Activation of Raf-1 through the Depletion of RKIP by Bcr-Abl Promotes CML Cell Proliferation

RKIP (Raf kinase inhibitor protein) regulates cell proliferation, apoptosis, and cell cycle through the inhibition of Raf-1, and it has been reported that the suppression of the RKIP expression promotes the proliferation of tumor cells. However, its function is little known in CML. We found that the expression of RKIP gene and protein was suppressed in CML cells. In this study, we have investigated the expression and the function of the RKIP in CML cell proliferation. [Methods] The cells used in this study were human CML cell lines, K562, Meg01, and SHG3 cells. Primary CML cells (ALDH^hicells) were obtained from the bone marrow of CML (CP) patients (n=12). Human normal ALDH^hi cells were isolated from bone marrow of healthy volunteers after obtaining informed consents. For analysis of RKIP mRNA expression, quantitative RT-PCR was performed in all cell lines treated with Abl kinase inhibitors (STI571, AMN107, and BMS354825). For proliferation analysis and the levels of Erk1/2 phosphorylation in CML cells, MTT assays, western blot and cell cycle analysis were performed in all cell lines transfected with Bcr-Abl, RKIP siRNA, or RKIP cDNA respectively. For colony analysis, the colonies of CFU-GEMM, CFU-GM, and BFU-E were counted in CML stem/progenitor cells transfected with RKIP siRNA or treated with Abl kinase inhibitors. [Results] In CML cell lines treated with Abl kinase inhibitors or transfection with Bcr-Abl siRNA, the expressions of RKIP mRNA and protein were significantly increased more than untreated cells. Moreover, in CML cells transfected with the RKIP siRNA, the cell proliferation inhibition by treatment of the Abl kinase inhibitors was weakened compared to RKIP siRNA-untransfected cells, and the phosphorylation levels of Erk1/2 and Raf-1 were markedly increased compared to theRKIP siRNA-untransfected cells. On the other hands, the overexpression of RKIP induced G1 cell cycle arrest through p27 and p21 accumulation, decreased the phosphorylation levels of ERK1/2 and Raf-1, and inhibited the proliferation in CML cells. In CML stem/progenitor cells obtained from patients with CML, the expression of RKIP mRNA was suppressed in 10/10 (100 %) of CML. The transfection with Bcr-Abl siRNA or treatment with Abl kinase inhibitors increased the expression of RKIP mRNA and protein, and the overexpression of RKIP decreased the counts of CFU-GEMM, CFU-GM and BFU-E. [Conclusion] Our results demonstrated that the Bcr-Abl suppressed the expression of RKIP, the depletion of RKIP induced Raf-1 activation, and induced the proliferation of CML cells through ERK1/2 phosphorylation. Moreover, in CML, the induction of RKIP expression inhibited the proliferation of CML stem/progenitor cells.

No relevant conflicts of interest to declare.