Hfe Modulates the Response to Erythopoietic Stress In Wt Mice by Two Distinct Mechanisms

Abstract 4251

A deficient hepcidin response to iron is the principal mechanism responsible for increased iron uptake from the diet leading to iron overload. In hereditary hemochromatosis (HH), mutations in the HFE gene lead to iron overload through abnormally low levels of hepcidin. Interestingly, hepcidin has been shown to respond to a variety of stimuli, including iron, hypoxia, erythropoiesis and inflammation, requiring integration of the respective signals for its regulation. Further studies showed that HFE/Hfe could also modulate cellular iron uptake by associating with the transferrin receptor-1 (Tfrc), a crucial protein for iron uptake by erythroid cells. In addition, some studies have reported altered erythropoietic values in HH patients. Despite these findings, the role of Hfe in erythropoiesis was never explored. We hypothesized that Hfe influences erythropoiesis by two distinct mechanisms: 1) limiting hepcidin expression, thereby increasing iron availability, under conditions of simultaneous iron overload and stress erythropoiesis; 2) participating directly in the control of transferrin-bound iron uptake by erythroid cells. To test this hypothesis we investigated the role of Hfe in erythropoiesis, aiming to uncover the relative contribution of each of the aforementioned mechanisms. When erythropoiesis was challenged by phlebotomy, Hfe-KO animals were able to recover faster from anemia (p≤0.05) than either normal or iron overloaded wt mice. In Hfe-KO mice, despite their increased iron load, downregulation of hepcidin in response to phlebotomy or erythropoietin administration was comparable to that seen in wt mice. In contrast, iron overloaded wt mice showed increased hepcidin expression both at steady state and after erythropoietic stimulation compared to wt or Hfe-KO mice. In phlebotomized mice fed a standard diet, analysis of serum iron and transferrin saturation indicated that wt mice on the standard diet were able to increase their serum iron very rapidly. After 24 hours, both wt and Hfe-KO mice had similar serum iron and transferrin saturation levels. On the other hand, wt mice kept on an iron deficient diet over the course of phlebotomy, were unable to overcome the phlebotomy-induced anemia. In contrast, Hfe-KO mice fed the low iron diet were able to recover from anemia, although at a slower pace than either Hfe-KO or wt mice on a standard diet. These data indicate that gastrointestinal iron absorption in both wt and Hfe-KO mice is a major factor leading to recovery from anemia, although the excess iron in the liver of Hfe-KO mice contributes to restoration of the red blood cell reservoir. Phlebotomy is the main tool utilized to treat iron overload in HH patients. However, our data suggests that this treatment leads to both mobilization of iron from stores and increased gastrointestinal iron absorption. These observations suggest that patients might benefit from a controlled iron diet or from supplementation with hepcidin or an hepcidin agonist to limit iron absorption. Next, we determined that Hfe is expressed in erythroid cells and that it interacts with Tfrc in murine erythroleukemia cells. Moreover, we discovered that the level of Tfrc expression in Hfe-KO cells is 80% of that seen in wt cells, as measured by flow cytometry. This observation, together with measurement of iron uptake using ⁵⁹Fe-saturated transferrin, indicated that Hfe-KO erythroid cells take up significantly more iron than wt cells. To confirm that Hfe plays a role in erythropoiesis independent from that in the liver, we transplanted Hfe-KO or wt bone marrow cells into lethally irradiated wt recipients and analyzed their recovery from phlebotomy. We observed that recovery from anemia was faster in Hfe→wt than in wt→wt and was associated with increased mean corpuscular hemoglobin levels, suggesting that lack of Hfe in the hematopoietic compartment can lead to increased hemoglobin production. In summary, our results indicate that lack of Hfe enhances iron availability for erythropoiesis by two distinct mechanisms. On the one hand, Hfe plays an important role in maintaining erythroid iron homeostasis by limiting the response of hepcidin to iron, particularly under conditions of erythropoietic stimulation. On the other hand, lack of Hfe contributes directly to increased iron intake by erythroid progenitors, even in the absence of iron overload.