铁死亡在阿尔茨海默病中的作用*
2024-07-31付世青杨杰王芳
【摘要】 铁死亡是一种新发现的细胞程序性死亡方式,主要由细胞内铁依赖性脂质过氧化物积累引起,在形态学、生物化学和遗传学上不同于以往报道的细胞凋亡、坏死和自噬。阿尔茨海默病(AD)是最常见的神经退行性疾病,其病理特征包括神经纤维缠结、老年斑及铁的异常沉积等,提示铁死亡可能参与其发病的进展。探讨铁死亡的发生机制及其在AD中的作用,以期为铁死亡在神经退行性疾病中的研究提供参考。
【关键词】 铁死亡 发生机制 神经退行性疾病 阿尔茨海默病
The Role of Ferroptosis in Alzheimer's Disease/FU Shiqing, YANG Jie, WANG Fang. //Medical Innovation of China, 2024, 21(18): -188
[Abstract] Ferroptosis is a newly discovered mode of programmed cell death, mainly caused by the accumulation of iron dependent lipid peroxides in the cell, which is different from apoptosis, necrosis and autophagy in morphology, biochemistry and genetics. Alzheimer's disease (AD) is the most common neurodegenerative disease, and its pathological features include neurofibrillary tangles, age spots and abnormal iron deposition, suggesting that ferroptosis may be involved in the progression of its pathogenesis. Exploring the mechanism of ferroptosis and its role in AD, in order to provide reference for the study of ferroptosis in neurodegenerative diseases.
[Key words] Ferroptosis Occurrence mechanism Neurodegenerative diseases Alzheimer's disease
First-author's address: Department of Physiology, Bijie Medical College, Bijie 551700, China
doi:10.3969/j.issn.1674-4985.2024.18.042
铁死亡是一种新型的、铁依赖性的细胞死亡方式,最早由Dixon等[1]于2012年提出,在细胞内,铁离子可以与细胞内的氧分子反应,产生有害的自由基,导致细胞氧化应激和损伤。此外,铁还可以干扰细胞内的能量代谢、DNA修复和蛋白质合成等基本生物过程,最终导致细胞死亡。近年来的研究发现,铁死亡在神经退行性疾病中发挥着极其重要的作用,并具有共同的调控机制。本综述将重点讨论铁死亡的发生机制及其在阿尔茨海默病(AD)中的作用,希望为AD发病的潜在机制和治疗提供有价值的策略。
1 铁与铁死亡
铁参与氧的运输和细胞呼吸、DNA合成和细胞分裂、细胞新陈代谢和神经传导,对维持机体功能和日常新陈代谢至关重要[2]。铁在体内以氧化状态循环的能力是其生物功能的基础,过量的铁可导致生物大分子氧化应激损伤及细胞功能障碍。随着年龄的增长,大脑中积累的铁将增加神经退行性疾病的风险[3]。
铁死亡是一种铁依赖的新型细胞死亡模式,与细胞凋亡、细胞坏死和自噬有明显区别。其主要机制是在亚铁或脂氧合酶的作用下,铁催化细胞膜上高表达不饱和脂肪酸的脂质体过氧化反应,从而诱导细胞死亡[1]。铁死亡的形态特征是线粒体萎缩,双层膜密度增加,线粒体内膜嵴消失,完整的细胞膜仍然存在,正常大小的线粒体内膜的嵴消失[4],但细胞核内没有染色质凝集[5]。大量研究表明,铁死亡还与细胞抗氧化系统中谷胱甘肽和谷胱甘肽过氧化物酶4(GPX4)的表达减少有关[6-8]。脂质过氧化物不能被GPX4催化的还原反应代谢,脂质在Fenton反应中被亚铁氧化生成大量活性氧促进铁死亡[9-10]。因此,铁死亡的实质是细胞内脂质氧化物代谢紊乱,在铁离子催化下异常代谢产生大量脂质破坏细胞内氧化还原平衡,攻击生物大分子,引发细胞程序性死亡。
2 铁死亡的发生机制
2.1 铁在神经元中的运输和储存
神经元的铁代谢相关蛋白1,即转铁蛋白受体蛋白1(TfR1)在神经细胞膜表面高表达[11];与脑毛细血管内皮细胞的转铁相似,铁通过网格蛋白介导的Tf/TfR1吞噬作用而进入神经元,并通过二价金属转运体(DMT1)以还原二价铁离子(Fe2+)的形式离开内含体释放到胞质中[12]。朊病毒蛋白(PrPC)作为DMT1的铁还原酶辅酶,以铁离子络合物的形式介导PrPc/DMT1在质膜中的摄取[13]。在大脑中,Fe2+通常在神经元的胞质中代谢,并以三价铁离子(Fe3+)的形式储存在铁蛋白中;当神经元缺铁时,铁蛋白可被溶酶体降解从而释放出储存的铁以满足神经元的正常生理需要[14]。铁代谢平衡在翻译水平上受到调节。铁调控蛋白2(IRP2)是一种RNA结合蛋白,在编码多种铁调控分子(包括DMT1和TfR1)的基因的非翻译区(UTR)中,IRP1和IRP2与铁反应元件(IREs)结合从而控制参与铁代谢相关蛋白的翻译。在缺铁状态下,IRP2和IREs的结合可最大限度地提高细胞内的铁含量。当铁含量增加时,细胞外铁调节途径(IRE/IRP系统)将被激活从而降低铁过载[15]。核受体辅激活因子4(NCOA4)可以降解铁蛋白并介导铁自噬,该过程使细胞内Fe2+增加和导致铁死亡[16]。铁响应元件结合蛋白2(IREB2)是铁死亡的调控因子,能上调铁代谢过程中细胞质中铁蛋白轻链和重链的表达,减轻铁死亡诱导剂erastin诱导的铁死亡[17]。核因子E2相关因子2(Nrf2)可降低TfR1的表达,调节铁代谢,维持细胞内铁平衡,限制活性氧(ROS)的产生,从而减少铁死亡[18]。
2.2 谷氨酸/半胱氨酸反转运蛋白在铁死亡中的作用
细胞通过谷氨酸/半胱氨酸反转运蛋白,即Xc-系统,包括12次跨膜蛋白转运体溶质运载家族7成员11(SLC7A11)和单通道跨膜调节蛋白溶质运载家族3成员2(SLC3A2)摄取胱氨酸在铁死亡发生中受到抑制[1]。因此,抑制Xc-系统会导致细胞内半胱氨酸的缺乏[19]。半胱氨酸在谷胱甘肽(GSH)的生物合成中发挥着重要作用。GSH作为GPX4的底物,是脂质修复功能所必需的,半胱氨酸缺乏引起的GSH耗竭会导致GPX4活性丧失,以及未修复的脂质过氧化物和铁毒性的积累[20]。GPX4可以将还原型GSH转化为氧化型谷胱甘肽(GSSG),进而把脂质过氧化氢还原为相应的醇或将游离过氧化氢还原为水[21]。硒(Se)是GPX4活性的关键调节因子,含Se的野生型GPX4可有效地将过氧化物还原为相应的醇,从而防止铁死亡[22]。不稳定铁池(LIP)是神经元中易解离铁离子的交换池,GSH则是Fe2+的天然配体,GSH结合LIP中的Fe2+以防止铁氧化,这不仅维持了Fe2+的溶解度,还阻止了Fe2+作为催化剂将生理上可用的氢催化成强氧化剂羟基自由基[23]。因此,直接抑制GSH合成可触发铁死亡。
2.3 脂质过氧化在铁死亡中的作用
ROS是导致铁死亡的重要因素,其主要来源包括氮氧化合物(NOXs)的产生和膜脂过氧化[1]。多不饱和脂肪酸(PUFAs)在胞质中的量和分布决定了细胞中脂质过氧化的程度及导致铁死亡,最易受影响的脂质是含有多不饱和脂肪酸(PUFA-PLs)的磷脂,它可导致细胞死亡[24]。游离PUFAs通过酯化形成膜磷脂,然后被氧化为铁离子信号合成脂质信号,特别是含有磷脂酰乙醇胺(PE)和花生四烯酸或肾上腺素部分的磷脂[25]。在膜脂代谢中,PUFAs被一类非血红素含铁蛋白脂质氧化酶(LOXs)特异性过氧化,最终导致铁死亡的发生[26]。
3 铁死亡与AD
AD的病理学特征除了β-淀粉样蛋白(Aβ)沉积和由tau蛋白组成的细胞内神经纤维缠结(NFTs)的积累外[27],铁在大脑中的异常沉积也是AD的一个共同特征,铁对AD的影响被归结于其与AD病理学的主要蛋白[淀粉样前体蛋白(APP)和tau蛋白]的相互作用和/或通过铁介导的促氧化分子(如羟基自由基)的生成[9];铁积累的潜在原因是组织内的衰老细胞随着年龄的增长而增加,而铁积累的衰老细胞引发炎症将导致与衰老相关的各种病症,铁的积累使衰老组织易受氧化应激的影响,导致细胞功能障碍和铁死亡[28]。此外,脑铁水平升高与AD进展和认知能力下降有关[29]。在300例AD病例的meta分析表明,大脑皮层多个区域的铁水平显著升高,且不同区域的铁水平存在差异[30]。铁的积累可能导致神经退行性变,可能是通过诱导氧化应激和铁死亡[31]。研究发现,脑铁水平、脑脊液铁蛋白和定量易感性图谱具有预测AD临床严重程度和认知能力下降的潜力[32]。通过对209例AD患者死后脑铁水平与死前12年认知能力下降之间的关系研究发现,AD患者脑内铁含量明显升高,且与认知功能明显相关;皮质部位的铁可能通过诱导氧化应激或铁死亡,或通过与炎症反应相关联,导致AD潜在蛋白病变使认知功能恶化[33]。
铁的积累可加速老年斑的沉积和神经纤维缠结的产生[34]。尸检证据和核磁共振成像分析证明,不仅在老年斑中存在大量铁沉积[35],而且在皮质tau蛋白聚集部位也存在大量铁沉积[36],这表明铁与老年斑和神经纤维缠结存在潜在的相互作用。铁代谢平衡紊乱是Aβ沉积的关键因素之一。细胞内铁浓度过高会增强IRE与IRP的相互作用,诱导APP上调,而裂解APP的α-和β-分泌酶受内切蛋白酶furin的调控,铁的过量能损伤furin的作用,使α-分泌酶被抑制,而β-分泌酶则被激活,导致Aβ生产增多[37]。有研究认为,在没有氧化还原金属剂的情况下,Aβ是无毒的,而Aβ的聚集需要金属的参与[3]。当细胞外铁增加时,可溶性的Aβ与Fe3+结合以清除多余的铁,且相互作用后则很难解离,Aβ可促进Fe3+还原成Fe2+,在此过程中释放的ROS更容易使Aβ迅速沉积并形成更多的老年斑[38]。铁与APP和Aβ的相互作用大大增加了老年斑形成的速度和程度,铁沉积可被纳入AD的“Aβ级联假说”[39]。铁还可与tau蛋白相互作用,AD患者大脑中可溶性tau蛋白的减少通过抑制FPN1的活性使脑铁沉积增加[40]。高铁饮食会导致小鼠的认知能力下降,神经元tau蛋白磷酸化异常增加,胰岛素信号通路的相关蛋白表达异常;补充胰岛素后可降低铁诱导的tau蛋白磷酸化,这表明铁沉积可能通过干扰胰岛素信号通路导致tau蛋白过度磷酸化[41]。
神经胶质细胞活化和神经炎症已被证明是AD病理学的突出特征[42]。小胶质细胞对大脑中铁水平升高较为敏感,当脑中铁水平升高时,小胶质细胞被激活,体积增大,长度减少[43]。铁可能通过核因子κB(NF-κB)介导的促炎因子激活小胶质细胞[44],激活后的小胶质细胞将表达更多的铁蛋白以清除细胞外的铁,导致细胞内铁潴留[45],肿瘤坏死因子-α(TNF-α)表达增加,最终以β-斑块形式沉积[46]。β-斑块与APP相互作用,促进Aβ的形成[47]。在铁水平升高的环境中,Aβ的形成会导致小胶质细胞中IL-1β的表达增加,加剧促炎效应[48]。另外,星形胶质细胞被增加的胶质纤维酸性蛋白(GFAP)激活后,释放炎症介质,诱导氧化应激,促进Aβ和tau蛋白缠结的形成,抑制了Aβ的清除[49]。在AD小鼠模型中,GSH在皮层中的表达减少,并与认知能力下降呈正相关,额叶和海马的GSH水平可作为预测AD和轻度认知障碍的生物标志物[33]。在AD患者的大脑中Nrf2的水平随着年龄的增长而降低,这使铁死亡更容易发生[50]。另外,在AD小鼠模型和AD患者大脑中,GPX4的表达都会降低,GPX4基因敲除小鼠表现出明显的海马神经元缺失和认知障碍[51-52]。这些结果表明,铁死亡在AD中起着关键作用,可导致神经元损伤和认知能力下降。因此,调节脑铁代谢和减少神经元铁死亡可能是治疗AD的一种有前景的方法。
4 展望
铁死亡是一种新发现的细胞死亡形式,表现为铁超载、脂质过氧化物和ROS的积累。越来越多的研究表明铁死亡在神经退行性疾病中发挥着重要作用[53]。在临床上,可通过补充外源性脂质促进细胞脂质过氧化、抑制GPX4和GSH的表达、补充过氧化氢和铁离子促进肿瘤细胞的Fenton反应等方法诱导铁死亡。在神经退行性疾病治疗反面可考虑针对铁死亡作为潜在靶点研发相关药物。
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(收稿日期:2024-01-27) (本文编辑:马娇)