Simulated microgravity disturbs iron metabolism and distribution in humans: Lessons from dry immersion, an innovative ground-based human model.

The objective of the present study was to determine the effects of dry immersion, an innovative ground-based human model of simulated microgravity and extreme physical inactivity, on iron homeostasis and distribution. Twenty young healthy men were recruited and submitted to 5 days of dry immersion (DI). Fasting blood samples and MRI were performed before and after DI exposure to assess iron status, as well as hematological responses. DI increased spleen iron concentrations (SIC), whereas hepatic iron store (HIC) was not affected. Spleen iron sequestration could be due to the concomitant increase in serum hepcidin levels (P < .001). Increased serum unconjugated bilirubin, as well as the rise of serum myoglobin levels support that DI may promote hemolysis and myolysis. These phenomena could contribute to the concomitant increase of serum iron and transferrin saturation levels (P < .001). As HIC remained unchanged, increased serum hepcidin levels could be due both to higher transferrin saturation level, and to low-grade pro-inflammatory as suggested by the significant rise of serum ferritin and haptoglobin levels after DI (P = .003 and P = .003, respectively). These observations highlight the need for better assessment of iron metabolism in bedridden patients, and an optimization of the diet currently proposed to astronauts.

Intermittent reloading does not prevent reduction in iron availability and hepcidin upregulation caused by hindlimb unloading.

NEW FINDINGS: What is the central question of this study? Could skeletal muscle be involved in microgravity-induced iron misdistribution by modulating expression of hepcidin, the master regulator of iron metabolism? What is the main finding and its importance? We demonstrate, in rats, that hepcidin upregulation is not a transient adaptation associated with early exposure to microgravity and that intermittent reloading does not limit microgravity-induced iron misdistribution despite having a beneficial effect on soleus muscle wasting. ABSTRACT: In humans, exposure to microgravity during spaceflight causes muscle atrophy, changes in iron storage and a reduction in iron availability. We previously observed that during 7 days of simulated microgravity in rats, hepcidin plays a key role in iron misdistribution, and we suggested that a crosstalk between skeletal muscle and liver could regulate hepcidin synthesis in this context. In the present study in rats, we investigated the medium-term effects of simulated microgravity on iron metabolism. We also tested whether intermittent reloading (IR) to target skeletal muscle atrophy limits iron misdistribution efficiently. For this purpose, Wistar rats underwent 14 days of hindlimb unloading (HU) combined or not combined with daily IR. At the end of this period, the serum iron concentration and transferrin saturation were significantly reduced, whereas hepatic hepcidin mRNA was upregulated. However, the main signalling pathways involved in hepcidin synthesis in the liver (BMP-small mothers against decapentaplegic (SMAD), interleukin-6-STAT3 and ERK1/2) were unaffected. Unlike what was observed after 7 days of HU, the iron concentration in the spleen, liver and skeletal muscle was comparable between control animals and those that underwent HU or HU plus IR for 14 days. Despite its beneficial effect on soleus muscle atrophy and slow-to-fast myosin heavy chain distribution, IR did not significantly prevent a reduction in iron availability and hepcidin upregulation. Altogether, these results highlight that iron availability is durably reduced during longer exposure to simulated microgravity and that the related hepcidin upregulation is not a transient adaptation to these conditions. The results also suggest that skeletal muscle does not necessarily play a key role in the iron misdistribution that occurs during simulated microgravity.

Spleen iron, molybdenum, and manganese concentrations are coregulated in hepcidin-deficient and secondary iron overload models in mice.

Iron excess increases the hepatic expression of hepcidin, the systemic iron metabolism regulator that favors iron sequestration in the spleen. Genetic iron overload related to hepcidin insufficiency decreases the spleen iron concentration and increases hepatic iron concentration, whereas during secondary iron overload, the hepcidin expression increases together with spleen iron concentration in addition to hepatic iron concentrations increase. Links between iron metabolism and other metals being suggested, our aim was to investigate, during iron overload, the relationships between the hepatic hepcidin expression level and the hepatic and splenic concentrations of iron, manganese, copper, zinc, and molybdenum, determined using inductively coupled plasma mass spectrometry. Hepcidin-deficient mice, secondary iron overload mice models, and their respective controls were studied. Spleen molybdenum and manganese concentrations paralleled the modulation of both spleen iron concentrations, increasing in secondary iron overload and decreasing in hepcidin deficiency related iron overload, as well as hepatic hepcidin mRNA expression. Our data suggest that iron, manganese, and molybdenum metabolisms could share mechanisms controlling their distribution that are associated to hepcidin modulation. In diseases with abnormal hepcidin levels, including chronic inflammation, special attention should be paid to those metals that can participate with the phenotype.-Cavey, T., Latour, C., Island, M.-L., Leroyer, P., Guggenbuhl, P., Coppin, H., Roth, M.-P., Bendavid, C., Brissot, P., Ropert, M., Loréal, O. Spleen iron, molybdenum, and manganese concentrations are coregulated in hepcidin-deficient and secondary iron overload models in mice.

Ceruloplasmin deficiency does not induce macrophagic iron overload: lessons from a new rat model of hereditary aceruloplasminemia.

Hereditary aceruloplasminemia (HA), related to mutations in the ceruloplasmin (Cp) gene, leads to iron accumulation. Ceruloplasmin ferroxidase activity being considered essential for macrophage iron release, macrophage iron overload is expected, but it is not found in hepatic and splenic macrophages in humans. Our objective was to get a better understanding of the mechanisms leading to iron excess in HA. A clustered regularly interspaced short palindromic repeats (CRISPR)/ CRISPR associated protein 9 (Cas9) knockout of the Cp gene was performed on Sprague-Dawley rats. We evaluated the iron status in plasma, the expression of iron metabolism genes, and the status of other metals whose interactions with iron are increasingly recognized. In Cp-/- rats, plasma ceruloplasmin and ferroxidase activity were absent, together with decreased iron concentration and transferrin saturation. Similarly as in humans, the hepatocytes were iron overloaded conversely to hepatic and splenic macrophages. Despite a relative hepcidin deficiency in Cp-/- rats and the loss of ferroxidase activity, potentially expected to limit the interaction of iron with transferrin, no increase of plasma non-transferrin-bound iron level was found. Copper was decreased in the spleen, whereas manganese was increased in the plasma. These data suggest that the reported role of ceruloplasmin cannot fully explain the iron hepatosplenic phenotype in HA, encouraging the search for additional mechanisms.-Kenawi, M., Rouger, E., Island, M.-L., Leroyer, P., Robin, F., Remy, S., Tesson, L., Anegon, I., Nay, K., Derbré, F., Brissot, P., Ropert, M., Cavey, T., Loréal, O. Ceruloplasmin deficiency does not induce macrophagic iron overload: lessons from a new rat model of hereditary aceruloplasminemia.

Testosterone perturbs systemic iron balance through activation of epidermal growth factor receptor signaling in the liver and repression of hepcidin.

UNLABELLED: Gender-related disparities in the regulation of iron metabolism may contribute to the differences exhibited by men and women in the progression of chronic liver diseases associated with reduced hepcidin expression, e.g., chronic hepatitis C, alcoholic liver disease, or hereditary hemochromatosis. However, their mechanisms remain poorly understood. In this study we took advantage of the major differences in hepcidin expression and tissue iron loading observed between Bmp6-deficient male and female mice to investigate the mechanisms underlying this sexual dimorphism. We found that testosterone robustly represses hepcidin transcription by enhancing Egfr signaling in the liver and that selective epidermal growth factor receptor (Egfr) inhibition by gefitinib (Iressa) in males markedly increases hepcidin expression. In males, where the suppressive effects of testosterone and Bmp6-deficiency on hepcidin expression are combined, hepcidin is more strongly repressed than in females and iron accumulates massively not only in the liver but also in the pancreas, heart, and kidneys. CONCLUSION: Testosterone-induced repression of hepcidin expression becomes functionally important during homeostatic stress from disorders that result in iron loading and/or reduced capacity for hepcidin synthesis. These findings suggest that novel therapeutic strategies targeting the testosterone/EGF/EGFR axis may be useful for inducing hepcidin expression in patients with iron overload and/or chronic liver diseases.

Mouse genetic background impacts both on iron and non-iron metals parameters and on their relationships.

Iron is reported to interact with other metals. In addition, it has been shown that genetic background may impact iron metabolism. Our objective was to characterize, in mice of three genetic backgrounds, the links between iron and several non-iron metals. Thirty normal mice (C57BL/6, Balb/c and DBA/2; n = 10 for each group), fed with the same diet, were studied. Quantification of iron, zinc, cobalt, copper, manganese, magnesium and rubidium was performed by ICP/MS in plasma, erythrocytes, liver and spleen. Transferrin saturation was determined. Hepatic hepcidin1 mRNA level was evaluated by quantitative RT-PCR. As previously reported, iron parameters were modulated by genetic background with significantly higher values for plasma iron parameters and liver iron concentration in DBA/2 and Balb/c strains. Hepatic hepcidin1 mRNA level was lower in DBA/2 mice. No iron parameter was correlated with hepcidin1 mRNA levels. Principal component analysis of the data obtained for non-iron metals indicated that metals parameters stratified the mice according to their genetic background. Plasma and tissue metals parameters that are dependent or independent of genetic background were identified. Moreover, relationships were found between plasma and tissue content of iron and some other metals parameters. Our data: (i) confirms the impact of the genetic background on iron parameters, (ii) shows that genetic background may also play a role in the metabolism of non-iron metals, (iii) identifies links between iron and other metals parameters which may have implications in the understanding and, potentially, the modulation of iron metabolism.

Ferroportin diseases: functional studies, a link between genetic and clinical phenotype.

Ferroportin (FPN) mediates iron export from cells and this function is modulated by serum hepcidin. Mutations in the FPN gene (SLC40A1) lead to autosomal dominant iron overload diseases related either to loss or to gain of function, and usually characterized by normal or low transferrin saturation versus elevated transferrin saturation, respectively. However, for the same mutation, the phenotypic expression may vary from one patient to another. Using in vitro overexpression of wild-type or mutant FPN proteins, we characterized the functional impact of five recently identified FPN gene mutations regarding FPN localization, cell iron status, and hepcidin sensitivity. Our aim was to integrate functional results and biological findings in probands and relatives. We show that while the p.Arg371Gln (R371Q) mutation had no impact on studied parameters, the p.Trp158Leu (W158L), p.Arg88Gly (R88G), and p.Asn185Asp (N185D) mutations caused an iron export defect and were classified as loss-of-function mutations. The p.Gly204Ser (G204S) mutation induced a gain of FPN function. Functional studies are useful to determine whether or not a FPN gene mutation found in an iron overloaded patient is deleterious and to characterize its biological impact, especially when family studies are not fully informative and/or additional confounding factors may affect bio-clinical expression.

GATA-4 transcription factor regulates hepatic hepcidin expression.

Hepcidin, a hormone mainly synthesized by hepatocytes and secreted in plasma, controls iron bioavailability. Thus, by inducing the internalization of the iron exporter ferroportin, it regulates iron release from macrophages, enterocytes and hepatocytes towards plasma. Abnormal levels of hepcidin expression alter plasma iron parameters and lead to iron metabolism disorders. Understanding the mechanisms controlling hepcidin (HAMP encodes hepcidin) gene expression is therefore an important goal. We identified a potential GATA-binding site within the human hepcidin promoter. Indeed, in hepatic HepG2 cells, luciferase experiments demonstrated that mutation of this GATA-binding site impaired the hepcidin promoter transcriptional activity in basal conditions. Gel-retardation experiments showed that GATA-4 could bind to this site. Co-transfection of a GATA-4 expression vector with a hepcidin promoter reporter construct enhanced hepcidin promoter transcriptional activity. Furthermore, modulation of GATA4 mRNA expression using specific siRNAs (small interfering RNAs) down-regulated endogenous hepcidin gene expression. Finally, we found that mutation of the GATA-binding site impaired the interleukin-6 induction of hepcidin gene expression, but did not prevent the bone morphogenetic protein-6 response. In conclusion, the findings of the present study (i) indicate that GATA-4 may participate in the control of hepcidin expression, and (ii) suggest that alteration of its expression could contribute to the development of iron-related disorders.

A novel N491S mutation in the human SLC11A2 gene impairs protein trafficking and in association with the G212V mutation leads to microcytic anemia and liver iron overload.

BACKGROUND: DMT1 is a transmembrane iron transporter involved in iron duodenal absorption and cellular iron uptake. Mutations in the human SLC11A2 gene coding DMT1 lead to microcytic anemia and hepatic iron overload, with unexpectedly low levels of plasma ferritin in the presence of iron stores. DESIGN AND METHODS: We report a patient with a similar phenotype due to two mutations in the SLC11A2 gene, the known p.Gly212Val (G212V) mutation and a novel one, p.Asn491Ser (N491S). To assess the expression of DMT1 in human liver, we studied the expression of the four DMT1 mRNA isoforms by real-time quantitative PCR in control human liver samples. We also studied the effect of G212V and N491S DMT1 mutations on RNA splicing in blood leukocytes and cellular trafficking of dsRed2-tagged-DMT1 protein in the human hepatic cell line HuH7. RESULTS: Our results showed that i) only the isoforms 1B-IRE and 1B-nonIRE were significantly expressed in human liver; ii) the G212V mutation did not seem to affect mRNA splicing and the N491S mutation induced a splicing alteration leading to a truncated protein, which seemed quantitatively of low relevance; and iii) the N491S mutation, in contrast to the G212V mutation, led to abnormal protein trafficking. CONCLUSIONS: Our data confirm the major role of DMT1 in the maintenance of iron homeostasis in humans and demonstrate that the N491S mutation, through its deleterious effect on protein trafficking, contributes together with the G212V mutation to the development of anemia and hepatic iron overload.

Metallomic profile in non-cirrhotic hepatocellular carcinoma supports a phenomenon of metal metabolism adaptation in tumor cells.

We have previously described a form of hepatocellular carcinoma (HCC) in non-cirrhotic liver (HCC-NC) developed by Peruvian patients. We analyzed the metallomic profile in hepatic tissues from two independent cohorts exhibiting HCC-NC. Clinical, histopathological data, and HCC and non-tumoral liver (NTL) samples of 38 Peruvian and 38 French HCC-NC patients, were studied. Twelve metals were quantified using ICP/MS: Mn, Fe, Cu, Co, Zn, As, Se, Rb, Mo, Cd, Pb, and Sn. Associations between metals and survival were assessed. Our data showed significant differences between cohorts. Mean ages were 40.6 ± 20, 67.5 ± 9 years old for Peruvians and French, respectively. Fifty percent of the Peruvian patients were positive for the HBsAg, versus 3% in French patients. Mn, Cu, Zn, As, Se, Rb, Mo, Cd, Sn metal concentrations were higher in NTL of Peruvians. Importantly, metal concentrations were lower in HCC areas compared to NTL tissues in both cohorts, except for Cu for which mean concentration was higher in HCC (p < 0.05). Se concentration in HCC was associated with extended survival only in Peruvians. Our data, obtained in Peruvian and French HCC-NC cohorts, highlights similarity in the metallomic profile of HCC compared to NTL during the hepatic tumorigenesis in these specific groups of patients.

A new mutation in the hepcidin promoter impairs its BMP response and contributes to a severe phenotype in HFE related hemochromatosis.

Low levels of hepcidin are responsible for the development of iron overload in p.Cys282Tyr HFE related hemochromatosis. Every genetic factor lowering the hepcidin gene expression could contribute to a more severe phenotype in HFE hemochromatosis. Based on this hypothesis, we identified a heterozygous nc.-153 C>T mutation in the hepcidin gene promoter sequence in a patient homozygous for the p.Cys282Tyr HFE mutation who presented massive iron overload, resisting to well conducted iron depletive treatment. Our results demonstrate that the nc.-153 C>T mutation, located within a BMP-RE (Bone Morphogenetic Protein-Responsive Element): i) decreases the transcriptional activity of the hepcidin promoter, ii) alters its IL-6 (Interleukin-6) total responsiveness, and iii) prevents the binding of the SMAD protein complex (1/5/8 and 4) to the BPM-RE. In conclusion, our results suggest that a mutation in the BMP-RE of hepcidin promoter may impact on human iron metabolism.

The POU transcription factor Oct-1 represses virus-induced interferon A gene expression.

Alpha interferon (IFN-alpha) and IFN-beta are able to interfere with viral infection. They exert a vast array of biologic functions, including growth arrest, cell differentiation, and immune system regulation. This regulation extends from innate immunity to cellular and humoral adaptive immune responses. A strict control of expression is needed to prevent detrimental effects of unregulated IFN. Multiple IFN-A subtypes are coordinately induced in human and mouse cells infected by virus and exhibit differences in expression of their individual mRNAs. We demonstrated that the weakly expressed IFN-A11 gene is negatively regulated after viral infection, due to a distal negative regulatory element, binding homeoprotein pituitary homeobox 1 (Pitx1). Here we show that the POU protein Oct-1 binds in vitro and in vivo to the IFN-A11 promoter and represses IFN-A expression upon interferon regulatory factor overexpression. Furthermore, we show that Oct-1-deficient MEFs exhibit increased in vivo IFN-A gene expression and increased antiviral activity. Finally, the IFN-A expression pattern is modified in Oct-1-deficient MEFs. The broad representation of effective and potent octamer-like sequences within IFN-A promoters suggests an important role for Oct-1 in IFN-A regulation.

Repression by homeoprotein pitx1 of virus-induced interferon a promoters is mediated by physical interaction and trans repression of IRF3 and IRF7.

Interferon A (IFN-A) genes are differentially expressed after virus induction. The differential expression of individual IFN-A genes is modulated by the specific transcription activators IFN regulatory factor 3 (IRF3) and IRF-7 and the homeoprotein transcription repressor Pitx1. We now show that repression by Pitx1 does not appear to be due to the recruitment of histone deacetylases. On the other hand, Pitx1 inhibits the IRF3 and IRF7 transcriptional activity of the IFN-A11 and IFN-A5 promoters and interacts physically with IRF3 and IRF7. Pitx1 trans-repression activity maps to specific C-terminal domains, and the Pitx1 homeodomain is involved in physical interaction with IRF3 or IRF7. IRF3 is able to bind to the antisilencer region of the IFN-A4 promoter, which overrides the repressive activity of Pitx1. These results indicate that interaction between the Pitx1 homeodomain and IRF3 or IRF7 and the ability of the Pitx1 C-terminal repressor domains to block IFN-A11 and IFN-A5 but not IFN-A4 promoter activities may contribute to our understanding of the complex differential transcriptional activation, repression, and antirepression of the IFN-A genes.

Regulation of virus-induced interferon-A genes.

Different members of the interferon regulatory factor (IRF) family are early activated by viral infection of eukaryotic cells. The IRFs participate in the virus-induced transcriptional regulation of different genes, including the multigenic interferon-A (IFN-A) family, members of which are involved in the establishment of an antiviral state, cell growth inhibition or apoptosis. This study presents the recent progress in the field of virus-induced transactivation and repression of IFN-A gene promoters. Data presented on the modular organization of IFN-A gene promoters and their transactivation dependent on IRF-3 and IRF-7 provide a new insight on the cooperativity mechanisms among the different IRF family members. Data on the transcriptional repression of virus-induced interferon-A promoters by the homeodomain protein Pitx1 contribute to our understanding of the complex differential transcriptional activation, repression and antirepression of the IFN-A genes.

Positive and negative control of virus-induced interferon-A gene expression.

Transcriptional regulation is a consequence of the combination of both activation and repression for establishing specific patterns of eukaryotic gene expression. The regulation of the expression of type I interferon (IFN-A and -B) multigene family is controlled primarily at the transcriptional level and has been widely studied as a model to understand the mechanisms of stable repression, transient expression and postinduction repression of genes. The positive and negative regulatory elements required for this on/off switch have been defined within a complex 5' upstream region of their transcription start site. The differential expression pattern of IFN-A genes is thought to involve both substitutions in the virus responsive element (VRE-A) and presence or absence of the distal negative regulatory element (DNRE) which is delimited upstream of the VRE-A. The interferon regulatory factors (IRF)-3 and -7 binding to the VRE-A and interacting as homodimers or heterodimers participate in the virus-induced transcriptional activation of IFN-A family. This data and the presence of homeodomain protein pituitary homeobox 1 (Pitx1) binding to the distal DNRE, negatively regulating the IRF-3 and IRF-7 activities and interacting physically with IRF-3 and IRF-7 contribute to our understanding of the complex differential transcriptional activation and repression of the IFN-A genes.

Molecular diagnosis of genetic iron-overload disorders.

Genetic iron overload has long been confined to the picture of classical hemochromatosis related to the HFE C282Y mutation (type 1 hemochromatosis). C282Y homozygosity affects approximately three people out of 1000 of the Caucasian population, representing one of the most frequent genetic predispositions. It has, however, rapidly become clear that the HFE C282Y mutation is not the sole culprit in genetic iron overload. Several novel mutations in HFE and other genes have been discovered and related to various entities, which are now known as types 2, 3 and 4 hemochromatosis. These diseases are far less frequent than the classical type 1 hemochromatosis but, by contrast, are not limited to the Caucasian population. Molecular diagnosis obviously plays a key role in the diagnostic strategy. In the future, it will undoubtedly enable not only identification of new diagnostic markers, but also provide potential molecular targets for pathophysiologically based innovative therapeutic approaches.

Natural and synthetic STAT3 inhibitors reduce hepcidin expression in differentiated mouse hepatocytes expressing the active phosphorylated STAT3 form.

During the inflammatory process, hepcidin overexpression favours the development of anaemia of chronic diseases which represents the second most common form of anaemia worldwide. The identification of therapeutic agents decreasing hepcidin expression is therefore an important goal. The aim of this study was to target the STAT3 signalling involved in the development of increased hepcidin expression related to chronic inflammation. In a co-culture model associating mouse hepatocytes and rat liver epithelial cells, the mRNA levels of hepcidin1, albumin, aldolase B, Cyp3a4, Stat3, Smad4 and iron regulatory genes were measured by real-time PCR. STAT3 and phosphorylated SMAD1/5/8 proteins were analysed by Western blot. At variance of hepatocyte pure culture, co-culture provided high levels of hepcidin1 mRNA, reaching 400% of the freshly isolated hepatocyte values after 6 days of culture. Hepcidin expression was associated with the maintenance of hepatocyte phenotype, STAT3 phosphorylation and functional BMP/SMAD pathway. Stat3 siRNAs inhibited the hepcidin1 mRNA expression. STAT3 inhibitors, including curcumin, AG490 and a peptide (PpYLKTK), reduced hepcidin1 mRNA expression even when cells were additionally exposed to IL-6. Hepcidin1 mRNA was expressed at high levels by hepatocytes in the co-culture model, and STAT3 pathway activation was controlled through STAT3 inhibitors. Such inhibitors could be useful to prevent anaemia related to hepcidin overexpression during chronic inflammation.

Purification of neuronal inclusions of patients with Huntington's disease reveals a broad range of N-terminal fragments of expanded huntingtin and insoluble polymers.

Huntington's disease resulting from huntingtin containing an expanded polyglutamine is associated with aggregates largely confined to neuronal inclusions, and with neuronal death. Inclusions are thought to originate from discrete N-terminal fragments of expanded huntingtin produced by specific endopeptidases. We have now purified the neuronal inclusions of Huntington's disease brain. When incubated in concentrated formic acid, purified inclusions release a polymer, an oligomer and a broad range of N-terminal fragments of expanded huntingtin. The fragments and the polymeric forms are linked to each other by non-covalent bonds as they are both released by formic acid, whereas the polymeric forms themselves are presumably stabilized by covalent bonds, as they are resistant to formic acid. We also demonstrate the presence in affected areas of the brain but not in unaffected areas of a broad range of soluble N-terminal fragments of expanded huntingtin not yet associated with the inclusions and which are likely to be the precursors of the inclusions. Fragmentation of expanded huntingtin in Huntington's disease must result from the operation of multiple proteolytic activities with little specificity and not from that of a specific endopeptidase; subsequent aggregation of the fragments by covalent and non-covalent bonds leads to the formation of the inclusions.