Albertine Donker

Chapter 1 22 Figure 3. Cellular iron homeostasis: key role of IRE/IRP system IRON REPLETE CELLS c-aconitase IRP1 Fe-S cluster assembly Inactive Active + High O 2 IRP2 IRP1 Proteasomal degradation TfR1 DMT1 CDC14A IRON DEFICIENT CELLS 3’ UTR IRE mRNA stabilization Fe uptake Ferritin Ferroportin DMT ALAS2 ACo2 HIF2α Fe storage and export 5’ UTR IRE translational repression The size of the LIP is determined by the rate of iron uptake, utilization, storage and export. In order to avoid both intracellular detrimental iron deficiency and iron toxic iron excess, a tight regulation is required. This regulation occurs post-transcriptionally and involves two cytoplasmic iron regulatory proteins, IRP1 and IRP2 that are able to bind to IRE’s. The IRE’s constitute of binding sites of the UTR’s of the mRNA’s encoding ferritin and TfR1. The binding of IRP’s to single IRE’ s in the 5′ UTR’ s of target mRNA's inhibits their translation, whereas IRP interaction with 3′ UTR IRE’s of TfR1 transcript increases its stability. On the contrary, without IRP binding to the 3′ UTR’s of TfR1 , the transcript is susceptible to endonuclease attack and degradation, leading to down-regulation of translation (yellow figure). Under conditions of iron starvation, IRP1 and IRP2 bind with high affinity to the IRE’s of the ferritin mRNAs, thereby inhibiting their translation and preventing intracellular storage or iron. Simultaneously the IRP’s stabilize the translation of TfR1 in order to stimulate cellular iron uptake. By contrast, in iron-replete cells IRE-binding activity of both IRP1 and IRP2 is diminished, resulting in the inhibition of further iron uptake and the stimulation of storage of intracellular iron within ferritin. A characteristic feature of IRP1 is the presence of a Fe/S cluster within its active site. Iron starvation promotes the loss of this Fe/S cluster that triggers a conformational switch, subsequently conferring IRE-binding capacity and conversion from holo-IRP, exhibiting cytosolic aconitase activity to the apo-IRP1 without cytosolic aconitase activity; hence the protein is bifunctional. On the contrary, IRP2 does not bind a Fe/S cluster, but is regulated at the level of protein stability. The IRE/IRP system also controls the expression of additional IRE-containing mRNAs, including the mRNA’s encoding the iron transporters DMT1 and FPN, the enzyme ALAS2 that catalyzes the first reaction for heme biosynthesis in erythroid progenitor cells, the enzyme AC02, the cell cycle regulator CDC14A, and HIF2α, a transcription factor that orchestrates molecular responses to hypoxia. Abbreviations: AC02 denotes citric acid cycle mitochondrial aconitase; ALAS2, 5-aminolevulinic acid synthase 2; CDC14a, Cell division cycle 14A; DMT1, dimetal transporter 1; FNP, ferroportin; HIF2α, hypoxia inducible factor 2 alpha; IRE, iron-responsive elements; IRP, iron-regulatory protein; LIP, labile iron pool; TfR, transferrin receptor; UTR, un-translated regions. Figure adapted from Hentze et al, 2010. 15