ENDOGENOUS FERRITIN PROTECTS CELLS WITH IRON-LADEN LYSOSOMES AGAINST OXIDATIVE STRESS

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.