Low-Level Expression of the Tumor Suppressor Bim in CML Cells: Role of BCR/ABL, Delineation of Underlying Signaling Pathways, and Re-Expression by Imatinib, AMN107, and Proteasome Inhibitors.

Chronic myeloid leukemia (CML) is a myeloproliferative disease in which BCR/ABL enhances survival of leukemic cells through modulation of pro- and anti-apoptotic molecules. Recent data suggest that pro-apoptotic Bim plays a role as a tumor-suppressor in myeloid cells, and that leukemic cells express only low amounts of this death activator. In the current study, we have investigated expression of Bim in primary CML cells and in the CML cell lines K562 and KU812, and in Ba/F3 cells inducibly expressing BCR/ABL on exposure to doxycycline (TonB.210-X). As assessed by Northern- and Western blotting, primary CML cells were found to express significantly lower amounts of Bim mRNA and Bim protein compared to normal bone marrow cells. The BCR/ABL-inhibitors imatinib (Novartis Pharma AG) and AMN107 (Novartis Pharma AG) were found to promote Bim expression in CML cells at pharmacologic concentrations. Correspondingly, BCR/ABL was found to down-regulate expression of Bim in TonB.210-X cells. The BCR/ABL-induced decrease in expression of Bim in leukemic cells was found to be a post-transcriptional event that depended on signaling through MEK, and was abrogated by the proteasome-inhibitor MG132. Interestingly, MG132 was found to up-regulate Bim-expression and to suppress the growth of Ba/F3 cells containing either wild-type BCR/ABL or various imatinib-resistant mutants of BCR/ABL including the T315I mutant that is resistant to all currently available ATP-competitive tyrosine kinase inhibitors (IC50: 30–100 nM). To confirm the role of Bim as a tumor suppressor in CML, a Bim specific siRNA was transfected into K562 cells. This siRNA was found to counteract imatinib- and MG132-induced cell death. In conclusion, our data identify BCR/ABL as a Bim-suppressor in CML cells and suggest, that re-expression of Bim by proteasome inhibition or by targeting of signaling pathways downstream of BCR/ABL may be an attractive therapeutic approach in imatinib-resistant CML.