Disbalanced Erythroid Ferroportin Expression Contributes to Ineffective Erythroid Output in Anemia of Chronic Disease

Iron is essential for proper red blood cell development. During erythroid maturation in the bone marrow iron dependency starts at the basophilic stage, and lasts until the reticulocyte stage. Thereby the majority of iron is delivered in a Tf-TfR dependent manner and is incorporated in hemoglobin of developing red cells. Just recently, it has been discovered that erythroblasts and mature red blood cells not only incorporate iron but also express the sole known iron exporter, ferroportin (Fpn), throughout their development, and thus can also effectively export iron that is not used for heme synthesis. Fpn surface expression is regulated via hepcidin, a hepatocyte-derived peptide, which binds directly to surface-Fpn, inducing its internalization and consequent degradation in lysosomes.

Among patients suffering from anemia of chronic disease (ACD), hepcidin is constantly induced due to long-lasting inflammation, leading to impaired iron export capacity. While splenic tissue iron overload, which is associated with the restricted capacity of red pulp macrophages to export recycled iron form degraded erythrocytes is well established, the effect of high hepcidin levels in ACD and its consequences on developing erythroblasts in the bone marrow has not been investigated.

First we induced chronic kidney disease (CKD) in C57BL/6 mice via an Adenine - diet. Alongside microcytic anemia, reduced Tf-saturation, increased hepcidin levels and splenic tissue iron overload, we found a ~38% increase in tissue iron content in bone marrows of CKD mice compared to control mice. In parallel, Western blot analysis revealed massively reduced Fpn protein levels in the bone marrow. Moreover, the individual iron-dependent erythroblast precursor populations (i.e. basophilic, polychromatic, orthochromatic erythroblasts and reticulocytes) showed higher levels of intracellular iron as measured by Calcein fluorescence via flow cytometry. We could further corroborate these results by Western blot analysis and flow cytometric work-up of reticulocytes and mature red blood cells in the blood stream, both revealing highly reduced Fpn protein levels on these cells in mice suffering from CKD.

Based on these findings we performed additional experiments to investigate the detrimental effect of iron overload on erythroid development and to exclude the possibility of a direct inflammation-regulated process in the CKD model. Therefor we established an iron-overload mouse model via repeated parenteral iron dextran applications. Despite significantly increased Tf-saturation levels as well as hepatic hepcidin levels (>4-fold) and reduced bone marrow Fpn protein levels among iron-treated mice, iron overload led to higher stress levels among erythroid precursor populations. In detail, we could demonstrate via flow cytometry that higher intracellular iron pools (measured by Calcein), correlated with higher levels of mitochondrial stress and higher levels of lipid peroxidation (determined by mean fluorescence intensity of MitoSOX and Bodipy 581/59, respectively). These data indicate that iron is a critical regulator of stress during erythroid development and can be regulated via the hepcidin-Fpn axis.

In conclusion, we clearly show that Fpn expression on erythroid precursor cells is inversely regulated to systemic hepcidin levels and affects the erythropoietic bone marrow iron content. Moreover our results reveal a novel model for ineffective erythroid output in patients suffering from ACD due to hepcidin-triggered Fpn internalistation. Based on our data, anti-hepcidin treatment strategies are promising to overcome restricted erythropoiesis, which will be evaluated in future experiments.