第二军医大学学报  2016, Vol. 37 Issue (8): 992-995   PDF    
主动脉疾病中血管平滑肌细胞表型转化调控机制研究进展
安朝, 徐志云     
第二军医大学长海医院胸心外科, 上海 200433
摘要: 主动脉疾病(aortic disease,AD)指由于主动脉壁病变所致的一类疾病,往往高度致命。目前非遗传性AD的发病机制尚未完全阐明。血管平滑肌细胞(vascular smooth muscle cells,VSMC)是主动脉壁中层的主要细胞成分,通常认为其存在收缩型与合成型两种表型,且可相互转化。VSMC由收缩型向合成型的过度转化在AD的发生发展过程起重要作用。尽管关于VSMC表型转化研究较多,但众多调控机制如何协调运作以及相互之间的关系仍然有待阐明。本文就目前已知的VSMC表型转化调控机制作一综述。
关键词: 主动脉疾病     血管平滑肌     平滑肌肌细胞     表型转化     表观遗传学    
Regulatory mechanism of vascular smooth muscle cell phenotype switch in aortic disease: an update
AN Zhao, XU Zhi-yun     
Department of Cardiothoracic Surgery, Changhai Hospital, Second Military Medical University, Shanghai 200433, China
Abstract: Aortic disease(AD) consists of a series of life-threatening diseases caused by aortic wall lesions. Up to now little has been known about the etiology of non-hereditary AD. Vascular smooth muscle cells(VSMC) are the major components of the medial aortic wall and can toggle between a contractile phenotype and a synthetic phenotype. The excessive transformation of VSMC from contractile state to synthetic state plays an important role in the development and progression of AD. Although there are many researches on the regulation of VSMS phenotype switch, how these regulatory pathways operate harmoniously and the what relationship between them remain to be elucidated. Here in this paper we reviewed the existing regulatory mechanisms of VSMC phenotype switch.
Key words: aortic diseases     vascular smooth muscle     smooth muscle myocytes     phenotype switch     epigenetics    

主动脉疾病(aortic disease,AD)包括急性主动脉综合征(acute aortic syndrome, AAS)、主动脉动脉瘤(aortic aneurysm,AA)等,是一类起病隐匿、高度致死性疾病[1]。除Marchesani综合征、Loeys-Dietz综合征等遗传性病因外,目前这类疾病的具体发病机制尚未完全阐明。主动脉壁中层血管平滑肌细胞(vascular smooth muscle cell, VSMC)数量减少及过度表型转化在AD的发生过程中扮演重要角色[2],VSMC过度表型转化直接导致主动脉壁中层细胞外基质(extracellular matrix, ECM)成分构成发生改变,并促进AD发生、发展。本文就目前AD中VSMC表型转化机制方面的研究进展进行综述。

1 VSMC表型转化与AD

主动脉中VSMC主要定位于主动脉壁中层,是主动脉壁中层的主要细胞成分[3]。不同于骨骼肌与心肌细胞,成人主动脉壁VSMC仍具有增殖分化能力。VSMC表型主要为收缩型与合成型:生理情况下VSMC表型主要为收缩型,主要特征为细胞内肌丝丰富,收缩功能较强,含α平滑肌肌动蛋白(α-SMA)、平滑肌肌球蛋白重链(SM-MHC)、平滑肌22α(SM22α)等特征性蛋白,几乎无分裂增殖能力,主要功能为参与维持主动脉壁的机械性能;当收缩型VSMC受内外因素刺激时可转化为合成型,主要特征为分裂增殖能力强,合成分泌功能旺盛,肌丝含量少,收缩功能弱或缺如,含骨桥蛋白(OPN)等特征性蛋白,主要功能为参与受损血管壁的修复[4]。Kimura等[5]报道,在胸主动脉夹层(thoracic aortic dissection,TAD)小鼠模型中,TAD组主动脉壁中层VSMC合成型较对照组明显增多,而收缩型减少、同时伴随细胞外弹性膜层数减少,断裂以及排列紊乱。Wang等[6]报道,在TAD患者中发现类似现象,并发现VSMC数量减少,同时由收缩型向合成型过度转化并大量分泌胶原与基质金属蛋白酶2(matrix metalloproteinase-2,MMP2),导致胶原沉积与细胞外弹性膜降解,最终导致AD发生。Blunder等[7]发现,主动脉壁中层VSMC减少及ECM构成异常与动脉瘤发生相关。这些研究提示VSMC的表型转化与AD的发生发展密切相关。

2 VSMC表型转化机制 2.1 多条信号通路参与VSMC表型转化调控

血小板衍生因子(platelet-derived growth factor,PDGF)可促进VSMC由收缩型向合成型转化,其广泛存在,正常血管壁表达PDGF及其受体水平较低,但当血管壁受损、高血压等病理情况下其表达可显著上调[8]。PDGF为二聚体,其二聚体可由两条高度同源的A链与B链构成,包括PDGF-AA、PDGF-BB、PDGF-AB。有研究表明,在大鼠颈动脉中灌注PDGF可促进VSMC迁移增殖并向合成型转化[9]。PDGF可以通过激活VSMC的PDGF受体,进而激活PI3K-Akt通路、ERK信号通路促进VSMC的表型转化[10]。Ishigaki等[11]在体外细胞研究中证明,PDGF可促进VSMC由收缩型向合成型转化。Owens等[12]报道,在小鼠的高血压模型中检测到PDGF mRNA表达水平较对照高2倍,并且这一差异在血压纠正后消失。

表皮生长因子(epidermal growth factor,EGF)可通过促进VSMC由收缩型向合成型转化[13]。VSMC表面分布EGF受体(epidermal growth factor receptor,EGFR)并可分泌EGF[14],EGF与EGFR结合后,EGFR发生构象变化形成二聚体,激活EGFR胞质内酪氨酸激酶结构域,发生自身磷酸化[15]。Igura等[16]报道,EGF在大鼠颈动脉血管受球囊损伤2 h后表达可较正常状态下升高12倍以上。VSMC中EGFR的自身磷酸化可激活多条通路,被激活的通路包括PI3K-Akt通路[16]、MAPK通路[17]、JAK-STAT通路[18]。这些通路的激活可使VSMC向合成型转化,其增殖、分泌、迁移能力均增强。Fiebeler等[19]报道, 高血压时VSMC中EGFR的反式激活促进了VSMC向合成型转化,导致血压水平进一步升高,而高血压是AD的重要危险因素。

Notch通路参与调控了VSMC表型转化。Notch受体被周围细胞细胞膜表面的Notch配体激活后可通过CBF-1/RBP-Jκ依赖与非依赖两种途径激活下游bHLH转录因子,进而调控VSMC表型转化[20]。Notch通路对VSMC表型的调节表现为双向性,Notch通路被激活后Notch受体的胞内部分可通过直接结合于α-SMA的启动子部分并促进其转录以维持VSMC的收缩表型,同时被激活的下游bHLH转录因子通过上调KLF4、抑制心肌素表达等途径在转录水平抑制α-SMA表达并促进VSMC向合成表型转化[21]

已证实转化生长因子β1(transforming growth factor-β1, TGF-β1)可维持VSMC的收缩表型,在体外TGF-β1可抑制由血清、PDGF、EGF等引起的VSMC表型转化与分裂增殖并促进α-SMA 、SM-MHC、SM22α等收缩型VSMC标志物表达[22-24]。TGF-β1与AD的发生密切相关,有研究表明在大鼠受损的主动脉血管壁中层中TGF-β1含量下降同时伴随VSMC收缩型标志物表达下降[25];Kimura等[5]报道在小鼠的TAD模型中观察到发生TAD的小鼠中TGF-β1通路下游信号转导蛋白Smad2表达下降,同时伴VSMC合成型异常增多,ECM构成改变,主动脉壁中层弹性膜层数减少,结构异常。

骨形态发生蛋白4(bone morphogenetic protein 4,BMP4)通路可通过上调心肌蛋白相关转录因子(myocd-related transcription factor,MRTF)进而下调KLF4来维持VSMC的收缩表型[26]。Chaterji等[27]报道CD138参与维持VSMC的收缩表型,敲减CD138的小鼠VSMC收缩型标志物α-SMA等表达显著下降,分裂增殖与合成能力增强,其认为CD138通过整合素ανδ3与ανδ5途径维持了VSMC的收缩表型。

2.2 ECM中多种成分参与VSMC表型调控

ECM中成分可通过整合素介导的多条信号通路参与细胞功能调控,层黏连蛋白可通过P38-MAKP信号通路维持VSMC的收缩表型,而纤连蛋白可通过ERK-MAKP信号通路促进VSMC向合成表型转化[28]。Qin等[29]报道,层粘连蛋白可使VSMC收缩型标志物SM22α等表达下调,并且可使VSMC由收缩型向合成型转化。Arcucci等[30]报道ECM中超氧化物歧化酶3(superoxide dismutase 3,SOD3)与Akt含量降低导致细胞外信号调节激酶1/2(extracellular signal regulated kinase 1/2, Erk1/2)磷酸化下降、MMP9含量升高,最终导致VSMC凋亡、弹性蛋白与胶原蛋白裂解,动脉瘤生成。Blunder等[7]报道,Ⅳ型胶原可促进VSMC收缩型标志物如α-SMA、SM-MHC表达,而Ⅰ型胶原则通过激活VCAM-1蛋白、转录因子NFAT等促进VSMC向合成型转化。

2.3 microRNA(miR)参与VSMC的表型调控

Cordes等[31]报道,miR-143与miR-145参与调控了干细胞向成熟VSMC分化的过程,其受血清应答因子(serum response factor,SRF)、心肌素、Nkx2.5调控,并且miR-143/145的高表达可维持VSMC的收缩表型,抑制VSMC向合成型分化。Davis-Dusenbery等[26]报道,TGF-β通路与BMP4通路均可调控下游的miR-143/145上调表达,进而下调KLF4,维持VSMC的收缩表型。miR-21在大鼠颈动脉损伤后的血管壁中高表达,并且细胞实验证实其促进VSMC向合成型转化[32]。TGF-β下游心肌素亦受miR-9调节,miR-9可协同KLF4抑制心肌素表达从而促进VSMC向合成型转化[33]

2.4 VSMC表型转化受表观遗传学机制调控

DNA甲基化与脱甲基化、组蛋白修饰等表观遗传学方面调节机制参与了VSMC表型转化的调控。Liu等[34]发现,TET2蛋白通过上调MYOCD与SRF启动子区域5-羟甲基胞嘧啶(5-hydroxymethylcytosine,5-hmC)水平促进MYOCD与SRF表达进而维持VSMC的收缩表型,同时VSMC合成型的促进因素KLF4则被抑制;敲除TET2则可促进VSMC向合成型转化。Qiu等[35]则报道TGF-β1通过上调P300/CBP蛋白促进SM22α基因所在区域组蛋白的乙酰化水平上调SM22α表达;过表达组蛋白乙酰化抑制蛋白如Twist1、E1A则抑制TGF-β1通路。

3 展望

VSMC是主动脉壁中的主要细胞成分,主动脉壁中VSMC由收缩型向合成型过度转化可导致主动脉壁VSMC功能相关蛋白减少,进而影响VSMC行使正常功能,最终可能导致主动脉壁弹性膜受损,ECM构成改变。这一过程可能在AD的发生、发展中起重要作用。目前已知多种机制共同导致了AD时VSMC的表型转化,但这些调控机制所构成的调控网络如何运作、是否有尚未阐明的其他机制作用于VSMC、VSMC的表型转化与AD发生的确切因果关系等仍有待进一步深入研究。明确VSMC表型转化的调控机制有助于寻找AD相关的治疗靶点,预防AD的发生,巩固AD的手术疗效。

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