Hepcidin-induced endocytosis of ferroportin is dependent on ferroportin ubiquitination

Abstract

Ferroportin exports iron into plasma from absorptive enterocytes, erythrophagocytosing macrophages, and hepatic stores. The hormone hepcidin controls cellular iron export and plasma iron concentrations by binding to ferroportin and causing its internalization and degradation. We explored the mechanism of hepcidin-induced endocytosis of ferroportin, the key molecular event in systemic iron homeostasis. Hepcidin binding caused rapid ubiquitination of ferroportin in cell lines overexpressing ferroportin and in murine bone marrow-derived macrophages. No hepcidin-dependent ubiquitination was observed in C326S ferroportin mutant which does not bind hepcidin. Substitutions of lysines between residues 229 and 269 in the third cytoplasmic loop of ferroportin prevented hepcidin-dependent ubiquitination and endocytosis of ferroportin, and promoted cellular iron export even in the presence of hepcidin. The human ferroportin mutation K240E, previously associated with clinical iron overload, caused hepcidin resistance in vitro by interfering with ferroportin ubiquitination. Our study demonstrates that ubiquitination is the functionally relevant signal for hepcidin-induced ferroportin endocytosis.

Figure 1. Hepcidin binding causes rapid ubiquitination of Fpn

A: HEK293 cells stably transfected with a Dox-inducible human Fpn-GFP were either not induced (−dox) or induced (+dox) to express Fpn-GFP, then treated with 1 μg/ml (360 nM) hepcidin for up to 2 h. Cell lysates were immunoprecipitated with anti-GFP Ab (ab6556), and blotted with anti-poly/monoUb Ab (FK2). The blot was reprobed with a different anti-GFP Ab (7.1/13.1) to confirm even loading. B: Cells were treated identically to those in A, but the blot was incubated with an Ab recognizing polyUb only (FK1). C: Mouse primary bone marrow-derived macrophages were treated with 1 μg/ml hepcidin for up to 1 h. Lysates were immunoprecipitated with anti-Fpn Ab (MTP11-A) and immunoblotted with FK2 anti-poly/monoUb Ab. The amount of immunoprecipitated Fpn was verified by probing the blot with another anti-mouse Fpn Ab (R1). D: HEK293 cells stably expressing Dox-inducible WT Fpn-GFP or C326S mutant which does not bind hepcidin were treated with 1 μg/ml hepcidin and processed as in A.

Figure 2. Lysine substitutions in the third intracellular loop of Fpn prevent hepcidin-dependent ubiquitination of Fpn without affecting iron-exporting function or hepcidin binding

A: Untransfected HEK293 cells (HEK) and HEK293 stably transfected with Dox-inducible K3R (K229R/K240R/K247R), K4R (K229R/K240R/K247R/K258R) or DEL (del229-269) Fpn-GFP mutant were incubated without (−dox) or with 0.5 μg/ml dox (+dox). After 24 h, protein lysates were assayed for ferritin levels. Vertical bars and error bars represent the mean and standard deviation of 4–7 experiments. *p=0.001, **p<0.001 by t-test. B: Cells were induced with Dox and treated with 10 μg/ml (3.6 μM) biotinylated hepcidin for 30 min. Protein lysates were immunoprecipitated with anti-GFP Ab (ab290), and biotinylated hepcidin bound to Fpn-GFP was detected with streptavidin-HRP. The amount of immunoprecipitated Fpn-GFP was determined by Western blotting with anti-GFP Ab (7.1/13.1). C: Cells were induced with Dox to express WT or mutant Fpn-GFP and treated with 1 μg/ml hepcidin for up to 2 h. Protein lysates were immunoprecipitated with anti-GFP Ab (ab6556), and immunoblotted with anti-poly/monoUb Ab (FK2) or anti-GFP Ab (7.1/13.1).

Figure 3. Lysine substitutions in the third intracellular loop of Fpn interfere with hepcidin-induced Fpn endocytosis and cellular iron retention

Stably transfected HEK293 cells were induced with Dox to express WT, K3R, K4R or del229-269 Fpn-GFP. Dox was washed off prior to adding hepcidin. A: Cells were treated with 1 μg/ml (360 nM) hepcidin and Fpn-GFP location assessed at 4 and 24 h by fluorescent microscopy. B: Cells were treated with indicated hepcidin concentrations. After 4 h, cell surface Fpn was detected by staining with anti-human Fpn Ab against an extracellular loop of Fpn (M1), followed by the secondary Ab conjugated to PE. Fluorescence was quantified by flow cytometry and expressed as % fluorescence of untreated samples. Each point represents the mean and standard deviation of 3 independent experiments. Each of the mutant curves (K3R, K4R or DEL) significantly differed from the WT (p<0.001, two-way ANOVA). C: Cells expressing WT or mutant Fpn-GFP were incubated without or with 0.2 μg/ml (72 nM) hepcidin for 24 h. Cell lysates were assayed for ferritin. Fold increase in ferritin after hepcidin treatment is shown as a mean and standard deviation of at least 6 separate measurements. *p=0.003 and **p=0.001 by Mann-Whitney Rank Sum Test for the comparison of the hepcidin-treated mutant with the hepcidin-treated WT cells. For K3R, K4R or DEL mutants, no significant difference in ferritin levels was observed between hepcidin-treated or untreated cells.

Figure 4. K240E human Fpn mutation causes hepcidin resistance

A: Stably transfected HEK293 cells were induced with Dox to express WT or K240E Fpn-GFP, and were treated with 1 μg/ml hepcidin for up to 2 h. Protein lysates were immunoprecipitated with anti-GFP Ab (ab6556), and immunoblotted with anti-poly/monoUb Ab (FK2) or anti-GFP Ab (7.1/13.1). B: HEK293 cells induced to express WT or K240E Fpn-GFP were treated with 1 μg/ml hepcidin and Fpn-GFP location assessed after 4 and 24 h by fluorescent microscopy. WT images are identical to those in Figure 6. C: HEK293 cells induced to express WT or K240E Fpn-GFP were treated with 0.2 μg/ml hepcidin for the indicated time. To quantify cell surface Fpn, cells were stained with anti-human Fpn Ab against an extracellular loop of Fpn (M1), followed by the secondary Ab conjugated to PE. Fluorescence was measured by flow cytometry and results expressed as % of the fluorescence of untreated samples. Means and standard deviations of 4 separate measurements are shown. K240E and WT were compared at each time-point by t-test. D: Cells expressing WT or K240E Fpn-GFP were incubated without or with 0.2 μg/ml hepcidin for 24 h, and protein lysates assayed for ferritin (n=10 for WT and 12 for K240E). WT values are the same as in Figure 3C. The means and standard deviations of fold increase in ferritin levels after hepcidin treatment are shown, p=0.003 by Mann-Whitney Rank Sum Test for the comparison of the hepcidin-treated K240E with the hepcidin-treated WT cells. E: Serum hepcidin, ferritin and hepcidin/ferritin ratio in two subjects with K240E mutation, compared to historical controls (Ganz et al., 2008).