In order to overcome chemotherapy resistance, many laboratories are searching for agents that increase the sensitivity of cancer cells to anticancer drugs. Arsenic trioxide (As(2)O(3)) is widely used in treating human acute polymyelocytic leukemia (APL). However, solid tumors and other leukemia cells such as U937 promonocytic leukemia cells are insensitive to As(2)O(3). Esculetin, a coumarin derivative, has previously induced cell cycle arrest and apoptosis of HL-60 cells as well as enhanced taxol-induced apoptosis in HepG2 cells, thereby displaying anticancer potential. In this study, esculetin inhibited proliferation and mitogen activated protein kinases (MAPKs) activation in human leukemia U937 cells. Since inhibitors of MAPKs have modulated the GSH-redox state and enhanced the sensitivity of leukemia cells to As(2)O(3)-provoked apoptosis, we monitored the effect of combining esculetin and As(2)O(3) (2.5 microM) on the GSH level. Our study showed that esculetin, PD98059 (MEK/ERK inhibitor), and SP600125 (JNK inhibitor) similarly enhanced the As(2)O(3)-induced GSH depletion. We found that the As(2)O(3) (2.5 microM) treatment slightly induced apoptosis and the pretreatment of esculetin enhanced the As(2)O(3)-provoked apoptosis significantly. In addition, esculetin enhanced the effect of As(2)O(3) on caspase activation in U937 cells. We compared the combined esculetin and As(2)O(3) treatment to the As(2)O(3) treated alone. The combined esculetin and As(2)O(3) treatment increased Bid cleavage, Bax conformation change and cytochrome C release. The study also indicated that esculetin enhanced the As(2)O(3)-induced lysosomal leakage and apoptosis. Furthermore, pretreatment with N-acetylcysteine (NAC) reduced these enhanced effects. Based on these studies, esculetin enhances the As(2)O(3)-provoked apoptosis by modulating the MEK/ERK and JNK pathways and reducing intracellular GSH levels. GSH depletion led to higher oxidative stress which activated lysosomal-mitochondrial pathway of apoptosis.