A short-term pre-exposure to dehydroascorbic acid (DHA) promotes U937 cell death upon exposure to otherwise non-toxic levels of peroxynitrite (ONOO−). Toxicity is mediated by a saturable mechanism and cell death takes place as a consequence of mitochondrial permeability transition. The following lines of evidence are consistent with the notion that the enhancing effects of DHA were related to mitochondrial events resulting in inhibition of complex III upon exposure to otherwise inactive concentrations of ONOO−. First, DHA, as well as bona fide complex III inhibitors, similarly enhanced toxicity and subsequent formation of H2O2 induced by ONOO− via a rotenone- or catalase-sensitive mechanism. Secondly, bona fide complex III inhibitors were ineffective in DHA-pre-loaded cells. In addition, respiration-deficient cells were resistant to toxicity elicited by ONOO− and their supplementation with increasing concentrations of DHA, although resulting in the accumulation of vitamin C levels identical with those observed in respiration-proficient cells, failed to affect ONOO− toxicity. Finally, oxygen-consumption experiments demonstrated that pre-exposure to DHA promotes the ONOO−-dependent inhibition of complex III. In conclusion, the above results collectively demonstrate that increasing the intracellular accumulation of vitamin C promotes mitochondrial events leading to ONOO−-dependent formation of H2O2 and resulting in a rapid necrotic response.
Abbreviations used: AsA, ascorbic acid; calcein-AM, calcein acetoxymethyl ester; DHA, dehydroascorbic acid; DHR, dihydrorhodamine 123; HQNO, 2-heptyl-4-hydroxyquinoline; MPT, mitochondrial permeability transition; Rho, rhodamine 123; TMPD, N,N,N´,N´-tetramethyl-p-phenylenediamine.