F-box and leucine-rich repeat protein 5 (FBXL5) is required for maintenance of cellular and systemic iron homeostasis

Transcriptome analysis of iron over-accumulating Arabidopsis genotypes uncover putative novel regulators of systemic and retrograde signaling

On account of its competence to accept and donate electrons, iron (Fe) is an essential element across all forms of life, including plants. Maintaining Fe homeostasis requires precise orchestration of its uptake, trafficking, and translocation in order to meet the demand for Fe sinks such as plastids. Plants harboring defects in the systemic Fe transporter OPT3 (OLIGOPEPTIDE TRANSPORTER 3) display constitutive Fe deficiency responses and accumulate toxic levels of Fe in their leaves. Similarly, ectopic expression of IRONMAN (IMA) genes, encoding a family of phloem-localized signaling peptides, triggers the uptake and accumulation of Fe by inhibiting the putative Fe sensor BRUTUS. This study aims at elucidating the mechanisms operating between OPT3-mediated systemic Fe transport, activation of IMA genes in the phloem, and activation of Fe uptake in the root epidermis. Transcriptional profiling of opt3-2 mutant and IMA1/IMA3 overexpressing (IMA Ox) lines uncovered a small subset of genes that were consistently differentially expressed across all three genotypes and Fe-deficient control plants, constituting potential novel regulators of cellular Fe homeostasis. In particular, expression of the the F-box protein At1g73120 was robustly induced in all genotypes, suggesting a putative function in the posttranslational regulation of cellular Fe homeostasis. As further constituents of this module, two plastid-encoded loci that putatively produce transfer ribonucleic acid (tRNA)-derived small ribonucleic acids are possibly involved in retrograde control of root Fe uptake.

Prenatal exome sequencing: A useful tool for the fetal neurologist

Prenatal exome sequencing (pES) is a promising tool for diagnosing genetic disorders when structural anomalies are detected on prenatal ultrasound. The aim of this study was to investigate the diagnostic yield and clinical impact of pES as an additional modality for fetal neurologists who counsel parents in case of congenital anomalies of the central nervous system (CNS). We assessed 20 pregnancies of 19 couples who were consecutively referred to the fetal neurologist for CNS anomalies. pES had a diagnostic yield of 53% (10/19) with most diagnosed pregnancies having agenesis or hypoplasia of the corpus callosum (7/10). Overall clinical impact was 63% (12/19), of which the pES result aided parental decision making in 55% of cases (6/11), guided perinatal management in 75% of cases (3/4), and was helpful in approving a late termination of pregnancy request in 75% of cases (3/4). Our data suggest that pES had a high diagnostic yield when CNS anomalies are present, although this study is limited by its small sample size. Moreover, pES had substantial clinical impact, which warrants implementation of pES in the routine care of the fetal neurologist in close collaboration with gynecologists and clinical geneticists.

Inflaming the Brain with Iron

Iron accumulation and neuroinflammation are pathological conditions found in several neurodegenerative diseases, including Alzheimer's disease (AD) and Parkinson's disease (PD). Iron and inflammation are intertwined in a bidirectional relationship, where iron modifies the inflammatory phenotype of microglia and infiltrating macrophages, and in turn, these cells secrete diffusible mediators that reshape neuronal iron homeostasis and regulate iron entry into the brain. Secreted inflammatory mediators include cytokines and reactive oxygen/nitrogen species (ROS/RNS), notably hepcidin and nitric oxide (·NO). Hepcidin is a small cationic peptide with a central role in regulating systemic iron homeostasis. Also present in the cerebrospinal fluid (CSF), hepcidin can reduce iron export from neurons and decreases iron entry through the blood-brain barrier (BBB) by binding to the iron exporter ferroportin 1 (Fpn1). Likewise, ·NO selectively converts cytosolic aconitase (c-aconitase) into the iron regulatory protein 1 (IRP1), which regulates cellular iron homeostasis through its binding to iron response elements (IRE) located in the mRNAs of iron-related proteins. Nitric oxide-activated IRP1 can impair cellular iron homeostasis during neuroinflammation, triggering iron accumulation, especially in the mitochondria, leading to neuronal death. In this review, we will summarize findings that connect neuroinflammation and iron accumulation, which support their causal association in the neurodegenerative processes observed in AD and PD.

Iron Dyshomeostasis Participated in Rat Hippocampus Toxicity Caused by Aluminum Chloride

Aluminum (Al) alters iron regulatory factors content and leads to the changes in iron-related proteins causing iron accumulation. But limited evidence ascertains this hypothesis. Therefore, our experiment was conducted and four groups of male Wistar rats were orally administrated of 0, 50, 150, and 450 mg/kg BW/d aluminum chloride (AlCl₃) for 90 days by drinking water, respectively. The cognitive function, pathological lesion of hippocampus, oxidative stress, as well as iron-related proteins and iron regulatory factors expression were detected. The results showed that AlCl₃ remarkably induced the oxidative stress and pathological lesion in the hippocampus and impaired the learning-memory ability. The contents of Al and iron increased in all AlCl₃-exposed groups. Meanwhile, the increased divalent metal transporter 1 (DMT1) expression enhanced iron import and the decreased ferroportin 1 (Fpn1) expression reduced iron export in AlCl₃-exposed groups. The iron accumulated and ferritin heavy chains (Fth) expression decreased in all AlCl₃-exposed groups led to an increase in free iron. The study also showed that iron regulatory factor iron regulatory protein 2 (IRP2) was decreased and hepcidin was increased in AlCl₃-exposed groups. The results indicated that AlCl₃ induces iron dyshomeostasis presenting as iron accumulation, the disordered expression of iron import, export, store, and regulatory proteins in rat hippocampus accompanied with oxidative stress, pathological lesion, and impaired learning-memory ability.

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.

Disruption of FBXL5-mediated cellular iron homeostasis promotes liver carcinogenesis

Hepatocellular iron overload elicited by ablation of the iron-sensing ubiquitin ligase FBXL5 promotes liver carcinogenesis induced by exposure to a chemical carcinogen or hepatitis virus, suggesting that FBXL5 is a previously unrecognized oncosuppressor in liver carcinogenesis in mice.

Hepatic iron overload is a risk factor for progression of hepatocellular carcinoma (HCC), although the molecular mechanisms underlying this association have remained unclear. We now show that the iron-sensing ubiquitin ligase FBXL5 is a previously unrecognized oncosuppressor in liver carcinogenesis in mice. Hepatocellular iron overload elicited by FBXL5 ablation gave rise to oxidative stress, tissue damage, inflammation, and compensatory proliferation of hepatocytes and to consequent promotion of liver carcinogenesis induced by exposure to a chemical carcinogen. The tumor-promoting outcome of FBXL5 deficiency in the liver was also found to be effective in a model of virus-induced HCC. FBXL5-deficient mice thus constitute the first genetically engineered mouse model of liver carcinogenesis promoted by iron overload. In addition, dysregulation of FBXL5-mediated cellular iron homeostasis was found to be associated with poor prognosis in human HCC, suggesting that FBXL5 plays a key role in defense against hepatocarcinogenesis.

Regulation of cellular iron metabolism: Iron-dependent degradation of IRP by SCFFBXL5 ubiquitin ligase

Because of essentiality and toxicity of iron in our body, iron metabolism is tightly regulated in cells. In mammalian cells, iron regulatory protein 1 and 2 (IRP1 and IRP2) are the central regulators of cellular iron metabolism. IRPs regulate iron metabolism by interacting with the RNA stem-loop structures, iron-responsive elements (IREs), found on the transcripts encoding proteins involved in iron metabolism only in iron depleted condition. It is also well-known that the ubiquitin system plays central roles in cellular iron regulation because both IRPs having the IRE binding activity are recognized and ubiquitinated by the SCF^FBXL5 ubiquitin ligase in condition of iron-replete. FBXL5, which is a substrate recognition subunit of SCF^FBXL5, senses iron availability via its hemerythrin-like domain. In this small article, current understanding of the roles of SCF^FBXL5-mediated degradation of IRPs played in cellular iron metabolism is discussed.

F-box and leucine-rich repeat protein 5 promotes colon cancer progression by modulating PTEN/PI3K/AKT signaling pathway

The hyper-activation of PI3K/AKT signaling is common in many kinds of malignant tumors and promotes cell growth. Moreover, FBXL5 is reported to play an important role in the progression of gastric cancer and cervical cancer. In this view, this study aims to explore the function of FBXL5 in the progression of colon cancer and determine if PI3K/AKT signaling pathway involves in this process. Western blotting, RT-PCR, and immunohistochemistry were used to detect the expression pattern of FBXL5 in colon cancer tissues and cell lines. Immunofluorescence, Duolink, and immunoprecipitation (IP) assays were performed to evaluate the interaction between FBXL5 and PI3K/AKT signaling. Results showed that FBXL5 was elevated in colon cancer tissues and cells, which had physical interaction with PTEN protein and negatively regulated its expression, whereas positively modulated PI3K, AKT and mTOR expression and their phosphorylation. Besides, FBXL5 promoted cell proliferation and tumorigenesis and inhibited apoptosis by modulating PTEN/PI3K/AKT signaling. In conclusion, this study demonstrated that FBXL5 functioned as an oncogene in the progression of colon cancer through regulating PTEN/PI3K/AKT signaling.

Transcriptome profile of rat genes in bone marrow-derived macrophages at different activation statuses by RNA-sequencing

The underlying mechanisms of macrophage polarization have been detected by genome-wide transcriptome analysis in a variety of mammals. However, the transcriptome profile of rat genes in bone marrow-derived macrophages (BMM) at different activation statuses has not been reported. Therefore, we performed RNA-Sequencing to identify gene expression signatures of rat BMM polarized in vitro with different stimuli. The differentially expressed genes (DEGs) among unactivated (M0), classically activated pro-inflammatory (M1), and alternatively activated anti-inflammatory macrophages (M2) were analyzed by using Gene Ontology and Kyoto Encyclopedia of Genes and Genomes enrichment analysis. In this study, not only we have identified the changes of global gene expression in rat M0, M1 and M2, but we have also made clear systematically the key genes and signaling pathways in the differentiation process of M0 to M1 and M2. These will provide a foundation for future researches of macrophage polarization.

Mutant huntingtin induces iron overload via up-regulating IRP1 in Huntington's disease

Background

Iron accumulation in basal ganglia accompanies neuronal loss in Huntington’s disease (HD) patients and mouse disease models. Disruption of HD brain iron homeostasis occurs before the onset of clinical signs. Therefore, investigating the mechanism of iron accumulation is essential to understanding its role in disease pathogenesis.

Methods

N171-82Q HD transgenic mice brain iron was detected by using Diaminobenzidine-enhanced Perls’ stain. Iron homeostatic proteins including iron response protein 1 (IRP1), transferrin (Tf), ferritin and transferrin receptor (TfR) were determined by using western blotting and immunohistochemistry, and their relative expression levels of RNA were measured by RT-PCR in both N171-82Q HD transgenic mice and HEK293 cells expressing N-terminal of huntingtin.

Results

Iron was increased in striatum and cortex of N171-82Q HD transgenic mice. Analysis of iron homeostatic proteins revealed increased expression of IRP1, Tf, ferritin and TfR in N171-82Q mice striatum and cortex. The same results were obtained in HEK293 cells expressing N-terminal of mutant huntingtin containing 160 CAG repeats.

Conclusion

We conclude that mutant huntingtin may cause abnormal iron homeostatic pathways by increasing IRP1 expression in Huntington’s disease, suggesting potential therapeutic target.

Ferredoxin reductase is critical for p53-dependent tumor suppression via iron regulatory protein 2

In this study, Chen and colleagues investigated the biological function of ferredoxin reductase (FDXR), a target of p53. They generated a Fdxr-deficient mouse model and found that the signal from FDXR to iron homeostasis and the p53 pathway was transduced by ferredoxin 2, a substrate of FDXR, and that p53 played a role in iron homeostasis and was required for FDXR-mediated iron metabolism, suggesting that the FDXR–p53 loop is critical for tumor suppression via iron homeostasis.

Ferredoxin reductase (FDXR), a target of p53, modulates p53-dependent apoptosis and is necessary for steroidogenesis and biogenesis of iron–sulfur clusters. To determine the biological function of FDXR, we generated a Fdxr-deficient mouse model and found that loss of Fdxr led to embryonic lethality potentially due to iron overload in developing embryos. Interestingly, mice heterozygous in Fdxr had a short life span and were prone to spontaneous tumors and liver abnormalities, including steatosis, hepatitis, and hepatocellular carcinoma. We also found that FDXR was necessary for mitochondrial iron homeostasis and proper expression of several master regulators of iron metabolism, including iron regulatory protein 2 (IRP2). Surprisingly, we found that p53 mRNA translation was suppressed by FDXR deficiency via IRP2. Moreover, we found that the signal from FDXR to iron homeostasis and the p53 pathway was transduced by ferredoxin 2, a substrate of FDXR. Finally, we found that p53 played a role in iron homeostasis and was required for FDXR-mediated iron metabolism. Together, we conclude that FDXR and p53 are mutually regulated and that the FDXR–p53 loop is critical for tumor suppression via iron homeostasis.

Modular transcriptional repertoire and MicroRNA target analyses characterize genomic dysregulation in the thymus of Down syndrome infants

Trisomy 21-driven transcriptional alterations in human thymus were characterized through gene coexpression network (GCN) and miRNA-target analyses. We used whole thymic tissue--obtained at heart surgery from Down syndrome (DS) and karyotipically normal subjects (CT)--and a network-based approach for GCN analysis that allows the identification of modular transcriptional repertoires (communities) and the interactions between all the system's constituents through community detection. Changes in the degree of connections observed for hierarchically important hubs/genes in CT and DS networks corresponded to community changes. Distinct communities of highly interconnected genes were topologically identified in these networks. The role of miRNAs in modulating the expression of highly connected genes in CT and DS was revealed through miRNA-target analysis. Trisomy 21 gene dysregulation in thymus may be depicted as the breakdown and altered reorganization of transcriptional modules. Leading networks acting in normal or disease states were identified. CT networks would depict the "canonical" way of thymus functioning. Conversely, DS networks represent a "non-canonical" way, i.e., thymic tissue adaptation under trisomy 21 genomic dysregulation. This adaptation is probably driven by epigenetic mechanisms acting at chromatin level and through the miRNA control of transcriptional programs involving the networks' high-hierarchy genes.

Regulation of iron homeostasis by the p53-ISCU pathway

Accumulation of iron in tissues increases the risk of cancer, but iron regulatory mechanisms in cancer tissues are largely unknown. Here, we report that p53 regulates iron metabolism through the transcriptional regulation of ISCU (iron-sulfur cluster assembly enzyme), which encodes a scaffold protein that plays a critical role in Fe-S cluster biogenesis. p53 activation induced ISCU expression through binding to an intronic p53-binding site. Knockdown of ISCU enhanced the binding of iron regulatory protein 1 (IRP1), a cytosolic Fe-S protein, to an iron-responsive element in the 5′ UTR of ferritin heavy polypeptide 1 (FTH1) mRNA and subsequently reduced the translation of FTH1, a major iron storage protein. In addition, in response to DNA damage, p53 induced FTH1 and suppressed transferrin receptor, which regulates iron entry into cells. HCT116 p53^+/+ cells were resistant to iron accumulation, but HCT116 p53^−/− cells accumulated intracellular iron after DNA damage. Moreover, excess dietary iron caused significant elevation of serum iron levels in p53^−/− mice. ISCU expression was decreased in the majority of human liver cancer tissues, and its reduced expression was significantly associated with p53 mutation. Our finding revealed a novel role of the p53-ISCU pathway in the maintenance of iron homeostasis in hepatocellular carcinogenesis.

Deciphering Mineral Homeostasis in Barley Seed Transfer Cells at Transcriptional Level

In addition to the micronutrient inadequacy of staple crops for optimal human nutrition, a global downtrend in crop-quality has emerged from intensive breeding for yield. This trend will be aggravated by elevated levels of the greenhouse gas carbon dioxide. Therefore, crop biofortification is inevitable to ensure a sustainable supply of minerals to the large part of human population who is dietary dependent on staple crops. This requires a thorough understanding of plant-mineral interactions due to the complexity of mineral homeostasis. Employing RNA sequencing, we here communicate transfer cell specific effects of excess iron and zinc during grain filling in our model crop plant barley. Responding to alterations in mineral contents, we found a long range of different genes and transcripts. Among them, it is worth to highlight the auxin and ethylene signaling factors Arfs, Abcbs, Cand1, Hps4, Hac1, Ecr1, and Ctr1, diurnal fluctuation components Sdg2, Imb1, Lip1, and PhyC, retroelements, sulfur homeostasis components Amp1, Hmt3, Eil3, and Vip1, mineral trafficking components Med16, Cnnm4, Aha2, Clpc1, and Pcbps, and vacuole organization factors Ymr155W, RabG3F, Vps4, and Cbl3. Our analysis introduces new interactors and signifies a broad spectrum of regulatory levels from chromatin remodeling to intracellular protein sorting mechanisms active in the plant mineral homeostasis. The results highlight the importance of storage proteins in metal ion toxicity-resistance and chelation. Interestingly, the protein sorting and recycling factors Exoc7, Cdc1, Sec23A, and Rab11A contributed to the response as well as the polar distributors of metal-transporters ensuring the directional flow of minerals. Alternative isoform switching was found important for plant adaptation and occurred among transcripts coding for identical proteins as well as transcripts coding for protein isoforms. We also identified differences in the alternative-isoform preference between the treatments, indicating metal-affinity shifts among isoforms of metal transporters. Most important, we found the zinc treatment to impair both photosynthesis and respiration. A wide range of transcriptional changes including stress-related genes and negative feedback loops emphasize the importance to withhold mineral contents below certain cellular levels which otherwise might lead to agronomical impeding side-effects. By illustrating new mechanisms, genes, and transcripts, this report provides a solid platform towards understanding the complex network of plant mineral homeostasis.

Deregulation of F-box proteins and its consequence on cancer development, progression and metastasis

F-box proteins are substrate receptors of the SCF (SKP1-Cullin 1-F-box protein) E3 ubiquitin ligase that play important roles in a number of physiological processes and activities. Through their ability to assemble distinct E3 ubiquitin ligases and target key regulators of cellular activities for ubiquitylation and degradation, this versatile group of proteins is able to regulate the abundance of cellular proteins whose deregulated expression or activity contributes to disease. In this review, we describe the important roles of select F-box proteins in regulating cellular activities, the perturbation of which contributes to the initiation and progression of a number of human malignancies.

FBXL5 modulates HIF-1α transcriptional activity by degradation of CITED2

CITED2 is a ubiquitously expressed nuclear protein exhibiting a high affinity for the cysteine-histidine-rich domain 1 (CH1) of the transcriptional co-activators CBP/p300. CITED2 is particularly efficient in the inhibition of the hypoxia-inducible factor-1α (HIF-1α) dependent transcription by competing with it for the interaction with the CH1 domain. Here we report a direct and specific interaction between CITED2 and the F-box and leucine rich repeat protein 5 (FBXL5), a substrate adaptor protein which is part of E3 ubiquitin ligase complexes mediating protein degradation by the proteasome. We demonstrated that depletion of FBXL5 by RNA interference led to an increase of CITED2 protein levels. Conversely, overexpression of FBXL5 caused the decrease of CITED2 protein levels in a proteasome-dependent manner, and impaired the interaction between CITED2 and the CH1 domain of p300 in living cells. In undifferentiated mouse embryonic stem cells, the overexpression of FBXL5 also reduced Cited2 protein levels. Finally, we evidenced that FBXL5 overexpression and the consequent degradation of CITED2 enabled the transcriptional activity of the N-terminal transactivation domain of HIF-1α. Collectively, our results highlighted a novel molecular interaction between CITED2 and FBXL5, which might regulate the steady state CITED2 protein levels and contribute to the modulation of gene expression by HIF-1α.

Iron-binding E3 ligase mediates iron response in plants by targeting basic helix-loop-helix transcription factors

Iron uptake and metabolism are tightly regulated in both plants and animals. In Arabidopsis (Arabidopsis thaliana), BRUTUS (BTS), which contains three hemerythrin (HHE) domains and a Really Interesting New Gene (RING) domain, interacts with basic helix-loop-helix transcription factors that are capable of forming heterodimers with POPEYE (PYE), a positive regulator of the iron deficiency response. BTS has been shown to have E3 ligase capacity and to play a role in root growth, rhizosphere acidification, and iron reductase activity in response to iron deprivation. To further characterize the function of this protein, we examined the expression pattern of recombinant ProBTS::β-GLUCURONIDASE and found that it is expressed in developing embryos and other reproductive tissues, corresponding with its apparent role in reproductive growth and development. Our findings also indicate that the interactions between BTS and PYE-like (PYEL) basic helix-loop-helix transcription factors occur within the nucleus and are dependent on the presence of the RING domain. We provide evidence that BTS facilitates 26S proteasome-mediated degradation of PYEL proteins in the absence of iron. We also determined that, upon binding iron at the HHE domains, BTS is destabilized and that this destabilization relies on specific residues within the HHE domains. This study reveals an important and unique mechanism for plant iron homeostasis whereby an E3 ubiquitin ligase may posttranslationally control components of the transcriptional regulatory network involved in the iron deficiency response.

Endogenous bone regeneration is dependent upon a dynamic oxygen event

Amputation of the digit tip within the terminal phalangeal bone of rodents, monkeys, and humans results in near-perfect regeneration of bone and surrounding tissues; however, amputations at a more proximal level fail to produce the same regenerative result. Digit regeneration is a coordinated, multifaceted process that incorporates signaling from bioactive growth factors both in the tissue matrix and from several different cell populations. To elucidate the mechanisms involved in bone regeneration we developed a novel multi-tissue slice-culture model that regenerates bone ex vivo via direct ossification. Our study provides an integrated multi-tissue system for bone and digit regeneration and allows us to circumvent experimental limitations that exist in vivo. We used this slice-culture model to evaluate the influence of oxygen on regenerating bone. Micro-computed tomography (µCT) and histological analysis revealed that the regenerative response of the digit is facilitated in part by a dynamic oxygen event, in which mutually exclusive high and low oxygen microenvironments exist and vacillate in a coordinated fashion during regeneration. Areas of increased oxygen are initially seen in the marrow and then surrounding areas of vasculature in the regenerating digit. Major hypoxic events are seen at 7 days postamputation (DPA 7) in the marrow and again at DPA 12 in the blastema, and manipulation of oxygen tensions during these hypoxic phases can shift the dynamics of digit regeneration. Oxygen increased to 21% oxygen tension can either accelerate or attenuate bone mineralization in a stage-specific manner in the regenerative timeline. These studies not only reveal a circumscribed frame of oxygen influence during bone regeneration, but also suggest that oxygen may be one of the primary signaling influences during regeneration.

Iron-sulfur cluster biogenesis in mammalian cells: New insights into the molecular mechanisms of cluster delivery

Iron-sulfur (Fe-S) clusters are ancient, ubiquitous cofactors composed of iron and inorganic sulfur. The combination of the chemical reactivity of iron and sulfur, together with many variations of cluster composition, oxidation states and protein environments, enables Fe-S clusters to participate in numerous biological processes. Fe-S clusters are essential to redox catalysis in nitrogen fixation, mitochondrial respiration and photosynthesis, to regulatory sensing in key metabolic pathways (i.e. cellular iron homeostasis and oxidative stress response), and to the replication and maintenance of the nuclear genome. Fe-S cluster biogenesis is a multistep process that involves a complex sequence of catalyzed protein-protein interactions and coupled conformational changes between the components of several dedicated multimeric complexes. Intensive studies of the assembly process have clarified key points in the biogenesis of Fe-S proteins. However several critical questions still remain, such as: what is the role of frataxin? Why do some defects of Fe-S cluster biogenesis cause mitochondrial iron overload? How are specific Fe-S recipient proteins recognized in the process of Fe-S transfer? This review focuses on the basic steps of Fe-S cluster biogenesis, drawing attention to recent advances achieved on the identification of molecular features that guide selection of specific subsets of nascent Fe-S recipients by the cochaperone HSC20. Additionally, it outlines the distinctive phenotypes of human diseases due to mutations in the components of the basic pathway. This article is part of a Special Issue entitled: Fe/S proteins: Analysis, structure, function, biogenesis and diseases.

Diversity of the metal-transporting P1B-type ATPases

The P1B-ATPases are integral membrane proteins that couple ATP hydrolysis to metal cation transport. Widely distributed across all domains of life, these enzymes have been previously shown to transport copper, zinc, cobalt, and other thiophilic heavy metals. Recent data suggest that these enzymes may also be involved in nickel and/or iron transport. Here we have exploited large amounts of genomic data to examine and classify the various P1B-ATPase subfamilies. Specifically, we have combined new methods of data partitioning and network visualization known as Transitivity Clustering and Protein Similarity Networks with existing biochemical data to examine properties such as length, speciation, and metal-binding motifs of the P1B-ATPase subfamily sequences. These data reveal interesting relationships among the enzyme sequences of previously established subfamilies, indicate the presence of two new subfamilies, and suggest the existence of new regulatory elements in certain subfamilies. Taken together, these findings underscore the importance of P1B-ATPases in homeostasis of nearly every biologically relevant transition metal and provide an updated framework for future studies.

Iron in Eukarya

Iron is essential for the normal physiological function of all organisms. In humans it is required for a plethora of biochemical roles including the transport of oxygen in the blood and energy production in the mitochondria. However, iron is also highly cytotoxic when present at high levels as it readily participates in oxidation-reduction reactions that lead to the generation of reactive oxygen species. One unique feature of iron biology is the lack of excretory mechanisms to remove excess iron from the body. Therefore, the concerted action of several genes and proteins working together to regulate the movement of iron across cell membranes, its storage in peripheral tissues and its physiological utilization in the body is essential for maintaining iron homeostasis. Humans are exposed to iron in a number of chemical forms (haem or non-haem; ferric or ferrous). This chapter will describe how humans acquire iron from their diet; the subsequent delivery of iron to its sites of utilization and storage; and how iron is recycled from effete erythrocytes for re-use in metabolism. Mutations in a number of the genes controlling iron metabolism have been identified and study of the pathological consequences of these mutations has allowed us to gain a greater understanding of how the body senses changes in iron status and coordinates its transport, storage and utilization to maintain homeostasis.

The IRP/IRE system in vivo: insights from mouse models

Iron regulatory proteins 1 and 2 (IRP1 and IRP2) post-transcriptionally control the expression of several mRNAs encoding proteins of iron, oxygen and energy metabolism. The mechanism involves their binding to iron responsive elements (IREs) in the untranslated regions of target mRNAs, thereby controlling mRNA translation or stability. Whereas IRP2 functions solely as an RNA-binding protein, IRP1 operates as either an RNA-binding protein or a cytosolic aconitase. Early experiments in cultured cells established a crucial role of IRPs in regulation of cellular iron metabolism. More recently, studies in mouse models with global or localized Irp1 and/or Irp2 deficiencies uncovered new physiological functions of IRPs in the context of systemic iron homeostasis. Thus, IRP1 emerged as a key regulator of erythropoiesis and iron absorption by controlling hypoxia inducible factor 2α (HIF2α) mRNA translation, while IRP2 appears to dominate the control of iron uptake and heme biosynthesis in erythroid progenitor cells by regulating the expression of transferrin receptor 1 (TfR1) and 5-aminolevulinic acid synthase 2 (ALAS2) mRNAs, respectively. Targeted disruption of either Irp1 or Irp2 in mice is associated with distinct phenotypic abnormalities. Thus, Irp1(-/-) mice develop polycythemia and pulmonary hypertension, while Irp2(-/-) mice present with microcytic anemia, iron overload in the intestine and the liver, and neurologic defects. Combined disruption of both Irp1 and Irp2 is incombatible with life and leads to early embryonic lethality. Mice with intestinal- or liver-specific disruption of both Irps are viable at birth but die later on due to malabsorption or liver failure, respectively. Adult mice lacking both Irps in the intestine exhibit a profound defect in dietary iron absorption due to a "mucosal block" that is caused by the de-repression of ferritin mRNA translation. Herein, we discuss the physiological function of the IRE/IRP regulatory system.

FBXL5 targets cortactin for ubiquitination-mediated destruction to regulate gastric cancer cell migration

Cortactin, an actin-interacting protein, is implicated in cytoskeletal architecture and often amplified in several types of cancer including gastric adenocarcinomas. Downregulation of cortactin decreases cell migration and invasion. However, how to regulate cortactin in gastric cancer remains largely unknown. Here, we report that FBXL5 interacts with and targets cortactin for ubiquitylation and subsequent proteasomal degradation. Furthermore, we showed that FBXL5-induced cortactin degradation is mediated by extracellular regulated signal kinase (ERK). Serine phosphorylation sites mutant, cortactinS405A/S418A, prevent FBXL5-induced cortactin degradation. Moreover, CortactinS405A/S418A exhibited stronger effects in promoting gastric cancer cell migration when compared to wild-type cortactin. Taken together, our data suggested a novel molecular mechanism for the negative regulation of cortactin by FBXL5 in gastric cancer cells migration.

Roles of F-box proteins in cancer

F-box proteins, which are the substrate-recognition subunits of SKP1-cullin 1-F-box protein (SCF) E3 ligase complexes, have pivotal roles in multiple cellular processes through ubiquitylation and subsequent degradation of target proteins. Dysregulation of F-box protein-mediated proteolysis leads to human malignancies. Notably, inhibitors that target F-box proteins have shown promising therapeutic potential, urging us to review the current understanding of how F-box proteins contribute to tumorigenesis. As the physiological functions for many of the 69 putative F-box proteins remain elusive, additional genetic and mechanistic studies will help to define the role of each F-box protein in tumorigenesis, thereby paving the road for the rational design of F-box protein-targeted anticancer therapies.

F-box and leucine-rich repeat protein 5 (FBXL5): sensing intracellular iron and oxygen

Though essential for many vital biological processes, excess iron results in the formation of damaging reactive oxygen species (ROS). Therefore, iron metabolism must be tightly regulated. F-box and leucine-rich repeat protein 5 (FBXL5), an E3 ubiquitin ligase subunit, regulates cellular and systemic iron homeostasis by facilitating iron regulatory protein 2 (IRP2) degradation. FBXL5 possesses an N-terminal hemerythrin (Hr)-like domain that mediates its own differential stability by switching between two different conformations to communicate cellular iron availability. In addition, the FBXL5-Hr domain also senses O2 availability, albeit by a distinct mechanism. Mice lacking FBXL5 fail to sense intracellular iron levels and die in utero due to iron overload and exposure to damaging levels of oxidative stress. By closely monitoring intracellular levels of iron and oxygen, FBLX5 prevents the formation of conditions that favor ROS formation. These findings suggest that FBXL5 is essential for the maintenance of iron homeostasis and is a key sensor of bioavailable iron. Here, we describe the iron and oxygen sensing mechanisms of the FBXL5 Hr-like domain and its role in mediating ROS biology.

The interplay between mitochondrial protein and iron homeostasis and its possible role in ageing

Free (labile or chelatable) iron is extremely redox-active and only represents a small fraction of the total mitochondrial iron population. Several studies have shown that the proportion of free iron increases with age, leading to increased Fenton chemistry in later life. It is not clear why free iron accumulates in mitochondria, but it does so in parallel with an inability to degrade and recycle damaged proteins that causes loss of mitochondrial protein homeostasis (proteostasis). The increase in oxidative damage that has been shown to occur with age might be explained by these two processes. While this accumulation of oxidative damage has often been cited as causative to ageing there are examples of model organisms that possess high levels of oxidative damage throughout their lives with no effect on lifespan. Interestingly, these same animals are characterised by an outstanding ability to maintain correct proteostasis during their entire life. ROS can damage critical components of the iron homeostasis machinery, while the efficacy of mitochondrial quality control mechanisms will determine how detrimental that damage is. Here we review the interplay between iron and organellar quality control in mitochondrial dysfunction and we suggest that a decline in mitochondrial proteostasis with age leaves iron homeostasis (where several key stages are thought to be dependent on proteostasis machinery) vulnerable to oxidative damage and other age-related stress factors. This will have severe consequences for the electron transport chain and TCA cycle (among other processes) where several components are acutely dependent on correct assembly, insertion and maintenance of iron-sulphur clusters, leading to energetic crisis and death.

Parallel SCF adaptor capture proteomics reveals a role for SCFFBXL17 in NRF2 activation via BACH1 repressor turnover

Modular cullin-RING E3 ubiquitin ligases (CRLs) use substrate binding adaptor proteins to specify target ubiquitylation. Many of the ~200 human CRL adaptor proteins remain poorly studied due to a shortage of efficient methods to identify biologically relevant substrates. Here, we report the development of parallel adaptor capture (PAC) proteomics and its use to systematically identify candidate targets for the leucine-rich repeat family of F-box proteins (FBXLs) that function with SKP1-CUL1-F-box protein (SCF) E3s. In validation experiments, we identify the unstudied F-box protein FBXL17 as a regulator of the NFR2 oxidative stress pathway. We demonstrate that FBXL17 controls the transcription of the NRF2 target HMOX1 via turnover of the transcriptional repressor BACH1 in the absence or presence of extrinsic oxidative stress. This work identifies a role for SCF(FBXL17) in controlling the threshold for NRF2-dependent gene activation and provides a framework for elucidating the functions of CRL adaptor proteins.

Iron metabolism in the CNS: implications for neurodegenerative diseases

Abnormal accumulation of brain iron has been detected in various neurodegenerative diseases, but the contribution of iron overload to pathology remains unclear. In a group of distinctive brain iron overload diseases known as 'neurodegeneration with brain iron accumulation' (NBIA) diseases, nine disease genes have been identified. Brain iron accumulation is observed in the globus pallidus and other brain regions in NBIA diseases, which are often associated with severe dystonia and gait abnormalities. Only two of these diseases, aceruloplasminaemia and neuroferritinopathy, are directly caused by abnormalities in iron metabolism, mainly in astrocytes and neurons, respectively. Understanding the early molecular pathophysiology of these diseases should aid insights into the role of iron and the design of specific therapeutic approaches.