王亞卉 李青松 符之瑄
[摘要] 眼表疾病的發(fā)病機(jī)制復(fù)雜多樣,其中角結(jié)膜細(xì)胞外基質(zhì)(ECM)代謝失衡是其發(fā)病的重要原因之一。轉(zhuǎn)化生長(zhǎng)因子-β1(TGF-β1)/Smads作為調(diào)控ECM產(chǎn)生、轉(zhuǎn)歸及代謝平衡的主要信號(hào)通路,參與了本病的發(fā)生發(fā)展。本文將主要從該通路與眼表疾病的關(guān)系方面做一綜述,以期為眼表疾病的治療提供新的依據(jù)。
[關(guān)鍵詞] 轉(zhuǎn)化生長(zhǎng)因子-β1(TGF-β1)/Smads信號(hào)通路;眼表疾??;細(xì)胞外基質(zhì)
[中圖分類號(hào)] R777.33 [文獻(xiàn)標(biāo)識(shí)碼] A [文章編號(hào)] 1673-7210(2019)05(a)-0050-05
[Abstract] The pathogenesis of ocular surface diseases is complex and diverse, among which the imbalance of ECM is one of the important causes. Transforming growth factor -β1 (TGF-β1)/Smads is involved in the development of the disease as a major signaling pathway regulating the production, outcome and metabolic balance of ECM. The relationship between this pathway and ocular surface diseases will be reviewed in this paper, which will provide a new basis for the treatment of ocular surface diseases.
[Key words] Transforming grouth facter-β1(TGF-β1)/Smads signaling pathway; Ocular surface disease; Extracellular matrix
正常視力的維持依賴于角結(jié)膜的透明度及形態(tài)規(guī)則,眼表疾病往往表現(xiàn)為角結(jié)膜結(jié)構(gòu)與功能的異常,研究認(rèn)為其與角結(jié)膜細(xì)胞外基質(zhì)(ECM)代謝失衡有一定的關(guān)聯(lián)[1-2]。轉(zhuǎn)化生長(zhǎng)因子-β1(TGF-β1)/Smads是目前公認(rèn)的調(diào)控ECM代謝的相關(guān)信號(hào)通路,主要由TGF-β1、TGF-β1受體(TGFβR)、Smads蛋白及其轉(zhuǎn)錄調(diào)節(jié)因子組成。研究發(fā)現(xiàn)翼狀胬肉、干眼、圓錐角膜等眼表疾病的發(fā)生發(fā)展都與該通路密切相關(guān),靶向調(diào)控該通路可能是一種新的治療思路與方法。
1 TGFβ1的生物學(xué)特性
TGF-β1是TGF-β超家族中的關(guān)鍵一員,其生物活性的發(fā)揮依賴于與TGFβR的特異性結(jié)合,主要表現(xiàn)為促進(jìn)損傷修復(fù)、抗炎、刺激成纖維細(xì)胞和肌成纖維細(xì)胞膠原基因mRNA水平表達(dá)增高以及促使成纖維細(xì)胞的分化、增殖等[3-4]。
2 Smads蛋白的特點(diǎn)
Smads蛋白是TGF-β1的下游信號(hào)轉(zhuǎn)導(dǎo)分子,根據(jù)其功能和特征的不同分為:受體調(diào)節(jié)型(R-Smads:Smad1、2、3、5及8)、共同型(Co-Smads:Smad4)和抑制型(Ⅰ-Smads:Smad6、7)[5]。R-Smads主要與絲氨酸/蘇氨酸蛋白激酶結(jié)合,并被其激活,Co-Smads與活化的R-Smads結(jié)合形成復(fù)合物轉(zhuǎn)移至細(xì)胞核中,Ⅰ-Smads則通過(guò)抑制R-Smads的磷酸化,阻斷信號(hào)通路的轉(zhuǎn)導(dǎo)[5]。
3 TGF-β1/Smads信號(hào)轉(zhuǎn)導(dǎo)
3.1 TGF-β1的激活
TGF-β1在體內(nèi)以三種非活性的形式存在,即與非活性相關(guān)肽(LAP)形成小的非活性復(fù)合物、與LAP、TGF-β結(jié)合蛋白(LTBP)形成大的非活性復(fù)合物以及與蛋白酶抑制物α2巨球蛋白(α2-M)結(jié)合形成的TGF-β1/α2-M[6]。其常見(jiàn)激活方式有共價(jià)鍵的斷裂或修復(fù)、TGF-β1的酶解激活以及整合素、糖基化、基質(zhì)金屬蛋白酶也可激活TGF-β1[6]。
3.2 Smads介導(dǎo)的信號(hào)轉(zhuǎn)導(dǎo)
下游介質(zhì)(Smad2、3)的激活是TGF-β1發(fā)揮生物學(xué)效應(yīng)的關(guān)鍵,活化后的TGF-β1首先與TGFβRⅡ結(jié)合而啟動(dòng)細(xì)胞內(nèi)信號(hào)傳導(dǎo),隨后,TGF-β1激活TGFβRⅠ激酶,導(dǎo)致Smad2、3的磷酸化,活化后的Smad2、3與Smad4形成復(fù)合物轉(zhuǎn)運(yùn)到細(xì)胞核中調(diào)控靶基因轉(zhuǎn)錄(圖1)[7]。Smad7則通過(guò)抑制Smad2、3磷酸化來(lái)阻斷TGF-β1信號(hào)的過(guò)度激活,從而調(diào)控ECM的代謝平衡(圖1)[8]。
3.3 信號(hào)轉(zhuǎn)導(dǎo)的分子調(diào)控
TGF-β1/Smads信號(hào)轉(zhuǎn)導(dǎo)受SARA、STRAP及PTD等相關(guān)分子的調(diào)控,其中SARA通過(guò)促使R-Smads與TGFβRⅠ結(jié)合而發(fā)揮正性調(diào)節(jié)作用,相反,STRAP則下調(diào)TGF-β1的生物學(xué)作用[9]。PTD調(diào)節(jié)Smad2的磷酸化,并促進(jìn)Smad2與Smad4的結(jié)合[10]。
3.4 核內(nèi)靶基因轉(zhuǎn)錄的分子調(diào)控
活化的Smads復(fù)合物在細(xì)胞核內(nèi)積聚,同時(shí)招募轉(zhuǎn)錄激活因子(CBP、p300等)和轉(zhuǎn)錄抑制因子(SnoN等)參與靶基因轉(zhuǎn)錄[11]。其中p300、CBP的高表達(dá)促進(jìn)Smads蛋白的激活,SnoN則通過(guò)與Smads蛋白復(fù)合物相互作用抑制信號(hào)轉(zhuǎn)導(dǎo)[11]。
3.5 與其他信號(hào)通路的交叉
TGF-β1/Smads信號(hào)通路與絲裂原活化蛋白激酶(MAPK)超家族相互交叉影響,其中p38MAPK、c-Jun末端激酶(JNK)和細(xì)胞外信號(hào)調(diào)節(jié)激酶(ERK)三者都可以促進(jìn)Smad2、3的磷酸化(圖1)[12],阻滯P38MAPK通路可抑制Smad3的激活和表達(dá)[13]。Jeon等[14]發(fā)現(xiàn)小分子過(guò)氧化物酶體增殖物激活受體γ配體通過(guò)阻斷TGF-β1誘導(dǎo)下p38MAPK的磷酸化抑制Smads通路的激活,為角膜纖維化的治療開(kāi)辟了新的道路。
4 TGF-β1/smads信號(hào)通路與眼表疾病
4.1 翼狀胬肉
翼狀胬肉是一種纖維化疾病,病變侵入角膜并覆蓋視覺(jué)軸,導(dǎo)致不規(guī)則的角膜散光,最終失明。研究[15]認(rèn)為其與紫外線輻射、免疫學(xué)機(jī)制及細(xì)胞調(diào)亡與異常增生等有關(guān),其中紫外線輻射是主要的致病因素。本病目前主要采取手術(shù)切除,但術(shù)后復(fù)發(fā)率高達(dá)88%[16]。翼狀胬肉組織中TGF-β1及Ⅰ型膠原蛋白的表達(dá)明顯增多[17-18],該病與TGF-β1/Smads通路功能紊亂密切相關(guān),通過(guò)下調(diào)TGF-β1的表達(dá)或補(bǔ)充Smad4可重建TGF-β1/smads信號(hào)轉(zhuǎn)導(dǎo)通路[19],除此之外,Smad3在翼狀胬肉組織中高表達(dá),抑制Smad3可能對(duì)治療該病術(shù)后復(fù)發(fā)有效[20]。在藥物治療方面,迷迭香酸能夠通過(guò)降低翼狀胬肉上皮細(xì)胞的細(xì)胞活力、減少Ⅰ型膠原蛋白的產(chǎn)生和下調(diào)TGF-β1/Smads信號(hào)傳導(dǎo)來(lái)改善翼狀胬肉上皮細(xì)胞的纖維化[21]。綜上,靶向調(diào)控TGF-β1/Smads通路可為治療原發(fā)性及術(shù)后復(fù)發(fā)性翼狀胬肉提供一種有效的方法。
4.2 干眼
干眼的發(fā)病病因復(fù)雜多樣,結(jié)膜杯狀細(xì)胞分泌黏蛋白維持眼表微環(huán)境穩(wěn)態(tài),若杯狀細(xì)胞數(shù)量減少或分化受阻都會(huì)導(dǎo)致干眼[22]。SPDEF調(diào)控結(jié)膜杯狀細(xì)胞的分化、增殖,研究發(fā)現(xiàn)條件性敲除K14細(xì)胞中TβRⅡ后眼表上皮細(xì)胞異常增生以及杯狀細(xì)胞數(shù)量增加,同時(shí)SPDEF表達(dá)也出現(xiàn)增加,這證明TGF-β1/Smads信號(hào)通路在結(jié)膜內(nèi)限制杯狀細(xì)胞增殖,其主要機(jī)制為p-Smad2、3與Smad4復(fù)合物結(jié)合到SPDEF啟動(dòng)子上阻止其轉(zhuǎn)錄[23]。干眼的發(fā)生還與淚腺細(xì)胞、結(jié)膜上皮細(xì)胞、角膜上皮細(xì)胞的凋亡有著重要關(guān)系[24-25],且TGF-β1在角膜上皮、角膜緣上皮、結(jié)膜基質(zhì)層中廣泛表達(dá)[26-27]。因此,該通路在調(diào)控干眼的發(fā)生發(fā)展方面可做進(jìn)一步探索。
4.3 結(jié)膜松弛癥
結(jié)膜松弛癥(conjunctivochalasis,CCh)的發(fā)病機(jī)制目前尚不明確,研究發(fā)現(xiàn)其與炎性反應(yīng)有關(guān),炎性因子的高表達(dá)激活基質(zhì)金屬蛋白酶(MMPs),促進(jìn)細(xì)胞外基質(zhì)(ECM)降解,從而加重結(jié)膜松弛[28-29],具體表現(xiàn)為MMP-1、3的增加以及Ⅰ/Ⅲ型膠原蛋白比例的失衡[30]。西醫(yī)目前主要采取手術(shù)切除,在中藥治療方面發(fā)現(xiàn)杞精明目湯能夠下調(diào)CCh成纖維細(xì)胞MMP-1、3的表達(dá)[31],且進(jìn)一步研究認(rèn)為該復(fù)方能夠下調(diào)MAPK信號(hào)通路相關(guān)蛋白的表達(dá),從而為中藥靶向治療CCh提供了新的思路與方法[32]。ECM代謝失衡是CCh主要的發(fā)病機(jī)制之一,CCh的發(fā)生是否受TGF-β1/Smads信號(hào)通路的調(diào)控,值得進(jìn)一步研究,可為本病的治療提供新的理論基礎(chǔ)。
4.4 角膜疾病
角膜是重要的屈光間質(zhì),角膜纖維化形成的特征在于肌成纖維細(xì)胞的出現(xiàn)和ECM的過(guò)度沉積,TGF-β1/Smads信號(hào)通路是調(diào)節(jié)角膜纖維化的常見(jiàn)途徑[33-34]。在基因水平上抑制角膜纖維化是目前最有效的方法,Gupta等[35]發(fā)現(xiàn)AAV5-Smad7在兔角膜上單一局部應(yīng)用后,角膜混濁和角膜纖維化的顯著減少。
4.4.1 角膜營(yíng)養(yǎng)不良 角膜營(yíng)養(yǎng)不良(corneal dystrophy,CDs)常常表現(xiàn)為復(fù)發(fā)性角膜糜爛和視力損害。CDs的標(biāo)志是TGF-β1誘導(dǎo)的基因蛋白(TGFBIp)在角膜基質(zhì)中呈年齡相關(guān)性漸進(jìn)性累積,最終導(dǎo)致角膜透明度破壞,視力受損[36-37]。本病是一種基因病變,臨床上缺乏有效的治療藥物,目前認(rèn)為鋰能夠抑制CDsp-Smad3的表達(dá)[38],曲尼司特顆粒能夠通過(guò)抑制角膜成纖維細(xì)胞TGFBIp的表達(dá)來(lái)抑制TGF-β1信號(hào),從而抑制p-Smad2/Smad3的表達(dá),延緩角膜混濁的發(fā)生[39],但二者的具體療效尚待進(jìn)一步研究。手術(shù)干預(yù)是目前主要的治療方法,但不是一勞永逸的方法,如何在基因水平上研發(fā)藥物靶向治療本病,則需要眼科研究者的深入探索。
4.4.2 圓錐角膜 圓錐角膜(keratoconus,KC)是一種遺傳性角膜變薄性疾病,其特點(diǎn)是由于角膜結(jié)構(gòu)的改變導(dǎo)致角膜完整性喪失和視力下降[40]。研究發(fā)現(xiàn)KC中Smad6、7顯著下調(diào)[41]。在基因水平上,TGF-β1信號(hào)通路的激活誘導(dǎo)TGFBIp發(fā)生改變,從而誘導(dǎo)膠原蛋白、層粘連蛋白、纖連蛋白及蛋白多糖的黏附[42]。目前沒(méi)有徹底治療KC的方法,Sharif等[43]發(fā)現(xiàn)角膜交聯(lián)術(shù)后抑制性因子Smad6、7的表達(dá)明顯降低,因此靶向調(diào)控TGF-β1/Smads信號(hào)通路可能對(duì)治療KC有效。
5 小結(jié)與展望
正常的眼表結(jié)構(gòu)與功能是獲得良好視覺(jué)質(zhì)量的前提條件,角結(jié)膜的形態(tài)是否規(guī)則與眼表疾病的發(fā)生密切相關(guān),ECM作為細(xì)胞之間的機(jī)械支持和連接結(jié)構(gòu),參與細(xì)胞之間的信號(hào)傳遞,因此調(diào)控角結(jié)膜ECM代謝平衡是治療眼表疾病的關(guān)鍵。綜上所述,靶向調(diào)控TGF-β1/smads信號(hào)通路可能對(duì)治療眼表疾病有一定療效,但目前對(duì)其作用機(jī)制研究尚少,有待進(jìn)一步探索,以為眼表疾病的治療開(kāi)拓新的思路與方法。
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(收稿日期:2018-10-31 本文編輯:封 華)