Iron deficiency down-regulates the Akt/TSC1-TSC2/mammalian Target of Rapamycin signaling pathway in rats and in COS-1 cells

Regulation of translation in response to iron deficiency in human cells

Protein synthesis is a highly energy-consuming process that is downregulated in response to many environmental stresses or adverse conditions. Studies in the yeast Saccharomyces cerevisiae have shown that bulk translation is inhibited during adaptation to iron deficiency, which is consistent with its requirement for ribosome biogenesis and recycling. Although iron deficiency anemia is the most common human nutritional disorder, how iron modulates translation in mammals is poorly understood. Studies during erythropoiesis have shown that iron bioavailability is coordinated with globin synthesis via bulk translation regulation. However, little is known about the control of translation during iron limitation in other tissues. Here, we investigated how iron depletion affects protein synthesis in human osteosarcoma U-2 OS cells. By adding an extracellular iron chelator, we observed that iron deficiency limits cell proliferation, induces autophagy, and decreases the global rate of protein synthesis. Analysis of specific molecular markers indicates that the inhibition of bulk translation upon iron limitation occurs through the eukaryotic initiation factor eIF2α and mechanistic target of rapamycin (mTOR) pathways. In contrast to other environmental and nutritional stresses, iron depletion does not trigger the assembly of messenger ribonucleoprotein stress granules, which typically form upon polysome disassembly.

Loss of miR-144/451 alleviates β-thalassemia by stimulating ULK1-mediated autophagy of free α-globin

Most cells can eliminate unstable or misfolded proteins through quality control mechanisms. In the inherited red blood cell disorder β-thalassemia, mutations in the β-globin gene (HBB) lead to a reduction in the corresponding protein and the accumulation of cytotoxic free α-globin, which causes maturation arrest and apoptosis of erythroid precursors and reductions in the lifespan of circulating red blood cells. We showed previously that excess α-globin is eliminated by Unc-51-like autophagy activating kinase 1 (ULK1)-dependent autophagy and that stimulating this pathway by systemic mammalian target of rapamycin complex 1 (mTORC1) inhibition alleviates β-thalassemia pathologies. We show here that disrupting the bicistronic microRNA gene miR-144/451 alleviates β-thalassemia by reducing mTORC1 activity and stimulating ULK1-mediated autophagy of free α-globin through 2 mechanisms. Loss of miR-451 upregulated its target messenger RNA, Cab39, which encodes a cofactor for LKB1, a serine-threonine kinase that phosphorylates and activates the central metabolic sensor adenosine monophosphate-activated protein kinase (AMPK). The resultant enhancement of LKB1 activity stimulated AMPK and its downstream effects, including repression of mTORC1 and direct activation of ULK1. In addition, loss of miR-144/451 inhibited the expression of erythroblast transferrin receptor 1, causing intracellular iron restriction, which has been shown to inhibit mTORC1, reduce free α-globin precipitates, and improve hematological indices in β-thalassemia. The beneficial effects of miR-144/451 loss in β-thalassemia were inhibited by the disruption of Cab39 or Ulk1 genes. Together, our findings link the severity of β-thalassemia to a highly expressed erythroid microRNA locus and a fundamental, metabolically regulated protein quality control pathway that is amenable to therapeutic manipulation.

Iron Limitation Restores Autophagy and Increases Lifespan in the Yeast Model of Niemann-Pick Type C1

Niemann-Pick type C1 (NPC1) is an endolysosomal transmembrane protein involved in the export of cholesterol and sphingolipids to other cellular compartments such as the endoplasmic reticulum and plasma membrane. NPC1 loss of function is the major cause of NPC disease, a rare lysosomal storage disorder characterized by an abnormal accumulation of lipids in the late endosomal/lysosomal network, mitochondrial dysfunction, and impaired autophagy. NPC phenotypes are conserved in yeast lacking Ncr1, an orthologue of human NPC1, leading to premature aging. Herein, we performed a phosphoproteomic analysis to investigate the effect of Ncr1 loss on cellular functions mediated by the yeast lysosome-like vacuoles. Our results revealed changes in vacuolar membrane proteins that are associated mostly with vesicle biology (fusion, transport, organization), autophagy, and ion homeostasis, including iron, manganese, and calcium. Consistently, the cytoplasm to vacuole targeting (Cvt) pathway was increased in ncr1∆ cells and autophagy was compromised despite TORC1 inhibition. Moreover, ncr1∆ cells exhibited iron overload mediated by the low-iron sensing transcription factor Aft1. Iron deprivation restored the autophagic flux of ncr1∆ cells and increased its chronological lifespan and oxidative stress resistance. These results implicate iron overload on autophagy impairment, oxidative stress sensitivity, and cell death in the yeast model of NPC1.

Ca2+/Calmodulin induces translocation of membrane-associated TSC2 to the nucleus where it suppresses CYP24A1 expression

Tuberous sclerosis complex 2 (TSC2) is a tumor-suppressor protein. A loss of TSC2 function induces hyperactivation of mechanistic target of rapamycin (mTOR). The C-terminal region of TSC2 contains a calmodulin (CaM) binding region and the CaM-TSC2 interaction contributes to proper mTOR activity. However, other downstream signaling pathways/effectors activated by the CaM-TSC2 complex have not been fully elucidated. In this study, we found that activation of Ca2+/CaM signaling resulted in the translocation of membrane-associated TSC2 to the nucleus and suppressed the transcriptional activity of the vitamin D receptor (VDR). TSC2 was released from the membrane in an activated CaM-dependent state in rat brain and HeLa cells. It subsequently formed a transcriptional complex to partially suppress the transcription of CYP24A1, a well-known VDR target gene. These data suggest, in part, that TSC2 attenuates VDR-associated transcriptional regulation via Ca2+/CaM signaling.

Iron: an underrated factor in aging

Iron is an essential element for virtually all living organisms, but its reactivity also makes it potentially harmful. Iron accumulates with aging, and is associated with many age-related diseases; it also shortens the lifespans of several model organisms. Blocking iron absorption through drugs or natural products extends lifespan. Many life-extending interventions, such as rapamycin, calorie restriction, and old plasma dilution can be explained by the effects they have on iron absorption, excretion, and metabolism. Control of body iron stores so that they remain in a low normal range may be an important, lifespan- and healthspan-extending intervention.

TOR functions as a molecular switch connecting an iron cue with host innate defense against bacterial infection

As both host and pathogen require iron for survival, iron is an important regulator of host-pathogen interactions. However, the molecular mechanism by which how the availability of iron modulates host innate immunity against bacterial infections remains largely unknown. Using the metazoan Caenorhabditis elegans as a model, we demonstrate that infection with a pathogenic bacterium Salmonella enterica serovar Typhimurium induces autophagy by inactivating the target of rapamycin (TOR). Although the transcripts of ftn-1 and ftn-2 encoding two H-ferritin subunits are upregulated upon S. Typhimurium infection, the ferritin protein is kept at a low level due to its degradation mediated by autophagy. Autophagy, but not ferritin, is required for defense against S. Typhimurium infection under normal circumstances. Increased abundance of iron suppresses autophagy by activating TOR, leading to an increase in the ferritin protein level. Iron sequestration, but not autophagy, becomes pivotal to protect the host from S. Typhimurium infection in the presence of exogenous iron. Our results show that TOR acts as a regulator linking iron availability with host defense against bacterial infection.

Iron, an essential nutrient for both hosts and pathogens, can impact host-pathogen interactions. Thus, depending on availability of iron, hosts may use distinct strategies to defend against bacterial infections. Using the model organism Caenorhabditis elegans, we show that autophagy is activated and required for resistance to S. Typhimurium infection under normal conditions. In the presence of exogenous iron, autophagy is suppressed, leading to an increase in the protein level of ferritin, which in turn protects worms against S. Typhimurium infection. Finally, our data demonstrate that TOR is a molecular switch for turning autophagy and ferritin on and off. Thus, our study reveals a previously unknown role of TOR in modulating the transition between autophagy and iron sequestration in response to a bacterial infection via sensing an iron cue.

Clioquinol improves motor and non-motor deficits in MPTP-induced monkey model of Parkinson's disease through AKT/mTOR pathway

Despite decades of research into the pathology mechanisms of Parkinson’s disease (PD), disease-modifying therapy of PD is scarce. Thus, searching for new drugs or more effective neurosurgical treatments has elicited much interest. Clioquinol (CQ) has been shown to have therapeutic benefits in rodent models of neurodegenerative disorders. However, it’s neuroprotective role and mechanisms in PD primate models and PD patients, especially in the advanced stages, are not fully understood. Furthermore, issues such as spontaneous recovery of motor function and high symptom variability in different monkeys after the same toxic protocol, has not been resolved before the present study. In this study, we designed a chronic and long-term progressive protocol to generate a stabilized PD monkey model showed with classic motor and non-motor deficits, followed by treatment analysis of CQ. We found that CQ could remarkably improve the motor and non-motor deficits, which were based on the reduction of iron content and ROS level in the SN and further improvement in pathology. Meanwhile, we also showed that ferroptosis was probably involved in the pathogenesis of PD. In addition, the study shows a positive effect of CQ on AKT/mTOR survival pathway and a blocking effect on p53 medicated cell death in vivo and in vitro.

Glycomacropeptide Prevents Iron/Ascorbate-Induced Oxidative Stress, Inflammation and Insulin Sensitivity with an Impact on Lipoprotein Production in Intestinal Caco-2/15 Cells

Background. Metabolic Syndrome (MetS), a major worldwide concern for the public health system, refers to a cluster of key metabolic components, and represents a risk factor for diabetes and cardiovascular diseases. As oxidative stress (OxS) and inflammation are the major triggers of insulin sensitivity (IS), a cardinal MetS feature, the principal aim of the present work is to determine whether glycomacropeptide (GMP), a milk-derived bioactive peptide, exerts beneficial effects on their expression. Methods. Fully differentiated intestinal Caco-2/15 cells are used to evaluate the preventive action of 2 mg/mL GMP against OxS and inflammation induced by the mixture iron-ascorbate (Fe/Asc) (200 μM:2 mM). The potency of GMP of decreasing the production of lipoproteins, including chylomicrons (CM), very-low-density lipoproteins (VLDL) and low-density lipoproteins (LDL) is also assessed. Results. The administration of GMP significantly reduces malondialdehyde, a biomarker of lipid peroxidation, and raises superoxide dismutase 2 and glutathione peroxidase via the induction of the nuclear factor erythroid 2–related factor 2, a transcription factor, which orchestrates cellular antioxidant defenses. Similarly, GMP markedly lowers the inflammatory agents tumor necrosis factor-α and cyclooxygenase-2 via abrogation of the nuclear transcription factor-kB. Moreover, GMP-treated cells show a down-regulation of Fe/Asc-induced mitogen activated protein kinase pathway, suggesting greater IS. Finally, GMP decreases the production of CM, VLDL, and LDL. Conclusions. Our results highlight the effectiveness of GMP in attenuating OxS, inflammation and lipoprotein biogenesis, as well as improving IS, the key components of MetS. Further investigation is needed to elucidate the mechanisms mediating the preventive action of GMP.

Cross talk between mineral metabolism and meat quality: a systems biology overview

Meat quality has an inherent complexity because of the multiple interrelated causative factors and layers of feedback regulation. Understanding the key factors and their interactions has been challenging, despite the availability of remarkable high-throughput tools and techniques that have provided insights on muscle metabolism and the genetic basis of meat quality. Likewise, we have deepened our knowledge about mineral metabolism and its role in cell functioning. Regardless of these facts, complex traits like mineral content and meat quality have been studied under reductionist approaches. However, as these phenotypes arise from complex interactions among different biological layers (genome, transcriptome, proteome, epigenome, etc.), along with environmental effects, a holistic view and systemic-level understanding of the genetic basis of complex phenotypes are in demand. Based on the state of the art, we addressed some of the questions regarding the interdependence of meat quality traits and mineral content. Furthermore, we sought to highlight potential regulatory mechanisms arising from the genes, miRNAs, and mineral interactions, as well as the pathways modulated by this interplay affecting muscle, mineral metabolism, and meat quality. By answering these questions, we did not intend to give an exhaustive review but to identify the key biological points, the challenges, and benefits of integrative genomic approaches.

A genome-wide transcriptional study reveals that iron deficiency inhibits the yeast TORC1 pathway

Iron is an essential micronutrient that participates as a cofactor in a broad range of metabolic processes including mitochondrial respiration, DNA replication, protein translation and lipid biosynthesis. Adaptation to iron deficiency requires the global reorganization of cellular metabolism directed to optimize iron utilization. The budding yeast Saccharomyces cerevisiae has been widely used to characterize the responses of eukaryotic microorganisms to iron depletion. In this report, we used a genomic approach to investigate the contribution of transcription rates to the modulation of mRNA levels during adaptation of yeast cells to iron starvation. We reveal that a decrease in the activity of all RNA polymerases contributes to the down-regulation of many mRNAs, tRNAs and rRNAs. Opposite to the general expression pattern, many genes including components of the iron deficiency response, the mitochondrial retrograde pathway and the general stress response display a remarkable increase in both transcription rates and mRNA levels upon iron limitation, whereas genes encoding ribosomal proteins or implicated in ribosome biogenesis exhibit a pronounced fall. This expression profile is consistent with an activation of the environmental stress response. The phosphorylation stage of multiple regulatory factors strongly suggests that the conserved nutrient signaling pathway TORC1 is inhibited during the progress of iron deficiency. These results suggest an intricate crosstalk between iron metabolism and the TORC1 pathway that should be considered in many disorders.

Anemia in infancy is associated with alterations in systemic metabolism and microbial structure and function in a sex-specific manner: an observational study

Background

Anemia is a term that describes low hemoglobin concentrations and can result from micronutrient deficiencies, infection, or low birth weight. Early-life anemia, particularly iron-deficiency anemia (IDA) is associated with several negative metabolic, developmental, and cognitive outcomes, some of which persist into adulthood.

Objective

The aim of this study was to investigate alterations in systemic metabolism and fecal microbial diversity and functionality associated with anemia and IDA in male and female infants from Iquitos, Peru.

Design

Cross-sectional stool and serum samples were collected from 95 infants (53 boys and 42 girls) at 12 mo of age. The fecal microbiome was assessed by using 16S ribosomal RNA gene sequencing, and the fecal and serum metabolomes were quantified using 1H-nuclear magnetic resonance.

Results

The prevalence of anemia was 64%, with a greater proportion of anemia in male infants attributed to iron deficiency. Metabolically, anemia was associated with decreased concentrations of tricarboxylic acid cycle metabolites in both sexes (males: succinate, P = 0.031; females: fumarate, P = 0.028). In addition, anemic male infants exhibited significantly lower serum concentrations of several amino acids compared with nonanemic male infants. Although no specific structural or functional differences in the microbiota were observed with anemia in general, likely due the heterogeneity of its etiology, IDA affected the microbiome both structurally and functionally. Specifically, the abundance of butyrate-producing bacteria was lower in IDA subjects of both sexes than in nonanemic, non-iron-deficient subjects of the same sex (females: Butyricicoccus, P = 0.041; males: Coprococcus, P = 0.010; Roseburia, P = 0.027). IDA male infants had higher concentrations of 4-hydroxyphenyllactate (P < 0.001) and putrescine (P = 0.042) than those without IDA, whereas IDA female infants exhibited higher concentrations of leucine (P = 0.011) and valine (P = 0.003).

Conclusions

Sexually dimorphic differences associated with anemia and IDA are suggestive of greater mitochondrial dysfunction and oxidative stress in male infants compared with female infants, and alterations in microbial structure and function may further contribute. Differences in metabolic pathways associated with anemia and IDA in each sex point to potential mechanisms for the associated lasting cognitive deficits. This trial is registered at clinicaltrials.gov as NCT03377777.

Effects of long-term exposures to low iron and branched-chain amino acid containing diets on aging skeletal muscle of Fisher 344 × Brown Norway rats

Aging skeletal muscle displays an altered iron status that may promote oxidative stress and sarcopenia. A diet containing low iron (LI) could reduce muscle iron status and attenuate age-related muscle atrophy. Supplemental branched-chain amino acids (BCAA) may also alleviate sarcopenia by promoting muscle protein synthesis and iron status improvement. This study examined individual and combined effects of LI and BCAA diets on anabolic signaling and iron status in skeletal muscle of aging rats. Twenty-nine-month-old male Fisher 344 × Brown Norway rats consumed the following control-base diets: control + regular iron (35 mg iron/kg) (CR; n = 11); control + LI (∼6 mg iron/kg) (CL; n = 11); 2×BCAA + regular iron (BR; n = 10); and 2×BCAA + LI (BL; n = 12) for 12 weeks. Although LI and/or 2×BCAA did not affect plantaris muscle mass, 2×BCAA groups showed lower muscle iron content than did CR and CL groups (P < 0.05). p70 ribosomal protein S6 kinase phosphorylation was greater in 2×BCAA and LI animals compared with CR animals (P < 0.05). Interactions between IRON and BCAA were observed for proteins indicative of mitochondrial biogenesis (peroxisome proliferator-activated receptor gamma coactivator 1 alpha) and oxidative capacity (cytochrome c oxidase subunit 2 and citrate synthase) (P < 0.05) wherein the combined diet (BL) negated potential benefits of individual diets. Antioxidant capacity, superoxide dismutase activity, and oxidative injury (3-nitrotyrosine, protein carbonyls, and 4-hydroxynonenal) were similar between groups. In conclusion, 12 weeks of LI and 2×BCAA diets showed significant impacts on increasing anabolic signaling as well as ameliorating iron status; however, these interventions did not affect muscle mass.

Neonatal mouse hippocampus: phlebotomy-induced anemia diminishes and treatment with erythropoietin partially rescues mammalian target of rapamycin signaling

BackgroundPhlebotomy-induced anemia (PIA) is common in premature infants and affects neurodevelopment. PIA alters hippocampal metabolism in neonatal mice through tissue hypoxia and iron deficiency. The mammalian target of rapamycin (mTOR) pathway senses the status of critical metabolites (e.g., oxygen, iron), thereby regulating hippocampal growth and function. We determined the effect of PIA and recombinant human erythropoietin (rHuEpo) treatment on mTOR signaling and expression of genes related to mTOR pathway functions.MethodsMice receiving an iron-supplemented diet were phlebotomized from postnatal day (P)3 to a target hematocrit of <25% by P7. Half were maintained at <25% until P14; half received rHuEpo from P7 to increase the hematocrit to 25-28%. Hippocampal phosphorylated to total protein ratios of four key mTOR pathway proteins were measured by western blotting at P14 and compared with non-phlebotomized, non-anemic control mice. mRNA levels of genes regulated by mTOR were measured by quantitative PCR.ResultsPIA suppressed phosphorylation of all mTOR proteins. rHuEpo restored AMP-activated protein kinase (AMPK) and AKT status, and partially rescued the mTOR output protein S6K. PIA and rHuEpo treatment also altered the expression of genes regulated by S6K.ConclusionPIA compromises and rHuEpo treatment partially rescues a pathway regulating neuronal DNA transcription, protein translation, and structural complexity.

FBXL5 Inactivation in Mouse Brain Induces Aberrant Proliferation of Neural Stem Progenitor Cells

FBXL5 is the substrate recognition subunit of an SCF-type ubiquitin ligase that serves as a master regulator of iron metabolism in mammalian cells. We previously showed that mice with systemic deficiency of FBXL5 fail to sense intracellular iron levels and die in utero at embryonic day 8.5 (E8.5) as a result of iron overload and subsequent oxidative stress. This early embryonic mortality has thus impeded study of the role of FBXL5 in brain development. We have now generated mice lacking FBXL5 specifically in nestin-expressing neural stem progenitor cells (NSPCs) in the brain. Unexpectedly, the mutant embryos manifested a progressive increase in the number of NSPCs and astroglia in the cerebral cortex. Stabilization of iron regulatory protein 2 (IRP2) as a result of FBXL5 deficiency led to accumulation of ferrous and ferric iron as well as to generation of reactive oxygen species. Pharmacological manipulation suggested that the phenotypes of FBXL5 deficiency are attributable to aberrant activation of mammalian target of rapamycin (mTOR) signaling. Our results thus show that FBXL5 contributes to regulation of NSPC proliferation during mammalian brain development.

Hepcidin inhibits Smad3 phosphorylation in hepatic stellate cells by impeding ferroportin-mediated regulation of Akt

Hepatic stellate cell (HSC) activation on liver injury facilitates fibrosis. Hepatokines affecting HSCs are largely unknown. Here we show that hepcidin inhibits HSC activation and ameliorates liver fibrosis. We observe that hepcidin levels are inversely correlated with exacerbation of fibrosis in patients, and also confirm the relationship in animal models. Adenoviral delivery of hepcidin to mice attenuates liver fibrosis induced by CCl₄ treatment or bile duct ligation. In cell-based assays, either hepcidin from hepatocytes or exogenous hepcidin suppresses HSC activation by inhibiting TGFβ1-mediated Smad3 phosphorylation via Akt. In activated HSCs, ferroportin is upregulated, which can be prevented by hepcidin treatment. Similarly, ferroportin knockdown in HSCs prohibits TGFβ1-inducible Smad3 phosphorylation and increases Akt phosphorylation, whereas ferroportin over-expression has the opposite effect. HSC-specific ferroportin deletion also ameliorates liver fibrosis. In summary, hepcidin suppresses liver fibrosis by impeding TGFβ1-induced Smad3 phosphorylation in HSCs, which depends on Akt activated by a deficiency of ferroportin.

The peptide hormone hepcidin is released from hepatocytes and regulates iron homoeostasis. Here, the authors show that hepcidin also regulates the activation of hepatic stellate cells (HSCs) in mouse models of liver fibrosis by reducing ferroportin expression and inhibiting the HSC response to TGFβ.

Iron chelation: an adjuvant therapy to target metabolism, growth and survival of murine PTEN-deficient T lymphoma and human T lymphoblastic leukemia/lymphoma

Iron is an essential nutrient, acting as a catalyst for metabolic reactions that are fundamental to cell survival and proliferation. Iron complexed to transferrin is delivered to the metabolism after endocytosis via the CD71 surface receptor. We found that transformed cells from a murine PTEN-deficient T-cell lymphoma model and from T-cell acute lymphoblastic leukemia/lymphoma (T-ALL/T-LL) cell lines overexpress CD71. As a consequence, the cells developed an addiction toward iron whose chelation by deferoxamine (DFO) dramatically affected their survival to induce apoptosis. Interestingly, DFO displayed synergistic activity with three ALL-specific drugs: dexamethasone, doxorubicin, and L-asparaginase. DFO appeared to act through a reactive oxygen species-dependent DNA damage response and potentiated the action of an inhibitor of the PARP pathway of DNA repair. Our results demonstrate that targeting iron metabolism could be an interesting adjuvant therapy for acute lymphoblastic leukemia.

An explanation of the pathophysiology of adverse neurodevelopmental outcomes in iron deficiency

Iron deficiency (ID) is a major public health problem worldwide among children aged 0-12 months. Several factors seem to contribute to the iron-deficient state in infancy, including insufficient antenatal and neonatal iron supplementation, exclusive breastfeeding, and early umbilical cord clamping after birth. The most concerning complications of ID, except for anemia, are related to altered long-term neurodevelopment. Clinical studies have shown a negative impact of ID anemia on fetal and neonatal behavior including impairments of motor maturity, autonomic response, memory/learning, and mood. ID-induced defects during infancy seem to persist later in life, even after ID treatment. The underlying mechanisms involve dysfunctional myelination, neurotransmission alterations, and altered synaptogenesis and/or dendritogenesis. The purpose of the present review is to summarize these mechanisms and to provide recommendations for future clinical research in the field.

A critical role for mTORC1 in erythropoiesis and anemia

Red blood cells (RBC) must coordinate their rate of growth and proliferation with the availability of nutrients, such as iron, but the signaling mechanisms that link the nutritional state to RBC growth are incompletely understood. We performed a screen for cell types that have high levels of signaling through mTORC1, a protein kinase that couples nutrient availability to cell growth. This screen revealed that reticulocytes show high levels of phosphorylated ribosomal protein S6, a downstream target of mTORC1. We found that mTORC1 activity in RBCs is regulated by dietary iron and that genetic activation or inhibition of mTORC1 results in macrocytic or microcytic anemia, respectively. Finally, ATP competitive mTOR inhibitors reduced RBC proliferation and were lethal after treatment with phenylhydrazine, an inducer of hemolysis. These results identify the mTORC1 pathway as a critical regulator of RBC growth and proliferation and establish that perturbations in this pathway result in anemia.

DOI:http://dx.doi.org/10.7554/eLife.01913.001

To multiply and grow, cells need to create more of the molecules—such as proteins—that make up their structure. This only happens if the cell has a good supply of the nutrients used to build the proteins.

Red blood cells are particularly sensitive to the supply of nutrients, especially iron, which is a key component of the hemoglobin molecules that enable the cells to transport oxygen around the body. A lack of iron can lead to a shortage of red blood cells and a condition called anemia. People with mild forms of anemia may feel tired or weak, but more severe forms of anemia can cause heart problems and even death.

A protein called mTOR forms part of a protein complex that helps alert the cells of many different organisms to the presence of nutrients. mTOR can add phosphate groups to ribosomes—the molecular machines that translate molecules of mRNA to build proteins. In 2012, researchers developed a technique called Phospho-Trap that can isolate these phosphorylated ribosomes from cells. Cells with an activated mTOR complex express more mTOR protein and in turn have more ribosomes that are modified. Examining the mRNA molecules associated with these ribosomes can reveal which proteins are produced in greater amounts in these cells.

Previous experiments using Phospho-Trap found the proteins that make up hemoglobin in unexpectedly high amounts in the mouse brain. Now, Knight et al.—and other researchers involved in the 2012 work—have established that the hemoglobin was not coming from the brain cells but from immature red blood cells circulating within the brain. These immature blood cells were found to have a highly active mTOR complex that promotes the production of hemoglobin and new blood cells.

Using genetic techniques in mice, Knight et al. found that the mTOR complex can cause anemia if it is underactive or overactive. Underactive mTOR complexes cause a type of anemia that produces small red blood cells and is usually triggered by a lack of iron. This made sense because mTOR is known to regulate both protein production and cell size. Boosting the activity of the mTOR complex leads to a type of anemia in which the cells are much larger than normal, and which is normally associated with inadequate amounts of folate and B12 vitamins.

When Knight et al. gave mice a drug that inhibits the mTOR protein, the mice developed anemia that resolved when the treatment stopped. However, mice that were given the mTOR inhibitor at the same time as a drug that destroys red blood cells, all died within days. Clinical trials are currently testing mTOR inhibitors as a possible cancer treatment; however, a common side effect of chemotherapy is that it stops new red blood cells being produced. Knight et al. suggest that the red blood cells of patients in these clinical trials must be closely monitored before deciding whether to continue the treatment further.

DOI:http://dx.doi.org/10.7554/eLife.01913.002

Mammalian target of rapamycin coordinates iron metabolism with iron-sulfur cluster assembly enzyme and tristetraprolin

Both iron deficiency and excess are relatively common health concerns. Maintaining the body's levels of iron within precise boundaries is critical for cell functions. However, the difference between iron deficiency and overload is often a question of a scant few milligrams of iron. The mammalian target of rapamycin (mTOR), an atypical Ser/Thr protein kinase, is attracting significant amounts of interest due to its recently described role in iron homeostasis. Despite extensive study, a complete understanding of mTOR function has remained elusive. mTOR can form two multiprotein complexes that consist of mTOR complex 1 (mTORC1) and mTOR complex 2. Recent advances clearly demonstrate that mTORC1 can phosphorylate iron-sulfur cluster assembly enzyme ISCU and affect iron-sulfur clusters assembly. Moreover, mTOR is reported to control iron metabolism through modulation of tristetraprolin expression. It is now well appreciated that the hormonal hepcidin-ferroportin system and the cellular iron-responsive element/iron-regulatory protein regulatory network play important regulatory roles for systemic iron metabolism. Sustained ISCU protein levels enhanced by mTORC1 can inhibit iron-responsive element and iron-regulatory protein binding activities. In this study, hepcidin gene and protein expression in the livers of tristetraprolin knockout mice were dramatically reduced. Here, we highlight and summarize the current understanding of how mTOR pathways serve to modulate iron metabolism and homeostasis as the third iron-regulatory system.

Neuronal-specific iron deficiency dysregulates mammalian target of rapamycin signaling during hippocampal development in nonanemic genetic mouse models

Iron deficiency (ID) is the most common nutrient deficiency worldwide, disproportionally affecting infants, children, and women of childbearing age. Although ID commonly occurs with anemia (IDA), nonanemic ID is 3 times more common than IDA in toddlers and also occurs in infants following gestational complications. Both conditions negatively affect motor, socio-emotional, and cognitive behaviors, suggesting that iron, apart from anemia, has a critical role in neurodevelopment. Here, the specific role of iron in regulation of mammalian target of rapamycin (mTOR) signaling (a kinase pathway that integrates metabolic supply and demand to regulate cell growth and morphology) was examined using 2 hippocampal, pyramidal cell-specific, nonanemic, genetic mouse models of ID: a CAMKIIα cre-loxP permanent knockout of divalent metal transporter-1 (DMT-1 CKO) and a CAMKIIα-tTA-driven reversible, overexpression of nonfunctional, dominant negative transferrin receptor-1 (DN TfR-1). In both models, mTOR activity, assessed by phosphorylation levels of key proteins, was upregulated during development by ID [S6K(Thr389) phosphorylation increased 87 and 57% in the DMT-1 CKO and DN TfR-1 models, respectively; P < 0.05]. This effect was shown to be iron-dependent, because iron repletion at postnatal d 21 normalized mTOR activity in the reversible DN TfR-1 model (62% reduction compared with unrepleted mice; P < 0.05). In the permanent DMT-1 CKO model, suppression of ID-induced mTOR hyperactivity by rapamycin administered during the sensitive period for iron improved Morris water maze performance despite ongoing ID (DMT-1 wild-type and DMT-1 CKO mice reached criterion in 3 d compared with 4 d necessary for vehicle-treated DMT-1 CKO mice; P < 0.05). Together, these findings implicate mTOR dysregulation as a cellular mechanism underlying the acute and persistent neurodevelopmental deficits that accompany early-life ID.

mTOR regulates cellular iron homeostasis through tristetraprolin

Iron is an essential cofactor with unique redox properties. Iron-regulatory proteins 1 and 2 (IRP1/2) have been established as important regulators of cellular iron homeostasis, but little is known about the role of other pathways in this process. Here we report that the mammalian target of rapamycin (mTOR) regulates iron homeostasis by modulating transferrin receptor 1 (TfR1) stability and altering cellular iron flux. Mechanistic studies identify tristetraprolin (TTP), a protein involved in anti-inflammatory response, as the downstream target of mTOR that binds to and enhances degradation of TfR1 mRNA. We also show that TTP is strongly induced by iron chelation, promotes downregulation of iron-requiring genes in both mammalian and yeast cells, and modulates survival in low-iron states. Taken together, our data uncover a link between metabolic, inflammatory, and iron-regulatory pathways, and point toward the existence of a yeast-like TTP-mediated iron conservation program in mammals.

Gestational-neonatal iron deficiency suppresses and iron treatment reactivates IGF signaling in developing rat hippocampus

Gestational-neonatal iron deficiency, a common micronutrient deficiency affecting the offspring of more than 30% of pregnancies worldwide, leads to long-term cognitive and behavioral abnormalities. Preclinical models of gestational-neonatal iron deficiency result in reduced energy metabolism and expression of genes critical for neuronal plasticity and cognitive function, which are associated with a smaller hippocampal volume and abnormal neuronal dendrite growth. Because insulin-like growth factor (IGF) modulates early postnatal cellular growth, differentiation, and survival, we used a dietary-induced rat model to assess the effects of gestational iron deficiency on activity of the IGF system. We hypothesized that gestational iron deficiency attenuates postnatal hippocampal IGF signaling and results in downstream effects that contribute to hippocampal anatomic and functional deficits. At postnatal day (P) 15 untreated gestational-neonatal iron deficiency markedly suppressed hippocampal IGF activation and protein kinase B signaling, and reduced neurogenesis, while elevating extracellular signal-regulated kinase 1/2 signaling and hypoxia-inducible factor-1α expression. Iron treatment beginning at P7 restored IGF signaling, increased neurogenesis, and normalized all parameters by the end of rapid hippocampal differentiation (P30). Expression of the neuron-specific synaptogenesis marker, disc-large homolog 4 (PSD95), increased more rapidly than the glia-specific myelination marker, myelin basic protein, following iron treatment, suggesting a more robust response to iron therapy in IGF-I-dependent neurons than IGF-II-dependent glia. Collectively, our findings suggest that IGF dysfunction is in part responsible for hippocampal abnormalities in untreated iron deficiency. Early postnatal iron treatment of gestational iron deficiency reactivates the IGF system and promotes neurogenesis and differentiation in the hippocampus during a critical developmental period.

The role of iron in learning and memory

Iron deficiency (ID) is the most common nutrient deficiency, affecting 2 billion people and 30% of pregnant women and their offspring. Early life ID affects at least 3 major neurobehavioral domains, including speed of processing, affect, and learning and memory, the latter being particularly prominent. The learning and memory deficits occur while the infants are iron deficient and persist despite iron repletion. The neural mechanisms underlying the short- and long-term deficits are being elucidated. Early ID alters the transcriptome, metabolome, structure, intracellular signaling pathways, and electrophysiology of the developing hippocampus, the brain region responsible for recognition learning and memory. Until recently, it was unclear whether these effects are directly due to a lack of iron interacting with important transcriptional, translational, or post-translational processes or to indirect effects such as hypoxia due to anemia or stress. Nonanemic genetic mouse models generated by conditionally altering expression of iron transport proteins specifically in hippocampal neurons in late gestation have led to a greater understanding of iron's role in learning and memory. The learning deficits in adulthood likely result from interactions between direct and indirect effects that contribute to abnormal hippocampal structure and plasticity.

Hepcidin induces HIV-1 transcription inhibited by ferroportin

BACKGROUND

Physiological regulation of cellular iron involves iron export by the membrane protein, ferroportin, the expression of which is induced by iron and negatively modulated by hepcidin. We previously showed that iron chelation is associated with decreased HIV-1 transcription. We hypothesized that increased iron export by ferroportin might be associated with decreased HIV-1 transcription, and degradation of ferroportin by hepcidin might in turn induce HIV-1 transcription and replication. Here, we analyzed the effect of ferroportin and hepcidin on HIV-1 transcription.

RESULTS

Expression of ferroportin was associated with reduced HIV-1 transcription in 293T cells and addition of hepcidin to ferroportin-expressing cells counteracted this effect. Furthermore, exposure of promonocytic THP-1 cells to hepcidin was associated with decreased ferroportin expression, increased intracellular iron and induction of reporter luciferase gene expression. Finally, exposure of human primary macrophages and CD4+ T cells to hepcidin and iron was also associated with induction of viral production.

CONCLUSION

Our results suggest that the interplay between ferroportin-mediated iron export and hepcidin-mediated degradation of ferroportin might play a role in the regulation of HIV-1 transcription and may be important for understanding of HIV-1 pathogenesis.