Siderophore-mediated iron trafficking in humans is regulated by iron

Maternal ketone supplementation throughout gestation improves neonatal cardiac dysfunction caused by perinatal iron deficiency

Iron deficiency (ID) is common during gestation and in early infancy and has been shown to adversely affect cardiac development and function, which could lead to lasting cardiovascular consequences. Ketone supplementation has been shown to confer cardioprotective effects in numerous disease models. Here, we tested the hypothesis that maternal ketone supplementation during gestation would mitigate cardiac dysfunction in ID neonates. Female Sprague-Dawley rats were fed an iron-restricted or iron-replete diet before and throughout pregnancy. Throughout gestation, iron-restricted dams were given either a daily subcutaneous injection of ketone solution (containing β-hydroxybutyrate [βOHB]) or saline (vehicle). Neonatal offspring cardiac function was assessed by echocardiography at postnatal days (PD)3 and 13. Hearts and livers were collected post-mortem for assessments of mitochondrial function and gene expression profiles of markers oxidative stress and inflammation. Maternal iron restriction caused neonatal anemia and asymmetric growth restriction at all time points assessed, and maternal βOHB treatment had no effect on these outcomes. Echocardiography revealed reduced ejection fraction despite enlarged hearts (relative to body weight) in ID offspring, resulting in impaired oxygen delivery, which was attenuated by maternal βOHB supplementation. Further, maternal ketone supplementation affected biochemical markers of mitochondrial function, oxidative stress and inflammation in hearts of neonates, implicating these pathways in the protective effects conferred by βOHB. In summary, βOHB supplementation confers protection against cardiac dysfunction in ID neonates and could have implications for the treatment of anemic babies.

Computational Modeling of Macrophage Iron Sequestration during Host Defense against Aspergillus

Iron is essential to the virulence of Aspergillus species, and restricting iron availability is a critical mechanism of antimicrobial host defense. Macrophages recruited to the site of infection are at the crux of this process, employing multiple intersecting mechanisms to orchestrate iron sequestration from pathogens. To gain an integrated understanding of how this is achieved in aspergillosis, we generated a transcriptomic time series of the response of human monocyte-derived macrophages to Aspergillus and used this and the available literature to construct a mechanistic computational model of iron handling of macrophages during this infection. We found an overwhelming macrophage response beginning 2 to 4 h after exposure to the fungus, which included upregulated transcription of iron import proteins transferrin receptor-1, divalent metal transporter-1, and ZIP family transporters, and downregulated transcription of the iron exporter ferroportin. The computational model, based on a discrete dynamical systems framework, consisted of 21 3-state nodes, and was validated with additional experimental data that were not used in model generation. The model accurately captures the steady state and the trajectories of most of the quantitatively measured nodes. In the experimental data, we surprisingly found that transferrin receptor-1 upregulation preceded the induction of inflammatory cytokines, a feature that deviated from model predictions. Model simulations suggested that direct induction of transferrin receptor-1 (TfR1) after fungal recognition, independent of the iron regulatory protein-labile iron pool (IRP-LIP) system, explains this finding. We anticipate that this model will contribute to a quantitative understanding of iron regulation as a fundamental host defense mechanism during aspergillosis.

IMPORTANCE Invasive pulmonary aspergillosis is a major cause of death among immunosuppressed individuals despite the best available therapy. Depriving the pathogen of iron is an essential component of host defense in this infection, but the mechanisms by which the host achieves this are complex. To understand how recruited macrophages mediate iron deprivation during the infection, we developed and validated a mechanistic computational model that integrates the available information in the field. The insights provided by this approach can help in designing iron modulation therapies as anti-fungal treatments.

Understanding the Potential and Risk of Bacterial Siderophores in Cancer

Siderophores are iron chelating molecules produced by nearly all organisms, most notably by bacteria, to efficiently sequester the limited iron that is available in the environment. Siderophores are an essential component of mammalian iron homeostasis and the ongoing interspecies competition for iron. Bacteria produce a broad repertoire of siderophores with a canonical role in iron chelation and the capacity to perform versatile functions such as interacting with other microbes and the host immune system. Siderophores are a vast area of untapped potential in the field of cancer research because cancer cells demand increased iron concentrations to sustain rapid proliferation. Studies investigating siderophores as therapeutics in cancer generally focused on the role of a few siderophores as iron chelators; however, these studies are limited and some show conflicting results. Moreover, siderophores are biologically conserved, structurally diverse molecules that perform additional functions related to iron chelation. Siderophores also have a role in inflammation due to their iron acquisition and chelation properties. These diverse functions may contribute to both risks and benefits as therapeutic agents in cancer. The potential of siderophore-mediated iron and bacterial modulation to be used in the treatment of cancer warrants further investigation. This review discusses the wide range of bacterial siderophore functions and their utilization in cancer treatment to further expand their functional relevance in cancer detection and treatment.

Rethinking IRPs/IRE system in neurodegenerative disorders: Looking beyond iron metabolism

Iron regulatory proteins (IRPs) and iron regulatory element (IRE) systems are well known in the progression of neurodegenerative disorders by regulating iron related proteins. IRPs are also regulated by iron homeostasis. However, an increasing number of studies have suggested a close relationship between the IRPs/IRE system and non-iron-related neurodegenerative disorders. In this paper, we reviewed that the IRPs/IRE system is not only controlled by iron ions, but also regulated by such factors as post-translational modification, oxygen, nitric oxide (NO), heme, interleukin-1 (IL-1), and metal ions. In addition, by regulating the transcription of non-iron related proteins, the IRPs/IRE system functioned in oxidative metabolism, cell cycle regulation, abnormal proteins aggregation, and neuroinflammation. Finally, by emphasizing the multiple regulations of IRPs/IRE system and its potential relationship with non-iron metabolic neurodegenerative disorders, we provided new strategies for disease treatment targeting IRPs/IRE system.

Systematic Surveys of Iron Homeostasis Mechanisms Reveal Ferritin Superfamily and Nucleotide Surveillance Regulation to be Modified by PINK1 Absence

Iron deprivation activates mitophagy and extends lifespan in nematodes. In patients suffering from Parkinson’s disease (PD), PINK1-PRKN mutations via deficient mitophagy trigger iron accumulation and reduce lifespan. To evaluate molecular effects of iron chelator drugs as a potential PD therapy, we assessed fibroblasts by global proteome profiles and targeted transcript analyses. In mouse cells, iron shortage decreased protein abundance for iron-binding nucleotide metabolism enzymes (prominently XDH and ferritin homolog RRM2). It also decreased the expression of factors with a role for nucleotide surveillance, which associate with iron-sulfur-clusters (ISC), and are important for growth and survival. This widespread effect included prominently Nthl1-Ppat-Bdh2, but also mitochondrial Glrx5-Nfu1-Bola1, cytosolic Aco1-Abce1-Tyw5, and nuclear Dna2-Elp3-Pold1-Prim2. Incidentally, upregulated Pink1-Prkn levels explained mitophagy induction, the downregulated expression of Slc25a28 suggested it to function in iron export. The impact of PINK1 mutations in mouse and patient cells was pronounced only after iron overload, causing hyperreactive expression of ribosomal surveillance factor Abce1 and of ferritin, despite ferritin translation being repressed by IRP1. This misregulation might be explained by the deficiency of the ISC-biogenesis factor GLRX5. Our systematic survey suggests mitochondrial ISC-biogenesis and post-transcriptional iron regulation to be important in the decision, whether organisms undergo PD pathogenesis or healthy aging.

The Effects of Human BDH2 on the Cell Cycle, Differentiation, and Apoptosis and Associations with Leukemia Transformation in Myelodysplastic Syndrome

Iron overload is related to leukemia transformation in myelodysplastic syndrome (MDS) patients. Siderophores help to transport iron. Type 2-hydroxybutyrate dehydrogenase (BDH2) is a rate-limiting factor in the biogenesis of siderophores. Using qRT-PCR, we analyze BDH2mRNA expression in the bone marrow (BM) of 187 MDS patients, 119 de novo acute myeloid leukemia (AML) patients, and 43 lymphoma patients with normal BM. Elevated BDH2mRNA expression in BM is observed in MDS patients (n = 187 vs. 43, normal BM; P = 0.009), and this is related to ferritin levels. Patients with higher BDH2 expression show a greater risk of leukemia progression (15.25% vs. 3.77%, lower expression; P = 0.017) and shorter leukemia-free-survival (medium LFS, 9 years vs. 7 years; P = 0.024), as do patients with a ferritin level ≥350 ng/mL. Additionally, we investigate the mechanisms related to the prognostic ability of BDH2 by using BDH2-KD THP1. The cell cycle analysis, surface markers, and special stain studies indicate that BDH2-KD induces differentiation and decreases the growth rate of THP1 cells, which is associated with the retardation of the cell cycle. Moreover, many genes, including genes related to mitochondrial catabolism, oncogenes, tumor suppressor genes, and genes related to cell differentiation and proliferation influence BDH2-KD THP1 cells. Herein, we demonstrate that BDH2 is involved in cell cycle arrest and the inhibition of differentiation in malignant cells. Furthermore, the high BDH2 expression in MDS patients could be suggestive of a poor prognostic factor. This study provides a foundation for further research on the roles of BDH2 and iron metabolism in the pathogenesis of MDS.

Iron Overload Damages the Endothelial Mitochondria via the ROS/ADMA/DDAHII/eNOS/NO Pathway

It has been recognized that iron overload may harm the body's health. Vascular endothelial cells (VECs) are one of the main targets of iron overload injury, and the mechanism involved was thought to be related to the excessive generation of reactive oxygen species (ROS). However, the subcellular and temporal characteristics of ROS generation, potential downstream mechanisms, and target organelles in VECs injured by iron overload have not been expounded yet. In this study, we elucidated the abovementioned issues through both in vivo and in vitro experiments. Mice were fed pellet diets that were supplemented with iron for 4 consecutive months. Results showed that the thoracic aortic strips' endothelium-dependent dilation was significantly impaired and associated with inflammatory changes, noticeable under brown TUNEL-positive staining in microscopy analysis. In addition, the serum content of asymmetric dimethylarginine (ADMA) increased, whereas nitric oxide (NO) levels decreased. Furthermore, the dimethylarginine dimethylaminohydrolase II (DDAHII) expression and activity, as well as the phosphorylation of endothelial nitric oxide synthase (eNOS) in aortic tissue, were inhibited. Human umbilical vein endothelial cells were treated with 50 μM iron dextran for 48 hours, after which the cell viability, NO content, DDAHII expression and activity, and phosphorylation of eNOS decreased and lactate dehydrogenase and caspase-3 activity, ADMA content, and apoptotic cells significantly increased. After the addition of L-arginine (L-Arg) or pAD/DDAHII, the abovementioned changes were reversed. By dynamically detecting the changes of ROS generation in the cytoplasm and mitochondria and interfering with different aspects of signaling pathways, we have confirmed for the first time that excessive ROS originates from the cytoplasm and activates the ROS-induced ROS release (RIRR) mechanism, leading to mitochondrial dysfunction. Together, our data suggested that excessive free iron ions produced excess ROS in the cytoplasm. Thus, excess ROS create one vicious circle by activating the ADMA/eNOS/DDAHII/NO pathway and another vicious circle by activation of the RIRR mechanism, which, when combined, induce a ROS burst, resulting in mitochondrial dysfunction and damaged VECs.

The significance, trafficking and determination of labile iron in cytosol, mitochondria and lysosomes

The labile iron pool (LIP) is a pool of chelatable and redox-active iron, not only essential for a wide variety of metabolic process, but also as a catalyst in the Fenton reaction, causing the release of hazardous reactive oxygen species (ROS) with potential for inducing oxidative stress and cell damage. The cellular LIP represents the entirety of every heterogenous sub-pool of iron, not only present in the cytosol, but also in mitochondria, lysosomes and the nucleus, which have all been detected and characterized by various fluorescent methods. Accumulated evidence indicates that alterations in the intracellular LIP can substantially contribute to a variety of injurious processes and initiate pathological development. Herein, we present our understanding of the role of the cellular LIP. To fully review the LIP, firstly, the significance of cellular labile iron in different subcellular compartments is presented. And then, the trafficking processes of cellular labile iron between/in cytosol, mitochondria and lysosomes are discussed in detail. Then, the recent progress in uncovering and assessing the cellular LIP by fluorescent methods have been noted. Overall, this summary may help to comprehensively envision the important physiological and pathological roles of the LIP and shed light on profiling the LIP in a real-time and nondestructive manner with fluorescent methods. Undoubtedly, with the advent and development of iron biology, a better understanding of iron, especially the LIP, may also enhance treatments for iron-related diseases.

LncRNA TP73-AS1 sponges miR-141-3p to promote the migration and invasion of pancreatic cancer cells through the up-regulation of BDH2

LncRNA TP73 antisense RNA 1T (TP73-AS1) plays an important role in human malignancies. However, the levels of TP73-AS1 and its functional mechanisms in pancreatic cancer metastasis remain unknown, and the clinical significance of TP73-AS1 in human pancreatic cancer is also unclear. In the present study, the levels of TP73-AS1 and its candidate target miR-141 in pancreatic cancer and adjacent normal tissue were detected using qRT-PCR. The association between TP73-AS1 levels and the clinicopathologic characteristics of pancreatic cancer patients were analyzed. The relationship between TP73-AS1 and miR-141, and miR-141 and its candidate target 3-hydroxybutyrate dehydrogenase type 2 (BDH2) was confirmed using dual-luciferase reporter assays. TP73-AS1 and/or miR-141 were knocked down using siRNA or an inhibitor in pancreatic cancer cells and cell migration and invasion then examined. The results showed that TP73-AS1 was up-regulated in pancreatic cancer tissue and cell lines. High levels of TP73-AS1 were correlated with poor clinicopathological characteristics and shorter overall survival. MiR-141 was a direct target for TP73-AS1, while BDH2 was a direct target for miR-141. The knockdown of TP73-AS1 significantly inhibited the migration and invasion of pancreatic cancer cells, while the miR-141 inhibitor significantly restored the migration and invasion. Therefore, TP73-AS1 positively regulated BDH2 expression by sponging miR-141. These findings suggest that TP73-AS1 serves as an oncogene and promotes the metastasis of pancreatic cancer. Moreover, TP73-AS1 could serve as a predictor and a potential drug biotarget for pancreatic cancer.

Iron transport and its regulation in plants

Iron is an essential element for plants as well as other organisms, functioning in various cellular processes, including respiration, chlorophyll biosynthesis, and photosynthesis. Plants take up iron from soil in which iron solubility is extremely low especially under aerobic conditions at high-pH range. Therefore, plants have evolved efficient iron-uptake mechanisms. Because iron is prone to being precipitated and excess ionic iron is cytotoxic, plants also have sophisticated internal iron-transport mechanisms. These transport mechanisms comprise iron chelators including nicotianamine, mugineic acid family phytosiderophores and citrate, and various types of transporters of these chelators, iron-chelate complexes, or free iron ions. To maintain iron homeostasis, plants have developed mechanisms for regulating gene expression in response to iron availability. Expression of various genes involved in iron uptake and translocation is induced under iron deficiency by transcription factor networks and is negatively regulated by the ubiquitin ligase HRZ/BTS. This response is deduced to be mediated by cellular iron sensing as well as long-distance iron signaling. The ubiquitin ligase HRZ/BTS is a candidate intracellular iron sensor because it binds to iron and zinc, and its activity is affected by iron availability. The iron-excess response of plants is thought to be partially independent of the iron-deficiency response. In this review, we summarize and discuss extant knowledge of plant iron transport and its regulation.

A comparison between whole transcript and 3' RNA sequencing methods using Kapa and Lexogen library preparation methods

BACKGROUND

3' RNA sequencing provides an alternative to whole transcript analysis. However, we do not know a priori the relative advantage of each method. Thus, a comprehensive comparison between the whole transcript and the 3' method is needed to determine their relative merits. To this end, we used two commercially available library preparation kits, the KAPA Stranded mRNA-Seq kit (traditional method) and the Lexogen QuantSeq 3' mRNA-Seq kit (3' method), to prepare libraries from mouse liver RNA. We then sequenced and analyzed the libraries to determine the advantages and disadvantages of these two approaches.

RESULTS

We found that the traditional whole transcript method and the 3' RNA-Seq method had similar levels of reproducibility. As expected, the whole transcript method assigned more reads to longer transcripts, while the 3' method assigned roughly equal numbers of reads to transcripts regardless of their lengths. We found that the 3' RNA-Seq method detected more short transcripts than the whole transcript method. With regard to differential expression analysis, we found that the whole transcript method detected more differentially expressed genes, regardless of the level of sequencing depth.

CONCLUSIONS

The 3' RNA-Seq method was better able to detect short transcripts, while the whole transcript RNA-Seq was able to detect more differentially expressed genes. Thus, both approaches have relative advantages and should be selected based on the goals of the experiment.

Evaluation of the iron regulatory protein-1 interactome

The interactions of iron regulatory proteins (IRPs) with mRNAs containing an iron-responsive element (IRE) is a major means through which intracellular iron homeostasis is maintained and integrated with cellular function. Although IRE-IRP interactions have been proposed to modulate the expression of a diverse number of mRNAs, a transcriptome analysis of the interactions that form within the native mRNA structure and cellular environment has not previously been described. An RNA-CLIP study is described here that identified IRP-1 interactions occurring within a primary cell line expressing physiologically relevant amounts of mRNA and protein. The study suggests that only a small subset of the previously proposed IREs interact with IRP-1 in situ. Identifying authentic IRP interactions is not only important to a greater understanding of iron homeostasis and its integration with cell biology but also to the development of novel therapeutics that can compensate for iron imbalances.

The actin-binding protein profilin 2 is a novel regulator of iron homeostasis

Pfn2 mRNA has a functional and conserved IRE in the 3′ untranslated region. Pfn2 knockout mice display an iron phenotype with iron accumulation in specific areas of the brain and depletion of liver iron stores.

[Roles of bacterial and mammalian siderophores in host-pathogen interactions]

Iron is an essential nutriment for almost all forms of life, from bacteria to humans. Despite its key role in living organisms, iron becomes toxic at high concentrations. In the body, to circumvent this toxicity, almost all the intracellular iron is bound to proteins (especially to ferritin, a protein able to bind up to 4000 atoms of iron) and a small proportion (0.2% to 3%) to low molecular weight ligands (less than 2 kDa) constituting a free iron pool able to ensure the traffic of intracellular iron. A number of small molecules (citrate, phosphate, phospholipid, polypeptide) able to chelate iron, with variable affinities, have been known for a long time. In 2010, two teams have identified new mammal endogen chelators able to bind iron with similar chemical properties as bacterial siderophores. Recently, a few publications emphasized that most of the free iron present in the body cells is indeed linked to these siderophores, which play a key role in infected-host protection mechanisms during bacterial infections, through iron homeostasis and oxidative stress regulation.

Electrophoretic mobility shift assay (EMSA) for the study of RNA-protein interactions: the IRE/IRP example

RNA/protein interactions are critical for post-transcriptional regulatory pathways. Among the best-characterized cytosolic RNA-binding proteins are iron regulatory proteins, IRP1 and IRP2. They bind to iron responsive elements (IREs) within the untranslated regions (UTRs) of several target mRNAs, thereby controlling the mRNAs translation or stability. IRE/IRP interactions have been widely studied by EMSA. Here, we describe the EMSA protocol for analyzing the IRE-binding activity of IRP1 and IRP2, which can be generalized to assess the activity of other RNA-binding proteins as well. A crude protein lysate containing an RNA-binding protein, or a purified preparation of this protein, is incubated with an excess of(32) P-labeled RNA probe, allowing for complex formation. Heparin is added to preclude non-specific protein to probe binding. Subsequently, the mixture is analyzed by non-denaturing electrophoresis on a polyacrylamide gel. The free probe migrates fast, while the RNA/protein complex exhibits retarded mobility; hence, the procedure is also called "gel retardation" or "bandshift" assay. After completion of the electrophoresis, the gel is dried and RNA/protein complexes, as well as free probe, are detected by autoradiography. The overall goal of the protocol is to detect and quantify IRE/IRP and other RNA/protein interactions. Moreover, EMSA can also be used to determine specificity, binding affinity, and stoichiometry of the RNA/protein interaction under investigation.

Inflammation and ER stress downregulate BDH2 expression and dysregulate intracellular iron in macrophages

Macrophages play a very important role in host defense and in iron homeostasis by engulfing senescent red blood cells and recycling iron. Hepcidin is the master iron regulating hormone that limits dietary iron absorption from the gut and limits iron egress from macrophages. Upon infection macrophages retain iron to limit its bioavailability which limits bacterial growth. Recently, a short chain butyrate dehydrogenase type 2 (BDH2) protein was reported to contain an iron responsive element and to mediate cellular iron trafficking by catalyzing the synthesis of the mammalian siderophore that binds labile iron; therefore, BDH2 plays a crucial role in intracellular iron homeostasis. However, BDH2 expression and regulation in macrophages have not yet been described. Here we show that LPS-induced inflammation combined with ER stress led to massive BDH2 downregulation, increased the expression of ER stress markers, upregulated hepcidin expression, downregulated ferroportin expression, caused iron retention in macrophages, and dysregulated cytokine release from macrophages. We also show that ER stress combined with inflammation synergistically upregulated the expression of the iron carrier protein NGAL and the stress-inducible heme degrading enzyme heme oxygenase-1 (HO-1) leading to iron liberation. This is the first report to show that inflammation and ER stress downregulate the expression of BDH2 in human THP-1 macrophages.

Endogenous siderophore 2,5-dihydroxybenzoic acid deficiency promotes anemia and splenic iron overload in mice

Eukaryotes produce a siderophore-like molecule via a remarkably conserved biosynthetic pathway. 3-OH butyrate dehydrogenase (BDH2), a member of the short-chain dehydrogenase (SDR) family of reductases, catalyzes a rate-limiting step in the biogenesis of the mammalian siderophore 2,5-dihydroxybenzoic acid (2,5-DHBA). Depletion of the mammalian siderophore by inhibiting expression of bdh2 results in abnormal accumulation of intracellular iron and mitochondrial iron deficiency in cultured mammalian cells, as well as in yeast cells and zebrafish embryos We disrupted murine bdh2 by homologous recombination to analyze the effect of bdh2 deletion on erythropoiesis and iron metabolism. bdh2 null mice developed microcytic anemia and tissue iron overload, especially in the spleen. Exogenous supplementation with 2,5-DHBA alleviates splenic iron overload in bdh2 null mice. Additionally, bdh2 null mice exhibit reduced serum iron. Although BDH2 has been proposed to oxidize ketone bodies, we found that BDH2 deficiency did not alter ketone body metabolism in vivo. In sum, our findings demonstrate a key role for BDH2 in erythropoiesis.

Neisseria gonorrhoeae modulates iron-limiting innate immune defenses in macrophages

Neisseria gonorrhoeae is a strict human pathogen that causes the sexually transmitted infection termed gonorrhea. The gonococcus can survive extracellularly and intracellularly, but in both environments the bacteria must acquire iron from host proteins for survival. However, upon infection the host uses a defensive response by limiting the bioavailability of iron by a number of mechanisms including the enhanced expression of hepcidin, the master iron-regulating hormone, which reduces iron uptake from the gut and retains iron in macrophages. The host also secretes the antibacterial protein NGAL, which sequesters bacterial siderophores and therefore inhibits bacterial growth. To learn whether intracellular gonococci can subvert this defensive response, we examined expression of host genes that encode proteins involved in modulating levels of intracellular iron. We found that N. gonorrhoeae can survive in association (tightly adherent and intracellular) with monocytes and macrophages and upregulates a panel of its iron-responsive genes in this environment. We also found that gonococcal infection of human monocytes or murine macrophages resulted in the upregulation of hepcidin, NGAL, and NRAMP1 as well as downregulation of the expression of the gene encoding the short chain 3-hydroxybutyrate dehydrogenase (BDH2); BDH2 catalyzes the production of the mammalian siderophore 2,5-DHBA involved in chelating and detoxifying iron. Based on these findings, we propose that N. gonorrhoeae can subvert the iron-limiting innate immune defenses to facilitate iron acquisition and intracellular survival.

Regulation of the cholesterol efflux transporters ABCA1 and ABCG1 in retina in hemochromatosis and by the endogenous siderophore 2,5-dihydroxybenzoic acid

Hypercholesterolemia and polymorphisms in the cholesterol exporter ABCA1 are linked to age-related macular degeneration (AMD). Excessive iron in retina also has a link to AMD pathogenesis. Whether these findings mean a biological/molecular connection between iron and cholesterol is not known. Here we examined the relationship between retinal iron and cholesterol using a mouse model (Hfe(-/-)) of hemochromatosis, a genetic disorder of iron overload. We compared the expression of the cholesterol efflux transporters ABCA1 and ABCG1 and cholesterol content in wild type and Hfe(-/-) mouse retinas. We also investigated the expression of Bdh2, the rate-limiting enzyme in the synthesis of the endogenous siderophore 2,5-dihydroxybenzoic acid (2,5-DHBA) in wild type and Hfe(-/-) mouse retinas, and the influence of this siderophore on ABCA1/ABCG1 expression in retinal pigment epithelium. We found that ABCA1 and ABCG1 were expressed in all retinal cell types, and that their expression was decreased in Hfe(-/-) retina. This was accompanied with an increase in retinal cholesterol content. Bdh2 was also expressed in all retinal cell types, and its expression was decreased in hemochromatosis. In ARPE-19 cells, 2,5-DHBA increased ABCA1/ABCG1 expression and decreased cholesterol content. This was not due to depletion of free iron because 2,5-DHBA (a siderophore) and deferiprone (an iron chelator) had opposite effects on transferrin receptor expression and ferritin levels. We conclude that iron is a regulator of cholesterol homeostasis in retina and that removal of cholesterol from retinal cells is impaired in hemochromatosis. Since excessive cholesterol is pro-inflammatory, hemochromatosis might promote retinal inflammation via cholesterol in AMD.

Sodium/hydrogen exchanger NHA2 is critical for insulin secretion in β-cells

NHA2 is a sodium/hydrogen exchanger with unknown physiological function. Here we show that NHA2 is present in rodent and human β-cells, as well as β-cell lines. In vivo, two different strains of NHA2-deficient mice displayed a pathological glucose tolerance with impaired insulin secretion but normal peripheral insulin sensitivity. In vitro, islets of NHA2-deficient and heterozygous mice, NHA2-depleted Min6 cells, or islets treated with an NHA2 inhibitor exhibited reduced sulfonylurea- and secretagogue-induced insulin secretion. The secretory deficit could be rescued by overexpression of a wild-type, but not a functionally dead, NHA2 transporter. NHA2 deficiency did not affect insulin synthesis or maturation and had no impact on basal or glucose-induced intracellular Ca(2+) homeostasis in islets. Subcellular fractionation and imaging studies demonstrated that NHA2 resides in transferrin-positive endosomes and synaptic-like microvesicles but not in insulin-containing large dense core vesicles in β-cells. Loss of NHA2 inhibited clathrin-dependent, but not clathrin-independent, endocytosis in Min6 and primary β-cells, suggesting defective endo-exocytosis coupling as the underlying mechanism for the secretory deficit. Collectively, our in vitro and in vivo studies reveal the sodium/proton exchanger NHA2 as a critical player for insulin secretion in the β-cell. In addition, our study sheds light on the biological function of a member of this recently cloned family of transporters.