Regulation of cellular iron metabolism

Abstract

Iron is an essential but potentially hazardous biometal. Mammalian cells require sufficient amounts of iron to satisfy metabolic needs or to accomplish specialized functions. Iron is delivered to tissues by circulating transferrin, a transporter that captures iron released into the plasma mainly from intestinal enterocytes or reticuloendothelial macrophages. The binding of iron-laden transferrin to the cell-surface transferrin receptor 1 results in endocytosis and uptake of the metal cargo. Internalized iron is transported to mitochondria for the synthesis of haem or iron-sulfur clusters, which are integral parts of several metalloproteins, and excess iron is stored and detoxified in cytosolic ferritin. Iron metabolism is controlled at different levels and by diverse mechanisms. The present review summarizes basic concepts of iron transport, use and storage and focuses on the IRE (iron-responsive element)/IRP (iron-regulatory protein) system, a well known post-transcriptional regulatory circuit that not only maintains iron homoeostasis in various cell types, but also contributes to systemic iron balance.

Figure 1. Hormonal regulation of iron efflux from duodenal enterocytes and reticuloendothelial macrophages by hepcidin

Enterocytes absorb inorganic or haem iron from the diet and macrophages phagocytose iron-loaded senescent red blood cells, or acquire iron by other mechanisms (see the main text). Both cell types release Fe²⁺ into the plasma via ferroportin (Fpn), which is incorporated into apo-Tf following oxidation to Fe³⁺ via hephaestin (Heph) or ceruloplasmin (Cp). Hepatocytes generate the iron-regulatory hormone hepcidin in response to high iron or inflammatory signals, which inhibits the efflux of iron via ferroportin and promotes its retention within enterocytes and macrophages.

Figure 2. Cellular iron uptake via the Tf cycle

Iron-loaded holo-Tf binds to TfR1 on the cell surface and the complex undergoes endocytosis via clathrin-coated pits. A proton pump acidifies the endosome, resulting in the release of Fe³⁺, which is subsequently reduced to Fe²⁺ by Steap3 and transported across the endosomal membrane to the cytosol by DMT1. Internalized iron is directed to mitochondria via mitoferrin for metabolic utilization (such as synthesis of haem and ISCs), and excess iron is stored in ferritin. A cytosolic fraction of redox-active intracellular iron constitutes the LIP. The apo-Tf–TfR1 complex is recycled to the cell surface, where apo-Tf is released to capture plasma Fe³⁺.

Figure 3. Typical IRE motif

A typical IRE motif consists of a hexanucleotide loop with the sequence 5′-CAGUGH-3′ (H could be A, C, or U) and a stem, interrupted by a bulge with an unpaired C residue (left) or an asymmetric tetranucleotide bulge (right); the latter is characteristic of ferritin IRE. Base-pairing between C¹ and G⁵ of the loop is functionally important.

Figure 4. Post-transcriptional control of cellular pathways by the IRE–IRP regulatory system

Translational-type IRE–IRP interactions in the 5′UTR modulate the expression of the mRNAs encoding H- and L-ferritin, ALAS2, m-aconitase, ferroportin, HIF-2α, β-APP and α-synuclein, which in turn control iron storage, erythroid iron utilization, energy homoeostasis, iron efflux, hypoxic responses and neurological pathways respectively. Conversely, IRE–IRP interactions in the 3′UTR stabilize the mRNAs encoding TfR1, DMT1, Cdc14A and MRCKα, which are involved in iron uptake, iron transport, the cell cycle and cytoskeletal remodelling respectively. Note that the regulation of DMT1, Cdc14A and MRCKα may require additional factors, and that the IREs in Cdc14A and MRCKα mRNAs are not phylogenetically conserved.

Figure 5. Crystal structure of IRP1

(A) c-Aconitase form (PDB code 2B3X); (B) IRE-binding form (PDB code 2IPY). A three-dimensional structure of this Figure is available at http://www.BiochemJ.org/bj/434/0365/bj4340365add.htm.

Figure 6. Under physiological conditions, IRP1 is regulated by a reversible ISC switch

Iron deficiency promotes ISC disassembly and a conformational rearrangement, resulting in the conversion of IRP1 from c-aconitase to an IRE-binding protein. The ISC is regenerated in iron-replete cells. Hypoxia favours maintenance of the ISC, whereas H₂O₂ or NO promote its disassembly. When the ISC-biogenesis pathway is not operational, iron leads to ubiquitination of apo-IRP1 by the FBXL5 E3 ligase complex (including Skp1, Cul1 and Rbx1), resulting in proteasomal degradation.

Figure 7. Iron and oxygen-dependent regulation of IRP2 stability by FBXL5

IRP2 is stable in iron deficient and/or hypoxic cells; under these conditions, FBXL5 undergoes ubiquitination and proteasomal degradation. An increase in iron and oxygen levels stabilizes FBXL5 by formation of an Fe–O–Fe centre in its haemerythrin domain, triggering the assembly of an E3 ubiquitin ligase complex together with Skp1, Cul1 and Rbx1. This complex ubiquitinates (Ub) IRP2, leading to its recognition by the proteasome and its degradation.