Ferroportin 1 (FPN1) is transmembrane protein involved in iron homeostasis. In the duodenum, FPN1 localizes to the basolateral surface of enterocytes where it appears to export iron out of the cell and into the portal circulation. FPN1 is also abundantly expressed in reticuloendothelial macrophages of the liver, spleen, and bone marrow, suggesting that this protein serves as an iron exporter in cells that recycle iron from senescent red blood cells. To directly test the hypothesis that FPN1 functions in the export of iron after erythrophagocytosis, FPN1 was stably expressed in J774 mouse macrophages by using retroviral transduction, and release of 59 Fe after phagocytosis of 59 Fe-labeled rat erythrocytes was measured. J774 cells overexpressing FPN1 released 70% more 59 Fe after erythrophagocytosis than control cells, consistent with a role in the recycling of iron from senescent red cells. Treatment of cells with the peptide hormone hepcidin, a systemic regulator of iron metabolism, dramatically decreased FPN1 protein levels and significantly reduced the efflux of 59 Fe after erythrophagocytosis. Subsequent fractionation of the total released 59 Fe into heme and nonheme compounds revealed that hepcidin treatment reduced the release of nonheme 59 Fe by 50% and 25% from control and FPN1-overexpressing cells, respectively, but did not diminish efflux of 59 Fe-heme. We conclude that FPN1 is directly involved in the export of iron during erythrocyte-iron recycling by macrophages. reticuloendothelial cell ͉ reticuloendothelial system A pproximately two-thirds of total body iron in the adult human is present in hemoglobin in circulating red blood cells. Mature erythrocytes circulate until senescent or damaged and are then phagocytosed predominantly by reticuloendothelial macrophages of the liver, bone marrow, and spleen. After erythrophagocytosis, hemoglobin is proteolytically degraded, and the resultant heme moiety is oxidatively cleaved to release free iron, which is then either stored within the macrophage or released into the circulation. This recycling of iron from senescent red cells supplies the bone marrow with Ϸ20 mg of iron per day for new red cell synthesis (1).Several lines of evidence implicate the involvement of the recently identified protein ferroportin 1 (FPN1) in iron recycling by the macrophage. FPN1, also known as IREG-1 and MTP-1, was initially identified as an iron-export protein located on the basolateral membrane of duodenal enterocytes (2-4). The abundant expression of FPN1 in reticuloendothelial macrophages of the liver, bone marrow, and splenic red pulp (5) suggests that it plays a similar iron-export role in macrophages as well. Genetic evidence for this proposed function is provided by a growing number of hemochromatosis patients identified with mutations in FPN1 (reviewed in ref. 6). The distinguishing clinical feature of these patients is iron accumulation in liver macrophages (Kupffer cells). Studies of cultured macrophages, which show marked increases in FPN1 after erythrophagocytosis ...