Survival of Cancer Stem Cells under Hypoxia and Serum Depletion via Decrease in PP2A Activity and Activation of p38-MAPKAPK2-Hsp27

Hypoxia and serum depletion are common features of solid tumors that occur upon antiangiogenesis, irradiation and chemotherapy across a wide variety of malignancies. Here we show that tumor cells expressing CD133, a marker for colorectal cancer initiating or stem cells, are enriched and survive under hypoxia and serum depletion conditions, whereas CD133− cells undergo apoptosis. CD133+ tumor cells increase cancer stem cell and epithelial-mesenchymal transition properties. Moreover, via screening a panel of tyrosine and serine/threonine kinase pathways, we identified Hsp27 is constitutively activated in CD133+ cells rather than CD133− cell under hypoxia and serum depletion conditions. However, there was no difference in Hsp27 activation between CD133+ and CD133− cells under normal growth condition. Hsp27 activation, which was mediated by the p38MAPK-MAPKAPK2-Hsp27 pathway, is required for CD133+ cells to inhibit caspase 9 and 3 cleavage. In addition, inhibition of Hsp27 signaling sensitizes CD133+ cells to hypoxia and serum depletion -induced apoptosis. Moreover, the antiapoptotic pathway is also activated in spheroid culture-enriched CD133+ cancer stem cells from a variety of solid tumor cells including lung, brain and oral cancer, suggesting it is a common pathway activated in cancer stem cells from multiple tumor types. Thus, activation of PP2A or inactivation of the p38MAPK-MAPKAPK2-Hsp27 pathway may develop new strategies for cancer therapy by suppression of their TIC population.缺氧与血清剥夺是多种恶性肿瘤经抗血管生成治疗、放疗或化疗后，实体瘤中出现的常见特征。本研究证实，表达CD133（结直肠癌起始细胞/干细胞标志物）的肿瘤细胞在缺氧及血清剥夺条件下得以富集并存活，而CD133阴性细胞则发生凋亡。CD133阳性肿瘤细胞可增强癌症干细胞及上皮间质转化特性。此外，通过筛选一系列酪氨酸激酶与丝氨酸/苏氨酸激酶通路，我们发现：在缺氧及血清剥夺条件下，Hsp27(Heat shock protein 27)在CD133阳性细胞中呈组成型激活状态，而CD133阴性细胞无此现象；但在正常生长条件下，两类细胞的Hsp27激活水平无显著差异。由p38MAPK-MAPKAPK2-Hsp27通路介导的Hsp27激活，是CD133阳性细胞抑制半胱天冬酶9与3切割的必要条件。此外，抑制Hsp27信号通路可使CD133阳性细胞对缺氧及血清剥夺诱导的凋亡敏感化。进一步研究显示，该抗凋亡通路在肺癌、脑癌及口腔癌等多种实体瘤细胞的球体培养富集的CD133阳性癌症干细胞中同样被激活，表明其是多瘤种癌症干细胞中共通的激活通路。因此，激活PP2A(Protein Phosphatase 2A)或抑制p38MAPK-MAPKAPK2-Hsp27通路，或可通过清除肿瘤起始细胞（Tumor-Initiating Cells, TIC）群，为癌症治疗开发全新策略。