Hydrogen peroxide inhibits activation, not activity, of cellular caspase-3 in vivo.

Oxidants such as H(2)O(2) can induce a low level of apoptosis at low concentrations but at higher concentrations cause necrosis. Higher concentrations of H(2)O(2) also inhibit the induction of apoptosis by chemotherapy drugs. One theory is that, at higher concentrations, H(2)O(2) causes direct oxidative inactivation of caspase-3 activity, thus preventing the apoptotic pathway from being used. We find that treatment of recombinant caspase-3 with H(2)O(2) can partially reduce its enzymatic activity: However, the following findings show that this does not occur in the cell. (1) The inhibition by H(2)O(2) of VP-16-induced apoptosis and cellular caspase-3 activity can be overcome by adding inhibitors of poly(ADP-ribose) polymerase (PARP) at sub-stoichiometric concentrations. (2) Delayed addition of H(2)O(2) to VP-16-treated cells prevents additional caspase induction but does not inhibit the caspase activity that has already been generated. (3) H(2)O(2) is a poor inhibitor of caspase-3 activity in cell lysates. (4) Addition of H(2)O(2) to cells inhibits activation of caspase-9, which is required for activation of caspase-3. We conclude that inhibition of caspase-3 activity in the cell occurs indirectly at a step located upstream of caspase-3 activation. H(2)O(2) acts in part by inducing DNA strand breaks and activating PARP, thus depleting the cells of ATP. When this pathway is blocked, even high concentrations of H(2)O(2) can induce caspase-9 and -3 activation and cause apoptosis.