵蛋白的表達(dá)水平和活性從而促進(jìn)EMT的發(fā)生，導(dǎo)致腫瘤發(fā)展。Zhao等[37]研究發(fā)現(xiàn)一種抑制腫瘤生長(zhǎng)的因子miRNA-34a及其下游靶基因Sirt1和Notch1通過(guò)調(diào)節(jié)EMT相關(guān)基因表達(dá)參與EMT過(guò)程，從而促進(jìn)慢性胰腺炎向胰腺癌的發(fā)展。Lin等[38]認(rèn)為鈣調(diào)蛋白家族成員之一Tagln2在胰腺癌的增殖和侵襲過(guò)程中起重要的調(diào)節(jié)作用，TGF-β/Smad信號(hào)通路可通過(guò)Smad受體磷酸化誘導(dǎo)Tagln2在胰腺細(xì)胞的高表達(dá)，從而促進(jìn)胰腺癌的轉(zhuǎn)移。

四、展望

胰腺癌具有間質(zhì)比例高、成分多等特征，在腫瘤發(fā)展過(guò)程中由于基因累積突變，間質(zhì)與腫瘤細(xì)胞相互作用，不斷改變腫瘤微環(huán)境以利于腫瘤生長(zhǎng)。TGF-β1信號(hào)通路參與了由正常胰腺導(dǎo)管上皮轉(zhuǎn)變?yōu)橐认侔┑倪^(guò)程。近年來(lái)學(xué)者們熱衷研究的TGF-β/Smad信號(hào)通路為探索胰腺癌侵襲轉(zhuǎn)移機(jī)制、尋找新的診斷分子標(biāo)記物和治療靶點(diǎn)提供了很有前景的研究方向。但由于TGF-β/Smad信號(hào)通路調(diào)控機(jī)制復(fù)雜，涉及相關(guān)基因數(shù)量眾多，眾多研究報(bào)道的結(jié)果仍眾說(shuō)紛紜。因此，還需要更多、更加深入的關(guān)于TGF-β/Smad信號(hào)通路介導(dǎo)胰腺癌EMT作用機(jī)制的研究，從而進(jìn)一步闡明胰腺癌發(fā)生發(fā)展的機(jī)制，為胰腺癌侵襲轉(zhuǎn)移的分子干預(yù)提供潛在的作用靶點(diǎn)，以期在基因水平實(shí)現(xiàn)治療目的。

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(本文編輯：屠振興)

10.3760/cma.j.issn.1674-1935.2017.01.019

200433 上海，第二軍醫(yī)大學(xué)長(zhǎng)海醫(yī)院消化內(nèi)科

李兆申，Email: zhsli@81890.net;黃浩杰，Email:pea1920@hotmail.com

2016-04-12)