Arsenic Trioxide and Melarsoprol Induce Programmed Cell Death in Myeloid Leukemia Cell Lines and Function in a PML and PML-RAR Independent Manner

Inorganic arsenic trioxide (As2O3) and the organic arsenical, melarsoprol, were recently shown to inhibit growth and induce apoptosis in NB4 acute promyelocytic leukemia (APL) and chronic B-cell leukemia cell lines, respectively. As2O3 has been proposed to principally target PML and PML-RARalpha proteins in APL cells. We investigated the activity of As2O3 and melarsoprol in a broader context encompassing various myeloid leukemia cell lines, including the APL cell line NB4-306 (a retinoic acid-resistant cell line derived from NB4 that no longer expresses the intact PML-RARalpha fusion protein), HL60, KG-1, and the myelomonocytic cell line U937. To examine the role of PML in mediating arsenical activity, we also tested these agents using murine embryonic fibroblasts (MEFs) and bone marrow (BM) progenitors in which the PML gene had been inactivated by homologous recombination. Unexpectedly, we found that both compounds inhibited cell growth, induced apoptosis, and downregulated bcl-2 protein in all cell lines tested. Melarsoprol was more potent than As2O3 at equimolar concentrations ranging from 10(-7) to 10(-5) mol/L. As2O3 relocalized PML and PML-RARalpha onto nuclear bodies, which was followed by PML degradation in NB4 as well as in HL60 and U937 cell lines. Although melarsoprol was more potent in inhibiting growth and inducing apoptosis, it did not affect PML and/or PML-RARalpha nuclear localization. Moreover, both As2O3 and melarsoprol comparably inhibited growth and induced apoptosis of PML+/+ and PML-/- MEFs, and inhibited colony-forming unit erythroid (CFU-E) and CFU granulocyte-monocyte formation in BM cultures of PML+/+ and PML-/- progenitors. Together, these results show that As2O3 and melarsoprol inhibit growth and induce apoptosis independent of both PML and PML-RARalpha expression in a variety of myeloid leukemia cell lines, and suggest that these agents may be more broadly used for treatment of leukemias other than APL.