Bortezomib Is More Effective to Side Population of RPMI8226 Myeloma Cells than Classical Anti-myeloma Agents

AIM: Cytotoxic chemotherapy-based treatment of multiple myeloma (MM) is not curative, and the disease eventually recurs. This is partially because although currently available anti-MM strategies are effective in targeting the bulk of tumor cells, they do not target the tumor-initiating subpopulation of cancer stem cells. This study investigated the prevalence and biological functions of side population (SP) cells in MM cell lines including RPMI8226, ARH77, MM.1R and IM 9. MATERIALS AND METHODS: Flow cytometry-based Hoechst 33342 staining was used to evaluate the existence of SP cells. In addition, the ability of SP cells to regenerate the original population was determined. RESULTS: The frequency of SP cells was heterogeneous. Most cell lines (ARH77, IM9, and MM.1R) contained fewer than 1% SP cells; however, RPMI8226 contained approximately 10% SP cells. Sorted SP cells showed a higher proliferative ability and clonogenicity than the MP in the RPMI8226 myeloma cell line. The activity of ATP-binding cassette subfamily G member 2 (ABCG2), which is associated with high rates of proliferation, was higher in SP cells. However, the expression of specific surface markers such as cluster of differentiation (CD)138, CD34, CD38, CD19, CD20, and CD27 did not differ between SP and MP cells. Bortezomib was the only agent that significantly affected proliferation of both SP and MP cells. CONCLUSION: Our studies demonstrated that the SP fraction of myeloma cells possessed clonogenic tumor-initiating potential and revealed new mechanisms of action for bortezomib on SP cells.