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Erythropoiesis, circulating iron and hepcidin
AB
CD
of mice treated with both erythropoietin and iron when compared with those treated with erythropoietin alone (Figure 8C). Interestingly, the decrease remained when Tfr1 levels were normalized to Gypa (Figure 8D), indicat- ing lower Tfr1 expression in individual erythroblasts and a reduction in iron demand following iron injection.
Discussion
Developing erythrocytes are by far the largest sink for iron in the body,10 so it is not surprising that erythropoiesis is a major regulator of hepcidin production. Our under- standing of how the erythroid mass in the bone marrow and spleen signal changes in HAMP expression in hepato- cytes has been advanced considerably with the recent dis- covery of erythroferrone, a protein produced by develop- ing erythrocytes and secreted into the circulation, which inhibits hepatic hepcidin production,11,12 As a result, iron absorption and storage iron release are increased, provid- ing more iron for erythrocyte development. However, other factors are also known to regulate hepcidin expres- sion. Of these, serum iron levels, particularly diferric transferrin, may be relevant to hepcidin expression fol- lowing stimulated erythropoiesis. We have previously
Figure 4. Circulating iron parameters immedi- ately after iron injection of erythropoietin-treat- ed mice. Six-week old male C57BL/6 mice were injected intravenously with 10 U/g body weight human erythropoietin. Five hours later, mice were intravenously injected with either 2.5 mg/g body weight ferric citrate or an equimolar amount of citrate as sodium citrate. Mice were eutha- nized 5 min after this final injection and blood taken for analysis. Total serum iron (A), transfer- rin saturation (B), total iron binding capacity (C) and relative diferric transferrin levels (D) were determined for each group. The relative diferric transferrin levels represent the percentage of transferrin in the diferric form expressed as a pro- portion of the values in mice injected with sodi- um citrate but not with erythropoietin. The data represent the mean ± SEM with the number of mice in each group indicated in parentheses along the x-axis. Con: control mice that were injected with sodium citrate; Fe: mice that were injected with ferric citrate; No Epo: mice that were not injected with erythropoietin; Epo: mice that were injected with erythropoietin; Apo: apo- transferrin; Mono: monoferric transferrin; Di: diferric transferrin. ***P<0.005.
shown that diferric transferrin correlates with inhibited Hamp expression in rats following treatment with the hemolytic agent phenylhydrazine,18 and have suggested that the level of diferric transferrin in the circulation might signal the iron demands of the erythroid marrow to the liver and influence hepcidin production.13,19
In the current study we have demonstrated for the first time that the rapid decrease in Hamp1 expression that occurs following erythropoietin injection in mice is pre- ceded by a transient decrease in circulating diferric trans- ferrin. The evidence in favor of serum iron and, therefore, diferric transferrin, influencing hepcidin production is strong. We have previously shown that reductions in serum iron levels correlate closely with the inhibition of Hamp expression in rats switched to an iron-deficient diet25 and following phenylhydrazine treatment.18 We and others have also demonstrated that increased serum iron levels stimulate hepcidin expression in mice.15,19 In addi- tion, a mechanism by which hepatocytes might detect the levels of circulating diferric transferrin has been described. It has been proposed that the hemochromatosis protein HFE on the surface of hepatocytes competes with diferric transferrin for binding to TFR1, with the HFE/TFR1 com- plex being favored when diferric transferrin levels are low.13,26 Thus, diferric transferrin would not be expected to
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