Retinoic acid and arsenic trioxide sensitize acute promyelocytic leukemia cells to ER stress.

Promyelocytic leukemia (APL) is characterized by the chromosomal translocation t(15:17). This translocation results in the expression of the chimeric protein PML-RARα that arrests the differentiation program driven by RARα, blocking the leukemic blasts at the promyelocytic stage. Pharmacological doses of Retinoic Acid (RA) are able to resume granulocytic differentiation and partially degrade PML-RARα. The association of RA with chemotherapy or with arsenic trioxide (ATO), which efficiently targets PML-RARα for degradation, results in high cure rates. Despite showing a considerably improved safety profile RA and ATO are not completely devoid of toxicity. We show here that granulocytic differentiation of human APL cells, driven by RA, generates mild ER stress, sufficient to render them very sensitive to small quantities of ER stress inducing drugs, like Tunicamycin (Tm). Indeed, RA-induced differentiation of human APL cell lines and primary blasts dramatically increases their sensitivity to Tm, at doses that are not toxic in the absence of RA. Importantly the combination of RA and Tm results not toxic on human bone marrow progenitors cells derived from healthy donors. We also show that the PERK pathway, triggered by ER stress, plays a major protective role and, by using a specific PERK inhibitor, we potentiated the toxic effect of the combination of RA and Tm. Moreover we found that small amounts of pharmacologically induced ER stress are also able to strongly increase ATO toxicity even in the absence of RA: the combination of ATO with Tm efficiently induces apoptosis in RA-sensitive and RA-resistant APL cell lines, at doses ineffective in the absence of ER stress. Eventually, we demonstrate that insurgency of oxidative stress, tightly linked with the UPR, is at the basis of the toxicity induced by Tm in combination with RA and/or ATO. In conclusion, our findings identify the ER stress-related pathways as potential targets in the search for novel therapeutic strategies in AML.