Conditional disruption of mouse HFE2 gene: Maintenance of systemic iron homeostasis requires hepatic but not skeletal muscle hemojuvelin

Gkouvatsos, Konstantinos; Wagner, John; Papanikolaou, George; Sebastiani, Giada; Pantopoulos, Kostas

doi:10.1002/hep.24547

Cited by 50 publications

(56 citation statements)

References 46 publications

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“…Expression of Hjv in hepatocytes of Hjv-null mice is able to correct the defects of iron metabolism (13). In agreement with this finding, studies in mice with tissue-specific Hjv knockdown demonstrate that hepatocyte-specific expression of Hjv is essential for hepcidin expression, whereas skeletal muscle Hjv is not (14,15).…”

supporting

confidence: 59%

Neogenin Interacts with Matriptase-2 to Facilitate Hemojuvelin Cleavage

Enns

Ahmed

Zhang

2012

Journal of Biological Chemistry

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Hemojuvelin (HJV) and matriptase-2 (MT2) are co-expressed in hepatocytes, and both are essential for systemic iron homeostasis. HJV is a glycosylphosphatidylinositol-linked membrane protein that acts as a co-receptor for bone morphogenetic proteins to induce hepcidin expression. MT2 regulates the levels of membrane-bound HJV in hepatocytes by binding to and cleaving HJV into an inactive soluble form that is released from cells. HJV also interacts with neogenin, a ubiquitously expressed transmembrane protein with multiple functions. In this study, we showed that neogenin interacted with MT2 as well as with HJV and facilitated the cleavage of HJV by MT2. In contrast, neogenin was not cleaved by MT2, indicating some degree of specificity by MT2. Down-regulation of neogenin with siRNA increased the amount of MT2 and HJV on the plasma membrane, suggesting a lack of neogenin involvement in their trafficking to the cell surface. The increase in MT2 and HJV upon neogenin knockdown was likely due to the inhibition of cell surface MT2 and HJV internalization. Analysis of the Asn-linked oligosaccharides showed that MT2 cleavage of cell surface HJV was coupled to a transition from high mannose oligosaccharides to complex oligosaccharides on HJV. These results suggest that neogenin forms a ternary complex with both MT2 and HJV at the plasma membrane. The complex facilitates HJV cleavage by MT2, and release of the cleaved HJV from the cell occurs after a retrograde trafficking through the TGN/Golgi compartments.

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supporting

confidence: 59%

Neogenin Interacts with Matriptase-2 to Facilitate Hemojuvelin Cleavage

Enns

Ahmed

Zhang

2012

Journal of Biological Chemistry

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“…Loss of HJV causes severe cases of iron loading in humans termed juvenile hemochromatosis (22), and mouse models confirmed that ablation of HJV (23,24), specifically in hepatocytes (25,26), leads to extreme iron overload due to depressed hepatic hepcidin expression. Furthermore, the ligand BMP-6 is essential for appropriate HJV-mediated hepcidin expression (27,28), and serine/threonine type I (predominately ALK3 (29)) and type II (ActRIIA and BMPRII (30)) receptors are required for transmission of this signal.…”

Section: Regulation Of Hepcidin Through the Bone Morphogenetic Proteimentioning

confidence: 93%

Regulation of Iron Metabolism by Hepcidin under Conditions of Inflammation

Schmidt

2015

Journal of Biological Chemistry

139

105

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Iron is a redox-active metal required as a cofactor in multiple metalloproteins essential for a host of life processes. The metal is highly toxic when present in excess and must be strictly regulated to prevent tissue and organ damage. Hepcidin, a molecule first characterized as an antimicrobial peptide, plays a critical role in the regulation of iron homeostasis. Multiple stimuli positively influence the expression of hepcidin, including iron, inflammation, and infection by pathogens. In this Minireview, I will discuss how inflammation regulates hepcidin transcription, allowing for sufficient concentrations of iron for organismal needs while sequestering the metal from infectious pathogens.Iron is crucial for many life functions in both eukaryotic hosts and prokaryotic pathogens. Due to its ability to readily accept or donate electrons, iron is a valuable cofactor in proteins essential in metabolic processes. However, when left unsupervised, iron can also react with oxygen to generate radical oxygen species that can damage all facets of a cell, leading to tissue damage and eventual organ failure. Therefore, it is crucial for organisms to maintain strict control over iron uptake and distribution to assure appropriate amounts for life requirements, yet regulate and sequester it tightly to prevent oxidative stress or microbial proliferation during infection. Herein, I will present an overview of how iron balance is maintained in vertebrate organisms and discuss the role of hepcidin, the master regulator of iron metabolism, in iron regulation during inflammation and infection. General Iron HomeostasisEach day the average human must absorb 1-2 mg of iron from the diet to offset unregulated losses from general bleeding, menstruation, or the sloughing of epithelial cells. Iron is a critical cofactor required for DNA synthesis, mitochondrial respiration, and various signaling pathways. Most crucially, almost 25 mg of iron per day is required for hemoglobin synthesis and the replacement of an estimated 200 billion RBCs. The vast majority of this iron pool is acquired through the recycling of senescent erythrocytes by macrophages of the reticuloendothelial compartment. Whole body iron homeostasis is thought to occur completely at the level of iron absorption, as no physiologically regulated means of iron excretion has been elucidated. Hence, influx of iron from the diet, and recycling of iron from aged or damaged RBCs, must be closely regulated to prevent iron-restricted erythropoiesis resulting in anemia or excess iron loading and subsequent tissue damage caused by the generation of radical oxygen species. Proper distribution of circulating iron to tissues such as the brain, heart, and skeletal muscle is crucial for the prevention of human disease states.Non-heme dietary reduced iron is admitted into the body through divalent metal transporter 1 (DMT1, Slc11a2) (1, 2), an iron transporter located on the apical membrane of duodenal enterocytes located in the first section of the small intestine ( Fig. 1). After uptake in...

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“…Soluble HJV has been implicated in suppressing hepcidin expression by acting as a decoy to compete with hepatic HJV for BMP6 (11,27). More recent studies in conditional knockout mice indicated that only the hepatic HJV is necessary for iron homeostasis (38,39). However, these results do not rule out the possibility that soluble HJV acts as a suppressor of hepcidin expression.…”

Section: Control Of Hepcidin Expressionmentioning

confidence: 97%

Iron regulation by hepcidin

Zhao

Zhang²,

Enns³

2013

J. Clin. Invest.

195

158

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Discovery and function of hepcidin in iron homeostasisHepcidin is a key peptide hormone that regulates iron homeostasis in chordates. Hepcidin was initially characterized as an antimicrobial peptide in a mass spectrometry-based search for cysteine-rich defensin-like peptides in blood (1) and in urine (2). However, both groups showed that hepcidin displays only a weak antimicrobial activity. Further, unlike defensins, which vary in sequence among species, hepcidin is highly conserved from zebrafish to humans. Shortly after hepcidin was described, subtractive hybridization studies comparing the livers of normal and iron overloaded mice established a connection between iron loading and increased hepcidin mRNA (3, 4). The fundamental insight into hepcidin's role in iron homeostasis came in 2004 with the discovery that hepcidin acts to lower iron in the blood by binding to and downregulating the iron transporter, ferroportin (FPN1) (5). FPN1 is the only known transporter that is responsible for the efflux of iron from cells. Downregulation of FPN1 by hepcidin in splenic and hepatic macrophages decreases the ability of macrophages to export recycled iron from senescent rbcs, which constitute the primary source of iron in the plasma. In addition, a high concentration of hepcidin in the blood decreases the transport of iron out of intestinal epithelial cells, further limiting iron in the blood. Control of hepcidin expressionHepcidin is synthesized, processed to an active form, and secreted predominantly by hepatocytes (6, 7). The expression of hepcidin is mediated through the bone morphogenetic protein (BMP) and JAK2/STAT3 signaling pathways (Figure 1). Under nonpathological conditions, iron levels in the body upregulate hepcidin expression. Although the underlying mechanisms are poorly understood, recent studies have documented the essential roles of hemojuvelin (HJV), hereditary hemochromatosis protein (HFE), transferrin receptor 2 (TfR2), and matriptase-2 (MT2, encoded by the gene TMPRSS6) in the process of hepcidin regulation in humans and animal models as well as of BMP6, neogenin, and BMP receptors (ActRIIA/ALK2/ALK3) in animal models (8)(9)(10)(11)(12)(13)(14)(15)(16)(17).Intact BMP signaling is essential for hepcidin expression. The canonical BMP-signaling pathway is initiated upon BMP binding to a BMP receptor complex on the cell surface, which activates the receptor kinase to phosphorylate the cytoplasmic proteins SMAD1, SMAD5, and SMAD8. These phosphorylated, receptor-regulated SMADs then form transcription factor complexes with SMAD4, consisting of receptor-regulated SMADs and SMAD4, that translocate into the nucleus to induce the transcription of target genes such as hepcidin (18). In mice, liver-specific disruption of SMAD4 or the BMP receptors ALK2 or ALK3 markedly decreased hepcidin expression, resulting in iron overload (15,19).BMP6. At least 20 BMPs are expressed in mammals. In vitro studies reveal that BMP2, -4, -5, -6, -7, and -9 can robustly induce BMP signaling and markedly increase hepcidin expres...

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Conditional disruption of mouse HFE2 gene: Maintenance of systemic iron homeostasis requires hepatic but not skeletal muscle hemojuvelin

Cited by 50 publications

References 46 publications

Neogenin Interacts with Matriptase-2 to Facilitate Hemojuvelin Cleavage

Neogenin Interacts with Matriptase-2 to Facilitate Hemojuvelin Cleavage

Regulation of Iron Metabolism by Hepcidin under Conditions of Inflammation

Iron regulation by hepcidin

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