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Blood. 2018 Feb 22;131(8):899-910. doi: 10.1182/blood-2017-05-786590. Epub 2017 Dec 13.

Structure-function analysis of ferroportin defines the binding site and an alternative mechanism of action of hepcidin.

Author information

1
Molecular Biology Interdepartmental Doctoral Program.
2
Department of Medicine, and.
3
Department of Molecular, Cellular, and Integrative Physiology, David Geffen School of Medicine, University of California, Los Angeles, Los Angeles, CA.
4
Department of Molecular and Cellular Physiology, University of Cincinnati College of Medicine, Cincinnati, OH.
5
Division of Hematology, Department of Pediatrics, Children's Hospital of Philadelphia, Philadelphia, PA.
6
Structural Biology Program, Centenary Institute, and.
7
Faculty of Medicine, University of Sydney, Sydney, Australia; and.
8
Department of Pathology, David Geffen School of Medicine, University of California, Los Angeles, Los Angeles, CA.

Abstract

Nonclassical ferroportin disease (FD) is a form of hereditary hemochromatosis caused by mutations in the iron transporter ferroportin (Fpn), resulting in parenchymal iron overload. Fpn is regulated by the hormone hepcidin, which induces Fpn endocytosis and cellular iron retention. We characterized 11 clinically relevant and 5 nonclinical Fpn mutations using stably transfected, inducible isogenic cell lines. All clinical mutants were functionally resistant to hepcidin as a consequence of either impaired hepcidin binding or impaired hepcidin-dependent ubiquitination despite intact hepcidin binding. Mapping the residues onto 2 computational models of the human Fpn structure indicated that (1) mutations that caused ubiquitination-resistance were positioned at helix-helix interfaces, likely preventing the hepcidin-induced conformational change, (2) hepcidin binding occurred within the central cavity of Fpn, (3) hepcidin interacted with up to 4 helices, and (4) hepcidin binding should occlude Fpn and interfere with iron export independently of endocytosis. We experimentally confirmed hepcidin-mediated occlusion of Fpn in the absence of endocytosis in multiple cellular systems: HEK293 cells expressing an endocytosis-defective Fpn mutant (K8R), Xenopus oocytes expressing wild-type or K8R Fpn, and mature human red blood cells. We conclude that nonclassical FD is caused by Fpn mutations that decrease hepcidin binding or hinder conformational changes required for ubiquitination and endocytosis of Fpn. The newly documented ability of hepcidin and its agonists to occlude iron transport may facilitate the development of broadly effective treatments for hereditary iron overload disorders.

PMID:
29237594
PMCID:
PMC5824336
DOI:
10.1182/blood-2017-05-786590
[Indexed for MEDLINE]
Free PMC Article

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