Ferroportin in Erythrocytes: Importance for Iron Homeostasis and its Role in Infection

Ferroportin (FPN), the only known vertebrate iron exporter, transports iron from intestinal, splenic, and hepatic cells into the blood to provide iron to other tissues and cells in vivo. Most of the circulating iron is consumed by erythroid cells to synthesize hemoglobin. Recently, we found that erythroid cells not only consume large amounts of iron, but also return significant amounts of iron to the blood. Erythroblast-specific Fpn knockout (Fpn KO) mice developed lower serum iron levels in conjunction with tissue iron overload and increased FPN expression in spleen and liver without changing hepcidin levels. Our results also showed that Fpn KO mice, which suffer from mild hemolytic anemia, were sensitive to phenylhydrazine-induced oxidative stress but were able to tolerate iron deficiency upon exposure to a low-iron diet and phlebotomy, supporting that the anemia of Fpn KO mice resulted from erythrocytic iron overload and resulting oxidative injury rather than a red blood cell (RBC) production defect. Moreover, we found that the mean corpuscular volume (MCV) values of gain-of-function FPN mutation patients were positively associated with serum transferrin saturations, whereas MCVs of loss-of-function FPN mutation patients were not, supporting that erythroblasts donate iron to blood through FPN in response to serum iron levels. Our results indicate that FPN of erythroid cells has an unexpectedly essential role in maintaining systemic iron homeostasis and protecting RBCs from oxidative stress, providing insight into the pathophysiology of FPN diseases. When malaria parasites invade red blood cells (RBCs), they consume copious amounts of hemoglobin, and severely disrupt iron regulation in humans. Anemia often accompanies malaria disease; however, iron supplementation therapy inexplicably exacerbates malarial infections. We recently found that the iron exporter ferroportin (FPN) was highly abundant in RBCs, and iron supplementation suppressed its activity. Conditional deletion of the Fpn gene in erythroid cells resulted in accumulation of excess intracellular iron, cellular damage, hemolysis, and increased fatality in malaria-infected mice. In humans, a prevalent FPN mutation,Q248H (glutamine to histidine at position 248), prevented hepcidin-induced degradation of FPN and protected against severe malaria disease. FPNQ248H appears to have been positively selected in African populations in response to the impact of malaria disease. Thus, FPN protects RBCs against oxidative stress and malaria infection.

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