2016, Vol. 37
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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