BACKGROUND: Overwhelming evidence suggests that tumor bulks are comprised of differentiated tumor cells and cancer stem cells (CSCs). The stem cell-like side-population (SP) cells account for a minor fraction of the total tumor cells, yet are apparently the cells capable of tumor initiation, growth, maintenance, and recurrence.OBJECTIVE: To identify potential stem cell-like cancer cells in a U87 human brain glioma cell line on the basis of dye efflux, clone formation, and multi-drug resistance capacity. DESIGN, TIME AND SETTING: The cellular and molecular biology experiment was performed at the Laboratory of Shanghai Institute of Hematology and Laboratory of Shanghai Institute of Endocrinology in Ruijin Hospital; in vivo contrast observational animal trial was performed at Experimental Animal Center, School of Medicine, Shanghai Jiao Tong University from June 2007 to May 2008. MATERIALS: The U87 cell line was provided by the Shanghai Institute of Cancer Research, Chinese Academy of Science; DMEM/F12 (1: 1) and fetal bovine serum were purchased from Gibco, Invitrogen, USA; human recombinant basic fibroblast growth factors were purchased from BD Bioscience, USA; Hoechst 33342, Verapamil, and methyl thiazolyl tetrazolium were purchased from Sigma, USA; phycoerythrin-labeled anti-human-CD133 was purchased from Milteny Biotec, Germany; SYBR~(R) PrimeScript~(TM) RT-PCR kit was purchased from TaKaRa Biotechnology, Dalian, China. METHODS: Monolayer cultured cells were harvested by 0.25% Trypsin-EDTA and suspended at a 1 × 10~6/mL dilution in PBS containing 2% FBS, and were stained with Hoechst 33342 dye, either alone or in combination with Verapamil. Following fluorescence-activated cell sorting, SP and non-SP subsets were cultivated with serum-containing (DMEM plus 10% fetal bovine serum) or serum-free culture medium [DMEM/F12 (1: 1) + 1× B27 supplement + 10 ng/mL basic fibroblast growth factors + 1× L-glutamine] to determine growth characteristics in vitro. Finally, single free U87 cells and subsets (SP or non-SP cells) were subcutaneously injected into the backs of 5-week-old nude mice for in vivo tumorigenicity.MAIN OUTCOME MEASURES: Cell morphology and clonogenicity were observed under inverted microscope; SP phenotype and fluorescent antibody labeling were analyzed by MoFlo(tm) flow cytometry; ABC transporter mRNA expression was evaluated by semi-quantitative real-time RT-PCR; efflux capacity for anti-neoplastic drugs from the U87 cell line and subsets was measured with the MTT assay, then detected by enzyme-linked immunosorbent assay at a wavelength of 490 nm; in vivo tumorigenicity in immunodeficient nude mice was evaluated by diameter size.RESULTS: During in vitro passages, human U87 cells maintained a stable SP fraction profile and exhibited the ability to form neurosphere-like clones. SP cell proliferation decreased compared with non-treated U87 cells. CD133 expression was reduced in the SP and non-SP cells. Freshly sorted SP fractions expressed higher levels of ABC drug transporter genes, and exhibited increased potential for cytotoxic drug resistance. The in vivo malignancy of U87 cells was largely dependent on non-SP cells in nude mice, and tumors that formed from the non-SP fraction developed faster and larger compared with tumors from the SP fraction.CONCLUSION: The SP cell component was a key factor that influenced mRNA expression and cytotoxic drug resistance. In particular, cancer stem cells or tumor-initiating cells were not exclusively enriched in the SP subset of the U87 cell line, and non-SP cells were even more tumorigenic.