Liver iron modulates hepcidin expression during chronically elevated erythropoiesis in mice

Combined spectroscopic, biochemical and chemometric approach toward finding of biochemical markers of obesity

BACKGROUND

Early-stage detection of subclinical obesity-driven systemic changes is a challenging area of medical diagnostics, where the most popular existing measures - such as body mass index - BMI - often fall short of providing a realistic estimate of adiposity and, therefore, of ongoing pathologies at the systemic, tissue and cellular level. In the quest for identifying new more robust diagnostic markers, whole-organ analysis of chemical elements is a promising approach for identifying candidate proxies of obesity status in the system.

METHODS

Total Reflection X-ray fluorescence (TXRF) coupled with biochemical assays, chemometrics and statistical validation was used as a new integrated pipeline for marker identification in external ear samples of obese animals. The specimens were taken from obese animals fed a high calorie diet as well as from lean intact animals fed a standard diet.

RESULTS

The most significant differences in the content of K, Fe, Br, and Rb between the studied groups of the animals were identified. However, with the methodology applied Rb was found the most robust biochemical discriminator of early-stage obesity effects, as validated by the logistic regression model. We observed no relationship between the levels of the elements consumed by the animals and their apparent content in the earlobe tissue samples.

CONCLUSIONS

Our preliminary study confirms that obesity alters tissue trace metal metabolism and shows the proposed new approach as an accurate and reliable methodology for detecting tissue elemental obesity-related alterations.

GENERAL SIGNIFICANCE

This result can be of practical significance for designing new point-of-care systems for obesity screening tests, taking advantage of direct/indirect Rb measurements.

Atorvastatin Inhibits Ferroptosis of H9C2 Cells by regulatingSMAD7/Hepcidin Expression to Improve Ischemia-Reperfusion Injury

Background

Ferroptosis plays a key role in cardiomyopathy. Atorvastatin (ATV) has a protective effect on ischemia-reperfusion (I/R) cardiomyopathy. The purpose of this study is to elucidate the mechanism of ATV in I/R injury.

Methods

H9C2 cells and cardiomyopathy rats were induced by hypoxia/reoxygenation (H/R) and I/R to construct in vitro and in vivo models. Cell viability was determined by CCK8. Cardiac histopathology was observed by HE staining. Transmission electron microscope (TEM) was used to observe the mitochondrial morphology. The reactive oxygen species (ROS) content in cells was analyzed by the biochemical method. ELISA was conducted to calculate the concentrations of total iron/Fe²⁺ and hepcidin. The expression of ferroptosis and SMAD pathway-related genes were detected by qPCR. Western blot was performed to detect the expression levels of ferroptosis and SMAD pathway-related proteins.

Results

In H9C2 cells, ATV reversed the decline in cell viability, mitochondrial shrinkage, and ROS elevation induced by erastin or H/R. The concentration of total iron and Fe²⁺ in H/R-induced H9C2 cells increased, and the protein expression of FPN1 decreased. After ATV treatment, the concentration of total iron and Fe²⁺ decreased, and the protein expression of FPN1 increased. The expression of the SMAD7 gene in H/R-induced H9C2 cells decreased, and the expression of the hepcidin gene increased, which were reversed by ATV. When SMAD7 was knocked down, ATV treatment failed to produce the above effect. ATV also improved ferroptosis in I/R rat myocardium through the SMAD7/hepcidin pathway.

Conclusions

ATV reversed the decline in H9C2 cell viability, mitochondrial shrinkage, and ROS elevation, and improved the myocardium ferroptosis through the SMAD7/hepcidin pathway in I/R rat.

Developmental changes in iron metabolism and erythropoiesis in mice with human gain-of-function erythropoietin receptor

Iron availability for erythropoiesis is controlled by the iron-regulatory hormone hepcidin. Increased erythropoiesis negatively regulates hepcidin synthesis by erythroferrone (ERFE), a hormone produced by erythroid precursors in response to erythropoietin (EPO). The mechanisms coordinating erythropoietic activity with iron homeostasis in erythrocytosis with low EPO are not well defined as exemplified by dominantly inherited (heterozygous) gain-of-function mutation of human EPO receptor (mtHEPOR) with low EPO characterized by postnatal erythrocytosis. We previously created a mouse model of this mtHEPOR that develops fetal erythrocytosis with a transient perinatal amelioration of erythrocytosis and its reappearance at 3-6 weeks of age. Prenatally and perinatally, mtHEPOR heterozygous and homozygous mice (differing in erythrocytosis severity) had increased Erfe transcripts, reduced hepcidin, and iron deficiency. Epo was transiently normal in the prenatal life; then decreased at postnatal day 7, and remained reduced in adulthood. Postnatally, hepcidin increased in mtHEPOR heterozygotes and homozygotes, accompanied by low Erfe induction and iron accumulation. With aging, the old, especially mtHEPOR homozygotes had a decline of erythropoiesis, myeloid expansion, and local bone marrow inflammatory stress. In addition, mtHEPOR erythrocytes had a reduced lifespan. This, together with reduced iron demand for erythropoiesis, due to its age-related attenuation, likely contributes to increased iron deposition in the aged mtHEPOR mice. In conclusion, the erythroid drive-mediated inhibition of hepcidin production in mtHEPOR mice in the prenatal/perinatal period is postnatally abrogated by increasing iron stores promoting hepcidin synthesis. The differences observed in studied characteristics between mtHEPOR heterozygotes and homozygotes suggest dose-dependent alterations of downstream EPOR stimulation.

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.

Coordination of iron homeostasis by bone morphogenetic proteins: Current understanding and unanswered questions

Iron homeostasis is tightly regulated to balance the iron requirement for erythropoiesis and other vital cellular functions, while preventing cellular injury from iron excess. The liver hormone hepcidin is the master regulator of systemic iron balance by controlling the degradation and function of the sole known mammalian iron exporter ferroportin. Liver hepcidin expression is coordinately regulated by several signals that indicate the need for more or less iron, including plasma and tissue iron levels, inflammation, and erythropoietic drive. Most of these signals regulate hepcidin expression by modulating the activity of the bone morphogenetic protein (BMP)-SMAD pathway, which controls hepcidin transcription. Genetic disorders of iron overload and iron deficiency have identified several hepatocyte membrane proteins that play a critical role in mediating the BMP-SMAD and hepcidin regulatory response to iron. However, the precise molecular mechanisms by which serum and tissue iron levels are sensed to regulate BMP ligand production and promote the physical and/or functional interaction of these proteins to modulate SMAD signaling and hepcidin expression remain uncertain. This critical commentary will focus on the current understanding and key unanswered questions regarding how the liver senses iron levels to regulate BMP-SMAD signaling and thereby hepcidin expression to control systemic iron homeostasis.

Epigallocatechin-3-Gallate Suppresses BMP-6-Mediated SMAD1/5/8 Transactivation of Hepcidin Gene by Inducing SMILE in Hepatocytes

Hepcidin, a major regulator of systemic iron homeostasis, is mainly induced in hepatocytes by activating bone morphogenetic protein 6 (BMP-6) signaling in response to changes in the iron status. Small heterodimer partner-interacting leucine zipper protein (SMILE), a polyphenol-inducible transcriptional co-repressor, regulates hepatic gluconeogenesis and lipogenesis. Here, we examine the epigallocatechin-3-gallate (EGCG) effect on BMP-6-mediated SMAD1/5/8 transactivation of the hepcidin gene. EGCG treatment significantly decreased BMP-6-induced hepcidin gene expression and secretion in hepatocytes, which, in turn, abated ferroportin degradation. SMILE overexpression significantly decreased BMP receptor-induced hepcidin promoter activity. SMILE overexpression also significantly suppressed BMP-6-mediated induction of hepcidin mRNA and its secretion in HepG2 and AML12 cells. EGCG treatment inhibited BMP-6-mediated hepcidin gene expression and secretion, which were significantly reversed by SMILE knockdown in hepatocytes. Interestingly, SMILE physically interacted with SMAD1 in the nucleus and significantly blocked DNA binding of the SMAD complex to the BMP-response element on the hepcidin gene promoter. Taken together, these findings suggest that SMILE is a novel transcriptional repressor of BMP-6-mediated hepcidin gene expression, thus contributing to the control of iron homeostasis.

Hepcidin secretion was not directly proportional to intracellular iron-loading in recombinant-TfR1 HepG2 cells: short communication

Hepcidin is the master regulator of systemic iron homeostasis and its dysregulation is observed in several chronic liver diseases. Unlike the extracellular iron-sensing mechanisms, the intracellular iron-sensing mechanisms in the hepatocytes that lead to hepcidin induction and secretion are incompletely understood. Here, we aimed to understand the direct role of intracellular iron-loading on hepcidin mRNA and peptide secretion using our previously characterised recombinant HepG2 cells that over-express the cell-surface iron-importer protein transferrin receptor-1. Gene expression of hepcidin (HAMP) was determined by real-time PCR. Intracellular iron levels and secreted hepcidin peptide levels were measured by ferrozine assay and immunoassay, respectively. These measurements were compared in the recombinant and wild-type HepG2 cells under basal conditions at 30 min, 2 h, 4 h and 24 h. Data showed that in the recombinant cells, intracellular iron content was higher than wild-type cells at 30 min (3.1-fold, p < 0.01), 2 h (4.6-fold, p < 0.01), 4 h (4.6-fold, p < 0.01) and 24 h (1.9-fold, p < 0.01). Hepcidin (HAMP) mRNA expression was higher than wild-type cells at 30 min (5.9-fold; p = 0.05) and 24 h (6.1-fold; p < 0.03), but at 4 h, the expression was lower than that in wild-type cells (p < 0.05). However, hepcidin secretion levels in the recombinant cells were similar to those in wild-type cells at all time-points, except at 4 h, when the level was lower than wild-type cells (p < 0.01). High intracellular iron in recombinant HepG2 cells did not proportionally increase hepcidin peptide secretion. This suggests a limited role of elevated intracellular iron in hepcidin secretion.

Elevated FGF23 and disordered renal mineral handling with reduced bone mineralization in chronically erythropoietin over-expressing transgenic mice

Fibroblast Growth Factor 23 (FGF23) is a phosphaturic factor causing increased renal phosphate excretion as well as suppression of 1,25 (OH)₂-vitamin D_3. Highly elevated FGF23 can promote development of rickets and osteomalacia. We and others previously reported that acute application of erythropoietin (EPO) stimulates FGF23 production. Considering that EPO is clinically used as chronic treatment against anemia, we used here the Tg6 mouse model that constitutively overexpresses human EPO in an oxygen-independent manner, to examine the consequences of long-term EPO therapy on mineral and bone metabolism. Six to eight weeks old female Tg6 mice showed elevated intact and C-terminal fragment of FGF23 but normal plasma levels of PTH, calcitriol, calcium and phosphate. Renal function showed moderate alterations with higher urea and creatinine clearance and mild albuminuria. Renal phosphate excretion was normal whereas mild hypercalciuria was found. Renal expression of the key proteins TRPV5 and calbindin D28k involved in active calcium reabsorption was reduced in Tg6 mice. Plasma levels of the bone turnover marker osteocalcin were comparable between groups. However, urinary excretion of deoxypyridinoline (DPD) was lower in Tg6 mice. MicroCT analysis showed reduced total, cortical, and trabecular bone mineral density in femora from Tg6 mice. Our data reveal that chronic elevation of EPO is associated with high FGF23 levels and disturbed mineral homeostasis resulting in reduced bone mineral density. These observations imply the need to study the impact of therapeutically applied EPO on bone mineralization in patients, especially those suffering from chronic kidney disease.

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.

Imbalance of erythropoiesis and iron metabolism in patients with thalassemia

Aim: This study aimed to evaluate the imbalance of erythropoiesis and iron metabolism in patients with thalassemia. Methods: 192 patients with non-transfusion-dependent thalassemia (NTDT), 94 patients with transfusion-dependent thalassemia (TDT) and 101 healthy controls were recruited between June 2013 and December 2016 in the Hematology Department, the First Affiliated Hospital of Guangxi Medical University. The groups were compared in terms of levels of erythropoiesis biomarkers [growth differentiation factor 15 (GDF15), erythropoietin (EPO) and soluble transferrin receptor (sTfR)] and of iron overload biomarkers [serum ferritin (SF), liver iron concentration (LIC) and cardiac T2*] and hepcidin. Results: The levels of GDF15, EPO, sTfR, LIC and SF were significantly higher in patients with thalassemia. The levels of GDF15 and EPO were significantly higher in patients with TDT compared to NTDT. Those with iron overload had higher EPO, GDF15, SF and sTfR levels compared with non-iron overload patients. Hepcidin levels and ratios of hepcidin to erythropoietic activity and to iron biomarker levels were lower in patients with β-thalassemia intermedia or hemoglobin (Hb) E/β-thalassemia than in patients with HbH disease. The hepcidin levels were correlated negatively with the levels of EPO, GDF15 and sTfR in patients with NTDT and TDT, but correlated positively with SF and Hb levels only in patients with TDT. Conclusions: Patients with thalassemia showed iron overload, reduced hepcidin levels, and a greater extent of ineffective erythropoiesis. The hepcidin levels were more strongly related to ineffective erythropoiesis compared with iron overload. The imbalance between erythropoiesis and iron metabolism differed across different thalassemia types.

Efficacy of iron supplementation may be misinterpreted using conventional measures of iron status in iron-depleted, nonanemic women undergoing aerobic exercise training

Background: Despite its known detrimental effects, iron deficiency remains the most common micronutrient deficiency in the world. Many interventions that aim to improve iron status involve physically active populations. Intense aerobic exercise training negatively affects iron status; however, the impact of regular moderate aerobic exercise on the effectiveness of iron supplementation remains unclear.Objective: This study aimed to determine whether aerobic training modifies the assessment of the effectiveness of iron supplementation in improving conventional iron status measures.Design: Seventy-two iron-depleted, nonanemic Chinese women [serum ferritin (sFer) <25 μg/L and hemoglobin >110 g/L] were included in an 8-wk, partially blinded, randomized controlled trial with a 2 × 2 factorial design including iron supplements (42 mg elemental Fe/d) or placebo and aerobic training (five 25-min sessions/wk at 75-85% of maximum heart rate) or no training. Linear mixed models were used to evaluate the relation between supplement type, training, and changes in iron status over time, measured by sFer, hemoglobin, soluble transferrin receptor (sTfR), and estimated total body iron.Results: After treatment, both the iron-supplemented trained and untrained groups showed significantly improved sFer, sTfR, and body iron values compared with either of the placebo groups. Similarly, trained participants had significantly higher aerobic fitness measures than untrained participants. Training modified the sFer response to supplementation (training by supplement interaction, P = 0.07), with the iron-supplemented trained group having significantly lower sFer than the iron-supplemented untrained group at week 8 (mean ± SD: 31.8 ± 13.5 and 47.6 ± 15.7 μg/L, respectively; P = 0.042), whereas there was no significant difference between the placebo trained and untrained groups (21.3 ± 12.2 and 20.3 ± 7.0 μg/L, respectively; P = 1.00).Conclusions: Regular aerobic training reduces the apparent effectiveness of iron supplementation in improving sFer and calls into question whether conventional measures of iron status accurately reflect iron metabolism in physically active, nonanemic women. This trial was registered at clinicaltrials.gov as NCT03002090.

Erythropoietin administration increases splenic erythroferrone protein content and liver TMPRSS6 protein content in rats

Erythroferrone (ERFE) and TMPRSS6 are important proteins in the regulation of iron metabolism. The objective of the study was to examine splenic ERFE and liver TMPRSS6 synthesis in rats treated with a combination of iron and erythropoietin (EPO). EPO was administered to female Wistar rats at 600U/day for four days, iron-pretreated rats received 150mg of iron before EPO treatment. Content of ERFE and TMPRSS6 proteins was determined by commercial antibodies. Iron pretreatment prevented the EPO-induced decrease in hepcidin expression. Content of phosphorylated SMAD 1,5,8 proteins was decreased in the liver by both EPO and iron plus EPO treatment. Fam132b expression in the spleen was increased both by EPO and iron plus EPO treatments; these treatments also significantly induced splenic Fam132a expression. ERFE protein content in the spleen was increased both by EPO and iron plus EPO to a similar extent. EPO administration increased TMPRSS6 content in the plasma membrane-enriched fraction of liver homogenate; in iron-pretreated rats, this increase was abolished. The results confirm that iron pretreatment prevents the EPO-induced decrease in liver Hamp expression. This effect probably occurs despite high circulating ERFE levels, since EPO-induced ERFE protein synthesis is not influenced by iron pretreatment.

Characterization of hepcidin response to holotransferrin in novel recombinant TfR1 HepG2 cells

Hepcidin is the key regulator of systemic iron homeostasis. The iron-sensing mechanisms and the role of intracellular iron in modulating hepatic hepcidin secretion are unclear. Therefore, we created a novel cell line, recombinant-TfR1 HepG2, expressing iron-response-element-independent TFRC mRNA to promote cellular iron-overload and examined the effect of excess holotransferrin (5g/L) on cell-surface TfR1, iron content, hepcidin secretion and mRNA expressions of TFRC, HAMP, SLC40A1, HFE and TFR2. Results showed that the recombinant cells exceeded levels of cell-surface TfR1 in wild-type cells under basal (2.8-fold; p<0.03) and holotransferrin-supplemented conditions for 24h and 48h (4.4- and 7.5-fold, respectively; p<0.01). Also, these cells showed higher intracellular iron content than wild-type cells under basal (3-fold; p<0.03) and holotransferrin-supplemented conditions (6.6-fold at 4h; p<0.01). However, hepcidin secretion was not higher than wild-type cells. Moreover, holotransferrin treatment to recombinant cells did not elevate HAMP responses compared to untreated or wild-type cells. In conclusion, increased intracellular iron content in recombinant cells did not increase hepcidin responses compared to wild-type cells, resembling hemochromatosis. Furthermore, TFR2 expression altered within 4h of treatment, while HFE expression altered later at 24h and 48h, suggesting that TFR2 may function prior to HFE in HAMP regulation.

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.

Recent Advances in Iron Metabolism: Relevance for Health, Exercise, and Performance

Iron is necessary for physiological processes essential for athletic performance, such as oxygen transport, energy production, and cell division. However, an excess of "free" iron is toxic because it produces reactive hydroxyl radicals that damage biological molecules, thus leading to cell and tissue injury. Therefore, iron homeostasis is strictly regulated; and in recent years, there have been important advancements in our knowledge of the underlying processes. Hepcidin is the central regulator of systemic iron homeostasis and exerts its function by controlling the presence of the iron exporter ferroportin on the cell membrane. Hepcidin binding induces ferroportin degradation, thus leading to cellular iron retention and decreased levels of circulating iron. As iron is required for hemoglobin synthesis, the tight link between erythropoiesis and iron metabolism is particularly relevant to sports physiology. The iron needed for hemoglobin synthesis is ensured by inhibiting hepcidin to increase ferroportin activity and iron availability and hence to make certain that efficient blood oxygen transport occurs for aerobic exercise. However, hepcidin expression is also affected by exercise-associated conditions, such as iron deficiency, anemia or hypoxia, and, particularly, inflammation, which can play a role in the pathogenesis of sports anemia. Here, we review recent advances showing the relevance of iron for physical exercise and athletic performance. Low body iron levels can cause anemia and thus limit the delivery of oxygen to exercising muscle, but tissue iron deficiency may also affect performance by, for example, hampering muscle oxidative metabolism. Accordingly, a hemoglobin-independent effect of iron on exercise capacity has been demonstrated in animal models and humans. Here, we review recent advances showing the relevance of iron for physical exercise and athletic performance.

Limiting hepatic Bmp-Smad signaling by matriptase-2 is required for erythropoietin-mediated hepcidin suppression in mice

Hepcidin, the main regulator of iron homeostasis, is repressed when erythropoiesis is acutely stimulated by erythropoietin (EPO) to favor iron supply to maturing erythroblasts. Erythroferrone (ERFE) has been identified as the erythroid regulator that inhibits hepcidin in stress erythropoiesis. A powerful hepcidin inhibitor is the serine protease matriptase-2, encoded by TMPRSS6, whose mutations cause iron refractory iron deficiency anemia. Because this condition has inappropriately elevated hepcidin in the presence of high EPO levels, a role is suggested for matriptase-2 in EPO-mediated hepcidin repression. To investigate the relationship between EPO/ERFE and matriptase-2, we show that EPO injection induces Erfe messenger RNA expression but does not suppress hepcidin in Tmprss6 knockout (KO) mice. Similarly, wild-type (WT) animals, in which the bone morphogenetic protein-mothers against decapentaplegic homolog (Bmp-Smad) pathway is upregulated by iron treatment, fail to suppress hepcidin in response to EPO. To further investigate whether the high level of Bmp-Smad signaling of Tmprss6 KO mice counteracts hepcidin suppression by EPO, we generated double KO Bmp6-Tmprss6 KO mice. Despite having Bmp-Smad signaling and hepcidin levels that are similar to WT mice under basal conditions, double KO mice do not suppress hepcidin in response to EPO. However, pharmacologic downstream inhibition of the Bmp-Smad pathway by dorsomorphin, which targets the BMP receptors, improves the hepcidin responsiveness to EPO in Tmprss6 KO mice. We concluded that the function of matriptase-2 is dominant over that of ERFE and is essential in facilitating hepcidin suppression by attenuating the BMP-SMAD signaling.

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.

Insights into the antimicrobial properties of hepcidins: advantages and drawbacks as potential therapeutic agents

The increasing frequency of multi-drug resistant microorganisms has driven research into alternative therapeutic strategies. In this respect, natural antimicrobial peptides (AMPs) hold much promise as candidates for the development of novel antibiotics. However, AMPs have some intrinsic drawbacks, such as partial degradation by host proteases or inhibition by host body fluid composition, potential toxicity, and high production costs. This review focuses on the hepcidins, which are peptides produced by the human liver with a known role in iron homeostasis, as well by numerous other organisms (including fish, reptiles, other mammals), and their potential as antibacterial and antifungal agents. Interestingly, the antimicrobial properties of human hepcidins are enhanced at acidic pH, rendering these peptides appealing for the design of new drugs targeting infections that occur in body areas with acidic physiological pH. This review not only considers current research on the direct killing activity of these peptides, but evaluates the potential application of these molecules as coating agents preventing biofilm formation and critically assesses technical obstacles preventing their therapeutic application.

Erythropoietin's inhibiting impact on hepcidin expression occurs indirectly

Under conditions of accelerated erythropoiesis, elevated erythropoietin (Epo) levels are associated with inhibition of hepcidin synthesis, a response that ultimately increases iron availability to meet the enhanced iron needs of erythropoietic cells. In the search for erythroid regulators of hepcidin, many candidates have been proposed, including Epo itself. We aimed to test whether direct interaction between Epo and the liver is required to regulate hepcidin. We found that prolonged administration of high doses of Epo in mice leads to great inhibition of liver hepcidin mRNA levels, and concomitant induction of the hepcidin inhibitor erythroferrone (ERFE). Epo treatment also resulted in liver iron mobilization, mediated by increased ferroportin activity and accompanied by reduced ferritin levels and increased TfR1 expression. The same inhibitory effect was observed in mice that do not express the homodimeric Epo receptor (EpoR) in liver cells because EpoR expression is restricted to erythroid cells. Similarly, liver signaling pathways involved in hepcidin regulation were not influenced by the presence or absence of hepatic EpoR. Moreover, Epo analogs, possibly interacting with the postulated heterodimeric β common EpoR, did not affect hepcidin expression. These findings were supported by the lack of inhibition on hepcidin found in hepatoma cells exposed to various concentrations of Epo for different periods of times. Our results demonstrate that hepcidin suppression does not require the direct binding of Epo to its liver receptors and rather suggest that the role of Epo is to stimulate the synthesis of the erythroid regulator ERFE in erythroblasts, which ultimately downregulates hepcidin.

Identification of erythroferrone as an erythroid regulator of iron metabolism

Recovery from blood loss requires a greatly enhanced supply of iron to support expanded erythropoiesis. After hemorrhage, suppression of the iron-regulatory hormone hepcidin allows increased iron absorption and mobilization from stores. We identified a new hormone, erythroferrone (ERFE), that mediates hepcidin suppression during stress erythropoiesis. ERFE is produced by erythroblasts in response to erythropoietin. ERFE-deficient mice fail to suppress hepcidin rapidly after hemorrhage and exhibit a delay in recovery from blood loss. ERFE expression is greatly increased in Hbb(th3/+) mice with thalassemia intermedia, where it contributes to the suppression of hepcidin and the systemic iron overload characteristic of this disease.