Bone marrow stromal cells support acute myeloid leukemia stem cells enhancing antioxidant defenses

Acute myeloid leukemia (AML) is a heterogeneus group of diseases due to chromosomal abnormalities and gene mutations causing the impairment of myeloid progenitors differentiation. Since leukemic blasts express mutant proteins and especially fusion proteins which cannot reach their native form, this intrinsic source of proteostasis alteration renders them more sensitive to pharmacologically induced proteotoxic stresses. Indeed, we previously demonstrated that in the presence of the differentiating agent Retinoic Acid (R), the combination of ER stress induced by Tunicamycin (T) and oxidative stress induced by Arsenic Trioxode (A) is effective in inducing cell death of AML cell lines and human primary blasts bearing the mutation ITD (Internal-Tandem-Duplication) of the tyrosine kinase receptor FLT3 (FLT3-ITD). Then, in order to increase the translational potential of the combined treatment as a new therapeutic strategy for AML, we replaced the ER stress inducer T with proteasome inhibitor Bortezomib (B), already used in clinical practice, as R and A. The results we obtained show that the triple combination Retinoic Acid, Bortezomib and Arsenic trioxide (RBA) is even more efficient than the combination RTA in causing cell death of FLT3-ITD+ AML cell lines and of leukemic stem cells isolated from patients. We showed that cell death induced by the combined treatment RBA is mostly due to generation of oxidative stress: ROS production results in a strong activation of the oxidative stress response but this is not enough to allow leukemic blast survival. Furthermore, we evaluated the efficacy of the combined treatment considering the bone marrow niche. We found that murine bone marrow stromal cells are able to protect leukemic blasts from the toxic effects of the combination RBA. In particular we found that stromal cells protect AML cells by attenuating oxidative stress induced by RBA thus we used high doses of ascorbic acid (vitC) as an adjuvant pro-oxidant agent to tamper with the protective effect of bone marrow stromal cells on AML cells. Indeed high doses of vit C have been tested in clinical trials as adjuvant for cancer treatment, with no toxic consequence. Finally, we obtained some preliminary evidences in vivo confirming the efficacy of the triple treatment RBA combined with ascorbic acid: these results open to a possible translational application of this combination in AML treatment.