2.1 杯状型与肠上皮细胞型
杯状型结肠癌患者无病生存期长,临床化疗一般优先使用伊立替康等药物。在基础研究中可使用HT29和LS1747等细胞系模拟杯状型结肠癌模型,这种类型具有杯状特异性MUC2和TFF3 mRNA高表达的特点[39]MUC2和TFF3主要存在于肠上皮的黏液层,并与杯状细胞一起分布于大肠和小肠[39]。MUC2是一种糖基化黏蛋白,由串连而不规则的重复序列组成,富含作为寡糖链潜在附着位点的丝氨酸和苏氨酸。目前已有专利研发出靶向MUC2蛋白的多肽[33]。TFF3是一种三叶因子,在肠黏膜的创面修复和愈合中起着重要的作用,已有研究证实microRNA-7-5p能够靶向至TFF3并调节结肠癌细胞的增殖[40]。但目前还没有专门靶向至MUC2和TFF3的制剂形式,可考虑利用多肽或microRNA-7-5p与伊立替康等结合制备成口服吸收的纳米粒,以靶向至杯状型结肠癌组织。
而肠上皮细胞型结肠癌特点为MUC2特异性基因的高表达,较少表达TFF3。因此,肠上皮细胞型结肠癌患者应服用针对MUC2基因高表达制备的靶向药物,从而使药物精准靶向至MUC2,减小对正常组织的伤害。
2.2 干细胞型
干细胞型结肠癌患者无病生存期短,伊立替康为该型患者的一线化疗药物。现有研究表明,可运用SW48、HCT8、SW620、HCT116、COLO320等细胞系模拟干细胞型结肠癌模型进行体外试验[6]。干细胞型结肠癌不仅具有高表达细胞外因子(Wnt)信号传导与干细胞、肌上皮基因、间充质基因的特点,还存在着低表达分化标志物的情况。CD44受体在结肠癌干细胞中也具有高表达[41]。与正常干细胞类似,结肠癌干细胞具有对称的细胞分裂无限自我更新的能力,并通过不对称分裂产生子代细胞[42-43]。因为结肠癌干细胞不进行终末分化,所以会产生无限增殖的后代,这个过程直接促进了实体瘤的形成[44]。Wnt信号通路是细胞增殖分化、胚胎和器官发育的关键调控环节和信号转导途径之一[45],在多种恶性肿瘤的生长过程中Wnt途径都被异常激活,证明了其与癌症的相关性。Wnt信号通路包括Wnt/β-连环蛋白、Wnt/Ca2+、Wnt/细胞极性通路等,其中Wnt/β-连环蛋白的生物信号就是卷曲受体[46]。Gurney A等[47]运用单克隆抗体OMP-18R5通过识别卷曲蛋白7与卷曲受体结合,并阻断Wnt信号传导,结果表明,该抗体能够选择性地抑制一系列Wnt/β-连环蛋白信号通路高表达肿瘤的生长。若将该单克隆抗体或者靶向CD44受体的靶分子HA与伊立替康等化疗药物结合,可能对干细胞型结肠癌有较好的治疗效果。
2.3 炎症型
炎症型结肠癌以趋化因子和干扰素(IFN)相关基因及MUC1的相对高表达为特征[48]。趋化因子是一种小细胞因子样肽,分子量为7~15 kDa,与其受体共同协调白细胞在稳态和炎症条件下的迁移。Li Z等[49]发现,microRNA-126可以通过靶向趋化因子受体4(CXCR4),抑制结肠癌细胞的侵袭和迁移。IFN是反映炎症状况的一个指标,与炎症反应密切相关,根据受体特异性和序列同源性可分为Ⅰ型和Ⅱ型:Ⅰ型IFN由多种IFN-α、IFN-β、IFN-ω和IFN-τ亚型组成,这些亚型结构相似,并可与常见的异二聚体受体结合;Ⅱ型IFN主要是由IFN-γ亚型组成[50]。但是目前尚未见针对IFN及IFN亚型的靶向药物或靶分子的研究。
2.4 过渡扩增型
过渡扩增型结肠癌分为CS-TA和CR-TA两类。CSTA类结肠癌患者无病生存期长,可过表达EGFR配体表皮调节素和双调蛋白,所以使用靶向EGFR的西妥昔单抗进行治疗可以获得很好的疗效。针对CS-TA类结肠癌的体外研究可使用NCL-H508、SW1116等细胞系[6]。而CR-TA类结肠癌患者无病生存时间短,具有FLNA高表达和对细胞间质上皮转化因子(cMET)抑制剂比较敏感的特点。体外研究CR-TA型结肠癌时可使用LS1034、SW948等细胞系[6]。近期有研究表明,敲除FLNA基因的细胞对抗肿瘤药物多西紫杉醇更加敏感[51]。癌细胞缺乏FLNA,易出现DNA损伤、G2/M期阻滞以及磷酸化组蛋白H2AX增加,进而促进细胞凋亡[52-53];FLNA还与血管内皮生长因子A共同参与了肿瘤血管生成[54]。但是目前靶向至FLNA受体的靶分子研究较少,尚未寻找到适宜的靶分子与cMET抑制剂结合以制备CR-TA靶向药物。
3 展望
随着结肠癌患者人数的逐年上升,病死率居高不下,再加上化疗药物缺乏选择性,在杀死癌细胞的同时也会杀死正常细胞,对患者的身体造成了极大的损害,因此准确靶向至结肠癌细胞的药物及靶分子的研发迫在眉睫。结肠癌靶分子在选择性识别结肠癌细胞受体上起着非常重要的作用。故寻找对结肠癌细胞更加敏感的靶分子从而提高药物的选择性,成为了结肠癌治疗的关键。目前,临床上已经出现了可以靶向至结肠癌细胞的药物,但仍然因为其对不同的结肠癌患者缺乏选择性使其应用受到限制。虽然将结肠癌患者分型并据此给予不同的靶向药能够达到事半功倍的效果,但是目前这个分型方法还没有应用到临床上,并且此分型方法也由于样本量的限制,还不够完善。为此,关于结肠癌的分型以及针对不同分型的靶向药物还有必要继续深入研究。近年来,国内外科研工作者将靶向结肠癌细胞的靶分子与化疗药物结合在一起,大大提高了靶向结肠癌药物的有效性和安全性。相信随着科研工作的进一步发展,一定会早日研发出准确有效靶向至各型结肠癌的药物。
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