Cardiac Hepcidin Expression Associates with Injury Independent of Iron

Identification of three mechanistic pathways for iron-deficient heart failure

Current understanding of iron-deficient heart failure is based on blood tests that are thought to reflect systemic iron stores, but the available evidence suggests greater complexity. The entry and egress of circulating iron is controlled by erythroblasts, which (in severe iron deficiency) will sacrifice erythropoiesis to supply iron to other organs, e.g. the heart. Marked hypoferraemia (typically with anaemia) can drive the depletion of cardiomyocyte iron, impairing contractile performance and explaining why a transferrin saturation < ≈15%-16% predicts the ability of intravenous iron to reduce the risk of major heart failure events in long-term trials (Type 1 iron-deficient heart failure). However, heart failure may be accompanied by intracellular iron depletion within skeletal muscle and cardiomyocytes, which is disproportionate to the findings of systemic iron biomarkers. Inflammation- and deconditioning-mediated skeletal muscle dysfunction-a primary cause of dyspnoea and exercise intolerance in patients with heart failure-is accompanied by intracellular skeletal myocyte iron depletion, which can be exacerbated by even mild hypoferraemia, explaining why symptoms and functional capacity improve following intravenous iron, regardless of baseline haemoglobin or changes in haemoglobin (Type 2 iron-deficient heart failure). Additionally, patients with advanced heart failure show myocardial iron depletion due to both diminished entry into and enhanced egress of iron from the myocardium; the changes in iron proteins in the cardiomyocytes of these patients are opposite to those expected from systemic iron deficiency. Nevertheless, iron supplementation can prevent ventricular remodelling and cardiomyopathy produced by experimental injury in the absence of systemic iron deficiency (Type 3 iron-deficient heart failure). These observations, taken collectively, support the possibility of three different mechanistic pathways for the development of iron-deficient heart failure: one that is driven through systemic iron depletion and impaired erythropoiesis and two that are characterized by disproportionate depletion of intracellular iron in skeletal and cardiac muscle. These mechanisms are not mutually exclusive, and all pathways may be operative at the same time or may occur sequentially in the same patients.

Hypoxia-inducible factor-prolyl hydroxylase inhibitor Roxadustat (FG-4592) reduces renal fibrosis in Dahl salt-sensitive rats

OBJECTIVE

Although hypoxia-inducible factor-prolyl hydroxylase (HIF-PH) inhibitors have been developed for the treatment of renal anemia, their effects on cardiac and renal dysfunction remain unknown. We previously reported on Dahl salt-sensitive rats, in a rat model of salt-sensitive hypertension, that exhibited anemia and impaired expression of duodenal iron transporters after the development of hypertensive cardiac and renal dysfunction. Therefore, we investigated the effects of Roxadustat (FG-4592), an HIF-PH inhibitor, on anemia, iron regulation, and cardiac and renal dysfunction in Dahl salt-sensitive rats.

METHODS

Six-week-old male Dahl salt-sensitive rats were fed a normal or high-salt diet for 8 weeks. A further subset of Dahl salt-sensitive rats, that were fed a high-salt diet, was administered Roxadustat for 8 weeks.

RESULTS

Dahl salt-sensitive rats fed a high-salt diet developed hypertension, cardiac and renal dysfunction, and anemia after 8 weeks of feeding. Roxadustat increased hemoglobin and serum erythropoietin levels in Dahl salt-sensitive rats fed a high-salt diet. With regard to the iron-regulating system, Roxadustat lowered hepatic hepcidin gene expression and increased the gene expression of duodenal iron transporters, such as cytochrome b and divalent metal transporter 1 , in Dahl salt-sensitive rats fed a high-salt diet. Roxadustat did not affect the development of hypertension and cardiac hypertrophy in Dahl salt-sensitive rats with a high-salt diet; however, Roxadustat treatment attenuated renal fibrosis in these rats.

CONCLUSIONS

Roxadustat ameliorated anemia with affecting the gene expression of the iron-regulating system, and did not affect cardiac hypertrophy but attenuated renal fibrosis in Dahl salt-sensitive rats fed a high-salt diet.

HMGB1 induces hepcidin upregulation in astrocytes and causes an acute iron surge and subsequent ferroptosis in the postischemic brain

Dysregulation of brain iron levels causes functional disturbances and damages neurons. Hepcidin (a peptide hormone) plays a principal role in regulating intracellular iron levels by modulating ferroportin (FPN, the only known iron exporter) through triggering its internalization and lysosomal degradation. We observed a significant and rapid iron surge in the cortices of ischemic hemispheres at 3 h after cerebral ischemia (middle cerebral artery occlusion, MCAO) that was maintained until 4 d post-MCAO. We showed upregulation of hepcidin expression in the brain as early as 3 h post-MCAO, mainly in astrocytes, and significant hepcidin accumulation in serum from 6 h post-MCAO, and these inductions were maintained for 1 day and 7 days, respectively. High mobility group box 1 (HMGB1), a prototypic danger-associated molecular pattern, accumulates markedly after transient MCAO and plays critical roles in damage aggravation via its proinflammatory effects. Here, we demonstrated that treatment with recombinant HMGB1 stimulated astrocytes to induce hepcidin expression in a TLR4- and CXCR4-dependent manner. Furthermore, hepcidin-mediated intracellular iron accumulation in neurons was confirmed by an experiment using N-methyl-D-aspartate (NMDA)-conditioned medium-treated primary astrocytes and fresh primary cortical neurons treated with hepcidin-containing astrocyte-conditioned medium. Moreover, HMGB1-mediated local hepcidin upregulation and subsequent local iron surge were found to cause ferroptosis in the postischemic brain, which was suppressed by the functional blocking of HMGB1 using intranasally administered HMGB1 A box or anti-HMGB1 antibody. These findings show that HMGB1 serves as a ferroptosis inducer by upregulating hepcidin in astrocytes and thus aggravates acute damage in the postischemic brain.

Pre-emptive iron supplementation prevents myocardial iron deficiency and attenuates adverse remodelling after myocardial infarction

AIMS

Heart failure (HF) after myocardial infarction (MI) is a major cause of morbidity and mortality. We sought to investigate the functional importance of cardiac iron status after MI and the potential of pre-emptive iron supplementation in preventing cardiac iron deficiency (ID) and attenuating left ventricular (LV) remodelling.

METHODS AND RESULTS

MI was induced in C57BL/6J male mice by left anterior descending coronary artery ligation. Cardiac iron status in the non-infarcted LV myocardium was dynamically regulated after MI: non-haem iron and ferritin increased at 4 weeks but decreased at 24 weeks after MI. Cardiac ID at 24 weeks was associated with reduced expression of iron-dependent electron transport chain (ETC) Complex I compared with sham-operated mice. Hepcidin expression in the non-infarcted LV myocardium was elevated at 4 weeks and suppressed at 24 weeks. Hepcidin suppression at 24 weeks was accompanied by more abundant expression of membrane-localized ferroportin, the iron exporter, in the non-infarcted LV myocardium. Notably, similarly dysregulated iron homeostasis was observed in LV myocardium from failing human hearts, which displayed lower iron content, reduced hepcidin expression, and increased membrane-bound ferroportin. Injecting ferric carboxymaltose (15 µg/g body weight) intravenously at 12, 16, and 20 weeks after MI preserved cardiac iron content and attenuated LV remodelling and dysfunction at 24 weeks compared with saline-injected mice.

CONCLUSION

We demonstrate, for the first time, that dynamic changes in cardiac iron status after MI are associated with local hepcidin suppression, leading to cardiac ID long term after MI. Pre-emptive iron supplementation maintained cardiac iron content and attenuated adverse remodelling after MI. Our results identify the spontaneous development of cardiac ID as a novel disease mechanism and therapeutic target in post-infarction LV remodelling and HF.

Duality of Nrf2 in iron-overload cardiomyopathy

Cardiomyopathy deeply affects quality of life and mortality of patients with b-thalassemia or with transfusion-dependent myelodysplastic syndromes. Recently, a link between Nrf2 activity and iron metabolism has been reported in liver ironoverload murine models. Here, we studied C57B6 mice as healthy control and nuclear erythroid factor-2 knockout (Nrf2-/-) male mice aged 4 and 12 months. Eleven-month-old wild-type and Nrf2-/- mice were fed with either standard diet or a diet containing 2.5% carbonyl-iron (iron overload [IO]) for 4 weeks. We show that Nrf2-/- mice develop an age-dependent cardiomyopathy, characterized by severe oxidation, degradation of SERCA2A and iron accumulation. This was associated with local hepcidin expression and increased serum non-transferrin-bound iron, which promotes maladaptive cardiac remodeling and interstitial fibrosis related to overactivation of the TGF-b pathway. When mice were exposed to IO diet, the absence of Nrf2 was paradoxically protective against further heart iron accumulation. Indeed, the combination of prolonged oxidation and the burst induced by IO diet resulted in activation of the unfolded protein response (UPR) system, which in turn promotes hepcidin expression independently from heart iron accumulation. In the heart of Hbbth3/+ mice, a model of b-thalassemia intermedia, despite the activation of Nrf2 pathway, we found severe protein oxidation, activation of UPR system and cardiac fibrosis independently from heart iron content. We describe the dual role of Nrf2 when aging is combined with IO and its novel interrelation with UPR system to ensure cell survival. We open a new perspective for early and intense treatment of cardiomyopathy in patients with b-thalassemia before the appearance of heart iron accumulation.

Cardiac Hamp mRNA Is Predominantly Expressed in the Right Atrium and Does Not Respond to Iron

Hepcidin is a liver-derived hormone that controls systemic iron traffic. It is also expressed in the heart, where it acts locally. We utilized cell and mouse models to study the regulation, expression, and function of cardiac hepcidin. Hepcidin-encoding Hamp mRNA was induced upon differentiation of C2C12 cells to a cardiomyocyte-like phenotype and was not further stimulated by BMP6, BMP2, or IL-6, the major inducers of hepatic hepcidin. The mRNAs encoding hepcidin and its upstream regulator hemojuvelin (Hjv) are primarily expressed in the atria of the heart, with ~20-fold higher Hamp mRNA levels in the right vs. left atrium and negligible expression in the ventricles and apex. Hjv−/− mice, a model of hemochromatosis due to suppression of liver hepcidin, exhibit only modest cardiac Hamp deficiency and minor cardiac dysfunction. Dietary iron manipulations did not significantly affect cardiac Hamp mRNA in the atria of wild-type or Hjv−/− mice. Two weeks following myocardial infarction, Hamp was robustly induced in the liver and heart apex but not atria, possibly in response to inflammation. We conclude that cardiac Hamp is predominantly expressed in the right atrium and is partially regulated by Hjv; however, it does not respond to iron and other inducers of hepatic hepcidin.

Triad role of hepcidin, ferroportin, and Nrf2 in cardiac iron metabolism: From health to disease

Iron is an essential trace element required for several vital physiological and developmental processes, including erythropoiesis, bone, and neuronal development. Iron metabolism and oxygen homeostasis are interlinked to perform a vital role in the functionality of the heart. The metabolic machinery of the heart utilizes almost 90 % of oxygen through the electron transport chain. To handle this tremendous level of oxygen, the iron metabolism in the heart is utmost crucial. Iron availability to the heart is therefore tightly regulated by (i) the hepcidin/ferroportin axis, which controls dietary iron absorption, storage, and recycling, and (ii) iron regulatory proteins 1 and 2 (IRP1/2) via hypoxia inducible factor 1 (HIF1) pathway. Despite iron being vital to the heart, recent investigations have demonstrated that iron imbalance is a common manifestation in conditions of heart failure (HF), since free iron readily transforms between Fe²⁺ and Fe³⁺via the Fenton reaction, leading to reactive oxygen species (ROS) production and oxidative damage. Therefore, to combat iron-mediated oxidative stress, targeting Nrf2/ARE antioxidant signaling is rational. The involvement of Nrf2 in regulating several genes engaged in heme synthesis, iron storage, and iron export is beginning to be uncovered. Consequently, it is possible that Nrf2/hepcidin/ferroportin might act as an epicenter connecting iron metabolism to redox alterations. However, the mechanism bridging the two remains obscure. In this review, we tried to summarize the contemporary insight of how cardiomyocytes regulate intracellular iron levels and discussed the mechanisms linking cardiac dysfunction with iron imbalance. Further, we emphasized the impact of Nrf2 on the interplay between systemic/cardiac iron control in the context of heart disease, particularly in myocardial ischemia and HF.

Molecular toxicological alterations in the mouse hearts induced by sub-chronic thiazolidinedione drugs administration

Thiazolidinediones are well-known anti-diabetic drugs. However, they are not widely used due to their cardiotoxic effects. Therefore, in this study, we aimed to determine the molecular toxicological alterations induced in the mouse hearts after thiazolidinedione administration. Balb/c mice received doses clinically equivalent to those given to humans of the most commonly used thiazolidinediones, pioglitazone, and rosiglitazone for 30 days. After that, RNA samples were isolated from the hearts. The mRNA expression of cytochrome (cyp) p450 genes that synthesize the cardiotoxic 20-hydroxyeicosatetraenoic acid (20-HETE) in addition to 92 cardiotoxicity biomarker genes were analyzed using quantitative polymerase chain reaction array technique. The analysis demonstrated that thiazolidinediones caused a significant upregulation (p < 0.5) of the mRNA expression of cyp1a1, cyp4a12, itpr1, ccl7, ccr1, and b2 m genes. In addition, thiazolidinediones caused a significant (p < 0.05) downregulation of the mRNA expression of adra2a, bsn, col15a1, fosl1, Il6, bpifa1, plau, and reg3b genes. The most affected gene was itpr1 gene, which was upregulated by pioglitazone and rosiglitazone by sevenfold and 3.5-fold, respectively. In addition, pioglitazone caused significant upregulation of (p < 0.05) hamp, ppbp, psma2, sik1, timp1, and ucp1 genes, which were not affected significantly (p > 0.05) by rosiglitazone administration. In conclusion, this study showed that thiazolidinediones induce toxicological molecular alterations in the mouse hearts, such as the induction of cyp450s that synthesize 20-HETE, chemokine activation, inflammatory responses, blood clotting, and oxidative stress. These findings may help us understand the mechanism of cardiotoxicity involved in thiazolidinedione administration.

In-depth review: is hepcidin a marker for the heart and the kidney?

Iron is an essential trace element involved in oxidation-reduction reactions, oxygen transport and storage, and energy metabolism. Iron in excess can be toxic for cells, since iron produces reactive oxygen species and is important for survival of pathogenic microbes. There is a fine-tuning in the regulation of serum iron levels, determined by intestinal absorption, macrophage iron recycling, and mobilization of hepatocyte stores versus iron utilization, primarily by erythroid cells in the bone marrow. Hepcidin is the major regulatory hormone of systemic iron homeostasis and is upregulated during inflammation. Hepcidin metabolism is altered in chronic kidney disease. Ferroportin is an iron export protein and mediates iron release into the circulation from duodenal enterocytes, splenic reticuloendothelial macrophages, and hepatocytes. Systemic iron homeostasis is controlled by the hepcidin-ferroportin axis at the sites of iron entry into the circulation. Hepcidin binds to ferroportin, induces its internalization and intracellular degradation, and thus inhibits iron absorption from enterocytes, and iron release from macrophages and hepatocytes. Recent data suggest that hepcidin, by slowing or preventing the mobilization of iron from macrophages, may promote atherosclerosis and may be associated with increased cardiovascular disease risk. This article reviews the current data regarding the molecular and cellular pathways of systemic and autocrine hepcidin production and seeks the answer to the question whether changes in hepcidin translate into clinical outcomes of all-cause and cardiovascular mortality, and cardiovascular and renal end-points.

Keeping heart homeostasis in check through the balance of iron metabolism

Highly active cardiomyocytes need iron for their metabolic activity. In physiological conditions, iron turnover is a delicate process which is dependent on global iron supply and local autonomous regulatory mechanisms. Though less is known about the autonomous regulatory mechanisms, data suggest that these mechanisms can preserve cellular iron turnover even in the presence of systemic iron disturbance. Therefore, activity of local iron protein machinery and its relationship with global iron metabolism is important to understand cardiac iron metabolism in physiological conditions and in cardiac disease. Our knowledge in this respect has helped in designing therapeutic strategies for different cardiac diseases. This review is a synthesis of our current knowledge concerning the regulation of cardiac iron metabolism. In addition, different models of cardiac iron dysmetabolism will be discussed through the examples of heart failure (cardiomyocyte iron deficiency), myocardial infarction (acute changes in cardiac iron turnover), doxorubicin-induced cardiotoxicity (cardiomyocyte iron overload in mitochondria), thalassaemia (cardiomyocyte cytosolic and mitochondrial iron overload) and Friedreich ataxia (asymmetric cytosolic/mitochondrial cardiac iron dysmetabolism). Finally, future perspectives will be discussed in order to resolve actual gaps in knowledge, which should be helpful in finding new treatment possibilities in different cardiac diseases.

The multifaceted role of iron in renal health and disease

Iron is an essential element that is indispensable for life. The delicate physiological body iron balance is maintained by both systemic and cellular regulatory mechanisms. The iron-regulatory hormone hepcidin assures maintenance of adequate systemic iron levels and is regulated by circulating and stored iron levels, inflammation and erythropoiesis. The kidney has an important role in preventing iron loss from the body by means of reabsorption. Cellular iron levels are dependent on iron import, storage, utilization and export, which are mainly regulated by the iron response element-iron regulatory protein (IRE-IRP) system. In the kidney, iron transport mechanisms independent of the IRE-IRP system have been identified, suggesting additional mechanisms for iron handling in this organ. Yet, knowledge gaps on renal iron handling remain in terms of redundancy in transport mechanisms, the roles of the different tubular segments and related regulatory processes. Disturbances in cellular and systemic iron balance are recognized as causes and consequences of kidney injury. Consequently, iron metabolism has become a focus for novel therapeutic interventions for acute kidney injury and chronic kidney disease, which has fuelled interest in the molecular mechanisms of renal iron handling and renal injury, as well as the complex dynamics between systemic and local cellular iron regulation.

Cardiomyocyte hepcidin: From intracellular iron homeostasis to physiological function

Cellular iron is required for the utilization of oxygen in the cell. Iron in iron-sulfur and heme groups is required for electron transfer and oxygen activation in oxidative phosphorylation, while labile free iron is required for oxygen activation by dioxygenases, and as a catalyst for redox signaling. At the same time, this reactivity with oxygen underpins the production of cell-damaging free radicals in the presence of excess iron. Because the cardiac cell is a major site of oxygen flux, it requires tight control of intracellular iron levels. Until recently, such control was thought to be mediated predominantly by the action of iron regulatory proteins. However, new evidence reveals that cardiomyocyte hepcidin is indispensable for the control of intracellular iron levels, normal metabolism and heart function. This new evidence highlights the need for better understanding of the regulation of cardiomyocyte hepcidin in health and disease.

Myocardial iron homeostasis and hepcidin expression in a rat model of heart failure at different levels of dietary iron intake

BACKGROUND

Up to 50% of patients with chronic heart failure (HF) have systemic iron deficiency, which contributes to symptoms and poor prognosis. Myocardial iron deficiency (MID) in HF patients has been recently documented, but its causes and consequences are unknown. The goal of our study was to address these questions in a well-defined rat HF model induced by volume overload due to aorto-caval fistula.

METHODS

Modulation of dietary iron content in a rat model of HF has been used to address how iron status affects cardiac iron levels, heart structure and function, and how the presence of HF affects cardiac expression of hepcidin and other iron-related genes.

RESULTS

MID developed in the rat model of heart failure. Iron supplementation did not normalize the myocardial iron content; however, it improved survival of HF animals compared to animals fed diet with normal iron content. We observed marked upregulation of hepcidin mRNA expression in HF animals, which was not associated with systemic or cardiac iron levels but strongly correlated with markers and parameters of heart injury. Identical iron-independent pattern was observed for expression of several iron-related genes.

CONCLUSIONS

MID is not caused by defective iron absorption or decreased systemic iron levels, but rather by intrinsic myocardial iron deregulation. Altered cardiac expression of hepcidin and other iron-related genes is driven by iron-independent stimuli in the failing heart.

GENERAL SIGNIFICANCE

Understanding of the causes and consequences of MID is critical for finding strategies how to improve cardiac iron stores and in HF patients.

Detection and quantitation of iron in ferritin, transferrin and labile iron pool (LIP) in cardiomyocytes using 55Fe and storage phosphorimaging

Dysregulated iron metabolism has a detrimental effect on cardiac function. The importance of iron homeostasis in cardiac health and disease warrants detailed studies of cardiomyocyte iron uptake, utilization and recycling at the molecular level. In this study, we have performed metabolic labeling of primary cultures of neonatal rat cardiomyocytes with radioactive iron coupled with separation of labeled iron-containing molecules by native electrophoresis followed by detection and quantification of incorporated radioiron by storage phosphorimaging. For the radiolabeling we used a safe and convenient beta emitter ⁵⁵Fe which enabled sensitive and simultaneous detection and quantitation of iron in cardiomyocyte ferritin, transferrin and the labile iron pool (LIP). The LIP is believed to represent potentially dangerous redox-active iron bound to uncharacterized molecules. Using size-exclusion chromatography spin micro columns, we demonstrate that iron in the LIP is bound to high molecular weight molecule(s) (≥5000 Da) in the neonatal cardiomyocytes.

Balance of cardiac and systemic hepcidin and its role in heart physiology and pathology

Hepcidin is the main regulator of iron metabolism in tissues. Its serum levels are mostly correlated with the levels of hepcidin expression from the liver, but local hepcidin can be important for the physiology of other organs as well. There is an increasing evidence that this is the case with cardiac hepcidin. This has been confirmed by studies with models of ischemic heart disease and other heart pathologies. In this review the discussion dissects the role of cardiac hepcidin in cellular homeostasis. This review is complemented with examination of the role of systemic hepcidin in heart disease and its use as a biochemical marker. The relationship between systemic vs local hepcidin in the heart is important because it can help us understand how the fine balance between the actions of two hepcidins affects heart function. Manipulating the axis systemic/cardiac hepcidin could serve as a new therapeutic strategy in heart diseases.

Expression of hepcidin and ferroportin in full term placenta of pregnant cows

Hepcidin (HEP) and ferroportin (FPN) play a central role in systemic iron homeostasis. The HEP/FPN axis controls both extracellular iron concentration and total body iron levels. HEP is synthesized mainly by hepatocytes and controls the absorption of dietary iron and the distribution of iron to the various cell types; its synthesis is regulated by both iron and innate immunity. FPN is a membrane protein and the major exporter of iron from mammalian cells, including iron recycling macrophages, iron absorbing duodenal enterocytes, and iron storing hepatocytes. HEP limits the pool of extracellular iron by binding FPN and mediating its degradation, thus preventing its release from intracellular sources. Here we investigated, for the first time, the molecular and morphological expression of HEP and FPN in placenta of pregnant cows at term. Their expression has been evaluated investigating their mRNAs by reverse transcriptase PCR (RT-PCR). Sequencing of related amplicons revealed a 100% identity with HEP and FPN sequences from Bos taurus as reported in the GeneBank (mRNASequence ID: NM_001114508.2 and ID: NM_001077970.1, respectively). HEP and FPN proteins have also been revealed by Western blot analysis and immunohistochemistry. The strongest immunoreactivity for both proteins was observed in the cytoplasm of the trophoblastic cells of the villi and the caruncular crypts of the placentome. Hep mRNA was more representative in caruncular rather cotyledonar areas; on the contrary, Fpn mRNA was more expressed in cotyledonar rather than in caruncular areas. Transcripts of ferritin, transferrin and its receptor have been also documented by real time RT-PCR. HEP and FPN placental proteins may play a dual role. HEP/FPN axis seems to have a central role in infections, with microorganisms within macrophages or that survive in the bloodstream or other cellular spaces. In addition, HEP may be responsible for iron flux regulation as a molecular bridge for iron trafficking and response to infection. FPN may also have a significant role for embryonic development, growth and organogenesis.