Insights on the role of the protein hemojuvelin in the maintenance of systemic iron homeostasis

Iron is an essential nutrient, involved in a wide range of biochemical activities. However, its flexible coordination chemistry and favorable redox potential may render this element potentially toxic. Thus, tight and accurate regulation of iron absorption in humans is critical to prevent systemic excess or deficiency. This complex task is accomplished by hepcidin, a liver-derived peptide hormone that binds to the iron transporter ferroportin and promotes its degradation. This results in a decrease of the iron efflux from duodenal enterocytes, macrophages and hepatocytes into the blood stream. Severalmolecules, such as HFE, TfRs, BMP6 and HJV, are being implicated in the regulation of hepcidin expression. HJV is encoded by the HFE2 gene and functions as a BMP co-receptor, activating hepcidin expression through the BMP/SMAD pathway. It is predominantly expressed in the skeletal muscles and at lower levels in the heartand the liver. Furthermore, a putative muscle-derived soluble Hjv is supposed to circulate in the plasma. Humans bearing pathogenic mutations of the HfE2 gene develop juvenile hemochromatosis, a disease characterized by profound hepcidindeficiency and early-onset severe iron overload. Mouse models with ubiquitous disruption of the expression of HJV recapitulate the main features of the disease. Previous studies have proposed an essential role for HJV in iron sensing, however, the implicated mechanisms have not been elucidated thus far. In this work we study the function of HJV within the iron metabolism regulatory mechanisms. In chapter II we investigate the role of HJV, expressed in different tissues, in the maintenance of iron homeostasis. We generate two novelmouse models with targeted disruption of the expression of Hjv in liver hepatocytes or skeletal muscle cells and we analyze their phenotype. We show that hepatic HJV suffices to regulate the expression of hepcidin and to maintainsystemic iron homeostasis whereas skeletal muscle HJV is dispensable for systemic iron metabolism. Furthermore, we do not observe any significant iron regulatory function of a putative muscle-derived soluble Hjv.In chapter III we focus our interest on the role of hepatic HJV in iron sensing to hepcidin. To this end, we analyze the molecular responses of HJV-/- and HJV+/+ mice to dietary iron manipulations. We demonstrate that HJV is not essential for iron sensing and it rather functions as an enhancer of the primary iron signal. Furthermore, we provide evidence of iron-dependent regulation of duodenal DMT1 and tissue specific function of hepcidin in the spleen and the duodenum of the studied HJV-/- mice.Le fer est un nutriment essentiel impliqué dans une vaste gamme d'activités biochimiques. Cependant, sa coordination chimique flexible et son potentiel redox favorable rendent cet élément potentiellement toxique. Ainsi, une régulation stricte et précise de l'absorption du fer chez l'homme est essentielle pour prévenir un excès ou une insuffisance systémique. Cette tâche complexe est réalisée par l'hepcidine, une hormone peptidique dérivée du foie qui s'attache au transporteur du fer ferroportine et favorise sa dégradation. Ceci mène à une diminution de l'efflux du fer des entérocytes duodénaux, des macrophages et des hépatocytes dans le flux sanguin. Plusieurs molécules, comme le HFE, le TfR2, leBMP6 et l'HJV, sont impliquées dans la régulation de l'expression de l'hepcidine. HJV est codée par le gène HFE2 et fonctionne comme un co-récepteur de BMP, en activant l'expression d'hepcidine par la voie BMP/SMAD. Elle est principalement exprimée dans les muscles squelettiques et à des niveaux inférieurs dans le coeur et le foie. En outre, un putatif HJV soluble dérivé des muscles est supposé circuler dans le plasma. Les humains portant des mutations pathogènes du gène HFE2 développent l'hémochromatose juvénile, une maladie caractérisée par une carence profonde en hepcidine et une surcharge sévère et précoce en fer. Des modèles de souris avec une perturbation omniprésente de l'expression de l'HJV récapitulent les principales caractéristiques de la maladie. Des études antérieures ont proposé un rôle essentiel pour HJV dans la détection du fer, cependant, les mécanismes impliqués n'ont pas été élucidés à ce jour. Dans ce travail, nous étudions la fonction de HJV dans les mécanismes de régulation du métabolisme du fer. Dans le chapitre II, nous étudions le rôle de HJV, exprimé dans différents tissus, dans le maintien de l'homéostasie du fer.Nous générons deux nouveaux modèles murins avec une perturbation ciblée de l'expression de HJV dans les hépatocytes ou des cellules musculaires squelettiques et nous analysons leur phénotype. Nous montrons que l'HJV hépatique est suffisante pour réguler l'expression de l'hepcidine et de maintenir l'homéostasie du fer systémique alors que l'HJV musculaire est non essentiel pour le métabolisme du fer systémique. De plus, nous n'avons pas observé une fonction significative d'une HJV soluble musculaire hypothétique liée à la régulation du fer.Dans le chapitre III, nous nous intéressons au rôle de l'HJV hépatique dans la détection des altérations du niveau de fer et la transmission subséquente de ce signal sur l'expression de l'hepcidine. À cette fin, nous analysons les réponsesmoléculaires des souris HJV-/- et HJV+/+ à des manipulations alimentaires de fer. Nous démontrons que HJV n'est pas essentielle pour la détection des variations des niveaux ferriques mais qu'elle fonctionne plutôt comme un amplificateur dusignal primaire de fer. En outre, nous fournissons des preuves d'une régulation du DMT1 dépendante du fer et une fonction de l'hepcidine spécifique au tissu, dans le duodénum et la rate des souris HJV-/ - étudiées

Hépatite B : nouvelles recommandations de prise en charge

Hepatitis B virus (HBV) infection is a major public health concern associated with major clinical complications, notably chronic liver disease that can progress with time to cirrhosis or even to hepatocellular carcinoma. The management of HBV-infected patients is complex and requires the close collaboration between the general practitioner and the specialist. This review presents an overview of recently published guidelines, from the European Association for the Study of the Liver, and suggests strategies for initial management and referral of HBV-infected patients for the general practitioner

Hemojuvelin deficiency promotes liver mitochondrial dysfunction and predisposes mice to hepatocellular carcinoma

Hemojuvelin (HJV) enhances signaling to the iron hormone hepcidin and its deficiency causes iron overload, a risk factor for hepatocellular carcinoma (HCC). We utilized Hjv(-/-) mice to dissect mechanisms for hepatocarcinogenesis. We show that suboptimal treatment with diethylnitrosamine (DEN) triggers HCC only in Hjv(-/-) but not wt mice. Liver proteomics data were obtained by mass spectrometry. Hierarchical clustering analysis revealed that Hjv deficiency and DEN elicit similar liver proteomic responses, including induction of mitochondrial proteins. Dietary iron overload of wt mice does not recapitulate the liver proteomic phenotype of Hjv(-/-) animals, which is only partially corrected by iron depletion. Consistent with these data, primary Hjv(-/-) hepatocytes exhibit mitochondrial hyperactivity, while aged Hjv(-/-) mice develop spontaneous HCC. Moreover, low expression of HJV or hepcidin (HAMP) mRNAs predicts poor prognosis in HCC patients. We conclude that Hjv has a hepatoprotective function and its deficiency in mice promotes mitochondrial dysfunction and hepatocarcinogenesis.Hemojuvelin (HJV), a BMP co-receptor promoting hepcidin expression in the liver, has a hepatoprotective function and its deficiency in mice triggers mitochondrial dysfunction and hepatocarcinogenesis

Iron-Dependent Regulation of Hepcidin in Hjv−/− Mice: Evidence That Hemojuvelin Is Dispensable for Sensing Body Iron Levels

<div><p>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.</p></div

List of primers used for qPCR or genotyping.

<p>List of primers used for qPCR or genotyping.</p

Effects of dietary iron manipulations on hepatic and splenic iron content.

<p>Livers and spleens from the Hjv−/− and wild type mice described in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0085530#pone-0085530-g001" target="_blank">Fig. 1</a> were used for histological detection of iron by staining with Perls’ Prussian blue, and for tissue iron quantification by the ferrozine assay. (A) Visualization of ferric deposits in representative liver sections (magnification: 10×). (B) Quantification of non-heme hepatic iron. (C) Visualization of ferric deposits in representative spleen sections (original magnification: 10×). (D) Quantification of non-heme splenic iron. Data in (B) and (D) are presented as the mean ± SEM. The p values were calculated by using one-way ANOVA with Bonferroni post-test correction. Detailed statistical analysis is provided in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0085530#pone.0085530.s006" target="_blank">Table S1</a>.</p

Hjv−/− mice exhibit elevated serum iron indices independently of dietary iron intake.

<p>Ten-week old male Hjv−/− and wild type mice (n = 10 for each group) in C57BL/6 background were placed on diets with variable iron content (low: 75–100 ppm; normal: 225 ppm; high: 225 ppm plus 2% carbonyl iron). After four weeks the animals were sacrificed and sera were analyzed for iron (A), transferrin saturation (B), and ferritin (C). Data are presented as the mean ± SEM. Statistical analysis is provided in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0085530#pone.0085530.s006" target="_blank">Table S1</a>.</p

Residual iron-dependent regulation of hepatic hepcidin mRNA expression in Hjv−/− mice.

<p>RNA was extracted from tissues of the Hjv−/− and wild type mice described in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0085530#pone-0085530-g001" target="_blank">Fig. 1</a> and used for qPCR analysis. (A) Expression of hepatic hepcidin mRNA. (B) Expression of splenic hepcidin mRNA. Note that absolute hepcidin mRNA levels in the spleen are >100 times lower than in the liver. Data are presented as the mean ± SEM. The p values were calculated by using one-way ANOVA with Bonferroni post-test correction. Detailed statistical analysis is provided in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0085530#pone.0085530.s006" target="_blank">Table S1</a>.</p

Serum and liver iron indices in wild type and Hjv−/− mice of 129S6/SvEvTac or C57BL/6 genetic background (n = 10 male C57BL/6 mice for each genotype; n = 5 male 129S6/SvEvTac mice for each genotype).

<p>p<0.05;</p><p>p<0.01;</p><p>p<0.001 vs 129S6/SvEvTac mice of the same genotype (Student’s t test).</p><p>All differences among wild type and Hjv−/− mice of the same strain are statistically significant (p values not shown).</p