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綜述

MicroRNAs通過BMP/TGF-β信號通路調(diào)節(jié)骨形成的研究進(jìn)展

張靜龔逸明

(復(fù)旦大學(xué)附屬中山醫(yī)院口腔科，上海200032)

The Progress of microRNAs in Osteogenisis by Regulating BMP/TGF-β Signaling Pathway

ZHANGJingGONGYiming

DepartmentofStomatology,ZhongshanHospital,FudanUniversity,Shanghai200032,China

1microRNAs在骨形成中的作用

microRNAs(miRNAs)是一類內(nèi)源性單鏈小分子(～22nt)非編碼RNA序列,通過抑制靶基因轉(zhuǎn)錄及下調(diào)蛋白表達(dá)在多種生理和病理過程中起到重要的調(diào)控作用[1]。越來越多的研究表明，miRNAs在干細(xì)胞工程中具有潛在的應(yīng)用價值，同時參與并調(diào)節(jié)骨形成和骨重建[2]，如：miR-96促進(jìn)MC3T3-E1細(xì)胞成骨分化，而miR-125 b可以通過下調(diào)Cbfβ表達(dá)抑制C3H10T1/2 細(xì)胞增殖和成骨分化[3-4], miR-21過表達(dá)的骨髓間充質(zhì)干細(xì)胞可以加速大鼠閉合性股骨骨折愈合[5],轉(zhuǎn)染miR-26a的脂肪來源干細(xì)胞結(jié)合羥基磷灰石(HA)支架能夠顯著促進(jìn)大鼠脛骨骨缺損中的新骨形成，因此結(jié)合miR-26a的骨替代體在骨缺損修復(fù)中具有可觀的治療潛能[6]。基質(zhì)干細(xì)胞具有自我更新和多分化潛能，在不同條件下可以分化多種細(xì)胞,包括成骨細(xì)胞、軟骨細(xì)胞和脂肪細(xì)胞[5]。miRNAs具有調(diào)節(jié)并決策基質(zhì)干細(xì)胞分化方向的能力。過表達(dá)miRNA-22能夠抑制靶基因HDAC6，進(jìn)而促進(jìn)人脂肪來源基質(zhì)干細(xì)胞成骨分化而抑制其成脂分化[7]。miR-146a過表達(dá)促進(jìn)胎兒股骨干細(xì)胞成骨分化而抑制其成軟骨分化[8]。以上研究均表明，miRNAs具有嚴(yán)格的調(diào)控機(jī)制，在成骨分化及骨形成方面發(fā)揮重要的調(diào)節(jié)作用。

2骨形成蛋白/轉(zhuǎn)化生長因子β(BMP/TGF-β)信號通路調(diào)節(jié)骨形成

2.1BMP/TGF-β信號通路組成及作用機(jī)制BMP/TGF-β信號通路廣泛參與多種生物學(xué)過程，在哺乳類動物骨形成中起到重要的調(diào)控作用[9]。BMP/TGF-β信號轉(zhuǎn)導(dǎo)包括經(jīng)典的Smads蛋白依賴的信號通路(包括BMP/TGF-β配體、受體和Smads)及不依賴Smads的非經(jīng)典通路(如p38MAPKs)。經(jīng)典的BMP/TGF-β信號通路首先與相應(yīng)的BMP或TGF受體結(jié)合，分別激活Smad 1/5/8或Smad 2/3，磷酸化的Smad 1/5/8或Smad 2/3與Smad 4蛋白結(jié)合形成復(fù)合體，該復(fù)合體進(jìn)入細(xì)胞核以直接或間接方式活化成骨特異性轉(zhuǎn)錄因子如(Runx2)，繼而促進(jìn)成骨分化和骨形成[9-10]。此外，BMP/TGF-β通路還可通過與其他成骨相關(guān)通路如 Wnt、MAPK、Notch、Hedgehog (Hh)等交互作用在骨組織代謝中發(fā)揮調(diào)節(jié)作用[11]。

2.2BMP/TGF-β信號通路對骨形成的影響B(tài)MP/TGF-β信號通路參與并精細(xì)調(diào)節(jié)骨形成過程，同時任何干擾BMP/TGF-β信號通路及相關(guān)調(diào)節(jié)因子的因素都可能造成骨形成障礙及發(fā)育不全。已有研究[12]表明， BMPs (BMP 2、BMP 4、BMP 6、BMP 7、BMP 9)能夠誘導(dǎo)干細(xì)胞成骨分化，促進(jìn)骨和軟骨形成，在骨愈合、生物工程及再生醫(yī)學(xué)方面具有重要的應(yīng)用潛能。其中，BMP 2和BMP 4缺失可導(dǎo)致小鼠四肢嚴(yán)重發(fā)育不全[13]。而TGF-β受體(TGFβR1和TGFβR2)的缺失造成小鼠長骨、軟骨缺陷；Smads蛋白如Smad 1敲除的小鼠則異位軟骨形成，成骨細(xì)胞增殖、分化受損[14-15]。

3miRNAs經(jīng)BMP/TGF-β信號通路對骨形成的調(diào)控作用

3.1調(diào)節(jié)Smads蛋白Smads蛋白在經(jīng)典的BMP/TGF-β信號通路中發(fā)揮重要的功能。Wu等[16]研究發(fā)現(xiàn)，在BMP 2誘導(dǎo)的成骨分化中，miR-30家族成員顯著下調(diào)；進(jìn)一步研究表明，miR-30家族成員直接作用于Smad 1和Runx2，進(jìn)而抑制骨形成。同樣，miR-199a可通過顯著抑制Smad 1的表達(dá)而抑制C3H10T1/2干細(xì)胞的早期軟骨分化[17]。而Li等[18]在BMP 2誘導(dǎo)的C2C12細(xì)胞成骨分化中發(fā)現(xiàn)，miR-133和miR-135表達(dá)下調(diào)，兩者分別作用于Runx2和Smad 5，協(xié)同抑制骨形成。miR-21通過抑制 Smad 7(BMP信號通路抑制劑)，促進(jìn)BMP 9誘導(dǎo)的小鼠多能細(xì)胞成骨分化[12]。Cheung等[8]發(fā)現(xiàn)，miR-146a過表達(dá)后，Smad 2/3下調(diào)，而Smad 2/3的抑制促進(jìn)Smad 1/5/8上調(diào)，進(jìn)而上調(diào)轉(zhuǎn)錄因子Runx2，促進(jìn)胎兒股骨干細(xì)胞成骨分化，但軟骨相關(guān)基因SOX9表達(dá)下調(diào)，導(dǎo)致軟骨形成被抑制。

3.2調(diào)節(jié)BMPs及其受體miRNAs可以通過調(diào)節(jié)BMPs及其受體調(diào)控骨形成。Liao等[19]研究發(fā)現(xiàn)，BMP-2 和miR-148b共表達(dá)的脂肪來源干細(xì)胞可以促進(jìn)其自身成骨分化，加速裸鼠顱骨缺損(直徑達(dá) 4 mm)的骨愈合和骨改建。在對Satb 2誘導(dǎo)小鼠骨髓基質(zhì)干細(xì)胞成骨分化的研究中發(fā)現(xiàn)，miR-27表達(dá)下降而其靶基因BMP 2、 BMPR1A等升高，過表達(dá)miR-27則顯著抑制骨形成，說明miR-27可能通過BMP/TGF-β信號通路調(diào)節(jié)骨形成[20]。同樣，miR-140-5p 和miR-654-5p 可通過直接抑制靶點BMP-2的表達(dá)而抑制基質(zhì)干細(xì)胞的成骨分化[21-22]。miR-542-3p 則可通過抑制BMP 7及其調(diào)控信號通路而抑制成骨細(xì)胞增殖和分化[23]。

miRNAs也可以通過調(diào)節(jié)BMP信號通路抑制因子調(diào)控骨形成。Zhang等[24]研究表明，miR-20a能下調(diào)抑制劑PPARγ、 Bambi 、 crim 1，進(jìn)而增強(qiáng)BMP/Runx2 信號通路，促進(jìn)人基質(zhì)干細(xì)胞成骨分化。miR-15b直接作用于Smurf 1，通過抑制其降解成骨轉(zhuǎn)錄因子Runx2而促進(jìn)成骨細(xì)胞分化[25]。

3.3調(diào)節(jié)TGF-β相關(guān)因子Mizuno等[26]發(fā)現(xiàn)，miR-210能夠促進(jìn)BMP-4誘導(dǎo)的成骨細(xì)胞分化。其機(jī)制是： miR-210通過抑制ACVR1B的表達(dá)而抑制TGF-β/activin信號通路，從而促進(jìn)ST2細(xì)胞骨形成。miR-29b 能直接下調(diào)TGFβ-3、 ACVR2A等成骨抑制因子，進(jìn)而促進(jìn)MC3T3細(xì)胞成骨分化[27]。miR-181a能通過抑制TGF-βI和TβR-I/Alk5而促進(jìn)C2C12和MC3T3細(xì)胞成骨分化[28]。以上研究表明，多種miRNAs可以參與調(diào)節(jié)BMP/TGF-β信號通路而對骨形成起到重要的調(diào)控作用。

4小結(jié)

綜上所述，在骨形成這一復(fù)雜、精細(xì)的調(diào)控網(wǎng)絡(luò)中，miRNAs發(fā)揮著不可或缺的重要調(diào)節(jié)作用。miRNAs通過參與并調(diào)節(jié)BMP/TGF-β信號通路影響骨形成，但其僅限于細(xì)胞和動物實驗研究中，而臨床應(yīng)用尚未得到證實。隨著研究的不斷深入，許多問題尚待解決：成骨相關(guān)miRNAs如何被啟動并作用于骨相關(guān)特異性轉(zhuǎn)錄因子? LncRNAs等非編碼RNAs是否參與調(diào)控骨形成過程？過表達(dá)或沉默的相關(guān)miRNAs對成骨調(diào)節(jié)的起效時間和維持時間及其在臨床骨缺損或發(fā)育異常上的應(yīng)用如何？總之，miRNAs的研究還處于起步階段，其在骨形成方面及其他領(lǐng)域都需要進(jìn)行更深入的研究和探索，進(jìn)而更好地服務(wù)于臨床。

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中圖分類號R783

文獻(xiàn)標(biāo)志碼A

通訊作者龔逸明，E-mail: gongymingi@aliyun.com