Characterization of Cancer Stem-Like Cells in a Novel STI571-Resistant Chronic Myeloid Leukemia Cell Line.

To characterize a novel chronic myeloid leukemia (CML) cell line and to further elucidate the mechanisms of resistance to STI571, a novel K562 cell line (K562/VP16) was achieved after exposure of the K562 cells in gradually increasing concentrations of VP16 over a period of several months. A small subpopulation (K562/VP16 SP) that was capable of excluding Hoechst 33342 in the K562/VP16 cell line was isolated by flowcytometry sorting. The rest of the K562/VP16 cells were classified as non-SP K562/VP16. The mechanisms involved in K562/VP16 SP cells that became resistant to STI571 were studied. We found that the levels of Bcr-Abl and Abl proteins were similar in the K562 cells and in non-SP K562/VP16 and K562/VP16 SP cells. The MDR-1 gene expression of the 170 kDa P-gp was detected in K562/VP16 non-SP and K562/VP16 SP cells but not in K562 cells. The expression levels of P-gp in the two K562/VP16 cell lines were similar. Furthermore, there were no mutations of Abl in the K562/VP16 SP cells. Compared with non-SP K562/VP16, the K562/VP16 SP cells were more resistant to STI571, and many multidrug resistance inhibitors could hardly reverse this resistance. In addition, in vivo study showed that the K562/VP16 SP cells induced tumorigenesis in mice, while the K562/VP16 non-SP cells failed to do so. These results suggest that a small side population K562/VP16 SP cells in K562/VP16 cell line, had high resistance to STI571 treatment and more tumorigenic than the K562 cells and it may represent the cancer stem cells of the K562/VP16 cell line.