GCN5 contributes to intracellular lipid accumulation in human primary cardiac stromal cells from patients affected by Arrhythmogenic cardiomyopathy.

Arrhythmogenic cardiomyopathy (ACM) is a genetic disease associated with sudden cardiac death and cardiac fibro-fatty replacement. Over the last years, several works have demonstrated that different epigenetic enzymes can affect not only gene expression changes in cardiac diseases but also cellular metabolism. Specifically, the histone acetyltransferase GCN5 is known to facilitate adipogenesis and modulate cardiac metabolism in heart failure. Our group previously demonstrated that human primary cardiac stromal cells (CStCs) contribute to adipogenesis in the ACM pathology. Thus, this study aims to evaluate the role of GCN5 in ACM intracellular lipid accumulation. To do so, CStCs were obtained from right ventricle biopsies of ACM patients and from samples of healthy cadaveric donors (CTR). GCN5 expression was increased both in ex vivo and in vitro ACM samples compared to CTR. When GCN5 expression was silenced or pharmacologically inhibited by the administration of MB-3, we observed a reduction in lipid accumulation and a mitigation of reactive oxygen species (ROS) production in ACM CStCs. In agreement, transcriptome analysis revealed that the presence of MB-3 modified the expression of pathways related to cellular redox balance. Altogether, our findings suggest that GCN5 inhibition reduces fat accumulation in ACM CStCs, partially by modulating intracellular redox balance pathways.

A fully human anti-BMP6 antibody reduces the need for erythropoietin in rodent models of the anemia of chronic disease.

Recombinant erythropoietin (EPO) and iron substitution are a standard of care for treatment of anemias associated with chronic inflammation, including anemia of chronic kidney disease. A black box warning for EPO therapy and concerns about negative side effects related to high-dose iron supplementation as well as the significant proportion of patients becoming EPO resistant over time explains the medical need to define novel strategies to ameliorate anemia of chronic disease (ACD). As hepcidin is central to the iron-restrictive phenotype in ACD, therapeutic approaches targeting hepcidin were recently developed. We herein report the therapeutic effects of a fully human anti-BMP6 antibody (KY1070) either as monotherapy or in combination with Darbepoetin alfa on iron metabolism and anemia resolution in 2 different, well-established, and clinically relevant rodent models of ACD. In addition to counteracting hepcidin-driven iron limitation for erythropoiesis, we found that the combination of KY1070 and recombinant human EPO improved the erythroid response compared with either monotherapy in a qualitative and quantitative manner. Consequently, the combination of KY1070 and Darbepoetin alfa resulted in an EPO-sparing effect. Moreover, we found that suppression of hepcidin via KY1070 modulates ferroportin expression on erythroid precursor cells, thereby lowering potentially toxic-free intracellular iron levels and by accelerating erythroid output as reflected by increased maturation of erythrocyte progenitors. In summary, we conclude that treatment of ACD, as a highly complex disease, becomes more effective by a multifactorial therapeutic approach upon mobilization of endogenous iron deposits and stimulation of erythropoiesis.

The haemochromatosis gene Hfe and Kupffer cells control LDL cholesterol homeostasis and impact on atherosclerosis development.

AIMS: Imbalances of iron metabolism have been linked to the development of atherosclerosis. However, subjects with hereditary haemochromatosis have a lower prevalence of cardiovascular disease. The aim of our study was to understand the underlying mechanisms by combining data from genome-wide association study analyses in humans, CRISPR/Cas9 genome editing, and loss-of-function studies in mice. METHODS AND RESULTS: Our analysis of the Global Lipids Genetics Consortium (GLGC) dataset revealed that single nucleotide polymorphisms (SNPs) in the haemochromatosis gene HFE associate with reduced low-density lipoprotein cholesterol (LDL-C) in human plasma. The LDL-C lowering effect could be phenocopied in dyslipidaemic ApoE-/- mice lacking Hfe, which translated into reduced atherosclerosis burden. Mechanistically, we identified HFE as a negative regulator of LDL receptor expression in hepatocytes. Moreover, we uncovered liver-resident Kupffer cells (KCs) as central players in cholesterol homeostasis as they were found to acquire and transfer LDL-derived cholesterol to hepatocytes in an Abca1-dependent fashion, which is controlled by iron availability. CONCLUSION: Our results disentangle novel regulatory interactions between iron metabolism, KC biology and cholesterol homeostasis which are promising targets for treating dyslipidaemia but also provide a mechanistic explanation for reduced cardiovascular morbidity in subjects with haemochromatosis.

Association of mitochondrial iron deficiency and dysfunction with idiopathic restless legs syndrome.

BACKGROUND: Restless legs syndrome is a sensorimotor neurological disorder of the limbs that impairs quality of life and disturbs sleep. However, there has been progress in understanding the disease involving the dopaminergic system as well as iron metabolism. The exact pathophysiological mechanisms of restless legs syndrome remain elusive. We tried to elucidate the underlying mechanisms in iron metabolism in restless legs syndrome subjects on a systemic, cellular, and mitochondrial level. METHODS: We conducted a study prospectively recruiting 168 restless legs syndrome patients and 119 age-matched healthy controls focusing on iron metabolism using human monocytes as surrogates. RESULTS: Evaluation of systemic iron metabolism parameters in the circulation showed no significant difference between patients and controls. We observed a significant reduction in mRNA levels of heme oxygenase 1 and mitochondrial iron genes like mitoferrin 1 and 2 in monocytes isolated from restless legs syndrome patients, indicating mitochondrial iron deficiency. Interestingly, we also observed reduced expression of iron regulatory protein 2 along with impaired activity of mitochondrial aconitase and reduced mitochondrial superoxide formation in restless legs syndrome subjects. Along this line, patients had reduced mitochondrial respiratory capacity that improved in restless legs syndrome subjects under treatment with dopaminergic drugs compared with untreated patients. CONCLUSIONS: Our data suggest that restless legs syndrome is linked to mitochondrial iron deficiency and associated impairment of mitochondrial function. This is partly corrected by treatment with dopaminergic drugs compared with untreated patients, which may be linked to an effect of dopamine on cellular iron homeostasis. © 2018 International Parkinson and Movement Disorder Society.

Pre-analytic evaluation of volumetric absorptive microsampling and integration in a mass spectrometry-based metabolomics workflow.

Volumetric absorptive microsampling (VAMS) is a novel approach that allows single-drop (10 µL) blood collection. Integration of VAMS with mass spectrometry (MS)-based untargeted metabolomics is an attractive solution for both human and animal studies. However, to boost the use of VAMS in metabolomics, key pre-analytical questions need to be addressed. Therefore, in this work, we integrated VAMS in a MS-based untargeted metabolomics workflow and investigated pre-analytical strategies such as sample extraction procedures and metabolome stability at different storage conditions. We first evaluated the best extraction procedure for the polar metabolome and found that the highest number and amount of metabolites were recovered upon extraction with acetonitrile/water (70:30). In contrast, basic conditions (pH 9) resulted in divergent metabolite profiles mainly resulting from the extraction of intracellular metabolites originating from red blood cells. In addition, the prolonged storage of blood samples at room temperature caused significant changes in metabolome composition, but once the VAMS devices were stored at - 80 °C, the metabolome remained stable for up to 6 months. The time used for drying the sample did also affect the metabolome. In fact, some metabolites were rapidly degraded or accumulated in the sample during the first 48 h at room temperature, indicating that a longer drying step will significantly change the concentration in the sample. Graphical abstract Volumetric absorptive microsampling (VAMS) is a novel technology that allows single-drop blood collection and, in combination with mass spectrometry (MS)-based untargeted metabolomics, represents an attractive solution for both human and animal studies. In this work, we integrated VAMS in a MS-based untargeted metabolomics workflow and investigated pre-analytical strategies such as sample extraction procedures and metabolome stability at different storage conditions. The latter revealed that prolonged storage of blood samples at room temperature caused significant changes in metabolome composition, but if VAMS devices were stored at - 80 °C, the metabolome remained stable for up to 6 months.

H2S during circulatory shock: some unresolved questions.

Numerous papers have been published on the role of H2S during circulatory shock. Consequently, knowledge about vascular sulfide concentrations may assume major importance, in particular in the context of "acute on chronic disease", i.e., during circulatory shock in animals with pre-existing chronic disease. This review addresses the questions (i) of the "real" sulfide levels during circulatory shock, and (ii) to which extent injury and pre-existing co-morbidity may affect the expression of H2S producing enzymes under these conditions. In the literature there is a huge range on sulfide blood levels during circulatory shock, in part as a result of the different analytical methods used, but also due to the variable of the models and species studied. Clearly, some of the very high levels reported should be questioned in the context of the well-known H2S toxicity. As long as "real" sulfide levels during circulatory shock are unknown and/or undetectable "on line" due to the lack of appropriate techniques, it appears to be premature to correlate the measured blood levels of hydrogen sulfide with the severity of shock or the H2S therapy-related biological outcomes. The available data on the tissue expression of the H2S-releasing enzymes during circulatory shock suggest that a "constitutive" CSE expression may play a crucial role of for the maintenance of organ function, at least in the kidney. The data also indicate that increased CBS and CSE expression, in particular in the lung and the liver, represents an adaptive response to stress states.

A Novel Algorithmic Approach to the Analysis of Multi-Electrode Array Signals of hiPSC-Derived Cardiomyocytes.

In order to perform in vitro cardiotoxicity screening of pharmacological substances, multi-electrode array systems are increasingly used to measure the extracellular field potentials of cell layers of human induced pluripotent stem cell cardiomyocytes. The analysis of the field potentials is usually performed using complex analysis software provided by the hardware manufacturers. In the case of the Cardiac Analysis Tool software from Axion Biosystems, inconsistencies were found in the results, which can significantly influence the cardiotoxicity screening results. In order to obtain more reliable results, a new algorithm was developed and implemented in an easy-to-use software tool, the INCardio Data Analysis Tool, which, due to its high degree of automation, can also be used by inexperienced users. The validation reveals differences in the results of the two tools both in depolarization spike amplitudes and in the time course of the field potential durations. The manual analysis of all signals affected by deviations shows that the results of the newly developed Data Analysis Tool are correct in all cases and can therefore be classified as more accurate and reliable than the reference analysis software.

VAMS-Based Blood Capillary Sampling for Mass Spectrometry-Based Human Metabolomics Studies.

Volumetric absorptive microsampling (VAMS) is a recently developed sample collection method that enables single-drop blood collection in a minimally invasive manner. Blood biomolecules can then be extracted and processed for analysis using several analytical platforms. The integration of VAMS with conventional mass spectrometry (MS)-based metabolomics approaches is an attractive solution for human studies representing a less-invasive procedure compared to phlebotomy with the additional potential for remote sample collection. However, as we recently demonstrated, VAMS samples require long-term storage at -80 °C. This study investigated the stability of VAMS samples during short-term storage and compared the metabolome obtained from capillary blood collected from the fingertip to those of plasma and venous blood from 22 healthy volunteers. Our results suggest that the blood metabolome collected by VAMS samples is stable at room temperature only for up to 6 h requiring subsequent storage at -80 °C to avoid significant changes in the metabolome. We also demonstrated that capillary blood provides better coverage of the metabolome compared to plasma enabling the analysis of several intracellular metabolites presented in red blood cells. Finally, this work demonstrates that with the appropriate pre-analytical protocol capillary blood can be successfully used for untargeted metabolomics studies.

Generation of human induced pluripotent stem cell line EURACi015-A from a patient affected by dilated cardiomyopathy carrying the Lamin A/C p.Glu161Lys mutation.

Dilated cardiomyopathy (DCM) is a common heart disorder caused by genetic and non-genetic etiologies, characterized by left ventricular dilatation and contractile dysfunction. Here, we created a human induced pluripotent stem cell line from peripheral blood mononuclear cells using non-integrating vectors from a patient carrying a heterozygous LMNA variant (c.481G > A, p.Glu161Lys, NM_170707.4). The obtained EURACi015-A line, showed the typical morphology of pluripotent cells, normal karyotype and exhibited pluripotency markers and a trilineage differentiation potential. This cell line can be successfully differentiated into cardiomyocytes and endothelial cells. This line represents a human in vitro model to study the genetic basis of DCM.

Modeling incomplete penetrance in arrhythmogenic cardiomyopathy by human induced pluripotent stem cell derived cardiomyocytes.

Human induced pluripotent stem cell derived cardiomyocytes (hiPSC-CMs) are commonly used to model arrhythmogenic cardiomyopathy (ACM), a heritable cardiac disease characterized by severe ventricular arrhythmias, fibrofatty myocardial replacement and progressive ventricular dysfunction. Although ACM is inherited as an autosomal dominant disease, incomplete penetrance and variable expressivity are extremely common, resulting in different clinical manifestations. Here, we propose hiPSC-CMs as a powerful in vitro model to study incomplete penetrance in ACM. Six hiPSC lines were generated from blood samples of three ACM patients carrying a heterozygous deletion of exon 4 in the PKP2 gene, two asymptomatic (ASY) carriers of the same mutation and one healthy control (CTR), all belonging to the same family. Whole exome sequencing was performed in all family members and hiPSC-CMs were examined by ddPCR, western blot, Wes™ immunoassay system, patch clamp, immunofluorescence and RNASeq. Our results show molecular and functional differences between ACM and ASY hiPSC-CMs, including a higher amount of mutated PKP2 mRNA, a lower expression of the connexin-43 protein, a lower overall density of sodium current, a higher intracellular lipid accumulation and sarcomere disorganization in ACM compared to ASY hiPSC-CMs. Differentially expressed genes were also found, supporting a predisposition for a fatty phenotype in ACM hiPSC-CMs. These data indicate that hiPSC-CMs are a suitable model to study incomplete penetrance in ACM.

Mitochondrial Respiration in Response to Iron Deficiency Anemia: Comparison of Peripheral Blood Mononuclear Cells and Liver.

Iron is an essential component for metabolic processes, including oxygen transport within hemoglobin, tricarboxylic acid (TCA) cycle activity, and mitochondrial energy transformation. Iron deficiency can thus lead to metabolic dysfunction and eventually result in iron deficiency anemia (IDA), which affects approximately 1.5 billion people worldwide. Using a rat model of IDA induced by phlebotomy, we studied the effects of IDA on mitochondrial respiration in peripheral blood mononuclear cells (PBMCs) and the liver. Furthermore, we evaluated whether the mitochondrial function evaluated by high-resolution respirometry in PBMCs reflects corresponding alterations in the liver. Surprisingly, mitochondrial respiratory capacity was increased in PBMCs from rats with IDA compared to the controls. In contrast, mitochondrial respiration remained unaffected in livers from IDA rats. Of note, citrate synthase activity indicated an increased mitochondrial density in PBMCs, whereas it remained unchanged in the liver, partly explaining the different responses of mitochondrial respiration in PBMCs and the liver. Taken together, these results indicate that mitochondrial function determined in PBMCs cannot serve as a valid surrogate for respiration in the liver. Metabolic adaptions to iron deficiency resulted in different metabolic reprogramming in the blood cells and liver tissue.

Metabolic Signature of Arrhythmogenic Cardiomyopathy.

Arrhythmogenic cardiomyopathy (ACM) is a genetic-based cardiac disease accompanied by severe ventricular arrhythmias and a progressive substitution of the myocardium with fibro-fatty tissue. ACM is often associated with sudden cardiac death. Due to the reduced penetrance and variable expressivity, the presence of a genetic defect is not conclusive, thus complicating the diagnosis of ACM. Recent studies on human induced pluripotent stem cells-derived cardiomyocytes (hiPSC-CMs) obtained from ACM individuals showed a dysregulated metabolic status, leading to the hypothesis that ACM pathology is characterized by an impairment in the energy metabolism. However, despite efforts having been made for the identification of ACM specific biomarkers, there is still a substantial lack of information regarding the whole metabolomic profile of ACM patients. The aim of the present study was to investigate the metabolic profiles of ACM patients compared to healthy controls (CTRLs). The targeted Biocrates AbsoluteIDQ® p180 assay was used on plasma samples. Our analysis showed that ACM patients have a different metabolome compared to CTRLs, and that the pathways mainly affected include tryptophan metabolism, arginine and proline metabolism and beta oxidation of fatty acids. Altogether, our data indicated that the plasma metabolomes of arrhythmogenic cardiomyopathy patients show signs of endothelium damage and impaired nitric oxide (NO), fat, and energy metabolism.

Dietary Iron Overload and Hfe-/- Related Hemochromatosis Alter Hepatic Mitochondrial Function.

Iron is an essential co-factor for many cellular metabolic processes, and mitochondria are main sites of utilization. Iron accumulation promotes production of reactive oxygen species (ROS) via the catalytic activity of iron species. Herein, we investigated the consequences of dietary and genetic iron overload on mitochondrial function. C57BL/6N wildtype and Hfe-/- mice, the latter a genetic hemochromatosis model, received either normal diet (ND) or high iron diet (HI) for two weeks. Liver mitochondrial respiration was measured using high-resolution respirometry along with analysis of expression of specific proteins and ROS production. HI promoted tissue iron accumulation and slightly affected mitochondrial function in wildtype mice. Hepatic mitochondrial function was impaired in Hfe-/- mice on ND and HI. Compared to wildtype mice, Hfe-/- mice on ND showed increased mitochondrial respiratory capacity. Hfe-/- mice on HI showed very high liver iron levels, decreased mitochondrial respiratory capacity and increased ROS production associated with reduced mitochondrial aconitase activity. Although Hfe-/- resulted in increased mitochondrial iron loading, the concentration of metabolically reactive cytoplasmic iron and mitochondrial density remained unchanged. Our data show multiple effects of dietary and genetic iron loading on mitochondrial function and linked metabolic pathways, providing an explanation for fatigue in iron-overloaded hemochromatosis patients, and suggests iron reduction therapy for improvement of mitochondrial function.

Oxidized LDL-dependent pathway as new pathogenic trigger in arrhythmogenic cardiomyopathy.

Arrhythmogenic cardiomyopathy (ACM) is hallmarked by ventricular fibro-adipogenic alterations, contributing to cardiac dysfunctions and arrhythmias. Although genetically determined (e.g., PKP2 mutations), ACM phenotypes are highly variable. More data on phenotype modulators, clinical prognosticators, and etiological therapies are awaited. We hypothesized that oxidized low-density lipoprotein (oxLDL)-dependent activation of PPARγ, a recognized effector of ACM adipogenesis, contributes to disease pathogenesis. ACM patients showing high plasma concentration of oxLDL display severe clinical phenotypes in terms of fat infiltration, ventricular dysfunction, and major arrhythmic event risk. In ACM patient-derived cardiac cells, we demonstrated that oxLDLs are major cofactors of adipogenesis. Mechanistically, the increased lipid accumulation is mediated by oxLDL cell internalization through CD36, ultimately resulting in PPARγ upregulation. By boosting oxLDL in a Pkp2 heterozygous knock-out mice through high-fat diet feeding, we confirmed in vivo the oxidized lipid dependency of cardiac adipogenesis and right ventricle systolic impairment, which are counteracted by atorvastatin treatment. The modulatory role of oxidized lipids on ACM adipogenesis, demonstrated at cellular, mouse, and patient levels, represents a novel risk stratification tool and a target for ACM pharmacological strategies.

Metabolic reprogramming of Salmonella infected macrophages and its modulation by iron availability and the mTOR pathway.

Iron is an essential nutrient for immune cells and microbes, therefore the control of its homeostasis plays a decisive role for infections. Moreover, iron affects metabolic pathways by modulating the translational expression of the key tricarboxylic acid cycle (TCA) enzyme mitochondrial aconitase and the energy formation by mitochondria. Recent data provide evidence for metabolic re-programming of immune cells including macrophages during infection which is centrally controlled by mTOR. We herein studied the effects of iron perturbations on metabolic profiles in macrophages upon infection with the intracellular bacterium Salmonella enterica serovar Typhimurium and analysed for a link to the mTOR pathway. Infection of the murine macrophage cell line RAW264.7 with Salmonella resulted in the induction of mTOR activity, anaerobic glycolysis and inhibition of the TCA activity as reflected by reduced pyruvate and increased lactate levels. In contrast, iron supplementation to macrophages not only affected the mRNA expression of TCA and glycolytic enzymes but also resulted in metabolic reprogramming with increased pyruvate accumulation and reduced lactate levels apart from modulating the concentrations of several other metabolites. While mTOR slightly affected cellular iron homeostasis in infected macrophages, mTOR inhibition by rapamycin resulted in a significant growth promotion of bacteria. Importantly, iron further increased bacterial numbers in rapamycin treated macrophages, however, the metabolic profiles induced by iron in the presence or absence of mTOR activity differed in several aspects. Our data indicate, that iron not only serves as a bacterial nutrient but also acts as a metabolic modulator of the TCA cycle, partly reversing the Warburg effect and resulting in a pathogen friendly nutritional environment.

Metabolic Signature of Dietary Iron Overload in a Mouse Model.

Iron is an essential co-factor for several metabolic processes, including the Krebs cycle and mitochondrial oxidative phosphorylation. Therefore, maintaining an appropriate iron balance is essential to ensure sufficient energy production and to avoid excessive reactive oxygen species formation. Iron overload impairs mitochondrial fitness; however, little is known about the associated metabolic changes. Here we aimed to characterize the metabolic signature triggered by dietary iron overload over time in a mouse model, where mice received either a standard or a high-iron diet. Metabolic profiling was assessed in blood, plasma and liver tissue. Peripheral blood was collected by means of volumetric absorptive microsampling (VAMS). Extracted blood and tissue metabolites were analyzed by liquid chromatography combined to high resolution mass spectrometry. Upon dietary iron loading we found increased glucose, aspartic acid and 2-/3-hydroxybutyric acid levels but low lactate and malate levels in peripheral blood and plasma, pointing to a re-programming of glucose homeostasis and the Krebs cycle. Further, iron loading resulted in the stimulation of the urea cycle in the liver. In addition, oxidative stress was enhanced in circulation and coincided with increased liver glutathione and systemic cysteine synthesis. Overall, iron supplementation affected several central metabolic circuits over time. Hence, in vivo investigation of metabolic signatures represents a novel and useful tool for getting deeper insights into iron-dependent regulatory circuits and for monitoring of patients with primary and secondary iron overload, and those ones receiving iron supplementation therapy.

Dopamine promotes cellular iron accumulation and oxidative stress responses in macrophages.

Iron is essential for many biological functions including neurotransmitter synthesis, where the metal is a co-factor of tyrosine hydroxylase, which converts tyrosine to dopamine and further to norepinephrine. As the shared chemical structure, called catechol, may potentially bind iron we questioned whether tyrosine derived hormones would impact on cellular iron homeostasis in macrophages, which are central for the maintenance of body iron homeostasis. Using murine bone marrow-derived macrophages (BMDMs), we investigated the effect of catecholamines and found that only dopamine but neither tyrosine, nor norepinephrine, affected cellular iron homeostasis. Exposure of macrophages to dopamine increased the uptake of non-transferrin bound iron into cells. The expansion of intracellular iron upon dopamine treatment resulted in oxidative stress responses as evidenced by increased expression of nuclear factor erythroid 2-related factor (Nrf2) and hypoxia inducible factor-1α. As a consequence, the transcriptional expression of stress response genes such as heme oxygenase-1 and the iron export protein ferroportin1 were significantly increased. Genetic deletion of Nrf2 abolished these effects of dopamine. Dopamine directly affects cellular iron homeostasis by increasing iron incorporation into macrophages and subsequently promoting intracellular oxidative stress responses. Our observations are of interest for disorders involving dopamine and iron dyshomeostasis such as Parkinson's disease and restless legs syndrome, partly enlightening the underlying pathology or the therapeutic efficacy of dopamine agonists to overcome neuronal iron deficiency.

Influence of collection tubes during quantitative targeted metabolomics studies in human blood samples.

BACKGROUND: Plasma and serum are the most widely used matrices in clinical studies. However, some variability in absolute concentrations of metabolites are likely to be observed in these collection tubes matrices. METHODS: We analyzed 189 metabolites using the same protocol for quantitative targeted metabolomics (LC-MS/MS AbsoluteIDQ p180 Kit Biocrates) in three types of samples, serum, plasma EDTA and citrate, of 80 subjects from the Cooperative Health Research In South Tyrol cohort (40 healthy elderly and 40 healthy young). RESULTS: The concentration levels were higher in serum than citrate and EDTA, in particular for amino acids and biogenic amines. The average Pearson's correlation coefficients were however always higher than 0.7 for these two classes of metabolites. We could also demonstrate that blank EDTA vacutainer tubes contain a significant amount of sarcosine. Finally, we compared the metabolome of young people against elderly subjects and found that the highest number of metabolites significantly changing with age was detected in serum. CONCLUSION: Serum samples provide higher sensitivity for biomarker discovery studies. Due to the presence of spurious amount of sarcosine in vacutainer EDTA tubes, plasma EDTA is not suitable for studies requiring accurate quantification of sarcosine.

Dietary iron loading negatively affects liver mitochondrial function.

Iron is an essential co-factor for several metabolic processes, including mitochondrial respiration, and mitochondria are the major sites of iron-utilization. Cellular iron homeostasis must be tightly regulated, as intracellular iron deficiency can lead to insufficient energy production, whereas iron overload triggers ROS (reactive oxygen species) formation via the Fenton reaction. So far little is known on how iron imbalances affect mitochondrial function in vivo and the impact of the genotype on that, we studied the effects of dietary iron loading on mitochondrial respiratory capacity in liver by comparing two genetically divergent mouse strains, namely C57BL/6N and FVB mice. Both mouse strains differed in their basal iron levels and their metabolic responses to iron loading as determined by expression of iron trafficking proteins (ferritin was increased in livers of animals receiving high iron diet) as well as tissue iron content (2-fold increase, FVB p = 0.0013; C57BL/6N p = 0.0022). Dietary iron exposure caused a significant impairment of mitochondrial oxidative phosphorylation, especially regarding OXPHOS capacity (FVB p = 0.0006; C57BL/6N p = 0.0087) and S-ETS capacity (FVB p = 0.0281; C57BL/6N p = 0.0159). These effects were more pronounced in C57BL/6N than in FVB mice and were paralleled by an iron mediated induction of oxidative stress in mitochondria. The increased susceptibility of C57BL6/N mice to iron loading may be due to reduced expression of anti-oxidant defense mechanisms and altered iron trafficking upon dietary challenge pointing to a role of genetic modifiers for cellular and mitochondrial iron trafficking. Finally, iron-mediated induction of mitochondrial oxidative stress and reduction of oxidative phosphorylation may underlie fatigue in subjects with iron loading diseases.