Myeloperoxidase Is Required for the Combined Apoptotic Effects of Arsenic Trioxide with Ascorbic Acid or Epigallocatechin-3-Gallate in Myeloid Leukemia Cells.

Arsenic trioxide (As₂O₃) is a drug used world-wide that selectively causes the death of acute promyelocytic leukemia (APL) cells by novel mechanisms and induces complete clinical remission in 90% of patients without significant toxicity. The outstanding success of As₂O₃ therapy in relapsed APL patients has not been replicated by an equivalent success in other types of acute myeloid leukemia (AML). We have studied As₂O₃ mechanism(s) of action and identified agents to use in combination to improve the use of As₂O₃ as a treatment for other types of AML. As₂O₃ produces higher levels of H₂O₂ and apoptosis in APL NB4 cells than in other AML cells at therapeutic concentrations of 1-2 uM. As₂O₃ does not induce apoptosis in HL-60 cells but is synergistic with ascorbic acid (AA) or epigallocatechin-3-gallate (EGCG). Both AA and EGCG produce H₂O₂ which is augmented by the addition of As₂O₃. Apoptosis induction by As₂O₃ in combination with AA or EGCG is inhibited by catalase and the antioxidant N-acetylcysteine. Myeloperoxidase (MPO), a major neutrophil enzyme, augments H₂O₂-induced apoptosis by converting it into more potent reactive oxygen species. HL-60, NB4, SKNO-1 and PLB985 cells which express high level of MPO, but not U937 and K562 cells without expression of MPO, are responsive to As₂O₃ plus AA or EGCG-induced apoptosis. HP-100 cells, a subclone of HL-60 cells without MPO expression, are resistant to both combination treatments. MPO stable transfection sensitizes K562 cells to apoptosis following treatment with As₂O₃ and AA or EGCG. NADPH oxidase is an enzyme complex which generates O₂⁻ and H₂O₂ in neutrophils. In APL NB4 cells, As₂O₃ induces expression of NADPH oxidase members and is thought to participate in H₂O₂ production required for As₂O₃ response. X-CGD cells, a subclone of PLB-985 with targeted disruption of the gp91^phox gene (