Disease relevance of IREB2

• During the late phase of hypoxia, IRP2 RNA-binding activity increases, becoming the dominant regulator responsible for decreasing Ft synthesis [1].
• DESIGN AND METHODS: Specific antibodies were used in RNA-supershift, immunoblotting and immunohistochemistry assays to evaluate IRP1 and IRP2 separately in monocytes, macrophages and duodenum of control subjects, and patients with HH or iron-deficiency anemia [2].
• Screening for mutations of the IRP2 gene in Parkinson's disease patients with hyperechogenicity of the substantia nigra [3].
• Knock-out IRP1 and IRP2 mice have shown that in latter mice brain iron accumulation precedes the neurodegeneration, ataxia and bradykinesia observed in these animals [4].
• HPLC-proven yersiniabactin-producing E. coli lacked modifications found in irp1 and irp2 in the human pathogen CFT073, and it is not clear whether CFT073 produces yersiniabactin [5].

Psychiatry related information on IREB2

• Preliminary demonstration of an allelic association of the IREB2 gene with Alzheimer's disease [6].

High impact information on IREB2

• Here, c-MYC is shown to repress the expression of the heavy subunit of the protein ferritin (H-ferritin), which sequesters intracellular iron, and to stimulate the expression of the iron regulatory protein-2 (IRP2), which increases the intracellular iron pool [7].
• This conclusion was confirmed with hybrid proteins in which IRP-1 surface loops were grafted into IRP-2 [8].
• Requirements for iron-regulated degradation of the RNA binding protein, iron regulatory protein 2 [9].
• Here, we demonstrate that a 73 amino acid sequence that corresponds to a unique exon in IRP2 contains a sequence required for rapid degradation in iron-replete cells [9].
• Sodium nitroprusside promotes IRP2 degradation via an increase in intracellular iron and in the absence of S nitrosylation at C178 [10].

Chemical compound and disease context of IREB2

• Iron regulates IRP2 by mediating its rapid proteasomal degradation, where hypoxia and the hypoxia mimetics CoCl2 and desferrioxamine (DFO) stabilize it [11].

Biological context of IREB2

• Here we describe the consequences of IRP regulation and show that iron homeostasis is regulated in 2 phases during hypoxia: an early phase where IRP1 RNA-binding activity decreases and iron uptake and Ft synthesis increase, and a late phase where IRP2 RNA-binding activity increases and iron uptake and Ft synthesis decrease [1].
• In mammalian cells, iron homeostasis is largely regulated by post-transcriptional control of gene expression through the binding of iron-regulatory proteins (IRP1 and IRP2) to iron-responsive elements (IREs) contained in the untranslated regions of target mRNAs [12].
• This second gene product, which we call IRP2, is expressed in many tissues, but its mRNA abundance and tissue distribution are different from IRP1 [13].
• The Caenorhabditis elegans genome does not contain an IRP2 homolog or identifiable IREs; its IRP1 homolog has aconitase activity but does not bind to mammalian IREs [12].
• Whereas IRP2 is known to be involved in Fe homeostasis, the role of IRP1 is less clear; it may provide a link between citrate and iron metabolisms and be involved in oxidative stress response [14].

Anatomical context of IREB2

• In most cell lines tested, levels of IRP2 are inversely regulated by iron levels due to iron-dependent regulation of the half-life of the protein [13].
• IRP2 levels increased when monocytes differentiated to macrophages [2].
• INTERPRETATION AND CONCLUSIONS: These findings of the first extensive investigation of the comparative expression of the two IRP in human tissues and blood cells indicate that IRP2 is the major regulator of intracellular iron homeostasis in humans [2].
• We have investigated changes in activity and expression of both IRP1 and IRP2 during phorbol 12-myristate 13-acetate (PMA)-induced differentiation of HL-60 cells [15].
• Here, we report that while IRP-1 is present at similar levels in both Alzheimer and control brain tissue, IRP-2 shows striking differences and is associated with intraneuronal lesions, including neurofibrillary tangles, senile plaque neurites and neuropil threads [16].

Associations of IREB2 with chemical compounds

• The pathway for IRP2 degradation involving 2-oxoglutarate-dependent oxygenase(s) does not require the E3 ubiquitin ligase activity of pVHL [17].
• Site-directed mutagenesis of IRP1 and IRP2 reveals that, although the binding affinities for consensus IREs are indistinguishable, the contributions of arginine residues in the active-site cleft to the binding affinity are different in the two RNA binding sites [18].
• Moreover, mutation of one cysteine (C178S) prevents NO-mediated degradation of IRP2 [19].
• Actinomycin D, cycloheximide, succinylacetone, and dimethyl-oxalylglycine antagonize IRP2 degradation in response to both FAC and SNP, suggesting a common mechanistic basis [10].
• It has been proposed that nitroprusside ([Fe(CN)5NO]2-) fails to donate iron into cells and that it promotes IRP2 degradation via S nitrosylation at C178 [10].

Physical interactions of IREB2

• An In vivo coimmunoprecipitation assay showed that IRP2 bound to YB-1 in the presence of iron and a proteasome inhibitor [20].
• Multiple, conserved iron-responsive elements in the 3'-untranslated region of transferrin receptor mRNA enhance binding of iron regulatory protein 2 [21].
• We show here that IRP2 can interact with pVHL in co-transfection/co-immunoprecipitation assays [17].
• Increased IRP1 and IRP2 RNA binding activity accompanies a reduction of the labile iron pool in HFE-expressing cells [22].
• Control experiments excluded IRP-1 binding to the IRP-2-specific sequence in vivo [23].

Regulatory relationships of IREB2

• Furthermore, pVHL is able to promote the ubiquitination and the decay of transfected IRP2 [17].
• Here we report that purified recombinant IRP2 inhibits translation of ferritin mRNAs with a molar efficacy equal to that of recombinant IRP1 [24].
• The mRNAs for TfR-1 and, potentially, FPN-1 are posttranscriptionally regulated by iron regulatory protein (IRP)-1 and IRP-2 [25].

Other interactions of IREB2

• We then examined the biological effect of the interaction of YB-1 and IRP2 on translational regulation [20].
• As an inappropriate up-regulation of total IRP activity has been found in the duodenum and monocytes of patients with hereditary hemochromatosis (HH), we investigated the respective roles of IRP1 and IRP2 in these settings [2].
• The concentration and activity of the iron regulatory proteins (IRP1 and IRP2) were analyzed to determine whether there was a functional deficit in the post-transcriptional regulatory mechanism for TfR expression [26].
• Four 5' flanking sequences, comprising the 5'-UTR and upstream promoter region, have been isolated and cloned from the 1-aminocyclopropane-1-carboxylate (ACC) oxidase gene family of white clover (Trifolium repens L.), and designated TR-ACO1p, TR-ACO2p, TR-ACO3p and TR-ACO4p [27].
• Cobalt, which mimics hypoxia by activation of hypoxia-inducible factor 1 (HIF-1), also increases IRP2 protein levels; however, cobalt-induced IRP2 lacks RNA binding activity [28].

Analytical, diagnostic and therapeutic context of IREB2

• Site-directed mutagenesis has demonstrated that specific cysteines within the IRP2 exon are required for iron-dependent degradation [9].
• Western blot analysis and supershift assays showed that this cytosolic protein is neither IRP1 nor IRP2 [29].
• RNA-based PCR amplification of ACC oxidase cDNAs from ethylene-treated corollas and wounded leaves revealed transcripts for ACO1, ACO3 and ACO4 indicating that a least three of these genes are transcriptionally active [30].
• Double immunofluorescence labeling showed that IRP1, IRP2, DMT1 and -IRE DMT1 were mostly expressed in GFAP positive astrocytes [31].

References