Ferroxidase activity is required for the stability of cell surface ferroportin in cells expressing GPI-ceruloplasmin

Abstract

Ferroportin (Fpn), a ferrous iron Fe(II) transporter responsible for the entry of iron into plasma, is regulated posttranslationally through internalization and degradation following binding of the hormone hepcidin. Cellular iron export is impaired in mice and humans with aceruloplasminemia, an iron overload disease due to mutations in the ferroxidase ceruloplasmin (Cp). In the absence of Cp Fpn is rapidly internalized and degraded. Depletion of extracellular Fe(II) by the yeast ferroxidase Fet3p or iron chelators can maintain cell surface Fpn in the absence of Cp. Iron remains bound to Fpn in the absence of multi-copper oxidases. Fpn with bound iron is recognized by a ubiquitin ligase, which ubiquitinates Fpn on lysine 253. Mutation of lysine 253 to alanine prevents ubiquitination and maintains Fpn-iron on cell surface in the absence of ferroxidase activity. The requirement for a ferroxidase to maintain iron transport activity represents a new mechanism of regulating cellular iron export, a new function for Cp and an explanation for brain iron overload in patients with aceruloplasminemia

The Role of Copper and Zinc Toxicity in Innate Immune Defense against Bacterial Pathogens

“…Fe transport and regulation are linked intimately with the inflammatory response, where removal away from sites of infection is the norm. Based on our current understanding of eukaryotic Fe homeostasis, movement of Fe out of cells depends on the efflux of ferrous iron via ferroportin, and this requires the ferroxidase activity of ceruloplasmin (97,98). Assembly of holo-ceruloplasmin in the host endoplasmic reticulum or Golgi relies on the transport of Cu via the Cutransporting ATP7B (99).…”

Measurement of serum hepcidin-25 levels as a potential test for diagnosing hemochromatosis and related disorders

“…19 Ceruloplasmin encoded by CP is a major ferroxidase in circulation, Kaneko et 4 and is believed to play a crucial role in maintaining stability of cell surface ferroportin. 20 Impaired regulation of ceruloplasmin negatively impacts ferroportin expression, which in turn blocks iron transport in patients.…”

“…39 Ceruloplasmin is a multicopper oxidase and plays a role in the mobilization and oxidation of iron from tissue stores with subsequent incorporation of ferric iron into transferrin. 18,19,20 According to the iron sensor hypothesis of TFR2, increasing concentrations of iron-saturated transferrin increase TFR2 protein levels in hepatocyte by protecting the receptor from degradation. And consecutively, TFR2 would be expected to increase the association with HFE and to stimulate of hepcidin transcription.…”

Role of External Loops of Human Ceruloplasmin in Copper Loading by ATP7B and Ccc2p

“…Synthesis of this isoform is via alternative splicing, causing the replacement of the five C-terminal amino acids of the secreted protein by 30 alternative residues that lead to the addition of the GPI anchor (4). The ferroxidase activity of Cp is required for proper iron homeostasis; lack of oxidase-active Cp leads to internalization and degradation of ferroportin (Fpn), the only known mammalian iron exporter (5). Furthermore, genetic defects of the Cp gene cause aceruloplasminemia, a rare autosomal iron overload disease with clinical manifestations, including retinal degeneration, diabetes mellitus, and neurological symptoms, which include ataxia, involuntary movements, and dementia (6).…”