The effect of ascorbic acid and ferric ammonium citrate on iron uptake and storage in lens epithelial cells.

Ferritin is the major intracellular iron storage protein which has been shown to protect cells against oxidative damage. Recent reports that an inherited abnormality in human ferritin synthesis is associated with early bilateral cataracts underscore the importance of understanding ferritin synthesis and iron storage in lens epithelial cells. We previously demonstrated that ascorbic acid greatly increases de novo synthesis of ferritin in lens epithelial cells. The objectives of the present study were to determine: (1) the effects of ascorbic acid and ferric ammonium citrate on iron uptake by canine lens epithelial cells from iron bound to transferrin and from ferric chloride and (2) the incorporation of this element into ferritin. Iron uptake by lens epithelial cells from 59ferric chloride was 20 times higher than from 59iron-transferrin and iron deposition into ferritin was 8-fold higher when 59ferric chloride was the source. Ascorbic acid had a stimulatory effect on iron uptake from transferrin and on incorporation of this element into ferritin. The ascorbic acid-induced increase of iron uptake required de novo protein synthesis but not specifically de novo ferritin biosynthesis. Although ferritin is not directly involved in iron uptake, the level of ferritin protein could control the pool of intracellular iron. The present results indicate that iron homeostasis in lens epithelial cells is affected mainly by changes in apoferritin synthesis, which is greatly stimulated by ascorbic acid, rather than by altering the rate of protein degradation, which is very slow in these cells under all circumstances. Ferric ammonium citrate activates iron uptake from transferrin in a wide range of cell lines by generation of free radicals. Ferric ammonium citrate also increased iron uptake from Tf in lens epithelial cells. Ferric ammonium citrate treated cells incorporated 5 times more iron and deposited 2 times more iron into ferritin than control cells. Increased incorporation of iron into ferritin was due to ferric ammonium citrate-induced stimulation of de novo ferritin synthesis rather than an increased rate of iron deposition into pre-existing ferritin. Ferric ammonium citrate had a different effect on iron uptake from ferric chloride; total iron uptake was not significantly increased while deposition into ferritin was significantly decreased. These results demonstrate that iron homeostasis in lens epithelial cells is regulated by ascorbic acid and by changes in the rate of de novo ferritin synthesis. In addition, the differences in iron uptake from transferrin and ferric chloride and its subsequent incorporation into ferritin suggests that the mechanisms by which iron is incorporated into ferritin are source dependent.
(Copyright 1998 Academic Press.)