Hypomethylating agents reactivate FOXO3A in acute myeloid leukemia

The deregulation of the DNA damage response (DDR) can contribute to leukemogenesis and favor the progression from myelodysplastic syndrome (MDS) to acute myeloid leukemia (AML). Since hypomethylating agents - notably azacitidine - constitute an efficient therapy for patients with high-risk MDS, we assessed whether such compounds can activate the DDR in malignant blasts. While azacitidine and decitabine had moderate effects on apoptosis and cell cycle progression, both agents induced profound changes in the expression and functionality of DDR-related proteins. Decitabine - and to a lesser degree azacitidine - induced the activation of checkpoint kinases Chk-1 and Chk-2, and the phosphorylation of the DDR-sensor H2AX. In addition, hypomethylating agents were found to cause the dephosphorylation of the transcriptional regulator forkhead box O3, best known as FOXO3A, whose phosphorylation has been related to poor prognosis in AML. The dephosphorylation of FOXO3A induced by azacitidine or decitabine in malignant blasts was accompanied by the translocation of FOXO3A from the cytoplasm to the nucleus. Upon stimulation with azacitidine, MDS/AML-derived, azacitidine-sensitive SKM-1S cells upregulated FOXO3A and the pro-apoptotic FOXO3A targets BIM and PUMA, and this effect was attenuated or abolished in azacitidine-resistant SMK-1R cells. Altogether, our results suggest that the re-activation of FOXO3A may contribute to the effects of hypomethylating agents in malignant blasts.