趙素潔， 陸 怡， 吳曉琰

復(fù)旦大學(xué)附屬華山醫(yī)院老年病科，上海 200040

·綜 述·

生長分化因子11在衰老中的研究進(jìn)展

趙素潔， 陸 怡， 吳曉琰*

復(fù)旦大學(xué)附屬華山醫(yī)院老年病科，上海 200040

生長分化因子11(growth differentiation factor 11, GDF11)是轉(zhuǎn)化生長因子β(transforming growth factor-β，TGF-β)家族的一員。GDF11在多種器官和組織中表達(dá)。生長發(fā)育期間，GDF11通過可逆性地阻滯細(xì)胞周期來調(diào)節(jié)發(fā)育進(jìn)程；在衰老方面，GDF11可延緩心臟、骨骼肌、大腦、骨骼、血管等多個(gè)器官和組織的衰老進(jìn)程。本文就GDF11在生長發(fā)育及衰老中的作用及其機(jī)制作一綜述，以期為GDF11在衰老方面的研究提供指導(dǎo)。

生長分化因子11；衰老；轉(zhuǎn)化生長因子β

轉(zhuǎn)化生長因子β(transforming growth factor-β，TGF-β)家族可以分為4個(gè)亞類：轉(zhuǎn)化生長因子類(TGFs)；激活素類(activins)；骨形成蛋白類(BMPs)；生長分化因子類(GDFs)，如GDF11和其結(jié)構(gòu)類似物GDF8；其他，如抗苗勒氏管激素(AMH)和GDF15[1]。與TGF-β家族其他成員一樣，GDF11和GDF8的前體在細(xì)胞內(nèi)合成后以成熟信號肽形式分泌到細(xì)胞外，與相應(yīng)細(xì)胞膜表面的Ⅰ型、Ⅱ型受體結(jié)合后活化細(xì)胞質(zhì)內(nèi)的信號分子，并將信號傳遞給細(xì)胞核，進(jìn)而調(diào)控基因的表達(dá)。GDF11和GDF8可結(jié)合Ⅰ型受體中的激活素受體B(activin typeⅠreceptor B, ActRⅠB)、轉(zhuǎn)化生長因子Ⅰ型受體(TGF-β receptor Ⅰ，TβRⅠ)，Ⅱ型受體中的激活素受體A(activin typeⅡreceptor A， ActRⅡA)、激活素受體B(activin typeⅡreceptor B，ActRⅡB)。GDF11和GDF8在細(xì)胞質(zhì)中激活的信號分子為Smad2/3 (smart mothers against decapentaplegic-2 and -3，Smad2/3)。細(xì)胞膜接受膜外信號分子傳遞的信號后選擇性磷酸化Smad2/3，Smad2/3把信號傳遞給細(xì)胞核，從基因水平調(diào)控細(xì)胞的一系列生物活動(dòng)[1-2]。

TGF-β家族在衰老方面發(fā)揮著至關(guān)重要的作用[3-4]。阻滯GDF11的同源類似物GDF8可以改善小鼠衰老相關(guān)的肌肉萎縮，并增加其胰島素敏感性[5]。近年來，研究[6]發(fā)現(xiàn)TGF-β/Smad通路通過激活p21基因誘導(dǎo)細(xì)胞周期阻滯來干預(yù)哺乳動(dòng)物胚胎發(fā)育期間的程序性衰老。

1 GDF11與GDF8的調(diào)節(jié)因子

TGF-β家族種類繁多，配體遠(yuǎn)多于受體，信號有條不紊的傳遞依賴多種調(diào)控因子。人GDF11前體蛋白有407個(gè)氨基酸，其二聚體由兩個(gè)GDF11單體組成，每個(gè)單體由109個(gè)氨基酸組成，成熟GDF11即為酶切后的GDF11前體C端成熟區(qū)[7]。GDF11可以通過誘導(dǎo)機(jī)體產(chǎn)生卵泡抑素(follistatin，F(xiàn)ST)來建立負(fù)反饋調(diào)節(jié)[8]。FST通過活化周期素依賴性激酶2/4(cyclin-dependent kinase2/4, cdk2/4)，上調(diào)細(xì)胞周期蛋白D1基因(cyclinD1)，下調(diào)p21基因，進(jìn)而抑制Smad2/3磷酸化，減弱GDF11對肌源性祖細(xì)胞(myosphere-derived progenitor cells ，MDPCs)的抑制作用[9]。生長分化因子相關(guān)血清蛋白(growth and differentiation factor-associated serum protein，GASP)包括GASP 1和GASP2，是GDF11/GDF8的天然抑制信號分子[2]。GASP、FST與GDF11相協(xié)調(diào)共同參與機(jī)體的發(fā)育[1-2,8]。

2 GDF11對組織器官生長發(fā)育及衰老的作用

GDF11基因作為1種保守基因，廣泛參與機(jī)體生長發(fā)育的調(diào)控，在衰老中的作用雖有爭議，但大部分研究顯示其有抗衰老作用(表1)。

表1 GDF11對生長發(fā)育及衰老的作用

2.1 GDF11對心肌、骨骼肌的作用 2013年，哈佛醫(yī)學(xué)院研究團(tuán)隊(duì)[10]通過外科手術(shù)使高齡小鼠與低齡小鼠共用一套血液循環(huán)，發(fā)現(xiàn)循環(huán)中的GDF11可緩解高齡小鼠的心肌肥大。他們還發(fā)現(xiàn)小鼠循環(huán)中的GDF11隨著年齡增長濃度降低[10,31]。在3月齡小鼠體內(nèi)，GDF11的mRNA表達(dá)量由高到低分別為脾、胸腺、腎、視網(wǎng)膜、骨骼肌、小腦、前腦、骨髓、心臟、小腸、肺、肝，且脾的表達(dá)量明顯高于其他組織[31]。按照0.1 mg/kg給高齡小鼠注射外源性重組GDF11蛋白(recombinant GDF11，rGDF11)，結(jié)果發(fā)現(xiàn)衛(wèi)星細(xì)胞的比例明顯增加，小鼠肌纖維結(jié)構(gòu)得到改善[14]。與GDF11結(jié)構(gòu)相似的GDF8在小鼠、牛、羊骨骼肌的生長中發(fā)揮明顯的抑制作用[32]。

2.2 GDF11對神經(jīng)系統(tǒng)的作用 GDF11對神經(jīng)發(fā)育的調(diào)節(jié)大多是抑制性作用，并呈現(xiàn)一定的時(shí)空特異性，即在不同組織及不同發(fā)育階段，GDF11對細(xì)胞增殖的作用不同。在胚胎視網(wǎng)膜發(fā)育階段，GDF11通過調(diào)節(jié)祖細(xì)胞中影響視網(wǎng)膜神經(jīng)節(jié)細(xì)胞(retinal ganglion cell，RGC)發(fā)生的Math5基因表達(dá)來間接影響RGC的發(fā)育[30]。在胚胎嗅神經(jīng)發(fā)生過程中，GDF11則是通過上調(diào)細(xì)胞周期依賴的激酶抑制劑p27激酶抑制蛋白1(p27 kinase inhibition protein1，p27Kip1)的水平來可逆性誘導(dǎo)定向祖細(xì)胞的周期阻滯，從而調(diào)節(jié)RGC的發(fā)育[18]。

然而，與GDF11胚胎神經(jīng)發(fā)育期間的抑制性調(diào)節(jié)作用不同，研究[19]發(fā)現(xiàn)，GDF11對衰老大腦則起促進(jìn)神經(jīng)再生的作用。22個(gè)月齡小鼠連續(xù)注射GDF11 4周后，共聚焦顯微鏡成像顯示大腦室下區(qū)(subventricula zone，SVZ)神經(jīng)細(xì)胞數(shù)量增多，此外血管數(shù)目和血容量明顯增加。之后，該研究團(tuán)隊(duì)發(fā)現(xiàn)，GDF11可促進(jìn)高齡小鼠嗅神經(jīng)及血管重建[19]；共生模型的老年小鼠神經(jīng)髓鞘得到修復(fù)和再生[16]；低齡小鼠的血液有改善高齡小鼠認(rèn)知的作用[33]。研究[17]發(fā)現(xiàn)，通過給阿爾茨海默病(Alzheimer′s disease，AD)小鼠移植低齡小鼠富含GDF11的脾臟，可以緩解其認(rèn)知和記憶能力減退。

對脊髓發(fā)育影響的研究[34]發(fā)現(xiàn)，GDF11可能經(jīng)GDF11-Smad2通路調(diào)控同源盒基因(Hox)在脊髓中的表達(dá)區(qū)域及延喙尾軸在脊髓尾部的位置；而FST作為GDF11的抑制性因素參與靶基因的調(diào)控。

2.3 GDF11對血液循環(huán)系統(tǒng)的作用 應(yīng)用酶聯(lián)免疫吸附法(enzyme linked immunosorbent assay，ELISA)、LC-MS/MS等多種方法檢測，在小鼠、大鼠、馬、羊等多種動(dòng)物中已檢測到GDF11。但是，血液中GDF11的濃度與年齡的關(guān)系目前沒有定論[10,31]。研究[26-27]發(fā)現(xiàn)，在血液透析患者體內(nèi)，血清GDF11高濃度與低血紅蛋白濃度有關(guān)；GDF11-ActRⅡB-Smad2/3 通路參與紅系造血，并與血液透析患者促紅細(xì)胞生成素(erythropoietin，EPO)無效性貧血有關(guān)。國內(nèi)有研究[35]報(bào)道，骨髓增生異常綜合征(myelodysplastic syndromes, MDS)患者血液中的GDF11濃度明顯高于健康人。此外，小鼠脾臟中GDF11高表達(dá)[31]可能與脾臟本身儲存大量的血小板有關(guān)。

有關(guān)GDF11與貧血關(guān)系的研究較深入。靶作用GDF11及其相關(guān)信號分子在治療β-地中海貧血方面已經(jīng)取得明顯的進(jìn)展[26-29]。GDF11-ActRⅡA通路與β-地中海貧血的病情進(jìn)展有密切的關(guān)系，GDF11在有核紅細(xì)胞中高表達(dá)，促進(jìn)終末期骨髓無效造血；人體GDF11及TGF-βs阻斷劑RAP-011、RAP-536(對應(yīng)小鼠體內(nèi)的ACE-011、ACE-536)，可以捕獲ActRⅡA配體，阻滯GDF11-ActRⅡA通路，達(dá)到改善貧血的效果[28]。

2.4 GDF11對血管的作用 國內(nèi)關(guān)于人臍靜脈內(nèi)皮細(xì)胞(human umbilical vein endothelial cells，HUVECs)的研究[36]表明，GDF11(50 ng/mL)處理細(xì)胞48 h可激活Smad1/5/8和Smad2/3信號通路，增加HUVECs中的NADPH氧化酶(NOX4)、磷酸化的c-Jun氨基末端激酶(p-JNK)和磷酸化的絲裂原活化蛋白激酶(p-AMPK)的含量；GDF11處理24 h后，噻唑藍(lán)(MTT)實(shí)驗(yàn)檢測顯示，細(xì)胞活性增加，72 h后細(xì)胞活性降低，但GDF11沒有改變細(xì)胞增殖和遷移能力。有研究[37]報(bào)道，GDF11可促進(jìn)內(nèi)皮祖細(xì)胞 (endothelial progenitor cells, EPCs) 分化，利于血管形成。GDF11能促進(jìn)腦內(nèi)毛細(xì)血管再生[19]。

2.5 GDF11對其他組織的作用 研究[38]發(fā)現(xiàn)，在小鼠胚胎中，NGN3+內(nèi)分泌祖細(xì)胞表達(dá)GDF11；GDF11通過抑制神經(jīng)原素3(neurogenin 3，NGN3)的表達(dá)來調(diào)節(jié)胰島細(xì)胞分化。GDF11-Smad2通路參與胰島β細(xì)胞的成熟，并影響胰島β細(xì)胞的數(shù)目[24]。GDF11與糖尿病的發(fā)生和發(fā)展也有密切聯(lián)系。2015年，骨骼衰老相關(guān)的研究[21]發(fā)現(xiàn)，GDF11一方面可調(diào)節(jié)間充質(zhì)干細(xì)胞(MSC)向脂肪細(xì)胞和成骨細(xì)胞分化的百分比，另一方面通過降低過氧化物酶體增生物激活受體-γ(peroxisome proliferators-activated receptor-γ，PPAR-γ)及促進(jìn)其類泛素化來緩解骨質(zhì)疏松。

3 總結(jié)與展望

GDF11通過調(diào)控祖細(xì)胞基因表達(dá)或通過可逆性阻滯對已分化細(xì)胞周期來影響機(jī)體發(fā)育，并在多個(gè)器官的衰老進(jìn)程發(fā)揮作用。多項(xiàng)報(bào)道提示，GDF11在胚胎神經(jīng)、骨骼、肌肉發(fā)生期間通過GDF11-ActRⅡA/ActRⅡB-Smad或GDF11-TβRⅠ-Smad通路發(fā)揮反饋性抑制作用[8,20,39-40]。GDF11在神經(jīng)發(fā)生、血管生成及動(dòng)脈粥樣硬化過程中[19,37]發(fā)揮的作用使GDF11通路有望成為治療老年癡呆、高血壓、心血管疾病、糖尿病等疾病的新靶點(diǎn)。干預(yù)GDF11與其類似物GDF8有望成為治療衰老相關(guān)的心肌肥大、肌肉萎縮等疾病的新思路。

細(xì)胞衰老本質(zhì)是由各種原因?qū)е碌牟豢赡娴募?xì)胞周期阻滯，這些原因可以是端?？s短、DNA損傷及強(qiáng)烈的致癌信號等，且細(xì)胞衰老也機(jī)體免于產(chǎn)生腫瘤的一個(gè)保護(hù)因素[41]。GDF11通過抑制細(xì)胞從G1期向S期轉(zhuǎn)變的進(jìn)程來抑制MDPCs增殖[9]；通過可逆性地誘導(dǎo)嗅神經(jīng)細(xì)胞周期阻滯來調(diào)節(jié)神經(jīng)發(fā)育[18]。臨床研究[42]發(fā)現(xiàn)，GDF11高水平與結(jié)腸癌患者預(yù)后差緊密相關(guān)，結(jié)腸癌組織中高表達(dá)的GDF11可能與TGF-β有相似作用，即將腫瘤細(xì)胞周期阻滯在G1期。TGF-β激活p21通路引起的細(xì)胞周期阻滯參與哺乳動(dòng)物胚胎發(fā)育期間細(xì)胞的程序性衰老[6]。

GDF11對機(jī)體的生長發(fā)育及衰老的調(diào)控貫穿自發(fā)育起始的整個(gè)生命歷程。GDF11及TGF-β家族信號網(wǎng)絡(luò)與衰老有密切的聯(lián)系，干預(yù)GDF11及TGF-β家族信號通路有望延緩衰老，相關(guān)機(jī)制有待更加深入的探索。

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[本文編輯] 姬靜芳

Advances on aging of growth differentiation factor 11

ZHAO Su-jie，LU Yi，WU Xiao-yan*

Department of Gerontology，Huashan Hospital，Shanghai Medical College of Fudan University, Shanghai 200040，China

Growth differentiation factor 11(GDF11), a member of the transforming growth factor β(TGF-β)family, is widely expressed in many tissues. GDF11 regulates multiple developmental processes by reversing cell cycle arrest and reverses the aging process of many tissues including heart, skeletal muscle, brain, bone and blood vessels. In this article, we review the role of GDF11 in aging considering the TGF-β family signaling pathway and the mechanism of GDF11 in growth and development.

GDF11; aging; transforming growth factor-β

2016-11-01 [接受日期] 2017-04-14

上海市科學(xué)技術(shù)委員會重點(diǎn)項(xiàng)目科技創(chuàng)新行動(dòng)(10431904000). Supported by the Key Program of Science and Technology Commission of Shanghai (10431904000).

趙素潔, 碩士生, 住院醫(yī)師. E-mail：sujiezhao2014@163.com

*通信作者(Corresponding author). Tel: 021-52887270, E-mail：wxygtj@ aliyun.com

10.12025/j.issn.1008-6358.2017.20161017

R 592

A