Apoptosis induced by arsenic trioxide in leukemia U937 cells is dependent on activation of p38, inactivation of ERK and the Ca2+-dependent productionof superoxide

The mechanism of the induction of apoptosis by arsenic trioxide (As2O3), wh ich was demonstrated recently to be an effective inducer of apoptosis in pa tients with leukemia, was examined in detail in human leukemia U937 cells, Upon treatment of U937 cells with 50 muM of As-2,O-3, complete inactivation of the kinases ERK1 and ERK2 was detected within 30 min. p38 was activated within 3 hr, and the maximum activity was detected at 6 hr, when DNA fragm entation remained undetectable. Experiments with transfected cells that exp ressed constitutively activated MEK1 and a specific inhibitor of p38 also s uggested that inactivation of ERKs and activation of p38 might be associate d with the induction of apoptosis by As2O3. In contrast to the inactivation of ERKs and the activation of p38, activation of INK by As2O3 appeared to protect cells against the induction of apoptosis. Treatment of U937 cells w ith As2O3 also caused the Ca2+ dependent production of superoxide and intra cellular acidification and a decrease in the mitochondrial membrane potenti al at the early stages of induction of apoptosis by As2O3. These changes pr eceded the release of cytochrome c from mitochondria and the activation of caspase-3. It should be possible to exploit the unusual characteristics of the mechanism of induction of apoptosis by As2O3 in U937 cells by making us e of synergistic effects of this compound with other inducers of apoptosis. (C) 2001 Wiley-Liss, Inc.