Multiple myeloma (MM) is characterized by the accumulation of a population of malignant plasma cells within the bone marrow. Cytotoxic chemotherapy-based treatment is not curative, and the disease eventually recurs. Although currently available anti-MM strategies are effective at targeting the bulk of tumor cells, it is not clear that these agents are targeting the tumor-initiating subpopulation, or cancer stem cells. Side Population (SP) cells are an enriched source of cancer-initiating cells with stem cell properties, which have been identified in solid tumors, as well as in hematopoietic malignancies. SP cells express high levels of various members of the ABC transporter family, which are responsible for their drug resistance. A recent our work demonstrated that SP cells in MM have shown to exhibit stem cell like characteristics as well as high tumorigenicity. Therefore, it is worthy to identify gene/proteins specifically expressed in MM SP cells, which could be essential therapeutic targets.
The aim of this study was to identify genes and transcripts that could serve as molecular markers for targeting the MM SP cells, and to identify candidate agents for the MM SP cells.
We used Hoechst 33342 dye to detect the MM SP in five MM cell lines (RPMI 8226, AMO1, KMS-12BM, KMS-11 and JJN3) and eight primary samples. We then tested whether the MM SP cells have stem-like characteristics and performed gene expression analysis to detect genes specifically expressed in the MM SP. On that basis, we tested candidate agents such as an aurora kinase inhibitor (VX-680), a histone methyltransferase inhibitor (DZNep), lenalidomide, thalidomide and a proteasome inhibitor (bortezomib) for their ability to target MM SP cells.
We found that clonogenic MM SP cells exhibit “stem cell-like” properties, including self renewal, differentiation and repopulation. Gene expression analysis of MM cell lines and primary samples revealed that, in SP cells, expression of genes related to G2/M phase (e.g. CDC2, CCNB1)-, microtubule attachment (e.g. BIRC5, CENPE, SKA1)-, mitosis or centrosomes (e.g. AURKB, KIF2C, KIF11, KIF15)-, proliferation (e.g. TOP2A, ASPM)-, polycomb (e.g. EZH2, EPC1)- and proteasomes(e.g. UBE2D3, UBE3C, PSMA5)- was significantly stronger in SP than non-SP cells. On that basis, we used VX-680, DZNep, lenalidomide, thalidomide and bortezomib against MM cells. Of these, bortezomib reduced the SP fraction most effectively due to its ability to reduce levels of target gene transcripts including phospho-histone H3, aurora kinase B and EZH2. Finally we tried to examine effects of those candidate agents to “clonogenic ability of SP”, and found that bortezomib possessed the most powerful effects for reduction of SP colonies. These results suggest that bortezomib has a broader range of targets than other agents and could include cell cycle, centrosome, polycomb and proteasome genes/proteins.
Our findings are i) the first to identify genes specifically expressed in the MM SP, ii) the first to provide a rationale for treating MM using agents against genes and encoded proteins that are specifically expressed in MM SP cells.
Iida:Janssen Pharmaceutical K.K.: Honoraria.
Asterisk with author names denotes non-ASH members.