Ceruloplasmin and Ferroportin interaction in macrophage iron efflux

Marlene PANZER^1,2, Herbert TILG¹, Heinz ZOLLER^1,2, Benedikt SCHAEFER¹, Sonja WAGNER^1,2, Elke PERTLER^1,2, Maria TROPPMAIR¹, Laura OBHOLZER¹, Verena PETZER³, Guenter WEISS⁴, Heribert TALASZ⁵

¹Internal Medicine I, Medical University of Innsbruck, Innsbruck, Austria
²Christian Doppler Laboratory for Iron and Phosphate Biology, Medical University of Innsbruck, Innsbruck, Austria
³Department of Internal Medicine V, Medical University of Innsbruck, Innsbruck, Austria
⁴Department of Internal Medicine II, Medical University of Innsbruck, Innsbruck, Austria
⁵Division of Medical Biochemistry, Protein Core Facility, Biocenter, Medical University of Innsbruck, Innsbruck, Austria

Macrophages are crucial for iron recycling from senescent red blood cells. The ferroxidase Ceruloplasmin (Cp) is an abundant protein in plasma and also implicated in iron export mainly from hepatocytes. In humans, recessive CP mutations cause aceruloplasminemia with iron overload in the liver and brain, but not the spleen. This study aims to investigate the function of ceruloplasmin in macrophage iron export, erythropoiesis and iron homeostasis.

Ferroportin-floxed mice with a macrophage-specific LysM-Cre recombinase (FPNDM/DM) were crossed with ceruloplasmin knockout (CP-/-) mice to generate CP-/-FPNDM/DM, FPNDM/DM, CP-/- and FPNflox/flox mice as controls. The impact on iron homeostasis and erythropoiesis was analyzed in 2 and 9-month-old female mice. Tissue iron, mRNA expression, blood count, red cell indices and plasma iron parameters were assessed. Erythropoiesis was analyzed by flow cytometry of the bone marrow.

Hemoglobin was significantly reduced in CP-/-FPNDM/DM and FPNDM/DM mice at 2 and 9 month, but only in 9 month old CP-/- mice. Plasma ferritin was elevated in CP-/-FPNDM/DM and FPNDM/DM and hepcidin-to-ferritin ratio was decreased. Transferrin saturation was reduced in all groups at 9 month. Iron was increased in the spleen of FPNDM/DM and CP-/-FPNDM/DM mice, whereas CP-/- and CP-/-FPNDM/DM mice had increased liver iron concentrations. In the bone marrow of CP-/- mice, iron was decreased at 9 months of age.

In conclusion, our findings suggest that ceruloplasmin is important for iron export from hepatocytes, whereas ferroportin is required for iron egress from macrophages. The phenotype of double knock out mice suggests limited functional interaction of ceruloplasmin and ferroportin.