Mechanisms of Ferritin Iron Mobilization in Macrophages and Hepatocytes

Abstract

We and others have demonstrated that to retrieve iron stored in ferritin, this large hollow protein (which carries iron‐ferrihydrite crystals) moves into lysosomes, where proteases must degrade its protein “shell” so that the iron may be solubilized and reenter the cytosolic labile iron pool. We have investigated two steps in this process, using hepatocyte and macrophage cell culture models as well as rodents, the first being that leading to ferritin autophagy by lysosomes, the other iron transfer from lysosome to cytosol. It has long been known that a portion of cellular ferritin contains covalently linked oligomers. To test whether these might be involved in ferritin autophagy to lysosomes, we isolated ferritin from liver and spleen of rats pre‐iron‐loaded to induce ferritin, after 5‐days of subsequent injections with saline (control), or erythropoietin (EPO) to induce ferritin iron mobilization. EPO‐treatment resulted in a marked drop in the proportion of ferritin oligomers compared to the controls. To determine whether DMT1 was mediating return of iron from lysosomes to cytosol, we preloaded J774 mouse macrophage lysosomes with iron dextran, and monitored the effects of a specific DMT1 inhibitor on formation of cytosolic ferritin. DMT1 inhibition lowered ferritin production. In mice with a conditional knockout of DMT1 in hepatocytes (provided by Mitchell Knutson), stimulation of erythropoiesis by bleeding mobilized less iron from liver, but not from spleen. We conclude that DMT1 is important for the lysosomal mobilization of ferritin iron, and that ferritin oligomers may preferentially enter lysosomes during autophagy.