Apoptosis induced by hydrogen peroxide is mediated by decreased superoxide anion concentration and reduction of intracellular milieu

Abstract Hydrogen peroxide (H 2 O 2 ) is considered to be a mediator of apoptotic cell death but the mechanism by which it induces apoptosis is unclear. Here, we show that cells undergoing apoptosis from exposure to H 2 O 2 display a significant decrease in intracellular concentration of superoxide (O − 2 ) which is associated with a reduction of the intracellular milieu, as measured by an increase in the GSH/GSSG ratio and a decrease in intracellular pH. The notion that a decrease in intracellular O − 2 concentration triggers apoptosis is supported by the observation that H 2 O 2 -mediated apoptosis could be retarded in cells in which the intracellular O − 2 concentration is maintained at or above the cellular baseline level by inhibition of the major O − 2 scavenger superoxide dismutase (Cu/Zn SOD). Taken together, our observations indicate that a decrease in the intracellular O − 2 concentration, reduction and acidification of the intracellular milieu constitute a signal for H 2 O 2 -mediated apoptosis, thereby inducing a reductive as opposed to an oxidative stress.