脑肿瘤干细胞体外分化过程中的回逆现象分析

Preliminary studies on in vitro differentiation reversion of brain tumor stem cells

目的探讨脑肿瘤干细胞(BTSCs)体外分化过程中的回逆现象,为研究其分化抑制机制奠定基础。方法利用CD133免疫磁珠筛选系统,从肿瘤组织中分离获得的CD133^-细胞(BTSCs)分成4组进行培养:(1)含10%胎牛血清(FCS);(2)10%FCS+丙戊酸钠注射液(VPA);(3)无FCS+生长因子;(4)无FCS+生长因子+VPA。取不同时间点上的细胞,相差显微镜观察其形态变化;流式细胞术检测与分化相关的标志物、细胞周期和DNA倍体变化:利用免疫激光共聚焦分析与分化相关标志物的共表达情况。结果无FCS条件下培养的BTSCs呈悬浮球状生长,高表达CD133和巢蛋白(nestin),不表达胶质纤维酸性蛋白(GFAP)和β-微管蛋白Ⅲ(β-TubulinⅢ)。G0/G1期细胞占大多数,G2/M期细胞接近0%,DNA都是异倍体,对VPA反应不敏感。含FCS培养的原本悬浮的细胞约4h开始贴壁,均呈圆形。此后逐渐向多形性分化,至7d时分化的细胞部分又返回至圆形,至10 d～21d时,有的还能重新恢复球形,并呈悬浮生长。培养3d、7d、10d和21d时,CD133、nestin阳性细胞数先降后升,GFAP^+和β-TubulinⅢ^-细胞数始终处于较低水平。含FCS培养液中加入VPA,细胞形态上未见上述的回逆现象,CD133和nestin表达的先降后升现象消失,GFAP和β-TubulinⅢ在第7天以后表达明显升高,但极大部分细胞共表达nestin。而神经干细胞(NSCs)在含FCS培养至10 d时,即以GFAP和β-TubulinⅢ表达为主,未见CD133^+细胞。此外,含血清培养时BTSCs仍以异倍体为主,含少量的G2/M期细胞,加VPA诱导后细胞周期和DNA倍体变化不明显。结论BTSCs在含血清条件下培养出现的多向分化表型不稳定,时有去分化所导致的回逆。加入诱导分化剂VPA培养,虽然能阻止回逆现象出现,并有代表星形胶质细胞和神经元标志物表达上升,但因其共表达nestin而仍属于未完全分化细胞,表明BTSCs分化始终处于受抑状态。

Objective To investigate in vitro differentiation reversion of brain tumor stem cells （BTSCs） for further study the inhibitory mechanisms of BTSCs differentiation. Methods CD133^＋ cells （BTSCs） were isolated from glioma specimens using the immunomagnetic bead separation system, and cultured in DMEM/F-12 medium supplemented with： （1） 10% fetal calf serum （FCS）, （2） 10% FCS ＋ sodium valproate （VPA） injection, （3） no FCS ＋ growth factors, and （4） no FCS ＋ growth factors ＋ VPA injection, respectively. The morphological changes of cells were observed under phase contrast microscope at different time points; the cell surface markers relating with cell differentiation, cell cycles and changes in DNA ploids were detected with flow cytometry （FCM）; co-expressions of cell surface markers relating with differentiation were analyzed with immunofluorescence staining and laser scanning confocal microscopy （LSCM）. Results BTSCs cultured grew with a spherical shape in a suspended way in FCS-free medium and the expression levels of CD133 and nestin were high, but expressions of glial fibrillary acidic protein （GFAP） and β-TubulinⅢ were negative. Cells in G0/G1 phase were the majority among these cells and in G2/M phase, nearly 0%. Besides, chromosomes of these cells were all heteroploids and the cells were insensitive to VPA. Having been cultured in FCS medium for 4 h, the suspended BTSCs spheres became adherent and turned to the round shape. Later on, the cells differentiated gradually into various shapes, until 7 d after in vitro culturing, the differentiated cells partially returned to the round shape and during the 10-21 d, some even regained the spherical shape and again grew in the suspended way. In addition, the results of FCM showed that the number of CD133 and nestin positive cells decreased first and then increased after having been cultured in FCS medium for 3, 7, 10 and 21 days, but the number of GFAP^＋ or β-TubulinⅢ^＋ cells were at low levels. On the contrary, when VPA was added to the FCS medium, the phenomenon of reversion in cell morphology and changes in CD133 and nestin expression levels disappeared. The GFAP^＋ and β-TubulinⅢ cells increased significantly after treatment with VPA for 7 d, but nestin was co-expressed with most of the GFAP^＋ or β-TubulinⅢ^＋ cells. After cultured in FCS medium for 10 d, the majority of neural stem cells （NSCs） were GFAP^＋ and β-TubulinⅢ^＋ cells with no CD 133^＋cells. Besides, the BTSCs cultured in FC S medium were heteroploids primarily including a few G2/M phase cells. After treatment with VPA, there were no significant differences in cell cycle and DNA ploids. Conclusion The phenotype of multi-directional differentiation of BTSCs in FCS medium was instable; the differentiated cells reversed frequently due to dedifferentiation. The differentiation inducer VPA inhibited differentiation reversion of BTSCs and facilitated the expressions of cell surface markers representing astrocytes and neurons, but the differentiation of BTSCs was incomplete for their co-expression with nestin, suggesting that the differentiation of BTSCs was still consistently inhibited.