Targeting PKCα alleviates iron overload in diabetes and hemochromatosis through the inhibition of ferroportin.

ABSTRACT: Ferroportin (Fpn) is the only iron exporter, playing a crucial role in systemic iron homeostasis. Fpn is negatively regulated by its ligand hepcidin, but other potential regulators in physiological and disease conditions remain poorly understood. Diabetes is a metabolic disorder that develops body iron loading with unknown mechanisms. By using diabetic mouse models and human duodenal specimens, we demonstrated that intestinal Fpn expression was increased in diabetes in a hepcidin-independent manner. Protein kinase C (PKC) is hyperactivated in diabetes. We showed that PKCα was required to sustain baseline Fpn expression and diabetes-induced Fpn upregulation in the enterocytes and macrophages. Knockout of PKCα abolished diabetes-associated iron overload. Mechanistically, activation of PKCα increased the exocytotic trafficking of Fpn and decreased the endocytic trafficking of Fpn in the resting state. Hyperactive PKCα also suppressed hepcidin-induced ubiquitination, internalization, and degradation of Fpn. We further observed that iron loading in the enterocytes and macrophages activated PKCα, acting as a novel mechanism to enhance Fpn-dependent iron efflux. Finally, we demonstrated that the loss-of-function of PKCα and pharmacological inhibition of PKC significantly alleviated hereditary hemochromatosis-associated iron overload. Our study has highlighted, to our knowledge, for the first time, that PKCα is an important positive regulator of Fpn and a new target in the control of iron homeostasis.

The hepatokine FGL1 regulates hepcidin and iron metabolism during anemia in mice by antagonizing BMP signaling.

ABSTRACT: As a functional component of erythrocyte hemoglobin, iron is essential for oxygen delivery to all tissues in the body. The liver-derived peptide hepcidin is the master regulator of iron homeostasis. During anemia, the erythroid hormone erythroferrone regulates hepcidin synthesis to ensure the adequate supply of iron to the bone marrow for red blood cell production. However, mounting evidence suggested that another factor may exert a similar function. We identified the hepatokine fibrinogen-like 1 (FGL1) as a previously undescribed suppressor of hepcidin that is induced in the liver in response to hypoxia during the recovery from anemia, and in thalassemic mice. We demonstrated that FGL1 is a potent suppressor of hepcidin in vitro and in vivo. Deletion of Fgl1 in mice results in higher hepcidin levels at baseline and after bleeding. FGL1 exerts its activity by directly binding to bone morphogenetic protein 6 (BMP6), thereby inhibiting the canonical BMP-SMAD signaling cascade that controls hepcidin transcription.

Hepcidin and Iron in Health and Disease.

Hepcidin, the iron-regulatory hormone, determines plasma iron concentrations and total body iron content. Hepcidin, secreted by hepatocytes, functions by controlling the activity of the cellular iron exporter ferroportin, which delivers iron to plasma from intestinal iron absorption and from iron stores. Hepcidin concentration in plasma is increased by iron loading and inflammation and is suppressed by erythropoietic stimulation and during pregnancy. Hepcidin deficiency causes iron overload in hemochromatosis and anemias with ineffective erythropoiesis. Hepcidin excess causes iron-restrictive anemias including anemia of inflammation. The development of hepcidin diagnostics and therapeutic agonists and antagonists should improve the treatment of iron disorders.

Fetal factors disrupt placental and maternal iron homeostasis in murine ß-thalassemia.

Pregnancy rates in ß-thalassemia are increasing but the risk of complications is higher; thus, better understanding of maternal and fetal iron homeostasis in this disorder is needed. HbbTh3/+ (Th3/+) mice model human ß-thalassemia. Both the murine and human diseases are characterized by low hepcidin, high iron absorption, and tissue iron overload, with concurrent anemia. We hypothesized that disordered iron metabolism in pregnant Th3/+ mice would negatively affect their unborn offspring. The experimental design included these groups: wild-type (WT) dams carrying WT fetuses (WT1); WT dams carrying WT and Th3/+ fetuses (WT2); Th3/+ dams carrying WT and Th3/+ fetuses (Th3/+); and age-matched, nonpregnant adult females. Serum hepcidin was low, and mobilization of splenic and hepatic storage iron was enhanced in all 3 groups of experimental dams. Intestinal 59Fe absorption was lower in Th3/+ dams (as compared with WT1/2 dams) but splenic 59Fe uptake was higher. Th3/+ dams had hyperferremia, which led to fetal and placenta iron loading, fetal growth restriction, and placentomegaly. Notably, Th3/+ dams loaded Th3/+ and WT fetuses, with the latter situation more closely mirroring human circumstances when mothers with thalassemia have relatively unaffected (thalassemia trait) offspring. Iron-related oxidative stress likely contributed to fetal growth impairment; enhanced placental erythropoiesis is a probable cause of placental enlargement. Moreover, high fetal liver iron transactivated Hamp; fetal hepcidin downregulated placental ferroportin expression, limiting placental iron flux and thus mitigating fetal iron loading. Whether gestational iron loading occurs in human thalassemic pregnancy, when blood transfusion can further elevate serum iron, is worth consideration.

Iron is a modifier of the phenotypes of JAK2-mutant myeloproliferative neoplasms.

JAK 2-V617F mutation causes myeloproliferative neoplasms (MPNs) that can manifest as polycythemia vera (PV), essential thrombocythemia (ET), or primary myelofibrosis. At diagnosis, patients with PV already exhibited iron deficiency, whereas patients with ET had normal iron stores. We examined the influence of iron availability on MPN phenotype in mice expressing JAK2-V617F and in mice expressing JAK2 with an N542-E543del mutation in exon 12 (E12). At baseline, on a control diet, all JAK2-mutant mouse models with a PV-like phenotype displayed iron deficiency, although E12 mice maintained more iron for augmented erythropoiesis than JAK2-V617F mutant mice. In contrast, JAK2-V617F mutant mice with an ET-like phenotype had normal iron stores comparable with that of wild-type (WT) mice. On a low-iron diet, JAK2-mutant mice and WT controls increased platelet production at the expense of erythrocytes. Mice with a PV phenotype responded to parenteral iron injections by decreasing platelet counts and further increasing hemoglobin and hematocrit, whereas no changes were observed in WT controls. Alterations of iron availability primarily affected the premegakaryocyte-erythrocyte progenitors, which constitute the iron-responsive stage of hematopoiesis in JAK2-mutant mice. The orally administered ferroportin inhibitor vamifeport and the minihepcidin PR73 normalized hematocrit and hemoglobin levels in JAK2-V617F and E12 mutant mouse models of PV, suggesting that ferroportin inhibitors and minihepcidins could be used in the treatment for patients with PV.

Human defensin-inspired discovery of peptidomimetic antibiotics.

SignificanceWe report the development of peptidomimetic antibiotics derived from a natural antimicrobial peptide, human α-defensin 5. By engaging multiple bacterial targets, the lead compound is efficacious in vitro and in vivo against bacteria with highly inducible antibiotic resistance, promising a useful therapeutic agent for the treatment of infections caused by antibiotic-resistant bacteria.

Characterization of erythroferrone structural domains relevant to its iron-regulatory function.

Iron delivery to the plasma is closely coupled to erythropoiesis, the production of red blood cells, as this process consumes most of the circulating plasma iron. In response to hemorrhage and other erythropoietic stresses, increased erythropoietin stimulates the production of the hormone erythroferrone (ERFE) by erythrocyte precursors (erythroblasts) developing in erythropoietic tissues. ERFE acts on the liver to inhibit bone morphogenetic protein (BMP) signaling and thereby decrease hepcidin production. Decreased circulating hepcidin concentrations then allow the release of iron from stores and increase iron absorption from the diet. Guided by evolutionary analysis and Alphafold2 protein complex modeling, we used targeted ERFE mutations, deletions, and synthetic ERFE segments together with cell-based bioassays and surface plasmon resonance to probe the structural features required for bioactivity and BMP binding. We define the ERFE active domain and multiple structural features that act together to entrap BMP ligands. In particular, the hydrophobic helical segment 81 to 86 and specifically the highly conserved tryptophan W82 in the N-terminal region are essential for ERFE bioactivity and Alphafold2 modeling places W82 between two tryptophans in its ligands BMP2, BMP6, and the BMP2/6 heterodimer, an interaction similar to those that bind BMPs to their cognate receptors. Finally, we identify the cationic region 96-107 and the globular TNFα-like domain 186-354 as structural determinants of ERFE multimerization that increase the avidity of ERFE for BMP ligands. Collectively, our results provide further insight into the ERFE-mediated inhibition of BMP signaling in response to erythropoietic stress.

Hypoferremia of inflammation: Innate host defense against infections.

Iron is an essential nutrient for microbes, plants and animals. Multicellular organisms have evolved multiple strategies to control invading microbes by restricting microbial access to iron. Hypoferremia of inflammation is a rapidly-acting organismal response that prevents the formation of iron species that would be readily accessible to microbes. This review takes an evolutionary perspective to explore the mechanisms and host defense function of hypoferremia of inflammation and its clinical implications.

Iron Homeostasis During Pregnancy: Maternal, Placental, and Fetal Regulatory Mechanisms.

Pregnancy entails a large negative balance of iron, an essential micronutrient. During pregnancy, iron requirements increase substantially to support both maternal red blood cell expansion and the development of the placenta and fetus. As insufficient iron has long been linked to adverse pregnancy outcomes, universal iron supplementation is common practice before and during pregnancy. However, in high-resource countries with iron fortification of staple foods and increased red meat consumption, the effects of too much iron supplementation during pregnancy have become a concern because iron excess has also been linked to adverse pregnancy outcomes. In this review, we address physiologic iron homeostasis of the mother, placenta, and fetus and discuss perturbations in iron homeostasis that result in pathological pregnancy. As many mechanistic regulatory systems have been deduced from animal models, we also discuss the principles learned from these models and how these may apply to human pregnancy.

Erythroid overproduction of erythroferrone causes iron overload and developmental abnormalities in mice.

The hormone erythroferrone (ERFE) is produced by erythroid cells in response to hemorrhage, hypoxia, or other erythropoietic stimuli, and it suppresses the hepatic production of the iron-regulatory hormone hepcidin, thereby mobilizing iron for erythropoiesis. Suppression of hepcidin by ERFE is believed to be mediated by interference with paracrine bone morphogenetic protein (BMP) signaling that regulates hepcidin transcription in hepatocytes. In anemias with ineffective erythropoiesis, ERFE is pathologically overproduced, but its contribution to the clinical manifestations of these anemias is not well understood. We generated 3 lines of transgenic mice with graded erythroid overexpression of ERFE and found that they developed dose-dependent iron overload, impaired hepatic BMP signaling, and relative hepcidin deficiency. These findings add to the evidence that ERFE is a mediator of iron overload in conditions in which ERFE is overproduced, including anemias with ineffective erythropoiesis. At the highest levels of ERFE overexpression, the mice manifested decreased perinatal survival, impaired growth, small hypofunctional kidneys, decreased gonadal fat depots, and neurobehavioral abnormalities, all consistent with impaired organ-specific BMP signaling during development. Neutralizing excessive ERFE in congenital anemias with ineffective erythropoiesis may not only prevent iron overload but may have additional benefits for growth and development.

Maternal prolactin or estrogen signaling in hepatocytes does not regulate iron homeostasis during pregnancy.

Not available.

Comparative analysis of the functional properties of human and mouse ferroportin.

Ferroportin (Fpn)-expressed at the plasma membrane of macrophages, enterocytes, and hepatocytes-mediates the transfer of cellular iron into the blood plasma. Under the control of the iron-regulatory hormone hepcidin, Fpn serves a critical role in systemic iron homeostasis. Whereas we have previously characterized human Fpn, a great deal of research in iron homeostasis and disorders utilizes mouse models. By way of example, the flatiron mouse, a model of classical ferroportin disease, bears the mutation H32R in Fpn and is characterized by systemic iron deficiency and macrophage iron retention. The flatiron mouse also appears to exhibit a manganese phenotype, raising the possibility that mouse Fpn serves a role in manganese metabolism. At odds with this observation, we have found that human Fpn does not transport manganese, so we considered the possibility that a species difference could explain this discrepancy. We tested the hypothesis that mouse but not human Fpn can transport manganese and performed a comparative analysis of mouse and human Fpn. We examined the functional properties of human Fpn, mouse Fpn, and mutant mouse Fpn by using radiotracer assays in RNA-injected Xenopus oocytes. We found that neither mouse nor human Fpn transports manganese. Mouse and human Fpn share identical properties with respect to substrate profile, calcium dependence, optimal pH, and hepcidin sensitivity. We have also demonstrated that Fpn is not an ATPase pump. Our findings validate the use of mouse models of ferroportin function in iron homeostasis and disease.

Placental Erythroferrone and Erythropoietin mRNA Expression is not Associated with Maternal or Neonatal Iron Status in Adolescents Carrying Singletons and Adult Women Carrying Multiples.

BACKGROUND: The iron regulatory hormones erythroferrone (ERFE), erythropoietin (EPO), and hepcidin, and the cargo receptor nuclear receptor coactivator 4 (NCOA4) are expressed in the placenta. However, determinants of placental expression of these proteins and their associations with maternal or neonatal iron status are unknown. OBJECTIVES: To characterize expression of placental ERFE, EPO, and NCOA4 mRNA in placentae from newborns at increased risk of iron deficiency and to evaluate these in relation to maternal and neonatal iron status and regulatory hormones. METHODS: Placentae were collected from 114 neonates born to adolescents carrying singletons (14-18 y) and 110 neonates born to 54 adults (20-46 y) carrying multiples. Placental EPO, ERFE, and NCOA4 mRNA expression were measured by RT-qPCR and compared with maternal and neonatal iron status indicators (SF, sTfR, total body iron, serum iron) and hormones. RESULTS: Placental ERFE, EPO, and NCOA4 mRNA were detected in all placentae delivered between 25 and 42 wk of gestation. Relationships between placental ERFE and EPO differed by cohort. In the multiples cohort, placental EPO and ERFE were positively correlated (P = 0.004), but only a positive trend (P = 0.08) was evident in the adolescents. Placental EPO and ERFE were not associated with maternal or neonatal iron status markers or hormones in either cohort. Placental NCOA4 was not associated with placental EPO or ERFE in either cohort but was negatively associated with maternal SF (P = 0.03) in the multiples cohort and positively associated with neonatal sTfR (P = 0.009) in the adolescents. CONCLUSIONS: The human placenta expresses ERFE, EPO, and NCOA4 mRNA as early as 25 wk of gestation. Placental expression of ERFE and EPO transcripts was not associated with maternal or neonatal iron status. Greater placental NCOA4 transcript expression was evident in women and newborns with poor iron status (lower SF and higher sTfR, respectively). Further research is needed to characterize the roles of these proteins in the human placenta. TRIAL REGISTRATION NUMBER: These clinical trials were registered at clinicaltrials.gov as NCT01019902 (https://clinicaltrials.gov/ct2/show/NCT01019902) and NCT01582802 (https://clinicaltrials.gov/ct2/show/NCT01582802).

The complex relationship between iron status and anemia in pregnant and postpartum women in India: Analysis of two Indian study cohorts of uncomplicated pregnancies.

Low hemoglobin is widely used as an indicator of iron deficiency anemia in India and other low-and-middle income counties, but anemia need not accurately reflect iron deficiency. We examined the relationship between hemoglobin and biomarkers of iron status in antenatal and postnatal period. Secondary analysis of uncomplicated singleton pregnancies in two Indian study cohorts: 1132 antenatal women in third trimester and 837 postnatal women 12-72 h after childbirth. Associations of hemoglobin with ferritin in both data sets, and with sTfR, TSAT, and hepcidin in the postnatal cohort were examined using multivariable linear regression. Multinomial logistic regression was used to examine the association between severity of anemia and iron status. Regression models were adjusted for potential confounders. Over 55% of the women were anemic; 34% of antenatal and 40% of postnatal women had low ferritin, but 4% antenatal and 6% postnatal women had high ferritin. No evidence of association between hemoglobin and ferritin was observed (antenatal: adjusted coefficient [aCoef] -0.0004, 95% confidence interval [CI] -0.001, 0.001; postnatal: aCoef -0.0001, 95% CI -0.001, 0.001). We found a significant linear association of hemoglobin with sTfR (aCoef -0.04, 95% CI -0.07, -0.01), TSAT (aCoef -0.005, 95% CI -0.008, -0.002), and hepcidin (aCoef 0.02, 95% CI 0.02, 0.03) in postnatal women. Likelihood of low ferritin was more common in anemic than non-anemic women, but high ferritin was also more common in women with severe anemia in both cohorts. Causes of anemia in pregnant and postpartum women in India are multifactorial; low hemoglobin alone is not be a useful marker of iron deficiency.

Iron loading induces cholesterol synthesis and sensitizes endothelial cells to TNFα-mediated apoptosis.

In plasma, iron is normally bound to transferrin, the principal protein in blood responsible for binding and transporting iron throughout the body. However, in conditions of iron overload when the iron-binding capacity of transferrin is exceeded, non-transferrin-bound iron (NTBI) appears in plasma. NTBI is taken up by hepatocytes and other parenchymal cells via NTBI transporters and can cause cellular damage by promoting the generation of reactive oxygen species. However, how NTBI affects endothelial cells, the most proximal cell type exposed to circulating NTBI, has not been explored. We modeled in vitro the effects of systemic iron overload on endothelial cells by treating primary human umbilical vein endothelial cells (HUVECs) with NTBI (ferric ammonium citrate [FAC]). We showed by RNA-Seq that iron loading alters lipid homeostasis in HUVECs by inducing sterol regulatory element-binding protein 2-mediated cholesterol biosynthesis. We also determined that FAC increased the susceptibility of HUVECs to apoptosis induced by tumor necrosis factor-α (TNFα). Moreover, we showed that cholesterol biosynthesis contributes to iron-potentiated apoptosis. Treating HUVECs with a cholesterol chelator hydroxypropyl-ß-cyclodextrin demonstrated that depletion of cholesterol was sufficient to rescue HUVECs from TNFα-induced apoptosis, even in the presence of FAC. Finally, we showed that FAC or cholesterol treatment modulated the TNFα pathway by inducing novel proteolytic processing of TNFR1 to a short isoform that localizes to lipid rafts. Our study raises the possibility that iron-mediated toxicity in human iron overload disorders is at least in part dependent on alterations in cholesterol metabolism in endothelial cells, increasing their susceptibility to apoptosis.

Enteral ferric citrate absorption is dependent on the iron transport protein ferroportin.

Ferric citrate is approved as an iron replacement product in patients with non-dialysis chronic kidney disease and iron deficiency anemia. Ferric citrate-delivered iron is enterally absorbed, but the specific mechanisms involved have not been evaluated, including the possibilities of conventional, transcellular ferroportin-mediated absorption and/or citrate-mediated paracellular absorption. Here, we first demonstrate the efficacy of ferric citrate in high hepcidin models, including Tmprss6 knockout mice (characterized by iron-refractory iron deficiency anemia) with and without adenine diet-induced chronic kidney disease. Next, to assess whether or not enteral ferric citrate absorption is dependent on ferroportin, we evaluated the effects of ferric citrate in a tamoxifen-inducible, enterocyte-specific ferroportin knockout murine model (Villin-Cre-ERT2, Fpnflox/flox). In this model, ferroportin deletion was efficient, as tamoxifen injection induced a 4000-fold decrease in duodenum ferroportin mRNA expression, with undetectable ferroportin protein on Western blot of duodenal enterocytes, resulting in a severe iron deficiency anemia phenotype. In ferroportin-deficient mice, three weeks of 1% ferric citrate dietary supplementation, a dose that prevented iron deficiency in control mice, did not improve iron status or rescue the iron deficiency anemia phenotype. We repeated the conditional ferroportin knockout experiment in the setting of uremia, using an adenine nephropathy model, where three weeks of 1% ferric citrate dietary supplementation again failed to improve iron status or rescue the iron deficiency anemia phenotype. Thus, our data suggest that enteral ferric citrate absorption is dependent on conventional enterocyte iron transport by ferroportin and that, in these models, significant paracellular absorption does not occur.

Maternal hepcidin determines embryo iron homeostasis in mice.

Iron disorders are associated with adverse pregnancy outcomes, yet iron homeostatic mechanisms during pregnancy are poorly understood. In humans and rodents, the iron-regulatory hormone hepcidin is profoundly decreased in pregnant mothers, which is thought to ensure adequate iron availability for transfer across placenta. However, the fetal liver also produces hepcidin, which may regulate fetal iron endowment by controlling placental iron export. To determine the relative contribution of maternal vs embryo hepcidin to the control of embryo iron endowment in iron-sufficient or iron-overloaded mice, we generated combinations of mothers and embryos that had or lacked hepcidin. We found that maternal, but not embryonic, hepcidin determined embryo and placental iron endowment in a healthy pregnancy. We further determined that inflammation can counteract pregnancy-dependent suppression of maternal hepcidin. To establish how essential maternal hepcidin suppression is for embryo iron homeostasis, we mimicked the range of maternal hepcidin activity by administering a hepcidin peptide mimetic to pregnant mice. This also allowed us to determine the effect of isolated maternal hepcidin excess on pregnancy, in the absence of other confounding effects of inflammation. Higher doses of hepcidin agonist caused maternal iron restriction and anemia, lower placenta and embryo weight, embryo anemia, and increased embryo mortality. Low agonist doses did not cause maternal anemia but still adversely affected the embryo, causing anemia, tissue iron deficiency (including in the brain), and decreased weight. Our studies demonstrate that suppression of maternal hepcidin during pregnancy is essential for maternal and embryo iron homeostasis and health.

Erythroferrone contributes to iron mobilization for embryo erythropoiesis in iron-deficient mouse pregnancies.

Erythroferrone (ERFE) is an erythroblast-secreted regulator of iron metabolism. The production of ERFE increases during stress erythropoiesis, leading to decreased hepcidin expression and mobilization of iron. Pregnancy requires a substantial increase in iron availability to sustain maternal erythropoietic expansion and fetal development and is commonly affected by iron deficiency. To define the role of ERFE during iron-replete or iron-deficient pregnancy, we utilized mouse models expressing a range of ERFE levels: transgenic (TG) mice overexpressing ERFE, wild-type (WT), and ERFE knockout (KO) mice. We altered maternal iron status using diets with low or standard iron content and performed the analysis at E18.5. Iron deficiency increased maternal ERFE in WT pregnancy. Comparing different maternal genotypes, ERFE TG dams had lower hepcidin relative to their liver iron load but similar hematological parameters to WT dams on either diet. In ERFE KO dams, most hematologic and iron parameters were comparable to WT, but mean corpuscular volume (MCV) was decreased under both iron conditions. Similar to dams, TG embryos had lower hepcidin on both diets, but their hematologic parameters did not differ from those of WT embryos. ERFE KO embryos had lower MCV than WT embryos on both diets. The effect was exacerbated under iron-deficient conditions where ERFE KO embryos had higher hepcidin, lower Hb and Hct, and lower brain iron concentration compared to WT embryos, indicative of iron restriction. Thus, under iron-deficient conditions, maternal and embryo ERFE facilitate iron mobilization for embryonic erythropoiesis.

Increased gene copy number of DEFA1/DEFA3 worsens sepsis by inducing endothelial pyroptosis.

Sepsis claims an estimated 30 million episodes and 6 million deaths per year, and treatment options are rather limited. Human neutrophil peptides 1-3 (HNP1-3) are the most abundant neutrophil granule proteins but their neutrophil content varies because of unusually extensive gene copy number polymorphism. A genetic association study found that increased copy number of the HNP-encoding gene DEFA1/DEFA3 is a risk factor for organ dysfunction during sepsis development. However, direct experimental evidence demonstrating that these risk alleles are pathogenic for sepsis is lacking because the genes are present only in some primates and humans. Here, we generate DEFA1/DEFA3 transgenic mice with neutrophil-specific expression of the peptides. We show that mice with high copy number of DEFA1/DEFA3 genes have more severe sepsis-related vital organ damage and mortality than mice with low copy number of DEFA1/DEFA3 or wild-type mice, resulting from more severe endothelial barrier dysfunction and endothelial cell pyroptosis after sepsis challenge. Mechanistically, HNP-1 induces endothelial cell pyroptosis via P2X7 receptor-mediating canonical caspase-1 activation in a NLRP3 inflammasome-dependent manner. Based on these findings, we engineered a monoclonal antibody against HNP-1 to block the interaction with P2X7 and found that the blocking antibody protected mice carrying high copy number of DEFA1/DEFA3 from lethal sepsis. We thus demonstrate that DEFA1/DEFA3 copy number variation strongly modulates sepsis development in vivo and explore a paradigm for the precision treatment of sepsis tailored by individual genetic information.

Effects of altitude and recombinant human erythropoietin on iron metabolism: a randomized controlled trial.

Current markers of iron deficiency (ID), such as ferritin and hemoglobin, have shortcomings, and hepcidin and erythroferrone (ERFE) could be of clinical relevance in relation to early assessment of ID. Here, we evaluate whether exposure to altitude-induced hypoxia (2,320 m) alone, or in combination with recombinant human erythropoietin (rHuEPO) treatment, affects hepcidin and ERFE levels before alterations in routine ID biomarkers and stress erythropoiesis manifest. Two interventions were completed, each comprising a 4-wk baseline, a 4-wk intervention at either sea level or altitude, and a 4-wk follow-up. Participants (n = 39) were randomly assigned to 20 IU·kg body wt-1 rHuEPO or placebo injections every second day for 3 wk during the two intervention periods. Venous blood was collected weekly. Altitude increased ERFE (P ≤ 0.001) with no changes in hepcidin or routine iron biomarkers, making ERFE of clinical relevance as an early marker of moderate hypoxia. rHuEPO treatment at sea level induced a similar pattern of changes in ERFE (P < 0.05) and hepcidin levels (P < 0.05), demonstrating the impact of accelerated erythropoiesis and not of other hypoxia-induced mechanisms. Compared with altitude alone, concurrent rHuEPO treatment and altitude exposure induced additive changes in hepcidin (P < 0.05) and ERFE (P ≤ 0.001) parallel with increases in hematocrit (P < 0.001), demonstrating a relevant range of both hepcidin and ERFE. A poor but significant correlation between hepcidin and ERFE was found (R2 = 0.13, P < 0.001). The findings demonstrate that hepcidin and ERFE are more rapid biomarkers of changes in iron demands than routine iron markers. Finally, ERFE and hepcidin may be sensitive markers in an antidoping context.