Normal systemic iron homeostasis in mice with macrophage-specific deletion of transferrin receptor 2

Bone marrow Tfr2 deletion improves the therapeutic efficacy of the activin-receptor ligand trap RAP-536 in β-thalassemic mice

β-thalassemia is a disorder characterized by anemia, ineffective erythropoiesis (IE), and iron overload, whose treatment still requires improvement. The activin receptor-ligand trap Luspatercept, a novel therapeutic option for β-thalassemia, stimulates erythroid differentiation inhibiting the transforming growth factor β pathway. However, its exact mechanism of action and the possible connection with erythropoietin (Epo), the erythropoiesis governing cytokine, remain to be clarified. Moreover, Luspatercept does not correct all the features of the disease, calling for the identification of strategies that enhance its efficacy. Transferrin receptor 2 (TFR2) regulates systemic iron homeostasis in the liver and modulates the response to Epo of erythroid cells, thus balancing red blood cells production with iron availability. Stimulating Epo signaling, hematopoietic Tfr2 deletion ameliorates anemia and IE in Hbbth3/+ thalassemic mice. To investigate whether hematopoietic Tfr2 inactivation improves the efficacy of Luspatercept, we treated Hbbth3/+ mice with or without hematopoietic Tfr2 (Tfr2BMKO/Hbbth3/+) with RAP-536, the murine analog of Luspatercept. As expected, both hematopoietic Tfr2 deletion and RAP-536 significantly ameliorate IE and anemia, and the combined approach has an additive effect. Since RAP-536 has comparable efficacy in both Hbbth3/+ and Tfr2BMKO/Hbbth3/+ animals, we propose that the drug promotes erythroid differentiation independently of TFR2 and EPO stimulation. Notably, the lack of Tfr2, but not RAP-536, can also attenuate iron-overload and related complications. Overall, our results shed further light on the mechanism of action of Luspatercept and suggest that strategies aimed at inhibiting hematopoietic TFR2 might improve the therapeutic efficacy of activin receptor-ligand traps.

Transferrin receptor 2 deficiency promotes macrophage polarization and inflammatory arthritis

OBJECTIVE

Rheumatoid arthritis is an inflammatory joint disease in which synovial iron deposition has been described. Transferrin receptor 2 (Tfr2) represents a critical regulator of systemic iron levels. Loss of Tfr2 function in humans and mice results in iron overload. As iron contributes to inflammatory processes, we investigated whether Tfr2-deletion affects the pathogenesis of inflammatory arthritis in an iron-dependent manner.

METHODS

Using a global and conditional genetic disruption of Tfr2, we assessed the relevance of Tfr2 in K/BxN serum-transfer arthritis (STA) and macrophage polarization.

RESULTS

Male Tfr2^-/- mice subjected to STA developed pronounced joint swelling, and bone erosion as compared to Tfr2^+/+ littermate-controls (P < 0.01). Furthermore, an increase of neutrophils and macrophages/monocytes was observed in the inflammatory infiltrate within the paws of Tfr2^-/- mice. To elucidate whether Tfr2 in myeloid cells has a direct role in the pathogenesis of arthritis or whether the effects were mediated via the systemic iron overload, we induced STA in Tfr2^fl/fl-LysMCre + mice, which showed normal iron-loading. Cre + female mice displayed increased disease development compared to Cre-controls. As macrophages regulate iron availability and innate immunity, we hypothesized that Tfr2-deficiency would polarize macrophages toward a pro-inflammatory state (M1) that contributes to arthritis progression. In response to IFN-γ stimulation, Tfr2^-/- macrophages showed increased expression of M1-like cytokines, IFN-γ-target genes, nitric-oxide production, and prolonged STAT1 activation compared to Tfr2^+/+ macrophages (P < 0.01), while pre-treatment with ruxolitinib abolished Tfr2-driven M1-like polarization.

CONCLUSION

Taken together, these findings suggest a protective role of Tfr2 in macrophages on the progression of arthritis via suppression of M1-like polarization.

Transferrin receptor 1-mediated iron uptake regulates bone mass in mice via osteoclast mitochondria and cytoskeleton

Increased intracellular iron spurs mitochondrial biogenesis and respiration to satisfy high-energy demand during osteoclast differentiation and bone-resorbing activities. Transferrin receptor 1 (Tfr1) mediates cellular iron uptake through endocytosis of iron-loaded transferrin, and its expression increases during osteoclast differentiation. Nonetheless, the precise functions of Tfr1 and Tfr1-mediated iron uptake in osteoclast biology and skeletal homeostasis remain incompletely understood. To investigate the role of Tfr1 in osteoclast lineage cells in vivo and in vitro, we crossed Tfrc (encoding Tfr1)-floxed mice with Lyz2 (LysM)-Cre and Cathepsin K (Ctsk)-Cre mice to generate Tfrc conditional knockout mice in myeloid osteoclast precursors (Tfr1^ΔLysM) or differentiated osteoclasts (Tfr1^ΔCtsk), respectively. Skeletal phenotyping by µCT and histology unveiled a significant increase in trabecular bone mass with normal osteoclast number in long bones of 10-week-old young and 6-month-old adult female but not male Tfr1^ΔLysM mice. Although high trabecular bone volume in long bones was observed in both male and female Tfr1^ΔCtsk mice, this phenotype was more pronounced in female knockout mice. Consistent with this gender-dependent phenomena, estrogen deficiency induced by ovariectomy decreased trabecular bone mass in Tfr1^ΔLysM mice. Mechanistically, disruption of Tfr1 expression attenuated mitochondrial metabolism and cytoskeletal organization in mature osteoclasts in vitro by attenuating mitochondrial respiration and activation of the Src-Rac1-WAVE regulatory complex axis, respectively, leading to decreased bone resorption with little impact on osteoclast differentiation. These results indicate that Tfr1-mediated iron uptake is specifically required for osteoclast function and is indispensable for bone remodeling in a gender-dependent manner.

RNF217 regulates iron homeostasis through its E3 ubiquitin ligase activity by modulating ferroportin degradation

RNF217 functions as a novel E3 ubiquitin ligase, mediating the degradation of the iron exporter FPN and regulating iron homeostasis.

Rnf217 expression is upregulated by Tet1-mediated demethylation; loss of Tet1 stabilizes FPN and impairs the response to iron overload.

Ferroportin (FPN), the body’s sole iron exporter, is essential for maintaining systemic iron homeostasis. In response to either increased iron or inflammation, hepatocyte-secreted hepcidin binds to FPN, inducing its internalization and subsequent degradation. However, the E3 ubiquitin ligase that underlies FPN degradation has not been identified. Here, we report the identification and characterization of a novel mechanism involving the RNF217-mediated degradation of FPN. A combination of 2 different E3 screens revealed that the Rnf217 gene is a target of Tet1, mediating the ubiquitination and subsequent degradation of FPN. Interestingly, loss of Tet1 expression causes an accumulation of FPN and an impaired response to iron overload, manifested by increased iron accumulation in the liver together with decreased iron in the spleen and duodenum. Moreover, we found that the degradation and ubiquitination of FPN could be attenuated by mutating RNF217. Finally, using 2 conditional knockout mouse lines, we found that knocking out Rnf217 in macrophages increases splenic iron export by stabilizing FPN, whereas knocking out Rnf217 in intestinal cells appears to increase iron absorption. These findings suggest that the Tet1-RNF217-FPN axis regulates iron homeostasis, revealing new therapeutic targets for FPN-related diseases.

Evidence for dimerization of ferroportin in a human hepatic cell line using proximity ligation assays

Mutations in the only known iron exporter ferroportin (FPN) in humans are associated with the autosomal dominantly inherited iron overload disorder ferroportin disease or type IV hereditary hemochromatosis (HH). While our knowledge of the central role of FPN in iron homeostasis has grown in the last 20 years, there exist some questions surrounding the structure and membrane topology of FPN with conflicting data on whether this receptor acts as a monomer or a multimer. To investigate and determine if FPN dimerization occurs in cells, we used novel tools including a variety of different FPN constructs expressing different tagged versions of the protein, a novel antibody that only detects cell surface FPN and proximity ligation assays. The results of the present study suggest that both the carboxy- and amino-termini of the FPN protein are intracellular. We also show that exogenously transfected FPN forms dimers; these dimers can be formed between the wild-type and mutant FPN proteins. This is the first study to examine the intracellular dimerization of FPN protein. Using proximity ligation assays, we show intracellular localization of FPN dimers and the interaction between FPN and hepcidin proteins as well. These results have important implications in the field of iron metabolism and add to our knowledge about FPN membrane topology and physiology of iron transport. This will be of importance in understanding the clinical implications of FPN mutations and of interest to future research aimed at targeting FPN expression to modulate iron homeostasis.

Transferrin Receptors in Erythropoiesis

Erythropoiesis is a highly dynamic process giving rise to red blood cells from hematopoietic stem cells present in the bone marrow. Red blood cells transport oxygen to tissues thanks to the hemoglobin comprised of α- and β-globin chains and of iron-containing hemes. Erythropoiesis is the most iron-consuming process to support hemoglobin production. Iron delivery is mediated via transferrin internalization by the endocytosis of transferrin receptor type 1 (TFR1), one of the most abundant membrane proteins of erythroblasts. A second transferrin receptor—TFR2—associates with the erythropoietin receptor and has been implicated in the regulation of erythropoiesis. In erythroblasts, both transferrin receptors adopt peculiarities such as an erythroid-specific regulation of TFR1 and a trafficking pathway reliant on TFR2 for iron. This review reports both trafficking and signaling functions of these receptors and reassesses the debated role of TFR2 in erythropoiesis in the light of recent findings. Potential therapeutic uses targeting the transferrin-TFR1 axis or TFR2 in hematological disorders are also discussed.

Dysregulated hepcidin response to dietary iron in male mice with reduced Gnpat expression

Exome sequencing has identified the glyceronephosphate O-acyltransferase (GNPAT) gene as a genetic modifier of iron overload in hereditary hemochromatosis (HH). Subjects with HFE (Homeostatic Iron Regulator) p.C282Y mutations and the GNPAT p.D519G variant had more iron loading compared with subjects without the GNPAT variant. In response to an oral iron challenge, women with GNPAT polymorphisms loaded more iron as compared with women without polymorphisms, reinforcing a role for GNPAT in iron homeostasis. The aim of the present study was to develop and characterize an animal model of disease to further our understanding of genetic modifiers, and in particular the role of GNPAT in iron homeostasis. We generated an Hfe/Gnpat mouse model reminiscent of the patients previously studied and studied these mice for up to 26 weeks. We also examined the effect of dietary iron loading on mice with reduced Gnpat expression. Gnpat heterozygosity in Hfe knockout mice does not play a role in systemic iron homeostasis; Gnpat^+/− mice fed a high-iron diet, however, had lower hepatic hepcidin (HAMP) mRNA expression, whereas they have significantly higher serum iron levels and transferrin saturation compared with wildtype (WT) littermates on a similar diet. These results reinforce an independent role of GNPAT in systemic iron homeostasis, reproducing in an animal model, the observations in women with GNPAT polymorphisms subjected to an iron tolerance test.

Hepatocyte-specific deletion of peroxisomal protein PEX13 results in disrupted iron homeostasis

Peroxisomes are organelles, abundant in the liver, involved in a variety of cellular functions, including fatty acid metabolism, plasmalogen synthesis and metabolism of reactive oxygen species. Several inherited disorders are associated with peroxisomal dysfunction; increasingly many are associated with hepatic pathologies. The liver plays a principal role in regulation of iron metabolism. In this study we examined the possibility of a relationship between iron homeostasis and peroxisomal integrity. We examined the effect of deleting Pex13 in mouse liver on systemic iron homeostasis. We also used siRNA-mediated knock-down of PEX13 in a human hepatoma cell line (HepG2/C3A) to elucidate the mechanisms of PEX13-mediated regulation of hepcidin. We demonstrate that transgenic mice lacking hepatocyte Pex13 have defects in systemic iron homeostasis. The ablation of Pex13 expression in hepatocytes leads to a significant reduction in hepatic hepcidin levels. Our results also demonstrate that a deficiency of PEX13 gene expression in HepG2/C3A cells leads to decreased hepcidin expression, which is mediated through an increase in the signalling protein SMAD7, and endoplasmic reticulum (ER) stress. This study identifies a novel role for a protein involved in maintaining peroxisomal integrity and function in iron homeostasis. Loss of Pex13, a protein important for peroxisomal function, in hepatocytes leads to a significant increase in ER stress, which if unresolved, can affect liver function. The results from this study have implications for the management of patients with peroxisomal disorders and the liver-related complications they may develop.

Transferrin and transferrin receptors update

In vertebrates, transferrin (Tf) safely delivers iron through circulation to cells. Tf-bound iron is incorporated through Tf receptor (TfR) 1-mediated endocytosis. TfR1 can mediate cellular uptake of both Tf and H-ferritin, an iron storage protein. New World arenaviruses, which cause hemorrhagic fever, and Plasmodium vivax use TfR1 for entry into host cells. Human TfR2, another receptor for Tf, is predominantly expressed in hepatocytes and erythroid precursors, and holo-Tf dramatically upregulates its expression. TfR2 forms a complex with hemochromatosis protein, HFE, and serves as a component of the iron sensing machinery in hepatocytes. Defects in TfR2 cause systemic iron overload, hemochromatosis, through down-regulation of hepcidin. In erythroid cells, TfR2 forms a complex with the erythropoietin receptor and regulates erythropoiesis. TfR2 facilitates iron transport from lysosomes to mitochondria in erythroblasts and dopaminergic neurons. Administration of apo-Tf, which scavenges free iron, has been explored for various clinical conditions including atransferrinemia, iron overload, and tissue ischemia. Apo-Tf has also been shown to ameliorate anemia in animal models of β-thalassemia. In this review, I provide an update and summary on our knowledge of mammalian Tf and its receptors.

Transferrin receptor 2 controls bone mass and pathological bone formation via BMP and Wnt signaling

Transferrin receptor 2 (Tfr2) is mainly expressed in the liver and controls iron homeostasis. Here, we identify Tfr2 as a regulator of bone homeostasis that inhibits bone formation. Mice lacking Tfr2 display increased bone mass and mineralization independent of iron homeostasis and hepatic Tfr2. Bone marrow transplantation experiments and studies of cell-specific Tfr2 knockout mice demonstrate that Tfr2 impairs BMP-p38MAPK signaling and decreases expression of the Wnt inhibitor sclerostin specifically in osteoblasts. Reactivation of MAPK or overexpression of sclerostin rescues skeletal abnormalities in Tfr2 knockout mice. We further show that the extracellular domain of Tfr2 binds BMPs and inhibits BMP-2-induced heterotopic ossification by acting as a decoy receptor. These data indicate that Tfr2 limits bone formation by modulating BMP signaling, possibly through direct interaction with BMP either as a receptor or as a co-receptor in a complex with other BMP receptors. Finally, the Tfr2 extracellular domain may be effective in the treatment of conditions associated with pathological bone formation.

Transferrin receptor 2 is a potential novel therapeutic target for β-thalassemia: evidence from a murine model

β-thalassemias are genetic disorders characterized by anemia, ineffective erythropoiesis, and iron overload. Current treatment of severe cases is based on blood transfusion and iron chelation or allogeneic bone marrow (BM) transplantation. Novel approaches are explored for nontransfusion-dependent patients (thalassemia intermedia) who develop anemia and iron overload. Here, we investigated the erythropoietin (EPO) receptor partner, transferrin receptor 2 (TFR2), as a novel potential therapeutic target. We generated a murine model of thalassemia intermedia specifically lacking BM Tfr2: because their erythroid cells are more susceptible to EPO stimulation, mice show improved erythropoiesis and red blood cell morphology as well as partial correction of anemia and iron overload. The beneficial effects become attenuated over time, possibly due to insufficient iron availability to sustain the enhanced erythropoiesis. Germ line deletion of Tfr2, including haploinsufficiency, had a similar effect in the thalassemic model. Because targeting TFR2 enhances EPO-mediated effects exclusively in cells expressing both receptors, this approach may have advantages over erythropoiesis-stimulating agents in the treatment of other anemias.

The peritoneum: healing, immunity, and diseases

The peritoneum defines a confined microenvironment, which is stable under normal conditions, but is exposed to the damaging effect of infections, surgical injuries, and other neoplastic and non-neoplastic events. Its response to damage includes the recruitment, proliferation, and activation of a variety of haematopoietic and stromal cells. In physiological conditions, effective responses to injuries are organized; inflammatory triggers are eliminated; inflammation quickly abates; and the normal tissue architecture is restored. However, if inflammatory triggers are not cleared, fibrosis or scarring occurs and impaired tissue function ultimately leads to organ failure. Autoimmune serositis is characterized by the persistence of self-antigens and a relapsing clinical pattern. Peritoneal carcinomatosis and endometriosis are characterized by the persistence of cancer cells or ectopic endometrial cells in the peritoneal cavity. Some of the molecular signals orchestrating the recruitment of inflammatory cells in the peritoneum have been identified in the last few years. Alternative activation of peritoneal macrophages was shown to guide angiogenesis and fibrosis, and could represent a novel target for molecular intervention. This review summarizes current knowledge of the alterations to the immune response in the peritoneal environment, highlighting the ambiguous role played by persistently activated reparative macrophages in the pathogenesis of common human diseases. Copyright © 2017 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

The liver in regulation of iron homeostasis

The liver is one of the largest and most functionally diverse organs in the human body. In addition to roles in detoxification of xenobiotics, digestion, synthesis of important plasma proteins, gluconeogenesis, lipid metabolism, and storage, the liver also plays a significant role in iron homeostasis. Apart from being the storage site for excess body iron, it also plays a vital role in regulating the amount of iron released into the blood by enterocytes and macrophages. Since iron is essential for many important physiological and molecular processes, it increases the importance of liver in the proper functioning of the body's metabolism. This hepatic iron-regulatory function can be attributed to the expression of many liver-specific or liver-enriched proteins, all of which play an important role in the regulation of iron homeostasis. This review focuses on these proteins and their known roles in the regulation of body iron metabolism.

Postmenopausal Iron Overload Exacerbated Bone Loss by Promoting the Degradation of Type I Collagen

117 postmenopausal women were divided into Normal, Bone loss (BL), and Osteoporosis group. Compared with Normal group (120.96 ± 43.18 μg/L), the serum ferritin (Fer) in BL (223.37 ± 130.27 μg/L) and Osteoporosis group (307.50 ± 161.48 μg/L) was significantly increased (p < 0.05). Fer level was negatively correlated with BMD (p < 0.01). TRACP levels in Osteoporosis group (4.37 ± 1.69 U/L) were significantly higher than Normal group (4.10 ± 1.60 U/L, p < 0.05). ALP levels in Osteoporosis group (112.06 ± 62.05 U/L) were significantly upregulated compared with Normal group (80.22 ± 14.94 U/L, p < 0.05). β-CTX and PINP were the degradation products of type I collagen. β-CTX levels in Osteoporosis group (667.90 ± 316.55 ng/L) were significantly increased compared with Normal group (406.06 ± 112.12 ng/L, p < 0.05). PINP levels in Osteoporosis group (78.03 ± 37.31 μg/L) were significantly higher than Normal group (37.60 ± 13.17 μg/L, p < 0.01). More importantly, there was a positive correlation between serum Fer and PINP (p < 0.01). Serum Fer showed a positive correlation of serum β-CTX (p < 0.01). The overloaded iron improved the degradation of type I collagen.