Effect of phlebotomy on hepcidin expression in hemojuvelin-mutant mice

Differing impact of the deletion of hemochromatosis-associated molecules HFE and transferrin receptor-2 on the iron phenotype of mice lacking bone morphogenetic protein 6 or hemojuvelin

Hereditary hemochromatosis, which is characterized by inappropriately low levels of hepcidin, increased dietary iron uptake, and systemic iron accumulation, has been associated with mutations in the HFE, transferrin receptor-2 (TfR2), and hemojuvelin (HJV) genes. However, it is still not clear whether these molecules intersect in vivo with bone morphogenetic protein 6 (BMP6)/mothers against decapentaplegic (SMAD) homolog signaling, the main pathway up-regulating hepcidin expression in response to elevated hepatic iron. To answer this question, we produced double knockout mice for Bmp6 and β2-microglobulin (a surrogate for the loss of Hfe) and for Bmp6 and Tfr2, and we compared their phenotype (hepcidin expression, Bmp/Smad signaling, hepatic and extrahepatic tissue iron accumulation) with that of single Bmp6-deficient mice and that of mice deficient for Hjv, alone or in combination with Hfe or Tfr2. Whereas the phenotype of Hjv-deficient females was not affected by loss of Hfe or Tfr2, that of Bmp6-deficient females was considerably worsened, with decreased Smad5 phosphorylation, compared with single Bmp6-deficient mice, further repression of hepcidin gene expression, undetectable serum hepcidin, and massive iron accumulation not only in the liver but also in the pancreas, the heart, and the kidneys.

CONCLUSION

These results show that (1) BMP6 does not require HJV to transduce signal to hepcidin in response to intracellular iron, even if the loss of HJV partly reduces this signal, (2) another BMP ligand can replace BMP6 and significantly induce hepcidin expression in response to extracellular iron, and (3) BMP6 alone is as efficient at inducing hepcidin as the other BMPs in association with the HJV/HFE/TfR2 complex; they provide an explanation for the compensatory effect of BMP6 treatment on the molecular defect underlying Hfe hemochromatosis in mice.

HJV and HFE Play Distinct Roles in Regulating Hepcidin

AIMS

Hereditary hemochromatosis (HH) is an iron overload disease that is caused by mutations in HFE, HJV, and several other genes. However, whether HFE-HH and HJV-HH share a common pathway via hepcidin regulation is currently unclear. Recently, some HH patients have been reported to carry concurrent mutations in both the HFE and HJV genes. To dissect the roles and molecular mechanisms of HFE and/or HJV in the pathogenesis of HH, we studied Hfe(-/-), Hjv(-/-), and Hfe(-/-)Hjv(-/-) double-knockout mouse models.

RESULTS

Hfe(-/-)Hjv(-/-) mice developed iron overload in multiple organs at levels comparable to Hjv(-/-) mice. After an acute delivery of iron, the expression of hepcidin (i.e., Hamp1 mRNA) was increased in the livers of wild-type and Hfe(-/-) mice, but not in either Hjv(-/-) or Hfe(-/-)Hjv(-/-) mice. Furthermore, iron-induced phosphorylation of Smad1/5/8 was not detected in the livers of Hjv(-/-) or Hfe(-/-)Hjv(-/-) mice.

INNOVATION

We generated and phenotypically characterized Hfe(-/-)Hjv(-/-) double-knockout mice. In addition, because they faithfully phenocopy clinical HH patients, these mouse models are an invaluable tool for mechanistically dissecting how HFE and HJV regulate hepcidin expression.

CONCLUSIONS

Based on our results, we conclude that HFE may depend on HJV for transferrin-dependent hepcidin regulation. The presence of residual hepcidin in the absence of HFE suggests either the presence of an unknown regulator (e.g., TFR2) that is synergistic with HJV or that HJV is sufficient to maintain basal levels of hepcidin.

mRNA regulation of cardiac iron transporters and ferritin subunits in a mouse model of iron overload

Iron cardiomyopathy is the leading cause of death in iron overload. Men have twice the mortality rate of women, though the cause is unknown. In hemojuvelin-knockout mice, a model of the disease, males load more cardiac iron than females. We postulated that sex differences in cardiac iron import cause differences in cardiac iron concentration. Reverse transcription polymerase chain reaction was used to measure mRNA of cardiac iron transporters in hemojuvelin-knockout mice. No sex differences were discovered among putative importers of nontransferrin-bound iron (L-type and T-type calcium channels, ZRT/IRT-like protein 14 zinc channels). Transferrin-bound iron transporters were also analyzed; these are controlled by the iron regulatory element/iron regulatory protein (IRE/IRP) system. There was a positive relationship between cardiac iron and ferroportin mRNA in both sexes, but it was significantly steeper in females (p < 0.05). Transferrin receptor 1 and divalent metal transporter 1 were more highly expressed in females than males (p < 0.01 and p < 0.0001, respectively), consistent with their lower cardiac iron levels, as predicted by IRE/IRP regulatory pathways. Light-chain ferritin showed a positive correlation with cardiac iron that was nearly identical in males and females (R(2) = 0.41, p < 0.01; R(2) = 0.56, p < 0.05, respectively), whereas heavy-chain ferritin was constitutively expressed in both sexes. This represents the first report of IRE/IRP regulatory pathways in the heart. Transcriptional regulation of ferroportin was suggested in both sexes, creating a potential mechanism for differential set points for iron export. Constitutive heavy-chain-ferritin expression suggests a logical limit to cardiac iron buffering capacity at levels known to produce heart failure in humans.

Sex differences in iron status and hepcidin expression in rats

Studies have shown that men and women exhibit significant differences regarding iron status. However, the effects of sex on iron accumulation and distribution are not well established. In this study, female and male Sprague-Dawley rats were killed at 4 months of age. Blood samples were analyzed to determine the red blood cell (RBC) count, hemoglobin (Hb) concentration, hematocrit (Hct), and mean red blood cell volume (MCV). The serum samples were analyzed to determine the concentrations of serum iron (SI), transferrin saturation (TS), ferritin, soluble transferrin receptor (sTfR), and erythropoietin (EPO). The tissue nonheme iron concentrations were measured in the liver, spleen, bone marrow, kidney, heart, gastrocnemius, duodenal epithelium, lung, pallium, cerebellum, hippocampus, and striatum. Hepatic hepcidin expression was detected by real-time PCR analysis. The synthesis of ferroportin 1 (FPN1) in the liver, spleen, kidney, and bone marrow was determined by Western blot analysis. The synthesis of duodenal cytochrome B561 (DcytB), divalent metal transporter 1 (DMT1), FPN1, hephaestin (HP) in the duodenal epithelium was also measured by Western blot analysis. The results showed that the RBC, Hb, and Hct in male rats were higher than those in female rats. The SI and plasma TS levels were lower in male rats than in female rats. The levels of serum ferritin and sTfR were higher in male rats than in female rats. The EPO levels in male rats were lower than that in female rats. The nonheme iron contents in the liver, spleen, bone marrow, and kidney in male rats were also lower (56.7, 73.2, 60.6, and 61.4 % of female rats, respectively). Nonheme iron concentrations in the heart, gastrocnemius, duodenal epithelium, lung, and brain were similar in rats of both sexes. A moderate decrease in hepatic hepcidin mRNA content was also observed in male rats (to 56.0 % of female rats). The levels of FPN1 protein in the liver, spleen, and kidney were higher in male rats than in female rats. There was no significant change in FPN1 expression in bone marrow. Significant difference was also not found in DcytB, DMT1, FPN1, and HP protein levels in the duodenal epithelium between male and female rats. These data suggest that iron is distributed differently in male and female rats. This difference in iron distribution may be associated with the difference in the hepcidin level.

Testosterone perturbs systemic iron balance through activation of epidermal growth factor receptor signaling in the liver and repression of hepcidin

Gender-related disparities in the regulation of iron metabolism may contribute to the differences exhibited by men and women in the progression of chronic liver diseases associated with reduced hepcidin expression, e.g., chronic hepatitis C, alcoholic liver disease, or hereditary hemochromatosis. However, their mechanisms remain poorly understood. In this study we took advantage of the major differences in hepcidin expression and tissue iron loading observed between Bmp6-deficient male and female mice to investigate the mechanisms underlying this sexual dimorphism. We found that testosterone robustly represses hepcidin transcription by enhancing Egfr signaling in the liver and that selective epidermal growth factor receptor (Egfr) inhibition by gefitinib (Iressa) in males markedly increases hepcidin expression. In males, where the suppressive effects of testosterone and Bmp6-deficiency on hepcidin expression are combined, hepcidin is more strongly repressed than in females and iron accumulates massively not only in the liver but also in the pancreas, heart, and kidneys.

CONCLUSION

Testosterone-induced repression of hepcidin expression becomes functionally important during homeostatic stress from disorders that result in iron loading and/or reduced capacity for hepcidin synthesis. These findings suggest that novel therapeutic strategies targeting the testosterone/EGF/EGFR axis may be useful for inducing hepcidin expression in patients with iron overload and/or chronic liver diseases.

Sex differences and steroid modulation of cardiac iron in a mouse model of iron overload

Iron cardiomyopathy is the leading cause of death in transfusional iron overload, and men have twice the mortality of women. Because the prevalence of cardiac iron overload increases rapidly during the second decade of life, we postulated that there are steroid-dependent sex differences in cardiac iron uptake. To test this hypothesis, we manipulated sex steroids in mice with constitutive iron absorption (homozygous hemojuvelin knockout); this model mimics the myocyte iron deposition observed in humans. At 4 weeks of age, female mice were ovariectomized (OVX) and male mice were castrated (OrchX). Female mice received an estrogen implant (OVX + E) or a cholesterol control (OVX), whereas male mice received an implant containing testosterone (OrchX + T), dihydrotestosterone (OrchX + DHT), estrogen (OrchX + E), or cholesterol (OrchX). All animals received a high-iron diet for 8 weeks. OrchX, OVX, and OVX + E mice all had similar cardiac iron loads. However, OrchX + E males had a significant increase in cardiac iron concentration compared with OrchX mice (P < 0.01), whereas the OrchX + T and OrchX + DHT groups only trended higher (P < 0.06 and P < 0.15, respectively). Hormone treatments did not impact liver iron concentration in either sex. When data were pooled across hormone therapies, liver iron concentration was 25% greater in males than females (P < 0.01). In summary, we found that estrogen increased cardiac iron loading in male mice, but not in females. Male mice loaded 25% more hepatic iron than female mice regardless of the hormone treatment.

Iron-dependent regulation of hepcidin in Hjv-/- mice: evidence that hemojuvelin is dispensable for sensing body iron levels

Hemojuvelin (Hjv) is a bone morphogenetic protein (BMP) co-receptor involved in the control of systemic iron homeostasis. Functional inactivation of Hjv leads to severe iron overload in humans and mice due to marked suppression of the iron-regulatory hormone hepcidin. To investigate the role of Hjv in body iron sensing, Hjv−/− mice and isogenic wild type controls were placed on a moderately low, a standard or a high iron diet for four weeks. Hjv−/− mice developed systemic iron overload under all regimens. Transferrin (Tf) was highly saturated regardless of the dietary iron content, while liver iron deposition was proportional to it. Hepcidin mRNA expression responded to fluctuations in dietary iron intake, despite the absence of Hjv. Nevertheless, iron-dependent upregulation of hepcidin was more than an order of magnitude lower compared to that seen in wild type controls. Likewise, iron signaling via the BMP/Smad pathway was preserved but substantially attenuated. These findings suggest that Hjv is not required for sensing of body iron levels and merely functions as an enhancer for iron signaling to hepcidin.

Mass spectrometry measurement of plasma hepcidin for the prediction of iron overload

BACKGROUND

Hepcidin has emerged as the primary regulator of iron homeostasis. Previous studies on assessing urinary hepcidin are limited. We developed a method for quantifying hepcidin-25 (Hep-25) in plasma using surface-enhanced laser-desorption-ionization time-of-flight mass spectrometry (SELDI-TOF/MS) and a 25-AA peptide as reference standard. The aims of the study were 1) to assess the performance of this method in different conditions of iron metabolism disorders; 2) to assess the diagnostic validity of non-invasive serum biomarkers in the identification of iron overload.

METHODS

Validation of the method was performed in 10 patients with type I hemochromatosis (HE) and in 177 subjects previously enrolled in a general population epidemiological study. Among the latter group, 17 had non-alcoholic fatty liver disease, 10 had chronic hepatitis C, and 150 subjects had normal ultrasound, normal liver function tests (LFTs), an alcohol intake < 20 g ethanol/day and were negative for the C282Y mutation. The following biomarkers were assayed in each case: plasma Hep-25, C282Y and H63D mutations of the HFE gene; serum iron, ferritin (SF), transferrin saturation, transaminases, γ-glutamyltransferase (GGT), glucose, insulin, total cholesterol, high-density lipoprotein (HDL)-cholesterol, low-density lipoprotein (LDL)-cholesterol and triglycerides.

RESULTS

Plasma Hep-25 concentrations were higher in HCV+ patients (26.3 ± 7.2 nmol/L) than in controls, and correlated positively with SF (p < 0.001). H63D heterozygous subjects revealed a pattern of iron overload that was significantly higher than H63D wild type subjects. Analyzing the data with the Biomarker Pattern 5.0.2. software to identify the most significant biomarkers for discriminating between HE cases and controls allowed us to produce an algorithm with four terminal nodes, which included glucose > 4.8 mmol/L and Hep-25/SF ratio ≤ 6.6 as the main splitters. These variables enabled the correct diagnosis of HE with 100% sensitivity, 93% specificity and an area under the receiver operating characteristic (ROC) curve of 0.993.

CONCLUSIONS

Our plasma Hep-25 mass spectrometry method yields measurements that reflect pathological and genetic influences; simple non-invasive biomarkers (Hep-25/SF ratio and glucose) can predict the presence of HE.

Regulation of hepcidin and iron-overload disease

Hepcidin, a 25-amino-acid antimicrobial peptide, is the central regulator of iron homeostasis. Hepcidin transcription is upregulated by inflammatory cytokines, iron, and bone morphogenetic proteins and is downregulated by iron deficiency, ineffective erythropoiesis, and hypoxia. The iron transporter ferroportin is the cognate receptor of hepcidin and is destroyed as a result of interaction with the peptide. Except for inherited defects of ferroportin and hepcidin itself, all forms of iron-storage disease appear to arise from hepcidin dysregulation. Studies using multiple approaches have begun to delineate the molecular mechanisms that regulate hepcidin expression, particularly at the transcriptional level. Knowledge of the regulation of hepcidin by inflammation, iron, erythropoiesis, and hypoxia will lead to an understanding of the pathogenesis of primary hemochromatosis, secondary iron overload, and anemia of inflammatory disease.

Interaction of the hereditary hemochromatosis protein HFE with transferrin receptor 2 is required for transferrin-induced hepcidin expression

The mechanisms that allow the body to sense iron levels in order to maintain iron homeostasis are unknown. Patients with the most common form of hereditary iron overload have mutations in the hereditary hemochromatosis protein HFE. They have lower levels of hepcidin than unaffected individuals. Hepcidin, a hepatic peptide hormone, negatively regulates iron efflux from the intestines into the blood. We report two hepatic cell lines, WIF-B cells and HepG2 cells transfected with HFE, where hepcidin expression responded to iron-loaded transferrin. The response was abolished when endogenous transferrin receptor 2 (TfR2) was suppressed or in primary hepatocytes lacking either functional TfR2 or HFE. Furthermore, transferrin-treated HepG2 cells transfected with HFE chimeras containing only the alpha3 and cytoplasmic domains could upregulate hepcidin expression. Since the HFE alpha3 domain interacts with TfR2, these results supported our finding that TfR2/HFE complex is required for transcriptional regulation of hepcidin by holo-Tf.

The Regulation of Hepcidin and Its Effects on Systemic and Cellular Iron Metabolism

Systemic iron homeostasis depends on the regulated expression of hepcidin, a peptide hormone that negatively regulates iron egress from intestinal cells and macrophages by altering the expression of the cellular iron exporter ferroportin. In doing so, hepcidin can control both the total body iron by modulating intestinal iron absorption as well as promote iron available for erythropoiesis by affecting the efficiency with which macrophages recycle iron from effete red blood cells. This review focuses on the systemic and cellular physiology of hepcidin regulation in relation to iron stores, erythropoiesis, inflammation, and hypoxia and how hepcidin regulation and dysregulation contributes to normal iron homeostasis and iron metabolism disorders.