The Ras/Raf-1/MEK/ERK pathway is constitutively activated in Bcr-Abl transformed cells, and Ras activity enhances the oncogenic ability of Bcr-Abl. On the hand, Raf kinase inhibitor protein (RKIP) inhibits activation of MEK by Raf-1 and its downstream signal transduction, resulting in blocking the MAP kinase pathway. Moreover, Raf-1 has been reported to regulate cell cycle progression. However, the mechanism by which Bcr-Abl promotes the cell cycle progression through Raf-1 is not completely understood. In the present study, we found that the expression of RKIP was suppressed in CML cells, and investigated the interaction between RKIP and Bcr-Abl in CML cells. In aldehyde dehydrogenase (ALDH)hi/CD34+ cells derived from CML patients, the inhibition of Bcr-Abl induced RKIP expression and reduced the phosphorylated-FOXM1 (pFOXM1) status, resulting in inhibited colony formation of Bcr-Abl+ progenitor cells. Moreover, overexpression of RKIP significantly decreased the colony numbers, reduced the pFOXM1 status, and reduced pFOXM-1 target genes such as Skp2, Cdc25B and KIS, and induced the expression of p27Kip1a and p21Cip1. Thus, Bcr-Abl represses the expression of RKIP, and continuously activates FOXM1, resulting in the proliferation of CML progenitor cells through the cell cycle modulation.
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