Arsenic trioxide (As2O3) caused apoptosis in U-937 human promonocytic cells. This effect was potentiated by the simultaneous addition of the glutathione (GSH) synthesis inhibitor DL-buthionine-(R,S)-sulfoximine or the protein kinase C activators 12-O-tetradecanoylphorbol-13-acetate (TPA) and bryostatin 1. In addition TPA decreased the intracellular GSH content, caused ERK activation, and potentiated the As2O3-provoked activation of p38 and JNK. The addition of N-acetyl-L-cysteine, the PKC inhibitor GF109203X, and the MEK/ERK inhibitors PD98059 and U0126 attenuated both apoptosis induction and GSH decrease, whereas the p38 inhibitor SB203580 and the JNK inhibitor SP600125 were ineffective. TPA also potentiated ERK activation and GSH depletion when added simultaneously to cadmium chloride (CdCl2) and doxorubicin. However, TPA only enhanced apoptosis in the case of CdCl2, which is a GSH-sensitive agent, whereas it reduced the toxicity of doxorubicin and other DNA-specific drugs. Finally, preincubation for 14-24 h with TPA did not potentiate but, instead, attenuated the As2O3- and CdCl2-provoked apoptosis. The same result was obtained by preincubation with bryostatin 1 and other differentiation inducers. It is concluded that TPA increases the apoptotic action of As2O 3, an effect mediated by ERK activation and GSH depletion. However, the increase in apoptosis is only effective in non-differentiated cells.