Importin β1 mediates nuclear factor-κB signal transduction into the nuclei of myeloma cells and affects their proliferation and apoptosis

Multiple myeloma (MM) is a plasma cell neoplasm that is currently incurable. The activation of nuclear factor-kappa B (NF-kappa B) signalling plays a crucial role in the immortalisation of MM cells. As the most important transcription factor of the canonical NF-kappa B pathway, the p50/p65 heterodimer requires transportation into the nucleus for its successful signal transduction. Importin beta 1 is the key transport receptor that mediates p50/p65 nuclear import. Currently, it remains unclear whether the regulation of importin beta 1 function affects the biological behaviour of MM cells. In the present study, we investigated the changes in p65 translocation and the proliferation and apoptosis of MM cells after treatment with small interfering RNA (siRNA) or an importin beta 1 inhibitor. The underlying mechanisms were also investigated. We found importin beta 1 over-expression and the excessive nuclear transport of p65 in myeloma cells. Confocal laser scanning microscopy and Western blot analysis results indicated that p65 nuclear transport was blocked after inhibiting importin beta 1 expression with siRNA and the importin beta 1-specific inhibitor importazole (IPZ). Importantly, electronic mobility shift assay results also verified that p65 nuclear transport was dramatically reduced. Moreover, the expression of the NF-kappa B signalling target genes involved in MM cell apoptosis, such as BCL-2, c-IAP1 and XIAP, were markedly reduced, as demonstrated by the RT-PCR results. Furthermore, the proliferation of MM cells was inhibited, as demonstrated by MIT assay results, and the MM cell apoptosis rate was higher, as demonstrated by the annexin V/propidium iodide (PI) double-staining assay results. Additionally, the percentage of S phase cells in the myeloma cell lines treated with IPZ was dramatically reduced. In conclusion, our results clearly show that importin beta 1 mediates, the translocation of NF-kappa B into the nuclei of myeloma cells, thereby regulating proliferation and blocking apoptosis, which provides new insights for targeted myeloma therapies. (c) 2015 Elsevier Inc. All rights reserved.