I. Toth et al., ASCORBIC-ACID ENHANCES IRON-INDUCED FERRITIN TRANSLATION IN HUMAN LEUKEMIA AND HEPATOMA-CELLS, The Journal of biological chemistry, 270(6), 1995, pp. 2846-2852
Ascorbate is an important cofactor in many cellular metabolic reaction
s and is intimately linked to iron homeostasis. Continuously cultured
cells are ascorbate deficient due to the lability of the vitamin in so
lution and to the fact that daily supplementation of media with ascorb
ate is unusual. We found that ascorbate repletion alone did not alter
ferritin synthesis. However, ascorbate-replete human hepatoma cells, H
ep3B and HepG2, as well as K562 human leukemia cells achieved a substa
ntially higher cellular ferritin content in response to a challenge wi
th iron than did their ascorbate-deficient counterparts grown under st
andard culture conditions. Most of the elevation in ferritin content w
as due to an increase in de novo ferritin synthesis of greater than 50
-fold, as shown by in vivo labeling with [S-35]methionine and immunopr
ecipitation. RNA-blot analysis showed only minor changes in steady sta
te levels of ferritin mRNA, suggesting that ascorbate enhances iron-in
duced ferritin synthesis primarily by post-transcriptional events. Tra
nsient gene expression experiments using chloramphenicol acetyltransfe
rase reporter gene constructs showed that the ascorbate effect on ferr
itin translation is not mediated through the stem-loop near the transl
ational start site that transduces ferritin synthesis in response to c
ytokines. The data suggest that ascorbate possibly modifies the action
of the iron-responsive element on ferritin translation, although more
precise structure-function studies are needed to clarify this issue.
These data demonstrate a novel role of ascorbate as a signaling molecu
le in posttranscriptional gene regulation. The mechanism by which asco
rbate modulates cellular iron metabolism is complex and requires addit
ional detailed investigation.