OBJECTIVETo study the cancer stem cell populations in esophageal cancer cell lines KYSE150 and TE1 and identify whether resulting stem-like cell spheres display radiation resistance characteristics.

METHODSSerum-free medium (SFM) suspension was used to culture the esophageal cancer stem cell lines and enrich the esophageal stem-like cell spheres. RT-PCR assay was used to detect the stem cell gene expression in the sphere cells. Radiosensitivity of the sphere cells and parental cells were evaluated by clone formation assay. Different cells after irradiation at different doses were tested to evaluate the changes of sphere formation, and cell cycle and CD44(+)CD271(+) expression of the sphere cells were also analyzed by flow cytometry before and after irradiation.

RESULTSCancer stem-like cell spheres were generated from KYSE150 and TE1 cells and enriched by culture in serum-free medium, and the number of spheres was increasing alone with the increase of cell passages. The numbers of spheres formed from the 1st, 2nd and 3rd generations of KYSE150 cells were 25 ± 2, 37 ± 2 and 47 ± 3, respectively. The numbers of spheres formed from the 1st, 2nd and 3rd generations of TE1 cells were 15 ± 3, 24 ± 3 and 36 ± 4, respectively. Certain doses of radiation increased the sphere formation rate. The average survival fraction (SF2) of the suspension-cultured KYSE150 stem-like cell spheres after 2 Gy irradiation were 0.81 ± 0.03 and 0.69 ± 0.04, while that of TE1 parental cells were 0.87 ± 0.01 and 0.80 ± 0.03 (P < 0.05 for all). In the esophageal parental KYSE150 and TE1 cells, arrest at G2 phase was induced after irradiation. After the same dose of irradiation, the inhibition of proliferation of the cancer stem cells was lower than that of the parent cells (P < 0.05). After 0, 4 and 8 Gy irradiation, the CD44(+)CD271(+) cell percentage of KYSE150 parental cells were (1.08 ± 0.03)%, (1.29 ± 0.07)% and (1.11 ± 0.09)%; the CD44(+)CD271(+) cell percentage of TE1 parental cells were (1.16 ± 0.11)%, (0.97 ± 0.08)% and (1.45 ± 0.35)% (P > 0.05 for all). After 0, 4 and 8 Gy irradiation, the percentage of CD44(+)CD271(+) cells of KYSE150 stem cell-like spheres were (35.83 ± 1.23)%, (44.90 ± 1.67)% and (57.77 ± 1.88)%, and that of TE1 stem cell-like spheres were (16.07 ± 0.91)%, (22.67 ± 1.12)% and (33.27 ± 1.07)%. Compared the 4 Gy and 8 Gy irradiated KYSE150 and TE1 stem-like cell spheres with the 0 Gy irradiated spheres, the differences were statistically significant (P < 0.05 for all).

CONCLUSIONSThe cancer stem cells in KYSE150 and TE1 spheres are more radio-resistant than their parental cells. It may suggest that cancer stem cell populations in the esophageal cancer cells are related to the mechanism of occurrence of radioresistance.