Apo- and holo-transferrin differentially interact with hephaestin and ferroportin in a novel mechanism of cellular iron release regulation

BACKGROUND

Apo- (iron free) and holo- (iron bound) transferrin (Tf) participate in precise regulation of brain iron uptake at endothelial cells of the blood-brain barrier. Apo-Tf indicates an iron-deficient environment and stimulates iron release, while holo-Tf indicates an iron sufficient environment and suppresses additional iron release. Free iron is exported through ferroportin, with hephaestin as an aid to the process. Until now, the molecular mechanisms of apo- and holo-Tf influence on iron release was largely unknown.

METHODS

Here we use a variety of cell culture techniques, including co-immunoprecipitation and proximity ligation assay, in iPSC-derived endothelial cells and HEK 293 cells to investigate the mechanism by which apo- and holo-Tf influence cellular iron release. Given the established role of hepcidin in regulating cellular iron release, we further explored the relationship of hepcidin to transferrin in this model.

RESULTS

We demonstrate that holo-Tf induces the internalization of ferroportin through the established ferroportin degradation pathway. Furthermore, holo-Tf directly interacts with ferroportin, whereas apo-Tf directly interacts with hephaestin. Only pathophysiological levels of hepcidin disrupt the interaction between holo-Tf and ferroportin, but similar hepcidin levels are unable to interfere with the interaction between apo-Tf and hephaestin. The disruption of the holo-Tf and ferroportin interaction by hepcidin is due to hepcidin's ability to more rapidly internalize ferroportin compared to holo-Tf.

CONCLUSIONS

These novel findings provide a molecular mechanism for apo- and holo-Tf regulation of iron release from endothelial cells. They further demonstrate how hepcidin impacts these protein-protein interactions, and offer a model for how holo-Tf and hepcidin cooperate to suppress iron release. These results expand on our previous reports on mechanisms mediating regulation of brain iron uptake to provide a more thorough understanding of the regulatory mechanisms mediating cellular iron release in general.

The biology of mammalian multi-copper ferroxidases

The mammalian multicopper ferroxidases (MCFs) ceruloplasmin (CP), hephaestin (HEPH) and zyklopen (ZP) comprise a family of conserved enzymes that are essential for body iron homeostasis. Each of these enzymes contains six biosynthetically incorporated copper atoms which act as intermediate electron acceptors, and the oxidation of iron is associated with the four electron reduction of dioxygen to generate two water molecules. CP occurs in both a secreted and GPI-linked (membrane-bound) form, while HEPH and ZP each contain a single C-terminal transmembrane domain. These enzymes function to ensure the efficient oxidation of iron so that it can be effectively released from tissues via the iron export protein ferroportin and subsequently bound to the iron carrier protein transferrin in the blood. CP is particularly important in facilitating iron release from the liver and central nervous system, HEPH is the major MCF in the small intestine and is critical for dietary iron absorption, and ZP is important for normal hair development. CP and HEPH (and possibly ZP) function in multiple tissues. These proteins also play other (non-iron-related) physiological roles, but many of these are ill-defined. In addition to disrupting iron homeostasis, MCF dysfunction perturbs neurological and immune function, alters cancer susceptibility, and causes hair loss, but, despite their importance, how MCFs co-ordinately maintain body iron homeostasis and perform other functions remains incompletely understood.

Association between iron metabolism and SARS-COV-2 infection, determined by ferritin, hephaestin and hypoxia-induced factor-1 alpha levels in COVID-19 patients

BACKGROUND

Due to the growing evidence of the importance of iron status in immune responses, the biomarkers of iron metabolism are of interest in novel Coronavirus Disease 2019 (COVID-19). The present prospective study was carried out to compare iron status indicated by levels of ferritin with the levels of two novel biomarkers related to iron homeostasis, hephaestin and hypoxia-inducible factors-1 (HIF-1α) in the serum of patients with COVID-19 in comparison with a control group.

METHODS AND RESULTS

Blood samples from 34 COVID-19 patients and from 43 healthy volunteers were collected and the levels of HEPH and HIF-1α were measured by ELISA and compared with levels of serum ferritin. COVID-19 patients had higher serum levels of ferritin than those levels in control group (P < 0.0001). Conversely levels of HIF-1α and HEPH in the COVID-19 group were significantly lower than those of control group (P < 0.0001 for both). An inverse correlation between hephaestin and ferritin as well as between HIF-1α and ferritin was found among all subjects (P < 0.0001), and among COVID-19 patients, but not to statistical significance.

CONCLUSION

Levels of hephaestin and HIF-1α were found to be inversely related levels of ferritin across all participants in the study, and to our knowledge this is the first report of hephaestin and HIF-1α as potential markers of iron status. Further studies are needed to corroborate the findings, utilizing a broader range of markers to monitor inflammatory as well as iron status.

Conditional knockout of hephaestin in the neural retina disrupts retinal iron homeostasis

Iron accumulation has been implicated in degenerative retinal diseases. It can catalyze the production of damaging reactive oxygen species. Previous work has demonstrated iron accumulation in multiple retinal diseases, including age-related macular degeneration and diabetic retinopathy. In mice, systemic knockout of the ferroxidases ceruloplasmin (Cp) and hephaestin (Heph), which oxidize iron, results in retinal iron accumulation and iron-induced degeneration. To determine the role of Heph in the retina, we generated a neural retina-specific Heph knockout on a background of systemic Cp knockout. This resulted in elevated neural retina iron. Conversely, retinal ganglion cells had elevated transferrin receptor and decreased ferritin, suggesting diminished iron levels. The retinal degeneration observed in systemic Cp-/-, Heph-/- mice did not occur. These findings indicate that Heph has a local role in regulating neural retina iron homeostasis, but also suggest that preserved Heph function in either the RPE or systemically mitigates the degeneration phenotype observed in the systemic Cp-/-, Heph-/- mice.

Expression of soluble, active, fluorescently tagged hephaestin in COS and CHO cell lines

Hephaestin (Hp) is a trans-membrane protein, which plays a critical role in intestinal iron absorption. Hp was originally identified as the gene responsible for the phenotype of sex-linked anaemia in the sla mouse. The mutation in the sla protein causes accumulation of dietary iron in duodenal cells, causing severe microcytic hypochromic anaemia. Although mucosal uptake of dietary iron is normal, export from the duodenum is inhibited. Hp is homologous to ceruloplasmin (Cp), a member of the family of multi copper ferroxidases (MCFs) and possesses ferroxidase activity that facilitates iron release from the duodenum and load onto the serum iron transport protein transferrin. In the present study, attempts were made to produce biologically active recombinant mouse hephaestin as a secretory form tagged with green fluorescent protein (GFP), Hpsec-GFP. Plasmid expressing Hpsec-GFP was constructed and transfected into COS and CHO cells. The GFP aided the monitoring expression in real time to select the best conditions to maximise expression and provided a tag for purifying and analysing Hpsec-GFP. The protein had detectable oxidase activity as shown by in-gel and solution-based assays. The methods described here can provide the basis for further work to probe the interaction of hephaestin with other proteins using complementary fluorescent tags on target proteins that would facilitate the fluorescence resonance energy transfer measurements, for example with transferrin or colocalisation studies, and help to discover more about hephaestin works at the molecular level.

Is brain iron trafficking part of the physiology of the amyloid precursor protein?

The amyloid precursor protein is so named, because a proteolytic fragment of it was found associated with a neuropathic disorder now known as Alzheimer's disease. This fragment, Aβ, along with tau makes up the plaques and tangles that are the hallmark of AD. Iron (and other first-row transition metals) is found associated with these proteinaceous deposits. Much research has focused on the relationship of the plaques and iron to the etiology of the disease. This commentary asks another question, one only more recently addressed namely, what is the physiologic function of the amyloid precursor protein (APP) and of its secretase-generated soluble species? Overall, the data make clear that APP and its products have neurotrophic functions and some data indicate one of these may be to modulate the trafficking of iron in the brain.

Deletion of hephaestin and ceruloplasmin induces a serious systemic iron deficiency and disrupts iron homeostasis

Multi-copper ferroxidases (MCFs) play important roles in cellular iron metabolism and homeostasis. In this study, we generated the hephaestin (Heph), ceruloplasmin (Cp) single and Heph/Cp double knockout (KO) mice to investigate the roles of MCFs in iron transport among system and vital organs in mice at 4 weeks and 6 months of age. Compared with wild-type (WT) mice, Heph/Cp mice at both ages presented with severe anemia and significantly lower iron level in the serum and spleen, but with significantly higher iron level in the liver, heart, kidney, and duodenal enterocytes. Furthermore, Heph/Cp mice displayed significantly lower level of hepcidin mRNA and transferrin receptor 1 (TFR1) protein expression, but significantly higher level of ferroportin 1 (FPN1) protein expression in the liver than WT mice at 6 months of age. Liver superoxide dismutase (SOD) and glutathione peroxidase (GPx) enzyme activities were significantly lower in Heph/Cp KO mice than WT mice at 6 months of age. Together, our results suggest that ablation of HEPH and CP could lead to severe systemic iron deficiency and local tissue iron overload, which disrupt the whole body iron homeostasis and impact on tissue functions.

Ceruloplasmin and hephaestin jointly protect the exocrine pancreas against oxidative damage by facilitating iron efflux

Little is known about the iron efflux from the pancreas, but it is likely that multicopper ferroxidases (MCFs) are involved in this process. We thus used hephaestin (Heph) and ceruloplasmin (Cp) single-knockout mice and Heph/Cp double-knockout mice to investigate the roles of MCFs in pancreatic iron homeostasis. We found that both HEPH and CP were expressed in the mouse pancreas, and that ablation of either MCF had limited effect on the pancreatic iron levels. However, ablation of both MCFs together led to extensive pancreatic iron deposition and severe oxidative damage. Perls’ Prussian blue staining revealed that this iron deposition was predominantly in the exocrine pancreas, while the islets were spared. Consistent with these results, plasma lipase and trypsin were elevated in Heph/Cp knockout mice, indicating damage to the exocrine pancreas, while insulin secretion was not affected. These data indicate that HEPH and CP play mutually compensatory roles in facilitating iron efflux from the exocrine pancreas, and show that MCFs are able to protect the pancreas against iron-induced oxidative damage.

Iron importers Zip8 and Zip14 are expressed in retina and regulated by retinal iron levels

Intracellular retinal iron accumulation has been implicated in the pathogenesis of age-related macular degeneration (AMD), the leading cause of irreversible blindness among individuals over the age of 50. Ceruloplasmin/hephaestin double knockout mice (Cp/Heph DKO) and hepcidin knockout mice (Hepc KO) accumulate retinal iron and model some features of AMD. Two canonical pathways govern cellular iron import - transferrin-bound iron import and non-transferrin bound iron import. In Cp/Heph DKO and Hepc KO iron-loaded retinas, transferrin-bound iron import is downregulated. Despite this effort to reduce cellular iron burden, iron continues to accumulate in these retinas in an age-dependent manner. Quantitative RT-PCR and Western analysis were used to quantify the expression of three ferrous iron importers, Dmt1, Zip8, and Zip14, in wild-type (Wt), Cp/Heph DKO, and Hepc KO retinas. Zip8 and Zip14 protein levels were analyzed using Western analysis in mice injected intravitreally with either apo- or holo-transferrin to elucidate one possible mechanism of Zip14 regulation in the retina. Both zip8 and zip14 were expressed in the mouse retina. Paradoxically, protein levels of non-transferrin bound iron importers were upregulated in both Cp/Heph DKO and Hepc KO retinas. Intravitreal holo-transferrin injection decreased Zip 14 protein levels. These data indicate that Zip8 and Zip14 may take up increasing amounts of non-transferrin bound iron in these two mouse models of retinal iron accumulation. Their upregulation in these already iron-loaded retinas suggests a vicious cycle leading to toxicity.

Intestinal Iron Transport and its Regulation

In recent years, a number of components of the iron absorption pathway have been identified, greatly increasing our understanding of this important process. These include two molecules involved in brush border iron uptake, the ferric reductase DcytB and the iron transporter DMT1, and two mediating iron transfer to the body, the iron transporter Ireg1 and the ferroxidase hephaestin (Hp). Analysis of the regulation of these molecules has provided us with valuable insights into how the body responds to changes in iron requirements, and has enabled us to re-examine how iron absorption is controlled, and in particular the mucosal block phenomenon. Evidence suggests that the block to absorption that follows a priming dose of iron is the result of elevated intracellular iron levels decreasing the expression of the brush border iron transporter DMT1. Based on these observations, it is possible to propose a general model for the regulation of iron absorption whereby the basolateral transfer step involving Ireg1 and Hp controls the rate of absorption. In this model, DMT1 expression, and hence, brush border uptake, is regulated by local iron levels that are, in turn, determined by the rate of basolateral transfer.

Low expression of ferroxidases is implicated in the iron retention in human atherosclerotic plaques

The effect of iron on the progress of atherosclerosis is still controversial. To explore the relationship between atherosclerotic plaques and iron metabolism and how iron is accumulated in plaque macrophages, we performed Oil red O staining to detect the lipid of the atherosclerotic plaques, enzyme-linked immunosorbent assay to detect the intracellular lipids (total cholesterol, free cholesterol) and serum hepcidin, Western-blot to examine the iron-related proteins, immunohistochemical and immunofluorescence assays to localize ferroportin 1 in macrophages. The contents of serum iron and transferrin saturation were measured. The results confrimed that atherosclerotic plaques were all lipid-rich. Compared to normal vessel wall, atherosclerotic plaques had significantly higher levels of ferritin and ferroportin 1. Strikingly, we found the much lower levels of ferroxidases ceruloplasmin and hephaestin in plaque tissue than the normal vessel, while the content of serum hepcidin, iron and transferrin saturation were similar in these two groups. The novel finding suggests that the inability of ferrous iron to be oxidized into ferric iron might be a potential mechanism for iron retention in plaques.

Quantitative study of iron metabolism-related genes expression in rat

OBJECTIVE

To investigate the multiple iron metabolism-related genes expression, its regulation by iron and the expression correlation among the genes in rat tissues.

METHODS

Two groups (n=30) of Sprague-Dawley female weanling rats were fed with a control diet and an iron deficient diet respectively for 4 weeks. All rats were then sacrificed, and blood and tissue samples were collected. The routine blood examination was performed with a veterinary automatic blood cell analyzer. Elemental iron levels in liver, spleen and serum were determined by atomic absorption spectrophotometry. The mRNA expression of genes was detected by real-time fluorescence quantitative PCR.

RESULTS

After 4 weeks, the hemoglobin (Hb) level and red blood cell (RBC) count were significantly lower in the iron deficient group compared with those in the control group. The iron levels in liver, spleen and serum in the iron deficient group were significantly lower than those in the control group. In reference to small intestine, the relative expression of each iron-related gene varied in the different tissues. Under the iron deficiency, the expression of these genes changed in a tissue-specific manner. The expression of most of the genes significantly correlated in intestine, spleen and lung, but few correlated in liver, heart and kidney.

CONCLUSION

Findings from our study provides new understandings about the relative expression, regulation by iron and correlation among the mRNA expressions of transferrin receptors 1 and 2, divalent metal transporter 1, ferritin, iron regulation proteins 1 and 2, hereditary hemochromatosis protein, hepcidin, ferroportin 1 and hephaestin in intestine, liver, spleen, kidney, heart, and lung of rat.

Ferroportin and exocytoplasmic ferroxidase activity are required for brain microvascular endothelial cell iron efflux

The mechanism(s) of iron flux across the brain microvasculature endothelial cells (BMVEC) of the blood-brain barrier remains unknown. Although both hephaestin (Hp) and the ferrous iron permease ferroportin (Fpn) have been identified in BMVEC, their roles in iron efflux have not been examined. Using a human BMVEC line (hBMVEC), we have demonstrated that these proteins are required for iron efflux from these cells. Expression of both Hp and Fpn protein was confirmed in hBMVEC by immunoblot and indirect immunofluorescence; we show that hBMVEC express soluble ceruloplasmin (Cp) transcript as well. Depletion of endogenous Hp and Cp via copper chelation leads to the reduction of hBMVEC Fpn protein levels as well as a complete inhibition of (59)Fe efflux. Both hBMVEC Fpn protein and (59)Fe efflux activity are restored upon incubation with 6.6 nm soluble plasma Cp. These results are independent of the source of cell iron, whether delivered as transferrin- or non-transferrin-bound (59)Fe. Our results demonstrate that iron efflux from hBMVEC Fpn requires the action of an exocytoplasmic ferroxidase, which can be either endogenous Hp or extracellular Cp.

Molecular mechanisms involved in intestinal iron absorption

Iron is an essential trace metal in the human diet due to its obligate role in a number of metabolic processes. In the diet, iron is present in a number of different forms, generally described as haem (from haemoglobin and myoglobin in animal tissue) and non-haem iron (including ferric oxides and salts, ferritin and lactoferrin). This review describes the molecular mechanisms that co-ordinate the absorption of iron from the diet and its release into the circulation. While many components of the iron transport pathway have been elucidated, a number of key issues still remain to be resolved. Future work in this area will provide a clearer picture regarding the transcellular flux of iron and its regulation by dietary and humoral factors.