A Small-Molecule Suppressant of Survivin YM155 Induces Cell Death Via Proteasomal Degradation of c-Myc in Multiple Myeloma Cells

[Background] Survivin is a member of the inhibitor of apoptosis protein (IAP) family with its dual roles in mitosis and apoptosis, and emerges as an attractive target for cancer therapy. YM155, a novel molecular targeted agent, suppresses survivin, which is overexpressed in many tumor types. However, the effect of this agent on multiple myeloma (MM) cells remains unclear. [Materials & Methods] Five human MM cell lines, RPMI8226, U266, KMS20, KMS28PE, and KMS34 were used in this study. Cell proliferation and cell death were evaluated by MTT assay and by flow cytometric analysis with annexin V/PI staining. Gene and protein expressions were analyzed with quantitative PCR (qPCR) and immunoblot, respectively. For proteasome inhibitory assay, cells were treated with YM155 and/or proteasome inhibitor MG132 for 6 hours. [Results] YM155 inhibited cell proliferation of these cells in a dose- and time-dependent manner. Annexin V assay showed that YM155 induced apoptosis in these cells. To better understand these effects of YM155 on MM cells, we evaluated the intracellular signaling and apoptosis-associated protein status. Immunoblot analyses showed that YM155 reduced not only survivin but also Mcl-1 and XIAP expressions. We also observed the activation of caspase-3 and PARP in YM155-treated cells, indicating that YM155 induces caspase-dependent apoptosis. In contrast, YM155 did not affect phosphorylation status of Erk1/2 and STAT3. Interestingly, YM155 suppressed c-Myc and IRF4 protein expressions, both of which are recognized as an important transcription factor in the pathogenesis of MM. In addition, qPCR assay showed that YM155 treatment did not reduce c-Myc mRNA level. On the other hand, proteasome inhibitor prevented the suppression of c-Myc expression by YM155 treatment, indicating a proteasomal degradation of c-Myc by YM155. [Conclusion] YM155 suppresses cell proliferation and survival in MM cells in part via not only inhibiting anti-apoptotic proteins such as survivin, Mcl-1 and XIAP but also suppressing c-Myc oncoprotein expression. Further study is needed to clarify the molecular mechanism of c-Myc degradation induced by YM155. Our results may provide a new concept in c-Myc-targeting therapy for MM because c-Myc oncoprotein have been considered undruggable.