Chronic myeloid leukemia progenitor cells require autophagy when leaving hypoxia-induced quiescence

Albeit tyrosine kinase inhibitors anti-Abl used in Chronic Myeloid Leukemia (CML) block the deregulated activity of the Bcr-Abl tyrosine kinase and induce remission in 90% of patients, they do not eradicate immature hematopoietic compartments of leukemic stem cells. To elucidate if autophagy is important for stem cell survival and/or proliferation, we used culture in low oxygen concentration (0.1% O₂ for 7 days) followed back by non-restricted O₂ supply (normoxic culture) to mimic stem cell proliferation and commitment. Knockdown of Atg7 expression, a key player in autophagy, in K562 cell line inhibited autophagy compared to control cells. Upon 7 days at 0.1% O₂ both K562 and K562 shATG7 cells stopped to proliferate and a similar amount of viable cells remained. Back to non-restricted O₂ supply K562 cells proliferate whereas K562 shATG7 cells exhibited strong apoptosis. Using immunomagnetic sorted normal and CML CD34⁺ cells, we inhibited the autophagic process by lentiviral infection expressing shATG7 or using a Vps34 inhibitor. Both, normal and CML CD34⁺ cells either competent or deficient for autophagy stopped to proliferate in hypoxia. Surprisingly, while normal CD34⁺ cells proliferate back to non restricted O₂ supply, the CML CD34⁺ cells deficient for autophagy failed to proliferate. All together, these results suggest that autophagy is required for CML CD34⁺ commitment while it is dispensable for normal CD34 cells.