Albertine Donker

Chapter 1 20 The size of the LIP is determined by the rate of iron uptake, utilization, storage and export. In order to avoid both intracellular detrimental ID and iron toxic iron excess, a tight regulation is required. This regulation occurs post-transcriptionally and involves two cytoplasmic iron regulatory proteins, iron regulatory protein 1 (IRP1) and iron regulatory protein 2 (IRP2) ( Figure 3 ). 11,15,16,47,48 The iron responsive elements (IRE’s) constitute of binding sites of the un-translated regions (UTR’s) of the mRNA’s encoding ferritin and TfR1. Under conditions of iron starvation, IRP1 and IRP2 bind with high affinity to the IRE’s of the ferritin mRNA's, 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 as well as other signals such as hydrogen peroxide and nitric oxide 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. On the contrary, IRP2 does not bind a Fe/S cluster, but is regulated at the level of protein stability. 48 The IRE/IRP system also controls the expression of additional IRE-containing mRNA's, including the mRNA’s encoding the iron transporters DMT1 and FPN, the enzyme 5-aminolevulinic acid synthase 2 (ALAS2) that catalyzes the first reaction for heme biosynthesis in erythroid progenitor cells, 8 the enzyme of the citric acid cycle mitochondrial aconitase, the cell cycle regulator cell division cycle 14A (CDC14A), and hypoxia inducible factor 2 alpha (HIF2α), a transcription factor that orchestrates molecular responses to hypoxia. 48 The highly complex and sophisticated IRE/IRP system integrates multiple and often opposing signals regarding iron- and oxygen status. As would be expected, the IRE/IRP system impairs translation of HIF2α and FPN in situations of ID since such responses would down-regulate erythropoietin (EPO)-mediated erythropoiesis and would protect enterocytes from intracellular ID. However, such a response might be threatening for iron deficient organisms because iron would be retained in enterocytes instead of being transferred to the plasma. This could be the reason that ID also simulates IRP-mediated stabilization of HIF2α and FPN in combination with HIF2α-mediated transcriptional activation of