表观遗传学在运动改善T2DM骨代谢中的作用机制

第三节 表观遗传学在运动改善T2DM骨代谢中的作用机制

体育活动和体育运动对人类的整体健康状况起着重要的作用。众所周知,经常运动有助于降低发生各种健康问题的风险,如心血管疾病,T2DM和癌症等。目前,部分研究试图找出DNA甲基化与T2DM骨代谢之间的关系以及其中的机制,但由于研究较少,其具体机制仍不清楚。

随着研究深入,表观遗传在影响机体健康中的作用也被逐渐挖掘,但其分子机制仍有待确定,目前确定了几种假定的候选机制,一种是DNA甲基转移酶的易错性,它不能以100%的保真度将母代DNA链的甲基化模式复制到子代DNA链;另一种是导致DNA甲基化异常的机制,它与碱基切除修复DNA修复系统有关,该机制假定hyper-methylated CpG区可能逐渐或迅速失去其甲基化标记,而该过程要么是在复制中DNA甲基化维持受到损害而被动消失,要么是通过碱基切除修复促进10-11-易位-甲基胞嘧啶双加氧酶(TET)主动氧化5-甲基胞喀啶(5-methylcytosine,5mC),这种被动和主动的去甲基化在介导机体健康中均发挥重要的作用。研究表明,运动能通过改变基因的表观遗传状态对疾病产生影响,这证明表观遗传可能是将宏观刺激与微观变化联系起来的桥梁。由于骨组织是力学敏感组织,那么运动可能通过改变骨形成和骨吸收基因的表观遗传状态来调节其表达,进而对骨量、骨微结构产生影响,最终改善T2DM骨代谢失衡。田雪文分别采用低氧运动和常氧运动来处理肥胖大鼠后发现,低氧运动能有效降血脂、调节脂代谢进而降低体质量、体脂,对相关因子mRNA和蛋白表达进行检测后发现,与脂代谢相关的基因表达出现显著变化,并用RRBS技术检测到这两种运动方式使大鼠全基因组中675个基因的CpG岛发生显著的甲基化变化,其中Wnt信号通路相关基因的mRNA表达和甲基化最明显。T2DM会引起高同型半胱氨酸血症,并伴随DNA甲基转移酶3a和DNA甲基转移酶3b的升高,同型半胱氨酸一方面能通过激活PP2A/FoxO1/MAPK级联反应,调控成骨细胞中的氧化还原机制并提高RANKL/OPG比值,增加破骨细胞活性;另一方面同型半胱氨酸还能通过刺激细胞内活性氧生成以直接刺激破骨细胞活性增加。小鼠进行4周游泳运动后DNA甲基转移酶3a和DNA甲基转移酶3b水平显著降低,并且同型半胱氨酸浓度降低,提示游泳运动能通过降低DNA甲基转移酶介导的甲基化水平来减少同型半胱氨酸生成,进而抑制PP2A/FoxO1/MAPK级联反应的激活,最终抑制骨吸收。但DEMINICE等研究表明,只有抗阻训练能有效降低同型半胱氨酸水平,而有氧运动并无作用,且同型半胱氨酸水平与运动量显著相关,与运动强度无关,造成两者研究结果矛盾的原因可能是运动时间的差异。另外,同型半胱氨酸可以增加BMSCs和成骨细胞中雌激素受体α基因启动子A区的甲基化程度,减少雌激素受体α基因转录,从而介导骨质疏松的产生。结合运动能有效降低同型半胱氨酸水平,推测运动能通过降低同型半胱氨酸来减少雌激素受体α基因甲基化水平,进而改善骨质疏松。

表观遗传中组蛋白修饰和非编码RNA介导运动对T2DM骨代谢影响的相关研究更少。Sirt1作为第Ⅲ类去乙酰化酶,是细胞衰老、能量代谢、骨骼重塑等环节的交汇点,对骨代谢稳定有重要的作用。Sirt1能直接或间接促进β-catenin在核内积累以激活Wnt通路进而促进骨形成:Sirt1直接去乙酰化β-catenin,使其大量聚集于细胞核并激活Wnt信号通路;Sirt1去乙酰化FoxO,减弱其对β-catenin的结合作用,使β-catenin积累并激活Wnt信号通路。运动通过Sirt1调控T2DM引起的骨代谢失衡。T2DM患者体内的高糖环境会抑制NF-κB抑制蛋白的降解,使NF-κB抑制蛋白对NF-κB的抑制作用降低,促进NF-κB转移入核并促进破骨细胞形成,而运动能逆转这一过程。Boyle等发现,T2DM大鼠经8周运动后,其骨中Sirt1表达上调,在乙酰化NF-κB的同时抑制其穿梭入核,导致下游RANKL/c-fos/NFATC1途径无法激活,使破骨细胞分化受阻并抑制骨吸收。在体外对高糖环境下的RAW264.7施加力学刺激时,检测到Sirt1蛋白表达上调,同时观察到破骨细胞生成减少且骨吸收能力下降,说明运动可能通过Sirt1/NF-κB/RANKL途径来调节骨代谢。微小RNA作为与骨代谢密切相关的生物标志物,可受力学刺激调控进而影响骨代谢过程。miR-21能通过下调程序性细胞死亡因子4蛋白并诱导RANKL表达进而促进c-fos的浓度升高,刺激破骨细胞生成,适当的体力活动可以抑制miR-21表达,减少RANKL的生成并促进OPG的生成,从而减少骨吸收。与miR-21促进骨吸收的作用不同,miR-214是通过抑制活化转录因子4、OSX、ALP、β-catenin等成骨因子表达来抑制骨形成的,在对成骨细胞施加3%的牵张强度,0.5 Hz,在4小时的持续牵张刺激下,采用周期为3天或7天,能有效降低miR-214表达进而减弱其对上述成骨因子的抑制作用,增强成骨作用。除微小RNA外,长链非编码RNA也发挥着作用。长链非编码RNA OGRU是一种具有1 816个核苷酸的新型长链非编码RNA,对力学刺激卸载非常敏感。研究表明,OGRU过表达会竞争性地结合miR-320-3p进而促进同源盒A10蛋白表达,而同源盒A10又能通过活化核心结合因子2来直接调节ALP、骨钙素等成骨基因表达,促进成骨细胞分化。因此,可推测微小RNA和长链非编码RNA介导运动对T2DM骨代谢产生影响。

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