pifithrin-mu and Multiple-Myeloma

Multiple myeloma (MM) is an incurable plasma cell malignancy in which p53 is rarely mutated. Thus, activation of the p53 pathway by a small molecule inhibitor of the p53-MDM2 interaction, nutlin, in MM cells retaining wild type p53 is an attractive therapeutic strategy. Recently we reported that nutlin plus velcade (a proteasome inhibitor) displayed a synergistic response in MM. However, the mechanism of the p53-mediated apoptosis in MM has not been fully understood. Our data show that nutlin-induced apoptosis correlated with reduction in cell viability, upregulation of p53, p21 and MDM2 protein levels with a simultaneous increase in pro-apoptotic targets PUMA, Bax and Bak and downregulation of anti-apoptotic targets Bcl2 and survivin and activation of caspase in MM cells harboring wild type p53. Nutlin-induced apoptosis was inhibited when activation of caspase was blocked by the caspase inhibitor. Nutlin caused mitochondrial translocation of p53 where it binds with Bcl2, leading to cytochrome C release. Moreover, blocking the transcriptional arm of p53 by the p53-specific transcriptional inhibitor, pifithrin-α, not only inhibited nutlin-induced upregulation of p53-transcriptional targets but also augmented apoptosis in MM cells, suggesting an association of transcription-independent pathway of apoptosis. However, inhibitor of mitochondrial translocation of p53, PFT-μ, did not prevent nutlin-induced apoptosis, suggesting that the p53 transcription-dependent pathway was also operational in nutlin-induced apoptosis in MM. Our study provides the evidence that nutlin-induced apoptosis in MM cells is mediated by transcription-dependent and -independent pathways and supports further clinical evaluation of nutlin as a novel therapeutic agent in MM.

Topics: Apoptosis; Apoptosis Regulatory Proteins; Benzothiazoles; Cell Line, Tumor; Cell Proliferation; Cell Survival; Cyclin-Dependent Kinase Inhibitor p21; Dose-Response Relationship, Drug; Flow Cytometry; Gene Expression Profiling; Gene Expression Regulation, Neoplastic; Humans; Imidazoles; Immunoblotting; Multiple Myeloma; Piperazines; Protein Binding; Protein Transport; Proto-Oncogene Proteins c-bcl-2; Proto-Oncogene Proteins c-mdm2; Reverse Transcriptase Polymerase Chain Reaction; Signal Transduction; Sulfonamides; Toluene; Transcription, Genetic; Tumor Suppressor Protein p53