Hemojuvelin is essential for dietary iron sensing, and its mutation leads to severe iron overload

A Recombinant Antibody Against ALK2 Promotes Tissue Iron Redistribution and Contributes to Anemia Resolution in a Mouse Model of Anemia of Inflammation

Patients with chronic inflammation are burdened with anemia of inflammation (AI), where inflammatory cytokines inhibit erythropoiesis, impede erythropoietin production, and limit iron availability by inducing the iron regulator hepcidin. High hepcidin hinders iron absorption and recycling, thereby worsening the impaired erythropoiesis by restricting iron availability. AI management is important as anemia impacts quality of life and potentially affects morbidity and mortality. The bone morphogenetic protein (BMP)-SMAD pathway is crucial for hepcidin regulation. Here, we characterized a research antibody against BMP receptor ALK2, RKER-216, and investigated its mechanism in suppressing hepcidin and improving anemia in acute/chronic inflammation. Additive effects of RKER-216 and recombinant human erythropoietin (rhEPO) on erythropoiesis and iron utilization were also explored. We showed that RKER-216 neutralized ALK2 activity by competing with the binding of BMP6. RKER-216 reduced hepcidin transcription in Hep3B cells, and a subcutaneous dose of RKER-216 at 3 mg/kg suppressed serum hepcidin and increased circulating iron for 3-4 days in wildtype mice. Moreover, RKER-216 decreased hepcidin by inhibiting SMAD1/5/9 signaling in lipopolysaccharide-mediated inflammation and liberated iron from the recycling pathway to alleviate anemia in mice with adenine-induced chronic kidney disease (CKD), a mouse model of AI. Finally, RKER-216 reversed iron-restricted erythropoiesis in CKD mice and supplied the iron requirement for complete resolution of anemia when coupled with rhEPO in addressing AI. Our data support that ALK2 is a key hepcidin regulator and that a neutralizing ALK2 antibody has the potential to restore iron homeostasis as monotherapy or in combination with rhEPO to ameliorate AI.

Hepcidin expression is associated with increased γ-secretase-mediated cleavage of neogenin in the liver

Neogenin (NEO1) is a ubiquitously expressed transmembrane protein. It interacts with hemojuvelin (HJV). Both NEO1 and HJV play pivotal roles in iron homeostasis by inducing hepcidin expression in the liver. Our previous studies demonstrated that this process depends on Neo1-Hjv interaction and showed that the Hjv-mediated hepcidin expression is correlated with the accumulation of a truncated and membrane-associated form of Neo1. In this study, we tested whether hepcidin expression is induced by increased γ-secretase-mediated cleavage of Neo1 in the liver. We found that Neo1 underwent cleavage of its ectodomain and intracellular domains by α- and γ-secretases, respectively, in hepatoma cells. Our in vitro studies suggest that γ-secretase is responsible for cleavage and release of the cytoplasmic domain of Neo1 in the Hjv-Neo1 complex. This process was enhanced by the inhibition of α-secretase proteolysis and by co-expression with the Neo1-binding partner, Alk3. Further in vivo studies indicated that Neo1 induction of hepcidin expression required γ-secretase cleavage. Interestingly, neither predicted form of γ-secretase-cleaved Neo1 was able to induce hepcidin when separately expressed in hepatocyte-specific Neo1 KO mice. These results imply that the function of Neo1 requires a de novo γ-secretase proteolysis. Additional studies revealed that in addition to the Hjv-binding domains, the function of Neo1 also required its C-terminal intracellular domain and the N-terminal immunoglobulin-like domains that are involved in Neo1 binding to Alk3. Together, our data support the idea that Neo1 induction of hepcidin is initiated as a full-length form and requires a de novo γ-secretase cleavage of Neo1's cytoplasmic domain.

Hemojuvelin-mediated hepcidin induction requires both bone morphogenetic protein type I receptors ALK2 and ALK3

ABSTRACT

Hemojuvelin (HJV) is a glycosylphosphatidylinositol-anchored protein of the repulsive guidance molecule family acting as a bone morphogenetic protein (BMP) coreceptor to induce the hepatic iron regulatory protein hepcidin. Hepcidin causes ubiquitination and degradation of the sole known iron exporter ferroportin, thereby limiting iron availability. The detailed signaling mechanism of HJV in vivo has yet to be investigated. In the current manuscript, we used an established model of adeno-associated virus (AAV)-mediated liver-specific overexpression of HJV in murine models of hepatocyte-specific deficiency of the BMP type I receptors Alk2 or Alk3. In control mice, HJV overexpression increased hepatic Hamp messenger RNA (mRNA) levels, soluble HJV (sHJV), splenic iron content (SIC), as well as phosphorylated small mothers against decapentaplegic protein (pSMAD1/5/8) levels. In contrast, in Alk2fl/fl;Alb-Cre and Alk3fl/fl;Alb-Cre mice, which present with moderate and severe iron overload, respectively, the administration of AAV-HJV induced HJV and sHJV. However, it did not rescue the iron overload phenotypes of those mice. Serum iron levels were induced in Alk2fl/fl;Alb-Cre mice after HJV overexpression. In phosphate-buffered saline-injected Alk3fl/fl;Alb-Cre mice, serum iron levels and the expression of duodenal ferroportin remained high, whereas Hamp mRNA levels were decreased to 1% to 5% of the levels detected in controls. This was reduced even further by AAV-HJV overexpression. SIC remained low in mice with hepatocyte-specific Alk2 or Alk3 deficiency, reflecting disturbed iron homeostasis with high serum iron levels and transferrin saturation and an inability to induce hepcidin by HJV overexpression. The data indicate that ALK2 and ALK3 are both required in vivo for the HJV-mediated induction of hepcidin.

Thyroid dysfunction alters gene expression of proteins related to iron homeostasis and metabolomics in male rats

Thyroid hormones (THs) are crucial in bodily functions, while iron is essential for processes like oxygen transport. Specialized proteins maintain iron balance, including ferritin, transferrin, ferroportin, and hepcidin. Research suggests that THs can influence iron homeostasis by affecting mRNA and protein expression, such as ferritin and transferrin. Our study focused on male rats to assess mRNA expression of iron homeostasis-related proteins and metabolomics in thyroid dysfunction. We found altered gene expression across various tissues (liver, duodenum, spleen, and kidney) and identified disrupted metabolite patterns in thyroid dysfunction. These findings highlight tissue-specific effects of thyroid dysfunction on essential iron homeostasis proteins and provide insights into associated metabolic changes. Our research contributes to understanding the intricate interplay between thyroid hormones and iron balance. By unveiling tissue-specific gene expression alterations and metabolic disruptions caused by thyroid dysfunction, our work lays a foundation for future investigations to explore underlying mechanisms and develop targeted strategies for managing iron-related complications in thyroid disorders.

Low circulatory Fe and Se levels with a higher IL-6/IL-10 ratio provide nutritional immunity in tuberculosis

Tuberculosis (TB) patients show dysregulated immunity, iron metabolism, and anemia. In this study, circulatory cytokines, trace metals, and iron-related proteins (hepcidin, ferroportin, transferrin, Dmt1, Nramp1, ferritin, ceruloplasmin, hemojuvelin, aconitase, and transferrin receptor) were monitored in case (active tuberculosis patients: ATB) and control (non-tuberculosis: NTB and healthy) study populations (n = 72, male: 100%, mean age, 42.94 years; range, 17-83 years). Using serum elemental and cytokine levels, a partial least square discriminate analysis model (PLS-DA) was built, which clustered ATB patients away from NTB and healthy controls. Based on the PLS-DA variable importance in projection (VIP) score and analysis of variance (ANOVA), 13 variables were selected as important biosignatures [IL-18, IL-10, IL-13, IFN-γ, TNF-α, IL-5, IL-12 (p70), IL-1β, copper, zinc, selenium, iron, and aluminum]. Interestingly, low iron and selenium levels and high copper and aluminum levels were observed in ATB subjects. Low circulatory levels of transferrin, ferroportin, and hemojuvelin with higher ferritin and ceruloplasmin levels observed in ATB subjects demonstrate an altered iron metabolism, which partially resolved upon 6 months of anti-TB therapy. The identified biosignature in TB patients demonstrated perturbed iron homeostasis with anemia of inflammation, which could be useful targets for the development of host-directed adjunct therapeutics.

Heart Ferroportin Protein Content Is Regulated by Heart Iron Concentration and Systemic Hepcidin Expression

The purpose of the study was to investigate the expression of ferroportin protein following treatments that affect systemic hepcidin. Administration of erythropoietin to C57BL/6J mice decreased systemic hepcidin expression; it also increased heart ferroportin protein content, determined by immunoblot in the membrane fraction, to approximately 200% of control values. This increase in heart ferroportin protein is very probably caused by a decrease in systemic hepcidin expression, in accordance with the classical regulation of ferroportin by hepcidin. However, the control of heart ferroportin protein by systemic hepcidin could apparently be overridden by changes in heart non-heme iron content since injection of ferric carboxymaltose to mice at 300 mg Fe/kg resulted in an increase in liver hepcidin expression, heart non-heme iron content, and also a threefold increase in heart ferroportin protein content. In a separate experiment, feeding an iron-deficient diet to young Wistar rats dramatically decreased liver hepcidin expression, while heart non-heme iron content and heart ferroportin protein content decreased to 50% of controls. It is, therefore, suggested that heart ferroportin protein is regulated primarily by the iron regulatory protein/iron-responsive element system and that the regulation of heart ferroportin by the hepcidin-ferroportin axis plays a secondary role.

Role of Iron in Aging Related Diseases

Iron progressively accumulates with age and can be further exacerbated by dietary iron intake, genetic factors, and repeated blood transfusions. While iron plays a vital role in various physiological processes within the human body, its accumulation contributes to cellular aging in several species. In its free form, iron can initiate the formation of free radicals at a cellular level and contribute to systemic disorders. This is most evident in high iron conditions such as hereditary hemochromatosis, when accumulation of iron contributes to the development of arthritis, cirrhosis, or cardiomyopathy. A growing body of research has further identified iron’s contributory effects in neurodegenerative diseases, ocular disorders, cancer, diabetes, endocrine dysfunction, and cardiovascular diseases. Reducing iron levels by repeated phlebotomy, iron chelation, and dietary restriction are the common therapeutic considerations to prevent iron toxicity. Chelators such as deferoxamine, deferiprone, and deferasirox have become the standard of care in managing iron overload conditions with other potential applications in cancer and cardiotoxicity. In certain animal models, drugs with iron chelating ability have been found to promote health and even extend lifespan. As we further explore the role of iron in the aging process, iron chelators will likely play an increasingly important role in our health.

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.

Effects of Excess Iron on the Retina: Insights From Clinical Cases and Animal Models of Iron Disorders

Iron plays an important role in a wide range of metabolic pathways that are important for neuronal health. Excessive levels of iron, however, can promote toxicity and cell death. An example of an iron overload disorder is hemochromatosis (HH) which is a genetic disorder of iron metabolism in which the body’s ability to regulate iron absorption is altered, resulting in iron build-up and injury in several organs. The retina was traditionally assumed to be protected from high levels of systemic iron overload by the blood-retina barrier. However, recent data shows that expression of genes that are associated with HH can disrupt retinal iron metabolism. Thus, the effects of iron overload on the retina have become an area of research interest, as excessively high levels of iron are implicated in several retinal disorders, most notably age–related macular degeneration. This review is an effort to highlight risk factors for excessive levels of systemic iron build-up in the retina and its potential impact on the eye health. Information is integrated across clinical and preclinical animal studies to provide insights into the effects of systemic iron loading on the retina.

Physiology and Inflammation Driven Pathophysiology of Iron Homeostasis-Mechanistic Insights into Anemia of Inflammation and Its Treatment

Anemia is very common in patients with inflammatory disorders. Its prevalence is associated with severity of the underlying disease, and it negatively affects quality of life and cardio-vascular performance of patients. Anemia of inflammation (AI) is caused by disturbances of iron metabolism resulting in iron retention within macrophages, a reduced erythrocyte half-life, and cytokine mediated inhibition of erythropoietin function and erythroid progenitor cell differentiation. AI is mostly mild to moderate, normochromic and normocytic, and characterized by low circulating iron, but normal and increased levels of the storage protein ferritin and the iron hormone hepcidin. The primary therapeutic approach for AI is treatment of the underlying inflammatory disease which mostly results in normalization of hemoglobin levels over time unless other pathologies such as vitamin deficiencies, true iron deficiency on the basis of bleeding episodes, or renal insufficiency are present. If the underlying disease and/or anemia are not resolved, iron supplementation therapy and/or treatment with erythropoietin stimulating agents may be considered whereas blood transfusions are an emergency treatment for life-threatening anemia. New treatments with hepcidin-modifying strategies and stabilizers of hypoxia inducible factors emerge but their therapeutic efficacy for treatment of AI in ill patients needs to be evaluated in clinical trials.

Runx3 regulates iron metabolism via modulation of BMP signalling

Objectives

Runx3, a member of the Runx family of transcription factors, has been studied as a tumour suppressor and key player of organ development. In a previous study, we reported differentiation failure and excessive angiogenesis in the liver of Runx3 knock‐out (KO) mice. Here, we examined a function of the Runx3 in liver, especially in iron metabolism.

Methods

We performed histological and immunohistological analyses of the Runx3 KO mouse liver. RNA‐sequencing analyses were performed on primary hepatocytes isolated from Runx3 conditional KO (cKO) mice. The effect of Runx3 knock‐down (KD) was also investigated using siRNA‐mediated KD in functional human hepatocytes and human hepatocellular carcinoma cells.

Result

We observed an iron‐overloaded liver with decreased expression of hepcidin in Runx3 KO mice. Expression of BMP6, a regulator of hepcidin transcription, and activity of the BMP pathway were decreased in the liver tissue of Runx3 KO mice. Transcriptome analysis on primary hepatocytes isolated from Runx3 cKO mice also revealed that iron‐induced increase in BMP6 was mediated by Runx3. Similar results were observed in Runx3 knock‐down experiments using HepaRG cells and HepG2 cells. Finally, we showed that Runx3 enhanced the activity of the BMP6 promoter by responding to iron stimuli in the hepatocytes.

Conclusion

In conclusion, we suggest that Runx3 plays important roles in iron metabolism of the liver through regulation of BMP signalling.

Ferroptosis as a new therapeutic opportunity for nonviral liver disease

Nonviral liver disease is a global public health problem due to its high mortality and morbidity. However, its underlying mechanism is unclear. Ferroptosis is a novel form of cell death that is involved in a variety of disease processes. Both abnormal iron metabolism (e.g., iron overload) and lipid peroxidation, which is induced by deletion of glutathione (GSH) or glutathione peroxidase 4 (GPX4), and the accumulation of polyunsaturated fatty acid-containing phospholipids (PUFA-PLs) trigger ferroptosis. Recently, ferroptosis has been involved in the pathological process of nonviral liver diseases [including alcohol-related liver disease (ALD); nonalcoholic fatty liver disease (NAFLD); hereditary hemochromatosis (HH); drug-, ischemia/reperfusion- or immune-induced liver injury; liver fibrosis; and liver cancer]. Hepatocyte ferroptosis is activated in ALD; NAFLD; HH; drug-, ischemia/reperfusion- or immune-induced liver injury; and liver fibrosis, whereas hepatic stellate cell and liver cancer cell ferroptosis are inhibited in liver fibrosis and liver cancer, respectively. Thus, ferroptosis is an ideal target for nonviral liver diseases. In the present review, we discuss the latest findings on ferroptosis and potential drugs targeting ferroptosis for nonviral liver diseases. This review will highlight further directions for the treatment and prevention of nonviral liver diseases.

Tissue-Specific Regulation of Ferroportin in Wild-Type and Hjv-/- Mice Following Dietary Iron Manipulations

Hepcidin is a liver‐derived peptide hormone that limits iron egress from tissues to the bloodstream. It operates by binding to the iron exporter ferroportin, which blocks iron transport and tags ferroportin for degradation. Genetic hepcidin inactivation leads to hereditary hemochromatosis, a disease of iron overload. We used wild‐type and Hjv‐/‐ mice, a model of hemochromatosis, to examine the expression of ferroportin and other proteins of iron metabolism in hepcidin target tissues. The animals were previously subjected to dietary iron manipulations. In Hjv‐/‐ mice, hepcidin messenger RNA correlated significantly with hepatic iron load (r = 0.8211, P < 0.001), but was substantially lower compared with wild‐type controls. Duodenal ferroportin and divalent metal transporter 1 (DMT1), as well as splenic and hepatic ferroportin, were overexpressed in these animals. A high‐iron diet (2% carbonyl iron) suppressed duodenal DMT1 levels in both wild‐type and Hjv‐/‐ mice; however, it did not affect duodenal ferroportin expression in Hjv‐/‐ mice, and only reduced it in wild‐type mice. In contrast, the high‐iron diet decreased splenic ferroportin exclusively in Hjv‐/‐ mice, whereas it induced hepatic ferroportin exclusively in wild‐type mice. Conclusion: Our data show that dietary iron differentially affects ferroportin expression in mouse tissues and are consistent with hepcidin‐dependent and hepcidin‐independent mechanisms for ferroportin regulation. In the Hjv‐/‐ mouse model of hemochromatosis, duodenal ferroportin remains unresponsive to iron but DMT1 is appropriately iron‐regulated.

Hepatocyte neogenin is required for hemojuvelin-mediated hepcidin expression and iron homeostasis in mice

Neogenin (NEO1) is a ubiquitously expressed multifunctional transmembrane protein. It interacts with hemojuvelin (HJV), a BMP coreceptor that plays a pivotal role in hepatic hepcidin expression. Earlier studies suggest that the function of HJV relies on its interaction with NEO1. However, the role of NEO1 in iron homeostasis remains controversial because of the lack of an appropriate animal model. Here, we generated a hepatocyte-specific Neo1 knockout (Neo1fl/fl;Alb-Cre+) mouse model that circumvented the developmental and lethality issues of the global Neo1 mutant. Results show that ablation of hepatocyte Neo1 decreased hepcidin expression and caused iron overload. This iron overload did not result from altered iron utilization by erythropoiesis. Replacement studies revealed that expression of the Neo1L1046E mutant that does not interact with Hjv, was unable to correct the decreased hepcidin expression and high serum iron in Neo1fl/fl;Alb-Cre+ mice. In Hjv-/- mice, expression of HjvA183R mutant that has reduced interaction with Neo1, also displayed a blunted induction of hepcidin expression. These observations indicate that Neo1-Hjv interaction is essential for hepcidin expression. Further analyses suggest that the Hjv binding triggered the cleavage of the Neo1 cytoplasmic domain by a protease, which resulted in accumulation of truncated Neo1 on the plasma membrane. Additional studies did not support that Neo1 functions by inhibiting Hjv shedding as previously proposed. Together, our data favor a model in which Neo1 interaction with Hjv leads to accumulation of cleaved Neo1 on the plasma membrane, where Neo1 acts as a scaffold to induce the Bmp signaling and hepcidin expression.

Physiological and pathophysiological mechanisms of hepcidin regulation: clinical implications for iron disorders

The discovery of hepcidin has provided a solid foundation for understanding the mechanisms of systemic iron homeostasis and the aetiologies of iron disorders. Hepcidin assures the balance of circulating and stored iron levels for multiple physiological processes including oxygen transport and erythropoiesis, while limiting the toxicity of excess iron. The liver is the major site where regulatory signals from iron, erythropoietic drive and inflammation are integrated to control hepcidin production. Pathologically, hepcidin dysregulation by genetic inactivation, ineffective erythropoiesis, or inflammation leads to diseases of iron deficiency or overload such as iron-refractory iron-deficiency anaemia, anaemia of inflammation, iron-loading anaemias and hereditary haemochromatosis. In the present review, we discuss recent insights into the molecular mechanisms governing hepcidin regulation, how these pathways are disrupted in iron disorders, and how this knowledge is being used to develop novel diagnostic and therapeutic strategies.

Hepcidin and Iron Metabolism in Experimental Liver Injury

The liver plays a pivotal role in the regulation of iron metabolism through its ability to sense and respond to iron stores by release of the hormone hepcidin. Under physiologic conditions, regulation of hepcidin expression in response to iron status maintains iron homeostasis. In response to tissue injury, hepcidin expression can be modulated by other factors, such as inflammation and oxidative stress. The resulting dysregulation of hepcidin is proposed to account for alterations in iron homeostasis that are sometimes observed in patients with liver disease. This review describes the effects of experimental forms of liver injury on iron metabolism and hepcidin expression. In general, models of acute liver injury demonstrate increases in hepcidin mRNA and hypoferremia, consistent with hepcidin's role as an acute-phase reactant. Conversely, diverse models of chronic liver injury are associated with decreased hepcidin mRNA but with variable effects on iron status. Elucidating the reasons for the disparate impact of different chronic injuries on iron metabolism is an important research priority, as is a deeper understanding of the interplay among various stimuli, both positive and negative, on hepcidin regulation. Future studies should provide a clearer picture of how dysregulation of hepcidin expression and altered iron homeostasis impact the progression of liver diseases and whether they are a cause or consequence of these pathologies.

The Potential Role of Ferroptosis in Alzheimer's Disease

Alzheimer's disease (AD) is the most prevalent cause of dementia, accounting for approximately 60%-80%of all cases. Although much effort has been made over the years, the precise mechanism of AD has not been completely elucidated. Recently, great attention has shifted to the roles of iron metabolism, lipid peroxidation, and oxidative stress in AD pathogenesis. We also note that these pathological events are the vital regulators of a novel regulatory cell death, termed ferroptosis-an iron-dependent, oxidative, non-apoptotic cell death. Ferroptosis differs from apoptosis, necrosis, and autophagy with respect to morphology, biochemistry, and genetics. Mounting evidence suggests that ferroptosis may be involved in neurological disorders, including AD. Here, we review the underlying mechanisms of ferroptosis; discuss the potential interaction between AD and ferroptosis in terms of iron metabolism, lipid peroxidation, and the glutathione/glutathione peroxidase 4 axis; and describe some associated studies that have explored the implication of ferroptosis in AD.

Genes Regulating Spermatogenesis and Sperm Function Associated With Rare Disorders

Spermatogenesis is a cell differentiation process that ensures the production of fertilizing sperm, which ultimately fuse with an egg to form a zygote. Normal spermatogenesis relies on Sertoli cells, which preserve cell junctions while providing nutrients for mitosis and meiosis of male germ cells. Several genes regulate normal spermatogenesis, some of which are not exclusively expressed in the testis and control multiple physiological processes in an organism. Loss-of-function mutations in some of these genes result in spermatogenesis and sperm functionality defects, potentially leading to the insurgence of rare genetic disorders. To identify genetic intersections between spermatogenesis and rare diseases, we screened public archives of human genetic conditions available on the Genetic and Rare Diseases Information Center (GARD), the Online Mendelian Inheritance in Man (OMIM), and the Clinical Variant (ClinVar), and after an extensive literature search, we identified 22 distinct genes associated with 21 rare genetic conditions and defective spermatogenesis or sperm function. These protein-coding genes regulate Sertoli cell development and function during spermatogenesis, checkpoint signaling pathways at meiosis, cellular organization and shape definition during spermiogenesis, sperm motility, and capacitation at fertilization. A number of these genes regulate folliculogenesis and oogenesis as well. For each gene, we review the genotype–phenotype association together with associative or causative polymorphisms in humans, and provide a description of the shared molecular mechanisms that regulate gametogenesis and fertilization obtained in transgenic animal models.

Cell-type-specific insights into iron regulatory processes

Despite its essential role in many biological processes, iron is toxic when in excess due to its propensity to generate reactive oxygen species. To prevent diseases associated with iron deficiency or iron loading, iron homeostasis must be tightly controlled. Intracellular iron content is regulated by the Iron Regulatory Element-Iron Regulatory Protein (IRE-IRP) system, whereas systemic iron availability is adjusted to body iron needs chiefly by the hepcidin-ferroportin (FPN) axis. Here, we aimed to review advances in the field that shed light on cell-type-specific regulatory mechanisms that control or modify systemic and local iron balance, and how shifts in cellular iron levels may affect specialized cell functions.

Bone morphogenic proteins in iron homeostasis

The bone morphogenetic protein (BMP)-SMAD signaling pathway plays a central role in regulating hepcidin, which is the master hormone governing systemic iron homeostasis. Hepcidin is produced by the liver and acts on the iron exporter ferroportin to control iron absorption from the diet and iron release from body stores, thereby providing adequate iron for red blood cell production, while limiting the toxic effects of excess iron. BMP6 and BMP2 ligands produced by liver endothelial cells bind to BMP receptors and the coreceptor hemojuvelin (HJV) on hepatocytes to activate SMAD1/5/8 signaling, which directly upregulates hepcidin transcription. Most major signals that influence hepcidin production, including iron, erythropoietic drive, and inflammation, intersect with the BMP-SMAD pathway to regulate hepcidin transcription. Mutation or inactivation of BMP ligands, BMP receptors, HJV, SMADs or other proteins that modulate the BMP-SMAD pathway result in hepcidin dysregulation, leading to iron-related disorders, such as hemochromatosis and iron refractory iron deficiency anemia. Pharmacologic modulators of the BMP-SMAD pathway have shown efficacy in pre-clinical models to regulate hepcidin expression and treat iron-related disorders. This review will discuss recent insights into the role of the BMP-SMAD pathway in regulating hepcidin to control systemic iron homeostasis.

Juvenile Hemochromatosis: A Case Report and Review of the Literature

Juvenile hemochromatosis (JH), type 2A hemochromatosis, is a rare autosomal recessive disorder of systemic iron overload due to homozygous mutations of HJV (HFE2), which encodes hemojuvelin, an essential regulator of the hepcidin expression, causing liver fibrosis, diabetes, and heart failure before 30 years of age, often with fatal outcomes. We report two Japanese sisters of 37 and 52 years of age, with JH, who showed the same homozygous HJV I281T mutation and hepcidin deficiency and who both responded well to phlebotomy on an outpatient basis. When all reported cases of JH with homozygous HJV mutations in the relevant literature were reviewed, we found—for the first time—that JH developed in females and males at a ratio of 3:2, with no age difference in the two groups. Furthermore, we found that the age of onset of JH may depend on the types of HJV mutations. In comparison to patients with the most common G320V/G320V mutation, JH developed earlier in patients with L101P/L101P or R385X/R385X mutations and later in patients with I281T/I281T mutations.

Repulsive guidance molecules lock growth differentiation factor 5 in an inhibitory complex

Repulsive guidance molecules (RGMs) are cell surface proteins that regulate the development and homeostasis of many tissues and organs, including the nervous, skeletal, and immune systems. They control fundamental biological processes, such as migration and differentiation by direct interaction with the Neogenin (NEO1) receptor and function as coreceptors for the bone morphogenetic protein (BMP)/growth differentiation factor (GDF) family. We determined crystal structures of all three human RGM family members in complex with GDF5, as well as the ternary NEO1-RGMB-GDF5 assembly. Surprisingly, we show that all three RGMs inhibit GDF5 signaling, which is in stark contrast to RGM-mediated enhancement of signaling observed for other BMPs, like BMP2. Despite their opposite effect on GDF5 signaling, RGMs occupy the BMP type 1 receptor binding site similar to the observed interactions in RGM-BMP2 complexes. In the NEO1-RGMB-GDF5 complex, RGMB physically bridges NEO1 and GDF5, suggesting cross-talk between the GDF5 and NEO1 signaling pathways. Our crystal structures, combined with structure-guided mutagenesis of RGMs and BMP ligands, binding studies, and cellular assays suggest that RGMs inhibit GDF5 signaling by competing with GDF5 type 1 receptors. While our crystal structure analysis and in vitro binding data initially pointed towards a simple competition mechanism between RGMs and type 1 receptors as a possible basis for RGM-mediated GDF5 inhibition, further experiments utilizing BMP2-mimicking GDF5 variants clearly indicate a more complex mechanism that explains how RGMs can act as a functionality-changing switch for two structurally and biochemically similar signaling molecules.

The functional role of hemojuvelin in acute ischemic stroke

Our study aimed to establish the role of hemojuvelin (HJV) in acute ischemic stroke (AIS). We performed immunohistochemistry for HJV expression in human brain tissues from 10 AIS and 2 non-stroke autopsy subjects. Plasma HJV was measured in 112 AIS patients within 48 h after stroke. The results showed significantly increased HJV expression in brain tissues from AIS patients compare to non-stroke subjects. After adjusting for clinical variables, plasma levels of HJV within 48 h after stroke were an independent predictor of poor functional outcome three months post-stroke (OR:1.78, 95% CI: 1.03-3.07; P = 0.038). In basic part, Western blotting showed that HJV expression in mice brains was apparent at 3 h after middle cerebral artery occlusion (MCAO), and increased significantly at 72 h. In cultured cortical neurons, expression of HJV protein increased remarkably 24 h after oxygen glucose deprivation (OGD), and small interfering RNAs (siHJV) transfected OGD neurons had a lower apoptotic rate. Importantly, 72 h post-MCAO, HJV knockout mice had significantly smaller infarcts and less expression of cleaved caspase-3 protein compared with wild-type mice. In summary, HJV participates in the mechanisms of post-stroke neuronal injury, and that plasma HJV levels can be a potential early outcome indicator for AIS patients.

Ablation of Hepatocyte Smad1, Smad5, and Smad8 Causes Severe Tissue Iron Loading and Liver Fibrosis in Mice

A failure of iron to appropriately regulate liver hepcidin production is central to the pathogenesis of hereditary hemochromatosis. SMAD1/5 transcription factors, activated by bone morphogenetic protein (BMP) signaling, are major regulators of hepcidin production in response to iron; however, the role of SMAD8 and the contribution of SMADs to hepcidin production by other systemic cues remain uncertain. Here, we generated hepatocyte Smad8 single (Smad8^fl/fl ;Alb-Cre⁺ ), Smad1/5/8 triple (Smad158;Alb-Cre⁺ ), and littermate Smad1/5 double (Smad15;Alb-Cre⁺ ) knockout mice to investigate the role of SMAD8 in hepcidin and iron homeostasis regulation and liver injury. We found that Smad8;Alb-Cre⁺ mice exhibited no iron phenotype, whereas Smad158;Alb-Cre⁺ mice had greater iron overload than Smad15;Alb-Cre⁺ mice. In contrast to the sexual dimorphism reported for wild-type mice and other hemochromatosis models, hepcidin deficiency and extrahepatic iron loading were similarly severe in Smad15;Alb-Cre⁺ and Smad158;Alb-Cre⁺ female compared with male mice. Moreover, epidermal growth factor (EGF) failed to suppress hepcidin in Smad15;Alb-Cre⁺ hepatocytes. Conversely, hepcidin was still increased by lipopolysaccharide in Smad158;Alb-Cre⁺ mice, although lower basal hepcidin resulted in lower maximal hepcidin. Finally, unlike most mouse hemochromatosis models, Smad158;Alb-Cre⁺ developed liver injury and fibrosis at 8 weeks. Liver injury and fibrosis were prevented in Smad158;Alb-Cre⁺ mice by a low-iron diet and were minimal in iron-loaded Cre^- mice. Conclusion: Hepatocyte Smad1/5/8 knockout mice are a model of hemochromatosis that encompasses liver injury and fibrosis seen in human disease. These mice reveal the redundant but critical role of SMAD8 in hepcidin and iron homeostasis regulation, establish a requirement for SMAD1/5/8 in hepcidin regulation by testosterone and EGF but not inflammation, and suggest a pathogenic role for both iron loading and SMAD1/5/8 deficiency in liver injury and fibrosis.

Animal Models of Normal and Disturbed Iron and Copper Metabolism

Research on the interplay between iron and copper metabolism in humans began to flourish in the mid-20th century, and diseases associated with dysregulated homeostasis of these essential trace minerals are common even today. Iron deficiency is the most frequent cause of anemia worldwide, leading to significant morbidity, particularly in developing countries. Iron overload is also quite common, usually being the result of genetic mutations which lead to inappropriate expression of the iron-regulatory hormone hepcidin. Perturbations of copper homeostasis in humans have also been described, including rare genetic conditions which lead to severe copper deficiency (Menkes disease) or copper overload (Wilson disease). Historically, the common laboratory rat (Rattus norvegicus) was the most frequently utilized species to model human physiology and pathophysiology. Recently, however, the development of genetic-engineering technology combined with the worldwide availability of numerous genetically homogenous (i.e., inbred) mouse strains shifted most research on iron and copper metabolism to laboratory mice. This created new opportunities to understand the function of individual genes in the context of a living animal, but thoughtful consideration of whether mice are the most appropriate models of human pathophysiology was not necessarily involved. Given this background, this review is intended to provide a guide for future research on iron- and copper-related disorders in humans. Generation of complementary experimental models in rats, swine, and other mammals is now facile given the advent of newer genetic technologies, thus providing the opportunity to accelerate the identification of pathogenic mechanisms and expedite the development of new treatments to mitigate these important human disorders.

Despite Genetic Iron Overload, Hfe-Hemochromatosis Mice Do Not Show Bone Loss

One of the most prevalent genetic iron overload disorders in Caucasians is caused by mutations in the HFE gene. Both HFE patients and Hfe‐mouse models develop a progressive accumulation of iron in the parenchymal cells of various tissues, eventually resulting in liver cirrhosis, hepatocellular carcinoma, cardiomyopathies, hypogonadism, and other pathologies. Clinical data and preclinical models have brought considerable attention to the correlation between iron overload and the development of osteoporosis in HFE/Hfe hemochromatosis. Our study critically challenges this concept. We show that systemic iron overload, at the degree present in Hfe^−/− mice, does not associate with the microarchitecture impairment of long bones, thus excluding a negative effect of iron overload on bone integrity. We further reveal that Hfe actions in osteoblasts and osteoclasts are dispensable for the maintenance of bone and iron homeostasis in mice under steady‐state conditions. We conclude that, despite systemic iron overload, Hfe^−/− mice present normal physiological bone homeostasis. © 2019 The Authors. JBMR Plus in published by Wiley Periodicals, Inc. on behalf of the American Society for Bone and Mineral Research.

Hemojuvelin is a novel suppressor for Duchenne muscular dystrophy and age-related muscle wasting

BACKGROUND

Muscle wasting occurs in response to various physiological and pathological conditions, including ageing and Duchenne muscular dystrophy (DMD). Transforming growth factor-β1 (TGF-β1) contributes to muscle pathogenesis in elderly people and DMD patients; inhibition of TGF-β1 signalling is a promising therapeutic strategy for muscle-wasting disorders. Hemojuvelin (HJV or Hjv as the murine homologue) is a membrane-bound protein that is highly expressed in skeletal muscle, heart, and liver. In hepatic cells, Hjv acts as a coreceptor for bone morphogenetic protein, a TGF-β subfamily member. The aim of this study was to investigate whether Hjv plays an essential role in muscle physiological and pathophysiological processes by acting as a coreceptor for TGF-β1 signalling.

METHODS

Conventional and conditional Hjv knockout mice as well as mdx and aged mice transfected with Hjv overexpression vector were used to study the role of Hjv in muscle physiology and pathophysiology. qRT-PCR, western blotting, and immunohistochemistry examinations were conducted to evaluate gene, protein, and structural changes in vivo and in vitro. Exercise endurance was determined using treadmill running test, and muscle force was detected by an isometric transducer. RNA interference, immunoprecipitation, and dual-luciferase reporter assays were utilized to explore the mechanism by which Hjv regulates TGF-β1 signalling in skeletal muscle.

RESULTS

Conventional and conditional Hjv knockout mice displayed muscle atrophy, fibrosis, reduced running endurance, and muscle force. HJV was significantly down-regulated in the muscles of DMD patients (n = 3, mean age: 11.7 ± 5.7 years) and mdx mice as well as in those of aged humans (n = 10, 20% women, mean age: 75.1 ± 9.5 years) and mice. Overexpression of Hjv rescued dystrophic and age-related muscle wasting. Unlike its function in hepatic cells, the bone morphogenetic protein downstream phosphorylated p-Smad1/5/8 signalling pathway was unchanged, but TGF-β1, TGF-β receptor II (TβRII), and p-Smad2/3 expression were increased in Hjv-deficient muscles. Mechanistically, loss of Hjv promoted activation of Smad3 signalling induced by TGF-β1, whereas Hjv overexpression inhibited TGF-β1/Smad3 signalling by directly interacting with TβRII on the muscle membrane.

CONCLUSIONS

Our findings identify an unrecognized role of HJV in skeletal muscle by regulating TGF-β1/Smad3 signalling as a coreceptor for TβRII. Unlike the TGF-β1/Smad3 pathway, HJV could be a reliable drug target as its expression is not widespread. Novel therapeutic strategies could potentially be devised to interfere only with the muscle function of HJV to treat DMD and age-related muscle wasting.

Deregulation of Hepatic Mek1/2⁻Erk1/2 Signaling Module in Iron Overload Conditions

The liver, through the production of iron hormone hepcidin, controls body iron levels. High liver iron levels and deregulated hepcidin expression are commonly observed in many liver diseases including highly prevalent genetic iron overload disorders. In spite of a number of breakthrough investigations into the signals that control hepcidin expression, little progress has been made towards investigations into intracellular signaling in the liver under excess of iron. This study examined hepatic signaling pathways underlying acquired and genetic iron overload conditions. Our data demonstrate that hepatic iron overload associates with a decline in the activation of mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (Erk) kinase (Mek1/2) pathway by selectively affecting the phosphorylation of Erk1/2. We propose that Mek1/2-Erk1/2 signaling is uncoupled from iron-Bmp-Smad-mediated hepcidin induction and that it may contribute to a number of liver pathologies in addition to toxic effects of iron. We believe that our findings will advance the understanding of cellular signaling events in the liver during iron overload of different etiologies.

Recent progress on inhibitors of the type II transmembrane serine proteases, hepsin, matriptase and matriptase-2

Members of the type II transmembrane serine proteases (TTSP) family play a vital role in cell growth and development but many are also implicated in disease. Two of the well-studied TTSPs, matriptase and hepsin proteolytically process multiple protein substrates such as the inactive single-chain zymogens pro-HGF and pro-macrophage stimulating protein into the active heterodimeric forms, HGF and macrophage stimulating protein. These two proteases also have many other substrates which are associated with cancer and tumor progression. Another related TTSP, matriptase-2 is expressed in the liver and functions by regulating iron homoeostasis through the cleavage of hemojuvelin and thus is implicated in iron overload diseases. In the present review, we will discuss inhibitor design strategy and Structure activity relationships of TTSP inhibitors, which have been reported in the literature.

Hepcidin and the BMP-SMAD pathway: An unexpected liaison

Hepcidin, the main regulator of iron metabolism, is synthesized and released by hepatocytes in response to increased body iron concentration and inflammation. Deregulation of hepcidin expression is a common feature of genetic and acquired iron disorders: in Hereditary Hemochromatosis (HH) and iron-loading anemias low hepcidin causes iron overload, while in Iron Refractory Iron Deficiency Anemia (IRIDA) and anemia of inflammation (AI), high hepcidin levels induce iron-restricted erythropoiesis. Hepcidin expression in the liver is mainly controlled by the BMP-SMAD pathway, activated in a paracrine manner by BMP2 and BMP6 produced by liver sinusoidal endothelial cells. The BMP type I receptors ALK2 and ALK3 are responsible for iron-dependent hepcidin upregulation and basal hepcidin expression, respectively. Characterization of animal models with genetic inactivation of the key components of the pathway has suggested the existence of two BMP/SMAD pathway branches: the first ALK3 and HH proteins dependent, responsive to BMP2 for basal hepcidin activation, and the second ALK2 dependent, activated by BMP6 in response to increased tissue iron. The erythroid inhibitor of hepcidin Erythroferrone also impacts on the liver BMP-SMAD pathway although its effect is blunted by pathway hyper-activation. The liver BMP-SMAD pathway is required also in inflammation to cooperate with JAK2/STAT3 signaling for full hepcidin activation. Pharmacologic targeting of BMP-SMAD pathway components or regulators may improve the outcome of both genetic and acquired disorders of iron overload and deficiency by increasing or inhibiting hepcidin expression.

Variation in the repulsive guidance molecule family in human populations

Repulsive guidance molecules, RGMA, RGMB, and RGMC, are related proteins discovered independently through different experimental paradigms. They are encoded by single copy genes in mammalian and other vertebrate genomes, and are ~50% identical in amino acid sequence. The importance of RGM actions in human physiology has not been realized, as most research has focused on non-human models, although mutations in RGMC are the cause of the severe iron storage disorder, juvenile hemochromatosis. Here I show that repositories of human genomic and population genetic data can be used as starting points for discovery and for developing new testable hypotheses about each of these paralogs in human biology and disease susceptibility. Information was extracted, aggregated, and analyzed from the Ensembl and UCSC Genome Browsers, the Exome Aggregation Consortium, the Genotype-Tissue Expression project portal, the cBio portal for Cancer Genomics, and the National Cancer Institute Genomic Data Commons data site. Results identify extensive variation in gene expression patterns, substantial alternative RNA splicing, and possible missense alterations and other modifications in the coding regions of each of the three genes, with many putative mutations being detected in individuals with different types of cancers. Moreover, selected amino acid substitutions are highly prevalent in the world population, with minor allele frequencies of up to 37% for RGMA and up to 8% for RGMB. These results indicate that protein sequence variation is common in the human RGM family, and raises the possibility that individual variants will have a significant population impact on human physiology and/or disease predisposition.

Iron, erythropoietin, and inflammation regulate hepcidin in Bmp2-deficient mice, but serum iron fails to induce hepcidin in Bmp6-deficient mice

The bone morphogenetic protein (BMP)-SMAD signaling pathway is a key transcriptional regulator of hepcidin in response to tissue iron stores, serum iron, erythropoietic drive and inflammation to increase the iron supply when needed for erythropoiesis, but to prevent the toxicity of iron excess. Recently, BMP2 was reported to play a non-redundant role in hepcidin regulation in addition to BMP6. Here, we used a newly validated BMP2 ELISA assay and mice with a global or endothelial conditional knockout (CKO) of Bmp2 or Bmp6 to examine how BMP2 is regulated and functionally contributes to hepcidin regulation by its major stimuli. Erythropoietin (EPO) did not influence BMP2 expression in control mice, and still suppressed hepcidin in Bmp2 CKO mice. Lipopolysaccharide (LPS) reduced BMP2 expression in control mice, but still induced hepcidin in Bmp2 CKO mice. Chronic dietary iron loading that increased liver iron induced BMP2 expression, whereas acute oral iron gavage that increased serum iron without influencing liver iron did not impact BMP2. However, hepcidin was still induced by both iron loading methods in Bmp2 CKO mice, although the degree of hepcidin induction was blunted relative to control mice. Conversely, acute oral iron gavage failed to induce hepcidin in Bmp6 ^-/- or CKO mice. Thus, BMP2 has at least a partially redundant role in hepcidin regulation by serum iron, tissue iron, inflammation and erythropoietic drive. In contrast, BMP6 is absolutely required for hepcidin regulation by serum iron.

Novel Liposome Eencapsulated Guanosine Di Phosphate based Therapeutic Target against Anemia of Inflammation

Hepcidin, master regulator of iron homeostasis, causes anemia under infectious and inflammatory conditions by reducing intestinal absorption of iron with decreased release of iron from macrophages and liver despite adequate iron stores leading to Anemia of Inflammation (AI). Many therapeutic trials have been carried out but none have been effective due to its adverse effects. In present study, we discover that Guanosine 5'-diphosphate (GDP) encapsulated in lipid vesicle (NH+) was found to inhibit NF-ҝB activation by limiting phosphorylation and degradation of IҝBα, thus, attenuating IL-6 secretion from macrophage cells. Moreover, the suppressed IL-6 levels down regulated JAK2/STAT3 pathway with decrease inflammation-mediated Hamp mRNA transcription (HepG2) and increase iron absorption (Caco2) in HepG2/Caco2 co-culture model. Analogous results were obtained in acute and chronic AI mice model thus, correcting haemoglobin level. These results proved NH + GDP as novel therapeutic agent to overcome limitations and suggests it as potential drug to ameliorate AI.

ALK3 undergoes ligand-independent homodimerization and BMP-induced heterodimerization with ALK2

The bone morphogenetic protein (BMP) type I receptors ALK2 and ALK3 are essential for expression of hepcidin, a key iron regulatory hormone. In mice, hepatocyte-specific Alk2 deficiency leads to moderate iron overload with periportal liver iron accumulation, while hepatocyte-specific Alk3 deficiency leads to severe iron overload with centrilobular liver iron accumulation and a more marked reduction of basal hepcidin levels. The objective of this study was to investigate whether the two receptors have additive roles in hepcidin regulation. Iron overload in mice with hepatocyte-specific Alk2 and Alk3 (Alk2/3) deficiency was characterized and compared to hepatocyte-specific Alk3 deficient mice. Co-immunoprecipitation studies were performed to detect the formation of ALK2 and ALK3 homodimer and heterodimer complexes in vitro in the presence and absence of ligands. The iron overload phenotype of hepatocyte-specific Alk2/3-deficient mice was more severe than that of hepatocyte-specific Alk3-deficient mice. In vitro co-immunoprecipitation studies in Huh7 cells showed that ALK3 can homodimerize in absence of BMP2 or BMP6. In contrast, ALK2 did not homodimerize in either the presence or absence of BMP ligands. However, ALK2 did form heterodimers with ALK3 in the presence of BMP2 or BMP6. ALK3-ALK3 and ALK2-ALK3 receptor complexes induced hepcidin expression in Huh7 cells. Our data indicate that: (I) ALK2 and ALK3 have additive functions in vivo, as Alk2/3 deficiency leads to a greater degree of iron overload than Alk3 deficiency; (II) ALK3, but not ALK2, undergoes ligand-independent homodimerization; (III) the formation of ALK2-ALK3 heterodimers is ligand-dependent and (IV) both receptor complexes functionally induce hepcidin expression in vitro.

Liver iron sensing and body iron homeostasis

The liver orchestrates systemic iron balance by producing and secreting hepcidin. Known as the iron hormone, hepcidin induces degradation of the iron exporter ferroportin to control iron entry into the bloodstream from dietary sources, iron recycling macrophages, and body stores. Under physiologic conditions, hepcidin production is reduced by iron deficiency and erythropoietic drive to increase the iron supply when needed to support red blood cell production and other essential functions. Conversely, hepcidin production is induced by iron loading and inflammation to prevent the toxicity of iron excess and limit its availability to pathogens. The inability to appropriately regulate hepcidin production in response to these physiologic cues underlies genetic disorders of iron overload and deficiency, including hereditary hemochromatosis and iron-refractory iron deficiency anemia. Moreover, excess hepcidin suppression in the setting of ineffective erythropoiesis contributes to iron-loading anemias such as β-thalassemia, whereas excess hepcidin induction contributes to iron-restricted erythropoiesis and anemia in chronic inflammatory diseases. These diseases have provided key insights into understanding the mechanisms by which the liver senses plasma and tissue iron levels, the iron demand of erythrocyte precursors, and the presence of potential pathogens and, importantly, how these various signals are integrated to appropriately regulate hepcidin production. This review will focus on recent insights into how the liver senses body iron levels and coordinates this with other signals to regulate hepcidin production and systemic iron homeostasis.

Inherited Disorders of Iron Overload

Dietary iron absorption and systemic iron traffic are tightly controlled by hepcidin, a liver-derived peptide hormone. Hepcidin inhibits iron entry into plasma by binding to and inactivating the iron exporter ferroportin in target cells, such as duodenal enterocytes and tissue macrophages. Hepcidin is induced in response to increased body iron stores to inhibit further iron absorption and prevent iron overload. The mechanism involves the BMP/SMAD signaling pathway, which triggers transcriptional hepcidin induction. Inactivating mutations in components of this pathway cause hepcidin deficiency, which allows inappropriately increased iron absorption and efflux into the bloodstream. This leads to hereditary hemochromatosis (HH), a genetically heterogenous autosomal recessive disorder of iron metabolism characterized by gradual buildup of unshielded non-transferrin bound iron (NTBI) in plasma and excessive iron deposition in tissue parenchymal cells. The predominant HH form is linked to mutations in the HFE gene and constitutes the most frequent genetic disorder in Caucasians. Other, more severe and rare variants are caused by inactivating mutations in HJV (hemojuvelin), HAMP (hepcidin) or TFR2 (transferrin receptor 2). Mutations in SLC40A1 (ferroportin) that cause hepcidin resistance recapitulate the biochemical phenotype of HH. However, ferroportin-related hemochromatosis is transmitted in an autosomal dominant manner. Loss-of-function ferroportin mutations lead to ferroportin disease, characterized by iron overload in macrophages and low transferrin saturation. Aceruloplasminemia and atransferrinemia are further inherited disorders of iron overload caused by deficiency in ceruloplasmin or transferrin, the plasma ferroxidase and iron carrier, respectively.

Hepcidin-mediated hypoferremic response to acute inflammation requires a threshold of Bmp6/Hjv/Smad signaling

Systemic iron balance is controlled by hepcidin, a liver hormone that limits iron efflux to the bloodstream by promoting degradation of the iron exporter ferroportin in target cells. Iron-dependent hepcidin induction requires hemojuvelin (HJV), a bone morphogenetic protein (BMP) coreceptor that is disrupted in juvenile hemochromatosis, causing dramatic hepcidin deficiency and tissue iron overload. Hjv^-/- mice recapitulate phenotypic hallmarks of hemochromatosis but exhibit blunted hepcidin induction following lipopolysaccharide (LPS) administration. We show that Hjv^-/- mice fail to mount an appropriate hypoferremic response to acute inflammation caused by LPS, the lipopeptide FSL1, or Escherichia coli infection because residual hepcidin does not suffice to drastically decrease macrophage ferroportin levels. Hfe^-/- mice, a model of milder hemochromatosis, exhibit almost wild-type inflammatory hepcidin expression and associated effects, whereas double Hjv^-/-Hfe^-/- mice phenocopy single Hjv^-/- counterparts. In primary murine hepatocytes, Hjv deficiency does not affect interleukin-6 (IL-6)/Stat, and only slightly inhibits BMP2/Smad signaling to hepcidin; however, it severely impairs BMP6/Smad signaling and thereby abolishes synergism with the IL-6/Stat pathway. Inflammatory induction of hepcidin is suppressed in iron-deficient wild-type mice and recovers after the animals are provided overnight access to an iron-rich diet. We conclude that Hjv is required for inflammatory induction of hepcidin and controls the acute hypoferremic response by maintaining a threshold of Bmp6/Smad signaling. Our data highlight Hjv as a potential pharmacological target against anemia of inflammation.

Relationship of serum haemojuvelin and hepcidin levels with iron level and erythropoietin requirement in prevalent hepatitis C virus positive haemodialysis patients

AIM

Iron overload is frequently reported in haemodialysis (HD) patients particularly those with chronic hepatitis C virus (HCV) infection. Soluble haemojuvelin (sHJV) has recently emerged as one of the significant regulators of iron homeostasis and hepcidin expression. The aim of the present study was to evaluate the potential associations of sHJV and hepcidin with inflammation, iron parameters and erythropoietin requirement in prevalent HD patients with HCV.

METHODS

Serum sHJV and hepcidin were measured in 60 prevalent HD patients with [group I (n = 30)] and without [group II (n = 30)] HCV, and controls (n = 30) by enzyme-linked immunosorbent assay. Parameters related to anaemia, iron metabolism, inflammation, sHJV and hepcidin were measured.

RESULTS

Serum hepcidin in HCV positive versus negative groups was 89.40 ± 46.08 ng/mL and 224.1 ± 72.36 ng/mL, P = 0.000, respectively, while sHJV was 245 ± 1.338 ng/mL and 254 ± 0.762 ng/mL, P = 0.147, respectively in positive versus negative patients. In group I, hepcidin correlated with serum ferritin (r = -0.512 P = 0.005) and transferrin saturation (TSAT%) (r = 0.572, P = 0.000) and sHJV correlated with ferritin (r = 0.40, P 0.000), TSAT% (r = 0.450, P = 0.002) and a significant correlation also existed between sHJV and hepcidin (r = -0.259, P = 0.045). In the regression analysis, ferritin and TSAT% were able to predict sHJV; (standardized β = 0.52, P 0.001) and (standardized β = 0.48, P 0.010). Ferritin and sHJV were also able to predict hepcidin (standardized β = 0.627, P = 0.006) and (standardized β = 0.300, P = 0.007) in group I.

CONCLUSION

Soluble haemojuvelin levels seem to be associated with iron overload parameters and hepcidin levels in HCV positive HD patients.

DNA methylation of hepatic iron sensing genes and the regulation of hepcidin expression

Production of the iron regulatory peptide hepcidin is tightly controlled by a network of proteins in hepatocytes that sense levels of iron in the circulation (as diferric-transferrin) and in tissues (in ferritin). Human studies show high variability in the normal range of serum hepcidin levels. We have postulated that this may, in part, be related to inter-individual variability in the expression of genes in the iron sensing pathway, potentially governed by epigenetic factors. Here, we have investigated whether genes encoding hepatic iron sensing proteins and hepcidin are regulated by DNA methylation. Experiments were performed on two human hepatoma cell lines, HepG2 cells and Huh7 cells. Basal expression of TFR2 and HAMP was significantly lower in Huh7 cells compared with HepG2 cells. Analysis of bisulphite-converted DNA from Huh7 cells revealed partial methylation of TFR2 (alpha transcript), which could result in gene silencing. Demethylation using 5-aza-2’-deoxycitidine (AZA) increased TFR2 mRNA expression in Huh7. PCR analysis of bisulphite-converted HAMP promoter DNA, using methylation-specific primers, revealed no differences between cell lines. However, HAMP mRNA expression in Huh7 was increased by AZA treatment, suggesting that methylation of one or more iron sensing genes may indirectly influence HAMP expression. Our study provides evidence that DNA methylation might control expression of HAMP and other hepatic iron sensing genes, and indicates that epigenetic influences on iron homeostasis warrant further investigation.

Erythroferrone: An Erythroid Regulator of Hepcidin and Iron Metabolism

Iron homeostasis ensures adequate iron for biological processes while preventing excessive iron accumulation, which can lead to tissue injury. In mammalian systems, iron availability is controlled by the interaction of the iron-regulatory hormone hepcidin with ferroportin, a molecule that functions both as the hepcidin receptor as well as the sole known cellular exporter of iron. By reducing iron export through ferroportin to blood plasma, hepcidin inhibits the mobilization of iron from stores and the absorption of dietary iron. Among the many processes requiring iron, erythropoiesis is the most iron-intensive, consuming most iron circulating in blood plasma. Under conditions of enhanced erythropoiesis, more iron is required to provide developing erythroblasts with adequate iron for heme and hemoglobin synthesis. Here the hormone erythroferrone, produced by erythroblasts, acts on hepatocytes to suppress hepcidin production, and thereby increase dietary iron absorption and mobilization from stores. This review focuses on the discovery of erythroferrone and recent advances in understanding the role of this hormone in the regulation of iron homeostasis during states of increased erythropoietic demand. Gaps in our understanding of the role of erythroferrone are highlighted for future study.

Smad7 deficiency decreases iron and haemoglobin through hepcidin up-regulation by multilayer compensatory mechanisms

To maintain iron homoeostasis, the iron regulatory hormone hepcidin is tightly controlled by BMP-Smad signalling pathway, but the physiological role of Smad7 in hepcidin regulation remains elusive. We generated and characterized hepatocyte-specific Smad7 knockout mice (Smad7^Alb/Alb ), which showed decreased serum iron, tissue iron, haemoglobin concentration, up-regulated hepcidin and increased phosphor-Smad1/5/8 levels in both isolated primary hepatocytes and liver tissues. Increased levels of hepcidin lead to reduced expression of intestinal ferroportin and mild iron deficiency anaemia. Interestingly, we found no difference in hepcidin expression or phosphor-Smad1/5/8 levels between iron-challenged Smad7^Alb/Alb and Smad7^flox/flox , suggesting other factors assume the role of iron-induced hepcidin regulation in Smad7 deletion. We performed RNA-seq to identify differentially expressed genes in the liver. Significantly up-regulated genes were then mapped to pathways, revealing TGF-β signalling as one of the most relevant pathways, including the up-regulated genes Smad6, Bambi and Fst (Follistatin). We found that Smad6 and Bambi-but not Follistatin-are controlled by the iron-BMP-Smad pathway. Overexpressing Smad6, Bambi or Follistatin in cells significantly reduced hepcidin expression. Smad7 functions as a key regulator of iron homoeostasis by negatively controlling hepcidin expression, and Smad6 and Smad7 have non-redundant roles. Smad6, Bambi and Follistatin serve as additional inhibitors of hepcidin in the liver.

Trace Elements and Healthcare: A Bioinformatics Perspective

Biological trace elements are essential for human health. Imbalance in trace element metabolism and homeostasis may play an important role in a variety of diseases and disorders. While the majority of previous researches focused on experimental verification of genes involved in trace element metabolism and those encoding trace element-dependent proteins, bioinformatics study on trace elements is relatively rare and still at the starting stage. This chapter offers an overview of recent progress in bioinformatics analyses of trace element utilization, metabolism, and function, especially comparative genomics of several important metals. The relationship between individual elements and several diseases based on recent large-scale systematic studies such as genome-wide association studies and case-control studies is discussed. Lastly, developments of ionomics and its recent application in human health are also introduced.

Differential regulation of hepcidin in cancer and non-cancer tissues and its clinical implications

Hepcidin is a crucial peptide for regulating cellular iron efflux. Because iron is essential for cell survival, especially for highly active cells, such as tumor cells, it is imperative to understand how tumor cells manipulate hepcidin expression for their own metabolic needs. Studies suggest that hepcidin expression and regulation in tumor cells show important differences in comparison with those in non-tumorous cells. These differences should be investigated to develop new strategies to fight cancer cells. Manipulating hepcidin expression to starve cancer cells for iron may prove to be a new therapy in the anticancer arsenal.

Hemojuvelin Predicts Acute Kidney Injury and Poor Outcomes Following Cardiac Surgery

Acute kidney injury (AKI) is detrimental after cardiac surgery. In this multicenter study, the novel biomarker hemojuvelin (HJV) was evaluated for AKI prediction following cardiac surgery. Urinary HJV, neutrophil gelatinase-associated lipocalin (NGAL), and urinary creatinine were measured in 151 patients after surgery. The outcomes of advanced AKI (KDIGO stages 2 and 3) and all causes of in-hospital mortality as the composite outcome were recorded. Areas under the receiver operator characteristic curves (AUC) and a multivariate generalized additive model (GAM) were applied to predict these outcomes of interest. Urinary HJV differentiated patients with/without AKI, advanced AKI or composite outcome after surgery (p < 0.001, by a generalized estimating equation) in this study. At three hours post-surgery, urinary HJV predicted advanced AKI (p < 0.001) and composite outcome (p < 0.001) with corresponding AUC values of 0.768 and 0.828, respectively. The performance of creatinine-adjusted HJV was also superior to NGAL in predicting advanced AKI (AUC = 0.784 and 0.694; p = 0.037) and composite outcome (AUC = 0.842 and 0.676; p = 0.002). The integration of HJV into the Cleveland Clinic score for advanced AKI led to a significant increase in risk stratification (net reclassification improvement [NRI] = 0.598; p < 0.001).