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低剂量辐照细胞损伤机制

2012-01-29贺永明崔凤梅

中国医学创新 2012年6期
关键词:电离辐射旁观者细胞周期

贺永明 崔凤梅

在哺乳动物中,低剂量辐射杀死细胞数量有限,并能迅速为新生细胞所修复而不留痕迹。辐射可致死,而亚致死伤害亦可获得迅速而有效的修复。遗憾的是,在修复过程中,受损细胞可产生修复错误,导致基因突变,产生某种形式的遗传学不稳定。遗传学强不稳定性可致正常细胞死亡或致肿瘤发生,而中等度不稳定性可为细胞所耐受甚至遗传给下一代。显然,人们关注“诊断剂量”,希望理解中等剂量照射时是否以及如何增加健康风险。充分理解电离辐射诱导的分子学过程,将使临床治疗更有效而风险更小。

过去曾认为,辐射损伤的唯一细胞效应与DNA损伤有关,但现有证据表明,辐射损伤可影响特定细胞靶点并产生级链反应。因此,现有研究已不再局限于直接DNA损伤和修复,而是扩展至间接作用,如适应反应、毒物兴奋效应、旁观者效应、遗传学不稳定性及遗传易感性。这些研究提供了大量的医学信息,为患者和从事核放射行业人员建立准确辐射保护标准提供了科学依据。近年来,辐射效应更是受到空间科学的驱动而方兴未艾。

1 旁观者效应

旁观者效应原指一种心理状态,在场的人越多,越是没有人站出来救助受害者。现在,放射学家使用这一术语描述直接受损细胞将这一损害传播至其他未直接受损细胞的能力。这一模型假定低剂量低LET照射可能比线性非阈值模型预期的危害更大[1]。旁观者效应的最新定义基于下列发现:暴露于低剂量 粒子细胞姊妹染色体交换频率增加至30%,但真正发生有效交换的不足1%。因此,电离辐射损伤来自直接照射的观点显然站不住脚[2~5]。但DNA损伤似乎为起动旁观者效应所必需,因为人们发现,DNA修复缺失的细胞,其毒性远大于DNA正常细胞[6]。

亦有报道显示,以受辐照细胞的条件培养基培育正常细胞,可诱导旁观者效应[7]。更为有趣的是,同样的实验条件下,适应性反应和旁观者效应可同时出现[8]。有些观察发现,旁观者效应可遗传至下一代[9,10]。粒子可特异性产生旁观者效应,并通过特异性激活因子使这一效应扩布开来;光子特异性诱导适应性反应。旁观者效应的动力学及机制,尤其是其时间和空间效应对旁观者信号传导还不清楚。

2 旁观者效应机制及途径

人们提出了多个机制用于解释放射诱导的旁观者效应。受辐照细胞分泌低分子量化学因子,这些化学因子影响了未受辐照的附近细胞,从而产生旁观者效应。另有理论认为,受辐照细胞分泌的小分子通过“缝隙连接”作用于邻近细胞产生旁观者效应。信号传导包括可溶性因子,如寿命短的氧自由基可起动应答及其他因子可调节缝隙连接[11],扩布并维持这一效应。质膜表面细胞因子或生长因子受体可将信号放大并传导至核内,受辐照细胞的另一些信号可激活质膜表面氧化酶[12],细胞终将更持久产生活性氧自由基。活性氧自由基可损伤DNA,刺激修复机制,激活特定检测点,使细胞暂时停留在G1期[9]。旁观者效应细胞表现为多种生物学后果,如遗传学和表观遗传学改变,基因表达改变,信号传导途径的激活以及其后代所表现出来的遗传效应[13,14]。

3 基因组不稳定性

电离辐射可使受照幸存细胞后代基因组不稳定,诱导其后代延迟死亡或致死性突变和诱变[15~17]。这一形式的基因组不稳定性与旁观者效应一样,可由活性氧自由基触发[18]。大量研究表明,电离辐射诱发的基因组不稳定性与旁观者效应紧密相关。研究表明,辐照幸存细胞后代染色体不稳定,可分泌可溶性因子诱导旁观细胞死亡和染色体不稳定。亦有旁观细胞后代延迟效应的报道,这些延迟效应包括在体和离体的染色体不稳定性以及后代死亡[19~23]。

4 辐照细胞功能失活

4.1 坏死、凋亡 已知几个机制参与调节辐照细胞失活,有细胞周期一过性停止、细胞死亡及细胞衰老等。治疗剂量的辐射可杀伤肿瘤细胞,而对正常细胞伤害小,但仍可造成严重组织(如大脑)损伤。放疗后肿瘤部位偶可立即出现一大片坏死组织,放疗后数周至数月内出现大片坏死组织则更为常见,此即放射性坏死。细胞凋亡经常发生。上世纪八十年代初,有人率先提出细胞凋亡是受照细胞死亡的一种重要形式[24],研究了细胞凋亡机制,并将细胞凋亡反应与放射敏感性联系起来[25]。Aldridge[26]新近研究表明,5 个人血细胞克隆生存率与凋亡反应关系明确。中等剂量照射诱导细胞凋亡率与细胞周期检点功能相关,恰好反映了单克隆生存反应。高放射敏感性细胞系迅速出现凋亡,而不敏感细胞系则很久才出现细胞凋亡。这些研究表明,从DNA损伤至触发细胞凋亡这段自身修复时间是决定放射敏感性的关键因素,至少造血细胞如此。相反,Kyprianou[27]研究表明,人前列腺癌细胞系Bcl-2的过度表达可显著延缓放射诱导细胞凋亡,并不影响细胞系的生存率。这一截然不同的结果反映了细胞凋亡在细胞死亡中的贡献因细胞类型不同而异。

DNA损伤诱导细胞凋亡受p53基因调控[28],但并非唯一调控因素。譬如,电离辐射并不能诱导p53野生型MCF-7细胞凋亡。正常情况下,p53基因通过上调p21,使细胞停留在G1期而减少细胞凋亡[29],而p53基因缺失或p53基因突变可促进细胞凋亡。此时,细胞在 G2期时发生凋亡[30~32]。细胞凋亡的机制值得深入研究。

4.2 线粒体灾难 研究表明,许多细胞系受到电离辐射后,早期并不出现细胞凋亡,但在晚期可因线粒体灾难而出现细胞死亡。细胞两种死亡模式功能上存在联系,线粒体灾难可视作因持续DNA损伤及细胞周期检点控制缺失所致一种caspase介导的细胞死亡的亚型[33]。有学者设想p53基因与线粒体灾难有关[34]。研究则表明,p53基因通过转录抑制机制调节细胞周期素B1水平[35]。研究野生型和无功能型p53鼠胚胎成纤维细胞结果表明,线粒体灾难可致细胞周期素B1水平上调。野生型和无功能型p53鼠胚胎成纤维细胞受照后可诱使细胞停留在S/G2期,但p53基因突变细胞出现线粒体灾难后继而细胞周期素B1升高,野生型p53基因细胞表现为细胞周期素B1轻度升高,线粒体灾难出现频率也低[36]。然而,这一放射诱导的延迟型细胞凋亡的分子机制可能相当复杂。其他的如乳房细胞,顺序过表达Fas,TRAIL(肿瘤坏死因子相关凋亡诱导配体)及肿瘤坏死因子-α,晚期过表达相应配体似也参与了早期细胞凋亡和晚期线粒体灾难[37]。细胞死亡受体过表达可为照射直接诱导,但也可以是线粒体灾难直接后果。细胞周期检点受阻或有丝分裂异常可致线粒体灾难。其他研究表明,其他调节因素可能控制了线粒体灾难,包括细胞周期特异性激酶(如Cdk1和Aurora),细胞周期检点蛋白及生存素。P53和p21wafl/cipl参与了细胞死亡配体表达[33]。这些问题值得深入研究。

4.3 衰老 衰老是指正常细胞经过有限次细胞分裂后增殖停止。衰老细胞仍然具有代谢和合成能力,并表现出特有的形态学和生化改变,如细胞变大变扁、细胞中颗粒增加,SA-gal活性增强等[38,39]。经典的细胞衰老是由于染色体两端端粒酶及其他结构的缩短所致[40]。衰老加速类似于经典的细胞衰老,表现在形态学、生化及生物学特征方面,但无端粒酶缩短现象。衰老加速系正常细胞和肿瘤细胞对潜在致癌影响的一种保护性、非程序性反应[41]。研究表明,电离损伤可模仿衰老现象,致正常细胞和肿瘤细胞细胞周期停止[42]。DNA损伤是细胞提前衰老的主要原因,但具体机制未明。可以肯定的是,电离损伤可促发癌细胞(受p21waf1/cip1诱导)p53介导的多种反应[43,44]。但研究表明,电离损伤诱导的人直肠癌HCT116细胞衰老(p53,p16或p21基因缺失)仅部分受抑制[45],这点不同于野生型HCT116细胞。因此,尽管p53和p21似乎参与了细胞衰老调节,但并非细胞衰老所必需,这些研究提示其他基因可能参与了肿瘤细胞的衰老,具体基因尚未明确。可以肯定的是,衰老可影响电离损伤后自我修复能力,电离诱导细胞衰老程度与该细胞对电离损伤敏感性有关[46]。有些研究表明,p53基因缺失可阻止细胞衰老,也可使放射治疗失效[47]。目前,人类已迈入深太空,电离诱导衰老反应尤其有现实意义,深入研究这一现象将有助于阐明太空慢性电离辐射如何影响宇航员组织和器官功能的。最后,阐明肿瘤衰老的基因和调节机制有助于设计新的治疗方案,以提高放疗的效果并减少副作用。

低剂量辐照受损细胞结局取决于能否彻底揭开低剂量辐照受损细胞分子机制。此外,许多生物学和流行病学问题亟待解决。

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