THE METABOLIC FATE OF THE PEPTIDE HORMONE HEPCIDIN

Abstract

Hepcidin (Hep), a 25-amino acid peptide hormone, is the principal regulator of plasma iron concentrations. Hepcidin's receptor is the iron channel ferroportin (Fpn), the only known cellular iron exporter in vertebrates. Fpn exports iron from duodenal enterocytes, macrophages and hepatocytes into plasma. Hep binding induces Fpn internalization and degradation, thus blocking iron efflux from cells into plasma. The aim of this study was to characterize the fate of Hep after binding to Fpn and determine its cellular catabolism and the mechanism of clearance. It has been known that Hep is rapidly excreted through the kidney, but we now describe that Hep is taken up by ferroportin-expressing cells and degraded together with its receptor. When Texas red-labeled Hep was added to Fpn-GFP expressing cells, confocal microscopy showed co-localization of red Hep and green Fpn. We assessed Hep degradation by flow cytometry and showed that Hep is almost completely degraded by 24h after addition. Lysosomal inhibitors and to a lesser extent the proteosomal inhibitor MG132 prevented degradation of both Fpn and Hep suggesting that the receptor-ligand complex is degraded by both degradation pathways. In addition, using radio-labeled Hep and HPLC analysis we show that Hep is not recycled, and that only degradation products are released from the cells. Together these results show that the hormone Hep after binding to its receptor Fpn, is internalized together with Fpn and trafficked to lysosomes where both are degraded. We confirmed this finding in vivo using PET/CT mouse imaging studies. In conditions of iron deficiency when Fpn is upregulated, I124-Hep accumulated in ferroportin-expressing tissues more than when animals were iron replete and Fpn expression was lower. These data suggest that both receptor-mediated endocytosis as well as renal clearance contribute to the removal of hecidin from the circulation.