MECHANISMS BY WHICH ASCORBIC-ACID INCREASES FERRITIN LEVELS IN CULTURED LENS EPITHELIAL-CELLS

A previous study demonstrated that ascorbic acid increased the concent ration of the iron storage protein, ferritin, in cultured lens epithel ial cells. The current study was designed to determine the mechanism b y which ascorbic acid exerts this effect. Ascorbic acid increased both ferritin mRNA levels (by about 30%) and translation of ferritin (de n ovo synthesis was increased up to 15-fold) within 6 hr. Cycloheximide completely abolished the ability of ascorbic acid to increase ferritin levels, whereas actinomycin D only decreased it by about 30%. Therefo re, the ascorbic-acid induced increase in ferritin concentration is du e mainly to an increase in ferritin synthesis at the translational lev el. This is a novel role for ascorbic acid. Addition of iron with asco rbic acid further increased de novo synthesis of ferritin, but this ad ditive effect was only noted at a later time point (20 hr). Factors wh ich decrease ferritin mRNA translation, such as the reducing agent dit hiothreitol or the iron chelator desferrioxamine, reduced the ascorbic acid effect on de novo ferritin synthesis. The effects of ascorbic ac id on ferritin mRNA levels may be mediated by its oxidation product, H 2O2, since, like ascorbic acid, H2O2 increased ferritin mRNA levels by 30%. However, in contrast to the ascorbic acid-induced increase in tr anslation df ferritin, H2O2 substantially decreased de novo ferritin s ynthesis. This effect of H2O2 could have physiological significance in eyes where concentrations of H2O2 in the aqueous humor are elevated. High levels of H2O2 could decrease the concentration of ferritin withi n the lens. Since ferritin sequesters iron and has been shown to decre ase oxidative damage by limiting the availability of iron to catalyse free radical reactions, H2O2-induced reduction in ferritin concentrati on in the lens could have deleterious effects. The ability of ascorbic acid to increase ferritin concentration in lens epithelial cells coul d provide an additional protective mechanism for this antioxidant vita min. The importance of ferritin to normal lens functioning is undersco red by the recent finding that humans with a dominantly inherited abno rmality in ferritin (C) 1997 Academic Press Limited.