Bim Is the Critical, and Bad and Bmf Are the Ancillary BH3-Only Proteins for Imatinib Mesylate-Induced Apoptosis of Bcr/Abl-Positive Leukemias.

Imatinib mesylate (imatinib) exerts the anti-Philadelphia-positive (Ph¹+) leukemia activity both by the inhibition of cell proliferation and by the induction of apoptosis. Recent studies demonstrate that the induction of cell death is essential for eradication of Ph¹+ leukemic clones in imatinib treatment; however, the molecular mechanisms have not yet been clearly described. By examining the effect of imatinib on parental K562 and subclones overexpressing either Bcl-2, Bcl-XL or a dominant interfering mutant of FADD/MORT1, which blocks death receptor apoptosis signalling, we found that imatinib triggers apoptosis exclusively via the Bcl-2 family-regulated intrinsic apoptotic pathway. We investigated the involvement of BH3-only proteins as apoptotic initiators in imatinib-induced cell death, because the cell life-or-death decision is arbitrated by the balance between pro-apoptotic BH3 only-proteins and anti-apoptotic Bcl-2 proteins. We found that imatinib treatment upregulated Bim in Ph¹+ leukemic cell lines and bcr-c-abl transformed murine fetal liver cells (FLCs)-derived cell lines both by transcriptional and post-translational mechanisms. Imatinib also activated Bad through dephosphorylation and upregulated Bmf transcriptionally. To examine the role of Bim in imatinib-induced apoptosis, we examined the cell killing activity of imatinib in subclones of K562 and BV173 Ph¹+ cells expressing abnormally reduced levels of Bim using stable RNA interference system. This revealed that the cell killing activity of imatinib largely dependent on Bim expression levels in these cell lines, although significant apoptosis was still evident. To further define the role of Bim, Bad and Bmf in imatinib-induced cell death, we examined the effect of imatinib on retrovirally bcr-c-abl transformed cell lines derived from FLCs from wild type C57BL/6, Bim-/-, Bad-/-, Bim-/-Bad-/- double KO and Bcl-2 transgenic fetuses. The bim-/-bcr-c-abl+ FLCs were shown to be more resistant to imatinib-induced cell death than wt.bcr-c-abl+ FLCs, however, bim-/-bcr-c-abl+ FLCs were eventually induced into cell death, indicating that Bim is not the only initiator of apoptosis. The bad-/-bcr-c-abl+ FLCs were also partially resistant to imatinib-induced cell death. Intriguingly, like in vav.bcl-2.bcr-c-abl+ FLCs, the cell death induction by imatinib (~5.0μM) was largely abrogated in bim-/-bad-/-bcr-c-abl+ FLCs, indicating that Bim collaborates with Bad for the apoptotic induction by imatinib. Importantly, we found that Bim was inducible by ex vivo imatinib treatment in primary Ph¹+ leukemic cells only from clinically good responders but not from patients refractory to imatinib treatment. Collectively, these results demonstrate that Bim is the critical but not the only initiator required for imatinib-induced apoptosis of Bcr/Abl-positive hematopoietic cells; Bad and Bmf may be the ancillary BH3-only proteins in this process. Our results provide evidence for the therapeutic significance of regulation of BH3-only proteins, particularly Bim, for the eradication of Ph¹+ leukemic cells.