shRNA-mediated Bmi-1 silencing sensitizes multiple myeloma cells to bortezomib

The introduction of bortezomib has resulted in a paradigm shift in the treatment of multiple myeloma (MM) and has contributed to the improved survival of patients with MM. Inevitably, resistance to therapy develops, and thus the clinical efficacy of bortezomib is hampered by drug resistance. The oncogene B-cell-specific Moloney murine leukemia virus insertion site-1 (Bmi-1) has been implicated in the pathogenesis of various human malignancies. Furthermore, RNA interference (RNAi)-mediated Bmi-1 silencing has been shown to sensitize tumor cells to chemotherapy and radiation. The role of Bmi-1 in influencing the response to bortezomib therapy has not been investigated to date. In the present study, Bmi-1 was silenced in two MM cell lines (U266 and RPMI8226) using short hairpin RNA (shRNA) targeting Bmi-1 (shBmi-1). A cell counting kit-8 (CCK-8) assay was performed to analyze cell proliferation and evaluate the 50% inhibitory concentration (IC50) values of bortezomib. Cell cycle progression and apoptosis were analyzed by flow cytometry (FCM), and the mRNA and protein expression of associated genes (Bmi-1, p14, p21, Bcl-2 and Bax) was quantified by RT-qPCR and western blot analysis, respectively. The IC50 values significantly decreased in the cells transfected with shBmi-1 (p<0.05). The depletion of Bmi-1 sensitized the MM cells to bortezomib, which increased the G, phase duration and enhanced bortezomib-induced apoptosis (p<0.05). The expression of p21 and Bax (apoptosis inducer) was upregulated, whereas that of the anti-apoptotic protein, Bcl-2, was downregulated in the Bmi-1-silenced cells (p<0.05). The depletion of Bmi-1 enhanced the sensitivity of MM cells to bortezomib by inhibiting cell proliferation and inducing cell cycle arrest and apoptosis. Our data suggest that Bmi-1 may serve as an important novel therapeutic target in MM.