Arsenic Trioxide Plus ABT-737 Is Synergistic to Induce Apoptosis in AML Cells by Repression of Mcl-1 Protein and Inactivation of Bcl-2 Protein

Arsenic trioxide as a single agent induces remission in acute promyelocytic leukemia (APL) patients with minimal toxicity but not in other types of acute myeloid leukemia (AML). In vitro, APL as compared to AML cells are more sensitive to arsenic trioxide-induced apoptosis. Arsenic trioxide-induced apoptosis in APL cells results from activation of a mitochondria-mediated pathway. The Bcl-2 antiapoptotic protein family members Bcl-2, Bcl-XL, Bfl-1 and Mcl-1 block mitochondria-mediated apoptosis. In studies of several AML cell lines, we detected high levels of Bcl-2 and Mcl-1 protein, but lower or no expression of Bcl-XL and Bfl-1. Arsenic trioxide treatment decreased the levels of Mcl-1 without inducing apoptosis in AML cells suggesting that Bcl-2 would be a key factor causing arsenic trioxide resistance. We therefore hypothesize that inhibitors of Bcl-2 might restore arsenic trioxide-induced programmed cell death. In this study, the apoptotic effects of arsenic trioxide, ABT-737 (a potent Bcl-2 inhibitor) and their combination were investigated in NB4, HL-60, U937 and K562 cells. Arsenic trioxide at 1–2 μM induced apoptosis only in NB4 cells but decreased the levels of Mcl-1 in all of the four cell lines. ABT-737 at concentrations lower than 5 μM induced apoptosis in NB4, HL-60 and U937 cells but not in K562 cells which had undetectable Bcl-2 levels. Arsenic trioxide (2 μM) plus ABT-737 (0.05–0.5 μM) synergistically induced apoptosis in HL-60 and U937 but not in K562 cells as determined by PARP cleavage and Annexin V staining. Our data suggest that inhibition of Mcl-1 expression by arsenic trioxide and inactivation of Bcl-2 activity by ABT-737 leads to the synergistic apoptosis observed with this combination and is the basis for a novel treatment for AML.