Genetic ablations of iron regulatory proteins 1 and 2 reveal why iron regulatory protein 2 dominates iron homeostasis (original) (raw)
Related papers
Post-transcriptional regulation of cellular iron homeostasis
Cellular iron homeostasis is achieved by the coordinate and reciprocal post-transcriptional regulation of transferrin receptor 1 (TfR1) and ferritin expression, which are key molecules for iron acquisition and storage, respectively. The mechanism involves interactions between cytoplasmic iron regulatory proteins (IRP1 and IRP2) and iron responsive elements (IREs) located within non-coding sequences of TfR1 and ferritin mRNAs. In iron-deficient cells, IRE/IRP interactions stabilize TfR1 mRNA and inhibit the translation of the mRNAs encoding H-and L-ferritin. The RNA-binding activities of IRP1 and IRP2 are induced in iron deficiency. In iron-replete cells, IRP1 assembles a cubane [4Fe-4S] iron sulfur cluster (ISC), which Correspondence/Reprint request: Dr. K. Carine Fillebeen et al. # % prevents the binding to IREs and converts the protein to a (cytosolic) c-aconitase. By contrast, IRP2 undergoes ubiquitination and degradation by the proteasome. IRPs respond not only to cellular iron ...
Cellular Iron Metabolism – The IRP/IRE Regulatory Network
2012
Non-haem non iron-sulphur proteins, these proteins can be of three types: Mononuclear non-haem iron enzymes such as catechol or Rieske dioxygenases, alpha-keto acid dependent enzymes, pterin-dependent hydrolases, lipoxygenases and bacterial superoxide dismutases Dinuclear non-haem iron enzymes, also known as diiron proteins, like the H-ferritin chain, haemerythrins, ribonucleotide redictase R2 subunit, stearoyl-CoA desaturases and bacterial monoxygenases Proteins involved in ferric iron transport, for instance the transferrin family that includes serotransferrin, lactotransferrin, ovotransferrin and melanotransferrin and are found in physiological fluids of many vertebrates. As previously mentioned, many proteins involved in very different cellular pathways contain iron. Therefore, cells require iron to function properly. However, mammals have no physiological excretion mechanisms to release an excess of iron and consequently, iron homeostasis must be tightly controlled on both the systemic and cellular levels to provide just the right amounts of iron at all times. If an adequate balance of iron is not achieved, it will cause a clinical disorder. Iron is therefore crucial for health. Iron deficiency leads to anaemia-a major worldwide public health problem-and iron overload is toxic and increases the oxidative stress of body tissues leading to inflammation, cell death, system organ dysfunction, and cancer (Hentze et al., 2010). Systemic iron homeostasis is regulated by the hepcidin/ferroportin system in vertebrates (Ganz & Nemeth, 2011). Hepcidin is a liver-specific hormone secreted in response to iron loading and inflammation and is the master regulator of systemic iron homeostasis. Increased hepcidin levels result in anaemia while decreased expression is a causative feature in most primary iron overload diseases. Transcription of hepcidin in hepatocytes is regulated by a variety of stimuli including cytokines (TNF-, IL-6), erythropoiesis, iron stores and hypoxia (De Domenico et al., 2007). At the molecular level, the binding of hepcidin to the iron exporter ferroportin (FPN) induces its internalization and degradation; and thus prevents iron entry into plasma (Nemeth et al., 2004). Cellular iron homeostasis is mainly controlled by a system composed of RNA binding proteins and RNA binding elements that constitutes a post-transcriptional gene expression regulation system known as the Iron Regulatory Protein (IRP) / Iron-Responsive Element (IRE) regulatory network (Hentze et al., 2010; Muckenthaler et al., 2008; Recalcati et al., 2010). This chapter will focus on the IRP/IRE regulatory network, addressing in depth, its role in the regulation of cellular iron homeostasis, its alterations in diseases and new research lines to be explored in the future. 2. Cellular iron homeostasis Cellular iron maintenance involves the coordination of iron uptake, utilization, and storage to ensure appropriate levels of iron inside the cell. Although transcriptional regulation of iron metabolism has been reported in the literature; cellular iron homeostasis is mainly controlled at the post-transcriptional level (Muckenthaler et al., 2008). In general, posttranscriptional regulation ensures a faster and easier way of controlling protein expression levels in mammalians by changing the rate of specific mRNA synthesis using repressor or www.intechopen.com Cellular Iron Metabolism-The IRP/IRE Regulatory Network 27 stabilizer proteins. Particularly in iron metabolism, this system involves the so-called IRP/IRE regulatory network. 2.1 The IRP/IRE regulatory network The Iron Regulatory Protein (IRP) / Iron-Responsive Element (IRE) regulatory network is a post-transcriptional gene expression regulation system that controls cellular iron homeostasis. This network comprises two RNA binding proteins called Iron Regulatory Proteins (IRP1 and IRP2) and cis-regulatory RNA elements, named Iron-Responsive Elements, or IRE, that are present in mRNAs encoding for important proteins of iron homeostasis.
Blood, 2011
Iron regulatory proteins (IRPs) 1 and 2 are RNA-binding proteins that control cellular iron metabolism by binding to conserved RNA motifs called iron-responsive elements (IREs). The currently known IRP-binding mRNAs encode proteins involved in iron uptake, storage, and release as well as heme synthesis. To systematically define the IRE/IRP regulatory network on a transcriptome-wide scale, IRP1/IRE and IRP2/IRE messenger ribonucleoprotein complexes were immunoselected, and the mRNA composition was determined using microarrays. We identify 35 novel mRNAs that bind both IRP1 and IRP2, and we also report for the first time cellular mRNAs with exclusive specificity for IRP1 or IRP2. To further explore cellular iron metabolism at a system-wide level, we undertook proteomic analysis by pulsed stable isotope labeling by amino acids in cell culture in an iron-modulated mouse hepatic cell line and in bone marrow-derived macrophages from IRP1- and IRP2-deficient mice. This work investigates cellular iron metabolism in unprecedented depth and defines a wide network of mRNAs and proteins with iron-dependent regulation, IRP-dependent regulation, or both.
Iron-regulatory proteins, iron-responsive elements and ferritin mRNA translation
The International Journal of Biochemistry & Cell Biology, 1999
Iron plays a central role in the metabolism of all cells. This is evident by its major contribution to many diverse functions, such as DNA replication, bacterial pathogenicity, photosynthesis, oxidative stress control and cell proliferation. In mammalian systems, control of intracellular iron homeostasis is largely due to posttranscriptional regulation of binding by iron-regulatory RNA-binding proteins (IRPs) to iron-responsive elements (IREs) within ferritin and transferrin receptor (TfR) mRNAs. The TfR transports iron into cells and the iron is subsequently stored within ferritin. IRP binding is under tight control so that it responds to changes in intracellular iron requirements in a coordinate manner by dierentially regulating ferritin mRNA translational eciency and TfR mRNA stability. Several dierent stimuli, as well as intracellular iron levels and oxidative stress, are capable of regulating these RNA±protein interactions. In this mini-review, we shall concentrate on the mechanisms underlying modulation of the interaction of IRPs and the ferritin IRE and its role in regulating ferritin gene expression.
Iron is an essential nutrient but also a potential biohazard. Elaborate homeostatic mechanisms have evolved to regulate dietary iron absorption at levels sufficient to satisfy metabolic needs and prevent the accumulation of metal excess. Internalized dietary iron enters the pool of plasma transferrin for delivery into the erythron and other tissues. Nevertheless, in healthy adults, the daily contribution of dietary iron for erythropoiesis is minimal and the vast majority of circulating transferrin-iron derives from macrophages, that eliminate senescent red blood cells and recycle their iron. Cellular iron uptake is mediated by endocytosis of iron-loaded transferrin upon binding to its transferrin receptor 1 (TfR1). Excess of intracellular iron that is not required for metabolic purposes is stored within ferritin. The expression of TfR1 and ferritin is coordinately and reciprocally controlled by a post-transcriptional mechanism. This involves two cytoplasmic iron regulatory proteins (IRP1 and IRP2), which interact with the iron responsive elements (IREs) of TfR1 and ferritin mRNAs. IRE/IRP interactions that occur in iron-deficient cells, stabilize TfR1 mRNA and inhibit ferritin mRNA translation. In iron-replete cells, IRP1 assembles an aconitase-type [4Fe-4S] 2+ cluster, which precludes IRE-binding. By contrast, IRP2 undergoes iron-dependent proteasomal degradation following ubiquitination. IRPs control the expression of additional mRNAs and respond not only to cellular iron levels but also to other stimuli, such as oxygen, oxidative stress and nitric oxide. The targeted disruption of both IRP1 and IRP2 in mice is associated with early embryonic lethality, underlying the physiological significance of the IRE/IRP regulatory system. While the ablation of IRP1 alone does not manifest any discernible pathology, IRP2(-/-) mice exhibit microcytic anemia and neurological defects. The ongoing development of mouse strains with spatial and temporal disruption of IRPs is providing further insight on their physiological functions.
Proceedings of the National Academy of Sciences, 1997
Iron regulatory proteins (IRPs) are cytoplasmic RNA binding proteins that are central components of a sensory and regulatory network that modulates vertebrate iron homeostasis. IRPs regulate iron metabolism by binding to iron responsive element(s) (IREs) in the 5 or 3 untranslated region of ferritin or transferrin receptor (TfR) mRNAs. Two IRPs, IRP1 and IRP2, have been identified previously. IRP1 exhibits two mutually exclusive functions as an RNA binding protein or as the cytosolic isoform of aconitase. We demonstrate that the Ba͞F3 family of murine pro-B lymphocytes represents the first example of a mammalian cell line that fails to express IRP1 protein or mRNA. First, all of the IRE binding activity in Ba͞F3-gp55 cells is attributable to IRP2. Second, synthesis of IRP2, but not of IRP1, is detectable in Ba͞F3-gp55 cells. Third, the Ba͞F3 family of cells express IRP2 mRNA at a level similar to other murine cell lines, but IRP1 mRNA is not detectable. In the Ba͞F3 family of cells, alterations in iron status modulated ferritin biosynthesis and TfR mRNA level over as much as a 20-and 14-fold range, respectively. We conclude that IRP1 is not essential for regulation of ferritin or TfR expression by iron and that IRP2 can act as the sole IRE-dependent mediator of cellular iron homeostasis.
Insights on Regulation and Function of the Iron Regulatory Protein 1 (IRP1)*
Hemoglobin, 2008
We show that wild type or mutant forms of IRP1 that fail to assemble a [4Fe-4S] cluster are sensitized for iron-dependent degradation by the ubiquitin-proteasome pathway. The regulation of IRP1 abundance poses an alternative mechanism to prevent accumulation of inappropriately high IREbinding activity when the ICS assembly pathway is impaired. To study functional aspects of IRP1, we overexpressed wild type or mutant forms of the protein in human H1299 lung cancer cells in a tetracycline-inducible fashion, and analyzed how this affects cell growth. While the induction of IRP1 did not affect cell proliferation in culture, it dramatically reduced the capacity of the cells to form solid tumor xenografts in nude mice. These data provide a first link between IRP1 and cancer.
Journal of Nutrition
Iron regulatory protein 1 (IRP1) and IRP2 are cytoplasmic RNA binding proteins that are central regulators of mammalian iron homeostasis. We investigated the time-dependent effect of dietary iron deficiency on liver IRP activity in relation to the abundance of ferritin and the iron-sulfur protein mitochondrial aconitase (m-acon), which are targets of IRP action. Rats were fed a diet containing 2 or 34 mg iron/kg diet for 1-28 d. Liver IRP activity increased rapidly in rats fed the iron-deficient diet with IRP1 stimulated by d 1 and IRP2 by d 2. The maximal activation of IRP2 was five-fold (d 7) and three-fold (d 4) for IRP1. By d 4, liver ferritin subunits were undetectable and m-acon abundance eventually fell by 50% (P < 0.05) in iron-deficient rats. m-Acon abundance declined most rapidly from d 1 to 11 and in a manner that was suggestive of a cause and effect type of relationship between IRP activity and m-acon abundance. In liver, iron deficiency did not decrease the activity ...
Iron regulatory protein 1 (IRP1) controls the translation or stability of several mRNAs by binding to iron responsive elements (IREs) within their untranslated regions. Its activity is regulated by an unusual iron-sulfur cluster (ICS) switch. Thus, in iron-replete cells, IRP1 assembles a cubane [4Fe-4S] cluster that prevents RNA-binding activity and renders the protein to cytosolic aconitase. We show that wild type or mutant forms of IRP1 that fail to assemble a [4Fe-4S] cluster are sensi-tized for iron-dependent degradation by the ubiquitin-proteasome pathway. The regulation of IRP1 abundance poses an alternative mechanism to prevent accumulation of inappropriately high IRE-binding activity when the ICS assembly pathway is impaired. To study functional aspects of IRP1, we overexpressed wild type or mutant forms of the protein in human H1299 lung cancer cells in a tetracycline-inducible fashion, and analyzed how this affects cell growth. While the induction of IRP1 did not affect cell proliferation in culture, it dramatically reduced the capacity of the cells to form solid tumor xenografts in nude mice. These data provide a first link between IRP1 and cancer.