Background: Cancer-initiating cells (CICs) are proposed to be responsible for the generation of metastasis and\r\nresistance to therapy. Accumulating evidences indicates CICs are found among different human cancers and cell\r\nlines derived from them. Few studies address the characteristics of CICs in cervical cancer. We identify biological\r\nfeatures of CICs from four of the best-know human cell lines from uterine cervix tumors. (HeLa, SiHa, Ca Ski, C-4 I).\r\nMethods: Cells were cultured as spheres under stem-cell conditions. Flow cytometry was used to detect\r\nexpression of CD34, CD49f and CD133 antigens and Hoechst 33342 staining to identify side population (SP).\r\nMagnetic and fluorescence-activated cell sorting was applied to enrich and purify populations used to evaluate\r\ntumorigenicity in nude mice. cDNA microarray analysis and in vitro radioresistance assay were carried out under\r\nstandard conditions.\r\nResults: CICs, enriched as spheroids, were capable to generate reproducible tumor phenotypes in nu-nu mice and\r\nserial propagation. Injection of 1 Ã?â?? 103 dissociated spheroid cells induced tumors in the majority of animals,\r\nwhereas injection of 1 Ã?â?? 105 monolayer cells remained nontumorigenic. Sphere-derived CICs expressed CD49f\r\nsurface marker. Gene profiling analysis of HeLa and SiHa spheroid cells showed up-regulation of CICs markers\r\ncharacteristic of the female reproductive system. Importantly, epithelial to mesenchymal (EMT) transition-associated\r\nmarkers were found highly expressed in spheroid cells. More importantly, gene expression analysis indicated that\r\ngenes required for radioresistance were also up-regulated, including components of the double-strand break (DSB)\r\nDNA repair machinery and the metabolism of reactive oxygen species (ROS). Dose-dependent radiation assay\r\nindicated indeed that CICs-enriched populations exhibit an increased resistance to ionizing radiation (IR).\r\nConclusions: We characterized a self-renewing subpopulation of CICs found among four well known human\r\ncancer-derived cell lines (HeLa, SiHa, Ca Ski and C-4 I) and found that they express characteristic markers of stem\r\ncell, EMT and radioresistance. The fact that CICs demonstrated a higher degree of resistance to radiation than\r\ndifferentiated cells suggests that specific detection and targeting of CICs could be highly valuable for the therapy\r\nof tumors from the uterine cervix.
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