Abstract
The reticuloendothelial system has a central role in erythropoiesis and iron homeostasis. An important function of reticuloendothelial macrophages is phagocytosis of senescent red blood cells. The iron liberated from heme is recycled for delivery to erythrocyte precursors for a new round of hemoglobin synthesis. The molecular mechanism by which recycled iron is released from macrophages remains unresolved. We have investigated the mechanism of macrophage iron efflux, focusing on the role of ceruloplasmin (Cp), a copper protein with a potent ferroxidase activity that converts Fe2+ to Fe3+ in the presence of molecular oxygen. As shown by others, Cp markedly increased iron binding to apotransferrin at acidic pH; however, the physiological significance of this finding is uncertain because little stimulation was observed at neutral pH. Introduction of a hypoxic atmosphere resulted in marked Cp-stimulated binding of iron to apotransferrin at physiological pH. The role of Cp in cellular iron release was examined in U937 monocytic cells induced to differentiate to the macrophage lineage. Cp added at its normal plasma concentration increased the rate of 55Fe release from U937 cells by about 250%. The stimulation was absolutely dependent on the presence of apotransferrin and hypoxia. Cp-stimulated iron release was confirmed in mouse peritoneal macrophages. Stimulation of iron release required an intracellular "labile iron pool" that was rapidly depleted in the presence of Cp and apotransferrin. Ferroxidase-mediated loading of iron into apotransferrin was critical for iron release because ferroxidase-deficient Cp was inactive and because holotransferrin could not substitute for apotransferrin. The extracellular iron concentration was critical as shown by inhibition of iron release by exogenous free iron, and marked enhancement of release by an iron chelator. Together these data show that Cp stimulates iron release from macrophages under hypoxic conditions by a ferroxidase-dependent mechanism, possibly involving generation of a negative iron gradient.
Highlights
Reticuloendothelial macrophages have a central role in regulating normal iron homeostasis [1]
We have investigated the mechanism of macrophage iron efflux, focusing on the role of ceruloplasmin (Cp), a copper protein with a potent ferroxidase activity that converts Fe2؉ to Fe3؉ in the presence of molecular oxygen
Regarding the mechanism of Cp-stimulated macrophage iron release, we show that the iron source is the intracellular labile iron pool (LIP), that Cp ferroxidase activity is required, and that development of a negative iron gradient is an essential feature of iron release from these cells
Summary
Ceruloplasmin; FPN1, ferroportin-1; LIP, labile iron pool; NTA, nitrilotriacetic acid; PBS, phosphatebuffered saline; PMA, phorbol myristoyl acetate; MOPS, 4-morpholinepropanesulfonic acid. Young et al [22] reported a 40% stimulation of iron release from human hepatocarcinoma HepG2 cells by Cp in the presence of apotransferrin. Cp increased iron release from iron-loaded HepG2 cells [23] and from central nervous system astrocytes [24], but the mechanism underlying the stimulation was unclear because it occurred even in the absence of apotransferrin. In this report we rigorously investigate the requirements for Cp stimulation of iron binding to apotransferrin and iron release from myeloid cells. Regarding the mechanism of Cp-stimulated macrophage iron release, we show that the iron source is the intracellular labile iron pool (LIP), that Cp ferroxidase activity is required, and that development of a negative iron gradient is an essential feature of iron release from these cells
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
