Role of Iron Sequestration in the Development of Anemia of CKD: Implications for Treatment Resistance

Background: Anemia is a common complication of CKD, and iron supplementation is the recommended first line of therapy. The efficacy of oral iron therapy in CKD is unsatisfactory, likely owing, at least in part, to elevated hepcidin. Hepcidin induction might limit iron availability for erythropoiesis through either reduced intestinal absorption or by enhanced sequestration. However, the relative contribution of iron malabsorption vs. sequestration to anemia of CKD remains unclear. Methods: CKD was induced in 8-week old wild type (WT) and macrophage-specific (LysM-Cre) ferroportin (Fpt) knockout (KO) mice using a 0.2% adenine diet for 8 weeks. We used oral carbonyl iron, iron sulfate, and iron citrate in doses comparable to those used in CKD patients. Iron content was quantified in the bone marrow, spleen, and liver by staining, a bathophenanthroline-based colorimetric method, and by MRI. Tissue ferritin was assessed by immunoblotting. Results: WT CKD mice developed anemia and their serum hepcidin was 3-fold higher than in healthy mice. There was a marked increase in the number of iron-positive cells in the bone marrow (Fig.) and a 2-fold increase in the spleen of WT CKD mice compared to healthy mice. Liver ferritin was induced in untreated WT CKD mice. Macrophage-specific Fpt KO CKD mice had more severe anemia and further increase of iron stores in the bone marrow, spleen, and liver, compared to WT CKD mice. Oral iron supplementation improved hemoglobin in WT CKD mice by 20.8% but increased hepcidin by 1.5-fold, spleen iron content by 74.8% and liver iron content by 12.5-fold compared to untreated CKD mice. Conclusion: In this model of CKD, we observed iron sequestration in the bone marrow, spleen, and liver in the absence of iron therapy. Oral iron supplementation further enhanced iron sequestration, disproportionate to the alleviation of anemia in CKD mice. Sequestered iron remained partially mobilizable via incompletely suppressed ferroportin. Funding: NIDDK Support, Private Foundation SupportBone marrow iron staining by Perls Prussian blue in healthy mice and in mice with untreated CKD after 8 weeks of control or adenine diet.