Treatment with 1-4 μM As2O3 slightly induced apoptosis in U-937 human promonocitic leukemia cells. This effect was potentiated by co-treatment with MEK/ERK (PD98059, U0126) and JNK (SP600125, AS601245) inhibitors, but not with p38 (SB203580, SB220025) inhibitors. However, no potentiation was obtained using lonidamine, doxorubicin, or cisplatin instead of As2O3. Apoptosis potentiation by mitogen-activated protein kinase (MARK) inhibitors involved both the intrinsic and extrinsic executionary pathways, as demonstrated by Bax activation and cytochrome c release from mitochondria, and by caspase-8 activation and Bid cleavage, respectively; and the activation of both pathways was prevented by Bcl-2 over-expression. Treatment with MEK/ERK and JNK inhibitors, but not with p38 inhibitors, caused intracellular glutathione (GSH) depletion, which was differentially regulated. Thus, while it was prevented by N-acetyl-L-cysteine (NAC) in the case of U0126, it behaved as a NAC-insensitive process, regulated at the level of DL-buthionine-(S,R)-sulfoximine (BSO)-sensitive enzyme activity, in the case of SP600125. The MEK/ERK inhibitor also potentiated apoptosis and decreased GSH content in As2O3-treated NB4 human acute promyelocytic leukemia (APL) cells, but none of these effects were produced by the JNK inhibitor. MEK/ERK and JNK inhibitors did not apparently affect As 2O3 transport activity, as measured by intracellular arsenic accumulation. SP600126 greatly induced reactive oxygen species (ROS) accumulation, while BSO and U0126 had little or null effects. These results, which indicate that glutathione is a target of MAP kinases in myeloid leukemia cells, might be exploited to improve the antitumor properties of As 2O3, and provide a rationale for the use of kinase inhibitors as therapeutic agents. © 2006 Wiley-Liss, Inc.