Inhibition Of PI3K/mTOR Overcomes Nilotinib Resistance In BCR-ABL1 Positive Leukemia Cells

Expression of the Philadelphia chromosome (Ph), i.e., the t(9;22) chromosomal translocation and the formation of the BCR-ABL1 fusion protein, is the hallmark of chronic myeloid leukemia (CML). Tyrosine kinase inhibitors (TKI), such as imatinib and nilotinib, have emerged as leading compounds to treat CML. Translocation t(9;22) does not only occur in CML, 20-30% of acute lymphoblastic leukemia (ALL) are also found to carry the Ph. However, TKIs are not as effective in the treatment of Ph+ ALL as in CML. In this study, the Ph+ cell lines JURL-MK2 and SUP-B15 were used to investigate TKI resistance mechanisms and sensitization of Ph+ tumor cells to TKI treatment. The annexin V/PI (propidium iodide) assay revealed that nilotinib induced apoptosis in JURL-MK2 cells, but not in SUP-B15 cells. There was no mutation in the tyrosine kinase domain of BCR-ABL1 in both cell lines. SUP-B15 cells were not generally irresponsive to TKI, evidenced by dephosphorylation of the BCR-ABL1 downstream target GAB2 (Grb-associated binder-2). The resistance to apoptosis after nilotinib treatment was accompanied by the constitutive and nilotinib irresponsive activation of the phosphoinositide 3-kinase (PI3K) pathway. Treatment of SUP-B15 with the dual PI3K/mammalian target of rapamycin (mTOR) inhibitor BEZ235 alone induced apoptosis in a low percentage of cells, combination of nilotinib and BEZ235 led to a synergistic effect. Caspase 3 and PARP (poly ADP ribose polymerase) cleavage confirmed that apoptosis was induced after combined treatment. The main role of PI3K/mTOR inhibitor BEZ235 and the reason for apoptosis in the nilotinib-resistant cells was the block of the translational machinery, leading to the rapid down-regulation of anti-apoptotic protein MDM2 (human homolog of the murine double minute 2, hMDM2). The current findings suggest that MDM2 may be a therapeutic target to increase TKI-mediated apoptosis and that the combination of PI3K/mTOR dual inhibitor and TKI inhibitor might turn out to be a novel strategy for TKI-resistant BCR-ABL1 positive leukemia.