Physiologic Hypoxia Promotes Maintenance of CML Stem Cells Despite Effective BCR−ABL1 Inhibition

ABL1 kinase inhibitors such as imatinib mesylate (IM) are effective in managing chronic myelogenous leukemia (CML) but incapable of eliminating leukemia stem cells (LSCs), suggesting that kinase−independent pathways support LSC survival. Given that the bone marrow hypoxic microenvironment supports hematopoietic stem cells, we investigated if hypoxia similarly contributes to LSC persistence. Importantly, we found that while BCR−ABL1 kinase remained effectively inhibited by IM under hypoxia, apoptosis became partially suppressed. Furthermore, hypoxia enhanced the clonogenicity of CML cells, as well as their efficiency in repopulating immunodeficient mice, both in the presence and absence of IM. HIF1−α, which is the master regulator of the hypoxia transcriptional response is expressed in the bone marrow specimens of CML individuals. In vitro, HIF1−α is stabilized during hypoxia and its expression and transcriptional activity can be partially attenuated by concurrent IM treatment. Expression analysis demonstrates at the whole transcriptome level that hypoxia and IM regulate distinct subsets of genes. Functionally, knockdown of HIF1−α abolished the enhanced clonogenicity during hypoxia. Taken together, our results suggest that in the hypoxic microenvironment, HIF1−α signaling supports LSC persistence independently of BCR−ABL1 kinase activity. Thus targeting HIF1−α and its pathway components may be therapeutically important for the complete eradication of LSCs. 24 samples consisting CD34+ bone marrow aspirates of 3 chronic phase patients that were subjected to 24h or 96h of DMSO/Normoxia (21% oxygen, 5% carbon dioxide) control, 2 µM Imatinib, hypoxia (0.5% oxygen, 5% carbon dioxide) or combined Imatinib/hypoxia treatments in triplicate cultures.