Retinoic Acid and proteotoxic stress induce myeloid leukemia progenitors cell death overcoming the protective effects of the bone marrow niche mesenchymal cells

The biological issue - Acute myeloid leukemia (AML) is a group of diseases due to chromosomal and genetic mutations that impair myeloid progenitor differentiation. Some leukemic blasts express mutant proteins sensitizing them to pharmacologically induced proteotoxic stresses. Indeed, our lab demonstrated that in the presence of the differentiating agent Retinoic acid, the ER stress inducer Tunicamycin and the oxidative stress inducer Arsenic Trioxide trigger cell death of AML cell lines and primary blasts bearing the FLT3-ITD mutation. Whereas Retinoic acid and Arsenic trioxide are the current therapy for acute promyelocytic leukemia (APL), Tunicamycin has never been used in clinical practice. Therefore, in order to develop a new therapeutic strategy, my Ph.D. project investigated the proteasome inhibitor Bortezomib, which induces ER stress and is already clinically approved for multiple myeloma and mantle cell lymphoma. Results - AML cell lines were treated with low doses of Retinoic Acid (R), Bortezomib (B), and Arsenic Trioxide (A). FLT3-ITD+ cell lines and primary blasts are highly sensitive to the triple combination RBA but not to the single agents. The cytotoxic effect is mostly due to the generation of oxidative stress and the suppression of the pro-survival branches of the Unfolded Protein Response (UPR). Additionally, the role of the bone marrow niche was examined. In an in vitro co-culture system, bone marrow stromal cells (BMSCs) protect AML from RBA toxicity by attenuating oxidative stress. However, pharmacological doses of ascorbic acid (Vit C) used as an adjuvant pro-oxidant agent, tamper with this protection. Importantly, combined treatment efficacy was confirmed in vivo. Furthermore, the combination of RBA with some promising targeted therapies also provided encouraging results. Conclusion - FLT3-ITD+ AML cells are sensitive to RBA combination. Notably, the bone marrow niche can protect leukemic blasts from oxidative stress, but additional Vit C hampers this protective effect. These results stress the importance of studying AML in the context of the bone marrow niche and open to possible applications in AML treatment.