Expression of hepcidin is down-regulated in TfR2 mutant mice manifesting a phenotype of hereditary hemochromatosis

Abstract

Transferrin receptor 2 (TfR2) is a membrane glycoprotein that mediates cellular iron uptake from holotransferrin. Homozygous mutations of this gene cause one form of hereditary hemochromatosis in humans. We recently reported that homozygous TfR2(Y245X) mutant mice, which correspond to the TfR2(Y250X) mutation in humans, showed a phenotype similar to hereditary hemochromatosis. In this study, we further analyzed the phenotype as well as iron-related gene expression in these mice by comparing the TfR2-mutant and wild-type siblings. Northern blot analyses showed that the levels of expression of hepcidin mRNA in the liver were generally lower, whereas those of duodenal DMT1, the main transporter for uptake of dietary iron, were higher in the TfR2-mutant mice as compared to the wild-type siblings. Expression of hepcidin mRNA in the TfR2 mutant mice remained low even after intraperitoneal iron loading. In isolated hepatocytes from both wild-type and TfR2 mutant mice, interleukin-6 and lipopolysaccharide each induced expression of hepcidin mRNA. These results suggest that up-regulation of hepcidin expression by inflammatory stimuli is independent of TfR2 and that TfR2 is upstream of hepcidin in the regulatory pathway of body iron homeostasis. IntroductionHereditary hemochromatosis (HH) is a group of genetic disorders that manifest iron deposition in a variety of organs such as the liver, pancreas, heart, and skin. If untreated, liver cirrhosis, heart failure, and diabetes can develop. Most HH is caused by mutations in the HFE gene. 1 The frequency of homozygous C282Y mutation of this gene is estimated to be 1 in 150 in people of northern European descent, 2 though its clinical penetrance is low. 3 Mutations in several other genes also produce an HH phenotype, including hemojuvelin (HFE2/HJV), 4 hepcidin (hepatic antimicrobial peptide: HAMP), 5 and transferrin receptor 2 (TfR2). 6 The phenotypes caused by mutations of these genes are similar, manifesting increased transferrin (Tf) saturations, periportal hepatic iron loading, and reticuloendothelial iron sparing. This observation suggests that the products of these genes are on a common pathway for regulation of iron homeostasis.TfR2 protein is a membrane glycoprotein that can interact with Tf. 7 Human TfR2 has at least 2 alternatively spliced transcripts, ␣ and ␤. TfR2-␣ is the membrane-bound form predominantly expressed in the liver, whereas TfR2-␤ is a form that consists of only the extracellular domain of TfR2-␣. Similar to TfR1, TfR2-␣ interacts with holo-Tf but not with apo-Tf at neutral pH. 8 Expression of TfR2 mRNA almost exclusively occurs in the liver and erythroid precursor cells. 9 Homozygosity for one of several mutations in the TfR2 gene, including the truncation mutation Y250X, has been associated with hereditary hemochromatosis in humans. 6 In addition, we recently reported that homozygous TfR2(Y245X) mutant mice, which correspond to the TfR2(Y250X) mutation in humans, showed hepatocellular iron deposition with elevated serum Tf saturations. 10 However,...

Transferrin Receptor 2: Evidence for Ligand-induced Stabilization and Redirection to a Recycling Pathway

“…Because TfR2 is expressed in hepatocytes, it is positioned to regulate the expression of hepcidin, a small peptide hormone synthesized and secreted by hepatocytes that controls systemic iron levels by modulating cellular iron efflux (Nicolas et al, 2001(Nicolas et al, , 2002Pigeon et al, 2001;Roetto et al, 2003;Nemeth et al, 2004). Consistent with this hypothesis, individuals and mice with disease-causing mutations in TfR2 fail to regulate hepcidin appropriately (Kawabata et al, 2005;Nemeth et al, 2005). TfR2 might, therefore, sense systemic iron levels through interaction with its ligand.…”

Defective trafficking and localization of mutated transferrin receptor 2: implications for type 3 hereditary hemochromatosis

“…It appears that the hepatocytes are the primary site for TfR2 expression and activity, because hepatocyte-specific deletion of TfR2 in mice leads to a similar iron overload phenotype as global deletion (17,18,25,28,33,35). TfR2 is also involved in the regulation of hepcidin (18,25,33), and it is likely that TfR2 activity is modulated by an interaction with transferrin. Two studies have shown that the TfR2 protein is stabilized by diferric transferrin (17,28).…”

“…Recently, we have shown that a liver-specific deletion of the mouse TfR2 gene is sufficient to induce iron overload in mice (35). These and other studies have shown that liver-expressed TfR2 is important for maintaining iron homeostasis, through its regulation of the iron-regulatory hormone hepcidin (6,18,33,35). At a cellular level, however, it is still not clear how TfR2 is involved in the regulation of iron metabolism.…”

Iron‐ and inflammation‐induced hepcidin gene expression in mice is not mediated by Kupffer cells in vivo†

“…Mutant forms of these proteins are responsible for different types of hemochromatosis, the most prevalent genetic disorder characterized by the deregulation of iron absorption (for review, see Fleming and Sly 36 and Pietrangelo 37 ). Interestingly, a lack of an appropriate hepcidin response to iron loading was demonstrated for the three types of hemochromatosis (HFE1, 19,20,38,39 TfR2, 40,41 and HFE2 42 ), suggesting that hepcidin is the common pathogenic denominator in iron overload syndromes. The role of these proteins in mediating hepcidin response to inflammation is less clear.…”