Superoxide dictates the mode of U937 cell ascorbic acid uptake and prevents the enhancing effects of the vitamin to otherwise nontoxic levels of reactive oxygen/nitrogen species.
Exposure of U937 cells to low micromolar levels of ascorbic acid or dehydroascorbic acid, while resulting in identical ascorbic acid accumulation, is unexpectedly associated with remarkably different responses to exogenous oxidants. We observed that otherwise nontoxic levels of hydrogen peroxide, tert-butylhydroperoxide or peroxynitrite promote toxicity in cells preloaded with ascorbic acid, whereas hardly any effect was detected in cells pretreated with dehydroascorbic acid. Further experiments performed with peroxynitrite in cells preloaded with ascorbic acid provided evidence for a very rapid nonapoptotic death, preceded by early Bax mitochondrial translocation and by mitochondrial permeability transition. The notion that conversion of extracellular ascorbic acid to dehydroascorbic acid prevents the enhancing effects on oxidant toxicity and nevertheless preserves the net amount of vitamin C accumulated was also established using ascorbate oxidase as well as various sources of superoxide, namely, xanthine/xanthine oxidase or ATP-driven NADPH oxidase activation. These findings suggest that superoxide-dependent conversion of extracellular ascorbic acid to dehydroascorbic acid represents an important component of the overall survival strategy of some cell types to reactive oxygen/nitrogen species.
Dipartimento di Scienze Biomolecolari Università degli Studi di Urbino Carlo Bo, 61029 Urbino, Italy.
Mara
M
Fiorani
Catia
C
Azzolini
Liana
L
Cerioni
Andrea
A
Guidarelli
Orazio
O
Cantoni
D001204:Ascorbate Oxidase / Q000378:metabolism; D001205:Ascorbic Acid / Q000378:metabolism* / Q000493:pharmacokinetics*; D002460:Cell Line / Q000187:drug effects; D003683:Dehydroascorbic Acid / Q000493:pharmacokinetics; D006801:Humans; D006861:Hydrogen Peroxide / Q000494:pharmacology; D053078:Membrane Potential, Mitochondrial / Q000187:drug effects; D030421:Peroxynitrous Acid / Q000494:pharmacology; D017382:Reactive Oxygen Species / Q000378:metabolism*; D013481:Superoxides / Q000378:metabolism*; D014969:Xanthine Oxidase / Q000378:metabolism; D051028:bcl-2-Associated X Protein / Q000378:metabolism; D020122:tert-Butylhydroperoxide / Q000494:pharmacology
C494826:BAX protein, human; D017382:Reactive Oxygen Species; D051028:bcl-2-Associated X Protein; D013481:Superoxides; D030421:Peroxynitrous Acid; D020122:tert-Butylhydroperoxide; D006861:Hydrogen Peroxide; D001204:Ascorbate Oxidase; D014969:Xanthine Oxidase; D001205:Ascorbic Acid; D003683:Dehydroascorbic Acid