Arsenic induces apoptosis of multidrug-resistant human myeloid leukemia cells thin express Bcr-Abl or overexpress MDR, MRP, Bcl-2, or Bcl-x(L)

We investigated the in vitro growth inhibitory and apoptotic effects of clinically achievable concentrations of As₂O₃ (0.5 to 2.0 μmol/L) against human myeloid leukemia cells known to be resistant to a number of apoptotic stimuli. These included chronic myelocytic leukemia (CML) blast crisis K562 and HL-60/Bcr-Abl cells, which contain p210 and p185 Bcr-Abl, respectively, and HL-60 cell types that overexpress Bcl-2 (HL-60/Bcl-2), Bcl-x(L) (HL- 60/Bcl-x(L)), MDR (HL-60/VCR), or MRP (HL-60/AR) protein. The growth- inhibitory IC₅₀ values for As₂O₃ treatment for 7 days against all these cell types ranged from 0.8 to 1.5 μmol/L. Exposure to 2 μmol/L As₂O₃ for 7 days induced apoptosis of all cell types, including HL-60/Bcr-Abl and K562 cells. This was associated with the cytosolic accumulation of cyt c and preapoptotic mitochondrial events, such as the loss of inner membrane potential (ΔΨm) and the increase in reactive oxygen species (ROS). Treatment with As₂O₃ (2 μmol/L) generated the activities of caspases, which produced the cleavage of the BH3 domain containing proapoptotic Bid protein and poly (ADP-ribose) polymerase. Significantly, As₂O₃-induced apoptosis of HL-60/Bcr-Abl and K562 cells was associated with a decline in Bcr-Abl protein levels, without any significant alterations in the levels of Bcl-x(L), Bax, Apaf-1, Fas, and FasL. Although As₂O₃ treatment caused a marked increase in the expression of the myeloid differentiation marker CD11b, it did not affect Hb levels in HL-60/Bcr-Abl, K562, or HL-60/neo cells. However, in these cells, As₂O₃ potently induced hyper-acetylation of the histones H3 and H4. These findings characterize As₂O₃ as a growth inhibiting and apoptosis-inducing agent against a variety of myeloid leukemia cells resistant to multiple apoptotic stimuli.