Retinoic acid and proteotoxic stress induce AML cell death overcoming stromal cell protection

Abstract

Acute myeloid leukemia (AML) patients bearing the ITD mutation in the tyrosine kinase receptor FLT3 (FLT3-ITD) present a poor prognosis and a high risk of relapse. FLT3-ITD is retained in the endoplasmic reticulum (ER) and generates intrinsic proteotoxic stress. We devised a strategy based on proteotoxic stress, generated by the combination of low doses of the differentiating agent retinoic acid (R), the proteasome inhibitor bortezomib (B), and the oxidative stress inducer arsenic trioxide (A). It exerts strong cytotoxic activity on FLT3-ITD+ AML cell lines and primary blasts isolated from patients, due to ER homeostasis imbalance and generation of oxidative stress. AML cells become completely resistant to the combination RBA when treated in co-culture with bone marrow stromal cells (BMSC). Nonetheless, we could overcome such protective effects by using high doses of ascorbic acid (Vitamin C) as an adjuvant. Importantly, the combination RBA plus ascorbic acid significantly prolongs the life span of a murine model of human FLT3-ITD+ AML without toxic effects. Furthermore, we show for the first time that the cross-talk between AML cells and BMSC upon treatment involves disruption of the actin cytoskeleton and the actin cap, increased thickness of the nuclei, and relocalization of the transcriptional co-regulator YAP in the cytosol of the BMSC. Our findings strengthen our previous work indicating induction of proteotoxic stress as a possible strategy in FLT3-ITD+ AML therapy and open to the possibility of identifying new therapeutic targets in the crosstalk between AML cells and BMSC, involving mechanotransduction and YAP signaling