Hydrogen peroxide generated at the level of mitochondria in response to peroxynitrite promotes U937 cell death via inhibition of the cytoprotective signalling mediated by cytosolic phospholipase A2

We have studied the relationships existing between delayed formation of H2O2 and activation of cytosolic phospholipase A2 (cPLA2), events respectively promoting toxicity or survival in U937 cells exposed to peroxynitrite. The outcome of an array of different approaches using phospholipase A2 inhibitors, or cPLA2 antisense oligonucleotides, as well as specific respiratory chain inhibitors and respiration-deficient cells led to the demonstration that H2O2 does not mediate toxicity by producing direct molecular damage. Rather, the effects of H2O2 were found to be upstream to the arachidonic acid (AA)-mediated cytoprotective signalling and in fact causally linked to inhibition of cPLA2. Thus, it appears that U937 cells exposed to nontoxic concentrations of peroxynitrite are nevertheless committed to death, which however is normally prevented by the activation of parallel pathways resulting in cPLA2-dependent release of AA. A rapid necrotic response, however, takes place when high concentrations of peroxynitrite promote formation of H2O2 at levels impairing the cPLA2 cytoprotective signalling.