Abstract
Multiple myeloma (MM) is a clonal disease of plasma cells that remains, for the most part, incurable despite the advent of several novel therapeutics. The elevated expression of p27 and its association with cell-cycle arrest is speculated to be one of the major mechanisms by which MM cells escape the cytotoxic effects of therapeutic agents. In this study, we demonstrated that RBX1 silencing could inhibit MM cell growth and promote cell drug resistance. RBX1 directly interacted with and triggered the ubiquitination and degradation of p27, ultimately causing p27 reduction. Additionally, cell growth and apoptosis analysis indicated that the role of RBX1 in regulating myeloma cell proliferation and drug resistance resulted from p27 accumulation, which occurred in a Thr187 phosphorylation-dependent manner. Furthermore, the cell-cycle analysis demonstrated that RBX1 overexpression induced cells to enter the cell cycle (S-phase) and partially inhibited chemotherapeutic drugs-mediated cell cycle arrest. Notably, the forced expression of RBX1 also inhibited the cell adhesion-mediated elevation of p27 and induced the accumulation of adherent cells in apoptosis, especially the proteolytic cleavage of caspase-3. Additionally, RBX1 knockdown significantly inhibited myeloma development in SCID-Hu mice and in a human MM xenotransplant model. Overall, these in vitro and in vivo experiments indicated that the RBX1-p27 axis could be a central molecular mechanism by which RBX1 functions as a tumor promoter and stimulates cell growth in chemotherapeutic drugs treated MM cells.