Regulation of type II transmembrane serine proteinase TMPRSS6 by hypoxia-inducible factors: new link between hypoxia signaling and iron homeostasis

Iron biology

Iron is a fundamental element for biological life, from bacteria to humans. Iron is essential for cell function and survival, energy production and metabolism, whereas increased levels cause oxidative stress. It is also a constituent of haemoglobin and thus it is necessary for oxygen transportation through the body. Given these multiple functions, the regulation of iron metabolism is complex and tight coupled with oxygen homeostasis at tissue and cellular levels, thanks to the interaction with the hypoxia inducible factor system. In patients with chronic kidney disease (CKD), iron deficiency significantly contributes to anaemia development. This frequently overlaps with chronic inflammation, causing iron- restricted erythropoiesis. To add further complexity, metabolic hyperferritinemia may, on one hand, increase the risk for CKD and, on the other, overlaps with functional iron deficiency. Excessive intracellular iron in certain cell types during CKD can also mediate cellular death (called ferroptosis), and contribute to the pathogenesis of kidney damage, atherosclerosis and vascular calcifications. This review is aimed at broadening the perspective of iron metabolism in the setting of CKD not just as a contributor to anaemia in CKD patients, but also as an important player with an impact on cell metabolism, renal fibrosis and the cardiovascular system.

Red Blood Cell Distribution Width is a Biomarker of Red Cell Dysfunction Associated with High Systemic Inflammation and a Prognostic Marker in Heart Failure and Cardiovascular Disease: A Potential Predictor of Atrial Fibrillation Recurrence

At the beginning of the 21st century, approximately 2.3 million US adults had atrial fibrillation (AF), and there has been a 60% increase in hospital admissions for AF. Given that the expectancy is a continuous increase in incidence, it portends a severe healthcare problem. Considerable evidence supports the immune system and inflammatory response in cardiac tissue, and circulatory processes are involved in the physiopathology of AF. In this regard, finding novel inflammatory biomarkers that predict AF recurrence after catheter ablation (CA) is a prime importance global healthcare problem. Many inflammatory biomarkers and natriuretic peptides came out and were shown to have predictive capabilities for AF recurrence in patients undergoing CA. In this regard, some studies have shown that red blood cell distribution width (RDW) is associated with the risk of incident AF. This review aimed to provide an update on the evidence of the RDW as a biomarker of red cell dysfunction and its association with high systemic inflammation, and with the risk of incident AF. Through the literature review, we will highlight the most relevant studies of the RDW related to AF recurrence after CA. Many studies demonstrated that RDW is associated with all cause-mortality, heart failure, cardiovascular disease, and AF, probably because RDW is a biomarker of red blood cell dysfunction associated with high systemic inflammation, reflecting an advanced heart disease with prognostic implications in heart failure and cardiovascular disease. Thus, suggesting that could be a potential predictor for AF recurrence after CA. Moreover, the RDW is a parameter included in routine full blood count, which is low-cost, quick, and easy to obtain. We provided an update on the evidence of the most relevant studies of the RDW related to AF recurrence after CA, as well as the mechanism of the high RDW and its association with high systemic inflammation and prognostic marker in cardiovascular disease and heart failure.

Iron deficiency and supplementation in heart failure

Non-anaemic iron deficiency (NAID) is a strategic target in cardiovascular medicine because of its association with a range of adverse effects in various conditions. Endeavours to tackle NAID in heart failure have yielded mixed results, exposing knowledge gaps in how best to define 'iron deficiency' and the handling of iron therapies by the body. To address these gaps, we harness the latest understanding of the mechanisms of iron homeostasis outside the erythron and integrate clinical and preclinical lines of evidence. The emerging picture is that current definitions of iron deficiency do not assimilate the multiple influences at play in patients with heart failure and, consequently, fail to identify those with a truly unmet need for iron. Additionally, current iron supplementation therapies benefit only certain patients with heart failure, reflecting differences in the nature of the unmet need for iron and the modifying effects of anaemia and inflammation on the handling of iron therapies by the body. Building on these insights, we identify untapped opportunities in the management of NAID, including the refinement of current approaches and the development of novel strategies. Lessons learned from NAID in cardiovascular disease could ultimately translate into benefits for patients with other chronic conditions such as chronic kidney disease, chronic obstructive pulmonary disease and cancer.

A functional interplay between the two BMP-SMAD pathway inhibitors TMPRSS6 and FKBP12 regulates hepcidin expression in vivo

The Activin A Receptor type I (ALK2) is a critical component of BMP-SMAD signaling that, in the presence of ligands, phosphorylates cytosolic SMAD1/5/8 and modulates important biological processes, including bone formation and iron metabolism. In hepatocytes, the BMP-SMAD pathway controls the expression of hepcidin, the liver peptide hormone that regulates body iron homeostasis via the BMP receptors ALK2 and ALK3, and the hemochromatosis proteins. The main negative regulator of the pathway in the liver is transmembrane serine protease 6 (TMPRSS6), which downregulates hepcidin by cleaving the BMP coreceptor hemojuvelin. ALK2 function is inhibited also by the immunophilin FKBP12, which maintains the receptor in an inactive conformation. FKBP12 sequestration by tacrolimus or its silencing upregulates hepcidin in primary hepatocytes and in vivo in acute but not chronic settings. Interestingly, gain-of-function mutations in ALK2 that impair FKBP12 binding to the receptor and activate the pathway cause a bone phenotype in patients affected by Fibrodysplasia Ossificans Progressiva but not hepcidin and iron metabolism dysfunction. This observation suggests that additional mechanisms are active in the liver to compensate for the increased BMP-SMAD signaling. Here we demonstrate that Fkbp12 downregulation in hepatocytes by antisense oligonucleotide treatment upregulates the expression of the main hepcidin inhibitor Tmprss6, thus counteracting the ALK2-mediated activation of the pathway. Combined downregulation of both Fkbp12 and Tmprss6 blocks this compensatory mechanism. Our findings reveal a previously unrecognized functional cross talk between FKBP12 and TMPRSS6, the main BMP-SMAD pathway inhibitors, in the control of hepcidin transcription.NEW & NOTEWORTHY This study uncovers a previously unrecognized mechanism of hepcidin and BMP-SMAD pathway regulation in hepatocytes mediated by the immunophilin FKBP12 and the transmembrane serine protease TMPRSS6.

Associated Effect of SLC40A1 and TMPRSS6 Polymorphisms on Iron Overload

Mutations in the ferroportin (FPN) gene SLC40A1 alter iron recycling and cause disturbances in iron homeostasis. The variants of TMPRSS6 contribute to the development of iron deficiencies. In this study, we determined the role of FPN and TMPRSS6 gene polymorphisms in the modulation of iron homeostasis based on biochemical parameters. PCR analysis and sequencing were performed to determine the single nucleotide polymorphisms (SNPs) SLC40A1 c.44−24G>C (rs1439816), SLC40A1 c.663T>C (rs2304704), and TMPRSS6 c.2207T>C (rs855791). Hemoglobin concentration and iron status were determined by standard procedures. We studied 79 iron-loaded individuals for SLC40A1 polymorphisms. Interestingly, 35/79 individuals with SLC40A1 SNPs also carried a TMPRSS6 c.2207T>C polymorphism. The biochemical values of the iron overloaded individuals were compared to those of the individuals carrying TMPRSS6 SNPs and the healthy individuals (wild-type group). The ferritin concentration, transferrin saturation % (TS%), and hemoglobin concentration were significantly higher in the participants with FPN SNPs than in the other three groups. The ferritin concentration and TS% were higher in participants with both SLC40A1 and TMPRSS6 SNPs than in the TMPRSS6 and wild-type groups, while hemoglobin concentration was significantly higher than that in the TMPRSS6 SNP group only. The participants with TMPRSS6 SNPs had significantly lower ferritin concentration, TS%, and hemoglobin concentration than all the other groups. SLC40A1 and TMPRSS6 SNPs might act in the opposite direction, preventing the development of severe iron overload, and the modulation of the iron status by TMPRSS6 SNPs might provide protection.

Novel insights into alcoholic liver disease: Iron overload, iron sensing and hemolysis

The liver is the major target organ of continued alcohol consumption at risk and resulting alcoholic liver disease (ALD) is the most common liver disease worldwide. The underlying molecular mechanisms are still poorly understood despite decades of scientific effort limiting our abilities to identify those individuals who are at risk to develop the disease, to develop appropriate screening strategies and, in addition, to develop targeted therapeutic approaches. ALD is predestined for the newly evolving translational medicine, as conventional clinical and health care structures seem to be constrained to fully appreciate this disease. This concept paper aims at summarizing the 15 years translational experience at the Center of Alcohol Research in Heidelberg, namely based on the long-term prospective and detailed characterization of heavy drinkers with mortality data. In addition, novel experimental findings will be presented. A special focus will be the long-known hepatic iron accumulation, the somewhat overlooked role of the hematopoietic system and novel insights into iron sensing and the role of hepcidin. Our preliminary work indicates that enhanced red blood cell (RBC) turnover is critical for survival in ALD patients. RBC turnover is not primarily due to vitamin deficiency but rather to ethanol toxicity directly targeted to erythrocytes but also to the bone marrow stem cell compartment. These novel insights also help to explain long-known aspects of ALD such as mean corpuscular volume of erythrocytes (MCV) and elevated aspartate transaminase (GOT/AST) levels. This work also aims at identifying future projects, naming unresolved observations, and presenting novel hypothetical concepts still requiring future validation.

Iron Mining for Erythropoiesis

Iron is necessary for essential processes in every cell of the body, but the erythropoietic compartment is a privileged iron consumer. In fact, as a necessary component of hemoglobin and myoglobin, iron assures oxygen distribution; therefore, a considerable amount of iron is required daily for hemoglobin synthesis and erythroid cell proliferation. Therefore, a tight link exists between iron metabolism and erythropoiesis. The liver-derived hormone hepcidin, which controls iron homeostasis via its interaction with the iron exporter ferroportin, coordinates erythropoietic activity and iron homeostasis. When erythropoiesis is enhanced, iron availability to the erythron is mainly ensured by inhibiting hepcidin expression, thereby increasing ferroportin-mediated iron export from both duodenal absorptive cells and reticuloendothelial cells that process old and/or damaged red blood cells. Erythroferrone, a factor produced and secreted by erythroid precursors in response to erythropoietin, has been identified and characterized as a suppressor of hepcidin synthesis to allow iron mobilization and facilitate erythropoiesis.

NRF2 and Hypoxia-Inducible Factors: Key Players in the Redox Control of Systemic Iron Homeostasis

Significance: Oxygen metabolism and iron homeostasis are closely linked. Iron facilitates the oxygen-carrying capacity of blood, and its deficiency causes anemia. Conversely, excess free iron is detrimental for stimulating the formation of reactive oxygen species, causing tissue damage. The amount and distribution of iron thus need to be tightly regulated by the liver-expressed hormone hepcidin. This review analyzes the roles of key oxygen-sensing pathways in cellular and systemic regulation of iron homeostasis; specifically, the prolyl hydroxylase domain (PHD)/hypoxia-inducible factor (HIF) and the Kelch-like ECH-associated protein 1/NF-E2 p45-related factor 2 (KEAP1/NRF2) pathways, which mediate tissue adaptation to low and high oxygen, respectively. Recent Advances: In macrophages, NRF2 regulates genes involved in hemoglobin catabolism, iron storage, and iron export. NRF2 was recently identified as the molecular sensor of iron-induced oxidative stress and is responsible for BMP6 expression by liver sinusoidal endothelial cells, which in turn activates hepcidin synthesis by hepatocytes to restore systemic iron levels. Moreover, NRF2 orchestrates the activation of antioxidant defenses that are crucial to protect against iron toxicity. On the contrary, low iron/hypoxia stabilizes renal HIF2a via inactivation of iron-dependent PHD dioxygenases, causing an erythropoietic stimulus that represses hepcidin via an inhibitory effect of erythroferrone on bone morphogenetic proteins. Intestinal HIF2a is also stabilized, increasing the expression of genes involved in dietary iron absorption. Critical Issues: An intimate crosstalk between oxygen-sensing pathways and iron regulatory mechanisms ensures that fluctuations in systemic iron levels are promptly detected and restored. Future Directions: The realization that redox-sensitive transcription factors regulate systemic iron levels suggests novel therapeutic approaches. Antioxid. Redox Signal. 35, 433-452.

Serum Erythroferrone During Pregnancy Is Related to Erythropoietin but Does Not Predict the Risk of Anemia

BACKGROUND

Maintaining adequate iron status during pregnancy is important for the mother and her developing fetus. Iron homeostasis is influenced by 3 regulatory hormones: erythropoietin (EPO), hepcidin, and erythroferrone (ERFE). To date, normative data on ERFE across pregnancy and its relations to other hormones and iron status indicators are limited.

OBJECTIVES

The objective of this study was to characterize maternal ERFE across pregnancy and at delivery and evaluate the utility of hepcidin, ERFE, and EPO in identifying women with increased iron needs.

METHODS

ERFE was measured in extant serum samples collected from 2 longitudinal cohorts composed of women carrying multiple fetuses (n = 79) and pregnant adolescents (n = 218) at midgestation (∼26 wk) and delivery (∼39 wk). Receiver operating characteristic curves were generated to characterize the predictive ability of serum ERFE, hepcidin, and EPO and their ratios to identify women at increased risk of iron deficiency and anemia.

RESULTS

In these pregnant women, mean ERFE was 0.48 ng/mL at both ∼25 wk of gestation and at delivery. ERFE was positively associated with EPO at midgestation (β = 0.14, P = 0.002, n = 202) and delivery (β = 0.12, P < 0.001, n = 225) but was not significantly associated with maternal hepcidin at any time point surveyed. Of all hormones measured at midgestation and delivery, EPO was best able to identify women with anemia (AUC: 0.86 and 0.75, respectively) and depleted iron stores (AUC: 0.77 and 0.84), whereas the hepcidin-to-EPO ratio was best able to identify women with iron deficiency anemia (AUC: 0.85 and 0.84).

CONCLUSIONS

Maternal ERFE was significantly associated with EPO but was not able to identify women with gestational iron deficiency. At term, the hepcidin-to-EPO ratio, an index that accounts for both iron status and erythropoietic demand, and EPO were the strongest indicators of maternal iron deficiency and anemia. This trial was registered at clinicaltrials.gov as NCT04517734 (https://clinicaltrials.gov/ct2/show/NCT04517734).

The hypoferremic response to acute inflammation is maintained in thalassemia mice even under parenteral iron loading

Background

Hepcidin controls iron homeostasis by inducing the degradation of the iron efflux protein, ferroportin (FPN1), and subsequently reducing serum iron levels. Hepcidin expression is influenced by multiple factors, including iron stores, ineffective erythropoiesis, and inflammation. However, the interactions between these factors under thalassemic condition remain unclear. This study aimed to determine the hypoferremic and transcriptional responses of iron homeostasis to acute inflammatory induction by lipopolysaccharide (LPS) in thalassemic (Hbb^th3/+) mice with/without parenteral iron loading with iron dextran.

Methods

Wild type and Hbb^th3/+ mice were intramuscularly injected with 5 mg of iron dextran once daily for two consecutive days. After a 2-week equilibration, acute inflammation was induced by an intraperitoneal injection of a single dose of 1 µg/g body weight of LPS. Control groups for both iron loading and acute inflammation received equal volume(s) of saline solution. Blood and tissue samples were collected at 6 hours after LPS (or saline) injection. Iron parameters and mRNA expression of hepcidin as well as genes involved in iron transport and metabolism in wild type and Hbb^th3/+ mice were analyzed and compared by Kruskal–Wallis test with pairwise Mann–Whitney U test.

Results

We found the inductive effects of LPS on liver IL-6 mRNA expression to be more pronounced under parenteral iron loading. Upon LPS administration, splenic erythroferrone (ERFE) mRNA levels were reduced only in iron-treated mice, whereas, liver bone morphogenetic protein 6 (BMP6) mRNA levels were decreased under both control and parenteral iron loading conditions. Despite the altered expression of the aforementioned hepcidin regulators, the stimulatory effect of LPS on hepcidin mRNA expression was blunt in iron-treated Hbb^th3/+ mice. Contrary to the blunted hepcidin response, LPS treatment suppressed FPN1 mRNA expression in the liver, spleen, and duodenum, as well as reduced serum iron levels of Hbb^th3/+ mice with parenteral iron loading.

Conclusion

Our study suggests that a hypoferremic response to LPS-induced acute inflammation is maintained in thalassemic mice with parenteral iron loading in a hepcidin-independent manner.

Amelioration of chronic kidney disease-associated anemia by vadadustat in mice is not dependent on erythroferrone

Vadadustat is an investigational hypoxia-inducible factor prolyl hydroxylase inhibitor that increases endogenous erythropoietin production and has been shown to decrease hepcidin levels, ameliorate iron restriction, and increase hemoglobin concentrations in anemic patients with chronic kidney disease (CKD). In studies of physiological responses to other erythropoietic stimuli, erythropoietin induced erythroblast secretion of erythroferrone (ERFE), which acts on the liver to suppress hepcidin production and mobilize iron for erythropoiesis. We therefore investigated whether vadadustat effects on erythropoiesis and iron metabolism are dependent on ERFE. Wild type and ERFE knockout mice with and without CKD were treated with vadadustat or vehicle. In both wild type and ERFE knockout CKD models, vadadustat was similarly effective, as evidenced by normalized hemoglobin concentrations, increased expression of duodenal iron transporters, lower serum hepcidin levels, and decreased tissue iron concentrations. This is consistent with ERFE-independent increased iron mobilization. Vadadustat treatment also lowered serum urea nitrogen and creatinine concentrations and decreased expression of kidney fibrosis markers. Lastly, vadadustat affected fibroblast growth factor 23 (FGF23) profiles: in non-CKD mice, vadadustat increased plasma total FGF23 out of proportion to intact FGF23, consistent with the known effects of hypoxia-inducible factor-1α and erythropoietin on FGF23 production and metabolism. However, in the mice with CKD, vadadustat markedly decreased both total and intact FGF23, effects likely contributed to by the reduced loss of kidney function. Thus, in this CKD model, vadadustat ameliorated anemia independently of ERFE, improved kidney parameters, and decreased FGF23. How vadadustat affects CKD progression in humans warrants future studies.

HIF1A: A Putative Modifier of Hemochromatosis

HFE-related hereditary hemochromatosis (HH) is characterized by marked phenotypic heterogeneity. Homozygosity for p.C282Y is a low penetrance genotype suggesting that the HFE-HH is a multifactorial disease resulting from a complex interaction involving a major gene defect, genetic background and environmental factors. We performed a targeted NGS-based gene panel to identify new candidate modifiers by using an extreme phenotype sampling study based on serum ferritin and iron removed/age ratio. We found an increased prevalence of the HIF1A p.Phe582Ser and p.Ala588Thr variants in patients with a severe iron and clinical phenotype. Accordingly, Huh-7 cells transfected with both variants showed significantly lower HAMP promoter activity by luciferase assay. The qRT-PCR assays showed a downregulation of hepcidin and an upregulation of the HIF1A target genes (VEGF, HMOX, FUR, TMPRSS6) in cells transfected with the HIF1A-P582S vector. We identified mutations in other genes (e.g., Serpina1) that might have some relevance in single cases in aggravating or mitigating disease manifestation. In conclusion, the present study identified HIF1A as a possible modifier of the HFE-HH phenotype cooperating with the genetic defect in downregulating hepcidin synthesis. In addition, this study highlights that an NGS-based approach could broaden our knowledge and help in characterizing the genetic complexity of HFE-HH patients with a severe phenotype expression.

Inflammation Mediated Hepcidin-Ferroportin Pathway and Its Therapeutic Window in Breast Cancer

Experimental and clinical data strongly support that iron is an essential element which plays a big role in cancer biology. Thus, hepcidin (Hp) and ferroportin (Fpn) are molecules that regulate and maintain the metabolism of iron. A peptide hormone hepcidin limits recycled and stored iron fluxes in macrophage and hepatic hepatocyte, respectively, to the blood stream by promoting degradation of the only iron exporter, Fpn, in the target cells. Moreover, the inflammatory microenvironment of breast cancer and altered hepcidin/ferroportin pathway is intimately linked. Breast cancer exhibits an iron seeking phenotype that is accomplished by tumor-associated macrophage (TAM). Because macrophages contribute to breast cancer growth and progression, this review will discuss TAM with an emphasis on describing how TAM (M2Ф phenotypic) interacts with their surrounding microenvironment and results in dysregulated Hp/Fpn and pathologic accumulation of iron as a hallmark of its malignant condition. Moreover, the underlying stroma or tumor microenvironment releases significant inflammatory cytokines like IL-6 and bone morphogenetic proteins like BMP-2 and 6 leading in aberrant Hp/Fpn pathways in breast cancer. Inflammation is primarily associated with the high intracellular iron levels, deregulated hepcidin/ferroportin pathway, and its upstream signaling in breast cancer. Subsequently, scholars have been reported that reducing iron level and manipulating the signaling molecules involved in iron metabolism can be used as a promising strategy of tumor chemotherapy. Here, we review the key molecular aspects of iron metabolism and its regulatory mechanisms of the hepcidin/ferroportin pathways and its current therapeutic strategies in breast cancer.

Regulation of Iron Homeostasis and Related Diseases

The liver is the organ for iron storage and regulation; it senses circulating iron concentrations in the body through the BMP-SMAD pathway and regulates the iron intake from food and erythrocyte recovery into the bloodstream by secreting hepcidin. Under iron deficiency, hypoxia, and hemorrhage, the liver reduces the expression of hepcidin to ensure the erythropoiesis but increases the excretion of hepcidin during infection and inflammation to reduce the usage of iron by pathogens. Excessive iron causes system iron overload; it accumulates in never system and damages neurocyte leading to neurodegenerative diseases such as Parkinson's syndrome. When some gene mutations affect the perception of iron and iron regulation ability in the liver, then they decrease the expression of hepcidin, causing hereditary diseases such as hereditary hemochromatosis. This review summarizes the source and utilization of iron in the body, the liver regulates systemic iron homeostasis by sensing the circulating iron concentration, and the expression of hepcidin regulated by various signaling pathways, thereby understanding the pathogenesis of iron-related diseases.

Therapeutic Advances in Regulating the Hepcidin/Ferroportin Axis

The interaction between hepcidin and ferroportin is the key mechanism involved in regulation of systemic iron homeostasis. This axis can be affected by multiple stimuli including plasma iron levels, inflammation and erythropoietic demand. Genetic defects or prolonged inflammatory stimuli results in dysregulation of this axis, which can lead to several disorders including hereditary hemochromatosis and anaemia of chronic disease. An imbalance in iron homeostasis is increasingly being associated with worse disease outcomes in many clinical conditions including multiple cancers and neurological disorders. Currently, there are limited treatment options for regulating iron levels in patients and thus significant efforts are being made to uncover approaches to regulate hepcidin and ferroportin expression. These approaches either target these molecules directly or regulatory steps which mediate hepcidin or ferroportin expression. This review examines the current status of hepcidin and ferroportin agonists and antagonists, as well as inducers and inhibitors of these proteins and their regulatory pathways.

Hepcidin and Anemia: A Tight Relationship

Hepcidin, the master regulator of systemic iron homeostasis, tightly influences erythrocyte production. High hepcidin levels block intestinal iron absorption and macrophage iron recycling, causing iron restricted erythropoiesis and anemia. Low hepcidin levels favor bone marrow iron supply for hemoglobin synthesis and red blood cells production. Expanded erythropoiesis, as after hemorrhage or erythropoietin treatment, blocks hepcidin through an acute reduction of transferrin saturation and the release of the erythroblast hormone and hepcidin inhibitor erythroferrone. Quantitatively reduced erythropoiesis, limiting iron consumption, increases transferrin saturation and stimulates hepcidin transcription. Deregulation of hepcidin synthesis is associated with anemia in three conditions: iron refractory iron deficiency anemia (IRIDA), the common anemia of acute and chronic inflammatory disorders, and the extremely rare hepcidin-producing adenomas that may develop in the liver of children with an inborn error of glucose metabolism. Inappropriately high levels of hepcidin cause iron-restricted or even iron-deficient erythropoiesis in all these conditions. Patients with IRIDA or anemia of inflammation do not respond to oral iron supplementation and show a delayed or partial response to intravenous iron. In hepcidin-producing adenomas, anemia is reverted by surgery. Other hepcidin-related anemias are the “iron loading anemias” characterized by ineffective erythropoiesis and hepcidin suppression. This group of anemias includes thalassemia syndromes, congenital dyserythropoietic anemias, congenital sideroblastic anemias, and some forms of hemolytic anemias as pyruvate kinase deficiency. The paradigm is non-transfusion-dependent thalassemia where the release of erythroferrone from the expanded pool of immature erythroid cells results in hepcidin suppression and secondary iron overload that in turn worsens ineffective erythropoiesis and anemia. In thalassemia murine models, approaches that induce iron restriction ameliorate both anemia and the iron phenotype. Manipulations of hepcidin might benefit all the above-described anemias. Compounds that antagonize hepcidin or its effect may be useful in inflammation and IRIDA, while hepcidin agonists may improve ineffective erythropoiesis. Correcting ineffective erythropoiesis in animal models ameliorates not only anemia but also iron homeostasis by reducing hepcidin inhibition. Some targeted approaches are now in clinical trials: hopefully they will result in novel treatments for a variety of anemias.

Natural selection on TMPRSS6 associated with the blunted erythropoiesis and improved blood viscosity in Tibetan pigs

Tibetan pigs, indigenous to Tibetan plateau, are well adapted to hypoxia. So far, there have been not any definitively described genes and functional sites responsible for hypoxia adaptation for the Tibetan pig. The whole genome-wide association studies in human suggested that genetic variations in TMPRSS6 was associated with hemoglobin concentration (HGB) and red cell counts (RBC). Here we conducted resequencing of the nearly entire genomic region (40.1 kb) of the candidate gene TMPRSS6 in 40 domestic pigs and 40 wild boars along continuous altitudes and identified 708 SNPs, in addition to an indel (CGTG/----) in the intron 10. We conduct the CGTG indel in 838 domestic pigs, both the CGTG deletion frequency and the pairwise r² linkage disequilibrium showed an increase with elevated altitudes, suggesting that TMPRSS6 has been under Darwinian positive selection. As the conserved core sequence of hypoxia-response elements (HREs), the deletion of CGTG in Tibetan pigs decreased the expression levels of TMPRSS6 mRNA and protein in the liver revealed by real-time quantitative PCR and western blot, respectively. We compared domestic pigs and Tibetan pigs living continuous altitudes, found that the blood-related traits with the increase of altitude, however, the HGB did not increase with the elevation in Tibetan pigs. Genotype association analysis results dissected a genetic effect on reducing HGB by 13.25 g/L in Gongbo'gyamda Tibetan pigs, decreasing mean corpuscular volume (MCV) by 4.79 fl in Diqing Tibetan pigs. In conclusion, the CGTG deletion of TMPRSS6 resulted in lower HGB and smaller MCV, which could reflect a blunting erythropoiesis and improving blood viscosity as well as erythrocyte deformability. It remains to be determined whether a blunting of erythropoiesis for TMPRSS6 or others genetic effects are the physiological adaptations among Tibetan pigs.

Hepcidin in chronic kidney disease anemia

Chronic kidney disease (CKD) is associated with several complications that worsen with progression of disease; anemia, disturbances in iron metabolism and inflammation are common features. Inflammatory response starts early, releasing pro-inflammatory cytokines, acute phase reactants and hepcidin. Hepcidin production is modulated by several factors, as hypoxia/anemia, erythropoietin and erythropoiesis products, transferrin saturation (TSAT) and liver iron levels, which are altered in CKD. Treatment of CKD anemia is based on pharmaceutical intervention, with erythropoietic stimulating agents and/or iron supplementation; however, in spite of the erythropoietic benefits, this therapy, on a regular basis, involves risks, namely iron overload. To overcome these risks, some therapeutic approaches are under study to target CKD anemia. Considering the actual alerts about risk of iron overload in dialysis patients, inhibition of hepcidin, the central key player in iron homeostasis, could be a pivotal strategy in the management of CKD anemia.

Signaling pathways regulating hepcidin

Since its discovery in 2001, there have been a number of important discoveries and findings that have increased our knowledge about the functioning of hepcidin. Hepcidin, the master iron regulator has been shown to be regulated by a number of physiological stimuli and their associated signaling pathways. This chapter will summarize our current understanding of how these physiological stimuli and downstream signaling molecules are involved in hepcidin modulation and ultimately contribute to the regulation of systemic or local iron homeostasis. The signaling pathways and molecules described here have been shown to primarily affect hepcidin at a transcriptional level, but these transcriptional changes correlate with changes in systemic iron levels as well, supporting the functional effects of hepcidin regulation by these signaling pathways.

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.

Hepcidin regulation in a mouse model of acute hypoxia

OBJECTIVE

During hypoxia, hepcidin expression is inhibited to allow iron mobilization to sustain erythropoietic expansion. We analyzed molecular mechanisms underlying hypoxia-induced hepcidin inhibition in an in vivo model of acute hypoxia.

METHODS

Mice were kept under normal or hypoxic conditions for 6 hours and 15 hours and treated with α-PDGF-BB antibody or PDGF-BB receptor inhibitor. Blood, liver, spleen, and bone marrow were collected to extract RNA and protein or to quantify EPO and PDGF-BB. mRNA and protein levels were assessed by RT-PCR and Western blot.

RESULTS

Hepcidin was strongly inhibited at 15 hours, and this downregulation followed erythropoiesis activation and upregulation of several growth factors. PDGF-BB, erythroferrone, GDF15, and TWSG1 were upregulated by hypoxia in the bone marrow, but not in spleen or liver. Inactivation of PDGF-BB or its receptor suppressed the hypoxia-induced hepcidin inhibition.

CONCLUSION

Spleen and liver are not involved in the early stages of hypoxia-induced hepcidin downregulation. Our data support the role of PDGF-BB and probably also of erythroferrone in the recruitment of iron for erythropoiesis in the hypoxia setting. The rapid normalization of all the erythroid factors against persistent hepcidin suppression suggests that other signals are involved that should be clarified in future studies.

What can we learn from ineffective erythropoiesis in thalassemia?

Erythropoiesis is a dynamic process regulated at multiple levels to balance proliferation, differentiation and survival of erythroid progenitors. Ineffective erythropoiesis is a key feature of various diseases, including β-thalassemia. The pathogenic mechanisms leading to ineffective erythropoiesis are complex and still not fully understood. Altered survival and decreased differentiation of erythroid progenitors are both critical processes contributing to reduced production of mature red blood cells. Recent studies have identified novel important players and provided major advances in the development of targeted therapeutic approaches. In this review, β-thalassemia is used as a paradigmatic example to describe our current knowledge on the mechanisms leading to ineffective erythropoiesis and novel treatments that may have the potential to improve the clinical phenotype of associated diseases in the future.

Hypoxia enhances H2O2-mediated upregulation of hepcidin: Evidence for NOX4-mediated iron regulation

The exact regulation of the liver-secreted peptide hepcidin, the key regulator of systemic iron homeostasis, is still poorly understood. It is potently induced by iron, inflammation, cytokines or H₂O₂ but conflicting results have been reported on hypoxia. In our current study, we first show that pronounced (1%) and mild (5%) hypoxia strongly induces hepcidin in human Huh7 hepatoma and primary liver cells predominantly at the transcriptional level via STAT3 using two hypoxia systems (hypoxia chamber and enzymatic hypoxia by the GOX/CAT system). SiRNA silencing of JAK1, STAT3 and NOX4 diminished the hypoxia-mediated effect while a role of HIF1α could be clearly ruled out by the response to hypoxia-mimetics and competition experiments with a plasmid harboring the oxygen-dependent degradation domain of HIF1α. Specifically, hypoxia drastically enhances the H₂O₂-mediated induction of hepcidin strongly pointing towards an oxidase as powerful upstream control of hepcidin. We finally provide evidences for an efficient regulation of hepcidin expression by NADPH-dependent oxidase 4 (NOX4) in liver cells. In summary, our data demonstrate that hypoxia strongly potentiates the peroxide-mediated induction of hepcidin via STAT3 signaling pathway. Moreover, oxidases such as NOX4 or artificially overexpressed urate oxidase (UOX) can induce hepcidin. It remains to be studied whether the peroxide-STAT3-hepcidin axis simply acts to continuously compensate for oxygen fluctuations or is directly involved in iron sensing per se.

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Hypoxia strongly induces hepcidin via STAT3 signaling.

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HIF1α is not involved in hepcidin regulation under hypoxia.

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Hypoxia enhances hydrogen peroxide-mediated hepcidin induction.

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Oxidases, such as NOX4 are powerful inducers of hepcidin.

The relationship between systemic iron homeostasis and erythropoiesis

Red blood cell production (erythropoiesis) is the single largest consumer of iron in the body; this need is satisfied by maintaining a sensitive regulation of iron levels. The level of erythropoietic demand regulates the expression of the iron hormone hepcidin and thus iron absorption. Erythropoiesis-mediated regulation of hepcidin is an area of increasing importance and recent studies have identified a number of potential regulatory proteins. This review summarizes our current knowledge about these candidate erythroid regulators of hepcidin and the relation between transferrin receptors and erythropoiesis.

Does Hypoxia Cause Carcinogenic Iron Accumulation in Alcoholic Liver Disease (ALD)?

Alcoholic liver disease (ALD) is a leading health risk worldwide. Hepatic iron overload is frequently observed in ALD patients and it is an important and independent factor for disease progression, survival, and the development of primary liver cancer (HCC). At a systemic level, iron homeostasis is controlled by the liver-secreted hormone hepcidin. Hepcidin regulation is complex and still not completely understood. It is modulated by many pathophysiological conditions associated with ALD, such as inflammation, anemia, oxidative stress/H₂O_2, or hypoxia. Namely, the data on hypoxia-signaling of hepcidin are conflicting, which seems to be mainly due to interpretational limitations of in vivo data and methodological challenges. Hence, it is often overlooked that hepcidin-secreting hepatocytes are physiologically exposed to 2–7% oxygen, and that key oxygen species such as H₂O₂ act as signaling messengers in such a hypoxic environment. Indeed, with the recently introduced glucose oxidase/catalase (GOX/CAT) system it has been possible to independently study hypoxia and H₂O₂ signaling. First preliminary data indicate that hypoxia enhances H₂O₂-mediated induction of hepcidin, pointing towards oxidases such as NADPH oxidase 4 (NOX4). We here review and discuss novel concepts of hypoxia signaling that could help to better understand hepcidin-associated iron overload in ALD.

Iron homeostasis: An anthropocentric perspective

The regulation of iron metabolism in biological systems centers on providing adequate iron for cellular function while limiting iron toxicity. Because mammals cannot excrete iron, mechanisms have evolved to control iron acquisition, storage, and distribution at both systemic and cellular levels. Hepcidin, the master regulator of iron homeostasis, controls iron flows into plasma through inhibition of the only known mammalian cellular iron exporter ferroportin. Hepcidin is feedback-regulated by iron status and strongly modulated by inflammation and erythropoietic demand. This review highlights recent advances that have changed our understanding of iron metabolism and its regulation.

Molecular pathogenesis and clinical consequences of iron overload in liver cirrhosis

BACKGROUND

The liver, as the main iron storage compartment and the place of hepcidin synthesis, is the central organ involved in maintaining iron homeostasis in the body. Excessive accumulation of iron is an important risk factor in liver disease progression to cirrhosis and hepatocellular carcinoma. Here, we review the literature on the molecular pathogenesis of iron overload and its clinical consequences in chronic liver diseases.

DATA SOURCES

PubMed was searched for English-language articles on molecular genesis of primary and secondary iron overload, as well as on their association with liver disease progression. We have also included literature on adjuvant therapeutic interventions aiming to alleviate detrimental effects of excessive body iron load in liver cirrhosis.

RESULTS

Excess of free, unbound iron induces oxidative stress, increases cell sensitivity to other detrimental factors, and can directly affect cellular signaling pathways, resulting in accelerated liver disease progression. Diagnosis of liver cirrhosis is, in turn, often associated with the identification of a pathological accumulation of iron, even in the absence of genetic background of hereditary hemochromatosis. Iron depletion and adjuvant therapy with antioxidants are shown to cause significant improvement of liver functions in patients with iron overload. Phlebotomy can have beneficial effects on liver histology in patients with excessive iron accumulation combined with compensated liver cirrhosis of different etiology.

CONCLUSION

Excessive accumulation of body iron in liver cirrhosis is an important predictor of liver failure and available data suggest that it can be considered as target for adjuvant therapy in this condition.

Soluble hemojuvelin in transfused and untransfused thalassaemic subjects

OBJECTIVE

The hemojuvelin-bone morphogenetic protein axis is the principal iron-dependent mechanism of hepcidin regulation. The determination of soluble hemojuvelin (sHJV) levels could allow for a better understanding of the pathophysiological mechanisms of hepcidin regulation in thalassaemia.

METHOD

We have assessed sHJV in 45 transfused and 15 untransfused thalassaemic patients in comparison with 15 healthy subjects, evaluating its relationships with some parameters of iron overload, anaemia and erythropoiesis.

RESULTS

Untransfused thalassaemic patients had more severe anaemia and erythropoietic activity, while in transfused patients, the transfused RBCs reduced % reticulocytes and sTfR, increased serum indices of iron overload and iron stores in the liver (low MRI T2* values). sHJV levels were higher in patients than in controls and in untransfused in comparison with transfused patients. In the transfused group, we also found that sHJV values are significantly related to serum ferritin, cardiac MRI T2* and growth differentiation factor 15 and are sensitive to hepatitis C virus infection.

CONCLUSION

These results suggest that sHJV synthesis seems to be affected by an erythropoietic/hypoxic signal in untransfused patients that have severe anaemia, while in regularly transfused subjects, it is influenced by iron stores.

The Human Ether-a-go-go-related Gene (hERG) Potassium Channel Represents an Unusual Target for Protease-mediated Damage

The human ether-a-go-go-related gene (hERG) encodes the pore-forming subunit of the rapidly activating delayed rectifier potassium channel (IKr), which is important for cardiac repolarization. Dysfunction of hERG causes long QT syndrome and sudden death, which occur in patients with cardiac ischemia. Cardiac ischemia is also associated with activation, up-regulation, and secretion of various proteolytic enzymes. Here, using whole-cell patch clamp and Western blotting analysis, we demonstrate that the hERG/IKr channel was selectively cleaved by the serine protease, proteinase K (PK). Using molecular biology techniques including making a chimeric channel between protease-sensitive hERG and insensitive human ether-a-go-go (hEAG), as well as application of the scorpion toxin BeKm-1, we identified that the S5-pore linker of hERG is the target domain for proteinase K cleavage. To investigate the physiological relevance of the unique susceptibility of hERG to proteases, we show that cardiac ischemia in a rabbit model was associated with a reduction in mature ERG expression and an increase in the expression of several proteases, including calpain. Using cell biology approaches, we found that calpain-1 was actively released into the extracellular milieu and cleaved hERG at the S5-pore linker. Using protease cleavage-predicting software and site-directed mutagenesis, we identified that calpain-1 cleaves hERG at position Gly-603 in the S5-pore linker of hERG. Clarification of protease-mediated damage of hERG extends our understanding of hERG regulation. Damage of hERG mediated by proteases such as calpain may contribute to ischemia-associated QT prolongation and sudden cardiac death.

Regulation of Hepcidin by Erythropoiesis: The Story So Far

Hepcidin is the master regulator of systemic iron homeostasis, facilitating iron balance by controlling intestinal iron absorption and recycling. Hepcidin levels are suppressed when erythropoiesis is stimulated, for example following acute blood loss, appropriately enhancing cellular iron export to the plasma to support production of new red blood cells. However, persistent increased and ineffective erythropoiesis, for example in thalassemia, results in sustained elevations in iron absorption, which cause iron overload with associated organ toxicities. The ligands, receptors, and canonical pathways by which iron loading and inflammation upregulate hepcidin expression have been largely established. However, although several mechanisms have been proposed, the means by which erythropoiesis causes hepcidin suppression have been unclear. The erythroid-derived hormone erythroferrone appears to be a convincing candidate for the link between increased erythropoiesis and hepcidin suppression. If confirmed to be clinically and physiologically relevant in humans, potentiation or inhibition of erythroferrone activity could be a crucial pharmaceutical strategy.

The Regulation of Iron Absorption and Homeostasis

Iron is an essential element in biology, required for numerous cellular processes. Either too much or too little iron can be detrimental, and organisms have developed mechanisms for balancing iron within safe limits. In mammals there are no controlled mechanisms for the excretion of excess iron, hence body iron homeostasis is regulated at the sites of absorption, utilisation and recycling. This review will discuss the discoveries that have been made in the past 20 years into advancing our understanding of iron homeostasis and its regulation. The study of iron-associated disorders, such as the iron overload condition hereditary haemochromatosis and various forms of anaemia have been instrumental in increasing our knowledge in this area, as have cellular and animal model studies. The liver has emerged as the major site of systemic iron regulation, being the location where the iron regulatory hormone hepcidin is produced. Hepcidin is a negative regulator of iron absorption and recycling, achieving this by binding to the only known cellular iron exporter ferroportin and causing its internalisation and degradation, thereby reducing iron efflux from target cells and reducing serum iron levels. Much of the research in the iron metabolism field has focussed on the regulation of hepcidin and its interaction with ferroportin. The advances in this area have greatly increased our knowledge of iron metabolism and its regulation and have led to the development of novel diagnostics and therapeutics for iron-associated disorders.

Four-Week Studies of Oral Hypoxia-Inducible Factor-Prolyl Hydroxylase Inhibitor GSK1278863 for Treatment of Anemia

Hypoxia-inducible factor prolyl hydroxylase inhibitors stabilize levels of hypoxia-inducible factor that upregulate transcription of multiple genes associated with the response to hypoxia, including production of erythropoietin. We conducted two phase 2a studies to explore the relationship between the dose of the hypoxia-inducible factor-prolyl hydroxylase inhibitor GSK1278863 and hemoglobin response in patients with anemia of CKD (baseline hemoglobin 8.5-11.0 g/dl) not undergoing dialysis and not receiving recombinant human erythropoietin (nondialysis study) and in patients with anemia of CKD (baseline hemoglobin 9.5-12.0 g/dl) on hemodialysis and being treated with stable doses of recombinant human erythropoietin (hemodialysis study). Participants were randomized 1:1:1:1 to a once-daily oral dose of GSK1278863 (0.5 mg, 2 mg, or 5 mg) or control (placebo for the nondialysis study; continuing on recombinant human erythropoietin for the hemodialysis study) for 4 weeks, with a 2-week follow-up. In the nondialysis study, GSK1278863 produced dose-dependent effects on hemoglobin, with the highest dose resulting in a mean increase of 1 g/dl at week 4. In the hemodialysis study, treatment with GSK1278863 in the 5-mg arm maintained mean hemoglobin concentrations after the switch from recombinant human erythropoietin, whereas mean hemoglobin decreased in the lower-dose arms. In both studies, the effects on hemoglobin occurred with elevations in endogenous erythropoietin within the range usually observed in the respective populations and markedly lower than those in the recombinant human erythropoietin control arm in the hemodialysis study, and without clinically significant elevations in plasma vascular endothelial growth factor concentrations. GSK1278863 was generally safe and well tolerated at the doses and duration studied. GSK1278863 may prove an effective alternative for managing anemia of CKD.

Hepcidin: A Promising Therapeutic Target for Iron Disorders: A Systematic Review

Iron is required for most forms of organisms, and it is the most essential element for the functions of many iron-containing proteins involved in oxygen transport, cellular respiration, DNA replication, and so on. Disorders of iron metabolism are associated with diverse diseases, including anemias (e.g., iron-deficiency anemia and anemia of chronic diseases) and iron overload diseases, such as hereditary hemochromatosis and β-thalassemia. Hepcidin (encoded by Hamp gene) is a peptide hormone synthesized by hepatocytes, and it plays an important role in regulating the systematic iron homeostasis. As the systemic iron regulator, hepcidin, not only controls dietary iron absorption and iron egress out of iron storage cells, but also induces iron redistribution in various organs. Deregulated hepcidin is often seen in a variety of iron-related diseases including anemias and iron overload disorders. In the case of iron overload disorders (e.g., hereditary hemochromatosis and β-thalassemia), hepatic hepcidin concentration is significantly reduced.Since hepcidin deregulation is responsible for iron disorder-associated diseases, the purpose of this review is to summarize the recent findings on therapeutics targeting hepcidin.Continuous efforts have been made to search for hepcidin mimics and chemical compounds that could be used to increase hepcidin level. Here, a literature search was conducted in PubMed, and research papers relevant to hepcidin regulation or hepcidin-centered therapeutic work were reviewed. On the basis of literature search, we recapitulated recent findings on therapeutic studies targeting hepcidin, including agonists and antagonists to modulate hepcidin expression or its downstream signaling. We also discussed the molecular mechanisms by which hepcidin level and iron metabolism are modulated.Elevating hepcidin concentration is an optimal strategy to ameliorate iron overload diseases, and also to relieve β-thalassemia phenotypes by improving ineffective erythropoiesis. Relative to the current conventional therapies, such as phlebotomy and blood transfusion, therapeutics targeting hepcidin would open a new avenue for treatment of iron-related diseases.

Effect of Erythropoietin, Iron Deficiency and Iron Overload on Liver Matriptase-2 (TMPRSS6) Protein Content in Mice and Rats

Matriptase-2 (TMPRSS6) is an important negative regulator of hepcidin expression; however, the effects of iron overload or accelerated erythropoiesis on liver TMPRSS6 protein content in vivo are largely unknown. We determined TMPRSS6 protein content in plasma membrane-enriched fractions of liver homogenates by immunoblotting, using a commercial antibody raised against the catalytic domain of TMPRSS6. Plasma membrane-enriched fractions were obtained by centrifugation at 3000 g and washing. TMPRSS6 was detected in the 3000 g fraction as a 120 kDa full-length protein in both mice and rats. Feeding of iron-deficient diet as well as erythropoietin treatment increased TMPRSS6 protein content in rats and mice by a posttranscriptional mechanism; the increase in TMPRSS6 protein by erythropoietin was also observed in Bmp6-mutant mice. Administration of high doses of iron to mice (200, 350 and 700 mg/kg) decreased TMPRSS6 protein content. Hemojuvelin was detected in the plasma membrane-enriched fractions of control animals as a full length protein of approximately 52 kDa; in iron deficient animals, the full length protein was partially cleaved at the N-terminus, resulting in an additional weak band of approximately 47 kDa. In livers from hemojuvelin-mutant mice, TMPRSS6 protein content was strongly decreased, suggesting that intact hemojuvelin is necessary for stable TMPRSS6 expression in the membrane. Overall, the results demonstrate posttranscriptional regulation of liver TMPRSS6 protein by iron status and erythropoietin administration, and provide support for the interaction of TMPRSS6 and hemojuvelin proteins in vivo.

New perspectives on the molecular basis of the interaction between oxygen homeostasis and iron metabolism

Oxygen and iron are two elements closely related from a (bio)chemical point of view. Moreover, they share the characteristic of being indispensable for life, while also being potentially toxic. Therefore, their level is strictly monitored, and sophisticated pathways have evolved to face variations in either element. In addition, the expression of proteins involved in iron and oxygen metabolism is mainly controlled by a complex interplay of proteins that sense both iron levels and oxygen availability (ie, prolyl hydroxylases, hypoxia inducible factors, and iron regulatory proteins), and in turn activate feedback mechanisms to re-establish homeostasis. In this review, we describe how cells and organisms utilize these intricate networks to regulate responses to changes in oxygen and iron levels. We also explore the role of these pathways in some pathophysiological settings.

Anaemia in kidney disease: harnessing hypoxia responses for therapy

Improved understanding of the oxygen-dependent regulation of erythropoiesis has provided new insights into the pathogenesis of anaemia associated with renal failure and has led to the development of novel therapeutic agents for its treatment. Hypoxia-inducible factor (HIF)-2 is a key regulator of erythropoiesis and iron metabolism. HIF-2 is activated by hypoxic conditions and controls the production of erythropoietin by renal peritubular interstitial fibroblast-like cells and hepatocytes. In anaemia associated with renal disease, erythropoiesis is suppressed due to inadequate erythropoietin production in the kidney, inflammation and iron deficiency; however, pharmacologic agents that activate the HIF axis could provide a physiologic approach to the treatment of renal anaemia by mimicking hypoxia responses that coordinate erythropoiesis with iron metabolism. This Review discusses the functional inter-relationships between erythropoietin, iron and inflammatory mediators under physiologic conditions and in relation to the pathogenesis of renal anaemia, as well as recent insights into the molecular and cellular basis of erythropoietin production in the kidney. It furthermore provides a detailed overview of current clinical experience with pharmacologic activators of HIF signalling as a novel comprehensive and physiologic approach to the treatment of anaemia.

Hepcidin: regulation of the master iron regulator

Iron, an essential nutrient, is required for many diverse biological processes. The absence of a defined pathway to excrete excess iron makes it essential for the body to regulate the amount of iron absorbed; a deficiency could lead to iron deficiency and an excess to iron overload and associated disorders such as anaemia and haemochromatosis respectively. This regulation is mediated by the iron-regulatory hormone hepcidin. Hepcidin binds to the only known iron export protein, ferroportin (FPN), inducing its internalization and degradation, thus limiting the amount of iron released into the blood. The major factors that are implicated in hepcidin regulation include iron stores, hypoxia, inflammation and erythropoiesis. The present review summarizes our present knowledge about the molecular mechanisms and signalling pathways contributing to hepcidin regulation by these factors.

Iron homeostasis: a new job for macrophages in adipose tissue?

Elevated serum ferritin and increased cellular iron concentrations are risk factors for diabetes; however, the etiology of this association is unclear. Metabolic tissues such as pancreas, liver, and adipose tissue (AT), as well as the immune cells resident in these tissues, may be involved. Recent studies demonstrate that the polarization status of macrophages has important relevance to their iron-handling capabilities. Furthermore, a subset of macrophages in AT have elevated iron concentrations and a gene expression profile indicative of iron handling, a capacity diminished in obesity. Because iron overload in adipocytes increases systemic insulin resistance, iron handling by AT macrophages may have relevance not only to adipocyte iron stores but also to local and systemic insulin sensitivity.

Iron and oxygen sensing: a tale of 2 interacting elements?

Iron and oxygen metabolism are intimately linked with one another.

Hypoxia controls iron metabolism and glutamate secretion in retinal pigmented epithelial cells

BACKGROUND

Blood-barrier systems are essential in controlling iron levels in organs such as the brain and eye, both of which experience hypoxia in pathological conditions. While hypoxia's effects on numerous iron regulatory and storage proteins have been studied, little is known about how hypoxia affects iron metabolism. Iron also controls glutamate production and secretion; therefore the effects of hypoxia on iron metabolism and glutamate secretion were studied in polarized retinal pigmented epithelial (RPE) cells.

METHODS

Primary canine RPE were cultured in Millicells to create polarized cell cultures. Iron uptake and efflux were measured in hypoxic and normoxic conditions. RPE were loaded with ⁵⁹Fe-transferrin. Glutamate concentrations in the cell conditioned media were also measured.

RESULTS

Hypoxia induced a large increase in iron efflux from RPE in the basolateral direction. Glutamate secretion occurred mainly in the basolateral direction which is away from the retina and out of the eye in vivo. Glutamate secretion was doubled under hypoxic conditions.

CONCLUSIONS

Hypoxia is known to induce oxidative damage. The current results show that iron, a key catalyst of free radical generation, is removed from RPE under hypoxic conditions which may help protect RPE from oxidative stress. Results obtained here indicate the importance of using polarized tight junctional cells as more physiologically relevant models for blood-barrier-like systems.

GENERAL SIGNIFICANCE

While the effects of hypoxia on iron efflux and glutamate secretion may be protective for RPE cells and retina, increased glutamate secretion in the brain could cause some of the damaging neurological effects seen in stroke.

Iron-refractory iron deficiency anemia (IRIDA)

Iron deficiency anemia is a common global problem whose etiology is typically attributed to acquired inadequate dietary intake and/or chronic blood loss. However, in several kindreds multiple family members are affected with iron deficiency anemia that is unresponsive to oral iron supplementation and only partially responsive to parenteral iron therapy. The discovery that many of these cases harbor mutations in the TMPRSS6 gene led to the recognition that they represent a single clinical entity: iron-refractory iron deficiency anemia (IRIDA). This article reviews clinical features of IRIDA, recent genetic studies, and insights this disorder provides into the regulation of systemic iron homeostasis.

The role of TMPRSS6/matriptase-2 in iron regulation and anemia

Matriptase-2, encoded by the TMPRSS6 gene, is a member of the type II transmembrane serine protease family. Matriptase-2 has structural and enzymatic similarities to matriptase-1, which has been implicated in cancer progression. Matriptase-2 was later established to be essential in iron homeostasis based on the phenotypes of iron-refractory iron deficiency anemia identified in mouse models as well as in human patients with TMPRSS6 mutations. TMPRSS6 is expressed mainly in the liver and negatively regulates the production of hepcidin, the systemic iron regulatory hormone. This review focuses on the current understanding of matriptase-2 biochemistry, and its role in iron metabolism and cancer progression. In light of recent investigations, the function of matriptase-2 in hepcidin regulation, how it is being regulated, as well as the therapeutic potential of matriptase-2 are also discussed.

Hepcidin and sports anemia

Iron is an important mineral element used by the body in a variety of metabolic and physiologic processes. These processes are highly active when the body is undergoing physical exercises. Prevalence of exercise-induced iron deficiency anemia (also known as sports anemia) is notably high in athletic populations, particularly those with heavy training loads. The pathogenesis of sports anemia is closely related to disorders of iron metabolism, and a more comprehensive understanding of the mechanism of iron metabolism in the course of physical exercises could expand ways of treatment and prevention of sports anemia. In recent years, there have been remarkable research advances regarding the molecular mechanisms underlying changes of iron metabolism in response to physical exercises. This review has covered these advances, including effects of exercise on duodenum iron absorption, serum iron status, iron distribution in organs, erythropoiesis, and hepcidin’s function and its regulation. New methods for the treatment of exercise-induced iron deficiency are also discussed.

Iron refractory iron deficiency anemia

Iron refractory iron deficiency anemia is a hereditary recessive anemia due to a defect in the TMPRSS6 gene encoding Matriptase-2. This protein is a transmembrane serine protease that plays an essential role in down-regulating hepcidin, the key regulator of iron homeostasis. Hallmarks of this disease are microcytic hypochromic anemia, low transferrin saturation and normal/high serum hepcidin values. The anemia appears in the post-natal period, although in some cases it is only diagnosed in adulthood. The disease is refractory to oral iron treatment but shows a slow response to intravenous iron injections and partial correction of the anemia. To date, 40 different Matriptase-2 mutations have been reported, affecting all the functional domains of the large ectodomain of the protein. In vitro experiments on transfected cells suggest that Matriptase-2 cleaves Hemojuvelin, a major regulator of hepcidin expression and that this function is altered in this genetic form of anemia. In contrast to the low/undetectable hepcidin levels observed in acquired iron deficiency, in patients with Matriptase-2 deficiency, serum hepcidin is inappropriately high for the low iron status and accounts for the absent/delayed response to oral iron treatment. A challenge for the clinicians and pediatricians is the recognition of the disorder among iron deficiency and other microcytic anemias commonly found in pediatric patients. The current treatment of iron refractory iron deficiency anemia is based on parenteral iron administration; in the future, manipulation of the hepcidin pathway with the aim of suppressing it might become an alternative therapeutic approach.

Iron homeostasis in the liver

Iron is an essential nutrient that is tightly regulated. A principal function of the liver is the regulation of iron homeostasis. The liver senses changes in systemic iron requirements and can regulate iron concentrations in a robust and rapid manner. The last 10 years have led to the discovery of several regulatory mechanisms in the liver that control the production of iron regulatory genes, storage capacity, and iron mobilization. Dysregulation of these functions leads to an imbalance of iron, which is the primary cause of iron-related disorders. Anemia and iron overload are two of the most prevalent disorders worldwide and affect over a billion people. Several mutations in liver-derived genes have been identified, demonstrating the central role of the liver in iron homeostasis. During conditions of excess iron, the liver increases iron storage and protects other tissues, namely, the heart and pancreas from iron-induced cellular damage. However, a chronic increase in liver iron stores results in excess reactive oxygen species production and liver injury. Excess liver iron is one of the major mechanisms leading to increased steatohepatitis, fibrosis, cirrhosis, and hepatocellular carcinoma.

Regulation of systemic iron homeostasis

PURPOSE OF REVIEW

The circulating peptide hepcidin modulates systemic iron balance by limiting the absorption of dietary iron and the release of iron from macrophage stores. Recent studies conducted in humans, animal models, and tissue culture systems have enhanced our understanding of the molecular mechanisms by which hepcidin levels are altered in response to iron stores, inflammation, and erythropoietic activity.

RECENT FINDINGS

The bone morphogenetic protein (BMP) type I receptors ALK2 and ALK3 play key, nonredundant roles in mediating hepcidin synthesis through the BMP signaling pathway. Actions of the hereditary hemochromatosis proteins HFE and transferrin receptor 2 may intersect with the BMP pathway. Hepcidin induction in response to inflammation requires cooperative BMP signaling. A variety of innate immune and infectious stimuli induce hepcidin expression. The hypoxia inducible factor pathway appears to suppress hepcidin indirectly through the capacity of erythropoietin to stimulate erythropoiesis.

SUMMARY

Study of the molecular mechanisms underlying the regulation of hepcidin synthesis has revealed complex biology. Improved understanding of the signaling pathways involved in hepcidin regulation may contribute to improved therapeutic outcomes for patients with genetic and acquired disorders that impact systemic iron balance.