p38 mitogen-activated protein kinase mediates bid cleavage, mitochondrial dysfunction, and caspase-3 activation during apoptosis induced by singlet oxygen but not by hydrogen peroxide

p38 mitogen-activated protein kinase is activated and involved in cleavage of caspase-3 during apoptosis induced by a number of stimuli. However, the signaling events triggered by p38 that result in caspase-3 activation are s till unknown. In human leukemia cells, two reactive oxygen species, singlet oxygen and hydrogen peroxide (H2O2), selectively stimulated the phosphoryl ation of p38, Preincubation of cells with SB203580, a specific inhibitor of p38, dose dependently inhibited DNA fragmentation induced by singlet oxyge n but not by H2O2. Protection from apoptosis by SB203580 correlated with in hibition of caspase-3, and several events that are associated with caspase- 3 activation, including Bid cleavage, decrease in mitochondrial transmembra ne potential and release of cytochrome c from mitochondria, whereas caspase -8 cleavage was not affected by this inhibitor. In contrast, blockade of ca spase-8 with Ile-Glu-Thr-Asp-fluoromethyl ketone is sufficient to prevent f ormation of DNA fragments and to inhibit all the above signaling events, wi th exception of p38 phosphorylation, in both singlet oxygen- and H2O2-treat ed cells. These data suggest that caspase-3 activation is regulated through redundant signaling pathways that involve p38 and caspase-8 acting upstrea m of Bid during singlet oxygen-induced apoptosis, whereas the activation of caspase-3 by H2O2 is only governed by a caspase-8-mediated apoptotic pathw ay.