Targeting Autophagy Potentiates Imatinib-Induced Cell Death in Philadelphia Positive Cells Including Primary CML Stem Cells.

Imatinib mesylate (IM), a potent ATP-competitive inhibitor of the BCR/ABL tyrosine kinase, has become standard therapy for patients with chronic myelogenous leukemia (CML). However, the main limitations of IM- and second generation tyrosine kinase inhibitor (TKI)-based therapy are the insurgence of resistance in patients and the intrinsic refractoriness of primitive Philadelphia-positive stem cells. Therefore, there is the need to develop new therapeutic approaches that, in combination with TKI, might be more effective in targeting the stem cell population and preventing the outgrowth of TKI-resistant CML cells. TKI-induced elimination of BCR/ABL-dependent intracellular signals is known to trigger apoptosis, but it is unclear whether this also activates additional cell death and/or survival pathways. We show that IM treatment induces autophagy in CML blast crisis cell lines, CML primary cells and p210BCR/ABL-expressing 32Dcl3 (32D) myeloid precursor cells, but not in 32D cells expressing v-Src or the IM-resistant T315I p210BCR/ABL mutant. IM-induced autophagy does not involve c-Abl, as it is also observed in cells co-expressing p210BCR/ABL and the IM-resistant T315I c-Abl mutant. Induction of autophagy is associated with endoplasmic reticulum-stress and is suppressed by depletion of intracellular calcium. By contrast, ectopic Bcl-2 expression does not block IM-induced autophagy. Suppression of autophagy by pharmacological inhibitors or siRNA-mediated knockdown of essential autophagy genes enhances cell death induced by IM in cell lines and primary CML cells, demonstrating that induction of autophagy has a pro-survival effect. Critically, the combination of TKI with autophagy inhibitors results in near complete elimination of phenotypically (CD34+38−) and functionally (colony forming cells) defined CML stem cells. Together, these findings suggest that autophagy inhibitors may enhance the therapeutic effects of TKI in the treatment of CML.