Lung cancer stem cells and lung cancer

doi:10.3969/j.issn.2095-4344.2014.28.023

Abstract

Abstract:

BACKGROUND: Lung cancers are highly heterogeneous and resistant to available therapeutic agents, with a five year survival rate of less than 15%. It has been difficult to determine the basis of lung cancer heterogeneity and drug resistance. Cancer stem cell model has attracted a significant amount of attention in recent years as a viable explanation for the heterogeneity, drug resistance, dormancy and recurrence and metastasis of various tumors.
OBJECTIVE: To summarize the current understanding of lung cancer stem cells, including their histological types and tumor growth areas, and to discusses the prognosis of lung cancer and its relationship with lung cancer stem cells, in an effort to eradicate these cells to combat lung cancer.
METHODS: In order to search relevant articles about the lung cancer stem cell and its relationship with lung cancer from PubMed and Sciencedirect databases (from 1990 to 2014), a computer-based search was performed, using the key words of “lung cancer, cancer stem cell, lung cancer stem cell, lung cancer occur, tumor heterogeneity, drug resistance, gene mutation, signal pathways” in English. After eliminating literatures which were irrelevant to research purpose or containing a similar content, 48 articles were chosen for further analysis.
RESULTS AND CONCLUSION: The cancer stem cell model has gained considerable support recently in context of lung cancers and stem-like cells that are associated with aggressive cancer behavior, metastatic progression, resistance to therapy and relapse. Since lung cancer stem cells are thought to consist of a heterogeneous population depending on the histology and site of tumors, and multiple signaling pathways might have to be targeted to effectively eliminate lung cancer stem cells for therapeutic benefit. It can be imagined that the multidisciplinary efforts currently under way to characterize and target stem-like cells in lung cancer will reap significant therapeutic benefits in the future.

中国组织工程研究杂志出版内容重点：干细胞；骨髓干细胞；造血干细胞；脂肪干细胞；肿瘤干细胞；胚胎干细胞；脐带脐血干细胞；干细胞诱导；干细胞分化；组织工程

全文链接：

Key words: lung neoplasms, neoplastic stem cells, tumor stem cell assay, neoplasms by histologic type

CLC Number:

R318

Liu Zhe-liang, Xiao Gao-ming, Chen Yue-jun, Wu Guan-yu. Lung cancer stem cells and lung cancer[J]. Chinese Journal of Tissue Engineering Research, 2014, 18(28): 4568-4572.

Figures/Tables 1

2.1 肺癌干细胞概述肺癌与吸烟高度相关，肺癌死亡病例占全球癌症死亡人数的最大比例[1-3]。长期大量吸烟者即使戒烟，其肺癌的发病风险仍然显著高于常人。新的肺癌确诊患者中有50%是烟民，其中许多人在确诊前5年或更早的时间就停止了吸烟[4]。世界卫生组织预测，到2030年每年将约250万人死于肺癌[1]。在美国，约85%确诊的肺癌患者在5年以内死于该病，这种死亡率自20世纪70年代以来就没有显著改变[5-6]。尽管关于癌症的知识越来越多，但针对肺癌的有效治疗手段仍很有限[7]。治疗原发病灶很少杜绝远处转移，这是癌症致死的主要原因[6]。这些现状强调需要更好地在细胞和分子水平上研究癌症的发生和转移，从而制定出新的相关治疗策略。在这种情况下，近几年已萌芽一种学术思想，即肿瘤形成于肿瘤细胞的一类子群，被称为肿瘤干细胞，它可以保持休眠状态，能够远处转移并抵抗化疗药物治疗。这个概念不同于以往的理论，认为所有的癌细胞具有类似的增殖能力和启动肿瘤生长、扩散的机会[8]。此外，肿瘤干细胞模型表明癌组织成分复杂，分化无序，被少量细胞亚群所驱动，它们具有自我更新能力，并且可以分化为构成肿瘤的异质成分[9]。因此，理论上，那些可以杀灭肿瘤干细胞或肿瘤起始细胞的药物就是潜在的高效抗癌剂。1997年，白血病干细胞的成功鉴别是第一个表明肿瘤干细胞存在的实验证据[10]。2003年在乳腺癌研究中第一次证明实体肿瘤起源于干细胞[11]。然而学术界对实体肿瘤中是否存在肿瘤干细胞争议不断[12]。最近有实验运用小鼠研究模型分别对大脑、皮肤和小肠肿瘤进行了研究[13-15]。这3个独立的研究团体提供了令人信服的证据表明，肿瘤干细胞是确实存在的，并负责维持实体器官中肿瘤的生长。 20世纪80年代，Carney等[16-17]第一次实验证明，在肺癌细胞中存在干细胞样克隆亚群。在这项开创性研究中，只有少量小细胞肺癌和肺腺癌标本在软琼脂培养基中表现出生成细胞克隆集落的能力[16-17]。将这些克隆集落移植到无胸腺裸鼠体内，显示出其致瘤性[16-17]。此后，研究人员试图通过各种实验方法来获取肺癌干细胞。目前，科学家主要通过流式细胞术分离肺癌干细胞特异的细胞表面标记物，或通过肺癌干细胞的生物学特征加以鉴定分离。国内外研究最多的是参照其他肿瘤干细胞(如：脑胶质瘤特征标记分子CD133，乳腺癌干细胞标记物CD44)，或多能干细胞的标记物(如：OTC4，SOX2，ALDH1等)对肺癌进行研究。目前，肺癌干细胞尚无特异性的标记物，研究比较多的标记物初步的结论包括：CD133更倾向于作为耐药的指标而非肺癌干细胞标记[18-19]；CD44，SOX2有望成为肺鳞癌干细胞的标记[20-22]；OCT4有望成为肺腺癌干细胞的标记[23-24]；ALDH1是多种肿瘤干细胞的通用标记物，其特异性有待进一步研究。虽然肺癌干细胞方面的研究取得初步的成果，但这离解决肺癌这一难题还远远不够。肺癌干细胞特异的标记物尚不明确，这些标记物对临床的病理分型、分化的联系有待进一步研究。 2007年肺癌干细胞的研究获得突破性进展，Ho等[25]首次发现利用Hoechst染料外排法，从多种人肺癌细胞系和人肺癌临床样本中分离出SP细胞，这部分细胞表现出与干细胞相似的特性，包括体外高致瘤率、细胞表面的ABC转运蛋白表达上调、人端粒末端转移酶表达增高等，说明SP细胞具有肿瘤干细胞特性。Kim等[26]2005年发现Kras基因能促进细支气管肺泡干细胞的分化，但Kras基因的活化也能促进肺腺癌的发生，并且两者发生部位都是细支气管肺泡结合处，这提示肺腺癌起源于细支气管肺泡干细胞的恶性转化。目前已有证据显示，肺癌干细胞与肺癌发生解剖部位对应的肺干细胞有相似的细胞学特征和表面标志[27-28]，但对于肺癌干细胞是否源于正常肺干细胞的恶性转化还缺乏直接证据，恶性转化的分子机制需要进一步的研究证明。这对揭示肺癌发生机制和对肺癌防治有重要的作用，并可为其他肿瘤发生机制研究提供参考。 2.2 肺癌干细胞与肺癌发生的关系相比其他类型的肿瘤干细胞，人们对肺癌干细胞生物学性能了解较少。这主要是由肺癌的复杂性所决定的，包括其表型多样性和气管内的起源解剖部位不同。肺癌可分为小细胞肺癌和3种类型的非小细胞肺癌，包括鳞状细胞癌、腺癌和大细胞癌。小细胞肺癌和鳞状细胞癌在呼吸道的近端区域发生，而腺癌起源于远端气道[28]。在呼吸道不同部位存在多样化的自我更新的肺上皮干细胞被认为是这种部位差别的原因。为了支持这一假说，有研究证实，小细胞肺癌，鳞状细胞癌和腺癌/支气管肺泡癌的发生部位与裸鼠气道干细胞龛的分布区域吻合[29-30]。运用基因修饰法诱导实验动物发生肺癌，可以证明肺癌起源于气道局部干细胞。经典的基因修饰包括转基因表达癌基因或敲除抑癌基因，它们在肺上皮细胞特异性启动子的控制下单独或组合运用[31]。这些基因修饰可以在大面积肺组织内产生相同的突变，因而理论上能够使整个肺都发生癌变。然而，实际上各类肺癌的发生部位只出现在气道的干细胞龛，从而证明肺癌起源于肺癌干细胞[32]。 2.2.1 小细胞肺癌和肺神经内分泌细胞小细胞肺癌预后较差，容易出现转移播散[33]。人类小细胞肺癌主要发生于中级细支气管，通常表达一系列神经内分泌细胞标志物，包括降钙素基因相关肽和其他通常表达于肺神经内分泌细胞内的标记物，而不表达其他细胞类型的相关标志物[34]。视网膜母细胞瘤基因(Rb)的缺失和p53的功能与人小细胞肺癌密切相关[35]。除了基因缺失，肺神经内分泌细胞增生的是肺特有的Rb失活模型[36]。同时缺失Rb基因和p53的成年小鼠，会出现进行性上皮增生，但只局限于神经上皮小体微环境[36]。重要的是，这些病变进展形成的转移性肿瘤类似人类小细胞肺癌[36]。总结上述研究结果，小细胞肺癌可能起源于肺神经内分泌细胞。在最近的一份报告，研究者在成年小鼠肺中运用特定细胞类型的Cre-loxP表达模型来确定小细胞肺癌的起源细胞[37]。Rb和p53的缺失通常发生在气管和支气管的克拉拉细胞、支气管肺泡干细胞、AT-2细胞和肺神经内分泌细胞，这表明肺神经内分泌细胞就是小细胞肺癌的致瘤前体细胞。此外，还有数据表明，在p53和Rb缺失的前提下，表面活性蛋白C阳性祖细胞可能诱发小细胞肺癌，尽管其癌变概率非常低[37]。 2.2.2 肺鳞癌和基底干/祖细胞由于不存在人类肺鳞癌的转基因小鼠模型，一般是采用化学诱导小鼠癌变为鳞状细胞癌[38]。致癌物诱导的小鼠鳞癌通常发生在近端气道到第二或第三级气道分叉处，很少累及远端气道，这与相应的干/祖细胞龛的位置相吻合[39]。组织学上，致癌物诱导鳞癌的第一步是广泛的基底细胞增生[40]。此外，肺鳞癌中的过度增生和癌前病变的特点就是基底干细胞特异性角蛋白14阳性[41]。目前的研究数据只能证明裸鼠的肺鳞癌模型与近端气道的基底祖细胞有直接关系，因此需要寻找更好的方法确认肺鳞癌的起源细胞。 2.2.3 肺腺癌和支气管肺泡干细胞支气管腺癌和细支气管肺泡细胞癌在吸烟和不吸烟患者中都是最常见的肺癌类型[33]。在小鼠模型中，腺癌起源于终末细支气管和肺泡的交界处，被称为“细支气管结”(BADJ)[26]。裸鼠移植瘤模型和人肺腺癌细胞都表达克拉拉细胞分泌性蛋白和/或表面活性蛋白C标记物，这表明克拉拉细胞或Ⅱ型肺泡细胞在功能上可以被认定为腺癌的起源细胞[26]。此外，在小鼠模型中克拉拉细胞分泌性蛋白或肺泡表面活性蛋白C启动子表达一系列致癌蛋白，如突变的表皮生长因子受体，活跃的K-ras基因(G12D)[26，42]，显性负转化生长因子[43]，或大T抗原等[44]，这些蛋白都可以导致细支气管肺泡细胞癌。如上所述，抵抗萘损伤、神经上皮小体无约束生长、克拉拉细胞分泌性蛋白和表面活性蛋白C的表达、Sca-1表达阳性以及支气管肺泡干细胞都出现在萘损伤小鼠肺的细支气管结区[26]。在同一研究中，通过细胞类型特异性表达模型，Kim等[26]已经证明了选择性的、剂量依赖性的支气管肺泡干细胞扩增和K-ras基因突变所导致的支气管肺泡。相反，最近有研究表明表面活性蛋白C阳性细胞在K-ras和p53缺失，或持续表达表皮生长因子受体条件下，可以导致肺泡细胞癌，这一过程无需支气管肺泡干细胞和参与[45]。然而，另一项研究表明，K-ras基因在克拉拉细胞和支气管肺泡干细胞中表达可导致肺上皮增生，在AT-2细胞中表达可导致肺腺癌[42]。 2.2.4 基因驱动与肺癌干细胞以上研究结果提供的证据表明，正常气管干细胞可以作为肺癌起源细胞。不过，一个有趣的研究，运用降钙素基因相关肽启动子驱动的转基因构建使H-ras基因特异性表达，导致支气管腺癌，但不形成小细胞肺癌[46]。同样，在最近的研究中，导入激活的H-ras基因能将神经内分泌型小细胞肺癌细胞彻底改变表型，而更类似于腺癌[47]。这些结果表明，不同基因病变的生理效应可以驱动同一目标细胞向不同方向分化。此外，还应当指出的是，所谓的“起源细胞”是用于描述那些自我更新的干细胞因获得致癌突变而不稳定增长。肺癌干细胞也可能来自于受限制的祖细胞，通过遗传或表观遗传机制获得自我更新的特性。最近的一份报告支持关于肺癌干细胞的这一假说。Sca-1阳性的支气管肺泡干细胞最初被认为可导致K-ras基因所驱动的细支气管肺泡癌，然而在肿瘤内，Sca-1阳性以及阴性细胞都获得了肿瘤干细胞的特性，并由其植入小鼠体内继发肿瘤的能力所证明[48]。因此，肿瘤干细胞应该具备更多的特异性标记物，而不是过分强调其作为特定细胞类型的起源，这表明从肿瘤中分离和鉴定肿瘤干细胞将是未来研究的主要挑战。"

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