Lipid raft-dependent endocytosis: a new route for hepcidin-mediated regulation of ferroportin in macrophages.

BACKGROUND: Expression of the iron exporter ferroportin at the plasma membrane of macrophages is enhanced by iron loading and is decreased by hepcidin. We previously showed that ferroportin is present in specific cell surface domains suggestive of lipid rafts. Herein, we have clarified the localization of ferroportin in macrophage membranes and tested whether raft-mediated endocytosis plays a role in hepcidin activity. DESIGN AND METHODS: Raft/detergent-resistant membranes from murine bone marrow-derived macrophages and J774a1 cells were analyzed by Western blotting. The effect of lipid raft- or clathrin-dependent endocytosis inhibitors was studied on hepcidin activity. For this purpose, after treatment, ferroportin expression was analyzed by fluorescence microscopy, Western blotting of total protein extracts or plasma membrane protein samples, and by quantitative immunofluorescence assay (In-Cell-Western).
RESULTS: Macrophage ferroportin was mostly detected in detergent-resistant membranes containing raft markers (caveolin 1, flotillin 1). Interestingly, iron overload strongly increased the presence of ferroportin in the lightest raft fraction. Moreover, lipid raft breakdown by cholesterol sequestration (filipin) or depletion (methyl-beta-cyclodextrin) decreased hepcidin activity on macrophage ferroportin. Cell surface biotinylation and immunofluorescence studies indicated that the process of both hepcidin mediated endocytosis and degradation of ferroportin were affected. By contrast, the inhibition of clathrin dependent endocytosis did not interfere with hepcidin effect.
CONCLUSIONS: Macrophage ferroportin is present in lipid rafts which contribute to hepcidin activity. These observations reveal the existence of a new cellular pathway in hepcidin mediated degradation of ferroportin and open a new area of investigation in mammalian iron homeostasis.