B. Garner et al., ENDOGENOUS FERRITIN PROTECTS CELLS WITH IRON-LADEN LYSOSOMES AGAINST OXIDATIVE STRESS, Free radical research, 29(2), 1998, pp. 103-114
Previous studies have shown that a variety of mammalian cell types, in
cluding macrophages, contain small amounts of redox-active iron in the
ir lysosomes. Increases in the level of this iron pool predispose the
cell, to oxidative stress. Limiting the availability of intralysosomal
redox-active iron could therefore represent potential cytoprotection
for cells under oxidative stress. In the present study we have shown t
hat an initial 6 h exposure of J774 macrophages to 30 mu M iron, added
to the culture medium as FeCl3, increased the lysosomal iron content
and their sensitivity to H2O2-induced (0.25 mM for 30 min) oxidative s
tress. Over time (24-72 h), however, the cells were desensitized to th
e cytotoxic effects of H2O2; most likely as a consequence of both lyso
somal iron exocytosis and of ferritin synthesis (demonstrated by atomi
c absorption spectrophotometry, autometallography, and immunohistochem
istry). When the cells were exposed to a second dose of iron, their ly
sosomal content of iron increased again but the cells became no furthe
r sensitized to the cytotoxic effects of H2O2 Using the lysosomotropic
weak base, acridine orange, we demonstrated that after the second exp
osure to iron and H2O2, lysosomes remained intact and were no differen
t from control cells which were exposed to H2O2 but not iron. These da
ta suggest that the initial induction of ferritin synthesis leads to e
nrichment of lysosomes with ferritin via autophagocytosis. This limits
the redox-availability of intralysosomal iron and, in turn, decreases
the cells' sensitivity to oxidative stress. These in vitro observatio
ns could also explain why cells under pathological conditions, such as
haemochromatosis, are apparently able to withstand high iron concentr
ations for some time in vivo.