RELEASE OF IRON BY HUMAN RETINAL-PIGMENT EPITHELIAL-CELLS

Retinal pigment epithelial cells, which form one aspect of the blood-retinal barrier, take up iron in association with transferrin by a typical receptor-mediated mechanism (Hunt et al., 1989. J. Cell Sci. 92:655-666). This iron is dissociated from transferrin in a low pH environment and uptake is sensitive to agents that inhibit endosomal acidification. The dissociated iron enters the cytoplasm as a low molecular weight (< 10 kD) component and subsequently binds to ferritin. No evidence for recycling of iron in association with transferrin was found. Nevertheless, much of the iron that is taken up is recycled to the extracellular medium, primarily from the low molecular weight pool. This release of iron is not sensitive to inhibitors of energy production or of vesicular acidification but is increased up to a maximum of about 40% of the total Fe-55 incorporated when cells are incubated with serum or the medium is changed. When a short loading time for Fe-55 from Fe-55-transferrin is used (i.e., when the low molecular weight pool is proportionately larger), a much larger fraction of the cell-associated radio-label is released than when longer loading times are used. The data suggest that a releasable intracellular iron pool is in equilibrium with the externalized material. The released iron may be separated into a high and a low molecular weight component. The former is similar on polyacrylamide gel electrophoresis to ferritin although it cannot be immune precipitated by anti-ferritin antibodies. The low molecular weight Fe-55 which is heterogeneous in nature can be bound by external apo-transferrin and may represent a form that can be taken up by cells beyond the blood-retinal barrier.