Deferoxamine promotes survival and prevents electrocardiographic abnormalities in the gerbil model of iron-overload cardiomyopathy

The role of iron overload and ferroptosis in arrhythmia pathogenesis

Ferroptosis is a newly discovered form of programmed cell death triggered by intracellular iron overload, which leads to the accumulation of lipid peroxides in various cells. It has been implicated in the pathogenesis and progression of various diseases, including tumors, neurological disorders, and cardiovascular diseases. The intricate mechanism underlying ferroptosis involves an imbalance between the oxidation and antioxidant systems, disturbances in iron metabolism, membrane lipid peroxidation, and dysregulation of amino acid metabolism. We highlight the key molecular mechanisms governing iron overload and ferroptosis, and discuss potential molecular pathways linking ferroptosis with arrhythmias.

An Alternative Mechanism of Subcellular Iron Uptake Deficiency in Cardiomyocytes

BACKGROUND

Systemic defects in intestinal iron absorption, circulation, and retention cause iron deficiency in 50% of patients with heart failure. Defective subcellular iron uptake mechanisms that are independent of systemic absorption are incompletely understood. The main intracellular route for iron uptake in cardiomyocytes is clathrin-mediated endocytosis.

METHODS

We investigated subcellular iron uptake mechanisms in patient-derived and CRISPR/Cas-edited induced pluripotent stem cell-derived cardiomyocytes as well as patient-derived heart tissue. We used an integrated platform of DIA-MA (mass spectrometry data-independent acquisition)-based proteomics and signaling pathway interrogation. We employed a genetic induced pluripotent stem cell model of 2 inherited mutations (TnT [troponin T]-R141W and TPM1 [tropomyosin 1]-L185F) that lead to dilated cardiomyopathy (DCM), a frequent cause of heart failure, to study the underlying molecular dysfunctions of DCM mutations.

RESULTS

We identified a druggable molecular pathomechanism of impaired subcellular iron deficiency that is independent of systemic iron metabolism. Clathrin-mediated endocytosis defects as well as impaired endosome distribution and cargo transfer were identified as a basis for subcellular iron deficiency in DCM-induced pluripotent stem cell-derived cardiomyocytes. The clathrin-mediated endocytosis defects were also confirmed in the hearts of patients with DCM with end-stage heart failure. Correction of the TPM1-L185F mutation in DCM patient-derived induced pluripotent stem cells, treatment with a peptide, Rho activator II, or iron supplementation rescued the molecular disease pathway and recovered contractility. Phenocopying the effects of the TPM1-L185F mutation into WT induced pluripotent stem cell-derived cardiomyocytes could be ameliorated by iron supplementation.

CONCLUSIONS

Our findings suggest that impaired endocytosis and cargo transport resulting in subcellular iron deficiency could be a relevant pathomechanism for patients with DCM carrying inherited mutations. Insight into this molecular mechanism may contribute to the development of treatment strategies and risk management in heart failure.

Synergistic cardioprotective effects of melatonin and deferoxamine through the improvement of ferritinophagy in doxorubicin-induced acute cardiotoxicity

Ferritinophagy is one of the most recent molecular mechanisms affecting cardiac function. In addition, it is one of the pathways by which doxorubicin, one of the anticancer drugs commonly used, negatively impacts the cardiac muscle, leading to cardiac function impairment. This side effect limits the use of doxorubicin. Iron chelators play an important role in hindering ferritinophagy. Antioxidants can also impact ferritinophagy by improving oxidative stress. In this study, it was assumed that the antioxidant function of melatonin could promote the action of deferoxamine, an iron chelator, at the level of ferritinophagy. A total of 42 male Wistar rats (150-200 g) were divided into seven groups (n = 6) which consisted of group I: control normal, group II: doxorubicin (Dox), group III: melatonin (Mel), group IV: deferoxamine (Des), group V: Mel + Dox, group VI: Des + Dox, and group VII: Mel + Des + Dox. Groups III, V and VII were orally pretreated with melatonin 20 mg/kg/day for 7 days. Groups IV, VI and VII were treated with deferoxamine at a 250 mg/kg/dose once on D4 before Dox was given. Doxorubicin was given at a 20 mg/kg ip single dose. On the 8th day, the rats were lightly anaesthetized for electrocardiography analysis and echocardiography. Serum samples were collected and then sacrificed for tissue sampling. The following biochemical assessments were carried out: PCR of NCOA4, IREB2, FTH1, SLC7A11, and GPX4; and ELISA for serum cTnI, serum transferrin, tissue GSH, and malondialdehyde. In addition, histopathological assessment of heart injury; immunostaining of caspase-3, Bax, and Bcl2; and physiological function assessment by ECG and ECHO were carried out. Doxorubicin-induced acute significant cardiac injury with increased ferritinophagy and apoptosis responded to single and combined prophylactic treatment, in which the combined treatment showed mostly the best results. In conclusion, using melatonin as an antioxidant with an iron chelator, deferoxamine, could hinder the hazardous cardiotoxic effect of doxorubicin. However, further studies are needed to detect the impact of higher doses of melatonin and deferoxamine with a prolonged treatment period.

Biomarkers in the clinical management of patients with atrial fibrillation and heart failure

Atrial fibrillation (AF) and heart failure (HF) are two cardiovascular diseases with an increasing prevalence worldwide. These conditions share common pathophysiologiesand frequently co-exit. In fact, the occurrence of either condition can 'cause' the development of the other, creating a new patient group that demands different management strategies to that if they occur in isolation. Regardless of the temproral association of the two conditions, their presence is linked with adverse cardiovascular outcomes, increased rate of hospitalizations, and increased economic burden on healthcare systems. The use of low-cost, easily accessible and applicable biomarkers may hasten the correct diagnosis and the effective treatment of AF and HF. Both AF and HF effect multiple physiological pathways and thus a great number of biomarkers can be measured that potentially give the clinician important diagnostic and prognostic information. These will then guide patient centred therapeutic management. The current biomarkers that offer potential for guiding therapy, focus on the physiological pathways of miRNA, myocardial stretch and injury, oxidative stress, inflammation, fibrosis, coagulation and renal impairment. Each of these has different utility in current clinincal practice.

Silencing of lipocalin-2 improves cardiomyocyte viability under iron overload conditions via decreasing mitochondrial dysfunction and apoptosis

This study aimed to investigate the mechanistic roles of LCN-2 and LCN-2 receptors (LCN-2R) as iron transporters in cardiomyocytes under iron overload condition. H9c2 cardiomyocytes were treated with either LCN-2 small interfering RNA (siRNA) or LCN-2R siRNA or L-type or T-type calcium channel (LTCC or TTCC) blockers, or iron chelator deferiprone (DFP). After the treatments, the cells were exposed to Fe³⁺ or Fe²⁺ , after that biological parameters were determined. Silencing of lipocalin-2 or its receptor improved cardiomyocyte viability via decreasing iron uptake, mitochondrial fission, mitophagy and cleaved caspase-3 only in the Fe³⁺ overload condition. In contrast, treatments with LTCC blocker and TTCC blocker showed beneficial effects on those parameters only in conditions of Fe²⁺ overload. Treatment with DFP has been shown beneficial effects both in Fe²⁺ and Fe³⁺ overload condition. All of these findings suggested that LTCC and TTCC play crucial roles in the Fe²⁺ uptake, whereas LCN-2 and LCN-2R were essential for Fe³⁺ uptake into the cardiomyocytes under iron overload conditions.

Iron Overload, Oxidative Stress and Calcium Mishandling in Cardiomyocytes: Role of the Mitochondrial Permeability Transition Pore

Iron (Fe) plays an essential role in many physiological processes. Hereditary hemochromatosis or frequent blood transfusions often cause iron overload (IO), which can lead to cardiomyopathy and arrhythmias; however, the underlying mechanism is not well defined. In the present study, we assess the hypothesis that IO promotes arrhythmias via reactive oxygen species (ROS) production, mitochondrial membrane potential (∆Ψm) depolarization, and disruption of cytosolic Ca dynamics. In ventricular myocytes isolated from wild type (WT) mice, both cytosolic and mitochondrial Fe levels were elevated following perfusion with the Fe3+/8-hydroxyquinoline (8-HQ) complex. IO promoted mitochondrial superoxide generation (measured using MitoSOX Red) and induced the depolarization of the ΔΨm (measured using tetramethylrhodamine methyl ester, TMRM) in a dose-dependent manner. IO significantly increased the rate of Ca wave (CaW) formation measured in isolated ventricular myocytes using Fluo-4. Furthermore, in ex-vivo Langendorff-perfused hearts, IO increased arrhythmia scores as evaluated by ECG recordings under programmed S1-S2 stimulation protocols. We also carried out similar experiments in cyclophilin D knockout (CypD KO) mice in which the mitochondrial permeability transition pore (mPTP) opening is impaired. While comparable cytosolic and mitochondrial Fe load, mitochondrial ROS production, and depolarization of the ∆Ψm were observed in ventricular myocytes isolated from both WT and CypD KO mice, the rate of CaW formation in isolated cells and the arrhythmia scores in ex-vivo hearts were significantly lower in CypD KO mice compared to those observed in WT mice under conditions of IO. The mPTP inhibitor cyclosporine A (CsA, 1 µM) also exhibited a protective effect. In conclusion, our results suggest that IO induces mitochondrial ROS generation and ∆Ψm depolarization, thus opening the mPTP, thereby promoting CaWs and cardiac arrhythmias. Conversely, the inhibition of mPTP ameliorates the proarrhythmic effects of IO.

Arrhythmias and Sudden Cardiac Death in Beta-Thalassemia Major Patients: Noninvasive Diagnostic Tools and Early Markers

Beta-thalassemias are a group of inherited, autosomal recessive diseases, characterized by reduced or absent synthesis of beta-globin chains of the hemoglobin tetramer, resulting in variable phenotypes, ranging from clinically asymptomatic individuals to severe anemia. Three main forms have been described: heterozygotes, homozygotes β+, and homozygotes β°. Beta-thalassemia major (β-TM), the most serious form, is characterized by an absent synthesis of globin chains that are essential for hemoglobin formation, causing chronic hemolytic anemia. Cardiac complications represent a leading cause of mortality in β-TM patients, although an important and progressive increase of life expectancy has been demonstrated after the introduction of chelating therapies. Iron overload is the primary factor of cardiac damage resulting in thalassemic cardiomyopathy, in which diastolic dysfunction usually happens before systolic impairment and overt heart failure (HF). Although iron-induced cardiomyopathy is slowly progressive and it usually takes several decades for clinical and laboratory features of cardiac dysfunction to manifest, arrhythmias or sudden death may be present without signs of cardiac disease and only if myocardial siderosis is present. Careful analysis of electrocardiograms and other diagnostic tools may help in early identification of high-risk β-TM patients for arrhythmias and sudden cardiac death.

QT Prolongation and Associated Ventricular Tachycardia due to Cardiac Iron Load in a Patient with Thalassemia Major

We report the case of a 23-year-old male with thalassemia major who developed long QT and continuous ventricular tachycardia (VT). Electrocardiography, echocardiography, and cardiac magnetic resonance imaging (MRI) were used for diagnosis and risk stratification. VT causes and treatments are presented and discussed. Ventricular arrhythmia can be treated by normalizing QT interval with high-dose beta-blocker therapy. However, MRI-compatible internal cardiac defibrillator implantation was performed due to the high risk in this patient.

Mass spectrometry and associated technologies delineate the advantageously biomedical capacity of siderophores in different pathogenic contexts

Siderophores are chemically diverse small molecules produced by microorganisms for chelation of irons to maintain their survival and govern some important biological functions, especially those cause that infections in hosts. Still, siderophores can offer new insight into a better understanding of the diagnosis and treatments of infectious diseases from the siderophore biosynthesis and regulation perspective. Thus, this review aims to summarize the biomedical value and applicability of siderophores in pathogenic contexts by briefly reviewing mass spectrometry (MS)-based chemical biology and translational applications that involve diagnosis, pathogenesis, and therapeutic discovery for a variety of infectious conditions caused by different pathogens. We highlight the advantages and disadvantages of siderophore discovery and applications in pathogenic contexts. Finally, we propose a panel of new and promising strategy as precision-modification metabolomics method, to rapidly advance the discovery of and translational innovations pertaining to these value compounds in broad biomedical niches. © 2018 Wiley Periodicals, Inc. Mass Spec Rev XX:XX-XX, 2018.

Involvement of cytosolic and mitochondrial iron in iron overload cardiomyopathy: an update

Iron overload cardiomyopathy (IOC) is a major cause of death in patients with diseases associated with chronic anemia such as thalassemia or sickle cell disease after chronic blood transfusions. Associated with iron overload conditions, there is excess free iron that enters cardiomyocytes through both L- and T-type calcium channels thereby resulting in increased reactive oxygen species being generated via Haber-Weiss and Fenton reactions. It is thought that an increase in reactive oxygen species contributes to high morbidity and mortality rates. Recent studies have, however, suggested that it is iron overload in mitochondria that contributes to cellular oxidative stress, mitochondrial damage, cardiac arrhythmias, as well as the development of cardiomyopathy. Iron chelators, antioxidants, and/or calcium channel blockers have been demonstrated to prevent and ameliorate cardiac dysfunction in animal models as well as in patients suffering from cardiac iron overload. Hence, either a mono-therapy or combination therapies with any of the aforementioned agents may serve as a novel treatment in iron-overload patients in the near future. In the present article, we review the mechanisms of cytosolic and/or mitochondrial iron load in the heart which may contribute synergistically or independently to the development of iron-associated cardiomyopathy. We also review available as well as potential future novel treatments.

Increased Ferritin Concentration and Risk of Atrial Fibrillation and Heart Failure in Men and Women: Three Studies of the Danish General Population Including 35799 Individuals

BACKGROUND

Moderately increased plasma ferritin, as a biomarker of iron overload, has been associated with higher rates of cardiovascular death and heart failure. However, the association of moderately increased plasma ferritin with risk of atrial fibrillation in the general population is unknown.

METHODS

We examined the association of plasma ferritin concentrations with risk of atrial fibrillation and heart failure in metaanalyses of 35799 men and women from 3 studies of the Danish general population: the Copenhagen City Heart Study, the Danish General Suburban Population Study, and the Copenhagen General Population Study.

RESULTS

Multivariable adjusted fixed effects odds ratios for atrial fibrillation were 1.23 (95% CI, 1.05-1.44; P = 0.005) in men for ferritin concentration ≥300 μg/L vs <300 μg/L, 1.13 (95% CI, 0.93-1.38; P = 0.22) in women for ≥200 μg/L vs <200 μg/L, and 1.19 (95% CI, 1.06-1.35; P = 0.005) in both sexes combined (P _{sex interaction} = 0.52). Corresponding fixed effects odds ratios for heart failure were 1.16 (95% CI, 0.98-1.37; P = 0.08) in men, 0.86 (95% CI, 0.67-1.10; P = 0.23) in women, and 1.05 (95% CI, 0.91-1.21; P = 0.45) in both sexes combined (P _{sex interaction} = 0.05). Multivariable adjusted fixed effects odds ratio for atrial fibrillation per step increase in ferritin concentrations was 1.13 (95% CI, 1.06-1.21; P _trend = 0.0005) in both sexes combined (P _{sex interaction} = 0.59); the corresponding value for heart failure was 1.03 (95% CI, 0.95-1.11; P _trend = 0.47) (P _{sex interaction} = 0.08). In sensitivity analyses, there was no evidence of U-shaped relationships between plasma ferritin concentrations and risk of atrial fibrillation or heart failure in men or women.

CONCLUSIONS

Increased ferritin concentration is associated with increased risk of atrial fibrillation in the general population.

Electrocardiographic Presentation, Cardiac Arrhythmias, and Their Management in β-Thalassemia Major Patients

Beta-thalassemia major (β-TM) is a genetic hemoglobin disorder characterized by an absent synthesis of globin chains that are essential for hemoglobin formation, causing chronic hemolytic anemia. Clinical management of thalassemia major consists in regular long-life red blood cell transfusions and iron chelation therapy to remove iron introduced in excess with transfusions. Iron deposition in combination with inflammatory and immunogenic factors is involved in the pathophysiology of cardiac dysfunction in these patients. Heart failure and arrhythmias, caused by myocardial siderosis, are the most important life-limiting complications of iron overload in beta-thalassemia patients. Cardiac complications are responsible for 71% of global death in the beta-thalassemia major patients. The aim of this review was to describe the most frequent electrocardiographic abnormalities and arrhythmias observed in β-TM patients, analyzing their prognostic impact and current treatment strategies.

Role of non-invasive assessment in prediction of preclinical cardiac affection in multi-transfused thalassaemia major patients

BACKGROUND

The principal cause of mortality and morbidity in β-thalassemia major (β-TM) is the iron overload as these patients receive about 20 times the normal intake of iron, which leads to iron accumulation and damage in the liver, heart, and endocrine organs. Chronically transfused patients used to die from cardiac iron overload in their teens and twenties. Monitoring of iron status through cardiac magnetic resonance imaging (CMRI) has replaced the conventional methods of assessment, yet this modality is not readily available in centers where the disease distribution is maximal. Objectives The aim of this work is to study some simple non-invasive tools and their abilities to predict preclinical cardiac affection reflecting cardiac iron deposition (CID) in multi-transfused β-TM patients taking the T2* CMRI as a gold standard reference test.

METHODS

One hundred consecutive multi-transfused, clinically stable β-TM patients with age ranging from 16 to 30 years (mean ± SD, 21.1 ± 3.9) were included in this study. Assessment of serum ferritin, serum hepcidin, and high-sensitivity C-reactive protein as well as cardiac assessment by echo-doppler and 24-hour Holter were used to predict CID, and consequently predict preclinical cardiac affection, in reference to CMRI results as the standard method of cardiac iron assessment.

RESULTS

According to CMRI results, patients were subdivided into a group of 42 patients with detectable myocardial iron (T*≤ 20 ms) and a group of 58 patients with no detectable myocardial iron (T* > 20 ms). No differences in age, gender, or distribution of splenectomized patients were observed between both groups. Patients with detectable myocardial iron received significantly higher number of transfusions per year than those with no detectable myocardial iron (mean ± SD, 14.6 ± 1.7 vs. 12.5 ± 1.7; P < 0.001) yet comparable levels of serum ferritin, serum hepcidin, and hepcidin/ferritin ratio (P > 0.05) were noted. Cardiac iron detection was associated with significantly lower heart rate (mean ± SD, 75 ± 6.1 vs. 80 ± 6.9; P < 0.001), lower left ventricular ejection fraction (LVEF) (mean ± SD 60.1 ± 3.2 vs. 70.1 ± 2.8; P < 0.001), and higher total number of premature ventricular contractions (PVCs) (median 78 vs. 14; P < 0.001). The group with CID comprised significantly more patients with left ventricular diastolic dysfunction (15/42, 35.7% vs. 3/58, 5.2%; P < 0.001); PVCs ≥10/hour (13/42, 31% vs. 2/58, 3.4%; P < 0.001); episodes of sinus pauses (6/42, 14.3% vs. 1/58, 1.7%; P < 0.05); episodes of high-grade atrio-ventricular block (5/42, 11.9% vs. 1/58, 1.7%; P < 0.05) compared to the group with no (CID). In presence of normal LVEF, detection of 10 or more PVCs per hour was the most predictive of cardiac iron loading with a positive predictive value of 86.7% and specificity of 96.6%, and the highest likelihood ratio (9.09). Detection of more than 22 PVCs/24 hours had the best sensitivity (81%) and the best negative predictive value (84%). The positive likelihood ratio of the studied parameters was highest in case of presence of PVCs ≥10/hour and lowest in case of average heart rate with a cut-off level of ≤77.5 bpm (9.09 and 1.46, respectively).

CONCLUSION

Our results support our hypothesis that monitoring β-TM patients with echo and Holter electrocardiogram can help in the detection of preclinical cardiac affection in centers lacking cardiac MRI; however, due to relatively low sensitivity they can not fully replace CMRI. Further work is needed to identify additional simple parameters that can form a diagnostic model with adequate sensitivity.

Liver iron quantification by 3 tesla MRI: calibration on a rabbit model

PURPOSE

To determine the feasibility of liver iron quantification by 3 Tesla (T) MRI using a novel iron overload rabbit model.

MATERIALS AND METHODS

Forty-two rabbits underwent iron dextran loading from 1 to 15 weeks. MRI signal intensity ratio (SIR) was measured using a gradient-echo sequence, and R2(1/T2) measured using an eight-echo spin-echo sequence at 3T. Ex vivo hepatic pathology was obtained for all rabbits studied. Postmortem assessments of liver iron concentration (LIC) were conducted in an atomic absorption spectrophotometer. MRI measures were fitted against LIC using linear regression for 30 of the iron-loaded rabbits. The remaining 12 iron-loaded rabbits were used to test the prediction accuracy of the derived models.

RESULTS

LIC was linearly correlated to both liver-to-muscle SIR (r = -0.845) and R2 (r = 0.965) in a range achieved in this study (LIC < 10 mg/g dry tissue) at 3T. By regression, the linear equations were determined as: Y1 = 10.581-5.924X1 (Y1 : LIC, X1 :SIR); Y2 = -1.273+0.103X2 (Y2 :LIC, X2 :R2). In the 12 test rabbits, the predicted LICs using the derived equations agreed well with the results obtained using the spectrophotometer.

CONCLUSION

Both SIR and R2 are highly correlated with LIC in a novel rabbit model. MRI quantification of liver iron overload is feasible at 3T.

Electrocardiographic consequences of cardiac iron overload in thalassemia major

Iron cardiomyopathy is a leading cause of death in transfusion-dependent thalassemia major (TM) patients and MRI (T2*) can recognize preclinical cardiac iron overload, but, is unavailable to many centers. We evaluated the ability of 12-lead electrocardiography to predict cardiac iron loading in TM. 12-lead electrocardiogram and cardiac T2* measurements were performed prospectively, with a detectable cardiac iron cutoff of T2*less than 20 ms. Patients with and without cardiac iron were compared using two-sample statistics and against population norms using age and gender-matched Z-scores. 45/78 patients had detectable cardiac iron. Patients having cardiac iron were older and more likely female but had comparable liver iron burdens and serum ferritin. Increased heart rate (HR) and prolonged corrected QT interval (QT(c)) were present, regardless of cardiac iron status. Repolarization abnormalities were the strongest predictors of cardiac iron, including QT/QT(c) prolongation, left shift of T-wave axis, and interpretation of ST/T-wave morphology. Recursive partitioning of the data for females using T-axis and HR and for males using QT, HR, and T-axis produced algorithms with AUROC's of 88.3 and 87.1, respectively. Bradycardia and repolarization abnormalities on 12-lead electrocardiography were the most specific markers for cardiac iron in thalassemia major. Changes in these variables may be helpful to stratify cardiac risk when cardiac MRI is unavailable. However, diagnostic algorithms need to be vetted on larger and more diverse patient populations and longitudinal studies are necessary to determine reversibility of the observed abnormalities.

Rapid monitoring of iron-chelating therapy in thalassemia major by a new cardiovascular MR measure: the reduced transverse relaxation rate

In iron overload, almost all the excess iron is stored intracellularly as rapidly mobilizable ferritin iron and slowly exchangeable hemosiderin iron. Increases in cytosolic iron may produce oxidative damage that ultimately results in cardiomyocyte dysfunction. Because intracellular ferritin iron is evidently in equilibrium with the low-molecular-weight cytosolic iron pool, measurements of ferritin iron potentially provide a clinically useful indicator of changes in cytosolic iron. The cardiovascular magnetic resonance (CMR) index of cardiac iron used clinically, the effective transverse relaxation rate (R(2)*), is principally influenced by hemosiderin iron and changes only slowly over several months, even with intensive iron-chelating therapy. Another conventional CMR index of cardiac iron, the transverse relaxation rate (R(2)), is sensitive to both hemosiderin iron and ferritin iron. We have developed a new MRI measure, the 'reduced transverse relaxation rate' (RR(2)), and have proposed in previous studies that this measure is primarily sensitive to ferritin iron and largely independent of hemosiderin iron in phantoms mimicking ferritin iron and human liver explants. We hypothesized that RR(2) could detect changes produced by 1 week of iron-chelating therapy in patients with transfusion-dependent thalassemia. We imaged 10 patients with thalassemia major at 1.5 T in mid-ventricular short-axis planes of the heart, initially after suspending iron-chelating therapy for 1 week and subsequently after resuming oral deferasirox. After resuming iron-chelating therapy, significant decreases were observed in the mean myocardial RR(2) (7.8%, p < 0.01) and R(2) (5.5%, p < 0.05), but not in R(2)* (1.7%, p > 0.90). Although the difference between changes in RR(2) and R(2) was not significant (p > 0.3), RR(2) was consistently more sensitive than R(2) (and R(2)*) to the resumption of iron-chelating therapy, as judged by the effect sizes of relaxation rate differences detected. Although further studies are needed, myocardial RR(2) may be a promising investigational method for the rapid assessment of the effects of iron-chelating therapy in the heart.

Iron overload decreases CaV1.3-dependent L-type Ca2+ currents leading to bradycardia, altered electrical conduction, and atrial fibrillation

BACKGROUND

Chronic iron overload (CIO) is associated with blood disorders such as thalassemias and hemochromatosis. A major prognostic indicator of survival in patients with CIO is iron-mediated cardiomyopathy characterized by contractile dysfunction and electrical disturbances, including slow heart rate (bradycardia) and heart block.

METHODS AND RESULTS

We used a mouse model of CIO to investigate the effects of iron on sinoatrial node (SAN) function. As in humans, CIO reduced heart rate (≈20%) in conscious mice as well as in anesthetized mice with autonomic nervous system blockade and in isolated Langendorff-perfused mouse hearts, suggesting that bradycardia originates from altered intrinsic SAN pacemaker function. Indeed, spontaneous action potential frequencies in SAN myocytes with CIO were reduced in association with decreased L-type Ca(2+) current (I(Ca,L)) densities and positive (rightward) voltage shifts in I(Ca,L) activation. Pacemaker current (I(f)) was not affected by CIO. Because I(Ca,L) in SAN myocytes (as well as in atrial and conducting system myocytes) activates at relatively negative potentials due to the presence of Ca(V)1.3 channels (in addition to Ca(V)1.2 channels), our data suggest that elevated iron preferentially suppresses Ca(V)1.3 channel function. Consistent with this suggestion, CIO reduced Ca(V)1.3 mRNA levels by ≈40% in atrial tissue (containing SAN) and did not lower heart rate in Ca(V)1.3 knockout mice. CIO also induced PR-interval prolongation, heart block, and atrial fibrillation, conditions also seen in Ca(V)1.3 knockout mice.

CONCLUSIONS

Our results demonstrate that CIO selectively reduces Ca(V)1.3-mediated I(Ca,L), leading to bradycardia, slowing of electrical conduction, and atrial fibrillation as seen in patients with iron overload.

Iron sufficient to cause hepatic fibrosis and ascites does not cause cardiac arrhythmias in the gerbil

Chronic iron overload associated with hereditary hemochromatosis or repeated red cell transfusions is known to cause cardiac failure. Cardiac arrhythmias have been incidentally noted in patients with iron overload, but they are often dismissed as being related to comorbid conditions. Studies with anesthetized iron-loaded gerbils using short recordings suggest a role for iron in the development of arrhythmias. Our goal was to characterize iron-induced arrhythmias in the chronically instrumented, untethered, telemetered gerbil. Electrocardiograms were recorded for 10 s every 30 min for approximately 6 months in iron-loaded (n=23) and control (n=8) gerbils. All gerbils in both groups showed evidence of frequent sinus arrhythmia. There was no difference in heart rate, electrocardiographic parameters, or number of arrhythmias per minute between groups. Gerbils rarely showed significant arrhythmias. Body weight and heart weight were not significantly different between groups, whereas liver weight increased with increasing iron dose in the treated group. Cardiac and hepatic iron concentrations were significantly increased in iron-loaded gerbils. Eight of 14 gerbils loaded to 6.2 g/kg body weight developed ascites. We conclude that an iron load sufficient to cause clinical liver disease does not cause cardiac arrhythmias in the gerbil model of iron overload.

Influence of iron chelation on R1 and R2 calibration curves in gerbil liver and heart

MRI is gaining increasing importance for the noninvasive quantification of organ iron burden. Since transverse relaxation rates depend on iron distribution as well as iron concentration, physiologic and pharmacologic processes that alter iron distribution could change MRI calibration curves. This article compares the effect of three iron chelators, deferoxamine, deferiprone, and deferasirox, on R1 and R2 calibration curves according to two iron loading and chelation strategies. Thirty-three Mongolian gerbils underwent iron loading (iron dextran 500 mg/kg/wk) for 3 weeks followed by 4 weeks of chelation. An additional 56 animals received less aggressive loading (200 mg/kg/week) for 10 weeks, followed by 12 weeks of chelation. R1 and R2 calibration curves were compared to results from 23 iron-loaded animals that had not received chelation. Acute iron loading and chelation-biased R1 and R2 from the unchelated reference calibration curves but chelator-specific changes were not observed, suggesting physiologic rather than pharmacologic differences in iron distribution. Long-term chelation deferiprone treatment increased liver R1 50% (P < 0.01), while long-term deferasirox lowered liver R2 30.9% (P < 0.0001). The relationship between R1 and R2 and organ iron concentration may depend on the acuity of iron loading and unloading as well as the iron chelator administered.

Antioxidant-mediated effects in a gerbil model of iron overload

INTRODUCTION

Iron cardiomyopathy is a lethal complication of transfusion therapy in thalassemia major. Nutritional supplements decreasing cardiac iron uptake or toxicity would have clinical significance. Murine studies suggest taurine may prevent oxidative damage and inhibit Ca2+-channel-mediated iron transport. We hypothesized that taurine supplementation would decrease cardiac iron-overloaded toxicity by decreasing cardiac iron. Vitamin E and selenium served as antioxidant control.

METHODS

Animals were divided into control, iron, taurine, and vitamin E/selenium groups. Following sacrifice, iron and selenium measurements, histology, and biochemical analyses were performed.

RESULTS

No significant differences were found in heart and liver iron content between treatment groups, except for higher hepatic dry-weight iron concentrations in taurine-treated animals (p < 0.03). Serum iron increased with iron loading (751 +/- 66 vs. 251 +/- 54 microg/dl, p < 0.001) and with taurine (903 +/- 136 microg/dl, p = 0.03).

CONCLUSION

Consistent with oxidative stress, iron overload increased cardiac malondialdehyde levels, decreased heart glutathione peroxidase (GPx) activity, and increased serum aspartate aminotransferase. Taurine ameliorated these changes, but only significantly for liver GPx activity. Selenium and vitamin E supplementation did not improve oxidative markers and worsened cardiac GPx activity. These results suggest that taurine acts primarily as an antioxidant rather than inhibiting iron uptake. Future studies should illuminate the complexity of these results.

Deferasirox and deferiprone remove cardiac iron in the iron-overloaded gerbil

INTRODUCTION

Deferasirox effectively controls liver iron concentration; however, little is known regarding its ability to remove stored cardiac iron. Deferiprone seems to have increased cardiac efficacy compared with traditional deferoxamine therapy. Therefore, the relative efficacy of deferasirox and deferiprone were compared in removing cardiac iron from iron-loaded gerbils.

METHODS

Twenty-nine 8- to 10-week-old female gerbils underwent 10 weekly iron dextran injections of 200 mg/kg/week. Prechelation iron levels were assessed in 5 animals, and the remainder received deferasirox 100 mg/kg/D po QD (n = 8), deferiprone 375 mg/kg/D po divided TID (n = 8), or sham chelation (n = 8), 5 days/week for 12 weeks.

RESULTS

Deferasirox reduced cardiac iron content 20.5%. No changes occurred in cardiac weight, myocyte hypertrophy, fibrosis, or weight-to-dry weight ratio. Deferasirox treatment reduced liver iron content 51%. Deferiprone produced comparable reductions in cardiac iron content (18.6% reduction). Deferiprone-treated hearts had greater mass (16.5% increase) and increased myocyte hypertrophy. Deferiprone decreased liver iron content 24.9% but was associated with an increase in liver weight and water content.

CONCLUSION

Deferasirox and deferiprone were equally effective in removing stored cardiac iron in a gerbil animal model, but deferasirox removed more hepatic iron for a given cardiac iron burden.

Desferri-Exochelin, a lipid-soluble, hexadentate iron chelator, effectively removes tissue iron

Chronic iron-overload is damaging to the heart, liver, and other organs. Better iron chelators are needed to treat this serious medical condition. The uptake and distribution of the lipid-soluble, hexadentate iron chelator desferri-Exochelin 772SM (D-Exo) is studied and its efficacy in removing iron from tissue in rodent models is evaluated. After an intravenous bolus of tritiated D-Exo to rats, counts rapidly disappeared from the blood and rapidly appeared in 15 organs studied, usually peaking within 15 min. There was considerable uptake in the heart and liver, 2 organs especially susceptible to damage from clinical iron overload. To assess actual decreases in cardiac and hepatic iron in response to D-Exo, mice loaded with 42 mg of iron dextran (2100 mg/kg) were studied. Untreated, iron-loaded mice sacrificed 9 weeks later had a 4-fold increase in cardiac iron and a 20-fold increase in hepatic iron compared with controls that were not iron-loaded. In iron-loaded mice treated with 7 mg of D-Exo intraperitoneally (i.p.) 4 days/week for 8 weeks (total 224 mg), tissue iron, measured by atomic absorption, was reduced by 20% in the liver and 25% in the heart (P < 0.01 for each organ). During the first 8 h after a D-Exo dose, iron was excreted in the urine. Mice treated with D-Exo gained weight normally and showed no evidence of toxicity. In conclusion, in this iron-overload mouse model, D-Exo administered intravenously or i.p. rapidly diffuses into multiple organs, including the heart and liver, and effectively removes iron without apparent toxicity.

Antioxidant and lysosomotropic properties of acute D-propranolol underlies its cardioprotection of postischemic hearts from moderate iron-overloaded rats

The benefits of acute D-propranolol (D-Pro, non-beta-adrenergic receptor blocker) pretreatment against enhanced ischemia/reperfusion (I/R) injury of hearts from moderate iron-overloaded rats were examined. Perfused hearts from iron-dextran-treated rats (450 mg/kg/week for 3 weeks, intraperitoneal administration) exhibited normal control function, despite iron treatment that elevated plasma iron and conjugated diene levels by 8.1-and 2.5-fold, respectively. However, these hearts were more susceptible to 25 mins of global I/R stress compared with non-loaded hearts; the coronary flow rate, aortic output, cardiac work, left ventricular systolic pressure, positive differential left ventricular pressure (dP/dt), and left ventricular developed pressure displayed 38%, 60%, 55%, 13%, 41%, and 15% lower recoveries, respectively, and a 6.5-fold increase in left ventricular end-diastolic pressure. Postischemic hearts from iron-loaded rats also exhibited 5.6-, 3.48-, 2.43-, and 3.45-fold increases in total effluent iron content, conjugated diene levels, lactate dehydrogenase (LDH) activity, and lysosomal N-acetyl-beta-glucosaminidase (NAGA) activity, respectively, compared with similarly stressed non-loaded hearts. A comparison of detection time profiles during reperfusion suggests that most of the oxidative injury (conjugated diene) in hearts from iron-loaded rats occurred at later times of reperfusion (8.5-15 mins), and this corresponded with heightened tissue iron and NAGA release. D-Pro (2 microM infused for 30 mins) pretreatment before ischemia protected all parameters compared with the untreated iron-loaded group; pressure indices improved 1.2- to 1.6-fold, flow parameters improved 1.70- to 2.96-fold, cardiac work improved 2.87-fold, and end-diastolic pressure was reduced 56%. D-Pro lowered total release of tissue iron, conjugated diene content, LDH activity, and NAGA activity 4.59-, 2.55-, 3.04-, and 4.14-fold, respectively, in the effluent of I/R hearts from the iron-loaded group. These findings suggest that the enhanced postischemic dysfunction and tissue injury of hearts from iron-loaded rats was caused by excessive iron-catalyzed free radical stress, and that the membrane antioxidant properties of D-Pro and its stabilization of sequestered lysosomal iron by D-Pro may contribute to the cardioprotective actions of D-Pro.

Iron dextran causes renal iron deposition but not renal dysfunction in angiotensin II-treated and untreated rats

BACKGROUND

Angiotensin II infusion into rats causes iron deposition in the kidney, which may augment the pro-proteinuric effects of this octapeptide. We have investigated whether administration of iron mimics the renal damage induced by angiotensin II.

METHODS

Rats were treated with iron dextran at a total dose of 960 mg/kg either with or without angiotensin II treatment at a dose of 0.7 mg/kg/day for 7 days. Protein expression of ferritin and heme oxygenase-1, an oxidative stress-sensitive gene, was determined by Western blot analysis and immunohistochemistry.

RESULTS

Administration of iron dextran did not significantly increase proteinuria or decrease creatinine clearance in the rats with or without angiotensin II treatment. Prussian blue staining showed that iron deposition was observed mainly in the glomerular and medullar regions in the iron dextran-treated rats, but in the tubular epithelial cells in angiotensin II-infused rats. Administration of iron dextran upregulated ferritin, but not heme oxygenase-1.

CONCLUSION

Iron dextran did not enhance or cause the renal dysfunction in the angiotensin II-treated or untreated rats, respectively. The distribution of deposited iron and presumably the type of iron compound administered may be important determinants of the development of renal injury.

Cardiac iron determines cardiac T2*, T2, and T1 in the gerbil model of iron cardiomyopathy

BACKGROUND

Transfusional therapy for thalassemia major and sickle cell disease can lead to iron deposition and damage to the heart, liver, and endocrine organs. Iron causes the MRI parameters T1, T2, and T2* to shorten in these organs, which creates a potential mechanism for iron quantification. However, because of the danger and variability of cardiac biopsy, tissue validation of cardiac iron estimates by MRI has not been performed. In this study, we demonstrate that iron produces similar T1, T2, and T2* changes in the heart and liver using a gerbil iron-overload model.

METHODS AND RESULTS

Twelve gerbils underwent iron dextran loading (200 mg . kg(-1) . wk(-1)) from 2 to 14 weeks; 5 age-matched controls were studied as well. Animals had in vivo assessment of cardiac T2* and hepatic T2 and T2* and postmortem assessment of cardiac and hepatic T1 and T2. Relaxation measurements were performed in a clinical 1.5-T magnet and a 60-MHz nuclear magnetic resonance relaxometer. Cardiac and liver iron concentrations rose linearly with administered dose. Cardiac 1/T2*, 1/T2, and 1/T1 rose linearly with cardiac iron concentration. Liver 1/T2*, 1/T2, and 1/T1 also rose linearly, proportional to hepatic iron concentration. Liver and heart calibrations were similar on a dry-weight basis.

CONCLUSIONS

MRI measurements of cardiac T2 and T2* can be used to quantify cardiac iron. The similarity of liver and cardiac iron calibration curves in the gerbil suggests that extrapolation of human liver calibration curves to heart may be a rational approximation in humans.

Deferoxamine prevents cardiac hypertrophy and failure in the gerbil model of iron-induced cardiomyopathy

To evaluate the effects of the iron chelator deferoxamine on the functional and structural manifestations of iron-induced cardiac dysfunction, we measured cardiac power, left ventricular systolic, and diastolic function as (dP/dt)max and (dP/dt)min, respectively, and left ventricular and septal wall thickness in isolated heart preparations derived from the Mongolian gerbil model of iron overload. We induced iron overload with weekly subcutaneous injections of iron dextran (800 mg/kg/wk); deferoxamine (DFO; 100 mg/kg) was administered twice daily by subcutaneous injection, 5 of 7 days each week; and control animals received weekly subcutaneous injections of dextran alone. Animals administered iron alone initially exhibited, at 5 weeks, increased cardiac power but by 12 to 20 weeks, cardiac power was severely diminished, with impairment of both systolic and diastolic function of the left ventricle and marked cardiac hypertrophy (P<.001 for all vs control animals). Administration of DFO with iron did not interfere with the initial augmentation of cardiac power at 5 weeks but prevented the subsequent deterioration in cardiac performance. After 12 to 20 weeks, gerbils given DFO with iron had mean values of cardiac power indistinguishable from those of control animals; both systolic and diastolic function were significantly enhanced not only in comparison with those of animals treated with iron alone but also with respect to controls. In addition, DFO prevented cardiac hypertrophy; mean ventricular and septal wall thickness in gerbils given DFO and iron were not significantly different from those in controls. In the gerbil model of iron overload, concurrent administration of DFO with iron prevents both the development of cardiac hypertrophy and the progressive deterioration in cardiac performance that are produced by chronic iron accumulation.

Optical mapping reveals conduction slowing and impulse block in iron-overload cardiomyopathy

Cardiac disease with arrhythmia or heart failure is the leading cause of death in patients with thalassemia major and a major complication of other forms of iron overload. Current antiarrhythmic treatment does not appear to alter the clinical course. Using a gerbil model of iron-overload cardiomyopathy, we previously observed a reduction in the fast inward sodium current in isolated cardiomyocytes. Electrocardiograms (ECGs) in the same gerbil model indicate PR-interval prolongation, QRS-interval widening, and arrhythmias. We hypothesize that such changes in the ECG in this model are the result of abnormal action-potential conduction at the level of the whole heart. To test this hypothesis, we took ECGs and recorded action potentials using high-resolution optical mapping from the anterior surface of 9 iron-overloaded and 9 age-matched control ventricular-paced, Langendorff-perfused gerbil hearts. The iron-overloaded gerbils received weekly iron-dextran injections of 800 mg/kg for 14 to 18 weeks. ECGs showed QRS- and PR-interval prolongation in iron-treated gerbils compared with that in controls. In addition, atrioventricular block was observed in 2 of 6 iron-treated gerbils but not in controls. Conduction velocity was significantly slower in iron-treated gerbils than in controls. At normal pacing rates, abnormal activation patterns caused by stable regions of conduction block were observed in iron-overloaded gerbils (33%) but not in controls. Such abnormal impulse conduction may be a mechanism of increased arrhythmia vulnerability in iron-overload cardiomyopathy.