Physiologic hypoxia promotes maintenance of CML stem cells despite effective BCR-ABL1 inhibition

C-abl oncogene 1, nonreceptor tyrosine kinase (ABL1) kinase inhibitors such as imatinib mesylate (imatinib) are effective in managing chronic myeloid leukemia (CML) but incapable of eliminating leukemia stem cells (LSCs), suggesting that kinase-independent pathways support LSC survival. Given that the bone marrow (BM) hypoxic microenvironment supports hematopoietic stem cells, we investigated whether hypoxia similarly contributes to LSC persistence. Importantly, we found that although breakpoint cluster region (BCR)-ABL1 kinase remained effectively inhibited by imatinib 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 imatinib. Hypoxia-inducible factor 1 alpha (HIF1-alpha), which is the master regulator of the hypoxia transcriptional response, is expressed in the BM specimens of CML individuals. In vitro, HIF1-alpha is stabilized during hypoxia, and its expression and transcriptional activity can be partially attenuated by concurrent imatinib treatment. Expression analysis demonstrates at the whole-transcriptome level that hypoxia and imatinib regulate distinct subsets of genes. Functionally, knockdown of HIF1-alpha abolished the enhanced clonogenicity during hypoxia. Taken together, our results suggest that in the hypoxic microenvironment, HIF1-alpha signaling supports LSC persistence independent of BCR-ABL1 kinase activity. Thus, targeting HIF1-alpha and its pathway components may be therapeutically important for the complete eradication of LSCs.